The effects of pressure on coal chars porous structure development

Fuel 172 (2016) 118–123

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The effects of pressure on coal chars porous structure development Natalia Howaniec Central Mining Institute, Department of Energy Saving and Air Protection, Pl. Gwarków 1, 40-166 Katowice, Poland

a r t i c l e

i n f o

Article history: Received 4 September 2015 Received in revised form 30 October 2015 Accepted 5 January 2016 Available online 11 January 2016 Keywords: Coal Char Pyrolysis Pressure Porous structure Underground coal gasification

a b s t r a c t The porous structure of chars determine their physical, mechanical and sorption properties, affecting the heat and mass transport in thermochemical processing and sorption capacity when the post-process residues are concerned. The available data on the pressure related porous structure development is limited, and the results may be often considered contradictory, unless complex interactions between various operating parameters are examined simultaneously. In the study the effects of pyrolysis pressure from 1 to 4 MPa on bituminous coal chars surface characteristics are presented. The enhanced porous structure development was observed with the increasing pyrolysis pressure up to 3 MPa, indicating a major role of pressure related swelling behavior of a parent coal in modeling the surface characteristics of chars under the operating parameters applied. The process parameters selected in order to better represent the conditions of the underground coal gasification differentiate the research from the previous literature, where high heating rates and short residence times are applicable. Therefore, the results complement the limited data available in the field concerned. They are also of both cognitive and practical importance in the considerations related to the thermochemical processing of coal as well as the sorption phenomena in the vicinity of the underground coal gasification. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Coal pyrolysis conditions are known to be influencing the structure of the resulting chars and therefore determining their properties in the subsequent gasification process. The porous structure, i.e. cavities, channels and interstices, pore shape, surface area and volume all determine the physical, mechanical and sorption properties of a material. While closed pores contribute to such properties as density, mechanical strength and thermal conductivity, open pores are responsible for flow and sorption phenomena [1]. All these features influence the heat and mass transport and therefore are essential for the assessment of a fuel performance in thermochemical processing. Numerous studies have been reported on the influence of various operating parameters of pyrolysis on the efficiency of this process, as well as on the effectiveness of the subsequent gasification. This vast literature includes analyses of the effects of pyrolysis temperature, pressure, gas atmosphere, heating rate, residence time and fuel composition on products yields and reactivity of chars [2–6]. An increase in temperature is reported to result in an enhanced gaseous products generation and reduced tars production, while higher pyrolysis pressure in general is claimed to suppress the volatiles and tars yields [7–11]. The effects of E-mail address: [email protected] http://dx.doi.org/10.1016/j.fuel.2016.01.028 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

pressure on volatiles and tars yields are also stated to depend on process temperature, though the trends observed in this respect are often contradictory [5,12,13]. However, only some research works go further and examine the changes in the structure of chars produced under various pyrolysis conditions in an attempt to find the relationships between the transformations of coal and the resulting porous structure of chars. The latter determine the transport properties, and in turn, affect the course and effects of the thermochemical processing by influencing thermal, chemical and mechanical behavior of chars. Here the studies on the temperature effects, particle size and chemical treatment on chars structure and reactivity in combustion and gasification processes prevail [14–22]. When pressure is considered, it is reported to influence coal performance in devolatilization, affecting the process products yield as described above, but also to have an effect on swelling behavior and fluidity of fuel particles. These properties are enhanced at elevated pressures [10,11,23]. Still, very few reports consider the influence of coal pyrolysis pressure on the surface characteristics of the resulting chars [10,11,16,24,25]. In general, higher pyrolysis pressure is reported to result in a decreased surface area of chars [11,26] but this depends also on other factors, e.g. coal rank, heating rates, residence times and pyrolysis temperature applied [10,24,25]. Hence, the increase in surface area and reactivity with pressure have also been reported [16,27]. Chars

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produced in hydropyrolysis at higher pressure are considered to be less reactive, while these generated under inert conditions show no influence of pressure on reactivity [12,28]. The main rationale for undertaking the research presented in the paper was therefore the limited and varying data on the pressure-related development of the porous structure of chars and its importance for recognition of essential aspects of gasification process. These include chars performance in pressurized gasifiers and underground georeactors, as well as the understanding and prediction of process products sorption in an underground post-gasification cavity. In the paper the analysis of the pyrolysis pressure of 1, 2, 3 and 4 MPa on bituminous coal chars surface structure, characterized in terms of the specific surface area, total pore volume, micropore volume and area as well as pore size distribution is presented. The main novelty of the approach comes from the experimental conditions applied: relatively low heating rate, high pyrolysis temperature and long residence time of chars at the final pyrolysis temperature. Such conditions were applied to reproduce the conditions of the in-situ coal gasification. They differentiate the study from the limited analyses of pressure effects on chars structure available in the literature, where heating rates and residence time applied differ by several orders of magnitude from the values selected in the study presented. The determination of the basic characteristics of the development of the porous structure of chars at elevated pressures may be also considered as the initial stage of simulation studies of the process products and byproducts sorption in an underground post-gasification cavity. These future studies may be considered as another aspect of the innovativeness resulting from the work presented since very few works on various gases sorption on coal have been reported so far [29,30]. Further, none of them has considered the sorption on the products of pressurized thermochemical processing of coal. 2. Experimental 2.1. Sample preparation Raw bituminous coal was sampled from the residual coal seam at the level of approximately 400 m and pre-treated in accordance with the relevant standard (PN-G-04502:2014–11). Next, the proximate and ultimate analyses were performed in accordance with the relevant national standards (see Table 1). Chars were produced in the high pressure thermogravimetric analyzer Rubotherm with a magnetic suspension balance mechanism (Fig. 1). A 1 g coal sample of particle size below 200 lm was put into the reactor crucible, heated to the temperature of

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1000 °C with the heating rate of 20 °C/min under the pressure of 1, 2, 3 or 4 MPa, respectively, in argon atmosphere and kept at the final temperature for 5 h. The respective thermograms are presented in Fig. 2. 2.2. Surface characteristics of chars The resulting coal chars were crushed and sieved to the fraction below 200 lm with the application of standardized sieves (PN-ISO 3310–1:2000) and outgassed under vacuum at 120 °C overnight prior to the volumetric characterization with the application of the Autosorb iQ Instrument (Quantachrome, USA). The analyzer is equipped with a high vacuum system with turbomolecular pump and low-pressure transducer, which makes possible micro-/mesopores analysis based on nitrogen isotherm. The nitrogen adsorption at 77 K was applied to determine the specific surface area, total pore volume, micropore volume and area as well as pore size distribution of the samples tested. The principle of the adsorption measurement is described elsewhere [33]. The specific surface area was quantified with the use of the multipoint Brunauer–Emmett–Teller (BET) method [31] according to PN-ISO 9277 standard and the respective international recommendations [1,32]. Pore size distribution was calculated with the application of the Density Functional Theory (DFT) method, commonly applied in the characterization of solid materials containing micro- and mesopores [33], while the micropore volume and micropore surface area with the V-t-deBoer method [34]. The volume adsorbed at the relative pressure of 0.99 was considered to represent the total pore volume. 3. Results and discussion As it can be seen from the thermograms, higher pressure resulted in an increased final weight loss of the samples tested, amounting to 34%, 35%, 38%, and 39%, respectively (see Fig. 2). This trend, although seemingly in contrary with some early observations, may become valid if the variation in pyrolysis conditions applied in the literature and in the present study are taken into account. The yield of volatiles and larger tar molecules decreases with process pressure, as a result of secondary reactions and mass transport limitations when short residence times of a few seconds are applied [7–11,35]. This may be quite different when the chars are exposed to the prolonged effects of high temperature and high pressure environment. Indeed, the effect of pressure on volatiles and tars yields is reported to be dependent also on process temperature [5,12,13] and residence time [16], though the tendencies

Table 1 Selected physical, chemical and petrographic data for coal tested. No

Parameter, unit

Value

Standards and analyzers applied

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Moisture W, %w/w Ash A, %w/w Volatiles V, %w/w Heat of combustion Qs, kJ/kg Calorific value Qi, kJ/kg Total sulfur St, %w/w Combustible sulfur Sc, %w/w Carbon C, %w/w Hydrogen Ht, %w/w Nitrogen N, %w/w Oxygen O, %w/w Fixed carbon, %w/w Vitrinite reflectance Ro,% Vitrinite V, %vol. Liptynite L, %vol. Inertinite I, %vol.

3.04 4.21 29.86 30,614 29,651 0.79 0.59 76.79 4.07 1.33 9.97 62.89 0.65 35 10 55

PN-G-04560:1998; automatic thermogravimetric analysers LECO: TGA 701 and MAC 500 PN-G-04560:1998; automatic thermogravimetric analysers LECO: TGA 701 and MAC 500 PN-G-04516: 1998; automatic thermogravimetric analysers: LECO TGA 701 and MAC 500 PN-81/G-04513, PN-ISO 1928:2002; LECO calorimeters: AC-600 and AC-350 PN-81/G-04513, PN-ISO 1928:2002; LECO calorimeters: AC-600 and AC-350 PN-G-04584:2001; automatic analyser TruSpec S by LECO PN-G-04584:2001; automatic analyser TruSpec S by LECO PN-G-04571:1998; TruSpecCHN analyser PN-G-04571:1998; TruSpecCHN analyser PN-G-04571:1998; TruSpecCHN analyser PN-G-04516:1998; calculated as: 100% – W – A – C – H – Sc – N PN-G-04516:1998 calculated as: 100% – W – A – V PN-ISO 7404-5:2002; polarization microscope Axio Imager D1 PN-ISO 7404-3:2001; polarization microscope Axio Imager D1 PN-ISO 7404-3:2001; polarization microscope Axio Imager D1 PN-ISO 740-3:2001; polarization microscope Axio Imager D1

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Venllaon Venllaon for overpressure

Process gas sampling

Gas Dosing System

Purge gas (Ar) inlet Reacon gas (Ar) inlet

High Pressure Thermogravimetric Analyzer

Purge gas (Ar) Reacon gas (Ar)

Reacon gas mixing Gas fow rate control

Cooling water out Cooling water in

Weight change measurements Temperature and pressure control

Process gas

Raw process gas outlet

Process Gas Cooling System

Overpressure outlet

Gas cooling and tar removal

Fig. 1. Schematic diagram of high pressure thermogravimetric analyser installation for pyrolysis experiments.

(b) 100

Time, hh:mm

90

90

05:30

05:00

04:30

04:00

03:30

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04:00

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03:00

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50

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60

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04:00

03:30

03:00

02:30

02:00

01:30

01:00

00:30

00:00

60

70

01:00

70

80

00:30

80

00:00

weight change, %

(d) 100

weight change, %

00:00

Time, hh:mm

(c) 100

50

02:30

02:00

01:30

60 50

05:30

05:00

04:30

04:00

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70

80

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90

weight change, %

weight change, %

(a) 100

Time, hh:mm

Time, hh:mm

Fig. 2. Thermograms of coal pyrolysis at 1000 °C and under pressure of: (a) 1, (b) 2, (c) 3, and (d) 4 MPa.

Table 2 Porous structure properties of coal and chars measured by nitrogen adsorption at 77 K. No

Sample

Multi point BET Specific surface area (m2/g)

DFT pore diameter (mode) (nm)

Average pore diameter (nm)

Total pore volume (cm3/g)

t-method micropore volume (cm3/g)

t-method micropore surface area (m2/g)

1 2 3 4 5

Char Char Char Char Coal

70.63 87.07 101.06 83.56 2.22

0.56 0.57 0.57 0.56 4.15

2.36 2.59 2.37 2.48 24.37

0.042 0.056 0.060 0.052 0.014

0.017 0.022 0.027 0.023 0.000

41.07 55.06 68.32 56.04 0.00

(1 MPa) (2 MPa) (3 MPa) (4 MPa)

claimed in this respect differ significantly. The effects of the pyrolysis process conditions on chars surface characteristics need to be therefore considered as the resultant of numerous interacting factors, created by the process parameters and experimental setup configuration. Under the conditions applied in the study presented

the prolonged exposure to high pressure and temperature seems to have had a considerable effect on the residual weight value, decreasing with the pressure applied from 66% to 61%. The results of characterization of the porous structure of coal and chars are presented in Table 2. The porous structure of the

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Fig. 3. Nitrogen adsorption (77 K) isotherms for coal chars produced in the pressurized thermogravimetric analyzer at 1000 °C and under: (a) 1, (b) 2, (c) 3, and (d) 4 MPa.

parent coal was composed of mesopores with the average pore diameter of over 24 nm and the mode pore diameter of approximately 4 nm. Such a structure is typical for high/medium volatile bituminous coal of high inertinite content (see Table 1) [20,36]. The specific surface area of coal was relatively low, but characteristic for natural coals (see Table 2) [20–22,37]. The nitrogen adsorption isotherms determined at 77 K for various chars tested were of type IV with a slight hysteresis over the

relative pressure 0.4, distinctive for capillary condensation in mesopores (see Fig. 3) [38,39]. The mode pore diameter was similar for all chars, irrespective of the pressure value applied (see Table 2), while some slight variations in the average pore diameter were observed. Both characteristic pore dimensions were an order of magnitude lower than the respective values reported for a parent coal. Over 60% of the surface area of chars was accessible in micropores of a diameter in the narrow range of 0.7–0.8 nm, while

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Fig. 3 (continued)

surface area, m2/g

(a)

100 80 60

specific surface area 40

micro pore surface area

20 0

1

2

3

4

pressure, MPa

(b) 0,07 pore volume, cm3/g

0,06 0,05 0,04

total pore volume

0,03

micro pore volume

0,02 0,01 0,00

1

2

3

4

pressure, MPa Fig. 4. Relationship between pressure applied in coal pyrolysis and (a) pore surface area and (b) pore volume of chars.

over 50% of the surface area in parent coal constituted pores of a diameter between 2 and 5 nm. The specific surface area of chars increased with the pressure applied in pyrolysis from 1 MPa to 3 MPa and decreased for 4 MPa. The most developed porous structure was reported for chars produced under the pressure of 3 MPa. These chars were characterized by the highest share of micropores, corresponding to over 45% of the total pore volume and 68% of the specific surface area (see Fig. 4). These findings are in agreement with the trend reported recently by Tremel et al. [16], who has observed that the surface area of chars prepared in an entrained flow gasifier under H2/N2 atmosphere, and determined with the application of the DFT model to carbon dioxide (273 K) isotherm data, increases with pressure applied from 0.5 to 2.5 MPa. This tendency was especially clear for short residence times (below 2 s). Although the early works

have shown that the carbon dioxide surface area of chars in general decreases with pyrolysis pressure [11,26], the strong dependence of this phenomena also on other factors, e.g. coal rank, heating rates, residence times and pyrolysis temperature should be emphasized [10,24,25]. The swelling properties of coal are also known to influence chars particles size, porosity and the resulting reactivity [23]. Plastic properties of coal are reported to be correlated with the content of volatiles and selected macerals, mainly vitrinite. Inertinite is claimed to exhibit high fusibility, similar to vitrinite, when exposed to elevated pressure [27]. The maximum swelling and fluidity have been proven for coals of 25–28% and 28–32% of volatiles content, respectively [25]. The intensification of swelling, influencing the surface characteristics of chars, with the increasing pyrolysis pressure has been also observed [10,40]. The maximum value of swelling ratio, mostly within the range of 1–2 MPa, followed by a drop with a further increase in pressure has been reported [10,23,41]. It may be therefore expected that good swelling and plastic properties of the parent coal tested, indicated by volatile, vitrinite and inertinite contents (Table 1) similar to the ones reported previously [25,27] could influence the observed enhancement of porous structure development with increasing pyrolysis pressure from 1 to 3 MPa. Thus, it may be concluded that coal characteristics, with predominantly important coal rank and swelling properties under elevated pressure, resulted in the observed development of the porous structure, which was enhanced within the increasing pressure range of 1–3 MPa, and slightly inhibited under 4 MPa. The relatively low heating rate and long residence time applied in the study may have also influenced the results observed. These findings may be confirmed by previous observations of the effects of pyrolysis pressure on chars surface characteristics, though the literature available is limited and the results are often contradictory. The results presented here contribute to the more-in-depth understanding of the influence of pressure on surface characteristics of chars, and in turn, on fuel behavior in the thermochemical conversion and sorption properties of the residual chars. They complement the limited data available in the field concerned also in terms of the experimental results, which are believed to be valid for the underground coal gasification conditions, with the lower heating rate and longer residence time than applicable in conventional gasifiers. Thus, the results are both novel and of practical importance in terms of the recognition of the in-situ coal gasification process and sorption phenomena in the vicinity of a postprocess cavern.

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4. Conclusions 1. The enhancement of the development of the porous structure of chars was reported with the increase in pyrolysis pressure, with the peak values of the specific surface area, micropore surface area, total and micropore volumes reported for chars produced under 3 MPa. This may imply that pressure related swelling behavior of a parent coal played a significant role in shaping the surface characteristics of chars under the experimental conditions applied in the study. The relatively low heating rate and long residence time may have also had an influence on the effects observed. An increase in the final weight loss in coal pyrolysis with pressure applied was also observed. 2. Further analyses of pressure related surface structure characteristics of chars are essential for the determination of chars performance in pressurized gasifiers or process products sorption in the vicinity of an underground coal gasification georeactor. 3. The studies concerning pressure influence on the porous structure development of chars reported so far are very limited, and the results are often ambiguous, since other parameters, to mention only coal rank, process temperature, heating rates, residence time, swelling behavior and plastic properties of a parent coal all affect the resulting chars properties.

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