The most efficient way so far to extract energy from renewable sources is combustion of solid fuel. Solid fuel furnaces of moderate capacity (5–10 MW) equipped with reciprocating grates are most popular. Grate combustion is a well-developed technology; however, to burn biofuel in this type of furnaces in the optimal and safe way, the fuel must be of high quality and have at least constant moisture content. However, increasing demand for biofuel results in increasing prices. To remain in the market and to stay competitive, heat producers choose to utilise moist biofuel of lower quality, whose moisture content can vary and reach up to 60% wt. The burning on the grate of such biofuel is complicated as the drying process occupies most of the space in the furnace. The purpose of this work was to analyse processes taking place in a furnace, such as: primary air supply, influence of flue gas recirculation and radiation from hot surfaces of the furnace to biofuel drying. Analysis of the data obtained would provide technical decisions facilitating optimal fuel combustion in a furnace without additional investments. Analysis of biofuel drying was performed in an experimental setup with a fixed fuel bed. The experiments were performed with wood chips and four different drying fluid temperatures. The results of experimental studies have shown that the drying rate of biofuels upper layers is strongly influenced by radiation from hot surfaces and the moisture content of the sample decreases by 18% wt.

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Gasification process is a fairly complicated matter and using pelletized biomass for the gasification mostly results in fuel agglomeration. The pelletized biomass moving from the pyrolysis zone to the oxidation zone sticks together in lumps and disrupts entire process. In order to determine the regularities of thermal deformations, experimental research of pelletized biomass thermal deformations during pyrolysis were performed in a horizontal pyrolysis reactor from 300-900 °C temperature capturing wood particle, wheat straw and wood pellet radial changes by a digital camera. Also the center temperature and the mass loss of samples were measured to determine cause of biomass thermal deformations. Observed results reveal that when increasing the pyrolysis temperature from 400-900 °C, the wheat straw and wood pellets expand at the beginning of pyrolysis process and after it start to shrink, while wood particle is only affected by shrinkage. The swelling effect of pelletized samples starts decreasing over 600-650 °C heating temperature and disappears when the temperature is higher than 850 °C. Biomass shrinkage intensifies exponentially as the heating temperature increases till 700-750 °C. However, the final shrinkage starts to decrease as the heating temperature increases from 700-750 to 900 °C due to swelling of formed char. Determined phenomenon of pelletized biomass swelling explains cause of fuel adhesion in pyrolysis zone of gasifier. Besides estimated regularities of biomass thermal deformations upon pyrolysis could be used to improve the existing numerical models of biomass pyrolysis.

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