EP0617785B1 - Method and regenerator for reheating gases - Google Patents

Abstract

PCT No. PCT/FR93/01025 Sec. 371 Date Apr. 28, 1994 Sec. 102(e) Date Apr. 28, 1994 PCT Filed Oct. 19, 1993 PCT Pub. No. WO94/10519 PCT Pub. Date May 11, 1994A method is provided for heating a gas in a regenerator with a heat accumulation mass consisting of a loose bulk material arranged in a ring between two coaxial cylindrical grids, a hot collection chamber, surrounded by the inner hot grid, for the hot gases and a cold collection chamber, enclosed between the outer cold grid, on the one hand, and the wall of the regenerator, on the other hand, for the cold gases, wherein the increase in the head loss during the heating phase is at least 5 times as great as the product rho .g.H, in which H is the height of the regenerator, rho is the density of the gas at a temperature of 20 DEG C. and g is the acceleration due to gravity, and the gas flow rate is at least equal to 300 m3N/h.m2 of surface area of the hot grid at standard pressure.

Description

The present invention relates to a method of heating gas in a regenerator with a mass of heat accumulation consisting of bulk material arranged in a ring between two cylindrical grids coaxial, a hot collection chamber, surrounded by the hot grid internal, for hot gases and a cold collection chamber, enclosed between the external cold grid on the one hand and the external wall of the enclosure of the regenerator on the other hand, for cold gases, as well as a regenerator of this type. A process and a regenerator in accordance with the preambles of the claims 1 and 4, respectively, are known for example from document DE-C-4 108 744.

In such a regenerator, the hot gases respectively the cold gases are driven in the radial direction through the mass of accumulation of heat, unlike otherwise usual air heaters, and done during the reheating phase, from the hot collection chamber inside the regenerator towards the external cold collection chamber, and in the opposite direction during the cold blowing of the regenerator. Gases at reheat can also be gas mixtures, which contain also parts of vapors, in particular water vapor.

A regenerator of this type is described in US-A-2,272,108. The quantitative realization, but not presented here of the application example given there, shows that a regenerator conforming to the description of this US patent would absolutely not work in the convenient. A qualitative assessment also shows that the gas velocity chosen for crossing the accumulation layer of heat was chosen far too low and furthermore that the size aforementioned grains of the bulk material the mass of accumulation of heat is too great. These values thus lead to a loss of gas charge far too low in the material bed. So the gas pressure decreases with height in the collection chamber cold, while this effect, also known as the "effect of chimney ", is negligible in the hot collection chamber. the application example, the pressure difference caused by this "chimney effect" is a multiple of the pressure drop in the bed matter, with the consequence that when the regenerator is heated, the gases of heating would circulate only in the upper region through the bed of matter, while in the lower region we should even expect reflux. When running in hot wind, so during cold blowing, the conditions are reversed, i.e. only the lower region of the bed of material would be exposed. These results necessarily lead to the conclusion that the regenerator described in patent US-A-2,272,108 would be entirely faulty.

The object of the invention is therefore to improve the process mentioned in the introduction as well as the regenerator described above, avoiding disadvantages caused by the chimney effect and in particular by increasing the power of the regenerator for a construction height significantly less of it.

In the context of the process described above, this objective is achieved by the fact that the increase in the pressure drop during the heating phase is at least 5 times as great as the product ρ.gH, in which H is the height of the regenerator, ρ is the density of the gas at a temperature of 20 ° C and g is the acceleration of gravity, that the gas flow is worth at least 300 m ³ N / hm ² of surface of the hot grid at pressure normal, and that the grain size of the bulk material is chosen less than 15 mm.

The implementation of this process according to the invention has shown that, unlike known air heaters, it is established in the material in bulk an entirely different temperature distribution because it is essentially linear in these while in the process proposed it is on the contrary in the form of S. This distribution in S of the temperature, shown in Fig. 1, includes first the advantage that the drop in temperature of the hot wind during blowing cold is very low, and besides that the variation in temperature average of the entire material bed is on the contrary very high with about 600 ° C. In air heaters known so far, the variation mean temperature is only about 100 ° C, hence it results that the S distribution of temperature stores about six times more thermal energy than the linear distribution of temperature. This result allows the mass to be reduced to around one sixth. heat buildup.

This solution also results in the chimney effect described above losing importance and even that it can be eliminated. It is advantageous that the difference Δ ^2p consisting of _hot ΔP (pressure drop of the regenerator at the end of the heating phase) and _cold ΔP (pressure drop of the regenerator at the start of the heating phase) is large compared to ρ .gH Quantitatively, efforts should be made to achieve Δ 2 p ρ. g . H = 10 at 20.

In another advantageous implementation of the process, the cold phase, that is to say the cold blowing, is carried out with an overpressure.

In this form of operation, necessary for example during the application of the process for heating blast furnace wind, the flow of gas to be heated increases in the P / P ₀ ratio, without the heat transfer degrading. If, for example, a blast furnace wind is produced at a pressure of 5 bar, the flow rate can reach 5000 m ³ N / hm ² , respectively 2500 kW / m ² . With a regenerator having a grid area of 20 m ² , a hot wind flow of 100,000 m ³ N / h can be produced.

On the contrary, the heating of the mass of heat accumulation will performed only at normal pressure, for economic reasons, and for this reason three regenerators must be heated simultaneously, while a fourth regenerator is in the process of cold blowing.

In another advantageous implementation of the method, running under load partial, the heating phase is carried out at full power, and breaks are observed after the cold blowing phase. This implementation of the process makes it possible to work with the power desired constriction, and the thermal equilibrium of the two phases is then established by the breaks after the cold blowing, and also to use for heating the regenerator a burner which has only a range of very limited adjustment, unlike the burners used until now in conventional wind heaters.

The other objective fixed to the invention is, in a regenerator intended for the implementation of the process, achieved by the fact that the outside diameter of the annular mass of heat accumulation is at most the twice its internal diameter, and that the grain size of the bulk material is chosen to be less than 15 mm.

This realization of the thickness of the heat accumulation layer influences the quantity Δ ^2p already explained above. This quantity is in fact small for a ratio of the diameters larger than that which is quoted. Calculations and tests have shown that this ratio should not exceed the value of 2.

Advantageously, the regenerator is heated with a premix burner.
The use of such a burner guarantees that the hot collecting chamber of the regenerator is entirely sufficient as a combustion chamber and that the combustion takes place not only without noise but also without pulsations.
Furthermore, the size of the regenerator is not adversely affected by the use of such a premix burner.

An exemplary embodiment of the burner is shown in FIG. 2 and will explained in detail below.

The regenerator 1 intended for the implementation of the method of the invention has an enclosure 2 having the shape of an upright cylinder, which can by example be supported by pillars 3.

The interior space of enclosure 2 is essentially divided by two grids 4 and 5 of cylindrical shape and arranged concentrically with distance from each other, in a cylindrical hot collection chamber 6 internal, an intermediate annular chamber 7 containing the accumulation mass heat consisting of bulk material, and a cold external annular collection 8 formed by the wall of the enclosure 2 with grid 5.

In the masonry foot 9 region of enclosure 2, arrivals are expected 10 for heating gases, which are produced by a premix burner 11, which in turn is supplied by a gas - air mixing tube 12.

The hot internal collection chamber 6 ends in the upper region enclosure 2 of regenerator 1 by a hot wind outlet 13, the external collection chamber 8 is connected to a chimney 14 for evacuation burnt gases, from which the heating gases can escape after they have passed through the heat storage agent in the intermediate chamber 7.

The gas-air mixing tube 12 is connected to a fan 15, which produces both air for the heating phase and for the cold blowing. In the heating phase, air is led through the tube of gas - air mixture 12 and mixed with heating gas, which has been introduced by the gas injector 16 into the gas-air mixing tube 12.

After the completion of the heating phase, the valves 17, 18 and 19 are closed, the valve 20 and the outlet 13 are on the contrary open, so that the cold blowing phase can begin. After the completion of the cold blowing phase, the open fittings are at again closed and the previously closed valves are open, so that the heating phase can start again.

The bulk material of the heat accumulating mass consists of a loading of granules with a grain size not exceeding 15 mm, and the outside diameter of the annular mass of heat accumulation is not more than double the inside diameter.

Although the mass of heat storage of this regenerator is reduced to approximately one sixth of the mass of heat storage of the usual air heaters and vertical circulation used until now, the same amount of thermal energy is accumulated ; this results from the distribution in S of the temperature according to FIG. 1. This temperature distribution is fundamentally different from that of known air heaters, where it is essentially linear. The distribution in S of the temperature offers two decisive advantages compared to the linear distribution, on the one hand the temperature drop of the hot wind during the cold blowing phase is very weak, and on the other hand the variation of the average temperature of the entire bed of material is very high, of the order of 600 ° C. However, the temperature distribution in S also depends not only on the prescribed grain size of the pellet charge but also on a determined minimum gas flow. This minimum flow corresponds to a power of 300 m ³ N / hm ² . This corresponds, for a wind temperature of 1200 ° C, to a specific power of 150 kW / m ² , below which it is not necessary to descend. As the power increases, the S-profile of the temperature is more and more clearly raised. A particularly advantageous operating point appeared for a flow capacity of 1000 m ³ N / hm ² , a pressure drop from 1000 to 1600 Pascal. An increase in the flow rate up to 2000 m ³ N / hm ² is possible without reducing the heat transfer, taking into account a pressure drop from 3000 to 5000 Pascal. This power limit is applicable to walking at normal pressure.

The operation under increased pressure has shown the surprising result, that the flow rate can be further increased, in fact in proportion to the absolute pressure, without the heat transfer data being degraded. If, for example, a blast furnace wind at 5 bar is produced, the flow can reach 5000 m ³ N / hm ² , respectively 2500 kW / m ² . It is thus possible to produce a flow of hot wind of 100,000 m ³ N / h with a regenerator having a grid surface of 20 m ² .

The fact that the regenerator is generally heated leads to normal pressure, three generators must be heated simultaneously, so a total of four regenerators are required to ensure continuous operation for the production of gas hot. These regenerators only have a diameter of 4 m for a height of 5 m, while air heaters of the same power used up to now have a diameter of 8 m and a height of 30 m.

A partial load step is actually only achievable by performing the heating phase at full power, but it must however possibly insert breaks after the cold blowing phase. This results from the fact that due to the small size of the regenerator, the use of a conventional burner for heating the regenerator is not not possible, because such a burner has a build volume larger than the regenerator itself. We therefore use a burner says premix, in which the heating gas and combustion air are intimately mixed with each other when cold, before ignition, and are only ignited after mixing. For a safe walk of such premix burner, it is necessary not to go below a minimum gas speed, to surely avoid a return of flame of the mixture. As a result, such a premix burner does not has only a very limited adjustment range.

The breaks which are therefore necessary in a dependent walk partial are preferably observed after the cold blowing of the regenerator.

Finally, it still appeared during the operation of such a regenerator that the temperature of the hot wind was only 20 ° C below the theoretical flame temperature and that it remained largely constant throughout the wind phase. This means that even in the case of a temperature drop, we have improved by a factor of 10, exactly as it is for the size. Thermal efficiency was increased from 65% for conventional air heaters to 95% for the regenerator according to the invention.

Claims (5)

1. Method of reheating a gas in a regenerator by a heat storage medium, consisting of loose material placed in the form of a ring between two coaxial cylindrical grids, a hot collecting chamber, surrounded by the hot inner grid, for the hot gases and a cold collecting chamber, enclosed between on the one hand, the cold outer grid and, on the other hand, the external wall of the regenerator chamber, for the cold gases, in which:

   a) during a phase called the heating phase, a heating gas is made to flow from the hot collecting chamber into the cold collecting chamber via the heat storage medium so as to heat the latter;

   b) during a phase called the cold blowing phase, the said gas to be reheated is made to flow from the cold collecting chamber into the hot collecting chamber via the heat storage medium so as to reheat the gas;

      characterized in that: ΔPhot - ΔPcold ≥ 5ρgH where

   AP_hot represents the head loss of the regenerator at the end of the said heating phase;

   ΔP_cold represents the head loss of the regenerator at the start of the said heating phase;

   H is the height of the regenerator;

   ρ is the density of the said gas to be reheated, at a temperature of 20°C;

   g is the acceleration due to gravity;

   in that the gas flow rate during the heating phase is greater than or equal to 300 Sm³ per hour and per m² of hot grid area at standard pressure and in that the particle size of the loose material is chosen to be less than 15 mm.
2. Method according to Claim 1, characterized in that the said cold blowing phase is carried out with an over pressure.
3. Method according to Claim 1 or 2, characterized in that the method is implemented according to an operation referred to as partial charging operation, in which the heating phase is carried out at full power and in which pauses are observed after the cold blowing phase.
4. Regenerator suitable for implementing the method of reheating a gas according to any one of Claims 1 to 3, comprising a heat storage medium consisting of loose material placed in the form of a ring between two coaxial cylindrical grids (4, 5) comprising a hot inner grid (4) and a cold outer grid (5), a hot collecting chamber (6), surrounded by the hot inner grid (4), for the hot gases and a cold collecting chamber (8), enclosed between, on the one hand, the cold outer grid (5) and, on the other hand, the wall of the chamber (2) of the regenerator, for the cold gases, characterized in that the external diameter of the annular heat storage medium is at most twice its internal diameter and in that the particle size of the loose material is chosen to be less than 15 mm.
5. Regenerator according to Claim 4, characterized in that it includes a premixing burner (11) for producing the said heating gas.

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