Abstract: Apparatus and method for the reduction of nitrogen oxides (NOx) in the emissions of a fuel burning fluidized bed combustion system. Using a staged combustion technique, an overfire air inlet is provided in the freeboard zone above the bed. In the first stage, residual oxygen (O.sub.2) in the off-gas just above the bed is held in the 0.4 to 0.8% range by the actual coal and air rates. In the second stage, carbon monoxide (CO) and unburned hydrocarbons, both present in the first stage off-gas, are combined with additional air using the overfire air inlet. CO levels in the final effluent are reduced with little or no increase in NOx emissions.

Abstract: The combustible component of a gas stream of low heating value is combusted using less than a stoichiometric amount of oxygen in the presence of an oxygenation catalyst and the heat energy in the combusted gas is utilized, for example, by expansion in a gas turbine.

Abstract: A combustion method which can reduce the emission of NOx and smoke. In sharp contrast to the conventional combustion method, the method of the invention can remarkably reduce the emission of both of NOx and the smoke simultaneously, by adopting a specific flow pattern of fuel and combustion air in the combustion chamber, the pattern has been obtained as a result of studies and experiments concerning the influence of the intensity of mixing of the fuel and the combustion air on the emission of NOx.

Abstract: A gaseous fuel-air premix combustion burner is disclosed wherein the production of oxides of nitrogen can be inhibited. By blowing secondary air onto at least a maximum temperature region of a secondary reaction region of a gaseous fuel-air premix combustion flame, it is possible to minimize the production of oxides of nitrogen not only in regions of the flame where the volume of primary air is smaller or larger than the theoretical volume of air but also in the vicinity of a region of the theoretical volume of air, whereby the area of a region where the production of oxides of nitrogen is reduced can be increased.

Abstract: When explosive gases are introduced into a combustion chamber care must be taken that no pre-ignition occurs. To this end, the gas is spatially subdivided into discrete volume elements, for example by means of a bundle of tubes having an internal diameter which does not exceed twice the quenching distance of the explosive gases. Such a division ensures that the wall distances of the gas portions of one volume element are greater than their quenching distances.

Abstract: A compact burner-boiler combination having an improved heating cycle for effecting substantial fuel saving and maximum boiler and combustion efficiency in a relatively simple and expedient manner. This is attained by a compact boiler construction utilizing an improved burner construction and method of combustion in which a liquid fuel is gasified by the heating products of combustion to form a homogeneous fuel gas-air mixture prior to combustion so as to effect a substantially stoichiometric burning of the fuel so as to result in a high velocity and a very high flame temperature (approx. 4100.degree. F.) and to cause the breakdown of a portion of the hydrocarbons into their basic component elements so as to make them available for diffusion burning.

Abstract: Disclosed is a process for combusting coal wherein the emission of SO.sub.x or the emission of SO.sub.x and NO.sub.x are minimized. The process comprises (a) providing a coal containing at least twice as much organic calcium than sulfur; (b) burning the coal at a temperature greater than about 1200.degree. C. under reducing conditions; (c) separating the solid effluents from the gaseous effluents; and (d) burning the gaseous effluents at a temperature from about 1000.degree. C. to 1500.degree. C. under oxidizing conditions.

Abstract: A method of producing a hot gas flow by combustion of preferably liquid fuels, by the following steps:(a) atomizing the liquid fuel to form fine particles;(b) further atomizing the thus generated fuel particles with the aid of combustion air supplied as atomizing medium, and reducing the mean fuel particle diameter to less than one-half of the mean diameter obtained in step (a);(c) burning the fuel-air mixture within a flame cone extending on an axis A, under stoichiometric conditions or with an excess of oxygen within an air duct;(d) introducing a stream of cold air to be heated or of another medium, optionally including components adapted to be oxidized, in a plane E extending substantially perpendicularly to axis A.Further, the invention relates to a hot gas generator or incinerator for carrying out the method, comprising a straight air conducting pipe which has coaxially positioned therein a burner for liquid fuels, including a two-stage atomizing assembly for the liquid fuel.

Abstract: A burner body defines a combustion zone and a secondary dilution zone. A first inlet opening leads to the combustion zone to admit fuel and air while a second inlet opening leads to the secondary dilution zone. The burner body contains a tertiary dilution zone connected to a third inlet opening. The secondary dilution zone is positioned downstream from the combustion zone and the tertiary dilution zone is positioned downstream from the secondary dilution zone.The flow areas of the first inlet opening and the second inlet opening is arranged to have a constant ratio. An air passage supplies air to the first, second and third inlet openings and a flow controller is positioned to control the air flow through the third inlet opening. When the flow controller is closed, air flows from the passage through the first and second inlet openings, while, when the flow controller is open, air flows from the passage through the first, second and third inlet openings.

Abstract: In order to burn a fuel of low ignition properties and low contents of volatile gases in steam generators: the combustion air is introduced in three batches at three successive places into the firebox. Preferably, the primary air is introduced at a temperature of at least about 500.degree., preferably between 500.degree. and 600.degree. C. so as to effect a pregasification of the fuel. The secondary air is then introduced at a temperature of about 350.degree. to 400.degree. C. to start the combustion of the pregasified fuel and the tertiary air is finally introduced at a temperature of only about 250.degree. to 300.degree. C. in order to complete the combustion. This temperature differentiation of heat ranges then will permit to withdraw theash as a dry discharge.

Abstract: A combustion method and apparatus for disposal of materials that release noxious, corrosive, and/or toxic gases when burned, particularly halogens and/or acidic gases, is disclosed herein. In the disclosed system, a waste material is burned, the gaseous products of that combustion are cooled, and the cooled gases are then contacted with an alkaline substance of a Periodic Table Group I.A or II.A element in a fluidized bed to scavenge the noxious, corrosive, and/or toxic components in the combustion products.

Abstract: Methods of operating combustors to produce lower emissions, particularly lower emissions of nitrogen oxides and CO, including supplying separate streams of air to primary and secondary combustion regions of a combustor, and expanding combustion products when passing same from said primary combustion region to said secondary combustion region. In preferred embodiments, unheated air can be used in said primary combustion region, and/or a second stream of air mixed with said combustion products during said expansion thereof.

Abstract: A burner apparatus comprising in axial alignment a cylindrical first section connected through a restrictive opening to a frustoconical second section having its largest diameter at the downstream end and an axial outlet opening at the downstream end. Tangential combustion supporting gas inlets aligned in the same rotational direction are located in the cylindrical section and at the downstream end of the frustoconical section. The outlet opening in the frustoconical second section is of greater diameter than the restrictive opening between the sections and is of lesser diameter than the upstream end of the frustoconical second section so that a stable flame is produced in the second section, there is an inflow of external gas through the outlet opening, and a flow of combustion supporting gas in an upstream direction along the wall of the second section. A method for burning fuel in combustion supporting gas, particularly adapted to produce carbon black, is also provided.

Abstract: Process and apparatus for the partial combustion of pulverized coal, in which the pulverized coal, dispersed in an inert carrier gas, is supplied to a burner, and oxygen is injected into the stream of pulverized coal at two longitudinally spaced locations in the burner for feeding into a gasification reactor.

Abstract: This invention relates to a new and novel method and apparatus for reacting nongaseous material with a gaseous reactant comprising introducing a first stream containing a nongaseous material into a reaction zone; simultaneously introducing a second stream containing a gaseous reactant into the reaction zone such that the gaseous reactant immediately contacts and reacts with the first stream thereby producing a gaseous product; forming a spiralling vortex within the reaction zone to cause substantial separation of gases, including the gaseous product, from the nongaseous material; forming and removing a third stream from the reaction zone containing the gaseous product which is substantially free of the nongaseous material before a major portion of the gaseous product can react with the nongaseous material; and forming and removing a fourth stream containing the nongaseous material from the reaction zone.

Abstract: An apparatus and method whereby fuel is burned in serially connected furnaces under controlled combustion temperature and airflow conditions so as to inhibit the formation of nitric oxides while achieving complete combustion of the fuel.

Abstract: In the complete and safe purification of waste gases which contain organic or inorganic combustible substances by thermal decomposition at temperatures between about 800.degree. C. and 1000.degree. C., wherein the amount of waste gas supplied to an inlet varies in time within wide limits and also the concentration of oxygen and combustible substances varies in such a way that the lower explosion limit is at times exceeded in the waste gas and wherein supplementary fuel is optionally added for the maintenance of the decomposition temperature, the improvement which comprises(a) adding to the waste gas to be decomposed fresh air or waste gas which contains no combustible substances in a constant amount which maintains the incoming flow rate W.sub.o defined in FIG.

Abstract: A fuel burner adapted to produce a stable, non-contaminating blue flame throughout a broad operating range. The burner includes a combustion zone into which is directly fed the combustion air. Air-atomized liquid fuel is fed into the combustion zone through a Venturi whose constricted throat communicates by way of a feedback passage to the combustion zone, whereby the ejector effect produced by the atomized fuel passing through the Venturi causes a portion of the hot combustion gas generated in the combustion zone to be drawn into the throat to intermix with the atomized fuel therein, thereby pre-vaporizing the fuel to insure full combustion thereof.

Abstract: A method for effecting combustion of carbonaceous fuel is operative to sustain the combustion at a predetermined rate or rates of total fuel demand, the fuel being distributed between two combustion stages to make up the required total feed rate. Thus one portion, usually about 20-70 percent, of the total fuel is fed to a thermal combustion stage. Sufficient air is supplied simultaneously to the thermal combustion stage, preferably about 125 to 185 percent of the stoichiometric amount of air, to effect substantially complete combustion of that portion of the fuel and produce a heated effluent. An additional amount of relatively cool air is mixed with this effluent, and the resulting cooled mixture continues through the mixing zone, the additional air being sufficient to inhibit preburning of the remaining portion of the fuel which is added downstream in the mixing zone.

Abstract: A liquid and/or gaseous fuel is burned in a tubular combustion chamber which has at least one baffle wall therein with a central aperture therethrough, comprises directing the fuel into the combustion chamber in a stream adjacent the baffle so as to cause it to flow through the aperture of the baffle and directing combustion air into the combustion chamber around and substantially parallel to the fuel stream so as to cause a portion of it to be directed against the baffle plate and whirl around adjacent the aperture and a further portion to pass directly through the aperture and into the whirling stream so as to provide a turbulent air stream which intermixes with the fuel both downstream and upstream of the baffle, igniting the fuel and air as it intermixes, and directing a separate combustion air stream into the intermixed fuel and combustion air on the opposite downstream side of the baffle.

Abstract: The complete combustion of hot gases of low calorific power is effected by introducing the gases into a combustion chamber at a pressure close to atmospheric pressure and at a temperature of between 600.degree. and 900.degree. C, by introducing at burner level primary air to a maximum of 80% of the stoichiometric proportion, by supplying secondary air near the base of the flame in an excess relation to the stoichiometric amount so that the flame temperature is between 1,000.degree. and 1,300.degree. C and by discharging the combustion products from the combustion chamber by means of a chimney.

Abstract: A combustor for a gas turbine engine which includes a fuel nozzle at the head end of the combustor, to provide a diffusion flame, and downstream inlet means at a plurality of axial dimensions of the combustor to inject pre-mixed lean fuel/air into the combustor for admission downstream from the diffusion flame resulting in a series of low temperature premixed flames to provide relatively high turbine inlet temperatures from the combustor with a minimum of thermally formed NOx compounds.

Abstract: A combuster is provided for utilizing a combustible mixture containing fuel and air, to heat a load fluid such as water or air, in a manner that minimizes the formation of nitrogen oxide. The combustible mixture passes through a small diameter tube where the mixture is heated to its combustion temperature, while the load fluid flows past the outside of the tube to receive heat. The tube is of a diameter small enough that the combustible mixture cannot form a flame, and yet is not subject to wall quench, so that combustion occurs, but at a temperature less than under free flame conditions. Most of the heat required for heating the combustible mixture to its combustion temperature, is obtained from heat flow through the walls of the pipe to the mixture.

Abstract: A process for the simultaneous combustion of nitrogen-rich fuels and nitrogen-poor fuels which results in the production of less nitrogen oxide emissions.

Abstract: A direct heating surface combustor is provided that utilizes a non-adiabatic flame to provide a low-emission combustion for gas turbines. A fuel-air mixture is directed through a porous wall, the other side of which serves as a combustion surface. A radiant heat sink disposed adjacent to and spaced from the combustion surface controls the combustor flame temperature in order to prevent the formation of oxides of nitrogen. A secondary air flow cools the heat sink. Additionally, up to 100% of secondary air flow is mixed with the combustion products at the direct heating surface combustor to dilute such products thereby reducing exit temperature. However, if less than 100% secondary air is mixed at the combustor, the remainder may be added to the combustion products further downstream.

Abstract: A gas cooking range of the smooth top type has four burners positioned under a single plate of heat-resistant glass/ceramic material; and a single igniter and safety control assembly is centrally positioned between the burners. The supply of gas to each of the burners flows through an ignition chamber where it is ignited, and it then flows through a combustion tube to a combustion chamber, where combustion is completed. Some air is mixed with the gas at the fuel supply control valve, and additional air is supplied through the ignition chamber. The burning gas mixture then flows through the combustion tube to the combustion chamber at the entrance of which an additional quantity of air is added to provide the remainder of air necessary for complete combustion.

Abstract: Reasons of ecology, health and prevention of corrosion require firing methods and furnaces with which the contents of soot and uncombustible gases such as carbon monoxide, hydrogen, hydrocarbons as well as nitrogen oxides and sulfur trioxides do not excess certain levels. This is obtained by a firing method in which a fuel is decomposed with deficient amounts of primary combustion air to combustible gases. Such gases are combusted by the admixture of secondary and tertiary combustion air amounts whereby a flame is obtained which is extended in space and time and, thus, the temperature of which does not rise above moderate values such as 1400.degree. centigrade. Prior to being exhausted, the combustion gases are thoroughly mixed so as to obtain perfect combustion of possibly subsisting combustible substances. Exhausting takes place with heat withdrawal so that cool and pure combustion gases enter the ambiency.

Abstract: Reasons of ecology, health and prevention of corrosion require firing methods and furnaces with which the contents of soot and uncombustible gases such as carbon monoxide, hydrogen, hydrocarbons as well as nitrogen oxides and sulfur trioxides do not excess certain levels. This is obtained by a firing method in which a fuel is decomposed with deficient amounts of primary combustion air to combustible gases. Such gases are combusted by the admixture of secondary and tertiary combustion air amounts whereby a flame is obtained which is extended in space and time and, thus, the temperature of which does not rise above moderate values such as 1400.degree. C. Prior to being exhausted, the combustion gases are thoroughly mixed so as to obtain perfect combustion of possibly subsisting combustible substances. Exhausting takes place with heat withdrawal so that cool and pure combustion gases enter the ambiency.

Abstract: A fuel combustion apparatus of a double-shell construction in which the inner shell is composed of an upstream small-diameter section, a downstream large-diameter section, and a conically-shaped connecting section between the two sections of different diameters, all the sections being aligned and connected together on a common axis. Fuel injection valve and a first air supply port equipped with swirl blades surrounding the valve are provided at the upstream end of the small-diameter section of the inner shell. The cylindrical wall of the small-diameter section is not formed with any air supply port, but second air supply ports shrouded with scoops are formed in the cylindrical wall portion of the large-diameter section close to the small-diameter section. On the upstream side of the first air supply port is installed means for controlling the rate of air supply to the same port.

Abstract: A method for combusting nitrogen-containing fuel by: combusting a first fuel-air mixture in the presence of a catalyst in a first stage, operated fuel-rich so that the amount of air in the first stage is substantially less than the amount needed for complete combustion; adding additional air to the effluent gas from the first stage to form a second mixture with an amount of air at least sufficient to combust fully the effluent from the first stage; and then combusting the second mixture in a second stage. The first mixture is sufficiently fuel-rich, and the second mixture contains sufficient additional air, so that the combustion temperature in the first stage is below a temperature that would result in any substantial formation of oxides of nitrogen or other fixed nitrogen compounds from atmospheric nitrogen present in the mixture being combusted, and the second stage temperature also is below that for substantial nitrogen oxide formation therefrom.

Abstract: Combustion chamber comprising a first combustion zone opening into a wider second combustion zone into which the combustion air is admitted through a practically frontal air entry comprising obstacles which impart a coefficient of blocking of about 60% to 80% to it. The first zone is supplied with air and fuel in substantially stoichiometric proportion in idling. At the other running rates, a supplementary fuel flow is injected into the chamber and burns in the second zone in the wake of the obstacles.

Abstract: A method of firing coal-powered boilers to produce both heat to operate the boiler at its full rated capacity, and also to produce a usable fuel gas generally equal in heat content to the heat consumed in the boiler to produce steam, said method consisting of burning pulverized coal in the presence of varying proportions of steam and oxygen. The boiler is over-fired with coal, but with an undersupply of oxygen, in order that the total available heat content of the coal is divided between heat released by the incomplete combustion thereof, which may be sufficient to operate the boiler at full steam capacity, and heat content contained in combustible flue gases, which may be utilized for many different purposes.

Abstract: Fuel is burned in a primary combustion chamber with less than the air required for stoichiometric combustion so that the combustion gases have a high carbon monoxide (CO) and hydrocarbon content and the temperature of the gases is held below that at which significant nitrogen oxides (NO.sub.2) would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide (CO.sub.2). The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone is maintained at a temperature below that at which nitrogen oxides (NO.sub.x) will be produced in significant quantities.

Abstract: NO.sub.x produced by combustion of nitrogen-containing fuels is reduced by a forced draft burner operating with below stoichiometric mixtures of air and fuel in a primary combustion chamber, combustion being completed by controlled injection of secondary air near the outlet of the chamber.

Abstract: Burner arrangements for staged combustion to decrease nitrogen oxides (NOx) during the combustion of hydrocarbon fuel with air. The burner arrangements have a combustion into which hydrocarbon fuel and less than 70% of the stoichiometric air are introduced, and a path for the secondary air around the combustion chamber. A flame holding means is provided in one end of the combustion chamber to stabilize the substoichiometric combustion so that a partially burned gas containing mainly H.sub.2 and CO as combustible components is obtained. The combustion chamber has an exit, and there is an exit from the secondary air path near the exit of the combustion chamber. A discharging means for the partially burned gas is provided at the exit of the combustion chamber to vary the pattern of the flow of said partially burned gas by converting the enthalpy of said partially burned gas into kinetic energy.

Abstract: A nozzle employed as a burner within a fluidized bed is coaxially enclosed within a tubular shroud that extends beyond the nozzle length into the fluidized bed. The open-ended shroud portion beyond the nozzle end provides an antechamber for mixture and combustion of atomized fuel with an oxygen-containing gas. The arrangement provides improved combustion efficiency and excludes bed particles from the high-velocity, high-temperature portions of the flame to reduce particle attrition.

Abstract: Fuel is burned in a primary combustion chamber with less than the air required for stoichiometric combustion so that the combustion gases have a high carbon monoxide (CO) and hydrocarbon content and the temperature of the gases is held below that at which significant nitrogen oxides (NO.sub.x) would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide (CO.sub. 2). The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone is maintained at a temperature below that at which nitrogen oxides (NO.sub.x) will be produced in significant quantities.

Abstract: New combustors, and methods of operating same, which produce lower emissions, particularly lower emissions of nitrogen oxides. Methods and means are provided for supplying separate streams of air to primary and secondary combustion zones of a combustor, for removing heat from said primary combustion zone, and reintroducing said heat into the combustor at a region spaced apart and downstream from said primary and secondary combustion zones.

Abstract: A catalyst chamber capable of converting the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and ammonia in the presence of a reducing atmosphere is inserted downstream of the boiler section of a fossil fuel burning power generator and there is added a system for extracting formed hydrogen sulfide from the fuel gas stream.After combustion of the primary fuel, typically sulfur bearing coal in excess air, a gaseous hydrocarbon such as methane, is combined with the products of combustion to at least scavenge the oxygen present to form a boiler effluent which ranges from slightly oxidizing to reducing in nature.Where the boiler effluent is reducing in nature, partial conversion of the oxides of sulfur and the oxides of nitrogen occur in the boiler, the balance occurring in the catalyst chamber.

Abstract: A burner for Rankine cycle engines which includes a combustor of the rotating atomizer type and a control system therefor which keeps the ratio of fuel and air supplied to the combustor at an optimum over a wide range of operation to maximize efficiency and minimize the emission of pollutants from the combustor.

Abstract: New combustors, and methods of operating same, which produce lower emissions, particularly lower emissions of nitrogen oxide and CO, are provided. Means and methods are provided for supplying separate streams of air to first and second combustion regions of a combustor, and expanding combustion products when passing same from said first combustion region to said second combustion region. The combustors of the invention are characterized by the relatively small volume of said first combustion region as compared to the total volume of said first combustion region and said second combustion region; and methods and means provided for introducing first and second streams of air to said first and second regions, respectively. In some embodiments of the invention unheated air is used in the first combustion region. In other embodiments of the invention heated air is used in said first combustion region.

Abstract: New combustors, and methods of operating same, which produce lower emissions, particularly lower emissions of nitrogen oxides and CO, are provided. A first stream of air is introduced, either radially or axially, to a first combustion region of the combustor. A second stream of air is introduced tangentially into said first combustion region. A third stream of air is introduced tangentially into a second combustion region of the combustor. The amount of said first stream of air admitted to the first combustion region is varied in accordance with the fuel flow to said first combustion region.

Abstract: A gas conditioning means and more particularly a gas conditioning means incorporating an improved method of providing sulfur trioxide for injection into a flue gas stream.

Abstract: The disclosure of this invention relates to a gas turbine engine combustion chamber including a flame tube having holes closed by a filling of a material of lower melting point than the flame tube material so that if in operation the temperature of the tube exceeds that of the filling the latter melt and cooling air can enter the tube through the holes.

Abstract: Carbonaceous fuel is thermally combusted with an amount of air substantially less than that needed for complete combustion to carbon dioxide and water of all the combustible components in the fuel to produce a gaseous first effluent containing a substantial proportion of carbon monoxide but little or no nitrogen oxides. The first effluent is mixed with additional air and at least a portion of the resulting mixture is passed into the presence of a solid oxidation catalyst for combustion at a temperature above the instantaneous auto-ignition temperature of the mixture but below nitrogen-oxide-forming temperatures. Heat is withdrawn from the initial thermal combustion stage and from the gases downstream of the catalyst for the purpose for which the furnace is being operated.

Abstract: Method and burner apparatus for burning fuels wherein an air-fuel premixture of a low fuel concentration formed by premixing a fuel with combustion air in an amount larger than the theoretical air amount necessary for combustion of said fuel is fed and burnt in a combustion zone, and simultaneously, a fuel and combustion air in an amount smaller than the theoretical air amount necessary for combustion of said fuel are fed separately and burnt in the vicinity of said combustion zone or an air-fuel premixture formed by premixing a fuel with a part of combustion air supplied in said amount and the remainder of said combustion air are fed separately in the vicinity of said combustion zone.

Abstract: A low emission combustion chamber in which a premixed, fuel rich-air mixture is discharged uniformly into the hot products of combustion of a pilot combustion zone to be vaporized thereby and to flow therewith as a non-autoignitable, vaporized, fuel rich-air mixture into the main combustion zone where combustion air is added thereto in swirling form for rapid mixing, autocombustion and burning therewith for low emission results.

Abstract: An emulsion oil containing oil and water in a ratio of 5 - 9.5:5 - 0.5 by volume is burned by two-stage combustion method or by the combination of two-stage combustion method and exhaust gas recirculation method.

Abstract: Fuel is burned in a primary combustion chamber with less than the air required for stoichiometric combustion so that the combustion gases have a high carbon monoxide (CO) and hydrocarbon content and the temperature of the gases is held below that at which significant nitrogen oxides (NO.sub.x) would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide (CO.sub.2). The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone is maintained at a temperature below that at which nitorgen oxides (NO.sub.x) will be produced in significant quantities.

Abstract: A steam generator consists of several units in which combustion air is mixed with fuel at furnace sections placed in series. An excess of combustion air is used in the first furnace section and the exhaust gases which include unused combustion air are led into another furnace where fuel is added in sufficient quantities to use at least a portion of the combustion air which was not used in said first furnace.