Analysis And Estimation Of Stoichiometric Air-Fuel Ratio And Methane Number For Natural Gas
Stoichiometric air-fuel ratio and methane number are important parameters to optimize natural gas fueled internal combustion engines for power and emissions. The increased production of high energy content natural gas from shale and other sources combined with an increasingly reticulated natural gas transportation system has resulted in widely variable fuel composition for many natural gas fueled engines. In some cases, this requires dynamic adjustments of engine controls to maintain emission compliance, maximize power production, and avoid pre-ignition/detonation conditions. The introduction of large quantities of shale gas and increased use of natural gas for electrical power generation has resulted in gas compositions varying significantly in some areas of the country. This can create challenges to optimum engine control. Incorporating calculations to add stoichiometric air fuel ratio control and methane number (MN) in the unit control system can lead to enhanced engine control methods for improved emission compliance and reliability. Air fuel ratio control is a key parameter in maintaining air emission limits, especially with respect to nitrogen oxides (NOx). The paper describes two different methods for calculating stoichiometric air-fuel ratio and one additional method for estimating air fuel ratio using only a four component gas composition. It is expected that the use of the (MN) can be used in engine controls to optimize operation to avoid detonation and power deration. Several different methods for calculating MN were reviewed and the problems and limitations of those methods are identified. Most of the MN calculation methods described in the paper were compared against measured MNs with some methods being more accurate than others. Most of the methods used to calculate MN are proprietary. The MN methods that are open and publically available have significant limitations in respect to the accuracy and the applicable gas compositions. Two new methods to cal
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