Oxygen Migration in Torrefaction of Eupatorium adenophorum Spreng

Article pubs.acs.org/EF

Oxygen Migration in Torrefaction of Eupatorium adenophorum Spreng. and Its Improvement on Fuel Properties Zhennan Han,†,‡ Xi Zeng,† Changbin Yao,*,† and Guangwen Xu*,† †

State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Torrefaction of Eupatorium adenophorum Spreng., a major invasive plant in southeast China, was investigated in a laboratory fixed bed at temperatures of 200−325 °C and residence time of 30 min for improving the properties of the biomass as fuel. During torrefaction, a large amount of oxygen was removed from biomass, which made the torrefied biomass more like coal. Oxygen in torrefaction products was characterized to study the quantity and approach of oxygen migration. In gas products, oxygen existed as CO2 and CO, and in the liquid product, it existed in forms of H2O and oxygen-containing compounds, such as acids, alcohols, aldehydes, ketones, furans, guaiacols, phenols, and extracts. The oxygen in the solid product presented as oxygencontaining functional groups, of which the proportion of C−OH and C−O−C obviously decreased with the increase in the torrefaction temperature. At low temperatures (200−250 °C), oxygen in biomass was transferred to H2O with traces of oxygen migrated to bio-oil. As the torrefaction temperature increased, a growing amount of oxygen in biomass migrated to bio-oil and gas but dehydration still dominated deoxidation. Carbon migration coupled with oxygen migration led to energy loss to decrease the energy yield of the torrefied product or the obtained fuel. From the perspective of deoxidation effectiveness and carbon loss during torrefaction, 250 °C was suggested to be the optimal temperature for torrefaction of E. adenophorum Spreng.

1. INTRODUCTION Lignocellulosic biomass is a carbon-neutral renewable energy source, which releases much less CO2 than that of fossil fuel in utilization. An annual production of about 170 billion tons in the world1 makes the exploitation and utilization of biomass important and attractive. However, because of the properties of high moisture, low energy density, and high oxygen content, it always faces the problems of low burning conversion, low flame temperature, and smoke generation during biomass combustion1,2 as well as poor syngas quality and high tar concentration in biomass gasification.3,4 Thus, developing an efficient upgrading technology for biomass fuels is necessary and essential. Torrefaction is a thermal pretreatment process for fuel, usually operating at temperatures of 200−300 °C in an inert atmosphere for several hours. Its reaction products mainly include torrefied biomass, condensed liquid, and a noncondensable gas mixture. By this process, hemicellulose is deeply decomposed, with cellulose and lignin partially decomposed. The moisture in biomass will also be removed, and the physicochemical characteristics for biomass will become consistent and uniform.5−7 These, in turn, improve the energy density and durability of biomass fuel; meanwhile, the cost of long-distance transportation and long-term storage can be decreased. Besides, the torrefied biomass has a much improved grinding performance, and its required specific grinding energy can be much decreased because torrefaction reduces fiber length and mechanical stability of the fuel.8,9 These improved characteristics benefit the further energy conversion process, and it has been proven that the torrefied biomass can generate electricity with similar efficiency as for coal10 and improve the syngas quality in gasification.11 © 2015 American Chemical Society

Therefore, torrefaction provides a promising method to improve biomass fuel properties8,12 and convert low-quality biomass into an upgraded renewable fuel for generating heat or power. These are predominately as a result of oxygen removal and the reduced O/C ratio in the torrefied biomass.13,14 Hence, oxygen migration to liquid and gas products plays a critical role in the torrefaction process and greatly affects the property of torrefied biomass. The temperature and reaction time have obvious effects on the yield and property of the solid product from torrefaction.7,15,16 Increasing the torrefaction temperature raises the carbon content and calorific value of the solid product, although its yield decreases. From the perspective of oxygen removal, the temperature may influence the oxygen migration quantity as well as approach. On the basis of the energy yield, the high-temperature environment would lead to drastic energy loss for the removal of massive volatiles.17 Thus, there must be optimal torrefaction conditions, and it is significant to investigate such conditions. Eupatorium adenophorum Spreng. (EAS) is widely distributed as an invasive plant in southwest China, and it is becoming a serious aggressive weed.18,19 By 2008, it has affected more than 14 million hectares of land in China. Hence, there is a large amount of EAS required to be used every year, and it can be considered as a potential biomass energy resource. The mature technology to use biomass mainly includes the production of activated carbon and bioenergy. As a perennial herb of the family Asteraceae,20 EAS contains more holocellulose and less lignin than woody biomass, making it unsuitable for preparation Received: June 12, 2015 Revised: October 12, 2015 Published: October 13, 2015 7275

DOI: 10.1021/acs.energyfuels.5b01318 Energy Fuels 2015, 29, 7275−7283

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Energy & Fuels

In each test, 9.0 ± 0.1 g of sample was loaded into the reactor tube. Before heating, a high-rate flow of nitrogen was used to purge the reactor for 10 min to remove air. Then, the gas inlet valve was regulated to ensure the flow of nitrogen at 100 mL/min. The heating device was then switched on to heat the sample from room temperature to the preset temperature (200−325 °C) at 10 °C/min. It was kept at the preset temperature for 30 min before stopping heating. The effect of the temperature on the torrefaction behavior was tested via a temperature interval of 25 °C. The torrefied biomass was taken from the reactor and weighted to calculate the solid yield. The volume of non-condensable gas was equal to that of replaced water in the graduated cylinder. After a test, the reactor, receiver, water condenser, and acetone scrubbing bottles were all washed with acetone for fully collecting the liquid product, including tar. The obtained liquid mixture was divided into two parts and weighted, respectively. One was analyzed by the Karl Fischer moisture titrator (870 KF Titrino plus) to determine the water yield, and the other was mixed with anhydrous magnesium sulfate to remove its moisture. In the latter case, the liquid mixture was further treated in a vacuum rotary evaporator to remove acetone and to recover the water-free bio-oil (tar), which was weighted to calculate the bio-oil yield. 2.2. Product Analysis and Characterization. The proximate analysis of biochar was performed according to the standard procedure of ASTM International (ASTM E870-82), while the ultimate analysis of biochar and bio-oil was carried out using an elemental analyzer (Vario EL III). A known-mass sample (approximately 1.0 g) was combusted in an adiabatic oxygen bomb calorimeter (SCLR-5000) to determine its higher heating value (HHV). For this measurement, analysis in triplicate proved the reproducibility in 5%. The so-called ordinary least squares (OLS) and partial least squares (PLS) models,24,25 as shown by the following eqs 1 and 2, were also used to calculate the HHV of the measured biomass. The symbols C, H, and N in the equations represent the mass percent of carbon, hydrogen, and nitrogen on a dry and ash-free basis, respectively.

of activated carbon for the low product quality and yield.21 Torrefaction intends to improve the properties of biomass for diversified uses,22,23 while it can also be applied to overcome the difficulties in using EAS as well as other biomasses. With torrrfaction of EAS, it can produce a big amount of biofuel. Thus, this study intends to improve the properties of EAS as a kind of solid biofuel by torrefaction. The improvement of oxygen migration on fuel properties in torrefaction and the optimal conditions for torrefaction are clarified.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. EAS obtained from Sichuan province of China was used as raw material for the torrefaction tests. Before experiments, the material was dried in an oven at a temperature of 105 °C for 24 h to remove its redundant moisture. Then, the biomass was milled to give an average particle size of 0.3 mm. The properties of the dried EAS are shown in Table 1. It is obvious that the material has a high content of volatile, while the fixed carbon was relatively low. The oxygen content in the material is higher than 40 wt %.

Table 1. Properties of EAS Proximate Analysis (wt %) moisture volatile matter (VM) fixed carbon (FC) ash Elemental Analysis (wt %, dafa) C H O N S HHV (MJ/kg, dbb) a

1.55 74.50 18.98 4.97 45.87 8.10 43.98 0.73 0.10 18.55

HHV (OLS model) = 1.87C 2 − 144C − 2802H + 63.8CH + 129N + 20147

daf = dry and ash-free basis. bdb = dry basis.

(1)

HHV (PLS model) = 5.22C 2 − 319C − 1647H + 38.6CH

Figure 1 shows a schematic diagram of the experimental facility. The facility consists of a reactor, a nitrogen cylinder, and a set of product collection units. The fixed-bed reactor is a quartz tube of 20 mm in inner diameter and was heated by an electric heater. The reaction temperature was controlled using a thermocouple immersed in the tested sample. Nitrogen from a cylinder provided the torrefaction circumstance. Tar in the generated gas was trapped through its passing through the water condenser and three gas bottles with acetone inside as the scrubbing agent. Further after filtration, the non-condensable gas was metered using the drainage gas collection method.

+ 133N + 21028

(2)

The HHV of bio-oil or tar (MJ/kg) was calculated using the following empirical equation from its C, H, and O mass content in percent (excluding sulfur).26 HHV = 0.352C + 0.944H + 0.105(S − O)

(3)

The composition of non-condensable gas was analyzed using a micro gas chromatograph (GC, Agilent 3000A). The chemical compounds in

Figure 1. Schematic diagram of the used fixed-bed torrefaction apparatus. 7276


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was higher than that of gas above 250 °C, indicating the intensive decomposition of not only hemicellulose but also other polymers, such as cellulose and lignin.7 Mass balance at different temperatures was calculated to validate the reliability of experimental data. The sum of product yields at different temperatures was all above 96%, because volatile is hard to recover completely and traces of aerosols should also be trapped by pipes and dryer during torrefaction. 3.2. Characterization of Torrefied Biomass. Table 3 shows the results of proximate and elemental analyses for untreated and torrefied biomass at different temperatures. For elemental analyses, the oxygen content was also determined using an elemental analyzer instead of by difference to obtain more accurate data. Nevertheless, not all elements in biomass could be detected using an elemental analyzer, such as chlorine, phosphorus, etc. Thus, the elemental analysis data were not in total of 100 wt %. As shown in Table 3, the contents of volatile and oxygen in the torrefied biomass sharply decreased with raising the temperature. This trend was reverse for the contents of fixed carbon, ash, and carbon. The hydrogen content decreased from 8.10 to 5.57 wt % corresponding to the temperature rise from 200 to 325 °C. The nitrogen and sulfur contents in the resulting char slightly increased with raising the temperature, but these elements were still at trace levels. Hence, the torrefaction temperature strongly affects the torrefied biomass composition. Moreover, the change in composition was sharpest in 250−300 °C. It is a consequence of the extensive removal of hydrogen and oxygen from biomass in the form of volatile, while some carbon is also released with torrefaction. Thus, burning the torrefied biomass would produce few organic compounds and less smoking,4,28 making the torrefied biomass more suitable to be a kind of clean fuel than raw biomass. Figure 2 shows the HHV of untreated and torrefied biomass at different temperatures. The data include results of measurement and estimation via both the HHV (OLS) and HHV (PLS) models. As a result of devolatilization, the HHV of torrefied biomass significantly increased with raising the temperature, which reached 26.74 MJ/kg at the tested highest temperature and was about 50% higher than that of raw biomass (18.55 MJ/kg). The HHV (OLS) and HHV (PLS) models well predicted the heating value of the torrefied biomass. The variation of HHV with the temperature was contributed to the increase in the carbon content of torrefied biomass, as a result from the removal of oxygen via torrefaction. On the other hand, from an energy density perspective, the energy in the C−C bond is higher than those in C−O and C− H bonds.8 The element analysis shows that the devolatilization by torrefaction considerably removed oxygen from biomass,

bio-oil or tar were identified using a gas chromatography−mass spectrometry (GC−MS) spectrometer (QP 2010 Ultra). The detector and injector temperature of the GC was 290 °C, and the column heating was first to 50 °C in 5 min, in succession to 280 °C at 6 °C/ min, and holding at 280 °C for 10 min. The mass spectrometer had a solvent delay time of 2.5 min and worked with an electron impact ion source temperature of 200 °C and a scanning range of m/z 35−500. Limited by GC, the adopted GC−MS analyzed only the tar factions with boiling points below 300 °C. X-ray photoelectron spectroscopy (XPS) spectra of untreated and torrefied biomass were obtained with Thermo ESCALAB 250Xi (Thermo Fisher Scientific) equipped with a monochromatic Al Kα Xray source (1486.6 eV). It was operated at 25 W in combination of the electron flood gun and ion bombarding for charge compensation. The XPS survey scan spectra were recorded in a pass energy of 100 eV and a binding energy range of 1200−0 eV by a step of 1 eV. Highresolution scanning of the C 1s region was performed in a step of 0.05 eV with a pass energy of 20 eV. Peak deconvolution of C1s spectra was accomplished using the XPSPEAK software. For this measurement, untreated and torrefied biomass was milled with ball mill to obtain the sample particle size below 75 μm and the XPS spectra were taken with the ground sample.27

3. RESULTS AND DISCUSSION 3.1. Overall Characteristics. Table 2 shows product yield distribution and mass balance of torrefaction at different Table 2. Product Distribution of Torrefaction at Different Temperatures product yield (wt %, dba)

a

temperature (°C)

solid

bio-oil

water

gas

mass balance (wt %, db)

200 225 250 275 300 325

89.66 84.98 75.37 57.77 45.70 38.56

1.95 3.57 9.44 18.30 24.60 28.58

4.17 6.35 9.37 14.99 17.36 18.24

0.63 1.44 3.10 7.13 9.21 10.69

96.41 96.34 97.28 98.19 96.87 96.07

db = dry basis.

temperatures. An increasing temperature leads to an obvious decrease in the yield of torrefied biomass but increases in the yields of bio-oil, water, and gas. The variation in torrefied biomass yield shows that the loss of the solid yield was about 18 and 12% in 250−275 and 275−300 °C, respectively. These yield losses are higher than those at the other tested temperatures. For the yields of bio-oil, water, and gas, there was the same trend of variation with the temperature. The results should be attributed to the rapid decomposition of hemicellulose at 250−300 °C. In addition, the yield of bio-oil

Table 3. Properties of Untreated and Torrefied Biomass at Different Temperatures proximate analysis (wt %, dba)

a

elemental analysis (wt %, dafb)

temperature (°C)

volatile

fixed carbon

ash

C

O

H

N

S

raw 200 225 250 275 300 325

76.05 73.19 70.96 65.09 53.53 40.35 34.73

18.98 21.47 23.40 28.33 38.04 48.91 53.26

4.97 5.34 5.64 6.58 8.43 10.74 12.01

45.87 47.51 48.78 51.62 56.73 62.26 67.46

43.98 43.68 42.48 39.92 35.47 28.01 25.13

8.10 7.49 7.71 7.39 6.66 5.80 5.57

0.72 0.75 0.75 0.82 0.96 1.05 1.14

0.09 0.18 0.12 0.11 0.11 0.12 0.16

db = dry basis. bdaf = dry and ash-free basis. 7277


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and liquid through devolatilization in torrefaction, thus greatly improving the fuel quality. 3.3. Oxygen in Torrefaction Products. 3.3.1. Gas Product. Figure 4 shows the yield and composition of gas

Figure 2. HHV of untreated and torrefied biomass at different temperatures.

thus increasing the carbon content in torrefied biomass and its HHV. Figure 3 shows the change in chemical composition as a result of torrefaction through the van Krevelen diagram. It gives

Figure 4. Effect of the torrefaction temperature on the yield and composition of the gas product.

generated in torrefaction at different temperatures. The gas is mainly composed of CO2, CO, and traces of CH4. The content of H2 is negligibly small, while CO2 is the dominant gas, agreeing with many literature reports.30,31 This result implies that oxygen presents in the gas product mainly as CO2 and secondarily as CO. The torrefaction temperature significantly affected the gas yield and composition and the oxygen amount in the gas. At low temperatures, the gas yield is very low and CO2 contributes about 80−90% (v/v) of the total gas production. The yields of CO2 and CO obviously increase at temperatures over 275 °C, and the CO2 fraction in the gas is reduced to about 60% (v/v) as a result of the quick increase in CO generation. Increasing the torrefaction temperature causes more oxygen to be transferred into the gas product from biomass. In torrefaction of biomass, CO and CO2 mainly come from the cleavage of hemicellulose and cellulose via the decarbonylation and decarboxylation reactions, respectively.32 Increasing the temperature facilitates the decomposition of hemicellulose and cellulose to increase the CO and CO2 yields. The variation trend in the CO and CO2 yields also corresponds to the trend of biomass mass loss. Nonetheless, there is no CH4 in the gas product until the temperature is over 275 °C, but the CH4 yield was always below 0.1% at the tested temperatures. These show that CH4 is generated at certainly higher temperatures than CO2 and CO, complying with the result that, in pyrolysis, CO2 is the first gas product.33−35 3.3.2. Liquid Product. In torrefaction, oxygen transfers into the liquid product from biomass in forms of H2O (water) and oxygen-containing bio-oil compounds. The amount of oxygen in H2O was calculated by weighing, and the type and amount of oxygen in bio-oil were analyzed using GC−MS and elemental analysis, respectively. Figure 5 compares the GC−MS analysis results for bio-oils from torrefaction at different temperatures. In the figure, the marked part I refers to alcohols, aldehydes, ketones, acids, and furans, part II refers to aromatics, including guaiacols and phenols, and part III refers to fatty acids, fatty alcohols, and

Figure 3. van Krevelen plot of torrefied biomass at different torrefaction temperatures.

the variation with the temperature in H/C and O/C atomic ratios of the torrefied biomass. The plotted data include those for the typical solid fuels, including anthracite, bituminous coal, lignite, and peat,29 in addition to the tested torrefied biomass from torrefaction. The biomass fuels contain more hydrogen and oxygen than that of all coals. There is an obvious decrease in the atomic O/C and H/C ratios when raising the torrefaction temperature. This means that biomass lost more oxygen and hydrogen in torrefaction, so that carbon was enriched in the torrefied biomass. Consequently, the torrefied biomass has condensed energy to be more like coal. When the torrefaction temperature is above 250 °C, the composition of torrefied biomass is similar to lignite and peat. Essentially, this indicates the transfer of oxygen existing in raw biomass into gas 7278


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in lignin,37 while the others are formed via decomposition of holocellulose. The species of furans, ketones, and aldehydes are from the ring-opening and rearrangement reactions of pyranose rings in holocellulose.38,39 The acid is almost acetic acid formed via either the cleavage of the acetyl group or the thermal decomposition of intermediates, such as hydroxyacetaldehyde and hydroxymethylfurfural.40 In this study, traces of sugars were observed in bio-oil. It is most likely due to the ash catalysis of sugars into small-molecular compounds, such as acids, water, and gas.41 Besides, the interaction between hemicelluloses and cellulose showed a negative effect on the generation of sugars but a positive effect on the generation of furans.42 The torrefaction temperature played a significant role on biooil composition and oxygen transfer into bio-oil. Guaiacols in bio-oil ranged from 0.8 to 41.37% (area percentage) and showed a soaring trend as the temperature increased. They reached the highest percent (area) at the torrefaction temperature above 250 °C, suggesting accelerated decomposition of lignin at this temperature. The amount of phenols was relatively stable when the temperature was above 225 °C, and the content of acids increased at first and then decreased with the rise of the temperature. The amount of furans remained at about 16% (area percent) at temperatures below 300 °C, but it tended to decrease at temperatures above 300 °C. The content of ketones and aldehydes increased with increasing temperature. Consequently, oxygen transferred into bio-oil from biomass in forms of acids, furans, ketones, and aldehydes at a low temperature, but more oxygen migrated into guaiacols at higher temperatures. Table 4 shows the result of elemental analysis for bio-oil from torrefaction at different temperatures. The elemental composition of bio-oils from different torrefaction temperatures is similar, and all oils are composed of C, H, O, and traces of N and S. The carbon content is relatively high because of plant wax and biomass extracts contained in bio-oil. It then gradually decreased when gradually more oxygen-containing compounds were generated at higher temperatures, including aldehydes, ketones, acids, and furans. On the other hand, aromatics are generated in large amounts from lignin decomposition, which caused the increase in bio-oil carbon content again at temperatures above 250 °C. The same trend of variation with the temperature was observed for the O/C ratio, whereas the oxygen content varies in the opposite trend. The higher oxygen content in oil causes its lower heating value, which is just half of that of diesel (46.04 MJ/kg). The lowest HHV is 19.97 MJ/kg at 250 °C of torrefaction, corresponding to the highest O/C ratio, lowest carbon content, and highest oxygen content. Thus, the torrefaction at this temperature greatly converted oxygen in biomass to bio-oil with less carbon loss than at the other temperatures. 3.3.3. Solid Product. Oxygen exists in biomass as oxygencontaining functional groups, and XPS was used to characterize the torrefied biomass at different temperatures. Figure 7 shows the C1s spectra, and all of them were further deconvoluted to represent different O-containing functional groups or carbons. According to Dorris and Gray,43 four types of C named CI, CII, CIII, and CIV can be categorized as aromatic/aliphatic carbon (C−C, CC, and C−H), ether or hydroxyl/phenol groups (C−OR), carboxyl carbon (CO and O−C−O), and carboxylic group/ester (−COO). The binding energy of CI is 284.6 eV, which may shift slightly as a result of the effect of neighboring atoms.44 Because of bonding between carbon and oxygen, the binding energy of C1s increases with raising the

Figure 5. GC−MS analysis of bio-oil generated at different torrefaction temperatures.

fused ring compounds. The last contains a large number of carbon, for example, C15H26O, C18H36O2, C20H40O, etc., which likely came from plant wax or extracts of biomass.36 At low temperatures, the bio-oil mainly consists of parts I and III to have little aromatics. With raising the temperature, part II gradually become the biggest amount. Therefore, the fraction of heavy oil increases and that of light oil decreases with raising the temperature. Figure 6 shows the major chemical species of bio-oil from torrefaction at different temperatures. Nonetheless, part III is ignored for its low amount. The oxygen-containing compounds include acids, furans, ketones, aldehydes, phenols, and guaiacols. Among them, the phenols and guaiacols are produced by cleavage of the weak α-ether and β-ether bonds

Figure 6. Major chemical species in bio-oil from torrefaction at different temperatures. 7279


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Energy & Fuels Table 4. Elemental Analysis of Bio-oil from Torrefaction at Different Temperatures elemental analysis (wt %) temperature (°C)

C

O

H

N

S

O/C

H/C

HHV (MJ/kg)

200 225 250 275 300 325

55.24 56.38 51.76 52.99 53.93 55.29

34.64 34.99 38.58 37.38 36.62 35.02

6.10 6.34 6.06 6.29 6.71 7.15

1.43 1.30 1.54 1.45 1.54 1.45

0.87 0.49 0.78 0.65 0.62 0.48

0.63 0.62 0.75 0.71 0.68 0.63

0.11 0.11 0.12 0.12 0.12 0.13

21.66 22.21 19.97 20.73 21.54 22.58

hydroxyl and ether linkage. The carbon CI shown by its RP varies in 52.23−75.39% for the torrefied biomass with raising the torrefaction temperature from 200 to 325 °C, while the RPs for CII and CIII both decreased with increasing the temperature. There was little variation in the RP for CIV. All of these show in fact the removal of oxygen (reducing CII and CIII) and enrichment of carbon (raising CI) with torrefaction, so that the torrefied biomass prepared at the higher temperature is more coal-like. Figure 8 shows the monomer structural unit of xylan (the main composition of hemicellulose), cellulose, and lignin. While the pyranose ring is the skeleton structure of hemicellulose and cellulose, the aromatic compound is the major composition of lignin. Hence, almost all carbon in hemicellulose and cellulose is bonded to oxygen, so that CI contributes mainly to lignin. Against this, CII contributes to ether and hydroxyl groups in hemicellulose, cellulose, and lignin. Because cellulose and hemicellulose are both polymers linked by pyranoses through glucoside keys, oxygen exists in biomass mainly in forms of hydroxyl and ether linkages. It is also evident that the proportion of ether and hydroxyl/phenol groups (CII) is much higher than that of CIII and CIV. In Table 5, the RP of CII obviously decreased with raising the torrefaction temperature until 275 °C, especially in 250−275 °C, which indicates the removal of hydroxyl and ether linkages. The proportion of CII varied little at temperatures over 275 °C. The C−OH bond in biomass presents as hydroxyl in holocellulose and phenolic hydroxyl in lignin. Below 275 °C, the yield of water is higher than that of bio-oil (Table 2), indicating that dehydration takes the leading role in removing oxygen at low temperatures. Some hydroxyl groups in biomass would also be transferred into alcohols and acids. Over 275 °C, the yield of bio-oil far exceeded water (Table 2) and guaiacols became the main composition of bio-oil (Figure 6) as a result of lignin decomposition. This shows that the phenolic hydroxyl groups begin to be removed and most of them are transfer into bio-oil. The removal of hydroxyl groups still occurs to form water. Generally, the C−O−C bond exists

Figure 7. C1s spectra of XPS analysis for untreated and torrefied biomass.

number of oxygen bonded to carbon. Therefore, the binding energies of CII, CIII, and CIV are higher than CI, and their energy difference with CI is 1.5, 2.9, and 4.2 eV, respectively. Table 5 shows the results from deconvoluting C1s, including the binding energy and relative proportion for each functional group. For raw biomass, the main groups are aliphatic/aromatic carbon (CI) and ether or hydroxyl/phenol groups (CII) with their relative proportions (RPs) of 52.23 and 37.40%, respectively. The RPs of CIII and CIV are small, just 6.94 and 3.42%, respectively. It is clear that raw biomass contains abundant oxygen-containing functional groups, especially

Table 5. Carbon Functionalities in Untreated and Torrefied Biomass from XPS Analysis CI

a

a

CII b

CIII

CIV

temperature (°C)

BE (eV)

RP (%)

BE (eV)

RP (%)

BE (eV)

RP (%)

BE (eV)

RP (%)

raw 200 225 250 275 300 325

284.62 284.72 284.73 284.75 284.76 284.76 284.80

52.23 55.07 57.89 63.51 72.51 74.50 75.39

286.19 286.26 286.27 286.30 286.31 286.25 286.32

37.40 34.67 31.03 28.05 20.70 20.12 19.72

287.75 287.84 287.60 287.98 287.80 287.91 287.81

6.94 5.99 6.82 5.34 1.49 0.59 0.82

288.80 288.84 288.80 288.96 288.83 288.91 288.95

3.42 4.28 4.27 3.09 5.30 4.79 4.07

BE = binding energy. bRP = relative proportion (area percent) of each functional group. 7280


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Figure 8. Monomer structural unit of (a) xylan, (b) cellulose, and (c) lignin.

Figure 9. Distributions of (a) oxygen and (b) carbon in torrefaction products.

in α-ether/β-ether bonds in lignin and the ring hemiacetal bond in the pyranose ring. During torrefaction, the pyranose ring is decomposed to furans, ketones, and aldehydes by the ringopening reaction39 and α-ether/β-ether bonds in lignin are broken into phenolic hydroxyl groups. As seen from the main chemical composition distribution of bio-oil in Figure 6, the former is dominant at low temperatures and is exceeded by the later at higher temperatures. The RP of CIII (CO and O−C−O) decreases with an increasing temperature. At 275 °C, the proportion reduced dramatically from 5.34 to 1.49%, and CIII almost disappeared from torrefied biomass at rather higher temperatures. The C O bond exists in aldehyde groups and can partly decompose into CO via decarbonylation, while the O−C−O bond with the pyranose ring usually breaks in torrefaction as a result of the high-reactivity bond between C-1 and O-6.45 The amount of carboxylic group/ester (CIV) remains to have a slight change in torrefaction. It seems that the carboxylic groups exist in the form of carboxylates by interacting with ash, which is more thermally stable than free carboxylic groups. 3.4. Oxygen and Carbon Migration. Figure 9a shows the allocation of oxygen in torrefied biomass, water, bio-oil, and gas produced by torrefaction at different temperatures. The sum of oxygen in all products is above 97%, proving the accuracy of the experiment. The proportion of oxygen in torrefied biomass dramatically decreased from 86.21 to 19.82% with raising the temperature from 200 to 325 °C. This high sensitivity to the temperature appeared especially at 250, 275, and 300 °C. The removed oxygen from solid biomass transferred into liquid and gas products in forms of H2O, CO2, CO, and oxygencontaining liquid compounds. Of them, oxygen migrated into

H2O takes the dominant part, and bio-oil and gas follow it in succession. The temperature plays a crucial role in oxygen transfer. At low temperatures, oxygen is removed mainly by producing H2O with traces of oxygen into bio-oil. With the higher temperature, more oxygen in biomass migrates into biooil and gas, and dehydration is the dominant reaction at the tested temperatures. Figure 9b shows carbon allocation in torrefied biomass, biooil, and gas, and the sum of carbon is all above 95%. The carbon dominantly exists in torrefied biomass, despite the fact that the proportion of carbon in torrefied biomass continuously decreased from 92.50 to 52.51% as the temperature elevated from 200 to 325 °C. This means that about half of the carbon in biomass is lost by torrefaction at the tested highest temperature. Surely, carbon removed from biomass mainly migrated to bio-oil and then to gas. The energy of biomass is attributed mainly to carbon, noting that the hydrogen content is very low. The removal of carbon means the loss of energy from the solid fuel, which becomes more obvious at the higher temperature. Thus, the energy yield with torrefied biomass decreased with the increase in the torrefaction temperature. In torrefaction, hemicellulose, cellulose, and lignin in biomass undergo progressive decomposition. On the basis of the element composition change in biomass, torrefaction refers to a process including drying and deoxidation combining with carbon and hydrogen migration. Deoxidation significantly changes the physical and chemical properties of biomass. For example, it transforms biomass from hydrophilic to hydrophobic38 and improves the stability as well as HHV of biomass. However, carbon migration causes a big part of energy in biomass to be transferred to bio-oil and torrefaction gas. To 7281


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Energy & Fuels

more coal-like and hydrophobic with a high carbon content and low oxygen content. In addition, the torrefaction temperature had a significant effect on oxygen migration, both quantity and approach. At a low temperature, oxygen was removed by producing H2O with traces of oxygen migrating to bio-oil. Within the tested temperatures (until 325 °C), dehydration dominated the ways of deoxidation, even though a growing number of oxygen in biomass migrated to bio-oil and gas at higher temperatures. Carbon migration coupled with oxygen migration led to energy loss and a decrease in the energy yield as a result of carbon loss. From the aspect of maximizing oxygen removal but minimizing carbon loss, 250 °C was suggested to be the optimal temperature for torrefaction of EAS based on the results of this study.

make torrefied biomass like coal in possibly the biggest degree and, meanwhile, avoid the excessive energy loss from the solid fuel, the torrefaction conditions should be optimized by simultaneously considering the deoxidation and carbon or energy loss. Figure 10 correlates the carbon and oxygen removals from biomass at different torrefaction temperatures. The dotted line

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: 86-10-8254886. E-mail: [email protected]. *Telephone/Fax: 86-10-8254886. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to financial support of the Natural Science Foundation of China (21306209), the National Basic Research Program of China (2014CB744303), and the Major Program of National Natural Science Foundation of China (U1302273).

Figure 10. Correlation of carbon and oxygen removals at different torrefaction temperatures.

represents the ratio of removed oxygen over removed carbon, which indicates the specific oxygen removal per percent of carbon loss from biomass. Usually, the higher oxygen removal corresponds to the higher carbon removal. The deoxidation of biomass is more obvious at 250, 275, and 300 °C than at the other temperatures. Especially at 275 °C, more than 22.73% oxygen was removed form biomass than at 250 °C, but carbon loss is also 14.53% more. It is clear that there is less carbon or energy loss if the specific oxygen removal ratio is higher. In fact, the removal of O and C from biomass is similar between 200 and 250 °C, but then the specific oxygen removal becomes smaller at rather higher temperatures. Thus, the torrefaction at 200, 225, and 250 °C should be more effective from the aspect of maximizing O removal but, meanwhile, minimizing the carbon loss. On the other hand, more than 80% of oxygen is still retained in the solid product at 200 and 225 °C, and the improvement on fuel characteristics is unconspicuous. Consequently, 250 °C seems to be the optimal torrefaction temperature for the tested biomass (EAS).

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REFERENCES

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4. CONCLUSION Torrefaction of EAS at temperatures of 200−325 °C and residence time of 30 min was performed in a fixed-bed reactor. Oxygen removal caused by devolatilization during torrefaction greatly improved the fuel properties of biomass. The oxygen in torrefaction products was characterized to study the quantity and approach of oxygen migration. In gas products, oxygen existed as CO2 and CO, and in the liquid product (called biooil), it existed in forms of H2O and oxygen-containing compounds, such as acids, alcohols, aldehydes, ketones, furans, guaiacols, phenols, and extracts. The oxygen in the solid product or torrefied biomass presented as oxygen-containing functional groups, among which the proportion of C−OH and C−O−C obviously decreased with the increase in the torrefaction temperature. After torrefaction, biomass become 7282


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Oxygen Migration in Torrefaction of Eupatorium adenophorum Spreng

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