Biomass combustion in pulverized-fuel boilers: use of apparent pyrolysis parameters in CFD codes

Blondeau, Julien [UCL] Jeanmart, Hervé [UCL] Ryckmans, Yves [Laborelec] Allard, Patrick [Laborelec] Rochaya, David [Laborelec]

Biomass combustion or co-combustion in pulverized-fuel boilers (pf-boilers) has become an attractive way to reduce CO2 emissions from power plants. Computational Fluid Dynamics (CFD) codes used for design or diagnostic purposes are most of the time inherited from the study of coal-fired pf-boilers. Due to the fundamental differences between coal and biomass combustions, such tools need to be adapted. As pyrolysis accounts for the major mass and energy releases during biomass combustion in pf-boilers,the present work focuses on this step. In order to relax the particles’ isothermicity assumption commonly used in CFD codes, that has been proven to be questionnable for biomass applications, a detailed pyrolysis kinetic scheme has been coupled to a physical model of a single biomass particle. In previous works, apparent kinetic parameters have been derived from this detailed single particle model. The use of these apparent parameters in existing CFD codes requires nomodification of the code structure nor any increase of the computational time. It allows for the consideration of particle internal heat transfer effects. This modifies the gaseous products emission rate from the particles. The influence of the proposed apparent parameters has been tested on a complete CFD simulation. The main results are presented here. A large impact on the resulting flame temperature has been observed, together with a modification of the predicted CO and NOx emissions. Comparison with onsite measurements shows that the proposed parameters improve the prediction capability of the code. The proposed apparent parameters method has moreover been extended to the prediction of the gaseous species emitted by the particles. Again, a parametrical adaptation of the CFD sub-models can be achieved on the basis of the detailed single-particle simulations.