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.

This materials and process engineering research project in the context of the hydrogen economy and reducing CO2 emissions is of economic and environmental benefit. Industrial partners may commercialise the new CO2 sorbent powders being researched, and hence become more competitive, bringing benefit to the UK economy. Environmental benefits of reduced carbon emissions and a cleaner process for energy production have impact on the general public. Within the project, the research assistant would gain new technical skills and develop their career through interaction with the industrial partners and professional dissemination activities, providing the basis for a career in the renewable energy sector. The inter-disciplinary research will provide examples of how advances in knowledge and understanding of solid sorbents can arise through academic collaborations between the materials and chemical engineering communities.

The hydrogen economy is a key component of the EU policy to reduce greenhouse gas emissions. Producing hydrogen in sufficient quantities to meet future demand in a sustainable manner is a major challenge in moving toward a hydrogen economy. The International Partnership for Hydrogen and Fuel Cells in the Economy highlighted that in order for Europe to meet its 2050 emissions targets (80 % CO2 reduction), fuel cell vehicles requiring hydrogen will be an essential element. Hydrogen can be produced by several methods. A clean and renewable method is to combine steam reforming and water gas shift reactions, utilising waste feed-stocks, such as cooking oil, pine waste and waste industrial products. Capture of CO2 from the product stream, H2+CO2, shifts the reaction equilibrium and improves hydrogen yield efficiency. This is usually achieved using CaO from limestone, as it absorbs CO2 effectively at the optimum temperatures required for steam reforming, ~ 600 C. For process optimisation, the carbonated sorbent should be capable of regeneration by heating to higher tempertures to release the captured CO2 without significant loss of sorption capacity.

Whilst steam reforming of biomass is a renewable source of energy, the CO2 released during sorbent regeneration must be captured for the process to be a truly 'green' energy. Thus it is important for the released CO2 from sorbent regeneration to be in a form that can be easily integrated with carbon capture and storage programmes. This requires high regeneration temperatures, ~ 950 C for CaCO3, to liberate a near-pure CO2 stream.

Unfortunately CaO degrades after mulitiple regeneration cycles at high temperatures due to loss of porosity and exposed surface area. Efforts to minimise this problem have focussed on adding ~ 20 wt% of refractory second-phase spacer particles. These strategies give improved durability at sub-optimal regeneration temperatures, below ~ 800 C, which produce a mixed gas output stream after regeneration. They are not well-suited to carbon capture and storage, as costly gas separation units would be required.

Our proposal to use partially stabilised zirconia in a novel manner to create microcracking and porosity after sorbent regeneration, if successful, will solve the problems of achieving high muticycle durability using kinetically and thermodynamically favoured high regeneration temperatures, ~ 950 C. Powders will be applicable to steam reforming of waste biomass and potentially for large-scale CO2 capture in fossil fuel based electricity generating plants, and other industries. Academic impact will arise from investigating the effects of the volume expanding tetragonal-monoclinic phase transition on cooling partally stabilised zirconia in untested ranges of microstructural porosity and chemical environments. The use of advanced transmission electron microscopy and spectroscopy to study sorbent carbonation-decarbonation reactions will also have impact.