Chemistry Reference
In-Depth Information
alkaline-earth content, is attacked, as is monumen-
tal iron and bronze. Organic materials such as paper,
leather and textiles, which retain water, absorb SO
2
causing the cellulose and proteins to dissolve in the
resulting acids.
Alteration of the combustion processes alone is
insufficient to control sulfur emission. Either pre-
treatment of fuels or desulfurisation of flue gas using
calcium carbonate is necessary to reduce emissions:
ment such as the addition of stoichiometric quanti-
ties of ammonia:
NO + NO
2
+ 2NH
3
Æ 2N
2
+ 3H
2
O
Road vehicle fuels contain negligible nitrogen and
the NO
x
arises solely through the high-temperature
combination of atmospheric N
2
and O
2
. Road trans-
port contributes approximately 50% and power sta-
tions 25% of the NO
x
emissions.
Precious metal catalysts are used to effect reduc-
tions in emissions of NO
x
, carbon monoxide and
unburnt hydrocarbons from lead-free petrol-vehicle
exhausts, converting these gases into water, nitrogen
and carbon dioxide.
Nitrogen dioxide is a pulmonary irritant and high
concentrations can produce oedema. It also con-
tributes synergistically to the effects of other pol-
lutants and participates in ozone formation and
photochemical smogs. The NO
2
in damp air converts
to nitric acid, leading to acid deposition with similar
effects to sulfuric acid.
Emissions of NO
x
in the UK gradually increased
until the early 1990s due to increased traffic but are
now declining slowly as vehicle emission regulations
take effect and power station emissions decline, with
the shift away from coal to other fuels. The UK
anthropogenic emissions of NO
x
were 2.43 Mt in
1970, 2.67 Mt in 1990 and 1.83 Mt in 1997. The N
2
O
emissions are dominated by agricultural sources and
in 1997 were 0.19 Mt [4].
CaCO
3
+ SO
2
+
1
/
2
O
2
Æ CaSO
4
+ CO
2
Apart from adding to global warming through CO
2
release, this process also consumes huge quantities
of limestone. Other processes include the addition of
ammonia to flue gases:
1
/
2
O
2
Æ (NH
4
)
2
SO
4
2NH
3
+ SO
2
+ H
2
O +
and the reduction of the SO
2
to elemental sulfur by
stoichiometric addition of hydrogen sulfide:
2H
2
S + SO
2
Æ 3S + 2H
2
O
Nitrogen species.
Natural sources of the nitrogen
oxides NO and NO
2
, referred to collectively as NO
x
,
introduce some 20-30 Mt of nitrogen per year into
the atmosphere, mainly from lightning-induced
combinations of atmospheric N
2
and O
2
, biogenic
sources in the soil and natural fires. Atmospheric
oxidation of ammonia, mainly from animal excreta,
also contributes to NO
x
. By comparison, anthro-
pogenic sources of NO
2
, which arise mainly from
combustion processes, contribute a further 20 Mt.
A third oxide of nitrogen, N
2
O, is released from
soils, especially those treated with fertilisers, and
from industrial sources, particularly nitric acid and
adipic acid production. The most abundant nitrogen
oxide in the atmosphere is N
2
O but it is rather un-
reactive and non-toxic.
The NO
x
production in stationary combustion
plants is affected by the fuel nitrogen content, burner
design, combustion temperature and air excess. The
emissions are mainly NO, with about 10% NO
2
,
although oxidation by atmospheric ozone shifts the
ratio so that most of the atmospheric burden is as
NO
2
.
Carbon monoxide.
Small amounts of CO occur in
the atmosphere naturally from volcanoes and atmos-
pheric methane oxidation. Carbon monoxide is,
however, a largely anthropogenic pollutant princi-
pally associated with motor vehicle emissions. These
contribute up to 98% of the CO in urban areas and
75% on average nationally in the UK. Emission from
properly adjusted stationary combustion processes is
negligible.
Carbon monoxide is removed from the atmos-
phere only slowly by reaction with hydroxyl radicals
formed mainly by the photolytic break-up of ozone
via the energetic singlet oxygen:
O
3
h
Æ O(singlet) + O
2
O(singlet) + H
2
O Æ 2OH
•
OH
•
+ CO Æ CO
2
+ H
•
The hydrogen radical then may react with CO
2
and
NO to regenerate the hydroxyl radical:
NO + O
3
Æ NO
2
+ O
2
The NO
x
emissions from stationary processes can be
reduced by about 50% by control of the combustion
process parameters. Further removal requires treat-
