Introduction of straw briquettes in AD plant considerably increases biogas production


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Biogas production is rapidly increasing worldwide, and the UK is no exception to this. Historically the biogas has been used for production of heat, electricity, in transport or finally upgraded and delivered to the natural gas grid. The latter is the end purpose for Kinetic Biofuel customer Farm Renewables on The Isle of Sheppey. Introducing straw briquettes into the reactor has provided for nearly 50% of the total biogas output.

The yearly amount of raw material used at the Sheppey plant amounts to approx. 6,000 tons of straw briquettes (85%); 10,000 tons of chicken litter (75%); 14,000 tons of maize silage (33%) and 6,000 tons of fruit waste (20%). Dry matter percentage in parenthesis. Straw is bought on a contract basis to ensure a reasonable price of approximately £50 per ton on average. All raw materials are fed into a bio-mixer at the mentioned proportion, and subsequently fed into the digester. This entire straw handling and feeding in solution is cost effective in terms of operation & management and in terms of pre-treating the straw allowing for unimpaired use of straw for AD.
Malcolm McAllister explains that he favours biogas plants based on agricultural residues, as he easily obtains raw materials from local farms and also returns the digestate to these same farms. In time, he wishes to consider options of producing briquettes for bedding material for subsequent return of the straw to the digester as imbedded in the poultry manure, or any other manure. This will ensure an optimal bedding material, being sanitized and therefore free of any potential risk of carrying pathogens to the animal houses, such as any flu virus or any other pathogen.

But why add straw briquettes?
One quick answer: Induced steam explosion! When the straw passes through the briquetting machine, an explosion is created and the cell structure is disrupted allowing for water to penetrate the straw.

This completely new hydrophilic, water loving, property of straw is fundamental to suspend and digest straw briquettes in biofuel reactors. Briquetted straw has an incredible absorption rate - 7-10 times more than normal straw, which is the basis for high yields and easy use in biogas plants.

Farm Renewables considered different technologies including conventional steam-explosion, but in the end, they decided on acquiring a briquetting line from Kinetic Biofuel including a complete straw handling system. The line included two BP6510 C.F. Nielsen briquetting presses with a total capacity of 3 tons per hour.
Although new legislation is on its way from 2023, mentioning environmental issues due to the amount of digestate left after biogas production, the future still looks bright as there are many advantages to using straw in biogas production.

Malcolm McAllister says:

"Undoubtedly straw briquettes are a very good feedstock it works well in the system. We just need to get better at managing the straw getting into the plant. We have implemented better reception of straw
with moisture control and we are considering buying a bale-dryer".
"I like the product (the straw briquettes) it is very good for the system, easy to feed and, compared to using steam explosion at our other plant, briquetting is very simple".

The amount of biogas/methane generated from straw in the biogas plant can be difficult to estimate precisely due to input of various raw materials. However, indications are that the yield from straw at the Farm Renewables plant so far is in the range of 200-250 Nm3 methane per ton. The upper limit is generally easy to achieve in conventional digesters (i.e., 300 I per kg VS).

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