Development of the new NO{sub}x reduction catalyst with hydrocarbons and the cleanup systems of exhaust gases for lean-burn gas engines

摘要

Development of cleanup technology for combustion waste is more and more necessary today. The emissions of stationary natural-gas-fueled engines can be purified by in-engine methods and by treatment of the exhaust gases. This paper describes the latter technologies. There are two conventional technologies for nitrogen oxides (NO{sub}x) reduction methods by the three-way catalyst and the selective catalytic reduction (SCR). The three-way catalyst operates only well within a narrow air-to-fuel ratio window, but when the exhaust gas is too lean, the NO{sub}x will not be removed. The SCR of NO{sub}x in exhaust gas has the advantage that the engine process itself does not have to be adapted and closely controlled as in case of extended lean-burn technologies. Ammonia or urea injected into the exhaust gas must be used as the reducing agent with conventional SCR system. However, the addition of a SCR system for the small or middle size cogeneration system, it would pose problems regarding cost and space for storage and injection of the reducing agent. Therefore, we have examining the catalyst which is able to reduce NO{sub}x with hydrocarbons (HC) containing in exhaust gas itself, and we developed the new HC-SCR catalyst. The de-NO{sub}x system using the HC-SCR catalyst has numerous advantages as follows: 1) Compactness and low cost: catalyst unit only 2) High efficiency: adaptation of lean-bum technology for engine operation 3) High performance: excellent catalytic activity and durability 4) Safeties: no use of ammonia as a reducing agent In this study, we report the catalytic activity of the new catalyst and propose a total cleanup system for exhaust gases of lean-burn gas engines. Alumina-supported silver (Ag/Al{sub}2O{sub}3) was used for the new catalyst. When the measurement was carried out with one reducing gas in the reactant gas each, propane and propane were most effective for NO{sub}x reduction, ethane and ethylene were secondly effective for NO{sub}x reduction. Practical test of the Ag/Al{sub}2O{sub}3 catalyst was carried out using a real exhaust gas from a 400 kW class lean-bum gas engine. The full-size catalyst was obtained by washcoating the catalyst powder on a metallic monolithic honeycomb substrate (size: 650 mmΦ × 324 mmL, 200 cells/inch{sup}2). When the engine was operated at 400 kWe output, temperature of the exhaust gas was 762 K and GHSV was 17635 h{sup}(-1). The NO{sub}x conversion was reached to 30 % and the catalytic activity was maintained after the operation for more than 2000 hr. Conventional alumina-supported platinum (Pt/Al{sub}2O{sub}3) catalysts were mounted to exhaust gas line for cleanup test. The emission of CO and aldehydes was in the exhaust gas, but it could be highly removed by the Pt/Al{sub}2O{sub}3 catalyst. Practical tests of this catalyst were carried out using 300 - 400 kW class lean-burn gas engines. GHSV of these catalysts were about 50,000 h{sup}(-1). The CO and aldehydes conversion were reached to more than 90 % and the catalytic activities were maintained after the operation for about 10,000 hr.