Characteristics of Water Contaminants from Underground Coal Gasification (UCG) Process—Effect of Coal Properties and Gasification Pressure

Abstract

One of the most important issues during UCG process is wastewater production and treatment. Condensed gasification wastewater is contaminated by many hazardous compounds. The composition of the generated UCG-derived wastewater may vary depending on the type of gasified coal and conditions of the gasification process. The main purpose of this study was a qualitative and quantitative characterization of the UCG wastewater produced during four different UCG experiments. Experiments were conducted using semi-anthracite and bituminous coal samples at two distinct pressures, i.e., 20 and 40 bar. The conducted studies revealed significant relationships between the physicochemical composition of the wastewater and the coal properties as well as the gasification pressure. The strongest impact is noticeable in the case of organic pollutants, especially phenols, BTEX and PAH’s. The most abundant group of pollutants were phenols. Conducted studies showed significantly higher concentration levels for bituminous coal: 29.25–49.5 mg/L whereas for semi-anthracite effluents these concentrations were in much lower range 2.1–29.7 mg/L. The opposite situation occurs for BTEX, higher concentrations were in wastewater from semi-anthracite gasification: 5483.1–1496.7 µg/L, while in samples from bituminous coal gasification average BTEX concentrations were: 2514.3–1354.4 µg/L. A similar relationship occurs for the PAH’s concentrations. The higher values were in case of wastewater from semi-anthracite coal experiments and were in range 362–1658 µg/L while from bituminous coal gasification PAH’s values are in lower ranges 407–1090 µg/L. The studies conducted have shown that concentrations of phenols, BTEX and PAH’s decrease with increasing pressure. Pearson’s correlation analysis was performed to enhance the interpretation of the obtained experimental data and showed a very strong relationship between three parameters: phenols, volatile phenols and CODcr.

Full text

Energies 2021, 14, 6533. https://doi.org/10.3390/en14206533 www.mdpi.com/journal/energies
Article
Characteristics of Water Contaminants from Underground Coal
Gasification (UCG) Process—Effect of Coal Properties and
Gasification Pressure
Magdalena Pankiewicz-Sperka
1,
*, Krzysztof Kapusta
1
, Wioleta Basa
1
and Katarzyna Stolecka
2
1
Department of Energy Saving and Air Protection, Główny Instytut Górnictwa (Central Mining Institute),
Plac Gwarków 1, 40-166 Katowice, Poland; kkapusta@gig.eu (K.K.); wbasa@gig.eu (W.B.)
2
Department of Power Engineering and Turbomachinery, Silesian University of Technology, Konarskiego 18,
44-100 Gliwice, Poland; katarzyna.stolecka@polsl.pl
* Correspondence: mpankiewicz@gig.eu; Tel.: +48-32-3246536; Fax: +48-32-3246522
Abstract: One of the most important issues during UCG process is wastewater production and treat-
ment. Condensed gasification wastewater is contaminated by many hazardous compounds. The
composition of the generated UCG-derived wastewater may vary depending on the type of gasified
coal and conditions of the gasification process. The main purpose of this study was a qualitative and
quantitative characterization of the UCG wastewater produced during four different UCG experi-
ments. Experiments were conducted using semi-anthracite and bituminous coal samples at two dis-
tinct pressures, i.e., 20 and 40 bar. The conducted studies revealed significant relationships between
the physicochemical composition of the wastewater and the coal properties as well as the gasifica-
tion pressure. The strongest impact is noticeable in the case of organic pollutants, especially phenols,
BTEX and PAH’s. The most abundant group of pollutants were phenols. Conducted studies showed
significantly higher concentration levels for bituminous coal: 29.25–49.5 mg/L whereas for semi-
anthracite effluents these concentrations were in much lower range 2.1–29.7 mg/L. The opposite
situation occurs for BTEX, higher concentrations were in wastewater from semi-anthracite gasifica-
tion: 5483.1–1496.7 µg/L, while in samples from bituminous coal gasification average BTEX concen-
trations were: 2514.3–1354.4 µg/L. A similar relationship occurs for the PAH’s concentrations. The
higher values were in case of wastewater from semi-anthracite coal experiments and were in range
362–1658 µg/L while from bituminous coal gasification PAH’s values are in lower ranges 407–1090
µg/L. The studies conducted have shown that concentrations of phenols, BTEX and PAH’s decrease
with increasing pressure. Pearson’s correlation analysis was performed to enhance the interpreta-
tion of the obtained experimental data and showed a very strong relationship between three param-
eters: phenols, volatile phenols and COD
cr
.
Keywords: underground coal gasification; SNG; UCG wastewater; environmental impact assess-
ment; correlation analysis; effluents
1. Introduction
Nowadays meeting the challenges of energy supply safety and provision of compet-
itive energy costs is one of the most important challenges in the energy sector today. De-
spite the current ecological trends towards shifting to renewable energy and green re-
sources, fossil fuels and coal will still be a major source of energy in a near future [1,2].
Coal has been and still is one of the most crucial primary energies and contributes approx-
imately 65% of the total fossil fuel reserves in the world [3]. It is estimated that 45% of
global energy demand will be covered by coal consumption by 2030 [2,4]. However, con-
ventional coal mining has become more difficult and controversial. Ecological and eco-
nomic factors stimulate searching for new ways and solutions for use of coal reserves. One
Citation: Pankiewicz-Sperka, M.;
Kapusta, K.; Basa, W.; Stolecka, K.
Characteristics of Water
Contaminants from Underground
Coal Gasification (UCG) Process.
Effect of Coal Properties and
Gasification Pressure. Energies 2021,
14, 6533. https://doi.org/10.3390/
en14206533
Academic Editor: Marek Laciak
Received: 4 August 2021
Accepted: 4 October 2021
Published: 12 October 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and institu-
tional affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
ativecommons.org/licenses/by/4.0/).

Energies 2021, 14, 6533 2 of 12
of them is underground coal gasification (UCG) which offers many potential advantages
over the traditional mining methods [5,6]. UCG is a method of in-situ (directly in the un-
derground coal seam) thermochemical coal conversion into a synthetic gas [7–9]. The basis
of the UCG process is direct injection of gasifying reagents to the ignited coal seam and
receiving the gas product at the surface [10]. Compared to traditional mining UCG process
has lower surface impact and hence may contribute to the reduction of air pollutants and
greenhouse gas emission [11]. There are several process techniques for the UCG described
in detail in the literature [11–14]. The final gas composition is mainly H2, CH4, CO and
CO2. The most desirable product for UCG process is methane, which strongly improve
calorific value of gas [1,15]. Methane is formed in methanation reaction and directly from
solid carbon in hydrogenation reaction [1]:
CO + 3H2 → CH4 + H2O (ΔH= −206 kJ/mol)
C + 2H2 → CH4 (ΔH= −91 kJ/mol)
Methane rich gas called synthetic natural gas (SNG) can be used as a chemical feed-
stock or as a fuel for power generation [1,16]. SNG seems to be a future fuel and an essen-
tial component in the energy production, which will make several energy-intense indus-
tries more efficient and sustainable, while reducing their carbon footprint. However,
every thermochemical coal processing technology is associated with environmental im-
pact assessment. One of the most important issues is wastewater production and its treat-
ment. The raw UCG product gas, apart from tar compounds and particulates (coal and
ash) contains water vapour, mainly derived from the evaporation of coal moisture, the
coal pyrolysis (pyrogenic water) or from hydrogen combustion. These gas components
tend to condense onto the cooler parts of the facilities, such as the internal surfaces of gas
pipelines or in the gas-treatment module particular devices (e.g., water scrubber). These
condensed processing wastewater is contaminated by many hazardous compounds such
as polycyclic aromatic hydrocarbons (PAHs), phenols, monoaromatic compounds includ-
ing benzene, toluene, ethylbenzene and xylene [10,17–20]. Heavy metals are another
group of UCG-derived contaminants [10,17,18]. Due to its specific nature, the UCG
wastewater requires an appropriately tailored treatment technique. In 1988 Bryant et al.
evaluate the biological treatability of wastewater from the UCG pilot installation in
Hanna, Wyoming [21]. Zhang et al. propose pretreatment of wastewater generated during
coal gasification by acidification demulsion [22]. A large number of toxic compounds pre-
sent in UCG wastewater are difficult to decompose if only biological methods are used
[23]. Thomas et al. presents the possibility of phenol removal from UCG effluents by using
coagulation-flocculation and the H2O2/UV Process [24]. Treatment of coal gasification
wastewater by catalytic oxidation with trace ozone is another promising technique [25].
In recent years there have been several new developments involving biological coupling
processes to treat coal gasification wastewater. Biological coupling treatment methods in-
cluding: conventional biological processes, the combination of adsorption and biotechnol-
ogy processes, biological enhancement technologies, co-metabolism technologies and the
combination of advanced oxidation and biotechnology [23–30]. The development of an
appropriate treatment method to remove pollutants from UCG wastewater is of utmost
importance for the successful implementation of this technology. However, the composi-
tion of the generated UCG-derived wastewater may vary depending on the type of gasi-
fied coal and conditions of the gasification process.
The main aim of the study was to conduct the qualitative and quantitative character-
ization of UCG wastewater generated during four different ex situ UCG experiments. The
effluents were collected during the experiments in order to correlate the compositions and
concentrations of produced contaminants with the coal properties (coal type) and gasifi-
cation conditions.

Energies 2021, 14, 6533 3 of 12
2. Materials and Methods
2.1. Coal Samples and UCG Experiments
The four UCG experiments were carried out in an ex situ UCG installation located in
the Clean Coal Technology Centre of the Central Mining Institute (Mikołów, Poland). The
experimental installation enables simulation of the UCG process in surface conditions.
The schematic view of the installation and wastewater sampling point are presented in
Figures 1 and 2.
Figure 1. Schematic view of the ex-situ high pressure UCG installation. Reproduced from K. Kapusta et al. [1]. (1) reagent
supply system, (2) gasification reactor, (3) tar sampling point, (4) water scrubber—wastewater sampling point, (5) air
cooler for process gas, (6,7) gas separators, (8) thermal combustor, (9) gas purification module for GC analysis.
Figure 2. Water scrubber—wastewater sampling point.
Experiments were conducted using two different coal samples. Coal samples were
gathered from two various locations. The first semi-anthracite “Six feet” coal was obtained
from an open cast coal mine near Merthyr Tydfil (South Wales, UK) and the second one
bituminous coal was obtained from the “Wesoła” coal mine located in Mysłowice (Upper
Silesia, Poland). Detailed parameters of used coals are presented in Table 1. The raw coal
samples were tested for 18 elements, including selected metals and metalloids being con-
sidered the most important for the aquatic environment. The results obtained are pre-
sented in Table 2.

Energies 2021, 14, 6533 4 of 12
Table 1. Characteristics of coals used for the UCG experiments.
Coal
Parameter “Six-Feet” Semi-Anthracite “Wesoła” Bituminous
As received
Total Moisture Wtr, % 1.15 ± 0.40 3.60 ± 0.40
Ash Atr, % 4.61 ± 0.30 8.74 ± 40
Volatiles Vr, % 9.92 ± 0.12 27.67 ± 0.50
Total Sulphur Str, % 1.55 ± 0.04 0.31 ± 0.02
Calorific value Qir, kJ/kg 33,416 ± 220 28,798 ± 200
Analytical
Moisture Wa, % 0.84 ± 0.30 2.18 ± 0.27
Ash Aa, % 4.62 ± 0.30 8.87 ± 0.63
Volatiles Va, % 9.95 ± 0.13 28.08 ± 0.92
Combustion Heat Qsa, kJ/kg 34,414 ± 228 30,317 ± 161
Calorific value Qia, kJ/kg 33,527 ± 221 29,258 ± 201
Total Sulphur Sa, % 1.55 ±0.04 0.31 ± 0.08
Carbon Cta, % 87.31 ± 0.66 75.35 ± 1.13
Hydrogen Hta, % 3.97 ± 0.28 4.61 ±0.40
Nitrogen Na, % 1.29 ± 0.12 1.20 ± 0.22
Oxygen Oda, % 0.50 ± 0.05 7.65 ± 0.1
Specific Gravity, g/cm3 1.35 ± 0.028 1.40 ± 0.018
Vitrinite reflectance, Ro, % 1.67 ± 0.03 0.91 ± 0.03
Vitrinite, V, vol.% 72 ± 6 59 ± 6
Liplinite, L, vol.% 0 ± 1 6 ± 4
Inertinite, I, vol.% 28 ± 3 35 ± 7
Mineral matter, MM, vol.% 2 ± 1 4 ± 3
Table 2. Concentrations of metals and metalloids in raw coals.
Element “Six-Feet” Semi-Anthracite “Wesoła” Bituminous
mg/kg (ppm)
As 10 0
B 14 18
Cd 0 1
Co 10 0.5
Cr 73 0.3
Cu 25 13
Hg 0.22 0.02
Mn 218 357
Mo 4 0.1
Ni 52 2.6
Pb 27 0.8
Sb 17 0.4
Se 0 2.2
Zn 14 8.1
% mass
Al 1.05 0.07
Fe 1.04 1.43
K 0.09 0.002
Ti 0.04 0.001

Energies 2021, 14, 6533 5 of 12
All gasification tests were conducted for a period of 96 h and under two distinct pres-
sure regimes—20 and 40 bar. The general summary of the UCG experiments conducted is
presented in Table 3.
Table 3. General summary of UCG experiments [1].
Coal Type Semi-Anthracite “Six
Feet” (South Wales, UK)
Semi-Anthracite “Six
Feet” (South Wales, UK)
Bituminous “Wesoła”
Coal (Upper Silesia,
Poland)
Bituminous “Wesoła”
Coal (Upper Silesia,
Poland)
Gasification Reagent O2/H2O O2/H2O O2/H2O O2/H2O
Gasification Pressure, bar 20 40 20 40
Experiment duration 96 96 96 96
Average Gas Production Rate,
Nm3/h 9.0 9.4 9.3 9.4
Gas Yield, Nm3/kg of coal
consumed 1.98 1.98 1.77 1.70
Gas calorific value, Q,
MJ/Nm3 11.7 12.1 9.2 10.4
Coal gasified, kg 436.1 455.5 504.0 530.2
Total wastewater production,
kg 46.5 38.6 67.3 55.2
To investigate the effect of coal type and gasification pressure the oxidant supply
rates were the same in all experiments. During first 24 h of the process, oxygen was used
as a gasifying agent, with constant flow5 Nm3/h. After 24 h the processes were carried out
with oxygen and water with flow ratio 5 Nm3/h and 2.5 kg/h respectively.
2.2. Post-Processing Water Sampling
The UCG effluents produced in water scrubber were collected after completion of
each gasification experiment. They represented the average sample of wastewater for
given gasification experiment. After sampling, the wastewater were transported to the
laboratory for chemical analyses. Coal tars and other undissolved residues were removed
by vacuum filtration (Whatman GF/C filters), and filtrates were subsequently stored at 4
°C until analysed.
2.3. Chemical Analyses
The chemical analyses were carried out according to standard analytical methods.
The conductivity, pH and CODCr (chemical oxygen demand) were determined as typical
nonspecific industrial wastewater parameters. Following inorganic parameters were also
determined: total ammonia nitrogen, chlorides, cyanides, sulphates, sulphides and 17
metal and metalloid trace elements (Mn, Fe, Sb, As, B, Cr, Zn, Al, Cd, Co, Cu, Mo, Ni, Pb,
Hg, Se, Ti). Organic analysis included benzene with its three alkyl homologues: toluene,
ethylbenzene and xylene (BTEX), total phenols and 15 polycyclic aromatic hydrocarbons
(PAHs). To determine pH and conductivity potentiometry and conductometry methods
were used according to PN-EN ISO 10523: 2012 and PN-EN 27888:1999 standards. CODCr
index was determined by spectrophotometric method according to PN-ISO 15705: 2005.
Ammonia nitrogen was determined by Flow Injection Analysis (FIA) with gaseous diffu-
sion and spectrophotometric detection according to PN-EN ISO 11732: 2007). The chlo-
rides were determined according to PN-ISO 9297: 1994. The cyanides and the volatile phe-
nols were determined by segment flow analysis (SFA) with spectrophotometric detection
according to PN-EN ISO 14403-2:2012 and PN-EN ISO 14402:2004. Sulphates were deter-
mined according to PN-ISO 9280: 2002. Flow Injection Analysis (FIA) with spectrophoto-
metric detection was used to determined sulphides. To determined metals and metalloid
trace elements inductively coupled plasma- optical emission spectroscopy (ICP-OES) was

Energies 2021, 14, 6533 6 of 12
used (PN-EN ISO 11885: 2009). For the BTEX and phenols analysis the Agilent Technolo-
gies 7890A chromatograph coupled with a static headspace auto sampler Agilent 7697A
and FID detector was applied. The chromatographic column was DB-5MS (30 m, 0.25 mm,
0.5 µm). For determination of PAHs high-performance liquid chromatography was ap-
plied using Agilent Technologies HPLC Series chromatograph equipped with fluores-
cence detector on Agilent ZORBAX Eclipse PAH column (3.0 mm × 250 mm, 5 µm).
2.4. Linear Correlation Analysis
Pearson’s correlation analysis was performed to enhance the interpretation of the ob-
tained experimental data. It is known as a valuable method of measuring the association
between variables data because it is based on the method of covariance. Pearson’s corre-
lation analysis gives information about the magnitude of the correlation and direction of
the relationship. The values of the Pearson coefficient “r” can fluctuate from −1 to 1. An r
= −1 indicates a perfect negative linear relationship, an r = 0 indicates no linear relation-
ship, and an r = 1 indicates a perfect positive linear relationship between variables. The
closer the indicator is to 1, the greater the correlation occurs. In statistical analysis, it is
assumed that the values >0.7 indicating significant correlation between the variables. In-
put data were physicochemical parameters of obtained wastewater samples from all four
UCG experiments.
3. Results and Discussion
The average physicochemical characteristics of the post processing water samples
obtained during all four UCG experiments are presented in the Table 4. Conducted study
revealed significant differences in the qualitative and quantitative characteristics of the
tested water samples. The differences obtained were related to both the type of the coal
used and the applied gasification pressure. The results of the Pearson’s correlation analy-
sis are presented in Table 5. The values of the Pearson coefficient >0.7 are bolded.
Table 4. Average values of physicochemical parameters determined in the UCG effluents from semi-anthracite and bitu-
minous coal experiments.
Parameters Unit
Semi-Anthracite Coal Bituminous Coal
20 Ba
r
40 Ba
r
20 Ba
r
40 Ba
r
pH pH 6.4 5.2 5.3 4.9
Conductivity µS/cm 1228.38 253.38 942 1006.71
CODCr mg/L O2 151.63 48.63 322.71 185.91
Ammonia nitrogen mg/L N 160.11 11.68 96.41 95.74
Chlorides mg/L 11.15 11.68 29.18 45.94
Cyanides mg/L 1.11 1.43 1.7 0.87
Total phenols volatile mg/L 8.45 0.87 17.04 24.46
Sulphates mg/L 33.51 47.66 42.86 52.97
Sulphides mg/L 1.04 0.04 0.97 0.02
Mn mg/L 0.017 0.021 0.018 0.012
Fe mg/L 0.823 0.284 0.131 0.245
Sb mg/L 0.036 0.121 0.064 0.013
As mg/L 0.036 <0.02 <0.01 <0.01
B mg/L 0.072 0.056 0.130 0.252
Cr mg/L 0.013 0.012 0.010 0.006
Zn mg/L 0.021 0.499 0.320 0.200
Al mg/L 0.031 0.046 0.029 0.023
Cd mg/L <0.0005 0.001 <0.0005 <0.0005
Co mg/L 0.004 0.003 <0.003 <0.003
Cu mg/L 0.005 0.010 0.009 0.002
Mo mg/L 0.005 <0.005 0.026 <0.005
Ni mg/L 0.098 0.312 0.051 0.027

Energies 2021, 14, 6533 7 of 12
Pb mg/L <0.005 0.064 0.046 0.060
Hg mg/L <0.0005 <0.0005 <0.0005 <0.0005
Se mg/L 0.016 0.017 0.036 0.027
Ti mg/L <0.0005 0.001 0.001 <0.0005
Total BTEX µg/L 5483.13 1496.73 2514.32 1354.37
Including benzene µg/L 4156.08 1341.43 2196.75 1059.07
Total PAH µg/L 1657.98 361.99 1090.34 407.2
Including Naphthalene µg/L 1321.25 320.88 905 305.74
Total Phenols mg/L 29.73 2.14 49.46 29.25
Table 5. Pearson correlation matrix—results of the linear correlation analysis of physicochemical parameters of UCG
wastewater.
pH Cond. CODCr NH4+ Cl− CN− Volatile Phenols SO42− S
2− Fe B Zn Al Ni Pb Se BTEX PAH Phenols
pH 1.00
Cond. 0.55 1.00
CODCr 0.20 0.56 1.00
NH4+ 0.63
0.99 0.53 1.00
Cl− −0.57 −0.07 0.23 −0.16 1.00
CN− 0.19 0.00 0.19 0.02 −0.11 1.00
Volatile phenols 0.39 0.77 0.87 0.75
0.07 0.18 1.00
SO42− −0.64 0.05 0.10 −0.02 0.33 −0.14 0.04 1.00
S2− 0.31 0.44 0.13 0.46 −0.14 0.02 0.15 0.13 1.00
Fe −0.10 −0.15 −0.14 −0.16 −0.03 0.04 −0.15 0.04 0.06 1.00
B −0.16 0.59 0.57 0.52 0.41 −0.25 0.65 0.40 −0.07 −0.13 1.00
Zn −0.57 −0.55 −0.44 −0.56 −0.02 −0.02 −0.46 0.30 −0.34 −0.20 −0.19 1.00
Al −0.49 −0.18 −0.13 −0.23 0.28 0.24 −0.10 0.59 −0.21 −0.06 0.02 0.53 1.00
Ni −0.26 −0.28 −0.35 −0.27 −0.17 0.23 −0.28 0.34 −0.07 0.45 −0.24 0.50 0.66 1.00
Pb −0.71 −0.30 −0.15 −0.35 0.36 −0.33 −0.22 0.60 −0.21 −0.04 0.22 0.58 0.58 0.38 1.00
Se 0.35
0.87 0.61 0.83 0.09 0.03 0.83 0.14 0.25 −0.10 0.66 −0.41 −0.11 −0.25 −0.14 1.00
BTEX 0.35 0.21 0.18 0.22 −0.16 0.35 0.21 −0.20 0.45 0.66 −0.21 −0.50 −0.21 0.16 −0.44 0.16 1.00
PAH 0.37 0.63 0.31 0.64 −0.07 −0.12 0.32 0.15
0.89 −0.09 0.16 −0.50 −0.25 −0.28 −0.24 0.37 0.35 1.00
Phenols 0.43 0.67
0.75 0.66 −0.09 0.31 0.83 −0.06 0.34 −0.08 0.38 −0.53 −0.18 −0.32 −0.34 0.63 0.39 0.50 1.00
3.1. Coal Type Effect
As can be seen from the Table 4 all analysed water samples exhibit high values of the
CODCr parameter, which is typical for effluents from the thermochemical processing of
coal. The much higher CODCr values were observed in water samples from gasification of
bituminous coal, ranged from 185.9 mg/LO2 to 322.7 mg/LO2, while for semi-anthracite coal
this parameter was in the range from 48.6 mg/LO2 to 151.6 mg/LO2. pH of analysed water
samples was slightly higher for semi-anthracite experiments, fluctuating within 5.2–6.4
level and 4.9–5.3 for bituminous coal. Ammonia nitrogen levels for bituminous coal re-
mained relatively constant from 95.7 mg/L to 96.4 mg/L, while for semi-anthracite coal
wastewater there was a wide concentration range from 11.7 mg/L to 160.1 mg/L. This sit-
uation is determined by pH values, which were in a wider range and fluctuated more
during the gasification of semi-anthracite coal. For chlorides there was the opposite situ-
ation and in effluents from gasification of semi-anthracite coal concentrations were in the
lower range 11.2–11.7 mg/L while for bituminous coal wastewater levels were higher and
fluctuated in a wider range from 29.2 mg/L to 45.9 mg/L. In all wastewater samples low
concentration levels of cyanides and sulphides were observed. Sulphates levels were rel-
atively higher for wastewater from bituminous coal gasification and were from 42.9 mg/L
to 53.0 mg/L while for semi-anthracite coal concentration values were in range of 33.5–
47.7 mg/L. The conducted studies have shown concentrations of metals and metalloids in
all studied water samples were at very low levels (Table 4). Among the 17 of metals and
metalloids, 9 of them (Mn, As, Cr, Cd, Co, Cu, Mo, Hg and Ti) were identified in concen-
trations below the lower detection limit or in an amount not exceeding 0.036 mg/L (for
As). For the rest metals and metalloids concentrations were above lower detection limits,
but still at very low levels. While in raw coals the highest values were for Mn and were

Energies 2021, 14, 6533 8 of 12
218 mg/kg and 357 mg/kg for semi-anthracite and bituminous coal respectively, the high-
est values in effluents were observed for Fe. For semi-anthracite wastewater concentra-
tions varied from 0.284 to 0.823 mg/L, while for bituminous effluents Fe levels were lower
and range from 0.131 mg/L to 0.245 mg/L. Concentrations of metals and metalloids occur-
ring in the raw coal do not directly affect the composition of the wastewater generated
during the UCG process. This is due to the fact that their concentrations are dependent on
the solubility of the individual elements, which varies with pH and the presence of other
compounds (background) in the sample. The wastewater which are formed during the
process is water coming from condensation onto the cooler parts of the installations (e.g.,
in particular devices of the gas-treatment module). Composition of obtained wastewaters
is therefore mainly determined by organic contaminants originating from the tars which
are generated during the gasification process. Therefore, the studies carried out confirmed
that type of coal used for gasification experiments has a significant impact on concentra-
tion levels of organic compounds. Among all pollutants, organic compounds (phenols,
BTEX, PAH) constituted the most significant group of contaminants in UCG wastewater
samples. Comparison of selected organic contaminants concentrations in the wastewater
from gasification experiments are presented in Figure 3.
Figure 3. Concentrations of selected organic contaminants in wastewater from gasification experiments—coal type impact.
As can be seen from the Figure 3 due to high water affinity, the most abundant group
of pollutants in analysed water samples were phenols. Conducted studies showed signif-
icantly higher concentration levels for bituminous coal wastewater, with values from
29.25 to 49.5 mg/L whereas for semi-anthracite effluents these concentrations were in
much lower range 2.1–29.7 mg/L. An analogous situation exists for volatile phenols, where
average concentrations in bituminous coal wastewater were 17 and 24 mg/L at 20 and 40
bar, while for semi-anthracite coal wastewater the average concentrations were propor-
tionally lower 8.45 and 0.87 at 20 and 40 bar respectively. However the opposite situation
occurs for BTEX levels. The conducted studies showed that higher concentrations occurs
in wastewater from semi-anthracite gasification. BTEX average amounts are 5483.1 µg/L
for 20 bar experiment and 1496.7 µg/L for 40 bar experiment, while in samples from bitu-
minous coal gasification average BTEX concentrations were in lower range 2514.3–1354.4
µg/L. A similar relationship can be found for the PAH’s concentrations. The higher values
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
50,000
Total Phenols
(20 bar)
Total Phenols
(40 bar)
Volatile phenols
(20 bar)
Volatile phenols
(40 bar)
BTEX (20 bar) BTEX (40 bar) PAH (20 bar) PAH (40 bar)
Concentration (µg/l)
semi-anthracite coal bituminous coal

Energies 2021, 14, 6533 9 of 12
362–1658 µg/L occurs in case of wastewater from semi-anthracite coal experiments. For
wastewater from bituminous coal gasification PAH’s values are in lower ranges 407–1090
µg/L.
3.2. Effect of Gasification Pressure
The conducted studies revealed some dependencies between coal gasification pres-
sure and physicochemical composition of analysed post-processing water samples. It was
observed that pressure affects such parameters as chloride and sulphate concentrations.
As can be seen from Table 4 chloride release increases along with increasing pressure,
especially for bituminous coal effluents, where chlorides levels were 29.18 mg/L and 45.94
mg/L for 20 and 40 bar respectively. The same situation occurs for sulphates concentra-
tions. For 20 bar pressure sulphates levels were 33.5 mg/L for bituminous coal and 42.9
mg/L for semi-anthracite coal effluents. When process pressure increased to 40 bar, con-
centrations were also higher and were 47.7 mg/L and 53.0 mg/L for bituminous and semi-
anthracite coal respectively.
Just as it was in the case of coal impact the impact of pressure is especially noticeable
in the case of organic compounds such as phenols, BTEX and PAH. Comparison of se-
lected wastewater organic contaminants from gasification of semi-anthracite and bitumi-
nous coal are presented in Figure 4.
Figure 4. Comparison of selected wastewater organic contaminants from gasification of semi-anthracite and bituminous
coal—pressure impact ((s)—semi-anthracite coal; (b)—bituminous coal).
The studies conducted have shown that concentrations of phenols decrease with in-
creasing pressure. When gasification pressure was lower (20 bar) phenols concentrations
were in the field of 29.7 mg/L and 49.46 mg/L for semi-anthracite and bituminous coal
respectively. Whereas in the case of the high-pressure experiments, there was more than
10-fold decrease in phenols concentration for hard coal and almost halved decrease for
bituminous coal, reaching values 2.14 mg/L and 29.25 mg/L respectively. This significant
decrease in the concentration of phenols with the increase in gasification pressure resulted
in a significant decrease in the value of CODCr parameter, which is strongly correlated
with the concentration of phenols (Table 5). The same situation occurred with the BTEX
values and with increasing pressure there were large decreases in BTEX concentrations.
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
50,000
Phenols (s) Phenols (b) BTEX (s) BTEX (b) PAH (s) PAH (b)
Concentrations (µg/l)
20 bar 40 bar

Energies 2021, 14, 6533 10 of 12
In the case of 20 bar hard coal gasification process, the average BTEX values in the studied
effluents were 5483.1 µg/L, while for the high-pressure 40 bar process these values de-
creased more than threefold to 1496.7 µg/L. For effluents from bituminous coal gasifica-
tion, the decrease was slightly lower, with BTEX values of 2514.2 µg/L at 20 bar and 1354.4
µg/L at 40 bar, respectively. The effect of pressure was also observed for PAH levels. As
the pressure increases, there is a large decrease in PAH concentration in the studied
wastewater samples from all four experiments. In the case of semi-anthracite coal experi-
ment there is a decrease from 1658.0 µg/L to 362 µg/L. In the case of bituminous coal the
difference is also significant, for the 20 bar experiment the PAH value was 1090.3 µg/L
while for the 40 bar experiment the average value was 407.2 µg/L. For all discussed or-
ganic compounds groups the same dependence occurs, with the increase of pressure their
concentration in the studied effluents decreases. It can be explained by volatility of these
compounds. At lower pressure more of them are dissolved in the water phase. However,
as the pressure increases, a greater release of the compounds into UCG gas takes place.
3.3. Pearson’s Correlation Analysis
Correlation analysis (Table 5) showed a strong relationship between the conductivity
of the studied effluents and the level of ammonia nitrogen. The Pearson’s correlation co-
efficient was 0.99 which indicates an almost linear relationship between these two param-
eters for all four gasification experiments. Furthermore, correlation analysis showed a
very strong relationship between three parameters: phenols, volatile phenols and CODcr.
The correlation coefficients were 0.87 and 0.75 for CODCr—phenols volatile and CODCr—
total phenols respectively. On the other hand, correlation analysis showed no significant
dependence between CODCr parameter and other toxic organic compounds concentra-
tions such as BTEX or PAH. Although high toxicity of these compounds, the general tox-
icity of gasification wastewater is mainly determined by concentration of phenols [17].
The main reason for this may be the levels of BTEX and PAH concentrations, which are
several times and in some cases even several dozen times lower than the levels of phenols.
For metals and metalloids no significant correlations were observed. This can be explained
by the low concentrations levels in studied wastewater samples. Only for Se correlation
analysis showed a high correlation coefficient between Se and conductivity (r = 0.87), Se
and NH4+ (r = 0.83) and Se—volatile phenols (r = 0.83).
4. Conclusions
The studies conducted revealed that the type of coal used and gasification pressure
have a significant impact on the wastewater parameters. The conducted studies on the
gasification effluents revealed significant relationships between the physicochemical com-
position of the wastewater and the coal properties as well as the gasification pressure.
Regarding the impact of the used coal, influence on parameters such as pH and chloride
can be observed. The pH of the obtained water samples was slightly higher for the semi-
anthracite coal, whereas chloride levels were higher for effluents from gasification of bi-
tuminous coal. The water samples from bituminous coal gasification showed significantly
higher levels of COD parameter. The studied water samples were characterised by a high
concentration of organic compounds, therefore the strongest impact is noticeable in the
case of these pollutants, especially volatile phenols, phenols, BTEX and PAH. Concentra-
tions of volatile phenols and phenols were much higher for bituminous coal. However,
for the BTEX and PAH levels, the opposite situation was observed and higher concentra-
tions were in the case of wastewater from gasification of semi-anthracite coal. Gasification
pressure has also noticeable impact on the composition of obtained gasification
wastewater. As can be seen from the presented data, there is a greater release of chlorides
along with increasing pressure, especially in the case of bituminous coal. The same situa-
tion also occurs for sulphates concentrations. As well as for the impact of the coal type,
gasification pressure impact is the most significant in the case of organic compounds. As

Energies 2021, 14, 6533 11 of 12
has been shown, their concentrations are inversely proportional to the gasification pres-
sure. The conducted analysis showed that among the three main groups of organic pollu-
tants: phenols, BTEX and PAHs, phenols were present at the highest concentrations.
Therefore, it can be assumed that phenolic compounds will have the greatest impact on
the toxicity level of the tested UCG wastewater. Correlation analysis showed also a strong
relationship between the conductivity of the studied water samples and the level of am-
monia nitrogen. The Pearson’s correlation coefficient for these two parameters was 0.99
which indicates an almost linear relationship between them. The conducted research has
shown that the composition of mineral matter of raw coals does not directly affect the
composition of the UCG wastewater. This is because the concentrations of metals and
metalloids are strongly pH dependent. Therefore, the composition of the obtained
wastewater is determined mainly by organic pollutants derived from tars, which are gen-
erated in the gasification process. The conducted research has shown that UCG
wastewater contains many hazardous pollutants and requires the selection of an appro-
priate treatment method, for example, such as for coking wastewater. The presented re-
sults can help in the development of an appropriate UCG wastewater treatment strategy
depending on the coal used and gasification parameters.
Author Contributions: M.P.-S.: Conceptualization, Writing—original draft; K.K.: Conceptualiza-
tion, Methodology, Project Administration, Funding Acquisition; W.B.: Experiments, Visualization;
K.S.: Writing—review and editing, Visualization. All authors have read and agreed to the published
version of the manuscript.
Funding: This work was a part of the MEGAPlus project supported by the EU Research Fund for
Coal Steel, under the Grant Agreement number 800774—MEGAPlus—RFCS-2017 and Polish Min-
istry of Science and Higher Education under Grant Agreement No. 3996/FBWiS/2018/2.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Supplementary data associated with this article can be found in the
online version at https://doi.org/10.3390/en13061334.
Conflicts of Interest: The authors declare no conflict of interest.
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