Oxygen-Enhanced Combustion Provides Advantages in Al-Melting Furnaces
In fuel-fired aluminum melting furnaces, O2-enhanced combustion can provide increased productivity via increased melt rate, melt energy cost savings, reduced emissions and reduced melt loss (increased aluminum recovery). In most cases, an O2 combustion system is selected for retrofit to an existing melt furnace originally utilizing a 100% combustion air system to accomplish at least one of the above objectives. O2 combustion systems can be either 100% oxy/fuel (no combustion air) or a combination air/oxy/fuel. Factors that influence the optimum type of O2 combustion system for a particular furnace include type of furnace (well-charged reverb, direct-charged reverb, rotary), type of charge materials (clean vs. oily or coated, thick vs. thin-wall), number and location of burners, level of combustion air preheat (cold air, recuperator or regenerative burners) and relative cost of fuel and O2.
General principles behind the benefits of O2 combustion and optimal application of O2 combustion technology for various types of aluminum melting furnaces are discussed in this article, and are supported with several furnace case studies.
Well-charged (flat bath) reverberatory aluminum scrap-melting furnaces-cold air combustion. Typically, preheated air systems (recuperator or regenerative burners) have not been used in these furnaces because salt fluxing and impurities in the scrap mix can cause severe corrosion or fouling in downstream recuperators. Dust and oxide particulate carryover together with salt fumes and scrap-based impurities can prematurely clog the regenerative burner beds, causing excessive maintenance delays. The capital cost, physical size and mechanical installation requirements for either a recuperator or a regenerative burner system can be prohibitive.
Oxygen combustion enhancement can significantly increase productivity and reduce overall melt energy cost in these melters, and the Pyretron variable ratio air/oxy/fuel system has achieved 20-50% melt rate increases in several installations. The use of 100% oxy/fuel burners can provide similar production increases in this type of furnace. However, higher flame temperatures and smaller flame volumes of 100% oxy/fuel (i.e., more localized heat distribution) pose a greater risk of localized overheating, which can lead to refractory damage and/or increased melt losses. It is important with any O2 combustion technique (especially 100% oxy/fuel) to control burner firing rate based on both melt and furnace roof temperatures to eliminate the risk of overheating.
Air Liquide has never observed an increase in melt loss using O2 enhanced combustion. Melt loss has remained the same or been reduced in Pyretron and other O2 combustion installations because faster melting reduces overall exposure to the combustion atmosphere.
In the past when natural gas prices were lower, use of O2 in aluminum melting was usually justified based on increased furnace productivity alone (when desired). Companies could tolerate the slight increase in net melt energy cost (total fuel + O2 cost/lb melted), because the cost penalty was greatly exceeded by the value of the incremental production. By comparison, with today's higher natural gas prices, use of O2 in aluminum melting usually provides an overall decrease in net melt energy cost with or without an accompanying productivity increase.
Table 1 compares calculated net melt energy cost (fuel + O2) and profitability for varying levels of O2 participation used to achieve a desired 33% production increase in an a 60 ton, 60 MMlb/yr, air/fuel, cold air, well-charged reverb melter equipped with O2 enhanced combustion. The typical Btu/lb and O2 scf/lb consumption values are based on Air Liquide's U.S. installation experience (actual values for individual furnaces may vary).
There are diminishing returns as O2 use is increased toward 100%. For example, increasing O2 participation from 25 to 75% doubles O2 consumption, but only reduces fuel consumption by 27%. Therefore, using as little O2 as possible to achieve the desired productivity increase minimizes melt energy cost (maximizes profitability). In other words, melt energy cost is lower with air/oxy/fuel than with 100% oxy/fuel at this particular fuel/ O2 cost ratio. Overall profitability improvement would typically provide a payback time of less than 1 year on the cost of implementing the Pyretron system in this example.
These results are shown graphically in Fig. 1 using 60°F (15°C) combustion air with production held constant at 60 MMlb/yr. Figure 5 shows as fuel cost increases, O2 use can be justified on melt energy cost savings alone (with or without a production increase; a productivity increase would only make O2 look more attractive). At fuel/O2 cost ratios below about 1.3, air/fuel typically provides the lowest melt energy cost (at lower natural gas prices of the past). Air/oxy/fuel provides the lowest melt energy cost for fuel/ O2 cost ratios between about 1.3 and 3.0 (today's higher natural gas prices). This is where the flexibility of the variable ratio Pyretron air/oxy/fuel system can really pay off; it can operate in a 25 to 75% O2 participation range (or higher and lower levels as necessary) to minimize overall melt energy cost (fuel + O2) for any fuel/ O2 cost ratio.
Use of 100% oxy/fuel theoretically provides the lowest melt energy cost at a fuel/ O2 cost ratio higher than about 3.0. However, potential savings are small and must be weighed against possible detrimental effects, such as higher flame temperature and smaller flame volume, which can lead to localized overheating with possible refractory damage and/or increased melt losses. Lower furnace pressure can increase air infiltration to negate any theoretical combustion efficiency gains.
Today's aluminum reverb melting energy costs usually exceed $0.01/lb, and can approach $0.02/lb. Melt energy cost usually represents the single highest conversion cost item for an aluminum melt/cast facility (not including raw materials). As such, it can represent a significant opportunity for savings. Use of O2 may be worth reconsidering with today's higher fuel (natural gas) costs of $6-10 versus $3-5. Pyretron systems have also been successful in furnaces using recycled oil (a lower cost alternative to natural gas).
Direct-charged reverberatory aluminum melting furnaces and rotary furnaces-cold air combustion. Average Btu and O2 consumption are lower for cold-air, direct-charged reverb melters than for well-charged furnaces. Direct flame/flue gas exposure to cold, high-surface area charge materials (scrap pile) provides both higher heat transfer rates and fuel efficiency, especially early in the batch heating cycle. Once the bath has become flat and heated up to pouring temperature, the furnace is poured (after required alloying and stirring) and recharged for the next melt cycle. Overall average fuel consumption for cold air/fuel burners is about 1,800 Btu/lb versus about 2,200 Btu/lb for well-charged (flat bath) melters. Rotary furnaces can have even higher fuel efficiency, because the hot refractory rotates to transfer more heat to the charge/bath via direct contact. (Direct charging oily scrap can further reduce effective Btu/lb requirements in rotary furnaces due to combustion of the evolved hydrocarbons.)
Similar to well-charged furnaces, air/oxy/fuel usually provides the lowest overall melt energy cost together with optimum overall heat transfer (highest productivity increase) for cold-air direct-charged reverbs and rotary furnaces. The only difference is slightly lower overall Btu and O2 consumption. An additional benefit of Pyretron air/oxy/fuel for these furnaces is its flexibility to adapt to different stages of the batch melting cycle. For example, O2 consumption can be further reduced during holding time periods (pouring, stirring and alloying) that have low firing rates, further reducing overall melt energy cost.
The melt rates in several rotary furnace installations processing both dross and scrap were increased by 50% or more using Air Liquide oxy/fuel and air/oxy/fuel burner systems, while melt losses remained the same or decreased. Table 2 summarizes experience with rotary-furnace installations in the U.S., showing calculated performance for a 10-ton rotary melting furnace (charging dry scrap) using O2 participation levels of 25 to 100% to accomplish a 50% melt rate increase. Similar to the well-charged reverb example in Table 1, air/oxy/fuel offers the lowest overall melt energy cost, and, therefore, the highest profitability. Similar results would be expected for processing dross.
The slightly lower flame temperature, higher flame volume and overall improved convective heat transfer characteristics of Pyretron air/oxy/fuel are more suitable than the smaller, hotter 100% oxy/fuel flame for direct-charged reverb melters and rotary furnaces. Pyretron air/oxy/fuel provides broader, more uniform heat distribution in rotary furnaces; the increased flame volume provides higher furnace pressure to minimize air infiltration. Oxy/fuel flames can tend to "tunnel" a hole through the scrap pile (especially light-gage scrap) in a direct-charged melter. Therefore, it is necessary to orient and control oxy/fuel burners carefully to avoid direct flame impingement and subsequent overheating and increased melt loss. The Pyretron air/oxy/fuel flame is better suited for direct flame impingement or direct over-firing of the scrap pile. Use of O2 also reduces flue gas volume (see Fig. 3), which is especially important in rotary furnaces, because dust loading and effective baghouse capacity is proportional to total burner flue gas volume.
Direct charging of oily (hydrocarbon-containing) scrap. In well-charged reverbs, scrap typically is dried (or delacquered) usually in a rotary kiln scrap dryer, to remove moisture including hydrocarbons (coatings, oils, etc.) before charging. Yield is improved by submerging into the well (especially light gage scrap) as opposed to exposing it to direct flame impingement. If scrap is not dried/delacquered before submerging in the well, the volatilized hydrocarbon bubbles can burst at the well surface, increasing molten metal surface area exposure to air, leading to increased melt loss.
Charging oily (e.g., oily turnings), coated and painted scrap directly into direct-charged aluminum reverbs and rotary furnaces provides additional hydrocarbon fuel value. In some rotary furnaces, taking advantage of this "free" energy source (burning evolved hydrocarbons) reduces purchased fuel requirement to as low as 1,000 Btu/lb, even with cold air combustion. However, careful control is required using this practice because overheating the overhead exhaust canopy and exhaust ductwork is possible if too much energy is released too quickly and/or if insufficient air is provided to combust rapidly evolving hydrocarbons. Using a combination of O2 and combustion air can help to more rapidly and effectively combust hydrocarbons in the furnace, and transfer the heat to the scrap charge. The author believes that injecting 100% O2 is too aggressive and could lead to localized overheating with excessive melt loss and/or refractory damage. An O2/combustion air mix can provide the right combination of burning speed and flame volume for broader heat distribution within the furnace.
Excessive O2 injection can lead to excessive melt loss. Therefore, Air Liquide developed feedback-based control technology (based on either temperature or exhaust-gas concentration, or both) to automatically inject the optimum amount of O2 (or air/ O2) to burn hydrocarbons in rotary and direct-charged reverbs (especially when charging oily scrap).
The company's most advanced feedback-control incorporates a diode laser-based exhaust gas concentration measurement technology (tunable diode laser, or TDL), which provides essentially instantaneous information on species concentration (CO, CO2, O2) as they evolve. This eliminates the extractive-sampling delay associated with conventional "probe with sample pump" analyzers (and eliminates sample probe, pump and filter-related maintenance concerns). Field demonstrations on full-scale melt furnaces show that this technology provides reduced fuel and O2 consumption, increased melt rate and reduced melt loss (1+% increase in aluminum recovery). Increased aluminum recovery is especially valuable because a 1% reduction in melt loss is equivalent to a 1% reduction in raw material (scrap) costs, or a 1% increase in product sales.
Direct-charged aluminum reverb melting furnaces-preheated combustion air. Some direct-charged furnaces melting relatively clean scrap and prime incorporate regenerative burners or a recuperator to preheat combustion air using energy from the furnace exhaust gases. Fuel efficiency can be increased in these furnaces to near the level attainable with O2 by preheating combustion air (see Fig. 2). Replacing an existing regenerative burner system or recuperator with air/oxy/fuel or oxy/fuel combustion is usually not economical. Use of O2 to supplement the existing preheated air combustion system rather than replace it can boost production or provide energy cost savings, or both.
For example, Pyretron systems were installed in two 185,000-lb direct-charged, round-top melters in a large U.S. aluminum rolling mill producing canstock. Each furnace employed a recuperator to preheat combustion air up to 750°F (400°C) to four air/gas burners firing directly on the prime/scrap charge mix (including coils). Table 3 shows that melt rate was increased by 20% using supplementary fuel and O2 (with preheated combustion air) through specially designed retrofit Pyretron air/oxy/gas burners. Net melt energy cost (natural gas + O2) was unchanged. (A net melt energy cost reduction would have resulted at today's natural-gas prices.) Supplementary O2 was turned off when the furnace reached flat-bath conditions, as burner firing rate automatically turned down to maintain temperature.
With this furnace, the main advantage of using O2 was the ability to input more Btus with both minimal additional flue gas volume and enhanced flame heat transfer characteristics (higher energy transfer/ available load surface area). This was especially important in this example due to fixed recuperator volume capacity.
The same concept could be applied to furnaces using regenerative burners. For example, some direct-charged scrap melting furnaces use regenerative burners operating in pairs. A regen burner pair, while providing high fuel efficiency (low Btu/lb) via high air preheat, can compromise the heat distribution pattern. Alternate firing between burners effectively has the pair operating as one large burner. Heat distribution is not optimum, especially in furnaces with only one regen burner pair, because the scrap pile is heated by only one (large) burner at a time. It could be cost- and/or space-prohibitive to use more than one pair of regen burners (which are quite large in size) in some furnaces. Use of relatively small, compact supplementary oxy/fuel burners to input heat at additional furnace locations can provide more uniform heat distribution, resulting in faster meltdown. Oxy/fuel burners introduce supplementary energy input at high thermal efficiency with minimum additional flue gas volume. The burners can be turned off after meltdown, during holding time periods (alloy, stir, cast).
Table 4 shows the calculated performance with supplementary 8 MMBtu/hr oxy/fuel firing to a 24-MMBtu/hr capacity regen burner pair in a hypothetical 60,000 lb direct-charged extrusion scrap melter. In this example, the regen burners are de-rated slightly to 20 MMBtu/hr with the supplemental oxy/fuel firing to provide more balanced heat distribution. The small additional firing capacity potentially allows acheiving a substantial melt rate increase with reduced flue gas volume and combustion air requirement. In practice, the firing rates of both the regen burners and supplemental oxy/fuel burners can be adjusted to obtain optimal heat distribution balance for the best combination of productivity and melt energy cost. There can be a slight increase in net melt energy cost (fuel + O2) due to the very high efficiency of the regen burners. But, the value of increased productivity far outweighs the slight energy cost penalty in situations where increased productivity is desired (payback time should be about 1 year or less). Careful positioning and control of supplementary oxy/fuel burners based on roof or flue gas temperature is required to avoid refractory damage or melt loss. IH