Description of the industrial waste heat utilisation with consideration of an optimised power production based on an ORC cycle
BIOS BIOENERGIESYSTEME GmbH, Graz
Industrial processes often produce large amounts of unused waste heat, which is cooled off with expensive cooling units or directly exhausted. Considering continuously raising energy prices, suitable measures for the utilisation of waste heat can contribute to the reduction the energy costs of industrial plants.
The efficient utilisation of waste heat reduces the CO2 emissions by substitution of fossil fuels. Moreover, valuable electric energy can be generated.
Industrial waste heat sources
Depending on the industrial process, different forms of waste heat at varying temperature levels are produced. Typical waste heat flows are:
- Flue gas / exhaust gas from furnaces, gas engines and thermal post combustion
- Exhaust air / hot air
- Pressurised hot water / hot water / waste water
- Steam / exhaust vapour
The temperature of waste heat ranges from hot flue gas streams with several hundred degrees to low temperature waste heat with 40 to 60°C.
Besides the temperature, the characteristic of waste heat flows strongly depends on the heat source and the industrial process, respectively. Therefore, a characterisation of the industrial waste heat flow (e.g. flue gas composition, dust load, contamination of water, steam quality, corrosion characteristics) is necessary.
The amount of waste heat available depends on the industrial process upstream the heat recovery units. Hence, significant short and long term load rate variations (night, weekend, season) are possible.
The design of optimised waste heat recovery plants requires a detailed evaluation of the available waste heat sources as well as potential heat consumers within or outside (e.g. district heating systems) the manufacturing plant. Moreover, the characteristics of each heat source and heat consumer (heat transfer medium and its properties, temperature, heat capacity/demand) needs to be considered.
Power production based on waste heat and further utilisation possibilities
Depending on the waste heat capacity and temperature level available, different technologies such as the steam turbine process, the screw-type steam engine or the ORC process are applicable. Steam turbine processes are mainly suitable for power outputs > 2 MW electric and for high temperature waste heat. Screw-type steam engines are applicable for lower power outputs but require high temperature waste heat as well. The steam for the steam turbine process or the screw-type steam engine is generated in waste heat boilers. The steam produced is then used in a conventional water-steam-cycle including a steam turbine or a screw-type steam engine for the production of electric power.
The ORC process was especially developed for the power production based on low temperature heat sources such as geothermal energy, waste heat utilisation and decentralised biomass CHP plants. Thus, the ORC process is especially suited for the power production based on industrial waste heat. In order to produce electricity waste heat is transferred to a thermal oil cycle and further on to the ORC unit by heat exchangers. The ORC unit produces electric energy which can be used to cover the auxiliary power demand of the manufacturing plant or for the feed-in into the public grid. Additionally, the ORC unit produces low temperature heat. Depending on the site constraints of the manufacturing plant the low temperature heat can be used as low temperature process heat (e.g. drying) or for the space heat supply.
Alternative possibilities of waste heat utilisation includes the heat extraction from waste heat flows for the internal space and process heat supply or external heat consumers (e.g. via a district heating system).
Besides waste heat from flue gas, exhaust gas and hot water flows other heat sources such as energy from waste or exhaust steam, exhaust air from hot air and drying processes as well as heat from engine and compressor cooling can be used as heat sources for other heat consumers.
Intelligent energy master plans even allow the utilisation of low temperature waste heat. (Energy concepts and -efficiency) Thus, concepts such as a multiple utilisation (energy cascade) of heat, heat storage and preheating processes (fresh water preheating with waste water) are applied. Further applications include the utilisation of low temperature waste heat for cooling with absorption chillers.
An efficient way to use low temperature waste heat is as an input into heat transformers based on absorption technology. Thereby, a part of the low temperature heat is raised to a higher temperature level and thus an utilisation for process and space heating is possible. However, the remaining part of the heat which has a rather low temperature needs to be cooled off.
Field of application
The industrial waste heat utilisation is especially relevant for industrial processes with high heat demands. This includes the following industry sectors:
- Iron and Steel industry
- Cement and building material industry
- Food and beverage processing industry
- Pulp and paper industry
- Chemical industry
- Petroleum industry
Realised projects and proposals under design
- Waste heat recovery for district heat utilisation and design of pipe network / BIOCHEMIE Kundl GmbH (Tyrol, Austria)
- Waste heat recovery by flue gas condensation / Holzindustrie KAINDL (Salzburg, Austria)
- Heat recovery from an existing CHP-plant / Domat (Grisons, Switzerland)
- Heat and power production by waste heat recovery of industrial flue gas streams based on an ORC cycle – RHI AG, Radenthein (Carinthia, Austria)
- Heat and power production by waste heat recovery of industrial flue gas streams based on an ORC cycle, Wietersdorf (Carinthia, Austria)
- Heat and power production by waste heat recovery of industrial waste heat based on an ORC cycle, Secunda (Mpumalanga, South Africa
- Steam generation with waste heat from an existing biogas plant with gas engine, Holsworthy (Devon, England)