Waste not, want not

Shell hopes ethanol produced from crop debris will be available commercially soon.

Shell Global Solutions plans to build a commercial-scale plant to produce ethanol from crop waste by 2008. It will use a process developed by Canadian enzymes specialist Iogen, with whom Shell has a technology joint venture.

Lionel Clarke, Shell’s head of strategic research for fuels technology, said the company is already the largest distributor of fuels derived from biomass. The technology has two strands: production of diesel from woodchips, as a fuel in its own right and for blending with refinery diesel; and the production of ethanol as a petrol additive.

Blending ethanol into petrol reduces carbon dioxide emissions, as the CO2 released is offset by the amount absorbed by plants. Under the UK’s Kyoto Protocol commitments, all fuel companies will have to blend five per cent biofuel into petrol and diesel by 2010.

Iogen’s technology is concerned with making ethanol. Producing ethanol from vegetable matter is not new; fermenting grain to produce alcohol then distilling it was one of the first chemical technologies. But making alcohol from the waste — the stalks and leaves of grain crops rather than the grain — is a different matter.

Making ethanol from grain is known as a first-generation process — the technology is well-established but it competes with food production for its raw material so is not seen as completely sustainable. Deriving alcohol from the non-food parts of the crop, known as second-generation biofuel, is more desirable. ‘It’s a win-win because you can make food from the grains and still use the rest for the fuel; it’s a much more efficient use of the land,’ said Clarke.

However, this is difficult. The grain from wheat, barley and corn is packed with simple sugars that are easy to convert into alcohol with the help of yeasts. But wheat, barley straw and corn stover — the stems, stalks and husks of the plant — are composed of cellulose, hemicelllulose and lignin, which are natural polymers made from the same sugars but joined in complex ways.

The polymers are extremely stable, which is why straw has been used for millennia as a roofing material. Industry observers estimate commercialisation of second-generation biofuels is at least 10 years away but Shell and Iogen think they are well ahead.

To get energy out of straw you can burn it, but to turn it into a fuel you must unlock the chemical energy — a more subtle transformation. The best way to do that is to use enzymes to digest the cellulose and lignin but, so far, no-one has found, or engineered, an enzyme that can do the job fast enough to make it commercially viable.

Iogen, which began by converting corn stover and wheat straw into easily digestible animal feed, has genetically engineered enzymes from a tropical fungus to convert cellulose into ethanol at an accelerated rate. Although crucial, the enzyme is only a part of the process because the structure of the biomass locks the cellulose in. To release it, Iogen has modified a process known as steam explosion, which was originally developed by the cotton industry.

Steam explosion involves saturating plant material with high-pressure steam, then releasing the pressure suddenly. This blows the plant material apart like popcorn.

Iogen’s process involves pre-treating the biomass with dilute sulphuric acid before the explosion. Then the fibres in the plant stems are separated, increasing the surface area of the biomass and making the component compounds more ‘accessible’ to the enzymes. This allows the cellulose portion of the straw, about 75 per cent by weight, to be converted into sugars by the engineered enzymes. These are then fermented using specialised yeasts to produce a ‘beer’ of dilute alcohol, which can be distilled by industrial processing into pure ethanol.

The process is fully integrated, with the lignin remnant of the hydrolysis burned to fuel steam generation. The company demonstrated it in 2004 by producing the first ethanol from corn stover and since then has been working on commercialising the process with Shell’s engineers.

The company has encountered technical and organisational problems in the scale-up, including supply chain difficulties, said Clarke. ‘But we are now designing a fully commercial scale plant, of 150,000-200,000 tonne/year capacity, which we’d expect to be operational by 2008,’ he said.