ADDRESSING HAZARDOUS AIR POLLUTANT AND ODOR ISSUES FOR NATURAL GAS FIRED LEAN BURN ENGINES

With the incorporation of oxidation catalysts for the control of carbon monoxide emissions from natural gas-fired internal combustion engines, the profile of odorous compounds in the exhaust can change. Specifically, nitric oxide emissions may be converted to nitrogen dioxide emissions, which have a much lower odor threshold and, hence, a greater potential for odor impacts off-site. Recenthealth riskbased regulations in California are targeting other trace level hazardous air pollutant(HAP) emissions from natural gas combustion sources, such as acrolein and acetaldehyde. While catalytic control technologies can also be effective for the control of these HAPs, the potential for downwind impacts of odors from the engine exhaust remains a problem. This case study of several large, natural gas-fired, lean burn engines located in an upscale residential community reflects thechallenges of controlling criteria pollutants and HAP emissions, while not exacerbating community odor complaints. Emission testing for odors, HAPs, and criteria pollutants (CO and NO-NO2) was performed under various engine operating conditions and with various catalyst bed space velocities. Acorollary reactor was installed on a slipstream of one engine's exhaust to evaluate theeffectiveness of different catalysts and space velocities. Removal efficiencies of the HAPs with variouscatalyst bed depths were correlated with NO/NO2 ratios and resulting odor concentrations. The factthatthe odor concentration of the exhaust often increased with better oxidation catalyst control led to the evaluation of other control scenarios such as a SCR/oxidation catalystcombination and increased stack height for better dispersion. Strobic Air™ fans were investigatedfor providingboth dilution and a significant increase in “effective” stack height without physical construction of tall stacks. Screening level health risk assessments were performed coupled with dispersion modeling ofodor impacts to the surrounding community. A combined approach of optimumcatalytic control,dilution, and higher “effective” exhaust stack release was demonstrated to be the most cost-effective solution.