An Overview of Exhaust Emissions Regulatory Requirements and Control Technology for Stationary Natural Gas Engines

Abstract

Abstract A practical overview of stationary natural gas engine exhaust emissions control technology and trends in emissions regulatory requirements is presented. Selective and non-selective catalytic reduction and lean burn technologies are compared. Particular emphasis is focused on implications of the Clean Air Act of 1990. Recent emissions reduction conversion kit developments and a practical approach to continuous monitoring are discussed. Introduction Measurement and control of stationary gas engine exhaust emissions began more than a decade ago when various agencies began proposing new regulations. Over that time, allowable emission levels have steadily declined around the world as manufacturers have developed new control technologies. Recently legislated requirements are forcing compliance with emissions levels that are frequently below the capability of commercially viable technology. The prechambered lean burn engine and catalytic reduction have emerged and remain as the dominant emission control technologies. This paper reviews current engine exhaust emissions requirements in the United States as well as in other developed countries. Particular emphasis is placed on implications of the Clean Air Act of 1990, passed by the U.S. Congress in November 1990. It also compares best available control technologies (BACT) for controlling exhaust emissions from two and four cycle engines, including lean burn combustion, selective and non-selective catalytic reduction, continuous emissions monitoring and best available retrofit control technology (BARCT). Specific examples are based on engines manufactured by the authors' company under the Ajax and Superior trademarks. EMISSIONS REGULATIONS - A WORLDWIDE PERSPECTIVE Historical U.S. Requirements In the late 1970's the United States Environmental Protection Agency (EPA) proposed New Source Performance Standards (NSPS) for stationary engines. Although NSPS were never enacted, the required emission levels were effectively superseded by various provisions of the Clean Air Act of 1977. In accordance with these regulations, state and local agencies require a stringent permitting process for new or modified engine installations. This process requires the use of best available control technology (BACT). This approach has proven to be a highly flexible and effective method of requiring the lowest emission levels available anywhere. Before regulation, typical NOx emissions for stationary natural gas engines ranged from 10 to 20 gm/BHP-hr (13 to 27 gm/kW-hr). The proposed 1970's NSPS for stationary engines would have required a NOx level of approximately 9.0 gm/BHP-hr (12 gm/kW-hr). P. 125^