Researchers at Missouri S&T analyze methods for improving melting efficiency.
A Global Casting Staff Report
The high temperatures required to melt steel result in significantly higher energy losses in comparison with melting other cast alloys. The energy costs associated with heat losses during melting are significantly higher for steel casting facilities than for those melting other alloys. Today’s steel casting facilities use induction furnaces (IF) and electric arc furnaces (EAF) to melt steel.
Siddhartha Biswas, Kent Peaslee and Simon Lekakh of Missouri University of Science & Technology, Rolla, Mo., conducted a benchmarking survey on current energy use among U.S. steel casting facilities. They investigated opportunities for energy improvement through a series of industrial trials involving chemical energy and ladle practice development.
Background
Furnace capacity, power supply, age of equipment, rate of production, melting schedule and operating practice all have major influences on energy consumption. A study of 19 North American steel casting facilities included a combination of historical data and industrial measurements on the type of melting equipment, refractory practices (Fig. 1), energy use and ladle practices. (See Tables 1, 2 and 3.)
A multiple regression analysis using commercially available statistics software allowed the researchers to evaluate the influence of the melting furnace (type, size, age and transformer power) and operating parameters such as tap temperature, tap to tap time and furnace productivity on the energy consumption for melting steel.
Successful energy management in steel casting facilities is difficult without monitoring energy consumption. Unfortunately, this is an area where the steel casting industry is poorly equipped. Only 38% of EAFs and 15% of IFs in operation are equipped with electric meters for monitoring electric consumption. More than one third of the plants surveyed have no equipment for monitoring their energy consumption during steel melting.
Fig 1. This chart shows the refractory linings used in melting furnaces in the 19 facilities surveyed.
Multiple regression analyses determined how operating practice variables and equipment type influence the energy consumption in kWh/ton for melting steel. The analysis showed the following independent variables had an influence on the energy consumption for melting steel (from strong to weak influence):
- Increasing “tap temperature” increased energy consumption (strong influence).
- Increasing “tap to tap time” increased energy consumption (strong influence).,
- EAF has lower energy consumption than IF (strong influence).
- Newer equipment decreased energy consumption (strong influence).
- Increasing “furnace capacity” decreased energy consumption (weak influence).
In addition to the statistical data collected, operators were asked to report what they considered to be major factors with the greatest influence on energy losses during melting at their facilities. The three most frequently cited were: refractory (75%), scheduling (70%) and casting yield (25%).
Procedure
The MS&T team visited five metalcasting facilities, observed the melting of several heats and calculated heat balances. Figure 2 shows an example of the heat balance from an electric arc furnace.
Fig. 2. This Sankey diagram (energy flows) depicts melting steel in a 15-ton EAF.
Supplemental chemical energy is one way to decrease electrical energy consumption and increase the efficiency and productivity of melting steel in EAFs. Many technologies can introduce supplemental chemical energy into the process. Preheating of the scrap charge and using oxyfuel burners can increase melting efficiency of the solid scrap charge. Two supplementary chemical energy methods, post-combustion of CO in the furnace to CO2 and exothermic heat from oxidation reactions to the melt, could increase energy efficiency during the flat bath period.
Opportunities to increase energy efficiency are greatest during the superheating and correction period because the electrical energy efficiency drops significantly when heating liquid steel with an open arc in air. A significant portion of the arc energy is reflected from the arc and bath surface to the sidewalls and roof where the energy is lost in heating (and often melting) refractory rather than steel. In addition to using chemical energy, there is a future potential for increasing arc efficiency by utilizing more energy efficient long arcs (higher voltage and lower current) with a foamy slag, to decrease the heat losses by blanketing the arc.
In an industrial trial, chemical energy from oxygen combustion of natural gas was introduced in a 4 ton EAF through installation of an oxyfuel
burner through the door. Effective combustion of natural gas provides energy to the solid charge during the melting period. The electrical energy consumption was decreased from 480-500 kWh/ton without oxyfuel burners to 400-420 kWh/ton with burners.
Direct injection of oxygen by a lance to the solid charge and melted steel can reduce electrical energy consumption by decreasing scrap melting time and direct generation of chemical energy from oxidation reactions in the melt. The introduction of coherent jet has decreased electrical energy consumption 10% and also reduces melt down time 13%.
Fig. 3. This Sankey diagram shows the decrease in electrical energy consumption by the addition of chemical energy (0.4% SiC in charge).
Scrap preheating systems, oxyfuel burners and postcombustion of CO require additional capital investment. By comparison, the addition of a material such as SiC, which produces exothermic reactions during the oxygen blow, does not require any capital investment (Fig. 3).
Because the heat of oxidation reaction is generated within the liquid steel, heat transfer efficiency from exothermic reactions should be nearly 100%. This expected efficiency is much higher than the typical 40% efficiency for post-combustion of CO above the bath. In the study, the amount of exothermic heat generated during oxygen boiling was increased by adding SiC with the solid charge. The energy and operational effects of adding enough SiC with the scrap charge to represent 0.4% to 0.6% of the charge weight was investigated in a 20-ton acid-lined EAF. The addition of SiC reduced electrical energy consumption by 7.1% and increased productivity by nearly 5%.
Effective ladle design, preheat practices and use are important for steel casting production. The tap temperature of the liquid steel typically is superheated 250F to 500F (121C to 260C) above the steel’s liquidus to compensate for heat losses during tapping and holding in small ladles with large surface area to volume ratios.
In spite of the relatively short time the steel is in contact with the ladle lining, the huge thermal gradients in the lining drive high values of heat flux through the refractory surface. Initial information about heat losses during steel ladling was taken from a survey of steel casting facilities and from industrial measurements at seven plants.
The influence of the thermal properties of different ceramic materials typically used for steel ladle linings on heat losses during use was analyzed. From this work, a new type of ladle lining was developed at Missouri University S&T. It was based on porous ceramics with the potential to significantly decrease heat losses and save considerable ladle preheat energy.
The data collected through the survey and trials was analyzed to determine the factors that were most important to energy losses in the ladle. One of the most important factors was found to be the ladle capacity. The tap temperature was found to be significantly lower for higher capacity ladles. A computational fluid dynamics (CFD) model was used to study the effects of ladle size and validate the industrial measurements.
The temperature of the liquid steel at tap typically varies between 2,950F (1,621 C) and 3,200F (1,760C) at steel casting facilities. These temperatures are close to the softening temperature of the complex Al, Ca, Si, and Mg oxide compounds which are often used for ceramic linings. Also, the high rate of chemical reactions between the lining and components of the liquid steel and slag takes place at these temperatures. As a rule, ladles are not fully soaked even when used multiple times and are therefore used under unsteady state heat transfer conditions. Even in cases where the lining is preheated prior to tap, a significant part of the heat energy from the liquid steel accumulates inside the lining during the first 5-30 minutes after tap.
Foundry ladle operations require special ceramic lining materials. A specially designed low density porous alumina castable was introduced. It has very low thermal conductivity and was determined to improve energy efficiency in the ladle (Table 4).
Results and Conclusions
Major opportunities for energy savings were identified as: improvement in scheduling and decreasing delays while liquid metal is in the furnace; addition of chemical energy for melting steel; and improvement in ladle practice. CFD modeling, and industrial and laboratory trials determined the effects of these changes in reducing electrical power consumption. This data will be used in the future for development of a spreadsheet type model to allow metalcasters to calculate energy usage and melt temperature losses.
This article is based on a research paper, “Increasing Melting Energy Efficiency in Steel Foundries,” presented at the 2012 AFS Metalcasting Congress.