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140 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Upcoming projects include the Mytum & Selby site, which is constructing a 25,000 tonnes per annum facility, due to be operatational by October 2009. Phase two would see this capacity increase to 75,000 tonnes per annum. 274 9.3 Hydrolysis and fermentation of lignocellulose (Biological 2nd Generation Biofuel) Wood waste, Agricultural / cereal waste Pre treatment Hydrolysis/ saccharification Fermentation Process Description Most ethanol is produced industrially by the fermentation of sugars by yeasts or bacteria although a small quantity is produced by the hydration of ethylene. The most common processes use simple sugars such as sucrose that the organisms can easily process. These sugars are found in food crops, either directly as in sugar cane and beet, or as starch in grain that is converted to sugar by the action of an enzyme. Wood and the stem material of plants - lignocellulosic material - are also composed to a large extent of sugars and are much more abundant. However these sugars are unsuitable for fermenting organisms and locked into the plant structure. Lignocellulosic material contains two sugars, cellulose and hemi-cellulose, both in polymer form. They are enclosed by coating of lignin, a compound with no sugars that gives the plant its structural strength. To obtain fermentable sugars it is first necessary to release the cellulose and hemi-cellulose from the lignin and then hydrolyse them to simple sugars. These can then be fermented by yeasts, or more usually, bacteria to produce ethanol. The generic flowsheet for this process has four sequential steps covering a pre-treatment step to release lignin and hydrolyse the hemicellulose, hydrolysis of the cellulose to simple sugars, fermentation, and purification of the ethanol produced. Of these steps pre-treatment and hydrolysis have proved technically the most difficult and the subject of most research and development. Pretreatment. The first operations are normally physical treatments to reduce the size and increase the surface area of the feedstock that improve its accessibility to the reagents used. Washing may also be necessary for some feedstocks, and steaming followed by solvent washing has been proposed to remove resins that may inhibit the later biological processes. Normally the breaking of the structure is achieved by a dilute acid that will break the lignin free from the structure to liberate the sugars, and at the same time hydrolyse the hemi-cellulose polymer to simpler 274 Mytum & Selby plans £45m waste-to-biofuel plant, Letsrecycle.com 13 th March 2009. Purification Ethanol
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. 141
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Figure 27 Orchid Environmental proc
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