CA1111338A

CA1111338A - Combustion process with reduced nitrogen oxides exhaust

Info

Publication number: CA1111338A
Application number: CA318,435A
Authority: CA
Inventors: Kikuo Tokunaga; Nobuaki Murakami; Minoru Hashiguchi; Yutaka Yamasita; Yasuro Takahashi; Masayasu Sakai
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1977-12-23
Filing date: 1978-12-21
Publication date: 1981-10-27
Also published as: GB2011051B; JPS5486823A; JPS624606B2; NL187768C; NL187768B; DE2855766A1; FR2412786A1; GB2011051A; NL7812304A; DE2855766C2; FR2412786B1

Abstract

ABSTRACT OF THE DISCLOSURE
A combustion process is improved so as to reduce a nitrogen oxides content in an exhaust gas. The improvement exits in that a fuel is incompletely burnt, then into an exhaust gas produced by the incomplete combustion is supplied a gas containing oxygen so that an oxygen concentration of about 0.3%
or less with respect to the amount of said exhaust gas may be obtained, and further a necessary amount of air for completely burning an unburned fuel content is supplied to its downstream to burn said unburned fuel content.

Description

3~8 1 The present invention relates to a combustion process for reducing nitrogen oxides (hereinafter represented by NOx) in an exhaust gas.
As principal ones of the com~ustion processes with reduced NO exhaust in the prior art, the following three kinds of processes have ~een known, and at present these three kinds of processes are employed either solely or in combination to achieve reduction of NOx in a combustion exhuast gas. In the following, these three kinds of processes will be explained, respectively, with reference to Figs. 1 to 3, and one example of the combination process will be explained with reference to Fig. 4.
(1) Over Fire Air Process (OFA Process):

.
In a combustion furnace 1 illustrated in Fig. 1, a fuel is fed through a piping 3 to a burner 4 to be injected into the furnace, and a combustion air A is divided into an air Al supplied through a piping 2 to a burner section 2a and an air A2 supplied to an OFA duct 2b pro~iaed at the downstream of a combustion zone. Owing to such a provision, a main combustion flame C becomes a low-temperature and inactive combustion flame lac~ing in air, and thereby production of NOx can be suppressed low.
Thereafterl an unburned fuel content remaining in a combustio~n exhaust gas due to the want of air is completely burnt in a zone 5 by the air A2 blown into the zone through the OFA ~uct 2b, and exhausted to the outside of the combustion system through a flue 6 as a low public nuisance gas G having a reduced NOx content.

(2) Gas Recirculation Process (GR Process):

In a combustion furnace 1 illustrated in Fig. 2, while ~' ~ .

,3~

1 a fuel is fed through a piping 3 to a burner 4 and is burnt with an air A supplied through a piping 2 to form a combustion flame C, a part Ga of an exhaust gas G flowing through a flue 6 is blown into a furnace bottom la at the upstream of the combustion flame C through a recirculation piping 8 with a recirculation gas fan 7 provided therein. Owing to this blowing o~ a recirculation gas, the recirculation gas consisting of a non-combustible gas diffuses in and mixes with the combustion flame C and suppresses production of NOX by lowering a flame temperature.
lQ (3) Recirculation Gas Mixing Process (GM Process):
In a combustion furnace illustrated in Fig. 3, a fuel fed through a piping 3 is burnt within the furnace with an air A supplied through a piping 2, and a part Ga of the exhaust gas G produced in the furnace is mixed with the combustion air A
in the piping 2 through a recirculation piping 8 with a recirculation gas fan 7 provided therein, A combustion flame C
burning with an air Ag mixed with the recirculation gas is suppressed to a combustion flame temperature lower then that before mixing oi the recirculation gas, and thereby production of N~x is suppressed low.
While this GM process is more effective than the previously described GR process because the former is a process of more positively mixing a recirculation gas, thexe is a fear that instability of combustion may be resulted, and therefore, the mixing proportion of the recirculation gas is limited to 20 ~ 30% of the amount of air.
(4) Combination Process:
In a combustion furnace illustrated in Fig. 4, a fuel fed through a piping 3 is burnt within the furnace forming a combus'ion flame C with a combustion air A supplied through a

3;~3 1 piping 2, and an exhaust gas G in this case is exhausted to the outside of the system through a flue 6. In this case, a part Ga of the exhaust gas G is sucked by a recirculation gas fan 7 to be returned to the interior of the furnace through a recircu-lation piping 8, and the part of gas Ga is further divided into parts Gal and Ga2, the sub-part Gal being supplied to a furnace bottom portion la at the upstream of the combustion stream C, and the sub-part Ga2 is mixed with the combustion air A. This exhaust gas mixed air Ag consisting of the combustion air A
mixed with the combustion exhaust gas Ga2 is further divided into a part Agl supplied to a burner 4 and a part Ag2 (over fire air) blown into the furnace at the downstream of the combustion flame C.
The above-mentioned combination combustion process aims at a multiple NO reduction effect of the OFA, GR and GM
processes, and at present it is deemed as the most advanced NOX
; reduction process.
Representing general effects of the aforementioned processes, they are indicated approximately by the values given in TABLE-l below.
TABLF-l . ~ .
: Kinds of t~ote 1) (Note 1) (Note 2) NOX amount in an NOX Reduction GR Amount GM Amount OFA Amount exhaust gas (ppm) Processes (%~ (%) (~) LNG Com- C Heavy bustion Oil Com-bustion _ NOX Reduction process not u~ O O O 200 250 _ . _. _ GR Process 30 0 0 140 175 Combined use of GR and GM 10 20 0 100 130 Processes . . _ _. . . .
30 Combined Use of GR, GM and 10 20 20 70 100 OFA Processes . ...... _ -3~8 (Note 1~ GR amount and GM amount are represented in ~ with respect to the amount o~ the gas produced by combustion.
(Note 2) O~A amount is represented in % with respect to the amount of combustion air.
More particularly, in LNG combustion, the amount of NOX in an exhaust gas in the case of not using any NOX reduction process is about 200 ppm, but by making use of the above-described various NOX reduction processes the amount of NOX is successively reduced, and in the case of combined use of the GR, GM and O~A processes, it can be reduced down to 7~ ppm;
also in C heavy oil combustion, the amount of NOX in an exhaust gas in the case o~ not using any NOX reduction process is about 250 ppm, but likewise in the case of combined use of the GR, GM and OFA processes, it can be reduced down to 100 ppm.
However, e~en with the above-described processes in the prior art it will be difficult to meet the re~ulation by-laws and ordinances for the NO exhaust that will be made more severein the future, and therefore, it has been required to develop a novel NOX reduction process for greatly improving the NOX reduction effect.
The inventors of this invention conducted a more detailed analysis and experimental researches on the above-described processes in the prior art, and as a result, have obtained the following knowledges:
(I) The reason why NOX can be reduced by employing the GR or the GM process or by employing these processes in combination~ is because the temperature of the combustion flame is lowered and thereby production of NOX is suppressed.
(II) The reason why NOX can be more reduced than the case of (I) above by employing the OFA process in combination 1 with the GR process and/or GM process, is as follows:
That is, in the main combustion section there occurs want of oxygen due to the fact that a part of combustion air is by-passed as over fire air, and as a multiple effect with the aforementioned process (I), N0x is further reduced.
(III) However, due to the want of oxygen, in the combustion exhaust gas are contained, in addition to the unburned fuel content such as C0, CnHm, etc., nitrogen compounds such as HCN, NH3, etc. If over fire air is blown into this combustion exhaust gas, jointly with the unburned fuel content such as C0, CnHm, etc., the nitrogen compounds such as HCN, NH3, etc. are also burnt, resulting in newly produced N0x, and therefore, this process cannot be a fully satisfactory process although there exists an N0x reduction effect as a whole in the combustion system.
For instance, in the case of C heavy oil combustion, although the amount of production of N0x in the main combustion section is an extrememly small amount of about 30 ppm, nitrogen compounds such as HCN, ~H3, etc. are also contained in the combustion exhaust at about 90 ppm, and when over fire air is blown into this combustion exhaust, additional NOX is produced at about 70 ppm by the combustion of the nitrogen compounds, so that the overall effect is limited to the reduction of N0x down to about 100 ppm as indicated in TABLE-l.
Therefore, the inventors of this invention further conducted many experimental researches for the purpose o~
suppressing the additional production of N0x caused by the blowing of the over fire air, and as a result, have completed the present invention on the basis of the discoveries that if a gas having an extremely low oxygen concentration is blown into 3;3~3 1 the combustion ~xhaust gas at 500 ~ 1700~C, preferably at 700 ~ 1700C, and more preferably at 1000 ~ 1700~C in the stage prior to the blowing of the over fire air, the nitrogen compounds such as HCN, NH3, etc. which are more reactive than the unburned fuel content such as CO, CnHm, etc. will instantly react with the oxygen in said blown gas to be decomposed into H2O, CO. CO2, N2, NOX, etc.; that in order to suppress this decomposition into NOX to minimum, it is only necessary to limit ; the oxygen concentration in said blown gas to an extrememy low concentration of the order that is equivalent to the amount of the nitrogen compounds such as HCN, NH3, etc. (normally an oxygen concentration of 0.3% or less with respect to the amount of the combustion exhaust gas); and that in the over fire section, production of NOX is almost not found because there exists no nitrogen compound such as HCN, NH3, etc. which is a ;~ source of production of additional NOX.
In orher words, the essence of the present invention ; exists in a combustion process with reduced nitrogen oxides exhaust, characterized by the steps of incompletely burning a fuel, then supplying a gas containing oxygen into an exhaust gas produced by the incomplete combustion so that an oxygen concentration of about 0. 3~ or less with respect to the amount of said exhaust gas may be obtained, and further supplying a necessary amount of air for completely burning an unburned fuel content to its downstream to burn said unburned fuel content.
The above-mentioned and other features and advantages of the present invention will become more apparent by reference to the following description of its preferred embodi-ments taken in con~unction with the accompanying drawings, in which:

3;3 8 1 Figs. 1 through 4 are diagrammatic views for illustrating the combustion processes with reduced NOX exhaust in the prior art.
Figs. 5 and 6 are diagrammatic views illustrating one preferred mode of embodiment of the present invention, Fig. 7 is a schematic view showing an outline of an apparatus used in the Example-l and Example-2 of the present invention, and Fig. 8 is a diagram showing the results obtained by Experiment (ii) in the Example-3 of the present invention.
One basic preferred embodiment of the process according to the present invention is illustrated in Fig. 5. Referring to this figure, reference numeral 1 designates a combustion furnace in a conventional boiler for electric power generation, steam generation, etc., and into this furnace are supplied main combustion air through a line 2 and a fuel (for example, a fossil fuel such as LNG, heavy oil, coal, etc. or pulp or the like) through a line 3, and the fuel is incompletely burnt. A
gas containing oxygen is supplied ~ia a line 5 into an exhaust gas pxoduced by the incomplete combustion so that an oxygen concentration of about ~.3~ or less with respect to said exhaust gas may be obtained. Thereby nitrogen compounds such as HCN, NH3, etc. contained in said exhaust gas are instantly decomposed into H2O, CO, CO2, N2, etc. while suppressing production of NOX as described above. It is to be noted that as the gas containing oxygen to be supplied through the line 5, air itself could be used (in this case it is only necessary to add a small amount of air so as to attain the above-specified oxygen concentration), also taking into consideration sufficient mixing with the exhaust gas, air diluted with steam or nitrogen 33~3 1 could be used, otherwise a part of the exhaust gas as recircu-lated could be used similarly to the above-described GR process or GM process, or else various oxygen-containing gases could be used.
Thereafter, a necessary amount of air for completely burning an unburned fuel content in said exhaust gas (that is, over fire air) is supplied through a line 4. Since nitrogen compounds such as HCN, NH2, etc. which act as a source oE
production of NOx do not exist in this exhaust gas, only the unburned fuel content is burnt, and production of NOx would not occur.
One preferred embodiment of the process according to the present invention as applied to a combined-process of the above-described GR process, GM process and OFA process is illustrated in the diagram~atic view in Fig. 6.
With reference to Fig. 6, a fuel fed to a combustion furnace 1 through a piping 3 is spray injected into the furnace by means of a burner 4, and is burnt with comhustion air A. On the other hand, a par-t of an exhaust gas G within a flue 6 is sucked into a piping 8 by means of a recirculation exhaust gas fan 7, and is then fed as divided into three parts Gal, Ga2 and Ga3. Among these, the part Gal is blown into a furnace bottom la and lowers the combustion temperature by diffusing into a combustion flame C, resulting in reduction of NOx (GR
process), the part Ga2 is mixed with the combustion air A to dilute an oxygen concentration as converted into an exhaust gas mixed combustion air Ag, and thereby lowers the combustion temperature, resulting in reduction of NOx (GM process). In addition, this exhaust gas mixed combustion air Ag is divided into a part Agl serving as a main combustion air and a part Ag2 3~
1 serving as an over fire air, both being blown into the ~urnace, and in the main combustion section the combustion becomes ineomplete combustion WhiCh produces a reducing atmosphere in which an unburned fuel content is mixed, resulting in reduction of NO (OFA process). Furthermore, the part Ga3 forming the above-specified oxygen-containing gas is blown int~ the exhaust gas produced by the incomplete combustion in the main combustion section at an intermediate section between the main combustion section and the over fire section so that an oxygen concentration of about 0.3~ or less with respect to the exhaust gas may be obtained. Thereby, as described above, the nitrogen compounds such as HCN, NH3, etc. in the exhaust ~as can be decomposed.
Thereafter, a necessary amount of air for completely burning the unburned fuel content in the exhaust gas is supplied by making use of said over fire air Ag2 to complete the combustion, and the exhaust gas G is discharged to the outside of the furnace through the flue 6.
As described above, the process according to the present invention can be applied to a low NO combustion furnace itself according to the incomplete combustion process in the prior art, furthermore since the necessary supplying rate of the oxygen-containing gas is small, an installation eost and a runnin~ cost required for the fan and pipings are very low, and effective NO reduction can be realized. Therefore, the process according to the present invention can be said to be an industrially useful combustion process with reduced NO
exhaust. It is to be noted that in a practical eombustion installation, the process according to the present invention can be effeetively applied even to the case where an unbalaneed eondition between a fuel and air within a furnace is inevitable 33~3 1 and locally there is a reducing atmosphere in which oxygen molecules do not exist despite of the overall air ratio exceeding 1.0, and a considerable amount of nitrogen compounds such as HCN, NH3, etc. is produced although the amount of production of NOX is small As described above, the process according to the present invention can be applied to the ca~e where the combustion in the main combustion section is incomplete combustion regardless of whether the overall air ratio i5 equal to, higher than or lower than 1.
Now practical examples of the combustion process according to the present invention will be described to show the effects and advantages of the invention in more detail.
Example-l:
. ~
- A small type combustion furnace generally shown in Fig. 7 was employed. In Fig. 7, reference numeral 101 designates a main combustion furnace, numeral 104 designates an over fire furnace, numeral 102 designates a supply line of a main combus-tion air,numeral 103 designates a fuel feed line, numeral 105 designates an over fire air supply line, numeral 107 designates zo a stack, numeral 109 designates a flue, numeral 108 designates a fan for sucking a part of an exhaust gas from the flue 109, and numeral 106 designates a line for supplying a part of the exhaust gas sucked by the fan 108 to the proximity of the terminal portion of the main combustion furnace 101 as an oxygen-containing gas.
As a fuel, C heavy oil (N content of 0.2 weight ~) was employed,a C heavy oil feeding rate through the line 102 was chosen to be 79 kg/H (exhaust gas rate of 1000 N m3/H), and air supply rates through the lines 103 and 105 were regulated so that an air ratio in the main combustion furnace of 0.7 and a 33~

1 residual oxygen concentxation in the flue 109 of 1% may be attained. An exhaust gas supply rate through the line 106 was varied over different values as shown in TABLE-2. It is to be noted that the exhaust gas temperature within the furnace 101 at the point of supplying the exhaust gas from the line 106 was 1100C, and the exhaust gas temperature within the furnace 104 at the point of supplying the over fire air from the line 105 was 1070C. Measurement for NOX was conducted in the flue 109.
- The results of measurement were as indicated in T~BLE-2.

Amount of the èxhaust gas added from line 106 _ rrepresented by a volume ratio of the amount~ NO in the of 2 in the added exhaust gas to the amount x of the exhaust gas within the furnace 1. flue 109 (Note) _ _ _ _ . . _. ~ ~
0 ppm (0 ~) 87 ppm 100 ppm (0.01 %) 65 ppm 200 ppm (0.02 ~) 10 ppm 500 ppm (0.05 ~) 20 ppm 1000 ppm (0.1 %) 60 ppm 3000 ppm (0.3 %) 77 ppm 5000 ppm (0.5 %) 80 ppm 1 % 85 ppm ¦ 2 % 86 ppm (Note) Since the residual 2 concentration in the flue 109 is 1%, in case where only 5~ of the exhaust gas in the flue 109 is added through the line 105, then the 2 concentration with respect to the amount of the exhaust gas within the furnace 101 will ~e 1~ x 5/100 = 0.05~ = 500 ppm.
0 ppm corresponds to the case where the fan 108 is stopped.
2~ corresponds to the case where the air supply through the line 105 is not present.

33~3 1 As will be apparent from TABLE-2, hy supplying a gas containing 2 f 3000 ppm (0.3%) with respect to the exhaust gas into the exhaust gas produced by the incomplete combustion, N0x in the exhaust gas was extremely reduced. Also, it was confirmed that in case where the supply of the over fire air throu~h the line 105 is stopped and all the amount of combustion air is supplied through the line 103, the effect of reducing N0x by adding 2 through the line 106 is eliminated.
Example-2:
. .
The same apparatus as the Example-l was employed.
At first, the supply of the over fire air through the line 105 was not effected, the main combustion air through the line 102 was supplied so as to attain an air ratio of 1.03, and com~ustion was performed while feeding pulp (N content of 0.1 weight %) through the line 102 at a rate of 80 kg~ As a result, with regara to 2 and nitrogen compounds within the main combustion furnace 101, the 2 concentration has a considerable deviation, 2 does not exist at the pulp injection section, among the nitrogen compounds N0x does not exist and most of the nitrogen compounds were present in the form of HCN and NH3. In addition, the N0 concentration in the flue 109 was 74 ppm.
Next, when 3~ of the exhaust gas within the flue 109 was supplied into said main combustion furnace 101 through the line 106, N0 in the flue 109 was reduced to 36 ppm;
Example-3:
A small type combustion furnace similar to that shown in Fig. 6 (dimensions of the combustion furnace 1: 2.1 m in wid~h, 2.5 m in depth and 7 m in height) was employed. The operating condition was selected as follows:

3~8 1 Fuel and its feed rate: C heavy oil, 1200 kg/H
Combustion air (A): room temperature about 18500 kg/H

Air ratio in the main 1.1 (including the amount of the combustion section: over fire air) Amount of over fire air (Ag2): 0 ~ 20% (Ag2/Ag x 100~) Amount of recirculation 30% of the exhaust gas (G) in the gas (Ga): flue 6 (about 6000 kg/H) [Ga/(G-Ga~ x 100%]
Among this recirculation gas, the amount blown into the furnace bottom la (Gal) 10~

the amount mixed with the combustion air (A) (Ga2) 20%

the amount blown into the front stage of the over fire section (Ga3) 0 or 15%
the amount of residual 2 of the exhaust gas (G) in the flue 6 about 2%
The arrangement of the burners and nozzles is as follows:
Arrangement of burners: one at each corner at a height of 1.2 m from the furnace bottom, four in total Arxangement of Gal nozzles: at a height of 0.3 m from the furnace bottom Arrangement of Ga3 nozzles: one at each corner at a height of 0.7 m from the position of - the burners, four in total Arrangement of Ag2 nozzles (over fire air nozzles): one at each corner at a height of 0.5 m from the position of the Ga3 nozzle, four in total.

(i) Combustion was effected under the above-described operatiny condition except for that the amount of over fire air (hereinafter represented by O~A) (Ag2) was set at 20%, the 3~ amount of exhaust gas blown into the furnace bottom la (herein-1 after represen~d by BGR) (Gal) was set at 10%, the amount of gas mixed with the comb~stion air (A) (hereinafter represented by GM) (Ga2) was set at 20%, and the amount of exhaust gas blown into the front stage of the over fire section (hereinafter represented by AGR) tGa3) was alternatively set at 0~ ICase-1) and at 15% (Case-2). The exhaust gas temperature at the AGR
blowing position was about 1320C when the AGR was 0~, and about 1160C when the AGR was 15%. The results are shown in TABLE-3 below.

.

. _ ~ _ o Z a) a~ a) a~
_ ~ o o h ~
O O ~ a) O o o o a~ o o O r ( q' o ~ J ~r U~ N ~ ~ ~) Sl ~ ~1 ~I h ~ ~ ~ I` o ~7 o ~' ~ O o~o~o O ~oo u~
o o o o . ~r _ _ ... _ ~ o o o o a~
l ~ ~ O ~ o~
O O O o o O , ~ ~ ~ ~ Q t~) .
C`l ~1 (`1 O O O O U~ rt llr) O N O N
C) 00 ~ Z> ~1 .
_ _ _ ~~ .. __._ ____ o`Po~d~
l C~ d~ ~ ~ ~ ~ ~ o~ ~ ~ ~ ~ ~ d~ ~ ~ ~ ~ P~
1~ ~_ Et ~ ~ XZ
~:
. ~ 0 Z ::C Z ~; Z m Z v :~
~:
U~
~ C0~
0 O ~n ¢ .,, ~ ~ 0 .,, o P~ r:C ~
o ~ ~ ~ ~ ~o.
o h ~ ~ q~ X
_ ~1 _~ ~ ' ~ .~ .
~ ~ u~
~ O ~
.,1 -rl ~ ~ ~ O
1~ ~ ~ U~ O ''I
0 ~1 ~ ~ U ~
h ~ ~ 0 .,1 0 ~ u~ ~n O ~ X 0 0 O t.) ~ 4 33~3 1 (Note) Since the residual amount f 2 f the exhaust gas G in the flue 6 is 2%, in each case where the AGR amount ~is 15%, the 2 amount with respect to the exhaust gas after the main combustion will be 2% x 15/100~ 0.3%.
As will be apparent from TABLE-3, in the case-l, while 20 ^~ 30 ppm o~ NOX, 70 ~ 80 ppm of HCN and 12 ppm of NH3 were contained in the exhaust gas after the main combustion, after OFA, ~0 was increased to 102 ppm, but on the contrary HCN and NH3 had been almost completely decomposed. Also in the case-2, while the exhaust gas after the main combustion had almost the same composition as the case-l, after AGR, HCN and NH3 were abruptly reduced and NOx was increased only by about 10 ppm at most, and further after OFA, HCN and NH3 had been almost completely decomposed and NO was increased by ahout 10 ppm and the total amount of NO was 53 ppm.
(ii) Combustion was effected under the above-described operating condition except for that the amount of OFA (Ag2) was ~aried in the range of O ~ 20%, the amount of BGR (Gal) was set at 10~, the amount of GM (Ga2) was set at 20%, and the amount of AGR (Ga3) 20 was alternatively set at 0~ or at 15~. The results are shown in TABLF- 4 .

~ oo oo l o ? ~ o o ~ ~ o E~ ~ I~ ~ ~ .
_ _._ _ _ cn o o o o u~
l ~ ~ ~ ? ~ ~ o E~ o o o o ~Q _ ~
u~ O O O ~n ,, o u~ u~ O a~
~ E~ _ ~ ~ ~o o .
I_ o o ,~ ~ o o ~; l 11') ~i ~1 r-i ~1 O
~ E~ .
CO ~ O ~D O C~
o . ~ o . ~ o . ~ o E~ o o ~ ~ ~
( oot ~ ~"~oo E~~ r~ o ~r____ Q _____ ~ ~o ~ ~ ~ ~ ~ o E~ ~ ~> ~
_ ~
. ~ o u~ ~ ~ ~
dP l O O O OO O ~ ~ ~ . ~ O
~r O E l ~-1 ~-1t~ ~ a) N ~1 ~ ~ 01 _ _ ~ ZO
1 l cs~ ~1 ~ ~
'¢ ~ E-l LO . t~) o ~ ~ ~ o E~

f3 ~ C~ro ~o E~ o __ - :

oP ~ ~ a~ & ~i Ei _. ~oP1 oPoP Q~ ~ oP Ql ~ d Q-bl ~ O ~ ~ X Z ~ X Z ~1 X Z
~ ~ ~O O O O O O O O
_ ~1 ~) ~ r Z ;~ Z ~ Z X
~ llS 1~) P:; 11;

51 g ~ _ Z ~ o ~ u~ ~1 ¢. ~ o ~ s~ ~ ~ a ~ O ~ a) ~: Q ~ a) ~ ~
~n o ~ m ~.,( ~: o ~ ~ ~ ~4 aJ a~ 4~ ~ O-r~ 4~ ~ ~ O
E~ t~
- - - - -- -t~
~ o ~
.,~ o ~) rl ~ ~) N U~ U~
rl ~ ~ O
a) ~ ~ o C) ~ X ~ O '1 . _ O V .

3~
I From TABLE-4 above, the variation of NOx values in the exhaust gas ~G) as a func-tion of the OFA amount (Ag2) is picked up and the relation is shown in the diagram in Fig. 8. In this figure, broken-line curve 1 indicates the aforementionea relation in the case of AGR amount of 0%, while solid-line curve 2 indicates the same relation in the case of AGR amount of 15~.
From Fig. 8 it is seen that the effect of AGR is large in the region where the OFA amount exceeds 10%, but it is small in the region ôf the OFA amount of 10~ or less.

Also, from TABLE-4 above, the following are obvious:

(a) The total amount of NOX and HCN after the main combustion can be reduced by increasing the OFA amount.
(b) If the OFA amount exceeds 10%, the 2 amount after the main combustion is reduced (that is, the com~ustion in the main combustion section becomes combustion under want of oxygen), so that NOX is abruptly reduced but HCN is increased.
This implies that since an air ratio is chosen as high as 1.1, in order to incompletely burn the fuel in the main combustion section for the purpose of reducing NOX, it is necessary to select a large OFA amount of 10~ or more, and that in this case production of HCN
is increased.' (c) The amount of increase of NOX after OFA is proportional to the amount of HCN after the main combustion.

(d) Most of the HCN after the main combustion is decomposed by AGR.
It is to be noted that the analytic instruments and - the analytic process employed in the gas analysis in Examples-l, -2 and -3 are as follows:

2 2 meter manufactured by NIHGN Glass Company 3~8 1 CO: Non-dispersive type infra-red analyser CnHm: H C meter type EHF 1002 manu~actured by YANAGIMOTO Company NOX: Chemi-Lumi NO meter manufactured by YANAGIMOTO
Company PDS process used in combination HCN: Infra-red spectro-photometer (IR 430) manufactured by SHIMAZU, Ltd.
~et type adsorption process (pyridine-pyrazolon process) NH3: Indophenol process.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A combustion process with reduced nitrogen oxides exhaust, characterized by the steps of incompletely burning a fuel, then supplying a gas containing oxygen into an exhaust gas produced by the incomplete combustion so that an oxygen concentra-tion of about 0.3% or less with respect to the amount of said exhaust gas may be obtained, and subsequently burning the unburned fuel content in the exhaust gas by supplying a necessary amount of air for its complete combustion.