Method for reducing NOx production during air-fuel combustion processes

Oxy-fuel combustion combined with air-fuel combustion to increase productivity of the combustion process and reduce nitrogen oxides in the products of combustion by maintaining fuel rich oxy-fuel combustion while combusting the air-fuel as close as possible at stoichiometric conditions.

FIELD OF THE INVENTION 
The present invention pertains to air-fuel combustion processes. 
BACKGROUND OF THE INVENTION 
The vast majority of combustion processes use air as an oxidizer in 
combustion with a fuel such as natural gas, fuel oil, propane, waste oils, 
other hydrocarbons and the like. It is also known that the performance of 
many air-fuel combustion processes can be improved by enriching the 
combustion air with oxygen. Enrichment of the combustion air increases 
both the flame temperature and the thermal efficiency while the flue gas 
volume decreases as the oxygen concentration in the air or oxidizer 
increases. The cost of high purity oxygen for enrichment can be offset by 
gains in productivity from the enhanced combustion. Low level enrichment 
of up to 35% total oxygen content in the oxidizer can generally be 
retrofitted to existing air-fuel systems with few modifications. 
Recently, the environmental impact of combustion processes has received a 
great deal of attention. It has been shown that the nitrogen oxides, known 
as NOx, are detrimental to the environment by producing smog, acid rain, 
and ozone in the lower atmosphere, the latter being a cause of global 
warming. In the United States the new Clean Air Act signifies the 
commitment of the Federal Government toward controlling pollution. The new 
regulations have spurred an increased interest in controlling NOx 
formation as a result of combusting air-fuel mixtures. 
It is also known that low level oxygen enrichment in combustion can cause a 
dramatic increase in NOx emissions. In industrial combustion processes, 
over 90% of the NOx emissions are in the form of a nitric oxide or NO. It 
has also been shown that at high levels of oxygen enrichment, e.g. above 
90% total oxygen content in the oxidizer, less NOx is produced than using 
air for the same firing rate. However, high levels of oxygen enrichment 
can be uneconomical in a given process and in fact may produce materials 
problems also. 
In the past, two strategies have been used to reduce NOx emissions. The 
first is to remove the NOx from the exhaust gases before they exit into 
the atmosphere. Post-treatment of the exhaust gases from the combustion 
process can be carried out by selective catalytic or non-catalytic 
reduction and various combinations of oxidation, absorption and reduction 
processes to scrub out the NOx. These processes generally involve high 
cost and a need to shut down the combustion process in the event of 
failure with the post-treatment equipment. The second method or strategy 
is to minimize NOx formation in the first place by modifying the 
combustion process in some manner. Examples of this second strategy 
include water or steam injection into the flame, reduced excess air in the 
combustion process and so-called low NOx burner designs. These 
technologies provide the user with a penalty in reduced thermal efficiency 
or only minimal NOx reductions. 
SUMMARY OF THE INVENTION 
The present invention provides a method of utilizing oxy-fuel combustion in 
combination with an existing air-fuel system to increase productivity 
while minimizing NOx formation. The oxy-fuel combustion is shielded by the 
air-fuel combustion and the oxy-fuel combustion is controlled so that the 
combustion takes place in a fuel-rich state.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The problem as stated above with utilizing oxygen enrichment in combustion 
is that there is a dramatic increase in NOx emissions. In most industrial 
combustion processes, over 90% of the NOx emissions are in the form of 
nitric oxide or NO. FIG. 1 shows the equilibrium predictions for NO for an 
adiabatic, stoichiometric, methane flame as a function of the oxygen 
concentration in an oxygen-nitrogen oxidizer, e.g. air. The units for 
nitric oxide or NO are pounds of NO per million BTUs gross firing rate of 
the fuel. FIG. 1 shows that NO dramatically increases at low levels of 
oxygen enrichment which trend has been verified experimentally in numerous 
tests conducted with enrichment of conventional air-fuel burners. 
Economics, process considerations and materials limitations more often 
than not make high levels of oxygen enrichment impractical, even though 
the high levels of enrichment produced less NOx than air for the same 
firing rate. In accord with the present invention, the process is most 
easily achieved by retrofitting an oxygen-fuel burner to an existing 
air-fuel system to increase productivity while minimizing NOx formation. 
As shown in FIG. 2, a dual fuel air-fuel burner shown generally as 10 
includes a first atomizing air passage 12 in the form of a tubular shape 
with an air inlet fitting 14 disposed concentrically around the first air 
tube is a fuel tube 16 having a fuel inlet 18 the fuel tube 16 being 
surrounded on its forward end with a concentric tube which defines a main 
combustion air inlet tube 20 with a combustion air inlet passage 22. On 
the forward end of the burner 10 is a burner mounting flange 24 containing 
a burner tile 26, the burner tile being fabricated from a ceramic 
material. The forward end of the main combustion air passage 20 terminates 
in a plate 28 with a plurality of air passages 30 disposed on axes 
parallel to longitudinal axis of the fuel passage 32 which is in the form 
of an annulus disposed around the annular termination of the atomizing air 
passage 34. Disposed concentrically within air passage 12 is an oxy-fuel 
burner shown generally as 36. Oxy-fuel burner 36 is a concentric tube 
burner having an outer tube 38 concentrically disposed around an inner 
tube 40. The inner tube is spaced from the outer tube by a plurality of 
radially spaced spacers 42 proximate the front end 44 of the burner. The 
oxy-fuel burner is adapted so that in one embodiment, fuel is admitted 
through a fitting 46 and is conducted around the outside of the inner tube 
40 and exits the front end of the burner 44. Oxygen is conducted down the 
center or oxidizer tube 40 by means of an oxidizer inlet fitting 48. 
Oxidizer tube 40 terminates at a location inwardly of the discharge end of 
the burner 36 so that proper combustion of the oxy-fuel mixture can take 
place. Alternatively, the oxygen and fuel passages can be reversed. 
Concentric oxy-fuel burners are well known in the art, one being a K-Tech 
burner offered for sale by Air Products and Chemicals, Inc. of Allentown, 
Pa. 
In operation the burner 10 is set up so that natural gas is delivered 
through fitting 18 and primary combustion air delivered through fitting 22 
so that combustion takes place forward of the front end of the oxy-fuel 
burner 36. The oxy-fuel burner is used to produce an oxy-fuel flame or 
combustion at the forward end 44 of the burner 36 by introducing oxygen 
into fitting 46 and fuel such as natural gas into fitting 48. In order to 
minimize the production of NOx, the oxy-fuel burner is operated in a 
fuel-rich condition. Fuel-rich is taken to mean an equivalence ratio 
between 1.2 and 1.35 preferably at about 1.33 when the ratio of the oxygen 
in the oxy-fuel burner to the fuel is divided into the ratio of oxygen and 
the oxy-fuel burner as if the oxy-fuel burner were operating under 
conditions of stoichiometry. Furthermore, operation is conducted so that 
the oxy-fuel flame is shrouded by the air-fuel flame. In the device of 
FIG. 2 this is easily accomplished by placing the oxy-fuel burner inside 
of the air-fuel burner so that the oxy-fuel combustion takes place inside 
of an envelope of the air-fuel combustion. Other configurations can be 
utilized wherein the oxy-fuel flame is separate from the air-fuel flame 
except that the two flames are merged after creation. 
By way of explanation it is well known that running a flame fuel-rich 
reduces NOx production because of the reduction in flame temperature and 
the reduced amount of available oxygen radicals. Due to the fact that 
there is not enough oxygen to fully oxidize the fuel, the maximum flame 
temperature is never achieved. In addition, thermodynamically the oxygen 
preferentially combines with carbon and hydrogen before combining with 
nitrogen to form nitrogen oxides. However, unburned hydrocarbon emissions 
increase under fuel-rich conditions which conditions are environmentally 
unacceptable and the overall thermal efficiency of the combustion process 
is reduced. 
An actual test with the set up of FIG. 2, high purity oxygen (e.g. 99% 
O.sub.2) was used in the oxy-fuel burner and natural gas containing better 
than 95% methane and 0.32% nitrogen by volume was used for the fuel. As 
shown in FIG. 3, the X axis shows how much of the total firing rate was 
created by using an oxy-fuel combustion supplement. For example, at 50% 
oxy-fuel, half the total firing rate was oxy-fuel and half was air-fuel. 
The equivalence ratio of the oxy-fuel burner was varied from 1.00 to 1.33 
where 1.0 is the stoichiometric ratio for methane. In FIG. 3 where no 
oxy-fuel (0% oxy-fuel) was used, the air-fuel equivalence ratio was 
varied. These curves show a dramatic decline in NO as the oxy-fuel flame 
became more fuel-rich. The curves also show that there will be peaks in 
the middle ranges of oxy-fuel supplementation, however, the precise peaks 
were not determined during these experimental runs. 
FIG. 4 shows the same data that was plotted in FIG. 3 except that the data 
is presented in the terms of overall oxygen concentration in the oxidizer 
so the test can be evaluated as if the oxygen was premixed with the air. 
For example, if the total firing rate is split evenly between the air-fuel 
and the oxygen-fuel burners and both burners are at stoichiometric 
conditions, the equivalent oxygen concentration for oxygen premixed with 
air is 34.6%. This curve is instructive because it can be readily compared 
to the theoretical NOx curves shown in FIG. 5. The curves of FIG. 5 are 
the equilibrium predictions assuming fuel and oxidizer are perfectly 
mixed. The curves of FIG. 5 shows that NOx is predicted to increase as the 
oxygen enrichment level increases toward 40%. However, the experimental 
data of FIG. 3 shows the NOx declines as enrichment goes from 30-40%. It 
is believed that this is due to separate flame zones that are created by 
inserting an oxy-fuel burner into an air-fuel burner. A conventional 
oxy-fuel burner would have a high flame temperature. 
The curves shown in the drawing are based on gross firing rate and do not 
include the effect of increased efficiency with more oxygen. This 
effectively lowers NOx even further if it is measured on a basis of 
(pounds) lbs NO/net MMBTu. 
The air-fuel burner tested had unusually low NOx because of its poor mixing 
characteristics. Most air-fuel burners are &gt;0.1 lb NO/MMBtu which means 
the invention may be even better for other burners. 
By running the oxy-fuel burner fuel-rich the flame temperature is 
dramatically reduced. In addition, the natural gas in the outer annulus of 
the oxy-fuel burner acts as a shroud which delays the inner high purity 
oxygen from reaching the nitrogen in the air-fuel burner. Since the flame 
is fuel-rich, most of the oxygen oxidizes the hydrocarbon fuel before it 
mixes with the air-fuel stream. It is believed that the combination of 
lower flame temperature, reducing conditions in the oxy-fuel flame 
temperature and shrouding of the inner high purity oxygen from the 
nitrogen in the air-fuel flame all contribute to the unexpected reduction 
of NOx in the exhaust gases. From the experimental data one could predict 
that the preferred conditions would occur when at least two thirds of the 
total firing rate is accomplished by virtue of using the oxy-fuel burner. 
In view of the experimental data and the conclusions reached above, the 
preferred operating conditions are running the oxy-fuel flame fuel-rich 
and the air-fuel flame approximately at stoichiometric conditions rather 
than the reverse. Using conditions reverse to the invention would only 
marginally reduce the flame temperature of the air-fuel flame and 
drastically increase the flame temperature of the oxy-fuel flame. In view 
of the fact that thermal NOx production in flames increases exponentially 
with temperature, the high peak flame temperatures that would result in 
the reverse practice of the invention would produce higher NOx than the 
invention. Although running both burners fuel-rich would reduce NOx, the 
thermal efficiency would suffer. Since oxygen enrichment has been proven 
to increase thermal efficiency, the present invention would enhance 
productivity even though the oxy-fuel flame is fuel rich because the 
reduction from being fuel-rich is far outweighed by oxygen enrichment. In 
most industrial furnaces, unburned hydrocarbon emissions will not be 
present due to air infiltration into the furnace and into the exhaust 
system which combusts any remaining fuel in the exhaust gases leaving the 
flame zone. 
The present invention can be easily retrofitted to existing systems at 
minimal cost. The present invention gives an incremental increase in 
productivity without the penalty of high NOx emissions which normally 
occur at low levels of oxygen enrichment. Oxy-fuel burners have previously 
been added to air-fuel furnaces to increase productivity, however, prior 
to the present discovery NOx emissions were not considered to be a 
problem. 
Having thus described our invention what is desired to be secured by 
Letters Patent of the United States is set forth in the appended claims.