Ind. Eng. Chem. Res. 1999, 38, 4175-4182
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Kinetics of Ammonia Decomposition in Hot Gas Cleaning Wuyin Wang, Nader Padban, Zhicheng Ye, Arne Andersson, and Ingemar Bjerle* Department of Chemical Engineering II, Lund University, P.O. Box 124, 221 00 Lund, Sweden
Reduction in the amount of ammonia in fuel gas from biomass gasification was studied. Experiments were carried out in a fixed-bed reactor at 200-1000 °C, 21 atm. A kinetic model for ammonia decomposition was developed. The partial pressure of hydrogen in the fuel gas was a key factor to model ammonia decomposition. Activation energies in the empty reactor, on carbon, and in a sand bed were similar, 130-140 kJ/mol. The frequency factors for carbon and sand were 10 times as large as for the empty reactor. The activation energy for a Ni-based catalyst was 111-113 kJ/mol. Carbon deposit deactivated the Ni-based catalyst. High temperature was found to be essential for avoiding carbon fouling and for achieving high ammonia removal efficiency. Estimation of the ammonia reduction for fuel gas showed that a moderate amount of ammonia could be removed by use of the Ni-based pellets at 800 °C. Introduction There has been increasing interest recently in the simplified integrated gasification combined-cycle (IGCC), which uses pressurized air-blown gasification and hotgas cleanup. This process was reported to be capable of providing high power-generation efficiency, high powerto-heat ratios for co-generation, and excellent environmental performance.1,2 Hot gas cleaning, which purifies the fuel gas produced at high temperature and high pressure by the gasification process, is essential for maintaining a high level of efficiency in the IGCC systems.3-7 One major problem in connection with this process is the formation of the NOx precursors, ammonia and hydrogen cyanide. NH3 and HCN produced from fuelbound nitrogen (fuel-N) during gasification can be converted to nitrogen oxides (NOx) through gas turbine combustion.3,5,6,8-10 Removing ammonia at high temperature before combustion has been investigated as one major alternative to that of combustion chamber modification for minimizing the NOx emission. Usually much more fuel-N is converted to ammonia than to hydrogen cyanide. According to Mojtahedi et al.,6 20-30% of the fuel-N in coal is converted to ammonia and to a lesser extent to HCN, whereas in biomass gasification 60-80% of the fuel-N can be converted to ammonia. Chambers et al.7 observed that the concentration of HCN is typically less than 1/10 that of NH3. In gasification, the conversion of fuel-N to ammonia is significantly higher in a pressurized process, a fluidized-bed gasifier, or at low freeboard temperatures, than in an atmospheric process, an updraft gasifier, or at high freeboard temperatures.5,8,9 The amount of ammonia in the product gas also depends on the fuel type and its fuel-N content. In gasifying peat, brown coal, and wood sawdust in a pressurized fluidized-bed gasifier, Leppa¨lahti et al.8,9 found 6000-9000 ppm, 2500-2800 ppm, and 330-450 ppm ammonia, respectively, to be formed. However, in a 15 MW(Th) pilotscale plant the measured value for ammonia was found to be 700-1500 ppm in gasification of coal but up to * Corresponding author. Tel.: 00 46 46 2228268; fax: 00 46 46 2228268; e-mail:
[email protected].
3000 ppm in biomass tests.6 In any case, high ammonia levels have been recorded in the fuel gas produced by gasification. As the obtaining of “energy from waste” becomes increasingly emphasized, use of waste with high fuel-N content can be introduced, so as to produce still higher ammonia levels in the fuel gas. In the gas turbine as much as 50% of the ammonia in the fuel gas can be converted to NOx when the gas is combusted to produce power. The thermal NOx generated in the gas turbine when operating below 1400 °C is less than 30 ppm.6 When coal gas is used, from 17 to 32% of the NH3 in the fuel gas can be converted to NOx even with use of low NOx combustion technology.7 At an NH3 content of less than 300 ppm, the coal gas can be burned so as to produce NOx emissions of less than 25 ppm at 16 mol % O2.7 However, in commercially operated IGCC plants, which are not existing yet, the emission levels are expected to be higher. Environmental regulations concerning NOx emissions have become increasingly stringent. In Europe, 50 mg NOx/MJ (50 ppm) has been set as the limit for new plants, and in the US equally stringent limits have been imposed.3,6 The catalytic decomposition of ammonia is commonly investigated regarding the extent to which the requirements for hot gas cleaning are met. Various materials were tested at high temperature for different gases. At above 800 °C in an inert atmosphere, Chambers et al.7 observed that CaO enhanced NH3 decomposition. The impact decreases with increasing pressure and at exposure to an H2-CO-CO2 atmosphere. At 900 °C a significant reduction of ammonia, with around 50% conversion, was reported by Mojtahedi and Abbasian3 on inert alumina for a simulated fuel gas, and by Simell et al.4 on SiC and dolomite for the product gas from a fluidized-bed gasifier. Leppa¨lahti et al.5 found silicon carbide, limestone, and dolomite to have no capacity to decompose ammonia in a product gas from an updraft gasifier. On the contrary, these materials increased the ammonia level by converting the tar-containing nitrogen to ammonia. Catalysts containing Fe, Co, Ru, Ir, and Ni have been shown to have high catalytic activity.10 At high temperatures ferrous materials such as iron sinter, iron
10.1021/ie990337d CCC: $18.00 © 1999 American Chemical Society Published on Web 10/13/1999
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Figure 1. Schematic diagram of the bench-scale HPFB.
pellets, and ferrous dolomite catalyze the reaction.4,5 A fairly low iron content is sufficient to create a catalytic effect. Nickel-based catalysts are those most applicable here. Their efficiency increases with the Ni content.3,4 The H2O/CO2 reforming of hydrocarbons and ammonia decomposition are both catalyzed, according to Hepola and Simell,10 by nickel catalysts on which tar, ammonia, and methane compete for the same active nickel sites. In addition to Ni-based catalysts, Mo-based catalysts are effective at 790 °C,3 although molybdenum vaporization may limit their use. Ru-based catalysts, although subject to sulfur poisoning, are likewise very effective.3 Considerable attention has been directed at the deactivation of Ni-based catalysts through sulfur poisoning. At less than 900 °C, NiS can form, rendering the catalysts inactive through reducing the active surface area.6 Compared with a nearly complete deactivation of catalysts by H2S at 550 °C, above 800 °C, much weaker deactivation was observed by Krishnan et al.11 Some types of Ni-based catalysts even showed negligible deactivation. To prevent the sulfur poisoning of Ni-based catalysts, the catalytic process should operate at as high a temperature as possible in practice.10 At high pressures the H2S effect is more severe.6 In pressurized tests, carbon deposit on the catalyst bed was found after use for 2-3 days.12 A dense layer of fine pyrolytic carbon (
