Great importance must be given during the transportation, receipt and storage of coal to ensure that its heating value is preserved and there is no deterioration on account of moisture addition enroute to the power plant or in the coal yard storage prior to its entry into the furnace of the boilers. In the case of imported coals higher moisture coals are cheaper and the increased generation cost due to moisture is offset by the cost advantage. The same is not true for Indian coals where there is no provision for cost accounting of total moisture except for upper limits.
Coal (popularly known as black diamond) is the primary energy source of the thermal power stations (TPS) which is the back bone of the Indian power sector. The installed capacity of the country is ~250 GW out of which ~140 GW is the share of coal based power generation (~57%). The power generation growth rate represented by compound annual growth rate (CAGR) is 8%.
Coal is contributing to ~1.5% of the GDP as it is the main energy source for power generation with a reserves of 275 billion tonnes (production capacity over 10 million t/y and up to a depth of 600 m) of which nearly 115 billion tonnes are proven. Coal will continue to dominate the electrical energy generation scenario for the next 20-30 years. Most of the indigenous coal is from government owned mines which account for 90% of the indigenous production. Out of this 88% is mined through open cast processes. Shaft mining is restricted to only high quality coals.
Coal follows a long route from the time it is mined till it is ultimately combusted in the utility boilers. The mechanism of supply of coal to the power plants is through the fuel linkage system based on fuel supply agreement between the colliery and the power utility. Nearly 60% of the coal is transferred from the mine to the power plant through Indian railways, 25% through trucks & the balance through dedicated transfer systems such as merry-go-round-systems, etc.
Coals and most other solid fuels being of variable heating value are priced based on the product of the quantity (tonnes) and the quality (gross heating value in kcal/kg).
Indian coals being of drift origin are of high ash (25-50%) with gross calorific values (GCV) in the range of 2300-4500 kcal/kg. Sulphur (<0.6%) is not a problem except in very few specific mines. Coals & most other solid fuels being of variable heating value are priced based on the product of the quantity (tonnes) and the quality (gross heating value-GCV in kcal/kg). For the purpose of computing quantities, the average coal GCV of indigenous coal is taken as 3500 kcal/kg and that of imported coal is taken as 6500 kcal/kg.
With the import of high GCV coal to sustain power generation on the rise, energy efficient utilization of coal resources is essential. Efficient use of coal calls for effective transfer, storage, monitoring and management to ensure that there are minimal losses in quantity or quality in the process of transfer from the mine to the boilers. Coal utilization efficiency (before it is used in the boilers,i.e., from mills till bunkers) is in the range of 80-98%.
Figure 1 shows the gradual rise in cost (FOB (freight on board)) of imported coal over the year.
Fig. 1: Rise in cost of imported coal
The main non chemically reactive ingredients in coal which result in the drop in GCV are ash and moisture.
Coal Quality- ASH & Total Moisture Content In Coal
Ash is technically the solid residual end product of combustion of coal and is realized only on combustion since it is an integral part of the overall coal lump. But due to the open cast mining process, besides the inherent ash the extraneous mineral matter (clay, sand, and stones generally referred as mud) also gets mixed up with the coal. This extraneous mineral matter is called as extraneous ash. Even though scientifically it is not ash, techno commercially it is called as extraneous ash because it is an incombustible component. Thus ash implies inherent ash which is the product of combustion & externally mixed earth. Extraneous mineral matter can be removed through washing processes typically the run of mine jig wet washing process. Inherent ash cannot be removed except by complex and cost chemical methods in small sample sizes at the laboratory scale.
Moisture in coal consists of inherent moisture (IM) and surface moisture (SM). Then total moisture (TM) is a sum of IM and SM. Inherent moisture is moisture which is an integral part of the coal seam in its natural state, including water in pores but excluding that in macroscopically visible fractures. Equilibrated moisture (in chemically equilibrated condition) or chemical moisture is taken as inherent moisture though it can be different for low grade coals. As per IS:1350 Part I – 1984) for moisture Equilibrated Moisture means the moisture content, as determined after equilibrating at 60% relative humidity (RH) & 40OC as per the relevant provisions (relating to determination of equilibrated moisture at 60% RH and 40OC) of BIS 1350 of 1959).
Surface moisture is the difference between total moisture and inherent moisture and is also called as excess moisture (EM).
TM= IM + SM
Total Moisture implies the total moisture content (including surface moisture) expressed as percentage present in Coal and determined on as-delivered basis. IM or equilibrated moisture is not in our control as it is governed by the thermodynamics of liquid-vapour equilibrium. SM is an added quantity and can vary in any range. Hence TM is affected by the criticality of SM. This brings down the GCV of coal (thermal content of coal) which reduces the output it delivers, reduced boiler efficiency and unit overall efficiency. Also, wet coal is difficult to handle & its movement in conveyors, chutes, hoppers, bunkers and pipes is considerably hindered making its grinding, milling and flow into the boiler very difficult.
As the coal quality decreases (ash and moisture increase) the cost of coal (Rs./Gcal) is likely to decrease as indicated in Table 1. However, the cost of energy generation increases as the boiler efficiency and hence the unit efficiency decreases (unit heat rate increases). Hence on the overall, there is trade off between increased cost of generation and decreased cost of coal leading to lower overall cost of energy produced.
Coals as mined are classified on the basis of the sum total of ash and moisture in equilibrium as in Figure 2.
The coal payments for indigenous collieries are being made on the basis of equilibrated moisture (inherent moisture at 60% RH & 40OC).
Fig. 2: Classification of coals
Effect of Moisture In Coal
Effect on heating value of coal
Figure 3 gives the decrease in GCV with moisture for a sample Indian coal of GCV of 2,000 to 7,000 kcal/kg. Figure 4 gives the drop in GCV of coal for 1% moisture increase.
Fig. 3: Decrease in GCV of coal with moisture
Fig. 4: Drop in GCV of coal for 1% moisture increase
Effect on coal movement and handling in the coal yard
While internal moisture affects the coal combustion process, external (mechanical) moisture gives rise to difficulties in handling (transfer and flow ability) of coal with severe capacity reduction of all equipment in the coal plant ranging from crushers to conveyors. External moisture also creates combustion difficulties by creating thermal lag during the combustion process.
Units tripping on mill choke up, load hunting due to insufficient flow from bunkers, raw coal feeder jam, etc, are quite common during this period.
Even though the bunker level may be full, only 30% of the bunker capacity can be utilized due to bonding of coal at the bunker periphery and flow is only through rat hole in the bunker centre. When there is a choke up, the procedure is usually to remove the blockage by poking through the bottom opening. Air blasters are sometimes being used. If the level of coal is over 30-40%, a through hole cannot be established to remove the choke up. The bunker level under this condition needs to be filled continuously to the optimal level of 30% to 50% depending on the coal wetness and risk of choke up. Full filling of the bunker can be resorted to only when there is no risk of choke up. Choke up on full level can be quite difficult to release.
Rainy season restricts the plant load ability due to the movement of sticky coal which contains clayey mineral matter. Retardation of coal flow through the systems results in capacity reduction. When the surface moisture of coal exceeds 6%, it becomes sticky in addition to the stickiness created by the clay content of the mineral matter leading to severe capacity restriction in the tipplers, conveyors, crushers, bunkers and mills. The effective flow able coal through bunkers gets restricted to only 20% of the bunker volume in its centre.
The effect of moisture on bulk density of coal is given in Figure 5 for various coal finenesses (% passing through 200 mesh or 75 μm).
Fig. 5: Effect of coal fineness (% through 75 microns) on the bulk density of coal
The stations need to gear up to the demands of the rainy season through several measures such as the following:
- Stocking of sufficient coals of sandy background which do not have serious sticky properties as compared to coals of clayey background.
- Use of washed coals of sandy background.
- Blending of raw coal (GCV=14.5 GJ/kg) with washed coals (GCV=17.5 GJ/kg) or imported coals (GCV=21 GJ/kg).
- Optimal (partial) filling of bunker levels.
Some of the solutions for wet coal handling are:
Management of coal yard
Management of coal yard
- Rain guards for conveyors
- Tarpaulins to cover wagons
- Providing slopes for drainage of water
- Concreting of storage yards and providing retaining walls'
- Rain water channeling, dredging and cleaning of flow passages
- Compacting by special compactors instead of bull dozers.
- Storage pile design improvement through compacting. Pyramidal shapes with drains on either side lead to low water absorption.
- Further the piles must not have surface depressions or pits.
- Used oil may be sprayed on coal yard instead of reselling. Alternatively it can be blended with fuel oil.
- Dome for storage of coal
- Provision for ground level tippling (non-pit type) of wagons.
Management of conveyors
- Increased conveyor angles
- Multi bladed cleaners
- Reduction in belt speeds
- Skirt board seals, baffle plates and centering plates at loading points
- Self cleaning screening system
- Well designed wash down drainage system
- Management of carry over return
- Conveyor belt sealing between chute and pan of vibratory feeder to prevent spillage.
Management of chutes and bunkers
- Deflector plates of Stainless steel (SS 304) to chutes
- Vibratory feeders/thumpers/rappers in place of static feeders
- Air blasters
- Chute modification to increase angle
- Widening of passages
- Water jet cleaning.
Many of the solutions described above are add-ons or modifications (to the already supplied coal handling and conveying equipment) done at the level of the power station. The coal handling and conveying technology needs to viewed holistically and specific products for handling wet coal need to be designed as the rainy season in India lasts for almost one third of the year in several regions. Figure 6 shows the bonding of wet coal with clayey mineral background.
Fig. 6: Bonding of high moisture coal in a coal yard
Basis for Sale of Coal
Indian collieries
Figures 7 & 8 show the experimental correlation between total moisture and surface moisture with inherent moisture in Indian coals mined in India. It can be seen that there is SM of 4-7% in Indian coals.
In the case of indigenous coals, the heating value for commercial purposes is based on equilibrated moisture which is equivalent to inherent moisture and the total moisture does not get reflected in the commercial heating value. In other words, surface moisture does not get accounted in the costing. The basis for payment at the collieries is the GCV on the basis of equilibrated moisture and the GCV drop due to surface moisture does not figure. The actual heating value of coal received for power generation will be lower than the commercial heating value as indicated in the graphs on equilibrated moisture and total moisture.
This matter must be taken up by the thermal power plants with the coal authorities. Hence, the realistic basis for payment would be the total moisture at the mining point. Addition of surface moisture enroute to the thermal power plant or moisture addition in the coal yard of the power plant must be to the account of the user.
Fig. 7: Correlation between surface moisture and equilibrated moisture in mined coal
Fig. 8: Correlation between total moisture and equilibrated moisture in mined coal
Imported coal
In the case of imported coal the basis for payment is defined on the basis of either equilibrated moisture or total moisture as per the agreement. The cost of imported coal decreases with increase in total moisture. Figure 9 gives the drop in GCV due to increase in total moisture of imported coals.
Fig. 9: Drop in GCV due to increase in total moisture of imported coals
Effects of Moisture in Coal on Power Station Performance
There are three cost effects of moisture in coal:
- Increase operation costs due to decreased boiler efficiency (Fig. 10) & decreased overall unit efficiency (increase in heat rate) (Fig. 11).
- Increase in operation and maintenance costs attributed to handling of wet coal.
- Decrease purchase cost of coal due to higher moisture and hence lower GCV.
The boiler efficiency decreases due to increase in moisture and the unit heat rate increases. This results in increased cost of generation.
Fig. 10: Decrease in boiler efficiency due to increased moisture content
Fig. 11: Increase in unit heat rate due to increased moisture
Cost Sensitivity of Moisture in Coal
Indian coal
The fuel supply agreements for Indian coals do not have any provision for accounting the effect of total moisture. Only equilibrated moisture (IM) gets factored in the pricing. The surface moisture and hence the total moisture (T M) does not get factored into the agreement. The only relief for indigenous coal users is that in the event that monthly weighted average surface moisture in coal exceeds 7% during the months from October to May and 9% during the months from June to September, the coal quantities delivered to the power plants will be adjusted for the resultant excess surface moisture, which shall be calculated in percentage by which the surface moisture exceeds the foregoing limits. This corresponds to a TM of approximately 12% in summer & 14% in rainy season which rarely happens. Hence, it can be said that the surface moisture effect is virtually not factored in the cost calculations. On this account Indian coal costs do not show sensitivity to total moisture as indicated in Figures 12 and 13.
Fig. 12: Sensitivity of Indian coal price (Rs./t) to total moisture
Fig. 13: Sensitivity of Indian coal price (Rs./Gcal) to total moisture
However, if the moisture effect is considered the price should decrease as given in Figures 14 and 15.
The coal pricing should be on the basis of TM as it gives a realistic picture of the energy content in the coal available for end use.
Fig. 14: Sensitivity of Indian coal price (Rs./t) to total moisture if the moisture effect is considered
Fig. 15: Sensitivity of Indian coal price (Rs./Gcal) to total moisture if moisture effect is considered
Imported coal
Figures 16 & 17 give the cost sensitivity of Imported coals to moisture in terms of Rs./t and Rs./Gcal.
The cost sensitivity of moisture in coal to generation cost and fuel cost component.
It is clearly seen that the cost impact due to actual decrease in energy efficiency is very small (Rs. 0.01 to 0.015/kWh) as compared to reduced fuel purchase cost component of generation cost (Rs. 0.36/kWh) because as the TM increases the price of coals decrease.
Fig. 16: Cost sensitivity of Imported coals to moisture
Fig. 17: Cost sensitivity of Imported coals to moisture
Conclusions
- Major capacity addition has been based on assumed coal supplies from indigenous sources. The decreasing quality as well as difficulties in mobilization for a CAGR of 8% has resulted in turning to imports (15-20%) for supplementing of the primary fuel requirements which is a good short term measure.
- Moisture in coal has a negative impact on the energy performance and all efforts are required from the mine till the coal is fired into the boilers, to ensure that moisture does not get added to the coal and its heating value is preserved.
- If the boilers are designed for operating on high moisture coals, the high moisture imported coals can be successfully fired in an economical fashion as the open market price of coals with higher moisture will be lower than coals with lower moisture. The overall cost of generation will be lower for coals with higher moisture contents. The increased generation cost due to lower boiler efficiency & unit overall efficiency (Rs. 0.015/kWh) will be completely offset by the component due to decreased purchase price of higher moisture coals (Rs. 0.38/kWh). Therefore, for imported coals the economics is in favour of operating on higher moisture coals based on the coal pricing.
- In the case of indigenous coals, the heating value for commercial purposes is based on equilibrated moisture which is equivalent to inherent moisture and the total moisture does not get reflected in the commercial heating value. In other words, surface moisture does not get accounted in the costing. The actual heating value of coal received for power generation will be lower than the commercial heating value as indicated in the graphs on equilibrated moisture and total moisture. This matter must be taken up by the thermal power plants with the coal authorities. Hence, the realistic basis for payment would be the total moisture at the mining point.
- However, in the case of both imported coals and indigenous coals, addition of surface moisture or moisture addition/deterioration in heating value in the coal yard of the power plant is to the account of the user and must be minimized. Besides just the heating value the difficulties in flow ability, crushing, pulverizing, and injection of coal into the furnace of the boiler present.
M Siddhartha Bhatt & N Rajkumar are from CPRI, Bangalore.
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