Now assume that the heat exchanger is  not used. Coal is burned with a mixture of cold air and oxygen. Excess of oxygen (air + pure oxygen combined) over theoretical for combustion will be kept same as with preheater. Calculate the amount of oxygen required in kg/hr to obtain the same fuel consumption as obtained with the heat exchanger

Pre-heater adds  sensible heat into furnace. When pre-heater is not used, nitrogen of air must be reduced so that heat taken out by nitrogen equal to

Let Y kg mol oxygen is required, this corresponds to 3.76Y of nitrogen

This problem illustrates the method of reduction of fuel by adopting technologies like preheating of air and oxygen enrichment of air. Both the technologies are in use to reduce the fuel consumption in industrial practice. Which technology suits has to be evaluated locally keeping in view the available resources and expertise.