(introduction...)

Biogas can be used in the same way as any other combustible gas. When biogas is mixed with air in the proportions of 1:20, highly explosive detonating gas forms. Leaky gas pipes in enclosed spaces constitute a hazard! However, there have been no reports of dangerous explosions caused by biogas. The calorific value of biogas is about 6 kWh/m³ - this corresponds to about half a litre of diesel oil. The net calorific value depends on the efficiency of the burners or appliances (see Table, Figure 38).

Fig. 38: Biogas: properties and utilization

Efficiency is high if, for example, a litre of water boils quickly. This takes longer if the burner is wrongly set. Efficiency is then poor. The air supply substantially determines the efficiency.

A gas pressure of 5-20 cm WG is best for cooking. Lamps require a pressure of about 10 cm WG.

The hydrogen sulphide in the biogas combines with condensate to form corrosive acids. Water-heating appliances and utensils and refrigerators are particularly at risk. The combustion chambers and burners should be made of cast steel, high-grade steel or enamel.

Biogas can be rid of sulphur by iron oxide filters

(FeO + H₂S ® FeS + H₂O; 2FeS + O₂ ® 2FeO + 2S).

With large volumes of gas, the filter material has to be replaced frequently and this becomes a laborious task. In this case filtration should be omitted and high-grade steel utensils should be used despite the higher cost. The gas does not have to be filtered for use in engines. The gas pressure may be low because the engine aspirates the gas. It is seldom worthwhile using the gas from simple plants to run engines.

Biogas cannot be economically liquefied. Gas pipes may be made of steel, copper, ruber or plastic. Rubber hoses and rigid PVC pipes quickly become porous and leaky when exposed to the sun and should therefore either be shaded or wrapped in some sort of protective material.

The longer the gas pipes are the greater is the decrease of pressure (Figure 39). The gas pipe must have an outlet for perspiration water (Figure 40).

Fig. 39: Pressure drop in gas pipes The gas pressure (p) falls with pipe length (L). The density relative to air is indicated by "s". The pressure drop depends on the pipe friction (c) and especially on the diameter of pipe (c = 2.24 in galvanized steel pipes (G.I.); c = 2.80 in smooth plastic pipes). The curve shows the gas flow (Q) in a galvanized steel pipe allowing for a pressure drop of p = 1.25 cm WC. Bends increase the pressure drop.

Fig. 40: Condensate trap Biogas contains water vapour. If the gas is cooled, condensate forms. It always collects at the lowest point in the pipe. It must be possible to drain condensate from this point, otherwise the pipe will be blocked. Water pockets must be avoided. Condensate may be drained by: (a) a gaslight water tap (ball valve), (b) a Patel type overflow water trap, or (c) a simple U-shaped trap (see also Figure 31). Approach (d) is recommended for cases involving a high groundwater level.