How to effectively reduce CH4 slip using CFD simulations?
One of many Bridge Technologies towards carbon neutral shipping, is the use of Liquefied Natural Gas (LNG) as a marine fuel. It offers a theoretical carbon dioxide (CO2) emissions benefit of up to 25%. However, natural gas consists of a high proportion of methane (CH4), whose global warming potential is 85 times greater than CO2. Therefore, the reduction of CH4 emissions is one of the main targets in the development process of modern LNG dual-fuel engines. Due to the complex interactions in the engine, reduction potentials are often not directly apparent. This is where modern simulation and calculation approaches can help and reveal hidden potentials. In this blog post, you can find out what such a solution approach might look like using the example of a medium-speed dual-fuel engine.
What CH4 reduction potentials are possible without structural modifications?
Current LNG dual-fuel concepts are based on the so-called diesel-gas process. In this process, liquefied natural gas is fed into the air path upstream of the combustion chamber and then ignited with a small amount of a pilot fuel (e.g. diesel). System-related methane emissions result from incomplete combustion as well as from what is known as "scavenging." In this case, methane flows from the inlet duct directly into the exhaust tract without participating in combustion. This effect is not intentional, but results from the engine-specific valve timing, which is optimized to ensure the smallest possible amount of residual gas in the combustion chamber. Scavenging losses can thus be reduced by adjusting the valve timing. However, this has a negative impact on combustion quality and thus efficiency.
Adaptation of gas injection parameters
The 3D-CFD simulation enables us to reproduce the entire dual-fuel working process in all three spatial dimensions, from gas injection to combustion. The developed overall engine model was carefully compared and validated with experimental data from our university partner (LKV/University of Rostock). Based on this, the FVTR team was able to quantify the potential for reducing methane emissions by varying certain engine operating parameters. A reduction of the gas injection duration, which results in a reduction of the methane storage in the air path, was found to be promising. This measure can greatly reduce methane scavenging, so the problem is being addressed from its source. As a technical and practical implementation measure, an increase in gas pressure was identified, since this leads to the desired effect of a reduction in the necessary injection duration and can be realized with the existing systems.
The CFD investigations were carried out for a full load operating point and showed a reduction potential of CH4 emissions by scavenging of more than 40 %, it should be noted without any system intervention, but only by adjusting an operating variable. In general, the pressure in the natural gas system is high enough to implement the measure even in current serial production engines. The results were afterwards confirmed by our university partner (LKV/University of Rostock). Experimentally, a reduction of approx. 30 % was achieved. An analysis of other load points showed a similar potential. No negative effects on nitrogen oxide (NOX) emissions or fuel consumption were detected.
Due to the high global warming potential of CH4, when converted to CO2 equivalent, the potential is approximately 10 g CO2/kWh. This means that a hypothetical engine power of 30 MW results in a CO2 saving of 300 kg ... per hour.
Through the consistent use of modern calculation and simulation approaches, we are able to map the motor process down to the smallest detail and thus identify hidden optimization potential.
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