Development of a commercially viable carbon capture and sequestration (CCS) technology for fossil fuel power generation is vital if the anticipated effects of global warning are to be avoided. Chemical-looping combustion (CLC) is an indirect combustion process that utilises a regenerable solid oxygen sorbent (oxygen carrier, OC), typically a metal oxide, to transfer oxygen from the combustion air to the fuel such that direct contact between air and fuel is avoided. CLC is a variant on an oxy-fuel carbon capture system that offers the potential for a much lower energy penalty as CO2 separation is achieved intrinsically such that additional energy-intensive gas separation steps are avoided. Our research focuses on the development and optimisation of OCs for CLC systems using biomass and biomass derived fuels. The development of a CLC process utilising biomass is of particular interest as it has the potential to result in negative CO2 emissions i.e. a net removal of CO2 from the atmosphere.
Thermochemical conversion of biomass typically results in the formation of significant quantities of refractory tar compounds which are difficult to combust and can lead to reduced fuel conversion efficiencies. Decomposition of the tars on the surface of the OC can result in severe coking and temporary deactivation. Coking of the OC also limits the overall CO2 capture efficiency of the process as regeneration of the OC in air produces CO2 which cannot be captured.
This thesis documents the progress made towards the development of a robust laboratory based system for testing the effects of biomass tars on the long term performance of a chemical-looping combustion process. The work completed in this thesis can be divided into two main areas: the first involved developing optimised fabrication strategies for the production of inexpensive iron-based oxygen carrier particles of high reactivity and robust physical characteristics that could be used in CLC systems utilising biomass as the fuel. The second research focus involved the development of a reactor and analysis protocol for studying the interactions between biomass pyrolysis tars and the cheap, synthetic iron-based oxygen carrier materials.
A range of pure iron oxide and iron oxide supported with 40 wt.% Al2O3 oxygen carrier materials were prepared via simple scalable fabrication techniques based on wet granulation for use in CLC systems utilising biomass or gasified biomass as a fuel. The oxygen carrier particles were subjected to rigorous testing using a range of analytical methods to assess their physical and chemical properties and suitability for use in large-scale systems. The effect of fabrication method and alumina precursor material used for producing the supported iron oxide materials were found to have a considerable effect on the physical characteristics and reactivity of the oxygen carrier material.
The reduction kinetics (the rate limiting step in the CLC of gaseous fuels) of the different OC materials prepared in this work were assessed using a thermogravimetric analyser (TGA). A simple particle model based on the concept of effectiveness factor was applied to determine the intrinsic kinetic information. Preparation of the Al2O3 supported iron oxide oxygen carrier material using a Al(OH)3 alumina precursor gave the most porous oxygen carrier material with the highest surface area. This oxygen carrier was also the most reactive particularly at temperatures above 973 K and demonstrated very good thermal stability at temperatures up to 1173 K. The activation energy of the oxygen carrier was found to increase from 73 kJ mol-1 for the temperature range 823-1073 K to 123 kJ mol-1 at temperatures of 1073-1173 K. The increase in the activation energy was attributed to further conversion of Fe3O4 to FeAl2O4 which was more pronounced at the higher temperature range. Here we propose that the formation of FeAl2O4 was beneficial, acting to enhance the thermal stability, reactivity and oxygen transfer capacity of the iron oxide based oxygen carrier material.
A new 500W laboratory-scale, two-stage fixed-bed reactor for simulating CLC with ex situ solid fuel gasification has been designed and constructed. Preliminary studies of the interactions between OC materials consisting of pure iron oxide and 60 wt.% Fe2O3 iron oxide supported on Al2O3 and a gas stream produced from the pyrolysis of biomass to emulate a fuel gas containing large quantities of tars were carried out. The presence of both OC materials at 973 K was found to significantly reduce the amount of biomass tars by up to 71 wt.% in the case of the 60 wt.% Fe2O3/40 wt.% Al2O3 OC material compared with analogous experiments in which the biomass tars were exposed to an inert bed of sand. Exposing the pyrolysis vapours to the oxygen carriers in their oxidised form favoured the production of CO2. The production of CO was favoured when the oxygen carriers were in their reduced forms. Both oxygen carrier materials were affected by carbon deposition. Carbon deposition was removed in the subsequent oxidation phase with no obvious deleterious effects on the reactivity of the oxygen carrier materials after exposure to the pyrolysis gases and vapours.