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Show overall boiler control system may be divided into a number of subsystems: unit load demand, boiler load demand, total fuel and 02-trim control. Individual burner control, fuel and air flow distribution. temperature proflle/NOx control, feedwater control, superheat temperature control, and reheat temperature control. Each of these subsystems should operate In close coordination with others and also with the sequential logic system which handles the processes of equipment startup and shutdown, emergency trips, interlocks, changes in modes of operation, set-points, etc. Enhancements to conventional boiler control systems anticipated for the LEBS application include: Burner Air/Coal Ratio Control. One of the important low emission strategies is the balancing of fuel-to- alr ratios of individual burners to achieve even distribution of temperature and gas composition in the horizontal plane of the furnace. Deviations of a few percent on a single burner could cause higher NOx. flame instability, or both. While there are acceptable methods to measure the air flows to individual burners, the challenge for the LEBS boiler is to find an Improved (reliable and accurate) method to measure coal flows to individual burners. Development and proper testing of these methods is an important future task in B & W s LEBS project. Furnace Temperature Control. Another important factor in reducing N O x emissions and maximizing boiler efficiency is the control of the gas temperature at the exits of the upper and lower furnaces. A n important step in achieving this task for the LEBS project is to develop a methodology to characterize the upper furnace temperature profile and Its variations. Flame Fingerprinting. Instrumentation resulting in a "flame signature" signal related to type of fuel. burner design, and operating conditions will be sought and/or developed to aid in the control of the low-NOx combustion process. Sensing of Unburned Carbon. Reliable measurements of unburned carbon and combustibles may be especially Important when operating the main coal burners at substoichiometric levels. These signals must be used as limiting factors (directional blocking) to prevent any further reduction of air/fuel ratio when unburned combustibles or C O exceed the allowable m a x i m u m values. SOx/PARTICULATES AND NOx SUBSYSTEM INTEGRATION The integration of the SOx/Particulates or N O x subsystems with the boiler/power plant discussed above is very important However, integration of S O x and N O x emissions control is also important. The SOx/Particulates and N O x subsystems integration is discussed below: Common Injection Locations. As boiler load decreases, the sorbent injection location for the LIDS (SOx/Partlculate Subsystem) furnace limestone injection process will change as the time/temperature profile in the boiler changes. At some point during load lowering, the base-load sorbent injection ports may become ineffective for achieving the needed residence time for S 0 2 removal. At low loads it m a y be possible to investigate incorporating the low-load sorbent Injection ports within the existing O F A ports used as a N O x emissions control strategy. Heat Absorption/Surface Arrangements. As mentioned in previous sections, the residence time/ temperature profile in the boiler and convection pass must be evaluated In terms of the furnace sorbent injection process for optimum S 0 2 removal as well as in terms of optimum N O x removal. In the LEBS program, a new boiler can be designed for the heat absorption profile and residence times in the boiler to allow maximum S 0 2 and N O x capture. Ammonia Enhancement. An option for a final "trim" in N O x removal is Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR). In both processes, ammonia is injected into the flue gas to react with N O x to yield nitrogen and water. Often, not all of the ammonia reacts with N O x causing what is referred to as ammonia "slip". When SCR or S N C R is followed by a scrubbing process - such as LIDS - the "slipped" ammonia becomes an enhancement to the S 0 2 removal in the scrubber. AIR TOXICS IMPACTS Title III of the Clean Air Act Amendments of 1990 established a list of 190 hazardous air pollutants and charged the U.S. Environmental Protection Agency (EPA) with the responsibility for regulating emissions of these substances. Electric utility plants are currently exempt from requirements pending the outcome of several risk assessment and emissions characterization studies. The EPA is scheduled to propose its plan for regulating electric utilities under Title III in a report to Congress in November, 1995. While the EPA's final approach is uncertain, some air toxics species issuing from utility stacks m a y be regulated - especially some of the high-risk compounds such as arsenic, beryllium, cadmium, chromium, and mercury, and/or compounds known to be emitted in large quantities such as hydrogen chloride and hydrogen fluoride. The final commercial LEBS power plant must be able to meet all air toxics regulations, federal and local, in place at that time. Again, integration of the various LEBS plant subsystems is being conducted with this air toxics emissions goal in mind. LIDS and Air Toxics Low Temperature Fabric Filtration. The LIDS sys- 7 Paper No. 11-13 |