Intense flue-gas recirculation from high-momentum jet flames improves temperature uniformity and lowers emissions.

High-momentum jet flames are widely used in different industrial applications, such as radiant-tube heating, to develop intensive flue gas recirculation for better forced convection heat transfer and temperature uniformity in the working chamber. Four major types of radiant-tube burner working-chamber geometries are: coaxial tubes (SER, or single-ended recuperative design), "U", "W" and "P" shapes. The main design advantages and performance of Eclipse's Auto-Recupe burner mounted inside an SER radiant tube are discussed. The burner uses high-momentum flame technology to meet industry requirements and environmental regulations.

Emissions output is an important aspect of contemporary thermal process technology especially in view of today's environmental regulations. New burner technology has been developed to increase thermal efficiency, raise flux rates and improve temperature uniformity. In addition, generation of nitrogen oxides (NOx) can be significantly reduced using staged air/gas mixing or staged combustion. Staged combustion is a combination of two technologies: rich combustion (under-stoichiometric condition) and lean combustion (over-stoichiometric condition).

Under a rich mixing condition, the combustion air provided to the mixing zone creates oxygen-starved zones, which lead to incomplete fuel oxidation and, as a result, less peak flame temperature. Both flame temperature reduction and lack of oxygen in the mixing/rich combustion zone lead to NOx reduction. Under lean mixing conditions, the combustion air provided to the mixing zone creates a flame having excess air above the stoichiometric level. The higher the excess air, the more heat is wasted in preheating that excess air, thus resulting in a lower flame temperature and reduced NOx emissions.

Staging of air causes a gradual air premix to the flue gas stream. Staging of gas causes a gradual gas premix to the full combustion air stream. This usually is accomplished by means of a special nozzle/ mixer design. Staging leads to the formation of rich or lean combustion zones. The first stage produces the richest or leanest mixture, the next one produces less rich or lean, and so on. The last stage in a staging process produces the formation of a stoichiometric mixture (or whatever percentage of excess air is required).

Initial momentum of the flame jet determines the intensity of flue-gas recirculation in the working chamber. The higher the momentum, the higher is the flue gas recirculation inside the chamber. This results in a larger amount of flue gases being returned into the flame envelope. Recirculating flue gases contain less oxygen than combustion air and they also have a lower temperature than a flame envelope, which lowers the total flame jet temperature, thus reducing emissions.

Fig 1 AR-Ultra SER radiant-tube burner (top) and cut-away view (bottom) showing burner design and flame/combustion products recirculating pattern

Burner design and principles of operation

The Auto-Recupe Ultra (AR-Ultra) single-ended recuperative radiant tube burner (Fig. 1) uses Eclipse's high velocity burner technology. A nozzle supplies air jets to the gas flow in a progressive manner, creating a staged air mixing along the gas flow path.

The burner consists of a housing having exhaust outlet and air and gas inlet blocks, each equipped with a flow metering orifice plate. The rear cover provides openings for a flame rod or ultraviolet (UV) scanner, spark rod and peepsight. Other components include a recuperator and a gas supply tube to connect the gas inlet block to a nozzle located inside the combustor.

The burner is mounted to the furnace wall using an adapting flange, which also is used to connect the radiant tube located inside the furnace chamber to the burner. The inner tube is assembled from ceramic elements and mounted inside the radiant tube. From the burner side, there is a mounting gap between the combustor outlet and the inner tube inlet. At the end of the radiant tube, a specially shaped ceramic element is installed to reverse the direction of the combustion products from inside the inner tube to the annulus gap formed between the inner tube and the radiant tube.

A high momentum flame jet creates intensive recirculation of the combustion products inside the radiant tube. Due to a Venturi effect, part of the stream is entrained back into the flame envelope through the gap between combustor outlet and the inner tube inlet, diluting the flame with cooler combustion products. Moving through the recuperator to the exhaust, combustion products preheat combustion air coming to the nozzle for gradual mixing with the gas flow. Radiant tubes re-radiate the heat from the flame and the recirculating combustion products to the furnace chamber including load, refractory walls and accessories.

Ceramic elements (instead of metal alloy) used to assemble the inner tube allow the AR-Ultra burner to operate at a higher heat flux (as high as 40.9 kW/m2, or 90 Btu/hr in2). The higher heat flux results in fewer burners being required on the furnace to achieve the same capacity. If increased furnace capacity is needed, AR-Ultra burners can intensify heat transfer, and, therefore, deliver more heat per second to the surface of the load. The flexible ceramic inner tube is able to withstand higher temperature levels and fluctuations and is more responsive to the radiant tube deviation and temperature expansion, extending the total burner service life.

Fig 2 Radiant tube-temperature uniformity versus furnace temperature in AR-Ultra burner of 150 mm diameter by 1,400 mm effective length Fig 3 Radiant tube-temperature uniformity versus furnace temperature in AR-Ultra burner of 200 mm diameter by 2,030 mm effective length

Burner temperature uniformity

Temperature uniformity of the radiant tube is an important criterion in estimating burner performance. Higher temperature uniformity provides more even heat distribution in the furnace chamber resulting in improved quality of the final product. Temperature uniformity is an expression of the difference between the hottest and the coolest spots on the radiant tube surface. Data shown in Fig. 2 and Fig. 3 show a tendency of improved tube temperature uniformity with increasing furnace temperature.

The trend is similar for the three fuels tested (natural gas, propane and butane). Tests were performed on radiant tubes having 150 mm (6 in.) outside diameter by 1,400 mm (55 in.) effective length (Fig. 2), and 200 mm (7.5 in.) outside diameter by 2,030 mm (80 in.) effective length (Fig. 3). Input was constant throughout the furnace temperature cycle and average heat flux was about 34 kW/m2 (75 Btu/in2 hr). The burner operated at 15% excess air from cold start up to the final furnace temperature. Temperature was measured using thermocouples placed evenly along the tube surface.

Fig 4 Single-burner laboratory batch furnace for burner-technology R&D

A single radiant tube was tested in the batch-type furnace having partial load simulation (Fig. 4). In continuous heat treating furnaces, the influence of a load on tube _T is different from one furnace type to another. Therefore data in Fig. 2 and Fig. 3 could be slightly different from one application to another.

Fig 5 NOx emission versus furnace temperature in AR-Ultra burner of 150 mm diameter by 1,400 mm effective length Fig 6 NOx emission versus furnace temperature in AR-Ultra burner of 200 mm diameter by 2,030 mm effective length

Furnace temperature and NOx emissions

A furnace thermocouple usually measures the temperature of the furnace atmosphere, which is influenced by radiation from the radiant tube and re-radiation from the load, wall and accessories. The largest temperature difference in the furnace system is between the temperature of the radiant tube (the energy source for heating the load) and the temperature of the load. The temperatures of the furnace wall and atmosphere are somewhere in between. The higher the furnace and radiant tube temperatures, the higher the NOx emissions in the exhaust (Fig. 5 and Fig. 6). Propane and butane combustion produces NOx levels approximately 30% higher than that produced by natural gas because of their higher flame temperatures.

Fig 7 NOx emission as a function of heat flux in AR-Ultra burner of 150 mm diameter by 1,400 mm effective length Fig 8 NOx emission as a function of heat flux in AR-Ultra burner of 200 mm diameter by 2,030 mm effective length

Effects of other factors on NOx emission

NOx emission data versus radiant heat flux, measured at a furnace temperature of 950C (1740F) are shown in Fig. 7 and Fig. 8. Heat flux was calculated based on fuel input, burner thermal efficiency and effective tube surface area. The higher the heat flux dissipated from the tube to the furnace chamber, the lower are the NOx emissions in the burner exhaust. This is due to flame quenching inside the inner tube causing the release of more radiant energy from the burner.

Flame jet outlet velocity from the combustor is an important factor in creating optimum recirculation within a radiant tube. Optimizing the tube temperature uniformity helps lower NOx emissions and increase burner efficiency. As shown in Fig. 9, increased velocity promotes NOx reduction by approximately 8% for natural gas and 4% for propane. The higher velocity also increases the rate of recirculation, resulting in even lower NOx emissions. This is due to the increased dilution of the fresh, hot flame jet with cooler returning combustion products.

Fig 9 Flame outlet velocity influence on NOx emission in AR-Ultra burner of 200 mm diameter by 2,030 mm effective length Fig 10 Influence of flue gas recirculation on relative NOx emission and relative tube temperature difference

Flue-gas recirculation and NOx emissions

Three factors that determine the amount of flue-gas recirculation possible in radiant tube burners are combustion outlet velocity, flow rate (input) and the size of the gap between the combustor outlet and the inlet to the inner tube. Test results in Fig. 10 show that when this gap is optimized in relation to the other two factors, NOx emissions can be reduced 10% and _T lowered by 30% over results achieved using a burner with no gap (no recirculation).

For more information: Contact Jim Roberts, Metals Market Manager, Eclipse Combustion, 1665 Elmwood Rd., Rockford, IL 61103; tel: 815-877-3031; fax: 815-877-3336; e-mail:jroberts@eclipse; Internet: www.eclipse