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Show ME~ de-NOX reburn process. The ultimate goal of the project was to validate and deploy l.n a full-s~ale utility boiler a concept that has been proven to be technically and econo~cally fea~lbl~ but required further experience with full-scale systems to ensure operational . practicality and broad utility and industrial acceptance in order that such technology IS selected for long term use. Cofunding of this project was provided by IGT S:MP .as well as several large gas/utility companies. In-kind funding was provided by DetrOIt Stoker Company and Cogentrix of Richmond. The City of Richmond supplied the natural gas line to the boiler at no cost to the project. :METHANE de-NOX CONCEPT FOR COAL-FIRED STOKERS Conventional reburning is a process in which additional fuel (e.g., natural gas) is injected into the products of complete combustion so as to eliminate the excess oxygen and provide hydrocarbon radicals. These radicals react with NOx as the source of oxygen, thereby reducing it to molecular nitrogen. The reburn process requires relatively high temperature and sufficient residence time. At the end of the reburn zone, overfire air is inj ected to create a burnout zone. This process has been successfully applied, using natural gas as a reburning fuel, to pulverized coal and cyclonic coal-fired combustors (5). Over 50% of NOx reduction was demonstrated during long term operation. If conventional reburning is applied to the stoker, then the combustion volume will be divided into four zones: two combustion zones (coal bed and combustion zone above the bed in the lower furnace), reburn and burnout zones. In this case at least three levels of injection are required, two for overflre air and one for natural gas as reburning fuel. The overall height of coal-fired stokers is usually not very large. For example, the elevation of Cogentrix boilers from the grate to platten superheater is only about 35 feet. It is practically impossible to install and ensure effective processing of two combustion zones, rebum and burnout zones in such furnace. The total residence time of combustion products in the furnace is about 2.5 seconds, but the reburn zone alone requires about 1 second for effective mixing and reacting of natural gas with the combustion products. The:METHANE de-NOX reburning process, developed by IGT with support from the gas industry and GRI, for stoker fired combustors is shown in Figure 1. In this case,. natural gas as a reburning fuel is injected just above the coal bed. Compared to conventional reburning the :METHANE de-NOX process requires only three zones instead of four, and two levels of injection instead of three. Injected natural gas not only reduces NOx formed in the coal bed, but also limits its formation because a significant portion of the NOx precursors are decomposed and react to form molecular nitrogen. A reduction in the number of zones and injection levels provides sufficient residence time for rebuming and burnout. The METHANE de-NOX process was evaluated on a pilot-scale combustor at Riley Stoker's facility (6), and twice field tested during 1991 at the Olmsted County Municipal Waste Combustion facility in Rochester, Minnesota (7), and during 1995 at an incineration plant in Japan. The tests at Olmsted County achieved an average reduction of 60% of NO x and 50% of CO compared to baseline levels when injecting 13% 'latural gas. In Japan, 55% of NOx reduction was achieved with 7% of natural gas. A significant increase in boiler efficiency (about 3%) was also demonstrated at the Olmsted Country 100 tonld MSW boiler. 4 |