NexGen Technologies for Mining and Fuel Industries

ISBN 978-93-85926-40-2

Mercury emissions for coal based power plants of India

Tarit Baran Das1, Ashis Mukherjee1, J. George2 and Gajanan Sahu3
1
Combustion Science and Technology, CSIR-CIMFR, Dhanbad, India
2
Industrial Biotechnology and Waste Utilization, CSIR-CIMFR, Dhanbad, India
3
Gasification and Liquefaction Research Group, CSIR-CIMFR, Dhanbad, India

ABSTRACT: Mercury is one of the most important environmental contaminants that have
aroused a global concern due to its toxicity, long range transport, persistence and bioaccumulation
in the environment. Globally and nationally, coal combustion is one of the major sources of
mercury emissions to the atmosphere. Future of electric power generation is under increasing
pressure as environmental regulations become more stringent worldwide. The revised norms
notified by the Ministry of Environment, Forests and Climate Change (MoEF and CC) in Decem-
ber, 2015 have made the pollution standards more stringent for new coal based power projects as
well as old plants. The emissions standard proposed for mercury will serve the purpose of meeting
national commitments towards Global Mercury Treaty, the Minamata Convention.
The paper presents the results of abundance of mercury content in Indian thermal coals which
are primarily used for the generation of electricity. About 165 number of coal samples covering
both sub-bituminous to bituminous coals have been collected from the major coal producing
regions of the country. A comprehensive analysis of onsite measurements for mercury emissions
and behavior in nine Indian coal-fired power plants has also been presented. The Ontario Hydro
Method was used to sample the mercury in flue gases, at the outlet of the APCDs.
The arithmetic mean of mercury concentration has been found to be 175 µg/kg for 165 coal-
samples. Excepting one instance, the mercury emissions figures for all the boiler units have been
found to be well within the national regulation limit of 30.0 µg/Nm3. Mercury mass balance
estimates reveal that during combustion, partitioning of mercury takes place in such a way that
almost two thirds of feed coal mercury gets emitted through stack as mercury vapour, whereas the
rest remain associated with the fine fly ash particles. Only a small portion is found to retain with
the bottom ash and mill rejects. Mercury speciation study revealed that the proportion of elemental
mercury in flue gas is significantly higher than that of the oxidized form.

1. INTRODUCTION
elemental mercury can travel thousands of

M ercury is a wide-spread persistent global

toxicant, traveling across international
borders through air and water. Mercury emis-
kilometers. When coal is burned in power
plants, the trace amount of mercury that it
contains passes along with the flue gas into the
sions are a global problem that knows no atmosphere.[1] The mercury eventually falls
national or continental boundaries.[1] The geo- back to earth in rain, snow, or as dry particles,
graphic scale of mercury’s environmental impact either locally or sometimes hundreds of miles
is highly dependent upon the form of mercury. distant. Once the mercury is deposited on land
Water soluble oxidized mercury generally depo- or in water, bacteria often act to change the
sits locally, while the more volatile, insoluble metal into an organic form, called methyl

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mercury, that easily enters the food chain and more than 99% of themercury releases into the
“bio accumulates.” At the upper reaches of the flue gas during coal combustion in the form of
food chain, some fish and other predators end elementalmercury (Hg°).[3,6] With flue gas
up with mercury levels more than a million temperature decreasing from the core burning
times higher than those in the surrounding area to theflue gas outlet of the boiler, Hg°
environment.[2] For the humans and wildlife reacts with other components in the flue gas (fly
that ultimately consume these species, these ash) and partially converted to gaseous oxidized
concentrations can be poisonous.
mercury (Hg2+).[4] Part of the gaseousmercury
Stationary combustion is by far the largest
adsorbs onto the surface of fly ash and forms
sector for mercury emissions in India.Coal
particulate-bound mercury (Hgp).[4]
burning is one of the most significant anthro-
The physicochemical forms of mercury en-
pogenic sources of mercury emissions to the
tering the pollution control equipment and the
atmosphere.[2] Coal does not contain high
variability in speciation in flue gas have been
concentrations of mercury, but the combination
identified as functions of the temperature,
of the large volume of coal burned and the fact
pressure, flue gas composition, mercury concen-
that a significant portion of the mercury present
tration and mode of occurrence in the feed coal,
in coal is emitted to the atmosphere yield large
concentration and characteristics of the ash, and
overall emissions from this sector.[3] The
length of time that the mercury emissions are
mercury content of coal varies widely, intro-
entrained in the flue gas.[6]
ducing a high degree of uncertainty in estimat-
Because of the environmental and human
ing mercury emissions from coal burning.[4]
health impact of mercury emissions, it is
Mercury content of coal varies largelyamong
important to quantify mercury emissions for
different coal producing areas, usually in the
policymakers, researchers, and the public to
range of 0.01–0.3 mg/kg.[5] A fundamental
make informed decisions about mitigating and
understanding of mercury transformations in
abating risk.[7]
combustion systems is necessary for selecting
Measuring mercury emissions from coal-
suitable mercury emission control technologies,
fired combustion sources can improve our
because the different forms of Hg have distinct understanding of the impacts that coal type and
removal characteristics. While elemental quality and air pollution control equipment
mercury (Hg°) is relatively difficult to capture configurations may have on mercury emis-
in pollution control devices installed on power sions.[7,8] The measurements are fundamental in
plants, whereas the oxidized (Hg2+) and parti- the development and validation of credible
culate (Hg(p)) forms are easily captured in emission inventories. Mercury measurements
devices such as ESP (for particulate control) can also be used to demonstrate the effective-
and FGD (for SO2 control). Different chemical ness of co-benefit/co control strategies for
kinetics models have been developed for mercury removal and inform the policy debate
prediction of mercury speciation and capture in for mercury emissions control and monitoring
pulverized coal fired boilers. The models programs.[8]
considered gas phase Hg reactions with Cl
species (Cl, Cl2, HCl, HOCl) coupled to 2. EXPERIMENTAL
heterogeneous reactions of Hg/Cl with un-
The present study was focused on two major
burned carbon in fly ash. The model also
tasks:
included an Hg oxidation mechanism on SCR
catalysts (Ti/V) installed on power plants for (a) Quality assessments and estimation of
the control of NO emissions, and an equili- mercury concentrations of the coals being
brium analysis for predicting Hg retention in fed to coal power plants;
FGD systems.[5] Because the combustion (b) Mercury emissions measurement at nine
temperature of coal in boiler is above 1000°C, pulverized coal fired power plants.

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2.1 Assessment of mercury contents in Indian and reduced by standard coning and quartering
coals method, pulverized to < 212 µm, bottled and
In the present study, representative run of mine stored for analyses.
coal samples from different commercial mines Field measurements on emissions and speci-
of eight major coalfields of India have been ation of mercury had been conducted in one
collected. Coal samples were also collected boiler units of power stations. Flue gas (up-
from the unloading points of about twenty large stream of ESP) sampling had been carried out
capacity thermal power utilities across the by adopting Ontario Hydro methodology.
country, each of which is specific to particular (ASTM D6784–2002). Flue gas was iso-kineti-
mine source. cally pulled through a quartz fibre filter and a
The coal samples received were crushed and series of chilled impinges. The filter retains
reduced through coning and quartering method. particulate present in the gas stream and vapor
About two kilograms of < 3.0 mm representative phase components of the flue gas are absorbed
coal samples were pulverized in a laboratory
in impinger liquids. The collection of flue gas
mill to pass 212 µm. Nearly 100 gms of each of
samples and that of Suspended Particulate
the samples were properly homogenized and
allowed to equilibrate in ambient air for 24 hours Matter (SPM) were performed simultaneously.
and then preserved in polythene bottles in an The duration of sample collection ranged
cooling incubator maintained at 10°C. between 2–3 hours.
Detailed characterization of each of the coal
samples was done in terms of mercury concen- 3. RESULTS AND DISCUSSIONS
tration, proximate analysis, ultimate analysis
and gross calorific value in accordance with 3.1 Mercury in coal samples
Indian Standard procedures.
The determination of mercury concentra- The quality of the 165 coal samples have been
tions of the coal samples was carried out using presented in the Table 1.
Tri-cell Direct Mercury Analyser, DMA-80 It is observed that the coal samples of the
(M/S Milestone, Italy). The determination of major coal producing regions of the country are
mercury combines the techniques of thermal
of moderate heat values and the ash yield
decomposition, catalytic conversion, amalgama-
ranges between 27.7 to 43.5 per cent. The
tion, and atomic absorption spectrophotometry.
average sulphur contents of the coal samples
vary between 0.39 per cent for BCCL to 1.07
2.2 Mercury emissions measurement in
per cent for SCCL. The dry mineral matter free
power plants
carbon and volatile matter indicate that BCCL
Direct emissions measurement was carried out coals are of low volatile bituminous type;
at three pulverized coal fired power plants whereas the coal samples of the seven other
(Power Plant A to Power Plant I). The unit coalfields are, in general, sub-bituminous or
generation capacities of the boiler units varied high volatile bituminous in nature.
from 110 MW to 660 MW.
All the nine power plants are equipped with Mercury content of the coals: From Table 1, it
Electrostatic Precipitator (ESP) for particulate is observed that there is a wide variation of
capture. Representative samples of crushed coal mercury concentrations in the coal samples
(20 mm top size), pulverized coal (< 212 µm), analyzed. The average mercury content of coals
fly ash and bottom ash, mill rejects were of the individual coal fields varies from
collected from one boiler unit of each of the 83 µg/kg to 219 µg/kg.
power stations. The composite fly ash samples The frequency distribution of mercury val-
were collected from the silos of the respective ues in the 165 coal samples of eight major coal-
power plants. The samples were dried, crushed fields of the country are presented in Figure 1.

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Table 1: Hg content in some Indian coals
Av. GCV Av. Av. Av. Hg
Av. sulphur Av. Range of
No. of (kcals/kg) volatile carbon conc.
conc.% (dry ash % Hg conc.
Coalfield samples (dry matter % (%) (µg/kg)
basis) (dry (µg/kg)
analysed basis) (dmmf (dmmf (dry
basis) (dry basis)
basis) basis) basis)
CCL 24 0.52 4530 37.1 37.5 81.6 84–324 203
MCL 35 0.50 3960 43.5 42.1 79.9 64–366 219
NCL 18 0.41 4710 34.7 40.9 80.8 59–544 209
SECL 27 0.70 5610 27.7 34.5 81.0 03–327 140
WCL 24 1.03 4640 37.5 37.6 80.6 54–421 209
SCCL 11 1.07 4770 36.3 38.5 81.3 86–237 147
ECL 18 0.55 5290 31.2 36.5 78.2 16–219 86
BCCL 8 0.39 4570 41.9 24.3 86.8 45–126 83

Fig. 1. Frequency distribution of Hg in Indian coals.

The distribution of mercury values indicates similar to that of feed coals, whereas mercury
that the mercury content of the majority of the contents of bottom ash samples are very less.
coal samples lies in the range of 100 µg/kg to Mercury speciation study revealed that the
250 µg/kg with an arithmetic average mercury proportion of elemental mercury in flue gas is
concentration of 175 µg/kg. significantly higher than that of the oxidized
form (Table 2). The mercury emissions figures
3.2 Mercury emissions from power plants for all the boiler units excepting one (Power
Plant F) have been found to be well within the
Table 2 presents the feed coal mercury content, national regulation limit of 30.0 µg/Nm3.
the total mercury emissions and speciation of Mercury mass balance estimates reveal that
mercury in the flue gas for the nine coal fired during combustion, partitioning of mercury
power plants. takes place in such a way that almost sixty per
It is observed from Table 2 that the mercury cent of feed coal mercury gets emitted through
concentrations of the feed coals of the nine stack as mercury vapour, whereas the rest
power plants vary from 93 ng/gm to 215 ng/gm remain associated with the fine fly ash par-
with an average value of 168 ng/gm. It is also ticles. Only a small portion is found to retain
observed that the fly ash mercury contents are with the bottom ash.

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Table 2. Hg emission from power plants
Flue Gas Speciation (% of Fraction
Hg contents of solid samples Hg total emitted of Hg
Power Capacity Conc. mercury) emitted
plants (MW) Feed Bottom
Fly ash SPM
coal ash μg/NM3 Hg2+ Hg° (ODF)
ng/gm ng/gm
ng/gm ng/gm
A 210 210 158 11 57 11.50 11.3 88.1 0.61
B 250 93 242 17 105 04.24 41.6 58.1 0.27
C 500 215 97 6 66 14.84 38.8 60.8 0.81
D 200/500 135 165 21 131 11.7 30.9 68.8 0.73
E 500 185 156 20 29 18.5 28.4 71.6 0.53
F 210/500 138 215 39 136 32.6 30.1 69.3 0.60
G 500 170 310 29 320 13.0 38.2 56.6 0.48
H 660/500 196 205 22 193 15.8 30.5 69.4 0.64
I 110 171 176 31 238 08.9 37.5 62.1 0.56
Average 168 192 22 142 14.6 31.9 67.2 0.58

The mercury release from the power sector has • Measurements in power plants indicate
been estimated using the following equations. considerable content of Hg is retained in
Emissions factor = Input factor × output the fly ash.
distribution factor to air • The average ‘Output Distribution Factor’
Estimated mercury = Activity rate × emissions for mercury to air for the nine power plants
release (ton/year) factor is 0.58.
Activity rate = Amount of coal fed to The average ‘Output Distribution Factor’
power plants in a year can be used to estimate mercury emission
(ton/year) factor and also mercury emissions from the
Input factor = Mercury input factor of sector.
coal (Hg concentration in
coal, g/ton) REFERENCES
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