How thermal energy is obtained from fossil fuel?

All fossil fuel contain potential energy. On combustion potential energy is released in the products of combustion. The products of combustion exchange energy with the sink to raise its temperature to the required value and then exit the system. The sensible heat in POC at the critical process temperature is not available to the furnace. The higher the process critical temperature higher would be the sensible heat in POC. This sensible heat in POC is very important from the point of view of fuel utilization. We define gross available heat (GAH) as

GAH =  Calorific value of fuel + sensible heat of reactants − Heat carried by POC   1)

GAH represents the heat available at the critical process temperature; it may not represent heat available to perform a given function due to the various types of losses. GAH may be used as a criterion for comparing different fuel-combustion system.

Once the furnace is designed and built, the heat losses are not within the control of the operator; it is    governed by the process critical temperature, refractory lining thickness and thermal conductivity of the refractory. Defining net available heat (NAH) as

NAH = GAH − Heat Losses                                                                                         2

NAH can be used as a criterion for comparing the smelting/melting/heating efficiency of different furnaces.

Variables affecting heat utilization

For a given furnace design and the daily heat requirements, GAH is fixed and it is required to supply this much of heat on per day basis, we can calculate

                                                                   3)

If heat supply is the critical factor in determining the process throughput then GAH can not determine the throughput, we have to consider the NAH       

                                                     4)

Heat utilization or fuel utilization according to equation 5 is inversely proportional to GAH/kg of fuel. We can derive the factors affecting heat utilization by considering eq.1

Air adjustment: Calorific value (CV) of fuel is the energy obtained on complete combustion of fuel with theoretical amount of air. Excess air, air leakage, furnace draft, fuel/air ratio will control the fuel consumption

Sensible heat of reactant; this heat directly adds to the furnace, fuel consumption will decrease.

POC temperature: an increase in POC temperature will increase fuel consumptionIncomplete combustion or un-burnt fuel; corresponding to incomplete combustion part of the CV of the fuel is lost with the products of incomplete combustion