The recent development of efficient electrostatic ReCyclone® systems allows their application to biomass boilers for strict legal limit compliance. The purpose of this paper is to present a model to predict in a more realistic way the collection efficiency of a gas cyclone with an electrostatic enhanced recirculation system. These systems consist in an optimized reverse-flow cyclone (col- lector) combined with partial recirculation of un-captured particles via a straight-through cyclone (concentrator). A DC electric field is applied to the concentrator to enhance particle recirculation to the cyclone collector, leading to very high collection efficiencies.
To model the system, a computer program (PACyc) was recently developed. The model, starts by estimating the collection efficiency of a system without agglomeration, building what is referred as the baseline grade-efficiency curve. To build this baseline curve, the PACyc model relies on previously published models to predict collection efficiency either for isolated cyclones, or for cyclones with mechanical/electrostatic recirculation.
After solving the particle trajectory in a turbulent flow field inside the cyclone, and by employing a fixed set of parameters, PACyc determines if a collision/agglomeration occurs. The result of this procedure is an altered grade-efficiency curve after agglomeration (a composite curve), since the initial particles will have a dynamic behavior inside the cyclone as a newly formed agglomerate. The calculated efficiency will significantly increase above theoretical predictions without agglomeration, and this can be observed in various experimental results, usually referred as “hook-like” grade- efficiency curves.
Some experimental results obtained in biomass boilers burning wood and coke are presented, together with the corresponding PACyc predictions on grade and global collection efficiencies.