1250
Ind. Eng. Chem. Res. 1999, 38, 1250-1257
KINETICS, CATALYSIS, AND REACTION ENGINEERING Steam Reforming of Gasification Gas Tar over Dolomite with Benzene as a Model Compound Pekka A. Simell,* Elisa K. Hirvensalo, and Visa T. Smolander Technical Research Center of Finland, VTT Energy, P.O. Box 1601, FIN-02044 VTT, Finland
A. Outi I. Krause Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland
Tar decomposition over a dolomite catalyst in gasification conditions was modeled using benzene as a tar model compound. The reactions of the gas main components were included in the models studied. Kinetic studies were carried out at 750-925 °C and under ambient pressure in a plug flow reactor using a mixture of simulated gasification gas. Operation conditions without external or internal mass-transfer limitations were applied. Mechanistic models of the LangmuirHinshelwood type describing benzene decomposition were developed and tested. Experimental results could be best described by a kinetic rate equation based on the assumption that singlesite adsorption of benzene was the rate-determining step and that adsorption of hydrogen inhibited benzene decomposition. Introduction The development of biomass and waste gasification processes based on gas utilization in a gas turbine or in an IC engine offers an environmentally sound way to produce electricity. One of the most crucial steps in the development work is the removal of tars that are always present in gasification gas as a side product and have a tendency to plug downstream process equipment. The use of catalysts at high temperature is technically and economically a very interesting method for gas cleaning. There are extensive literature references available on catalytic tar removal from gasification gas by nickel catalysts1-7 and dolomites.5,8-15 The authors of these papers have shown that almost complete tar decomposition to gaseous components can be achieved at approximately 900 °C temperatures. Carbonate rock catalysts are active only if calcined, and thus it is essential that the partial pressure of CO2 is always lower than the equilibrium decomposition pressure of CaCO3.15,16 This means that in gasification gas (ca. 15 vol % CO2) the operation temperature of the dolomite catalysts has to be above approximately 800 °C at atmospheric pressure. The most important tar-decomposing reaction in gasification gas over dolomite seems to be that with steam according to our previous studies of the subject,17 * Corresponding author. E-mail:
[email protected].
and thus the decomposition reactions can be described similarly as steam reforming reactions of hydrocarbons.17 In gasification gas the presence of reaction products (CO, CO2, H2) can also contribute to the decomposition rate, and their effect should be taken into account. According to steam reforming chemistry the main gas components can also interact via the watergas shift reaction. There is, however, scarce information available about the detailed kinetics of tar decomposition and about reactions of the main gas components over dolomite in gasification gas atmosphere. Espena¨s and Waldheim18 studied naphthalene decomposition kinetics in a gasification gas mixture at 820-880 °C and observed that H2 and H2O were the only main components that affected the reaction rate. A power-law type of rate equation was derived for the two types of dolomite studied. Taralas et al.19 studied steam reforming of n-heptane over dolomites, CaO, and MgO at 700-800 °C. In their paper, hydrocarbon decomposition was assumed to follow first-order kinetics. The present work is a continuation of our previous work where CO2 reforming of benzene as a tar model compound was studied.20 We describe the kinetics of the decomposition reaction of the tar model compound in gasification gas and of the reaction of the main gas components over dolomite in a temperature range relevant to hot gas cleaning conditions. Benzene was used for the tar model compound because it represents
10.1021/ie980646o CCC: $18.00 © 1999 American Chemical Society Published on Web 02/27/1999
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1251 Table 1. Experimental Conditions in the Test Series Performed and Catalyst Weight wcat for Uncalcined Dolomite test series
T (°C)
wcat (g)
vp (dm3n/min)
benzene (ppmv)
H2O (vol %)
H2 (vol %)
CO (vol %)
CO2 (vol %)
1 2 3 4
750-900 800-900 825-925 750-950
0.1-1 0.08-0.41 2-11.8 0.54-10
0.77-2.1 0.52-2.48 0.70-2.84 0.8-2.66
40-400 90-3600 80-1500 830-3400
10 0.1-7.8 10-12.4 1.0-12.7
7.5-10.1 0-12.6
8.8-12.1 0-10.5
8.9-13.6 0-13.4
Table 2. Chemical Composition of the Bed Materials (wt %) dolomite SiC
Ca
Mg
Fe
Ti
Mn
Al
Si
P
Ba
Na
K
O + res
20.9
12.8
