Ingenieria qumica

A system approach to biogas technology
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Components of a biogas system

Biogas technology is a complete system in itself with its set objectives (cost effective production of energy and soil nutrients), factors such as microbes, plant design, construction materials, climate, chemical and microbial characteristics of inputs, and the inter-relationships among these factors. Briefdiscussions on each of these factors or subsystems are presented in this section.

Biogas

This is the mixture of gas produced by methanogenic bacteria while acting upon biodegradable materials in an anaerobic condition. Biogas is mainly composed of 50 to 70 percent methane, 30 to 40 percent carbon dioxide (CO2) and low amount of other gases as shown in Table 1.

|Table 1: Composition of biogas|
|Substances |Symbol |Percentage |
|Methane |CH4 |50 - 70 |
|Carbon Dioxide |CO2 |30 - 40 |
|Hydrogen |H2 |5 - 10 |
|Nitrogen |N2 |1 - 2 |
|Water vapour |H2O |0.3|
|Hydrogen Sulphide |H2S |Traces |
| | | |
|Source: Yadav and Hesse |

Biogas is about 20 percent lighter than air and has an ignition temperature in the range of 650 degrees to 750 degrees C. It is an odourless and colourless gas that burns with clear blueflame similar to that of LPG gas (Sathianathan, 1975). Its calorific value is 20 Mega Joules (MJ) per m3 and burns with 60 percent efficiency in a conventional biogas stove.

Methanogenic bacteria or methanogens

These are the bacteria that act upon organic materials and produce methane and other gases in the process of completing their life-cycle in an anaerobic condition. As livingorganisms, they tend to prefer certain conditions and are sensitive to micro-climate within the digester. There are many species of methanogens and their characteristics vary.

The different methane forming bacteria have many physiological properties in common, but they are heterogeneous in cellular morphology. Some are rods, some cocci, while others occur in clusters of cocci known as sarcine. Thefamily of methanogens (Methanobacteriacea) is divided into following four genera on the basis of cytological differences (Alexander, 1961):

• A. Rod-shaped Bacteria
o (a) Non-sporulating, Methanobacterium
o (b) Sporulating, Methanobacillus
• B. Spherical
o (a) Sarcinae, Methanosarcina
o (b) Not in sarcinal groups, Methanococcus

A considerablelevel of scientific knowledge and skill is required to isolate methanogenic bacteria in pure culture and maintain them in a laboratory. Methanogenic bacteria develop slowly and are sensitive to a sudden change in physical and chemical conditions. For example, a sudden fall in the slurry temperature by even 2o C may significantly affect their growth and gas production rate (Lagrange, 1979).Biodigesters

The biodigester is a physical structure, commonly known as the biogas plant. Since various chemical and microbiological reactions take place in the biodigester, it is also known as bio-reactor or anaerobic reactor. The main function of this structure is to provide anaerobic condition within it. As a chamber, it should be air and water tight. It can be made of various construction materialsand in different shape and size. Construction of this structure forms a major part of the investment cost. Some of the commonly used designs are discussed below.

Floating drum digester. Experiment on biogas technology in India began in 1937. In 1956, Jashu Bhai J Patel developed a design of floating drum biogas plant popularly known as Gobar Gas plant. In 1962, Patel's design was approved by...