BCIA’s annual program of postgraduate research scholarships is part of our commitment to strategic investment in skills development to secure the scientific, engineering and trades expertise required for the development of new low-emissions brown coal technologies.

To date, BCIA has awarded six research scholarships to PhD candidates at top-ranking Australian universities.

In this edition of Perspectives on Brown Coal we will hear from BCIA scholarship recipient, Hirra Azher from Melbourne University.

Water Recovery from Brown Coal using Membrane Separation Processes

Coal fired power stations have high water requirements, and extraction of this water from river systems can lead to environmental stresses, particularly in the drought conditions which are all too common in Australia. Victorian brown coal has a very high water content – so what are the opportunities to recover useful water from the flue gas? Hirra Azher updates us on the progress of her PhD work below.

By Hirra Azher, Melbourne University PhD Candidate and BCIA Scholarship Awardee

Conventional coal-fired power plants produce large amounts of flue gases containing CO2, N2, O2, water vapour, as well as small amounts of nitric oxides (NOx), sulphur dioxides (SOX) and fly ash. The amounts of these components depend on a number of things including coal grade.

Australian brown coal is known to have a low sulphur content compared to other coals globally. The SO2 emissions are dispersed via the stack with no need for a flue gas desulfurisation (FGD) unit hence the flue gases are emitted at a high temperature. This offers the potential to recover water as low pressure steam.

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Fig 1. Schematic diagram of a coal-fired power plant with a membrane unit to recover and recycle water vapour.

My research focuses on investigating membranes with the potential to recover water from high temperature brown coal flue gases to complement the Australian coal-fired power plants. As well as water, the CO2 permeation properties of membranes will also be investigated. The presence of CO2 in the recycled boiler feed water must be avoided, as CO2 will make it acidic and cause corrosion issues in the boiler.

So far, an experimental rig (see Fig 2 and photos below) has been custom designed and built for the purpose of measuring steam and CO2 permeability of a Nafion membrane at high temperature.

Nafion is a polymer that has been extensively researched for Polymer Electrolyte Membrane fuel cell application. It shows good water uptake and transport in the fuel cell which also makes it promising for flue gas dehydration purposes.

The effects of temperature and feed water content on water and CO2 permeability through Nafion 115 have been studied. It has been found that CO2 permeability through the membrane is considerably lower than the water permeability (as desired) however both permeabilities increase with increased feed water content.

Permeability is a function of diffusion and solubility of the component. At high water concentrations, the diffusion of water through the membrane increases resulting in increased permeability.

Both water and CO2 permeability decrease with increasing temperature and this is due to the reduced solubility of the components at high temperatures. This was validated with sorption work done on Nafion on a different rig and the experimental results were modelled to gain a better understanding of effect of temperature and feed water content on membrane permeation properties.

Future work includes publication of the Nafion work as well as investigation of high temperature water and CO2 permeation properties of Sulphonated Polyether ether ketone (SPEEK); a polymer that is being extensively researched by the University of Twente group for low temperature flue gas dehydration purposes. This work will provide a fundamental understanding of the type of polymeric membranes that have the potential to recover high purity, low pressure steam from high temperature brown coal flue gases which can be recycled back as boiler feed water and have potential for heat recovery.

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Fig 2. Schematic of High Temperature Water Permeation Experimental Rig.

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Above: High temperature water permeation experimental rig from the outside
Right: A side view of the membrane cell inside the experimental rig.

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