EFFECTS OF WATER WITH BIOGAS

Biogas digesters where water is a constraint
This digester, developed by the Central Institute of Agricultural Engineering, Bhopal, India, is a modification of the fixed-dome type and it allows fresh undiluted cattle dung to be used. The modified design requires very little or no water for mixing with the cattle dung, generates about 50% more biogas for each kilogram of dung loaded into the system, and does not require slurry drying time before it can be used as fertiliser.

The main changes to a conventional fixed dome digester are an increase in the bore of the inlet feed, greater reinforcement of the chamber to withstand the higher gas pressures, an enlarged slurry chamber outlet and a smooth widened outlet channel to streamline the flow of the slurry (Shyam, 2001).

Compact biogas digester using waste foodstuffs
For those without cattle or within urban centres, a conventional digester may not be appropriate. The Indian Appropriate Rural Technology Institute (ARTI) has introduced a small biogas digester that uses starchy or sugary wastes as feedstock, including waste flour, vegetable residues,waste food, fruit peelings, rotten fruit, oil cake, rhizomes of banana, canna (a plant similar to a lily but rich in starch), and non-edible seeds. The compact plants are made from cut-down high-density polythene (HDPE) water tanks, which are adapted using a heat gun and standard HDPE piping. The standard plant uses two tanks, with volumes of typically 0.75 m3 and 1 m3. The smaller tank is the gas holder and is inverted over the larger one which holds the mixture of decomposing feedstock and water (slurry).

Figure 3: Compact biogas digester Download the full PDF version to see this diagram.
The feedstock must be blended so that it is smooth using a blender powered by electricity or by hand. Two kilograms of such feedstock produces about 500 g of methane, and the reaction is completed with 24 hours.

An inlet is provided for adding feedstock, and an overflow for removing the digested residue. The digester is set up in a sunny place close to the kitchen, and a pipe takes the biogas to the kitchen. (ARTI, 2006)

Larger-scale biogas plants
Industrialised countries commonly use biogas digesters where animal dung, and increasingly fuel crops, are used as feedstock for large-scale biogas digesters. Brazil and the Philippines lead the world in crop-based digesters using sugar-cane residues as feedstock.

Interest and public support in biogas has been growing in most of the European countries. After a period of stagnation, caused by technical and economical difficulties, the environmental benefits and increasing price of fossil fuel have improved the competitiveness of biogas as an energy fuel. This has been seen in both small and large scale plants in Denmark, Germany (with over 3000 plants producing 500MW electricity and 1000MW of heat) and Switzerland, and as a transport fuel in Sweden (where vehicles using biomass were voted environmental cars of the year in 2005). There have been interesting biogas projects in the UK, Ireland, and the Netherlands. Despite this, the use of biogas in Europe is modest in relation to the raw-material potential, and biogas produces only a very small share of the total energy supply.

Several countries are experimenting with dedicated biogas energy crops, such as newly bred grass varieties (Sudan grass and tropical grass hybrids) or biogas ‘super maize’ developed in France. The crops are developed in such a way that they ferment easily and yield enough gas when used as a single substrate. Biogas crops can be used whole, which allows for the use of far more biomass per hectare.

When produced on a large scale, biogas can be fed into the natural gas grid and enter the energy mix without consumers being aware of the change. A select number of European firms have already begun doing so, while farmers who generate excess biogas on their farms make use of incentives to sell the electricity they generate from it to the main power grid. In Germany, electricity from biogas is an integral part of the energy market. In 2005, biogas units produced 2.9 billion kilowatt-hours of electricity .
India is planning to deal with one of its major problems – air pollution from transport, through the use of compressed biogas (CBG). Since over 70% of the world's longterm (2030) growth in demand for automotive fuels will come from rapidly developing countries like India this is highly relevant and is currently in the research phase (Biopact)