Electrochemical Removal of Biogas Impurities with Absorption
Authors: Borgquist Sebastian , Villadsen Sebastian Nis Bay , Fosbøl Philip Loldrup , Abildskov Jens 
Published: December 2024

Introduction

As climate changes are becoming an increasingly important topic worldwide, alternatives to fossil fuels are continuously being developed.

While the market for green power sources, such as wind, hydro, nuclear and solar will continue to grow in the future, there will be a market for hydrocarbon based fuels for use in areas such as heating and fuel production for many years to come.

Biogas offers an alternative to traditional fossil hydrocarbon fuels and biogas sector is expected to grow rapidly over the next decades. Biogas is produced by anaerobic fermentation of biological material (typically waste products). In Denmark the biogas market is growing rapidily. Currently, approximately 40% of the gas in the natural gas grid is biomethane, which is produced from upgrading biogas. The goal of the Danish government is to have only biomethane in the natural gas grid in 2030.

Raw biogas consists mainly of methane (CH4) and carbon dioxide (CO2), but also contains several impurities, such as H2S, mercaptans, ketones and terpenes. Most abundant among these impurities is hydrogen sulfide (H2S) which can be found in concentrations between 100-10000 parts per million (ppm). Other impurities in the biogas can generally be found in concentrations between a few ppm and several 100 ppm. It is important to remove impurities from the biogas, since they may be corrosive, poisonous and might destroy catalysts used in further treatment of the gas. Additionally, upgrading biogas to biomethane requires the removal of more than 99% of sulfur components from the gas.

Several technologies for cleaning of biogas currently exists, but they typically focus purely on H2S and are generally fairly expensive. Furthermore the current technologies are not able to quickly adapt to changes in impurity concentration, which is an issue, since the concentration of impurities may vary several 1000 ppm may be seen when the feed biomass is changed.

Process

A new desulfurization process for biogas cleaning has been developed at The Technical University of Denmark (DTU). The process consumes no chemicals and uses only electricity to oxidise and remove H2S from biogas.

The process is an electrochemical absorption process. In an absorber column chlorine is utilized to oxidise impurities and thereby remove them from the gas phase. At the same time the chlorine is reduced to chloride ions. The solvent from the absorber is then pumped into an electrochemical cell where a current is applied to regenerate the chlorine at the anode. At the same time hydrogen gas is produced at the cathode of the electrochemical cell. In this way, no chemicals are consumed by the process, since the chlorine can be continuously reused by supplying power to the electrochemical cell.

Results

The electrochemical absorption process has several advantages compared to conventional biogas desulfurization technologies. First of all the process is highly adaptable, since the applied current can be adjusted to match the incoming level of impurity in the biogas. The process has proven capable of removing H2S to undetectable levels on both a laboratory and pilot scale. Additionally, the desulfurization process was shown to remove the vast majority of sulphur compounds form biogas. The process removes the impurities without adding oxygen to the biogas, which would prevent it from being injected to the natural gas grid. The cost of the process seems to be competitive with currently used desulfurization procceses.

Including H2S, the process removed 99.96% of sulphur from the biogas. Additionally, 94.77% of the VOC were removed from the biogas. After the cleaning process, the biogas lives up to Danish regulations and can be injected into the natural gas grid (after removal of CO2).