Reaction Engineering.
Research of the Combustion Group is concerned with the sustainable
generation of energy by gasification and combustion in fluidised
beds, particularly novel techniques to combust or gasify solid fossil
fuels to capture the CO2 for sequestration. Fluidised beds
tolerate changes in the fuel, and operate economically at relatively
small scales. A particular interest is Chemical Looping (CL). In
basic CL, gaseous fuel is oxidised by a solid metal oxide, 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
theCO2 is separated from the nitrogen in air. Hitherto,
nothing has been published on CL with solid fuels directly, because
the fuel and reduced oxide cannot be separated during oxidation.
Dennis's group is the first to publish a suitable technique for this
(UK Patent Application 0605762.4). This is being extended to produce
clean hydrogen from biomass, with pure CO2 for
sequestration. A variant is particularly important because it allows
the upgrade of low quality syngas produced, e.g., by the air
gasification of biomass, to pure H2 of a suitable quality
for use in directly in current fuel cells. It would also allow
distributed production of H2 because its capital and
operating costs are unlikely to be as scale-dependent as is the
production of H2 from conventional steam reforming.
Another project involves coal being hydrogasified to CH4
in such a way that it removes CO2 from the gas and
maximises theyield of H2. A recent discovery is the
formulation of a durable, high-capacity sorbent for CO2,
capable of many cycles of sorption and desorption. This discovery is
contrary to the expectations of many other researchers, primarily
because they have used conventional approaches to this problem. There
is also research with Johnson Matthey examining the reactions between
NOx and soot at conditions relevant to the exhausts of
diesel engines.
Granular System. 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, an area in which we have a global lead. The significant advantage of MR over the other two techniques is that it can image the distribution of solids as well as their velocities and, potentially, 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. It is expected to extend these studies to larger beds by using complementary studies of MRI with either PEPT (in collaboration with University of Birmingham) or ECT (with Ohio State University).
Algae. The transport fuels area has received a recent significant boost by the interest of major oil companies in the use of algae to generate lipid, and in some cases hydrocarbons, suitable for use in fuel. The algae can either be grown autrophically (viz. photosynthesising using CO2 and light) or heterotrophically (viz. using sugars or waste streams as carbon source, in the dark) or using both modes. The Group is part of a major initiative (with direct funding from industry) involving also the Departments of Plant Sciences, Biochemistry and Engineering, and a major oil company, to scale-up and intensify algal production and downstream harvesting so that it is sustainable and economically competitive. This will require very close liaison between bioscientists and engineers and a radical re-think about reactor design and operation. In all of this work, we are alert to the need for critical thinking on sustainability, quantified using Life Cycle Analysis (LCA). Research on the scale-effect of biodiesel production in the UK using LCA and these techniques will be applied to algal production and in other areas. This is particularly exciting since it involves similar research at the University of Cape Town (UCT), with which there has been a strong undergraduate teaching link on sustainability since 2003.
Modelling and Other Techniques. For fluidised bed, vibrating beds and other granular flow reactors, Discrete Element Modelling (DEM) has been used (in collaboration with Dr. Scott, Engineering Dept.) 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. It is being extended to detailed reactions for the first time.