CALORIFIC VALUE-DEFINE

Calorific Value of a fuels:

     The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight ( or volume in the case of a sample of gaseous fuels ) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 degree K.

         It is usually expressed in Gross Calorific Value (GCV) or Higher Heating Value (HHV) and Net Calorific Value (NCV)or Lower Calorific Value (LHV). The difference being the latent heat of condensation of the water vapour produced during the combustion process. Gross calorific value assumes all vapour produced during the combustion process is fully condensed. Net calorific value assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value. The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal while calorific value of fuel oils are much more consistent.

   Energy content of the Indian Coal is expressed in "Useful Heating Value" (UHV)              basis Indian coal (non-coking) is classified by grades (A-G) defined on the basis of Useful Heat Value (UHV). UHV is an expression derived from ash and moisture contents for non-cocking coals as per the Government of India notification.

UHV is defined by the formula:UHV kcal/kg = (8900-138�[percentage of ash content +percentage of moisture content])

           In the case of coal having moisture less than 2% and volatile content less than 19%, the UHV shall be the value arrived as above, reduced by 150 kcal/kg for each 1% reduction in volatile content below 19% fraction pro-rata. Both moisture and ash shall be determined after equilibrating at 60% relative humidity and 40�C temperature as per relevant clauses of the Indian Standard Specification No. IS:1350-1959.

        The quality of coal depends upon its rank and grade. The coal rank arranged in an ascending order of carbon contents is Lignite → sub-bituminous coal → bituminous coal → anthracite

      Indian coal is of mostly sub-bituminous rank, followed by bituminous and lignite (brown coal). The ash content in Indian coal ranges from 35% to 50%.

     Chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analysis. The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon. Elemental or Ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen. The calorific value Q, of coal is the heat liberated by its complete combustion with oxygen. Q is a complex function of the elemental composition of the coal. Gross Calorific value Q is mostly determined by experimental measurements. A close estimate can be made with the
 Dulong formula:

 Q = (144.4 %[C])+(610.2 %[H])-(65.9 %[O])+(0.39 %[O]2) Q  

is given in kcal/kg or Btu/lb. Values of the elements C,H, and O, are calculated on a dry ash-free coal basis. 

Empirical Relationship of GCV, UHV, and NCV UHV: 

Useful heat value          = 8900 - 138(A+M) GCV.
Gross Calorific Value  = (UHV + 3645 -75.4 M)/1.466 NCV.
Net Calorific Value      = GCV - 10.02M UHV, GCV, NCV in kcal/kg,
                                            “A” is %age Ash; “M” is %age Moisture.