Waste heat recovery using water vapour recompression
The investigations of waste heat in industry led to a new technology for steam generators, which can be integrated into existing steam networks. The base technology is water vapour recompression (WVR) which can be applied at different levels of a steam network and offers short payback times. The following case study shows the potential for an integration at a company level.
At a Swiss manufacturer, the process of shrink sleeving is used to apply foils onto cups. Steam is used as heating agent steam as it allows fast processing times and the required hygiene. Every week, 4’000 litres of oil and approximately 1’200 m3 of fresh water are needed for this process. Waste heat is rejected as a mixture of air and steam with a temperature of 70⁰C and almost 100% relative humidity. Plant and system optimization showed that the waste heat cannot be used for other processes or heating at the manufacturing site. Water vapour recompression can be implemented quite easily as an add-on system in parallel to the existing oil-furnace for steam production. The WVR-system will then recover the waste heat from the process and condense most of the demineralized water, which can be reused in the system. The steam can be supplied again to the existing steam lines. This automatically reduces the amount of steam generated by the fossil-fuelled furnace, which would mostly serve as a backup and for system-start-up.
Table: Savings potential of WVR compared to the existing system at steady state operation
|Current system*)||System using WVR**)||Savings|
|CO2 Emissions||219 gCO2/ kgSteam||8 gCO2/ kgSteam||96 %|
|Energy cost||0.050 CHF/ kgSteam||0.036 CHF/ kgSteam||28 %|
|Water usage||4 L / kgSteam||0.5 L / kgSteam||87 %|
*) Assumptions: furnace efficiency of 90%, oil emits 0.28 kgCO2/kWh, oil price: 0.70 CHF/L, reverse osmosis with water-ratio of 1:4
**) Assumptions: waste heat used at 40 ⁰C, electricity mix (CH) emits 0.037 kgCO2/kWh, electricity price: 0.164 CHF/kWh, water re-usage of 50%
Since currently there is a lack of knowledge of these WVR systems, the topic will be further investigated in the second phase of the SCCER-EIP.