GB2395722A

GB2395722A - The prevention or removal of deposits from heating and ancillary surfaces

Info

Publication number: GB2395722A
Application number: GB0227695A
Authority: GB
Inventors: Ulf Walden; Rainer Hoffman
Original assignee: POLARCHEM INTERNAT Ltd
Current assignee: POLARCHEM INTERNAT Ltd
Priority date: 2002-11-27
Filing date: 2002-11-27
Publication date: 2004-06-02
Also published as: GB0227695D0

Abstract

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 atomised form by means of at least one injection device, a liquid additive which excludes nitrates of any alkali metal and which comprises an aqueous solution of ammonium nitrate, 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 a metering device in order to maintain the exit gas temperature of the equipment above the acid dewpoint level.

Description

- 1 Method for the prevention of deposits or removal of deposits from

heating and ancillary surfaces The present invention relates to a method for 5 preventing the formation of deposits on heating and ancillary surfaces and/or for assisting the removal of deposits from heating and ancillary surfaces, which deposits are formed as a result of the combustion of gaseous, liquid and solid fuels. In particular, the 10 invention relates to the prevention and/or removal of chloride deposits from heating and ancillary surfaces, such as boiler and economiser tubes in turbo-

compressors, turbo-chargers, exhaust-boilers, gas turbines, process furnaces and other equipment. The 15 present invention is also particularly concerned with the prevention and/or removal of chloride deposits from heating and ancillary surfaces in incinerators for municipal, clinical and/or industrial waste.

20 As used herein, the general term "deposits" refers to deposits which are formed on heating and ancillary surfaces from the products resulting from the combustion of gaseous, liquid and solid fuels and from the combustion of municipal, clinical and/or 25 industrial waste.

As used herein, the term "chloride deposits" refers to deposits which comprise chloride ions and which are formed on heating and ancillary surfaces from the 30 products resulting from the combustion of gaseous liquid and solid fuels and from the combustion of municipal, clinical and/or industrial waste.

The incineration of domestic refuse and the 35 destruction of plastics materials containing chlorinated hydrocarbons in particular gives rise to

1 1 - 2 the presence of chlorine and hydrogen chloride gas in the flue gases. There are numerous reactions of hydrochloric acid inside the boiler which, in combination with the sulphur oxides which may also be 5 present, lead to corrosion and baked-on deposits. The sulphur dioxide corrosion phenomena which are more common in fuel oil, coal and lignite combustion are thus substantially reduced, although the oxidation of primarily occurring SO2 to form SO3 and the resultant 10 formation of sulphuric acid is promoted by the presence of hydrochloric acid. However, these compounds are quite stable and, in the absence of water, they remain gaseous and undecomposed as long as the temperature does not drop below the dew point 15 which, in refuse incineration, is substantially higher than in oil firing.

The hydrogen chloride and chlorine generating reactions can be approximately described as follows: a) Formation of hydrogen chloride and chlorine.

HC1 is formed in the decomposition of chlorinated hydrocarbons. Chlorine is formed according to the pattern of the DEACON process: 25 2 HCl + O2 ' C12 + H2O The eutectic mixture FeCl + CaCl2 serves as catalyst.

b) Secondary reaction with sulphur oxides and decomposition upon admission of water.

30 Sulphonation of hydrochloric acid: SO2 + HC1 HSO2C

Decomposition: HSO2C1 + H2O H2SO3+ HC1

Formation of sulphuryl chloride: 35 SO2 + C12 SO2C12

Decomposition: SO2C12+ 2H2O H2SO4+ 2HCl

Formation of chlorosulfonic acid: SO3 + HC1 -> HSO3C1

Decomposition: HSO3C1 + H2O -> H2SO4 + HC1

c) Reactions with nitrogen oxides: Hydrochloric acid and chlorine react with the oxides of nitrogen in the same manner as the oxides of sulphur. The reactions which result in nitrosyl 10 chloride are only one example of the many possible reactions. These compounds are stable in the absence of water: N2O3 + 2HC1 -> 2NOC1 + H2O

or 15 2NO + C12 2NOC1

or HNO3 + 3 HC1 NOC1 + C12 + H2O

Decomposition: NOC1 + H2O HNO2 + HC1

20 Nitrosyl chloride decomposes thermally above 700 C: 2NOC1 NO + C12

Deposits are generally hard and adherent. When deposits are formed on steel or other metals then, 25 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 30 the equipment.

The deposits may also increase the corrosion of metal surfaces by trapping corrosive agents, such as acid sulphates and chlorides, therein. At high

- 4 temperatures, in the absence of liquid water, these compounds do not normally cause corrosion and it is usual to maintain the combustion gases at a temperature above that temperature (the "dew point") 5 at which liquid water 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 gas is above the "dew point". Thus water may condense on the deposits 10 allowing the acid sulphates and chlorides to cause -

corrosion of the metal.

Chloride deposits can cause particularly bad corrosion of metal surfaces. Not only do they trap corrosive 15 sulphate and chloride compounds as explained above, they may also themselves form acidic chlorides when water condenses on them which can also corrode the metal surfaces.

20 The applicant's earlier application EP-A-0058086 provided a method for preventing deposits on or removing deposits from heating and ancillary surfaces of boilers and like equipment, which method comprised continuously or intermittently introducing into the 25 combustion chamber of the equipment or into the flue gas stream in atomised form by means of at least one injection device, a liquid additive comprising an aqueous solution of ammonium nitrate, at least one nitrate of an alkali metal, optionally at least one 30 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 a metering device in order to maintain the exit gas temperature of the equipment above the acid dewpoint level.

5 Nitrates are used in the post-combustion process of EP-A-0058086 as a source of oxides which neutralise sulphur oxides produced in the combustion process. For example

10 Mg(NO3) 2. 6H2O = MgO + NO + NO2 + O2 + 6H2O 2KNO3 = K2O + NO +NO2 + O2

MgO + SO3 = MgSO4 K2O + SO3 = K2SO4

15 In addition, the nitrogen oxides produced as a result of the decomposition of the nitrates also react with the sulphur oxides to form the anhydride of nitrosylsulfuric acid. This compound generally leaves the combustion chamber intact and does not cause any 20 corrosion.

Since this earlier application, the change in design of boilers, furnaces and heaters to permit firing at higher temperatures, together with the change in the 25 composition or quality of fuel oil has led to new problems in the superheaters of boilers and in the convection zone of furnaces and heaters during de-

coking. It has led to more severe fouling and the formation of much harder deposits as a result of 30 reactions between vanadium, sodium and potassium. In particular, the increased occurrence of hard, adherent chloride deposits cannot be adequately treated by the use of the compositions and methods of EP-A-0058086.

As explained above, the formation of chloride deposits

is a particular problem in the incineration of municipal, clinical and industrial waste.

Whereas previously deposits could be removed by use of 5 compositions as described in EP-A-0058086, it has now been found that the use of such compositions in modern equipment actually increases the problem of deposit formation, in particular chloride deposit formation.

This is believed to be because the use of such 10 compositions (especially the use of potassium nitrate which is the preferred alkali metal nitrate) results in a decrease in the melting point of the deposits.

Once melted the deposits then spread over and stick to the equipment surfaces making them adhere more 15 strongly. In practice an increase in the melting point of the deposits is desired. The size of the increase needed in order to prevent melting is further exaggerated by the abovementioned tendency for modern equipment to operate at higher firing temperatures.

The applicant has now discovered that it is possible to achieve the desired aim of increasing the melting point of the deposits so as to prevent their formation and/or so as to facilitate their removal by the use of 25 a composition which comprises an aqueous solution of ammonium nitrate and a nitrate of an alkaline earth metal and which does not include a nitrate of an alkali metal.

30 Accordingly 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 atomised form by means of at least one injection device, a liquid additive which excludes nitrates of any alkali metal and which comprises an 5 aqueous solution of ammonium nitrate, one or more nitrates 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; 10 monitoring the dewpoint of the flue gas stream and adjusting, as necessary, the amount and composition of the additive supplied to a 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: Component % by weight 20 Ammonium nitrate up to 45% Alkaline earth metal nitrate up to 75% Indicator trace Water (optional) up to 55% 25 Preferably, the amount of ammonium nitrate is from 1 to 45% by weight, more preferably, from 10 to 35% by weight, and most preferably, 15 to 25 % by weight. For example, in one embodiment, the amount of ammonium nitrate is 18 to 20% by weight.

Preferably, the amount of alkaline earth metal nitrate is from 1 to 75% by weight, more preferably, from 10 to 60% by weight, and most preferably, 35 to 55% by

- 8 weight. For example, in one embodiment, the amount of alkaline earth metal nitrate is from 40 to 50% by weight. 5 Preferably, the amount of indicator is from 0.01 to 3% by weight, more preferably, from 0.1 to 1% by weight.

The liquid additive may contain water, for example, in an amount of up to 55% by weight. Preferably, the 10 amount of water is from 25 to 50% by weight, more -

preferably, from 30 to 45% by weight. For example, in one embodiment, the additive comprises 35 to 45% water by weight.

15 It should be understood, however, that the exact composition of the additive may be varied depending on the type of fuel being combusted, and/or the nature of the deposits that are likely to be formed on the heating and ancillary surfaces. The exact composition 20 of the additive may also depend on whether the additive is introduced continuously or intermittently into the combustion chamber of the equipment.

The liquid additive may require further dilution with 25 water prior to use in the method of the present invention. The amount of water required to dilute the liquid additive before use will depend on a number of factors, including, for example, the size of the equipment to be treated. The amount of water may also 30 depend on whether the additive is introduced continuously or intermittently into the combustion chamber of the equipment. In one embodiment, one part of the liquid additive may be diluted with 2 to 20 parts of water, for example, 5 to 15 parts of water.

In a preferred embodiment, the liquid additive comprises (in % by weight) : Ammonium Nitrate: 18 - 22%, for example, 20% Magnesium Nitrate: 45 55%, for example, 50% 5 Water: 25 -35%, for example, 30% Indicator: trace The liquid additive above may be particularly suitable for intermittent dosing. Before use, one part of the 10 above additive may be mixed with 3 to 5, for example, 4 parts of clean water.

In another preferred embodiment, the liquid additive comprises (in % by weight): 15 Ammonium Nitrate: 16 - 20%, for example, 18% Magnesium Nitrate: 35 - 45%, for example, 40% Water: 35 -40%, for example, 42% Indicator: trace 20 The liquid additive above may be particularly suitable for continuous dosing. Before use, one part of the above additive may be mixed with 5 to 7, for example, 6 parts of clean water.

25 In a preferred embodiment of the present invention, the alkaline earth metal nitrate is magnesium nitrate.

Alternatively or additionally, calcium nitrate is employed. In one embodiment, a mixture of calcium nitrate and magnesium nitrate is employed. The weight 30 ratio of calcium nitrate to magnesium nitrate may be 0.5 to 2:1, for example, 1:1.

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 7 to 9. In a preferred embodiment the indicator is thymol blue or bromothymol blue.

5 In another preferred embodiment the pH value of the additive is adjusted to within the range of from 7 to 9 by the addition of an alkali. Preferably said alkali does not comprise potassium.

10 The liquid additive may further comprise a nitrateof a metal selected from Groups 3 to 12 of the Periodic Table. Preferably, the nitrate is a nitrate of a transition metal, for example, one selected from the group consisting of: Sc, Ti, V, Cr. Mn, Fe, Co, Ni, 15 Cu. and Zn. More preferably, the nitrate is an iron nitrate. For example, ferric nitrate may be employed.

The amount of the Group 3 to 12 metal nitrate employed may be O to 5% by weight, for example, 0.01 to 3% by weight, more preferably, from 0.2 to 1. 5% by weight.

20 In a preferred embodiment, 0.5% by weight ferric nitrate is included in the liquid additive. This provides the additive with anti-corrosion properties.

The liquid additive may also comprise a sulphate of a 25 metal selected from Groups 3 to 12 of the Periodic Table. Preferably, the sulphate is a sulphate of a transition metal, for example, one selected from the group consisting of: So, Ti, V, Cr. Mn, Fe, Co, Ni, Cu. and Zn. More preferably, the sulphate is an iron 30 sulphate. For example, ferrous sulphate may be employed. The amount of the Group 3 to 12 metal sulphate employed may be O to 5% by weight, for example, 0.01 to 3% by weight, more preferably, from 0.2 to 1.5% by weight. In a preferred embodiment, 0.5%

- 11 by weight ferrous sulphate is included in the liquid additive. This provides the additive with anti-

corrosion properties.

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

During the operation of the method the additive (preferably, once diluted with water) is fed into the 10 combustion chamber or into the 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 preferably introduced by means 15 of one or more injection lances.

In another preferred mode of operating the present invention the nitrates are provided as aqueous solutions in separate tanks. The required amounts of 20 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.

The method described above is found to exhibit 25 improved deposit removal/prevention when used in conjunction with modern heating equipment. In particular it exhibits improved chloride deposit removal/prevention. Even when deposits are formed, the present method results in the formation of deposits 30 which are soft, dry and powdery rather than hard and/or sticky. This makes them easier to remove with steam, air or sonic soot blowers and other cleaning devices.

A further advantage of the use of compositions of the present invention is that, because nitrates of an alkaline earth metal decompose at a lower temperature than nitrates of an alkali metal, the composition can 5 be used to treat turbo chargers fed from the exhaust gases of diesel engines where the temperature can be as low as 250 C. The presence of potassium nitrate would not allow this because it does not decompose until the temperature exceeds about 333 C.

The present invention is particularly advantageous in relation to the disposal of municipal, industrial and hospital waste which is not disposable in landfill sites due to its carbonaceous content and which must 15 be incinerated. Municipal waste contains different forms of plastic material, many of which produce hydrochloric acid upon incineration resulting in the formation of chloride deposits. A particular advantage of the present invention is the unexpected improvement 20 in the prevention of formation and the ease of removal of sticky chloride deposits which result in both fouling and corrosion.

A further unexpected advantage of the method of the 2S present invention is that it results in a reduction in the emission of dioxins and furans in the residue from combustion. In another aspect, the present invention also provides 30 for the use of a liquid additive which excludes nitrates of any alkali metal and which comprises an aqueous solution of ammonium nitrate, at least one nitrate of an alkaline earth metal, and an indicator, in a combustion process for the purpose of preventing

the formation of, or facilitating the removal of, chloride deposits on heating and ancillary surfaces of boilers and like equipment.

5 The present invention will be further described by way of the following examples: Examples

Example 1

10 Municipal waste was treated in an incineration unit at a rate of 9 tons per hour. After 1000 hours of operation, the incineration unit was shut down and cleaned. Samples of the deposits formed on the inner walls of the incinerator were obtained for visual 15 inspection. Figure 1 illustrates a sample taken at a depth of 14.5 m from the mouth of the incineration unit. Figure 2 illustrates a sample taken at a depth of 17.5 m.

20 After cleaning, the incineration unit was re-started.

In addition, a liquid additive was continuously introduced into the unit via two injection points.

The liquid additive employed was formed from the 25 following concentrate: Ammonium nitrate - 18% by weight Magnesium Nitrate - 40% by weight Water 42% by weight 30 Indicators - trace Before use, 2 parts of this concentrate were diluted with 5 parts of water. 0.2 litres of concentrate was employed for each ton of waste treated.

After 6 months of operation, the incinerator was shut-

down for cleaning. It was found that the cleaning time was reduced by 75%. Samples of the deposit formed on the inner walls of the incinerator were also obtained 5 for visual inspection. Figure 3 illustrates a sample taken at a depth of 14.5 m, whilst Figure 4 illustrates a sample taken at a depth of 17.5 m. As can be seen from a comparison of Figures 1 and 3, the deposits formed in the presence of the additive were 10 smaller and more friable than the deposits formed in the absence of the additive. A similar effect can be seen from a comparison of Figures 2 and 4.

The deposits were also analysed for their chlorine and 15 SO3 content. Figure 5 illustrates the data obtained from the samples shown in Figures 1 and 3. As shown in Figure 5, the chlorine content of the deposits was significantly reduced by the use of the additive. The SO3 content of the deposits was increased by the use 20 of the additive. Figure 6 illustrates the data obtained from the samples shown in Figures 2 and 4. As shown in Figure 6, the chlorine content of the deposits was significantly reduced by the use of the additive. The SO3 content of the deposits was 25 increased by the use of the additive.

Example 2

Municipal and clinical wastes were treated in an incinerator having a capacity of 20 tons of waste per 30 hour. After 1000 hours of operation, the incineration unit was shut down and cleaned. A sample of the deposits formed on the inner walls of the incinerator was obtained for visual inspection.

After cleaning, the incineration unit was re-started.

In addition, a liquid additive was continuously introduced into the unit via 8 injection points.

5 The liquid additive employed was formed from the following concentrate: Ammonium nitrate - 18% by weight Magnesium Nitrate - 40% by weight 10 Water - 42% by weight Indicators - trace Before use, 1 part of this concentrate was diluted with 14 parts of water. 0.1 litres of concentrate was 15 employed for each ton of waste treated.

After 6 months of operation, the incinerator was shut-

down for cleaning. It was found that the cleaning time was significantly reduced. A sample of the deposit 20 formed on the inner walls of the incinerator was also obtained for visual inspection. It was found that the deposits formed in the presence of the additive were smaller and more friable than the deposits formed in the absence of the additive.

The deposits were also analysed for their chlorine and SO3 content. It was found that the chlorine content of the deposits was significantly reduced by the use of the additive. The SO3 content of the deposits was 30 shown to have increased by the use of the additive (see Figure 7).

Example 3

Municipal waste was treated in an incineration unit at

a rate of 15 tons per hour. In addition, a liquid additive was continuously introduced into the unit via 4 injection points.

5 The liquid additive was formed from the following concentrate: Ammonium nitrate - 18% by weight Magnesium Nitrate - 40% by weight 10 Water - 42% by weight Indicators - trace Before use, 0.2 parts of this concentrate was diluted with 2.3 parts of water. 0.2 litres of concentrate was 15 employed for each ton of waste treated.

It was found that it was easier and quicker to clean the incinerator when the liquid additive was used during the incineration process compared to when the 20 incineration was carried out in the absence of the liquid additive.

Comparative Example A A number of metal tubes were weighed, and disposed in 25 an incinerator. The incinerator was then employed in the normal manner to treat waste. After 100 hours of operation, the tubes were removed from the incinerator and re-weighed. The difference in the final and initial weights of the tubes was indicative of the 30 extent of corrosion caused by the incineration process. Example 4

The procedure of Comparative Example A was repeated.

In addition, however, a liquid additive was continuously injected into the incinerator.

The additive was formed from the following 5 concentrate: Ammonium Nitrate - 15% by weight Magnesium Nitrate - 25% by weight Calcium Nitrate - 20% by weight Ferric Nitrate - 0.5% by weight 10 Water - 39.5% by weight Indicators - traces The dosing for the first 48 hours was made at a rate of 0.6 litres per ton of waste. Water was also added 15 at a rate of 2.4 litres per ton of waste. Thereafter, 0.2 litres of the additive was employed per ton of waste. Water was employed at a rate of 2.3 litres per ton of waste.

20 After 100 hours of operation, the tubes were removed from the incinerator and re-weighed. It was found that the loss of weight observed with the tubes of this example was less than that observed with the tubes of Comparative Example A. This indicated that the degree 25 of corrosion observed in the presence of the additive was less than that observed in the absence of the additive. This was confirmed by a visual inspection of the tubes.

30 Example 5

The procedure of Comparative Example A was repeated.

- 18 The additive was formed from the following concentrate: Ammonium Nitrate - 15% by weight Magnesium Nitrate - 25% by weight 5 Ferrous Sulphate - 20% by weight Ferric Nitrate - 0.5% by weight Water - 39.5% by weight Indicators - traces 10 The dosing for the first 48 hours was made at a rate of 0.6 litres per ton of waste. Water was also added at a rate of 2.4 litres per ton of waste. Thereafter, 0.2 litres of the additive was employed per ton of waste. Water was employed at a rate of 2.3 litres per 15 ton of waste.

After 100 hours of operation, the tubes were removed from the incinerator and re-weighed. It was found that the loss of weight observed with the tubes of this 20 example was less than that observed with the tubes of Comparative Example A. This indicated that the degree of corrosion observed in the presence of the additive was less than that observed in the absence of the additive. This was confirmed by a visual inspection of 25 the tubes.

Claims

i - 19 Claims

1. A method for preventing deposits on or removing deposits from heating and ancillary surfaces of 5 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 atomised form by means of at least one injection device, a liquid additive which excludes 10 nitrates of any alkali metal and which comprises an aqueous solution of ammonium nitrate, 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 15 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 a metering device in order to maintain the exit gas temperature of the equipment above the acid dewpoint level.

2. A method as claimed in claim 1 wherein the additive which is used in the present invention is comprises the following ingredients: Component % by weight 25 Ammonium nitrate up to 45\ Alkaline earth metal up to 75% nitrate Indicator trace Water up to 55%

3. A method as claimed in claim 1 or claim 2 wherein the alkaline earth metal nitrate is magnesium nitrate and/or calcium nitrate.

4. A method as claimed in any one of the preceding claims wherein the indicator is thymol blue or bromothymol blue.

5 5. A method as claimed in any one of the preceding claims wherein the nitrates are provided as aqueous solutions in separate tanks, the required amounts of each of these ingredients being supplied in the desired amount to a common tank for mixing prior to 10 introduction into the combustion chamber or flue gas.

6. A method as claimed in any one of the preceding claims wherein the combustion process is that which occurs in turbo chargers fed from the exhaust gases of 15 diesel engines.

7. A method as claimed in any one of the preceding claims wherein the combustion process is the incineration of municipal, industrial and hospital 20 waste

8. A method as claimed in any one of the preceding claims wherein the combustion process is the incineration of municipal, industrial and hospital 25 waste.

9. A method as claimed in any one of the preceding claims, wherein the additive further comprises ferric nitrate.

10. The use of a liquid additive which excludes nitrates of any alkali metal and which comprises an aqueous solution of ammonium nitrate, at least one nitrate of an alkaline earth metal, and an indicator,

- 21 in a combustion process for the purpose of preventing the formation of, or facilitating the removal of, chloride deposits on heating and ancillary surfaces of boilers and like equipment.

11. The use of a liquid additive which excludes nitrates of any alkali metal and which comprises an aqueous solution of ammonium nitrate, at least one nitrate of an alkaline earth metal, and an indicator, 10 in a combustion process for the purpose of reducing the emission of dioxins and/or furans.

12. A method as described herein and with reference to the accompanying drawings and Examples.

13. Uses as described herein and with reference to the accompanying drawings and Examples.