From Tech Trends August 1995

Gasification Treatment for Soils

By Marta K. Richards, EPA National Risk Management Research Laboratory

Texaco, Inc. has developed waste gasification as an innovative extension of their conventional fuels gasification technology. The Texaco Gasification Process (TGP) was evaluated by EPA's SITE (Superfund Innovative Technology Evaluation) Program to determine the applicability of the TGP to treatment of a hazardous waste-contaminated soil. Since the TGP is operating at temperatures above the melting point of the inorganic materials in the feed stream, the solid residuals form a glassy slag, while the TGP converts carbonaceous organic materials into a gas mixture, called syngas, consisting primarily of hydrogen and carbon monoxide. This is accomplished by reaction with a limited amount of oxygen (partial oxidation) in a refractory-lined gasifier at temperatures in excess of 2,200 F and at pressures greater than 250 psig. According to Texaco, these temperatures and pressures are sufficient to destroy hydrocarbons and organics in the feed and prevent the formation of undesirable organic by-products associated with other coal/fossil fuel conversion processes. The TGP produces syngas as a chemical intermediate product which can be used in the production of hydrocarbons, ammonia, methanol and other chemicals. Alternatively, the syngas can be combusted directly in a gas turbine to produce electricity.

The process works as follows. Waste feed, along with coal, oil and/or coke is ground and mixed with water in a high-solids' concentration slurry (generally 55 to 70% by weight). The water serves as a reactant, a temperature moderator and a transport medium. Liquid and gas fuels, which can be fed directly to the gasifier, need no pretreatment but may require water or steam moderators. For the SITE demonstration at the Texaco Montebello Research Laboratory in South El Monte, California, the slurry feed consisted of coal, water, waste soil from the Purity Oil Sales Superfund Site in Fresno, California, and clean soil. The composite slurry was spiked with heavy metals (lead and barium) to ensure that the feed contained sufficient levels of contamination to fail the Toxicity Characteristic Leaching Procedure (TCLP) test. The addition of approximately 6.42 lbs. per hour of chlorobenzene, a volatile organic, was fed into the gasifier with the slurry to facilitate the calculation of the Destruction and Removal Efficiency (DRE).

In the SITE demonstration, slurry was fed into the gasifier, along with oxygen, through an injector nozzle. The chlorinated species in the feed formed hydrogen chloride in the raw syngas; the hydrogen chloride dissolved into the scrubber water before being purged from the process. The sulfur in the feed formed hydrogen sulfide or carbonyl sulfide in the product gas. Commercial acid gas and sulfur removal processes scrubbed these sulfides from the product gas. The residual ash was recovered as a glassy slag.

The DRE for chlorobenzene was greater than 99.99%. The average composition of the combustible dry syngas from the TGP was 37% hydrogen, 36% carbon monoxide and 21% carbon dioxide. No organic compound, other than methane (55 parts per million) was above 0.1 ppm. On the average, the primary TGP solid product, coarse slag, complied withthe TCLP regulatory requirement for lead [5 milligrams per liter (mg/L)] and the TCLP and California Waste Extraction Test (WET) regulatory requirements for barium (100 mg/L). The coarse slag did not meet the WET standard for lead (5 mg/L). Volatile heavy metals, such as lead, tend to partition and concentrate in the secondary TGP solid products -- fine slag and clarifier solids. These secondary products did not comply with the TCLP and WET regulatory requirements for lead but did meet the standards for barium.

For more information, call Marta Richards at EPA's National Risk Management Research Laboratory at 513-569-7692. A Technology Capsule (Document No. EPA/540/R-94/514a) can be ordered by calling 513-569-7562). An Innovative Technology Evaluation Report should be available by Fall.