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Show and S03 occurs. For example, concentrations of 5 ppm each of NH3 and S03 have a sulfate formation temperature of 41 O°F. Hence, solid sulfate deposition is likely to occur in the air heater section where flue gas temperatures drop below 400°F. These deposits are of concern as they can lead to excessive pressure drop across the air heater, requiring off-line cleaning of the air heater wheel in the case of regenerative air heater designs. As indicated previously I the potential benefit of incorporating the urea within a lime hydrate is the removal and neutralization of any S03 present within the flue gas by the lime. As the calcium sulfite/sulfate reaction products are stable at relatively high temperatures, the S03 is potentially removed from the flue gas and no longer available to react with any associated ammonia emissions in the temperature regime where the ammonium sulfates would be thermodynamically stable. Particulate Emissions Since the lime-urea hydrate is a dry solidi the particulate loading will Increase In proportion to the sorbent injection rate and the baseline coal ash content. As previously mentioned I only the lime contributes to the particulate loading since the urea decomposes into the gas phase upon injection into the furnace. The increase in particulate loading as a function of coal ash content and NOx emission level is shown in Figure 5. For a sorbent injection rate corresponding to a N/NOx ratio of 1.0, the particulate loading Is only Increased nominally 180;0 for a unit firing a 10% ash coal with baseline NOx emissions of 750 ppm. In comparison, furnace sorbent injection programs directed at S02 control Increase particulate loadings nominally 70% at a Ca/S ratio of 2 with baseline S02 emissions corresponding to a 2.5% sulfur coal. As these significantly higher particulate loadings on electrostatic precipitators can normally be accommodated with flue gas conditioning (I.e. humidification), the particulate emissions increase associated with lime-urea hydrate for NOx control are considered manageable. LIME-UREA ECONOMICS In order to evaluate the cost effectiveness of the technology I an economic assessment was conducted, which focused on three prinCiple areas of analysis. The first area was directed toward an estimation of the cost to produce a lime-urea hydrate relative to commercially produced calcium hydroxide. The second area utilized this lime-urea sorbent cost information in an assessment of the capital and levelized costs associated with a utility boiler retrofit. Finally I the third area compared the cost of the lime-urea hydrate technology with alternative approaches for NOx control. In order to establish a representative sorbent cost for the lime-urea hydrate, a delivered cost of base lime hydrate (with no urea addition) of S70/ton (Ireland et aL, 1988) and a delivered urea price of S180/ton (Chemical Marketing Reporter I 1989) -10- |