Calorific Value

Calorific vale is the potential enrgy contained in the fuel. The calculation of calorific value is illustrated in the following on the basis of gasification of 1 kg of carbon. The calorific value of gaseous fuel when air is used for gasification is  

 

 of C available in producer gas is 

That means  of calorific value of C represents sensible heat and heat losses.

Calorific value of producer gas when produced by gasifying carbon with a mixture of air+steam is

when standard state of the product is gas.

Calorific value of producer gas as expressed in % of  calorific value of

We observe that the gasification of C with a mixture of air and steam increases the calorific value of producer gas which is mainly due to addition of hydrogen.

The calorific value of producer gas obtained by a mixture of oxygen and steam is almost similar to that of air + steam. But the volume of producer gas is only 62% that of air + steam and 48% that of air only. Benefit of using steam is self evident.

Is it possible to decompose all steam to hydrogen and C to CO?

Typically, gas producer, operates in a counter-current mode i.e. coal is charged from top and a mixture of air and steam is blown through the coal bed simultaneously and continuously from the bottom. The flow rates of air and steam are adjusted so that the heat evolution in the reaction of  with C of coke/coal balances the heat absorption due to endothermic reaction of decomposition of steam. Thus

• Not all steam decomposes to  i.e. some amount of steam remains undecomposed and
• Not all C is converted to CO.

The extent of decomposition of steam to hydrogen and conversion of C to CO depends on temperature, residence time of the reactants in the reactor, reactivity of carbon of coal, reaction surface area and etc. The gaseous fuel produced after gasification with a moisture of air +s team and carbon will always contain undecomposed steam and carbon dioxide besides other components.

For further reading

1. A. Butts Metallurgical problems.
2. O.P.Gupta: elements of fuels, furnaces and refractories
3. S.C.Koria Video lecture on fuel,furnace and refractory, lecture no. 7
4. S.C.Koria Video lecture in the course on material and heat balance in metallurgical processes.