Salt precipitation and scale control in supercritical water oxidation—part B: commercial/full-scale applications

Abstract

Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each company's SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.