|Biogas Plants (GTZ, 1988)|
|5. Design of biogas plants|
Simple biogas plants are usable only conditionally in tropical uplands or in temperate climatic zones. At latitudes as high as only 25 - 30°, gas production in winter generally falls to about half the summer level.
Whether it is worthwhile to heat a plant must be decided on an individual-case basis. In Europe, large-scale plants use up 20-30 % of their gas production for heating. Practicable heating systems for simple plants have not yet been developed.
Utilization of solar energy in the mixing tank (Figure 23) and insulation by covering with straw are insufficient where frost occurs. Floating drums have the highest heat losses. Underground fixed-dome plants maintain more even but generally lower temperatures. Fixed-dome plants with floating gasholders (Figures 3 and 52) may be a valid solution for cold regions although more expensive. Good results are obtained with roofed-over biogas plants. However, the cost of a "greenhouse" superstructure is relatively high. It is worthwhile only where low temperatures are combined with high insolation. Good results have been obtained by placing the plant under a compost heap. If the digester is surrounded externally by soft insulation, the wall cannot be "relieved of its load" by the earth pressure (see Figure 17).
Again, insulation must always remain dry. The only exception is special insulation with closed pores. Biogas plants are completely shut down in winter in the north of China; they are used for only six to eight months per year.
Where frost occurs, mixing and filling tanks must be roofed over. Transport of feed material is difficult in snow. It is essential to consider in detail how the plant is to be operated before commencing construction. Energy is particularly expensive in cold regions. This is why biogas plants have to be used in these regions. Unfortunately, appropriate types of simple plants have not yet been developed.