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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
*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
219 Elsevier Science Publishers Ltd, England. Printed in
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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
Conventional 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 900C (+ 0.5C), using heating rates
ranging from 5 to 80C/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 000C 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- ] 41 ! I char L_
1
cle size a reactor specially designed for isothermal
gas
7 ~'
cooling oil water
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 700C ( + 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
LHV 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 analys is
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 600C ( 5C). 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 900C
( + 5C), 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 400C ( +
5C), 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 900C (+ 5C), 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
-
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
pyrolysis char
aevo laO l l za t lon
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 200C, 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 200C), 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).
Experimental TG runs: l , 5 K/rain; 2, 20 K/min; 3, 80 K/min.
Theoretical curve: 4, 7 50 K/rain.
of the pyrolysis products. The rate achieves signi- ficant
values at temperatures between 300 and 600C.
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 temperature 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
550C. The char- coal yield decreases as the temperature increases.
The production of the liquid fraction has a maxi- mum at
temperatures between 350 and 450C. 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 500C 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
Yie lds ' of products (%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 orange II/g of char )
i P + 1. i
i 3- -V , ,~ -~7 6 I
i
0.001 0.01 0,1 Ceq (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 450C
Charcoal from C H 0 N Ash HHV LHV
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
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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 95C 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 150C.
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 700C 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 (700-1000C), 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 900C
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 850C) 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 CH 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
-
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 800C 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
-
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 450C: 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 550C.
The pyrolysis products are fed to a tubular entrained-bed reac- tor
inserted in a furnace and operated at tempera-
-
230 G. Maschio, C. Koufopanos, A. Lucchesi
tures between 750 and 850C. 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-
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