Fly ash is more controversial. Its potentially hazardous nature is due primarily to the volatile toxic metals that it contains. Metals that have low boiling points (Cadmium, Zinc, Lead, Mercury, etc.) tend to concentrate and accumulate in the fly ash, and the ash will frequently fail the EPA test for toxicity. Federal and state regulations have been moving toward stricter disposal regulations for fly ash. Most states now require that fly ash be placed in a regulated monofill or a hazardous landfill.

Various efforts have been made at implementing technologies that would stabilize ash by mixing it in cement, polymers, and various clay-based substances. None of these has been approved as an official treatment method.

Melting ash at appropriately high temperatures with fluxing media (vitrification) has been demonstrated to form a monolithic glass/ceramic solid that normally passes the EPA Toxicity Characteristic Leaching Procedure (TCLP) tests. If the solid residue passes the TCLP test, it is not a hazardous waste. If a use can be found for the solid residue, and it can be continually put to use, it does not meet the definition of a waste.

The process used to destroy organic materials is pyrolysis and gasification. The process is accomplished in a slight vacuum and in the absence of air, and uses metered amounts of steam to convert the solid/liquid organic materials into a synthetic fuel gas (syngas). Breakdown of the solid/liquid organic material is accomplished inside a refractory lined vessel with high-density ultraviolet (UV) and infrared (IR) energy produced by a plasma torch. The syngas, composed mainly of Hydrogen and Carbon Monoxide, represents a significantly smaller volume of gas than that produced from full combustion of the organic material, and contains less particulate. Destruction of organic materials is more thorough, providing higher Destruction and Removal Efficiency (DRE) than accomplished by incineration.

The amount of organic material in ash is a result of the efficiency of the incinerator that produced the ash. When treating ash with a TDR System, all carbon, unburned organic material, and toxic and hazardous organic residues in both bottom and fly ash will be totally destroyed. An efficient incinerator will leave very little carbonaceous material, and treating ash of that type with a TDR System will result in very low production of syngas. TDR treatment of ash with little or no carbonaceous material is basically a melting/vitrification process directed toward the inorganic nature of the waste.

In all cases, there will be a small gas stream that results from the TDR Process. Even if there is no carbonaceous material to gasify, there will be a low volume torch gas that flows through the process, and ash usually contains some moisture that produces steam. The volume of gas coming from TDR vitrification of ash will be extremely small, and if the system is located near the incinerator that is producing the ash, the off-gas can be piped into the incinerator scrubber system, eliminating any direct atmospheric emissions from the TDR Process.

The TDR vitrification process can be used for bottom ash or fly ash independently, or they can be mixed. The process goal is to melt the ash into a monolithic glassy/ceramic material that is resistant to leaching when cooled. Fly ash is usually dominated by metal particulate, and may not have sufficient glass-forming materials to produce a low melting, low viscosity melt. In this case, fluxing agents, or glass-formers, (i. e., soda ash and sand) may need to be added.

Bottom ash, especially from WTE’s will contain a substantial amount of glass forming material (scrap glass, sand, ceramics, etc.) negating any requirement for adding flux. Mixing bottom and fly ash usually results in a glassy ceramic residue that can be tapped from the processing vessel into a water tank where it shatters into gravel-sized material that can be used as construction aggregate or fill material. There is very little value in the glassy residue, but the cost avoidance associated with landfill disposal can be substantial.

Bottom ash, especially from WTE’s may also contain a substantial amount of metal. Most of this metal will melt and accumulate as a separate layer in the bottom of the processing vessel. The metal layer can be tapped into molds, cooled and recycled as scrap.