Elsevier

Applied Energy

Volume 205, 1 November 2017, Pages 316-322
Applied Energy

Sorption-enhanced water gas shift reaction for high-purity hydrogen production: Application of a Na-Mg double salt-based sorbent and the divided section packing concept

https://doi.org/10.1016/j.apenergy.2017.07.119Get rights and content

Highlights

  • Na-Mg double salt-based sorbent was used for high-temperature CO2 sorption.

  • Divided section packing concept was applied to the SE-WGS reaction.

  • High-purity H2 was produced from the SE-WGS reaction with divided section packing.

  • High-purity H2 productivity could be further enhanced by modifying packing method.

Abstract

Hydrogen is considered a promising environmentally benign energy carrier because it has high energy density and produces no pollutants when it is converted into other types of energy. The sorption-enhanced water gas shift (SE-WGS) reaction, where the catalytic WGS reaction and byproduct CO2 removal are carried out simultaneously in a single reactor, has received considerable attention as a novel method for high-purity hydrogen production. Since the high-purity hydrogen productivity of the SE-WGS reaction is largely dependent on the performance of the CO2 sorbent, the development of sorbents having high CO2 sorption capacity is crucial. Recently, a Na-Mg double salt-based sorbent has been considered for high-temperature CO2 capture since it has been reported to have a high sorption capacity and fast sorption kinetics. In this study, the SE-WGS reaction was experimentally demonstrated using a commercial catalyst and a Na-Mg double salt-based sorbent. However, the SE-WGS reaction with a one-body hybrid solid, a physical admixture of catalyst and sorbent, showed poor reactivity and reduced CO2 sorption uptake. As a result, a divided section packing concept was suggested as a solution. In the divided section packing method, the degree of mixing for the catalyst and sorbent in a column can be controlled by the number of sections. High-purity hydrogen (<10 ppm CO) was produced directly from the SE-WGS reaction with divided section packing, and the hydrogen productivity was further improved when the reactor column was divided into more sections and packed with more sorbent.

Introduction

Due to the rapidly increasing global population and accelerated industrial development, energy demands have soared over the last few decades. Conventional energy production and conversion processes are based on the combustion of fossil fuel feedstocks [1], which cause massive emissions of anthropogenic greenhouse gases resulting in alarming environmental problems. Thus, the development of eco-friendly energy processes is required, but these processes are not easily adopted due to their economic infeasibility. Hydrogen is considered as an alternative energy carrier because it can be easily produced from various feedstocks and also because it possesses a higher energy density than conventional fossil fuels [2]. Approximately 50 million tons of hydrogen are already produced annually, and hydrogen is widely used in a variety of applications including the chemical synthesis of ammonia and urea, hydrocracking and hydrotreating in oil-upgrading, and in fuel for vehicles [3]. Hydrogen can also be used in many ways for power generation, especially in fuel cell systems [4].

Steam methane reforming (SMR) reaction processes are commonly used for bulk hydrogen production. Furthermore, water gas shift (WGS) reaction processes using synthesis gas feeds produced by the gasification of carbonaceous feedstocks, such as coal and biomass, are becoming increasingly common [5], [6], [7]. In the WGS reaction, carbon monoxide and steam react to produce hydrogen and carbon dioxide. Since the hydrogen produced from the WGS reaction contains massive amounts of byproduct CO2, unreacted CO, and steam, the product gas stream is subjected to separation processes such as pressure swing adsorption after steam condensation to obtain high-purity hydrogen [8], [9]. In particular, hydrogen for polymer electrolyte membrane fuel cells must be more than 99.9% pure and contain less than 10 ppm CO. Recently, to improve the performance of the WGS reaction, the sorption-enhanced reaction (SER) concept was applied [10], [11]. In the sorption-enhanced WGS (SE-WGS) reaction, the WGS reaction and CO2 removal by sorption are carried out simultaneously in a single reactor, and the thermodynamic equilibrium limitation of the WGS reaction can be circumvented based on the Le Chatelier principle. Since fuel-cell grade high-purity hydrogen can be directly produced from the SE-WGS reaction, the cost of separation can be reduced, and a more compact process can be developed. The SER process is also considered to be a promising pre-combustion CO2 capture technology, and combined SE-WGS and integrated gasification combined cycle (IGCC) technology has been highlighted as a prominent solution for mitigation of CO2 emissions in coal-based power plants [12], [13]. The application of SE-WGS reaction process in IGCC has benefits in terms of operating costs compared to the combination of IGCC and the conventional Selexol process [14].

For the successful implementation of SE-WGS reactions, sorbents that can capture high-temperature CO2 are crucial. Hydrotalcite, calcium oxide, Na2O-modified alumina, modified MgO, and alkali metal zirconate have been applied as high-temperature CO2 sorbents for the SE-WGS reaction [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Hydrotalcite and modified hydrotalcite-like sorbents have been particularly successfully applied to SE-WGS reaction processes, but their CO2 sorption uptakes are relatively low (0.13–1.4 mol/kg) at high (or intermediate) temperature, resulting in rapid saturation with CO2 and reduced hydrogen productivity. To increase the CO2 sorption uptake of these sorbents, several approaches, such as addition of alkali metals, modification of the synthesis method, and changing the component ratio, were considered [26], [27]. However, these sorbents need further examination before application to SER processes.

Recently, double salt-based sorbents were reported as promising materials for high-temperature CO2 capture, showing characteristic properties for CO2 sorption [28], [29]. In our previous studies, Na-Mg and K-Mg double salt-based sorbents were synthesized and have been tested for high-temperature CO2 capture, and they showed unique and promising CO2 sorption performances [30], [31]. In particular, the Na-Mg double salt-based sorbent exhibited fast sorption kinetics, outstanding cyclic stability, and high CO2 sorption capacity (∼3 mol/kg) between 200 and 400 °C, which corresponds with the temperature required for the WGS reaction.

In this study, the SE-WGS reaction was experimentally investigated using a commercial catalyst and a Na-Mg double salt-based sorbent. The Na-Mg double salt-based sorbent was first applied to the SE-WGS reaction. Pellets of a one-body hybrid solid of catalyst and sorbent, a catalyst-only solid, and a sorbent-only solid, were prepared by compressing the corresponding powder. The prepared pellets were packed into a reactor column at different catalyst-to-sorbent ratios and with different packing methods, and their effects on the SE-WGS reaction performance and hydrogen production were investigated. In particular, the reduced catalytic reactivity and CO2 sorption performance using one-body hybrid solid pellets having direct contact between the catalyst and sorbent are reported for the first time, and the divided section packing concept is suggested and tested as a solution to this problem.

Section snippets

Sample preparation

To experimentally demonstrate WGS and SE-WGS reactions, commercial ShiftMax 210 (Cu/ZnO/Al2O3) catalyst powder manufactured by Clariant (formerly Süd-Chemie) and a Na-Mg double salt-based CO2 sorbent were used. The Na-Mg double salt-based sorbent was prepared by a precipitation method [30]. For the synthesis of this sorbent, appropriate amounts of sodium carbonate (Na2CO3; Sigma-Aldrich, >99.5%) and magnesium nitrate hexahydrate (Mg(NO3)2·6H2O; Sigma-Aldrich, ACS reagent, 99%) were suspended in

Characteristics of prepared samples

Crystalline structures of the catalyst before and after WGS reaction, the pristine Na-Mg double salt-based sorbent, and the one-body hybrid solid having a catalyst-to-sorbent ratio of 0.25 before and after the SE-WGS reaction were identified using XRD analysis. The XRD spectra in Fig. 3a indicates that the pristine commercial catalyst is mainly composed of CuO, ZnO, and Al2O3. The characteristic peaks of CuO in the catalyst were fully converted to those of Cu after the WGS reaction at 375 °C (

Conclusions

A Na-Mg double salt-based CO2 sorbent was applied to the SE-WGS reaction for high-purity H2 production. First, a one-body hybrid solid, a physical admixture of commercial catalyst and CO2 sorbent, was prepared, and its characteristics were analyzed. From XRD analysis, the one-body hybrid solid was confirmed to contain both catalyst and sorbent crystalline components, resulting from the homogeneous mixing of the catalyst and the sorbent by ball-milling. However, the reactivity and CO2 sorption

Acknowledgements

This work was supported by a New & Renewable Energy Core Technology Program (No. 20153030041170) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government’s Ministry of Trade, Industry & Energy, and grants from the Korea Institute of Energy Research (B7-2424).

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