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High efficiency and low emission factor
For more than ten years, the author has been involved with the implementation of combined heat and power (CHP) plants at beverage producers. During these years, the regulatory framework has changed several times and in some cases fundamentally.
A look back
Since the 1990s, combined heat and power plants (CHP) with combined electricity and heat generation have been particularly well known in the brewing and beverage industry. Plant efficiencies of up to almost 100 per cent make the technology attractive. Waste heat utilisation as hot water is the simplest type of heat extraction. However, the separate extraction of waste gas energy for the generation of saturated steam in waste heat boilers has also been implemented.
Since 2003, the Combined Heat and Power Act (KWK-G) has regulated subsidies for operators. Some plants were highly lucrative due to these subsidies. Over time, changes in the various levies and subsidies have caused the economic viability of large industrial plants in particular to decline bit by bit.
The combination of low electricity prices, the complete calculation of the EEG levy and the upcoming CO2 taxes finally led to the fact that since 2020, well-running plants in beverage companies could no longer be operated economically. The currently rising energy prices now make the operation of larger CHP plants in particular sensible again.
Current conditions in Germany
The CHP Act 2020 applies to the operation of plants. According to this, operators of plants with an electrical output of up to 100 kW receive CHP surcharges for both own consumption and feed-in. Larger plants receive these surcharges only for electricity fed into the grid. The EEG levy for plants with an electrical output of up to 1,000 kW is 40 percent; for larger plants, the so-called claw-back model applies, according to which the levy increases to 100 percent depending on the hours of use.
Certain industries, such as producers of fruit and vegetable juices, can be exempted from the EEG levy on self-generated electricity. However, the EEG levy loses significance in the analysis because it is to be abolished in 2023 according to the plans of the new coalition. In return, the CO2 levy on fuel will rise continuously, from currently 30 EUR/t (2022) to at least 55 EUR/t (2025). A further increase was suspended in the current coalition agreement "for social reasons" - but it should be expected in the medium term. Electricity and energy tax are due for CHP plants; the energy tax on the fuel used can be reimbursed to operators upon application and under certain conditions.
Until electricity from renewable energy sources is available in sufficient quantities, CHP plants offer emission factors that are significantly lower than the current German electricity mixImplications for the brewing and beverage industry
As a rule, companies do not operate or plan CHP plants out of pure idealism, but try to provide the necessary thermal and electrical energy at the lowest possible cost. The high degree of utilisation of the fuel used is a fundamental argument for the use of this technology. Unfortunately, levies and subsidies from regulatory interventions do not provide good security for investment decisions due to frequent changes.
CHP plants currently still function by burning fossil fuels. At first glance, this does not sound good from an environmental point of view. However, it can be shown that the CO2 emission of CHP electricity from natural gas when using the heat produced is significantly lower than that of the current electricity mix (values are taken from a current project planning): with an electrical efficiency of 44.0 per cent and a thermal efficiency of 44.4 per cent, there are 11.6 per cent system losses via radiation from motors, pipelines or with the exhaust gas. If the emissions of the fuel used are divided proportionally between the (generated) electrical and thermal work, the emission factor for self-generated electricity is 0.227 t CO2/MWh. This value is below the emission factor of the current German electricity mix (2020) of 0.366 t CO2/MWh.
Projects from the field
In terms of emissions, CHP plants can therefore currently contribute to improving the CO2 balance. Whether the plants can also be operated with good economic efficiency depends strongly on the investment requirements and here especially on the investment costs for the use of thermal energy. The calculation must be made on a case-by-case basis. Three plants from the plans of Ingenieurbüro Hebmüller GmbH are presented below to show the range of possible heat uses.
CHP with waste heat boiler and use for bottle washing machines
This project was realised at a large German mineral water company. A CHP unit with an electrical output of 1282 kW was installed. The heat is extracted separately for exhaust gases and the engine. The waste gas heat (nominally 699 kW, temperature > 400 °C) is fed to a waste heat boiler, which generates saturated steam at a pressure of 1 bar(Ü) and contributes to covering the base load in the plant.
The engine waste heat heats the cleaning caustic of a total of five bottle washers via a heat accumulator. Such an application becomes possible when lye temperatures in the washing machines ensure low return temperatures in the heating circuit and thus a sufficient cooling effect for the motor.
The system has been running with an efficiency of about 90 per cent since 2016. A further increase would be possible if the flue gas upstream of the chimney were cooled down further by means of a condensing heat exchanger.
Gas turbine for combustion air preheating
The second project shows a principally different heat utilisation by connecting the CHP plant with a steam boiler. The electricity is not generated by a gas engine, but by means of a gas turbine whose exhaust gas with a temperature of about 300 °C and a residual oxygen content of 17.5 percent is fed to the burner of the existing steam boiler as combustion air. The preheated combustion air reduces the fuel requirement of the boiler burner. This implementation requires a special boiler burner.
The project was implemented at Pott's Brauerei in Oelde and was awarded CHP of the Month (July 2018) by the CHP Info Centre.
CHP with waste heat boiler and utilisation of engine waste heat
This project is currently in the planning phase. A total of three CHP modules with a combined electrical output of approx. 4500 kW are planned. The exhaust gas waste heat of 1700 kW is extracted as described above via a three-pass waste heat boiler with a capacity of 2600 kg saturated steam (6 bar(Ü)).
The engine waste heat (3000 kW) is collected in an energy storage unit and converted into process cooling by means of an absorption chiller (ACM). In an ACM, the thermal energy supplied is the driving force of the refrigeration process, in contrast to the electrical drive of compression refrigeration machines. The intended machine has a cooling capacity of approx. 2000 kW.
The cold is fed into the process via a glycol-water mixture as a refrigerant and used for product cooling, e.g. in the area of flash pasteurisation units (flash pasteurisation units). The system has an efficiency of approx. 90 percent. The generation of cold through the use of heat is called combined heat, power and cooling (CHC) and, by reducing the load on the existing compression refrigeration systems, additionally reduces the amount of electricity required. However, the relatively low efficiency of converting heat into cold must be critically evaluated in every project.
Due to the high efficiency loss, it only appears effective in exceptional cases to convert existing CHP plants to hydrogen combustion. In this respect, CHP represents a sensible transition technology for the coming yearsProspects
The emission factor of self-generated CHP electricity is lower than that of the current electricity mix. However, this will continue to decrease in the coming years due to the expansion of renewable energies. In the European Green Deal, climate neutrality was agreed by 2050; for this to happen, electricity generation must be completely converted to renewable energies and also contribute to the provision of fuel that is still needed, e.g. in the form of hydrogen or e-fuel. Although CHP modules can already be converted to run on hydrogen with minor modifications, the double loss of efficiency when converting electrical energy into hydrogen and burning it to generate electrical power means that this approach only makes sense in exceptional cases. On the way to climate neutrality, cogeneration will be a sensible transitional technology for the next few years. The coalition agreement of the new government explicitly mentions gas-fired power plants for this purpose. We can look forward to seeing whether the legislator will also give appropriate consideration to gas-fired CHP plants for industry in the concrete implementation.
Sources
- https://www.bafa.de/DE/Energie/Energieeffizienz/Kraft_Waerme_Kopplung/kraft_waerme_kopplung_node.html (accessed 14.12.2021).
- https://www.bhkw-infozentrum.de/beispiele/bhkw_des_monats_06_2018.pdf (accessed 14.12.2021).
- N.N.: Mehr Fortschritt wagen, Koalitionsvertrag 2021–2025 zwischen der SPD, Bündnis 90/Die Grünen und FDP, Berlin 2021.
