Effect of Moisture in Coal on Station for Indian Thermal Power Plants

The best way to prevent coal from moisture in power plant is to use Alcox space frame structure

Indian coals inherently contain low inherent moisture (IM) (5.5-9.5 % with an average of around 7.5 %) and moisture addition is through the surface moisture (SM) due to sources external to the coal. On the other hand imported coals contain very high IM (15-25 %) but negligible SM. Presence of surface moisture (SM) is a major liability to the power generating process and its control needs to be understood on a broader national perspective. Great importance must be given during the transportation, handling, transfer and storage of coal to ensure that its heating value is preserved and there is no deterioration on account of SM addition enroute to the power plant or in the coal yard storage prior to its entry into the furnace of the boilers. It is the SM which affects the power plant operations. Total moisture and inherent moisture in coal analyzed to make further progress. In the case of imported coals, higher total moisture (TM) coals (higher inherent moisture and almost no SM) are cheaper and the marginally increased generation cost due to IM is offset by the cheaper purchase price. The same is not true for Indian coals where there is no provision for cost accounting of SM except for upper limits. The impact of 1% TM (without considering the weight effect) is -4.2 kcal/kWh on design unit heat rate, -8.2 kcal/kWh on operating unit heat rate, -0.0226 p.u. of plant load on Station load, -4.0% on Plant load factor, +0.2263% of plant load on Auxiliary power, +0.0079 kg/kWh on Specific coal consumption, +1.1426 ml/kWh on Specific fuel oil consumption and +15.21 kcal/kWh on Station heat rate.

+0.0079 kg/kWh on Specific coal consumption,+1.1426 ml/kWh on Specific fuel oil consumption and

+15.21 kcal/kWh on Station heat rate.

1.0 INTRODUCTION

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 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. By 2016, the percentage of coal requirement for power generation in India is likely to go to 82% of the total coal production (520 million tonnes/year) and the import is likely to go to 38% (200 million tonnes/year). The increase in coal Consumption is not only because of new capacity addition but also because of deterioration of coal quality (in terms of its heating value) hence the effect of moisture content in coal should be calculated to minimize its effect.

Coal follows a long route from the time it is mined till it is ultimately combusted in the utility boilers. The commercial 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 and the balance through dedicated transfer systems such as merry-go-round-systems, etc.

Coals and most other solid fuels being of variable heating values are priced based on the product of the quantity (tonnes) and the quality (gross heating value in kcal/kg).

Indian coals for power generation applications are 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 and most other solid fuels being of variable heating value are priced based on the product of the Quantity (tones) and the quality (gross heating value-GCV in kcal/kg). The largest coal producer in India is Coal India which is selling coals based on 17 grades [G(n)] with a grade bandwidth of 300 kcal/kg as follows:

                     ….(1)

For the purpose of computing bulk 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

The main non-chemically reactive ingredients in coal which result in the drop in GCV are ash and moisture.

2.0 TOTAL MOISTURE IN COAL

Moisture in coal consists of inherent moisture (IM) and surface moisture (SM). The total moisture (TM) is a sum of IM and SM. IM 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 coal (in chemically equilibrated condition) (EM) or chemical moisture is taken as IM though it can be different for low grade coals. As per IS:1350 Part I (2000) [1] for moisture EM means the moisture as determined after equilibrating at 60% relative humidity (RH) and 40 ºC as per the provisions (relating to determination of equilibrated moisture in coal at 60% RH and 40 ºC) of BIS 1350

.. (2)

SM is the difference between TM and IM and is also called as excess moisture. SM is the physically visible moisture exterior to the coal and is seen as wetness or common man’s perception of moisture. IM is not visible to the eye. A coal or lignite with very high IM but no SM can still be bone dry when it is seen or touched. Wet coal generally implies a coal with high SM manifested as physically visible wetness.

TM implies the total moisture content (including SM) expressed as percentage of coal and determined on as-delivered basis. IM or EM is not in our control as it is governed by the thermodynamics of liquid-vapour equilibrium and the chemical processes which has led to the coal formation. 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 and its movement in conveyors, chutes, hoppers, bunkers and pipes is considerably hindered making its grinding, milling and flow into the boiler very difficult.

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 (IM at 60 % RH and 40 ºC).

Generally Indian coals are low in TM (6-10 %) and the high moisture in Indian coals is due to physical addition of SM during the process or mining, transfer or handling. On the other hand imported coals contain high TM as high as 25-40 % but most of it is in the form of IM and there is virtually no SM. Therefore the coals are absolutely dry to handle even though they contain high TM just like lignite. Imported coals do not present any difficulty in pre-combustion processing like crushing, transfer, etc. and the loadability of the generating units are not affected due to transfer and flow related issues outside the boilers

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.

The drop in GCV (kcal/kg) of coal for 1 % increase in TM is given by,

….(3)

….(4)

Figures 5 and 6 show the experimental correlation between TM and SM with IM in Indian coals mined in India. It can be seen that there is SM of 4-7 % in Indian coal.

14(%)12y = -0.827x + 11.9210Moisture86Suraface42055.566.57.588.5Equilibriated Moisture (%)
FIG. 5 CORRELATION BETWEEN SM AND EQUILIBRATED MOISTURE IN MINED COAL.

2018(%)16Moisture14Total1210y = 0.173x + 11.92855.566.577.588.599.510
Equilibriated Moisture (%)

FIG. 6 CORRELATION BETWEEN TM AND EQUILIBRATED MOISTURE IN MINED COAL.

According to a technical study by NTPC, R & D, the maximum drop in GCV of coal in a coal yard is around 600 kcal/kg in an year [2]. The drop of GCV in the coal yard according to some studies [3,4] is in-between 1.4 %/year (70 kcal/kg/year) in winter like weather and 2.1 %/year (105 kcal/ kg/year) in summer like weather. In majority of the cases, the number of days of storage is varying from 9 to 30 and hence the drop in GCV should not exceed a value of 150 kcal/kg.

The GCV of both receipt coal (receiving end coal at the thermal station) as well as bunkered coal (coal fed into the boilers) can be either represented as either Air Dried Basis (ADB) which can be treated as EM or IM basis where the GCV is determined by drying the sample in air under equilibrated conditions, or as Air fired basis (AFB) wherein the moisture effect is subtracted. The experimental different between the two is around 280-350 kcal/kg with an average difference of 315 kcal/kg which represents the effect of excessive SM (~5.6 to 7.0 %). The experimental correlation between the two is [5]:

….(5)

In all experimental bombs calorimetric it is only GCV (ADB) which is determined. Based on the initial SM prior to drying the GCV (AFB) is computed based on the experimental correlation between GCV, ash and ΔTM. The magnitude of the difference between the receipt coal GCV and bunkered coal GCV should be within 150 kcal/ kg of the receipt coal GCV implying that the SM should be contained to within 3 %. The drop or difference between the receipt coal GCV and the bunkered coal GCV indicates the drop in GCV at the coal handling plant of the TPS which is predominantly due to addition of SM. Cost wise it would tantamount to higher cost of coal (Rs./ Gcal).

EFFECTS OF MOISTURE IN COAL ON POWER STATION PERFORMANCE

1. Performance of the power block (unit performance)
   

The effects of moisture in coal on the power block (boiler-turbine-generator unit) are decreased in boiler efficiency (Figure 7), decreased overall unit efficiency (increase in heat rate) (Figure 8) and decrease in unit load.

The boiler efficiency decreases due to increase in moisture and the unit heat rate increases. This results in increased cost of generation. 

The sensitivity of boiler efficiency and unit heat rate to TM without considering the weight effect is given in Table 1. The effect of weight of TM on the GCV has not been considered, i.e., if the TM is increased by 10 % the weight of coal will be only 90 %. If the coal of GCV of 3500 kcal/kg has an increase of TM of 10 %, then the weight of coal will be 90 % or the GCV will be lower by 350 kcal/kg by virtue of the weight. This effect has not been considered here because once the coal enters the premises of the thermal power plant, then the SM addition will only increase in weight addition to the extent of the addition of external moisture and does not decrease the GCV. For computation of specific fuel consumption (SFC) the original weight only will be considered and the added new weight is general not considered.

According to one study [6] the impact of 1 % increase of moisture on boiler efficiency is 0.2

% for coals and 0.27 % for lignite’s and on unit heat rate it is 0.43 % which comes to around 10-11 kcal/kWh

TABLE 1

SENSITIVITY OF BOILER EFFICIENCY AND UNIT HEAT RATE TO TM WITHOUT CONSIDERING THE WEIGHT EFFECT

Sl.	Particular	Value
No.
01	Boiler efficiency	-0.123
	(design) % per 1 % inc. in TM
02	Boiler efficiency	-0.270
	(operating) (same unit)
03	Unit heat rate (design)	4.2
	kcal/kWh per 1 % inc. in TM
04	Unit heat rate (operating)	8.2
	(same unit)

3.2 Effect of coal movement and handling

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

and flow is only through rat hole in the bunker center. 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 SM 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 9 for various coal fine nesses (% passing through 200 mesh or 75 μm) (source: US

Bureau of Mines) [7].

FIG. 9 EFFECT OF COAL FINENESS (% THROUGH

75 MICRONS) ON THE BULK DENSITY OF COAL

(SOURCE: US BUREAU OF MINES STUDY ON COAL)

The stations need to gear up to the demands of the rainy season through several measures [8] such as the following:

yStocking of sufficient coals of sandybackground which do not have serious sticky properties as compared to coals of clayey background.

y Use of washed coals of sandy background.

y Blending of raw coal (GCV=~3500 kcal/kg)with washed coals (GCV=~4200 kcal/kg) or imported coals (GCV=~5000 kcal/kg).

yOptimal (partial) filling of bunker levels.

Some of the solutions for wet coal handling are [8]:

• 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 coaly 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 speed
• Skirt board seals, baffle plates and 3.3 Effect of TM on station performance
  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
  

C. 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 10 shows the bonding of wet coal with clayey mineral background.

Moisture effects have been studied in the form of dips in the station performance during monsoon months [9,10]. While these studies focused on month wise performance they did not exclusively focus on TM or SM. In the present study the effect of SM leading to increased TM for Indian coals have been presented.

This is due to combined effect of the effect of moisture on the power block as well as the coal movement and handling. During monsoon months when there is severe rainfall SM addition to coal happens at mining site, loading end and enroute from the mine as well as in-plant during the movement from the wagon tipplers till the bunkers. The combined effect results in dips in the station load, increased fuel oil consumption, increased heat rate and increased station auxiliary power. These effects are given in the following figures.

Table 3 gives the sensitivity of station performance parameters to increase in TM. The weight effect is not considered here because the SM addition is after the coal is initially weighed. When the TM is increased by 10 %, the coal weight will be reduced to 90 % thereby the GCV will be lower on a weight basis. If the original GCV is 3500 kcal/kg the new GCV will be reduced by 350 kcal/kg. This basis is not considered because SM addition will result in addition of weight of the original coal and does not physically reduce the total heat content.

FIG. 10 BONDING OF HIGH MOISTURE COAL IN A COAL YARD.

FIG. 11 SENSITIVITY OF STATION INSTANTANEOUS

LOAD TO TM.

FIG. 12 SENSITIVITY OF STATION AUXILIARY POWER

TO TM

FIG. 13 SENSITIVITY OF STATION MONTHLY PLF TO

TM.

FIG. 14 SENSITIVITY OF STATION MONTHLY

SPECIFIC FUEL CONSUMPTION TO TM.

FIG. 15 SENSITIVITY OF STATION MONTHLY

SECONDCARY FUEL OIL CONSUMPTION TO TM

FIG. 16 SENSITIVITY OF STATION MONTHLY HEAT

RATE TO TM.

The sensitivity indices of station performance parameters to TM without considering the weight effect are given in Table 2.

782

TABLE 2

THE SENSITIVITY INDICES OF STATION

PERFORMANCE PARAMETERS

1. COST SENSITIVITY OF MOISTURE CONTENT IN COAL
   

1. Indian coal
   

In the case of indigenous coals, the heating value for commercial purposes is based on equilibrated moisture which is equivalent to IM and the TM does not get reflected in the commercial heating value. In other words, SM 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 SM 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 TM.

The realistic basis for payment would be the TM at the mine loading end (sending end). Addition of SM enroute to the thermal power plant or moisture addition in the coal yard of the power plant is to the account of the user.

The fuel supply agreements for Indian coals do not have provision for accounting the effect of SM. Only equilibrated moisture (IM) gets factored in the pricing. The SM and hence the TM does not get factored into the agreement. The only relief for indigenous coal users is that in the event that monthly weighted average SM 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 SM, which shall be calculated in percentage by which the SM exceeds the foregoing limits. This corresponds to a TM of approximately 12 % in summer and 14 % in rainy season which rarely happens. Hence it can be said that the SM effect is virtually not factored in the cost calculations. On this account Indian coal costs do not show sensitivity to TM as indicated in Figures 17 and 18.

FIG. 17 SENSITIVITY OF INDIAN COAL PRICE (RS./T)

TO TM

FIG. 18 SENSITIVITY OF INDIAN COAL PRICE (RS./

GCAL) TO TM.

The coal pricing should be on the basis of TM at the mine loading end (sending end) as it gives a true picture of the energy content in the coal purchased.

Based on the coal prices cited above, there is an increase in generating cost by Rs. 0.198 /kWh due to increased TM by 10 % over the normal value and an additional increase in cost of fuel oil and auxiliary power of Rs. 0.10/kWh giving an increased cost of generation by Rs. 0.298/kWh at the station level.

4.2 Imported coal

Imported coals contain high TM by virtue of high

IM but very low SM. Hence handling difficulties do not arise. In the case of imported coal the basis for payment is defined on the basis of either equilibrated moisture or TM as per the agreement. The cost of imported coal decreases with increase in TM or increase in IM.

Figures 19 and 20 give the cost sensitivity of Imported coals to moisture in terms of Rs./t and Rs./Gcal.

FIG. 19 COST SENSITIVITY OF IMPORTED COALS TO TM

FIG. 20 COST SENSITIVITY OF IMPORTED COALS TO TM.

Table 3: Cost sensitivity of imported coals to increased cost due to increased unit heat rate and decreased purchase cost by increase of TM by 10 % over the normal value.

TABLE 3

COST SENSITIVITY OF IMPORTED COALS

It is seen that the cost impact due to 10 % increase in TM over the normal value actual is a Rs. 0.11/ kWh for increased heat rate. This is very small as compared to the reduced fuel purchase cost component of the generation cost (Rs. 0.36/kWh) (for similar 10 % increase in TM over the normal value) because as the TM increases the price of coals decrease.

5.0 CONCLUSIONS

Looking at the above advantages, there will be huge savings on the operational cost as well substantial increase in power generation due to improvement of GCV, reduction in surface moisture and more workable coal. Moreover, the structure will prevent loss of minerals which go to waste due to lump formation caused by absorption of moisture in open storage. Hence, it will save huge cost to the exchequer and will improve the efficiency of power plants and prevent loss of minerals

.

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Design of coal storage shed