The laser's hot for burning

It's rather ironic, but waste disposal is one of the largest producers of industrial emissions and the emissions it produces can be among the nastiest.

Unlike chemical processes, waste disposal plants have to cope with inconsistent input streams. Sometimes, the stream will contain compounds which do not burn efficiently, or compounds containing halogen atoms which smother combustion. This makes it very difficult for operators to set the conditions inside the furnace. As a result, temperatures inside the furnace can drop by as much as 300K, leading to incomplete combustion and the production of compounds even more hazardous than those that should have been destroyed.

Aiming to solve this problem, Ashot Nazarian, of the Science Applications International Corporation (SAIC) in Gaithersburg, Maryland, has developed a device called the laser-driven thermal reactor (LDTR). This acts as a sophisticated sensor, analysing the heating characteristics of a waste disposal plant feed stream so that conditions can be kept at the optimum level.

The LDTR is a small metal sphere, up to 25mm across, into which small samples of the process stream can be pumped (see diagram above right). Infra-red lasers on opposite sides of the sphere supply intense, but controllable, heat, and can raise the temperature of the sample as high as 2000K at a rate of several hundred degK per second. Thermocouples inside the sphere provide a precise temperature profile of the temperature as the sample heats up and cools down.

This heat burst, Nazarian explains, can start chemical reactions and high-temperature decomposition, and 'represents the effects of multiple physical and chemical properties, in a dynamic environment characteristic of real-world reactors.'

Nazarian proposes a system where LDTRs monitor the feed of a waste destruction plant (see diagram below). The data from the LDTRs is analysed in 'near-real time' by the control system, which is connected to plasma generators at the entrances to both the kiln and the afterburner. Normally, the plant will be set up for complete combustion of an average feed, with the plasma generators switched off. But when the LDTRs detect that the feed requires more energy, the control system initiates a burst of plasma. As well as providing energy, this sprays free radicals into the combustion zones, which sets off new reactions and further increases the rate of destruction. Once the situation returns to normal, the plasmatrons switch off.'These adjustments permit more complete hazardous waste reduction; more efficient use of energy; and more constant emissions reduction in the off-gas stream,' Nazarian comments.