Burner Design

Turbulent Jet Diffusion Flames

The flame in the rotary cement kiln, riser, or calciner is for the most part produced by a turbulent diffusion jet- During the past century, scientists have paid far less attention to diffusion flames than they have to pre- mixed flames, despite the fact that the majority of industrial flames involve the simultaneous mixing and combustion of separate streams of fuel and air. The problem with analysing diffusion flames is that there is no fundamental property, like flame speed, which can be measured and correlated, even the mixture strength has no clear meaning.

When any jet mixes into its surroundings, steep concentration gradient are set-up in the neighbourhood of the orifice. Further downstream, tur- bulent mixing causes these gradients to become less severe but then rapid and random oscillations and pulsations occur. Only after the jet has largely decayed can any approximation to homogeneity be seen.

The particular fuel/air mixing pattern is determined by the mechanical and diffusion flux. Chemical reaction rates are of little importance except in the tail of the flame where chars can take a significant time to burn.

Rotary Kiln Burners

Rotary kiln burners are different from most other industrial burners in that only a proportion of the combustion air passes through the burner and is therefore under the control of the burner designer. Most of the air comes from the product cooler and the aerodynamics of the flow is dependent on others.

The most commonly used methods for designing rotary cement kiln burners are:

kinetic energy: where the cross-sectional area of the burner nozzle or nozzles are generally based on the formula: PAV², Primary Flow x

(Velocity)²

140 • Cement Plant Operations Handbook

momentum flux: where the cross-sectional area of the burner nozzle or nozzles are generally based on the formula: Primary Air Flow x Velocity expressed as % m/s.-% = primary airflow as a percentage of the stoichiometric air requirement

jet entrainment (see Figure 9.2): where the cross-sectional area of the burner nozzle or nozzles are derived from more complex calculation but generally related to:

me = mass flow of entrained secondary air mo = mass flow-rate of fuel and primary air

through the burner ma = mass flow of secondary air

The first two approaches assume the mixing between the fuel and air is unaffected by the secondary air and confinement of the rotary kiln- The jet entrainment approach determines the degree of external recircula- tion as the burner fuel jet mixes with the secondary air as shown in Figure 9.14.

Figure 9.14: Mixing and Recirculation Downstream of a Confined Jet

All kiln and calciner burners except processing jet burners are jet entrainment burners. The first two methods for designing burners usu- ally results in a very high primary air velocity (>300 m/s) that employs 5-10% of the stoichiometric air requirement. The jet entrainment method usually requires more mass flow of primary air at a lower velocity to provide enough momentum for external recirculation.

Cement Plant Operations Handbook • 141

(

)

m +

The mass flow and velocity of primary air is a central debate in the excess oxygen. This is one of the main rcasunfi why NOx emissions are

reduced with low primary air burners. Hence, there are competing forces between minimizing the amount of primary air and excess air that must be taken into consideration when designing a kiln burner.

Flame Stability excess of 80m/s are susceptible to severe instabilities. Despite all these potential

hazards, few kiln burners have adequate means of ensuring good flame stability. The most effective technique is to form an internal

recirculation zone just in front of the gas nozzle. Burning gas is carried

swirl on the primary air

swirl on both fuel and primary air.

Bluff bodies suffer overheating caused by the flame and therefore tend to be unreliable over long periods. Swirl on the gas can give good results, but tends to be less effective with high primary air flows and velocities. Swirl on the primary air is a very effective way of ensuring flame stability, but quite high levels of swirl are required to achieve effec- tive stability, and this can have adverse side effects on the overall flame characteristics such as causing flame impingement on the refractory.

The most effective method of ensuring flame stability is to use limited swirl on the both the fuel (in the case of gas only) and primary air. This ensures excellent flame stability and predictable burner performance over a wide range of operating conditions.

Except on some relatively primitive burners, such as some commercial- ly available rotary kiln burners, re-radiation from the hot walls should rarely be used as the primary means of flame stabilisation, more posi- tive methods are preferred.

Flash Calciner Burners

Many calciner burners are simply open ended pipes projecting through the walls of the vessel. The burning fuel is in intimate contact with the product. Flame stability is not normally an issue, since the incoming combustion air is normally preheated to above the gas ignition temper- ature. Hence, sophisticated swirl and bluff body devices are generally

unnecessary. However, such simple burners can suffer from a number of disadvantages including poor fuel/air mixing and uncontrolled heat transfer, which can adversely affect product quality. Most calciner burn- ers of this type produce large quantities of CO, typically over 1000 ppm and sometimes up to several per cent.

More sophisticated calciner burners are scaled down kiln burners, and like kiln burners should be matched to the calciner aerodynamics to optimise performance. Some calciner aerodynamic flow patterns give serious combustion problems because of poor combustion airflow. In these cases the burner alone cannot ensure the fuel/air mixing is opti- mised. The airflow must also be improved to ensure optimum combus- tion and heat transfer efficiency.

Cement Plant Operations Handbook • 143

Gas Burners

Rotary kiln oil burners are similar to conventional gas burners with an oil

sprayer replacing the gas gun. Some primary air is always used.

Because the cement kiln requires a precisely controlled heat up, opera- tors should use a high performance twin fluid atomiser with a wide turndown (8:1). Excellent flame stability is achieved using an ae rody- namic swirler flame stabiliser.

Flash Calciner Oil Burners

Oil burners for flash calcincrs vary from open ended pipes spewing oil into the vessel to sophisticated burners employing twin fluid atomisers,

The open ended pipes tend to produce large drop sizes (over 1000 micron) which cannot bum-out during their residen ce time in the ves- sel. This un-burned fuel either contaminates the product or ends up in the duct collector. It results in both efficiency losses and product deteri- oration. The open ended pipe maywell have been satisfactory for the lighter fuels of the 1960s but they are totally unsuitable for the high asphaltene oils of today. High performance, internal mixing, twin fluid atomizers are essential if all the fuel is to be burnt within the vessel. As for gas burners, the oil burners should be designed and optimised using modelling.

Coal Burners

Where the ash contmination can be tolerated, coal is the best fuel for rotary kilns owing to its very high emissivity, which results in high rates of heat transfer to the charge. Generally, the lower cost of coal gives it a significant economic advantage compared with other fuels. Coal is however, rather more difficult to handle than oil or gaseous fuels, since it is a solid material of varying composition and calorific value.

Regardless of the design of burner, coal must be dried and ground before being supplied to the kiln. As for oil and gas burners, a coal burn- er is a critical component in a rotary kiln. The variable nature of pul- verised coal requires a flexibility of burner design to allow the use of differing grades of fuel.

Many rotary kiln coal burners are simple open ended pipes, and apart from the inconvenience of having to insert a temporary oil burner to warm-up the kiln, an open ended pipe can given an excellent perfor- mance. Unlike oil and especially gas, it is quite safe to rely on re-radia- tion from the kiln walls when coal firing owing to the low ignition temperature of most coals.

Cement Plant Operations Handbook • 145

With the conversion of many rotary kilns from oil firing to coal firing in the transfer of vibrational energy from one molecule to the next, and in fluids it occurs in addition as a result of the transfer of kinetic