Matt Hale on the forgotten element of anaerobic digestion - incidental heat
Anaerobic digestion (AD) produces many valuable and useful products, including biogas (which can then be turned into heat, electricity or biomethane gas) and digestate, a biofertiliser rich in nutrients and organic matter.
However, many farm AD plants also produce incidental heat, which can be captured and used within the AD process or for other on-site operations.
Wasted heat is becoming increasingly important, not only from an economic point of view, put also politically. In the UK the Renewable Heat Incentive (RHI) is the key policy driver to encourage the utilisation of heat from renewable sources, including incidental heat from the AD process, while some European countries now specify targets for the use of heat from AD plants.
Sources of heat
Surplus heat produced by biogas combustion in a combined heat and power (CHP) unit is the most common heat source within an AD plant. Other processes that result in residual, usable heat include: digestate pre-heating; digester heating (especially in summer when less heating is required); pasteurisation (either before or after digestion); and electricity generation (e.g. via CHP). For example, a minimum temperature of 70°C may be required for pasteurisation, leaving 30-40°C of ‘leftover’ heat, which has historically been wasted to the atmosphere. This heat could instead be put to good use elsewhere within the AD process. Equally, heat left over from concentrating digestate could be used within pasteurisation.
Heat exchangers take heat from one process or place and transfer it to another. In practice, they allow the heat from a liquid or gas to pass to another liquid or gas without the two having to mix together or come into direct contact. Common everyday examples include domestic radiators (which transfer heat from a boiler to a room) and car radiators (which take heat away from the engine).
Heat exchangers are used for numerous applications, including space heating; cooling; air conditioning; sewage treatment; food processing; and in chemical industries. Increasingly, their potential role in the AD sector is being recognised, with more plants specifying their inclusion at the design stage or retrofitting them, either to improve overall process efficiency or to use heat that would otherwise be wasted.
Types of heat exchanger
There are different types of heat exchanger and it is important that the right type is selected for a particular application. Two of the most common types in use today are plate heat exchangers and tubular heat exchangers. However, within these broad categories there are many different models and refinements and it is important to understand what is being offered.
It is therefore advisable to consult a specialist who can explain the benefits of different types and perhaps offer different solutions. For example, viscous fluids such as digestate can quickly foul tubes and surfaces. For this reason, scraped-surface heat exchangers are usually recommended, as they will constantly remove such fouling. However, another option is to use a tube design, which will minimise fouling in the first place. HRS corrugated tube heat exchangers are designed so that the constant swirling of the fluid in the tube prevents sediment and clogging.
Whichever system is proposed, it is important to compare running costs, including maintenance and cleaning, over the full life of the plant – downtime caused by regular dismantling or cleaning can quickly eat into any capital savings made at the time of purchase.
Potential uses for heat in the AD process
So, having identified a source of heat, what can be done with it? There are a number of options with the AD process, including: preheating feedstock, which can help to speed up the digestion process or improve gas production; for pasteurising, for example to meet PAS 110 requirements for digestate or to ensure crop hygiene; and to improve the quality and reduce the volume of digestate. Using surplus heat in an HRS digestate concentration system (DCS), for example, can reduce digestate volumes by around 60%, bringing considerable savings in storage, transport and application to farmland, while retaining all the nutritional benefits.
Finally, using waste heat to upgrade biogas to biomethane, for use as a transport fuel or for injection into the gas grid, is also becoming increasingly common and helps to fulfil AD’s potential as a diverse energy source. What’s more, depending on the exact technology used, as much as 75% of the heat used for biogas upgrading can then be recovered.
Other uses for heat
Captured heat can be used almost anywhere, provided that it is economically and practically feasible to transfer it. Even low-temperature water can be used to reduce the amount of additional heating required, for example by a boiler.
There are many uses of heat at an AD plant whether it’s farm or food based. For farms, heat can be used for space heating of greenhouses and polytunnels to drying crops or biomass fuels. Many livestock buildings require heat, particularly for pig and poultry production, and where farms have diversified to create office or business centres there is often the scope to install district heating systems. And at a food plant, heat can be used for space heating and cooking, to heating liquids to aid processing as well as pasteurising and sterilising foodstuffs.
An endless cycle?
It may be tempting to think that you can keep recapturing the heat used over and over again, but unfortunately, this isn’t possible. However, what is possible is to reuse some of the leftover heat to improve operational efficiency. Systems that do this, such as an HRS digestate pasteuriser, can often deliver efficiency savings of 40% or more compared to traditional pasteurisers without heat recovery.
With careful planning and a long-term approach that considers the full life of an AD plant – not just initial purchase prices – the individual heat loads of each process within an AD operation can be greatly reduced by using recaptured heat, resulting in improvements in both efficiency and product quality.
Matt Hale is with HRS Heat Exchangers.