Method for the thermal treatment of raw meal using an electric heating device, and corresponding system

Abstract

The invention relates to a method for thermally treating raw meal in order to produce burnt lime and to a corresponding system. According to the invention, raw meal is thermally treated at an electrically heated heating contact. This leads to an exhaust gas which is free of nitrous gases and other organic constituents. The pure exhaust gas can be subjected to sequestration without any further special purification processes.

Description

[0001] Process for the thermal treatment of raw meal using an electric heating device and corresponding plant

[0002] The invention relates to a process for the thermal treatment of raw meal for the production of burnt lime and a corresponding plant.

[0003] To produce burnt lime (CaO), it is known to finely grind lime (CaCO3) from natural deposits into so-called raw meal, suspend it in a gas stream, dry it, and then subject it to thermal treatment. Burnt lime (CaO), along with other substances, is a starting material for the production of cement clinker, but also for the production of soda. Burnt lime is also used as a base in various chemical processes and in inorganic binders. Ground, burnt lime has different names depending on the industry. In the cement industry, a flour made from a mixture of burnt lime and silicate-containing rock is called 'raw meal'. In other industries, pure burnt lime is also referred to as 'raw meal'.For the calcination, i.e., thermal treatment of lime (CaCOs) in the form of raw meal, it is usually placed in an entrained-flow reactor into which a flame protrudes or, depending on the further process, hot exhaust gases from a downstream rotary kiln are fed. During this thermal treatment, not only is carbon dioxide (CO2) formally driven out of the natural lime (CaCOs), which is an endothermic process, but due to the high activation energy, a comparatively high temperature of approximately 800°C is necessary to trigger the thermolysis of the lime (CaCOs). To generate the high temperature, fuel must be burned, and the flame enters the entrained-flow reactor. The exhaust gases from the calcination process contain carbon dioxide (CO2) from the burnt lime (CaCOs) and also carbon dioxide (CO2) from the combustion of fossil fuels.In addition to carbon dioxide (CO2), the exhaust gas also contains nitrous gases (NOx) and other components, such as volatile organic compounds (VOCs). To sequester carbon dioxide (CO2), which is recognized as harmful to the climate, the exhaust gas must be subjected to a purification process to separate atmospheric nitrogen (N2), the nitrous gases, and the VOCs from the exhaust gas before sequestration.

[0004] It would be desirable to conduct the process for the thermal treatment of lime in such a way that the complex exhaust gas purification prior to sequestration is not necessary.

[0005] The object of the invention is therefore to provide a process for the thermal treatment of lime in which exhaust gas purification prior to sequestration of carbon dioxide (CO2) is unnecessary.

[0006] The object of the invention is achieved by heating the raw meal at an electrically heated heating contact. The corresponding system comprises a reactor within which the electrically heated heating contact is arranged, which is designed such that the raw meal slides over the electrically heated heating contact. Further advantageous embodiments of the system are specified in the subclaims to claim 2.

[0007] According to the concept of the invention, the raw meal, regardless of whether it is mixed with silicate-containing rock, as is common in the cement industry, or whether the raw meal is pure ground lime, is heated at an electrically heated heating contact. In order to transfer the necessary amount of heat to the raw meal, it can be advantageous in the design of the system according to the invention for the speed at which the raw meal is conveyed past the electrically heated heating contact to be slowed down by a structural arrangement of sliding surfaces as an electrically heated heating contact.For this purpose, in a first embodiment of the system according to the invention, the electrically heated heating contact can be provided with several electrically heated sliding surfaces arranged vertically one above the other, offset in pairs, so that the raw meal slides from top to bottom between the paired sliding surfaces, as if in cascades, with the sliding surfaces being arranged in the reactor. The several electrically heated sliding surfaces arranged vertically one above the other, offset in pairs, act like baffles that slow the flow of raw meal as the raw meal trickles from top to bottom over the sliding surfaces.

[0008] In a further embodiment of the system according to the invention, the sliding surfaces can have openings through which gas produced during the thermal treatment flows and additionally heats the raw meal sliding over the sliding surfaces. The openings are open to the side and towards the sliding raw meal so that the raw meal does not fall through the openings. The gas coming from below, however, flows through the openings and thus also flows through the sliding raw meal. During this mixing, heat present in the gas is transferred to the raw meal, so that it is additionally heated. The flow of the hot exhaust gases from the thermal treatment at lower levels of the electrically heated heating contact thus means recuperation of the heat electrically introduced into the raw meal.

[0009] In a further embodiment of the system according to the invention, the electrically heated heating contact can be composed of several vertically arranged, wave-shaped sliding surfaces arranged parallel to one another. The wave-shaped sliding surfaces have a similar effect to the previously mentioned paired, offset sliding surfaces. However, the wave-shaped sliding surfaces are arranged so closely to one another that, in the vertically winding path formed between two wave-shaped sliding surfaces, a mixture of trickling of the raw meal and swirling of the raw meal by gas flowing upwards from below, which is generated during the thermal treatment, occurs.

[0010] In yet another embodiment of the plant according to the invention, the reactor can be tubular and the electrically heated heating contact is embedded in the base of the tubular reactor, wherein the tubular reactor has a horizontal or inclined arrangement, within which a screw conveyor is arranged. The screw conveyor moves the raw meal over the electrically heated heating contact until the fired raw meal is ejected at the end of the reactor via a rotary valve. In order to seal the reactor, in which the screw conveyor is arranged, from the atmosphere, it is advantageously provided that the raw meal is also fed to the reactor via a rotary valve. The supply and removal of the raw meal via a rotary valve in all reactor types mentioned here has the advantage that the exhaust gas produced is a relatively pure exhaust gas consisting of carbon dioxide (CO2), which only contains water (H2O) as a further component.Since the water can be easily condensed, the resulting exhaust gas is suitable for sequestration, for example by storing it in aquifers or caverns as permanent storage facilities.

[0011] All reactor types presented here feature a vapor outlet. The vapors, which consist primarily of carbon dioxide (CO2) and water (H2O), still contain suspended raw meal. The suspended raw meal can be separated in a cyclone separator. The exhaust gas exiting the cyclone separator can also be passed through a heat exchanger to extract the heat from the vapors as process heat. Since this heat is not very high, the heat extracted from the vapors is suitable for drying other materials.

[0012] The invention presented here, the electrical firing of raw meal, whether with silicate-containing rock or as pure lime flour, initially appears very costly and therefore unattractive. However, if electrical energy can be generated from process heat elsewhere or from renewable energies, then electrical firing, combined with the necessary sequestration of the carbon dioxide (CO2) produced during firing, is a highly attractive option, both from an economic and climate-balance perspective.

[0013] The invention is explained in more detail with reference to the following figures. It shows:

[0014] Fig. 1 shows a plant for the thermal treatment of raw meal using an electric heating device in a first embodiment,

[0015] Fig. 2 shows a plant for the thermal treatment of raw meal using an electric heating device in a second embodiment,

[0016] Fig. 3 shows a plant for the thermal treatment of raw meal using an electric heating device in a third embodiment,

[0017] Fig. 4 shows a plant for the thermal treatment of raw meal using an electric heating device in a fourth embodiment.

[0018] Figure 1 outlines a first embodiment of a plant 100 for the thermal treatment of raw meal 10 using an electric heating device. Raw meal 10 is fed via a rotary valve 101 into a reactor 105, where the raw meal 10 trickles over several electrically heated sliding surfaces 110 arranged vertically one above the other, offset in pairs. The sliding surfaces 110 are provided on the rear, i.e., underside, with an electric resistance heater 111, which heats the sliding surfaces 110. The sliding surfaces 110 serve to protect the resistance heater 111 located underneath, so that the abrasive raw meal does not wear the resistance heaters 111. In this embodiment, the reactor 105 is designed to be gas-tight.Vapors 115 and carbon dioxide (CO2), both produced during the electrical firing of lime (CaCO3), leave the reactor 105 together with the burned or calcined raw meal 11 at the lower base 106 of the reactor 105 and are fed into a cyclone separator 107. In the cyclone separator 107, the burned or calcined raw meal 11 is separated from the vapors and carbon dioxide (CO2). The finished product, i.e., the burnt or calcined raw meal 11, leaves the cyclone separator 107 at its lower coarse material outlet 108. The gas, i.e., the vapors and carbon dioxide (CO2), leaves the cyclone separator 107 at its upper fine material / gas outlet 109. The gas discharged there is further cleaned by a dust separator 112, separating out the fine, burnt raw meal 12. The cleaned exhaust gas, the vapors 115, and the carbon dioxide (CO2) are then discharged via a flow regulator 114 and a compressor 116 for suction and sequestration.The cloud symbol shown here for the vapors 115 does not indicate that the exhaust gas is discharged into the atmosphere; rather, the exhaust gas is subjected to a sequestration process not shown in detail here. The fractions of the finished product, namely the burnt or calcined lime 11, 12, are combined and sent for further use.

[0019] Figure 2 shows a second embodiment of a plant 200 for the thermal treatment of raw meal 10 using an electric heating device. The plant 200 differs from the embodiment shown in Figure 1 in that the sliding surfaces 210 have openings through which vapors 215 and carbon dioxide (CO2), both generated on the lower sliding surfaces 210, can flow through the upper sliding surfaces 210. The exhaust gas, namely the hot vapors 215 and the hot carbon dioxide (CO2), flows through the raw meal 10 on the upper sliding surfaces 210, thereby additionally heating the raw meal trickling there. This additional heating of the raw meal 10 means recuperation of the waste heat generated at the temperature required for the thermolysis of the lime (CaCO3) to burnt lime (CaO). To prevent the raw meal from falling through the openings, the openings are provided with a compartment area D in the sliding direction.This partial roof area D is shown more clearly in the enlarged view of detail A. Unlike reactor 105 in the embodiment shown in Figure 1, the reactor 205 shown here is provided with a gas vent at the top 204 of reactor 205. This gas vent arrangement results in a countercurrent gas flow within reactor 205, which is represented by vertical arrows drawn in reactor 205. The gas flows upward into the gas vent in the top 204 of reactor 205, while the finished burnt or calcined product falls into the base 206 of reactor 205. There, the finished burnt or calcined lime is discharged through a rotary valve 213. The burnt or calcined lime 11 separated in the dust separator 207 is combined with the product from the rotary valve 213. The exhaust gas, on the other hand, flows through a flow regulator 214 and compressor 216 for further sequestration.

[0020] Figure 3 shows a third embodiment of a plant 300 for the thermal treatment of raw meal 10 using an electric heating device. Unlike the cascade-like sliding surfaces 110 and 210 in the two previously mentioned embodiments, this embodiment provides for the sliding surfaces 310 to be closely spaced, wave-shaped sliding surfaces 310 as electrically heated heating contacts, which form a meandering or undulating gap between them, running essentially from top to bottom. Within the gap between two sliding surfaces 310, a mixing movement is formed by a trickling of the raw meal 10 and a gas countercurrent, in which the raw meal 10, which is present with the already partially burnt or calcined lime 11, is swirled up, similar to a countercurrent reactor.The gas counterflow has a heat-recovering effect, which was already discussed for the plant design in Figure 2. It is important here that the distance between the two sliding surfaces 310 is not too wide, so that a separation cannot occur between the trickling raw meal and the upward flow of the exhaust gas. On the other hand, the distance must not be so small that the raw meal 10 falls as if through a pipe, rolling against the upward flowing gas, so that the gas and the freely trickling raw meal 10 mutually block each other's path. The sliding surfaces 310 are connected on one side to resistance heaters 311. The gas flows upward into the gas outlet in the head 304 of the reactor 305, while the finished burnt or calcined product falls into the base 306 of the reactor 305. There, the finished burnt or calcined lime is discharged through a rotary valve 313.The burnt or calcined lime 11 separated in the dust separator 307 is combined with the product from the rotary valve 313. The exhaust gas, on the other hand, flows through a flow regulator 314 and compressor 2316 for further sequestration.

[0021] Finally, Figure 4 shows a plant 400 for the thermal treatment of raw meal 10 using an electric heating device 410 in a fourth embodiment. A screw conveyor 450 is arranged in the reactor 405, which is depicted as a tubular reactor 405 horizontally or inclined ('inclined' not shown here). This screw conveyor 450 transports the raw meal 10, which is introduced into the reactor 405 via a rotary valve 401, over the sliding surfaces 410 embedded in the lower wall 451, which are electrically heated from below by resistance heaters 411. This embodiment has the advantage that the residence time of the raw meal 10 on the sliding surfaces 410 can be adjusted by adjusting the speed of the screw conveyor 450 as an electrically heated heating contact. Vapors 415 and carbon dioxide (CO2) are also produced as exhaust gases in this reactor 405.This exhaust gas is routed via gas outlets 460 to a dust separator 412, which separates raw meal 11 still suspended in the exhaust gas. The remaining exhaust gas consisting of vapors 415 and carbon dioxide (CO2) is then subjected to sequestration. The finished fired or calcined product from the rotary valve is combined with the separated product from the dust separator 412.

[0022] LIST OF REFERENCE SYMBOLS

[0023] Raw meal, unburned 200 plant

[0024] Raw meal, burnt 201 rotary valve

[0025] Raw meal, burnt 204 head (reactor)

[0026] 205 reactor

[0027] Plant 206 feet (reactor)

[0028] Rotary valve 207 cyclone separator

[0029] Heat exchanger 208 coarse material discharge

[0030] Reactor 209 fines / gas outlet

[0031] Foot (Reactor) 210 Sliding surface

[0032] Cyclone separator 211 resistance heating

[0033] Coarse material outlet 213 Rotary valve

[0034] Fines / Gas outlet 214 flow regulator

[0035] Sliding surface 215 Brüde

[0036] Resistance heater 216 compressor

[0037] dust collector

[0038] Rotary valve 300 system

[0039] Flow controller 301 rotary valve

[0040] Breed 304 Head (Reactor)

[0041] Compressor 305 Reactor

[0042] 306 Foot (reactor) sliding surface 414 Flow controller resistance heating 415 Vapour rotary valve 416 Compressor flow controller 450 Screw conveyor vapour 451 Compressor wall 460 Gas discharge system A Detail rotary valve D Partial roof area dust collector

Claims

PATENT CLAIMS 1. Process for the thermal treatment of raw meal for the production of burnt lime, characterized by - Heating the raw meal (10) on an electrically heated heating contact.

2. Plant (100, 200, 300, 400) for carrying out the method according to claim 1, comprising a reactor (105, 205, 305, 405) within which the electrically heated heating contact is arranged, which is designed such that the raw meal (10) slides over the electrically heated heating contact.

3. System according to claim 2, characterized in that the electrically heated heating contact consists of several pairs of offset heating elements arranged vertically one above the other and electrically heated Sliding surfaces (110) so that the raw meal (10) slides from top to bottom as if via cascades between the sliding surfaces (110) arranged in pairs, wherein the sliding surfaces (110) are arranged in the reactor (105).

4. Plant according to claim 3, characterized in that the sliding surfaces (210) have openings through which gas generated during the thermal treatment flows and additionally heats the raw meal (10) sliding over the sliding surfaces (210).

5. System according to claim 4, characterized in that the electrically heated heating contact consists of several vertically arranged and wave-shaped sliding surfaces (310) which are arranged parallel to one another and form an undulating gap between them in pairs.

6. Plant according to claim 2, characterized in that the reactor (405) is tubular and the electrically heated heating contact is embedded in the bottom of the tubular reactor (405), wherein the tubular reactor (405) has a horizontal or inclined arrangement, within which a screw conveyor (450) is arranged, which moves the raw meal (10) over the electrically heated heating contact.

7. Plant according to one of claims 2 to 6, characterized in that the reactor (105, 205, 305, 405) has at least one discharge opening for the vapors produced during the thermal treatment, which is connected to a dust separator (212, 312, 412) which separates raw meal (10) from the vapors.

8. Plant according to claim 7, characterized in that a heat exchanger (102) is arranged in the gas path, which dissipates heat from the vapors (115) emerging from the gas.

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