GB2040308A - Process for manufacturing smokeless agglomerate fuels, smokeless agglomerate fuels thus prepared and oven for use in this process - Google Patents

Abstract

Process for manufacturing smokeless agglomerate fuels from agglomerates of carbonaceous material, in particular coal and/or coke particles and a bituminous binder, which comprises subjecting the agglomerates to an oxidizing heat treatment in at least three stages using hot gas and then cooling the agglomerates, the first stage being a heating and drying step, the second stage being an oxidation step, and the third stage being a finishing oxidation step. An oven suitable for the process may be a travelling grate oven having 3 or more zones.

Description

SPECIFICATION Process for manufacturing smokeless agglomerate fuels, smokeless agglomerate fuels thus prepared and oven for use in this process.

This invention relates to a process for manufacturing smokeless agglomerate fuels from agglomerates of carbonaceous material, in particular coal and/or coke particles and a bituminous binder, to smokeless agglomerate fuels thus prepared and to an oven for use in this process.

Air pollution from various fuels increasingly becomes the object of attention in several countries, and for instance the British "Clean Air Act" has laid down requirements for fuels for domestic heating as regards their smoking properties. Agglomerates of carbonaceous particles and bituminous binder, e.g., "green briquettes", may burn with evolution of smoke, depending upon the nature and the amount of volatile matter. Therefore they are usually subjected to some kind of heat treatment, e.g., a dry distillation, optionally preceded by an oxidizing treatment. Thus, it is known from French Patent 1,047,584 and its additions 63,415, 66,133 and 67,980 to render smokeless by oxidation small agglomerates of carbonaceous substance. That process comprises two heating stages at different temperatures, followed by a cooling stage.In this process the reaction rate is mainly controlled by the oxygen content and/or the temperature of the treating gases, thus requiring complicated gas dosing means. Moreover, it is not mentioned to which degree the carbonaceous substance is rendered smokeless.

In the present invention the problem of controlling the reaction rate has been solved by recognizing that given a mass of agglomerates -threevariables affect the reaction rate: the speed, the temperature, and the oxygen content of the treating gas. By maintaining a certain temperature and oxygen content, the gas speed, which is easily regulated by electric fans, governs the reaction rate. Moreover, it has been found that a special sequence of three of more oxidizing stages results in a great flexibility of the process. In addition to considerably lowering the tar content the mechanical strength, expressed in, e.g., crushing strength, abrasion resistance and resistance to dropping of the agglomerates is substantially improved.

According to this invention, smokeless agglomerate fuels are manufactured from agglomerates of carbonaceous material, in particular coal and/or coke particles and a bituminous binder by subjecting the agglomerates to an oxidizing heat treatment in at least three stages using hot gas and then cooling the agglomerates, the first stage being a heating and drying step, the second stage being an oxidation step. By smokeless is meant a tar content below 1.4%w, which corresponds with the requirements laid down in the British Clean Air Act for domestic heating.

The process of this invention is now described in more detail: The agglomerates of, e.g., coal and/or coke particles and bituminous binder may be in the form of briquettes, extrudates, pellets, etc., such as the agglomerates disclosed in British Patent Specification 1,498,494 and in French Patent Specification 7,538,325. It is possible to use coal duff, coke breeze, pulverized anthracite, or any coal fine, carbonaceous product or waste, pure or mixed with other matter. The binder may be any conventional bituminous material, such as coal tar pitch, petroleum bitumen or pitch, ethylene cracker residual pitch and so forth. Preferred is a petroleum bitumen, in particular a hard bitumen, penetration, e.g., 1-15 dmm, softening point 8Q-95"C, such as a high vacuum bitumen or a cracked bitumen.

Semi-blown or fully blown precipitation, in particular propane bitumen, which may contain a fluxing oil, can also be used.

In the first stage of the heat treatment the agglomerates are heated up to, e.g., 250"C. They lose their residual water and an exothermic reaction, i.e., the oxidation of the binder, starts. The temperature of the gas just before it strikes the agglomerates is adjusted to about 250-350"C, preferably 250-300"C, but a lower or higher setting is also possible, provided the residual water is lost and the exothermic reaction is started, respectively no spontaneous combustion of agglomerates occurs. The purpose of the gas flow in the first stage is primarily to deliver the necessary calories.

Second stage: because of the exothermic reaction the temperature of the agglomerates rises, and exceeds the gas inlet temperature. To prevent the agglomerates from catching fire, it is necessary to moderate the conditions, e.g., by lowering the gas inlet temperature or preferably by water-spraying. The purpose of the gas flow is now to take away heat created inside the agglomerates. Accordingly, the gas outlet temperature becomes higher than the inlet temperature. Usually the gas inlet temperature is set between Sand 50"C lower, preferably 20"C lower than in the first stage, but it is possible to maintain the temperature and moderate the reaction rate by a higher gas speed, a lower oxygen content and/or the addition of water, as will be explained.

For every oxygen concentration of the gas there is an equilibrium gas speed, where the heat created by the exothermic reaction is carried away by the gas. If the gas speed is lower than this equilibrium speed the exothermic reaction will run out of control and the agglomerates will start burning, while too high a gas speed will result in the agglomerates not being correctly desmoked within a given time, as their core temperature is not sufficiently high. It is also possible, and sometimes advisable, to inject water and/or air to slow down the reaction rate. In the case of water its evaporation lowers the temperature and secondly the resulting steam lowers the partial oxygen concentration, thus slowing the reaction rate in two ways.

The third stage of the heat treatment is meant to desmoke the remaining unreacted parts of the agglomerates. As the exothermic reaction in the second stage proceeds, the rate slows, because the concentration of unreacted material decreases. Likely the outer layers of the agglomerates already have reacted with oxygen, while the cores yet have to react, because the diffusion of oxygen takes some time. In order to complete the reaction quickly, the inlet gas temperature is raised again, e.g., to the same value as in the first stage. Any other temperature is possible, in principle, but the same remarks about this as in the second stage apply.

Sometimes it is necessary to add a fourth, fifth, etc. stage of thermal treatment, to remove or convert the last remnants of potentially smoke-producing matter. The temperature of the last stage will almost always be higher than in the penultimate stage, for reasons discussed above in the third stage. The total treatment time is normally less than 2 hours, usually about 80 minutes, e.g., four stages of 20 minutes each.

Suitable layer thicknesses of the agglomerates are 10 to 50 cm.

Preferably the gas speed and oxygen content of the hot gas are such that the core temperature of the agglomerates does not exceed 420"C in any of the stages, and does not drop below 200"C in the second or any following stage. Thus, the oxidizing heat treatment may be carried out in a travelling grate oven, using air and/or fumes as a vehicle to exchange calories with agglomerates. In the stages 2 and higher the exothermic reaction has to be controlled by a minimum gas speed through the layer to prevent the agglomerates from catching fire. The gas speed leaving a 40 cm thick layer of egg-shaped briquettes of 30 g must, e.g., exceed 1.6 m/s for an oxygen contentof 17to 18%v. Oxygen contents higherthan 10%vare preferred.

It is obvious that the fumes evolved during the heat treatment can be combusted, condensed or recycled to avoid pollution of the environment.

Finally, a cooling step is required for the safe handling, storage and use of the agglomerates. In this way the exothermic reaction is stopped, and catching fire of hot agglomerates is prevented. It is feasible to cool the agglomerates with air and/or water. Air cooling is time- and space-consuming. Spraying with water is also possible, but by far the quickest and most space-saving method is immersion in a water tank. The temperature of the water and the immersion time can be varied to control the final temperature and the water content of the agglomerates. When the agglomerates are allowed to cool further in the air, part of the water that was taken up during the immersion will evaporate.Typical but not limitative values are: temperature of hot agglomerates: 350"C, after 7 minutes of water cooling: 1 00C, temperature of the water tank: 80-85 C.

The apparatus in which the heat treatment is carried out can be derived from any known furnace or oven, operating batchwise or continuously and may be a circulating furnace, a travelling grate oven, a tunnel furnace, etc. A travelling grate oven is preferred. The oven comprises at least 3 zones, preferably 3 separated compartments, being regulated separately with burners, water nozzles, etc., preferably in fixed position. A fluidized bed type oven may also be used. If the hot gas is flowing through a horizontal layer of agglomerates from below, it is observed that the agglomerates in the upper layer reach the highest temperatures, because of the generation of heat in the lower layers. Of course, this is the reverse of the first stage where the gas is delivering heat and the upper layer will be cooler initially.

The resulting agglomerates are particularly suitable for domestic use.

EXAMPLES Example! (comparative Example) Ground and dried anthracite was fed into a heated mixer and heated to 70-800C, then 5.5%w of H 80/90 bitumen binder (softening point 80/90 C, penetration 6-15 dmm) at 220"C was added to the coal. After mixing for 10 minutes at 80"C, the mixture was pressed. 46 kg of small briquettes (about 9cm3, 11.2 g, 4.3%w water) were produced. The desmoking was carried out in a two-zone pilot oven without atmosphere control, i.e., in air. The briquettes were put in a metallic basket (70 x 70 x 15 cm), bed thickness was 14 cm. They were heat-treated for 20 minutes in zone 1 at 250"C and for 40 minutes in zone 2 at 230"C (gas inlet temperatures).

The upper layer reached 31 5"C at the end of the treatment. After forced coolng in air the briquettes were analyzed; weight 10.8 g, water content nil, residual tar content 1.6%w and thus do not meet the requirement of the British Clean Air Act as indicated hereinbefore.

Example lI The experiment of Example I was repeated, but the heat treatment consisted of three stages: 20 minutes at 250'C, 20 minutes at 230"C and 20 minutes at 2850C. The upper layer reached 360"C at the end of the treatment. The residual tar content was 1.2%w.

The rise in temperature at the end of the heat treatment in Example II apparently served to remove 25% more tar in the same time.

Example //l The process of the invention was carried out under the conditions tubulated. The mechanical properties of the "green" vs. the heat-treated briquettes were measured. Quenching was by immersion in water.

Example Ill A Example Ill B Composition Binder content, parts per hundred 5.5 6.1 Type of binder 85/2 bitumen H 80/90 bitumen Type of coal, %w anthracite 60 anthracite 100 meager coal 15 steam coal 15 fettlings 10 Properties of green briquettes Water content, %w 4.5 4.7 Average crushing strength, kg 74 40 Standard deviation (30 briquettes) kg 7.2 9.8 Resistance to dropping % passing 5 mm sieve after 1 > c 5m 5.8 8.7 2 x 5 5m 14.2 21.4 3 x 5m 22.5 31.5 Resistance to abrasion, after 100 revolutions % passing 5 mm sieve 8.8 9.1 Tar content, %w 3.18 3.39 Weight volume density Conditions of treatment Number of zones 3 4 Gas inlet temperature, C 290-270-310 300-280-300-320 Oxygen content, %v 17 19.5 Resistence time in zones, min. 40-20-20 20-20-20-20 Properties treated briquettes Weight, g 44.4 33.1 Volume, ml 34.2 26.7 Apparent density, g/ml 1.30 1.24 Water content, %w Average crushing strength, kg 129 79 Standard deviation (30 briquettes) kg 12.7 15.7 Resistance to dropping % passing 5 mm sieve after 1 x 5m 1.8 3.0 2 x 5m 4.5 7.4 3 x 5m 8.7 11.8 Resistance to abrasion, after 100 revolutions % passing 5 mm sieve 4.8 5.0 Tar content, %w 1.0 1.0 These briquettes are virtually smokeless, and show improved mechanical properties.

Claims (16)

1. Process for manufacturing smokeless agglomerate fuels from agglomerates of carbonaceous material, in particular coal and/or coke particles and a bituminous binder, which comprises subjecting the agglomerates to an oxidizing heat treatment in at least three stages using hot gas and then cooling the agglomerates, the first stage being a heating and drying step, the second stage being an oxidation step, and the third stage being a finishing oxidation step.

2. Process according to claim 1, wherein the inlet temperature of the hot gas in the first stage lies between 250 and 350"C.

3. Process according to claim 1 or 2, wherein the inlet temperature of the hot gas in the second stage is up to 50"C lower than in the first stage.

4. Process according to any one of claims 1 -3, wherein the inlet temperature of the hot gas in the third stage is higher than in the secod stage.

5. Process according to any one of claims 1-4, wherein the number of stages is four, and wherein the inlet temperature of the hot gas in the fourth stage is higher than in the third stage.

6. Process according to any one of claims 1-5, wherein hot gas is passing through a layer of agglomerates of a thickness of 10 to 50 cm.

7. Process according to any one of claims 1-6, wherein the gas speed and the oxygen content of the hot gas are such that the core temperature of the agglomerates does not exceed 420"C in any one of the stages, and does not drop below 200"C in the second stage or any following stage.

8. Process according to any one of claims 1-7, wherein the oxygen content of the gas is higher than 10%v.

9. Process according to any one of claims 1-8, wherein the cooling of the agglomerates is effected by water and/or air.

10. Process according to claim 9, wherein cooling of the agglomerates is effected by quenching them in water.

11. Process according to any one of claims 1-10, wherein the bituminous binder is a petroleum bitumen having a pentration of 1-15 dmm and a softening point of 80-95"C.

12. Process according to any one of claims 1-11, wherein the bituminous binder is a high-vacuum bitumen or a cracked bitumen.

13. Process as claimed in claim 1, substantially as hereinbefore described with special reference to the Examples.

14. Smokeless agglomerate fuel manufactured according to any one of the preceding claims.

15. An oven, in particular a travelling grate oven, for use in the process of claims 1-13, comprising at least 3 zones.

16. Oven according to claim 15, wherein the zones are separated compartments being regulated separately.