Literature review on CNG / H2 mixtures for heavy-duty CNG vehicles
Sustainably produced hydrogen (H2) is seen as a low carbon fuel for transportation. Likewise compressed natural gas (CNG) could reduce CO2 emissions from vehicles, especially if it is produced from renewable sources such as biomethane. Being both gaseous, H2 and CNG can easily form a mixture (H2/CNG), and the existing gas network could potentially be used for a smooth transition from fossil natural gas to a mixture of renewable CH4 and H2, thereby achieving a low carbon energy supply for internal combustion engines (ICEs). Concawe commissioned DNV a literature review to assess the benefits, drawbacks and barriers of using H2/CNG mixtures in ICEs, with a focus on heavy duty vehicles.
The report reveals that the European heavy-duty CNG vehicle market is moving from lean burn (excess air, the technology that was most used in pre-Euro V vehicles) to stoichiometric engines (current technology of choice in Euro VI vehicles). In general, the literature primarily focusses on the effect of H2 addition to CNG in lean-burn engines, limited information being available on stoichiometric heavy-duty engines.
When 20% vol. H2 is mixed with CNG, the literature shows:
– Engine efficiency gains between 0% and 13% in a spark-ignited lean burn engine, strongly dependent on engine parameter settings; in a stoichiometric engine, the efficiency gain is unknown;
– Greenhouse gases emissions reductions between 8% (if no efficiency gain) and 20% (13% efficiency gain)
– Vehicle driving range reductions between 24% (if no efficiency gain) and 14% (13% efficiency gain), due to the unfavourable compressibility factor of H2 compared to CH4.
The tolerance to H2 content in the natural gas grid is heterogeneous in Europe, ranging between 0.1% (Belgium, UK) to up to 10% (Germany, on a case-by-case basis, depending on the grid specificities). In the whole supply chain connected to the gas grid, CNG vehicles are among the end-use applications with the lowest tolerance to H2 addition (2% vol. max) because the steel tanks (Type 1) used in the legacy fleet may suffer from H2 embrittlement. For this reason, the H2 limit of 2% vol. max is fixed in the European standard EN 16723-2 for CNG as automotive fuel. However, new vehicles are no longer concerned by this issue thanks to new tank types made of non-steel composite materials. The report identifies knowledge gaps and research needs regarding the compatibility of engines and retail stations with higher rates of H2 in CNG which concern the aftertreatment system, the spark plugs, the lubricating oil, the CNG tank and fuel lines, the injectors, the knock management and rating, the engine calibration and the sensors; the effects on engine-out emissions and combustion stability are also listed.
Moreover, the report compares the merits of use of pure H2 both in fuel cell electric vehicles (FCEVs) and in ICEs. On the one hand, FCEVs show better fuel economy and no pollutant emissions compared to ICEs. But on the other hand, they require a very high degree of purity of H2 at the retail station and have a higher total cost of ownership (TCO).