Hydrous pyrolysis/oxidation: in-ground thermal destruction of organic contaminants

摘要

Experimental work with organic solvents at Lawrence Livermore National Laboratory has suggested that in situ thermal oxidation of these compounds via hydrous pyrolysis forms the basis for a whole new remediation method, called hydrous pyrolysis oxidation. Preliminary results of hydrothermal oxidation using both dissolved 0(sub 2) gas and mineral oxidants present naturally in soils (e.g., MnO(sub 2)) demonstrate that TCE, TCA, and even PCE can be rapidly and completely degraded to benign products at moderate conditions, easily achieved in thermal remediation. Polycyclic aromatic hydrocarbons (PAHS) have an even larger thermodynamic driving force favoring oxidation, and they are also amenable to in situ destruction. Today, the principal treatment methods for chlorinated solvent- and PAH-contaminated soil are to remove it to landfills, or incinerate it on site. The most effective method for treating ground water, Dynamic Underground Stripping (Newmark et al., 1995), still involves removing the contaminant for destruction elsewhere. Hydrous pyrolysis/oxidation would eliminate the need for long-term use of expensive treatment facilities by converting all remaining contaminant to benign products (e.g., carbon dioxide, water, and chloride ion). The technique is expected to be applicable to dense non-aqueous phase liquids (DNAPLS) and dissolved organic components. Soil and ground water would be polished without bringing them to the surface. This would dramatically decrease the cost of final site closure efforts. Large-scale cleanup using hydrous pyrolysis/oxidation may cost less than $10/yd. The end product of hydrous pyrolysis/oxidation is expected to be a clean site. The delivery concept for hydrous pyrolysis/oxidation utilizes the established experience in heating large volumes of ground developed in the Dynamic Underground Stripping Demonstration (Newmark et al., 1995). Steam and possibly oxygen are injected together, building a heated, oxygenated zone in the subsurface. When injection is halted, the steam condenses and contaminated groundwater returns to the heated zone. It mixes with the condensate and oxygen, destroying any dissolved contaminants. This avoids many of the mixing problems encountered in other in situ oxidation schemes. In other oxidation schemes, an oxidizing reagent is injected into the subsurface resulting in the displacement of the contaminant. Without a return process such as the steam condensation, the contaminant and oxidant never mix. Using hydrous pyrolysis/oxidation, DNAPLs and dissolved contaminants may be destroyed in place, without surface treatment. This will improve the rate and efficiency of remediation by rendering the hazardous materials into benign ones via a completely in situ process. Because the subsurface is heated during this process, hydrous pyrolysis/oxidation also takes advantage of the large increase in mass transfer rates which make contaminant more available for destruction, such as increased diffusion out of silty sediments. Many remediation processes are limited by the access of the reactants to the contaminant, making mass-transfer limitations the bane of remediation efforts in low-permeability media. In preparation for testing this method at Lawrence Livermore National Laboratory (TCE in groundwater) and at a Southern California pole treating site (fire product with PAH and pentachlorophenol)