Date on Master's Thesis/Doctoral Dissertation

8-2009

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Chemical Engineering

Committee Chair

Willing, Gerold A.

Subject

Hydrogen sulfide; Digester gas; Animal waste--Biodegradation

Abstract

This thesis is an examination of the viability of a low pH hydrogen peroxide scrubbing process for removing H 2 S acid gas present in typical biogas streams generated from dairy farm anaerobic digesters. Biogas ranges in composition based on the feedstock manure used in the anaerobic digestion process but typically consists of methane (50-60%), carbon dioxide (40-50%), and trace amounts of hydrogen sulfide and ammonia. Hydrogen sulfide is of prime concern because it is an odorous, poisonous, and highly corrosive gas which can impede use in power generation applications for biogas such as boilers, internal combustion engines, microturbines, fuel cells, and stirling engines. Thus, removal of hydrogen sulfide is highly recommended. Desirable attributes for a gas purification system include low capital cost, low operational costs, minimal preventative maintenance, minimum energy inputs, and ease of use. H 2 O 2 is a highly selective oxidant that does not produce toxic and corrosive by-products and has been shown to be a convenient way of eliminating oxidizable pollutants such as hydrogen sulfide gas from air or other gas streams. Based on these criteria, an experimental approach was used to investigate the feasibility of using an acidic H 2 O 2 scrubber for the removal of H 2 S from synthetic biogas. Two test reactors were constructed, each setup with multiple configurations of packing volume, H 2 O 2 concentration, and liquid volume. Synthetic biogas was introduced into the reactors and data was collected including liquid pH, liquid oxidation reduction potential, and H 2 S concentration of exit gas during experiments. In total there were over twenty separate experiments conducted between the bench scale experiments, 1st scrubber trials, and 2nd scrubber trials. The results of these experiments demonstrate that a low pH H 2O 2 scrubbing system shows commercial viability for the removal of H 2 S from biogas. Functional oxidation of H 2 S was achieved with removal efficiencies of 99% in certain reactor configurations. Bench scale experiments indicate that highest oxidation reduction potential of hydrogen peroxide solutions occurs in the acidic pH range between pH 3-5. Key operating parameters observed for functional oxidation of H 2 S gas were the bubble diameter of inlet biogas and gas residence time. Increased residence times and smaller mean inlet bubble diameters led to maximum removal efficiencies. The research was conducted in the University of Louisville Food Processing Laboratory and used as proof-of-concept for claims made in United States Patent Application 20090130008. These initial results indicate that future work is warranted for examining suitability of using a commercial scale acidic hydrogen peroxide scrubbing vessel as an H 2 S removal technology in biogas purification.

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