Waste not , want not
Chris Horsley , Process Engineering Director at Babcock Wanson , looks at the innovative development of Supercritical CO2 technologies in industrial waste heat recovery
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Heat recovery has become increasingly popular within industry , driven by high energy prices , environmental regulations , and a desire to reduce carbon footprint . However , sites and processes vary dramatically , so heat recovery systems are frequently bespoke in nature or , at the very least , require extensive modification of existing equipment and expert installation .
To truly make heat recovery appealing to users , the energy efficiency of these systems needs to be improved to reduce ROI time , and installation made far simpler .
The EC Horizon 2020 grant funded I-ThERM project , coordinated by Brunel University London , set out to do just this . The project was divided into three areas , with Babcock Wanson involved in the recovery of heat from temperatures above 300 ° C using a Supercritical Carbon Dioxide System .
When Carbon Dioxide is held at , or above , its critical temperature ( 31 ° C ) and critical pressure ( 73.8 bar ) it turns into a fluid state , known as Supercritical Carbon Dioxide ( sCO2 ). sCO2 ’ s thermal stability and non-flammability , makes direct heat exchange from high temperature sources ranging between 350 ° C - 800 ° C possible . Furthermore , its physical footprint is simple and compact , and it has a low toxicity and environmental impact . All of which make it potentially appealing when it comes to heat recovery systems .
Designing and building a Supercritical Carbon Dioxide Heater presented a host of new challenges , as not only does the heater have to contend with very high temperatures , it would also be used for testing so needed to be adaptable .
Following several briefings and learning more about the process and the testing regime , we built a specialist Direct Process Air Heater , using our MIXBLOC Heater as the starting point as it is designed to provide high temperature air with no flue gas losses . MIXBLOC can operate using the process air flow as the source of combustion air , or an independent combustion air fan .
The heater was designed for operation at up to 780 ° C air temperature with heater lockout set at 800 ° C . Burner turndown ratio was retained at 5:1 . The process fan size was specifically selected to account for the higher head loss associated with operation at higher working temperatures and with a heat exchanger located close to the heater outlet .
A final discharge cooling system was included , comprising of air cooler with variable speed fan to allow the final exhaust to be cooled to around 350 ° C irrespective of the heat absorbed into the sCO2 .