Waste Coal Fines Reburn for NOx and Mercury Emission Reduction (2008)

Injection of coal-water slurries (CWS) made with both waste coal and bituminous coal was tested for enhanced reduction of NO{sub x} and Hg emissions at the AES Beaver Valley plant near Monaca, PA. Under this project, Breen Energy Solutions (BES) conducted field experiments on the these emission reduction technologies by mixing coal fines and/or pulverized coal, urea and water to form slurry, then injecting the slurry in the upper furnace region of a coal-fired boiler. The main focus of this project was use of waste coal fines as the carbon source; however, testing was also conducted using pulverized coal in conjunction with or instead of waste coal fines for conversion efficiency and economic comparisons. The host site for this research and development project was Unit No.2 at AES Beaver Valley cogeneration station. Unit No.2 is a 35 MW Babcock & Wilcox (B&W) front-wall fired boiler that burns eastern bituminous coal. It has low NO{sub x} burners, overfire air ports and a urea-based selective non-catalytic reduction (SNCR) system for NO{sub x} control. The back-end clean-up system includes a rotating mechanical ash particulate removal and electrostatic precipitator (ESP) and wet flue gas desulfurization (FGD) scrubber. Coal slurry injection was expected to help reduce NOx emissions in two ways: (1) Via fuel-lean reburning when the slurry is injected above the combustion zone. (2) Via enhanced SNCR reduction when urea is incorporated into the slurry. The mercury control process under research uses carbon/water slurry injection to produce reactive carbon in-situ in the upper furnace, promoting the oxidation of elemental mercury in flue gas from coal-fired power boilers. By controlling the water content of the slurry below the stoichiometric requirement for complete gasification, water activated carbon (WAC) can be generated in-situ in the upper furnace. As little as 1-2% coal/water slurry (heat input basis) can be injected and generate sufficient WAC for mercury capture. During July, August, and September 2007, BES designed, procured, installed, and tested the slurry injection system at Beaver Valley. Slurry production was performed by Penn State University using equipment that was moved from campus to the Beaver Valley site. Waste coal fines were procured from Headwaters Inc. and transported to the site in Super Sacks. In addition, bituminous coal was pulverized at Penn State and trucked to the site in 55-gallon drums. This system was operated for three weeks during August and September 2007. NO{sub x} emission data were obtained using the plant CEM system. Hg measurements were taken using EPA Method 30B (Sorbent Trap method) both downstream of the electrostatic precipitator and in the stack. Ohio Lumex Company was on site to provide rapid Hg analysis on the sorbent traps during the tests. Key results from these tests are: (1) Coal Fines reburn alone reduced NO{sub x} emissions by 0-10% with up to 4% heat input from the CWS. However, the NO{sub x} reduction was accompanied by higher CO emissions. The higher CO limited our ability to try higher reburn rates for further NO{sub x} reduction. (2) Coal Fines reburn with Urea (Carbon enhanced SNCR) decreased NO{sub x} emissions by an additional 30% compared to Urea injection only. (3) Coal slurry injection did not change Hg capture across the ESP at full load with an inlet temperature of 400-430 F. The Hg capture in the ESP averaged 40%, with or without slurry injection; low mercury particulate capture is normally expected across a higher temperature ESP because any oxidized mercury is thought to desorb from the particulate at ESP temperatures above 250 F. (4) Coal slurry injection with halogen salts added to the mixing tank increased the Hg capture in the ESP to 60%. This significant incremental mercury reduction is important to improved mercury capture with hot-side ESP operation and wherever hindrance from sulfur oxides limit mercury reduction, because the higher temperature is above sulfur oxide dew point interference.