EP0368834B1 - Aqueous combustion catalysts and fuels - Google Patents

Description

The invention relates to aqueous combustion catalysts for improved combustion of carbon and / or hydrocarbon-containing substances, such as heating oils, heating gases and solid fuels and / or for removing soot or tar deposits on surfaces of the combustion chamber. The invention further relates to the use of such catalyst solutions and fuels.

In order to avoid soot deposits on solid surfaces on the fire side, the most complete combustion possible of the fuels used containing carbon and hydrocarbons is necessary. This can be achieved by a high excess of air, i.e. that much more air is supplied to the fuels than is necessary for their stoichiometric combustion. Although this brings about an improvement in the complete combustion, it also has the disadvantage of an uneconomical use of the fuels, since unnecessarily large amounts of air have to be heated and the amount of hot exhaust gases is increased. It is therefore endeavored to design the combustion of fuels containing carbon and hydrocarbons without forming soot or minimizing them in the case of near-stoichiometric combustion, which of course in practice is not completely or not always possible. A gradual change in the nozzle injection in the case of liquid fuels also negatively influences an initially perfect combustion setting and increases the formation of soot.

Soot formation adversely affects combustion due to the loss of heat generation as well as the reduced heat transfer, since soot has a high thermal insulation effect.

Combustion catalysts have therefore been used for a long time in order to both prevent or minimize the formation of soot and to burn off soot and tar deposits that have already formed at the lowest possible temperatures. The improved combustion of tars and soot already formed is most effectively achieved by transition metal compounds, which are known to exist in different valences. So far mainly used metal compounds are those of copper, manganese, cobalt, zinc and chromium (chromates). In particular, copper compounds or metallic copper powder significantly reduce the combustion temperature of soot and tars. However, they have the disadvantage of acting as toxic components in the exhaust gas, as well as causing a significantly increased risk of corrosion on iron materials. Copper is much more positive than iron in the electrochemical voltage series, so that in the case of copper deposits, local elements are formed on the iron materials commonly used in heating technology, which corrode the iron-containing structural parts. This is significantly increased by the presence of sulfuric acid or sulfurous acid, which are formed when burning sulfur-containing fuels.

Zinc compounds are less toxic than copper and also pose no significant risk of corrosion for ferrous materials, but are practically hardly effective as a combustion catalyst for soot and tars.

Chromium has so far been used for the present purposes mainly in the form of chromates, in order to benefit both from the combustion-catalytic effect of the metal ion and to exploit the oxidation potential of the chromates on carbon. Due to the carcinogenic effects of some chromium compounds, however, these are eliminated according to today's environmental protection requirements. The same applies to nickel and cobalt compounds, although cobalt in particular is able to have a good combustion catalytic effect.

Manganese, in particular as permanganate, has good combustion-catalytic properties and, with -1.70 volts as MnO₂ + 2H₂O = MnO₄⁻ + 4 H⁺ + 3e⁻, a high oxidation potential, but manganese compounds are also toxic and have even been classified as carcinogenic.

Iron is a non-toxic transition metal, which is often used in oil-soluble form (iron soaps, ferrocene, for example) as a combustion catalyst in liquid fuels. The injection of aqueous iron salt solutions, such as iron sulfates, into the flame has also been proposed or handled. Despite these positive properties for combustion catalysis, iron compounds were an insufficient tool to burn soot and tars that had already formed. The reason lies in the relatively low oxidation potential of trivalent iron compounds, namely -0.77 volt Fe⁺⁺ = Fe⁺⁺⁺ + e⁻.

Calcium compounds - although calcium as an alkaline earth is not known to be a transition metal - have also been recommended for removing soot deposits. Calcium has only a slight combustion catalytic effect and is only likely to exert this effect at high combustion temperatures due to the formation of hydroxyl ions in the flame. However, as alkaline earth, it has a neutralizing effect on sulfuric and sulfuric acid, provided that it is not bound to thermally stable and non-combustible anions.

Hydrogen peroxide was also used as a metal ion-free cleaning agent for fire-side carbon deposits. This oxidizing agent is also recommended in combination with free alkalis, such as alkali hydroxides, carbonates and silicates, whereby the soot is oxidized by H₂O₂ and the acid neutralization by the strong aqueous alkali solutions; there is no combustion catalytic effect.

Examples of published documents are GB-PS 1 252 624 for hydrogen peroxide and strong aqueous alkali solutions, DE-OS 3 023 520 for the addition of calcium compounds (calcium phosphates in powder form), DE-PS 2 413 520 for deblocking agents based on metal compounds containing copper powder , US Pat. No. 4,287,090 for aqueous catalytically active metal salt solutions consisting of manganese acetate, calcium nitrate and copper acetate, DE-OS 2 911 259 as a means of cleaning on the fire side with alkali iodate or alkali periodate solutions together with copper carbonate as oxidation aid and ammonia or alkali carbonates for neutralizing acidic ash components.

Potassium nitrate and ammonium nitrate were mentioned as chemical soot killers in DE-OS 2 228 467 (without a catalytic combustion effect). GB-PS 1 303 552 recommends ammonium nitrate in addition to potassium nitrate and / or sodium nitrate for the binding and condensation of sulfuric acid from the combustion exhaust gases.

In a number of other fire-side cleaners of heating surfaces, supporting oxidants or catalytic combustion aids such as ammonium nitrate, ammonium chromate or zinc and copper salts (DE-OS 1 810 424) are mentioned to remove the soot and tar fractions.

Years ago, the Swedish Nobel works brought out an agent for keeping heating surfaces clean under the brand SP-SOTIN, which, according to its own statements, contains alkali nitrates (no combustion catalyst).

All of the means mentioned aim at the removal of soot and tars, but can only insufficiently meet today's requirements for environmentally friendly heat transfer, which if possible takes place without major losses. The catalytic Combustion by suitable metals, with good effectiveness, has the disadvantage of toxins in the exhaust gas (Cu, Ni, Co, Cr, Mn), while the oxidation potential of environmentally friendly metals such as Fe, Ca is insufficient. When using oxidizing agents such as nitrates, iodates or peroxides there is only a one-off effect and soot and tar can be deposited again without hindrance after consumption of the active oxygen from these substances and worsen the heat balance and pollute the environment due to soot emissions.

It should be noted that e.g. a soot thickness as a deposit of only 1 mm can increase the exhaust gas temperature by approx. 70 ° C and entails additional fuel oil (heating) consumption of 5%.

The object of the invention is therefore to provide a combustion catalytic converter which is non-toxic, catalytically highly effective for combustion at the lowest possible temperatures of soot and tars due to atmospheric oxygen, and which has a high oxidation potential in its higher-quality form. Furthermore, there should be a neutralization potential after combustion or thermal decomposition.

Lanthanides, in particular cerium compounds, in their tetravalent form have been found to be suitable as such a combustion catalyst. Water-soluble cerium compounds are non-toxic and cerium nitrate hexahydrate has, for example, an LD50 _(oral-rat) of 4200 mg / kg, compared to pure table salt LD50 _(oral-rat) of only 3000 mg / kg.

Tetravalent cerium compounds are strong oxidizing agents, the oxidation potential of Ce (IV) / Ce (III) is approx. 1.6 volts, ie it is only slightly below that of permanganate to manganese dioxide with approx. 1.7 volts and more than twice as much high than for Fe (III) / Fe (II).

Oil-soluble cerium compounds have previously been used as fuel oil additives for improved combustion, in accordance with DE-OS 2 729 365 and US Pat. No. 4,462,810. However, these are exclusively oil-soluble and non- aqueous solutions of cerium compounds. In addition, the cerium is there in its trivalent form as an additive and not tetravalent.

Both known applications are therefore not intended for the fire-side cleaning of soot and tars and are also not suitable.

From EP-A1 270 719 it has become known to use cerium ions in the form of carboxylic acid salts in acidic solution or in powder form as a combustion aid. However, such solutions with a pH <7 have the disadvantage of increased corrosion, and there is also no neutralization potential for any acids that are present or form during combustion. An increase in the pH value was countered by the difficulty that cerium precipitates in an alkaline environment.

With regard to the possible cerium compounds and the value of the cerium, for example, Comprehensive Inorganic Chemistry in five Volumes, Volume 4, Lanthanides, Transition Metal Compounds, Pergamon Press, pages 97-101, and further on Chemistry With Inorganic Qualitative Analysis, second edition, Academic Press, page 1033.

For cleaning on the fire side, it has also proven to be advantageous if, after burning the soot or tar or after thermal decomposition of the tetravalent aqueous cerium solution applied, an alkalinity reserve is present in order to neutralize acids from the burned fuels. This neutralization of inorganic acids, such as sulfuric and sulfurous acid and organic, such as acetic acid, propionic acid, formic acid, reduces the deposition of moist and adherent, carbon-containing deposits on the solid surfaces, in particular those for heat transfer, and inhibits the corrosive action of these acids on the Materials. The use of aqueous tetravalent cerium solutions in the presence of the above-mentioned alkalinity reserve offers the advantage that these combustion-catalytically active solutions can also be adjusted around the neutral point (pH = 7) or can be present in weakly acidic or mildly alkaline aqueous solutions.

The invention is primarily characterized in that the aqueous catalyst solution has a pH of at least 7 and in a catalytically effective amount of tetravalent cerium (Ce⁴⁺) and alkali, alkaline earth and / or magnesium compounds, which after their combustion or thermal decomposition react alkaline, and contains complexing agents for this Ce⁴⁺. Further advantageous features of the invention can be found in the claims and the description.

Compared to the strongly acidic solutions e.g. from Cernitrat this provides appropriate corrosion protection for the metallic surfaces treated with it. Aqueous solutions with a strongly alkaline reaction also offer risks of attack for metallic and in particular mineral building materials, such as chamottes, as well as burns for the user during application.

The following examples are intended to explain the present invention in more detail.

example 1

In 77.4 parts by weight (GT) of water 6 GT trivalent cerium nitrate = Ce (NO₃) ₃.6H₂O are dissolved and converted into the tetravalent form with 2.2 GT 35% hydrogen peroxide, corresponding to about 2 wt .-% Cer⁴⁺ in the finished solution. Furthermore, 3 pbw of citric acid and 2 pbw of a sodium salt of maleic acid / acrylic copolymer with an average molecular weight of 70,000 were added as a complexing agent to the above solution and brought to a pH of approx. 9 with 4.4 pbw of caustic potash (KOH). To improve the wetting on the sooty and tarred solid surfaces, 2 parts by weight of a synthetic alcohol with 13 carbon atoms and 9 moles of ethylene oxide addition are added to this aqueous solution. A stable, clear solution of red-brown color is created.

This solution is sprayed onto sooty and tarred surfaces. Subsequent heating of these surfaces in normal use leads to a catalytically favored burning off of the carbon and hydrocarbon-containing deposits (soot, tar).

The aforementioned solution can also advantageously be applied to heat transfer surfaces and other soot-endangered surfaces after the respective mechanical cleaning, which is mandatory in many countries, and serves both as corrosion protection and in particular as catalytically effective surface coating, which largely inhibits sooting and taring.

After burning off the organic substances (also the citric acid and the maleic acrylic copolymer), approx. 1.9% tetravalent cerium remains, predominantly as CeO₂, and approx. 5% caustic alkalis (from 4.4% KOH and the sodium salt of the copolymer) from 100% of the aforementioned solution for the purpose of the catalytic combustion of soot and tars and for the neutralization of mineral acids, mainly sulfur and sulfuric acid as well as organic acids, such as acetic acid (from wood burning).

Example 2

The aqueous catalyst solution according to Example 1 is diluted 1:10 with water and periodically applied to heat transfer surfaces and other surfaces to be sooted by means of a mechanical, built-in spray device. It turns out that - depending on the heating load and the degree of sootiness - a periodic spraying in 2- to 24-hour intervals is sufficient to prevent soot deposits and to achieve the best possible heat transfer and thus utilization of the fuels. This method is mainly for large systems, i.e. industrial plants and caloric power plants.

Example 3

The aqueous catalyst solution according to Example 1 is diluted 1:20 with water, which means that the finished dilution contains about 1,000 ppm of tetravalent cerium ions. Of these, fine stone spraying into the combustion air of larger heating and boiler systems in the ratio of 1 GT diluted aqueous solution per 50 GT residue heating oil as a combustion catalyst brought in. Due to the relatively low proportion of 20 ppm tetravalent cerium in relation to heavy heating oil, the soot number is reduced by 3 points according to Bacharach.

Example 4

10 pbw of a tetravalent water-dispersible cerium hydrate are finely distributed in 79 pbw of water. To this end, 10 pbw of a sodium polyacrylate with an average molecular weight of 4000 are dissolved, and 1 pbw of nonylphenol with 7 moles of ethylene oxide addition.

The cerium hydrate contains 89% CeO₂. The particle size is 15 micrometers, the crystal size (XRD) is approximately 9 nanometers. The Ce⁴⁺ content is 7.2 wt .-%, based on the dispersion.

The above-mentioned aqueous dispersion is sprayed onto tar deposits in wood-burning systems and burned off by means of a brief, increased supply of air. It turns out that the catalytic effect of the tetravalent cerium can also be used to almost completely burn off the harmful and fire-hazardous tar residues in this form.

Example 5

10 pbw of trivalent cerium nitrate are dissolved in 70 pbw of water and converted into the tetravalent form with 3 pbw of 35% hydrogen peroxide (corresponding to about 3.4% by weight of cerium). Furthermore, 10 pbw of magnesium nitrate, 5 pbw of polyacrylates with a molecular weight of 2,000 (on average) are dissolved and brought to a pH of 8 using potassium hydroxide solution or caustic potash.

This solution is applied to sooty and tarred surfaces and improves the combustion of these harmful coatings similar to Example 1. For oily and greasy soot coatings, the addition of wetting agents is recommended. Nonionic or anionic surface-active substances are advantageously used, such as ethylene oxide addition products, alkanesulfonates, alkylarylsulfonates. Cationactive wash-active substances are also suitable, but they often lower the interfacial tension between the aqueous catalyst solution and the oily / tarry residues to a lesser extent and are also less economical.

When soot and tars are burned off, the magnesium nitrate is also thermally decomposed and, as a basic oxide / hydroxide, the mineral acids as well, but also organic acids, which are formed when burning sulfur-containing heating oils and coal or in wood, straw etc., accordingly neutralize.

Example 6

The solution according to Example 5 is prepared analogously, with magnesium nitrate being replaced by barium nitrate. Here too, an alkalinity reserve occurs due to thermal decomposition, as well as the combustion catalysis mentioned.

Example 7

Analogous to Examples 5 and 6, with calcium nitrate being used instead of magnesium nitrate.

Example 8

A solution according to Example 1 is prepared, the Caustic potash is replaced by lithium hydroxide monohydrate. Lithium hydroxide not only has an acid-neutralizing effect, but also a combustion-catalytic effect which supports the tetravalent cerium.

Example 9

5 pbw of cerium nitrate (Ce (NO₃) ₂.6H₂O are dissolved in 77 pbw of water and converted into the tetravalent cerium form with 2 pbw of hydrogen peroxide (35%) (approx. 1.7% by weight of cerium). 25 pbw of a 40th % solution of the tetrasodium salt of ethylenediaminetetraacetic acid (EDTA) and 1 part by weight of wetting agent, which is used as an aqueous combustion catalyst analogously to the abovementioned examples.

Example 10

Composition according to Example 9, the sodium nitrilotriacetate (NTA) being used instead of EDTA.

Example 11

40 pbw of cerium nitrate (trivalent) are dissolved in 60 pbw of water and brought into the tetravalent form with 20 pbw of 35% hydrogen peroxide (about 13.3% by weight of cerium). Furthermore, 60 pbw of oxyacetic acid (glycolic acid) are added and adjusted to a pH of 7 with potassium hydroxide solution or caustic potash. The solution is used in the sense of the aforementioned examples as an aqueous combustion catalyst for liquid, solid and also gaseous fuels.

Example 12

10 pbw of cerium nitrate (trivalent) are dissolved in 60 pbw of water and brought into the tetravalent form with 3 pbw of 35% hydrogen peroxide (about 3.3% by weight of cerium). Furthermore, 7 pbw of potassium carbonate and 20 pbw of a 40% solution of tetrapotassium EDTA are added. This solution is mechanically dispersed in the finest particle size in a ratio of 1: 1000 parts by weight of heating oils. The W / O emulsion is burned, soot formation can be largely avoided. SO₃ or H₂SO₄ is largely bound by the neutralizing effect of KOH, so that even when burning sulfur-rich heating oils, the weakly acidic and therefore less aggressive and corrosive SO₂ or sulfurous acid are mainly present in the exhaust gas.

Advantageously, the content of the cerium ions in the catalyst solution according to the invention is generally between 0.1 and 15% by weight of the amount of catalyst solution and the metal ion content of the alkaline compound is advantageously between 0.1 and 30% by weight, based on the amount of catalyst solution. In the liquid or solid fuel, the Cer⁴⁺ ion content is preferably between 1 and 100 ppm, based on the amount of fuel.

The catalyst solution according to the invention is at a pH of at least 7 in the neutral or basic range. This takes into account the required corrosion protection. The complexing agent is preferably present in the catalyst solution in such an amount that precipitation of the cerium as a hydroxide is prevented. The quantity range for the complexing agent can be between 1 and 40% by weight, based on the total amount of catalyst, preferably between 2 and 10% by weight.

Claims (14)

1. Aqueous catalyst solution for improved combustion of carbon- and/or hydrocarbon-containing substances, such as fuel oils, fuel gases and solid fuels and/or for removing soot or tar deposits on surfaces of the combustion chamber, in which the catalyst solution contains cerium salts, characterized in that the aqueous catalyst solution has a pH value of at least 7 and contains, in a catalytically effective amount, tetravalent cerium (Ce⁴⁺⁺) and compounds of alkali metals, alkaline earth metals and/or magnesium that after their combustion or thermal decomposition react alkaline, as well as complexing agents for this Ce⁴⁺.
2. Catalyst solution according to claim 1, characterized in that organic potassium, sodium or lithium salts or soaps are included as alkaline-reacting compounds.
3. Catalyst solution according to claim 1, characterized in that organic calcium, magnesium and/or barium compounds are included as alkaline-reacting compounds.
4. Catalyst solution according to claim 1, characterized in that nitrates of alkali metals, alkaline earth metals and/or magnesium are included as alkaline-reacting compounds.
5. Catalyst solution according to claim 1, characterized in that carbonates of alkali metals, alkaline earth metals and/or magnesium are included as alkaline-reacting compounds.
6. Catalyst solution according to claim 1, characterized in that polycarboxylates of alkali metals, alkaline earth metals and/or magnesium are included as alkaline-reacting compounds.
7. Catalyst solution according to claim 1, characterized in that hydroxycarboxylates of alkali metals, alkaline earth metals and/or magnesium are included as alkaline-reacting compounds.
8. Catalyst solution according to one of the preceding claims, characterized in that the catalyst solution contains Ce⁴⁺ in amounts between 0.1 and 15 wt%, relative to the total amount of catalyst solution, and that the alkaline-reacting compound is included in amounts between 0.1 and 30 wt%, relative to the total amount of catalyst solution.
9. Catalyst solution according to one of the preceding claims 1 to 8, characterized in that the complexing agent is included in amounts of 1-40 wt%, relative to the total amount of catalyst solution.
10. Catalyst solution according to claim 9, characterized in that the complexing agent is included in amounts of 2-10 wt%.
11. Use of the aqueous catalyst solution for improved combustion of carbon- and/or hydrocarbon-containing substances according to claims 1-10, characterized in that it is introduced continuously or periodically into the combustion chamber, preferably by spraying in.
12. Use of the aqueous catalyst solution for improved combustion of carbon- and/or hydrocarbon-containing substances according to claims 1-10, characterized in that it is dispersed or emulsified in liquid fuel before the combustion.
13. Use of the aqueous catalyst solution for improved combustion of carbon- and/or hydrocarbon-containing substances according to claims 1-10, characterized in that it is applied to solid fuel before the combustion.
14. Liquid or solid fuel, especially fuel oil or coal, characterized in that it contains the aqueous catalyst solution according to one or several of claims 1 to 10 in such an amount that the Ce⁴⁺ content is between 1 and 100 ppm, preferably between 15 and 35 ppm, relative to the amount of fuel.