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CBT 2007:Coal Mining Task Force (CMTF), Asea Pacific Partnership on Clean Coal Development & Climate, Indian Institute of Coal Management, Ranchi, August 22-24, 2007

Optimum Beneficiation of Global Coals

Dr Kalyan Sen, Emeritus Professor, Bengal Engineering and Science University, Howrah-3.

Abstract
The globalization of trade has opened up the possibility of importing low ash coals, both coking and non-coking varieties, from China, Indonesia, Australia, etc. Some of these coals are of different origin, and hence some of the basic coal characteristics are also significantly different.
The question comes up is how to utilize these indifferent types of coals with highest economical gains and ensuring acceptable useful properties.
Introduction
Indian coals are generally of high ash content and difficult to wash due to “drift” origin. Though the production of such coals is significant, their utilization is lacking due to absence of proper beneficiation and blending technologies.

Blending of raw high ash coals with low ash imported coals is not always recommendable as the total amount of inert materials remains the same and the resultant blend is hardly ever of acceptable consistent quality.
The paper deals with the optimization parameters which should be considered during optimum beneficiation of Indian and imported coals.
Washing Characteristics
Coal is a heterogeneous substance. Pure coal is lighter (sp. Gr. <1.30) and minerals generally associated with it are heavier (sp Gr >1.80). Generally raw coal is a heterogeneous mixture of particles having different proportions of pure coal and associated mineral matter. Thus, the sp. Gr. of individual particles vary from <1.30 to >2.00. A good coal is composed of significant amount of lighter particles as shown in the figure 1. An inferior coal has little amount of lighter particles but much higher proportion of intermediate particles known as middlings.

 

 

 

 

 

 

 

Fig.1

 

 

Fig 2 shows the resultant cumulative ash content of good coals (say Western Coals) and inferior Indian coals. The curve is much steep for Indian coals whereas moderately sloped for good western coals.

Fig.2.
Apparently it seems that it may not be possible to extract any washed coal of low ash from Indian source, as such yield will be less than 10% in the above example.
Optimization parameter

It has been proved by both theoretical treatise and practical experimentation that useful property of overall cleans does not depend on the cumulative ash but depend mainly on the maximum characteristic ash at the sp.Gr. of cut. It has also been proved that if the characteristic ash at the sp.Gr. of cut is restricted to 26 to 28%, the coking property of the inherently matured coking coal can be guaranteed (ref. CFRI, 1994).
Thus, if both the high ash Indian coals and the low ash imported coals are washed by restricting the characteristic ash from 26 to 28%, the overall washed coal amount will be maximum and at the same time it will assure the best coking property achievable from the given coals.
It is understood that the overall clean coal ash from individual Indian coal could be as high as 15 to 17% and the same for western coals could be 8 to 10%, but the overall coking and other reactive properties will be matching. This explains the general practice followed in both the countries and the universal acceptability of the theory of “equalization of Characteristic ash at the point of cut” as the optimization parameter.

Fig.3

Similarly, it has been proved that the maximum characteristic ash at the point of cut for combustion of non-coking power grade coals in Pulverized Fuel thermal power plants should be 50 to 55% (ref. CFRI,2001). It is also understood that the overall washed coal ash content can be 25% in case of imported coals and about 34% for Indian coals, but the limiting combustible property and the efficiency of combustion will be more or less comparable in the both the cases.
The value of the optimization parameter “characteristic ash” depends mainly on the maturity of coal, kinetics of the downstream process and should be determined on actual case study.