Why would a high porosity of the fuel lead to more fines when combusting a solid biofuel?

Porosity is defined as the volume of void spaces (open pores) in a solid material, expressed as a fraction or percentage of the total volume. In the context of solid biofuels, porosity plays a significant role in their combustion characteristics since it affects the fuel's density, heat transfer, and chemical reactivity. Fines, on the other hand, are small particles or dust resulting from the combustion process, which can cause operational issues in boilers and air pollution when released in the environment.

The formation of fines during the combustion of solid biofuels is influenced by several factors, including the fuel composition, moisture content, particle size, combustion temperature, and combustion residence time. Porosity is related to these factors, as it directly affects the fuel's specific surface area and reactivity, which, in turn, influence the formation of fines.

A high porosity solid biofuel has a larger specific surface area available for combustion reactions, which results in faster and more efficient combustion processes. This increased reactivity leads to rapid devolatilization of the biofuel, which can result in increased formation of particulate matter such as fines. Furthermore, highly porous biofuels have low mechanical strength due to their higher void volume fraction and weaker solid matrix, making them more susceptible to fragmentation during combustion. This fragmentation can lead to the generation of more fines, which can create operational challenges and environmental issues when not adequately managed. In summary, high porosity in solid biofuels results in increased fines production because of the enhanced reactivity due to the larger specific surface area and the low mechanical strength of the fuel, which makes it susceptible to fragmentation during combustion.