Furnaces – Process – Supplying fluid
Reexamination Certificate
2000-06-22
2001-11-20
Bennett, Henry (Department: 3749)
Furnaces
Process
Supplying fluid
C110S346000, C110S297000, C110S233000, C110S246000, C110S345000, C432S106000
Reexamination Certificate
active
06318278
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for calcining an ore-based material, in which process:
the material is passed through a precalcination device equipped with at least one fuel injector supplied with at least one fuel so as to form a fuel-injection zone at the outlet of the fuel injector and supplied with oxidizing agent by the products of combustion from a rotary kiln located downstream of the precalcination device with respect to the direction in which the material flows, then
the at least partially calcined material is passed into the rotary kiln which, at its downstream end, is equipped with a primary combustion unit.
2. Description of the Related Art
The manufacture of cement passes through an intermediate stage involving the manufacture of a product known as clinker. Clinker is a product which is obtained by firing ore-based material, particularly clay and limestone. The material, in the form of a powder, may be supplied to a rotary kiln either in dry form (in a dry process) or in the form of a water-based paste (or slurry) (wet process). The composition of the clinker is generally carefully controlled in order to obtain the desired proportions of the various inorganic materials, particularly calcium carbonate, silica, alumina, iron oxide and magnesium carbonate. After being placed in a kiln, the material that is the precursor to the manufacture of clinker is first of all dried out and heated. Next, this material undergoes calcination in which the carbonates of the various minerals are converted into the oxides of these minerals by the removal of carbon dioxide. While the temperatures are still high, the minerals thus obtained react chemically with each other to essentially produce calcium silicates and calcium aluminates. This last process is known as the clinkering process and takes place in the hot zone of a rotary kiln. The resulting clinker is then cooled and ground then mixed with additional ingredients to form a cement such as portland cement.
The clinker-manufacturing process has, in the past, been performed in rotary kilns which typically have diameters of 3 to 5 m and lengths of 60 to 200 m. Improvements to the process have been made by decarburizing or calcining a variable fraction of the raw meal in a stage in the process preceding the rotary kiln, allowing the use of shorter and more thermally efficient rotary kilns. A process stage such as this may be performed in preheater towers (or suspension preheaters), in LEPOL grates or in flash calcination devices.
The extent to which the raw meal is decarburized before it enters the rotary kiln is typically 10 to 45% in the case of suspension preheaters and LEPOL grates and 90 to 95% in the case of flash calcination devices. The energy required for the highly endothermic decarburization stage is supplied by introducing a fraction of fuel into the calcination zone.
Thus, processes for the manufacture of clinker are generally carried out in plants which comprise, in succession:
a precalcination device into which the material is introduced and in which drying-out, if necessary, then heating and some of the calcination of the material are carried out, and
an inclined rotary kiln into which the partially calcined material is introduced and in which calcination is completed, followed by the clinkering reaction.
Types of precalcination device other than those mentioned hereinabove may be calcination chambers or devices known by the name of riser ducts.
In all that follows, the terms “upstream” and “downstream” are to be understood as being with respect to the direction in which the material in such a plant flows.
One or more burners are arranged at the downstream end of the rotary kiln to supply the calorific energy needed for this kiln to operate. The flue gases produced by the burners downstream of the rotary kiln flow against the flow of the material in the plant and supply some of the calorific energy needed for the operation of the precalcination device. Additional energy is provided to this precalcination device by one or more burners.
In general, the search is on to limit the cost of the manufacture of clinker and to improve the processes used for the manufacture of clinker.
Hence, documents U.S. Pat. Nos. 5,572,938 and 5,580,237 relate to the burners downstream of the rotary kilns and propose that the injectors of these burners be modified so that oxygen-injection lances can be introduced thereto. The solutions described in these documents make it possible, with high-quality fuel, to improve the production efficiencies and/or reduce the production of pollutants.
However, these solutions still lead to the emission of relatively significant pollutants.
Elsewhere, the search is on to use low-quality fuels for supplying the calorific energy needed for the operation of clinker-manufacturing plants.
Low-quality fuels are to be understood as meaning fuels which have net calorific values (NCVs) lower than 15 MJ/kg, or water content by mass in excess of 20%. This category also covers fuels which contain less than 20% by mass of volatilizable substances or substances which cannot be reduced to small-sized particles or droplets. In respect of this last criterion, a fuel thus reduced, in which the proportion by mass of particles or droplets of a size exceeding 200 &mgr;m is greater than 75%, is considered as being a low-quality fuel.
Industrial waste, such as waste water or solid waste, for example of plastics or cardboard, constitutes low-quality fuels that can be used in the manufacture of clinker.
Clinker manufacturers are looking to increase their consumption of low-quality fuels given their very low costs, these manufacturers even sometimes being paid to incinerate industrial waste such as waste water.
However, the use of such fuels in large quantity poses problems because the flames produced with these fuels are unable to meet the thermal constraints required in the correct implementation of clinker-manufacturing processes.
The object of the invention is to solve these various problems by providing a process for calcining an ore-based material which, in particular, allows the manufacture of clinker at low cost, particularly using low-quality fuels, while at the same time limiting the emission of pollutants.
SUMMARY OF THE INVENTION
To this end, the subject of the invention is a process for calcining an ore-based material in which the material is passed through a precalcination device equipped with at least one fuel injector supplied with at least one fuel so as to form a fuel-injection zone at the outlet of the fuel injector and supplied with oxidizing agent by the products of combustion from a rotary kiln located downstream of the precalcination device with respect to the direction in which the material flows, then the at least partially calcined material is passed into the rotary kiln which, at its downstream end, is equipped with a primary combustion unit, wherein at least one oxygen-rich fluid is injected near to the fuel-injection zone, the oxygen rich fluid having an oxygen concentration by volume that is higher than that of the products of combustion from the rotary kiln and which pass through the precalcination device, so that the oxygen-rich fluid can supply from 1% to 40%, and preferably from 1% to 10%, of the stoichiometric amount of oxygen needed for the combustion of the fuel injected by the fuel injector.
According to some particular embodiments, the process may exhibit one or more of the following features, taken in isolation or in any technical feasible combination:
60% to 99% of the stoichiometric amount of oxygen needed for the combustion of the fuel are provided by the products of combustion from the rotary kiln;
the oxygen concentration by volume of the products of combustion from the rotary kiln is greater than or equal to 1%;
the oxygen-rich fluid is a mixture of some of the products of combustion and a gas containing at least about 20% oxygen;
some of the products of combustion are drawn off and air or oxygen-enriched air and/or industrially p
Alves Dora Sophia
Borissoff Thierry
Dugue Jacques
Marin Ovidiu
Milosavljevic Ivan
Bennett Henry
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Rinehart K. B.
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