Reduction of Ultrafine Particle Emissions in Wood Fired Cookstove by using a Turbulent Agglomerator

Abstract

Emission of fine and ultrafine particles from wood fired cook stoves are a major concern worldwide especially in the many countries where biomass cookstove are used on daily basis. Due to their small size these particles can reach deep within the lungs and cause various health issues. The effect of insertion of a turbulent agglomerator in the cookstove chimney on the number of small particles emitted has been investigated. This device consists of a number of properly designed obstacles in the flow increasing the turbulence level and enhancing collisions and agglomeration of particles. The device is suitable because it requires little extra investment and does not require an external energy source.
First an analytical model of the flow through a Plancha type cookstove was made in MATLAB, in order to obtain estimates for the flow properties at the chimney inlet. Next, CFD simulations were performed using Ansys Fluent R3 and the grid generator program CFMesh+ software. Eulerian-Lagrangian (EL) simulations of dispersed multiphase flow were made to determine the particle dynamics within the flow domain. Particle agglomeration was studied using Eulerian-Eulerian (EE) simulations in combination with the population balance equation (PBE) for particle size. Appropriate agglomeration kernels were implemented via user-defined functions. A key feature taken into account is that agglomeration of particles for particles smaller than the Kolmogorov scale is determined by Brownian motion whereas turbulence takes over as driver for agglomeration for particles larger than the Kolmogorov
scale. The turbulence properties were obtained using the standard k-ϵ model with standard wall functions.
To check the operation of the CFD model, a first application of the EE approach was made to a turbulent agglomerator with extra air injection via a jet in the side wall. This was studied in a simplified 2D geometry. In the investigated case it was observed that the turbulence caused by the jet has larger effect than the turbulence of the obstacles.
Next, the EL and EE CFD models were applied to a agglomerator type for which experimental data are available. These simulations concern a 3D domain representing chimney with obstacles creating turbulence. To be relevant for the cookstove chimney case operation at a lower flow rate than the experiment had to be studied. Wakes behind obstacles play an important role. Large particles have higher probability to bypass the wakes. The wakes provide zones with larger mean residence time where the agglomeration process can have more effect. The simulations using the PBE provided quantitative results for the evolution of the volume fraction of different size classes of particles. It was found that for the considered chimney (lower velocity, hence lower turbulence level) the turbulent agglomeration effect is too weak to obtain a sufficient removal efficiency of submicron particles (under 1%). In case of higher particle load higher removal efficiency can be achieved.