Co-firing
semicoke and coal in utility boilers is a promising approach
to cleanly and efficiently utilize semicoke, while the co-combustion
characteristics have yet to be fully understood, including the effects
of the blending method and secondary air distribution mode on NO
x
emission and combustion efficiency. In the
present work, both full-scale experiments and simulation were carried
out to investigate the co-combustion characteristics of semicoke and
coal in a 300 MW coal-fired utility boiler with a focus on the blending
method and air distribution. The results indicated that co-firing
semicoke would reduce the furnace temperature and that both NO
x
emission and incomplete combustion heat
loss increased with the blending amount of semicoke. Hence, to limit
the excessive deterioration of boiler performance, the blending amount
of semicoke was recommended to be below 50% for the in-furnace blending
method and below 33% for the out-furnace blending method. When the
in-furnace blending method was employed, the semicoke and coal were
recommended to be fed from burners in alternate layers, in particular,
without semicoke fed from the top and/or bottom burners. The NO
x
emission was proved to positively correlate
with the average height of semicoke burners, while the burnout ratio
increased at first and then decreased with the increasing average
height. The in-furnace blending method with optimal burner allocation
was proved to have advantage over the out-furnace blending method.
The secondary air distribution of the pagoda down type could reduce
NO
x
generation and unburned carbon due
to the formation of a high-temperature and strong reducing atmosphere
within the furnace.
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.
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