Co-gasification of Alkaline Black Liquor and Coal in Supercritical

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Co-gasification of Alkaline Black Liquor and Coal in Supercritical Water at High Temperatures (600−750 °C) Changqing Cao,*,† Youyou He,† Gaoyun Wang,† Hui Jin,† and Ziyang Huo‡ †

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China Queensland Micro- and Nanotechnology Centre, Griffith University, Brisbane 4111, Australia

‡

ABSTRACT: The catalytic activity of the alkali compounds in alkaline black liquor can be used in supercritical water gasification (SCWG) of coal to lower the reaction temperature and improve the hydrogen production, and the black liquor can be handled simultaneously. In the present study, the gasification features of coal/black liquor blends at high temperatures (600−750 °C) were studied through thermodynamic analysis and experimental study. A synergetic effect was found during co-gasification of coal and black liquor, where both the gasification efficiency and hydrogen production was improved by efficiently using the alkali catalyst in black liquor. The highest improvement of the gasification was found at the blending ratio of about 50:50. Both the thermodynamic analysis and experiments indicated that higher temperature favors the hydrogen production. The gasification efficiency was improved with temperature and the maximum carbon conversion of 79.46% was achieved at 750 °C. The dilution of the black liquor/coal blends favors the gasification by improving the gasification efficiency and H2 production. The prolongation of reaction time enhanced the gasification, but its influence was insignificant when it was above 10 min. The initial pressure of the reactor and the reactant amount impacted the gasification results in different ways. This study may fill the research gaps in SCWG of coal/black liquor blends at higher temperatures and assist in its further development. at a lower temperature (700 °C) when K2CO3 was used as the catalyst.14 Taking advantage of the fluidized-bed reactor, the needed gasification temperature can be further reduced. We achieved nearly complete gasification of different ranks coals (Zhundong coal, Shenmu bitumite, Hami lignite and Yimin lignite) at 620−690 °C with K2CO3 as the catalyst.9 However, the usage of high-price alkali catalyst will improve the operating cost of this process, especially as the alkali recovery is still a challenge until now. To reduce the cost of alkali catalyst, some alkali-containing wastewater is used as inexpensive additives to catalyze SCWG of coal. Black liquor generated from pulping process contains a lot of alkali, which can be used as inexpensive alkali resource for conventional gasification of coal15 and petroleum coke16 as well as SCWG of coal.17 Its gasification in SCW has been investigated in much research,18−22 which revealed that it is a promising and innovative treatment method of black liquor. Through this treatment, the pollutant of black liquor can be eliminated and clean hydrogen-rich gas can be produced simultaneously. Therefore, black liquor can also be converted into clean energy when being used as the alkali resource during SCWG of coal. Besides, some other advantages can be expected in the co-gasification of coal and black liquor: (1) The water content of black liquor is usually higher than 80 wt %, so its addition can reduce the water consumption for coal-water slurry preparation. (2) Black liquor can serve as the dispersant for coal-water slurry and improve the slurryability,23,24 which can reduce the consumption of the other additives. (3) SCWG of black liquor can be integrated with the pulping process and

1. INTRODUCTION Coal has been used as the major energy resource in the world for quite a long time. In particular, in China, about 70% of power is still produced from coal-fired power plant in recent decades.1 However, coal combustion can generate much pollutant, including SO2, NOx, and fine particles, which can cause severe environmental problems.2−4 Therefore, a clean and efficient coal conversion technology is highly desired today. Supercritical water gasification is an innovative energy conversion method working at the temperatures above 374 °C and the pressure above 22.1 MPa. It takes advantage of the unique properties of supercritical water (SCW), such as low dielectric constant, high diffusivity, and low viscosity and solubility of organics and gases to transform biomass, organic wastes, and coal into hydrogen-rich gases.5−8 Less of these previously-mentioned pollutants were generated in the supercritical water gasification (SCWG) of coal for the special reacting environment, which can significantly reduce the investment of the decontamination equipment and operating cost. Some investigations on SCWG of coal have been done and some encouraging results were achieved.9−12 Therefore, SCWG is a promising coal utilization method, which can ease the growing problems of environment pollution. For the unique property and constitution of coal, a high temperature was required to realize its complete gasification. Ge et al.13 achieved complete gasification of lignite (10 wt %) at 950 °C in the capillary quartz reactor, and even higher temperature (980 °C) was needed for complete gasification of bituminous coal (10 wt %). The high gasification temperature can bring challenges to the reactor construction and improve its construction cost. The use of alkali catalyst was proved to be able to enhance the gasification and lower the gasification temperature. For example, complete gasification of lignite (2 wt %) was achieved © XXXX American Chemical Society

Received: October 9, 2017 Revised: November 16, 2017

A

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Component Analysis of Air-Dried Coal and Black Liquor Solid ultimate analysis (%) black liquor Hongliulin coal a

proximate analysis (%)

C

H

Oa

N

S

M

A

V

FC

31.07 68.87

2.96 4.80

28.33 11.82

0.55 1.02

0.93 1.41

3.81 7.8

32.35 4.29

45.69 36.65

18.91 51.26

By difference. characterizations, ultimate analysis was carried out with PerkinElmer CHNS/O element analyzer, and proximate analysis was carried out with a proximate analyzer (SDTGA5000) manufactured by Sundy Science and Technology CO., Ltd. 2.2. Experiments. We carried out the experiments using a batch reactor with an internal volume of 10 mL. It is made of Inconel-625, which allows it to work at high temperatures and pressures up to 750 °C and 30 MPa. The reactor material was reported to also benefit the gasification of black liquor.20 A pressure sensor and K-type thermocouples were inserted in the reactor to monitor the system pressure and reactant temperature, respectively. For each experiment, a certain amount of black liquor was first loaded into the reactor with a syringe, and a certain amount of coal powder was added directly. Then the reactor was sealed and shaken several times for uniformly mixing. Next, it was purged with argon more than three times to eliminate the residual air and avoid its influence on the gasification. After that, the reactor was heated quickly by being put into an oven preheated to the set temperature. The average heating rate of the reactant is about 30 K/min. When the temperature reached the set point, the pressure was controlled in the range of 23−26 MPa by adjusting the amount of the reactant. The variation of the pressure in this range can ensure both the supercritical pressure of water and the safety operation of the reactor. The reaction temperature was manipulated by a PID controller with a deviation of ±5 °C. After the gasification, the reactor was quenched in cold water to terminate the reaction quickly. When the reactor was cooled to the ambient temperature, the gas product was collected for composition analysis with a gas chromatography (GC) and volume measurement with a wet-type gas flow meter. More detailed description of this reactor can be found in our previous study.29 2.3. Analysis Method. The composition of the collected gas product (H2, CO, CO2, CH4, C2H4, and C2H6) was analyzed by gas chromatography (Agilent 7890A) equipped with a thermal conductivity detector (TCD) and a Plot C-2000 capillary column. The column worked in the oven with programmed temperatures. It worked at 60 °C for 1.5 min first and then was heated to 150 °C a rate of 40 °C/min, and it was holding at this temperature for 5 min. Argon with a purity above 99.999% was used as the carrier gas at a flow rate of 5 mL/min. The composition of the sulfur-containing gases was not detected in this study for the relatively low sulfur content in the reactants. 2.4. Data Interpretation. In this study, the gasification efficiency (GE), carbon gasification efficiency (CE), and gas yield were used to evaluate the gasification performance of coal and black liquor blends. They were defined as the following equations:

produce not only hydrogen but also power and heat for the pulp mill and the surrounding communities.25,26 The cogasification of coal and black liquor can improve the scale of the gasification system and improve the energy self-sufficient of the pulping process, which can also supply more energy for the mill and the communities. As a result, we investigated the influence of black liquor addition on SCWG of coal in our previous study.17 It was found that both the presence of inherent alkali and lignin in black liquor can improve the gasification of coal and increase the hydrogen production. In particular, the alkali acted as effective catalyst and improved the gasification efficiency. However, only low-temperature (550 °C) cogasification was tested for the constraint of the reactor at that time, so the carbon gasification efficiency was extremely low (10%−50%).17 According to the literature, the efficient gasification of both coal and black liquor required higher temperatures.9,20,22 Therefore, the investigation of high-temperature gasification of coal/black liquor blends is required to provide more information on high-efficiency gasification features, which is also essential to the practical application of this technology. In this study, we investigated co-gasification of black liquor and coal using a batch reactor made of Inconel-625, which allows us to conduct the experiments at high temperatures up to 750 °C. Alkaline wheat straw black liquor was chosen because straw is still an important pulping resource in the countries that lack of forest and wood resources, such as China and India.27,28 First, thermodynamic analysis of the cogasification based on the principle of Gibbs energy minimization was studied to understand the influence of the reaction temperatures on the SCWG of coal/black liquor blends from the theoretical aspect. The influence of reaction temperature and the total concentration of coal/black liquor blends on the gasification was estimated. Then the influence of the mixing ratio of black liquor and coal was investigated experimentally to find the optimal preparation method. With the optimal mixing ratio of black liquor and coal, the influence of the main operating parameters, including the reaction temperature, total concentration of the mixture, reaction time, initial pressure of the reactor, and the reactant amount, was studied to get the gasification features at higher temperatures.

GE =

2. EXPERIMENTS AND METHODS

total mass of gas product × 100% mass of ash‐free coal/black liquor blends solids (1)

2.1. Feedstock. Black liquor from soda pulping of wheat straw collected from a local pulp mill was used in this study because wheat straw is a major pulping raw material in China. Black liquor mainly contains hemicellulose and lignin and their derivatives, which is the residue after the cellulose was extracted from raw biomass. It also contains the alkalis that used in the pulping process and therefore has a high pH value (12.4). The total solid content of the black liquor is 9.5 wt %, and component analysis of black liquor solid are listed in Table 1. A typical bituminous coal collected from Hongliulin of Shaanxi Province was used as another feedstock in this study. It was ground and screened to the particle diameters under 150 um before being used. The ultimate and proximate analysis of the air-dried Hongliulin coal were also performed and listed in Table 1. For their

CE =

carbon content of gas product × 100% carbon content of coal/black liquor blends

gas yield =

(2)

mol of the gas product mass of ash free solids of coal/black liquor blends (3)

2.5. Thermodynamic Analysis. Thermodynamic analysis of SCWG of black liquor and coal blends was performed with Aspen Plus software using the principle of minimizing Gibbs energy. Because there are no available data for coal and black liquor, their B

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Influence of temperature on the (a) composition and (b) yield of the gas product from SCWG of coal/black liquor blends at equilibrium state (blending ratio of 50:50; total concentration of 9.5 wt %; 25 MPa).

Figure 2. Influence of coal/black liquor ratio on the (a) gas composition and (b) CE during their co-gasification in supercritical water (700 °C; 30 min; total concentration of 9.5 wt %). The dotted line in panel b represents the theoretical CE of the blends. characterization results listed in Table 1 were input in the software to estimate the thermophysical properties. According to the research experience and gasification results, H2, CO, CH4, CO2, C2H4, and C2H6 were assumed as the possible product components. The calculation was under the assumption of complete gasification of coal/ black liquor blends. The property method based on the Peng− Robinson equation of state coupled with the Boston−Matuias α function (PR−BM) was used to calculate the thermodynamic and transport properties under the studied conditions. This method has been used in the thermodynamic analysis of the reactions occurred in supercritical water by several researchers and was reported to be convincible in calculation.30−32 The detailed description of the thermodynamic analysis method can be found in our previous study.17

increased with the temperature. This may be because water− gas shift reaction is an exothermic reaction, which was inhibited at higher temperature.33,34 Thus, the consumption of CO was reduced, which increased the CO fraction in the gas product. With the increase of H2 fraction, the H2 yield increased remarkably with the temperature. It worth noting that the yield of CO2 increased with the temperature, but its fraction in the gas product decreased. This may be because the increase of H2 yield was more significant than the CO2 yield, which resulted in the decrease of CO2 fraction in the gas product. The predicted results agreed with the thermodynamic analysis result of SCWG of black liquor alone,22 in which hydrogen production was also promoted at higher temperatures. This result indicated that co-gasification of black liquor and coal at higher temperatures (600−750 °C) in this study will be distinct from the results obtained in our previous study that operated at 550 °C.17 From the prediction, more H2 and less CH4 will be produced from gasification at higher temperatures. For example, the H2 fraction of the gas product was 58.16% at 750 °C, which is more than two times of that achieved at 550 °C (28.92%). However, the CH4 fraction in the gas product was decreased from 30.95% to 7.19%, when the temperature increased from 550 to 750 °C. As a result, it is necessary to study co-gasification of coal and black liquor at higher temperature to find the unique gasification performance.

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Analysis. The influence of temperature on the gasification performance of coal and black liquor blends with a ratio of 50:50 was predicted through thermodynamic analysis. The studied temperatures ranged from 500 to 800 °C, which covers the temperatures studied in this work (600−750 °C) and the previous study (550 °C)17 for comparison. The results showed that the temperature had a significant influence on the gas product composition from the co-gasification (Figure 1). With the increasing of temperature, the H2 fraction increased significantly, and the fraction of CO2 and CH4 decreased. The fraction of CO in the gas product was extremely low throughout the studied temperatures and C

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Influence of reaction temperature on co-gasification of black liquor and coal in supercritical water (30 min; total concentration: 9.5 wt %; black liquor to coal ratio: 50:50).

3.2. Influence of Mixing Ratio. To determine the suitable coal/black liquor ratio, we gasified them with different blending ratios at 700 °C. The increase of black liquor fraction in the blends increased the H2 fraction and decreased the CO fraction in the gas product (Figure 2). It can be attributed to the high alkali content in black liquor, which was widely reported to be able to catalyze the water−gas shift reaction (eq 4).35−37 Therefore, the addition of black liquor and the increase of its fraction improved the H2 production and decreased the CO production. However, the influence on the gas composition by adding black liquor was relatively smaller than our previous study on co-gasification of lean coal and black liquor at 550 °C.17 We assumed that this difference was related to the higher temperature used in this study. From the thermodynamics analysis (Figure 1), high temperature prohibited H2 production in SCWG and helped quickly approach the equilibrium results. As a result, the improvement brought by the catalytic effect of the alkali was not as significant when the temperature is higher: H 2O + CO → H 2 + CO2

Figure 2b. However, the realistic CE of the blends from the experiments are higher than the calculated ones. It indicated a synergetic effect between coal and black liquor during cogasification, where the existence of coal and black liquor enhanced the gasification of each other under this condition. As reported in our previous study,17 both the alkali and the lignin in black liquor can promote the coal gasification in supercritical water. Among them, the alkali played the role of catalyst to promote the gasification and inhibit the char formation. The main organic component of black liquor, lignin, also promoted the gasification because its decomposed intermediates can promote the extraction of the substances coated outside the coal particle and favor the coal gasification. However, the existence of coal may also promote the gasification of black liquor. As reported by Jin et al.,10,38 a porous char can be formed in SCWG of coal after the removal of the volatiles. The char may provide a good reaction surface for the gasification and played the catalytic role of activated carbon in SCWG of biomass as reported,39,40 which improved the gasification efficiency of black liquor. From Figure 2b, the maximum promotion of the gasification by mixing coal and black liquor was achieved when the ratio was about 50:50. With less black liquor presented in the blends, inadequate alkali catalyst was supplied by black liquor and the coal gasification was not sufficiently catalyzed and promoted. While the black liquor fraction is too high, the alkali brought by the black liquor may exceed the demand and cannot be used sufficiently. Our previous study17 obtained a similar result, although different rank of coal (lean coal) was used, which found that 50% of black liquor has been adequate to supply the needed alkali catalyst for the gasification and more addition of black liquor had negligible influence on the gas composition. Both studies indicated that this is the optimal blending ratio for coal and black liquor for co-gasification. 3.3. Influence of Temperature. A main goal of this study is to investigate the gasification performance of coal/black liquor blends at higher temperatures. Taking advantage of the batch reactor made of Inconel-625, we gasified the blends of coal and black liquor with a ratio of 50:50 at 600−750 °C. The reaction temperature showed a significant influence on the gasification efficiency and a slight influence on the gas composition (Figure 3). The increase of temperature decreased the fractions of the alkanes (CH4 and C2H6) in the product. It may be because high temperature can further promote their reforming reactions (6 and 7) because both are endothermic

(4)

It worth noting that the fraction of ethylene (C2H4) was lower than the ethane (C2H6) fraction throughout this study. As shown in Figure 2a, all the fractions of C2H4 were lower than 0.13%, while the fractions of C2H6 were in the range of 0.09−1.28%. This result has been noticed and discussed in our previous studies.17,22 We assumed that ethylene may be tended to be transformed into ethane through hydrogenation reaction in the hydrogen-rich reaction atmosphere during gasification. The thermodynamic analysis also supported this point as shown in section 3.4, where more C2H6 was generated than C2H4 under the same reaction conditions. Given that the gasification performance of coal and black liquor during co-gasification was not influenced by each other, the theoretical CE of the blends (CEblends) can be calculated by their separate gasification efficiencies under the same reaction condition: CE blends = x BL· CE BL + (1 − x BL) ·CEcoal

(5)

where xBL is the fraction of black liquor in the blends; and CEBL and CEcoal are the CE of black liquor and coal, respectively, under the same reaction conditions during their separate gasification. Thus, the theoretical CE of the blends can be calculated through eq 5 and should be distributed in a line that go through CEBL and CEcoal as the blue dotted line shown in D

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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Boucard et al.18 found that the existence of nano-CeO2 can promote the conversion of black liquor in SCW and inhibit the charcoal generation. The preparation and development of suitable catalyst for co-gasification of coal and black liquor may also help to lower the required temperature. 3.4. Influence of Total Concentration. Concentration is another important parameter in SCWG, which will determine the scale of the treatment plant. The influence of the total concentration of coal and black liquor in the range of 5.0− 17.3% was explored with a mixing ratio of 50:50. It needs to be mentioned that there is an upper limit of the total concentration of the mixture when the black liquor concentration and the mixing ratio are fixed. The highest concentration can be obtained by directly adding certain amount of coal powder into black liquor without dilution. In this study, the as-received black liquor has 9.5 wt % solid content, so 9.5 g of coal can be mixed with 100 g of black liquor to get a mixing ratio of 50:50. In this case, the total concentration is calculated to be 17.35 wt %, which is also the highest total concentration tested in this study. Under the studied conditions, the increase of the total concentration reduced the H2 fraction and increased the CH4 fraction in the gas product. This change trend with the concentration agreed with the thermodynamic analysis results as shown in Figure 4. It indicated that this change of gas

reactions. Additionally, the H2 fraction was relatively high, which was in the range of 54.73%−57.07%. This result was partly in agreement with thermodynamic analysis, which showed that the H2 fraction in the gas product was in the range of 46.5%−58.16% at 600−750 °C (Figure 1a). However, the H2 fraction was not increased with the temperature as predicted by the thermodynamic analysis. We assumed that the high alkali content of black liquor catalyzed the WGS reaction in SCWG and help to achieve the high H2 production even at lower temperatures: 2H 2O + CH4 → 4H 2 + CO2

(6)

4H 2O + C2H6 → 7H 2 + 2CO2

(7)

The variation of temperature showed a more-significant impact on the gasification efficiencies. The CE of the blends increased from 36.88% to 79.46% when the reaction temperature increased from 600 to 750 °C. This may be because both coal and black liquor are mainly composed of polycyclic aromatic hydrocarbons, which were proven to be hard to decompose in SCWG.10,33,41 Therefore, higher reaction temperature is required to sufficiently decompose them and further improve the gasification efficiency. The important role of the temperature can also be seen from the comparison of this study to the previous study.17 Though the low flowability of the reactant in the batch reactor used in this study may be unfavorable to the mass transfer of the reactant comparatively, the CE of the mixture at 600 °C in this study (36.88%) was still higher than that achieved in the previous study that conducted at 550 °C (31.00%). Besides the high temperature adopted here, the longer reaction time (30 min) may also contributed to this result. The GE also increased significantly with the temperature. With the participation of water in the reaction, the GE of the blends was higher than 100% at 700 and 750 °C, and the maximum GE reached 185.43% at 750 °C. The gasification still did not approach to complete even at 750 °C. Maybe gasification at higher temperature can help to obtain this goal. However, the temperatures tested here has been too high for the current technology of reactor construction and will bring challenges to the practical application. Therefore, some other alternative methods are required to achieve the complete gasification at applicable temperatures. First, optimization of the reactor construction may help to resolve this problem to some extent. For example, using continuous reactor can enhance the mass transfer of the reactant for their intense movement during gasification, which will help to improve the gasification efficiency. Especially in fluidized-bed reactor, the coal particles can move up and down under the force of gravity, drag force, and buoyancy force when the reactor is properly constructed. The back mixing of the coal particles can be realized with the variation of the reactor inner diameters and the coal particles diameters during reaction, which will prolong the residence time in the fluidized-bed reactor.42 Our previous study showed that the usage of fluidized-bed reactor can realize the complete gasification of different rank coals at around 620−690 °C with the catalysis of alkali.9 It is lower than the temperature required in the batch reactor with alkali catalyst (700 °C)14,29 and that in quartz capillary reactors without catalyst (950 °C).13 Therefore, the usage of fluidized-bed reactor will lower the required temperature for complete gasification of coal and black liquor blends. The other method is to use proper catalyst to improve the gasification efficiency at lower temperatures. For example,

Figure 4. Influence of total concentration on the gas composition from co-gasification of coal/black liquor blends predicted by thermodynamic analysis (700 °C; 25 MPa; black liquor to coal ratio of 50:50).

product composition was mainly influenced by the thermodynamic features of the reaction. When the total mixture concentration was diluted to 5 wt %, the highest H2 fraction of 59.26% was obtained, which is also the highest H2 fraction obtained in this study. In addition, the alkane gas products (CH4 and C2H6) increased with the mixture concentration. For these gas components, steam reforming may be the main reactions for their consumptions and may be inhibited for the low water fraction in gasifying high-concentration feedstock. Thus, the fractions of CH4 and C2H6 were improved with the increasing of the total concentration, which was also in agreement with the thermodynamic analysis results (Figure 4). Corresponding to the thermodynamic analysis, the fraction of C2H4 was extremely low in the gasification and considerably lower than the fraction of C2H6. The total concentration also showed a great influence on the gasification efficiency of the blends of coal and black liquor. As shown in Figure 5b, both the CE and GE was reduced with the E

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Influence of total concentration of coal/black liquor blends on co-gasification performance in supercritical water (700 °C, 30 min, black liquor to coal ratio of 50:50).

Figure 6. Influence of the reaction time on SCWG of black liquor and coal blends (700 °C, black liquor/coal = 50:50; total concentration = 9.5 wt %).

increase of the total concentration. The CE and GE was reduced from 73.76% and 163.74% to 43.94% and 86.16%, respectively, when the total concentration was increased from 5.17 to 17.12 wt %. This result was assumed to be associated with the increase of the mass-transfer resistance in gasification of the blends with high concentration. Especially in the batch reactor used here, the mass-transfer resistance may be higher because the reactants cannot move and flow intensively in the reactor. Therefore, the intermediates and products generated around the coal particles cannot be taken away in time, which may inhibit the further reactions of the coal particle. On the contrary, the dilution of the mixture reduced the mass-transfer resistance and promoted the gasification. When the coal−black liquor mixture was diluted to 5 wt %, the highest CE of 73.76% was obtained. 3.5. Influence of Reaction Time. The influence of reaction time in the range of 5−40 min on co-gasification of coal and black liquor was studied at 700 °C. The variation of the reaction time in this range showed slight influence on the composition of the gas product (Figure 6). The prolongation of reaction time decreased the CO2 fraction and increased the CH4 fraction slightly. The H2 fraction increased slightly with reaction time in the range of 5−20 min, but it was almost independent of the reaction time when it was longer than 20 min. Meanwhile, the CO fraction was increased, and fractions of C2H4 and C2H6 was decreased slightly. The decreasing C2H4

and C2H6 fractions in the gas product may be because their reforming reactions were carried out more thoroughly with longer reaction time. Similar results were also noticed in our previous study on SCWG of black liquor in a continuous flow reactor.19 We assumed that this may be because the reactions between the gases and intermediates were very fast, especially with the alkali in black liquor as catalyst. As a result, the composition of the gas product can reach the equilibrium states quickly after the gases and intermediates were generated. The variation of the reaction time showed a greater influence on GE and CE, particularly in lower ranges (Figure 6b). When the reaction time increased from 5 to 10 min, CE increased from 43.98% to 56.25%. Further prolongation of reaction time above 10 min still increased the gasification efficiency, but the influence was very slight. We speculated that the prolongation of the reaction time below 10 min can promote the decomposition of the organics in coal and black liquor and release more carbon in the gas product to increase CE. After 10 min of reaction, some refractory components may be generated and cannot be decomposed under this reaction condition even with longer reaction times. As a result, the prolongation of the reaction time above 10 min had limited impact on the gasification. Under this circumstance, improving the reaction temperature or adding some proper catalyst may help to further promote the decomposition of the refractory organic substances and improve the gasification efficiency. Compared F

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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Table 2. Gas Composition and CE of Coal and Black Liquor Blends in Supercritical Water with Different Reactant Amount and Initial Pressures (700 °C; 30 min; Black Liquor to Coal Ratio of 50:50; Total Concentration of 9.5 wt %) gas composition (%) no.

reactant amount (g)

initial pressure (MPa)

final pressure (MPa)

H2

CO

CH4

CO2

C2H4

C2H6

CE (%)

1 2 3 4 5

0.90 1.05 1.05 1.05 1.20

5.98 4.03 4.97 5.94 4.02

24.11−26.99 17.96−21.01 22.70−26.00 26.53−27.63 18.88−21.39

56.28 50.32 54.73 54.45 47.41

0.79 0.66 0.88 0.79 0.72

10.74 14.92 11.71 11.13 16.82

31.40 33.15 31.75 32.95 34.13

0.09 0.05 0.06 0.06 0.00

0.70 0.89 0.86 0.63 0.93

53.82 51.97 59.03 68.03 41.26

pressure of about 6 MPa and increased with the reactant amount. The different influence of the reactant amount between these cases may be attributed to the different influence mechanism of pressure on subcritical and supercritical water gasification. Though the thermodynamic analysis showed that the gasification pressure had limited influence on the gas product, Demirbas et al.49 found that the increase of pressure ranged from 23−48 MPa can promote the gasification in SCWG for increasing the mass transfer and the solvent diffusion rates of water. While in the subcritical water gasification, the pressure had a different influence on the gasification. Lu et al.50 also reported that the gasification pressure showed distinct influence on the gasification efficiency between sub- and supercritical water gasification. In any case, the CE and H2 fraction in the product achieved at subcritical pressure area were all lower than that achieved at supercritical pressure area. This result showed the advantage of SCWG and the importance to improve the gasification pressure to above the critical pressure.

to the literature, this study takes longer time to reach this steady state of reaction. For example, Antal et al.43 found that glucose can be completely gasified in only 28 s at 600 °C and 34.5 MPa. Lee et al.44 also reported that the yields of all the gas product were almost independent of the residence time when it is longer than 12 s in SCWG of glucose at 700 °C. The longer reaction time taken in this study may be because both coal and black liquor are harder to decompose than glucose. However, the batch reactor used here may also inhibit the mass transfer and reduced the reaction rate, which will take longer time to get the steady state. 3.6. Influence of Reactant Amount and Initial Pressure of the Reactor. For the batch reactor, both the reactant amount and initial pressure of the reactor will influence the gasification pressure. Additionally, the change of the reactant amount will also change the water density during gasification, which was reported to be able to affect the reaction kinetics of lignin SCWG.45,46 In this study, we investigated both their influence on the gasification at 700 °C for 30 min with the total concentration and mixing ratio being kept constant. The results showed that both of them influenced the gasification performance (Table 2). With the reactant amount of 1.05 g, the increase of the initial pressure increased the H2 fraction and decreased the CO2 and CH4 fractions in the gas product. In particular, when the initial pressure increased from 4.03 to 4.97 MPa, the H2 fraction increased from 50.32% to 54.45%. However, the increase of the initial pressure from 4.97 to 5.94 MPa had almost no influence on the gas composition. From the records, we found that the final pressure was in the range of 17.96−21.01 MPa when initial pressure was 4.03 MPa, which did not reach the critical pressure of water (22.1 MPa). While the initial pressure was 4.97 and 5.94 MPa, the gasification pressures were all above the critical pressure. Referring to the literatures,41,47,48 the chemical and physical properties of water changed dramatically near its critical point. Therefore, the reactions and gasification performance can be very different between super- and subcritical water gasification. As shown in Table 2, the variation of the initial pressure also changed the CE, which increased with the initial pressure. The reactant amount showed different influence on the gasification performance with different initial pressures. With an initial pressure of about 4 MPa, higher CE and H2 fraction in the gas product were achieved with less reactant loaded in the reactor. While the initial pressure is about 6.0 MPa, higher CE was achieved with a larger reactant amount, but the gas product composition was almost unchanged. We supposed that this distinctive influence was also related to the different final pressures. With the initial pressure of 4 MPa, the increase of reactant amount from 1.05 to 1.20 g increased the final gasification pressure, but they were still in the subcritical area (17.96−21.39 MPa). On the contrary, the final pressure was in the supercritical area (24.11−27.63 MPa) with the initial

4. CONCLUSIONS In this study, gasification performance of black liquor/coal blends in supercritical water at 600−750 °C was studied using a batch reactor. A synergetic effect was found during the cogasification of coal and black liquor. The presence of black liquor improved coal gasification for providing alkali as the catalyst. The presence of coal makes full use of the alkali catalyst and generates porous char that may be active to improve the gasification of black liquor. The synergetic effect was most evident when the blending ratio of coal and black liquor was about 50:50. Both the thermodynamic analysis and experimental results showed that more hydrogen can be produced at higher temperatures. The experiments also showed that the increase of temperature significantly enhanced the gasification and improved the gasification efficiency. A maximum CE of 79.46% was achieved in co-gasification of coal and black liquor with a mixing ratio of 50:50 at 750 °C. The decrease of the total concentration of the blends improved the gasification efficiency and the H2 fraction in the gas product, but it reduced the CH4 fraction, which is consistent with the thermodynamic analysis. Highest H2 fraction in the product (59.26%) was obtained when the total concentration was 5 wt %. The prolongation of reaction time promoted the gasification, but the influence became smaller when it was longer than 10 min. Both the initial pressure and reactant amount in the batch reactor influenced the gasification pressure and the gasification results. The increase of the initial pressure improved the CE and H2 production with the same reactant amount. The increase of the reactant amount showed different influence on the gasification result when the final gasification pressure was in the subcritical and supercritical areas. G

DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Phone: +86-29-82660996. Fax: 86-29-82669033. ORCID

Changqing Cao: 0000-0001-8599-7820 Hui Jin: 0000-0001-9216-7921 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the China National Key Research and Development Plan Project (no. 2016YFB0600100) and the National Natural Science Foundation of China (no. 51606150 and 51527808) are gratefully acknowledged.

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DOI: 10.1021/acs.energyfuels.7b03044 Energy Fuels XXXX, XXX, XXX−XXX