Patent Application
20120214115
The present invention relates to a method for heating a blast furnace stove for use with a blast furnace.
The combustion air supplied to a blast furnace is typically preheated using a stove, comprising refractory material which is heated using a burner. When the material is hot enough, combustion air is passed through the stoves to pre-heat it before injection into the blast furnace. Usually, several stoves are operated in parallel and cyclically so that at least one stove is operated for heating combustion air while the refractory material of at least one stove is heated.
Conventionally, the top gas leaving the blast furnace has a temperature of around 110-120° C. and contains about 20-25% each of CO and CO2. Typically, 3-5% H2 and some H2O will also be present, but the other major constituent of the top gas is N2 (typically 45-57%). The gas constitutes a low grade fuel, having a relatively low heating value, and is commonly used to fuel the stoves.
The top gas is normally combusted using air-fuel burners in the stoves. In order to ensure the necessary high air blast temperatures needed by the blast furnace, it is well known to enrich the top gas with a high calorific value gas, such as coke oven gas or natural gas. The combustion of such additional fuel leads to larger overall emissions of carbon dioxide from the plant, and is therefore not desirable.
It is also known to oxygen enrich the combustion air used in stack burners. Usually, the enrichment levels needed to reduce or eliminate the need for additional, high-calorific fuels are such as to result in a final oxidant oxygen content in the combustion air of around 28-30%.
Such methods may in some cases render peak flame temperatures high enough to damage the refractory material of the stove.
The blast furnace itself is a highly efficient counter-current reactor that has evolved over many years. It is approaching the limits of thermodynamic efficiency. Moreover, the blast furnace and its ancillary equipment, such as stoves, are the largest energy consumers in an integrated iron and steel works. Furthermore, the energy consumed in iron making is the dominant factor determining the carbon consumption of the integrated steel making process, and therefore the emissions of carbon dioxide. Therefore, it would be desirable to increase thermal efficiency of blast furnace stoves.
In addition to the problem of high peak temperatures mentioned above, too low flame temperatures or heat input rates will lead to long heating cycles, which is undesirable. In other words, the flame temperature needs to be moderated.
The present embodiments solve the above described problems and make it possible to achieve other advantages as will be described below.
Thus, the present embodiments relate to a method for heating a blast furnace stove by combusting a fuel with a lower heating value (LHV) of 9 MJ/Nm3 or less in a combustion region in which there is maintained a stable visible flame, arranged in a combustion chamber in the stove, and causing the combustion gases to flow through and thereby heat refractory material in the stove, and is characterized in that the fuel is combusted with an oxidant comprising at least 85% oxygen, and in that combustion gases are caused to be recirculated into the combustion region to dilute the mixture of fuel and oxidant therein sufficiently for the flame not to damage the refactory material.
In the following, the invention will be described in detail, with reference to exemplifying embodiments of the invention and to the appended drawings, in which:
Each stove 100 comprises refractory material in the form of ceramic bricks or the like, which is first heated and then used to heat blast air which is fed into the blast furnace.
When operated in refractory material heating mode (“on gas” mode), the top gas is combusted in the stove 100 with the oxidant, and the combustion gases are fed to a flue gas treatment device 150, possibly including a conventional carbon capture step.
When operated in blast air heating mode (“on blast” mode), air is led through the refractory material in the opposite direction, and then on to the blast furnace 120.
The stoves 100 are operated cyclically, so that at any point in time at least one stove is operated on blast and the rest of the stoves are operated on gas.
When operated on gas, top gas and air is fed into a combustion zone of the combustion chamber 101, in which combustion takes place, via an air burner 108. The burner 108 comprises a fuel inlet 105 and an air inlet 104. The hot combustion gases then stream up through the chamber 101, past the dome 103 and down through the refractory material 102, thereby heating the latter. When exiting through the port 106, the temperature of the combustion gases is conventionally about 200-350° C.
When the refractory material has reached a predetermined temperature, the operation is switched to on blast operation. Then, air is introduced through the port 106, streams through the hot refractory material 102, via the dome 103 and the combustion chamber 101, and out through an outlet port 107. At this point, the blast air has a typical temperature of 1100-1200° C.
It is preferred, in the context of the present invention, to heat the stove with blast furnace top gas, as described above. It is furthermore preferred to use top gas from a blast furnace to which blast air is provided from the stove. This allows for the arrangement of the stove near the blast furnace, is energy efficient and leads to low total emissions from the plant.
However, it is to be understood that the present invention can be equally advantageously applied to stoves heated with other low-grade fuels. By way of example, typical chemical compositions (percentage values) and lower heating values (LHV) are provided in Tables I and II, respectively, for blast furnace top gas and converter off-gas.
According to the present invention, the stove is heated with a gaseous fuel the LHV value of which is not higher than 9 MJ/Nm3. Use of such low-grade fuel will draw maximum benefit from the possible cost benefits of the present invention. The fuel may comprise a certain addition of another, more high-grade fuel, as long as the LHV value of the mixture is equal to or less than 9 MJ/Nm3. In order to minimize cost and emissions, it is however preferred not to add high grade fuels prior to combustion.
According to the present invention, such a low-grade fuel is used for heating the stove by combusting it, not with air or slightly oxygen-enriched air, but with an oxidant comprising at least 85% by weight, preferably at least 95% by weight, oxygen, where the oxidant most preferably is industrially pure oxygen having an oxygen content of essentially 100%.
This will increase fuel efficiency, since the nitrogen ballast present in air does not need to be heated. Moreover, by reducing the nitrogen ballast in the combustion products, the necessary flame temperatures can be attained without the need to supplement the low-grade fuel gas with high calorific fuels. The reduced energy demand will facilitate increased power generation and/or lead to a reduced need for import gas, thus improving fuel management.
Normally, using an oxidant with such large oxygen contents would lead to peak temperatures high enough to damage the dome and refractory material of the stove.
However, it is possible to use this type of oxidant under condition that the stove combustion gases are recirculated into the combustion zone to such extent that the mixture of fuel and oxidant therein is diluted sufficiently for the combustion in the combustion region to form a stable, visible flame at temperatures that do not damage the dome and the refractory material.
That “combustion gases are recirculated into the combustion region” herein refers to combustion gases located outside of the combustion region being recirculated back into the combustion region. Such combustion gases may originally be located inside the combustion chamber itself, but outside of the part of the combustion chamber occupied by the region in which combustion mainly takes place (the “combustion region”). Thus, in this case combustion gases are in fact recirculated within the combustion chamber. Alternatively, such combustion gases may be recirculated from outside of the combustion chamber back to the combustion region.
As will be described in further detail in the following, the dilution of the reactants may be achieved either by creating heavy turbulence inside the combustion chamber using high-velocity lancing of oxidant, possibly using a staged combustion scheme, and/or the recycling of flue gases from the stove back into the combustion zone.
In accordance with the invention, it is possible to achieve sufficiently low peak flame temperatures so as not to damage the refactory material of the stove.
Additionally, when a high-oxygen oxidant is used to combust low-grade fuels such as blast furnace top gas, the CO2 contents of the combustion gases become considerably higher as compared to when using air or slightly oxygen-enriched air as the oxidant. Since conventional carbon capture techniques tend to be considerably cheaper per unit captured CO2 when the treated gas contains a larger share of carbon dioxide, this leads to considerable cost savings when using such a carbon capture step to treat the stove combustion gases.
In all embodiments described herein, the total amount of oxidant per time unit is balanced against the amount of supplied low-grade fuel, so as to create the desired combustion conditions in terms of stoichiometry.
It is preferred that each lance 310, 311, 312 supplies oxidant to the combustion zone at high velocity, preferably at least 200 m/s, more preferably at least sonic velocity. Such high-velocity lancing leads to heavy turbulence in the combustion chamber, in turn entraining combustion gases into the combustion zone and thereby diluting the flame so as to render it diffuse with a peak temperature that does not damage the refractory material of the stove.
According to one preferred embodiment, a lance 310 is arranged with its orifice in close proximity to the orifice of the fuel inlet 305. According to another preferred embodiment, a lance 311 is arranged at a position at a distance from the orifice of the fuel inlet 305. Depending on the geometry of the combustion chamber 301, one of these arrangements, or a combination of both, may provide the best recirculation of combustion gases into the combustion zone. A supplementary lance 312, arranged further downstream in relation to the other lance or lance 310, 311, can be used to provide a staged combustion process, whereby the total flame volume can be made even larger. Naturally, more than one lance of each of the described types 310, 311, 312 may be arranged to complement each other. In case the oxidant is lanced in close proximity to the fuel inlet 305, it is preferred to also lance oxidant further downstream so as to create a staged combustion process.
Low grade fuel is supplied via a supply conduit 411, a supply device 412 and an inlet 413. Oxidant is supplied via a supply conduit 414, a supply device 415 and a lance comprising an orifice 416. The lance is arranged so that its orifice 416 is arranged adjacent to the fuel inlet 413. Preferably, the lance runs coaxially to the fuel inlet 413, as depicted in
The stove 400 can be part of a standing iron making plant and adapted to operate in accordance with the invention from a conventional mode of operation in which air is used to support combustion of the blast furnace gas, in which the blast furnace gas is supplemented by a coke oven gas or natural gas and in which there is no recirculation of the combustion products with the stove 400.
According to a preferred embodiment, an existing, conventional, air burner, which was used to heat the existing stove 400 previously, is in an initial step replaced by an oxyfuel burner 410 comprising the above described fuel inlet 413 and oxidant lance. An “oxyfuel” burner herein refers to a burner driven with a fuel and an oxidant, wherein the oxidant comprises a large part oxygen, preferably at least 85% oxygen, more preferably at least 95% oxygen.
According to an alternative, preferred embodiment, the existing air burner described above is, in an initial step, supplemented with one or several high-velocity oxidant lances as described above, and the air supply is terminated.
As described above, such high velocity lancing yields heavy turbulence inside the combustion chamber 301, 401, leading to sufficiently low peak flame temperatures for the refactory materials in the stove not to be damaged.
However, the mass flow rate of the combustion gases will be lower when using a high-oxygen oxidant as compared to when using air as the oxidant. This would lead to smaller convective heat transfer to the refractory material and hence longer heating cycle times. Therefore, when converting an existing stove for high-oxygen oxidant operation, flue gases are recycled from the stove back into the combustion zone as described below in connection to
Thus,
During on gas operation, the combustion gases leave the stove 500 through a port 506. However, part of the combustion gases are recycled back to the combustion region in the combustion chamber 501 via a recycling device 511. The feedback device 511 may include a propelling device, such as a fan, to feed the recycled combustion gas to the combustion chamber 501.
The recycling device 511 is also arranged to mix the recycled combustion gas with a high-oxygen oxidant of a composition as described above, provided via a supply conduit 512. The mixing may take place using conventional diffusers. The mixture of recycled combustion gas and oxidant is then supplied to the combustion chamber 501 via an inlet 513. A low-grade fuel, such as top gas from the blast furnace, is provided, via a supply conduit 514, a supply device 515 and an inlet 516. In the combustion zone, the fuel is hence combusted with the oxidant in the presence of the combustion gases that have been recycled into the combustion zone after they have already past the stove 500. This way, the flame in the combustion chamber 501 is diluted.
Using such flue gas recycling, it is possible to reach convective heat transfer rates high enough so as to be able to maintain the heating cycle time of an existing stove in which a method according to the present invention is applied. This is achieved by recycling a sufficient amount of combustion gases to maintain the gas mass or thermal energy flow per time unit through the stove 500, at a level which is at least the same as the gas mass or thermal energy flow per time unit which was used when the existing stove was operated, prior to conversion to operation according to the present invention, using a low-oxygen oxidant with no recycling.
As previously mentioned, the method according to the invention replaces air combustion of a calorifically enriched low calorific value fuel gas with oxy-fuel combustion, in which the flame is diluted by recirculating flue gas, by example by high impulse mixing of the combustion space using lances for the injection of the oxidant. The need for a high cost high calorific value booster fuel gas is eliminated and the stove is fuelled using blast furnace gas alone. The stoves typically account for around 10% of the total energy demand for integrated steel-making and some 18% of the energy delivered to the stoves is lost in the flue gas. Recycling flue gas reduces this energy loss and lowers the amount of energy that must be supplied to the stove from combustion of a fuel gas. The method according to the invention therefore combines some of the benefits of waste heat recovery with those of oxy-fuel combustion.
Consider a hypothetical example of a 1500 m3 working volume blast furnace operating with a productivity of about 2.2 t/m3/d. Such a furnace would produce some 138 tonnes of hot metal per hour and based on typical blast volumes, might be expected to consume 138,000 Nm3/h of hot blast. To achieve a hot blast temperature of 1200° C. would require a stove burner flame temperature about 150° C. higher and some 230 GJ/h would be required to heat the air to this temperature. For a stove efficiency of around 80% this means the energy input to the stoves would be around 290 GJ/h or 145 GJ/h for each stove assuming two stoves are ‘on-gas’ simultaneously. It is well established that for normal stove operating conditions, about 18% of the energy input to the stoves exits in the flue gas. It has been estimated that for the conditions considered this would result in a flue gas temperature of around 250° C.
These conditions have been used to establish hypothetical heat and mass balances for 3 modes of operation, “air-fuel” (i.e. conventional operation without flue gas recirculation; “oxygen-enriched” (i.e. as “air-fuel”, but with the air enriched in oxygen) and “flue gas recycle” (i.e. in accordance with the method of the invention). The calculations have been done to ensure a constant flame temperature and constant mass flow of combustion products, so that conditions for convective heat transfer are maintained. Flame stoichiometry has, in each case, been adjusted to ensure 1% excess oxygen in the flue gas. The results are compared in Table 3.
It can be seen that for the conditions considered, oxygen enrichment of the air supplied to the stoves reduces, but does not eliminate, the amount of coke oven gas employed. Blast furnace gas flow is increased to ensure the heat input of 145 GJ/h is maintained. The CO2 content of the flue gas increases marginally due to the elimination of some nitrogen from the system.
The introduction of flue gas recirculation removes the need for calorific enrichment of the fuel gas. This is because a further modest increase in the flow of blast furnace gas, combined with the recovery of the sensible heat contained in the flue gas, is sufficient to enable the desired flame temperature to be reached. It is to be understood that with flue gas recirculation, the oxidant is not air but a gas mixture containing at least 85% by volume of oxygen or essentially pure oxygen. (The calculated results shown in Table 3 are based on the latter). The energy input from combustion is decreased by about 4% due to recovery of energy from the recycled flue gas.
Air is eliminated and combustion is sustained by the use of industrial oxygen. Importantly it can be seen that the CO2 content of the flue gas has increased from the initial 23% to 41%. This equates to 50 tonnes of CO2 per hour for a single stove or 100 tonnes for the two stoves ‘on-gas’. 75 tonnes of this would be available for carbon capture and sequestration whilst the remainder is recycled.
For the hypothetical case under consideration it is reasonable to assume that the 138 tonnes of hot metal produced each hour is converted to 150 tonnes of slab or other metal product which accounts for the likelihood of scrap additions during steel-making.
Applying industry benchmark figures, it can be estimated that the entire integrated steel plant would generate about 280 tonnes of CO2 per hour. Hence, for the example considered, recycling flue gas to the stoves (assumed to be Cowper stoves) makes some 27% of the plant-wide CO2 emissions available for carbon capture.
Whilst simple heat and mass balances, such as those detailed in Table 1, serve to illustrate the main advantages achievable by the method according to the invention, they do not completely reflect the benefits. In particular, they do not take account of the improved heat transfer conditions generated on switching from air-fuel to oxy-fuel combustion. For this purpose a dynamic model that accounts for changes to the overall heat transfer coefficient as a function of composition, temperature and mass flow in the refactory checker work can be used. A number of modelling studies of hot blast stoves, have shown that the heat transfer taking place can be accurately represented by an overall or ‘lumped’ heat transfer coefficient that combines the effects of convection and radiation. So that for the gassing cycle
α=αc+αr
α=convective heat transfer coefficient,
and;
αr=radiative heat transfer coefficient
The convective heat transfer coefficient is related to the mass flow rate and may be calculated from the Sieder-Tate or Hausen equations. The radiant heat transfer coefficient is derived from the Stefan-Boltzmann law which can be expressed in the form;
εg=emissivity of the gas which is a function of composition and temperature and may be derived from grey gas models or from Hottel charts.
αg=absorptivity of the gas
Tg=gas temperature.
TB=mean temperature of local checker work.
A zonal model that incorporates such principles and accounts for heat transfer to and within the checkers (refactory) has been used to make a more detailed assessment of the benefits. The baseline for comparison is operational data from a set of modern Cowper stoves generating an industry benchmark hot-blast temperature of 1250° C. The results are shown in Table 4.
It is interesting to compare these cases in a little more detail:
The conventional operations show that the stove uses a significant level of natural gas enrichment to generate a high blast temperature of 1248° C.
All three examples (“Oxygen Cases” 1, 2 and 3) are in accordance with the inventors. In ‘Oxygen Case 1’ the model has been run retaining the same blast temperature, blast volume, and stack temperature as in the conventional operations. This case generates comparable results to a steady state heat balance because although radiation heat transfer to the (refractory) checker bricks has improved, the benefit of this is disguised by forcing the model to retain a constant stack temperature. In fact, since the heat capacity of the CO2 contained in the recycled flue gas, is higher than that of the nitrogen it is replacing, the overall impact is that slightly more energy is needed to maintain a constant dome (and blast) temperature. Nevertheless, replacing expensive natural gas with a cheaper fuel source is sufficient to compensate for both the higher energy input and the cost of the oxygen consumed. It is worth pointing out that the overall heat transfer coefficients calculated by the model show a 13.5% increase relative to air fuel combustion near the top of the checkers, but even at the lower temperatures towards the base of the checkers the overall heat transfer coefficient had increased by some 8.5%.
In ‘Oxygen Case 2’ the enhanced heat transfer conditions have been accounted for by allowing the stack gas temperature to re-equilibrate to a lower temperature. It can be seen that since more heat is retained in the checkers, the stack temperature drops by some 25° C. The net effect is that it is possible to lower gassing cycle time whilst retaining the same blast temperature. The total energy input for a 3 stove operation is marginally reduced but blast temperature and volume is maintained even at a lower stack gas volume. This is an important feature that could be exploited under plugged stove conditions.
In ‘Oxygen Case 3’ the reduction in stack gas temperature by increasing the firing rate until the original stack gas temperature has been exploited by increasing the firing rate until the original stack gas temperature has been restored. It is apparent that the firing rate can be increased by almost 10%. This is enough to increase the blast temperature by some 13° C., enough to result in significant coke savings at the blast furnace.
Computational fluid dynamic (CFD) modelling has been used alongside the dynamic heat balance, to develop a detailed understanding of temporal and spatial variations of temperature, velocity and concentration that occur during a complete stove cycle. Some relevant CFD results are presented in
Referring now to
According to an alternative preferred embodiment, enough combustion gases are recycled to essentially maintain or increase the thermal energy throughput through the refractory material. This takes into consideration the different heat capacities for various inert components in the combustion gases. In this case, it is also preferred that enough combustion gases are recycled so that the flame temperature is essentially maintained or decreased.
As is also shown in Table 3, the CO2 contents of the flue gases vented from the stove 500 are much higher—41% as compared to 23% in the conventional operation mode. The costs per unit weight captured CO2 for conventional carbon capture techniques is significantly decreased as the CO2 concentration increases from low levels up to a level of roughly 50-60%. Concentrations increased beyond this limit will provide smaller gains. As a result, the costs for a carbon capture step for treating the stove flue gases may be reduced significantly per unit weight captured CO2 when a high-oxygen oxidant is used in accordance with the present invention.
According to a preferred embodiment, an existing, conventional, air burner, which was used to heat the existing stove 500 previously, is in an initial step replaced by a fuel inlet 516 and an inlet for recycled combustion gases 513, and the fuel is then combusted with the above described high oxygen oxidant. To this end, it is preferred that the oxidant is submitted by premixing with the recycled combustion gases. It is alternatively preferred that such premixing is combined with one or several lances as described above.
Oxidant is supplied via an oxidant supply conduit 613 and an oxidant supply device 614 to an oxidant lance arranged so that the orifice 615 of the lance is arranged adjacent to an orifice 612 for supply of recycled combustion gases, supplied from the recycle device 611. Preferably, the oxidant lance runs coaxially with the recycled combustion gas inlet 612. In a way which is similar to the function of the coaxial lance orifice 416 as described in connection to
The embodiment shown in
As indicated above, it is furthermore preferred that the stove 300, 400, 500, 600 is connected to a respective carbon capture step 350, 450, 550, 650, which may be conventional per se, separating the carbon dioxide contents of the combustion gases vented from the stove before the combustion gases are released into the environment.
When the age of a blast furnace stove approaches its expected useful life, it is preferred to apply one of the herein described embodiments, or a combination of several of them, to the stove.
This way, the useful life of the stove may be prolonged, operating it with lower flame temperatures, with maintained production rates in terms of blast air, better fuel economy and lower emissions.
Thus, a method according to the present invention will allow a blast furnace stove to be operated only on a low grade fuel such as blast furnace top gas, with no need for higher calorific value fuel enrichment and no risk for temperature-induced stove damage, while producing flue gases that are better suited for carbon capture. In addition, it allows the useful life of a stove to be prolonged.
If sufficient recycling of combustion gases is used, it is also possible to achieve the same amount and quality of blast air in an existing stove which is converted, according to what has been described above, for operation with a high-oxygen oxidant, and which stove is provided with the combustion gas recycling arrangement described in connection to
For example, any one of the methods for creating recirculation of combustion gases as described in connection to
Moreover, the ejector-propelled recirculated combustion gases method as described in connection to
Also, the ejector-propelling of pre-mixed or non-pre-mixed recycled combustion gases as described in connection to
Heat may be recovered from combustion gases that are not recycled. Additionally or alternatively, the combustion gas may be subjected to carbon capture.
The method according to the invention may be applied to Kalugin blast furnace stoves as an alternative to the stores illustrated in the drawings.
It will be understood that the embodiments described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result.
