Hydrogen from Coal in a Single Step - American Chemical Society

Jun 11, 2005 - Department of Mechanical Engineering and Energy Processes, and Coal Research Center, Southern Illinois. University, Carbondale, Illinoi...
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Hydrogen from Coal in a Single Step Kanchan Mondal,† Krzystof Piotrowski,‡ Debalina Dasgupta,† Edwin Hippo,† and Tomasz Wiltowski*,†,‡ Department of Mechanical Engineering and Energy Processes, and Coal Research Center, Southern Illinois University, Carbondale, Illinois 62901

The production of high-purity hydrogen via steam gasification of coal has been investigated. The separation of CO from H2 in the gasification products is achieved by CO oxidation to CO2 followed by uptake of the CO2 by a suitable removal agent. This uptake of CO2 increases the extent of the water gas shift reaction and enhances the yield and purity of H2. In addition to the water gas shift reaction, the oxidation is enhanced by the use of a solid oxygen transfer agent (Fe2O3) in the hydrogen enrichment pass. Subsequently, the reduced oxygen transfer agent is reoxidized (and thus regenerated) in the presence of air and the heat liberated via the exothermic reaction is utilized to regenerate carbon dioxide removal agent. In this study, the effect of process variables on coal gasification and hydrogen enrichment has been evaluated. Fixed bed gasification studies using coal and coal-Fe2O3 and coal-CaO mixtures were conducted to evaluate the kinetics of gasification and separation effectiveness of the process. Finally, a bench scale fluidized bed reactor was employed to study the efficacy of the simultaneous gasification-hydrogen enrichment process. The reactions were conducted in the temperature range of 670-900 °C at atmospheric pressures. The results from the fundamental studies, the fixed bed reactor studies, and the fluidized bed reactor studies are presented. Introduction Hydrogen is an important raw material in the chemical industries such as in the manufacture of ammonia and methanol. The possibility of H2 as a future energy source in the heating, electric power, and transportation sectors will cause a huge increase in the H2 demand. Currently, the primary route for H2 production is the conversion of natural gas and other light hydrocarbons. Coal and petroleum coke may also serve as raw materials for H2 production in the future. The first step of forming H2 from coal is gasification1-7 followed by the water gas shift reaction.8-10 Reforming reactions are highly endothermic and thermodynamically favored by high temperature and low pressure. On the other hand, the water gas shift reaction is favored by low temperature and is not pressure dependent. Due to the overall endothermic nature, gasification is generally conducted at high temperatures. Thermochemical conversion of carbonaceous raw materials to high yield H2 via the use of catalysts is gaining attention from several researchers.11-12 Calcium-based catalysts are also known to promote steam gasification at relatively moderate reaction conditions. Thermogravimetric studies and high-temperature X-ray diffraction analysis to investigate the catalytic effects of Ca-based compounds13 showed that the addition of calcium compounds decreased the reaction temperature and increased the gasification rates. In their investigations, Balasubramanian et al.14 found that the addition of NiO/Al2O3 and CaO successfully achieved near-equilibrium conditions for reforming of CH4, water gas shift, and the separation of CO2 simultaneously in a single reactor at 550 °C. * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Mechanical Engineering and Energy Processes. ‡ Coal Research Center.

The CO2 generated, a greenhouse gas with a potential to contribute to global warming, is generally released to the atmosphere. With stringent environmental regulation already in place and the requirement for zero emissions in the future, it is important to develop means to capture/sequester CO2 from the process. In the current context, in situ capture of CO2 not only provides a chance to sequester the greenhouse gas, but also increases the conversion to and the purity of the H2 stream by removing the thermodynamic limitations at a given condition. Thus, separation of the CO2 and H215-20 needs to be achieved. Separation of H2 from the coal gasification products also supports existing H2 markets (such as refineries and power production) and makes a H2 economy a distinct possibility. It must also be noted that CO in the H2 gas stream acts as a poison to the catalysts used in electrodes employed in fuel cells, necessitating its removal. Given that a CO2 acceptor is available, near-zero CO content in the outlet gas may also be achieved. Existing technologies utilize the water gas shift reaction for the conversion of CO to CO2. The CO2 is then separated from H2 by absorption (Selexol, Rectisol, A-MDEA, Purisol, Sulfinol, UCARSOL, amines), adsorption (molecular sieves, activated carbon), cryogenic separation, and membrane separation (polymer, metal, facilitated transport membrane, molecular sieves), all of which are costly alternatives. In addition, a methanation step may be required to follow the amine scrubbing step to reduce the COx to trace levels. In the pressure swing adsorption technology, no further purification is required, but at the cost of H2 yield. The first attempt of using CaO in a “CO2 acceptor process” was conducted by Curran et al.21 and McCoy et al.22 In these studies, only 50% of CO and CO2 was immobilized in the solids. However, in later studies in a microautoclave,23,24 product gases consisting of primarily H2 and CH4 were obtained. Up to 84% pure H2 stream was reported by Lin et al.16 in a pressurized bed

10.1021/ie048974d CCC: $30.25 © 2005 American Chemical Society Published on Web 06/11/2005

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reactor. The pressurized bed reactor provided higher rates of CH4 decomposition, resulting in higher H2 contents. A new method, Hydrogen Production by Reactions Integrated Gasification (HyPr-Ring), that combines the gas production and separation reactions in one reactor was suggested by Lin et al.23 In this process the energy required for the endothermic reforming reactions were provided by the heat of CO2 absorption. Wang and Takarada25 reported that complete fixation of CO2 with Ca(OH)2 could be achieved for a Ca/C molar ratio of 0.6 (stoichiometry dictates the ratio to be 1) along with significantly enhanced decomposition of tar and char. Kuramoto et al.26 also investigated the C-CaO-H2O system, using subcritical steam (50-150 °C). They reported an increase in the CH4 content due to the addition of Ca(OH)2, along with the yields of H2. The overall conversion rate of CO to CO2 is expected to be enhanced by the inclusion of an oxygen donor in the reaction zone. The steam reforming rates of CH4 and other hydrocarbons released during coal pyrolysis may also be enhanced by the choice of a suitable oxygen donor that may double as a catalyst. Thermodynamic analysis also shows that the FeO-Fe2O3 transition has ideal enthalpy characteristics for the water gas shift (WGS) reaction necessary to convert syngas to H2. However, Fe2O3 is found to oxidize H2 at a much faster rate (nearly 3-10 times) as compared to CO.27-32 This would result in a reduction in the yield of H2. The reduction of iron oxide and various ores containing iron oxide have been studied in the past. Various controlling mechanisms have been suggested in their research such as random nucleation,29 phase boundary,30 or a combination of both.31 Thus, the effect of Fe2O3 on simultaneous coal gasification and H2 separation needs to be investigated. In the past, Nikanorova and Antonova33 reported an increase in H2 yield by 1.5 times due to the addition of iron oxide to lignite prior to gasification with a final purity of 60%. The addition of the iron oxide during atmospheric pressure mild gasification of Shenmu bituminous coal was also found to significantly increase the H2 yield while decreasing the CO content.34 The nonvolatile components present in tars cause several engineering problems along with increasing the need for a hot gas purification step in gasification processes. Among the alternatives, catalytic steam gasification appears to be attractive. Past studies35 have shown limestone to be an effective catalyst for gasification of model compounds found in tars. The results were confirmed during investigations on real tars.36,37 Researchers38,39 have also discussed the positive effects of the presence of iron oxide on the decomposition of coal and other hydrocarbons. In addition, Li et al.40 reported that Fe2O3-CaO mixtures were efficient in sulfur removal in IGCC processes through multiple desulfurization-regeneration cycles. Thus, selection of Fe2O3 and CaO as the oxygen transfer compound (OTC) and carbon dioxide removal material (CDRM), respectively, should provide additional benefits in H2 production via gasification. The focus of this research is also to investigate the reaction conditions that result in the oxidation of CO to CO2. CO2 would then be removed by CaO. Fixed bed studies were conducted for parametric (steam, temperature, Fe2O3 loading, CaO loading) evaluation on H2 yield and purity in a fixed bed reactor. Finally, additional experiments were conducted in a fluidized bed reactor to verify and compare the results obtained in a

fixed bed reactor. The Gibbs free energy data show that the reduction of hematite to wustite is favored at high temperatures. However, the capture of CO2 via lime is generally favored at temperatures less than 900 °C.44 Nonetheless, the temperature window of 650-900 °C affords thermodynamically and kinetically44 favorable reaction conditions for both the oxidation of CO to CO2 using Fe2O3 and the subsequent capture of CO2 by lime. Process Concept The basic idea of the process is the simultaneous use of (a) an oxygen transfer compound (OTC) to enhance the conversion rates of CO to CO2 along with the water gas shift reaction present in syngas and (b) capture of CO2 using an appropriate carbon dioxide removal material (CDRM) leading to the production of high purity H2 stream. The net of these reactions is exothermic, so additional CO2 and H2 are produced by CH4 reforming. Subsequently, the reduced OTC is oxidized (and thus regenerated) in the presence of air/oxygen and the heat liberated via the exothermic reaction is utilized to regenerate CDRM. Thus, the products of this system would result into three separate streams of (a) high purity H2 for use in fuel cells, (b) sequestration ready CO2, and (c) high temperature (pressure) oxygen depleted air for use in gas turbines. In this paper, we investigate Fe2O3 as the OTC and CaO as the CDRM. The following are the reactions expected to take place in the reactor. The heats of reaction are calculated for 800 °C.

Hydrogen Enrichment Stage C + H2O f CO + H2 (steam reforming of coal) +135.74 kJ/mol (1) CH4 + H2O f CO + 3H2 (steam reforming of methane) +206 kJ/mol (2) CO + H2O f CO2 + H2 (water gas shift reaction) -33.07 kJ/mol (3) 2CO f CO2 + C (Boudouard reaction) -169.44 kJ/mol (4) CO + Fe2O3 f 2FeO + CO2 (CO oxidation) -4.75 kJ/mol (5) CaO + CO2 f CaCO3 (CaO carbonation) -168.69 kJ/mol (6) CaO + H2O f Ca(OH)2 (CaO hydration) -94.05 kJ/mol (7) Ca(OH)2 + CO2 f CaCO3 +H2O

-74.67 kJ/mol (8)

CH4 + CO2 f CO + 3H2 (dry reforming of methane) +260.76 kJ/mol (9) C + 2H2 f CH4 (methanation) -21.657 kJ/mol (10) H2 + Fe2O3 f 2FeO + H2O (H2 oxidation) +28.97 kJ/mol (11) C + Fe2O3 f CO + 2FeO (solid catalyzed C oxidation) +9.39 kJ/mol (12)

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Oxide Regeneration Stage FeO + O2 f Fe2O3 CaCO3 f CaO + CO2

-281.4 kJ/mol

(13)

+167.6 kJ/mol (14)

Thermodynamic Analysis Equilibrium gas compositions were calculated for four reaction systems, namely, (a) C-H2O, (b) C-CaO-H2O, (c) C-Fe2O3-H2O, and (d) C-CaO-Fe2O3-H2O, at 1 bar at temperatures ranging from 450 to 900 °C. Combining eqs 1 and 3, one can see that 1 mol of C requires 2 mol of water for complete conversion of C to CO2. Thus, for each of the reaction systems the ratio of C:H2O was set at 0.5. Since 1 mol of C produces 1 mol of CO or 1 mol of CO2, the ratios of C:CaO and C:Fe2O3 were set at 1 (one) for relevant cases. Figure 1 plots the equilibrium composition of the product gases in each reaction system. It is seen that steam gasification of C (case a) produces a stream with a maximum H2 content of 57%. The CO2 content is observed to decrease while that of CO increases with temperature. This is expected since the water gas shift reaction is exothermic. Due to the increase in the extent of CH4 reformation (eq 2) and decreased levels of methanation (eq 10), the CH4 content decreases with temperature. Ideally, 3 mol of gas (with a H2 purity of 66.67%) should be produced per mole of C gasified. A maximum of 1.36 mol of gas/mol of C was produced in the range of 700-775 °C for case a. On adding CaO, the maximum equilibrium moles of dry gas in the exit stream was 1.46 per mole of C. However, one must consider that approximately 1 mol of CO2 would be removed from the gaseous stream by CaO per mole of C gasified. It is clearly seen from Figure 1b that the

utilization of CaO could potentially produce a gaseous stream with more than 90% H2 purity. However, the equilibrium H2 concentration in the product gases is observed to decrease while that of CO and CO2 increases with an increase in temperature (greater than 650 °C). The use of Fe2O3 during C gasification (Figure 1c) decreased the H2 content in the dry gas and the yield of moisture-free gases. This is primarily attributed to the oxidation of H2 to water. The amount of CO2 produced is also higher than in the C-H2O system due to the CO oxidation by Fe2O3. It is also observed from the figure that the CH4 content in the gaseous mixture is lower than both earlier reaction systems. The use of both CaO and Fe2O3 during steam gasification of C (Figure 1d) provided a high yield of equilibrium dry gas mixture with the highest H2 content (98.3-99.4%). The gas yield ranged from 1.15 to 1.7 mol/mol of C. Experimental Section Materials. Pulverized Utah coal was used for this study. The fixed carbon content was 73.30%, while the ash and moisture contents were 7.85% and 8.68%, respectively. The size ranges of CaO (from Mississippi Lime Company, MO), iron oxide (Fisher Scientific, Chicago, IL), and silica sand used were 75-106 µm. Calcium carbonate was obtained from Sigma Aldrich (St. Louis, MO). Fixed Bed Experiments. Pulverized coal sample with initial weight of 0.2 g, as received, was placed so that it constituted a “packed bed” in a quartz tubular reactor of inner diameter of 1.25 cm (Figure 2a). The length of the reactor was 45 cm. CaO/Fe2O3 was also packed inside the quartz tube. The amounts used in

Figure 1. Equilibrium dry gas compositions for reaction systems: (a) C-H2O, (b) C-CaO-H2O, (c) C-Fe2O3-H2O, and (d) C-CaOFe2O3-H2O.

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Figure 2. Schematic of (a) fixed bed reactor system and (b) fluidized bed reactor.

each experiment are discussed in the relevant sections. Nitrogen was used as a carrier gas. Samples were heated to the desired temperature and steam was introduced. In all cases, the total gas flow rate was maintained at 100 mL/min. The reaction temperatures for individual cases are discussed in the relevant sections. A “cold” trap was located before the sampling outlet to condense all tar components. Samples were collected at regular intervals for analysis in a Gow-Mac Series 600 gas chromatograph. Fluid Bed Experiments. The schematic of the tubular fluidized bed reactor (fabricated with Inconel 880HT) along with the dimension are illustrated in Figure 2b. The oxides and sand mixture (total mass of 60 g in most cases) to be fluidized were inserted into the reactor, and the reactor was heated to the desired temperature in the nitrogen flow at atmospheric pressure. The actual masses of each oxide used are discussed in the appropriate sections. The furnace temperature was set to the predetermined temperature (discussed in the relevant sections). After required thermal conditions had been stabilized, steam was introduced into the reactor to obtain the predetermined steam partial pressures. The flow of generated steam and nitrogen was adjusted to obtain the total gas mixture linear flow rate equal to 15 times the minimum fluidization velocity (0.092 slpm). Steam content in the reactor was maintained at the predetermined partial pressure. Two grams of pulverized coal samples (100% passing 75 µm) was injected into the reactor by use of the solids delivery system driven by nitrogen. Immediately after coal injection, the outlet gas samples were collected at 1-min

Figure 3. Influence of temperature on (a) coal pyrolysis in N2 and (b) steam gasification of coal.

intervals and the outlet volumetric flow rate was recorded. Collected gas samples were analyzed using gas chromatography (GOW-MAC 600). Results and Discussion Laboratory Scale Fixed Bed Studies. The influence of reaction temperature, steam content, and the loading of Fe2O3 and CaO on the rates of reactions and product gas concentration profile was evaluated in a fixed bed reactor. All the data were analyzed based on the cumulative data obtained for 10 min. Effect of Temperature. Reaction temperature is an important parameter affecting the performance of coal gasification and subsequent water gas shift reaction. Gasification reactions are endothermic, while the WGS reaction is exothermic. Therefore, there exists an optimal temperature range wherein coal gasification would produce a maximum gaseous yield with low CO content as observed under equilibrium conditions (Figure 1a). The effect of temperature in a fixed bed reactor was evaluated in (a) pyrolysis with nitrogen (100 mL/min) only and (b) steam gasification (with a water flow rate of 1 mL/min). During pyrolysis in N2, it is observed (Figure 3a) that the cumulative CO2 concentration at the end of 10 min decreases from 10% at 750 °C to nearly zero at 900 °C. The CH4 concentration is also observed to decrease (from 60% at 750 °C to 19% at 900 °C). The CH4 content in the gas during pyrolysis is primarily a result of devolatilization of this component. It is also seen from Figure 3a that CO and H2 contents increase from 30% to 80% and from 2% to 20%, respectively, when the

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Figure 4. Conversion (residence time ) 10 min) as a function of temperature.

temperature in the reactor is increased from 750 to 900 °C. The increase in the gasification, devolatilization, and dry reforming rates partially contribute to this increase in CO and H2 and the decrease in CO2 concentration. At reaction temperatures greater than 900 °C, the CH4 concentration increases to 73% (950 °C) while the CO2 concentration is observed to be practically zero. The CO2 produced is completely consumed by dry reforming, resulting in the presence of the excess CH4 in the product stream. The CH4 produced may also be partially attributed to the methanation reactions of CO and C with H2 (eqs 9 and 10). A corresponding decrease in CO and H2 contents is also observed. In the presence of 91% steam (Figure 3b), the CO2 concentration increases from 10% to 22% when the reactor temperature was increased from 700 to 850 °C. This is accompanied by a significant increase in H2 concentration (from 2.5% to 43%), but only a moderate increase in CO content (from 30.5% to 38%). The presence of steam in the reaction zone results in an increase in the C conversion rates (Figure 4). For example, at 900 °C, a 2-fold increase in coal conversion is observed. The increase in the CO production by this enhanced gasification is reflected by the increase in CO2 (due to WGS) in the overall product. The CH4 concentration in the product stream decreases from 57% at 700 °C to 8% at 850 °C. The authors believe that this decrease is primarily due to steam reforming of CH4. At reaction temperatures above 850 °C, the CO2 concentration decreases while the CO content is observed to increase rapidly with temperature. This observation can be partially explained by the predominance of CO2consuming reactions with an increase in temperature in comparison to the water gas shift reaction (exothermic). Similar trends are also observed for experiments with different steam partial pressures. Increasing the water flow rate (steam to fuel ratio) results in an increase in the H2 and CO2 contents in the product, while those of CO and CH4 decrease. For example, at 800 °C, the H2 and CO2 concentrations are found to increase by 12 and 1.83 times, respectively, while the CO and CH4 concentrations are observed to decrease by more than 1/2 and 1/3, respectively, when a water flow rate of 1 mL/min is employed as compared to the experiments with no steam. Similar changes in the product gas profile were obtained by Kim et al.41 Gil et al.42 and Pfiefer et al.43 also reported the increase in CO2 and H2 and a decrease in CO and CH4 contents with an increase in the steam:fuel ratio.

The rise in temperature as well as the presence of steam leads to an increase in the coal conversion (Figure 4). Two regimes of conversion are observed for experiments with and without steam. When steam is present in the reactor system, a 43% increase in conversion is obtained when the temperature is raised from 700 to 800 °C. However, an additional 103% increase in conversion is observed when the temperature is increased to 900 °C. The results show that, for the given coal, gasification temperatures should be in the range of 800-900 °C. The kinetic investigations of the steam gasification of coal with 1 mL/min water revealed (plots not shown) that the estimated rate constants at 850 °C (0.0051 min-1) and 900 °C (0.0078 min-1) are about 2 and 3 times the value for the rate constant obtained at 800 °C (0.0031 min-1), respectively. The Arrhenius parameters of activation energy and frequency factor are estimated to be 96.678 kJ mol-1 and 2.62 s-1, respectively. Effect of Steam Content. The effect of the partial pressure of steam was also studied by increasing the flow rate of water. The results from the experiments conducted at 850 °C are shown in Figure 5. It is observed that both the CO and CO2 contents increased by 77% and 80%, respectively, while those of H2 and CH4 decreased by 39% and 50%, respectively, on introducing water at 0.5 mL/min (85%). In this regime, steam reforming of coal and CH4 predominate and the steam is not sufficient for extensive WGS; as a result, the CO content is observed to increase sharply while that of H2 decreases. However, on further increasing the steam partial pressures to 91% (1 mL/min water), the CO content decreases from a maximum of 58% to 43% at 91% steam partial pressure. Simultaneously, the H2 content is observed to increase, indicating the required flow rate of steam was achieved for the water gas shift reaction to predominate. No change in the outlet gas compositions is observed at flow rates greater than 1.3 mL/min (95% steam). It is clearly seen from Figure 6 that the addition of steam significantly enhances the conversion rates up to a value of 1.3 mL/min. A 282% increase in the coal conversion is obtained when the water flow rate is increased from 0 to 1.3 mL/min. On the basis of these data, it is concluded that greater than 91% steam should be used for maximizing the H2 yield and content via the WGS reaction. Effect of Solid Oxygen Carrier Loading. Experiments were conducted at several loadings of Fe2O3 at a temperature of 850 °C and a water flow rate of 1 mL/ min (91% steam). The influence of the oxygen carrier loading on the product gas composition, yield, and coal conversion are shown in Figure 7. The H2 and CO2 contents are observed to increase with Fe2O3 to coal ratio, while a decrease in the CO content and CH4 content is observed (Figure 7a). A 167% increase in the H2 content to 56% occurs when the Fe2O3 to coal ratio is increased from 0 to 5. In the same range of Fe2O3 to coal ratio, a decrease in the CO content from 57% to 3% is observed. Previous researchers41 have reported that an increase in the O2 supply resulted in a decrease in the H2 content and an increase in the concentrations of CO and CO2. These contradicting data at lower Fe2O3: coal ratio are a result of the enhanced water gas shift reaction that results in an increase in the H2 content and a decrease in the CO content. The CO content is further decreased by the direct oxidation of CO by Fe2O3. A corresponding decrease in the CO2 content is

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Figure 5. Effect of steam content (based on water flow rate) on product profile. Inset: Steam partial pressure vs water flow rate.

Figure 6. Influence of steam content (water flow rate) on conversion.

obtained. However, on increasing the Fe2O3 to coal ratio to a value greater than 5, a decrease in H2 concentration is observed accompanied by a sharp increase in the CO content. The decrease in H2 content for experiments employing an Fe2O3:coal ratio greater than 5 is probably due to the oxidation of H2 to steam This decrease in the H2 content is reflected in a corresponding increase in the relative CO concentration, even though CO is also oxidized by Fe2O3 (albeit at a slower rate). The CO2 content in the product stream decreased gradually after reaching a maximum value when the Fe2O3:coal ratio was 2.5. The decrease in CO2 may be a result of dry reformation of CH4 and is confirmed by the gradual decrease in the CH4 content. It is also possible that CH4 is partially oxidized to CO and H2 in the presence of excess Fe2O3. The conversion of coal is found to increase with Fe2O3: coal ratio, while the char formation (as determined by the C in the product gases) is observed to decrease (Figure 7b). The coal conversion is enhanced by greater than 17.5 times when the Fe2O3 to coal ratio is increased from 0 to 10. Thus, it can be concluded that the optimal ratio of Fe2O3:coal should be between 2.5 and 5. At these values, the conversions are increased by 4.7 and 8.5

Figure 7. Effect of iron oxide loading on (a) gasification product profiles and (b) coal conversion.

times compared to the experiments in which no Fe2O3 was added. The ln(C/C0) vs Fe2O3:coal ratio appears to be fairly linear. It can be represented as

ln

[Fe2O3]0 C ) -k C0 C0

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Differentiating and rearranging the above equation, we obtain

dC ) -kC d[Fe2O3]0/C0 We can deduce from the above information that not only does Fe2O3 enhance the conversion rates (including C conversion and devolatalization), but it does so in the manner of a catalyst rather than a reactant. However, the monotonic increase of C in the outgases while the CH4 content remained fairly constant shows that the sum of CO and CO2 yield is directly proportional to the Fe2O3:coal ratio, or

dC ) -k[Fe2O3]/C0 d[Fe2O3]/C0 Thus, it can be concluded that Fe2O3 indirectly causes some oxidation of C. The rate constants of steam reforming of coal with (Fe2O3:coal ) 0.065) and without Fe2O3 were determined assuming that the steam reforming of coal can be modeled by first-order reaction kinetics. The estimated rate constant at 850 °C was 0.0523 min-1 (no Fe2O3), which increased to 0.124 min-1 (with Fe2O3). Effect of Addition of Calcium Oxide. The addition of CaO in the reaction mixture is found to significantly improve the cumulative purity of H2 and the conversion at the end of 10 min. Thermogravimetric analysis (not shown) revealed the CaO used in this study could capture 64% of its theoretically calculated maximum capacity. Parts a and b of Figure 8 contain the data on the composition of the product gases and coal conversion, respectively. These experiments were conducted at 850 °C. Greater than 70% purity of H2 is achievable with more than 50% conversion at a CaO:coal of 48. It must be noted that the conversion was only 4.7% when no CaO was added. Thus, the addition of 50 times as much CaO as compared to the coal mass improves the conversion by more than 18 times. The conversion is further enhanced by nearly twice when the ratio of CaO: coal is increased to 80. When a ratio of 80 was employed, the conversion increases to 89% with 61% H2 in the product gases. The maximum H2 purity observed with coal gasification only was 40%, which increased to a maximum of 55% due to the addition of Fe2O3. In the experiment conducted with a CaO:coal ratio of 48, the CH4 content and the CO content were found to be the minimum. The CO content also decreases (from 42% without any CaO addition to 17% when the ratio used was 48) due to the removal of CO2 from the gases by CaO. The increase in the extent of the WGS results in the production of more H2 and CO2. The decrease in CH4 content is attributed to the dry reforming of CH4 on the CaO by CO2. The CaO carbonation-calcination occurs dynamically at these temperatures. The CH4 molecule in the vicinity of CaCO3 is reformed by the CO2 released by these sites (eq 9). However, when a CaO:coal ratio of 80 is employed, the concentration of CaCO3 sites on the surface decreases, thereby decreasing the degree (probability) of CH4 dry reforming. As a result, the CH4 content is observed to increase at these ratios. Although 90% of the C in coal (data not shown) was found to have reacted, the C in the exiting gaseous product was found to be only 0.023 g (Figure 8b), which represents only

Figure 8. Effect of calcium oxide loading on (a) gasification product profiles and (b) coal conversion and carbon in the product.

Figure 9. Typical product distribution as a function of time in a fixed bed reactor (670 °C, 1 mL/min water flow rate (91% steam), 0.2 g of coal, and Fe2O3:coal and CaO:coal ratios of 2 and 50, respectively).

19% of the total C gases formed. The remaining C was captured as CaCO3. Figure 9 shows a typical profile product composition as a function of time. The experiments were conducted at 670 °C, with 1 mL/min water flow rate, and Fe2O3: coal and CaO:coal ratios of 2 and 50, respectively. Fluidized Bed Studies. The influence of process parameters such as temperature, steam partial pressure, Fe2O3 loading, and CaO loading on the gaseous product profile during coal gasification in a fluidized bed is presented in the following. Figure 10a shows the

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Figure 10. Product characteristics as a function of (a) steam partial pressure (2 g of coal, 60 g of sand), (b) temperature in a fluidized bed reactor (2 g of coal, 20 g of CaCO3, 40 g of CaO, 85% steam), (c) iron oxide loading (40 g of CaO, 870 °C, 85% steam), and (d) CaO loading in a fluidized bed reactor (2 g of coal, 20 g of Fe2O3, 870 °C, 85% steam).

product profile and the total gas output under different steam partial pressures at 670 °C. Sixty grams of sand was used as the fluidized solids. A gradual rise in the total gas output is observed with an increase in steam partial pressure up to to 75% steam. A sudden increase in the total gas yield is observed when the steam pressure is increased to 85%. The H2 content, however, does not change (61%) when the steam partial pressure is increased from 75% to 85%. Both CH4 and CO decrease with an increase in the amount of steam present in the reaction zone. Steam reformation of the produced CH4 is primarily responsible for the decrease in the CH4 content in the product gases, while an increase in the water gas shift reaction causes the decrease in the CO content with steam pressure. This is accompanied by an increase in the CO2 content. The results show that 85% steam is ideal for these studies. Higher steam partial pressures caused operational problems, and thus the previously determined optimal condition of 91% steam was not used. The effect of temperature on the product gas profile and the total gas output at steam partial pressure of 75% is shown in Figure 10b. Two grams of coal was injected into a fluidized bed of solids containing 20 g of Fe2O3 and 40 g of CaO. The total gas produced at the end of 15 min increased drastically at temperatures greater than 770 °C. A maximum H2 content (61%) was obtained at 770 °C, and decreased to 54% at 870 °C. It is known that, at 900 °C, the calcination of CaCO3 is favored over the carbonation of CaO. This results in the

reduction of the observed CO2 removal capacity of the CaO and is reflected in the sudden increase in the CO2 content at 870 °C. Rates of CH4 reformation reactions, on the other hand, increase. Nonetheless, the high gas output (due to coal conversion) at 870 °C translates into a higher yield of H2 and has been considered suitable for further studies. Figure 10c contains the data on the product profile and the total gas output as a function of Fe2O3 loading at 870 °C and 85% steam using 40 g of CaO. It is observed that the H2 content decreased by 14% on increasing the Fe2O3 content in the reactive mixture from 20 to 30 g. A maximum of 70% H2 is observed when 20 g of Fe2O3 is employed. On the other hand, the total gas output increases by only 6%. In addition, the CO2 and CH4 contents are also observed to increase at the highest Fe2O3 loadings of 30 g. Thus, 20 g of Fe2O3 is sufficient for enhancing the conversion of CO to CO2. The results from experiments conducted with 20 g of Fe2O3 and varying loads of CaO are shown in Figure 10d. The temperature in the reactor was maintained at 870 °C, and 85% steam was introduced for these experiments. Increasing the CaO loading has a positive effect on the H2 purity. As observed in the figure, increasing the CaO loading from 10 to 20 g resulted in 40% enhancement in purity (from 50% to 70%). Simultaneously the CH4 and CO2 in the product gases are observed to decrease. Further increase in the CaO content did not significantly enhance the H2 purity.

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Figure 11. Typical product distribution as a function of time in a fluidized bed reactor (2 g of coal, 40 g of CaO, 20 g of Fe2O3, 870 °C, 85% steam).

However, it improves the coal gasification characteristics by increasing the gas output by an additional 10%. Conclusions The preliminary studies show that experiments conducted in fluid bed at 870 °C, 85% steam, with 40 g of CaO and 20 g of Fe2O3 would yield a gas with the highest H2 purity along with maximum coal conversion (Figure 11). The use of Fe2O3 alone had a negative effect on the H2 yield, while the use of CaO alone increased the H2 yield and purity. In addition, the presence of CO2 showed a strong negative correlation with CH4. This is primarily due to the dry reforming of CH4 in the presence of CO2. This is also corroborated with the positive correlation between CH4 and CaO (since CaO may remove excess CO2, resulting in the decrease in the extent of dry reforming of CH4). The negative correlation observed between CH4 and Fe2O3 is significant in that Fe2O3 also helps in demethanation. The optimal temperatures and steam partial pressures appear to be 800-850 °C and 85%, respectively. The Fe2O3:coal and CaO:coal ratios should be maintained in the ranges of 2.5-5 and ∼50, respectively. Acknowledgment The authors are pleased to acknowledge the U.S. Department of Energy (Contract Nos. DE-FC2600FT40974 and DE-FC26-03NT41842) and General Electric for their financial support. Literature Cited (1) Cox, A. W. Hydrogen from coalsProspects and challenges. Energy World 2004, 321, 10. (2) Ruby, J.; Johnson, A. A.; Ziock, H. Gasification for carbon capture and zero emission. SME Annu. Meet. Prepr. 2004, 961968. (3) De Biasi, V. FutureGn IGCC to convert coal into hydrogen and electric power. Gas Turbine World 2003, 33 (4), 12-14 +16. (4) Pierce, J. Gasification of coal could provide clean energy. Engineer 2003, 292, 10. (5) Vamvuka, D. Gasification of Coal. Energy Explor. Exploit. 1999, 17 (6), 515-582. (6) Hauserman, W. B. High Yield Hydrogen Production by Catalytic Gasification of Coal or Biomass. Int J. Hydrogen Energy 1994, 19 (5), 413-419.

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Received for review October 20, 2004 Revised manuscript received May 8, 2005 Accepted May 12, 2005 IE048974D