assessment of trace elements leaching

8/7/2019 Assessment of Trace Elements Leaching....

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Published in: Jr. of Environ. Science & Engg. 49(1): 77-86.

Assessment of Trace Elements Leaching of Coal CombustionResidues from Bokaro Thermal Power Station

Gurdeep Singh, Ritesh Kumar and Prabhat Kumar

Professor and Head

Centre of Mining Environment

Indian School of Mines Dhanbad-826 004

ABSTRACT

The leaching behaviour of coal combustion residues (CCRs) viz. fly ash, bottom

ash and pond ash from Bokaro Thermal Power Station (BTPS), Jharkhand was

investigated by open column percolation experiment. The study aims to determine the

long-term leaching of trace/heavy elements CCRs from BTPS. Results of potentiometric

analysis of leachates reflected that CCRs are slightly acidic to alkaline but overall on the

long-term basis these are alkaline in nature. From the long-term leaching study of

approximately two years it has revealed that of the twenty three elements those were

analysed by open column percolation experiments, only Ca, Mg, Na and K were found to

be leaching throughout the study period though its concentrations reduced considerably

with time. Other elements such as Mn, Fe, Cu, Zn, Pb leached at significant concentration

levels for sometime but found to be absent on long-term basis. Elements such as As, Cr,

Cd, Ni, Al, Co, B etc. were not found to be present in the leachate. This study establishes

that CCRs from BTPS are environmentally benign with respect to leaching of traceelements.

(Key words: CCRs, fly ash, bottom ash, pond ash, BTPS, trace elements and leachates)

INTRODUCTION

India is the second most populous country in the world and it is expected that its

population would reach 1.15 billion by 2010 (US Census Bureau, 1999). With the

population showing sharp rise and modernization taking its hold in the every nook and

corner of the society, the demand of electricity has also shown a sharp increase. The

increasing population and industrialization has placed a tremendous pressure on the energy

sector. Coal is the prime source of electricity generation and it accounts for about 70% ofelectricity generation (Anon, 2001/02). Indian coal is rich in ash content containing ash

between 30-60%. Due to high ash content in Indian coal electricity generation is also

resulting in the production of huge quantity of CCRs. Energy on one hand is important for

the development and growth of economy, whereas on the other hand huge quantity of

CCRs poses several environmental and other related problems.

Most of the power plants in India follow the wet system of disposal. In this system

of disposal, fly ash collected in the hoppers is taken to the collector tank where it is mixed

with water to form slurry and via pipeline sent to the disposal ponds called ash ponds.

Similarly, Bottom ash falling under gravity as clinker is first grinded to below 25mm size,

is mixed with water and then hydraulically transported to disposal ponds along with the fly

ash.


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Presently, in India more than 90MT/annum CCRs are being generated from the 85

existing thermal power plants (TPPs) of which about 80% is fly ash and the rest is bottom

ash (Sharma Mansavi et. al., 2001). Disposal of such a huge quantity of CCRs is a major

problem from environmental point of view. Such a huge quantity of CCRs, besides having

several environmental implications of its own, demands for huge tract of land for its

disposal. In India nearly 65,000acres of land is needed for the disposal of CCRs (SharmaMansavi et. al., 2001). Contamination of surface and ground water due to trace/heavy

metal leaching in the disposal environment is one of the major problems associated with

the holding of CCRs in the ash ponds. Utilization of CCRs is the prime need and is

approached for from the viewpoint of resource conservation and minimal disposal.

Centre of Mining Environment at Indian School of Mines has been continuing

studies on environmental aspects of CCRs including disposal and utilisation aspects

(Kumar, 1996, Singh and Kumar, 1996, Vibha, 1998, Jain, 1998, Kumar, 1999, Singh, and

Vibha, 1999, Singh, 2000). This paper presents levels of trace elements leaching from

CCRs of Bokaro Thermal Power Station and envisages the level of contamination of

surface and ground water with trace metals present in CCRs.

STUDY AREA

Bokaro Thermal Power Station (BTPS) is located in Jharkhand State at a distance

of 55km to the west of Dhanbad city. It is located on the banks of the river Konar in

Bokaro District. BTPS is the first low-grade coal burning power station constructed by

Damodar Valley Corporation. BTPS consists of plant A with 4 units and plant B with

three units. The first unit at BTPS was put in service on February 21, 1953.

The details of power generation at various units are shown in Table 1.

Table 1: Details of Generating Units at BTPS

Capacity (MW) Name of

ManufacturersUnit

Original Present Boiler TG

Year ofCommissioning

Special Features

Plant A

1 57.5 45 CE GE Feb., 1953

2 57.5 45 CE GE Aug., 1953

3 57.5 45 CE GE Oct., 1953

4 75 40 MAN MAN Apr., 1960

Units have twin

boilers

All the units have

two stages Fly

Ash collected by

MDC

Plant B

1 210 210 ABL BHEL March, 1986

2 210 210 ABL BHEL Nov., 1990

3 210 210 ABL BHEL August, 1993

Fly Ash controlled

by EP

CW through CT

pond

Source: www .dvcindia .org

EXPERIMENTAL METHODOLOGY

Standard sampling, leaching and analysis methods were used for the environmentalcharacterisation of CCRs.


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SAMPLING

Standard sampling methods were used for collecting coal combustion residues

samples from Bokaro Thermal Power Station. In all, five samples viz. FA#A, FA#B,

BA#A, BA#B and AP were taken for the purpose of leaching study. FA#A was taken from

the front field and FA#B from the backfield of ESPs hoppers. Similarly, BA#A and BA#B

was collected from the front field and the backfield respectively. Ash pond samples were

taken from the existing ash pond. These samples were collected on five different days and

a final homogenized sample in each case was prepared while mixing appropriate portions.

Open Percolation Column Experiments

In open percolation column experiments, deionized water is percolated through a

packed column of CCRs (fly ash, bottom ash and pond ash packed separately in different

columns) in the presence of oxygen at a rate, which depends on the natural permeability of

the material. The open columns for leaching experiments were made of PVC pipes 10 cm

in diameter and 60 cm in length. The column set-up involved packing the CCRs material

at a optimum moisture and density conditions as determined by the proctor test. The CCRsmaterial was packed into the column in 5 cm lifts with a 5 cm x 5 cm wooden rod, about

120 cm long. Each packed material was scarified, by lightly scraping the top with a long

thin rod to ensure proper interlocking of the material. The top 7.5 cm of the column was

left unpacked to allow for the addition and maintenance of the leaching medium. About

200 ml of leaching medium (deionized water) was added to the top of the column once

every alternate day to maintain sufficient supply of water to the packed CCRs material.

The top end of the column was exposed to the atmosphere and the bottom end was

connected to quarter inch tubing. The columns discharged the leachates through this

tubing into 250-ml polypropylene beakers. The leachates were collected in these beakers

and analysed.

ANALYSIS OF THE LEACHATE SAMPLES

The leachates from open column percolation experiment were subjected to

potentiometric and elemental analysis.

LEACHATE ANALYSIS

The leachates obtained from open column percolation experiment were

potentiometrically analysed for pH using Cyber Scan 510 pH meter and conductivity and

TDS using Cyber Scan 200 conductivity meter.

ELEMENTAL ANALYSIS OF LEACHATES

After the potentiometric determination leachate samples were filtered and acidified

with 2ml of nitric acid and then preserved in polypropylene sampling bottles. The samples

were kept in a refrigerator until further analysis. Analysis of samples were carried out as

per the standard procedure recommended by the American Public Health association

(APHA, 1985). Sodium and potassium were determined using Systronic flame photometer

Model No. 128. Trace elements were determined using Atomic Absorption

Spectrophotometer (AAS), GBC-902 coupled with graphite furnace, hydride generator and

computer data station. Working solutions standards were prepared according to instruction

given in the operating manual of the GBC-902 AAS.


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RESULTS AND DISCUSSIONS

Summarised results of the leachate analysis are presented in the Table 2 along with

its comparison with the Indian Standards for Effluent Discharge in the Inland Surface

Water and On Land for Irrigation (IS: 2490).

pH of the leachates were observed in the range of 6.39 to 10.551 in FA#A, 6.40 to9.10 in FA#B, 5.80 to 9.54 in BA#A, 5.32 to 8.99 in BA#B and 6.25 to 9.03 in AP. These

results reflect that the pH of all the five samples leachates are neutral to alkaline in nature

and generally observed within the permissible limits as per the Indian Standards (IS:

2490). This alkaline nature of CCRs can be made use of in amendment of acidic soils and

derelict mined lands.

Conductivity and TDS of all the five samples showed a similar trend with initial

high value and then decreasing gradually with time. Conductivity is directly related to

TDS and it mainly represents the availability of surfacial element (sodium, potassium and

calcium) ions. This is further revealed from the results of the leachate analysis where

sudden decrease of conductivity gives rise to a decrease in concentration levels of the

surfacial elements. These elements tend to be washed away upon their first contact with

water (first flush phenomenon) and are scarcely available again. Conductivity of leachates

was observed in the range 0.159 to 0.750 in FA#A, 0.202 to 0.788 in FA#B, 0.310 to

1.303 in BA#A, 0.075 to 0.852 in BA#B and 0.161 to 0.920 mmhos/cm in AP. TDS of

leachate was observed in the range 79 to 375 in FA#A, 102 to 399 in FA#B, 155 to 652 in

BA#A, 37 to 462 in FA#B and 81 to 460 ppm in AP. The TDS content of all the leachates

was within the permissible limit of 2100 mg/l as per IS: 2490. Initial higher values of

conductivity and TDS are due to the washout of the surfacial elements. As the surfacial

elements got washed out, the values of conductivity and TDS were found to decrease. The

potentiometric analyses of the five samples are presented graphically in Fig. 1 to Fig. 3.

Leachates from five samples were analysed for twenty-three elements andsummarised results along with its comparison with IS: 2490 are presented in Table 2.

Elements such as barium, boron, arsenic, nickel, cadmium, cobalt, aluminum, antimony,

silver, mercury, chromium, vanadium, Selenium and molybdenum were below the

detection limit in the entire study period. Selenium was found to be leaching on negligibly

few occasions in case of FA#B, BA#A and BA#B samples. Ca, Mg Na and K were

observed to be leaching throughout the study period though their concentration reduced

considerably in the long run. Fe, Cu, Zn, Mn and Pb did not show regular leaching trend.

The concentration of Cu and Zn was within the permissible limit as per IS: 2490. The

concentration of lead was above the permissible limit in all the five samples.

The high concentration of Pb was observed in the initial few months of the studyperiod. Thereafter, the level showed a decreasing trend and finally reduced to BDL.

Similarly, Fe was found to be leaching almost in the entire study period. Mn also showed

initial regular leaching then intermittent leaching and finally reduced to BDL. In all the

nine elements high concentration was observed in the initial study period and finally either

the concentration reduced considerably as in the case of Na, K, Ca and Mg or the

concentration reduced to the BDL value as in the case of Fe, Mn, Cu, Zn and Pb. Initial

high concentration of trace elements may be accounted due to the first flush phenomenon

and due to high solid to liquid ratio. Presence of trace elements in CCRs can be made use

of as a source of nutrients for plants in agriculture application and during reclamation of

the abandoned ash pond. The elemental analyses of the leachate samples are presented

graphically in Fig. 4 to Fig. 12.


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Table 2: Summary of the Leachate Analysis of CCRs Samples from BTPS

(IS:2490, 1981)Parameter FA#A FA#B BA#A BA#B AP

Inland

SurfaceWater

On Land

forIrrigation

pH 6.39-10.51 6.40-9.10 5.80-9.54 5.32-8.99 6.25-9.03 5.5-9.0 5.5-9.0

Conductivity 0.159-0.750 0.202-0.788 0.310-1.303 0.075-0.852 0.161-0.920 -- --

TDS 79-375 102-399 155-652 37-426 81-460 2100 --

Iron BDL-0.740 BDL-0.692 BDL-1.924 BDL-0.762 BDL-1.369 -- --

Lead BDL-0.420 BDL-0.396 BDL-0.462 BDL-0.412 BDL-0.462 0.1 --

Magnesium BDL-36.00 BDL-38 2.00-50.13 BDL-40.00 BDL-44.00 -- --

Calcium 1.060-66.90 1.839-42 2.931-42.00 1.13-44.00 5.321-48.00 -- --

Copper BDL-0.089 BDL-0.068 BDL-0.246 BDL-0.137 BDL-0.047 3 --

Zinc BDL-0.288 BDL-0.372 BDL-0.247 BDL-0.045 BDL-0.045 5 --

Manganese BDL-0.037 BDL-0.046 BDL-0.043 BDL-0.039 BDL-0.048 -- --

Sodium 5-56 3-49 BDL-51 3-37 3-47 -- 60

Potassium 4-42 2-36 2-26 2-31 2-33 -- --

Chromium BDL BDL BDL BDL BDL 2 --

Nickel BDL BDL BDL BDL BDL 3 --

Cobalt BDL BDL BDL BDL BDL -- --

Cadmium BDL BDL BDL BDL BDL 2 --

Selenium BDL BDL BDL BDL BDL 0.05 --

Aluminium BDL BDL BDL BDL BDL -- --

Silver BDL BDL BDL BDL BDL -- --

Arsenic BDL BDL BDL BDL BDL 0.2 2

Boron BDL BDL BDL BDL BDL 2 2

Barium BDL BDL BDL BDL BDL -- --

Vanadium BDL BDL BDL BDL BDL -- --

Antimony BDL BDL BDL BDL BDL -- --

Molybdenum BDL BDL BDL BDL BDL -- --

Mercury BDL BDL BDL BDL BDL 0.01 --

BDL-Below detectable limit i.e.= 0.001 mg/l; Conductivity in mmhos/cm; TDS in ppm & concentration in ppm


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CONCLUSION

From the long-term leaching study it is found that coal combustion residues

leachates as generated from the open column percolation experiment as such do not pose

any adverse environmental impact in its disposal system. This open column percolation

experiment closely resembles coal combustion residues leaching in the field. In thecolumn there are chances that material will develop preferential pathways for the water

due to cracks. This possibility exists in the field too. This test is the best of all the leaching

study experiments as it test mimic the field situation and thereby gives the true picture of

what is happening in the field. The physical set up of the open columns more closely

resembles with because the flow of the leaching medium is influenced by the gravity alone

and solid to liquid ratio is more close to the field situation. This is the most efficient of all

the experimental procedures in predicting the effects on ground water.

Overall, the coal combustion residues would not seem to pose any environmental

problem during its utilization and/or disposal and can be used in a benign manner for

various purposes. As a bulk utilization these coal combustion residues can be used formine backfilling and this way putting back the material from where it came can solve the

problem of disposal.

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Impact of Srubber Sludge on Ground Water at an Abandoned Mine Site.Environmental Monitoring and Assessment, 50: 1-13.

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Thermal Power Station; an M. Tech. thesis submitted to Indian School of Mines,

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Dhanbad, 2000.

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Carbondale, July.


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Paul, Bradely, C. and Gurdeep Singh, 1995. First World Mining Congress. Oxford and

IBH Publishing Co. Ltd., New Delhi Proceeding, pp 1015-1030.

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Fig. 1: Open Column Leachate Analysis for pH

Fig. 2: Open Column Leachate Analysis for Conductivity


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Fig. 3: Open Column Leachate Analysis for TDS

Fig. 4: Open Column Leachate Analysis for Sodium


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Fig. 5: Open Column Leachate Analysis for Potassium

Fig. 6: Open Column Leachate Analysis for Calcium


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Fig. 7: Open Column Leachate Analysis for Magnesium

Fig. 8: Open Column Leachate Analysis for Manganese


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Fig. 9: Open Column Leachate Analysis for Copper

Fig. 10: Open Column Leachate Analysis for Iron


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Fig. 11 Open Column Leachate Analysis for Zinc

Fig. 12: Open Column Leachate Analysis for Lead

assessment of trace elements leaching

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