The use of ozone in a continuous cyclical swing mode regeneration of Fe-EDTA for a clean biogas process from a swine farm waste

Abstract

Hydrogen sulfide in biogas causes environmental problem such as acid rain, therefore several methods are employed in the process to eliminate hydrogen sulfide in biogas. A process for hydrogen sulfide removal in clean biogas production has been examined at a swine farm. Ferric chloride – Ethylenediaminetetraacetic acid solution was used to oxidize hydrogen sulfide to elemental sulfur and ozone was used in the regeneration of ferrous ion to ferric ion. The experiments were conducted using 2 bubbling columns with the same diameter and height, which were 52 mm and 610 mm, respectively. The process was run in a continuous cyclical swing mode. The flow rate of biogas was fixed at of 0.5 L/min and hydrogen sulfide concentrations in the biogas were in the range of 4000–5000 ppm. The initial pH of the Ferric chloride – Ethylenediaminetetraacetic acid solution was varied from 6.5 to 9. Experimental results in the hydrogen sulfide removal part, indicated that Ferric chloride – Ethylenediaminetetraacetic acid solution achieved a maximum 99.5% hydrogen sulfide removal efficiency at pH 8, with a criteria that hydrogen sulfide in the biogas must be less than 100 ppm. In the regeneration part, the regeneration of ferric ion from ferrous ion using ozone in air stream was successfully obtained. The regeneration of ferric ion from ferrous ion was completed within 10 min and the pH turned to pH 8. The regenerated Ferric chloride – Ethylenediaminetetraacetic acid solution was reused several times and still showed high ability for hydrogen sulfide removal. The key parameter, that had to be monitored, was pH, which was relevant to ferrous precipitation. The goal of the process was achieved since hydrogen sulfide was reduced to not exceed 100 ppm, without altered methane concentration in the biogas. Ozone was successfully applied in a continuous cyclical swing mode for the first time in the hydrogen sulfide removal in the biogas production.