GB2172982A

GB2172982A - Hot blast stoves

Info

Publication number: GB2172982A
Application number: GB08606899A
Authority: GB
Inventors: Stanley Smith; Andrew Hardie
Original assignee: Davy McKee AG
Current assignee: Davy McKee AG
Priority date: 1985-03-25
Filing date: 1986-03-20
Publication date: 1986-10-01
Also published as: DE3609893A1; GB8606899D0; GB2172982B

Abstract

A hot blast stove for a blast furnace comprises a shell defining a vertical portion and a dome, and a lining within said shell, said lining comprising a plurality of courses of refractory bricks 20 arranged one course on top of another, in which the bricks of each said course are laid with gaps 21 at intervals in the course to allow for expansion of the bricks to maximum design temperature in operation and in which the gaps 21 in each said course are sized so that at maximum design temperature for that course the bricks thereof are held under compression one against another, as a result of thermal expansion. <IMAGE>

Description

SPECIFICATION Stoves This invention relates to blast furnace internal combustion chamber stoves.

Blast furnace internal combustion chamber stoves are brick lined regenerators enclosed in a cylindrical steel shell with a flat bottom and a dome shaped top. The purpose of such a stove is to preheat air for supply to a blast furnace.

A typical stove comprises two main parts, namely a combustion chamber in the form of a vertical passageway in which cleaned blast furnace gas is burnt and a chequer chamber.

The combustion chamber extends from a level near the bottom of the stove and the hot products of combustion from the burner at the bottom of the combustion chamber pass upwards and are then deflected downwards for subsequent passage through the chequer chamber by passage round the dome at the top of the stove. The chequer chamber is filled with chequerwork which is built up from special bricks which together form a multiplicity of small passageways through which the products of combustion pass in a generally downward direction after having passed round the dome from the combustion chamber. The products of combustion leave the stove through a waste gas outlet at the base of the chequer chamber. The combustion chamber, the chequer chamber and the dome each have a lining built up from appropriately shaped refractory bricks.

Once the dome and chequerwork have been heated to the selected operating temperature gassing is stopped, the control valves are adjusted and then air is drawn into the stove from the bottom of the chequer chamber and is passed through the stove in the reverse direction to that of the combustion products during the heating phase of the stove operating cycle. In this way the air for the blast furnace is heated to the desired high temperature. After the chequerwork has cooled to a determined level the blast air flow is switched to another stove and gassing is reinstated to reheat the dome and chequerwork. Once a stove with a silica lining has been put into operation it should be kept hot thereafter as the lining is prone to serious damage if the silica is allowed to cool below about 700"C.

It will thus be seen that the various parts of the interior of the stove are subjected to a considerable variation in thermal conditions in the course of its operating cycle. This wide variety of thermal conditions experienced by the refractory fill of a stove demands that the stove designer pays particular attention to the expansion properties of the selected refractory materials of the lining in both the vertical and radial directions. Traditional stove design has made allowance for the expansion of the refractory bricks of the lining at the expected operating temperatures.Hence it is normal to construct the lining of the combustion chamber, of the dome and of the chequer chamber from appropriately shaped refractory bricks laid in courses and held one to another with an appropriate mortar, whilst providing expansion gaps in each course to allow for expansion of the bricks from ambient temperature to their maximum design temperature. Such expansion gaps are often filled with combustible material, such as paper or an expanded plastics material, for example foamed polystyrene, or with compressible ceramic fibre during construction.

The traditional approach to refractory lining of stoves has been to calculate the expansion gaps so that, at maximum design temperature, the individual bricks are not stressed so as to avoid the risk of the lining breaking up in operation of the stove due to thermal stresses.

For example, silica bricks may expand by about 1% to about 1.5% between ambient temperature and maximum design dome temperature, depending upon the source of silica used, whilst alumino-silicate bricks may expand by up to about 1.0% over the corresponding temperature range. Hence in conventional designs of stove the expansion gaps typically total from about 10 mm to about 15 mm per metre of course in the case of silica lining bricks, and up to about 10 mm per metre of course for alumino-silicate lining bricks, when the expansion gaps are to be filled during construction with a combustible material. Somewhat larger gaps are used if a compressible ceramic fibre felt is used as packing for the expansion gaps during construction.

Although this traditional design philosophy is often satisfactory, it has been observed on occasions that such expansion gaps have resulted in parts of the lining moving in operation, thus leaving the insulation and shell exposed to hot gases.

There is accordingly a need to provide an improved form of stove refractory construction for internal combustion chamber stoves for blast furnaces in which the lining does not suffer from the risk of movement.

The present invention accordingly seeks to provide a hot blast stove for a blast furnace with an improved form of lining which obviates the afore-mentioned disadvantages.

According to the present invention there is provided a hot blast stove for a blast furnace comprising a shell defining a vertical portion and a dome, and a lining within said shell, said lining comprising a plurality of courses of refractory bricks arranged one course on top of another, in which the bricks of each said course are laid with gaps at intervals in the course to allow for expansion of the bricks to maximum design temperature in operation and in which the gaps in each said course are sized so that at maximum design temperature for that course the bricks thereof are held under compression one against another as a result of thermal expansion.

The bricks forming the lining to the dome are shaped and laid in courses so as to form a part spherical shape. Preferably at least a majority of the courses of bricks, and even more preferably all of the courses of bricks, of the lining of the dome are laid with expansion gaps between and within courses such that at maximum design temperature for each such course the bricks are held under compression against one another as a result of thermal expansion. Hence in operation of the stove the bricks of the lining to the dome experience compressive dome stresses, including both compressive hoop stresses in a horizontal plane and also similar compressive stresses in vertical longitudinal planes along the arc of the dome. The total compressive dome stresses should lie within a limiting stability band, particularly in the case of the bricks of the courses at and adjacent the base of the dome.In this way the stability of the lining of the dome is improved.

In the vertical portion of the hot blast stove the courses are laid one on another with expansion gaps in each course. Preferably a hot blast stove according to the invention is constructed so that at least a majority of the courses of bricks, and even more preferably all the courses of bricks, that form the lining to the vertical portion are provided with expansion gaps within each course so that in each course the bricks thereof are subjected in operation of the stove to a level of compressive hoop stresses which is calculated to be tolerable by the bricks and shell. In this way the stability of the lining of the vertical portion of the stove is improved.

The bricks in each course are usually made of the same material. However, different courses within the stove lining may be made of bricks of different refractory materials. The bricks within the courses can be laid in the usual way with the aid of an appropriate mortar to bond the bricks one to another. Typically the mortar bonds between adjacent bricks are nominally 2 mm in thickness.

In conventional stove design the mortar has generally been regarded as inert and no allowance has been made for the mortar in calculating the expansion gaps. Investigation has however shown that the fluid properties of the mortar at elevated tempratures should not be neglected. Thus the development of specific refractory mortars has led to an understanding of their fluid behaviour at elevated temperatures and of the significance of this fluid property in the overall expansion allowance policies.

A hot blast stove in accordance with the invention may have a lining constructed entirely of alumino-silicate bricks or a lining constructed partly from courses of alumino-silicate bricks and partly from courses of silica bricks.

Different courses may be made from bricks of different alumino-silicate compositions, although the bricks of each individual course will usually be made from bricks of a single composition. The choice of type of brick will depend at least to some extent upon the maximum design temperature to which the relevant part of the lining will be exposed in use.

Hence alumino-silicate bricks with different alumina contents may be used at different levels in the lining. Generally speaking silica bricks will be used for those parts of the lining which are expected to be exposed in use to the highest temperatures.

Silica bricks effectively cease to expand above a temperature in the range of from about 600"C to about 700"C depending upon the source of silica, whereas alumino-silicate bricks expand continuously to the upper temperature limit of their capability.

In the case of a single course of silica bricks the expansion gaps used in the stove of the invention may total, for example, between about 50% and about 80% of the total gaps used in conventional designs. Thus, for example, for a silica with an expansion of 1% between ambient temperature and maximum design temperature (e.g. about 1 2000C to about 1500 C) the expansion gaps for a course of such silica bricks in accordance with the invention may be from about 5 mm to about 8 mm per metre of course, where these are to be filled with combustible material. If a grade of silica is used which expands by 1.5% over the corresponding temperature range, then the expansion gaps in accordance with the invention may be from about 7.5 mm to about 12 mm per metre of course of such silica bricks.

In the case of a single course of aluminosilicate bricks the expansion gaps will depend on the quality of the brick, its creep properties and the range of temperatures to which it is to be exposed, which may be, for example, from about 200"C up to the dome operating temperature, that is to say from about 1 2000C to about 1500"C. In accordance with the invention the expansion gaps may total from about 50% to about 80% of the corresponding figure used for such conventional designs.

For example, in the case of an alumino-silicate brick which expands 1% between ambient and working temperature, the expansion gaps for a conventional design will total about 10 mm per metre of course, if they are filled with combustible material; in accordance with the invention the expansion gaps for a course of such bricks may total from about 5 mm to about 8 mm per metre of course. Larger gaps will be needed if pads of ceramic fibre felt are used to fill the expansion gaps during construction.

It will thus be seen that the expansion gaps provided in the lining of the -stove of the invention are significantly smaller than usual.

In order that the invention may be clearly understood and readily carried into effect, a preferred embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein: Figure 1 is a vertical longitudinal section through a hot blast stove constructed according to the invention; Figure 2 is a horizontal section through the stove of Figure 1; Figure 3 is an enlarged horizontal section showing the detailed construction of part of the lining of the stove of Figure 1; and Figure 4 is a side view of part of the lining of the stove of Figure 1.

Referring to the drawings, a hot blast stove comprises an outer steel shell 1 and a lining 2 of refractory material. It includes a vertical portion having a combustion chamber 3 and a chequer chamber 4, separated one from another by a dividing wall 5. The structure is surmounted by a dome 6. Line 7 indicates the top of the burner (not shown) in the combustion chamber 3 and line 8 is the centre line of the hot blast main (not shown). Chequer chamber 4 is filled with the usual chequerwork built up from chequer bricks which are each conveniently hexagonal in horizontal section with a plurality of vertical passages formed therein and with a plurality of vertical partcircular or multi-sided grooves in their periphery to form with similar grooves on adjacent bricks further vertical passageways for combustion products.The top part 9 of the chequerwork is constructed of silica or alumino-silicate bricks, whilst the lower parts 10, 11, 12 and 13 are made of aluminosilicate bricks selected to meet the temperature conditions at each of these levels in the stove. The chequerwork is supported on the usual grids 14 above the stove floor 15.

Figure 3 illustrates the construction of a typical lining of the chequer chamber in more detail; the lining of the combustion chamber is constructed in a similar fashion. Reference numerals 16, 17 and 18 indicate three layers of bricks of differing grades of insulation material which are interposed between the lining 2 and the shell 1. Reference numeral 19 indicates an expansion gap, which is filled during construction with combustible material (e.g.

foamed polystyrene), located between the innermost layer 18 of insulation material and the lining of interlocked silica or alumino-silicate bricks 20. The bricks 20, whether made of silica or of alumino-silicate, are laid in courses and are cemented one to another with a mortar. The nominal joint thickness of the mortar layer is in each case 2 mm. The bricks may be of any convenient length, for example from 200 mm to 450 mm. Typically the mortar has a clay content. This can be in the form of a heat setting refractory cement in the case of silica bricks. In the case of alumino-silicate bricks it can be a heat setting cement or an air drying cement.

Figure 4 illustrates how every course of bricks 20 is interrupted at regular intervals, as shown at 21, by small gaps. These represent from about 50% to about 80% of the theoretical brick expansion allowance in the case of silica bricks. For alumino-silicate bricks the construction is similar except that the expansion gaps will reflect the creep properties of the brick; hence the expansion gaps, in the case of alumino-silicate bricks, are generally from about 50% to about 80% of the theoretical brick expansion allowance. During construction these gaps are filled by spacers, made for example of foamed polystyrene, which burn during heating to operating temperature. As shown gaps 21 are provided every four bricks 20 of each course; however, other spacings can be used, e.g. a gap every 7 bricks of each course, depending upon the size of the bricks used.

In construction of the lining of the dome, expansion gaps similar to the gaps 21 are left in each course of bricks; in addition corresponding gaps are left at intervals between courses.

Instead of filling the expansion gaps with a combustible material, such as polystyrene foam, these can alternatively be filled with compressible ceramic fibre pads. In this case the expansion gaps have to be larger in order to accommodate the compacted fibre at working temperatures. For example a 10 mm thick ceramic fibre pad may compact upon heating to maximum design temperatures to between about 4 mm and about 5 mm in thickness.

In order to assess the degree of expansion allowance required for the thermal conditions under consideration, the thermomechanical properties of the materials are first assessed experimentally by determining the thermal expansion of a typical brick between ambient temperature and maximum design temperature, the modulus of elasticity under varying loading conditions over that temperature range, and the Poisson ratio. This assessment is carried out under carefully controlled conditions in a temperature controlled experimental furnace on bricks and on bricks and mortar combinations.

From such results it is possible to calculate the stress levels both in the refractory lining and also in the shell. Typically the expansion allowance provided by the expansion gaps incorporated during construction of the lining in accordance with the invention ranges from about 50% to about 80% of the theoretical expansion allowance that is required to give a conventional "no stress" lining. The precise expansion allowance should be selected in dependence on the expected operating temperature to be encountered in operation and on the mechanical properties of the materials se lected for use.

It will thus be appreciated by those skilled in the art that the hot blast stoves of the present. invention have a gas tight lining with an acceptably low loading being exerted on the refractories and on the shell.

Claims

1. A hot blast stove for a blast furnace comprising a shell defining a vertical portion and a dome, and a lining within said shell, said lining comprising a plurality of courses of refractory bricks arranged one course on top of another, in which the bricks of each said course are laid with gaps at intervals in the course to allow for expansion of the bricks to maximum design temperature in operation and in which the gaps in each said course are sized so that at maximum design temperature for that course the bricks thereof are held under compression one against another as a result of thermal expansion.

2. A hot blast stove according to claim 1, in which the bricks in each course are all made of the same material.

3. A hot blast stove according to claim 1 or claim 2, in which different courses are made of bricks of different refractory materials.

4. A hot blast stove according to any one of claims 1 to 3, in which there is at least one course of aluminosilicate bricks.

5. A hot blast stove according to claim 4, in which the gaps in said at least one course of alumino-silicate bricks total from about 50% to about 80% of the theoretical expansion allowance for such bricks.

6. A hot blast stove according to any one of claims 1 to 5, in which there is at least one course of silica bricks.

7. A hot blast stove according to claim 6, in which the gaps in said at least one course of silica bricks total from about 50% to about 80% of the theoretical expansion allowance for such bricks.

8. A hot blast stove according to any one of claims 1 to 7, in which the gaps in each course are filled during construction with spacers of combustible material.

9. A hot blast stove according to any one of claims 1 to 7, in which the gaps in each course are filled during construction with pads of compressible ceramic fibre felt.

10. A hot blast stove according to any one of claims 1 to 9, in which the courses of bricks forming a lining to the dome are laid with gaps at intervals between them, the gaps between the courses being so sized that at maximum design temperature for the respective courses the bricks of the lining to the dome are held against each other by compressive dome stresses due to thermal expansion.

11. A hot blast stove according to any one of claims 1 to 10, in which the courses of bricks forming a lining to the vertical portion have gaps therein at intervals which are so sized that in operation of the stove the bricks thereof are held under compression one against another by compressive stresses.