With the harvest in full swing and baling well underway, conversations are once again turning to the straw market. Prices and demand have fluctuated in recent years, with measures such as the Straw Incorporation Measure (SIM) putting a floor under the market and providing an alternative to baling and selling.

However, an increasingly important outlet for straw across Europe is for biomethane production. As straw is classified as a residue, or by-product, of grain production, its use is not considered to compete with animal feed under EU sustainability rules. Its high energy content also makes it an attractive feedstock for anaerobic digestion (AD), although it remains a challenging material to process.

While the first generation of agricultural AD plants relied heavily on dedicated energy crops such as maize silage, new developments are centred on agricultural residues, manures and food processing by-products. According to James Dorman of 2050 R&G Consultancy and Developments, who spoke at the recent World Biogas Expo in the UK, government policy has driven this transition, with support mechanisms placing greater emphasis on waste-derived feedstocks.

Dorman explained that the UK’s Renewable Heat Incentive (RHI) marked a significant shift by requiring a proportion of biogas feedstock to originate from wastes or residues. That principle has since been carried into the Green Gas Support Scheme (GGSS), strengthening markets for livestock manures, food processing by-products and crop residues.

Challenging

Among these, cereal straw represents one of the largest untapped feedstocks, particularly across eastern England where significant volumes are chopped and returned to the field because transporting it to livestock-dense regions is uneconomic. However, Dorman stressed that its physical and chemical characteristics make it one of the most challenging agricultural feedstocks for AD.

“Straw is designed by nature to be robust,” he said. “It has to support the cereal head until harvest, so it’s waxy, brittle and contains high levels of lignin. None of those characteristics are particularly favourable for anaerobic digestion.”

Unlike grass silage or food waste, straw has a tough lignocellulosic structure that is difficult for microbes to break down. As a result, hydrolysis is often the rate-limiting step in AD.

Hydrolysis is the first biological stage of AD, where specialised bacteria break complex organic matter into soluble compounds before conversion into methane. If hydrolysis is slow, overall gas production is restricted

Briquetting

To overcome this barrier, Dorman explained that mechanical densification through briquetting offers one of the most effective pre-treatment options currently available.

During briquetting, straw is subjected to high mechanical pressure, generating frictional heat that partially fractures the lignocellulosic structure. This increases pore space and exposes a greater proportion of cellulose fibres to the microbes.

“The mechanical forces and temperature effectively open up the structure,” he explained. “Once the straw enters the digester, hydrolysis becomes much more efficient because the bacteria can access more of the available organic material.”

Laboratory studies have shown that biomethane yields of 250- 300m³ per tonne of volatile solids from pre-treated straw are achievable.

However, Dorman cautioned that achieving similar performance at commercial scale depends on feedstock quality, pre-treatment, biological process control, moisture content, particle size, storage conditions and digester loading rates.

Large-scale project

Dorman is applying these principles in a large-scale biomethane development in Lincolnshire, where straw will form part of a feedstock mix exceeding 300,000 tonnes annually. The fully permitted project has secured both feedstock supply agreements and long-term biomethane offtake contracts.

Digestate from the facility will be further processed into a commercial granular fertiliser, with its nutrient value enhanced using mineral nutrients to produce a standardised agricultural product.

The plant is expected to produce more than 7,000m³ of biomethane per hour. Feedstock procurement will largely operate through agricultural contractors, .

Innovation

Looking beyond digestion itself, Dorman believes combine harvesters and forage harvesters could integrate additional processing functions, reducing the number of handling operations required before straw reaches its final destination.

Every additional operation, whether baling, stacking, loading, transporting or secondary processing, adds cost to the supply chain. Integrating chopping, conditioning or densification into harvesting operations could reduce logistics costs while delivering a product better suited to AD.

Comment: could straw be a biogas feedstock in Ireland?

Technically, yes. In fact, straw is already being used, and has been proposed, as a feedstock at least one operational biogas plant in Ireland.

Straw has many of the characteristics of a suitable feedstock, but it also presents challenges.

It requires pre-treatment to improve digestibility, meaning that a relatively large-scale plant is generally needed to make its use economically viable.

This could make straw an attractive feedstock in Ireland’s tillage heartlands, such as County Wexford, where straw is produced in significant quantities.

However, straw is already a valuable commodity in Ireland.

The SIM has effectively established a price floor for straw.

The mushroom sector is also competitive and has a strong demand for quality straw, while the Irish livestock sector also relies heavily on straw for bedding and feed.

Consequently, biogas plants would need to offer a sufficiently high price to attract straw away from these established markets.

So yes, straw can work as a biogas feedstock, but it’s an expensive one and the viability of a dedicated plant could be tricky.

There may, however, be more efficient and cost-effective ways to harness the biogas potential of straw.

One option would be a structured straw-for-manure exchange scheme, whereby biogas operators purchase straw for or with livestock farmers, who in turn supply the resulting farmyard manure to the biogas plant.

Having already been processed by animals, the manure is a more suitable feedstock for AD, while still allowing the livestock sector access to the straw it requires.

The Government could also consider adapting the SIM. Although the SIM is already under budgetary pressure, a reduced payment could potentially be offered where straw is diverted to biogas production rather than incorporated into the soil.

In both cases, the digestate produced by the biogas plant would be returned to farmers, recycling nutrients back to agricultural land and supporting a more circular farming system.

A typical 50 GWh biomethane plant fuelled entirely by straw would require approximately 30,000 tonnes of straw annually.

So, with an average yield of 12 round 4x4 bales (approx 150kg each) per acre, this equates to around 200,000 round bales, or the straw harvested from approximately 16,700 acres of winter barley each year.

If the tillage industry is looking for new opportunities to safeguard its future, this could be one.