Organic waste streams, agricultural by-products and manure are valuable resources for the production of biogas. HoSt offers a total biogas plant solution for the anaerobic digestion of organic waste streams. But what is biomass, anaerobic digestion and biogas? What has to happen to convert the one into the other? And last but not least: what are the characteristics of a HoSt biogas plant? 

Biogas is a renewable alternative for the fossil fuel natural gas, since the energy in biogas can be turned into biomethane, electricity and heat. Biogas is the gas that is produced during the decomposition of organic materials, such as: vegetable and fruit waste, manure, straw and sludge, by several subsequent chemical and biological processes. This method is known as digestion or fermentation.

Fermentation is another word for digestion. Besides digestion for production of biogas, fermentation takes place in various other situations, such as:

  • Leavening of yeast bread
  • Alcoholic fermentation
  • Production of yoghurt, sauerkraut, salami, soy sauce etc.
  • Digestive system of ruminants

HoSt offers a total biogas plant solution for the anaerobic digestion of organic waste streams. With extensive experience, in-house knowledge and expertise since 1991, HoSt successfully realized hundreds of biogas projects worldwide.

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What is biomass?

Biomass is a collective name for all kinds of organic material and organic waste. Biomass can be turned into energy by, among others, anaerobic digestion. Possible alternatives are combustion and gasification of the biomass. In the biogas installations by HoSt, organic waste and agricultural side streams are anaerobically digested.

Biomass consists of water and dry matter, so everything that isn’t water. Dry matter consists of an organic part: carbon compounds that can be converted to biogas, and an inorganic (low-energy) part, such as salts and sand.

Generating energy from organic material and utilizing this energy, is possible with a biogas plant.

These can also be called anaerobic digestion installations or digesters.

In a biogas installation, the decomposition process is able to proceed in a controlled manner.

This way, the biogas potential of the organic material is utilized in an optimized setting, and the biogas is produced as efficiently as possible.

What is digestate?

Because biomass is not all organic matter that can be converted to biogas, after digestion, a high-quality end product remains: digestate. Digestate is a mixture of biomass, from the moment it is in the digester, and consists of water, micro-organisms, minerals and some remaining not-digested organic matter. The water and inorganic part of the biomass remain in the digestate.

HoSt is able to separate and process the digestate to clean, dischargeable water and chemical fertilizer extract. Other or additional possibilities for digestate treatment are: resource recovery, drying and hygienisation/validation.

BIOMASS

Biomass is a collective name for all kinds of organic material and organic waste and is the fuel for a biogas plant.

DIGESTATE

Digestate is the part of the biomass that has not (yet) been converted to biogas and is suitable as fertilizer or for recovery of resources, such as struvite.

BIOGAS

Biogas is a mixture of approximately 60% methane (CH4) and 40% carbon dioxide (CO2). Biogas can be converted to energy and to food-grade CO2.

Processes in a biogas installation

During the process of anaerobic digestion, or microbiological fermentation, the organic matter goes through several processes for controlled decomposition of biogas production. In most cases, this happens in an anaerobic digester: a closed tank, in which the conversion of organic matter takes place, in absence of oxygen. In short: the carbon compounds (proteins, carbohydrates and fats) from the biomass are converted in a mixture of approximately 60% of methane and 40% of carbon dioxide: biogas.

Designing a biogas installation

HoSt does not only design biogas plants, but offers a complete solution: from feasibility studies, advice, and project development to building, assembly, and maintenance. The designing process of a HoSt biogas plant starts roughly with three questions:

The biomass menu

What do you put in the digester?

The output

What do you want to get out of it?

Digestion

How does the biomass behave in the digester?

The biomass menu

Designing a biogas plant starts with settling on a base menu: which biomass is available in the area. Also it is important to know if the demand lies mostly in processing waste and biomass, or in the production of energy. Putting together a menu that serves the demand and that is chemically and biologically functional, is an important step in designing a digestion system.

For the AD plants of HoSt, ingoing biomass may be solid or liquid, as long as the biomass inside digester (the digestate) is liquid. This means that HoSt uses the process of wet digestion. Another way to digest biomass is through dry digestion.

The output

The output of a biogas plant basically consists of biogas and digestate.  What is done with this after it gets out of the digester, depends on a few things, like:

What is the purpose of the output?
  • Own use
  • For sale
What is (economically) interesting in this area?
  • Electricity and heat
  • Biomethane (renewable natural gas)
What are the available options for the digestate?
  • Fertilizing land
  • Resource recovery

Applications of biogas

Biogas upgrading

The produced biogas can be purified in a biogas upgrading plant to clean the biogas to make green gas, also called biomethane or renewable gas. The contaminations in the biogas are removed in this process, and the methane and carbon dioxide are separated. The methane rich part is biomethane and can be fed into the natural gas network or be compressed to be used as renewable CNG to fuel cars. This is a great advantage of biomethane: it is an energy carrier that is easy to store and to use at a different place. The CO2-rich part can be cleaned and liquefied to be used in greenhouses, cooling industry, food industry etc.

Heat and power

The alternative is a biogas-CHP (combined heat and power, or engine). In combined heat and power system, the biogas can be combusted to generate electricity (green electricity) and heat. The electricity can be fed to the grid, and the heat can be used in the facilities of the client, and/or to heat the digesters.

A HoSt biogas plant

HoSt biogas plants are built up from several parts. The most recognizable part is the digesting tank with the spherical roof. However, our biogas plants are way more than that. HoSt successfully realized hundreds of biogas projects worldwide. The characteristics of our biogas installations, and the steps of the process are described below.

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Solid biomass: this is delivered and stored in silo’s or similar, until it can be fed into the bio digester.

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Liquid biomass: this can be pumped directly into the digester from the source, or delivered by trucks and stored in tanks.

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Solid feeding system: a system for the input of solid biomass into the digester. There are several options, which are suitable for a broad spectrum of biomasses. HoSt can advise about this.

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Liquid biomass storage: one or more tanks or silos for pre-storage of liquid biomass. Depending on the kind of biomass which is meant to go into the tank or silo, HoSt will advise to put in a mixer, isolation and/or heating system.

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Digesting tank(s): the tank(s) in which the bio-fermentation takes place. Typical parts are the mixers, spherical gas tight (insulated) double membrane roof which serves as gas storage, and various sensors as part of the control system.

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Flare: the flare burnes the surplus of gas. For example during maintenance, malfunction, or overproduction. It depends on local laws and regulations whether a flare is needed or not.

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Biogas upgrading: alternative for biogas CHP. A biogas upgrading system using membrane technology to purify the biogas to biomethane which is a renewable gas of natural gas quality. The CO2 can be purified and liquefied, for use in food industry and cooling technology among others.

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Biogas CHP: alternative for biogas upgrading. This biogas-fired combined heat and power system is a gas engine in which the biogas is converted into electricity and heat.

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Digestate treatment: the part of the biomass that has not been converted to biogas, also known as digestate, can be treated.

Biogas projects of HoSt

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HoSt has extensive experience in the engineering and construction of anaerobic digestion plants for a large range of companies and sectors. Depending on the input material available and your wishes, HoSt can give advice about the best option for an installation. Contact us for advice and information.

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