Installing a Jenbacher J620 engine at William Grant & Sons’ distillery in Girvan in Scotland to burn biogas from the anaerobic digestion of waste materials will cut the plant’s carbon dioxide emissions while providing 3 MW of power, as well as steam for distillery processes, finds Piers Evans.

Not even combined heat and power (CHP) can soothe all the headaches triggered by Scotland’s celebrated whisky industry. But cogeneration could certainly ease those brought on by the sector’s tightening environmental standards.

Producing Scotch whisky generates several co-products that – in an untreated form – provide highly unwelcome additions to Scotland’s scenic landscapes.

Residues of the grain fermented to produce the ‘mash’ from which whisky is distilled are known as ‘draff’. From the copper stills where the mash is first heated comes ‘pot ale’. Out of the spirit stills where whisky undergoes its final distillation comes ‘spent lees’. The CHP potential of each has recently been examined in a research project at the Energy Systems Research Unit (ESRU) at the University of Strathclyde in Glasgow.

Draff is resistant to biochemical breakdown and has a moisture content of 70–80%. But large distilleries – or combinations of distilleries – could economically dry draff to form a solid biomass fuel with gross calorific value (GCV) of about 21,000 kJ/kg.

Pot ale from the wash stills for initial distillation contains yeast cells and dissolved barley solids. With a high chemical oxygen demand (COD) of 65–71 kg/litre, pot ale is unsuited for disposal untreated but can be appropriate for producing biogas through anaerobic digestion (AD).

Spent lees have a lower COD concentration, typically of about 3–5 kg/litre. But this co-product can be mixed with pot ale to produce a liquid with a COD of about 35 kg/litre that is suited for biogas production in an AD plant.

As Scottish whisky’s global markets expand, the quantities of co-products are also surging – up to 1600 million litres of pot ale and 187,000 tonnes of draff each year, according to a recent estimate.

CHP is already a key contender for averting pollution and carbon dioxide emissions from these co-products while also generating power and heat. In Speyside, for instance, a 7.2 MWe biomass CHP plant is due to fire up in the third quarter to 2012, burning draff from 16 local distillers alongside wood. As well as heating 9000 local homes, the heat from the plant at Rothes will be used to convert pot ale into a syrup for making animal feed.

But William Grant & Sons – a world-renowned distiller behind several of Scotland’s best-known whiskies – is applying an alternative CHP approach to its power, heat and environmental needs through generating biogas from its liquid by-products.

The independent family-owned distillery at Girvan, in the southwest of Scotland, dates back to 1887 and now produces 25 batches every week, each originating with a mixture of barley and hot water. But with the delivery of a 3.3 MW J620 GE Jenbacher engine in October this year from Clarke Energy in Liverpool, the distiller is aiming for energy independence through generating 7.2 MW of electricity, as well as steam from its waste products.

Until recently the plant’s mash-derived wastewater has been discharged into the Firth of Clyde, where its high COD presents an environmental hazard. Mash wastewater will now go to a Biolak AD system from German company Von Nordenskjold. The AD process strips away 95% of the by-product’s organic compounds while producing biogas.

Gas is stored in a gas holder where it undergoes a desulphurization wash and then feeds three 1.3 MW J420 GE Jenbacher engines along with the newly installed 3.3 MW J620 engine, which can run at more than 43% efficiency on biogas. A lean NOx system ensures TA Luft verified emissions within 500 mg/m3. The engines are designed to undergo major overhauls after eight years – or 60,000 hours of operation – which should involve about 60% of their original cost.

The total capacity of 7.2 MW will meet the brewery’s 4.8 MW demand with 2 MW exported to the grid, earning two Renewable Obligation Certificates (ROCs) per MWh.

Steam is another valued product of the installation, in which heat will be recovered as steam from the J620’s exhaust. The plant’s processes consume about 70 kg/h of steam – of which it currently produces only about 10%.

While clearing up its environmental headaches – and those of the fish in the Firth of Clyde – William Grant & Sons can look forward to a swift financial return. Its total investment of £15 million is set to be paid back in four years.

William Grant & Sons produces several famous whiskies at its plant in Girvan

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