###### 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 (NO\({}_{x}\)) 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 NO\({}_{x}\) generation characteristics of the utility boiler were extensively analyzed. The simulation results indicated that as the blending ratio of semi-coke increased, the NO\({}_{x}\) emission at furnace outlet decreased. The blending methods (in-furnace versus out-furnace) had certain impacts on the NO\({}_{x}\) 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 NO\({}_{x}\) 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, NO\({}_{x}\) 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 NO\({}_{x}\) generated from the supercritical wall-fired boiler could be reduced with an increase of the OFA height. s 2019 9 889 300 000 0000 00000 00000 00000 00000 000000 000000 000000 000000 0000000 0000000 0000000 0000000 00000000 00000000 00000000 000000000 000000000 000000000 00000  
which has the characteristics of low volatile content and high heat value. Large particles of pyrolysis semi-coke can be used in chemical applications; however, small particles and powdered pyrolysis semi-coke are difficult to utilize in traditional chemical industry. With the booming coal industry in China, the coal grading conversion increases and the production of semi-coke is promoted accordingly. A large amount of semi-coke is urgently needed to be burned and utilized. Hence, the use of semi-coke as a power fuel in the field of power generation is a feasible solution. The combustion instability, poor ignition, low burnout ratio, and high NO\({}_{x}\) emission are common problems during the combustion of semi-coke, inducing the difficulty to achieve efficient and clean utilization [8; 9; 10; 11; 12; 13; 14]. In previous studies, the circulating fluidized bed (CFB) boilers were mainly used to explore the combustion of semi-coke [15; 16; 1]. However, the vast majority of boilers in operation in China are pulverized coal fired boilers. Consequently, except for CFB boilers, the pulverized-coal fired boilers are also another preferable option for the combustion of semi-coke in large-scale. However, the direct combustion of semi-coke in existing power plants is usually greatly difficult. Therefore, it is a feasible strategy to co-fire semi-coke and coal. With the shortage of high-quality coal resources and enhancement of environmental awareness, extensive research has been conducted focusing on the NO\({}_{x}\) generation and combustion characteristics of co-firing solid fuels [17; 18; 19]. [PERSON] et al.