Comparison of metal elution from cavern residue after underground coal gasification and from ash obtained during coal combustion

Comparison of metal elution from cavern residue after underground coal gasification and from ash obtained during coal combustion

Fuel 158 (2015) 733–743 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Comparison of metal elution f...

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Fuel 158 (2015) 733–743

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Comparison of metal elution from cavern residue after underground coal gasification and from ash obtained during coal combustion Aleksandra Strugała-Wilczek a,⇑, Krzysztof Stan´czyk b a b

Department of Environmental Monitoring, Central Mining Institute, Plac Gwarków 1, 40-166 Katowice, Poland Department of Energy Saving and Air Protection, Central Mining Institute, Plac Gwarków 1, 40-166 Katowice, Poland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The first research on metal elution

from residues after in situ UCG were conducted.  Hydro-geochemical background typical for groundwater was exceeded in eluates.  Highly toxic elements (Sb, Cd, Hg, As) were not detected in any of water extracts.  Metals were classified into groups according to their mobility to water phase.  Ashes from four UCG processes were compared with ashes from coal combustion.

a r t i c l e

i n f o

Article history: Received 13 August 2014 Received in revised form 29 May 2015 Accepted 3 June 2015 Available online 10 June 2015 Keywords: Underground coal gasification Ash Char Metals Groundwater pollution

a b s t r a c t The aim of this paper was to determine the capability for metal elution that may constitute potential contamination to the water environment from post-process cavern residue after underground coal gasification (UCG). Samples of raw coals that had been subjected to gasification (three samples of hard coal, the coal mine ‘‘Bobrek’’, the experimental mine ‘‘Barbara’’ and the coal mine ‘‘Piast’’, and a sample of brown coal from the coal mine ‘‘Bełchatów’’) were analysed for their physicochemical properties. Similar tests were performed with samples of ash and char that remained in the cavern after experiments associated with underground coal gasification. For comparison, samples of raw coals were also burned under controlled conditions in air, in a specially designed furnace, until their complete incineration. Each of these materials was subjected to an elution test using deionised water. An analysis of the physicochemical composition of the water extracts for the selected parameters was carried out. The qualitative–quantitative composition of the water extracts was related to the hydro-geochemical background characteristics of groundwater. The effect of the degree of coal conversion on the ability of metals to elute into the water environment was discussed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author. Tel.: +48 32 259 2853; fax: +48 32 2592273. E-mail address: [email protected] (A. Strugała-Wilczek). http://dx.doi.org/10.1016/j.fuel.2015.06.009 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

The origins of studies on the technology of underground coal gasification (UCG) date back to the 1930s, and their results have been presented in several review papers [1–4]. This process

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acquires energy through in situ coal bed gasification and consists in bringing the appropriate gasifying agent (air, oxygen, water vapour) to the ignited bed and then collecting the generated gas on the surface. Gas, a by-product of industrial value, can be used as synthesis gas or can be used to produce electricity and heat [5]. The process of underground coal gasification involves the use of this material with the lowest possible burden on the natural environment. One of the main advantages of UCG is the possibility of using deeply deposited coal beds or residual beds, which for economic or technological reasons are not suitable for operation using traditional methods [6]. Due to unique geological conditions, the diversified deposition of coal resources and the variability of the process itself, UCG is such a complex process that its implementation requires a number of detailed research studies and analyses, including studies on both the bed intended for gasification as well as its immediate surroundings before, during and after exploitation of the deposit. Particularly interesting and still poorly investigated is the determination of the potential risk of contamination of the natural water environment near quenched post-process caverns. This risk results directly from the coal gasification process, during which solid, liquid and gas products are generated. These products contain significant concentrations of organic and inorganic substances, including substances that are toxic and hazardous to the environment [7–12]. Any oxidation process (including gasification) involving hard coal or brown coal results in the generation of by-products in the form of ash, slag and char. The quality and quantity of the by-products generated in the gasification process depends on the quality and quantity of the mineral components of the combusted fuel, the gasifying agent used and the conditions under which the gasification process occurs. The main source of inorganic impurities is the eluate formed after contact with the residues of coal gasification, whereas the tar formed during the process is mainly responsible for the formation of organic pollutants and ammonia [4]. In general, the elements present in coal, depending on the weight of the dry mass of fuel, can be divided into major elements (forming the organic component of fuel with contents on the order of a few % by weight); ash-forming elements such as Al, Ca, Fe, K, Mg, Na, and Si (at concentrations ranging from approximately 1000 ppm to several weight%); and trace elements such as As, B, Cd, Cr, Hg, Pb, Se, Zn, Cl, and F (at concentrations below 1000 ppm) [13]. The organic matter in coal consists mainly of elements such as carbon, hydrogen, nitrogen, sulphur and oxygen in proportions that depend on the type and degree of coalification of coal. The mineral content of solid fuels can be classified into two groups based on origin: syngenetic (internal, which accompanies the formation of coal beds from the beginning) and epigenetic (external, non-homogeneous mixture containing organic coal). During thermal processing, coal mineral matter undergoes profound physicochemical changes, resulting in the formation of, e.g., slags and ashes composed mainly of the oxides of Si, Al, Ti, Fe, Ca, Mg, Na, K, and P. The trace elements present in coal have been studied by a number of authors to examine their behaviour at the different temperatures of pyrolysis, gasification and combustion; to determine their affinity for organic and inorganic substances; or to classify them with respect to their volatility or importance to the environment [14–19]. After the completion of the process of underground coal gasification, a portion of solid products such as char, ash and slag remains in the cavern where the quenching occurred, and the complete removal of these solid products is not possible. The reference literature presents studies whose aim was to determine how different metals behave under the conditions of underground coal gasification or pyrolysis [20–22]. Such studies, among others, have

attempted to determine the feasibility of exploiting coal seams by means of underground coal gasification for commercial purposes [23]. However, there is a lack of research that provides a definite answer to the question regarding whether, after the UCG process, a cavern is a neutral place and does not pose a threat to the natural environment. A report published by the EPA in 2009 [24] documents that from the residue stored after the thermal treatment of coal (from ash or sludge), harmful substances can be eluted at concentrations that pose a serious threat to human health. An elution test of 73 samples of coal ash conducted by the EPA showed that from some of the ashes under certain conditions, harmful substances can be released, whereby hundreds or even thousands of times the adopted standards for drinking water are exceeded, and some of these substances, such as arsenic or selenium, elute at the level classifying water as a dangerous waste substance. Water flowing into the post-reaction space in contact with post-process cavern residue may also be assumed, under certain circumstances, to elute significant amounts of toxic substances. Therefore, it is extremely important to monitor the surroundings of geo-reactors for the elution of pollutants into groundwater, either directly before or after gasification, as well as during a sufficiently long period after the termination of the process. Safe storage of waste, including the residues of the underground coal gasification process, is a serious problem from a technical point of view and requires knowledge regarding not only the chemical composition of the pollutants themselves but also the extent of their elution. Because geo-reactors are situated in the vicinity of natural geological formations of varying structure and properties, there is always a risk that, despite proper prior hydrogeological examination, the thermal and mechanical factors accompanying the process of UCG significantly alter rock mass parameters and thus enable the migration of eluted substances into the environment (for example, with newly formed cracks, crevices or sinks). Because each waste remaining in the environment represents a potential threat, the possibility of quantifying the waste is even more important. During UCG risk of water pollution in the vicinity of the site of operation is negligible, because the pressure of the process is lower than the pressure in the overburden and contaminants are retained inside the cavern. After completion of the process, groundwater may flow into the post-UCG area and have contact with the post-process residues, eluting impurities from waste and carrying them into the surrounding aquifers (on the principle of circulation of groundwater). Due to the physicochemical transformations that waste is subject to with the passage of time in the above-described environment, it is important to find a proper method that would not only allow us to assess pollutants’ susceptibility to elution and degree of toxicity but also determine the anticipated effects of long-term storage. A number of elution tests for solid waste have been developed. These include, among others, the ‘‘EP toxicity test’’ used in the United States; the Toxicity Characteristic Leaching Procedure (TCLP) method, based on US EPA Method 1311; the Synthetic Precipitation Leaching Procedure (SPLP) method, based on US EPA Method 1312 or the method recommended by the Swiss Office for the Protection of the Environment (BUS); and the elution test based on TVA AS.1991 for the determination of hazardous substances in waste. The last test enables, in a relatively objective way, the effects of the long-term storage of solid waste to be predicted. In the case of UCG post-process waste, it is important to adopt the proper method for assessing elution. In many of the methods used to assess solid waste, the degree of elution and the type of chemicals eluted from UCG post-process cavern residues are crucial factors in determining the quality and safety of natural soil

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and the water environment. Over time, the material remaining in a post-process cavern can, under oxidation–reduction conditions, be subjected to many types of reactions and geochemical transformations, generating a series of new compounds that are subject to elution and that may migrate into surrounding water. Due to the nature of the analysed samples, this paper proposes a method of elution that allows us to estimate the amount of substances released into the environment and to assess their toxicity relative to the values established by national ministerial regulations [25] and European regulations [26,27]. A modified elution test was used in this study under quasi-static and dynamic conditions based on the Polish and European standards PN-Z-15009 (1997) and PN-EN 12457-2 (2006) and on the suggestion of the Swiss standard TVA AS.1991 that samples should be tested in their natural state, without further processing. Therefore, uncrushed samples were obtained to prepare water extracts in the form in which they appear in post-process caverns after conducting experiments associated with underground coal gasification. Because the process of underground coal gasification is accompanied by the generation of waste with properties similar to those of ash, the ash obtained after the complete combustion of the samples of the same coal subjected to gasification was adopted as a reference material. These studies were undertaken mainly from the viewpoint of water management, especially groundwater management. An analysis of water extracts from the four samples of ash and char collected from the quenched reactor after carrying out four experiments of underground coal gasification [5,28,29] was performed. Three samples of hard coal and a sample of brown coal were examined. In addition, samples of raw coals were burned completely under controlled conditions in air. Each char and ash sample after UCG and burned coal was then subjected to elution. An analysis of the physicochemical composition of the water extracts obtained was carried out by comparing the amount of metal eluted from individual samples. 2. Experimental 2.1. Samples preparation and analyses The starting raw materials for the study were three samples of hard coal (origin: the experimental mine ‘‘Barbara’’ in Mikołów, Poland; the hard coal mine ‘‘Bobrek’’ in Bytom, Poland; and the hard coal mine ‘‘Piast’’ in Bierun´, Poland) and one sample of brown coal from the coal mine ‘‘Bełchatów’’ in Bełchatów, Poland. For samples of raw coal collected from seams, technical and elemental analyses were carried out. Hard coal from the experimental mine ‘‘Barbara’’ was subjected to in situ underground gasification, whereas the other three coals were gasified in the ex situ surface installation, simulating the process of UCG (Table 1). The details of the experiments are available in [5,28,29]. Each of the four raw coal samples was subjected to controlled combustion in air. For this purpose, each coal sample was placed on a cast iron grill in a specially designed furnace built of refractory bricks, and the coal sample was burned until complete

Table 1 Conditions for carrying out in situ [29] and ex situ UCG experiments [5,28].

a

Origin of coal

Type of Gasifying experiment agent

Max. temperature of the process (°C)

Experimental mine ‘‘Barbara’’ Hard coal mine ‘‘Bobrek’’ Hard coal mine ‘‘Piast’’ Brown coal mine ‘‘Bełchatów’’

In situ Ex situ Ex situ Ex situ

800–1200a 1300 1300 900

Air/oxygen Oxygen Oxygen Oxygen

Estimated temperature; inability to measure during the UCG.

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incineration; when necessary, the combustion was supported by a stream of compressed air from a cylinder. For all samples of raw coal, char and ash collected from the quenched post-reaction cavern, and the ash from the burned coal, the analytical moisture content and the content of major and trace elements (Sb, As, B, Cr, Zn, Al, Cd, Co, Mn, Cu, Mo, Ni, Pb, Hg, Se, Ti, Fe) were determined. The analyses were performed in the accredited laboratory of the Central Mining Institute in accordance with the current European standards. The moisture was determined by a gravimetric method of analysis, according to standard PN-Z-15008-2 (1993). The contents of Al, Fe, Ti, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb and Zn were determined by X-ray fluorescence spectrometry with wavelength dispersion (ZSX Primus II by Rigaku, Tokyo, Japan). Boron and selenium concentrations were determined using an Optima 5300 DV atomic emission spectrometer by Perkin Elmer (Waltham, MA, USA) with excitation via inductively coupled plasma–optical emission spectrometry (ICP–OES) after mineralization of the samples in aqua regia and hydrofluoric acid. The mercury content was determined by atomic absorption spectrometry with cold vapour generation (CV-AAS) using an SMS 100 apparatus (Perkin Elmer, Waltham, MA, USA).

2.2. Elution test Each sample of char and post-processing ash (Fig. 1a and b) and the ash from the burned coal was subjected to an elution test with deionised water (conductivity <0.1 lS/cm). The modified elution test was adopted under quasi-static–dynamic conditions based on Polish and European standards PN-Z-15009 (1997) and EN 12457-2 (2006) and the Swiss standard TVA AS.1991. Uncrushed samples were used to prepare water extracts in the form in which they appear in a post-process cavern after carrying out the experiments for underground coal gasification. The average weight of a laboratory sample was approximately 100 g. For the char and ash remaining after the gasification process, the proportional amounts of variously sized parts were selected from the raw material to provide the best representative sample. A weighed sample was mixed with deionised water in a 1:10 ratio and then shaken on a GFL3040 laboratory shaker (GFL, Burgwedel, Germany) at a rotational speed of 15 rpm for 2 h. After being allowed to stand (4 h), the sample was again shaken for 1 h and then allowed to stand under static conditions for 16 h. The sample was then shaken for 1 h. The total duration of the elution test was 24 h. The resulting eluates were filtered under reduced pressure through a membrane filter with a pore diameter of 0.45 lm (Millipore Merck, Darmstadt, Germany), minimizing the contact between the sample and air, and its physicochemical composition was analysed. The physicochemical composition of the water extracts was analysed with particular emphasis on the selected parameters highlighted in the legislation on groundwater quality [25–27]. The electrical conductivity of the eluate samples was measured according to PN-EN 27888 (1999) using a PHM 240 conductivity meter by Radiometer (Copenhagen, Denmark). The pH was measured potentiometrically, using a CDM pH meter 83 (Radiometer, Copenhagen, Denmark) in accordance with PN-C-04540/01 (1990). The contents of metals and non-metals (Sb, As, B, Cr, Zn, Al, Cd, Co, Mn, Cu, Mo, Ni, Pb, Se, Ti, and Fe) in the eluate were determined by plasma emission spectrometry (ICP–OES) according to PN-EN ISO 11885 (2009) using an Optima 5300 DV by Perkin Elmer (Waltham, MA, USA). The mercury content was determined by atomic absorption spectrometry with cold vapour generation (CV-AAS) according to our own developed procedure using an SMS 100 (Perkin Elmer, Waltham, MA, USA).

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(a) char from the UCG process

(b) ash from the UCG process

Fig. 1. Exemplary samples of test materials (the origin of the coal: experimental mine ‘‘Barbara’’).

3. Results and discussion 3.1. Behaviour of the metals during gasification and combustion Table 2 shows the results of the technical and elemental analyses of the examined coals in the starting coal samples. The results of the analysis of moisture and trace and major elements in char and the ash produced during the gasification as well as the ash resulting from the total incineration of the coal sample, are collected in supplementary material. Some metals are more volatile than others, and during the combustion or gasification of coal, they are released into the air or remain in the solid combustion products, such as fly ash or bottom ash. The concentrations of elements indicate which elements are highly volatile and show that their concentration diminished from coal to ash (Hg). Data (in supplementary material) also indicate which of the elements are rather resistant and show that their concentrations were enriched after combustion (e.g. Fe, Ni, Ti, Cu, Al, B, Sb, and As). Moreover, we can compare the concentrations of metals in the ash from UCG and from the bottom ash from coal combustion. Elements such as Al, Fe and Mn showed similar behaviour in both processes, but for most of the elements, quite different behaviour was observed (As, B, Cr, Co, Cu, Mo, Ni, and Ti). There was a significant difference in the Pb concentration in ash derived from coal combustion and in ash derived from UCG. Similarly, the concentration of Zn was higher in bottom ash, but only in the case of hard coal. Moreover, collected data show the differences in the metals and non-metals contents of the hard coal and brown coal. A much higher concentrations were observed in the hard coals, and in the brown coal, only the Se and Hg contents were higher. Xu et al. [15] showed that the distribution and contents of trace elements in organic and inorganic matter of coal affect the quality

of the semi-finished products of combustion. These elements may have an affinity for organic matter or inorganic substances, or they may be split between the two (as in the case of beryllium, 30% and 70%, respectively). The following are 100% related to the mineral composition: Zn, Co, Li, Mn, Ni, Pb and V, and the following are 100% related to the organic composition: Cd, Cr and Cu [16]. Due to the elements’ behaviour in thermal processes, Scotto et al. [14] divided dispersed, trace and rare elements into two groups: – Mn, Cr, V, Ni, Co, Ge, and Ca – During the thermal decomposition and combustion of coal at 800 °C, these elements remain in the ash in the environment of the basic elements, bonded in the form of silicates, aluminosilicates, and carbonates. At higher combustion temperatures, these elements pass into the fly ash in the form of aerosols. – As, Li, Pb, B, Be, Zn, Cd, and Hg – As, Li, Hg, and Pb sublime at a temperature below 800 °C, whereas the other elements sublime between 900 and 1300 °C. Elements such as As, Li, Cd, and Hg exhibit strong toxicity. In the gaseous form, these elements mix with other pollutants (NOx, COx, SOx, and organic substances) and form toxic aerosols. Additionally, Erickson [17] divided trace elements according to their volatility: – group I (non-volatile elements) – Eu, Hf, Mn, La, Rb, Sc, Sm, Th, and Zr; – group II (moderately volatile elements) – As, Cd, Pb, Sb, Zn, Ga, Ge, Sn, and Te; – group III (volatile elements) – Hg, Br, Cl, and F. – Additionally, Erickson classified some elements into two groups:

Table 2 A technical and elemental analyses of raw coals. Parameter

Analytical state Ash Aa Volatile matter content Va Heat of combustion Qas Calorific value Qai Total sulphur Sa Content of hydrogen Hat Content of carbon Cat Content of nitrogen Na Index of fixed carbon1 1

Unit

% % kJ/kg kJ/kg % % % % %

Origin and type of raw coal Experimental mine ‘‘Barbara’’ hard coal, type 31.2

Hard coal mine ‘‘Bobrek’’ hard coal, type 32.1

Hard coal mine ‘‘Piast’’ hard coal, type 31.2

Brown coal mine ‘‘Bełchatów’’ brown coal

16.5 29.8 24 258 23 192 0.5 3.7 58.0 0.9 47.3

10.2 33.2 30 327 29 242 1.0 4.7 73.6 1.2 54.5

5.73 34.0 29 264 28 078 0.75 4.37 74.26 1.02 54.4

8.6 42.8 20 161 18 955 1.9 3.9 50.7 1.3 34.1

An index of fixed carbon calculated as 100-Wa–Aa–Va.

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– group I and II – Co, Cr, Cu, Be, Ni, Ba, Mo, Cs, Ta, U, V, and W; – group II and III – B, Se, and I. Vejahati et al. [18] classified the elements generated during the high-temperature conversion of coal and are of particular importance for the environment as follows: – group I (elements of the greatest significance): As, B, Cd, Hg, Mo, Pb, and Se; – group II (elements of medium importance): Cr, Cu, Ni, V, and Zn; – group III (elements of minor importance): Ba, Co, Ge, Li, Mn, Sb, and Sr; – group IV (elements of great importance, but present in very low concentrations): Be, Sn, Te, and Tl. Research conducted by Clarke [30] also allowed us to divide trace elements into three classes, depending on their behaviour in combustion or gasification processes, including the boiling points of individual trace element species: – group I (low volatility elements): Eu, Hf, La, Mn, Rb, Sc, Sm, Th, and Zr; – group I/II: Ba, Be, Bi, Co, Cr, Cs, Cu, Mo, Ni, Sr, Ta, U, V, and W; – group II (moderately volatile elements): As, Cd, Ga, Ge, Pb, Sb, Sn, Te, Tl, and Zn; – group II/III: Be, Se, and I; – group III (volatile elements): Hg, Br, Cl, and F. Clarke [30] noted that some elements might belong to two groups because in the various coals, they are linked to the mineral composition in different ways. The presence of trace elements in coals is highly diverse, and the distribution of trace elements is directly responsible for the differences in the behaviour of these elements during heat treatment [31]. The elements present in coal may remain in the products of combustion or gasification, and the concentration ratio of the products depends on both the type of ash and the element itself. This phenomenon can be described in terms of the relative enrichment ratio (RER), also called the relative enrichment factor, according to the relationship (ci a/ci C) ⁄ (AC)/100, where ci a is the concentration of this element in the combustion (gasification) products, ci C is the concentration of the carbon element in coal, and the AC is the percentage content of ash in coal [32]. Generally, the elements that, in the bottom ash, have an RER value 1 belong to class III (highly volatile elements, with a strongly reduced condensing ability on the surface of submicron particles of ash). Elements with an RER value <0.7 belong to class IIa, IIb or IIc. They are volatile and undergo condensation. An RER value close to unity indicates the nonvolatile properties of elements (class I). Based on the RER value, Meij classified the elements as follows: – class Ti; – class – class – class – class

I: Al, Ca, Ce, Cs, Eu, Fe, Hf, K, La, Mg, Sc, Sm, Si, Sr, Th, and IIc: Ba, Cr, Mn, Na, and Rb; IIb: Be, Co, Cu, Ni, P, U, V, and W; IIa: As, Cd, Ge, Mo, Pb, Sb, Tl, and Zn; III: B, Br, C, Cl, F, Hg, I, N, S, and Se [32].

For each of the metals present in the material resulting from the gasification and combustion of coal, the value of the relative enrichment ratio (RER) was calculated (supplementary material). A general increase in the RER value can observed in going from the post-process UCG char and the post-process ash to the ash obtained after burning the coal, but this trend is characteristic of only the samples of hard coal, not the brown coal sample.

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In terms of volatility, based on the RER value and the boiling point of individual elements, the elements were classified as follows: Group I elements exhibit the lowest volatility and remain in ash. Group II elements are moderately volatile elements, present both in ash as well as in the gaseous phase. These Group II compounds can then condense on the surface of ash particles as the gas cools. Group III elements are highly volatile and do not show a tendency to condense, and many of the elements can be classified into more than one group. Metal volatility is determined both by the properties of a given element as well as the bonding structure it adopts (sulphides and organic matter exhibit high metal volatility), as well as the type of coal and high-temperature treatment used. In the case of gasification, the coal bed is heated more slowly than in the combustion process, which affects the behaviour of metals. Thermodynamic models suggest that trace metals are more volatile under reducing gasification conditions then in an oxidising environment, likely because gaseous volatile compounds (such as chlorides, sulphides and hydroxides) are more stable under reducing conditions. In an oxidising environment, metals tend to transform into less volatile compounds, e.g., oxides and sulphates [33]. The so-called chalcophile elements, such as As, Cd, Mo, Pb, Sb, Se, As, Zn, and Cu, which have a strong affinity for sulphur, are considered to be the most volatile elements under combustion conditions because they occur in the form of sulphides or sulphide minerals. However, under the reducing conditions of gasification, volatile compounds may differ significantly from those generated in the combustion process [34]. In turn, lipophilic components (such as Co, Cr, Ti, V, Al, Ti or Mn) that may easily react with oxygen slowly form volatile compounds of silicates, aluminosilicates, oxides, and other salts of oxyacids. The phenomenon of metal release is further complicated by the chemical and physical adsorption of the compounds resulting from the process [31]. In the case of hard coals (Fig. 2), most of the metals examined tended to undergo changes in their volatile properties depending upon the type of gasification (combustion) product formed. This behaviour was observed for Fe, Zn, B, Mn, Ni, Cu, Sb, Al, Ti, Pb, Cr, and As, which can be classified into group III based on their contents in the char, whereas their contents in the UCG post-process ashes and the ashes after coal combustion indicate group II or group I behaviour, respectively. Some metals can be assigned to two groups. For example, in the case of chars, groups III and II include Al, Fe, Cu, As, B, Co, Zn, and Mo. In the post-process ashes, groups III and II are represented by Sb and Zn and groups II, and I are represented by Mo, B, Cr, Al and Ni. The ashes resulting from the burning of coal contained elements belonging to both groups II and III as well as elements belonging to groups I and II. Additionally, Fe, Cu, Sb, Zn and Mn could be assigned to both groups I and II. An extreme case was that of Cr, which could be classified into group I or III. Similar behaviour for Cr was observed by Huang et al. [31], who justified the assignment by the presence of the metal not only in the organic matter of coal (in which it is easier to sublime, therefore making Cr eligible to belong to group III), but also in the mineral portion, which in turn led to the classification of Cr as a group I element (nonvolatile elements). Hg, Se and Cd belong to the group of metals that sublime below 800 °C; thus, their concentrations in samples collected after the thermal decomposition processes are lower than those in the starting samples. Arsenic, despite its similar properties, shows no such tendencies and may occur in UCG post-process ashes, as confirmed by Liu et al. [20]. In terms of volatility, the metals in the hard coals studied were distributed differently than in brown coal, which has a much higher moisture content and contains substances that are readily released upon heating. Following heating, coal bed cracks and loose structures were formed with an increased surface area due

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Fig. 2. Classification of major elements and trace elements due to their behaviour during the underground gasification and combustion of coals.

to the reaction, as a result of which a considerably higher amount of substances could be released from brown coal than from hard coal. Thus, the properties of the metals showed a tendency to shift toward those of metals in group III (Fig. 2). 3.2. The reference values for eluates Boundary values for different classes of groundwater quality [25], taking their hydro-geochemical background into consideration, are presented in supplementary material. The values given for metals refer to their dissolved form. In some classes of water quality, the limit values are the same due to the lack of a sufficient basis for their differentiation. In these cases, during the assessment of a water class, the highest quality class should be chosen among those that have the same limit value. Depending on its physicochemical composition, groundwater is characterised as exhibiting a good or poor chemical status. A good chemical status indicates that the water possesses group I (very good), II (good) or III (satisfactory) quality. This state is achieved when the physicochemical elements are formed only through natural processes occurring in groundwater and fall within the range of concentrations of the groundwater studied (hydro-geochemical background), indicating no impact due to human activities. The increase in the value of certain physicochemical elements may be the result of natural processes occurring in groundwater or possibly a very weak effect due to human activity. Groundwater of quality class IV (unsatisfactory) or V (bad) is characterised by increased values of physicochemical elements that arise from both natural processes and significant human activity. Excessive concentrations of parameters obtained in eluates relative to the adopted upper limits of the hydro-geochemical background for groundwater are written in italics and the values above the worst class limits for groundwater are written in bold (Table 3). 3.3. Distribution coefficient of the studied metals For each element detected in the water extracts obtained from the elution tests, the distribution coefficient kd was calculated and defined as the ratio of the amount of substances in the solid phase (in mg/kg) to the concentration of the substances in solution (in mg/L) (supplementary material). From this definition, it follows that the lower the value of the distribution coefficient is, the greater the relative mobility of the dissolved substance under examination is [7]. Based on the kd value, the analysed elements were divided into three groups (Fig. 3). To facilitate the evaluation,

the distribution coefficients were logarithmised, and as criteria for membership to these groups, the following ranges were adopted: – group III: log10(kd) < 3 – mobile elements with a high capacity for elution to an aqueous phase. – group II: log10(kd) > 3 and log10(kd) < 4 – moderately mobile (semi-mobile) elements. – group I: log10(kd) > 4 – faintly-mobile elements, a small amount of which is eluted into the aqueous phase. Group III comprises Mo, B, Se and Pb, which are characterised by the smallest distribution coefficients. The kd values in the case of Zn allowed us to classify Zn into groups II and III. Iron undoubtedly belongs to group I, as do Al, Cr and Ni; the latter three, however, can be classified into group II as well based on their mobility. The graphs (Figs. 4–6) show the contents of the selected metals in the water extracts of post-process char, the UCG-induced ashes as well as the ashes of the burned coals with respect to the top hydro-geochemical background values characteristic for groundwater. 3.4. Elution of metals classified as mobile – group III (Fig. 4) 3.4.1. Molybdenum A high elution ability was observed in the case of molybdenum, which has the highest boiling point among the analysed elements (in addition to boron), hence its presence in the materials remaining after the gasification and combustion of coal. Excessive concentrations of this metal relative to the adopted upper limit of the hydro-geochemical background for groundwater were recorded in water extracts. Molybdenum passed into the aqueous phase in far greater concentrations from the investigated ashes than it did from char. In the case of char, a slightly excessive concentration of Mo was observed for the material derived from the coal mine ‘‘Piast’’, and concentrations 4 times and 10 times the limit were observed for the material derived from the hard coal mine ‘‘Bobrek’’ and brown coal, respectively. Post-process ashes and ashes obtained after burning coal exceeded the background concentration of molybdenum by factors of 20 and 50 (experimental mine ‘‘Barbara’’), 80 times (hard coal mine ‘‘Bobrek’’), 20 and 300 times (hard coal mine ‘‘Piast’’) and 60 and 13 times (brown coal mine ‘‘Bełchatów’’), respectively. 3.4.2. Boron Boron concentrations exceeding the background concentration occurred in all water extracts derived from the ashes of the burned

Table 3 A physicochemical composition of water extracts obtained from elution tests of char and post-process ash from four experiments of underground coal gasification and coal ashes from burned coal. Physicochemical element

Inorganic elements Antimony Sb Arsenic As Boron B Chromium Cr Zinc Zn Aluminium Al Cadmium Cd Cobalt Co Manganese Mn Copper Cu Molybdenum Mo Nickel Ni Lead Pb Mercury Hg Selenium Se Titanium Ti Iron Fe

Experimental mine ‘‘Barbara’’ (water extract)

Hard coal mine ‘‘Bobrek’’ (water extract)

Hard coal mine ‘‘Piast’’ (water extract)

Brown coal mine ‘‘Bełchatów’’ (water extract)

Char

Ash

Ash from burned coal

Char

Ash

Ash from burned coal

Char

Ash

Ash from burned coal

Char

Ash

Ash from burned coal

lS/cm

8.05 65.35

10.9 595

6.9 2700

11.5 1190

12 3600

10.6 2360

11.6 964

12.6 8570

10.2 3860

8.2 1210

11.0 4590

12.0 2380

mV

160

16

290

(–)104

(–)485

(–)20

(–)121

(–)556

(–)15

144

(–)473

(–)58

mgSb/dm3 mgAs/dm3 mgB/dm3 mgCr/dm3 mgZn/dm3 mgAl/dm3 mgCd/dm3 mgCo/dm3 mgMn/dm3 mgCu/dm3 mgMo/dm3 mgNi/dm3 mgPb/dm3 mgHg/dm3 mgSe/dm3 mgTi/dm3 mgFe/dm3

<0.005 <0.01 0.12 <0.001 0.0195 0.205 <0.0005 <0.001 0.00255 <0.002 <0.0005 <0.003 <0.003 <0.0005 <0.005 0.0055 0.108

<0.005 0.045 0.42 0.013 0.01 0.15 <0.0005 <0.0005 <0.002 <0.003 0.056 0.0034 0.026 <0.0005 <0.005 <0.002 0.0044

<0.03 <0.05 8.25 <0.003 0.20 <0.01 <0.0005 0.40 1.37 <0.005 0.17 0.13 <0.005 <0.0007 <0.005 <0.005 0.0075

<0.01 <0.01 0.14 <0.001 0.011 0.82 <0.0005 <0.001 <0.002 <0.001 0.013 <0.0005 <0.002 <0.0005 <0.01 <0.003 0.0059

<0.01 <0.005 0.035 <0.001 0.019 1.30 <0.001 <0.002 <0.003 0.004 0.25 <0.003 <0.003 <0.001 0.026 <0.005 0.0066

<0.03 <0.03 5.1 0.0045 0.014 3.40 <0.0005 <0.003 <0.002 <0.003 0.24 <0.003 <0.005 <0.0005 0.067 <0.005 0.0063

<0.01 <0.005 0.25 <0.001 0.015 2.24 <0.0005 <0.001 <0.002 <0.001 0.0032 <0.001 <0.001 <0.0005 0.0072 <0.002 0.0086

<0.01 <0.005 0.05 <0.001 0.017 0.093 <0.001 <0.001 0.0021 <0.003 0.058 <0.002 <0.005 <0.001 0.043 <0.01 0.024

<0.01 <0.03 33 0.0049 0.016 54.4 <0.0005 <0.001 <0.002 <0.003 0.89 <0.003 <0.005 <0.0005 0.022 <0.01 0.009

<0.01 <0.01 0.114 <0.001 0.037 1.04 <0.0005 <0.002 0.0072 <0.003 0.028 <0.003 <0.004 <0.0005 0.0165 <0.003 0.00115

<0.01 <0.03 0.13 <0.001 0.13 0.92 <0.001 <0.002 0.0037 <0.005 0.19 0.019 0.072 <0.0005 0.063 <0.01 0.022

<0.005 <0.01 0.13 0.03 0.039 3.35 <0.0005 <0.001 <0.002 <0.003 0.039 <0.003 0.022 <0.0005 <0.01 <0.005 0.013

pH

´ czyk / Fuel 158 (2015) 733–743 A. Strugała-Wilczek, K. Stan

General elements Reaction Electrolytic conductivity at 20 °C Reduction potential

Unit

739

740

´ czyk / Fuel 158 (2015) 733–743 A. Strugała-Wilczek, K. Stan

– in addition to the coal properties themselves—may be related to the high volatility of selenium and the high temperatures of the UCG process. Increased concentrations of selenium relative to the hydro-geochemical background values of groundwater were determined in the eluates of the two chars (hard coal mine ‘‘Piast’’ and brown coal mine ‘‘Bełchatów’’) and three post-process ashes (hard coal mine ‘‘Bobrek’’ 5 times, hard coal mine ‘‘Piast’’ 9 times, and in the case of brown coal over 10 times). In the ashes from the burned hard coal from the coal mine ‘‘Bobrek’’, there was 10 times more selenium eluted, and in the ashes from the hard coal mine ‘‘Piast’’, 4 times more selenium than the acceptable concentration was eluted. It is difficult to explain why such considerable amounts of this metal occurred in the eluates precisely because of the low boiling point of the element.

Fig. 3. A distribution of the elements in terms of mobility for elution into the water phase.

hard coals (for material from the experimental mine ‘‘Barbara’’, the concentration was 15 times higher; for material from the hard coal mine ‘‘Bobrek’’, the concentration was slightly higher; and for material from the coal mine ‘‘Piast’’, the concentration was 66 times higher); however, no significant amounts of boron were observed in the UCG post-process waste extracts. 3.4.3. Selenium Selenium was detected in all water extracts except those prepared from material from the experimental mine ‘‘Barbara’’ (which

3.4.4. Lead The presence of lead was detected in three eluates: UCG ash from the experimental mine ‘‘Barbara’’ and from the UCG ashes (a concentration exceed 7 times the established limit) and ash from the burned brown coal. The concentration of lead found in the samples examined, despite the low boiling point, attests to the fact that this metal is only subject to partial sublimation, remaining bound to coal mineral matter (in the ash) and possibly subject to elution, posing a threat to the water environment. 3.5. The elution of metals classified as moderately mobile – group II (Fig. 5) All of the metals classified into group II may also be included in class III (Zn) or I (Al, Cr, Ni).

Fig. 4. Mobile elements content in the water extracts in relation to the hydro-geochemical background values of groundwater.

´ czyk / Fuel 158 (2015) 733–743 A. Strugała-Wilczek, K. Stan

741

Fig. 5. Semi-mobile elements content in the water extracts in relation to the hydro-geochemical background values of groundwater.

Fig. 6. Faintly-mobile metal content in the water extracts in relation to the hydrogeochemical background values of groundwater.

times) the standards adopted for groundwater. In the case of aluminium, both UCG-generated ashes and UCG-induced chars showed comparable elution abilities. An extract from the char obtained by the gasification of coal derived from experimental mine ‘‘Barbara’’ showed twice the standard concentration of aluminium; that obtained from hard coal mine ‘‘Bobrek’’ showed 8 times the standard concentration; and extracts from the ‘‘Piast’’ and ‘‘Bełchatów’’ coal mines showed 20 times the standard concentration. Extracts from the post-process ashes of the experimental mine ‘‘Barbara’’ and hard coal mine ‘‘Bobrek’’ showed high aluminium contents, and the extract from the coal mine ‘‘Piast’’ showed a concentration that exceeded the standard by a factor of 20. The highest concentrations of aluminium occurred in the eluates from the ashes of the burned coal: the ‘‘Bobrek’’, ‘‘Piast’’, and ‘‘Bełchatów’’ coal mines showed concentrations that exceeded the standard by 30 times, 550 times, and 30 times, respectively.

3.5.1. Zinc Despite the significant zinc concentration observed in the solid materials examined, concentrations exceeding the standard were observed only in the two water extracts – the ash from burned coal from the experimental mine ‘‘Barbara’’ (4 times) and the post-processing ash from the brown coal (more than 2 times). Zinc is a relatively volatile metal, and when exposed to high temperatures, zinc transitions to a vapour phase.

3.5.3. Chromium Chromium was detected in all water extracts from the ashes of the burned coals as well as in the water extract from the UCG ash of coal from the experimental mine ‘‘Barbara’’. In this extract, and in the eluate from brown coal ash, the concentration of chromium exceeded the standard. Due to the low volatility of chromium, one must consider the likely risks associated with the elution of the metal from the waste that remains after the thermal treatment of coal.

3.5.2. Aluminium The greatest amounts of aluminium (next to molybdenum) were eluted into eluates, even though Al is characterised by a low mobility. The aluminium content in the examined solid materials was so large that even a small percentage of elution caused the metal concentration in the aqueous phase to exceed (several

3.5.4. Nickel Nickel was detected in the eluates obtained from the post-processing ash and the ash of burned coal from the experimental mine ‘‘Barbara’’ (the nickel concentration exceeded the hydro-geochemical background value by as much as 26 times) and in the eluate of the ashes obtained by the UCG of brown coal

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from the ‘‘Bełchatów’’ mine, whose nickel concentration exceeded the background value by a factor of 4. This element transitions to the gaseous phase above the processing temperature; hence, nickel occurred in the waste after the combustion (gasification) of coal. 3.6. Elution of metals classified as faintly-mobile – group I (Fig. 6) 3.6.1. Iron Despite the low mobility of iron, its presence was detected in all of the water extracts analysed, which may have been due to its high content in the examined coals and their derivatives after thermolysis (which, in turn, can be explained by the high boiling point of the metal). Iron is well tolerated by organisms and therefore has high acceptable concentrations that were not exceeded. 3.7. Elution of other metals The metals Mn, Ti, As, Co and Cu, which occurred only in single water extracts, were not classified, similarly to Sb, Cd and Hg, which were not detected at the detection level of the measuring instrument in any of the examined eluates. Cd, Hg, As and Sb are highly toxic metals, and their presence in the waters that were in contact with the UCG post-process waste would constitute a major threat; however, as metals with volatile properties that likely already occur during the gasification (combustion) of coal, these metals can then pass to the environment and therefore are not eluted from the process residues. Cu and Ti have relatively high boiling points, which may suggest that in the gasification or incineration processes conducted, they did not transition to the gaseous phase but remained in the solid phase and were therefore not mobile, which may have been due to their incorporation into a water-insoluble structure. A significantly high cobalt content was observed in the extract of the ash derived from the burned coal of the experimental mine ‘‘Barbara’’, where the concentration of Co exceeded the groundwater background value by a factor of approximately 400; however, this metal was not eluted from post-process waste. Cobalt transitions to the vapour phase above the burning temperature employed in this study; therefore, its presence in the waste combustion (gasification) of coal is possible. Manganese also exceeded the characteristic values in the eluate from the ash of the burned coal derived from the experimental mine ‘‘Barbara’’.

4. Conclusions An elution method for substances formed during underground coal gasification was proposed. Increased or significantly excessive hydro-geochemical background values characteristic of groundwater were exceeded in the eluates obtained as a result of contact between deionised water and post-process coal chars, ashes and ashes from burned coal. The physicochemical composition of the eluates obtained, simulating water after coming into contact with a post-process cavern, clearly indicates anthropogenic influences, and increased concentrations of eluted substances caused the groundwaters’ quality to be classified as class V (the worst); this result may indicate the existence of a real risk of water environmental pollution in the vicinity of geo-reactors even after the completion of the UCG process. The major and trace metals present in coal after the completion of the UCG process may be eluted after gasification by incoming groundwater. Water pollution containing trace elements represents a particularly important issue because water plays a key role in the transport of chemical components between different elements of the environment. The metals eluted from the residues

after the gasification or combustion of coal include Al, Mo, B, Se, Cr, Zn, Pb, Ni, Co, and Fe and, to a lower extent, Mn, Ti and Cu. The majority of the metals examined showed a tendency to elute to a higher extent from ash than from char (Mo, B, Se, Cr, Zn, Pb, Ni, and Co). Metals present in the ash derived from burned coal (B, Al, Cr, Co and Mo – with the exception of brown coal ash) exhibited a greater affinity for the aqueous phase than did the metals present in the ash from UCG. Sb, Cd, Hg and As (highly toxic elements) were not detected in any of the extracts examined, suggesting that a fraction of the volatile metals in the coal combustion stage transition to the gaseous phase; thus, they are not eluted from the process residues. Studies of the water extracts of various materials (char and post-process ash and ash from burned coal) allowed us to estimate how the degree of coal conversion affects the ability of individual elements to elute into the environment. Inorganic substances were more difficult to elute from the structure of the char remaining in the cavern than in the ash obtained after the complete combustion of coal (char contains fewer inorganic substances per unit mass than does ash). The exception was the behaviour of aluminium, which could be explained by the presence of a significant amount of this element in the starting materials (possibly in the form of aluminosilicates). A greater number of metals were eluted from brown coal than from hard coals. For the metals present in the products of the gasification and combustion of coal, the values of enrichment factors (RERs) were estimated, and based on these values, an attempt was made to classify the metals into three groups in terms of volatility. The properties of most of the tested metals, depending on the type of combustion or gasification product examined, differed from those of the most volatile metals in group III (in the case of chars) and from those of the slightly to moderately volatile metals in groups II and I (in the case of ashes). The most volatile metals are undoubtedly Hg, Se and Cd. Some of the examined metals could be classified into two different groups which may be associated with the type of coal studied, the characteristics of a given element, the type of structure into the element is incorporated (organic or mineral matter), and the processing conditions of thermolysis. The classification of metals into groups according to their mobility during their elution with water based on the estimated distribution coefficients was proposed. Group III metals (most mobile) included Mo, B, Se, Pb, and Zn. Group II (moderately mobile metals) consisted of Zn, Al, Cr and Ni. Certain metals’ weak capacity for elution (group I: Al, Cr, Ni, and Fe) should not be underestimated because due to their high content in the solid material, the amount of eluted substances may greatly exceed the accepted standards for groundwater (as is the case for aluminium). Based on the results of this study, one can conclude that the intensity of metal elution from gasification residues depends not only on the degree of coal conversion and the type of structure into which the metals are bonded (mineral and organic coal matter) and the volatility of the various metals but also on the gasification (combustion) conditions, such as the gasifying agent used, temperature and operating pressure.

Acknowledgements The results presented in this work were obtained within the framework of HUGE2 (Hydrogen Oriented Underground Coal Gasification for Europe) – a European research project financed by the Research Fund for Coal and Steel under contract no. RFCR-CT-2011-00006. Part of the research was supported by the Polish Ministry of Science and Higher Education, Project No. 11420433-332.

´ czyk / Fuel 158 (2015) 733–743 A. Strugała-Wilczek, K. Stan

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.06.009.

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