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Show 1.0 INTRODUCTION The 1990 Clean Air Act Amendments (CAAA) require retrofit N O x control for existing combustion sources in ozone nonattainment areas, to the extent that the U.S. E P A and individual states determine what N O x reductions are needed for attainment of the National Ambient Air Quality Standards (NAAQS). The states must prepare attainment plans that include specific source control retrofit programs and milestones to reduce ozone levels in all nonattainment areas. Natural gas prime movers (gas-fueled turbines and both rich- and lean-burn reciprocating internal combustion engines) used in the compression, transmission, and storage of natural gas can be significantly impacted by this regulatory process. This is because N O x emission levels from many existing uncontrolled reciprocating engines and turbines qualify them as "major" sources. In addition, emissions from new and modified prime movers trigger new source review (NSR) requirements in ozone nonattainment areas; the trigger levels vary depending upon the classification of each nonattainment area. 2.0 PRIME MOVER EQUIPMENT TYPES AND INVENTORY The natural gas industry uses a variety of prime movers in natural gas compression, transmission, and storage facilities. The power capacity of these units ranges from less than 300 bhp to greater than 30,000 bhp. The smaller units, those below 1000 bhp, are typically used to generate power for general use and emergency equipment and for gas gathering. Figure 1 illustrates the current inventory of natural gas prime movers. Best estimates indicate that there are nearly 7,600 prime movers in operation. Of these, approximately 1,150 are gas turbines and the rest are reciprocating internal combustion engines. Engines are further distinguished between rich and lean burn reciprocating engines. Rich-burn engines are characterized by near-stoichiometric combustion; i.e. the engine exhaust has little or no excess oxygen (less than 1 percent) because fuel is burned with a theoretical amount of air. Lean-fuel engines operate with medium to high levels of excess air in the exhaust. The inventory clearly indicates that lean-burn reciprocating engines account for the majority of all prime movers. Reciprocating engines include nearly 350 models spanning over 50 years of design. Gas turbines were introduced by in the mid '50s and now include nearly 60 different models. Tables 1 through 4 list the principal models of gas turbines and reciprocating engines, respectively. Also shown are average N O x emission levels. Total N O x emissions from all prime movers are estimated to be approximately 1.1 million tons per year and are distributed among the major equipment categories as shown in Figure 1(c). Several engine models, particularly reciprocating engines, have been in services for many years. In fact, thirty percent of all prime movers are older than 30 years (Reference 1). Their long duty service is the result of a regimented maintenance program and the high cost of engine replacement. The total horsepower is estimated to be about 20 million and is nearly divided between gas turbines and reciprocating engines. Rich-burn reciprocating engines account for less than 10 percent of the total horsepower. Nearly all prime movers operate with little or no supervision and mostly in remote locations. Because of variability in natural gas demand, many prime movers experience variability in load that can make application of certain retrofit N O x controls particularly difficult. 2 1-4 |
