ARTICLE pubs.acs.org/IECR
Experimental Study of Biomass Pyrolysis Based on Three Major Components: Hemicellulose, Cellulose, and Lignin Tingting Qu, Wanjun Guo, Laihong Shen,* Jun Xiao, and Kun Zhao Thermoenergy Engineering Research Institute, Southeast University, Nanjing 210096, Jiangsu Province, China ABSTRACT: Fast pyrolysis of cellulose, xylan, and lignin was experimentally conducted between 350 and 650 °C in a tube furnace, and the effect of temperature on pyrolysis products (char, noncondensable gas, and bio-oil) was investigated. The yields of char, noncondensable gas, and bio-oil were quantified using gas chromatography and gas chromatography with mass spectrometry. The noncondensable gas mainly consists of CO, CO2, CH4, and H2. The bio-oil includes acids, ketones, aldehydes, esters, benzenes, alcohols, alkenes, phenols, alkanels, carbohydrates, etc. The results show that cellulose is the principal source of carbohydrates and phenols are the basis of the bio-oil from lignin, while the bio-oil from xylan mainly consists of acids, ketones, aldehydes, and phenols. The char yields for the three components decrease with an increase in temperature, and the gas yields and bio-oil yields increase with an increase in temperature, reach a maximum at a certain temperature, and then decrease after that temperature. The maximum biooil yields for cellulose, xylan, and lignin are 65, 53, and 40%, respectively; and their corresponding temperatures are 400, 450, and 500 °C, respectively. To investigate a relationship between biomass and three major components (hemicellulose, cellulose, and lignin), the pyrolysis of three typical biomass samples (rice straw, corn stalk, and peanut vine) was also studied, and the additivity law is adopted to predict the product components of biomass pyrolysis based on the content of hemicelluloses, cellulose, and lignin. The results show that the additivity law can predict reasonably the trend of product yields of biomass samples from their composition of hemicelluloses, cellulose, and lignin.
1. INTRODUCTION Biomass residues and wastes already provide ∼14% of the world’s primary energy supplies and are considered as the fourth largest energy source. Bioenergy offers cost-effective and sustainable opportunities with the potential to meet 50% of world energy demands during the next century and to meet the requirement of reducing carbon emissions from fossil fuels.13 Biomass pyrolysis plays a significant role in biomass conversion. Fast pyrolysis can convert low-grade biomass into high-quality liquid fuel or high-value-added chemicals. Many researchers have investigated the pyrolysis of different biomass samples, such as olivekernel, almondshell, sawdust, straw, corncob, straw, oreganum stalks, etc.47 Liu et al.8 investigated the influence of pyrolysis temperature, biomass particle size, and vapor residence time on the product distribution using corn straw in a benchscale fluidized bed reactor. Results showed that pyrolysis temperature and vapor residence time had an obvious effect on biooil yield, while their interaction and biomass particle size had an unnoticeable effect on it. Acikgoz et al.9 studied fast pyrolysis of a sample of linseed on a fixed bed focusing on the effect of pyrolysis temperature, heating rate, particle size, and sweep gas flow rate on the product yields and their compositions. The maximum biooil yield of 57.7 wt % was obtained at a final pyrolysis temperature of 550 °C, over a particle size range of 0.61.8 mm, and at a sweep gas (N2) flow rate of 100 cm3/min. Liu et al.10 studied the effects of fast pyrolysis parameters on the product distribution in a fluidized bed reactor using sawdust. The biomass pyrolysis behavior is crosswise affected by many factors such as temperature, particle size, heating rate, feed rate, and the nature of the biomass, and the results810 showed that temperature is one of the most important factors for biomass pyrolysis product distribution. r 2011 American Chemical Society
Biomass mainly consists of three components, which have very different thermal behaviors. The products of biomass pyrolysis may be considered to be the overall performance of the pyrolysis of three main components, extractives, and ash. Therefore, it should be possible to predict the pyrolysis product distribution according to the component proportion in a biomass. Cellulose, as the predominant component, was studied by assessing its pyrolysis kinetics and pyrolysis product compositions.1115 Tan et al.16 studied the distribution of gaseous, liquid, and solid products between 350 and 850 °C using the three components by a thermogravimetric analysis (TGA) and heat radiation reactor. Fu et al.17 revealed important information about thermal behavior of the various components (hemicelluloses, cellulose, and lignin) of the biomass using rice straw and maize stalk in a tubular reactor with a Fourier transform infrared (FTIR) analyzer. Yang et al.18 used TGA to investigate the different roles of the three components in pyrolysis. The results showed that the interaction among the three components could be neglected for the synthesized samples containing two or three of the biomass components, and a linear relationship existed between the weight loss and the proportion of hemicellulose (or cellulose) in the specified temperature range. Two sets of multiple linear-regression equations were established for predicting the component proportions in a biomass and the weight loss during biomass pyrolysis via TGA. Their calculations for the synthesized samples were Received: December 21, 2010 Accepted: August 12, 2011 Revised: July 22, 2011 Published: August 12, 2011 10424
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Table 1. Properties of Samples cellulose
hemicellulose
lignin
rice straw
corn stalk
peanut vine
64.35
Proximate Analysis (wt %, as received basis) volatile
89.87
73.37
63.39
61.28
70.74
fixed carbon
4.96
13.1
29.99
14.22
16.75
17.5
ash
0.21
2.83
2.63
17.43
3.15
6.58
moisture
4.96
10.7
3.99
7.07
9.36
11.58
C
39.63
38.03
61.45
35.22
39.24
35.07
H
5.31
5.31
5.54
4.57
4.92
4.89
N O
0 49.88
0.03 43.1
0.92 23.98
0.79 34.92
0.81 42.52
1.26 40.62
S
0
0
1.5
0
0
0
cellulose
37
42.7
44.99
hemicellulose
16.5
23.2
18.23
lignin
13.6
17.5
11.76
extractives
13.1
9.8
19.26
ash
19.8
6.8
5.76
low-heating vale (LHV)a (MJ/kg)
13.4
13.4
20.6
12.5
13.6
12.2
Ultimate Analysis (wt %, as received basis)
Composition of Lignocelluosic Material (wt %)
a
LHV = [4.19(81 C + 300 H + 26 S 26 O) 2500(9 H + moisture)/100]/1000.39
consistent with the experimental measurements. Couhert et al.19 tried to establish a link between the components of a biomass and the gas yield and composition of its pyrolysis at a temperature of 950 °C with a gas residence time of ∼2 s. They found that the interactions occurred among the three components, and mineral matter in biomass influenced its pyrolysis process. Hosoya et al.20 found that a significant interaction occurred in celluloselignin pyrolysis and a comparatively weak interaction occurred in cellulosehemicellulose pyrolysis during wood pyrolysis at 800 °C. Although the pyrolysis of the biomass and the three main components were investigated by many researchers, the information about the relationship between the pyrolysis of the three components and biomass is not sufficient, and a satisfactory relationship between the biomass and its components has not yet been established. In this paper, we conducted a systematic study of the three main components and three biomass samples to investigate the relationship between biomass pyrolysis and the three components (hemicellulose, cellulose, and lignin) of pyrolysis. The effect of temperature on the product distributions of the gas, bio-oil, and char in biomass pyrolysis is also examined in this study.
2. MATERIALS AND METHODS 2.1. Materials. In this study, hemicelluloses, cellulose, and lignin are experimentally used in the investigation of biomass pyrolysis. Microcrystalline cellulose was obtained from Shanghai kayon Biological Technology Co., Ltd. Hemicellulose is complex and not commercially available. Xylan from oat spelts (Sigma Co.) was used as a hemicellulose model in the study. Alkali lignin with a purity of 97% (Sigma Co.) was used as the lignin source. Three agricultural wastes (rice straw, corn stalk, and peanut vine) were also studied. Biomass was crushed, sieved to 0.50.6 mm, and dried for 2 h at 100 °C to remove moisture. The properties of
the biomass samples, hemicelluloses, cellulose, and lignin are listed in Table 1. 2.2. Experimental Setup. Fast pyrolysis of the samples was conducted in a bench-scale tubular reactor (inside diameter of 80 mm, length of 340 mm), as shown in Figure 1. The experimental apparatus mainly consisted of a tubular reactor, a temperature control device, a heat exchanger, an oilpot, etc. In each experiment, after the reactor reached the designated temperature (as a result of the electric heater with the help of a temperature controller), nitrogen with a flow rate of 300 mL/min was flushed into the reactor to maintain an inert atmosphere for thermal decomposition of the samples before the experiment. Afterward, a sample of 10 g of raw material was placed in a porcelain boat, and the boat was rapidly pushed into the pyrolysis reactor. The sample was quickly heated in the furnace and reached the furnace temperature in ∼1 s. From the reactor, the gas passed through two spherical condenser tubes where most of the condensable gas condensed and collected in the oilpot. The condensation medium was an ice/water mixture. A water washing device was used to remove the rest of the bio-oil. After that, the gas passed through a desiccator that was filled with porous silica gel, where the water was captured. Following the desiccator, the gas passed through a flow meter and then was collected in gas bags. The pyrolysis process was almost complete within 50 min. The gaseous products collected in gas bags were analyzed by gas chromatography (GC) (Agilent 6890N) to quantify the concentrations of H2, CO, CO2, CH4, CnHm (C2H6 and C3H8), and N2. The chemical composition of the bio-oil was analyzed via gas chromatography and mass spectrometry (GCMS) (Agilent 7890A/5975C). GC was performed using a Varian Cp-sil 8cb capillary column [30 m 0.25 m (inside diameter), 0.25 μm film thickness]. Helium (99.999%) was used as the carrier gas with a flow rate of 3 mL/min. The oven temperature was programmed from 40 to 180 °C at a heating rate of 5 °C/min and then to 280 °C at a heating rate of 20 °C/min. MS was conducted with the following operation conditions: transfer line, 230 °C; ion 10425
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Figure 1. Schematic diagram of a fast pyrolysis reactor.
Figure 2. Effect of temperature on product distribution.
source, 230 °C; and electron energy, 70 eV. The ultimate analysis of biomass raw materials was performed using a EURO EA3000 elemental analyzer (Euro Vector SPA). The amount of char containing inert ash was determined by weighing the solid remaining in the porcelain boat after pyrolysis. The total volume of the gaseous product was measured by the flow meter. Although the collection of bio-oil in the oilpot was not complete, the bio-oil yield could be determined on the basis of gas and solid yields.
3. RESULTS AND DISCUSSION 3.1. Distribution of the Pyrolysis Products of the Three Components. The pyrolysis temperature is crucial for the
production of bio-oil, gas, and char from biomass pyrolysis in a
tubular fixed bed. Research has shown that for most biomass samples the maximum bio-oil yield was affected by temperature.21 In this work, the pyrolysis temperature was varied from 350 to 650 °C. Results referring to only the pyrolysis product yields are presented to demonstrate the effect of pyrolysis temperature. The pyrolysis product yields are shown as a function of the pyrolysis temperature, as indicated in Figure 2. Temperature had a remarkable effect on pyrolysis product yields. The trend of bio-oil yield from the three components is similar. The bio-oil yield increases first, reaches a maximum at a certain temperature, and then decreases with temperature. This can be explained as follows. When the temperature is low, the volatile matter slowly evaporates and the carbonization reaction dominates. The release of the volatile matter begins quickly with the increase in temperature and produces a maximum yield of bio-oil at a certain temperature. Afterward, cracking of unstable components of the volatile matter results in a decreased bio-oil yield. As shown in Figure 2, the maximum oil yields from cellulose, xylan, and lignin are 65, 53, and 40%, respectively, which were obtained via heating at 400, 450, and 500 °C, respectively. The comparison of the overall production of bio-oil of the three components indicates that the yield of bio-oil from cellulose was the largest and the yield of bio-oil from lignin was the smallest. A maximum oil yield of 65% from cellulose seems to be reasonable, considering the results of other pyrolysis studies. Kojima22 obtained a maximum bio-oil yield via cellulose pyrolysis of 63% at 400 °C in a fluidized bed. Luo et al.23 obtained a bio-oil yield of 58.6% at 450 °C in a fixed-bed reactor. A maximum bio-oil yield of 78% was obtained at 600 °C (TRS) by Liao et al., but the actural reaction temperature was far below 600 °C.24 As the temperature exceeded 550 °C, the bio-oil yield from cellulose decreased at a rate faster than the rate of decrease of the yield of bio-oil from xylan and lignin (Figure 2). It is a clear indication that bio-oil from cellulose easily decomposes with an increase in temperature. This is in agreement with experimental results of Tan et al.16 The reason is that the bio-oil from lignin pyrolysis mainly consists of polycyclic aromatic hydrocarbons and phenolic compounds, xylan oil mainly consists of acids, furfurals, etc., 10426
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Industrial & Engineering Chemistry Research and cellulose oil mainly consists of levoglucosan and glycolaldehyde, as well as small amounts of ketone and acid compounds. The product of levoglucosan is very sensitive to temperature, and it can be easily decomposed at high temperatures; substances such as phenol and furfural had relatively higher thermal stabilities.13 A moderation temperature of 427677 °C (TRS) and a gas residence time of less than 1 s are necessary to produce a bio-oil with a high yield of levoglucosan, and a higher temperature aids the formation of hydroxyacetaldehyde, acetol, formaldehyde, and furfural.25 Therefore, at the high temperature, the yields of the products of secondary cracking for oils from both lignin and xylan are much smaller than that for cellulose. The temperature has a significant role in promoting the production of gas from biomass pyrolysis. Generally, the gas yields increase with an increase in temperature as shown in Figure 2. Among the three components, the yield of gas from lignin pyrolysis is the smallest in the whole temperature range. The yield of gas from xylan pyrolysis is the highest below 560 °C, whereas the yield of gas from cellulose pyrolysis increases rapidly with temperature and reaches a maximum when the temperature exceeds 560 °C. This can be attributed to the release of volatile matter and secondary decomposition of bio-oil. The char yields decrease significantly as the pyrolysis temperature is increased from 350 to 650 °C. As indicated in Figure 2, the comparison among the overall distribution of char yields of the three components reveals that the char yield from lignin pyrolysis is the highest, and that from cellulose is the lowest, only 11% at 650 °C. These phenomena can be explained by the physical structures of the three components. Lignin is a natural amorphous polymer consisting of phenylpropane units, and their precursors are three aromatic alcohols, namely, p-coumaryl, coniferyl, and sinapyl alcohols. The aromatic constituents of these alcohols in the polymer are called p-hydroxyphenyl, guaiacyl, and syringyl moieties, respectively.26,27 Sharma et al.28 studied the characterization of alkali lignin char and its reactivity toward the formation of polycyclic aromatic hydrocarbons (PAHs). They found that the chars lost both hydroxyl and aliphatic groups with an increase in temperature, and the increase in aromatic character is quick above 450 °C, resulting in an aromatic carbon content of ∼70% at high temperatures. Aromatic monomers exhibited good thermal stability, which led to a high char yield. On the other hand, the content of fixed carbon in the lignin is 29.99%, as shown in Table 1, which is significantly higher than that in cellulose or hemicellulose. 3.2. Characteristics of Gases from the Three Components of Pyrolysis. The effects of temperature on the gas compositions of the three components of pyrolysis were also investigated, as indicated in Figure 3. The pyrolytic gases of the three components are rich in CO and CO2, while the amounts of C2H6 and C3H8 released are generally small. Cellulose has a higher CO2 volume fraction, which decreases evidently with pyrolysis temperature. The volume fraction of CO increases remarkably with temperature, reaches a maximum of 47% (which is greater than that from xylan and lignin pyrolysis) at ∼500 °C, and then decreases with temperature. The H2 volume fraction increases rapidly when the temperature is above 550 °C. The volume fraction of CO2 from xylan pyrolysis is the highest, and there is a substantial decrease in the CO2 volume fraction, from 60 to 40%, over the pyrolysis temperature range of 500550 °C. The volume fraction of CO fluctuates around 30% with temperature. The volume fraction of H2 is small when the temperature is lower than 500 °C but increases remarkably over the pyrolysis
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Figure 3. Effect of temperature on gas composition.
temperature range of 500550 °C, reaches a maximum of 29% at 550 °C, and then remains nearly constant with a temperature increase. The gas compositions of lignin are evidently different from those of cellulose and xylan, as shown in Figure 3ac. The volume fraction of CH4 from lignin pyrolysis is the largest. In general, CO2 is mainly released from the cracking and re-forming of functional groups of carboxyl (CdO) and COOH, while CO is mainly from the cracking of carbonyl (COC) and carboxyl (CdO) groups. H2 is mainly from the cracking and deformation 10427
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Figure 4. Effect of temperature on bio-oil composition.
of CdC and CH groups, while CH4 is mainly from the cracking of methoxy-O-CH3 groups. The volume fraction of CO from cellulose pyrolysis is the largest, because of the higher carbonyl content of cellulose. The volume fraction of CO2 from hemicellulose pyrolysis is the largest because of the higher carboxyl content of hemicelluloses. The largest volume fraction of CH4 is observed in lignin pyrolysis, which is attributed to a greater content of methoxyl in lignin.14 3.3. Characteristics of Oils from the Three Components of Pyrolysis. To analyze the influence of temperature on the characteristics of pyrolysis products, bio-oils from the three components of pyrolysis at 400, 500, and 600 °C were analyzed using GCMS. A variety of substances were detected, such as phenol, 4-ethyl-2-methoxyphenol, 4-ethylphenol, propanoic acid, 3-methyl-1,2-benzenediol, 5-methyl-2-furancarboxaldehyde, maltol, cyclopentanone, 1,1-dimethylhydrazine, etc. In this paper, more than 200 chemical substances were classified as acids, ketones, aldehydes, esters, benzenes, alcohols, alkenes, phenols, alkanes, carbohydrates, and others in the way indicated by Graham Solomons and Fryhle.29 As shown in Figure 4, almost the same constituents of oils from the three components of pyrolysis are observed, but the content of each constituent obviously differs with temperature. The bio-oil from cellulose pyrolysis contains amounts of carbohydrates compared with that from xylan and lignin pyrolysis, and levoglucosan is the main constituent of carbohydrates. The relative peak area of levoglucosan could reach g40%. In the bio-oil from xylan pyrolysis, acids and phenols are detected and a small amount of carbohydrates is found. In comparison with the bio-oil from cellulose and xylan pyrolysis, the level of phenols in bio-oils from lignin pyrolysis is up to 75%, which is larger than those from cellulose and xylan pyrolysis. Meanwhile, it also contains some aldehydes, ketones, and esters. Carbohydrates are not detected in the bio-oil from lignin pyrolysis in this study. These differences are due to the chemical structure of the three components. In general, cellulose makes a significant contribution to carbohydrate formation. Levoglucosan was produced by cellulose macromolecules of β-(1,4)-glycosidic bond cleavage and intramolecular rearrangement, whereas levoglucosenone is formed by dehydration of levoglucosan.30 Xylan makes a significant contribution to the formation of acids that are derived from the acetyl groups and uronic acid side chains.31,32 Lignin makes a significant contribution to phenols, which is due to the dehydration of OH groups in the alkyl side chain of the lignin basic units and the breakage of ether linkages contained in the main chains.33,34
Figure 5. Effect of temperature on product distribution.
3.4. Pyrolysis of Biomass Materials. For comparison with the pyrolysis of the three components, three familiar biomass samples (rice straw, corn stalk, and peanut vine) were used. The effects of pyrolysis temperature on product yields, gas compositions, and bio-oil compositions are investigated in the study. Similar product distributions are attained from the biomass pyrolysis process as shown in Figure 5. The bio-oil yields increase with temperature, then reach a maximum at a certain temperature, and then decrease after that temperature. The yields of char decrease with an increase in temperature, and that of the noncondensable gas increases gradually. The yields of products from the three biomass samples were different, such as the optimal bio-oil yields. The maximal bio-oil yields for rice straw, corn stalk, and peanut vine are 43, 51, and 48%, respectively, which are obtained at 400, 450, and 450 °C, respectively. This is due to the fact that the thermal decomposition of hemicellulose and cellulose terminates at ∼400 °C. However, lignin decomposes slowly over a much wider temperature range of 180900 °C,14 and extractives, which consist of fats and proteins, decompose in a manner similar to that of lignin, but at a higher rate.35 The yield of bio-oil from rice straw pyrolysis decreases gradually because of the cracking of bio-oil in the pyrolysis vapors into gas and secondary bio-oil, and the sum of the additional release of bio-oil from decomposition of lignin and extractives (mostly yielding char) is not significant above 400 °C. Similarly, the yields of bio-oil both from corn stalk and peanut wine pyrolysis decrease above 450 °C. This can also explain the difference between the temperatures for the maximal bio-oil yields for the three biomass samples. The pyrolysis of cellulose could produce more bio-oil; the pyrolysis of lignin could produce more char, while hemicellulose is also helpful to bio-oil production, as shown in Figure 2. Compared with the properties of the three biomass materials as indicated in Table 1, the sum contents of cellulose and hemicelluloses in corn stalk or peanut vine are higher than those in rice straw. Also, the content of ash in rice straw is the highest of the three biomass samples. In general, the higher the content of 10428
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Figure 7. Effect of temperature on bio-oil composition.
Figure 6. Effect of temperature on gas composition.
minerals in biomass, the lower the yield of bio-oil and the higher the yield of char.36 Therefore, the lowest yield of bio-oil is obtained from rice straw. Although the content of lignin in rice straw is the lowest, the yield of char is the highest in the whole temperature range because of its high ash content. As indicated in Figure 6a, the volume fraction of CO2 is the highest in noncondensable gas from rice straw pyrolysis and decreases almost linearly in the temperature range of 500600 °C. More CO is produced for cellulose pyrolysis, as
shown in Figure 3, and the content of cellulose in rice straw is the lowest. Therefore, it leads to the increase in the CO2 volume fraction. This is why the CO volume fraction decreases with an increase in temperature until 550 °C is reached. The volume fraction of CO2 from rice straw pyrolysis declines when the temperature is above 550 °C. It is ascribed to the volume fractions of CO2 from the three components of pyrolysis that decrease with temperature. In addition, the high content of ash in rice straw has an effect on the gas yield.36 As for the distribution of CH4 and H2, it is found that the volume fraction of CH4 increases with temperature and the volume fraction of H2 increases when the temperature is above 550 °C. H2 is not detected in gases from corn stalk pyrolysis as indicated in Figure 6b, and the same is true of lignin pyrolysis as shown in Figure 3c. In this study, it is difficult to detect a H2 concentration via the limited measurement methods when the CO2 volume fraction is high. Figure 6c shows that CO2 is also the main gas for peanut vine pyrolysis. A higher yield of more than 70% CO2 is obtained when the temperature is below 500 °C and decreases sharply with temperature. The volume fraction of CO remains at a value of ∼20% over the whole temperature range. The CH4 volume fraction increases slowly when the temperature is above 400 °C. The H2 volume fraction increases rapidly above 550 °C, and it reaches 40% at 650 °C. Extractives that consist of fats and proteins behave like lignin in the pyrolysis process.35 The sum yield of lignin and extractives from peanut vine is the highest of the three biomass materials. Research14,37 has shown that lignin pyrolysis produces a high concentration of H2 and CH4, although H2 is not detected in this work because of the high CO2 content of lignin pyrolysis. The volume fraction of CmHn (C2H6 and 10429
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C3H8) is always low in the pyrolysis of the three biomass materials. Figure 7 shows that the main constituents of bio-oil obtained from the three-biomass pyrolysis are the same at different temperatures, but there is a difference in their relative contents. The constituents of bio-oil from rice straw pyrolysis are mainly acids, ketones, phenols, and small amounts of aldehydes, esters, benzenes, alkenes, and carbohydrates. Bio-oil from corn stalk pyrolysis consists of more aldehydes, acids, ketones, and phenols, while the relative peak area of aldehydes is the largest, accounting for 2535%. Meanwhile, the average relative peak area of acids and phenols is ∼20%. The contents of acids, ketones, and phenols found are higher in bio-oils from peanut vine pyrolysis, and the relative peak area of acids accounts for ∼35% at 650 °C. The average relative peak areas of ketones and phenols are both ∼25%. Those of aldehydes and carbohydrates are relatively smaller. Bio-oil from peanut vine pyrolysis also contains small amounts of esters, alcohols, alkenes, etc. Compared with the constituents of bio-oil from the threebiomass pyrolysis as shown in Figure 7, some carbohydrate is detected in the bio-oils from pyrolysis of rice straw, corn stalks, and peanut vines, with a minimal value for pyrolysis of rice straw and a maximal value for pyrolysis ofpeanut vine. It may be the reason that the content of cellulose in peanut vine is the highest while the content in rice straw is the lowest, as shown in Table 1, and cellulose is the principal source of carbohydrates, as shown in Figure 4. In comparison with pyrolysis of lignin, more aldehydes are produced in the pyrolysis of xylan and cellulose as indicated in Figure 4. Therefore, a higher content of aldehydes in bio-oil from corn stalk pyrolysis is caused by the higher content of xylan and cellulose, which is consistent with the theoretical analysis. A large amount of phenols existing in bio-oil from lignin pyrolysis leads to a certain amount of phenols in bio-oils from the three-biomass pyrolysis. The average relative peak area of phenols is ∼20% in bio-oils from pyrolysis of rice straw and corn stalk. The maximal average relative peak area of phenols is around 25% in bio-oil from pyrolysis of peanut vine. The sum content of lignin and extractives in peanut vine is the highest, so the content of phenols from pyrolysis of peanut vine is the highest, which is also consistent with theoretical analysis (as shown in section 3.5). A certain amount of acids is obtained in bio-oils from the threebiomass pyrolysis, whereas more acid is found in bio-oil from pyrolysis of corn stalks because of its higher content of hemicellulose. 3.5. The Component Additivity for Predicting Product Yields of Biomass. To develop a reasonable relationship between biomass and the three major components (hemicellulose, cellulose, and lignin), we introduce the additivity law to predict the components of the biomass pyrolysis product based on the content of hemicelluloses, cellulose, and lignin. The three components in rice straw, corn stalk, and peanut vine are assumed to be unaffected by each other in the pyrolysis process, and extractives are added to the lignin content.35 Ash is considered as an inert material in the pyrolysis process and is left in the char. The pyrolysis product yields are calculated from the following equation: Yi ¼ aYi;cellulose þ bYi;xylan þ cYi;lignin þ Aj
ð1Þ
where Yi (weight percent) is the correlated product yield for any given biomass in this study (i = 1, 2, or 3, denoting char, gas, or bio-oil, respectively). Also, a, b, and c are the initial fractions of
Figure 8. Comparison of additivity results with experimental results.
cellulose, hemicellulose, and lignin in the biomass, respectively, and the product yields of cellulose, hemicellulose, and lignin are Yi,cellulose, Yi,xylan, and Yi,lignin, respectively. Aj is the ash content of each biomass sample, which is used only in the calculation of char (i = 1). In other words, Aj is equal to 0 in the gas (i = 2) and biooil (i = 3) calculations. Comparing the experimental and calculational results, we find the trend of product distribution is similar in the temperature range of 350650 °C as shown in Figure 8. The char yields for 10430
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Figure 9. Comprehensive simulation diagram for the pyrolysis process using Aspen Plus.
the three components decrease with an increase in temperature; the gas yields increase, and the bio-oil yields increase with temperature, then reach a maximum at a certain temperature, and then decrease after that temperature. Although the overall trends of calculational results are in agreement with the experimental data, there is a quantitative discrepancy. It may be ascribed to the following. Extractives are added to the lignin content, resulting in a higher content of lignin in the calculated process. Lignin pyrolysis produces the highest char yield and the lowest gas yield, so the calculated value of char from biomass pyrolysis is larger than the experimental value. Also, the calculated value of gas is smaller than the experimental value. Meanwhile, the pyrolysis of celluloses extracted from different biomass samples has a similar product distribution, but the distribution becomes different for both hemicellulose and lignin extracted from different biomass samples. Lignin could even present a very different product distribution in the same family.26,33 In this study, hemicellulose was obtained from oat spelts by Sigma, while lignin was alkali lignin with a purity of 97%. The chemical structures of the three components used in this study are different from the ones extracted from the biomass samples. The three components in a biomass possess a compact organizational structure with a determinate polymerization, resulting in a higher thermal stability. Moreover, the reactions among various components are not independent of each other in the biomass pyrolysis process, and they actually occur in biomass pyrolysis. Further, the pyrolysis product distribution is affected by the mineral material in the ash of the biomass. Therefore, further investigation is needed to study the influence of interactions among the ash, the extractives, and the three components in the process of pyrolysis. 3.6. Simulation of the Pyrolysis Process. To explain further the results of the model as shown in eq 1 and the experimental data, we implemented a simulation of biomass pyrolysis process with Aspen Plus. This simulation is also a basis for investigation of the effects of pyrolysis temperature and the composition of biomass on bio-oil yield, and the optimal temperature for enhanced bio-oil yield for any biomass from their compositions can be deduced with the simulation. The process design for the simulation of biomass pyrolysis is shown in Figure 9. The two stages of biomass pyrolysis and pyrolysis product separation are simulated. The pyrolysis module is composed of a separator (sep1) and a pyrolyzer (decomp). The separator block is merely a decomposer, which corresponds to the Sep block of Aspen Plus; its function is to separate biomass into cellulose,
hemicelluloses, lignin, extractives, and ash. The pyrolyzer block is a Ryield reactor; its function is to calculate the product yield by FORTRAN subroutine on the basis of experimental data of the three-component pyrolysis. Pyrolysis products from the threecomponent pyrolysis at 400, 500, and 600 °C were assessed on the basis of the experimental results described above. The gaseous product in the pyrolyzer is separated from the char by the cyclone and then passes through a cooler (condense), where the oil product in the gaseous product is collected. The pyrolysis product CARB is defined as a mixture of char and ash. The noncondensable gas consists of CO, CO2, H2, CH4, C2H6, and C3H8. The chemical composition of the bio-oil is very complex; it contains hundreds of chemical compounds. At present, there is no standard characterization procedure for bio-oils. Therefore, on the basis of their constituent components, including carbon, hydrogen, and oxygen, the bio-oils from the pyrolysis of three components (hemicellulose, cellulose, and lignin) are defined as CH5O2, CH4O2, and C2H3O, respectively. The other assumptions are made on the basis of the application of Aspen Plus: (1) the process is steady state, (2) char contains only carbon and ash, and (3) particles are uniform in size. Operating conditions and primary parameters in the simulation are as follows: room temperature, 20 °C; biomass flow rate, 1000 kg/h; pyrolysis temperature range, 400600 °C; pressure, 0.1 MPa; feedwater inlet temperature, 15 °C; and outlet cold fuel gas temperature, 75 °C. The flow rate of H2O is variable with the pyrolysis temperature and biomass. Three biomass samples (rice straw, corn stalk, and peanut vine) were used as raw materials, and the pyrolysis product distributions of three biomass samples from the simulation are shown in Figure 10. For the three kinds of biomass, the char yield decreases with an increase in temperature, the gas yield increases, and the bio-oil yield reaches a maximum at a certain temperature. Figure 10a shows the simulation results compared with experimental results for the pyrolysis product distribution of rice straw. The char yield of rice straw obtained from the simulation is slightly smaller than the experimental result, and the gas yield from the simulation is in good agreement with the experimental result. A high ash content of 19.8% in rice straw had an influence on the pyrolysis products. The minerals in ash are beneficial for both char and gas production.38 Furthermore, hemicellulose and lignin used in this study are different from those extracted from actual biomass. Comparison of simulation prediction with experimental data for the pyrolysis product distribution of corn stalk is shown in Figure 10b. The char yield of corn stalk obtained 10431
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reasonable to predict the pyrolysis product distribution of different biomass samples by this simulation according to its composition of the three components, extractives, and ash.
4. CONCLUSIONS In this study, fast pyrolysis of cellulose, xylan, lignin, and three biomass samples (rice straw, peanut vine, and corn stalk) was conducted. The overall trends of the product distributions are consistent with a temperature increase. The char yields decrease with an increase in temperature; the gas yields increase, while the bio-oil yields increase with temperature, then reach a maximum at a certain temperature, and then decrease after that temperature. The maximum bio-oil yields for cellulose, xylan, and lignin are 65, 53, and 40%, respectively, and the corresponding temperatures are 400, 450, and 500 °C, respectively. Lignin produces the highest char yield, with a highest yield of 69% at 350 °C, and the yield could even reach 40% at 650 °C. The lowest char yield is obtained from cellulose pyrolysis (only 11% at 650 °C). The maximal bio-oil yields for rice straw, corn stalk, and peanut vine are 43, 51, and 48%, respectively, obtained at 400, 450, and 450 °C, respectively. The char yield obtained from rice straw is higher than that from peanut vine or corn stalk in this study. GCMS analysis shows that bio-oil includes acids, ketones, aldehydes, esters, phenols, benzenes, alkenes, carbohydrates, etc. The composition of bio-oil varies depending on the feedstock. Aids are mainly from hemicelluloses, whereas carbohydrates are mainly from cellulose and phenols from lignin. Therefore, the content of carbohydrates in bio-oil from peanut straw pyrolysis is larger because of a higher content of cellulose; bio-oil from corn stalk pyrolysis contains more aldehydes for the higher content of xylan and cellulose, while bio-oil from peanut straw pyrolysis contains plenty of phenols for its high sum content of lignin and extractives. In comparison with the experimental results, the overall trends of our calculations are in agreement with the experimental data, but there is a quantitative discrepancy. The additivity law can predict the trend of product yields of biomass samples from their composition of hemicelluloses, cellulose, and lignin. It is also possible to simulate the pyrolysis process to predict the pyrolysis product distribution using Aspen Plus. Therefore, the optimal temperature for an enhanced bio-oil yield for any biomass from its composition can be deduced. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-25-83795598. Fax: +86-25-83793452. E-mail: lhshen@ seu.edu.cn. Figure 10. Comparison of simulation results with experimental results.
from simulation is smaller than the experimental value, and the gas yield from the simulation is greater than the experimental values. Figure 10c shows better agreement between simulation prediction and experimental data for the pyrolysis product distribution of peanut vine in the temperature range of 400600 °C. The simulation results are in good agreement with the experimental results, which demonstrates that the mathematical model can successfully predict the biomass pyrolysis product distribution. On the basis of this discussion, it is
’ ACKNOWLEDGMENT We appreciate financial support from the National Key Basic Research Development Program (2007CB210208 and 2010CB732206). ’ REFERENCES (1) Zhang, L. H.; Xu, C. B.; Champagne, P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Convers. Manage. 2010, 51 (5), 969–982. 10432
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