Bioenergy has long been a promising solution to reduce the greenhouse gas (GHG) emissions from the transportation, electric power, and natural gas sectors, and thus, it has been promoted by policies in the interest of climate change mitigation globally. In the United States, environmental policies such as the federal Renewable Fuel Standard (RFS) and state-level Low Carbon Fuel Standards (LCFSs) and Renewable Portfolio Standards (RPSs) are in effect to regulate transportation fuels and electricity generation. While purpose-grown or energy-dedicated biomass feedstocks seem to be important to reduce GHG emissions under these policies, at present they typically provide only incremental reductions. To achieve a significant reduction of GHG, utilizing waste or residual biomass feedstock is likely required. The field of industrial ecology as long promoted waste valorization as a pathway for sustainable production systems. This concept has also been popularized more recently as part of circular economy.The goal of this dissertation is to analyze applications through the lens of circular economy in California based on life cycle assessment (LCA) and techno-economic analysis (TEA). Studies presented in this dissertation contribute to current knowledge by developing a methodology for biomass feedstock classification to support regulatory practices, identifying the circular economy systems around bioenergy pathways, and evaluating the systems in terms of environmental and economic impacts.
The dissertation is comprised of three primary studies. In the first study, a classification system was developed for biomass feedstocks. Two decision trees were developed which are different in inclusion of an economic threshold, and they were applied to classify four biomass feedstocks: corn stover, distillers corn oil (DCO), palm fatty acid distillate (PFAD), and wheat slurry. The results showed that all four tested feedstocks were classified as the same categories by both decision trees, but there was a chance that one feedstock may be classified differently depending on its economic value. A Strengths, Weaknesses, Opportunities, and Threats (SWOT) analysis was also performed for each feedstock case, and the results showed that there could be a challenge applying these decision trees to other biomass feedstocks that have not widely been used for biofuel production previously or that have recently emerged as biofuel feedstocks.
In the second study, two electricity generation systems using different agricultural residues, rice residues and almond residues, as feedstocks were evaluated for their economic performance, and the primary solid waste stream from these generation processes, ash, was analyzed to examine opportunities for valorization as a supplementary cementitious material (SCM). According to the results, both almond shell and rice hull were economically feasible feedstocks for electricity generation, and rice hull was selected as a target feedstock due to the abundance of rice hulls in California, better economic performance, and higher ash production per unit of electricity generation. While concrete mixtures with rice hull ash (RHA) showed lower environmental impacts than Portland cement or cement mixtures with the most common SCM, fly ash, these mixtures also showed changes in performance including reduced strength and improved chloride permeability, indicating that RHA can be used as an SCM if performance requirements for the application are met.
In the third study, LCA and TEA were performed to quantify the economic and environmental impacts of a system utilizing a waste stream from an anaerobic digester as a nutrient source for microalgae and further generating energy from the microalgal biomass. Three pathways were analyzed, producing biodiesel, electricity, or together from microalgae, respectively, and the pathway for electricity generation showed better economic and environmental performance than the other pathways.