High-Efficiency Gasification of Wheat Straw Black Liquor in

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High-Efficiency Gasification of Wheat Straw Black Liquor in Supercritical Water at High Temperatures for Hydrogen Production Changqing Cao,*,† Lichao Xu,‡ Youyou He,† Liejin Guo,† Hui Jin,† and Ziyang Huo§ †

State Key Laboratory of Multiphase Flow in Power Engineering and ‡School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China § Queensland Micro- and Nanotechnology Center, Nathan Campus, Griffith University, Brisbane 4111, Australia ABSTRACT: Supercritical water gasification (SCWG) is an innovative handling method for black liquor, which can eliminate its pollution and produce hydrogen simultaneously. In this study, we investigated SCWG of wheat straw soda black liquor with higher temperature and longer reaction time for efficient gasification in a batch reactor. The influence of temperature (600−750 °C), reaction time (10−50 min), and black liquor concentration (2.5−9.5 wt %) were studied. Higher temperature, longer reaction time, and lower concentration promoted hydrogen production and carbon conversion. For black liquor with an initial concentration of 9.5 wt %, maximum carbon conversion of 94.10% was achieved at 750 °C. Higher carbon conversion (98.17%) was obtained when black liquor was diluted to 2.5 wt %. From thermodynamic analysis and experimental results in literature, we compared low- and high-temperature SCWG of black liquor and found that high-temperature SCWG has better opportunities. On the basis of gasification results, potentially 0.042, 1.12, and 12.00 million tons of hydrogen are estimated to be produced annually from SCWG of black liquor in a typical pulp mill, China, and the world, respectively. Producing so much hydrogen by SCWG of black liquor instead of natural gas reforming can reduce roughly 0.51, 13.63, and 146.54 million tons of CO2 emission annually. More CO2 emission can be reduced when SCWG is used to replace coal gasification. The results suggest SCWG of black liquor can make a great contribution to solving the energy shortage and global warming problems. water (T > 374 °C, P > 22.1 MPa) to provide an excellent reaction environment for gasification of biomass and organic waste.9−13 It is more suitable to treat high moisture content organic wastes because the drying process is not needed. Yoshida et al.14 found that SCWG is more efficient than other conversion technologies for biomass when the feedstock has a high moisture content (>40 wt %). Weak black liquor has a moisture content above 80%, so SCWG is a more energy-efficient treatment method. Additionally, a homogeneous reaction environment can be formed during SCWG, which benefits the conversion of organics with lower mass-transfer resistance. On the other hand, alkali is reported to be an effective catalyst in SCWG,15−20 so the inherent alkali of black liquor can serve as catalyst.21 Besides, the above-mentioned pollutants including NOx, SOx, and fine particles generated from combustion or conventional gasification are not generated in SCWG, even when the feedstock contains N and S elements, due to the unique reaction environment.22,23 Therefore, SCWG is considered to be a promising handling method for black liquor, where both goals of pollution elimination and hydrogen production can be realized simultaneously. Therefore, SCWG of black liquor has attracted much attention from researchers around the world. Sricharoenchaikul24 assessed SCWG of black liquor with a quartz capillary reactor and achieved a maximum carbon conversion efficiency of 84.8% at 650 °C and 120 s reaction time. Higher temperature was not tested, probably because the temperature was restricted by the

1. INTRODUCTION With growing populations and rising living standards, the consumption and production of paper has grown rapidly in recent decades. Black liquor is a byproduct and also the main pollutant from the pulping process for papermaking, which mainly contains degraded lignin, hemicellulose, and some extractives generated from delignification of wood or straw. It also contains residues of the inorganic cooking chemicals used in the pulping process, such as NaOH and Na2CO3 in the case of soda pulping process.1 Thus, black liquor is a wastewater with high COD (chemical oxygen demand) content, high alkalinity and pH values, camel color, and offensive smell. It can cause severe environmental pollution without proper treatment. Black liquor is conventionally treated with Tomlinson recovery, which has a history of over 80 years. However, several drawbacks were recognized in the utilization, including lower energy efficiency; generation of NOx, SOx, and fine particles; and safety problems in operation for salts melting during combustion.1−3 For example, weak black liquor has a concentration of 10−20 wt %, and it has to be condensed to a concentration above 70% before combustion in the boiler.4 Thus, much energy will be consumed and greatly lower the energy efficiency. Gasification is also suggested as an alternative treatment method of converting black liquor into valuable syngas.3,5−8 It has several advantages over combustion, but the conventional gasification still needs energy-intensive evaporation processing. Furthermore, the high reaction temperature and introduction of air can also result in generation of the above-mentioned pollutants. Supercritical water gasification (SCWG) is an alternative treatment method for black liquor developed in recent decades. This technology utilizes the unique physicochemical properties of supercritical © 2017 American Chemical Society

Received: November 13, 2016 Revised: February 17, 2017 Published: March 6, 2017 3970

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Energy & Fuels Table 1. Component Analysis of Black Liquora proximate analysisb

ultimate analysis

a

c

C

H

O

N

S

M

A

V

FC

pH

31.07

2.96

29.09

0.55

0.93

3.81

32.35

45.69

18.91

12.4

Given as weight percent, air-dried basis. bM, moisture; A, ash; V, volatile matter; FC, fixed carbon. cBy difference. designed parameters are 800 °C and 30 MPa. A K-type thermocouple was inserted in the reactor to detect the reaction temperature. At each experiment, a certain amount of black liquor or diluted black liquor was loaded into the reactor. The loading amount was determined to make the pressure reach above the critical point of water when it is heated to the reaction temperature. The pressure was detected by a pressure sensor, and the results showed that it was maintained in the range 23−26 MPa during gasification in this study. The variation of pressure in this range is proved to have little influence on gasification in supercritical water by both thermodynamic study and experimental results.11,28 After loading of the reactant, the reactor was purged with argon three times to eliminate air in the reactor, and it was pressurized to 4−6 MPa with argon as the initial pressure to avoid water boiling during heating and assist gas product collection after gasification. Then the reactor was heated to the reaction temperature by putting it into a preheated oven. The average heating rate of the reactant was about 30 °C/min. The reaction temperature was controlled by a proportional−integral− derivative (PID) controller with a deviation of ±5 °C. After gasification, the reactor was quenched in water for fast cooling to terminate the reaction. When the reactor was cooled to room temperature, the gas product was collected through a valve on the top of the reactor for further analysis. Some experiments were performed three times, and the average value is regarded as the final result. The results showed good repeatability and small standard deviation. The composition of the collected gas was analyzed on an Agilent 7890A gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). A Plot C-2000 capillary column, purchased from Chromatographic Technology R&D Center of Lanzhou Institute of Chemical Physics, was utilized to separate the gas components. It was operated at 60 °C for 1.5 min, heating at a ramp of 40 °C/min to 150 °C and holding for 5 min. High-purity helium (purity >99.999%) served as the carrier gas at a flow rate of 5 mL/min. After GC analysis, the volume of the gas product was measured by a wet-type precision gas meter. The thermodynamic equilibrium calculation of SCWG of black liquor was conducted by use of Aspen Plus (Aspen Technology, Inc.), based on the principle of minimizing Gibbs free energy. As there are no available data for black liquor in the software, we input the ultimate and proximate analytic results for black liquor (Table 1) and estimated its properties through the model provided by the software. A yield-type rector was introduced to decompose black liquor into elementary components (C, H2, and O2) and separated ash by use of a separator. Then the elementary components were fed into RGIBBS reactor for equilibrium calculation. Peng−Robinson equation of state with Boston−Mathias α function (PR-BM EOS) was used to calculate the physical properties. This equation of state is widely used for calculating the equilibrium results of supercritical water gasification and oxidation with acceptable results.29−31 In the calculations, H2, CO, CO2, CH4, H2O, C2H4, and C2H6 were considered as possible reaction products according to research experience with SCWG. It should be pointed out that complete gasification of black liquor is assumed in the thermodynamic analysis. The detailed calculating method can be found in our previous study.32 In this study, gasification efficiency (GE, percent), carbon gasification efficiency (CGE, percent), and gas yield (moles per kilogram) were used to evaluate the conversion efficiency of black liquor in SCWG. They are defined by the following equations:

reactor material (SiO2), which can react with the alkali in black liquor at higher temperatures and result in breakage of the reactor. Boucard et al.25 investigated sub- and supercritical water gasification of black liquor at lower temperature (350−450 °C). They found that nano-CeO2 can improve the conversion of black liquor and inhibit the generation of charcoal. Huet et al.26 investigated SCWG of black liquor at 430−470 °C. They achieved maximum carbon conversion (34%) at 470 °C for 60 min and concluded that a higher temperature is needed to convert all the organics into gases. De Blasio et al.27 gasified black liquor in SCW by use of a stainless steel reactor and an Inconel625 reactor, respectively, to explore the influence of the reactor wall. They found that both increasing temperature and Inconel reactor can promote gasification. Over 80% hot gas efficiency and a carbon gasification efficiency close to 70% were obtained in the Inconel reactor at 700 °C. In our previous study, we investigated SCWG of alkaline wheat straw black liquor in a tubular reactor.21 Maximum COD removal efficiency of 88.69% was reached at 600 °C. However, all gasification results reported in the literature are far from complete gasification, which is important to its industrial application. From the literature, the use of a suitable catalyst can enhance black liquor gasification. But most catalysts are easily deactivated by the complex components of black liquor, including the alkali and sulfur content. The other effective way to get higher conversion efficiency is increasing the temperature and reaction time. In this study, SCWG of black liquor with higher temperature and longer reaction time was studied in an effort to obtain higher gasification efficiencies. First, we conducted thermodynamic analysis to calculate the equilibrium state results. The influence of temperature and black liquor concentration on gasification was studied theoretically from the view of thermodynamics. Then we gasified alkaline black liquor from wheat straw in a batch reactor at temperatures from 600 to 750 °C. The influence of temperature, reaction time, and concentration were studied. Experimental results along with thermodynamic analysis are discussed to further understand the reaction. On the basis of thermodynamic analysis and experimental results from this study, as well as related literature on SCWG of black liquor, we compared low- and high-temperature SCWG for black liquor treatment. Finally, we estimated the potential hydrogen production and CO2 emission reduction from SCWG of black liquor in a modern pulp mill, China, and the whole world, based on the gasification results.

2. EXPERIMENTAL METHODS The black liquor used in this study comes from soda pulping of wheat straw, which was collected from a pulping mill in Shaanxi Province in China. It mainly contains degraded lignin and hemicellulose and their derivatives. It also contains plenty of alkali derived from the pulping process and therefore has a high pH value (12.4). The black liquor contains 9.5 wt % air-dried solids. Ultimate and proximate analyses of the black liquor solid are listed in Table 1. Ultimate analysis was carried out on a PerkinElmer CHNS/O element analyzer, and proximate analysis was carried out with a proximate analyzer. The experiments were carried out in a batch reactor made of Ni-based alloy Inconel 625. The internal volume of the reactor is 10 mL, and the

GE =

total mass of gas product × 100 mass of ash − free black liquor solid

CGE = 3971

carbon content of gas product × 100 carbon content of black liquor

(1)

(2)

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Figure 1. Calculated variation of (A) gas composition and (B) GE and HHV of the gas product at equilibrium states with reaction temperature (pressure = 25 MPa; concentration = 9.5 wt %).

Figure 2. Calculated variation of (A) gas composition and (B) GE and HHV of the gas product at equilibrium states with black liquor concentration (temperature = 700 °C; pressure = 25 MPa).

gas yield =

moles of gas product mass of ash − free black liquor solids

product. When the temperature was above 600 °C, hydrogen fraction reached above 40%, and maximum H2 fraction of 61.34% was achieved at 800 °C. It needs to be mentioned the curves of gas composition in Figure 1 are not so smooth at the temperatures around the critical point of water (374 °C). This is probably related to the dramatic change in thermophysical properties of water near the critical point, which cannot be accurately predicted by the EOS used in this study (PR-BM). This is also a problem for other equations of state and thermophysical property calculating methods. However, the results at other temperatures can still help us to understand the temperature dependence of gasification. Moreover, the heating value of the gas product also changed with temperature (Figure 1B). HHV of the gas product decreased from 17.29 MJ/Nm3 at 300 °C to 8.55 MJ/Nm3 at 800 °C, which was reduction by more than half. This result can be attributed to the increase in H2 fraction and the decrease in CH4 fraction, as the HHV of H2 (12.75 MJ/Nm3) is lower than that of CH4 (39.82 MJ/Nm3). As a result, the gas produced from SCWG of black liquor at lower temperatures had a higher HHV. The change of temperature also influenced the GE under thermodynamic equilibrium states. As mentioned above, complete gasification was assumed in thermodynamic analysis (CGE = 100%). In SCWG, water also participated in the gasification besides the black liquor organics. As a result, GE as

(3)

The higher heating value (HHV, megajoules per normal cubic meter) of the gas product was calculated to indicate the calorific value of the gas product. It can be calculated by multiplying the gas composition and the HHV of each gas species, as in the following equation:33 HHV = 12.745ωH2 + 12.635ωCO + 39.816ωCH4 + 63.397ωC2H4 + 70.305ωC2H6

(4)

where ωi is the fraction of gas species i in the gas product.

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Analysis. Thermodynamic analysis of SCWG of black liquor was conducted on the basis of Gibbs energy minimization. The influence of temperature over a wide range of 300−800 °C was calculated to study the temperature dependence of gasification (Figure 1). Temperature shows a significant influence on the predicted results at equilibrium states. At lower temperatures, the main components of the gas product are CH4 and CO2, and the H2 content is low. When the temperature increased, the fraction of H2 increased and CH4 fraction decreased. Their fractions approach each other at 505 °C, with values around 26%. Above this temperature, hydrogen becomes the dominant combustible component of the gas 3972

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Figure 3. Experimentally determined influence of reaction temperature on (a) composition of gas product and (b) gasification efficiencies (concentration = 9.5 wt %; reaction time = 30 min; error bars represent standard deviation, SD).

Although the increase in black liquor concentration can improve the heating value of the gas product and enlarge the processing capacity of a specified system, too-high concentration is not recommended. The weak black liquor from pulping process has a concentration of about 10−20%, so drying and condensation processes will be needed to obtain higher concentrations. Thus, a lot of energy will be consumed, which will lower the energy efficiency. Additionally, high-concentration black liquor will be more difficult to gasify from the reaction kinetics aspect. As a result, an optimal black liquor concentration can be selected through balancing energy loss (in the case of low concentration) and drying energy consumption (for highconcentration gasification). 3.2. Influence of Temperature. The influence of temperature at high levels (600−750 °C) on gasification of original black liquor (concentration = 9.5 wt %) was studied (Figure 3). The gas product mainly contains H2, CO2, and CH4, with trace amounts of CO, C2H4, and C2H6, and the temperature showed a significant influence on gas composition. The fraction of H2 increased and the fractions of CO2 and CH4 decreased with increasing temperature. This change agrees with the predicted results by thermodynamic analysis (Figure 1A). High temperature may accelerate the methane reforming reaction, which is an endothermic reaction. Thus, more CH4 can be converted into H2. Moreover, the gas product has a low CO content, which is below 2.20% at all the studied conditions. It can be attributed to the inherent alkali content of black liquor. Black liquor contains not only organics but also cooking chemicals. The black liquor used here comes from soda pulping, where NaOH was used as a cooking chemical. Therefore, it also contained NaOH, as well as Na2CO3 and NaHCO3 produced from NaOH by reaction with CO2, which can be certified by the high pH value (12.4) measured. In several reports,18−20,37,38 these alkaline compounds are found to be effective catalysts in SCWG. They can improve carbon conversion and accelerate water−gas shift reaction, where CO can be converted into CO2 and H2. Thus, the CO content of gas product from alkaline black liquor was extremely low. This may benefit the end use of the gas product, because the gas product should be processed to eliminate CO before being used in some areas. For example, extremely low CO content is essential for use in some fuel cells, as CO can deactivate the electrodes in fuel cells.39,40 In the gas product, fractions of both C2H4 and C2H6 are also low and decrease with temperature, as predicted by thermody-

defined in eq 1 can be higher than 100% and the value can reflect the amount of water participating in gasification. As shown in Figure 1B, all GE values at the studied temperatures were higher than 138.75% and increased with temperature. Maximum GE of 197.34% was achieved at 800 °C. This indicates that more water participated in the gasification at higher temperatures and contributed to the gas product, and water with almost the same weight as black liquor solid participated in the gasification at 800 °C. On the basis of predicted gas compositions, SCWG of black liquor can be classified into two categories. The first one is to gasify black liquor at lower temperatures (350−500 °C) to produce methane as the main product. In this case, catalyst and longer reaction time are usually required to improve the reaction rate. Some researchers25,26,34,35 have studied it, but the gasification efficiency was still low even with catalyst. The other one is to gasify black liquor at higher temperatures (>600 °C) to produce hydrogen-rich gases. At such temperatures, shorter reaction times can fulfill the requirement for efficient gasification even without a catalyst. The literature21,24,27,32,36 reported on this process and obtained higher CGE. From thermodynamic analysis, high-temperature gasification can get a higher GE as more water participates in the gasification, while low-temperature gasification can produce gas with a higher calorific value. They will be compared in more detail on the basis of thermodynamic analysis and experimental results of this study and related literature in section 3.2. In this study, we gasified black liquor at higher temperatures (600−750 °C) and longer reaction times in a batch reactor to obtain higher gasification efficiency and maximum production of hydrogen. Concentration is another important parameter for black liquor gasification, which can directly influence the handling capacity for a certain SCWG system. Thermodynamic analysis shows that the variation of concentration in the range 0.5−50 wt % has a significant influence on gas product composition (Figure 2A). With increasing concentration, the H2 fraction decreased and the CH4 fraction increased. Thus, the heating value of the gas product increased with concentration, which increased from 8.13 to 14.93 MJ/Nm3 as the concentration increased from 0.5 to 50 wt %. On the other hand, increasing concentration reduced the water fraction in the reactor and thus water participation in gasification. Thus, GE was reduced from 206.86% to 144.76% when the concentration increased from 0.5 to 50 wt % (Figure 2B). 3973

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Figure 4. Experimentally determined influence of reaction time on (a) gas product composition and (b) gasification efficiencies during SCWG of black liquor at 600 °C (error bars represent SD).

Figure 5. Experimentally determined influence of reaction time on (a) gas product composition and (b) gasification efficiencies during SCWG of black liquor at 700 °C (error bars represent SD).

dramatically with temperature. For the received black liquor without dilution, CGE increased from 71.21% to 94.10% when the temperature increased from 600 to 750 °C. CGE of 94.10% is also the highest CGE of black liquor as reported in literature. Complete gasification is more closely approached but is still not reached. It may be achieved at higher temperatures, but further increasing temperature is restricted by the reactor in this study. On the other hand, the use of continuous reactor may be also helpful to obtain complete gasification, as the mass-transfer resistance can be reduced by the flowing of the reactant. 3.3. Influence of Reaction Time. The influence of reaction time on SCWG of black liquor was investigated for gasification at 600 and 700 °C. The reaction time also showed a significant influence on gas composition and gasification efficiencies. At 600 °C (Figure 4), the fraction of H2 increased and the fractions of CH4 and CO2 decreased with reaction time. Hydrogen production from black liquor is increased significantly, and its fraction increased from 46.14% to 60.12% when the reaction time was prolonged from 10 to 50 min. Meanwhile, hydrogen yield increased from 10.39 to 25.82 mol/kg. Longer reaction time also promoted gasification and improved gasification efficiencies. CGE increased from 59.55% to 84.20% as the reaction time increased from 10 to 50 min. At 700 °C (Figure 5), the gas composition showed a similar changing performance with reaction time at the beginning. When the reaction time was longer than 30 min, gas composition and gasification efficiencies reached steady states and changed less

namic analysis. We assumed the increase in temperature improved the decomposition and reforming of C2H4 and C2H6, which were transformed into light gas species. It is notable that the fraction of C2H4 was always lower than that of C2H6 in both the experimental study and thermodynamic analysis. For example, C2H4 fraction is in the range 0.12−0.21%, while C2H6 is in the range 1.26−1.60% under the studied conditions. A similar result was also found in SCWG of coal and other biomass and was discussed in our previous study.32 It is probably because a reductive reaction atmosphere was formed in the reactor for high hydrogen content during SCWG, which can promote transformation of C2H4 into C2H6 through hydrogenation. The increase in temperature enhanced gasification and improved GE and CGE significantly (Figure 3b). GE increased from 104.29% at 600 °C to 153.38% at 750 °C. It is noteworthy that GE was above 100% at all studied temperatures. As discussed before, water also participated in gasification and contributed to the gas product. Thus, the weight of the gas product was higher than that of the black liquor organics, and GE as defined in eq 1 was higher than 100%. The increase in temperature may not only improve black liquor decomposition but also promote more water to participate in gasification. Though GE was higher than 100% for the studied condition, it is still lower than the GE predicted by thermodynamic analysis (167.77∼193.96%) because complete gasification was still not realized. Comparatively, CGE is a better indicator to evaluate the conversion efficiency of black liquor, which also increased 3974

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Figure 6. Experimentally determined influence of black liquor concentration on (a) gas product composition and (b) gasification efficiencies at 700 °C for 30 min (error bars represent SD).

with reaction time. From kinetic studies,21 the reaction rate at 700 °C is higher than that at 600 °C, so a shorter time is needed to reach equilibrium status. At 700 °C, the reactions reached equilibrium status more quickly, and further prolonging the reaction time contributed less to change the gasification results. During gasification, some refractory compound may be formed that cannot be decomposed at this temperature even with longer reaction time. In the studied conditions, CGE of black liquor at 700 °C is always higher than that at 600 °C. The CGE at 700 °C for 30 min is higher than that at 600 °C for 50 min, reflecting that temperature is a very important parameter in black liquor gasification. The fractions of C2H4 and C2H6 decreased with reaction time, as adequate time is needed to convert C2H4 and C2H6. The content of C2H4 is still lower than C2H6 content because of transformation of C2H4 to C2H6 by hydrogenation in the hydrogen-rich reaction atmosphere. On the other hand, their content in the gas product at 700 °C was significantly lower than that at 600 °C. For example, the fractions of C2H6 were in the range 0.29−0.76% at 700 °C, while they are in the range 1.26∼1.60% at 600 °C for the studied reaction time ranges. This shows that both longer reaction time and higher temperature can reduce their content by accelerating their reforming and decomposition reactions. In the future, a continuous reactor should be used in industrial utilization instead of a batch reactor. In that case, long reaction time will increase the volume of the reactor and the construction cost. The reaction time of 10−50 min used in this study is too long for a continuous reactor. The frequently used reaction time of SCWG in continuous reactors is always in the range of several to tens of seconds.21,27 On the other hand, such long reaction time may not be needed in the continuous reactor. The flowing and moving of the reactant and intermediates in the reactor can improve mass transfer and reaction rate. Acquiring its reaction rate equation may be helpful to determine the required reaction time and guide the design of reactors for SCWG of black liquor. 3.4. Influence of Concentration. To investigate the influence of black liquor concentration (Figure 6), the original black liquor with a concentration of 9.5 wt % was diluted to different concentration with deionized water. The variation of concentration influenced gas composition, and the influence is similar to that predicted by thermodynamic analysis (Figure 2). Dilution of black liquor increased the fraction of H2 and decreased CH4 and CO in the gas product. When black liquor

was diluted to 2.5 wt %, H2 fraction in gas product reached 75.23%, which was the highest H2 fraction in the study. We believe the reactions involved with water were improved for high water fraction in the reactor in the case of low-concentration gasification. For example, low concentration will favor the water−gas shift reaction and methane reforming reactions, which can consume CO and CH4 and produce hydrogen. Also, dilution of black liquor favored the steam reforming of C2H4 and C2H6 and decreased C2H4 and C2H6 fractions in the gas product. When the concentration is diluted to 2.5 wt %, the content of C2H4 was not detected, being below the lower limit of GC detection. Diluting black liquor also increased gasification efficiencies and hydrogen production. Maximum CGE of 98.17% was achieved in gasifying 2.5 wt % black liquor, which further approached complete gasification. Under this condition, 62.38 mol of H2 can be produced from 1 kg of ash-free black liquor solid. The enhancement of gasification may also occur because of the high contact between water and the organics in black liquor, which favors reaction between them. The high content of water may also accelerate the diffusion of generated intermediates distributed around the organics and further promote reactions between water and organics. For the studied black liquor concentration, GE was in the range 135.04∼174.57%. It was still lower than that calculated by thermodynamic analysis (187.44∼205.19%), as complete gasification was still not obtained. Dilution of black liquor favors gasification and hydrogen production, but it can also reduce the energy efficiency of the SCWG system. In gasification of diluted black liquor, more water will be involved in the gasification and result in more aqueous residue. As the energy contained in the product and the aqueous residue cannot be fully recovered, there will be more heat loss in this case. On the other hand, the treated black liquor concentration can also affect the scale of black liquor treatment system. For a given amount of black liquor solid, a larger-scale system is needed to gasify low-concentration black liquor, which will increase the construction cost. As a consequence, the optimal treatment concentration should be evaluated by fully considering gasification performance, energy efficiency, and construction cost of the treatment system. 3.5. Comparison with Previous Works. Many researchers have studied the advantages of SCWG for black liquor treatment over conventional methods and have published several reports (Table 2). From these publications, black liquor is difficult to 3975

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gasify efficiently, which is probably related to the unique properties of black liquor. Its main component is lignin and its derivatives, which are reported to be harder to decompose in SCW than cellulose and hemicellulose.41 Generally speaking, the research on SCWG of black liquor can be classified into two types: low-temperature gasification with catalyst25,26,34,35 and high-temperature gasification without catalyst.21,24,26,27,36 In the former, methane-rich gases can be produced, while hydrogenrich gases are generated from the latter. The targeted product composition is in accord with the thermodynamic analysis. Low-temperature gasification works in the temperature range 300−450 °C. With its unique components and properties, black liquor is difficult to decompose at these temperatures. Therefore, the reported conversion efficiency is extremely low. As reported by Huet et al.,26 the CGE of black liquor is below 34% at 430− 470 °C without catalyst. To overcome the issue of low reaction rate, catalyst is used to promote the gasification. Boucard et al.25 found that nano-CeO2 is active in catalyzing SCWG of black liquor, but the gasification efficiency is still low (600 °C) even without catalyst. From Table 2, CGE higher than 50% was achieved in several reports.21,24,27,36 It reached 70% in continuous reactors with shorter residence times27,36 and reached up to 84.8% in a quartz capillary reactor for a longer reaction time.24 In this study, we obtained higher CGE by adopting higher temperature and longer reaction time. Maximum CGE of 94.10% was achieved for the received black liquor with a concentration of 9.5 wt %, which is also the highest CGE reported. When black liquor is diluted to 2.5 wt %, much higher CGE (98.17%) was reached, which is close to complete gasification. Both treatment processes have their own features and advantages. For low-temperature gasification, cheaper reactor material can be employed and thus reduce the construction cost of the system. However, the required longer reaction time may enlarge the reactor scale, and the use of catalyst is essential to overcome the issue of low reaction rate. Both of these will improve the operating cost at the same time. Especially, most catalysts are likely to be deactivated by the complex components of black liquor. For example, alkali is reported to be able to enforce the corrosion of Ni-based materials in SCW,18,32 so it may also result in the corrosion of catalyst or catalyst support and thus reduce catalyst activity. Additionally, sulfur contained in some black liquor can poison some metal catalysts, such as Ru and Rh catalysts.42−44 These problems will shorten the life of the catalyst and further increase the operating cost. An effective and durable catalyst may be able to be developed in the future and solve these problems, which would make a great contribution to commercializing this technology. For high-temperature SCWG, the main challenge is the usage of expensive material in reactor construction for high working temperature, which will result in high construction cost. In a word, these challenges should be considered thoroughly when choosing a suitable process to treat black liquor. Given the current research results, high-temperature

375−650 °C; 5−120 s 600 °C; 25 MPa; 2 min 500−700 °C; 25 MPa

400−600 °C; 25 MPa; 4.94−13.71 s

550 °C; 25 MPa

350 and 450 °C; 25 MPa; 15 or 60 min

300−450 °C; 20 MPa 430−470 °C; 24−27 MPa; 2−63 min

Sricharoenchaikul24 Rönnlund et al.36 De Blasio et al.27

Cao et al.21

Cao et al.32

Boucard et al.25

Sealock et al.34,35 Huet et al.26

High-Temperature Gasification without Catalyst 2.5−9.5 wt % alkaline straw black batch CGE = 59.55−94.10% for 9.5 wt % black liquor; max CGE of 98.17% achieved for 2.5 wt % liquor black liquor wood kraft black liquor quartz capillary max carbon conversion of 84.8% achieved at 650 °C for 120 s 10−20 wt % sulfate black liquor semibatch very good gasification properties; CGE of 50−70% reached at 10−20 wt % kraft black liquor tubular CGE 30−70% at 500−700 °C; max CGE of ∼70% achieved at 700 °C; over 80% hot gas efficiency alkaline wheat straw black liquor tubular max CGE of 61.45% and COD removal efficiency of 88.69% achieved without dilution at 600 °C 10 wt % alkaline straw black liquor fluidized-bed CGE of 49.74% achieved Low-Temperature Gasification with Catalyst kraft black liquor batch, nano-CeO2 CGE of 5% (15 min) and 30% (60 min) achieved at 450 °C; use of nanocatalyst improved catalyst conversion kraft black liquor researchers remarked that black liquor is less reactive than other biomass alkaline black liquor batch max CGE of 34% achieved at 470 °C and 60 min 600−750 °C; 10−50 min this study

reactor type black liquor experimental conditions ref

Table 2. Comparison of Experimental Results for SCWG of Black Liquor from This Study and Related Literature

summary of results

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DOI: 10.1021/acs.energyfuels.6b03002 Energy Fuels 2017, 31, 3970−3978

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Energy & Fuels Table 3. Potential Hydrogen Production and CO2 Emission Reduction Annually When SCWG of Black Liquor Is Used pulp production, Mt modern pulp mill China whole world

45

0.63 16.83c 180.93c

black liquor solid production, Mt

hydrogen production, t

CO2 reduction,a Mt

CO2 reduction,b Mt (coal)

1.10 29.45 316.63

6.96 × 10 1.86 × 106 2.0 × 107

0.51 13.63 146.54

2.04 54.54 586.36

4

a Compared with hydrogen production from natural gas reforming. bCompared with hydrogen production from coal gasification. cStatistic data for 2015 from FAO.46

54.54, and 586.36 Mt, respectively, for the reference pulp mill, China, and the world. It is worth emphasizing that this is a rough estimation, as it did not take into account energy consumption during the treatment, such as energy consumption in raw material preparation, gas separation and hydrogen purification, feedstock pumping, and reactor heating. Omission of the energy requirement was supposed to cause a great error in calculating the CO2 emission reduction, particularly because SCWG is an energy-intensive process for its high-temperature and high-pressure reaction conditions. However, the calculation error can be reduced when it is considered that most energy consumed in gasification can be recovered in practical application. For example, the energy of the high-temperature and high-pressure product can be recovered to generate low-pressure and high-pressure steam and electric power for the pulping process and the district heating network. Thus, the calculation error from the assumption of omitting the energy requirement can be greatly reduced. On the other hand, some black liquor has been used through combustion or conventional gasification nowadays to cover the energy use of the pulp mill and surrounding cities. It will reduce the amount of black liquor available for hydrogen production from SCWG and the corresponding CO2 emission reduction. Thus, there will be a conflict of interest in different usages of black liquor. Additionally, the conventional treatment of black liquor, including combustion and conventional gasification, has already contributed to CO2 emission reduction.49,50 This contribution should be considered and compared to reveal the accurate advantages of SCWG of black liquor on CO2 emission reduction. More detailed and rigorous estimation based on system analysis and life-cycle assessment will be conducted in the future.

SCWG is much closer to commercial large-scale utilization than low-temperature SCWG. 3.6. Potential for Hydrogen Production and CO2 Emission Reduction. On the basis of the gasification results, the potential for hydrogen production in a typical pulping plant, China, and the whole world were estimated. A typical modern pulp mill, developed in Sweden with a pulp capacity of 0.63 million tons (Mt)/year, was used as reference.45 In the pulping process, about 1.7−1.8 t of black liquor solids can be generated for 1 t of pulp production.3,47 With the mean value of 1.75 taken as reference, the black liquor solid produced from this pulp mill was calculated to be 1.10 Mt/year. According to the statistical data of the Food and Agriculture Organization of the United Nations (FAO),46 about 16.83 and 180.93 Mt of pulp were produced for papermaking in China and the world, respectively, in 2015. Similarly, annual productions of black liquor solid in China and the world are calculated to be 29.45 and 316.63 Mt, respectively. As described above, maximally 41.78 mol of hydrogen can be produced from 1 kg of ash-free black liquor solid when the original black liquor (undiluted) was gasified. When it is considered that black liquor solid contains 32.35% ash (Table 1), SCWG of 1 kg of black liquor solid can produce about 31.57 mol of hydrogen. On the basis of this result, the potential hydrogen produced from SCWG of black liquor was estimated (Table 3). Annually about 69.61 kt, 1.86 Mt, and 19.99 Mt of hydrogen can be produced from SCWG of black liquor in the reference pulp mill, China, and the whole world. It needs to be mentioned that this is a rough calculation because this technology is still not fully developed for commercialization, and hydrogen loss may exist in real utilization. But it still can show that use of SCWG for black liquor treatment can make a big contribution to solving the growing problems of energy shortage and fossil fuel depletion. Nowadays, hydrogen is mainly produced from fossil fuel, and natural gas reforming and coal gasification are the main methods. Replacing them with SCWG of black liquor to produce hydrogen can reduce the consumption of fossil fuel and CO2 emission. Kothari et al.48 reported that CO2 emission from producing 1 kg of H2 through natural gas reforming is about 7.33 kg. Black liquor is derived from biomass, which is considered as carbon-neutral from the viewpoint of whole life cycle usage. Therefore, CO2 produced from SCWG of black liquor can be considered to be environmentally neutral. Under this assumption, the CO2 emission reduction by using SCWG of black liquor to produce hydrogen can be calculated. About 0.51, 13.63, and 146.54 Mt of CO2 emissions can be reduced by producing hydrogen from black liquor in the reference pulp mill, China, and the world, respectively (Table 3). Coal gasification is another main hydrogen production method, which can generate more CO2 emission (29.33 kg of CO2/kg of H2).48 When it is replaced with SCWG of black liquor, more CO2 emission can be reduced. Similarly, CO2 emission reduction is calculated to be about 2.04,

4. CONCLUSION In this study, SCWG of black liquor at higher temperature was investigated through thermodynamic analysis and experiments. Both thermodynamic analysis and experimental results showed that higher temperature can increase the H2 fraction and decrease the CH4 fraction in gas product, which reduces the heating value of the gas product. The increase in temperature improved the gasification efficiency, and maximum CGE of 94.10% was achieved at 750 °C from the original black liquor (9.5 wt %). Dilution of black liquor favors gasification by improving hydrogen production and gasification efficiency. Maximum CGE of 98.17% was achieved when black liquor was diluted to 2.5 wt %, and the hydrogen yield reached 62.38 mol/kg. Longer reaction time also favored gasification at both 600 and 700 °C, which increased the H2 content in gas product and the gasification efficiency. On the basis of this study and related literature, we compared SCWG of black liquor at low temperatures (300−450 °C) and high temperatures (>600 °C). Judging from current research situations, high-temperature SCWG to produce hydrogen has better opportunities than lowtemperature SCWG to produce methane. With the gasification results, the potential for hydrogen production from SCWG of 3977

DOI: 10.1021/acs.energyfuels.6b03002 Energy Fuels 2017, 31, 3970−3978

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Energy & Fuels

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black liquor in a reference pulp mill, China, and the world were estimated, which were 69.6 kt, 1.86 Mt, and 20.00 Mt, respectively. To produce these amounts of hydrogen, roughly 0.51, 13.63, and 146.54 Mt of CO2 emissions can be reduced by using SCWG of black liquor instead of natural gas reforming. When SCWG is used to replace coal gasification to produce hydrogen, more CO2 emission can be reduced.

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AUTHOR INFORMATION

Corresponding Author

*Telephone +86 29 82660996; fax +86 29 82669033; e-mail cq. [email protected]. 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 This work was financially supported by the National Natural Science Foundation of China (Contracts 51606150 and 51236007) and Shaanxi Science & Technology Co-ordination & Innovation Project (2015TZC-G-1-10).

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DOI: 10.1021/acs.energyfuels.6b03002 Energy Fuels 2017, 31, 3970−3978