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Effect of pressurized torrefaction pretreatments on biomass CO2 gasification Li Xiao, Xianqing Zhu, Xian LI, Zong Zhang, Ryuichi Ashida, Kouichi Miura, Guangqian Luo, Wenqiang Liu, and Hong Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01485 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

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Effect of pressurized torrefaction pretreatments on biomass CO2 gasification Li Xiaoa, Xianqing Zhua, Xian Lia,*, Zong Zhanga, Ryuichi Ashidab, Kouichi Miurac, Guangqian Luoa, Wenqiang Liua, and Hong Yaoa,* a

State Key Laboratory of Coal Combustion, Huazhong University of Science and

Technology, Wuhan 430074, China; b

Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan;

c

Institute of Advanced Energy, Kyoto University, Kyoto 615-8510, Japan

*Correspondence:

Xian Li State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China Tel: +86-27-87545526

Fax: +86-27-87545526

Email: [email protected] Hong Yao State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China Tel: +86-27-87545526

Fax: +86-27-87545526

Email: [email protected]

Manuscript submitted to Energy & Fuels

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Abstract It is well known that too much tar formed is one of the most urgent problems for the biomass gasification process. In our previous work, it was found the pressurized torrefaction can effectively improve the fuel property of the biomass. In this paper, the effect of torrefaction pretreatments under atmospheric pressure (AP), gas pressure (GP), and mechanical pressure (MP) at 200 ~ 300 oC on the biomass gasification were studied. It was found that the O/C value and higher heating value of the GP semi-char were respectively as low as 0.27 and as high as 26.2 MJ/kg. The fixed carbon content of the GP semi-char was higher than those of the other two semi-chars. The GP torrefaction significantly reduced the tar formation during the biomass gasification compared with AP and MP torrefactions. Also, the H2 and CO yields of GP semi-char gasification were much higher than those of AP and MP semi-chars and raw biomass. Thus, the GP torrefaction was the most effective method for the gasification tar reduction and gasification behavior improvement of the biomass among the three torrefaction methods studied in this work.


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1. Introduction With the growth of emerging economies and increasing population, the continued use of fossil fuels has resulted in a fast increase in global energy consumption. Due to the seriously associated environmental issues and the dwindling fossil sources, it has to explore and utilize alternatives such as renewable resources. As an alternative carbon source, biomass offers some more attractive benefits over fossils: biomass is the only source of abundant, concentrated source of non-fossil carbon which is available on earth. Due to the structural and chemical complexity of biomass, various advanced processes have been employed. Gasification has appeared as one of the most promising technologies, because it sufficiently expands the advantages of biomass, such as low ash content, low sulfur content, and low nitrogen content. However, it is well known that too much tar formed during the gasification is one of the most urgent problems for the biomass gasification process. Because the tar, condensing at low temperature (lower than its dew point), can result in blockage of the pipelines and fouling of downstream equipments.1 Besides, the tar in the producer gas, which ranges from 0.5 g/Nm3 to 100 g/Nm3, must be removed before utilizing the producer gas.2, 3 This results in significant energy waste. Various methods have been developed for the tar removal during the biomass gasification, such as thermal cracking method and in-situ catalytic cracking method during the gasification process, and mechanical method and downstream cracking method after the gasification process, as reviewed by Devi et al.4 These methods are to remove the tar after it formed. There are some disadvantages for the tar removal methods mentioned above, such as reduction of the heating value of the producer gas, increment of operating cost, deactivation of catalysts, requirement of complex process, etc.5-8


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Most of the tar is formed at the initial stage of biomass gasification from volatile matter in the biomass.9, 10 If this part of volatile matter was removed before the gasification, the tar formation can be suppressed effectively. Torrefaction is one of the most effective technologies for the volatile matter removal of biomass, which treats the biomass at around 200 ~ 300 oC under inert atmosphere.11,

12

The volatile matter and oxygen-containing

functional groups in the raw biomass can be removed effectively through dehydration, decarboxylation,

and

decomposition

of

hemicellulose

and

cellulose

during

the

torrefaction.13-17 It is the so-called biomass upgrading. Most studies on torrefactions were conducted under atmospheric pressure. Several researchers also studied the effects of pressure on the biomass torrefaction or pyrolysis. Wannapeera and Worasuwannarak18 studied the influence of torrefaction pressure on the leucaena upgrading at 200 ~ 250 oC. They found that the tar yield of the torrefied leucaena pyrolysis at 800 oC decreased with the increase of torrefaction pressure, which was due to the promotion of cross-linking reactions during the torrefaction. Besides, the yield, higher heating value (HHV), and combustion rate of torrefied leucaena char increased with the gas pressure rising from 0.1 MPa to 4 MPa at the same temperature, indicating the fuel quality of leucaena was improved by the torrefaction under gas pressure. Mahinpey et al.19 performed the pyrolysis of wheat straw in a tubular reactor at 500 oC under different pressures (0.069 ~ 0.276 MPa). They found more liquid products were formed with the increase of pressure. Also, the compositions of the tar and the structure of biochars changed significantly. Basile et al.20 reported that the heat consumed during the pyrolysis of biomass reduced when the gas pressure increased from 0.1 MPa to 4 MPa. They concluded that the biomass pyrolysis


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reactions might change from endothermic to exothermic with the rising of pyrolysis pressure. In our previous work, we pretreated the biomass waste by a “hot press” under 10 MPa of mechanical pressure at the temperature below 300 oC. The pretreated biomass was then carbonized at 900 oC.21, 22 It was found that the mechanical pressure promoted the oxygen removal and the cross-linking reactions of –OH group. Furthermore, the tar yield was significantly reduced, and gaseous products (such as CO and H2) were enhanced during the carbonization. Also, the yield of pretreated biomass char obtained by the carbonization at 900 o

C was almost 4 times of the raw biomass char yield. So, the biomass torrefaction under pressure may have advantages for the tar reduction and

improvement of fuel property for biomass gasification, compared with the biomass torrefaction under atmospheric pressure. However, there was rather few research on the gasification of the biomass torrefied under pressure. In this paper, the biomass was torrefied under gas pressure and mechanical pressure at the temperature ranging from 200 oC to 300 oC to prepare semi-chars. The fuel properties and gasification behaviors of the semi-chars, and the tar formation during the gasification were investigated in detail. The gasification of the semi-char obtained by the torrefaction under atmospheric pressure was also performed for the comparison purpose. Besides, the mechanism of the suppression of tar formation by the pressurized torrefactions was investigated preliminarily.

2. Experimental Section 2.1. Raw material A rice straw (RS), used as the raw biomass sample in this study, was collected from South


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China village. The raw RS, ground and sieved to 90 ~ 212 µm, was dried at 105 oC for more than 5h before the experiment. Table 1 shows the ash composition of RS, which was determined by X-ray fluorescence (XRF, EDAX, EAGLE III). 2.2. Torrefaction The torrefactions of the RS were performed under atmospheric pressure, N2 pressurized reactor, and mechanically pressurized reactor to prepare semi-chars. The torrefaction under atmospheric pressure were carried out using a fixed bed reactor, as shown in Figure 1(a). A self-designed horizontal quartz tube (80 cm in length and 5 cm in inner diameter) was used as the fixed bed reactor. About 2.0 g of the dried RS were placed in the middle of the reactor at room temperature. The sample was then heated to the torrefaction temperature (200 oC, 250 oC or 300 oC) at the heating rate of 10 oC/min. During the torrefaction 0.2 L/min of high-purity nitrogen were introduced into the reactor continuously to ensure the inert atmosphere. After holding for 15 min at the torrefaction temperature, the sample was pulled to the water-cooled zone for quickly cooling. It was cooled below 200 oC in less than 10 seconds. The semi-char obtained was weighted at room temperature. The non-condensable gaseous products were collected by a gas bag. The weights of the gaseous products were calculated according to their concentrations and the volume determined by the accumulative flow meter. The weight of liquid products, including H2O and condensable organics, were calculated by difference. The torrefaction under gas pressure were performed in a batch autoclave reactor (500 cm3 in volume), as shown in Figure 1(b). About 15 g of the dried RS were placed in the autoclave, which was then purged sufficiently and charged by N2 of 2.5 MPa at room temperature. The


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sealed autoclave was heated to the torrefaction temperature (200 oC, 250 oC or 300 oC) at the heating rate of 10 oC/min. The pressure at the torrefaction temperature was around 5.0 MPa. After held for 15 min at the torrefaction temperature, the autoclave was taken out from the furnace and immersed in water bath in order to be cooled below 200 oC in less than 30 seconds to stop the reactions. The weights of the semi-char, liquid, and gaseous products were determined by the same method as described above. The torrefaction under mechanical pressure were performed using a “hot press” reactor, as shown in Figure 1(c). The stainless steel reactor was placed in a vertical quartz tube. About 2.0 g of the dried RS, loaded between the two molds, was heated by an infrared image furnace (Shinku-Riko, RHL-P610P) to torrefaction temperature (200 oC, 240 oC or 280 oC) at the heating rate of 10 oC/min, where it was held for 15 min. N2 was introduced into the tube continuously during the experiment to ensure the inert atmosphere. The mechanical pressure on the molds was fixed at 10 MPa during the torrefaction. After the holding time (15 min), the reactor was taken out from the furnace and blown by electric fan in order to be cooled below 200 oC in less than 60 seconds. The semi-char yields were obtained by weighting the solid samples after the torrefaction. The yields of the liquid products were almost zero and ignored in this work. The yields of gas products were calculated by 100% − semi-char yield. The cylindrical semi-chars were ground and sieved to particles (< 212 µm) for the gasification tests. The torrefaction under mechanical pressure at 240 oC and 260 oC were performed in our laboratory. It was found that the results obtained at the two temperature points were rather similar to each other. So, the results obtained under 240 oC were used for the comparison


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with the atmospheric pressure and gas pressure torrefactions at 250 oC in this work. The effect of the temperature difference of 240 oC and 250 oC was ignored. The torrefaction under mechanical pressure under 300 oC was also tried. However, the rice straw turned to soft and overflowed from the molds. The torrefaction under mechanical pressure at the temperature of 280 oC was succeeded. In this paper, AP, GP, and MP represent the semi-char obtained at atmospheric pressure by fixed bed reactor, gas pressure by batch autoclave reactor, and mechanical pressure by “hot press” reactor, respectively. LT, MT, and ST represent the torrefaction at low temperature (200 oC), middle temperature (240/250 oC), and severe temperature (280/300 o

C), respectively. For example, “LT-AP” was short for the semi-char prepared at 200 oC by

the fixed bed reactor. 2.3. CO2 gasification CO2 gasification of raw RS and semi-chars were performed by the fixed bed reactor, as shown in the Figure 1(a). The quartz crucible containing about 0.3 g of sample was rapidly pushed to the middle of the reactor, after the temperature of reactor reached the desired temperature of 900 oC. The holding time was 20 min to ensure complete gasification of the sample. During the gasification process, 0.25 L/min of high-purity carbon dioxide and 0.25 L/min of high-purity nitrogen were introduced into the reactor continuously. The weights of the main gaseous products were calculated by their concentrations determined by a gas chromatograph and the volume of the gaseous products. The liquid products, including water and tar, were collected by a condenser (two liquid nitrogen-cooled U-shaped glass tubes). The weights of the liquid products were determined by weighting the condensers before and


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after the experiment at room temperature. After the gasification, the condensers were washed carefully with dichloromethane (CH2Cl2) for several times to collect the liquid products. 2.4. Product analyses The elemental compositions of raw RS and solid products were determined by an elemental analyzer (Vario, CHN EL-2). The chemical structures and the surface functional groups of raw RS and solid products were estimated by Fourier transform infrared (FTIR) spectrometer (Bruker, Vertex 70). The samples were mixed with KBr powder at the same ratio (1:100) and ground for FTIR spectroscopy analyses. The IR spectra ranging from 4000 to 400 cm-1 were measured with 64 scans and a resolution of 4 cm-1. The gaseous products obtained during torrefactions and CO2 gasifications were analyzed by a gas chromatograph (GC, Agilent 3000A) to determine the concentrations of the main components. The tar was analyzed by gas chromatography/mass spectrometry (GC/MS, Agilent 7890A/5975C). High-purity helium (1.0 ml/min) was used as carried gas and the split ratio was 10:1. The GC separation was conducted by using a HP-5MS capillary column. The injection volume was 1 µL, and the injector temperature was 250 oC. The GC oven temperature was programmed from 40 oC (holding 2min) to 180 oC (holding 2min) at the heating rate of 5 oC/min, then to 300 oC (holding 10min) at the heating rate of 20 oC/min. The mass spectrometer was operated in EI mode (70 eV), and scanned per 0.5 second within the electron range of (m/z) 30-500 amu. Identification of chromatographic peaks was completed referring to the NIST MS library.

3. Results and Discussion


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3.1. Product distributions of biomass torrefactions Temperature is one of the most important parameters of the biomass torrefactions. Figure 2(a) ~ (c) shows the product distributions of RS torrefactions at 200 ~ 300 oC. The yields of semi-chars decreased with the increase of torrefaction temperature. The yields of liquid and gas increased with the increase of torrefaction temperature for all of the three torrefactions. The effect of temperature on AP torrefaction was most significant. The yield of LT-AP semi-char (88.0%) was almost twice of that of ST-AP semi-char (44.8%). The effect of temperature on GP torrefaction was relatively smaller. The LT-GP semi-char yield and ST-GP semi-char yield were 51.3% and 35.2%, respectively. The difference between GP semi-char yield and AP semi-char yield was decreased with torrefaction temperature increase. At the same torrefaction temperature, the MP semi-char yield was the highest, and the GP semi-char yield was the lowest. It was found the gas pressure promoted the formation of liquid and gaseous products during the biomass torrefaction.19 It should be due to the slight hydrothermal liquefaction or slight gasification reactions occurred subsequently between the biomass and H2O or CO2 which formed by the biomass torrefaction.23 Because the H2O and CO2 formed were remained in the sealed reactor during the GP torrefaction. Furthermore, the pressure can promote the reactions between the biomass and H2O or CO2 in the reactor. On the contrary, the volatile matter (including H2O and CO2) formed were released quickly during the AP and MP torrefactions. These should be the reasons for the lower GP semi-char yield. Moreover, it is interesting to find that the product distribution of MT-GP torrefaction was almost same as that of ST-AP torrefaction, indicating that the gas pressure could reduce the torrefaction temperature.


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3.2. Carbon and oxygen distributions in products of biomass torrefaction High oxygen content causes low fuel properties of biomass, such as low energy density, self-ignition tendency, high hydroscopicity, and so on.24-26 These are the main problems for the practical utilization of biomass as fuel. So, the oxygen removal is important for the fuel property improvement of biomass. It is also one of the main purposes of biomass torrefaction. Carbon and oxygen distributions of the torrefaction products were calculated according to the yields and elemental compositions of the products. The results are shown in Figure 2(d) ~ (i). The carbon and oxygen distributions to the semi-chars decreased with the increase of the torrefaction temperature. Although more than 90% of carbon was remained in the semi-chars obtained by MP torrefactions at the three temperatures, only 10.9%, 25.1%, and 46.5% of oxygen were respectively removed, much less than the oxygen removal of AP and GP torrefactions at the same temperature. These results suggest that the mechanical pressure torrefaction was not an effective method for the oxygen removal of biomass. Most of the oxygen in the biomass was removed as gaseous or liquid products by AP and GP torrefactions. At the same torrefaction temperature, the carbons remained in the GP semi-chars were less than AP semi-chars. But as high as 66.5%, 77.9%, and 85.1% of oxygen was removed during by GP torrefactions at LT, MT, and ST, much higher than those of AP torrefactions at the same temperature. These results indicate that the gas pressure torrefaction was most effective for oxygen removal of the biomass among the three torrefaction methods. 3.3. Fuel properties of semi-chars Table 2 shows the proximate analyses, ultimate analyses, and higher heating values (HHVs) of the semi-chars and RS. The HHV was calculated by the Dulong equation:27


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HHV=0.3381C+1.4418H-0.1802O

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(1)

where C, H, and O represent the percentage of carbon content, hydrogen content, and oxygen content (on dry and ash-free basis), respectively. The volatile matter contents of the semi-chars decreased significantly with the increase of the torrefaction temperature. On the other hand, the fixed carbon contents increased substantially with the increase of the torrefaction temperature. The volatile matter contents of the GP semi-chars were much lower than those of the AP and MP semi-chars at the same temperature. On the other hand, the fixed carbon contents of the GP semi-chars were almost twice of those of the AP and MP semi-chars. It indicates that the volatile matter removal by gas pressure torrefaction was much more effectively than that by the other two torrefaction methods, especially at lower temperature. The carbon and oxygen contents of the semi-chars were respectively higher and lower than those of the raw RS. With torrefaction temperature increase, the carbon contents of the semi-chars increased and the oxygen contents decreased significantly, resulting in that the HHVs of the semi-chars increased greatly with the temperature increase for each torrefaction. The carbon contents and oxygen contents of the GP semi-chars were significantly higher and lower than those of the AP and MP semi-chars at the same temperature, respectively. It results in the much higher HHVs of the GP semi-char than those of AP and MP semi-chars at the same temperature, especially at the middle temperature (MT). The heat recovery was also calculated according to the yield and HHV of the semi-char by the equation: heat recovery = HHVsemi-char × yieldsemi-char/HHVraw RS. The heat recovery to MT-GP semi-char was as high as 71%, only slightly lower than that to the MT-AP semi-char, although the yield and carbon


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distribution of the MT-GP semi-char were obviously lower than those of MT-AP semi-char, as shown in Figure 2(b). The HHVs of GP semi-chars increased slightly with the temperature rising from 250 oC to 300 oC. But the GP semi-char yields decreased obviously with the temperature rising from 250 oC to 300 oC, as shown in Figure 2(b) ~ (c). It implies that much more energy was lost with the GP torrefaction temperature rising from 250 oC to 300 oC. Therefore, severe torrefaction (300 oC) was unsuitable for gas pressure torrefaction of biomass. The H/C vs. O/C diagram of raw RS and semi-chars are shown in Figure 3. The O/C values of the semi-chars were lower than those of the raw RS, except LT-AP and LT-MP semi-chars. With the increase of torrefaction temperature, the H/C and O/C values of the semi-chars decreased significantly. Furthermore, the O/C values of the GP semi-chars were significantly lower than those of the AP and MP semi-chars at the same temperature. The O/C value of MT-GP semi-char was 0.27, even smaller than the O/C values of ST-AP and ST-MP semi-chars. The results discussed above indicate that the MT-GP torrefaction was the most effective method for volatile matter and oxygen removal, and upgrading of the RS. Therefore, it was focused on the gasification behaviors of the semi-chars obtained at middle temperature by the three torrefactions hereafter. 3.4. FTIR analysis of torrefaction semi-chars The FTIR spectra of raw RS and the semi-chars produced by the three torrefaction methods at middle temperature are compared in Figure 4. It can be observed that the FTIR spectra of MT-AP or MT-MP semi-chars were similar to that of raw RS, indicating that the


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chemical structure of biomass was nearly unchanged during atmospheric pressure and mechanical pressure torrefactions. On the other hand, the wide band centered at 3406 cm-1 attributed to –OH group and peak at 1740 cm-1 attributed to >C=O in the spectrum of MT-GP semi-char were much smaller than those of raw RS, MT-AP or MT-MP semi-chars, indicating that a part of the oxygen-containing functional groups in the biomass was removed by dehydration and decarboxylation reactions during the MT-GP torrefaction. It was consistent with the low oxygen content of the MT-GP semi-char, as shown in the Table 2. It is not enough to investigate the chemical structure difference among the semi-chars only through FTIR. Further characterization of the chemical structure and composition of the semi-chars need to be performed for understanding the mechanism of this process. 3.5. Yields and compositions of tars Reduction of tar is one of the bottlenecks for the biomass gasification. The liquid product (tar and H2O) contents of CO2 gasifications of RS and MT semi-chars at 900 oC are shown in Figure 5. The liquid product yields of semi-char gasifications were lower than that of raw RS gasification. The MT-GP semi-char gasification produced almost only one-quarter (26.8%) liquid products of the raw RS gasification, also much less than those of the MT-AP and MT-MP semi-char gasifications. These results were mainly attributed to the three reasons. Firstly, a part of the volatile matter in the biomass was removed by the torrefactions, especially for GP torrefaction, as shown in table 2. It was found that the higher tar yield of biomass gasification mainly due to the higher volatile matter content of the raw biomass. 9, 10 Secondly, the mineral matter (such as K and Ca) contents in the GP semi-char were much higher than those of the raw RS, as shown in Table 1 and 2. The ash contents of raw RS and


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MT-GP semi-char were 9.6% and 18.7% (on dry basis), respectively. It is well known that some minerals, such as K and Ca, can catalyze the tar cracking reactions during the biomass gasification. Thirdly, a part of volatile matter in the biomass was converted to fixed carbon by cross-linking reactions by the torrefactions, which were accelerated by GP torrefaction (as shown in Figure 6). This also caused the reduction of the volatile matter content in the semi-chars. The harmfulness of tar was not only associated with the content of tar but also the compositions and properties, which greatly influenced the condensation performance.1,

28

Main compositions of the tars from raw RS and MT semi-char CO2 gasifications are shown in table 3. Most of the identified compounds were polycyclic aromatic hydrocarbons (PAHs) with 2-6 rings. Only slight difference between the compositions of MT-GP tar and MT-AP tar was observed. However, obvious difference was found among the compositions of raw RS tar, MT-GP tar, and MT-MP tar. Naphthalene was considered as one of the tar components which were difficult to be decomposed or removed.29 The percentage of naphthalene in raw RS tar was around 10 times of that in semi-char tars, indicating the raw RS tar is more difficult to be removed compared with the semi-char tar. Besides, the heavy PAHs (4-6 rings) content of the MT-GP tar was 56.66%, much less than that of the MT-MP tar which was as high as 79.05%, as shown in Table 3. Too much heavy PAHs would increase the dewpoint of tar, resulting in more tar condensation in downstream equipments.28 So, the MT-GP tar was less harmful than MT-MP tar. 3.6. Yields and compositions of gaseous products Figure 7 shows the yields of main syngas components (H2, CO, CH4, and C2Hn) of CO2


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gasifications of RS and MT semi-chars at 900 oC. The H2 and CO yields of the raw RS, MT-AP semi-char, and MT-MP semi-char gasifications were similar to each other. However, the H2 and CO yields of MT-GP semi-char gasification were as high as 1.6 and 2.2 times of those of the raw RS gasifications. If they were converted to raw biomass basis, the H2 and CO yields were 80.0 ml per gram raw RS (ml/g) and 754.2 ml/g, just slightly smaller to those of the raw biomass gasification which were 113.3 ml/g for H2 and 797.9 ml/g for CO. This can compensate the low semi-char yield of the MT-GP torrefaction, which was only 43.0% as shown in Figure 2(b). The high H2 and CO yields of MT-GP semi-char gasification should be attributed to the high fixed carbon content of the MT-GP semi-char. The fixed carbon content of the MT-GP semi-char was as high as 40.5%, much higher than those of MT-AP semi-char, MT-MP semi-char, and raw RS, even higher than those of ST-AP and ST-MP semi-chars, as shown in Table 2. Furthermore, the fixed carbon contents of the MT semi-chars on raw biomass basis were shown in Figure 6. It shows the fixed carbon content of the MT-GP semi-char on raw biomass basis was higher than that of the raw biomass, indicating that a part of the volatile matter in the raw biomass was converted as fixed carbon by MT-GP torrefaction. The fixed carbon contents of the MT-AP and MT-MP semi-chars on raw biomass basis were similar to that of the raw biomass, indicating that the volatile matter in the biomass was just removed as liquid or gaseous products by MT-AP and MT-MP torrefactions. This shows another advantage of the GP torrefaction, which can convert the volatile matter as fixed carbon in biomass under mild condition.

4. Conclusions


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The effect of torrefactions under atmospheric pressure, gas pressure, and mechanical pressure on biomass CO2 gasification was studied. The O/C value of the gas pressure semi-char obtained at around 250 oC was less than half of those of raw biomass and the semi-chars obtained by the other two torrefaction methods at the same temperature. The fixed carbon contents and HHVs of the gas pressure semi-chars were much higher than those of the other two semi-chars. The gas pressure torrefaction significantly suppressed the tar formation and enhanced H2 and CO yields of the biomass CO2 gasification. Thus, the gas pressure torrefaction was an effective method for the gasification tar reduction and gasification behavior improvement of biomass.

5. Acknowledgments The authors would like to thank the financial support from National Natural Science Foundation of China (21306059), the National Program of International Science and technology Cooperation (2015DFA60410), the National Natural Science Foundation of China (51306063), and the Foundation of State Key Laboratory of Coal Combustion of China. The authors would also like to thank the Analytical and Testing Center of Huazhong University of Science and Technology, for providing the test equipments.

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Energy & Fuels

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(4) Devi, L.; Ptasinski, K. J.; Janssen, F. J. Biomass Bioenerg. 2003, 24, 125-140. (5) Bridgwater, A. V. Fuel 1995, 74, 631-653. (6) Dinkelbach, L. Report ECN: ECN-C-00-084 2000. (7) Gil, J.; Caballero, M. A.; Martín, J. A.; Aznar, M.; Corella, J. Ind. Eng. Chem. Res. 1999, 38, 4226-4235. (8) Hallgren, A. Publishable Final Report (TPS AB), European Commission JOULE III Programme, Project no; JOR3-CT97-0125 1997. (9) Di Blasi, C. Prog. Energy Combust. Sci. 2008, 34, 47-90. (10) Font Palma, C. Appl. Energ. 2013, 111, 129-141. (11) Bridgeman, T. G.; Jones, J. M.; Shield, I.; Williams, P. T. Fuel 2008, 87, 844-856. (12) Uslu, A.; Faaij, A. P. C.; Bergman, P. C. A. Energy 2008, 33, 1206-1223. (13) Chen, W.; Kuo, P. Energy 2011, 36, 803-811. (14) Chen, W.; Peng, J.; Bi, X. T. Renew. Sust. Energ. Rev. 2015, 44, 847-866. (15) Wannapeera, J.; Fungtammasan, B.; Worasuwannarak, N. J. Anal. Appl. Pyrol. 2011, 92, 99-105. (16) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Fuel 2007, 86, 1781-1788. (17) Chew, J. J.; Doshi, V. Renew. Sust. Energ. Rev. 2011, 15, 4212-4222. (18) Wannapeera, J.; Worasuwannarak, N. J. Anal. Appl. Pyrol. 2012, 96, 173-180. (19) Mahinpey, N.; Murugan, P.; Mani, T.; Raina, R. Energ. Fuel 2009, 23, 2736-2742. (20) Basile, L.; Tugnoli, A.; Stramigioli, C.; Cozzani, V. Fuel 2014, 137, 277-284. (21) Miura, K.; Nakagawa, H.; Okamoto, H. Carbon 2000, 38, 119-125. (22) Miura, K.; Nakagawa, H.; Ashida, R.; Sakuramoto, Y.; Nakagawa, K. Abstracts of Papers of the American Chemical Society 2005, 1685-1686. (23) Toor, S. S.; Rosendahl, L.; Rudolf, A. Energy 2011, 36, 2328-2342. (24) Chen, W.; Peng, J.; Bi, X. T. Renew. Sust. Energ. Rev. 2015, 44, 847-866.


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(25) Stelt, M. J. C.; Gerhauser, H.; Kiel, J. H. A.; Ptasinski, K. J. Biomass Bioenerg. 2011, 35, 3748-3762. (26) Chew, J. J.; Doshi, V. Renew. Sust. Energ. Rev. 2011, 15, 4212-4222. (27) Mott, R. A.; Spooner, C. E. Fuel 1940, 19, 242. (28) Bergman, P. C.; van Paasen, S. V.; Boerrigter, H. Expert Meeting, Strasbourg, France 2002. (29) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Ind. Eng. Chem. Res. 2005, 44, 9096-9104.


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Tables Table 1. XRF analysis of RS ash after ashing at 400 °C Ash composition [%] Samples RS

MgO

SiO2

P2O5

SO3

K2O

CaO

MnO

2.4

54.7

3.8

6.3

23.0

8.2

1.6


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Table 2. Proximate analyses, ultimate analyses, HHVs, and yields of RS and semi-chars Proximate analysis

Ultimate analysis

Semi-char

Heat

yield

recovery

[wt%,

[%,

d.a.f.]

d.a.f.]

HHV [wt%, d.b.]

[wt%, d.a.f.]

Samples

[MJ/kg, VM

FC

Ash

C

H

N

O[diff.]

O/C

H/C

d.a.f.]

RS

75.2

15.2

9.6

48.9

5.2

0.9

45.0

0.69

1.28

15.9

100

100

LT-AP

74.3

15.5

10.2

47.5

6.0

0.9

45.6

0.72

1.51

16.5

88.0

91.1

LT-GP

49.8

33.8

16.4

61.0

5.6

1.3

32.1

0.39

1.09

22.8

51.3

73.4

LT-MP

74.1

15.5

10.4

48.0

5.9

0.9

45.2

0.71

1.47

16.6

94.0

97.9

MT-AP

68.8

19.9

11.3

50.8

5.6

1.0

42.7

0.63

1.31

17.5

77.0

84.6

MT-GP

40.8

40.5

18.7

68.4

5.2

1.5

24.9

0.27

0.92

26.2

43.0

70.7

MT-MP

69.0

17.8

13.2

51.3

6.0

0.9

41.8

0.61

1.40

18.4

85.2

98.4

ST-AP

43.0

35.5

21.5

65.3

5.0

1.3

28.4

0.33

0.91

24.1

44.8

67.7

ST-GP

30.6

47.5

21.9

73.2

5.1

1.6

20.0

0.21

0.84

28.6

35.2

63.2

ST-MP

57.8

27.5

14.7

59.3

5.5

1.2

34.0

0.43

1.12

21.9

73.4

100.9


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Table 3. GC-MS analysis results of the tars from RS and MT semi-char CO2 gasifications at 900 oC Percent (%) Compounds RS

MT-AP

MT-GP

MT-MP

Styrene

1.54

-

-

-

Phenol

1.74

-

-

-

Indene

3.57

-

-

-

Naphthalene

19.91

2.43

1.70

0.97

Quinoline

0.69

-

-

0.60

-

-

-

0.11

2-methyl-naphthalene

15.33

8.60

5.02

0.91

Biphenyl

3.77

2.37

1.53

1.34

Biphenylene

8.12

6.68

5.65

3.52

-

-

-

0.53

1-naphthalenecarbonitrile

1.15

1.46

1.69

0.84

Dibenzofuran

0.64

0.82

0.90

0.46

-

-

-

0.22

1H-phenalene

0.38

-

-

-

Fluorene

4.10

4.23

3.80

2.55

Anthracene

10.78

16.37

17.20

7.07

Carbazole

0.27

0.41

1.37

0.33

1-phenyl-naphthalene

0.39

0.56

0.59

0.53

2-methyl-phenanthrene

1.19

0.42

1.84

-

1-methyl-anthracene

0.37

1.73

0.52

-

4H-cyclopenta[def]phenanthrene

1.59

2.43

2.63

1.78

2-phenyl-naphthalene

1.29

1.84

1.53

0.97

Fluoranthene

6.07

10.25

11.38

-

Pyrene

4.20

6.99

7.90

7.67

-

0.40

0.47

-

1.41

2.73

3.07

1.03

Hexadecane

2-(1-methylethenyl)-naphthalene

Fluorene-9-methanol

Benzo[b]naphtho[2,3-d]furan 1-methyl-pyrene


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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11H-benzo[a]fluorene

1.04

1.59

3.86

-

11H-benzo[b]fluorene

0.82

1.32

-

-

4-methyl-pyrene

0.21

-

-

-

Cyclopenta[cd]pyrene

3.77

-

1.91

-

Benzo[ghi]fluoranthene

0.28

0.82

-

-

Triphenylene

1.82

11.60

9.92

12.64

1,1'-binaphthalene

0.44

3.53

1.95

-

-

3.04

2.34

-

Benzo[k]fluoranthene

0.79

-

-

-

Benzo[a]pyrene

0.89

-

4.25

24.81

Benzo[e]pyrene

1.44

6.24

5.30

31.12

Indeno[1,2,3-cd]pyrene

-

0.43

0.89

-

Dibenzo[def,mno]chrysene

-

0.71

0.79

-

Light PAHs (2-3 rings)

69.84

46.05

40.28

19.18

Heavy PAHs (4-6 rings)

24.77

52.08

56.66

79.05

Eicosane


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions Figure 1. Schematic diagram of (a) fixed bed reactor; (b) batch autoclave reactor; (c) “hot press” reactor Figure 2. Product distributions of LT(a); MT(b); ST(c), carbon distributions of LT(d); MT(e); ST(f), and oxygen distributions of LT(g); MT(h); ST(i) of RS torrefactions Figure 3. H/C vs. O/C diagram of RS and semi-chars Figure 4. FTIR spectra of the MT-AP, MT-GP, MT-MP semi-chars and raw RS Figure 5. Liquid product contents of raw RS and MT semi-chars CO2 gasification at 900 o

C

Figure 6. The fixed carbon and volatile matter contents of the MT semi-chars on raw RS basis Figure 7. Yields of H2, CO, CH4, and C2Hn of CO2 gasification of RS and MT semi-chars at 900 oC


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a) N2

Cooling water

Quartz reactor

Thermocouple

Accumulative flow meter

Sample

Cotton wool filter

Gas bag

╳

Electrical furnace Liquid nitrogen

(b)

Pressure gauge Thermocouple Valve Electrical furnace

Autoclave

Sample

Load (P=10 MPa)

(c) Reactor tube

IR furnace

Molds

Sample

Thermocouple

Figure 1. Schematic diagram of (a) fixed bed reactor; (b) batch autoclave reactor; (c) “hot press” reactor.


Energy & Fuels

(b)

(a)

Liquid

60 40 Semi-char

20

LT-GP

Gas

80

40 Semi-char

20 0

LT-MP

Liquid

60

MT-AP

(d)

60 Semi-char

40 20

LT-AP

LT-GP

80

Liquid

Semi-char

40 20 0

MT-AP

MT-GP

Oxygen distributions (wt%, d.a.f.）

Gas

Liquid

40

Semi-char

LT-AP

LT-GP

LT-MP

ST-MP

80

Liquid

60 40 Semi-char

20

ST-AP

ST-GP

ST-MP

(i) 100

Gas

80 60 Liquid

40 20 Semi-char

0

ST-GP

Gas

0

MT-MP

100

80

0

ST-AP

(h)

(g)

20

Semi-char

20

(f)

60

LT-MP

100

60

40

100 Gas


0

Liquid

60

(e)

Liquid

80

Gas

80

0

MT-MP

100

Gas

Carbon distributions (wt%, d.a.f.）

Carbon distributions (wt%, d.a.f.）

100

MT-GP

Carbon distributions (wt%, d.a.f. ）

LT-AP

100 Product distributions (wt%, d.a.f.）

Product distributions (wt%, d.a.f.）

Product distributions (wt%, d.a.f.）

Gas

80

0

(c)

100

100


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Page 26 of 31

MT-AP

MT-GP

MT-MP

Gas

80 60 Liquid

40 20 Semi-char

0

ST-AP

ST-GP

ST-MP

Figure 2. Product distributions of LT(a); MT(b); ST(c), carbon distributions of LT(d); MT(e); ST(f), and oxygen distributions of LT(g); MT(h); ST(i) of RS torrefactions


Page 27 of 31

2.0 – H2O

Biomass

– CH4

1.5

1.0

Peat – CO2

Coal

Atmoic H/C ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

RS

0.5

0.0 0.0

LT-AP

0.2

LT-GP

LT-MP

MT-AP

MT-GP

MT-MP

ST-AP

ST-GP

ST-MP

0.4 0.6 Atmoic O/C ratio

0.8

1.0

Figure 3. H/C vs. O/C diagram of RS and semi-chars


Energy & Fuels

MT-GP Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

MT-MP MT-AP Rice straw

4000

3600

3200

2800 2400 2000 1600 -1 Wavenumber (cm )

1200

800

400

Figure 4. FTIR spectra of the MT-AP, MT-GP, MT-MP semi-chars and raw RS


Page 29 of 31

300

3

Liquid Content (g/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

200

100

0

RS

MT-AP

MT-GP

MT-MP

Figure 5. Liquid product contents of raw RS and MT semi-chars CO2 gasification at 900 o

C


Energy & Fuels

100 msemi-char/mraw RS (wt%,d.a.f.）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 VM

20 FC

0 RS

MT-AP

MT-GP

MT-MP

Figure 6. The fixed carbon and volatile matter contents of the MT semi-chars on raw RS basis


Page 31 of 31

2500 Gas yield (mL/g sample, d.a.f.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

RS

MT-AP

MT-GP

CO

CH4

MT-MP

2000 1500 1000 200 150 100 50 0

H2

C2Hn

Figure 7. Yields of H2, CO, CH4, and C2Hn of CO2 gasification of raw RS and MT semi-chars at 900 oC


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