Pulverized coal combustion: Flame attachment and nitrogen oxide emissions.

摘要

To fully utilize coal as a long-term energy source, pollution prevention technologies must be developed to mitigate the negative environmental and health impacts of coal combustion. NO_x emissions are of particular concern due to their role in forming ground-level ozone, photochemical smog, fine particulates and acid rain.; A systematic evaluation of near-flame aerodynamics was conducted to determine how burner operating parameters and oxygen partial pressure influence flame attachment and coal ignition, two properties essential for proper low NO _x burner operation. A laboratory scale (17kW) 2m tall, 0.5m diameter electrically heated furnace and axial burner with adjustable secondary combustion air annuli and primary fuel jets were used in the study. Transport air oxygen partial pressure (PO₂), coal particle size distribution, primary and secondary jet velocity, and wall temperature were varied independently to determine the effect of each variable on flame attachment and NO_x .; NO_x emissions from the furnace were similar to those from full-scale tangentially-fired boilers. The tendency for flame attachment increased with velocity ratio (Θ), wall temperature, PO₂, and coal fines. Θ's greater than 1 were required for stable combustion. Increasing Θ reduced flame standoff distances and NO_x for always-detached flames. NO_x increased with Θ for always-attached flames.; Increasing PO₂ reduced NO_x by up to 50% by promoting flame attachment. However both oxygen enrichment and increasing fines had little impact on NO_x for always-attached and always-detached flames.; Wall temperature and excess air in leakage were the dominant variables affecting NO_x. Furnace exhaust oxygen levels increased when operating under a slight vacuum with corresponding increases in NO_x. Emissions for detached flames increased with wall temperature 3 times faster than attached flames. Emissions data obtained from the furnace under slight positive furnace pressure increased linearly with wall temperature.; A novel dual flame was produced at high Θ and reduced PO₂ consisting of one flame attached to the burner and one stabilized 18&inches; below the burner. This configuration is similar to staged combustion but without separate over-fire air. Emissions from the dual flame were significantly below those observed from conventional Type-O attached and detached flames.