Heating – Work chamber having heating means – Having means by which work is progressed or moved mechanically
Reexamination Certificate
1999-11-04
2001-02-06
Ferensic, Denise L. (Department: 3749)
Heating
Work chamber having heating means
Having means by which work is progressed or moved mechanically
C432S011000, C432S012000
Reexamination Certificate
active
06183246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the heating of continuously charged furnaces, and in particular to a method of heating furnaces intended to raise to a high temperature, as uniformly as possible, steel-making products which may have a large cross section, for example slabs, billets, blooms or ingots, and to a heating (or reheat) furnace of this kind.
2. Description of the Related Art
The temperature of steel-making products is raised in this way for example so that these products can be rolled, because steel is more malleable at high temperature and better lends itself to the operation.
The furnaces for which this method is intended may be beam-type furnaces, continuous pusher-type furnaces, and rotating-hearth furnaces in particular.
The invention also relates for example to furnaces for carrying out heat treatments “on the fly”, particularly for part-finished or finished products (strip, tubes, wire, miscellaneous components).
Ideally, a furnace that performs well is a furnace which delivers a practically uniform temperature with good productivity, forming little scale (or oxides) on the surface, because scale, which is removed just before rolling, corresponds to a significant loss of material, and no adhering scale, thus avoiding the phenomena of “stress cracking” or burning of the products, and which produces a low amount of oxides of nitrogen and carbon dioxide.
The continuously charged furnaces to which the invention pertains generally stretch longitudinally between a product-charging end and a discharging end, the products being conveyed from one end to the other so that they pass right along the internal space of the furnace.
Along this internal space, these furnaces comprise, in succession, zones which fulfil different functions, sometimes immediately identifiable from the existence of internal partitions or particular roof profiles, but sometimes having no distinct physical demarcation.
More specifically, starting from the charging end, conventional furnaces of this type include, first of all, a portion which has no burners, then a portion which has air/fuel burners extending approximately as far as the discharge end.
The portion with burners thus comprises one or more heating zones, for example, from the upstream end in the downstream direction, a preheat zone, a heating zone proper, and an equalization zone near the discharge end from which the heated products are directed towards a rolling installation, for example; the flames developed by the burners allow the products in the furnace to be heated directly or indirectly using heat from the wall of the furnace. The essential method by which heat is transmitted is by radiation in the heating and equalization zones (accounting for more than 90%).
BRIEF DESCRIPTION OF THE INVENTION
It is because combustion at the burners using an oxidizing agent such as air releases a significant volume of flue gases at a high temperature (about 1200° C.), that it has been deemed advantageous to provide, on the charging end side, a burner-free zone in which the flue gases are circulated towards the charging end so that they can be removed, having, in theory, had a high proportion of their energy “drained” on the in-coming cold products. However, although the burner-free port-ion allows a significant amount of the energy present in the flue gases to be used up, it is still advantageous to recuperate these flue gases so that some of their energy can be used to preheat the combustion air, using an appropriate recuperation apparatus.
It may be noted, on the one hand, that the air/fuel ratio is set so that there is a slight excess of air so as to ensure complete combustion and thus avoid any formation of unburnt substances and, on the other hand, that the temperature in the burner-free so-called flue-gas recuperation or drainage zone is markedly lower (900° C. to 1000°) than in the rest of the furnace, which means that the convective-heating contribution in this zone ceases to be negligible (about 30%); at the present time, there is barely any scope for increasing the temperature in this zone because the energy losses would be prohibitive.
SUMMARY OF THE INVENTION
The object of the invention is to overcome this drawback, and the invention therefore consists in a method of heating for raising steel-making products to a high temperature in a furnace of the continuously charged type, in which the products are made to pass from a charging end to a discharging end, this furnace exhibiting at least one heating zone equipped with heating air/fuel burners which may be doped with oxygen but the combustion of which gives off a significant volume of flue gases typical of combustion using air, on the discharge end side, and a so-called flue-gas recuperation or drainage zone, on the charging end side, in the region of which the flue gases are removed, the method being characterized in that at least one fuel body in the gaseous state is incorporated into the flue gases and oxygen gas is introduced upstream of that possibly doped air/fuel burner which is situated furthest upstream when referring to the direction in which the products are made to pass, and at least some of the fuel body in the gaseous state is burnt, thus raising the temperature in the recuperation zone.
By virtue of these features, there are obtained a shift of the heat flux in the furnace in favour of the recuperation zone and, in particular, a reduction in the volume of combustion air, a reduction in the energy developed in the heating and equalization zones, the advantage of additional energy developed in the recuperation zone, a reduction in the volumetric flow of flue gases and, in particular, of the flue gases leaving the furnace, a reduction in the formation of the oxides of nitrogen by virtue of the decrease in the partial pressures of oxygen and of nitrogen and in the temperature in the heating and equalization zones, and better temperature uniformity in the products leaving the heating zone.
The method may additionally exhibit one or more of the following features:
in order to incorporate at least one fuel body in the gaseous state into the flue gases, at least one air/fuel burner is set to a sub-stoichiometric air/fuel ratio and flue gases containing unburnt substances are produced in the furnace;
in order to incorporate at least one fuel body in the gaseous state into the flue gases, at least one oxy-fuel burner is set to a sub-stoichiometric oxygen/fuel ratio and flue gases containing unburnt substances are produced in the furnace;
in order to incorporate at least one fuel body in the gaseous state into the flue gases, this fuel body is injected separately from or together with an injection of oxygen into the heating zone or into the inlet to the recuperation zone (in the direction of travel of the flue gases);
oxygen is introduced using at least one means chosen from the following group of means: at least one jet of oxygen is injected, giving it a high impulse perpendicular to the overall direction of the flue gases in the flue-gas recuperation or drainage zone; a series of small jets of oxygen distributed uniformly over a section of the furnace is injected; a series of small jets of oxygen distributed uniformly along the recuperation or drainage zone is injected; at least one jet of oxygen which is made to swirl is injected; at least one top-up oxy-gas burner is set to run super-stoichiometrically;
oxygen is introduced at the inlet to the recuperation zone;
oxygen is introduced into the recuperation zone;
air and fuel are introduced at the burners of the heating zone with a sub-stoichiometric air/fuel ratio corresponding to a value in the range from 0.95 to 0.99;
the air/fuel ratio at the burners of the heating zone is adjusted so that there are no unburnt substances leaving the openings of the furnace;
the pressure is set to a low level, preferably to a depression of a few millimetres' water column;
the oxygen flow rate is set to suit the total rate at which fuel is introduced into the furnace and to suit the combustion
Burns Doane , Swecker, Mathis LLP
Ferensic Denise L.
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Wilson Gregory A.
LandOfFree
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