Accepted Manuscript Synergistic combination of biomass torrefaction and co-gasification: reactivity studies Yan Zhang, Ping Geng, Rui Liu PII: DOI: Reference:

S0960-8524(17)31511-0 http://dx.doi.org/10.1016/j.biortech.2017.08.197 BITE 18806

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Bioresource Technology

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24 July 2017 24 August 2017 30 August 2017

Please cite this article as: Zhang, Y., Geng, P., Liu, R., Synergistic combination of biomass torrefaction and cogasification: reactivity studies, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech. 2017.08.197

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Synergistic combination of biomass torrefaction and co-gasification: reactivity studies Yan Zhang*, Ping Geng, Rui Liu

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian City, 116024, China

Abstracts Two typical biomass feedstocks obtained from woody wastes and agricultural residues were torrefied or mildly pyrolized in a fixed-bed reactor. Effects of the torrefaction conditions on product distributions, compositional and energetic properties of the solid products, char gasification reactivity, and co-gasification behavior between coal and torrefied solids were systematically investigated. Torrefaction pretreatment produced high quality bio-solids with not only increased energy density, but also concentrated alkali and alkaline earth metals (AAEM). As a consequence of greater retention of catalytic elements in the solid products, the chars derived from torrefied biomass exhibited a faster conversion than those derived from raw biomass during CO2 gasification. Furthermore, co-gasification of coal/torrefied biomass blends exhibited stronger synergy compared to the coal/raw biomass blends. The results and insights provided by this study filled a gap in understanding synergy during co-gasification of coal and torrefied biomass. 1

Keywords: Biomass, Torrefaction, Char, Co-gasification, Synergy. * Corresponding author. Tel./fax: +86 411 84709684. E-mail address: [email protected] (Y Zhang).

1. Introduction 1.1. General background Among all renewable energy sources, biomass is the primary one in use and currently accounts for approximately 10% of primary energy demand in worldwide. Compared with coal, biomass has advantages of being CO2-neutral, with lower sulfur and nitrogen contents. However, the direct use of biomass as a fuel entails several technical and economic drawbacks, due to its inherent characteristics. These characteristics include its high moisture content, low calorific value, and low bulk density, which together result in poor conversion efficiency as well as high costs for its collection, storage, and transportation. Co-processing biomass and coal in existing coal-fired boilers and gasifiers can mitigate the aforementioned drawbacks of biomass to a certain extent [Masnadi et al. 2015]. However, the fibrous nature of raw biomass makes it difficult to process in already existing pulverized coal milling systems [Al-Mansoura and Zuwalab 2010]. The pulverized particles from raw biomass are coarse and slender in nature with a low sphericity value [Phanphanich and Mani, 2011]. Such lower sphericity reduces the flowability of coal/biomass powder blends and therefore limits the blending percentage of biomass to 5–10 wt% [Maciejewska et al.

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2006]. Consequently, there is a need for pretreatment technologies to modify raw biomass to accommodate existing coal-fired boilers or gasifiers. In recent years, the use of torrefaction for energy applications including combustion and gasification has generated significant interest around the world. Torrefaction is an efficient pretreatment technology that can improve the properties of biomass and therefore addresses at least partially some solutions to the above-mentioned problems. Torrefaction is a mild thermochemical process that is generally performed in an inert atmosphere in the temperature range of 200–300 °C for several minutes or hours [Wannapeera et al. 2011; Phanphanich and Mani,2011; Peng et al. 2012]. During torrefaction, the fibrous structure of biomass is partially or entirely broken down. The weakened fibrous structure improves the grindability of the torrefied biomass and enables it to be co-milled or milled in existing pulverized coal-firing facilities [Phanphanich and Mani,2011; Bridgeman et al. 2010]. This is a very attractive feature allowing cost-savings as it eliminates the need for capital expenditures for expensive infrastructure. Additionally, the energy density of torrefied biomass is similar to that of some coals [Phanphanich and Mani,2011], which reduces the storage and transportation costs of the processed material. Overall, the potential benefits of torrefaction technology to improve the grindability of the solid products and increase the energy density make this a very attractive strategy for improving the use of biomass. However, more in-depth scientific research and technical development is required before these benefits are fully demonstrated.

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1.2. Current status from biomass torrefaction to gasification and co-gasification

Following up with biomass torrefaction research, some researchers have undertaken investigations into the feasibility and performance of the torrefied biomass in gasification. Among these, a group of researchers experimentally performed to compare gasification performance between raw and torrefied biomass in fixed-bed [Sarkar et al. 2014; Xiao et al.2015; Ceron et al. 2016; Dudyński et al. 2015], fluidized-bed [Kulkarni et al. 2016; Kwapinska et al. 2015; Pinto et al. 2017; Berrueco et al. 2014; Prins et al. 2006] and entrained-bed gasifiers [Couhert et al. 2009; Weiland et al. 2014; Chen et al. 2011]. Literature review on this issue is illustrated in Table 1. Both positive and negative influences of torrefaction on the gasification process have been reported. For example, Cerone et al. [2016] investigated gasification of wood chips and torrefied wood chips in an updraft fixed-bed pilot plant. They found that the use of torrefied feedstock resulted in a significant reduction to about 1/5 of the tar load in the syngas and a 44% increase of the thermal power of the plant when compared to the performance obtained using raw wood. Positive effects of torrefaction on gasification performance were also reported in a bubbling fluidized-bed gasifier [Pinto et al. 2017; Berrueco et al. 2014], a bench-scale atmospheric [Weiland et al. 2014] and a pilot-scale pressurized [Chen et al. 2011] entrained-bed gasifiers. Another group of studies, however, reported negative impacts of torrefaction on gasification. Prins et al. [2006] observed a reduction in the overall gasification efficiency for the torrefied wood compared with that of the raw biomass. Kulkarni et al. [2016] reported that the energy 4

content and cold gas efficiency were higher for raw pine than with torrefied samples as a gasification fuel in a bench-scale bubbling fluidized bed gasifier. Thus, they concluded that the change in biomass composition due to torrefaction did not improve syngas composition and energy value. Couhert et al. [2009] also observed that the chars from torrefied wood were less reactive towards steam than the char from wood during steam gasification at 1200 ºC in an entrained-bed gasifier. In addition to above gasification studies, limited studies have been reported to investigate the influence of torrefaction on the char reactivity using either a thermogravimertry analysis (TGA) [Guo et al. 2017; Fisher et al. 2012; Tran et al. 2016] or a single particle reactor [Karlström 2015]. Typical results and findings of these studies are shown in Table 2. Different experimental conditions as well as varied biomass samples lead to some conflicting conclusions. Fisher et al. [2012] conducted steam gasification of the chars from raw and torrefied willow, who found that the reactivity of the torrefied-willow char was lower than that of raw char. Trans et al. [2016] observed that CO2 reactivity of chars from torrefied birch and spruce decreased with increased torrefaction temperature. Otherwise, Karlstrom et al. [2015] reported that the CO2 reactivity of the char from torrefied olive stones was lower than that of the char from raw fuel, while the char reactivity of the torrefied straw was higher than the char reactivity of the raw straw. Guo et al. [2017] found that the gasification reactivity of the chars from both the torrefied grape marc and the torrefied macroalgae was lower than that of the chars from their corresponding raw fuels at 800°C in CO2, while the reactivity for the torrefied macroalgae char was found to be higher than that of the raw macroalgae char at 1000°C. The above contradictory 5

conclusions suggest there are still complexities of this process that are poorly understood, and we still do not know the determinants of whether torrefied biomass exhibits enhanced gasification performance than the parent untreated material. Clarifying this question is of particular importance since the performance of the feedstocks during gasification greatly contributes to the overall process efficiency. In contrast with single-handed biomass gasification process, co-gasification of biomass and coal maybe a more viable option for syngas production. Saw and Pang [2013] suggested that co-processing of coal and biomass in existing coal-based systems can offer the economies of plant scale that can reduce specific operating costs to allow better use of biomass than for the case of constructing new decentralized plants fed exclusively with biomass. Likely, the same benefit can be acquired from co-processing coal and torrefied biomass. If the torrefied biomass is co-fired in a coal-fired boiler and/or co-gasified in a coal-fired entrained-bed gasifier, the grindability of the torrefied biomass is considered to be crucial in whether torrefaction is useful for these applications. This is not a prerequisite for the process of co-gasifying torrefied biomass with coal in fixed-bed and fluidized-bed gasifiers, where severe size reduction is not required but char reactivity and synergistic interactions between coal and torrefied biomass remain concerns. In reviewing recent work on the topic of co-gasification, most studies have been performed using raw biomass and coal as feedstocks, and both synergy [Zhang et al. 2016a, 2016b, 2016c; Jeong et al. 2014; Howaniec et al. 2013; Krerkkaiwan et al.2013 Masnadiet al. 2015] and inhibition [Habibi et al. 2012; Ellis et al. 2015; Yu et al. 2015] were observed during the co-gasification of coal and biomass. 6

Little information was reported about the co-gasification of the torrefied biomass with coal. There is insufficient information on synergy in the co-gasification of coal and torrefied biomass. 1.3. Objective of this study

Given the aforementioned uncertainties and issues, the first objective of this paper was to address the influence of torrefaction on char gasification reactivity. The second objective was to fill the research gap in understanding the synergy during co-gasification of coal and torrefied biomass. It was found that biomass torrefaction not only increased the energy density of the solid products, but also played a role in retaining more catalytic elements in torrefied solids and their derived chars. Additionally, synergistic effects were observed in the co-gasification of coal with raw and torrefied biomass, and the torrefied biomass exhibited stronger synergy than its raw fuels. Investigation of this study can provide useful insight into upgrading biomass through the combination of torrefaction and coal gasification systems. The results from this study will contribute to further work to integrate the process of biomass torrefaction with coal gasification, which will be reported in forthcoming papers.

2. Material and method

2.1. Feedstocks

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The biomass samples used in this study were prunings of metasequoia (PM), a woody species, and corn stalk (CS), an agricultural residual. The PM samples with thickness near 10 mm were carefully selected, which is a size similar to that of common biomass pellets. The raw biomass feedstocks were cut into pieces of 10–15 mm long for torrefaction studies. Bituminous coal (BC) was used for co-gasification studies with raw biomasses or their torrefied solid products. The particle sizes of BC were below 200 m. Both biomass and coal samples were pre-dried in an oven for 2 h at 107 °C and then kept in a desiccator for subsequent use. The proximate and ultimate analyses of coal and biomass samples are shown in Table 3.

2.2. Torrefaction reactor and operating conditions

The torrefaction tests were performed in a fixed bed reactor. A cylindrical quartz tube (with an inner dimeter of 64 mm and a length of 500 mm) was set vertically inside an electric furnace. The sample was placed on a ceramic porous bed that was fixed in the quartz tube and located at the central heating zone of the furnace. The sample loads were controlled at 40 g (±5%) for PM and 20 g (±10%) for CS. The use of different sample loads was to ensure that all samples have nearly the same bed height in the reactor, since CS has a lower bulk density than PM. The reactor was purged with nitrogen gas (200 mL/min) from the top of the reactor for 20 min prior to and during the torrefaction test. The sample in the reactor was heated at a heating rate of 10 °C/min up 8

to three different torrefaction temperatures (250, 270, and 290 °C). An extended mild pyrolysis temperature of 340 °C was also used for comparison. The samples were maintained at the final temperature for different residence times (30 to 90 min). The torrefied biomass samples were denoted as A(T)R, A representing the abbreviation of the biomass samples, e.g. PM or CS, T indicating the torrefaction temperature, and R indicating the residence time. The gas stream produced from torrefaction was first cooled using a condenser-west tube. Cold water (approximately 10 °C) at a flow rate of 1000 mL/min was flowed through the condenser to cool down the flow stream from the reactor allowing the recovery of condensable liquids contained in the torrefaction gas.

2.3. Determination and analyses of the products

The mass yields of the torrefied solids and the condensates were measured by gravimetric method after each test. The compositions of the solid products were determined by proximate and ultimate analyses and their higher heating value (HHV) was measured in a bomb calorimeter.

2.4. Char preparation and CO2 gasification of raw and torrefied biomass

A simultaneous thermal analyzer (Mettler-Toledo TGA/DSC-1100LF) was used in this study. Raw or torrefied biomass sample was preliminary pulverized below 200 m and then pyrolyzed under a nitrogen flow of 200 mL/min and heating rate of 50 °C/min 9

to 900 °C, and maintained at this temperature for 10 min to prepare char sample. For a special comparison, a slow heating rate of 20 °C/min and long holding time of 40 min at 900 °C were also used. For subsequent CO2 gasification test, 10 mg of the char sample was filled in an alumina crucible and heated to 900 °C with a heating rate of 50 °C/min under a nitrogen flow of 200 mL/min. The isothermal gasification of the char was initiated by switching to a CO2 flow of 200 mL/min and completed until weight loss reached constant. In all cases, baseline correction was conducted by subtracting a “brank” signal that was recorded with an empty crucible from the TGA data of the sample determined under the same conditions. Char conversion (x) and isothermal reactivity (r, s-1) of the char during the isothermal gasification step were calculated by equations given by Zhang et al. [2016c].

2.5. Determination of catalytic elements in raw and torrefied biomass chars

Calcium, potassium, and sodium contents in raw and torrefied biomass-based chars were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The sample was treated by acid digestion method before ICP-AES analysis. Concentrated HNO3 (68%) and HClO4 (30%) were used as digestion reagents. The digestion was performed at 180 ± 5 °C. No solid residues were found in the final digested solution. Therefore, metallic elements determined in this way should be their total amounts existed in biomass or char sample. The results are shown in Table 4.

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2.6. Co-gasification of coal with raw and torrefied biomass in CO2

The TGA instrument used in this study has an elliptical alumina sample holder that is divided into reference and sample zones, which enables the simultaneous use of two crucibles for TGA tests. A congruent-mass thermogravimetric analysis mode was adopted to study co-gasification of coal with raw or torrefied biomass. The detailed procedures of this method have been reported elsewhere [Zhang et al 2016a; 2016b]. Briefly, the following two experimental runs were conducted: in the first TGA run, 10 ± 0.1 mg of coal and 10 ± 0.1 mg of bio-solids were placed into two crucibles. This set of two samples was denoted by a dash connection, for example, BC-PM. The two crucibles were then placed in the sample and reference zones of the elliptical holder for subsequent TGA gasification test. In the second TGA run, two crucibles were filled with the coal/bio-solids blends with a mass-blending ratio of 1:1. Each of them contains 5 ± 0.1 mg of coal and 5 ± 0.1 mg of bio-solids. Both the total sample mass (20 mg) and the individual masses of coal (10 mg) and bio-solids (10 mg) were exactly congruent with those used in the first TGA run. This set of two samples was denoted by a slash connection, for example, BC/PM. The two crucibles were then located on the sample and reference zones of the elliptical holder. Gasification conditions and procedures were the same as those described in Section 2.4.

3. Results and discussions

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3.1. Mass yields of the torrefied solids and condensates

In this study, only PM was used systematically to investigate the influence of temperature and residence time on yields and properties of the torrefied solids and condensates. Fig.1 shows the effects of both temperature (Fig.1a) and residence time (Fig.1b) on solid and condensates yields under torrefaction. The tested temperatures for this test runs were 250, 270, 290, and 340 ºC. The mass yields almost linearly reduced from 85.6 to 63.2 wt% for the solid products and linearly increased from 7.8 to 26.1 wt% for the condensates as the torrefaction temperature increased from 250 to 290 ºC (Fig.1a). At the more severe pyrolysis temperature of 340 ºC, the yields of both the solid and the condensates were out of these linear dependences. These observations implied that the reaction mechanism of PM at 340 ºC might differ from that at a temperature range of 250–290 ºC, because carbonization may occur at 340 ºC but not at temperatures below 290 ºC. Thus, torrefaction is defined as a mild thermochemical process that is performed in the temperature range of 200–300 °C [Phanphanich et al 2011; Peng et al. 2012]. The effect of residence time on solid and condensates yields under torrefaction was measured and is shown in Fig.1b. A distinct feature in the data shown in this figure is that the mass yields of the solids and the condensates were maintained at almost a constant level when the residence time increased from 60 to 90 min. This result supports the view proposed by Wannapeera et al. [2011], who stated that the thermal degradation of biomass is rapid at torrefaction times less than 1 h, and becomes very slow beyond 1 12

h. The dependence of biomass origins on the mass yields of the solid and liquid products was investigated by conducting torrefaction of PM and CS at 270 ºC for 60 min. The two biomass samples used in this study represent two different families of biomass: wood and agricultural residue. The mass yield of torrefied solid was 73.8 wt% for PM (wood) and 38.4 wt% for CS (agricultural residue), while the condensate yield was 23.9 wt% for CS and 15 wt% for PM. The results agree with the experimental observations that the decomposition rate of agricultural residues is comparatively higher than that of woody biomass due to its higher hemicellulose content, thus resulting in lower mass yield [Bergman et al. 2005].

3.2. Properties of the torrefied solids

Proximate and ultimate analyses and HHV of raw and torrefied biomass were measured and are summarized in Table 3. The energy yield (EY) of the torrefied biomass calculated based on solid mass yield and its HHV is also listed in this table. It can be seen from Table 3 that the moisture content of all torrefied solid products was quite similar, varying in a narrow range of 1.6–3.3 wt%. Generally speaking, torrefaction treatment significantly decreased the volatile mater (VM) and oxygen content of raw biomass, and consequently increased the content of both elemental carbon and fixed carbon (FC) as well as the HHV value of the solid products. As a result of this balance between volatile release from raw biomass and the carbon accumulation 13

in the solid product, the EY value of the torrefied biomass decreased with increased severity of torrefaction. For example, the EY value of the torrefied PMs under all applied conditions decreased from 94.7 to 68.8%. On the other hand, comparing the properties of the solid products derived from PM and CS, at a torrefaction temperature of 270 ºC and with a residence time of 60 min, the HHV value of CS(270)60 was higher than that of PM(270)60. This result can be explained by the fact that the CS released more volatiles (23.9 wt%) than PM (15 wt%) during torrefaction. As a result, the CS resulted in lower mass and energy yields of the solid product.

3.3. Influence of torrefaction conditions on char reactivity

The next concern is to investigate how the torrefaction pretreatment influences char gasification reactivity. For this purpose, raw and torrefied biomass chars generated at a pyrolysis temperature of 900 ºC were comparatively studied for CO2 gasification. The results are presented in Figs. 2. Fig. 2a shows x–t plots of PM-derived chars during CO2 gasification at 900 ºC, which provides a direct comparison of the reaction times required for completed char conversion. The x–t plots in Fig. 2a indicate that the torrefied PM chars gasified faster than the raw PM char, and the higher the torrefaction temperature of PM, the faster the conversion of the produced PM char. The x–t plots of raw and torrefied CS chars are shown in Fig. 2b. Again, the CS(270)60- char showed a faster conversion than CS-char. The above results strongly suggest that the biomass torrefaction pretreatment produced more reactive char material compared to raw 14

biomass char material. On the other hand, comparison of Figs. 2a and 2b indicates that CS-derived chars gasified much faster than the corresponding PM-derived chars. The times required for complete char conversion were ca. 8 min and ca. 6.7 min for CS-char and CS(270)60-char, respectively, significantly faster than the ca.12 min and ca. 10.3 min for PM-char and PM(270)60-char, respectively. Regarding the results presented in Fig. 2, of particular concern is why torrefaction pretreatment produced more reactive chars in comparison with raw biomass chars. Zhang et al. [2008] studied CO2 gasification of 14 biomass chars derived from different origins, including forestry wastes, agricultural residuals, and food-processing wastes. They observed that char reactivity was predominantly determined by the amounts of indigenous AAEM in bio-chars, in particular, calcium, potassium, and sodium. It is presumed that the difference in reactivity between raw and torrefied biomass chars may be correlated with the relative amounts of catalytic elements. To test this, ICP-AES analysis was performed to determine calcium and potassium contents in several typical chars. For this analytical run, torrefied CS and PM chars were compared with their parent chars (CS-char and PM-char). The results are summarized in Table 4. It is seen that the PM-derived chars are generally richer in calcium than potassium, whereas the CS-derived chars are more dominant in potassium than calcium. Additionally, there is an opposite trend between CS- and PM-related chars in Table 4. Namely, K concentration of CS (270)60-char comparing to CS-char increased from 1.63% to 4.60, while there was no significant difference between their Ca contents. In contrast, Ca concentration of PM (270)60-char comparing to PM-char increased from 1.47% to 4.48, 15

while their K content has no obvious difference. The results in Table 4 strongly suggest that torrefaction pretreatment may facilitate the retention of catalytic elements in the solid products. This may be reasonably explained by the releasing behaviors of volatile matter and catalytic elements during torrefaction and char preparation processes. Table 3 shows that the ash content of the torrefied PM increased with increasing torrefaction temperature. This is a reasonable trend because the yield of the torrefied solid decreases with increasing torrefaction temperature as indicated in Fig 1a. This fact means that volatilization of volatile organic compounds and water is much more predominant than the release of mineral matter (mainly AAEM) from PM during torrefaction. Chen et al. [2016] and Deng et al. [2017] reported that only 5–8% of potassium released during torrefaction and mild pyrolysis of three agricultural residuals at 300–500 ºC. On the other hand, the concentrations of AAEM in the torrefied PM related semi-chars (Table 4) are well correspond to the ash contents in the corresponding torrefied PM (Table 3). Namely, the torrefied PM with high ash content produce a semi-char concentrated in AAEM. However, this result seems not to be so reasonable. For example, as shown in Table 3, the volatile matters in the torrefied PM solids reasonably decrease with increasing torrefaction severity. As a consequence, the yields of PM related semi-chars increase with increasing torrefaction temperature during char preparation at 900 ºC (Table 4). Assuming the release of AAEM from all studied materials could be negligible during pyrolysis, the concentration of AAEM in their semi-chars should decrease with increasing char yield (or torrefaction temperature). The analytical data in Table 4 was contrary to this assumption. These unexpected results strongly suggested that the release 16

of AAEM during char preparation was non-negligible. Jiang et al. [2012] studied the release characteristics of AAEM during biomass pyrolysis at 900 ºC, and found that 53–76% of alkali metals and 27–40% of alkaline earth metals are released in the pyrolysis process. The more the evolution of the volatile matter, the greater the release of the AAEM. Their experimental findings provided a reasonable explanation for the unexpected results obtained in this study. For example, PM-char and PM(340)60-char were formed undergoing volatilization of approximate 75 and 41wt% volatiles from raw PM and PM(340)60 (Table 3), respectively, during high temperature pyrolysis at 900 ºC. It is inferred that the release ratio of AAEM from PM(340)60 should be lower than that from raw PM during this high temperature heating step. This inference is supported by the analytical data shown in Table 4, where the torrefied chars were found to be rich in calcium, potassium, and sodium compared to their original ones. These AAEM species can then play catalytic roles in the subsequent char gasification, which explains why the torrefied bio-chars are more reactive than the original ones. The experimental findings obtained above were in contrast to some conclusions reported in literature as listed in Table 2, where some researchers observed that the reactivity of char from torrefied biomass was lower than that of char from raw biomass. Except for biomass origins and gasifying agents, we noticed that some thermal events were somewhat different between ours and literature, i.e., heating rate, holding time, and gasification temperature. The char samples used in the above experiments were obtained by identical pyrolysis conditions at a 50 ºC/min heating rate and 10 min holding time at 900 ºC. Okuno et al. [2003] reported that some AAEM species rapidly 17

released with increasing the holding time at 800 ºC. This means that char reactivity will possibly decrease with the release of AAEM. Does the long-time heating result in a subversive conclusion? A comparative study was conducted to clarify this question. Raw PM and PM(340)60 were pyrolyzed with a slow heating rate of 20 °C/min to 900 °C and a long holding time of 40 min to prepare char samples (denoted by PM-SL and PM(340)60-SL, respectively). These conditions were similar to those reported in literature [Fisher et al. 2012, Tran et al. 2016]. The two char samples were then subjected to isothermal gasification at either 800 °C or 900°C in CO 2. The results indicated that the reactivity of PM(340)60-SL char was more reactive than PM-SL char, regardless of both heating rate and holding time for char preparation. A few more notes should be added here regarding the criterion of comparing kinetic data among different samples. Zhang et al. [2016a] reported that the gasification reactivity of chars exhibited remarkable sample-mass dependence when using TGA for kinetic study. The sample-mass dependence of the reactivity may cause illusions in reactivity comparison among different samples. Note that the same initial mass loading of raw and torrefied biomass will produce different amount of char in a combined pyrolysis/gasification TGA mode. In the present study, char samples were previously prepared. The difference in initial mass loading among different char samples were carefully controlled within  1 mg. Therefore, the reactivity data determined in this study are comparable and reliable.

3.4. Synergistic effect during the co-gasification of torrefied biomass and coal 18

The co-gasification of coal and biomass has been extensively studied in recent years. Most studies have focused on the presence of possible interaction or synergy between the co-processed materials. Zhang et al. [2016a; 2016b] developed a congruent-mass thermogravimetric analysis that showed conclusive evidence of synergy between coal and biomass. It was concluded that the presence of indigenous potassium species in raw biomass was a direct consequence of the synergy during co-gasification of coal and biomass [Zhang et al. 2016b; 2016c]. Moreover, the synergy degree greatly depended on the coal and biomass origins [Zhang et al. 2016b]. There have been very few recent studies on synergy between coal and torrefied biomass during co-gasification. The results reported in Figs. 3 and 4 will fill this research gap. Fig. 3a shows the x–t curves of separate BC-PM and blended BC/PM during CO2 gasification at 900 ºC, which were determined by the congruent-mass thermogravimetric analysis method described earlier [Zhang 2016a; 2016b]. The x-t curve of individual PM (10 mg) is also shown in the same figure for comparison. It should be noted that exactly the same total sample mass (20 mg) and the individual masses of BC (10 mg) and PM (10 mg) were used for the separate and blended TGA gasification tests. As shown in Fig. 3a, the gasification time required for char conversion up to x = 0.95 is 21.2 min for the separated gasification (BC-PM), but only 13.3 min for the blended co-gasification (BC/PM). The results provide evidence of synergy for the BC/PM blend. Zhang et al. [2016b] proposed a synergy index (SI) to assess interactions between coal and biomass during co-gasification. A larger magnitude 19

of SI corresponds to a greater degree of synergy of the blends. According to the data shown in Fig. 3a, the SI value for BC/PM co-gasification was calculated to be 1.59 (indicated in Fig. 3a). The separate and blended gasification tests of BC with torrefied PM were examined. Only PM(270)60 was selected for comparative study. The results are shown in Fig. 3b. The individual gasification of PM(270)60 is also shown in the figure. The time required to achieve x = 0.95 for BC-PM(270)60 was the same as that of BC-PM (21.2 min), The time required to achieve x = 0.95 was only 11.2 min for their blends (BC/PM(270)60), faster than the time for BC/PM (13.3 min). According to Fig. 3b, the SI value for BC/PM(270)60 was 1.73, larger than that of BC/PM blends (1.59 in Fig. 3a). This stronger synergy effect is attributed to higher AAEM content in the PM(270)60 (Table 4). Fig. 4 represents x–t plots of separate and blended gasification tests for BC and CS related sample combinations. The x–t plots in Fig. 4 are universally similar to those in Fig. 3. Apparently, the time required to achieve x = 0.95 for separate gasification of BC-CS and BC-CS(270)60 are almost the same as the time required for BC-PM and BC-PM(270)60. However, the times for co-gasification of the blends were 9.5 min for BC/CS (Fig. 4a) and 8.1 min for BC/CS(270)60 (Fig. 4b). The SI values for BC/CS and BC/CS(270)60 co-gasification tests were calculated to be 2.23 and 2.41, respectively. Again, the results provided a conclusive evidence in regard to synergy for BC/CS and BC/CS(270)60 systems. The torrefied CS exhibited stronger synergy than raw CS in their blends with BC. Furthermore, comparison between Figs. 3 and 4 indicated that CS 20

and CS(270)60 had stronger synergy effect than PM or PM(270)60. On the other hand, there seems to be an apparent inhibition effect at early stages of co-gasification between coal and torrefied solids as shown in Figs. 3b and 4b, where the blend samples have slower conversion rates than separated ones. The results are found to be reproducible, but conclusive explanations for this scenario need further study.

4. Conclusions

Torrefaction treatment remarkably decreased the volatile mater and oxygen content of raw biomass, which produced high-quality solid fuels with both increased energy density and concentrated AAEM. Due to the catalytic effects of concentrated AAEM in torrefied solids, the chars derived from torrefied biomass exhibited higher CO2 gasification reactivity than those derived from raw biomass. The synergy index for the torrefied PM and CS was found to be higher than that of their corresponding raw fuels. Furthermore, raw and torrefied CS exhibited stronger synergy than raw and torrefied PM during co-gasification in CO2.

5. Recommendations

As char gasification is the rate-determining step in biomass gasification, high reactivity of the torrefied bio-chars will certainly contribute to the improvement of the 21

gasification efficiency. However, from the perspective of the overall process efficiency, this high reactivity does not necessarily mean that the torrefied biomass is a better choice than raw biomass as a gasification feedstock. There is a balance in gasification performance between volatile matter and solid char. The homogeneous gas phase reactions between volatile matter and gasifying agents are much faster than heterogeneous gas-solid reactions between chars and reactant gases. Further efforts should be made to addresses whether the added energy consumption and operational cost required for a torrefaction step can be compensated by technical and economical benefits of the torrefied solid products (energy saving for grinding, decreased costs for storage and transportation, and enhancement of char reactivity). Although a major benefit of biomass torrefaction is the improvement in the grindability of the torrefied solids, this may be only beneficial to the case of co-processing torrefied biomass with coal in a pulverized coal-fired boiler or a coal-fired entrained-bed gasifier. To date, there is no evidence that synergy occurs in these entrained-flow systems during co-processing coal and raw or torrefied biomass. TGA is a common analytical choice to investigate thermal behavior and kinetics of coal and biomass during various thermal events. A reliable kinetic data of coal and biomass or their blends is crucial to the designing and operation of industrial systems. Evidence of synergy between coal and torrefied biomass observed by TGA in this study may provide a useful insight into a more rational approach of co-pelletizing or -briquetting torrefied biomass with coal for fixed-bed or fluidized-bed gasification, other than pulverized co-firing or entrained-flow gasification. If synergy occurs between coal and 22

torrefied biomass, this suggests an attractive possibility strategy to improve the overall efficiency of the co-processing system. Further studies should be performed to elucidate the contribution of this synergy to the overall process efficiency.

Acknowledgements The authors are grateful to the financial supports from the National Natural Science Foundation of China (Grant numbers 51376031 and 51676028).

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Figure Captions Fig. 1. Mass yields of solid and condensates from torrefaction of PM (a) at different temperatures for 60 min and (b) at 270 °C for different residence times. Fig. 2. The x–t plots of (a) raw and torrefied PM chars and (b) raw and torrefied CS chars during CO2 gasification at 900 °C. Fig. 3. The x–t plots of (a) separate gasification of BC-PM and co-gasification of BC/PM and (b) separate gasification of BC-PM(270)60 and co-gasification of BC/PM(270)60, at 900 °C in CO2. Fig. 4. The x–t plots of (a) separate gasification of BC-CS and co-gasification of BC/CS and (b) separate gasification of BC-CS(270)60 and co-gasification of BC/CS(270)60, at 900 °C in CO2.

Table 1. Comparison of the gasification performance between raw and torrefied biomass Torrefaction

Auothe

Gasification

Feedstock

Fuel size (mm)

T(°C)/t( min)

Gasif ier type

Scale

T(°C)

Agent

Sarkar

Switch- grass

4

230,270/ 30

FB

small

700,800, 900

air

Xiao

rice straw

0.09-0. 212

200,240, 300/15

FB

small

900

CO2

r

29

Positive and negative effects of torrefacti on on gasificati on Torrefied biomass resulted in lower H2 and CO yields, lower sygas LHV, CCE, and CGE. The torrefacti on suppress ed the tar formatio n and enhanced H2 and CO

Eucalyptus

12-18

270/50

FB

pilot

640-740

air, air/ste am, O2/ste am

Dudyńs ki

wood pellets

raw:7× 30; tottrefi ed: 5×25

no descripti on

FB

indust rial

790-100 0

air

Kulkar ni

pine

0.85

no descripti on

FLB

bench

935

O2 + N2

Kwapin ska

Miscanthus×giga ntenus

raw:1.3 ; torrefie d: 0.72

250/240

FLB

bench

660-850

air /N2

Pinto

rice husk

original size

200-300 /30,45,6 0

FLB

bench

850

O2/ste am

Prins

beech willow