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Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource
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Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource Changqing Cao*, Yi Zhang, Linhu Li, Wenwen Wei, Gaoyun Wang, Ce Bian State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
article info
abstract
Article history:
Supercritical water gasification (SCWG) is a new treatment of black liquor (BL) for both
Received 3 August 2018
energy recovery and pollution management. To provide more energy for the pulp mill, it is
Received in revised form
proposed to use the pulping raw material as supplementary energy source because it is
25 September 2018
readily available, inexpensive and renewable. In this study, co-gasification of BL and wheat
Accepted 1 October 2018
straw (WS) in supercritical water was investigated. The synergistic effect was observed in
Available online xxx
the co-gasification because the addition of wheat straw can make better use of the alkali in BL. The maximum improvement of the gasification by the synergistic effect was obtained
Keywords:
with the mixing ratio of 1:1. The influences of the temperature (500e750 C), reaction time
Supercritical water
(5e40 min), mixture concentration (5.0e19.1 wt%), mixing ratio (0e100%) and the wheat
Black liquor/wheat straw mixture
straw particle diameter (74e150 mm) were studied. It was found that the increase of tem-
Co-gasification
perature and reaction time, and the decrease of concentration and wheat straw particle
Synergistic effect
size favored the gasification by improving the hydrogen production and gasification efficiency. The highest carbon gasification efficiency of 97.87% was obtained at 750
C.
Meanwhile, the H2 yield increased from 12.29 mol/kg at 500 C to 46.02 mol/kg. This study can help to develop a distributed energy system based on SCWG of BL and raw biomass to supply energy for the pulp mill and surrounding communities. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Black liquor is the major waste byproduct from the paper and pulp-making process. It possesses great threat to the public health and ecological environment for its high organic content and pH value, as well as the dark caramel color. Hence, the handling of BL gained a lot of attention all over the world. Several kinds of handling methods were invented and studied, including recovery boiler, sedimentation/flotation, anaerobic/ aerobic treatment, membrane filtration and ozonation [1,2]. Among them, Tomlinson recovery boiler is the most
commonly used treatment method of black liquor nowadays globally [2,3]. With this method, the organics of BL are combusted to recover the energy and the inorganics are recovered from the residual ash and recycled in cooking process. It has been used for several decades for the high handling performance, but some drawbacks were found during its development and application. First, the weak black liquor (10e20 wt% solid content) must be condensed to strong black liquor (solid content>70 wt%) before being combusted in the boiler [4,5]. A lot of energy were consumed in the evaporation process, which accounted for approximate 37% of the total energy consumption of a typical conventional pulp mill [6]. Second,
* Corresponding author. E-mail address: [email protected] (C. Cao). https://doi.org/10.1016/j.ijhydene.2018.10.006 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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the melting of the alkali salts for the high combustion temperature also brings challenges to the safety and operations of the system. Additionally, some polluting emissions, such as NOx, SO2 and fine particles were generated during black liquor combustion [2,7,8]. With increasing concerns on the environment pollution and energy shortage nowadays, a clean and efficient treatment method of black liquor was desired. SCWG is a promising transformation method of biomass [9e12], organic wastes [13e15] and coal [16e18]. It can utilize the unique properties of supercritical water (T > 374 C, P > 22.1 MPa) to transform them into hydrogen-rich gases. For the special reaction environment and conditions, SCWG can overcome the above-mentioned problems of conventional BL handling methods. First, black liquor evaporation is not needed since the reaction was occurred in water, which can save much energy consumption and improve the energy efficiency. Therefore, it has great advantage in treating the wet biomass, especially for the black liquor with over 80% water content. Second, the problems brought by alkali melting can be avoided because of the low reaction temperature and special reaction environment. Additionally, no pollutants (NOx, SO2 and fine particles) were reported to be generated in the reactions in SCW even the feedstock contains N and S elements [19e21]. Thus, the investment and operating cost can be reduced by avoiding the subsequent decontamination processes. As a result, many researchers in the world focused on SCWG of BL and did a lot of work on this topic [22e27]. Sricharoenchaikul [22] conducted the experiments on SCWG of BL from kraft pulping and obtained the highest carbon gasification efficiency (84.8%) at 650 C. Blasio et al. [25] found that using the reactor making of nickel-based alloy can further boost the gasification of kraft black liquor and achieved high hot gas efficiency (>80%) at 700 C. Previously, we found that SCWG cannot only decontaminate soda black liquor from wheat straw by reducing its COD (Chemical Oxygen Demand) concentration, pH value and chroma, but also produce hydrogen-rich gases (about 50 vol%) [23]. At high temperature (750 C), the maximum CE of 94.1% can be achieved in SCWG of soda black liquor when its concentration is 9.5 wt% [26]. Therefore, SCWG is a promising clean and efficient handling method of black liquor. The recovered energy from BL was encouraged to be used in pulping process in both conventional treatment methods [3,28,29] and SCWG [30e32]. We analyzed a system coupling with pulp mill and BL SCWG, which cogenerated hydrogen, low pressure steam, medium pressure steam and power for the pulp mill and surrounding communities [30,32]. As a large portion of the energy was recovered as hydrogen, some external energy needs to be imported to keep the running of the system. To produce more energy and realize the energy self-sufficient of the pulp mill, the supplementary energy resource can be co-gasified with black liquor. Previously, we found that coal can be a supplement energy source as it can make full use of the alkali salt in BL as the catalyst [33,34]. Besides, the pulping raw material (biomass) is also available. Compared with coal, biomass is a renewable energy resource and can increase the renewable energy usage. Also, it is cheaper and more convenient to be acquired in the pulp mill. Several kinds of biomass were used as the raw material in pulping process, including wood, grass, bamboos and straws
[5]. Among them, wheat straw was widely used in the countries like China and India, which are short of forest and wood resource [35,36]. When wheat straw is co-gasified with black liquor as the supplementary energy resource, its gasification can also be boosted for the alkali content in BL because alkalis were proven to be an effective catalyst for biomass SCWG [37e40]. Additionally, the mixing of WS powder can be a more energy-efficient way to increase the concentration of black liquor instead of evaporation. The proper rise of the feedstock concentration was proven to increase the energy and exergy efficiency of the SCWG system by reducing the system scale and the heat loss by our previous study [32]. As a result, SCWG of WS/BL mixture can promote the energy conversion from both the aspects of reaction kinetics and the system optimization. However, to the best of our knowledge, co-gasification of BL and WS in SCW has not been studied to date. In the present study, co-gasification of alkaline BL and WS was studied with an autoclave to assess the gasification features. The interact effect of black liquor and wheat straw was firstly studied and the optimal mixing ratio of them was obtained. Additionally, the effect of some important technical parameters (reaction temperature, reaction time and mixture concentration of BL/WS) on the co-gasification performance was studied. And the gasification of black liquor with wheat straw powder with different particle diameters was also studied to get the optimal wheat straw preparing method. This study may help to understand the co-gasification of BL and the raw biomass and develop a distributed energy system based on SCWG to supply energy for the pulp plant and surrounding communities.
Experiments Black liquor from soda pulping of wheat straw in a pulp mill in Shaanxi Province of China was used as the feedstock as wheat straw is widely used as pulping raw biomass in China. The pH value of the BL is as high as 12.4 and the solid content is measured to be 10.62 wt%. The characterization of the BL solid can be found in Table 1. The wheat straw used in this study was collected from Gaoling District of Shaanxi Province, which was ground and sieved before being used. The wheat straw with particle diameters between 74 and 150 mm was chosen as the feedstock in this study. Its characterization results were also shown in Table 1. The elemental and proximate analysis were performed with a CHNS/O elemental analyzer (Perkin Elmer 2400 II) and a proximate analyzer (SDTGA5000, Sundy Enterprise Co. Ltd., China) respectively. The experiments were carried out with an autoclave making of Inconel 625 superalloy with an internal volume of 10 ml, whose design temperature and pressure were 750 C and 30 MPa respectively. Before the experiment, a given mass of BL and WS powder was loaded into the reactor, which was then shaken repetitively for uniform mixing. After being sealed, the reactor was swept with Ar gas for over 3 times to exclude the influence of the air resided in the reactor. Then the reactor was put into a vertical preheated oven for heating. The reaction temperature was measured by a K-type thermocouple, which showed that the average heating rate was up to 30 K/min and its deviation was about ±5 C. As the
Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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Table 1 e Characterization of WS and BL solids (air-dried basis). Elemental composition, wt%
Black liquor Wheat straw a
Proximate analysis, wt%
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygena
Moisture
Ash
Volatile
Fixed carbon
27.9 40.89
3.20 5.42
0.87 0.94
3.48 0.96
13.79 36.43
3.82 7.08
46.93 8.29
38.22 73.78
14.85 17.93
By difference.
temperature was raised to the setting value, the reactor pressure ranged from 23 to 26 MPa with the calculated amounts of reactants through the equation of state. Once the reaction was finished, we took out the reactor and quenched it in cold water to be cooled to ambient temperature rapidly. Then the produced gas was quantitatively measured with a gas flow-meter and the composition was analyzed with a gas chromatography (GC). Finally, the reactor was opened to collect the liquid residues for COD analysis. We analyzed the gas product compositions quantitatively using a GC (Agilent 7890 A) equipped with a thermal conductivity detector. The gas species were separated by a Plot C-2000 capillary column working under programmed temperatures. The carrier gas used in this study is pure argon. The COD concentration of aqueous products was determined by a Lovibond ET99718 COD measuring system. The experimental procedure and methods were described in more detail in our previous studies [26,34]. To evaluate the co-gasification performance of BL and WS, some parameters were used in the present study, including the gas yield, carbon gasification efficiency (CE), gasification efficiency (GE) and hydrogen gasification efficiency (HE). They can be calculated as follows: GE ¼
Total mass of gas product 100% Mass of ash-free WS=BL blends solid
(1)
CE ¼
Carbon content of gas product 100% Carbon content of WS=BL blends
(2)
HE ¼
Hydrogen content of gas product 100% Hydrogen content of WS=BL blends solids
(3)
Gas yield ¼
Mole of the gas product Mass of WS=BL blends solid
(4)
Results and discussion The effect of the BL/WS mixing ratio The co-gasification performance of BL/WS with various mixing ratio was studied at 700 C with the reaction time of 30min. The total concentration was fixed at 10.62 wt% for the convenience to compare with the gasification of as-received black liquor. From the results, the gas products were mainly composed of H2, CH4, CO2 with trace amount of CO, C2H6 and C2H4. The content of the last three gas species was relatively low, whose total mole fraction is less than 3.56% throughout the study. In Fig. 1 (a), the variation of the BL/WS mixing ratio had a great impact on the CE and GE. In the
cases of separate gasification, the CE and GE of the black liquor were 71.47% and 122.54% respectively, which were higher than those of wheat straw (66.27% and 102.26% respectively). It indicated that black liquor was easier to be gasified under this reaction condition, which seems contradictory to the theoretical results predicted from the components and SCWG characteristics. Wheat straw is a typical biomass that mainly composed of cellulose, hemicellulose and lignin [41,42]. The cellulose of wheat straw was extracted during pulping process, so the byproduct, black liquor mainly contains lignin and hemicellulose. As reported, lignin was more difficult to be gasified than cellulose and hemicellulose in the SCW [43]. From this point of view, black liquor should be more difficult to be gasified than wheat straw, which is opposite to the results of the study. We assumed that the higher gasification efficiency of black liquor was related to its inorganic content, which is the alkalis derived from the pulping process. These alkalis were shown to be effective catalyst in both conventional conversion [44,45] and SCWG of biomass [37e40], which can enhance the degradation of the lignin and hemicellulose in black liquor. Therefore, higher gasification efficiency of BL than WS was achieved under the same reaction conditions. The synergistic effect was observed in co-gasification of WS and BL in supercritical water, that is, both the present of WS and BL improved the gasification of each other. If there were no interactions in the co-gasification, their theoretical gasification efficiency should be distributed on the dashed line as shown in Fig. 1(a) as discussed in our previous study [34]. However, the realistic GE and CE were larger than the theoretical ones, indicating that the synergistic effect was existed during the co-gasification. Similar with the co-gasification of BL/coal [34], the synergistic effect of BL and WS during their co-gasification in SCW may mainly related to the alkali components in black liquor. In a word, the addition of BL improved the WS gasification by bringing high content of alkali, which performed as the catalyst to improve the gasification performance [37e40]. On the other hand, the addition of wheat straw can make better use of the alkali catalysts that excess the required amount for black liquor gasification. Hence, both the gasification of WS and BL were boosted in the cogasification. It is notable that the synergistic effect of WS and BL in SCWG found in this study was smaller than that between coal and BL as observed previously [33,34]. We considered that this was mainly because of the different compositions and properties of coal, wheat straw and black liquor organic materials. Wheat straw and black liquor are more similar in composition than coal and black liquor because the BL is sourced from WS. The distinct formation and property of coal and BL may result
Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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Fig. 1 e Effect of BL fraction in BL/WS mixture on the GE and CE (a) and gas yield (b) of the co-gasification (total concentration: 10.62 wt%; reaction time: 30min; temperature: 700 C). The dashed line in (a) indicates the theoretical GE and CE of the BL/ WS mixture.
in the distinct reaction intermediates and reaction environment, which influenced the decomposition and gasification of each other [33]. While in co-gasification of BL and WS, the reaction intermediates and reaction environment in SCWG may be similar for their similar composition and property. Thus, the influence on the gasification performance of BL and WS was less significant than the co-gasification of BL and coal. The variation of WS/BL ratio also changed the gas yields from the co-gasification. Though the gasification performance was enhanced when BL was mixed, the yields of some gas products, including H2, CO2 and CH4 were reduced when the addition of black liquor was increased (Fig. 1(b)). For example, the H2 yield decreased from 25.29 to 17.33 mol/kg when the fraction of black liquor in the mixture increased from 25% to 100%, and the CO2 yield decreased from 17.82 to 10.84 mol/kg in the meantime. This is also contradicted with the theoretical results predicted from the reaction mechanisms. Judging from the catalytic activity of alkali brought by black liquor, the H2 and CO2 production should be improved by catalyzing the water-gas shift reaction [37e40], that consumes CO and generates H2 and CO2. We assumed that the reduced yields of these gases were related with the unique composition of the black liquor and the calculation method of gas yield. As black liquor had higher ash content (46.93%, Table 1) than wheat straw (8.29%), the increase in the BL/WS ratio raised the ash content that cannot be converted into gas products. Therefore, the gas yields from the same mass weight of BL/WS mixture were decreased when the BL/WS ratio was increased. In fact, the present of BL had positive impact on the gas product compositions. For example, the addition of the BL indeed reduced the CO fraction of the gas products, which declined from 1.57% to 0.23% as the fraction of BL increased from zero to 100%. The reduction of the CO fraction of the gas products favored its end use in some cases where the CO fraction was strictly limited, such as the PEMFC [46,47].
The effect of reaction temperature Temperature is an important parameter in SCWG because the reactions in SCWG were mainly endothermic reactions, including decomposition, hydrolysis and steam reforming reaction. In this study, we investigated the influence of reaction temperature ranged from 500 C to 750 C with the total
concentration of BL/WS mixture of 10.62 wt%. The mixing ratio was fixed at 1:1 because it was the optimal ratio for cogasification of BL and WS (Section 3.1). The change of reaction temperature had a great impact on the gasification efficiencies, including GE, CE and HE. From Fig. 2(a), the GE, CE and HE increased dramatically with the increase of temperature. They all had more than two-fold increase when the temperature was increased from 500 C to 750 C, which increased from 66.71%, 41.22% and 65.18%e 162.05%, 97.87% and 206.98% respectively. Notably, the HE had a larger improvement than GE and CE, which had almost three-fold increase. Additionally, it was higher than 100% when the temperature was higher than 600 C, which can be attributed to the participation of water in SCWG. The decomposition or other reactions involving water released hydrogen element in the gas products and make the hydrogen content in gas products be higher than that in BL/WS solids. For the same reason, the GE was also higher than 100% when the temperature was higher than 600 C. The significant improvement in GE and HE also indicated that more water was involved in the reactions and contributed to the gas production. Additionally, the improvement of the gasification efficiencies was more significant at higher temperature, especially when it is higher than 650 C. The GE increased from 109.50% to 162.05% when the temperature raised from 650 C to 750 C. When the temperature was raised to 750 C, nearly complete gasification was achieved, and the CE reached up to 97.87%. This value was also higher than the highest CE as reported of separate gasification wheat straw and black liquor. The relatively high CE of black liquor with similar concentration was achieved in our previous study [26], where CE up to 94.10% was obtained at 750 C when the concentration was 9.5 wt%. The higher CE achieved in this study can be attributed to the synergistic effect of BL and WS. The addition of wheat straw made better use of the alkali salt in black liquor, and it also brought other components like cellulose that is easier to be gasified in SCW than lignin in black liquor. Therefore, higher gasification performance was achieved in their cogasification than the separate gasification. The reaction temperature also showed a great impact on the gas product composition and gas yield (Fig. 2(b)). With more gas produced at higher temperatures, the total gas yield
Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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C
5
C
°
°
Fig. 2 e Effect of reaction temperature on the GE, CE, HE and COD concentration of the aqueous residue (a) and the gas yield (b) (mixing ratio: 1:1; mixture concentration: 10.62 wt%; reaction time: 30min).
increased from 28.78 to 86.24 mol/kg when it raised from 500 C to 750 C. Meanwhile, hydrogen had a most significant improvement with the temperature, which increased almost four times. The yield of CO2 was also improved, but the improvement was much lower than hydrogen. This also led to the increase of H2 fraction and the decrease of CO2 fraction. The CO yield was relatively low at all the tested temperatures, which was around 0.62 mol/kg for all the tested conditions. This result was probably related with the alkali salts in BL, including NaOH, Na2CO3 and NaHCO3. As reported [37e40], they were effective in catalyzing SCWG, which can promote the water-gas shift reaction (H2O þ CO ⟶ H2þCO2) and resulted in CO consumption. This may also be the reason for the increase of H2 and CO2 yield. As mentioned above, the low CO content of the gas product can be beneficial to its application and end use. Black liquor is considered as a great threat to the environment and human health. One of its threats is the high COD content, which can cause great pollution to the water resource if it is discharged without proper treatment. The COD concentration of the studied black liquor is about 106000 mg/L. The mixture of BL/WS may have a higher COD concentration because the same amount of wheat straw can consume more oxygen for chemical oxidation because of its lower ash content than black liquor solid. After the treatment of SCWG, the COD concentration was tremendously reduced (Fig. 2(a)). The COD concentration of the liquid residue was reduced to 4386.33 mg/l when they were gasified at 500 C, which is only 4.13% of the COD concentration of the black liquor with the same concentration. With the increasing temperature, the COD content was reduced quickly. The COD concentrations of the liquid residues were all below 672.15 mg/L when the temperature was higher than 650 C. This value was still higher than the water discharge standard on COD concentration for pulping industrial in China, which is lower than 200 mg/L [48]. However, the COD concentration can be further reduced in the realistic applications, where preheated water was used to realize the fast heating of the organic reactant to inhibit the side reactions [39]. Thus, the COD concentration can be diluted to a certain extent corresponding to the mixing ratio of preheated water and feedstocks. Then it may fulfill the requirement of the discharge standard.
The effect of reaction time Reaction time is another crucial parameter in SCWG, especially for the reactions in autoclave. This is because the mass transfer resistance was higher than the continuous reactor where the reactants can flow in the reactor. The effect of the reaction time ranged from 5 to 40 min on co-gasification of BL/ WS with the mixing ratio of 1:1 was investigated at 700 C (Fig. 3). The prolonging reaction time <30 min improved the gasification efficiency. The GE, CE and HE increased from 91.34%, 59.47% and 97.34% to 112.18%, 73.70% and 135.93% respectively with the reaction time increasing from 5 to 30min. It is notable that the total gas yield achieved at 700 C with a reaction time of 5 min was almost the same with that achieved at 600 C with a reaction time of 30 min (shown in Fig. 2). It showed that temperature was a prior parameter and this gasification extent can be reached quicker at higher temperatures. It also indicated that the reaction temperature must be considered in determining the optimal reaction time. However, the reaction time had seldom influence on the gasification when it is longer than 30min. The GE, CE and HE were almost unchanged while the time was prolonged from 30 to 40 min (Fig. 3 (a)). We proposed that 30min was long enough for the decomposition of the loaded BL and WS, so further prolonging the reaction time had little impact on the gasification. The prolongation of the reaction time in the range of 5e30 min also improved the yields of H2, CH4 and CO2 (Fig. 3(b)). The H2 yield had the largest improvement with the reaction time among all the gas products, which increased from 16.10 to 35.33 mol/kg when it was extended from 5 to 30 min. However, when the reaction time increased from 30 to 40min, the H2 yield decreased slightly while the CH4 yield increased from 7.76 to 9.71 mol/kg. This is probably because further prolonging the time above 30 min had little impact on the reactions that generate hydrogen, such as organics decomposition, steam reforming and water-gas shift reaction. However, it can still influence some hydrogen-consuming reactions, like methanations (Eqs. (5) and (6)), which generates more CH4. Therefore, the prolongation of the reaction time favored the production of CH4, but it inhibited the hydrogen production.
Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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Fig. 3 e Effect of reaction time on the GE, CE and HE (a) and the gas yields (b) in co-gasification of BL and WS (Mixing ratio: 1:1; Total concentration: 10.62 wt%; Reaction temperature: 700 C).
3H2þCO/CH4þH2O
(5)
The effect of mixture concentration
4H2þCO2/CH4þ2H2O
(6)
A main advantage of BL and WS co-gasification is that it can improve the feedstock concentration, the handling and energy-production capacity of a certain system. However, the variation on the feedstock concentration will have a great impact on the co-gasification of BL and WS. In the present study, we investigated the effect of the mixture concentration of BL/WS on the co-gasification. The mixing ratio was fixed at 1:1 for it is the optimal mixing ratio for their co-gasification. When the mixing ratio and the concentration of the black liquor were fixed, the highest mixture concentration can be achieved by adding a certain amount of wheat straw powder into the black liquor without dilution. For example, we can add 10.62 g wheat straw powder into 100 g black liquor (10.62 wt%) to get a mixture with a mixing ratio of 1:1. In this way, the mixture concentration is calculated to be 19.2 wt%, which is also the upper limit of the studied concentration. From Fig. 4, the gasification efficiencies, including GE and CE were decreased with the rise of the concentration. The highest CE of the mixture of 98.89% at 700 C was obtained when the concentration is 5 wt%, which is even higher than that achieved in co-gasification with a concentration of 10.62 wt% at 750 C (Fig. 2). Meanwhile, the GE and HE reached 161.81% and 231.94% respectively, which were also the highest values in this study. When the concentration was increased to 19.1 wt%, the GE and HE were decreased to 112.14% and
The results showed that 30 min is required to complete most of the reactions and reach the equilibrium state. This reaction time is much longer than that needed in SCWG of biomass with continuous systems, which only need several to dozens of seconds to complete the gasification. For example, Antal et al. [49] obtained complete gasification of 0.1 M glucose in SCW in 28 s at 34.5 MPa and 600 C with a continuous reactor. Lee et al. [50] also found that the residence time had seldom impact on the gas yields when it was longer than 10.4 s in gasification of 0.6 M glucose in SCW in a tubular continuous reactor. The difference on the required reaction time can be mainly attributed to the different mass-transfer resistance of the two different reactors. Compared with the batch reactors, the reactants, reaction intermediates and the products in continuous reactor can flow through the reactor and take away the products from the reactants, which will favor the reactions for gas production. The great gap between the required time to finish the reactions also indicated the importance of improving the mass-transfer efficiency in SCWG of BL and WS. In the future, more work needs to be done to study the optimal residence time for co-gasification of BL and WS with continuous reaction systems.
Fig. 4 e Effect of the mixture concentration of BL/WS on the GE, CE and HE (a) and the gas yield (b) during co-gasification (mixing ratio: 1:1; reaction temperature: 700 C; reaction time: 30min). Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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Fig. 5 e Effect of WS particle size on the GE and CE (a) and gas fraction (b) of co-gasification of BL and WS (mixing ratio: 1:1; total concentration: 10.62 wt%; reaction time: 30min; reaction temperature: 700 C).
136.97% respectively, which is only a half of that achieved with the concentration of 5 wt%. It is notable that the HE obtained with all the studied concentration was above 100 wt%, indicating that water was involved in the reactions and contributed to the gas production. With the decrease of the concentration, there will be more water existed in the reactor and the collision frequency of the water with reactants and intermediates was improved. Thus, the reactions involved water will be enhanced and the gasification performance and H2 production were boosted. The variation of the mixture concentration also changed the gas product composition and yield. From Fig. 4(b), the total gas yield was reduced dramatically with the increasing concentration of BL/WS mixture. When the concentration increased from 5 to 19.1 wt%, the total gas yield decreased from 63.71 to 34.71 mol/kg, which was reduced by half. And 34.71 mol/kg was also the minimum total gas yield obtained in this study. The hydrogen fraction was also reduced with the increasing concentration, which decreased from 56.09% to 39.53% when the concentration was increased from 5 wt% to 19.1 wt%. And 39.53% was also the lowest hydrogen fraction that obtained in this study. Though diluting the BL/WS mixture is beneficial to the gasification from the reaction aspects, it may reduce the energy efficiency and increase the investment cost of the related energy system in realistic applications. For a given amount of BL and WS solids, the dilution of the feedstock can increase the system scale and the construction cost of the system, which may reduce the energy efficiency owing to more energy losses [32]. Therefore, determination of the optimal handling concentration should consider not only its impact on the gasification performance, but also the energy and economic analysis of the realistic application scenarios.
The effect of biomass particle diameter In this study, the wheat straw was ground and sieved to get the wheat straw particles with different sizes. In this study, three kind of wheat straw particles with different particle diameters (<74 mm, 74e107.5 mm and 107.5e150 mm) were gasified with black liquor to study the influence of the particle size of wheat straw. The results showed that the gasification was
favored with the smaller wheat straw particles size, and the GE and CE were improved when the particle size decreased (Fig. 5(a)). The GE improved from 88.76% to 102.3% and the CE improved from 81.39% to 92.58% when the particle diameter decreased from >107.5 mm to <74 mm. We supposed that the decrease of the particle diameter can improve the specific surface area of the particle and improve the contact of the wheat straw particle with other reactants and intermediates. Therefore, the reactions of the particle involved with SCW and the reactive reactant, such as hydrolysis and steam reforming were accelerated, so the gasification was improved with smaller particle size. On the other hand, the impact of the WS particle size on the gas product components was very slight (Fig. 5(b)). The main gas products were H2, CH4, CO2 and C2H6, which were in the range of 46.41%~50.28%, 11.05%~13.17%, 34.84%~39.50% and 1.63%~2.07% respectively. The change of the wheat straw particle size not only influence the gasification performance of BL/WS mixture, but also affect the economic efficiency of the process in practical application. The decrease of the wheat straw particle size is favorable to the co-gasification, but it will increase the energy consumption of the grinding process. As a result, an appropriate particle size should be selected by balancing its influences on the gasification efficiency, energy consumption and the operating cost.
Conclusion In this study, we proposed a new treatment method of black liquor, where it was co-gasified with wheat straw to improve the energy supply for the pulping process and make better use of the alkalis in black liquor. Their co-gasification performance in supercritical water was studied experimentally with an autoclave. During co-gasification, the synergistic effect of black liquor and wheat straw was found. It was mainly because the present of black liquor offered the alkali salt as the catalyst for wheat straw, and the present of wheat straw made better use of the alkali catalyst. The optimal mixing ratio of BL/WS for their co-gasification was found to be 1:1, where the highest improvement of the gasification efficiency by the synergistic effect was obtained. The increase in the
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reaction temperature also improved the gasification efficiency, where the highest CE of 97.87% for BL/WS mixture was achieved at 750 C. The COD concentration of the aqueous residues was reduced to below 672.15 mg/L when the temperature was above 650 C. The prolongation of the reaction time from 5 to 30min improved the gasification efficiency and the hydrogen production, but it had limited influence on the gasification when it was longer than 30min. The dilution of the BL/WS mixture also enhanced the gasification and improved the hydrogen production. The highest HE and CE of 231.94% and 98.89% respectively was achieved when it was diluted to 5 wt%. Reducing wheat straw particle sizes favored the gasification by improving the gasification efficiency, but it will also increase the energy consumption of wheat straw grinding. To realize the practical applications of cogasification of BL and WS, the investigation based on continuous reaction system should be conducted in the future. Meanwhile, the economic analysis based on life cycle assessment and system analysis may also be helpful to optimize the co-gasification process and promote the applications.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (No. 51606150), the China National Key Research and Development Plan Project (No. 2016YFB0600100) and Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JQ5172 and 2017ZDXM-GY-067) for funding this study.
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Available online at www.sciencedirect.com
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Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource Changqing Cao*, Yi Zhang, Linhu Li, Wenwen Wei, Gaoyun Wang, Ce Bian State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
article info
abstract
Article history:
Supercritical water gasification (SCWG) is a new treatment of black liquor (BL) for both
Received 3 August 2018
energy recovery and pollution management. To provide more energy for the pulp mill, it is
Received in revised form
proposed to use the pulping raw material as supplementary energy source because it is
25 September 2018
readily available, inexpensive and renewable. In this study, co-gasification of BL and wheat
Accepted 1 October 2018
straw (WS) in supercritical water was investigated. The synergistic effect was observed in
Available online xxx
the co-gasification because the addition of wheat straw can make better use of the alkali in BL. The maximum improvement of the gasification by the synergistic effect was obtained
Keywords:
with the mixing ratio of 1:1. The influences of the temperature (500e750 C), reaction time
Supercritical water
(5e40 min), mixture concentration (5.0e19.1 wt%), mixing ratio (0e100%) and the wheat
Black liquor/wheat straw mixture
straw particle diameter (74e150 mm) were studied. It was found that the increase of tem-
Co-gasification
perature and reaction time, and the decrease of concentration and wheat straw particle
Synergistic effect
size favored the gasification by improving the hydrogen production and gasification efficiency. The highest carbon gasification efficiency of 97.87% was obtained at 750
C.
Meanwhile, the H2 yield increased from 12.29 mol/kg at 500 C to 46.02 mol/kg. This study can help to develop a distributed energy system based on SCWG of BL and raw biomass to supply energy for the pulp mill and surrounding communities. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Black liquor is the major waste byproduct from the paper and pulp-making process. It possesses great threat to the public health and ecological environment for its high organic content and pH value, as well as the dark caramel color. Hence, the handling of BL gained a lot of attention all over the world. Several kinds of handling methods were invented and studied, including recovery boiler, sedimentation/flotation, anaerobic/ aerobic treatment, membrane filtration and ozonation [1,2]. Among them, Tomlinson recovery boiler is the most
commonly used treatment method of black liquor nowadays globally [2,3]. With this method, the organics of BL are combusted to recover the energy and the inorganics are recovered from the residual ash and recycled in cooking process. It has been used for several decades for the high handling performance, but some drawbacks were found during its development and application. First, the weak black liquor (10e20 wt% solid content) must be condensed to strong black liquor (solid content>70 wt%) before being combusted in the boiler [4,5]. A lot of energy were consumed in the evaporation process, which accounted for approximate 37% of the total energy consumption of a typical conventional pulp mill [6]. Second,
* Corresponding author. E-mail address: [email protected] (C. Cao). https://doi.org/10.1016/j.ijhydene.2018.10.006 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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the melting of the alkali salts for the high combustion temperature also brings challenges to the safety and operations of the system. Additionally, some polluting emissions, such as NOx, SO2 and fine particles were generated during black liquor combustion [2,7,8]. With increasing concerns on the environment pollution and energy shortage nowadays, a clean and efficient treatment method of black liquor was desired. SCWG is a promising transformation method of biomass [9e12], organic wastes [13e15] and coal [16e18]. It can utilize the unique properties of supercritical water (T > 374 C, P > 22.1 MPa) to transform them into hydrogen-rich gases. For the special reaction environment and conditions, SCWG can overcome the above-mentioned problems of conventional BL handling methods. First, black liquor evaporation is not needed since the reaction was occurred in water, which can save much energy consumption and improve the energy efficiency. Therefore, it has great advantage in treating the wet biomass, especially for the black liquor with over 80% water content. Second, the problems brought by alkali melting can be avoided because of the low reaction temperature and special reaction environment. Additionally, no pollutants (NOx, SO2 and fine particles) were reported to be generated in the reactions in SCW even the feedstock contains N and S elements [19e21]. Thus, the investment and operating cost can be reduced by avoiding the subsequent decontamination processes. As a result, many researchers in the world focused on SCWG of BL and did a lot of work on this topic [22e27]. Sricharoenchaikul [22] conducted the experiments on SCWG of BL from kraft pulping and obtained the highest carbon gasification efficiency (84.8%) at 650 C. Blasio et al. [25] found that using the reactor making of nickel-based alloy can further boost the gasification of kraft black liquor and achieved high hot gas efficiency (>80%) at 700 C. Previously, we found that SCWG cannot only decontaminate soda black liquor from wheat straw by reducing its COD (Chemical Oxygen Demand) concentration, pH value and chroma, but also produce hydrogen-rich gases (about 50 vol%) [23]. At high temperature (750 C), the maximum CE of 94.1% can be achieved in SCWG of soda black liquor when its concentration is 9.5 wt% [26]. Therefore, SCWG is a promising clean and efficient handling method of black liquor. The recovered energy from BL was encouraged to be used in pulping process in both conventional treatment methods [3,28,29] and SCWG [30e32]. We analyzed a system coupling with pulp mill and BL SCWG, which cogenerated hydrogen, low pressure steam, medium pressure steam and power for the pulp mill and surrounding communities [30,32]. As a large portion of the energy was recovered as hydrogen, some external energy needs to be imported to keep the running of the system. To produce more energy and realize the energy self-sufficient of the pulp mill, the supplementary energy resource can be co-gasified with black liquor. Previously, we found that coal can be a supplement energy source as it can make full use of the alkali salt in BL as the catalyst [33,34]. Besides, the pulping raw material (biomass) is also available. Compared with coal, biomass is a renewable energy resource and can increase the renewable energy usage. Also, it is cheaper and more convenient to be acquired in the pulp mill. Several kinds of biomass were used as the raw material in pulping process, including wood, grass, bamboos and straws
[5]. Among them, wheat straw was widely used in the countries like China and India, which are short of forest and wood resource [35,36]. When wheat straw is co-gasified with black liquor as the supplementary energy resource, its gasification can also be boosted for the alkali content in BL because alkalis were proven to be an effective catalyst for biomass SCWG [37e40]. Additionally, the mixing of WS powder can be a more energy-efficient way to increase the concentration of black liquor instead of evaporation. The proper rise of the feedstock concentration was proven to increase the energy and exergy efficiency of the SCWG system by reducing the system scale and the heat loss by our previous study [32]. As a result, SCWG of WS/BL mixture can promote the energy conversion from both the aspects of reaction kinetics and the system optimization. However, to the best of our knowledge, co-gasification of BL and WS in SCW has not been studied to date. In the present study, co-gasification of alkaline BL and WS was studied with an autoclave to assess the gasification features. The interact effect of black liquor and wheat straw was firstly studied and the optimal mixing ratio of them was obtained. Additionally, the effect of some important technical parameters (reaction temperature, reaction time and mixture concentration of BL/WS) on the co-gasification performance was studied. And the gasification of black liquor with wheat straw powder with different particle diameters was also studied to get the optimal wheat straw preparing method. This study may help to understand the co-gasification of BL and the raw biomass and develop a distributed energy system based on SCWG to supply energy for the pulp plant and surrounding communities.
Experiments Black liquor from soda pulping of wheat straw in a pulp mill in Shaanxi Province of China was used as the feedstock as wheat straw is widely used as pulping raw biomass in China. The pH value of the BL is as high as 12.4 and the solid content is measured to be 10.62 wt%. The characterization of the BL solid can be found in Table 1. The wheat straw used in this study was collected from Gaoling District of Shaanxi Province, which was ground and sieved before being used. The wheat straw with particle diameters between 74 and 150 mm was chosen as the feedstock in this study. Its characterization results were also shown in Table 1. The elemental and proximate analysis were performed with a CHNS/O elemental analyzer (Perkin Elmer 2400 II) and a proximate analyzer (SDTGA5000, Sundy Enterprise Co. Ltd., China) respectively. The experiments were carried out with an autoclave making of Inconel 625 superalloy with an internal volume of 10 ml, whose design temperature and pressure were 750 C and 30 MPa respectively. Before the experiment, a given mass of BL and WS powder was loaded into the reactor, which was then shaken repetitively for uniform mixing. After being sealed, the reactor was swept with Ar gas for over 3 times to exclude the influence of the air resided in the reactor. Then the reactor was put into a vertical preheated oven for heating. The reaction temperature was measured by a K-type thermocouple, which showed that the average heating rate was up to 30 K/min and its deviation was about ±5 C. As the
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Table 1 e Characterization of WS and BL solids (air-dried basis). Elemental composition, wt%
Black liquor Wheat straw a
Proximate analysis, wt%
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygena
Moisture
Ash
Volatile
Fixed carbon
27.9 40.89
3.20 5.42
0.87 0.94
3.48 0.96
13.79 36.43
3.82 7.08
46.93 8.29
38.22 73.78
14.85 17.93
By difference.
temperature was raised to the setting value, the reactor pressure ranged from 23 to 26 MPa with the calculated amounts of reactants through the equation of state. Once the reaction was finished, we took out the reactor and quenched it in cold water to be cooled to ambient temperature rapidly. Then the produced gas was quantitatively measured with a gas flow-meter and the composition was analyzed with a gas chromatography (GC). Finally, the reactor was opened to collect the liquid residues for COD analysis. We analyzed the gas product compositions quantitatively using a GC (Agilent 7890 A) equipped with a thermal conductivity detector. The gas species were separated by a Plot C-2000 capillary column working under programmed temperatures. The carrier gas used in this study is pure argon. The COD concentration of aqueous products was determined by a Lovibond ET99718 COD measuring system. The experimental procedure and methods were described in more detail in our previous studies [26,34]. To evaluate the co-gasification performance of BL and WS, some parameters were used in the present study, including the gas yield, carbon gasification efficiency (CE), gasification efficiency (GE) and hydrogen gasification efficiency (HE). They can be calculated as follows: GE ¼
Total mass of gas product 100% Mass of ash-free WS=BL blends solid
(1)
CE ¼
Carbon content of gas product 100% Carbon content of WS=BL blends
(2)
HE ¼
Hydrogen content of gas product 100% Hydrogen content of WS=BL blends solids
(3)
Gas yield ¼
Mole of the gas product Mass of WS=BL blends solid
(4)
Results and discussion The effect of the BL/WS mixing ratio The co-gasification performance of BL/WS with various mixing ratio was studied at 700 C with the reaction time of 30min. The total concentration was fixed at 10.62 wt% for the convenience to compare with the gasification of as-received black liquor. From the results, the gas products were mainly composed of H2, CH4, CO2 with trace amount of CO, C2H6 and C2H4. The content of the last three gas species was relatively low, whose total mole fraction is less than 3.56% throughout the study. In Fig. 1 (a), the variation of the BL/WS mixing ratio had a great impact on the CE and GE. In the
cases of separate gasification, the CE and GE of the black liquor were 71.47% and 122.54% respectively, which were higher than those of wheat straw (66.27% and 102.26% respectively). It indicated that black liquor was easier to be gasified under this reaction condition, which seems contradictory to the theoretical results predicted from the components and SCWG characteristics. Wheat straw is a typical biomass that mainly composed of cellulose, hemicellulose and lignin [41,42]. The cellulose of wheat straw was extracted during pulping process, so the byproduct, black liquor mainly contains lignin and hemicellulose. As reported, lignin was more difficult to be gasified than cellulose and hemicellulose in the SCW [43]. From this point of view, black liquor should be more difficult to be gasified than wheat straw, which is opposite to the results of the study. We assumed that the higher gasification efficiency of black liquor was related to its inorganic content, which is the alkalis derived from the pulping process. These alkalis were shown to be effective catalyst in both conventional conversion [44,45] and SCWG of biomass [37e40], which can enhance the degradation of the lignin and hemicellulose in black liquor. Therefore, higher gasification efficiency of BL than WS was achieved under the same reaction conditions. The synergistic effect was observed in co-gasification of WS and BL in supercritical water, that is, both the present of WS and BL improved the gasification of each other. If there were no interactions in the co-gasification, their theoretical gasification efficiency should be distributed on the dashed line as shown in Fig. 1(a) as discussed in our previous study [34]. However, the realistic GE and CE were larger than the theoretical ones, indicating that the synergistic effect was existed during the co-gasification. Similar with the co-gasification of BL/coal [34], the synergistic effect of BL and WS during their co-gasification in SCW may mainly related to the alkali components in black liquor. In a word, the addition of BL improved the WS gasification by bringing high content of alkali, which performed as the catalyst to improve the gasification performance [37e40]. On the other hand, the addition of wheat straw can make better use of the alkali catalysts that excess the required amount for black liquor gasification. Hence, both the gasification of WS and BL were boosted in the cogasification. It is notable that the synergistic effect of WS and BL in SCWG found in this study was smaller than that between coal and BL as observed previously [33,34]. We considered that this was mainly because of the different compositions and properties of coal, wheat straw and black liquor organic materials. Wheat straw and black liquor are more similar in composition than coal and black liquor because the BL is sourced from WS. The distinct formation and property of coal and BL may result
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Fig. 1 e Effect of BL fraction in BL/WS mixture on the GE and CE (a) and gas yield (b) of the co-gasification (total concentration: 10.62 wt%; reaction time: 30min; temperature: 700 C). The dashed line in (a) indicates the theoretical GE and CE of the BL/ WS mixture.
in the distinct reaction intermediates and reaction environment, which influenced the decomposition and gasification of each other [33]. While in co-gasification of BL and WS, the reaction intermediates and reaction environment in SCWG may be similar for their similar composition and property. Thus, the influence on the gasification performance of BL and WS was less significant than the co-gasification of BL and coal. The variation of WS/BL ratio also changed the gas yields from the co-gasification. Though the gasification performance was enhanced when BL was mixed, the yields of some gas products, including H2, CO2 and CH4 were reduced when the addition of black liquor was increased (Fig. 1(b)). For example, the H2 yield decreased from 25.29 to 17.33 mol/kg when the fraction of black liquor in the mixture increased from 25% to 100%, and the CO2 yield decreased from 17.82 to 10.84 mol/kg in the meantime. This is also contradicted with the theoretical results predicted from the reaction mechanisms. Judging from the catalytic activity of alkali brought by black liquor, the H2 and CO2 production should be improved by catalyzing the water-gas shift reaction [37e40], that consumes CO and generates H2 and CO2. We assumed that the reduced yields of these gases were related with the unique composition of the black liquor and the calculation method of gas yield. As black liquor had higher ash content (46.93%, Table 1) than wheat straw (8.29%), the increase in the BL/WS ratio raised the ash content that cannot be converted into gas products. Therefore, the gas yields from the same mass weight of BL/WS mixture were decreased when the BL/WS ratio was increased. In fact, the present of BL had positive impact on the gas product compositions. For example, the addition of the BL indeed reduced the CO fraction of the gas products, which declined from 1.57% to 0.23% as the fraction of BL increased from zero to 100%. The reduction of the CO fraction of the gas products favored its end use in some cases where the CO fraction was strictly limited, such as the PEMFC [46,47].
The effect of reaction temperature Temperature is an important parameter in SCWG because the reactions in SCWG were mainly endothermic reactions, including decomposition, hydrolysis and steam reforming reaction. In this study, we investigated the influence of reaction temperature ranged from 500 C to 750 C with the total
concentration of BL/WS mixture of 10.62 wt%. The mixing ratio was fixed at 1:1 because it was the optimal ratio for cogasification of BL and WS (Section 3.1). The change of reaction temperature had a great impact on the gasification efficiencies, including GE, CE and HE. From Fig. 2(a), the GE, CE and HE increased dramatically with the increase of temperature. They all had more than two-fold increase when the temperature was increased from 500 C to 750 C, which increased from 66.71%, 41.22% and 65.18%e 162.05%, 97.87% and 206.98% respectively. Notably, the HE had a larger improvement than GE and CE, which had almost three-fold increase. Additionally, it was higher than 100% when the temperature was higher than 600 C, which can be attributed to the participation of water in SCWG. The decomposition or other reactions involving water released hydrogen element in the gas products and make the hydrogen content in gas products be higher than that in BL/WS solids. For the same reason, the GE was also higher than 100% when the temperature was higher than 600 C. The significant improvement in GE and HE also indicated that more water was involved in the reactions and contributed to the gas production. Additionally, the improvement of the gasification efficiencies was more significant at higher temperature, especially when it is higher than 650 C. The GE increased from 109.50% to 162.05% when the temperature raised from 650 C to 750 C. When the temperature was raised to 750 C, nearly complete gasification was achieved, and the CE reached up to 97.87%. This value was also higher than the highest CE as reported of separate gasification wheat straw and black liquor. The relatively high CE of black liquor with similar concentration was achieved in our previous study [26], where CE up to 94.10% was obtained at 750 C when the concentration was 9.5 wt%. The higher CE achieved in this study can be attributed to the synergistic effect of BL and WS. The addition of wheat straw made better use of the alkali salt in black liquor, and it also brought other components like cellulose that is easier to be gasified in SCW than lignin in black liquor. Therefore, higher gasification performance was achieved in their cogasification than the separate gasification. The reaction temperature also showed a great impact on the gas product composition and gas yield (Fig. 2(b)). With more gas produced at higher temperatures, the total gas yield
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C
5
C
°
°
Fig. 2 e Effect of reaction temperature on the GE, CE, HE and COD concentration of the aqueous residue (a) and the gas yield (b) (mixing ratio: 1:1; mixture concentration: 10.62 wt%; reaction time: 30min).
increased from 28.78 to 86.24 mol/kg when it raised from 500 C to 750 C. Meanwhile, hydrogen had a most significant improvement with the temperature, which increased almost four times. The yield of CO2 was also improved, but the improvement was much lower than hydrogen. This also led to the increase of H2 fraction and the decrease of CO2 fraction. The CO yield was relatively low at all the tested temperatures, which was around 0.62 mol/kg for all the tested conditions. This result was probably related with the alkali salts in BL, including NaOH, Na2CO3 and NaHCO3. As reported [37e40], they were effective in catalyzing SCWG, which can promote the water-gas shift reaction (H2O þ CO ⟶ H2þCO2) and resulted in CO consumption. This may also be the reason for the increase of H2 and CO2 yield. As mentioned above, the low CO content of the gas product can be beneficial to its application and end use. Black liquor is considered as a great threat to the environment and human health. One of its threats is the high COD content, which can cause great pollution to the water resource if it is discharged without proper treatment. The COD concentration of the studied black liquor is about 106000 mg/L. The mixture of BL/WS may have a higher COD concentration because the same amount of wheat straw can consume more oxygen for chemical oxidation because of its lower ash content than black liquor solid. After the treatment of SCWG, the COD concentration was tremendously reduced (Fig. 2(a)). The COD concentration of the liquid residue was reduced to 4386.33 mg/l when they were gasified at 500 C, which is only 4.13% of the COD concentration of the black liquor with the same concentration. With the increasing temperature, the COD content was reduced quickly. The COD concentrations of the liquid residues were all below 672.15 mg/L when the temperature was higher than 650 C. This value was still higher than the water discharge standard on COD concentration for pulping industrial in China, which is lower than 200 mg/L [48]. However, the COD concentration can be further reduced in the realistic applications, where preheated water was used to realize the fast heating of the organic reactant to inhibit the side reactions [39]. Thus, the COD concentration can be diluted to a certain extent corresponding to the mixing ratio of preheated water and feedstocks. Then it may fulfill the requirement of the discharge standard.
The effect of reaction time Reaction time is another crucial parameter in SCWG, especially for the reactions in autoclave. This is because the mass transfer resistance was higher than the continuous reactor where the reactants can flow in the reactor. The effect of the reaction time ranged from 5 to 40 min on co-gasification of BL/ WS with the mixing ratio of 1:1 was investigated at 700 C (Fig. 3). The prolonging reaction time <30 min improved the gasification efficiency. The GE, CE and HE increased from 91.34%, 59.47% and 97.34% to 112.18%, 73.70% and 135.93% respectively with the reaction time increasing from 5 to 30min. It is notable that the total gas yield achieved at 700 C with a reaction time of 5 min was almost the same with that achieved at 600 C with a reaction time of 30 min (shown in Fig. 2). It showed that temperature was a prior parameter and this gasification extent can be reached quicker at higher temperatures. It also indicated that the reaction temperature must be considered in determining the optimal reaction time. However, the reaction time had seldom influence on the gasification when it is longer than 30min. The GE, CE and HE were almost unchanged while the time was prolonged from 30 to 40 min (Fig. 3 (a)). We proposed that 30min was long enough for the decomposition of the loaded BL and WS, so further prolonging the reaction time had little impact on the gasification. The prolongation of the reaction time in the range of 5e30 min also improved the yields of H2, CH4 and CO2 (Fig. 3(b)). The H2 yield had the largest improvement with the reaction time among all the gas products, which increased from 16.10 to 35.33 mol/kg when it was extended from 5 to 30 min. However, when the reaction time increased from 30 to 40min, the H2 yield decreased slightly while the CH4 yield increased from 7.76 to 9.71 mol/kg. This is probably because further prolonging the time above 30 min had little impact on the reactions that generate hydrogen, such as organics decomposition, steam reforming and water-gas shift reaction. However, it can still influence some hydrogen-consuming reactions, like methanations (Eqs. (5) and (6)), which generates more CH4. Therefore, the prolongation of the reaction time favored the production of CH4, but it inhibited the hydrogen production.
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Fig. 3 e Effect of reaction time on the GE, CE and HE (a) and the gas yields (b) in co-gasification of BL and WS (Mixing ratio: 1:1; Total concentration: 10.62 wt%; Reaction temperature: 700 C).
3H2þCO/CH4þH2O
(5)
The effect of mixture concentration
4H2þCO2/CH4þ2H2O
(6)
A main advantage of BL and WS co-gasification is that it can improve the feedstock concentration, the handling and energy-production capacity of a certain system. However, the variation on the feedstock concentration will have a great impact on the co-gasification of BL and WS. In the present study, we investigated the effect of the mixture concentration of BL/WS on the co-gasification. The mixing ratio was fixed at 1:1 for it is the optimal mixing ratio for their co-gasification. When the mixing ratio and the concentration of the black liquor were fixed, the highest mixture concentration can be achieved by adding a certain amount of wheat straw powder into the black liquor without dilution. For example, we can add 10.62 g wheat straw powder into 100 g black liquor (10.62 wt%) to get a mixture with a mixing ratio of 1:1. In this way, the mixture concentration is calculated to be 19.2 wt%, which is also the upper limit of the studied concentration. From Fig. 4, the gasification efficiencies, including GE and CE were decreased with the rise of the concentration. The highest CE of the mixture of 98.89% at 700 C was obtained when the concentration is 5 wt%, which is even higher than that achieved in co-gasification with a concentration of 10.62 wt% at 750 C (Fig. 2). Meanwhile, the GE and HE reached 161.81% and 231.94% respectively, which were also the highest values in this study. When the concentration was increased to 19.1 wt%, the GE and HE were decreased to 112.14% and
The results showed that 30 min is required to complete most of the reactions and reach the equilibrium state. This reaction time is much longer than that needed in SCWG of biomass with continuous systems, which only need several to dozens of seconds to complete the gasification. For example, Antal et al. [49] obtained complete gasification of 0.1 M glucose in SCW in 28 s at 34.5 MPa and 600 C with a continuous reactor. Lee et al. [50] also found that the residence time had seldom impact on the gas yields when it was longer than 10.4 s in gasification of 0.6 M glucose in SCW in a tubular continuous reactor. The difference on the required reaction time can be mainly attributed to the different mass-transfer resistance of the two different reactors. Compared with the batch reactors, the reactants, reaction intermediates and the products in continuous reactor can flow through the reactor and take away the products from the reactants, which will favor the reactions for gas production. The great gap between the required time to finish the reactions also indicated the importance of improving the mass-transfer efficiency in SCWG of BL and WS. In the future, more work needs to be done to study the optimal residence time for co-gasification of BL and WS with continuous reaction systems.
Fig. 4 e Effect of the mixture concentration of BL/WS on the GE, CE and HE (a) and the gas yield (b) during co-gasification (mixing ratio: 1:1; reaction temperature: 700 C; reaction time: 30min). Please cite this article in press as: Cao C, et al., Supercritical water gasification of black liquor with wheat straw as the supplementary energy resource, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.006
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Fig. 5 e Effect of WS particle size on the GE and CE (a) and gas fraction (b) of co-gasification of BL and WS (mixing ratio: 1:1; total concentration: 10.62 wt%; reaction time: 30min; reaction temperature: 700 C).
136.97% respectively, which is only a half of that achieved with the concentration of 5 wt%. It is notable that the HE obtained with all the studied concentration was above 100 wt%, indicating that water was involved in the reactions and contributed to the gas production. With the decrease of the concentration, there will be more water existed in the reactor and the collision frequency of the water with reactants and intermediates was improved. Thus, the reactions involved water will be enhanced and the gasification performance and H2 production were boosted. The variation of the mixture concentration also changed the gas product composition and yield. From Fig. 4(b), the total gas yield was reduced dramatically with the increasing concentration of BL/WS mixture. When the concentration increased from 5 to 19.1 wt%, the total gas yield decreased from 63.71 to 34.71 mol/kg, which was reduced by half. And 34.71 mol/kg was also the minimum total gas yield obtained in this study. The hydrogen fraction was also reduced with the increasing concentration, which decreased from 56.09% to 39.53% when the concentration was increased from 5 wt% to 19.1 wt%. And 39.53% was also the lowest hydrogen fraction that obtained in this study. Though diluting the BL/WS mixture is beneficial to the gasification from the reaction aspects, it may reduce the energy efficiency and increase the investment cost of the related energy system in realistic applications. For a given amount of BL and WS solids, the dilution of the feedstock can increase the system scale and the construction cost of the system, which may reduce the energy efficiency owing to more energy losses [32]. Therefore, determination of the optimal handling concentration should consider not only its impact on the gasification performance, but also the energy and economic analysis of the realistic application scenarios.
The effect of biomass particle diameter In this study, the wheat straw was ground and sieved to get the wheat straw particles with different sizes. In this study, three kind of wheat straw particles with different particle diameters (<74 mm, 74e107.5 mm and 107.5e150 mm) were gasified with black liquor to study the influence of the particle size of wheat straw. The results showed that the gasification was
favored with the smaller wheat straw particles size, and the GE and CE were improved when the particle size decreased (Fig. 5(a)). The GE improved from 88.76% to 102.3% and the CE improved from 81.39% to 92.58% when the particle diameter decreased from >107.5 mm to <74 mm. We supposed that the decrease of the particle diameter can improve the specific surface area of the particle and improve the contact of the wheat straw particle with other reactants and intermediates. Therefore, the reactions of the particle involved with SCW and the reactive reactant, such as hydrolysis and steam reforming were accelerated, so the gasification was improved with smaller particle size. On the other hand, the impact of the WS particle size on the gas product components was very slight (Fig. 5(b)). The main gas products were H2, CH4, CO2 and C2H6, which were in the range of 46.41%~50.28%, 11.05%~13.17%, 34.84%~39.50% and 1.63%~2.07% respectively. The change of the wheat straw particle size not only influence the gasification performance of BL/WS mixture, but also affect the economic efficiency of the process in practical application. The decrease of the wheat straw particle size is favorable to the co-gasification, but it will increase the energy consumption of the grinding process. As a result, an appropriate particle size should be selected by balancing its influences on the gasification efficiency, energy consumption and the operating cost.
Conclusion In this study, we proposed a new treatment method of black liquor, where it was co-gasified with wheat straw to improve the energy supply for the pulping process and make better use of the alkalis in black liquor. Their co-gasification performance in supercritical water was studied experimentally with an autoclave. During co-gasification, the synergistic effect of black liquor and wheat straw was found. It was mainly because the present of black liquor offered the alkali salt as the catalyst for wheat straw, and the present of wheat straw made better use of the alkali catalyst. The optimal mixing ratio of BL/WS for their co-gasification was found to be 1:1, where the highest improvement of the gasification efficiency by the synergistic effect was obtained. The increase in the
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reaction temperature also improved the gasification efficiency, where the highest CE of 97.87% for BL/WS mixture was achieved at 750 C. The COD concentration of the aqueous residues was reduced to below 672.15 mg/L when the temperature was above 650 C. The prolongation of the reaction time from 5 to 30min improved the gasification efficiency and the hydrogen production, but it had limited influence on the gasification when it was longer than 30min. The dilution of the BL/WS mixture also enhanced the gasification and improved the hydrogen production. The highest HE and CE of 231.94% and 98.89% respectively was achieved when it was diluted to 5 wt%. Reducing wheat straw particle sizes favored the gasification by improving the gasification efficiency, but it will also increase the energy consumption of wheat straw grinding. To realize the practical applications of cogasification of BL and WS, the investigation based on continuous reaction system should be conducted in the future. Meanwhile, the economic analysis based on life cycle assessment and system analysis may also be helpful to optimize the co-gasification process and promote the applications.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (No. 51606150), the China National Key Research and Development Plan Project (No. 2016YFB0600100) and Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JQ5172 and 2017ZDXM-GY-067) for funding this study.
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