Page 329 - Advanced thermodynamics for engineers

318 CHAPTER 14 CHEMICAL KINETICS

The amount of carbon monoxide reduces as the mixture gets leaner, until misfire occurs in the com-
bustion device.
Sulfur dioxide (SO 2 ) is formed by oxidation of the sulfur in the fuel.
Carbon particulates are formed during diffusion combustion because the oxygen in the air is not
able to reach all the carbon particles formed during the pyrolysis process while they are at a suitable
condition to burn.

14.6 THE EFFECT OF POLLUTANTS FORMED THROUGH CHEMICAL
KINETICS
14.6.1 PHOTOCHEMICAL SMOG
Photochemical smogs are formed by the action of sunlight on oxides of nitrogen, and the subsequent
reactions with hydrocarbons. Photochemical smogs were first identified in Los Angeles in the mid-
1940s, and brought about the stringent legislation introduced in California to control the emissions
from automobiles. These smogs act as bronchial irritants and can also irritate the eyes.
Among the pollutants involved in photochemical smogs are ozone, nitrogen dioxide and perox-
yacyl nitrate (PAN). The nitrogen dioxide, and other oxides of nitrogen, are primary pollutants pro-
duced by dissociation in combustion reactions, and both ‘prompt’ and ‘thermal’ NO x can be involved
in the reactions. Ozone and PAN are secondary pollutants produced by the action of sunlight on the
primary pollutants and the atmosphere. Glassman (1986) describes in some detail the chemical chain
that results in photochemical smog.
Oxides of nitrogen can be measured using non-dispersive infra-red equipment and the values are
3
often quoted in parts per million (ppm), g/m (at normal temperature and pressure), g/kWh or g/mile (if
applied to a vehicle engine).

14.6.2 SULFUR DIOXIDE EMISSIONS
Sulfur emissions can only occur if there is sulfur in the fuel. The levels of sulfur in diesel fuel and
petrol are being reduced at the refinery in the developed world, and the sulfur levels are often around
0.5% or lower. Sulfur dioxide (SO x ) emissions from vehicles are not a major problem. In the devel-
oping world, the levels of sulfur might be significantly higher and then the SO x emissions from ve-
hicles become a more serious problem. A further advantage of reducing the sulfur content of diesel
fuels is that this reduces the synergy between the sulfur and the particulates, and reduces the quantity
of particulates produced. The levels of sulfur dioxide produced in some power plant can be extremely
high. Diesel engines running on ‘heavy fuel’, which has a high percentage of sulfur, will produce
significant quantities of sulfur dioxide. Power stations running on heavy fuels or some coals will also
produce large quantities of sulfur fumes, which smell noxious, cause corrosion and form acid rain.
Many coal-burning power stations are now being fitted with flue gas desulfurisation, a process by
which the sulfur dioxide reacts with limestone to form gypsum. These processes are suitable for
stationary plant, but do consume copious amounts of limestone.
Sulfur in fuel has been a problem for a long time and caused high levels of acid rain in the UK up
till recently. The move away from coal to natural gas for electricity generation, and the imposition of
smokeless zones in urban areas have both reduced the problem to a more acceptable level. Sulfur can
be removed from crude oil by catalytic hydrodesulfurisation, but is often left in the residual oils which