Assessing the catalytic effect of coal ash constituents on the CO2 gasification rate of high ash, South African coal

Fuel Processing Technology 92 (2011) 2048–2054

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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c

Assessing the catalytic effect of coal ash constituents on the CO2 gasification rate of high ash, South African coal Burgert B. Hattingh a,⁎, Raymond C. Everson a, Hein W.J.P. Neomagus a, John R. Bunt a, b a b

Energy Systems, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa Sasol Technology (Pty) Ltd, PO Box 1, Sasolburg, 1947, South Africa

a r t i c l e

i n f o

Article history: Received 2 December 2010 Received in revised form 20 May 2011 Accepted 2 June 2011 Keywords: South African coal Characterization CO2 gasification Inherent catalytic effect Ash constituents

a b s t r a c t The catalytic effect of inorganic species, within the ash, on the CO2 gasification of three South African coals containing similar carbon-structural properties (elemental, structural and petrographical properties) was assessed. The reactivity of the coals with a particle size between 150 and 250 μm was determined in a thermo gravimetric analyser. The reactivity was measured at temperatures between 900 and 1000 °C, pressures between 1 and 10 bar, and fractions of CO2 between 10 and 30%. For the selected coals, the reactivity decreased with ash content, and was found to be dependent on the composition of the ash. Specifically, the reactivity increased with calcium and magnesium content and alkali index. © 2011 Elsevier B.V. All rights reserved.

1. Background and Introduction The importance of coal today, apart from its role as an important energy source, lies in its ability to be converted into useful chemical products as well as liquid fuels [1]. The conversion of coal to these products involves the initial step of gasification which primarily consists of initial rapid pyrolysis (devolatilization) of the coal to form char, tar and gases; and the subsequent gasification of the formed char to produce “syngas” [2–4]. The gasification process includes reactions with oxygen, steam and CO2 [5] and therefore gasification processes are largely influenced by the concentration of these gases within the gasifier as well as the hydrodynamics involved. In addition coal type and coal characteristic properties also play an important role during gasification. The correlation of coal characteristic properties such as carbon content, volatile matter, maceral content and rank with coal reactivity has generally been discussed in literature [6–8]. Furthermore it has been concluded that the major factors which control the reactivity of the coals are: (1) maceral- and microlithotype composition [6–11], (2) concentration of active sites or reaction surface area (micropore surface area) [10,12,13], (3) accessibility of the particular reactant gas to the active sites (porosity) [14] and (4) the catalytic effect of the

⁎ Corresponding author. Tel.: + 27 18 299 1545; fax: + 27 18 299 1535. E-mail address: [email protected] (B.B. Hattingh). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.06.003

inorganic constituents (the presence of catalytically active species such as Ca-, K-, Na- and Mg bearing species present in the ash) [16–20]. Currently a particular interest has been shown in assessing the catalytic activity of inorganic species on coal gasification. It is believed that the presence of inorganic constituents within coal is a direct consequence of geochemical factors such as erosion, ion exchange, chelation, diagenetic change and epigenetic input, thus contributing to very diverse forms of inorganic species [21]. Low rank coals therefore contain a relatively large amount of Na, Mg, Ca, K and Sr which are organically associated within organic acid groups and chelates, as well as mineral grains [22]. The macromolecular structure of lignites and sub-bituminous coals are characterized by a significant amount of carboxylic acid groups. A fraction of the H+ ions in these carboxylic acid groups have however been exchanged with metal cations such as K +, Na+, Ca 2+, Mg2+ and Fe3+. It is believed that the presence of these metal cations on the surface of the coal increases the reactivity of coals in gasification [5,23–26]. For higher rank coals the inorganic components are mainly concentrated in discrete mineral grains of quartz, kaolinite, illite, pyrite and calcite [21]. Furthermore the catalytic effect of inorganic species on coal/char reactivity varies with gasifying atmosphere and for the specific case of CO2 gasification; Ca 2+ plays an important catalytic role [11,27,28]. Carbon dioxide (CO2) gasification experiments conducted by Adanez and Diego [20] on both raw and demineralized chars indicated that the conversion rate of the raw chars was larger than for the demineralized chars, indicating that mineral matter plays a role in the reactivity during lignite gasification. Sun et al. [15] observed the same during the CO2 gasification of Shenmu maceral chars (a Chinese coal)

B.B. Hattingh et al. / Fuel Processing Technology 92 (2011) 2048–2054

with and without the addition of K2CO3 as catalyst. Even demineralized maceral chars impregnated with catalyst showed an increasing trend in reactivity [15]. Jaffri and Zhang [29] also illustrated the effect of Ca2+ and Na + loading on the CO2 gasification reactivity of Fujian anthracite. The catalytic effect of metal species such as Ca2+, Mg2+, K + etc., in oxidizing conditions, was also confirmed by other investigators [16–19]. In addition it is believed that the catalytic effect on the gasification of higher rank coals is much less than for lower rank coals [21]. Numerical indices such as the alkali index (AI) [30] have been defined in an attempt to describe the catalytic efficiency of inorganic species. A few mechanisms have been proposed in order to describe the catalytic pathway of certain inorganic species [16,23,31–33]. One such pathway includes the carbonation of the Ca–C interface, followed by a two step oxygen exchange mechanism [16]. According to Floess et al. [23] the three fold increase in reaction rate obtained during CO2 gasification due to calcium can be attributed to the lowering of the activation energy for chemisorption of CO2. A large number of the catalytic gasification studies consist however of removing the original mineral matter by accustomed demineralization techniques and impregnating/loading the demineralized coal/ char with inorganic additives such as K2CO3 and Na2CO3. The catalytic effect of a certain mineral species is therefore investigated by externally adding or removing catalyst. In addition a drawback however of this procedure is that the method and acids used during the demineralization process can also modify the chemical functionalities and morphology of the carbon matrix [34]. The loading of catalyst without inherent mineral removal also provides the additional complication of interaction between the inherent mineral matter and the loaded catalyst [35]. The purpose of this paper is to report the inherent (already contained minerals within the coal) catalysis on gasification based on the comparison between the effects of the different parent properties of the coal. For this, the CO2 gasification of three coals, having similar chemical- and petrographical features, was investigated. 2. Material and methods 2.1. Coals Three coals; XA, XB and XC all originating from the South African Highveld no. 4 seam were chosen for this investigation. The choice of the three coals was based on their similarity in rank and petrographical nature, therefore attributing any differences between the coals to their mineralogical properties. 2.2. Characterization of the coals All three coals were characterized on a chemical, petrographical and structural basis. Standard procedures and methods were used to conduct the respective characteristic analyses. The important coal properties derived from these analyses, as well as the respective standards used are summarized in Table 1. Discussion of these results will be given in Section 3.1. Gas adsorption with CO2 was conducted to determine the micropore surface area using a Micrometrics ASAP 2010 Analyser. The samples were initially degassed at 25 °C for 48 h prior to the adsorption at 0 °C. A saturation pressure (P0) of b660 mm Hg was used for all the gas adsorption experiments. The values of micropore surface area were referred to on an ash-free basis for comparative purposes, due to the fact that some of the coals contained large amounts of mineral matter with different adsorption properties than the organic fraction [36]. XRF and XRD analyses were used to determine the composition of ash- and mineral constituents respectively. The ASTM 3682 method was used in the identification of the different inorganic species present in the ash. For the XRD analyses the coal powder samples were prepared using a back loading preparation method. A PANalytical X'Pert Pro powder diffractometer with X'Celerator detector and variable divergence- and receiving slits with Fe

2049

Table 1 Chemical, petrographic and surface properties of the three coals. Standard Chemical properties Proximate analysis (wt.%) Inherent Moisture (a.d.b) Ash (a.d.b) Volatile Matter (d.a.f) Fixed Carbon (d.a.f) Gross calorific value (MJ/kg, m.m.b) Gross calorific value (MJ/kg, m.m.f) Ultimate analysis (wt.%, d.a.f) Carbon Hydrogen Nitrogen Oxygen Total Sulfur (organic and inorganic) Petrographic properties Mean random reflectance (%) Maceral analysis (vol.%, m.m.f) Vitrinite Liptinite Inertinite Carbominerite (vol.% m.m.b) Minerite (vol.%, m.m.b)

SABS 925 SABS ISO 1171:1997 SABS ISO 562:1998 By difference SABS ISO 1928

XA

XB

XC

6.5 20.5 30.5 69.5 22.3

4.3 27.9 30.5 69.5 20.6

3.4 42.9 34.6 65.4 15.6

30.5

30.4

30.1

70.9 5.0 1.7 21.8 0.5

71.6 4.9 1.8 20.9 0.9

65.7 5.3 1.7 26.1 1.3

ISO 12902

By difference ISO 19759

ISO 7404–5:1994 ISO 7404–3:1994

ISO 7404–4:1988 ISO 7404–4:1988

Surface properties CO2 surface area (m2/g, d.a.f)

0.57

0.62

0.61

22.4 5.9 71.8 14.0 7.0

20.2 4.8 75.9 17.0 9.0

21.7 4.3 73.9 21.0 25.0

195.0

180.7

197.0

filtered Co-Kα radiation was used to measure the diffraction of each sample. The phases were identified with the X'Pert Highscore plus software and the relative phase amounts (wt.%) were estimated using the Rietveld method [37] (Autoquan Program). This quantification method was first proposed for use in multicomponent systems by Bish and Howard [38] and entails the refinement of the least-squares fitting of the diffractogram until the best fit is obtained between the raw data and the entire calculated pattern [37]. Details of this procedure are provided elsewhere [37]. Any amorphous phases, if present, were not taken into account in the quantification. 2.3. CO2 reaction rate measurements The coals were respectively crushed and screened before gasification experiments. A particle size distribution of 150–250 μm was used for investigating the CO2 reactivity of each coal. The CO2 gasification reaction rate was investigated in a Bergbau-Forshung GMBH7 high pressure thermo gravimetric analysis system (TGA). A detailed description of the apparatus is provided elsewhere [39]. Reaction rate measurements consisted of lowering a platinum gauze basket containing 45 mg of sample into a well-mixed region which was controlled at the specific reaction temperature and reactant partial pressure, while recording the mass loss over a period of time. The sample basket was only lowered in the reaction zone once it reached the desired isothermal reaction temperature. Gas composition was controlled with the aid of two Brooks 5850 mass flow controllers. A similar experimental protocol was followed for the pressurized experiments, with the exception that the reaction chamber was pressurized after reaching the isothermal reaction temperature and prior to sample lowering. The reaction pressure was controlled manually with the gas outlet valve of the TGA system. Table 2 gives a summary of the experimental conditions used for the investigation. A Response Curve Methodology (RCM) was implemented in contrast to the conventional way of combining variables in order to optimize the amount of runs to adequately describe the CO2 gasification of the three coals [40,41]. RCM is an experimental technique that

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3. Results and discussion

Table 2 Experimental conditions.

3.1. Coal characteristic properties

Variable

Range specification or composition

Mean particle size Operating temperatures Operating pressures Gasifying agents CO2 Composition Gas flow rate Sample mass used

200 μm 900, 950 and 1000 °C (isothermal operation) 1, 5.5 and 10 bar (abs.) CO2 in Nitrogen 10, 20 and 30 mol% 1.8 NL/min 45 mg

combines both statistical and mathematical techniques to analyze, model and optimize problems in which an experimental response is influenced by several variables [40,41]. Furthermore the desired response can be mathematically expressed as follows: yr = f ðξ1 ; ξ2 ; ξ3 ; ::ξk Þ + εr :

ð1Þ

After obtaining response data, an empirical model, either first- or second-order can be derived to adequately describe the response surface. Response surface methodology has been successfully applied in numerous investigations such as in the description of a full scale gasifier [42], supercritical ethanol technology [43] and the biochemical research field [44]. Numerous experimental designs are available for fitting response surfaces, with the most common one being the CCD (Central Composite Design) method. For this investigation a facecentered central composite design method (CCD) was implemented. This consisted of a full factorial 2 q design to which an additional star design was added [40,41]. A center point (repeated 3 times) was further added to test for variability in the CCD. The total number of experiments for q factors (independent variables) is given as: q

NT = 2 + 2q + Nc :

ð2Þ

Nc is the amount of center point runs, which is either taken as 3 or 5. Fig. 1 graphically illustrates the 3 factor, 3 level, face-centered CCD (CCF) experimental design used for this investigation. The CCD experiments were conducted in a random order to prevent any systematic bias that could have occurred during experimentation. The TGA experimental results obtained in the format of mass loss versus time were converted accordingly to conversion (X)-time profiles (on an ash free basis) with the following equation: X=

ðm0 −mV Þ−mt ðm0 −mV Þ−mash

ð3Þ

Eq. (3) is used with the assumption that no product gas in the form of CO is produced from mineral transformations or mineral reactions with CO2.

The chemical, petrographic and structural properties of each coal is summarized in Table 1, while the results of the mineral- and ash analyses, as obtained from XRD- and XRF (on a loss of ignition free basis) analysis, are presented in Tables 3 and 4 respectively. The amount of crystalline minerals present in the three coals was quantified with the application of the Rietveld method [37,38]. All three coals were classified as bituminous medium rank C-D coals according to the ISO 11760 standard. Furthermore all three coals were characterized as inertinite-rich coals. From the results it is evident that all three coals have similar elemental-, surface- and petrographical features, while the largest differences between the coals, from a coal characteristic perspective can be attributed to their mineralogical properties. From the results all three coals are characterized as high ash coals (SABS ISO 1171:1997) with ash contents (d.b) of 21.9 wt.%, 29.2 wt.% and 44.4 wt.% respectively for coal XA, XB and XC. The distribution of mineral matter in these coals is reflected in the results obtained from carbominerite (included minerals) and minerite (excluded minerals) analyses. From the results of Table 1 it is evident that coal XA contains a higher proportion (approximately 67%) of minerals associated with the carbonmatrix than the other two coals. The total mineral matter of coal XC however seemed to be distributed evenly. Detailed mineral distributions can however be obtained with the aid of QEMSCAN analysis. All three coals are rich in kaolinite and quartz with coal XA having the lowest total amount of these two species, which is common for typical Highveld seam 4 coals [45]. Coal XA contains relatively small amounts of anatase and siderite, while negligible amounts of siderite were present in the ash of coals XB and XC. The ash species of each coal is reported in Table 4 on both a wt.% and on an ash amount basis (a.m.d, g/100 g coal). The ashes of the three coals are rich in SiO2 and Al2O3 (which corresponds to the high levels of kaolinite and quartz in all three coals) with coal XA containing the least amount of SiO2, while coal XC contains the least amount of Al2O3. Large differences in SiO2 content were observed between the three coals. No significant differences (wt.% and a.m.b) were observed in the amounts of Fe2O3 and K2O present in the ash of coals XA and XB. The amounts of these species were however slightly higher for coal XC. This slightly higher amount of Fe2O3 present in the ash of coal XC can be related to the higher amount of pyrite present in this coal in comparison to coals XA and XB. A large difference was however found between the amounts of CaO (mainly derived from calcite and dolomite) present in the ash of each coal. Although not quantified in this investigation, it has been shown that dolomite and calcite moieties tend to propagate as cleats in organic rich particles [45]. Of all three coals the largest amount of calcium species was confined to coal XA (15.3 wt.%).

(1000, 1, 30)

(1000, 1, 10)

T Table 3 Mineral composition (graphite-free basis) of coals XA, XB and XC (wt.%).

(1000, 5.5, 20) (950, 1, 20) (1000, 10, 10)

yCO2 (950, 5.5, 20)

(1000, 10, 30) (950, 5.5, 30)

(950, 5.5, 10) (900, 1, 10)

(900, 1, 30)

(950, 10,20)

Factorial point (950, 1,20)

(900, 10, 10)

P

Star point (900, 10, 30)

Center point

Fig.1. CCF experimental design for three factors (T, P, yCO2) with three levels.

MINERAL ANALYSES (XRD) Species

Chemical formula

XA

XB

XC

Anatase Calcite Dolomite Kaolinite Muscovite Pyrite Quartz Rutile Siderite

TiO2 CaCO3 CaMg(CO3)2 Al2Si2O5(OH)4 KAl2(AlSi3O10)(OH2) FeS2 SiO2 TiO2 FeCO3

0.18 6.32 13.95 54.32 2.21 2.04 19.64 1.05 0.28

1.22 3.22 9.30 48.61 8.52 1.27 27.01 0.78 0.07

0.79 2.86 6.88 50.40 10.60 2.43 24.90 1.14 0.00

B.B. Hattingh et al. / Fuel Processing Technology 92 (2011) 2048–2054

A

Table 4 Ash composition of coals XA, XB and XC as wt.% and on an ash mass basis (g/100 g coal). Ash analyses (XRF) XA

XB

XC

wt.%

a.m.b (g)

wt.%

a.m.b (g)

wt.%

a.m.b (g)

SiO2 Al2O3 Fe2O3 P2O5 TiO2 CaO MgO K2O Na2O SO3

44.00 25.95 1.32 1.71 1.64 15.28 4.31 0.33 0.91 4.53

9.46 5.58 0.28 0.37 0.35 3.29 0.93 0.07 0.20 0.97

52.80 26.04 1.89 0.25 1.90 8.80 3.04 0.42 0.56 4.26

14.52 7.16 0.52 0.07 0.52 2.42 0.84 0.11 0.15 1.17

64.12 23.96 3.25 0.17 1.14 2.38 1.19 1.51 0.28 2.05

28.47 10.64 1.44 0.08 0.51 1.05 0.53 0.67 0.12 0.91

Conversion (X)

Species

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1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

XA_1000°C_20% CO2_5.5bar XB_1000°C_20% CO2_5.5bar XC_1000°C_20% CO2_5.5bar

0

25

50

75

100

125

150

175

200

Time (min)

B

3.3. Influence of ash content and ash constituents on reactivity To explain the substantial difference between the reaction rates of the three coals, the influence of the different coal properties on reactivity was assessed. In order to do so, qualitative plots of the dimensionless relative reactivity (Rrel) and the respective coal properties were constructed (as illustrated in Fig. 2). The relative reactivity for each coal was calculated from its initial reactivity (R0) relative to the coal with the highest initial reactivity (in this case coal XA). Initial- and relative reactivity is respectively defined by Eqs. (4) and (5) as:

R0 =

dX j dt t = 0

ð4Þ

XA_950°C_20% CO2_5.5bar XB_950°C_20% CO2_5.5bar XC_950°C_20% CO2_5.5bar

0

50

100

150

200

250

300

350

400

Time (min)

C Conversion (X)

Typical conversions of carbon versus time results are shown in Fig. 2 on a comparative basis for coals XA, XB and XC at three different temperatures (900 °C, 950 °C and 1000 °C), a constant a pressure of 5.5 bar, and a CO2 composition of 20 mol%. All the results in Fig. 2 as well as all the other results at the conditions given in Table 2 confirmed expected trends, in terms of an increase of reaction rate with temperature, pressure and CO2 composition. From Fig. 2 it can be seen that Coal XA shows the fastest reaction rate, followed by coal XB and coal XC. It was also evident from the results that coal reactivity increased with increasing temperature, CO2 composition and pressure, although only slight increases in reactivity with increase in pressure was observed. This general trend was confirmed for all the different experimental conditions (not shown). The effects of experimental conditions on coal reactivity is also reflected in the probability parameters (F- and p-values) as determined from an analysis of variance test (ANOVA) performed by a CCD statistical evaluation software package (Design Expert® 7) on the experimental results. A summary of the obtained probability results are shown in Table 5. The quadratic model equation was found to empirically predict the influence of each experimental variable. A full description of statistical terminology regarding F- and p-values is provided by Montgomery and Myers [40,41]. In principle the higher the F-value the greater the effect of the variable, thus from the above results temperature was the dominating factor affecting coal reactivity, while pressure with the lowest F-value had the smallest influence on the reactivity. p-Values smaller than 0.05, indicate that the variables have a significant effect on the response (coal reactivity in this case). The p-values of temperature for all three coals have the smallest values, again confirming the prominent effect of temperature.

Conversion (X)

3.2. Reactivity comparison between the three coals

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

XA_900°C_20% CO2_5.5bar XB_900°C_20% CO2_5.5bar XC_900°C_20% CO2_5.5bar

0

100

200

300

400

500

600

700

800

Time (min) Fig. 2. Comparison between the CO2 reaction rate of coals XA, XB and XC at (A) 1000 °C, (B) 950 °C and (C) 900 °C.

Rrel =

dX =dt j t = 0 × 100: dX =dt j XA;t = 0

ð5Þ

The average relative reactivity of each coal was used in the construction of the qualitative plots. The values of initial- and relative reactivities for all three coals at the respective CCD experimental conditions are summarized in Table 6. No systematic trends (in terms of catalytic effect) could be observed between the relative reactivities of the three coals and operating Table 5 F- and p-values for the different experimental factors influencing CO2 gasification. Experimental condition

XA

XB

XC

F-value

p-value Prob N F

F-value

p-value Prob N F

F-value

p-value Prob N F

Temperature CO2 composition Pressure

491.08 130.48 15.16

b 0.0001 b 0.0001 0.0059

257.15 24.68 12.27

b 0.0001 0.0016 0.0100

314.34 50.87 36.47

b0.0001 0.0002 0.0005

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120

Table 6 Summary of initial reactivities of coals XA, XB and XC at different CCD conditions. Coal initial reactivity × 102 (min–1) and relative reactivity

T (°C)

1000 1000 1000 1000 1000 950 950 950 950⁎ 950⁎ 950 950 900 900 900 900 900 Average Rrel

P (bar)

yCO2 (mol%)

XA R0

Rrel

R0

Rrel

R0

Rrel

1 1 5.5 10 10 1 5.5 5.5 5.5 5.5 5.5 10 1 1 5.5 10 10

30 10 20 30 10 20 30 20 20 20 10 20 30 10 20 30 10

4.34 1.90 4.29 6.30 2.08 1.46 2.48 1.64 1.70 1.83 1.07 1.84 0.75 0.48 0.85 1.10 0.52

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

2.26 1.14 2.02 2.57 1.60 0.65 1.28 0.92 0.89 0.95 0.62 0.90 0.32 0.26 0.49 0.64 0.36

52 60 47 41 77 45 52 57 52 52 58 49 43 54 58 58 69 54 (± 4.3)

1.32 0.67 1.45 2.04 1.17 0.48 0.89 0.73 0.65 0.71 0.51 0.78 0.28 0.19 0.35 0.56 0.30

30 35 34 32 56 33 36 45 38 39 48 42 38 40 41 51 58 41 (±3.9)

XB

XC

⁎ Repeatability runs.

conditions. In all cases coal XA has the largest initial- and relative reactivity followed by coals XB and XC. From Table 6 it was however evident that the initial relative reactivities of the three coals did not change significantly with increase in pressure, The reason for the little effect of pressure at higher pressure, might be attributed to the fact that most of the active sites on the coal particles have been occupied and saturated at 1 bar; and that a further increase in pressure completely saturated the active sites and had no significant effect on reactivity. This trend was also the general conclusion of Kühl et al. [45]. For temperature and CO2 composition this was however not the case. For the correlation between ash content and average relative reactivity, the ash content on a dry, volatile matter free basis was used. The reason pertaining due to the fact that the ash value changes after devolatilization and CO2 reactivity was only measured on the formed in-situ char. Coal reactivity decreased with increasing ash value. This however differed from studies done by Samaras et al. [46] and Skodras and Sakellaropoulos [28]. CO2 reactivity experiments conducted on acid-treated Greek lignites by Samaras et al. [46] indicated that reactivity increased with increasing ash content. CO2 gasification studies done by Skodras and Sakellaropoulos [28] on five different lignites however revealed that reactivity increased up till a certain ash value, where after it decreased with increasing amounts of ash. Furthermore Sakawa et al. [30] established in their studies that total ash content presents a poor correlation with reaction rate. This inconsistency between the effects of ash on reactivity possibly indicates that the reactivity of a specific coal is not solely ash amount dependent, but more dependent on the inherent constituents of the ash. Therefore the abundance of a certain constituent can enhance the inherent catalytic effect of the ash. A comparison between initial reactivity and the mineral contents present in the ash revealed the individual influence of each species. Comparing between the predominant mineral species (calcite, dolomite, kaolinite and quartz) in the coals and average relative reactivity indicated an increasing reactive behavior for increasing amounts of the Ca-rich mineral species (calcite and dolomite). In addition average relative reactivity increased for increasing CaO and MgO contents, which indicated an enhanced catalytic effect for coal XA, which contained the largest amount (wt.% and a.m.b) of these species. The collective influence of CaOand MgO content is illustrated in Fig. 3. Although CO2 reactivity increased with increasing Na2O and K2O contents (not shown here), the inherent catalytic effect of these

100

Average Rrel

Experimental conditions

80 60

XA 40

XB 20

XC

0

3.5

11.64

19.1

CaO + MgO content (wt.%) Fig. 3. Comparison between the CO2 relative reactivity of coals XA, XB and XC with CaO + MgO content.

species was neglected due to their low concentration in the ash [47]. The overall observed trends however corresponded to the catalytic effects of Ca, Mg, Na and K observed by numerous other authors [16– 19,28,46,48–50]. Although, conventionally, many authors [9,51] describe the effect of each ash constituent separately, it does however provide the problem of ignoring any interaction that could have existed between the different species in the ash as well as the combined effect of all the different constituents. To overcome this problem some indices have been proposed to predict or explain combustion and/or gasification behavior. The alkali index (AI) [30] is a parameter frequently used to describe the overall influence of catalytically active species within the ash and is defined as the ratio of the sum of the fraction of the basic compounds in the ash (CaO, MgO, K2O, Na2O and Fe2O3) to the fraction of the acidic compounds (SiO2 and Al2O3) in the ash, multiplied by the ash value (Eq. (6)). The acidic compounds are normally of non-catalytic nature.  AI = ash% ×

CaO + K2 O + MgO + Na2 O + Fe2 O3 Al2 O3 + SiO2

 ð6Þ

Many authors however report the catalytic efficiency of the ash as the ratio of non-catalytic constituents, i.e.: Al2O3 to SiO2 present in the ash [52,53]. The influence of both the AI and Al2O3/SiO2 ratio of each coal on its average relative reactivity is depicted in Fig. 4. From the Figure it is clear that coal reactivity increases with both increasing AI and Al2O3/SiO2 value. The increased reactivity of coal XA can thus be explained by the large amount of catalytic active species such as CaO and MgO etc., as well as the lower amount of non-catalytic SiO2 present in the ash, thus confirming the relatively strong catalytic effect of the ash of coal XA. Similar trends were observed by Skodras and Sakellaropoulos [28], Sakawa et al. [30] and Zhang et al. [54]. The slower reactive nature of coal XC is therefore confined to its lower concentration of catalytic active species in the ash. It has however been shown that at high gasification temperatures the melting of alumina-silicates and silicates can effectively hinder the carbon reaction rate due to the coverage and blockage of pores within coal particles [55,56]. In retrospect, the lower reactivity of coal XC could therefore also be affected by the blockage of more available active sites by the melting of the larger amount of silicate species present in this coal. The influence of minerals is also reflected in the volatile matter content (d.b), calorific value (d.b) and fuel ratio (Fixed carbon/Volatile matter ratio) of each coal. Volatile matter content, fuel ratio and calorific value are important parameters affecting the carbon burnout behavior, ignitability and flame stability of a certain coal. Apart from the catalytic activity of the minerals, it is also expected that the increased reactive nature of coal XA can therefore be attributed to its

120

120

100

100

80 60 XA

40

Average Rrel

Average Rrel

B.B. Hattingh et al. / Fuel Processing Technology 92 (2011) 2048–2054

80 60 XA

40 XB

XB

20 0

20

XC

0.37

0.49

2053

XC

0

0.59

4.2

Al2O3/SiO2 ratio

5.2

6.5

Alkali Index

Fig. 4. Comparison between the CO2 average relative reactivity of coals XA, XB and XC with the Al2O3/SiO2 ratio and the Alkali index (AI).

higher heating value, larger volatile matter content and higher fuel ratio [30,49–51,54,57], which leads to enhanced carbon burnout and ignitability. In addition the higher specific heat capacity of included minerals in comparison to organic material also contributes to a decrease in reactivity [26]. Due to this phenomenon a mineral rich particle will subsequently absorb more heat, reducing the temperature and consequently reducing the reactivity. Therefore, in this case, the strong correlation between reactivity and calorific value (CV, d.b.) suggests that the effect of the included minerals cannot be neglected. In addition to its lower concentration of catalytic active species, and large amount of silicate moieties, the lower reactivity of coal XC can therefore also be attributed to the loss of heat due to its larger mineral content [26]. Based on the above observations the predominance of inherent catalytic active species such as Ca 2+ can enhance the reactivity of a particular coal. Equally an increase in the presence of non-active ash constituents can significantly reduce the heating value and available reactive surface area, which can consequently lead to a lower reactive nature of the coal, such as is strongly the case for coal XC. Although species such as SiO2 and Al2O3 do not significantly contribute to the catalytic activity of the ash, their presence in large amounts can however from a general perspective lead to lower carbon burnout behavior and reactivity.

4. Conclusions The effect of ash constituents on the CO2 gasification reactivity of three typical South African coals was investigated. A difference in the CO2 reactivity of the three coals could not be explained by the elemental-, structural- or petrographical properties of the coals due to the similarity that existed between the properties of these coals. The ash contents and ash constituents did however show a difference. It was postulated that this could however provide a reason pertaining to the difference in reactivity. CO2 reactivity decreased with increasing ash value, but it was however suspected that this could not only be attributed to only the amount of ash, but also to the constituents present within the ash. It was observed that relative reactivity increased with increasing calcite, dolomite and CaO content, which corresponds with literature describing the important catalytic role of Ca 2+ during CO2 gasification. The enhanced reactivity of Coal XA could therefore be explained by the large concentration of CaO and MgO present in its ash. This was however the opposite for coal XC, where the presence of large amounts of non-catalytic active species dictated the lower reaction rate of the coal. The catalytic activity of each coal's ash was quantified with proposed indices in literature such as the alkali index and the Al2O3/SiO2 ratio. As expected, an increase in coal reactivity was observed for an increase in these ratios/indices. The subsequent reactive nature of each coal is also related to the effects of

total mineral contents as reflected in carbon burnout behavior and coal ignitability. Nomenclature Abbreviations a.d.b air dry basis A.I. alkali index a.m. b Ash mass basis d.a.f dry, ash-free basis m.m.b mineral matter basis m.m.f mineral matter free basis

Greek symbols εr error ξk input variable Symbols m0 mash mV mt NT Nc P R0 Rrel t T X yCO2 yr

initial coal mass ash mass devolatilized mass mass at a certain time t total number of experiments amount of center point runs. pressure initial reactivity relative reactivity time temperature carbon conversion molar composition of CO2 response variable

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