Biogas desulfurization - Technology selection guide

Hydrogen sulfide (H?S) is a natural byproduct of the anaerobic digestion process in biogas production. During this process, organic matter is broken down by microorganisms in the absence of oxygen. H?S is formed when sulfur-containing compounds in the feedstock are reduced. Here’s a brief overview of how this happens:

Sources of Hydrogen Sulfide in Biogas:

A. Sulfur-Containing Feedstock: Organic materials such as manure, sewage sludge, food waste, and agricultural residues often contain sulfur compounds (e.g., proteins, sulfates, and sulfites). During anaerobic digestion, these sulfur compounds are converted into H?S.

B. Sulfate-Reducing Bacteria (SRB): SRB, such as?Desulfovibrio?and?Desulfotomaculum, play a key role in H?S formation. These bacteria reduce sulfates (SO?2?) and other sulfur compounds to H?S in the absence of oxygen:

C. Decomposition of Organic Sulfur Compounds: Proteins and amino acids (e.g., cysteine and methionine) in the feedstock are broken down by hydrolytic and acidogenic bacteria, releasing H?S.

Factors Influencing H?S Formation:

1. Feedstock Composition: High sulfur content in feedstock (e.g., manure, certain industrial wastes) leads to higher H?S production.
2. Digester Conditions: pH: Neutral to slightly alkaline conditions favour H?S formation. Temperature: Mesophilic (30-40°C) and thermophilic (50-60°C) conditions can influence microbial activity and H?S production. Retention Time: Longer retention times may increase H?S formation.
3. Microbial Activity: The presence and activity of sulfate-reducing bacteria (SRB) directly impact H?S levels.

Hydrogen sulfide formation in biogas is a natural process driven by the breakdown of sulfur-containing compounds during anaerobic digestion. Proper management and treatment are essential to mitigate its negative effects and ensure safe biogas utilization.

Below are some of the process widely used for abatement of hydrogen sulfide in the industry.

A.????? Chelated Iron Process:

The?Chelated Iron Process?is a chemical method used for?biogas desulfurization, specifically to remove?hydrogen sulfide (H?S)?from biogas. This process is particularly useful in anaerobic digestion systems, where biogas produced contains H?S, which is corrosive and harmful to equipment and the environment. The chelated iron process is efficient, cost-effective, and environmentally friendly.

How the Chelated Iron Process Works:

1.????? Oxidation of H?S:

o?? Biogas containing H?S is passed through a scrubbing solution containing?chelated iron (Fe3?).

o??The chelated iron acts as an oxidizing agent, converting H?S into elemental sulfur (S?) and water (H?O):

2.????? Regeneration of Chelated Iron:

o?? The reduced iron (Fe2?) is then regenerated back to its oxidized form (Fe3?) by introducing oxygen (air) into the solution:

o??This regeneration step allows the chelated iron solution to be reused, making the process sustainable.

?3.????? Sulfur Recovery:

o?? The elemental sulfur formed during the process is separated from the solution, typically by filtration or settling.

o?? The recovered sulfur can be further processed or disposed of safely.

?Key Features of the Chelated Iron Process:

·??????? High Efficiency:?Capable of removing >99% of H?S from biogas.

·??????? Selective:?Targets H?S without affecting other biogas components like methane (CH?).

·??????? Regenerative:?The chelated iron solution is continuously regenerated and reused.

·??????? Environmentally Friendly:?Converts H?S into non-toxic elemental sulfur, which can be reused or safely disposed of.

·??????? Operational Flexibility:?Can handle varying H?S concentrations and biogas flow rates.

?The chelated iron process is a reliable and sustainable solution for biogas desulfurization, ensuring compliance with environmental regulations and protecting downstream equipment from H?S-related damage.

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B.????? Alkali Scrubbing for Biogas Desulfurization

Alkali scrubbing is a chemical process used to remove hydrogen sulfide (H?S) from biogas. It involves passing biogas through an alkaline solution, such as sodium hydroxide (NaOH), or sodium carbonate (Na?CO?), which reacts with H?S to form non-volatile compounds.

a.????? Key Points:

????????? i.????? Reaction:?H?S reacts with the alkali to form sulphides or bisulfides:

b.????? Process:?? Biogas is introduced into a scrubbing tower bottom where alkali solution is sprayed from the top of the column, in a counter-current flow for better contact. These counter-current columns are typical of the packed type. H?S is absorbed and chemically converted, leaving the biogas clean.

c.?????? Advantages:

??????? i.????? High H?S removal efficiency (up to 99%).

??????? ii.????? Simple and cost-effective for small to medium-scale applications.

????????iii.????? Can be regenerated in some systems.

d.????? Disadvantages:

????????? i.????? Alkali consumption requires periodic replenishment.

????????? ii.????? Spent solution disposal or treatment is needed.

????????? iii.????? pH control is critical for optimal performance.

e.????? Applications:

????????? i.????? Used in biogas upgrading for renewable natural gas (RNG), wastewater treatment, and agricultural/industrial biogas systems.

????????? ii.????? Alkali scrubbing is a reliable and efficient method for biogas desulfurization, ensuring safe and clean biogas for energy use.

?C.????? FerroSorp-Based Biogas Desulfurization

Ferrosorp is a dry desulfurization method that uses iron hydroxide (Fe(OH)?) based adsorbents to remove hydrogen sulfide (H?S) from biogas. It is a simple, cost-effective, and widely used technique, especially for small to medium-scale biogas systems.

Key Points:

1.????? Mechanism:

o?? H?S reacts with iron oxide/hydroxide to form iron sulfide (FeS) or iron disulfide (FeS?):

o?? The reaction is irreversible with out air, and the adsorbent is consumed over time.

o?? FerroSorp with oxygen in biogas and relative humidity about 90% provides 2-3 times more loading rate and in situ regeneration of catalyst which significantly brings down overall cost of operation.

2.????? Process:

o?? Biogas is passed through a bed of Ferrosorp material (e.g., pellets or granules).

o?? H?S is adsorbed and chemically bound, leaving the biogas clean.

3.????? Advantages:

o?? Simple and easy to operate with no liquid waste.

o?? High H?S removal efficiency (up to 99%).

o?? Low maintenance and suitable for small-scale applications.

o?? Ideal choice for polishing irrespective of biogas flow rate where downstream process is very sensitive to hydrogen sulfide.

4.????? Disadvantages:

o?? Adsorbent is consumed and needs periodic replacement over a period of time.

o?? Limited capacity for high H?S concentrations or large biogas volumes.

o?? Spent material requires proper disposal.

5.????? Applications:

o?? Commonly used in agricultural biogas plants, small-scale digesters, and wastewater treatment facilities.

Ferrosorp-based desulfurization is a reliable and efficient method for removing H?S from biogas, ensuring safe and clean biogas for energy production.

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D.????? Biochemical Scrubbing Process:

The?Biological Desulfurization Process, is a sustainable and efficient method for removing?hydrogen sulfide (H?S)?from biogas using?sulfur-oxidizing bacteria. This process is widely used in anaerobic digestion systems, landfill gas treatment, and other biogas-producing facilities. It is an environmentally friendly alternative to chemical desulfurization methods.

?How the Process Works:

1.????? Absorption of H?S:

o?? Biogas containing H?S is introduced into a?scrubber?(e.g., a packed column or bubble column).

o??The H?S is absorbed into an alkaline scrubbing solution (typically sodium hydroxide, NaOH, or sodium carbonate, Na?CO?), forming a sulfide-rich solution:

2.????? Biological Oxidation:

o?? The sulfide-rich solution is transferred to a?bioreactor?containing?sulfur-oxidizing bacteria?(e.g.,?Thiobacillus?species).

o?? These bacteria oxidize the sulfide (HS?) to elemental sulfur (S?) or sulfate (SO?2?) in the presence of oxygen:

o?? The process can be controlled to favour the production of elemental sulfur, which is easier to handle and has commercial value.

o?? The scrubbing solution, now free of sulfide, is regenerated and recycled back to the absorber for reuse.

3.????? Sulfur Recovery:

o?? Elemental sulfur is separated from the bioreactor effluent by settling or filtration.

o?? The recovered sulfur can be used as a raw material in various industrial applications or agricultural applications.

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Key Features of the Biochemical Scrubbing Process:

·??????? High Efficiency:?Removes >99% of H?S from biogas.

·??????? Environmentally Friendly:?Uses natural biological processes and produces minimal waste.

·??????? Cost-Effective:?Low operating costs due to the use of renewable biological catalysts.

·??????? Selective:?Targets H?S without affecting methane (CH?) or other biogas components.

·??????? Flexible:?Can handle varying H?S concentrations and biogas flow rates.

Every biogas purification process has an optimal operating range where the total cost of ownership (TCO) is minimized. This optimal range varies depending on the biogas flow rate and hydrogen sulfide (H?S) concentration. Key factors influencing H?S removal technology selection are the H?S loading and downstream processing requirements.

To assist in technology selection, we've developed a guide that recommends the solution with the lowest TCO. Our TCO calculation includes capital expenses and operating costs projected over a 20-year plant lifespan. We advise users to focus on the trends in TCO (calculated using our provided formula) rather than absolute numbers, as the trends remain consistent regardless of the specific calculation method.

For a free copy of our technology selection program, please visit www.avenirenergia.net or contact us at [email protected].