Axial Transport and Residence Time of MSW in Rotary Kilns

Axial transport and residence time of MSW in rotary kilns

Part I. Experimental

S.-Q. Li a,b,*, J.-H. Yan a, R.-D. Li a, Y. Chi a, K.-F. Cen a

aDepartment of Energy Engineering, Zhejiang University, Hangzhou 310027, PR ChinabDepartment of Thermal Engineering, Tshinghua University, Beijing, 100084, PR China

Received 17 October 2000; received in revised form 28 December 2001; accepted 31 December 2001

Abstract

Experiments on the influences of operational variables on the axial transport of both heterogeneous municipal solid waste (MSW) and

homogenous sand are conducted in a continuous lab-scale rotary kiln cold simulator. Compared with sand, the residence time of MSW has a

relatively large discrepancy with the ideal normal distribution due to the trajectory segregation of MSW components. The residence time at

different axial zone is quite different due to the varied bed depth profile along the kiln length. MSW has a longer mean residence time (MRT)

and a lower material volumetric flow (MVF) than sand because of the higher hd than sand. The increment of both rotating speed and kiln

slope reduces MRT, and increases MVF. Exit dam has a significant impact on the MRT and the influence of internal structure group

consisting of various axial ribs and circular ribs is mainly determined by the height of circular ribs. Inside wall roughness also has effect on

MRT through changing the bed regimes. For a case with the certain inlet and exit bed depths, the product of MRT and MVF holds at a

constant within the limits of experimental errors in spite of the changing experimental variables.

D 2002 Published by Elsevier Science B.V.

Keywords: Rotary kiln; MSW; Axial transport; Mean residence time; Material volumetric flow

1. Introduction

Rotary kilns have been widely employed in chemical and

metallurgical industries as heterogeneous noncatalytic gas–

solid reactors. The typical applications include drying or

heating of wet solids, mixing or grinding of powders,

calcining of limestone, clinkering of cementitious materials,

reducing of iron ore or ilmenite, etc. [1–3]. Rotary kilns

continue to find new applications in such gas–solid reac-

tions, despite challenges from newer and more specialized

reactors such as fluidized bed and spouted bed.

In recent years, rotary kilns have played an important

role in the thermochemical treatment of municipal solid

wastes (MSW). Rotary kiln system is one of the most

promising incineration processes since it can simultane-

ously treat wastes as liquids or solids of various shapes

and sizes and easily achieve the flexible adjustment by

altering kiln inclination, rotational speed, etc. Rotary kiln

as a primary gasification chamber, followed by a secon-

dary combustion chamber, can fulfil the complete destruc-

tion and detoxification of hazardous wastes, meanwhile

minimize emissions of dioxins and heavy mental. All these

unique features enable rotary kiln irreplaceable in MSW

incineration. ‘Siemens Schwelbrenn’, ‘Noell Conversion’

and ‘Westinghouse O’Connor’ processes are updated rep-

resentatives of rotary kiln incinerators [4,5].

Pyrolysis, on the other hand, is an attractive alternative to

incineration as a waste treatment option with respect to

minimum environmental emissions and maximum resource

recovery [6,7]. Rotary kiln pyrolyser also has many unique

advantages over other types of reactors. For instance, slow

rotation of inclined kiln enables the well mixing of wastes,

thereby the more uniform pyrolytic products. Also, the

flexible adjustment of residence time can make pyrolysis

reaction perform at a perfectly optimum condition conven-

iently. With a view to different resource recovery option,

rotary kiln can be properly designed to yield mainly the

synthesis gas, e.g., ‘Landgard’ Process [8], or to make the

high calorific tars as well as porous carbon black, e.g.,

‘Kobe Steel’ Process [9].

Mean residence time (MRT) of solids through rotary kiln

is one of the most important parameters, which not only

directly influences mass and heat transfer, but also deter-

0032-5910/02/$ - see front matter D 2002 Published by Elsevier Science B.V.

PII: S0032 -5910 (02 )00014 -1

* Corresponding author. Tel.: +86-10-62782108.

E-mail address: [email protected] (S.-Q. Li).

www.elsevier.com/locate/powtec

Powder Technology 126 (2002) 217–227

mines chemical reaction degree of gas and solid phase. In

order to optimize the design and operation of rotary kiln, it

is necessary to develop the simplified empirical expressions

to enable the proper predicting of the volumetric flow of

material (MVF) as well as MRT. Sullivan et al. [10]

originally conducted the experimental research on the sol-

ids’ MRT in rotary cylindrical kiln and derived the empirical

equation of MRT correlating various operational variables,

kiln geometry parameters and material properties. Subse-

quently, Vahl and Kingma [11] and Kramers and Kroock-

ewit [12] made further experiments on the holdup as well as

MRT in a horizontal or inclined cylinder, respectively. The

effect of internal structures is one of remarkable research

community. Chatterjee et al. [13] studied the effect of ring

formation, Matchett and Sheikh [14] studied the effect of

both number and angle of axial flight, and Rutgers [15]

considered the influences of shapes of kiln entrance and exit

end faces. Furthermore, the residence time distribution

(RTD) in rotary drums were researched by Abouzeid and

Fuerstenau [16] and Sai et al. [17] adopting tracer stimulus-

response techniques, or by Wes et al. [18] using atomic

absorption spectroscopy methods. In addition, as the prac-

tical field-scale rotary kiln was concerned, Groen et al. [19]

performed corresponding investigations in a high-temper-

ature kiln, while Schofield and Glikin [20] studied them in

an intensive gas-flow fleeting kiln.

More recently, Wightman and Muzzio [21] emphasized

that a research community focusing on the segregation of

multimixed particles in rotary cylinder. Donald and Rosse-

man [22] firstly performed experimental studies in a hori-

zontal system and identified three patterns of segregation:

radial, axial and end longitudinal. Gupta et al. [23] described

qualitative mechanisms of axial segregation, stating that a

difference in the dynamic reposing angles of two pure

components is a necessary (though not sufficient) condition

of band formation. Nakagawa et al. [24] recently employed

magnetic resonance imaging to study axial segregation.

Boateng and Barr [25] and Bridgewater et al. [26] studied

the mechanism of radial segregation, respectively.

However, previous researches on axial transport in rotary

kilns are mostly concentrated on the studies of small

cementitious and metallurgical particles, which are rela-

tively homogeneous in nature. Although rotary kilns have

been extensively used as reactors for MSW incineration or

pyrolysis, so far, there have been few attempts on extrap-

olating the experiences and correlation developed from

homogeneous materials to heterogeneous MSW. In this

part, comparative studies are conducted between homoge-

neous sand and irregular MSW in a rotary kiln cold

simulator (I.D. 0.3� 1.8 m). Impacts of material character-

istics (in terms of the dynamical angle of repose), kiln

geometry characteristics (i.e., roughness of kiln wall, exit-

end dam and internal structures) and operational parame-

ters (i.e., kiln inclination and rotational speed) on both

MRT and MVF are examined. Simplified formulas of MRT

and MVF are proposed on the basis of the experiment

results in Part II of this work.

2. Experimental

2.1. Setup

A cold simulator of rotary kiln, 0.3 mm in diameter and

1.8 mm in length, shown schematically in Fig. 1, was

employed for the experiments. The cylinder was made of

plexiglass so that the solid motion can be viewed. The

rotational speed is variable within the range of 0.5–10 rpm

(revolution per minute). The angle of kiln inclination can be

easily adjusted between 0j and 5j by altering the height of

the supporter at kiln inlet end. The feed rate of materials was

adjusted to a certain amount that keeps the inlet depth of the

solids on a desired value during each run. That is, the inlet

depth of solid bed responds to the feed rate of materials,

which is practically equal to the flow rate of materials under

the steady state, one to one, under the same operational

conditions. Therefore, the inlet bed depth instead of material

feed rate were selected as one of the operating parameters,

which was kept at 70 mm in all runs.

In order to study the impact of internal structures on solid

material motion, axial ribs and circular ribs were specially

designed, as shown in Fig. 2. The grouped types and

Fig. 1. Schematic of rotary kiln cold simulator ((1) Funnel, (2) Belt conveyor, (3) Tracer addition point, (4) Feed chute, (5) Rotary cylinder, (6) Position

plate, (7) Belt wheel, (8) Position wheel, (9) Jockey wheel, (10) Slope angle adjustor, (11) Supportor, (12) Varible motor, (13) Exit chute, (14) Sample

collector).

S.-Q. Li et al. / Powder Technology 126 (2002) 217–227218

geometric factors of the different kinds of axial ribs and

circular ribs are listed in Table 1.

2.2. Materials

Two categories of materials were employed for experi-

ments. One was reconstituted MSW consisting of 49.9 wt.%

wood chips, 17.0 wt.% paper plates and 33.1 wt.% waste

tyres. The mixture has irregular shape, size and heteroge-

neous property. Also, homogeneous sand was used as

another category for a contrast, which has higher density,

regular shape and similar size. The physical properties of

both kinds of materials are given in Table 2.

As was reported in earlier literatures [11–20], the bulk

characteristics of solids in terms of the dynamic angle of

repose, hd, exert significant influences on the transport and

mixing of the solids in the kiln. Here hd is measured

according to the Rotating Drum-Method (Henein et al.

[27]). This measurement is done under one of the most

general bed rotation cases of the kiln, rolling regime, and it

can reflect the real dynamical bulk characteristics of solids

in kiln. For all hd measuring of various materials, the kiln

rotates at 4 rpm and fill ratio of solids in the kiln is about

15%–20%. hd of sand is about 29.7j while that of MSW

mixture is 48.5j.In order to study the influence of wall roughness on

MRT and MVF, the inside kiln wall are covered by the

finer or coarser emery cloths. The wall friction factor of

solids is defined as the tangent function of the wall friction

angle. The latter is measured by a special shear-plate-

analyzer with an easy adjusting shear angle. A plate, which

has the same roughness with the tested wall, is fixed on

the adjustable shear plate. A layer of tested solids is laid

on the plate. Then, the plate is gradually tilted until the

solids begin to fall along it. At that time, the slope of the

plate with respect to the horizontal line is just the wall

friction angle of solids. As shown in Table 2, compared to

the smooth inside wall, the friction coefficient, f, increased

dramatically (69–243%) with finer emery cloth setting.

However, f increased only about 6–25% from the finer

emery cloth to the coarser one.

2.3. Experimental methods

To determine MRT and MVF, the system must be

adjusted to achieve the steady state, which is reached when

the output of materials is equal to the feed rate of materials.

The steady-state flow rate of materials, volumetric or molar,

was measured by collecting sample successively within a

certain time and quantifying it. As is widely known, the

residence time of solids through rotary kiln is not a constant,

but a probability distribution. Hence, the mean and variance

of residence time are experimentally obtained by the stim-

ulus-response techniques of tracers. Generally, the method

for RTD measurement of MSW is hardly available in current

literatures, though that of homogeneous sand has been

described in detail [16–18]. In this work, experiments were

taken by introducing the dyed tracers consisting of 9 wood

chips, 15 paper plates and 36 waste tyres. The ratios of three

kinds of tracers are 47.9, 16.9 and 35.2 wt.%, which are

quite similar to those of original mixed wastes (i.e., 49.9

wt.% wood chips, 17.0 wt.% papers and 33.1 wt.% tyres). In

fact, it is difficult to feed all tracers to kiln inlet end at the

same time. Also, it is infeasible to label every tracer and

measure its time one by one in such a short time interval.

Thus, all tracers are divided into three groups and dyed red,

white and yellow (each of which consists of 3 wood chips, 5

papers and 12 tyres). As the steady state is reached, three

groups of tracers are successively fed to the kiln inlet and

the corresponding inlet time for each group is recorded. At

the kiln outlet, they were collected after a certain time

interval, until all tracers finished their excursion through

the kiln. Meanwhile, the residence time of tracers in each

sample interval was recorded (here, the inlet-time differ-

ences of three group were taken into account). The mean

and variance of residence time of the tracers are expressed

below:

MRTcXI

i¼1

tiEðDtiÞ; ð1Þ

r 2cXI

i¼1

ðti �MRTÞ2EðDtiÞ; ð2Þ

Table 1

The grouped types and geometric factors of internal structures

Group No. Axial ribs Circular ribs

Number Height

(mm)

Number Height

(mm)

Exit dam 1 – – 1 30

Exit dam 2 – – 1 50

12b-4n 12 20 4 30

12b-7n 12 20 7 30

12n-4n 12 10 4 30

12b-4b 12 20 4 50

Fig. 2. Schematic of internal structures ((1) Cylinder, (2) Circular ribs, (3)

Longitudinal ribs).

S.-Q. Li et al. / Powder Technology 126 (2002) 217–227 219

where I is the sequence of sampling interval, Dti is the

interval of the ist sampling interval, and ti is the retention

time of tracers in the ist interval. E(Dti) can be expressed as

the ratio of the number of tracers in ist sampling interval to

that of the total tracers, i.e.:

EðDtiÞ ¼ NðDtiÞ=XI

i¼1

NðDtiÞ: ð3Þ

Usually, the relative variance is used to express the

dispersing extent of RTDs, which satisfies the relation:

r 2r ¼ r 2=MRT2: ð4Þ

3. Results and discussions

Table 3 summarizes the detailed experimental results for

MRT (together with r and rr) and MVF of the MSW in

rotary kiln simulator with various rotating speeds, kiln

slopes, exit-end dams, internal structures and walls of

different roughness. In the following sections, the effect of

each variable both on MRT and MVF will be discussed

accordingly.

3.1. Residence time distribution of MSW and sand

The previous studies on residence time of solids in

rotary kiln are scarcely concentrated on the heterogeneous

MSW, but on the homogeneous particles instead. For

instance, Abouzeid and Fuerstenau [16] concluded that

residence time of dolomites in rotary kiln is approximately

subjected to a normal distribution by employing the axial

dispersion model. The comparison between experimental

results and theoretical calculation for RTD of both sand

and MSW are shown in Figs. 3 and 4, respectively. As for

sand, the probability of tracers by experiment in each

sample interval (Dti) fits well with the theoretical normal

distribution function. However, it is noted that, for MSW,

there exists a relatively large discrepancy between exper-

imental value and theoretical curve, other than rr2 of

MSW is much larger than that of sand under the same

condition. It can be explained that, as tracers consist of

three components with various shapes, sizes and densities,

the variance in residence time would arise from axial

segregation instead of axial mixing (particle collision). In

fact, the axial segregation causes the deviation of meas-

ured RTD from the normal distribution (this view will be

further verified in Part II of this work). In addition, the

alternate band formation of the various components (i.e.,

the visible axial segregation) that has been studied and

emphasized in a batch kiln system by some investigators

[21–24] does not occur in this experiment. According to

Donald and Rosseman [22], the alternate band formation

in batch system may not arise in continuous system where

the length of system is not adequate for particles to

demix. Gupta et al. [23] stated that a difference in hdof all pure components at a particular rotation speed is

one of the necessary (though not sufficient) conditions of

band formation. From Table 2, the hd difference among

three components of MSW is not significant. The rotating

speed is only an order of magnitude smaller than that in

the study of Gupta et al. Thus, it is induced that the axial

segregation of MSW in kiln won’t be violent enough to

form alternate bands, especially for such a system with a

limited ratio of length to diameter (L/D = 6).

The detailed r and rr of RTD of MSW under various

rotating speeds, kiln slopes, exit-end dams, internal struc-

tures and wall roughness are given in Table 3. Much

valuable information can be obtained as follows. (1)

Increasing rotating speed or kiln slope leads to relatively

slight increment of r, while rr varies or keeps in a

narrow range from 0.02 to 0.05. (2) The usage of exit

dam can also increases r or rr; but the impact on

variance is less appreciable than that on MRT. (3)

Employment of internal structures promotes both r and

rr remarkably by one order of magnitude (e.g., rr from

range [0.02, 0.05] to range [0.2, 0.4]). However, it must

be stated that the measuring error of RTD’s r and rr is

quite high due to the segregation of MSW properties.

Meanwhile, the measuring precision of both MRT and

MVF can doubtlessly reach an expected level because of

their statistical averaged characteristics. Therefore, more

attention is paid to discussions on MRT/MVF rather than

r/rr in the following paper.

Table 2

Summary of properties, bulk characteristic and wall friction factors of materials

Materials Shapes Bulk density

(kg/m3)

True density

(kg/m3)

Sizes (mm) hd (j) f1 f2 f3 * *

Wood chips Cylindrical 371.5 646.0 U25� 30 47.3 0.525 0.902 0.941

Paper plates Tabulate 104.5 691.7 30� 30� 3 51.9 0.563 1.930 2.331

Waste tyre Arcuate 278.7 1020.0 10� 5� 30 52.9 0.421 0.941 1.102

Mixed MSW* – 225 777.6 – 48.5 0.480 1.003 1.251

Sand Nodular 1342 2660 1.0–2.0 29.7 0.407 0.724 0.768

* Mixed MSW consist 49.9 wt.% woods, 17.0 wt.% papers and 33.1 wt.% tyres.

** f1, f2, f3 are wall friction factor of solids with none, finer and coarser emery cloth setting on inside wall.


3.2. Axial velocity distribution along kiln length

For end-open system, as the bed depth and the fill ratio

of solids in cross-section are different at the different axial

position, the axial cascading velocity of solids is not

constant along the kiln axis. That is, the residence time

in different zone along kiln length is quite different. Figs.

5 and 6 give the axial velocity of sand and MSW under

different axial points, respectively. It can be seen that the

axial velocity of particles increases along the axial direc-

tion. It is due to the decrement of the bed depth or the fill

ratio along the kiln axis. Thus, according to the mass

conservation theory, d(quA)/dx = 0, the axial velocity along

kiln length increases gradually. By the way, as for the

practical rotary kiln reactor, it has various reaction zones

along the axis and the solids have different properties in

every zone. Thus, it is essential to know the detailed

residence time of the solids in each zone. However, up

to now, nearly all the experimental/theoretical works of

solid transport are concentrated on the overall residence

time through the kiln inlet to outlet. The residence time of

solid passing a special reaction zone can be obtained by

integration of the bed axial velocity along the age of this

zone, ti ¼ mzizi�1dz=uðzÞ where z represents kiln axis and i

Table 3

Overall experimental data for MRT and MVF of MSW with different variables

Run number Internal structure Rotated rate (rpm) Inclination (j) MRT (min) MVF (l/min) r rr

1 Smooth wall 2 2.40 11.53 1.56 0.26 0.023

2 3 2.40 8.25 2.62 0.20 0.024

3 4 2.40 5.58 3.73 0.18 0.026

4 4 1.81 7.05 2.53 0.23 0.043

5 4 0.62 11.28 2.18 0.54 0.049

6 6 2.40 4.07 4.24 0.16 0.042

7 8 2.40 3.27 5.50 0.12 0.037

8 Finer emery cloth setting 3 2.40 10.98 2.04 2.65 0.24

9 4 2.40 8.87 3.09 2.02 0.23

10 4 1.81 8.78 2.09 1.75 0.20

11 4 0.62 16.80 1.16 3.84 0.24

12 6 2.40 6.20 4.58 1.19 0.19

13 8 2.40 4.20 4.62 0.85 0.20

14 Coarser emery cloth setting 3 2.40 11.83 1.60 2.10 0.18

15 4 2.40 8.40 2.80 1.70 0.20

16 4 1.81 9.47 1.96 2.23 0.24

17 4 0.62 12.30 1.69 3.82 0.23

18 6 2.40 6.15 3.82 1.43 0.23

19 8 2.40 4.95 4.27 0.97 0.20

20 Exit dam 1 (30 mm) 3 2.40 13.15 1.64 0.43 0.033

21 4 2.40 9.93 2.44 0.37 0.037

22 4 1.81 12.27 1.47 0.37 0.030

23 8 2.40 5.92 4.22 0.30 0.051

24 Exit dam 2 4 2.40 14.80 1.56 0.67 0.045

25 12b-4n 2 2.40 19.67 1.27 4.48 0.23

26 3 2.40 13.90 1.71 3.29 0.24

27 4 2.40 9.75 2.16 2.26 0.23

28 4 1.81 12.67 2.00 3.56 0.28

29 4 0.62 24.67 1.18 8.26 0.34

30 12b-7n 2 2.40 16.95 1.02 4.36 0.26

31 3 2.40 15.90 1.69 4.11 0.27

32 4 2.40 11.62 2.33 2.45 0.21

33 4 1.81 15.12 2.13 4.34 0.29

34 4 0.62 21.67 0.98 7.32 0.33

35 12n-4n 2 2.40 19.67 1.33 5.57 0.28

36 3 2.40 12.08 1.84 3.24 0.27

37 4 2.40 9.78 2.71 2.14 0.22

38 4 1.81 15.20 1.91 4.67 0.31

39 4 0.62 22.37 1.20 6.89 0.33

40 12b-4b 2 2.40 24.92 0.87 6.27 0.25

41 3 2.40 15.85 1.47 2.94 0.19

42 4 2.40 12.03 2.04 2.61 0.22

43 4 1.81 15.95 1.38 3.92 0.25

44 4 0.62 26.75 0.84 7.65 0.27

* The inlet depth of solid bed in all runs is 70mm (23% of inner diameter).


the zone’s sequence. Axial velocity, u(x), can be calculated

through the empirical correlations (Lebas et al. [28],

Perron and Bui [29]).

3.3. Influences of particle characteristics on MRT and

MVF

The comparison of MRT and MVF between sand and

MSW under the same conditions is shown in Figs. 7 and 8,

respectively. The MRT of MSW is greater than that of sand

for all runs. From the regress curve in Fig. 7, it is obtained

that the former is about 1.43 times of the latter. Contrarily,

MVF of MSW is less than that of sand with the multiple

of 0.625 (1/1.48). From Table 2, it can be seen that sinhdof MSW is 1.50 times of that of sand. Thus, the conclusion

is drawn: the material’s characteristics exerts its influences

on the MRT and MVF mainly in terms of hd; MRT

increases approximately in linear fashion as sinhd of

material increases, while MVF is subjected to the inverse

proportional function of sinhd. These conclusions will be

verified subsequently by the theoretical analysis in Part II

of this work.

3.4. Influences of rotating speed and kiln slope

The impact of rotating speed on the MRT and MVF of

heterogeneous MSW is shown in Fig. 9. As rotational speed

increases from 2 to 8 rpm, MRT decreases nearly in inverse

proportional fashion of rotating speed, while MVF increases

gradually. These conclusions are consistent with those

acquired from the homogenous small particles by others

[11,30]. It may be explained that the axial transport of solids

mainly occurs in the active layer of bed surface, while solids

in the stagnant region under bed surface only turn around the

kiln axis without any axial displacement. As the rotational

speed increases, the times of a particle entering the active

layer per unit time increases, which further results in the

increase of the particle’s axial displacement per unit time

(namely, particle’s axial velocity) [18,31]. Therefore, MRT

decreases and MVF increases.

Fig. 3. Residence time distribution of sand.

Fig. 4. Residence time distribution of MSW.

Fig. 5. Axial speed distribution of sand along kiln axis.

Fig. 6. Axial speed distribution of MSW along kiln axis.


Fig. 10 indicates the effect of kiln slope on the transport

behavior of MSW. When kiln slope angle increases from

0.62j to 2.40j, the MRT decreases in an approximately

linear fashion from 11.28 to 5.58 min, while MVF rises

from 2.18 to 3.73 l/min. It is possible that the increasing

kiln inclination causes the increment of the gravitational

force component in the axial direction of individual

particle during its cascading, i.e., the increment of the

solid axial velocity, which finally causes MRT to decrease

and MVF to increase.

3.5. Influences of exit-end dams

The exit-end dam exerts significant influences on the

MRT and MVF of solids in a rotary kiln. As shown in Fig.

11, MRT of MSW and sand with the 30-mm dam (about

10% of inner kiln diameter) are 78.0% and 71.4% longer

than that with no end constriction, respectively. As for a

higher 50-mm dam (about 16.7% of inner diameter), the

corresponding augment is 165% and 138% for MSW and

sand, respectively. The higher height of dam has, the more

remarkable effect it has on MRT. The affecting extent made

by the 50-mm dam is almost twice of that made by the 30-

mm dam. The reasons for above conclusions lie in two

aspects. First, the usage of exit dam reduces the slope of the

solid bed and then the axial cascading velocity of particles.

On the other hand, it causes the increment of bed depth in

kiln. This increment of flow area in cross-section will

decrease axial cascading velocity, either. Finally, the combi-

nation of two reasons above cause the remarkable increase

of MRT. In addition, MVF decreases when employing exit-

end dam. As for the 30-mm dam, the reduction of MVF of

MSW and sand are 34.5% and 27.9%, respectively, and for

the 50-mm dam, the corresponding reduction is 58.3% and

35.2% (shown in Fig. 12). It is doubtless that the usage of

exit dam is an effective method to control the MRT and

MVF of solids. However, it is noted that exit dam has no

such apparent impacts on relative variance rr as it has on

MRT, as seen from Table 3.

Fig. 7. Comparison of MRT between MSW and sand.

Fig. 8. Comparison of MVF between MSW and sand.

Fig. 9. Effect of rotational speed on MSW transport behavior.

Fig. 10. Effect of kiln slope angle on MSW transport behavior.


3.6. Influences of the internal structures

The internal structures inevitably affect the axial trans-

port of solids [13,14]. The impacts of internal structures on

MRT are different with various groups consisting of a

certain number of axial ribs or circular ribs. Figs. 13 and

14 illustrate the influences of four groups of internal

structures (listed in Table 2) on the MRT of MSW and sand,

respectively. It is found that all these four kind of internal

structures seriously increase MRT of solids. The detailed

conclusions are drawn: as the number of circular ribs in an

internal structure group increases (12b-4n! 12b-7n), the

MRT in 12b-7n case is slightly longer than that in 12b-4n

case for both MSW and sand; as the height of circular ribs

increases from 30 to 50 mm (12b-4n! 12b-4b), the incre-

ment of MRT from 12b-4n to 12b-4b case is more remark-

able. However, with the increasing height of axial ribs from

10 to 20 mm (12n-4n! 12b-4n), the MRT changing ten-

dency of MSW and sand is inconsistent or inexplicit.

According to above, it is concluded that influences of the

internal structure group on MRT are dependent on the height

of circular ribs, while the impacts of the height of axial ribs

is inexplicit. The influence of circular ribs on MRT can be

explained by their similarity to the exit dam whose influence

has been already tested to be remarkable. The impact of

axial ribs on MRT is quite complicated, which not only

changes the solid’s dynamic angle of repose, but also kicks

up some particles from the bed surface to the freeboard

space. These conclusions can be verified by the experi-

ments. For instance, the 30-mm exit dam promotes MRT

with 78.0%; however, the internal structure groups labeled

12b-4n, 12b-7n and 12n-4n, whose circular ribs is also 30-

mm height, only promote the MRT in range of 75% to 108%

with MSW under an condition of rotating speed at 4 rpm

and inclination at 2.40j (Fig. 14).

Since the exit dam (regarded as one special circular rib)

does not exert the same apparent effects on rr as it does

on MRT, here, the great promotion by one-order of magni-

Fig. 11. Effect of exit end dam on MRT.

Fig. 12. Effect of exit end dam on MVF.

Fig. 13. Effect of various internal structures on MRT of MSW.

Fig. 14. Effect of various internal structures on MRT of sand.


tude of rr may be attributed to the presence of axial ribs

(Table 3).

3.7. The influences of inside wall roughness

The inside wall’s roughness, designated by the wall

friction factors of solids ( f), has significant effects on axial

transport. Fig. 15 presents the variation of MRT with

the various f. When the inside wall is smooth with f at

0.480, MRT is rather short. When f increases to 1.003

(viz. the finer emery cloth is set on inside wall), MRT

increases significantly. However, with further increase of f

from 1.003 to 1.251 (the coarser emery cloth setting), the

increment of MRT is not apparent. It can be explained that

the variation of f greatly changes the bulk characteristics

(in terms of variation of hd) and further changes the bed

regimes of solids. As observed, the solid bed in a case of

smooth wall may perform at a slumping regime, in which the

solids cascade as periodic ‘avalanche’ through the kiln

and have a short MRT. When f increases to 1.003, hdincreases from 48.5j to 58j and the rolling regime is well

formed. The variation of both hd and bed regimes greatly

increases MRT. When f continues to increase to 1.251, hdslowly increases up to about 59.5j, the rolling regime also

dominates the bed behavior (that is, the advanced cataract-

ing regime is not yet formed). Thus, the variation of MRT

is small. In addition, the impact of f on MVF of MSW is

shown in Fig. 16. MVF decreases gradually with the

increasing f, and its changing tendency is contrary to that

of MRT.

3.8. The relationship between MRT and MVF

As discussed above, when one of the variables, such as

rotational speed, kiln slope, dynamic angle of repose and

internal-structures, changes, the variation of MRT has the

reverse tendency with that of MVF. It is apparent that the

product of MRT and MVF is just the holdup of solids in

kiln, which is expressed as Holdup =MRT�MVF. Fig. 17

shows the product of MRT and MVF (i.e. holdup) under

different run, in which the values of MRT and MVF are

obtained from Table 3. In a case of smooth wall with open

exit end, in spite of the changing of rotational speed or kiln

slope, the product of MRT and MVF keeps around 19.7 l,

which implies that the overall fill ratio of solids to kiln

vessel is a constant at 15%. As far as the internal-structures,

such as 12b-4n, 12b-7n and 12n-4n are considered, it holds

around 25 l (that’s, overall fill of solids reaches about

18f 19%). The employment of 12b-4n, 12b-7n or 12n-4n

not only similarly enlarges the exit-end bed depth from 0 to

30 mm (while the bed depth at kiln inlet holds 70 mm), but

also expands the bulk characteristics of solids along all kiln

length. Thus, the holdup increases from 19.7 to 25.3 l.

Finally it is drawn that the overall fill ration or holdup of

solids within kiln is just relevant to the inlet bed depth, exit

bed depth and usage of internal structures, but independent

of some operational parameters such as rotational speed or

kiln slope. This conclusion is much meaningful to the scale-

up or design rotary kiln reactor.

Fig. 17. Relationship between MRT and MVF.Fig. 15. Effect of inside wall roughness on MRT of MSW.

Fig. 16. Effect of inside wall roughness on MVF of MSW.


4. Conclusion

(1) The reconstituted MSW consists of 49.9 wt.% wood

chips, 17.0 wt.% paper plates and 33.1 wt.% waste tyres.

The dynamic angle of repose (hd) reflects the bulk character-istics of solids in kiln. hd of MSW is 48.5j and hd of the

contrastive sand is about 29.3j. The value of hd increases

with the enhancement of wall roughness, but is independent

of the drum rotation speed.

(2) The distribution of residence time of MSW arises

from the axial segregation of different components, but not

the axial collisions. It results in a relatively large discrep-

ancy of experimental RTD with ideal normal distribution.

The variance (r or rr) of MSW is greater than that of sand.

It is noted that the known phenomenon of alternate band

formation does not occur in such a continuous system.

(3) The axial cascading velocity of particles increases

along the axial direction due to the decrement of the bed

depth or the fill area along the kiln axis. Thus, it is essential

to know the detailed residence time of the solids in each

divided zone besides that of the whole kiln, which implies

incoming research intensive.

(4) The difference of the MRT/MVF between the hetero-

geneous MSW and regular sand is related to their dynamic

angles of repose. MRT is approximately a proportion

function of sin hd, while MVF is an inverse proportion

function of sin hd.(5) Increasing either rotating speed or kiln slope results in

the decreasing MRT and increasing MVF. These variables

are both considered as flexible parameters to adjust the kiln

peformance in the practice. The r of RTD shows the same

fashion as MRT with various rotating speed or kiln slope,

while rr keeps in a narrow range from 0.02 to 0.05.

(6) The exit dam has remarkable impact onMRTof solids;

thus, it can also be used as an adjusting tool of the kiln. Impact

of internal structures, which are composed of axial ribs and

circular ribs, on MRT mainly depends on height of circular

ribs. However, exit dam (or circular ribs) does not exert the

same apparent impact on rr as it does on MRT. It is implied

that the axial ribs will have great effect on rr.

(7) The effect of roughness of inside wall on MRT and

MVF can be explained by that the variation of f between the

wall and the solids directly changes the bulk characteristics

of the solids in kiln and further changes the motion regime

of the bed.

(8) For a case with given inlet and exit bed depths, the

holdup in terms of the product of MRT and MVF holds at a

constant within the limits of experimental errors. The

presence of internal structures increases the holdup of

solids.

Acknowledgements

This research was supported mainly by Nation Natural

Science Funds of China (No. 50076037) and partially by

Zhejiang provincial National Science Funds of China (No.

RC99041). We are grateful to Dr. A. -M. Li for helpful

discussion about rotary kiln transport processes. The

contribution of Dr. J. T. Huang and Z. X. Zhang to this

work is gratefully acknowledged.

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Axial Transport and Residence Time of MSW in Rotary Kilns