The aluminium smelting process requires
large amounts of electrical energy. Typically, 13 to 15 megawatt
hours (DC) is required to produce one metric tonne of hot, molten
aluminium. Being energy intensive, most of the world's smelters are
located in areas where there are abundant energy resources -
whether fossil fuels (natural gas, coal and oil), nuclear fuels or
hydro-electric power. Many primary aluminium smelters have their
own power generation plants to ensure an uninterrupted power
supply.
The burning of fossil fuels typically emits carbon dioxide
(CO2), carbon monoxide (CO), sulphur dioxide
(SO2), oxides of nitrogen (NOx) and
particulate matter (most commonly respirable particulates,
PM10). Other emissions, like ozone and volatile organic
compounds, tend to be less significant.
Power generation at DUBAL
DUBAL's location in the Middle East ensures an abundant source
of energy, primarily from natural oil and gas. Our smelter complex
includes a captive combined cycle power plant with a power
generation capacity of 2,350 megawatts (at 30°C), making our
operations self-sufficient in terms of energy requirements.
Through combustion, chemical energy in fuel (either natural gas
or distillate) is converted into heat energy. This passes through a
gas turbine where it is converted to mechanical energy. This
mechanical energy is then converted to electrical energy using
generators. The resulting alternating current (AC) is converted
into direct current (DC) by rectifier transformers, for use in the
aluminium reduction process.
Currently, our power plant has 23 gas turbines and seven steam
turbines; and supplies on-site power for DUBAL as well as
externally to nearby customers. The gas turbines have heat recovery
steam generators that produce the steam, which further provides
fuel-free energy to generate power from our seven steam turbines.
Further utilisation of waste steam after two of the steam turbines
is used by our desalination
plant's evaporator units.
Save in the event of shortages or emergencies, the DUBAL power
plant is operated on natural gas (the 'cleanest' type of fuel),
which contributes to reduced air emissions. We also deploy the best
available technologies to further reduce air emissions during the
power production process. For instance, our gas turbines are also
fitted with dry, low nitrogen oxide (NOx) burners, which
allow the gas to be burnt at a lower temperature, resulting in
lower NOx emissions. Between 2007 and 2009, the volume
of DUBAL's NOx emissions had declined by 27 per
cent.
In addition, three of our gas turbines have water injections
fitted for liquid fuel operation, to reduce NOx
emissions.
Emissions of ozone-depleting substances through our power
generation activities include minor amounts of refrigerant gases
(commonly chlorofluorcarbons or CFCs). Over the years, we have
taken several actions to eliminate our ozone-depleting emissions.
As a result, the total CFC-11 equivalent emitted was limited to
0.12 metric tonnes in 2009.
Water desalination at
DUBAL
The power plant at DUBAL produces an enormous amount of hot
combustion exhaust gases, which we harness as an energy source to
drive our thermal water desalination plant, where up to 30 million
imperial gallons (140 Mℓ) of fresh water can be produced per day.
This fresh water is used for our on-site needs, with the excess
being sold to a number of external customers, including the city of
Dubai via Dubai Electricity & Water Authority (DEWA).
The DUBAL desalination plant makes fresh water from seawater
using six Multi-Stage Flash (MSF) evaporators, each consisting of
19 stages and capable of producing
5 million gallons per day. Seawater/brine is heated to a
temperature of 90ºC to 110ºC, using steam from the power plant. The
hot brine then enters the flash chambers, which are under vacuum.
The high temperature and vacuum causes some of the brine to
vaporise (flash). The vapours are then re-condensed into distilled
water via condenser tubes, through which cold seawater is
passed.
Some of the distilled water collected from each stage is
retained for industrial use (either by DUBAL or external
customers). The balance of the distilled water is pumped to a
blending plant where chemicals are added to make it suitable for
drinking. The blended water is stored in a reservoir, from where it
is pumped to customers, either directly or via tankers filled at
the DUBAL Water Filling Station.