Process and apparatus for emissions reduction from waste incineration

A process for combustion of the combustible material includes introducing the combustible material into the combustion chamber, advancing the combustible material through the combustion chamber, supplying combustion air to the combustion chamber for drying and partially combusting the combustible material and final ash burnout in a primary combustion zone, and removing ash products from the combustion chamber. The fuel or fuel/carrier fluid mixture is supplied into the combustion chamber to create an oxygen deficient secondary combustion zone for NO.sub.x reduction and other nitrogen bearing compounds decomposition. An oxidizing fluid is supplied into the combustion chamber above the oxygen deficient secondary combustion zone for thorough mixing with combustion products and at least partial burnout of combustibles in an oxidizing tertiary combustion zone. A furnace for combustion in accordance with this process is also disclosed wherein a combustion chamber is configured such that combustible material can be advanced from a drying zone, to a combustion zone, to a burnout zone, and then into an ash pit. An air source provides air for drying, combustion and burnout in a primary combustion zone. Fuel or a fuel/carrier fluid mixture is injected above the primary combustion zone to create an oxygen deficient secondary combustion zone, to reduce NO.sub.x and decompose other nitrogen bearing compounds entering the secondary combustion zone. An oxidizing fluid is injected into the combustion chamber above the oxygen deficient secondary combustion zone.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process and apparatus for combustion of waste 
such as municipal solid waste (MSW), refuse derived fuel (RDF) or other 
comparable solid waste; the process results in simultaneous reduction in 
nitrogen oxides (NO.sub.x), carbon monoxide (CO), total hydrocarbons 
(THC), dioxins (PCDD), furans (PCDF), and other organic emissions. 
2. Description of the Prior Art 
Most of the existing processes and apparatuses for combustion of waste 
include a combustion chamber equipped with a sloped or horizontal stoker 
grate that reciprocates or travels to move the waste from the waste inlet 
side of the combustor to the ash removal side of the combustor. A portion 
of the combustion air, generally equivalent to 1.0 to 1.3 of the waste 
stoichiometric requirement, is supplied under the stoker grate. Such 
combustion air is typically called undergrate air, or UGA, and is 
distributed through the stoker grate to dry and burn the waste present on 
the stoker grate. The waste is first dried on the drying portion or drying 
grate of the stoker grate, then combusted on the combustion portion or 
combustion grate of the stoker grate. The residual waste that primarily 
includes ash and carbon is then decarbonized or burned on the burnout 
portion or burnout grate of the stoker grate. The bottom ash is then 
removed through an ash pit. To assure carbon burnout, a high level of 
excess air, compared to the amount required for carbon burnout, is 
maintained at the burnout grate. In addition to other species, the 
products of waste drying, combustion and burnout contain products of 
incomplete combustion (PIC's) such as carbon monoxide (CO) and total 
hydrocarbons (THC), oxides of nitrogen (NO ), such as NO, NO.sub.2, 
N.sub.s O and other nitrogen bearing compounds such as NH.sub.3, HCN and 
the like. 
The majority of NO.sub.x evolved from the stoker grate is believed to form 
from the oxidation of nitrogen bearing compounds and a smaller portion 
forms from the oxidation of molecular nitrogen. 
Additional air or overfire air is usually introduced above the stoker grate 
and mixed with the products evolved from the stoker grate to burn out the 
combustibles. The excess air level downstream of the overfire air 
injection is generally in the range of 60% to 
Nitrogen bearing compounds that evolve from the waste react with oxygen in 
and downstream of the overfire air injection zone, forming significant 
additional NO.sub.x. Because of the low combustion temperatures in and 
downstream of the overfire air injection, most of the NO.sub.x formed in 
this zone is by the oxidation of nitrogen bearing compounds (less than 
about 10% are formed in this zone by the oxidation of molecular nitrogen). 
Based on measurements by the inventors, typical mass burn operations would 
result in about 30% of the total NO.sub.x formed on the stoker and about 
70% in and downstream of the overfire air injection. 
In most cases, a boiler is an integral part of the combustor to recover the 
heat generated by MSW combustion. In some cases, cooled flue gases from 
downstream of the boiler are recirculated back into the combustion zone to 
reduce oxygen concentration and to lower combustion temperatures and thus 
are believed to decrease oxides of nitrogen formation. A disadvantage of 
flue gas recirculation (FGR) is generally a higher concentration of 
products of incomplete combustion within the flue gases and within the 
stack gases because of reduced combustion efficiency. 
U.S. Pat. No. 3,781,162 teaches an apparatus for mixing recirculated flue 
gases with combustion air before the gases reach an igniter. The '162 
patent discloses combustion without recirculating vitiated air from over a 
burnout grate for overfiring. The '162 patent teaches neither fluid 
swirling in the combustion chamber nor injecting fuel above a stoker 
grate. 
U.S. Pat. No. 3,938,449 discloses a waste disposal facility which uses a 
rotary kiln that differs from a stoker. The rotary kiln includes a hollow, 
open-ended circular tube body mounted for rotation about its circular 
axis. Hot flue gases are recirculated to dehydrate the waste material and 
remove oxygen. The '449 patent does not disclose fluid swirling in the 
combustion chamber or fuel injection downstream of the primary waste 
combustion zone. 
U.S. Pat. No. 4,336,469 teaches a method of operating a magnetohydrodynamic 
(MHD) power plant for generating electricity from fossil fuel. The MHD 
combustor has a first stage which operates substoichiometrically, second 
stage natural gas injection, and third stage air injection for complete 
combustion. The '469 patent does not disclose the use of vitiated air from 
the combustor for overfiring and does not disclose fluid swirling within 
the combustion chamber. The '469 patent discloses a dwell chamber 
downstream of the MHD generator for reducing nitrogen oxides, but does not 
disclose nitrogen bearing compound decomposition. 
U.S. Pat. No. 4,672,900 teaches a tangentially-fired furnace having 
injection ports for injecting excess air above a fireball of the 
combustion chamber to eliminate the flue gas swirl as the flue gas flows 
into a convection section. The furnace uses pulverized coal as a fuel. 
Secondary air is tangentially injected into the furnace and swirls in the 
direction opposite of the flue gas swirl. The '900 patent does not suggest 
the use of recirculated vitiated air from the main combustor for 
overfiring, fluid swirling within the combustion chamber, or fuel 
injection downstream of the primary combustion zone. 
U.S. Pat. Nos. 4,013,399, 4,050,877 and 3,955,909 teach reduction of 
gaseous pollutants in combustion flue gas. The '909 patent discloses 
two-stage combustion within a combustion chamber. Heat removal occurs in 
the first, second or both combustion stages to reduce nitrogen oxides. 
Secondary combustion air is injected or diffused through tubes into the 
stream of gaseous combustion products flowing from a primary combustion 
chamber to promote mixing and complete combustion without an excessive 
amount of secondary air. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a process and apparatus for 
combustion of combustible materials such as MSW, RDF or other comparable 
solid combustible material where fuel, preferably natural gas, is injected 
above the burning combustible material providing a sufficient temperature, 
from about 1600.degree. F. to about 2400.degree. F., and a sufficient 
length of time, from about 1.0 sec to about 4.0 sec, to create a secondary 
combustion zone in which nitrogen bearing compounds entering the secondary 
combustion zone are decomposed to N.sub.2 and secondary combustion air or 
overfire air injected above the secondary combustion zone is used to 
reduce other emissions such as carbon monoxide (CO), total hydrocarbons 
(THC), dioxins (PCDD), and dibenzofurans (PCDF), without forming 
significant additional NO.sub.x. 
It is another object of this invention to inject a carrier fluid, such as 
steam, water, nitrogen and/or recirculated flue gases from the boiler exit 
into the secondary combustion zone to enhance mixing, and improve 
temperature and composition uniformity in the secondary combustion zone. 
It is another object of this invention to remove a portion of the 
combustion products from above the burnout grate or from above the burnout 
zone, which normally enter the secondary combustion zone, to increase 
temperature and improve temperature and composition uniformity in the 
secondary combustion zone, to decrease the necessary amount of fuel, to 
reduce NO.sub.x emissions and to improve combustible burnout in a tertiary 
combustion zone downstream of the secondary combustion zone. 
It is another object of this invention to provide a process and apparatus 
for combustion of solid combustible materials using a combination of low 
excess air or substoichiometric combustion of solid combustible materials 
in certain zones within the combustion chamber above the drying and 
primary combustion zones, using flue gas recirculation or other carrier 
fluid upstream and/or downstream of the combustion chamber, using fuel 
injection or injection of a mixture of fuel and recirculated flue gases or 
other carrier fluid to provide a secondary combustion zone downstream of 
the primary combustion zone or above the burning combustible material for 
decomposing nitrogen bearing compounds and NO.sub.x, and using secondary 
combustion air or overfire air injection above the secondary combustion 
zone for final burnout of remaining combustibles in a tertiary combustion 
zone. 
It is another object of this invention to remove a significant portion of 
the combustion products, or vitiated air, from above or downstream of the 
burnout zone and mix it with fresh air and/or oxygen for reinjection 
downstream of the secondary combustion zone. 
It is yet another object of this invention to provide a process and 
apparatus for combustion of solid combustible materials where recirculated 
flue gases or another carrier fluid are injected downstream of the primary 
combustion zone, or above the stoker grate, into the secondary combustion 
zone which thus creates turbulent flow for enhanced mixing, nitrogen 
bearing compounds decomposition and NO.sub.x reduction. Decomposition of 
nitrogen bearing compounds and NO.sub.x reduction is further enhanced by 
tangentially injecting fuel, a fuel/recirculated flue gas mixture, a 
fuel/other carrier fluid mixture, recirculated flue gases or other carrier 
fluid above the stoker grate to create multiple swirl zones. Similarly, 
combustible burnout is increased by tangentially injecting oxidant 
downstream of the secondary combustion zone. 
These objects are accomplished in accordance with one embodiment of this 
invention in which combustible material is injected into a plurality of 
walls which define a combustion chamber of a stoker-type furnace having at 
least one drying grate, at least one combustion grate and at least one 
burnout grate. At least one ash pit is located downstream of the burnout 
grate, within the combustion chamber. Integral to the furnace and disposed 
downstream of the stoker grate is a boiler or other heat recovery device 
in which heat in the flue gases is used for generating steam or providing 
thermal energy for some other process. 
At least one combustible material inlet is located in at least one wall of 
the combustion chamber in a position such that the combustible material is 
introduced into the combustion chamber onto the drying grate. At least one 
conduit is in communication with a primary combustion air or undergrate 
air source and a space beneath the grates. Primary combustion air injected 
into the combustion chamber from beneath the grates is used to 1) dry the 
combustible material on the drying grate, 2) combust the dried combustible 
material which has been moved by combustible material advancement means 
from the drying grate to the combustion grate to form a primary combustion 
zone immediately above the combustion grate, and 3) burn out any 
uncombusted material remaining in the ash from the combustion grate which 
has been moved by combustible material advancement means onto the burnout 
grate. Ash from the burnout grate is deposited into the ash pit. Through 
an opening in a wall of the combustion chamber, a fuel and/or a carrier 
fluid such as steam, water, nitrogen or recirculated flue gases from the 
boiler or heat recovery section of the furnace is introduced into the 
combustion chamber directly above the primary combustion zone, forming a 
secondary combustion zone. Oxygen concentrations within this secondary 
combustion zone are maintained below a level which promotes the formation 
of NO.sub.x ; that is, the secondary combustion zone is an oxygen 
deficient zone with respect to nitrogen, including nitrogen in nitrogen 
bearing compounds, in the zone. In this zone, nitrogen bearing compounds 
from the primary combustion zone are decomposed, significantly reducing 
the amount of NO.sub.x produced in the oxygen deficient secondary 
combustion zone. Through still another opening in a wall of the combustion 
chamber, overfire air comprising at least one of vitiated air withdrawn 
from above the burnout grate in the combustion chamber and fresh air is 
introduced into the combustion chamber directly above the oxygen deficient 
secondary combustion zone forming an oxidizing tertiary combustion zone. 
Combustion of carbon monoxide, hydrogen, unburned hydrocarbons and other 
combustibles entering this zone from the oxygen deficient secondary 
combustion zone is completed in this oxidizing tertiary combustion zone. 
Using the process and apparatus of this invention, NO.sub.x in the flue 
gases is reduced by about 50% to about 70%. 
In a preferred embodiment of this invention, fluids injected into the 
oxygen deficient secondary and oxidizing tertiary combustion zones are 
injected through nozzles positioned in a wall of the combustion chamber 
such that the fluids are injected into the combustion chamber tangentially 
with respect to the combustion chamber walls. In yet another preferred 
embodiment of the invention, the fluids are injected tangentially or 
radially into the combustion chamber at an angle with respect to the 
horizontal. 
In one embodiment of this invention, mounted within an opening formed in a 
combustion chamber wall, preferably above the burnout grate, is a fan, 
blower, compressor or other type of air moving or compressing apparatus 
inlet through which vitiated air from above the burnout grate is 
withdrawn, compressed and reinjected through a nozzle into the combustion 
chamber above the oxygen deficient secondary combustion zone, forming an 
oxidizing tertiary combustion zone. In another embodiment of the 
invention, the vitiated air is mixed with fresh air or industrial grade 
oxygen from a nitrogen/oxygen separator and then injected into the 
combustion chamber. In still another embodiment, only fresh air or 
industrial grade oxygen is injected into the combustion chamber above the 
oxygen deficient secondary combustion zone, forming an oxidizing tertiary 
combustion zone. 
The amount of overfire air, that is, vitiated air and/or fresh air or 
industrial grade oxygen, injected into the combustion chamber to form an 
oxidizing tertiary combustion zone is an amount sufficient to provide 
about 3% to about 12% oxygen concentration within the oxidizing tertiary 
combustion zone. 
In one preferred embodiment according to this invention, the average oxygen 
level, relative to fuel and combustible materials in the combustion 
chamber, in the oxygen deficient secondary combustion zone is an amount 
equivalent to about 0.6 to about 1.3 of a stoichiometric requirement for 
complete combustion of said fuel and combustible materials. In another 
preferred embodiment, the oxygen concentration downstream of the overfire 
air inlet is about 3% to about 12%. In yet another preferred embodiment, 
flue gases are recirculated for drying and preheating the combustible 
material. 
In another embodiment of this invention, fuel is injected within the 
combustion chamber, above the stoker grate, to provide an oxygen deficient 
secondary combustion zone for decomposing nitrogen-bearing compounds as 
well as reducing NO.sub.X in the combustion products entering the oxygen 
deficient secondary combustion zone. The fuel, which does not contain 
significant quantities of fuel-bound nitrogen, can be in a solid, liquid 
or gaseous form. A preferred fuel is natural gas. The fuel injected into 
the combustion chamber above the stoker grate represents about 5% to about 
40% of the combustible material heating value. The fuel is injected above 
the stoker grate into the oxygen deficient secondary combustion zone in an 
amount sufficient to maintain an average oxygen level equivalent to about 
0.6 to about 1.3 of a stoichiometric requirement for complete combustion 
of fuel and combustible material in the combustion chamber. In one 
embodiment of this invention, about 5% to about 30% of the flue gases from 
the boiler exhaust are recirculated back into the oxygen deficient 
secondary combustion zone. In another embodiment of this invention, 
another carrier fluid such as steam, water, or industrial grade nitrogen 
in an amount comprising about 1% to about 40% by weight of the total flue 
products from the furnace is injected into the oxygen deficient secondary 
combustion zone. 
Vitiated air is ejected from above the burnout grate portion and injected 
into the combustion chamber, above the oxygen deficient secondary 
combustion zone. In one embodiment of this invention, the ejected vitiated 
air is mixed with fresh air or industrial grade oxygen prior to injection. 
Overfire air is supplied into the combustion chamber through at least one 
overfire air inlet above the oxygen deficient secondary combustion zone 
for thorough mixing and at least partial burnout of combustibles contained 
within the combustible material combustion products in a tertiary 
combustion zone, which is downstream of the oxygen deficient secondary 
combustion zone. In another embodiment according to this invention, 
overfire air representing about 5% to about 50% of a total air supply to 
the combustion chamber is injected above the oxygen deficient secondary 
combustion zone to provide an oxidizing zone. 
In one embodiment of this invention, natural gas, recirculated flue gases, 
or a mixture of natural gas and recirculated flue gases or other carrier 
fluid is injected into the combustion chamber above the stoker grate and 
overfire air is injected downstream thereof. Any of the fluid streams can 
be tangentially or radially injected into the combustion chamber, or can 
be injected into the combustion chamber at an angle with respect to the 
horizontal. 
A furnace or apparatus for combustion of solid combustible materials in 
accordance with the process of this invention includes a plurality of 
walls which define a combustion chamber. In one embodiment of the present 
invention, a stoker grate having at least one drying grate portion, at 
least one combustion grate portion, and at least one burnout grate portion 
is located in a lower portion of the combustion chamber. At least one ash 
pit is located downstream of the burnout grate portion, within the 
combustion chamber. 
At least one solid combustible material inlet is located in at least one 
wall of the combustion chamber, in a position such that the combustible 
material is introduced into the combustion chamber on the drying grate 
portion. At least one conduit is in communication with an undergrate air 
source or a primary combustion air source and a space beneath the stoker 
grate and is used to supply undergrate air through the stoker grate, or 
through another combustion chamber design. 
In one embodiment of this invention, at least one nozzle for injecting 
fuel, a fuel/carrier fluid mixture, or carrier fluid alone is sealably 
secured to at least one wall of and is in communication with an oxygen 
deficient secondary combustion zone within the combustion chamber, above 
the stoker grate. In a preferred embodiment, each of these nozzles is 
positioned such that the fluids are tangentially injected into the 
combustion chamber above the stoker, with respect to the combustion 
chamber walls. At least one overfire air nozzle is used to supply overfire 
air into the combustion chamber above the oxygen deficient secondary 
combustion zone. Each overfire air nozzle is sealably secured to the 
combustion chamber wall in a position such that the overfire air is 
injected into combustion products within the combustion chamber. In yet 
another preferred embodiment, each overfire air nozzle is positioned such 
that overfire air is also tangentially injected, with respect to the 
combustion chamber walls, into the combustion chamber above the oxygen 
deficient secondary combustion zone. Each overfire air nozzle is in 
communication with the combustion chamber. 
In one embodiment, at least one overfire air nozzle for injecting vitiated 
air, a vitiated air/fresh air mixture or a vitiated air/industrial grade 
oxygen mixture is sealably secured to at least one wall of and is in 
communication with the combustion chamber above the oxygen deficient 
secondary combustion zone. In a preferred embodiment, each overfire air 
nozzle is positioned such that a fluid is tangentially or radially 
injected into the combustion chamber above the oxygen deficient secondary 
combustion zone, at an angle with respect to the horizontal. In yet 
another preferred embodiment, the fluid is tangentially injected, with 
respect to the combustion chamber walls, through the overfire air inlet 
into the combustion chamber above the oxygen deficient secondary 
combustion zone. 
These and other objects and features of the invention will be more readily 
understood and appreciated from the description and drawings contained 
herein.

DESCRIPTION OF PREFERRED EMBODIMENTS 
For purposes of this invention, NO.sub.x is oxides of nitrogen or nitrogen 
oxides, such as NO, NO.sub.2, and N.sub.2 O; nitrogen bearing compounds 
are compounds such as HCN and NH.sub.3 that can be oxidized to NO.sub.x, 
in the presence of oxygen. The primary combustion zone is the zone in 
which combustion of the combustible material occurs, primarily in the 
vicinity immediately above the combustion grate. The secondary combustion 
zone is the volume of the combustion chamber downstream of the primary 
combustion zone into which products of combustion from the primary 
combustion zone flow. The tertiary combustion zone is the volume of the 
combustion chamber downstream of the secondary combustion zone into which 
derivative flue products from the secondary combustion zone flow. The term 
"combustible material" as used in this specification and in the claims 
means any suitable material which can be burned. However, without 
intending to limit its scope in any manner, "combustible material" used in 
the process and apparatus of this invention will typically be municipal 
solid waste (MSW), refuse derived fuel (RDF), and/or other comparable 
solid waste. It is conceivable that waste may also have glass, metal, 
paper and/or plastic material removed from the composition, such as in the 
case of RDF, and still be used as combustible material in the furnace of 
this invention. The term "carrier fluid" as used in this specification and 
claims means any fluid suitable for injection into a combustion chamber 
for enhancing mixing and improving temperature and composition uniformity 
within the combustion chamber. Without intending to limit its scope in any 
way, "carrier fluids" typically used in the process and apparatus of this 
invention are flue gases, steam, water, air and industrial grade nitrogen. 
Finally, the term "oxygen deficient" as used throughout this specification 
and in the claims means insufficient oxygen to promote the conversion of 
nitrogen bearing compounds to NO.sub.x. 
The apparatus for combustion of combustible material in accordance with one 
embodiment of this invention, furnace 10, is shown in a diagrammatic 
cross-sectional side view in FIG. 1. A plurality of walls 12 define 
combustion chamber 15. A stoker grate positioned within combustion chamber 
15, preferably in a lower portion thereof, comprises at least one drying 
grate portion 20, at least one combustion grate portion 25, and at least 
one burnout grate portion 30. At least one ash pit outlet 35 is located 
within combustion chamber 15, positioned to receive ash from burnout grate 
portion 30. At least one combustible material inlet means 37 is positioned 
in wall 12 above the grate such that the combustible material enters 
combustion chamber 15 and flows onto drying grate portion 20. The 
combustible material is advanced by combustible material advancement means 
from drying grate portion 20, over combustion grate portion 25, over 
burnout grate portion 30, and into ash pit outlet 35. 
Undergrate air supply means comprises at least one undergrate air conduit 
40 in communication with an undergrate air source and a space beneath at 
least one of drying grate portion 20, combustion grate portion 25, and 
burnout grate portion 30. Undergrate air conduit 40 is used to supply 
undergrate air beneath and then through the grate. An undergrate air 
source and at least one space beneath the stoker are in communication with 
undergrate air conduit 40 and are also used to provide undergrate air 
beneath and the through the grate. Undergrate air is the primary source of 
air for combustion of combustible material in combustion chamber 15. 
Combustion of the combustible material occurs in combustion chamber 15 
primarily in the vicinity immediately above combustion grate portion 25, 
forming a primary combustion zone. 
At least one fuel/carrier fluid nozzle 43 is secured to wall 12 and in 
communication with combustion chamber 15 Each fuel/carrier fluid nozzle 43 
is positioned on wall 12 such that fuel/carrier fluids are injected into 
combustion products within combustion chamber 15. At least one overfire 
air nozzle 45 is sealably secured to wall 12 and in communication with 
combustion chamber 15. Each overfire air nozzle 45 is secured to wall 12 
in such a position that a fluid, preferably vitiated air, is injected into 
combustion chamber 15, above the oxygen deficient secondary combustion 
zone. In a preferred embodiment according to this invention, each overfire 
air nozzle 45 and each fuel/carrier fluid nozzle 43 is either positioned 
or has internal mechanical components known in the art for tangentially or 
radially injecting each respective fluid into combustion chamber 15, above 
the oxygen deficient secondary combustion zone and the stoker grate, 
respectively. It is apparent that internal baffles, internal or external 
nozzles, or the like, can be used to tangentially or radially direct the 
fluid into combustion chamber 15. Thus, fluid swirl which enhances mixing 
can be accomplished in combustion chamber 15 having any type of cross 
section, even a rectangular cross section, as shown in FIG. 3. 
Referring to FIG. 3, overfire air nozzles 45 can be positioned at angles 
relative to wall 12 such that at least one swirl, preferably multiple 
swirls, are formed within combustion chamber 15. It is apparent that the 
fluid can be injected into combustion chamber 15 at an angle with respect 
to the horizontal by positioning secondary air nozzle 45 at an angle with 
respect to the horizontal, as shown in FIG. 2. 
In one embodiment of this invention, exhaust means for exhausting vitiated 
air from above burnout grate portion 30 comprises at least one induced 
draft fan 33 mounted within exhaust opening 32, preferably above burnout 
grate portion 30. Induced draft fan 33 is used to exhaust vitiated air 
from above burnout grate portion 30, within combustion chamber 15. In 
another embodiment of this invention, induced draft fan 33 and a discharge 
nozzle are used to inject vitiated air into combustion chamber 15, 
downstream of the oxygen deficient secondary combustion zone. In a 
preferred embodiment, the vitiated air is mixed with fresh air or 
industrial grade oxygen from a nitrogen/oxygen separator (not shown) 
injected through air inlet means 34 into vitiated air duct 31 and then the 
mixture is injected into combustion chamber 15 through overfire air nozzle 
45, forming an oxidizing tertiary combustion zone downstream of the oxygen 
deficient secondary combustion zone. The temperature of the oxidizing 
tertiary combustion zone preferably is between about 1600.degree. F. and 
about 2400.degree. F. The amount of vitiated air and/or fresh air or 
industrial grade oxygen injected through overfire air nozzle 45 is 
sufficient to provide an oxygen concentration preferably of about 3% to 
about 12% within the oxidizing tertiary combustion zone. 
Exhaust opening 32 can be positioned at any suitable location within wall 
12, above burnout grate portion 30, preferably within the top section of 
wall 12, as shown in FIG. 1. Vitiated air duct 31 is sealably secured to 
wall 12 around exhaust opening 32. It is apparent that fan 33 can be a 
blower, a suction nozzle of a compressor, or any other type of suitable 
air compressing device or blower means. 
In accordance with another embodiment of this invention, each of the 
hydrocarbon fuel, flue gases recirculated from the boiler section of the 
furnace and other carrier fluids is injected independently of each other 
into combustion chamber 15 and mixed therein to form an oxygen deficient 
secondary combustion zone. 
In a process in accordance with this invention, combustible material is 
introduced through combustible material inlet 37 into combustion chamber 
15 and onto drying grate portion 20 of the grate. The combustible material 
is further advanced, preferably by reciprocating motion and gravity over 
combustion grate portion 25 and burnout grate portion 30. Undergrate air 
is supplied beneath and then through drying grate portion 20, combustion 
grate portion 25 and burnout grate portion 30 for drying and combusting 
the combustible material. Ash products are removed from combustion chamber 
15 through ash pit outlet 35 which is located downstream of burnout grate 
portion 30, within combustion chamber 15. Fuel is injected into combustion 
chamber 15 above the stoker grate to form an oxygen deficient secondary 
combustion zone of increased temperature for decomposing nitrogen bearing 
compounds as well as reducing NO.sub.x entering the oxygen deficient 
secondary combustion zone and improving combustible burnout downstream of 
the oxygen deficient secondary combustion zone. The fuel can be in either 
a solid, liquid or gaseous form, preferably containing insignificant 
amounts of fuel-bound nitrogen. In a preferred embodiment, the fuel is 
natural gas. The fuel represents about 5% to about 40% of the combustible 
material heating value. The fuel, either alone or mixed with recirculated 
flue gases and/or other carrier fluids, is injected through at least one 
fuel/carrier fluid nozzle 43, as shown in FIG. 1, to provide an average 
oxygen level equivalent to about 0.6 to about 1.3 of a stoichiometric 
requirement for complete combustion of combustible material and fuel 
within combustion chamber 15, above the stoker grate. Recirculated flue 
gases, representing about 5% to about 30% of the flue gases at the boiler 
exhaust, or other carrier fluid, such as steam, water, air, or industrial 
grade nitrogen in an amount preferably between about 5% and about 25% by 
weight of the total flue products from the furnace may be injected into 
the oxygen deficient secondary combustion zone to enhance mixing and 
improve temperature and gas composition uniformity. 
In one embodiment of this invention, vitiated air is ejected from above 
burnout grate portion 30, mixed with fresh air or industrial grade oxygen 
at fresh air nozzle 34, and injected as overfire air into combustion 
chamber 15 above the oxygen deficient secondary combustion zone. The 
overfire air is preferably injected through at least one overfire air 
nozzle 45 secured to wall 12 and in communication with combustion chamber 
15, above the oxygen deficient secondary combustion zone. 
Overfire air is supplied into combustion chamber 15 through at least one 
overfire air nozzle 45 for thorough mixing and at least partial burnout of 
combustibles contained within the combustible material combustion 
products. In a preferred embodiment of this invention, overfire air is 
tangentially or radially injected, with respect to wall 12, into 
combustion chamber 15, above the oxygen deficient secondary combustion 
zone. In one embodiment of this invention, overfire air providing an 
oxygen concentration of about 3% to about 12% in an oxidizing tertiary 
combustion zone is injected above the oxygen deficient secondary 
combustion zone. 
Residence times, preferably of about 1 to about 4 seconds, for combustion 
products within the oxygen deficient secondary combustion zone must be 
sufficient to permit decomposition of nitrogen bearing compounds and 
reduction of NO.sub.x. The preferred residence time of about 1 to about b 
4 seconds is due to the relatively low temperatures in waste combustors. 
However, it is apparent that the residence time may vary according to the 
specific combustible material, amount of fuel injected and the combustor 
operating temperature. 
In another preferred embodiment according to this invention, the ejected 
vitiated air is mixed with fresh air prior to injection into combustion 
chamber 15, above the oxygen deficient secondary combustion zone. An 
oxygen level, relative to fuel and combustible materials, in the oxygen 
deficient secondary combustion zone in the combustion chamber is an amount 
equivalent to about 0.6 to about 1.3 of a stoichiometric requirement for 
complete combustion of said fuel and combustible materials and the oxygen 
concentration downstream of overfire air nozzle 45 is about 3% to about 
12%. In another embodiment according to this invention, flue gas is 
recirculated for drying and preheating combustible material on the drying 
grate portion 20. 
In still another preferred embodiment according to this invention, natural 
gas, carrier fluids, a natural gas/carrier fluid mixture, and/or overfire 
air, all generally referred to as a fluid, can be tangentially or radially 
injected, with respect to wall 12, into combustion chamber 15, above the 
stoker. In another embodiment according to this invention, the fluid can 
be injected into combustion chamber 15 above the stoker grate, at an angle 
with respect to the horizontal, as shown in FIG. 2. 
This invention uses a combination of low excess air or substoichiometric 
combustion of the combustible material on the stoker grate. Natural gas or 
any other solid, liquid, or gaseous fuel that, preferably, does not 
contain significant fuel-bound nitrogen and/or carrier fluid is injected 
into combustion chamber 15 above the stoker grate into an oxygen deficient 
secondary combustion zone in an amount sufficient to maintain an average 
oxygen level equivalent to about 0.6 and about 1.3 of the stoichiometric 
requirement for complete combustion of fuel and combustible materials 
above the stoker grate resulting in decomposition of nitrogen bearing 
compounds to N.sub.2 and reduction in NO.sub.x formation Overfire air is 
injected above the oxygen deficient secondary combustion zone to provide a 
relatively strong mixing zone which assures high efficiency/low pollutant 
emission combustion within combustion chamber 15, providing low air 
emissions such as CO, THC, PCDD and PCDF. 
While in the foregoing specification this invention has been described in 
relation to certain preferred embodiments thereof, and many details have 
been set forth for purpose of illustration, it will be apparent to those 
skilled in the art that the invention is susceptible to additional 
embodiments and that certain of the details described herein can be varied 
considerably without departing from the basic principles of the invention.