Continuous high-purity hydrogen production by sorption enhanced steam reforming of ethanol over modified lithium silicate

Abstract Sorption-enhanced steam reforming of ethanol (SE-SRE) with in-situ CO2 removal is an environmentally friendly and sustainable approach for hydrogen production. Researches on continuous production of high-purity H2 by SE-SRE over the modified Li4SiO4 sorbent were conducted using two parallel reactor in this work. The low cost Li4SiO4 derived from rice husk ash (RHA) is a promising high-temperature CO2 sorbent. However, the poor adsorption kinetics of RHA-Li4SiO4 sorbent at low CO2 concentration is the major challenge. The metallic elements (K, Ca, Al, Mg) were employed to modify the RHA-Li4SiO4 for efficient CO2 capture. The developed sorbents were characterized and tested to study the role of dopants on the crystal, textural, microstructure and CO2 adsorption kinetics and cyclic stability. Results indicated that K doping effectively inhibited the growth of crystal aggregation and resulted in a fluffy morphology with abundant pores and higher specific surface area, while the addition of Ca, Al and Mg formed a nubby structure with larger particle size. K-doped RHA-Li4SiO4 exhibited the best CO2 uptake properties and the optimal K doping molar content was 0.02 with the maximum capture capacity of 34.16 wt%, which is higher than 27.1 wt% of pure RHA-Li4SiO4. Then, the effect of operating conditions on the enhancement behaviors was considered in the SE-SRE system. High-purity H2 (above 96%) was achieved by coupling K(0.02)/RHA-Li4SiO4 sorbent with Ni-based catalyst under the optimum condition (T: 525 °C, liquid hourly space velocity: 0.9 mL/(g·h), sorbent/catalyst: 4 and steam/carbon: 8.0). The adsorption activity of K(0.02)/RHA-Li4SiO4 maintained at a high level in ten SE-SRE/regeneration cycles. Finally, a scheme including two parallel fixed-bed reactors was designed and operated periodically for continuous production of high-purity H2. The reaction switching time was shown to depend strongly on the pre-breakthrough time and operating conditions. As the reaction switching time was 40 min, the products were always only H2 and CH4 (no CO and CO2 appear) and the H2 purity remained above 90% during 400 min, confirming high purity hydrogen stream can be obtained continuously.