Catalytic oxidation of methane could solve one of the big emission challenges for gas engines. Turbochargers will play a role in making the technology work
A new technology being explored by MTU could help minimise methane slip from gas engines. In the process, it could bring together some recent development strands at the Friedrichshafen-based enginebuilder and turbocharger company.
If it were not for methane slip, gas engines would be a much better greenhouse gas emissions solution. Even at a range of 100 years, methane is 28 times more potent than CO2 in its contribution to warming. The multiple is even higher, around 84 times, over a 20-year period. So, while using LNG in marine engines dramatically reduces SOx and NOx emissions, the reduction in CO2 emissions is somewhat offset by methane slippage.
For MTU this is a challenge well worth exploring. Over recent years the company has invested significantly in the pure-gas version of its 4000 engines. The first marine versions of the lean-burn engines are due to begin operation imminently in the Netherlands, on board two new ferries owned by Rederij Doeksen.
Lean-burn engines (which admit gas fuel under low pressure and then ignite it) are more prone to methane slip than high-pressure engines, which inject fuel as a high-pressure spray and so enable more complete combustion of methane. As a result, MTU is exploring two measures. As well as exploring a high-pressure dual-fuel engine concept – a relative rarity in the four-stroke sector – it is also investigating ways to reduce methane slip in its lean-burn, pure-gas engines.
One such method is known as catalytic oxidation. The process is currently used to limit carbon monoxide and formaldehyde production where emissions legislation dictates, on stationary engines. They are also emerging for methane, the main component of LNG, but the precious metal-based catalyst technologies are very sensitive to water and sulphur dioxide in exhaust gas. These hinder the methane conversion rate under the temperature conditions after the turbocharger, where the catalyst unit is usually positioned.
“Moving the catalyst upstream of the turbocharger improves catalytic conversion performance because of higher temperatures; but it also brings additional challenges”
Moving the catalyst upstream of the turbocharger improves catalytic conversion performance because of higher temperatures. But it also brings additional challenges, including faster heat-induced degradation of the precious metal catalyst, efficiency decreases and a more complicated mechanical integration.
MTU has studied a full-size pre-turbine catalyst prototype designed for high methane conversion rates in lean-burn gas engines. By varying conditions including mass flow, temperature and catalyst volume the company gained a better understanding of catalyst performance. The tests involved 400 hours of engine operation and the results were compared with additional experiments on a synthetic gas test bench.
The results confirmed the potential effectiveness of a catalyst installed in a pre-turbine location. But deterioration of the catalyst due to heat remains a challenge. MTU looked at increasing the catalyst volume to compensate for thermal aging but noted that this had an impact on system integration space, cost and efficiency as well as engine dynamics.
Engine dynamics are a particular issue for pre-turbine catalysts. The catalyst substrate requires heating to work effectively and, combined with inflow and outflow components involved, this acts as an additional heat sink that can slow down cold start of the engine, as the exhaust gas reaching the turbocharger takes longer to heat up.
Another one of MTU’s technologies could offer a solution to this dynamic response challenge. In 2017 the company bought a company that had developed an electrical assistance system for a turbocharger, using electricity to drive the turbocharger when air flow was insufficient. This electrical assistance was intended to take the place of traditional starter air blowers. MTU found that, given suitably selected levels of electrical assistance, dynamic losses due to the pre-turbine catalyst were not only compensated, but the performance enhanced.
Both concepts are in the initial investigation phase: catalytic oxidation of methane, whether before or after the turbocharger, is a relatively immature technology; and using electrical assistance on engines as big as those installed on ships remains some way off. But MTU’s investigations show how combining innovative technologies, including turbocharger functions, can unlock the emissions solutions shipping will soon need.