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High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture
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High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture Ehsan Mostafavi a, Nader Mahinpey a,⇑, Moshfiqur Rahman b, Mohammad Hashem Sedghkerdar a, Rajender Gupta b a b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada
h i g h l i g h t s
17
H2 was produced by ash-free coal
18
catalytic steam gasification with CO2 capture. H2 purity up to 85% (on dry basis with N2) was achieved in a fixed bed reactor. The maximum gasification rate occurred with 20 wt% catalyst and Ca/ C ratio of 2. Carbon conversion of 97% was measured by elemental CHNS analysis of the residues. CaO improves the H2 molar fraction while K2CO3 enhances the overall reaction rate.
19 20 21 22 23 24 25 26 27 28 29 30 31
g r a p h i c a l a b s t r a c t
Ash Free Bienfait lignite (BL) Coal
High–Purity Hydrogen
Catalytic Steam Gasification of Ash-free BL Coal with Sorbent CO2 capture (at atmospheric and 675ºC conditions)
Condensed Steam and Tars
AFC Preparation Stages
Reaction Stage
Tar Separation/Gas Analysis Stages
34
a r t i c l e 3 4 6 8 37 38 39 40 41 42 43 44 45 46 47
i n f o
Article history: Received 28 September 2015 Received in revised form 8 March 2016 Accepted 10 March 2016 Available online xxxx Keywords: Ash-free coal Catalytic steam gasification High-purity hydrogen production Dry-sorption CO2 capture
a b s t r a c t This study is aimed at the improvement of process conditions for catalytic steam gasification of ash-free coal (AFC) integrated with CO2 capture. A novel modified catalytic hydrogen production reaction integrated gasification (M-HyPr-RING) process was proposed for high-purity H2 production from steam gasification of AFC. AFC was prepared from lignite coal by solvent extraction with maximum yields of 67% on a dry ash free basis at 400 °C using hydrotreated heavy aromatic hydrocarbons with a coal-to-solvent ratio of approximately 1:10. The gasification temperature and sorbent-to-carbon (CaO/C) ratio were maintained constant for all experiments at 675 °C and 2, respectively, as determined by our previous findings. The steam gasification experiments were conducted on AFC in a fixed-bed reactor in the presence of different catalyst loadings (5, 10 and 20 wt% of potassium carbonate) with and without sorbent. A maximum hydrogen molar fraction of 85% (dry basis with nitrogen) was observed with catalyst loading of 20% and CaO/C of 2 after 5 min of steam gasification under mild process conditions. Ó 2016 Published by Elsevier Ltd.
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⇑ Corresponding author at: Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada. Tel.: +1 (403) 210 6503; fax: +1 (403) 284 4852. E-mail address: [email protected] (N. Mahinpey).
1. Introduction
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Growing concerns on the deteriorating effects of atmospheric concentrations of carbon dioxide (CO2), as the major greenhouse gas emitted through anthropogenic activities [1–3], have resulted
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http://dx.doi.org/10.1016/j.fuel.2016.03.026 0016-2361/Ó 2016 Published by Elsevier Ltd.
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in worldwide efforts to develop efficient zero-carbon emission technologies to implement carbon capture and sequestration (CCS) in power plants [1]. Hydrogen, as a clean fuel, has recently been receiving a lot of attention. In particular, hydrogen production from gasification has become an interesting research topic in the last three decades [3]. Despite the numerous works performed in this area, there are still research questions that require additional in-depth studies with respect to operational conditions, such as temperature, pressure and steam-to-carbon and sorbent-to-carbon ratios [4]. A variety of carbonaceous fuels can be gasified through advanced gasification processes for the production of hydrogen [5], among which low rank coals (e.g. lignites) are of particular interest due to their lower heating values (LHVs) and their high reactivity. The most recommended gasifying agent for the production of H2 is pure steam. In the presence of steam, the H2 content can reach as high as 40–60 volume percent (on a dry basis) in the gasification product gas [6]. It has also been asserted that nearly pure H2 is achievable by adding calcium-based sorbents to the gasification process [7]. Although CO2 removal using a solid sorbent (dry method) is an energy-intensive process, the integration of the reaction and separation phases into one single stage has advantages over the conventional method (steam gasification without CO2 sorbent), such as process simplification, enhanced energy efficiency, increased reactant conversion, and higher product yields [7,8]. With respect to the CO2 capture, there are three main processes: post-combustion, pre-combustion and oxy-combustion technologies [5,9–11] being the most studied. As the name suggests, postcombustion capture refers to CO2 capture from the flue gases. In practice, amine-based CO2 wet scrubbing has proven to be the only mature process; however, it is associated with efficiency loss and demands high capital and operational costs [12]. CO2 capture can also be classified by process types. Among these methods, high-efficiency dry sorption is a promising method, although it is energy-intensive [9]. Ca based materials are the best candidates for CO2 capture sorbents, due to their abundance, low cost and high CO2 uptake at gasification temperatures [10]. In contrast to post-combustion, pre-combustion CO2 capture removes CO2 from process gas streams before the burning stage. Through this conventional process, a carbonaceous fuel, such as coal or natural gas, is converted via gasification to syngas, with carbon monoxide (CO) and H2 as the major components. The syngas is then routed to a shift reactor where the CO reacts with steam to produce H2 and CO2. Finally, the CO2 is separated from H2 in the amine scrubber column, resulting in a hydrogen rich stream. At the other end of the spectrum, sorption-enhanced hydrogen production from coal is a novel pre-combustion capture technique, which integrates steam gasification, shift conversion (or water–gas shift reaction) and sorbent CO2 capture reactions into one stage [7]. Lopez Ortiz et al. [13] were among the first to study the production of H2 from methane (CH4) reforming over commercial nickel-based catalyst through a single step in a laboratory-scale reactor. The integrated reactions took place more rapidly than the noncatalyzed process, so that equilibrium could be reached. H2 with a purity of 95% on a dry basis was obtained by applying this technique. Lin et al. invented a novel gasification method for sorptionenhanced H2 production from coal, which is known as hydrogen production reaction integrated novel gasification, HyPr-RING [7,8,14,15]. The HyPr-RING method combines the steam gasification of coal, the water–gas shift (WGS) reaction and CO2 capture into one stage in the same reactor. In more recent efforts, Lin et al. utilized synthesized calcium oxide (CaO, <325 mesh, Wako Pure Chemical Industries, Osaka, Japan) in a reactor to capture CO2 from gasification product gas at temperatures ranging from
600 to 800 °C [8]. The individual and overall main reactions in the HyPr-RING process are: Steam char gasification:
C þ H2 O ! CO þ H2 WGS reaction: CaO carbonation:
CaO þ CO2 ! CaCO3
DHo298 ¼ 41:5 kJ=mol DHo298 ¼ 178 kJ=mol
ð1Þ
138 140
ð2Þ
142 141 143 145
ð3Þ
147 146 148 150
ð4Þ
152 151 153 155
Therefore, the overall reaction is:
C þ CaO þ 2H2 O ! CaCO3 þ 2H2
DHo298 ¼ 87:5 kJ=mol
In such a continuous operation, the saturated CO2 carrier sorbent (calcium carbonate, CaCO3) should be regenerated to CaO in the calciner at temperatures of 850–900 °C. The heat required for regeneration of CaCO3 can be obtained from the combustion of unreacted carbon remained from the gasification reaction. The negative heat of the overall reaction shows that the process needs virtually no heat from the combustion of coal in the gasifier and that the heat of the carbonation and WGS reactions are available as part of the heat required for steam gasification. Lin et al. [16] confirmed that the major gas product from integrated hydrocarbon gasification and CO2 separation at 700 °C and 25 MPa was H2. It is noteworthy that there is a heat demand for the sorbent regeneration (recalcination; as shown in the following reaction) which makes the process energy intensive to some extent. Nonetheless, the temperature difference between the cold and hot streams offers the opportunity for a heat integration between the calcination and gasification/carbonation section.
CaCO3 ! CaO þ CO2
DHo298
136 137
DHo298 ¼ 132 kJ=mol
CO þ H2 O ! CO2 þ H2
135
¼ 178 kJ=mol
ð5Þ
In another study, Lin et al. [8] investigated the effect of pressure (in the range of 0.1 to 6.0 MPa) on gas products for Taiheiyo coal that had undergone the HyPr-RING process. According to the results, pressure increased the production of H2 accompanied by CH4 decomposition. Thermodynamic studies of the HyPr-RING process by coal gasification at 650 °C and 3 MPa showed a mixture of flue gas consisting of 91% H2 and 9% CH4 on a dry basis and nitrogen (N2) free. Potassium carbonate (K2CO3) has been known as a superior catalyst for gasification since its first application in an Exxon pilotscale gasifier [17,18]. However, a challenging issue is the recycling of the catalyst. Several approaches have been examined to mitigate catalyst deactivation. Wang et al. [19] proposed the addition of calcium hydroxide (Ca(OH)2) during the char preparation stage. The synergistic catalytic effect between calcium and potassium has been studied in a few publications. For instance, it has been determined that the synergistic effects between potassium and calcium is due to the formation of bimetallic compound, K2Ca(CO3)2 [20]. Hydrothermal pretreatment of coal with Ca(OH)2 was observed to allow for more catalyst, as a water-soluble compound, to remain after gasification with ash [21]. Our previous study on mixed catalyst sorbent composite also proved the synergic effect of catalyst and sorbent on coal gasification [4]. Calcium compounds catalyze tar-cracking reactions along with the WGS, resulting in enhanced hydrogen production. Increasing the temperature accelerates the Boudouard and reverse water–gas shift (RWGS) reactions. Consequently, a reaction temperature of 675 °C was found to be the best performance temperature, due to the CO2 capture effect of CaO [22]. The main drawback of potassium-catalyzed gasification is that the interaction between the catalyst and the minerals in the raw coal deactivates the catalyst and creates water-insoluble potassium compounds, such as potassium aluminosilicates. Therefore,
Please cite this article in press as: Mostafavi E et al. High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.03.026
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the catalyst becomes virtually unrecoverable. Nevertheless, this adverse impact does not happen to ash-free coal (AFC), as known as HyperCoal or ashless coal; and, the catalytic gasification reaction rates of AFCs were found to be higher than that of the parent raw coals [23]. Removing mineral from coal by thermal extraction with organic solvents to prepare AFC appears to be a promising technique, particularly for low rank coals that have smaller heating values [22]. After adding catalyst to the gasification process, AFCs have been found to be as reactive as high rank raw coals. Moreover, catalytic gasification can be performed at lower temperatures [24]. Above all, the catalyst can be recovered and reused in multiple cycles. Wang et al. produced high-yield H2 by catalytic steam gasification of coal using K2CO3 [23]. Surprisingly, only a slight loss in the catalyst activity occurred after the catalyst was recycled twice. Hence, the catalytic steam gasification of AFCs has proven to be a potential technology for high-efficiency H2 production. In a similar study, the catalyst of hydrogenation of HyperCoals was successfully recycled five times with insignificant deactivation [25]. In practice, the power generation of IGCC processes operates similar to that of natural gas combined cycles (NGCC). IGCC power plants are expected to achieve higher efficiencies (50% or even more) compared to those of conventional coal-fired power plants (35%). Consequently, this improved efficiency remarkably reduced the CO2 emissions per unit electrical power generated (by 40%), according to the U.S. Department of Energy [26]. IGCC power plants were brought to market in 1980s. In the U.S., the 292 megawatts (MW) Wabash River Coal Gasification Plant in Indiana and 313 MW Polk Power Station in Florida were among the first commercial IGCC plants. However, most of the gasification plants around the world use other types of feedstocks, such as heavy oil, petroleum coke and tar. These include the heavy oil based Exxon-Singapore Sarlux and Shell-Pernis IGCC plants and the petroleum coke based Texaco-El Dorado and Motiva Delaware IGCC plants [26,27]. Given the advantages of gasification in producing syngas for a range of chemical processes and in terms of overall energy efficiency and environmental factors in power generation, there has been considerable work on the gasification of a wide range of feedstocks. Despite these efforts, there are insufficient experimental data to be able to identify the optimal conditions for conducting catalytic steam gasification integrated with efficient CO2 capture. Coal has the largest reserves of all fossil fuels in the world [28], with abundant reserves in Canada. Coal will most likely continue to play an important role in the global energy supply, because it is abundant and cheap [29]. It is the second largest source of energy after natural gas in the U.S., while biomass is the fifth largest source of energy [30]. Coal plays an important role among the fossil fuels in the energy supply, due to its abundance and availability. Canada possesses 0.1% of the world’s total coal deposits, with a ranking of the 10th largest reserves of coal, 90% of which have been discovered in Alberta, British Columbia and Saskatchewan. Canada’s coal reserves will last for a century at the current production rate [31]. On the other hand, the demand for hydrogen gas (H2) production as a clean source of energy is on the rise. Currently, H2 is widely used in petroleum refining and upgrading applications [32]. Accordingly, with the increasing trends of oil sand extraction particularly in Canada, an increasing trend in H2 demand is inevitable [33]. It is noteworthy that while H2 is known as a clean fuel it is primarily produced from steam reforming of fossil fuels i.e. natural gas [34,35]. The steam gasification reaction converts carbon to H2 and CO as follows:
CðsÞ þ H2 O ! CO þ H2
DH298 ¼ 132 kJ=mol
Similar to SMR, the steam gasification of char is endothermic; hence, the use of an external heating source is unavoidable. Again, the CO can be further converted to H2 and CO2 through the WGS reaction. Although H2 production from coal can be commercially performed, the process is more complex and more costly than H2 production from natural gas via the SMR process. However, given the abundance of coal, investigation into and development of advanced clean coal technologies are worthwhile. Finally, CO2 acceptor process particularly for sorption enhanced H2 production from methane and coal (through SMR and HyPr-RING, respectively) has been under the spotlight since 2002, although the original idea was proposed by Curran et al. in 1966. Despite some complexity that is added by the pretreatment of raw coal to prepare AFC, there are process simplifications (because of the gasification, simultaneous shift reaction and CO2 separation in a single-step process) that include elimination of the need for heat exchangers between catalyst beds as well as the absorption and stripping units required for CO2 removal (or the pressure swing adsorption unit). This was mentioned by C. Han and D.P. Harrison for the case of methane [36]. This paper offers a novel M-HyPr-RING process for sorption enhanced H2 production. It also provides in-depth insight on the best conditions for high-purity H2 production by catalytic steam gasification of AFC, prepared from Bienfait lignite (BL), coal integrated with CO2 capture using calcined natural Cadomin limestone in a horizontal fixed-bed reactor under mild process conditions. During the gasification experiments, the molar fractions of H2 and CO2 were closely monitored using a micro GC. In order to identify the optimal catalyst loading, the effects of the catalyst wt% with and without sorbent on catalytic coal gasification were investigated. Moreover, the solid residues from all coal gasification experiments were analyzed to determine their compositions and the carbon conversion of each experiment using a thermogravimetric analyzer (TGA) and an elemental (carbon, hydrogen, nitrogen, sulfur) analyzer. The carbon conversions obtained from the results of TGA and CHNS analysis on the residues were compared with the results of gas chromatography for the molar fraction of hydrogen to confirm the optimal catalyst wt% with and without sorbent. The optimal catalyst wt% was determined to achieve the maximum values of the H2 molar fraction, the carbon conversion, and the reaction rate from steam gasification of AFC in the presence of CO2 sorbent.
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2. Experimental section
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2.1. Materials
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In this study, AFC prepared from BL coal (Bienfait lignite, Saskatchewan, Canada) was used as the carbonaceous material in the gasification experiments. The proximate analysis of the BL coal was performed according to the ASTM D-3172 standard using a TGA (TG 209, F1 Iris, Netzsch). In addition, the ultimate analysis of the coal sample was obtained using a Perkin Elmer elemental analyzer (CHNS/O Analyzer Series III, 2400). Tables 1 and 2 present the proximate and ultimate analyses for raw BL coal and BL-AFC on a dry basis.
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Table 1 Proximate analyses of raw BL coal and BL-AFC (wt%, dry ash-free (daf) basis).
a b
Sample
VMa
Ash
FCb
BL coal BL-AFC
37.6 59.6
16.5 0.2
45.9 40.2
VM: Volatile matters. FC: Fixed carbon.
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354 Table 2 Ultimate analyses of raw BL coal and BL-AFC-BL (wt%, dry ash-free (daf) basis).
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Sample
C
H
N
S
O
BL coal AFC-BL
58.0 88.5
3.4 4.8
1.8 2.6
1.0 0.8
35.8 3.3
Natural Cadomin limestone from western Canada was calcined at 850 °C under an N2 atmosphere and utilized as a sorbent for CO2 capture. The particle size of the calcined limestone (CaO) was less than 250 lm, which was prepared by CANMET Energy. The chemical analysis of Cadomin limestone can be found elsewhere [37]. The coal was crushed by a milling machine to a fine powder with an average diameter of about 5 lm and then mixed with CaO powder. The K2CO3 was 99.99% pure (ACS reagent >99%, Sigma–Aldrich. Pure grade N2 (99.9%) was supplied by Praxair. As shown in Fig. 1, the BL-AFC was prepared by solvent extraction [38]. Typically, an autoclave reactor (0.5 L) was loaded with dry pulverized coal (<150 lm) and mixed with solvent (coal-tosolvent ratio of 1:10). The coal-solvent slurry was heated to 400 °C under N2 (10 bar) with continuous agitation at this temperature for 1 h. Hot filtration at 100 °C was carried out using filter paper (1.0 lm) to separate the dissolved liquid and the solid residue. The residue was washed several times with tetrahydrofuran (THF) and then dried under vacuum at 80 °C. The dissolved product (filtrate) was slowly added to hexane (filtrate-to-hexane ratio of 1:40) with continuous stirring to precipitate the coal-derived organic components. The precipitate (AFC) was collected by filtration and dried under vacuum. The extraction yields were calculated based on dry ash free basis (daf) from the weight of the residue using the following Eq. (1).
ðFeed coal ðdafÞ Residue ðdafÞÞ Extraction Yield ðwt:%; dafÞ ¼ Feed coal ðdafÞ 100 ð6Þ
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The extraction yields of AFC from BL lignite coal is 67.0% and the ash content of BL-AFC sample was determined by ASTM D3174 method and was found to be 0.07%. A sample containing 5 g of AFC was used in each experiment. The CaO was added to the coal sample at two different molar ratios to the carbon in BL-AFC (Ca/C = 0, 2). The experiments were conducted on AFC in a fixed-bed reactor in the presence of different catalyst loadings (K2CO3 at 5, 10 and 20 wt%) with and without sorbent. N2 was fed at a flow rate of 400 mL/min into the reactor as a purge gas for pyrolysis. During the heating step, the gas samples from the exhaust of the reactor were taken at increments of 50 °C from 300 °C to 675 °C using an Agilent micro GC-TCD 490 (gas chromatograph/thermal conductivity detector) to keep a record of the composition variations during pyrolysis. The temperature was then kept constant at 675 °C for 10 min to ensure that it was stable, after which water was injected into the helical coil to start gasification [39,40].
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2.2. Experimental setup
375
Fig. 2 illustrates the schematic diagram of the experimental setup. A stainless steel fixed-bed tubular reactor (length of 100 cm and nominal inner diameter of 2.54 cm) was used in a horizontal electric furnace to conduct the gasification experiments. A spiral coil (length of 600 cm and inner diameter of 0.32 cm) was attached to the outside of the reactor tube and placed in the
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Fig. 1. Schematic diagram for the preparation of AFC by solvent extraction [38].
Please cite this article in press as: Mostafavi E et al. High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.03.026
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Mass flow controller ȹ
Needle valve
Water injection pump N2 Water Steam
N2
Product gas/Steam
Coal/CaO/K2CO3 powder
Condenser
Micro GC Product gas
Fig. 2. Schematic diagram of the experimental setup for the catalytic steam gasification of BL-AFC [40].
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furnace, in order to produce superheated steam (675 °C at 1 atm) by pumping water. Therefore, the required steam for gasification was produced by employing the coil installed inside the same furnace. The exit gas and wall temperatures were measured with two K-type thermocouples installed inside and outside of the reactor tube. A mass flow controller was used to maintain the flow rate of N2 at 400 mL/min passing through the reactor while conducting the experiments. The resultant gases produced from the steam gasification of the BL-AFC were routed to a condenser where the moisture and condensable vapors were collected. The tube connecting the reactor to the condenser was electrically trace-heated to prevent tar deposition and blockage. The product gas was finally conducted to the Agilent micro GC-TCD 490 in order to measure the composition.
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2.3. Experimental procedure
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In this work, 5 g of BL-AFC were used in all experiments. The amounts of catalyst added to the coal sample at specific weight ratios of 5%, 10%, and 20% with a constant sorbent-to-carbon ratio in the BL-AFC coal of 2 (Ca/C = 2) for the experiments with sorbent. The weight percentage of the catalyst was determined based on the K+ ions to the fixed carbon of the coal [41]. The Ca/C ratio of 2 was selected based on our previous study [40]. The feedstock, including AFC, catalyst and sorbent powders, were mixed thoroughly and placed in the middle of the reactor. The temperature was raised at an average heating rate of 30 °C/min under a pure N2 atmosphere for pyrolysis of the sample. All gasification experiments were performed at 675 °C, which was determined in our previous experiments to be the optimal temperature when sorbent was added [37,39,40], and near atmospheric pressure (15 psig). Once the reactor reached the desired gasification temperature of 675 °C, it was maintained for 5 min to ensure that the temperature was stable. The water pump was then turned on at a constant rate of 1 mL/min into the hot coil, so that steam (approximately 75% with the balance N2) was generated and injected to the reactor. Sequences of 15 samplings were taken by the micro GC-TCD, each taking 150 s to measure the variations in the resultant gas molar fraction with gasification time. The molar fractions of H2, CH4, CO, and CO2 were continuously measured using the micro
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GC-TCD during the pyrolysis and gasification stages of the experiment. All experiments were performed three times, and the results showed an uncertainty of less than 5%. The average heating rate of 30 °C/min was applied in all tests.
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3. Results and discussions
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3.1. Effect of catalyst loading on hydrogen production
426
In the present study, the effect of catalyst loading, as one of the main defining process variables of steam gasification of AFC, was investigated. Two sets of experiments were performed with varying amounts of catalyst and either with or without sorbent. Three different catalyst concentrations (5, 10 and 20 wt%) with constant molar ratios of CaO/C of 0 and 2 (without and with sorbent, respectively) were chosen for the steam gasification of BL-AFC. The catalyst only (sorbent free) steam gasification of coal experiments were carried out to attain results for the base case. The composition trends of gases with time can be well explained by knowledge of the key reactions occurring in the gasification process. In the HyPr-RING process, steam char gasification, WGS reaction and CaO carbonation (as shown in Eqs. (1)–(3)) are the main reactions that occur due to the presence of the CO2 sorbent [7,8]. The WGS reaction plays an important role and is considered as the key reaction. The use of sorbent moves the WGS reaction forward, leading to higher H2 production. The composition of product gas from pyrolysis and steam gasification for catalyst only (sorbent free) experiments at 675 °C is shown in Fig. 3(a). All the results reported in this research were on a dry basis with H2 being the major component in the pyrolysis gas and small CH4, and CO fractions. With the study’s main emphases on hydrogen production and CO2 capture, only H2 and CO2 trends are depicted here. Pyrolysis began at approximately 450 °C and was accelerated by increasing the temperature. By introducing steam to the system, the H2 molar fraction increased drastically with time. The hydrogen molar fraction peaked after a few minutes of gasification and then gradually declined over time. However, the molar fraction of CO2 increased moderately in the pyrolysis and then declined toward the end of the pyrolysis stage. The increase in the CO2 frac-
427
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100 90
(a)
80
Molar Percent (%)
N2
Steam/N2
5 wt.% catalyst only, H2 10 wt.% Catalyst only, H2
70
20 wt.% Catalyst only, H2
60
10 wt.% catalyst only, CO2
5 wt.% catalyst only, CO2 20 wt.% catalyst only,CO2
50 40 30 20 10 0 0
10
20
30
40
Time (min)
50
60
70
100
(b)
90
Molar Percent (%)
80
N2
Steam/N2
5 wt.% Catalyst, [CaO]/[C]=2,H2 10 wt.% Catalyst, [CaO]/[C]=2,H2
70
20 wt.% Catalyst, [CaO]/[C]=2H2
60
5 wt.% Catalyst, [CaO]/[C]=2,CO2
50
20 wt.% Catalyst, [CaO]/[C]=2,CO2
10 wt.% Catalyst, [CaO]/[C]=2,CO2
40 30 20 10 0 0
10
20
30
40
50
60
70
Time (min) Fig. 3. Variations of molar fractions of H2 and CO2 produced during the catalytic pyrolysis and steam gasification of BL-AFC at 675 °C (a) without sorbent and (b) with sorbent.
464
tion was due to the consumption of fresh AFC by gasification and WGS reactions. After a while, the deactivation of catalyst became important [42], since there was no sorbent in place in this case. The decline was inevitable, due to the shift in mechanism from kinetics to mass transfer on both catalyst and coal surfaces and their respective active sites.
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3.2. Effect of sorbent loading on CO2 capture capability
466
Increasing the amount of catalyst (wt%) had a positive effect on hydrogen production, reaching more than 60% with 20% of catalyst. The maximum reaction rate, slope or rate of change in the molar fraction occurred for AFC gasification with a catalyst loading of 20%. With lower amounts of catalyst (5% and 10%), the hydrogen production was decreased with lower initial reaction rates. There was a significant jump in the H2 fraction when the catalyst was increased from 10% to 20%. However, there was no significant change (only a 4% increase in the hydrogen peak fraction) when the catalyst loading was increased from 20% to 30%, indicating that further addition of catalyst did not have a significant effect on gasification. When the catalyst loading was increased from 5% to 20%, the CO2 fraction grew from 10% to just under 20%. The increasing trend in the CO2 molar fraction was accompanied by an increasing trend in the H2 molar fraction. This supports the idea that the WGS is the
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key reaction in this gasification process. The CO2 production passes through a smooth maximum and then decelerates toward the end of gasification. The overall behavior of CO2 was similar to that of H2, except that H2 had higher and sharper peaks. After about 10 min of gasification, the H2 fraction started to decline. As expected, faster declines occurred for higher catalyst loadings, since most of the char had been gasified during the high-speed initial gasification stage. In other words, sharper peaks in hydrogen production were followed by sudden declines. This can be explained by the gradual char consumption and ash layer formation around the coal particles. However, lower catalyst loadings resulted in slower decreasing trends, as less char had already reacted with steam and more char was still available to be converted to hydrogen. As can be observed in Fig. 3(b), the addition of sorbent significantly changed the H2 production for the same experimental conditions. All the H2 molar fraction increased with sorbent. In fact, CO2 capture emerged as the main factor in hydrogen production. This phenomenon can be explained by the shifting forward of the WGS caused by CO2 removal. This enhancement also improved the char gasification reaction. CO2 capture not only increased the maximum achievable H2 fraction, but also it enhanced the char conversion, as the larger areas under the curve in Fig. 3(b) suggest. The highest H2 molar fraction (almost 85%) was achieved with a catalyst loading of 20% and a sorbent-to-carbon ratio of 2. The maximum achievable H2 fraction increased nearly 1.4 times in
Please cite this article in press as: Mostafavi E et al. High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.03.026
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the presence of CaO compared to that of without the sorbent (85% compared to 60%). In contrast to the H2 fraction obtained from catalyst only (sorbent free) cases, the differences among H2 molar fractions were not well distinguished. This may be due to the deterring effect of CaO on the catalyst activity [20]. The formation of an oxygenated intermediate on the char surface during pyrolysis has been postulated to be an important factor in maintaining the char reactivity [19]. In addition, calcium species act as a deactivation deterrent [20]. Hydrogen produced can be calculated from the total molar flow rate of gas, which in turn, is estimated from the N2 molar flow fraction and the molar fraction of H2 in each sampling time (every 2.5 min; 150 s) and the total concentration in the reactor under the actual pressure and temperature using the following formula:
522 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572
Z nH2 ¼
0
7
purity H2 production could be attributed to the use of catalyst and naturally active Cadomin limestone with suitable surface properties to capture CO2 compared to a synthesized CaO. The CH4 measured in the product gas from coal pyrolysis with sorbent was less than that in the product gas from the coal pyrolysis without sorbent. The reduction of CO due to the WGS reaction may enhance CH4 steam reforming to H2:
CH4 þ H2 O ! CO þ 3H2
ð8Þ
CO2 was formed through the WGS reaction, and CH4 was formed through the following reaction known as hydrogasification:
C þ 2H2 ! CH4
ð9Þ
573 574 575 576 577 578 579
580 582 583 584 585
586 588
t
yH2 C total F total dt
ð7Þ
In other words, Xc values calculated based on hydrogen produced (carbon gasified) and measured by gas analysis during gasification process were compared with those estimated from the residual ash burn-off tests. The residual tests predict lower carbon conversions, which indicate that there are some other carbon containing gases (carbonaceous compounds) that are not measured by the gas analysis. For instance at 675 °C, the Xc approximated from the area on the curve was 87% and the Xc from residual tests was 98%. The difference is caused by the formation of small amounts of CH4 and CO and some tars as well as measurements deviations of less than 6% associated with gas analyzer under the same experimental conditions. It should be noted that, in terms of hydrogen production, the experiments with 10 wt% of catalyst and a sorbent-to-carbon ratio of 2 presents the highest area under the curve and, hence, the highest amounts of H2 mole produced. The fastest initial reaction rate was obtained for catalyst loading of 20% with sorbent. This confirms that sorbent had a more important role on the H2 molar fraction, whereas the catalyst amount affected the initial gasification reaction rate. The presence of sorbent decreased the CO2 molar fractions to less than 3% for all experiments with catalyst loadings of 5–20%; and, as shown in Fig. 3(b), the molar fraction curves were very close to each other. Higher H2 molar fractions coincided with lower CO2 fractions. These results demonstrate that CO2 removal favored the WGS reaction to shift toward increased hydrogen production, according to Le Chatelier’s principle. As a result, any suppression in CO2 production was equivalent to enhancement in H2 production. The decrease of H2 production after 30 min in the presence of sorbent was generally faster than the decrease in cases without the sorbent. In practice, the maximum value of the H2 molar fraction stemmed from a shift from kinetics to mass transfer limited regime. Clearly, when there was sorbent in the reaction system, the mass transfer resistance was caused by the carbonated layer around the sorbent, as well as by the ash layer around the char particles [40]. In the absence of sorbent, the lower reactivity of char particles along with catalyst deactivation caused the downward trend and shorter effective gasification time. Eventually, the deceleration of char gasification reaction suppressed the gasification process, in particular the WGS reaction that took place in series with the char gasification. This results in the leveling off of the CO2 molar fractions. Lin. et al. in managed to produce H2 at a molar fraction of 85.3% (on a dry basis, N2 free) by steam gasification of a subbituminous coal (Taiheiyo coal, Japan) integrated with sorbent CO2 capture at 700 °C. This was an excellent achievement with a novel process [7]. Nonetheless, a H2 molar fraction of 85.5% (on a dry basis with N2) was achieved at 675 °C in this current work, which is equivalent to more than 99% H2 on a dry basis and N2 free. The higher
3.3. Characterization of residual solid from steam gasification
589
The residual solid materials from the BL-AFC gasification experiments were weighed and analyzed with TGA ash burn-off tests, in order to measure the unconverted carbon and CaCO3 contents and to evaluate the carbon conversion. In each burn-off test, approximately 30 mg of solid residue underwent a calcination heat-up ramp from ambient temperature to 900 °C at a heating rate of 20 °C/min under pure N2 atmosphere with a flow rate of 50 mL/ min. After 10 min of isothermal treatment at 900 °C, the N2 gas was turned off and immediately changed to air for burning off the residue to ash. Fig. 4(a) and (b) presents the weight loss behavior of residual samples over the period of time measured by TGA. The results for the residual samples from catalyst only (sorbent free) gasification experiments are shown in Fig. 4(a). There were two major weight loss steps. The first step indicates the release of moisture, while the second step indicates the combustion of unconverted carbon. The hydrophilic characteristic of K2CO3 results in more moisture content with increased catalyst loading. A larger difference in weight loss was observed due to the moisture content when the catalyst amount was increased from 10% to 20%. The highest moisture (wt%) contains belongs to a catalyst loading of 30%. The small step before the combustion could be attributed to pyrolysis, as there could still have been some volatiles remaining in the residual samples. Another probable reason is the decomposition of K2CO3 [43] at less than 900 °C (898 °C). In Fig. 4(a), the larger distinctive step represents the combustion of unconverted carbon. The largest step occurred with catalyst loading of 5%; and, the second longest step, which was only slightly shorter than that with the 5% catalyst loading, occurred with catalyst loading of 10%. A sudden decrease in the amount of unconverted carbon occurred when the catalyst loading was increased from 10% to 20%; and, the smallest amount of unconverted carbon was observed for the 30% catalyst loading. Therefore, it can be concluded that increasing the catalyst loading decreased the unconverted carbon in the residual samples. This implies that more carbon was converted to hydrogen, leaving less carbon in the residual sample. This confirms the positive effect of catalyst loading on hydrogen production, which is in contrast with our findings previously mentioned in Fig. 3. Fig. 4(b) illustrates the burn-off results for gasification experiments with both catalyst and sorbent. The moisture was converted to Ca(OH)2, due to the presence of CaO; therefore, no weight loss behavior was observed at the beginning of TGA tests. Hence, the early step of moisture was eliminated and replaced with a smooth step at higher temperatures. This small step was related to decomposition of Ca(OH)2.
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Time (min) Fig. 4. TGA ash burn-off results of the solid residues obtained from the catalytic steam gasification of BL-AFC at 675 °C: (a) without sorbent and (b) with sorbent. 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665
The large step of weight loss indicates the decomposition of CaCO3, followed by carbon combustion. The portion of weight loss that occurred before the temperature reached 900 °C represents the combustion of unconverted carbon; whereas the portion after 900 °C and the gas switch from N2 to air indicates the combustion of unconverted carbon. In other words, first the Ca(OH)2 decomposed; the CaCO3 was then calcined to CaO; and, finally, the unconverted carbon was burnt to ash. The first portion of the step quantified the amount of Ca(OH)2; and, the second portion determined the amount of carbon captured and fixed to the sorbent structure, as well as the carbon content of the residual sample. The overall trends of weight loss for all three different solid residual samples were similar. The largest decrease (second step) in weight loss occurred for gasification with sorbent and catalyst loading of 5%. This suggests that more unconverted carbon remained in the residual sample and less carbon was gasified to hydrogen. Increasing the catalyst loading from 5% to 10% while maintaining the sorbent-to-carbon ratio of 2 resulted in a slight decrease in the length of the second step. Further increases in the catalyst loading caused a major reduction in the carbon content of the residual sample. The catalyst loading of 20% appeared to be the best ratio, as shown in Fig. 4(b). The presence of sorbent led to slight differences in unconverted carbon of the residual samples. Hence, the effect of sorbent on the gasification performance of AFC, in terms of carbon conversion, was more pronounced than that of catalyst. Fig. 5 shows the measured unconverted carbons contained in the solid residual samples after coal gasification with and without the sorbent for CO2 capture and with four catalyst-to-carbon ratios
using a Perkin Elmer elemental analyzer (CHNS/O Analyzer Series III, 2400). As shown in this figure, the amount of unconverted carbon decreased with increased catalyst loading into the steam gasification. In the steam gasification of AFC without sorbent (CaO/C of 0), the unconverted carbon content of the solid residual sample decreased with increased catalyst loading, indicating that the steam gasification of coal improved with increased catalyst loading. Similarly, the measured unconverted carbon (carbon content) of the solid residual samples after steam gasification of coal with sorbent decreased with increased catalyst loading. According to Fig. 5, the presence of sorbent dramatically improved the steam gasification of coal, due to the capture of CO2 during the process that drives the WGS reaction forward, leading to higher coal conversion and H2 production. In addition, the CaO-based sorbent may have had a catalytic effect [24,44,45] on the steam gasification of the coal and tar cracking reactions [46,47]. Thus, the mixture of catalyst and sorbent enhanced carbon conversion [4]. The results from ultimate analyses were in good agreement with those of the TGA burn-off tests (Fig. 4(a) and (b)), confirming the enhancement in gasification by both the catalyst and sorbent, as shown in 2. Although the presence of catalyst improved the steam gasification process, the effect of sorbent was more significant due to the numerous positive roles that sorbent played in the process, including CO2 capture and catalytic effects. The final conversions were estimated as a measure of the degree of completion of the reactions. The carbon conversions for
Please cite this article in press as: Mostafavi E et al. High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.03.026
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Fig. 5. Comparison of the ultimate analysis results on the amount of unconverted carbon in solid residues from the catalytic steam gasification of BL-AFC using a CHNS analyzer.
694 695
all catalytic steam gasification experiments were calculated based on:
696 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720
Xc ¼
mc0 mcr 100 mc0
ð10Þ
where X is the conversion of coal, mc0 is the mass of the carbon in the AFC loaded into the reactor, and mcr is the unconverted carbon mass in residues remained from the steam gasification experiments [40]. The calculated values for carbon conversions are listed in Table 3. In the first series of experiments, which were catalyst only without sorbent, the conversions rose from 70% to 80% by adding more catalyst. In steam gasification of the BL-AFC with sorbent, the carbon conversions were even higher, due to the sorptionenhanced gasification. The high oxygen contents shown in Fig. 5 can be due to the large amounts of CaCO3 in the residues obtained from gasification experiments. During the CHNS/O tests, the CaCO3 is decomposed to CO2 and CaO at 900 °C causing a weight loss that also counts as oxygen. This oxygen makes up the major portion of the residues for the with-sorbent experimental cases. For the without-sorbent (catalyst only) cases, the compositions of the residues are substantially different from those of the with-sorbent cases, as there is no CaCO3. However, there are small amounts of K2CO3 in the residues, which is decomposed even below 200 °C to K2O. Hence, calcium and potassium carbonates are the probable causes of high oxygen content measurements of the residues.
Table 3 Carbon conversion from catalytic steam gasification of BL-AFC with different catalyst loadings, with and without sorbent. Experimental ratios
Carbon conversion
5% Catalyst, [CaO]/[C] = 0 10% Catalyst, [CaO]/[C] = 0 20% Catalyst, [CaO]/[C] = 0 30% Catalyst, [CaO]/[C] = 0 5% Catalyst, [CaO]/[C] = 2 10% Catalyst, [CaO]/[C] = 2 20% Catalyst, [CaO]/[C] = 2
72.0 76.5 79.4 93.2 96.1 97.1 97.9
Increasing the catalyst loading up to 20% (with a CaO/C ratio of 2) improved the conversion to where it reached 97%. Moreover, adding sorbent to the gasification reaction system enhanced the conversion by approximately 20%. Therefore, the maximum achievable conversion was above 97%. The maximum achievable carbon conversion and hydrogen fraction in the product gas were improved from our previous study [40] to 97% and 86%, respectively. In the optimal case of catalyst loading of 20% and a CaO/C ratio of 2, the steam gasification of coal proceeded almost to completion, as evidenced by less than 3% carbon in the residual samples. The reaction rate increased dramatically, such that the H2 peak appeared in less than 5 min. However, the H2 fraction peaked after more than 10–12 min for the raw parent coal with sorbent [40], because the WGS was far from equilibrium due to lower temperatures [44]. The presence of catalyst in the solid mixture increased the overall reaction rate (char gasification and WGS reaction rates); therefore, more char was gasified to hydrogen, and more carbon conversion was achieved under the same experimental conditions (i.e., flow rate, pressure, temperature, and gas residence time in the reactor tube).
721
4. Conclusions
742
The main focus of this work was the determination of the effects of catalyst loading and sorbent, which were investigated with the aim of high-purity hydrogen production via catalytic steam gasification of AFC and integrated CO2 capture capability of the sorbent. A novel M-HyPr-RING process for a catalytic sorption enhanced H2 production was proposed. Different catalyst loadings with and without sorbent were used in order to further understand the factors involved in the catalytic steam gasification of AFCs. Based on our experimental results the following conclusions have been drawn:
743
(1) The effect of a sorbent on the hydrogen production yield was more significant than that of a catalyst. Sorbent considerably enhanced hydrogen production, whereas catalyst accelerated the gasification reaction rate.
753
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(2) The hydrogen molar fraction of 85% (dry basis with nitrogen) was observed with catalyst loading of 20% and a sorbent-tocarbon ratio (CaO/C) of 2 after 5 min of steam gasification. The experiments indicated trace amounts of CO and CH4 along with other hydrocarbons are present in the product gas. (3) Reaction rates for steam gasification of BL-AFC were more than twice as fast as that of raw BL coal under the same experimental conditions. (4) The maximum reaction rate and hydrogen molar fraction occurred at catalyst loading of 20% and a CaO (calcined Cadomin limestone)/C ratio of 2 at 675 °C. A maximum carbon conversion of 97% was achieved. (5) Residue tests revealed that unconverted carbon trends were in the same direction as hydrogen production. (6) The TGA ash-burn off results of solid residues verified that the majority of CO2 was fixed to the sorbent structure.
774 775
Acknowledgements
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The financial supports from Carbon Management Canada (CMC) and Natural Sciences and Engineering Research Council (NSERC) for this research are greatly appreciated. The authors wish to thank Dr. Davood Karami, Ms. Maria Fernanda Oliviera, and Ms. Negar Manafi for their valuable assistance in the ultimate analysis tests on the AFC and residues, as well as data analysis in this work. The authors would also like to extend their sincere gratitude to Dr. Vasilije Manovic and Dr. Edward Anthony from CANMET Energy for preparing the Cadomin limestone.
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[1] Herzog HJ. Peer reviewed: What future for carbon capture and sequestration? Environ Sci Technol 2001;35:148A–53A. [2] Rao AB, Rubin ES. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ Sci Technol 2002;36:4467–75. [3] Rezaiyan J, Cheremisinoff NP. Gasification technologies: a primer for engineers and scientists. CRC Press; 2005. [4] Mostafavi E, Mahinpey N, Manovic V. A novel development of mixed catalyst– sorbent pellets for steam gasification of coal chars with in situ CO2 capture. Catal Today 2014;237:111–7. [5] Basu P. Biomass gasification and pyrolysis: practical design and theory. Academic Press; 2010. [6] Corella J, Toledo JM, Molina G. Steam gasification of coal at lowmedium (600–800 °C) temperature with simultaneous CO2 capture in a bubbling fluidized bed at atmospheric pressure. 2. Results and recommendations for scaling up. Ind Eng Chem Res 2008;47:1798–811. [7] Lin S. Hydrogen production from coal by separating carbon dioxide during gasification. Fuel 2002;81:2079–85. [8] Lin S, Harada M, Suzuki Y, Hatano H. Continuous experiment regarding hydrogen production by coal/CaO reaction with steam (I) gas products. Fuel 2004;83:869–74. [9] Cooper CD, Alley FC. Air pollution control: a design approach. 4th ed. Long Grove: Waveland Press; 2011. [10] Florin N, Harris A. Hydrogen production from biomass coupled with carbon dioxide capture: the implications of thermodynamic equilibrium. Int J Hydrogen Energy 2007;32:4119–34. http://dx.doi.org/10.1016/j. ijhydene.2007.06.016. [11] Figueroa JD, Fout T, Plasynski S, McIlvried H, Srivastava RD. Advances in CO2 capture technology—the U.S. Department of Energy’s Carbon Sequestration Program. Int J Greenh Gas Control 2008;2:9–20. http://dx.doi.org/10.1016/ S1750-5836(07)00094-1. [12] Kanniche M, Gros-Bonnivard R. Pre-combustion, post-combustion and oxycombustion in thermal power plant for CO2 capture. Appl Therm Eng 2010. [13] Lopez Ortiz A, Harrison DP. Hydrogen production using sorption-enhanced reaction. Ind Eng Chem Res 2001;40:5102–9. http://dx.doi.org/10.1021/ ie001009. [14] Lin S-Y, Suzuki Y, Hatano H, Harada M. Developing an innovative method, HyPr-RING, to produce hydrogen from hydrocarbons. Energy Convers Manage 2002;43:1283–90. http://dx.doi.org/10.1016/S0196-8904(02)00014-6. [15] Lin S-Y, Suzuki Y, Hatano H, Harada M. Hydrogen production from hydrocarbon by integration of watercarbon reaction and carbon dioxide removal (HyPrRING Method). Energy Fuels 2001;15:339–43.
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[16] Lin S, Harada M, Suzuki Y, Hatano H. Process analysis for hydrogen production by reaction integrated novel gasification (HyPr-RING). Energy Convers Manage 2005;46:869–80. [17] Nahas NC. Exxon catalytic coal gasification process. Fuel 1983;62:239–41. [18] Jüntgen H. Application of catalysts to coal gasification processes. Incentives and perspectives. Fuel 1983;62:234–8. [19] Wang J, Yao Y, Cao J, Jiang M. Enhanced catalysis of K2CO3 for steam gasification of coal char by using Ca(OH)2 in char preparation. Fuel 2010;89:310–7. [20] Jiang M-Q, Zhou R, Hu J, Wang F-C, Wang J. Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: influences of calcium species. Fuel 2012;99:64–71. [21] Jiang M, Hu J, Wang J. Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: effect of hydrothermal pretreatment. Fuel 2013;109:14–20. [22] Kim J, Choi H, Lim J, Rhim Y, Chun D, Kim S, et al. Hydrogen production via steam gasification of ash free coals. Int J Hydrogen Energy 2013;38:6014–20. [23] Wang J, Sakanishi K, Saito I, Takarada T, Morishita K. High-yield hydrogen production by steam gasification of Hypercoal (ash-free coal extract) with potassium carbonate: comparison with raw coal. Energy Fuels 2005;19:2114–20. [24] Shuai C, Hu S, He L, Xiang J, Sun L, Su S, et al. The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas. Int J Hydrogen Energy 2014;39:15506–16. [25] Koyano K, Takanohashi T, Saito I. Catalytic hydrogenation of HyperCoal (ashless coal) and reusability of catalyst. Energy Fuels 2009. [26] National Energy Technology Laboratory. Gasifiers and gasification technology for special applications and alternate feedstocks; 2015. [27] Hassanzadeh Khayyat A. A regenerative process for CO2 removal and hydrogen production in igcc. Chicago: Illinois Institute of Technology; 2007. [28] Vejahati F, Xu Z, Gupta R. Trace elements in coal: associations with coal and minerals and their behavior during coal utilization – a review. Fuel 2010;89:904–11. http://dx.doi.org/10.1016/j.fuel.2009.06.013. [29] Habibi R. Co-gasification of Biomass and Non-biomass Feedstocks by A. University of Calgary; 2013. http://dx.doi.org/10.1017/CBO9781107415324. 004. [30] The International Energy Agency. Key World Energy Statistics; 2015. [31] McCelland, Stewart. The Canadian Encyclopedia; 2000.
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Fuel journal homepage: www.elsevier.com/locate/fuel 5 6 3 4 7 8 9 10 12 11 13 1 5 316 2
High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture Ehsan Mostafavi a, Nader Mahinpey a,⇑, Moshfiqur Rahman b, Mohammad Hashem Sedghkerdar a, Rajender Gupta b a b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada
h i g h l i g h t s
17
H2 was produced by ash-free coal
18
catalytic steam gasification with CO2 capture. H2 purity up to 85% (on dry basis with N2) was achieved in a fixed bed reactor. The maximum gasification rate occurred with 20 wt% catalyst and Ca/ C ratio of 2. Carbon conversion of 97% was measured by elemental CHNS analysis of the residues. CaO improves the H2 molar fraction while K2CO3 enhances the overall reaction rate.
19 20 21 22 23 24 25 26 27 28 29 30 31
g r a p h i c a l a b s t r a c t
Ash Free Bienfait lignite (BL) Coal
High–Purity Hydrogen
Catalytic Steam Gasification of Ash-free BL Coal with Sorbent CO2 capture (at atmospheric and 675ºC conditions)
Condensed Steam and Tars
AFC Preparation Stages
Reaction Stage
Tar Separation/Gas Analysis Stages
34
a r t i c l e 3 4 6 8 37 38 39 40 41 42 43 44 45 46 47
i n f o
Article history: Received 28 September 2015 Received in revised form 8 March 2016 Accepted 10 March 2016 Available online xxxx Keywords: Ash-free coal Catalytic steam gasification High-purity hydrogen production Dry-sorption CO2 capture
a b s t r a c t This study is aimed at the improvement of process conditions for catalytic steam gasification of ash-free coal (AFC) integrated with CO2 capture. A novel modified catalytic hydrogen production reaction integrated gasification (M-HyPr-RING) process was proposed for high-purity H2 production from steam gasification of AFC. AFC was prepared from lignite coal by solvent extraction with maximum yields of 67% on a dry ash free basis at 400 °C using hydrotreated heavy aromatic hydrocarbons with a coal-to-solvent ratio of approximately 1:10. The gasification temperature and sorbent-to-carbon (CaO/C) ratio were maintained constant for all experiments at 675 °C and 2, respectively, as determined by our previous findings. The steam gasification experiments were conducted on AFC in a fixed-bed reactor in the presence of different catalyst loadings (5, 10 and 20 wt% of potassium carbonate) with and without sorbent. A maximum hydrogen molar fraction of 85% (dry basis with nitrogen) was observed with catalyst loading of 20% and CaO/C of 2 after 5 min of steam gasification under mild process conditions. Ó 2016 Published by Elsevier Ltd.
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⇑ Corresponding author at: Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada. Tel.: +1 (403) 210 6503; fax: +1 (403) 284 4852. E-mail address: [email protected] (N. Mahinpey).
1. Introduction
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Growing concerns on the deteriorating effects of atmospheric concentrations of carbon dioxide (CO2), as the major greenhouse gas emitted through anthropogenic activities [1–3], have resulted
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http://dx.doi.org/10.1016/j.fuel.2016.03.026 0016-2361/Ó 2016 Published by Elsevier Ltd.
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in worldwide efforts to develop efficient zero-carbon emission technologies to implement carbon capture and sequestration (CCS) in power plants [1]. Hydrogen, as a clean fuel, has recently been receiving a lot of attention. In particular, hydrogen production from gasification has become an interesting research topic in the last three decades [3]. Despite the numerous works performed in this area, there are still research questions that require additional in-depth studies with respect to operational conditions, such as temperature, pressure and steam-to-carbon and sorbent-to-carbon ratios [4]. A variety of carbonaceous fuels can be gasified through advanced gasification processes for the production of hydrogen [5], among which low rank coals (e.g. lignites) are of particular interest due to their lower heating values (LHVs) and their high reactivity. The most recommended gasifying agent for the production of H2 is pure steam. In the presence of steam, the H2 content can reach as high as 40–60 volume percent (on a dry basis) in the gasification product gas [6]. It has also been asserted that nearly pure H2 is achievable by adding calcium-based sorbents to the gasification process [7]. Although CO2 removal using a solid sorbent (dry method) is an energy-intensive process, the integration of the reaction and separation phases into one single stage has advantages over the conventional method (steam gasification without CO2 sorbent), such as process simplification, enhanced energy efficiency, increased reactant conversion, and higher product yields [7,8]. With respect to the CO2 capture, there are three main processes: post-combustion, pre-combustion and oxy-combustion technologies [5,9–11] being the most studied. As the name suggests, postcombustion capture refers to CO2 capture from the flue gases. In practice, amine-based CO2 wet scrubbing has proven to be the only mature process; however, it is associated with efficiency loss and demands high capital and operational costs [12]. CO2 capture can also be classified by process types. Among these methods, high-efficiency dry sorption is a promising method, although it is energy-intensive [9]. Ca based materials are the best candidates for CO2 capture sorbents, due to their abundance, low cost and high CO2 uptake at gasification temperatures [10]. In contrast to post-combustion, pre-combustion CO2 capture removes CO2 from process gas streams before the burning stage. Through this conventional process, a carbonaceous fuel, such as coal or natural gas, is converted via gasification to syngas, with carbon monoxide (CO) and H2 as the major components. The syngas is then routed to a shift reactor where the CO reacts with steam to produce H2 and CO2. Finally, the CO2 is separated from H2 in the amine scrubber column, resulting in a hydrogen rich stream. At the other end of the spectrum, sorption-enhanced hydrogen production from coal is a novel pre-combustion capture technique, which integrates steam gasification, shift conversion (or water–gas shift reaction) and sorbent CO2 capture reactions into one stage [7]. Lopez Ortiz et al. [13] were among the first to study the production of H2 from methane (CH4) reforming over commercial nickel-based catalyst through a single step in a laboratory-scale reactor. The integrated reactions took place more rapidly than the noncatalyzed process, so that equilibrium could be reached. H2 with a purity of 95% on a dry basis was obtained by applying this technique. Lin et al. invented a novel gasification method for sorptionenhanced H2 production from coal, which is known as hydrogen production reaction integrated novel gasification, HyPr-RING [7,8,14,15]. The HyPr-RING method combines the steam gasification of coal, the water–gas shift (WGS) reaction and CO2 capture into one stage in the same reactor. In more recent efforts, Lin et al. utilized synthesized calcium oxide (CaO, <325 mesh, Wako Pure Chemical Industries, Osaka, Japan) in a reactor to capture CO2 from gasification product gas at temperatures ranging from
600 to 800 °C [8]. The individual and overall main reactions in the HyPr-RING process are: Steam char gasification:
C þ H2 O ! CO þ H2 WGS reaction: CaO carbonation:
CaO þ CO2 ! CaCO3
DHo298 ¼ 41:5 kJ=mol DHo298 ¼ 178 kJ=mol
ð1Þ
138 140
ð2Þ
142 141 143 145
ð3Þ
147 146 148 150
ð4Þ
152 151 153 155
Therefore, the overall reaction is:
C þ CaO þ 2H2 O ! CaCO3 þ 2H2
DHo298 ¼ 87:5 kJ=mol
In such a continuous operation, the saturated CO2 carrier sorbent (calcium carbonate, CaCO3) should be regenerated to CaO in the calciner at temperatures of 850–900 °C. The heat required for regeneration of CaCO3 can be obtained from the combustion of unreacted carbon remained from the gasification reaction. The negative heat of the overall reaction shows that the process needs virtually no heat from the combustion of coal in the gasifier and that the heat of the carbonation and WGS reactions are available as part of the heat required for steam gasification. Lin et al. [16] confirmed that the major gas product from integrated hydrocarbon gasification and CO2 separation at 700 °C and 25 MPa was H2. It is noteworthy that there is a heat demand for the sorbent regeneration (recalcination; as shown in the following reaction) which makes the process energy intensive to some extent. Nonetheless, the temperature difference between the cold and hot streams offers the opportunity for a heat integration between the calcination and gasification/carbonation section.
CaCO3 ! CaO þ CO2
DHo298
136 137
DHo298 ¼ 132 kJ=mol
CO þ H2 O ! CO2 þ H2
135
¼ 178 kJ=mol
ð5Þ
In another study, Lin et al. [8] investigated the effect of pressure (in the range of 0.1 to 6.0 MPa) on gas products for Taiheiyo coal that had undergone the HyPr-RING process. According to the results, pressure increased the production of H2 accompanied by CH4 decomposition. Thermodynamic studies of the HyPr-RING process by coal gasification at 650 °C and 3 MPa showed a mixture of flue gas consisting of 91% H2 and 9% CH4 on a dry basis and nitrogen (N2) free. Potassium carbonate (K2CO3) has been known as a superior catalyst for gasification since its first application in an Exxon pilotscale gasifier [17,18]. However, a challenging issue is the recycling of the catalyst. Several approaches have been examined to mitigate catalyst deactivation. Wang et al. [19] proposed the addition of calcium hydroxide (Ca(OH)2) during the char preparation stage. The synergistic catalytic effect between calcium and potassium has been studied in a few publications. For instance, it has been determined that the synergistic effects between potassium and calcium is due to the formation of bimetallic compound, K2Ca(CO3)2 [20]. Hydrothermal pretreatment of coal with Ca(OH)2 was observed to allow for more catalyst, as a water-soluble compound, to remain after gasification with ash [21]. Our previous study on mixed catalyst sorbent composite also proved the synergic effect of catalyst and sorbent on coal gasification [4]. Calcium compounds catalyze tar-cracking reactions along with the WGS, resulting in enhanced hydrogen production. Increasing the temperature accelerates the Boudouard and reverse water–gas shift (RWGS) reactions. Consequently, a reaction temperature of 675 °C was found to be the best performance temperature, due to the CO2 capture effect of CaO [22]. The main drawback of potassium-catalyzed gasification is that the interaction between the catalyst and the minerals in the raw coal deactivates the catalyst and creates water-insoluble potassium compounds, such as potassium aluminosilicates. Therefore,
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the catalyst becomes virtually unrecoverable. Nevertheless, this adverse impact does not happen to ash-free coal (AFC), as known as HyperCoal or ashless coal; and, the catalytic gasification reaction rates of AFCs were found to be higher than that of the parent raw coals [23]. Removing mineral from coal by thermal extraction with organic solvents to prepare AFC appears to be a promising technique, particularly for low rank coals that have smaller heating values [22]. After adding catalyst to the gasification process, AFCs have been found to be as reactive as high rank raw coals. Moreover, catalytic gasification can be performed at lower temperatures [24]. Above all, the catalyst can be recovered and reused in multiple cycles. Wang et al. produced high-yield H2 by catalytic steam gasification of coal using K2CO3 [23]. Surprisingly, only a slight loss in the catalyst activity occurred after the catalyst was recycled twice. Hence, the catalytic steam gasification of AFCs has proven to be a potential technology for high-efficiency H2 production. In a similar study, the catalyst of hydrogenation of HyperCoals was successfully recycled five times with insignificant deactivation [25]. In practice, the power generation of IGCC processes operates similar to that of natural gas combined cycles (NGCC). IGCC power plants are expected to achieve higher efficiencies (50% or even more) compared to those of conventional coal-fired power plants (35%). Consequently, this improved efficiency remarkably reduced the CO2 emissions per unit electrical power generated (by 40%), according to the U.S. Department of Energy [26]. IGCC power plants were brought to market in 1980s. In the U.S., the 292 megawatts (MW) Wabash River Coal Gasification Plant in Indiana and 313 MW Polk Power Station in Florida were among the first commercial IGCC plants. However, most of the gasification plants around the world use other types of feedstocks, such as heavy oil, petroleum coke and tar. These include the heavy oil based Exxon-Singapore Sarlux and Shell-Pernis IGCC plants and the petroleum coke based Texaco-El Dorado and Motiva Delaware IGCC plants [26,27]. Given the advantages of gasification in producing syngas for a range of chemical processes and in terms of overall energy efficiency and environmental factors in power generation, there has been considerable work on the gasification of a wide range of feedstocks. Despite these efforts, there are insufficient experimental data to be able to identify the optimal conditions for conducting catalytic steam gasification integrated with efficient CO2 capture. Coal has the largest reserves of all fossil fuels in the world [28], with abundant reserves in Canada. Coal will most likely continue to play an important role in the global energy supply, because it is abundant and cheap [29]. It is the second largest source of energy after natural gas in the U.S., while biomass is the fifth largest source of energy [30]. Coal plays an important role among the fossil fuels in the energy supply, due to its abundance and availability. Canada possesses 0.1% of the world’s total coal deposits, with a ranking of the 10th largest reserves of coal, 90% of which have been discovered in Alberta, British Columbia and Saskatchewan. Canada’s coal reserves will last for a century at the current production rate [31]. On the other hand, the demand for hydrogen gas (H2) production as a clean source of energy is on the rise. Currently, H2 is widely used in petroleum refining and upgrading applications [32]. Accordingly, with the increasing trends of oil sand extraction particularly in Canada, an increasing trend in H2 demand is inevitable [33]. It is noteworthy that while H2 is known as a clean fuel it is primarily produced from steam reforming of fossil fuels i.e. natural gas [34,35]. The steam gasification reaction converts carbon to H2 and CO as follows:
CðsÞ þ H2 O ! CO þ H2
DH298 ¼ 132 kJ=mol
Similar to SMR, the steam gasification of char is endothermic; hence, the use of an external heating source is unavoidable. Again, the CO can be further converted to H2 and CO2 through the WGS reaction. Although H2 production from coal can be commercially performed, the process is more complex and more costly than H2 production from natural gas via the SMR process. However, given the abundance of coal, investigation into and development of advanced clean coal technologies are worthwhile. Finally, CO2 acceptor process particularly for sorption enhanced H2 production from methane and coal (through SMR and HyPr-RING, respectively) has been under the spotlight since 2002, although the original idea was proposed by Curran et al. in 1966. Despite some complexity that is added by the pretreatment of raw coal to prepare AFC, there are process simplifications (because of the gasification, simultaneous shift reaction and CO2 separation in a single-step process) that include elimination of the need for heat exchangers between catalyst beds as well as the absorption and stripping units required for CO2 removal (or the pressure swing adsorption unit). This was mentioned by C. Han and D.P. Harrison for the case of methane [36]. This paper offers a novel M-HyPr-RING process for sorption enhanced H2 production. It also provides in-depth insight on the best conditions for high-purity H2 production by catalytic steam gasification of AFC, prepared from Bienfait lignite (BL), coal integrated with CO2 capture using calcined natural Cadomin limestone in a horizontal fixed-bed reactor under mild process conditions. During the gasification experiments, the molar fractions of H2 and CO2 were closely monitored using a micro GC. In order to identify the optimal catalyst loading, the effects of the catalyst wt% with and without sorbent on catalytic coal gasification were investigated. Moreover, the solid residues from all coal gasification experiments were analyzed to determine their compositions and the carbon conversion of each experiment using a thermogravimetric analyzer (TGA) and an elemental (carbon, hydrogen, nitrogen, sulfur) analyzer. The carbon conversions obtained from the results of TGA and CHNS analysis on the residues were compared with the results of gas chromatography for the molar fraction of hydrogen to confirm the optimal catalyst wt% with and without sorbent. The optimal catalyst wt% was determined to achieve the maximum values of the H2 molar fraction, the carbon conversion, and the reaction rate from steam gasification of AFC in the presence of CO2 sorbent.
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2. Experimental section
318
2.1. Materials
319
In this study, AFC prepared from BL coal (Bienfait lignite, Saskatchewan, Canada) was used as the carbonaceous material in the gasification experiments. The proximate analysis of the BL coal was performed according to the ASTM D-3172 standard using a TGA (TG 209, F1 Iris, Netzsch). In addition, the ultimate analysis of the coal sample was obtained using a Perkin Elmer elemental analyzer (CHNS/O Analyzer Series III, 2400). Tables 1 and 2 present the proximate and ultimate analyses for raw BL coal and BL-AFC on a dry basis.
320
Table 1 Proximate analyses of raw BL coal and BL-AFC (wt%, dry ash-free (daf) basis).
a b
Sample
VMa
Ash
FCb
BL coal BL-AFC
37.6 59.6
16.5 0.2
45.9 40.2
VM: Volatile matters. FC: Fixed carbon.
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354 Table 2 Ultimate analyses of raw BL coal and BL-AFC-BL (wt%, dry ash-free (daf) basis).
329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353
Sample
C
H
N
S
O
BL coal AFC-BL
58.0 88.5
3.4 4.8
1.8 2.6
1.0 0.8
35.8 3.3
Natural Cadomin limestone from western Canada was calcined at 850 °C under an N2 atmosphere and utilized as a sorbent for CO2 capture. The particle size of the calcined limestone (CaO) was less than 250 lm, which was prepared by CANMET Energy. The chemical analysis of Cadomin limestone can be found elsewhere [37]. The coal was crushed by a milling machine to a fine powder with an average diameter of about 5 lm and then mixed with CaO powder. The K2CO3 was 99.99% pure (ACS reagent >99%, Sigma–Aldrich. Pure grade N2 (99.9%) was supplied by Praxair. As shown in Fig. 1, the BL-AFC was prepared by solvent extraction [38]. Typically, an autoclave reactor (0.5 L) was loaded with dry pulverized coal (<150 lm) and mixed with solvent (coal-tosolvent ratio of 1:10). The coal-solvent slurry was heated to 400 °C under N2 (10 bar) with continuous agitation at this temperature for 1 h. Hot filtration at 100 °C was carried out using filter paper (1.0 lm) to separate the dissolved liquid and the solid residue. The residue was washed several times with tetrahydrofuran (THF) and then dried under vacuum at 80 °C. The dissolved product (filtrate) was slowly added to hexane (filtrate-to-hexane ratio of 1:40) with continuous stirring to precipitate the coal-derived organic components. The precipitate (AFC) was collected by filtration and dried under vacuum. The extraction yields were calculated based on dry ash free basis (daf) from the weight of the residue using the following Eq. (1).
ðFeed coal ðdafÞ Residue ðdafÞÞ Extraction Yield ðwt:%; dafÞ ¼ Feed coal ðdafÞ 100 ð6Þ
356
The extraction yields of AFC from BL lignite coal is 67.0% and the ash content of BL-AFC sample was determined by ASTM D3174 method and was found to be 0.07%. A sample containing 5 g of AFC was used in each experiment. The CaO was added to the coal sample at two different molar ratios to the carbon in BL-AFC (Ca/C = 0, 2). The experiments were conducted on AFC in a fixed-bed reactor in the presence of different catalyst loadings (K2CO3 at 5, 10 and 20 wt%) with and without sorbent. N2 was fed at a flow rate of 400 mL/min into the reactor as a purge gas for pyrolysis. During the heating step, the gas samples from the exhaust of the reactor were taken at increments of 50 °C from 300 °C to 675 °C using an Agilent micro GC-TCD 490 (gas chromatograph/thermal conductivity detector) to keep a record of the composition variations during pyrolysis. The temperature was then kept constant at 675 °C for 10 min to ensure that it was stable, after which water was injected into the helical coil to start gasification [39,40].
357
2.2. Experimental setup
375
Fig. 2 illustrates the schematic diagram of the experimental setup. A stainless steel fixed-bed tubular reactor (length of 100 cm and nominal inner diameter of 2.54 cm) was used in a horizontal electric furnace to conduct the gasification experiments. A spiral coil (length of 600 cm and inner diameter of 0.32 cm) was attached to the outside of the reactor tube and placed in the
376
Fig. 1. Schematic diagram for the preparation of AFC by solvent extraction [38].
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Mass flow controller ȹ
Needle valve
Water injection pump N2 Water Steam
N2
Product gas/Steam
Coal/CaO/K2CO3 powder
Condenser
Micro GC Product gas
Fig. 2. Schematic diagram of the experimental setup for the catalytic steam gasification of BL-AFC [40].
395
furnace, in order to produce superheated steam (675 °C at 1 atm) by pumping water. Therefore, the required steam for gasification was produced by employing the coil installed inside the same furnace. The exit gas and wall temperatures were measured with two K-type thermocouples installed inside and outside of the reactor tube. A mass flow controller was used to maintain the flow rate of N2 at 400 mL/min passing through the reactor while conducting the experiments. The resultant gases produced from the steam gasification of the BL-AFC were routed to a condenser where the moisture and condensable vapors were collected. The tube connecting the reactor to the condenser was electrically trace-heated to prevent tar deposition and blockage. The product gas was finally conducted to the Agilent micro GC-TCD 490 in order to measure the composition.
396
2.3. Experimental procedure
397
In this work, 5 g of BL-AFC were used in all experiments. The amounts of catalyst added to the coal sample at specific weight ratios of 5%, 10%, and 20% with a constant sorbent-to-carbon ratio in the BL-AFC coal of 2 (Ca/C = 2) for the experiments with sorbent. The weight percentage of the catalyst was determined based on the K+ ions to the fixed carbon of the coal [41]. The Ca/C ratio of 2 was selected based on our previous study [40]. The feedstock, including AFC, catalyst and sorbent powders, were mixed thoroughly and placed in the middle of the reactor. The temperature was raised at an average heating rate of 30 °C/min under a pure N2 atmosphere for pyrolysis of the sample. All gasification experiments were performed at 675 °C, which was determined in our previous experiments to be the optimal temperature when sorbent was added [37,39,40], and near atmospheric pressure (15 psig). Once the reactor reached the desired gasification temperature of 675 °C, it was maintained for 5 min to ensure that the temperature was stable. The water pump was then turned on at a constant rate of 1 mL/min into the hot coil, so that steam (approximately 75% with the balance N2) was generated and injected to the reactor. Sequences of 15 samplings were taken by the micro GC-TCD, each taking 150 s to measure the variations in the resultant gas molar fraction with gasification time. The molar fractions of H2, CH4, CO, and CO2 were continuously measured using the micro
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398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420
GC-TCD during the pyrolysis and gasification stages of the experiment. All experiments were performed three times, and the results showed an uncertainty of less than 5%. The average heating rate of 30 °C/min was applied in all tests.
421
3. Results and discussions
425
3.1. Effect of catalyst loading on hydrogen production
426
In the present study, the effect of catalyst loading, as one of the main defining process variables of steam gasification of AFC, was investigated. Two sets of experiments were performed with varying amounts of catalyst and either with or without sorbent. Three different catalyst concentrations (5, 10 and 20 wt%) with constant molar ratios of CaO/C of 0 and 2 (without and with sorbent, respectively) were chosen for the steam gasification of BL-AFC. The catalyst only (sorbent free) steam gasification of coal experiments were carried out to attain results for the base case. The composition trends of gases with time can be well explained by knowledge of the key reactions occurring in the gasification process. In the HyPr-RING process, steam char gasification, WGS reaction and CaO carbonation (as shown in Eqs. (1)–(3)) are the main reactions that occur due to the presence of the CO2 sorbent [7,8]. The WGS reaction plays an important role and is considered as the key reaction. The use of sorbent moves the WGS reaction forward, leading to higher H2 production. The composition of product gas from pyrolysis and steam gasification for catalyst only (sorbent free) experiments at 675 °C is shown in Fig. 3(a). All the results reported in this research were on a dry basis with H2 being the major component in the pyrolysis gas and small CH4, and CO fractions. With the study’s main emphases on hydrogen production and CO2 capture, only H2 and CO2 trends are depicted here. Pyrolysis began at approximately 450 °C and was accelerated by increasing the temperature. By introducing steam to the system, the H2 molar fraction increased drastically with time. The hydrogen molar fraction peaked after a few minutes of gasification and then gradually declined over time. However, the molar fraction of CO2 increased moderately in the pyrolysis and then declined toward the end of the pyrolysis stage. The increase in the CO2 frac-
427
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100 90
(a)
80
Molar Percent (%)
N2
Steam/N2
5 wt.% catalyst only, H2 10 wt.% Catalyst only, H2
70
20 wt.% Catalyst only, H2
60
10 wt.% catalyst only, CO2
5 wt.% catalyst only, CO2 20 wt.% catalyst only,CO2
50 40 30 20 10 0 0
10
20
30
40
Time (min)
50
60
70
100
(b)
90
Molar Percent (%)
80
N2
Steam/N2
5 wt.% Catalyst, [CaO]/[C]=2,H2 10 wt.% Catalyst, [CaO]/[C]=2,H2
70
20 wt.% Catalyst, [CaO]/[C]=2H2
60
5 wt.% Catalyst, [CaO]/[C]=2,CO2
50
20 wt.% Catalyst, [CaO]/[C]=2,CO2
10 wt.% Catalyst, [CaO]/[C]=2,CO2
40 30 20 10 0 0
10
20
30
40
50
60
70
Time (min) Fig. 3. Variations of molar fractions of H2 and CO2 produced during the catalytic pyrolysis and steam gasification of BL-AFC at 675 °C (a) without sorbent and (b) with sorbent.
464
tion was due to the consumption of fresh AFC by gasification and WGS reactions. After a while, the deactivation of catalyst became important [42], since there was no sorbent in place in this case. The decline was inevitable, due to the shift in mechanism from kinetics to mass transfer on both catalyst and coal surfaces and their respective active sites.
465
3.2. Effect of sorbent loading on CO2 capture capability
466
Increasing the amount of catalyst (wt%) had a positive effect on hydrogen production, reaching more than 60% with 20% of catalyst. The maximum reaction rate, slope or rate of change in the molar fraction occurred for AFC gasification with a catalyst loading of 20%. With lower amounts of catalyst (5% and 10%), the hydrogen production was decreased with lower initial reaction rates. There was a significant jump in the H2 fraction when the catalyst was increased from 10% to 20%. However, there was no significant change (only a 4% increase in the hydrogen peak fraction) when the catalyst loading was increased from 20% to 30%, indicating that further addition of catalyst did not have a significant effect on gasification. When the catalyst loading was increased from 5% to 20%, the CO2 fraction grew from 10% to just under 20%. The increasing trend in the CO2 molar fraction was accompanied by an increasing trend in the H2 molar fraction. This supports the idea that the WGS is the
458 459 460 461 462 463
467 468 469 470 471 472 473 474 475 476 477 478 479 480 481
key reaction in this gasification process. The CO2 production passes through a smooth maximum and then decelerates toward the end of gasification. The overall behavior of CO2 was similar to that of H2, except that H2 had higher and sharper peaks. After about 10 min of gasification, the H2 fraction started to decline. As expected, faster declines occurred for higher catalyst loadings, since most of the char had been gasified during the high-speed initial gasification stage. In other words, sharper peaks in hydrogen production were followed by sudden declines. This can be explained by the gradual char consumption and ash layer formation around the coal particles. However, lower catalyst loadings resulted in slower decreasing trends, as less char had already reacted with steam and more char was still available to be converted to hydrogen. As can be observed in Fig. 3(b), the addition of sorbent significantly changed the H2 production for the same experimental conditions. All the H2 molar fraction increased with sorbent. In fact, CO2 capture emerged as the main factor in hydrogen production. This phenomenon can be explained by the shifting forward of the WGS caused by CO2 removal. This enhancement also improved the char gasification reaction. CO2 capture not only increased the maximum achievable H2 fraction, but also it enhanced the char conversion, as the larger areas under the curve in Fig. 3(b) suggest. The highest H2 molar fraction (almost 85%) was achieved with a catalyst loading of 20% and a sorbent-to-carbon ratio of 2. The maximum achievable H2 fraction increased nearly 1.4 times in
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the presence of CaO compared to that of without the sorbent (85% compared to 60%). In contrast to the H2 fraction obtained from catalyst only (sorbent free) cases, the differences among H2 molar fractions were not well distinguished. This may be due to the deterring effect of CaO on the catalyst activity [20]. The formation of an oxygenated intermediate on the char surface during pyrolysis has been postulated to be an important factor in maintaining the char reactivity [19]. In addition, calcium species act as a deactivation deterrent [20]. Hydrogen produced can be calculated from the total molar flow rate of gas, which in turn, is estimated from the N2 molar flow fraction and the molar fraction of H2 in each sampling time (every 2.5 min; 150 s) and the total concentration in the reactor under the actual pressure and temperature using the following formula:
522 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572
Z nH2 ¼
0
7
purity H2 production could be attributed to the use of catalyst and naturally active Cadomin limestone with suitable surface properties to capture CO2 compared to a synthesized CaO. The CH4 measured in the product gas from coal pyrolysis with sorbent was less than that in the product gas from the coal pyrolysis without sorbent. The reduction of CO due to the WGS reaction may enhance CH4 steam reforming to H2:
CH4 þ H2 O ! CO þ 3H2
ð8Þ
CO2 was formed through the WGS reaction, and CH4 was formed through the following reaction known as hydrogasification:
C þ 2H2 ! CH4
ð9Þ
573 574 575 576 577 578 579
580 582 583 584 585
586 588
t
yH2 C total F total dt
ð7Þ
In other words, Xc values calculated based on hydrogen produced (carbon gasified) and measured by gas analysis during gasification process were compared with those estimated from the residual ash burn-off tests. The residual tests predict lower carbon conversions, which indicate that there are some other carbon containing gases (carbonaceous compounds) that are not measured by the gas analysis. For instance at 675 °C, the Xc approximated from the area on the curve was 87% and the Xc from residual tests was 98%. The difference is caused by the formation of small amounts of CH4 and CO and some tars as well as measurements deviations of less than 6% associated with gas analyzer under the same experimental conditions. It should be noted that, in terms of hydrogen production, the experiments with 10 wt% of catalyst and a sorbent-to-carbon ratio of 2 presents the highest area under the curve and, hence, the highest amounts of H2 mole produced. The fastest initial reaction rate was obtained for catalyst loading of 20% with sorbent. This confirms that sorbent had a more important role on the H2 molar fraction, whereas the catalyst amount affected the initial gasification reaction rate. The presence of sorbent decreased the CO2 molar fractions to less than 3% for all experiments with catalyst loadings of 5–20%; and, as shown in Fig. 3(b), the molar fraction curves were very close to each other. Higher H2 molar fractions coincided with lower CO2 fractions. These results demonstrate that CO2 removal favored the WGS reaction to shift toward increased hydrogen production, according to Le Chatelier’s principle. As a result, any suppression in CO2 production was equivalent to enhancement in H2 production. The decrease of H2 production after 30 min in the presence of sorbent was generally faster than the decrease in cases without the sorbent. In practice, the maximum value of the H2 molar fraction stemmed from a shift from kinetics to mass transfer limited regime. Clearly, when there was sorbent in the reaction system, the mass transfer resistance was caused by the carbonated layer around the sorbent, as well as by the ash layer around the char particles [40]. In the absence of sorbent, the lower reactivity of char particles along with catalyst deactivation caused the downward trend and shorter effective gasification time. Eventually, the deceleration of char gasification reaction suppressed the gasification process, in particular the WGS reaction that took place in series with the char gasification. This results in the leveling off of the CO2 molar fractions. Lin. et al. in managed to produce H2 at a molar fraction of 85.3% (on a dry basis, N2 free) by steam gasification of a subbituminous coal (Taiheiyo coal, Japan) integrated with sorbent CO2 capture at 700 °C. This was an excellent achievement with a novel process [7]. Nonetheless, a H2 molar fraction of 85.5% (on a dry basis with N2) was achieved at 675 °C in this current work, which is equivalent to more than 99% H2 on a dry basis and N2 free. The higher
3.3. Characterization of residual solid from steam gasification
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The residual solid materials from the BL-AFC gasification experiments were weighed and analyzed with TGA ash burn-off tests, in order to measure the unconverted carbon and CaCO3 contents and to evaluate the carbon conversion. In each burn-off test, approximately 30 mg of solid residue underwent a calcination heat-up ramp from ambient temperature to 900 °C at a heating rate of 20 °C/min under pure N2 atmosphere with a flow rate of 50 mL/ min. After 10 min of isothermal treatment at 900 °C, the N2 gas was turned off and immediately changed to air for burning off the residue to ash. Fig. 4(a) and (b) presents the weight loss behavior of residual samples over the period of time measured by TGA. The results for the residual samples from catalyst only (sorbent free) gasification experiments are shown in Fig. 4(a). There were two major weight loss steps. The first step indicates the release of moisture, while the second step indicates the combustion of unconverted carbon. The hydrophilic characteristic of K2CO3 results in more moisture content with increased catalyst loading. A larger difference in weight loss was observed due to the moisture content when the catalyst amount was increased from 10% to 20%. The highest moisture (wt%) contains belongs to a catalyst loading of 30%. The small step before the combustion could be attributed to pyrolysis, as there could still have been some volatiles remaining in the residual samples. Another probable reason is the decomposition of K2CO3 [43] at less than 900 °C (898 °C). In Fig. 4(a), the larger distinctive step represents the combustion of unconverted carbon. The largest step occurred with catalyst loading of 5%; and, the second longest step, which was only slightly shorter than that with the 5% catalyst loading, occurred with catalyst loading of 10%. A sudden decrease in the amount of unconverted carbon occurred when the catalyst loading was increased from 10% to 20%; and, the smallest amount of unconverted carbon was observed for the 30% catalyst loading. Therefore, it can be concluded that increasing the catalyst loading decreased the unconverted carbon in the residual samples. This implies that more carbon was converted to hydrogen, leaving less carbon in the residual sample. This confirms the positive effect of catalyst loading on hydrogen production, which is in contrast with our findings previously mentioned in Fig. 3. Fig. 4(b) illustrates the burn-off results for gasification experiments with both catalyst and sorbent. The moisture was converted to Ca(OH)2, due to the presence of CaO; therefore, no weight loss behavior was observed at the beginning of TGA tests. Hence, the early step of moisture was eliminated and replaced with a smooth step at higher temperatures. This small step was related to decomposition of Ca(OH)2.
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Time (min) Fig. 4. TGA ash burn-off results of the solid residues obtained from the catalytic steam gasification of BL-AFC at 675 °C: (a) without sorbent and (b) with sorbent. 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665
The large step of weight loss indicates the decomposition of CaCO3, followed by carbon combustion. The portion of weight loss that occurred before the temperature reached 900 °C represents the combustion of unconverted carbon; whereas the portion after 900 °C and the gas switch from N2 to air indicates the combustion of unconverted carbon. In other words, first the Ca(OH)2 decomposed; the CaCO3 was then calcined to CaO; and, finally, the unconverted carbon was burnt to ash. The first portion of the step quantified the amount of Ca(OH)2; and, the second portion determined the amount of carbon captured and fixed to the sorbent structure, as well as the carbon content of the residual sample. The overall trends of weight loss for all three different solid residual samples were similar. The largest decrease (second step) in weight loss occurred for gasification with sorbent and catalyst loading of 5%. This suggests that more unconverted carbon remained in the residual sample and less carbon was gasified to hydrogen. Increasing the catalyst loading from 5% to 10% while maintaining the sorbent-to-carbon ratio of 2 resulted in a slight decrease in the length of the second step. Further increases in the catalyst loading caused a major reduction in the carbon content of the residual sample. The catalyst loading of 20% appeared to be the best ratio, as shown in Fig. 4(b). The presence of sorbent led to slight differences in unconverted carbon of the residual samples. Hence, the effect of sorbent on the gasification performance of AFC, in terms of carbon conversion, was more pronounced than that of catalyst. Fig. 5 shows the measured unconverted carbons contained in the solid residual samples after coal gasification with and without the sorbent for CO2 capture and with four catalyst-to-carbon ratios
using a Perkin Elmer elemental analyzer (CHNS/O Analyzer Series III, 2400). As shown in this figure, the amount of unconverted carbon decreased with increased catalyst loading into the steam gasification. In the steam gasification of AFC without sorbent (CaO/C of 0), the unconverted carbon content of the solid residual sample decreased with increased catalyst loading, indicating that the steam gasification of coal improved with increased catalyst loading. Similarly, the measured unconverted carbon (carbon content) of the solid residual samples after steam gasification of coal with sorbent decreased with increased catalyst loading. According to Fig. 5, the presence of sorbent dramatically improved the steam gasification of coal, due to the capture of CO2 during the process that drives the WGS reaction forward, leading to higher coal conversion and H2 production. In addition, the CaO-based sorbent may have had a catalytic effect [24,44,45] on the steam gasification of the coal and tar cracking reactions [46,47]. Thus, the mixture of catalyst and sorbent enhanced carbon conversion [4]. The results from ultimate analyses were in good agreement with those of the TGA burn-off tests (Fig. 4(a) and (b)), confirming the enhancement in gasification by both the catalyst and sorbent, as shown in 2. Although the presence of catalyst improved the steam gasification process, the effect of sorbent was more significant due to the numerous positive roles that sorbent played in the process, including CO2 capture and catalytic effects. The final conversions were estimated as a measure of the degree of completion of the reactions. The carbon conversions for
Please cite this article in press as: Mostafavi E et al. High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.03.026
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Fig. 5. Comparison of the ultimate analysis results on the amount of unconverted carbon in solid residues from the catalytic steam gasification of BL-AFC using a CHNS analyzer.
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all catalytic steam gasification experiments were calculated based on:
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Xc ¼
mc0 mcr 100 mc0
ð10Þ
where X is the conversion of coal, mc0 is the mass of the carbon in the AFC loaded into the reactor, and mcr is the unconverted carbon mass in residues remained from the steam gasification experiments [40]. The calculated values for carbon conversions are listed in Table 3. In the first series of experiments, which were catalyst only without sorbent, the conversions rose from 70% to 80% by adding more catalyst. In steam gasification of the BL-AFC with sorbent, the carbon conversions were even higher, due to the sorptionenhanced gasification. The high oxygen contents shown in Fig. 5 can be due to the large amounts of CaCO3 in the residues obtained from gasification experiments. During the CHNS/O tests, the CaCO3 is decomposed to CO2 and CaO at 900 °C causing a weight loss that also counts as oxygen. This oxygen makes up the major portion of the residues for the with-sorbent experimental cases. For the without-sorbent (catalyst only) cases, the compositions of the residues are substantially different from those of the with-sorbent cases, as there is no CaCO3. However, there are small amounts of K2CO3 in the residues, which is decomposed even below 200 °C to K2O. Hence, calcium and potassium carbonates are the probable causes of high oxygen content measurements of the residues.
Table 3 Carbon conversion from catalytic steam gasification of BL-AFC with different catalyst loadings, with and without sorbent. Experimental ratios
Carbon conversion
5% Catalyst, [CaO]/[C] = 0 10% Catalyst, [CaO]/[C] = 0 20% Catalyst, [CaO]/[C] = 0 30% Catalyst, [CaO]/[C] = 0 5% Catalyst, [CaO]/[C] = 2 10% Catalyst, [CaO]/[C] = 2 20% Catalyst, [CaO]/[C] = 2
72.0 76.5 79.4 93.2 96.1 97.1 97.9
Increasing the catalyst loading up to 20% (with a CaO/C ratio of 2) improved the conversion to where it reached 97%. Moreover, adding sorbent to the gasification reaction system enhanced the conversion by approximately 20%. Therefore, the maximum achievable conversion was above 97%. The maximum achievable carbon conversion and hydrogen fraction in the product gas were improved from our previous study [40] to 97% and 86%, respectively. In the optimal case of catalyst loading of 20% and a CaO/C ratio of 2, the steam gasification of coal proceeded almost to completion, as evidenced by less than 3% carbon in the residual samples. The reaction rate increased dramatically, such that the H2 peak appeared in less than 5 min. However, the H2 fraction peaked after more than 10–12 min for the raw parent coal with sorbent [40], because the WGS was far from equilibrium due to lower temperatures [44]. The presence of catalyst in the solid mixture increased the overall reaction rate (char gasification and WGS reaction rates); therefore, more char was gasified to hydrogen, and more carbon conversion was achieved under the same experimental conditions (i.e., flow rate, pressure, temperature, and gas residence time in the reactor tube).
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4. Conclusions
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The main focus of this work was the determination of the effects of catalyst loading and sorbent, which were investigated with the aim of high-purity hydrogen production via catalytic steam gasification of AFC and integrated CO2 capture capability of the sorbent. A novel M-HyPr-RING process for a catalytic sorption enhanced H2 production was proposed. Different catalyst loadings with and without sorbent were used in order to further understand the factors involved in the catalytic steam gasification of AFCs. Based on our experimental results the following conclusions have been drawn:
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(1) The effect of a sorbent on the hydrogen production yield was more significant than that of a catalyst. Sorbent considerably enhanced hydrogen production, whereas catalyst accelerated the gasification reaction rate.
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(2) The hydrogen molar fraction of 85% (dry basis with nitrogen) was observed with catalyst loading of 20% and a sorbent-tocarbon ratio (CaO/C) of 2 after 5 min of steam gasification. The experiments indicated trace amounts of CO and CH4 along with other hydrocarbons are present in the product gas. (3) Reaction rates for steam gasification of BL-AFC were more than twice as fast as that of raw BL coal under the same experimental conditions. (4) The maximum reaction rate and hydrogen molar fraction occurred at catalyst loading of 20% and a CaO (calcined Cadomin limestone)/C ratio of 2 at 675 °C. A maximum carbon conversion of 97% was achieved. (5) Residue tests revealed that unconverted carbon trends were in the same direction as hydrogen production. (6) The TGA ash-burn off results of solid residues verified that the majority of CO2 was fixed to the sorbent structure.
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Acknowledgements
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The financial supports from Carbon Management Canada (CMC) and Natural Sciences and Engineering Research Council (NSERC) for this research are greatly appreciated. The authors wish to thank Dr. Davood Karami, Ms. Maria Fernanda Oliviera, and Ms. Negar Manafi for their valuable assistance in the ultimate analysis tests on the AFC and residues, as well as data analysis in this work. The authors would also like to extend their sincere gratitude to Dr. Vasilije Manovic and Dr. Edward Anthony from CANMET Energy for preparing the Cadomin limestone.
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