to read the full report - Ecolateral by Peter Jones

Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
Xylose and 5 carbon sugars. Some cellulose will also be converted to 6 carbon sugars, but most will be
hydrolysed in the following stage.
Physical methods have also been proposed for pre-treatment, usually in combination with dilute acid
treatment, the most common being steam explosion where the slurry in the pre-treatment stage is flashed
to low pressure through a choke causing the cell structure to burst during decompression. The same
explosive decompression effect has been demonstrated using ammonia and CO2 as the medium.
A consequence of the acidic conditions is that some of the sugars will be converted to furfans and other
compounds that may inhibit the fermentation organisms in a later stage. A key development challenge for
process designers is to minimise the production of inhibitors whilst maximising the yield of simple sugars.
Acid will need to be recovered and sugars washed from the process and the liquor neutralised following
pre-treatment. Lignin is also removed at this stage. Most developers propose to use this as a fuel for
power and process heat generation.
Cellulose Hydrolysis. Pre-treatment is followed by a second hydrolysis stage where the cellulose is
converted to 6 carbon sugars. Historically this has been achieved by acid hydrolysis using a higher
temperature but lower concentration acid than in the pre-treatment step. An alternative route is to use
concentrated acids which improves the yield of sugar but is critically dependent for its economics on
effective recovery and reuse of the acid. This is technically possible but difficult and involves complex
engineering.
Acid based processes have been proven for many years but only now are demonstrations being planned
in the USA and the EU. This lack of progress has been historically due to the expensive equipment
necessary and the low price of competing ethanol from sugar and grain. Only with the recent interest in
ethanol as a biofuel has development restarted on this process.
Following hydrolysis the process will include separation stages to wash out the sugars and remove
fermentation inhibitors such as heavy metal contaminants and organic products of hydrolysis followed by
neutralisation with lime if acid has been used.
In the last decade, an alternative, enzyme hydrolysis route has been proposed and developed to pilot
stage. This technology has the potential to achieve high conversion rates with low production of inhibitors
due to the mild reaction conditions. The key development necessary for commercialisation is to reduce
the cost of the enzyme cellulase by an order of magnitude. Enzyme based processes are still in the
development phase but offer the prospect of being economically more attractive than other options if their
further development is successful. Demonstrations are planned in Canada, USA and EU.
Fermentation.
The sugars from either hydrolysis route contain five and six carbon atoms and need to be fermented in a
microbial process rather than the yeast process used for simple sugars from sugar beet, sugar cane and
grains. Typically these are proprietary recombinant organisms whose metabolisms have been tailored to
the sugars in the process. We understand work is also being carried out on advanced yeasts.
Development status
A substantial research programme has been underway for over a decade in the USA and EU to
commercialise this technology. 275 This has resulted in several demonstration plants, but as yet no fully
commercial installations. 276 Progress has been made however and the US have announced a target of
producing ethanol from lignocellulose to the same price as that from corn by 2012.
275 IEA (2008) From first to second generation biofuel technologies: An over view of current industry and R,D & D activities
276 www.abc-energy.at/biotreibstoffe/demoplants.php.
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