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Appendix 1  Chemistry of Simple Digestion

A basic analysis of the process by which food materials are converted to biogas provides an understanding of the energy released by this process. Calculations are provided for the conversion of two basic food materials: sugar and starch. A simple first order rate model is also presented, which allows the gas production from a biogas plant to be estimated, based on the plant working volume and the daily feed volume.


Appendix 2  Biogas Plant Design Details

Various designs of a biogas plant have been developed by the different biogas extension projects. Since these plants have been designed to do similar jobs, basic dimensions are similar. Earlier designs used longer retention time, and therefore larger volumes. The required retention time is affected by the local ambient temperature.

The mathematical formulae required to calculate various volumes are given. Actual measurements may be slightly different from those calculated from the formulae, as the shapes produced when plants are built may not fit exactly to the shapes defined by the formulae.

Appendix 3  Building a Masonry Biogas Plant

Biogas units made in large numbers in Asia are made from masonry by manual labour, using cylindrical or spherical shapes. The basic building techniques are defined and use basic skills that need to be gained by the masons who are doing the work. Similar approaches need to be used for the different designs, although there are particular approaches required for each different design.

Appendix 4  Basic gas pipe fitting

The biogas from a biogas plant is only beneficial if it can be piped to where it is needed and used efficiently. Gas leaks from piping are often a cause of operators deciding that their biogas plants are not working. Biogas is being produced, but it is lost through the leaks. Biogas contains water vapour, which can condense in pipes and block them, so measures need to be designed for the water to be removed. As gas flows down a pipe, friction between the gas and the walls of the pipe causes pressure losses, which can be calculated.

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