HEAT DRYING BIOSOLIDS: ECONOMICS, ENERGY, AND SAFETY CONSIDERATIONS

Heat drying systems evaporate water from biosolids, creating a product that is free of pathogens (Class A). This paper discusses three key factors in selecting and designing a drying system and provides guidelines and recommendations to address those factors. The factors are economics, energy, and safety.The capital cost of a biosolids drying system can be significant, often higher than alternative disposal systems. This higher cost needs to be balanced against the generally superior product and risk reduction associated with that product. In addition to capital costs, the energy demand of drying systems is relatively high, and every effort to reduce this cost will increase the feasibility of a project.While water evaporation theoretically requires 960 Btu per pound, various inefficiencies and energy losses, such as heating the dry fraction, heat transfer losses, stack heat losses, burner inefficiency, and equipment radiation losses, combine to raise the heating requirements to 1,300 to 1,600 Btu/lb. Consequently, designers and manufacturers seek opportunities for reducing heat losses and recovering heat. These opportunities include:Recovery and reuse of heat from the dryer exhaust gas to heat the dryer combustion air or other processes within the plant, such as digestion.Use of digester gas to fuel the dryer burner.Use of exhaust gas from an engine or turbine to directly heat the biosolids or to heat the oil or steam in an indirect dryer.Two important and equally serious safety concerns result from the possibility of biosolids combustion and biosolids dust explosion.Biosolids are composed primarily of carbohydrates, proteins and fats and will burn readily in the solid form. Warm organic material containing even small amounts of moisture and an available oxygen supply can generate sufficient biological activity to produce smoldering or even combustion. Control measures for the auto-heating problem include reducing the temperature of the final product, minimizing oxygen content in the gas contacting the product, and nitrogen blanketing.The organic dust associated with dried biosolids has caused explosions at a few installations. Preventing the formation of dust is the first line of defense against explosions.Both combustion and explosion mitigation can also be achieved by design and operating strategies such as oxygen sensors within the drying units, thermocouple trees to monitor silo temperatures, and infra-red detectors in product conveyors