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Temperature control for biodigestion plants

14 November 2016

Once running, biodigestion plants are generally pretty self-sufficient. However they do, from time to time, need a helping hand by way of warming or cooling. Food Processing reports. 

Since the UK’s Anaerobic Digestion Strategy was published in 2011 there has been an acceleration in biodigestion development. Launched with the support of the Department for Education, Environment, Food and Rural Affairs (DEFRA), the objective of the strategy was to boost the use of anaerobic digestion (AD) to produce renewable energy, reduce greenhouse gas emissions, and divert otherwise unusable food and farm waste from landfill.

AD involves harnessing natural biological breakdown processes, with bacteria operating in an oxygen-free environment and producing methane. The energy produced can then be used to power generators to produce renewable electricity or heat.

According to DEFRA’s latest Anaerobic Digestion Strategy and Action Plan 2011, since 2011 the number of AD plants has more than doubled across the UK – rising to 140 in 2015, with a further 200 plants having received planning permission. 

In the early days, plants were mainly used to process sewage. However, a significant part of the expansion over the past five years includes adoption for the treatment of food waste. 

Plants currently operating have capacity to process more than 10 million tonnes of waste a year – around 2.3 million tonnes of this is food waste. 
To support this expansion, Carrier Rental Systems is supplying high-performance temporary cooling and heating equipment for use in the UK’s energy-from-waste biodigester facilities. When new plants are commissioned, an external energy source is required to warm the feed stock within the biodigester, which initiates biological activity. Once fully under way, the process becomes self-perpetuating.

To provide this temporary heating source, Carrier Rental Systems can supply packaged boiler equipment, buffer tanks and associated pipework on a hire basis. 

A recent example of this application involved a boiler hire package for a biodigester project in Scotland. A 250kW boiler was used over a six-week period during the commissioning phase of the plant to gradually raise the temperature in the digester tanks to 45°C, at which point the system produces enough power to initiate and self-support the onsite generator. 
Additional heating
Additional heating may be required at other stags in the biodigestion lifecycle. For example biological activity in a digester can stall, due to a problem with the feedstock or other external conditions that cause the system to cool. In a recent example, Carrier Rental Systems supplied a temporary boiler package to a major water treatment plant in Dartford to address just such a problem.
Once installed and commissioned, the boiler was again soon generating sufficient hot water to restart the biological process and production of methane was quickly re-established. The warming intervention overcame the need to remove the inactive contents from the bio-digester and start the process from scratch, saving time, money and lost productivity.

There will also be a need for chillers, to cool the contents of the biodigester in order to arrest biological activity before maintenance is carried out. In a recent case, Carrier Rental Systems supplied a high efficiency 100kW chiller to an energy-from-waste plant in Reading, which was undergoing major maintenance work. The chiller was connected to the plant’s main process heat exchanger, enabling rapid cooling of the biodigester contents as they passed through. This quickly reduced the temperature within the reactor vessel, progressively arresting biological processes, enabling the maintenance work to commence.

Looking to the future, DEFRA anticipates that growth in the use of anaerobic biodigesters is likely to continue, driven by the attraction of renewable energy and the rising costs of sending organic and food waste to landfill. The economic case is further bolstered by the growth in the demand for digestate, the material that remains after processing, for use as a fertiliser. This not only further reduces landfill costs, but gives waste producers a second source of revenue.

At a time when controlling food waste is becoming increasingly important, anaerobic digestion offers a solution that is not only good for the environment, but can make commercial sense, too.

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