PARTICULATE CHARACTERIZATION AND ULTRA LOW-NOx BURNER FOR THE CONTROL OF NO{sub x} AND PM{sub 2.5} FOR COAL FIRED BOILERS

Abstract

In response to the serious challenge facing coal-fired electric utilities with regards to curbing their NO{sub x} and fine particulate emissions, Babcock and Wilcox and McDermott Technology, Inc. conducted a project entitled, ''Particulate Characterization and Ultra Low-NO{sub x} Burner for the Control of NO{sub x} and PM{sub 2.5} for Coal Fired Boilers.'' The project included pilot-scale demonstration and characterization of technologies for removal of NO{sub x} and primary PM{sub 2.5} emissions. Burner development and PM{sub 2.5} characterization efforts were based on utilizing innovative concepts in combination with sound scientific and fundamental engineering principles and a state-of-the-art test facility. Approximately 1540 metric tonnes (1700 tons) of high-volatile Ohio bituminous coal were fired. Particulate sampling for PM{sub 2.5} emissions characterization was conducted in conjunction with burner testing. Based on modeling recommendations, a prototype ultra low-NO{sub x} burner was fabricated and tested at 100 million Btu/hr in the Babcock and Wilcox Clean Environment Development Facility. Firing the unstaged burner with a high-volatile bituminous Pittsburgh 8 coal at 100 million Btu/hr and 17% excess air achieved a NO{sub x} goal of 0.20 lb NO{sub 2}/million Btu with a fly ash loss on ignition (LOI) of 3.19% and burner pressure drop of 4.7 in H{sub 2}O for staged combustion. With the burner stoichiometry set at 0.88 and the overall combustion stoichiometry at 1.17, average NO{sub x} and LOI values were 0.14 lb NO{sub 2}/million Btu and 4.64% respectively. The burner was also tested with a high-volatile Mahoning 7 coal. Based on the results of this work, commercial demonstration is being pursued. Size classified fly ash samples representative of commercial low-NO{sub x} and ultra low-NO{sub x} combustion of Pittsburgh 8 coal were collected at the inlet and outlet of an ESP. The mass of size classified fly ash at the ESP outlet was sufficient to evaluate the particle size distribution, but was of insufficient size to permit reliable chemical analysis. The size classified fly ash from the inlet of the ESP was used for detailed chemical analyses. Chemical analyses of the fly ash samples from the ESP outlet using a high volume sampler were performed for comparison to the size classified results at the inlet. For all test conditions the particulate removal efficiency of the ESP exceeded 99.3% and emissions were less than the NSPS limits of {approx}48 mg/dscm. With constant combustion conditions, the removal efficiency of the ESP increased as the ESP voltage and Specific Collection Area (SCA) increased. The associated decrease in particle emissions occurred in size fractions both larger and smaller than 2.5 microns. For constant ESP voltage and SCA, the removal efficiency for the ultra low-NO{sub x} combustion ash (99.4-99.6%) was only slightly less than for the low-NO{sub x} combustion ash (99.7%). The decrease in removal efficiency was accompanied by a decrease in ESP current. The emission of PM{sub 2.5} from the ESP did not change significantly as a result of the change in combustion conditions. Most of the increase in emissions was in the size fraction greater than 2.5 microns, indicating particle re-entrainment. These results may be specific to the coal tested in this program. In general, the concentration of inorganic elements and trace species in the fly ash at the ESP inlet was dependent on the particle size fraction. The smallest particles tended to have higher concentrations of inorganic elements/trace species than larger particles. The concentration of most elements by particle size range was independent of combustion condition and the concentration of soluble ions in the fly ash showed little change with combustion condition when evaluated on a carbon free basis.