Physical and combustion characteristics of biomass particles prepared by different milling processes for suspension firing in utility boilers

The physical and combustion properties of roller-milled and hammer-milled wood pellets for suspension-firing are characterized both experimentally and numerically. The particle size and shape distribution is measured based on dynamical digital image processing. The traditional coal roller mills produce larger biomass particles than hammer mills do. The true density of the particles, measured by ethanol displacement method, does not remarkably vary with the different mills. Combustion tests of the milled biomass particles are performed in a single particle combustion reactor under conditions close to suspension-fired boilers. The ignition, devolatilization and burnout times of the milled particles under different combustion conditions are analysed. A one-dimensional transient model, properly accounting for the particle-ambient flow interaction and appropriately addressing the key sub-processes within the particle, is also developed to predict the combustion behaviours. The good agreement between the model predictions and the experimental results serves as a cross validation of both the tests and the model. Without changes in the infrastructure, biomass pelletizing may not be a viable pre-treatment for large-scale biomass combustion in the existing pulverized coal-fired utility boilers.