GB2126493A - Fluidised catalytic combustion - Google Patents

Abstract

In a fuel burning method for heating working media (e.g. heat exchange fluid or substances to be dried) a stoichiometric mixture of fuel and air is supplied through a fluidized bed of a complete-oxidation catalyst (4), with the temperature of the fluidized bed of the complete-oxidation catalyst (4) maintained in the range from 670 to 1070 K by variation of the flowrate of working medium (5). A fuel burning apparatus comprises a vertical casing (1) fully filled with the fluidized bed of the complete-oxidation catalyst (4) limited at its bottom by a gas-distribution screen (2) serving to supply air therethrough and an horizontally- disposed sectionalizing screen (3) rated at an open area of 50 to 80 per cent and provided with holes, each measuring two to ten diameters of the grains of the complete- oxidation catalyst (4). <IMAGE>

Description

SPECIFICATION Fuel burning method and apparatus for performing it This invention relates to an improved fuel burning method for heating working media, and in apparatus for performing such a method.

The invention is applicable in all fields of engineering which utilizes energy generated by the burning of fuel.

Burning of organic fuel is a basic process in modern engineering. The energy thus generated is used to meet the major part of demands for heat, electric power and mechanical work. The great majority of production processes in industry, agriculture, construction engineering and transport consume energy generated by fuel burning. The grave experience of the last decade has shown everyone that high fuel prices mean high prices of all products. Considering these circumstances as well as limited resources of organic fuel extracted by rational methods, mankind faces a vital worldwide problem of finding alternatives for today's industrial development by which world fuel consumption rises twofold every 1 2 to 1 5 years.Along with the recent advance in utilization of novel types of power resources (such as atomic and nuclear), the problem of optimum utilization of organic fuel is still urgent today, and will probably remain vital for quite a long time.

A fuel burning method for heating working media is herein proposed, consisting in that a fuel/air mixture is supplied through a fluidized bed of complete-oxidation catalyst, with a stoichiometric fuel/air mixture, and with the temperature of the fluidized bed of completeoxidation catalyst maintained in the range 670 to 1070 K by variation of the working medium flow rate.

The fuel burning method according to the invention provides for high-rate low-temperature oxidation of fuels unaccompanied by formation of toxic components (such as carbon monoxide and nitrogen oxides) peculiar to high-temperature combustion, and also for a high heat utilization efficiency (up to 90-95 per cent).

It is expedient that the complete-oxidation catalyst be in the form of spherical grains having a diameter of 0.4 to 0.2 cm and a bulk density of 1 to 2 g/cu.cm.

The foregoing structure of complete-oxidation catalyst is favourable for minimizing the wear thereof resulting from abrasion in the boiling bed, for setting a sufficiently high rate of heat exchange, and for separating the complete-oxidation catalyst aerodynamically from the working medium in hard (powder) state during drying procedure.

It is preferable that the fluidization number of the fluidized bed of complete-oxidation catalyst is not less than 3.

If the fluidized number is above 3, heat transfer in the bed of catalyst is improved sufficiently, and the grains of stoichiometric concentrated fuel/air mixture do not overheat during oxidation.

When, the fuel burning method is employed for drying working media, it is expedient that the fluidization velocity of the complete-oxidation catalyst is either equal to, or more than the terminal velocity of particles of the working medium.

The foregoing fluidization velocity of the complete-oxidation catalyst ensures a high rate of heat exchange between the catalyst grains, and between the grains and working medium, with the result that thermal overheating of the complete-oxidation catalyst grains is precluded, and stoichiometric concentrated fuel/air mixtures can be employed in the process.

Also herein proposed is an apparatus for performing the fuel burning method, comprising a vertical case fully filled with a fluidized bed of complete-oxidation catalyst, limited on the bottom thereof by a gas-distribution screen serving to feed air therethrough, wherein, according to the invention, the vertical case incorporates an horizontally-disposed sectionalizing screen rated at an open area of 50 to 80 per cent and provided with holes each measuring two to ten diameters of complete-oxidation catalyst grains.

The above-mentioned apparatus is particularly useful for drying various powder materials, including materials not stable in oxygeneous media, such as coal, sulphide concentrates, ores, and so forth.

It is possible that tubular heat-exchange surfaces can be arranged all over the internal space of the vertical case to permit heating and evaporation of liquid, including water and hydrocarbon liquids.

The term "fuel" hereinafter used implies any gaseous, liquid or solid material which chemically reacts with oxygen contained in the air, and generates heat.

The term "heat utilization efficiency" defines the percentage of heat transferred from heating working media.

The term "working medium" defines any liquid, solid or gaseous material which is heated, evaporated or transformed from one state to another using the heat generated by catalytic oxidation of fuel.

The invention wili now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows a fuel burning apparatus serving for drying working media, according to the invention; Figure 2 shows a temperature profile of the fuel burning apparatus of Fig. 1; Figure 3 shows a fuel burning apparatus serving for heating and evaporation of liquids, according to the invention; Figure 4 shows a temperature profile of the fuel burning apparatus of Fig. 3.

Referring now to Fig. 1, the apparatus for burning fuel and heating a working medium according to the invention comprises a vertical casing 1 limited at the bottom by a gasdistribution screen 2, rated at an open area of 2 to 5 per cent. The vertical casing 1 accommodates an horizontally-disposed sectionalizing screen 3 rated at an open area of 50 to 80 per cent and having holes measuring two to ten diameters of grains of a complete-oxidation catalyst 4 which fully fills the vertical case. The complete-oxidation catalyst 4 is in the form of spherical grains having a diameter of 0.04 to 0.2 cm and a bulk density of 1 to 2 g/cu.cm.The complete-oxidation fuel catalysts 4 comprise oxides included into the secondary subgroups of groups I and VIII of the periodic table, with said oxides deposited on mechanically-strong spherical grains of ceramic carrier based on oxides of aluminium, iron, silicon or aluminosilicates, with a specific surface area of 10 to 200 sq.m/g, with a bulk weight of 0.6 to 0.9 g/cu.cm, and with a content of active constituent oxides amounting to 5 to 30 per cent by weight of grains of complete-oxidation catalyst 4. The active constituents may both be in the form of individual oxides of these metals, and their mixtures or compounds comprised of these oxides alone, or of these oxides and carrier material.In the operating state, the fluidized grain bed of complete-oxidation catalyst 4 fills the sectionalizing screen immersed therein and forms two zones of freely boiling bed, one located below the sectionalizing screen 3, and the other disposed above it. The height of the bottom zone is adjusted in such a way that the process of catalytic oxidation of fuel is completed therein (commonly, the height of the bottom zone is 0.2 to 0.6 m).

Turning to Fig. 2, the temperature profile of the apparatus according to Fig. 1 consists of two isothermal zones, that is, a bottom zone wherein the temperature is maintained in the range from 670 to 1070 K required for complete fuel oxidation, and a top zone wherein the temperature depends on heating conditions of a working medium 5, for example, from 420 to 770 K. The intermittent temperature region is in the internal space of the casing occupied by the sectionalizing screen 3 (Fig. 1).

In the apparatus shown in Fig. 1, heat is transferred from the top zone of the fluidized bed of the complete oxidation catalyst 4 by introducing the working medium 5, such as wet pulverized material, from a bin 6 through a batching device, for example, a screw feeder 7, directly to the surface of the fluidized bed of the complete-oxidation catalyst 4. An essential requirement in the operation of the apparatus lies in aerodynamic separation of the catalyst 4 and introduced hard particles of the working medium 5. In practice, the dimensions and density of the completeoxidation catalyst 4 and working medium 5 are chosen in such a manner that the velocity of entrainment of particles of the working medium 5 is twice or thrice below that of the grains of the complete-oxidation catalyst 4.The particles of the working medium 5 are removed from the vertical casing 1, are deposited in a cyclone 8, and are separated from the flow of flue gases.

The fuel burning method is achieved in the apparatus as follows: the complete-oxidation catalyst 4 (Fig. 1) contained in the casing 1 is heated by any suitable prior-art method to a temperature of 520 K. For starting the apparatus, it is sufficient to heat only a definite part of the catalyst 4 adjacent the gas-distribution screen 2 and a fuel supply and distribution device 9.

Air required for fluidization of the bed of complete-oxidation catalyst 4 is injected under the screen 2, and the device 9 supplies a fuel, for example in liquid state, with a boiling point of 470 to 620 K. If the fuel/air mixture is oxidized at a high rate, and if the bed of the catalyst 4 is in a fluidized state, the temperature quickly rises to an operating range of from 670 to 1070 K, whereupon the supply of liquid fuel at a boiling point of 470 to 620 K is cut off, the device 9 starts feeding an operating fuel either in gaseous, liquid or hard state, and a stoichiometric mixture of fuel and air is supplied through the fluidized bed of the complete-oxidation catalyst 4 during further operation.

The pulverized working medium 5 is supplied from the bin 6 through the feeder 7 to the top section of the vertical casing 1, with the fuel supply rate adjusted in such a way that the temperature in the top zone of the fluidized bed of the catalyst 4 is maintained in the range from 420 to 770 K, and the temperature in the bottom zone below the sectionalizing screen 3 is maintained in the range from 670 to 1070 K.

One of the principal problems arising during catalytic oxidation of concentrated air/fuel mixtures consists in finding appropriate service conditions under which the grains of the complete-oxidation catalyst 4 do not overheat significantly, and no hot spots occur in the fluidized bed of the catalyst 4 such as may result in its de-activation. This problem is quite essential for the following reasons: 1. For the major part of fuels, the adiabatic heating temperature in the combustion of stoichiometric fuel/air mixtures amounts to more than 2300 K.

2. The present-day hard-phase catalysts based on metal oxides are thermally stable at temperatures not exceeding 1000 to 1 200 K. It therefore follows that when heat transfer in the bed of the catalyst 4 is impaired even temporarily, some part of the complete-oxidation catalyst 4 undergoes irreversible de-activation. For these reasons, the primary consideration in design of the method of the present invention was given to determination of conditions under which the above-mentioned irreversible de-activation is impossible. This object is accomplished in the apparatus wherein the bed of the catalyst 4 is subjected to free boiling in the bottom zone of the casing 1 at linear velocities (fluidization numbers) at least thrice exceeding the initial velocity of fluidization of grains of the catalyst 4.Under such conditions, the heat transfer rates in the bed are so high that no significant over-heating and de-activation of the complete-oxidation catalyst 4 occur.

The use of the sectionalizing screen 3 permits reducing of the heat transfer in the fluidized bed from 10,000-20,000W/m.K (free boiling bed) to 200-500 W/m.K (fluidized bed incorporating the sectionalizing screen 3 of said dimensions). The decrease in heat transfer of the fluidized bed is ascribed to deceleration of the grains which pass the cells of the sectionalizing screen 3. With the heat transfer in the fluidized bed of the sectionalizing screen 3 maintained at this level, with heat generated in the bottom zone of the apparatus, and with heat removed in the top zone of the apparatus, there is a considerable temperature difference across the sectionalizing screen amounting to 20-100 K per cm in height of the sectionalizing screen 3, whereby the fuel burning apparatus provided for a non-isothermal temperature profile (Fig.

2).

The use of the fuel burning method and apparatus for performing it in the case of treatment of a hardphase working medium, that is, drying of wet discrete porous material, is illustrated in the following examples.

EXAMPLE 1 The vertical casing 1 having an integral diameter of 250 mm and incorporating the gas distribution screen 2 with an open area of 5 per cent is filled with 25 kg of the copperchromium complete-oxidation bead catalyst 4 (with beads measuring 2 to 2.5 mm, and with the bulk weight equal to 1000 kg/cu.m). The initial fluidization velocity of this catalyst 4 is equal to 0.9 m/s at a fume gas temperature of 870 K. The preheated fluidized bed of completeoxidation catalyst 4 at a temperature of 420 K is supplied from below with air and fuel in stoichiometric proportion (ez = 1.0 to 1.1). The air flow rate is adjusted so as to fluidize the catalyst 4, and is equal to 1 20 normal cu.m/hr. The linear operating velocity of the fluidized agent in the section of the apparatus casing 1 is equal to 2.2-2.8 m/s.In the bottom part of the boiling fluidized bed of the complete-oxidation catalyst 4, heat is generated at a high rate due to catalytic oxidation of fuel. The temperature maintained herein is equal to 870 K. The material to be dried, that is porous alumosilicate absorbent with an initial moisture content of 1 7 per cent by weight, is introduced at a rate of 1000 kg/hr. The fractional composition of adsorbent being dried is below 0.04 cm. The terminal velocity of adsorbent grains is equal to 1.2 m/s. In the top zone of the fluidized bed, heat is removed at a high rate by virtue of evaporation of water from the surfaces of particles in direct contact with grains of the completeoxidation catalyst 4. The realized operating conditions permit drying the material at a temperature of 380 to 400 K for a final moisture content of 3 to 4 per cent by weight.The heat utilization efficiency in the burning of fuel for performance of the wet material drying process described in this example is equal to 80 per cent.

EXAMPLE 2 The procedure is identical to that of Example 1, but the material being dried is in the form of an adsorbent having an initial moisture content of 35 per cent by weight and the same fractional composition, with the material introduced at a rate of 600 kg/hr. The material is dried at a temperature of 393 to 423 K for a final moisture content of 4 to 5 per cent by weight. The heat utilization efficiency is equal to 90 per cent.

In an alternative embodiment of the method according to the invention, fuel is burnt for heating and evaporation of liquid working media, such as water, water solutions, hydrocarbon liquids and other media.

In this case, the apparatus of Fig. 1 is furnished with a tubular heat-exchanger surface 10 disposed all over the interior space of the apparatus. This fuel burning apparatus is illustrated in Fig. 3. The apparatus comprises the vertical casing 1 limited on the bottom by the gasdistribution screen 2, the device 9 serving to introduce fuel into the fluidized bed of the complete-oxidation catalyst 4, and the sectionalizing section 3 serving to separate the bed into two zones, including a high-temperature bottom zone wherein fuel is burnt catalytically at a temperature of 670 to 1070 K, and a top zone wherein the flue gases cool down at a temperature of 420 to 570 K, and incorporates coils of the tubular heat-exchange surfaces 10 wherein liquid being heated is disposed.It is expedient that combustion products are passed upwardly in countercurrent to the liquid flowing downwards inside the heat-exchange surfaces 10 for maximizing the heat transfer and ulilization efficiency of the apparatus.

The apparatus is started in the same way as in the above case of heating hard working media, with the difference that the liquid being heated is supplied into the tubular heat-exchange surfaces 10 after the apparatus starts operating under steady-state conditions and the operating temperature is in the range from 670 to 1070 K.

Fig. 4 presents a temperature profile related to the height of the fluidized bed of the catalyst 4 in the apparatus shown in Fig. 3.

Referring to the graph of Fig. 4, the sectionalizing screen 3 (Fig. 3) permits of operating the apparatus under non-isothermal conditions which are most advantageous because the temperature from 670 to 1070 K essential for instense catalytic oxidation of fuel is maintained in the bottom zone, and the temperature of 420 to 570 K required for appropriate cooling of combustion products and for reaching a heat utilization efficiency above 90 per cent is maintained in the top zone.

The use of the fuel burning method and apparatus for the performance of it in the heating and evaporation of liquids is illustrated by the following examples.

EXAMPLE I The fuel used is furnace oil of the following elementary composition: H C S O-N 11.2 87.4 0.5 0.9 10.5 87.6 0.7-1 1.0 The fuel is burnt in the apparatus described hereinabove. The furnace oil preheated to a temperature of 350 K is supplied by means of a plunger pump. The flow rate amounts to 5.6 kg/hr for fuel and to 63 cm.m/h for air.

The complete-oxidation catalyst is preheated by means of an electric heater to a temperature of 600 to 700 K, then diesel fuel is supplied to elevate the temperature of the fluidized bed of the catalyst 4 to 750-800 K during a period of several minutes. After the above-mentioned temperature is reached, the diesel fuel supply is cut off, and the furnace oil supply is started.

When the temperature is in the range from 750 to 800 K, the furnace oil is ignited steadily, and then the temperature in the bed is raised to the limits from 670 to 1070 K.

It has been found that when the rates of fuel and air supply are adjusted as indicated above, the values a estimated by measuring the supply and by chromatographic analysis of effluent gases are practically equal and amounts to 1.1. The gas mixture above the fluidized bed of catalyst 4 has been subjected to analysis.

The nitrogen oxide content has been estimated by prior art photocalorimetric techniques using the Griess-llosvay reagent.

The resulting composition of effluent gas mixture is as follows: CO2 = 13.8 per cent; 02= 2.2 per cent; NOx = 70 ppm; CO = 1 0 - per cent, maximum; CH4= 10-3 per cent, maximum.

If the temperature in the bed is more than 920 K and a is more than 1, sulphur contained in the fuel is quantitatively oxidized to SO3.

The heat utilizatin efficiency 77 amounts to 92 per cent.

EXAMPLE 2 The apparatus is filled with 70 litres of catalyst comprising 5 percent of copper chromite, and the rest is y-A12O3 (with a bulk weight of 0.76 g/cu.cm). On completion of the preheating procedure, the operating conditions are: the air flow rate is 60 cu.m/hr; the diesel fuel flow rate is 6.39 litres/hr; the water flow rate inside the heat-exchange surfaces is 1100 litres/hr; the inlet water temperature is 300 K, and the outlet water temperature is 345 K. The temperature in the freely boiling bed is 800 K, and the temperature at the temperature at the outlet of the apparatus is 450 K. The heat utilization efficiency is 85 per cent.

EXAMPLE 3 The procedure is identical to that of Example 1, with the air flow rate equal to 100 cu.m/hr, with the fuel flow rate equal to 10.8 litres/hr, and with the water flow rate equal to 2025 litres/hr (At = 38). The temperature in the bottom zone is equal to 830 K, the outlet temperature is 570 K, and the heat utilization efficiency is equal to 85 per cent.

EXAMPLE 4 The apparatus is filled with 50 litres of catalyst comprising 30 per cent of copper chromite, and the rest is y-AI203 (with a bulk weight of 1.1 g/cu.cm). On completion of the preheating procedure, the operating conditions are: the air flow rate is 103 cu.m/hr; the diesel fuel flow rate is 9.8 litres/hr; and the water flow rate is 3400 litres/hr (At = 22). The temperature in the bottom zone of the apparatus is 895 K, and the outlet temperature is 400 K. The heat utilization efficiency is equal to 89 per cent.

EXAMPLE 5 The procedure is identical to that of Example 1, with the air flow rate equal to 80 cu.m/hr, with the diesel fuel flow rate equal to 6.7 litres/hr, and with the methane flow rate equal to 0.8-1.3 cu.m/hr. The temperature in the bottom zone is from 900 to 970 K, and the outlet temperature is 770 K. No methane has been revealed in reaction products, and the CO2 content of flue gases has been estimated at 14.2 to 15.0 per cent.

EXAMPLE 6 The fuel used is brown coal with the following parameters: (a) Particle size 0.04 to 0.1 cm (b) Operating moisture content 1 6 per cent (c) Operating ash content 1 2.3 per cent (d) Volatile-matter content based on dry solids 37.5 per cent (e) Ash content based on dry solids 0.9 per cent (f) Sulphur content based on dry solids 0.7 per cent.

For burning the coal, the apparatus described hereinabove is used. The coal is batched and introduced into the bottom part of the casing 1 by means of pneumatic transport.

The catalyst heating and fuel burning procedures, and the analysis of flue gases are identical to those described in Example 1, with the air flow rate amounting to 630 cu.m/hr and coal supply rate amounting to 1 20 kg/hr.

The excess air ratio is determined by measuring the effluent gas oxygen content, and is equal to 1.02-1.1. The steady-state combustion of coal occurs at temperatures above 770 K.

The resulting composition of effluent gas mixture is as follows: CO2 = 18.0 to 19 per cent; O2 = 0.4 to 2 per cent; NOx= 80 to 140 mg/cu.m; CO = 1 0 - per cent, maximum; CH4 = 10-3 per cent, maximum.

All sulphur contained in the fuel is fixed to ash. Dispersed ash is removed by the effluent gas flow. The temperature profile related to the bed height is maintained as in the case with the profile of Example 4.

The heat utilization efficiency 7 amounts to 92 per cent.

Thus, in the fuel burning method and apparatus for performing it, according to the invention, the working medium is in contact with the fluidized bed of complete-oxidation catalyst capable of burning all fuel without excess air at relatively low temperatures in the range 700 to 1000 K.

To preclude overheating of the catalyst grains on the bottom part of the fluidized bed to which stoichiometric fuel/air mixture is supplied, there is a freely boiling zone wherein the gas flow velocity significantly (twice or thrice) exceeds the initial fluidization velocity of the catalyst grains. The major part of the fuel undergoes catalytic oxidation in this zone.

In order to reduce the temperature on the surface of the bed, and, hence, the effluent gas temperature, a special function sectionalizing screen arranged in the central part of the bed limits the heat exchange between the top and bottom part of the bed. This feature permits maintaining the effluent gas temperature at a lower level of 370 to 500 K, and providing optimum temperature conditions in the bottom part of the bed for catalytic oxidation of fuel (670 to 1070 K). Thus, the low temperature of effluent gases provides for a high heat utilization efficiency (up to 96 per cent), and the high velocity of catalytic oxidation provides for a high heat release rate and small dimensions of the fuel burning apparatus.

As noted above, a further and no less important problem within the scope of the present invention lies in minimizing the quantity of toxic ingredients in flue gases. Such ingredients are carbon monoxide (CO), sulphur oxides and nitrogen oxides.

1. Catalytic oxidation of carbon monoxide to CO2 occuring at a temperature of 500 K and more proceeds very intensely, so that the complete oxidation process requires just about one volume of oxidation catalyst per 20,000 volumes of oxidized gas mixture per hour. Even at minimum excess air rates (cut1.02), no carbon monoxide is revealed in the flow of effluent gases.

2. The development of processes for burning fuel in a boiling bed of inert heat-transfer medium is fostered chiefly because there is a possibility of fixing sulphur in non-volatile compounds, for example: SO2 + 1/2 02= MeOoMeSO4, where Me = Ca- or Mg.

However, since this reaction requires a free oxide of an alkaline-earth metal, it must be made at a temperature above 1100 K at which the original carbonate compounds are thermally decomposed, for example: CaCOCaO + CO2.

During catalytic oxidation of fuel in the proposed temperature ranges from 670 to 1070 K for the bottom zone and from 420 to 570 K for the top zone, the reaction SO2 + 1/2 02SO3 proceeds in the catalyst at a high rate and degree of completeness with the result that even at minimum content of oxygen in effluent gases (an1.05), sulphur dioxide is transformed to sulphur trioxide. In this case, vapours of sulphuric acid are condensed on the particles of the boiling bed in the top (low-temperature) zone of the apparatus, and sulphuric acid can be fixed to alkaline-earth metals forming non-volatile compounds, including carbonate compounds: H2SO4 + MeCO3 o MeSO4 + CO2.

Thus, during the catalytic oxidation of fuel under the foregoing temperature conditions, sulphur oxides are fully arrested and the temperature is not to be raised to 1100 K at which the carbonates of alkaline-earth metals decompose.

3. The content of nitrogen oxides in effluent gases primarily depends on the temperature level of the fuel burning process. Under the conditions specified in the prototype method, this nitrogen oxide content is fairly high and is far in excess of present-day tolerances. Thus, at a temperature of 1123 K and a = 1.1, the NO content amounts of 120 mg/cu.m, and at 1323 K is equal to 460 mg/cu.m, whereas the tolerances established in the USA limit the NO content to 100-150 ppm (200-300 mg/cu.m). In the temperature range used in the fuel burning method and apparatus according to the invention, the nitrogen oxide content does not generally exceed 100 ppm.

Claims (8)

1. A fuel burning method for heating working media consisting in that a stoichiometric mixture of fuel and air is supplied through a fluidized bed of complete-oxidation catalyst, with the temperature of the fluidized bed of complete-oxidation catalyst maintained in the range from 670 to 1070 K by variation of working medium flow.

2. A fuel burning method as claimed in claim 1, wherein the completesxidation catalyst is in the form of spherical grains measuring 0.04 to 0.2 cm in diameter and having a bulk density of 1 to 2 g/cu.cm.

3. A fuel burning method as claimed in claim 1, wherein the fluidized bed of completeoxidation catalyst has a fluidization number of not less than 3.

4. A fuel burning method for drying working medium as claimed in claims 1 and 3, wherein the fluidization velocity is equal to, or more than, the terminal velocity of particles of the working medium.

5. Apparatus for performing the fuel burning method claimed in claims 1 and 4, comprising a vertical casing fully filled with the fluidized bed of complete-oxidation catalyst; a gasdistribution screen serving to limit the vertical casing at the bottom thereof and to supply a stoichiometric mixture of fuel and air therethrough; and an horizontally disposed sectionalizing screen installed inside the vertical case, rated at an open area of 50 to 80 per cent and having holes, each measuring two to ten diameters of grains of the complete-oxidation catalyst.

6. Apparatus as claimed in claim 5, incorporating tubular heat-exchange surfaces arranged inside the vertical casing all over its internal space.

7. A fuel burning method substantially as hereinabove described with reference to the accompanying drawings.

8. Apparatus for burning fuel substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.