Biogas

The high sulfur content in biogas plants create several issues

The process of biogas production is a series of complex stages aimed at producing methane from waste. Traditional biogas plants can use different types of waste as feedstock, including, for instance, biomass. Since the feedstock is renewable, biogas plants produce green energy, contributing to a circular economy. According to the World Bioenergy Association, the sector of biogas production is one of the fastest growing among all biofuel sectors. The average growth of biogas production was 11.2 % in the last year, reaching an overall production of 58.7 Nm3. Almost half of this amount was produced in Europe where approximately 17000 biogas plants are in operation. Germany is the leading country in terms of number of biogas plants, with a total of 11000 installation, followed by Italy with 1600 and France with 800. 

Ozone can be useful in several parts of the biogas production process. Among other possibilities, we identified three main applications for ozone in biogas plants:

  • Feedstock ozonation
  • H2S reduction for methane upgrading
  • Odor control
biogas, ozone, feedstock ozonation, H2S reduction, odor control, odor, digester

An overview of possible ozone applications in the biogas production process.

Application type

Benefit

Result

Feedstock ozonation

Large polymers broken down into monomers by oxidation

Up to 150 % higher substrate conversion to biogas

H2S reduction for methane upgrading

General H2S reduction in the digester

Lower H2S concentration entering the methane upgrading

Odor control

Removal of odorous and corrosive contaminants

Reduced odor and H2S

A Common Issue - Sulfur

In each stage of the biogas production process, sulfur compounds and ions are commonly present. These compounds are often responsible for a variety of issues affecting the overall performance. For instance, high amounts of sulfur ions in the feedstock enhance the activity of sulfur reducing bacteria (SRB) in the digester, inhibiting the activity of methane-producing microorganisms such as archaea. As a result, the yield of methane production is reduced while the production of reduced-sulfur compounds, such as H2S, is favored. When high concentrations of H2S are released from the digester, the process may occur in odor and corrosion problems. In particular, odor problems are common in biogas plants, since H2S has one of the lowest odor threshold known, making the human nose very sensitive even to trace concentrations.

Feedstock pretreatment 

Prior the anaerobic digestion stage, a pretreatment step is often implemented. The purpose of this step is reducing the workload of the hydrolytic fermentative bacteria by making the substrate more easily biodegradable. This includes properties such as increasing the surface area, dissolving complex matter, reducing crystallinity in polymers such as cellulose etc. The most common types of pretreatment applied today are summarized in the tables below.

Common pre-treatments for the feedstock prior to anaerobic digestion.

PHYSICAL PRETREATMENTS

Mechanical

Milling to reduce particle size, increasing the biomass availability. Leads to increasing complexity increasing process sensitivity and cost.

Thermal

Heating (200oC), disrupting hydrogen bonds (chemical macro-structures), increasing the biomass availability. Leads to a high energy demand.

CHEMICAL PRETREATMENTS

Alkali

Alkali treatment over several weeks, easing the degradation of ligno-cellulosic compounds. Requires large amounts of chemicals, chemical handling and leading to a slow process.

Ozonation

Inline ozonation, significantly increasing the biodegradability of stable organic matter, potentially tripling biogas production.

BIOLOGICAL PRETREATMENTS

Microbiological

Compostation, an aerobic pretreatment step forming hydrolytic enzymes, facilitating the first step of the anaerobic digestion.

Fungal

Pure cultures of an aerobic fungi during a 4 day incubation time resulted in up to 40% more biogas, and an increase in the grade.

Benefits of feedstock ozonation

Ozone is known for its highly oxidative properties, and has been shown to be able to degrade parts of the complex organic matter used as feedstock for an anaerobic digester. For example, when applying ozone to a feedstock of waste activated sludge, the effects are multiple. When ozone is properly dosed, the substrate conversion to biogas can be greatly increased, as shown by the figure below.

ozone, biogas, biodegradability, substrate conversion

The effect of ozone treatment on substrate biodegradability for waste activated sludge. TS=Total solids.

For many cases such as of biogas production from waste activated sludge, the results show a strong positive impact of the ozone treatment. This results is due to the high content of aerobic bacteria and non-degraded organic matter in this type of feedstock. Under these conditions, ozone quickly oxidizes any unsaturated bond, forming radicals which continue to oxidize other organic matter. This reaction mechanism leads to a higher biodegradability of the feedstock, which is translated to a higher biogas production. The increase of biogas production is proportional to the ozone injection in the system. The more ozone is used, the higher the biodegradability and the methane yield. As shown in the study above by Bougrier et. al. at “Laboratoire de Biogechnologie de l’Environnement”, the ozone pretreatment improves the biogas production, with an optimum around 0.15 g O3 per g total solids, resulting in an increase of approx.150 % in the biogas production, compared to the untreated feedstock. 

Anaerobic digestion

After the pretreatment stage, the feedstock enters the digester for the biochemical conversion.  In this unit, several type of microorganism reacts with the feedstock in different stages. All stages are anaerobic i.e. in absence of oxygen. These are summarized in the table below.

The steps of anaerobic digestion stage.

Step

Description

Hydrolysis

Breaking down of large substrate, such as cellulose and proteins to glucose and amino acids

Fermentation

Formation of volatile organic acids and alcohols

Acetogenesis

Formation of acetate, carbon dioxide, and hydrogen

Methanogenesis

Formation of methane and carbon dioxide

The methane production through anaerobic digestion may be performed with a single-stage or two-stage process. In the first case, all the reactions steps above are carried out in the same reactor. The substrate is converted into methane with a concentration of 50-55 % in the biogas, depending on the type of substrate. In two-stage process only the reaction steps until the acetogenesis are carried out in the first digester. The following methane production is performed in a second stage, called methane upgrading stage. By splitting the process in two stages, it is possible to increase the methane concentration in the biogas up to 70 %. Therefore, the system becomes more efficient and the costs for the following biogas purification are reduced. 

Benefits of H2S reduction by ozonation in the digester

When the biogas is produced in a two-stage process, it is possible to reduce the H2S concentration before the methane upgrading by injecting ozone in the digester. In this way, the H2S is reduced even before the methane production, resulting in a more efficient system and lower costs for the final biogas cleaning. Since the digestion process is anaerobic, the ozone injection needs to be carefully controlled for not compromising the process conditions. Normally, the ozone is injected in the air pocket above the bio-bed in the digester unit. Alternatively, it can be injected in an intermediate tank between the digester and the methane upgrading unit.

Read more on H2S reduction for methane upgrading.

Biogas after treatment

In the biogas after the methane upgrading, high concentrations of hydrogen sulfide are often present, creating issues for the following process steps. Corrosion is one of the issues to be considered, since very high H2S levels may corrode pipes and process instrumentation, resulting in costs up to several thousand € per year. Odor is also one of the major problems related to the methane upgrading off-gas. This is due to the very high sensitivity of the human nose to hydrogen sulfide, since the odor receptors are triggered for concentrations in the part-per-billion (ppb) range. Therefore, even a small leak in the process lines or an opening in a process step may create an odor issue for a large area, since the emitted gas needs to be diluted up to 200 000 times before the odor is masked.

Benefits of odor control with ozone-based solutions

Starting from traditional solutions, Ozonetech developed innovative ozone-based solutions for odor emissions, greatly improving the cost effectivity of the process and keeping high standards in terms of removal grade. A summary of the operational costs of a traditional solution against an Ozonetech’s ozone-based solution is presented in the figure below.

biogas, odor, operating costs, ozone, solution

Operating costs for a H2S removal system: comparison between a traditional solution (left) and a range of Ozonetech's solutions (right).

From the figure above, it can be clearly seen that the operating costs of an Ozonetech's solution are more than four times lower than the ones of a traditional solution, even for sub-optimal configurations. The lower operational costs result in great economic savings every year, for a value up to 100 000 €/y. The figure above also highlight the flexibility of our solution. Since each biogas process is different and has different requirements, it is important to adjust our system to the needs of the customers. Therefore, every solution is carefully designed and tailor-made for maximizing the benefits for the customer, as the operating costs in the presented case. Besides the great costs savings, the Ozonetech solution ensures a high-efficiency and robust performance throughout its lifetime, resulting in an effective odor removal for a very long time.

A common Ozonetech’s solution for odor control is composed by several stages, including the RENA Pro ozone units and the Nodora catalytic filters.

Read more on Odor Control and Nodora catalytic filters

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