Session: 17-01-01: Research Posters

Paper Number: 150849

150849 - Inverse Prediction of Multimode Heat Transfer in a Fibrous Porous Substrate During Direct Solar Methane Pyrolysis 

Utilization of sustainable energy processes is gaining increasing importance in climate risk mitigation, and solar energy is widely utilized in many different forms around the world. In addition to direct generation of electricity, solar energy can convert conventional fossil fuels into alternative low-carbon, carbon-neutral or potentially even carbon-negative fuels. Methane comprises major proportions of biogas and natural gas supplies with broad applications, but it is also the second-largest contributor to greenhouse gas emissions. Additionally, location incongruity of methane generation and consumption makes methane storage and long-distance transportation necessary, typically requiring energy-intensive processes such as liquefaction. As such, conversion of methane into higher-value chemicals enabled by renewable energy sources is regarded as a potential means of utilizing this abundant resource in a sustainable manner. 
Porous media have wide application in renewable energy conversion processes, such as solar-thermal fuels production and decarbonization. Heat transport mechanisms within porous media can be highly complex, particularly under extreme conditions encountered in concentrated solar thermal reactors in which direct measurement of temperature is challenging. This work reports the application of an inverse method in estimating the temperature distribution of a fibrous porous medium in a direct solar methane pyrolysis process. Finite-difference heat transfer methods are constructed to solve the heat diffusion equation with temperature-dependent effective thermal conductivity in the computational domain. An inverse optimization algorithm is then applied to determine the parameterized conductivity. The substrate is located in the solar-thermal experimental system such that its back side (the side without direct solar irradiation) temperature is experimentally measured using an infrared camera. This back-side temperature distribution is then applied as an input to the inverse problem. The Melder-Mead method is implemented to identify the minimum of an objective function and to solve for the 2D domain's temperature distribution. Facilitated by the foregoing inverse approach, the contribution of thermolysis and photolysis to graphitic carbon production can be elucidated. Specifically, we devise an experiment with a shadow mask shielding the direct solar irradiation to the central part of the substrate, excluding the direct photon involvement in this region. An obvious variation of fiber diameter change rate is observed across the shadowed and the exposed region, whereas the experimentally measured back-side temperature is relatively uniform which might lead to a conclusion that the growth rate and the quality of the graphite are mainly induced by photon contributions. However, after implementing the inverse temperature prediction, the reaction zone temperature across the shadowed and exposed region displays a similar trend of variation to the fiber growth rate. To further quantify the analysis, a simplified kinetics model based on observed graphite deposition rates and the predicted temperature is constructed, showing a consistent activation energy with and without direct photon participation. In conclusion, temperature instead of photon effect is the predominant factor in this renewable energy conversion and fuel production process.

Presenting Author: Timothy Fisher University of California, Los Angeles

Presenting Author Biography: Hengrui Xu joined UCLA in Fall 2022 as a PhD student in Mechanical Engineering. He earned his Bachelors degree with honors in Energy and Power Engineering from Xi’an Jiaotong University in 2022. At UCLA, Hengrui is working on the project titled Direct Solar Conversion of Biogas to Hydrogen and Solid Carbon.

Authors:

Hengrui Xu University of California, Los Angeles
Timothy Fisher University of California, Los Angeles