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).
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).
Biogas appliances are domestic appliances. They serve a practical purpose. However, they are also relevant to the self-image of the housewife or master of the house. The biogas plant will be looked after better the higher the prestige value of the gas appliances. For this reason even simple, inexpensive gas appliances made in the village should be of appealing design. They must be not only cheap but also, and in particular, "modern".
Most households cook on two flames. Two-flame burners are preferred. The burners (Figure 41) should be set initially and then fixed. Efficiency will then remain at a high practical level.
In villages without electricity, lighting is a basic peed and a status symbol. However, biogas lamps have low efficiency. This means that they also get very trot.' If they hang directly below the roof, there is a fire risk. The mantles do not last long. It is important that the gas and air in a biogas lamp be thoroughly blended before the mixture reaches the gas mantle, and that the air space around the mantle be adequately warm.
Particular problems also arise with biogas-operated refrigerators. The composition of biogas varies substantially from day to day. The gas pressure fluctuates excessively with the amount of gas stored even in a floating-drum plant. Special' stable-burning jets are therefore needed - especially if the refrigerator is thermostatically controlled and the flame burns only when required. On every ignition there is a risk of the flame going out. Gas will then discharge without burning. The gas supply must therefore automatically be cut off if the flame goes out.
A gas appliance specialist must always be called in where biogas is to be used in refrigerators!
For use of biogas in engines see Figure 42.