April 15, 2010

FOCUS | Demolition & environmental engineering

Geosyntec looks at smoldering combustion for site remediation

MARY BAXTER

correspondent

Cleaning up contaminated soil or groundwater usually means excavation and a substantial investment. A company that has licensed a new technology developed by researchers in London and Edinburgh is proposing an alternative: smoldering combustion — the same principle at work in a charcoal barbecue.

Called STAR (self-sustaining treatment for active remediation), the process uses the contaminants themselves to fuel the slow burn that eventually eradicates them, says Gavin Grant, a senior engineer with Geosyntec’s Guelph office. The company’s SiREM lab division, which is commercializing the technology, specializes in site remediation and management.

The process works because the contaminants are embedded in the soils, which retain the energy of the combustion.

The soil is essential for the process to work. “If not for the soil, the heat wouldn’t be retained and you would lose all of that heat energy to the air you wouldn’t be able to feed the system,” Grant explains. It’s highly effective, he says.

“Essentially, the chemicals are destroyed and turned into carbon dioxide and water.”

During the combustion process, temperatures will reach about 1,200 C. That’s not hot enough to damage soils, typically composed of silicon dioxide, he adds.

Study results published in the American Chemical Society’s November 2009 issue of Environmental Science & Technology, note that more research is needed to determine the impact of the process on soil.

The results also indicate the process uses oxygen to sustain the burn.

Once the contaminants are destroyed or the oxygen source is removed, the process stops. The results suggest that the process can function in a variety of soil types.

Grant says it has mainly been applied to non-aqueous phase liquids such as coal tar, creosotes and petroleum hydrocarbons, which have the most energy available and have few other remedial alternatives.

But the process could be applied to many other chemicals ranging from PCBs, to mineral oils and chlorinated solvents, he says.

Emissions released are primarily carbon dioxide, carbon monoxide and water.

There are some other compounds released in the vapour phase “which essentially volatize,” such as light hydrocarbons, he says. The concentrations are small in comparison to the amounts of carbon dioxide and carbon monoxide released.

Contaminated soil before and after remediation.

“We typically capture them and treat them with activated carbon,” Grant says.

How emissions are dealt with depends on factors such as the site conditions, depth of the contamination and contaminant types. “Not all of these compounds contain highly volatile materials,” Grant says. But the technology for treating these is well understood and “off the shelf.”

Jason Gerhard, a professor at the University of Western Ontario and Jose Torero, a professor at the University of Edinburgh invented the process about six years ago, says Grant.

Last fall, SiREM completed two pilot tests. More tests are planned this spring. These include treating a 20 by 60 foot area of soil contaminated with coal tar at a former creosol manufacturing plant and applying the process ex-situ to petroleum hydrocarbons recovered from flare pits — a type of contamination associated with oil and gas recovery.

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