OCR Text |
Show fired by Canadian utility boilers and coals which have the potential to be utility fuels was complied. The coal data base was evaluated to select ten coals which have the best potential to be used a reburning fuels from the standpoints of lowest NO" emissions, lowest impact on boiler performance, and highest probability of use by a Canadian utility. Finally, a series of screening studies was conducted in which the performance of the ten selected coals was evaluated in a 1.0 MMBtulhr downfired reactor. In the next phase of the study, four coals will be selected for more detailed parametric evaluation of the impacts of process parameters on the coal reburning process. Furnace heat transfer models will be applied to the data to assess process scale-up constraints. In the following, the general impacts of process parameters and fuel properties on the performance of the coal reburn process will be described, and the results of the initial screening studies will be presented. Coal Reburning Rebuming is a combustion modification technology which removes NO x from combustion products by using fuel as a reducing agent. The fundamental princi pie of this technology- that CH fragments can react with NO to fonn molecular nitrogen-has been studied for over two decades. The reburn process has been extensively studied at bench, pilot and full scale to identify the parameters expected to control the process performance 1. 4. 5. The results of these studies have shown that the most critical parameters which impact the process perfonnance are: the primary NO x level, the reburn zone temperature and residence time, and the reburn zone stoichiometry. In general, rebuming effectiveness improves with increasing primary NO x level, and with increasing reburn zone temperature and residence time. The optimal stoichiometry for the rebuming zone is typically 0.9; where rebuming zone stoichiometry is defined as the ratio of the total air supplied to the primary and reburning zones to the total stoichiometric requirements of the primary and reburning fuels. In practical application of the process, mixing of the rebuming fuel and overf'rre air is also an important consideration. In the use of coal as a reburning fuel, a significant question is the extent to which fuel bound nitrogen and other coal properties influence the NO x reduction effectiveness. From previous studies of the use of coal as a rebuming fuel, it appears that the suitability of a coal for reburning depends upon fuel volatility, nitrogen content, and nitrogen reactivity. Fuel volatility impacts the availability of fuel in the reburn zone and, hence, the evolution of radical species. As a result, more volatile fuels would 3 |