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

Chang’An Wang,Defu Che,Qiang Lv,Jingwen Zhang,Lin Zhao,Pengqian Wang,Qinqin Feng,Yongbo Du

doi:10.3390/app9050889

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.

Highlights

Nowadays, coal plays a crucial role in energy resources worldwide, especially in China, and will still hold an important position in future
The results showed that the combustion characteristics of the in-furnace units were better than those of the out-furnace case
Andand the the validity of the the numerical calculation model must first be verified by the comparison to the in-situ test data

Summary

Introduction

Coal plays a crucial role in energy resources worldwide, especially in China, and will still hold an important position in future. The proven reserves of coal resources in China are as high as 140 billion tons, of which low-rank coal accounts for about 50% [1,2,3,4,5]. Due to the fact that the content of moisture in low-rank coal is high, the efficiency of direct combustion for power generation is relatively low. The grading conversion utilization of low-rank coal can improve this situation. Semi-coke is the solid by-product that low-rank coal pyrolyzes under low temperature [6,7,8], Appl.

Methods

Discussion

Conclusion