Gas Fired Particulate Melting Apparatus and Method

This invention relates to a liquid fuel fired melting furnace for particulate material and more particularly it relates to a furnace apparatus with a shaft preheater section mounted above a melting chamber wherein the surface of a lower portion of a free-standing pile of preheated feedstock is melted.

The initial objective of this invention disclosed herein was to solve various problems relating to the melting of blast furnace slag and other siliceous raw materials for the production of mineral wool insulation. Normally the melting process was carried out in a cupola or shaft type furnace into the top of which the raw material was fed along with a solid fuel, such as a metallurgical coke. The fuel was ignited and combustion air was circulated upwardly from the bottom of the furnace through the mixture of raw material and fuel. This caused the fuel to be consumed, heat to be produced and the raw material to be melted. The melted material gravitated to the bottom of the furnace where it was drawn off and formed into mineral wool. Additional raw material and fuel were fed intermittently or continuously into the top of furnace to replenish the portion of the mixture which was consumed or melted.

A recent problem with the process is that the cost of the high quality coke required to produce a suitable product has risen significantly. Another problem which has plagued prior art processes using solid carbonaceous fuel is that the intimate contact between the melted material and the unburned fuel, particularly at the high temperatures involved, resulted in contamination of the product by the fuel. At times the flue gases which were emitted from the furnace contained contaminants, especially if a sulfur containing solid fuel were used, and thus had to be treated by anti pollution devices before they could be discharged into the atmosphere. Another critical problem was the lack of operational controls. After an effective particle size had been selected along with the proper ratio of fuel in the charge, the only remaining controllable variable in the process was the flow rate of combustion air through the furnace. All of these factors added to the cost of production or difficulty of operation.

Natural gas appeared to be an ideal alternative fuel because it is a relatively clean burning fuel and thus will not contaminate the feedstock material or create an air pollution problem if it is used properly. Also in most instances it is more economical to use than other sources of heat. However, when slag or a similar particulate material was melted in a shaft furnace by itself, rather than in a mixture containing a solid fuel, it tended to coalesce randomly in the shaft thereby impeding the uniform upward flow of hot combustion gases through the load and also impeding the uniform downward flow of the feedstock.

Accordingly it is a general object of this invention to solve the aforementioned problems and to provide an improved liquid fuel fired furnace apparatus and related method for economically melting flowable particulate material. It is another object to provide such a furnace and method which have more controllable variables and thus are operationally more flexible. It is still another object of this invention to provide a melting furnace with a gas fired heating system which effectively distributes and efficiently utilizes the products of combustion to preheat and melt particulate feedstock, such as blast furnace slag.

The particulate melter disclosed herein is a liquid fuel fired furnace apparatus having a vertically disposed cylindrical preheater section or shaft supported over the open top of a coaxially disposed melting chamber. Flowable particulate feedstock is introduced into the top of the vertically disposed preheater shaft to form a foraminous column of feedstock which is preheated to an incipient softening point temperature by gases flowing upwardly from the melting chamber. The bottom of the column of feedstock in the shaft is supported on the top of a freestanding pile of feedstock contained in the surrounding melting chamber. The melting chamber has downwardly diverging sidewalls which are sized and designed with respect to the bottom end of the preheater shaft such that the sidewalls remain spaced from the surface of the freestanding pile of feedstock. Liquid fuel burners direct hot combustion products towards and around the base and middle portions of the feedstock pile causing the surface of these portions to be melted. Gas at a temperature below the softening point temperature of the feedstock is introduced around the top portion of the pile to prevent hot gases in the lower portions of the melting chamber from prematurely softening or melting the feedstock in this region. A reservoir for holding the molten material is located at the lower end of the melting chamber floor.

Preferably the system for generating and distributing the heat includes a liquid fuel fired heat exchanger for producing heated air and cooled combustion gases. The heated air is conveyed to high temperature burners in the melting chamber while the cooled gases are conveyed to the top portion of the melting chamber.

FIG. 1 is a schematic elevational view of the melting furnace apparatus with portions of it shown in cross section.

Referring to the drawing the liquid fuel fired melting apparatus 10 has a vertically disposed hollow cylindrical preheater section 12 which is open at both ends and made of suitable refractory material. The open bottom end of the preheater section is mounted concentrically on the open too of a generally frustoconical shaped chamber 14 defined by refractory sidewalls which extend outwardly and downwardly from the open end of the preheater section. A high density wear resistant refractory ring 16 having a sharp inner edge and a horizontally disposed bottom surface exposed to the chamber is provided on the inside corner of the bottom of the preheater section 12 to enhance an abrupt separation of the feedstock from contact with the surrounding wall. An annular manifold 18 for injecting cooled combustion gases is disposed around the top section 20 of the chamber. The manifold 18 has a plurality of inwardly and downwardly directed et apertures 22 extending through the inclined inner surface of the melting chamber adjacent to the separator ring 16. The lower portion of the chamber 14 where the actual melting occurs is divided into an intermediate section 24 and a bottom section 26. High temperature liquid fuel burners 28 fire tangentially into each of these lower sections. The intense heat produced by the burners causes the surface of the freestanding pile of preheated feedstock to melt and cascade to the floor 30 of the melting chamber. Preferably the floor 30 is sloped towards one side so that the melted material will gravitate to the lower end thereof where it can be drawn off or collected in a reservoir or forehearth 32 for additional processing.

Heat is generated and distributed through the furnace by a means which includes a first combustion system for producing heated air and cooled combustion gases. It has a liquid fuel fired burner 34 incorporated into a heat exchanger 40. Air is fed under pressure to the burner 34 and also through one side of the heat exchanger 40 from a common source, such as a compressor or blower 42. The combustion products or gases, which are cooled as they travel through the other side of the heat exchanger, are conveyed to annular manifold 18 via conduit 44. In the process the air flowing through the first side of the heat exchanger becomes heated. This heated air is transferred from the heat exchanger via conduit 46 to a second combustion system which includes high temperature melting chamber burners 28. Fuel is fed to both combustion systems through a main fuel supply pipe 50 and then to the respective systems through branch lines 52, 54. Another branch line 56 may be used to inject reducing gas to a distributor dome 60 projecting upwardly from the center of the melting chamber floor 30 into the bottom portion of the freestanding pile of preheated feedstock. Preferably the fuel used in the systems is natural gas, but if conditions warrant then other liquid fuels could be used.

The operating process involves the introduction of a flowable particulate feedstock into the top of the preheater section to form a foraminous column of feedstock therein. The bottom of the column is supported on the top of a freestanding pile of preheated feedstock disposed on the floor of the surrounding melting chamber. High temperature burners fire into the lower portions of the melting chamber causing the feedstock on the surface of the pile in these regions to be melted. The melted material gravitates to the sloped floor and then travels down the slope of the floor to a reservoir located at its lower end where it may be processed further or drawn off for immediate use.

Cooled combustion oases from the heat exchanger of the first combustion system are introduced through an annular manifold on the top of the melting chamber to envelope the top portion of the feedstock pile and prevent the hot combustion gases and radiation from the lower section of the melting chamber from melting or softening the feedstock in the top of the pile. The flow rate of the cooled combustion gases into the chamber may be varied during operation as well as the combustion rate of the high intensity burners so as to maximise melting efficiency.

The column of feedstock in the preheater is heated to an incipient softening point temperature by an upwardly flowing mixture of cooled combustion gases from the too of the melting chamber and hot combustion gases from the lower portions of the melting chamber. The temperature profile in the preheater increases from top to bottom and from outside to inside whereas the temperature profile in the pile of feedstock in the melting chamber increases from inside to outside. A reducing gas may be introduced into the bottom center of the pile to provide additional process control.

Although the invention is described with respect to a single illustrated embodiment, it is to be understood that modification can be made without departing from the scope of the invention which is defined by the appended claims.