Biogas – A Renewable Energy Resource

People across the world are debating the existence and effects of climate change, even when its impact is already evident. The use of renewable energy is the most logical solution to tackle climate change. Anaerobic digestion and biogas generation both provide us the platform to tackle multiple environmental issues such as solid waste management, carbon emission, and energy crisis, and also contribute in lowering the adverse impact of climate change.

Anaerobic digestion is a phenomenon known to us since ancient times, brewing beer being one such example. Though the process occurs naturally in the environment, its full potential is yet to be harnessed. Alessandro Volta, an Italian physicist, was the first person to discover methane generated from marshlands. This discovery led to the onset of biogas production and energy generation across the world. Currently, Germany is the leading country to exploit the benefits of biogas generation and biomass to energy conversion. But the first successful biogas generation plant in India was built at a leper colony in Bombay (now Mumbai), India, in 1859. The scenario in India, however is not so bright as in Germany, as here a more decentralized version of biogas plants are in operation. India has approximately 5 million domestic biogas plants that cater to the domestic needs of cooking gas and water heaters. Biogas generation technology has now gone to a new level and India needs to adopt these new practices to make a leap towards sustainable energy. The key steps involved in Biogas production are:

Substrate generation:

The choice of substrate determines the quantity of biogas generated. If the substrate used has high fat content, then on an average, 80% biomass can be converted into biogas; similarly a high protein content will lead up to 70% gas generation, while carbohydrate rich substrate will give 60% gas production. Any kind of organic waste can be utilized for biogas production such as agricultural wastes, animal manure, energy crops, domestic sewage as well as industrial waste with high organic matter content. Before feeding the substrate into the anaerobic digester, it’s a prerequisite to process the substrate in order to facilitate efficient anaerobic digestion. The common processing steps involved are mechanical crushing and chemical treatment with acids and alkali.

Microbial Digestion in Anaerobic Digester

Anaerobic digestion can occur in anaerobic lagoons, land fills and enclosed system of anaerobic digester. The drawback of open systems like anaerobic lagoons and land fills is that a large amount of biogas generated is lost to the environment. The heart of an efficient anaerobic digestion process is the anaerobic digester which provides a better control on the process and biogas yield. The different types of anaerobic digesters widely used for varied applications are

    • Stirred tank bioreactors – Agricultural waste and animal manure

    • Up flow anaerobic Sludge blanket and Extended granular sludge blanket Reactors – Effluents with dry matter less than 2%

    • Plug flow Reactors – Animal and Agricultural wastes and dry matter content of 30-60%

    • Garage System – Domestic solid waste with dry matter above 70%

The machinery required for undergoing the anaerobic digestion process is provided by a battery of micro organisms. The Anaerobic digestion process is illustrated in the image below.

As the final process of acetogenesis and methanogenesis are sensitive to acidic pH, therefore it’s of utmost importance to maintain the pH throughout the process, by utilizing suitable buffering systems. The other operation parameters that influence the smooth functioning of the anaerobic digester are the organic loading rate, temperature and trace mineral salts.

Biogas and Biomethane generation

The final products of anaerobic digestion are biogas and digestate. The general composition of biogas is as follows

Compound

Formula

In Percentage

Methane

CH 4

50–75

Carbon dioxide

CO 2

25–50

Nitrogen

N 2

0–10

Hydrogen

H 2

0–1

The digestate obtained is of high nutritive value which can be used as organic manure for agricultural activities. Biogas in crude form contains traces of hydrogen sulphide which can be corrosive to the supply pipes and Combined heat and Power (CHP) generators. Therefore biogas is further purified to Biomethane using certain upgrades in the existing digester. Biomethane contains 97% methane and has higher fuel efficiency compared to biogas. Biomethane can be fed to a CHP engine which can generate electricity as well as hot water for domestic purposes. A CHP engine works on the principle of generating combined heat and power. The energy generated by combustion of biomethane can be used to run turbines of a generater for producing electricity, and the heat exchange system provides us with a source of hot water for domestic and industrial application.

Though renewable energy is a topic known to all , its relevance has not yet impressed upon the masses. Solar energy and wind energy are the front runners in production of renewable energy but Biogas generation is catching up. The main advantage of Biogas generation is its use as a stored energy which can compensate for the time when both solar and wind energy are not available. The potential of electricity generation from biogas can suffice the needs of rural regions in India, which have been poorly connected to the electrical grids. With the new government policies and widespread awareness, it’s an assurance that the scenario of biogas generation in India will undergo a major boost in the coming years.

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