GB2080700A - Catalytic combustion system with fiber matrix burner - Google Patents

Abstract

A combustion system for combusting an air-fuel mixture with high efficiency and low NOx emissions which includes a catalytic burner (10) comprising an annular matrix of high temperature resistant fibers (12) randomly oriented and packed together to a density of 12-16 lb/ft<3>, an optimum range for maintaining combustion in a shallow outer heterogeneous reaction zone (16) at a temperature below the adiabatic flame temperature of the mixture, and also below the use temperature of the fiber, by outward radiant heat transfer from the zone; the matrix forming a heat insulation barrier (18) to maintain the matrix temperature at the inlet side (14) below the ignition temperature of the mixture, to prevent flashback. Strands of a more active catalytic material may be interspersed through the matrix. The matrix can be used within the combustion chamber of a firetube boiler system. <IMAGE>

Description

SPECIFICATION Catalytic combustion system with fiber matrix burner This invention relates in general to catalytic com bustion technology, and in particular relates to catalytic combustion systems such as firetube boil ers and burners for use therein.

Conventional firetu be boiler systems incorporat ing diffusion flame burners achieve high combustion efficiency (low CO and HC) but produce relatively high nitrogen oxide emissions (NOx). Typically, conventional diffusion flame burners cannot achieve NOX emissions below 100 ppm (volume basis at 0% excess oxygen for gaseous and liquid fuels without significant nitrogen content). In view of the wide spread use of firetube boiler systems of this type the adverse consequences of atmospheric pollution and photochemical smog are significant.

Radiant burners have previously been developed which employ discrete fuel-airjets to heat up a ceramic block which in turn radiates energy. Burners of this type are not applicable to catalytic combustion in a firetube boiler system as in the present invention.

It is a general object of the invention to provide a new and improved fuel-air combustion system and method operating at high combustion efficiency {low CO and HC), high overall system efficiency, and low thermal NOX emissions.

Another object is to provide a combustion system and method suitable for use in firetube boilers with high overall boiler efficiency and reduced atmospheric pollutants and photochemical smog.

Another object is to provide a combustion system and method of the type described which is capable of operating at near-stoichiometric mixtures with catalytic bed temperatures below the adiabatic flame temperature of the mixture and within the temperature limitations of the bed material.

Another object is to provide a catalytic combustion system and method of the type described which operates at low excess air levels to limit stack gas energy losses for maintaining high overall efficiency while also maintaining the combustion bed temperature at acceptably low levels to minimize thermal NOX emissions.

The invention in summary comprises a catalytic combustion system incorporating a burner formed by a matrix of high temperature resistant fibers with interstitial spaces in the matrix forming a flowpath Tor the fuel-air mixture. The density of the fibers and thickness of the burner are within an optimum range so that the mixture combusts at a heterogeneous reaction zone along a shallow layer on one side of the burner. Heat is transferred primarily by radiation from the reaction zone so that the temperature of the body is maintained below its use temperature and below the adiabatic flame temperature of the mixture.

Figure lisa fragmentary cross-section of a burner matrix according to the invention.

Figure 2 is a chart depicting the approximate temperature profile within the matrix as a function of depth through the thickness of the burner of Figure 1.

Figure 3 is a schematic diagram of a firetube boiler system incorporating the burner matrix of Figure 1.

Figure 4 is a perspective view, partially broken away and exploded, of the firetube boiler system of Figure 3.

Figure 5 is a chart depicting combustion surface temperature for a burner matrix of the invention as a function of theoretical air for different face velocities of the fuel-air mixture.

Figure 6 is a chart depicting NOX emissions for the burner matrix used in Figure 5 as a function of theoretical air for different face velocities of the fuel-air mixture.

The catalytic burner system of the invention incorporates a burner 10 of porous matrix composition as illustrated in the cross-section of Figure 1. The material of the matrix is comprised of randomly oriented fibers 12 of a high temperature resistant material such as alumina silicate. The fibers are packed to an optimum density in the range of substantially 12 to 16 Ib/ft3 to form the desired shape and thickness, e.g.

a flat pad as in Figure 1. With this range of densities the interstitial spaces between the fibers provides a flow path for the fuel-air mixture over the entire extent of the matrix pad. In Figure 1 the inlet side 14 is shown on the left of the pad with the mixture moving through the pad for combustion at a heterogeneous reaction zone 16 on the right hand side.

The fiber matrix composition of the burner pad has relatively poor internal heat conductivity so that the upstream portion 18 of the matrix forms a heat insulation barrier such that reaction zone 16 is established along a shallow layer at a depth of only a few millimeters on the outlet side of the pad. The shallow depth ofthe reaction zone produces significant heat transfer away from the zone primarily by radiation with some transfer by convection. The rate of this radiative transfer is such that the surface temperature of the fiber material in the reaction zone is maintained below the adiabatic flame temperature of the fuel-air mixture and also below the "use" temperature ofthe fiber material. In comparison, many conventional combustors with relatively thick cores, e.g.

honeycomb bed combustors, result in a deep combustion zone with high peak temperatures within the core. The substantially lower surface temperature of the matrix materials in the present invention thereby permits operation at near-stoichiometric mixtures with relatively low NOX emissions and high combustion efficiencies as compared to combustors of conventional design.

An important feature of the invention is that the problem of flashback into the incoming fuel-air mixture is minimized. The poor internal heat conduction of the fiber matrix and the shallow depth at the reaction zone prevents temperature rise on the surface at the inlet side which could otherwise lead to detonations and destruction of the flame. The approximate temperature profile for the burner pad of Figure 1 is illustrated in the graph of Figure 2. The temperature at the surface on the inlet side and through the major depth 18 ofthe pad is substantially ambient or close to the temperature of the incoming mixture.

Approaching the heterogeneous reaction zone 16 the temperature rises sharply. Rapid transfer of heat by radiation from the downstream surface is rep resented by the downturn at the tail of the temperature curve.

The thickness of the burner pad is selected in accordance with the particular fiber matrix density.

With a fiber matrix density of 12 Ib/fxss a pad thickness of substantially one inch is optimum; a thicker pad would result in increased pumping requirements and reduce system efficiency while a thinner pad would increase the risk of flashback. Similarly, a matrix with a density of higher than the optimum would undesirably increase pumping requirements while a density below the optimum range would not contain the reaction zone to the shallow layer which produces the desired radiative heat transfer for maintaining the low temperatures of the matrix in the reaction zone.

A burner pad suitable for use in the invention can comprise a pad of Cerablanket fibers (alumina silicate) sold by the Johns-Manville Company with a density of approximately 12 Ibiff. This material has a use temperature in the range of 16000 F to 2500 F with an optimum heat release rate per unit area of 80,000-150,000 Btulhr-flz. A fiber material of this character can be formed into the desired matrix shape, e.g. a cylindrical shell, by forming a wet slurry of the fibers by means of conventional vacuumforming techniques.

The catalytic activity of the burner pad can be improved by the addition of materials having a higher degree of catalytic activity, e.g. strands of a catalytic metal such as chrome wire can be interspersed through the matrix. In addition, the matrix can be formed in two or more separate layers, each having different densities or different compositions.

Thus, for controlling flashback the layer on the upstream side could be of a composition which is less catalytic than the downstream layer, and the strands of catalytic metal could be contained in only the downstream layer.

Figures 3 and 4 illustrate the burner system of the invention incorporated into a firetube boiler 20. In the system catalytic burner 22 is comprised of a fiber matrix formed into a cylindrical shell configuration.

The matrix material comprises Cerablanket fibers sold by the Johns-Manville Company with a density of approximately 12 iblfl3, and the radial thickness of the cylinder wall 24 is approximately one inch. The downstream end of the cylinder is capped by a circular fiber matrix pad 26 of a composition, density and thickness similar to that of the cylindrical wall of the burner. The upsteam end of the cylinder is sealed by a flange 28 of the firetube boiler combustion chamber 30.

The fuel-air mixture is directed into the burner through a perforated manifold tube 32 extending concentrically within the burner. The wall of the mat rix is rigidly supported by radial spokes 34 extending from the manifold tube.

Burner 22 is mounted coaxially within firetube combustion chamber 30 with the radial spacing bet ween the outer surface of the matrix and the com bustion chamber surface 36 in the range of 1 to 5 inches. Typically the inner diameter of the combustion chamber is in the range of 14to 25 inches and the chamber length is in the range of 3 to 15 feet.

The fuel for the firetube boiler system may be gaseous, e.g. natural gas, or vaporized liquid. Theb fuel is premixed with air and forced under pressute into the manifold tubes of the burners. The burner may operate on diesel fuel in which case the fuel ffi partially vaporized by preheating the air stream to 425" F prior to mixing with the fuel.

The premixed air-fuel mixture is directed under a positive pressure through manifold tube 32 and the interior volume 38 of the burner. The mixture is forced outwardlythrough-the interstitial spaces in the matrix and emerges from the outer surface where it is artificially ignited. Heterogeneous combustion is uniformly established in reaction zone 16 overthe entire outer surface of the cylinder and end cap 26 to a depth of a few millimeters of the matrix.

Heat is transferred outwardly from the reaction zone primarily by radiation with some contribution by convective transfer. This heat transfer limits the matrix surface temperature to less than 25009 the use temperature of the fiber material, while the adiabatic flame temperature of the reactive mixture exceeds 3500" F. The heat radiation is absorbed by the surrounding metal wall 36 of the firetube combustion chamber, and the wall conducts heat to the boiler water 40 to heat the water orto raise steam. Flue gases are forced to the end of the firetube and out through the flue passage 42 or into additional firetube passes.

In another example of the invention the operating results demonstrate the low emission characteristics and the ability to radiatively transfer heat to surrounding surfaces. In this example, the burner element is comprised of the previously-described Cerablanketfiber material at a density of 12 Ibift3.

The element is in the shape of a fiat circular disc of 6 inch diaeneter and 1 inch thickneo The disc is moureerlwithin a conduit perpendlcular to the flow of premixed air-fuel reactants. The mixture is forced under positive pressure through the disc and is i9nieed at the downstream faceto establish the heterogeneous reaction zone to a depth of a few mil liwneters The operating resllts of the flat circular disc bemer configuration using natural gas fuel and varied inlet flow velocity conditions are depicted in Figures 5 and 6.Figure 5 depicts the matrix surface temperature of the heterogeneous reaction zone as a function of theoretical air for three different facEvei- ocities. Curve 44 depicts the results at a face velocity of 2 ft/sec, curve 46 at a face velocity of 1 fasec, And curve 48 at a face velocity of 112 fUsec. The graph demonstrates that lower velocities result in lower surface temperatures at the lower excess air levels (100 to 130% theoretical air) where firetube boilers normally operate. The graph of Figure 6 demonstrates that this lower surface temperature is advantageous in reducing NOX emissions (ppm) as a function of percent theoretical air, with curve 50 depicting the results ata face velocity of 2 ft/sec, curve 52 at a velocity of 1 ft/sec, and curve 54 at a velocity of 1/2 ft/sec. As demonstrated by the curve 54 in Figure 6, NOX emissions of less than 10 ppm at 115% theoretical air are possible. Additionally, high com bustion efficiency (low CO and HC) can be achieved simultaneously with low NOX emissions.

The low surface temperatures represented in Fig urn 5 also demonstrate the effectiveness of radiative heat transfer away from the heterogeneous reaction 'zone. The indicated temperatures of 2200 F to 2500 F at 100% theoretical air are within the use tempera ture capabilities of the alumina silicate fibers which comprise the matrix, and these temperatures are also far below the mixture's adiabatic flame temper ature of 3500" F. These results demonstrate that the optimum flow velocity for the particular matrix mat erial which was employed is in the range of 1/3 to 1/2 ft/sec.

The foregoing demonstrates that the invention provides a catalytic burner with important operating advantages, particularly for firetube boilers. Com bustion in the fiber matrix of the burner takes place at relatively low temperatures with active heat trans ferto achieve low emissions of nitrogen oxides. The matrix can be fabricated in a variety of configura tions, such as a cylindrical shell compatible with the combustion chamber of a firetube boiler. Good combustion efficiency is achieved and high overall boiler efficiency realized in that the burner can oper ate at near-stoichiometric conditions. The burner can be operated on a variety of gaseous and liquid (with pre-vaporization) fuels.

Claims (8)

1. Acombustorforburning an air-fuel mixture with high combustion efficiency and low NOX emis sions comprising the combination of a burner body in a hollow cylindrical shell configuration and com prised of a matrix of high temperature resistant fib ers with interstitial spaces between the fibers form ing a flow path for the mixture, with the fibers packed to a density in the range of 12 to 16 Ib/fta so that combustion of the mixture is sustained at a heterogeneous reaction zone along an outer layer of the shell whereby heat transfers outwardly by radia tion from the reaction zone to maintain the tempera ture of the matrix in the zone below the adiabatic flame temperature of the mixture and also below the use temperature of the fibers.

2. A combustor as in claim 1 which includes inlet manifold means for injecting the mixture within the shell for flow outwardly through the interstitial spaces of the matrix.

3. A combustor as in claim 2 including a cylindri cal combustion chamber having a wall radially spaced about the outer surface of the shell for 'absorbing radiant energy transferred from the reac tion zone of the matrix.

4. A method of combusting a fuel-air mixture with high combustion efficiency and low thermal NOX omissions, comprising the steps of directing a fl.ow of the mixture through interstitial spaces in a matrix of high temperature resistant fibers packed to a density in the range of 12to 16 lb1, combusting the mixture at a heterogeneous reaction zone along a layer of the matrix on the side downstream of the flow, and radiating heat from the reaction zone at a rate which maintains the temperature of the matrix in the zone below the adiabatic flame temperature of the mixture and also below the use temperature of the fibers.

5. A method as in claim 4 in which heat conduction from the reaction zone through the matrix in a direction upstream of the flow is at a rate so that the temperature of the upstream side of the matrix is below the ignition temperature of the mixture for preventing flashback into the upstream flow of gases.

6. A method as in claim 4 in which the stoichiometry of the mixture is in the range of 100 to 130% theoretical air.

7. A combustor as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in FIGURES 1,3 and 4 of the accompanying drawings.

8. A method as claimed in claim 4 substantially as hereinbefore described with reference to the accompanying drawings.