하수 슬러지의 효율적인 연료화를 위한 증기압축-유중증발 결합 공정의 특성 해석

The ban of ocean dumping of sewage sludge has raised a growing interest on energy recovery technologies. For fuel production from the highly wet material, energy-efficient drying becomes the key technology. This study investigates a process combining ⅰ) the fry-drying of sludge for quick and complete drying and ⅱ) the steam compression for efficient recovery of latent heat in steam evaporated from sludge during fry-drying. Through heat and mass transfer calculations, this study identified the key design parameters and issues to be addressed for actual operation of the process. For recovery of the latent heat in steam, two types of condenser were considered: spray tower condenser and tube-type heat exchanger. The spray tower condenser allows direct contact of compressed steam at 2-5 bars with the cold oil sprays circulated from the oil bath. This can maximize the contact area between the two phases and therefore recover the latent heat in a compact volume. However, the flow rate of oil circulation should be about 10 times larger than that of steam in order to achieve an overall energy efficiency of 50% for sludge with a moisture content of 80%. This is due to the huge latent heat of steam compared to the heat capacity of oil. Such large circulation of oil from the bath to the condenser is not practical in terms of a stable operation of the bath as well as the electricity input required for a pump. When the tube-type heat exchanger is used for heat recovery, it is installed inside the oil bath to directly heat up the oil. As the saturation temperature of steam in the tube increases with pressures, steam compression into high pressures can reduce the length of the tube required for heat recovery. For a 100 ㎏/hr throughput of wet sludge into an oil bath with a volume of 1.0 ㎥, estimated energy efficiencies achievable were 61.81% at 5 bar of steam compression and 43.88% at 2 bar. It was concluded that the energy efficiency of the fry-drying process combined with steam compression is essentially limited by the size of condenser or the flow rate of the heat transfer medium (oil). In-bath heat exchanger is favorable for the stability during operation, but requires further process optimization.