Heating of Steel in Reheating Furnace Jun01

Heating of Steel in Reheating Furnace...

Heating of Steel in Reheating Furnace Reheating furnace is important equipment in the process of hot rolling. It is the heart of any hot rolling mill. Reheating of steel is a continuous process. The steel material to be rolled is charged at the entrance of the reheating furnace. The steel material is pushed forward on the hearth of the furnace by means of a pusher machine whose ram is in direct contact with the steel material. The steel material is pre-heated, heated and soaked as it passes through pre-heating, heating and soaking zone of the reheating furnace. At the end of the soaking zone of the furnace, the steel material is discharged from the furnace by ejector for rolling in the rolling mill. The temperature of the heated steel material at the time of discharged depends on several factors and it can vary in the range of 1100 deg C to 1250 deg C. The size of the reheating furnace is normally expressed as the capacity to supply the rolling mill with sufficiently hot steel, in tons per hour. Steel materials with different material compositions, dimensions, and charging temperatures can reside in the furnace simultaneously. The reheating furnace used for heating the steel materials is normally considered to be having high energy consumption. It also emits good amount pollutants in the atmosphere because the process used for heat generation is the combustion process. Reheating process has considerable influence on the economics of the working of the rolling mill. There are usually three types of continuous reheating furnaces used in the rolling mills. These are (i) pusher type furnace, (ii) walking hearth furnace, and (iii) walking beam furnace. Pusher type furnaces have some disadvantages which includes (i) frequent damage of refractory hearth, (ii) skid marks...

Mill Scale

Mill Scale Mill scale is the product of oxidation which takes place during hot rolling. The oxidation and scale formation of steel is an unavoidable phenomenon during the process of hot rolling which involve reheating of steel in a reheating furnace, multi-pass hot rolling and air-cooling in the inter-pass delay times and after rolling.  Mill scale is usually removed by process water used for descaling, roll and material cooling, and by other methods. It is subsequently separated by gravity separation techniques. The formation of oxide scale not only results in a significant loss of yield of steel, but also deteriorates the surface quality of the steel product caused by rolled-in scale defects or roughened surface. In addition, the presence of a hard scale layer on the steel can have an adverse effect on roll wear and working life. The amount of mill scale generated in a rolling mill depends on the type of the reheating furnace and on the practice of rolling adopted in the mill. It is generally in the range of 1 % to 3 % of the weight of the steel rolled. Mill scale mill scale is a layered and brittle material, composed of iron oxides with wustite as a predominant phase. It is normally considered as waste material. From the chemical and physical analysis performed on the mill scale, and with respect to the environmental concerns, mill scale is considered to be non-dangerous waste and normally considered as a green waste. Scale formed during the heating of steel to rolling temperatures in the reheating furnace is known as primary scale. This primary scale is removed generally by hydraulic descaling before hot rolling. The removal of the primary scale formed during the reheating operation before hot rolling is usually done for...

Combustion System of a Reheating Furnace Jul01

Combustion System of a Reheating Furnace...

Combustion System of a Reheating Furnace The main function of a reheating furnace is to raise the temperature of the semi-finished steels (billets, blooms, slabs or rounds) typically to temperatures between 1000 deg C and 1250 deg C, until it is plastic enough to be rolled to the desired section, size or shape in the hot rolling mill. The reheating furnace must also meet specific requirements and objectives in terms of the heating rates for metallurgical and productivity reasons. In the reheating furnace there is a continuous flow of material which is heated to the desired temperature as it travels through the furnace. Hot rolling operations require high quality reheated semi-finished steels at the lowest possible cost and at the optimal production rate of the rolling mill. The reheating furnaces used for heating the semi-finished steels in a hot rolling mill consume a large quantity of the energy and simultaneously generate large quantity of pollutants. Because of this, there is a necessity to look into the ways for the reduction of energy consumption as well as pollutants and hence the costs. This can be done by improving the fuel efficiency of the reheating furnaces. The combustion system of the reheating furnace has a major influence on both the quality of the reheated semi-finished steel product and on the amount of fuel needed for the reheating. The important expectations from a reheating furnace today are not only to lower the emission of the pollutants and the energy consumption, but also to have the improved high quality of the heated steel product, reliability, uniform temperature, heat flux and safety of the equipment and personnel. All these are the key factors which have considerable effect on the combustion system of the reheating furnace. The three basic things...

Oxy- Fuel Combustion and its Application in Reheating Furnace Jan13

Oxy- Fuel Combustion and its Application in Reheating Furnace...

Oxy- Fuel Combustion and its Application in Reheating Furnace Steel reheating is an energy intensive process requiring uniform temperature distribution within reheating furnaces. Historically, recuperators have been used to preheat combustion air, thereby conserving energy. More recent innovations include oxygen (O2) enrichment and the use of regenerative burners, which provide higher preheat air temperatures than recuperators. These processes have limitations such as equipment deterioration, decreasing energy efficiency over time, high maintenance costs, and increased NOx emissions with increased air preheat temperature, unless special equipment is used. Three things are necessary for the starting and sustenance of combustion. These are fuel, oxygen and sufficient energy for ignition. The efficiency of the combustion process is highest if fuel and oxygen can meet and react without any restrictions. But during heating practice, besides efficient combustion, transfer of heat is also of practical considerations. Normal air used for combustion contains nitrogen (N2) and argon (Ar) besides oxygen. In an air – fuel burner the burner flame contains nitrogen from the combustion air. A significant amount of the fuel energy is used to heat up this nitrogen. The hot nitrogen leaves through the stack, creating energy losses. Hence air does not provide optimum conditions for combustion as well as heat transfer. Heat absorbed by nitrogen either gets wasted or is to be recovered for the purpose of energy conservation. Present day best air- fuel heating system in the reheating furnace need at least 310 M Cal for a ton of steel for achieving the right temperature of the steel product for rolling. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame. Introduction of an innovative oxy...

Types of Burners in Reheating Furnaces Jan08

Types of Burners in Reheating Furnaces...

Types of Burners in Reheating Furnaces Presently steel industry is facing major challenges of the continuous reduction of the  environmental emissions while improving the economic viability of the processes. Reheating furnaces due to their high-energy consumption is one area where attention of steel industry is needed. They are important equipment which contribute to the productivity and energy efficiency of the steel plant. Modern reheating furnaces are walking beam furnaces where the beams lift the charge steel material (billets, blooms or slabs) inside the furnace and move it to the next position. The heating is done in these furnaces by direct firing of the fuel both from above and below of the steel charge material, using roof burners (to heat the roof which then radiates the heat to the steel charge material) and/or long flame burners (side walls or/and front wall). Common fuels used in the reheating furnaces are mixed gas of low calorific value (blast furnace gas mixed with the coke oven gas and the converter gas), coke oven gas, heavy oil, low sulphur heavy stock (LSHS), or natural gas. The important  parameters for a reheating furnace includes  the combustion system consisting of the fuel used, supply and the technological condition of the combustion air, the burners and their location for proper heat distribution, preheating of combustion air and fuel gas, waste heat recovery system, air fuel ratio, control of furnace draft, the furnace exhaust system, and furnace parameters measurement and control system.  The furnace combustion system besides supporting the required productivity level of the furnace, ensures efficient utilization of heat input, lower heat losses, efficient recovery of waste heat, minimal damage to the furnace refractories, and lower generation of green house gases specially NOx. The furnace combustion system also ensures a furnace atmosphere...