Pyrolysis Promising Route

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Bioresource Technology 42 (1992) 219-231 Pyrolysis, a Promising Route for Biomass Utilization G. Maschio Dipartimento di Chimica Industriale, Universith di Messina, Salita Sperone 31 CP 29, 1-98166 S'Agata di Messina, Italy C. Koufopanos* & A. Lucchesi Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universit~ di Pisa, Pisa, Italy (Received 30 July 1991; revised version received 15 January 1992; accepted 4 February 1992) Abstract The pyrolysis of biomass is a thermal treatment which results in the production of char, liquid and gaseous products. In this laboratory the pyrolysis process has been studied experimentally using apparatus of different scales. In particular, the influence of the main process parameters on the yields and characteristics of the products has been investigated. On the basis of these results the differences between conven- tional and fast pyrolysis can be discussed. The most attractive product of conventional pyrolysis is charcoal, as the handling and use of bio-oil presents some problems due to its charac- teristics. The pyrolysis gas is a medium BTU gas and can be easily burnt. Fast pyrolysis minimizes charcoal production. This process gives as the main product a high yieM of a medium BTU gas rich in hydrogen and carbon monoxide. The feasibility of the process on an industrial scale is discussed. Key words: Biomass, pyrolysis, thermochemical conversion, wood, ligno-cellulosic components. INTRODUCTION Amongst the different processes which have been proposed for the energetic utilization of biomass, pyrolysis remains one of the most promising. The *Presentaddress:Hellenic CementResearchCentre. 15, K. Pateli,Gr-14123 Likovrisi, Greece. Bioresource Technology 0960-8524/92/S05.00 © 1992 Great Britain term 'pyrolysis', is defined as the thermal treat- ment of biomass, in the absence of oxygen, which results in the production of solid (charcoal), liquid (tar and an aqueous solution of organics) and gaseous products. Pyrolysis is interesting, not only as an independent process leading to the produc- tion of energetically-dense products, but also as an intermediate step in a gasification or combus- tion process. A large number of research projects in the field of thermochemical conversion of biomass and particularly on biomass pyrolysis have been carried out (Knight, 1979; Sorer & Zaborsky, 1981; Bridgwater & Beenackers, 1985; Bridg- water & Van Swaaij, 1987; Bridgwater, 1988; Beenackers et al., 1989; Bridgwater & Bridge, 1991; Diebold, 1991 ). The results of this research have proved the feasibility of this technology. Many results regarding the identification of the wide spectrum of substances produced and their physico-chemical characterization are now avail- able. The problems associated with the realization of the process and the utilization of the products have been made evident. Bridgwater (1988) in a recent review analyzes the state of the art of dif- ferent pyrolysis technologies. Different interesting approaches to the efficient solution of the scale- up problems have appeared (Bridgwater & Bridge, 1991; Diebold, 1991 ). During the past 15 years many different pyroly- sis processes have been researched and developed in USA (Knight et al., 1986; Diebold & Power, 1988; Kovac et al., 1987); Diebold ( 1991 ) reviews the development of pyrolysis reactor concepts in the USA. However, the complexity of the process 219 Elsevier Science Publishers Ltd, England. Printed in

Transcript of Pyrolysis Promising Route

Page 1: Pyrolysis Promising Route

Bioresource Technology 42 (1992) 219-231

Pyrolysis, a Promising Route for Biomass Utilization G. Maschio

Dipartimento di Chimica Industriale, Universith di Messina, Salita Sperone 31 CP 29, 1-98166 S'Agata di Messina, Italy

C. Koufopanos* & A. Lucchesi

Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universit~ di Pisa, Pisa, Italy

(Received 30 July 1991; revised version received 15 January 1992; accepted 4 February 1992)

Abstract

The pyrolysis of biomass is a thermal treatment which results in the production of char, liquid and gaseous products.

In this laboratory the pyrolysis process has been studied experimentally using apparatus of different scales. In particular, the influence of the main process parameters on the yields and characteristics of the products has been investigated. On the basis of these results the differences between conven- tional and fast pyrolysis can be discussed.

The most attractive product of conventional pyrolysis is charcoal, as the handling and use of bio-oil presents some problems due to its charac- teristics. The pyrolysis gas is a medium BTU gas and can be easily burnt. Fast pyrolysis minimizes charcoal production. This process gives as the main product a high yieM of a medium BTU gas rich in hydrogen and carbon monoxide.

The feasibility of the process on an industrial scale is discussed.

Key words: Biomass, pyrolysis, thermochemical conversion, wood, ligno-cellulosic components.

INTRODUCTION

Amongst the different processes which have been proposed for the energetic utilization of biomass, pyrolysis remains one of the most promising. The

*Present address: Hellenic Cement Research Centre. 15, K. Pateli, Gr-14123 Likovrisi, Greece.

Bioresource Technology 0960-8524/92/S05.00 © 1992 Great Britain

term 'pyrolysis', is defined as the thermal treat- ment of biomass, in the absence of oxygen, which results in the production of solid (charcoal), liquid (tar and an aqueous solution of organics) and gaseous products. Pyrolysis is interesting, not only as an independent process leading to the produc- tion of energetically-dense products, but also as an intermediate step in a gasification or combus- tion process.

A large number of research projects in the field of thermochemical conversion of biomass and particularly on biomass pyrolysis have been carried out (Knight, 1979; Sorer & Zaborsky, 1981; Bridgwater & Beenackers, 1985; Bridg- water & Van Swaaij, 1987; Bridgwater, 1988; Beenackers et al., 1989; Bridgwater & Bridge, 1991; Diebold, 1991 ). The results of this research have proved the feasibility of this technology. Many results regarding the identification of the wide spectrum of substances produced and their physico-chemical characterization are now avail- able. The problems associated with the realization of the process and the utilization of the products have been made evident. Bridgwater (1988) in a recent review analyzes the state of the art of dif- ferent pyrolysis technologies. Different interesting approaches to the efficient solution of the scale- up problems have appeared (Bridgwater & Bridge, 1991; Diebold, 1991 ).

During the past 15 years many different pyroly- sis processes have been researched and developed in USA (Knight et al., 1986; Diebold & Power, 1988; Kovac et al., 1987); Diebold ( 1991 ) reviews the development of pyrolysis reactor concepts in the USA. However, the complexity of the process

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220 G. Maschio, C. Koufopanos, A. Lucchesi

requires a variety of solutions suitable for the particular needs of each application.

It is reported (Bridgwater & Van Swaaij, 1987; Beenackers et al., 1989) that, considering the oil prices of 1988, the cost of an energy unit pro- duced via pyrolysis is double that derived from fuel oil. However, the trend of oil prices indicates that significant variations may occur even in a short period. A significant increase in the price of oil is a likely scenario. As a consequence, the development of pyrolysis and other thermo- chemical processing of biomass can play an important role in the programming of short-term strategies.

In the past few years (Elliot, 1985; Bridgewater & Bridge, 1991) much effort has been focused on the optimization of the operating conditions in order to obtain the most favorable yields of products and to improve their quality. In this study particular attention has been paid to improving the characteristics of the bio-oil. By changing the operating conditions during pyroly- sis we can modify the actual course of reactions and, thus, modify the final product distribution. In particular the kinetics of the process are influ- enced by the values of the main process para- meters: temperature, solid residence time, compo- sition of feedstock, particle size and heating rate. It has been shown (Koufopanos et al., 1989, 1991), that the heating conditions strongly affect the progress of the process. High heating rates (above 1000 K/s), which are employed in flash pyrolysis, minimize the yields of solid pyrolysis products and maximize those of liquid products (Scott & P iskorz, 1982; Antal, 1983 ).

Depending on the operating conditions, the pyrolysis processes can be divided into three sub- classes: Conventional Pyrolysis, Fast Pyrolysis and Flash Pyrolysis.

The range of the values of the main operating parameters are summarized in Table 1.

The evolution of fast- and flash-pyrolysis tech- nologies must be attributed to the fact that the utilization of liquid fuels is very attractive (Antal, 1983; Scott et al., 1985; Radlein et al., 1987).

Some interesting results concerning the improve- ment of bio-oil characteristics by using fast- or flash-pyrolysis followed by a secondary upgrading process have been reported in the literature (Knight et al., 1986; Bridgwater & Bridge, 1991). The characteristics of the bio-oil produced and the economics of the process suggest further research developments in this field.

In our laboratory, the pyrolysis process has been systematically studied using different experi- mental apparatus of laboratory-, bench-, and large (pilot)-scale (Lucchesi & Maschio, 1987; Koufo- panos et al., 1989, 1991). Some significant results are presented here. The large amounts of experi- mental data, regarding the yields and the charac- terization of the pyrolysis products as well as pyrolysis reactor design and performance, offer a basis for the assessment of the process and pro- pose the most attractive paths to follow. As these data concern both conventional and fast pyrolysis, the differences between these two versions of pyrolysis can be discussed, their boundaries can be explored and possible interpretations of their behavior can be provided. This work helps to fill the gap existing between the research and the application of biomass conversion technologies.

METHODS

Experimental apparatus

Convent ional pyrolysis The conventional pyrolysis process was studied experimentally in apparatus of different scale and type. A first series of experimental runs was carried out in order to investigate the influence of temperature, composition and biomass particle size on the rate of pyrolysis. Pulverized biomass particles (d< 0"5 mm) were pyrolyzed in a thermo- balance (Mettler TA 3000).

TG runs were carried out on samples up to 150 mg in a temperature range from 200 to 900°C (+ 0.5°C), using heating rates ranging from 5 to 80°C/rain. In order to analyze the effect of parti-

Table 1. Range of the main operating parameters for pyrolysis processes

C. pyrolysis Fast pyrolysis Flash pyrolysis

Operating temperature (°C) 300-700 600-1000 800-1000" Heating rate (°C/s) 0.1-1 10-200 >I 1000 Solid residence time (s) 600-6 000 0.5-5 < 0.5 Particle size (mm) 5-50 < 1 Dust

"Up to 2 000°C with solar furnaces.

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Pyrolysis of biomass 221

Fig. 1. change determination. 1, sample; 2, metallic rod; 3, furnace; 4, helical coil; 5, flow meter; 6, quenching; 7, gas exit; 8, balance.

:i 1 r I

carrier ',,, / N gas _..._,.._1.,,

Experimental apparatus for isothermal mass- ] 4 1 ! I char L _ 1

cle size a reactor specially designed for isothermal

gas

7 ~'

cooling oil w a t e r

Fig. 2. Flow diagram of the bench-scale pyrolysis reactor. 1, Biomass feeder; 2, moving bed reactor; 3, electric oven; 4, char collector; 5, hot cyclone; 6, heat exchanger; 7, entrain- ment separator.

mass-change determination (Koufopanos et al., 1989, 1991 ) was used.

The experimental runs were performed in a device designed for this purpose (Fig. 1). A tubu- lar reactor (38 mm inside diameter) was inserted into an electrically-heated tubular furnace. The sample material was placed in a stainless-steel wire mesh basket hung on a metallic rod con- nected to a balance in order to determine the weight loss of the sample.

Isothermal mass-change determination was carried out on samples of different size (from sawdust of 0.3-0"5 mm up to cylinders of diam- eter 20 mm and length 100 mm) in a temperature range from 200 to 700°C ( + 1 °C).

In order to analyze the overall performances of the process (yields of pyrolysis products and global kinetics rate) a series of experimental runs was carried out in a semi-batch bench-scale reac- tor (Lucchesi & Maschio, 1986).

The apparatus, shown schematically in Fig. 2, consisted of a moving-bed reactor (inner diameter id= 100 ram, height h - -500 ram) placed in an electrically heated oven. The biomass was fed to the reactor by a screw feeder and deposited on a rotating grate, where it met a countercurrent gas stream introduced at the bottom of the reactor. The purpose of the gas was to remove the volatile products of the pyrolysis and carry them outside the reactor. The gaseous stream leaving the reac- tor passed through a hot cyclone where the entrained char dust was separated. The liquid products were condensed in a heat exchanger. The gas flow was measured with a gas meter and analyzed on line by gas chromatography. The char was extracted from the reactor through the rotat- ing grate and collected under the reactor.

The experimental runs were carried out with different lignoceUulosic materials (Table 2) at

Table 2. Chemical and elemental analysis and heating values of tested materials (% wt)

Biomass Hemicellulose Cellulose Lignin Extractives Ash C H 0 N Moisture % wt

L H V MJ/I,g

dry biomass

Wood 19-4 47-5 24'0 7"5 1'6 47'8 5'1 45"4 0"1 Hazelnut shells 24.1 27.5 40.7 3.9 1.0 44.5 5.0 49.0 0.5 Oiivehusks 21.1 22.2 45-0 8"1 3-6 47.5 5'8 37.5 1.5 Corn-cobs 31'8 51"2 14-8 1"2 1-0 45"8 6-2 46-7 0"3 Wheat straw 40"0 24.0 21.0 8-0 8"0 42.3 5"3 43"9 0"3 Lucerne pressed cake 45.5 13.7 21.3 10.6 7"6 45.4 5'5 39-3 2"9

33 10

9 12

7 8

17 17 19 17 17 18

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222 G. Maschio, C. Koufopanos, A. Lucchesi

I /!

©

Gas

> >

>

6

9 10 11

to a n a l y s i s

Fig. 3. Flow diagram of the entrained-bed reactor for fast pyrolysis. 1, burner; 2, furnace; 3, entrained bed reactor; 4, biomass hopper; 5, feeder; 6, water pump; 7, vaporizer; 8, heat exchanger; 9, cooler; 10, gas/solid separator; 11, water condenser; 12, gasometer; 13, chimney.

temperatures ranging from 300 to 600°C ( ± 5°C). The pyrolysis residence times ranged from 2 to 30 min. The yields of pyrolysis products were deter- mined gravimetrically by weighing the different fraction of products (char, tar and aqueous frac- tions), and of the gaseous fraction by a gas-meter. The accuracy in the determination of yields was about 5%.

Fast pyrolysis The fast pyrolysis process was carried out in a tubular entrained-bed reactor (EBR, Fig. 3) mounted in a furnace (Lucchesi & Maschio, 1987).

The biomass was fed to the reactor entrained by a gas stream (inert gas or steam) by a specially designed feeder. The reactor consisted of a spiral coiled tube (id = 10 ram, l= 20 000 mm) inserted in the furnace. In the reactor the temperature of the stream increased rapidly and the pyrolysis of the suspended biomass particles and a consider- able reforming of tar produced took place. The gaseous products, with charcoal and residual tar, leaving the reactor were cooled in a heat exchanger. After the separation of the solids the gas-flow rate was measured in a gas meter and a sample of the gas analyzed on-line by gas chromatography. Finally, the gas was burned in a flare.

The yields of the products and their composi- tions were determined in an operating tempera- ture range between 700 and 900°C ( + 5°C), under

an inert gas (nitrogen) flow. Heating rates higher than 300 K/s were achieved.

The yields of pyrolysis products were deter- mined gravimetrically by weighing the different fraction of products (char, tar and aqueous frac- tions), the yields of gaseous fraction were deter- mined by the gas-meter. The accuracy in the determination of yields was about +_ 5%.

Pilot plant The feasibility of the process and the problems associated with scale-up were examined using a pilot plant installed in the experimental area of the Department of Chemical Engineering of the University of Pisa and constructed with financial support from ENEA (Italy). This unit could pro- cess about 20 kg of biomass per hour (Lucchesi & Maschio, 1986). Figure 4 shows a flow diagram of the pilot plant.

The entire process (pyrolysis/gasification) was carried out using two reactors in series. The first step of the procezs, in which a conventional pyro- lysis was performed at about 400°C ( + 5°C), was carried out in a continuous-screw reactor (d= 200 mm, 1--- 1500 mm) indirectly heated. The second step, in which fast pyrolysis and/or gasification of char was performed in an operating temperature ranging from 700 to 900°C (+ 5°C), was carried out in a tubular entrained bed reactor (d= 52 mm, l= 20 000 mm) mounted in a furnace.

In the entrained-bed reactor the temperature of the stream leaving the pyrolysis reactor increased rapidly (about 350 K/s) and a fast pyrolysis and gasification of the suspended char particles, simul- taneously with a considerable reforming of the tar, was involved. The heat necessary for the process was supplied indirectly by the combustion of pulverized biomass in a special swirl-burner.

Experimental data concerning the pyrolysis step was obtained by sampling (with an isokinetic tube) the stream, containing the char and gaseous phase, leaving the first reactor. The char was separated in a hot filter and the tar and aqueous phase condensed in a cooler. After these separa- tions the gas phase was analyzed on-line by a gas chromatograph.

RESULTS

Conventional pyrolysis Conventional pyrolysis is defined as the pyrolysis which occurs under a slow heating rate. This condition permits the production of solid, liquid

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Pyrolysis of biomass 223

W

c, ", T I

W~ w

A?

II Fig. 4. Flow diagram of the pilot plant for pyrolysis/gasification. T 1 and T2, Biomass hopper; PR, pyrolysis reactor; GR, gasi- fication reactor; C1, swirl burner; C2, gas burner; R1 and R2, heat exchanger; QT, quenching tower; F, flake; B1, B2 and B3, blowers, P, water pump; K, chimney; M, motor.

I

0.8

0,6

0.4

0.2

Residual weight lraction I

F

I

pre-pyrolysts

pyrolys i s c h a r

a e v o l a O l l z a t l o n

s

0 - - i

0 100 200 300 400 500 600 700 Temperature (°C)

Fig. 5. Thermogram of pyrolysis of biomass (hard wood). Heating rate = 20 K/min.

and gaseous pyrolysis products in significant por- tions.

Lignocellulosic materials of different chemical composition were tested (Table 2).

Kinetics Preliminary experimental runs were carried out using the thermobalance TA 3000 and isothermal reactor (IMCR), the pyrolysis conversion was followed by measuring the weight-loss rate.

Figure 5 shows a typical thermogram obtained with the TA 3000; three ideal stages can be dis- tinguished. The first, which occurred between 120 and 200°C, can be called pre-pyrolysis. During this stage some internal rearrangement (water elimination, bond breakage, appearance of free radicals, formation of carbonyl, carboxyl and hydroperoxide groups) takes place (Shafizadeh, 1982). A small weight loss was observed caused mainly by the release of H20, CO and CO2 (Koufopanos et al., 1989).

When a lignocellulosic material is heated at low temperatures (below 200°C), even for a long time period, a small weight loss occurs and no signifi- cant external modifications of the material are observed. However, after this treatment, the inter- nal structure of the biomass is changed. Thus, the pyrolysis yields of this material will be different from the yields of a material which has not received this preliminary treatment. This means that pre-pyrolysis is important for the progress of the whole process.

The second stage of the solid decomposition corresponds to the main pyrolysis process. It pro- ceeds with a high rate and leads to the formation

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224 G. Maschio, C. Koufopanos, A. Lucchesi

Residual weight fraction

1

0.8

0.6

0.4 r

0,2

0 100

1 ,2 3 ' ,4

200 300 400 500 600 Temperature (°C)

Fig. 6. Effect of heating rate on pyrolysis rate (olive husks). Exper imenta l TG runs: l , 5 K/rain; 2, 20 K/min; 3, 80 K/min. Theore t ica l curve: 4, 7 50 K/rain.

of the pyrolysis products. The rate achieves signi- ficant values at temperatures between 300 and 600°C.

During the third stage, the char decomposes at a very slow rate and the solid residue reaches an asymptotic value. This continuous devolatilization of the char due to the further cleavage of the C--H and C--O bonds, results in the carbon enrichment of the residual solid.

Different rates and temperatures for the three stages have been observed when lignocellulosic materials with different chemical composition were tested (Koufopanos et al., 1989, 1991 ).

It is interesting to determine how the heating rate affects the pyrolysis rate. Figure 6 shows typi- cal thermograms obtained under different heating rates. The experimental curves 1, 2 and 3, obtained with the TA 3000 thermobalance, repre- sent pyrolysis runs under slow heating conditions. The heating rate effect can be interpreted in terms of temperature and residence time effects (Hajall- gol et al., 1982; Shafizadeh, 1982; Koufopanos et al., 1989). However, thermograms for fast pyroly- sis conditions (heating rates above 15 K/s) show higher final conversion levels. This means that higher gas and/or volatile yields can be achieved by fast pyrolysis. This behavior is described by the curve 4 (Fig. 6), which was derived theoreti- cally by the prediction of a mathematical model discussed in a previous paper (Koufopanos et al., 1989) and is also in agreement with experimental literature data (Ekstrom & Resfelt, 1980).

The phenomena governing the pyrolysis of a single biomass particle are both chemical and physical. The chemical phenomena consist of a series of complex (primary and secondary) chemi- cal reactions. The physical phenomena are heat and mass transfer. Depending on the operating

conditions, the process may be controlled by either chemical and/or physical phenomena. In particular the particle size of the biomass plays a fundamental role concerning the processes of heat and mass transfer (Koufopanos et al., 1991 ).

In order to investigate the effect of the particle size experimental runs on cylindrical particles of wood were carried out in the IMC Reactor.

The effect of overall transfer phenomena and of secondary reactions, with respect to the intrin- sic kinetics (primary reactions) of pyrolysis, can be shown using the ratio r, defined as:

r = Overall pyrolysis rate/Intrinsic pyrolysis rate

where the numerator represents the overall pyrolysis rate, determined thermogravimetrically in the IMC reactor, for particles of different size at various operating temperatures, while the denom- inator represents the intrinsic pyrolysis rate, determined by the same technique, for pulverized biomass where the effect of transfer phenomena and secondary reactions can be neglected.

Figure 7 shows the ratio r versus particle size at different operating temperatures. The results suggest that for particles with dimensions below 1 mm the intrinsic kinetics practically govern the process. In this case the internal transfer pheno- mena can be neglected. As the particle-size and temperature increase the process becomes con- trolled by both chemical and physical phenomena.

Yields and characterization of products Several types of biomass were tested in the semi- batch, moving-bed, pyrolysis reactor described in Fig. 2. Pyrolysis of biomass provides three main products: char, liquid and gaseous products. Their yields depend mainly on the chemical composi- tion of the feedstock and the operating tempera- ture.

Overall/Intrinsic pyrolysis rate

0.8

I

600

0 ~_ . . . . . . . . . . . . . 0 2 4 6 8 10 12 14 16 18 20

diameter (ram)

Fig. 7. Effect of tempera ture and particle size on the over- all pyrolysis rate (hard wood).

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Pyrolysis of biomass 225

Table 3. Yields of conventional pyrolysis products (% wt)

Temperature (~C) 350 400 500 550

Biomass: Wood

Char 29"0 26.1 22"9 21.1 Tar fraction 11"5 11.4 10"1 9.4 Aqueous fraction 34.0 33"5 33"3 31.7 Gas 25"5 28.7 32"7 36.4

Biomass: Olive husks

Char 36.2 31-5 28.2 22.3 Tar fraction 12.2 11.5 9-2 7.4 Aqueous fraction 27"4 27.5 23"5 19"2 Gas 24.2 29-4 39"5 48"2

Table 3 shows the yields of pyrolysis products for biomass with different chemical compositions; wood represents a biomass rich in cellulosic com- pounds while olive husks are rich in lignin.

Figure 8 shows the typical distribution of the main products of the pyrolysis of olive husks over a wider range of operating temperatures.

The most interesting temperature range for the production, in significant amounts, of the pyroly- sis products is between 350 and 550°C. The char- coal yield decreases as the temperature increases. The production of the liquid fraction has a maxi- mum at temperatures between 350 and 450°C. At higher temperatures, the rather large molecules present in the liquid are broken down to produce smaller molecules which enrich the gaseous phase. Thus, when the temperature exceeds 500°C a rather sudden decrease of the liquid yield is observed and gas production is favored.

In lignin-rich biomass charcoal production is favored, but a higher pyrolysis time is required to reach the final conversion.

Charcoal The characteristics of the charcoal derived from different types of biomass are presented in Table 4.

The heating value of charcoal is lower than that of common coal, but higher than that of many other solid fuels (e.g. lignite). With ecological

Y i e l d s ' of product s (%wt )

Char

0,6 Gas

0.4

0.2

100 200 300 400 500 600 700

Temperature (*C) Fig. 8. Yiclds of conventional pyrolysis products. Bio- mass =olivc husks.

Xeq ( nag of o r a n g e II/g of c h a r )

i P + 1. i

i 3- - V , , ~ -~7 6 I

i

0.001 0.01 0,1 C e q ( g / l )

Fig. 9. Adsorbent properties of charcoal of different bio- mass (adsorption isotherms of orange Il). Biomass: 1, straw; 2, olive husks; 3, lucerne; 4, pine cone. Activated carbon: 5, analytical; 6, technical.

criteria, charcoal is a very interesting fuel due to its low ash (types with a rather high ash content, like wheat straw, are exceptions), sulfur and nitrogen content. The bulk density of charcoal ranges from 150 to 300 kg/m 3.

Charcoal has also good adsorbent properties towards dyes and, consequently, it can be used as a substitute for activated carbon. As shown in Fig. 9, charcoal obtained from the different biomass

Table 4. Elemental analysis (% by wt) and heating values (MJ/kg) of charcoal produced by conventional pyrolysis at 450°C

Charcoal from C H 0 N Ash HHV L H V

Wheat straw 66.4 2-7 11.1 0.6 17.1 25 24 Lucerne pressed cake 61.6 2.9 13"8 2.4 16"3 23 22 Olive husks 64.7 5.2 11'6 2.4 7"5 28 23 Pine cones 82-9 2"7 10"6 0.2 1"7 31 30 Wood 72-2 3.0 17.4 0.4 2"6 28 27

Chlorine and sulfur content very low.

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226 G. Maschio, C. Koufopanos, A. Lucchesi

Xable 5. Chemical analysis of organic substances contained in the fractions of liquid pyrolysis products

(a) Tar fraction

Substance % by wt Substance % by wt

Methanol Acetic acid Furfural Methyl furfural Guaiacol 4-Methyl guaiacol 4-Ethyl guaiacol m + p-Cresol 2,4-Xylenol Vanillic alcohol Vaniilic acid Eugenol 3-Methoxy,4-hydroxyphenyl ethylcarbinol Phenol 4-Propyl guaiacol Guaiacol propionate

0.9-1.2 O-Cresol 3'5-4 4-5 Coniferyl alcohol 1-2 3-4 3-Methoxy-4, 5-dihydroxyphenyl ketone 3-4 1-2 4.5- 5 2, 6 -Methoxy-4-propenylphenol 1 - 1" 5 4-5 3-4 Methyl formate < 0" 10 5-5.5 Acetone < 0'10 1' 5- 2" 5 Acetaldehyde < 0.10 9-10 Methyl acetate < 0-10 9-5-10.5 2, 5-Methyl furan < 0"10 2"5-3 Propionic acid < 0" 10 6- 8 2-Methyl- 5 -ethylfurfural < 0" 10 3-4 2-Hydroxy-3-methyl cyclopentanone < 0.10 4-4.5 2-2'5 Other organic compounds (< 0"05%) 6-7

(b) Aqueous fraction

Methanol Acetic acid Furfural Methyl furfural Guaiacol 4-Methyl guaiacol 4-Ethyl guaiacol Acetone 2, 4-Xylenoi Vanillic alcohol Vanillic acid Propionic acid Phenol Acetaldehyde Methyl acetate Ethyl acetate

1.8-2.1 9'4-11.3 0"9-1 0"2-0"3 0"2-0-3 0"2-0.3 0'1-0-15 0.5-0-75 0-1-0"15 0-7-1"1 0"9-1'5 0'6-0"75 0.3-0.4 0"1-0"2 0"3-0"4 0"1-0'2

O-Cresol 0"1-0'15 Cyclopentanone 0.3-0.4 3-Methoxy-4, 5-dihydroxyphenol ketone < 0" 10

2, 6-Methoxy-4-propenylphenol Methyl formate

<0'10 <0"10

2, 5-Methyl furan < 0' 10 Guaiacol propionate < 0' 10 Other organic compounds (< 0.05%) 4-5 Water 74-78

Table 6. Elemental analysis (% by weight) and heating values (MJ/kg) of bio-oil pro- duced by conventional pyrolysis at 450°C

Bio-oil from C H 0 N Ash HHV LHV

Wheat straw 65.2 8.2 24.2 0.5 0.2 27 24 Lucerne pressed cake 64.7 5.4 25-3 3.8 0.3 27 25 Olive husks 64.5 7.7 19.9 3.1 0.3 30 28 Pine cone 66"1 7-5 23.1 0"6 0"1 27 25 Wood 52"3 6"7 38.4 0.2 0.1 23 21

Chlorine and sulfur content very low.

types tested, with the except ion of pine-cones, has adsorben t proper t ies comparab le to those of commercia l act ivated-carbons.

Liquid. The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase containing a wide variety of organo-oxygen com- pounds of low molecular weight and a non- aqueous phase containing insoluble organics (mainly aromatics) of high molecular weight

(Garrett, 1979; Sofer & Zaborsky , 1981; Luc- chesi & Maschio, 1986). This phase is called bio- oil or tar and is the p roduc t of greatest interest. Convent ional pyrolysis gives a relatively low yield of bio-oil.

A wide spec t rum of organic substances is con- tained in the liquid fractions (Table 5). The analy- sis of these substances was carr ied out using a gas-chromatograph coupled with a mass spectro- meter.

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Pyrolysis of b iomass 2 2 7

The elemental analysis and the heating values of bio-oils derived by the pyrolysis of different biomass types are presented in Table 6. The heat- ing value of 'bio-oil' and the rather high density (930-1050 kg/m 3) makes it a rather energetically dense fuel. It has a high oxygen content. On the other hand, it contains very low amounts of ash and, essentially, no sulfur.

The handling of the bio-oil and its use as fuel presents some problems connected with its corro- sive nature, which is mainly due to the presence of a low pH aqueous phase.

Accelerated corrosion tests on steel strips were carried out on untreated and centrifuged bio-oils. The steel strips were immersed in samples of bio- oil and kept at 95°C for 168 h. The strips placed in untreated bio-oil showed a weight loss of 5.5%. Those placed in the bio-oil which had been centri- fuged to remove the aqueous phase showed a weight loss five times lower.

The bio-oil is soluble in polar organic solvents, such as acetone, but only slightly soluble in sol- vents such as heptane and gasoil. Its viscosity varies between that of fuel oils No. 4 and No. 6. The viscosity increases strongly on exposure to air. This instability of the bio-oil is caused by poly- merization. Bio-oil is also heat sensitive and begins to decompose at temperatures above 150°C.

The aqueous phase presents environmental problems because of high BOD and COD levels and its disposal presents problems. The quantity of the aqueous phase can be reduced by a prelimi- nary drying of the biomass, the residual water produced by pyrolysis, containing about 20% by weight of organic compounds, can be incinerated in the combustion unit of the pyrolysis plant.

Gases. Gaseous products leaving a conventional pyrolysis process contain mainly carbon mon- oxide and dioxide, hydrogen and less methane, small quantities of hydrocarbons with low mole- cular weight and some organic vapors. Table 7

shows the gas composition obtained in the pyroly- sis of hazel nut shells carried out in the moving bed reactor (Fig. 2). The heating value of the pyrolysis gas is about 10 to 15 MJ/Nm 3. For tem- peratures above 700°C a great increase in the hydrogen content is observed, this is probably due to gasification phenomena.

Fast pyrolysis In the previous paragraphs, it has been observed that fast heating-rates minimize the final charcoal yield. So, if the aim is the production of mainly gaseous and/or liquid products, a fast pyrolysis is recommended.

It seems that under fast heating certain inter- mediate compounds cannot be formed and the pyrolysis products are directly produced (Lede et al., 1980; Soltes et al., 1981; Scott & Piskorz, 1984; Kothari & Antal, 1985). This can explain why the final product distribution changes in fast pyrolysis.

The achievement of fast heating-rates (above 200 K/s) requires high operating temperatures (700-1000°C), very short contact times (less than 4 s) and very fine particles (smaller than lmm).

Fast pyrolysis was studied in the bench-scale, tubular, entrained-bed reactor (Fig. 3) of a capac- ity of about 1 kg/h. Experimental runs were carried out with temperatures of 700 to 900°C by feeding the reactor with crushed hazel-nut shells

(particle size 0.2-0.4 mm). Due to the turbulent flow conditions inside the tubes and the high heat transfer coefficients because of radiation, the bio- mass particles achieved high temperatures in a short residence time (1-2 s). Thus, the experi- mental runs were carried out under heating rates higher than 300 K/s.

The experimental results showed high conver- sion levels, measured in terms of weight-loss. The pyrolysis reached an asymptotic value in conver- sion (about 90% at 850°C) and a further increase in temperature did not significantly improve it. Table 8 shows the yield of the pyrolysis products.

Table 7. Chemical composition of gaseous products of the conventional pyrolysis of hazel nut shells at different operating temperatures (% voi.)

Temperature (%') 400 550 650 750 800 850

H 2 7"3 10 18 33 37 40"5 CO 2 25"2 20 15 16"4 11 7"6 C2H 4 0'7 1 1 0"6 0"2 - - C2H 6 0"9 4 3 1"2 0"4 0"1 C H 4 13"4 13 16 13"5 10"7 8"5 CO 30"0 41 35 25"2 33"5 37"1 Organic compounds 22.5 12 10"5 8.0 7"0 5-5

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228 G. Maschio, C. Koufopanos, A. Lucchesi

Table 8. Yields of the products obtained by fast pyrolysis. Biomass: hazel nut shells

Temperature (°C) Char Gas Tar (%) (kg/kg biomass) (Nm~/kg biomass)

700 0'28 0"68 7 720 0"27 0-73 6"8 750 0"2,4 0"79 6"1 800 0"16 0"90 4"5 850 0"11 1"21 3'7 900 0"11 1"28 2"1

Table 9. Chemical composition of gaseous products of the fast pyrolysis of hazel nut shells (% vol.)

Temperature (°C) 700 750 800 820 850 900

H2 19"1 31"6 35"7 35"5 36'5 42"5 CO 2 15"1 15-6 10"7 11'2 11 11"6 C2H 4 4"0 1'4 0"7 0"6 0"4 0"3 CzH 6 1"1 0"6 0"2 0"1 - - - - CH4 14'0 12'5 10"3 - 11'2 9"5 7'8 CO 26"0 23"3 32"1 32"2 37'3 32"4 Organic compounds 18.2 15.0 10.5 8'9 8"0 5-0

The high temperatures favor some secondary reactions such as cracking and tar reforming. On the other hand, above 800°C partial gasification processes are involved and, as a consequence, an enrichment of gas production is obtained.

The compositions of the gaseous streams for different pyrolysis temperatures are given in Table 9. The gas was rich in hydrogen and carbon monoxide. The content of methane and organic compounds decreased as the temperature increased. The gas yield was about 1.3 Nm3/kg biomass and its heating value about 14000 kJ/ Nm 3. This value makes the gas produced by fast pyrolysis a medium BTU gas. This is a better- quality gas than that produced by conventional pyrolysis.

The char produced by fast pyrolysis is consid- ered a marginal product, as its yield does not exceed 15% wt. Due to its higher ash content, it is less valuable than the conventional pyrolysis char- coal.

TECHNICAL ASSESSMENT OF PYROLYSIS PROCESSES

The main part of a pyrolysis plant is the reactor. The design of the reactor presents the problem of heating materials of poor thermal conductivity (such as lignocellulosic materials) to high tem- peratures.

Heat can be supplied in the following three ways (Baile & Doner, 1977; Bridgwater & Van Swaaij, 1987; Bridgwater, 1988):

- - Indirect heating: transfer of heat through the reactor wall.

- - Direct heating: an inert heating medium, such as an inert gas, solid steel balls, recir- culating hot sands, molten metals and salts can be used as a heat carrier.

- - Partial oxidation: a little oxygen is intro- duced into the reactor and heat is provided by the partial combustion of the pyrolysis products.

The most common pyrolysis systems employ one of the following types of reactor: fluidized bed, entrained bed, multiple hearth, rotary kiln, moving-bed with co-current or counter-current flow.

The choice of the reactor type and heating system affects the final product distribution (Baile & Doner, 1977; Jones, 1978; Solantausa, 1990; Bridgwater & Bridge, 1991). This choice is strongly affected by the characteristics of the raw materials available. So, if the raw material is avail- able as powder or fine particles then a fast pyroly- sis in an entrained- or fluidized-bed-reactor is suggested. If, however, the raw material is avail- able in the form of large particles then the employment of a fast-pyrolysis process in inadvis- able due to the high costs necessary for reducing

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Pyrolysis of biomass 229

the particle size. In this case, conventional pyroly- sis in a moving-bed or rotary-kiln reactor is recommended.

The experience acquired in our laboratory has proved that the high temperatures required can be efficiently achieved by indirect heating. When this is adopted, higher final product yields can be obtained and the dilution of the pyrolysis gases by combustion products can be avoided.

The most attractive products of conventional pyrolysis are the charcoal and the bio-oils. The charcoal can be used as solid fuel, as a raw material in the metallurgy industry and also as a substitute for active carbon. The handling and use of the bio-oils present several difficulties due to their characteristics. Problems associated with disposal of the aqueous phase must be faced. This phase is characterized by a high BOD and COD and is not environmentally acceptable. The pyro- lysis gas is a medium BTU gas and can be easily burnt (Lucchesi & Maschio, 1986).

After the above considerations and taking into account the technological and economical status of the process, one can assert that the most interest- ing application of conventional pyrolysis could be for the production of charcoal (carbonization). This approach seems to have more opportunities in places where sufficient amounts of raw material are available at a local level (e.g. farms, forests, pulp industries, etc.).

A simple flow sheet describing this process is proposed in Fig. 10. A preliminary drying of the biomass is recommended, thus, the volume of the pyrolysis reactor and the water content of the liquid fraction is reduced. The reactor can be a

5

!mill Fig. I0.

3

1 Char

Simplified flow diagram for a pyrolysis plant. 1, rotary drier; 2, pyrolysis reactor; 3, char screw cooler; 4, combustion chamber; 5, dust collector; 6, chimney.

moving-bed or, as in the pilot-plant, a continuous- screw reactor with indirect heating. The operating conditions recommended are a temperature of 350 to 450°C: higher temperatures could decrease the char yield. Heating rates are 20 to 40 K/min. Rather large particle-dimensions must be adopted (for cylindrical particles the diameters range from 50 to 200 mm). As the analysis of the previous paragraphs suggests, large particles favor charcoal production and the secondary reactions which enrich the charcoal in carbon (Koufopanos et al., 1991). Also, by using large particles the cost of size reduction is minimized. On the other hand, longer pyrolysis time periods are required (40 to 80 rain).

The volatile products are directly fed into a combustion chamber where they are burnt. The heat released is used for the heating requirements of the process. Appropriate design and opera- tion of the combustion chamber eliminates the ecological problem of the disposal of the aqueous phase.

If, however, optimization of bio-oil yield becomes the goal of the process particular atten- tion must be focused on the separation of the bio- oils from the aqueous phase and the gases. If condensation of the volatile products occurs in an appropriate temperature range the water princi- pally remains in the gaseous phase, which can be easily burnt.

The quality of the volatile products can be improved by upgrading processes (Baile & Doner, 1977; Bridgwater & Beenackers, 1985; Solan- tausta, 1990; Bridgwater & Bridge, 1991). How- ever, the economics of these processes do not favor their immediate application.

If gas production is the target of the process, fast pyrolysis can lead to the production of a medium BTU gas of good quality, since organic vapors or tars are present only in traces.

The major problem of fast pyrolysis is the implementation of the process on a large scale. The use of fluidized- or an entrained-bed-reactor (Bridgwater & Van Swaaij, 1987; Bridgwater, 1988) has been reported.

An efficient way to realize the process, as the results obtained by a demonstration plant operat- ed in our laboratory have proved (Lucchesi & Maschio, 1987), is the use of two reactors in series: the first is a rotary type reactor, indirectly heated, used to perform the conventional pyroly- sis at temperatures about 550°C. The pyrolysis products are fed to a tubular entrained-bed reac- tor inserted in a furnace and operated at tempera-

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230 G. Maschio, C. Koufopanos, A. Lucchesi

tures between 750 and 850°C. The volatile and gaseous products of pyrolysis can substitute for the carrier gas. Besides, the residence of the vola- tiles at high temperatures leads to their cracking and to the enrichment of the gaseous stream. The water produced would participate with the carbon of the pyrolysis charcoal in the gasification reac- tions.

If the aim is the production of fuel gas, fast pyrolysis can be more advantageous than gasifica- tion. The pyrolysis gas has a greater heating value mainly because of its higher methane content. A second advantage is that lower temperatures are required than for gasification.

quent cost of grinding. The problem of scale-up with respect to particle size and heat exchange has already been noted.

ACKNOWLEDGEMENTS

The research was partly supported by ENEA (Comitato Nazionale per la Ricera e lo Svilpuppo delrEnergia Nucleare e delle Energie Alternative) Rome, Italy. We gratefully acknowledge financial support, in the form of a grant to C. Koufopanos, from the Commission of the European Commu- nities.

CONCLUSIONS

There may exist some doubt about whether it is more convenient to transform the biomass into fuels or to burn it directly. The conversion of bio- mass leads to the formation of fuels with a higher energy density than the original. The fuels pro- duced are intended to be similar to conventional fuels, so that they can directly or after mixing be fed to conventional combustion apparatus. The combustion of biofuels can be done under better controlled conditions than for the direct combus- tion of biomass and with smaller environmental problems. Estimations carried out in our labora- tory (Lucchesi & Maschio, 1986) have indicated that conventional pyrolysis is energetically more efficient than combustion by at least 15%.

The first conclusion emerging from this work regards conventional pyrolysis. The most valuable product is the charcoal. Some difficulties in hand- ling and processing bio-oils arise from their corrosiveness and instability. Processes to upgrade the bio-oil after production, e.g. by hydrogenation, are presently very costly. On the other hand, charcoal exhibits good characteristics as a fuel and also as active carbon. The present status of conventional pyrolysis technology sug- gests as the most attractive application the car- bonization of biomass (optimization of charcoal production).

Fast pyrolysis minimizes charcoal production. The realization of this process in an entrained- bed reactor gives as the main product a medium BTU gas. The high temperature achieved favors the cracking of volatiles, and a minimum produc- tion of liquid.

At present fast pyrolysis in entrained-bed reac- tors requires very small particle-size with a conse-

REFERENCES

Antal, M. J. (1983). In Advances in Solar Energy, ed. K. W. Boer & J. A. Duffle. American Solar Energy Society, pp. 61-111.

Baile, R. C. & Doner, D. M. (1977). Pyrolysis and assessment of pyrolysis systems. A1ChE Symp. Ser., 73 ( 162), 102-19.

Beenackers, A. A. C. M., Bridgwater, A. V. & Van Swaaij, W. P. M. (1989). Status and opportunities for thermo- chemical biomass conversion in the European Commun- ity. In Energy from Biomass, Proc. 3rd Contractors' Meeting, ed. G. Grassi, D. Pirrwitz & H. Zibetta. Elsevier Applied Science, London, UK, pp. 18-24.

Bridgwater, A. V. & Beenackers, A. A. C. M. (1985), In Proc. 3rd EC Conference 'Energy from Biomass', Vol. 3, ed. W. Palz, J. Coombs & D. O. Halts. Elsevier Applied Science, London, UK, pp. 247-62.

Bridgwater, A. V. & Van Swaaij, W. P. M. (1987). In Proc. 4th EC Conference 'Energy from Biomass', Vol. 4, ed. G. Grassi, B. Belmont, J. F. Moile & H. Zibetti. Elsevier Applied Science, London, UK, pp. 235-51.

Bridgwater, A. V. (1988). In Research in Thermochemical Biomass Conversion, ed. A. V. Bridgwater & J. L. Kuester. Elsevier Applied Science, London, UK.

Bridgwater, A. V. & Bridge, S. A. (1991). A review of bio- mass pyrolysis and pyrolysis technologies, In Biomass Pyrolysis Liquid Upgrading and Utilization, ed. A. V. Bridgwater & G. Grassi. Elsevier Applied Science, London, UK, pp, 11-92,

Diebold, J. (1991). Development of pyrolysis reactor con- cepts in the USA. In Biomass Pyrolysis Liquid Upgrading and Utilization, ed. A. V. Bridgwater & G. Grassi. Elsevier Applied Science, London, UK, pp. 341-50.

Diebold, J. P. & Power, A. J. (1988). Engineering aspects of the vortex pyrolysis reactor to produce primary pyrolysis oil vapors for use in resins and adhesives. In Research in Thermochemical Biomass Conversion, ed. A. V. Bridg- water & J. Kuester. Elsevier Applied Science, London, UK, pp. 609-28.

Elliot, D. C. (1985). Comparative analysis of gassification/ pyrolysis condensates. In Proc. 1985 Biomass Thermo- chemical Conversion Contractors' Meeting, Minneapolis, MN, 15-16 October, p. 361.

Ekstrom, C. & Rensfelt, E. (1980). Flash pyrolysis of bio- mass in Sweden, In Proc. ,Specialists" Workshop on Fast Pyrolysis of Biomass, Copper Mountain, CO, October, SERI/CP-622-1096.

Garrett, D. E. (1979). Sol. Energy. Res. Inst., SERI TP-33- 285.

Page 13: Pyrolysis Promising Route

Pyrolysis of biomass 231

Hajailgol, M.' R., Howard, J. B., Longweil, J. P. & Peters, W. A. (1982). Product composition and kinetics for rapid pyrolysis of cellulose. Ind. Eng. Chem., Proc. Res. Dev., 21,457-65.

Lede, J., Versaro, F. & Villermaux, J. (1980). Rev. Phys. Appl., 15, 535-43.

Lucchesi, A. & Maschio, G. (1986). Study on the pyrolysis of agricultural wastes, Study Contract No. ESE-R-082-I-(S), Commission of the European Communities.

Lucchesi, A. & Maschio, G. (1987). Studi sulla pirolisi di bio- masse, Final Report of ENEA.

Jones, J. (1987). Converting solid wastes and residues to fuel. Chem. Eng., Jan., 87-94.

Knight, J. A. (1979), Progress in Biomass Conversion, Vol. 1, Academic Press, New York, NY.

Knight, J. A., Gorton, C. W., Kovac, R. J. & Newman, C. J. (1986). Entrained flow pyrolysis of biomass. In Proc. 1085 Biomass Thermochemical Conversion Contractor's Meet- ing, Pacific Northwest Laboratory, pp. 99-113.

Kothari, V. & Antal, M. J. (1985). Fuel, 64, 1487-94. Koufopanos, C., Maschio, G. & Lucchesi, A. (1989). Kinetic

modelling of the pyrolysis of biomass and biomass compo- nents. Can. J. Chem. Eng., 67, 75-84.

Koufopanos, C., Papayannakos, N., Maschio, G. & Lucchesi, A. (1991). Modelling of the pyrolysis of biomass particles. Studies on kinetics, thermal and heat transfer effects. Can. J. Chem. Eng., 69,907-15.

Kovac, R. J., Gorton, C. W., O'Neil, D. J. & Newman, C. J. (1987). Low pressure entrained flow pyrolysis of biomass to produce liquid fuels. In Proc. 1987 Biomass Thermo- chemical Conversion Contractors' Review Meeting, Atlanta, GA, 20-21 May, p. 23.

Radlein, D., Pirkorz, J. & Scott, D. S. (1987). Lignin derived oils from the fast pyrolysis of biomass. J. Anal. AppL Pyrolysis, 12, 51-9.

Shafizadeh, F. (1982). Introduction to pyrolysis of biomass. J. Anal AppL Pyrolysis, 3, 283-305.

Sofer, S. S. & Zaborsky, O. R. (1981). Biomass Conversion Processes for Energy and Fuels, Plenum Publishing Co., New York, NY.

Soltes, E. J., Wiley, A. & Kenny, L. ( 1981). Biomass pyrolysis - - Towards an understanding of its versatility and poten- tials. Biotechnology and Bioengineering Symp. Ser., 11, 125-36.

Scott, D. S. & Piskorz, J. (1982). Can. J. Chem. Eng., 60, 666-74.

Scott, D. S. & Piskorz, J. (1984). Can. J. Chem. Eng., 62, 404-12.

Scott, D. S., Piskorz, J. & Radlein, D. (1985). Ind. Eng. Chem. Proc. Des. Devel., 24,581-8.

Solantausta, Y. (1990). Techno-economic assessment of selected biomass liquefaction processes, Research Report, Technical Research Centre of Finland, Espoo.