Combustion of hot gases of low calorific power

The complete combustion of hot gases of low calorific power is effected by introducing the gases into a combustion chamber at a pressure close to atmospheric pressure and at a temperature of between 600.degree. and 900.degree. C, by introducing at burner level primary air to a maximum of 80% of the stoichiometric proportion, by supplying secondary air near the base of the flame in an excess relation to the stoichiometric amount so that the flame temperature is between 1,000.degree. and 1,300.degree. C and by discharging the combustion products from the combustion chamber by means of a chimney.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for the complete combustion of hot 
gases of low calorific power, such as reducing smoke of rich gases and 
by-products of the carbonisation of coal, which gases are available under 
a pressure close to atmospheric pressure, which process uses at least one 
burner in a combustion chamber having means for introducing primary air 
and means for introducing secondary air. The invention also relates to a 
combustion chamber for carrying out the process. 
2. Description of the Prior Art 
The by-products of a coal pyrolysis installation, such as a rotary furnace, 
or a rotary hearth furnace, or a direct-fired furnace, are in a gaseous 
state and entrain fine solid products, such as carbon black, soot or coal 
dust, and condensible products such as tars or benzenes. 
In conventional pyrolysis processes these by-products are generally 
subjected to sufficient cooling to achieve complete condensation, thereby 
making it possible to obtain in the carbonisation plant the negative 
pressure necessary for applying suction to the by-product with the aid of 
known means, such as extractor fans. If on the other hand it is desired to 
burn these by-products immediately at the outlet of the pyrolysis furnace, 
difficulties are encountered in transferring the gases from the pyrolysis 
furnace to the combustion furnace, in achieving complete combustion of the 
suspended particles, and finally in achieving the negative pressure 
necessary for the extraction of the gases because of the high temperature. 
There are innumerable disappointing inventions relating to the collection 
of charging gases in coke oven plants. In particular, techniques aiming to 
burn the impurities lead to only very partial results, combustion being 
incomplete. Furthermore, equipment is periodically destroyed by explosions 
or local overheating. 
A combustion chamber for the combustion of lean gases is known from U.S. 
Pat. No. 2,920,689. Air for combustion is supplied by a fan and divided 
into primary air and secondary air which are respectively supplied through 
valves to nozzles from which they pass out at high speed and with 
turbulence. 
Furthermore, from French Pat. No. 2,193,178 a process is known for the 
extinction of stoichiometric combustion products by the injection of air 
with a view to limiting to 525.degree. C. the temperature of the gases 
discharged to the atmosphere. Moreover, an arrangement comprising two 
combustion chambers followed by an extinction chamber is known from French 
Pat. No. 2,065,890. 
These arrangements have the major disadvantage of entailing the formation 
of a high-intensity turbulent flame, thereby making it necessary to use 
expensive refractory materials and to take special precautions against the 
risk of extinction and explosion, for example by maintaining a pilot 
flame. Moreover, stoichiometric proportions are difficult to maintain for 
gases whose calorific power may vary, as is frequently the case with 
numerous lean gases occuring as by-products of industrial processes, 
particularly when they contain crackable constituents at high temperature 
of high calorific power, such as carbon black and higher hydrocarbons. 
SUMMARY 
The aim of the invention is to propose a process and a combustion chamber 
making it possible to achieve complete combustion and perfect smoke 
removal without risk of explosion or damage to the plant, while producing 
a negative pressure favourable to the collection and evacuation of the 
gases. 
In the process of the invention this aim is achieved through the fact that 
the gases are introduced into the combustion chamber at the available 
pressure close to atmospheric pressure and at a temperature between 
600.degree. and 900.degree. C., the primary air is introduced at burner 
level in an amount limited to a maximum of 80% of the stoichiometric 
proportion, the secondary air is supplied near the base of the flame and 
in an excess in relation to the stoichiometric amount such that the flame 
temperature is between 1,000.degree. and 1,300.degree. C., and the 
combustion products are discharged outside the combustion chamber by means 
of a chimney. 
In this manner it is ensured that the carbon black and the high 
hydrocarbons will be completely burned thanks to perfect combustion 
ensuring good consumption of smoke. This result is interesting when it is 
known that it is difficult to achieve complete combustion of crackable 
products having high calorific power when they are mixed with considerable 
gaseous ballast. Combustion is achieved practically without turbulence. 
It is advantageous to introduce the gases into the combustion chamber at a 
temperature of about 750.degree. C. and to adjust the flame temperature to 
about 1,100.degree. C. 
An advantageous expedient is to introduce a part of the secondary air 
through apertures arranged to effect cooling by bathing the walls of the 
combustion chamber. 
In this way it is possible to obtain high combustion temperatures without 
having to make the combustion chamber of expensive refractory material. 
The combination of the bathing of the walls by fresh air and the absence 
of turbulence is in fact very favourable for maintaining a low wall 
temperature. 
The combustion chamber according to the invention comprises in combination: 
a vertical wall connected at the top to a chimney; a base plate in which 
is disposed at least one burner comprising a nozzle supplying gas at a 
pressure close to atmospheric and means of supplying forced primary air 
for combustion, the said base plate in addition being provided with 
peripheral nozzles directed towards a point situated above the burner and 
having means of supplying forced secondary air for combustion; control 
means controlling the flow of air for combustion in dependence on the 
temperature of the upper portion of the chamber, and control means 
controlling the distribution of primary and secondary air flows in 
dependence on the temperature of the flame. 
In this way, the secondary air forms a kind of arch surrounding the flame 
and effects perfect smoke-consuming combustion. 
It is advantageous for the base plate to be provided in addition with 
vertical peripheral apertures admitting air for bathing the vertical 
walls, and for the base plate to have a projection surrounding the 
peripheral nozzles of the burners.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is illustrated a combustion chamber which has 
vertical walls and which is given the general reference 1, this chamber 
being connected at the top to a natural draught chimney 3. A burner 12 and 
air inlet 14 are disposed in the base plate 11 of the chamber 1, as will 
be explained below. 
A pipe (not shown) supplies hot gases having low calorific power to the 
nozzle 13 of the burner 12 which includes the air inlet duct 14 supplying 
primary air for combustion delivered by a fan 21, the flow of which is 
measured by a diaphragm 15 and is regulated by a damper 16 controlled by 
first regulating means shown as a thermocouple 17 measuring the 
temperature at the top of the chamber 1, which is connected to the chimney 
3. 
The secondary air is introduced into the chamber through oblique convergent 
nozzles 18 directed towards the flame appearing at the tip of the burner 
12. The Applicants' experience has shown that an angle .alpha. close to 
40.degree. is advantageous. The nozzles 18 are supplied with air by means 
of a wind box 19, which in turn is fed with air by a fan (not shown) or by 
a branch of the primary air supply pipe, this branch being adjustable by 
means of a damper. 
In either case the flow of secondary air is controlled by second regulating 
means shown as a thermocouple 20. 
The control means effect regulation in such a manner that the conditions 
previously indicated for the definition of the main characteristic of the 
invention are achieved, that is to say for introduction of the gases at a 
temperature between 600.degree. and 800.degree. C., primary air limited to 
80% of the stoichiometric proportion, an excess of secondary air such that 
the flame temperature is between 1,000.degree. and 1,300.degree. C. 
Preferred adjustments will be 750.degree. C. for the admission temperature 
and 1,100.degree. C. for the flame temperature. A peripheral projection 10 
surrounds the nozzles 18 and improves the directivity of the secondary 
air. 
A further quantity of secondary air may be introduced into the combustion 
chamber 1 through apertures 22 distributed around the periphery of the 
base plate 11. These apertures 22 are disposed vertically, so that the air 
which passes through them passes into the combustion chamber 1 so as to 
bathe the vertical walls of the chamber 1 and thus cool them. It is 
advantageous for the apertures 22 to be adjustable. It is preferable for 
the air to enter the apertures 22 under the action of the negative 
pressure prevailing in the chamber. Alternatively, the air can be supplied 
to the apertures 22 by means of a fan or of a branch from the wind box 19. 
A fan 23 makes it possible to blow cold air through a pipe 25 at the base 
of the chimney 3 so that, by adjusting a damper 26 controlled by a 
thermocouple 24, an outlet temperature of the chamber lower than 
600.degree. C. can be obtained. 
A heat exchanger 4 makes it possible to use the sensible heat of the smoke 
during at least part of the operating periods. To this end it is possible 
to produce a forced circulation through the heat exchanger 4 by means of a 
branch pipe 41, with the aid of an exhaust fan 43 provided with a damper 
44 controlled by the thermocouple 24 and finally through an independent 
chimney 42. 
The installation may be completed by any usual subsidiary devices, for 
example starting burners (not shown) to enable the combustion chamber to 
reach its operating temperature, flame detection cells, and any other 
usual safety device. 
The process may be applied to gases such as those defined in the preamble, 
without this constituting a limitation; it may for example advantageously 
be applied to the gases coming from a coal pyrolysis plant carrying out 
the process described in French Patent Application No. 74 22402 of June 
27, 1974, having the title "Process for the production of pulverulent coke 
and reactive coke in grains". 
Experiments carried out by the Applicants surprisingly show that, when 
applied to pyrolysis gases at 750.degree. C. which are obtained from the 
process mentioned above, the process of the invention makes it possible to 
obtain at the outlet of the chimney 3 an exhaust to the atmosphere which 
is practically invisible and is even invisible in a wreath of water 
vapour. 
In this case the gases had a net calorific value, including sensible heat, 
of 2600 kcal/kg at 750.degree. C. The termal load of the chamber attained 
25000 th/h, and produced in the gas supply pipe a negative pressure of 
about 2 millibars for a negative pressure of the combustion chamber of 3 
millibars. The combustion conditions were very stable with a primary air 
flow of 18,000 cubic meters per hour, a central secondary air flow of 
22,000 cubic meters per hour, and a peripheral secondary air flow of 
20,000 cubic meters per hour. 
FIG. 2 shows a variant of the apparatus illustrated in FIG. 1, comprising 
three burners 12. The same reference numerals designate the same parts as 
in FIG. 1. 
It will be observed that the process utilise an internal regulation 
arrangement which it would appear helpful to recapitulate: 
The air for combustion is regulated by the first regulating means 17 in 
dependence on the temperature measured in the top part of the combustion 
chamber; 
The distribution between primary air and secondary air in the burner is 
regulated by the second regulating means 20 in dependence on the 
temperature of the flame. In practice it will be easy to regulate it in 
dependence on the temperature radiated at burner level onto the walls, 
this temperature being taken as image temperature of the flame 
temperature; 
The secondary air bathing the walls is preferably regulated by action on 
the base plate apertures in such a manner that the two previously 
mentioned regulations remain within a good operating range; 
In the case of recuperation of the sensible heat of the smoke, the thermal 
load of the arrangement is regulated by action on the damper of the 
exhaust fan. 
Finally, the particular advantages obtained by operating in accordance with 
the invention should be noted: Complete or partial recuperation of the 
latent or sensible calories of low-value gaseous by-products is effected, 
while creating a utilisable negative pressure in their circuit; 
These gaseous by-products, which are naturally dirty and polluting, can be 
burned by recuperating the heat in large capacity units giving rise to 
remarkably little or even to no pollution. 
In an alternative of the smoke circuit it is possible to produce the entire 
draught through the heat exchanger 4 by means of the exhaust fan 43. In 
this case, the chimney is a forced draught chimney, all the smoke from the 
combustion chamber 1 being drawn into the heat exchanger 4. In this 
alternative the flame temperature will be advantageously adjusted at 
1200.degree. C.