Importance of experimental measurements and simulations for "sludge-to-energy" systems

Abstract

It has recently been found that sewage sludge is a suitable and widely applicable alternative energy source which belongs to renewables. Several options for sludge-to-energy exploitation in sludge treatment sections of waste-water-treatment plants (WWTP) can be considered. Among the key factors influencing the choice of sludge treatment technology are the sludge heating value, its composition and specifically, the fraction of organic compounds. A crucial parameter in evaluation of mass and energy fluxes is the level of attainable de-watering, which depends on several factors which should be investigated. Thermal properties of sewage sludge belong among the most important input data for a calculation of characteristic parameters of a heat exchanger for sludge preheating. Results of measurements, performed in order to obtain the necessary data, are presented in this paper. Temperature dependences of the following sludge properties have been measured: dynamic viscosity, density and specific heat capacity for temperature range from 20°C to 50°C. Sludge samples were taken from two sewage plants which differ in the type of inlet waste water. The results of measurement are presented in diagrams. Temperature dependences of the above-mentioned quantities were measured for digested, mixed-raw and pasteurized sludge. Acquired data were used for preliminary assessment of heat exchanger water-sludge. This novel type of heat exchanger will be used for determining the influence of temperature on the degree of sewage sludge dewatering. For this specific task, a made-to-measure heat exchanger had to be designed. From the evaluated alternatives choices, a conception of a helical plate heat exchanger has been selected. The application of this heat exchanger and dependence of heat exchanger design on sludge properties are discussed.