Peat beneficiation by wet carbonization

Abstract

Peat, as it exists in nature, contains about 90% water and has a heating value of about 19.8 MJ/kg (dry basis). The water in peat is very difficult to remove by mechaninal means. The current technology can dewater peat to a moisture content of only about 67 wt.%. Wet carbonization is a process whereby carbonaceous materials are heated under pressure for a period of time to produce a beneficiated product. In the case of peat, a low-sulfur solid fuel with enhanced heating value, improved dewaterability, and better handling characteristics is produced. A comprehensive study on the effects of operating conditions on the wet-carbonization characteristics of Minnesota peat was conducted. This study was jointly sponsored by the U.S. Department of Energy/Pittsburgh Energy Technology Center and Minnesota Power and Light Company. Tests were conducted in a 360-kg/h continuous process research unit. The operating conditions studied were temperatures ranging from 175 to 260°C; residence times of 1–60 min; atmosphere, steam and air; product filtrate recycle; and slurry concentration, 6.1–9.5 wt.%. The effluent process water was treated by IGT's two-phase anaerobic digestion process for energyrecovery. This process maximizes the production of methane and minimizes hydraulic retention times in the digesters. An integrated wet-carbonization process to produce a solid peat fuel with energy and by-product recoveries from the liquid effluent streams has been developed. This paper discusses the work accomplished in this study.