Reducing your carbon footprint with heat exchangers

02 October, 2023

A second example shows how heat left over from one process can be recaptured to be used elsewhere. Many anaerobic digestion (AD) plants use heat exchangers to pasteurise the digestate produced during the AD process, so that it can be sold as an agricultural fertiliser. The ‘surplus’ heat which is generated after the system has been running for two hours is used to preheat the digestate, reducing total heat load and improving overall plant efficiency by increasing the amount of generated energy, which is available for export or other uses, as opposed to being required for pasteurisation.

Finally, combining multiple heat exchanges can often provide the greatest energy benefits, for example, in a multi-effect evaporation system such as the HRS DCS Digestate Concentration System. This uses heat exchangers and evaporation to reduce the volume and increase the concentration of sludges and digestate. The first evaporation stage heats liquid digestate and uses a cyclone separator; the steam produced from this first cycle (usually available at 70˚C) is then used as the heating media for the second effect, whereby the process is repeated. The subsequent steam (usually available at 60˚C) is then used as the heating media for the third cycle. The number of effects is determined by the level of dry solids required and the amount of surplus heat available, up to a maximum of four cycles. After the final stage the steam is condensed back to water and this heat is used to pre-heat the incoming product before the first stage of evaporation.

Heat recovery is not limited to systems dealing with liquids. HRS recently supplied a large G Series gas-to-gas heat exchanger to recapture heat from the high temperature exhaust gases leaving a large chemical reactor. This recovered heat is then used to help pre-heat the chemicals entering the reactor to around 500 °C. As well as helping to improve energy efficiency at the plant, the new unit has been designed to cope with challenging operating conditions in order to provide a suitable working life.

These examples show that where the situation allows, HRS corrugated heat exchangers have significant potential to reduce the energy consumption (and therefore GHG emissions) of thermal processes in a wide range of industries. The capital costs of including energy recovery in a heat exchanger system is likely to be higher than similar systems without heat recovery, but these will be recovered over the working life of the unit, particularly at today’s high energy prices.

1 https://www.iea.org/commentaries/how-energy-efficiency-will-power-net-zero-climate-goals

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