983857 - Biofilter CFD Modeling for Atmospheric Emissions Control of a Heating System Based on Sawdust Pellets

The use of waste biomass, e. g., sawdust, is an alternative for heating and industry in countries with large areas of radiata pine and eucalyptus forests. Indeed, the Chilean demand for domestic, industrial, and district heating has been increasing in recent years, standing at 75 thousand tons per year and with a projection to increase by 400 % until 2030. However, burning hydrophilic sawdust pellets and wooden logs generates particle matter, NOx, and polycyclic aromatic hydrocarbons (PAHs) emissions that have placed this type of fuel as one of the most contributing sources of pollutants into the atmosphere in southern Chile. According to current emission inventories, residential biomass combustion contributed more than 95% of total particulate matter emissions. It has become a limitation for biomass as a source of energy and a challenge for applying Waste-to-Energy technologies to obtain biochars with greater calorific value and less ash. On the other hand, PAHs are the most problematic pollutants group to reduce in the solid and gas phase due to its high molecular weight. Thus, it is important to study a system that allows the reduction of those pollutants by improving the solid fuel and the pellet stove and developing an efficient pollution control system, like a PAHs biofilter.
This research studied the emission of benzo(a)pyrene (BaP), as a model compound of PAHs, during the combustion of commercial pellets in a stove. The PAHs emissions, determined by gas chromatography, were between 17.8 and 22.4 μg/kg of pellets with an operating temperature range of 350 to 450 °C. A biofiltration system treated the exit combustion gas for pollutant abatement. Vermiculite was used as a support medium coated with a microbial consortium composed of Fusarium solanis and Rhodococcus erythropolis. The system showed an efficiency close to 80% and an elimination capacity of 650 gm-3h-1.
On the other hand, for the CFD modeling, the geometry was obtained by a mathematical algorithm of random generation for bed packed in bioreactors. Meshing and numerical solution of the model were carried out by a new transient solver created and compiled in OpenFOAM, called woodyFoam. The resolution of the velocity - pressure field using SimpleFOAM showed a maximum error of 6%, and for the concentration of the BaP the error was 9%. The maximum substrate utilization rate was 0.18 gm-3s-1, with a Monod constant of 19.5 gm-3. The specific surface area value, determined by mathematical regression from the Ergun equation, was 157 m2/m3. In conclusion, biofiltration is an effective solution to reduce the PAHs emissions of burning biomass at the residential and industrial levels.