Cement Energy and Environment

' BURNERS FOR ALTERNATIVE FUELS This article first appeared in International Cement Review in March 2011 and is reprinted with kind permission of Tradeship Publications Ltd, United Kingdom. Email: info((l)CemNet.com Website: www.CemNet.com Alternative fuels offer major cost and environmental benefits but they almost always have some negative side-effects on process stability, quality and capacity. Neil Taylor and Raine lsaksson look at some basic kiln burner requirements needed to make the best use of alternative fuels. Neil Taylor and Raine lsaksson, lsaksson-Taylor Management Consultants, Australia/Sweden Alternative fuels can introduce species which cause buildups and blockages, and then add insult to injury by creating reducing conditions which make the recirculation problems worse. Their generally poor combustion characteristics also make the kiln flame longer which lowers thermal efficiency. The longer flame usually changes the clinker mineralogy and reactivity, affecting cement properties like strength , workability and colour. Alternative fuels also introduce more false air, moisture and ash into kiln system, which increases variability and reduces kiln capacity. But these problems can be made much more manageable by ensuring that the alternative fuels are burned efficiently through a suitable burner. The combustion process The main burner converts the energy in the fuel into high– grade heat, suitable for making clinker of the right quality. It combines the pulverized or nebulised fuel with the hot secondary air to . create an intimate fuel/air mix which can burn quickly and efficiently. With a moderately efficient burner and traditional fuels this process is usually effective enough to allow substitution rates of-20 per cent with many alternative fuels without creating problems. But with higher fuel substitution rates the proportion of high-grade fuel is reduced, lowering the average flame temperature to the point where it becomes difficult to consistently make good-quality clinker. To increase alternative fuel substitution beyond this level makes much more severe demands on the burner. Although there are many different types of kiln burners available, almost all commercial designs, with the exception of Alternative fuels also introduce more false air, moisture and ash into the kiln system, which increases variability and reduces kiln capacity. But these problems can be made much more manageable by ensuring that the alternative fuels are burned efficiently through a suitable burner. .. ... .... -- .. .. -···::::::: aeeondatyalr L:J - prlmllfY air ---. ~­ ·· -- .. :::::: seeondaryalr Figure 1· typical cha ractenst1cs of a rewculatory kdn flame the precessing jet burner, are jet entrainment burners. In a typical jet entrainment burner, the fuel and air are mixed by injecting a jet of primary air and fuel into the kiln at a high enough velocity to draw the hot secondary air into the flame envelope. The friction and turbulence created by this process helps to intimately mix the fuel with the hot secondary air, promoting rapid combustion when ignited (see Figure 1). The importance of adequate burner momentum The intensity of the air/fuel mixing process is determined mainly by the mass and velocity, or momentum, of the primary air jet. In some gas-fired burners the jet velocity is provided by the gas jets themselves rather than primary air, but the entrainment principle is similar. With low jet momentum, air– fuel mixing and combustion is less efficient, creating a longer flame and raising both carbon monoxide levels and gas temperatures at the back-end of the kiln . With a high– momentum burner, the more intense jet causes more vigorous and rapid mixing, so combustion is largely complete before the plug flow zone. This shortens and stabilizes the flame and improves heat transfer in the burning zone, and usually enhances clinker reactivity. If 19