Influence of torrefaction pretreatment on biomass gasification technology
State Key Laboratory of Clean Energy Utilization, Zhejiang University
, Hangzhou 310027,
Torrefaction is a slow pyrolysis process that is carried out in the relatively low temperature range of 220-300C. The influence of torrefaction as a pretreatment on biomass gasification technology was investigated using a bench-scale torrefaction unit, a bench-scale laminar entrained-flow gasifier, and the analysis techniques TGA-FTIR and low temperature nitrogen adsorption. A series of experiments were performed to examine the characteristics of the torrefaction process, the properties of torrefaction products, and the effects of torrefaction on gas composition, cold gas efficiency and gasification efficiency. The results showed that during the torrefaction process the moisture content of biomass were reduced, and the wood fiber structure of the material was destroyed. This was beneficial to storage, transport and subsequent treatments of biomass in large scale. For solid products, torrefaction increased the energy density, decreased the oxygen/carbon ratio, and created a more complex pore structure. These improved the syngas quality and cold gas efficiency. Combustible gases accounted for about 50% of non-condensable gaseous torrefaction products. Effective use of the torrefaction gases can save energy and improve efficiency. Overall, biomass torrefaction technology has good application prospects in gasification processes.
Energy is the most important basis for economic and social
development. With large-scale industrial development, the
total exploitable amount of fossil fuel is declining, and
environmental pollution is increasing. With the advantages of
being clean and CO2 neutral, biomass is the only renewable
energy source that can fix carbon, and biomass has
gradually won worldwide attention. However, as a result of its
dispersion, low energy density, low bulk density and high
moisture content, the costs of logistics and transport are
increased. Those factors make large-scale utilization of
biomass for bioenergy production inefficient and
uneconomic. Consequently, it is necessary to enhance the
characteristics of biomass feedstocks through pretreatment.
At present, biomass pretreatments include drying,
pelletisation, pyrolysis and torrefaction. While drying is a relatively
mature conventional technology, the moisture content of
biomass is as high as typically about 10 wt% after drying
. Dried biomass will re-absorb water and start to
decompose. In addition, drying has little benefit for the
improvement on the properties such as low energy density and bulk
density, high oxygen content and grindability. As a slow
pyrolysis process at moderate temperatures under
atmospheric pressure, torrefaction can solve these problems.
Using torrefaction technology, the energy density and bulk
density of biomass are increased, and the costs of
transportation and storage reduced. Moreover, because of its high
process efficiency (94%) compared with pelletisation (84%)
and pyrolysis (64%), torrefaction is potentially the best
method for improving the economics of the overall
production chain for bioenergy .
Most research has been focused on the mechanism,
feedstock and products of torrefaction. At the operating
tem The Author(s) 2011. This article is published with open access at Springerlink.com
perature (220300C), the main process of torrefaction is
hemicellulose pyrolysis. The feedstocks include mainly
herbaceous and ligneous plants. The results show that under
the same torrefaction conditions, the conversion of
agricultural residues is higher than that of the latter because of the
high content of volatile matter and hemicellulose in
agricultural residues. A comparison between wood and
agricultural residues in gas composition indicates that the latter is
characterized by higher CO2 production , whereas wood
samples yield a higher proportion of solid products .
Deciduous trees are more active than coniferous trees. This
may be explained because the hemicellulose fraction in
deciduous wood contains mainly xylan, which is much more
reactive than the mannan found in coniferous wood .
Because of the fibrous structure and the tenacity of ligneous
plants, the power consumption for grinding is higher  but
the grindability can be improved through torrefaction .
Prins et al.  analyzed the torrefaction products of four
kinds of biomass. Bridgeman et al.  used three kinds of
biomass to investigate the effects of torrefaction on solid
fuel qualities and combustion properties.
Compared with combustion and pyrolysis, gasification
technology has higher efficiency and more
environmentfriendly performance, and is considered to be a very
important route to large-scale utilization of biomass. However,
little attention has been paid to the influence of torrefaction
on the performance of the biomass gasification process.
Hence, based on entrained flow gasification technology, the
aim of the present study was to investigate the combination
of torrefaction technology and the gasification process. A
series of experiments were carried out to examine the
characteristics of the torrefaction process, the properties of
torrefaction products, and the effects of torrefaction on syngas
quality and gasification efficiency. Gasification technology
is widely used in China for centralized gas supply, and
much attention has been paid to the straw resources of the
country. In the future, gasification technology will be
extensively applied for power generation, combined heat and
power generation and indirect liquefaction for liquid fuel
production to relieve the energy crisis. And there are
abundant resources of forestry and wood processing waste. In the
experiments reported herein sawdust was chosen as the
feedstock for torrefaction and gasification. The
experimental results provide a practical and theoretical basis for
large-scale use of bioenergy.
Sawdust was pulverized and screened to
Table 1 Ultimate and Proximate analysis
obtain particles with size in the range 2030 mesh.
Proximate and ultimate analyses of the biomass sample are
shown in Table 1.
(ii) Biomass torrefaction. The bench-scale torrefaction
unit is shown schematically in Figure 1. The diameter of the
reactor was 30 mm. During torrefaction about 10 g of
sawdust was heated under a continuous N2 flow to the desired
temperature (230, 250, 270 or 290C) at a heating rate of
10C min1, and maintained at this temperature for 20 or 30
min. The non-condensable gaseous products were collected
after cooling and cleaning, then the solid and gaseous
products were analyzed.
(iii) Entrained flow gasification. Gasification tests for
the torrefied solid products were carried out using a
bench-scale high-temperature entrained flow gasification
system, shown schematically in Figure 2. A corundum pipe
with effective height 1500 mm and inner diameter 60 mm
was used as the gasifier. The biomass throughput for this
reactor was 5 g min1. The gasification temperature was
controlled at 1200C with an oxygen/biomass ratio of 0.3.
This gasification system had the advantages of short
residence time and high gasification rate.
(iv) TG-FTIR analysis. The mechanism of the
torrefaction process was investigated using a thermogravimetric
analyzer (Mettler-Toledo TGA/SDTA85le) and a Fourier
transform infrared spectrometer (Nicolet NEXTXUS 670).
An inert atmosphere was maintained using nitrogen
(purity>99.99%) as a carrier gas with flow rate 50 mL min1.
The sample was heated at 10C min1 from 25C to the
desired temperature (230, 250, 270 or 290C), and maintained
at that temperature for 30 min. To analyze the relative
concentrations of the volatile pyrolysis products based on the
intensity of absorption peaks, an accurately weighed 5.6 mg
wood sample was used for each test.
(v) Pore structure analysis. The pore structures of
sawdust and torrefied sawdust particles were determined by the
low-temperature nitrogen adsorption method. Isothermal
Figure 1 Schematic diagram of the torrefaction unit. 1, N2; 2, tube
furnace; 3, temperature control system; 4, quart glass reactor; 5, condenser; 6,
filter; 7, gas analysis system.
Figure 2 Schematic diagram of the gasification system. 1, N2; 2, O2; 3,
mass flow controller; 4, preheater; 5, feeder; 6, furnace; 7, gasifier; 8, ash
bucket; 9, cyclone; 10, water scrubber; 11, filter; 12, pump; 13, GC gas
analysis system; 14, temperature control system.
adsorption and desorption were conducted at the saturation
temperature of liquid nitrogen (196C), using nitrogen
(purity>99.99%) as the adsorption medium, at relative
pressure (P/Po) between 0.01 and 0.995 (P and Po are
low-temperature adsorption equilibrium pressure and
saturation pressure, respectively). The adsorption-desorption
isotherms obtained were used to determine the pore
structures of samples through Brunauer-Emmett-Teller (BET)
theory and Barrett-Joyner-Halenda (BJH) theory.
Results and discussion
Characteristics of torrefaction
Figures 3 and 4 illustrate the influence of torrefaction
temperature and residence time on the final mass yield of solid
residue. After water evaporation in the initial 06 min,
sawdust seldom showed weight loss. During the isothermal
period, the weight loss became apparent as a result of the
large quantity of volatiles that were produced. Judging from
the peak heights and areas in the DTG curves, increasing
temperature had a marked effect on the decomposition of
the sample: the maximum mass loss rate occurred at higher
temperatures for samples that had undergone torrefaction at
higher temperatures. This was the main stage of the
torrefaction process. Finally, the tendency for weight to be lost
gradually disappeared, and the remaining material
decomposed slowly until the end of pyrolysis.
Figure 5 shows FTIR spectra obtained during sawdust
torrefaction at 290C, and indicates the effect of residence
time on the torrefaction process. According to the DTG
curve at 290C, eight representative time/temperature points
were selected: before, at and after the first peak; between
Mass loss during torrefaction at different final temperatures.
Mass loss rate during torrefaction at different final temperatures.
the two peaks; before, at and after the second peak; and at
the stage of the slow decomposition, as follows. 1.337 min,
38.4C; 2.406 min, 49.1C; 6.082 min, 85.8C; 14.036 min,
165.4C; 22.373 min, 248.7C; 25.695 min, 282.0C;
28.739 min, 290C; 52.599 min, 290C. During the heating
stage at low temperature (1.337, 2.406 and 6.082 min) only
the characteristic H2O peak appeared, and became gradually
more intense as the temperature increased. This peak was
associated with the evolution of free water. At the stage of
slight weight loss (14.036 min), the characteristic peak of
CO2 appeared together with weak peaks of carbonyl
compounds and aromatics. These observations can be explained
by the release of small molecular compounds such as H2O,
CO and CO2, caused by depolymerization, recombination
and modification of samples . At the main stage of the
torrefaction process (22.373, 25.695 and 28.739 min), in
addition to the characteristic peaks of H2O and CO2 there
were strong absorption peaks that were assigned to CH
stretching vibration (30002650 cm1), the carbonyl C==O
double bond stretching vibration (18501600 cm1), CH
in-plane bending vibration, and CO and CC skeletal
vibrations (1500900 cm1). Those bands are attributed to
alkanes, carbonyl compounds, aromatics and phenols
associated with the production of tar and other substances, and
damage and fracture of the fiber structure. Because H2O
was fully evaporated in the previous stages, the intensity of
characteristic peaks except for H2O increased and then
decreased with increasing residence time. At the
decomposition stage of residue (52.599 min), CO2 was the main
gaseous product. These results were in accordance with the DTG
Because the main process of torrefaction occurred in the
isothermal stage, the time points corresponding to the
maximum mass loss rates in this stage at different
torrefaction temperatures were selected to analyze the effect of
temperature on the torrefaction process. From the results
shown in Figure 6, it seemed that the volatile constituents
were mainly H2O and CO2 at low temperatures. As the
temperature increased, the intensity of characteristic peaks
of alkanes, carbonyl compounds, aromatics and phenols
were gradually enhanced, indicating that increasing
temperature could accelerate decomposition of fiber structures.
Torrefaction is a slow pyrolysis process, during which
biomass experiences dehydration, devolatilization,
depolymerization and carbonization. The experimental results
discussed above showed that H2O was one of the main
products generated by drying at low temperature and by
dehydration reactions between organic molecules at high temperature
. The dehydration reaction results in the destruction of
Figure 5 Effect of residence time on torrefaction.
Figure 6 Effect of temperature on torrefaction.
hydroxide radicals, which causes the loss of capacity to
form hydrogen bonds with water. Furthermore, formation of
non-polar unsaturated structures also occurs , making
the torrefied biomass more hydrophobic and reducing its
tendency to weathering, cracking or self-combusting, which
is advantageous for storage and transportation. In addition,
the fibrous structure of biomass was destroyed because of
decomposition to alkanes, aldehydes, ketones, carboxylic
acids, alcohols and other macromolecules. Consequently,
the grindability and feeding properties, especially for
entrained flow gasification, can also be improved.
Effect of torrefied solid product on gasification
(i) Properties of torrefied solid product. Experiments were
carried out to study the effect of the torrefaction
temperature (230, 250, 270 or 290C) and residence time (20 or 30
min) on the solid product. According to the TG data, the
changes in weight loss were small when the residence time
was more than 30 min. Hence it was thought that a
residence time of more than 30 min would not have a
significant effect on torrefaction.
Proximate and ultimate analyses of solid products are
shown in Table 2. Compared with raw sawdust, the
moisture and volatiles content of torrefied fuels decreased
greatly with increasing temperature and residence time,
whilst large increases were observed in the ash and fixed
carbon content. The sulfur and nitrogen contents remained
almost constant. The hydrogen content decreased rapidly at
higher temperatures (270 and 290C) because of the
volatilization of water and release of hydrocarbons (such as CH4
and C2H6) only at high temperature. The increase of carbon
was as high as 27.8% compared with raw sawdust, and the
largest percentage decrease of oxygen was 29.1%. This was
because the oxygen-containing functional groups with high
activity and low activation energy were easy to crack or
recombine to release CO and CO2 . The increased
calorific value illustrated the effect of these changes on the
energy content. The above information together with the TG
and DTG data suggested that torrefaction could
significantly improve the physical density, energy density and
bulk density of biomass feedstock to effectively use storage
space and reduce costs of transportation .
The pore structures of torrefied sawdust at different
temperatures with residence time 30 min were measured by the
low-temperature nitrogen adsorption method. The data in
Table 3 show that because of the release of gaseous and
volatile products, the total pore volumes of torrefied fuels
were higher than that of raw sawdust. Figure 7 displays the
influence of torrefaction temperature on the specific surface
area and average pore diameter of samples. When the
temperature was relatively low (230C), the specific surface
area and pore diameter of torrefied fuel changed little
compared with raw sawdust. It can be inferred that the release of
H2O and CO2 as the main products at lower temperature has
Table 2 Ultimate and proximate analysis of torrefied fuels
Residence time Temperature
Heating value (kJ kg1)
Table 3 Total pore volume of raw sawdust and torrefied sawdust
Total pore volume (cm3 g1)
Figure 7 Dependence of specific surface area and average pore diameter
on torrefaction temperature.
Figure 8 Pore size distribution.
little effect on the pore structure. At 250C the pores were
enlarged and more open pores were generated as a result of
the increased speed of volatilization of gaseous products. At
the same time volatile tar in semi-precipitated state may
plug some pores to form new pores. That effect complicated
the pore structure and led to the decreased average pore size
and the increased specific surface area. When the
temperature reached 270 or 290C, carbonyl compounds, aromatics,
phenols and other tar substances cracked further and
released some light compounds. Simultaneously, some pores
were closed and restructured resulting in increased average
pore size and reduced specific surface area. That was
attributed to softening, deformation and carbonization of the
particles and the plastic deformation of the pores. In addition,
the decrease of specific surface area was partly related to
the higher ash content of solid products . Figure 8
shows that the pore size distribution curves of raw sawdust
and torrefied sawdust produced under different torrefaction
temperatures were similar. However, the volume of pores
with diameter 20100 nm was higher for torrefied particles
than for raw samples. These results suggested that
torrefaction can improve the pore structure of feedstocks. Torrefied
particles with the largest specific surface area and smallest
pore size were produced by torrefaction at 250C.
(ii) Effect of torrefied solid product on the gasification.
Gasification is a typical heterogeneous reaction. Through
external and internal diffusion, the reactant gases are
absorbed on the surface of solid particles to react with
biomass. The above analysis indicated that the torrefied
sawdust had improved physical and chemical characteristics
compared with raw sawdust. Hence the gasification
characteristics of the torrefied fuels were investigated under
conditions of high heating rate and high temperature. The
results are shown in Figures 9 and 10.
The non-condensable gaseous products were mainly CO,
H2 and CO2, CH4 content was negligible. Torrefaction
lowered the moisture and oxygen content to reduce the
oxygen/carbon ratio. Hence, compared to gasification of raw
sawdust, the H2 and CO content increased, whilst CO2
decreased. The H2 content at 270 or 290C was lower than at
250C. It was known that H2 content depended on the oxygen
Figure 9 Effect of torrefied fuels on syngas composition.
Figure 10 Effect of torrefied fuels on cold gasification efficiency.
content as well as on the hydrogen content of torrefied
fuels. The release of H2O during torrefaction led to hydrogen
loss. In general, different torrefaction temperatures and
residence times had limited effects on the syngas
Cold gas efficiency was improved by torrefaction, and it
increased slightly with increasing residence time. Higher
with low O/C ratios. The highest cold gasification
efficiency was found for sawdust torrefied at 250C. This result
was in accordance with the measured pore structure of
torrefied sawdust, which indicated that the torrefied fuel
produced at 250C had the largest specific surface area and
smallest pore size.
The cold gas efficiency is given by
(QH2 yH2 + QCO yCO + Qi yi ) V / 22.4
where Q is the heating value, y is the gas volume fraction, i
represents hydrocarbons and V is the total volume of
Effect of torrefaction gases on gasification
(i) Composition of torrefaction gases. Based on the
qualitative FTIR analysis, the composition of torrefaction gases
was determined quantitatively using gas chromatography.
Figure 11 shows that the non-condensable torrefaction
gases included CO2, CO, C3H6 and trace amounts of H2 and
C2H6. The combustible gases accounted for about 50% of
the non-condensable gases. As the torrefaction temperature
increased, CO2 and CO concentration increased; the CO2
content was higher than that of CO. As the most active of
the three lignocellulose components in biomass,
hemicellulose decomposes at temperatures ranging from 200 to
250C. The decomposition of cellulose occurs at 240
350C, and lignin is the last component to decompose at
still higher temperatures (280500C) . Hence the main
thermal decomposition process during torrefaction is the
pyrolysis of hemicellulose because of its poor heat stability.
Xylan is the predominant hemicellulose found in biomass,
and contains a large number of furfural acid side chains,
from which the removal of a carboxyl group occurred to
release a large amount of CO2 during the pyrolysis process
. The concentration of CH4, which originates mainly
gasification efficiencies can be achieved for torrefied fuels
Figure 11 Effect of torrefaction temperature and residence time on the composition of torrefaction gases.
from the large number of methoxy groups in lignin ,
was too low to be detected. The H2 content was also close to
zero, indicating the release of hydrogen in the form of water
(ii) Effect of torrefaction gases on the gasification
efficiency. The gasification efficiency is given by
Figure 12 shows that the gasification efficiency of
torrefied sawdust was lower than that of raw sawdust. This
difference was mainly because of the energy loss caused by the
release of gaseous and volatile products during torrefaction;
those products were not used in the gasification
experiments. Furthermore, the devolatilization was strengthened
by increasing torrefaction temperature and residence time.
Hence, in practical applications of torrefaction, to improve
the overall gasification efficiency the carbon and the energy
in the torrefaction gases should be utilized effectively.
As discussed above, there are positive and negative
influences of torrefaction on the gasification process.
Compared with other pretreatments, torrefaction has strong
advantages and very promising market potential. The
combination of torrefaction and gasification process is equal to
two-stage pyrolysis and gasification technology, but the
energy consumption of torrefaction is lower than that of
high-temperature pyrolysis . And the power required for
grinding wood can be saved as high as 50%85% .
Because of the diversity and seasonality of biomass, fuel
flexibility is required for biomass utilization technology for
sustainable development. It was found that torrefaction process
can make different types of biomass, including commercial
timber, agricultural waste and energy crops, have quite
similar physical and chemical properties. Seasonal
influences on the properties of feedstocks are also reduced .
Based on the idea of the combination of torrefaction and
gasification, experiments were carried out to show that the
torrefaction process had a strong competitive advantage and
good application prospects in gasification processes.
(1) Torrefaction can completely dry raw biomass, destroy
the fibrous structure, and release torrefaction gases.
Increased temperature and residence time enhances the
decomposition of fiber structures, and facilitates storage,
transportation, grinding, briquetting and pneumatic feeding.
(2) Torrefaction can increase the energy density and
reduce the oxygen/carbon ratio of sawdust, and form porous
structures. Sawdust particles torrefied at 250C have the
largest specific surface area and smallest pore size. In the
gasification of torrefied fuels, syngas quality and cold gas
efficiency are improved compared with raw sawdust.
Figure 12 Effect of torrefaction on gasification efficiency.
Gasification of sawdust torrefied at 250C gives the best
(3) The non-condensable torrefaction gases include
mainly CO2, CO and C3H6, which are the predominant
pyrolytic products of hemicellulose. Flammable gas accounts
for about 50% of the total non-condensable gases. As a
result of the energy loss caused by the unused torrefaction
gases, the total gasification efficiencies of torrefied fuels are
lower than that of raw sawdust. In practical applications of
torrefaction, the torrefaction gases should be used
effectively to improve the gasification efficiency.
This work was supported by the National Basic Research Program of
China (2007CB210200) and the National Natural Science Foundation of