Catalytic gasification of a Powder River Basin coal

Fuel Processing Technology 161 (2017) 145–154

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Research article

Catalytic gasification of a Powder River Basin coal with CO2 and H2O mixtures ZhangFan a,b, FanMaohong a,⁎, HuangXin c, Morris D. Argyle d, ZhangBo c, Brian Towler a, ZhangYulong b a

Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA Chemistry & Physics Center, National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, PR China School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, PR China d Department of Chemical Engineering, Brigham Young University, Provo, UT 84602, USA b c

a r t i c l e

i n f o

Article history: Received 22 December 2016 Received in revised form 7 March 2017 Accepted 7 March 2017 Available online 27 March 2017 Keywords: Coal gasification CO2 Catalysts Gas products

a b s t r a c t CO2 capture and utilization caused more and more public attention because of its environmental impact. The objective of this work was to study the catalytic gasification of a sub-bituminous coal from the Powder River Basin (PRB) using a CO2 and H2O mixture as the gasifying agent. Sodium and iron, in the form of inexpensive compounds, were chosen to catalyze the coal gasification. The experiments were conducted between 700 and 900 °C in a fixed-bed laboratory gasifier under the atmospheric pressure. Results show that the added catalysts had an effect on the composition of gas products in a way different from it in pure steam gasification. Fitting models for kinetic data were examined and the results show that addition of 3 wt% Na or 3 wt% iron led to a 28.0% or a 19.5% reduction in the activation energy of the gasification reaction, respectively. The sodium and iron in the coal char were characterized and the results show that sodium is more evenly distributed than iron during gasification. Suggestions for improving the catalysts' performance in coal gasification were proposed accordingly. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Due to the serious impact of accelerating global climate change, the United States and China signed the U.S.-China Joint Announcement on Climate Change in 2014, which imposed significant restrictions on the coal industry. Integrated gasification combined cycle (IGCC) processes have been proposed as a better method for coal based electricity generation due to its higher efficiency on CO2 capture compared to conventional coal-fired plants [1–4]. To avoid emissions to the atmosphere, CO2 captured during IGCC process is proposed to be injected into oceans or deep underground geological formations [5,6], which involves a significant cost issue. Additionally, CO2 generated from coal combustion or the water gas shift reaction in an IGCC process is usually at a moderately high temperature (~500–600 °C) [7,8]. Discharging the high-temperature CO2 to the environment wastes not only the captured material, but also the energy resource. Gasification is the core of coal-based IGCC systems. CO2 can be used as a gasifying agent during coal gasification, as shown in (R1), which is also known as the Boudouard reaction: C ðsÞ þ CO2 ðgÞ⇌2CO ðgÞ ΔH ° rxn ¼ þ172:67 kJ=mol:

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

http://dx.doi.org/10.1016/j.fuproc.2017.03.010 0378-3820/© 2017 Elsevier B.V. All rights reserved.

ðR1Þ

Conventionally, the energy needed to achieve the gasification temperature is obtained from exothermic combustion reactions prior to gasification, while high temperature CO2 separation technologies make it possible to use the high-temperature CO2 as a reactant in coal gasification [8], which would result in less carbon being consumed by coal combustion and lower CO2 emissions. Using high-temperature CO2 as a gasifying agent could create a win-win scenario for environmental protection and for the economics of IGCC. H2O is another gasifying agent usually used in coal gasification [1,4, 5], as shown in (R2): C ðsÞ þ H2 O ðgÞ⇌CO ðgÞ þ H2 ðgÞ

ΔH ° rxn ¼ þ131:46 kJ=mol:

ðR2Þ

Compared to CO2 gasification, steam gasification produces both H2 and CO. The study of coal gasification with a mixture of CO2 and H2O is necessary for CO2 utilization in coal gasification, and has already attracted researchers' attention [9–11]. For example, Huang et al. [9] studied the kinetics of coal gasification in CO2 and H2O mixtures at atmospheric pressure and Xu [12] studied the composition of the gas products of coal gasification in CO2 and H2O mixtures as a function of decreasing steam in the gasifying agent. To address the slow reaction kinetics of CO2 gasification, catalytic gasification has been widely studied due to the availability and efficiency of low cost catalysts [5,13]. As two of the few catalysts that are both

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effective and relatively inexpensive, sodium and iron have previously been considered for use in catalytic coal gasification [1–5]. Ye et al. [14] confirmed that Na2CO3 could accelerate considerably the rate of gasification of low-rank coal at atmospheric pressure and temperatures between 714 °C and 892 °C. FeCO3 was also proven to be an effective catalyst for coal gasification by other researchers [5,15]. Catalysts could also play an important role in altering the composition of the gasification products through catalyzing the water gas shift (WGS) reaction (R3) during coal gasification with H2O [2,4]. CO ðgÞ þ H2 O ðgÞ⇌CO2 ðgÞ þ H2 ðgÞ ΔH ° rxn ¼ −41:3 kJ=mol:

ðR3Þ

The WGS reaction without catalyst occurs significantly when the reaction temperature is higher than 600 °C, while iron oxide is an active catalyst for the WGS reaction. Popa et al. [2] studied steam coal gasification with iron catalysts and suggested that H2 can be generated by the WGS reaction on FeO (wustite) with surface defects, as follows: 3FeO þ H2 O→Fe3 O4 þ H2 Fe3 O4 þ CO→3FeO þ CO2 Monterroso et al. [4] studied the effect of composite iron-sodium catalysts on steam coal gasification and verified that the overall production of H2 with a pure sodium catalyst was lower than that of a pure iron catalyst. Although much research has been performed on coal gasification with CO2 and H2O mixtures, supplemental work still needs to be undertaken. This research was designed to study the catalytic effects of sodium and iron on the gasification of Wyoming Powder River Basin coal with CO2 and H2O mixtures. The gaseous product composition and catalytic characteristics of sodium and iron in coal gasification were examined. 2. Experimental

(IWI) method. The specific preparation procedure has been described in detail elsewhere [2]. The weight percentage of sodium or iron added to the coal samples was calculated on a dry and ash-free basis. The gasification experiments were performed in a fixed-bed laboratory gasifier at gasification temperatures of 700 °C, 750 °C, 800 °C, 850 °C, and 900 °C at ambient pressure, which is ~75 kPa at the elevation (~2200 m) of our laboratory. The gasification apparatus used in this work is shown in Fig. 1. The flow rates of N2 (1a, UHP, Praxair), used as an inert purge gas, and CO2 (1b, UHP, Praxair), used as one gasifying agent were controlled by mass flow controllers (2a, Porter Instruments Series 201, with 2b, a 4 channel PCIM4 controller). The flow rate of the second gasifying agent, H2O, was controlled by a water pump (3, Scientific Systems-Lab Alliance Series 1) with its accessories (4, 5). N2, CO2, and H2O were preheated in tubing wrapped in heating tapes (6), in which the water was transformed to steam before entering the reactor. Temperatures of the heating tapes and the reactor were monitored by thermocouples (7). The coal sample (11) was loaded and retained by ceramic wool (10) in the middle of the reactor (9) manufactured from a stainless steel tube. After purging with N2, the reactor was heated by a tube furnace (8, Thermolyne 21100) at 10 °C/min to gasification temperatures in an N2 atmosphere to pyrolyze the samples. When approaching the desired reaction temperature, CO2 and H2O were introduced into the reactor at specified flow rates to initiate the gasification reaction. The flow rate of the gasifying agents was tested at 900 °C with 3 wt% Na to make sure that the gasification reaction was not limited by the gasifying agents. The tar and unreacted water in the gas products were condensed by a water-cooled condenser (14) and collected in a vial (13). The water was further removed from the gas products by passing through a desiccant-filled water trap (17). After that, the gas products flowed into an on-line gas chromatograph (18, Agilent 3000A micro GC) with two micro-columns (MolSieve 5A PLOT and 4 m PoraPlot U) to separate H2, CO, N2, CO2, and light hydrocarbons, prior to concentration analysis using a calibrated thermal conductivity detector (TCD). When no CO was detected by the micro GC, the gasification was considered to be complete. The residual coal ash in the reactor was then collected and analyzed.

2.1. Gasification experiments 2.2. Sample characterization Raw coal from Wyoming Powder River Basin was ground to particles and passed through a 200 mesh screen before use. Na2CO3 and FeCO3 were mixed with the coal particles by the incipient wetness impregnation

The proximate analysis of the raw coal was performed according to ASTM D5142 and D5016, while the ultimate analysis of the raw coal

Fig. 1. Schematic diagram of the catalytic coal gasification apparatus: (1a) N2, (1b) CO2, (1c) Ar; (2) mass flow controllers; (3) water pump; (4) pressure gauge; (5) back pressure regulator; (6) heating tapes; (7) thermocouples and temperature scanner; (8) tube furnace; (9) reactor; (10) ceramic wool; (11) coal sample; (12) temperature controller for heating tapes; (13) tar and water collector; (14) water-cooled condenser; (15) pressure relief valve; (16) bubble meter to verify flow rate; (17) water-trap; (18) micro GC; and (19) data acquisition system.


and the char samples were done using ASTM methods D5373, D5016, and D4239. SEM images were obtained by an FEI Quanta 450 field emission scanning electron microscope with a Schottky field emission gun at an accelerating voltage of 20 kV. Simultaneous energy dispersive X-ray spectroscopy (EDS) was performed to analyze the sodium distribution and catalytic availability in the samples. The samples for SEM tests were collected before and after pyrolysis, as well as after 30 min of gasification, and after complete gasification. X-ray diffraction (XRD) analyses were performed on powdered coal and char samples in a Rigaku Smartlab X-ray diffraction system using the Cu Kα1 line (1.5406 Å) obtained at 40 kV/40 mA, with 2θ ranging from 20° to 80° with 0.020° steps and a dwell time of 0.05 s/step. 3. Results and discussion The relevant results of the proximate and ultimate analyses are listed in Table 1. Before reaching the desired gasification temperatures, coal samples in the reactor undergoes a pyrolysis process, in which coal releases coal tar and some light gas species, and transforms to coal char. The effects of sodium and iron on the pyrolysis of Powder River Basin coal at the conditions of this work have been studied elsewhere [2,15]. These results indicated that sodium or iron plays a role in the composition of formed coal chars. The ultimate analyses of coal chars after pyrolysis at different conditions are listed in Table 2. The distribution of sodium or iron was changed during coal pyrolysis. As shown in Fig. 2, the XRD results suggested that a portion of sodium combined with the inherent elements in coal and existed in the form of aluminosilicates, while iron was reduced during coal pyrolysis and existed as metallic iron after pyrolysis. In this work, the coal samples with added sodium or iron were subjected to pyrolysis under identical conditions, and then the gasifying agents CO2 and H2O were introduced into the reactor to initiate gasification. As steam gasification is much easier than CO2 gasification [16], a CO2/H2O mole ratio of 7:3 was chosen to ensure that CO2 participates in the gasification reaction.

147

Table 2 Ultimate analyses of the coal chars after pyrolysis at different conditions. wt% Raw coal 3 wt% Na 3 wt% Fe (DAF 700 °C 800 °C 900 °C 700 °C 800 °C 900 °C 700 °C 800 °C 900 °C basis) C H N S Ash O

90.89 1.88 1.68 0.4 – 5.14

93.12 1.1 1.64 0.36 – 3.78

94.86 0.67 1.58 0.42 – 2.47

92.38 1.39 1.73 0.29 – 4.21

93.01 0.81 1.92 0.36 – 3.9

93.98 0.5 1.87 0.39 – 3.25

92.11 1.01 0.84 0.84 – 5.21

93.96 0.73 0.89 0.92 – 3.50

95.58 0.72 0.94 1.20 – 1.57

with increasing gasification temperature as those without catalysts, but the trend of CO yields is just the opposite, as shown in Fig. 3b. In the reaction system, CO2 gasification (R1) and steam gasification (R2) occur simultaneously as soon as they come into contact with the coal char. However, the steam concentration in the system declines more quickly, as steam gasification is much faster than CO2 gasification at the same conditions [12]. With the H2O/CO2 mole ratio of 3:7 in this work, the steam is expected to become depleted quickly, while H2 becomes relatively abundant, as the gasifying agents passed through the coal char in the fixed-bed reactor. With relatively high concentrations of CO2 and H2, equilibrium considerations make it more likely that the WGS reaction would be reversed, as shown in the reverse water gas shift (RWGS) reaction (R4): CO2 ðgÞ þ H2 ðgÞ→CO ðgÞ þ H2 O ðgÞ

ΔH ° rxn ¼ þ41:1 kJ=mol:

ðR4Þ

The RWGS reaction is endothermic and easily reaches thermal equilibrium at high temperature [17]. In Popa et al.'s research on pure steam gasification [2], the amount of steam in the system was sufficient to favor the WGS reaction. Thus, CO2 and additional H2 were produced from WGS reaction, which played an important role in the resultant composition of gas products during char gasification [2]. In this work, the difference is that H2O, which was generated via the RWGS reaction, was more active than CO2 and was available to react with carbon in the system to cause additional steam gasification [18]:

3.1. Catalytic effect on the H2 and CO yields

C ðsÞ þ H2 O ðgÞ⇌CO ðgÞ þ H2 ðgÞ

During char gasification, CO and H2 were detected as the main products, while the CH4 yield was less than 1 mol% of the gaseous products, and thus was not studied in detail in this work. Fig. 3 shows the H2 yield and the CO yield after normalization based on the number of carbon moles obtained from the non-catalytic and catalytic coal gasification. The H2 molar yields with catalysts display the same decreasing trend

The sum of (R4) and (R2) is CO2 gasification via the Boudouard reaction:

Table 1 Proximate and ultimate analyses of raw Wyodak coal. As received (wt%)

Moisture free (wt%)

DAF (wt%)

Proximate analysis Moisture Ash Volatile matter Fixed carbon Total

17.81 7.17 36.56 38.46 100

– 8.72 44.48 46.8 100

– – 48.73 51.27 100

Ultimate analysis Moisture Hydrogena Carbon Nitrogen Sulfur Oxygena Ash Total

17.81 2.8 59.17 0.76 0.35 11.95 7.16 100

– 3.4 71.99 0.93 0.43 14.54 8.71 100

– 3.72 78.87 1.01 0.47 15.93 – 100

a

Hydrogen and oxygen values reported do not include hydrogen and oxygen in the free moisture associated with the samples.

ðR2Þ

C ðsÞ þ CO2 ðgÞ⇌2CO ðgÞ

ðR1Þ

In this respect, H2 plays a catalytic-like role in this process. Therefore, unlike the situation in pure steam gasification, we propose that the WGS reaction (or the RWGS reaction) has little effect on the gas composition in the present study. Thus, the H2 yield in each gasification test was approximately determined by the amount of carbon consumed by H2O (R2). For example, examining the tests at 700 °C (H2 yield per mole carbon: 0.75 mol; CO yield per mole carbon: 1.25 mol) and 900 °C (H2 yield per mole carbon: 0.41 mol; CO yield per mole carbon: 1.59 mol) in detail, the consumed carbon is calculated as follows: 700 °C Moles/mol: Moles/mol: 900 °C Moles/mol: Moles/mol:

C (s) 0.75 C (s) 0.25 C (s) 0.41 C (s) 0.59

+ + + +

H2O (g) 0.75 CO2 (g) 0.25 H2O (g) 0.41 CO2 (g) 0.59

→ → → →

CO (g) 0.75 2CO (g) 0.5 CO (g) 0.41 2CO (g) 1.18

+

H2 (g) 0.75

+

H2 (g) 0.41

The results suggest that 75 mol% of the carbon was consumed by steam at 700 °C, while only 41 mol% was steam gasified when the reaction temperature increased to 900 °C. Since high temperature is

148


Fig. 2. XRD patterns of the coal samples after pyrolysis at 800 °C: (a) raw coal; (b) coal samples with 3 wt% Na; (c) coal samples with 3 wt% Fe.

beneficial to both CO2 gasification and the RWGS reaction, more carbon was consumed by CO2 at 900 °C. This explains why the H2 yield decreased while CO yield increased with increasing gasification temperature in the experiments with no catalysts in this study. With the addition of catalysts, the H2 yield obtained at each temperature was lower than its counterpart without the catalyst. CO2 gasification was spurred by sodium or iron, and thus resulted in more carbon consumed by CO2. Steam gasification is also believed to be promoted by the catalysts, but it is restrained as the limiting reactant in the reaction. The declining trend of H2 yield with increasing temperature was unaltered under the conditions with catalysts, as sodium or iron should accelerate CO2 gasification rather than inhibit it. Fig. 3 also shows that the gap between the H2 yields obtained from non-catalytic and catalytic conditions is diminished with increasing reaction temperature. The reason for this observation may be that the catalytic effect was more prominent on gasification rates at low temperatures and became less important relative to thermal reactions as the gasification rate increased at high temperatures. This is consistent with the research of Popa et al., who found that the catalytic effect would be weakened as the reaction temperature increased during coal gasification [2]. Interestingly, there is a difference in the trajectory of the descending trends of H2 yields between the iron and sodium. This phenomenon indicates that sodium and iron have different catalytic characteristics during the gasification. The H2 yields obtained with sodium was lower than that with iron. As described previously, this suggests that more carbon was consumed by CO2 during the gasification with sodium. Thus, compared to iron, sodium has a better catalytic effect on coal gasification under the tested conditions. Additional evidence and its explanation will be provided below in the discussion of the kinetic and characterization results.

3.2. Catalytic effect on the gasification kinetics Catalytic coal gasification kinetics has been studied by a large number of research groups [5,19–27]. The random pore model (RPM), the shrinking core model (SCM), and their modified forms are commonly used to evaluate the kinetics of coal gasification [5,20,21]. RPM assumes that the internal surfaces of the pore structures are the dominant reaction interfaces, and pays attention to the evolution of pore structure during the char gasification, since the cylindrical pores would enlarge gradually and coalesce eventually as the internal surfaces eroded by the gasifying agents [5,19]. According to RPM, the gasification rate can be expressed as [5,21]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dX ¼ krp ð1−X Þ 1−ψ ln ð1−X Þ dt

ð1Þ

where X is the fractional carbon conversion at time t, krp is the rate constant for reaction on the pore surfaces, and ψ is the pore surface parameter. This model has successfully been used in modeling some char gasification reactions due to its ability to model the maximum reaction rate [21–23]. In most of these cases, the char used in the gasification experiments was prepared under rapid and high temperature pyrolysis conditions, which is beneficial to pore generation [24], as shown in Fig. 4. In this work, the heating rate of pyrolysis was 10 °C/min, which is significantly lower than those in the aforementioned cases, and thus led to diverse surface topography. Fig. 5 shows SEM images of the coal char surface obtained after pyrolysis at 800 °C. The particle exterior is much smoother than those obtained with rapid and high temperature pyrolysis conditions. Furthermore, the sodium or iron added by the

Fig. 3. Effect of catalysts on the H2 and CO yields in char gasification as a function of reaction temperature and catalyst: (a) H2 yield; (b) CO yield.


149

Fig. 4. SEM images showing typical pore structures of coal char obtained from rapid pyrolysis during gasification (a: 3-second residence time at 1400 °C in a drop tube furnace (DTF) [22]; b: 6-second residence time at 1000 °C in a DTF [23]; c: high heating rate at 800 °C [25]).

IWI method is more likely to adhere to the exterior of the coal particles. Thus, most of the active sites created by added catalysts should distribute on the external particle surface, leading to the gasification reaction mainly occurring on the external surface of the coal chars. Therefore, the shrinking core model was more suitable than RPM in this study. The shrinking core model (SCM) proposes that gasifying agent is consumed either on the exterior of the coal particle or within the pores close to the particle exterior surface [19]. The reaction moves from the particle surface to the interface of the shrinking, unreacted core through a layer of ash or/and catalyst, and progresses toward the center of the coal particle [21]. SCM can be expressed as follows [25–27]: dX ¼ ki ð1−X Þm dt

ð2Þ

where X denotes the fractional carbon conversion at time t, ki is the reaction rate constant, and m is a shape factor (for spheres m = 2/3, for cylinders m = 1/2, and for flat plate m = 0) [21]. When m = 2/3 is selected, it assumes that the char particle is an aggregation of small grains that are spheres of uniform size [5]. Then the model, named the grain model, is shown as follows [5,15]: 2 dX ¼ kg ð1−X Þ =3 dt

ð3Þ

The grain model is the most appealing form derived from SCM, and is applied to coal gasification kinetics by many researchers [2,5,15]. Unfortunately, the char particles generally do not maintain spherical shapes

during catalytic gasification, and the results are more likely an integration of several shapes [13]. Therefore, the original SCM was chosen as more suitable than the grain model for this work. The SCM fitting results for the kinetic data collected during non-catalytic and catalytic gasification are shown in Fig. 6. The model fits, shown as solid lines through the data points, are very good, as shown by the near-unity determination (R2) values in Table 3, which also lists the corresponding kinetic parameters. The values of the exponent, m, range from nearly 0.9 to below 0.5, decreasing as function of increasing gasification temperature. The data confirm that the general SCM, as opposed to the grain model assumption of m = 0.667, is suitable for most of the gasification tests under the conditions in this work. These results also show that sodium has a better catalytic performance than iron during the char gasification under the tested conditions. As shown in Table 3, the values of shape factor (m) obtained from raw coal gasification were ~0.667, indicating that grain model can also well fit the kinetic data of raw coal gasification, which coincides with other researchers' results [2,5,15]. However, the value dispersion of shape factors (m) of 3 wt% Na and 3 wt% Fe suggests that SCM is more suitable than grain model for the catalytic conditions set in this work. The fitting results also reveals that the amount of active sites provided by the added catalysts decreased with carbon conversion, and the decrease trend roughly followed the shrinking core model. During the coal gasification, coal ash formed by the inherent elements in coal, such as Si and Al, would exposed to the reaction interface gradually as the exterior carbon was consumed. It is possible that the coal ash would entrap the active Na, and can also hinder the contact of active Fe with unreactive carbon, leading to the decrease in the active sites

Fig. 5. SEM images showing the char particles without catalyst obtained from slow pyrolysis during gasification: (a) 30 μm scale bar; (b) 5 μm scale bar.

150


Fig. 6. Shrinking core model fits for conversion rate vs. conversion (dX/dt − X) data obtained during gasification: (a) raw coal, (b) 3 wt% Na and (c) 3 wt% Fe.

Table 3 The kinetic parameters and coefficient of determination (R2) values obtained by SCM. T (°C)

700 750 800 850 900

Raw coal

3 wt% Na m

R2

k (∗10−4 s−1)

m

R2

k (∗10−4 s−1)

m

R2

0.657 1.06 1.79 2.61 3.50

0.7574 0.6669 0.6146 0.6447 0.6236

0.99877 0.9988 0.99898 0.99995 0.99954

1.46 2.17 3.12 4.02 4.91

0.8699 0.7016 0.6129 0.5342 0.4362

0.9967 0.9973 0.9954 0.9889 0.9774

1.12 1.73 2.51 3.36 4.41

0.8595 0.7739 0.6728 0.6160 0.5658

0.9972 0.9989 0.9941 0.9973 0.9967

resulting from the addition of Na or Fe. In the gasification of a demineralized coal, the active sites provided due to the introduction of catalysts are supposed to accumulate on the reaction interface, and would have a more significant impact on the model fitting. The combination of Na with Si and the inhibition effect of coal ash on Fe will be discussed in the following catalyst characterization part. As shown in Fig. 7, the sensitivity of the rate constant to reaction temperature during catalytic and non-catalytic gasification was characterized by the reaction activation energy obtained from Arrhenius plots:

ln k ¼ −

3 wt% Fe

k (∗10−4 s−1)

Ea þ ln A RT

ð4Þ

where A is the pre-exponential factor, Ea is activation energy, R is the ideal gas constant, and T is the reaction temperature. The results show that the apparent activation energy of the gasification reaction decreases from 80.8 kJ/mol for the uncatalyzed system to 58.2 kJ/mol with 3 wt% Na, a 28.0% decline, and to 65.1 kJ/mol with 3 wt% Fe, a 19.5% decline. In comparison, Kwon et al. [28] studied the char-CO2 gasification with different ranks of coal using a non-reactive coal model. Their results show that the apparent activation energies varied from 79.0 to 155.5 kJ/mol. Thus, sodium and iron clearly promoted the gasification kinetics of Powder River Basin coal with CO2 and H2O mixtures by reducing the activation energy relative to the uncatalyzed reaction.

or Char−M þ CO2 →Char−M−O þ CO

ðR6Þ

Char−M−O→Char−C−O þ Char−M

ðR7Þ

Char−C−O→Char þ CO

ðR8Þ

where M refers to a metal active site that has been added to the coal as a catalyst. To understand the distribution and catalytic availability of sodium and iron in coal char, XRD and SEM/EDS were employed to characterize the char particles with sodium and iron, respectively. 3.3.1. XRD results of the char samples with catalysts Fig. 8a shows the XRD results of the samples with sodium catalysts obtained in gasification, and their corresponding information are listed in Table 4. A portion of sodium combines with the aluminum, silicon, or calcium from the inherent minerals in coal, existing as aluminosilicate or calcium silicate. Unassociated sodium is thought to be highly mobile at gasification temperatures, and could redistribute to form active sites

3.3. Characterization of catalysts in char samples The catalytic effect of sodium or iron on coal gasification has been widely confirmed and studied [1–4,29,30]. The catalytic mechanism is generally described as the added metal element contacting with gasifying agent (H2O and/or CO2) produces hydrogen and/or carbon monoxide and a layer of corresponding metallic oxides on the surface of char particles (R5/R6); then, this oxide layer reacts with adjacent carbon, which can reduce the metallic oxides to metal and generate carbon monoxide (R7), followed by the desorption of the carbon monoxide (R8), as shown in the following reactions [29,30]: Char−M þ H2 O→Char−M−O þ H2

ðR5Þ

Fig. 7. Arrhenius plot showing the effect of 3 wt% Na during PRB coal gasification.


151

Fig. 8. XRD patterns of the coal samples with (a) 3 wt% Na or (b) 3 wt% Fe at different gasification stages at 900 °C: (1) before gasification; (2) during gasification; (3) after gasification.

evenly on the reaction interface of the char, rather than aggregate in a crystalline phase that can be detected by XRD [31,32]. Thus it is reasonable that no Na, Na2O, or Na2CO3 was observed in the XRD patterns. The unassociated sodium played a dominant role in catalyzing the reaction, while the formation of aluminosilicate or calcium silicate compounds with the sodium was adverse to its catalytic effect, since sodium was detained in the non-catalytic compounds [33,34]. Fig. 8b shows the XRD results of the samples with iron obtained during gasification. Austenite (Fe, C), metallic iron, FeO, and Fe3O4 were observed in the XRD patterns. The peaks representing austenite (Fe, C) were observed in the initial stage of reaction, while its intensity gradually weakened and disappeared. At the end of gasification, iron

was oxidized by the gasifying agents, and existed as Fe3O4. Compared to sodium, iron is less mobile at the gasification conditions. Thus, it is difficult for all of iron to move onto the surface of the unreacted char core, but more likely for part of iron to break away from the unreacted char as the reaction progresses and stay with the external ash layer. Then, the metallic iron, which isolated from carbon, would be oxidized by H2O or CO2 by some degree. Thus, FeO and Fe3O4 were observed during gasification. It is possible that the oxidation-reduction cycle of FeO and Fe3O4 occurred during gasification as follows [35]:

3FeO þ CO2 →Fe3 O4 þ CO

ðR9Þ

152


Table 4 Primary peaks and PDF numbers of the species identified in the XRD patterns. Name

Main peaks (2θ)

PDF number

SiO2 Ca2Al2SiO7 NaAlSiO4 Na6Al4Si4O17 Na2CaSiO4 CaS Na1.75Al1.75Si0.25O4

20.856, 26.638, 36.541, 50.137 23.986, 29.131, 31.422, 52.094 21.315, 23.180, 27.383, 34.882 21.085, 34.742, 50.048, 62.444 33.783, 48.512, 60.448 31.406, 41.683, 55.833, 65.504, 74.423 21.110, 25.330, 26.759, 30.671, 31.570, 34.386, 34.962, 39.908, 51.210, 61.994, 63.177 23.519, 33.523, 38.696, 48.157, 59.981 43.472, 50.673, 74.677 44.673, 65.021 36.342, 42.193, 61.164, 73.129 30.157, 35.521, 43.172, 57.098, 62.703

85-0798 35-0755 35-0424 10-0033 24-1069 08-0464 49-0004

Na4CaSi3O9 Austenite (Fe, C) Fe FeO Fe3O4

Fe3 O4 þ H2 →3FeO þ H2 O

37-0282 23-0298 06-0696 46-1312 88-0315

ðR10Þ

As aforementioned, the generated H2O should participate in the steam gasification (R2), thus promoting the char gasification with CO2.

right panels. Most of the added sodium and iron appears to aggregate at concentrated spots in the char samples. The difference is that a fraction of sodium spread among the char particles uniformly with low concentration, while almost all of added iron concentrated at certain particles. The phenomenon is also based on the fact that sodium is highly mobile at the temperatures and redistributes rapidly in the coal matrix during gasification [30,36]; in contrast, iron is not regarded as highly mobile as sodium under 900 °C [4]. Some sodium was attracted by the elements from the inherent minerals in coal, and combined together, leading to the formation of the high-concentrated sodium locations [37]. Non-uniform distribution of catalysts inhibits the catalytic efficiency. Sodium distribution in coal char mainly depends on the coal property and reaction temperature [36–38]. Coal with low ash content and appropriate gasification temperature contribute to the catalytic performance of sodium. Iron distribution solely relies on the catalyst loading method in this work. An improved loading method would benefit to iron distribution, and further its catalytic performance. Nevertheless, sodium had a better distribution compared to the iron during char gasification, which may partly explain the reason that sodium with the same weight percentages had a better catalytic effect in this work. 4. Conclusion

3.3.2. SEM characterization of the char samples with catalysts Fig. 9 shows the high magnification SEM images (~20 μm scale bar) of particles with high sodium or iron concentration obtained during gasification, and corresponding EDS images of catalyst and carbon. Particles with sodium or iron have different surface morphology. In accordance with the XRD results, part of added sodium combined with silicon, leading to the aggregation of sodium, and its high mobility contributes to the formation of regular spheroids like the one shown in Fig. 9a. Sodium on the spheroid surface may still have a catalytic effect on the char which adhered to it [30], such as the char fragments shown in Fig. 9a. However, the internal sodium of the spheroid would lose the opportunity and thus be detrimental to catalytic gasification. As shown in Fig. 9b, it is likely that some iron was attached on the char particle, which occurred during sample preparation. According to the XRD results, iron on the particle surface would exist as FeO or Fe3O4, which formed iron clusters, as shown in the image. Another portion of iron would be on the reaction interface and undermine the unreacted char core. Fig. 10 shows the low magnification SEM images of the char samples obtained during gasification at 900 °C in the left panels, while the corresponding sodium or iron distribution obtained by EDS is shown in the

During coal gasification with a H2O and CO2 mixture, H2 yield was decreased by added sodium or iron catalysts and by increasing reaction temperature. The coal char gasification kinetic data were analyzed, with the shrinking core model (SCM) shown to be suitable for catalytic conditions with coal char obtained from slow pyrolysis. Sodium aluminosilicate and sodium calcium silicate were detected by XRD of the char samples in gasification, which were detrimental to its catalytic performance. Iron in the reacted samples was identified as metallic iron, FeO and Fe3O4, which are beneficial to catalytic gasification. SEM results show that sodium was better distributed compared to iron during the char gasification. Both sodium and iron were found to be promising catalysts for the gasification of Power River Basin coal, with sodium having better catalytic performance under the tested conditions. Acknowledgements This research was supported by the Wyoming Clean Coal Program, China Scholarship Council (201206430023), the U.S. Department of Energy, FMC Inc., and SIDCO Minerals Inc. The authors thank Mr. Aaron

Fig. 9. SEM/EDS images of the char particles with 3 wt% Na (a) or 3 wt% Fe (b) during gasification at 900 °C.


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Fig. 10. SEM/EDS images of the char samples with 3 wt% Na (a) and 3 wt% Fe (b) during gasification at 900 °C.

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