OCR Text |
Show oxide (NO), with a small amount of nitrogen dioxide (N02). However, in the atmosphere, the NO is readily oxidized to N02, a brown gas. NO and N02 (NOx) have several environmental consequences: 1. The brown color of N02 contributes to stack opacity and atmospheric visibility reduction. 2. N02 is an ingredient in photochemical smog. 3. NO~ can react with water in the atmosphere producing nitric acid Wh1Ch contributes to acid precipitation. 4. NO reacts readily with ozone and this reaction has the potential to deplete the earth's ozone layer. Recognition of these NOx related problems has led to promulgation of numerous emission control regulations. Beginning in 1971, EPA New Source Performance Standards have required NO~ emission controls on new large boiler installations. Existing units are often controlled via State Implementation Plans. Also the Prevention of Significant Deterioration (PSD) regulations set limits on existing units undergoing modifications. In 1990, the Clean Air Act was revised to tighten emission regulations in many areas including NOx emissions. There is a need for a cost effective retrofit NO control technology to meet these requirem ts. The ideal NO control technofogy will be compatible with all types of co Jstion systems andwil1 be synergistic with other emission control technologies, especially those which control S02. Gas Reburning (GR) is an NOx control technology which meets these requirements. It can be retrofitted to virtually any type of combustion system and can be integrated with other control technologies to achieve high levels of combined NOx and SO control. This paper discusses the GR technology and gives a status report on field evaluations of GR integrated with Sorbent Injection and Low NOx Burners which are now in progress. GAS REBURNING (GR) The concept of NOx reduct i on by f1 ames has been recogn i zed for over a decade ,2. Reburning for in-furnace NO~ control has been applied to boilers in Japan 3,4 and work is being supported 1n the United States by the Gas Research Institute, the U.S. Environmental Protection Agency, the Department of Energy and the E1 ectri c Power Research Inst i tute6 - 9 • ~ • Figure 1 ill ~ trates the basic reburning process with natural gas as the reburning fuel. Tt normal fuel is burned in conventional burners under fue1- lean conditions. A reburning fuel is injected downstream of the main combustion zone to reduce NOx • The combust i on air for the reburni ng fuel is injected separately further downstream. The overall process can be divided into three zones in series: 1. M 1n Combustion Zone Approximately 80-85 percent of the heat is released in this zone under fuel-lean conditions producing NOx• 2 |