High temperature aerosol formation and emission minimisation during combustion of wood pellets

Sammanfattning: Combustion of solid biomass under fixed bed conditions is a common technique to generate heat and power in both small and large scale grate furnaces (domestic boilers, stoves, district heating plants). From both an environmental and economical perspective, one of the most interesting alternatives to replace oil and electricity for heating of family houses and apartments in Sweden is wood pellets made of stem wood sawdust from pine and spruce. Unfortunately, combustion of biomass (also wood pellets) will produce particle emissions that may have a negative implication on the human health. The smallest (< 1.0 microns) particles are very hard to separate in ordinary cleaning devices. However, for large plants, advanced gas cleaning devices are an acceptable option and therefore a relative high amount of particles in the flue gas before cleaning may be tolerated. For small appliances such as wood pellets burners and stoves advanced gas cleaning devices is not an option due to its relative high costs. Hence, the only way to reduce emissions of small particles small scale equipments is therefore to reduce the formation of particles already in the combustion process. The aim with this work was to study the formation mechanisms and the influence from different operating and fuel parameters on particle emission during combustion of wood pellets. The results from this work may then be used as a basis for design with aim to minimise the particle emissions already in the combustion process. To address these issues, experiments were carried out in an 8-11 kW updraft fired wood pellets reactor that has been custom designed for systematic investigation of particle emissions. To investigate the formation mechanisms, particle samples were withdrawn from the centre line of reactor through 10 sampling ports by a rapid dilution sampling probe. The corresponding temperatures at the sampling positions were in the range of 200-1450 ¢ªC. The particle sample was size segregated in a low pressure impactor, allowing physical and chemical resolution of the fine particles. The chemical composition of the particles was investigated by SEM/EDS and XRD analysis. Furthermore, the experimental results were compared to theoretical models of particle formation and growth. When the influence from fuel and operating parameters was investigated, particle samples were withdrawn isokinetically in the flue gas stack after the reactor. The particle mass were analysed with traditional filtering technique. The particle mass and number size distribution were analysed by low pressure impactor and a scanning mobility particle sizer. The chemical composition of the particles was furthermore analysed with SEM/EDS. The results in this work show that combustion generated particles during wood pellets combustion is produced from several mechanisms resulting in a bimodal or a trimodal size distribution. The largest particles (> 10 microns) are produced from residual fly ash particles (refractory metals) that have left the fuel bed and been carried by the gas upwards. The finest particles (< 1 microns) are produced from two mechanisms, vaporisation and condensation of easily volatile ash elements (K, Na, S, Cl, Zn and in some case also P) and from incomplete combustion (i.e. soot particles). The middle mode between the coarse and the fine mode (~1 microns), is produced from a combination of refractory oxides, unburned carbon and condensable inorganic species. In general the fine mode (< 1 microns) dominates the mass and number concentration of the total particle emissions in the flue gases after the combustion chamber followed by the coarse mode (> 10 microns). The mass concentration of the middle mode (~1 microns) are significantly lower then both the fine and the coarse mode. The particle concentration in the flue gas channel is affected by both operating and fuel parameters. The results showed that both the temperature and the flow pattern in the combustion zone affect the particle emissions. Increasing combustion temperature yields decreasing emissions of coarse fly ash (> 10 microns) and soot particles, however, the emissions of submicron fly ash particles increases simultaneously. Increased mixing rate in the combustion chamber will also decrease the emissions of soot particles. In addition to the operating conditions, significant differences in particle emissions were found between different biomass fuels. For the particles that were dominated by ash elements the particle emissions were correlated to the ash concentration in the unburned fuel. However, if the combustion condition allowed for organic particles, the sooting tendency of each fuel becomes important. Furthermore, the results showed that in general the fuel type affects the particle emissions stronger than the influence from different operating and construction parameters.