Performance evaluation of multiple oxidation catalysts on a lean burn natural gas engine

Abstract

Emission from lean burn natural gas engines used for power generation and gas compression are major contributors to air pollution. Two-way catalysts or oxidation catalysts are the common after-treatment systems used on lean burn natural gas engines to reduce CO, VOCs and formaldehyde emissions. The performance of the oxidation catalysts is dependent on operating parameters like catalyst temperature and space velocity. For this study, a part of the exhaust from a Waukesha VGF-18 GL lean burn natural gas engine was flowed through a catalyst slipstream system to access the performance of the oxidation catalysts. The slipstream is used to reduce the size of the catalysts and to allow precise control of temperature and space velocity. Analyzers used include Rosemount 5-gas emissions bench, Nicolet Fourier Transform Infra-Red spectrometer and HP 5890 Series II Gas Chromatograph. The oxidation catalysts were degreened at 1200°F (650°C) for 24 hours prior to performance testing. The conversion efficiencies for the emission species varied among the oxidation catalysts tested from different vendors. Therefore, the performance of all the oxidation catalysts is not the same for this application. Most oxidation catalysts showed over 90% maximum conversion efficiencies on CO, VOCs and formaldehyde. Saturated hydrocarbons such as propane were difficult to oxidize in an oxidation catalyst due to high activation energy. High VOC oxidation was noticed on all catalysts, with maximum conversion efficiency at 80%. VOC reduction efficiency was limited by propane emission in the exhaust for the catalyst temperatures tested. Additional formulations need to be developed for oxidation catalysts to increase VOC reduction efficiency. Oxidation of NO to NO2 was observed on most oxidation catalysts; this reaction is favored based on chemical equilibrium. Variation in space velocity showed very little effect on the conversion efficiencies. Most species showed over 90% conversion efficiency during the space velocity sweep. The oxidation catalysts showed increasing CH2O conversion efficiency with decreasing space velocity. No change on performance of the oxidation catalysts on conversion of emission species was noticed for varying space velocities after conversion efficiencies reached 90%. Thus, adding more catalyst volume may not increase the reduction efficiency of emission species. Varying cell density showed very little effect on performance of the oxidation catalysts. The friction factor correlation showed the friction factor is inversely proportional to cell density.