Recovering energy from fluegas

Alcoa of Australia (www.alcoa.com.au) has developed a process to recover sensible and latent heat from the fluegas (FG) produced by Bayer alumina calciners, and use the heat to evaporate Bayer spent liquor. Peter Hay, of Alcoa’s Technology Delivery Group, at Alcoa’s Kwinana Refinery, in Western Australia, says the recovered heat can also be applied to seawater desalination. Calcination consumes 25 to 40% of the refinery’s total energy input, and produces large quantities of FG that is generally vented to the atmosphere, says Hay.

Energy is recovered from the FG by progressive cooling. Only sensible heat is recovered between 165°C and the dew point. Below the dew point, the energy recovered is mainly the latent heat of the water condensed. The higher-grade, sensible heat is only about 10% of the practical recoverable energy, considering 50°C to be the heat sink practical limit. Water is recovered after the dew point is reached. Since considerable refining infrastructure is invested to collect and store fresh water — an essential raw material for the Bayer process — any process that can reduce fresh water usage has significant value to the refining operation, says Hays.

The basic components of the heat recovery process (flowsheet) are: fluegas quench to cool the FG to its dew point and to wet and discharge the fugitive dust in the FG; counter-current contacting tower to heat water and cool the FG; induced draft fan to overcome pressure losses in the FG circuit; indirect exhaust FG heater to ensure fluegas buoyancy and dispersion; falling film evaporator, to exchange heat between the heated water in the shell with the spent liquor film flowing inside the tubes thereby evaporating the spent liquor; and indirect heat exchanger, to condense and recover high quality condensate from the evaporated liquor.

For every metric ton of smelting-grade alumina produced, about 0.2 GJ of sensible heat is recovered from FG exit temperatures between 85 and 165°C, and about 0.6 m.t. of water is recovered from FG exit temperatures between 57 and 82°C. Hay and co-inventor Dean Ilievski, also of Alcoa’s Technology Delivery Group, have applied for a patent on the process.