Analysis and characteristics of tars collected during a pilot-scale underground coal gasification (UCG) trial

Fuel 208 (2017) 595–601

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Full Length Article

Analysis and characteristics of tars collected during a pilot-scale underground coal gasification (UCG) trial

MARK

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Marian Wiatowski , Krzysztof Kapusta, Krzysztof Stańczyk Główny Instytut Górnictwa (Central Mining Institute), Plac Gwarków 1, 40-166 Katowice, Poland

A R T I C L E I N F O

A B S T R A C T

Keywords: Underground coal gasification UCG Pilot trial CO2 gasification Tar

This paper presents the results of six tar sample analyses obtained during 60 days of an underground coal gasification (UCG) trial in the “Wieczorek” mine in Poland. The tars were sampled periodically during coal gasification by four different reagents: oxygen enriched air, air, a mixture of air and carbon dioxide, and air with nitrogen. The tar samples were analysed over a wide range of physico-chemical parameters typical for the assessment of the quality of coke-oven tar. It was observed that due to tar fractionation, only approximately 40% of the produced tar condensed on the surface. The tar obtained during coal gasification by air mixed with carbon dioxide demonstrated analysis results different than the other tars. The resulting tars were characterized by a high average calorific value of 39 MJ/kg, which is higher than that for the typical coke-oven tar (approximately 36.0 MJ/kg). A positive test of miscibility of tar from UCG with coke-oven tar enables consideration of their coprocessing. It was estimated that the temperature in the pyrolysis zone could be in the range of 700–1000 °C. This study also revealed that tars were subjected to secondary reactions in which cracking processes dominated over the gasification time.

1. Introduction Underground coal gasification (UCG) is a technology that can gasify coal located underground into combustible gas. In this method, through the injection well drilled from the surface to the ignited coal seam, a gasifying agent is supplied, while through the production well, the process gas is discharged to the surface. A gasifying agent may be used air, oxygen and water or steam. The obtained process gas consists of hydrogen, carbon monoxide and carbon dioxide, methane and nitrogen. The raw gas obtained in UCG contains some quantities of tar compounds originating from coal pyrolysis. Beside the tar, the raw gas also contains some amount of particulate matter as coal dust, ash and char. The contaminated gas is not suitable for further direct utilization and therefore it must be cleaned. In the purification stage, gas tars mixed with water are condensed in the surface gas treatment plant as a result of gas cooling [1,2]. Although tar is a by-product in the UCG process, after cleaning it can be used as a valuable raw material for chemical applications [3]. The quantity of tars transferred with process gas to the surface depends on process conditions and may differ depending on local conditions. Calculation of the amounts of tars contained during U.S. UCG trials [4] showed that tars contents changed over a wide range, from 1.5 g/m3 (Hoe Creek III) to 15.5 g/m3 (Rawlings I). In the former Soviet

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Corresponding author. E-mail address: [email protected] (M. Wiatowski).

http://dx.doi.org/10.1016/j.fuel.2017.07.075 Received 7 December 2016; Received in revised form 17 July 2017; Accepted 19 July 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.

Union, the experiment of underground coal gasification on an industrial scale [5] at the Podzemgaz South Abinsk Station showed that the tar content in the raw gas amounted to 0.5–6.0 g/m3. In Poland, during two UCG trials [6,7] conducted at the experimental “Barbara” mine at a depth of 30 m, the amounts of condensed tar in gas obtained on the surface were 0.9 g/Nm3 and 0.45 g/Nm3. The typical tars from UCG trials had different properties when compared to typical coal tars from industrial coking plants. Tars from UCG had lower density, a lower viscosity rather similar to that of oil [8], and different chemical and fractional composition [9]. The amount of tar generated in the UCG is usually 13–25 times less than from coking of coal [5]. Although the UCG process was demonstrated many times in a pilotscale, information on coal tars produced during this process is still scarce. The most comprehensive data on UCG tars originate from two U.S. trials, i.e., Hanna IVB [10] and Rocky Mountain I [11]. The studies carried out showed that tar produced in UCG undergoes many physical and chemical transformations before reaching the surface [12–14], resulting in changes in its properties. The process of tar generation begins when the temperature in a coal seam reaches the value of 350–400 °C and finishes at approximately 1000 °C. According to a scheme of two-step coal pyrolysis [15,16] tar formation can be divided into two stages. In the first stage, thermal decomposition of the volatile components contained in the raw coal

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main aim of the insulation was to reduce the amount of condensable components underground during the gasification process. In the lowest points of the installation, where the tar-water condensates were suspected to accumulate, three separators were installed, denoted as S1, S3, S4. In the surface part of the installation, raw gas was subjected to purification.

occurs. In the second stage, tar components are subjected to secondary chemical processes such as partial combustion and cracking. The level of secondary cracking depends mainly on the residence time of tar in the reactor and the temperature. The longer tar stays in the reactor and the higher the temperature, the greater the level of cracking. Compositional data [10] for UCG tars suggest that a considerable amount of secondary cracking occurs underground and during tar transporting with hot gases from the reaction zone to the surface. As a result of this process in UCG tars, carbon and hydrogen content as well as one and two-ring aromatic compounds may change as gasification progresses [10,11]. Data obtained from U.S. trials [10,17] also showed that during the gas flow, fractionation of tar compounds in the underground channels occurred. Fractionation occurs according to boiling point mainly on long transmission pipelines due to temperature differences between the beginning and end. For this reason, heavier components condensed close to the coal seam, while lighter components moved with process gas to the surface. This effect for UCG is undesirable but may be reduced by an appropriate choice of gasification parameters. This article presents results of the investigation on properties of coal tars generated during a pilot-scale UCG experiment conducted in one of the operating coal mines in Poland (mine “Wieczorek”) [18,19] The tar samples were collected periodically over the 60-day gasification trial at different gasification conditions.

2.2. Gasification experiment A description of the main phases of the gasification trial is presented in Table 2. Phase I involved process initiation by ignition and application of the oxygen enriched air (OEA). In phases II–IV, the main research on the gasification conditions were conducted. During phase II of the experiment, operating parameters of the reactor were tested over long time intervals. In phase III, gasification with CO2 as gasification reagent was tested. Concentrations of the CO2 in the gasification agent (air) were in the range from 6 to 16.5% by volume. Phase IV of the experiment consisted of reheating the reactor after the CO2 stage. In phase V, the nitrogen proportion in the gasifying agent was gradually increased, so as to initiate the extinguishing procedure. The average gas compositions obtained in the particular phases of gasification are presented in Table 3. 2.3. Sampling and preparation of tar samples

2. Experimental

UCG tar samples were periodically collected from the surface gas processing plant (wastewater tank denoted as WT in Fig. 1). The collected samples represented the condensable part of the tar gasification by-products separated in the particular gas cooling devices. To obtain samples representative of the different gasification conditions, the samples were composited over the particular phases of the gasification process (Table 4). After collection and averaging, the tar samples were transported to the analytical laboratory. The samples were of liquid and non-homogenous consistency and contained different amounts of the upper aqueous layer. Very small differences in the density of water and tar substances made the preparation procedure difficult and tedious. Thus, great importance for the results obtained, it was a method of preparing samples for analysis leading to obtain a representative sample of tar. The procedure for preparing the sample for analysis included

2.1. Description of the test site The UCG reactor was built in coal seam no. 501, near the “East” shaft of the “Wieczorek” mine located in Katowice, Poland. Seam no. 501 in this region is characterized by a thickness between 5 and 6 m and a dip angle of 6°. The average depth of the seam is approx. 465 m. The overlying strata of the seam are composed mainly of low-permeability shale and sandstone. Long-term exploitation of coal in seam 501 and in other neighbouring coal seams resulted in drying of the rock mass in the area of the planned experiment. Hydrogeological studies showed that the cube of rock mass, in which the UCG reactor was located, was not involved in the regional groundwater flow. Characteristics of coal in seam no. 501 are presented in Table 1. A simplified flow chart of the UCG installation [18] is shown in Fig. 1. Gasification reagents (oxygen, air, carbon dioxide) were supplied from surface tanks through atmospheric evaporators (E), and the gasification air was supplied by rotary blowers D1 or D1R (reserve). The reagent water was supplied by the mine's underground waterworks. The raw UCG gas was transported to the surface through a gas pipeline of total length 760 m. The underground part of the gas pipeline and its high-temperature section on the surface were completely insulated. Insulation thickness of the pipeline was determined so that the temperature on the outside layer of the insulation did not exceed 40 °C. The

• placement of the entire sample in a large glass beaker for the visual assessment of aqueous and tar (oil) layers, • removal of water by decantation and separation of tars for further tests, • determination of the water contents in the prepared tar samples. During the preparation, pre-heating was avoided to prevent evaporation of the light tar fractions. 2.4. Tar samples analysis

Table 1 Proximate and ultimate characteristics of coal (Seam No. 501). Parameter

Value

Proximate analysis, (air dried) Volatile matter, (wt%) Moisture W, (wt%) Ash A, (wt%) Fixed carbon, (wt%) Lower calorific value W, (MJ/kg)

29.9 3.04 4.25 62.89 28.920

Ultimate analysis, (air dried) Carbon C, (wt%) Hydrogen H, (wt%) Nitrogen N, (wt%) Sulphur S, (wt%) Oxygen and other, from difference O, (wt%)

76.79 4.07 1.33 0.79 9.73

Analyses were carried out in the Institute For Chemical Processing of Coal in Zabrze and in the Central Mining Institute in Katowice. The tar samples were analysed for the following physical and chemical parameters typical for the assessment of the quality of coke-oven tar:

• Water content by Karl-Fischer method (own laboratory procedure), • Toluene insoluble components (TI) – according to PN-82/C-97057 standard, • Quinoline insoluble components (QI) according to PN-C97058:1999 standard, • Ash content, according to PN-77/C-97065 standard, • Residues after coking according to PN-88/C97071 standard, • Coking number according to PN-93/C-97093 standard, • Ignition temperature by Marcusson method, 596

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CT

O2

E

N2

E

CO2

E

CP

C

HE21 HE22 HE23

HE01

SV

HE11

TS

WP01

RAS

CO

I

D1

PM

DN100 DN300 in shaft

DN150

I

air

PS WT DN300 N2-I

H2O

SD

S1

S3

S4

G

Fig. 1. Simplified scheme of the UCG pilot plant: O2 – liquid oxygen tank; N2 – liquid nitrogen tank, CO2 – liquid carbon dioxide tank; D1, D1R – rotary blower; E – evaporator; G – UCG reactor; S1, S2, S3 – tar/water separators; C – cyclone separator; N2-I – nitrogen pipe to safety dam; SD – safety dam; HE01 – gas/air heat exchanger; SV – Venturi scrubber; HE11 – gas condensation unit; HE21, HE22, HE23 – final gas cooling/purification stages; TS – tar swirl separator; WP01 – suction fan; RAS – reversible adsorption system; PS1/PS2 – diaphragm waste water pump; CO – flare stack; CT – cooling tower; WT – waste water tank. Table 2 Stages of the UCG trial. Stage No.

Table 4 List of the collected tar samples and process conditions.

Gasification agent

I II III

Air + oxygen Air Air + carbon dioxide IV Air V Air + nitrogen VI Nitrogen Total: 1343 h

The time interval of the gasification stage (h)

Duration (h)

0–193 193–888 888–1008

193 695 120

1008–1181 1181–1343 Extinguishing and cooling down

173 162

Day of the experiment

Sampling date

Gasification reagent

1 2 3 4 5 6

6 14 27 34 39 55

06.07.2014 14.07.2014 27.07.2014 03.08.2014 09.08.2014 24.08.2014

Air + oxygen Air Air Air Air + carbon dioxide Air + nitrogen

• Content of polycyclic aromatic hydrocarbons (PAH) according to PN-C-82056:2000 standard, • Fractional distillation according to PN-C – 97055:2001 standard, • Viscosity according to PN-EN ISO 3219:2000 standard, • Content of BTX and phenols by gas chromatography (GC), • Elemental analysis C, H, N, S.

Table 3 Gas composition (dry basis). Stage

Sample No.

Gas component (vol%) H2

CO

CH4

C2H6

H2S

CO2

N2

O2

I 193 h II 695 h III 120 h IV 173 h V 162 h

15.90 12.09 7.43 10.20 8.34

17.54 15.34 14.95 14.95 11.24

1.22 2.06 1.92 2.03 1.81

0.03 0.04 0.06 0.06 0.06

0.10 0.15 0.09 0.14 0.15

6.43 10.04 18.34 9.12 7.75

58.63 69.76 57.04 63.27 70.56

0.15 0.00 0.17 0.23 0.09

Total 1343 h

10.99

14.40

1.80

0.05

0.13

9.35

63.20

0.08

3. Results and discussion 3.1. Tar content in the process gas To reduce the amount of tar condensed in the surface treatment plant, the average process gas temperature before purification was maintained at approximately 75 °C. At this temperature, the total mass of collected water-free tar in the waste-water tank (Fig. 1) was 2.68 tonnes. During this process, the 230.5 tonnes of coal was gasified, and the total volume of 1,055,867 Nm3 of gas after cleaning was produced. On this basis, the calculated amount of tar contained in the

• Density, • Calorific values, • Miscibility test with coke-oven tar, 597

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process gas on the surface was 2.54 g/Nm3. After finishing the gasification, the installation was dismantled, including the underground part, which was transported on the surface. The inspection of the unscrewing underground installation showed a large amount of tar contained in the separators S1–S3 (Fig. 1) and adhered inside output pipeline diameter DN300. Based on these data, the estimated amount of water-free tar remaining underground was approximately 4 tonnes. Thus, the total amount of tar produced in this process was approximately 6.68 tonnes, and only approximately 40% of this tar was transported with the process gas to the surface. The rest remained in an installation underground due to tar fractionation.

the pycnometric method. The densities of the tested UCG tars were in the range between 1.050 and 1.095 g/cm3. These values are considerably smaller than those characteristic of coke-oven tar, i.e., approximately 1.20 g/cm3. As seen from Table 5, the densities of the UCG tars differed slightly between the particular stages of the UCG process and a decreasing trend over gasification time was observed. Other parameters of the tested samples, especially the high content of quinoline insoluble (QI) and ash, indicate a large amount of impurities in the form of dust and macromolecular agglomerates contained in the tars. The contents of toluene insoluble matter (TI) are similar to the values of QI. The only exception is sample no. 6, which is characterized by considerably higher TI values than values of QI (similarly to coke-oven tar). Such observation can be attributed to the technological conditions in which the tars were formed, i.e., in more inert atmosphere (similar to the coking process). The lowest values of TI and QI were obtained for sample no. 5, deriving from the CO2 gasification stage. The ash contents in the tested tar samples were quite high (0.58–3.04 wt%) relative to the typical coke-oven tar (0.01–0.05 wt%). The observed large differences in the ash content for the individual samples confirm their high heterogeneity. The coking behaviour, expressed both by the yields of coking residues (LK at 870 °C) and by the coking values (CV by Condradson method at 550 °C) change significantly between the tested tar samples (Table 5). The lowest values for both parameters were obtained for the tar collected during the CO2 gasification stage. Contrary to the typical coke-oven tar, the highest ash content for sample no. 2 (over 3%) does not correlate positively with its coking value. Similarly, sample no. 1, with the highest CV number, is characterized by very low ash content (below 0.9%). This additionally confirms the significant heterogeneity of the tested tars. Ignition temperatures for the tested tars ranged between 80 and 98 °C. These relatively high values, compared to the coke-oven tars (65–80 °C), can be attributed to high water contents. The strong positive correlations between the ignition temperature and the tar moisture are especially evident for samples no. 1 and no. 3.

3.2. Physical properties of tars 3.2.1. Technical analysis Prior to implementation of the agreed upon research program, initial tests on selection of the most suitable method for determination of water content were conducted. Application of the azeotropic method, commonly used for coal tars, was excluded, owing to the fact that heating the tar samples to the boiling point of an azeotropic mixture with toluene or xylene would distil the light tar fractions, and possibly initiate the polymerization/polycondensation process. It would substantially affect the properties of the dehydrated tar samples. It was therefore decided that for the determination of water content, a KarlFischer method would be applied, which involves only minor heating of the samples. This titration method is based on the quantitative reaction of water with iodine and sulphur dioxide in the presence of a lower alcohol and an organic base (pyridine). The samples delivered from the UCG test were a mixture of organic tar substances and water. During sample preparation, it was possible to remove only the upper layer of water by decantation. Further physical separation of the water and tar, due to their similar density and mutual solubility, was impossible. As a consequence, very high water contents, determined by the Karl-Fischer method (i.e., up to 40%), were characteristic of the tested tars (Table 5). Such high water contents can be explained by the technological solutions applied both during the gasification process and in the gas treatment plant on the surface. The lowest water content was determined for sample no. 5, derived from the CO2 gasification (stage III), which may indicate a different nature of the tar resulting from the gasification conditions applied. Due to the variable properties and significant heterogeneity of the tar samples and questionable reproducibility of the preparation procedure, the results obtained for other parameters (aside from density and ignition temperature), were expressed as water-free state. This allowed the comparison of results for the individual samples, regardless of the water content. Determination of tar density by the aerometric method was not feasible due to the high heterogeneity of the samples (water suspensions). Hydrometers immersed in the samples showed no tendency to sink. Therefore, the approximate values of density were determined by

3.2.2. Calorific values and miscibility test with coke-oven tar The results of calorimetric studies for the tested tar samples are presented in Table 6. All the tested samples are characterized by high calorific values, i.e., between 36.2 and 43.4 MJ/kg, relative to the typical coke-oven tar (approximately 36.0 MJ/kg). To determine the potential utilization of tar from UCG, the miscibility tests one of selected tar sample (sample no. 3) with coke-oven tar were performed. For this purpose, mixtures of coke-oven tar with 10, 20 and 30% of the UCG tar were conducted. Tests were carried out at room temperature. Prepared mixtures were left to stand and after 24 and 48 h their appearance and stratification tendencies were evaluated. Miscibility tests have shown susceptibility to forming homogeneous

Table 5 Results of technical analysis of tar samples. Parameter

Moisture content Density1 Toluene insoluble Quinoline insoluble Ash Residues after coking Coking value Ignition temperature1 1

Symbol

W d TId QId Ad LKd CVd Tzap

Unit

(wt%) (g/cm3) (wt%) (wt%) (wt%) (wt%) (wt%) (°C)

Sample No. 1

2

3

4

5

6

33.3 1.095 18.7 20.1 0.89 21.0 24.3 98

38.3 1.088 15.3 14.2 3.04 17.9 18.3 93

20.7 1.081 5.9 8.7 0.58 11.2 11.9 85

24.4 1.071 8.6 8.0 1.69 10.9 11.9 82

10.4 1.076 4.2 4.6 0.72 7.0 7.9 86

24.5 1.050 23.8 6.1 1.95 7.3 9.1 80

Determined for wet sample.

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100

Sample No.

1 2 3 4 5 6

Heat of combustion (wet sample) Qir (MJ/ kg)

Calorific value (dry sample) Qid (MJ/kg)

Calorific value (dry ash free sample) Qidaf (MJ/kg)

23.263 23.684 29.873 26.946 32.870 31.379

34.877 38.386 37.671 35.643 36.685 41.562

36.202 39.782 39.331 37.190 38.710 43.389

Percent evaporated (vol %)

Table 6 Results of calorimetric studied for tar samples.

mixtures of the tested tars with a coke-oven tar, with no tendency to stratification and the formation of an aqueous layer. As a consequence, the concepts of possible co-processing of these tars can be considered. Utilization of the tars as components of heavy fuel oils requires separate studies, especially to determine their impact on emission levels.

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

90 80 70 60 50 40 30 20 10 0 100

150

200

250 300 o Temperature ( C)

350

Fig. 2. Distillation curves obtained for the tested coal tar samples.

3.2.3. Tar distillation To better assess the fractional composition of the tar samples, the distillation was carried out in two variants, i.e., fractional distillation and simple distillation (distillation curve). The results are presented in Table 7 and in Fig. 2, respectively. Distillation of the test samples was conducted according to the standard course of 200 ml of sample. During the distillation, the formation of a yellow raid on the distillation head and the release of white fumes were observed, which may indicate partial thermal decomposition of the samples. The distillation residue had no pitch character and did not tend to weaken, making it impossible to determine its softening point. Distillation of the test samples of tar confirms the high water content. Water content determined by Karl-Fischer correlates well with the fraction up to 107 °C. No fraction boiling in the range of 108 to 170 °C in almost all samples (except sample no. 2), confirms the thesis that lightweight components contained in the tar distilled together with water as an azeotropic mixture. Comparing the total fractions of oil in the tars, the highest yields were obtained for sample no. 5 (61.4%) and the lowest for sample no. 1 (29.3%). Apart from sample no. 5, which was of a different nature, significant differences in oil content between samples no. 1 and no. 2 (30% level) and the other, which have a fraction of oil in the range of 45–55% were identified. This explains the different rheological properties of these samples. For samples no. 1 and no. 3, the distillation was completed below 360 °C due to their thermal decomposition. The remaining samples distilled to a temperature of 360 °C.

Table 8 Results of rheological tests - changes in tar viscosity with the temperature. Temperature (°C)

Viscosity (Pa*s)

25 30 40 50 60 70

3.2.4. Rheological properties Viscosity measurements were performed on a rotational viscometer Haake VT 550 with rising temperature in the range of 25–70 °C, higher

Sample No. 1

2

3

4

5

6

0.2063 0.1079 0.0784 0.0636 0.0514 0.0338

0.2017 0.1010 0.0655 0.0562 0.0417 0.0373

0.0729 0.0573 0.0400 0.0271 0.0244 0.0184

0.0725 0.0493 0.0358 0.0278 0.0166 0.0138

0.0434 0.0310 0.0198 0.0129 0.0111 0.0071

0.0271 0.0203 0.0121 0.0117 0.0107 0.0089

0,24 0,22 0,20 0,18 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 0,00

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

20

30

40

50

o

60

70

Temperature ( C) Fig. 3. Viscosity vs. temperature for the tested UCG tar samples.

Table 7 Yields of particular boiling fractions during fractional distillation of the coal tars (vol%). Boiling range (°C)

temperatures were not applied because of the risk of thermal decomposition occurring in the sample. The results obtained are shown in Table 8 and in Fig. 3, respectively. The viscosity tests proved significantly higher values for samples no. 1 and no. 2, the remaining samples have a viscosity much lower and at a similar level. The effect of temperature on the viscosity is rather low, much lower than that of coke-oven tars. Fig. 3 shows that a decreasing trend of tar viscosity over gasification time is observed. The decrease of tar viscosities over gasification time resulted from secondary cracking reactions to which tars were subjected. As a result of this process, side chains were removed from large and branched molecules, resulting in a reduction in viscosity of tars.

Sample No. 1

2

3

4

5

6

Up to 107 108–170 170–270 270–330 330–360 Total of oil fractions

37.0 – 13.2 12.1 4.4 29.7

42.6 6.3 5.6 11.9 8.4 32.2

25.0 – 16.3 18.9 9.6 44.8

30.0 – 16.4 13.7 12.6 42.7

12.0 – 25.2 19.9 16.3 61.4

25.6 – 25.3 18.7 10.2 54.2

Residues Losses

30.2 3.1

24.7 0.5

27.8 2.4

26.4 0.9

25.8 0.8

19.8 0.4

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Table 9 Elemental analysis of tested UCG-derived tars calculated on the dry basis, (wt%).

Table 10 Contents of BTX, phenols and naphthalene derivatives in the tested tar samples, (wt%).

Sample No.

Nitrogen Carbon Hydrogen Sulphur Oxygen* H/C ratio

Compound

1

2

3

4

5

6

1.0 87.1 3.66 0.7 8.54 0.50

1.1 89.3 3.51 0.9 6.29 0.47

0.8 91.0 4.22 0.8 3.98 0.56

0.9 88.6 4.16 0.9 6.34 0.56

1.0 88.5 5.23 0.7 5.57 0.71

1.2 93.4 4.21 0.7 1.69 0.54

* From the difference to 100%.

3.3. Chemical properties of tars 3.3.1. Elemental analysis Elemental analysis provides information about the content of individual elements in the tar samples. The nitrogen and sulphur content, beside some variations during the entire experiment, did not undergo major changes. A decrease in oxygen content (except for samples 3 and 4) and an increase in hydrogen and carbon are observed. The hydrogen content in tars increases faster than the amount of carbon, as evidenced by the increase of the H/C ratio. The described phenomena resulted from secondary reactions to which tars are subjected, with the gasification process causing changes in their chemical composition. The secondary reactions of tars are stimulated by enlargement of the cavity dimensions, which increases the tar residence time in the hot zone of the reactor, a rise of the gasification temperature, and the presence of a large amount of steam inside the reactor. The presence of steam results from water evaporation contained in the gasified coal seam and from water flow into the reactor from the surrounding underground strata [18]. The results of elemental analysis show that the tar samples under study were characterized (Table 9) by relatively small hydrogen contents compared to coke-oven tar (5–6 wt%), and compared to tars generated during other field-scale UCG operations, e.g., the Rocky Mountain trial (8.9–9.6 wt%) and Hanna IVB trial (8.79 wt%). This can be attributed to inefficient recovery of low boiling alkyl hydrocarbons in the applied gas processing plant, which escaped the treatment plant untrapped. A significantly higher hydrogen content was characteristic of sample no. 5 (gasification with CO2), which confirms its different physicochemical nature. It can be concluded that this sample contains lower amounts of unsaturated and aromatic structures. This could result from a limited dealkylation and hydrogen loss under the lower temperature conditions. Some researchers concluded that gasification in the presence of CO2 inhibits secondary processes of thermal decomposition of tars, which would explain the obtained results [20,21].

Sample No. 1

2

3

4

5

6

Benzene Toluene Xylenes Indene 2-Methylnaphthalene 1-Methylnaphthalene Phenol 2,4-Dimethylphenol 2-Methylphenol 3-Methylphenol 4-Methylphenol

< 0.01 < 0.01 < 0.01 0.34 0.68 0.23 0.15 0.19 0.40 0.21 1.25

< 0.01 < 0.01 < 0.01 0.23 0.76 0.66 0.16 0.18 0.39 0.17 1.38

< 0.01 < 0.01 < 0.01 0.30 1.28 0.34 0.36 0.36 0.76 0.37 2.44

< 0.01 < 0.01 < 0.01 0.29 1.14 0.25 0.27 0.37 0.79 0.40 2.20

< 0.01 < 0.01 < 0.01 0.38 1.92 0.81 0.65 0.79 1.52 0.96 2.89

< 0.01 < 0.01 < 0.01 0.75 2.42 0.91 0.57 0.75 1.55 0.92 4.21

Total

3.45

3.93

6.21

5.71

9.92

12.08

Table 11 Contents of PAHs in the tested tar samples, (wt%). Compound

Sample No. 1

2

3

4

5

6

Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b+k)fluoranthene Benzo(e)pyrene Benzo(a)pyrene Perylene Dibenzo(a,h)anthracene + Indeno(1,2,3-cd)pyrene Benzo(g,h,i)perylene

1.25 2.49 0.45 1.42 2.73 1.11 1.58 1.23 0.20 0.19 0.15 0.05 0.06 0.10 0.08

1.38 2.80 0.39 1.63 2.87 1.08 1.43 1.19 0.18 0.20 0.14 0.05 0.06 0.10 0.08

2.44 3.45 0.64 2.55 4.88 1.48 2.04 1.71 0.29 0.29 0.18 0.06 0.07 0.12 0.10

2.20 3.09 0.56 2.41 4.44 1.35 1.80 1.55 0.26 0.26 0.16 0.05 0.06 0.10 0.05

2.89 2.84 0.58 2.62 4.21 1.92 1.53 1.30 0.26 0.24 0.14 0.05 0.06 0.09 0.06

4.21 2.71 0.46 2.69 3.38 1.73 1.20 1.04 0.17 0.16 0.08 0.02 0.03 0.04 0.02

0.05

0.07

0.07

0.04

0.04

0.02

Total

18.33

17.71

25.52

22.58

21.14

20.24

individual compounds increases as the gasification progresses. In tested tars samples, the sum of these compounds was 3.45 wt% on the 6th day of the process, and almost four times higher, 12.08 wt%, on the 55th and final day of the process. The increase in the content of these aromatic compounds was a result of a secondary reaction of the liquid products (cracking, cyclization, aromatization) leading to the creation of stable aromatic structures. In the secondary tars, changes in the cracking processes dominate, particularly in the heaviest components, as evidenced by the decrease in the content of the distillation residues and the increase in the content of oil products (Table 7). Table 11 shows that as gasification progresses, the content of polycyclic aromatic hydrocarbons (PAHs), aside from naphthalene, initially increased (samples 1–3) and then started to decrease (samples 4–6). Generally, the PAHs determined in the highest concentrations were acenaphthylene (2.49–3.35 wt%) and phenanthrene (2.73–4.88 wt%). Contents higher than 1% were also determined for naphthalene, anthracene, fluorene, fluoranthene and pyrene. Sample no. 6 contains the highest amounts of naphthalene, which makes it similar to coke-oven tar. The total content of PAHs was at a level similar to the coke-oven tars, but no explicit domination of naphthalene was observed as for a typical coke-oven tar (9–11 wt%). Using the data in Tables 10 and 11, the temperature range for tar formation (pyrolysis zone) can be estimated. Monocyclic aromatic compounds are generally formed at temperatures of 500–600 °C and start to decompose at 700 °C. Naphthalene and anthracene are formed

3.3.2. Individual chemical compounds The samples were analysed by gas chromatography (GC) for the content of light aromatics (benzene and its derivatives – BTX), polycyclic aromatic hydrocarbons (PAHs), naphthalene and phenols. The analyses were performed using the internal standard method with detection by flame ionization and mass spectrometer. The results are summarized in Tables 10 and 11. As Table 10 shows, for all tested tar samples, monoaromatic hydrocarbons (BTX) were not identified, which makes the samples very distinct from typical coke-oven tars. The content of phenols is relatively high. In this group of compounds, 4-methylphenol was determined in the highest concentrations (1.25–4.21 wt%). Regarding the contents of phenolic compounds, samples no. 1 and no. 2 are quite different from the remaining tars, since they are characterized by the lowest concentrations of phenols (approx. 2 wt%). The highest value of total phenols was determined in sample no. 6 (more than 4 wt%). Analysing the data of Table 10 shows that the total sum of the 600

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H/C ratio, and increase of the oil fractions sum and light aromatic compound content (phenols and naphthalene derivates), as well the decline in the amount of residues after distillation, it was observed that as gasification progresses, tars increasingly were subjected to secondary reactions that dominate the process of heavier components cracking.

at a temperature of 700–1000 °C and decompose in the range of 1000–1100 °C [22–25]. Taking into account the continuous increase of the monocyclic aromatics (phenol and its derivatives) and naphthalene contents, it can be conjectured that the temperature was higher than 700 °C. The content of anthracene also generally increased but at the end of gasification for the last sample (sample no. 6) began decreasing most likely due to thermal decomposition at temperatures above 1000 °C. Considering this data and the tendency for initial growth and then decline of other PAH contents, it can be concluded that the temperature in the pyrolysis zone was not too high, ranging between 700 and 1000 °C. This may be due to the use of air as the gasifying medium, and the high content of steam in the process gas. During 60 days of coal gasification, approximately 65 tonnes of contaminated water was condensed at the surface cooling plant (after subtraction of tar). Evaporation during the gasification process of such an amount of water must result in a lower gasification temperature. This water originated from evaporation of moisture contained in the gasified coal and/or evaporation of the naturally inflowing water into the UCG reactor. Part of it may also be the result of uncontrolled combustion of UCG gas components containing hydrogen. Of all analysed tar samples, it is evident that tar no. 5 has different properties than the others. The most important of these differences include

Acknowledgements The research presented in this paper was performed as a part of the project “Elaboration of coal gasification technology for a high efficiency production of fuels and electricity”, supported by the Polish National Centre for Research and Development under contract no. SP/E/3/ 7708/10. References [1] Blinderman MS, Anderson B. Underground coal gasification for power generation: efficiency and CO2-emissions. In: 12th international conference on coal science, Cairns, Australia; November 2–6; 2003, paper No. 13C1. [2] Wang GX, Wang ZT, Feng B, Rudolph V, Jiao JL. Semi-industrial tests on enhanced underground coal gasification at Zhong-Liang-Shan coal mine. Asia-Pac J Chem Eng 2009;4:771–9. [3] Li C, Suzuki K. Resources, properties and utilization of tar. Resour Conserv Recycl 2010;54(11):905–15. [4] Cena RJ, Thorness CB. Underground coal gasification data base. Livermore CA 94550: Lawrence Livermore National Laboratory; 1981. [5] Pavlovich LB, Strakhov VM. Producing hydrocarbons by the underground gasification of coal. Coke Chem 2013;56:349–55. [6] Wiatowski M, Stańczyk K, Świądrowski J, Kapusta K, Cybulski K, Krause E, et al. Semi-technical underground coal gasification (UCG) using the shaft method in Experimental Mine “Barbara”. Fuel 2012;99:170–9. [7] Wiatowski M, Kapusta K, Świądrowski J, Cybulski K, Ludwik-Pardała M, Grabowski J, et al. Technological aspects of underground coal gasification in the Experimental “Barbara” Mine. Fuel 2015;159:454–62. [8] King SB. Composition of selected fractions from coal tars produced from an underground coal gasification test. ACS Fuel Div 1997;22(2):169–84. Preprints. [9] Gunn RD. Problems solved and problems not solved in UCG. Fuel 1977;22(4):64–75. [10] Barbour FA, Cummings RE. Comparison of coal tars generated by pyrolysis of Hanna coal and UCG Hanna IVB coal tars. Larmie, Wyoming: Western Research Institute; 1986. [11] Barbour FA., Campbell SL., Covell JR, Analysis of coal tars collected from Rocky Mountain 1 ELW and CRIP Modules, Western Research Institute Laramie, Wyoming, Work Performed Under Cooperative Agreement DE-FC21-86 MC11076 for US DOE; June 1988. [12] Akbarzadeh H, Chalaturnyk RJ. Structural changes in coal at elevated temperature pertinent to underground coal gasification: a review. Int J Coal Geol 2014;131:126–46. [13] Klebingat S, Azzam R, Schulten M, Quicker P, Kempka T, Schlüter R, et al. Quantitative tar pollutant production and water solubility prognoses in the frame of Underground Coal Gasification (UCG). 19. Tagung für Ingenieurgeologie mit Forum für junge Ingenieurgeologen. München; 2013. [14] Brutto AW, Bazmi AA, Zahedi G. Underground coal gasification: From fundamentals to applications. Prog Energy Combust Sci 2013;39:189–214. [15] Michael A, Serio MA, Hamblen DG, Markham JR, Solomon PR. Kinetics of volatile product evolution in coal pyrolysis: experiment and theory. Energy Fuels 1987;1:138–52. [16] Gregg DW, Edgar TF. Underground coal gasification. AIChE J 1978;24(5). [17] Philips NP, Muela CA. In-situ coal gasification: status of technology and environmental impact. Radian Corporation P.O. Box 9948, Austin, Texas 78766. Prepared for: U.S. Environmental Protection Agency, Office of research and Development, Washington D.C. 20460. [18] Mocek P, Pieszczek M, Świądrowski J, Kapusta K, Wiatowski M, Stańczyk K. Pilotscale underground coal gasification (UCG) experiment in an operating Mine “Wieczorek” in Poland. Energy 2016;111:313–32. [19] Mocek P, Gil I, Świądrowski J. Instalacja procesowa dla hybrydowej technologii podziemnego zgazowania węgla. Przem. Chem. 2014;93(1):66–9. (In Polish). [20] Luo K, Zhang C, Zhu S, Bai Y, Li F. Tar formation during coal pyrolysis under N2 and CO2 atmospheres at elevated pressures. J Anal Appl Pyrol 2016;118:130–5. [21] Vreugdenhil BJ, Zwart RWR. Tar formation in pyrolysis and gasification. ECN Biomass, Coal and Environmental Research, ECN report number ECN-E-08-087; 2009, p. 1–37. http://www.ecn.nl/docs/library/report/2008/e08087.pdf. [22] Lowry HH. Chemistry of coal utilization: 2nd Suppt.; June 1966. [23] Bruinsma OSL, Geertsma RS, Bank P, Moulijn JA. Gas phase pyrolysis of coal-related aromatic compounds in a coiled tube flow reactor. 1. Benzene and derivatives. Fuel 1988;67:327–33. [24] Bruinsma OSL, Tromp PJJ, de Sauvage Nolting HJJ, Moulijn JA. Gas phase pyrolysis of coal-related aromatic compounds in a coiled tube flow reactor. 2. Heterocyclic compounds, their benzo and dibenzo derivatives. Fuel 1988;67:334–40. [25] Liu P, Le J, Wang L, Pan T, Lu X, Hang D. Relevance of carbon structure to formation of tar and liquid alkane during coal pyrolysis. Appl Energy 2016;183:470–7.

• the lowest water content, • the highest content of total oil fractions, • the lowest amounts of inert substances defined by the content of toluene insoluble (TI) and quinoline insoluble (QI) matter, • the lowest carbon and a higher hydrogen content after gasification, • the lowest content of aromatic and unsaturated structures, a low viscosity, • a relatively high content of phenols. A detailed explanation of the differences in the observed properties of tar no. 5 in relation to the other tars would require additional structural tests such as those of 1H and 13C NMR spectroscopy and FT-IR spectroscopy. These studies, however, exceed the scope of this paper and can be the subject of a separate publication. 4. Conclusions 1. In this process, the mass of condensed water-free tar collected in the surface gas processing plant was approximately 40% of the total estimated tar produced. The rest remained in the underground part of the installation due to the tar fractionation phenomena. 2. The physicochemical properties of the UCG tars were significantly influenced by the process conditions (gasification reagent). It should be emphasized that the tar obtained during the CO2 gasification stage (sample no. 5) has very distinct properties compared with tar samples obtained in other gasification stages. 3. Sample no. 6 has a physicochemical character most similar to the typical coke-oven tar, which results from a much higher percentage of TI compared to QI, and from the highest concentration of naphthalene from all samples, although this concentration is still more than two times lower than that of a coke-oven tar. 4. Tars from UCG are characterized by a high calorific value in the range of 36–43 MJ/kg, which poses the possibility of their use as components of heavy fuel oils. A positive test of miscibility with coke-oven tar allows consideration of their co-processing. Utilization of tars from UCG was not the main goal of this paper, therefore this requires additional studies. 5. Based on changes in the content of one and two-ring aromatic compounds and PAHs and their temperature of formation, it was estimated that the temperature in the pyrolysis zone could be in the range of 700–1000 °C. 6. On the basis of the decrease in the viscosity of tars, increase of the 601