What is a Biogas Plant? Experiences from Kenya

A biogas plant is an anaerobic digester that produces biogas from animal wastes or energy crops.

By Merculine Maoncha Rabera, Nakuru/Kenya

Made by Samsung DVC

Eco club students from Menengai high school in Nakuru Kenya at a biogas plant at the Egerton university

Energy crops are cheap crops grown for the purpose of biofuels, rather than food. Biofuels are liquid, gaseous, or solid fuel made from live or recently dead organic material known as biomass, as opposed to fossil fuels, which are composed of ancient biological materials. Biogas is a type of biofuel created via anaerobic, or oxygen-free, digestion of organic matter by bacteria. A biogas plant is composed of a digester and a gas holder.

 How does a biogas plant work?

In the collecting pit, slurry and other substrates are stored temporarily, and shredded and mixed when needed. The term „substrate“ is used to designate the solid or liquid biomass that is used in the digester to generate biogas.

The digester is the core of the plant. It is a container in which the biomass is decomposed by microorganisms in darkness and under anaerobic conditions. Subsequently, the products of this decomposition are metabolized by methanogenic bacteria into methane and carbon dioxide.

biogas2

The digester is usually heated to about 38 to 42 °C (mesophilic) or 55 °C (thermophilic) and stirred regularly.

The generated biogas is collected either under the gas-tight cover directly above the substrate or in an external gas storage tank.

It can be conducted directly to a CHP station (combined heat and power station), where it is burnt in a combustion engine for the generation of electricity and heat.

Alternatively, the biogas can be dried and purified in a gas treatment unit. In this biogas treatment or purification, the methane content of the biogas is raised substantially and it reaches the quality of natural gas (biomethane).

Biomethane can subsequently be fed directly into the natural gas distribution system.

After the substrate has been fermented in the digester, it is first deposited in the fermentation residue store. From here, it is finally used as a high-quality biological fertilizer. The fermentation residue can also be separated and dried.

biogas3

Uses of  Biogas

  • Cooking.
  • Lighting.
  • Moving automobiles.
  • Energy Production: The most common use of biogas plants is the production of electricity. Biological wastes are contained within a large container that directs the methane released by the waste into a chamber where the methane can be burned. The power produced by this burning is used to turn turbines that create electricity. This electricity can be used on site or can be introduced to a municipal power grid and used in the same way as any other electricity source. An additional side benefit is that the waste, following the burning of the methane it releases, can be used as fertilizer for agricultural crops.
  • Heating. A side effect of burning waste methane for electricity is that the burning produces heat in addition to the electricity created by the turbines. This heat can be captured and used for space heating or other purposes. The technology takes advantage of something that is wasted in massive quantities in most industrial applications: the incidental heat that is created by all processes of burning or mechanical motion. Increasingly, newer and more efficient industrial processes are realizing that utilizing this heat can reduce pollution and simultaneously save money.

How to Make Electricity Using Biogas

Biogas refers to gases that are derived from the composition of organic materials such as manure and plant remains. These gases can be used as fuels and also to produce electricity. The main composition of biogas is methane. Biogas possesses chemical energy, and therefore electricity from biogas comes as a result of converting this chemical energy to mechanical energy and finally into electricity. This is done by the use of transducers such as generators and turbines that convert energy from one form to another. This electricity can be used both domestically and commercially since it can be made in small and large scale.

Things You’ll Need

  • Biogas source
  • Gas engine
  • AC generator

Instructions

  • 1 Connect the biogas source to the inlet of the gas engine. The biogas source may be a cylinder that contains pressurized gas or directly from a digester, which is the means of decomposing the organic material. The gas engine is designed to work in a similar manner to that of a car, since it is composed of pistons within which the gas is burnt and used to rotate a shaft, converting the chemical energy in the biogas into mechanical energy through motion.
  • 2 Connect the gas engine to the AC generator in such a way that the rotating shaft powers the AC generator. The motion transferred to the AC generator produces electricity through magnetism.
  • 3 Connect the AC generator to cables that transfer electricity to a chargeable battery for storage or directly to a power distribution grid for consumption. Open the tap from the gas source and fire up the gas engine. Take precautionary measures such as having a fire extinguisher nearby in case of an accidental fire, as methane is a very volatile gas.
  • 4 Step up the electricity with a transformer to reduce the power lost as electricity is being transmitted through the cables. Connect the system to a bulb to test whether electricity is being produced.

 The Advantages of a Biogas Plant

Biogas plants are a relatively recent invention—used for the treatment of farm and crop waste. Biogas is generated by recycling waste from crops—including from maize or biodegradable wastes, such as sewage. The gas is produced in an oxygen deficient environment within these plants and contains 60 percent methane.

  • Production of Energy. Biogas has a calorific value of around 6kWh/m3—this is equivalent to half a liter of diesel. The calorific value is determined by the amount of energy produced by a biogas plant. The methane content of the gas produced is significantly high and is popular as a type of fuel. One m3 of biogas corresponds to 1.3 kg of wood, 1.1 m3 of city gas and 0.24 m3 of propane.
  • Fertilizer. A polythene biogas digester—found within biogas plants—can produce fertilizer from natural organic waste. The nitrogen rich fertilizer is obtained by processing cow dung. The nitrogen content of fertilizer made by using polythene biogas digesters is three times higher than for the fertilizer obtained by conventional methods.
  • Reduction of Workload. The use of biogas can reduce workloads in rural areas. It is a quick, cheap and convenient method to produce fertilizers for many people.
  • Positive Health Effects. Biogas has a positive effect on rural health conditions. Biogas plants help to decrease the occurrence of respiratory diseases. A considerable decrease in cases of asthma, eye infections and lung diseases have been noted in areas where there have been implementation of many biogas plants. These plants are also responsible for killing pathogens, such as cholera, typhoid and paratyphoid.
  • Environmental benefits. Plants considerably reduce the greenhouse effects on the earth’s atmosphere by recycling waste. They also protect many natural resources, as they work to prevent waste, as well as provide a more environmentally friendly energy source.