Conservation of energy resources:

Natural energy resources are limited and every attempt must be made to conserve them. One of the ways could be to substitute the lower quality fuel. The quality of the fuel can be judged by its capability to generate flame temperature. Quality of the fuel is the relative term. The fuel which produces higher adiabatic flame temperature on complete combustion with stoichiometric amount of air is of higher quality than a fuel which produces lower flame temperature. The following table gives the adiabatic flame temperature for some fuels (the readers can calculate the flame temperature as illustrated in lectures 12 and 13 of this course. Also you may consult the video lectures of the fuel,furnace and refractory course)

According to the above table it can be said that fuel oil is high quality fuel than producer gas. If it is possible to substitute the lower quality fuel to perform a high temperature operation, high quality fuel can be conserved. The question is to what extent this substitution is possible.  

In combustion of fuel, flame temperature of the products of combustion controls the furnace temperature. Whereas heat requirement of a process can be met by increasing the amount of fuel temperature requirement needs fuel of a particular quality for given combustion conditions.  This is the basis of calculation the percent substitution of lower quality fuel.