Influence of Geometrical Features of Solid Biofuels on the Implementation of the Combustion Process

Abstract

ABSTRACT The article discusses the influence, in a broad spectrum, of geometric features of various solid biofuels (pellets and briquettes in several sizes) of plant origin (wheat straw, rye straw, oat straw, birch, and pine sawdust) on the combustion process, in particular on the ignition time, flame duration and fuel afterburning in a low-power boiler. The research used a test stand, an essential element of which was the standard grate boiler with a cuboidal combustion chamber surrounded by a water jacket. This type of device allowed for the combustion of fuels with various geometrical features – pellets and briquettes – in the same combustion chamber. The particles of the combusted fuel had a surface area (S) from 281 to 13188 mm2, a volume (V) from 345 to 115 787 mm3 and a density (ρ) from 602 to 1127 kg·m−3, which resulted in a total combustion total time from 600 to 1500 s. The duration of individual combustion stages of the tested biofuels was proportional to the product of the square of the volume/area quotient and the fuel density. It was found that the coefficients C0 (start combustion time coefficient), C1 (flame combustion time coefficient), C2 (afterburning combustion time coefficient) of their duration increase with the decrease in the value of the product of the square of the ratio of the volume to the area and the fuel density, which is characteristic for pellets. The distribution of the fuel ignition, flame combustion, and afterburning constants as a function of the product of the square of the volume/area quotient and the fuel density takes the power distribution with the fit values of 0.9017; 0.9779; 0.9322 respectively. The conducted correlation tests showed quite strong dependencies between the geometrical features of the fuel and the time of its combustion. At the same time, the correlations between the ash content in fuels and the duration of individual combustion stages did not show solid, regular relationships. The analysis of the course of their combustion concerning the duration of the various stages of combustion regarding variable geometrical features given to formed biofuels allows predicting the course of their combustion. It is essential at the level of exploitation of heating devices. From the perspective of this research fuels with a higher value of the square of the quotient of the volume to the area and the fuel density would be optimal.