Some of the most important Physics Topics include energy, motion, and force.
What is Biomass Energy and How Does Biomass Energy Work?
The dead parts of plants and trees, and the waste material of animals are called biomass. Biomass is the organic matter which is used as a fuel to produce energy. Biomass includes wood, agricultural wastes (crop residues) and cow-dung. Biomass contains chemical energy in the form of carbon compounds.
Dried biomass (like wood) is the oldest source of heat energy which is still widely used as a fuel for domestic purposes. Actually, biomass is another form in which solar energy manifests itself. This is because all the plants and trees which provide biomass (like wood) used sun’s energy to grow. Even the animal wastes (like cow-dung) are given by cattle who grew by consuming plant food made with the help of sunlight energy.
Since the fuels like wood, agricultural wastes and cow-dung are all plant and animal products, so they are called biomass (or biofuels). Please note that biomass is a renewable source of energy because it is obtained from plants (or animals) which can be produced again and again.
The Case of Wood and Charcoal
Wood is biomass. When wood is burnt, heat is produced. So, wood has been used as a fuel for a long time (Wood that is burnt as a fuel is called firewood). Wood is a renewable source of energy. Wood is obtained by cutting down the trees.
Now, if we can ensure that enough trees are planted in place of cut down trees, then a continuous supply of firewood can be obtained. The traditional use of wood as a fuel has many disadvantages. For example:
- the burning of wood produces a lot of smoke which pollutes the air, and
- the calorific value (or heat value) of wood is low, being only 17 kJ/g. This means that wood produces less heat per unit mass, on burning.
Due to improvement in the technology for using the conventional sources of energy, wood can be converted into a much befter fuel called charcoal. Charcoal can be prepared from wood as follows : When wood is burnt in a limited supply of air, then water and other volatile materials present in it get removed and a black substance ‘charcoal’ is left behind.
Thus, wood minus volatile material is charcoal. Charcoal is mainly carbon (C). Charcoal is mainly used as a fuel for domestic purposes. Charcoal is a better fuel than wood because of the following reasons:
- Charcoal has a higher calorific value than wood. That is, charcoal produces more heat on burning than an equal mass of wood. For example, the calorific value (heat producing value) of charcoal is higher, being about 33 kilojoules per gram, whereas that of wood is low, being only 17 kilojoules per gram.
- Charcoal does not produce smoke while burning whereas wood produces a lot of smoke on burning and pollutes the air.
- Charcoal is a compact fuel which is easy to handle and convenient to use.
The Case of Cow-Dung and Biogas
Cow-dung is biomass. It is also known as ‘cattle dung’ or ‘animal dung’ or just ‘dung’. Cow-dung is the ‘excreta’ of cattle such as cows and buffaloes, etc. Cow-dung is usually semi-solid. In our villages, dried cow-dung cakes have been traditionally used as a fuel for cooking food. When cow-dung cakes are burnt, they produce heat. This heat is used for cooking food, etc. It is, however, not good to burn cow-dung directly as a fuel because of the following disadvantages:
- Cow-dung contains important elements like nitrogen and phosphorus (called nutrients), which are required by the soil to support crops. So, burning of dung in the form of dung cakes destroys the useful nutrients which can otherwise be used as a manure in agriculture.
- Dung cakes produce a lot of smoke on burning which causes air pollution.
- Dung cakes do not burn completely, they produce a lot of ash as residue.
- Dung cakes have low calorific value (low heat producing value).
Since there are many disadvantages in using cow-dung as a fuel directly, it is better to prepare biogas (or gobar gas) from cow-dung. And this gas can then be used as a smokeless fuel. After extracting the biogas, the spent cow-dung can be used as a manure because it still contains all the ‘nutrient elements’ which were present in it initially.
Only the organic matter of cow dung is decomposed and converted into biogas. The other elements like nitrogen and phosphorus, etc., remain intact. In this way, the nutrients like nitrogen and phosphorus can be returned to the soil in the form of manure. When used in this way, cow- dung gives us a double advantage :
- it gives us a clean fuel called biogas, and
- the spent dung can be used as a manure.
Before we discuss biogas in detail, it will be good to know the meaning of the term “anaerobic degradation”. The decomposition which takes place in the absence of oxygen is called anaerobic degradation. Anaerobic degradation is carried out by the micro-organisms called anaerobic bacteria (which do not require oxygen to decompose complex organic compounds like those present in cow-dung). Let us discuss the biogas now.
Biogas is not a single gas. It is a mixture of gases. Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide. The major constituent of biogas is methane, which is an extremely good fuel. In fact, biogas contains up to 75 per cent of methane gas which makes it an excellent fuel.
Biogas is produced by the anaerobic degradation of animal wastes like cow-dung (or plant wastes) in the presence of water. This degradation is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen. Cow-dung and plant wastes contain a lot of complex carbon compounds like carbohydrates, proteins and fats.
The anaerobic bacteria degrade (decompose) these carbon compounds to form methane gas, which is the main constituent of biogas. Some other gases like carbon dioxide, hydrogen and hydrogen sulphide are also formed. Cow-dung is known as ‘gobar’ in Hindi. Since biogas is prepared mainly from cow-dung, so biogas is popularly known as ‘gobar gas’ in our villages. We will now describe a biogas plant which is also known as gobar-gas plant.
A biogas plant consists of a well-shaped, underground tank T called digester, which is made of bricks, and has a dome-shaped roof D, also made of cement and bricks (Figure). The digester is a kind of sealed tank in which there is no air (or oxygen). The dome of the digester tank acts as a gas-holder or storage tank for the biogas. There is a gas outlet S at the top of the dome having a valve V.
On the left side of the digester tank is a sloping inlet chamber I and on the right side is a rectangular outlet chamber O, both made of bricks. The inlet chamber is for introducing fresh dung slurry into the main digester tank whereas the outlet chamber is for taking out the spent dung slurry after the extraction of biogas. The inlet chamber is connected to a mixing tank M while the outlet chamber is connected to the overflow tank F.
We will now describe the working of a biogas plant. Cow-dung and water are mixed in equal proportions in the mixing tank M to prepare the slurry. This slurry of dung and water is fed into the digester tank T through the inlet chamber I.
The digester tank is filled with dung slurry upto the cylindrical level (as shown in Figure), the dome being left free for the collection of biogas. It takes about 50 to 60 days for the new gas-plant to become operative (start functioning). During this period, the cow-dung undergoes degradation by anaerobic bacteria in the presence of water (but absence of oxygen) with the gradual evolution of biogas.
This biogas starts collecting in the dome. As more and more biogas collects in the dome, it exerts pressure on the slurry in the digester tank, and forces the spent slurry to go into overflow tank F (Figure), through the outlet chamber O. From the overflow tank, the spent slurry is removed gradually. The spent dung-slurry, left after the extraction of biogas, is rich in nitrogen and phosphorus compounds and hence forms a good manure.
Once the gas-plant becomes operative, more fresh dung slurry is added to the digester tank regularly and this leads to the continuous biogas production. The biogas which has ‘collected in the dome of the digester tank is taken out through the outlet S and supplied to village homes through a network of pipes to be used as a cooking gas.
Please note that though mostly we use cow-dung in biogas plants, but human excreta (human feces) can also be added alongwith cow- dung in biogas plants. Agricultural wastes, vegetable wastes, poultry droppings, and paper scrap can also be used for producing biogas. In some of the cities of our country we have sewage gas plants (Sewage is the dirty drain water containing human excreta). Biogas can also be obtained by the action of anaerobic bacteria on domestic sewage in the absence of air.
The biogas obtained from the degradation of domestic sewage is also called sewage gas. So, at many places in our country, domestic sewage is digested in large sewage gas plants to produce biogas as well as manure. The large scale utilisation of biowastes and sewage materials for producing biogas can provide us a safe and efficient method of waste disposal besides supplying energy and manure. The important uses of biogas are given below :
1. Biogas is used as a fuel for cooking food. When biogas is burned in a gas stove, it produces a lot of heat. This heat is used for cooking food and for other domestic heating purposes. Biogas is a good domestic fuel because of the following reasons :
(a) Biogas burns without smoke and hence does not cause air pollution.
(b) Biogas has a high calorific value. That is, biogas produces a large amount of heat per unit mass.
(c) Biogas burns completely without leaving behind any residue (unlike wood, charcoal or coal). So, it is a clean fuel.
(d) There is no storage problem for biogas as it is supplied by pipes directly from the gas plant.
(e) Biogas is cheaper than most common fuels.
2. Biogas is also used for lighting.
3. Biogas is used as a fuel to run engines. At many places, the engines of water pumping sets used for irrigation are run on biogas, instead of diesel.
4. Biogas is used for generating electricity.
Biogas is a renewable source of energy. Since India has a large population of cattle and other farm animals (called livestock) to provide animal dung, therefore, biogas can become a steady source of energy in our rural areas. Before we end this discussion on biomass as a source of energy, we would like to compare the biomass and hydroelectricity as sources of energy.
- Biomass is a renewable source of energy and hydroelectricity is also a renewable source of energy,
- The use of biomass by burning causes air pollution but the use of hydroelectricity does not cause any pollution.
- Biomass gives heat energy which can be used for cooking and heating only. On the other hand, . hydroelectricity can run all types of electrical appliances.
- Biomass energy can be obtained without using any special device but hydroelectricity can be produced only by establishing hydro-power plants.