Program for Computational Reactive Mechanics (PCRM)

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.

Indian coal is classified by grades⁸ 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. The ultimate analysis of coal used in power plants in India is readily not available. Ultimate analysis of D, E, and F grade coal is obtained as personal communication from Central Fuel Research Laboratory (CFRI), Jharkhand, India. Ultimate analysis of coal used at Dadri, Rihand, Singrauli, Chandrapur, and Dahanu power plants is obtained as personal communication from National Energy Technology Laboratory, Pittsburgh, USA. The Neyveli Lignite Corporation provided by personal communication, the ultimate analysis for lignite used at the Neyveli and Kutch power plants. These are presented in Tables 6A & 6B.

Table 6A: Elemental analysis, moisture content, and grades of typical Indian coals

Table 6B: Elemental analysis and moisture content of the coal used at the seven power plants in India

Importance of the quality of coal is illustrated by the comparison of coal used at Chandrapur Thermal Power Plant (India) and the Ohio (USA) coal. The ultimate analyses of the two coal types are given in Table 8. The Calorific value of Ohio coal is almost twice to that of Chandrapur coal. This means that roughly twice the Indian coal compared to Ohio coal is needed to generate same quantity of steam (electricity). Assuming 30% thermal conversion efficiency in converting thermal energy from coal to the electrical energy, coal used for generating a unit of electricity is (Kg/KWH) 0.77 for Indian coal and 0.36 for Ohio coal.

Table 8: Comparison (Ultimate analysis) of Chandrapur and Ohio coal