CA1090693A - Method of at least partially burning a hydrocarbon and/or carbonaceous fuel - Google Patents

Abstract

Abstract A method of at least partially burning a hydrocarbon and/or carbonaceous fuel in order to yield hot gaseous products of relatively low pollutant content, in which the fuel is partially burned in a first stage flame under such conditions that the partially combusted fuel has a tempe-rature in the range 800 to 1600°C (e.g., about 1150°C) and is sub-stantially free of smoke and/or carbon, bringing the partially combusted fuel at 800 to 1600 C into contact with a substantially non-volatile catalyst (19) which is active for reducing the amount of NOx in the partially combusted fuel, and then at least partially burning the partially combusted fuel, after contact with the catalyst (19) in a second stage flame to yield the desired low pollutant hot gaseous products. Preferably one or both of the said flames burns in contact with a substantially non-volatile catalyst (25, 23) to reduce still further the NOt content of the final gaseous products. The preferred non-volatile catalysts (19, 25, 23) comprise either a mixture of iron and chromium oxides or cobalt oxide.

Description

~ Method of at least partially burning a Hydrocarbdn and/or Carbonaceous , Fuel The present invention rela~es to a method of at least partially burning a hydrocarbon and/or carbonaceous fuel.
The growing concern about both energy conservation and atmospheric pollution demands that combustion systems be operated with maximum combustion efficiency (with m m imum excess air) and without any emission of pollutants. The two goals, however, are not readily compatible, and indeed have proved difficult to realize. This is because any reduction in excess air to promote combustion efficiency increases smoke, carbon monoxide and unburned hydrocarbons, and often also oxides of nitrogen (NOX) .
The present invention relates to a system and method in which both of the foregoing objectives can be achieved, not just with relatively clean-burning gaseous and light distillate fuels but also with fuel oils. The system and method basically involve staged combustion, as described below, in which improvements have been made in the design of the combustion system, and catalysts used in different stages. Con-ventional staged combustion involved firing of all the fuel in the first stage with a sub-stoichimetric quantity of air and the injection in the second stage of sufficient air to co~plete combustion. Such systems have been claimed to reduce NOX by ca SO~, but they usually render the control of other pollutants, notably carbon, more difficult.

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1090~93 Accordi~3g to the invention, in one aspect, fuel is partly burned in a first stage flame to yield hot substantially carbon (or smoke-)free partly-combusted fuel or first stage product, the latter being contacted with a catalyst which is active for promoting the conversion of nitrogen oxide(s) ir. the partly-combusted fuel or first stage product to nitrogen, the partly-combusted fuel (or first stage product), after contact with the catalyst, then being at least partially burned in a second stage flame to yield hot gaseous products having a low nitrogen oxide (and other pollutants) content. The invention, in another aspect, comprises 10 a method of burning a hydrocarbon and/or carbonaceous fuel comprising the following steps in sequence:
(a) partially burning the fuel in a first stage flame and pro-ducing a substantially carbon-free or smoke free partially com-busted gas phase fuel at a temperature of at least 800 C;
(b) contacting the said partially combusted gaseous phase fuel with a solid, substantially non-volatile catalyst which is active for reducing the amount of nitrogen oxidets) in the partially combusted fuel, the contacting being effected at a temperature of at least 800C; and (c) at least partially burning the partially combusted uel in a second stage flame to yield hot gaseous products of relatively low pollutant content.
The temperature at which the partly-combusted fuel or first-stage product contacts the catalyst may be in the range of from 800 to 1600C, preferably 900 to 1400C, more preferably 1000C to 1300C, a]though the actual temperature will depend upon the fuel and on the amount of oxygen -supplied for combustion in the first stage flame, and on the amount of nitrogen introduced with the oxygen.
In order to reduce still further the nitrogen oxide content of the products, a catalyst to reduce or inhibit the formation of nitrogen oxide may be located within the flame of the first stage and/or within the flame of the second stage. Preferably, a catalyst for reducing or inhibiting N0x formation is located within the flames of both stage.
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'` ~: " ,. , ~' , " ,, 1090~93 The catal~st f(,r the. cr each, stage, is preferably so disposed within the flame as to contact or be contacted by the hottest region of the flame usually a~ betwcen 30~ and 45% of the flame length from its upstream end.
The catalyst employed between the stages, and the catalyst(s) used in the first and/or second stages may be the same or different. They are preferably selected f om substances consisting of or containing the elements or compounds (e.g., the oxides) of chromium, iron, cobalt, nickel, moly~ denum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, stron~ium, neodymium, vanadium, alkali metals or any mixture thereof. A preferred catalyst, for cost-effectiveness, i~
Fe/Cr, although cobalt may alternatively be fa~oured. The total amount of o~ygen supplied in relation to the fuel may be a substoichiometric amolnt so that the hot gaseous products contain combustible material, or a slubstantially stoichiometric amount so that the hot gaseous products substantially comprise a flue gas containing no free oxygen or com-bustible material, or a superstoichiometric amount so that a flue gas containing free excess oxygen is produced.
The part-combustion in the first stage is preferably performed in any way which produces partly combusted fuel (or first stage products) which is free of, or susbtantially free of, carbon or smoke. The second stage (part-) combustion is also preferably performed in a manner which produces substantially no carbon or smoke.
In a preferred method of effecting the first and/or second stage (part-) combustion without the production of smoke and/or carbon, a technique, herein referred to as the "anti-smoke technique", is pre-ferably employed. In the practice of the anti-smoke technique, the periphery of the frame is laterally confined or bounded for at least part of its length by a surrounding combustion chamber which reduces the greatest cross-sectional dimensions of at least a portion of the thus laterally confined part of the flame relative to the cross-sectional dimensions that the flame would otherwise have were it not laterally confined. Preferably the cross-sectional dimensions of the flame are reduced (preferably where the flame cross-sectional dimensions are j .

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~4~ 10~93 greatcst~l and t!~ sion~l reducti~n is prefer2bly in thc range ~f from l to 6.5 cms, more preferably 1.25 to 6.35 cms, and preferably no more than ~0~ of the unconfined cross-sectional plane. The confinement of part of the flame by the combustion chamber causes deflection and reflection~ by the hot walls of the combustion chamber, of reactive flame species that would otherwise be quenched. The recirculation of reactive flar.le spccies back into the flame to promote efficient flame reactions may be further promoted by the provision of one or more baffles of suitable shape, which are contacted by the respective flame.
Preferably such baffles extend unwardly from the wall of the combustion chamber across no more than part of the cross-s~ction of the flame. The anti-smoke technique is more fully described la~er in this specification. -Preferably, the total pressure drop through the combustion chamber is no greater than 25.4 cms of water.
! The second stage combustion may be effected by mixing an oxygen-co~taining gas (such as air) into the hot partly combusted fuel or first-stage products following contact with the N0 ~reducing catalyst between the first and second stage combustion stages. ~-By effecting the part-combustion in the first stage flame as described, the N0 production in the absence of all the catalysts which - might be employed in the practice of this invention is up to about 60%
less than the N0x production by single stage combustion, By employing catalysts active for reducing the N0x content of hot ga8es between the fir6t and second flame stages, and also within the first and second ~lame stages, a reduction in the N0x concentration of over 90~ to an innocuously low concentration may be realized. The foregoing combustion w~y be effected at very high fuel to oxygen ratios with markedly reduced carbon or smoke formation from the first stage flame when the anti-smoke technique i8 employed, and combustion with substantially no excess air may be effected in the second stage flame with substantially no carbon or smoke formation particularly, but not essentially, when the anti-smoke technique is applied to the second stage flame.
When the fuel contains sulfur, operation with no, or very little, exceæs air reduces the formation of S03, most of the sulfur appearing in ,.~
'~j i93 the l`lue ~.~;e~ as 5~)2 wi~h a correspondingly advantageous increase in the acid de~ point temrerature and potential improvements for a~ditional heat recovery.
The invention is llOW described in more detail, and ~ith refere3ce to tbe accompanying drawings, in which:-Figure 1 is a schc1.natic diagrammatic cross-sectional elevation of the principal parts o. an apparatus for effecting combustion or part-combustion of a fuel in accordance with the method of the invention;
Figure lA is a schematic diagram~atic cross-sectional elevation of a variant of the apparatus shown in Figure l;
Figure ~ is a graphical representation of the effect of catalysts on nitrogen oxides formation at different fuel/air ratios for different positions of the catalysts between the ~irst and second stage flames;
Figure 3 is a graphical representaLior. of the effect of a catalyst in the second stage fl~me on the N0x content of the flue gas at differ-ent fuellair ratios;
Figure 4 is a graphical representation of the effect of a catalyst in the first stage flame on the N0x content of the flue gas at different fuel/air ratios;
. Figure:5 is a graphical representation of the effects of different types of burner on Bacharach smoke number at different fuel/air ratios;
Figure 6 is a graphical representation of the effects of different di.ameters of combustion chamber on Bacharach smoke number at different fuel/air ratios;
Figure 7 is a graphical representation of the effects of dirferent combustion chamber lengths on Bacharach smolce number using light fuel oil at different fuel/ai.r ratios;
Figure 8 is a graphical representation like that of figure 7 but using gas oil as fuel;
Figure 9 is a graphical representation of the effects of baffles at different dis~ances from the burner on Bacharach smoke N0 for different fuel/air ratios; and Figure 10 is a graphical representation of the effects of baffles on the composition of the first stage flame products at different fuel/air ratios.

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~' ~ , -6- 1~93 1n Fi~ cs 2, 3 a~d;4, the ordinate shows the N0 concen.ration in parts pe~ mi1 iie& ~pm) by volume corrected to 3% 2 Referrino first to Fi~ure 1, the schematic diagram shows apparatus, generally illdica~ed by reference 10, compri.sing a burner ll in which fuel i8 burned to produ~e a flame (not indicated), a cylindrical re-fractory-lir.cd conbustion chamber 12 which receives the flame produced by the burner at one end, and from ~-hich combustion or part-co~bustion products arc discharged at the other end to a ~lue pipe 13. The com-bus~ion cl-ambe-L is provided with a number of viewing ports 14. Fuel, preferably a commonly-used liquid fuel such as light fuel oil or gas o;l, i8 passed into the b~rner 11 via injector tube 15 and air is pa~sed into an a.~nular air supply ring 16 via conduit 17. The air mixes with atomized oil and the latter burns in the burner 11 to produc~ a flame ~hich passes to the upstream end of the combustion chamber 12. Recir-culation of some of the burning gases takes place around the air ring, the recirculation serving to improve the quality of combustion.
The apparatus 10 depicted is arranged for combustion of the fuel in two stages, the firæt stage combustion being effected under fuel-rich conditions, and the partial combustion flame entering the upstream end 2d of the combustion chamber 12 via an orifice bounded by walls 18 treated BO as to contain ~ substantially non~volatile cataly~t active for reducing carbon and/or smoke. In this instance the catalyst is either a mixture of iron and chromium oxides or cobalt oxide.
The internal diameter of the combustion chamber 12 is chosen to be less than the natural or unconfined maximum diameter of at least a portion of the first stage flame produced by the burner 11 so that reac~ive flame species which promote combustioll reactio~s are deflected and reflected back into the first stage flsme by the hot walls of th~
combustion cha~ber 12 rather than being quenche~, thercby enhancing the efficiency o partial combustion of the fuel in the first stage flame.
The hot substantially carbon-free combustible gases st a temper-a~ure in the range of from 1000 to 1300C, e.g. about 1250Cl producad in the first stage flame, pass down the combustion chamber into contact with an orificed refrac~ory member 19 extending across the chamber 12 ' ~ -- - ~' . , , ~ . . - ' , `

_7_ 1090~3 and whi~h is imp.~in7~t~ ;witil, and coa~ed with, substan~ially non-voia~iie ca.cll~sts ~ilicll are active for reducing the content of NO in the gases. In ~his instance, thc catalyst is either a mixture of iron ~nd chromium oxidcs or of cobalt oxide impregnated in and suppor~ed on a silica (approximatcly 33~)~ alumina (approximately 65~) refractory material. The member 19 has a large number of orifices 20 therethrough to minimize the pressure drop. The gases after passagc through .be orifices 20 in the member 19 are mixed with air furnished from an air injection head 21 which is connectcd to a source of a;r by a conduit 22 and ignites to form a second stage flame (no~ shown~ which may burn with insufficient, sufficient or an excess of air for complete combustion of the combustible gases. The diameter of the combustion chamber 12 is chosen to be narrower than the natural or unconfined diameter of the second stage flame so as to promote efficient (part-) combustion with substantially no carbon or smoke formation even at low air/gas ratios.
The second stage flame contacts a refractory member 23 having a large number of perforations 24 therethrough which member is impregnated and coa~ed with a substantially non-volati]e catalyst ~hich is active or reduc;ng the Nx content of the flame gases and/or for inhibiting the formation of NOx. The catalyst may be, for example, either a mixture of iron and chromium oxides or cobalt oxide.
The resulting hot gases, which may be reducing, neutral or oxidizing, are of low carbon or smo~e content and of low NOx content.
The advantageous effects caused by collfining at least part of each of the first ~nd second stage fl~mes by the combustion chamber so as to reduce t:heir diameters may be enhanced by providing one or more baffles which iS or are contac~ed by one or both of ~he flames. Such baffle(s) promote recirculation of reactive species in the flame(s) and thereby enhance the efficiency of the flame reactions and thereby reduce the formation of NOX and also reduce the formation of smoke andlor carbon.
The or each baffle may take any form provided the pressure drop caused thereby ;s acceptably low. One such baffle 25 in the first stage flame is in the form of an annulus with a central hole. The baffle 25 is most effective for enhancing the flame reactions when it is disposed less than half way down thé first stage flame ftom i~s upstream end at the exit from the burner 11~ and more preferably ~hen i~ is disposed about 33Z of the first stage flame leng~h from the burner exit.

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' ~090~i93 Refcr~ c~ is now m~de to Fi~ure lA in whcih all parts sre the same as in Figure 1 ~ h the following exceptions:
(l) the fir*t stage flame con-acts two refractory baffles 25a and 25b of annu]ar shape and which are separated by a distance greater than thc (lGcal) internal diamcter of the combustion chamber 1~.
Prefera~y ~he upstream baffle 25a is located at a distance in the range of from 25~ ~o 33% of the length of the first stage flame from the upstream end of the flame at its exit from the bùrner 11 and en~rance into the combustion chamber 12, and the downstream baffle 25b is located at a distance in the range of from 50~ to 672 of the length of the first stage flame from its upstream end.

(2) the second stage flame cortacts an ellipsoidal body 23a com-prised of refrac~ory material (e.g. alumina-silica) impregnated and ¦ co~ted with a substantially non-volatile catalyst which is active for reducing the NO content of the flame gases and/or for inhibit-¦ ing the formation or N0x. The catalyst may comprise any one or more of the metals hereinbefore stated to be useful for the fore- .
going purposes, but is preferably either a mixture of iron and chromium oxides or cobalt oxide for cost-effectiveness. The ellipsoidal body 23a is aligned with its long 8XiS on thè axis of the combustion chamber and supported by refractory coated radial metal wires 26 preferably with at least a major portion of the body - 23a less than half-way down the second stage flame from its up-stream end (which commences at the air outlets of the air injection head 21).
In the embodiments of both figures 1 and 2, the baffle(s) 25 or 25a and/or 25b may comprise a susbtantially non-volatile catalyst active for reducing and/or inhibiting the formation of N0x in the flame.
The invention and expedients which may optionally be employed 30- therewith (as described with reference to Figure 1) is now described in relation to various desiderata and factors influencing them.
Control of nitrogen oxides (~x) ~
~0 are reduced most effectively when catalysts are used in the interstage zone. Less, but significant further reductions are achieved by placing catalysts within the first and second stage flsmes.

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.' ' ' 9 1090~93 The d~t~ L~ r~ ot~ for ~ syste~ where ei~her light ~u~l oil or ~e~ o;l, do~ d ~ h ~ridinc ~o increase the N0 forming po~ent;al to the l:ighes~ likely ~.o be ~ncotmtered from any petroleu~ fuel, was fired with a ~urner of the type which recirculates a part of the burnt gas into the fresh char~e. The combustion chamber ~as lined with silica/
alumina refr~ctory. It was 229 cms (90 in) long and 20.3 cms (8 ir..) internal diame~r.
Catalyst in interstage zone:-The effect of an iron/chromium ox;de cdtalyst, coated on a per-forated alumin~/silica refractory block (7.. 6 cms (3 in.) thick with numerous 1.25 cms (0.5 in..) diameter cbannels across it), when placed at different positions in the interstage zone, is shown in Figure 2 .~nd Table 1. The c~talyst markedly reduced N0 . Its effect increased the further downstream it was placed, and also as the proportion of fuel-to-air was increased. Table 1 also shows that cobalt o~ide, while an effective catalyst, is a less effective catalyst than iron/chrGmiu~
oxide.
TA~LE 1 .
Influence of inters~age catalyst at different d;stances from the primary burnerS and-at different air consumption rates - (Fuel: Li.ght fuel oil Firing rage, 11.35 ljh (2.5 gall/h) .
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Position of Air N0 catalyst from Actual/ Reduction Catalyst the primary burner. .. Stoichiometric...... ~.
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Fe/Cr 167.6 cms (66 ins.) 0.75 62 0.~0 41 0.85 25 132.3 cms (52 ins.) 0.75 - 53 0.~0 34 - 0.~5 21 101.5 cms (40 ins.) 0.7S 44 0.80 30 . ......... .. :. ............... 0.85. . .. . .. 1~
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Co132.3 cms (52 ins.) 0.75 45 0.80 30 . ... . . . . . ............. O~S .. . .. . . 1~
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' 10~ 93 Cat~ st il~ t; s~(ond st~ge flame:
An iron/chromil)m v~ide tr~a~cd refractQry block similar to that in the iDterstage zone, ~ut only half its thickness 3.81 cms, (1.5 in.), w~s placed 7.6 cms (3 ins.) downstream of the air injector. As shown in Figure 3, this brou~ht about a fur~her redtution in N0 . For instance, with the primary air 73% of the stoichiometric and the overall air input stoichiom2tric, the second stagc catalyst reduced N0 from 140 ppm to 120 ppm - a 147 reduction.
A different design of catalyst suyport in the form of an elong-ated el]ipsoid wi~h ca 15-25 cms (6 ins.) and ;.08 cms ~2 ins.) axes, supported ca 5.08 cms (2 ins.) ~owr~stream fro~ the burner (air injector 21 of Fig. 1) alou~ the central axis - produced virtually the same effect as tlle perforated block catalyst.
Catalyst in the first stage flame:
The use of iron/chromium catalyst in the first s~age flame fired by li~ht fuel oil at a rate of 11.35 litres per hour (2.5 Imperial gallons per hour), in a manner similar to that in the second stage flame, substantially reduced N0 (Figure 4). For instance, whilst the inter-stage catalyst, with primary air 74~ of stoichiometric, reduced N0 from 20 - 275 ppm to 115 ppm (58% reduction) the additional first stage catalyst reduced N0 to 70 ppm (757 reduction). At this primary air level, second stage combustion with a catalyst therein, produced 100 ppm of ~0 at an overall stoichiometry of 1Ø A conventional combustion system, with the high nltrogen content fuel used, ~ould generate well over 1000 ppm of N0.
Control of sulphur trioxide:
Fonmation of sulphur trioxide from any sulphur impurity in the flame is highly undesirable because of its corrosive nature. Combustion according to the invention greatly reduces S03 formation. For instance, whereas in a conventional system, operating at 2.17 excess-oxygen, light fuel oil ~i~h 2.57 sulphur content gave rise to 44 pp~ of S03; the mod;fied combustion system, without staging or the use of catalysts, produced 34 ppm of sulphur trioxide (23Z reduc~ion) at the same stoichio-metry. Staging reduced~sulphur trioxide from ~4 ppm to 26 ppm. Thc use '~

lOgO~;93 ~of t1~e ineors~s~ ~atalys~ rcduced i1 to ~O ppm, and that of the firs~
and ~ecor~i s~ ta;~s:s to 18 ppm. S,nce, unli~e the conventional syste~n, stap,ed combustion can be operated smoke-free under stoichio-metric conditivns, this further greatly reduces su~phur trioxide to only 5 ppm - that is ca ~O% overall reduction.
Control of smoke, carbon monoxide, hydrogen and unburned hydrocarbons in the second stage:
For comple~e combustioll of smoke, carbon ~onoxide, hydro~cn and unburned hydrocarbons in t-he second stage, withcut any excess air, and in a limited space~ tborough mixino of the products of the first stage with the added a;r should be promoted. The necessary expedients for such mixing are cs~entially the same as those employed for the first stage. Vsing the same relatively narro~ combustion chamber with baff'es in the same manrler as in the first stage, satisfactory combustion was succ/essfully effected with stoichiometric air on an overall basis.
Smo~e emission in the flue gas from the second stage was not greater than 3 Bacharach Number - that is, no smoke was visible in the stack gas. Carbon monoxide, hydrogen and unburned hydrocarbons were absent.
And, as previously stated, both NOX and sulphur trioxide had been reduced by about 9OX.
The foregoing description is given to illustrate the invention and is not limitative of its 5cope.
Reducing Carbon and/or Smoke Forma~ion (Using the "anti-smoke ~echnique"):
In ~he following descript;on (and related Figures 5 to lO of the drawings) there is now described a combustion technique which may be employed with the method of the invention in order to reduce still further the amount of pollutant in the gas products of part o~ full combustion and to improve the efficiency of utili7ation of the fuel. As discloæed above, the combustion technique comprises confining a portion of the first stage flame and~or the second stage flame by means of a laterally bounding combustion ch2mber so that cross-sectional dimensions - of the flame(s) are reduced relative to the cross-sectional dimensions of the flame(s), in the same cross-sectional planes~when the flame~s) are not laterally bo~nded or confined.

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., ' ' It is preferred ~hat the ~reate~t natural or unconfined cross-section~] dim~n~ion~ .g. dian~eter) of the flame be reduced by con-tainment in the combustion ch.lmber by from 1.25 cms or thereabcuts to 6.3 cms or thereabouts, more preferably from 1.8 cms or thereabouts to 5.1 cms or thereabouts. In many cases, the reduction of the flame's greatest cross-sectional dimensions may suitably be from c.2.5 cms (c.l.0 inch) or thereabouts to c.3.8 cms (c.1.5 inches) or thereabcuts.
It is preferrcd that the cross-sectional dimension should not be reduced by ~.ore than 25%.
At least 50~ of the natural length of the flame is preferably contained or confined by the combustion chamber Rnd more prefPrably 60%
or more (e.g. 70~). Better improvements in combustion may be realized when the combustion chamber laterally confines the upstream part (towards the burner) rather than the downstream part of the flame. The com-bustion chamber may confine the first stage flame starting from a position either at the exit from the burner or spaced d~wnstream there-from. It may be convenient to attach the combustion chamber to the burner or burner support.
The combustion ch~mber may contain at least one internal fixed baffle for;promoting recirc~lation of reactive species in the flame.
The, or each, baffle may have any convenient form such as a refractory r;ng or annulus (in the case of cylindrical combustion chambers) as exemplified by baffle 25, ~'igure 1, extending inwardly from the peri-phery at the internal wall.
The baffle(s) should cause the s~alles~: pressure drop which is economically acceptable for the realized improvement in combustion. For most cases, a pressure drop of up to 5 cms (c.2.0 inches) water will be acceptable. With one baffle, the pressure drop will usually tend to be about 2.5 cms (c.l.0 inch) water in most cases.
When there are two or more baffles, they should be separated by a distance equal to at least the cross-sectional dimension ~e.g. diameter or e~uivalent) of the co~bustion chamber.

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lU90~93 The location o, .hc bafflc(s) in the fla ~ ~ends to influence the improvement in combustion. With one baffle, th~ baffle should be located preferably less ~han half-way down the ~.~tal length of the flame from the burner, e.g. about 33~ of the total flame length from the burner.
h~en two baffles are employed, it is prefemred that the upstream baffle is locfl~c:d from ~5% to 33~ of the length ~f the flame downstream of the base of the flame, e.g. at the burner i~ ~he case of the first stage flame, and the downstream baffle from 5~ ~o 67~ of the flame length from the flame base.
With three baffles, the location of the up~tream and middle baffles is preferably in the same range as for two baff~es, the downstream baffle being located within the flame at any di's~tance downstream of the middle baffle but separated therefrom by a dis~ ~ ce no smaller than the internal diameter (or its equivalent) of the conlbustion chamber.
While the combustion chamber tends to incre~se the length of the flame, each baffle reduces the flame length so ~hat a shorter combustion chamber can be used to effect the same improven~nt in combustion. One baffle alone can reduce the flame length by up ~D 25~, e.g. lS to 20~, while three. baffles can reduce the flame leng~llby up to 50%.
The technique is particularly useful in redHucing the amount of air (or other oxygen-containing gas) required to el~Doninate, or reduce to an acceptable level, smoke or carbon, so that it i'~ possible to burn relative-ly heavy fuel oils and solid fuels substoichicn~rically to produce hot, clean reducing gas relatively efficiently. The 1~echnique may also be employed with lighter fuels, e.g. naphthas up ~D liquefied petroleum gas containing more than 2~ of butane~snd may also ~e employed in the com-bustion of any of the foregoing fuels to produc~e a substantially smoke-and carbon-free hot neutral (i.e. neither oxidiz~ing nor reducing) gas useful in processes requiring inert gas blanke~ing and in power gener-ation. With regard to the latter, the absence ~f excess air tends to reduce the formation of S03 from sulpnur in t~ fuel whereby greater heat recovery may be effected without the risk ~ sulphuric acid cor-rosion. Moreover, the production of nitrogen 09~ des also tends to be reduced in the substantial absence of excess a~.
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The techn;~ue ~i]~ now be des~ribed ir connection with tbe pro-ducticn of hot, clean, reducing gases.
Hot reducing .~m~spherefi are extensively generated for heat treat-ment of metals. Their far wider use in uture has been forecast for injection into the bosh ~one of blast furnaces and eventually for the production of raw steel by direct reduction of iron ore.
Currently reducing atmospheres are generated mostly by partial combustion of gaseous fuels - natural gas, to~n gas, propane/butane - in the presence of a catalyst. The operation requires a careful control of fuel and catalyst quality and maintenance of the optimum operating conditions to ensure p-;evention of carbon formation and deposition on the catalyst.
Much work has already been carried out on improved designs for minimizing carbon formation. What has now been discovered is that a far greater reduction in carbon-forming tendency can be achieved by careful design of the combustion chamber. With a suitably designed combustion chamber, in conjunction with a suitable burner, a clean, highly reducing atmosphere can be generated from, e.g., liquid fuels of wide composit-ional range, without the aid of a catalyst.
In the past, improvements in combustion havë arisen from burner modifications which have been chiefly based on promoting better mixing ~f the fuel and air feed and/or on the injection into the fuel-air feed of water/steam~prod~lcts of combustion. In contrast, it has been dis-covered that combustion chamber modifications that increase mixing and recirculation of the flame reactants and products in the flame itself can greatly reduce carbon formation. The combustion chamber design parameters which promotc mixing and recirculation have been systemat-ically investigated, and it has been found that these entail:
- optimum choice of combustion chamber diameter (or equivalent cross-sectional dimension) - optimum choice of chamber length - provision of suitable baffles in the chamber ~' ' ' . . .' ', ' ' " ` , ~O90~i93 Thesc featurcs are ~asy to incorporate and unlike burner modifi-ca~ions, do i.o~ requi~e any complex and expensive subsidiary control devices.
The effcct of combustion chamber modifications was investigated with a burner which rccircula~es a part of the combustion products into the fuel-air feed described with refcrence to Figure 1. This burner produces considerably less carbon than a typical medi~ pressure air atomizing burller. Comparative results with this burner firing into a refractory lined combustion chamber of conventional size (61 cms (34 inches) diameter and 127 cms (50 inches) long) are shown in Figure 5, wherein curv~ A shows the performance of the medium pressure air atomi-zing burner, and curve B the performance of the exhaust gas recircul-ation burner. ~oth bunlers burned light fuel oil at a rate of 9.09 l/h (2 gallons per hour).
The influence of different parameters on the efficiency of com-bustion was investigated, as described below.
Influence of Combustion Chamber Diameter:
Using the exhaust gas recirculation burner described with reference to Figure 1, i~s smoke emission performance in the 61 cms internal diameter combustion chamber was compared with that in a 20.3 cms (8 ins.) i~ternal dia~eter chamber of the same length. The narrower dia-meter chosen was about 2.5 cms (1 in.) smaller than the flame diameter at its widest. The results ~Figure 6) show the markedly less carbon forming tendency in the narrower chamber (curve B) compared with the carbon forming tendency in the wider chamber (curve A). The results were obtained with the same light fuel oil and fuel feed rate (9.09 litres/hour) as used in the tests of Figure 5.
Studies were next carried out in a still narrower (12.7 cms (5 ins.) internal diameter) chamber. But now the flame len~th has become far too long to effect complete combustion within the 127 cms (50 ins.) long chamber.

- 1090~93 Influenc. o' (;oin'austion Chamber Leng~h:
l~ith the 20.3 cms (8 ins.) internal diameter chamber, the influence of incre~sir.g the length of the combustion chamber from 127 cms (50 ins.) to 190 cms (75 ins.) and then 229 cms (90 ins.) is shown in Figure 7 for light fuel oil and in ~igure 8 for gas oil,at a fuel burning rate of about 11.37 litres/hour (2.5 gallons/hour). In Figure 7, curve A
shows the perfor~ance using the 127 cms long combustion chamber, curve B
that of the 19~ cms long cha~ber and curve C the performance of the 229 cms long chamber. It is seen that an increase in length reduces the carbon-forming tendency, although the effect is less marked than that of reducing the diameter.
Influence of Baffles:
In the 20.3 cms (8 ins.) diameter 127 cms (5~ ins.) long combustion chamber, three anmllar refractory baffles each like baffle 25 of Figure 1, each with 6.25 cms hole in the centre, were spaced in the combustion chamber at 40.6 cms (16 ins.), 76.2 cms (30 ins.) and 122 cms (48 ins.) from the burner. The burner consumed 11.37 litres/hour (2.5 gallons/hour) of gas oil. These baffles, as shown by curve B of Figure 9, consider-ably reduced carbon formation rela~ive to the non-baffle~chamber of curve A. The effect, in fact, is greater than that of increasing the length of the flame~ by confinement in a narrow combustion chamber and --thus provides an irlexpensive way of reducing carbon forming tendency even with short combustion chambers.
In general, therefore, carbon formation can be ~arkedly redu.ed by reducing the chamber diameter (or equivalent cross-sectional dimension), increasing the chamber length and by providing baffles in the chamber.
Of the three factors, the effect of diameter is most marked. However, care has to be taken that the diameter is not reduced excessively -preferably by not more than ~.5 to 5.1 cms (one-2~ inches), or by more than 25~ of that of the unrestricted~unconfinéd flame, since otherwise the resulting excessive inhibition of flame reactions and aerodynamic factors can necessitate the use of an impracticably long combustion chambe~ for completion of the flame processes. For a given length constraint, carbon formation can be considerahly reduced by the inser-; tion of baffles in the cha~ber. Again, care is necessary that these are ,.;,~, , ~'' ' .
. ` ~
. .

1090~;93 not ?laced sa sl~scly as to inhibit the combustiol- processes to any si~nificarit exten~-. In the system described he~e by way of examp]e, the posi~ion and size of baff]es and th~ diameter of the chamber are par-ticularly suiLed for maintaining a stable flame even with barely 50% of the stoichiomctric air. In conventional systems stable flames are usual]y hard ~o maintain with air less than 70~ of the stoichiometric.
Successfu~ maintenance of flame process with air barely 50~ of the stoichiome~ric amount produces a highly reducing atmosphere. The amounts of C0 and ~12 that are formed, along with C02~are shown in Figure 10 for gas oil fired at a rate of 11.37 litres¦hour ~2.5 Imperial gallons/
hour) into the 20.3 cms (8 ins.) diameter, 127 cms (50 ins.) long chamber fitted with 3 baffles (curve B~ compared with the inferior reducing gas quality produced in the absence of baffles (curve A).
For some applications it is desirable to generate even greater amounts of C0 and H2 and correspondingly less of C02 and H20. This can be readily achieved by passing the product gas through a bed of incandescent coal in accordance with well known practice.
In the practice of the invention, the pressure drop caused by the combustion chamber, interstage catalyst (and its support), second stage air iniector, together with any baffles to be contacted by one or both stage flames should be sufficiently low not to necessitate any signifi-cant modification to the burner supplying the flame to the combustion chamber. Preferably, the combustion chamber should not cause a pressure drop greater than 25.5 cms H20. More preferably, the pressure drop should be less than 12.25 cms water, and may be in the range of from 2 to 10 cm~ water, e.g. about 6 cms water.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of at least partially burning a hydrocarbon and/or carbonaceous fuel comprising the following steps in sequence:
(a) partially burning the fuel in a first stage flame and pro-ducing a substantially carbon-free or smoke-free partially com-busted gaseous phase fuel at a temperature of at least 800°C;
(b) contacting the said partially combusted fuel with a solid, substantially non-volatile catalyst which is active for reducing the amount of nitrogen oxide(s) in the partially combusted fuel, the contacting being effected at a temperature of at least 800°C;
and (c) at least partially burning the partially combusted fuel in a second stage flame to yield hot gaseous products of relatively low pollutant content.

2. A method according to claim 1 comprising contacting at least one of the first and second stage flames with a respective solid, substantially non-volatile catalyst which is active for reducing or inhibiting the formation of nitrogen oxide(s) in the flame(s).

3. A method according to claim 2 in which the hottest region(s) of the flame(s) contacts the said catalyst.

4. A method according to claim 2 or claim 3 in which the or each catalyst contacts the respective flame at a region from 30% to 45% of the length of the flame from its upstream end.

5. A method according to claim 1, 2, or 3 in which the or each catalyst comprises a metal selected from iron, chromium, cobalt and a mixture of at least tow of the foregoing.

6. A method according to Claim 1 in which at least one of the first stage flame and the second stage flame is laterally confined for at least part of its length by a surrounding combustion chamber which reduces the cross-sectional dimensions of at least a portion of the laterally confined part of the flames relative to the cross-sectional dimensions of the flame(s) when not laterally confined.

7. A method according to claim 6 in which the cross-sectional dimensions of the said portion of the laterally confined part of the flame(s) are reduced by not more than 20% relative to the cross-sectional dimensions of the flame(s) when not laterally confined.

8. A method according to claim 6 or claim 6 in which the cross-sectional dimensions of the said portion of the laterally confined part of the flame(s) are reduced by an amount in the range of from 1.00 to 6.50 cms.

9. A method according to claim 1 in which at least one of the first stage flame and the second stage flame contacts at least one baffle disposed for promoting recirculating or reactive species in the flame(s) contacting the said baffle(s).

10. A method according to claim 9 in which the or each respective flame contacts a baffle at a distance less than half way down the length of the flame from its upstream end.