Treatment of exhaust gases from kilns

Details Treatment of exhaust gases from kilns Treatment of exhaust gases from kilns

1999-10-22

2001-04-03

Wilson, Gregory (Department: 3749)

Heating

Tumbler-type rotary - drum furnace

Having combustion products generated in or fed to drum

C432S014000, C110S246000, C110S346000

Reexamination Certificate

active

06210154

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of reducing the emission of nitrogen oxides from a mineral-burning process, especially although not exclusively a cement-manufacturing process, in which the mineral raw material is passed through a preheater region before it is passed into a rotary kiln. The present invention also relates to apparatus in which such a method may be carried out, and further relates to the mineral products of such mineral-burning processes.
BACKGROUND OF THE INVENTION
Kilns in which mineral matter is subjected to burning are commonly fired by means of a so-called fossil fuel, such as pulverised coal, natural gas or fuel oil, or a fuel derived therefrom, such as coal gas. However, the combustion of such a fuel in the presence of a combustion-supporting gas, usually air, may give rise to the generation of nitrogen oxides (NO
x
), the emission of which into the atmosphere is undesirable for environmental reasons. Such nitrogen oxides are generated from nitrogen compounds that are present in the fuel; also, if the combustion temperature is sufficiently high, in particular above 1200° C., nitrogen oxides would also be generated by the reaction of nitrogen and oxygen that are present in the combustion-supporting air. The patent literature contains a number of proposals for ways in which the emission of NO
x
from mineral-burning processes may be reduced, for example as disclosed in the following documents.
DE-A-3 426 296 discloses a process for reducing the content of NO
x
in the exhaust gas of a rotary kiln in the manufacture of cement, in which air that has been heated during the cooling of the cement clinker is passed, a combustion air, through the kiln, a calciner and a cyclone preheater in counter-current to the cement raw material. The firing in the calciner is reducing in a first zone but, on account of supply of heated combustion air, is oxidising in a subsequent zone. Part of the gas leaving the kiln is passed through the reducing zone in the calciner, in which nitrogen oxide is reduced to nitrogen and fresh formation of nitrogen oxide is very largely obviated. Thereafter, this part of the gas is mixed with the other part of the gas leaving the kiln and the mixture is passed through the oxidising zone of the calciner. The said other part of the gas, on account of its content of oxygen, will enable the carbon monoxide, which is formed in the reducing zone, to be completely burnt. A similar method is disclosed for processes in which the air is passed through the kiln and then through a grid preheater.
U.S. Pat. No. 4,808,108 discloses a method of heat-treating fine-grained material, in which the material is preheated in a preheating zone using the hot exhaust gases from a precalcination zone, the preheated material is then further heated in the precalcination zone by the combustion of additional fuel, and the material subsequently undergoes burning in a combustion zone, in particular a rotary kiln, and the exhaust gases from the combustion zone are passed into the precalcination zone. The fuel that is combusted in the precalcination zone is delivered at two locations that are spaced one from the other. The burnt material that issues from the combustion zone may be cooled in a cooler by means of air and the air which is exhausted from the cooler may then be passed into the precalcination zone at two spaced locations, each adjacent a respective location at which the fuel enters the precalcination zone. The portion of the fuel that is added to the precalcination zone at the point nearer to the rotary kiln and the content of oxygen in the gases in this region are controlled so as to achieve a sub-stoichiometric combustion resulting in the establishment of a carbon monoxide content between 0.05 and 1 per cent. Under these reducing conditions, the nitrogen oxides contained in the exhaust gases from the rotary kiln, which are passed into the precalcination zone, are largely decomposed to nitrogen, with the formation of carbon dioxide. The material introduced into the precalcination zone from the preheating zone has a catalytic effect on the removal of the nitrogen oxides and also serves to regulate the temperature since the deacidification thereof takes up the heat produced by the combustion. In the other region where fuel is introduced into the precalcination zone, the combustion of the fuel also gives rise to the combustion of the carbon monoxide emerging from the region of the precalcination zone adjacent the kiln.
EP-A-0 391 463 discloses a method for the reduction of NO
x
emission from apparatus for burning mineral raw materials, especially cement raw materials, by means of a nitrogen-containing solid fuel such as coal. The apparatus comprises a burning zone, in particular a rotary kiln, a precalcining zone for calcining the raw materials prior to their entry into the burning zone, and a multi-string, multi-stage preheater system for preheating the raw materials by heat exchange with the exhaust gases from the burning zone and/or from the precalcining zone. One preheater string, the kiln string, is in direct connection with a kiln riser pipe and another preheater string, the calciner string, is connected to the precalcining zone. Material from the burning zone is cooled in a cooling zone, at least some of the exhaust air from which is passed to the precalcining zone as combustion air. Preheated raw material from the kiln string and the calciner string is subjected to precalcination in the precalcining zone and thereafter the bulk of the precalcined raw materials is fed to the burning zone. A nitrogen-containing solid fuel such as coal is fed into the kiln riser pipe in an amount as to constitute from 20 to 50 per cent of the total amount of solid fuel supplied to the apparatus. The nitrogen introduced into the kiln riser pipe by means of the fuel reacts with the nitrogen oxide (NO
x
generated in the burning zone and present in the exhaust gas therefrom, thereby forming harmless nitrogen (N
2
). The non-combusted fuel, which now contains less nitrogen, is separated from the gases in the lowermost preheater stage of the kiln string and is conveyed to and combusted in the calcining zone.
WO-A-97/30003 discloses a method for reducing the NO
x
emission from a kiln plant for the heat treatment of raw materials, in which plant nitrogen-containing fuel can be fired in at least three different zones. In a first such zone, in particular a kiln, an amount c of fuel is burned and to this zone there is fed an oxygen-containing gas. In a second such zone, especially a calciner, an amount b of fuel is burned and to this zone there is fed NO-containing exhaust gas from the other two zones, the exhaust gases from the said second zone being removed from the kiln plant. In the third such zone, in particular a burning compartment, an amount a of fuel is burned and to this zone there is fed at least a portion of the raw materials and also an oxygen-containing gas. The oxygen-containing gas fed to the said second and third zones may be exhaust air from a cooler in which the burned material from the kiln is cooled. The amount of fuel b+a is determined by the requirements of the raw-material treatment; also, the amounts of fuel b and a are adjusted until a minimum NO content is achieved in the exhaust gases from the said second zone. The temperatures in the second and third zones should be as high as the process permits but below 1200° C. In certain embodiments the temperature in the second zone is from 900 to 1150° C. and the temperature in the third zone is from 1000 to 1200° C.
There have also been proposals for the disposal of waste materials, for example used tyres, in kilns and/or associated plant for the heat treatment of mineral matter, in particular kilns and/or associated plant for the production of cement.
Every year large numbers of pneumatic tyres, in particular automobile tyres, are discarded. For example, it was reported in Rock Products, October 1980, that over 200 million automobile tyres were being discarded every year in the Uni

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