INTEGRATED COAL PYROLYSIS WITH CO2 REFORMING OF METHANE
Jiahe Liu, Haoquan Hu, Lijun Jin, Pengfei Wang
State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 129 street, Dalian116012, P.R. China
Abstract: In our previous work, a novel integrated process of coal pyrolysis and CO2 reforming of methane for improving tar yield was put forward and the tar yield could be about 1.6 and 1.8 times as that in coal pyrolysis under H2 and N2, respectively. In this paper, the effect of CO2/CH4 ratio, CH4 flow rate, holding time on tar, water yield and CH4 conversion, and the effect of pyrolysis temperature on char yield and gas composition were investigated in an atmospheric fixed-bed reactor containing upper catalyst layer and nether coal layer for the integrated process. The results indicated that the tar and water yield increases with the increase of CO2/CH4 ratio, CH4 flow rate, holding time, respectively. The highest tar yield was obtained at 1:1 CO2/CH4 ratio, 400 ml/min CH4 flow rate, 30 min holding time. And the char yield，decreasing with increasing pyrolysis temperature, is higher than that under H2 and N2 at the same pyrolysis temperature. Keywords: Coal; pyrolysis; CO2 reforming of methane; tar; coal char
1. INTRODUCTION Hydropyrolysis can improve tar yield and quality effectively, compared with coal pyrolysis under N2 atmosphere, but it is handicapped by the high cost of hydrogen. Therefore the use of methane as a low-cost hydrogen substitute in hydropyrolysis reactions could make the coal conversion to liquid fuel competitively, especially at the time with high petroleum price. Maier et al. (1994) investigated bituminous coal pyrolysis under CH4, H2, and N2 with and without N2O at temperatures range from 580 to 750 C in the presence of lithium oxide as catalyst. A relatively increase in alkylated aromatics and condensable liquids in the presence of CH4 indicated the reaction of coal with CH4. Smith et al. (1989) studied the reaction of IBCSP coal No.5 being treated with various gas mixtures in the microbalance reactor. The results showed that weight loss was larger when the coal sample was treated with CH4/NO and CH4/O2, and the yield of C2 and C3 hydrocarbons was higher than that under other atmospheres. Liu et al (2005) reported an integrated process of coal pyrolysis with
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catalytic partial oxidation of methane for coal pyrolysis with methane. When Yanzhou and Datong coal pyrolysis was carried out in stream of mixture gas CH4/O2 (4:1) over Ni/Al2O3 catalyst instead of same amount of H2 at 2 MPa and 650 or 700 oC, the tar yield increases about 70% and 125%, respectively. In our previous studies, integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalysts was carried out, and the tar yield was 1.6 times and 1.8 times as that in pryolysis under H2 and N2 when Pingshuo coal pyrolysis was performed at 750 C (Liu et al., 2008). In this work, the effect of CO2/CH4 ratio, CH4 flow rate, holding time on tar yield, water yield, and CH4 conversion was studied in detail. Meanwhile, the effect of pyrolysis temperature on char yield and gas composition was also investigated. 2. EXPERIMENTAL 2.1 Coal Sample. A Chinese bituminous coal, Pingshuo (PS) coal, with particle size of -100 mesh, was used in the experiments. The proximate and ultimate analyses of coal sample were shown in Table 1. Table 1 The proximate and ultimate analyses of Pingshuo coal sample Proximate analysis/wt.%
Ultimate analysis/wt.%,daf
Mad
Ad
Vdaf
C
2.23
17.93
37.19
80.41
H 5.20
O*
N
S
11.95
1.38
1.06
* by difference 2.2 Coal Pyrolysis. CH4
CO2
Quartz wool
Catalyst Coal bed Outlet
Fig. 1. Configuration of the fixed-bed reactor The experiments of coal pyrolysis were performed in an atmospheric fixed-bed reactor as shown in Figure 1. The reactor contains upper catalyst layer and nether coal layer. About 1 g catalyst and 5 g coal were placed in the two layers separately. The gas mixture of CH4 and CO2 with a volume ratio of 0.25-1.25 was fed into the reactor from the upside, and CH4 flow rate was fixed at 400 ml/min. The reactor was heated to a desired temperature (500-800 C) within 10 min and held at the temperature for 0-30 min. Details of products collection can be found elsewhere (Hu et al., 2004), and CH4 conversion was defined as: CH4 conversion (%) = (Ftotal,inCin-Ftotal,outCout)/Ftotal,inCin×100% where Ftotal,in and Ftotal,out represent the inlet and outlet flow rate of the total gas, Cin and Cout represent the inlet
and outlet concentration of CH4 in the total gas, respectively. 2.3 Catalyst Preparation. The Ni/MgO catalyst was prepared by incipient wetness impregnation method. The Ni loading in the catalysts were 10 wt%. After being dried at 120 C for 24 h and calcined in air at 800 C for 4 h, the catalyst powder was pressed, crushed and sieved to 20-40 mesh. Finally, the catalysts were reduced in H2 at 850 C for 4 h before use. 3. RESULTS AND DISCUSSION 3.1 Effect of CO2/CH4 ratio. Figure 2 shows the tar, water yield and CH4 conversion in pyrolysis of PS coal under CH4/CO2 at different CO2/CH4 ratio. With increase CO2/CH4 ratio in a range of
