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Supercritical water gasification of sewage sludge and combined cycle for H2 and power production – A thermodynamic study
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Supercritical water gasification of sewage sludge and combined cycle for H2 and power production e A thermodynamic study Dwi Hantoko a, Mi Yan a,*, Ekkachai Kanchanatip a,b, Muflih A. Adnan c, Antoni a, Ishrat Mubeen a, Fauziah Shahul Hamid d a
Institute of Energy and Power Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, PR China b Center of Excellence in Environmental Catalysis and Adsorption, Faculty of Engineering Thammasat University, Pathumthani 12120, Thailand c Department of Chemical Engineering, Islamic University of Indonesia, Yogyakarta 55584, Indonesia d Institute of Biological Sciences, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia
highlights Co-gasification
of
graphical abstract sludge
and
lignite coal for H2 production and power generation. Integrated system consisted of SCWG and gas/steam turbine with heat recovery. Higher temperature (700
C) and
lower feed concentration (10 wt%) favored H2 yield. Lignite
coal
addition
reduced
minimum feed input to realize auto-thermal condition. 9.06 kg of H2 and 48.37 kW electricity were obtained from 100 kg sludge at 700 C.
article info
abstract
Article history:
An integrated system of supercritical water gasification (SCWG) and combined cycle has
Received 26 May 2019
been developed for H2 production and power generation. Sewage sludge and lignite coal
Received in revised form
were selected as raw material in this simulation. The effects of feed concentration (10
20 July 2019
e30 wt%) and lignite coal addition (0e50 wt%) on syngas yield and H2 yield were also
Accepted 25 July 2019
investigated in the temperature range of 500 Ce700 C. Several heat exchangers were
Available online 22 August 2019
considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed
* Corresponding author. E-mail addresses: [email protected] (D. Hantoko), [email protected] (M. Yan). https://doi.org/10.1016/j.ijhydene.2019.07.210 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Keywords: Sewage sludge Supercritical water Gasification Hydrogen Combined cycle Thermodynamic
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concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve selfsufficient energy at 500 C, 600 C, and 700 C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H2 yield were obtained at 700 C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H2 were obtained after H2 separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H2 production. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, sewage sludge with high organic matter derived from urban or industrial wastewater treatment plant has been considered as a potential energy source to meet the demand of the energy, as well as, to solve the waste treatment problems. It is estimated that the rate of sewage sludge production is around 0.1e30.8 kg per population equivalent per year [1]. In China, the production of sewage sludge exceeded 30 million tons in 2015 and it is expected to reach 60e90 million tons in 2020 due to the rapid development of urbanization and industrialization [2]. It can be converted to energy through biochemical and thermochemical process. Unfortunately, biochemical process e.g. fermentation, takes longer time and has lower conversion rate than its counterpart of thermochemical process [3]. For conventional thermal treatments, such as incineration, pyrolysis and gasification, a pre-drying procedure is unavoidably required, which leads to the most of energy invested and released during these thermal treatments as the energy is being consumed to remove the moisture retained in sewage sludge. Therefore, exploring innovative and sustainable technologies for sewage sludge treatment are urgently required. The considerations of choosing suitable technology for energy recovery from sewage sludge involves technical and economic feasibility, environmental sustainability, marketing facts and public acceptance. Compared with the conventional thermochemical process (e.g. combustion, pyrolysis and gasification), supercritical water gasification (SCWG) seems to be a more suitable technique to convert sewage sludge with high moisture content (80e90 wt%) into syngas with high content of hydrogen (H2) without the need for drying pre-treatment. As a result, the energy and cost required for drying process can be omitted. In addition, SCWG process requires shorter reaction time compared with anaerobic or biological treatments. Owing to the unique properties of water in supercritical condition (374.3 C and 22.1 MPa), such as high diffusivity, low viscosity, low density, low dielectric constant, the organic matters in sewage sludge, organic wastes and coal can be dissolved and converted into H2-rich syngas [4,5]. Besides, supercritical water provides excellent reaction environment for the gasification of organic constituents [6]. It has been reported that organic matters in sewage sludge were almost completely
hydrolyzed and decomposed into gaseous product at 425 C [7]. SCWG was also assigned to give significant environmental benefits on the treatment of municipal and animal wastes, e.g., odor elimination, BOD removal, and pathogen kill [8]. Therefore, the utilization of SCWG process can replace the ineffective and uneconomical conventional techniques in dealing with biowastes, containing high amount of moisture, especially sewage sludge. Additionally, low-rank coals (lignite and sub-bituminous coal) are regarded as an energy resources due to its abundant availability compared to other fossil fuels. Low-rank coals contained high volatiles which are rich in aliphatic -CO- and -C-C- structures [9]. Therefore, they present high reactivity during thermo-chemical conversions. In some countries including China and Indonesia, low-rank coals (lignite and sub-bituminous coal) are being progressively utilized due to the increase in energy demand and high price of high-rank coals [10]. For combustion in pulverized coal power plant, low-rank coal needs additional pre-treatment to deal with the high moisture content (25e65 wt%) [9]. These pretreatment diminished the overall energy efficiency of the power plant. In addition, coal-fired power plant may generate much pollutant such as; SO2, NOx and fine particles, which can cause environmental problems [11]. In this regard, SCWG is gaining more attention as an alternative and efficient technique for utilization of low-rank coal. Consequently, some research on SCWG of coal have been conducted to explore the potential of energy recovery [12e15]. It has been reported that high carbon content in coal was beneficial in increasing gaseous product, improving gasification efficiency which will reduce energy loss [16]. However, due to the unique property and structure of coal, higher temperature is required to achieve complete gasification. It is worth noting that high SCWG temperature may not only increase the reactor construction cost but also bring the challenges to the safety issue. Some researchers investigated co-gasification of coal and biomass and they found a synergetic effect on hydrogen production and gasification efficiency at lower temperature than coal itself [12,17]. In this study, the addition of lignite coal is expected to promote syngas production from SCWG of sewage sludge. Only few studies placed concerns on the process design of SCWG, particularly those associated with the energy
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self-sufficiency. Ortiz et al. [18] had proposed the first design of supercritical water reforming of glycerol for power production by expanding the high-pressure syngas. The process design was equipped with several heat exchangers, as well as, oxygen injection into reformer to achieve autothermal conditions. Fiori et al. [19] studied the SCWG of biomass for H2 production. The energy required to sustain the SCWG reactor was supplied by burning the remaining gases, after H2 membrane separator. The addition of auxiliary methane is needed if the system cannot achieve self-sustained condition. Likewise, some fraction of the produced H2 from the separator unit can be burnt in the furnace to realize the energetic selfsustainability, as reported by Ortiz et al. [20]. As a consequence, the yield of H2 will be reduced because of the internal consumption. Moreover, the use of auxiliary methane in SCWG process required high operating cost. Process integration and optimization in conventional gasification of empty fruit bunch [21], rice husk [22], and black liquor [23] for syngas production have been reported. The potential use of liquid metal oxides as an oxygen carrier for chemical looping gasification was also thermodynamically and experimentally studied to produce high value of syngas [24,25]. Additionally, the design configuration of fuel/electricity production pathways with real-biomass [26], glycerol [27], black liquor [28,29], sugarcane [30], and coal-water slurry [31] via SCWG have also been investigated. Concerning energy efficiency, integrated gasification with combined cycle (IGCC) system has been taken into account to have higher energy efficiency. High power generation efficiency (approximately 60%) can be achieved from macroalgae by employing IGCC system [32]. In summary, the integrated system of gasification and power generation is more efficient than standalone gasification for syngas production. Considering the points mentioned above, an integration process of SCWG and combined cycle-based power generation was proposed. Although, the drying process can be ignored by SCWG, the system still requires large energy input to attain energy self-sufficiency. The feedstock, sewage sludge and low-rank coals (e.g. lignite coal) have been used in this study due to their high moisture content. To the best of the author0 s knowledge, there has been no study on integrating SCWG and combined cycle system for H2 production and power generation, especially for sewage sludge application. Moreover, the concept of the proposed design also attempted to recover the energy or heat emitted throughout the whole process by employing several heat exchanger networks. Hence, the effect of feed concentration and lignite coal addition on the H2 yield and net produced power were investigated. The proposed integrated system is expected to be an alternative for H2 production and power generation from sewage sludge.
Proposed integrated system of SCWG and combined cycle Basic concept of the of the integrated system A conceptual design of the proposed integrated system for hydrogen production and power generation is presented in Fig. 1. The integrated system mainly consists of SCWG and
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combined cycle. The solid line and dashed line represent the material and energy flows, respectively. The process scheme has been designed to be as simple as possible to be applicable to the actual demo-scale or industrial-scale plant. Nevertheless, the requirement for the energetic self-sustainability was also considered. The possibility of the self-sufficient energy for the small-scale system can be used as a concept for future process development. The lignite coal was added into the feed stream in order to fulfill the energy requirement in the SCWG process. In the SCWG reactor, sewage sludge and lignite coal were converted to syngas which mainly contains H2, CO, CO2 and CH4. The H2-rich syngas was directly flowed to the hydrogen separator without hydrogen enrichment. Meanwhile, the remaining gases from hydrogen separator was fed to the combustor in combined cycle module to generate electricity. The combination of SCWG and combined cycle system could improve the total energy efficiency. Some parts of the heat from combined cycle were used to recover the energy in the SCWG reactor through heat exchanger network system.
Description of the integrated system The process modeling and calculation related with the mass and energy balances of the proposed system was developed using Aspen Plus V8.8 (Aspen Technology, Inc). The detailed schematic process flow diagram of the proposed integrated system is illustrated in Fig. 2. The detailed properties and status parameter of the streams shown in the flow diagram can be found in the Supplementary File. In this process, sewage sludge and lignite coal were adopted as a feedstock for SCWG. The feedstock and water were mixed (MX1), then pumped (P1) to the supercritical pressure. The mixture was then preheated in preheater (HE1 and HE2) before it flowed into the SCWG reactor. In SCWG reactor, which consisted of two blocks (RYIELD and RGIBBS), the feed was converted to syngas. The RYIELD block with FORTRAN subroutine (defined using a calculator block) was used to decompose the feedstock (non-conventional component) into C, H2, O2, N2, and S elements (mixed component) based on the ultimate and proximate properties. In order to simulate the chemical equilibrium at a specific temperature and pressure, the RGIBBS block was employed based on Gibbs free energy minimization. The produced syngas mainly contained H2, CO, CO2, and CH4. The main chemical reactions typically occur in the SCWG process are summarized in Table 1 [33e35]. The sensible heat of syngas from the SCWG reactor was utilized to preheat the mixture which was done by a heat exchanger (HE1). In this process, the main heat source for SCWG process was supplied by the exhausted hot gas from gas turbine through heat exchanger (HE2). Subsequently, the resulting stream from SCWG reactor was cooled down to 60 C in order to separate the water (WTSEP). In this unit, about 85%e95% of water was recovered. The recovered water can be recirculated into the SCWG reactor as a reactant. The recirculation of water was not considered in this simulation in order to simplify the model. After the water separator, the produced syngas flowed to turbo expandergenerator (TURB-EXP) to expand the high-pressure syngas, which was modeled by the turbo-expander generator of
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Fig. 1 e Conceptual design of the proposed integrated system for H2 production and power generation.
Fig. 2 e Simplified schematic flow diagram of supercritical water gasification for H2 production and power generation.
Table 1 e Summary of the chemical reactions in supercritical water gasification [33e35]. Reaction type
Stoichiometric reaction equation
Simplified overall reaction of SCWG CHxOy þ (2-y)H2O / (2y þ x/2)H2 þ CO2 Water gas shift reaction CO þ H2O 4 CO2 þ H2 Boudouard reaction C þ CO2 4 2CO Steam methane reforming CH4 þ H2O 4 CO þ 3H2 Dry reforming CH4 þ CO2 4 2CO þ 2H2 Methane formation C þ 2H2 4 CH4 Methanation of CO2 CO2 þ 4H2 4 CH4 þ 2H2O Methanation of CO CO þ 3H2 4 CH4 þ H2O CO2 þ 2H2 4 C þ2H2O Hydrogenation of CO2 Hydrogenation of CO CO þ H2 4 C þ H2O
General Electric (GE), Oil and Gas Company as a technical reference. The extracted energy from this unit can be used to drive the generator for producing electricity. Turbo expandergenerator is commonly used in industrial applications to recover energy from the fluid process, i.e. natural gas processing, integrated gasification combined cycle and petrochemicals industries [18]. The resulting stream was sent to the H2 separator (H2-SEP) without being shifted for hydrogen enrichment. A shift reaction can converts CO to hydrogen, however, additional heat is required in the shift reactor for the endothermic reaction [36]. The separation of H2 was achieved by means of the Hysep Technology model (HYSEP Modul Type 1308, ECN, palladium membrane filter). High purity of H2 can
DH298K (MJ/kmol) DG298K (MJ/kmol) Reaction number Endothermic 42 þ172 þ206 þ247 74 165 206 90 131
29 120 142 171 51 114 142 63 91
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10
be obtained. Hence, the feed stream should be heated using flue gas from steam turbine through heat exchanger (HE3). The remaining gases mainly contained CH4, CO, and CO2 (without H2) were transferred to gas turbine packages. The amount of air used for syngas combustion was adjusted according to the flow rate of gas entering the combustor by setting the design specification in a Design Spec block. Finally, the gas turbine drove the generator for power generation. The high temperature flue gas was used to supply the heat needed in SCWG reactor. The rest of its heat was utilized to generate the steam in the heat recovery steam generator (HRSG). The flow rate of steam was accordingly controlled to drive the steam turbine (ST). At last, the flue gas was passed into the
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CO2 absorption unit (ABS-CO2) before being released to the atmosphere. About 90% of CO2 was assumed to be absorbed in this unit [37]. The absorbed CO2 can be directed to the series of downstream facilities such as cooler and compressor to achieve the desired conditions.
Process modeling and analysis General conditions The Gibbs free energy minimization is a well-known method to evaluate the performance of gasification process. Due to the complexity of SCWG reaction mechanisms, this approach is more preferable to predict the possible components of the product without the knowledge of reactions involved. A mixture of sewage sludge and lignite coal at different ratios were considered as a feedstock. The proximate and ultimate analysis based on the work of Chen et al. [7] and Atimtay et al. [38] with modifications are listed in Table 2. The heating value of the feedstocks were calculated based on the proximate and ultimate analysis [39]. A flow rate of 100 kg/h feedstock was used in all simulations. The stream class used in Aspen Plus was MIXCINC and the property method was Peng-Robinson with Boston-Mathias modification (PR-BM). The previous study mentioned that PR-BM equation of state-provided highly accurate SCWG simulation with Pearson0 s correlation coefficient (R2) and root mean squared error (RMSE) of 0.99 and 0.25, respectively [40]. In this study, the assumptions were made as follows: (i) SCWG reactor consisted of RYield and RGibbs, (ii) ambient temperature was 25 C, (iii) all of the heat exchangers were countercurrent, (iv) minimum temperature approach in all heat exchangers was 10 C, (v) heat loss was negligible, (vi) the air used contained 79 mol% nitrogen and 21 mol% oxygen and (vii) there was no air contaminant inside the SCWG reactor. To simplify the simulation, the effect of pressure drop and mass transfer were not considered. The assumed conditions for SCWG and combined cycle modules are summarized in Table 3.
Table 2 e Proximate and ultimate analysis of sewage sludge and lignite coal. Properties
Sewage sludge [7]
Proximate analysis (wt.% db)a Fixed carbon Ash Volatile matter Ultimate analysis (wt.% daf)b Carbon Hydrogen Nitrogen Sulfur Oxygenc Lower heating value (MJ kg1) a b c
Lignite coal [38]
9.41 28.96 61.63
44.65 25.62 29.73
53.74 4.79 6.57 1.48 33.42 19.91
74.72 5.81 2.65 2.28 14.54 26.61
On a dry basis (db). On a dry and ash-free basis (daf). By difference (O% ¼ 100% ash% C% H% N% S%).
Table 3 e Assumed conditions for SCWG and combined cycle modules. Properties Supercritical water gasification Temperature (oC) Pressure (MPa) Feed concentration (wt%) Coal to sewage sludge ratio (wt%) Pump efficiency (%) Heat exchanger min. temp. approach (oC) Combustor and gas turbine Compressor isentropic efficiency (%) Gas turbine inlet temperature (oC) Gas turbine isentropic efficiency (%) Discharge pressure (MPa) HRSG and steam turbine HRSG outlet pressure (MPa) Steam turbines isentropic efficiency (%) Discharge pressure (MPa)
Value
Ref./Note
500, 600, 700 25 10e30 10e50 80 10
This study This study This study This study [20,41] [42]
90 1500 90 0.15
[23] [42] [42] [23]
12 90
[43]
0.15
[23]
Model validation The system validity was partially evaluated for the SCWG reactor unit by comparing the results of the simulation with those of the experimental results reported by Byrd et al. [44]. The SCWG reactor is a considerably complex unit operation compared to other unit operations that are typically controlled by irreversible reaction (combustion in combined cycle) and physical parameters (heat exchanger, compression, and expansion and separator). The model validation was performed by comparing the simulated syngas yield (Sim) and the experimental data results (Exp), as shown in Table 4. The Pearson0 s correlation factor (R2) and the root mean squared error (RMSE) were applied to evaluate the difference between the simulation and experiment data results. It can be observed that the results of our simulation were in good agreement with the experimental results. The equilibrium yield of H2 (R2 ¼ 0.98) and CH4 (R2 ¼ 0.97) well-fitted with the experimental results. While the equilibrium yield of CO (R2 ¼ 0.79) and CO2 (R2 ¼ 0.82) did not fit very well as the correlation coefficient was lower, however, they were in acceptable level. In addition, the relative error of the predicted H2 composition in the present simulation was lower than 5%. The difference between the predicted gas yields and experimental gas yields can be explained by the use of short residence time (1 s) in the experimental condition, as reported by Byrd et al. [44]. It is worth mentioning that the predicted composition of syngas from the present study was also close to the syngas composition as reported by other studies [45e47].
Performance evaluation The performance of the integrated SCWG process was determined in terms of hydrogen yield, total syngas yield, hydrogen production efficiency, power generation efficiency and total efficiency. The hydrogen yield and the total syngas yield were calculated using Eq. (1) and Eq. (2), respectively. In this case, m_ H2 , m_ total syngas , and m_ feed are the mass flow rate of hydrogen, mass flow rate of syngas, and the mass flow rate of the feed,
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Table 4 e Simulated syngas yield (Sim) vs the experimental data results (Exp) for SCWG of glycerol by Byrd et al. [44] at various temperatures and feed concentrations. Syngas yield (mol/molglycerol)a
Parameter conditions H2 Temp. (oC)
CO
CO2
Exp
Sim
Exp
Sim
Exp
Sim
Exp
Sim
5 5 5 15 20 30 35 40
5.12 5.81 6.54 4.13 3.94 2.87 2.60 2.18
6.08 6.52 6.69 4.61 3.79 2.72 2.24 2.07 0.48 0.98
0.49 0.29 0.29 0.72 0.81 0.92 0.95 0.94
0.20 0.08 0.03 0.51 0.71 0.97 1.09 1.14 0.20 0.97
0.02 0.01 0.09 0.04 0.17 0.21 0.24 0.24
0.12 0.16 0.20 0.36 0.37 0.38 0.38 0.38 0.18 0.79
2.34 2.50 2.34 2.21 2.42 2.07 1.93 1.79
2.68 2.76 2.77 2.13 1.92 1.65 1.52 1.48 0.37 0.82
700 750 800 800 800 800 800 800 RMSE R2 a
CH4
Feed (wt.%)
Sim ¼ simulated syngas yield; Exp ¼ experimental data results [44].
respectively. The feed concentration was defined as the ratio of the amount of feed (mixture of sewage sludge and lignite coal) (kg) to the total amount of feed and water (kg), according to our previous work [34]. P P P N exp sim exp sim R2 ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h P i h P iffi P P 2 n exp2 ð expÞ2 N sim2 ð simÞ
Syngas and hydrogen production
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 ðexp simÞ RMSE ¼ N "
# _ H2 kg m Hydrogen yield ¼ _ feed kgfeed m
(1)
# _ total syngas m kg ¼ Syngas yield _ feed kgfeed m
(2)
"
The total energy efficiency (h total ) of the proposed integrated system was calculated as the sum of the hydrogen production efficiency (h H2 ) and power generation efficiency (h power ). These values can be written as follows: h total ¼ h H2 þ h power h H2 ¼
_ H2 LHVH2 m _ feed LHVfeed m
hpower ¼
Wout Win _ feed LHVfeed m
lignite coal in the feed from 10 wt% to 50 wt%. A constant pressure was maintained at 25 MPa, since pressure has insignificant effect on the equilibrium results of SCWG [40,48]. The temperature of the SCWG was varied from 500 C to 700 C.
Fig. 3 shows the yield of syngas versus feed concentration at different SCWG temperature. It can be seen that yield of syngas slightly decreased as the feed concentration increased. Likewise, the yield of hydrogen decreased significantly, as can be seen in Fig. 4. The presence of water plays an important role to complete the SCWG reactions. The low feed concentration indicated that more water is available during the SCWG process, while high feed concentration implied that less water is existing during the reactions. The lack of water inhibited the steam methane reforming reaction (R4) and water-gas shift reaction (R2). The steam reforming reaction promotes the formation of H2, which opposed to the methanation reactions (R6 e R8). This finding was in good agreement with the previous reported works on the SCWG of sewage sludge in batch and fluidized bed reactor [7,49]. Among the other chemical
(3)
(4)
(5)
where LHVH2 and LHVfeed are the lower heating value of hydrogen and lower heating value of feed, respectively. Wout is the total generated power from expander, gas turbine and steam turbine. Win is the internal power consumption during the processes, including pumping and compression.
Results and discussion In this study, the model was investigated with the feed concentration in a range of 10 wt% to 30 wt% with the ratio of
Fig. 3 e Effect of feed concentration (only sewage sludge) on the syngas yield at different SCWG temperatures.
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reactions in SCWG, steam reforming, water-gas shift, and methanation reaction are considered to dominate the change in syngas composition. In addition, the reaction temperature strongly affected the yield of syngas and the yield of hydrogen, as presented in Figs. 3 and 4, respectively. With increasing temperature from 500 C to 700 C, the syngas and hydrogen yield significantly increased from 147.79 kg/100kgfeed and 1.99 kg/100kgfeed to 178.08 kg/100kgfeed and 9.06 kg/100kgfeed, respectively. Carbon conversion was in the range of 98e99%, which was higher than that obtained from the previous study [34]. This result revealed that higher temperature favored the endothermic reactions where SCWG reactions take place. This was further confirmed by Withag et al. [48] showing that the overall gasification reactions were endothermic reactions. However, the higher temperature in the SCWG reactor would lead to more energy requirement or more energy sources to sustain the process. The discussion on the limit of feed concentration at different SCWG temperature for sustaining the process is given in the next section. Fig. 5 shows the effect of the addition of lignite coal on syngas yield at 700 C. The addition of lignite coal had a positive effect on the enhancement of syngas production. For instance, at feed concentration of 10 wt% and temperature of 700 C, the syngas yield increased from 178.08 kg/100kgfeed to
200.81 kg/100kgfeed when the ratio of the addition of lignite coal increased from 10 wt% to 50 wt%. This can be explained by the fact that the addition of lignite coal increased the carbon content in the feed, as well as, the heating value of the coal and sludge mixture. At the same time, thermodynamic analysis on SCWG of coal and black liquor showed that the addition of coal could improve the production of hydrogen in SCWG of black liquor at equilibrium condition [13]. Since coal contained more carbon and less oxygen than black liquor, more water was involved in SCWG of coal to give the required oxygen to produce CO and CO2 than in SCWG of black liquor. As a consequence, more H2 would be released from water in SCWG of coal than black liquor. A similar results on the H2 production was also found in this study because of the addition of lignite coal into SCWG of sewage sludge. Cao et al. [12] found a similar trend in their experimental work on cogasification of alkaline black liquor and coal in supercritical water. A strong synergetic effect was observed when the blending ratio of coal and black liquor was about 50:50. They observed that the presence of black liquor enhanced the performance of coal gasification due to the presence of alkali content in black liquor which acted as catalyst, while, the presence of coal facilitated the full use of the alkali catalyst to improve SCWG of black liquor. The addition of lignite coal also had a considerable influence on the syngas composition, as shown in Fig. 6. It can be observed that the H2 concentration decreased with the increase of coal addition. It was due to the fact that adding more lignite coal resulted in a higher O2 concentration in the feed of SCWG process. Consequently, the presence of higher amount of O2 converted H2 into H2O. A similar trend was reported by Adnan et al. [41], which showed lower H2 concentration in the co-gasification of Indonesian coal and microalgae. A previous study also reported that the yield of H2 could also be influenced by the C:H ratio and oxygen content in the fuel [40]. The maximum H2 yield could be obtained when the feedstock had a low C:H ratio and low oxygen content. If the ratio of C:H was between 6 and 10, then H2 yield turned out to be less dependent on both the C:H ratio and oxygen content. However, H2 yield became more dependent on the oxygen content when the C:H ratio was greater than 10. In addition, the yield of H2 was dependent on both the C:H ratio and the oxygen content, if C:H ratio was lower than 6. By varying the coal addition from 0 wt%
Fig. 5 e Effect of feed concentration on the syngas yield at different lignite coal addition (at 700 C).
Fig. 6 e Effect of feed concentration on the syngas composition at various lignite coal addition at 700 C.
Fig. 4 e Effect of feed concentration on the hydrogen yield at different SCWG temperatures.
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to 50 wt%, the C:H ratio increased from 11.23 to 12.17, while the oxygen content decreased from 33.42% to 23.46%, respectively.
Limit of feed concentration for autothermal condition The analysis of the energy requirement played an important role in the process modeling. The conditions to reach the selfsustained process with no auxiliary fuel was investigated for the temperature range of SCWG mentioned above. To achieve this target, a sensitivity analysis was carried out by changing the feed concentration and lignite coal addition from 10 wt% to 30 wt% and 10 wt% to 50 wt%, respectively, for each SCWG temperature. By this approach, the conditions from the energy
Fig. 7 e Effect of feed concentration on the energy flow to achieve autothermal condition at different SCWG temperatures: (a) 500 C, (b) 600 C, (c) 700 C.
deficit in the SCWG reactor to energy excess were obtained, corresponding to the cases with lower and higher feed concentration. In addition, a number of heat exchangers were properly installed in the process to minimize the energy loss and to achieve the optimum heat integration network system. The minimum temperature approach and countercurrent flow were used in the heat exchanger system. Fig. 7 shows the flow of energy that represented energy demand or energy production by the SCWG reactor. The heat flow in SCWG reactor corresponds to the heat consumed or generated by SCWG reactor which is determined by the operating temperature of the reactor and the concentration of the feed. The negative values implied that the reactor needs to be sustained by an external source of thermal energy. On the other hand, the positive value indicated that the production of thermal energy was obtained and the process was selfsustainable. According to our calculation, an energy deficit was observed when the feed concentration (only sewage sludge) was lower than 14.25 wt% at 500 C, 18.75 wt% at 600 C, and 25.50 wt% at 700 C and the heat surplus was obtained if the feed concentration was higher than those values
Fig. 8 e Effect of feed concentration (only sewage sludge) on H2 yield and net produced power at different SCWG temperatures: (a) 500 C, (b) 600 C, (c) 700 C.
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Table 5 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (500 C, 25 MPa). Component
Produced works (kW) Expander Gas turbine Steam turbine Comsumed works (kW) Pumps Compressor Total net power (kW)
Feed concentration (wt%) 10
15
20
25
30
7.30 94.50 10.18
6.69 97.86 12.57
6.37 99.63 17.84
6.17 100.73 23.45
6.04 101.48 28.57
11.82 38.02 62.13
7.66 39.64 69.82
5.72 40.50 77.62
4.65 41.03 84.68
3.98 41.39 90.72
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sewage sludge, this value was lower than the value calculated in the present study (approximately 25 wt% of feed concentration at 700 C). It might be explained by the use of the higher temperature of flue gas (directly from combustor) (ca. 1700 C) to preheat the feed before entering to the SCWG reactor, which helps the process to be rapidly sustained. In this simulation, the flue gas from the gas turbine had lower temperature than that used by Fiori et al. [19]. Ortiz et al. [50] also reported that their supercritical water reforming of glycerol at 800 C and 24 MPa could achieve self-sufficient process with 21.78 wt% feed concentration. In their study, the highpressure syngas was expanded to produce power, while some part of produced hydrogen in the furnace was proposed to compensate the deficit energy in the reformer [20]. It is
Table 6 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (600 C, 25 MPa). Component
Produced works (kW) Expander Gas turbine Steam turbine Comsumed works (kW) Pumps Compressor Total net power (kW)
Feed concentration (wt%) 10
15
20
25
30
9.88 81.21 8.82
8.50 88.83 11.29
7.73 92.96 16.50
7.24 95.57 22.20
6.89 97.38 27.48
11.77 31.49 56.65
7.61 35.22 65.78
5.67 37.24 74.27
4.60 38.52 81.88
3.94 39.41 88.40
(see Fig. 7(aec). By comparing Figs. 4 and 7, it is worth noting that higher temperature resulted in higher H2 production, but higher thermal energy was required, consequently, higher feed concentration was needed to sustain the process. For instance, the highest H2 yield was obtained at the temperature of 700 C, the process could not thermally self-sustain with a feed concentration below 25 wt%. It can also be seen that the addition of lignite coal had slightly effect on the thermal energy requirement, and no significant effect was found at 700 C. Fiori et al. [19] calculated a biomass concentration in the feed (22.9 wt% for sewage sludge, 20.5 wt% for glycerol, 18.3 wt % for microalgae, 16.6 wt% for grape marc, and 11.4 wt% for phenol) to achieve autothermal condition in their SCWG process, at 700 C and 300 bar. The main heat sources for SCWG reactor was supplied by the heat from combustion of remaining gases (after hydrogen separation). For the case of
Table 7 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (700 C, 25 MPa). Component
Produced works (MW) Expander Gas turbine Steam turbine Comsumed works (MW) Pumps Compressor Total net power (MW)
Feed concentration (wt%) 10
15
20
25
30
13.43 61.94 6.78
11.18 74.65 8.97
9.84 82.07 13.62
8.95 86.93 19.37
8.30 90.37 24.91
11.70 22.09 48.37
7.53 28.34 58.92
5.57 32.00 67.96
4.50 34.39 76.35
3.85 36.09 83.64
Fig. 9 e Effect of feed concentration on the H2 efficiency, power efficiency, and total efficiency at different SCWG temperatures.
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Fig. 10 e Typical of mass and energy diagram of the integrated system of SCWG and combined cycle power generation.
worth mentioning that operating with low feed concentration (15e25 wt%) is advantageous for both energy self-sufficiency and H2 production.
Energy analysis Fig. 8(aec) shows the effect of feed concentration on H2 yield and net produced power at different SCWG temperature. Although the yield of hydrogen decreased with the increase of feed concentration, more power was produced due to the higher fraction of CH4 in syngas (after H2 separation) was sent to the combined cycle module. For instance, the H2 yield increased from 0.59 kg/100kgfeed to 1.99 kg/100kgfeed, whereas, the net produced power decreased from 90.72 kW/100kgfeed to 62.13 kW/100kgfeed, when the feed concentration (only sewage sludge) increased from 10 wt% to 30 wt%, respectively at 500 C. The increase in temperature then caused an increase in H2 yield, thus resulting in the decrease in the net produced power, as shown in Fig. 8(aec). The detailed simulation results of internal power consumption and power generation at different temperature is presented in Tables 5e7. The internal power consumptions consisted of pumps and compressor work, while the power production was obtained from the expander, steam turbine and gas turbine unit. The pump duty decreased, i.e., approximately from 11 kW/100kgfeed to 4 kW/100kgfeed, with increasing feed concentration. It can be explained by the big amount of water at lower feed concentration which required more energy in the pumping process. The power consumption in this process was mainly dominated by the duty of compressor which was more than 70% of the total power consumption (see Table 6). The duty of compressor was significantly influenced by the feed concentration and SCWG temperature. The higher feed concentration and lower temperature produced more CH4 in the remaining gas (after H2 separator), resulting in the higher compressor duty to supply the air required for complete combustion. Fig. 9(aec) shows the efficiency of the proposed system at different feed concentration (only sewage sludge) and SCWG temperature. Higher temperature and lower feed concentration resulted in higher total efficiency. The decline in total efficiency was mainly caused by the decrease in H2 efficiency as the production of H2 significantly decreased with increasing feed concentration (Fig. 9(a)). On the other hand,
the power efficiency increased when the feed concentration increased which was in line with the power generation, as presented in Fig. 9(b). The total efficiency of 23.31% consisted of H2 efficiency of 12.07% and power efficiency of 11.23% was obtained at 500 C and 10 wt% of feed concentration. Increasing SCWG temperature from 500 C to 700 C increased the total H2 efficiency from 12.07% to 54.73%, which caused an increase in total efficiency from 23.32% to 63.48%. An effect of the increase in feed concentration was apparently observed on the reduction of total efficiency from 63.48% to 32.84% at 700 C. This trend was mainly attributed to the significant decrease in H2 production from 9.06 kg/100kgfeed to 2.93 kg/ 100kgfeed. Given the above results, the typical mass and energy diagram of the integrated system can be presented in Fig. 10. To keep the running of the system in autothermal condition, minimum feed concentration need to be maintained. In some case, an auxiliary fuel i.e., natural gas can be an alternative to achieve autothermal condition. Heat exchanger network may be optimized to recover the thermal energy and to minimize the energy loses. The produced H2 can be fed to Proton Exchange Membrane (PEM) fuel cells to produce electricity. The electricity produced in PEM fuels cells and combined cycle module is party used for internal processes, then the net electricity can be utilized via connection to a power grid.
Conclusion An integrated system consisting of SCWG and combined cycle based power generation has been proposed to utilize sewage sludge effectively for the production of H2 and power generation. The modeling and calculations of the proposed system was done by Aspen Plus V8.8 simulator. Sensitivity analysis has been carried out for different SCWG temperature (500e700 C), feed concentration (10e30 wt%), and lignite coal addition (10e50 wt%). The system can be energetically sustained without lignite coal addition by controlling the minimum feed concentration within the range of 15 wt% to 26 wt% at an operating temperature range of 500 Ce700 C. The utilization of lignite coal enhanced the syngas production and facilitated the system to achieve self-sufficient energy, but did not have effect on the H2 production. When the temperature of SCWG increased from 500 C to 700 C, the yield of syngas and H2 increased from 147.79 kg/100kgfeed and 1.99 kg/
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100kgfeed to 178.08 kg/100kgfeed and 9.06 kg/100kgfeed, respectively. The total efficiency of the integrated system was 63.48% at 700 C and 10 wt% feed concentration. The higher SCWG temperature increased the production of H2 and enhanced the total efficiency, however, an external energy supply was needed to keep the system in autothermal condition. On the other hand, the net produced power and power efficiency increased with increasing feed concentration. The proposed system is expected to be an alternative for energy recovery from sewage sludge.
Acknowledgments The authors acknowledged the financial support provided by the State International Cooperation Project, China (Grant No. 2016YFE0202000 and 2017YFE0107600) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LY17E060005). The authors would like to thank Prof. Agamuthu Pariatamby (University of Malaya, Malaysia) for his guidance during the revision of the paper. The authors also thank Mr. Petric Marc Ruya, for his kind assistance during the simulation.
Nomenclature Symbols SCWG daf db GT ST HE HRSG IGCC LHV PR-BM RMSE Exp Sim
Supercritical water gasification dry ash free dry basis gas turbine steam turbine heat exchanger heat recovery steam generator Integrated gasification with combined cycle Lower heating value Peng-Robinson and Boston-Mathias root mean squared error Experimental data results (mol/molglycerol) Simulated syngas yield (mol/molglycerol)
Greek letters h efficiency (%) · mass flow rate (kg/h) m Subscripts feed feed (mixture of sewage sludge and coal) glycerol glycerol hydrogen H2 in internal power consumption out output power power power total syngas total produced syngas total total
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.210.
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references
[1] Syed-Hassan SSA, Wang Y, Hu S, Su S, Xiang J. Thermochemical processing of sewage sludge to energy and fuel: fundamentals, challenges and considerations. Renew Sustain Energy Rev 2017;80:888e913. [2] Zheng C, Ma X, Yao Z, Chen X. The properties and combustion behaviors of hydrochars derived from cohydrothermal carbonization of sewage sludge and food waste. Bioresour Technol 2019;285:121347. [3] Tyagi VK, Lo S-L. Sludge: a waste or renewable source for energy and resources recovery? Renew Sustain Energy Rev 2013;25:708e28. [4] Safari F, Javani N, Yumurtaci Z. Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts. Int J Hydrogen Energy 2018;43:1071e80. [5] Yan M, Liu J, Hantoko D, Kanchanatip E, Grisdanurak N, Cai Y, et al. Hydrogen-rich syngas production by catalytic cracking of tar in wastewater under supercritical condition. Int J Hydrogen Energy 2019;44:19908e19. [6] Akiya N, Savage PE. Roles of water for chemical reactions in high-temperature water. Chem Rev 2002;102:2725e50. [7] Chen Y, Guo L, Jin H, Yin J, Lu Y, Zhang X. An experimental investigation of sewage sludge gasification in near and super-critical water using a batch reactor. Int J Hydrogen Energy 2013;38:12912e20. [8] Ro KS, Cantrell K, Elliott D, Hunt PG. Catalytic wet gasification of municipal and animal wastes. Ind Eng Chem Res 2007;46:8839e45. [9] Yu J, Tahmasebi A, Han Y, Yin F, Li X. A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization. Fuel Process Technol 2013;106:9e20. [10] Tahmasebi A, Zheng H, Yu J. The influences of moisture on particle ignition behavior of Chinese and Indonesian lignite coals in hot air flow. Fuel Process Technol 2016;153:149e55. [11] Duan F, Chyang C-S, Hsu S-H, Tso J. Combustion behavior and pollutant emissions of batch fluidized bed combustion. J Taiwan Inst Chem Eng 2013;44:1034e8. [12] Cao C, He Y, Wang G, Jin H, Huo Z. Co-gasification of alkaline black liquor and coal in supercritical water at high temperatures (600e750 C). Energy Fuels 2017;31:13585e92. [13] Cao C, Guo L, Yin J, Jin H, Cao W, Jia Y, et al. Supercritical water gasification of coal with waste black liquor as inexpensive additives. Energy Fuels 2015;29:384e91. [14] Chen Z, Gao L, Zhang X, Han W, Li S. High-efficiency power generation system with integrated supercritical water gasification of coal. Energy 2018;159:810e6. [15] Jin H, Chen Y, Ge Z, Liu S, Ren C, Guo L. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40:16096e103. [16] Yu J, Jiang C, Guan Q, Gu J, Ning P, Miao R, et al. Conversion of low-grade coals in sub-and supercritical water: a review. Fuel 2018;217:275e84. [17] Youssef EA, Nakhla G, Charpentier PA. Co-gasification of catechol and starch in supercritical water for hydrogen production. Int J Hydrogen Energy 2012;37:8288e97. rrez Ortiz FJ, Ollero P, Serrera A, Galera S. Process [18] Gutie integration and exergy analysis of the autothermal reforming of glycerol using supercritical water. Energy 2012;42:192e203. [19] Fiori L, Valbusa M, Castello D. Supercritical water gasification of biomass for H2 production: process design. Bioresour Technol 2012;121:139e47.
24470
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 4 5 9 e2 4 4 7 0
rrez Ortiz FJ, Ollero P, Serrera A, Galera S. Optimization [20] Gutie of power and hydrogen production from glycerol by supercritical water reforming. Chem Eng J 2013;218:309e18. [21] Aziz M, Prawisudha P, Prabowo B, Budiman BA. Integration of energy-efficient empty fruit bunch drying with gasification/combined cycle systems. Appl Energy 2015;139:188e95. [22] Darmawan A, Fitrianto AC, Aziz M, Tokimatsu K. Integrated system of rice production and electricity generation. Appl Energy 2018;220:672e80. [23] Darmawan A, Ajiwibowo MW, Yoshikawa K, Aziz M, Tokimatsu K. Energy-efficient recovery of black liquor through gasification and syngas chemical looping. Appl Energy 2018;219:290e8. [24] Sarafraz MM, Jafarian M, Arjomandi M, Nathan GJ. Experimental investigation of the reduction of liquid bismuth oxide with graphite. Fuel Process Technol 2019;188:110e7. [25] Sarafraz MM, Jafarian M, Arjomandi M, Nathan GJ. Potential use of liquid metal oxides for chemical looping gasification: a thermodynamic assessment. Appl Energy 2017;195:702e12. [26] Wan W. An innovative system by integrating the gasification unit with the supercritical water unit to produce clean syngas for solid oxide fuel cell (SOFC): system performance assessment. Int J Hydrogen Energy 2016;41:22698e710. [27] Patcharavorachot Y, Saebea D, Authayanun S, Arpornwichanop A. Hydrogen and power generation from supercritical water reforming of glycerol and pressurized SOFC integrated system: use of different CO2 adsorption process. Int J Hydrogen Energy 2018;43:17821e34. [28] Cao C, Guo L, Jin H, Cao W, Jia Y, Yao X. System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production. Energy 2017;132:238e47. [29] Cao C, He Y, Chen J, Cao W, Jin H. Evaluation of effect of evaporation on supercritical water gasification of black liquor by energy and exergy analysis. Int J Hydrogen Energy 2018;43:13788e97. [30] Safari F, Tavasoli A, Ataei A. Gasification of sugarcane bagasse in supercritical water media for combined hydrogen and power production: a novel approach. Int J Environ Sci Technol 2016;13:2393e400. [31] Chen Z, Zhang X, Han W, Gao L, Li S. Exergy analysis on the process with integrated supercritical water gasification of coal and syngas separation. Appl Therm Eng 2018;128:1003e8. [32] Aziz M. Power generation from algae employing enhanced process integration technology. Chem Eng Res Des 2016;109:297e306. [33] Adnan MA, Susanto H, Binous H, Muraza O, Hossain MM. Feed compositions and gasification potential of several biomasses including a microalgae: a thermodynamic modeling approach. Int J Hydrogen Energy 2017;42:17009e19. [34] Hantoko D, Su H, Yan M, Kanchanatip E, Susanto H, Wang G, et al. Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: effects of operating parameters. Int J Hydrogen Energy 2018;43:17620e32.
[35] Kraisornkachit P, Vivanpatarakij S, Amornraksa S, Simasatitkul L, Assabumrungrat S. Performance evaluation of different combined systems of biochar gasifier, reformer and CO2 capture unit for synthesis gas production. Int J Hydrogen Energy 2016;41:13408e18. [36] Aziz M. Integrated hydrogen production and power generation from microalgae. Int J Hydrogen Energy 2016;41:104e12. € gg MB. CO2 [37] Peters L, Hussain A, Follmann M, Melin T, Ha removal from natural gas by employing amine absorption and membrane technologyda technical and economical analysis. Chem Eng J 2011;172:952e60. [38] Atimtay AT, Topal H. Co-combustion of olive cake with lignite coal in a circulating fluidized bed. Fuel 2004;83:859e67. [39] Sheng C, Azevedo JLT. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenergy 2005;28:499e507. [40] Louw J, Schwarz CE, Knoetze JH, Burger AJ. Thermodynamic modelling of supercritical water gasification: investigating the effect of biomass composition to aid in the selection of appropriate feedstock material. Bioresour Technol 2014;174:11e23. [41] Adnan MA, Hossain MM. Co-gasification of Indonesian coal and microalgae e a thermodynamic study and performance evaluation. Chem Eng Process Process Intensif 2018;128:1e9. [42] Aziz M. Integrated supercritical water gasification and a combined cycle for microalgal utilization. Energy Convers Manag 2015;91:140e8. [43] Darmawan A, Hardi F, Yoshikawa K, Aziz M, Tokimatsu K. Enhanced process integration of black liquor evaporation, gasification, and combined cycle. Appl Energy 2017;204:1035e42. [44] Byrd AJ, Pant KK, Gupta RB. Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. Fuel 2008;87:2956e60. [45] Voll FAP, Rossi CCRS, Silva C, Guirardello R, Souza ROMA, Cabral VF, et al. Thermodynamic analysis of supercritical water gasification of methanol, ethanol, glycerol, glucose and cellulose. Int J Hydrogen Energy 2009;34:9737e44. rrez Ortiz FJ, Ollero P, Serrera A. Thermodynamic [46] Gutie analysis of the autothermal reforming of glycerol using supercritical water. Int J Hydrogen Energy 2011;36:12186e99. [47] Castello D, Fiori L. Supercritical water gasification of biomass: a stoichiometric thermodynamic model. Int J Hydrogen Energy 2015;40:6771e81. [48] Withag JAM, Smeets JR, Bramer EA, Brem G. System model for gasification of biomass model compounds in supercritical water e a thermodynamic analysis. J Supercrit Fluids 2012;61:157e66. [49] Chen Y, Guo L, Cao W, Jin H, Guo S, Zhang X. Hydrogen production by sewage sludge gasification in supercritical water with a fluidized bed reactor. Int J Hydrogen Energy 2013;38:12991e9. rrez Ortiz FJ, Ollero P, Serrera A, Galera S. An energy [50] Gutie and exergy analysis of the supercritical water reforming of glycerol for power production. Int J Hydrogen Energy 2012;37:209e26.
Available online at www.sciencedirect.com
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Supercritical water gasification of sewage sludge and combined cycle for H2 and power production e A thermodynamic study Dwi Hantoko a, Mi Yan a,*, Ekkachai Kanchanatip a,b, Muflih A. Adnan c, Antoni a, Ishrat Mubeen a, Fauziah Shahul Hamid d a
Institute of Energy and Power Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, PR China b Center of Excellence in Environmental Catalysis and Adsorption, Faculty of Engineering Thammasat University, Pathumthani 12120, Thailand c Department of Chemical Engineering, Islamic University of Indonesia, Yogyakarta 55584, Indonesia d Institute of Biological Sciences, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia
highlights Co-gasification
of
graphical abstract sludge
and
lignite coal for H2 production and power generation. Integrated system consisted of SCWG and gas/steam turbine with heat recovery. Higher temperature (700
C) and
lower feed concentration (10 wt%) favored H2 yield. Lignite
coal
addition
reduced
minimum feed input to realize auto-thermal condition. 9.06 kg of H2 and 48.37 kW electricity were obtained from 100 kg sludge at 700 C.
article info
abstract
Article history:
An integrated system of supercritical water gasification (SCWG) and combined cycle has
Received 26 May 2019
been developed for H2 production and power generation. Sewage sludge and lignite coal
Received in revised form
were selected as raw material in this simulation. The effects of feed concentration (10
20 July 2019
e30 wt%) and lignite coal addition (0e50 wt%) on syngas yield and H2 yield were also
Accepted 25 July 2019
investigated in the temperature range of 500 Ce700 C. Several heat exchangers were
Available online 22 August 2019
considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed
* Corresponding author. E-mail addresses: [email protected] (D. Hantoko), [email protected] (M. Yan). https://doi.org/10.1016/j.ijhydene.2019.07.210 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Keywords: Sewage sludge Supercritical water Gasification Hydrogen Combined cycle Thermodynamic
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concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve selfsufficient energy at 500 C, 600 C, and 700 C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H2 yield were obtained at 700 C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H2 were obtained after H2 separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H2 production. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, sewage sludge with high organic matter derived from urban or industrial wastewater treatment plant has been considered as a potential energy source to meet the demand of the energy, as well as, to solve the waste treatment problems. It is estimated that the rate of sewage sludge production is around 0.1e30.8 kg per population equivalent per year [1]. In China, the production of sewage sludge exceeded 30 million tons in 2015 and it is expected to reach 60e90 million tons in 2020 due to the rapid development of urbanization and industrialization [2]. It can be converted to energy through biochemical and thermochemical process. Unfortunately, biochemical process e.g. fermentation, takes longer time and has lower conversion rate than its counterpart of thermochemical process [3]. For conventional thermal treatments, such as incineration, pyrolysis and gasification, a pre-drying procedure is unavoidably required, which leads to the most of energy invested and released during these thermal treatments as the energy is being consumed to remove the moisture retained in sewage sludge. Therefore, exploring innovative and sustainable technologies for sewage sludge treatment are urgently required. The considerations of choosing suitable technology for energy recovery from sewage sludge involves technical and economic feasibility, environmental sustainability, marketing facts and public acceptance. Compared with the conventional thermochemical process (e.g. combustion, pyrolysis and gasification), supercritical water gasification (SCWG) seems to be a more suitable technique to convert sewage sludge with high moisture content (80e90 wt%) into syngas with high content of hydrogen (H2) without the need for drying pre-treatment. As a result, the energy and cost required for drying process can be omitted. In addition, SCWG process requires shorter reaction time compared with anaerobic or biological treatments. Owing to the unique properties of water in supercritical condition (374.3 C and 22.1 MPa), such as high diffusivity, low viscosity, low density, low dielectric constant, the organic matters in sewage sludge, organic wastes and coal can be dissolved and converted into H2-rich syngas [4,5]. Besides, supercritical water provides excellent reaction environment for the gasification of organic constituents [6]. It has been reported that organic matters in sewage sludge were almost completely
hydrolyzed and decomposed into gaseous product at 425 C [7]. SCWG was also assigned to give significant environmental benefits on the treatment of municipal and animal wastes, e.g., odor elimination, BOD removal, and pathogen kill [8]. Therefore, the utilization of SCWG process can replace the ineffective and uneconomical conventional techniques in dealing with biowastes, containing high amount of moisture, especially sewage sludge. Additionally, low-rank coals (lignite and sub-bituminous coal) are regarded as an energy resources due to its abundant availability compared to other fossil fuels. Low-rank coals contained high volatiles which are rich in aliphatic -CO- and -C-C- structures [9]. Therefore, they present high reactivity during thermo-chemical conversions. In some countries including China and Indonesia, low-rank coals (lignite and sub-bituminous coal) are being progressively utilized due to the increase in energy demand and high price of high-rank coals [10]. For combustion in pulverized coal power plant, low-rank coal needs additional pre-treatment to deal with the high moisture content (25e65 wt%) [9]. These pretreatment diminished the overall energy efficiency of the power plant. In addition, coal-fired power plant may generate much pollutant such as; SO2, NOx and fine particles, which can cause environmental problems [11]. In this regard, SCWG is gaining more attention as an alternative and efficient technique for utilization of low-rank coal. Consequently, some research on SCWG of coal have been conducted to explore the potential of energy recovery [12e15]. It has been reported that high carbon content in coal was beneficial in increasing gaseous product, improving gasification efficiency which will reduce energy loss [16]. However, due to the unique property and structure of coal, higher temperature is required to achieve complete gasification. It is worth noting that high SCWG temperature may not only increase the reactor construction cost but also bring the challenges to the safety issue. Some researchers investigated co-gasification of coal and biomass and they found a synergetic effect on hydrogen production and gasification efficiency at lower temperature than coal itself [12,17]. In this study, the addition of lignite coal is expected to promote syngas production from SCWG of sewage sludge. Only few studies placed concerns on the process design of SCWG, particularly those associated with the energy
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self-sufficiency. Ortiz et al. [18] had proposed the first design of supercritical water reforming of glycerol for power production by expanding the high-pressure syngas. The process design was equipped with several heat exchangers, as well as, oxygen injection into reformer to achieve autothermal conditions. Fiori et al. [19] studied the SCWG of biomass for H2 production. The energy required to sustain the SCWG reactor was supplied by burning the remaining gases, after H2 membrane separator. The addition of auxiliary methane is needed if the system cannot achieve self-sustained condition. Likewise, some fraction of the produced H2 from the separator unit can be burnt in the furnace to realize the energetic selfsustainability, as reported by Ortiz et al. [20]. As a consequence, the yield of H2 will be reduced because of the internal consumption. Moreover, the use of auxiliary methane in SCWG process required high operating cost. Process integration and optimization in conventional gasification of empty fruit bunch [21], rice husk [22], and black liquor [23] for syngas production have been reported. The potential use of liquid metal oxides as an oxygen carrier for chemical looping gasification was also thermodynamically and experimentally studied to produce high value of syngas [24,25]. Additionally, the design configuration of fuel/electricity production pathways with real-biomass [26], glycerol [27], black liquor [28,29], sugarcane [30], and coal-water slurry [31] via SCWG have also been investigated. Concerning energy efficiency, integrated gasification with combined cycle (IGCC) system has been taken into account to have higher energy efficiency. High power generation efficiency (approximately 60%) can be achieved from macroalgae by employing IGCC system [32]. In summary, the integrated system of gasification and power generation is more efficient than standalone gasification for syngas production. Considering the points mentioned above, an integration process of SCWG and combined cycle-based power generation was proposed. Although, the drying process can be ignored by SCWG, the system still requires large energy input to attain energy self-sufficiency. The feedstock, sewage sludge and low-rank coals (e.g. lignite coal) have been used in this study due to their high moisture content. To the best of the author0 s knowledge, there has been no study on integrating SCWG and combined cycle system for H2 production and power generation, especially for sewage sludge application. Moreover, the concept of the proposed design also attempted to recover the energy or heat emitted throughout the whole process by employing several heat exchanger networks. Hence, the effect of feed concentration and lignite coal addition on the H2 yield and net produced power were investigated. The proposed integrated system is expected to be an alternative for H2 production and power generation from sewage sludge.
Proposed integrated system of SCWG and combined cycle Basic concept of the of the integrated system A conceptual design of the proposed integrated system for hydrogen production and power generation is presented in Fig. 1. The integrated system mainly consists of SCWG and
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combined cycle. The solid line and dashed line represent the material and energy flows, respectively. The process scheme has been designed to be as simple as possible to be applicable to the actual demo-scale or industrial-scale plant. Nevertheless, the requirement for the energetic self-sustainability was also considered. The possibility of the self-sufficient energy for the small-scale system can be used as a concept for future process development. The lignite coal was added into the feed stream in order to fulfill the energy requirement in the SCWG process. In the SCWG reactor, sewage sludge and lignite coal were converted to syngas which mainly contains H2, CO, CO2 and CH4. The H2-rich syngas was directly flowed to the hydrogen separator without hydrogen enrichment. Meanwhile, the remaining gases from hydrogen separator was fed to the combustor in combined cycle module to generate electricity. The combination of SCWG and combined cycle system could improve the total energy efficiency. Some parts of the heat from combined cycle were used to recover the energy in the SCWG reactor through heat exchanger network system.
Description of the integrated system The process modeling and calculation related with the mass and energy balances of the proposed system was developed using Aspen Plus V8.8 (Aspen Technology, Inc). The detailed schematic process flow diagram of the proposed integrated system is illustrated in Fig. 2. The detailed properties and status parameter of the streams shown in the flow diagram can be found in the Supplementary File. In this process, sewage sludge and lignite coal were adopted as a feedstock for SCWG. The feedstock and water were mixed (MX1), then pumped (P1) to the supercritical pressure. The mixture was then preheated in preheater (HE1 and HE2) before it flowed into the SCWG reactor. In SCWG reactor, which consisted of two blocks (RYIELD and RGIBBS), the feed was converted to syngas. The RYIELD block with FORTRAN subroutine (defined using a calculator block) was used to decompose the feedstock (non-conventional component) into C, H2, O2, N2, and S elements (mixed component) based on the ultimate and proximate properties. In order to simulate the chemical equilibrium at a specific temperature and pressure, the RGIBBS block was employed based on Gibbs free energy minimization. The produced syngas mainly contained H2, CO, CO2, and CH4. The main chemical reactions typically occur in the SCWG process are summarized in Table 1 [33e35]. The sensible heat of syngas from the SCWG reactor was utilized to preheat the mixture which was done by a heat exchanger (HE1). In this process, the main heat source for SCWG process was supplied by the exhausted hot gas from gas turbine through heat exchanger (HE2). Subsequently, the resulting stream from SCWG reactor was cooled down to 60 C in order to separate the water (WTSEP). In this unit, about 85%e95% of water was recovered. The recovered water can be recirculated into the SCWG reactor as a reactant. The recirculation of water was not considered in this simulation in order to simplify the model. After the water separator, the produced syngas flowed to turbo expandergenerator (TURB-EXP) to expand the high-pressure syngas, which was modeled by the turbo-expander generator of
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Fig. 1 e Conceptual design of the proposed integrated system for H2 production and power generation.
Fig. 2 e Simplified schematic flow diagram of supercritical water gasification for H2 production and power generation.
Table 1 e Summary of the chemical reactions in supercritical water gasification [33e35]. Reaction type
Stoichiometric reaction equation
Simplified overall reaction of SCWG CHxOy þ (2-y)H2O / (2y þ x/2)H2 þ CO2 Water gas shift reaction CO þ H2O 4 CO2 þ H2 Boudouard reaction C þ CO2 4 2CO Steam methane reforming CH4 þ H2O 4 CO þ 3H2 Dry reforming CH4 þ CO2 4 2CO þ 2H2 Methane formation C þ 2H2 4 CH4 Methanation of CO2 CO2 þ 4H2 4 CH4 þ 2H2O Methanation of CO CO þ 3H2 4 CH4 þ H2O CO2 þ 2H2 4 C þ2H2O Hydrogenation of CO2 Hydrogenation of CO CO þ H2 4 C þ H2O
General Electric (GE), Oil and Gas Company as a technical reference. The extracted energy from this unit can be used to drive the generator for producing electricity. Turbo expandergenerator is commonly used in industrial applications to recover energy from the fluid process, i.e. natural gas processing, integrated gasification combined cycle and petrochemicals industries [18]. The resulting stream was sent to the H2 separator (H2-SEP) without being shifted for hydrogen enrichment. A shift reaction can converts CO to hydrogen, however, additional heat is required in the shift reactor for the endothermic reaction [36]. The separation of H2 was achieved by means of the Hysep Technology model (HYSEP Modul Type 1308, ECN, palladium membrane filter). High purity of H2 can
DH298K (MJ/kmol) DG298K (MJ/kmol) Reaction number Endothermic 42 þ172 þ206 þ247 74 165 206 90 131
29 120 142 171 51 114 142 63 91
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10
be obtained. Hence, the feed stream should be heated using flue gas from steam turbine through heat exchanger (HE3). The remaining gases mainly contained CH4, CO, and CO2 (without H2) were transferred to gas turbine packages. The amount of air used for syngas combustion was adjusted according to the flow rate of gas entering the combustor by setting the design specification in a Design Spec block. Finally, the gas turbine drove the generator for power generation. The high temperature flue gas was used to supply the heat needed in SCWG reactor. The rest of its heat was utilized to generate the steam in the heat recovery steam generator (HRSG). The flow rate of steam was accordingly controlled to drive the steam turbine (ST). At last, the flue gas was passed into the
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CO2 absorption unit (ABS-CO2) before being released to the atmosphere. About 90% of CO2 was assumed to be absorbed in this unit [37]. The absorbed CO2 can be directed to the series of downstream facilities such as cooler and compressor to achieve the desired conditions.
Process modeling and analysis General conditions The Gibbs free energy minimization is a well-known method to evaluate the performance of gasification process. Due to the complexity of SCWG reaction mechanisms, this approach is more preferable to predict the possible components of the product without the knowledge of reactions involved. A mixture of sewage sludge and lignite coal at different ratios were considered as a feedstock. The proximate and ultimate analysis based on the work of Chen et al. [7] and Atimtay et al. [38] with modifications are listed in Table 2. The heating value of the feedstocks were calculated based on the proximate and ultimate analysis [39]. A flow rate of 100 kg/h feedstock was used in all simulations. The stream class used in Aspen Plus was MIXCINC and the property method was Peng-Robinson with Boston-Mathias modification (PR-BM). The previous study mentioned that PR-BM equation of state-provided highly accurate SCWG simulation with Pearson0 s correlation coefficient (R2) and root mean squared error (RMSE) of 0.99 and 0.25, respectively [40]. In this study, the assumptions were made as follows: (i) SCWG reactor consisted of RYield and RGibbs, (ii) ambient temperature was 25 C, (iii) all of the heat exchangers were countercurrent, (iv) minimum temperature approach in all heat exchangers was 10 C, (v) heat loss was negligible, (vi) the air used contained 79 mol% nitrogen and 21 mol% oxygen and (vii) there was no air contaminant inside the SCWG reactor. To simplify the simulation, the effect of pressure drop and mass transfer were not considered. The assumed conditions for SCWG and combined cycle modules are summarized in Table 3.
Table 2 e Proximate and ultimate analysis of sewage sludge and lignite coal. Properties
Sewage sludge [7]
Proximate analysis (wt.% db)a Fixed carbon Ash Volatile matter Ultimate analysis (wt.% daf)b Carbon Hydrogen Nitrogen Sulfur Oxygenc Lower heating value (MJ kg1) a b c
Lignite coal [38]
9.41 28.96 61.63
44.65 25.62 29.73
53.74 4.79 6.57 1.48 33.42 19.91
74.72 5.81 2.65 2.28 14.54 26.61
On a dry basis (db). On a dry and ash-free basis (daf). By difference (O% ¼ 100% ash% C% H% N% S%).
Table 3 e Assumed conditions for SCWG and combined cycle modules. Properties Supercritical water gasification Temperature (oC) Pressure (MPa) Feed concentration (wt%) Coal to sewage sludge ratio (wt%) Pump efficiency (%) Heat exchanger min. temp. approach (oC) Combustor and gas turbine Compressor isentropic efficiency (%) Gas turbine inlet temperature (oC) Gas turbine isentropic efficiency (%) Discharge pressure (MPa) HRSG and steam turbine HRSG outlet pressure (MPa) Steam turbines isentropic efficiency (%) Discharge pressure (MPa)
Value
Ref./Note
500, 600, 700 25 10e30 10e50 80 10
This study This study This study This study [20,41] [42]
90 1500 90 0.15
[23] [42] [42] [23]
12 90
[43]
0.15
[23]
Model validation The system validity was partially evaluated for the SCWG reactor unit by comparing the results of the simulation with those of the experimental results reported by Byrd et al. [44]. The SCWG reactor is a considerably complex unit operation compared to other unit operations that are typically controlled by irreversible reaction (combustion in combined cycle) and physical parameters (heat exchanger, compression, and expansion and separator). The model validation was performed by comparing the simulated syngas yield (Sim) and the experimental data results (Exp), as shown in Table 4. The Pearson0 s correlation factor (R2) and the root mean squared error (RMSE) were applied to evaluate the difference between the simulation and experiment data results. It can be observed that the results of our simulation were in good agreement with the experimental results. The equilibrium yield of H2 (R2 ¼ 0.98) and CH4 (R2 ¼ 0.97) well-fitted with the experimental results. While the equilibrium yield of CO (R2 ¼ 0.79) and CO2 (R2 ¼ 0.82) did not fit very well as the correlation coefficient was lower, however, they were in acceptable level. In addition, the relative error of the predicted H2 composition in the present simulation was lower than 5%. The difference between the predicted gas yields and experimental gas yields can be explained by the use of short residence time (1 s) in the experimental condition, as reported by Byrd et al. [44]. It is worth mentioning that the predicted composition of syngas from the present study was also close to the syngas composition as reported by other studies [45e47].
Performance evaluation The performance of the integrated SCWG process was determined in terms of hydrogen yield, total syngas yield, hydrogen production efficiency, power generation efficiency and total efficiency. The hydrogen yield and the total syngas yield were calculated using Eq. (1) and Eq. (2), respectively. In this case, m_ H2 , m_ total syngas , and m_ feed are the mass flow rate of hydrogen, mass flow rate of syngas, and the mass flow rate of the feed,
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Table 4 e Simulated syngas yield (Sim) vs the experimental data results (Exp) for SCWG of glycerol by Byrd et al. [44] at various temperatures and feed concentrations. Syngas yield (mol/molglycerol)a
Parameter conditions H2 Temp. (oC)
CO
CO2
Exp
Sim
Exp
Sim
Exp
Sim
Exp
Sim
5 5 5 15 20 30 35 40
5.12 5.81 6.54 4.13 3.94 2.87 2.60 2.18
6.08 6.52 6.69 4.61 3.79 2.72 2.24 2.07 0.48 0.98
0.49 0.29 0.29 0.72 0.81 0.92 0.95 0.94
0.20 0.08 0.03 0.51 0.71 0.97 1.09 1.14 0.20 0.97
0.02 0.01 0.09 0.04 0.17 0.21 0.24 0.24
0.12 0.16 0.20 0.36 0.37 0.38 0.38 0.38 0.18 0.79
2.34 2.50 2.34 2.21 2.42 2.07 1.93 1.79
2.68 2.76 2.77 2.13 1.92 1.65 1.52 1.48 0.37 0.82
700 750 800 800 800 800 800 800 RMSE R2 a
CH4
Feed (wt.%)
Sim ¼ simulated syngas yield; Exp ¼ experimental data results [44].
respectively. The feed concentration was defined as the ratio of the amount of feed (mixture of sewage sludge and lignite coal) (kg) to the total amount of feed and water (kg), according to our previous work [34]. P P P N exp sim exp sim R2 ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h P i h P iffi P P 2 n exp2 ð expÞ2 N sim2 ð simÞ
Syngas and hydrogen production
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 ðexp simÞ RMSE ¼ N "
# _ H2 kg m Hydrogen yield ¼ _ feed kgfeed m
(1)
# _ total syngas m kg ¼ Syngas yield _ feed kgfeed m
(2)
"
The total energy efficiency (h total ) of the proposed integrated system was calculated as the sum of the hydrogen production efficiency (h H2 ) and power generation efficiency (h power ). These values can be written as follows: h total ¼ h H2 þ h power h H2 ¼
_ H2 LHVH2 m _ feed LHVfeed m
hpower ¼
Wout Win _ feed LHVfeed m
lignite coal in the feed from 10 wt% to 50 wt%. A constant pressure was maintained at 25 MPa, since pressure has insignificant effect on the equilibrium results of SCWG [40,48]. The temperature of the SCWG was varied from 500 C to 700 C.
Fig. 3 shows the yield of syngas versus feed concentration at different SCWG temperature. It can be seen that yield of syngas slightly decreased as the feed concentration increased. Likewise, the yield of hydrogen decreased significantly, as can be seen in Fig. 4. The presence of water plays an important role to complete the SCWG reactions. The low feed concentration indicated that more water is available during the SCWG process, while high feed concentration implied that less water is existing during the reactions. The lack of water inhibited the steam methane reforming reaction (R4) and water-gas shift reaction (R2). The steam reforming reaction promotes the formation of H2, which opposed to the methanation reactions (R6 e R8). This finding was in good agreement with the previous reported works on the SCWG of sewage sludge in batch and fluidized bed reactor [7,49]. Among the other chemical
(3)
(4)
(5)
where LHVH2 and LHVfeed are the lower heating value of hydrogen and lower heating value of feed, respectively. Wout is the total generated power from expander, gas turbine and steam turbine. Win is the internal power consumption during the processes, including pumping and compression.
Results and discussion In this study, the model was investigated with the feed concentration in a range of 10 wt% to 30 wt% with the ratio of
Fig. 3 e Effect of feed concentration (only sewage sludge) on the syngas yield at different SCWG temperatures.
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reactions in SCWG, steam reforming, water-gas shift, and methanation reaction are considered to dominate the change in syngas composition. In addition, the reaction temperature strongly affected the yield of syngas and the yield of hydrogen, as presented in Figs. 3 and 4, respectively. With increasing temperature from 500 C to 700 C, the syngas and hydrogen yield significantly increased from 147.79 kg/100kgfeed and 1.99 kg/100kgfeed to 178.08 kg/100kgfeed and 9.06 kg/100kgfeed, respectively. Carbon conversion was in the range of 98e99%, which was higher than that obtained from the previous study [34]. This result revealed that higher temperature favored the endothermic reactions where SCWG reactions take place. This was further confirmed by Withag et al. [48] showing that the overall gasification reactions were endothermic reactions. However, the higher temperature in the SCWG reactor would lead to more energy requirement or more energy sources to sustain the process. The discussion on the limit of feed concentration at different SCWG temperature for sustaining the process is given in the next section. Fig. 5 shows the effect of the addition of lignite coal on syngas yield at 700 C. The addition of lignite coal had a positive effect on the enhancement of syngas production. For instance, at feed concentration of 10 wt% and temperature of 700 C, the syngas yield increased from 178.08 kg/100kgfeed to
200.81 kg/100kgfeed when the ratio of the addition of lignite coal increased from 10 wt% to 50 wt%. This can be explained by the fact that the addition of lignite coal increased the carbon content in the feed, as well as, the heating value of the coal and sludge mixture. At the same time, thermodynamic analysis on SCWG of coal and black liquor showed that the addition of coal could improve the production of hydrogen in SCWG of black liquor at equilibrium condition [13]. Since coal contained more carbon and less oxygen than black liquor, more water was involved in SCWG of coal to give the required oxygen to produce CO and CO2 than in SCWG of black liquor. As a consequence, more H2 would be released from water in SCWG of coal than black liquor. A similar results on the H2 production was also found in this study because of the addition of lignite coal into SCWG of sewage sludge. Cao et al. [12] found a similar trend in their experimental work on cogasification of alkaline black liquor and coal in supercritical water. A strong synergetic effect was observed when the blending ratio of coal and black liquor was about 50:50. They observed that the presence of black liquor enhanced the performance of coal gasification due to the presence of alkali content in black liquor which acted as catalyst, while, the presence of coal facilitated the full use of the alkali catalyst to improve SCWG of black liquor. The addition of lignite coal also had a considerable influence on the syngas composition, as shown in Fig. 6. It can be observed that the H2 concentration decreased with the increase of coal addition. It was due to the fact that adding more lignite coal resulted in a higher O2 concentration in the feed of SCWG process. Consequently, the presence of higher amount of O2 converted H2 into H2O. A similar trend was reported by Adnan et al. [41], which showed lower H2 concentration in the co-gasification of Indonesian coal and microalgae. A previous study also reported that the yield of H2 could also be influenced by the C:H ratio and oxygen content in the fuel [40]. The maximum H2 yield could be obtained when the feedstock had a low C:H ratio and low oxygen content. If the ratio of C:H was between 6 and 10, then H2 yield turned out to be less dependent on both the C:H ratio and oxygen content. However, H2 yield became more dependent on the oxygen content when the C:H ratio was greater than 10. In addition, the yield of H2 was dependent on both the C:H ratio and the oxygen content, if C:H ratio was lower than 6. By varying the coal addition from 0 wt%
Fig. 5 e Effect of feed concentration on the syngas yield at different lignite coal addition (at 700 C).
Fig. 6 e Effect of feed concentration on the syngas composition at various lignite coal addition at 700 C.
Fig. 4 e Effect of feed concentration on the hydrogen yield at different SCWG temperatures.
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to 50 wt%, the C:H ratio increased from 11.23 to 12.17, while the oxygen content decreased from 33.42% to 23.46%, respectively.
Limit of feed concentration for autothermal condition The analysis of the energy requirement played an important role in the process modeling. The conditions to reach the selfsustained process with no auxiliary fuel was investigated for the temperature range of SCWG mentioned above. To achieve this target, a sensitivity analysis was carried out by changing the feed concentration and lignite coal addition from 10 wt% to 30 wt% and 10 wt% to 50 wt%, respectively, for each SCWG temperature. By this approach, the conditions from the energy
Fig. 7 e Effect of feed concentration on the energy flow to achieve autothermal condition at different SCWG temperatures: (a) 500 C, (b) 600 C, (c) 700 C.
deficit in the SCWG reactor to energy excess were obtained, corresponding to the cases with lower and higher feed concentration. In addition, a number of heat exchangers were properly installed in the process to minimize the energy loss and to achieve the optimum heat integration network system. The minimum temperature approach and countercurrent flow were used in the heat exchanger system. Fig. 7 shows the flow of energy that represented energy demand or energy production by the SCWG reactor. The heat flow in SCWG reactor corresponds to the heat consumed or generated by SCWG reactor which is determined by the operating temperature of the reactor and the concentration of the feed. The negative values implied that the reactor needs to be sustained by an external source of thermal energy. On the other hand, the positive value indicated that the production of thermal energy was obtained and the process was selfsustainable. According to our calculation, an energy deficit was observed when the feed concentration (only sewage sludge) was lower than 14.25 wt% at 500 C, 18.75 wt% at 600 C, and 25.50 wt% at 700 C and the heat surplus was obtained if the feed concentration was higher than those values
Fig. 8 e Effect of feed concentration (only sewage sludge) on H2 yield and net produced power at different SCWG temperatures: (a) 500 C, (b) 600 C, (c) 700 C.
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Table 5 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (500 C, 25 MPa). Component
Produced works (kW) Expander Gas turbine Steam turbine Comsumed works (kW) Pumps Compressor Total net power (kW)
Feed concentration (wt%) 10
15
20
25
30
7.30 94.50 10.18
6.69 97.86 12.57
6.37 99.63 17.84
6.17 100.73 23.45
6.04 101.48 28.57
11.82 38.02 62.13
7.66 39.64 69.82
5.72 40.50 77.62
4.65 41.03 84.68
3.98 41.39 90.72
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sewage sludge, this value was lower than the value calculated in the present study (approximately 25 wt% of feed concentration at 700 C). It might be explained by the use of the higher temperature of flue gas (directly from combustor) (ca. 1700 C) to preheat the feed before entering to the SCWG reactor, which helps the process to be rapidly sustained. In this simulation, the flue gas from the gas turbine had lower temperature than that used by Fiori et al. [19]. Ortiz et al. [50] also reported that their supercritical water reforming of glycerol at 800 C and 24 MPa could achieve self-sufficient process with 21.78 wt% feed concentration. In their study, the highpressure syngas was expanded to produce power, while some part of produced hydrogen in the furnace was proposed to compensate the deficit energy in the reformer [20]. It is
Table 6 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (600 C, 25 MPa). Component
Produced works (kW) Expander Gas turbine Steam turbine Comsumed works (kW) Pumps Compressor Total net power (kW)
Feed concentration (wt%) 10
15
20
25
30
9.88 81.21 8.82
8.50 88.83 11.29
7.73 92.96 16.50
7.24 95.57 22.20
6.89 97.38 27.48
11.77 31.49 56.65
7.61 35.22 65.78
5.67 37.24 74.27
4.60 38.52 81.88
3.94 39.41 88.40
(see Fig. 7(aec). By comparing Figs. 4 and 7, it is worth noting that higher temperature resulted in higher H2 production, but higher thermal energy was required, consequently, higher feed concentration was needed to sustain the process. For instance, the highest H2 yield was obtained at the temperature of 700 C, the process could not thermally self-sustain with a feed concentration below 25 wt%. It can also be seen that the addition of lignite coal had slightly effect on the thermal energy requirement, and no significant effect was found at 700 C. Fiori et al. [19] calculated a biomass concentration in the feed (22.9 wt% for sewage sludge, 20.5 wt% for glycerol, 18.3 wt % for microalgae, 16.6 wt% for grape marc, and 11.4 wt% for phenol) to achieve autothermal condition in their SCWG process, at 700 C and 300 bar. The main heat sources for SCWG reactor was supplied by the heat from combustion of remaining gases (after hydrogen separation). For the case of
Table 7 e Detailed simulation results at different feed concentration in SCWG of sewage sludge (700 C, 25 MPa). Component
Produced works (MW) Expander Gas turbine Steam turbine Comsumed works (MW) Pumps Compressor Total net power (MW)
Feed concentration (wt%) 10
15
20
25
30
13.43 61.94 6.78
11.18 74.65 8.97
9.84 82.07 13.62
8.95 86.93 19.37
8.30 90.37 24.91
11.70 22.09 48.37
7.53 28.34 58.92
5.57 32.00 67.96
4.50 34.39 76.35
3.85 36.09 83.64
Fig. 9 e Effect of feed concentration on the H2 efficiency, power efficiency, and total efficiency at different SCWG temperatures.
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Fig. 10 e Typical of mass and energy diagram of the integrated system of SCWG and combined cycle power generation.
worth mentioning that operating with low feed concentration (15e25 wt%) is advantageous for both energy self-sufficiency and H2 production.
Energy analysis Fig. 8(aec) shows the effect of feed concentration on H2 yield and net produced power at different SCWG temperature. Although the yield of hydrogen decreased with the increase of feed concentration, more power was produced due to the higher fraction of CH4 in syngas (after H2 separation) was sent to the combined cycle module. For instance, the H2 yield increased from 0.59 kg/100kgfeed to 1.99 kg/100kgfeed, whereas, the net produced power decreased from 90.72 kW/100kgfeed to 62.13 kW/100kgfeed, when the feed concentration (only sewage sludge) increased from 10 wt% to 30 wt%, respectively at 500 C. The increase in temperature then caused an increase in H2 yield, thus resulting in the decrease in the net produced power, as shown in Fig. 8(aec). The detailed simulation results of internal power consumption and power generation at different temperature is presented in Tables 5e7. The internal power consumptions consisted of pumps and compressor work, while the power production was obtained from the expander, steam turbine and gas turbine unit. The pump duty decreased, i.e., approximately from 11 kW/100kgfeed to 4 kW/100kgfeed, with increasing feed concentration. It can be explained by the big amount of water at lower feed concentration which required more energy in the pumping process. The power consumption in this process was mainly dominated by the duty of compressor which was more than 70% of the total power consumption (see Table 6). The duty of compressor was significantly influenced by the feed concentration and SCWG temperature. The higher feed concentration and lower temperature produced more CH4 in the remaining gas (after H2 separator), resulting in the higher compressor duty to supply the air required for complete combustion. Fig. 9(aec) shows the efficiency of the proposed system at different feed concentration (only sewage sludge) and SCWG temperature. Higher temperature and lower feed concentration resulted in higher total efficiency. The decline in total efficiency was mainly caused by the decrease in H2 efficiency as the production of H2 significantly decreased with increasing feed concentration (Fig. 9(a)). On the other hand,
the power efficiency increased when the feed concentration increased which was in line with the power generation, as presented in Fig. 9(b). The total efficiency of 23.31% consisted of H2 efficiency of 12.07% and power efficiency of 11.23% was obtained at 500 C and 10 wt% of feed concentration. Increasing SCWG temperature from 500 C to 700 C increased the total H2 efficiency from 12.07% to 54.73%, which caused an increase in total efficiency from 23.32% to 63.48%. An effect of the increase in feed concentration was apparently observed on the reduction of total efficiency from 63.48% to 32.84% at 700 C. This trend was mainly attributed to the significant decrease in H2 production from 9.06 kg/100kgfeed to 2.93 kg/ 100kgfeed. Given the above results, the typical mass and energy diagram of the integrated system can be presented in Fig. 10. To keep the running of the system in autothermal condition, minimum feed concentration need to be maintained. In some case, an auxiliary fuel i.e., natural gas can be an alternative to achieve autothermal condition. Heat exchanger network may be optimized to recover the thermal energy and to minimize the energy loses. The produced H2 can be fed to Proton Exchange Membrane (PEM) fuel cells to produce electricity. The electricity produced in PEM fuels cells and combined cycle module is party used for internal processes, then the net electricity can be utilized via connection to a power grid.
Conclusion An integrated system consisting of SCWG and combined cycle based power generation has been proposed to utilize sewage sludge effectively for the production of H2 and power generation. The modeling and calculations of the proposed system was done by Aspen Plus V8.8 simulator. Sensitivity analysis has been carried out for different SCWG temperature (500e700 C), feed concentration (10e30 wt%), and lignite coal addition (10e50 wt%). The system can be energetically sustained without lignite coal addition by controlling the minimum feed concentration within the range of 15 wt% to 26 wt% at an operating temperature range of 500 Ce700 C. The utilization of lignite coal enhanced the syngas production and facilitated the system to achieve self-sufficient energy, but did not have effect on the H2 production. When the temperature of SCWG increased from 500 C to 700 C, the yield of syngas and H2 increased from 147.79 kg/100kgfeed and 1.99 kg/
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 4 5 9 e2 4 4 7 0
100kgfeed to 178.08 kg/100kgfeed and 9.06 kg/100kgfeed, respectively. The total efficiency of the integrated system was 63.48% at 700 C and 10 wt% feed concentration. The higher SCWG temperature increased the production of H2 and enhanced the total efficiency, however, an external energy supply was needed to keep the system in autothermal condition. On the other hand, the net produced power and power efficiency increased with increasing feed concentration. The proposed system is expected to be an alternative for energy recovery from sewage sludge.
Acknowledgments The authors acknowledged the financial support provided by the State International Cooperation Project, China (Grant No. 2016YFE0202000 and 2017YFE0107600) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LY17E060005). The authors would like to thank Prof. Agamuthu Pariatamby (University of Malaya, Malaysia) for his guidance during the revision of the paper. The authors also thank Mr. Petric Marc Ruya, for his kind assistance during the simulation.
Nomenclature Symbols SCWG daf db GT ST HE HRSG IGCC LHV PR-BM RMSE Exp Sim
Supercritical water gasification dry ash free dry basis gas turbine steam turbine heat exchanger heat recovery steam generator Integrated gasification with combined cycle Lower heating value Peng-Robinson and Boston-Mathias root mean squared error Experimental data results (mol/molglycerol) Simulated syngas yield (mol/molglycerol)
Greek letters h efficiency (%) · mass flow rate (kg/h) m Subscripts feed feed (mixture of sewage sludge and coal) glycerol glycerol hydrogen H2 in internal power consumption out output power power power total syngas total produced syngas total total
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.210.
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references
[1] Syed-Hassan SSA, Wang Y, Hu S, Su S, Xiang J. Thermochemical processing of sewage sludge to energy and fuel: fundamentals, challenges and considerations. Renew Sustain Energy Rev 2017;80:888e913. [2] Zheng C, Ma X, Yao Z, Chen X. The properties and combustion behaviors of hydrochars derived from cohydrothermal carbonization of sewage sludge and food waste. Bioresour Technol 2019;285:121347. [3] Tyagi VK, Lo S-L. Sludge: a waste or renewable source for energy and resources recovery? Renew Sustain Energy Rev 2013;25:708e28. [4] Safari F, Javani N, Yumurtaci Z. Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts. Int J Hydrogen Energy 2018;43:1071e80. [5] Yan M, Liu J, Hantoko D, Kanchanatip E, Grisdanurak N, Cai Y, et al. Hydrogen-rich syngas production by catalytic cracking of tar in wastewater under supercritical condition. Int J Hydrogen Energy 2019;44:19908e19. [6] Akiya N, Savage PE. Roles of water for chemical reactions in high-temperature water. Chem Rev 2002;102:2725e50. [7] Chen Y, Guo L, Jin H, Yin J, Lu Y, Zhang X. An experimental investigation of sewage sludge gasification in near and super-critical water using a batch reactor. Int J Hydrogen Energy 2013;38:12912e20. [8] Ro KS, Cantrell K, Elliott D, Hunt PG. Catalytic wet gasification of municipal and animal wastes. Ind Eng Chem Res 2007;46:8839e45. [9] Yu J, Tahmasebi A, Han Y, Yin F, Li X. A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization. Fuel Process Technol 2013;106:9e20. [10] Tahmasebi A, Zheng H, Yu J. The influences of moisture on particle ignition behavior of Chinese and Indonesian lignite coals in hot air flow. Fuel Process Technol 2016;153:149e55. [11] Duan F, Chyang C-S, Hsu S-H, Tso J. Combustion behavior and pollutant emissions of batch fluidized bed combustion. J Taiwan Inst Chem Eng 2013;44:1034e8. [12] Cao C, He Y, Wang G, Jin H, Huo Z. Co-gasification of alkaline black liquor and coal in supercritical water at high temperatures (600e750 C). Energy Fuels 2017;31:13585e92. [13] Cao C, Guo L, Yin J, Jin H, Cao W, Jia Y, et al. Supercritical water gasification of coal with waste black liquor as inexpensive additives. Energy Fuels 2015;29:384e91. [14] Chen Z, Gao L, Zhang X, Han W, Li S. High-efficiency power generation system with integrated supercritical water gasification of coal. Energy 2018;159:810e6. [15] Jin H, Chen Y, Ge Z, Liu S, Ren C, Guo L. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40:16096e103. [16] Yu J, Jiang C, Guan Q, Gu J, Ning P, Miao R, et al. Conversion of low-grade coals in sub-and supercritical water: a review. Fuel 2018;217:275e84. [17] Youssef EA, Nakhla G, Charpentier PA. Co-gasification of catechol and starch in supercritical water for hydrogen production. Int J Hydrogen Energy 2012;37:8288e97. rrez Ortiz FJ, Ollero P, Serrera A, Galera S. Process [18] Gutie integration and exergy analysis of the autothermal reforming of glycerol using supercritical water. Energy 2012;42:192e203. [19] Fiori L, Valbusa M, Castello D. Supercritical water gasification of biomass for H2 production: process design. Bioresour Technol 2012;121:139e47.
24470
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 4 5 9 e2 4 4 7 0
rrez Ortiz FJ, Ollero P, Serrera A, Galera S. Optimization [20] Gutie of power and hydrogen production from glycerol by supercritical water reforming. Chem Eng J 2013;218:309e18. [21] Aziz M, Prawisudha P, Prabowo B, Budiman BA. Integration of energy-efficient empty fruit bunch drying with gasification/combined cycle systems. Appl Energy 2015;139:188e95. [22] Darmawan A, Fitrianto AC, Aziz M, Tokimatsu K. Integrated system of rice production and electricity generation. Appl Energy 2018;220:672e80. [23] Darmawan A, Ajiwibowo MW, Yoshikawa K, Aziz M, Tokimatsu K. Energy-efficient recovery of black liquor through gasification and syngas chemical looping. Appl Energy 2018;219:290e8. [24] Sarafraz MM, Jafarian M, Arjomandi M, Nathan GJ. Experimental investigation of the reduction of liquid bismuth oxide with graphite. Fuel Process Technol 2019;188:110e7. [25] Sarafraz MM, Jafarian M, Arjomandi M, Nathan GJ. Potential use of liquid metal oxides for chemical looping gasification: a thermodynamic assessment. Appl Energy 2017;195:702e12. [26] Wan W. An innovative system by integrating the gasification unit with the supercritical water unit to produce clean syngas for solid oxide fuel cell (SOFC): system performance assessment. Int J Hydrogen Energy 2016;41:22698e710. [27] Patcharavorachot Y, Saebea D, Authayanun S, Arpornwichanop A. Hydrogen and power generation from supercritical water reforming of glycerol and pressurized SOFC integrated system: use of different CO2 adsorption process. Int J Hydrogen Energy 2018;43:17821e34. [28] Cao C, Guo L, Jin H, Cao W, Jia Y, Yao X. System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production. Energy 2017;132:238e47. [29] Cao C, He Y, Chen J, Cao W, Jin H. Evaluation of effect of evaporation on supercritical water gasification of black liquor by energy and exergy analysis. Int J Hydrogen Energy 2018;43:13788e97. [30] Safari F, Tavasoli A, Ataei A. Gasification of sugarcane bagasse in supercritical water media for combined hydrogen and power production: a novel approach. Int J Environ Sci Technol 2016;13:2393e400. [31] Chen Z, Zhang X, Han W, Gao L, Li S. Exergy analysis on the process with integrated supercritical water gasification of coal and syngas separation. Appl Therm Eng 2018;128:1003e8. [32] Aziz M. Power generation from algae employing enhanced process integration technology. Chem Eng Res Des 2016;109:297e306. [33] Adnan MA, Susanto H, Binous H, Muraza O, Hossain MM. Feed compositions and gasification potential of several biomasses including a microalgae: a thermodynamic modeling approach. Int J Hydrogen Energy 2017;42:17009e19. [34] Hantoko D, Su H, Yan M, Kanchanatip E, Susanto H, Wang G, et al. Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: effects of operating parameters. Int J Hydrogen Energy 2018;43:17620e32.
[35] Kraisornkachit P, Vivanpatarakij S, Amornraksa S, Simasatitkul L, Assabumrungrat S. Performance evaluation of different combined systems of biochar gasifier, reformer and CO2 capture unit for synthesis gas production. Int J Hydrogen Energy 2016;41:13408e18. [36] Aziz M. Integrated hydrogen production and power generation from microalgae. Int J Hydrogen Energy 2016;41:104e12. € gg MB. CO2 [37] Peters L, Hussain A, Follmann M, Melin T, Ha removal from natural gas by employing amine absorption and membrane technologyda technical and economical analysis. Chem Eng J 2011;172:952e60. [38] Atimtay AT, Topal H. Co-combustion of olive cake with lignite coal in a circulating fluidized bed. Fuel 2004;83:859e67. [39] Sheng C, Azevedo JLT. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenergy 2005;28:499e507. [40] Louw J, Schwarz CE, Knoetze JH, Burger AJ. Thermodynamic modelling of supercritical water gasification: investigating the effect of biomass composition to aid in the selection of appropriate feedstock material. Bioresour Technol 2014;174:11e23. [41] Adnan MA, Hossain MM. Co-gasification of Indonesian coal and microalgae e a thermodynamic study and performance evaluation. Chem Eng Process Process Intensif 2018;128:1e9. [42] Aziz M. Integrated supercritical water gasification and a combined cycle for microalgal utilization. Energy Convers Manag 2015;91:140e8. [43] Darmawan A, Hardi F, Yoshikawa K, Aziz M, Tokimatsu K. Enhanced process integration of black liquor evaporation, gasification, and combined cycle. Appl Energy 2017;204:1035e42. [44] Byrd AJ, Pant KK, Gupta RB. Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. Fuel 2008;87:2956e60. [45] Voll FAP, Rossi CCRS, Silva C, Guirardello R, Souza ROMA, Cabral VF, et al. Thermodynamic analysis of supercritical water gasification of methanol, ethanol, glycerol, glucose and cellulose. Int J Hydrogen Energy 2009;34:9737e44. rrez Ortiz FJ, Ollero P, Serrera A. Thermodynamic [46] Gutie analysis of the autothermal reforming of glycerol using supercritical water. Int J Hydrogen Energy 2011;36:12186e99. [47] Castello D, Fiori L. Supercritical water gasification of biomass: a stoichiometric thermodynamic model. Int J Hydrogen Energy 2015;40:6771e81. [48] Withag JAM, Smeets JR, Bramer EA, Brem G. System model for gasification of biomass model compounds in supercritical water e a thermodynamic analysis. J Supercrit Fluids 2012;61:157e66. [49] Chen Y, Guo L, Cao W, Jin H, Guo S, Zhang X. Hydrogen production by sewage sludge gasification in supercritical water with a fluidized bed reactor. Int J Hydrogen Energy 2013;38:12991e9. rrez Ortiz FJ, Ollero P, Serrera A, Galera S. An energy [50] Gutie and exergy analysis of the supercritical water reforming of glycerol for power production. Int J Hydrogen Energy 2012;37:209e26.