OCR Text |
Show Removal of sulfur dioxide from combustion streams has typically been done by flue gas scrubbers. It is well known that this method has led to problems with operability, removal effectiveness, and impact on plant thermal performance. Current performance data show that fluidized-bed combustion has no trouble in meeting S02 pollutant emissions limits even with high-sulfur fuels, e.g., coal with 4.5 percent sulfur. In the fluidized-bed combustion process, the calcium carbonate in the limestone or dolomite in the FBC burner is first calcined to form fresh lime, CaO. The freshly formed lime is chemically active, and reacts with the sulfur dioxide in the presence of oxygen to form calcium sulfate by the following overall chemical reaction: CaO + S02 + \ 02 •* CaS04 Combustion temperature levels inside the FBC bed typically range from 1450° to 1550°F for atmospheric pressure beds and about 1750°F for pressurized (10 atm) beds. Effective sulfur dioxide retention within the fluidized-bed combustion process itself is a major reason why fluidized-bed combustion appears to be the best candidate for combustion of high-sulfur fuels. Nitrogen oxide emissions from coal-fired boilers have plagued the utility industry. The oxides of nitrogen are known toxicants. There are two sources of oxides of nitrogen: nitrogen compounds in fuels (known as fuel-bound nitrogen) and airborne nitrogen (known as atmospheric nitrogen). Since the average bed temperature in a fluidized-bed combustion burner (1450° to 1750°F) is not as high as in conventional boilers or incinerators, oxides of nitrogen are formed almost entirely from fuel-bound nitrogen in FBC. The mechanics of the formation of oxides of nitrogen inside a fluidize bed are very complicated. At present, it is understood that the following processes occur in, or are pertinent to operation of, FBC: o Decomposition of nitrogen compounds into amines. o Oxidation of decomposed nitrogen compounds to oxides of nitrogen in the high temperature, oxygen-rich zone near the distributor. o "Relaxation" or reduction of the oxides of nitrogen back to molecular nitrogen and oxygen under reducing conditions. Relaxation takes place in-bed and/or over-bed. Reduction is affected by the presence of carbon particles and carbon monoxide. o Slugging bed conditions with large gas bubbles are thought to contribute to formation of oxides of nitrogen. Generally, nitrogen oxide concentrations in FBC flue gas are higher than equilibrium chemical calculations predict; but there is evidence which implies that, given sufficient residence time, the oxides of 11-8 |