Reduction of combustion effluent pollutants

Injection of additive such as ammonia or calcium compounds along with a small amount of hydrocarbon, preferably methane or natural gas, in a relatively high temperature region of the effluent stream for effectively reducing pollutants such as NO.sub.x and SO.sub.x. Preferably, injection is achieved by atomization of a liquid-form additive or additive solution with a small amount of gaseous hydrocarbon.

FIELD OF THE INVENTION 
This invention relates to reduction of pollutant species in the effluent of 
combustion systems by injection of additives to the effluent. 
BACKGROUND OF THE INVENTION 
Concern over the emission of acid rain precursors, NO.sub.x and SO.sub.x, 
from stationary combustion processes is likely to lead to increasingly 
tight regulations. In certain areas, no single method of in-furnace or 
post-combustion reduction of these pollutants suffices. For reducing 
NO.sub.x emission, methods of post-combustion flue gas treatment include 
the injection of ammonia in the presence of a sufficient amount of O.sub.2 
to selectively reduce NO.sub.x from the combustion effluent. Reduction of 
SO.sub.2 may be affected by injection of calcium compounds. 
SUMMARY OF THE INVENTION 
An object of the invention is to reduce the emission of combustion 
process-produced pollutants, such as bound nitrogen species, e.g. 
NO.sub.x, and sulfuric acid precursors, e.g., SO.sub.x, by the injection 
of additives into the effluent. In the case of bound nitrogen species, the 
additives promote reaction pathways that lead to the harmless species, 
N.sub.2. In the case of SO.sub.x, the additives react with SO.sub.x and 
the sulfur is retained in a relatively harmless form. It is also an object 
to make these injections in such a way that the pollutants may be reduced 
in the dynamic environment of rapidly changing temperatures and chemical 
species concentrations in the turbulent flow of practical combustion 
systems, such as waste incinerators and power generation facilities, 
without the additives themselves contributing substantial pollutants to 
the flow. 
It has been discovered that these objects can be achieved by injection of 
additives along with certain small amounts of hydrocarbons, preferably 
methane or natural gas, in a relatively high temperature region of the 
combustion product stream, preferably by atomization of a liquid-form 
additive or additive solution with a small amount of gaseous hydrocarbon. 
Injection at relatively high temperatures allows for high reaction rates 
of the additive with pollutant precursors over a relatively wide 
temperature range which is an especially important consideration for 
effluent streams which are cooling rapidly such as those found in 
commercial power generation combustors and waste incinerators. The use of 
a hydrocarbon at low concentrations also enhances the kinetics of the 
reactions, further enhancing the efficiency of the reactions in a rapidly 
cooling environment, without substantial production of detrimental 
byproducts such as CO, often associated with hydrocarbon injections. 
Injection is carried out in a single stage, such that the additive and 
hydrocarbon are present in the same physical region of the effluent, 
exposed simultaneously to substantially the same temperature regime. For 
example, the use of an injection scheme in which the additive is atomized 
with the hydrocarbon enhances the efficiency of the reduction by promoting 
intimate mixing of the hydrocarbon and the additive, without a 
non-reactant atomizing medium such as high pressure steam or air, which 
both dilute the reactants and locally cool the effluent, thereby reducing 
the temperature, inhibiting reaction kinetics. Employing the invention, 
over 90% reduction in the amount of NO.sub.x may be achieved. 
The invention is effective in reducing not only NO.sub.x, but other species 
containing bound nitrogen, i.e., the total bound nitrogen (TBN) which are 
further potential sources for the formation of NO.sub.x by oxidation. As 
discussed herein TBN includes, the sum of species such as HCN, HNCO, NCO, 
NH.sub.3, NH.sub.2, NH, N, NO, N.sub.2 O, N.sub.2 H.sub.2, N.sub.2 H, 
nitrogenous combustion products, and unburned nitrogenous fuel 
contaminants. Their conversion to molecular nitrogen may be achieved by 
atomization of an ammonia-producing compound with hydrocarbon. 
In a first aspect, the invention features a method for the reduction of TBN 
in the NO.sub.x containing combustion products of a combustion process. 
The method includes injecting in the fuel lean region of the combustion 
products an ammonia-producing additive and a hydrocarbon. The 
concentration ratio (or molar ratio on a volume to volume basis (ppm/ppm)) 
of the hydrocarbon to the additive is between 0.2 and 0.01 and the 
injection occurs in a temperature range above about 1700.degree. F. 
In various embodiments, the hydrocarbon is an unsubstituted, saturated 
hydrocarbon, such as methane or natural gas. The ratio of hydrocarbon to 
additive is about 0.15 and 0.01, preferably, about 0.1 and 0.03. The 
temperature is about 1750.degree.-2100.degree. F., more preferably, 
1800.degree.-1950.degree. F. The amount of additive is selected such that 
the molar ratio of additive to NO.sub.x in effluent is about 2.0 or less, 
preferably, about 1.0 to 1.5. The effluent has a temperature decay on the 
order of about 500.degree. F./sec or more. The injection is achieved by 
atomizing additive with hydrocarbon. The additive is a liquid and the 
hydrocarbon is a gaseous hydrocarbon. The additive is ammonia, urea, 
cyanuric acid, or ammonium hydroxide and solutions and mixtures thereof. 
In another aspect, the invention features a method for modification of the 
effluent of a combustion process by providing in a fuel lean region of the 
effluent a mixture of a gaseous hydrocarbon and an additive effective in 
reducing a desired combustion product from the effluent by atomization of 
the additive with gaseous hydrocarbon to create a locally hydrocarbon rich 
region about the additive. 
In various embodiments, the gaseous hydrocarbon is natural gas or methane. 
The additive is a liquid-form, ammonia-producing additive, reactive with 
effluent to reduce total bound nitrogen emission. The ammonia-producing 
compound is selected from ammonia, urea, cyanuric acid, ammonium hydroxide 
and aqueous solutions and mixtures thereof. The hydrocarbon is about 5 to 
15% by weight of the mixture. The mixture is injected into effluent where 
effluent has a temperature in the range from about 1300.degree. to about 
2100.degree. F. 
For reduction of SO.sub.x emission, the additive is a calcium containing 
compound. The calcium containing compound is selected from the group 
consisting of calcium acetate, lime, hydrated lime, limestone and mixtures 
and solutions thereof. The hydrocarbon is natural gas. The mixture is 
injected into the effluent where the effluent has a temperature in the 
range of about 2800.degree. F. to 1700.degree. F., e.g., greater than 
about 2300.degree. F. The atomization is carried out with a high momentum 
atomizer. Hydrogen peroxide or CH.sub.4 is injected into effluent 
downstream of the additive injection. 
In another aspect, the invention features a method for reduction of 
SO.sub.x pollutants in the effluent of a combustion process by injecting 
in a fuel lean region of the effluent a calcium containing compound for 
reaction with SO.sub.x in the effluent, in combination with a hydrocarbon 
in sufficient amounts to form a locally fuel-rich region about compound, 
the injection occurring in a temperature range of about 1700.degree. F. to 
2800.degree. F. 
In various embodiments, the effluent remains in an overall fuel lean 
condition after injection. The additive is selected from calcium acetate, 
lime, hydrated lime, limestone and mixtures and solutions thereof. The 
temperature is greater than about 2300.degree. F. The hydrocarbon is 
natural gas. 
In another aspect, the invention features a method for the reduction of 
pollutants in the effluent of a combustion process by injecting in a fuel 
lean region of the effluent a calcium containing compound for reaction 
with SO.sub.x in the effluent, in combination with a hydrocarbon in 
sufficient amounts to form a locally fuel-rich region about compound, 
while maintaining the effluent in an overall fuel lean condition, the 
injection occurring in a temperature range of about 1700.degree. F. to 
2300.degree. F. and injecting in the fuel-ratio lean region of effluent an 
ammonia producing additive and a hydrocarbon consisting of methane or 
natural gas, the molar ratio of additive to NO.sub.x being between about 
1.0 and 1.5, the concentration ratio of hydrocarbon to additive being 
between about 0.15 and 0.01, and the injection occurring in a temperature 
range of about 1750.degree. to 2100.degree. F. 
In another aspect, the invention features a method for the reduction of 
NO.sub.x and SO.sub.x pollutants in the effluent of a combustion process, 
comprising injecting in a fuel lean region of effluent calcium acetate in 
combination with a hydrocarbon in a temperature range of about 
1700.degree. or above. 
Other embodiments and features follow.

STRUCTURE AND OPERATION 
Referring to FIG. 1, a commercial, coal burning boiler used for steam 
raising for power generation or for industrial process steam is shown to 
include a combustion chamber 6 with an assembly of burners 2. The region 
of the combustion chamber 6 immediately adjacent to the burners 2 is 
called the burner zone 4. The furnace walls consist of steam generating 
tubes 8 which absorb heat by radiation mainly from the flame to produce 
saturated steam. At the exit from the combustion chamber, the combustion 
products exit the nose 22, enter a duct 10, including superheaters 9 and 
tube banks of convective heat exchangers. The superheaters 12 include 
convective steam superheaters and reheaters for the preparation of the 
steam for use in steam turbine (not shown). Economizers 14 are for 
preheating the boiler feed-water and air preheaters 16 for preheating the 
combustion air. Past the preheater 16 the flue gas enters the gas cleaning 
equipment 18, e.g., a scrubber and gas solid separator. The cleaned gas is 
then exhausted to the environment through the stack 20. In a typical 
commercial-sized system, e.g., 300 MWe, the duct is 30-40 feet wide, and 
effluent flow rates are in the range of 6.times.10.sup.5 SCFM. The boiler 
is typically fired hydrocarbon fuels such as oil, natural gas, or coal as 
illustrated. It will also be understood the invention is applicable to 
other fuel utilization or combustion systems such as waste incinerators. 
In the region 4 where combustion is initiated, a mixture of fuel and a 
small stoichiometric excess of oxygen are present (fuel lean). At the exit 
of the furnace region 6, near the nose 22, combustion is substantially 
complete and the effluent from the combustion system is also typically 
fuel lean, (i.e., the oxygen present is in excess of the stoichiometric 
amount needed to fully oxidize the sum of hydrocarbons present in the 
effluent) e.g. about 0.5 to 7% stoichiometric excess oxygen is present. As 
the combustion products pass through the boiler 14 as indicated by arrows 
24, the temperature falls. For example, at the position indicated as 
T.sub.1, the combustion products have a temperature of about 2700.degree. 
F., cooling to, for example, a temperature of about 2300.degree. F. at 
position T.sub.2 near the furnace region exit, The products leaving the 
furnace region as effluent cool to about 2000.degree. F. at position 
T.sub.3, near the superheaters and to about 1700.degree. F. at a position 
T.sub.4, before the economizers 8, and finally, to about 300 .degree. F. 
at position T.sub.5, downstream of the scrubber 18. In typical combustor 
systems, the temperature of the effluent may be cooling rapidly, e.g., in 
the range of hundreds of degrees per second, e.g., 500.degree. F./sec or 
above. 
An atomization stage 31, is provided for injection of an ammonia producing 
compound to reduce bound nitrogen species, e.g., NO.sub.x, and an 
atomization stage 33 is provided for injection of a calcium sorbent to 
reduce SO.sub.x species. In stage 31, ammonia-producing material such as 
liquid ammonia, is provided from a supply 36, along with a small amount of 
natural gas from a supply 38 to a series of atomizer nozzles 39, such that 
the natural gas atomizes the ammonia-producing compound in a high momentum 
spray. The high momentum is important for the effective admixing of the 
additive to the combustion products by effecting sufficient penetration of 
the spray into the duct. In stage 33 a calcium sorbent, such as calcium 
acetate, for elimination of SO.sub.x is provided from a supply 30, along 
with natural gas from a supply 32 to a series atomizer nozzles 34, such 
that the natural gas atomizes the sorbent to form a high momentum spray. 
The nozzles are typically operated to produce droplet sizes in the range 
of about 20-100 .mu.m. Larger droplet sizes are employed for higher 
effluent flow rates. The number, type and size of the nozzles is selected 
based on the desired penetration and entrainment of the flue gas into the 
effluent stream. The nozzles are provided with sufficient pressure to 
extend the spray across the duct width, and operating conditions selected, 
based on duct width and effluent flow rate. High momentum pressure 
atomization nozzles 34, 38 need not extend into the duct, but rather are 
flush with the duct wall, to eliminate slagging of the nozzles by exposure 
to effluent solids such as ash. 
A particular feature of the invention is that additives effective in 
reducing desired combustion products are injected into the fuel lean 
region of the effluent by atomization of the additive with a small amount 
of hydrocarbon, to create a locally fuel-rich region about the additive 
while the effluent remains, overall, fuel lean. In particular embodiments, 
the additive is a liquid-form material, either a pure additive or an 
additive solution or slurry, and the atomizing hydrocarbon is a gas. The 
additive may also be particulate in nature and be entrained in the air 
flow. The atomization arrangement allows for rapid mixing of the additive 
with the hydrocarbon in the effluent stream and allows the gaseous 
hydrocarbon to promote evaporation of liquid additives. The injection can 
be made without substantial dilution of the additive and hydrocarbon 
reactants or unnecessary cooling of the effluent. Preferably, the 
atomizing medium is a gas, i.e., it is a compressible medium, used to 
impart momentum to the additive, preferably a liquid, as the additive 
mixes with the liquid and is accelerated during passage through a nozzle 
while expanding to atmospheric pressure. Most preferably, the atomizing 
medium is an unsubstituted (containing only hydrogen and carbon) and 
saturated (containing only single bonds) hydrocarbon that is in the gas 
phase at atmospheric temperature and pressure. Most preferably, the medium 
is natural gas. Natural gas is typically available from central supply 
sources at high pressure, (e.g., p.about.40 psi) which permits high 
atomization quality to be obtained even using relatively small amounts of 
natural gas (e.g., less than 20% by weight of the total mixture injected). 
Referring now to FIG. 2, a divergent nozzle for atomization of additives 
with hydrocarbons includes a relatively wide mixing stage 35, in which, 
e.g. natural gas, and the additive are mixed and pressurized and a 
divergent stage 37, terminating in an exit orifice 40, in which gas 
pressure is converted to jet momentum. A nozzle used in experiments 
discussed herein below may be fastened to a tubing member by threads has 
an overall length L.sub.1, about 0.990 inch. The mixing region 35 has a 
width of about W.sub.1, about 1.180 inch and length L.sub.2 about 0.446 
inch. The divergent region 37 has an overall length L.sub.3 of about 0.323 
inch, a first stage of length L.sub.4, about 0.123 inch, and a width of 
W.sub.2, about 0.031 inch and a second stage width of W.sub.3, about 0.052 
inch. For typical applications nozzle pressure at the inlet is about 40 
psi. The nozzle is generally constructed for a desired additive to 
hydrocarbon ratio. Other nozzles, such as Laval nozzles may also be used. 
Further discussion of Laval nozzles, along with optimal interrelationship 
of the dimensions and performance is discussed in Perry et al., Chemical 
Engineering Handbook, McGraw-Hill, 1973, page 529. A feature of the Laval 
nozzle is that substantially all of the pressure energy is convected to 
kinetic energy of the flow. The dimensions and operating conditions of the 
above nozzle are useful for experimental sized-applications. Nozzle size 
may be scaled with the combustor. 
Referring to FIGS. 3 and 3a, a nozzle of the Y-jet design is illustrated in 
cross-section and end-on view. Additive passes through a central lumen 42 
while atomizing medium is introduced through multiple outer lumens 44 
located about the central lumen. The central lumen and outer lumens 
intersect in a "Y" configuration to enable mixing of the atomizing medium 
and the additive in the outlet tube 46 so that a spray is generated at the 
exit opening 48. Typically, six or eight exit openings are provided. 
It will be understood that other nozzle designs may be used, provided 
sufficiently high momentum is induced to the injected species, and the 
nozzle may be operated for atomization using the amount of atomizing 
medium and additive, as described herein. 
TBN Reduction 
The ammonia-producing compound may be, e.g., liquid ammonia, urea, ammonium 
hydroxide, cyanuric acid and aqueous solutions and mixtures thereof. The 
additive may also be pre-mixed with an agent such as a hydrocarbon or 
other reactive species. Atomizing the ammonia-producing compound with a 
hydrocarbon atomizing medium produces a locally fuel rich environment in 
which the ammonia vaporizes. The conditions for the NO.sub.x -NH.sub.i 
reactions proceed toward the conversion of NO.sub.x to N.sub.2. The 
hydrocarbon reacts to produce radicals that in turn react with ammonia to 
produce species such as NH.sub.2 and NH, which in turn react with the 
NO.sub.x in the effluent to produce amino species which are reduced to 
form mainly N.sub.2. In addition, in the locally fuel-rich region created 
by the atomization, the NO.sub.x reacts with hydrocarbon fragments such as 
CH.sub.i radicals, e.g. NO+CH.fwdarw.HCN+O, or NO+CH.sub.2 .fwdarw.HCN+OH. 
The cyanides then form amines via intermediates such as HNCO and NCO, with 
the eventual formation of molecular nitrogen, N.sub.2. The presence of the 
proper radical pool generated from the breakdown of the hydrocarbon 
atomizing medium, preferably natural gas, also results in the destruction 
of residual NH.sub.3 by the reaction pathways that enable reaction of the 
amino species with each other to form N.sub.2, thus reducing NH.sub.3 
emission and reaction byproducts (e.g. N.sub.2 O, HCN, HNCO), so called 
"slippage". 
Injection of the ammonia producing additive by atomization with the 
hydrocarbon takes place at a position along the effluent stream where the 
effluent temperature is in the range above 1700.degree. F., preferably, 
between 1750.degree. to 1950.degree. F., most preferably between about 
1800.degree.-1900.degree. F. The amount of ammonia producing additive to 
be injected is based on the amount of NO.sub.x in the effluent stream. 
Typically, the mole ratio of the additive to NO.sub.x is about 2.0 or 
less, preferably between about 1.0 to 1.5. It will be understood that mole 
ratios of greater than 2.0 can be used, without excessively high ammonia 
slippage. Typically, the hydrocarbon makes up about 0.5 to 15%, most 
preferably about 5%, by weight of the injected gas mixture, depending, 
e.g., on the form of the additive (e.g., pure ammonia or aqueous 
solution). The amount of hydrocarbon is selected based on the amount of 
ammonia producing species to be injected. Typically, the concentration 
ratio of hydrocarbon to additive is about 0.2 or less, preferably about 
0.1 or less, most preferably about 0.05 to 0.01. Typically, TBN reduction 
is greater than 70%, more typically greater than 80% or 90% (concentration 
percentage based on NO.sub.x concentration at combustion chamber exit.) 
The high efficiency of TBN reduction as described herein is attributed to 
proper molar ratios of additive to NO.sub.x and hydrocarbon to additive 
that enhance the kinetics of the NO.sub.x reducing reactions in effluent 
streams with rapidly changing temperature. Atomization of the additive 
with the hydrocarbon also enhances efficiency by providing efficient 
mixing, while minimizing cooling and dilution effects. The following 
computer-generated chemical kinetic simulations in FIGS. 4-11 illustrate 
the principle. The data were computed by the Chemkin kinetic program, 
available from U.S. DOE-Sandia National Laboratory. 
Referring to FIGS. 4 and 5, plots of TBN concentration versus temperature 
are illustrated for various ratios of hydrocarbon to methane additive. 
Small amounts of methane, e.g., methane/ammonia ratio of about 0.046, can 
be used to effect low TBN at temperatures above about 1700.degree. F., 
whereas high concentrations in this temperature range do not as 
effectively reduce TBN. In addition, the temperature range in which small 
TBN levels can be kept below 500 ppm is wide, approximately 300.degree. F. 
or more (1700.degree.-2000.degree. F.). Further, compared to the use of 
high concentrations (methane/ammonia=1.0, 0.5), for low hydrocarbon to 
additive ratios, the steepness of the TBN increase at temperatures below 
the minimum is relatively mild. This result allows more efficient TBN 
reductions in effluent streams with rapidly falling temperature and under 
conditions of part load (turn down) when the effluent temperature is lower 
than at full load at a position in the duct. Referring to FIGS. 6 and 7, 
similar trends are illustrated, respectively, for plots of NO.sub.x and 
NH.sub.3 as a function of temperature. (Experimental data, shown in FIG. 
13, illustrates the favorable effect of small amounts of hydrocarbon 
injection.) 
Referring to FIG. 8, a plot of N.sub.2 O concentration as a function of 
temperature, as in the above figures as shown. The plot illustrates that 
for high methane/ammonia ratios, the amount of N.sub.2 O is nearly double 
that for low ratios of the invention. Low N.sub.2 O production is of 
importance for the reduction of the depletion of stratospheric ozone and 
the greenhouse effect. In sum, the above Figures illustrate that a small 
amount of methane in combination with ammonia is effective in maintaining 
low TBN (FIGS. 4 and 5) while at the same time enabling low ammonia slip 
(FIG. 7) without detrimental N.sub.2 O (FIG. 8) and NO.sub.x emission 
(FIG. 6). 
Referring now to FIG. 9, the TBN concentration as a function of residence 
time is plotted for an injection of additive with ratios of 
methane/ammonia=0.046 and 0.0 (pure ammonia) at a temperature of 
1900.degree. F. A small amount of hydrocarbon, produces a substantial 
minimum after about 40 milliseconds (msec), compared to results without 
methane which took over 50 milliseconds to reach the same TBN reduction. 
Thus, a small amount of hydrocarbon with ammonia additive, as described, 
increases the kinetic rate of TBN reducing reactions. A similar plot is 
shown in FIG. 10, with injections at 1800.degree. F. In this case, near 
the most preferred temperature range, the TBN reaches a minimum in about 
100 msec, whereas injections without hydrocarbon do not produce similar 
TBN reduction in over 200 msec. 
Referring now to FIG. 11, the temperature decay in an experimental effluent 
stream described further in Example 1, is illustrated. The decay is 
approximately 1022.degree. F./sec. In systems such as waste incinerators 
and power-generating combustors, temperature decay is typically in the 
range of about 500.degree. F./sec. The highly turbulent flow may be 
accomplished by atomization with hydrocarbons. 
SO.sub.x Reduction 
For reduction of SO.sub.x species in the effluent, may also create pockets 
or zones of varying temperature injection of a sorbent such as calcium 
containing compounds Calcium compounds are preferably supplied in water 
soluble form or slurry and include, for example, lime, hydrated lime, 
calcium carbonate or calcium acetate but may also be particulate in 
nature, e.g., lime particles. The CaO crystallites formed by nucleation of 
e.g., calcium acetate following atomization are finely dispersed, 
providing a larger surface area to mass ratio, than typically achievable 
with hydrated lime or calcium carbonate injection. Increased sorbent 
utilization results. 
The calcium compounds react with SO.sub.x in the effluent to form calcium 
sulfite (CaSO.sub.3), which further oxidizes to form calcium sulfate 
(CaSO.sub.4) as temperature is lowered. If the sorbent is in liquid form, 
it is introduced into the effluent by atomization, preferably with natural 
gas. However, if the sorbent is in a solid form, the hydrocarbon atomizing 
medium is used to carry the particles into the duct. The fuel-air ratio is 
locally fuel rich by the addition of the natural gas (and the acetate in 
the sorbent in the case of calcium acetate) which favors the NO.sub.x 
reducing reactions. In the case of calcium acetate, reduction in NO.sub.x 
may be achieved in addition to SO.sub.x. 
Referring to FIG. 12, a known thermodynamic equilibrium phase diagram of 
CaO, CaSO.sub.4 and CaS is illustrated. The percentage lines in the 
diagram refer to the sulfur capture efficiency as a function of 
temperature and fuel equivalence ratio. (Stability relationships are 
discussed further in Torrez-Ordonez, R., Department of Chemical 
Engineering, Ph.D. Thesis, Massachusetts Institute of Technology, 
Cambridge, Mass., 1987.) As the diagram indicates, calcium sulfate, 
CaSO.sub.4, the desired product of SO.sub.x and the sorbent is stable at 
temperatures below about 2300.degree. F. under fuel lean conditions Above 
2300.degree. F. calcium sulfate decomposes to calcium oxide (CaO) and 
SO.sub.x is released to the effluent. However, under fuel-rich conditions, 
CaS is stable at higher temperature. An aspect of the invention is that 
atomization with hydrocarbon may be made in a temperature range that is 
low enough for the products of sulfation (CaSO.sub.3 or CaSO.sub.4) to 
remain stable. A particular advantage of atomization of calcium-producing 
compounds with hydrocarbons is the increase in the temperature range, 
above the stable range for calcium sulfation products in fuel lean 
environments. By atomization with a hydrocarbon, a locally hydrocarbon 
rich environment is provided about the calcium particles which inhibits 
decomposition of CaS, thus retaining the captured sulfur. After 
equilibrium to the fuel-lean condition downstream of the injection, the 
temperature has cooled sufficiently to maintain stability. Injections may 
therefore be made at temperatures of up to about 2800.degree. F. High 
temperature injection of the calcium sorbent in this manner allows 
injection further upstream and hence greater residence time and more 
efficient SO.sub.x reduction. A preferred injection temperature is just 
above the decomposition temperature of calcium sulfate, e.g., about 
2300.degree. F. However, it will be understood that injection may be made 
at lower temperatures, e.g., down to about 1600.degree. F. At the lower 
temperature range, the calcium sorbent may also be co-injected with an 
NO.sub.x reducing additive by atomization with a hydrocarbon. 
In particular embodiments, the hydrocarbon employed for atomization is 
natural gas and may be from about 5% to 95% by weight of the mixture of 
calcium compound and hydrocarbon. At the higher ratios of methane 
injection, an overall fuel-rich condition might exist, thus creating 
reburn conditions, which are highly conducive to the reduction of NO.sub.x 
to N.sub.2. In the process sulfur capture would occur through the combined 
injection of, e.g., calcium acetate with hydrocarbon gas. The amount of 
the calcium compound injected is selected based on the Ca/S mole ratio to 
be maintained in the duct. Typically, the Ca/S mole ratio is maintained in 
the range of about 1 to 6, more typically about 1 to 4. 
The invention is further illustrated by the following examples. 
EXAMPLE 1 
An experimental study of NO.sub.x reduction by atomization of NH.sub.3 
using a hydrocarbon was carried out at the MIT combustion research 
facility (MIT-CRF). The Combustion Research Facility, is designed to 
facilitate detailed experimental investigations of industrial-type 
turbulent diffusion flames; it consists of a 1.2 m.times.1.2 m 
cross-section 10 m long combustion tunnel equipped with a single burner 
having a 3 MW thermal input, multi-fuel firing capability. During 
experiments the combustion tunnel is normally comprised of 15 of 0.30 m 
wide, refractory-lined water-cooled sections. The velocity of effluent 
flow in the region of injection was about 6 m/sec. The system was operated 
at 0.65 MW for these experiments. The experimental burner is more fully 
described in Beer et al. in "Laboratory Scale Study of Coal-Derived Liquid 
Fuels" EPRI Report AP4038, 1985. Injections were made with a nozzle as 
described in FIG. 2. 
In the study, NH.sub.3 was atomized with natural gas into the flue gas duct 
at a temperature of about 2000.degree. F. Referring to FIG. 13 and Table I 
below, NO.sub.x measurements made for several levels of CH.sub.4 are 
given. In the Table, CO concentration is also provided. 
TABLE I 
______________________________________ 
Initial NO.sub.x Conc. = 730 ppm 
NH.sub.3 /NO.sub.x Ratio = 1.6 
NO.sub.x Conc. at exit 
CH.sub.4 Conc. (ppm) 
of process CO Conc. (ppm) 
______________________________________ 
0 240 28 
82 200 48 
112 180 62 
186 210 67 
298 230 77 
969 250 87 
______________________________________ 
In this example, for a NO.sub.x concentration of 730 ppm, the optimum 
ratios of CH.sub.4 /NH.sub.3 for conversion to N.sub.2 is in the range of 
about 0.03-0.2. The NO.sub.x concentration decreases initially then 
increases with increasing amount of hydrocarbon. In this example, the 
maximum NO.sub.x reduction, at minimum, was about 75% of the original 
NO.sub.x level. The level of CO was not excessive. 
EXAMPLE 2 
Experiments carried out at the MIT Combustion Research Facility, under 
conditions similar to Example 1 showed that by atomizing calcium sorbent 
with hydrocarbon a sulfur capture close to 100% can be obtained. FIG. 14 
illustrates that at a Ca/S ratio of about 2.0, the sulfur in the gas phase 
was completely removed. At lower ratios of Ca/S the sulfur capture 
efficiency dropped, e.g., to reach a level of 66% at a ratio of 0.9. 
Injection was in the fuel lean zone below 2200.degree. F. for the products 
of sulfation (CaSO.sub.3 or CaSO.sub.4) to remain stable. It was also 
observed that for an initial NO.sub.x concentration of 540 ppm NO.sub.x 
was reduced to 250 ppm with the injection of calcium acetate, with 2500 
ppm of methane and a 2:1 molar ratio of calcium to sulfur. The initial 
SO.sub.2 concentration was around 600 ppm. The effluent was overall fuel 
lean under these conditions. Injections were made with a nozzle as 
discussed in FIG. 2. 
Other Embodiments 
Injection of the NO.sub.x reducing additive or the SO.sub.x capturing 
additive can be made at several points along the furnace or incinerator. 
This method can ensure that the temperature at which the injection occurs 
always falls within the previously specified range as operating conditions 
may change. Some systems may only require one of the injection stages 
illustrated in FIG. 1, e.g., for injection of ammonia-producing compounds. 
In some embodiments it may be advantageous to inject oxidant compounds 
such as hydrogen peroxide (H.sub.2 O.sub.2) or CH.sub.4 downstream of the 
injection of the hydrocarbon atomization to complete oxidation of the 
hydrocarbon. Since the fuel/air mixture ratio at the additive injection 
point is overall fuel-lean, the residual oxygen in and the temperature of 
the flue gas generally ensures the complete burn-out of the combustible 
species added. However, if the CO and hydrocarbon concentrations increase 
in the process, the injection of an oxidative species such as H.sub.2 
O.sub.2 would eliminate such a problem. 
Other embodiments are within the following claims.