Waste incinerating method and apparatus with counter-current exhaust gas flow

In a method of incinerating waste material in a parallel flow process wherein hot exhaust gases are conducted above a combustion chamber grate in the same direction as the material is moved on the grate, different temperatures zones are maintained to which air is admitted from the bottom through the grate such that a temperature of less than 900.degree. C. is maintained in a first zone which is a drying zone, a temperature of about 1000.degree. C. is maintained in a second zone which is an evaporization and vaporizing zone, a temperature of about 900.degree. C. is maintained to in a third zone, which is the final combustion zone and a temperature less than 900.degree. C. is maintained in a fourth zone which is a sintering zone. Additional air is supplied to the combustion zones 1 through the side walls of the combustion chamber. A dividing wall is disposed above the grate over which the hot combustion gases are conducted to a discharge opening so that heat is transferred to the dividing wall for radiation back onto the waste material on the grate.

BACKGROUND OF THE INVENTION 
The present invention resides in a method of incinerating waste material 
which needs to be treated thermally, such as garbage, on a grate of an 
incineration apparatus wherein, for performing the method in accordance 
with the principle of parallel flow heating, primary air is admitted to 
the grate from below to a combustion chamber. 
Thermal treatment of waste material is important in the framework in the 
concept of integrated waste material economics. However, waste material is 
still burnt with a relatively large amount of excess air. More than 3 
Nm.sup.3 air are required per kg of combustible material with a heat 
content of about 8 MJ/Kg. Actually, 6 Nm.sup.3 have been used until not 
long ago. By this time, the specific air use number has been reduced to 
about 5 Nm.sup.3. 
Cost efficient waste material incineration plants have not been developed 
since, up to this date, no processes have become available which would 
prevent the formation of NO.sub.x at the primary combustion site to such a 
degree that no exhaust gas cleaning equipment is required in the exhaust 
gas flue which does not need any means for reducing the NO.sub.x 
emissions. Although there is a limit value of 200 mg NO.sub.x /Nm.sup.3 
for the combustion of waste materials the public expects the NO.sub.x 
emission values to be less than half that amount. Consequently, the design 
target to be achieved for the primary side NO.sub.x emission reduction is 
100 mg/Nm.sup.3. 
U.S. Pat. No. 3,808,986 discloses an apparatus for the incineration of 
waste material. The purpose and the design for this apparatus are to 
increase the combustion temperatures in order to reduce the amount of the 
components which can normally not be burned. This leads to exhaust gas 
temperatures of much over 1000.degree. C. and consequently to the 
formation of relatively large amounts of NO.sub.x in the exhaust gas. This 
however is not acceptable with the ever stricter exhaust gas quality 
requirements. Other apparatus known in the art which operate in a center- 
and counter current flow fashion generally have high NO.sub.x emission 
values in the range of 200 to over 400 mg/Nm.sup.3. 
Further incineration processes and apparatus using a parallel flow 
principle for the combustion of waste materials are known from DE 42 19 
231 C1 and from Thome-Kozmiensky: Thermische Abfallbehandlung, Berlin, EF 
Verlag fur Energie-und Umwelttechnik, 1994, pages 160 to 163. In this 
process wherein secondary air is supplied to the combustion chamber from 
the top, a temperature profile is generated in the combustion zone above 
the grate which provides for uniformly increasing temperatures of from 
700.degree. C. at the beginning of the grate to 1300.degree. C. at the end 
of the combustion zone ahead of the combustion gas exhaust downstream of 
the combustion chamber area. Such an arrangement however also leads to 
undesirably high NO.sub.x emission values which was not recognized and 
generates a problem that has not been addressed by those designs. 
It is therefore the object of the present invention to provide a method 
with which the NO.sub.x content in the exhaust gas of such incinerators is 
reduced purely by measures to the combustion chambers. The invention is 
based on the understanding that this can be achieved with a temperature 
reduction to below 900.degree. C. at the end of the combustion chamber 
where the exhaust gas leaves the combustion chamber. Even such an object 
is novel. 
SUMMARY OF THE INVENTION 
In a method of incinerating waste material in a parallel flow process 
wherein hot exhaust gases are conducted above a combustion chamber grate 
in the same direction as the material is moved on the grate, different 
temperature zones are maintained on the grate to which air is admitted 
from the bottom through the grate such that a temperature of less than 
900.degree. C. is maintained in a first zone which is a drying zone, a 
temperature of about 1000.degree. C. is maintained in a second zone which 
is an evaporization and vaporization zone, a temperature of about 
900.degree. C. is maintained in a third zone, which is the final 
combustion zone and a temperature of less than 900.degree. C. is 
maintained in a fourth zone which is a sintering zone. Additional air is 
supplied to some of the combustion zones through the side walls of the 
combustion chamber. A dividing wall is disposed above the grate and the 
hot combustion gases are conducted to a discharge opening over the 
dividing wall to which heat is transferred in the process for radiation 
back onto the waste material on the grate. 
With the method according to the invention to thermal NO.sub.x formation at 
the end of the combustion process can be prevented by combustion chamber 
design measures resulting in a reduction of the the exhaust gas 
temperature from zone to zone. Not only is the specific combustion chamber 
air volume reduced, but the combustion gas temperature at the exhaust end 
is substantially below 900.degree. C. whereby NO.sub.x formation is 
relatively low. There is no need for an admission of secondary air for 
after-burning which would increase the exhaust gas temperature. It is 
important that with the method and apparatus according to the invention a 
controlled temperature field or profile is obtained that is generated 
within the combustion chamber. 
The exhaust gas is maintained by special building components within the 
combustion chamber in temperature ranges exactly controlled by the 
addition of supplemental air. The exhaust gas flows in parallel with the 
movement of the solid material on the grate through the zones at the end 
of which it is deflected upwardly and then again flows back over the 
building components in a direction opposite to the gas flow over the 
grate. With the guided flow pattern of the exhaust gases, the combustion 
chamber building components assume the temperature of the exhaust gases 
and act as infrared radiation emitters similar to the way such radiation 
is provided by the hot exhaust gas body present in a counter-current 
combustion process over the drying zone. The combustion conditions in the 
present parallel flow configuration are the same as those in a 
counter-current incinerator without its disadvantages that is without-the 
need for additional NO.sub.x removal measures. With the method and 
apparatus according to the invention, the advantages of both combustion 
processes are combined in an advantageous manner. 
Details of the method will be described below on the basis of the 
accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows schematically an embodiment of an incineration apparatus 
wherein waste is burnt on a grate in a parallel flow combustion principle. 
The primary air is supplied to the grate from the bottom that is from the 
under-flow zones a to d. The hot exhaust gas 2a-2e is conducted, in 
parallel flow with the solid material flow on the grate 1, above the grate 
1 in the combustion direction 6. The exhaust gas is guided by components 
3, 4 which are ceramic plates which may have the same length as the 
combustion zone 5 of the primary area I and which will be described in 
detail later. The combustion occurs in subsequent zones which have 
accurately defined temperatures as will be described in detail later on 
the basis of FIGS. 2 and 3. Subsequently, the hot exhaust gas is deflected 
upwardly around the end portion of the ceramic plate and 4 into the 
secondary area II and, in a countercurrent flow 2d, 2e above the 
components 3, 4. In the apparatus shown in FIG. 1, the counter current 
flow extends over the combustion zone back essentially to the waste 
material entrance area 8 before it is discharged through the exhaust 
opening 12. As a result, the heat of the exhaust gases is transferred to 
the components 3, 4 and is radiated therefrom toward the grate 1 over the 
full length of the components 3, 4, whereby the heat is transferred to the 
material on the grate 1 and, consequently, is utilized. A shorter heat 
transmission length over only a part of the components is possible by 
arranging the exhaust opening at a different location. 
The central element of a waste incineration apparatus in which the method 
is performed is the combustion chamber 10, which consists of the primary 
area I and the secondary area II and is delineated at its top by a heat 
insulating wall 9 as shown in the enlarged view of FIG. 2. The detail 
features shown in FIG. 2 are the same as those shown in FIG. 1. The same 
components are therefore designated by the same reference numerals even if 
they are not specifically given. In the lower part of the primary area I, 
there is the incineration grate 1 above which there is the combustion zone 
5 comprising individual zones as indicated in FIG. 2. The combustion of 
the material supplied through the waste material chute 11 at the beginning 
8 of the grate 1 occurs by parallel flow combustion wherein the material 
to be burnt moves in the combustion direction 6, that is, along with the 
combustion, to an ash discharge end 14. The combustion gases 2 formed in 
the combustion process flow in the direction of the arrows 2a to 2e in the 
secondary area II from the combustion zone 5 to the discharge opening 12 
into the flue 13. In the exemplary apparatus as shown in FIG. 1, the 
discharge opening 12 is disposed--as seen in the direction of the 
combustion 6--about above the beginning of the combustion zone 5 on top of 
the grate 1 in the upper wall 9 of the combustion chamber 10 downstream of 
the secondary area II and leads to the flue 13 disposed thereabove. 
However, the discharge opening 12 may be arranged in the area II further 
toward the front when seen against the direction of combustion. 
Preferably, a heat conductive and heat storing intermediate wall is 
disposed above the grate 1 below the top wall 9 of the combustion chamber 
10. The intermediate wall has about the same length as the combustion zone 
5 and consists of individual ceramic plates 3 and 4 arranged one after the 
other and supported on ledges 15 (FIG. 1) formed on the side walls 17 of 
the combustion chamber 10. The intermediate wall separates the primary 
area I from the secondary area II. The last ceramic plate 4 in the 
direction of combustion 6--is inclined toward the grate 1. The 
intermediate ceramic plates 3, 4 are sealed tightly between the side walls 
17 and the front wall 16 of the combustion chamber 10 and extends in the 
direction of combustion 6,--about up to the end 7 of the grate 1 or the 
combustion zone 5. The lower part of the side walls 17, which limits the 
primary area I or respectively, the combustion zone 5, is designated by 
the reference numeral 18. 
Behind the intermediate wall 3, 4, again as viewed in the direction of 
combustion 6, there is the flow reversal area 2c for reversing the flow 
direction of the exhaust gases 2a, 2b From here the combustion gases flow 
in the opposite direction 2d and 2e over the intermediate wall 3, 4 to the 
discharge opening 12. Above the intermediate wall 3, 4 beginning already 
in the flow reversal area 2c is the secondary area II. The plates 3, 4 
consist for example of an aluminum oxide ceramic material and have, with a 
grate width of 80 cm, a thickness of 25 to 35 mm. They are highly heat 
conductive and reflective to provide good heat transfer through the plates 
from the exhaust gas area 2d, 2e, and, by heat radiation back to the 
combustion zone 5. 
With the method according to the invention, the combustion in the primary 
area I takes place in four subsequent zones A, B, C, and D, which are 
disposed above the respective air supply zones a, b, c and d below the 
grate 1 as shown in FIG. 2. There are three mechanisms by which NO.sub.x 
is formed: 
1. From the nitrogen contained in the waste fuel; generally the waste 
material includes about 1% nitrogen in chemical compounds 
2. NO.sub.x is promptly formed with the nitrogen in the combustion air, 
3. Thermal NO.sub.x is formed as under 2, by nitrogen in the air in the 
exhaust gas flow after leaving the combustion chamber at high temperatures 
and the formation of flames. The method, according to the invention is 
concerned mainly with this type of NO.sub.x formation, that is, the method 
attempts to keep the exhaust gas temperatures in that area low. 
To this end, the temperatures in the various zones A, B, C and D are 
controlled in a quite special way as shown by the graph of FIG. 3: 
In a first zone A of the combustion chamber 10 above the grate 1, that is 
the drying and pyralysis zone of the waste material in the primary area I, 
an average temperature of less that 900.degree. C. is maintained. 
In a second zone B, the degasification and gasification zone of the fuel, a 
controlled average temperature in the range of maximally 1000.degree. C. 
is maintained which is higher than that maintained in the zone A. 
It is important that, afterwards in a third zone C, the burning zone of the 
waste material, a temperature is maintained, which is again lower than the 
temperature maintained in the zone B, that is, it is in the range of 
950.degree. C. down to below (860.degree. C.) whereas then, in a fourth 
zone D, the sinter zone, the temperature is still lower, that is, it is in 
the range from below (860.degree. C.) to (680.degree. C.) 
This means that the temperature generally decreases in the flow direction 
of the gases above the waste material bed. The desired temperature 
profiles are achieved by admitting the combustion air selectively by zones 
from the zones a, b, c, and d below the grate through the grate and 
controlling the combustion air flow volume for the zones A and B in such a 
way that the combustion in the fuel material bed is in an under 
stoichiometric range. Because of an insufficient oxygen supply during 
combustion on the primary side, substantial amounts of CO are generated by 
the material in this area, that is, CO amounts of 100 g/Nm.sup.3 are 
generated. The CO acts as reducing agent with regard to the NO.sub.x 
already formed, whereby elemental nitrogen is generated. In addition, a 
multitude of radical reactions may take place which may have an influence 
on the NO.sub.x reduction. The under-stoichiometric combustion can be 
controlled selectively by increasing waste material or fuel addition or 
the throttling of the combustion air permitted to flow through the 
respective zones. 
Furthermore, additional air is supplied to the combustion chamber 10 
through the side wall or walls, mainly in the zones A and B below the 
installation components 3 and 4, that is, into the combustion chamber 
above the grate 1. This additional air is called enveloping air 20. It has 
a lower or about the same temperature as the combustion chamber 
temperature. This additional air forms on air envelope in the side wall 
area. The enveloping air promotes the gas phase reactions in the zones A 
and B. It is important however, that in the area above the installation 
components 3 and 4 in the secondary area, that is after the fourth zone D, 
no additional secondary air is admitted. 
For an explanation of the addition of the enveloping air 20, FIG. 4 shows, 
in a cross-sectional view, a side wall of the apparatus, the cross-section 
being taken at the level of the combustion chamber 10. The side wall 17 
includes a cooling air channel 19 through which cooling air 22 is 
conducted, pressurized by means of a blower which is not shown, in 
parallel to the combustion direction 6 for cooling the side walls. In a 
partial area 18 (FIGS. 1, 4) adjacent the primary area I and the channel 
19, the side wall 17 is porous so that enveloping air 20 from the channel 
19 can pass into the primary area I. The air passages may be formed by 
porosity, small channels or other passages 21. The partial area 18 of the 
side wall 17 with the porosity or the passages is preferably disposed 
mainly in the area of the zones A and B. 
By providing a predetermined porosity for air and/or by a variation of the 
cooling air pressure, the amount of enveloping air 20 can be controlled as 
desirable. The temperature of the enveloping air depends on how much it is 
heated within the wall. 
The primary area I is delimited at the bottom by the grate 1 and at the top 
by the ceramic plate installations 3 and 4 and toward the sides by the 
lower side walls 18 consisting of a fire brick lining. In the primary area 
I, the side wall 18 is--as already described--air permeable to permit the 
enveloping air 20 to pass. The air permeability can be achieved by a 
predetermined uniform and adjustable air passage rate of the wall itself 
or certain wall parts. This is particularly advantageous for the 
arrangement described above wherein the enveloping air 20 as supplied 
through the cooling passages 22 in the side wall 17. However, the 
enveloping air 20 can be supplied by other sources through particular or 
several openings in the wall. 
FIG. 3B shows graphically the temperature in the various zones and FIG. 3A 
gives certain characteristic values of the combustion process. The values 
are actual values determined during tests in a waste material incineration 
plant. The curves with the round measuring points show the temperatures in 
the primary area I that is in the zones A, B C, and D at the measuring 
points T70 to T75 and the curves with the square measuring points indicate 
the temperatures in the exhaust gas tract at the measuring points 
T105-T107. The solid measuring points show the temperatures without 
addition of the enveloping air 20 and the empty points show the 
temperature values as desirable in accordance with the invention with the 
addition of enveloping air 20. The curves clearly show that the desired 
temperature reduction in the rear zones C and D is being achieved. In this 
connection, volume ratios of enveloping air to primary air of 1/5 to 1/6 
(that is, 14-17% enveloping air participation in the total air supply) 
with the combustion temperatures shown and enveloping air temperatures of 
about 500.degree. C. to 750.degree. C. have been found to be particularly 
advantageous. 
In the zones A, B, C and D of the primary area I all the processes such as 
drying, degasifying, gasifying, sinter reactions and gas phase reactions 
occur above the material bed. In the tests given in FIGS. 3A and 3B the 
primary air addition was controlled in stages from the zones a, b, c, and 
d below the grate. With a fuel material consumption of about 170 kg/h, the 
primary air addition was 100 Nm.sup.3 /h each in zone A and D, and 200 
Nm.sup.3 /h in each of zones B and C. To support the gas phase reaction in 
zones A and B above the material bed on the grid 1, the enveloping air is 
admitted to the combustion chamber along the limiting side walls 18 at the 
rate of 100-120 Nm.sup.3 /h. Because the enveloping air is admitted 
through passages in the hot side walls, the enveloping air is heated to 
the desired temperature of 500.degree. C. to 750.degree. C. by the time it 
enters the primary space I. In this range, the temperature can be adjusted 
by air flow control measures. 
The secondary space II is immediately adjacent the primary space I. As 
already mentioned, no additional combustion air is admitted in the 
secondary space II. For the chemical reaction occurring in this space such 
as the remaining CO conversion, the oxygen supplied with the primary air 
and the enveloping air is sufficient. 
During the tests with the apparatus described with parallel flow that is 
with the installation components 3 and 4 and with the addition of the 
enveloping air 20 from the side wall cooling system, a complete combustion 
with carbon monoxide values of less than 5 mg/Nm.sup.3 were reached. With 
the special air addition in the zones A, B, C and D, the temperature 
distribution in the gas canal can be controlled as desired. 
With the process according to the invention, the desired downstream 
temperatures of 870.degree. C. to 930.degree. C. can be achieved which 
results in an NO.sub.x reduction in the exhaust gas and, in addition, 
provides for a quite complete combustion of the exhaust gases. 
Tests have shown the following results given in FIG. 3A: 
Combustion without enveloping air: about 170 mg/Nm.sup.3 NO.sub.x in the 
exhaust gas flow. 
(temperature curve including solid points) 
Combustion with enveloping air: about 55 mg/Nm.sup.3 NO.sub.x in the 
exhaust gas flow. 
In each case there was a mass flow in m.sub.Br of 171 kg/h and oxygen 
additions of 9.0 or respectively, 10.8%, each without equipment for the 
removal of nitrogen from the exhaust gases. As already mentioned apparatus 
based on the state of the art, which operate with center and counter 
current flows have generally high NO.sub.x emission values in the range of 
200 to above 400 mg/Nm.sup.3. 
It has been shown accordingly with the new process that, simply with 
combustion chamber design measures, the emission limits expected to be 
mandated of substantially below 200 mg/Nm.sup.3 can be more than with the 
method and the apparatus according to the invention without additional 
NO.sub.x removal from the exhaust gas in the exhaust system.