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Routes for Beneficiation of high ash Indian Coals

 

POSSIBLE ROUTES FOR BENEFICIATING HIGH ASH INDIAN COALS FOR THERMAL POWER STATION

N.S.Das, A. Chowdhury, S. Biswas, K.M.P. Singh and Kalyan Sen
Central Fuel Research Institute, Dhanbad 828108, India.
Abstract
The wide variations in nature and type of Indian coals formed during lower Gondawana period and categorized as difficult-to-wash due to its typical mineral matter association with the organic matrix call for suitable beneficiation techniques as the preferred option for quality upgradation.

The present paper addresses the major barriers in cleaning high ash Indian coals, discusses different approaches, the choice and limitations of the different process units for producing clean coal for power generation and suggests the need for identifying an optimum level of washing.
 
INTRODUCTION
With the share of over 60% in commercial energy sector, coal has become India’s energy security. The relative abundance of coal in India compared to other fossil fuels makes it a natural choice as the primary source of fuel, be it for steel making, power generation or for other uses. The reserves to production ratio at current rate of production for coal is around 230 years as against 15 and 23 years respectively for indigenous oil and natural gas, making domestic coal a very secure energy source for the country. As India’s urbanisation and industrialisation accelerate and more of the rural population has access to electricity, energy consumption is likely to be doubled in next 10 years and so will be the requirement of coal.

Indian Gondwana coals failed to hit complete coal-forming sequences in most of the basins. The depositions remained mostly as low rank sub-bituminous and non-coking type. The vast reserves of the non-coking coal in the country are usually high moisture, high ash, high volatile and sub-bituminous types (Ro% ranging between 0.45 to 0.65).

Typical characteristics of the coals dissimilar to that of other coals of Gondwana formation and Laurasian coals of carbonaceous origin impart wide heterogeneity and the resulting variations in chemical and petrographic makeup have direct effect on their utilisation potential. Due to the unique geological origin, Indian coals exhibit both favourable and adverse characteristics during their use as a fuel for power generation:

 

Favourable Qualities:	Constraints:
low sulfur content (< 0.6%) high ash fusion temperature (>11000C)   low iron content low chlorine content (< 0.1%) low toxic trace elements High reactive inertinites	Genetic: refractory nature of the coal high inertinite content high moisture content Operational: mechanised mining in large OCP Organisational: Multiple sources/ linkages

Most of the coals fed to the power plants are inertinite rich. Fortunately, considerable portions of the inertinites have good reactivity towards combustion. As a result, the coals when prepared by reducing the ash and moisture level to an extent comparable to European coals show better combustion characteristics.
 
With the advent of more and more open cast mechanised mining (accounts for 85-90% of total production of power grade coal), the quality of the coals are deteriorating gradually. Almost 80-85% of non-coking coals mined at present are of high ash content (35-45% or more). The average calorific value of raw coal has reduced significantly from an average of 5000 kcal/kg in 1970 to an average of 3000-3400 kcal/kg in 2003.The quality is further deteriorated due to the multiplicity of coal sources fed to the power plants.
 
Increased consumption of thermal coals to meet the projected demand of electricity in the foreseeing future calls for conscientious efforts and rational approach for the optimal utilization of this non-renewable asset in the entire coal–energy chain, right from mining to end-use. The overall improvement in the system to meet the quality and quantity requirements of the consumers in cost effective manner would involve improvement of the different sub-processes in the chain. Needless to say, processing non-coking coal/ power coal through beneficiation will come out as best economic option for achieving consistency in quality supply to TPS and for subsequent improvement in cleaner energy production.
Present Status of Coal Beneficiation in India
Coal washing becomes one of the action areas identified for achieving the overall efficiency of the above-mentioned energy chain and for reduction of GHG emission per kWh of electricity generated. As a timely measure, National Thermal Power Corporation(NTPC), the largest coal consumer in India has entered into an agreement with CCL to take washed coal from Piparwar washery (cap. 6.5 Mty) for its Dadri TPS. BSES has installed its own washery (cap.2.5 Mty) at Dipika mine (SECL) for its power plant; Bina deshaling plant (cap. 4.5 Mty) has been set by NCL to supply washed coal to Indraprastha TPS.  A few more plants like, Kalinga CPP (cap. 8.0 Mty) in Talcher coalfield is also coming up in near future. Coal India Limited has recently decided to convert some of the existing coking coal washeries of BCCL and CCL namely, Dugda-1 (2.4 Mty), Kargali (cap. 2.0Mty ) and Gidi (cap. 2.0 Mty) into power coal washeries.
Common barriers for beneficiation of Indian non-coking coal
The importance of coal beneficiation in the country in improving the overall efficiency and environmental performance of the coal-energy chain has been established beyond doubt. Even then, indigenous production of washed coal is far behind the quality and quantity requirement for end uses. As a matter of fact, there has been little progress in setting of coal beneficiation capacity because of the different barriers that act as impediments in making coal washing an integral component of coal-energy chain.
Economic barrier
From producers’ point of view:
Additional cost for washing.
Shortcoming in meeting the supply target at the desired level of quality parameters. This would require increase in the production capacity of raw coal.
Present system of grading and pricing of coal. As there is hardly any incentive for supplying coal of higher heat value covered in the same grade, the producers have no motivation to wash the lower grade coals.
Coal companies are not in a position to invest in coal preparation plants because of overall resource constraints – capital, manpower, land, water, etc.
From users’ point of view:
Reluctance of the consumers to enter into a commercially enforceable long-term agreement with the producer to pay the additional cost of washing. This additional cost of washing needs to be evaluated in terms of efficiency improvement in overall 'coal-energy chain'.
Lack of appreciation of the main benefits of washing, in the light of greater energy content and lower emission level per unit of energy of the clean coal.
Some power plants located far away from the coal sources and who were earlier considering washed coal as low cost option are now finding imported coal cheaper in terms of calories delivered. This applies particularly to the plants located near the coast
In general, State Electricity Boards (SEB) are not equipped to take the responsibility of setting up washeries.
Policy Barrier
Present regulatory framework relating to private investments in production, preparation and marketing of coal needs to be reviewed to make it investor friendly
Lack of the proper ambience for inviting Private Sectors involvement in washery set up.  A gamut of issues such as, model contracts for supply of coal to washery, railway linkage to supply washed coal, price structure of washed coal are some of the major impediments.
Imbalance due to existing duty structure on  imported coal and increase in price on domestic coal makes washing of Indian coals less competitive
The system of linkages
Technological barrier
High inherent ash content: The average quality of non-coking coal progressively deteriorated due to inferior grades of the coal reserves and the high degree of mechanised mining introduced in large OCP. Besides, majority of the coal seams are inter-banded and the bands within the same seam or between the seams are thin. These dirts are disseminated in all the size fractions of mined coal and the coal needs rigorous washing
Difficult washability characteristics of present day’s mined coal need deep beneficiation involving more cost.
High moisture content of washed coal: Majority of the Indian non-coking coal is having moisture content in the range of 6-10%. As moisture gets added in the wet beneficiation  process, this limits the choice of process/ technology of beneficiation
Loss of coal in washery rejects, depending upon the nature of coal, depth of  cleaning and the process of cleaning
No suitable techno-economic outlet of  utilizing the rejects of rather medium ash (in the range of 50-70% )
Disposal / utilisation of rejects: Rejects containing carbons can lead to spontaneous combustion in the heap. High ash level can lead to ground water contamination due to leaching etc.
Though blending of high ash domestic coal with low ash washed coal is an option for coal feed,  these could not be the long-term solution as proper blending facilities/ bunker is seldom found at the power plant site
Beneficiation of high ash Indian coals:
One of the technological barriers that have been identified is the non-availability of techno-economic washing circuit for coals of widely varying washability characteristics. After all, there can not be any single washing scheme that can be preached (practiced economically) to beneficiate all Indian coals to the levels desired by different consumers. The existing commercial practice is to beneficiate the coal to the extent of removing the obvious dirt and stones only, notwithstanding the fact that the product still preserves sufficient quantity of non-combustibles detrimental to the boilers.
Choice of washers
Owing to the close association and intermixing of the coal and non-combustibles, there is a little distinctive difference in the relative density of coaly shale and shaly coal and the separation process for Indian non-coking coal becomes difficult. Choice of washing circuit has to be made depending on the degree of intermixing and the separation criteria.

It is essential, therefore, to take utmost care of the constraints, before selection of washers, during planning and designing any circuit:

Widely fluctuating quality of ROM coal
High amount of NGM at cut density: a possible solution to overcome the misplacement due to high NGM at desired cut density is incorporation of highly efficient washing unit, though it involves slightly higher capital & operation cost.
Moisture in the raw coal and its effect in washed product: high moisture content in non-coking coal (8-10% even as high as 22% during monsoon) and its increase by 2-4% (above equilibrated moisture i.e. 6-10% at 60 % RH, 40 OC) by washing, generally limit the choice of process technology. Wet beneficiation increases the moisture content but can be regulated by proper dewatering processes. 
Frequent deterioration in the quality of feed from multiple sources: where the coal is coming from multiple sources, washing depends on the composite washability of feed coal. The cut density and amount of rejection has to be controlled from pre-simulated results of all the possible compositions. CFRI has worked out such user-friendly software for a few deshaling plants.
Non availability of modern state of art equipment for high ash coals.
Possible routes for beneficiation of Power coals (table 1)

Level	Scheme	Methodology	Scope of ash reduction	Cleans,%	Moist. over ROM coal
I	Dry benefciation	Separation of stone/shale depending on the difference in breakage potentiality	3-4%	>90	Nil
II	Crushing, screening & coarse coal cleaning	Coarse coal only; usually +13/30mm is washed in Jig & finer size by passed and mixed with washed product	<10%	70-90	2-3%
III	Crushing, screening and total cleaning	Coarse and intermediate sizes are washed at optimum efficiency levels, using HM washers/ improved Jig; fine coal by Spiral	10-15	50-85	3-4%

Table 1: Possible schemes of preparation of power coal
Level I
Dry beneficiation, though needs further technological and R&D backup, appears to be an economically acceptable process for improving the quality without increasing the moisture content. Particularly, where the raw coal quality deteriorates because of the admixture of overburden stones only and where there is a distinct difference in hardness between coal and non-coal impurity, ash reduction by 3-4% can be achieved by dry destoning the ROM lumps by Rotary Breaker at the mine site.

Level II
At this level of beneficiation, the finer fraction (generally of lower ash content), is allowed to bypass the washing circuit. Since most of the free dirt and banded materials are included in the coarser fraction, beneficiation of this fraction only and mixing of the washed product with the untreated smaller fraction gains acceptance for maintaining presently desired ash at 34% and total moisture level at 10-13%.

The availability of ROM Jig opens the possibility of using a simple and economic process of beneficiating ROM coal of size as large as 400mm at pit head. The savings in railway freight due to elimination of rejects to the tune of 10 to 15% at the pit head may compensat the washing cost. This type of Jig is being used presently at Bina, NCL for coals like in Tura North and Purewa seams of south Karanpura (feed size 250-30mm). The other non-coking coal washeries that follow this circuit is at Piparwar, WCL (BATAC Jig, feed size 100-13mm). In case stone in ROM coal are hard to crush to the Jig feed size (250-100mm), Rotary Breaker in combination with Jig may be advantageous in comparison to other crushers like Gyratory or Feeder Breakers.

Obvious constraint to level II beneficiation is the presence of the dirt and stones coming from the untreated finer fraction and increasing high ash and moisture content of the lower fraction in the coals that are received at the washeries. Earlier studies at CFRI on the washability characteristics of some non-coking coals showed that the amount of free dirt/ sand having relative density greater than 1.80-1.85 may be very high and about 20-45% of the finer fractions (5-10% of the whole coal) is included in the prepared coal. This enhances the wear and tear of the boiler tubes and auxiliary unit and indirectly enhances the emission.

Level III
Availability of more efficient and improved H.M. Cyclone circuit in the global market makes it possible for deep beneficiation of the ‘difficult-to-wash’ power coals less than 30% ash level. Bilaspur washery installed by BSES for its own power plants, though simplifies to level II washing scheme, has introduced large diameter HM cyclone for coarse fraction (50-13mm) and Spiral for fines (-2/3mm). It is claimed that 1000mm diameter Cyclone has the ability to provide high capacity (500-550tph), efficient separation (Ep <0.03) while treating coal as coarse as 100 to 1 mm.

Beneficiation of finer fractions (both intermediate and fines) is one of the pertinent points in opting between level I, II and III. The results of the study of a typical power coal are shown in figure II. It is observed that the practically achievable yield of washed coal by different levels of beneficiation (using different washers) varies significantly.

It is interesting to note that use of HM washer in either level II or level III has an achievable maximum limit of ash content (<31%) in the washed coal. Although this washer is effective in producing low ash (<25%) cleans with better yield, it is not desirable to use this circuit for producing washed coal of ash beyond 30-31%, as a jig will be a much simpler unit to operate.
It is evident that each level has its own range of choice depending upon raw coal characteristics and quality demand. A combination of different washing circuits may also be required for higher ash, 'difficult-to-wash' coal. Moreover, the washing circuit should have the optimum flexibility of either bypassing or washing a part of intermediate/ finer fractions.
Proposed strategy of beneficiation

In first two cases, the desired overall ash content and the level of moisture content are maintained but the quality of the washed coal in terms of its end utilization in power plants may not always be assured. It needs to be mentioned that restriction of ash level to 34% in washed coal is in no way linked to the optimum quality parameter of boiler feed, but is an outcome of the initiatives taken by MOEF to reduce the existing level of pollution, the load in transportation, etc. The limit to which a coal needs washing or preparation has to be justified not from the overall qualities like ash content, etc.but from the specific quality parameters,like 'characteristic' ash, calorific value, etc., that have relation with the total combustible reactives and therefore, may govern the actual combustion behaviour of the prepared coal.

The concept of characteristic ash (l) which has been established previously as an optimum quality parameter for coking coal washing may be applied to select the specific gravity of cut in power coal beneficiation and to predict the combustion quality improvement thereby. The characteristic ash content (precisely, the elementary ash of the desired specific gravity fraction in F/S analysis) is one of the inherent properties of coal and washing a coal at a specific gravity determined from specified l value will minimize the presence of particles having ash values beyond this limit. From the detailed studies made by CFRI on Indian non-coking coals and their behaviour to ignition/combustion, it is proposed that the level of washing should be limited to that density of cut at which the washed product is devoid of particles of ash higher than 50-55%. The ash vs. relative density lines (figure II) for the coal samples studied show a working range of 1.70 to 1.85 as cut density for washing these coals while restricting the maximum ash of the particles in the clean product to the limit of 50-55%.
Conclusion 
Beneficiation of Indian non-coking coals is likely to become an integral component for coal based power generation in the future. Considering the wide heterogeneity of the washability characteristics of non-coking coals of different origin, no single washing scheme can be suggested. In order to deal efficiently with the changed feed characteristics, the existing washeries are required to be suitably modified/ modernised to optimise plant performance in terms of capacity utilisation and yield of clean coal of better quality.

The point that needs to be stressed is that the beneficiation strategy should be evolved not on the basis of reducing the overall ash and moisture content to a particular level but should be addressed by linking the level of washing with the performance of the coal in boilers. The quality parameters one needs to consider is the presence of requisite amount of reactive combustibles with a limit on the maximum ash content of the coal particles in cleans to ensure its better performance in boilers.

 

 

Reference

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