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Show COFIRE TECHNOLOGY United States industry operates over 2000 solid-fuel fired boilers for process steam, space heating and cogeneration. The majority are coal and wood-fired stokers, with a significant but smaller population of stokers fired with combustible wastes or mixed fuels, as well as non-stoker firing types, such as fluidized beds and pulverized coal. The fixed bed method of firing used in most solid-fuel industrial boilers can introduce severe operational and environmental constraints. The solid-bed method of combustion is prone to smoking and is typically slow to respond to load changes. The boiler efficiency is lower than with other firing types because of high levels of excess air used to suppress incipient smoking, and high carbon loss in ash. Despite these challenges, it is important to industry to maintain and, if possible, enhance the capability of the stoker resource base. Gas cofiring has been attempted sporadically over the years to address operational limitations with coal and wood-waste fired boilers. The results with these early gas burner retrofits was mixed. Typically, the gas flame was not engineered to interact with the solid-fuel combustion, but merely to be a startup and standby source of heat. Accordingly, the benefits achieved with gas cofiring were often modest and not necessarily keyed to the most important site-specific economic drivers for the plant. Also, in the early applications, conventional low pressure register burners were used. These sometimes operated off the existing forced draft fan and had weak penetration into the boiler flame and also very limited turndown. A n additional complication was that the burners were usually placed high in the furnace over concern of grate overheating. The combined characteristics of low penetration, low turndown, and remote placement usually relegated the use of the burner to warmup and standby. In 1994, GRI formed a team of ARCADIS (formerly Acurex Environmental), Coen, gas utilities, and boiler operators to develop and implement gas cofire for coal-fired stokers. The basic objective was to use gas as a process modification to resolve combustion problems with solid-fuel fired boilers. In this way, gas was introduced as a value-added combustion enhancement, rather than as a replacement fuel to the cheaper solid fuel. At the start of the project, surveys with over 50 boiler operators showed that the industrial boiler sector was coping with a variety of performance and environmental problems. The most prevalent problem was opacity control which was strongly linked to attempts to obtain high and low-load turndown, or routine load swings. Other major issues were coping with fuel variability and achieving environmental compliance. To address these operational issues, a cofire hardware package was developed by Coen Company and the project team. The criteria for the new cofire system was twofold: • Achieve maximum operating benefits from a small amount of gas, typically 5 to 15 percent of total heat input, • Design a flexible package readily adaptable to a wide variety of stoker designs and to a diversity of desired benefits The basic cofire system, which has been developed and used since 1994, is shown on Figure 1. 2 |