Disposal
of Coal Water
Introduction:-
Natural
gas produced from coal beds (coal-bed methane, CBM) accounts for
about 7.5 percent of the total natural gas production in the United
States. Along with this gas, water is also brought to the surface.
The amount of water produced from most CBM wells is relatively high
compared to conventional natural gas wells because coal beds contain
many fractures and pores that can contain and transmit large volumes
of water. In some areas, coal beds may function as regional or local
aquifers and important sources for ground water. The water in coal
beds contributes to pressure in the reservoir that keeps methane gas
adsorbed to the surface of the coal. This water must be removed by
pumping in order to lower the pressure in the reservoir and stimulate
desorption of methane from the coal. Over time, volumes of pumped
water typically decrease and the production of gas increases as coal
beds near the well bore are dewatered. The need to decrease CO2
emissions favors the increased use of natural gas as an alternative
to coal. The contribution of CBM to total natural gas production in
the United States is expected to increase in the foreseeable future
(Nelson, 1999). Estimates of the amount of recoverable CBM have
increased from about 90 trillion cubic feet (TCF) 10 years ago to
about 141 TCF, spurred by advances in technology, exploration, and
production (Nelson, 1999). As the number of CBM wells increases, the
amount of water produced will also increase. Reliable data on the
volume and composition of associated water will be needed so that
States and communities can make informed decisions on CBM
development. Most data on CBM waters have been gathered at two
historically large production areas, the San Juan Basin in Colorado
and New Mexico (sparse data) and the Black Warrior Basin in Alabama
(extensive data). Rapid development in basins with limited data on
CBM waters—i.e., the Powder River Basin in Wyoming and Montana—is
currently a concern of producers; land owners; Federal, State, and
local agencies; coal mining companies; and Native Americans.
Volumes
and Compositions of CBM Water:-
As
the ratio of water to gas, varies widely among basins with CBM
production. Causes of variations include the duration of CBM
production in the basin, originaldepositional environment, depth of
burial,and type of coal. Relatively recent regulations concerning
disposal and withdrawal of produced water have led to more accurate
reporting of water data. Volume data for produced water from specific
coal beds has the potential to provide information on exploration and
production of CBM. Compositional data is commonly limited to the
major dissolved ion species in water (cations and anions), whereas
information on trace metals and isotopic composition is sparse.
Generally,
dissolved ions in water coproduced with CBM contain mainly sodium
(Na), bicarbonate (HCO3),
and chloride (Cl). The composition is controlled in great part by the
association of the waters with a gas phase containing varying amounts
of carbon dioxide (CO2)
and methane. The bicarbonate component potentially limits the amount
of calcium (Ca) and magnesium (Mg) through the precipitation of
carbonate minerals. CBM waters are relatively low in sulfate (SO4)
because the chemical conditions in coal beds favor the conversion of
SO4
to sulfide. The sulfide is removed as a gas or as a precipitate. The
total dissolved solids (TDS) of CBM water ranges from fresh (200 mg/L
or parts per million) to saline (170,000 mg/L) and varies among and
within basins. For comparison, the recommended TDS limit for potable
water is 500 mg/L, and for beneficial use such as stock ponds or
irrigation, the limit is 1,000–2,000 mg/L. Average seawater has a
TDS of about 35,000 mg/L. The TDS of the water is dependent upon the
depth of the coal beds, the composition of the rocks surrounding the
coal beds, the amount of time the rock and water react, and the
origin of the water entering the coal beds. Trace-element
concentrations in CBM water are commonly low (<1 mg/L) as are
volatile organic compounds (Gas Research Institute, 1995; Rice,
2000). In general, most CBM water is of better quality than waters
produced from conventional oil and gas wells.
Fate
of CBM Water:-
Water
coproduced with methane is not reinjected into the producing
formation to enhance recovery as it is in many oil fields.Instead, it
must be disposed of or used for beneficial purpose:
The
choice depends in large part on the composition of the water.
Important composition information should include TDS (often equated
to the amount of “salt” a water contains), pH, concentrations of
dissolved metals and radium, and the type and amounts of dissolved
organic constituents. If, with minor to no treatment, the water is of
sufficient quality, it may be used with caution to supplement area
water supplies. This water must meet requirements under several
Federal and State regulations, including the Clean Water Act, the
Safe Drinking Water Act, and the Resource Conservation and Recovery
Act. If the water does not meet Federal and State standards for
reuse, or if the cost of treatment is excessive, the water is
disposed of by injection into a compatible subsurface formation or by
surface discharge. Disposal of CBM water is also regulated by Federal
and State agencies and must meet criteria for each type of disposal.
For example, subsurface injection requires compatibility studies of
the proposed injection formation and the water that is injected,
whereas discharge to surface streams must meet daily effluent limits
on constituents such as chlorides along with other criteria. For any
CBM field, the cost of handling coproduced water varies from a few
cents per barrel to more than a dollar per barrel and can add
significantly to the cost of gas production. In some areas, the
volumes of water produced and the cost of handling may prohibit
development of the resource.
Chemistry:-
The
chemistry of oxidation of pyrites, the production of ferrous ions and
subsequently ferric ions, is very complex, and this complexity has
considerably inhibited the design of effective treatment options.
Although
a host of chemical processes contribute to acid mine drainage, pyrite
oxidation is by far the greatest contributor. A general equation for
this process is:
2FeS2(s)
+ 7O2(g)
+ 2H2O(l)
= 2Fe2+(aq)
+ 4SO42−(aq)
+ 4H+(aq)
The
oxidation of the sulfide to sulfate solubilizes the ferrous iron
(iron(II)), which is subsequently oxidized to ferric iron
(iron(III)):
4Fe2+(aq)
+ O2(g)+
4H+(aq)
= 4Fe3+(aq)
+ 2H2O(l)
Either
of these reactions can occur spontaneously or can be catalyzed by
microorganisms that derive energy from the oxidation reaction. The
ferric irons produced can also oxidize additional pyrite and oxidize
into ferrous ions:
FeS2(s)
+ 14Fe3+(aq)
+ 8H2O(l)
= 15Fe2+(aq)
+ 2SO42−(aq)
+ 16H+(aq)
The
net effect of these reactions is to release H+,
which lowers the pH and maintains the solubility of the ferric ion.
Traditionally,
the character of acid mine drainage is determined by its acidity
(mg/L), which is measured by titrating AMD with sodium hydroxide
solution from the AMD initial pH till pH 8.3. Then calculate the
moles of NaOH that consumed by one liter of AMD, and transfer the
mole number into the weight of CaCO3.
It is the value of acidity (mg/L) of AMD. Hence, the direct meaning
of acidity is: weight of CaCO3
needed to neutralize the pH of 1 liter AMD.
However,
acidity can not best represent AMD’s characters. Some AMDs have
same acidity values, even same pH value, but of different properties.
Because the AMD acidity includes two components: hydrogen ions and
dissolved metal ions. This can be seen clearly from the AMD acidity
titration curves.
The
AMD acidity titration curve is shaped like a staircase. Vertical part
shows the process OH- ions neutralizing H+ ions, which increases the
pH of water. Horizontal part indicates OH- ions precipitate metal
ions into metal hydroxides, which will act as a buffer, using
hydroxide from the titrant, keeping the pH constant for a brief time
until a specific metal has completely precipitated.
Most
metal hydroxides (except Na and K) are insoluble in water and have
specific solubility products. When pH reaches certain level the metal
ions will precipitate and be eliminated from the water. This forms
the stair steps of the titration curve.
In
different AMD the metal concentration may range from 500mg/L to
0.1mg/L. If assuming the highest concentration allowed for each metal
is 0.1mmole/L, use metals’ precipitation products, we can calculate
the criteria pH for each metal in water. Higher than the pH criteria
the metal concentration is lower than 0.1mmole/L.
The
following table shows the experimental and theoretical pH criteria of
metals in AMD
Metal
Fe+3
Al+3
Cu+2/Mn
Zn+2/Ni+2
Fe+2
pH1
<3.2 3-4.5 5-6.5 6.5-8 5.5-6.5
pH2
2.93
4.43 6.60/9.33 7.83/8.22 8.95
pH1
values are experiment data pH2 values are calculated from metal
hydroxide solubility products, assuming metal concentration is
0.1mmole/L.
In
real situation the following factors can affect the metal
precipitation process, therefore in the experiment pH of
precipitation spans around 1-1.5:
1.
There are microbial and organic in the natural AMDs, which affect
metal ion activities greatly.
2.
During the titration process Fe+2
ions are quickly oxidized into Fe+3
in the air and precipitate as more insoluble Fe(OH)3.
This lowers Fe+2
ions precipitation pH. (usually AMD titrations are not conducted
under N2
flow).
3.
For Cu+2,
Ni+2
and Zn+2,
there are competitions between precipitates of hydroxide and
carbonate.
4.
In low pH AMDs Mn+2
ions are stable, as pH increases during the titration, Mn+2
is oxidized and precipitated as MnO2,
This also lowers Mn precipitation pH.
5.
When iron and manganese occur together (which is relatively common),
manganese will not be removed from solution until all the ferrous
iron is removed. This is because ferrous iron reduces - and
redissolves - insoluble manganese oxides. Once iron is removed,
manganese can be precipitated, either by raising water pH above 9.5
(rapid reaction) or by promoting microbial oxidation at
neutral-alkaline pH (slower reaction). As indicated above, manganese
oxide precipitates tend to promote manganese precipitation at lower
pH (ca. 7.5-8.5), but this reaction is somewhat slower than direct
precipitation at elevated pH.
The
major advantage of titration curve method is: It is simple,
convenient and low cost. It can show water problem quickly and
clearly. It helps to explain the AMD forming procedure and helps to
find the treat method.
Effects
on pH:-
In
some acid mine drainage systems temperatures reach 117 degrees
Fahrenheit (47 °C), and the pH can be as low as -3.6.
Acid
mine drainage causing organisms can thrive in waters with pH very
close to zero. Negative pH occurs when water evaporates from already
acidic pools thereby increasing the concentration of hydrogen ions.
About
half of the coal mine discharges in Pennsylvania have pH under 5.[4]
However, a significant portion of mine drainage in both the
bituminous and anthracite regions of Pennsylvania is alkaline,
because limestone in the overburden neutralizes acid before the
drainage emanates.
Acid
mine drainage has recently been a hindrance to the completion of the
construction of Interstate 99 near State College, Pennsylvania, but
this acid rock drainage didn't come from a mine: pyritic rock was
unearthed during a road cut and then used as filler material in the
I-99 construction. A similar situation developed at the Halifax
airport in Canada. It is from these and similar experiences that the
term acid rock drainage has emerged as being preferable to acid mine
drainage, thereby emphasizing the general nature of the problem.
Yellow
boy:-
When
the pH of acid mine drainage is raised past 3, either through contact
with fresh water or neutralizing minerals, previously soluble
Iron(III) ions precipitate as Iron(III) hydroxide, a yellow-orange
solid colloquially known as yellow boy[5]. Other types of iron
precipitates are possible, including iron oxides and oxyhydroxides.
All these precipitates can discolor water and smother plant and
animal life on the streambed, disrupting stream ecosystems (a
specific offense under the Fisheries Act in Canada). The process also
produces additional hydrogen ions, which can further decrease pH. In
some cases, the concentrations of iron hydroxides in yellow boy are
so high, the precipitate can be recovered for commercial use in
pigments.
Trace
metal and semi-metal contamination:-
Many
acid rock discharges also contain elevated levels of potentially
toxic metals, especially nickel and copper with lower levels of a
range of trace and semi-metal ions such as lead, arsenic, aluminium,
and manganese. In the coal belt around the south Wales valleys in the
UK highly acidic nickel-rich discharges from coal stocking sites have
proved to be particularly troublesome.
Treatment:-
In
the United Kingdom, many discharges from abandoned mines are exempt
from regulatory control. In such cases the Environment Agency working
with partners such as the Coal Authority have provided some
innovative solutions, including constructed wetland solutions such as
on the River Pelenna in the valley of the River Afan near Port Talbot
and the constructed wetland next to the River Neath at Ynysarwed.
Although
abandoned underground mines produce most of the acid mine drainage,
some recently mined and reclaimed surface mines have produced ARD and
have degraded local ground-water and surface-water resources. Acidic
water produced at active mines must be neutralized to achieve pH 6-9
before discharge from a mine site to a stream is permitted.
Methods
Lime
neutralization:-
By
far, the most commonly used commercial process for treating acid mine
drainage is lime precipitation in a high-density sludge (HDS)
process. In this application, slurry of lime is dispersed into a tank
containing acid mine drainage and recycled sludge to increase water
pH about ~9. At this pH, most toxic metals become insoluble and
precipitate, aided by the presence of recycled sludge. Optionally,
air may be introduced in this tank to oxidize iron and manganese and
assist in their precipitation. The resulting slurry is directed to a
sludge-settling vessel, such as a clarifier. In that vessel, clean
water will overflow for release, whereas settled metal precipitates
(sludge) will be recycled to the acid mine drainage treatment tank,
with a sludge-wasting side stream. A number of variations of this
process exist, as dictated by the chemistry of ARD, its volume, and
other factors. Generally, the products of the HDS process also
contain gypsum and untreated lime, which enhance both its
settleability and resistance to re-acidification and metal
mobilization.
Less
complex variants of this process, such as simple lime neutralization,
may involve no more than a lime silo, mixing tank and settling pond.
These systems are far less costly to build, but are also less
efficient (i.e., longer reaction times are required, and they produce
a discharge with higher trace metal concentrations, if present). They
would be suitable for relatively small flows or less complex acid
mine drainage.
Calcium
silicate neutralization:-
A
calcium silicate feedstock, made from processed steel slag, can also
be used to neutralize active acidity in AMD systems by removing free
hydrogen ions from the bulk solution, thereby increasing pH. As the
silicate anion captures H+
ions (raising the pH), it forms monosilicic acid (H4SiO4),
a neutral solute. Monosilicic acid remains in the bulk solution to
play many roles in correcting the adverse effects of acidic
conditions. In the bulk solution, the silicate anion is very active
in neutralizing H+
cations in the soil solution. While its mode-of-action is quite
different from limestone, the ability of calcium silicate to
neutralize acid solutions is equivalent to limestone as evidenced by
its CCE value of 90-100% and its relative neutralizing value of 98%.
In
the presence of heavy metals, calcium silicate reacts in a different
manner than limestone. As limestone raises the pH of the bulk
solution and heavy metals are present, precipitation of the metal
hydroxides (with extremely low solubilities) is normally accelerated
and the potential of armoring of limestone particles increases
significantly. In the calcium silicate aggregate, as silicic acid
species are absorbed onto the metal surface, the development of
silica layers (mono- and bi-layers) lead to the formation of
colloidal complexes with neutral or negative surface charges. These
negatively charged colloids create an electrostatic repulsion with
each other (as well as with the negatively charged calcium silicate
granules) and the sequestered metal colloids are stabilized and
remain in a dispersed state - effectively interrupting metal
precipitation and reducing vulnerability of the material to armoring.
Carbonate
neutralization:-
Generally,
limestone or other calcareous strata that could neutralize acid are
lacking or deficient at sites that produce acidic rock drainage.
Limestone chips may be introduced into sites to create a neutralizing
effect. Where limestone has been used, such as at Cwm Rheidol in mid
Wales, the positive impact has been much less than anticipated
because of the creation of an insoluble calcium sulfate layer on the
limestone chips, binding the material and preventing further
neutralization.
Ion
exchange:-
Cation
exchange processes have previously been investigated as a potential
treatment for acid mine drainage. The principle is that an ion
exchange resin can remove potentially toxic metals (cationic resins),
or chlorides, sulfates and uranyl sulfate complexes (anionic resins)
from mine water. Once the contaminants are adsorbed, the exchange
sites on resins must be regenerated, which typically requires
expensive reagents and generates a brine that is difficult to
dispose. A South African company claims to have developed a patented
ion-exchange process that treats mine effluents (and AMD)
economically, but such claims remain unsubstantiated at present.
Constructed
wetlands:-
Constructed
wetlands systems have been proposed during the 1980s to treat acid
mine drainage generated by the abandoned coal mines in Eastern
Appalachia.[14] Generally, the wetlands receive near-neutral water,
after it has been neutralized by (typically) a limestone-based
treatment process. Metal precipitation occurs from their oxidation at
near-neutral pH, complexation with organic matter, precipitation as
carbonates or sulfides. The latter results from sediment-borne
anaerobic bacteria capable of reverting sulfate ions into sulfide
ions. These sulfide ions can then bind with heavy metal ions,
precipitating heavy metals out of solution and effectively reversing
the entire process.
The
attractiveness of a constructed wetlands solution lies in its
relative low cost. They are limited by the metal loads they can deal
with (either from high flows or metal concentrations), though current
practitioners have succeeded in developing constructed wetlands that
treat high volumes (see description of Campbell Mine constructed
wetland) and/or highly acidic water (with adequate pre-treatment).
Typically, the effluent from constructed wetland receiving
near-neutral water will be well-buffered at between 6.5-7.0 and can
readily be discharged. Some of metal precipitates retained in
sediments are unstable when exposed to oxygen (e.g., copper sulfide
or elemental selenium), and it is very important that the wetland
sediments remain largely or permanently submerged.
An
example of an effective constructed wetland is on the Afon Pelena in
the River Afan valley above Port Talbot where highly ferruginous
discharges from the Whitworth mine have been successfully treated.
Precipitation
of metal sulfides:-
Most
base metals in acidic solution precipitate in contact with free
sulfide, e.g. from H2S
or NaHS. Solid-liquid separation after reaction would produce a base
metal-free effluent that can be discharged or further treated to
reduce sulfate, and a metal sulfide concentrate with possible
economic value.
As
an alternative, several researchers have investigated the
precipitation of metals using biogenic sulfide. In this process,
Sulfate-reducing bacteria oxidize organic matter using sulfate,
instead of oxygen. Their metabolic products include bicarbonate,
which can neutralize water acidity, and hydrogen sulfide, which forms
highly insoluble precipitates with many toxic metals. Although
promising, this process has been slow in being adopted for a variety
of technical reasons.
