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Title:
METHOD AND APPARATUS FOR BURNING HIGHLY REACTIVE GASEOUS FUEL
Document Type and Number:
WIPO Patent Application WO/1995/023315
Kind Code:
A1
Abstract:
Method comprising the following steps: a) mixing the fuel gas with another gas which contains et least 3 % oxygen, in such an amount that the amount of oxygen in the gas mixture thus obtained is at least 60 % of the stoichiometric amount required for complete combustion of the fuel gas, b) passing the gas mixture thus obtained through a burner body (1) having several outlet orifices (2), c) maintaining a flame above the burner body (1), which flame comprises several flame jets (3), the fuel gas being a highly reactive fuel gas which is passed through the burner body (1) in such a way that the spacing between the flame jets (3) is between 4.5 and 20 mm and the combustion is carried out at a ratio of the outflow velocity of the gas mixture and the flame speed of between 5.0 and 45.0. The burner body is preferably a ceramic burner plate (1), whereof the total area of the outlet orifices (2) forms from 0.5 to 30 %, preferably from 1 to 4 % of the total area of the burner plate (1) which is provided with round outlet orifices (2) having a diameter of from 0.5 to 1.5 mm. The highly reactive fuel gas is preferably hydrogen or a hydrogen/methane mixture containing more than 90 % hydrogen.

Inventors:
VAN DER DRIFT ABRAHAM (NL)
Application Number:
PCT/NL1995/000059
Publication Date:
August 31, 1995
Filing Date:
February 14, 1995
Export Citation:
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Assignee:
STICHTING ENERGIE (NL)
DRIFT ABRAHAM V D (NL)
International Classes:
F23C9/00; F23D14/02; F23D14/58; F23D14/74; (IPC1-7): F23D14/12; F23D14/14
Domestic Patent References:
WO1993007420A11993-04-15
Foreign References:
US5102329A1992-04-07
US3936003A1976-02-03
US4316881A1982-02-23
EP0536706A21993-04-14
US4421474A1983-12-20
Other References:
PATENT ABSTRACTS OF JAPAN vol. 14, no. 107 (M - 0942) 27 February 1990 (1990-02-27)
Download PDF:
Claims:
CLAIMS
1. Method for burning a fuel gas, which method comprises the following steps: a) mixing the fuel gas with another gas which contains at least 3% oxygen, in such an amount that the amount of oxygen in the gas mixture thus obtained is at least 60% of the stoichiometric amount required for complete combustion of the fuel gas, b) passing the gas mixture thus obtained through a burner body having several outlet orifices, c) maintaining a flame above the burner body, which flame comprises several flame jets, characterized in that the fuel gas is a highly reactive fuel gas which is passed through the burner body in such a way that the spacing between the flame jets is between 4.5 and 20 mm and the combustion is carried out at a ratio of the outflow velocity of the gas mixture and the flame speed of between 5«0 and 450.
2. Method according to Claim 1, wherein the spacing between the flame jets is between 6 and 10 mm.
3. Method according to Claim 1 or 2, wherein the ratio of the out flow velocity of the gas mixture and the flame speed is between 15 and*& 35.
4. Method according to any one of Claims 13, wherein the burner body is a ceramic burner plate, whereof the total area of the outlet ori¬ fices forms from 0.5 to 30%, preferably from 1 to 4%, of the total area of the burner plate.
5. Method according to any one of Claims 14, wherein the ceramic burner plate is provided with round outlet orifices having a diameter of from 0.5 to 1.5 m .
6. Method according to any one of Claims 15, wherein the highly reactive fuel gas comprises pure hydrogen or a mixture of hydrogen and another fuel gas, the mixture containing at least 70%, preferably at least 80% of hydrogen gas and the other fuel gas being selected from methane and natural gas.
7. Method according to any one of Claims 16, wherein the fuel gas in step a) is mixed with air.
8. Method according to Claim 7. wherein the combustion is carried out with an excess of air of from 0 to 0%, preferably of from 10 to 30%.
9. Apparatus for burning a fuel gas, which device comprises a ceramic burner body, the burner body being provided with several spaced outlet orifices, characterized in that the distance between the outlet orifices is between 4.5 and 15 mm and the total area of the outlet ori fices is from 0.5 to 6%, preferably from 1 to 4%, of the total area of the burner body.
10. Apparatus according to Claim 9. wherein the burner body is a ceramic burner plate.
11. Apparatus according to Claim 10, wherein the ceramic burner plate is provided with round outlet orifices having a diameter of from 05 to 1.5 mm.
12. 12 Method for burning a highly reactive fuel gas, characterized in that said combustion is carried out under such conditions that recirculation of the gases produced during combustion is obtained.
13. Method according to claim 12, carried out by a method according to any of the claims 18 and/or with an apparatus according to any of the claims 9~H.
Description:
Method and apparatus for burning highly reactive gaseous fuel

The present invention relates to a method for burning a fuel gas, which method comprises the following steps: a) mixing the fuel gas with another gas which contains at least 3% oxygen, in such an amount that the amount of oxygen in the gas mixture thus obtained is at least 60% of the stoichiometric amount required for complete combustion of the fuel gas, b) passing the gas mixture thus obtained through a burner body having several outlet orifices, c) maintaining a flame above the burner body, which flame comprises several flame jets.

The US Patent 4,919.609 discloses such a method for burning natural gas. This involves the use of a burner in the form of a highly porous ceramic crucible having a porosity of approximately J0% or more, which is provided with expanded metal composed of iron. With this burner, an output of more than 3.000 kW/m 2 can be achieved, because the presence of the expanded metal - an essential element of said burner - promotes the recirculation of the gases produced during combustion, as a result of turbulent flue gas circulation zones being formed, which stabilize the flame. Nothing is mentioned in this article, however, concerning the combustion of highly reactive fuels or concerning the N0 X emission of such combustion.

U.S. patent 5.102,392 describes a burner and a method for burning a fuel gas with low N0 X emission. According to this reference, a burner plate comprises slot issues to which the fuel gas and combustion air are separately fed and from which they are separately discharged into the combustion chamber, thereby preventing flame propagation into the burner. According to this reference, stable ignition of the fuel - i.e. a common fuel gas - is sustained by recirculation of hot combustion products in the areas between slots downstream of the burner plate. To achieve this, the spacing between the slots should be 5 to 8 inches. Also, this burner provides very low N0 X resulting from the low residence time of the nitrogen (from the combustion air) in the high intensity flame and the avoidance of hot spots within the flame due to the turbu¬ lence and uniform air-to-fuel ratio throughout the combustion chamber volume.

However, this reference is not related to the combustion of

highly reactive fuel gases, i.e. fuel gases containing more than 70 % hydrogen, in which said highly reactive fuel gas is premixed with the combustion air.

In addition, the Dutch Patent Application 9200620 by Applicant discloses a ceramic burner for burning natural gas, which is formed from a highly porous ceramic foam, by means of which a large pressure drop and reduced N0 X emission can be achieved, the reduced N0 X emission being a result of a lower combustion temperature. Studies by Applicant have shown that these burners, having 60 pores per inch, can be used for burning hydrogen/natural gas mixtures containing at most Sθ% hydrogen, with an excess of air of 30% , a halving of the N0 X emission to approximately -.0 ppm being achievable.

The ceramic natural-gas burners described in said prior art cannot be used, however, for burning highly reactive fuels such as pure hydrogen or hydrogen/methane mixtures containing more than 70% of hydro¬ gen with a low excess of air (less than 50$), since a higher flame tem¬ perature is obtained and as a result the emission of nitrogen oxides (N0 X emission) increases greatly.

Moreover, owing to the higher reactivity of the fuels a higher flame speed is obtained, and as a result flashback of the flame in the burner takes place sooner. Said flashback can be prevented by increasing the gas/air. velocity through the burner, but in the burners from the prior art this leads to incomplete combustion or even blow-off of the flame. Consequently, the natural-gas burners from the prior art cannot be used for burning, for example, hydrogen mixtures and hydrogen/natural gas mixtures containing a minimum of 70% hydrogen, because this produces unduly high N0 X emission and/or an unstable flame.

The object of the present invention is therefore to provide a method for burning highly reactive fuel/air mixtures, low N0 X emission being achieved at the same time.

A further object of the invention is to provide such a method in which stable and complete combustion without flashback or blow-off of the flame is achieved. These objects are attained according to the invention by means of a method of the above-described type, characterized in that the fuel gas is a highly reactive fuel gas which is passed through the burner body in such a way that the spacing between the flame jets is between 4.5 and 15 mm and the combustion is carried out at a ratio of the outflow

velocity of the gas mixture and the flame speed of between 5-0 and 4 -0.

In the method according to the invention, the distance between said flame jets is such that recirculation is achieved of the gases produced during combustion. By virtue of said internal flue gas circula- tion, which according to the invention amounts to some tens up to perhaps even some hundreds of per cent, an N0 X emission is* achieved, surprisingly, which is much lower than the emission to be expected on the basis of the flame temperature.

In addition, by virtue of the ratio of the outflow velocity of the gas mixture and the flame speed, a stable flame is obtained without blow-off and without flashback.

The invention further relates to an appliance for burning a fuel gas in accordance with the abovementioned method, which appliance comprises a burner body which is provided with several spaced outflow orifices, characterized in that the distance between the outflow orifices is between 4.5 and 20 mm and the total area of the outflow orifices is from 0.5 to 30% , preferably from 1 to 4% of the total area of the burner body.

The invention will be explained in more detail with reference to the following description and figures, in which:

Figure 1 is a sectional side-view of a burner plate of the invention, which schematically depicts the flame jets and the internal flue gas circulation according to the invention;

The Figures 2 - 5 are schematic top views of burner plates which can be used with the method according to the invention;

Figure 6 is a graph which depicts the N0 X emission for the combustion of 100% hydrogen as a function of the number of outlet ori¬ fices for various values of the excess of air and the output;

Figure 7 is a graph which depicts the N0 X emission for the combustion of the hydrogen/methane mixture as a function of the per¬ centage of hydrogen for various values of the excess of air and the out¬ put;

Figure 8 is a graph which depicts the N0 X emission for the combustion of 100% hydrogen as a function of the distance between the outlet orifices;

Figure 9 is a graph which depicts the H 2 emission for the combustion of 100% hydrogen as a function of the porosity of the burner plate for various values of the excess of air and the output;

Figure 10 is a graph which depicts the CO emission for the

combustion of hydrogen/methane mixtures as a function of the output for various values of the hydrogen/methane ratio and the excess of air;

Figure 11 is a graph which depicts the CH, conversion for the combustion of hydrogen/methane mixtures as a function of the output for various values of the hydrogen/methane ratio and the excess of air;

Figure 12 is a graph which depicts the N0 X emission as a func¬ tion of the ratio between the outflow velocity of the gas mixture and the flame speed;

Figure 13 is a schematic sectional side-view of the open burner set-up used in the examples;

Figure 14 is a schematic sectional side-view of the enclosed burner set-up used in the examples;

Figure 15 is a schematic top view of the burner plate used in the examples. The spacing between the flame jets in this application is to be understood as the distance between the middle (centre, centre line) of the flame jet and the middle of the nearest flame jet, as depicted in Figure 1.

The flame speed is to be understood, in this application, as the laminar flame speed of a freely propagating adiabatic flame.

Highly reactive fuel gases are to be understood, in this appli¬ cation, as mixtures of hydrogen and customary fuel gases, which contain at least 70% and preferably at least 80% hydrogen.

The customary fuel gas is in this context preferably selected from hydrocarbons such as methane, ethane, propane, butane, ethene, propene, butene, acetylene and the like, although it is also possible, for example, to use petrol vapour, methanol, ethanol and the like.

The highly reactive fuel gas is more preferably a mixture of at least 80% hydrogen and natural gas or methane. Further, according to a specific preferred embodiment of the invention, pure hydrogen gas is used.

The highly reactive hydrogen-containing fuel gases used in the invention have a high reactivity (high flame speed) and a high flame tem¬ perature, compared with, for example, natural gas. Consequently, these mixtures cannot be burnt by means of the highly porous natural-gas burners from the prior art without encountering flashback and/or unacceptably high N0 X emission.

With the method according to the invention, however, stable combustion is achieved, surprisingly, with an N0 X emission of less than

10 ppm and generally even less than 5 ppm, whereas on the basis of the flame temperature an N0 X emission of more than 100 ppm would be expected. Thus, when 100% hydrogen is burnt with an excess of air of 10% theoreti¬ cally, an adiabatic flame temperature of 2350 K and an N0 X emission of approximately 400 ppm are to be expected, while these values for the combustion of 90% hydrogen/10% methane with an excess of air of 10%, are 2300 K and approximately 300 ppm, respectively.

According to the invention, a burner is used, of which but a small fraction of the surface is permeable, for example a burner having a small number of small straight-through outlet orifices.

As a result, a high outflow velocity of the gas mixture is achieved, compared with known highly porous natural-gas burners, for a comparable output, as a result of which flashback is prevented and at the same time recirculation of the gases produced during combustion is obtained, the N0 X emission decreasing as a result. This internal flue gas circulation is depicted schematically in Figure 1, where

1 is the burner plate, 2 are the outlet orifices, 3. are the flame jets and 4 is the centre line of the flame jet, 5. being the recirculation.

The formation of N0 X during the combustion of nitrogen- containing fuel gases generally proceeds via equilibrium reactions in the flame, such reactions being generally known to those skilled in the art. One example.is the Zeldovich mechanism: 0 + N 2 - NO + N N + 0 2 - NO + 0 Without limiting the scope of the invention it is assumed that the reduction in the N0 X emission in the case of the method according to the invention is a consequence of the shift of the abovementioned equi¬ libria to the left, which shift is achieved by the recirculation of the gases produced in the flame. Nor can it be precluded that a lowering of the flame tempera¬ ture, a shorter residence time in the flame and further known factors are contributing, separately or in combination, to the reduction in the N0 X emission.

With the invention it was found, however, that if the distance between the flame jets becomes so small that the recirculation flows interfere with one another, the N0 X emission increases greatly. This can be seen, inter alia, in Figure 8, which depicts the N0 X emission for the combustion of 100% hydrogen as a function of the distance between the outlet orifices (round straight-through holes of diameter 1 mm) for an

outflow velocity of the gas mixture of 25 m/s and 30 m/s and an excess of air of 10% and 30%. As the "kink" in this graph shows, the N0 X emission increases greatly below a certain spacing.

The distribution of the flame jets is therefore such that the recirculation flows cannot interfere with one another. This means, in general, that the distance between the flame jets is not less than 4.5 mm and preferably not less than 6 mm. The maximum distance between the flame jets is not essential and is generally 20 mm, preferably 10 mm.

In the method according to the invention, the highly reactive fuel gas is mixed, in a first step, with another gas which contains at least 3%. preferably at least 10%, oxygen. In the process, air or oxygen- enriched air is preferably used.

The oxygen-containing gas is admixed in such an amount, that the amount of oxygen present after mixing is at least 60%, preferably at least 80%, of the stoichiometric amount required for complete combustion of the fuel gas.

The combustion is preferably carried out with an excess of air of 0-50%, and more preferably 10-30%. Said excess of air can also be expressed in the parameter n, the ratio between the amount of oxygen present (in moles) and the amount of oxygen (in moles) which is required for complete combustion of the fuel gas. According to the invention, n is at least 0.8, preferably 1.0 - 1.5 and more preferably 1.1 - 1.3.

The premixed combustible gas mixture is then passed through a burner body having various outlet orifices, while maintaining, above the burner body, a flame which consists of several flame jets. The term flame jets refers to the separate small flames, indicated by 3. i n Figure 1, which together form the entire flame above the burner.

These flame jets generally correspond to the outlet orifices in the burner body, the shape, the distribution and the spacing of the flame jets being defined by the shape, the distance and the mutual distribution of the outlet orifices. It is also possible for an assembly of various very small outlet orifices to produce one flame jet.

The burner body and the outlet orifices may have any shape desired, as long as the desired ratio of the outflow velocity of the gas mixture and the flame speed is achieved, and the desired recirculation of the gases produced during combustion is obtained.

Thus, the outlet orifices may have the form of narrow slits, irregularly shaped openings, a plurality of concentric circles or assemblies of very small holes (for example in rows running parallel).

Further possibilities will be evident to those skilled in the art. The outlet orifices are preferably straight-through small holes, in particular round holes, which are distributed uniformly over the burner body, for example in the form of a star, a grid, rows running parallel, or concentric circles.

The shape of the burner body is likewise not essential. Thus, the body may take the shape of a cylinder, a cone or a sphere, or a sec¬ tion thereof, as will be evident to those skilled in the art. Preferably, however, a burner plate is used. A number of possible burner plates is depicted in the Figures

2, 3. 4 and 5> Further possibilities will be evident to those skilled in the art, inter alia the burner plate depicted in Figure 15, which is used in the examples.

Figure 2 depicts a burner plate 6 which is provided with straight-through outlet orifices 2 which are assembled in the shape of a star.

Figure 3 depicts a burner plate 8 which is provided with slit- shaped outlet orifices 9. running parallel.

Figure 4 depicts a burner plate 10 which is provided with con- centric circular outlet orifices 11.

Figure 5 depicts a burner plate 12 which is provided with a number of very small outlet orifices 13. which are assembled as rows running parallel. In this case, each row of the small outlet orifices 13 will produce a separate flame jet. The burner body may further be made of any material suitable for the purpose, including metals such as iron, copper and the like. The burner body is preferably a ceramic burner body and more preferably a ceramic burner plate.

In the method according to the invention, the combustion of the highly reactive air/fuel mixture is carried out at a ratio between the gas velocity and the flame speed of from 5.0 to 45.0. If this ratio is greater than 45.0, the flame is blown off, whereas with a ratio of less than 5.0 flashback occurs.

The flame speed is a variable which can be calculated theoreti- cally and can be used to describe combustion. This parameter is known to those skilled in the art, , as are methods for determining it. Reference is made, inter alia, to Kirk-Othmer Encyclopedia of Chemical Technology,

Third Edition, Volume 4, John Wiley & Sons, 1982, pp. 284-289.

The flame speed generally depends on the fuel mix used, the

excess of air and the output supplied. The precise value of the flame speed may differ, however, depending on the method used for the determi¬ nation thereof, as is generally known to those skilled in the art.

According to the invention, the outflow velocity of the gas mixture, in this application also referred to as the gas/air velocity, is controlled by the size and distribution of the outlet orifices (in particular, the ratio between the area of the outlet orifices and the total area of the burner plate) , the amount of fuel gas fed in per unit time, and the excess of air (these last two values, together with the composition of the fuel gas, also determine the output supplied). These parameters are controlled in such a way, according to the invention, that the desired outflow velocity is obtained, which is within the scope of those skilled in the art.

In this arrangement, the area of the outflow orifices is 0.5 ~ 6%, preferably 1-4%, of the total area of the burner plate. In the case of the conventional ceramic natural-gas burners, said area forms more than 30% and often more than 60% of the total area.

The size of the outlet orifices is not critical. If the outlet orifices are too large, however, the risk of flashback becomes greater and, depending on the number of orifices, the flame speed may decrease unduly. If the orifices are too small, the pressure drop across the plate becomes too. high. The outlet orifices are therefore preferably, for example, round holes having a diameter of less than 5 mm, more preferably of less than 3 mm. In the case of slit-shaped orifices, the width of the slit is less than 5 mm, preferably less than 3 mm. Therefore, in general at least one of the dimensions of the orifices should be smaller than 5 mm, preferably smaller than 3 mm, and with a general minimum of 0.001 mm, preferably 0.01 mm.

According to a specific embodiment of the invention, the burner is a ceramic burner plate which is provided with round outlet orifices having a diameter of 0.5 - 1-5 mm. said holes being uniformly distributed over the area at 2 holes/cm 2 , for example in the form of a star, a grid or concentric circles.

With the method according to the invention, the burner may also modulate, for example in a ratio of at least 1:5.

If combustion of the highly reactive fuel mixture is carried out according to the method of the invention, the output achieved essentially depends on the gas flow rate and is generally between 500 and 2000 kW/m 2 .

For a ratio of the outflow velocity of the gas mixture and the flame speed of between 15 and 35 it is possible, given a burner plate having straight-through holes of diameter 1 mm, which are uniformly dis¬ tributed over the burner plate at 2 holes/cm 2 , and with an excess of air of 10% (n = 1.1), to obtain 1300-3000 kW/m 2 , and 950-2100 kW/m 2 for an excess of air of 30% (n = 1.3) • This produces an N0 X emission- of between 3 and 6 ppm. A mixture of 90% H 2 and 10% CH/. can be burnt in a stable manner by means of this burner plate, at n = 1.1, between 900-2100 kW/m 2 . 80% H 2 /20% CH ή gives 1400-3200 kW/m 2 at n = 1.1. Figure 7 depicts the N0 X emission for the combustion of hydrogen/methane mixtures as a function of the percentage of hydrogen for various values for the output and the excess of air (ceramic burner plate having 2.42 outlet orifices/cm 2 , dia¬ meter 1 mm) .

In the context of the invention, a reduction of the N0 X emission is obtained by virtue of internal flue gas circulation. This will lower the free-radical concentrations at the point of N0 X formation, via an increase in the mass. In this context it is important to note that Levinsky & van der Mey [Gas. July/August 1993. PP« 4l2-4l6] make a difference between thermal N0 X in and downstream of the flame front. The former, in terms of magnitude, certainly cannot be neglected, given the high free-radical concentrations (0 radicals) at the point of the flame front in the case of premixed flames. Moreover, an 0 radical concentra¬ tion in the flame front of 100 times the 0 concentration downstream of the flame is seen as a reasonable assumption. The driving force for internal flue gas circulation is the dynamic pressure differential created by velocity. Therefore: degree of recirculation: R • * - Δp Employing known relationships between pressure drop (Δp) , velocity, out¬ put (P) , excess of air (n) etc. it is possible to arrive at the following approximation

par 2

in which the porosity is a measure for the ratio between the total area of the outlet orifices and the total area of the burner plate. For a ceramic burner plate having straight-through holes with a diameter of 1 mm it is the case that: porosity in [%] = O.785 [number of holes/cm 2 ]

According to the invention the porosity of the burner plate is therefore 0.5~30%, preferably 1-4%.

Since it is assumed that the formation of N0 X is inversely pro¬ portional to the degree of recirculation it is fair to say that:

[NO por p . n

This could possibly explain that the N0 X emission increases with an increase in the porosity, a reduction in the excess of air and/or a reduction in output, as is shown by Figure 6, in which the N0 X emission for the combustion of hydrogen is depicted as a function of the number of outlet orifices per cm 2 (round straight-through outlet orifices of dia- meter 1 mm) for various values of the output and the excess of air. Like¬ wise, as Figure 8 shows, an increase in the N0 X emission is produced if the distance between the outlet orifices (uniformly distributed round holes of diameter 1 mm) decreases with constant porosity, as a result of the interference between the recirculation flows. The fraction of unburnt fuel which results according to the

Figures 9 and 10 can also be explained. Flue gas recirculation is employed in many commercial burners in order to increase the stability of the flame. The mechanism is simple: the hot gases flowing back ignite the as yet unburned gases at an early stage. Consequently, the flame remains attached to the surface in spite of its high speed.

This is also observed in the case of the invention: even at outflow velocities of 20 m/s and still higher, the flame remains attached to the surface, while the flame speed is only approximately from 1 to 2 m/s. At the same time, if there are too many holes, the flame is found to blow off: at 4.94 holes/cm 2 (spacing 4.5 mm) the flame nearly blows off at 2000 kW/m 2 , the velocity of gas and air per hole being approxi¬ mately 20 m/s. With 2.42 holes/cm 2 at 1000 kW/m 2 (velocity likewise approximately 20 m/s) , however, the flame remains firmly attached, as is the case for 1.28 holes/cm 2 at 500 kW/m 2 . This strongly suggests the presence of flue gas recirculation.

All this explains the increase in the fraction of unburned fuel - that is to say the fuel slip - in the case of measures which reduce the degree of flue gas recirculation (lowering output, increasing porosity), as can be seen from the Figures 9. 10 and 11, which depict the H 2 emission for the combustion of 100% hydrogen, and the CO emission and the

CH ή conversion, respectively, as a function of the porosity of the burner plate at various values of the excess of air and the output.

Although raising the excess of air increases the degree of recirculation, this nevertheless has the effect of raising the unburned fraction. The reason for this is the lowering of the flame temperature.

In addition to these factors, the mechanism applying- to a sur¬ face burner also operates: at a low gas/air velocity the plate will radiate, and the risk of flashback increases. A certain minimum velocity is therefore desirable to prevent this effect. An increase in the velocity will, on the one hand, lead to greater "blow-off behaviour", but will, on the other hand, via flue gas recirculation, enhance the stability of the flame (attachment) . This lowers the surface temperature and enhances the flame stability and consequently the fuel slip.

The ratio between the outflow velocity of the gas mixture, the gas/air velocity, and the flame speed is a measure for the ratio between the degree of flue gas recirculation and surface temperature increase. Stability ratio = (gas/air velocity)/(flame speed). In other words, for a low ratio, the so-called "surface burner mechanism" will predominate and the surface will therefore become hot and it will be possible, therefore, for flashback to occur. For a high ratio, the internal flue gas recirculation predominates. If the ratio is too high, the flame will blow off.

In Figure 12, the said ratio is plotted graphically against the gas/air velocity. This shows that for orifices having a diameter of 1 mm it is possible for flashback to occur at a ratio of 7 or less. If the value is greater than 15, the flame can be operated stably. If the value is greater than 35. however, there is the risk of blow-off, depending on the exact conditions. These ranges may differ for other diameters of the outlet orifices, without moving outside the scope of the invention. Admixture of methane goes hand in hand with a decrease in the reaction rate. Thus the laminar flame speed drops by approximately 40% if

10% of the H 2 is replaced by CH ή . The flame speed is halved if 20% of the

H 2 is replaced by CH / ,. The gas/air velocity, however, hardly changes. It is therefore clear that stabilization of a CH/,/H 2 flame presents problems if the burner has been adapted for stable combustion of 100% H 2 . For 4.94 holes/cm 2 and 1000 kW/m 2 , n = 1.3, admixture of 20% CH ή is possible (the gas/air velocity is then 10 m/s) .

It is striking that measures which increase the degree of recirculation cause a reduction in the N0 X emission as well as a reduc-

tion in the fraction of unburned fuel. These measures are: increasing the output, reducing the porosity and/or increasing the distance between the various holes.

An N0 X emission of less than 5 ppm is found to require a distance of at least 7 mm between the various holes. The effect of a change in the excess of air can be ascribed to the change in •flame tem¬ perature going hand in hand with it. Lowering n results in a higher N0 X emission and a lower fuel slip.

A minimum gas/air velocity of from 10 to 15 m/s is required to prevent flashback at 100% H 2 . The fuel slip, however, is considerable (a few hundred ppm) in the vicinity of this limit.

Although the above theoretical explanation is given to further elucidate the teaching of the present invention, and to enable a man skilled in the art to provide further embodiments of the present inven- tion not explicitly incorporated herein - which embodiments of course also are encompassed by the present invention - the above teaching should not be construed as limiting the invention in any way.

The invention further relates to an appliance for burning a fuel gas, said appliance comprising a burner body, the burner body being provided with several outlet orifices, the distance between the outlet orifices being between 4.5 and 15 mm and the total area of the outlet orifices being from 0.5 to 6%, preferably from 1 to 4%, of the total area of the burner body.

The burner body in this arrangement is, as specified above, preferably a ceramic burner body and more preferably a ceramic burner plate. In particular, the burner body is a ceramic burner plate which is provided with uniformly distributed straight-through round outlet ori¬ fices having a diameter of from 0.5 to 1.5 mm. The appliance may possibly also contain other known elements of burner appliances, such as a fuel gas supply, an air supply and a means for premixing the air and the fuel gas. All these and other elements and also their application will be evi¬ dent to those skilled in the art.

The invention will now be explained with reference to the following non-limiting examples. In the examples, the open burner set-up of Figure 13 and the enclosed burner set-up of Figure 14 were used.

Figure 13 depicts a burner set-up with a supply 14 for the air/fuel gas mixture. This mixture is passed through the orifices 16 of a burner pate 15, said burner plate being clamped in the clamping construe-

tion 17. The combustion of the fuel gas takes place in the combustion zone 18.

Figure 14 depicts an enclosed burner set-up, the fuel gas/air mixture being supplied via supply 19 and then being passed through the orifices 21 of a burner plate 20. The combustion takes place in the combustion zone 22. Then the discharge gases are passed over a heat exchanger 23 and discharged via discharge 24. Discharge 24 is provided with a flue gas sampling arrangement 25. The set-up further comprises cooled walls 26, a peep hole for an IR pyrometer 27, a pyrometer 28 operating with suction, and thermocouples 29.

In the examples, the following abbreviations are used:

Q = output supplied in kW/m 2 ; n = excess of air;

%H 2 = percentage of hydrogen in the fuel gas; %0 2 = percentage of oxygen in the flue gas (= measure for the excess of air) ;

CO = measured amount of carbon monoxide after combustion in ppm;

C0 2 % = measured amount of C0 2 in the flue gas in per cent;

N0 X = N0 X emission in ppm; C X H X = hydrocarbon slip in ppm;

H 2 = H 2 slip in ppm; dP = pressure drop across the plate in Pascal

Tplate°C = temperature of the plate in °C.

Example I The combustion of 100% H 2 was carried out with a modified

Stettner plate of 64 x 46 mm with 24 uniformly distributed holes having a diameter of 1 mm (0.82 holes/cm 2 ) . The measures were carried out with the burner set-up depicted in Figure 13• The emissions were measured by means of a probe (a water-cooled double tube for isokinetic flue gas sampling, which was positioned closely above the surface of the plate) , and the heat exchanger used was a quartz tube having a diameter of 100 mm and a length of 200 mm.

Start-up with Q = 1000 kW/m 2 , n = 1.3, with 100% H 2 was laborious; the holes at the outside had to be lit separately. The results are depicted in Table 1.

TABLE 1

Using the abovementioned burner, attempts were also made, at Q = 1000 kW/m 2 , n = 1.3, to admix the fuel gas with methane to obtain a mixture of 90% H 2 and 10% CH / ,.

Ignition at this setting is very laborious; many long flames are produced, and as a result blow-off nearly occurs. With this setting, no emissions could be measured.

Example II To improve ignition of the burner, a plate was used which had more holes, namely 39 holes having a diameter of 1 mm, which were distributed uniformly over the surface (= 1.32 holes/cm 2 ) . This plate is depicted in Figure 15 and comprises a ceramic plate 30 which is provided with round outlet orifices 31 (diameter 1 mm) . Owing to the reduced number of holes, the velocity per hole decreases and the flames shorten. Ignition is likewise improved, without an increase in the N0 X emission.

TABLE 2

Q n %H 2 0 2 N0 X dP

1500 1.3 100 5.9 2.8 3230

1500 1.15 100 3.5 3-9 2760 1500 1.1 100 2.4 4.3 2620

1000 1.3 100 6 2.8 1490 1000 1.13 100 3 3.5 1160 1000 1.1 100 2.3 3-4 1080

700 1.3 100 5.8 3-6 750 700 1.1 100 2.4 5-2 570

500 1.3 100 5.6 3.6 390

The abovementioned measurements demonstrate that a variable output does not cause changes in the N0 X emission. This also shows that possible cooling at the wall is not the most important cause of the low N0 X emission.

Example III

The combustion of 100% H 2 was carried out with a burner plate of 90 x 90 mm " with 104 uniformly distributed holes of diameter 1 mm (1.28 holes/cm 2 ) . This combustion was carried out in the sealed set-up depicted in Figure 14. The results are shown in Table 3.

TABLE 3

Q n 0 2 N0 X H 2 slip dP

1500 1.29 5-6 1.7 0 6550 1500 1.09 2.2 1.9 30 5110

1000 1.3 5-7 1.8 70 2950 1000 1.1 2.5 1.9 30 2290

500 1.29 5-6 2.7 210 660 500 1.11 2.5 3-9 70 540

It was further found that, using the plate of 0 x 90 mm and 104 holes used in this example, start-up could readily be achieved in the "open" set-up as depicted in Figure 13 at Q = 1000 kW/m 2 , n = 1.3.

Attempts were also made, using this plate in the sealed set-up depicted in Figure 14, to burn a mixture of 90% H 2 and 10% CH / ,. Start-up proceeded smoothly, but very long flames were produced on the plate, which visibly touched the heat exchanger, which resulted in high CO emission. The results are shown in Table 4 below.

TABLE 4

Q n 0 2 N0 X CO C0 2 C X X dP

1500 1.12 2.7 2.7 140 3.5 4.6 5300

It is therefore not possible, using the plate described in this example, to replace some of the hydrogen by methane. Because of the great reduction in flame speed, blow-off of the flame occurs.

Example V

In this example, combustion of 100% hydrogen and hydro- gen/methane mixture was carried out in the "open" set-up depicted in Figure 13, with a ceramic burner of 90 x 90 mm with 400 uniformly dis¬ tributed holes of diameter 1 mm (= 4.94 holes/cm 2 ). The results were as follows: at Q = 1000 kW/m 2 , n = 1.3, using 100% H 2 a pressure drop of approximately 300 Pascal was produced. The plate began to radiate so fiercely that the risk of flashback became very high;

at Q = 2000 kW/m 2 , n = 1.3, 100% H 2 could be burnt stably. Approximately 60% of the surface burns radiantly, there is a visible flame approximately 30 cm long on the burner. The holes at the edges of the plate do not contribute to the combustion; - at Q = 1000, n = 1.3, it was possible for 90%/10% and 8θ%/20% mixtures of H 2 and CH, to be burnt stably. In the centre of the plate a radiant patch is visible, the holes at the edges do not contribute.

The results above show that a burner with 400 holes of diameter 1 mm on an area of 90 x 90 cm 2 cannot be properly operated with 100% hydrogen. The gas velocity is too low to prevent flashback and thus ensure safe operation. At high outputs, the gas velocity is high enough, but the high output is the cause of the excessive heating of the plate.

Example VI In this example, a ceramic burner plate was used with an area of 90 x 90 cm 2 with 196 uniformly distributed holes of diameter 1 mm. With this burner it was possible for 100% hydrogen to be burnt stably. In the "open" set-up of Figure 13, the following results were obtained: Q=1000 kW/m 2 , n = 1.3, 100% H 2 : - stable operation; all holes are burning, and the pressure drop is 740 Pascal; Q=1000 kW/m 2 , n = 1.3, 95% H 2 and 5% CH/,: stable operation; all holes are burning, and pretty small blue flames are visible with an approximate length of 3 cm; Q=1000 kW/m 2 , n = 1.3. 90% H 2 and 10% CH / ,: stable operation; the holes at the edges are not burning, although pretty small blue flames are visible with an approxi¬ mate length of 3 cm; Q = 500 kW/m 2 , n = 1.3, 100% H 2 : - all the holes are able to burn; the plate is radiating so fiercely in the centre that the experiment was stopped because of possible flashback.

Then this plate was used to carry out the combustion of 100% H 2 and of H 2 /CH, mixture in the sealed set-up of Figure 14. The results were as follows:

When combustion was carried out with 100% H 2 , the holes at the edges do not visibly contribute. When combustion was carried with 95% H 2 , an improvement is visible, and at 90% H 2 all the holes take part. In addition, the considerable H 2 slip for the outputs at 100% H 2 is striking. This H 2 slip is not obtained with a 90 x 90 cm 2 plate having 104 holes of diameter 1 mm. This may possibly be explained by more frequent recirculation of the flue gases in the case of a plate having 104 holes (no hydrogen slip) , as a result of which the hydrogen more frequently has the opportunity of burning. It is also striking that the temperature of the plate, for outputs higher than 500 kW/m 2 , is lower at n = 1.1 than at n = 1.3« A possible explanation for this is that more flue gases are recirculated at n = 1.3. and the plate surface is consequently warmed up.

The results above show that it is possible, by means of the method according to the invention, for 100% H 2 as well as CH,/H 2 mixtures containing at least 80% H 2 to be burnt stably with a low N0 X emission. The invention is not limited, however, to the combustion of hydrogen or hydrogen/methane mixtures.




 
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