Title:
THREE-STAGE COMBUSTION
United States Patent 3727562


Abstract:
A three-stage combustion process is disclosed in which solid carbonaceous fuel is burned with a deficiency of air in a first zone leaving unburned fuel and producing a flue gas rich in CO. The unburned fuel is separated from the first zone flue gas and burned in a second zone with an excess of air to produce a flue gas containing excess oxygen. The flue gases from the first and second zones are then burned in a third zone to complete the combustion process with the total amount of air used in the first and second zones being approximately the stoichiometric amount required for complete combustion.



Inventors:
BAUER W
Application Number:
05/207353
Publication Date:
04/17/1973
Filing Date:
12/13/1971
Assignee:
LUMMUS CO,US
Primary Class:
Other Classes:
431/10
International Classes:
F23C6/04; (IPC1-7): F23B1/00
Field of Search:
110/1P,22R,28R,28F,106 431
View Patent Images:
US Patent References:
3421824METHOD OF BURNING INDUSTRIAL FUELS1969-01-14Herbst
3358624Coal burning arrangement for mhd generators1967-12-19Way
3228451Method of burning fuels1966-01-11Fraser et al.



Primary Examiner:
Sprague, Kenneth W.
Claims:
What is claimed is

1. A method of burning a solid carbonaceous fuel comprising the steps of:

2. A method as recited in claim 1 wherein the total amount of air introduced into said first zone and said second zone is approximately equal to that theoretically required for burning under conditions corresponding to stoichiometric conditions.

3. A method as recited in claim 2 wherein the total amount of air introduced into said first and second zones is no greater than 1 percent in excess of that theoretically required for burning under conditions corresponding to stoichiometric conditions.

4. A method as recited in claim 1 wherein said portion of said fuel remaining unburned after said first combustion zone is between 20 percent and 70 percent of the fuel and wherein the percentage of fuel reacted in said first combustion zone which produces CO is between 40 percent and 80 percent.

5. A method as recited in claim 4 wherein said portion of said fuel remaining unburned after the first combustion zone is about 60 percent of the fuel and wherein the ratio CO/CO2 in said first zone flue gas is about 7:3.

Description:
BACKGROUND OF THE INVENTION

The conventional burning of solid carbonaceous fuels and in particular pulverized coal requires the use of excess air in order to limit the loss of unburned carbon. It is quite normal to use up to 15 percent excess air in order to limit the loss of carbon to less than 1 percent. The use of excess air, however, has a number of disadvantages. The increased gas flow due to the quantity of excess air increases the heat loss by the discharge of the gases up the stack. The increased gas flow also increases the required dimensions of passages within the furnace and boiler and of gas ducts and the stack. A further disadvantage of the increased gas flow is that it increases the difficulty and expense of removing dust and pollutants from the flue gas. The excess oxygen which is present in the flue gas enters into corrosion producing reactions which damage the boiler tubes and other equipment.

The normal procedure for burning pulverized coal is to introduce the coal and air together into one burning zone wherein the combustion process goes substantially to completion. This complete combustion within one zone (excess air actually drops temperature) creates significantly high flame temperatures. It has been shown that these high flame temperatures lead to the creation of nitrogen oxides which are air pollutants. The production of the nitrogen oxide is a temperature dependent reaction, i.e., nitrogen oxides are produced at higher temperatures. It is, therefore, desirable to keep flame temperatures lower in order to reduce the production of these pollutants.

Another problem which can occur when there is excess oxygen in the flue gas is that certain air pollution control techniques are more difficult. For example, in the process of removing sulfur oxides from flue gas with copper oxide, the excess oxygen results in the conversion of copper sulfite to copper sulfate imposing a heavy penalty on the subsequent regeneration of the absorbent and undesirable additional water formation in the regeneration gas.

SUMMARY OF THE INVENTION

An object of the present invention is to burn solid carbonaceous fuels and in particular pulverized coal in such a manner that substantially all of the fuel is burned with little or no excess air. A further object is to burn these fuels in a manner which will limit maximum flame temperatures. The objects of the invention are accomplished by carrying out the combustion process in three stages with the first stage being carried out with a deficiency of air and the second step being carried out with an excess of air. The flue gas from the first stage containing a substantial quantity of CO and the flue gas from the second stage containing excess oxygen are then combined and burned in the third stage. The total quantity of air used for the process is substantially equal to the total amount of air required for complete combustion of the fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between operating parameters.

FIG. 2 illustrates the overall arrangement of the present invention.

FIG. 3 is an elevation view in cross section of a cyclone reactor.

FIG. 4 is an end view of the reactor of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is based on three-stage combustion of solid carbonaceous fuels and in particular pulverized coal which will be hereinafter used as the specific illustration of the fuel being burned. The first stage combustion is carried out with a deliberate deficiency of air and therefore a deficiency of oxygen so as to only partially burn the carbon in the fuel and to produce a flue gas rich in CO. The unburned pulverized coal from the first stage is separated from the first stage flue gas and then conducted to the second stage. In the second stage, the coal is burned with a deliberate excess of air so as to substantially completely burn the carbon and produce a second stage flue gas containing CO2 and free oxygen as well as some ash. The hot flue gases from both the first and second stages are then combined in a third stage in which the CO in the first stage gas reacts with the free oxygen in the second stage gas.

The reactions in the three stages may be represented by the following equations: ##SPC1##

Where:

x = fraction of carbon reacted in the first stage

y = fraction of carbon reacted in the first stage which generates CO

From the above equations an expression for the fractional excess O2, z, in the second stage is as follows:

z = xy/[2(1-x)]

FIG. 1 shows the relationship of y and z with x as a parameter. FIG. 1 also includes a dashed curve relating the CO content of the first stage flue gas with y. The selection of the variables x, y and z will be dependent upon various technical and economical considerations. For example, increasing the value of x has the effect of increasing the flame temperature in the first stage and decreasing the flame temperature in the second stage. It also decreases the solids flow from the first to the second stage, thus requiring a larger first stage and a smaller second stage. Increasing the value of x has the further effect of increasing the second stage excess air. Another factor to be considered in the selection of the design variables is that the temperatures in both the first and second stages must be kept below the ash fusion temperatures.

The concentration of combustibles (CO) in the flue gas from the first stage should be maintained above a level of about 10 volume percent to assure rapid combustion in the third stage. It can be seen in FIG. 1 that this minimum 10 percent CO content requires a value of y in excess of 0.4. However, depending on the combustibility of the fuel, its particle size, and its price, it is also desirable to maintain a sufficient excess of air in the second stage to assure good fuel utilization. For example, for the combustion of pulverized bituminous coal, 20 percent excess air in the second stage should assure virtually complete combustion. Selecting a slightly higher value for the excess air in the second stage, namely 23.3 percent, suitable values for the variables might be x = 0.4 and y = 0.7. The combustion of 40 percent of the carbon in the first stage generates a first stage flue gas containing CO and CO2 in the ratio of 7:3 with there being no remaining free oxygen. For these conditions, the volume percent of combustibles in the first stage flue gas is about 20 percent as indicated in FIG. 1. A suitable range for the value of x is 0.3 to 0.8 while the range for the value of y is 0.4 to 0.8.

The combustion in the second stage between the remaining fuel, which amounts to 60 percent of the carbon, and the second stage air substantially completely burns the carbon and produces a flue gas containing CO2 and essentially no CO along with the excess air. The flue gases from the first and second stages are then combined and the excess air in the second stage flue gas is ideally just sufficient to completely convert the CO in the first stage flue gas to CO2 producing a final or third stage flue gas containing no free oxygen and no CO.

FIG. 2 illustrates a system in which the present invention may be carried out. The first stage combustion is preferably carried out in a cyclone reactor 10 which may be of any desired type. A typical cyclone reactor is illustrated in FIGS. 3 and 4. The combustion air for the process is supplied by means of the forced draft fan 12. The air is then heated in the air preheater 14 and carried by means of the duct 16 to the various points of use. A portion of this preheated air is supplied to the air swept pulverizer 18 in which the coal is prepared to the proper size. The pulverized coal along with the primary combustion air is then forced by means of the exhauster fan 20 and duct 22 tangentially into the primary coal burner 24 of the reactor 10 as shown more clearly in FIGS. 3 and 4. Another portion of preheated air is introduced tangentially into the main cyclone chamber 26 of the reactor 10 by means of the duct 28. The illustrated reactor 10 is lined with refractory having water tubes embedded therein for cooling. The pulverized coal and the combustion air move cyclonically through the reactor and the flue gas exits through throat 30 into the duct 32 while the unburned carbon exits through the stand pipe 34. The essential requirement for the first stage reactor is the unburned coal must be able to be separated from the flue gas.

The unburned pulverized coal withdrawn from the reactor 10 into the stand pipe 34 is then conveyed by means of a high velocity flow of preheated air from duct 36 and blower 37 through the duct 38 into the second stage reactor 40. This second stage reactor is also illustrated as being a cyclone reactor similar to the reactor 10 but this second stage reaction may be carried out in apparatus other than cyclone reactors as will be pointed out hereinafter. The second stage combustion air is introduced tangentially into the second stage reactor 40 through duct 42 and any remaining ash and whatever carbon might remain unburned are withdrawn through the cleanout pipe 44. The flue gas from the second stage reactor 40 containing the excess oxygen is withdrawn through the duct 46.

The flue gases from the first and second stages are then conveyed by their respective ducts 32 and 46 into a suitable mixing device. This device, for example, may comprise a burner 48 such as conventional burners for the combustion of gaseous fuels. The burner 48 is attached to the furnace 50 such that the third stage combustion takes place within the furnace to generate steam. The flue gas from the third stage combustion after passage through the boiler is then passed through the air preheater 14 and to the stack or perhaps to intermediate treating equipment such as precipitators or pollution control apparatus.

The following table is an example of the material flows for a firing system illustrating the invention:

Firing Rate: 108,970 lb/hr of Coal (9% Ash)

Heat Release = 1,470 MM

BTU/hr Flows M lb/hr Flows lb-Mol/hr Ash C C O2 N2 CO CO2 __________________________________________________________________________ 1st stage IN 9.81 99.16 8,264 2,150 8,160 OUT 9.81 59.47 4,956 8,160 2,316 992 2nd stage IN 9.81 59.47 4,956 6,102 23,160 OUT 9.81 .14 12 1,158 23,160 4,944 3rd stage IN 1,158 31,320 2,316 5,936 OUT 31,320 8,252 __________________________________________________________________________ Total Flue Gas 39,572 2nd Stage Excess O2 23.4%

although the second and third stage reactors have been illustrated as being separate units 40 and 50, the only essential requirements is that the second and third stage reactions be carried out in apparatus which provides two separate combustion zones. For example, the unburned coal discharged from the reactor 10 into the stand pipe 34 may be blown directly into the lower portion of a furnace such as the furnace 50 together with the second stage combustion air. The second stage combustion will then take place in this lower furnace portion. The flue gas from the first stage reactor 10 is then introduced at a higher elevation in the furnace 50 such that the third stage combustion also takes place in the furnace but in a separate zone.

While a preferred embodiment of the present invention has been illustrated and described, it will be understood that this is merely illustrative and that the invention may be carried out using apparatus and techniques other than that disclosed.