|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Indian experience with treated straw as feed|
Table 3 shows the calculation of energy efficiency for different methods of using dung as fuel. The following factors are taken into consideration.
|Traditional open fire-place||Closed fireplace with chimney||Biogas production|
|Energy content (MJ) of:|
|Resulting animal products and services||4.6||4.6||4.6|
|Gas produced from dung||15.4|
|Useful energy obtained from dung (MJ)||5.5||11.0||9.2|
|Deduction for energy equivalent of nitrogen lost (MJ)||- 1.5||- 1.5|
|Total useful energy recovered from straw||8||14.1||13.8|
|Efficiency of processing (%)||8.6||14.1||13.8|
1. 6.9 kg straw containing 90 per cent dry matter and having a rate of combustion of 16 MJ/kg dry matter contains 100 MJ combustible energy. This amount of straw is approximately the daily consumption by an adult bovine.
2. The efficiency of animal production on a typical straw diet is calculated (see Annex 4) to be 4.6 per cent. This value is for the diet as a whole, the value of the straw component is probably slightly less.
3. 18.3 kg of dung containing 20 per cent dry matter and having a rate of combustion of 15 MJ/kg dry matter (the digestibility of straw energy averages 45 per cent) = 55 MJ.
4. 18.3 kg wet dung x 37 I biogas/kg wet dung x 0.61 CH4/l biogas x 0.038 MJ/l CH4 = 15.4 MJ.
5. The efficiency of burning dung in an open fireplace is estimated to be 10 per cent, in a close fireplace with chimney, 20 per cent, and in a gas burner, 60 per cent (2).
6. Assuming that 50 per cent of the nitrogen contained in straw is recovered in dung, the manufacturing energy value of this nitrogen is:
7. Total useful energy = energy contained in animal products and services + useful heat energy - deduction for energy equivalent of nitrogen lost.
8. Efficiency of processing = (total useful energy obtained x 100) / (energy content of original straw)