Clean air act

US researchers have built a plasma filter for the military to pump clean air into a building during a chemical or biological attack.

The filtration system uses plasma to neutralise chemicals like sarin gas released into a Tokyo subway in 1995, as well as biological agents like anthrax.

Plasma is an ionized gas where the positive and negative charges of the particles are roughly equal, and is perhaps most commonly seen in neon lighting. Plasma filtration has traditionally been used to remove toxic gases such as fluorine produced during industrial processes such as pulping paper or electronics manufacture.

Ken Rappe, Pacific Northwest National Laboratory’s senior development engineer said plasma technology is more reliable and easier to operate than conventional air filtration devices. ‘It offers chemical, biological and particulate protection in a single compact unit,’ he said.

Conventional biological and particulate filtration use High Efficiency Particulate Air (HEPA) filters, but they age quickly and become unreliable, said Rappe; and chemical filtration systems are bulky and relatively complex because the gas needs to be heated to allow reactions to occur then cooled before it passes out. ‘Plasma facilitates a combustion-type environment without needing significant heating,’ he said.

In the plasma filter, reactive particles — or radicals — form between a pair of electrodes and destroy up to 90 per cent of the toxic substances that enter the device. The charged particles also facilitate electrostatic precipitation by clinging to dangerous particulates. A series of stages then follow to neutralise the remaining toxic agents.

An acid gas stage neutralises by-products like phosphorus and fluorine from sarin gas, for example. The plasma reaction also produces ozone that needs to be removed by catalysts. A final ‘carbon polishing’ stage removes the last few per cent of toxic substances.

The prototype for the US Department of Defence produces around 10 cu feet of breathable air/min so is aimed at mobile housing and field hospitals. A key challenge for the researchers is scaling up the device for larger buildings or aircraft. ‘One of the prime obstacles is taking this unit up to a significantly larger flow,’ said Rappe.

‘Scaling up non-thermal plasma can be tricky. We’ll have to change the configuration of the unit.’ The team also plans further testing of live agent chemicals.

Pacific Northwest National Labs