A theoretical and experimental study on biochar as an adsorbent for removal of acid gases (CO₂ and H₂S)

Bamdad, Hanieh (2019) A theoretical and experimental study on biochar as an adsorbent for removal of acid gases (CO₂ and H₂S). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Biochar, a carbon-rich material that is obtained from forestry wood residues through thermochemical conversion in the absence of oxygen (i.e. pyrolysis), is a potential alternative to commercial adsorbents for acid gas treatment. Acid gases (CO₂ and H₂S) are present in landfill gases, fossil fuel gases, and mining operations. These gases must be treated to improve environmental safety and limit operational issues such as pipeline corrosion. Common processes for removal of acidic gases from landfill, flue, and natural gas streams include amine absorption processes, which are energy and space intensive due to required regeneration, and solid adsorbents (which can be costly to produce and dispose of). In this work, CO₂ adsorption using biochar as a solid adsorbent was investigated. Use of biochar as an adsorbent for acid gas removal is relatively novel. The specific objectives included; characterize the biochar structure (i.e. chemical, physical, and morphological) through a series of analyses; determine the operating conditions for obtaining maximum adsorption capacity; modify the biochar surface to determine impact on adsorption; and develop a molecular model to simulate the adsorption process to determine if it can be used as a tool in experimental design. Chapter one gives an overview of the conceptual framework of acid gas purification and outlines the objectives, the scopes, and the significance of this study along with a summary of the thesis chapters. Chapter two provides a literature review to identify different types of biochar production methods, reaction conditions (e.g. temperature and residence time), and woody biomass as one of possible feedstock materials. The biochar was compared with commercial adsorbents and the results indicated biochar could be used as a feasible alternative to activated carbon as it is environmentally friendly and a low-cost adsorbent. In addition, the impact of production conditions on biochar properties were investigated and it was found that carbon, hydrogen content, and surface area were significantly affected by pyrolytic temperature. The reported isotherms in the literature were compared and the Freundlich isotherm was the best fit with the biochar. The application of molecular modeling to describe adsorption process and different simulation methods were studied. The biochar for this research was produced from three different woody biomasses: softwood (sawdust and bark (Balsam fir)) and hardwood (Ash wood) through fast pyrolysis at 400-500⁰C and then compared in terms of chemical and physical properties in chapter three. Chapter four looks at the impact of three operating conditions, temperature, inlet feed flow rate, and CO₂ concentration, on biochar adsorption capacity and the interaction of these parameters were evaluated using response surface methodology. The operating conditions for maximizing CO₂ uptake were determined and the Freundlich isotherm best represented the equilibrium adsorption and the pseudo first-order was selected as a kinetic model. Thermodynamic analysis indicated the adsorption process was spontaneous and exothermic. Further, we found that biochar derived from “waste” materials had better adsorption capacity relative to commercial zeolite. Chapter five describes chemical modification of the biochar using two novel methods of amine functionalization and the maximum adsorption capacity was measured at the conditions obtained in chapter four. The results indicated functionalization decreased the pore volume, surface area, and subsequently the adsorption capacity of the biochar. In order to enhance capacity, the biochars (unmodified and chemically modified) were thermally activated via air diluted with nitrogen at a moderate 560⁰C. Some nitrogen functionality retained in the biochar structure even after activation. The synthesized N-enriched biochar followed by thermal activation was found to have much higher adsorption capacity as compared with commercially available activated carbon (Norit CA1) and recent carbon based adsorbents in the literature. Chapter six is dedicated to molecular modeling and linking the experimental results with simulations. The effect of various functional groups on adsorption of CO₂/H₂S on biochar surface was investigated. It was found that the presence of functional groups promotes CO₂ adsorption on the surface with exothermic adsorption energy. As expected, the DFT calculations showed amine functional groups enhanced CO₂ adsorption with more exothermic adsorption likely because of stronger bonding compared to other functional groups. The thermodynamic outcomes (Enthalpy and Gibbs free energy) validated that the affinity of the chars for CO₂ is on the same order of magnitude as H₂S. The simulated thermodynamic parameters and IR vibrational frequencies were calculated and both showed reasonable agreement with experimental results (chapter four and five). The results of this study would be helpful for developing future work, on the scale-up of the adsorption system, further modification of the biochar, CO₂ sequestration, regeneration, and atomic-level design of carbon surfaces.

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