Detoxicated gas contains little or no carbon monoxide. It is distributed to the consumption through pipelines at a pressure of at least about 1 lb/in2. The quality of the gas rated in terms of its calorific value i.e. the amount of heat that is produced with one cubic foot of gas is burned with air. The calorific value ranges from 450 to 500 B.T.U per cubic foot.
Because of its carbon monoxide content, town gas is highly poisonous, and because of its content of combustible gases it is highly explosive when mixed with air. If several cubic yards of gas are allowed to escape into a room, an explosive mixture will be formed, which can be ignited even by a tiny electric spark, for instance, be caused by a door bell or a telephone ringing.
Town gas is a fuel gas. The term coal gas is sometimes applied to it, denoting that it is produced mostly from coal. One way to produce this gas is to heat rough coal to a temperature of 1000° - 1200°C, out of contact of air, in a chamber called retort, which may be of the inclined type. In this process, up to about 500 cu. Ft. of town gas can be produced from 100 lb of dry coal with a low ash content. With remains of the coal after extraction of the gas is called coke.
In other gases mainly processes of the coal is not heated by the external application of heat, but is converted into gas by partial combustion with oxygen and chemical reaction, with water vapor. The crude gas produced must be carefully purified: in particular, it is necessary to remove volatile sulphur and nitrogen compounds. In many cases water gas or producer gas obtained in this way is used as an admixture to a gas. Fuel gases from oil refineries or natural gas are playing an increasingly important part in town gas supply. Despite its dangerous character, town gas is unlikely to entirely supersede by electricity in the foreseeable future.
Gas will be able to hold its own because of its relatively low cost and the convenience with which it can be distributed through pipes. At the same time, the large quantities of carbon dioxide and by no means inconsiderable quantities of sulphur dioxide, which are formed in the combustion of gas, add to the pollution of the atmosphere of our towns