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High-purity hydrogen via the sorption enhanced steam methane reforming reaction over a synthetic CaO-based sorbent and a Ni-catalyst Marcin Broda, Vasilije Manovic, Qasim Imtiaz, Agnieszka Marta Kierzkowska, Edward J. Anthony, and Christoph Rüdiger Mueller Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es305113p • Publication Date (Web): 22 Apr 2013 Downloaded from http://pubs.acs.org on April 29, 2013
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Environmental Science & Technology
High-purity hydrogen via the sorption-enhanced steam methane reforming reaction over a synthetic CaO-based sorbent and a Ni catalyst. Marcin Broda†, Vasilije Manovic‡, Qasim Imtiaz†, Agnieszka M. Kierzkowska†, Edward J. Anthony¦ and Christoph R. Müller†,*
† Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092 Zurich, Switzerland ‡
CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Canada K1A 1M1
¦School of Applied Science, Cranfield University, Bedfordshire, England MK43 0AL KEYWORDS: sorbent-enhanced steam methane reforming, CO2 capture, calcium oxide, nano-structured materials, hydrogen, nickel catalyst
Abstract Sorbent-enhanced steam methane reforming (SE-SMR) is an emerging technology for the production of high-purity hydrogen from hydrocarbons with in-situ CO2 capture. Here, SESMR was studied using a mixture containing a Ni-hydrotalcite-derived catalyst and a synthetic, Ca-based, calcium aluminate-supported CO2 sorbent. The fresh and cycled materials were characterized using N2 physisorption, X-ray diffraction and scanning and transmission electron microscopy. The combination of a Ni-hydrotalcite catalyst and the synthetic CO2 sorbent produced a stream of high-purity hydrogen, i.e., 99 vol.% (H2O- and N2-free basis). The CaO conversion of the synthetic CO2 sorbent was 0.58 mol CO2/mol CaO after 10 cycles, which was more than double the value achieved by limestone. The favourable CO2 capture characteristics of the synthetic CO2 sorbent were attributed to the uniform dispersion of CaO on a stable nano-sized mayenite framework, thus retarding thermal 1 ACS Paragon Plus Environment
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sintering of the material. On the other hand, the cycled limestone lost its nano-structured morphology completely over 10 SE-SMR cycles due to its intrinsic lack of a support component.
1. Introduction Globally, around 50 million tons of hydrogen are produced per year whereby the production is expected to increase over the next years.1 Hydrogen is used in various industrial processes, such as the synthesis of ammonia or the refining of crude oil and is also an attractive, environmentally benign energy carrier, e.g., for fuel cell applications.2-4 The first commercial technology for the industrial production of hydrogen was electrolysis of water, introduced in the late 1920s.5 However, in the late 1960s processes allowing the synthesis of hydrogen from fossil fuels were introduced, with steam methane reforming (SMR) being nowadays the dominating hydrogen production technology.5 In the SMR process, methane and steam are first catalytically reformed into hydrogen and carbon monoxide (1) at 8501000°C and 15-30 bar. For economic reasons, nickel supported on a metal oxide is typically used as the catalyst for the SMR reaction.6 However, the effluent gas of the steam reformer still contains ~12% CO. Thus, the moderately exothermic water-gas-shift (WGS) reaction (2) is employed to further convert CO into CO2.7 The WGS reaction is performed in two stages: a high-temperature water-gas shift at 350-400 °C (Fe-based catalyst), followed by a lowtemperature water-gas-shift reaction at ~200 °C (Cu-based catalyst). CH4 + H2O ↔ CO + 3H2
∆H025 °C = +206 kJ/mol
(1)
CO + H2O ↔ CO2 + H2
∆H025 °C = -41 kJ/mol
(2)
A typical composition of the effluent gas leaving the low-temperature water-gas-shift reactor (dry basis) is approximately 76% H2, 17% CO2, 3% CO and 4% CH4.8 To further reduce the concentration of CO, additional steps, e.g., preferential oxidation (PROX) are required.6 To 2 ACS Paragon Plus Environment
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remove the CO2 of the H2-rich gas, stream scrubbing with amines can be employed. However, amine scrubbing is an energy intensive CO2 removal technology.9 Thus, to avoid the operational complexity of the SMR process and the high costs associated with the amine-based CO2 capture technology, a new concept, i.e., sorptionenhanced steam methane reforming (SE-SMR), has been proposed.10-12 Here, a solid CO2 acceptor material and a reforming catalyst are mixed to allow the in-situ removal of CO213, e.g., via the carbonation reaction of CaO (3). ∆H025 °C = -178 kJ/mol
CO2 + CaO ↔ CaCO3
(3)
Subsequently, the material is regenerated via the reverse reaction (3), i.e., the calcination reaction (3). The summation of reactions (1-3) gives CH4 + 2H2O + CaO ↔ CaCO3 + 4H2
∆H025 °C = -13 kJ/mol
(4)
As a consequence, high yields of H2 (~ 95%, on a dry and N2-free basis) can be obtained at comparatively low reaction temperatures (450-750 °C), thus eliminating the need for the shift reactors and subsequent purification steps.6 In previous studies different CO2 sorbents, such as sodium zirconate (Na2ZrO3)14, lithium silicate (Li4SiO4)15 or hydrotalcite16, have been assessed for the SE-SMR reaction. Using a combination of experimental measurements and techno-economic modelling, Ochoa-Fernández et al.17 demonstrated that CaO, due to its high CO2 absorption capacity and fast carbonation kinetics, is the most attractive CO2 sorbent for the SE-SMR reaction. However, using CaO derived via calcination of naturally occurring limestone has a serious shortcoming, i.e., its poor cyclic CO2 capture stability. For example, Broda et al.18 studied the SE-SMR reaction using a mixture of a Ni-hydrotalcite-derived catalyst (47 wt % of Ni) and limestone and reported that the production of H2 decreased with cycle number due to the decreasing cyclic CO2 capture capacity of limestone. This decrease in the CO2 uptake of limestone with increasing number of carbonation and calcination cycles was observed previously and has been attributed to thermal sintering (the Tammann 3 ACS Paragon Plus Environment
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temperature of CaCO3 is 533 °C19) resulting in the destruction of pore volume within pores of diameters
