Compressed biogas

This technology can be applied in vehicles (typically heavy duty) powered by an internal combustion engine fuelled by biomethane. The biomethane is stored in a tank kept at high pressure (20-25 MPa). Technical challenges that apply to this technology mostly relate to methane slip (leakage) in the engine, which occurs at the inlet manifold (before the actual air supply to the cylinder) and from the cylinder due to incomplete combustion. Low pressure pre-mixed injection engines have higher slip (5% unburnt methane in the exhaust gas) than direct injection high pressure gas engines (reference: Advanced Motor Fuels Technology Collaboration Programme Annex 51). For this technology to deliver net emissions reductions, the methane would need to be produced from renewable sources (e.g. biomethane or synthetic methane) and burned in direct injection high pressure gas engines, as use of fossil methane in current engine technologies has no CO2 emissions benefit relative to diesel powertrains. Further technology development to avoid gas slip from the manifold are also required.
Exhaust gas after-treatment catalysts should also be enhanced to compensate for the fact that oxidising of methane molecules is difficult, especially at low temperatures (e.g. during engine warm-up, or when the engine is running at low loads). Indeed, since after-treatment of stoichiometric combustion is a far more cost-effective option for reducing methane and other pollutant emissions than both direct injection and diesel lean combustion, most current research efforts focus on enhancing after-treatment systems. As an example, http://www.h2020-longrun.eu/