Method and equipment for heat recovery

Abstract

The present invention relates to a method and equipment for recovering heat from exhaust gas removed from an industrial process, such as an electrolysis process for the production of aluminum. Heat is recovered by means of an extraction/suction system, where the exhaust gas contains dust and/or particles. The heat is recovered as the exhaust gas being brought into contact with heat-recovery elements. Flow conditions and the design of the heat recovery elements are such that the deposits of the dust and/or particles on the surfaces stated are kept at a stable, limited level. In preferred embodiments, the heat-recovery elements have a circular or an extended, elliptical cross-section and may be equipped with fins or ribs.

Claims

1. A method for recovering heat from an exhaust gas which contains dust and/or particles and which is removed from an industrial process, the method comprising: providing the exhaust gas from production of aluminum in an electrolysis plant; providing equipment including: (i) a transport channel for conveying the exhaust gas containing dust and/or particles; (ii) a extraction/suction system for driving the exhaust gas through the transport channel; and (iii) a plurality of hollow elements for allowing a heat-recovery medium to flow therethrough, the hollow elements being disposed in the transport channel, and a longitudinal axis of the hollow elements being perpendicular to a direction of flow the exhaust gas; providing the heat-recovery medium in the hollow elements; driving the exhaust gas containing the dust and/or particles through the transport channel at a flow speed higher than 10 m/s and less than 25 m/s and with a temperature of 120° C. to 600° C. such that the exhaust gas is brought into contact with an outer surface of the hollow elements, the temperature at the outer surface of the hollow elements being above a temperature of approximately 70° C.; conveying the exhaust gas through a gas cleaning plant, wherein the hollow elements are disposed upstream of the gas cleaning plant; and achieving a balance between particle deposition and particle removal on heat-transfer surfaces of the hollow elements, wherein after said achieving said balance between particle deposition and particle removal on heat-transfer surfaces of the hollow elements a fouling factor of the equipment is below 0.01 m.sup.2K/W, and further comprising cleaning the exhaust gas after the exhaust gas is brought into contact with the outer surface of the hollow elements, wherein the exhaust gas is raw gas from one or more electrolysis cells, wherein the exhaust gas is raw gas from production of aluminum in an electrolysis plant, wherein the hollow elements are enclosed by walls that form the transport channel, and the transport channel is coupled to the extraction/suction system, and wherein the hollow elements are disposed upstream of the extraction/suction system.

2. The method of claim 1, wherein the flow speed of the exhaust gas is 12 m/s.

3. The method of claim 1, wherein the heat-recovery medium is one of water, steam, and air.

4. The method of claim 1, wherein the hollow elements have circular cross-sections.

5. The method of claim 1, wherein the hollow elements have extended oval cross-section and the longitudinal axis of the cross-section is substantially coincident with the direction of flow of the exhaust gas.

6. The method of claim 1, wherein the hollow elements have ribs or fins for improved heat recovery.

7. The method of claim 1, wherein the hollow elements are made of galvanized carbon steel.

8. The method of claim 1, wherein said driving of exhaust gas is performed such that the flow speed of the exhaust gas passing over the hollow elements is maintained higher than approximately 11-13 m/s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in further detail in the following using examples and figures, where:

(2) FIG. 1 shows results from tests with elliptical, finned tubes,

(3) FIG. 2 shows a calculation of the heat exchanger volume for 120° C. inlet temperature to the heat recovery system, 6.5 MW thermal power. Exhaust gas flow rate: 440,000 Nm.sup.3/h, inlet temperature to the fan: 80° C. Results for the permitted pressure drop in the heat recovery system of 3000 Pa are indicated,

(4) FIG. 3 shows a calculation of the heat exchanger volume for 180° C. inlet temperature to the heat recovery system, 6.5 MW thermal power. Gas flow rate: 176,000 Nm.sup.3/h, inlet temperature to the fan: 80° C. Results for the permitted pressure drop in the heat recovery system of 4,000 Pa are indicated,

(5) FIG. 4 shows test equipment for a heat recovery system embodiment with elliptical, finned tubes.

(6) FIG. 5 shows an embodiment of the equipment used in the method for recovering heat from an exhaust gas.

DETAILED DESCRIPTION OF THE INVENTION

(7) The heat recovery system may consist of one or more hollow elements such as tubes with a circular or elliptical/oval cross-section, with or without fins fitted on the outside of the tubes, see FIG. 4. The tubes may be made of carbon steel that has been treated in a galvanisation process. Other materials may also be relevant for this application, such as aluminium. The external surfaces of the tubes that will be in contact with particles/dust may also be treated in accordance with relevant surface treatment techniques to produce an increased slip effect. Relevant slip coatings may also be included in such treatment techniques.

(8) The exhaust gas flows on the outside of the tubes and perpendicular to the axial direction of the tubes. The tubes are packed in a regular pattern with the center-to-center tube distance adjusted so that the mass flux (mass flow rate per unit of flow cross-section) and momentum of the exhaust gas are kept at a level at which a balance is achieved between particle deposition and particle removal on the heat-transferring surfaces. The heat recovery system is enclosed by side walls and thus forms a channel through which the exhaust gas flows. No special requirements are made for the coolant that flows inside the tubes. For example, the coolant may consist of liquid/steam or gas such as water/steam or air.

(9) To achieve a balance between particle deposition and particle removal, there must be a certain minimum mass flux and momentum for the exhaust gas. This threshold is both geometry-specific and process-specific. Tests are carried out to identify the threshold value for some specific geometries (Ø36 mm circular tubes, Ø36 mm circular tubes with Ø72 mm annular fins, 14×36 mm elliptical tubes with rectangular fins) in a small-scale test setup. In the test, real exhaust gas from aluminium production is used, with particle concentration and particle distribution typical for this process.

(10) The net particle/dust deposition on the heat transfer surfaces is controlled by the transport of particles/dust to the surface, adhesion at the surface and entrainment/removal from said surface. The transport to the surfaces is influenced by the concentration of particles in the gas, together with convection, diffusion, and phoresis for small particles, while momentum forces and inertia forces are more dominant for larger particles. The adhesion to the surface is influenced among other effects by van der Waal bonding forces, capillary forces, phoresis, and gravity. Entrainment/removal of particles/dust from the surface is influenced by shear forces in the flow, grinding and collisions caused by larger particles that hit the surface, together with gravity forces. A balance between particle deposition and particle entrainment/removal is achieved by the fact that the mechanisms causing the entrainment/removal of particles are augmented to a level that balances the deposition mechanisms. For a given system these mechanisms can be expressed by characteristic gas velocities, whereby various velocities will give corresponding net thickness of the fouling layer. Said layer will insulate against the heat transfer. These characterising gas velocities can in principle be established by theoretical calculations, but will in practice be determined by experiments and measurements, due of the complexity of the issue. An optimised velocity will be a velocity that, for the given system, renders an acceptable reduction in heat transfer caused by fouling at stable conditions, without rendering a too high pressure drop. In the experiments carried out, acceptable raw gas velocities were measured to be approximately 12 meters/second or higher.

(11) The exhaust gas temperature in the tests was approximately 130° C., and the tube wall temperature approximately 70° C. An example of test results in shown in FIG. 1 (elliptical tubes with rectangular fins), where the resistance to heat transfer on account of the deposit layer (fouling factor) is shown as a function of time for various free stream gas mass fluxes. A stable state (no change in the fouling factor) is typically achieved after 50-500 hours of operation at a gas velocity of approximately 11-13 m/s (equivalent to approximately 9.5-11 kg/m.sup.2s). [For the tests shown in FIG. 1, stable conditions occurred at a gas velocity of approximately 11 m/s (10 kg/m.sup.2s) after approximately 400 hours of operation.]

(12) The reduction in heat transfer under stabilised conditions is compensated for by a moderate increase in the heat-transfer surface, typically 25-40% in relation to a clean heat-transfer surface. At the same time, the pressure drop for the exhaust gas through the heat recovery system is kept at an acceptable level. These goals are achieved via a combination of tube/fin geometry, tube packing and flow conditions.

(13) Examples of dimensioning heat recovery systems for recovery of 6.5 MW heat from exhaust gas at 120° C. and 180° C. are shown in FIG. 2 and FIG. 3. These examples are based on given pressure drop correlations and an assumed total pressure drop in the heat recovery systems equivalent to a power demand in the fans of 10%, respectively 5% of the energy recovered. In these examples, only designs with exhaust gas velocities (the velocity in the open flow cross-section in the heat recovery system) over approximately 11-13 m/s (9.5-11 kg/m.sup.2s) will achieve stable conditions. The other designs will experience unacceptably high deposits over time. As the figures show, only elliptical finned tubes will allow a velocity high enough for stable conditions to be achieved at the specific pressure drops.

(14) The relationship between mass flux and momentum for the exhaust gas and stabilised coating resistance (fouling factor) is a function of exhaust gas temperature and composition, plus particle concentration and distribution. At the same time, the pressure drop is a function of tube and fin geometry, tube packing, exhaust gas temperature and speed and total heat-transfer surface. The relationships demonstrated so far are therefore not universal. Whether a heat recovery system can operate with stable coating conditions and acceptable pressure drop depends on the process (temperature level, particle characteristics, requirements for thermal efficiency for the heat recovery system, etc.). The relationships found are, however, regarded as typical for applications for heat recovery from exhaust gas from aluminium production based on prebaked electrode technology.

(15) Although the present invention has been defined on the basis of prebake technology, the principles of the present invention may also be applied in connection with systems that use so-called Søderberg technology, and other industrial processes, exemplified by ferrosilicon smelting industry and waste incineration.

(16) In the examples, tubes with circular and oval (elliptical) cross-sections have been mentioned. However, in other embodiments, it is possible to operate with an external geometry of the tubes where the tubes have been optimised with respect to particle deposition, heat transfer and pressure drop. For example, the cross-section of the tubes may principally be designed as a wing section.

(17) Moreover, electrostatic or other similar methods may also be used to counteract deposit formation on the heat recovery equipment.

(18) Further technical design adjustments can be carried out based upon the characteristics of the exhaust gas the heat shall be recovered from. This can by example involve the choice of material used in the recovery unit or its surface treatment, in particular in relation to recovering heat from humid or corrosive gases.

(19) Further design adjustments with regard to the geometry of the recovery unit, the velocity of the exhaust gas at the surface thereof and other flow dependent issues can be carried out based upon the characterising features of the exhaust gas to be treated, such as gas velocities and temperatures. The density and the dimensions of the dust/particles in the exhaust gas may also be of importance with regard to the design of the heat recovery unit.