MILD Combustion of Ethylene and Natural Gas

Combustion is the dominant source of energy supplied to the world. This has provided many benefits to the world, but has also resulted in significant pollution, and a dwindling supply of readily accessible fuels. Due to the many applications that cannot yet be replaced with alternative sources, it is necessary to reduce both the pollution produced and the amount of fuel consumed. One proposed method of achieving both aims is to use Moderate or Intense Low-oxygen Dilution (MILD) combustion, which has been shown to reduce both pollution and fuel consumption. In typical practical systems, exhaust gases are recirculated, which simultaneously preheats the reactants and dilutes the local oxygen (O2) concentration. This produces a distributed reaction zone, and a more uniform temperature distribution and heat distribution. As a result, peak flame temperature is reduced, which suppresses the production of pollutants such as oxides of nitrogen. Previous studies of MILD combustion have been limited in their range of O2 concentrations considered, typically considering concentrations of 3 %, 6 % and 9 %. Those works have shown that there are nonmonotonic changes in combustion behaviour with decreasing O2. However, the lack of resolution in O2 concentrations has limited the refinement of the transition point from high temperature, low oxygen combustion to MILD combustion. It is also well known that different fuels have different combustion characteristics. This thesis compiles two papers, presenting results and findings from an experimental study, supplemented with chemical reaction modelling. The first part consists of a study on the behaviour of ethylene (C2H4) flames in a variety of low oxygen, high temperature environments, in order to further understand the transition from high temperature, low oxygen combustion to MILD combustion. The use of C2H4, being a common intermediate product of hydrocarbon combustion, allows these results to be applied to understanding the intermediate phases of the hydrocarbon combustion. The second part consists of a comparative study between C2H4, and natural gas, along with blends of both fuels. This will provide more detailed insights into the behaviour of the intermediate phases of hydrocarbon combustion under MILD conditions. The results showed that, when C2H4 is burnt in preheated low O2 conditions, there are non-monotonic trends in lift-off height, and hydroxyl radical (OH) and formaldehyde (CH2O) formation heights, with reducing O2 concentration. These results also allowed the refinement of the peak to between 4 % and 5 % O2. The results also indicated that the OH number density and flame temperature have a monotonic trend in lift-off height with increasing oxidiser temperature. However, in the 3 % O2 oxidisers, the CH2O was shown to decrease with increasing oxidiser temperature. Since the 3 % O2 cases all satisfied the requirements for MILD combustion, this is evidence of the temperature having a significant role on the chemistry. Comparing the different fuels, it was found that the composition of the fuel plays a significant role in the lift-off characteristics, which was attributed to the large variation in ignition delay. Pure natural gas flames were significantly lifted. However, adding C2H4 to the fuel supply significantly reduced the lift-off. These changes are due to changes in the chemical reaction pathways which compete between those for CH4 and those for C2H4. The reduction in local O2 associated with MILD combustion was shown to have a significant effect on the reaction pathways. Fuel composition was found to have little effect on jet spread rate. The insights gained by this study allow greater understanding of MILD combustion. An improved understanding of the effect of the local O2 concentration is provided by this work, along with a greater understanding of the intermediate stages of the MILD combustion of many hydrocarbon fuels. The findings of this study will allow a wider implementation of MILD combustion technology in combustion devices.Thesis (M.Phil.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 202