EP0135785B1 - Process for manufacturing coal or coke briquets - Google Patents

Abstract

1. A process for the manufacture of coal or coke briquettes, characterized in that the fine-grained fuel, heated to a temperature of 100 +- 20 degrees C, is sprinkled with an aqueous solution, at 70 to 100 degrees C, especially 80 to 95 degrees C, of a fully saponified polyvinyl alcohol having a viscosity of 40 to 100 mPas in aqueous solution, the mixture of fuel and binding agent solution, optionally with further addition of heat, is homogenized and is pressed into briquettes with removal of vapours, wherein fully saponified polyvinyl alcohols are those having a degree of saponification or hydrolysis of the underlying solvolysed polyvinyl acetate exceeding 97 mol %, especially of 98 - 100 mol %, and the polyvinyl alcohol is used in the form of an aqueous solution comprising 10 to 15% dry substance.

Description

The common binders for hard coal briquettes are pitch and bitumen. Because briquettes with these binders do not burn without smoke and soot and break down too quickly due to softening of the binder, so that unburned coal falls through the grate, there has been no lack of attempts to improve hard coal briquettes by choosing other binders.

A new briquette that meets the requirements of the domestic fire market made of a comfort fuel must have good loading, weather resistance and fire resistance, it should burn free of smoke and soot and the combustion gases must be free of pollutants.

Loading strength means that the briquettes already have such high strength about 20 minutes after pressing that they can withstand the mechanical stresses when loading them into wagons, buckets, trucks and the like. Ä. have grown without breaking. This strength in the cold state (cold strength) should increase with a greater time interval from the pressing, but in no case should it drop noticeably again, in order to ensure that briquettes are resistant to trade and consumers.

The weather resistance indicates whether and to what extent the briquette strength can decrease due to the influence of air humidity and precipitation when stored outdoors. If it also increases again due to drying, for example due to solar radiation, it should not, however, fall below a point pressure resistance according to DIN 23081 of 50 kg.

The stability of the briquettes in the fire is ultimately the goal of any briquetting. Since fine-grained material, such as washed anthracite fine coal, fine coke of any kind and similar fine-grained fuels, cannot be burned in domestic furnaces, they are made into pieces by pressing with a binder and are therefore usable like nut coal. It would make no sense for the briquettes to fall apart at the start of the combustion and to pass through the grate and into the ash pan as unburned material.

The smoke-free and soot-free combustion as well as the freedom from pollutants in the combustion exhaust gases has meanwhile become a requirement of environmental protection. Emission values of briquettes are expected which do not significantly exceed those of anthracite nut coals and crushed cokes.

A new binder should be easy to work with and should not cause any nuisance to the workforce or the surroundings of the briquette factory. If a briquette factory with a conventional procedure already exists, the new binder should not require extensive renovation or even the new construction of the briquette factory for market and cost reasons.

Numerous organic and inorganic binders have been known for decades (see Jünemann, Brikett-Industrie pp. 140 - 147). However, they could not be introduced in practice because the briquettes could no longer be produced with the usual devices, but instead required difficult processing techniques, which made the briquettes' production costs too high. In many cases, the briquette qualities also proved to be inadequate. The disadvantage of all inorganic binders is that they increase the ash content of the briquettes quite a bit.

As an organic binder, i.a. also suggested molasses. However, due to its water solubility and long solidification time, it proved to be unusable for modern briquette processes and could not be introduced in the past due to its poor weather resistance. In order to improve the resistance to weather influences, it is also known to add sulfuric acid to the molasses or substances which form sulfuric acid when it is displaced, but this is already prohibited today because of the increase in the sulfur dioxide content in the exhaust gas.

More recently, experiments with adhesive plastics, such as latex-like aqueous plastic preparations, thermoplastics and polyurethanes, have been carried out, but they also did not meet practical requirements (see Glückauf Research Books 36 (1975) pp. 156-61). The same applies to specially selected, namely cationic, polyurethanes which, according to DE-OS 30 17 599, were used with up to 19% of a natural or synthetic binder, in particular on a bitumen basis. DE-OS 31 14 141 also suggests using polyvinyl alcohol as a binder in combination with the cationic polyurethane. However, tests by the applicant have shown that this does not achieve a satisfactory Velade strength, since the setting takes place very late and the weather resistance is also insufficient.

It has now surprisingly been found that certain types of polyvinyl alcohol, with special incorporation even without other additives previously regarded as essential, enable the production of coal or coke briquettes, in particular hard coal briquettes, which meet the conditions mentioned above and above all have good stability in fire.

The invention relates to a process for the production of coal or coke briquettes, in which the fine-grained fuel heated to a temperature of 100 + 20 ° C with an aqueous 70 to 100 ° C, in particular 80 to 95 ° C hot solution of a fully saponified polyvinyl alcohol a viscosity of 40 to 100, preferably 50 to 70 mPas, sprayed, the mixture of fuel and binder solution, if necessary with further heat supply, homogenized and compressed to briquettes with good dewatering. The viscosity data relate - as usual - to a 4% aqueous solution at 20 ° C.

This polyvinyl alcohol is preferably added in an amount which makes up 0.3 to 2% by weight, in particular 0.5 to 1.5% by weight, based on the dry substance of coal or coke.

"Fully hydrolyzed" polyvinyl alcohols are those with a degree of saponification or hydrolysis of the underlying solvolysed polyvinyl acetate of over 97 mole percent, in particular 98 to 100 mole percent. In this The crystallinity of the polyvinyl alcohol rises sharply in the area and this leads to drastic differences in the property profile compared to other z. B. only partially saponified polyvinyl alcohols. The fully saponified polyvinyl alcohols with high viscosities suitable for the purposes of the invention are e.g. B. as MOWIOL R types 56 - 98 and 66 - 100 (HOECHST) on the market.

For a homogeneous distribution of the aqueous binder solution with the fine-grained fuel, on the one hand, it is necessary that this solution is not too viscous, but on the other hand, no more water should be introduced than is absolutely necessary. The polyvinyl alcohol is therefore added in aqueous solution.

To prepare the binder solution to be used, the fully saponified polyvinyl alcohol of the required viscosity range is expediently dissolved in hot water. This gives an aqueous solution with 10 to 15% dry matter, which is sprayed hot on the coal and then in a suitable mixing unit, such as. B. a double mixing screw, is mixed well with the coal.

In the production of briquettes made of anthracite coal with an aqueous binder solution that contains only the specially selected fully saponified polyvinyl alcohol, but no adhesive plastic, such as. B. contained a cationic polyurethane, briquettes have already been achieved with a very good loading strength and very good final strength. The stability in the water was sufficient immediately after production, but improved noticeably after 24 hours. The heat resistance, i.e. Stability of the briquettes in the fire, can still be used for domestic stoves without too much unburned coal falling through the grate in the ash pan during combustion, but appears to be in need of improvement in particular for use in heating boilers, so that these smokeless briquettes with a very low emission value also can be used there.

Regardless of the bad experiences that have been made in the past with molasses as a binder, it has surprisingly been found that the combination of the special polyvinyl alcohol in combination with molasses achieves a very significant improvement in the heat resistance while maintaining the good briquette properties mentioned. With the method according to the invention, which fits well into the usual briquette production, low-smoke hard coal briquettes can thus be produced which fully meet the conditions of a comfort fuel mentioned at the beginning. Accordingly, the coal or coke briquettes contain 1 to 3, preferably 1.5 to 2.5% by weight of molasses and 0.3 to 2, preferably 0.5 to 1.5% by weight of a fully hydrolyzed polyvinyl alcohol with a viscosity of 40 to 100, preferably 50 to 70 mPas, each based on the dry matter of coal or coke.

Tubular or beet molasses are useful for the purposes of the invention. Beet molasses is obtained with a dry substance of 85 to 89%, with tubular molasses the water content is usually somewhat higher. Molasses has a high content of carbohydrates, especially sugar, as well as non-sugar substances, especially amino acids and nitrogen-free carboxylic acids in the order of about 40%. In addition there is an ash content of 10 to 16% on average, which mainly consists of potassium, sodium, calcium and magnesium salts. The heat resistance can also be further improved by adding up to 2% by weight, in particular 1.2 to 1.8% by weight, of powdered calcium carbonate, based on the dry matter of coal or coke. You can also replace part of the molasses with calcium carbonate. For this purpose, the fine-grained fuel sprinkled with the binder solution from the polyvinyl alcohol and possibly the molasses is sprinkled with the calcium carbonate in the specified amount, the mixture of fuel and the additives, if appropriate with further heat supply, homogenized and, with dewatering, pressed into briquettes.

In order to be fully effective in the mixture, the calcium carbonate should be as fine as possible and have a grain size below 100 µm. For reasons of cost, cheaper products, such as ground limestone, e.g. B. lime slate, marble, marl, chalk, stalactites or limestone tuff are preferred. The limestone can also contain a certain percentage of dolomite. Precipitated calcium carbonates are also usable for the purposes of the invention, which accumulate as a waste product in chemical processes and do not even need to be dried, because even with the incorporation of a slurry in the mixing units, sufficient homogenization with the fine-grained fuel can still be achieved, and because requires the use of the aqueous binder solution according to the invention without dewatering.

The weather resistance of the briquettes can be increased by spraying the freshly pressed briquettes with a high-percentage, in particular saturated sodium carbonate solution or by briefly immersing them in them.

As coal or coke for the production of the briquettes according to the invention, various fine-grained fuels, such as. B. anthracite coal, fine coke, flame coal, fat and lean coal, charcoal, lignite and charcoal are used.

The following procedures are used to check the quality characteristics of the briquettes described at the beginning:

The cold strength is checked by randomly removing 10 to 50 briquettes from production. Each briquette is loaded according to DIN 23081 between two stamps of at least 15 mm diameter with a stamp feed of at least 8 mm / min until it breaks. The breaking load in kg is given as the cold strength, the mean being calculated from the individual values.

The stability of the briquettes in water is checked as a measure of the weather resistance. Immediately after pressing and then at certain intervals, 10 briquettes are placed in a water bath. After a dwell time of 15 minutes, they are removed and their strength immediately according to the method of Cold resistance tested. The start of water bathing is the time of the water resistance.

The heat resistance of the briquettes is examined to determine their stability in the fire. 5 briquettes are placed in a muffle furnace heated to 600 ° C and removed after certain time intervals. Then they are loaded on the test press immediately after the cold strength process until they break.

As a measure of this fire resistance, combustion tests are carried out in the house furnace in accordance with the test regulations of DIN 18 890. The ash content is determined on the basis of the grate diarrhea. The soot-tar number is also identified as an indication of the emission. The pollutants in the exhaust gases are measured using the method described by Ahland / Merten in VDI reports No. 358/1980.

example 1

1 kg of a fully saponified polyvinyl alcohol with a degree of hydrolysis of 98.4 mol% and a viscosity of 56 mPas (commercial product MOWIOL 56-98) is dissolved in 10 l of water at 95 ° C. and then in each case so much molasses that the finished briquette is mixed has molasses content (in percent by weight) indicated in Table 1.

The aqueous binder solution is sprayed in an amount of 10 l / 100 kg of coal at a temperature of 80 to 95 ° C. onto washed anthracite coal, which in turn is heated. For the various tests, up to 3% molasses, based on the dry substance, was stirred into this binder solution. There are approximately 10 minutes between metering and pressing on conventional briquette presses, in which the briquette material is homogenized in a mixing unit and at the same time the water is easily escaped. The briquettes are then processed into egg briquettes in conventional double-roll presses. The various strengths, emission values and the analysis of the exhaust gases were then carried out as described above.

The loading strength, 20 minutes after pressing, was 415 N, well above the minimum value of 300 N to be demanded.

The final strength was excellent at 950 N. With a soot-tar number of 18, the emission value when burning the briquettes clearly fell below the upper limit of low-smoke fuels of 200. The stability in water was 350 N, but the values became much better 24 hours after pressing with over 500 N.

The increase in the strength of the briquettes with a constant polyvinyl alcohol content of 1% and increasing proportions of 1 to 3% molasses, in each case based on the dry substance, are shown in Table 1. The percentage increase in strength was given in brackets behind the strength values. For these experiments, the required amount of molasses was stirred into the binder solution.

Example 2

Until the 10% solution of the specified polyvinyl alcohol type was sprayed onto the coal, the procedure was as in Example 1, but then powdered calcium carbonate was sprinkled into the mixing unit in an amount which corresponded to the percentages given in Table 2. As can be seen, the addition of calcium carbonate significantly improves the heat resistance of the briquettes made with polyvinyl alcohol as a binder. In this case too, the percentage increase in strength was given in brackets after the heat resistance values after the different times.

Example 3

In this case, according to Example 1, briquettes produced only with the polyvinyl alcohol mentioned as a binder were immersed in a saturated sodium carbonate solution for a very short time immediately after the pressing. This procedure corresponds to an atomization. As a result of this measure, the determination of the weather resistance after 24 hours resulted in an improvement in strength of about 15% compared to the untreated briquettes.

Claims (6)

1. A process for the manufacture of coal or coke briquettes, characterized in that the fine-grained fuel, heated to a temperature of 100 ± 20°C, is sprinkled with an aqueous solution, at 70 to 100°C, especially 80 to 95°C, of a fully saponified polyvinyl alcohol having a viscosity of 40 to 100 mPas in aqueous solution, the mixture of fuel and binding agent solution, optionally with further addition of heat, is homogenized and is pressed into briquettes with removal of vapours, wherein fully saponified polyvinyl alcohols are those having a degree of saponification or hydrolysis of the underlying solvolysed polyvinyl acetate exceeding 97 mol %, especially of 98 - 100 mol %, and the polyvinyl alcohol is used in the form of an aqueous solution comprising 10 to 15 % dry substance.

2. A process according to claim 1, characterized in that the polyvinyl alcohol is added in a proportion which amounts to 0.3 to 2 % by wt., especially 0.5 to 1.5 % by wt., referred to the dry content of coal or coke.

3. A process according to claim 1 or 2, characterized in that the fully saponified polyvinyl alcohol has a viscosity of 50 to 70 mPas.

4. A process according to claim 1, characterized in that molasses are added to the polyvinyl alcohol solution in a proportion which amounts to up to 3 % by wt., preferably 1.5 to 2.5 % by wt., referred to the dry content of coal or coke.

5. A process according to one or more of claims 1 to 4, characterized in that calcium carbonate is added to the fine-grained fuel, onto which the binding agent solution of the polyvinyl alcohol and possibly the molasses has been sprayed, the calcium carbonate being in a proportion which amounts to up to 2 %, especially 1.2 to 1.8 % by wt., referred to the dry content of coal or coke, the mixture of fuel and the additives, optionally with further addition of heat, is homogenized and is pressed to form briquettes with good removal of vapours.

6. A process according to one of claims 1 to 5, characterized in that the newly pressed briquettes are sprinkled with a high-percentage, especially saturated sodium carbonate solution or are briefly dipped into this solution.