WATER GAS
693
Decrease in pressure and increase in temperature favour the formation of carbon monoxide. On the other hand, decrease in temperature and increase in pressure promote the formation of carbon dioxide. For all practical purposes it must be remembered that CO2 will be at a maximum when oxygen >is in excess of carbon. The importance of ensuring adequate air will be appreciated from a consideration öf the following thermochemical equations :■—
(1) C + O2 = CO2 + 14,600 B.Tli.U. per Ib. of carbon.
(6) 2C + Oa = 2 00 + 4,400 B.Th.U. „
The quantities of carbon monoxide and dioxide formed are to an extent deter-mined by the eqiülibrhim obtaining in the reversible system given below, but in technical Chemical processes reaction velocity is nearly always a more important factor than Chemical equilibrium:—
o + CO2^2CO.
In the above reaction equilibrium is reached with some rapidity from either side at temperatures above 930° C., but at temperatures below 800° C. the rate ot reaction is extremely low.
Temperature. Carbon monoxide. Carbon dioxide.
800° C 86-4 per cent. 13-6 per cent.
850° C 93-2 „ „ 6-8 „ „
900° C. 96-6 „ „ 34 „ „
925° C 97-5 „ „ 2-5 „
1,000° C 994 „ „ 0-6 „ „
1,100° C 99-8 „ „ 0-2 „ „
By the continuance of the “ blow ” for a definite period, usually rather more than half that of the gasmaking “ run,” the temperature of the bed and of the surrounding brickwork is restored. The part played by the substantial firebrick lining must not be overlooked. These walls, once they are heated through, form a reserve of heat which. is available to be drawn upon during the gasmaking period.
The main, reactions which. occur during the interaction between steam and coke have already been considered, but it is important to study thcir thermal nature as the principle upon. which water-gas plant is operated is dependent upon this. All three reactions are endothermic, as is shown by the following:—
(«) C 4-2H2O = CO2 + 2 H2 — 2,850 B.Th.U. per Ib. of carbon consumed.
(6)0 + H2O = CO + (H2—4,350
(c) CO2+ C =2 CO —5,850 „
The energy changes shown above, however, are based on the assumption. that the substances are reacting at zero temperature and finish, withthe products at that temperature. In the production of water gas, however, the reactions occur at high, temperatures.