Supercritical Water Gasification of Coal with Waste Black Liquor as

Publication Date (Web): December 2, 2014 ... potential of black liquor to be an inexpensive resource of alkali catalyst in supercritical water gasific...
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Supercritical Water Gasification of Coal with Waste Black Liquor as Inexpensive Additives Changqing Cao,†,‡ Liejin Guo,*,† Jiarong Yin,† Hui Jin,† Wen Cao,† Yi Jia,‡ and Xiangdong Yao†,‡ †

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

‡

ABSTRACT: Black liquor is a major wastewater generated from the pulping process that has a detrimental impact on the environment. This work assessed the potential of black liquor to be an inexpensive resource of alkali catalyst in supercritical water gasification of coal through thermodynamic analysis and experimental study. The experiments were performed in a fluidized-bed reactor at 550 °C and 25 MPa, and the products were characterized by gas chromatography, X-ray fluorescence, X-ray diffraction, and gas chromatography-mass spectrometry. In the gasification of coal and black liquor mixtures, the presence of coal can improve the H2 production under the equilibrium state. Both the inherent alkalis and lignin in black liquor played a role of improving the gasification efficiency of coal. The alkalis also accelerated the water−gas shift reaction and increased the H2 fraction. The high total mixture concentration inhibited the gasification, and the reactor was plugged with a concentration of 25 wt %. The presence of black liquor fixed more sulfur in the solid residues, but it aggravated the corrosion of the 316 SS reactor. The aqueous product mainly contained alkylphenols, cyclopentanone, and their derivatives. A simple influencing mechanism of lignin on coal gasification was proposed: the decomposition of lignin prior to coal can generate some phenolic compounds, which can promote the extraction of the substances coated outside the coal particle and favor the further reaction between the coal and water. proportion of NaOH as the catalyst at 600−700 °C and 12−105 MPa and found that H2 and CH4 were the major product gases.18,19 Wang and Takarada9 investigated SCWG of low-rank coals with Ca(OH)2 as the catalyst. They found that this catalyst promoted the decomposition of char and resulted in an increase in the yields of H2 and CH4. However, the high price of alkalis can increase the operating cost of the SCWG process, especially as the alkalis recovery is still a challenge to date. Exploiting an alternative alkali resource with a low price can help to resolve this problem. In this study, black liquor is selected to be the alkali resource to catalyze SCWG of coal. Black liquor is a wastewater generated from the pulping process and mainly contains lignin, hemicellulose, and alkalis. Its handling is a hot topic in environmental research worldwide for the detrimental impact on environment and human health of black liquor. Our previous study20 on SCWG of black liquor in a continuous tubular reactor showed that the dual goals of energy recovery and pollution reduction were achieved, and the inherent alkalis of black liquor played a role of catalyst in gasification. The utilization of waste black liquor instead of commercial alkali as the catalyst in SCWG of coal will reduce the operating cost. Additionally, the resource processing of black liquor can be realized simultaneously. According to the assessment by Ro et al.,21 SCWG has some significant environmental advantages on treating wet wastes compared to the conventional treating method, such as BOD removal, odor elimination, and pathogen kill. Meanwhile, the abundant water content in black liquor can be utilized in the preparation of coal−water slurry, which can

1. INTRODUCTION Nowadays, coal is the most abundant fossil energy resource in the world. Most coal is combusted in boilers to recover the energy through steam generation. However, coal combustion generates many pollutants, such as SO2, NOx, and fine dusty particles, which are harmful to the environment and human health. Alternatively, supercritical water gasification (SCWG) is an innovative and clean conversion technology of coal and biomass using a thermochemical method. This technology utilizes the unique physicochemical properties of supercritical water (T > 374 °C, P > 22.1 MPa) to provide an excellent reaction environment for the gasification of organic substances.1−6 The main product of SCWG is hydrogen, which is an environmentally clean energy as its combustion only generates water. Besides, the aforementioned pollutants generated from coal combustion are not generated in SCWG.7,8 As a result, SCWG of coal has attracted much attention of researchers around the world.9−17 However, the complete gasification of coal requires a much higher temperature than for the biomass due to the unique property and structure. According to our previous study on SCWG in a capillary quartz reactor, a relevant high temperature up to 950 °C is needed for complete gasification of coal.15 It is remarkable that the high temperature may not only increase the construction cost of the reactor but also present a great challenge to operational safety of the SCWG system. So it is highly desirable to explore a method to lower the gasification temperature. In the last few decades, several kinds of catalysts were used to improve the gasification efficiency and lower the gasification temperature of coal SCWG. Among them, alkalis are widely reported to be effective catalysts. Lin et al. studied the gasification of Taiheiyo coal in supercritical water with Ca(OH)2 and a small © 2014 American Chemical Society

Received: September 18, 2014 Revised: November 26, 2014 Published: December 2, 2014 384

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Table 1. Characterizations of Black Liquor, Coal, and Dealkaline Lignina ultimate analysis

a

proximate analysis

species

C

H

Ob

N

S

M

A

V

FC

black liquor coal lignin

34.43 55.82 45.97

3.12 3.16 5.07

29.38 5.49 19.25

0.86 0.78 0.12

0.84 2.40 3.77

5.96 2.44 13.68

25.41 29.91 12.14

51.07 18.50 48.14

17.56 49.15 26.04

wt%, air dried basis. bBy difference. at 150 °C. High-purity helium was used as the carrier gas at a flow rate of 5 mL/min. The solid residues were dried at 105 °C for 12 h before being analyzed. The elemental analysis of the solid residue was conducted in a Bruker S4 PIONEER X-ray fluorescence spectrum (XRF) using Ru target and 4 kW maximum powers. Powder X-ray diffraction (XRD) patterns of the solid residues were collected to estimate the distribution of the inorganic substances in the residues. It was performed on X’pert PRO MPD X-ray diffractometer (PANalytical) using Ni-filtered Cu Kα radiation. The instrument was operated at 40 kV and 40 mA, using a normal scan rate of 10°/min, in the 2θ range of 10−80°. The composition of the aqueous residue was analyzed by gas chromatography−mass spectrometry (GC-MS). Before the analysis, the aqueous samples were treated with a solid-phase extraction technique. First, 5 mL of aqueous residue was filtered through preconditioned Agilent C18 column. After drying of the column, the target compounds adsorbed on the column were eluted by 2 mL of ethyl acetate. Then, the treated liquid samples were analyzed by GC-MS (HP 6890 series GC system and 5973 mass selective detector). An HP-Innowax capillary column was used, which was 30 m long, 0.25 mm in diameter, and 0.25 μm in the film thickness. Helium was used as the carrier gas at a flow rate of 1.2 mL/min. The National Institute of Standards and Technology (NIST) mass spectroscopy library software was used to identify the compounds. 2.4. Data Interpretation. The gas yield, carbon gasification efficiency (CE), and gasification efficiency (GE) were determined to evaluate the gasification performance of coal and black liquor in supercritical water. Their definitions are

reduce the water consumption. In particular, this technique may also provide new opportunities for achieving a highly efficient and cost-effective SCWG process for practical application. In this work, we present a cost-effective and environmentfriendly method, which capitalizes on the black liquor waste as an additive, for the catalytic enhancement in SCWG of coal in a fluidized-bed reactor. First, the influence of the coal/black liquor ratio was investigated through thermodynamic analysis and experimental study. Second, the influence of the total mixture concentration of black liquor and coal was also investigated. Meanwhile, the solid residues were characterized to further study the influence of the presence of black liquor. Then the influence of dealkaline lignin on coal gasification was studied to investigate the role of the organic substances of black liquor in coal gasification. Finally, the aqueous residues from gasification of different feedstocks were characterized in detail to further study the influencing mechanisms.

2. EXPERIMENTAL SECTION 2.1. Feedstock. Black liquor from soda pulping of wheat straw was collected from a pulping plant in Shaanxi Province, China. It contains plenty of alkalis derived from the pulping process and has a high pH value up to 12.1. The received black liquor contains 11 wt % air-dried matters. The lean coal used in this study was produced from Shanxi Province, China. Before being used, the coal was grounded and sieved to a particle size range of 92−105 μm. The dealkaline lignin purchased from Tokyo Chemical Industry Co., Ltd. was used as the model organics of black liquor. The component analysis of the air-dried black liquor solid, lignin and coal was performed (Table 1). Sodium carboxymethyl cellulose (CMC) was used as the suspending agent in preparing the slurry of the insoluble feedstocks, including coal and lignin. In this study, 1.5 wt % CMC was mixed with coal or lignin powder to generate the uniform slurry to realize the continuous feeding. 2.2. Experimental Procedures. The experiments were carried out in a supercritical water fluidized-bed reactor made of 316 stainless steel (316SS). The bed diameter and freeboard diameter of the reactor were 30 and 40 mm, respectively, and the total length was 915 mm. A distributor was located at the bottom of the reactor, and the preheated water flowed through the distributor to form the fluidization state. The feedstock was fed into the reactor above the distributor and mixed with the preheated water to realize fast heating of the feedstock. The higher heating rate can suppress the formation of macromolecular compounds at lower temperatures in the heating progress.22 The temperatures mentioned in this study refer to the fluid temperature in the reactor, which was detected by K-type thermocouples. The temperature was controlled by PID control system with a deviation of ±5 °C. The pressure of the system was controlled by a back pressure regulator. After the regulator, the products were separated into gas and liquid products in a separator. The gas product was measured by a wet-type gas flow meter and then collected for further analysis. A more detailed description of this system can be found in the references.17,23 2.3. Analysis Method. The gas compositions were analyzed by an Agilent 7890A gas chromatography equipped with a thermal conductivity detector (TCD). A Plot C-2000 capillary column was used to separate the gas components, which 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

gas yield =

mol of the gas product , mol/kg ash‐free and dry weight of the feedstock (1)

CE =

content of carbon in gas product × 100% content of carbon in feedstock

(2)

GE =

total mass of gas product × 100% mass of the dry solids of feedstock

(3)

The mixing ratio of black liquor and coal in the mixture was evaluated by the black liquor fraction in the mixture α:

α=

mBLc BL mBLc BL + mcoal

(4)

where mBL and mcoal are the mass of black liquor and coal, respectively, and cBL is the concentration of black liquor. Both black liquor and coal can be gasified in supercritical water, so both of them contributed to the GE calculated above. To evaluate the alteration of coal conversion with the presence of black liquor, we introduced the GE of coal in the mixture, GEcoal. It was calculated by subtracting the GE of black liquor under the same conditions from the total GE, with the assumption that the GE of black liquor was not influenced by coal. It can be calculated by

GEcoal =

GE − (α × GE BL) 1−α

(5)

where GE is the total GE of coal and black liquor, and GEBL is the GE of black liquor under the same reaction conditions with the same concentration with α as defined by eq 4. 385

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Figure 1. Flowsheet of the simulated process of SCWG of coal and black liquor mixture.

Figure 2. Gas fraction (a) and gas yield (b) from SCWG of coal and black liquor with different mixing ratios at equilibrium state (T = 550 °C, P = 25 MPa, total concentration = 10 wt %).

Figure 3. Experimental results of SCWG of coal−black liquor mixture with different black liquor fractions: (a) gas fractions; (b) gasification efficiencies (T = 550 °C; P = 25 MPa; total concentration = 10 wt %). 2.5. Thermodynamic Analysis. The thermodynamic equilibrium calculation of SCWG of coal with black liquor was conducted by the Aspen Plus software (Aspen Technology, Inc.) based on the principle of minimizing Gibbs free energy. The feedstock streams, including black liquor and coal, were specified as nonconventional components in the software. The ultimate and proximate analyze results were input to calculate their properties because there are no available data in this software. In the modeling using Aspen Plus, the nonconventional component cannot be fed directly into the RGIBBS reactor. We introduced two yield-type reactors (DECOMP1&2) to convert black liquor solids and coal into elementary components (C, H2, and O2) and ash (Figure 1). The generated ash was separated, and the remaining components along with water were fed into the RGIBBS reactor to calculate the equilibrium composition. The Peng−Robinson equation of state with Boston-Mathias alpha function (PR-BM) was used to calculate the physical properties. This equation of state was widely used and accepted for predicting supercritical water gasification and oxidation.24−27 Given the previous studying result on SCWG and the

composition of the feedstock, H2, CO, CO2, CH4, H2O, C2H4, and C2H6 were considered as the possible reaction products. The N- and Scontaining products were ignored because the low content of N and S elements had no significant influence on other gas products, and we mainly focused on the above-mentioned gas species in this study.

3. RESULTS AND DISCUSSION 3.1. Chemical Equilibrium Prediction. The equilibrium results of cogasification of coal and black liquor with different mixing ratios at 550 °C and 25 MPa were predicted (Figure 2). The main compositions of the gas product under this condition were H2, CO2, and CH4 with a trace amount of CO, C2H4, and C2H6. The yield and fraction of C2H6 and C2H4 from gasification of coal-black liquor mixture were relatively low. Both of their fractions in the gas product were below 0.0025% and increased with the black liquor fraction in the mixture. 386

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Figure 4. Influence of total mixture concentration on SCWG of coal and black liquor: (a) gas yield; (b) gasification efficiencies (T = 550 °C; P = 25 MPa; black liquor fraction = 25 wt %).

liquor. A similar result is obtained in SCWG of coal by other researchers. Vostrikov et al. studied SCWG of coal in a continuous tubular reactor at 400−760 °C where no C2H4 was detected but only C2H6 in the gas product.13 This is probably because the C2H4 content was below the detection limit of their analyzing instrument. Wang et al.9 and Kumabe10 also obtained a similar result in SCWG of coal by using an autoclave reactor, but they did not discuss this difference in particular. We speculated that C2H4 may be transformed into C2H6 by the hydrogenation reaction under the hydrogen-rich environment during gasification. This result was also in agreement with the equilibrium results as described above. Although the experimental result was far from the equilibrium result due to the incomplete gasification, the yield of C2H6 at the equilibrium state was higher than C2H4 at different mixing ratios of black liquor and coal. The presence of black liquor also improved the GE. As shown in Figure 3b, the GE increased almost three times, from 15.18 to 60.40% when the black liquor fraction increased from 0 to 50 wt %. Black liquor was much easier to be gasified than coal, and its GE (76.62%) was much higher than that of coal (15.18%). The higher GE with more black liquor can be partly attributed to the higher GE of black liquor. On the other hand, it may be also because the presence of black liquor promotes the gasification of coal. Figure 3b shows that the addition of black liquor improved the GE of coal in the mixture (GEcoal), which increased from 15.18 to 44.11% as the black liquor fraction increased from 0 to 50 wt %. The promotion of coal gasification can mainly be attributed to the alkalis in the black liquor. Except for accelerating the WGS reaction as mentioned before, alkalis can also improve the decomposition of the organics. Sinag et al.22 believe that the alkalis could promote the formation of active hydrogen, which can suppress the char/coke generation and improve the gas yield. On the other hand, the presence of some organic substances shows a positive effect on supercritical water treatment of coal. For example, the addition of cellulose was found to be able to supply hydrogen for coal liquefaction in supercritical water and improve the yield and heating value of the product.33 Onsri et al.34 also found that adding the used tire can enhance the conversion of coal and increase the oil yield. Accordingly, it is assumed that the organic substances in black liquor may also influence the coal SCWG positively. In order to test this assumption, we performed the gasification of coal and dealkaline lignin in the next section. The change of the black liquor fraction at lower levels had a more significant influence on coal gasification. As shown in Figure 3, the H2 fraction increased and CO fraction decreased sharply with the increasing of the black liquor fraction in the range of 0−50 wt %. While in the range of 50−100 wt %, the

The results of separate gasification of coal and black liquor are significantly distinct from each other. This is in line with the study reported by Fiori et al.,28 which showed that different equilibrium compositions can be achieved from SCWG of different types of biomass (glycerol, microalgae, sewage sludge, grape marc, phenol). In this study, the H2 yield from coal was higher than 70 mol/kg, whereas that from black liquor was only 12.7 mol/kg. This difference is probably because of the distinct elements that comprise coal and black liquor. As shown in Table 1, coal contains more carbon and less oxygen than black liquor. In the thermodynamic analysis, it was assumed that complete gasification of coal and black liquor was achieved. Therefore, more water would be involved in coal gasification to provide the required oxygen to generate CO and CO2 than in black liquor gasification in the equilibrium state. Thus, more H2 will be released from water in SCWG of coal than black liquor. The H2 yield and fraction consequently increased with the coal fraction in the gasification of coal−black liquor mixture. In other words, the addition of coal in black liquor favors hydrogen production in the term of thermodynamics. 3.2. Influence of Black Liquor on SCWG of Coal. Coal was gasified with different fractions of black liquor as the additives at 550 °C and 25 MPa with the total concentration being kept at 10 wt %. Under this condition, the gas product from coal mainly contained H2, CO, CH4, and CO2, with a small amount of C2H4 and C2H6. When black liquor was added, the fractions of H2 and CO2 in the gas product increased, and the CO fraction decreased (Figure 3a). As the black liquor fraction increased to 50 wt %, the H2 fraction increased from 34.03% to 44.88%, while the CO fraction decreased from 20.14% to 0.14%. This result can be attributed to the effect of the inherent alkalis in black liquor. The black liquor was derived from soda pulping and contained plenty of alkalis, including NaOH, Na2CO3, and NaHCO3. In the presence of these alkalis, sodium formate (HCOONa) can be generated as the intermediate product in SCWG, through which the water−gas shift (WGS) reaction can be accelerated.22,29−32 The acceleration of the WGS reaction led to the increase of H2 and CO2 fractions, and the decrease of CO fraction. In addition, some alkalis with hydroxyl ion, such as NaOH and KOH, can work as CO2 absorber and shift the WGS reaction equilibrium toward enhanced H2 production.29 This catalytic effect was further enhanced when the black liquor fraction increased because more alkali catalyst was introduced with black liquor. The increase of the black liquor fraction reduced the fractions of C2H4 and C2H6 as well. The inherent alkalis in black liquor may also promote the reforming of these gases into smaller molecular gases. It is notable that the fraction of C2H6 was higher than C2H4 independent of the mixing ratio of coal and black 387

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may precipitate and accumulate in the reactor during SCWG. These precipitated inorganics may combine with the char generated in the gasification and plug the reactor after long-time gasification. The plugging problem should be seriously considered and solved by appropriate technologies in cogasification of coal and black liquor for their high ash content. 3.4. Characterization of the Solid Residues. The element compositions of the raw coal as well as the solid residues from SCWG of black liquor, coal, and their mixtures were analyzed by XRF (Table 2). The major elements of all the solid residues were

change of the black liquor fraction almost had no influence on the composition of the gas product. Similarly, the influence of the black liquor fractions in a higher range on GEcoal was not as significant as that in the lower range. This result indicated that 50 wt % black liquor has provided adequate catalytic activity under this condition, and further addition is not necessary. 3.3. Influence of Total Concentration of Coal and Black Liquor Mixture. The coal−black liquor mixture with different total concentrations in the range of 10−25 wt % was gasified at 550 °C and 25 MPa with the black liquor fraction being fixed at 25 wt % (Figure 4). The increase of total concentration decreased the H2 fraction and increased CO fraction in the gas product. As shown in Figure 4a, the H2 fraction decreased from 39.34% to 29.12% as the concentration increased from 10 to 25 wt %, while the CO fraction increased from 6.17% to 7.68%. WGS reaction is considered as an important reaction in deciding the final gas composition. In the case of higher concentration, the corresponding lower water proportion may decline the contact of CO with water and cannot enable the sufficient WGS reaction. The inhibition of WGS reaction resulted in a higher CO fraction and lower H2 fraction in the gas product. The increase of total concentration also reduced the gas yield and GE. The total gas yield is 12.07 mol/kg when the total concentration is 10 wt %. When the concentration increased to 25 wt %, the gas yield was decreased to 6.65 mol/kg, which is only a half of that with 10 wt % concentration. Meanwhile, the GE was reduced from 39.21% to 24.28% when the concentration increased from 10 to 25 wt % (Figure 4b). It can be concluded that the increase of total concentration impacts negatively by reducing the GE and H2 yield. These results are in agreement with the separate SCWG of black liquor20 and coal,11,17 where the increase of the concentration suppressed the gasification. In the preparation of coal−black liquor slurry, the most durable method is to blend a certain amount of coal powder with black liquor without adding water. This method can take advantage of a higher water content of black liquor and reduce the water consumption. In this method, the total concentration is only determined by the black liquor concentration and the set mixing ratio. The concentration of the black liquor used in this study was 11 wt %, and the black liquor fraction in the mixture was chosen to be 25 wt %, so the coal−black liquor slurry with a concentration of 33.08 wt % can be prepared using this method. This concentration is also the maximum total concentration that can be achieved when the black liquor fraction is fixed to be 25 wt %. Gasifying high-concentration feedstock efficiently is important in scaling up of the SCWG process because this is essential to achieve high economic and energy efficiencies.35 However, the gasification of coal−black liquor mixture with a high concentration encountered some problems in this study. The reactor was plugged in gasifying the coal−black liquor mixture with a total concentration of 25 wt % after 40 min. The plugging problem of SCWG reactor is also reported by other researchers and is mainly due to the salt precipitation and char formation.1 One goal of designing the fluidized-bed reactor used in this study was to solve the plugging problem. The previous study23 showed that this goal was attained to a certain extent, and 30 wt % glucose and 18 wt % corn cob were successfully gasified without a plugging problem. The plugging problem that occurred here is assumed to be related to the unique composition of the reactants. As shown in Table 1, both coal and black liquor had a high ash content (>25 wt %). Most inorganic compounds are insoluble in SCW for the relatively low dielectric constant of supercritical water.2,36 Therefore, the ash in coal and black liquor

Table 2. Elemental Composition of the Solid Residues (wt%) from Different Feedstocks As Determined by XRFa

Mn P Cl Cr Ni Ti Mg K Fe Ca Na S Al Si O CH

raw coalb

coal

25 wt % BL + coal

50 wt % BL + coal

black liquor

c 0.0094 0.0091 c c 0.2070 0.2750 0.1900 0.4490 1.3300 0.0480 0.1780 4.0500 7.6700 35.9000 49.7000

0.0093 0.0163 0.0094 0.0121 0.0054 0.1270 0.3620 0.2650 0.2120 0.9950 1.9700 0.0718 3.5000 6.7400 30.7000 55.0000

0.0064 0.0500 0.0258 0.0380 0.0431 0.1330 0.2470 0.6550 0.3230 0.9520 1.7300 0.4650 2.0100 3.7800 22.8000 66.7000

0.0074 0.1020 0.0584 0.0258 0.0238 0.1100 0.2420 0.8780 0.2730 1.0100 2.2300 0.5840 2.3200 4.1900 26.2000 62.0000

0.0115 0.0820 0.0597 0.1080 0.2060 0.1610 0.1630 0.9120 0.6370 0.5890 2.4800 0.9680 2.5200 3.0500 23.1000 64.9000

T = 550 °C; P = 25 MPa; total concentration = 10 wt %. bRaw coal. Others are the solid residues from SCWG of different feedstock. cNot detected. a

C, H, and O, which accounted for over 85 wt %. They probably derived from the unreacted organic substances or the char generated in gasification. The high content of these elements was in line with the low GE as described above. Besides C, H, and O, the solid residues also contained plenty of Si. Its content was above 3 wt % for all the solid residues. This may be related to the composition of the reactant. In general, black liquor from wheat straw pulping contained more Si than wood pulping black liquor.37 Table 2 showed that the raw coal also had a high Si content. The Si element in these feedstocks may precipitate in SCWG and result in the high Si content of the solid residues. The real biomass like wheat straw always contains certain amounts of K,38 so the black liquor from wheat straw also contains K. This may be the reason for the increasing K content in the residues with increasing black liquor fraction. The Na content in the solid residues was also high with the presence of black liquor. Most of the sodium may derive from the alkalis in black liquor, which come from the soda pulping process. It is worth noting that the addition of black liquor increases the content of Ni, Cr, Mn, and Fe elements. As shown in Table 2, these elements except Fe were not detected in the raw coal, and only trace amounts of them were detected in the solid residues from coal gasification. While in the presence of black liquor, they were detected in the residues and their contents increased with the black liquor fraction. We noticed that the 316SS reactor of this study also contained these elements and accordingly assumed that these elements derived from the reactor. This 388

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indicated that the presence of black liquor aggravated the corrosion of the reactor and generated some metal species. The enhanced corrosion of the reactor may be related to the inherent alkalis of black liquor. Sinag et al.22 found that the presence of alkalis can force the corrosion of the Inconel-625 reactor in SCWG. In this study, the alkalis in black liquor may also play this role on the 316SS reactor. Although the corrosion problem in SCWG is not considered as severe as supercritical water oxidation because of the distinct reaction environment,1 this result suggests that more attention should be paid to long-time SCWG with the presence of black liquor. The solid residues were also analyzed by XRD (Figure 5). The main minerals detected were analcime (NaAlSi2O6·H2O),

Table 3. Gas Fractions and Gasification Efficiencies of SCWG of Coal−Black Liquor Mixture and Coal−Lignin Mixturea feedstock gas fraction, % H2 CO CH4 CO2 C2H4 C2H6 total gas yield, mol/kg GE, % GEcoal, % a

coal

coal + 25 wt % lignin

lignin

coal + 25 wt % black liquor

black liquor

34.03 20.14 14.73 28.43 0.70 1.97 4.70

32.22 12.79 15.90 36.83 0.45 1.82 6.90

25.26 10.43 17.63 44.78 0.42 1.48 9.81

39.34 6.17 14.33 38.05 0.52 1.59 12.07

44.88 0.14 14.17 39.35 0.41 1.04 25.23

15.18 15.18

23.43 19.59

34.96

39.21 26.65

76.62

T = 550 °C; P = 25 MPa; total concentration = 10 wt %.

change the H2 fraction significantly. Compared with lignin and coal, black liquor contains plenty of alkali. The alkali can accelerate the WGS reaction and generate more H2 and less CO. As a result, the maximum H2 fraction (44.88%) and minimum CO fraction (0.14%) in this study were achieved in SCWG of only black liquor. The addition of lignin also had an influence on the GE of coal. The GE of lignin was 34.96%, which was over twice the GE of coal under the same conditions. The presence of lignin improved the GE of coal. The GE of coal with 25 wt % lignin was 19.59%, which was higher than the gasification of only coal. As the black liquor contained alkalis, it showed a much higher influence on the GE of coal than lignin. The GE of coal with 25 wt % black liquor reached 26.65%. This indicates that the alkalis in black liquor have a greater impact on the GE of coal than lignin. In short, both alkalis and lignin in black liquor favored the gasification of coal, with alkalis showing the greater increase on GE. To study the influencing mechanism of lignin on SCWG of coal, the composition of the aqueous residues from SCWG was analyzed and compared below. 3.6. Characterization of the Aqueous Residue. The aqueous residues can be regarded as the intermediates in SCWG, and their analysis can help understand the reaction mechanism. In the present work, the aqueous residues were analyzed by GCMS. As introduced by other researchers,39,40 the total ion chromatogram (TIC) area fraction of each detected compound in the total peak area was used for comparing the relative content of the compounds. As shown in Table 4, the main compositions of the aqueous products from SCWG of coal were alkylphenols, cyclopentanone, and their derivatives. The most abundant components were three different cresols, including m-, p-, and o-cresols. The peak area percentages of these cresols were higher than other compounds. With the addition of black liquor, cyclopentanone, and the derivatives decreased slightly and the alkylphenols increased. Moreover, several new compounds, including 2-ethylbenzaldehyde, 2-ethyl-6-methylphenol, and 2,3dihydro-1H-inden-1-ol, 3,4-dimethylphenol were detected in the aqueous residues when lignin or black liquor was mixed. The main components of aqueous residue from lignin gasification were also alkylphenols, cyclopentanone, and the derivatives. A similar result was reported by Wahyudiono et al. in supercritical water degradation of lignin at 400 °C and 30 MPa in a batch reactor.41 They also detected the cyclopentanone and phenolic

Figure 5. XRD patterns of the solid residues from SCWG of (A) coal, (B) coal + 25 wt % black liquor, (C) coal + 50 wt % black liquor, and (D) black liquor at 550 °C and 25 MPa.

anorthite (CaAl 2 Si 2 O 8 ), quartz (SiO 2 ), nepheline (Na3KAl4Si4O16), and vishnevite (Na6.5K1.02Ca0.12(Si6Al6O24)(SO4)0.96(H2O)2). The sulfurated mineral (vishnevite) was not found in the solid residue from coal gasification but was found in the solid residues from the gasification of coal−black liquor mixture. The intensity of the vishnevite peak increased with the fraction of black liquor. This result was in agreement with the XRF analysis result. As shown in Table 2, the S content in the solid residues from coal was extremely low (0.07 wt %) though the raw coal contained 2.4 wt % S element. With the presence of black liquor, the S content in the solid residue increased and reached up to 0.58% when the black liquor fraction was 50 wt %. The S in coal was assumed to be distributed in gas and liquid products in gasification of only coal. While in the presence of black liquor, the inorganic compounds from black liquor may react with S in the coal and form insoluble solid compounds like vishnevite. These results indicated that the addition of black liquor can promote the sulfur fixation in SCWG of coal and reduce the subsequent sulfur-eliminating treatment of the gas and liquid product before final utilization. 3.5. Influence of Lignin on SCWG of Coal. Lignin is the main organic component of black liquor, so the study of cogasification of coal and lignin can assist in understanding the role of organics of black liquor in coal gasification. This was performed with a total concentration of 10 wt % at 550 °C and 25 MPa. As shown in Table 3, the gasification of lignin generated a gas richer in CO2 and with a lower fraction of H2 and CO than that of coal. Adding lignin to coal increased the CO2 fraction and decreased the CO fraction in the gas product. This is probably because lignin contains more oxygen than coal (Table 1). The oxygen content may participate in reactions and convert carbon into CO2 rather than CO. The H2 fraction in the gas product from lignin was lower than coal, but the addition of lignin did not 389

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Table 4. GC-MS Characterization of the Aqueous Residues from SCWG of Different Feedstocksa component

area percentage, %

RT,min

compound

formula

case 1

case 2

case 3

case 4

case 5

10.13 10.37 10.98 15.56 16.09 20.93 21.70 31.85 34.25 35.94 36.15 36.33 37.68 38.12 38.27 38.43 39.27 40.12 40.20 42.22 43.74 45.40

cyclopentanone 2-hydroxycyclopent-2-enone 3-methylcyclopentanone 2-cyclopenten-1-one 2-methylcyclopent-2-enone 3-methylcyclopent-2-enone 2,3-dimethylcyclopent-2-enone 2,6-dimethylphenol 2-methylphenol (o-cresol) 2-ethylphenol 4-methylphenol (p-cresol) 3-methylphenol (m-cresol) 1-methoxycycloheptatriene 2-isopropylphenyl methylcarbamate 4-ethylphenol 3-ethylphenol 3,4-dimethylphenol 2-ethyl-6-methylphenol 2-propylphenol 2-ethylbenzaldehyde 2,3-dihydro-1H-inden-1-ol 6-methyl-4-indanol

C5H8O C5H6O2 C6H10O C5H6O C6H8O C6H8O C7H10O C8H10O C7H8O C8H10O C7H8O C7H8O C8H10O C11H15NO2 C8H10O C8H10O C8H10O C9H12O C9H12O C9H10O C9H10O C10H12O

8.68 5.38 1.17

7.94 3.87 2.03

3.56 1.56 1.22

2.28 0.88 0.42 0.61 0.78

1.60 19.27

4.02 2.731 0.94 17.55

1.31 0.41 0.23 0.40 0.40 1.26 1.49 0.74 23.28 0.81 15.44 18.02 1.47 1.38 9.45 5.47 3.98 1.56 1.32 0.92 1.92 1.32

13.38 9.30 2.09 3.91 1.92

18.73 1.21 13.01 11.77 1.38 2.12 16.95 5.86 2.98 2.01

10.59 9.1 0.99 1.28 10.26 3.16 1.96 0.58

1.06 1.63

5.44 0.79 19.98 0.60 14.14 14.49 1.08 1.14 5.96 3.23 2.33 0.94 0.66 0.63 1.00 0.37

T = 550 °C; P = 25 MPa; Total concentration = 10 wt %. The solid residue of supercritical water gasification of (case 1) coal; (case 2) 12.5 wt % black liquor + 87.5 wt % coal; (case 3) 50 wt % black liquor + 50 wt % coal; (case 4) 25 wt % lignin +75 wt % coal; (case 5) lignin.

a

Figure 6. Reaction of coal particle in supercritical water along with the intermediate products from lignin.

intermediate products may cover the outer layer of the coal and prevent the contacting of the unreacted coal and SCW, which can inhibit further reaction between coal and SCW. While in cogasification of coal and lignin, lignin is probably decomposed prior to coal and generates some phenolic compounds as listed in Table 4. According to Aida et al.44 and Kershaw,45 some phenolic compounds can promote the extraction and decomposition of coal in supercritical water. The phenolic compounds generated from lignin may have also played this role in favoring the extraction of the intermediates from coal. This can release the intermediate products from the coated coal and refresh the reactive site for SCW, and further improve the GE. This proposed reaction process can also reveal how the organics in black liquor improve SCWG of coal.

compounds in the aqueous residue and found that the cresols were the main component. Karagoz et al.42 found these compounds were also the main products of hydrothermal treatment of real biomass (sawdust) at 180−280 °C, which were probably generated from the lignin component of sawdust. Lignin and coal are quite different in structure, so they have different performances in supercritical water. Lignin is defined as a biopolymer in which hydroxyphenylpropane such as pcoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol are connected with ether and C−C bonds. And coal is usually described as an ensemble of functional groups organized into tightly bound aromatic ring clusters connected by weaker aliphatic and ether bridges.43 Therefore, lignin is supposed to be easier to decompose than coal. The above experimental results also showed that the GE of lignin was higher than that of coal under the same reaction conditions. Accordingly, lignin was supposed to decompose prior to coal in cogasification of coal and lignin. On the basis of the analyzing results, a simple influencing mechanism of lignin on SCWG of coal was proposed. Coal probably cannot dissolve in supercritical water like some organic compounds and still present as a solid particle. As shown in Figure 6, the organics in the outer layer of the coal particle may react with SCW first and release the produced gases. Some other

4. CONCLUSION Hydrogen production from SCWG of coal with alkaline black liquor as an additive has been studied by using a fluidized bed reactor. The thermodynamic analysis showed that the addition of coal can improve the H2 production in SCWG of black liquor at the equilibrium state. The experimental study showed that the alkalis in black liquor catalyzed the water−gas shift reaction and increased the GE of coal. When 50 wt % black liquor was added 390

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into coal, the CO fraction decreased from 20.1% to below 1%, the H2 fraction increased from 34.03% to about 45%, and the GE of coal increased from 15.18% to 44.11%. Besides alkalis, the lignin in black liquor also showed a slight positive influence on coal gasification. In cogasification of coal and black liquor, the increased total concentration reduced the GE. A plugging problem occurred in SCWG of 25 wt % coal−black liquor mixture after 40 min. The addition of black liquor also played the role of sulfur fixation in coal gasification, but it aggravated the corrosion of the 316 SS reactor. The main components of the aqueous residue from coal and black liquor gasification were alkylphenols, cyclopentanone, and their derivatives. In cogasification of coal and black liquor or lignin, the lignin can be decomposed prior to the coal. The phenolic intermediates generated from lignin can promote the extraction of the substances on the outside of the coal particle and improve the coal gasification. This study demonstrated that waste black liquor can be an efficient and inexpensive additive in SCWG of coal.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-29-82663895. Fax: 86-29-82669033. E-mail: [email protected]. 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 (Contract No. 51121092, 51236007) and Australian Research Council (Project No. LP 110100337). We are very grateful to Dr. Lijing Ma for the XRD and XRF analysis of the sample, as well as to Dr. Monica Lagos from Control Technology International for the language refining.

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