The new 44th German Emission Control Act (Bundes-Immissionsschutzverordnung; BImSchV) noticeably restricts the limit values for biogas cogeneration unit emissions; operators are facing particularly strict emissions limit values for nitrogen oxide (NOx). MAN Engines is prepared for the new limit values. Since early 2018, as a cooperation partner in a project initiated and financed by the Bavarian State Ministry of the Environment and Consumer Protection, it has been successfully testing its modular exhaust gas aftertreatment system at a biogas plant in the field. The project about minimising NOx emissions during biogas cogeneration unit operation is being carried out by the Technical University of Munich (TUM)
On 20th June 2019, the 44th BlmSchV came into force and establishes new emissions limit values for biogas cogeneration units. The long-anticipated amendment restricts the limit values for nitrogen oxide (NOx) in particular. While up until recently values of 0.5g/Nm³ were permissible, they cannot exceed 0.1g/Nm³ in the future. “Thanks to the efficient combustion properties of our MAN gas engines, we have been able to comply with these limit values in-engine so far. However, the new restriction of limit values is forcing engine manufacturers and plant operators to find new solutions,” explained Günther Zibes, Head of Power at MAN Engines.
MAN Engines found these solutions in-house. The engine manufacturer has been implementing its modular exhaust gas aftertreatment system (EAT) – which was derived from large-scale truck production – in agricultural engineering since 2015. It is based on Selective Catalytic Reduction technology, and reduces nitrogen oxide using an aqueous urea solution (AdBlue©). Engineers from the MAN Engine Competence Centre in Nuremberg carried the concept over to industrial gas engines for cogeneration units. They have been testing the application with biogas initially in the field since March 2018. For this purpose, an entirely new cogeneration unit container was set up at the Götz biogas plant in Markt Indersdorf, Dachau, by the unit builders, Elektro Hagl KG from Geisenfeld. “We made the conscious decision to install a completely new cogeneration unit for the field trial, in order to re-evaluate the entire system from the ground up,” said operator Josef Götz. The selected engine was an MAN E3262 LE202 turbo unit. The charged V12 engine with 530 kWel at 1500 rpm is particularly well-suited for the field trial as it was especially developed for operation with biogas, and modified by Elektro Hagl.
The EAT is housed in a container superstructure over the cogeneration unit. This consists of one exhaust gas aftertreatment system per cylinder bank. In the SCR mixer, a 32.5 percent aqueous urea solution (AdBlue©) is added to the exhaust gas flowing through, to create a homogeneous mixture. In the following SCR catalytic converter, the nitrogen oxides within are broken down to non-hazardous nitrogen with the formation of water. In the downstream oxidation catalytic converter, which is a part of the modular SCR system, oxygen is used to turn carbon monoxide to carbon dioxide, and formaldehyde into carbon dioxide and water.
“Without the SCR system it would no longer be possible to comply with the new 0.1 g/Nm³ limit values for nitrogen oxides,” confirmed Josef Götz. He also commented on economically unknown factors which the field trials were to determine: “We still don’t know exactly how high the AdBlue© fluid consumption will be or how often the catalytic converters need to be replaced. Additionally, more information needs to be collected regarding the effect the SCR system has on the entire engine operation and wear and tear.”
Both SCR catalytic converters are currently in continuous operation. Depending on the application, a cogeneration unit can reach up to 8,700 operating hours a year. A conventional passenger car catalytic converter, in comparison, is only designed to achieve approximately 3,000 to 4,000 operating hours in its total service life. Günther Zibes is highly satisfied with the course of the field trials so far, however. “The service lives of the installed catalytic converters are completely positive and the AdBlue© fluid consumption level is below our expectations.”
The deciding factor for efficient implementation of SCR technology is precisely controlling the injection quantity of AdBlue© fluid depending on the raw NOx emissions in relation to the limit value of 0.1g/Nm3, which varies greatly due to environmental influences and fluctuating gas quality. The exhaust gas aftertreatment system also poses new possibilities for operators. A modified operating mode of the engines makes up for the disadvantages posed by load change, increasing efficiency slightly. In the ideal scenario, this would allow the system to sustain itself at least in part, from a cost perspective. The results of the field trial will be used to determine economic calculation models, which contain the total costs to be expected by operators. TUM is carrying out the scientific work. In order to make test measurements easier, the spacial conditions around the SCR unit were purposefully dimensioned generously. The space-consuming AdBlue© fluid storage tank is located outside the container. The entire project was initiated and financed by the Bavarian State Ministry of the Environment and Consumer Protection.
The four-year field trial is based on the long-term partnership between MAN Engines and Götz Agrardienst GmbH. The biogas plant operator has been relying on MAN Engines for over 20 years, as their cogeneration unit gas engines are known for their low-pollutant emissions. The engine manufacturer’s Nuremberg-based Engine Competence Centre develops industrial units for special gases such as biogas or sewage gases, in performance classes of 68 kW to 580 kW, as well as natural gas engines with 37 kW to 580 kW. About half of cogeneration units in Germany within this performance segment run on MAN engines.
Götz currently has eight MAN units in operation, with an installed total power of 3 MW. They are distributed amongst three sites, and provide among other things, the Markt Indersdorf elementary and middle school, as well as the adjacent indoor swimming pool with heat via a cogeneration unit located on-site. Furthermore, biogas units are considered CO2-neutral. The carbon dioxide released was previously removed from the air by plants, which are the basis for the biomass. Josef Götz has calculated a CO2 abatement potential of 7,600 t per year for his plant.
Even Günther Zibes is convinced by the technology: “MAN has been selling engines specifically designed for gas applications since 1979. We regard biogas cogeneration units as an important part of the energy revolution, in order to fulfil the specified targets for CO2 reduction. This is why we are proud of the fact that we are able to provide modular exhaust gas aftertreatment to our partners. This is a future-proof solution for complying with the restricted limit values.” According to the 44th BImSchV, these limit values are to take effect from 2023. Existing plants have a grace period up to 2029.
|Engine speed||rpm (Hz)||1500 (50)|
|ISO standard power1||kW||550|