Study on increasing the energy efficiency of a dryer used for the conversion of sewage sludge to biofuel

The thermal energy content of sewage sludge (SS) is close to that of low-grade fuels. Drying is the most energy-consuming process in the conversion of sludge to biofuel. However, the energy efficiency of the drying system is not sufficiently emphasized. In this study, the energy efficiency of the sludge drying process is investigated using an experimental contact-type dryer. The effects of wall temperature, rotation speed, and waste heat usage alternatives are investigated. Waste heat usage has the highest impact on energy efficiency. Flue gas feeding increases the drying rate and reduces the total drying duration. Energy efficiency is increased with increasing temperatures. The rotation speed does not considerably affect energy efficiency. Sludge passes through 3 distinct phases regarding the specific energy consumption (SEC). In the first phase, the sludge temperature increases to the saturation temperature. A rather long period of constant energy consumption follows this phase. Most of the water evaporation and drying period occurs in this phase. SECvalues begin to increase in the last phase when humidity is in the range of 0.5–1 (30–50% wb). Considering the first drying stage, a 32–20% higher evaporation rate is obtained with waste heat recovery application depending on the temperature.