Biogas purification, enrichment and utilization

What is biogas?

Biogas is a fuel that is created when organic matter decomposes

It is very common to be created in landfills, where tons of solid waste get broken down into biogas.

Almost all forms of organic material can be used to produce biogas. However, waste water, manure, energy crops and organic industrial waste are the most common feedstocks.

This biogas contains a high percentage of methane which is very flammable gas and its emission would damage the atmosphere. Then it must therefore be collected securely

The main management methods related to biogas:

• Biogas Disposal: using torch burners for the disposal of gases. The energy contained in the gas can also be utilised in many different ways.
• Biogas Enrichment: Conversion of the biogas to renewable electricity and useful heat with cogeneration / combined heat and power.

Biogas Enrichment

Biogas produced in both solid and liquid waste treatment plants (biomethanization plants) and wastewater treatment stations, is a mixture of:

• Methane: 50-70%
• Carbon Dioxide 30-45%
• Hydrogen Sulphide
• Water
• Siloxanes
• other impurities

This mixture, with this content of methane, doesn’t allow it to be released into the atmosphere due to its high pollutant potential (it is one of the main greenhouse gases), its high calorific value means it can be harnessed for electric power production (cogeneration).

Thus, the production and reuse of biogas enables this type of installation to be ever more self-sufficient in terms of energy. Biogas enrichment can be reused onsite or in connection to the network of natural gas supply.

Nevertheless, one of the primary impediments to the use of biogas for electric power production is determined by the nature of the impurities that accompany the biogas. These impurities include the following: H2S and Siloxanes. CO2 and H2O aren’t real impurities but it’s necessary to remove them for results optimization.

According on this fact we try to summarize the main technologies used for each one:

A. Hydrogen Sulphide (H2S): is one of the substances that most often contaminate biogas, and in the greatest amounts.

It is a corrosive compound that attacks both the civil works of the installations where it is produced and the equipment responsible for producing the electrical energy.

Its concentration in the biogas can vary between 1,000 and 20,000 ppm (parts per million in volume), but in order to be used in electrical energy cogeneration systems the H2S concentrations must be below 400 or 500 ppm.

Technologies:

• Backwashing with pressurized water: This technology separates the carbon dioxide and hydrogen sulfide in a single step through a completely automated process of utmost efficiency.
• Chemical Oxidation: The oxidation of hydrogen sulphide is in scrubbers, connected in series. In the first stage, it is neutralized with an acidic solution (H2SO4) and then, in the second stage, an alkaline solution of NaClO and NaOH is used to cause the chemical oxidation.

  This option entails high consumptions of reagents in addition to presenting technical difficulties due to the presence of other chemical species (carbonatation of the CO2).

• Biological Oxidation: This option uses trickling filters where the surface of the filter filler material forms a biofilm comprising sulfide-oxidizing bacteria.

  These bioreactors enable the H2S to be eliminated at an extremely low operating cost, without the use of chemical reagents (which is an economic, safety and environmental advantage), and offer a consistently high disposal efficacy.

  Although the process is biological, these systems have proven to be very stable operating for long periods of time and adapt to the variability of the pollutant load to be broken down.

  The investment costs of a biological desulfurization process are slightly lower than those for the chemical system. Nevertheless, where the difference is very noticeable is in the operation costs, as chemical reagents are not used and there is hardly any waste.

  This factor makes it economically viable to convert traditional chemical systems to biological.

B. Carbon Dioxide (CO2): This product is not a real impurity. But it is necessary to separate it if we need to obtain more concentrated methane gas.

Technologies:

• Backwashing with pressurized water: This technology separates the carbon dioxide and hydrogen sulfide in a single step through a completely automated process of utmost efficiency.
• To precipitate completely CO2 and H2S it’s possible to add Ca(OH)2, obtaining CaCO3 and CaS.

C. Water (H2O): At the digestor exit the biogas is saturated with water and it could be necessary to dry it. Then, it’s possible to refrigerate the pipe and to collect the water.

Technologies:

• Refrigeration: To eliminate all the water it’s necessary to use a drying agent like Al2O3 or silica gel.

D. Siloxanes: They are a family of silicon compounds which are in biogas. Their crystallisation process causes important abrasion problems in equipments.

Technologies:

• Active Coal adsorption: the siloxanes can be reduced to ppb(v) using this technology.

Biogas Purification

Backwashing with pressurized water

The enrichment of biogas to the quality of natural gas through backwashing with pressurized water is the most flexible technology available for the treatment of biogas, regardless of quality and quantity.

This technology is used to enrich biogas and separate the carbon dioxide and hydrogen sulfide in a single step through a completely automated process of utmost efficiency.

Advantages of Biogas Enrichment with Backwashing with Pressurized Water:

• Plants are made in standard modules with different capacities and are easy to implement
• CO is removed from the biogas through scrubber technology using pressurized water
• No chemicals products are consumed
• No prior desulfurization is required
• No heat demand
• 99% recovery efficiency of methane
• High flexibility despite the variations of CH4 content

Biogas is compressed up to 7 bars and then washed in a counter current flow of water in a water scrubber. Carbon dioxide and hydrogen sulphide have much greater water solubility than methane and will dissolve in water.

During this process, the water used for biogas purification is sent to a desorption column where it is regenerated by separating it from the carbon dioxide and hydrogen sulfide.

After this process, three results are obtained:

• Wash water is cooled to a low temperature, so it can be reused in the scrubber.
• The biogas, now clean, is dried (first in a coalescing filter and later in two adsorption columns in parallel with low dew points) and can now be reused.
• The air coming from the adsorption column, which is loaded with carbon dioxide, hydrogen sulfide and traces of methane, must be treated before it is emitted in order to meet current regulations. Regenerative Thermal Oxidation (RTO) is the best technology for meeting emissions values that fit the rules of every country.

Regenerative Thermal Oxidation (RTO) systems are characterized devices called regenerators that recover the heat from purified gases. These regenerators have ceramic elements that accumulate the heat from the gases leaving the oxidation chamber.

Through a system of valves, consecutive operating cycles are set whereby the cleaned gases at an elevated temperature (about 800 °C) transfer their heat to ceramic masses so that the cold contaminated gases that enter the installation can adsorb this heat in the next cycle.

Thus, the inert bed acts as a preheater and regenerator, with the ability to recover up to 95% of the heat produced in the oxidizing reaction depending on crossing airflow.

We can find different types of RTO equipment:

• Two chambers (with or without clearing).
• Three chambers.

The principal characteristics of this equipment are:

• Minimal fuel consumption, permitting high efficiency heat recovery.
• Low operating and maintanence costs
• High purification efficiency
• Longer equipment life
• Reliable equipment with highly proven results

Uses of Biogas

The majority of biogas plants are equipped with cogeneration facilities that produce electricity and heat. Sometimes, not all excess heat can be utilized, therefore biogas plants are not used to their full potential.

In these cases, the alternative is the production of biomethane that offers interesting economic variables.

Through the use of biogas enrichment technologies, CO is eliminated from the biogas very efficiently and biomethane is produced with a quality equivalent to that of natural gas (CH4 97-99%).

Additionally, biomethane is a renewable gas of high quality that can be injected directly into existing natural gas. Some of the uses are:

• Combustion in facilities located far away from production (combined cycle)
• Biogas for direct consumption in households or industry
• Biofuels for vehicles
• Green energy