Selecting efficient parameters for the coke charge of shaft-type melting furnaces

Abstract

It is shown based on studies of the changes in the composition of the gases and temperature in the preliminary charge that the performance indices of coke-fired shaft furnaces are determined by the conditions under which the oxidizing zone is formed so as to produce the greatest possible amount of heat. It is shown that the size of this zone increases with an increase in the rate of filtration of the air blast, enrichment of the blast with oxygen, and the use of low-activity coke in the form of lumps of limited size. To maximize the reheating of the melt, it is necessary to increase the average size of the lumps and the flow rate of the air blast in order to increase The most commonly used source of heat in shaft melting furnaces of the cupola type (iron-foundry and mineral- wool cupolas and shaft furnaces used in nonferrous metallurgy) is metallurgical coke. The coke undergoes combustion in a air-blast flow supplied from an outside source. Among the distinguishing features of such furnaces: the furnace is of limited height (no higher than 4.5-5.0 m); the horizontal section of the layer of charge materials is also of limited dimensions (no more than 2.0-3.0 m); the injected gas jets are of low velocity (no higher than 75-80 m/sec); the charge materials do not undergo extensive physicochemical transformations. The quantity and distribution of the solid fuel in the working space of these fur- naces depends on the conditions under which the initial components are charged and on the required heating rate. As new portions of coke that enter the furnace with the charge descend in the furnace, they are ignited by a red-hot bed of coarse coke (the preliminary charge) that is introduced into the lower part of furnace beforehand. The upper part of this bed burns in a flow of air that is distributed by tuyeres or an outside source (1). Thanks to the high strength and porosity of the coke, it also acts as a drainage system that facilitates uniform distribution of the hot gases over the cross section of the furnace. In addition, it collects drops of the melt that filter through the supportive carbon-based packing and removes them from the working space of the furnace. In accordance with the theoretical postulates in (2), the combustion of solid carbon is an aggregation of complex physicochemical processes that take place in multiple stages: preparation of the fuel (drying and thermal decomposition accompanied by the release of volatile matter); combustion of the gaseous products and the coke residue. Numerous experimental studies (3, 4) have shown that the layered process of solid-fuel combustion depends both on the hydrodynamic conditions and the thermal conditions that exist. These conditions are interdependent and should be examined in relation to the specific conditions of the given production process. The most widely used method of feeding an air blast into shaft furnaces - the use of tuyeres - entails the use of high- velocity jets formed by tuyeres that are of a certain size and are evenly distributed about the perimeter of the furnace (5). Travelling at a high velocity (up to 30-50 m/sec) and having a high kinetic energy (up to 30-35 kW), the air blast propagates