ARTICLE pubs.acs.org/IECR
Calcium Looping Process (CLP) for Enhanced Steam Methane Reforming Shwetha Ramkumar,† Nihar Phalak,† and Liang-Shih Fan*,† †
William G. Lowrie Department of Chemical and Biomolecular Engineering, 140 West 19th Avenue, 125 Koffolt Laboratories, The Ohio State University, Columbus, Ohio 43210, United States ABSTRACT: Carbon dioxide (CO2) capture using calcium-based sorbents for post- and pre-combustion applications has the potential to become a viable technology. When applied to a pre-combustion system, the presence of calcium sorbents facilitates process intensification by combining the CO2 removal step with the reactions generating the fuel gas [syngas, hydrogen (H2), etc.] in a single step. Such a process is also capable of producing a high-purity sequestration-ready CO2 stream. The enhanced steam methane reforming (SMR) using the Calcium Looping Process (CLP) has been investigated in this work. The CLP comprises three reactors: the carbonation reactor or carbonator where the thermodynamic constraint of the reforming and water gas shift (WGS) reaction is overcome by the incessant removal of the CO2 product resulting in the production of high-purity H2, the calciner where the calcium sorbent is regenerated and a sequestration-ready CO2 stream is produced, and the hydrator where the regenerated sorbent is reactivated to improve its multicyclic performance. The exothermic carbonation and WGS reaction convert the highly endothermic SMR into a heat neutral process, thus reducing the temperature of reforming from >900 to 650 °C. Experiments conducted using methane in a bench-scale fixed bed reactor have indicated that high purity H2 (∼9599%, dry basis) can be produced using the CLP with in situ CO2 capture. Attempts to maintain the sorbent reactivity over multiple cycles using hydration have yielded encouraging results.
1. INTRODUCTION Rising energy demand coupled with the growing concerns of global warming due to unabated carbon dioxide (CO2) emissions have brought the development of cleaner and more efficient technologies for energy generation into focus.1 Even though renewable energy sources are expected to grow rapidly, it is predicted that 78% of the U.S. energy needs in 2035 will be satisfied by fossil fuels like coal and natural gas.2 Recent research efforts have also focused on the large-scale production of hydrogen (H2) and making H2 as the primary fuel for generating electricity, powering automobiles, etc.3 While the advantages of using H2 as a fuel zero CO2 emission and excellent calorific value are undisputable, the source of large quantities of H2 that would be required to achieve a global switchover will most likely be fossil fuels, especially natural gas. In such a scenario, improving the efficiency of H2 production from fossil fuels with the integration of CO2 capture becomes very important. Currently, natural gas is a cost-effective feed for making H2 due to wide availability, ease of handling, and high hydrogen-tocarbon ratio as compared to coal.3 Although historically, U.S. has relied on imports to satisfy its natural gas requirements, the recently reported increase in the estimates of shale gas reserves are likely to drive U.S. production, lower price, and cut imports.2 As per the available data, 30% growth in domestic gas production will outpace the 16% growth in consumption.2 This indicates that natural gas will remain the leading source of H2 even as technologies based on new sources are being developed. Steam methane reforming (SMR) is the most efficient and economical process that exists today to convert natural gas to H2.4 Natural gas or methane (CH4) is first passed through a zinc oxide (ZnO) bed to remove the sulfur and is then reacted with steam in a reformer at 800900 °C and 2030 atm in the r 2011 American Chemical Society
presence of a nickel-based catalyst to convert it into carbon monoxide (CO) and H2. This reaction is highly endothermic and requires substantial quantities of supplemental fuel to be burnt to provide this heat. The product gas from the reformer is then sent to a single-stage or dual-stage water gas shift (WGS) reactor in which bulk of the CO is catalytically converted to H2. The H2rich gas is further purified in a pressure swing adsorber (PSA) and the PSA-tail gas is combusted in the reformer. In the case where CO2 capture is necessary, processes like amine scrubbing are employed downstream of the WGS reactor. Some drawbacks of SMR are high steam requirement in the reformer and WGS reactors and high energy requirement in the endothermic reformer. With CO2 capture, the tolerance level for several contaminants in the product gas is even lower to protect the solvents that are employed in the capture process, negatively impacting the overall efficiency.5 To improve the efficiency of this process, process intensification by combining two or more reactions in a single stage reactor has been suggested. Specifically, H2 production by the equilibrium-limited reforming and WGS reactions can be enhanced by in situ removal of CO2 using different sorbents.68 Previous studies have shown that presence of a CO2 acceptor along with a reforming catalyst can result in the production of 97% H2 (dry basis) from CH4 in a single step without using any shift catalysts.8,9 Hydrotalcites, lithium zirconate (Li2ZrO3), lithium silicate (Li4SiO4), sodium zirconate (Na2ZrO3), calcium oxide (CaO), etc. and their synthetically modified versions are Received: August 3, 2011 Accepted: October 28, 2011 Revised: October 10, 2011 Published: October 28, 2011 1186
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potential candidates for such applications.10,11 However, the abundance of naturally occurring CaO precursors limestone (CaCO3) and dolomite (CaCO3.MgCO3) and the very high CO2 sorption capacity of CaO as compared to other natural or synthetic sorbents, make CaO an ideal candidate for such a process.12 Several studies have been reported in which different calcium sorbents have been used to enhance H2 production via reforming of hydrocarbons. Brun-Tsekhovoi et al. have investigated the catalytic steam-reforming of hydrocarbons in a fluidized bed of catalyst, also containing CaO and have reported a H2 purity of about 9498% (mole) on a dry basis with trace amounts of CO and CO2 in the product gas.13 A similar, but more comprehensive study, was performed by Balasubramaniam et al. where the effects of different parameters like temperature, steam-to-methane ratio, acceptor-tocatalyst ratio, etc. were investigated.5 Johnsen et al. conducted the sorption-enhanced SMR using dolomite over multiple cycles and found that even though the overall reaction rate remained unaffected, the CO2 uptake capacity of the sorbent reduced with increasing number of cycles.14 Modeling studies on design of such reactors and integration of this novel concept in a natural gas-to-H2 plant have also been discussed in separate studies.1517 The CaOassisted reforming of alcohols has also been reported.18 The Calcium Looping Process (CLP), developed at The Ohio State University (OSU), enables high purity H2 production from coal-gasified syngas with in situ CO2 and sulfur capture in a single step.7,1922 Process overview and experimental results from the bench-scale testing have been reported elsewhere.23,24 However, it is also possible to integrate the reforming of hydrocarbons along with CO2 and sulfur capture in the CLP. This study provides details of such a process scheme. Experiments have been performed with CH4 in a fixed-bed reactor, and the results are reported here.
2. PROCESS OVERVIEW AND THERMODYNAMIC ANALYSIS The CLP comprises three reactors—the carbonation reactor or carbonator, where the thermodynamic constraint of the reforming and WGS reaction is overcome by the incessant removal of the CO2 product and high-purity H2 is produced along with CaCO3; the calciner where the CaO is regenerated and a sequestrationready CO2 stream is produced; and the hydrator where the regenerated sorbent is reactivated to improve its recyclability.23 In the carbonator, hydrocarbon reforming, WGS and CO2 removal occur simultaneously. The steam reforming of the hydrocarbons occurs in the presence of the reforming catalyst, and the CO2 produced by the combined reforming and WGS reaction is removed by CaO. The concomitant carbonation of the CaO, leading to the formation of the CaCO3, drives the equilibrium-limited WGS and reforming reactions in the forward direction, according to Le-Chatelier’s principle. The reactions occurring in the carbonator are as follows Reforming :
Cx Hy þ xH2 O f xCO þ ðy = 2 þ xÞH2 ð1Þ
WGSR :
CO þ H2 O f H2 þ CO2
ΔH ¼ 41 kJ=mol
ð2Þ Carbonation :
CaO þ CO2 f CaCO3
ΔH ¼ 178 kJ=mol
ð3Þ
Figure 1. Thermodynamic analyses of the reforming and WGS reactions in the presence and absence of a calcium-based CO2 acceptor.
CaCO3 is then decomposed in the calciner, to obtain CaO and a pure CO2 stream Calcination :
CaCO3 f CaO þ CO2
ΔH ¼ 178 kJ=mol
ð4Þ In the hydrator, the regenerated CaO undergoes hydration to form Ca(OH)2, according to the following reaction Hydration : CaO þ H2 O f CaðOHÞ2
ΔH ¼ 109 kJ=mol
ð5Þ The CaO and H2O participating in reactions 1-3 are obtained by the dehydration of the above product. A major advantage of the CLP is the internal heat integration that it provides to the reforming of hydrocarbons. The exothermic carbonation and WGS reactions convert the highly endothermic reforming of hydrocarbons into a heat neutral process. An example of a system for the conversion of CH4 to H2 in the CLP carbonator is provided below
As seen from Figure 1, the equilibrium constant for the combined reforming and WGS reaction is very poor up to 700 °C, which explains why these reactions are traditionally conducted at higher temperatures (800900 °C) and under pressure. However, due to the integration of CO2 capture with these reactions, the overall reaction is closer to equilibrium in a comparatively lower temperature range of 500750 °C. Operation at lower temperature is likely to result in cost-savings due to lower consumption of supplemental fuel. Moreover, since in situ CO2 removal drives the H2-producing reactions forward, excess steam requirement is reduced. Finally, WGS catalysts are eliminated resulting in further cost savings. If the CLP carbonator operates in the temperature range of 500750 °C, the hydration of CaO will not be thermodynamically favorable. From the hydration-dehydration equilibrium curve, it is observed that the hydration reaction is favored at atmospheric pressure only below 500 °C.23 Hence even if steam 1187
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and CaO coexist in the carbonator, the steam will not compete with CO2 for reaction with CaO. All the available steam will be thus available for reforming and WGS reactions. The three stages of the CLP have been investigated in this study with the focus on improving the overall efficiency of the hydrocarbons to H2 conversion process. The results of this study can also be extended to a novel coal-to-liquids process configuration.25
3. EXPERIMENTAL METHODS 3.1. Chemicals, Sorbents, and Gases. The reforming catalyst was obtained from Sud-Chemie Inc. and consisted of nickel oxide. CaO was obtained from precipitated calcium carbonate (PCC) precursor. Detailed procedure of the synthesis of PCC is provided elsewhere.26 Pure CH4 (99.9%) was used as the hydrocarbon feed in all tests. 3.2. Bench-Scale Experimental Setup. The combined reforming, WGS, and carbonation reaction was investigated in a bench-scale fixed bed reactor, described in a previous work.23 Pure CH4 was metered into the reactor through a mass flow controller (MFC). From the MFC, the CH4 flows into the steam generating section which also serves to preheat the CH4 entering the reactor. The reactor (diameter 0.2500 , length 1000 ), which is heated by a tube furnace (ThermoLyne 79300), is provided with a pressure gauge and a thermocouple to monitor the pressure and temperature, respectively. The reactor consists of two concentric sections. The inner (packed bed) section is filled with solid particles consisting of only catalyst or catalyst and sorbent mixture, depending on the type of investigation. The outer section preheats the gas before it contacts the bed of solids. The packed bed section of the reactor is detachable which enables easy removal and loading of the solid particles. The gas leaving the reactor enters a back pressure regulator. After exiting the back pressure regulator, the gas is cooled in a heat exchanger using chilled water to condense the unreacted steam. The product gas at the exit of the heat exchanger is dried in a desiccant bed and is sent to a set of gas analyzers (California Analytical Instruments Model 200 and 600 FID) capable of determining the concentrations of CO, CO2, CH4, and H2 in the gas stream. A schematic diagram of this setup has been provided in ref 23. 3.3. Reforming Reaction in the Presence of Reforming Catalyst. The reforming of CH4 was investigated in the presence of the reforming catalyst. For these tests, 5 g of finely powdered (
