Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents

Abstract

The steam gasification of biomass, in the presence of a calcium oxide (CaO) sorbent for carbon dioxide ( CO 2 ) capture, is a promising pathway for the renewable and sustainable production of hydrogen ( H 2 ) . In this work, we demonstrate the potential of using a CaO sorbent to enhance hydrogen output from biomass gasifiers. In addition, we show that CaO materials are the most suitable sorbents reported in the literature for in situ CO 2 capture. A further advantage of the coupled gasification- CO 2 capture process is the production of a concentrated stream of CO 2 as a byproduct. The integration of CO 2 sequestration technology with H 2 production from biomass could potentially result in the net removal of CO 2 from the atmosphere. Maximum experimental H 2 concentrations reported for the steam gasification of biomass, without CO 2 capture, range between 40%-vol and 50%-vol. When CaO is used to remove CO 2 from the product gas, as soon as it is formed, we predict an increase in the H 2 concentrations from 40%-vol to 80%-vol (dry basis), based on thermodynamic modelling and previously published data. We examine the effect of key variables, with a specific focus on obtaining fundamental data relevant to the design and scale-up of novel biomass reactors. These include: (i) reaction temperature, (ii) pressure, (iii) steam-to-biomass ratio, (iv) residence time, and (v) CO 2 sorbent loading. We report on operational challenges related to in situ CO 2 capture using CaO-based sorbents. These include: (i) sorbent durability, (ii) limits to the maximum achievable conversion and (iii) decay in reactivity through multiple capture and release cycles. Strategies for enhancing the multicycle reactivity of CaO are reviewed, including: (i) optimized calcination conditions, and (ii) sorbent hydration procedures for reactivation of spent CaO. However, no CaO-based CO 2 sorbent, with demonstrated high reactivity, maintained through multiple CO 2 capture and release cycles, has been identified in the literature. Thus, we argue that the development of a CO 2 sorbent, which is resistant to physical deterioration and maintains high chemical reactivity through multiple CO 2 capture and release cycles, is the limiting step in the scale-up and commercial operation of the coupled gasification- CO 2 capture process.