PLASMA ZAPS POLLUTION

The blue light: microwave discharge in a gas flow (right) significantly cleans up pollutant streams, such as from Chineham incinerator (below)

Incineration of waste is increasingly seen as an economical alternative to landfill in the UK. The advantages of incineration are that it achieves typical reductions in waste volume by 90 per cent and in mass by 66 per cent.

However, there is some concern that incineration generates more harmful pollutants than other disposal methods. The quantity of these pollutants can be reduced by pulsed corona discharge, also known as plasma treatment.

The European Commission has part-funded a joint research project with AEA Technology to find out if VOCs, dioxins and furans could be cut.

At AEAT we built a mobile demonstration unit and made measurements at the Chineham municipal incineration plant, near Basingstoke, Hants, that has a design capacity of 43 000t/a of waste and produces 90 000Nm3/h of gas.

We obtained VOC destruction values of approximately 85 per cent. And conversion of nitrogen oxide into acid species was found to be 95 per cent.

Chineham was closed at the end of November 1996, along with all of the older municipal solid waste incinerators in the UK that could not meet the new EC limits of 20mg/Nm3 of VOCs and 1ng/Nm3 of dioxins/furans. Our project is soon to be moving to a new site.

EQUIPMENT AND SET UP

We built the pulsed corona power supply and reactor equipment on a 12m trailer unit inside a screened enclosure to ensure high voltage safety and to stop any pulsed electromagnetic radiation escaping to the outside world. The trailer was subdivided into four bays:

{{* Control room;* Power supply and monitoring system;* Pulser bay with capacitor charging supply and transformer; and* Reactors with connecting pipework.}}

Four reactors were installed in series to make a single module. A fan drew about 60Nm3/h from the off-gas stream, from between the electrostatic precipitator and the stack base. After treatment gas was returned higher up.

Through glass pipework we observed the discharge to be stable and uniform across the full diameter of the reactor.

With off-gas at around 200iC, gas flow start-up had to be graded. We achieved this state by using a computer controlled fluxing valve which bled together the off-gas and outside air into the optimum mix.

EVIDENCE OF DESTRUCTION

Because the incinerator was relatively clean, we were able to investigate the destruction of toluene as a VOC dopant. Toluene has the advantage of having an infrared spectrum mid-way between the characteristic absorption peaks of water and CO2. We observed a reduction in the intensity of the toluene resonance accompanied by an increase in the CO2 absorption, showing that oxidation had occurred. At an off-gas flow rate of 60Nm3/h, we obtained a maximum VOC reduction of 84+/-8 per cent.

Initial experiments were performed in air to determine the best operating conditions for the equipment, but conflicting requirements were found. If the destruction percentage of 100ppm toluene in air is plotted as a function of peak voltage, then more destruction is seen at the higher voltages. However, destruction efficiency - the percentage destruction of toluene obtained for the input of 1J/Nm3 of off-gas - is a more useful parameter. If the destruction efficiency is plotted as a function of peak voltage then the reverse trend is seen, it being more economical to operate at the lowest applied voltages.

At an off-gas flow of approximately 60Nm3/h, our tests showed that NOx was 95.1+/-0.5 per cent removed by the pulsed corona discharge system, by oxidation of NO to NO2 followed by conversion to acids. Similar results showed SO2 was removed but detector interference prevented quantitative measures.

At the same time as the NOx was measured, the gas stream was monitored for VOCs. The VOC monitor recorded a broad peak in such emissions with two periods of sharp decrease by 84+/-4 per cent when the corona reactors were switched on. The peak was produced by the burning of medical waste in the incinerator at that time with its high content of plastics.

Dioxin/furan 3-hour sampling was also carried out simultaneously and the percentage reduction ratio was estimated to be 85+/-5 per cent, for a direct corona discharge input power of approximately 50kJ/Nm3.

Further work on the measurement of absolute dioxin and furan levels in the input and output pipes is scheduled in the on-going experimental program. Work is in progress to improve the accuracy of the dioxin and furan measurements and to determine whether VOC, dioxin and furan emissions can be cut economically at rates above 1000Nm3/h.

Another task is to look at the effects of the corona discharge on the process efficiency in the varying off-gas environment. Power supply technology is also to be developed towards a fully industrialised system, with a view to construction of full scale plants. The AEATteam is also developing microwave discharge plasma systems for exhaust gas clean-up in the chemical, pharmaceutical and semiconductor industries.

This article was adapted from a paper presented by Carlow, King and McAdams (all of AEATechnology) at Advanced Oxidation Technologies in the US, 1996

AEA TECHNOLOGY ENTER 290