Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
1999-08-24
2002-06-25
Yeung, James C. (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S010000, C431S158000, C431S174000, C431S353000
Reexamination Certificate
active
06409499
ABSTRACT:
This invention relates to a method of combustion, in particular to combustion methods which minimize the production of nitrogen oxides (NO
x
), and to apparatus therefor.
There has long been a drive to reduce or minimize the emission of pollutants such as carbon monoxide (CO), NO
x
, volatile organic compounds (VOC), dioxins, soot and so on, caused during the combustion of fuels. In recent years environmental legislation has been a consistent impetus, although of late the reduction of harmful emissions has been linked to increased combustion efficiency.
Commercial combustion systems have concentrated on three main methods in order to control emissions: the reduction of in-flame pollutant formation; the promotion of in-flame pollutant destruction, and external pollutant destruction. Each of these methods has both virtues and disadvantages.
Various methods (all of which rely on the kinetic control of the formation of a particular pollutant) have been used to limit the in-flame generation of species such as NO
x
, CO, various aromatic and polycyclic hydrocarbons. One particularly well-known practice for reducing NO
x
emissions is “staging”, a method whereby either the fuel or the oxidant is delivered in stages into different parts of the combustion zone, or flame. With staging, the combustion zone is divided into separate regions (usually two) with differing local stoichiometries, at which the rate of formation of certain pollutants (NOX in particular) is very small.
Although effective in reducing NO
x
emissions, staging has its limitations due to poor flame stability and mixing, therefore poor combustion efficiency and temperature distribution, and restricted turndown capability. Accordingly, most staging processes are only a compromise between adequate mixing and emission reduction.
In-flame pollutant destruction is not very common, due to poor understanding of the chemical reactions involving in-flame species and their reversibility. One method employing in-flame destruction techniques involves soot formation and destruction. Here, the formation of soot in the flame is promoted, primarily so as to increase radiative heat transfer but also in order to reduce the formation of other pollutants, the soot being destroyed before it can leave the flame by oxidizing agents such as oxygen or OH radicals. There have also been limited trials to demonstrate the in-flame destruction of species such as NOX by injecting active chemicals such as urea into the flame. Overall, however, in-flame pollutant destruction is not practiced widely in commercial combustion.
External pollutant destruction is fairly widely adopted, because of its relative simplicity and its independence of the particular combustion system used. Generally, active chemicals are introduced into the hot waste gases before or after they exit the combustion chamber or the furnace. Methods used include ammonia injection for NO
x
destruction, external “reburning” using hydrocarbons such as methane, and oxygen injection for oxidation of CO and VOC. The disadvantages of such methods are that they are costly and inefficient—because by their very nature they are reactive, since even the most modern systems rely on a waste gas analyzer to control the amount and rate of chemical injection depending on the sensed pollutant concentration.
The present invention therefore provides a method of combustion characterized by the steps of:
a) mixing preferably thoroughly, flows of a first fluid fuel and a first fluid oxidant to produce a first, substantially homogeneous, sub-stoichiometric stream close to, at or below the lower flammability limit at ambient pressure;
b) mixing preferably thoroughly, flows of a second fluid fuel and a second fluid oxidant to produce a second, substantially homogeneous, super-stoichiometric stream close to, at or above the upper flammability limit at ambient pressure;
c) combusting said first and second streams separately and substantially simultaneously to produce first and second products of combustion, combustion of the second stream being sustained;
d) mixing preferably thoroughly, the first and second products of combustion upstream of nozzle means so as to produce a substantially homogeneous mixed stream, and
e) passing the mixed stream through the nozzle means and causing the mixed stream to combust downstream thereof, whereby the first and second fluid fuels are substantially combusted.
One significant advantage of such a method is its capability of reducing emissions to so-called “ultra low” levels. For NO
x
this limit lies below 10 ppm, and is not achievable with conventional combustion methods.
In order to achieve ultra low emissions, the method of the invention relies on the combustion of a fuel (gaseous, liquid or solid) with an oxidant (oxygen or oxygen-enriched air) in two separate spatial locations, or zones, with different stoichiometries. The stoichiometric ratio is defined as the ratio between the actual fuel/oxidant ratio and that fuel/oxidant ratio required for complete combustion. In the case of methane, the stoichiometric mixture for complete combustion comprises about 33% methane in oxygen; so, the stoichiometric ratio of a mixture containing one volumetric part of methane to two parts of oxygen will have a value of one. A substoichiometric (or rich) mixture will contain less oxidant than the amount theoretically required for complete combustion, and will have a stoichiometric ratio of more than one. The reverse applies for a superstoichiometric (or lean) mixture. In this invention, the actual stoichiometries in each zone are different, and each zone has a specific function.
One zone is used to combust a substoichiometric mixture of oxidant and fuel close to or at the lower flammability limit, while the other, separate zone is used to combust a superstoichiometric mixture at or even above the upper limit of flammability and to stabilize the flame thereat. At the conclusion of combustion in each zone, the mixtures of hot products leaving each zone are mixed, externally of the zones, to produce a final flame and to achieve complete combustion - whilst producing ultra low emission levels.
Under given ambient conditions there are upper (or rich) and lower (or lean) limits of flammability, and within these limits self propagation of a flame can occur only after the mixture has been ignited. For example, the lower flammability limit of methane in air or oxygen is reached at about 5% methane. The upper limit is about 15% methane for air and 61% methane for oxygen. At room temperature and atmospheric pressure (1 atmosphere, or 1.013 bar, or 7.6×10
2
torr, or 1.013×10
5
Pa, or 1.033 kgf cm
−2
) in an oxygen atmosphere no flame will propagate where there is less than 5% or more than 61% methane. This also defines the safety limits of methane/oxygen mixtures. In general, the flammability limits are widened with increasing pressure and/or temperature. Very preferably, the compositions of the lean and rich streams are, if not coincident with, very close to the relevant flammability limit, principally so as to achieve the ultra low emission levels but also to minimize problems of flame propagation where the composition is above the upper flammability limit, or below the lower flammability limit. For oxygen/methane mixtures, for example, the compositions are preferably between about 3% and 6% and between about 61% and 68%.
A stoichiometric fuel/oxidant mixture will combust to produce a large proportion of carbon dioxide (CO
2
) and water (in the case of methane/oxygen) and will show the highest flame temperature achievable with such mixtures (in practice the maximum temperature is found by combusting mixtures slightly above stoichiometric (i.e. stoichiometric ratio slightly greater than one) due to kinetic effects). If the mixture to be combusted is markedly sub-superstoichiometric, lower temperatures are achieved, with the lowest temperatures being found at the flammability limits (flame temperatures at the flammability limits can be as low as 1200° C., or less). At the flammabil
Cohen Joshua L.
Pace Salvatore P.
The BOC Group plc
Yeung James C.
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