Fundamentación de la estrategia de mejora

None of the chemical reactions can take place until the oxygen in the air is brought into contact with the gas.

All combustion processes therefore take place in the following stages:

MIXING→IGNITION→CHEMICAL REACTION→DISPERSAL OF PRODUCTS

The rate of combustion is dependent on the slowest of the above stages. In most industrial combustion systems, the mixing is slow whilst the other steps are fast. The rate and completeness of the combustion process is therefore controlled by the rate and completeness of fuel/air mixing. Insufficient fuel/air mixing produces un-burned CO in the flue gases, wasting fuel energy potential. For good combustion, it is neces- sary to ensure that adequate air is supplied for complete mixing and that the burner is designed to mix the fuel and air streams effectively .and efficiently. The rate and completeness of combustion is controlled by the rate of completeness of the fuel air mixing. Hence, the saying of combustion engineers:

"If it's mixed, it's burnt." Fuel/Air Mixing

For most burners, fuel/air mixing occurs as a result of Jet entrainment. Figure 9.1 shows a free jet issuing from a nozzle in an ambient medium.

Figure 9.1 : Entrainment of Secondary Air Into a Free Jet

Friction occurs between the boundary of the jot and its surroundings, causing the surrounding fluid to be locally accelerated to the jet veloci- ty. The accelerated air is then pulled into the jet thus expanding it. This

process is momentum controlled and continues until the velocity of the jet is the same as that of its surroundings. The greater the momentum of the jet, the more of the surrounding fluid that will be entrained into it. In the case of the free jet it can entrain as much of its surrounding medium as it requires to satisfy its entrainment capacity and it is able to expand unimpeded in doing so.

In the case of a confined jet however, such as in a rotary kiln flame, the jet is now constrained in two ways. The quantity of surrounding fluid being fed to the kiln ie., the secondary air, is controlled and limited, In addition, the expansion of the jet is now bound by the physical presence of the kiln shell.

If the confined jet has momentum in excess of that required for the com- plete entrainment of the secondary stream, then jet recirculation will occur. The secondary air stream is initially pulled into the jet as described above for a free jet. A point is then reached, however, when all the available air has been entrained into the flame. At this stage, the jet will pull back exhaust gase a from further up the kiln and draw them into the flame in order to overcome this excess momentum. This phe- nomenon, known as external recirculation, is illustrated in Figure 9.2.

Figure 9.2 : Idealised Recirculating of a Confined Jet

The Role of Primary Air

Primary air has two major roles in burners: controlling the rate of fuel/air mixing assisting with flame stability.

The Effect of Primary Air on Fuel/Air Mixing

The primary air itself mixes very rapidly with the fuel at the nozzle, but the remaining air (secondary air) must be entrained into the primary air and fuel jet as described above. The rate of entrainment is de pendent on the ratio of the momentum between the combined primary air and fuel jet and the momentum of the secondary air. Thus, the higher the flow- rate and velocity of primary air, the more rapid the fuel/air mixing. The flame characteristics are determined by this momentum ratio, and burners can be designed to give specific flame characteristics by the use of combustion modelling.

The presence or absence of recirculation has a great effect on the char- acteristics of the flame. A moderate degree of recirculation is a positive indication that fuel/air mixing is complete, whilst its absence is a clear

Table 9.1: Characteristics of Flames With and Without External Recirculation

Flame with

recirculation

Flame without recirculation

Fuel/air mixing Good Poor

Reducing/oxidising conditions

Oxidising conditions exist throughout the flame.

Strongly reducing conditions occur in fuel rich parts of the flame. Oxidizing conditions exist elsewhere.

Flame impingement None. Recirculating

gases protect bricks and clinker from flame impingement.

Flame impingement occurs on the brickwork/clinker at the point where the jet expands to hit the kiln (11°). Impingement is severe where a low primary air/secondary air ratio occurs.

Carbon monoxide level

CO only produced at levels of oxygen below 0.5%

CO produced at levels of oxygen as high as 2-4%

Heat release pattern Rapid mixing gives high flame temperature near the nozzle and a short burning zone.

Poor mixing gives gradual heat release with long flame

Flame stability Good flame shape with stable heat release pattern

Heat release pattern considerably affected by changes in secondary air temperature, excess air, fuel quality, etc.

indication that not all of the secondary air has been entrained into the primary jet- In this case, the production of significant levels of carbon

monoxide is normal.

Furthermore, in the absence of recirculation there is a tendency for the

flame to expand until it impinges on the brickwork. Hot reducing gases will then be in direct contact with (he refractory brick, tending to wash them away and causing their subsequent failure. The recirculating gases from a high momentum ratio flame, however, provide a 'cushion' of cooler neutral gases which prevents this direct impingement of the flame on the brickwork.

A high momentum recirculatory burner jet will also produce a more responsive and stable flame that is more controllable, hence making the operation of the plant itself easier. The characteristics of kiln flames with and without external recirculation are summarised in Table 9-1,

Secondary Air Aerodynamics

Since the secondary air has to be entrained into the fuel/primary air jet the secondary air aerodynamics can have a huge effect on the fuel/air mixing. The secondary air flow patterns are considerably affected by the inlet ducting.

In the case of rotary kilns the flow is considerably determined by the design of the cooler uptake and hood system, or in the case of planetary coolers, by the cooler throats. To obtain the optimum potential perfor- mance from any kiln, it is absolutely essential that the aerodynamic

characteristics of the kiln are fully taken into account when designing the burner. Extensive tests of kiln aerodynamics have been conducted by using water/bead model tests, computational fluid dynamics and full size investigations. One example of the aerodynamics for a grate cooler kiln is illustrated in Figures 9.3 and 9.4. The asymmetry in the air- flow pattern can be clearly seen.

In most cases burner design cannot overcome certain air-flow patterns and therefore, modification is often required to the geometry of the equipment. For example, in Figures 9.3 and 9.4 the tertiary air off-take on the rear of the hood has to be relocated to eliminate the poor aero- dynamics.

Figure 9.3 : Typical Aerodynamic from a Grate Cooler

Figure 9.4 : Close-up of Aerodynamics in the Burning Zone

However , in some cases, the solution may be as simple as changing the location of the burner tip in Figures 9.5 and 9.6

Similar effects are observed with riser ducts and flash calciners ( Figures 9.7 and 9.8 )

The sharp angle entry from the kiln to the riser gives a highly asym- metric airflow giving poor fuel/air mixing and an intense recirculation

zone on one side. Figure 9.9 shows how the velocity profile from the gases exiting the kiln dominates the distribution of particles injected in the riser. CO and temperature confirmed these predict-

tions.Figure 9.10 shows how the problem was corrected by tailoring the fuel injection to suit the aerodynamics

For complete combustion and uniform heat transfer, it is essential to have an even distribution of fuel throughout the cross-section of a fur- nace. Modelling techniques like the ones shown above are becoming an important tool in the cement industry for processs optimization.

Effect of Excess Air on Fuel Consumption

Although the effect of excess air level on overall efficiency for thermal processes has been understood for many years, it is surprising how little attention is paid to this matter even today. The reduction in efficiency which occurs as the oxygen level is increased is caused by the requirement to heat the excess oxygen and nitrogen passing through the system firstly to flame temperature and ultimately to exhaust gas temperature.

In cement plants, the increased air flow through the coolers causes a reduction in the secondary air temperature, and therefore a reduction in the flame temperature, thus requiring even more fuel to heat the charge to the required process temperature. The total increase in fuel con- sumption is much greater than that necessary to heat the excess air to back-end temperature alone.

The effect of excess air on kiln thermal efficiency is very considerable. Figure 9.11 shows the relationship between the oxygen level and the measured daily heat consumption for a cement kiln. A clear trend is apparent and increasing the oxygen level from 1% in the kiln to 5% causes an increase in the heat consumption of more than 10%.

Figure 9.1.1: Effect of Excess air on Heat consumption

If the excess air level in a flame is reduced below a certain level, carbon monoxide is produced. This in turn also leads to an increase in the fuel consumption, due to the incomplete combustion of carbon, (Figure 9.12). The better the fuel/air mixing, the lower the excess air level at which these emissions occur.

Figure 9.12 : Effects of Kiln Oxygen on Flue Gas Heat Loss

Excess air also has a dramatic effect the flame length and heat profile in the kiln. Many operators tend to believe the flame length increases as the draft from the ID fan increase. The Opposite is the true. Figure 9.13 shows a typical relationship between flame length and excess air for an

optimized kiln with good fuel/air mixing and one with poor fuel/air mixing.

Figure 9.13 : Flame Length Versus Excess Air Two important characteristics of Figure 9.13 are :

magnitude of Flame length to excess air levels Responsiveness of the flame to excess air levels.

The optimised flame can produce a length of 30 meters at 1.5% excess oxygen

whereas the poor flame produces the same flame length at 3.5% excess oxygon. In addition, the responsiveness of the optimised flame allows the operator to fine tune the length of the flame with minor adjustments to the excess air, whereas the poor flame requires a much broader range of excess air.