Graduation Semester and Year

2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

L Melanie Sattler

Abstract

The conversion of carbon dioxide into methane at a low temperature has great potential to reduce current international environmental issues like global warming and will create new sources of renewable energy. The overall goal of this research is to increase renewable energy production from landfill gas using an unconventional process – electrocatalytic methanation of carbon dioxide – which has not been implemented for landfill gas yet. The electrocatalytic methanation follows the Waseda process, which applies an electric field in the presence of a Ruthenium-supported cerium oxide (CeO2) catalyst. The specific objectives of this research were: 1. To explore the impact of independent variables (time, power, heat application, catalyst preparation method, electric field type, degradation of catalyst over time, performance of reactivated used catalyst) on the conversion of carbon dioxide (CO2) to methane (CH4) using synthetic landfill gas, as well as test the process at room temperature. 2. To test the Waseda method on real landfill gas, using the values of Objective 1 that showed maximum CO2 to CH4 conversion, and 3. To conduct a life cycle environmental and cost analysis for the conversion of CO2 in landfill gas to CH4, using the Waseda process. Ruthenium-supported cerium oxide (CeO2) catalyst was prepared by the impregnation method. Landfill gas, hydrogen gas, and argon gas were passed through the catalyst bed in a quartz glass tube in a ratio of 1: 1.09: 1.87, with an electric field imposed by two copper electrodes. Experiments were conducted to vary the parameters listed in Obj. 1. During the experiment for Objective 2, 54.53 W was applied to impose a strong electric field in the presence of 0.33 gm of catalyst evenly spread on the catalyst bed. A life cycle environmental analysis was conducted using Sustainable Minds software (ISO 14025) to estimate the total environmental impact and carbon footprint by life cycle stages of the process, including raw materials acquisition, use, transportation, and end of life. A life cycle cost analysis using present worth and a 2% interest rate was done for a 20-year lifetime [Macrotrend, 2020]. The methane concentration increased by 34. 5% in the presence of the catalyst and electric field (199.9V and 0.183A) in the baseline experiment over the initial measured methane concentration of the inlet (sample) gas. The impact of different power (25 V to 240 V) was analyzed, and the maximum wattage applied for this experiment was 54.53 W, which increased methane conversion 3% over the baseline experiment. However, applied heat in the presence of the EF did not improve the methane conversion. The method of catalyst preparation did not impact the conversion significantly. Using a non-uniform electric field reduced the conversion compared to a uniform electric field. For Objective 2, it was found that electrocatalysis increased the methane fraction of real landfill gas by up to 43% compared to the initial inlet gas composition measured. The overall environmental impact of the lab-scale Waseda process is -51.5 mPts and -20.4 kg CO2-equivalents per kg of methane generated. A millipoint (mPt) is 1 1/1000th of the annual environmental load (i.e. entire production/consumption activities in the economy) that one person in the US produces. In Sustainable Minds, this assumes that hydropower (the form of renewable energy with the lowest environmental impact and CO2 footprint in Sustainable Minds) is used to provide electricity needed for the Waseda process, and that the methane generated is used to generate power that replaces power from the standard US grid. Using the lab-scale apparatus, this assumption is necessary for the process to yield environmental benefits. An industrial-scale version of the equipment would have lesser environmental and CO2 impact per kg of methane generated, due to efficiencies of scale. The total cost of the process was estimated to be $13.99/kg of methane. The cost of producing 1 kWh of electrical energy from the methane generated was estimated to be $3.21. This cost would be reduced considerably if the small lab-scale apparatus were scaled up to commercial size.

Keywords

Electrocatalytic conversion, Renewable energy, Landfill gas, Carbon dioxide to methane conversion

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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