Operational Energy Efficient Improvement Practices in Furnace and Mill
Operational improvement are the most vital factors for fuel saving and improvement in the quality of heating. Any improvement or the modification in the design of furnaces has to be appropriately adopted in the operational practice to gam the benefits. Successful implementation of most of operational improvement depends upon the operating personnel. Such important practices are:
Optimal Operation of Combustion System
The basic requirement for good combustion of fuel in reheating furnace are :
- • Complete combustion of fuel,
- • Deshed flame configuration,
- • Minimum oxygen hi the outgoing product of combustion,
- • No occurrence of overheating of furnace elements,
- • Minimum pollution.
The excess ah hi flue gas should be kept at minimum level. The excess ah' should be set in such a way that there is no unbumt fuel in the flue gas. To achieve complete combustion of fuel with minimum excess air, factors such as type of burner, fuel, combustion ah pressure. It’s preheat temperature is also veiy important factor. The walking beam furnaces have oxygen analyzer to trim and control the oxygen in flue gas. The heat loss is higher at higher excess air resulting in higher fuel consumption. The scale loss is also high at higher excess ah. Fig. 14.3 shows the rate of fuel saving against excess air-fuel ratio (m).
Optimization of Thermal Regimes
The basic requirement of heating of stock in the reheating furnaces is that the metal should reach the desired level of temperature within the permissible tolerances. It is also necessary to make the metal ready only when it is to be discharged for rolling. Heating the metal much before it reaches discharge end leads to high fuel consumption and scale loss, as the stock remains in high temperature zone for a longer period. In case of continuous pusher type or walking beam furnaces, it is necessary to maintain required temperature profiles along the length of the furnace in such a way that the metal will get ready when it reaches to the discharge end. In case of batch-operated furnaces, the stock is heated at prescribed rate of rise of temperature (known as RAMP heating mode or step heating mode) with the help of programmable temperature controllers.
Furnace Pressure Regimes
The pressure inside the reheating furnace is normally maintained at 0.5 to 1.0 nun WC, to avoid digression of atmosphere air. This is achieved by operating the chimney damper in auto/computer mode in walking beam. The pressure inside the reheating furnaces is normally maintained positive at skid level in soaking zone. When the furnace pressure exceeds the normal limits, flame shoots out of the doors causing heat loss and may damage to the furnace walls. More detrimental effect is the loss of combustion air. Thus, both high and low furnace pressure will result in increased fuel consumption.
Hot Charging of Material
Hot charging of the material is one of the most efficient fuel saving measures and now being used worldwide. However, this requires installation of suitable facilities in the mill and also requires the good synchronization of rolling with the feeding mill. In hot charging, the available sensible heat of the stock can reduce the heat requirement in the furnace considerably, thus improving the fuel saving. The walking beams furnace has facilities to choose the charging temperature of slabs/billets/blooms and accordingly, then computer choose the heating curves and set zones temperatures for optimum heating. Fig. 14.4 shows how hot charging makes furnace energy efficient.
It is important to note that the benefits of hot charging are tangible only when the thermal regime is adjusted suitably, otherwise overheating/melting of stock takes place. The regimes can be adjusted only when a considerably number of blooms/slabs are charged continuously. It is practically impossible to work out regimes for hot charging where it is intermittent and temperature of input stock is widely varying. In some plants to supply consistently uniform temperature of stock, hot boxes are used.
Fig. 14.4 Potential for Fuel Saving by Hot Charging of Slabs/Blooms.
Furnace productivity is one of the important factors, which affects the specific fuel consumption in the furnace. There is an optimal level of productivity at which the furnace should be operated to derive maximum thermal efficiency. The under loading of the furnace results in high fuel consumption. To avoid the under loading, it is essential to run minimum numbers of furnaces.