FR2689615A3 - Fuel gas burners mfd. form ceramic foam - Google Patents

Abstract

A ceramic fuel gas burner mfd. from individual segments of ceramic foam. Ceramic foam fuel gas burner plates are mfd. by joining segments horizontally and/or vertically with metal frameworks, glue or simple butt joints to form various shapes and sizes. The segments are mfd. by a replication method where a suspension of ceramic material, contg. e.g. one or more of clay, alumina and talc cpds., is impregnated in a polyurethane foam master shape, followed by sintering. Multilayer mfr., with different porosities through the thickness to promote good pre-mixing of gas and air, is possible by suitable mfr. of the master. The edges of the segments are pref. sealed with ceramic to prevent sideways gas leakage. Thermal stresses are minimised by allowing easy movement of the segments within a framework and lower strength materials may be used. By choice of the gas flow, high flames heat by convection while keeping the plate relatively cool. Low flames use both convection and radiation and the upper surface may be coated to promote radiation.

Description

 "Bru ^ their ceramic".

 The invention relates to a ceramic burner comprising at least one flat segment of ceramic foam, disposed in a housing.

 Such a ceramic burner is described in French patent publication 1,175,620. The ceramic burner is constructed from a number of flat segments of ceramic foam material. These are all supported by a grid made of material; stainless steel. For the outer delimitation, the outermost segments are, laterally and from the top, closed by a metal limitation.

It is known that ceramic burners, obtained from ceramic foam, can be used for the production of energy from (natural) gas in households, buildings, industrial and horticultural installations and power plants. electricity. Combustion takes place temporarily with combustion systems which, in a certain number of situations, are modernized to lower the emission of NO (100-200 ppm). To achieve a more marked reduction in the emission of NOX (NMP) and a fuel economy, it is necessary to develop burners, at competitive prices, which - can be regulated on a large field, for a premix
constant and a small excess of air, - achieve a low NOX emission, - provide a desired and regular charge of the surface
heated, - lead to a compact installation volume, - are inexpensive.

 Conventional burners cannot meet these requirements.

 In US Patent 4,900,245, an infrared heater for a liquid immersion apparatus is described which has a cylindrical ceramic foam burner which is made from a crosslinked foam having a porosity of about 16 to 40 pores per centimeter. This burner can be provided with a coating which opposes a flashback. The burner consists of a core through which the gas is supplied and a peripheral envelope of ceramic foam. By form, manufacturing is far from simple. When heating large areas, breakage or deformation can easily occur, causing sealing problems.

 It appeared that the fixing of at least the peripheral segments in the manner of French patent 1,175,620 is unnecessarily complicated and simply complicates replacement of the segments and poses sealing problems.

 The object of the invention is to avoid these drawbacks.

 This object is solved by a ceramic burner described above by the fact that the housing comprises a cassette supporting the segment on three sides and by the fact that the segment is arranged in a self-supporting manner. By the arrangement in a cassette housing, the ceramic burner can be changed quickly and easily.

 According to a preferred embodiment of the invention, the ceramic foam material consists of different segments fixed to each other by a flexible refractory cement and made of such a ceramic foam material. These segments are smaller in size and can therefore be made more simply in solid form. The desired burner can be mounted by a combination of elements, both horizontally and vertically. According to a preferred embodiment, the segments are laterally provided with a ceramic seal so that no lateral gas outlet can occur.

 Thanks to the freedom of movement of the segments, the thermal stresses are reduced, which makes high resistance of the material less necessary.

 The ceramic foams may furthermore be suitably provided with a coating which opposes a flashback and which promotes radiation.

 When forming in layers having different pore sizes, a desired combustion can also be favored.

The ceramic segments can be glued to each other, but also be pinched together. A solid bond can also be obtained if the plates are joined in a horizontal plane by a straight edge.

 The burner according to the invention has satisfactory mechanical resistance, unlike, for example, ceramic fiber burners. Excess noise is absent and the risk of a flashback, which exists in the case of perforated ceramic burners, is absent here while a field of action much greater than in the case of metallic filaments is possible . In the burners according to the invention, there is no noise overload, no flashback, a very large field of application is possible (100 -> 6000 kW / m2) for a constant excess of air included between 0.8 and 1.6. The manufacturing techniques are simple, flexible shaping as regards the geometry and the material / heat transfer is possible by means of the assembly.

 The materials which can be used make very high temperatures possible.

 Given their very wide field of application, the burners according to the invention can be used as modulating burners, which leads to a gas saving of 10 to 15% (and therefore to a reduction in the emission of CO2). The NOX emission is low, that is to say about 5 ppm for 200 kW / m2 and n = 1.3; the burners can be manufactured by simple techniques.

 CO2 dosing and heat distribution by a single burner is possible for horticulture.

 In industry, the use of a cylindrical burner is possible among other things for the implementation of a retrofit in a boiler. For the processing industry (furnaces for example for the production of H2 and ethene), a compact construction with a volume reduction of 25-50% is possible.

 In a particular embodiment of a combustion device, such as a central heating boiler, this comprises a burner, as described above, with ceramic segments arranged horizontally, the gases to be burned being supplied from the top of the segments and combustion taking place on the underside of the segments. Obviously the segments can also be placed vertically or obliquely.

 By a burner according to the invention (see FIGS. 1 and 2) gas and air are mixed beforehand completely and supplied in a well distributed manner. The mixture passes through the burner. An ignition takes place after the burner. Depending on the flow: high or low, long or respectively short flames are obtained. In the case of long flames, the plate remains relatively cold and the performance is comparable to a conventional premix burner. In the case of short flames, the exit surface of the plate becomes radiant.

Thus, a quantity of heat is supplied by radiation, which leads to a lowering of the average temperature in the combustion zone and thus also of the rate of formation of NO,. The area capable of radiating can be enlarged by the use of a highly porous layer. As already mentioned, the burner can be placed for a large field of application with maximum conversion efficiency.

The excess air (n) can be adjusted between 0.8 and 1.6. The burner pressure drop characteristic can already be determined in the project phase by the choice of starting materials and machining techniques and the size of the pores.

The combustion principle can be implemented both on a small scale (water heaters, central heating boilers) and on a large scale (industrial boilers and furnaces), with the advantages: a reduction in NOX emission, a wide range of adjustment and therefore a better adjustable heat production, which leads to energy saving. By the use of ceramic burners, a saving of material is also possible thanks to a more compact construction of the installation. The properties of a burner according to the invention based on a foam with 20-26 pores per centimeter, measured under conditions of a central heating boiler (n = 1.3) placed horizontally, burning down, are
Specific load kW / m2 100-2000 (n = 1.3)
Pressure drop on Ps 100 + 20 (1500 kW / m2,
the plate n = 1.3) *
Excess flair% 30 (can be changed)
Methane conversion%> 99.99
Surface temperature C '1350 (maximum) **
NO emission (O% OJ ppmv 5-30
CO emission (0% OQ ppmv <50
CU emission (0% O ss ppmv <10 * Another pressure drop (higher or lower)
can be done by changing the parameters in
the manufacturing process.

** Other materials give higher maximum temperatures
high.

 A ceramic foam burner according to the invention can be prepared for example by manufacturing plates. The technique used is then the replication method, ceramic powder + additives being mixed until the formation of a ceramic suspension which is combined with an organic foam which is impregnated, then worked, burned and sintered, after which the foam ceramic is obtained. Variations in the size of the pores and the number of pores per centimeter in the organic foam can be compensated by precise control of the amount of suspension provided so that the plates meet the desired criterion of pressure drop.

 A suitable ceramic foam is obtained for example when an organic foam having 24 pores per centimeter is impregnated with a suspension in which clay, aluminum oxide and talc form the main elements. The amount of suspension must be such that the ceramic foam receives a density of between 100 and 700 kg / m, from which, for a thickness of 1.5 mm of the plate, gives the desired pressure drop. The sintering temperature must be such that the crystalline phases in the material consist mainly of cordierite, mullite and aluminum oxide.

 The following parameters of ceramic foam can be adjusted: thickness, shape, porosity, pressure drop, mechanical resistance, radiation properties (by coating), laminated construction with different porosities, type of material (temperature up to for example 22000C). Depending on the use, the number of pores per centimeter can be corrected between 4 and 40 poes / cm.

 Usually, flat plates are obtained which can be sawn to measure, small cylinders can be obtained from such plates by drilling. Cylinder segments can also be prepared using the same preparation method followed by curved sintering. Curved or oval geometries can also be fabricated as desired.

 As a fixing and connection technique, a cassette provided with a seal (see FIG. 3) can be used, but the plates can also be glued (see FIGS. 4 and 5), or finally they can be assembled by means of '' a saw cut having what is called a straight cut (see Figure 6). Any geometry and size can be made in such a way using 1) a simple metal frame, 2) the basic geometry, 3) the fixing / bonding techniques.

Example I
Magnesium aluminum silicate, suspension 1
8 g of dispersing agent (HNO 23M) are weighed and this is supplemented to 287 g with demineralized water. To this solution, 134 g of Mg (OH), 2,234 g of Alto2.3 and 345 g of SiO are successively added. A suspension having an average particle size of less than 50 μm is ready. The suspension is homogenized by grinding for 3 hours in a ball mill.

 50 g of the ready suspension are weighed and, by the addition of nitric acid, they are brought to a viscosity of 0.12 Pas. The quantity of suspension is brought to a piece of PUR foam (12 p / cm and 36 p / cm) and it is distributed homogeneously over the foam. This is conveniently done by introduction using a rolling pin. After drying, burning and sintering at 13500C, a cordierite foam is obtained which is suitable as a burner plate.

Example II
Magnesium aluminum silicate, suspension 2
5 g of dispersing agent (HNO 13M) and 5 g of dispersing agent (water glass, sodium silicate) are weighed and this is made up to 300 g with demineralized water. 500 g of clay are added to this. Depending on the chemical composition of the clay and the chemical composition to be achieved, we add to this 0-200 g of AlO, p-200 g of
Mg (OH) pet 0-200 g of SiO. 2 The choice of the chemical composition of the last components is not essential. The correct quantities of Al, Mg and Si which are added are essential. A suspension with an average particle size of less than 50 m is ready. The suspension is homogenized by grinding for 3 hours in a ball mill. A suitable foam is obtained by the procedure indicated in Example I.

Example III
5 g of dispersing agent (HNO 1 g) and 5 g of dispersing agent (water glass, sodium silicate) are weighed and this is completed with demineralized water up to 300 g. 500 g of clay are weighed and added, having the composition of kaolinite (AlO2.32SiO 2HOa. A suspension having an average particle size of less than 50 days is ready. The suspension is homogenized by grinding for 3 hours in a mill A suitable foam is obtained by the procedure described in Example I.

Claims (9)

 1. Ceramic burner, comprising at least one flat segment of ceramic foam disposed in a housing, characterized in that the housing comprises a cassette which supports the segment on three sides and in that the segment is arranged in a self-supporting manner.  2. Burner according to claim i, in which different segments of ceramic material, fixed to each other on the cold side by a flexible refractory cement, are arranged in the cassette.  3. Burner according to one of claims 1 and 2, wherein the segments are joined in a horizontal plane.  4. Burner according to one of claims 1, 2 and 3, wherein the segments are assembled to one another in a vertical plane.  5. Burner according to one of claims 1 to 4, wherein the segments are assembled both horizontally and vertically.  6. Burner according to one of claims 1 to 5, wherein the burner, seen in the direction of flow of the gas, consists of different layers having different pore sizes.  7. Burner according to one of claims 1 to 6, wherein the segments are provided laterally with a ceramic seal.  8. Burner according to one of claims 1 to 7, wherein the plates are joined in a horizontal plane by a straight cut.  9. Combustion device comprising a burner according to one of the preceding claims, with segments arranged horizontally, in which the gases to be burned are supplied from the top of the ceramic foam and combustion takes place on the underside of the segments.