Home Running Small-scale

Examples of the use of the models

These examples are taken from: “Small-scale, Rural Biogas Programmes”.

Example 1 - a plant using animal dung

The first order rate model can be used to provide an estimate of the gas production from a biogas plant. Using a  fixed dome biogas plant with a working volume (V) of 6 m3, fed with dung from 2 cattle and 2 buffalo, the wet mass of daily feed is 48 kg (using data from Table 4).

The dry mass is 48 × 18% = 8.64 kg and the volatile solids content is 8.64 × 77% = 6.65 kg

Normally, the dung is mixed with the same amount of water. Assuming a density for dung of 1000 kg m-3, the daily feed volume (v) is 0.096 m3. The concentration of volatile solids in the feed material is therefore S0 = 6.65/0.096 = 69.3 kgVS m-3.

The Retention Time (R)  = 6/0.096 = 62.5 days.

Using the values of C and k from Table 3: g = 2.24 m3 of biogas a day.

Example 2 - a plant using food wastes

Using a KVIC biogas plant with a working volume (V) of 34 m3, fed with 300 kg of wet food waste a day:

Using the data from the first line of Table 5, the dry mass is 300 × 41% = 123 kg and the volatile solids content is 123 × 64% = 78.7 kg

Mixing the food waste 1:4 with water and assuming a density of 1000 kg m-3, the daily feed volume (v) is 1.2 m3. The concentration of volatile solids in the feed material is therefore S0 = 78.7/1.2 = 65.6 kg.VS m-3.

R = 34/1.2 = 28.3 days.

Using the values of C and k from Table 3 for 34°C: g = 35.3 m3 of biogas a day.