At Burns Energy Systems we take energy efficiency issues very seriously. Efficiency is never easy to evaluate as it is necessary to measure the potential energy savings against productivity and environmental constraints. Often improvements in the thermal efficiency of a process are accompanied by extra maintenance and capital cost, may require specialized training for the operators and may require the operator to have a higher standard of education than would normally be required of an operator of less high-tech equipment. At Burns Energy Systems we can help you evaluate how to implement a program of comprehensive energy saving measures with in house training and with the assessment of your process. (Refer to Mass-Energy Balance for details.)
It is easiest to illustrate Burns Energy Systems approach by using an example from the Aluminum Industry. We could cite examples from the forging, galvanizing and zinc oxide industries however Aluminum melting in a gas fired reverberatory furnace poses some interesting challenges that we use here for illustration.
In this series of furnaces the refractory surfaces above the liquid metal are the most important heat transfer surfaces. An aluminum furnace with a fresh charge of scrap has a very large total solid aluminum surface area available to accept heat, however, as the metal melts the exposed surface of the scrap gets smaller, eventually approaching the surface area of the bath when the aluminum has fully melted. If the burner system is controlled from a bath thermocouple the burners will operate at full fire until liquid metal is sensed at the required set point. If the furnace is operated from a roof thermocouple the melting rate will be limited by the heat transfer rate associated with the set point temperature of the refractories above the melt. It is common practice to set this higher than the required bath temperature and then turn this down as the metal melts, usually at the operator's discretion. Under these conditions the roof refractory can become extremely hot, storing up a lot of extra heat. When the burners are eventually turn down the melt continues to increase in temperature, until the refractory above the melt and the bath temperature equalize. The bath thermocouple will therefore record an increase in temperature until the extra stored heat in the refractories above the melt has been transferred to the bath or is lost as operational heat losses. This is wasteful of energy and can cause metallurgical problems.
Thermal head control is used to limit the temperature overshoot. The idea is to anticipate the reduction of the surface area of the metal as it melts, and to revert to bath temperature control at a specific operator selected furnace condition. This minimizes the temperature overshoot and saves a significant quantity of energy at the same time. To get the best from this system it is necessary for the operator to understand how heat is exchanged inside the furnace and how the furnace set point temperatures can be optimized to get the best performance. This involves training and some operator trial-and-error before achieving the best performance. A more sophisticated version of thermal head control, requiring less operator interference but more setup time, allows the melt rate to be maximized. The system uses an algorithm that takes an input signal from the furnace flue to detect the decrease in the aluminum surface area. The system anticipates this reduction and automatically reduces the roof set point without operator interference. At a preset value the bath temperature sensor becomes the source of the primary signal, with the above melt sensor becoming the source of the secondary signal. This system would be ideal for a large furnace operating under very similar daily conditions but would not be appropriate for furnaces were conditions change radically from day to day.
In the above example fuel savings are achieved by maximizing the melt rate and limiting the roof temperature overshoot. This costs only some training and a modification to the furnace controls. Hardware additions may be limited to the installation of some extra flow valves and some electronic control panel modifications. The problem is that the savings will only be achieved if the operator can be properly trained to work with the system and if the system is properly maintained. The savings are impossible to guarantee because so much depends on the furnace operator. It might be possible to provide a blanket statement that 15% fuel savings are common, but to set up a before-and-after trial to verify this would be next to impossible. For a start it is often difficult to obtain an accurate value for the mass of material being loaded, and of the liquid metal that is discharged.
The next place to look for savings is in the use of the waste heat at the furnace flue. Typical would be the use of waste heat for drying or preheating. Caution is necessary as too much preheating for thin wall aluminum materials will cause all the potential fuel saving to be lost due to excessive oxidation. Very thin materials must be subducted as quickly as possible to protect the solid surface from oxidation. Load heat recuperation and material drying can be very effective methods of saving energy if the material is bulky enough not to be excessively oxidized. Hybrid shaft melters for scrap metal castings would be an example of this technique. The savings are measurable and the modifications required are usually confined to the handling system. Space and material flow may prevent this method of waste heat recovery, which leads on to the use of high efficiency burners.
Recuperative and regenerative burners are two technologies than can be used to recover waste heat from the furnace flue by using some of the waste heat to preheat combustion air. The elevated temperature combustion air is returned to the furnace chamber via the burner. Again there are a few problems. For the recuperative case, heat exchanges have to work reliably and must be sized to accommodate deterioration over time. They require fuel air ratio control equipment to maintain stable conditions at the burner, and they must be repaired and replaced as they do not usually have an indefinite life. Because of these costs it is vital to understand how much energy is being saved and how much is being lost as the recuperator gradually deteriorates. Without this assessment of fuel savings it is common to see recuperators being removed from a system because of the operational requirements for a repair or replacement.
Regenerative burners are less susceptible to corrosion than metallic recuperators, however, they have a number of other issues that must be considered when in operation. They are initially more expensive to install as they must be operated in pairs and require individual control for both burners. Deterioration can still occur, however in this case the contamination of the regenerator beds is the problem. Burners must be operated in blow mode in the event that excessive dusting is occurring inside the furnace and they must be periodically cleaned. The heat transfer medium tends to deteriorate with time and maintenance is required.
Finally both of these technologies produce higher than normal flame temperatures and this means NOx may be a problem. Not withstanding these issues the recuperative system will operate at combustion efficiency of about 62% and the regenerative system will be operating at about 85%.
It is possible to tailor furnace design to a particular process. For instance a reverberatory melter operates best when it is melting, but it is not usually an efficient holder. Capital cost may be saved by operating a combined melter/holder but the ongoing operating costs will be much higher than a system setup to efficiently melt, hold, and distribute the molten metal to the point where it is needed in discrete purpose-designed volumes.
At A. H. Burns Energy Systems we have a total set of solutions and years of experience with all of these issues. We are able to offer engineering consultancy to help you evaluate your own needs and to set up a plan for implementing some of these ideas. We have innovative technologies that can be applied such as the use of immersion burners systems for metal distribution filtering and holding. We have load recuperative, preheated air and regenerative technologies. We apply PINCH technology to determine the best use of waste heat in a process stream and we evaluate how heat can be captured and reused to the best effect.
We have used the above example of an Aluminum melting application only as an illustration. We apply the same logic and engineering standards to galvanizing applications, forging, ZnO, etc. We are happy to discuss your particular problems. We can provide consultancy services or a comprehensive equipment package to reduce the energy burden of your process.