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Combustion group

Research Group Activities

The Research Group has three major, interrelated, strands of activity:

1)    Innovative combustion and gasification of fossil and renewable fuels in novel power cycles capable of capturing their carbon content as a pure stream of CO2 suitable for sequestration.

The focus is primarily on fluidised bed reactors, since they are important in novel cycles using fossil fuels for power generation, designed to capture and sequester the CO2. Fluidised combustors are also important in power generation using biomass fuels, since they give advantages over other combustors in (i) tolerance to changes in fuel, (ii) the catalysis of reactions or capture of pollutants by the bed material, (iii) high heat transfer, and (iv) economic operation at relatively small scales. A particular interest is Chemical Looping (CL). In basic CL, gaseous fuel is oxidised by a solid metal oxide (or oxygen carrier), MeO, in one reactor. The exit gas yields almost pure CO2 after the steam is condensed. The reduced metal oxide, Me, is transferred to an oxidation reactor and regenerated. Thus, the fuel is combusted, but the CO2 is separated from the nitrogen in air. CL with solid fuels is more difficult because fuel and carrier are not easy to separate during carrier regeneration. Ensuring durability of the MeO over many redox cycles is an important problem being studied by the Group. We also work on the use of iron oxides to produce clean hydrogen from biomass, uncontaminated with COx and suitable for fuel cells. Multi-institutional, UK-China collaborations, elucidating the materials-science of looping-materials, are used to relate process performance to structure.

Gasifying coal to CO and H2 allows the enhanced watergas-shift (WGS) process, using solid CaO to remove the carbon content as CaCO3, to produce hydrogen for fuel. Subsequent heating of CaCO3 releases the CO2 for sequestration. The absorption capacity of CaO from natural minerals falls markedly with number of forward and reverse steps, but we have discovered ways of reducing this fall. The Group has also created synthetic sorbents which decay in capacity much less than natural sorbents.

Other research has examined (i) the kinetics of fuel devolatilisation, (ii) theory and experiment on mass transfer around combusting, or gasifying particles in fluidised beds of oxygen carrier, (iii) the reaction between SO2 and calcium-based sorbents and (iv) attrition and break-up of oxygen carriers in fluidised beds

2)    Physics of granular flows. Processes in (1) depend on efficient gas-solid reactions and solids flow. A primary interest is fluidisation, investigated using innovative combinations of new (e.g. Magnetic Resonance Imaging, MRI) and existing (e.g. Particle Image Velocimetry, Electrical Capacitance Tomography) experimental techniques, underpinned by theoretical approaches (e.g. Discrete Element Modelling, DEM).

Granular systems are visually opaque, making observations difficult inside three-dimensional (3D) systems. Accordingly, most previous experimental studies have been restricted to so-called two-dimensional (2D) fluidised beds, but the fluid dynamics are influenced by the walls and the findings cannot be confidently generalised to 3D beds. Until recently, only two experimental techniques have existed for the investigation of opaque, two-phase granular systems in 3D; X-radiography and electrical capacitance tomography (ECT), both of which present significant problems. In collaboration with Prof. Gladden, we have developed magnetic resonance (MR) imaging to observe fluidised beds internally. The significant advantage of MR over other techniques is that it can image the distribution of solids as well as their velocities and can image the gas. Recent research has shown the unique capability of ultra-fast (viz. acquisition times of 1-25 ms) MR measurements to give detailed, quantitative information on complex phenomena occurring in 3D gas-fluidised beds. As a result, a number of areas of long-standing controversy have finally been settled. For example, on whether gas issues from the holes in the distributor as a stream of bubbles or as a permanent jet. This is important, practically, because the distributor region can markedly affect overall performance in industrial units. For scale-up, we have conducted the first comparison of MRI with Electrical Capacitance Tomography, ECT (with Prof. L-S. Fan, Ohio State) on fast-fluidisation. We have also cross-validated MRI with X-ray imaging (Dr P. Lettieri, UCL) and also with PEPT (Dr D. Parker, Birmingham U.). For fluidised beds, vibrating beds and other granular flow reactors, Discrete Element Modelling (DEM) has been used and involves coupling the equations of motion of each individual particle with the equations describing the fluid mechanics of the gas and solving the whole simultaneously for incremental steps in time, allowing for the mechanics of deformation of the particles as they collide. The purpose is to understand, e.g. the movement of reacting fuel particles, which is only possible with a realistic description of both gas and solid phases present. A very important facet is to validate the theoretical predictions with experimental results from MR and PIV, an aspect neglected by other modellers because of the inaccessibility of 3D systems to direct observation. An important achievement has been to validate our DEM (in both cartesian and cylindrical geometries) with MR.

3)    Sustainable transport biofuels. This embraces reaction engineering studies of the Fischer-Tropsch (FT) synthesis and methanation using CO and CO2. There is also research on using chemical looping materials for selective oxidation reactions, e.g. oxidative dehydrogenation.

The research is largely concerned with understanding the fundamental interactions between mass transfer and intrinsic chemical kinetics, particularly as liquid are formed in FT. The research is aided by use of apparatus such as a spinning-basket reactor, run in batch mode, and a purpose-build packed bed reactor equipped with a recycle to simulate the superficial velocities obtained in large-scale units.

4)     Sustainability. We undertake system-level, LCA studies of, e.g. proposed routes for converting biomass to fuel or the impact of switching from a fossil-derived feedstock to a biomass-derived one.

Life-cycle analyses of biofuels have established important benchmarks, e.g. algal growth using closed reactors is unlikely ever to be carbon-neutral, and has studied the impact lignocellulosic ethanol production in the EU. We also interact closely with the Foreseer Project (Dr J. Allwood, which works on the interaction between land use, water use and energy demand on a regional and country basis.

We publish regularly in these fields, including attendance at major international conferences, and have significant links with researchers around the world.

Note to Applicants

  1. Any vacancies for post-doctoral positions in the Group will be advertised on the Vacancies page
  2. We do NOT offer undergraduate internships.
  3. Applicants for a PhD will require a good, First-Class (or equivalent), 4-year degree in chemical engineering, and will need to demonstrate excellent proficiency in: transport processes, mathematics, reaction engineering and thermodynamics.
  4. All internal applicants for a PhD (whether undergraduate or MPhil), must, in addition, gain > 70% in the Part IIB Course: Advanced Transport Processes.
  5. Very high standards of written and spoken English are required. Short-listed applicants will be interviewed.