Review on the influence of biomass properties on suspension boiler wood particle combustion – What can be learned from laboratory and modelling studies

In recent years, several PF (Pulverized Fuel) power plant boilers originally designed for coal firing have been converted to use wood pellets as fuel. In most cases the conversions have been successful, and includes modifications of fuel storage facilities, mill equipment, burners and flue gas cleaning systems. Two mayor issues have to be dealt with when applying wood on PF power boilers, the high alkali rich wood ash and the relatively large wood particle size (up to 2mm) that may lead to poor burn out and unstable flames. The alkali rich ash may lead to excessive deposit formation, corrosion and poisoning of SCR units. Several wood-fired PF power plants use today coal fly ash as an additive to remedy the alkali related boiler issues. The combustion of the relatively large wood particles have been handled by making some modifications of the mill and the burners. However, PF wood boilers sometimes experiences problems related to the wood pellet fuel, which is not easily understood based on the standard fuel analysis often conducted on the received pellets. Typically appears problems with poor burn out (high loss of ignition of bottom or fly ash) or problems with obtaining adequate ignition such that the swirlstabilized flames are lifted from the near quarl area.
Many studies have dealt with the combustion of wood at conditions resembling the conditions in wood stoves, grate or fluid bed boilers. However, the conditions in PF boilers are somewhat different with local temperatures up to 1500°C, maximum residence times of less than 5 seconds and the use of wood particles with a maximum size of 2 mm. In recent years, some laboratory studies have investigated wood combustion at the relevant PF conditions, and in this work, we have tried to summarize the obtained knowledge with respect to how wood properties influence the combustion process in PF boilers. Our ambition is to understand which wood fuel parameters that mainly influences burn out and flame stability. In the future this may lead to recommendations for which fuel properties that should be analyzed to predict the PF combustion behavior of wood. This study provides information on particle properties that influences char yield, ignition, devolatilisation time and char combustion in wood PF boilers based on the present publish knowledge.