Wasted heat is becoming increasingly important , not only from an economic point of view , put also politically . In the UK the Renewable Heat Incentive 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 which 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 )

• Electricity generation ( e . g . via CHP )

• Biogas upgrading to biomethane ( heat is required for the process , but up to 75 % of it can be recaptured and reused )

• Digestate concentration

A minimum temperature of 70oC may be required for pasteurisation , leaving 30-40oC of ‘ left over ’ 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 . What is a heat exchanger ?

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 come into direct contact . Common

everyday examples include domestic radiators and car radiators .

Two of the most common types are Plate Heat Exchangers and Tubular Heat Exchangers . However , within these broad categories there are many different models 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 , HRS corrugated tube heat exchangers are designed so that the constant swirling of the fluid in the tube prevents sediment and clogging when used with certain materials like digestate .

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 ; for pasteurising ; to reduce the volume of digestate or even to upgrade biogas to biomethane , for use as a transport fuel or for injection into the gas grid .

Other uses for heat

There are many uses of heat on farms , from 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 .

In other parts of the world more unusual uses for heat have also been seen , such as aquaculture for fish production ; further electricity generation through the use of Organic Rankine Cycle ( ORC ) technology and Kalina Cycle low temperature generation systems ; or transporting heat in containers which utilise latent heat storage technology .

With careful planning and a long term approach which 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 .