Brown heat

With the golden age of landfill coming to an end, thermal desorption could be a major aid to developers in the regeneration of Brownfield sites. John Haven reports

The Government has set a target of 60% for the use of previously developed land in the building of new homes. The industry has surpassed this by achieving 70%. But is this sustainable? The drastic decline in hazardous waste sites, together with new Hazardous Waste legislation and the introduction of Waste Acceptance Criteria, makes development more complex, costly and uncertain. The practice of blending material on-site to reduce its hazardousness, while widespread, is specifically banned, so it’s probably only a matter of time before the Environment Agency acts.

So, what to do? The whole thrust of the environmental legislation is to recover value from waste, and reduce damage to the environment. In this context, legislation is restricting landfill, by making landfill sites more expensive to operate and increasing landfill tax year-on-year.

Contaminated soil treatment is becoming increasingly cost-effective and thermal desorption is one of a range of technologies with a role to play in this emerging market.

A variety of uses

Developed in the 1980s, it’s a simple technology which has treated many tens of millions of tonnes of contaminated material. Thermal desorption is typically used on hydrocarbon-contaminated sites, such as coke works and refineries, but it has wider uses. The technology comes in the form of mobile plant capable of processing 10-35 tonnes/hour, and fixed plant that can treat 80 tonnes/hour.

It consists of a two-stage process. First, the contaminated materials are heated in a rotating drum to a temperature above the boiling point of the contaminants to be removed, changing them to a gaseous state. In the second stage, the gases are collected and treated using either a thermal or chemical abatement system.

There are two common forms of plant set-up. ‘Hot’ units have the baghouse located after the afterburner, while ‘cold’ units have the baghouse in front of the afterburner. These have different benefits and capabilities, but both treat materials to the same standards.

Contaminated material is blended prior to treatment to ensure a uniform feedstock. It is then screened to remove oversize particles (generally >40mm), and pre-treatment additives applied as necessary.

The material is then loaded into a hopper, where it is weighed and fed into the dryer at a pre-arranged rate. The material is heated as the dryer rotates to achieve the required treatment temperature (usually up to 6,000ºC) to ensure contaminants are desorbed from the soil matrix. The soil then exits the dryer, is cooled, and water added to return the soil’s moisture content to a normal range. The soil is essentially sterilised and divested of any organic matter.

After the burn

The off-gases then enter the abatement system where they are, most often, thermally oxidised in an afterburner designed to provide the correct temperature and residence time to ensure complete destruction of the contaminants, and prevent the production of dioxins. The afterburner oxidises the hydrocarbon contaminants in the gas stream by heating to temperatures above those required for auto-ignition such that most hydrocarbon contaminants are converted to carbon dioxide and water vapour. The treated gas is then cooled and, if required, scrubbed of other chemicals (e.g. chlorinated or sulfonated compounds) before solid particulates in the gas stream are filtered out. The treated gases are then exhausted to atmosphere.

Because of its ability to remove organic carbon, thermal desorption is a suitable pre-treatment method for complex wastes to meet the new limits on carbon content limits for waste to landfill. In short, it is a mature and highly effective treatment process aligned to legislative and market drivers. The benefits it can deliver mean the role of thermal desorption in the UK is set to grow.

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