Hydrogen sulfide in biogas: Generation and abatement guide

 

Hydrogen sulfide in biogas: Generation and abatement guide

Hydrogen sulfide (H₂S) 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. H₂S 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:

1. 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 H₂S.
2. Sulfate-Reducing Bacteria (SRB):
• SRB, such as Desulfovibrio and Desulfotomaculum, play a key role in H₂S formation.
• These bacteria reduce sulfates (SO₄²⁻) and other sulfur compounds to H₂S in the absence of oxygen:

3. 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 H₂S.

Factors Influencing H₂S Formation:

1. Feedstock Composition:
• High sulfur content in feedstock (e.g., manure, certain industrial wastes) leads to higher H₂S production.
2. Digester Conditions:
• pH: Neutral to slightly alkaline conditions favour H₂S formation.
• Temperature: Mesophilic (30-40°C) and thermophilic (50-60°C) conditions can influence microbial activity and H₂S production.
• Retention Time: Longer retention times may increase H₂S formation.
3. Microbial Activity:
• The presence and activity of sulfate-reducing bacteria (SRB) directly impact H₂S 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.

1.      Chelated Iron Process:

The Chelated Iron Process is a chemical method used for biogas desulfurization, specifically to remove hydrogen sulfide (H₂S) from biogas. This process is particularly useful in anaerobic digestion systems, where biogas produced contains H₂S, 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 H₂S:

o   Biogas containing H₂S is passed through a scrubbing solution containing chelated iron (Fe³⁺).

o  The chelated iron acts as an oxidizing agent, converting H₂S into elemental sulfur (S⁰) and water (H₂O):

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 H₂S from biogas.

·        Selective: Targets H₂S without affecting other biogas components like methane (CH₄).

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

·        Environmentally Friendly: Converts H₂S into non-toxic elemental sulfur, which can be reused or safely disposed of.

·        Operational Flexibility: Can handle varying H₂S 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 H₂S-related damage.

 

2.      Alkali Scrubbing for Biogas Desulfurization

Alkali scrubbing is a chemical process used to remove hydrogen sulfide (H₂S) from biogas. It involves passing biogas through an alkaline solution, such as sodium hydroxide (NaOH), or sodium carbonate (Na₂CO₃), which reacts with H₂S to form non-volatile compounds.

a.      Key Points:

                                                       i.      Reaction:      H₂S reacts with the alkali to form sulphides or bisulfides:

 

b.      Process:

                                                       i.      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.

                                                      ii.      H₂S is absorbed and chemically converted, leaving the biogas clean.

c.       Advantages:

                                                       i.      High H₂S 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.

 

3.      Ferrosorp-Based Biogas Desulfurization

Ferrosorp is a dry desulfurization method that uses iron oxide (Fe₂O₃) or iron hydroxide (Fe(OH)₃) based adsorbents to remove hydrogen sulfide (H₂S) 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   H₂S reacts with iron oxide/hydroxide to form iron sulfide (FeS) or iron disulfide (FeS₂):

o   The reaction is irreversible, and the adsorbent is consumed over time.

2.      Process:

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

o   H₂S is adsorbed and chemically bound, leaving the biogas clean.

3.      Advantages:

o   Simple and easy to operate with no liquid waste.

o   High H₂S removal efficiency (up to 99%).

o   Low maintenance and suitable for small-scale applications.

4.      Disadvantages:

o   Adsorbent is consumed and needs periodic replacement.

o   Limited capacity for high H₂S 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 H₂S from biogas, ensuring safe and clean biogas for energy production

 

4.      Biochemical Process:

The Biological Desulfurization Process, is a sustainable and efficient method for removing hydrogen sulfide (H₂S) 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 H₂S:

o   Biogas containing H₂S is introduced into a scrubber (e.g., a packed column or bubble column).

o The H₂S is absorbed into an alkaline scrubbing solution (typically sodium hydroxide, NaOH, or sodium carbonate, Na₂CO₃), 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 (S⁰) or sulfate (SO₄²⁻) 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.

 Key Features of the Biochemical Scrubbing Process:

·        High Efficiency: Removes >99% of H₂S 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 H₂S without affecting methane (CH₄) or other biogas components.

·        Flexible: Can handle varying H₂S 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 (H₂S) concentration. Key factors influencing H₂S removal technology selection are the H₂S 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 info.ocen@yahoo.com.

 

BiogaSmart-PWS: A Revolutionary Leap in Biogas Purification

Harnessing the Power of Innovation

We are proud to introduce BiogaSmart-PWS, a cutting-edge, patented smart biogas purification technology (Patent Number 555135). This revolutionary solution is designed to elevate the efficiency and reliability of biogas purification processes to unprecedented levels.

Unmatched Performance and Reliability

BiogaSmart-PWS offers a compelling array of features and benefits:

• Unmatched Methane Recovery:
  • Achieve up to a remarkable 99% methane recovery rate, maximizing energy yield from biogas.
• Consistent High-Purity Methane:
  • Delivering a minimum of 96% pure methane 24/7/365, ensuring consistent and reliable performance, regardless of operational conditions.
• Ultra-Pure CO2:
  • Produce up to 99% pure biogenic CO2 (moisture-free) at the regeneration package vent, ideal for various industrial applications, such as enhanced oil recovery (EOR) and greenhouse gas reduction initiatives.
• Zero Air Contamination:
  • Unlike traditional technologies, BiogaSmart-PWS employs advanced techniques to prevent air intrusion into the product stream, maintaining product purity and optimizing energy output.
• Smart Predictive Control (SPC):
  • Benefit from minimal operator intervention and reduced human error with our sophisticated control system. SPC continuously monitors and adjusts system parameters, ensuring optimal performance and efficiency.
• Microbial-Free Regeneration:
  • Eliminate the need for frequent cleaning and maintenance with our microbial-free regeneration package, saving time, labor, and costs.
• Foolproof Operation:
  • Our system is designed to withstand operator errors, ensuring continuous operation even in unexpected situations. This robust design minimizes downtime and maximizes productivity.

The Future of Biogas Purification

BiogaSmart-PWS represents a significant advancement in biogas purification technology. By addressing the limitations of traditional methods, this innovative solution offers a sustainable and cost-effective approach to harnessing the full potential of biogas.

We are committed to driving innovation and sustainability. By leveraging cutting-edge technology, we are empowering industries to maximize the benefits of biogas, reduce their environmental impact, and contribute to a greener future. 

For Additional Information feel free to connect at info.ocen@yahoo.com and +91 8600008213 or just drop a ping vai contact form.

Compressed biogas price - INR per kg or INR per MMBTU ? How it compare with each other.

 

The Compressed Biogas or Biomethane is mostly sold or purchased at a value equivalent to a kilogram of gas complying with IS16087:2016 Rev 1 as of date. Looking at the global trend and energy pricing in India we are valuing the gas based on energy content i.e. INR per MMBTU. The latest rate as per OMC is about INR 1470 per MMBTU and an additional INR 2 per Kg for direct pipe injection or INR 8 per kg for Injection via Cascade. The compression charges per kg will be calculated on which basis needs to be explored. We expect these numbers to be converted to identical energy units of INR per MMBTU for simplicity in near future. 

There are a lot of discussions happening in and around this and we strongly felt to elaborate on this so that all the stakeholders of this value chain lower down will be able to evaluate the various gas prices via different selling options to the common level before taking call for a product off-take mechanism.

Let’s quantify the energy value of CBG first and then convert various forms for better understanding.

The Energy of CBG is mainly from methane and the balance of the gasses is mostly fillers. The GCV of the pure methane is about 55.5 MJ/kg or 13265 Kcal/kg. 

As per IS 16087:2016 Rev 1 the appx.  96% v/v Methane, 3.5% Carbon Dioxide, up to 0.5% Oxygen, up to 20 mg/m3 Hydrogen sulfide and 5 mg/m3 water.  The composition here is on a volumetric basis and once converted to a weight basis the methane is estimated to be about 90% w/w of the CBG.

The appx energy value of CBG is about 11938.5 Kcal/kg or about 8955 Kcal/sm3 at an estimated density of about 0.75 kg/sm3 w.r.t. above mentioned specification.

1 BTU is equivalent to 0.252 Kcal. Or 1 MMBTU is about 252164 Kcal and with 11938.5 Kcal per KG CBG it can be estimated that each MMBTU will have about 252164/11938.5 i.e. 21.12 unit.

So with the given specification of CBG and further calculation, there are 21.12 kg CBG per MMBTU of the energy. Now let's convert about gas pricing in the same units for better understanding.

               The base price of the gas: 1470 Rs/MMBTU

               Compression Cost: 21.12 x 2 : 42.24 Rs/MMTU (Direct Injection or Via DCU in Nearest Line) ---1

               Compression Cost: 21.12 x 8 : 168.96 Rs/MMTU (Sale at nearest Retail outlet via Cascade in Nearest Line)---2

The landed price of the gas per kg to the developer will be 71.60 Rs/kg of CBG in direct injection case and 77.6 Rs/kg of CBG in Injection via Cascade in the nearest line.

Please note these values are specific to gas composition at the outlet of the gas purification plant so to make it simple we have developed an excel program. Please connect with us for a free copy of the program. A screenshot of the same is attached herewith for reference.

Cow dung for CBG production: Opportunity and challenges in India

India is a country with a rich history and with a very special connection to agriculture in our hearts. Agriculture is one of the prominent pillars of the Indian economy over the centuries. As it evolved over the years, animal husbandry became an integral part of Indian farmer's income. As per data about 150 to 200 million animals actively contribute to the Indian farmer’s household income in various ways and provide them with a decent cash flow in times of uncertainties apart from value-added fertilizer to the farms.

Cattle dung is one of the readily available household items across the country and fulfilling the requirement of fuels and fertilizer over the years. The dung is basically a partially digested portion of relatively hard-to-digest components from food. The actual specification is highly dependent on the type of food they eat and the purpose of the animal milking or non-milking ones etc but not limited to. It is a privilege to come from a farming background with a close association with dairy at home and working in Biogas in the recent past developed a decent level of confidence to put some data on cow dung and its potential.

The chemical composition of cow dung is mainly cellulose, Hemicellulose and lignin portion of the food with rich flora of healthy ruminococcus micro-organisms. It is the perfect mix for generation of biogas and it might be the reason it energized millions of household over the years in india and across the globe at small, medium and large scale over the years. For a simplicity one can call cow dung a perfectly pretreated lignocellulosic feedstock for biogas production which generally gives really good digestion efficiency on ground. Typically dung has about 15-20% w/w dry matter with 50-60% volatile solid, balance lignin and Ash. Once can expect 50-60% digestion efficiency of the key constituent which result in the yield of 40-50 sm3/MT of as it cow dung.

A significant portion of cow dung stored in open undergoes rapid digestion to form very good manure which make it one of the most value-added fertilizer rich in NPK and soil conditioner. At the same time this also make it one of the prominent source of GHG emission in the world with a significant contribution and which is quite alarming. All the efforts world putting in toward 1.5 Degree target and dung can not be ignored. The common practice of using cow dung due to highly decentralized production is making Upla or Dry Fuel which serve as a source of heat to rural households apart from stocking at the place and using as fertilizer multiple times a year. This activity contributes to GHG emissions and demands more scientific ways of extracting energy from it via biogas generation. The government of India's policy for Biogas highly encourages this route at every scale of biogas generation.

Cow dung is seen one of the largest sources of compressed biogas in Indian and across the globe considering bioenergy production while protecting the environment and at the same time providing value-added organic fertilizer to boost farmer’s income. India over 25 MMT of recoverable dung can shoulder a responsibility to protect the environment while recovering the CBG and biofertilizers which will have significant forex saving. The CBG value chain and the government is providing all required assistance for the conversion of cow dung to CBG and fertilizer.

For any CBG project availability, access and affordability of the potential feedstock are key attribute which will make the project viable and sustainable. The project which are viable and sustainable will drive agricultural economy in a long term by actively contributing the farmer’s income while solving the environmental & energy issues of the country. As CBG is been pushed really hard by government in a very positive sense, most of the bulk generator looking at this as an opportunity for generating assured income in longer horizon. This actually pushed the feedstock prices at very challenging level at some cases where viability of a project is in jeopardy.

Here I would like to present a simple math for all the key stakholders of the value chain to think about the value of the dung w.r.t. product and given a choice I will not be surprised to hear the feedstock pricing roadmap from authorities in line with sugar policy which arrive a Fair price depending on the final output and its market value which make the feedstock a win-win proposition to all the stakeholders.  

Let’s take cow dung as a case but it equally applies to all the Feedstocks. A typical CBG potential of the cow dung is about 16-20 kg/ MT as per IS 16087:2016 Rev 1. The market price of the CBG/CNG at Delhi is about INR 80 per kg at some point of time (say) which estimated to result in revenue of about 1300 to 1600 Rs per MT of Cow dung. The avg cost of conversion on account of power, Operation and maintenance, transport, administrative and miscellaneous expense is about Rs. 15-20 translate to about 300 to 400 Rs/MT of cow dung. The above Number will left arrive at about 900 to 1200 per MT when gas sold by B2C method of product evacuation. If the product is sold at SATAT Base rate then 500 to 700 is left which suppose to suffice the cost of feedstock and pay for loan for capital investment ensuring a adequate profit which make this a viable business case for any developer.

For CBG business the total cost of production more than 55% of the Revenues from Gas and Fertilizers looking at present business value chain from feedstock sourcing to product evacuation make really tough call to take by any developer. The values could be here and there but the trend is worth to be noted which demands a logical rationale to arrive at a fair value for feedstock with long term visibility to make CBG a fuel of the future moving forward. Any addition to this is welcome from all of you so that we would be putting fact to the table which will empower developers to take informed decision on right mix of feedstock for their proposed business ventures.