Cement Energy and Environment

there is enough axial jet momentum, hot combustion gases from further up the kiln are also drawn back towards the root of the flame. This external recirculation of hot gas confines the flame and helps protect the refractory and clinker from flame impingement. The more robust flame created by the higher momentum burner jet also makes the kiln easier to control at lower excess air levels. And the better air-fuel mixing with a higher momentum burner allows more low-grade fuel to be used while keeping the average flame temperature high enough to make good– quality clinker. Air-fuel mixing is also improved by using a swirl or radial air channel to create an internal recirculation zone (IRZ) at the root of the flame. This encourages more rapid mixing of the fresh fuel with secondary air and burning gas from further up the flame envelope, promoting more rapid combustion and anchoring the flame to the burner. This internal recirculation also allows the burner to achieve efficient combustion with lower overall momentum rates. But while increased swirl air levels can shorten the flame they can also produce higher peak flame temperatures, elevated NOx levels and refractory damage if the swirl angle or air flow rates are too high. Typical features of multichannel burners firing alternative fuels Directly fired kilns are usually fitted with a monochannel firing pipe which carries both the fuel and primary air through the same duct, but axial ai~ets swirl or radial air --.;;...-*_ pilot burner . : pneumoswirler liquid fuel nozzle cooling plate spare duct Figure 2: typical features of a solid fuel multichannel burner used for firing alternative fuels most kilns are now indirectly fired using a multichannel burner with separate ducts for and air fuel. While both types of burner can be optimized to use alternative fuels, the multichannel burner is preferred because it has much better flexibility for satisfying operational, quality and environmental requirements. Figure 2 shows some basic features of a typical multichannel burner designed for firing alternative fuels. Axial air jets are now used in most burner designs rather than an annular opening because the individual jets allow easier penetration of the hot secondary air into the flame envelope for better mixing with the fuel. Primary air jet velocities are potentially much greater with high pressure blowers than fans, so less air volume is needed to obtain a given momentum, although power costs are also higher for blowers than with single stage fans. For best flexibility the blower or fan should be fitted with a variable speed drive and online pressure and flow metering and the equipment should have adequate flow, pressure and power reserves to allow proper optimization. Coal/petcoke channel discharges the fuel from 25 and up to 40m/s for more volatile fuels. A variable-speed drive allows optimization and changes in fuel blends to be managed. Fuel metering using mass-flow technology provides the best operational stability. Swirl or radial air is typically supplied in separate channel , preferably using a separate fan to the axial circuit rather than a common fan with damper control to each duct. A stand-alone fan with variable speed drive, online pressure and flow measurement provides the best flexibility for flame optimization. The swirl channel is typically positioned closer to the burner axis than the coal channel , but on some burners axis than the coal channel, but on some burners it is on the outside of the channel to delay mixing and reduce NOx. Some designs merge both swirl and axial channels into an integrated swirling airflow before the air leaves the burner tip or they use a series of jets which can be mechanically adjusted to alter the jet direction and overall swirl on the burner. Pneumoswirler is a device to provide moderate expansion of 20