High quality syngas production from catalytic gasification of woodchip char

Energy Conversion and Management 151 (2017) 457–464

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High quality syngas production from catalytic gasification of woodchip char ⁎

MARK

Shuang Jia, Siyun Ning, Hao Ying , Yunjuan Sun, Wei Xu, Hang Yin Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab. for Biomass Chemical Utilization, Key and Open Lab. of Forest Chemical Engineering, SFA, Key Lab. of Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Woodchip char High quality syngas Catalytic gasification Potassium salts

This work aims on the production of high quality syngas suitable for widely applications from catalytic woodchip char gasification in a lab-scale fixed bed reactor. Using the absorption of the char to load catalysts was conducted in this study. The results showed that the char conversion and hydrogen yield were increased significantly after impregnation with potassium salts. The catalytic activity of the four catalysts is in the order: K2CO3 ≈ KOH > CH3COOK > KCl. In addition, when 6 wt% of KOH solution was used to impregnate, the maximum hydrogen yield of 197.2 g/kg char can be obtained, but further increasing the mass fraction of impregnation solution had an adverse impact on hydrogen yield and H2/CO. Moreover, the influence of particle size, reaction temperature and gasifying agent were investigated. The particle size of the char had little effect on gasification characteristics. Increasing temperature and steam amount were beneficial to improve char conversion and hydrogen yield, and the high quality syngas (H2 + CO) with the ratio of 91.8% was obtained at 950 °Cwhile the ratio of H2/CO decreased from 3.7 to 1.65 with increasing temperature. Additionally, continuously increasing steam had little influence on gasification consequences and was poor in economy. Introducing oxygen can improve the char conversion and adjusted the ratio of H2/CO in an economical way, but higher ER would weaken the quality of syngas.

1. Introduction

Biagini et al. [12,13] studied the gasification of agricultural residues including corn cobs, vine pruning and rice husks, and the process was operated in a demonstrative downdraft gasifier with air as gasifying agent. In their research, the H2/CO ratio of the syngas was lower than 1, and the maximum ratio of 0.86 was obtained by vine pruning gasification. Similarly, in the research of Weiland et al. [14] the syngas H2/ CO ratio was in the range of 0.54–0.57, and they performed in an oxygen gasification of wood power. For improving the hydrogen content in syngas, a serial of measures were conducted, such as catalytic steam gasification [15], sorption enhanced [16], two stage gasification [17,18] and feedstock pretreatment by torrefaction or microwave [19–21], and the ratio can be increased to 1–2 according to the methods. To satisfy more applications, syngas adjustment by water gas shift reaction is required [22]. The produced gas must be purified before adjustment because the impurities could affect the downstream process. In the gases, the mainly pollution is tar, and the condensed tar can plug pipelines and cause inactive of catalyst [23,24]. This evidently causes more operational problems. Biochar, as a solid product of pyrolysis or gasification, has some unique characteristics like high water holding capacity and energy density. In present, char can be used to combust for supplying heat or

Syngas, as a key precursor for fuel products, has a variety of applications according to the tailoring of the ratio of H2/CO. Employing the Fischer-Tropsch synthesis, liquid fuels such as dimethyl ether (DME) and methanol can be synthesized from syngas. The ratio should be 1 when dimethyl ether is the target product, but synthesis methanol and long chain alkanes require the ratio of 2 or higher [1–3]. Furthermore, improving hydrogen content by water gas shift reaction and membrane separation, the gas can be applied in fuel cells or synthetic natural gas [4,5]. However, current syngas mainly derived from fossil fuels, is not a sustainable way. Developing the renewable and environment friendly energy resources is crucial, which not only reduces the dependency on fossil fuels, but also mitigates the greenhouse gas emissions [6]. Recently, the conversion of biomass to syngas has attracted a lot of attention. The process is considered as CO2 neutral due to the released CO2 absorbing by photosynthesis of green plants, which may be a promising alternative to fossil fuels [7,8]. Among the thermo-chemical conversion of biomass, gasification is a potential technology which can convert biomass into synthesis gas efficiently [9,10]. But the ratio of the H2/CO in the syngas usually keeps at a lower level, especially in the gasifying agent of air and oxygen [11].

⁎

Corresponding author. E-mail address: [email protected] (H. Ying).

http://dx.doi.org/10.1016/j.enconman.2017.09.008 Received 7 July 2017; Received in revised form 23 August 2017; Accepted 4 September 2017 0196-8904/ © 2017 Elsevier Ltd. All rights reserved.

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concentrations of the impregnation solution, reaction temperature, gasifiying agent (steam and oxygen) were investigated. In our previous study, char steam gasification for hydrogen rich syngas was investigated [46]. The latent energy required for evaporation makes steam the more costly gasifying agent, especially in a long time reaction. Hussein et al. [47] investigated the effect of oxygen on chicken manure steam gasification, and found that adding oxygen was beneficial to carbon conversion and played an important role in adjusting syngas quality. Thus, we added oxygen in gasifying agent and studied the effect of oxygen addition in char catalytic steam gasification.

used as sorptive media and horticultural potting substrates [25,26]. Compared to the raw biomass, most of volatile and oxygen in char are removed, in which carbon content is accumulated significantly. Thus, introducing steam as gasifying agent into char gasification process not only boosts hydrogen yield, but also obtains high quality of syngas, and the problem of tar can be avoided [27]. But the heterogeneous reaction rate of char gasification is slow and kinetic limitation. This results in an amount of studies surrounding the mechanism and reactivity research of char gasification in order to provide a theory basis for gasifier design during biomass gasification [28,29]. Nevertheless, the studies using char as feedstock to produce high quality syngas are relative scarce. Waheed et al. [30]obtained bio-char from sugar cane bagasse, and hydrogen rich syngas was produced by high temperature steam gasification of bio-char, in which H2/CO ratio was higher than 4. Ding et al. [31] reported the similar result, and they obtained high H2/CO ratio syngas from coal char steam gasification. Gai et al., [32,33] in an investigation of gasification of sewage sludge which pretreated via hydrothermal carbonization and low temperature pyrolysis, concluded that high hydrogen yield and energy efficiency were observed after feedstock pretreatment. Considering the low reactivity of char gasification, adding catalyst to improve char reaction is indispensable otherwise high temperature is required, which certainly increases the risk of equipment loss. Among all the catalysts, GroupⅠmetals are best catalysts for enhancing char gasification, particularly lithium and potassium [34]. Kirtania et al. [35] investigated the kinetic of wood char gasification with different alkali salts, and they pointed out that K2CO3 (0.5 M) had high catalytic activity which was most suitable for catalytic char gasification. Sadhwani et al. [36] compared four metal catalysts (Na, Mg, K and Ca), and they impregnated pine char with metal acetate which was dissolved in ultrapure water. The result showed that K-char has the highest reactivity. In the study of Mostafavi et al. [37], they blended catalyst (K2CO3) with sorbent (CaO) for pellets, and the maximum hydrogen concentration of 80% in syngas was obtained by coal chars gasification. They also obtained high purity hydrogen by ash-free coal gasification integrated with CO2 capture with K2CO3 as catalyst [38]. Furthermore, the mechanism of alkali catalysts has been widely reported, which can be described as follows: [39–42]

K2CO3 + C→ K2 O+ CO2 + C→ 2 K+ CO2 + CO

(1)

2 K+ 2nC → 2KCn

(2)

2KCn + 2H2 O→ 2nC + 2KOH + H2

(3)

2KOH + CO2 → K2CO3 + H2 O

(4)

2. Materials and methods 2.1. Materials Pine woodchip char used in this research was obtained from the carbonization furnace at a local plant in Jiangsu province, China. The particle size of the woodchip char is higher than 2.5 mm with the bulk density of 0.18 g/cm3 and the BET surface area is 190.4 m2/g. The reasons for selecting woodchip char as feedstock were attributed to its absorption capacity and had no loss by entrainment during the gasification process. Table 1 summarized the results of proximate and ultimate analysis of the char. CHNS elemental analysis was conducted by Vario Micro Cube Element Analyzer. In addition, the higher heating value (HHV) of the char was calculated based on the elemental composition of feedstock as follows Eq. (5) [48].

HHV = (349.1 C+ 1178.3 H+ 100.5S−103.4O−15.1N−21.1ASH)/1000 MJ /kg

(5)

2.2. Loading of catalysts Woodchip char was impregnated with four potassium salts (KOH, K2CO3, KCl, and CH3COOK). The mass fraction of 4 wt% of solutions was prepared by dissolving quantitative amount of the potassium salts in water and stirred until complete miscibility. Then, 4 g of the woodchip char was impregnated in 100 g of the prepared solutions to absorb 3 h and accompanied by stirring during the process. After that, the char was took out by filtration and subsequently placed in a oven to dry at 105 °C. In addition, for further study the effect of mass fraction of catalysts on gasification characteristics, four different mass fractions (2 wt%, 4 wt%, 6 wt%, 9 wt%) of KOH solution were investigated. 2.3. Experimental apparatus and procedure

Among this reactions, the formation of active intermediate of potassium is essential, and the reaction (1) is favorable at temperature higher than 1000 K [42]. Afterward, the intermediate reacts with steam for producing potassium hydroxide, which subsequently reacts with carbon dioxide. As a naturally porous material, bio-char has absorption capacity and can be used as the carrier of catalysts. Phuhiran et al. [43] pointed out that some oxygen functional groups contained in char have been proved the ion-exchange capacity with catalysts, which can disperse catalysts within the char matrix. Cao et al. [44] and Yao et al. [45] reported similar studies, and they used coal char and bio-char to support nickel, respectively. In this paper, we take advantage of the absorption of the char to load catalysts, which can increase the catalytic contents in woodchip char and no other researchers have done similar studies. To the best of our knowledge, char in small particle is easily loss by entrainment due to the low reactivity, particularly in fluidized bed [29]. Therefore, we investigated the effect of particle size on gasification characteristics in an updraft fixed-bed gasifier first. In the next step, wood chip char was impregnated in four potassium salts solution, and the effect of different potassium salts on gasification characteristic was performed. Moreover, several operation parameters including different

Fig. 1 shows the schematic diagram of a updraft fixed-bed system, in which a quartz tube (length of 840 mm, inner diameter of 55 mm, heating zone length of 400 mm) as a reactor was placed in a vertical electric furnace. The temperature in this investigation was conducted in the range of 800-950 °C at atmospheric pressure, and type K thermocouples were used to monitor temperature. Steam was generated by a peristaltic pump forcing water into a water evaporator. Oxygen addition was controlled by a mass flow meter with the ER of 0.1–0.4. For each run, 2 g of woodchip char which has absorbed catalysts was Table 1 Proximate and ultimate analysis of woodchip char. Proximate analysis (wt.%, dry basis)

Ultimate analysis (wt.%, dry ash free basis)

Volatile matter

Fixed carbon

Ash

C

H

Oa

N

HHV (MJ/ kg)

11.4

86.34

2.26

87.2

2.45

10.3

1.17

31.2

a

458

Calculated by difference.

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Fig. 1. The schematic diagram of laboratory setup.

loaded into the reactor. Quartz basket was used to hold the bio-char. In the initial of each test, the system was swept by nitrogen to drive oxygen out, afterward heated up to the set-point temperature with a heating rate of 10 °C/min. When the temperature became stable, the quartz basket was placed into reactor and subsequently introduced gasifying agent. When reaction time over, gasifying agent was cut off, and pure nitrogen was purged into whole system for fully collecting the piping-retained product gas, and the flow rate of nitrogen was 1 L/min for 5 min. The outgoing gas flowed through a condenser and collected in a gas bag for analyzing off-line by Shimadzu GC-2014 (Porapak Q column and molecular sieve 5 A column) equipped with thermal conductivity (TCD) and flame ionization detectors (FID) to detect the concentration of components (H2, CO, CO2, N2, O2) and hydrocarbon (CH4, C2+) gases. To ensure the reliability of the data, each experiment was performed at least twice. The uncertainty of the results was estimated to be within ± 5%. The experimental woodchip char conversion is defined as the following equation:

X=

W0−Wt W0 × (1−A)

Table 2 Experimental results of particle size on gasification characteristics. Particle size

< 0.56 mm

0.56–0.9 mm

0.9–1.25 mm

2.5 mm

Hydrogen yield (g/kg char) Woodchip char conversion (%)

242.4

243.1

251.9

249.6

89.1

89.5

91.1

92.1

Gas composition (N2 free and dry basis) H2 (vol.%) 62.7 CO2 (vol.%) 21.2 CO (vol.%) 14.9 CH4 (vol.%) 1.2 H2/CO molar ratio 4.22 H2 + CO (vol.%) 77.6

62.6 20.8 15.4 1.1 4.06 78

62.8 20.5 15.6 1.1 4.02 78.4

62.4 20.1 16.4 1.1 3.81 78.8

Carbon balance (wt%) Carbon in syngas Carbon in residual char Carbon balance

90.6 10.9 101.5

93.4 9.3 102.7

94.3 8.2 102.5

90.2 11.6 101.8

Furthermore, it is worth to note that hydrogen concentration higher than 62.4% was obtained by char gasification, and the H2/CO ratio is higher than 3, which means the high quality of syngas produced by char gasification and it can suit wide applications without adjustment process. Compared to the biomass gasification, hydrogen concentration was promoted significantly by char steam gasification, because biomass gasification is limited by thermodynamic equilibrium which causes hydrogen concentration ranging in 40–60% [49]. More importantly, the CH4 concentration in gases keeps at a low level, which is also beneficial for downstream utilization. Methane can be considered as a pollution and would cause carbon deposition and deactivation of catalysts in the process of F-T synthesis. When the gas is applied in fuel cell, it also demand low concentration of methane [50]. In addition, methane reforming is a strong endothermic reaction, and additional energy is required, which is unfavorable in economy. Thus, the low concentration in methane results the more benefits in next step. Due to the accumulation of carbon in woodchip char, the primary reaction during gasification is water gas reaction (Eq. (7)) accompanied by water gas shift reaction (Eq. (8)) and Boudourd reaction (Eq. (9)). Both water gas reaction (Eq. (7)) and water gas shift reaction (Eq. (8)) produced hydrogen, which can explain the high concentration of hydrogen. In addition, there have some methanation reaction (Eq. (10)) and methane reforming reaction (Eq. (11)) [51], but both reactions are not significant due to the removal of volatile in woodchip char. In general, methane produced by tar and hydrocarbons reforming, little

(6)

where W0 is weight of the initial sample, Wt is the sample weight end of the reaction, and A is the ash content of sample. 3. Results and discussion 3.1. Effect of particle sizes As mentioned above, aerodynamic escaping of small particles may occur during the gasification process. In addition, size reduction by milling is costly and energy requirement. Thus, it is necessary to investigate the effect of particle sizes on gasification characteristics. In this study, the particle sizes between smaller than 0.56 mm, 0.56–0.9 mm, 0.9–1.25 mm and greater than 2.5 mm were selected as object. All the runs were performed at a temperature of 900 °C, a steam flow rate of 0.4 g/min and gasification time of 50 min without catalysts. The effect of particle size is shown in Table 2. It can be observed that there are few differences in the woodchip char conversion and gas composition with different particle sizes. This result indicates that the influence of particle size on gasification characteristic can be almost negligible. Zhai et al. [27]studied the characteristic of rice husk char gasification and found similar results. They concluded that although small particle size has an advantage over in heat and mass transfer rate, it is not obviously when the temperature elevates to 900 °C. 459

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Fig. 2. Effect of different catalysts on (a) gas composition and (b) hydrogen yield and H2/CO ratio.

catalysts on gas composition, hydrogen yield and H2/CO ratio. As observed in Fig. 2b, char adsorbing in potassium salt solution is a feasible pathway, and there are obvious changes in hydrogen yield. K2CO3 and KOH showed a similar and higher catalytic ability and obtained hydrogen yield of 193.5 g/kg char and 191.9 g/kg char, respectively. Compared to the without catalyst, hydrogen yield increased by 63.2% and 61.8%. The activity of CH3COOK is lower than K2CO3 and KOH, and hydrogen yield of 171 g/kg char was obtained. In addition, with the pretreatment in KCl solution, a mildly ascending trend in hydrogen yield was observed, and few differences in gas composition compared with no catalyst. These results imply the catalytic activity is in the order: K2CO3 ≈ KOH > CH3COOK > KCl. Zhang et al. [49]. selected K2CO3, CH3COOK and KCl as catalyst precursors to enhance carbon conversion efficiency, and they concluded when temperature was higher than 700 °C, the catalytic activity of CH3COOK was lower than K2CO3 because the pores may be blocked during CH3COOK decomposition. Similarly, they also confirmed KCl has little catalytic activity, which can difficultly be activated under the operational conditions. Although, adding catalysts can improve char conversion and hydrogen yield while hydrogen concentration presented a decrease trend. With the catalysts of K2CO3 and KOH, hydrogen concentration

formation of methane means the clean and high quality of syngas can be obtained by char gasification.

Water gas reaction: C+ H2 O→ H2 + CO

ΔH298K = 131 kJ/mol

Water gas shift reaction: CO + H2 O↔ H2 + CO2

(7)

ΔH298K

= −40.9 kJ/mol Boudourd reaction: C+ CO2 → CO ΔH298K = 162 kJ/mol Methanation reaction: C+ 2H2 ↔ CH 4 ΔH298K = −75 kJ/mol

(8) (9) (10)

Methane reforming reaction: CH 4 + H2 O↔ 3H2 + CO ΔH298K = 206.3 kJ/mol

(11)

3.2. Effect of catalysts To examine the effect of different potassium salts on char gasification, woodchip char (> 2.5 mm) was subjected to impregnate with four potassium salts (K2CO3, KOH, CH3COOK, KCl) to absorb catalyst, as base cases at a temperature of 850 °C, a steam flow rate of 0.4 g/min and the reaction time of 25 min. Fig. 2 presents the influence of 460

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3.3. Effect of reaction temperature

decreased from 65.1% to 60.9% and 61.1%, CO2 concentration decreased from 18.6% to 13.6% and 12.4%, and CO concentration increased by 69.8% and 76.7%, respectively. Reactions (1)–(4) describe the mechanism of catalytic gasification, and water gas reaction (Eq. (7)) is the summation of this four reactions. The reactivity of char gasification was improved significantly, resulting in a high hydrogen yield. However, catalysts also facilitate Boudourd reaction (Eq. (9)) in the meantime. Bouraoui et al. [28]and Perander et al. [52] in the investigation of CO2 gasification of bio-char with potassium as catalyst, suggested that gasification rate increased dramatically with the increase of K content. Thus, significantly increasing of CO concentration can be seen from Fig. 2a. Additionally, the decrease of hydrogen concentration and the increase of CO concentration results in the declining of H2/CO ratio. As shown in Fig. 2b, the H2/CO ratio decreased to 2.41 and 2.37 with the catalysts of K2CO3 and KOH. Both K2CO3 and KOH show superior in catalytic woodchip char steam gasification. The maximum syngas (H2+CO) of 86.9% was obtained when KOH used as catalyst, that because KOH can react with CO2 to form potassium carbonate. This reaction is expected to promote the quality of syngas as the useless content was removed. Furthermore, considering the char gasification rate and economy, the mass fractions of the potassium salt solution requires to be discussed. In this research, we elected KOH as object to study the effect of different mass fractions on gas characteristics. The experimental results depicted in Fig. 3 with the temperature of 850 °C and steam flow rate of 0.4 g/min. As shown in Fig. 3, with the increase of mass fraction, hydrogen and CO2 concentration decreased from 65.1% and 18.6% to 60% and 11.5%, respectively. The reduced content was complemented by the increase of CO concentration that increased from 14.6% to 27.9%. This finding can be explained by the increase of catalyst loading. As can be seen from Fig. 4, the ash content was elevated from 2.26 wt% to 12.34 wt% with the increase of impregnation solution concentrations from 0 wt% to 9 wt%, which means more potassium salts loaded on the char. However, increasing amount of catalysts would cause the decrease of the fixed carbon content, which result in the decrease of hydrogen yield. The maximum hydrogen yield of 197.2 g/kg char was obtained with 6 wt% of the KOH mass fraction. Moreover, it is worth to mention that slight changes in woodchip char conversion were observed when KOH mass fraction was higher than 6 wt% (Fig. 4). Based on the above analysis, the mass fraction of KOH solution controlled at 6 wt% is more suitable.

Temperature is one of the most significant factors for thermochemical conversion of woodchip char because the main reactions are endothermic, and it can affect gasification rate and thermal equilibrium directly. Fig. 5 illustrates the effect of rising temperature on gas composition. The experiment was conducted at a steam flow rate of 0.4 g/ min and 4 wt% mass fraction of KOH solution. Results showed that raising reaction temperature from 800 °C to 950 °C resulted in the rapid decrease in hydrogen and CO2 concentration from 64.4% and 17.4% to 57.1% and 7.7%, respectively. CO concentration increased from 17.4% to 34.7%, and CH4 concentration hovered around a constant low level. The primary reaction (Eq. (7)) is endothermic reaction. Thus increasing temperature is beneficial to improve char conversion and hydrogen product. As can be seen from Table 3, hydrogen yield increased from 156.8 g/kg char to 215 g/kg char, and almost all the char converted into gases at the temperature of 950 °C. However, Boudourd reaction (Eq. (9)) is also an endothermic reaction, and high temperature would shift equilibrium to the right side. In addition, water gas shift reaction (Eq. (8)) is an exothermic reaction, which is unfavorable at high temperature. This implied the decrease of hydrogen and CO2 concentration and the sharply increases of CO concentration. Similar detected trend reported by Acharya et al. [53], and they attributed the reducing of hydrogen concentration to the promotion of reverse water-gas shift at high temperature. Furthermore, it is interesting to note that high quality of syngas can be obtained at high temperature, and the ratio of syngas (H2 and CO) was higher than 90% presented in Table 3. However, a negative influence is the lower ratio of H2/CO, which decreased to 1.65 at the temperature of 950 °C. Higher temperature requires more energy input and a higher level of gasifier, which increases the running cost certainly. Thus, in consideration of economy and gasification rate, proper temperature within 850–900 °C is suggested for char catalytic gasification. 3.4. Effect of gasifying agent Steam gasification is known for producing higher quality syngas with high concentration of hydrogen. The main aim of adding oxygen in gasifying agent was to enhance the rate of reaction and evolve the quality of syngas. The results of the influence of gasifying agent illustrates in Table 4. During char gasification, element H is mainly derived from H2O. Fig. 3. Effect of different mass fractions on gas composition and hydrogen yield.

461

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Fig. 4. Woodchip char conversion rate and ash content in the char.

0.8 g/min. In addition, woodchip char conversion also presented a same trend, and no significant changes were observed when steam flow rate was higher than 0.4 g/min. In the meantime, higher steam flow rate would result in the increase of CO2 concentration, which is not a desired composition. As a consequence, the steam flow rate of 0.4 g/ min is more suitable in this research. Hussein et al. [47] studied the influence of oxygen addition on chicken manure steam gasification and reported gasification reaction rates enhanced by oxygen. In the study undertaken by Chen et al. [54], same observation was found, and carbon conversion increased by 37.7% with increasing ER from 0.025 to 0.1. Similarly, in our study, increasing ER from 0 to 0.4, woodchip char conversion increased from 89.4% to 99.1%. Eqs. (11) and (12) are reactions that oxygen participates, which are strong exothermic reaction. Thus, with the rising of ER, more char and hydrogen reacted with oxygen. The decrease of hydrogen yield and concentration was attributed to this reason. Additionally, the H2/CO ratio can be adjusted by adding oxygen, and the ratio presented a decline tendency with the raising of ER. It is an efficient pathway to obtain the wide range of H2/CO ratio to satisfy more applications without consuming additional energy. However, it has to note that excessive oxygen can weaken the quality of syngas due to the gradually increase of CO2 concentration.

Fig. 5. Effect of reaction temperature on gas composition.

Table 3 Effect of temperature on gas characterization. Temperature

800 °C

850 °C

900 °C

950 °C

Hydrogen yield (g/kg char) Woodchip char conversion (%) H2/CO molar ratio H2 + CO (vol.%)

156.8 61.2 3.70 81.7

191.9 89.4 2.37 86.9

209.2 97.0 1.82 90.2

215 99.7 1.65 91.8

Char oxidation: C+ O2 → CO2

(12)

Hydrogen oxidation: H2 + 1/2O2 → H2 O

(13)

3.5. Integrated biomass utilization system In convention, biomass utilization process includes gasification, purification, syngas adjustment and downstream applications [55]. The present of tar and other pollution would weaken the downstream catalyst activity. Therefore, purification is unavoidable, but tar removal process is costly meanwhile damaging the equipment surfaces. Thus, a novel and simplified process for high quality syngas is proposed (Fig. 6b). In this process, biomass is pretreated in a carbonization furnace firstly, in which by-products including tar and fuel gas are produced, and it can be burned to supply heat for biomass carbonization. In this stage, the waste heat also can be transferred into carbonize biomass. Secondly, the char impregnates in potassium salt solutions to absorb catalyst. Afterward, the char loaded with catalysts is introduced into gasifier with steam or steam-O2 as gasifying agent. After separation of

Thus the quantity of steam directly affects the hydrogen content. As displayed in Table 4, hydrogen yield and concentration increased from 76.2 g/kg char and 50.0% to 173.8 g/kg char and 57.0% with steam flow rate increasing from 0.2 g/min to 0.8 g/min. The increase of steam flow rate enhanced the water gas reaction (Eq. (7)) and water gas shift reaction (Eq. (8)) which contribute the lift of hydrogen content. The other benefit of raising steam flow rate is the increase of H2/CO ratio, which increased from 1.54 to 2.62. However, higher quantity of steam means more energy is consumed to generate steam. Therefore, taking energy consumption into consideration, the limitation of the steam amount is inevitable. As we can see, when steam flow rate raised from 0.2 g/min to 0.4 g/min, hydrogen yield increased almost 2 times while it only increased by 12.4% with further increasing steam flow rate to 462

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Table 4 Effect of gasifying agent on gasification characteristics. Reaction conditions

Temperature: 850 °C; mass fraction of KOH: 4 wt%; reaction time: 25 min

ER Steam flow rate (g/min) Hydrogen yield (g/kg char) Woodchip char conversion (%)

0 0.4 191.9 89.4

Gas composition (N2 free and dry basis) H2 (vol.%) 61.1 CO2 (vol.%) 12.4 CO (vol.%) 25.8 CH4 (vol.%) 0.7 H2/CO molar ratio 2.37 H2 + CO (vol.%) 86.9

0.1

0.2

0.3

0.4 116.6 99.1

0.2 0.2 76.2 55.9

173.3 94.8

154.6 96.4

130.3 98.6

0.4 154.6 96.4

0.6 161.2 97.5

0.8 173.8 99.7

58.0 16.0 25.3 0.8 2.29 83.2

53.9 17.5 27.9 0.6 1.93 81.8

50.2 21.7 27.5 0.6 1.83 77.7

48.1 26.9 24.2 0.8 1.99 72.3

50.0 16.6 32.6 0.8 1.54 82.6

53.9 17.5 27.9 0.6 1.93 81.8

55.2 19.0 25.2 0.7 2.19 80.4

57.0 20.6 21.8 0.6 2.62 78.8

gasification was an efficient way for producing clean and high quality syngas. The particle sizes of the char had little influence on gasification characteristic. For catalytic char gasification, take advantage of the adsorption of char to load catalyst and subsequently gasification was a feasible method. The catalytic activity of four potassium salts used in the experiment was in the order: K2CO3 ≈ KOH > CH3COOK > KCl. The influence of the mass fraction of the potassium salt solution was investigated and maximum hydrogen yield was obtained with 6 wt% of the KOH solution while further increasing mass fraction caused the decrease of hydrogen concentration and H2/CO ratio. The influence of operational conditions including reaction temperature and gasifying agent were investigated. Higher temperature was beneficial for the yield of hydrogen and purity of syngas, and the maximum syngas (H2 + CO) ratio of 91.8% was obtained at 950 °C. However, high temperature would cause the decrease of hydrogen concentration and H2/CO ratio. Steam introduced had a positive influence on hydrogen content, but excessive steam quantity had slight influence on gasification characteristic. Adding oxygen in gasifying agent can improve the gasification rate, and the ratio of H2/CO can be adjusted without additional energy consumption. In addition, a new integrated biomass utilization system was proposed, through which high quality syngas with a wide range of H2/CO ratio can be obtained, and the gases can satisfy many applications with a simplified process.

CO2 and H2O, the high quality syngas can be directly used for synthesizing liquid fuels or natural gas, or be used to produce high purity hydrogen by WGC and membrane separation. In addition, based on our experiment analysis, when steam was used as gasifying agent, higher ratio of H2/CO was obtained, which is suitable widely applications. Introducing oxygen is beneficial for char conversion and can adjust H2/ CO ratio to satisfy the specific utilization. In the investigation of Muradov et al. [56], they obtained liquid fuels via Fischer-Tropsch synthesis from the syngas produced by steam-O2 gasification of charred pinewood pellets. They selected char as feedstock to avoid the additional complicating factors such as tar and sulfurous removal. Compared to the conventional route, purification and adjustment can be simplified, and both of them are energy consumption process. Moreover, biomass gasification in fluidized-bed gasifier requires the small particles to increase the surface area and gasification efficiency. Nevertheless, in the proposed strategy, the woodchip char without grinding used as feedstock can reduce the energy consumption for milling. Based on the aforementioned analysis, it may be a promising way to convert biomass into high quality syngas. 4. Conclusions In this study, woodchip char catalytic gasification to produce high quality syngas was investigated in a lab-scale fixed-bed gasifier. Char

Fig. 6. Integrated biomass utilization system (a) conventional route and (b) a new process.

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Acknowledgments [27]

Support by the Forestry Science and Technology Extension Project ([2016] 15), the National Science and Technology Support Plan (2015BAD21B05), and the Central Public-interest Scientific Institution Basal Research Fund (CAFINT2014K04) is acknowledged.

[28]

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