Influence of particle shape and internal thermal gradients of biomass particles on pulverised coal/biomass co-fired flames

The practice of employing pulverised coal/biomass co-firing in power plants is gradually increasing. This is mainly because of the benefits associated in reducing the coal based CO2 and biomass based SOx and NOx emissions. However, biomass is difficult to mill due to its fibrous texture and this results in the presence of large particles of different shapes which influence the combustion characteristics. Existing computational fluid dynamics (CFD) models often ignore thermal gradients within the particles and this leads to inaccuracy in the combustion process modelling. In this paper, a CFD sub-model for the heat transfer within large particles is developed. The model is validated for the heating up, moisture release and devolatilisation of single wood particle measurements that are available in the literature. The impact of fuel particle sizes on the combustion characteristics has been investigated in terms of ignition, devolatilisation and char combustion in a co-firing case of an industrial combustion test facility. The predictions, while considering the internal thermal gradients with particle size and shape distribution, were identified to be in excellent agreement with measured data. The code was worked well when coupled with ANSYS FLUENT and with a negligible amount of extra time for the computations.

► A thermal conversion model for biomass fuel particles is proposed.
► Proposed model is validated for large woody biomass particles.
► Importance of the internal thermal gradients within particle is addressed.
► Influence of shape and size of biomass particle on combustion is addressed.
► An industrial test case is simulated and modelling improvements are highlighted.