OCR Text |
Show SELECTION OF METHODS FOR CONTROLLING EMISSIONS Particulate Matter Particulate matter in the coal diesel exhaust gas is of concern fo.r two reasons. First, relatively large particles (over 10 micron) can erode the engine's turbocharger, causing performance and maintenance problems. Therefore, most of these particles will be removed upstream of the turbocharger using a cyclone separator. Second, particulate matter controls must be implemented to attain accepted emissions standards. Commercially-viable, enginegrade, CWS is expected to contain 1 to 2 wt% ash (dry basis). With the demonstrated high engine combustion efficiency (97 to 99 % carbon burnout), uncontrolled particulate emissions have been measured at about 1 to 3 Ib/MBtu. Coal-fired utility boiler New Source Performance Standards (NSPS) are being used as an approximate target for emission levels. Achieving the NSPS level of 0.05 Ib/MBtu requires a reduction in particulate matter of about 95 to 98%. A conventional bag filter was selected as the final particulate removal device because it is capable of high collection efficiencies (over 99 %) and because it is an integral component of the duct injection S02 control technique used. NQx Measured emissions of NOx from the coal-fueled engine are about half those of conventional diesel engines, due in part to the suppression of flame temperature by water in the slurry and to optimization of the injection process. However, even the coal-fueled diesel NOx emission levels of 600 ± 200 ppm at 11 % ~ (1.8 ± 0.6Ib/MBtu) must be significantly reduced to make the engine competitive with other power generation options. The final reduction of N~ to NSPS levels is achieved by SCR. The NOx control approach employs a combination of in-cylinder combustion modification (25 to 50 % reduction to 0.9 to 1.2 Ib/MBtu) and post-combustion, exhaust gas treatment (80 to 90% reduction to 0.2 to 0.25Ib/MBtu). Selective Catalytic Reduction (SCR) was chosen as the post-combustion method since it is the only commercially available method that satisfies the performance targets. Three alternative technologies, rebuming, Selective Non-Catalytic Reduction (SNCR), and NO reduction on engine exhaust particulate, were evaluated in laboratory research programs because they offered the potential for much lower cost than SCR. However, none proved readily feasible. £02 Engine-grade CWS has a sulfur content of about 0.5 to 1.5 wt% (dry basis), which yields S~ levels in the untreated engine exhaust has of about 150 to 450 ppm at 11 % ~ (0.7 to 2.1Ib/MBtu). A goal of 70% S~ removal was established based upon the NSPS requirements for coal-fired utility boilers. Table 1 lists those S~ control technologies that were evaluated in detail: spray drying with hydrated lime sorbent, duct injection on calcium-based sorbents, and duct injection of sodium-b~ sorbents. Estimates of levelized busbar and initial capital cost were developed for FGD processes applied to a 12 MWe engine system. This analysis indicates that the duct injection offers significant economic benefits over spray dryers. 2 |