biomass size reduction machines

Size reduction machines for enhanced biogas production

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Introduction

Biogas is the source of energy that is mainly used for generation of heat or electricity in various

countries and is used as car fuel as well. Main constituent in biogas is methane which has high

calorific value and is cleaner fuel. Biogas production processes are clean, environment friendly

and economic.

Lignocelluloses are major products of different waste streams coming from food processing,

agriculture, forestry and municipals. Biological degradations of this polymer are carried out by

several enzymes such as amylase, cellulase, before further fermentation or digestion to e.g.

ethanol or biogas. However these polymers should be available for the enzymes to achieve

biodegradation. The inherent properties of these lignocellulosic wastes namely, crystallinity of

cellulose, protection by lignin and hemicelluloses make them resistant to the enzymatic attack.

Enzymatic digestibility is related to accessible surface area for cellulose.

Increase in population is leading to increase in fuel consumption and waste generation. The use

of non-renewable energy resources like coal, gasoline has limitation. Hence we are bound to

explore the alternate energy resources such as bio-fuels which has multiple advantages like less

polluting energy source and proper utilization of waste-biomass, solar energy, wind energy etc.

out of which the biofuel production seems to be economically feasible and efficient. In addition

to the energy production, biogas plants are helpful in dealing with the problems of waste

management and help to provide clean environment and decreasing pollution.

To cater the rising energy demands the production of biofuels should be increased and

pretreatment is one of the available solutions. The pretreatment can enhance the bio-digestibility

of the wastes for biogas production and increase accessibility of the enzymes to the materials. It

results in enrichment of the difficult biodegradable materials, and improves the yield biogas from

the wastes.


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1. Biogas Production

Biogas production from biomass takes place through anaerobic digestion. Anaerobic digestion is

the complex microbial process taking place in the absence of oxygen. The process converts

organic feedstock into the biogas, which contains about 50-70% methane, 50-30% CO2 and

small amounts of H2S depending upon the feedstock characteristics.

Anaerobic Digestion involves four following stages, Acidogenesis, Acetogenesis,

Homoacetogenesis and Methanogenesis.

Acidogenesis: It involves hydrolysis of complex polymers (proteins, polysaccharides)

into monomers and oligomers e.g. Sugars, amino acids, peptides and fermentation of

complex polymers to yield hydrogen and carbon dioxide and acetates and also it involves

fermentation of monomers, oligomers to yield propionate, butyrate and fatty acids.

Acetogenesis: It is the conversion of propionates, butyrates and fatty acids into Hydrogen

and carbon dioxide and acetates.

Homoacetogenesis: Homoacetogenesis involves the conversion of Hydrogen and Carbon

dioxide into acetates.

Methanogenesis: It is the final stage of the biogas production process yielding methane

and carbon dioxide from acetates and methane and water from hydrogen and carbon

dioxide.

Table 1: Legends for Fig.1

Legend Description Legend Description

1.1 Hydrolytic bacteria 4 Methanogenic

bacteria

1.2 Fermentative bacteria 4.1 Hydrogenotrophic

methanogenic bacteria

2 Acetogenic bacteria 4.2 Aceticlastic

methanogenic bacteria

3 Homoacetogenic

bacteria


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Fig.1 AD Process (Colleran E. 1991a)

2. Enhancing Biogas Production

There are two major ways of enhancing production,

1. Proper Pre-Treatment

Lignocelluloses are major components of feed-biomass. Lignocelluloses are very stable in nature

and hence bacterial or enzymatic attack is not very effective. Hence proper pretreatment of the

lignocelluloses becomes important process in order to achieve maximum biogas production from


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the feed. In the process of pretreatment, the biomass is processed by various techniques to

remove the lignin protection, reduce the stability and make biomass accessible for enzymatic

attack.

Crystallinity, accessible surface area, protection by lignin and hemicelluloses and degree of

cellulose polymerization are the effective parameters in the pretreatment of lignocelluloses.

(Taherzadeh and Karimi, 2008)

2. Maintaining the Optimum Operational Parameters

Biogas production can be enhanced by operating the biogas plant at optimum temperatures, pH,

loading rate, agitation etc. Any change in these can adversely affect the production efficiency of

the biogas plant.

pH: It is one of the important factors that affect the growth of microbes in anaerobic

fermentation. During the anaerobic process, CO2 and volatile fatty acids evolved, affect

the pH of the digester. Jain and Mattiasson (1998) found that the CH4 production

efficiency was more than 75% above pH 5.

Temperature: Temperature inside the digester plays an important role in biogas

production process. Anaerobic fermentation can be carried out in different temperature

ranges. But the anaerobes are most active in mesophilic (30-40oC) and thermophilic (50-

60oC) temperature ranges. (Mital, 1996; Umetsu et al.,1992; Maurya et al., 1994;

Takizawa et al., 1994). Methanogemes are sensitive to sudden thermal changes (Garba,

1996) therefore sudden changes in temperatures should be avoided.

Agitation/Mixing: Mixing devices such as scraper, gas recirculation can help increasing

the biogas production.

In this report Pre-Treatment method mainly Physical Pretreatment along with Size Reduction

Equipments to enhance the biogas production is discussed in detail.


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3. Pre-Treatment Techniques

3.1 Necessity of Pre-Treatment

To understand the necessity of the pretreatment we need to consider the structure of the feed

biomass. Kratky and Jirout (Chem Eng. Technol, 2011) have explained the structure of biomass

which is very easy to understand. They explained it comparing with the reinforced concrete

pillars, cellulose fibers analogous to the metal rods and lignin to the cement matrix. Lignin is

tightly bounded to the carbohydrates by covalent and hydrogen bond. Hence lignin protection

makes the biomass very stable and reluctant to enzymatic attack which results in longer

degradation times and degradation to the lower extent, as low as up to 20 per cent. (Pandey A.,

2009)

Following figure makes the structure of biomass and necessity of pretreatment more clear:

Fig. 2: Effect of Pretreatment on Biomass (Teherzadeh and Karimi, 2008)


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3.2 Factors Affecting the Rate of Biodegradation:

Teherzadeh and Karimi (2008) studied the factor affecting the biodegradation rates and

concluded that following factors play the key role in rate of biodegradation.

1. Crystallinity of Cellulose

The major part of cellulose exists in crystalline form. The crystal form is less accessible

compared to the amorphous part. Therefore it is seen that high crystalline cellulose is resistant to

enzymatic attack and hence less the crystallinity more is the digestibility of lignocelluloses. (Fan

et al.)

2. Accessible Surface Area

Lignocellulosic materials have two types of surface area namely internal surface area and

external surface area. Capillary structures of cellulose fibers determine the internal surface area

while size and shape of the particles decide the external surface area. Drying of fibers result in

irreversible shrinkage of capillary structure reducing the accessible surface area, whereas wet

celluloses has higher area compared to dry. Accessible surface area and its interaction with

enzymes can be limiting in enzymatic hydrolysis.

3. Lignin

Lignin is responsible for structural integrity and rigidity of lignocellulose. Lignin content is

responsible for lignocellulosic materials to resist the enzymatic attack. Hence the delignification

process results in increased enzyme activity.

4. Hemicellulose

Hemicellulose is also a barrier for the enzymatic attack on the cellulose. Removal of

hemicelluloses results in improved the enzymatic hydrolysis.


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Fig.3: Lignocellulosic Complexes in Plant Cell Walls (Bochmann and Montgomerry, 2013)


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4. Methods of Pretreatment

Pretreatment is a process to remove the lignin and hemecellulosic protection of cellulose, make it

available for enzymatic attack. The process converts the complex organic structures into simpler

molecules which are more susceptible to microbial degradation.

Pre-treatment methods can be classified in to three major categories:

1. Physical Pre-Treatment

2. Chemical Pre-Treatment

3. Biological Pre-Treatment

Also, the combinations of the two or more pretreatment techniques such as Physico-Chemical,

Bio-Chemical Pretreatments can be used to achieve the desired pretreatment.

An economic and effective pretreatment results in enhancement of the solubilization of cellulose

polymer into monomer sugars without any degradation product formation.

Pandey A. (2009) suggested the criteria for selection of proper pretreatment techniques which

takes into account the following factors:

Cost of Operation

Energy Demand

Emissions (Pollution)

In addition to the above mentioned factors, physical properties of feed biomass is also an

important while selecting the suitable pretreatment technique such as initial and final size of

biomass, moisture content etc.

In the following table, we see the comparison between the major three pretreatment techniques.


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Table 2: Comparative study of different pretreatment techniques

Physical Pretreatment Chemical Pretreatment Biological pretreatment Ref.

Aim

To Increase

Surface Area

Decrystallization

Reduce

polymerization*

Remove Lignin and

Hemicelluloses

protection

Increment in cellulose

accessibility for enzyme

attack.

Conversion of

lignocellulosic

biomass into more

accessible

components

Harun

et al.,

Mechanism

Using Size

Reduction

Equipments the

above mentioned

characteristics are

achieved.

Hydrolysis of

hemicelluloses,

Conversion of lignin

into soluble fragments.*

Swelling of fibrous

celluloses leads porous

biomass.*

Use of microbes to

degrade the

cellulose protection

Sun et al.

Sun

Y.,Cheng

J.,2002

Advantages Enhanced biogas production.

Disadvantages High energy

requirements

Toxicity and

corrosiveness of

chemicals cause many

problems to the process

as well as equipments.

Recovery of chemicals

is costly and difficult.

Longer process

time

Large space

requirement

Continuous

monitoring and

maintaining pro-

microbe

conditions*

Singh et

al.


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Examples

Milling

Shredding

Microwave

Extrusion etc

Acid Pretreatment

Alkaline pretreatment

etc

Use of fungi or

actinomycets

Chrysosporium on

cotton stakes results

lignin degradation

5. Physical Pretreatment Techniques

So far we have seen the necessity of pretreatment, criteria and different techniques for selection

of suitable pretreatment. Now here onwards we will look into various Physical pretreatment

equipments that are responsible for the comminuting the lignocelluloses in order to enhance the

biogas production.

Particle size reduction increases the available specific surface area, reduce the degree of cellulose

polymerization and its crystallinity which increases total hydrolysis yield by 5-25% and reduces

the digestion time by 23-59% (Hendriks and Zeeman, 2009).

The size reduction is usually achieved by chipping, grinding, milling or their combinations. The

energy requirement is the most important parameter which governs the economics of the

pretreatment and it is dependent on final particle size, type of the mill, biomass characteristics.

Yu et al. concluded that in the grinding process, almost all the energy is lost as heat and only up

to 1% energy is actually utilized for size reduction. Hence selection of the mill also becomes

important factor from the economic point of view.

6. Size Reduction Equipments

6.1 Shredders

Screw, Knife and Hammer Shredders and their combinations are most commonly used for

compact, difficult to handle feed stock such as food and food wastes, bones, grass, paper etc.

Most of the biomass grinders operate in rotary action. In addition, Igathinathane et al. developed

a linear sheer action knife based grid which they used for shredding of the corn stakes and switch


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grass and reported the lower energy consumption for the same duty by any other equipment. The

reported values of power requirement for linear-sheer action knife shredder are as follows:

For size reduction of corn stake and switch grass respectively

From 203mm to 100mm: 0.48 kWht-1

0.19 kWht-1

From 203mm to 50mm: 1.69 kWht-1

0.49 kWht-1

Whereas the values reported by Schell and Hardwood (1994) for energy consumption to reduce

the papers to final size of 4cm is 15.2 kWht-1

Fig. 4: Rotary Shredder Schematic (Kratky and Jirout, 2011)

Various types of knife shredders are considered to be universal for biomass size reduction.

(Kratky and Jirout, 2011)

In the above figure we can see the rotary shredder. The biomass is loaded in between the two

rotary blades. It undergoes sheering and reduction in size takes place. The comminuted product

is then taken out by the screens, opening and sent for further processing.

Feed

Screen


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The following Image, Fig.5 makes the installation and working of rotary shredder clear.

Fig. 5: Rotary Shredder

6.2 Ball Milling

Ball milling is also one of the effective lignocellulosic size reduction machines. In ball mill the

reduction is done by the impact as the ball drop down from the shell walls near top.

Modifications are possible as per the requirement such as compartment ball mills where different

sizes of the balls work in different compartment.

Compressive forces and sheering reduce the crystallinity and degree of cellulose polymerization

as well. Milling at elevated temperatures is also possible in ball milling which increases the

hydrolysis effectivity considerably compared to that at normal temperatures.


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Silva et al. found that the milling process time is associated with the hydrolysis effectivity.

Increase in milling time resulted increase in hydrolysis effectivity. Koullas (1992) studied the

effect of process time for wheat straw size reduction. He reported that the conversion of

saccharide during hydrolysis without treatment was 17.7% and after the milling for 2 hours

process time it was increased to 61.6%.

Fig.6 Ball Mill

Hence we can conclude that the ball milling is a time consuming operation, longer the process

time, better the hydrolysis effectivity.

6.3 Vibration Mill

Sun and Cheng found vibratory ball milling to be more effective than the ordinary ball milling

process in breaking down the cellulose crystallinity. The effectiveness of the vibratory mill

depends on grinding elements, the lignin structure and the frequency of the vibration.

The working of vibratory mill is similar to that of the ball mill with an addition of the vibrating

shell instead of rotating shell.

The mill consists of a pot and a motor. The whole apparatus is supported by a spring which

allows the vibrations generated by the motor.


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Following image shows the schematic of the vibratory mill:

Fig. 7: Schematic of Vibratory Mill (Kobayashi et al., 2008)

The pot contains the grinding balls which are most of the times made up of steel and the feed

biomass. The final size is determined by the frequency of the shaker/ vibrating motor.

6.4 Knife Milling

According to Teherzadeh and Karimi, 2008, the knife milling operation is usually applied with

dry biomass, moisture content not exceeding 15%, wet basis. Knife milling is limited for

comminuting dry biomass such as straws, grasses, fodder crop wastes etc.

Mani et al. (2004) concluded that the energy requirement of knife mill is the least among

available options for disintegration of the dry biomass.

The knife mill consists of a blades or knives mounted on rotor shaft. A rotor rotates the rotor

shaft and the knife grid. The feedstock comes into the shell usually from the top portion of the

mill. The mill also consists of stationary bars known as cutting bars and due to which the feed

traps in between the knife and cutting bar and the size reduction takes place. Final particle size is

controlled by feed rate, rotor RPM and the type of the drum screen. Energy requirement depends

on all the above parameters along with mounting angle and bevel angle of the knife.


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Fig. 7: Knife Mill

Fig. 8: Schematic of Knife Mill

Feed

Outlet Screen

Knife


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6.5 Hammer Milling

The main advantage of the hammer mills is their high size reduction ratio as well as simple

adjustment of particle sizes. Hammer mills are easy to operate, cheap and offer better particle

size control.

Fig. 9: Hammer Mill Schematic

As we can observe the figure above; the hammer mill consists of a rotor, hammers mounted on

rotor and a breaker plate. As the rotor rotates, the hammers attached to it impact the inlet feed

against a breaker plate and the reduction in size of the feedstock takes place. The product is then

taken out from the screens lying at the bottom of the mill.


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We can alter the screen size, feed rate and the hammer revolution in order to achieve desired size

reduction. But there is limitation to the rotation speed of the hammer tip. Lower hammer tip

speeds will allow the material to impinge on the screen at greater angle and cause coarser

material to pass through whereas higher speeds will make the material move parallel to the

screen surface resulting the partial opening. (Bitra et al. 2008)

Yu et al. reported the specific energy demand for wheat straw having moisture content of 8.3%,

wet basis, for grinding of 20-50mm long feed using multiple screen sizes as follows:

Table 3: Energy Required by Hammer Mill (Yu et al, 2003.)

Screen Size (mm) Energy Required (kWht-1

)

0.794 51.55

1.588 39.59

3.175 10.77

6.6 Disc Mill

In disc mills, the material is comminuted between the discs and flows to the radially outwards

direction due to the rotational motion. The material is fed through the center, coaxial to the

rotation axis and the due to acceleration it goes through multiple rotating blades.

Compression and sheer are the predominant mechanism of fragmentation of the feedstock.

Disc mills are built in single disc or double disc with profiled as well as straight blades

Millet et al. studied the energy requirements of the disc mill at room temperature and at 200oC

and observed 100 kWht-1

which was 7.5 to 8.5 times lesser than that at room temperatures to

achieve 0.5mm final particle size for wood chips.


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Fig. 9: Schematic of Disc Mill

6.7 Colloid Mill

Colloid mills are mainly used for solid suspended in liquid havin more than 15% moisture

content on wet basis. (Taherzadeh and Karimi, 2008)

Colloid machines consist of two discs placed close to each other and revolving in opposite

direction. The feed is let in through the center and travels through the discs undergoing the shear.


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Fig. 10: Schematic of Disc Mill

Schell and Hardwood came up with the energy requirements of the wet disc mill and that are

comparable to the hammer mill. Hideno and Inoue (2009) compared the energy requirements of

wet disc mills with the ball mill for disintegration of the rice straw having final size of 2-3mm

and came to the conclusion that wet disc milling is applicable as continuous and massive

treatment process.

The energy requirements as well as process time are functions of biomass characteristics,

suspension viscosity, rotational speed, gap size and the design of disc.

6.8 Two Roll Mill

The two roll mill is very basic machine for the reduction in crystallinity, degree of

polymerization of celluloses. It consists of two surface rolls of cast iron placed horizontally with


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an adjustable clearance between them. The feedstock is fed between them and pressed. This

results in reduction in crystallinity and increase in bulk density.

But it may happen that reduction in surface area might take place due to the agglomeration of the

particles. Size determining parameters are roll clearance, roller temperature, process time and

roller speed.

6.9 Extrusion

The typical extruder comprises of three compartments, namely Feed, Transition and compression

and metering. Extruders have the capability to provide high shear, rapid mixing and rapid heat

transfer.

Extrusion is a continuous treatment hence extruders can be used easily in large scale operations.

Extruder parameters such as screw speed, barrel temperature, compression ratio are important

factors that influence sugar recovery from biomass. (Karuranithy and Muthukumarappan, 2009)

Fig. 11 Twin Extruder patented by Berger and Gelus, 2005

Key: (1): Input, (2) extruder I, (3) acid solution, (4) tempered barrel, (5)

extruder II, (6) pressure-reducing valve, (7) expansion chamber


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6.10 High Shear Effective Machines

These are the alternative equipments for the disintegration of the biomass feed having high

moisture contents, usually greater than 15-20%, wet basis. This process can be used for

suspended solids in various liquids such as liquid ammonia, water, aqueous solutions of acid or

alkali reagents, enzyme solutions containing cellulose etc. (Kratky and Jirout, 2011)

The design of these machines is very similar to that of disc mills or pumps with differences in

process parameters. These machines consist of a stationary and multiple rotating discs. Rotating

discs are usually perforated. This design leads to generation of local cavitation and pressure

fluctuation.

Fig. 12: Machines using developed cavitation

6.11 Freeze Pretreatment

Chang et al (2011) studied the effect of freeze pretreatment on the biomass. They mixed rice

straw with the buffer solution, a solution selected on the basis of its ability of expansion on

cooling. When the solution is cooled, due to the expansion in the volume of buffer solution,

breakage in structures of cellulose takes place. The process has no adverse effects to the

environment.

1 2


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7. Conclusion

Biomass size reduction techniques and size reduction machines for the pretreatment of biomass

were studied thoroughly.

In order to improve the biogas production, pretreatment is of immense importance. To select the

most suitable pretreatment it is necessary to consider the structure of biomass, moisture content

of feedstock and initial and final sizes of the biomass.

It is observed that the Ball milling and vibratory milling are the universal size reduction

equipments since they can handle dry as well as wet biomass for various temperature ranges. But

there are limitations because the processes are time consuming.

Biomass having more than 15-20% moisture content is pretreated using extrusion, wet disc mills

and high shear effective machines. The extrusion process is of great benefit since it offers high

shear rate, faster heat transfer and is a continuous process whereas disc milling is a batch

process, disintegration of biomass in large quantity is difficult in case of disc or colloid milling.

Typically Dry Biomass, moisture content not exceeding 15%, are disintegrated commonly by the

Hammer milling, knife milling, two roll milling. In the case of hammer mills and knife mills

small sizes of output biomasses can be achieved at the lower energy inputs and hence these

machines are widely used in size reduction techniques.


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8. References

Chang, K. L., Thitikorn-amorn, J., Hsieh, J. F., Ou, B. M., Chen, S. H.,

Ratanakhanokchai, K., Chen, S. T. (2011). Enhanced enzymatic conversion with freeze pretreatment of rice straw. Biomass and Bioenergy, 35(1), 9095. doi:10.1016/j.biombioe.2010.08.027

Igathinathane, C., & Ph, D. (2008). Linear Knife Grid Application for Biomass

Size Reduction, 0300(06).

Kobayashi, N., Guilin, P., Kobayashi, J., Hatano, S., Itaya, Y., & Mori, S. (2008).

A new pulverized biomass utilization technology. Powder Technology, 180(3), 272283. doi:10.1016/j.powtec.2007.02.041

Mani, S., Tabil, L. G., & Sokhansanj, S. (2006). Effects of compressive force,

particle size and moisture content on mechanical properties of biomass pellets

from grasses. Biomass and Bioenergy, 30(7), 648654. doi:10.1016/j.biombioe.2005.01.004

Schell, D. J., & Harwood, C. (1994). Milling of lignocellulosic biomass - Results

of pilot-scale testing. Applied Biochemistry and Biotechnology, 45-46(1), 159168.

Sun, Y., & Cheng, J. (2002). Hydrolysis of lignocellulosic materials for ethanol

production: a review q, 83, 111.

Taherzadeh, M. J., & Karimi, K. (2008). Pretreatment of lignocellulosic wastes to

improve ethanol and biogas production: A review. International Journal of Molecular Sciences (Vol. 9, pp. 16211651). doi:10.3390/ijms9091621

Womac, A., Igathinathane, C., & Bitra, P. (2007). Biomass pre-processing size

reduction with instrumented mills. , 0300(07). Retrieved from http://biomassprocessing.org/Publications/2-Papers_presented/ASAE Paper

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