Energy Conservation in Secondary Aluminum Processing Through Flame Shaping
Oxyfuel melting of aluminum scrap has been used for nearly 30 years. Industrial gas companies that provide combustion technology led its commercialization, while justification for implementing oxyfuel technology was mainly driven by productivity improvements. Depending on the type of scrap, process and equipment used, furnace throughput typically increased between 10 and 25%. Higher throughput offset the higher operating costs stemming from oxygen consumption.
Historically, there has been little to differentiate oxyfuel technologies from each other. They all reduced the energy consumption, but scrap processors were forced to take advantage of oxyfuel's inherent capability to deliver increased productivity to realize economic benefits. Recognizing potential limitations of such benefits, BOC recently introduced its patented Flat Jet BurnerT to the aluminum industry, which has changed the rules of the game.
Flat Jet is unique because it successfully transfers heat to the bath via direct flame radiation. A review of traditional oxygen based combustion technology will shed some light on the unique qualities of this new technology.
Traditional methods of oxygen application in reverbatory furnaces consisted of bulk enrichment, a supplemental sidewall burner and the "burner-in-a burner" modification. These approaches are somewhat germane to the oxyfuel industry, and each has benefits and limitations.
Bulk enrichment is the most basic form of oxygen for combustion enhancement. Oxygen is added as an inoculent into the combustion air stream to increase the oxygen content from that in air (21%) to a slightly higher level (typically 22 to 25%). This effectively lowers the nitrogen content of the blended stream, increases flame temperature and reduces sensible heat losses.
Oxygen enrichment increases flame temperature (figure 1), and increasing flame temperature rapidly increases heat transfer rates, both radiative and convective.
In the case of a supplemental sidewall burner, a stand-alone oxyfuel burner is strategically mounted in the sidewall of the reverbatory furnace. The traditional installation method is to use a high-momentum burner set at a steep angle to the bath (figure 3). The purpose is to aim the burner at a critical point of impingement on the molten bath to superheat that region. The location of flame impingement usually is a cold spot or an area near the metal pump arch. The metal being drawn into the well is superheated to improve heat transfer to the scrap.
This technology is used as an energy boost in conjunction with an existing air-fuel combustion system. One of the advantages of the system is that it can be operated only when needed, then shut down during holding and tapping. Savings can be achieved using good melting practice at very high production rates, and when some metal oxidation can be tolerated. Limitations of the method are that the amount of oxyfuel energy that can be added is limited because the flame is focused on a small area, and incremental dross formation can occur if the burner energy is too great. It is common to limit the oxyfuel burner to no greater than 40% of the total power input to the system. If oxyfuel firing rates are further increased, the reverbatory action of the furnace will be upset, which can result in cold refractories. Under these circumstances, the refractories, instead of functioning as a source of radiation energy, will extract heat from the bath.
BOC abandoned this technology in the late 1980s (although still in practice on a limited basis) for two reasons: 1) the high momentum oxyfuel burner impinging on the bath can adversely affect yield, and 2) the technology is effective only at very high production rates. If melt rate turndown is required, the economics of the system become unfavorable.
Today, the most common form of this technology in use often is referred to as the burner-in-a-burner technique. An oxyfuel burner is installed down the center of a standard air-fuel burner. The two burners are then operated in tandem and the total fuel is ratioed between the two sections.
The burner-in-a-burner system increases flame temperature and shortens the length of the flame. The expected improvement over bulk enrichment is that the oxyfuel burner will pyrolyze the fuel in the air-fuel burner, resulting in a more luminous flame. A higher degree of flame radiation results in increased heat transfer. Overall higher levels of oxygen can be added using this method compared with that using the bulk enrichment method, but burner block characteristics usually limit the maximum oxygen level.
Bulk enrichment and burner-in-a-burner methods make up what the industrial-gas industry categorizes as oxyfuel technology. Published economic evaluations based on these generic techniques usually indicate the superiority of the burner-in-a-burner method. This is based on the fact that the oxygen is shut off during tapping and holding, whereas with a 100% oxyfuel burner, oxygen is needlessly consumed during the low-fire period with no benefit being achieved. It is further concluded that the burner-in-a-burner method delivers approximately the same heat transfer capabilities as does 100% oxyfuel. This wrongly implies that higher flame temperature and reduced flue losses will not increase combustion efficiency.
All Oxyfuel Methods Are Not Equal
The Flat Jet Burner system is unique compared with all other systems due to its heat-transfer characteristics (figure 4). Whereas standard systems rely on heating the entire furnace to higher temperatures, the Flat Jet transmits flame radiation directly into the bath. The gas mixing characteristics of the Flat Jet allow installation at minimum elevation above the bath, which is a major contribution to its superior heat-transfer characteristics. The large flame coverage produced using the burner further enhances efficiency. The Flat Jet incorporates a nonaxisymmetric design that does not deliver uniform heat in all directions, but instead, allows it to radiate a disproportionate amount of energy downward toward the bath.
The high heat transfer capabilities of the Flat Jet result in further unexpected benefits at turndown. Because the Flat Jet can produce a stable, luminous flame at very low power settings, fuel and oxygen consumption at the low-fire, or hold period, is extremely low. In addition, the ability of Flat Jet to maintain high thermal efficiency during the holding portion of the melt cycle makes the combustion system simple, and costs remain low.
Recent natural gas prices have in some cases doubled and even tripled the energy expense of operating the furnace. Also, a softening aluminum market temporarily diminishes the value of the additional production, increasing the pressure to reduce costs in a narrower margin environment. While the conventional tubular type burners operating with various enrichment levels all rely on a productivity gain to produce the economic benefits, the Flat Jet burner delivers unsurpassed efficiencies and convincing operating cost savings over the entire preferred productivity range. Table I compares the Flat Jet burner energy unit cost with those of the conventional air-fuel and air oxyfuel systems for the same productivity levels. An average size plant processing 10-million lb (4,500 t) of scrap per month will save approximately $10,000 in operating cost (including oxygen) by implementing a Flat Jet burner system.
Significant process modeling of Flat Jet flames and reverbatory type furnaces has helped to improve equipment design and process performance, which has been validated through actual production results. Furnaces of different configurations can be compared on a fair basis using a fuel-intensity value, defined as the total heat input to the system per pound of aluminum cast, measured as a function of the hearth surface area. Figure 5 compares fuel intensity values for several furnaces showing the superior performance of Flat Jet versus air-fuel systems.
The economics of the oxy-fuel alternatives to a conventional firing system depend on the relative price difference between natural gas and oxygen. Evaporated liquid offers supply and demand flexibility, but does not have the economic advantage of on-site produced oxygen. Conventional oxyfuel and air-oxyfuel burners use intermittent firing techniques in the reverberatory furnace for maximum efficiency. Common practice involves using different levels of enrichment or switching to the 100% air-fuel mode during low-fire periods. While this certainly helps to minimize oxygen consumption and is recommended in the case of using evaporated-liquid supply, it does not work well for the economics where on-site generated oxygen is considered. Operating the generator at substantial turndown and quick ramp-ups triggers evaporated liquid consumption and causes the unit oxygen price to be high. Consequently, the total cost of the on-site generated oxygen often exceeds the cost of evaporated liquid for the same application.
The concept of the Flat-Jet burner is based on the continuous and uniform radiative heat transfer from the flame to the molten metal bath. The steady flow of oxygen to the burner provides optimum conditions to maximize the use of the on-site generator and ensure the minimum cost of the supplied oxygen. Because the on-site supply reduces the cost of oxygen molecule by 30 to 40%, a secondary plant processing 10-million lb of aluminum per month would be saving an additional $30,000 per month over the cost of air-fuel-an equivalent of 1/4 of the melting furnace energy cost.
The versatility of the Flat-Jet burner to increase furnace productivity or reduce operating cost, if desired, has been accepted by the secondary aluminum processing industry. Since its introduction to the industry, more than ten different secondary aluminum companies have installed Flat Jet burners on 14 different side-well furnaces, including a recent installation at a secondary smelter site in the Los Angeles, Calif., basin, which not only provides the customer with cost reduction benefits, but also establishes the Flat Jet burner at the forefront of environmentally practical combustion solutions.
For more information: Mark Tomaszewsky is manager, Nonferrous & Recycling, BOC Gases, 5 Mountain Rd., Murray Hill, NJ 07974; tel: 908-508-3874; fax: 908-771-1148; e-mail: mark.tomaszewsky @us.gases.boc.com