Groundwater Pollution from Underground Coal Gasification

Groundwater Pollution from Underground Coal Gasification

Dec. 2007 Journal of China University of Mining & Technology J China Univ Mining & Technol Vol.17 No.4 2007, 17(4): 0467 – 0472 Groundwater Poll...

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Dec. 2007

Journal of China University of Mining & Technology

J China Univ Mining & Technol

Vol.17

No.4

2007, 17(4): 0467 – 0472

Groundwater Pollution from Underground Coal Gasification LIU Shu-qin, LI Jing-gang, MEI Mei, DONG Dong-lin School of Chemistry and Environmental Engineering, China University of Mining & Technology, Beijing 100083, China Abstract: In situ coal gasification poses a potential environmental risk to groundwater pollution although it depends mainly on local hydrogeological conditions. In our investigation, the possible processes of groundwater pollution originating from underground coal gasification (UCG) were analyzed. Typical pollutants were identified and pollution control measures are proposed. Groundwater pollution is caused by the diffusion and penetration of contaminants generated by underground gasification processes towards surrounding strata and the possible leaching of underground residue by natural groundwater flow after gasification. Typical organic pollutants include phenols, benzene, minor components such as PAHs and heterocyclics. Inorganic pollutants involve cations and anions. The natural groundwater flow after gasification through the seam is attributable to the migration of contaminants, which can be predicted by mathematical modeling. The extent and concentration of the groundwater pollution plume depend primarily on groundwater flow velocity, the degree of dispersion and the adsorption and reactions of the various contaminants. The adsorption function of coal and surrounding strata make a big contribution to the decrease of the contaminants over time and with the distance from the burn cavity. Possible pollution control measures regarding UCG include identifying a permanently, unsuitable zone, setting a hydraulic barrier and pumping contaminated water out for surface disposal. Mitigation measures during gasification processes and groundwater remediation after gasification are also proposed. Key words: groundwater pollution; underground coal gasification; strata CLC number: TP 39

1

Introduction

Underground coal gasification has been shown to be both technically and economically feasible. Many studies on UCG technologies have been carried out and up to now these has been widely tested under various situations in China and abroad[1–3]. UCG is partly environmental friendly due to no discharge of tailings, reduced sulfur emission and reduced discharge of ash, mercury and tar[4]. However, the underground gasification cavity is a source of both gaseous and liquid pollutants. They are created as a by-product of the gasification and pyrolysis processes, and may further react with the surrounding strata or dissolve in nearby groundwater. The risk of groundwater pollution from UCG depends on whether the contaminants can migrate beyond the immediate reaction zone to more sensitive groundwater areas. The transport of aqueous phase contaminants depends on the permeability of in-situ rocks, the geological set-

ting of the gasification reactor and the hydrogeology of the area. Studies of groundwater pollution from UCG have always been carried out based on UCG field tests. The former Soviet Union is the only country for UCG commercialization. Study results of large-scale UCG projects conducted during the late 1950’s and early 1960’s have revealed that groundwater contaminants, resulting from gasification, to be widespread and persistent, even up to five years after production had ceased. There are other reports stating that phenols were found with another aquifer in the former Soviet Union which extended over an area of 10 km2 [5]. The first available environmental data on UCG came from later United States trials, mainly regarding the Hanna and Hoe Creek UCG trails, for which groundwater contamination monitoring was conducted before, during and after gasification. The results showed that UCG at shallow depths can pose a significant risk to groundwater in adjacent strata.

Received 12 March 2007; accepted 05 June 2007 Projects 20207014 and 50674084 supported by the National Natural Science Foundation of China Corresponding author. Tel: +86-10-62331897; E-mail address: [email protected]

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Journal of China University of Mining & Technology

A European UCG trial at a depth of 550 m was monitored for water contamination in surrounding boreholes, in the drinking water supply of the local community and in the local river for a period from the initial site access to five years after the trial was completed. No environmental contamination was detected at any of its monitoring points. The only evidence of contamination was in the water of the cavity itself, which is co-produced with the gas and brought to the surface. The chinchilla project in Australia took place under the supervision of the Queensland Environmental Protection Agency (EPA) but detailed results have not, so far, been made available. Questions were raised about the post gasification process in the absence of a reliable large water supply. China is the third country for large-scale UCG studies since 1984. Up to now, more than nine UCG field tests have been conducted and their gas products have been successively used for combustion and power generation. Unfortunately, groundwater pollution has never been considered due to a lack of attention and the cost for environmental monitoring. With the commercialization of UCG technology environmental impact studies, especially of groundwater pollution prevention and control, become necessary and urgent. Our paper identifies possible ways for underground water pollution from UCG, reviews the identified organic and inorganic pollutants, analyzes the fate of the contaminants and proposes possible strategies for groundwater pollution control.

2

Principle of Groundwater Pollution Caused by UCG

Ground water pollution around UCG zones is mainly caused in one of the following ways: dispersion and penetration of the pyrolysis products of the coal seam to the surrounding rock layers, the emission and dispersion of high contaminants with gas products after gasification and migration of residue by leaching and penetration of groundwater. In addition, the escaped gases such as carbon dioxide, am-

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monia and sulfide may change the pH value after being dissolved, which subsequently affect the demand for chemical and biological oxygen of groundwater. During the gasification process, air or oxygen is injected with high pressure equal to or greater than the surrounding hydrostatic pressure. Some of the gas products are therefore lost to the surrounding permeable media and perhaps to overlying strata, as a result of cracks in the overburden, as shown in Fig. 1a. It may contain some higher molecular weight organic substances that are produced during pyrolysis of the coal seam. The more volatile the product, the farther it is transported out into the surrounding coal or strata before condensing or dissolving in the groundwater. The coal ash left in the burn zone remains largely isolated from the groundwater during gasification. At the completion of gasification, groundwater begins to invade the gasifier. Since the initial temperature of the gasification cavity is high, most of the returning water is vaporized and may be vented to the surface via the processing wells. As the cavity cools and fills with water, the residual ash is leached, leading to an increase in pH and in the concentration of many inorganic species. During this period, thermally driven convection currents transport some of the non-volatile inorganic contaminants from the ash out into the surrounding formation. This explains the large buildup of these contaminants observed radially near the burn zone after gasification. Afterwards, the concentrations of many contaminants will continue to change as a result of adsorption on the coal and strata or reactions among different species. The coal seam and strata serve, to some extent, as a natural groundwater cleaning system. Finally, after a certain period of time, the natural groundwater flow through the seam is re-established and a contaminant plume slowly develops, as shown in Fig. 1b. The extent and concentration of this plume depend primarily on the strength of the contaminant source, the groundwater flow velocity, the degree of lateral and longitudinal dispersion and the adsorption and reactions of the various contaminants.

(a) During UCG

(b) After UCG

Fig. 1

Groundwater pollution

Subsidence of the overburden above the UCG burn cavity also can cause serious groundwater pollution

problems. An example of this phenomenon was found at the Hoe Creek UCG test site in the United States

LIU Shu-qin et al

Groundwater Pollution from Underground Coal Gasification

where aquifer cross connection occurred during gasification operations. The obvious problem is the transmission of pollutants generated in the burn zone through fractures caused by subsiding overburden into overlying aquifers. However, this should be avoided during the phase of chosing the study site.

3

Identified Contaminants Around UCG Zone

3.1

Organic contaminants

It was accepted that the major organic groundwater contaminants, in terms of mass release to groundwater through UCG processes, are phenolic compounds. First reports arrived from groundwater monitoring results from the former Soviet Union. Groundwater samples taken from the burn cavity at a lignite gasification site near Fairfield, Texas also indicated that the major organic components were phenols[6–7]. Table 1 presents these data and shows that the minor organic contaminants of concern are the PAHs and heterocyclic compounds. Table 1

Organic profiles of groundwater samples near Fairfield UCG site (µg/L) Contents of the organics

Component Total phenolics Total heterocyclics

concern for its concentrations occurring at some of the sites and its designation as a human carcinogen. The Felix 1 coal seam aquifer at the Hoe Creek sites exhibited the highest benzene occurrence: concentrations as high as 3000 µg/L have been measured. But benzene contamination is generally confined to within 9 m of the gasification cavities in the affected aquifers. It persists in the groundwater and remained largely unaffected by the pumping, which suggests a non-dissolved source that is continuing to leach benzene into the groundwater. 3.2 Inorganic contaminants Several studies have identified changes in inorganic substances in groundwater due to the gasification process. Table 2 shows the data for the Tennessee Colony site in Texas which are representative of the type of increase in inorganic groundwater constituents at other UCG sites[8]. Table 2

Component

Concentrations of the inorganics After gasification

TDS

293

1462

Na+

78

136

Ca

2+

Soon after gasification

One year after gasification

SO42

7

100000

20

HCO3

2200

Changes of concentrations of inorganics substances in groundwater (mg/L) Before gasification

Before gasification Not detected

469

ˉ

ˉ

ˉ

8

94

5

625

275

385

Not detected

Cl

9

338

7

163

Two-ring PAHs

2

105

9

NH3

Three-ring PAHs

1

22

5

F

0.2

5.3

Four-ring PAHs

Not detected

7

Not detected

B



2.2

Five-ring PAHs

Not detected

3

Not detected

Total organics

10

103

34

Data reported by Campbell et al were obtained in an extensive study of a large-scale field operation of the Hoe Creek site in Wyoming[15]. Similar levels of these contaminants were also found in data from a groundwater quality study, which presents data from burn cavity water and surrounding monitor wells where groundwater samples were kept to be collected near UCG sites up to 15 months after the end of gasification. Results indicated that the contaminants consisted of phenols, aromatic carboxylic acids, aromatic hydrocarbons, ketones, aldehydes, pyridines, quinolines, isoquinolines and aromatic amines. Concentrations ranged up to about 50×106 with large variations both in the relative concentrations of acidic, neutral and basic constituents and in the concentrations of individual compounds. Naphthalene, o-xylene, 2-methyl pyridine and o-cresol were consistently present in high concentrations and were identified as UCG contaminant-indicator compounds that appear to be particularly useful for monitoring purposes. Benzene is another organic substance of particular

ˉ

The soluble ash components are seen to increase the total dissolved solid concentrations in the cavity water. The materials include a wide array of ionic species, mainly calcium, sodium, sulfate and bicarbonate. There are, however, many other inorganic substances leached into the groundwater which are of interest, even through they are present in smaller amounts. These include aluminum, arsenic, boron, iron, zinc, selenium, hydroxide and some radioactive materials such as uranium. Since it is generally agreed that inorganic contaminants tend to increase due to ash leaching, the field data have been further supported by laboratory ash leaching experiments[9–10]. Differences in results are probably due to coal and ash composition, gasifier temperatures, sampling techniques and natural water quality. Only one parameter, pH, showed very large variations among investigators. Ash from Wyoming sub-bituminous coal experienced very high pH changes upon leaching. Ash from Texas lignite showed little change in pH. Again, these differences could be accounted for by the inherent differences between coal and lignite, or may be site-specific.

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Journal of China University of Mining & Technology

Fate of Groundwater Contaminants

4.1 Migration with natural groundwater flow As stated before, after gasification the water in the coal seam, adjacent to the cavity, flows into it, picking up pollutants from the ash and char zone. The water level in the cavity gradually rises. Once the cavity is full, the pre-burn groundwater regime reestablishes itself and polluted water in the cavity then migrates into the coal seam. Mathematical modeling was regarded as a useful method to study the movement and fate of these pollutants over time and this had been carried out as part of the UCG test in Hanna, Wyoming. A two-dimensional, finite element groundwater flow model had been developed to study the movement of water in a coal seam in which large cavities were created by UCG[11–12]. Water quality modeling has to be preceded by flow modeling in order to determine velocities as a function of time. The model was applied to a series of UCG burns in Hanna. Estimates of the time to fill the cavities were obtained. Comparisons of measured and computed potential head are presented at different points in the coal seam. Flow modeling can thus be used to predict the movement of water into and out of UCG cavities while the output of velocities is necessary for water quality modeling. Flow modeling is also necessary to determine the time when the flow first starts to occur out of the cavity. 4.2 Self-restoration Many organic substances left underground after gasification are very likely to be extracted and transferred to underground water. Phenol is the most obvious organic pollutant due to its relatively high water solubility. The data, obtained from the UCG field site in the former Soviet Union have indicated that pollutants tend to decrease both with time and distance from the burn cavities[13]. The decrease of phenolic concentrations in groundwater, as a function of distance from the burn cavity and the time after gasification, are shown in Table 3[14]. Table 3 Decrease of phenolic concentrations in groundwater with distance from the burn cavity and the time after gasification (mg/L) Time after gasification (d)

Distance from the burn cavity (m)

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concentration over time[15]. It is concluded from the monitoring result of the UCG test in the former Soviet Union that the time for the existence of organic substances is short, but that of inorganic substances and H2S can extend one year. Groundwater quality is restored after two years[13]. The phenomena observed above can be explained by natural means in which the groundwater concentrations can be reduced. Most interpretations of field data attribute the improved water quality to adsorption and ion exchange properties of surrounding strata, precipitation reactions, dilution and dispersion by natural ground-water flow and biological conversion reactions[13]. It should be noted that only the last process, biological conversion, is ultimately useful in the final destruction or conversion of harmful contaminants to harmless products. The conceptual model of pollutant generation and transport proposed by Campbell, et al appears valid[15]. They hypothesized that the contaminants migrate as a function of the specific interaction of the contaminants with surrounding strata. Thus, substances with little affinity for solid surfaces, such as many of the entirely dissolved solid (TDS) components, will not be retained by the surrounding strata and will travel at the rate of groundwater flow with little diminution from peak concentrations. Substances with a medium to large affinity for solid surfaces will travel much more slowly and have significant decreases from peak concentrations as time progresses. Initial studies on the interactions between polluted groundwater and coal or strata have been carried out by Humenick, et al[16]. The results showed that pulverized lignite has a higher capacity for sorption of contaminants than either of the two adjacent clay seams. Adsorption of organic matter by clay and lignite is an effective removal mechanism, however some total organic carbon (TOC) may not be adsordable. The transport of inorganic cations is strongly affected by ion exchange. Divalent cations were consistently released from clay and lignite and replaced by monovalent cations. Ammonia was significantly adsorbed by lignite and clay. Anions exhibited less interaction with clay and lignite. Chloride and sulfate may be considered as conservative ions with respect to sorption. Alkalinity decreased upon contact with lignite and to a smaller extent with clays.

3

15

30

3

8

0.09

0.008

83

0.6

0.03

0.004

182

0.09

0.007

<0.001

5.1 Mitigation measures

280

0.04

0.003

<0.001

762

0.02

0.001

<0.001

Active pressure control, in which the cavity pressure is held in equilibrium with its surroundings, as shown in Fig. 2, was tested for the first time in the European trial. It was found that gas escape, which is the driving force for contaminant dispersal, could be substantially reduced. The main mitigation measures

Inorganic contaminants, including cations such as ˉ Na+, K+, Mg2+, NH4+, etc and anions such as SO42 , ˉ ˉ HCO3 , Cl , etc, also showed a large decrease in

5

Groundwater Pollution Control

LIU Shu-qin et al

Groundwater Pollution from Underground Coal Gasification

during UCG operation include: 1) the use of operational monitoring systems that can detect gas losses and ensure that reactor pressures are maintained below hydrostatic pressure; 2) ensuring that wells and boreholes used in the process are adequately sealed; 3) maintaining a “cone of depression” in the groundwater around the reactor.

Fig. 2

Sketch of active pressure control during UCG

The post operational phase is also a critical condition for groundwater contamination as pressure and contaminants can build up when the cavity returns to equilibrium. The recommended strategy for this condition is: 1) to minimize post-burn contaminant generation from pyrolysis products by accelerating the cooling of the cavities and preventing pressure build up post gasification; 2) to maintain the flow of groundwater towards the cavities by pumping water from the cavities and hence maintaining a hydrostatic gradient towards the reactor areas and 3) to maximise the removal of potential organic and inorganic groundwater contaminants from the underground strata by pumping and treating contaminants. 5.2 Groundwater remediation Combined air sparge and bioremediation has been conducted from 1995 to 1996 regarding a former UCG test site in northeastern Wyoming in the United States, where UCG testing from 1976 through 1979 contaminated three water-bearing units with benzene. Air sparging was selected as a method to strip dissolved benzene, volatilize the non-dissolved benzene source material and to provide oxygen for increasing aerobic bacteria populations. Indigenous bacteria populations were stimulated with the addition of ammonium phosphate. The project was designed to take advantage of its hydrogeological environment to produce a cost-effective approach to groundwater remediation. Groundwater pumping was used to manipulate subsurface air flow, nutrient transport and biomass management. The results of the demonstration showed that substantial reduction in benzene concentration across the demonstration area. Benzene concentration reductions greater than 80% were observed two months after demonstration operations

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were suspended. 5.3 Groundwater pollution control An important parameter in determining the risk in a UK context is to identify a zone of permanently unsuitable (PU) groundwater. This is defined as a block of strata where the water quality and/or yield are so poor that groundwater in that area cannot realistically be regarded as an environmentally or economically significant aquifer. If a PU zone were to be established, the assessment of the pollution risk should focus on ensuring that there is no connection with the zone with overlying aquifers. Although relaxations of the ground water regulations using the concept of PU ground water have been applied occasionally for hydrocarbon exploration, the principle needs to be tested, in the context of UCG. The concept of containment could be applied to a situation where little or no ground-water flow occurs in the coal seam. One strategy would be to monitor the periphery of the burn zone to insure that contaminants were not migrating from the contaminated area. Possibly a hydraulic barrier could effect containment of pollutants. Researchers have proposed several control technologies for containment, such as a hydraulic bypass around the contaminated zone, placing adsorbent clays within the cavity and placing a grout curtain around the contaminated zone. However, actual cost and effectiveness have not been established because of the absence of field data. Another concept for control would be pumping the contaminated water from the cavity and surrounding area and treating or disposing of the water on the surface. This alternative would be effective for removal of highly mobile contaminants. These contaminants would consist for the most part of the material generated by ash leaching, such as the lighter, more soluble organic matter and most of the ammonia. The remaining material would be the more insoluble, less mobile condensed organic matter around the periphery of the burn zone.

6

Conclusions

1) UCG shows the risk to groundwater pollution mainly due to gas dispersion to the surrounding permeable strata under high pressures and the possible leaching of residue by natural groundwater flow after gasification. 2) The identified pollutants include typical organic and inorganic substances, which change slightly at different UCG field sites. 3) The monitoring results show that contaminants decrease over time and with the distance from the burn cavity. Surrounding strata around UCG cavity serves as a filtration to stop the migration of the pollutants, which is caused by the adsorption of pollutants by coal and surrounding strata.

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4) Groundwater pollution from UCG can be controlled by pressure mitigation, identifying a permanently unsuitable zone, setting a hydraulic barrier and pumping contaminated water out for surface disposal. Groundwater remediation in the cavity after gasification is also a useful measure for pollution control.

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Acknowledgements Financial support for this work from the Natural Science Foundation of China (No. 20207014, 50674084) and the Hi-tech Research and Development Program of China (S-863) (No. 20001AA 529030) are gratefully acknowledged.

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