Biorefineries developed to produce ethanol from cellulose sources such as trees and fast-growing plants could get a significant economic boost from the sale of high-value chemicals – such as vanillin flavoring – that could be generated from the same feedstock.
Revenue from these ‘side stream’ chemicals could help make ethanol produced by biorefineries cost competitive with traditional fossil fuels.
So heard the recent 232nd national meeting of the American Chemical Society (ACS)when a researcher from the Georgia Institute of Technology described green chemical processes that could produce chemicals worth up to US$25 per pound (0.45kg) from the same feedstock used to produce ethanol.
The presentation was part of a session ‘Green Chemistry for Fuel Synthesis and Processing’ held during the ACS meeting.
“It seems unlikely that fuel from a biorefinery – at least in the beginning – is going to be as cost-effective as fuel from traditional fossil sources” said Charles Eckerta professor in the Georgia Tech School of Chemical and Biomolecular Engineering. “To make the biorefinery sustainablewe must therefore do everything we can to help the economics. If we can take a chemical stream worth only cents per pound and turn it into chemicals worth many dollars per poundthis could help make the biorefinery cost effective.”
To help make that happenEckert and collaborators Charles LiottaArthur RagauskasJason HallettChristopher KitchensElizabeth Hill and Laura Draucker are exploring the use of three environmentally-friendly solvent and separation systems – gas-expanded liquidssupercritical fluids and near-critical water – to produce specialty chemicalspharmaceutical precursors and flavourings from a small portion of the ethanol feedstock.
“These are novel feedstocks for chemical production” Eckert noted. “They are very different from what we've dealt with before. This gives us different challengesand provides a rich area for interdisciplinary research.”
Using near-critical water and gas-expanded liquidsEckert and his colleagues have already demonstrated the production of vanillinsyringol and syringaldehyde from a paper mill black liquor side stream. They have also proposed a process that would generate levulinic acidglucaric acid and other chemicals from the
pre-pulping of wood chips. That process would use an alcohol-carbon dioxide mixturefollowed by depolymerisation and dehydration in near-critical water.
Research aimed at producing high-value products from cellulose feedstocks is being done through the AtlantIC Alliancea coalition of three research institutions in the USA and the UK. The alliancewhich includes Oak Ridge National LaboratoryImperial College and Georgia Techseeks to solve the complex issues involved in economically producing ethanol fuel from cellulose materials such as wood chipssawgrasscorn stovers – and even municipal waste.
“The feedstock would likely be different in different geographic locationsdepending on what was readily available” Eckert noted. “In the southeastwe have abundant forest resources. In the westsources would include sawgrasscorn stovers and similar plant materials.
In the UKthere is strong interest in producing fuels from municipal wastes.”
The Alliance is taking a comprehensive approach to the biorefineryconducting studies of how to maximise plant growth through genetic engineeringdeveloping new microbial techniques for digesting celluloseand applying environmentally-friendly chemical processes for reactions and separations. The organisers decided to pursue only non-food sources as their feedstock.
Using tunable solvent systems in the biorefinery would avoid the generation of wastes associated with processes that depend on strong acids - which must be neutralised at the end of the reaction (Fig.1).
For instancenear-critical water – familiar H2O but at 250–300ºC under pressure –º separates into acid and base components that can be used to dissolve both organic and inorganic chemicals. When the pressure is removedthe water returns to its normal properties.
Gas-expanded liquidssuch as carbon dioxide in methanolprovide a flexible solvent whose properties can be adjusted by changing the pressure.
When the reaction is overthe pressure is releasedallowing the carbon dioxide to separate from the methanol.
Supercritical fluidssuch as carbon dioxide under high pressuresimplify separation processes. Separation of the carbon dioxide from chemicals dissolved in it requires only that the pressure be reducedallowing the carbon dioxide to return to its gaseous state.
Though many challenges remain before biorefineries can be designed and builtEckert says it is important to invest now in this renewable source of energy and chemicals.
“To make the biorefinery work will require a major effort that must be well co-ordinated among everybody working on it” he said. “The biorefinery is one of several answers that we need to pursue as part of a national energy strategy. Our future economic
well-being requires us to deal with the energy issue."
In 2004Eckert and Liotta received a Presidential Green Chemistry Challenge Award for their development and promotion of benign tunable solvents that couple reaction and separation processes.
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