Article pubs.acs.org/EF
High-Pressure Gasification of Coal Water Ethanol Slurry in an Entrained Flow Gasifier for Bioethanol Application Jong-Soo Bae,†,‡ Dong-Wook Lee,† Se-Joon Park,† Young-Joo Lee,† Jai-Chang Hong,† Ho Won Ra,† Choon Han,‡ and Young-Chan Choi*,† †
Clean Fuel Department, High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research (KIER), 71-2, Jang-dong, Yuseong, Daejeon 305-343, Republic of Korea ‡ Department of Chemical Engineering, Kwangwoon University, 447-1, Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea ABSTRACT: In this study, the gasification performance of coal water ethanol slurry (CWES) was estimated in a pilot-scale entrained flow gasifier. CWES was prepared by adding ethanol into coal water slurry (CWS) to estimate the gasification performance of bioethanol-contained coal water slurry. In comparison to CWS gasification, gasification with CWES showed a higher performance, such as composition of hydrogen and carbon monoxide, carbon conversion, cold gas efficiency, and total flow rate of syngas. In general, the gasification performance is directly associated with the carbon content of CWS, and it is very difficult to increase the carbon content of CWS while maintaining slurry viscosity to the atomizable level (2000 cP). However, in the case of CWES, the carbon content of slurry can be readily improved and the viscosity of slurry can be reduced at the same time. Thus, the improved carbon content of CWES led to a higher gasification performance than that of CWS. Moreover, if bioethanol is actually used for the preparation of CWES, carbon dioxide emission from gasification could be considerably diminished.
1. INTRODUCTION Despite growing interest globally in new renewable energy resources as potential replacements for fossil fuels, most energy used at present is still derived from fossil fuels, such as petroleum and coal. Petroleum offers relatively convenient usage compared to other energy sources, but it has problems of an unstable supply because of the finite nature of oil resources and geographical maldistribution and, furthermore, has tended to show significant price hikes. On the other hand, coal, with relatively rich reserves and an even distribution around the world, has been viewed as an alternative to petroleum.1−3 As an energy resource, coal is mostly used for thermal power generation. Recently, the coal gasification process, as used in the integrated gasification combined cycle (IGCC), has been receiving much attention as a way to increase energy efficiency and reduce the emissions of harmful substances, such as NOx and SOx.4,5 Coal gasification, one of the core technologies of IGCC, produces syngas that mostly consists of hydrogen and carbon monoxide, which can be turned into synthetic oil, synthetic natural gas (SNG), dimethyl ether (DME), and chemicals by adjusting the composition of hydrogen and carbon monoxide in the syngas.6−9 Coal gasifiers are classified into fixed-bed, fluidized-bed, and entrained flow gasifiers. Entrained flow gasifiers can also be classified into dry-coal gasification, such as Shell, Prenflo, etc., and coal slurry gasification, such as Texaco, E-gas, etc.10−12 The main reactions of coal gasification are shown in eqs 1−7. The exothermic reactions shown in eqs 1−3 provide the heat for the endothermic reactions shown in eqs 4 and 6.13
C(s) + O2 (g) → CO2 (g)
(2)
CO(g) + 1/2O2 (g) → CO2 (g) ΔH °298 K = −283.0 kJ/mol
© 2012 American Chemical Society
(3)
C(s) + H 2O(g) → CO(g) + H 2(g) ΔH °298 K = 131.3 kJ/mol
(4)
CO(g) + H 2O(g) → CO2 (g) + H 2(g) ΔH °298 K = −41.1 kJ/mol
C(s) + CO2 (g) → 2CO(g)
(5)
ΔH °298 K = 172.4 kJ/mol (6)
C(s) + 2H 2(g) → CH 2(g)
ΔH °298 K = −74.9 kJ/mol (7)
In general, the gasification performance is directly associated with the carbon content of coal water slurry (CWS), and it is very difficult to increase the carbon content of CWS while maintaining slurry viscosity to the atomizable level (2000 cP). To prepare CWS with high coal content, Zhou et al. increased the hydrophobicity of coal by coating coal particles with chemicals.14 Wu et al. prepared slurry by blending different kinds of coal.15 Other groups have improved the viscosity using different solvents in the preparation of CWS. Notably, Shen et al. prepared high-concentration slurry through high extraction
C(s) + 1/2O2 (g) → CO(g) ΔH °298 K = −110.6 kJ/mol
ΔH °298 K = −393.5 kJ/mol
Received: June 27, 2012 Revised: August 10, 2012 Published: August 10, 2012
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dx.doi.org/10.1021/ef301079z | Energy Fuels 2012, 26, 6033−6039
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Article
efficiency and swelling of coal particles.16 Svoboda et al. also prepared coal oil-in-water slurry (COWS) and evaluated the characteristics of gasification in the fluidized bed.17 In our previous study, coal water alcohol slurry (CWAS) was prepared with six kinds of alcohol additives, such as methanol, ethanol, isopropanol, 1-pentanol, 1-hexanol, and 1-octanol. As a result, in the case of preparing CWAS through the use of ethanol, a relatively hydrophilic alcohol, as an additive, there was a reduction of viscosity in CWAS as well as an increase in the heating value with the increase of the amount of the additive.18 In this study, the coal water ethanol slurry (CWES) was used as a fuel for a pilot-scale entrained flow gasifier (coal feeding rate of 1 ton/day).
2. EXPERIMENTAL SECTION 2.1. Preparation of CWS and CWES. Shenhua coal (bituminous coal) provided by the Shenhua Group Corporation of China, was used for the coal in this study. All of the coal was ground into tiny particles of 75 μm or less to prepare CWS and CWES. A proximate analysis of coal was carried out using TGA-701 (LECO), and an elemental analysis was conducted using a TruSpec elemental analyzer (LECO) and a SC-432DR sulfur analyzer (LECO). A Parr 6320EF calorimeter (PARR) was employed to measure higher heating values. The qualitative features of coal in these analyses are shown in Table 1.
Figure 1. Schematic diagram of an entrained flow gasifier at Korea Institute of Energy Research (KIER). castable designed to be used at an operating temperature of 1800 °C and at 25 bar. A slurry-injecting burner and a liquefied petroleum gas (LPG) preheating burner were installed at the upper part of the gasifier, and the temperature was measured at three regions inside the gasifier and the castable. The slag generated after the gasification reaction was removed by an ash lock hopper at the lower part. The syngas was cooled by quenching water at the lower part of the gasifier. The syngas was then passed through a refiner and then sent to a flare stack after analyzing its flow and other characteristics.
Table 1. Proximate and Ultimate Analyses of Raw Coal feed coal seam source proximate analysisa (wt %) moisture volatile matter ash fixed carbon ultimate analysisb (wt %) C H N O S high heating value (kcal/kg) a
Shenhua coal Shenhua group corporation
3. RESULTS AND DISCUSSION 3.1. Temperature and Pressure Profiles of CWS and CWES. The temperature of the gasifier is one of the most important factors in coal slurry gasification, because it is determined by a combination of endothermic and exothermic reactions in the chain reactions of gasification. In other words, the temperature of the gasifier directly influences the flow and the composition of syngas. Also, the pressure in the gasifier can be automatically controlled by the pressure control valve (between dust filter and scrubber). That is, the stable pressure of the gasifier means that the syngas flow rate is nearly constant without fluctuation. Therefore, monitoring the temperature and pressure during operation of the gasifier is a very important task, because they reflect the overall gasification performance. The pressure and temperature profiles in the gasification of CWS and CWES are shown in Figures 2 and 3, and operating conditions of the gasification are shown in Table 2. The gasifier was preheated to approximately 1100 °C using a LPG burner, and after feeding CWS or CWES into the gasifier, syngas starts to be produced, whereby pressurization proceeds. Isobaric operation of the gasifier at 20 bar was conducted using an automated pressure control valve. In the case of CWS gasification, it took approximately 110 min for gasifier pressure to reach 20 bar, whereas, for CWES gasification, it took 65 min for gasifier pressure to reach 20 bar. CWS and CWES gasification was operated for 170 and 500 min, respectively. As shown in the profiles of pressure and temperature, in the case of CWS gasification, stabilization at 20 bar took more time than CWES gasification, and the temperature and pressure profiles of CWES gasification were much more stable than those of CWS gasification. 3.2. Comparison of the Gasification Performance between CWS and CWES. The gasification performance of CWS with a coal content of 54 wt % was evaluated in a pilotscale entrained flow gasifier. Figure 4 shows the slurry feeding
12.83 27.65 8.73 50.79 82.56 4.52 0.91 11.88 0.13 6880
As-received basis. bDry and ash-free basis.
As for the preparation of CWS and CWES, viscosity of 2000 cP or under was maintained while maintaining the maximum coal content, so that the coal particles could be atomized when the slurry was injected into the burner.19 The viscosity of CWS and CWES were measured using a viscometer (TVC-5, Toki Sangyo Co.) at a temperature of 20 °C and a rotor rotation rate of 20 rpm. CWS was prepared with the same coal content of 54 wt %, as in the previous studies,18 with the addition of naphthalenesulfonic acid (PC1000) as a dispersant and NaOH as an electrolyte in rates of 1 and 0.1 wt % of coal, respectively, with the purpose of stabilizing the rheological behavior of slurry.20 In the case of CWES, a slurry with a high concentration and high heating value was prepared using ethanol as an additive, with a coal content of 57 wt % and with the addition of PC1000 and NaOH in the same rates as CWS. In this case, ethanol was added in a rate of 7 wt % of the coal concentration to maintain the viscosity of the slurry below 2000 cP. 2.2. Experimental Apparatus. For the gasification processes of CWS and CWES (Figure 1), a coal slurry manufacturing device, gasifier, gas refiner (such as scrubber and dust filter), automatic control system, and syngas analysis system (the details of which were described in the previous study) were used.21 The gasifier has an internal diameter of 250 mm and a height of 1400 mm and an external 6034
dx.doi.org/10.1021/ef301079z | Energy Fuels 2012, 26, 6033−6039
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Figure 3. Temperature of the reactor and castable: (a) CWS and (b) CWES gasifier.
Figure 2. Pressure change of (a) CWS and (b) CWES gasifier.
Table 2. Operation Conditions of CWS and CWES Gasification
rate and the O2/coal ratio according to operating time. Under atmospheric pressure, CWS was injected at a rate of 69.85 kg/h and with an O2/coal ratio of 1.03. The gasifier was pressurized to 20 bar by gradually closing the pressure control valve at the back end of the gasifier and increasing the CWS feeding rate up to 87.22 kg/h and the O2/coal ratio up to 1.20. After the gasifier pressure reached 20 bar, the pressure control valve was automatically operated to maintain 20 bar of the gasifier pressure. Figure 5 shows the syngas flow rate with changing the slurry feeding rate and the pressure. The syngas production rate was 37 N m3/h under atmospheric pressure. With an increase in the pressure of the gasifier and the slurry feeding rate, the average syngas flow rate was increased up to 53 N m3/h at 20 bar. In particular, when the pressure of 20 bar was reached at a reaction time of approximately 120 min, the syngas flow rate fluctuated severely. This is likely because the syngas production rate was temporarily increased as the pressure of the gasifier rose to 22 bar at around 100 min and then decreased because of the automated pressure control at 20 bar. When the pressure of the gasifier was relatively stabilized after 130 min, however, the syngas flow rate fluctuated again because of the increase in the temperature inside the gasifier at the same time, as shown in Figure 3a. As shown in Figure 6, when the temperature in the gasifier rose suddenly, the concentration of carbon dioxide in the syngas rose abruptly at around 130 min, thereby decreasing the heating value of syngas, as shown in Figure 7. From these
CWS
CWES
1 0.1
