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On the website of tomato producer Guy & Wright (established 1918), the company’s self-description includes the legend: “being complete idiots; building our own AD plant.” The story below expands on the designation.

Feedstock for the AD process includes cocoa powder, citrus fruits, grain and potatoes.

Fifteen years ago, when the first 1800m3 digester was built on the 100-acre site in Hertfordshire, England, John Jones (great grandson of Mr Guy) could have been forgiven for wondering what on earth he’d taken on. As if producing hundreds of tons of top-quality tomatoes each year wasn’t enough to think about, he was now getting to grips with that very steep biogas learning curve of feedstocks, temperatures and digestates.

A decade and a half on, with a second, 7000m3 digester, no energy bills, and excess power to sell (enough to power the equivalent of 1500 homes), Guy & Wright is a shining example of how to survive and thrive though diversification into renewable energy.

None of us could have predicted the sudden, massive leap in energy prices that are such a challenge today, but even in the early 2000s, rising fuel costs were already a big concern for John Jones. In the heat-thirsty production of tomatoes, he knew he had to act in order to protect and develop the family business.

At first, five natural-gas-powered 115kW micro-turbines were installed to produce hot water, electricity and CO2. This process enabled Guy & Wright to apply for ROCs (Renewable Obligation Certificates), which at the time, allowed generators of renewable energy to sell on and receive a premium, as well as the wholesale electricity price. But with those gas prices rising steeply, Guy & Wright soon reached the point of no return, investing in a 500kW CHP (combined heat and power) engine from Edina; converting three of the five turbines to run on biogas.

The investment in carefully-sourced equipment also saw the start of what has become a long and productive relationship with Börger; best known for its rotary lobe pumps, but also makers of key farming/biogas kit.

In 2008, a Börger Multi-crusher was put into operation to reduce feedstock particles down to 8mm to enhance the AD process. One might expect there to be no shortage of waste from growing so many tomatoes, but John Jones’ son, Rob, who now runs the biogas operation, soon saw that tomato leaves were not only extremely difficult to break down, but also low in calorific value.

‘Milkshake’ consistency’

So, as the never-ending fine-tuning of the AD plant continues, locally imported waste now includes citrus fruits, potatoes, grain and cocoa powder – plus processed DAF sludge from an ice cream manufacturer. This provides liquid to help create a ‘milkshake’ consistency that the digesters will benefit from far more than a consistent supply of more solid material.

“The Börger Multi-crusher certainly proves itself as a very durable and effective piece of kit for the demands of an AD plant”, said Rob Jones. “We keep one Multi-crusher as a spare so that in any eventuality, we can keep operating – with two always on the go, plus an additional unit now on order. They work very well for us”.

Based on the proven Börger Rotary Lobe Pump, the Multi-crusher chops coarse material to ensure that downstream machines and pumps operate smoothly. The Multi-crusher homogensies mediums at throughput volumes of up to 320 m³/h / 1,400 usgpm/h. In addition to food waste, it can handle fibres, pieces of wood, plastics, membranes and textiles across a wide range of applications.

The team at Guy & Wright added: “Investing in our first CHP was a real turning point, and as we’ve grown the biogas plant, converting (covering) our old lagoon into a secondary, 7000m3 digester*, we’ve not hesitated to invest in more Multi-crushers from Börger”.

The covered lagoon at Guy & Wright produces enough gas to run two of three CHP engines – and also provides retention times of up to six months, compared to most biogas plants where it is just 30 days. Every last bit of gas is extracted”.

Guy & Wright secured another Börger Multi-crusher when it began taking in liquid animal bi-products, for which they also needed a (7.5kW) Börger pump. Utilising this type of bi-product (via a new pasteuriser) has provided another important string to the Guy & Wright bow, enhancing biogas yields by having a feedstock with a high calorific value and also less digestate to deal with.

Two biomass boilers are also now in the fleet of machinery, providing much-needed additional heat to the nursery during winter. Hot water is stored in a buffer tank so that it can be used on demand. Guy & Wright have also become the first company in the biogas industry to take exhaust gas from a CHP and convert it into CO2 for the glasshouses. The gas is cleaned by a special system of catalyst bricks that absorb harmful gases, leaving the resulting CO2 (which is piped into the glasshouses) at perfectly safe levels. This also aids the photosynthesis of the tomatoes, resulting in more plentiful flowers and fruits.

The firm turned to Börger again to address the dwindling capacity of the plant’s open lagoon. Working together with four nearby farms, there had been problems with blockages during spreading with an umbilical system, but that’s all changed for the better now, thanks to the purchase of a Börger Bioselect Separator.

Using a purely mechanical process, liquid is separated from solids in the medium, so that nutrient-rich (PAS 110-approved at a maximum of 2mm) organic matter can go back to the land as a top-quality fertiliser. The setup combines a separation machine and two Börger Rotary Lobe Pumps, with the separator being load-triggered. The feed pump only conveys the volume that the Bioselect is able to process. The high-density solids discharge pump determines the degree of thickness.

“We no longer lose capacity in our lagoon”, explained the team at Guy & Wright. “This is due totally to the Börger Separator, which protects it. Works an absolute treat”.

*Outside of Malaysia, the covered lagoon digester at Guy & Wright is believed to be the largest in the world.

One claimed “first” in the biogas industry for Guy & Wright was the taking of exhaust gas from a CHP and converting it into CO2 for glasshouses.