Process challenges in rotary kiln‐based incinerators in soil remediation projects

Abstract

AbstractMost Superfund sites undergoing thermal remediation consist of high‐moisture content (15 to 40 percent) soils having light organic contamination (low‐heat content). In the early 1980s, the rotary kiln incinerators employed at these Superfund sites were relatively small mobile hazardous waste incinerators (HWI) built on standard‐size semi‐trailers. As the industry matured over the past ten years, competition has driven the industry to larger and more efficient rotary kiln systems that minimize the cost per ton of soil treated by increasing the throughput rates and shortening on‐site processing time.Because these units are transported from site to site by truck or a combination of rail and truck, the rotary kiln outside shell diameters have peaked to approximately 13.5 feet. Now the focus has shifted to increasing the processing capacity of these existing, maximized fixed‐sized systems. Such actions include the use of oxygen‐based combustion systems, which increase the throughput and efficiency by improving heat transfer and reducing the volume of combustion gas requiring treatment. However, despite the experience and expertise gained by the participants in the thermal remediation industry, many of the same process challenges still remain. Combustion gas particulate entrainment from a rotary kiln HWI is a key issue when thermally treating soils because of the slagging potential in the secondary combustion chamber (SCC) and/or the potential to overload the downstream incinerator wastewater treatment system (IWTS). Excessive infiltration air into the rotary kiln can reduce the rate of heat transfer to the solids bed and correspondingly reduce the overall processing rate. More stringent ash quality requirements require higher ash temperatures and longer solids retention time at temperature. Estimating metal emissions remains a challenge due to the limited availability of data on metal behavior in combustion systems. New and more stringent particulate, nitrogen oxides (NOx), metals, and dioxins/furans emissions performance requirements present additional challenges, as does a trend in towards zero‐liquid discharge from the air pollution control system.This article discusses these process challenges facing the industry and potential solutions, based on data from IT's Hybrid Thermal Treatment System® HTTS® and other sources.