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20 November 2009
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I don't want to go into the political aspects of whether Cyprus should
have either an onshore or an offshore regasification scheme for liquid
natural gas or compressed natural gas, but I will touch on the technical
aspects. My view is that Cyprus should not use natural gas at all for
electricity generation and I'll explain why.
It is claimed that natural gas is the least polluting fossil fuel. If you
count only the gas, as it is delivered to the burner or the turbine, this is
true. Nevertheless, for every kilogram of gas burnt in a boiler or a
turbine, 2.74 kg of carbon dioxide are emitted into the atmosphere - yes,
very nearly 2¾ kg. This translates to 0.20 kg of CO2/kWh
generated in an average power station. This compares with about 0.28 kg for
the currently used fuel oil, depending on the quality, and 0.34 kg for coal.
Unfortunately, this comparison is unfair, because the natural gas itself is
a bad greenhouse gas, 20-50 times worse than CO2. The crunch is
that the extraction, purification, liquefaction, transport and
regasification all create emissions of methane, so that, if you count the CO2
equivalence from the wellhead to the power station, the emissions may be
worse than even coal-burning and certainly worse than oil. For a full
technical explanation, with typical figures,
please consult
here.
Production of natural gas
To have natural gas available at a Cyprus power station, there is a
series of operations. When reading this, please remember that each time
there is an escape of gas, the greenhouse gas in the atmosphere balance
sheet becomes more positive (i.e., climate change becomes more pronounced).
We can take, as an example, Qatar as our starting point, as this is one of
the nearest production units. The first thing is that a well is drilled. As
the borer reaches a "pocket" of gas under considerable pressure, large
quantities will escape until the well can be cased and "Christmas treed".
The "Christmas tree" is a complex pipework system designed to crudely
separate the gas from water, oil and solid matter such as sand. It is
regularly purged until only gas escapes. After this, as a general rule, many
such wells are drilled within a radius of a few 100 m and a web of low
diameter pipes connect the Christmas trees to a purification plant. These
pipes are jointed and leaks do occur, along with small leaks from around the
casing.
After purification, which also causes emissions in normal exploitation,
the natural gas is typically 99% pure methane. It is then compressed and
sent via a pipeline to a liquefaction plant where it is converted into
liquid natural gas (LNG) at -163°C and close to atmospheric pressure. The
compression, transport and liquefaction all entail considerable energy
consumption (more CO2 emissions) and fugitive emissions of gas.
The LNG is then stored at low pressure in special double-walled,
nickel-steel tanks with vents to prevent a build-up of pressure. The LNG is
pumped into tankers with similar tanks. The action of connecting and
disconnecting the tanks also involves escapes of gas.
Up to recently, the boil-off from the tanks was used to propel the tanker
to its destination. This is no longer possible because the improved tank
insulation means less boil-off and the modern ships burn fuel oil (emitting
more CO2!). The boil-off is possibly used to drive the
ship's generators. However, the question arises as to what happens when the
tanker's fuel consumption is reduced, such as in port or waiting in the
roads. As the tanks are low pressure types, the only solution is to vent the
gas to atmosphere.
At the ship's destination, the LNG is discharged into tanks, similar to
those used for the storage after liquefaction. From there, it is pumped into
a regasification plant which heats it until the required amount of gas is
produced to meet the demand. The regasification plant also treats boil-off
gas from the storage tanks and, in some cases, from the tanker. It sounds
simple, but the plant is, in reality, quite complex to ensure that supply
equals demand.
All stages involve fugitive leaks and energy consumption as explained
op. cit., so that the 2.74 kg CO2 per kg of gas easily can
reach 4 kg CO2equivalent or more, by the time it is burnt.
Advantage of LNG
There is one significant advantage of using LNG. If it is used in a small
gas turbine, it can be rapidly brought on line in the event of a sudden,
unforeseen, increase in demand, such as may be occasioned by an abrupt and
unexpected change in the weather or an historic event causing everyone to
rush to switch on their television. Conventional thermal plants have a
longer lag time in adjusting to changes.
If variable renewable energy becomes significant, then this rapid
response time becomes doubly advantageous if it suddenly clouds over or the
wind stops blowing.
Problems specific to Cyprus
Cyprus will be a small consumer of LNG, compared to the major ones. The
time taken for a tanker to discharge its cargo from the moment of stopping
in the roads to the time that it moves off again is little different if it
unloads 15,000 or 250,000 m3 of LNG, typically one day. It is
clear that it is not profitable for a large tanker, costing $300 million, to
stop to partially unload one-tenth or less of its cargo. This implies that
only small LNG-tankers can supply Cyprus. However, there is a problem here;
small tankers are not welcome at the loading quays because it is similarly
more profitable for the supertankers to load. This means that the small
tankers may have to wait in the roads for many days or weeks before there is
a gap between the larger vessels. This obviously means higher costs,
reflected in the price paid for the LNG, but it also means that a larger
strategic buffer stock is required on the island, as it is impossible to
allow a tank to be completely emptied, because any air drawn into a tank,
will form a potentially explosive mixture.
Another problem is the site for the regasification plant. Proposals have
been made to have either a fixed or a floating unit. The latter is more
expensive but theoretically can be completed sooner. I say 'theoretically'
because no one knows the lead time as it is untried technology or even
whether it can be done with the necessary safety features. However, it
occurs to me a floating unit in a known earthquake region seems very
hazardous, if seismic activity broke it free from its moorings. Also, it
would complicate the unloading process from the tanker, because the relative
movement from swell would be greater than with a land-based plant.
All-in-all, speaking as an engineer, I feel that a conventional land-based
plant would seem the better option.
Compressed natural gas
The transport and storage of compressed natural gas is much more
hazardous because of the pressures involved, sometimes up to 800 bar,
compared to no pressure for LNG. The advantage is that the temperature is
ambient. We all know that we have difficulty keeping a liquid (water) at
modest pressures of a few bars into a distribution system. Water is a lot
easier for leak prevention than CNG because it has a high surface tension,
while the gas, with tiny molecules, has none. In reality, it is difficult to
have a low pressure NG pipe system totally leak-free; it is virtually
impossible with a high pressure one. Each leak contributes to greenhouse gas
emissions and is a potential fire hazard.
Conclusion
I don't believe that LNG or CNG is a good option for power generation in
Cyprus. It is true that it will appear to reduce greenhouse
gas emissions by about 28 percent, within the island, to generate a given
quantity of electricity, compared to using fuel oil. The truth is that will
increase them but this will happen mostly outside Cyprus. This
is an ostrich-like attitude pretending it doesn't happen because it is not
in our own backyard.
So what is a good option for this country? To start with, a fixed
thermal power station
burning household rubbish would add nearly 10 percent to our capacity.
This is a tried, tested and successful technology, used in many places round
the world. It offers the additional advantage that it reduces landfill
requirements by 80-90 percent, the ashes also being sterile. The downside is
that the plant is about 30 percent more expensive in capital costs than a
conventional thermal station, although this is rapidly amortised by the fact
that the fuel is not only free, it is a source of revenue. The second
echelon is, of course,
variable renewables, such as wind and solar. The third echelon seems to
me to be the inevitable one, which will not be popular:
nuclear fission. If a latest generation EuroPR power station is
envisaged rapidly, it could be in service just in time for when the energy
demands of the island will require that 1.6 GW extra capacity, about 2015,
but it will require action now to prevent a penury of electricity. This will
keep prices down, as well.
Whatever options are taken, there is another factor to consider: we shall
require more
water. Without going into the causes of this, further desalination
plants will become increasingly necessary. These are very energy-intensive
and if the energy comes from fossil fuels, we shall never keep up with our
Kyoto Protocol and EU commitments. As I see it, nuclear power is the only
way we can keep up with our ever-increasing demand for good water.
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