Hearth Coverage

Optimization of rolling schedule can increase the hearth coverage of a furnace, leading to increased furnace throughput and reduced energy consumption. However, the potential reduction in energy consumption will depend significantly on design of furnace and the feasibility of improving hearth coverage. Fig. 14.5 shows the effect of hearth coverage on furnace productivity and specific fuel consumption.

Effect of Hearth Coverage on Furnace Productivity and Specific Fuel Consumption

Fig. 14.5 Effect of Hearth Coverage on Furnace Productivity and Specific Fuel Consumption.

Use of Coil Box

During rolling in the finishing stands of the Hot Strip Mill (HSM), the tail end of the transfer bar will experience a temperature drop. Normally, it is necessary to compensate for this drop of temperature by increasing the speed of mill during rolling, which results in increased energy use. The coil box reduces temperature loss and reverses the transfer bar between the roughing and finishing mills, thus eliminating the temperature rundown and reducing the electrical energy requirement of mill by -10%. The use of a coil box also allows lower drop out temperature from the furnace, which would result in a ~5% reduction in fuel saving.

Unfired Preheat Zone

The length of modem walking beam reheating furnaces is in the range of 40-50 m and there is 18-20 in unfired preheating zone. One of the biggest influences on furnace efficiency is the length of unfired preheat zone. Within this zone, excess energy in the waste gas is transferred to the slab/blooms/ billets, while still retaining sufficient energy to allow economic preheating of combustion air. In a conventional furnace operating with cold charge slabs/ blooms, the waste gas temperature on the entry to the recuperator is hi the range of 850-1000°C, while for a furnace with 20 m unfired preheat zone this is reduced to the order of 700°C. It means that energy released to the waste gases can be reduced by -30%, leading to an overall reduction in energy use of-10%. The benefits of a long unfired preheating zone decreases when the temperature of charged slabs/blooms increases.

Recovery of Waste Energy

After the transfer of sensible heat into the slabs/blooms/billets being heated in unfired zone of walking beam, the next largest energy recovery can be from the sensible heat in cooling water and in waste gas. Efficient furnaces will operate such as to minimize the losses to cooling water and the waste gas encompasses conventional energy conservation technologies, such as increased levels of skid pipe insulation, as well as standard combustion improvement such as excess ah control and combustion air and fuel pre-heating via recuperator. Of these latter techniques, combustion air preheating is most commonly practiced. The advantages of preheating of combustion air are saving in fuel, increase in flame temperature. Fig. 14.6 shows the rate of fuel saving by preheating of combustion ah. Energy recovery from skid system can be used to heat water or to produce steam depending on the necessities of the mill. It also shows the fuel savmg of 20% at the combustion temperature 400°C and the waste gas temperature of 900°C.

Rate of Fuel Saving by Preheating Combustion Air

Fig. 14.6 Rate of Fuel Saving by Preheating Combustion Air.

Stack Efficiency

The stack efficiency is a measure of the amount of energy that is carried away with the exhaust gases. The higher process temperature, the higher exhaust temperature and therefore, the lower the stack efficiency. The stack efficiency is approximately the difference between the flame temperature and the exhaust temperature; divided by the flame temperature.

Thermal Cover of Roll Table in the Mill

Thermal cover along the tables between the roughing and finishing mills will save -70% of the transfer bar temperature drop, considering a transfer bar thickness of 25 mm, a roughing mill exit temperature of 1060°C and a finishing mill entry temperature of 1030°C. Thermal cover may also allow lower furnace drop out temperatures in association with coil box.

Computer/combustion Control Model

Computer/combustion control model optimize the sensible heat that slabs/ blooms/billets absorbs and can be applied along the frill length of a furnace to achieve the desire drop out temperature with minimal fuel use. When firing zones are not fully isolated and, therefore, not subject to individual control, this leads to inefficient fuel use due to uncontrolled flow of waste gas within the furnace atmosphere. Computer models can combine information concerning fuel CV, excess air in furnace atmosphere and firing zone temperatures as well as other parameters such as slab/bloom/billets entry temperature, maximum hearth coverage and mill status etc. The main advantage of process control system is their ability to optimize the ramping of furnace set point temperatures over time during unscheduled delays on mill. There will be reduction in scale build up during period of low production, as facilities will be available for lowering the furnace set point temperature. Energy consumption will also be reduced by 3-7%.

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