7.5 SKILLS

7.5 SKILLS

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SKILLS


Postgraduate Research Updates
Chemical Looping Combustion of Victorian Brown Coal
By Sharmen Rajendran, PhD Student in Energy, Fuels and Reaction Engineering Group, Department of Chemical Engineering, Monash University

Combustion of brown coal in an oxygen atmosphere can produce a pure stream of CO2 suitable for sequestration. In the second issue of Perspectives on Brown Coal we heard about a BCIA funded project looking at pilot scale oxy fuel combustion. In this issue, Sharmen Rajendran provides an overview of his BCIA-funded research into chemical looping, an alternate (and potentially lower cost) means of providing oxygen for combustion.

Victorian brown coal is an economical and important source of energy for the state of Victoria and accounts for approximately 85 per cent of all generated electricity. At the current rate of consumption, these brown coal reserves are expected to last for more than 500 years.

Victorian brown coal is an excellent fuel as it contains low levels of ash and is highly reactive. However, the use of this low energy density fuel has a significant drawback in that it generates high levels of greenhouse gases, particularly CO2, which are responsible for global warming.

There is therefore a strong incentive for the research and development of technologies which will allow for cleaner utilisation of Victorian brown coal. This can be accomplished in combination with Carbon Capture and Storage (CCS) technologies which aim to produce a concentrated stream of CO2 which can be sequestered, mostly geologically, to reduce the impact on the atmosphere.

It is possible to produce a concentrated stream of CO2 by firing the coal in an oxygen-rich atmosphere. In oxy-fuel combustion, oxygen is provided by an Air Separation Unit (ASU), however this has a high operating cost as it involves cryogenic separation of oxygen from air.

Chemical Looping Combustion (CLC) is an alternative way to provide oxygen for combustion, and shows potential to reduce the relative cost of oxy-combustion.

The CLC process has been significantly researched for use with gaseous fuels such as natural gas and synthesis gas and is a relatively simple process to implement, but investigation with solid fuels, coal in particular, was initiated only recently.

However, the vast majority of fossil fuels are in the form of coal, and the use of solid fuels in the CLC process brings about an extra level of complexity.


The operating principle of the CLC process is shown in Figure 1 below. The process utilises two reactors which are known as the Air Reactor (AR) and Fuel Reactor (FR), compared to a single reactor typically found in power plants. The most important facet of this technology is the oxygen carrier which is typically a transition element metal oxide such as NiO, CuO and Fe2O3.

The fuel is fed into the FR together with the oxygen carrier and is fluidized by gasification agents such as CO2 or steam. The CO2 or steam then reacts with the coal and forms gasification products consisting predominantly of CO and H2. These gases then react with the oxygen carrier causing it to be reduced and subsequently generate CO2 and H¬2O which then exit the FR. This flue gas stream is then cooled down to condense the steam to generate a concentrated stream of CO2 which is sequestration ready.

Once the oxygen carrier has been reduced, it enters the AR, fluidized by air, and undergoes oxidation to return to its initial higher oxidation state. The oxidized oxygen carriers are then recirculated into the FR for another reaction cycle.



Figure 1: Schematic representation of the CLC process


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