New Approach to Extend Durability of Sorbent Powders for Multicycle High Temperature CO2 Capture in Hydrogen

Research into solid adsorbents for CO2 is motivated by their potential advantages over liquid amine, membrane or cryogenic separation techniques in mid-high temperature CO2 separation, for example, in hydrogen production via steam reforming/gasification of waste biomass where production yields are increased through the use of a sorbent powder such as CaO that chemically binds the CO2 from the mixed product stream and shifts the reaction thermodynamics to increase hydrogen output. There are also applications in large scale CO2 capture involving integration with fossil fuel fired power stations, and other industries.

This materials engineering based proposal addresses the major problem facing utilisation of powder sorbents such as CaO for high temperature applications, including hydrogen production by sorbent enhanced steam reforming (SESR) of waste biomass. A decay in CO2 capture performance due to changes in the structure of the powder bed (densification) during regeneration at high temperatures prevents full exploitation of this promising technology in SESR and large scale CO2 capture applications.

Significant powder densification occurs after heat-treatments at > 800 C to release CO2 and regenerate the sorbent. This leads to loss of porosity and sorbent surface area, causing a serious decay in CO2 capture performance. Developments in recent years, for example, adding refractory spacer particles are only successful for non-optimal regeneration conditions (e.g. < 850 C in inert atmospheres). The powders to be developed in this 18 month feasibility study will exploit a novel means of counteracting densification and loss of surface area, aiming to achieve regeneration at 950 C (much higher than for existing sorbents) in atmospheric conditions without significant decay in CO2 sorption capacity. An important advantage of the new powders is that a near-pure CO2 stream will be generated during regeneration at 950 C, producing output streams suited to integration with CO2 storage and/or utilisation programmes; this contrasts to the mixed gas streams generated at lower temperatures using existing materials.

The new approach to the durability problem is to disperse ultrafine particles of partially stabilised zirconia (PSZ) in the sorbent matrix. The PSZ particles undergo a phase transition on cooling after regeneration which results in an increase in particle (crystallite) volume. Resulting strains generated in the surrounding, partially sintered, sorbent matrix will cause microcracks and secondary strain fields to develop which will open up pore channels for ingress of gasses. Loss of CO2 capture capacity in the subsequent sorption step will thus be mitigated, even for technologically favoured high regeneration temperatures (950 C), leading to increased multi-cycle sorbent efficiency, and increased hydrogen yield in SESR. The anti-densification mechanism will also be evaluated for an alternative CO2 sorbent, Na2ZrO3.