FEATURE HVAC
BETTER UTILISING HEAT from renewable sources
Many AD plants in the food sector produce incidental heat which can be captured and used within the process or for other on-site operations, says Matt Hale of HRS Heat Exchangers
A
naerobic digestion (AD) produces many valuable and useful products
including biogas (which can then be turned into heat, electricity or biomethane gas) and digestate which is rich in plant nutrients and organic matter and can be used as a valuable biofertiliser. However, many AD plants in the food sector 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 but 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 (before or after digestion) Electricity generation (eg 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 70C may be
required for pasteurisation, leaving 30-40C of ‘left over’ heat which has historically been wasted to the atmosphere but could be put to good use elsewhere within the AD process or for other processing tasks within the food production environment. 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
20 MAY 2016 | FACTORY EQUIPMENT
mix together or come into direct contact. Everyday examples include domestic radiators (which transfer heat from a boiler to a room) and car radiators (which take heat away from the engine). Two of the most common types in use today are plate and tubular. However, within these broad categories there are many different models and refinements and it is important to understand what is being offered. It is advisable to consult a specialist who can explain the benefits of different types and perhaps offer alternative 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 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 Having identified a source of heat, what can be done with it? There are several options with the AD process including: preheating feedstock; for pasteurising; to reduce the volume of digestate or
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 materials like digestate
even to upgrade biogas to biomethane for use as a transport fuel or for injection into the gas grid.
FURTHER USES FOR HEAT There are many uses of heat in the food industry from space heating and cooking to heating liquids to aid processing as well as pasteurising and sterilising foodstuffs. Large sites may have significant office and staff facilities and there may be scope to install district heating systems in these areas. In other parts of the world more unusual uses for heat have also been seen such as further electricity generation through the use of Organic Rankine Cycle (ORC) technology and Kalina Cycle low temperature generation systems; and 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.
HRS Heat Exchangers T: 01923 232 335
www.hrs-heatexchangers.com
/ FACTORYEQUIPMENT
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