Biomass Pelletising Critical Analysis

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IEE/09/758/SI2.558286 - MixBioPells

WP 3.2 / D 3.1.

Crit ical review on the pelletizing technology

Author 1: Markku Kallio; VTT

Date: 11.5.2011

Supported by the European Commission under the EIE programme

The sole responsibility for the content of this report lies with the authors. It does not necessarilyreflect the opinion of the European Communities. The European Commission is not responsible for

any use that may be made of the information contained therein.

Critical review on the

pelletizing technology

Contents 1. INTRODUCTION ........................................................................................................................4 2. RAW MATERIALS .....................................................................................................................6

2.1 Ash content of the raw materials....................................................................................10 3. PROCESSES FOR RAW MATERIAL.......................................................................................13

3.1 Densifying of raw material ............................................................................................14 3.2 Preheting of raw material...............................................................................................15

4. PREPROCESS OF PELLETING................................................................................................16 4.1 Reception of raw material..............................................................................................17 4.2 Screening contaminants of raw material.........................................................................18 4.3 Grinding of particle size of raw material........................................................................18

4.3.1 Expander...............................................................................................................20 4.3.2 Fractionation.........................................................................................................20 4.4 Drying of raw material...................................................................................................21

4.4.1 Emissions of drying ..............................................................................................22 4.5 Conditioning..................................................................................................................23 4.6 Additives of pellets........................................................................................................25

5. PELLETING PROCESS.............................................................................................................27 5.1 Pellet mill......................................................................................................................28 5.2 Some properties of pellets mill.......................................................................................30

5.2.1 Measures of die.....................................................................................................31 5.2.2 Roller wheels ........................................................................................................32 5.2.3 Moisture content ...................................................................................................32 5.2.4 Pelleting temperature ............................................................................................33 5.2.5 Pelletising pressure ...............................................................................................34 5.2.6 Density of the material..........................................................................................35 5.2.7 Particle size of the material ...................................................................................35 5.2.8 Efficiency of the pellet production ........................................................................35

6. COOLING OF PELLETS ...........................................................................................................36 7. SCREENING OF FINE PARTICLES.........................................................................................37 8. QUALITY OF PELLETS ...........................................................................................................37 9. STORAGE OF PELLETS...........................................................................................................39 10. PRODUCTION COSTS............................................................................................................40 11. CONCLUSION.........................................................................................................................41 12. LITERATURE..........................................................................................................................44

1. INTRODUCTIONFuel pellets are usually made from sawdust, wood chips and wood shavings. After experience woodis the best material for pellets in small scale use. Softwood pellets can be very high quality if theyare made with care. EN 14961-standard determines the quality of pellets made of several rawmaterials. Different kinds of high efficiency and low emission burners have been constructed forwood pellets.

On the other hand, in Europe there is lack of wood, so wood is more and more expensive rawmaterial for pellets. In northern Europe wet residual wood without pelletizing has been used as fuelin large CHP-plants of towns and industry. The maximal potential of dry sawdust and wood shavingshas been reached, and wood-based raw material has to be dried, which increases the productioncosts. Particleboard industry uses part of the potential raw material of wood pellets in their

processes. The economical fluctuations effect on capacities of sawmills and furniture industry, andalso causes lack of woody raw material during economic depression. In several European countriesthe forest industry and also the forest areas are much smaller compared to Scandinavian region andthere is a great potential for pellets from alternated materials.

The European wood pellets consumption exceeds 8 million tons annually (1 m3 = 3 MWh, 4.7MWh/t). Over 1.0 million t of wood pellets are imported to Europe from Canada and USA, and theCanadian wood pellet production is growing. Russian wood pellet production is also increasing. A

Russian factory, the capacity of which is one million tons has been built to Vyborg on the coast ofBaltic Sea. Part of the imported wood pellets is used in large CHPs in Belgium, Netherlands,Denmark and Sweden. Quality of these pellets can be lower than that of retail market produced

pellets. Price of wood pellets is about 200 – 300 €/t in small scale use and in large scale use 110 –150 €/t.

Figure 1. World pellet production and use (Pino Vivanco 2008).

Also other biomass residues than sawdust, wood chips and shavings are already used for pellet

production. Residual wood materials have been burned in large scale CHP-plants in severalEuropean countries. Straw pellets of 80 000 t/a (2009, Vattenfall A/S in Denmark) has been made inDenmark for several years (beginning 2004) for Amager power plant. The Perä-Seinäjoki pelletfactory of Vapo Oy produces annually 60 000 t peat and peat-wood pellets. In Sweden bark pelletsare produced in Norrköping. Enhanced wear of pelletizing machinery, caused by the impurities ofalternative raw materials is one drawback of the use of bark. Small diameter wood from forest, firstand second thinning has been pelletized in Northern Sweden (Haaker 2006). By the side of use indistrict heat plants and CHPs alternative pellets have also been used to increase the low heat contentof residue fuel, e.g. in winter time.

In Europe the production of alternative pellets was 352 000 t/a and production capacity 809 000 t/a

in 2008. Denmark, Poland (production growing, sunflower shells), Czech Republic (ECOVERCompany, patent, production license) and Finland (peat) are countries, where the production ofalternative pellets has started. In Denmark the annual production of straw pellets exceeds 100 000t/a. Other countries mentioned produce about 50 000 t/a. In Germany the pellets production isestimated to be 20 000 t/a, which is produced by at least 10 local, small factories. The producedstraw pellets are mostly used e.g. for littering or animal feeding, and the use of them for heat andenergy production is insignificant. Pusch AG plans to set up decentralised MBP productioncapacities. Basis will be a licensing system, in which special pellet production equipment is given tofarmers who produce MBP from local agricultural and waste materials (Bastian 2009).

Several studies of agricultural pellets, both production and use, has been done in Europe. Most of the

studies handle grass or straw based fuels. The production and use of switchgrass and alfalfa (Porteret al. 2008) as an alternate heating fuel has been studied in several researches in Canada. Researchesof pellets, made from mixed raw materials, are very few. According to the experiments with smallscale pellet burners, there has been great difficulties in heating with mixed pellets or pellets fromalternative materials. Usually the amount of ash, formed in combustion, is high, and the ash meltsand creates difficulties in the air supply of the burner.

There are many advantages of densified fuel pellets:• The amount of dust produced is minimised,• The fuel is free flowing, which facilitates material handling and rate of flow control,• The energy density is increased, easing storage and transportation,

• The capital cost for storage is reduced,• Higher uniformity and stability permits more efficient combustion control,• There are less particulates produced during the combustion process,• There are considerable reductions in labour for feedstock handling,• Risk of fire is reduced considerably (Porter et al. 2008).

An often mentioned hurdle for biomass utilization are the logistics inherent to an agricultural product; harvesting, moisture, storage, transportation, quality uniformity etc. Typically, biomass isdelivered to the bio refinery in bulk by railroad cars or by trucks in the form of chopped forage or

baled hay. One way for handling of biomass crops more efficiently is densifying them into bales, pellets, cubes or briquettes to reduce the bulk volume of the material. Although each method has pro’s and con’s, pelleting seems to have the greatest number of advantages. Although pelleting adscosts, pelleted material is floodable and allows the fuel to handled and stored easily and transported

more economically. In addition, pelleted biomass is very homogenous fuel. Pelletizing decreases the

moisture content and allows the pellets to be burned more efficiently (Porter et al. 2008).

2. RAW MATERIALS

In the Northern and Southern parts of Europe possible raw materials for mixed or alternative pelletsare different. In the following list of possible raw materials:

Residues from agriculture (straw, corn cob etc.),

Cultivated energy (grass, cardoon etc.),

Woody residues (prunes from differ sources),

Olive- and rape pressings (residual oil content), Grape-pressings and citrus fruits (stones and stems),

Residues from processing other agricultural commodities (such as coffee and tobacco),

Residues from landscape gardening (grass etc.).

In the project it will be estimated the local relevance of the raw materials in the considered regions.Part of the potential materials include materials which would be disposed, of which the pellet

producer might get a fee, and the another part is cultivated material, for which the pellet producerhas to pay. Bulk density of several agricultural raw materials is low, so they should be pelletisedlocally or pre-densified to avoid high transport coasts.

Residues from agriculture

Figure 2. A conservative estimation of straw potential resources in Europe (million ton of dry matter).Source: personal estimation of Pastre (2002), on the basis of Eurostat figures for the primarydata, to which have been applied several coefficients: ratio production/residue, moisture content

coefficient and availability coefficient (15%). "Total straw" refers to common wheat, durum wheat,rye, meslin, barley, oats, maize, rice and other cereals.

In Europe a special focus should be put on straw (Pastre 2002) mainly for the reasons: straw is the most important agricultural residue in the EU, estimated to be nearly 23 million

tons of dry biomass (present European use of pellets under 10 million tons), figure 2,

great amounts of straw is annually available, it is economically attractive raw material for pellets, straw corresponds to the most tested and pelletised agricultural residue.

Figure 3. Net straw surplus/deficit (1000 tonnes/region) estimated from national studies and cattle breedingdata (Eurostat: Spain – year 2002, Bulgaria – year 2001, other countries - year 2003), Edwards etal. 2006.

Straw is an important fuel at some regions of Europe, as the map of figure 3 shows. Besides differentstraw species (“Total straw” in the figure 2), there are also other straw like specific agriculturalresidues.

Low bulk density creates difficulties besides transport, also in storage because of the storage spaceneeded, and present pelleting machines have been designed for wood raw material, so whenalternative materials are used press works on the lower efficiency than it could.

Straw and grass contain high amounts of potassium and sodium. During combustion, alkalies reactwith silica and cause slagging and fouling problems in conventional combustion equipment designedfor burning wood at higher temperatures. Volatile alkalies also lower the fusion temperature of ash:in conventional combustion equipment having furnace gas exit temperatures above 800 °C,combustion of agricultural residues causes slagging and deposits on heat transfer surfaces. Speciallydesigned boilers with lower furnace exit temperatures or low operation temperature can reduceslagging and fouling from combustion of these fuels (Wach & Bastian 2009).

In Jena it was the First International fuel straw-congress (www.tll.de/ainfo/html/stro0408.htm) in

Cultivated energy

Different grasses have used already as fuel in Europe locally. In Northern Europe as fuel and rawmaterial of pellets and mixes reed canary grass (RCG). RCG is a plant which can grow in same

place several years and it is harvested annually in the spring. Dry yield of the harvest is about 5 – 12t /ha (yield depends on several different circumstances). Moisture content is then about 10 – 15%.The amount of water soluble components (Cl, K) diminishes during the winter. RCG is a modest

plant. It grows in old peat production areas and waste lands with very small amounts of fertiliser and

water. It is usually baled with baling machines of the farm and stored in well covered stockpiles.Stockpile is in the terrain and a truck transport the bales to the market. A drawback in storing is that bales can moisture. RCG is easy to pelletise or briguetaise.

In Central-Europe it is used Miscanthus giganteus as a raw material of biofuels in Europe since theearly 1980s. It can grow to heights of more than 3.5 m in a growth season. Its dry weight annualyield can reach 15-25 tonnes per hectare. Miscantus exhibits greater photosynthetic efficiency andlower water use requirements than other kinds of plants. It has very low nutritional requirements – ithas high nitrogen use efficiency and therefore is capable of growing well on barren land without theaid of heavy fertilization.

The rapid growth, low mineral content, and high biomass yield of Miscanthus make it a favoritechoice as a biofuel. After harvest it is used as a source of heat and electricity, or converted into

biofuel products such as pellets.

Miscanthus outperforms other grasses, such as switchgrass, which yields around 7-11 t/ha of biomass. Switchgrass is popular grass for biofuel in prairie area of North-America. It is sometimescalled "Elephant Grass" and thus confused with the African grass Pennisetum purpureum, alsocalled "Elephant Grass."

Cardoon (Cynara cardunculus L.) is an herbaceous species indicated as one of the most suitableenergy crop for southern European countries. Crop dry yield was not different between the twocultivars and it was rather stable with a mean value averaged from year 3 to 11 t/ha. The chemicalanalysis of cardoon biomass showed with good calorific value (15 MJ /kg) but with an ash content(13.9% d.w.) higher than other herbaceous energy crops. The cultivation results confirmed cardoon’sgood biomass yield and favourable energy balance even in cultivation systems characterised bylimited water input. Moreover future works are necessary in order to improve cardoon biomassquality and to evaluate the possibility of using it in blends with other biomass sources (Angelini etal. 2009).

Woody residues

In Italy woody residues, as dry substance, are more than 700 kt/a, with 30% resulting from pruningactivities related to the cultivation of about 360km2 of vine yards and 450 km2 of olive-groves; theseresidues are concentrated in the hilly part of the region close to the Adriatic seacoast (DiGiacomo &

Taglieri 2009). A properly localized wood pellets production plant could use these residues as a sub

raw material.In Italy woody residues (mainly pruning off-cuts from vineyards and olive groves) are about 3.5mill. t/a, 85% unused (Di Blasi et al 2007).

In the Northern Europe it is used forest residues and stumps as a fuel, but those are chipped orcrushed.

Olive- and rape pressings

In particular, during the production of olive oil it is possible to recover olive pits as a by-product for

energy production for use as fuel in domestic boilers or in large industrial plants for cogeneration(Pattara et al. 2010). On the other hand, the olive pit (Robles Fernández et al. 2009,) is a competitorto pellets. It is cheaper and needs only conditioning, but not any manufacturing process. Italian olive

pit production is estimated 277,000–519,000 t during 1999 – 2007 (Pattara et al 2010).

In Central- and Northern Europe it is pelletised rape residues as mix with wood and other straw-materials. In Denmark there are 4 pellet factories, which use rape as a raw material.

In Ukraine and some other European countries pellets are made of sunflower residues ( pellets-wood.com /agripellets-b351.html).

Grape-pressings and citrus fruits

Nut shells and fruit stones (about 0.2 mil. t/a in Italy), although not widely available on a nationalscale, can be significant on a local basis.

Residues from processing other agricultural

In industry it becomes different by-products as coffee waste, cigar waste, corn waste etc. These can be pelletised as mix and in Denmark (Nikolaisen et al. 2002, 2005) it has done several experimentsthose raw materials. In the experiments included combustion results have not been promising. These

by-products of industry become a lot in every year.

Residues from landscape gardening

Probably it is so small that no commercial meaning. In the appendix 1 has been told pelletising ofsuch rawmaterials.

________

As mentioned, mixing is still rare. Mixing a feed ingredient or biomass material having high natural binding capacity with the base feed to improve the strength and durability of the densified productshas been exploited in several studies (Kaliyan & Morey 2009b). Bradfield and Levi (1984) found

that pure wood of hardwoods (red maple, southern red oak, sweetgum, tupelo, white oak, and yellow poplar) did not produce pellets and they blocked the pellet-mill die for the conditions with or withoutsteam addition. However, mixing 15–35% bark with the pure wood produced pellets with about 93–

99% durability. Holm et al. (2006) mixed pine and beech, and brewers spent grains (BSG) were tested

as a possible organic additive in combination with beech dust. A mixture of 85% (wt) beech and 15%(wt) BSG could be pelletized and the durability of the pellets was increased. Further, when BSG wasadded to the beech dust inorganic powder additives (30% (wt) dry basis) could also be added without any

problems. These inorganic additives could not have been added to the beech dust alone without causinggreat problems in the pellet mill.

In Central-Europe the number of biogas digesters has grown, as well as the amount of the soliddigestate. Digestate can be dried up and pelletized, even for energy use. This kind of alternative forthe raw material of energy pellets is described e.g. in the presentation of Fürstaller et al. (2010).

Agricultural fuel pellets are often referred as fuel pellets without specification. However, the sources

for quality variations are incalculable. There are large differences even between softwoods andhardwoods, between different tree species, and between different parts of trees. The climatic andseasonal variations affect the raw material properties, as well as the length of the storage period andthe type of storage (Lehtikangas 1999).

2.1 Ash content of the raw materials

A drawback of the agricultural fuels is the ash content and the behaviour of them during

combustion. Nitrogen, sulphur and chlorine contents of several alternative raw materials are shownin table 1, and the chemical ash contents of different cereals in table 2. Diversity of agricultural

residues and their mixes are great and it has been tried to find a solution for the ash smelting properties of the alternative pellets through scientific studies and reasoning in production.

Agricultural biomass ashes have in general a low melting point in relation to e.g. coal ash due totheir specific contents of silica (Si), calcium (Ca), potassium (K), chlorine (Cl), sulphur (S) and othermain elements. A significant degree of melting produces slag whereas partial melting may be the

primary cause of agglomeration, sintering and deposit formation (Hjuler 2007). Ash meltingtemperature is raised by Si and Ca, lowered by K. With the tertiary drawings, figure 4, it iscontributed and forecasted of the behavior of mixes in heating. Ash problems will usually appear atleast in left corner of the drawing, where large Cao + MgO contents are.

From table 1 it can be seen that wood-based raw materials include much less nitrogen, sulphur andchlorine compared to alternative raw materials. Those chemicals can create bad combinations duringthe combustion. Hay and grass have the highest values. From the table 2 one can see that content ofsome chemical components can vary significantly between different cereals (SiO2, K 2O, CaO), andsome figures are very equal (P2O, SO3, Na2O). For combustion high content of calcium and lowcontent of potassium is good for raw materials.

Table 1. Composition of shown the nitrogen, sulphur and chlorine contents of several alternative

raw materials (Van Loo. & Koppejan, 2008).

Nitrogen (N) Sulphur (S) Chlorine (Cl)mg/kg (d.b)Woodchips (spruce) 900-1700 70-1000 50-60Woodchips (poplar,willow)

1000-9600 300-1200 100

Bark (spuce) l000-5000 100-2000 100-370Straw (winter wheat) 3000-5000 500-1100 1000-7000

Miscanthus 4000-17000 200-2000 500-4000Triticale (cereals) 6000-14000 1000-1200 1000-3000Hey 10000-24000 2000-6000 2500-20000

Needles (spruce) 11000-17000Grass 4000-36000 800-7000 2600-20000Waste wood 1000-39000 300-2000 300-4000Olive residues 77700-19400 920-1200 1000-3300

Table 2. Compositions of straw ashes in Finland (Wilen et al. 1986).

Component Wheat, % Rye, % Barley, % Oat, %SiO2 78.2 61.7 44.7 37.3K 2O 6.6 19.2 37.1 40.3CaO 5.0 7.4 9.3 12.3P2O 3.3 3.7 3.8 4.1MgO 3.6 2.8 2.5 3.0Al2O3 2.0 2.1 0.4 0.8Fe2O3 1.5 1.5 0.5 0.5SO3 1.4 1.3 1.4 1.4

Na2O 0.3 0.3 0.3 0.3

Presence of chlorine (Cl) in the deposits may lead to the severe metal corrosion. “Intelligent” fuel blending, known from the coal combustion, and introduction of mineral matter (additives) is oneway to recognize possible methods of reducing the above problems. CEN Denmark (Sander 1997)has recommended the maximum target values of 0.2% K and 0.1% Cl for efficient use of biofuelsfor power generation. The Pellet Fuels Institute recommends that chlorine levels should be below300 pp (0.03%) (Campbell 2007).

With the harvesting time it is possible to affect to the amount of water-soluble chemicals, and hencealso to the heating properties. The chlorine (Cl) and potassium (K) content of perennial grassfeedstocks is reduced if a late-season or overwintering harvest management regime is practiced.Burvall (1997) found an 86% reduction in chlorine content of reed canary grass when it was over-wintered in Sweden. Also switch grass is harvested in early October and it was found to contain0.95% potassium (K), while over-wintered switch grass harvested in mid-May was found to contain

just 0.06% potassium (Goel et al., 2000). Same phenomena works also to straw when leached,

"yellow" straw turns into "grey" straw and the content of water-soluble potassium and chlorine

decrease (Wach & Bastian 2009).

Figure 4. Visualisation of biomass ash compositions by using a ternary diagram (Hjuler 2007). Silicon (Si) ispresent in appreciable amounts in the ash, the higher the ratio of calcium plus magnesium (Ca +

Mg) relative to potassium plus sodium (K +Na), the higher the fusion temperature.

Silica (SiO2) is a common chemical found in grasses, deposited in the leaves, leaf stems andinflorescences of plants (Lanning & Eleuterius, 1989). Lanning and Eleuterius (1987) working inKansas prairie stands, found switch grass silica contents to be lowest in stems and higher in leafsheaths, inflorescences and leaf blades. High silica contest increase wear in the pellet machines.Producing switch grass (Porter et al. 2008) with lower silica levels increases energy contents,reduces abrasion on metal stove pans and reduces ash.

The melting temperature of wheat ash deviates from those of other cereals. In addition, the meltingtemperature of cereals ash is affected by the soil composition and by the fertilization. The ash

melting temperatures of different cereals are presented in Table 3.

Table 3. Melting temperatures of straw ash (Wilen et al. 1986).

Stage of melting Temperature range, 0C

Wheat Rye Oat Barley Turnip rape

Initial deformation 900-1050 800- 850 750- 850 730- 800 1150-1250

Hemisphere 1300-1400 1050-1150 1000-1100 850-1050 1250-1500

Flow temperature 1400-1500 1300-1400 1150-1250 1050-1200 1300-1500

Agricultural and other raw materials contain far more contaminants (2 – 10%) than sawdust (0.3 –0.5%). The ash content of barley, rye and oat is about 5 % and that of wheat even 6 - 7%. Also with

wood based raw materials the ash content can be high for natural reasons and the sand can increase

the amount of ash during the handle. The amount of the contaminants is affected by the harvestingmethods, which is technological significance for pellet manufacturing.

For example the shorter the stubble – the stem portion left standing – the more likely the inclusion ofcontaminants. On the other hand, just taking the uppermost portion drops the straw yield per unitfield area. The Køge straw pellet plant in Denmark experience some problems with respect to thesomewhat high contaminant proportion of straw. Although a stone trap is installed to separate thevaluable raw material from the foreign objects, it does not always work at 100% and these results indamages on the knives of the straw bales cutter, grinding and wear the press (Pastre 2002).

According to the survey of web-pages and the discussions with experts, the pelletizing of

Mediterranean raw materials is low at the moment. Factories is used different raw materials; straw,corn stalks, residues of maize, pruning of trees (Spinelli et al 2010), plants extirpated and alsovarious types of wood (mixed with other biomasses). Mixes will be done after season and source.Olive residues as 100% pure or mixed might be one possibility. Olive residues have a rather high

bulk density and heating value. They are used as mixed fuel in bigger plants, but without processing pellets. It is also difficult to find a pellet stove able to burn pelleted olive residues. Table 4 showssome characteristics for grape and olive residues.

Table 4. The some characteristics of the grape and olive residues.

Grape residues Olive residues

Nominal top size, mm 16 3,15Density as received, kg/m3 440 600Density of dry matter, kg/m3 140 500-550

Databases, such as the biomass database of the University of technology of Vienna:www.vt.tuwien.ac.at/biobib, the biomass database of IEA Task 32:www.ieabcc.nl/database/biomass.php and Phyllis biomass database:www.ecn.nl/phyllis/dataTable.asp, summarize the ash contents and other characteristics of severalraw materials.

Using of raw material with low K, Cl, Na and S contents is of particular importance for achieving

high-quality biomass fuels and lowering particulate emissions during biomass combustion. Themajor factors affecting the level of these compounds are fertilization practices, choice of species,stem thickness, time of crop harvest, relative maturity of the cultivar, and the level of precipitation ina region (Samson et al., 2005). Biomass-fuelled boilers have traditionally been developed for wood,which is lower in ash and chlorine levels. However, new multi-fuel boilers have been developed thatcan burn agricultural and wood biomass more and more effectively.

3. PROCESSES FOR RAW MATERIAL

In Canadian publications (Porter et al. 2008, Samson et al. 2008) it is told well about the working atfield and that subject is mostly left outside this publication.

3.1 Densifying of raw material

By the side of ash content, the difference of the bulk densities of raw materials forms another

main problem between wood- and straw pellets.

Bulk density of straw and grass raw material is low, whether it is chopped or in bales, varying between 50 to 150 kg/m3, so also the energy density of raw material becomes low. It is possible torecompress the raw material before the transport or even before the pelleting process. In the feedindustry it has been used two pelleting mills in series for compressing (Payne 1994). If strawmaterial can be recompressed the transport cost will be lower, and also traditional wood pellet pressand other process devices work better and more efficiently with the densified rawmaterial.Densifying biomass allows the material to be handled and stored more easily. The delivery option

for the densified biomass will be determined by the distance of the transportation radius.

In a Canadian report (Porter et al. 2008) production, handling and delivery of agriculturalrawmaterial (straw and grass) has divided in three possible choices.

1. Loose, chopped material

When the material (chopped hay, straw or grass) is harvested, it is recommended to put multiplewindrows together to use the chopper efficiently in the field. If the cut length of a forage chopper isset small enough (length of cut) it is possible to produce fairly fine and more desirable product forthe pellet plant. It might be possible to eliminate the wet hammer mill redundant process in the plant(Porter et al. 2008).

Current choppers can chop 30 to 100 dry tons/hour at a length as short as 7 mm. The forage ischopped by a self-propelled chopper and blown into a separate truck or wagon. The chopped forageis then field stored in a simple bunker or large plastic tube in the corner of a field that is accessible

by road for later transportation to the plant (Porter et al. 2008).

2. BalesDensities of bales can vary between 120 to 180 kg/m3. From a logistics standpoint, it makes the mostsense to make bales 2.4 m wide (the width of a semi-truck). This makes transportation, stacking andhandling convenient for a material handling loader. Bales are bound either with twine or wire and

are big enough to require mechanical/hydraulic loaders. They weigh 320-550 kg. Most are 0.9 x 0.9x 2.4 or 1.2 x 1.2 x 2.4 m3. Although round bales may be cheaper to produce per ton, with biomassfuel it is typically more efficient and safer (don’t roll) to produce large square bales fortransportation and storage logistics. Typical balers are capable of baling 10 to 20 t/h (Porter et al.2008).

Different dimensions of bales are being used in Scandinavia and Europe than in Canada. Measuresand weights of popular bales are following:

straw in round bales Ø1,5 m x 1,2 m, 240 kg, medium scale square bales 0,7 m x 1,2 m x 2,4 m, 280 kg and large scale square bales 1,2 m x 1,3 m x 2,4 m, 520 kg.

3. Cubes

Cubes are a Canadian speciality, used especially in feed transport. Cubes were created for a specialtymarkets, to make the transportation of western hay possible. To make cubes, hay is dehydrated andformed into low density cubes. Typically cubes are approximately 4 cm in size, and the cubes are notvery durable (Porter et al. 2008).

Cubes were made with equipment containing a ring die with one pressing wheel. Structure of the dieis special. John Deere built 400 mobile units thirty years ago, figure 5. Also a stationary machineexists. The cubes (Nelson&Nelson, 1980) can be made in a variety of densities and are extremelydurable, stable and highly resistant to disintegration or breakage under normal conditions. Once incube form the densified organic material can be rapidly loaded with bulk loading and handlingequipment for eventual storage, transport and consumption. Cubes might be suitable for use as

industrial fuels.

Figure 5. Cuber on the field picking up Alfalfa (Anon., 1978).

3.2 Preheting of raw material

Torrefaction and other heating processes have been used to change the physical and chemical properties of raw material.

Transforming the characteristics of the straw by heat was described years ago on the internet page ofFAO´s (www.fao.org/docrep/005/y1936e/y1936e0d.htm).

In FAO example the principles of the heat-explosion combine the heat reaction and the mechanical processing. Under the action of steam at 170°C, straw lignin is broken down and partly hydrolysed.During the explosion, the particles impact each other inside the tube and at the same time the watertrapped within cells rapidly expands to a gaseous state and physically tears apart the brittle cell wall.

Straw is shredded into fine particles, greatly increasing its surface area. The quality of the treatedstraw is considerably improved.

After heat-explosion, the physical properties of the straw have been changed, and the intake of the

entire crop is increased by 50 to 90 per cent. The digestibility is increased more than 50 per cent.

The same article describes also the ammonisation of straw and the changing of properties of straw bythat way.

Production of high quality pellets with regard to durability and water resistance were successfullydeveloped (Gunnerman 1977, Shen 1987). These methods were based on the pre-treatment of thegrind with a high temperature steam and long residence time in a press to reach a maximumsoftening effect, and consequently a maximum agglomeration effect of the material. Also in Norway,the Cambi Bioenergi Vestmarka did experiments with steam. The raw material (sawdust) was

preconditioned by heating a steam-compression reactor. After a certain exposure time the pressure

was reduced, causing the material to „explode“. This process works only under certain definedconditions. After this procedure the wood came out from the vessel in the form of wood fibre, whichwas very wet and brown in colour. In practice (Lehtikangas 1999), however, the methods for

production of "perfect" pellets were too expensive and the producers had to compromise between thequality and the production costs.

Present torrefaction process is a globally studied research area, table 5, and commercial applicationsare coming or exist already on the markets. On the other hand, torrefaction process will be anexpensive, not suitable extra unit in a small production, and ash problems will probably notdisappear completely during the torrefaction process. According to Kiel (2011) minimum plantcapacity should be 5 – 15 t/h. Most of the existing torrefaction results are from wood materials.

Table 5. Torrefaction technology developers in Europe (Kiel 2011).

Reactor technology Technology developersRotary drum CDS (UK), Torrcoal (NL), BioEndev (SE),

EBES (AU), BIO3D (FR)Multiple heart furnace CMI-NESA (BE)Screw reactor BTG (NL), Boilake (NL), FoxCoal (NL)Torbed reactor Topell (NL)Moving bed reactor ECN (NL), Thermya (FR), Buller (CH)

Belt reactor Stamproy (NL)

4. PREPROCESS OF PELLETING

Pelletizing process for wood, straw and other raw materials, consists of few basic sub-processes:feeding of the raw material, drying, pelleting, cooling and screening.

From the physical, chemical and botanical perspective, straw, grass and other agricultural materialsdiffers from wood. Even the pelleting capabilities of wood species differ from each other. This

creates differences in the pelleting process and also in the product quality and suitability for heating purpose. It is not so much a question of superior to wood, rather, it is a challenge of optimal processing of different raw materials.

4.1 Reception of raw material

At a plant straw is generally received as bales weighing up to 500 kg, and there might be somedifficulties in feeding a bale into the system if it is not well planned. A schematic drawing of aDanish plant suggest how some problems related to bales and raw materials handling can be solved,figure 6.

For pellet production the straw etc. material in bales should be first debaled and chopped. The lengthof the chopped stem particles is between 25 to 75 mm. The bigger the press, the coarser the rawmaterial can be. Bale type might effects on the throughput of the feedstock. Hammer mill at the end

of the feeding system grinds the raw material into fine particles.

Figure 6. Danish Pneumatic feeding system of straw in Studstrup power plant.

The straw storage facility at Studstrup is split into two sections, with a capacity of 560 Hesston balesof 1.2 x 1.3 x 2.4 m, each weighing 450-600kg. The straw delivery trucks are unloaded by anoverhead crane. The crane unloads twelve bales in one batch. During unloading the bales areweighed, the moisture content is measured using microwave techniques and the data is stored on acentral logistics computer (Van Loo & Koppejan 2008.).

In small scale pellets production it is possible to build very elegant feeding system. Complete balecan be fed straight in the grinder in a Finnish solution (http://louhetar.fi/biobotnia/).

Regarding straw, high concentrations of bacteria and inhalable toxins have also been seen in thestorage area and near shredders. To minimise the exposure to micro-organisms, employees mustwear respiratory protection. Straw has higher dust potential than wood chips, but usually the contentof microorganisms is higher per mg of wood dust than with straw dust. It seems that impacts onworkers’ health have not been studied extensively (Pastre 2002).

4.2 Screening contaminants of raw material

Magnets and screens for contaminants are normally used at different stages before grinding.Contaminants (metals, stones and other foreign material) are removed from raw material before

pelleting process. Ferrous metals are separated with a magnet from the conveying belt. Bigger stonesand other material are picked away manually before the process or a stone trap is installed toseparate the raw material from the foreign objects. An increased wear of machinery is createdgrowth of contaminants and contaminants damage the machines of the process.

4.3 Grinding of partic le size of raw material

After the material is debaled and contaminants have been separated, it flows to grinding, in mostcases with a hammer mill. After grinding the particle size is adjusted to a uniform maximumdimension, which is approximately 50 - 85 % or less of the minimum thickness of the pellet to be

produced, e.g. for alfalfa pellet approximately one-half the diameter of the pellet being produced.

A hammer mill, and sometimes a roller mill form two common components in grinding technology.The roller mills crushes the material between pairs of cylinders, while a hammer mills uses the

beating action of rotating hammers or steel strips to reduce the size of material by breaking andsplitting. The screen around (surrounding) the blades determine particle size distribution. The

particles exit through a screen with openings of chosen shape and size distribution, and with only

little control of particle geometry. To mill down native grasses, bales, a large throat area and a largesurface area of the screen are needed. A lot of power is required to achieve throughput productiontonnages. In hammer mill the material for pellets is decreased to a distribution of some millimetresafter the screen. Grinding and conditioning are said to be of great importance for achieving highquality pellets.

A number of studies have examined the impact of the length of chop on the pellet process. Overall ithas been realized that fine grinding produces denser pellets and increases the throughput capacity ofmachines as the material passes through the machine more easily (Dobie, 1959). Fine choppedmaterial provides a greater surface area for moisture addition during steam treatment. Mostcommercial alfalfa and switch grass pellet mills use hammer mills with ø 2.4 - 2.8 mm screen to

produce a suitable length of chop. The number of hammers, the screen holes in design, and hammertip speed also affect the fineness and uniformity of the grind when used in commercial installations(Porter 2008).

Figure 7. Typical particle size distribution of wheat straw grinds at various screen sizes (Mani et al. 2004).

An important consideration is, that does the finely grind material need more energy. Mani et al(2004) experiments with a hammer mill of the ø3.2 mm screen the energy consumption wasapproximately 25-30 kWh/t while the ø1.6 mm screen increased the energy consumption to 55-60kWh/t.

Larger holes have been used in Europe in screens of the production machines. In Nikolaisen et al.(2002) work the straw of wheat and other materials were grinded. A hammer mill with sieve of ø4mm and rotation speed of 1450 1/min was used. In the study the diameter of pellets was 12 mm.

Hutla et al. (2004) used screen of ø5 mm with pellets of diameter 10 mm. In mobile pelletizingmachine of Biobotnia Oy the screen is ø18 mm. In study of Narra et al. (2010) the raw materialswere straw of winter varieties of rye and wheat. In comparison test of the hammer mill the materialwas grinded until it was small enough to fall through the sieve having an aperture size of ø6 mm.

Reece et al. (1985) reported that corn ground using 3.18 mm, 6.35 mm and 9.53 mm hammer millscreen sizes produced pellet durability of 91.0, 91.3, and 92.5%, respectively. Also in his study(Bergström et al. 2008) saw dust of Scots pine found the small differences of the particle size (screenopenings ø1.0, 1.9, 4.0 and 8.0 mm). Thus it seems that less energy could be used if only oversized

particles are grinded before pelletizing.

In a production model Amandus Kahl –company has integrated a grinder to pelleting press.

In grinding wet material outside, especially in wintertime, material can plug the holes of the screen

and also create other problems, e.g. flakes on the screen of the machine. As a drawback with straw orgrass might be that a conveyor screw of the mill can stop the feeding into grinding machine iffibrous particle size too long. Long particles might wind around the screw conveyors.

Two machines, a chopper and a grinding machine, and also a little more manpower is needed forstraw-based materials it is needed. This makes producing of the straw pellets more expensivecompared to wood pellets in grinding stage. Production of straw pellets might need less kWh ingrinding and drying complete. Some alternative materials wear more the grinder (bark) than wood.

4.3.1 Expander

An expander consists of a conveying screw with mixing bolts mounted inside a barrel. The screwexerts shearing, mixing, and transport action into the feed. This moves the feed to a moving cone atthe outlet of the expander, thus creating an annular shaped gap. The position of the cone is controlled

by the power take-up of the expander drive. The expander is capable of raising the temperature ofthe feed material to above 100 °C through mechanical shear, without adding moisture, thusgelatinizing the starch better and improving the binding characteristics of the feed and producing

better quality feed pellets (Behnke 2006).

In German reports (Nguyen Trung Cong 2005, Narra et al. 2010) it has been described the use twin-screw for breaking and compressing agrimaterials before pelletising (without conditioning). Narra etal. (2010) pelletized rye and wheat straws. Pellets after the hammer mill (HM) and the twin screw

extruder (EX) were compared.

The twin screw extruder works with the principle of defibration. The material was brought with tworotating screws through the barrel and compacted against a die. The material got ground in closecontact between the barrel walls and the rotating screws which caused frictional effects and led toshearing forces. A destruction of the material’s cells occurred through the processing of moistureenriched material at high temperatures (80 °C – 130 °C) under pressure. Through high mechanicalenergy and high shear forces, the materials physical size (particle size) and chemical properties werechanged. Through the extrusion process, the straw surface gets partially destroyed, which has effecton the lignin content and on the wax surface (Narra et al. 2010).

DeFrain et al. (2003) evaluated an expander as an alternative to steam conditioning (66 °C) to pelletthe feed containing raw soybean hulls and corn steep liquor. They found that although the expanderincreased the pellet production (by 250 kg/h) and pellet durability (by 1–2%), the expanderconsumed about 4 times more energy (about 150–180 MJ/t) than the pellet mill (about 40–50 MJ/t).Therefore, they concluded that the additional energy expenditure did not justify the expander use asan alternative method of thermal processing for this feed mixture.

4.3.2 Fractionation

Process of physical separation of leaf tissue from the stem tissue is used in Canada for fractionation.

Leaves have a higher percentage of ash and contain many nutrients harmful to boiler steel. Ash candecrease the heat value of fuel, clinker up boilers and require higher volume ash removal. Normally,switchgrass ranges between 4-8% ash. Separating out the leaves from the stem produces a fuel that is

lower in ash content, has less clinker formation, longer boiler life and less ash to remove after

burning. The fractionated leaf material is itself a potential value-added product for use as a soilamendment due to its nutrient content (Porter et al. 2008).

4.4 Drying of raw material

Biomass feedstock need often to be dried prior to the conversion process, such as pellet production, pyrolysis or synthesis gas production. A number of different dryer types may be suited for the purpose, and the final choice should be made after careful consideration of operational and economicfactors specific to the application (Fagernäs et al. 2010).

Straw delivered to the pellet production plant is in the form of air-dried bale, with moisture contentof typically 15 - 20%. Moisture of wet sawdust is up to 50 - 60%. Moisture content of straw andwood pellets after the pelletizing process is 7 to 12%. If straw material is used in the pellet

production the need for drying the material is small. In mixes with wet materials, e.g. wood and reedcanary grass, drying might be needed.

At small scales costs are likely to dictate either a batch perforated-floor technology using heated air,or a simple band conveyor using exhaust gas or heated air. At intermediate scales, the rotary dryerwill probably continue to dominate, with band convey or designs being a possible alternative. Atlarger scales in steam cogeneration applications, the use of steam dryers may offer efficiencyadvantages. It makes recovery of low pressure steam or hot water for district heating possible. Theclosed system assures zero gaseous emissions. In stand-alone applications a low investment isusually emphasised, and correspondingly less energy efficient solutions like flue gas dryers (drumdryers) or band dryers are preferred (Fagernäs et al. 2010).

Temperatures in the dryer vary depending on the residence time. Drum dryers and equal “slow”dryers the raw material temperature should not exceed 200°C in order to eliminate the risks of thesubstance losses during starting pyrolysis. For the same reason, small and homogeneous particles arefavourable for an optimal drying process. Large particles imply a risk for pyrolysis on the surfacewhen the inner parts of the particle are insufficiently dried (Lehtikangas 1999).

Rotary dryers may accept large and variable particle size fuels, but flash- and belt dryers usuallyrequire crushing of the fuel to a particle size below 10 mm. The material will have a bulk density inthe range 50-400 kg/m3, depending on type and moisture content. Usually the bulk material willhave only moderate flow properties, but will readily permit through-circulation of the dryingmedium (Fagernäs et al. 2010).

Traditional dryers are divided to direct and indirect dryers according to the way of heat transfer. Indirect dryers the matter which transfers the moisture away is usually hot gas. The gas is in directcontact with the substance to be dried. These kinds dryers are e.g. rotary drum-, flash- and fluidised

bed dryers. Heat conducts through a layer to the dried material in the indirect driers. Drying surfaceis heated with electricity, oil or steam circulating in the tubes of drying unit. Drying substance is not

in direct contact to dried matter.

Table 6. Table Performance data for dryers applied for biomass (Fagernäs et al. 2010).

Table 6 continues.

Several sophisticated solutions (Vidlund 2004, Andersson et al. 2006) for drying the raw materialshave been developed for large scale pellet factories. Usually the process is integrated to another

process or drying stages are separated to several phases. There are plenty of information on differentdrying solutions in the literature and the internet. More precise analyse of drying would include

much more pages of the content of overview and it is not reasonable.

4.4.1 Emissions of drying

Organic compounds are released (Fagernäs et al. 2010) in drying biomass materials as a result ofvolatilization, steam distillation and thermal destruction, and cause emissions into the air orwastewaters. Studies on the emissions have been reported, e.g. in references (Fagernäs 1992,Fagernäs & Sipilä 1996 and Spets & Ahtila 2004 in Finland and Danielsson (2001), Johansson(1997), Karlsson (2002), Johansson (2002), Ståhl (2004), Granström (2002) and Granström (2001) inSweden.

Organic emissions can be classified as volatile organic compounds (VOCs) and condensablecompounds. In addition, there are particulate emissions. At low drying temperatures (under 100 °C)the compounds emitted consist mainly of monoterpenes and sesquiterpenes. The VOCs are of

environmental concern since they are known to form ground level ozone in the presence of nitrogen

oxides (Vidlund 2004). Photo-oxidants are also harmful to humans, as they cause irritation in therespiratory tract and insensitive parts of the lungs. The condensable organic compounds, such asfatty acids, resin acids and higher terpenes, emitted at over 100 °C, might condense on equipmentsurfaces and thus cause technical problems. They can also form “blue haze”, a discoloration of theexhaust plume, as the flue gases are cooled down after the chimney. The “blue haze” can representan odour and visual nuisance as well as a potential safety hazard. The thermal destruction of woodmaterials starts at about 150 °C with the destruction of hemicelluloses, when alcohols, acids andaldehydes are released. In the beginning of drying, thermal decomposition is slight, but the rate ofloss accelerates rapidly as temperature is increased further. Such degradation represents an energyloss to the overall process (Fagernäs et al. 2010).

Clean-up equipment for the exhaust gas stream depends on applicable emissions criteria andregulation, which vary greatly with location. Solid particulates may usually be dealt with cyclones or bag filters. Blue haze is composed largely of sub-micron aerosols, and these are notoriously difficultto remove with conventional gas cleaning techniques. As a general rule low material temperatures(<100 °C) should be maintained when possible (Fagernäs et al. 2010).

4.5 Conditioning

When the raw material properties are more or less uncontrollable the process variables can be usedfor steering the pelletising in the desired direction. Steam or water exhibit bonds via capillarysorption between particles and on the other hand they induce the thermal softening of the materials.However, too much moisture makes the feedstock slippery and it slides through the holes too easily,thereby reducing pellet quality. Decreased durability can also be obtained due to the steamexpansion after pellets leave a die. Materials that are too dry may plug the holes in the die if theresistance from the holes exceeds the roller force (Lehtikangas 1999).

In conditioning stage it is possible to add steam (and/or water) to the raw material in order to softenthe material fibres prior to densification. Conditioning also softens the lignite and hemi-cellulose ofthe raw material for a more suitable pelleting process and achieving better strength for pellets. As aresult is better physical quality of pellets and lower percentage of fines. Steam and residency time of

conditioning create a more pliable fibre and the production capacity usually increases. Conditioningalso covers the addition of determined binding agents or other additives. It gives to good mixing ofseveral raw materials and extra substances without any negative influence on the pellet quality.

At present the most common technique used is steam conditioning. In wood pelletizing steam issprayed at 90 – 150 °C to conditioning chamber. Pressure is usually 5 - 10 bar, sometimes evenhigher. Proportion of the steam is about 5% from the weight of raw material and process time is 1 –4 seconds. There exist also feed expanders, which can be used to expand conditioning. Inside theconditioning chamber a cascade mixer mixes steam, additives and raw material (figure 8) for

pelleting.

Steam conditioning/preheating the feed may require considerable energy. For example, Skoch et al.(1981) estimated that steam conditioning to increase the temperature from 27 to 80 °C consumedabout 26 kWh/t. Steam add in its pelleting operations improves pellet durability. Added steam

provides heat and moisture and it also helps to reduce energy consumption during pelleting. Steam

also activates natural binders and lubricants in the biomass.

Figure 8. Technical description of the conditioner (Manual of Sprout-Matador).

It is said that straw does not accept steam as well as wood residues. The time to allow the water to penetrate the switchgrass stems is 5 times longer than wood. It is critical to size the ripeningchambers proportionately to allow adequate time for the material to re-wet into the product – not justthe surface. When the feedstock is warmer and slightly moist, it is more pliable and then whencompressed by the roller into the die, the resulting pellet is a firm, glassy pellet of high density andquality. This pellet however is not durable until cooled.

Water resistance might become (Pastre 2002) from the existence of a thin wax layer on straw, whichnormally protect it against rain and insects attacks. After a certain period of time, the wax layerdisappears naturally and straw loses its shiny aspect, especially if straw bales have been outsidewithout cover.

If a two component mix is processed there must be two controlled feed-ins, and a double conditionerwith an effective mixing is needed. Similar method is used in feed industry to expand conditioning.

4.6 Additives of pellets

Additives are used in pellet production for better binding, lubrication or to decrease combustion problems.

A binder (or additive) can be a liquid or solid forming a bridge, film, matrix, or causes a chemicalreaction to make strong inter-particle bonding. Steam conditioning or preheating is essential to

provide heat and moisture to activate the inherent or added binders. Selection of binders mainlydepends on cost and environmental friendliness of the binders. When strength, durability or heatingvalues of pellets do not match with the quality standards or marketing requirements, additives areadded to the feed to increase the pellet quality or to minimize the pellet quality variations (Kaliyan &Morey 2010).

Binding agents have been investigated and used as a means to increase fuel pellets durability andthus reducing the dust and fines generated during their transport and handling. Examples of possible

binding agents for wood pellets include starch, molasses, natural paraffin, plant oil, lignin sulphateand synthetic agents.

Small amount of starch, fewer than 2%, increases the strength of pellets. E.g. potato- and maizestarches are used. It is question of expenses and of the result in quality when starch or other additives

are used. In experiments some positive experiences from the growing durability was noticed whenstarch was increased.

Binding agents (such as maize or rice) can also be used in order to decrease abrasion. This sort ofaddition is quite common in Austria for instance. However, a recent publication found that even theuse of binding agents does not always result in low abrasion values, and that abrasion must thereforedepend on several other parameters (Obernberger & Thek 2002).

Experiments have been carried out in Sweden on increasing lignin sulphate to raw material. Pelletshave been produces even from pure lignin sulphonate (Öhman et al. 2006). In the latter case the ideawas that if pulp and paper production cellulose is upgraded to ethanol. After that the by-product

lignin can be pelletised and used as a fuel. The binding quality of lignin sulphate is good and pelletscan be used in small scale use. Lignin increases the sulphur and ash content, which can increasesulphur emissions and cause problems with the combustion equipment.

Lignin acts as a binder in situ in the feed material. At elevated temperatures, lignin softens and helpsthe binding process. Bradfield and Levi (1984) reported that as lignin plus extractives contentincreased above a threshold level of 34% in wood samples, pellet durability decreased. They

postulated that the auto-adhesive action of thermally softened, non-crystalline wood polymers waslike that of mastic with little internal strength of its own. Initially, pellet durability increased as thenon-crystalline wood polymers acted as an adhesive between crystalline zones. However, above athreshold ‘‘excessive mastic’’ between crystallites reduced the strength and durability of the pellets.

Fibre can be classified as water-soluble and water-insoluble fibres. Water-soluble fibres increase theviscosity of the feed and positively affect the structural integrity of the pellets. Water-insoluble

fibres may entangle and fold between particles or fibres (Rumpf 1962). Hill and Pulkinen (1988)

found that increasing crude fibre content from 18.5–26.5% increased the durability of alfalfa pellets by about 5%.

Hill and Pulkinen (1988) concluded that addition (by weight) of any of the following six binders didnot improve the alfalfa pellet durability over the control: 4% bentonite, 1.5% Perma-Pel(lignosulfonate), 1.5% lignosite 458, 4% of neutralized liquid lignosite, 4% of liquid molasses, and40% of ground barley grain. Tabil et al. (1997) observed that the durability and hardness of pelletswith hydrated lime were highest for all three chop qualities. This may probably be due to theformation and subsequent hardening of calcium carbonate. Furthermore, pea starch increased pelletdurability without necessarily increasing hardness. They suggested that adding 0.5% of eitherhydrated lime or pea starch would be sufficient to improve the durability and hardness of pellets

made from low quality chops (Kaliyan & Morey 2009a).

Biomass combustion appliance manufacturers often recommend the addition of lime (CaO) to reduceclinker formation and slagging. Hartmann et al. (2007) found that this in practice reduced particulateloading by approximately 15%. Ronback et al. (2007) found 2% fine limestone mixed with oat grainfuel reduced total particulate loading by 15% and reduced total dust formation by 28%. Limestonecreates a chemical compound such as CaSO4 which has a higher melting temperature, thus thesespecies stay in the bottom ash. The authors felt this additive would be most valuable in largercombustion systems where the increased ash content of the fuel would have minimal negativeimpacts on combustion efficiency. The limestone also has the added benefit of reducing HClformation (Porter et al. 2008).

Nikolaisen et al. 2002 used molasses 2 – 5% as binding agent in their experiments. In theexperiments also anti-slagging additives, like limestone, kaolinite, Ca- and Al-oxcides, was used.Wilen et al. (1986) used in their experiments kaolin and talk as anti-slagging additives. Kaolin, 3%,worked rather well increasing the smelting temperature of celesters.

Different kind oils, e.g. rapeoil (Nikolaisen et al. 2002) and pineoil (Kallio et al. 2005), has beenused as lubricant and also dust binding. Inclusion of fat/oil (animal or vegetable based) in feedresults in lower pellet durability. This is because fat acts as a lubricant between the feed particles,and between the feed and the pellet-mill die-wall. Due to low friction in the die, pressure in the die isdecreased which would result in pellets with lower durability.

By studying the chemical structure of the ash it has been developed additives, which decrease theinfluence of harmful chemicals in heating. Prof. Martti Aho at VTT has participated to the work, inwhich the effect of chlorine has been decreased in combustion. Also as a Swedish – Chinese co-operation has been developed additives to prevent the sintering of agrobiomass (Xiong, S. et al.2007).

It can be underlined that modification of operating conditions – changing the thickness of dies, pressing time, pressing temperatures and pressure – can help to improve pellet quality without binding increasing agents. Kaliyan & Morey (2010) studied “Natural binders” through the micro-

structural analyses. The micro-structural analyses (i.e., light microscopy, scanning electronmicroscopy, and UV auto-fluorescence imaging) of corn stover and switchgrass briquettes and pellets showed that the natural binders in these biomass materials created ‘‘solid bridge” type

bonding between particles in the briquettes and pellets. The potential natural binding components in

these biomass materials are water soluble carbohydrates (2.2–7.9% d.b.), lignin (8.8–9.2% d.b.), protein (3.6–3.9% d.b.), starch (0.4–1.0% d.b.), and fat (0.7– 0.9% d.b.). The natural binders in the biomass can be expressed or activated (softened) under high pressures in the presence of moisture(e.g., water soluble carbohydrates) and in some cases increased temperature (e.g., lignin, protein,starch, and fat). When pressure is removed and the binder cools, it hardens or ‘‘sets up” forming

bridges or bonds between particles, which has the effect of binding them together and making theresulting product more durable. Furthermore, activating (softening) the natural binding componentsthrough moisture and temperature in the range of glass transition is essential to produce highlydurable briquettes and pellets.

In certain European countries, addition of some binders is prohibited. In Austria, biological additives

rich in starch content (e.g., maize and rye flour) of only 2% (by weight) are allowed for wood pellet production (Obernberger & Thek 2004). Under the German emission control legislation molasses asa residue from sugar production, natural paraffin or starch are allowed (Viak 2000). Apart fromincreasing the product’s hardness, additives are also conceivable as a means of improving somechemical characteristics, for example slag formation (during combustion) can be hindered by usingkaolin or calcium and magnesium oxides.

5. PELLETING PROCESS

The handling of raw material has been introduces in the former sections of the report. Usually thecomplete process consists of drying, grinding, pelleting, cooling, screening, figure 9. Next sectiondescribes the core process of pelleting process, pelleting.

Figure 9. To the pelleting process belongs drying, grinding, pelleting, cooling and screening (Source:Hannes Tuohiniitty, www.pellettienergia.fi).

5.1 Pellet mill

After conditioning, the particles are continuously fed into the pellet mill. A pellet press is(Lehtikangas 1999) composed of a die and generally of two or three rollers. Loose milled material isfed into the pelleting cavity. The rotation of the die and roller pressure forces material through thedie holes, compressing the material into pellets. Pellets are cut off when coming out from the die orthey can be cut with adjustable knives to a desired length. The density of the pellets depends on thefrictional forces which are controlled, e.g. by the length and the diameter of the apertures in the die,the condition of the die and rollers, the roller adjustment and the raw material properties.

There are several different pelleting methods. Those are schematic described in figure 10.

Figure 10. Different possibilities to pelletize raw materials (Näslund et al. 2003).

Ring die presses are the most popular in the pellet industry. From the basic method it has been

several developments. Ring die may rotate or be static, and the power transition becomes either thedie or wheels. In small pelleting machines it is limited space inside the die, the diameter of rollerwheels becomes small and it is difficult to build up good montage. Rotating die shall take all

pressing power with a weak montage, and it becomes stressed and lifetime shortens. The distributionof raw material is better than in static die; raw material rotates on the circle of die for centrifugalforce. Change time of the die is faster in a static die.

The rollers, which are usually inside the die, have also tested outer circle of ring die. Because spaceis enough, it is easier to build large rollers, which are easy to maintain. Feeding the rawmaterial

becomes more complicated. Further developed EcoTre –press is fabricated present day in Slovakia.

The flat die type is another popular press in Europe. It has a circular perforated disk on which two ormore rollers rotate. Rollers create force of press and the material goes through the holes and will pelleting. The structure of flat die is solid. Main axel rotates the big roller wheels around the die.Feeding of raw material happen upward down and distribution on the die is regular. Biggest flat diemachines produce 8 t/h and smallest few kilos per hour.

At least in Sweden and Germany extruders have used for pelletizing in small scale. In severallaboratory scale of pelleting experiments in Germany it is used Hosokawa pelletisers.

In Germany the PUSCH AG provides a new concept for the decentralized production of mixed biomass pellets from agricultural and woody raw materials. Based on the license concept

“agrarSTICK®”. A hydraulic press and PM6-28 -plant offers less work and cost intensive pretreatment of the raw materials. Therefore, the production of different mixed biomass pellets can be done without changing the press die.

Figure 11. Mobile pellet mill of Biobotnia Oy with a ring die. A whole bale can be put in hammer mill (Source:http://louhetar.fi/biobotnia/video).

Usually a static pellet plant is built somewhere good place. In pelletizing agricultural materials it has become an idea to build a mobile system, especially the areas, where large field areas are lacking..Biobotnia Oy in Finland operates nowadays with a mobile system. The pellet factory has been built

in a truck, figure 11. Also in Germany there is a commercial mobile construction

(www.energievomland.de/). 5.2 Some properties of pellets mill

A number of properties are commonly known to affect the quality of pelleting. They can be dividedfrom raw material depending and mechanical properties. From the raw material depending propertiesinclude moisture content of the material, density of the material, particle size of the material,characteristics of the materials and different kind agents. The mechanical main factors that improvethe pelleting process are steam conditioning, optimised die measures and pelleting pressure.

A review of some these factors and the basics of the binding process are described in next section.

Table 7. Pelletising researches done with different raw materials.

Raw materials Press Other data Combustionincluded

Source

Cereals and rapestraw

Mobile pelletising unit,small scale Kahl

flat die

Additives, kaolin etc. Small scale (farmers),screw stoker 10 - 30 kW

and 300 kW fluidized bed, gasifier

Wilen et al. 1996

Wheat mixes,additives

Full scale SproutMadator M12

ring die

12/108p96

12 mixes REKA –boiler, 30 kW Nikolaisen et al. 2002

Wheat Lab scale Kahl,8/40 flat die

Additives; limestone,starch, fibre, mollasses

Kiesewalterin &Röhrictin 2004

Hemp Small scale ringdie

Good bulk density,moisture, bad

hållfasthet

Other researches Finell et all. 2006

Reed canarygrass

Small scale ringdie

Difficulties in pelletting

Other researches Öberg et al. 2006Larsson 2008

Lignin High heating value etc. Mainly combustion Öhman et al. 2006Peat Mobile

pelletising unit,middle scaleKahl flat die

Raw material formobile too wet (35 -

Middle and big scale boilers

Oravainen 2003,Lahtinen

Rape straw and barley, mixes Lab scale,Amandus Kahl14-174 flat die

Barley was easy to pelletise. Rape straw- pellets did not slag.

small scale 20 kWstoker & boiler Kallio & Kallio 2007

Wheat, rapestraw, wood

species, mixes

Lab scaleHosokowa

Much analyses ofmaterial

Boilers 10 – 100 kW Rombrecht 2007

Mixes, wood -straw, wood –miscanthus,

straw -miscanthus

Unknown Mixes of rawmaterials, diff. 20%

Two lab scale devices Schneider et al. 2011

Mixes of mash,shea nut shell,

coffee-, cigar, pectin waste,

Full scale SproutMadator M12

ring die 8/90/P50(eff. Canal

13 Mixes Analyses, later in 2008combustion

(Hinge, J., 2008,Capablo et al. 2009)

Nikolaisen et al. 2005

Pelletising experiments with different sizes presses and integrated combustion tests have been

carried out in several countries. Some researches of the alternated pellets are gathered in the table 7.In most cases the alternative raw material has been straw. Appendix 1 describes the pelletising properties of several other rawmaterials.

During pelletizing the dust of the air can be dangerous. Face mask is good to use so that small particles do not enter the lungs during inhaling. It has been some researches of the subject with wooddust.

5.2.1 Measures of die

Measures of die are very important in pelleting process. Dies need to be selected based on the

feedback of the process. If a wrong choose has been done, it might happened that pellets does notform. A balance needs to be found between pellet durability and throughput when dies are chosen.Two main measures are dimension (D) of the aperture and length of pelleting channel (L).Dimension is in European wood industry usually either 6 or 8 mm. For agricultural pellets L/D-ratiocan be larger. For example (Nikolaisen et al. 2002) used their experiments diameter of 12 mm (L/D8/1). The prolonged residence time in a die gives positive effects on the durability of the pellets. Forflat die machine the length of whole is shorter than ring die machine, L/D for spruce and pine is withflat die 4 – 5:1 (Graf 2000) and with ring die 8 – 10:1 (Näslund et al. 2003).

Figure 12. Examples of different apertures of die (Näslund et al. 2003).

Production experience in commercial plants with pelletizing highly fibrous herbaceous biomass likeoat hulls and warm season grasses has found that L/D of the die should be approximately 8.5 - 9:1and the dehydrated alfalfa industry (10:1) (Porter et al. 2008).

Commercial pellet mills can operate at speeds of 60–500 rpm. Rapid die rotation tends to overloadthe pellet motor due to the high fiber content of the forage. In wood pelleting with industry scale

machines (die Ø 800-1000 mm) speeds are about 4 – 5 m/s (Näslund et al. 2003).

Main problems occurring during the pelleting process correspond to die blocking, die breakage, roll

cracking, overheating, high energy costs and at times poor end-product quality and high maintenanceneeds. Fitted with a manual or an electrically controlled air cylinder, virtually all overloads and blockages can be eliminated (Pastre 2002).

Same mills (Pastre 2002) are used for pelleting straw and wood, but from one product to another,dies and rollers have to be changed. Most of the machines allow a few hours’ removal and reset up.With straw the life span of die and rollers can be reduced by up to 20% that of wood. Due to thehigher silica content the straw has a higher abrasion wear.

The condition of the rollers (Lehtikangas 1999), and condition and speed of the die, as well as thedie specifications, including length and diameter of the hole, play a role in influencing the quality

properties of pellets.

5.2.2 Roller wheels

Roller wheels are the second main component in the pellet press. In a pellet press there is two orthree rollers. As big rollers will be manufactured as there is room for them inside the die. Largediameter rollers have longer pressing time. Larger rollers rotate easily and risk of slide decreases. Inthe rollers there is a pattern on the surface to get a better grip of the raw material. The surface wearsand the roller has to be changed after regular time shifts to prevent the sliding.

Gap between roller and die is also an important variable making good pellets. Robohm and Apelt

(1989b) and Robohm (1992) studied the effect of the distance (gap) between the roller and the die onthe strength and durability of pellets in a flat-die press, and a ring-die press. For both press types,increasing gap-size (about 2–2.5 mm) increased pellet hardness and durability. A further increase ingap-size (about 4–5 mm) caused decrease for pellet hardness and durability. The initial increase in

pellet quality was due to a dense layer of material compressed through the die as a result of increasedshear and prolonged pre-compression. A further increase in gap-size resulted in decreased stabilityof the feed mash on the edge of the roller and die because of sideways leaking of the feed mash(Kaliyan & Morey 2009a). In present mills the gap between dies and roller is adjust hydraulically.

In flat die press the roller wheels are as large as possible. Because the die is better supported thanring die it is smaller risk of bending. Also the hydraulic control between rollers and die is easier tomake.

An important task for rollers is to lubricate bearing regularly and also keep them clean as possible.Inside the die temperature can increase to 125 – 150°C and there is much dust.

5.2.3 Mois ture content

Water as moisture in the biomass is one of the most useful agents that are employed as a binder andlubricant (Kaliyan & Money 2010). Moisture content (Nielsen 2009) is a central property for woods

physical strength and stiffness. Also the former drying history interacts on the situation of moisture.

Several studies (Kaliyan & Money 2009b) have showed that strength and durability of the densified products increased with increasing moisture content until an optimum is reached.

With the help of heat, water induces a wide range of physical and chemical changes such as thermal

softening of biomass, denaturation of proteins, gelatinization of starch, and solubilisation andconsecutive recrystallization of sugars and salts (Thomas et al., 1998). These physic-chemicalchanges affect binding properties of the biomass particles. The optimum moisture content for

biomass densification may range from 8% to 20% (w.b.) (Kaliyan & Morey, 2009b). At highmoistures (>20% w.b.), coherent biomass briquettes/pellets may not be produced because the cellstructure remains largely intact at high moisture levels due to the incompressibility of high moisture

biomass particles (Pickard et al., 1961). Before conditioning after Örberg (2007) the optimal watercontent for straw and reed canary grass is about 15%.

Water content minimizes (Nielsen 2009) the stiffness of wood by lowering the physical strength ofthe cell walls of material. The polymer matrix in the wall is softened by water that occupies cross

linking hydrogen bonding sites in the cellulose and hemicellulose and decreases the glass transitiontemperature of lignin and hemicellulose. Softening of lignin is not a distinct transition that occurs ata well-defined temperature for wood. The lignin gradually (Back & Salmen 1982) changes from ahard and glassy to a soft and rubberlike state with increasing temperature and moisture content.Lignin and hemicellulose were found to be amorphous thermoplastic materials which would undergo

plastic deformation at low compaction pressures for temperatures in the range of their glasstransition temperatures (Back & Salmen, 1982).

In corn stover and switchgrass, the glass transition (i.e., softening) occurs from 50 to 113°C. Themean glass transition temperature for both corn stover and switchgrass is 75°C for the moisturecontent range of 10–20% (w.b.) (Kaliyan & Morey, 2009a). Irvine (1984) found that the glass

transition temperature of lignin ranged from 60 to 90°C. Therefore, the briquetting/ pelletingconditions causing glass transition in biomass particles may activate (soften) the biomass cellcontents/natural binders.

Amount of water and heat may therefore have several roles in this connection, because it decreasesthe bonding strength and the friction. Also, water may increase the plasticity, which will minimizethe energy for the particle deformations that may be involved.

5.2.4 Pelleting temperature

In the experiments the temperature of the pelletised material and the temperature of the die can beseparated. The temperature of sawdust can be the result of heating by steam addition or by the heatfrom the pellet mill itself. The temperature of the industrial die is approximately 125 °C, which iscaused by the sawdust's friction with the press channel walls, and may also be affected by thesawdust temperature (Nielsen 2009).

Modest increasing the temperature of the die also caused the pellet strength to increase. Canadianskeeps 85 °C the minimum temperature required to produce durable agripellets. It has been doneseveral studies; those show the advantages of higher pelleting temperatures. Working to produce 6.3mm pellets using 3 herbaceous feedstocks, Shaw and Tabil (2007) also found temperatures of 100°C were superior to 80 °C temperatures in improving pellet durability. The state of “glass transition

temperature” is defined as the temperature at which the material softens due to coordinatedmolecular motion and is critical to densification (, 1995).

The results show that increasing the temperature decrease the energy requirement for all the

pelletizing components. Temperature decrease the sawdust's stiffness and viscosity and thereby theenergy required for compression and flow. In the pellet mill, this temperature could be increased bymeans of heating the sawdust before it enters the pellet mill. This approach is utilized in pellet millswith steam addition. Additionally, the friction in the press channel was dependent on the die's andthe pellet's temperature. For species with high friction such as beech the friction may be highlydependent on this temperature and ways of increasing the pelletizing temperature could be way toincrease the pellet mill capacity (Nielsen 2009).

5.2.5 Pelletis ing pressure

Pelleting has been studied and compression test conducted using a single pelleting unit. In the

experiments properties of wheat, barley, canaola, oat, corn stover and switchgrass straws weredetermined at compressive forces, particle sizes, moisture contents, bulk densities and chemicalcompositions etc.

In the study of Adaba et al. (2009) were determined pressing characteristics of barley, canola, oatand wheat straw. Main results are shown in the table 8.

Table 8. Effect of compressive forces (pressures) on compact density and specific energy required

for compression and extrusion of agricultural straw compacts (Adaba et al. 2009).

From the table 8 it is possible to see (Adaba et al. 2009), that although the total specific energyincreased significantly with pressure, the compact density of barley and wheat did not increase

above a pressure of 63.2 MPa. Similarly, the compact density for canola and oat did not changeabove a pressure of 94.7 MPa. Therefore, a pressure of 63.2 MPa for barley and wheat straw, and a

pressure of 94.7 MPa for canola and oat straw produced the highest density compacts with minimal

specific energy consumption values.Differences of the results could be due to the fact that the bulk densities for all four straw sampleswere statistically not different (P > 0.05); however, the geometric mean particle diameter of groundoat straw was significantly smaller than the other three ground samples. This resulted in larger

plunger displacement values and consequently, higher specific energy values.

The percentage extrusion values reported in Adabas et al.´s study are higher than the values reported by Shaw (2008) for wheat and poplar biomass, while significantly lower than those reported by Maniet al. (2006a) for corn stover; Mewes (1959) and Bellinger & McColly (1961) for hay.

5.2.6 Density of the material

Typical bulk density of grasses and straws is 90 - 150 kg/m3 and after pelleting 650 - 700 kg m3. The bulk density of wood is about two to three times higher than with straw as a raw material. Higherdensity would result lower transportation costs, reduced storage volume and easier handling. Oliveresidues have a higher bulk density, about 600 kg/m3, when dried.

A trouble of agricultural materials is feeding of the raw material to the pellet process. In the pellet press it is not possible to feed equal amounts straw and wood. There are articles about this, and it has been noticed also in the experiments of VTT. In laboratory scale feeding system about 10 - 15 kg/hstraw could be fed. With wood the amount was 30 – 40 kg/h. As an example Pastre (2002) tells that

with the same pellet press of 250 kW, a 4 t/h output can be expected for wood pellets, and a 5 t/houtput for straw pellets while it would amount 20 t/h for feeding granulates (which do not meet thesame quality standards).

Double pelleting has been used in feed industry, for ex. cattle feed consisting of high fiber content.Robohm and Apelt (1989a) found that specific energy required for double pelleting was about 8–13kWh/t higher than that of the single pelleting system.

5.2.7 Particle s ize of the material

Particle size, perhaps also the shape, has not a great effect on power demand as Bergström et al.(2008) study with wood showed. Sawdust particles approach spherical form, whereas straw is a farmore fibrous material. It seems that particles behave rather equal in pellet production, for ex. need of

power equal, compare Køge example, VTT´s and SLU´s experiments. When straw is grinded tosmall particles it does not differ much about the wood particles, only the bulk density of agriculturalraw material is lower.

5.2.8 Efficiency of the pellet production

The output of pellet presses ranges from a few hundreds kilograms up to 10 ton/h and power demandof the presses are from 50 – 130 kWh/t. The most common mills produce 2 - 4 t/h. In the Køge plant,energy consumption (Pastre 2002) for the pelleting process is said to be equivalent to 4% of theenergy content for straw pellets, and only 2% for wood pellets made from wood chips. But this is

because 85% of the steam used in the dryer for wood chips is recovered and reused as a heat source.

Without this particularity, wood pellets energy consumption be expected to be higher to the straw

pellets one.In wood production it is acceptable efficiency to whole production line 130 - 200 kWh/t withoutartificial drying. Lange (2007) promises as low as ±85 kWh/t for whole wood pellets productionline. In experimental studies with a small flat die pelleting presses productivity has been 70 – 100kWh/t (for ex. Kallio & Kallio 2004). The figure of efficiency goes downwards to about 40 – 50kWh/t in big pellet presses. A complete large scale pelleting line would typically range a 600 - 800kWh power for straw processing in Køge. For a pellet mill of 250 kWh with straw feed an output is5 t/h and productivity 50 kWh/t.

6. COOLING OF PELLETS

Due to the upstream conditioning (with hot water or steam) and the friction in the pellet mill, the pellets leave the mill with temperatures of 100 °C and more. Also the moisture content can be 12 –18%. In cooling they are air quenched down to 20 - 25 °C and to 8 - 12%. The commonly usedcoolers are counter flow coolers. For very small pellet mills with low throughputs a subsequentcooler is not necessary. Period of utilization and required electric power has asked information from

pellet producers and cooler manufacturers, respectively.

If pellets include too much moisture inside the pellets steam explosions occur and pellets will be

fragile or broken. In the figure 13 it is shown pellets after bad process conditions.

Figure 13. Pellets may break in cooling stage if they include too much moisture (Payne 1994).

7. SCREENING OF FINE PARTICLES

The residual fines are screened to separate pellets. Fines are harmful in use. Fines are generally re-used in the process and re-pelletised. Some process lines are operated with under pressure in order tominimise dust escape from the process and improve the working environment.

During the pelleting process straw and other agricultural materials might generate more fines thanwood. Operation conditions of pellet process have a great influence. It is reported (Jannasch et al.2001) that, at the exit point of the press sawdust pellets present a 3 - 4% of fines, whereas for straw,fines proportion could amount 5 - 10%. It is also usually expected a 5% loss of dry material in mostalfalfa pelleting systems.

8. QUALITY OF PELLETS

Pelleting increases bulk density, energy density and decreases the moisture content of pellets. Bulkdensity increases from 100-150 to 500-700 kg/m3. Pelleting straw, crass and other alternative rawmaterials moisture content passes from 20-30% to 10%. It is important that amount of small particlesis low. From table 9 it can be noticed that straw pellets have a lower bulk density and a lower energydensity than wood pellets.

Table 9. General characteristics of raw materials for pellets (Van Loo. & Koppejan, 2008).

There are also considerable differences in combustion quality characteristics between biomass fuels.Agrifibers are generally more difficult to burn than wood chips. They have a lower heat value(switchgrass is approximately 5% lower in heat value than wood) and higher content of chlorine,alkali and ash. Improving biomass quality of agricultural raw material depends on minimizing their

nutrient, ash, and moisture content, and the emissions of particulate matter during combustion

(Porter et al. 2008).For controlling the market quality of pellets it has been done standards in Europe. “Multipartstandard EN14961 6 – Pellets” is published in 2010. In the standard (Alakangas 2010) it is included

both wood pellets for non-industrial use (part 2) and non-woody pellets for non-industrial use (part6). Pellets standards are targeted for non-industrial use in small-scale appliances, such as, householdsand small commercial and public sector buildings.

In “specification and classes” (prEN 14961-1) classification is based on origin, source, major tradedforms and properties. Hierarchical classification system is in table format: 1 Woody biomass, 2herbaceous biomass, 3 Fruit biomass and 4 Biomass blends and mixtures.

There are special requirements for chemically treated biomass. Chemical treatment defined as anytreatment with chemicals other than air, heat or water (e.g. glued, painted, coated, lacquered orotherwise treated wood, without halogenated compounds and heavy metals).

Classification is ”flexible”, and hence the producer or the consumer may select property from each property class. The classification does not bind different characteristics with each other and the fuelsupply chain shall be unambiguously traceable back over the whole chain.

Figure 14. After CEN modified tumbling can method in the laboratory of Enas OY in Fnland (photo M. Kallio).

For most commonly traded forms the standard includes 15 property classes. Some of the propertiesare normative (mandatory), e.g. The origin and the source have always to be stated. Normative

properties vary depending on both origin and traded form. Moisture content (M), and ash content (A)is necessity for all fuels. Some properties are informative (voluntary), but they are recommended to

During storage wood pellets create carbon monoxide (Swaan 2002). Silo or storage has to be well

ventilated before a person goes inside it. It has been some lethal accidents with wood chips and pellets in a closed space.

Temperature increase in organic material during storage is a well-known phenomenon with woodchips, pellets. About 30 000 m3 of pellets burned in a silo in the harbour of Rotterdam (Ljunblom2004). The temperatures are started to be monitored for the threat of fire, and large storages havewired with temperature detectors. Also the changes of indicator gases (ec. carbon monoxide and -dioxide) will be monitored. Inert gases can be used for air-condioning the silos. Different theorieshave been presented during the past years on why big stockpiles and silos heat up, e.g. moisturedifferences, fatty acids etc.

Microbial activity is absolutely one of the most important reasons for increasing temperatures(Kubier 1987). Respiration of living parenchyma cells is another process where heat is released andis considered by many researchers to be the initial cause of heating of fresh wood chips (e.g.Assarsson 1969, Feist et al. 1971). The temperature development implies biological, chemical and

physical changes in the raw materials and has to be considered when the importance of raw materialquality for pellets quality is discussed (Lehtikangas 1999).

The significance of these processes on pellets quality is, however, not totally known.

10. PRODUCTION COSTS

The following section gives an idea of the order of magnitude of pelletising costs. The straw pellets production costs are rather similar to the wood pellets. Regional differences of the raw material costsmay lead to a greater variability of straw pellets prices than to those of wood pellets.

Capital investment costs per ton decrease with greater capacity. The economies of scale for a pelletmill are shown in figure 15. The data in figure 15 shows that pellet mills maximize efficiency whenthey produce more than 5 - 12 tons of pellets per hour.

Let’s think about the production costs of a ton of wood pellets. Overwhelming the most expensive phases of production are raw material and drying in wood pellets production (Lange 2007). Withagricultural materials it is possible to decrease just these costs significantly. Precise descriptions ofthe costs can be found in the book of Obernberger and Thek (2010).

Figure 15. Pelleting cost versus plant size. In the bottom line is the capital costs/t, the middle line is theperating costs /t and the top line is total costs/t (Mani 2006b).

11. CONCLUSION

Development of renewable energy is a central aim of the European Commission's energy policy.Several reasons stand for this: renewable energy has an important role to play in reducing carbondioxide (CO2) emissions - a major Community objective. Increasing the share of renewable energyin the energy balance enhances sustainability. Renewable energy also helps to improve the securityof energy supply by reducing the Community's growing dependence on imported energy sources.Renewable energy sources are expected to be economically competitive with conventional energysources in the medium to long term (Anon. 2002). The renewable energies, biomass fuels already

play an important role in several European countries

In pellet production there is a shortage of woody raw materials in several countries. Also the price ofthe wood raw material increases. In Denmark and southern European countries it is the potential oflow forestry. So, agricultural residues could be largely used in the future for raw materials of pelletsmanufacturing. It is therefore of great importance to study the characteristics of this new category ofraw materials, paying special attention to the problems that they may trigger both at production andutilisation level. The information gathered in this report points out both positive and negativesubjects affecting agricultural pellets (with a special focuses on straw pellets) in comparison withwood pellets.

Pellets and agripellets have several positive aspects as fuels compared to firewood, wood chips and briquettes:

Pellets have a high energy content per volume unit, 4 – 5 MWh/to increased bulk density (500-700 kg/m3)o lower transportation costs,

o less storage is needed.

Low moisture contento favouring a long conservation,o less loss of product during storage,o advance to use of wet wood dust.

Small variations in fuel qualityo facilitating material handling,o rate of flow control,o cheaper and simple feeding equipment.

Dust freeo reducing dust explosion potential,o minimizing particle emission.

Uniformo more efficient control of combustion.

Homogeneous compositiono fully automatic heating operation,o complete combustion,o little repairs and high annual time of using.

An increased energy density in combustion,o better control possibilities,o higher energy efficiency.

Low emissions during combustion

Can be used for trimming of fuels in small and large heat centres. cheap price of raw material.

Naturally agricultural raw materials and -pellets have also some drawbacks compared to wood pellets:

The supply reliability and quality of the raw material,o soil, climatic conditions and fertilising.o growing season, with lower production in abnormally dry years.

Impurities of raw materialo straw, bark etc. would present a higher abrasive power,o An increased wear of the parts of mill.

Moistureo in winter wet and snowy bales,o the screen of hammer mill jams,o quality of pellets will be lower.

The low bulk volume,o high transportation costs,o demand for large storage capacitieso difficulties in pelletisig.

The fibre structure is different compared to saw dusto fibre rotates around the rotating feeding/handling devices,o bridging problems,o variations in material flow in production.

More difficult handling process at factory than with wood dust,o even the risk of dust explosion..

Mixed pellets

o need for two feeding in lines. In pressing process

o variation of power consumptions,o uneven feed.

Price of raw material can be increase, Difficulties in combustion, emissions (another report of the MixBioPells -research).

Technically production straw and other alternative raw materials can be pelletised without majordifficulties when the proper moisture content and pressing temperature exist. Feedstock moisturealso appears to have an important effect on improving pellet density and durability. As water softenslignin, moisture can improve durability if densification temperatures are low. To produce durable

pellets, several precautions are required (Porter et al. 2008): The moisture content of grass material should be 10-13%, The material should be finely ground using a screen of at least 2.8 mm ( Canadian

recommendation), in Europe it is used coarse screens, usually 4 – 5 mm and even muchcoarser up to 18 mm.

The pellet die should have L/D (length/diameter) of 8.5-9:1 (Canadian recommendation), inEurope ring dies 8:1 – 10:1 are used (results are few), diameter of aperture is 10 – 12 mm.

Steam should be used for conditioning and for increasing the temperature of the raw material.

Olive and grape residues are typical Mediterranean materials. Especially olive residues have beenused as a fuel. They have rather high bulk density and high heating value. On the other hand, theolive might be a competitor to pellets in larger plants. It is cheaper and needs only conditioning, butnot any manufacturing process.

Improvements could come from the fuel preparation stage, with the addition of some specific anti-slagging agents (e.g. kaolin, Ca(CO3)) or the mixing with sawdust to present final characteristicmore convenient with regard to combustion and ash issues (Pastre 2002).

Co-firing of agricultural pellets with other fuels is also an interesting alternative, both technicallyand economically. For straw pellets the small scale markets of devices are still very limited, butsome manufacturers already propose multi-fuel grate boilers in the range of 10-60 kW. In all cases,attention must be paid to the flue gas cleaning systems. Pellets made from agricultural residues (andin general other ash-, N-, K and Cl-rich fuels) should be used primarily in large scale combustion

plants equipped with sophisticated combustion control systems and flue gas cleaning systems,whereas wood pellets should be preferred for residential heating. Assuming that economic aspectsconcerning the agripellets energy option are favourable, the agripellet market for small-scale use willdevelop only if equipment manufacturers are encouraged to develop novel, safe and affordablecombustion solutions (Pastre 2002).

It is essential to further optimize (Porter et al. 2008) the alternative pelleting systems in order to becompleted on commercial pelleting systems. Parameters, such as time of harvest, the residence timeof high temperature saturated steam, impact of various L/D dies, and the impact of increasing pellet

diameter on pellet bulk density and durability require further assessment to more fully optimizeagripellets production and pellet quality.

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Appendix1/1

Table 1_1. Summary of the Potential for producing pellets from alternative materials (Steward

2007).

Material Cost

Ease of pelleting

Ash Sulphur Pellet Potential

Landfilltimber

Could be low ornegative cost Hard to moderate

Low V.low Very cheap but may needrape cake to improve pelletquality. Risk ofcontamination?

‘Gardenwaste’

Low Easy

Med/high Low butmay vary

May be too mixed andunpredictable but low cost,suitable for power stations?

Hemcore High – good marketsin animal bedding Moderate

Med Low Probably too expensive duetocompeting uses (animal

bedding, insulation) Oilseed rapestraw

Baling cost only? Hard to moderate

High High Tends to be ploughed in?Could be a good material,low cost, but ash andsulphur high

Oilseed rape

£90/tonne (Med/high?) High Expensive, competing

markets forcing price up, butmay begood as a blend to facilitateuse of other materials

Miscanthus £45/odt but may needto be higher to attractgrowers Moderate

Low/med Low May need to be expensive toencourage production

potential for high production. Ash andslagging may be a problem

Wheat straw Locally about£45/tonne delivered

but can be muchhigher Hard to moderate

Very high Med High ash, high pricevariability, may be other

demands when needed.

Wheat grain £95 per tonne +delivery Easy

Med ND Expensive but best used as a binder

Willow £45/odt but probablynearer£90/odt to attractgrowers Hard to moderate

Low/med ND Need to dry (drying costs?).May be expensive as needhigh price to encourage

production. However, potential for high production

Appendix 1/2

Table 1_2. Pelleting and combustion properties of non-wood materials (Steward 2007)

Pellet quality Combustion AshMaterial

Ease of pelleting Length,

mmDensity,

t/m3 MC,%

Lighting Burning % Quality

HemcorePale

Moderate 3 – 12 0.67Hard

8.4 Easy Verygood

2.7 Grey/buff powdery20% sinter

Hemcore +20% rapecake

Moderate/easy

8 – 20 0.67Hard

8 Easy Verygood

3.33 Palegrey/buff

powdery 5-10% sinter

Miscanthus Moderate 2 – 12 0.62Hard

8 Easy Good 1.07 Mid/darkgrey 35%sinter –(very hard)

Miscanthus+ 20%camelinacake

Moderate/easy

8 – 20 0.7Fairlyhard

8.8 Easy Good 1.83 Mid grey35% sinter(soft)

agriculture/etc residues

Easy 20 0.7

Hard Slow 5.1 Wormy, no

sinter

Oat straw Not easy 5 – 15 0.54Soft,

crumbly

10.4 Easy Verygood

3.7 Dark grey,wormy 40%sinter

Rape straw Not easy 8 – 15 0.67 5.21Wheat straw Hard to

Moderate10 – 20 0.64

F. hard

12.2 Easy Verygood

6.1 Black 15 -20% sinter

Wheat straw +

20%camelina

12 – 20 0.7

F. hard

Easy Very

5.4 Black, 15%

sinterWheat grain Fairly

Easy15 – 20 0.76

Hard11.2 Very

hardModerate to poor

2.46 Black, 40%soft sinter

Willow Moderateneededoil

Up to20 mm

0.66 Easy Good 1.0

Steward, A., 2007. An Investigation of the Feasibility of Preparing Fuel Pellets from a Range of

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