Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction
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):
CO2 sorption:
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,
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
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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