GROSS CALORIFIC VALUE A GUIDE TO OPTIMUM WASHING

GROSS CALORIFIC VALUE –A GUIDE TO OPTIMUM WASHING OF POWER COALS Chandan Das, Senior Research Fellow and Dr Kalyan Sen, Prof. Emeritus Bengal Engineering & Science University, Shibpur 2 nd Asian Mining Congress, Kolkata, 17 -19 th Jan. 2008

Basic Issue q Requirement of power plants Washed coal with consistent quality q. Quality Parameters : GCV, Ash and Moisture. AR (as received) basis

Introduction • About 70% coal produced in India is consumed in thermal power plants. • Indian non-coking coals : ü ü Permian era, optimum maturity and petrological composition High Volatile Matter low Sulphur, Mercury, Nitrogen and Chloride, etc. Burn efficiently Ø Large amount of Stones and obvious dirt lead to loss of sensible heat Ø Erosion in coal handling, crushing, grinding, ash handling, etc.

Gross Calorific Value (GCV) • Gross calorific value of coal depends upon its chemical composition, which is governed by geological environment during its coalification process, mainly on its maturity. • Combustible elements present in coal : Carbon, Hydrogen, Sulphur, etc • Non-combustible mineral contents : Silica, Iron, Aluminum, Calcium, Magnesium, etc.

GCV-Ash Relationship

GCV – Ash Relationship • Close relation between ash content vs Gross Calorific Value (GCV) of coal For Central Coalfield (CC) coals : GCV at “zero” ash (pure) coal varies from 7000 to 8000 kcal/kg, av. being 7800 kcal/kg GCV becomes “zero” (completely inert) at very high ash content i. e. about 80 to 90%, av. being 88%. • It is also proved that both ash and GCV contents are related to the Relative Density (RD) of coal.

Yield & Quality of Washed Coal • Fractional yield is a random property – varies from seam to seam and day to day • Cumulative yield at any cut point depends upon the Limiting GCV vs sp gr of cut and Efficiency of Washers • If the Particle of Limiting GCV is combustible in a given furnace, then all other particles are ensured to burn

Optimum Result – an example • To limit GCV of particles to 3000 kcal/kg, the corresponding characteristic ash will be about 55% at RD of cut 1. 875. • The cumulative GCV of theoretical cleans will be 5200 kcal/kg and ash content 30% with cumulative yield of 85%.

Conclusion • Concept of limiting GCV can optimize coal washing • It is suggested that if the worst particle can be effectively burnt in a given system of combustion, all better quality particles can be used up at ease. • The actual cumulative yield of cleans and their quality will depend upon the raw coal supplied from day to day and efficiency of washing. However, the efficiency of combustion will remain practically the same. • GCV is suggested to be used as a major factor to grade the coal as well as to optimize