Mathematical modelling and optimisation of lignite and wheat straw co-combustion in 350 MWe boiler furnace

Abstract

In this paper pulverised lignite-fired 350 MWe boiler furnace is selected for numerical simulations performed by using in-house developed computer code to deepen understanding of complex processes during direct co-firing with wheat straw. The CFD code is significantly upgraded to accommodate simulation of lignite and wheat straw particle reactions and interactions with gas phase, and to allow analysis of particle behavior under real conditions inside the furnace. Parametric analysis is done with emphasis on the thermal share, size and shape of biomass particle, method of biomass feeding into the furnace and the fuel distribution over the burner tiers. In the most favorable co-firing case (with 10% of wheat straw thermal ratio and particle diameter of 500 μm), the higher furnace exit gas temperature for 8 ˚C and lower NOx emission of 18.2% are achieved, compared with pure lignite combustion case. The optimal co-firing case provides relatively low percentage of wheat straw particles falling into the hopper (9.57%) and relatively high mass burnout of biomass particles at the furnace outlet (91.81%). Non-spherical shape of wheat straw particles is found to affect the fuel trajectories and flame significantly. The results of parametric analysis could support implementation of biomass co-firing technology in existing coal-fired power plants, to increase energy efficiency and mitigate environmental pollutants.