Disposal of Coal Water

   

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:

2FeS_2(s) + 7O_2(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) + O_2(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:

FeS_2(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.