WO1991019147A1 - Method and apparatus for control of carbon baking furnaces - Google Patents

Abstract

A method for the control of the oxygen/fuel ratio in a carbon baking furnace is provided wherein the oxygen level in one section of the furnace is obtained and used to determine any changes in at least one of the control parameters flue gas flow rate, fuel injection level and air injection level. The oxygen level may be obtained by direct measurement or by inference from a succession of temperature measurements under different fuel injection conditions. An improved furnace including means (7) for the controlled addition of supplementary air to the flue of one or more of the pre-heat sections (300) downstream of the first firing section (201) is also provided.

Description

METHOD AND APPARATUS FOR CONTROL OF CARBON BAKING FURNACES

This invention relates to a method and apparatus for the control of a carbon baking furnace. The invention is particularly concerned with the so-called ring furnaces commonly used in baking carbon bodies to form the anodes for use in the electrolytic production of aluminium.

Ring furnaces have been used for many years in the baking of carbon anodes. These furnaces include a plurality of pits defined by a network of longitudinally extending flue walls and transversely extending separating walls. Two longitudinally extending networks of pits are linked at each of their respective ends to form a closed loop or ring from which the furnace derives its name. The walls are built of refractory material and the longitudinally extending walls are hollow thus forming flues through which a suitable gas flow is maintained to heat or cool the carbon bodies in the adjacent pits. The transverse walls separate transversely extending groups of pits generally referred to as sections and the furnace may thus be considered as made up of groups of adjacent pit sections extending along the length of the furnace.

Each pit section is subjected in use to different heating conditions in order to bake the green or unbaked carbon bodies into the desired anode materials. The condition in each section is altered progressively between controlled limits until an upper limit is reached. After a predetermined time, the major factors causing progressive alteration of the conditions in each section are moved to an adjacent section. This type of movement takes place along the or each length of the furnace in which the carbon bodies are to be baked and is referred to as movement in the fire direction. Thus, the carbon bodies do not move through the furnace but the furnace conditions are intermittently and progressively changed in each successive pit section so that any one section passes through a packing stage, to several pre-heating stages in which the temperature of the bodies is progressively increased, to several firing stages in which the temperature of the bodies is progressively further increased, to several cooling stages in which the temperature of the bodies is progressively decreased, and finally to an unloading stage.

Any given section will have adjacent on one side, a section which is being treated at the stage last followed by the given section, and on the other side a section which is being treated at the stage next to be followed by the given section. The successive sections under treatment together make up an active zone of the furnace and each furnace will normally have sufficient sections for at least two active zones to be operated at the same time.

The heating or cooling of the carbon bodies at the various stages is, as indicated above, brought about by heat transfer through the furnace walls from the gases flowing through the flues. The flue gases are removed before a section in which the carbon bodies are initially packed into the furnace so this section thus has no flow of gases through its adjacent flue walls. However after this section the carbon bodies are heated in the successive pre-heating stages by flue gases which have passed through the firing stages. Following the pre-heating stages the carbon bodies are baked by heat from the flue gases containing gases which are burned to increase the temperature of the firing stages. After the firing stages, the carbon bodies are cooled in the successive cooling stages by an incoming air flow which enters the flues at the final cooling stage.

In the preceding paragraph, the operation of the ring furnace has been described with reference to the treatment of the carbon bodies which are being baked in the furnace. However, as the carbon bodies are not themselves moved from section to section of the furnace, it is more conventional to consider the sections in relation to the air and fuel gas flows and the heat which is generated or removed in each section. When considered in this manner, the construction and operation of the furnace by movement in the fire direction can be seen to include a succession of pre-heat sections in which the green carbon bodies are progressively raised in temperature, a succession of firing or baking sections in which the temperature is progressively raised further, and a succession of cooling sections in which the temperature is progressively lowered. The flue gases move in the fire direction from the cooling sections in at least the coldest of which cooling and combustion air enters the flues and moves towards the other end of the group of sections within which the action of heating, baking and cooling the carbon bodies takes place.

As the air which is the initial flue gas moves from the first cooling section, it is raised in temperature as it approaches the first firing section. In each successive firing section, fuel such as natural gas or fuel oil (hereinafter sometimes referred to simply as gas) is injected into the flues where it burns in the incoming heated air stream. As the fuel combustion proceeds, the oxygen content of the air stream is decreased and the temperature of the successive firing sections also decreases. After the last firing section, combustion continues in the flues with the burning of volatile materials which are driven from the carbon bodies and the pitch used to bind them. These volatile materials migrate through the flue walls to burn with the remaining oxygen in the air stream. In the last of the pre-heat sections, most of the residual heat is removed from the low oxygen air stream and serves to raise the temperature of the carbon bodies towards the temperature at which volatile materials will be driven from them.

As the combustion of the fuel added to the furnace in the heating sections and of the volatile materials driven from the carbon bodies in the pre-heat sections will depend on the amount of oxygen available, control of the oxygen content of the gas stream or draft drawn through the flues by the exhaust outlet is an important factor in maintaining the desired conditions throughout the furnace. The primary control over the draft is achieved by adjustment of the negative pressure exerted at the exhaust outlet. Increased negative pressure draws more air into the flues and increases the amount of oxygen available over a given time at any point in the flue. However, the oxygen content of the flue gases is affected not only by the cooling and combustion air admitted at the end cooling section but also by ambient air which may leak into the furnace due to imperfect sealing of various access apertures such as inspection ports and fuel inlets or through cracks in the furnace refractory.

In order to balance the numerous factors which affect the furnace operation and to make adjustments which will optimise both the furnace operation and the product quality, control systems have been devised which analyse data continuously or intermittently derived from observation of various furnace conditions. The analysis of the data is used to actuate appropriate control mechanisms which alter the furnace conditions towards predetermined optimum conditions. Control systems are commonly computer operated. Various methods and apparatuses have been proposed to assist in the control of the furnace conditions and thereby produce the desired control over the quality of the baked products. In International Patent Application PCT/FR87/00213 of

Aluminium Pechiney, the gas flow rate in each flue is adjusted by the use of flap valves on each suction nozzle of the exhaust manifold. The flap valves are responsive to fluctuations in temperature and negative pressure measι:rements in the flue and to opacity measurements made on the smoke from each flue.

In United States Patent 4,354,828 assigned to South Wire Company and National Steel Corporation, the flue and pit temperatures are measured to produce a control signal which operates valves varying the air/fuel mixture of each burner in the baking stages of the furnace. This control mechanism is used to adjust the temperature in each flue for which the burner mixture is controlled. However, these and other prior proposals are not entirely satisfactory and it is an object of the present invention to improve the control of carbon baking furnaces, thus leading to improved product quality and consistency.

In accordance with the present invention we provide a method for the control of the oxygen/fuel ratio in a carbon baking furnace by obtaining the oxygen level in one section of the furnace by measurement or inference and using this level to determine any changes in at least one of the flue gas flowrate, fuel injection level and air injection level. Controlling the oxygen/fuel ratio by changing one or more of these parameters enables the operator to control the temperature in the various sections of the furnace.

According to one aspect of the present invention, several successive series of temperature measurements is made in the flue of the first firing section. The first series of temperature measurements are made under normal gas injection conditions and are used to calculate an average temperature over a selected short period. Gas injection is continued at the same rate and then after a selected time a second series of temperature measurements is taken to provide the average temperature over the same selected time interval. An average temperature rise is then calculated to give the average temperature rise for the flue under normal gas injection conditions. The amount of gas injected is then increased above the previously prevailing level and maintained at the higher level for a selected short period. The change in temperature at the end of the period of increased gas injection is measured and this measured temperature change is used to infer the oxygen level at the front gas injection section. Thus, a small or negative increase in temperature indicates low or zero oxygen levels and a large increase indicates adequate oxygen levels.

Accordingly, the extent and direction of the measured temperature change can be used as a parameter to calculate the necessary change in the flow rate in the flue of the first firing section, in the gas injection level at one or more gas injection points located at or upstream from the first firing section or in the amount of supplementary air being added before the first firing section and after the first pre-heat section.

An additional or alternative measurement which can be used to control the oxygen:fuel ratio in accordance with the present invention is that of the oxygen level in all or some of the flue lines at the first, second or third pre-heat section downstream of the first firing section. The oxygen level as measured is then used to calculate any necessary adjustment of the oxygen level and this may be achieved by adding supplementary air to one or more of the pre-heat sections downstream of the first firing section. The addition of supplementary air will promote the efficient burning of pitch volatiles and enable operation at a lower negative draft pressure which in turn reduces leakage of outside air into the flues and lowers the flow rates in the f lues .

The controlled addition (herein called "injection") of supplementary air at the preheat sections provides an added temperature control over that normally achieved by adjusting the flow rate in the flues. It will be appreciated that excessive reduction of the flow rate to decrease the rate of temperature increase may lead to incomplete combustion of the pitch volatiles in the preheat sections because insufficient oxygen is being provided in these sections at the required time. The addition of supplementary air counters the effect of a reduced flow rate but an appropriate balance must be struck between the increased combustion from the oxygen added with the cold supplementary air and the reduction in temperature which is also caused by the introduction of this cold air. Oxygen level can also be changed by altering the flow rate of air in the flue, by changing the quantity of fuel being injected and by changing the amount of unwanted air leaking into the furnace.

Accordingly, the invention further provides a carbon baking furnace in which at least one means for the controlled addition of supplementary air is provided for the flue of one or more of the pre-heating sections downstream of the first firing section. The means for the controlled addition of supplementary air may be simply a valved inlet allowing ambient air to be drawn into the flue by the negative pressure generated through the exhaust outlet.

The invention will now be more fully described with reference to the accompanying drawing. The drawing is a diagrammatic sectional representation of one active zone of a ring furnace showing the range of heating conditions to which the initially green anodes are subjected through to their removal from the furnace in fully baked condition. It will be appreciated that while the heating and cooling sequence to which the anodes are subjected will always be basically the same as that illustrated, variations may be made, for example in the number of sections used for each purpose.

The active zone of the illustrated baking furnace includes a cooling zone 100 having five adjacent cooling sections 101, 102, 103, 104 and 105, a firing zone 200 having four adjacent firing sections 201, 202, 203 and 204, and a pre-heat zone 300 having three pre-heat sections 301, 302 and 303. Air at ambient temperature enters the furnace flue system through a cooling inlet such as manifold 1 and flows through the flues in a tortuous path as indicated by the curved arrows. Manifoled 1 is shown at the entry of cooling section 105 but a further inlet may be provided at subsequent cooling sections in the fire direction. Firing frames 2, 3, 4 and 5 are provided for the injection of gaseous or liquid fuel into the firing sections. The flue gases leave the furnace through an exhaust outlet such as manifold 6. The general direction of forward movement of the flue gases is referred to as the direction of the fire and is indicated by arrow 10.

As previously indicated, the green anodes initially charged into the furnace pits are progressively treated by changing the heating/cooling conditions by moving the exhaust manifold 6, the gas (or other fuel) injection firing frames 2, 3, 4, 5, and the cooling manifold 1 progressively in the "direction of fire" 10 as illustrated in the drawing. The draft within the flues is initially provided by air which enters through the cooling manifold 1. The cooling manifold 1 preferably injects air under positive pressure into the last cooling section 105 and the air flows forward either under its own positive pressure or because it is drawn forward by the negative pressure exerted by the exhaust manifold 6 at the other end of the active zone of the furnace. The draft through the flues is controlled so that there is negative pressure within the flues in at least the firing or baking sections 200 and in the preheat sections 300.

The air which initially enters is cold and has at least its normal oxygen content. The temperature of the cooling air increases as it moves through the cooling sections 100 towards the firing sections 200 due to heat transfer from the anodes in the adjacent pits, thus progressively cooling the baked anodes. The air reaching the firing sections 200 is thus elevated in temperature to such a degree that it will support the combustion of gaseous fuel injected into the firing sections 200 through the firing frames 2, 3, 4, 5 connected to each such section 204, 203, 202 and 201. If a liquid fuel is used, the temperature of the incoming air is such as to support both the combustion and vaporisation of this fuel. The temperature of the flue walls in the latter firing section 204 (that is the rearmost firing section in the direction of the fire) may be raised to approximately 1225 C by combustion of the incoming fuel in the relatively high oxygen content incoming air. In the second firing section 202 the oxygen content of the forwardly moving flue gases, has been reduced and the temperature within the flue will also be lower than that in the third and fourth firing sections 203 and 204. The temperature and oxygen content of the flue gas falls further in the first firing section 201 so that the temperature of the flue gases leaving the first firing section 201 may have fallen to about 1000°C. In the preheat sections 300 the temperature of the unfired anodes is progressively raised by the hot flue gases which have a relatively low oxygen content after much of the oxygen has been used in the combustion process of the firing sections 200. However, as the temperature of the unfired anodes is progressively raised, volatile materials in the pitch which is used to bind the carbon material forming the anodes together are released and burns in the residual oxygen of the flue gases. The temperatures of the flue walls in the first preheat section 301 where the unfired anodes are first subjected to the heated flue gases may be in the range 200 to 500°C. In this first preheat section 301, all the heating of the unfired anodes takes place by extraction of the residual heat from the flue gases.

In the second preheat section 302 the temperature may rise to between 500 and 800°C and the anodes are heated both by the incoming flue gases and the combustion of the pitch volatiles which are driven off as the anode temperatures are raised. In the third preheat section 303 of the illustrated embodiment, the flue wall temperature may reach 800 to 1000°C due to the combined action of the incoming flue gases and the combustion of further pitch volatiles.

The flue gases are removed via the exhaust manifold 6 after passing through the flues of the first preheat section 301. The furnace section 401 preceding the first preheat section is packed with unfired anodes after the fired anodes from the previous pass of the fire have been unloaded from that section. The section 401 packed with unfired anodes then becomes the first preheat section when the manifolds and firing frames are next moved forward.

The condition of the flue gases in any active zone of the furnace can obviously be controlled by adjustment of the amount of air supplied through the cooling manifold and extracted through the exhaust manifold as well as by the amount of fuel gas injected into each firing section. As noted above, various prior proposals have been made to influence the control of the flue gas conditions by appropriate adjustment of these air and fuel parameters. Adjustment of these flue gas parameters will obviously also enable adjustment of the flue gas temperature which is critical in controlling the way in which the anodes are baked.

In accordance with one aspect of the present invention, additional control is provided by a predetermined series of temperature measurements and fuel level adjustments which are used to provide a basis for adjustment of the flue gas flow rate. The temperatures are measured in at least one flue of the first firing section using an appropriate measuring device such as in infra red pyrometer or a thermocouple. The temperature measuring device is preferably inserted into a peep-hole normally provided for visual inspection of the flue interior. The first series of temperature measurements are taken over a limited period, for example over a 1 to 60 second period, and the average temperature over this period is calculated. The next series of temperature measurements is taken a short time later, preferably within 1 to 10 minutes, for example after 3 minutes, and the average temperature is again calculated. The difference between the average temperature initially calculated and the second average temperature calculated is then taken to represent the average temperature rise for the flue under normal gas injection conditions. At selected temperature intervals, preferably at intervals between 3 and 100 minutes, more preferably every 15 minutes, the amount of gas injected at the first firing section is increased above its normal maximum level. Extent of the increase may be between 10 and 50%, preferably 30% of the normal maximum level. The increased amount of gas injection is maintained for a selected period, for example between 1 to 15 minutes, preferably 3 minutes, and the temperature change resulting from the increased gas injection level is measured at the end of, or at some stage before or after the end of, the period for which the increased amount of gas is injected.

The change in temperature due to the increased gas injection as compared with the temperature rise due to normal gas injection levels is related to the amount of oxygen in the flue at the first firing section. A small increase or a decrease suggests a low or zero oxygen level but a large increase suggests an adequate oxygen level. The extent and direction of the change can thus be used to influence a decision as to changes which may be made in the flow rate in the flue in the first firing section or in the amount of gas injected at any of the gas injection points at or upstream of the first firing section. When supplementary air is being added as described below, a deficit in suggested oxygen level at the first firing section indicates that too much supplementary air is being added and the amount of supplementary air must be reduced.

As indicated by the supplementary air injection frame 7 in the drawing, additional air may be deliberately introduced into one or more of the preheat sections 300 immediately forward of the firing sections 200. As explained above, the introduction of this so-called supplementary air can be used to assist in the complete combustion of the pitch volatiles and hence in the control of the temperature and necessary flow rate in the flues of these preheat sections. Increasing the amount of supplementary air reduces the rate of temperature rise in the section or sections where pitch burn is occuring, thus reducing temperature gradients and helping to prevent anode cracking and improving anode quality. Changing the amount of supplementary air also allows more flexibility in the control logic for the fire control.

By using supplementary air, the flue can be operated at lower negative pressures while still maintaining sufficient oxygen levels to burn the pitch volatiles and any remaining fuel gas. Efficient burning of the pitch volatiles is important but there must, of course, be sufficient oxygen for their combustion. Inefficient combustion of pitch volatiles causes pitch condensation in the waste gas system requiring expensive clean outs, fires in the waste gas system causing equipment damage, and polluting emissions of pitch volatiles to the outside air. On the other hand, more efficient pitch volatile combustion means that less fuel gas or oil is required and the fuel efficiency of the furnace may therefore be improved.

It is therefore a further feature of the present invention to assist the control of the flow rate and negative pressure in the preheat sections by measuring the oxygen level in at least one of these sections. The measurement of the oxygen level is carried out by an appropriate probe, preferably a zirconia oxygen probe mounted in situ as at 8 in the illustrated embodiment or used to determine the oxygen content of flue gases sampled from the relevant section.

If the measurement of oxygen level shows that the oxygen level is more than is appropriate for full pitch volatile combustion, the furnace can be operated at a lower negative pressure, thereby reducing inleakage of air which is difficult to avoid completely. The flow rate of flue gases can also be reduced thereby reducing the rate of temperature rise in the preheat sections which is desirable to reduce temperature gradients and assist in the prevention of anode cracking and in the improvement of anode quality. The measurement of oxygen level is used to set a minimum flue negative pressure which is recalculated by the fire control logic every 0.1 to 900 seconds, preferably every 1 second. The minimum flue negative pressure set point is the minimum value that is required to give the minimum target oxygen value. The minimum target oxygen value is generally in the range of 0.1 to 7%, preferably 3%. The calculation includes the current value of flue negative pressure, the current oxygen level and the current minimum oxygen target for each flue.

Claims

1. A method for the control of the oxygen/fuel ratio in a carbon baking furnace in which carbon bodies are successively pre-heated, fired and cooled in first and successive pre-heating, firing and cooling sections made up from a plurality of cells defined by hollow walls of refractory material, the adjacent hollow walls being connected to form flues allowing the flow of flue gases between a cooling inlet through which air is injected and an exhaust outlet through which used flue gases are withdrawn, characterised in that the oxygen level in one section is obtained by measurement or inference and used to determine any changes in at least one of the flue gas flow rate, fuel injection level and air injection level

2. A method as claimed in claim 1 characterised in that a first series of temperature measurements is made in at least one flue of the first firing section under a given set of fuel injection conditions and used to calculate a first average temperature, fuel injection is continued under the given set of conditions, a second series of temperature measurements is made under the given set of gas injection conditions and used to calculate a second average temperature, an average temperature change is calculated from the first and second average temperatures, the amount of fuel injected is increased to and maintained at a level above the level of the given set of fuel injection conditions, the change in temperature resulting from the increased fuel injection is measured, the change in temperature between the calculated average temperature change and the measured change is used to infer the oxygen level at the first firing section, and the oxygen level so inferred is used to determine any changes in flue gas flow rate, fuel injection level or air injection level.

3. A method as claimed in claim 1 or claim 2 characterised in that the oxygen level in at least one flue of a pre-heat section is measured and the measured oxygen level is used to determine any changes in flue gas flow rate, fuel injection level or air injection level.

4. A method as claimed in claim 2 characterised in that supplementary air is added to at least one pre-heat section.

5. A method as claimed in claim 3 characterised in that supplementary air is added to at least one pre-heat section.

6. A method as claimed in claim 2 characterised in that the flow rate is altered by altering the withdrawal rate of used flue gases through the exhaust outlet.

7. A method as claimed in claim 3 characterised in that the flow rate is altered by altering the withdrawal rate of used flue gases through the exhaust outlet.

8. A method as claimed in claim 4 characterised in that the flow rate is altered by altering the withdrawal rate of used flue gases through the exhaust outlet.

9. A method as claimed .in claim 5 characterised in that the flow rate is altered by altering the withdrawal rate of used flue gases through the exhaust outlet.

10. A carbon baking furnace including first and successive pre-heating, firing and cooling sections made up from a plurality of cells defined by hollow walls of refractory material, the adjacent hollow walls being connected to form flues allowing the flow of flue gases between a cooling inlet through which air may be injected and an exhaust outlet through which used flue gases may be withdrawn, characterised in that at least one means for the controlled addition of supplementary air is provided for the flue of one or more of the pre-heating sections downstream of the first firing section.