Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

doi:10.1016/j.ijhydene.2012.04.160

International Journal of Hydrogen Energy

Volume 37, Issue 15, August 2012, Pages 11227-11236

https://doi.org/10.1016/j.ijhydene.2012.04.160 Get rights and content

Abstract

Sorption enhanced water gas shift reaction (SEWGS) based on calcium looping is an emerging technology for hydrogen production and CO₂ capture. SEWGS involves mainly two reactions, the catalytic WGS reaction and the bulk carbonation of CaO with CO₂, and the solid product is CaCO₃, and the Ca(OH)₂ may be formed from the reaction of CaO with H₂O with the presence of steam in gas phase. The effect of Ca(OH)₂ and CaCO₃ on the catalytic WGS reaction and carbonation reaction was studied in a fluidized bed reactor. It was found that the hydrated sorbent and CaCO₃ did not show any catalytic reactivity toward WGS reaction at 400 °C. When the temperature was increased to 500 °C and 600 °C, the catalytic reactivity of hydrated sorbent was recovered partially, but this will depend on the steam fraction in gas phase, the recovery of fresh CaO surface from dehydration of Ca(OH)₂ may be the reason of catalytic reactivity recovery. CaCO₃ can catalyze the WGS reaction at the high-temperature (>600 °C), this may due to the CaCO₃ decomposition and recarbonation processes in which the CaO is transiently formed. The possible mechanism was discussed.

Highlights

► Study the effect of Ca(OH)₂ on the SEWGS reaction in a fluidized bed reactor. ► Discern the effect of Ca(OH)₂ dehydration on SEWGS reaction. ► Study the effect of CaCO₃ on the WGS reaction and the morphology changing. ► Analyze the possible mechanism involved in the SEWGS reactions.

Introduction

The process of addition of CO₂ sorbents into conventional water gas shift reactor is called sorption enhanced water gas shift process (SEWGS). According to Le Chatelier's principle, if CO₂ is removed as soon as it is formed, the water gas shift reactions proceed beyond the conventional thermodynamic limits and more CO would be converted to hydrogen and high purity of hydrogen could be produced in single-step, the simplification process improves energy efficiency, increases reactant conversion and product yield [1], [2], [3], [4]. The important reactions in the SEWGS are:

Water Gas Shift (WGS): $CO + H_{2} O = {CO}_{2} + H_{2} \begin{matrix} Δ H_{298} = - 41.5 kJ / mol \end{matrix}$

CO₂ sorption: $CaO + {CO}_{2} = {CaCO}_{3} \begin{matrix} Δ H_{298} = - 178 kJ / mol \end{matrix}$

Recently, Fan et al. [5] suggested that SEWGS could remove simultaneously carbon dioxide, sulfur, and chloride impurities at high-temperatures during the generation of high-purity hydrogen, and investigated H₂ production with contaminant removal in the SEWGS process in the presence of CaO sorbent and catalyst [6]. In the SEWGS concept, CaO will be used repeatedly, after the SEWGS step, CaO must be regenerated in regenerator with following reaction, ${CaCO}_{3} = CaO + {CO}_{2} \begin{matrix} Δ H_{298} = 178 kJ / mol \end{matrix}$

If WGS catalyst is used in the shift reactor, the shift catalyst will be transferred into the regenerator with sorbent, catalyst oxidation and sintering will result in the serious deactivation of its catalytic activity, therefore, a sorbent/catalyst separation step will be necessary before sorbent calcination. In order to avoid some problems associated with catalyst such as sorbent/catalyst separation, catalyst deactivation in the presence of H₂S, catalyst sintering etc, attempt to achieve the SEWGS without shift catalyst were investigated by Harrison et al. [7] and Ramkumar et al. [8], they found CaO can catalyze the WGS reaction. However, in the papers published by Harrison et al. and Ramkumar et al., they all used stainless steel reactors, which were demonstrated to have a catalytic action on the WGS. Escobedo Bretado [9] used a quartz tube fixed-bed reactor to avoid the effect of steel reactor on the WGS and found that dolomite had catalytic action on the WGS and the chemical species responsible for the activity toward the WGS in calcined dolomite was possibly the MgO. From the thermodynamic point of view, Ca(OH)₂ may be formed in the SEWGS step with the presence of steam in gas phase with following reaction $CaO + H_{2} O = Ca {(OH)}_{2} \begin{matrix} Δ H_{298} = - 65 kJ / mol \end{matrix}$

The carbonation of Ca(OH)₂ with CO₂ has been investigated recently by researchers aim to resolve the loss in CaO carrying capacity by the hydration of CaO [10], [11], [12], [13], however, there is a significant difference between the carbonation reaction used for the CO₂ capture from flue gas and the SEWGS reaction, the knowledge of Ca(OH)₂ carbonation can not be directly used for the SEWGS reaction, and the effect of Ca(OH)₂ on the SEWGS reaction is not clear. At the same time, CaCO₃ product will be formed on the CaO surface during SEWGS reaction, it is reported that CaCO₃ product layer will increase the diffusion resistance and slow the carbonation rate [14], [15], [16], but there is less information from the literature concerning about the effect of CaCO₃ on the catalytic WGS reaction.

The purposes of this study were as follows: (1) to study the effect of Ca(OH)₂ formed during sorbent hydration step on the variation of product gases evolution profiles with reaction time at different temperature in order to discern the effects of Ca(OH)₂ on SEWGS, (2) to study the effects of CaCO₃ on the WGS reaction, and (3) to analyze the possible mechanism involved in the SEWGS reactions.

Section snippets

Samples

Limestone was used as both CO₂ sorbent and WGS reaction catalyst in this study. The particle size was 200–500 μm, and the composition of limestone was measured using XRF spectrometry (Rigaku ZSX Primus Π), as shown in Table 1. The sample mass used for each fluidized bed experiment was almost same, about 60 g.

In order to investigate the effect of steam on surface morphology of CaCO₃, single crystal of CaCO₃ was used. The single crystal CaCO₃ samples were sheets with 5 mm long and wide and 0.5 mm

WGS reaction on CaCO₃ surface

In this section, the effect of CaCO₃ surface on the WGS reaction was studied at different temperature, and the experimental details can be found from Procedure A in experiment introduction. Fig. 1 shows the calculated equilibrium pressure of CO₂ over CaCO₃ for various temperatures. The calculation indicates that no CaCO₃ decomposition occurs when the CO₂ fraction is 19 vol% and the temperature is lower than 800 °C. Fig. 2 shows the WGS reaction occurs on the surface of limestone from 400 °C to

Discussions

The SEWGS process involves CaO hydration, WGS catalytic reaction and carbonation reaction, the interaction of H₂O molecule with CaO surface plays an important role for these reactions. There is a long history for the research of H₂O interaction with solid surface, while the interaction mechanism from atomic level point of view remains still to be a challenge [18], [19]. When H₂O molecule is adsorbed by CaO surface, there are possibly physisorption (van der Waals interaction), molecule

Conclusions

The catalytic activity of CaO, CaCO₃ and Ca(OH)₂ toward to WGS reaction has been investigated. It was observed that limestone can catalyze the WGS reaction when the temperature is higher than 600 °C, the decomposition-recarbonation mechanism may be responsible for this catalytic activity. Hydrated sorbent can not catalyze the WGS reaction at 400 °C, when the temperature was increased to 500 °C and 600 °C, the decomposition of Ca(OH)₂ happens, and the conversion of CO was observed and decrease

Acknowldgement

This research was supported by the National Natural Science Funds of China (No. 51061130535), and by the National Basic Research Program of China (No. 2011CB707301).

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The catalytic activity per gram of CaO at 630 °C is typically an order of magnitude lower than the activity of conventional Cu-based catalysts at the same conditions [62]. Clearly, CaO is not a preferred catalytic material, nevertheless it has significant activity, in line with the facts that MgO is also active for RWGS [60] and that CaO is active for WGS, as reported in a study on sorption enhanced WGS [63]. CaO and MgO [59] are much more active for RWGS than Al2O3 in thermal operation.
This study reports on synergy between Dielectric Barrier Discharge plasma and calcium oxide as a catalyst for the Reverse Water Gas Shift (RWGS). Any effect of the presence of the catalyst on the contribution of plasma chemistry, i.e. chemical conversion in the plasma, is minimized by using a fixed bed of α-Al₂O₃ particles as a blank experiment and adding a small amount of calcium oxide, with similar dielectric constant, particle size and shape. This approach results in constant plasma power and ensures also that the residence time and specific energy input remain unchanged. Furthermore, synergy is determined based on reaction rates at fixed conditions, i.e. concentrations and temperature, based on kinetic equations derived from integral experiments, describing both thermal operation and plasma operation and both in absence and presence of calcium oxide. This approach has not been applied so far to study plasma-catalysis synergy.
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With the increasing concerns on fossil fuel depletion, environment deterioration, and global warming, great efforts have been devoted to the concept of “hydrogen economy” and the mitigation of CO₂ emission. Among different technologies, enhanced water gas shift processes have exhibited significant importance not only for the production of high purity H₂ but also for their efficient CO₂ separtion. By highlighting the novel materials and practical applications, we present a state-of-art review on the enhanced water gas shift processes towards improved CO₂ removal and H₂ production via Le Chatelier’s principle. First, we discussed detailed utilization of membrane reactors based on assortments of H₂-permselective membranes with a particular emphasis on their advantages and weaknesses towards CO₂ separation. We also introduced the unique development of solid CO₂ adsorbents, followed by their potential applications for intermediate-CO₂ capture. Furthermore, the integrated combination of membrane assisted/sorption enhanced WGS processes are featured in this promising field. Finally, we critically reviewed recent implementations and applications with challenges and outlooks on the basis of the current development.
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Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

Abstract

Highlights

Introduction

Section snippets

Samples

WGS reaction on CaCO3 surface

Discussions

Conclusions

Acknowldgement

Fuel

Particuology

Chem Eng Sci

Surf Sci

Int J Hydrogen Energy

Chem Eng Sci

Fuel Process Technol

Production of hydrogen

U S

Investigation of the enhanced water gas shift reaction using natural and synthetic sorbents for the capture of CO2

Ind Eng Chem Res

Sorption-enhanced hydrogen production: a review

Ind Eng Chem Res

Calcium looping process for enhanced catalytic hydrogen production with integrated carbon dioxide and sulfur capture

Ind Eng Chem Res

Calcium Looping Process (CLP) for enhanced noncatalytic hydrogen production with integrated carbon dioxide capture

Energy Fuels

Hydrogen production by absorption enhanced water gas shift (AEWGS)

Int J Hydrogen Energy

Investigation of attempts to improve cyclic CO2 capture by sorbent hydration and modification

Ind Eng Chem Res

Ca(OH)2 superheating as a low-attrition steam reactivation method for CaO in calcium looping applications

Ind Eng Chem Res

High temperature carbonation of Ca(OH)2

Ind Eng Chem Res

WGS reaction on CaCO₃ surface

Investigation of the enhanced water gas shift reaction using natural and synthetic sorbents for the capture of CO₂

Investigation of attempts to improve cyclic CO₂ capture by sorbent hydration and modification

Ca(OH)₂ superheating as a low-attrition steam reactivation method for CaO in calcium looping applications

High temperature carbonation of Ca(OH)₂