Numerical simulation of biomass high-ratioco-firing in a pre-pyrolysis pulverized coal industrial boiler

Co-firing of carbon-neutral biomass with coal is the most economic and promising technology to reduce CO₂ emission from coal combustion currently. Limited by the biomass distribution feature and collection cost in China, the biomass co-firing in large-scale coal power plants can only be at a small ratio of biomass. However, for the industrial boilers used for distributing energy system, it is able to co-fire biomass at a high ratio and even purely burn biomass. Biomass combustion in the grate furnace and circulated fluidized bed has been widely adopted, in contrast, the study and application of biomass co-firing in the pulverized coal industrial boilers of higher efficiency and automation degree at a high-ratio is rare. In this paper, the biomass high-ratio co-firing in a new developed pre-pyrolysis pulverized coal industrial boiler is studied. The effects of biomass co-firing ratio on temperature distribution, wall heat flux, specie distribution and NO_x emission are discussed. The CFD modeling results show that biomass co-firing or even pure combustion follows the similar temperature and specie distribution of pure coal combustion. With the increase of biomass co-firing ratio, the combustion in the primary zone is strengthened, however, due to the higher moisture content in biomass, the flame is overall delayed, the oxygen consumption rate decreases, and the volatile and CO concentrations increases in the primary combustion zone. Because of the dual effects from flame delay and higher nitrogen content in biomass, with the increase of biomass co-firing ratio, the NO_x emission increases first and then decreases. This study indicates that the pre-pyrolysis pulverized coal combustion technology is feasible to co-fire biomass at a high ratio and even hundred percent. When the nitrogen content of biomass is controlled, the NO_x emission can be significantly reduced by biomass co-firing.