above a source of primary pollutants. In such condition the air at
ground level is normally very still this causes the pollutants to get
trapped near to ground level. Also sunlight is needed because in
some of the reactions a photon of electromagnetic radiation is
needed to give the molecule the energy that it needs to break its
bonds*1. These reactions are know as photochemical reactions
and are represented by the symbol hv this is derived from the
formula E=hv where E is the energy (in this case needed to brake
the bond), h is planks constant and v is the frequency of the
photon absorbed.
v< 1 Hz
435 O2 + O O3
NO2 hv NO + O O3 + NO O2 + NO2
fig 2.
Fig 2 shows the reactions going on in relatively unpolluted areas
of the troposphere it shows that the reactions balance each other
out and that a stable background level of O3 is normally present.
O3 can be converted back in to O2 and O when a photon of
wavelength under or equal to 310nm is absorbed
O3 hv O2 + O*
The O* molecule formed in this reaction is of a very high energy
and will reacted with water to stabilise its self.
O* + H2O 2OH
However when the air becomes polluted with the common
primary pollutants the VOCs react with OH radicals to form water
and eventually the peroxy radical RCH2O2 where R is a
hydrocarbon chain.
RCH3 + OH RCH2 + H2O
RCH2 + O2 RCH2O2
The newly formed radical will readily react with NO to form NO2
RCH2O2 + NO RCH2O + NO2
this reaction uses up the NO used to ‘clear away’ the O3 fig2 and
produces more NO2 which creates O3 fig2. This causes a rise in
the concentration of ozone. Furthermore two OH radicals are
created from the decomposition of O3 which in turn causes two
more NO2 molecules, these react with light to make another two
ozone molecules. So this assumption can be drawn:
photochemical
O3 2O3
smog formation
therefore wherever the conditions are right for photochemical
smog to form the levels of O3 will double.
Longannet coal fired power plant like all other produces
sulphur dioxide and NOx. The amounts of these chemicals which
are produced must be limited because of the affect on the
atmosphere.
There are a few ways of reducing the production of SO2 but
the one used at Longannet is the sea water scrubbing method.
This method was chosen as the BPEO (best practical
environmental option) because of the availability of sea water and
there is no solid waste products to be tipped or sold. Also other
methods such as the limestone process need other materials
which need to be mined or quarried.
Fig 3.
The diagram shows the processes involved in the
desulphurisation of the flue gas. The pH of the sea water changes
from around 7.5 to 6 when the sulphur dioxide dissolves to
sulphite ions. The solution is then aerated to oxidise the sulphite
to a sulphate which can then be safely deposited into the ocean.
The NOx, emissions must also be kept to a minimum and the
BPEO for the reduction of NOx gases is a low NOx burner with gas
reburn facilities. A low NOx burner works by lowering the
temperature in the furnace so that nitrogen and oxygen in the air
are less likely to react, also the nitrates in the fuel are less likely to
oxidise due to the lower temperature and availability of oxygen.
This has to be controlled because if too little air gets in then there
will be incomplete combustion forming carbon monoxide and
giving out less energy making the plant less efficient.
As fig4 shows the mixture is now entered into the reburning
zone as it rises up the furnace here any NOx formed is reduced to
N2. This happens because an injection of natural gas the alkanes
react with the Nox to form safe products.
CH4(g) + 4NO(g) 2N2(g) + CO2(g) + 2H2O(g)
Any unburnt alkanes and CO molecules are fully oxidised in the
third and coolest part of the furnace. The combustion of the gas
here also adds to the heating of the water and aids the efficiency
of the whole plant because the reaction is exothermic. This was
chosen as the BPEO as it is the most environmentally friendly
choice.
Research into the formation of photochemical smog is an
important matter for all scientists at the moment. Chemists in
particular have to suggest reasons for the reactions that take
place and the speed at which they do so. For this reason several
area of study have arisen. These are mainly to find out what is in
the smog by monitoring the troposphere pollutants and by using
controlled reaction in lad condition or in large smog chambers.
Craig Davis 9033