Continuous high-purity hydrogen production by sorption enhanced steam reforming of ethanol over modified lithium silicate
Introduction
Energy shortage and environmental issues due to emissions of pollutants from combustion of fossil fuels are accelerating the developments of clean, high efficient and renewable energies [1,2]. Hydrogen is considered as a promising alternative to fossil fuels in view of the abundant availability of hydrogen bearing substances in nature, its high energy content (120.7 kJ/g), and its combustion without any environmental pollution. Currently, hydrogen is mainly produced from steam methane reforming (SMR) process in industry. To achieve sustainability and fuel flexibility, the use of renewable hydrocarbons resources for hydrogen production is urgent. Ethanol is a potential candidate with good features of wide availability, non-toxicity, high energy density, storage and handling safety.
Hydrogen production by steam reforming of ethanol (SRE) has been extensively studied due to the potentially high yield to hydrogen (6 mol of H2 per mol of ethanol fed) [[3], [4], [5], [6], [7]]. The ideal complete SRE reaction is described as C2H5OH + 3H2O → 6H2 + 2CO2, which is highly endothermic and usually operated at high temperatures (>500 °C), atmospheric pressure and water excess. In fact, SRE is a very complex process and many reactions could take place simultaneously or successively: decomposition of ethanol (Eq. (1), methane steam reforming (Eq. (2) and Eq. (3)) and water-gas shift (Eq. (4)).C2H5OH →CO + CH4 + H2 (H298K = 49.0 kJ mol−1)CH4 + H2O ↔ CO + 3H2 (H298K = 206.3 kJ mol−1)CH4 + 2H2O ↔ CO2 + 4H2 (H298K = 165.1 kJ mol−1)CO + H2O ↔ CO2 + H2 (H298K = −41.2 kJ mol−1)
Furthermore, the SRE process inevitably produces a large amount of CO2 that is the primary greenhouse gases blamed for global warming. The sorption-enhanced steam reforming (SESR) for generating high-purity H2 and capturing CO2 simultaneously is an emerging area of research [8]. During the technology, CO2 is removed in situ by solid sorbents, which shifts the normal thermodynamic equilibrium toward hydrogen production and increasing hydrocarbons conversion. Compared with the traditional SRE technology, sorption enhanced steam reforming of ethanol (SE-SRE) not only avoids the number of processing steps required for subsequent CO2 separation, but also reduces the operating temperature attributed to the energy compensation from the exothermic reaction of CO2 sorption. Although many attempts on SE-SRE have been made [[9], [10], [11], [12], [13]], there are still a lot of challenges to make the new process more economically feasible.
Selection of CO2 acceptor has great impact on the SE-SRE process. The desired sorbents should have several characteristics including high CO2 selectivity, adequate sorption/desorption kinetics, large CO2 capture capacity and excellent cyclic stability under the conditions of steam reforming. Among various materials, CaO and lithium orthosilicate (Li4SiO4) are the most promising high-temperature (400–700 °C) solid sorbents [14,15]. CaO has high CO2 uptake capacity and wide availability, but severe sintering problem impedes its widespread application [16,17]. Li4SiO4 possesses lower desorption temperature and superior durability but more expensive of raw materials and poor adsorption kinetics. The high cost of Li4SiO4 may be overcome by producing sorbent using some cheap and environmental-friendly raw materials such as fly ashes [18,19], wood ashes [20] and vermiculite [21]. In a previous research [22], three waste materials (rice husk ash, fly ash, kaolin) as silicon source were examined for preparing Li4SiO4 sorbent. Results showed that rick husk ash with large specific surface area and loose structure is the optimal silicon source. Also, the renewable rice husk ash (RHA) contains nearly 95 mass % silica [[23], [24], [25], [26]]. Controlling burning is an economical and effective method of extracting silica from RHs for further possible use. However, there is less information about the influence of thermal treatment about rice husk on the phase composition and microstructure of the silica as well as reactivity of its resulting sorbents. On the other hand, the incorporation of hetero-elements (Na, F, Fe, Ti, Y, etc.) has been proposed to address the shortcoming of poor CO2 adsorption kinetics of Li4SiO4 at low CO2 concentration [[27], [28], [29], [30], [31], [32], [33]]. While extensive research has not been conducted, some impurity phases (i.e., CaO, Al2O3, K2O, MgO) existing in RHA would be expected to affect the adsorbent morphology, structure and species formation, resulting in different CO2 adsorption properties.
The most important demands in the industrial application of the SE-SRE technology is the continuous production of high-purity hydrogen stream. There have been experimental studies dealing with high-purity hydrogen via SE-SRE [[34], [35], [36]]. However, due to the sorbent regeneration and catalyst reduction, high-purity H2 can be only generated intermittently on a single reactor rather than continuously. Continuous hydrogen production with in-situ CO2 removal requires parallel fixed bed reactors in an alternating operation mode, or a fluidized state with the catalyst/sorbent particles continuously transferred between reformer/carbonator and reduction/calciner. Lysikov et al. [37] achieved continuous H2 production by SR-SRE process over CaO sorbent in dual fixed bed reactors, but the process was complicated and energy intensive because the sorbent was periodically regenerated under temperature swing adsorption and pressure swing adsorption modes. Dou et al. [38] reported an approach on continuous sorption-enhanced steam reforming of glycerol by moving-beds in the continuous flow phase, in which the sorbent and catalyst always run in nearly fresh state. However, CaO sorbents in continuous moving way showed decay in their reactivity due to thermal sintering or attrition, resulting in a short time for high-purity H2 production.
In this work, we studied the application of modified RHA-Li4SiO4 as a CO2 sorbent in the SE-SRE process for continuous production of high-purity H2 under two parallel fixed-bed reactors. The effect of pretreatment of rice husk under different combustion temperatures on the microstructure of silica and CO2 adsorption performance of RHA-Li4SiO4 was first studied. Different metallic elements (K, Ca, Al, Mg) were doped into the obtained RHA-Li4SiO4 to improve its chemical properties. The role of dopants on the crystal, textural, microscopic structure and reactivity was determined. Then, the optimized sorbent was coupled with Ni-based catalyst in the SE-SRE process to selectively removes CO2 from the product gas, promoting the equilibrium shift to H2 production. We investigated the effect of key operating parameters (temperature, liquid hourly space velocity, ratio of water to ethanol, the amount of sorbent) on the hydrogen production. Finally, a scheme including two parallel fixed-bed reactors was designed and operated periodically for continuous production of high purity hydrogen under atmospheric pressure.
Section snippets
Materials
A Ni-based catalyst was chosen for ethanol steam reforming due to its low cost and high C–C rupture ability. 35 wt% Ni was supported on the Al2O3 substrate by using the impregnation method. A more detailed description of the preparation process can be seen in our previous article [22]. The catalyst and sorbent were ground and sieved to (0.15–0.3) 10−3 m size range. Rice husk ash was used as silica source for producing RHA-Li4SiO4 sorbents. Rice husks obtained from Jiangsu in China were first
Effects of thermal treatment on the RHA-Li4SiO4 sorbent
The rice husk ashes calcined at different temperatures were characterized by XRD and results are given in Fig. 2a. When the rice hull was pretreated by citric acid before combustion, any diffraction peaks are not observed in the RHA (700) powder. RHA(800) and RHA(900) samples only exhibit a broad diffraction peak between 10 and 30°, indicating the silica contained in these husk ashes was completely amorphous structures. According to previous studies, the combustion temperature at 973 K or above
Conclusions
The present study focused on the continuous production high-purity H2 via SR-SRE over the K-doped Li4SiO4 sorbent using two parallel reactors. The low cost Li4SiO4 was first synthesized from rice husk ashes burned at different temperatures. The thermal pretreatment of the raw RHA had an effect on the crystallinity and morphology of silica, thereby altering the adsorption reactivity. The obtained RHA-Li4SiO4 sorbent was further modified by several metallic elements (K, Ca, Al, Mg) to capture CO2
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) (51576085) and the Fund from Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180507184519927).
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