Modelling and Performance Investigation of Anode-off Gas Combustion in an IC Engine for Power Generation

Abstract

In the GasDrive project, a solid oxide fuel cell and a reciprocating gas engine are used to provide electrical and mechanical power for a ship. One of the goals of the GasDrive project is to broaden the limits between knock and misfire for the gas engine. Extending the operating limit could be achieved by addition of anode-off gas. Before anode-off gas can be used in combustion together with natural gas in the GasDrive project, the combustion characteristics of anode-off gas must be known. In this thesis, a literature review is conducted in order to explore similar gasses. One such gas is producer gas, on which both experimental and numerical studies have been conducted by other researchers. To investigate the performance of anode-off gas, an altered in-cylinder model was used. The model is verified using producer gas as a fuel, with the experimental data from producer gas combustion in an IC engine. The model functions properly and is used to investigate anode-off gas combustion.
In modelling the combustion of anode-off gas, multiple parameters are kept constant and similar to that of producer gas. These parameters include the Vibe parameters, engine geometry, air excess ratio, T1, p1, and stoichiometric gas mass fraction from the previous cycle. The combustion duration relative to producer gas is modelled based on the ratio of turbulent flame speeds. This turbulent flame speed is a combination of laminar flame speed and turbulence intensity. At first the SOC is kept constant as well, only altering the EOC for different combustion durations. The maximum power output is 79 kWe for anode-off gas with a 50% fuel utilization rate. The maximum efficiency is 20%. The peak pressure is 104 bar, which is higher than for producer gas, but falls within the limits of what the engine can handle. The peak temperature is 2847 K, which is more than 700 K higher than producer gas. The engine, it is believed, can not handle this temperature. The lowest peak temperature is reached for anode-off gas with a 85% fuel utilization rate and is 2652 K. This is still outside normal engine operating temperatures. Combustion of anode-off gas under stoichiometric conditions in an IC engine is therefore not feasible. Increasing the air excess ratio could resolve this issue, but can not be done in the current model without having experimental data on anode-off gas. The fuel consumption is 6-18 times higher than conventional fuels, also showing that anode-off gas combustion would not be an advantage over conventional fuels. More power at lower temperatures can be reached when retarding start of combustion, achieving MBT timing. Peak power is achieved using anode-off gas with a 50% fuel utilization rate and is 82 kWe. The corresponding peak temperature is 2822 K, which is also outside normal operating conditions of IC engines. Based on the modelling results, it can be said that combustion of anode-off gas is possible in a IC engine, but power outputs will be lower than conventional fuels and fuel consumption will be higher.