Solar drying of granulated waste blends for dry biofuel production

In the paper, results of drying biofuels from sewage sludge using solar energy are presented. Drying rates of biofuels made from sewage sludge and coal slime (PBS), sewage sludge and meat and bone meal (PBM), and sewage sludge and sawdust (PBT) with 15-mm and 35-mm granule particle size were studied. Tests were performed in a solar greenhouse dryer equipped with a specially designed mixing system. Experiments were aimed at determining the drying time of biofuels under various weather conditions in the southwestern part of Poland. In summer, in order to determine the best conditions for drying, tests were performed using various parameters, i.e., layers of various thickness, such as 5, 10, and 20 cm, and various mixing intensity (no mixing, mixing 3 and 5 times/day). In spring and the fall, 10-cm thick layers combined with 5 times mixing of fuels per day were used. The performed tests demonstrated that it is beneficial to dry fuels in 10-cm thick layer. In spring and the fall, PBS and PBM biofuels laid out in layers with just such thickness showed moisture content reduced to less than 10% after 8 days, while the PBT biofuel reached the same level after 14 days. In summer, the same result may be obtained for all the biofuels after 4 days on average. The presented original method of solar drying of biofuels obtained from sewage sludge and other waste may be used in wastewater treatment plants which process sewage sludge into fuels without incurring any additional costs for supplying heat.