WO2012091576A1

WO2012091576A1 - Electric arc-resistance furnace

Info

Publication number: WO2012091576A1
Application number: PCT/PL2010/000125
Authority: WO
Inventors: Jerzy Tomeczek; Tadeusz WlŚNIEWSKI; Jerzy BIALIK
Original assignee: EUROPEAN SILICON spόłka z o.o.
Priority date: 2010-12-27
Filing date: 2010-12-27
Publication date: 2012-07-05

Abstract

The object of the invention is an electric arc-resistance furnace, in particular for the manufacture of concentrated silicon alloys using the method of silicon dioxide reduction with coal, the system of the furnace including an expansion gas turbine.The furnace according to the invention comprises a hearth 1' and a post-reaction gas capture hood 1 situated above the hearth, with electrode columns equipped with water coolers installed in the said hood. Post-reaction gases released from the top surface of the charge are directed through an arrangement of three groups of nozzles, that is nozzles 18 in the roof of the hood 1, nozzles 2' in water coolers of the electrode equipment 2, and nozzles 3' at the ends of charging pipes 3 installed in the hood roof. The thermal energy obtained from post-reaction gas combustion is used to heat compressed air in heat exchangers 4 and 4' situated in the hood and in the flue delivering the dust-laden exhaust gas to the cooling 9 and filtration 10 systems. The filtered exhaust gas is discharged to a stack, while the heated high-temperature compressed air flows to expansion gas turbine 5, where it performs mechanical work by expanding, this work being used to drive compressor 7 and electric generator 8. In the compressed air system, upstream of the expansion gas turbine 5, a natural gas combustion chamber 6 is installed.

Description

Electric arc-resistance furnace

The object of the invention is an electric arc-resistance furnace, in particular for the manufacture of concentrated silicon alloys using the method of silicon dioxide reduction with coal, wherein the chemical and physical energy of post- reaction gas is used to generate electric energy using an expansion gas turbine.

The solution using the post-reaction gases energy to generate electric energy using Clausius-Rankine water cycle and a steam turbine is well-known. In this solution the post-reaction gas is burned only in the air sucked into the hood from the surroundings through partly open process ports. This results in lengthy combustion of post-reaction gas and partial flushing of heating surfaces in the hood by the flame. This manifests itself in an intensive dust deposition on low- temperature heating surfaces and is a source of pipe corrosion. This solution is also expensive in terms of capital expenditures.

The Mannesmann Demag Metallgewinnung solution using a steam boiler for electric energy generation is also known. In this solution post-reaction gas is burned in the hood only in the stream of air sucked in from the surroundings through partly open process ports in side walls.

The Elkem solution using Clausius-Rankine water cycle and a condensing or backpressure steam turbine is also known. In this solution low-temperature water heating surfaces are also installed in the hood, on which dusts are intensively deposited, making stable operation of the plant difficult to attain. Combustion of post-reaction gas occurs in the stream of air sucked in from the surroundings, which does not provide conditions for rapid combustion of post-reaction gas in the hood.

The German patent specification DE3120908 discloses a solution, wherein nozzles installed in the roof of the hood feed combustion air axially to the hood near its wall, setting thereby the gases inside the hood in a toroidal motion. There are also nozzles of another type installed in the roof, designed to feed protective gas protecting the electrodes against contact with the oxidizing atmosphere in the hood.

The patent publication WO99/41560 presents a solution, wherein the roof of an electric furnace is cooled in order to reduce heat losses to the environment in the process of furnace charge melting. This roof, however, does not fulfil the function of a combustion chamber as it is designed to prevent atmospheric air entry into the furnace interior.

Another patent publication, W099/36581 , presents a solution, wherein the process gas, comprising mainly carbon monoxide, fed from a melting furnace is burned in a rotary furnace to which natural gas is also supplied, and the flue gas formed is transferred to an economizer boiler. It is also suggested that process gas be used upon treatment to fire burners of a water boiler or to feed a gas turbine and generate electric power. The use of this gas to drive a gas turbine requires cooling the gas prior to its treatment, which consumes energy.

The solution according to the present invention is devoid of these drawbacks.

In the solution according to the present invention the post-reaction gas is burned in the hood of the furnace in a stream of combustion air supplied by nozzles installed in the hood roof, in components of electrical column equipment and at ends of charging pipes under the hood roof. The temperature of exhaust gas from the hood is regulated by controlled opening of process ports allowing air to be sucked in from the furnace hood surroundings. The heat released in the process of post-reaction gas combustion is used to heat the compressed air flowing through a series of heat exchangers in the hood and in flues delivering the exhaust gas from the hood to the filtration system. The heated compressed air then flows to an expansion gas turbine, where it generates mechanical power converted by a generator into electric energy.

The arc-resistance furnace according to the invention is equipped with a hearth and a hood for collecting post-reaction gas, the hood holding electrode columns with water coolers. Post-reaction gas released from the top surface of the furnace charge is burned in combustion air supplied by an arrangement of three groups of nozzles: those arranged in the hood roof direct the swirled air downwards, countercurrent to post-reaction gas flow; those arranged in water coolers of electrode equipment direct the air horizontally; and those at the ends of charging pipes situated in the hood roof direct the air downwards. The temperature in the hood is controlled by means of a stream of air sucked into the hood interior through partly open process ports, whereas the thermal energy obtained from post-reaction gas combustion is used to heat compressed air in heat exchangers situated in the hood and in the flue delivering the exhaust gas to the cooling and filtration systems. The filtered exhaust gas is discharged to a stack, while the heated high-temperature compressed air flows to an expansion gas turbine, where it performs mechanical work by expanding, this work being used to drive a compressor and the electric generator. In the compressed air system, upstream of the expansion gas turbine, a natural gas combustion chamber is installed, the operation of which stabilises inlet parameters of the gaseous medium entering the gas turbine, and hence the amount of electric energy generated by the generator. Each of the main nozzles installed in the hood roof consists of blades that swirl part of the air and of a central opening guiding the remaining part of the air axially downwards countercurrent to the process gas. The process port closure consists of two parts: upper part for complete closing of the port, lower part with a shaped opening, the two parts connected with each other by a spring and moved by an actuator.

The solution disclosed herein is decidedly less expensive than solutions known hitherto.

In the solution according to the invention the heat generated in the process of post-reaction gas combustion heats the compressed air flowing through a series of heat exchangers. Substituting water in pipes with compressed air raises the temperature of heating surfaces eliminating thereby the deposition of troublesome dust on the low-temperature heating surfaces in the hood and the flue. Introduction of combustion air into the hood through the arrangement of three groups of nozzles creates advantageous conditions for burning the post-reaction gas. This is particularly important when using coal as the reducer, as the hydrocarbons evolved during coal carbonization are characterized by prolonged combustion. This air also lends to the equalization of gas temperature in the hood, promoting the crystallization of S1O₂ formed by combustion of SiO before contacting the surfaces that take over the heat. The main air nozzles in the hood roof are fitted with blades that swirl part of the air, ensuring thereby effective mixing of the air with the process gas within the hood space. The use of the natural gas combustion chamber stabilises the inlet parameters of the gaseous medium entering the gas turbine at the various stages of the process in the arc-resistance furnace.

The object of the invention is shown as an example of its embodiment in drawings, wherein Fig. 1 represents a schematic diagram of the electric arc- resistance furnace, whereas Fig. 2 shows a schematic diagram of the main combustion air nozzles and their arrangement in the hood for collecting post- reaction gas, and Fig. 3 shows the closures of the process ports in the furnace according to the invention.

As shown in Fig. 1 , the electric arc-resistance furnace, in particular for the manufacture of concentrated silicon alloys using the method of silicon dioxide reduction with coal, is equipped with a hearth V and a hood 1 for collecting post- reaction gases, the hood holding electrode columns with water coolers. The post- reaction gases released from the top surface of the furnace charge are burned in combustion air supplied in stoichiometric quantity necessary to completely burn all post-reaction gases by an arrangement of three groups of nozzles: nozzles 18 arranged in the roof of the hood 1 direct the swirled air downwards, countercurrent to post-reaction gas flow, nozzles _ arranged in water coolers of electrode equipment 2 direct the air horizontally, and nozzles _ mounted at the ends of charging pipes situated in the hood roof direct the air downwards. The direction of the flow of combustion air supplied by this arrangement of nozzles is indicated by dotted arrows. The temperature in the hood of the furnace is controlled by means of air sucked into the hood interior through partly open process ports 11. The thermal energy obtained from post-reaction gas combustion is used to heat compressed air in heat exchangers 4 and situated in the hood and in the flue delivering the exhaust gases to the cooling 9 and filtration 10 systems. The filtered exhaust gas is discharged to a stack. The heated high-temperature compressed air flows to expansion gas turbine 5, where it performs mechanical work by expanding, this work being used to drive compressor 7 and electric generator 8. In the compressed air system, upstream of the expansion gas turbine 5, a natural gas combustion chamber 6 is installed, the operation of which stabilises inlet parameters of the gaseous medium entering the turbine, and hence the amount of electric energy generated by the generator 8. Stabilization of these parameters at the various stages of the process in the arc-resistance furnace is an important factor in generating electric power of proper quality.

The design of an embodiment of the main combustion air nozzles 18

mounted in the hood roof is shown in Fig. 2. The nozzle consists of blades 12 that swirl part of the air and of a central opening 13 guiding the remaining part of the air axially downwards countercurrent to the process gas released from the top surface of the furnace charge. Swirling of part of the combustion air ensures better mixing of the air stream with process gas, and hence more thorough combustion of the latter in the space of the hood.

The design of an embodiment of the closures of the process ports 11 in the furnace is shown in Fig. 3. The port closure consists of two parts 14 and 15, connected with each other by spring 16. The upper part 14 of closure has a solid surface that enables complete closing of the port, whereas the lower part 15 has an appropriately shaped opening to ensure linear characteristics of the flow of air sucked from the outside during the lifting and lowering of the closure by actuator 17. The temperature of exhaust gas in the hood is regulated by sucking of air from the hood surroundings into the furnace by controlled lifting and lowering of the process port closures. Large cross section area of these ports impedes precise control of the air stream sucked in, and thereby does not allow maintaining stable parameters of the exhaust gas leaving the hood, which has an adverse effect on the operation of the expansion gas turbine 5.

Claims

1. An electric arc-resistance furnace, in particular for the manufacture of concentrated silicon alloys using the method of silicon dioxide and iron oxides reduction with coal, the furnace comprising a hearth filled with a charge blend and a gas capture hood situated above the hearth, post-reaction gases released from the reduction processes proceeding in the furnace hearth being burned within the space of the hood, the furnace being connected via a system of flues with a filtration system and equipped with air nozzles supplying combustion air to the space between the charge surface and the gas capture hood, said air nozzles being situated in the hood roof, in the ends of charging pipes situated under the roof, in components of electrode column equipment, characterized in that series of heat exchangers (4) and (41) are installed in the space under the hood and in flues delivering the gases after combustion to the filtration system, hot exhaust gas flowing around the said heat exchangers through which hot compressed air flows taking away physical heat from the hot gas and, once heated, flows to the expansion gas turbine (5), where mechanical power is generated, subsequently converted by the generator (8) into electric power.

2. Electric furnace according to claim 1 , characterized in that in the

compressed air system, upstream of the expansion turbine (5), a natural gas combustion chamber (6) is installed, the operation of the said chamber stabilizing the thermal energy of gas upstream of the turbine and hence stabilizing the quantity of electric power generated by the generator.

3. Electric furnace according to claim 1 , characterized in that the air nozzles installed in the hood roof supplying combustion air to the space between the charge surface and the roof of the hood (1) are equipped with blades (12) that swirl part of the air flowing out of the nozzles, while the remaining part of air flows out axially through a central opening (13).

4. Electric furnace according to claim 1 , characterized in that the closure of the process ports comprises two parts, of which the upper part (14) comprises a solid surface enabling complete closure of the ports, whereas the lower part (15) has a shaped opening that ensures linear characteristics of the flow of air sucked in from outside during the opening and closing of process ports.