Download Control of emissions from oil fired plants

CONTROL
OF
EMISSIONS
FROM
OIL
FIRED
PLANTS
~·I.
-".
3ZRRI.
INTRODUCTION
Liquid petroleum
industrial
fuels available
on the South African
plants are LPG, kerosine,
Typical characteristics
gas oil and hea.y furnace oil.
of these fuels are:Kerosine
L.P.G.
0
Density ex 20 C
Viscosity
Sulphur
Gas Oil
H.P.P.
0,55
0,80
0,83
0,2
1,2
2,6
% mass max.
0,018
0,25
0,5
3,5
typical
0,010
0,18
0,3
3,0
21,450
19,900
19,700
18,700
27,440
46,285
45,820
41,105
o
cs. ex 37,8 C
Gross calorific value Btu/lb.
MJ/l.
EMISSIONS
arkeL for firing
FROM
The emissions
OIL
FIRED
0,95
250 (max , )
PLANTS
from plants fired by these fuels which have to be conside-
red from the air pollution
or accepted as potential
aspect and either discounted
nuisances
and measures
as innocuous
developed
to eliminate
the nuisance are:a)
Carbon dioxide
b)
Carbon monoxide
c)
Water vapour
d)
Soot and unburnt hydrocarbons
e)
Emissions
f)
Sulphur dioxide
g)
Sulphur trioxide and acid smuts
h)
Nitrogen
CARBON
from process materials
oxides
DIOXIDE
All fossil fuels contain carbon.
bustion
The product obtained by complete com-
of this carbon is carbon dioxide.
air pollution and from considerations
38
Both from the standpoint
of fuel econo~y it is essential
of
to burn all the carbon to carbon dioxide,
potential
of the fuel is not developed
otherwise
the full heating
i.e. the plant operates
at a low
efficiency.
Some concern has been expressed
increasing
scale will result in an increase
tent of the atmosphere
fossil fuels on an ever
in the carbon dioxide
con-
and this might have an effect on the world heat
It is difficult
balance.
that burning
to express any opinion on this matter,
so little factual data is available
on the scale of the problem
since
or if
indeed there is one.
Some recent figures which have been quoted are
that the concentration
of carbon dioxide
is increasing
year and the level in 1972 was 324 ppm.
fate of carbon dioxide does not however
is tempting
pointing
which is a well established
shown to promote quicker
an obviously
CARBON
desirable
seem to restrain
technique
comment so it
into the discussion
of the air in greenhouses
on the
by
by carbon dioxide
in many parts of Europe,
a~d greater growth of vegetation
has been
and fruits -
result.
MONOXIDE
Carbon monoxide
is not an important
plants although much attention
from internal
combustion
and handled
pollutant
engines.
In industries
in the process,
standpoint.
If oil fired equipment
as to produce
large amounts
smoke, soot and unburnt hydrocarbons
are necessarily
great care is taken to confine the
aspect but also from the safety
is so badly maintained
of carbon monoxide,
and adjusted
then copious amounts
will be concurrently
There is no excuse for such a situation
of equipment
oil fired
such as the iron and
of carbon monoxide
product not only from the pollution
proper adjustment
from industrial
is now being given to low level emissions
steel industry where large amounts
produced
The lack of information
to throw a further red herring
out that enrichment
by 1.3 ppm. per
of
produced.
since adequate maintainance
will solve this problem without
and
recourse
to special techniques.
Since one pound of carbon burnt to carbon dioxide
14 000 Btu whereas when only partially
liberates
just over
bur.nt to CO, the heat liberation
39
is 3 960 Btu or 28% of the potential,
a situation
who permits such
to develop is throwing money away as well as annoying the
Air Pollution
WATER
any industrialist
authorities.
VAPOUR
i
Because
petroleum
fuels contain hydrogen, water vapour is produced when
they are burnt.
Water vapour as a pollutant
of the immense amounts occuring naturally
unusual however
for photographs
steam.
SOOT
HYDROCARBONS
AND
as horrible
of such photographs
is really innocuous
UNBURNT
in the atmosphere.
of stacks emitting
appears to be fume to be published
Close inspection
can be discounted
and freedom
examples
often indicates
of pollution.
modern burner
with a "stack solids" content in the flue gases
"Stack solids" means fine
soot, ash from the oil and solid unburnt hydrocarbons.
is therefore better th~n 99.8% complete.
are extremely
fuels with
from soot and unburnt hydrocarbons.
of less than 0.2% by mass of fuel burnt.
efficiency
of what
that the "pollutant"
When burning heavy furnace fuel, a good well maintained
can operate continuously
It is not
large amounts
There is no problem in burning LPG and the two distillate
complete cleanliness
in view
finely dispersed
and are liberated
tely invisibly and without nuisance
Combustion
These stack solids
from the stack comple-
except when acid smut formation
(described below) occurs.
PROCESS
Emissions
EMISSIONS
from materials
from air pollution
being processedalthough'extremely
considerations
important
are inherent to the process whatever
fuel is being used and not relevant to the subject of this paper.
SULPHUR
DIOXIDE
,
Liquid petroleum
fuels, similarly to solid fuels contain sulphur to a
lesser or greater extent.
of industrial
40
Inspection
of the typical sulphur contents
fuels above indicates that the content only becomes signi-
It should, however, be recognised
ficant in the HFF grade.
that be-
cause of the higher calorific value of oil and the greater efficiency
oil fired equipment
a 3% sulphur oil is approximately
equivalent
of
to a
1.5% sulphur coal.
When oil is burnt, the sulphur is converted
to sulphur dioxide.
There
are two schools of thought about how the emission
of sulphur dioxide
should be handled.
by the Americans
Continental
emitted
The first school represented
countries
and
insists that the absolute amount of sulphur dioxide
to the atmosphere
should be limited.
Such restrictions
were met
in the first place by using fuel oils derived from crude oils naturally
low in sulphur.
emptive
Such crudes however,
are in limited supply and the pre-
demands for low sulphur fuels for ordinary burning purposes from
these countries
absorbed
been better utilised
supplies
of low sulphur oil which
in industries
sulphur fuels are technically
would have
such as the steel industry where low
desirable.
In the event, demands for low
sulphur fuels have now reac~ed such a level that they cannot be met and
in many countries the restrictions
on sulphur dioxide emission
to be relaxed to permit burning higher sulphur oils.
fuel oil is not available
and this approach
prevails,
have had
If low sulphur
the next logical
step is to require removal
of sulphur from fuel oil or removal
dioxide
The latter, by the way, is the only option
from flue gases.
available
to coal.
The costs of these operations
For example, a plant to desulphurise
1 million
of sulphur
is staggeringly
tons/year
high.
Kuwait oil to
1% sulphur will cost about R15 000 000 and add at least R5,7 / ton or
0,57 cents / litre to the price of oil.
A flue gas desulphurisation
plant to remove sulphur dioxide resulting
from~burning
1 million
tons
will cost about R18 000 000 and add R5,8 per ton of oil burnt to the
operating
costs.
The second school, represented
by the U.K. takes the view that it is not
the absolute amount of sulphur dioxide emitted which is important
natural means exist to remove this from the atmosphere
level concentration
but the ground
where humans and plants live and breathe.
leads to what is called the "tall stack" policy.
since
This
Stack he'ight.s, gas
41
efflux velocities
and exit temperatures
plume of effluent
gases is dispersed
phere without
causing localised
are set at such levels that the
harmlessly
into the upper atmos-
high sulphur dioxide
The success of this policy has been very marked
level concentrations
are steadily
dropping
nage of high sulphur oil being burnt.
when atmospheric
dispersing
inversion
to a low sulphur oil whilst
SULPHUR
in the U.K. where ground
despite the increasing
ton-
The tall stack is not effective
occurs as this prevents
in the upper atmosphere
concentrations.
the flue gas plume
but this can be overcome by changing
infrequent
conditions
prevail.
TRIOXIDE
Of the sulphur dioxide produced by burning
say 3% is oxidised
oil a small proportion
further to sulphur trioxide,
the anhydride
up to
of sul-
phuric acid.
When surfaces
i.e. ducts, fan impellers
with flue gases are operating
sulphuric
acid condenses
surface is not resistant
concurrently
at temperatures
to attack,
occurs.
corrosion
in contact
below the acid dew point,
If the material
to air pollution
burden to the industrialist
experienced
chimneys
on these surfaces.
as such is of no significance
it is a financial
and casings,
of the
This corrosion
authorities
even though
but if smut emission
is
and this is very likely, it becomes a prime
item of concern.
ACID
SMUTS
Smuts are produced when fine carbon wh~ch would
innocuously
and unnoticeably
and agglomerates.
Changes
result in agglomerates
otherwise be discharged
from stacks, adheres to the wet acid film
of load, wind direction,
becoming
detached
gas velocity
can
from the surface and carried
out of the chimney to fall in the neighbourhood.
ELIMINATION
Smut emission
OF
is associated
solids, presence
42
SMUTS
with three factors:-
of sulphur trioxide
presence
of unburnt
in the flue g~ses and relatively
low
temperatures
of surfaces
effort has been devoted to indentifying
the development
of methods
The first approach
possible
quality
the factors responsible
for eliminating
so that the quantity
equipment,
adjusted
by the installation
of good
and maintained.
The second step is to prevent the formation
this cannot be prevented,
is as near complete as
of solids carried out in the gases is re-
This can be achieved
properly
and to
the problem.
is to ensure that combustion
duced to the minimum.
Much research
in contact with these gases.
of sulphur trioxide
to prevent acid deposition.
or if
Three methods
have been found satisfactory.
a) Combustion
Control:-
eliminated
Oxidation
of S02 and S03 is virtually
when oil is burnt at low excess air levels.
pean power stations habitually
less than 1%.
Smokeless
levels can be achieved
operate with excess oxygen
operation
at these low excess air
at the low combustion
common in power station boilers.
intensities
Competition
aged boiler field is tending to increase
in the pack-
combustion
ty in this type of plant and not all burners
boilers
Euro-
intensi-
fitted to these
can operate with both low stack solid production
and
low excess air.
b) Maintenance
of surfaces
of surface temperatures:-
If the temperature
in contact with flue gases is maintained
the acid dewpoint,
acid cannot conden~e.
above
This is achieved
by stopping the influx of cold air into the flue gases, reducing the heat loss from surfaces by insulation
creasing the temperature
c)
eutralisation:basic compounds
oxides.
of water entering
Sulphur trioxide
such as ammonia,
Ammonia
addition
or by in-
the economiser.
can be neutralised
dolomite
by
or magnesium
is the simplest but is best ap-
plied to plant operating under steady load conditi~ns.
43
CHIMNEY
DESIGN
With unlined
Chimneys present a special problem.
steel chimneys,
the
rate of heat loss is high and provision must be made to ensure the maintenance of adequate
internal
skin temperatures.
S.A. it will probably be satisfactory
to fit cladding.
the steel chimney is sheathed with aluminium
the aluminium
sheathing and the chimney.
areas where low temperature
In coastal regions of
leaving an air gap between
On the Reef and in other
winter conditions
chimney insulation will be called for.
In this method
are experienced,
full
If a brick chimney already
exists, an internal flue or flues can be inserted in the brick chimney,
the gap between flue and chimney being filled with loose insulation.
When gases are emitted from chimneys at low velocity,
"downwash"
can
occur, waste gases being drawn down the lee side of the chimney.
gas velocity
also allows cold air to enter the chimney and cool the up-
flowing gases and chimney wall.
ced and smut formation
To eliminate
A low
("Inversion") •
can occur.
downwash and inversion,
times the average wind speed.
efflux velocity
Plume rise is redu-
should be 1,5
Modern chimney design allows for an
mls
of about 12
efflux velocities
(40
f/s) at full load.
If this velo-
city cannot be obtained by natural draught, fans will be required.
NITROGEN
OXIDES
In the high temperature
zones of a furnace or boiler fired with either
liquid or solid fossil fuels, nitrogen and oxygen combine to produce
nitrogen
oxides.
in the presence
terials.
These compounds
can react with unburnt hydrocarbons
of sunlight to produce acrid smelling and irritating ma-
Various methods
of minimising
which show promise are being investigated
gases, two stage combustion,
cessary to incorporate
ces.
the production
e.g. recirculation
low excess air combustion
the necessary
of nitrogen
equipment
oxides
of flue
and it may be ne-
in some particular
instan-
In general, however, the tall stack policy referred to earlier,
which has been successful
44
in controlling
ground level concentrations
of
sulphur dioxide will also satisfactorily
tration
of nitrogen
control the ground level concen-
oxides.
CONCLUSION
Methods
have been developed
emissions
to eliminate
from oil fired plants.
trol of emissions
is now a matter
any nuisance
caused by the
Since the means are available,
of wholehearted
acceptance
con-
of the phi-
losophy that control is desirable.
ACKNOWLEDGEMENT
Permission
to present this paper has been given by BP Southern Africa
(Pty) Ltd.
45