World Cement - November 2018

November 2018

40 \

World Cement

the kiln hearth area, reaching about 70% burnout

before moving with the bed material into the kiln.

A small to moderate improvement in tyre chips

burnout can be achieved by adjusting their injection

angle, so that their trajectories are kept within

the higher upwards gas velocity regions. Further

burnout improvement can be secured through the

installation of a venturi-type refractory-bump in

the region of the chips trajectories, giving rise to

gas velocities in the range of 40 – 45 m/sec. over

a distance of 2 – 3 m. The typical venturi solution

results in a 0.5 – 1.0 mbar pressure loss, which

is offset by a higher TSR of tyre chips, as well

as a reduced need for air blasting. The latter is

minimised because the improved tyre chip burnout

within the calciner reduces the carbon in hot meal,

eliminating frequent build-ups in the calciner.

The same observations apply to lower-density

solid AF, but adjustments need to be made to the

chip trajectories to ensure that they remain in a

region of strong upward gas flow along the riser

duct length (Figure 2a). When the chip trajectories

and the required upward gas velocities combine

to ensure upward entrainment of the solid AF and

its burnout within the suspension mode, both CO

and NO emissions are not an issue. On occasion,

meal split may be required, should solid chips

disperse less into the upward gas flow and so

have the propensity to cause hot spots near the

calciner walls. These hot spots can be eliminated

by introducing meal via two meal inlets, so that an

enlarged region of moderate temperature is created

(Figure 2b).

When introducing lower-density AF (e.g. fluff), it

is important to design the AF inlet(s) so that

chips/particles of the material are not trapped in

lower velocity regions, in particular in recirculation

zones, as entrapped unburnt fuel volatiles increase

CO formation.

In the following example,

a mixed solid AF

(fine plastics, tissues, paper, and wood) was

introduced via a gravity-fed screw feeder. As the

AF injection velocities were extremely low, the AF

chips did not mix with the gas stream and ended

up in a low velocity region. In consequence, this

region became fuel-rich through the depletion of

oxygen (Figure 3a). The reducing conditions led to

CO emissions of nearly 2500 ppmv. Use of a swirl

burner allowed better dispersion of particles and

a reduction of CO to below 800 ppm (Figure 3b).

The burner momentum was designed such that the

AF chips/particles occupy the centre of the flow

without burning in the vicinity of the opposite wall,

risking damage to the calciner refractory.

Calciner optimisation: separate line

calciners

A similar AF firing strategy is also applicable to

separate line calciners (SLC), with the exception

that it is more desirable to have a higher expansion

ratio between the riser and the calciner ducts (see

the difference in expansion ratio in Figures 4a and

4b). It has been shown that this design feature

creates an external recirculation zone (ERZ) within

the calciner expanded duct section, which, due to

the evolving fuel rich conditions, assists the early

ignition of AF particles.

This also reduces the

concentrations, a tendency absent in lower

expansion ratio (Figure 4b) calciner designs, which,

in specific cases, have several times higher NO

emission levels (350

–

1400 ppm).

A higher duct expansion ratio assists in burning

both higher and lower density AF in separate

line calciners, as the bigger chips are recirculated

within the lower velocity and higher velocity

regions, eventually experiencing increased burnout.

However, the propensity for high CO formation

exists if the ERZ extends further upstream and

Figure 4. Two separate line calciner configurations: (a)

high expansion ratio, (b) lower expansion ratio.

Figure 5. (a) Volatiles from the RSP and (b) mixing

profiles (red = kiln riser gases, blue = tertiary air).

Volatiles do not mix well with the kiln riser gases.

(a)

(b)

(a)

(b)