Milling and Physical Properties of Wood Pellets for Suspension-Fired Power Plants

The utilization of sustainably produced wood pellets in the existing coal suspension-fired power plants can offer a cost-efficient option of mitigating greenhouse gas emissions. However, the fibrous and non-brittle structure of wood poses challenges with regard to the size reduction process of wood pellets in the existing coal mills.

There is a lack of understanding of the pellet grinding behavior and morphology (i.e., size and shape) of milled pellet particles, and how the particle morphology can be related to the wood pellet processing history. New knowledge in this area has the potential to promote efficient and fast conversion of coal-fired power plants to the firing of milled pellets.

For this purpose, the study investigated the pellet grinding behavior in lab- and industrial-scale mills. The grinding behavior was determined by measuring the specific grinding energy and analyzing the morphology of milled and internal pellet particles. New methods were introduced to characterize the grinding behavior of wood pellets by laboratory testing, and to investigate if it is possible to predict grinding results in power plant coal mills.

The study used two industrial pellet qualities, and two pellet types made from Austrian pine (softwood) and European beech (hardwood) stem wood. Pellets were characterized according to standardized methods. The study has found that the industrial pellet production process has a larger impact on modifying the pre-densified wood particle shape than the pellet grinding process.

The pellet grinding behavior can be related to the mill type and pellet processing history (including feedstock type and internal pellet particle size distribution). It was shown that grinding of raw and pelletized beech produces finer, rounder and less elongated particles, and requires a lower grinding energy than pine.

The proposed laboratory roller mill-classifier system has the potential to assess the grinding properties of different pellet qualities. The comparison with grinding results from industrial mills showed that similar particle size reduction ratios can be achieved. This work presents relevant experimental data that provide an understanding of the morphology changes occurring during the industrial pelletization process.

It also has great value for power plant operators to maximize the pellet grinding capacity in the existing coal mills.