The reduction of bed temperature in fixed-bed biomass combustion is an effective measure to lower pollutant emissions. Air staging and bed cooling solutions are active strategies to decrease the fuel bed temperature. This work presents a CFD study of a biomass fixed-bed combustion plant that is equipped with an internal cooling bed system. Eight different cases are calculated to analyze the effect of the total airflow, air staging ratios and bed cooling system on biomass combustion. The findings are validated against experimental data from the literature. The results show good accordance between the numerical results and the experimental data. The primary airflow rate has the biggest influence on the bed’s maximum temperatures. The internal bed cooling system is able to achieve an average bed temperature reduction of 21%, slowing the biomass thermal conversion processes. Bed cooling techniques can be combined with air staging and primary airflow reduction to reduce bed temperatures in order to reduce pollutant emissions and other undesirable phenomena, such as fouling or slagging.

In this work, the three-dimensional CFD study of a fuel-flexible small-scale biomass combustion system with low primary air conditions is presented. The Eulerian Biomass Thermal Conversion Model (EBiTCoM) developed by the GTE research group of the University of Vigo was upgraded by including the detailed Ranzi-Anca-Couce (RAC) pyrolysis scheme, an updated gas reaction scheme with tar cracking and the latest advances in Eulerian bed motion dynamics. Thus, it was developed a comprehensive model capable of simulating combustion systems with large particle beds and complex geometries. Two case studies using fuels with different moisture contents (8% and 30% in mass) were simulated and the results validated against experimental data from literature. The species spatial distribution above the solid fuel bed and the temperature profiles inside the bed and in the primary zone were modelled with a good accuracy. It was shown that the fuel moisture content has a strong influence on the pyrolysis and drying zones and, therefore, in the volatile spatial distribution, as well as on the temperatures in the upper part of the bed and above it. However, the average gas composition and the maximum temperature are almost unaffected by the moisture content, since in both cases, the bottom part of the bed consists of a thick layer of char being reduced to CO, as in a gasifier. The presented model is a powerful tool for developing novel combustion technologies that achieve low emissions with a low primary air ratio in the fuel bed.