Use of Hydrogen and Hydrogen Mixtures in Gas Engines and Potentials of NO_x Emissions

In the utilization of gas mixtures with high amounts of H₂ there is a great number of applications of such special gases, for example several gases that result from pyrolysis or the gasification of biomass or thermally utilizable waste substances. What is special about gases containing H₂ is the shifting of the lean-burn limit towards greater amounts of excess air than is the case with natural gas. This effect causes the mean combustion chamber temperatures to sink and the NO_x emissions are reduced to a very low level. Depending on the amount of hydrogen and other gas components it is possible to attain NO_x values of under 5 ppm. Also very interesting is the property of these H₂-rich gas mixtures to have a neutral influence on the degree of efficiency (even with extremely high amounts of excess air). The background of this property lies in the considerably higher laminar flame speed of hydrogen. Especially in the lower and medium load range this effect can be utilized directly; in this regard it was possible to measure an efficiency of up to 2% points better with operation using pure hydrogen compared with NG. Higher BMEPs are also only possible to a limited extent with extreme lean-burn operation because the knocking limit is reached. Furthermore, the dimensioning of the turbocharger is becoming more and more difficult because the exhaust gas temperature upstream from the turbine sinks and as a result also the thermal energy is available only to a limited degree. When dealing with high amounts of H₂, from the standpoint of operational reliability it is necessary to modify the mixture formation before the TC position to the pressure side position upstream from the intake valve, because otherwise load fluctuations could lead to undesired rich mixtures in the inlet side. As a result, backfiring could occur that could also cause engine damage and that could be hazardous for personnel. From the viewpoint of GE Jenbacher H₂ technology can be applied relatively quickly to reduce NO_x emissions. Especially when considering the “life cycle costs”, this potential solution is superior to concepts functioning on the basis of stoichiometric combustion. The next step that can be mentioned is the concept of fuel-reforming integrated in the engine — here a part of the exhaust gas energy is used to reform a relatively small amount of natural gas to a CH₄/H₂/CO mixture. With this concept, alongside the dramatic reduction of NO_x emissions to the level of fuel cells, the degree of efficiency can be improved by about 2 to 3% points by means of “energy shifting”.