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From his family’s beef farm in Co Roscommon, Eanna Tiernan went on to start his career in renewable energy with ABP Group’s Munster Protein and wind farm developers. Now, he wants to combine both with Alchemy Utilities, a new business aiming to bring dry anaerobic digestion (AD) to Irish farms.
The main difference with traditional “wet” AD is that slurry is heated to between 70°C and 80°C before going into the digester. “From 9t of slurry, we can extract 3t of water and bring it to between 20% and 22% dry matter,” Tiernan said. According to him, such drying offers the advantage of easier nutrient extraction, and smaller machines and digestate volumes.
Engineers have been pioneering the technology at a plant in Finland, where the dried slurry is mixed with other dry matter such as straw or silage to reach the right carbon and nitrogen balance and fed into a digester that fits in a 40ft container. “They’re modular in design and easy to maintain and to put on farm. All you need is a 25x25m concrete base,” said Tiernan.
He added that a block of three digesters can process 9t of slurry per day and generate 55m3 of gas per hour containing 55% to 60% of biomethane. Part of the gas is used to heat the slurry and the rest can be used in a combined heat and power (CHP) generator or exported for purification and sale such as injection into the national gas network.
Future legislation
Nitrogen is extracted through the process by mixing with sulphuric acid. “We’re looking at a central facility to process the resulting ammonium sulphate. We’re talking to a fertiliser manufacturer,” said Tiernan. He expects that future legislation will impose a proportion of organic matter in industrial fertiliser, increasing demand for the product.
Another difference with the way AD has been developed so far is that Alchemy is planning to own the plants and partner with farmers around the country to find sites and feedstuffs for them. “We will pay rent or other benefits to host farmers,” Tiernan said. Fertiliser vouchers in exchange for the nutrients extracted from the slurry are an option. The company will also employ local staff to load and run the plants, who could be part-time farmers.
The challenges to the technology’s deployment are the need to establish a supply chain for the collection and marketing of gas and fertiliser, and possibly the extra-rich final digestate
While this ownership model means farmers will not need to raise the finance for a typical €700,000 three-digester and CHP plant, Alchemy must fund the fleet of machines itself. Its three English directors have prior experience with the biomass company Arbour Electrogen in the UK and have raised bonds on the Frankfurt stock exchange, with further financing arrangements under negotiation, Tiernan said. “There are incomes streams from both gas and fertiliser,” he added.
Tiernan said Alchemy has concluded agreements with three farmers to install its first units in Ireland. The first digester is due to begin operation on a chicken farm this summer. A beef farm and a pig farm importing slurry from a neighbouring dairy herd are also lined up, and the company is exploring options with breweries for their by-products.
“We see this as a solution to slurry tanks, filling up,” Tiernan said. “The back end of last year was so bad that many farmers couldn’t get slurry out. We can alleviate this by contracting farmers to take a couple of loads.”
The challenges to the technology’s deployment are the need to establish a supply chain for the collection and marketing of gas and fertiliser, and possibly the extra-rich final digestate: “We can see problems with spreading it directly on to land with phosphate and nitrate,” Tiernan said.
Another hurdle is that the Government’s support scheme for renewable heat due to open this year will not subsidise the exporting of biogas to external customers such as the national gas network. Yet, Tiernan said Alchemy’s founders remained convinced after setting up the company here several years ago that Ireland has potential to roll out dry AD.
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