NOx Emission Control in Coal Combustion Systems

Nitrogen oxides (NOx), principally nitric oxide and nitrogen dioxide, arise from the reaction of fuel-bound nitrogen and thermal oxidation of atmospheric nitrogen in coal-fired furnaces. These pollutants contribute to photochemical smog, tropospheric ozone formation and acid deposition, and are regulated under stringent environmental standards worldwide. Control strategies fall into two broad categories. Primary measures alter combustion conditions to minimise NOx formation in situ, encompassing staged combustion (air and fuel staging), reburning, flue‐gas recirculation and advanced low‐NOx burner designs. Secondary measures treat flue gases post‐combustion via selective non‐catalytic reduction (SNCR) or selective catalytic reduction (SCR), converting NOx into molecular nitrogen. Progress in computational fluid dynamics coupled with detailed kinetic mechanisms has enhanced understanding of radical‐mediated pathways, the roles of temperature, stoichiometry and residence time, and the influence of novel reburning fuels. Practical applications span pulverised coal boilers, circulating fluidised beds and entrained‐flow reactors. Recent trends include the integration of alternative carbonaceous reburning fuels (for example waste plastics), synergistic combinations of deep air staging with injected reducing agents (ammonia, pyrolysis gas), and hybrid O2/CO2 combustion atmospheres. Through global collaboration, these developments demonstrate substantial NOx abatement while preserving thermal efficiency and economic feasibility in existing and new coal‐fired installations.

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NOx Emission Control in Coal Combustion Systems publication trend

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Technical terms

Nitrogen oxides (NOx): Reactive gases (NO and NO₂) formed during combustion of nitrogenous fuels, major contributors to air pollution and acid rain.

Reburning: A staged combustion technique in which a secondary, fuel‐rich zone reduces NOx formed in the main combustion stage through reactions with hydrocarbon radicals.

Air staging: A primary control method that divides combustion air into primary and secondary streams to create oxygen‐lean zones, thereby suppressing thermal NOx formation.

Fuel staging: The sequential introduction of fuel at different points to establish reducing zones that limit NOx generation in the main flame region.

Selective non‐catalytic reduction (SNCR): A post‐combustion process injecting ammonia or urea into hot flue gas (850–1 100 °C) to convert NOx to N₂ without using a catalyst.

Selective catalytic reduction (SCR): A secondary abatement technique passing flue gas over a catalyst bed (250–450 °C), using ammonia or urea to achieve high‐efficiency NOx conversion to nitrogen.

References

1. Controlling NOx emission from boilers using waste polyethylene as reburning fuel. Chemical Engineering Journal (2021).
2. NOx Emissions and Nitrogen Fate at High Temperatures in Staged Combustion. Energies (2020).
3. Effects of Ammonia Solution and Pyrolysis Gas on NOx Emission from a 75 t/h Pulverized Coal Boiler. Catalysts (2022).