OCR Text |
Show Selective Catalytic Reduction (SCR) S C R is an exhaust gas treatment control technology that relies on the property of ammonia to selectively reduce N O x emissions over a catalyst at gas temperatures in the range of about 450 to 1,100 °F. The commercially available technology is capable of N O x reductions in excess of 90 percent, depending on the application and fuel. Experience of S C R on pipeline prime movers is minimal. Early demonstrations of the technology on lean-burn reciprocating engines uncovered significant technical obstacles to performance control with load change and catalyst durability. Many of these early systems have been removed. Today, S C R systems are found primarily on new large gas base-loaded turbines such as those found in cogeneration plants in some states, operating in combination with water or steam injection to achieve controlled N O x emissions as low as 3.75 ppm @ 15% 02. Although catalyst technology has advanced significantly in recent years, SCR remains a high-cost and often impractical option for pipeline prime movers, either gas turbines or lean-burn reciprocating engines. Besides cost and increased maintenance requirements, the principal drawbacks are primarily technical. With variable-load prime movers, S C R catalysts are subject to significant temperature transients that can dramatically affect reagent utilization, N O x reduction efficiency, N H 3 emissions, and catalyst life. Recent experience on pipeline reciprocating engines has not been favorable, indicating that further advances in catalyst formulations capable of resisting high-temperature sintering and low-temperature loss of performance are needed to make S CR technology viable for widespread retrofit on pipeline prime movers. Dry Low NOx Combustors (DLNC) Once fully demonstrated, DLNC technology will be the only technology considered both technically and economically feasible for existing gas turbine pipeline prime movers. D L N C offers both low N O x levels and minimal operational impacts. At present, however, D L N C s are commercially available only for new utility size gas turbines achieving emission levels as low as 9 ppm, or about 0.17 lb/hp-hr. Retrofit D L N C s with controlled emission levels in the range of 25 to 42 p p m @ 1 5 % 0 2 for smaller gas turbines are still under development, and are projected to become available by the mid 1990s. Table 7 lists gas turbine models for which D L N C development is active. These control levels represent N O x reductions in the range of 70 to 80 percent from current uncontrolled emissions and comparable to N O x levels attained with WI. Major gas turbine manufacturers estimate that 40 percent of existing gas turbine prime movers will be candidate for lean premix D L N C retrofits once the technology is fully demonstrated. Solar, G E , and Rolls Royce engines would contribute most of the N O x reduction potential that would result from industry-wide retrofits. The cost effectiveness of a typical D L N C retrofit on a pipeline turbine is estimated in the range of $2,000 to $4,000 per ton of N O x removed, much lower than the other alternatives W I or SCR. Clean-Burn Conversion for Uncontrolled Lean Combustion Reciprocating Engines The conversion to clean-burn combustion requires increasing the A/F ratio by adding a turbocharger and aftercooler to a naturally aspirated or blower scavenged engine or replacing and existing turbocharger and inter- or aftercooler with a larger capacity unit. The retrofit will also require replacement of the intake manifold and filtration system, exhaust manifold, and carburetor to accommodate the larger air flows. The retrofit kit sold by the major engine manufacturers 8 1-4 |