Multi-stage process for combusting fuels containing fixed-nitrogen chemical species

Fuels containing fixed-nitrogen chemical species are combusted in a multi-stage process. The process which converts substantially all of the fixed-nitrogen into molecular nitrogen (and thus avoids the formation of significant amounts of nitrogen oxides from the fixed-nitrogen) consists of four steps: (a) mixing said fuel with at least one first oxidizing agent in amounts such that the equivalence ratio of said fuel to said oxidizing agent is at least about 1.4; (b) partially combusting the mixture resulting from step (a) in at least one first stage at a first temperature of about 1850.degree. to about 2150.degree. K., with a residence time of at least 0.03 second; (c) mixing the combustion products resulting from step (b) with at least one second oxidizing agent in an amount such that the equivalence ratio of combustion products to the total amount of oxidizing agents in the mixture will be about 1.0 or less, such mixing taking place under conditions such that the temperature of the mixture will not exceed about 1800.degree. K.; and (d) completely combusting the mixture resulting from step (c) in at least one second stage at a second temperature of less than about 1800.degree. K.

SUMMARY OF THE INVENTION 
The present invention relates to a multi-stage process for combusting a 
fuel containing fixed-nitrogen chemical species which comprises the steps 
of: (a) mixing said fuel with at least one first oxidizing agent in 
amounts such that the equivalence ratio of said fuel to said oxidizing 
agent is at least about 1.4; (b) partially combusting the mixture 
resulting from step (a) in at least one first stage at a first temperature 
of about 1850.degree. to about 2150.degree. K., with a residence time of 
at least 0.03 second; and more preferably 0.15-0.25 second; (c) mixing the 
combustion products resulting from step (b) with at least one second 
oxidizing agent in an amount such that the equivalence ratio of combustion 
products to the total amount of oxidizing agents in the mixture will be 
about 1.0 or less, such mixing taking place under conditions such that the 
temperature of the mixture will not exceed about 1800.degree. K.; and (d) 
completely combusting the mixture resulting from step (c) in at least one 
second stage at a second temperature of less than about 1800.degree. K. 
It is well known that common fuels such as coal, coal liquids, diesel oils, 
bunker oils, crude oils, shale oils, natural gas, etc. contain varying 
amounts of fixed-nitrogen chemical species. It is also well known that 
combustion of such fuels will produce varying amounts of nitrogen oxides 
(e.g. 150-1500 ppm), depending on the type and quantity of fixed-nitrogen 
chemical species as well as the furnace and burner arrangements. 
It is axiomatic that it would be desirable to minimize the formation of 
nitrogen oxides without any significant impairment of the combustion 
efficiency. This desirable result has been achieved by means of the 
instant multi-stage combustion process, since this process results in the 
conversion of substantially all of the fixed-nitrogen chemical species 
contained in the fuel into innocuous molecular nitrogen (rather than 
nitrogen oxides) without any significant concommitant impairment of 
combustion efficiency. 
THE PRIOR ART 
U.S. Pat. No. 3,048,131 teaches a two-stage method for combusting 
nitrogen-containing fuels in order to minimize the production of NO.sub.x 
species in the combustion products. However the results achieved by the 
instant process surpass those of this patent. Moreover, this patent 
contains no teaching whatsoever of the four critical steps (outlined 
above) of this process. 
The M.S. thesis by Howard W. Chou entitled "Fate of Ammonia In Fuel Rich 
Flames" (deposited in the library of the Massachusetts Institute of 
Technology on Oct. 25, 1976) indicates the desirability of combusting 
fuels under fuelrich conditions (i.e. high equivalence ratios) and at 
elevated temperatures. However, the Chou thesis does not indicate the 
necessary residence times for the first stage combustion. Moreover, Chou 
did his work at flame temperatures corresponding to adiabatic or less and 
at equivalence ratios greater than unity. In contrast thereto, this 
process involves three interrelated parameters in the first stage: high 
temperatures (e.g. 1850.degree.-2150.degree. K.), high equivalence ratios 
(e.g. at least 1.4) and minimum residence times (e.g. at least 0.03 
second). 
Other relevant prior art processes are summarized in the paper entitled 
"Mechanisms and Kinetics/NO.sub.x Formation" by A. F. Sarofim et al. which 
was presented at the 69th annual meeting of the American Institute of 
Chemical Engineers on Nov. 30, 1976 and is incorporated herein by 
reference. 
DETAILS OF THE PRESENT INVENTION 
This combustion process is multi-stage in nature, i.e. it involves one or 
more first stages and one or more second stages. The combustion process 
may be practiced with any desired type of combustion chamber/burner, so 
long as the chamber/burner is capable of being utilized in accordance with 
the four critical steps outlined above. Further, the same combustion 
chamber(s) employed in the second stage(s) may be the same as or different 
from that employed in the first stage(s). 
The first step of this process involves mixing a fuel with a first 
oxidizing agent. The fuel may be a solid, a liquid, a gas or a mixture 
thereof such as the common fuels previously mentioned. The quantity and 
type of fixed-nitrogen chemical species contained in the fuel is 
relatively unimportant; however, most common fuels contain less than about 
5 wt. % of such chemical species. 
Typically, the first oxidizing agent is air; however oxygen, oxygen mixed 
with an inert gas such as helium, etc. may also be employed instead of 
air. If desired, the air may be enriched with oxygen, e.g. 6-15 wt. % of 
oxygen may be added to the air, based on the weight of the additional 
oxygen plus air. Further, it may also be useful to preheat the air to a 
temperature in the range of 450.degree. to 1100.degree. K. and preferably 
600.degree.-900.degree. K., prior to its admixture with the fuel. If 
desired, the fuel may also be preheated to similar temperatures prior to 
admixture with the air. 
The amount of oxidizing agent mixed with the fuel is such that an 
equivalence ratio of at least about 1.4 is obtained; preferably, the 
equivalence ratio is in the range of 1.4 to 2.2, most preferably 1.6 to 
2.0. The equivalence ratio (usually referred to as .phi.) is defined as: 
##EQU1## 
For complete combustion (e.g., oxidation of carbon monoxide to carbon 
dioxide), .phi. should be equal to or less than 1.0. Where .phi. has a 
value equal to or greater than 1.4, carbon will be oxidized to carbon 
monoxide plus carbon dioxide. It should also be noted that while a 
condition of .phi..ltoreq.1.0 is desirable from a complete combustion 
point of view, such condition favors conversion of fixed-nitrogen chemical 
species into nitrogen oxides. Thus by combusting in at least two stages in 
which the first stage(s) .phi..ltoreq.1.4 and the second stage(s) 
.phi..ltoreq.1.0, both minimization of the formation of nitrogen oxides 
and maximization of complete combustion are obtained. 
The mixture of fuel and first oxidizing agent may be formed externally to, 
or within, a suitable combustion chamber. In the second step of this 
process, the mixture is partially combusted (i.e. carbon is oxidized to 
carbon monoxide plus carbon dioxide) in at least one first stage. The 
combustion temperature of this first stage is maintained in the range of 
about 1850.degree. to about 2150.degree. K., preferably 1900.degree. to 
2050.degree. K. Further, the residence time of the fuel and oxidizing 
agent during the combustion reaction is maintained at a level of at least 
about 0.03 second, preferably 0.05 second, e.g. 0.2 second. 
The combustion products resulting from the second step (i.e. the first 
stage combustion) are then mixed with a second oxidizing agent (which may 
be the same as or different from the first oxidizing agent employed in the 
first step). Typically, the second oxidizing agent is also air, but it may 
be any of the other choices enumerated above for the first oxidizing 
agent. The amount of oxidizing agent employed in the third step is such 
that the equivalence ratio of combustion products to the total amount of 
oxidizing agents (i.e. any remaining first oxidizing agent plus the added 
second oxidizing agent) is equal to or less than 1.0, e.g. 0.80-0.99. 
Since an equivalence ratio of 1.0 or less favors the formation of NO.sub.x 
species at elevated temperatures, it is necessary that the mixing in the 
third step take place under conditions such that the temperature of the 
combustion products-second oxidizing agent mixture is maintained at a 
level not in excess of about 1800.degree. K., e.g. 
1200.degree.-1750.degree. K. This may be readily accomplished by several 
techniques, e.g. cooling of the combustion chamber, transfer of the 
combustion products to a different "cold" combustion chamber, cooling of 
the combustion products (e.g. by suitable heat exchangers) to a 
temperature of less than 1300.degree. K. Furthermore, cooling may not 
necessarily be required, e.g. the temperature of the combustion products 
in relation to the temperature, requisite amount, and rate of mixing, of 
the second oxidizing agent may be such that the temperature will at all 
times be below about 1800.degree. K. 
The mixing of combustion products and second oxidizing agent may, as in the 
case of the first step, take place external to, or within the same or 
different combustion chamber as was employed in the first stage combustion 
(i.e. the second step). Further, where the second oxidizing agent is 
chosen to be air, the air may be enriched with the same levels of 
additional oxygen as mentioned above with respect to the first oxidizing 
agent (provided that an equivalence ratio of .ltoreq.1.0 is maintained for 
the mixture). 
The second oxidizing agent (e.g. air) may be preheated and/or enriched with 
oxygen as was the case with respect to the first oxidizing agent. Further, 
the second oxidizing agent may be diluted with combustion products and/or 
inert gases prior to and/or during admixture with the combustion products 
resulting from the first stage. These alternatives, however, are subject 
to the proviso that the equivalence ratios and maximum temperatures 
outlined above must nevertheless be maintained. 
In the fourth step, the mixture of combustion products and second oxidizing 
agent is completely combusted in at least one second stage (in the same or 
different combustion chamber as that employed in the first stage 
combustion). The term "completely" combusted is used herein to denote that 
partially oxidized combustion products (e.g. carbon monoxide) resulting 
from the first stage combustion are further oxidized to their highest 
oxidation state (e.g. carbon dioxide). The fourth step is carried out at a 
temperature of less than about 1800.degree. K., preferably 1200.degree. to 
1750.degree. K. The residence times for the second stage depend on the 
fuel but are not critical, i.e. they need be only long enough to oxidize 
substantially all of the carbon monoxide and other remaining combustibles 
(from the first stage combustion) into carbon dioxide. Typically, 
residence times of 0.1 to 1.0 second will be sufficient for the second 
stage combustion. For boilers or process furnaces, residence times of 0.5 
to 1.0 sec. are anticipated. For gas turbines, the second stage residence 
time will be shorter, typically less than 0.1 sec.

EXAMPLE 1 
In this example and in Example 2 below, attention was focused on the first 
stage of the combustion process. Once the conversion of fixed-nitrogen 
chemical species to molecular nitrogen has been maximized by the first 
stage process conditions of this invention, completing the combustion in 
the second stage (at equivalence ratios of 1.0 or less) and at lower 
temperatures (about 1750.degree. K. or less) presents no problem vis-a-vis 
minimization of NO.sub.x formation. 
The apparatus employed in Examples 1 and 2 consisted of an 
electrically-heated vertical muffle-tube furnace; the furnace was 
constructed of zirconia and was 5.40 cm. I.D..times.60.64 cm. long (the 
heated zone was 35.56 cm. long). The reactants (i.e. premixed fuel and 
air) were fed to the bottom of the muffle-tube through a porous plug flat 
flame burner of b 2.54 cm. diameter (the face of the burner was located 
4.92 cm. below the heated zone). Gas samples for analysis of O.sub.2, NO 
and NO.sub.x were withdrawn through a water-cooled stainless steel probe 
that was axially located 41.91 cm. above the burner face. Gas samples for 
analysis of HCN and NH.sub.3 were withdrawn from the cool burner exhaust 
duct, approximately 99 cm. above the burner face. 
In Example 1, methane was doped with approximately 5,000 ppm NO and mixed 
with air so as to result in a mixture having an equivalence ratio of 1.7. 
The results in terms of the output of the sum of NO, NH.sub.3 and HCN as a 
mole percent of the input NO versus various adiabatic flame temperatures 
are shown in Table I below. 
TABLE I 
______________________________________ 
Output, Mole % Flame Temp., .degree.K. 
______________________________________ 
30.7 1762 
17.3 1824 
10.5 1884 
8.04 1940 
6.57 1994 
6.32 2045 
______________________________________ 
EXAMPLE 2 
Example 1 was repeated under the same conditions, except that the methane 
was doped with 4376 ppm NH.sub.3 (instead of the 5000 ppm NO). Table II 
set forth below indicates the wall temperature of the combustion chamber, 
the adiabatic flame temperature, the equivalence ratio and the output, 
mole fraction of input NH.sub.3 appearing as the sum of NH.sub.3, NO plus 
HCN in the combustion products. 
TABLE II 
______________________________________ 
Wall Adiabatic Output, 
Temp., Flame Equivalence 
Mole 
.degree.K. 
Temp., .degree.K. 
Ratio Fraction 
______________________________________ 
1658 2232 1.01 0.703 
1658 2191 1.13 0.584 
1658 2121 1.22 0.522 
1658 2052 1.31 0.411 
1658 1981 1.40 0.266 
1658 1915 1.49 0.253 
1658 1849 1.58 0.233 
1658 1705 1.78 0.409 
1658 1570 1.99 0.433 
1658 1450 2.19 0.408 
______________________________________ 
EXAMPLES 3-7 
The experimental apparatus described in Example 1 was used with some 
modifications to obtain the results presented in the examples that follow. 
All of the experimental data were obtained in a NH.sub.3 doped, premixed 
methane-nitrogenoxygen system that was reacted in an isothermal combustor 
where the wall temperature was matched to the theoretical adiabatic 
combustion temperature of the flame corresponding to each set of test 
conditions. The 2.54 cm diameter porous flat flame burner of Example 1 was 
replaced by a 5.0 cm diameter porous plate flat flame burner in the bottom 
end of the vertical furnace reactor. Most of the data were obtained using 
a vertical tubular furnace constructed of zirconia with dimensions of 5.40 
cm I.D. by 60.64 cm long (the heated zone was 35.56 cm long). In a few 
test runs, a furnace constructed of alumina with the same dimensions was 
used without any noticeable effect on the reaction products. Combustion 
reaction product compositions were determined by withdrawing samples 
isokinetically through a quartz lined water cooled stainless steel probe 
that was inserted into the furnace from the top and could be moved by 
external actuation to predetermined axial positions along the center of 
the furnace. The tip of the sampling probe was positioned to correspond to 
the reaction time requirements specified for the isothermal plug flow 
reaction zone in each series of test. 
EXAMPLE 3 
The experimental results were obtained at a constant reaction time of 300 
msec isothermally at a temperature of 1850.degree. K. to determine the 
effect of the equivalence ratio on the total fixed nitrogen content of the 
combustion products. FIG. 1 presents the results in graphical form as a 
plot of the sum of total fixed nitrogen expressed as the percent of the 
input NH.sub.3 (600 ppm) added to the premixed methane-nitrogen-oxygen 
mixture. In these experiments, the amount of oxygen used was adjusted to 
provide the selected conditions of equivalence ratio and adiabatic flame 
temperature for each test run. 
The results of FIG. 1 clearly show that a minimum occurs in the value of 
the total fixed nitrogen (sum of the concentrations of NO, NH.sub.3 and 
HCN) at a value of the equivalence ratio of about 1.75. This minimum value 
is about 20% of the input ammonia. 
EXAMPLE 4 
The results of Example 4 were obtained again using premixed reactants of 
methane, oxygen, nitrogen and ammonia. As in Example 1, the input ammonia 
concentration was 600 ppm. For this set of test runs. the reaction 
temperature in the furnace was controlled at 1950.degree. K. 
Consistent with the teaching of the present invention, the minimum value of 
the fixed nitrogen concentration (again expressed as the percent of the 
input NH.sub.3) decreased to a lower value at 1950.degree. K. compared to 
the results obtained at 1850.degree. K. As shown by the results in FIG. 2, 
the minimum value of the fixed nitrogen is about 10% of the input NH.sub.3 
at a value of the equivalence ratio of about 1.8. Comparison of FIGS. 1 
and 2 shows that the entire curve of fixed nitrogen plotted as a function 
of the equivalence ratio is shifted to lower values by raising the 
temperature. 
EXAMPLE 5 
The effect of reaction temperature was studied further at 2050.degree. K. 
using methane, oxygen, nitrogen and ammonia mixtures with an input 
concentration of 600 ppm NH.sub.3. As shown by the result in FIG. 3 which 
presents fixed nitrogen plotted vs. equivalence ratio at 2050.degree. K. 
in 300 msec, a further reduction in fixed nitrogen can be achieved by this 
increase in temperature at about .phi.=1.85 to a value of about 8% of the 
input NH.sub.3. 
The trend for the value of .phi. corresponding to minimum fixed nitrogen to 
shift to higher values with increasing temperature is consistent with 
thermodynamic equilibrium calculations for such reactant mixtures. 
EXAMPLE 6 
In this set of experiments, mixtures of methane, oxygen, nitrogen and 600 
ppm NH.sub.3 were reacted at a temperature of 1850.degree. K. and a value 
of .phi.=1.7, as a function of reaction time up to 300 msec. The .phi. 
value was selected to correspond to the minimum in fixed nitrogen 
previously determined for 300 msec and 1850.degree. K. in Example 3. 
FIG. 4 shows the results of these time resolved measurements. Following an 
initial transient buildup in fixed nitrogen, presumably due to some 
further fixation of atmospheric nitrogen, the fixed nitrogen concentration 
of the fuel rich combustion products decreases rapidly with reaction time 
for about the first 100 msec. It is seen that after about 50 msec, about 
30% of the NH.sub.3 input remains as fixed nitrogen. After 100 msec, the 
fixed nitrogen decreases to about 27% and after about 200 msec, 22% of the 
NH.sub.3 input. 
EXAMPLE 7 
A similar set of time resolved measurements as in Example 6 were made, but 
at a higher temperature of 1950.degree. K. Again reactant mixtures of 
premixed methane, oxygen, nitrogen and 600 ppm NH.sub.3 were used. At a 
reaction temperature of 1950.degree. K., the .phi. value selected was 1.8 
to correspond to the minimum in fixed nitrogen observed in the results 
presented in Example 4. 
The results are presented in FIG. 5. After an initial transient increase in 
the concentration of fixed nitrogen species, their concentration decreases 
even more rapidly with reaction time than at the lower temperature of 
Example 6. Because of the initial larger increase in fixed nitrogen, the 
actual values of fixed nitrogen concentration are seen to be higher at 
1950.degree. K. than at 1850.degree. K. up to a reaction time of about 60 
msec (at that reaction time the fixed nitrogen is about 30% of the 
NH.sub.3 input). At 100 msec the fixed nitrogen is about 22% of the 
NH.sub.3 input, at 200 msec it is about 15%, and at 300 msec it is about 
10% of the NH.sub.3 input.