ISSN 1068-364X, Coke and Chemistry, 2009, Vol. 52, No. 1, pp. 5–8. © Allerton Press, Inc., 2009.Original Russian Text © L.V. Kopeliovich, 2009, published in Koks i Khimiya, 2009, No. 1, pp. 6–9.

5

Literature data on the coking pressure of coal andcoal batches were mainly obtained in semiindustrialfurnaces with a mobile wall. There are very few data onthe coking pressure in industrial furnaces.

For different coals, the coking (splitting) pressure,measured in similar conditions, is significantly differ-ent [1, 2]. The relation between the coking pressure andtraditional laboratory characteristics of coal is not clear[3]. The coking pressure is greatest for some coals withreduced clinkering properties (such as K2 coal and thetechnological group OS6) and some coke-grade coals,as noted in [1, 2]. Bituminous coal, with maximumswelling and thickness of the plastic layer, does not cre-ate dangerous coking pressures. In other words, theswelling of the plastic material and the thickness of thecoal’s plastic layer are not directly related to the cokingpressure.

The coking pressure is determined by the gas per-meability of the barriers at the boundaries of the plasticlayer, predominantly on the hot side; the gas permeabil-ity does not depend primarily on the cracking of thesemicoke layer [2].

In our view, the coking pressure of coal is deter-mined primarily by the magnitude and rate of intolera-ble deformation of the plastic coal layer and by its rheo-logical (elastoviscous) properties.

There are ample literature data on the viscosity andlimiting shear stress of coal and coal batches in theplastic state. Among the many methods of determiningthe viscosity, the most common is the Giesler plastom-eter [4]. The near-limiting shear stress is determined bythe method in [5]. The information on the other rheo-logical properties is limited but suggests the need formore profound study [6].

To determine the rheological properties of coal inthe plastic state, a viscosimeter with a descending coax-ial cylinder has been created [7]. We know that instru-ments of this type are used to determine the viscosity ofoil bitumens. The basic component of the viscosimeter(shown in the figure) is steel cylinder

1

, which contains

a sample of the material to be tested. The lower thick-ened cylindrical section of rod

2

is positioned coaxiallyin the sample layer. Rod

2

is suspended on a thread,which passes through two pulleys

3

and

3a.

An adjust-able counterweight

4

is attached to the other end of thethread. One of the pulleys is equipped with an instru-ment for measuring the angle of rotation

5

and a scale

6

.Before the experiment, the sample and the cylinder

are placed in a tubular electrical furnace

7

, while therod in the sample is in equilibrium. Then the sample isheated in a specified manner. The temperature is mea-sured at the bottom of the cylinder by means of thermo-couple

8

, connected to a galvanometer. A load is peri-odically applied to the cylinder (rod), by removing andrestoring part of the initial counterweight at specifiedtime intervals.

Since the force on the moving cylinder of the instru-ment is periodic, the almost periodic motion of the cyl-inder under the action of this force may be distin-guished from the aperiodic motion together with theswelling plastic mass of the coal. To this end, the meanposition of the cylinder is determined at each moment,over a period corresponding to the period of variation ofthe force on the cylinder. Then the cylinder displace-ment in the viscosimeter (the difference between thecylinder position and this mean) is calculated at eachmoment.

The cylinder displacement

x

in the plastic mass ofthe coal under the action of a constant force typicallydepends on the time

t

as follows

(1)

where exp is for exponential function and the other des-ignations are variables and coefficients.This time dependence is characteristic of a body with arheological model of the form [8]

N

–

N

/

H

. (2)

Here

N

denotes a Newtonian body (an ideal viscous liq-uid), whose mechanical model corresponds to a cylin-

x b0 b1t C t/trel–( ),exp+ +=

COAL

Differences in the Coking Pressure of Coal

L. V. Kopeliovich

Eastern Coal-Chemistry Institute, Yekaterinburg, Russiae-mail: vuhin@nexcom.ru

Received December 6, 2008

Abstract

—A method is developed for formulating a rheological model that describes the plastic mass of coal;the parameters of the plastic mass in the state of maximum deformability are considered. This method is usedin investigating coal from the Pechorsk, Donetsk, and Karagandinsk basins.

DOI:

10.3103/S1068364X09010025

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2009

KOPELIOVICH

der with a liquid-filled piston;

H

is a Hookean solid (anideal elastic body), whose mechanical model is aspring; elements separated by a minus sign are in series;those separated by a slash are in parallel.

In deriving formulas for the parameters of thismodel, we assume that, in the preceding stage of load-ing, elastic deformation is completely developed. Thisis close to the truth, since the characteristic time for thedevelopment of elastic deformation (the relaxation time

t

rel

=

η

/

G

, where

η

is the viscosity of the second New-tonian body;

G

is the shear modulus of the Hookeanbody) is considerably less than the loading period, forthe given angles. In that case

(3)

Here

F

is the amplitude of the applied-force oscilla-tions;

η

1

is the viscosity of the first Newtonian body;

L

is the length of the mobile cylinder; and

while

α

is the ratio of the internal diameter of themotionless cylinder to the diameter of the mobile cylin-der. The other notation has already been explained.

To calculate the rheological parameters of the coalin the plastic state, the coefficients in Eq. (1) are deter-mined by familiar mathematical methods and com-pared with the coefficients in Eq. (3). Experiments at

x b0 Ft/Kη1 2F/KG( ) Gt/η–( ).exp–+=

K2πL

αln α2 1–

α2 1+---------------–

------------------------------;=

the same angle are performed with several loads on thecylinder.

If the dependence of

η

and/or

η

1

on the load is sta-tistically significant, the corresponding viscous ele-ment is regarded as a Bingham body (a liquid thatbegins to flow only at stress exceeding some limitingshear stress), rather than a Newtonian body. The param-eters of this model (the viscosity

η

or

η

1

and the limit-ing shear stress

θ

or

θ

1

) are determined from the depen-dence of the effective viscosity on the load, as follows.

As we know, for a Bingham body, the shear stress is

τ

=

θ

+

η

d

ν

/d

x

,

where d

ν

/d

x

is the gradient of the shear velocity.The dependence of the periodic component of the

Bingham body’s drag force

F

on the periodic compo-nent of the viscosimeter’s cylinder velocity

ν

may bewritten analogously

F

=

K

1

θ

+

K

ην

.

Here

where

r

is the radius of the mobile cylinder.Knowing the effective viscosity of the material

under test at two force levels

F

1

and

F

2

(

η

1

and

η

2

,

K1

2/3πLr 2 α2 1+( ) α3 1+

α2 1+--------------- 3/2 α 1+( )–+

αln α2 1–

α2 1+---------------–

---------------------------------------------------------------------------------------------------------,=

1

2

3

4

56

7

8

3a

To galvanometer

Instrument for determining the rheological properties of coal in the plastic state.

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DIFFERENCES IN THE COKING PRESSURE OF COAL 7

respectively), we calculate the rheological parametersof the Bingham body

η

= (

F

1

–

F

2

)/(

F

1

/

η

1 – F2/η2),

θ = (F1/K1)(1 – η/ η1).

For a Bingham body in parallel with a Hookeanbody, the analogous formulas may be obtained for theelastic modulus and the limiting shear modulus. In theformula for θ, K1 is replaced by 4K1.

By this method, a rheological model of the coal’splastic mass is constructed for each loading or unload-ing period of the cylinder. This model is a combinationof elements with purely elastic properties, with purelyviscous properties, and with friction and behaves underthe action of the load in the same way as the coal’s plas-tic mass. This model may be determined most accu-rately for the period of maximum deformability of theplastic mass. For this period, we have obtained rheolog-ical models of the plastic mass for several coals fromthe Kuznetsk, Pechorsk, Donetsk, and Karagandinskbasins [7].

The following conditions are assumed here: piecesize of the coal, ≤3 mm; compaction of the sample by avibrator after loading; heating rate 3 deg/min; loadingtime 1–2 min. The variation coefficients of individualdeterminations of the viscosity and shear modulus are25 and 45%, respectively.

The characteristics of some of the coals are given inTable 1, and the parameters of the corresponding mod-els (means of 4–8 determinations) in Table 2.

There are no data on the coking pressure for thecoals considered in the present work. However, on thebasis of literature data we may assume that only Taib-inskaya coal may be characterized by considerable cok-ing pressure. In a state of maximum deformability, fourof the five coals behave as non-Newtonian fluids; theirrheological models include the first Newtonian body,corresponding to Eq. (1). Therefore, the stress appliedto the plastic mass of these coals may relax to zero. Wewould expect that such coal would not develop a largecoking pressure.

In contrast, however, KS coal in the state of maxi-mum deformability has the properties of a solid: itsrheological model does not include the first Newtonianbody, and the stress applied to the coal’s plastic massdoes not completely relax. Therefore, we would expectthat, in certain conditions, this coal may develop con-siderable coking pressure, as confirmed by literaturedata.

Thus, the large coking pressure of some coal may beexplained on the basis of a rheological model of itsplastic mass.

CONCLUSIONS

(1) A method of developing rheological models ofcoal samples has been outlined, on the basis of deter-mining the parameters for the plastic mass of coal in thestate of maximum deformability.

Table 1

Shaft, enrichment plant; basin Rank of coal Ad, % Vdaf, % Plastic-layer thickness, mm

Kirov; Kuznetsk G 8.5 40.4 14

Vorgashorskaya 3; Pechorsk GZhO 8.5 33.2 11–12

Severnaya; Pechorsk Zh 8.3 29.9 21

Verkhne-Duvanskaya; Donetsk Zh 7.8 30.2 27

Taibinskaya; Kuznetskii KS 9.9 16.9 8

Table 2

Shaft, enrichment plantRheological parameters

η1, Pa s η, Pa s G, Pa θ, Pa

Kirov 34000 2100 86 60

Vorgashorskaya 3 470000 – 7700 –

Severnaya 290 – – –

Verkhne-Duvanskaya 130 – – –

Taibinskaya – 140000 6900 –

Note: A bar means that these parameters do not apply to the corresponding coal.

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KOPELIOVICH

(2) By this method, some coals from the Kuznetsk,Pechorsk, Donetsk, and Karagandinsk basins have beeninvestigated.

(3) The results indicate that that, if the rheologicalmodel of the coal’s plastic mass does not include a vis-cous element in series with the other elements, consid-erable coking pressure of the coal is possible.

REFERENCES

1. Erkin, L.I. and Latskaya, M.P., Coking Pressure for Coalfrom the Eastern Regions of the Soviet Union, Podgo-tovka i koksovaniya uglei (Preparation and Coking ofCoal), Sverdlovsk, 1971, issue IX, pp. 112–125.

2. Gryaznov, N.S., Osnovy teorii koksovaniya (Fundamen-tals of Coking Theory), Moscow: Metallurgiya, 1976.

3. Loisin, R, Foch, P., and Boyer, A., Coke Quality and Pro-duction, London: Butterworth, 1989.

4. ISO TC 27/SC 5N 359 Standard: Coal: Determining thePlastic Properties by the Giesler Method Using a Plas-tometer with Constant Torque, 2003.

5. Graznov, N.S., Plasticheskoe sostoyanie i spekanie uglei(Plastic State and Clinkering of Coal), Sverdlovsk: Met-allurgizdat, 1962.

6. Fitzgerald, D., Fuel, 1957, vol. 36, no. 4, pp. 389–394.7. Kopeliovich, L.V. and Gryaznov, N.S., Rheological

Properties of Coals in the Plastic State, Khim. Tverd.Topl., 1981, no. 2, pp. 37–42.

8. Reiner, M., Rheology, Berlin: Springer, 1958.