Page 330 - Advanced thermodynamics for engineers

14.6 THE EFFECT OF POLLUTANTS FORMED THROUGH CHEMICAL KINETICS 319

are used for marine propulsion, power generation, etc. The use of orimulsion, a fuel obtained by water
extraction of low-grade oils, also brings worries of high sulfur fuels.
Sulfur in the fuel tends to be oxidised into SO 2 during the combustion process. This can then form
sulfurous (H 2 SO 3 ) and sulfuric (H 2 SO 4 ) acids, which have corrosive effects on exhaust stacks,
chimneys, etc., before being released into the surrounding atmosphere to form acid rain. It is possible
to use flue gas desulfurisation to reduce the levels of SO x emitted to the atmosphere, but such systems
are expensive to install and run. It has been stated that if all the SO 2 emitted from the UK power
stations were converted to sulfuric acid the supply of this acid would exceed the UK demand.

14.6.3 PARTICULATE EMISSIONS
These are generated mainly by devices which rely on diffusion burning (see Chapter 15), in which a
fuel rich zone can produce carbon particles prior to subsequent oxidation. Hence, particulate emissions
tend to come from diesel engines, gas turbines and boiler plant – the particulates generated in gasoline
direct injection engines are also becoming a cause for concern. The carbon formed during combustion
can be an asset in a boiler because it increases the radiant heat transfer between the flame and the tubes
of the boiler: however, it is necessary to ensure that the soot emissions from the stack are not excessive.
The carbon formed in diesel engine and gas turbine combustion is not beneficial and it increases the
heat transfer to the components, increasing the need to provide cooling. In the past, particulates were
grouped under the broad term soot, which was measured by trapping dry soot on a filter paper to give a
Bosch smoke number, assessed by the ‘reflectance’ of a filter paper on which the soot has been
collected. The words particulates and soot were often used synonymously, but there is a difference in
nature between these emissions. Stone (2010) refers to Eastwood (2008) and explains that particulates
contain more than simply the dry soot, they often contain polycyclic aromatic hydrocarbons (PAH)
that have condensed. The PAH compounds have a tendency to be carcinogenic. While old, or poorly
maintained engines used to produce visible black smoke, modern engines produce particles much
smaller than 100 nm: these are invisible because they are close to the wavelength of light. While such
particles are very small they can be injurious to the lungs: while larger particles (>100 nm) get trapped
in the nasal passages the smaller ones (x10 nm) can be trapped deep in the lungs. The level of par-
ticulates is strongly affected by the amount of sulfur in the fuel, and increases with sulfur content.
Particulates are measured by trapping the particles on glass-fibre filter papers placed in a dilution
tunnel, and then weighing the quantity; they might be quoted in g/kWh.

14.6.4 GREENHOUSE EFFECT
The greenhouse effect is the name given to the tendency for the carbon dioxide (CO 2 ) in the atmo-
sphere to permit the passage of visible (short wavelength) light, while absorbing the long wavelength
infra-red transmission from objects on Earth. The effect of this is to cause the temperature of the Earth
to increase. The evidence for global warming is not conclusive because the temperature of the Earth
has never been constant, as shown by the Ice Ages of the past. There have been significant variations in
temperature through the last few centuries, with warm summers at times and the freezing over of the
Thames at others. It will take some time to prove conclusively whether global warming is occurring.
What is indisputable is that mankind has released a large amount of carbon from its repository in
hydrocarbons back into the atmosphere as CO 2 . The most convenient form of energy available to man