Modular exhaust gas aftertreatment system for biogas CHP units

On 20th June 2019, the 44th BImSchV came into force and established new emissions limit values for biogas CHP units. The long-anticipated amendment restricts the limit values for nitrogen oxide (NO_x) in particular. While up until recently values of 0.5 g/Nm³ were permissible, they cannot exceed 0.1 g/Nm³ in the future. Thanks to the efficient combustion properties of the MAN gas engines, MAN has been able to comply with these limit values in-engine so far. However, the new restriction of limit values has been forcing engine manufacturers and plant operators to find new solutions.

MAN Engines found this solution in-house. The engine manufacturer has been implementing its modular exhaust gas aftertreatment system (AGN) – which was derived from large-scale truck production – in agricultural machinery since 2015. It is based on Selective Catalytic Reduction (SCR) technology and reduces nitrogen oxide using an aqueous urea solution (AdBlue^© fluid). Engineers from the MAN Engine Competence Centre in Nuremberg carried the concept over to industrial gas engines for CHP units. They have been testing the application with biogas initially in the field since March 2018. For this purpose, the equipment manufacturer Elektro Hagl KG from Geisenfeld set up an entirely new CHP container unit at the Götz biogas plant in Markt Indersdorf near Dachau, Germany. “We made the conscious decision to install a completely new CHP unit for the field trial in order to re-evaluate the entire system from the ground up,” said plant operator Josef Götz. The selected engine was an MAN E3262 LE202 turbo unit. The charged V12 engine with 530 kW_el 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.}

SCR system ensures compliance with limit values

The AGN was housed in a container superstructure over the CHP unit. It consists of one exhaust gas aftertreatment system per cylinder bank. In the selective catalytic reduction (SCR) mixer, a 32.5 percent urea solution (AdBlue^© fluid) is added to the exhaust gas flowing through to create a homogeneous mixture. In the downstream 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 into 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 CHP unit can reach up to 8,700 operating hours per 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. The service lives of the installed catalytic converters are completely positive and the AdBlue^© fluid consumption level was lower than expected.

The deciding factor for efficient implementation of SCR technology is precisely controlling the injection quantity of urea solution depending on the raw NO^x emissions in relation to the limit value of 0.1 g/Nm³, which varies greatly due to environmental influences and fluctuating gas quality. The AGN system also poses new possibilities for operators: A modified operating mode of the engines compensates for the disadvantages posed during charge cycle and effects a slight increase in efficiency. 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 have been used to determine economic calculation models, which contain the total costs to be expected by plant operators. The TUM has been carrying out the scientific work. To facilitate test measurements, the spatial conditions around the SCR unit were deliberately designed to provide ample room to manoeuvre. The space-consuming AdBlue^© fluid storage tank is located outside the container. The entire project has been 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 CHP 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 gas in the power range from 68 kW to 580 kW as well as natural gas engines with 37 kW to 580 kW. About half of THE CHP units in Germany within this power 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 CHP 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.

MAN has been selling engines specifically designed for gas applications since 1979 and regards biogas CHP units as an important part of the energy transition in order to fulfil the specified targets for CO₂ reduction. According to the 44th BImSchV, these limit values are to take effect from 2023. Existing plants have a grace period up to 2029.