Fig. 1. Schematic of direct-fired self-recuperative burner
High energy cost is a major concern for most heat treating operations. As a result, the industry is focused on reducing fuel con-sumption through energy efficient burner design and heat recovery technologies. Direct-fired industrial heating is the current trend to increase process efficiency with the use of a recuperator. Recuperators work by recovering the waste heat of furnace exhaust gas to preheat combustion air. By preheating the combustion air, more heat is available to be transmitted directly to the load. Heating the furnace load more efficiently can reduce the energy costs of the process and increase the productivity of the operation. For years the primary type of recuperator used in direct-fired industrial applications involved a stand-alone recupera-tor. More recently, the advent of direct-fired self-recuperative burner technology has offered a more cost effective solution for either retrofitting an existing furnace or for new furnace construction. With the increased use of recuperation, NOx emissions need to be taken into consideration. Production of thermal NOx can increase significantly as combustion air preheat increases, however, modern low NOx design versions of direct-fired self-recuperative burners can address these concerns.
Stand-Alone Recuperators
Although several designs of stand-alone recuperators are available for preheating combustion air, metallic convection type recuperators are most common. Installation typically requires no floor space as these recuperators can be installed within the ductwork above grade. However, due to relatively high exhaust gas differential pressures, an induced draft fan is typically required which may require additional floor space. The metallic construction typically limits exhaust temperatures to 1800°F (980°C) with reported maximum combustion air preheat of 1000°F (540°C). As a result of the metal construction, the recuperator is susceptible to thermal expansion, stress and hot corrosion leading to leakage and shortened life, and can require a dilution air system for exhaust gas over-temperature protection. Metallic convection recuperators are relatively large per unit of heat recovered, and typically require the installation of relatively expensive insulated piping to supply the preheated combustion air to the burners located on the furnace.Direct-Fired Self-Recuperative Burners
A new alternative in recuperative technology is the direct-fired self-recuperative burner (Fig. 1). The total air supplied is split into a combustion air and an eductor air stream. The ambient combustion air enters the burner via a single air connection on the burner housing and moves across the inside of a finned heat exchanger picking up heat from the exiting exhaust gases, similar to a counter-flow heat exchanger. The hot exhaust gases are pulled across the finned recuperator as a result of the suction pressure generated by the eductor. This highly efficient eductor typically allows for removal of a nominal 90% of the exhaust gases from the furnace. The ambient air introduced at the eductor also acts to cool the exhaust gases exiting the burner to an acceptable discharge temperature prior to being vented into an exhaust removal system.The unique design features of direct-fired self-recuperative burner technology offer several advantages:
The Ecomax® burner is one such proven direct-fired self-recuperative type burner available for high temperature furnace applications in the US market. This burner is jointly offered by Hauck Manufacturing Company and LBE GmbH. With five size burners available, the maximum burner input ranges from 135,000 to 950,000 Btu/hr (40 to 278 kW) based on natural gas higher heating value. When firing into a furnace maintained at 2100°F (1150°C), the nominal combustion air preheat temperature achieved ranges from 840 to 1110°F (450 to 600°C).
Fig. 2. Ecomax® 5 air preheat at maximum input
Fuel Savings
In order to determine the fuel savings that can be achieved from a direct-fired self-recuperative burner vs. that of a non-recuperative (cold air) burner, the specific furnace operating conditions must be known. In our example we have selected a typical high temperature furnace operating at 2000°F (1093°C), and the largest size Ecomax® 5M burner operating at it's maximum burner input of 950,000 Btu/hr (278 kW), 10% excess air, and a 90% eductor suction rate. While operation at the maximum input is the ‘worst case' scenario for a direct-fired self-recuperative burner, even under these conditions it will be shown that fuel savings can still be substantial.The air preheat temperature must first be determined at the given operating conditions. From a graph of actual data of furnace temperature vs. air preheat temperature (Fig. 2), at a furnace temperature of 2000°F (1093°C) the resulting air preheat temperature is 780°F (415°C). This air preheat can then be used to determine the available heat of a direct-fired self-recuperative burner via available combustion reference handbooks or software [1]. Likewise, the available heat for a cold air burner can also be determined for comparison (Fig. 3). At a furnace temperature of 2000°F (1093°C), the available heat of the Ecomax® 5M burner is 56% vs. 42% for that of a cold air burner. Since available heat is in essence a measure of burner efficiency which directly correlates to fuel savings, the direct-fired self-recuperative burner operating under these conditions would result in a realized fuel savings of 33%. In conjunction with the initial capital equipment investment, this fuel savings can then be utilized to determine the payback period for justification of a direct-fired self-recuperative burner retrofit or new furnace project.
Fig. 3. Available heat and fuel savings for Ecomax® 5 burner
Low NOx Design Solutions
Because direct-fired self-recuperative burners can achieve high air preheat temperatures, NOx emissions are an inher-ent concern. The standard Ecomax® burner incorporates certain design features that aid in the reduction of NOx emis-sions. Inside the burner, the preheated combustion air stream is divided into two streams; a small percentage of the preheated air is directed to mix with the natural gas fuel for initial combustion inside the combustion chamber, with the remainder of the preheated air being mixed with the former at the exit of the burner to complete combustion of the re-maining unburned fuel gas. This process is termed partial premix combustion. In addition, as hot exhaust gases are pulled toward the outer annular opening of the finned recuperator, a small portion of the exhaust gas becomes entrained into the flame. This process is termed internal flue gas recirculation.In the event that the standard direct-fired self-recuperative burner is unable to meet more stringent NOx emission requirements for a specific furnace project, a low NOx version of the Ecomax® burner is available which operates in a flameless (termed Invisiflame™) combustion mode. Above the auto-ignition temperature of natural gas of approximately 1400°F (760°C), the control scheme switches from the standard firing mode to a low NOx mode, which further delays combustion until mixing is completed inside the furnace. This low NOx burner is also pulse fired in an on/off mode at a set firing rate using temperature feedback control. NOx emissions are reduced significantly in the low NOx mode vs. the standard firing mode, with ultra-low NOx being achievable at mid-range process temperatures. For example, at a 2000°F (1093°C) furnace temperature with an input of 950,000 Btu/hr (278 kW) at 10% excess air, NOx adjusted to 3% oxygen dry volume basis in standard mode is approximately 150 ppm, and drops to 30 ppm in the low NOx mode for the Ecomax® burner.
