Making A Biodigestor

BIOGAS AND LIQUID BIOFUELS
Introduction
Background
Biomass residues can also be converted into various non-solid fuel forms. These fuels are
referred to as biogas and liquid biofuels. The aim of this conversion process is to improve the
quality, specific energy content, transportability, etc., of the raw biomass source or to capture
gases which are naturally produced as biomass is microbiologically degraded or when
biomass is partially combusted. Biogas is a well-established fuel for cooking and lighting in a
number of countries, whilst a major motivating factor in the development of liquid biofuels has
been the drive to replace petroleum fuels. In this fact sheet we will be looking at some of these
fuels, their applications and the conversion technologies used to derive them.In Europe and the United States, as well as in several developing countries, there is a move
toward cultivating energy crops specifically for the production of biomass as a fuel. The
potential for energy production from biomass throughout the world is enormous and as fossilbased fuels become scarcer and more expensive, as carbon emission levels are becoming of
greater concern and as peoplerealise the benefits of developing integrated energy supply
options, then biomass could begin to realise its full potential as an energy source.

Technical
Biogas
Biogas is produced by means of a process known as anaerobic digestion. It is a process
whereby organic matter is broken down by microbiological activity and, as the name suggests,
it is a process which takes place in the absence ofair. It is a phenomenon that occurs
naturally at the bottom of ponds and marshes and gives rise to marsh gas or methane, which
is a combustible gas.
There are two common man-made technologies for obtaining biogas, the first (which is more
widespread) is the fermentation of human and/or animal waste in specially designed digesters.
The second is a more recently developed technology forcapturing methane from municipal
waste landfill sites. The scale of simple biogas plants can vary from a small household system
to large commercial plants of several thousand cubic metres.
The digestion of animal and human waste yields several benefits:
•
the production of methane for use as a fuel.
•
the waste is reduced to slurry which has a high nutrient content which makes an ideal
fertiliser;in some cases this fertiliser is the main product from the digester and the biogas
is merely a by-product.
•
during the digestion process bacteria in the manure are killed, which is a great benefit to
environmental health.
Two popular simple designs of digester have been developed; the Chinese fixed dome digester
and the Indian floating cover biogas digester (shown in figures 1& 2). Thedigestion process is
the same in both digesters but the gas collection method is different in each. In the floating
cover type, the water sealed cover of the digester is capable of rising as gas is produced and
acting as a storage chamber, whereas the fixed dome type has a lower gas storage capacity
and requires good sealing if gas leakage is to be prevented. Both have been designed for use
withanimal waste or dung.

Biogas & Liquid Fuels

Intermediate Technology Development Group

Figure 1: Fixed Dome Digester

The waste is fed into the digester via the inlet pipe and undergoes digestion in the digestion
chamber. The temperature of the process is quite critical - methane producing bacteria
operate most efficiently at temperatures between 30 - 40° C or 50 - 60° C - and incolder
climates heat may have to be added to the chamber to encourage the bacteria to carry out
their function. The product is a combination of methane and carbon dioxide, typically in the
ratio of 6:4. Digestion time ranges from a couple of weeks to a couple of months depending on
the feedstock and the digestion temperature. The residual slurry is removed at the outlet and
can be used as a...