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.

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.

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.

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 riormally 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.

GB-A-1378882 discloses a method for the removal of deposits from surfaces which comprises applying to a heated surface an agent which comprises a mixture of ammonium nitrate and at least one nitrate of an alkali metal, an alkaline earth metal, copper, zinc or an organic base, the agent not including therein an oxidisable organic material, sulphur or a sulphur compound.

FR-A-1357992 discloses an agent for the chemical removal of soot which comprises a nitrate of an alkali metal or an alkaline earth metal and one or more combustible materials in a proportion such that the mixture is overbalanced in favour of oxygen. The combustible material may be charcoal or sugar.

EP-A-0042367 discloses a method for reducing the sulphur context of exit gases resulting from the combustion of sulphur bearing fuels, which method comprises the step of igniting the sulphur bearing fuel in the presence of at least one alkaline earth metal nitrate, alkali metal nitrate or ammonium nitrate.

EP-A-0053085 discloses a method for cleaning the surfaces of an installation in which a dispersion, for example of a mixture of potassium nitrate and ammonium nitrate, is entrained by the combustion gases whereby it reacts with the deposits of carbon and minerals which encrust the surfaces of the installation and displacing the particles resulting from the reactions by the use of sound waves so that they become entrained in the air flow or combustion gases or are deposited in the ashes of the installation.

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 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 maintain the exit gas temperature of the equipment above the acid dewpoint level.

The additive which is used in the present invention may be composed as follows:

The preferred alkali metal nitrate for use in the additive is potassium nitrate and the preferred alkaline earth metal nitrate is magnesium nitrate.

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.

The dewpoint of the flue gas may be monitored by any conventional dewpoint meter.

The reactions will occur in the neutralization of gases containing sulfur oxides are as follows:-

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.

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.

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.

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.

The present invention will be further described with reference to the following Examples.

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.

• 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

The heating and ancillary surfaces were free of solid deposits, and the surfaces could be dry cleaned using compressed air.

The flue-gas temperature rise during the operation was only 6°C as compared with 35°C according to the previous operating records.

A reduction in flue-gas heat loss of 2%, was achieved which was equivalent to a fuel saving of about the same order.

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.

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.

The acid dew point was reduced to 102°C accordingly, the steam-operated air-preheater could be used without fear of corrosion.

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.

A VKW-Lentjes Benson boiler was treated.

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.

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.