Three energy companies have proposed cleaner burning systems in waste to electricity generation, under a UK-funded competition.
Waste offers the potential for an unlimited renewable energy resource, and is already used in simple power generation.
For example, methane derived from decomposing manure or municipal rubbish can be burned in a gas engine. But that only makes an incomplete use of the biomass, most of which is left as compost.
Solid biomass can also be burned in incinerators, to drive a steam turbine, but most energy is lost as heat to generate steam.
The use of waste in more advanced, efficient power generation has suffered difficulties from the production of heavy tars in the production process.
“The presence of tars in the product gas is a big problem in the commercial utilisation of biomass product gas as source of sustainable energy,” said a research note published by the Energy Research Centre of the Netherlands (ECN), one of the technology providers in the UK competition
“Tar is formed in the gasifier and comprises a wide spectrum of organic compounds. Heavy tars condense out as the gas temperature drops and causes major fouling, efficiency loss and unscheduled plant stops.”
The aim of gasification is to reduce any form of biomass, ranging from wood to waste, into a high energy gas comprising hydrogen and carbon monoxide, also called syngas.
That is achieved by heating the biomass under restricted oxygen.
In theory, the syngas can be burned in a gas turbine or reciprocating engine, generating electricity without the wasteful step of steam generation. Alternatively, it can be converted into renewable diesel fuels including jet fuel, through a well established Fischer Tropsch process already applied to coal and natural gas.
The trouble is the removal of impurities produced from heating the biomass, and especially dust, charcoal and heavy tars which otherwise clog the power generation and plant machinery.
The British competition, run by the publicly and privately funded Energy Technologies Institute, will choose between three companies which have submitted designs to demonstrate syngas cleaning.
The three companies are Advanced Plasma Power, Royal Dahlman and Broadcrown.
The Advanced Plasma Power process uses an electric torch called a plasma arc to zap the impurities, cracking them into smaller molecules such as methane.
“The intense heat from the plasma arc – in excess of 8,000C – and the strong ultraviolet light of the plasma result in the complete cracking of tar substances and the breakdown of char materials. The cracking creates a clean syngas,” the company said.
“It is the presence of these contaminants in waste gasification processes which has, hitherto, been the major obstacle in deploying the gas engines and turbines necessary to achieve higher electrical generating efficiencies.”
The second company, Royal Dahlman, is a technology supplier in the petrochemical and gas turbine industry, and is using an entirely different syngas cleaning approach, developed by Netherlands-based ECN.
The cleaning process is based on exploiting the dew point of tars, using special oils to condense out the tars as the syngas is cooled, in a multi-step process.
The third company, Broadcrown, was more coy about its technology approach.
The ETI will decide in the next few months the winning project design, which will then be built, tested and in operation by 2016, with an electricity generating capacity of between 5 and 20 megawatts.
This article was produced by the RTCC