The Process and Techniques of Anaerobic Digestion

 

Biogas production involves complex interactions between various micro-organisms such as enzymes and bacteria. There are four phases in its production.

 

Four Phases to produce Biogas
 
Table showing AD process

Phase One - Hydrolysis

During this phase the long chain organic compounds (e.g. proteins, fats, carbohydrates) are split into more simple organic compounds (e.g. amino acids, fatty acids, sugars) through bacterial action.

Phase Two - Acidogenesis

The products of hydrolysis are subsequently metabolized in the acidogenesis phase by acidogenic bacteria and broken down into short chain fatty acids (e.g. acetic acid, propionic acid, butyric acid, valeric acid) and alcohol. Acetic acid, hydrogen and carbon dioxide are also created and act as initial products for methane formation. The relationship between the products depends on the hydrogen partial pressure, i.e. the concentration of hydrogen.

Phase Three - Acetogenesis

The organic acids and alcohols are broken down from acetogenic bacteria into acetic acid, hydrogen and carbon dioxide which are the source compounds for biogas production.

Phase Four - Methanogenesis

The products from the previous phases are converted into methane and carbon dioxide by methanogenic micro organisms (archaea). The end product is a combustible gas called biogas.

 
Anaerobic Digestion - Temperature
 

The digestion of biomass is highly affected by environmental conditions. The process temperature is an important factor/parameter that influences the speed and stability of the AD process and consequently the gas production. The bacteria which are involved in the whole digestion process have different temperature optimums. Essentially two temperature ranges can be distinguished:

  • between 32 and 45°C (called mesophilic)
  • between 50 and 65°C (called thermophilic).

Methane bacteria experiences optimum growth between 38-45°C (mesophilic). Many biogas facilities operate within this temperature range due to high gas yields and good process stability.

Thermophilic digestion at 55°C can be an advantage if animal by-products or organic waste are added. In this case a disinfection is requiered (one hour at 70°C) and the thermophilic temperature range could be considered for operating a plant. It could cause a higher gas yield but the process is more sensitive to disturbances or irregularities.

As the bacteria do not produce sufficient heat alone, the digester must be insulated and externally heated, using CHP, for example.

To find out more about Inhibitors of the AD process please download the following pdf: Digestion Inhibitors.pdf (size 155 KB)

 
Substrates
 

Biogas is produced through the breakdown of organic materials. The origin of the substrates vary, from pig or cattle slurry, energy crops (e.g. grain, grass silage, canteen waste, vegetable oil, municipal solid waste (MSW) from households to organic solid waste from industry (e.g. slaughterhouse, food waste).

Waste products from industry produce biogas and therefore offer interesting opportunities for agriculture/farming. By using organic wastes or residues such as distillers pulp, grease or food wastes, the natural material cycles (carbon and nitrogen) is closed and provides a recirculation of the nutrients into agriculture.

All kinds of chopped biomass can be used for biogas production. Their main components are:

  • Carbohydrates
  • Proteins
  • Fats
  • Cellulose and Hemicelluloses

Materials with a high lignin content, for example any kind of wood, are not suitable for biogas production. The fundamental materials used in agricultural biogas plants are cattle and pig slurry.

  
Energy crop co-substrates like grass-silage, maize-silage or grain (wheat, triticale etc.) can increase biogas yield substantially (see gas yield below). Due to particularly high yields per hectare small amounts of chopped energy crops (grass-silage app. 20 mm, maize-silage 6-8 mm) can be added continuously by a solid feeding system.Image of grass silageImage of maize silage
 Grass SilageMaize Silage

Biogas yields

The biogas yield depends essentially on the composition of the used substrates. It is not substrate specific but depends largely on the ambient conditions in the digester (e.g. temperature, inhibitors etc). It is therefore possible that the same input materials/substrates could have different gas yields.

For specific gas yields please download the following table: Gas Yields Table.pdf (size 63.7 KB)

 
Techniques
 

A biogas plant consists mainly of the following components:

  • Liquid manure storage (with pump) and feeding system/solid containers for solid biomass
  • Disinfection unit if necessary (not shown in picture below)
  • Pit storage digester(s) with stirrers and foil covering
  • Storage tank or lagoon
  • Motor or cogeneration unit (CHP integrated)
Image of biogas plant with co-fermentation

Slurry and possibly a small amount of solid biomass are stored temporarily in the liquid manure store and if necessary they can be diluted or mixed. Co-digestion plants feeding liquid material and a high amount of solid biomass need special feeding systems depending on the solid materials (e.g. grass or maize silage). The liquid material is pumped from the liquid store into the digester. It is recommended that the feeding takes place continuously, for example, 12 times a day.

If, for example, animal by-products or food wastes are used they have to be disinfected (for one hour at 70oC) before or after the digestion process.

The heated digester is the main item of the plant. For a successful process the digester must be water and gas-proof and opaque. Special mixers (depending on input material) ensure a well mixed and homogeneous material in the digester so that bacteria and substrate are in close contact to get a high gas yield. An over-pressure / under-pressure protection system and foil covering (gas holder) are integrated in the digester.

For pictures of plant and plant components please download the following document: Pictures of different plant and plant components.pdf (size 405.6 KB)

When the substrate is digested, it is pumped to the storage tank (residue repository or lagoon) and can be perfectly used as a fertiliser for farmland. In order to utilise further produced biogas the storage tank could also be covered with a foil.

The produced biogas is cooled and cleaned from biogas ingredients such as H2S called desulphurisation. Cooling and cleaning is highly recommended due to the very corrosive effect of hydrogen sulphide (H2S) when it interacts with water (H2SO4). If the concentration of H2S is over a certain limit (over 200 ppm) for a long time it could damage the whole motor/cogeneration unit (engine breakdown) or components in the gas pipe. Repairs to the engine are very expensive. The H2S content in the biogas strictly depends on the substrates and the operation process (temperature etc).

The gas could finally be used in a cogeneration unit to produce electricity and heat (CHP) constantly for up to 8,000 hours a year.

 
Cogeneration unit
 

The cogeneration unit converts the biogas generated into electricity and heat (Combined Heat Power, CHP). It is a decisive component when it comes to the efficiency of the biogas plant. It constantly produces a stable amount of electricity and heat for up to 8,000 hours a year and has an overall efficiency rate of up to 90%.

Gas Otto or diesel engines are developed particularly for the biogas operation, which are comparable with four stroke engines from motor vehicles. Diesel engines work according to the diesel principle. As biogas does not catch fire with compression, ignition oil (max. 10% of the fuel power) is injected in order to produce a combustable gas mixture.

Mechanical power generated by the motor through combustion is converted into electricity by a generator. Electricity could be used for powering the facilities (e.g. farm) and could be fed to the public power supply system. Profits from power suppliers are an additional source of income (Renewable Energy Feed In Tariff) for many operators of biogas plants. Heat can be supplied to a close or a long-distance heating network such as a farm building, nursery or school.

High-quality motors (gas and diesel engines) have an installed electrical capacity from 45 kW up to 1400 kW, an electric efficiency rate between 30-42% and a thermal efficiency rate around 25-50%. All efficiency rates depend on the engine and gas conditions such as gas humidity, gas quality etc.

Cogeneration Engine/unit

Cogeneration engine_1Cogeneration unit

Gas and diesel cogeneration unit

Digester with Cogeneration unit (container unit)

Digester with cogeneration unit_1Digester with cogeneration unit_2
 
Process Parameters
 

Process parameters are indicators in order to operate a biogas plant technical and biological stable. Two important indicators of several more are explained in the following:

One parameter is the hydraulic retention time (HRT). It indicates the average time the added material remains in the fermenter before being pumped in the storage tank. The HRT is calculated by the utilisable volume of the digester and the amount of organic material loaded daily. The recommended retention time, normally 35 to 80 days, depends highly on substrate and plant components (e.g. covered storage tank). The aim is to attain the maximum gas yield or the complete digestion of the organic matter. So if the organic material, such as energy crops, are used in the fermentation process, a long retention time and therefore an appropriate size of digester should be considered.

Volume load or organic loading rate is also an important parameter. It indicates how many kilograms of organic dry solids are loaded per m3 of digester volume and unit of time. The organic loading rate is important for the plant components (esp. mixer/agitator) and for the bacteria. If the organic loading rate is too high (over 4.0 kg DS/m3d) technical components like mixers or pumps could be damaged or you need an earlier maintenance than calculated due to an overload. The bacteria could also be stressed by too much feeding. Consequently no biogas production will take place and the digestion process stops completely.

For an example how to calculate the retention time and the organic loading rate please download the following document: How to calculate the retention time and the organic loading rate.pdf (size 432.7 KB)

For the GHG abatement through AD please download the following document: Greenhouse gas abatement through AD.pdf (size 169.2 KB)

 
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