EP0048802B1 - Process for controlling the quality of coke - Google Patents

Description

The invention relates to a method for controlling the quality of coke from coal feed mixtures which are coked in the coke oven by indirect heating with the exclusion of air, the coal feed mixture undergoing plasticization during the coking process when passing through the temperature range of 320-550 ° C., which increases with the cooking time shifted from the furnace wall to the center of the furnace.

Mixtures are usually used for coking. I.e. The coking coals are first ground in order to achieve certain different grain sizes. When mixed, the different grain sizes give a grain spectrum that significantly influences the bulk density of the coking coals in the coke oven. It has been shown that optimal grain spectra can usually only be achieved by mixing different ground grain sizes.

The quality of the coking coal used for the feed mixtures has an even greater influence on the coke quality than the bulk density. Therefore, various test methods have been developed with the aim of assessing the coking behavior of the coal in advance. Significant properties result from the petrographic structure of coal (types of coal, strips) of their chemical composition (elemental composition, ash content, volatile constituents, bitumen content, state of oxidation) and their physical / chemical properties (softening behavior, bloating, shrinking, driving, degassing behavior).

Baking capacity is also of major importance. Baking ability is understood to mean the ability of the coal to change to a plastic state when heated in a vessel which allows the coal to expand freely and to form a baked, lumpy coke when heated further. Coal with good baking power not only provides baked, but even melted coke. The baking capacity defined in this way is measured either by the swelling index or by the Roga baking number.

A coal with a good baking capacity can still produce a less high-quality coke under the operational coking conditions than a poorly baking coal. Therefore, the coking capacity has to be considered in a special way. The coking capacity of a coal is assessed either by the dilatometer test or by determining the Grayking coke type. To determine the course of dilation, the change in length of a conical carbon compact is measured at a heating rate of 0.05 ° C / sec.

After all, measuring plastic behavior has so far been of some importance. The plastic behavior is understood as the ability of the coal to change into a plastic state within the temperature range between about 350 and 550 ° C.

Modified rotary viscometers are used in the usual methods for measuring plasticity. The result of the measurement, however, is not the viscosities of the softened coal mass in the physical sense, but quantities influenced by the apparatus, which are composed of the internal friction of the solid, liquid and gaseous phases and the elasticity variables of these three phases. Such measurements are usually laboratory measurements. This also applies to measurements of the degassing behavior, the driving pressure and the shrinkage, which are also used to determine the quality of the coal used. The large number of test methods commonly used here, which are commonly used in practice, for determining the amount of coal used and for determining the quality of coke shows that previously, targeted control of the quality of coking coal as the basis for high-quality coke was only possible with multiple measurements and particular difficulties.

The object of the invention is therefore to simplify the control of the required quality of the input material. The invention is based on the consideration that essential parameters for the coke quality have a common effect in a partial area of the coking process. This is the plastic area in further knowledge, which becomes clear when considering the coking process of hard coal in indirectly heated horizontal chamber furnaces.

Coking produces heating gases that are burned in the heating trains adjacent to the ovens. The heat is transferred from the neighboring heating trains through the furnace walls to the furnace lining, i.e. the insert coal mixture, transferred. Due to the indirect heating on both sides, temperature fields arise with a decreasing temperature level towards the center of the trim. The temperature fields are usually symmetrical to the center of the furnace, where the lowest temperature prevails. With increasing cooking time, the temperature front progresses from the walls to the middle of the oven, accompanied by a steep temperature gradient during the transition from the already coked to the unchanged charcoal. This transition area, in which all temperatures of 100-1000 ° C are run through, is locally limited and at the beginning of the coking process (pyrolysis) only extends over a few mm in the mainly 450 mm wide furnace chamber.

In the critical transition area, the gradual coking of the home carbon takes place, whereby two essential temperature ranges can be distinguished. One temperature range extends from about 320-480 ° C. In the temperature range, the coal softens and forms a quasi-plastic state. When through When the temperature range is exceeded, a large number of decomposition and polymerization reactions take place, creating the conditions for integrating inert components and pre-embossing the coke quality.

The other area is above the reconsolidation of the furnace stock with a contraction maximum around 600 ° C. In this area, the coke structure is formed with further flavoring of the stock.

The reactions taking place in both areas are dependent on the type of coal used and the grain size of the feed components, but the invention is based on the fact that the type of coal and grain size are largely predetermined in normal coking plant operation and, for the respective coke quality, rather the dependence of the coking on the operating conditions and The focus is on the pretreatment of the feed or the use of coking agents. I.e. According to the invention, the extent and the uniformity of the reactions taking place in these critical transition areas is of crucial importance.

The temperature gradient is decisive for the course of these reactions. A high temperature gradient means a high local heating rate at which the reactions taking place can only develop inadequately or, depending on their kinetics, are even overflowed. This leads to great inhomogeneity in the reaction process and results in an inhomogeneous coke structure. In addition, under such conditions there is not enough time to release plastic mass in sufficient quantity for the incorporation of the inert particles and to ensure mass transport with the aim of enveloping these particles. The duration of the plasticity, which correlates clearly with the heating rate i.e. decreasing with increasing heating rate is too short.

In the post-plastic area, a high temperature gradient leads to increased mechanical stresses, which lead to increased crack formation and thereby increase the undesirable portion of small coke.

In contrast, a reduction in the temperature gradient leads to a wider plastic zone with a locally lower heating rate. This extends the duration of the plasticity, which above all promotes the cementing of the individual particles, which is important for coke production. The reactions pre-embossed in the coal have more time to take effect and to develop quasi equilibrium states according to their reaction kinetics.

As the width of the plastic zone increases and the reaction proceeds more intensely, the amount of gas in the bubble in the plastic zone increases; the gas pressure rises. At first approximation, an approximately inversely proportional relationship is seen between the gas bubble size and the pressure with the same grain size of the insert coal. Increasing pressure causes smaller gas bubbles, which can result in a smaller pore diameter. This behavior of the gaseous decomposition products decisively overlaps the influence of the grain size on the swelling behavior of the bodies and the resulting welding of the surrounding particles.

In addition, the diffusion paths for the released gases increase with increasing width of the plastic zone. As a result, the gas pressure is maintained over a longer period of time, which can lead to greater homogeneity in the gas bubble distribution as in the previous generation and further reactions of gaseous decomposition products.

FR-A 752 468 refers to the plastic zone, but the temperature range of 650-750 ° C has been selected. This is the area of reconsolidation with a contraction maximum around 600 ° C. In this area, the coke structure is formed with further flavoring of the stock. The reactions taking place in the plastic zone and in the post-plastic area are dependent on the type of coal used and the grain size of the feed components, but the invention is based on the fact that the type of coal and grain size are largely predetermined in normal coking plant operation and rather the dependence on the respective coke quality Coking of the operating conditions and the pretreatment of the input material or the use of coking-active auxiliaries is the main reason. At the same time, the invention is based on the fact that the reactions in the plastic zone largely predetermine the phase of the reconsolidation, but the reverse is not ensured.

According to the invention, the quality of the coke is therefore controlled in that the distance of the plastic zone from the furnace wall is measured at predetermined time intervals, compared with a setpoint and the deviation from the setpoint by adding preheated coal and / or carbo and / or petrostatic coal binder is balanced.

According to the invention, the width of the plastic zone is the decisive factor influencing the coke quality. Your measurement enables a precise prediction of the coke quality that is set and is therefore to be used as a central control variable for the provision of the components used, the course of the coking process and for the resulting coke quality as a target variable.

The addition of carbo- and / or petro-derived binders according to the invention reduces the softening temperature of the furnace stock, but hardly changes its reconsolidation temperature. A larger temperature range is created for the plastic zone. This results in a spatial expansion of the plastic zone, so that the essential effects explained above in the duration of the plasticity, in the course of the reactions, in the gas bubble size and the homogeneity of the gas bubble distribution.

The addition of preheated coal according to the invention also results in a reduction in the temperature gradient. As already stated, this causes the plastic area to widen as a function of the distance from the chamber wall, this effect gaining in size and importance with increasing cooking time. The lower heating rate associated with the lower temperature gradient is also of great importance for the post-plastic area (contraction maximum at about 600 ° C). The mechanical stresses are reduced. This reduces the tendency to crack. The output on blast furnace coke increases as a success. Furthermore, the abrasion resistance (MIO) and the piece strength (M _4o ) increase according to ISO. The average pore diameter of the coke is significantly reduced. At the same time, the average wall thickness of the cell walls of the coke structure increases, which increases the structural strength of the coke.

In a further embodiment of the invention, the width of the plastic area is measured using the temperature distribution in the furnace stock. I.e. Temperature measurements are carried out at closely spaced intervals from the chamber wall to the center of the chamber. The resulting temperature values are compared with the temperature setpoints that determine the minimum width of the plastic area. If the temperature falls below the specified target temperature, coal binders and / or preheated coal are added to the feed mixture in subsequent furnace batches until the desired target temperatures (minimum temperatures) are reached in the furnace. Corresponding to the plastic area that changes during the coking process, the target temperatures corresponding to the limit values of the plastic area must be observed at different points in the cross-section of the furnace in each coking phase. The setpoint temperature can remain constant and assume one of the values between 320 and 480 ° C. A value at the lower temperature limit is preferably selected.

The spatial distances can be taken into account by a number of temperature measuring devices distributed over the width of the furnace chamber or by a single temperature measuring device that can be moved back and forth across the width of the furnace chamber. Thermocouples can be used as temperature measuring devices, which are preferably provided with protective tubes and inserted into the furnace stock before the coking begins. With a suitable choice of material for the protective tubes, longer service lives can be achieved.

The thermocouples can be placed in an electrical comparison circuit with setpoint devices, e.g. Potentiometer, bring. The differential voltage can then be taken as a measure of the necessary addition of preheated coal and / or advantageous coal binder.

Exceeding the target values specified according to the invention is harmless, since with the addition of preheated coal and advantageous coal binders, only a limited enlargement of the plastic zone can be achieved. The limit for the admixture of preheated coal results from the maximum preheat temperature of 250 ° C, the limit for the coal binder from the maximum admixable amount, which should not exceed a certain ratio of carbon substance to the amount of plasticizing additives, taking into account the coke quality.

Embodiment

Typically, every new feed mixture is tested in trial furnaces before it is used on an industrial scale. In the exemplary embodiment, coking experiments are carried out in 350 kg test furnaces (FB = volatile components) with an insert mixture consisting of 67% gas flame coal (36% FB waf) and 33% table carbon (16% FB waf) before being used in the operating ovens of the coking plants.

During the coking process, the temperature changes in the furnace batch run symmetrically towards the center of the coke oven. It is therefore sufficient to measure the area between a chamber wall and the center of the furnace. In the exemplary embodiment, protective tubes are installed in the oven at intervals of 20 mm, which protrude from above into the furnace bed when the furnace is full and during the coking process allow the introduction of thermocouples and temperature measurements in the furnace stock up to half of the furnace batch.

Within the first ten hours of the coking process, the temperature is measured at the prepared measuring points every hour. The resulting actual values are compared with the target values belonging to the hourly measurement. It is also possible to choose other time intervals for the temperature measurement. At time intervals that lie between the predetermined half-hourly and hourly measurements, a linear extrapolation is carried out between the predetermined target values of the neighboring half-hourly or hourly temperature measurements.

The target temperature is a temperature between 320 and 480 ° C. In other words, depending on the progress of the coking process, after one, two, four, etc. hours, there is a place in the furnace half covered by the temperature measurement, at which the temperature of the charge should be at least 320 ° C. Exceeding this temperature is harmless. On the other hand, if the temperature falls below the target temperature, there is no guarantee that high-quality blast furnace coke will be produced.

It is assumed that at the beginning of the coking process two plastic zones are created, which start on the furnace walls and scream away Tender coking move towards each other and combine in the middle of the chamber at the end of the cooking time. In the exemplary embodiment, the target temperature always indicates the target position at the boundary of the plastic zone facing the center of the chamber. At the boundary of the plastic zone facing the chamber wall, there is naturally a higher temperature as a result of the indirect heating and the temperature drop towards the center of the chamber. According to the invention, the control variable in the exemplary embodiment is the boundary of the plastic zone facing the center of the chamber. In some cases, the other boundary of the plastic zone can also be selected as the control variable. Then the target temperature is 480 ° C and the minimum distance from the chamber wall is equal to the minimum distance for a target temperature of 320 ° C minus the minimum width specified according to the invention for the plastic zone.

In the exemplary embodiment, the following minimum distances from the chamber wall result after four, six, eight and ten hours of cooking time for the target temperature 320 ° C.

• Statt 84mm nach vier Stunden nur 60mm, statt 130 mm nach sechs Stunden nur 104 mm, statt 167 mm nach acht Stunden nur 145 mm und statt 206 mm nach 10 Stunden nur 184 mm.

In the exemplary embodiment, the measured distances when using moist feed mixture differ from the minimum distances in the table above as follows:

• Instead of 84mm after four hours only 60mm, instead of 130mm after six hours only 104mm, instead of 167mm after eight hours only 145mm and instead of 206mm after 10 hours only 184mm.

According to the invention, the difference between the minimum distances and the desired distances is reduced, for example, by adding Carbopech and / or preheated coal. The addition of Carbopech brings a maximum 0.5-0.8 mm reduction in the difference per percent of Carbopech admixture. An upper limit of around 15% Carbopech can be assumed. Above 15%, the coke quality changes back to negative because the amount of plastic components in relation to the remaining carbon structure becomes too large (depending on the type of coal).

In the exemplary embodiment, the following approximations to the specified minimum distances of the 320 ° C limit value could be measured by adding 15% Carbopech at the cooking intervals of four, six, eight and ten hours selected here: 69 mm instead of 60 mm after four hours, instead of 104 mm after six Hours 114mm, instead of 145 mm after eight hours 156 mm and instead of 184 mm after ten hours 190 mm.

The differences to the minimum distance required to achieve the specified target values after the carbopech was added could be eliminated by adding preheated coal to the moist feed. In the exemplary embodiment, for economic reasons, instead of Carbopech, only the addition of preheated coal is used. The addition of preheated coal brings about 4 mm approximation per 10% addition of preheated coal of 220 ° C preheating temperature. This generally applies to all preheating temperatures above 200 ° C.

There are slight deviations below 200 ° C due to the greater dependence of the bulk density on the preheating temperature.

The target values specified according to the invention apply not only to the insert coal selected in the exemplary embodiment but also to other insert coal or insert mixtures, since the influence of the thermal conductivity of the insert material on the course of the temperature fields is small.

Claims (4)

1. A process for controlling the quality of coke from coal mixtures which are coked in a coke oven by indirect heating in the absence of air, in which the coal mixtures undergo plastification during the coking process in passing through the temperature range of 320-550°C, which changes with increasing coking time from the oven wall to the centre of the oven, characterised in that the distance of the plastic zone from the oven wall in predetermined time intervals is measured, compared with a theoretical value, and the variation from the theoretical value is compensated by the addition of pre-heated coal and/or carbo- and/or petro-derived coal binders.

2. A process according to claim 1, characterised by at least two measurements separated in time by at least one hour.

3. A process according to claim 1 or 2, characterised in that points of measurement lie divided in spatial relationship over the breadth of the coke oven chamber (oven border).

4. A process according to one of claims 1-3, characterised in that the temperature range of plastification, on which the measurement of the plastic zone is based, is between 350 and 480°C.