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

The steam gasification of biomass, in the presence of a calcium oxide (CaO) sorbent for carbon dioxide (CO2) 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 CO2 capture. A further advantage of the coupled gasification-CO2 capture process is the production of a concentrated stream of CO2 as a byproduct. The integration of CO2 sequestration technology with H-2 production from biomass could potentially result in the net removal of CO2 from the atmosphere. Maximum experimental H-2 concentrations reported for the steam gasification of biomass, without CO2 capture, range between 40%-vol and 50%-vol. When CaO is used to remove CO2 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) CO2 sorbent loading. We report on operational challenges related to in situ CO2 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 CO2 sorbent, with demonstrated high reactivity, maintained through multiple CO2 capture and release cycles, has been identified in the literature. Thus, we argue that the development of a CO2 sorbent, which is resistant to physical deterioration and maintains high chemical reactivity through multiple CO2 capture and release cycles, is the limiting step in the scale-up and commercial operation of the Coupled gasification-CO2 capture process. (C) 2007 Elsevier Ltd. All rights reserved.