2014 Theses Doctoral
Parametric and Mechanistic Studies of Biomass Conversion to High-Purity Hydrogen with Integrated Carbon Fixation
Due to the increasingly detrimental impacts of the global fossil fuel-driven energy economy, technological solutions that can mitigate the deleterious emissions from fossil fuel conversion or that can lessen societal dependence on fossil fuels are urgently required. The conversion of biomass, a renewable energy feedstock, into energy and fuels that are fungible with those derived from fossil fuels would help supplant some of the global fossil fuel consumption with sustainable energy generation. However, one of the main disadvantages of biomass as an energy feedstock when compared to fossil fuels is its low energy density. The majority of thermochemical biomass conversion technologies therefore focus on converting a low energy density feedstock in biomass to a higher energy density end product. Due to the operating parameters involved in these processes, they are typically accomplished on larger and more centralized scales by skilled operators. Few technologies exist that utilize biomass in a sustainable manner under a distributed energy framework, which would allow energy consumers to use locally available resources and waste material to generate energy.
The alkaline thermal treatment of biomass has recently been proposed as a novel method for producing high purity H₂ with suppressed COₓ formation under moderate reaction conditions (i.e., 573 K and ambient pressure). Essentially, biomass, which in this study were the model compounds of glucose and cellulose, is reacted with an alkali metal hydroxide, such as NaOH, in such a molar proportion that all of the carbon and oxygen embodied in the reactants is fixed as an alkali metal carbonate, while all of the elemental hydrogen is released as pure H₂ gas. Thus, fuel cell ready H₂ can be produced from biomass in a single reactor. This technology has great potential for sustainable bioenergy production since it can handle a wide range of feedstocks including biomass and biogenic wastes with high water content. In addition to having the potential to be a distributed energy generation technology, the alkaline thermal treatment of biomass could help meet increasing industrial demand for H₂ in a more sustainable manner, as 96% of current H₂ generation is derived from fossil fuels.
The alkaline thermal treatment technology is also relatively unexplored; thus, the effects of parameters such as feedstock type, reaction temperature, heating rate, NaOH:Biomass ratio, method of reactant mixing, flow of steam, and concentration of steam flow, on the gaseous and solid products formed are not fully understood. This study was undertaken to quantify the effects of these non-catalytic variables on the alkaline thermal treatment reaction and to elucidate potential reaction pathways in order to better evaluate the potential of the alkaline thermal treatment technology as a viable biomass conversion technology.
In the study of the alkaline thermal treatment of glucose, NaOH did play an important role in suppressing COₓ formation while facilitating H₂ production and promoting CH₄ formation. The non-catalytic alkaline thermal treatment of glucose in the absence of steam flow resulted in a maximum H₂ conversion of about 27% at 523 K with a stoichiometric mixture of NaOH and glucose. The solids analysis confirmed the presence of Na₂CO₃ in the solid product, indicating the inherent carbon management potential of the alkaline thermal treatment process. The addition of steam flow increased conversion to H₂ from 25% to 33%, while decreasing total CH₄ formation 5 fold.
After the investigation of the alkaline thermal treatment applied to glucose, cellulose was studied as a feedstock because it is the predominant component of lignocellulosic biomass, the target feedstock source for second generation biofuels. Like in the glucose study, it was found that H₂ and hydrocarbon formation occurred with the addition of NaOH to cellulose under thermal treatment, while the further addition of steam enhanced H₂ production and suppressed hydrocarbon formation. Both the enhancement of H₂ conversion and the suppression of hydrocarbon formation with the addition of steam flow was found to be more significant for cellulose than it was for glucose, with in the cellulose case H₂ conversion doubling from 25% to 48%, and CH₄ formation falling 35 times from the no steam flow case. Also like the glucose study, much of the carbon and oxygen present in the reactants were converted to Na₂CO₃.
With the knowledge gained about the effects various reaction parameters had on the alkaline thermal treatment reaction, a study of the reaction pathways of the alkaline thermal treatment of cellulose reaction was undertaken. Compounds formed at intermediate temperatures were identified, tested for gaseous production when reacted with NaOH, and the gas product formation rate trends of these reactions were compared with those trends observed from the alkaline thermal treatment of cellulose reaction. The intermediates identified included sodium carboxylate salts, namely sodium formate, sodium glycolate, and sodium acetate, among others. The reactions of these compounds with NaOH were found to yield H₂ and CH₄, with the gaseous formation rate trends being similar to trends observed for the alkaline thermal treatment reaction for cellulose in certain temperature regions. Particular focus was placed on sodium glycolate, which was an intermediate found in high concentration and that reacted with NaOH to produce both H₂ and CH₄. The formation of Na₂CO₃ at intermediate temperatures was also studied, and the comparison of Na₂CO₃ conversion to H₂ conversion at intermediate temperatures revealed that H₂ and Na₂CO₃ formation do not always occur at the 2:1 H₂:Na₂CO₃ molar ratio implied by the proposed stoichiometry of the alkaline thermal treatment reaction for cellulose. The aforementioned studies were conducted both in the presence and absence of steam flow to study its influence on the reaction.
Finally H₂ formation kinetic studies were performed on the alkaline thermal treatment of cellulose system as well as the H₂-producing sodium carboxylate salt reaction systems. Sodium formate and sodium oxalate were found to have better selectivity toward H₂ formation and their reactions were more kinetically favored than sodium glycolate with NaOH. A comparison of the isothermal H₂ kinetics between the cellulose and sodium glycolate systems at higher temperatures, however, revealed that H₂ conversion in the alkaline thermal treatment of cellulose appeared to be limited by the rate of conversion of sodium glycolate. From the results of these studies, recommendations are made for future research directions aimed at improving the alkaline thermal treatment of cellulose reaction.
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More About This Work
- Academic Units
- Earth and Environmental Engineering
- Thesis Advisors
- Park, Ah-Hyung
- Ph.D., Columbia University
- Published Here
- October 3, 2014