Evaluating the Feasibility of Selected Thermochemical Conversion Pathway as Energy-Producing Negative Emissions Technologies

Negative emissions technologies remove and store CO2 from the atmosphere. Bioenergy with carbon capture and storage that combines biomass conversion and CO2 capture and storage is a negative emissions technology that has been intensively studied to meet climate goals, but existing literature has mostly focused on combustion to produce bio-electricity. This dissertation evaluates the feasibility of thermochemical conversion technologies (hydrothermal treatment, pyrolysis, and gasification) of selected biomass as energy-producing negative emissions technologies. A combination of machine learning approaches, life cycle assessment, and economic analysis are used to assess possible combinations of feedstock properties and reaction conversion conditions. Key results include energy return on investment, net global warming potential, minimum product selling price, and levelized cost of carbon. Results show that machine learning via random forest modeling offers good predictive accuracy for product quantity (yield) and quality (carbon content, energy content, etc.) for all of the evaluated thermochemical platforms.

With regard to life cycle assessment and economic assessments, the proposed hydrothermal treatment coupled with carbon capture and storage platform constitutes a net-energy producing negative emissions technology (energy return on investment > 1, net global warming potential < 0) for some combinations of feedstock characteristics and reaction conditions. The best overall energy and climate change performances are achieved for hydrothermal treatment coupled with carbon capture and storage of lignocellulosic biomass at low conversion temperatures. Results for the proposed slow pyrolysis platform reveal a strong tradeoff between environmental and economic performance, whereby best overall energy and climate change performances are achieved via pyrolysis of lignocellulosic biomass at high temperature, whereas best minimum product selling price corresponds to pyrolysis of sludge at low temperature.

Finally, life cycle assessment and economic results are incorporated into a decision framework that enables decision-makers to choose an optimal pathway under uncertainty. Thermochemical conversion technologies, including hydrothermal treatment, pyrolysis, gasification, and conventional combustion, can be applied to so many feedstocks and reaction conditions. Different combinations of technologies, conditions, and feedstocks are assessed to determine their environmental and financial performance. Then, optimal actions are determined based on the levelized cost of carbon computed under different carbon credit scenarios.