Abstract: A fan assembly includes an internal housing having an outer internal housing, and an inner internal housing partially disposed within the outer internal housing. The inner internal housing defines an internal cavity, and has a curved exterior surface. Stator blades extend between the inner internal housing and the outer internal housing.

Abstract: Reverse steam generator for a lead-cooled fast reactor. The reverse steam generator comprises a cylindrical body with a bundle of heat exchange tubes located inside, the ends of the heat exchange tubes being fixed in tube sheets with intermediate support grids; inlet and outlet spherical chambers for supplying liquid metal coolant; a lower branch pipe for inlet water; and an upper branch pipe for a steam outlet. The cylindrical body is arranged horizontally and is curved in a Z-shape with a difference in height. The bundle of heat exchange tubes is also made in a Z-shape, repeating the bend of the cylindrical body.

Abstract: A dew point corrosion resistant heat exchanging system having a plurality of hollow heat transferring tubes through which cooler ambient air or hot combustion product gasses flow. The other of the air or gas flows across the outer surfaces of the tubes, and heat is transferred from the hot gasses to the ambient air, thus heating the air. A portion of the tubes includes an inner liner forming an air pocket chamber between the liner and the outer wall of the tube. The air pocket chamber provides heat transfer advantages that maintain the tubes at a temperature above the dew point of the gasses in the system, thus inhibiting corrosion of the tubes.

Abstract: A circular columnar body made of ceramics and including a plurality of through-holes extending in a single axial direction Z and having circular or elliptical cross sectional shapes orthogonal to the axial direction at a density of 2 to 50 pieces per square inch in a cross section orthogonal to the axial direction is once hollowed out in a direction X orthogonal to the axial direction Z by a cylinder, and the circular columnar body can also be once again hollowed out in the direction orthogonal to the axial direction Z and the direction X by the cylinder.

Abstract: The invention relates to an exhaust gas heat exchanger for transferring heat between the exhaust gas of a combustion engine of a motor vehicle and a coolant. The exhaust gas heat exchanger has a housing with connections for supplying and discharging coolant. According to the invention, one connection is formed by a pipe section which has a joining surface, formed on the outer circumference thereof, for fixing on the housing of the exhaust gas heat exchanger and a transfer opening, which is aligned transversely to the longitudinal axis (LA) of the pipe section and communicates with a passage opening in the housing.

Abstract: A plurality of independently flow rate-controlled flows of oxidant may be preheated at a heat exchanger (or both oxidant and fuel at separate heat exchangers) by heat exchange with a hot shell-side (heat transfer) fluid. The separate flows of hot oxidant are directed to associated separate burners where they combust with flows of fuel to produce hot combustion gases. The hot combustion gases are used to preheat the hot shell-side fluid at a recuperator or regenerator.

Abstract: Flue gas is a by-product of many energy and industrial plants and is typically emitted through a chimney stack. If the flue gas temperature in the chimney stack drops below the flue gas dew point, condensation of water vapor and acid gases ensues. These gases are very corrosive for chimney stacks designed to operate in a dry condition. The Flue Gas Reheat System of the present invention continuously and proactively manages flue gas chimney stack temperatures above the dew point in order to optimize emission control and effectuate energy efficiency improvements in industrial plants. Waste heat is harvested from the exterior surfaces of existing steam and pollution control equipment through conduction, convection and radiation. This heat is transferred to a working fluid. The working fluid is then directly mixed with the flue gas prior to the flue gas entering the chimney stack to raise the temperature of (or re-heat) the flue gas above its dew point to maintain a dry chimney stack condition.

Abstract: In an oxyfuel combustion boiler system, nitrogen gas separated by an air separation unit (ASU) is supplied as carrier gas to a pulverizer for drying and pulverization of fuel. A fluid mixture of the nitrogen gas from the pulverizer with pulverized fuel is supplied to a powder separation device where the pulverized fuel is separated. The separated pulverized fuel is mixed with the primary recirculated flue gas and supplied to a burner.

Abstract: A fired heater unit is disclosed having a radiant section having a high emissivity coating therein. The fired heater unit also includes a burner for mixing fuel and preheated air to produce flue gas to be used in heating operations within the unit, and a preheater for heating ambient air with the flue gas to produce the preheated air for use in the burner. Other process streams are heated with the flue gas in the radiant section and a convection section operatively connected to the radiant section prior to venting the flue gas to atmosphere. The fired heater unit, and associated processes, result in a reduction of NOx emissions over conventional systems and methods.

Abstract: In an oxygen mixer for an oxygen combustion boiler, circulating exhaust gas heated in a rotary preheater is mixed with oxygen. Resultant mixed gas is guided to a boiler furnace through a combustion gas duct having bends. At the bend closest to the rotary preheater in the combustion gas duct, an oxygen injection nozzle is arranged to have a plurality of injection ports for injection of oxygen into circulating exhaust gas.

Abstract: A method and system for generating electrical power is provided in which a high pressure synthesis gas stream generated in a gasifier is partially oxidized in an oxygen transport membrane based reactor, expanded and thereafter, is combusted in an oxygen transport membrane based boiler. A low pressure synthesis gas slip stream is split off downstream of the expanders and used as the source of fuel in the oxygen transport membrane based partial oxidation reactors to allow the oxygen transport membrane to operate at low fuel pressures with high fuel utilization. The combustion within the boiler generates heat to raise steam to in turn generate electricity by a generator coupled to a steam turbine. The resultant flue gas can be purified to produce a carbon dioxide product.

Abstract: Coal is properly dried and the efficiency of a boiler is increased while suppressing any reduction in efficiency of the boiler and the generation of hydrocarbon gas components. Employed are a coal dryer that dries coal, a boiler in which the coal that is dried in the coal dryer is mixed with air for combustion and burned, an air heater that heats the air for combustion that is supplied to the boiler using combustion exhaust gas discharged from the boiler, a precipitator that collects dust included in the combustion exhaust gas, a desulfurization plant that performs a desulfurization process on the combustion exhaust gas, and heat recovery equipment that heats a heating medium using the combustion exhaust gas. The coal dryer dries the coal using the heat energy of the heating medium that has been heated in the heat recovery equipment.

Abstract: A system for the thermal preparation of reclaimed road material containing PAHs, or of oil sludge having a loading station for pre-comminuted reclaimed road material or oil sludge, a rotary kiln in which the reclaimed road material or the oil sludge is burned, while oxidation air is fed in, a dust separator to which the flue gases produced in the rotary kiln are fed, an after-burner to which the flue gases leaving the dust separator are fed, and a filter to which the flue gases arising in the after-burning means are fed. These components are divided into a plurality of modules, each of which includes a weight and dimension such that each may individually be transported using conventional road transport means, in particular low loaders or heavy goods vehicles.

Abstract: In a boiler, a heat exchanger provided in one of two flow paths formed by partitioning a downstream portion of a flow path for exhaust gas discharged from a furnace has a smaller total heat transfer area than a heat exchanger provided in the other of the two flow paths; the exhaust gases discharged from the flow paths to outside the boiler are introduced, without being mixed, into an air heater downstream of the boiler; and, in the air heater, the heat of the exhaust gases is transferred to the primary air and the secondary air so as to heat combustion air.

Abstract: A fired heater includes a fired heater body with an air inlet and a flue gas outlet, and a flue gas waste heat recovery system communicated with the fired heater body and including at least two heat exchange chambers. A first port of each of the heat exchange chambers can be communicated with the flue gas outlet or the air inlet, and a second port of each of the heat exchange chambers can be communicated with the outside air or a fume extractor. When the first port of at least one heat exchanger chamber is communicated with the flue gas outlet and the second port thereof is communicated with the fume extractor, the first port of at least another heat exchange chamber is communicated with the air inlet and the second port thereof is communicated with the outside air.

Abstract: Integrated polymeric-ceramic membrane-based oxy-fuel combustor. The combustor includes a polymer membrane structure for receiving air at an input and for delivering oxygen-enriched air at an outlet. An oxygen transport reactor including a ceramic ion transport membrane receives the oxygen-enriched air from the polymer membrane structure to generate oxygen for combustion with a fuel introduced into the oxygen transport reactor.

Abstract: The present invention provides a burner system which uses waste fuels, especially waste plastic fuels. Burner size is minimized by having multiple combustion chambers concentrically located around a rotating screw conveyor. Heat efficiency is improved by having air passages disposed around the combustion chambers, thus preheating air for the combustion prior to its delivery to the combustion chambers, while simultaneously thermally insulating the combustion chambers against the environment. Waste fuel is transported from a fuel hopper to the combustion chambers by a rotating screw conveyor having the spiraling auger blades. Speed of screw conveyor rotation controls the consumption of waste fuel and, thus, the amount of thermal energy generated in the burner. The burner system includes an intelligent control system for controlling operation of the burner system, so that the burner system performs at optimum efficiency, safely and with minimum operator intervention.

Abstract: This disclosure includes a system and method of controlling fuel combustion including providing a system, measuring a property, and providing oxygen and fuel in response to the property. The system includes a furnace arranged and disposed to receive fuel and oxygen and combust the fuel and the oxygen to form a combustion fluid, a plurality of heat exchanger sections arranged and disposed to receive heat from the combustion fluid, and a plurality of oxygen injectors arranged and disposed to controllably provide oxygen to the combustion fluid to adjust composition of the combustion fluid and temperature of the combustion fluid. The property measured is selected from the group consisting of temperature of the combustion fluid, composition of the combustion fluid, temperature of the heat exchanger sections, and combinations thereof and is performed in close proximity to the oxygen injectors.

Abstract: An exemplary embodiment provides a regenerative burner apparatus. The apparatus includes a burner housing having a gas channel and a single-stage heat regenerator equipped with a housing enclosing a fluid-porous heat regenerative media bed. A first gas passageway in the housing directly interconnects the gas channel and the lower surface of the media bed. A second gas passageway in the housing interconnects an opening in the housing communicating with the exterior and the upper surface of the media bed. This arrangement allows hot waste combustion gases to pass upwardly through the media bed so that any condensable contaminant in the gases condenses to a liquid and flows out of the bed under gravity before becoming solid and clogging the bed. The liquid contaminant may then be removed from the regenerator from a position below the media bed.

Abstract: An air preheater 100 is described having an air damper assembly 162 that partially restricts an air inlet 130 and a flue gas damper assembly 152 that partially restricts flue gas inlet 124 during periods of reduced boiler load. Restricting the flue gas inlet 124 reduces the effective surface area of the preheater causing more heat to pass to the cold end of the air preheater 100, reducing acid condensation and fouling. Restricting the gas inlet 124, increases gas velocity, thereby eroding accumulations in the air preheater 100, also reducing fouling. Restricting the air inlet 130 reduces the effective heat transfer surface area of the air preheater, which raises the gas temperature in the cold end of the air preheater and thereby reduces acid condensation and fouling.

Abstract: This bio-fuel furnace includes a combustion unit and separate fuel feed unit mounted atop the combustion unit. The combustion unit includes a cylindrical burn pot, a preheat module, an expansion bowl and a primary heat exchanger supported by an inner frame and enclosed by an outer casing. The preheat module is a rectangular box having an internal spiral baffle. The burn pot is seated within a central opening in the preheat module. The expansion bowl is seated atop the preheat module and the primary heat exchanger is mounted above the expansion bowl. An inlet fan blows inlet air through the preheat module and into the burn pot and an outlet fan draws exhaust air around the primary heat exchanger from the combustion unit. The fuel feed unit has an internal storage bin and a rotating scoop arm deposits the bio fuel into the combustion unit.

Abstract: The present invention relates to limiting emissions of nitrogen oxides into the environment.

Abstract: This invention discloses the synergistic integration of solid fuel combustion, low NOx control technologies (such as Low NOx Burners, reburning and Advanced Reburning) with partial in-duct gasification of coal or other solid fuels. For partial gasification, the solid fuel can be transported and injected by recycled flue gas stream at 600-800° F. in the reburning zone or in the upper section of the main combustion zone of a boiler. This allows the fuel to be preheated and partially pyrolyzed and gasified in the duct and then injected into the boiler as a mixture of coal, gaseous products, and char. Gasification increases coal reactivity and results in lower carbon-in-ash levels. As an option, the gaseous and solid products can be split using a cyclone separator. Splitting the gasified fuel stream will allow the volatile matter to be used for reburning and the fixed carbon to be injected into the high-temperature main combustion zone.

Abstract: System for drying a damp biofuel, includes a boiler (1) for combustion of the fuel. Further, the system includes a first heat drying chamber (2), a drying gas flow (3) heated by the thermal energy of the combustion gases from the boiler and/or by steam, the gas flow being passed into the first heat drying chamber, and a fuel supply (4) for passing the fuel into the first heat drying chamber. The system includes a second heat drying chamber (5), an intermediate heating unit (6) for heating the drying gas flow before the second heat drying chamber, an intermediate supply (7) for passing the fuel from the first heat drying chamber into the second heat drying chamber, a boiler supply (8) for passing the fuel from the final heat drying chamber into the boiler and an outlet (9) for passing the flow of drying gas from the final heat drying chamber into the boiler.

Abstract: In a method of supplying air to a pulverized solid fuel burner passing through the burner cowl of a furnace, the fuel is dried and then conveyed by means of a given large quantity of hot air, which may or may not be depleted in oxygen in a separator in which the fuel is extracted from it. The fuel is then conveyed to the burner by means of a first portion only of the air from which the fuel has been removed. This conveyor air portion is used as primary combustion-supporting air for burning the fuel in the burner, into which the first portion of air, mixed with the fuel it conveys, is fed via an annular duct. The other portion of the hot air purified in the separator is used at least as additional primary combustion-supporting and flame adjustment air and is fed into the burner peripherally and centrally relative to the first portion of air charged with fuel.

Abstract: A method and apparatus for low-NOx combustion of a pulverized solid fuel in which combustion products from a partial oxidation combustor are mixed with a pulverized solid fuel, thereby preheating the pulverized solid fuel and resulting in devolatilization of at least a portion of the pulverized solid fuel. The preheated pulverized solid fuel and the devolatilization products are then burned in a burner firing directly into a combustion chamber.

Abstract: A fluidized bed combustion system and method in which a recycle heat exchanger is disposed integrally with the furnace of a fluidized bed combustor. The recycle heat exchanger includes a plurality of stacked sections for receiving the recycled solids and are arranged in such a manner that the recycled solids are introduced into an upper level of the sections and pass through these sections to a lower level of sections before returning to the furnace. A portion of the stacked sections contain heat exchange surfaces for removing heat from the solids therein while another portion does not. The solids in the various sections are selectively fluidized to control the flow of the solids through the sections to control the temperature of the solids accordingly.

Abstract: The present invention involves a regenerative thermal oxidation apparatus for purifying gases by thermal oxidation and for exchange of heat between inleting and outleting gases. The apparatus has a plurality of thermal chamber units, each such unit having a gate with at least one gate orifice movably located in a linear fashion against a base surface so as to alternatively align with various inlet and outlet openings. The improvement involves the use of a plurality of seals located between the gate and the openings with sufficient sealing capabilities between the gate and the openings to maintain each chamber at an elevated pressure, i.e. above atmospheric pressure, and so as to reduce leakage of gases from each of the chambers and reduce condensation during operation. The invention is also directed to the method utilizing this apparatus.

Abstract: The present invention is directed to an improved regenerative thermal oxidizer and a method of purifying gases by raising gases to their auto ignition temperature and by exchanging heat by inleting and outleting gases. The reactor has at least three regenerative thermal chamber units and these units have on a single surface at least three openings, a purge opening, an inlet opening and an outlet opening, which are connected to recycled outlet gases for purging, and to inlet and outlet gases respectively. Set against the purge, inlet and outlet openings is a slidable gate which has a single opening which moves through cycles so that its single opening coincides with the inlet opening of its chamber unit, or coincides with one or more spaces which are closed between the inlet and outlet opening, or with the purge opening, or coincide with the outlet opening. In other words, the gate cycles between inleting gases, no gas movement, purge position, no gas movement and outleting gases.

Abstract: The present invention is directed to a regenerative thermal oxidizer apparatus and a method for purifying gases by thermal oxidation and by exchange of heat by inleting and outleting gases. The oxidizer has at least three regenerative thermal chamber units and these units have on a single surface both an inlet opening and an outlet opening which are typically connected to inlet and outlet gases. Set against the inlet and outlet openings is a slidable gate which has a single opening which moves through cycles so that its single opening coincides with the inlet opening of its chamber unit, or a space which is closed between the inlet and outlet opening, or coincide with the outlet opening. In other words, the gate cycles between inleting gases, no gas movement, and outleting gases. There are means providing for moving the gate through the aforesaid cycle and, as mentioned, inlet ducts and outlet ducts connected to their respective openings.

Abstract: This invention relates to a regenerating, hazardous waste gas fume and odor incinerator. The incinerator comprises a combustion chamber (12) and at least three heat exchanger regenerator beds (26, 28, 30) through which the waste gases pass selectively for preheating on their way to the combustion chamber and the processed, heated gases are passed through for cooling before exhausted. All three regenerator beds are cycled from a cooling position in which they absorb heat from the previously processed gases to a second position where they preheat waste gases en route to the combustion chamber to an inactive position where they do not receive gases either from the waste supply or from the combustion chamber. The waste gases, after being preheated, are directed to the combustion chamber through a plenum chamber (14) in which is located a combustion burner (16). The preheated waste gases are directed through the flame of the burner into the combustion chamber for ignition and for combustion therein.

Abstract: A process and apparatus for removing moisture from smoke produced in a chemical reaction, such as a combustion reaction, by circulating outside air in heat-exchange relationship with smoke at a point near the chamber where the smoke is produced, thereby heating the air and cooling and partially dehydrating the smoke. A portion of the air which is thus heated is passed in further heat-exchange relationship with smoke and is then directed toward a smoke discharge stack where it is mixed with the partially dehydrated smoke.

Abstract: An emergency exhaust system (59) is provided for a mobile incineration system (10). A hinged gate (76) controls access to the emergency exhaust system (59), and operates only upon the sensing of a malfunction in one of a plurality of critical incineration system components (114, 36, 56) by sensors/relays (122-126). A malfunction as sensed by Sensors/relays (122-126) causes the power to be cut through switch (98) to the main incineration system components (114, 36, 56, 46, 116, 118, 22). In a preferred embodiment, the emergency exhaust system (59) comprises a plurality of water sprayers (64) to cool the volatile waste combustion products, and a granulated activated carbon column (68) to remove hazardous substances.

Abstract: A compact incinerator for biomass has an upright combustion chamber coaxially surrounded by a passage through which the biomass is fed from a separator in which the primary air is separated from the biomass, to the combustion chamber. An afterburner chamber surrounds this shaft so that flue gases from the combustion chamber can pass through the shaft and be filtered by the biomass while heating the biomass before afterburning is effected in the afterburner chamber. The primary air from the separator is preheated in a preheater in indirect heating exchange by the flue gas in the same coaxial apparatus which also includes, coaxially with the combustion chamber, a flue gas/solids separator for removing solids from the flue gas before it enters the preheater.

Abstract: A treatment method for the sulfur- and particle-containing hot waste gas of an incineration process where refuse is burned in the presence of air comprises the steps of preheating air with the hot waste gas, passing the hot waste gas through one side of a heat exchanger, passing air through the other side of the heat exchanger and thereby heating the air and cooling the waste gas, and desulfurizing the cooled waste gas. At most a portion of the heated air is fed directly to the incinerator and there used to combust the refuse. Another portion of the heated air is mixed with the cool desulfurized gas. The mixture of cool desulfurized gas and the other portion of the heated air is then released to the atmosphere. The preheating of the air is done in a second heat exchanger separate from the first-mentioned heat exchanger and traversed by the waste gas and air. In addition particles are filtered from the waste gas between the heat exchangers.

Abstract: A regenerative incinerator comprising at least two vertical heat exchanger chambers each having horizontal grates in an intermediate region thereof, a combustion chamber above the heat exchanger chambers connected at its bottom side to the top side of the heat exchanger chambers and an inlet duct connected into a region below the grates of each of the heat exchange chambers. A first valve resides in each of the inlet ducts and an exhaust duct is connected to another region below the grate of each heat exchanger chamber. A second valve resides in each exhaust duct and a timing means is provided having alternating first and second cycles, the first cycle of the timing means opening a portion of the valves and closing the second valves of the heat exchanger chambers having open first valves and opening a second valve of another heat exchanger chamber having a closed first valve and the second cycle of the timing means closing the first valve and opening the second valve of the heat exchanger chambers.

Abstract: The furnace system has a heating chamber wherein a rich combustion mixture is fed. The mixture has been preheated within a regenerator and the flue gas is ducted to another regenerator which is also fed with secondary air to more completely oxidize the flue gas. Part of the hot flue gas leaving the second regenerator is tapped and recirculated to the heating chamber and the remaining flue gas being fully oxidized is vented to the atmosphere. When the first mentioned regenerator becomes too cold to preheat the combustion mixture, given valves are opened and closed so that the combustion mixture first is preheated within the second regenerator, then burned, and directed to the heating chamber and the incomplete combustion flue gases are now ducted to the first mentioned regenerator to be completely oxidized as described before, and, in turn, to heat this regenerator.

Abstract: A combination structure interconnecting and sealing means for joining an air duct with the end plate of a heat exchanger core to allow for variations in spatial dimensions resulting from thermal expansion. The coupling arrangement comprises a circumferential flange attached, as by welding, to the circular duct. The periphery of the flange is provided with radial slots which are engaged underneath T-shaped clips attached to the heat exchanger plate. Within the circumference of the flange and generally in line with the duct and associated manifold section of the end plate is a U-shaped bladder member extending entirely around the joint and welded to provide a seal between the end of the duct and the plate. Similar bladder members are provided between adjacent units making up an overall heat exchanger core.

Abstract: In a pulverized fuel-fired steam generator, a steam air heater is disposed in the air inlet duct to the main air heater. Steam from an auxiliary in-plant source or from the boiler itself is passed through the steam air heater thereby increasing the temperature of the air entering the main air heater. The flow rate of steam passed through the steam air heater is regulated to maintain the flue gas temperature entering dust collection equipment located in the flue gas stream downstream of the main air heater above the dew point of water vapor and the condensation point of sulfuric acid during start-up and low load operations.

Abstract: An energy storage and distributing device having a heat storage chamber arranged for retaining heat which can be received from a solar collector arrangement or from a furnace or furnaces disposed within the chamber itself. Mounted on the chamber is a plenum arrangement disposed for directing warm fluid to a space to be heated, such as a room or rooms of a house, while a fluid distribution system is associated with the plenum and the chamber for alternately directing a fluid current directly into the plenum and directly into the chamber, and thereby supplying the plenum selectively with warm fluid from one of the chamber or from a source disposed outside the chamber. In this manner, solar energy, and the like, can be fed directly to the plenum, or stored in the chamber, or if solar energy is not available for whatever reason, and insufficiient heat energy is present in stored form within the chamber, a furnace or furnaces can be employed to supply the requisite heat energy.

Abstract: A heat exchanger which includes a plurality of elongated, parallel, vertical tubes for directing a hot fluid through a cross flowing fluid, particularly suited for handling larger quantities of fluid which contains abrasives. Each of the continuous tubes are secured at opposite ends to tube sheets. One tube sheet provides vertical support for the tubes, the other tube sheet is allowed to float. The tubes are thus unrestrained in the vertical direction and are permitted to freely expand in the presence of the hot fluid.

Abstract: An arrangement for supplying air to a boiler-furnace whereby an axial flow fan being used as a source of primary air may selectively take suction air from the atmosphere or from the discharge side of a fan being used to provide secondary air to the boiler. This arrangement will provide an increased inlet pressure for the primary air fan to thereby achieve increased stability and high system pressure for low load operation, thereby permitting the effective use of an axial flow fan as a source of primary air when such usage would otherwise be unjustified.

Abstract: A heat recovery system is provided for recovering heat from flue gases and the like and heating combustion air or the like in the form of a flue gas conduit receiving hot flue gas from a furnace or the like, a metallic recuperator receiving hot flue gas from said flue gas conduit, a ceramic regenerator selectively receiving at least a part of the hot flue gas leaving the metallic recuperator, by-pass means in parallel with the ceramic regenerator receiving any of the flue gas leaving the metallic recuperator which does not go through the ceramic regenerator, stack means receiving the flue gas from the by-pass means and the ceramic regenerator means for discharge to the atmosphere, means delivering air to be heated in parallel to the ceramic regenerator and the metallic recuperator, and conduit means receiving and combining heated air from the ceramic regenerator and the metallic recuperator for delivery to a point of use.

Abstract: A process and apparatus for the thermal destruction of solid fuel material in a gravity-type multiple hearth furnace having groups of two or more superimposed hearths. Solid fuel is fed to the upper hearth of each group of hearths. Auxiliary fuel is selectively burned in the lower hearth of each group of hearths to maintain the discharge temperature of the exhaust gas flowing from each said upper hearth above the ignition temperature of said gases so that each group of hearths is operated as a combined drying and burning zone. Preheated air for combustion is introduced into the lower hearth of the lowermost group of hearths so that oxygen available for combustion increases as the solid fuel moves downwardly. Ash is removed from the lower hearth of at least the lowermost group of hearths.