Principal Investigator: Dunn-Rankin, Derek . Institution: University of California-Irvine Proposal No: CBET-0932415
This research focuses on the combustion science involved in burning gas hydrates, a major potential energy resource. Gas hydrates are ice-like crystalline solids that encapsulate guest gas molecules like methane. A significant storehouse of methane is in the form of methane hydrates on the sea floor and in the arctic permafrost. These fuel hydrates represent an important intersection of energy and environmental concerns. Methane has a beneficial potential impact on climate change as a fuel because it makes less CO2 than any other hydrocarbon, but if released, its negative impact is significant because unreacted methane is a powerful greenhouse gas. Combustion of hydrate deposits is being considered both to release the natural gas and for direct energy conversion. In addition, hydrates are being examined as efficient forms for storage and transport of natural gas. The potential fire hazards of such uses can be significant.
This project will measure the combustion properties of methane, propane, and mixed gas hydrates. Although there is a broad scientific literature and understanding of the physical properties of hydrates and their formation kinetics, there is very little research on their combustion properties. Sample photographs of burning fuel hydrates demonstrate flammability, but these photographs contain none of the important standard combustion-behavior information. Fuel gas hydrates will be molded into pellets mounted on thin supports or dispersed as a snow. The hydrates will then be ignited for measuring key combustion properties such as surface regression rate, water melted versus evaporated, flame standoff distance, combustion temperatures and the formation of soot. The results will be contrasted with the classical results for the combustion of single-component droplets and porous-sphere diffusion flames. One of the key differences between conventional fuels and a hydrate form is that for hydrates the internal kinetics controlling gas release and diffusion to the surface are likely to be important.
Beyond the focused contribution to graduate education and research, the project will reach and encourage younger students through three programs at the University of California Irvine: LSAMP (Alliance for Minority Participation), UROP (Undergraduate Research Opportunities), and COSMOS (Summer Science Camp). At least one LSAMP student will participate on the project each year, and undergraduate research is an integral part of the work. The burning ice experiments developed in this project will be used in the precollege UCI COSMOS program, a summer residential activity for talented high school students interested in science and technology.
Natural gas hydrates are crystalline solids composed of ice and gas which are also known as clathrates. The gas molecules are trapped in ice cavities that are composed of hydrogen-bonded water molecules. Gas hydrates form at an elevated pressure and low temperature. The studies of gas hydrates have attracted many researches throughout the centuries because hydrates have unique properties, such as melting temperatures slightly above that of pure ice. Recently, hydrate storage of gases has been found favorable in that they permit lower storage space and lower pressure for safety. Methane hydrates, for example, have an energy density equivalent to that of a highly compressed gas and density only slightly lower than that of liquefied natural gas. Natural gas hydrate might also serve as a future alternative source of energy. Hydrates are found in substantial amounts in the earth sediments beneath the permafrost and in ocean bottom sediments. Research into these natural fuel hydrates has mainly focused on hydrate formation and dissociation; this work is one of the few on the combustion characteristics of hydrates. Our research activities during the last three years included experimental, analytical, and numerical studies to understand the different elements that might influence hydrate formation and combustion, including the geometry of the hydrate cake. Based on the results from our measurements, the hydrate geometry and the packing factor have a great influence on the amount of hydrate formed. For example, spherical methane hydrates formed 60% less clathrate compared to powder methane hydrate samples. Also the geometry of the sample affects the burning characteristics, including flame height and the burning rate. During the experimental phase we found that the formation of pure methane hydrate takes around 2 weeks for 50 g sample. Including a small amount of surfactant reduces dramatically the formation time, but it also affects the burning behavior. We found that with less than 250 ppm surfactant, there was only moderate burning changes with a reasonable formation time.. Along with the experimental studies, analytical and numerical models were created to describe the connection between the energy release from the methane combustion and the hydrate dissociation. From the analytical results, we confirmed the experimental observation that in order to have a continuous burn of the sample, the water from the hydrate dissociation needs to drain. In the numerical model, the velocity of the moving thermal dissociation interface between water melt and the hydrate solid is approximately 1 mm/s (consistent with the experimental results). Also in the numerical model in situ hydrate dissociation due to both depressurization and a thermal source were investigated. A comparison of the two shows that the depressurization method is a steady way of dissociating hydrates, but that thermal dissociation is faster at the start until the insulating water layer grows. Hence, the fastest dissociation would occur using a thermal source and ensuring that all the water from the dissociation drains away. The project supported 4 (two of them from underrepresented groups) graduate student researchers, 10 undergraduate student researchers, and international visitors interested in this topic. The work serves as the foundation for continuing research in the exploitation of hydrates for energy applications.
