[0001] The present invention relates to a method for the prevention or removal of soot and
other deposits from heating and ancillary surfaces, which deposits are formed as a
result of the combustion of gaseous, liquid and solid fuels, and also to the reduction
of corrosion of these surfaces by the lowering of the acid dewpoint of the combustion
gases. In particular the invention relates to the removal of deposits from and the
prevention of corrosion of heating and ancillary surfaces such as boiler and economiser
tubes, in turbo-compressors, turbo-chargers, exhaust- .boilers, gas turbines, process
furnaces and other equipment.
[0002] By the term "deposits" as used herein is meant the deposits formed from the products
resulting from the combustion of gaseous, liquid and solid fuels. The deposits may
consist of carbon, in the form of soot, or may comprise sulphur and/or sulphur compounds
or the ash of fuels, which are generally argillaceous compounds when the fuel is coal
or a product thereof, and vanadium compounds when the fuel is an oil.
[0003] The deposits are.generally hard and adherent. When deposits are formed on steel or
other metals, then because the thermal conductivity of the deposits is low compared
with that of the steel or other metal, the rate of heat transmission from the flame
or hot gases to the water or other fluid which is being heated is reduced, thereby
reducing the efficiency of the equipment.
[0004] The deposits may also increase the corrosion of metal surfaces by trapping corrosive
agents, such as acid sulphates, therein. At high temperatures, in the absence of liquid
water, these sulphur compounds do not normally cause corrosion and it is usual to
maintain the combustion gases at a temperature above that temperature (the "dew point")
at which liquid can form. However, when there is a deposit on a metal surface the
temperature at the metal surface itself may be below the "dew point" whilst the temperature
of the gases is above the "dew point". The deposit thus increases the risk of corrosion.
[0005] The present invention provides a method for preventing deposits on or removing deposits
from heating and ancillary surfaces of boilers and like equipment, which method comprise:
continuously or intermittently introducing into the combustion chamber of the equipment
or into the flue gas stream in atomized form by means of at least one injection device
a.liquid additive comprising an aqueous solution of a mixture of ammonium nitrate,
at least one nitrate of an alkali metal, optionally at least one nitrate of an alkaline
earth metal and an indicator, monitoring the pH value of the additive and, if necessary
adjusting the pH value to within the range of from 7 to 9, monitoring the dewpoint
of the flue gas stream and adjusting as necessary, the amount and composition of the
additive supplied to the metering device in order to adjust the dew point of the flue
gas stream to the desired level .
[0006] The additive which is used in the present invention may be composed as follows:

[0007] The preferred alkali metal nitrate for use in the additive is potassium nitrate and
the preferred alkaline earth metal nitrate is magnesium nitrate.
[0008] The indicator may be any suitable indicator which will indicate the pH of the additive,
thereby indicating whether or not the pH is within the range of from 7 to 9. The pH
of the additive is preferably in the range of from 8 to 9 and thymol blue or bromothymol
blue may be used to give an indication of a pH within this range. The pH may be adjusted
as necessary by the addition of-a base, e.g. potassium hydroxide.
[0009] The dewpoint of the flue gas may be monitored by any conventional dewpoint meter.
[0010] The reactions will occur in the neutralization of gases containing sulfur oxides
are as follows:-

100 Mg (N03)2 gives 16g MgO

16 g MgO neutralizes 32gSO
3

lOOg KNO
3 gives 47g K
20

47g K
20 neutralizes 40g SO
3
[0011] In addition, the nitrogen oxides produced as a result of the decomposition of the
nitrates affect the reactions occurring. The nitrogen oxides thus react with the sulphur
oxides to form the anhydride of nitrosylsulphuric acid. This is a stable compound
excluding water or up to a sulphuric acid concentration of 80%. Even at:.a 70% sulphuric
acid concentration (corresponding to a flue gas temperature of about 100°C) decomposition
is slight. The compound leaves the combustion chamber undecomposed and does not cause
any corrosion.
[0012] During the operation of the method of the invention the additive is fed into the
combustion chamber or flue gas stream, the composition of the additive and the volume
thereof being adjusted to suit the plant being treated and the problems associated
with the particular fuel being burnt. The additive is effective both at high temperatures,
i.e. >1000°C, and at low temperatures, i.e. >100°C.
[0013] In a preferred mode of operating the present invention aqueous solutions of the nitrates
are provided in separate tanks. The required amounts.of each of these ingredients
is then supplied in the desired amount to a common tank for mixing prior to introduction
into the combustion chamber or flue gas..
[0014] In operating the method of the invention a reduction in the temperature of the flue
gases usually occurs and this leads to a reduction in flue-gas heat loss with a concurrent
fuel saving.
[0015] The present invention will be further described with reference to the following Examples.
EXAMPLE 1 0058086
[0016] A STEINMULLER water-tube boiler was treated with the aim of decreasing the heating-surface
contamination and thus keeping the flue-gas temperature almost constant during a 3-month
seasonal operation.

Composition of the additive
[0017]

[0018] Addition: 02 litres of the additive per ton of fuel Injection point: combustion chamber
1 injection lance Injection appliance: fully automatic, time-controlled pneumatic
operation .
Results
[0019] The heating an ancillary surfaces were free of solid deposits, and the surfaces could
be dry cleaned using compressed air.
[0020] The flue-gas temperature rise during the operation was only 6°C as compared with
35°C according to the previous operating records.
[0021] A reduction in flue-gas heat loss of 2%, was achieved which was equivalent to a fuel
saving of about the same order.
EXAMPLE 2
[0022] A MAN water-tube boiler was treated with the aim of keeping the heating and ancillary
surface deposit rate as low as possible over a 12 month period and of reducing the
dew point from 134°C to about 105°C.

Composition of the additive;
[0023]

[0024] Addition: continuous, i.e. 0.4 litres of the additive per ton of fuel oil.
[0025] Injection point: combustion chamber 1 injection lance Injection appliance: semi-automatic;
pump operation Results: The heating and ancillary surfaces were up to 85% free of
solid deposits. The surfaces could be dry cleaned by means of compressed air and no
wet cleaning was required in the combustion chamber.
[0026] The acid dew point was reduced to 102°C accordingly, the steam-operated air-preheater
could be used without-fear of corrosion.
[0027] The flue-gas temperature of initially 172°C adjusted itself to 145°C; i.e. the flue-gas
heat loss was reduced by about 1.5%, equivalent to a fuel saving of about the same
order and, additionally, a steam saving in the air-preheater, so that an improvement
in efficiency of about 2.5% overall was achieved.
EXAMPLE 3
[0028] A VKW- Lentjes Benson boiler was treated.
Composition of the Additive per litre
[0029]

Flue gas parameters
[0030]

Addition
[0031] Addition of the additive took place continuously at a rate of 4.9 litre by means
of injection lances securely installed in the combustion chamber. Pump units fed the
additive from the storage containers to the injection lancers. It was thus possible
to match the output of the pump units to the desired reduction in acid dew point.
' Results
[0032] The additive was used over a relatively long period of time. The acid dew point was
lowered to 60-62°C and the flue gas temperature was set at 147°C. It was possible
to blow or dust off the residues on the heating and ancillary surfaces with compressed
air when the boiler was taken out of operation. The lowering of the sulphuric acid
dew point by about 60°C enabled an economy to be made in the steam required for the
air preheater, without any fear of corrosion.
1. A method for preventing deposits on or removing deposits from heating and ancillary
surfaces of boilers and like equipment, which method.comprises continuously or intermittently
introducing into the combustion chamber of the equipment or into the flue gas stream
in atomized form by means of at least one injection device a liquid additive comprising.an
aqueous solution of a mixture of ammonium nitrate, at least one nitrate of an alkali
metal, optionally at least one nitrate of an alkaline earth metal and an indicator,
monitoring the pH value of the additive and, if necessary adjusting the pH value to
within the range of from 7 to 9, monitoring the dewpoint of the flue gas stream and
adjusting as necessary, the amount and composition of the additive supplied to the
metering device in order to adjust the dew point of the flue gas stream to the desired
level.
2. A method as claimed in-- claim 1 wherein the alkali metal nitrate is potassium
nitrate.
3. A method as claimed in claim 1 or claim 2 wherein the alkaline earth metal nitrate
is magnesium nitrate.
4. A method as claimed in any one of the preceding claims wherein the indicator is
thymol blue or bromothymol blue.
5. A method as claimed in any one of the preceding claims wherein the pH value of
the additive is adjusted to . within the range of from 7 to 9 by the addition of an
alkali.
6. A method as claimed in any one of the preceding claims wherein the injection device
is an injection lance.
7. A method as claimed in any one of the preceding claims wherein aqueous solutions
of the nitrates are,provided. in separate tanks.
8. A method as claimed in claim 7 wherein the aqueous solutions of the nitrates are
supplied in the desired amounts to a common tank for mixing prior to their introduction
into the combustion chamber or flue gas.