Direct Carbon Fuel Cells More Efficient Use For Waste Biomass?

Current productive uses for waste biomass include gasification for power generation and liquid fuels, co-firing with coal for power generation, or fermentation to bio-gas for power generation. Now scientists at the University of St. Andrews in the UK are claiming that using waste biomass in direct carbon fuel cells (DCFCs) will yield twice as much power per tonne of waste. Specifically, they have tested waste medium density fibreboard (MDF) from furniture manufacturers.

Irvine's team first treat shredded MDF with 500 ºC heat in a nitrogen atmosphere to drive off water and volatile gases. This is a cost-effective process that leaves the material in an energy-dense and lighter form that is easier to move to where it's needed, says Irvine.

The treated MDF is then powdered and mixed with lithium and potassium carbonates, which are the electrolytes in the fuel cell. At temperatures between 500 and 800 ºC, these act as chaperones that encourage the carbon in the MDF to combine with the oxygen flowing into the cell to produce carbon dioxide and free up the electrons that provide electrical current.

The finished cell has a power density of around 100 milliwatts per square centimetre at a current density of 200 milliamps per square centimetre, meaning a square cell 10 centimetres on each side could generate 2 volts at a current of 20 amps. This is comparable to other prototype direct carbon fuel cells that could one day be used in large-scale power generation.

Identifying new feedstocks to power DCFCs is vital for the technology's success, says Dian-Xue Cao at Harbin Engineering University in China, who welcomes the team's studies into unconventional energy sources. _NS

Other waste biomass such as agricultural and forestry waste could be similarly treated for use in DCFCs. If electricity is the primary desired product, the overall efficiency of direct carbon fuel cells may well be greater than using biomass gasification, torrefaction with co-firing, or pyrolysis plus gasification.

Of course if you want both power and process heat, nothing beats IGCC plus CHP at this time. Specific applications will supply unique constraints.

Overall, the principle of matching local and regional feedstocks with local and regional needs and technologies makes the most sense -- just as smart builders tend to look for local materials that suit the regional climate and specific demands of the plant.