Performance of Na₂CO₃-CaO sorbent in sorption-enhanced steam methane reforming

Highlights

• Sodium doped CaO was used as CO₂ sorbent during methane reforming.

• High purity H2 (93.5 vol.%) was obtained over 10 reaction-regeneration cycles.

• The stability of the sorbent showed an inverse relation with the crystallite size.

• The presence of sodium favored the sintering of calcium oxide particles.

Abstract

The addition of alkali molten salts has been reported as a strategy to overcome the sintering problem presented by calcium oxide in CO₂ capture systems. In this work, the influence of sodium doping in a CaO sorbent was investigated in the sorption enhanced steam methane reforming process (SE-SMR). The goal was to increase the stability of the sorbent and, consequently, the efficiency of the process. For that, a Na-containing sorbent was prepared using the precipitation technique and a pure calcium oxide was obtained by calcination of CaCO₃. The sorbents were physically mixed with 10 % Ni/Al₂O₃ catalyst and tested in 10 cycles of SE-SMR at 600 °C and CH₄:H₂O equals to 4. In general, both materials showed 100 % of CH₄ conversion and H₂ molar fraction of 93.5 vol.%. However, regarding the stability over the SE-SMR cycles, it was evidenced that the addition of sodium decreased the duration of pre-breakthrough comparing with the non-doped material. The XRD, SEM, and TGA results allowed us to observe an inverse relationship of particle diameter and CO₂ capture performance. Na₂CO₃-CaO presented a larger average crystallite size compared to the pure CaO which led to a higher probability of the CaCO₃ layers to inactivate the calcium oxide and, consequently, caused a strong sintering effect. Besides the presence of sodium, the precipitation method and the synthesis conditions could have favored the low initial CO₂ uptake and poor stability of the Na₂CO₃-CaO sorbent.

Introduction

The use of hydrogen as a fuel has attracted interest due to many advantages such as waste-free combustion and high energy mass density. Hydrogen is also used as a raw material in several processes such as ammonia, urea and methanol synthesis, hydrocracking and hydroprocessing in refineries, metallurgical processes, and glass production [1,2]. There are many routes to obtain hydrogen, for instance, reforming of both fossil and renewable hydrocarbons, electrolysis of water, ammonia dissociation, and partial oxidation [3]. The most conventional way to obtain hydrogen is the steam methane reforming (SMR) because it is a relatively simple, low cost, and high efficiency process compared with other processes [[4], [5], [6]]. Nevertheless, the SMR is a highly endothermic process occurring at high temperatures (above 700 °C), which can emit around 8-ton CO₂ per ton H₂ produced [1,7].

Thereby, considerable research effort has been made in the last decades trying to improve the hydrogen production process with considerable low CO₂ emissions, since it is one of the main greenhouse gases. One technology that has received widespread attention is sorption-enhanced steam methane reforming (SE-SMR) that combines the steam reaction with simultaneous CO₂ capture in one vessel [3,5,8,9]. The principle of the SE-SMR process is shifting the equilibrium of the reversible reactions (Eqs. 1 and 2) based on Le Chatelier’s principle to produce high purity hydrogen with in-situ CO₂ capture with a sorbent material [[8], [9], [10]]. The key reactions of the process are the reform of methane with steam (Eq. 1), water gas shift reaction (WGSR) (Eq. 2), and CO₂ capture (Eq. 3):

$$\begin{array}{cc}\hfill \text{C}{\text{H}}_{4(\text{g})}\text{+}{\text{H}}_2{\text{O}}_{(\text{v})}\leftrightarrow \text{C}{\text{O}}_{(\text{g})}\text{+ 3}{\text{H}}_{2(\text{g})}\hfill & \hfill \text{Δ}{\text{H}}_{\text{298K}}^{°}=\text{+206 kJ/mol}\hfill \end{array}$$

$$\begin{array}{ccc}{\begin{array}{c}\mathrm{C}\mathrm{O}\end{array}}_{\left(\mathrm{g}\right)}{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{v}\right)}& \leftrightarrow {\mathrm{C}\mathrm{O}}_{2\left(\mathrm{g}\right)}+{\mathrm{H}}_{2\left(\mathrm{g}\right)}& \mathrm{\Delta }{\mathrm{H}}_{298\mathrm{K}}^{°}=-41\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

$$\begin{array}{cc}\hfill \text{Ca}{\text{O}}_{(\text{s})}\text{+ C}{\text{O}}_{2\left(\text{g}\right)}\leftrightarrow \text{CaC}{\text{O}}_{3(\text{s})}\hfill & \hfill \text{Δ}{\text{H}}_{\text{298K}}^{°}=-\text{178 kJ/mol}\hfill \end{array}$$

Some advantages presented by SE-SMR are higher efficiency in H₂ production, smaller emission of by-products (CO₂ and CO) to the atmosphere, and the possibility to capture relatively pure CO₂ suitable for sequestration or use in different processes. Furthermore, in this type of process, the elimination of individual reactor for water-gas shift and further H₂ purification can be possible, using 20%–25% less energy compared to the conventional SMR [5,11,12].

Many materials are used as CO₂ sorbents such as zeolites, activated carbon, metal-based sorbents, ceramic materials (e. g. hydrotalcite, CaO, MgO, Li₂ZrO₃, Li₄SiO_4, and Na₂ZrO₃), and organic materials (e.g. amines and Metal-Organic Frameworks (MOF’s) [13]. Among all of these materials, calcium oxide is the most studied one. The wide availability in nature as limestone or dolomite, low-cost raw material, high CO₂ sorption capacity, and adequate kinetics reaction are the advantages presented by CaO [14,15]. Another important characteristic is the high temperature for CO₂ adsorption observed for calcium oxide which is usually required for SE-SMR (e. g. 600 °C–800 °C). On the other hand, the main disadvantage of CaO is the fast deactivation and loss of CO₂ capacity by sintering phenomenon and blocking pores during the operation time. Consequently, some approaches have been developed to overcome the drawback related to the stability of sorbent such as the use of different calcium precursors, incorporation of inert support, additional treatments (e.g. hydration, thermal and chemical pretreatments), preparation technique (e.g. wet-mixing, co-precipitation, sol-gel, dry-mixing, etc.) and the doping with alkali metal components for double salt formation [[16], [17], [18], [19], [20], [21], [22]].

Focusing on the doping approach of CaO based sorbents, Al-Mamoori et al. [16] studied the development of a series of double salts such as potassium-promoted calcium (K − Ca) and sodium-promoted calcium (Na − Ca) adsorbents for CO₂ capture. In general, they concluded that the addition of K and Na improved the performance of CaO since these materials presented high CO₂ sorption capacity, fast kinetics, and good stability (only for K-Ca) above 300 °C. Lee et al. [17] performed a comparative study of sorption and regeneration kinetics of synthesized CaO doped with Na₂CO₃ and conventional CaO sorbent. In this study, the authors concluded that the addition of sodium carbonate in calcium sorbent can improve the cyclic stability of CO₂ sorption with fast kinetics.

Based on these studies that only approached the carbonation/calcination performance, the objective of this work was to investigate the applicability and feasibility of sodium doping in the Ca-based sorbent for sorption-enhanced steam methane reforming. Further, the physico-chemical properties of the sorbents were evaluated to explain the performance of the physical mixture between Ni/γ-Al₂O₃ and the synthesized sorbent in the cyclic SE-SMR.