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Cofiring sludge with coal based on existing power plants is a promising method to realize harmless recycling treatment for sludge disposal. However, the influences of sludge addition on NOx emission are still controversial. Moreover, the optimal operating strategies for the cocombustion boiler need further investigations. In this study, the influences of sludge blending ratio, moisture content, sludge inlet position, and air-staging parameter on the combustion characteristics and NOx emission behaviors in a 330-MW boiler were analyzed numerically. The results show that with the increase of sludge blending ratio, the flue gas temperature in the primary zone decreases. The overall burnout ratio declines with the addition of sludge due to the higher ash and moisture contents. Although the sludge contains more N than coal, the NOx emissions in the cofiring case are lower than those in the coal-fired case due to the lower temperature and stronger reducing atmosphere in the primary zone. Reducing the moisture in the sludge is beneficial to the combustion within the furnace. Nevertheless, because of the higher temperature and lower CO concentration within the furnace, the NOx concentration at the furnace outlet increases. For wall-fired boilers, feeding the sludge from the lower burners should be avoided considering the potential surge in NOx emission. When the excess air ratio in the primary zone increases from 0.74 to 0.96, the NOx concentration at the furnace outlet rises by 58%. Hence, although relatively lower excess air ratio in the primary zone could bring adverse impacts on the burnout behaviors, air-staging combustion is still necessary when cofiring sludge in the coal-fired boiler. [ABSTRACT FROM AUTHOR]

[Display omitted] • The co-firing of coal and municipal sludge with high moisture content was studied. • The more sludge co-fired, the less NO x emission. • There are higher flue gas temperature and NO x emission at lower moisture content. • Co-firing sludge from the medium-upper burner can reduce the layers of burner. • The shrunk-middle type is the optimum scheme of SA distribution. Using the current coal-fired power plant to treat sludge with large annual production is an efficient sludge treatment method. Nevertheless, the majority of the previous studies concentrated on sludge co-firing with low blending ratio and moisture content in utility boiler. Further analysis and discussion are necessary regarding the impact of sludge with high blending ratio and high moisture content on co-firing performance as well as the potential interactions between high moisture content sludge and coal during the co-firing process. The simulation results of the verified computational fluid dynamic (CFD) model can be regarded as numerical experiments. In the present research, CFD was used to analyze the co-firing characteristics of municipal sludge and coal in a 600 MW tangentially fired utility boiler. The impacts of sludge moisture content, blending ratio, blending strategy, and secondary air (SA) distribution on co-combustion characteristics and NO x emissions were investigated, and their potential synergistic effects were assessed. The co-firing of coal and sludge has little impact on the furnace outlet temperature while potentially lowering NO x emissions, provided that the net calorific value of the feeding of the mixture of sludge and coal is kept constant. The reduction of sludge moisture content can significantly improve co-combustion performance, but it also facilitates NO x production. Blending sludge from the position of the medium-upper burners can reduce the number of layers needed to burn the sludge in practical coal-fired boiler. Furthermore, the SA distribution has a major impact on the furnace outlet flue gas temperature and NO x content. The shrunk-middle type is the optimum scheme for SA distribution. This study contributes to a deeper knowledge of the actual co-firing of sludge and coal in the utility boiler and provides a possible theoretical foundation for the widespread, clean, and effective use of sludge. [ABSTRACT FROM AUTHOR]

The technology of oxy-fuel co-combustion is beneficial to consume the large amount of ultra-low volatile coal-based solid fuels (UCF). Moreover, the oxy-fuel co-combustion of various UCFs could also promote the development of coal-chemical industry and reduce the carbon emission in power plants. However, the co-gasification behaviors of various coal-based solid fuels blends at initial oxy-fuel co-firing stage are still vague, which need to be further studied. In the present, the co-gasification features of various coal-based solid fuels blends during initial oxy-fuel co-combustion process were investigated by oxy-fuel co-combustion system experimentally. The rise of O 2 concentration could obtain low Brunner−Emmet−Teller surface area and high Barrett-Joyner-Halenda average pore owing to the collapse of internal structures. The nitrogen gases (X–N) yields show a decreasing trend with the UCF proportion raised. The increasing temperature could promote the gas release of sample. The ash droplets observed on the surface of blend were formed by reactions between oxygen and carbon on the sample surface in 3% O 2 /97% CO 2 atmosphere, which are enlarged with temperature significantly. The present research could be beneficial for efficiency utilization of low-rank coal, together with the reduction of NO x emission and realization of carbon neutrality. • The co-gasification features at initial stage of oxy-fuel combustion were examined. • The synergistic effects of blends during co-gasification process were elucidated. • Tiny oxygen concentration results in collapsing of carbon and ash skeletons. • The addition of UCF could lead to the decreases of X–N yields in blends. • Ash droplets observed on the blend surface are enlarged with the T increased. [ABSTRACT FROM AUTHOR]