Numerical Investigation on Co-firing Characteristics of Semi-Coke and Lean Coal in a 600 MW Supercritical Wall-Fired Boiler

Abstract

Semi-coke is one of the principal by-products of coal pyrolysis and gasification, which features the disadvantages of ignition difficulty, low burnout rate, and high nitrogen oxides (NOx) emission during combustion process. Co-firing semi-coke with coal is a potential approach to achieve clean and efficient utilization of such low-volatile fuel. In this paper, the co-firing performance of semi-coke and lean coal in a 600 MW supercritical wall-fired boiler was numerically investigated which has been seldom done previously. The influences of semi-coke blending ratio, injection position of semi-coke, excess air ratio in the main combustion zone, the co-firing method, and over fire air (OFA) arrangement on the combustion efficiency and NOx generation characteristics of the utility boiler were extensively analyzed. The simulation results indicated that as the blending ratio of semi-coke increased, the NOx emission at furnace outlet decreased. The blending methods (in-furnace versus out-furnace) had certain impacts on the NOx emission and carbon content in fly ash, while the in-furnace blending method showed more flexibility in co-firing adjustment. The injection of semi-coke from the upper burners could significantly abate NOx emission at the furnace outlet, but also brought about the rise of carbon content in fly ash and the increase of outlet temperature. Compared with the condition that semi-coke and lean coal were injected from different burners, the burnout ratio of the blend premixed outside the furnace was higher at the same blending ratio of semi-coke. With the excess air ratio in the main combustion zone increased, NOx concentration at the furnace outlet was increased. The excess air ratio of 0.75 in the main combustion zone was recommended for co-firing 45% semi-coke with lean coal. The operational performance of the boiler co-firing semi-coke was greatly affected by the arrangement of OFA as well. The amount of NOx generated from the supercritical wall-fired boiler could be reduced with an increase of the OFA height.

A Review of the Numerical Modeling of Pulverized Coal Combustion for High-Efficiency, Low-Emissions (HELE) Power Generation

“…For studies on HELE boilers with blending fuels, the cofiring characteristics of brown coal and semicokes, which have the disadvantages of low combustibility, high unburnt carbon, and NOx emissions, have been presented in particular. , The use of semicokes as a substitute for black coal has been observed to improve the combustion time of bituminous coals, owing to the higher oxygen content in the semicokes. Furthermore, the decrease in NOx emissions was achieved as the semicoke ratio increased when the semicoke had a lower nitrogen content than coal.…”

Experimental study on multi‐fuel combustion of lean coal and by‐product gas in a 75t/h tangentially‐fired boiler

“…As anticipated, there are also some notable studies in other areas, in addition to multi-fuel combustion in common above. For example, Wang et al 27 simulated the co-firing of semi-coke and lean coal in a 600 MW supercritical wall-fired boiler. They presented that blending a proper amount of semi-coke can reduce the emission of NO despite slight increase in the carbon content of fly ash.…”

Relative Environmental, Economic, and Energy Performance Indicators of Fuel Compositions with Biomass

“…Fuels, derived from waste (e.g., biomass), such as biogas, syngas, pyrolysis oil and spirits, are renewable energy sources [1,2]. They are in high demand nowadays due to high rates of fossil fuel depletion [3]. The employment of reactors and gasifiers with biomass is economically feasible, no matter whether heat and power are produced in small or large volumes.…”