Formation of CH4 during K2CO3-Catalyzed Steam Gasification of Ash

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Formation of CH4 during K2CO3 catalyzed steam gasification of ashfree coal. Influence of catalyst loading, H2O/H2 ratio and heating protocol. Jan Kopyscinski, Charles A. Mims, and Josephine M. Hill Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01538 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 18, 2015

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Manuscript ID: ef-2015-01538f R1, revised

Formation of CH4 during K2CO3 catalyzed steam gasification of ash-free coal. Influence of catalyst loading, H2O/H2 ratio and heating protocol. Jan Kopyscinski3*, Charles A. Mims2 and Josephine M. Hill1 1

Department of Chemical & Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, Canada

2

Department of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College St., Toronto, M5S 3E5, Canada

3

Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, H3A 0C5, Canada *Email: [email protected], Tel.:+1 514-398-4276

Abstract Potassium catalyzed steam gasification experiments of ash-free coal were conducted in a drop-down reactor to study the influence of the catalyst loading, temperature, H2O/H2 ratio and heating protocol. The higher the catalyst loading the faster the carbon conversion, however, at an initial K/C ratio of 0.1 a saturation effect (decrease in reactivity per potassium atom) was observed. At higher gasification temperatures this saturation effect was more pronounced and likely caused by increased potassium mobility. The catalyst accelerated predominately the oxygen transfer reactions and not the methane formation. Methane is produced via direct hydrogenation of the carbon surface. The selectivity to methane and carbon monoxide was increased by reducing the H2O/H2 ratio, which would represent conditions in a gasifier away from the gas inlet. Lastly, the heating protocol influenced mainly the initial rate of gasification up to 20% carbon conversion, beyond that the rates were similar. 1

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1

Introduction

A significant amount of research has been and continues to be done on alkali catalyzed steam gasification of coal and other carbonaceous materials with the aim of producing a high calorie gas1– 13

. The gasification reaction is assumed to proceed via an oxygen-reduction cycle in which steam is

dissociated on a reduced catalyst site (*) leading to an oxidized site (O-*), see Eq. (1). The oxygen is then transferred to a free carbon surface (Cf) producing a C-O complex, which then desorbs as CO, see Eqs. (2) and (3), respectively. CO2 on the other hand is produced via the water-gas-shift reaction – combination of Eqs. (1) and (4) – and/or by continuous oxidation of the C-O complex, see Eq. (5). H2O + * ↔ O-* + H2

(1)

O-* + Cf → C-O + *

(2)

C-O → CO

(3)

CO + O-* ↔ CO2-* ↔ CO2 + *

(4)

C-O + O-* ↔ CO2 + *

(5)

The oxygen exchange reactions – i.e., dissociation of steam on the active site, see Eq. (1) and reaction of CO with an oxidized-site representing the water-gas-shift reaction, see Eq. (4) – are assumed to be in equilibrium, whereas the combination of Eqs. (2) and (3) is considered to be rate limiting1,4,6,9. Besides the formation of H2, CO and CO2, CH4 can be formed as well during steam gasification of carbonaceous materials. Most research, however, has been conducted under conditions where methane formation was negligible (high temperature, low pressure and low initial H2 partial pressure). The production of methane at temperatures >800°C is limited by the thermodynamic equilibrium. The use of a catalyst, however, allows lower operating temperatures

2


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Manuscript ID: ef-2015-01538f R1, revised that would favor methane formation and the production of substitute natural gas in subsequent conversion steps; a process that again has become important14. In detail, methane that is formed within the gasifier does not need to be produced in succeeding methanation reactors. Thus, the overall efficiency is larger than that of high temperature non-catalytic gasification processes (producing syngas containing predominately CO and H2) with subsequent methanation. Two different mechanisms have been proposed for methane production during gasification15,16. The first mechanism (noncatalytic) assumes a stepwise hydrogenation of the free carbon site, see Eq. (6). Cf + 2 H2 ↔ C-H4 ↔ CH4

(6)

The second mechanism is based on the dissociation of CO over a free carbon (Cf) or catalyst (*) site leading to a reactive carbon intermediate (C↓), which is then hydrogenated to methane, see Eqs. (7) and (8), respectively. The oxygen atom is rejected as H2O. CO + Cf + H2 ↔ C-C↓ + H2O

(7)

C-C↓ + 2 H2 ↔ Cf + CH4

(8)

During the gasification reactions, the catalytic activity of alkali catalysts, such as potassium, decreased due to the interaction with mineral matter in the coal ash to form inactive alkali aluminosilicates17,18. Thus, since the early 2000s, special attention has been given to ash-free coals in order to avoid/reduce catalyst deactivation in the context of hydrogen/syngas production via steam gasification11–13,19–21. In addition the reduction/removal of the coal ash (so-called coal beneficiation) led to new applications such as direct carbon fuel cells22,23. In regards to catalysed steam gasification, the effect of coal rank (lignite, bituminous and subbituminous coal)11,21, catalyst type (K2CO3, NiO)5, catalyst loading (0-25 wt%)21, temperature (600-

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Manuscript ID: ef-2015-01538f R1, revised 800°C)12,13,20,24, steam partial pressure19 and H2O/CO2 ratio24 on the carbon conversion and gas production (CO, CO2, H2) of various ash-free coals have been investigated. However, data are scarce for methane formation (most studies have been focused on hydrogen production), the influence of the H2O/H2 ratio and heating protocol. These parameters have been investigated for highly porous model carbons (no ash) and coal chars (with ash) only1,9,15,25. Ash-free coal produced from solvent extraction process has a very small surface area, thus the results might differ significantly. H2O/H2 ratio is a good indicator of the spatial position within the gasifier. That is, near the gasifier inlet the reaction gas consists mainly of steam (i.e., high H2O/H2 ratio), but farther away from the reactor inlet, more steam is converted and more hydrogen and carbon monoxide are produced (i.e., low H2O/H2 ratio). However, with increasing hydrogen content the gasification rate declines. The influence of heating protocol (i.e., experimental procedure and preparation of catalystcarbon sample) was investigated for K2CO3 catalyzed CO2 gasification via thermogravimetric analysis26,27. The effectiveness of the catalyst during carbon gasification depended not only on the catalyst loading (K/C ratio), but also on the degree and method of catalyst reduction. The latter effect was attributed to the reduction of potassium carbonate to an active surface complex during the initial heating. The catalyst reduction was inhibited by the gasification gas (i.e., CO2). The heating procedure in laboratory reactor setups (i.e., fixed bed, thermogravimetric analyzer) might not represent an industrial gasifier. In our previous work we developed a drop-down furnace, in which the carbon sample (e.g., ash-free coal and catalyst) was dropped into the hot reactions zone as it would be in a fluidized bed gasifier. The present study continues the work and investigates the dependency of catalyst loading, H2O/H2 ratio in the reaction gas, and heating protocol on the carbon conversion and gas production rates during gasification of ash-free coal using a drop-down reactor.

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2

Experimental

The ash-free coal (AFC) samples were produced by solvent extraction from Canadian subbituminous coal (Genesee coal, Alberta)26,28. Briefly, dry coal ( 99.0%, dP < 90 µm). During this procedure, the solid mixture adsorbed moisture from the air leading to agglomeration. The moisture content increased to a maximum of 6 wt% for the highest catalyst loading. Furthermore, as the sample adsorbed moisture the crystalline structure of K2CO3 changed to K2CO3·1.5 H2O and K4 H2(CO3)3·1.5 H2O as explained in detail in Kopyscinski et al. 201428. Wet impregnation methods could not be used as the solvent-extracted AFC samples were hydrophobic in nature and had a very low surface area (~10 m2 g-1 by CO2 adsorption at 0°C). Samples with 5-40 wt% K2CO3 and the same amount of AFC (16 mg) were produced and designated as AFC_X. For example, AFC_20 contains 16 mg AFC + 4 mg K2CO3 (i.e., 20 wt% K2CO3). The proximate and ultimate analyses of the ash-free coal, its char and the corresponding raw coal are summarized in Table 1. The ash-free coal had volatile matter and fixed carbon contents of 57.5 wt% and 42.5 wt%, on a dry basis, respectively. The amount of remaining ash was on the

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Manuscript ID: ef-2015-01538f R1, revised order of 700 mgash/kgAFC as previously published26. In terms of the elemental analysis, dry ash-free coal had a carbon content of 88.9 wt%, a hydrogen content of 5.0 wt%, a nitrogen content of 2.8 wt%, a sulfur content of 0.1 wt% and an oxygen content of 3.2 wt%. The char of the ash-free coal had carbon, hydrogen, nitrogen, sulfur and oxygen contents of 96.4 wt%, 1.7 wt%, 1.7wt%, 0.1 wt% and 0.1 wt%, respectively26. Compared to the corresponding raw coal the oxygen content of the ash-free coal has been substantially reduced, which has been explained by the decomposition of oxygen-functional groups that are commonly found in coal (e.g., carboxylic - COOH) during the solvent extraction process28. The steam gasification experiments were carried out in a fixed bed reactor in which the sample was dropped in the hot-reaction zone, which mimics fast heating and injection into a fluidized bed gasifier. The setup and experimental procedure have been described elsewhere20. Briefly, the empty reactor was heated under Ar atmosphere (40 mlN min-1, Linde 99.9999%) to the desired temperature (i.e., 600-725°C). Then, the sample was dropped via a specially designed valve and channel system into the quartz reactor. The gas released during the devolatilization process was condensed and sent to the exhaust. After a further holding time of 150 min, the H2O/H2 mixture (total gas flow rate of 40 mlN min-1 with various H2O/H2 ratios) was introduced into the reactor, while the heating gas Ar was bypassed. At this time the gasification time was set as t = 0. After the condenser a slip-stream of the dried gas was analyzed with a calibrated gas chromatograph (SRI 8610c), containing two channels. The first channel consisted of a continuously operating methanizer – flame ionization detector (FID) module in which CO and CO2 were converted to CH4 and subsequently quantified. The total carbon released from the char during the gasification reaction was measured with a temporal resolution of 5 Hz. The second channel consisted of a 1 m Haysep-D column to separate H2, CO, CO2 and CH4, a thermal conductivity detector and methanizer-FID to quantify the gas

6


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Manuscript ID: ef-2015-01538f R1, revised composition with a temporal resolution of 3.5 min. No deactivation of the methanizer was detected during the analysis/experimental period. In this work, the influence of catalyst loading (i.e., 5-40 wt% K2CO3), temperature for AFC_10 and AFC_20 (i.e., 600-800°C), H2O/H2 ratio (i.e., 1-9) and two different heating protocols on the gasification behavior were investigated. The experimental procedure for heating protocol (A) was described above, and this protocol allows separation of the devolatilization from the gasification process as the sample was dropped in the reactor under inert gas conditions (Ar atmosphere) with a long holding time. In heating protocol (B), the sample was dropped in the reactor under reaction gas conditions (i.e., T = 625°C and H2O/H2 = 4). Here the devolatilization and gasification processes overlapped. The gasification rates were measured under gas compositions far away from the gas-carbon equilibrium (i.e., ∙ ⁄ ratio was always 50 times smaller than Kp of the gasification reaction, C + H2O ↔ CO + H2). The experiments were conducted at a pressure of 2.5 barabs. The gasification results (i.e., conversion versus time) of repeated experiments for the same target loading and temperature varied within 10%. The reasons for the deviations were the small sample weight and the solid dry mixing process (i.e., nonhomogeneous catalyst distribution). The difference between the same repeated experiments, however, was very small compared to the change in conversion for different experimental conditions (e.g., higher temperature, catalyst loading, and H2O/H2 ratio). The carbon conversion of the char was calculated based on the total carbon measured in a given time interval with a methanizer flame ionization detector as described in detail previously 20. Briefly, the carbon conversion in a given time interval ∆ti was determined based on the FID signal I versus time t from Channel 1 with Eq. (9).

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= ∑

∙

∙

+

(9)

where ∆I⋅∆ti is the area in the time interval i, ∑ Δ ∙ Δ is the total area that represent the initial total carbon content in the char, and Xi and Xi-1 are the carbon conversions in the interval i and the previous interval i-1. For the first interval i =1, and the conversion is X0 = 0.

3 3.1

Results and Discussion Influence of catalyst loading

The influence of the catalyst loading during the steam gasification at 625°C with H2O/H2 = 4 and heating protocol (A) is illustrated in Figure 1 a-b in terms of carbon conversion and gasification rate of the char, respectively. The catalyst loading was increased from 5-40 wt%, which corresponded to an initial K/C ratio in the char of 0.02 to 0.28, respectively. After an induction time of approximately 2-3 min, the conversion varied significantly between the experiments with the amount of catalyst strongly influencing the gasification behavior. For example, 50% carbon conversion was achieved after 100 min for the AFC_5 sample, while 31, 16 and 11 min were needed for AFC_10, AFC_20 and AFC_40, respectively. For complete conversion, more than 300 min were necessary for the AFC_5 and only 50 min for AFC_40. As discussed in our previous work20, the induction period might be due to the formation of oxidized sites (O-*) and/or COcomplexes on the surface as described in Eqs. (1) and (2), respectively. The gasification rates depicted in Figure 1 b show a huge increase at small conversions, which might be associated with the development of porosity. For the AFC_5 sample, the rate was constant between 10-80% carbon conversions (dX/dt ∼0.05 min-1), whereas a distinct maximum in the rate (dX/dt ∼0.6 min-1) was observed for AFC_40 at 20% conversion. Thereafter, the rate declined almost linearly until the carbon was completely converted, due to (1) collapsing of pores or (2)

8


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Manuscript ID: ef-2015-01538f R1, revised decreasing catalyst activity due to either saturation of potential sites and/or formation of larger less active potassium-clusters. Evaporation of potassium can be neglected at this temperature26. A constant steam gasification rate between 10-70% carbon conversions was also observed for coal and model carbons15. The gasification rates relative to that for AFC_10 (K/C = 0.047) at 20% and 50% carbon conversion are depicted in Figure 2. Between K/C ratios of 0.02 to 0.08 the gasification rate increased steeply; more precisely, when the K/C ratio doubled the rate increased more than threefold. Then the curve flattened and a further increase of the K/C ratio from 0.11 to 0.28 increased the rate at 20% conversion by a factor of less than 2. At 50% carbon conversion the increase in the rate was even smaller. This decrease in reactivity per potassium atom at high catalyst loading might be attributed to the formation of larger potassium-clusters perhaps accompanying the saturation of potential active carbon sites. Earlier studies with K2CO3 impregnated coal char and model carbon showed a similar saturation behavior with a linear increase in the gasification rate until a K/C ratio of ~0.1; at higher ratios, gasification rates did not change significantly15. The carbon based gas production rates of CO2, CO and CH4 during steam gasification of AFC_5 to AFC_40 are depicted in Figure 3 a-c, respectively. The gas production rate was defined as millimole per minute of species i per gram carbon in the char (mmol min-1 gC-1). The initial CO2 formation rate - at less than 2% carbon conversion - was significantly lower compared to the CO2 formation rates between 10-90% conversions; whereas the opposite trend was observed for the CO and CH4 formation rates. Increased catalyst loading increased the gas production rates of CO2, CO and CH4 (Figure 3). However, the total amount of gas produced was independent from the catalyst loading.

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Manuscript ID: ef-2015-01538f R1, revised Above 10% carbon conversion, the formation rates of CO2 and CO exhibited a qualitatively similar behavior resulting in an almost constant CO2/CO ratio of 10.9 that was independent of the catalyst loading (not shown). This ratio is similar to, but slightly higher than the value predicted by equilibrium calculations (CO2/CO = 9.3) at 625°C and H2O/H2 = 4. The reason for the higher CO2 production might be the oxidation of the C-O complex to CO2 instead of desorption as CO, see reactions (5) vs. (3). At the onset of the gasification, 27% of the carbon was converted to CH4 for the AFC_5 sample, while at higher catalyst loading (e.g., AFC_40) the initial CH4 selectivity was approximately 10% (Figure 4). The CH4 selectivity decreased with progressing carbon conversion to values of less than 1.5%. This result indicates that the potassium predominately catalyzes the gasification and watergas-shift reactions instead of the methane formation. Methane was most likely formed by direct hydrogenation of the initial active carbon substrate – see Eq. (6) – and not via the methanation of CO, which would include CO dissociation, carbon deposition and subsequent hydrogenation. In addition, the declining CH4 selectivity with progressing carbon conversion suggests that the CH4 formation is decoupled from the CO and CO2 production as the surface is mostly covered with oxygen instead of hydrogen. 3.2

Influence of temperature

The temperature dependency of the steam gasification of AFC_0, AFC_10 and AFC_20 is depicted in Figure 5. The addition of 10 and 20 wt% K2CO3 to ash-free coal resulted in a decrease of the observed activation energy from 268 to 166 and 142 kJ mol-1, respectively. However, at temperatures higher than 675°C possible mass transfer limitations and product inhibition were observed for the AFC_20 sample. For example at 725°C, the CO2 content in the product gas was as high as 7 vol% relative to the H2 content, thus CO2 inhibition on the gasification rate could not be

10


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Manuscript ID: ef-2015-01538f R1, revised excluded. A stable CO2-* intermediate might be formed on the surface, reducing the number of sites for the oxygen transfer and, thus, reducing the oxygen coverage on the surface. In addition, catalyst saturation at T > 700°C was evident, as the rates for AFC_10 and AFC_20 were similar. At higher temperatures, formation of potassium clusters due to increased potassium mobility and high K/C ratio might also have occurred. In-situ Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) experiments in which K2CO3 catalyst was added on one side of a rectangular model carbon showed clearly an improved potassium mobility with increasing temperature29. A similar decrease in the activation energy from 256 to 160 kJ mol-1 was observed for model carbon gasified without and with the addition of alkali catalyst30. An activation energy of 120 kJ mol-1 was determined by means of thermogravimetry during steam gasification of 12 wt% K2CO3 mixed with ash-free coal31. This study also observed mass transfer limitations at temperatures larger than 700°C, which is comparable to our findings. 3.3

Influence of H2O/H2 ratio

The H2 inhibition during steam gasification at 625°C for the AFC_20 sample is depicted in Figure 6 and Figure 7 in terms of carbon conversion and relative gasification rate, respectively. With a H2O/H2 ratio of 9 (36 mlN min-1 H2O and 4 mlN min-1 H2) the coal sample was completely gasified after 30 min, whereas the gasification time increased to more than 2 h in the 50:50 H2O-H2 mixture. The high H2O/H2 ratio and fast gasification rate represent the conditions closer to the inlet of the gasifier. Traveling through the bed, the steam will be converted and hydrogen produced such that the H2O/H2 ratio, and corresponding gasification rate, will decrease. An increase of the H2 content from 10 to 20 vol% in the H2O-H2 feed mixture – i.e., decrease of the H2O content from 90 to 80 vol% – reduced the gasification rate by ~40% (Figure 7). A further increase in the H2 content to a 50:50 H2O-H2 mixture decreased the gasification rate to

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Manuscript ID: ef-2015-01538f R1, revised approximately 20% of the rate obtained at a H2O/H2 ratio of 9 to 1. In other words, when the H2O/H2 ratio was doubled the gasification rate increased by a factor of 1.6-1.7. Thus, the rate was not directly proportional to the H2O/H2 ratio ( ≠ ∙

! "

) as shown for Spherocarb32. In contrast,

!

the gasification rate in the present study was proportional to a more complicated function such as

∝∙

! " ! " $ ! ⁄%

where K1 is the equilibrium constant from reaction (1). The difference might

be explained, by the surface area and method of catalyst mixing. Spherocarb is highly porous and the catalyst was added via impregnation method, while the ash-free coal has a very small surface (10 m2 g-1; CO2 adsorption) and the catalyst was added via dry mixing. The CH4 and CO formation rates as a function of carbon conversion were essentially independent of the H2O/H2 ratio, whereas the CO2 formation rate decreased with increasing H2 content (Figure 8). More specifically, the CO2 formation rate decreased by a factor of ~5 when the H2O/H2 ratio was reduced from 9 to 1. Thus, the CO2/CO ratio decreased as well following the equilibrium of the water-gas-shift reaction (&'() ∙

!" !

=

*"

), as depicted and summarized in Figure 9 a and Table

*"

2, respectively. As mentioned above, a smaller H2O/H2 ratio represents a spatial position in a commercial gasifier that is farther away from the inlet. Thus, with progressing steam conversion and hydrogen production, the CO selectivity increased as well. In addition, due to competitive adsorption of hydrogen on the free carbon site, the CH4 selectivity increased significantly (Figure 9 b). With decreasing H2O/H2 ratio, the total amounts of CO and CH4 formed increased from 3.5 to 20 mmol gC-1 and 1 to 5 mmol gC-1, respectively, whereas the CO2 formation decreased from 80 to 60 mmol gC-1 (Table 1). These results confirm that methane is formed predominately by hydrogenation of the surface carbon (see Eq. 6) and not via dissociation of CO and subsequent hydrogenation of the active carbon intermediate (see Eqs. 7 and 8).

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Manuscript ID: ef-2015-01538f R1, revised 3.4

Influence of heating protocol

Figure 10 shows the normalized total carbon signal (FID) for the first eight minutes after the sample drop (AFC_20) at 625°C under (A) Ar atmosphere and (B) H2O-H2 mixture. The potassium containing carbon sample heated up very fast as the maximum carbon peak for both runs was observed at 0.5 min. Up to 2 min, the carbon signals were similar, indicating that the main devolatilization depends only on the carbon sample and temperature, but not on the gas atmosphere. From 2 min onward, the carbon signal differed significantly showing the start of gasification under protocol (B). The carbon signal obtained during protocol (A) declined asymptotically towards zero during the 150 min of soaking in the Ar atmosphere. During this time predominantly CO2 and little CO originating from the carbonated group were released as observed in our previous study. In contrast, the total carbon signal from the sample heated under protocol (B) was much higher and exhibited a second maximum at 3-4 min. The time for complete gasification was approximately 50 min (Figure 11 a), whereas an additional ten minutes were required to gasify the sample using protocol (A). The local maximum in the carbon signal during protocol (B) is embodied in the high gasification rate between 5 and 20% carbon conversion (Figure 11 b). At its maximum, the observed gasification rate during protocol (B) was approximately 3 times higher than that obtained during protocol (A). Thereafter, from 20% conversion onwards the rates for both runs were almost identical. The high rates obtained under protocol (B) are most likely a result of the release and gasification of functional groups from the carbon surface in form of CO2 and CO, see Figure 12 b. For protocol (A) these functional groups were removed prior to the gasification during the 150 min holding time in Ar. Thus, the observed gasification rate at low carbon conversion was much smaller; but once the functional groups were removed the gasification rates were similar and independent of the heating protocol. This effect was confirmed by the measurement of the gas

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Manuscript ID: ef-2015-01538f R1, revised formation rates as depicted in Figure 12. Here from 30% carbon conversion onwards, the CO2 formation rates were almost identical for protocol (A) and (B). However, the CO and CH4 formation rates up to 60% carbon conversion were much higher for the experiment following protocol (B). Integrating these values over the gasification time results in the total gas production based on the carbon in the char, see Table 3. The total amount of CO was doubled from 6.8 to 14.9 mmol gC-1, while the value for CO2 decreased from 74.1 to 67.2 mmol gC-1 when the sample was dropped into the H2O/H2 atmosphere (Protocol B) instead of into an inert gas (Protocol A). The behavior during catalyzed CO2 gasification of a similar ash-free coal was very different and depended greatly on the heating protocol over the whole conversion range. Here the CO2 gasification rate was much faster for protocol (A) than for protocol (B), which was attributed to the reduction of the potassium catalyst and could also reflect much lower catalyst mobility in CO2 atmospheres than in steam. In order to provide some more evidence of the hypothesized saturation effect and the formation of larger less active potassium-clusters ex-situ and in-situ Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis are in progress and subject of future publications.

4

Conclusions

•

The amount of K2CO3 in the ash-free coal influenced its gasification behavior and rate. Increasing the K2CO3 amount increased the gasification rates until a saturation effect occurred at initial K/C ratios of approximately 0.1 (similar to other carbon samples). In addition, the more catalyst the higher the CO, CO2 and CH4 formation rates. However, K2CO3

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Manuscript ID: ef-2015-01538f R1, revised predominately catalyzed the gasification and water-gas-shift reaction, as with higher catalyst loading the methane selectivity decreased. •

Methane is formed via direct hydrogenation of the carbon surface and not via dissociation of CO followed by the hydrogenation of the carbon intermediate.

•

Adding K2CO3 to ash-free coal lowered the activation energy from 260 kJ mol-1 to values between 140-165 kJ mol-1. At higher catalyst loading and temperatures larger than 675°C possible mass transfer limitations and product inhibition occurred.

•

Decreasing the H2O/H2 ratio decreased the gasification rate significantly, but increased the CH4 selectivity as more hydrogen adsorbs on the carbon surface.

•

The heating protocol had little influence on the overall gasification rate. Yet, dropping the catalyzed ash-free coal into a H2O/H2 atmosphere increased significantly the selectivity towards CO in the product gas.

Acknowledgements The authors would like to acknowledge the financial support from Carbon Management Canada (CMC), and Dr. Gupta and Dr. Rahman from the University of Alberta for providing the ash-free coal.

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List of Tables Table 1 Properties of Genesee raw coal, ash-free coal and char of the ash-free coal. Table 2 Influence of H2O/H2 ratio on the total gas production (mmol per gram carbon in char) during steam gasification of AFC_20 at 625°C. Table 3 Influence of the heating protocol ratio on the total gas production (mmol per gram carbon in char) during steam gasification of AFC_20 at 625°C with H2O/H2 = 4/1.

Table 1 Properties of Genesee raw coal, ash-free coal and char of the ash-free coal. Proximate analysis (wt% dry)

Elemental analysis (wt% daf)

VM

FC

Ash

C

H

N

S

Oa

GEN-Raw

31.5

38.3

30.2

73.1

4.3

1.0

0.4

21.2

AFC

57.5

42.5

700 mg/kgb

88.9

5.0

2.8

0.1

3.2

AFC-char

n/a

n/a

n/a

96.4

1.7

1.7

0.1

0.1

VM = volatile matters, FC = fixed carbon, dry = dry basis, daf = dry and ash-free oxygen content by difference b Determined by ICP-MS, n/a not analyzed a

Table 2 Influence of H2O/H2 ratio on the total gas production (mmol per gram carbon in char) during steam gasification of AFC_20 at 625°C. H2O/H2

H2

CO

CH4

CO2

Total Carbon

Sum

mmol gC-1

CO2/CO* obs.

equi.

9/1

159

3.5

1.1

78.7

83.3

242

22.5

21.0

4/1

152

6.8

1.4

74.1

82.3

234

10.9

9.3

2/1

141

15.3

2.6

65.3

83.2

225

4.3

4.7

1/1

129

19.7

4.6

59.2

83.5

213

3.0

2.3

* obs. = observed, equi. = equilibrium CO2/CO volume ratio (for water-gas-shift reaction)

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Manuscript ID: ef-2015-01538f R1, revised Table 3 Influence of the heating protocol ratio on the total gas production (mmol per gram carbon in char) during steam gasification of AFC_20 at 625°C with H2O/H2 = 4/1. Protocol

H2

CO

CH4

CO2

Total Carbon

Sum

mmol gC-1

CO2/CO* obs.

equi.

A

152

6.8

1.4

74.1

82.3

234

10.9

9.3

B

146

14.9

1.7

67.2

83.8

230

4.5

9.3

* obs. = observed, equi. = equilibrium CO2/CO volume ratio (for water-gas-shift reaction)

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List of Figures Figure 1. Influence of catalyst loading on (a) the carbon conversion and (b) gasification rate during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Figure 2. Normalized gasification rates (i.e., relative to that of AFC_10, K/C = 0.047) at 20% and 50% carbon conversion as a function of initial K/C ratio during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. Figure 3. Influence of catalyst loading on the gas production rates of (a) CO2, (b) CO and (c) CH4 as a function of carbon conversion during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. Figure 4. Selectivity to methane as a function of carbon conversion during steam gasification of AFC_5, AFC_10, AFC_20 and AFC_40 at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. Figure 5. Arrhenius plot based on the gasification rate at 50% carbon conversion for AFC_0, AFC_10 and AFC_20 (Heating protocol (A), H2O/H2 = 4/1). Figure 6. Influence of H2O/H2 ratio on the carbon conversion of AFC_20 gasified at 625°C (Gas flow 40 mlN min-1, Heating protocol (A)). Figure 7. Influence of H2 content in the H2O-H2 feed mixture (H2O/H2 ratio) on the relative gasification rates at 20% and 50% carbon conversion for AFC_20 (Gas flow 40 mlN min-1, Heating protocol (A) and at 625°C). Note: Dashed lines for guidance only.

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Energy & Fuels

Manuscript ID: ef-2015-01538f R1, revised Figure 8. Influence of H2O/H2 ratio on the gas production rates of (a) CO2, (b) CO and (c) CH4 as a function of carbon conversion during steam gasification of AFC_20 at 625°C (Heating protocol (A)). Note: Dashed lines for guidance only. Figure 9. Influence of H2O/H2 ratio on (a) the CO2/CO ratio and (b) the methane selectivity as a function of carbon conversion during steam gasification of AFC_20 at 625°C with heating protocol (A). Note: Dashed lines for guidance only. Figure 10. Normalized total carbon signal (flame ionization detector - FID) after drop of AFC_20 into reactor at 625°C under protocol (A) or (B). Protocol B with H2O/H2 = 4/1). Figure 11. Comparison of heating protocols (A) and (B) on (a) carbon conversion and (b) gasification rate of AFC_20 at 625°C. Figure 12. Carbon based gas production rates and CO2/CO ratios during gasification of AFC_20 at 625°C using (a) Protocol (A) and (b) Protocol (B). Note: Dashed lines for guidance only.

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Figure 1. Influence of catalyst loading on (a) the carbon conversion and (b) gasification rate during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). 218x291mm (300 x 300 DPI)


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Energy & Fuels

Figure 2. Normalized gasification rates (i.e., relative to that of AFC_10, K/C = 0.047) at 20% and 50% carbon conversion as a function of initial K/C ratio during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. 109x77mm (300 x 300 DPI)


Energy & Fuels

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Figure 3. Influence of catalyst loading on the gas production rates of (a) CO2, (b) CO and (c) CH4 as a function of carbon conversion during steam gasification of ash-free coal at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. 225x360mm (300 x 300 DPI)


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Figure 4. Selectivity to methane as a function of carbon conversion during steam gasification of AFC_5, AFC_10, AFC_20 and AFC_40 at 625°C (Heating protocol (A) and H2O/H2 = 4/1). Note: Dashed lines for guidance only. 113x81mm (300 x 300 DPI)


Energy & Fuels

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Figure 5. Arrhenius plot based on the gasification rate at 50% carbon conversion for AFC_0, AFC_10 and AFC_20 (Heating protocol (A), H2O/H2 = 4/1). 133x116mm (300 x 300 DPI)


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Figure 6. Influence of H2O/H2 ratio on the carbon conversion of AFC_20 gasified at 625°C (Gas flow 40 mlN min-1, Heating protocol (A)). 114x81mm (300 x 300 DPI)


Energy & Fuels

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Figure 7. Influence of H2 content in the H2O-H2 feed mixture (H2O/H2 ratio) on the relative gasification rates at 20% and 50% carbon conversion for AFC_20 (Gas flow 40 mlN min-1, Heating protocol (A) and at 625°C). Note: Dashed lines for guidance only. 123x95mm (300 x 300 DPI)


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Figure 8. Influence of H2O/H2 ratio on the gas production rates of (a) CO2, (b) CO and (c) CH4 as a function of carbon conversion during steam gasification of AFC_20 at 625°C (Heating protocol (A)). Note: Dashed lines for guidance only. 225x356mm (300 x 300 DPI)


Energy & Fuels

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Figure 9. Influence of H2O/H2 ratio on (a) the CO2/CO ratio and (b) the methane selectivity as a function of carbon conversion during steam gasification of AFC_20 at 625°C with heating protocol (A). Note: Dashed lines for guidance only. 198x243mm (300 x 300 DPI)


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Figure 10. Normalized total carbon signal (flame ionization detector - FID) after drop of AFC_20 into reactor at 625°C under protocol (A) or (B). Protocol B with H2O/H2 = 4/1). 114x81mm (300 x 300 DPI)


Energy & Fuels

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Figure 11. Comparison of heating protocols (A) and (B) on (a) carbon conversion and (b) gasification rate of AFC_20 at 625°C. 219x294mm (300 x 300 DPI)


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Figure 12. Carbon based gas production rates and CO2/CO ratios during gasification of AFC_20 at 625°C using (a) Protocol (A) and (b) Protocol (B). Note: Dashed lines for guidance only. 99x42mm (300 x 300 DPI)


Formation of CH4 during K2CO3-Catalyzed Steam Gasification of Ash

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