National Grid project will turn rubbish into gas substitute

National Grid has announced a demonstration project to turn commercial rubbish into a natural gas substitute using plasma technology.

The scheme will be based at the Advanced Plasma Power (APP) Gasplasma plant in Swindon, where technology to produce a form of methane called bio-substitute natural gas (bio-SNG) will be added to an existing waste-to-energy plasma facility.

This attempt to find an alternative to declining UK fossil-fuel supplies that also reduces carbon emissions, could divert commercial waste from landfill or incinerators while producing recycled materials and fewer pollutants than similar processes.

The UK needs to develop a way to decarbonise its heat supply, as well as its electricity if it is to meet its 2050 emissions targets, said National Grid’s future distribution networks manager Marcus Stewart.

‘We see long-term use for gas as heating and if you can make that gas greener, then that helps us hit the targets,’ he told The Engineer.

‘Bio-SNG has the advantage of utilising a feedstock that is abundant in the UK — waste — and converts that into energy. So that does provide greater energy security than having to import energy from abroad.

‘The other element is that we have good process engineering skills in the UK and we could develop the technology in the UK and export it abroad.’

Existing processes that produce energy from commercial and industrial waste tend to generate electricity by burning the waste to drive steam turbines or first converting it to a low-quality synthetic gas (syngas) that is then burnt.

These methods have relatively low energy efficiencies and produce large amounts of bottom-ash, as well as potentially harmful by-products such as heavy metals, which have to be disposed of.

An alternative is to use a plasma process, where bolts of electricity conducted through ionised gas (plasma) produce intense heat and UV light that break the waste down into constituent elements to form syngas, a mixture of carbon monoxide and hydrogen.

APP, using technology created by its sister company Tetronics, has combined a traditional gasifier with plasma equipment to produce a higher-quality syngas that can be burnt more efficiently in gas turbines or engines — or now converted into methane for the grid.

APP’s process converts commercial waste into high-quality syngas, which can then be converted into methane.

‘We started treating waste directly in a plasma vessel, but the parasitic loss was high and it was hard to control,’ APP chief executive officer Rolf Stein told The Engineer.

‘All gasifiers produce a pretty tarry syngas high in condensable tars, which if used in gas engines or turbines would foul them very quickly and couldn’t conceivably be used in fuel cells or for bio-SNG manufacture.

‘So we came up with a way of using the plasma to condition the gas by cracking all these long-chain molecules [to produce] a very clean, energy-rich syngas.’

The energy efficiency of the process depends on the fuel, but Stein said it could be more than 30 per cent — compared to more than 20 per cent for combustion processes — and burning clean syngas in engines or turbines has an efficiency of 40–50 per cent, compared to up to 25 per cent for steam turbines.

The company says a full-scale plant processing 150,000 tonnes of waste per year could provide electricity for around 15,000 homes and heat for 700 homes.

By diverting waste from landfill or incineration, the process effectively prevents more carbon dioxide from entering the atmosphere than it produces.

It also leaves about 15 per cent of the waste as a recyclable by-product, compared to 25 per cent leftover bottom-ash from combustion processes, and cuts harmful residues from gas cleaning from two per cent to 1.5 per cent.

National Grid has invested several hundred thousand pounds in the project as a way to stimulate the creation of an bio-SNG market, in the same way a bio-methane market has been seeded with organic waste processing plants.

‘We’re trying to demonstrate the scope of bio-SNG to market in the hope that others will develop the technology further to commercial scale,’ said Stewart.

APP eventually plans to combine the system with gas-powered fuel cells for even more efficient on-site electricity production at waste management facilities.

‘We are cheaper than incineration at comparable scale. Incinerators need to be larger to operate economically and that defeats the objective of the proximity principle both from a waste treatment and distributed energy/heat perspective,’ said Stein.