the prediction of the fusibility of coal blends

Fuel Processing Technology, 37 (1994) 25~269 255 Elsevier Science B.V., Amsterdam

The prediction of the fusibility of coal blends

R i c h a r d S a k u r o v s , Leo J . L y n c h * and T.P. M a h e r

CSIRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde 2113, NSW (Australia)

(Received March 25, 1993; accepted in revised form September 28, 1993)

A b s t r a c t

Procedures are outlined by which the thermoplastic properties of coal blends-fluidity, as measured by Gieseler plastometry, and extent of fusion, as measured by proton magnetic resonance thermal analysis (PMRTA) - can be predicted from those of their component coals on the assumption that the thermoplastic properties of the blend are the appropriately weighted average of the properties of the component coals at every temperature. Account is taken of the influence of inert material on measured fluidity in the Gieseler plastometry model and thus the model can be applied to blends which have inert material added. The extent to which the observed thermoplastic behaviour of a blend deviates from these linear models is in principle a measure of any interactive effects that occur. Blends involving four Australian bituminous coals of different rank and fusibility were prepared so that the effect of a wide range of thermoplastic behaviour of the component coals on blend properties could be more clearly delineated. The coals and their blends were characterised by Gieseler plastometry and PMRTA. The maximum fluidity and PMRTA maximum fusion of the blends of coals of different rank were predicted by the models to be less than the weighted average of the component coals, and generally agreed with observation. The fluidity and fusibility of blends containing the higher rank, high fluidity coal and the two lower rank coals were significantly greater than expected by the model which is interpreted as evidence of an interactive effect between these coals that increases the fusibility of blends formed from them.

1. INTRODUCTION

M o s t coa l s h a v e to be b l e n d e d in o r d e r to p r e p a r e a c o n s i s t e n t p r o d u c t .

H a v i n g t h e a b i l i t y to p r e d i c t t h e c h a r a c t e r i s t i c s of t he b l e n d f r o m t h o s e of t h e

b l e n d c o m p o n e n t s w o u l d r e d u c e t i m e s p e n t on t r i a l s to p r o d u c e b l e n d s t h a t

m e e t g i v e n spec i f i c a t i ons . O n e c o m m e r c i a l l y s i g n i f i c a n t c o a l s p e c i f i c a t i o n is

f u s i b i l i t y t h e m a x i m u m e x t e n t to w h i c h a coa l b e c o m e s f luid d u r i n g h e a t i n g

to p y r o l y s i s t e m p e r a t u r e s .

T h e r e a r e t w o d i f f e r e n t m e t h o d s w h i c h c a n be u s e d to e s t i m a t e d i r e c t l y t h e

f u s i b i l i t y o f coa ls : G i e s e l e r p l a s t o m e t r y a n d p r o t o n m a g n e t i c r e s o n a n c e the r -

m a l a n a l y s i s ( P M R T A ) . G i e s e l e r p l a s t o m e t r y def ines a n d q u a n t i f i e s t he "v is -

c o s i t y " (or f lu id i ty ) of t h e fused coa l ; t h e f u s i b i l i t y o f t he c o a l is i n d i c a t e d

as t h e m a x i m u m f lu id i ty of t h e coa l d u r i n g h e a t i n g a t 3 ° m i n - 1 . G i e s e l e r

* Corresponding author.

0378-3820/94/$07.00 © 1994 Elsevier Science B.V. SSDI 0378-3820(93)E0102-K

256 R. Sakurovs et al./Fuel Processing Technol. 37 (1994) 255-269

plas tomet ry is a well-established s tandard method (ASTM D2639, AS2137) and some empir ical studies of the re la t ionships between the maximum fluidity of a blend and tha t of the component coals have been performed [1-7]. PMRTA est imates the ex ten t to which a coal is fused at any stage dur ing its hea t ing in terms of its p ro ton magnet ic resonance (1H NMR) signals; the maximum exten t to which a coal is fused dur ing hea t ing at 4 ° m i n - 1 is used to charac te r i se its fusibil i ty [8, 9]. PMRTA is a re la t ive ly new test method and has not been previously used to inves t iga te the re la t ionships be tween the fusibil i ty of blends and those of the components .

Predic t ing the fusibil i ty of a blend of coals would be a re la t ive ly t r ivial ma t t e r if the t empera tu re at which the fluidity of a coal was a maximum was cons tan t for all coals, and there were no in te rac t ions between coals: the fusibil i ty of a blend of coals would then be an appropr ia te ly weighted average of the fusibil i ty of the blend components and would vary l inear ly with blend composit ion. This behav iour has been observed in several studies of blends con ta in ing coals with similar t empera tu res of maximum fluidity [1 3], al- t hough Kos ina [4] has produced a plot of log maximum fluidity vs. blend composi t ion of two coals with similar t empera tu res of maximum fluidity but different maximum fluidity tha t shows posi t ive devia t ion from l ineari ty .

The t empera tu re at which the fusibil i ty of a coal is de te rmined var ies with coal rank. Thus, at the t empera tu re at which the fluidity of a blend of coals of ve ry different r ank is a maximum, the fluidities of the component coals will be at less t han the i r maximum, and there fore the fusibil i ty of the blend would be expected to be less t han tha t of the weighted average of those of the components if there were no specific in te rac t ion between the component coals. This reduc t ion in fusibil i ty from the weight average of maximum fluidity of the components has been observed in several studies of blends of coals of different r ank [1, 5-7].

In o ther studies, Sirgado and Verduras [10] and Colombo and Ruiz [11] have proposed that , if a weight ing according to mean ref lec tance of the coal is used, the logar i thm of the maximum fluidity is additive.

In this paper, two new models for predic t ing the fusibil i ty of blends of thermoplas t ic coals given the fusibil i ty of the component coals, one based on Gieseler p las tomet ry and the o ther on PMRTA, are described and tested. The resul ts obta ined by the two independent methods are compared.

2. EXPERIMENTAL

Four Aus t ra l i an b i tuminous coals t ha t differed s ignif icant ly in r ank and fusibi l i ty were chosen: coal MVHF - a medium volati le, high fluidity, vi t r ini te- r ich coal; coal MVMF a medium-volat i le medium-fluidity iner t in i te- r ich coal; coal H V H F - a high-volati le, high-fluidity, v i t r in i te-r ich coal; coal H V LF - a high-volati le, low-fluidity, iner t in i te- r ich coal. It was expected tha t coals with widely different proper t ies would provide more s t r ingent tests on any blending model t han coals of similar composit ion. However , the s tudy has not ye t been extended to include low-volati le b i tuminous coals.

R. Sakurovs et al./Fuel Processing Technol. 37 (1994) 255-269 257

All binary blends with 1 : 1 weight ratios were made from samples of these coals whose relevant analytical data are listed in Table 1.

In a second experiment, fresh samples of the HVHF and MVMF coals (analytical data also listed in Table 1) were also obtained, and blended in the ratios 1:4,2: 3, 1:1, 3:2 and 4:1.

In order to minimise possible effects of weathering on the measured fusibility, Gieseler determinations on each blend set were completed within two weeks, and prior to PMRTA measurements, the subsamples were stored at -18°C.

2.1. P M R T A

In a standard PMRTA pyrolysis the sample (~0.2 g) is heated under flowing nitrogen from room temperature to 600°C at 4 ° min-1. During heating the variat ion of the totally unsaturated 1H NMR transverse magnetisation signal intensity with time [I(t)] is recorded at 1-2 min intervals (corresponding to a 4 °C-8 °C temperature resolution of the data).

The signal amplitude at zero time [I(0)] is proportional to the amount of hydrogen in the sample. Thus, the variat ion in signal amplitude with temperature during a pyrolysis experiment yields information about the amount of hydrogen remaining within the specimen during the experiment [8, 12].

The rate of decay of the 1H NMR signal amplitude with time is strongly affected by the degree and extent to which the specimen is mobile.

A parameter, designated M2T16, calculated from the IH NMR signals is used to characterise the extent of thermoplastic fusion of the specimen during heating [9, 12, 13]. M2T16 values can vary from close to 0 for a very mobile material to 55 kHz 2 for a totally rigid material such as high-rank coal at room temperature. The M2T16 parameter is l inear in that the M2T16 value of a blend of coals is the hydrogen-weighted average of the M2T16 values of the component coals if there is no interact ion between them.

Plots of I(0) and M2T16 vs. temperature for a typical predried bituminous coal are shown in Fig. 1. At low temperatures the MET16 value is high because the material is almost completely rigid. With increasing temperature the M2T16 value of the material decreases, providing a measure of the increasing molecular mobility in the sample. This decrease becomes rapid as the thermoplastic state is approached and the MET16 value reaches a minimum. It then increases as mobile material escapes as volatile matter from the coal matrix and the residual "metaplast" cokes. At even higher temperatures the M2T16 values are consistent with the material becoming a fully rigid semi-coke.

The minimum MET16 value for the coal and the temperature at which it occurs (Train) can be obtained from the plot (Fig. 1). The minimum value of M2T16 is used to characterise the fusibility of the coal (the smaller the minimum M2T16 value, the greater its fusibility). The temperatures at which M2T16 decreases most rapidly and increases most rapidly are obtained from the differential of M2T16 vs. temperature (Fig. 1), and are referred to as the PMRTA softening temperature (Tsof) and the PMRTA solidification temperature (T~o,), respectively. These parameters and their definitions were chosen to describe


<

o

o

~,~ ~ ~ . ~ , ~

0 ~

0 " ~

Q ~ ~ "~ " ~ ~ "4~ " ~ 4o . ~ ~ " ~ . . . ~ , ~ ~,~.~ ~ ~ ~ . ~

~ ' ~ o 0 ~ ~'~ ~ ~r.f3~r_f3 ~ ~ ~,~ ~ .~

II

A

8 II o


(a) lOO

80

2 ' 10

60 t -

O ) t -

"E 40

E ~- 20

0 I I I I I

200 400 600 Temperature (°C)

(b) 50

45

(o 40

~ 35

30

25

,

200 400 Temperature (°C)

[i 0.4 J !

] t

\', , ~ ®

-0.2 /Train

60O

Fig. 1. Plot of hydrogen remaining (a) and M2T16 (b) values vs. temperature for a high- volatile bituminous coal. The definitions of the PMRTA softening temperature (Tsof), the temperature of minimum M2T16 (Tmi,), the solidification temperature (Tso,) and the PMRTA fusibility index (minimum M2T16 value) (Mml,) are also indicated.

the t he rm op l a s t i c i t y of coal in an ana logous fash ion to the Giese ler plas- t o m e t r y method.

3. RESULTS

3.1. The Gieseler blending model

Lloyd and Yates [14] h a v e shown t h a t the l oga r i t hm of the fluidity of a m a t e r i a l as m eas u red in a Giese ler p l a s tome te r decreases as the vo lume f r ac t ion of fine iner t m a t e r i a l mixed wi th it increases . F rom the i r s tudies of addi t ion of p e t r o l e u m coke to coals the log(fluidity) (lf) is reduced by 0.34 uni t s for each 0.1 inc rease in vo lume f rac t ion of iner t ma t e r i a l up to 0.5 vo lume fract ion. As the vo lume f rac t ion of solids is inc reased the r e l a t ionsh ip be tween


volume fraction of solids and If deviates increasingly from linearity [14]. The mixture has no measured fluidity when the volume ratio of solid to liquid is so high that the liquid is trapped within the interstices of the solids [14].

Guided by these observations, this model estimates the fluidity of a blend of coals, explicitly correcting for the presence of inert material in the mixture assuming the relationship between If and the volume fraction of inert material present is linear. This model can therefore be used to predict the behaviour of blends containing infusible components.

Any model that aims to predict Gieseler fluidity of blends requires simplify- ing assumptions. Some of the assumptions are reasonably well founded on empirical evidence but others are clearly approximations of uncertain validity. For this model the following assumptions were made.

(i) There is no interaction between the component coals during pyrolysis. (This is the assumption which can be tested for particular blends given an otherwise linear model.)

(ii) The logarithm of Gieseler fluidity is linearly additive. At a given temperature, and in the absence of inert material, the logarithm of the fluidity of a blend of coals is the mass average of the logarithm of the fluidities of the blend components. Empirical support for this assumption is provided by Lin and Hong [3] and BCRA [1].

(iii) Each coal added into the blend has a fusible component and a non-fusible component.

(iv) The infusible material can itself be divided into a mineral matter component and an organic component.

(v) The volume fraction of mineral matter is calculated from the ash yield assuming the mineral matter to be twice as dense as coal.

(vi) The organic infusible component is assumed to be a fixed fraction of the inertinite content. Depending on the source of the coal, and the technique used to measure inertinite fusibility, this fraction can range between 0.5 and 1, but is commonly assumed to be 2/3 [15], the value chosen here. The model is not sensitive to the value chosen for this fraction. For example, changing the value of this fraction from 2/3 to 1 changed the predicted characteristic temperatures by less than 3 °C and the log (maximum fluidity) by less than 0.1 units in the cases considered here.

(vii) The ratio of fusible/non-fusible material during heat treatment is constant; fluidity is a function of the rheology (viscosity) of the fluid phase only.

(viii) The volume fractions of each blend component are constant during pyrolysis. That the component coals lose different amounts of volatile matter during Gieseler plastometry, thereby changing the volume fractions of the component coals and thus their contribution to the total Gieseler fluidity of the blend, is ignored.

(ix) Gieseler fluidity values below the normal plastic range (lf < -1 ) can be estimated by extrapolation. Formulation and application of a linear blending model requires that Gieseler fluidity values exist for all temperatures at which each constituent coal has measurable Gieseler fluidity. Since fluidities of less than 0.1 dial division per minute ( l f < - l )


are not measured by the standard Gieseler technique, the Gieseler fluidity temperature profiles or pyrograms of the component coals in general do not fully overlap. Thus, fluidities at temperatures where the coal does not have a measurable fluidity must be obtained by extrapolation. Justification for such extrapolations is based on the Gieseler plastometry data on the coals in this study which indicate that at softening temperatures the increase in fluidity is approximately one log unit per 22 ° and at solidification temperatures the decrease is one log unit per 7 ° . The rates of change were used to extrapolate fluidities outside the Gieseler plastic range.

In order to obtain values of If of the blends and components at the same temperatures, If values were obtained by interpolation, using a cubic spline fit of the averaged If values of duplicate experiment to 2 ° intervals.

At each temperature: (a) the If value of each of the component coals is converted to an infusible-free

If value, by adding to the fluidity value the product of a proport ionali ty constant k (set to 3.43 from the data of Lloyd and Yates [14]) and the volume fraction of the solids present in the component;

(b) the If values of the "infusible-free" blend are calculated as a volume- weighted average of the fluidities of the fusible materials present;

(c) the effect of the presence of infusible mat ter is restored, by subtracting from the fluidity of the blends the product of k and the average volume fraction of solids.

This procedure results in a linear model for Gieseler plastometry for coal blends whereby the If of the blend (lfb) at any given temperature is predicted to be

[xi(1-si) l f i ]+k xisl - (xis~) ]fb --i= 1 i i = 1 (1)

n

1 - ~ X i 8 i i = 1

In this equation, for the ith component of a blend, xi is its volume fraction, lfi is its measured or extrapolated If at the temperature and sl is the fraction of the component which consists of infusible material. Only lfl varies with temperature.

Equation (1) was applied to calculate the expected fluidities of the blends. If the effect of the presence of infusible material is neglected or the volume

fraction of infusible material in all of the blend components is constant, eq. (1) simplifies to

lfb = ~ xilf~ (2) i = l

Equation (2) has been independently suggested elsewhere [16, 17]. By plotting lfb values computed according to eq. (1) against temperature,

predicted fluidity pyrograms of blends are obtained. The predicted values of log (maximum fluidity) (lmf) and of the temperature of maximum fluidity are obtained by parabolic interpolat ion of the resul tant fluidity pyrograms. The


predic ted sof ten ing t e m p e r a t u r e ( the t e m p e r a t u r e a t which the fluidity first exceeds 1 ddpm) and sol idif icat ion t e m p e r a t u r e (the t e m p e r a t u r e a t which mot ion stops, which was app rox ima ted by the t e m p e r a t u r e a t which dial m o v e m e n t first decreases below 0.1 ddpm) are ob ta ined by l inear i n t e rpo la t ion f rom the predic ted fluidity pyrograms .

3.2. Results for the Gieseler blending model

In Figs. 2 and 3 measu red Giese ler fluidity p y r o g r a m s of the ind iv idua l coals and the i r 1 :1 blends a re compared wi th those pred ic ted by eq. (1) for the

4

~ 3

, . -2

¢,

0 ~ 0

-1 I I

350 550

~______~ ~ MVMF

I I


Fig. 2. log (Gieseler fluidity) plotted against temperature for the medium-volatile, medium- fluidity (MVMF) coal, high-volatile, low-fluidity (HVLF) coal, a 1:1 mixture and that predicted for the 1 : 1 mixture using eq. (1) ( . . . . ).

3

~- 2

~o

o = J 0

I

350 400 450

HVHF

i 50O I

550 Tempera ture (°C)

Fig. 3. log (Gieseler fluidity) plotted against temperature for the medium-volatile, high- fluidity (MVHF) coal, the high-volatile, high-fluidity (HVHF) coal, a 1 : 1 mixture and that predicted for the 1 : 1 mixture using eq. (1) (- - ~ .


" 0 " - 4

E

0 _J

0.0 0.2 0.4 0.6 0.8 1.0 Weight fraction of HVHF coal

Fig. 4. log (maximum fluidity) of a blend of the medium-volatile, medium-fluidity (MVMF) coal, the high-volatile, high-fluidity (HVHF) coal from blend set 2 predicted using eq. (1), plotted against blend composition. (O)=Actual results.

TABLE 2

Differences between results for Gieseler plastometry of blends and those predicted using eq. (1)

Blend composition Predicted-measuredGieseler results

HVLF HVLF MVHF MVMF Gso f Gtmf G~ol Glmf (%) (%) (%) (%) (°c) (oc) (°c)

50 50 0 0 - 4 - 7 - 4 -0.1 50 0 50 0 +2 +2 +1 -0.7 50 0 0 50 + 1 - 1 + 4 - 0.4 0 50 50 0 +4 --9 - 5 -0.7 0 50 0 50 +1 0 +3 --0.2 0 0 50 50 + 1 - 6 0 - 0.2

M V M F / H V L F and M V H F / H V H F blends. These d a t a for M V M F / H V L F b lend show good a g r e e m e n t be tween expe r i m e n t and the l i n e a r p r e d i c t i on model wh ich is the case for mos t b lends i n v e s t i g a t e d in th is s tudy. The re su l t s for the M V H F / H V H F b lend on the o the r h a n d i n d i c a t e t h a t the m e a s u r e d m a x i m u m f lu id i ty is g r e a t e r t h a n p red ic t ed by th is model . F i g u r e 4 shows a p lo t of p r ed i c t ed and m e a s u r e d lmf vs. b lend compos i t i on for the second M V M F / H V H F b lend set. The p red ic t ed lmf va lues de v i a t e from l i nea r i t y , but ag ree wi th the m e a s u r e d values .

Tab le 2 l i s ts the d i f ferences be tween the Giese le r p l a s t o m e t r y p a r a m e t e r va lues p r ed i c t ed for the b lends us ing eq. (1) and the m e a s u r e d values .

These d i f ferences for the c h a r a c t e r i s t i c t e m p e r a t u r e s a re a l l less t h a n 10 ° and most less t h a n 5 ° and do no t i n d i c a t e s ign i f i can t differences .

In con t r a s t , the d i f ferences be tween p red i c t ed and m e a s u r e d f lu id i t ies a re s ign i f i can t for some b lends (Table 3). The 1 : 1 M V H F / H V H F and M V H F / H V L F

264 R. Sakurovs et al./Fuel Processing Technol. 37 (1994) 255 269

TABLE 3

Differences between predicted and measured results for Gieseler plastometry of a series of blends of MVMF and HVHF coals

HVHF coal in blend (%)

Predicted-measured Gieseler results

Gsof Gtmf Gso~ Glmf CC) (°c) (°C)

20 - 7 - 3 - 4 -0.1 40 - 2 +1 - 1 -0.3 50 - 7 - 6 - 1 -0.2 60 - 7 - 3 - 2 0 80 - 8 - 7 - 4 -0.1

blends have fa r g r ea t e r fluidities t h a n predicted. This in pr inc ip le sugges ts t ha t there m a y be some in t e r ac t i on be tween these coals when they are co-pyrolysed t h a t inc reases blend fusibil i ty.

In summary, the results shown here indicate tha t the s tandard Gieseler charac- ter is t ics of b lends of the M V M F coal wi th the o the r coals can be adequa te ly predic ted us ing eq. (1), and as a corol lary , any in t e rac t ions be tween these coals t ha t affect the i r fusibi l i ty as m eas u r ed by Giese ler p l a s tome t ry are weak.

In cont ras t , b lends of M V H F coal wi th the two lower - rank coals show e i ther t ha t they i n t e r ac t on co-pyrolysis to inc rease the i r co-fusibili ty, or t ha t the model a s sumpt ions do not apply to these blends.

3.3. The P M R T A blending model

The only a s sumpt ions necessa ry in der iv ing a l inear model for P M R T A of coal b lends are (i) the componen t s of the blend do not i n t e r ac t dur ing the co-pyrolysis , and (ii) the 1H N M R signal ampl i tude is p ropor t iona l to the to ta l hydrogen in each coal of the blend dur ing the hea t ing and pyrolysis .

The a s sumpt ions which are needed for the Giese ler p l a s tome t ry l inear model are not requi red for P M R T A because: (i) a ccoun t can be t a k e n of any differen- t ial loss of vola t i les by the different coals by us ing the res idua l hydrogen con ten t ( f ) p y r o g r a m and (ii) MzT16 is a l inear pa rame te r , so t ha t its va lue for a (non- in te rac t ing) blend is s imply the ave r age of the M2~16 va lues of the components , weighted for the a m o u n t of hyd rogen in each c o m p o n e n t (which is g iven in i t ia l ly by the dry-basis hyd rogen con ten t and mon i to red dur ing heating by the IH N M R measurements ) .

Hence the M2T16 va lue of a blend (M2T16b) a t a g iven t e m p e r a t u r e in the absence of i n t e r ac t ive effects is the hydrogen-weigh ted ave rage of the M2T16 va lues of the components :

n xiHifiM2T16i

M2T16b = i= 1 (3)

L xiHifi i=1

R. Sakurovs et al./Fuel Processing Technol. 37 (1994) 255 269 265

where for the ith component, M2116i is its M2T16 value, xl is the weight fraction, Hi the dry-basis hydrogen content and fl is t he / (0 ) signal intensity recorded at any temperature expressed as a fraction of the/ (0) signal intensity observed at the start of the run. fi provides the corrections for any differential loss of volatiles by the component coals during the pyrolysis. Both fi and Meal61 are functions of temperature.

PMRTA of a coal specimen provides the fi and MET16i values simultaneously. Plots of M2T16b values against temperature can be used to provide the PMRTA characterist ics of the blend.

In practice the M2T16 and the hydrogen remaining (f) values obtained for the PMRTA analyses are smoothed by a cubic spline and then interpolated to give values at 5 ° intervals from 20 °C to 580°C. This permits the parameters for the component coals and their blends to be compared at common temperatures.

The predicted PMRTA temperatures of softening, minimum M2T16 and solidification of the blends were obtained by interpolating the cubic spline of the M2T16 pyrograms generated by eq. (3).

3.4. Resul ts for the P M R T A blending model

Examples of the predicted and measured MET16 pyrograms of 1 : 1 blends are shown: the MVMF/HVLF blend (Fig. 5) and the MVHF/HVHF blend (Fig. 6). The differences between the predicted PMRTA characterist ic temperatures and minimum M2T16 values and the measured values of the 1 : 1 blends are listed in Table 4.

There is no evidence for significant interact ion between the coals in the binary blends containing the MVMF coal or the blend of the two high-volatile coals that affects the maximum extent to which the coal is fused. However, the measured minimum M2T16 value is significantly lower than that predicted for

( D

I -

50

40

30

I

MVMF

I I ] I I t


Fig. 5. M2T16 values plotted against temperature for the medium-volatile, medium-fluidity (MVMF) coal, the high-volatile, low-fluidity (HVLF) coal, a 1 : 1 mixture, and that predicted for the 1 : 1 mixture by eq. (3) ( ).


( O 4o

30 Blend ~ j / - - - - - ~ HVHF

50

i I I I I I i


Fig. 6. M2T16 values plotted against temperature for the medium-volatile, high-fluidity (MVHF) coal, the high-volatile, high-fluidity (HVHF) coal, a 1 : 1 mixture, and that predicted for the 1 : 1 mixture by eq. (3) (- ).

TABLE 4

Differences between predicted and measured PMRTA parameters for blends of coals (blend set 1)

Blend Predicted measured PMRTA results

Minimum M2v16 Tsor Tmi, Tso, (°C) (°C) (°C)

M V H F / M V M F 1 : 1 0 + 6 - 1 + 5 M V H F / H V H F 1 : 1 + 2.2 - 7 - 1 + 1 MVHF/HVLF 1 : 1 + 1.9 - 3 +4 + 10 M V M F / H V H F 1:1 +0.9 - 7 - 3 - 3 M V M F / H V L F 1 : 1 - 0 . 3 - 1 + 4 + 13 HVHF/HVLF 1 : 1 - 1.1 - 3 + 1 0 M V H F / H V H F 1 : 3 + 1.0 - 2 - 4 - 2 M V H F / H V H F 1 : 1 +2.5 - 7 - 6 +6 M V H V / H V H F 3 : 1 + 2.4 - 10 - 2 0 Mean differences between

duplicates" 0.61 3.4 2.2 2.5

"Based on PMRTA of 100 bi tuminous coals.

t h e M V H F / H V L F a n d t h e M V H F / H V H F b l e n d s i n d i c a t i n g t h a t f o r t h e s e

b l e n d s t h e c o a l s i n t e r a c t t o p r o d u c e a m o r e f u s i b l e m i x t u r e . P M R T A s t u d i e s o f

f r e s h 1 : 1, 1 : 3 a n d 3 : I b l e n d s o f M V H F a n d H V H F c o a l s c o n f i r m t h e d i f f e r e n c e

( T a b l e 4). T h e g r e a t e r t h a n p r e d i c t e d f u s i b i l i t y o f t h e b l e n d s a g r e e s w i t h t h e

G i e s e l e r p l a s t o m e t r y r e s u l t s f o r t h e s e s a m e c o a l s a n d b l e n d s .

T a b l e 4 s h o w s t h a t t h e r e is n o s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e p r e d i c t e d a n d

m e a s u r e d t e m p e r a t u r e o f m i n i m u m M2~16 v a l u e a n d t h e a g r e e m e n t b e t w e e n

p r e d i c t e d a n d m e a s u r e d s o f t e n i n g t e m p e r a t u r e s is good . T h e c o n s i s t e n t l y


less than predicted measured solidification temperatures for the HVLF/MVHF and the HVLF/MVMF blends (Table 4) suggest that HVLF coal may accelerate the solidification of these blends.

4. DISCUSSION

The thermoplastic phenomenon of bituminous coals involves a number of overlapping processes [18] including the following ones: 1. A fusion or physical softening of extensive, aromatic-rich structures in the

coal material. 2. Pyrolytic rupture of covalent bonds involving crosslinks between and ap-

pendages to the basic condensed ring aromatic units of the coal structure. The products of this react ion are low molecular weight fragments including the volatile products and those which contribute to the residual fluid "metaplast". The high concentrat ion of thermally generated radicals en- sures the high reactivity of this fluid metaplast.

3. Possible solvation of the still-solid aromatic-rich domains of the coal material by the already fused material [19].

4. Spontaneous radical condensation reactions that result in solidification. The thermoplastic property of a coal is thus related in a complex fashion to

the rates and extent of these reactions. Moreover for blends of coals it is possible that the individual processes are affected by the presence of species generated from the other coal materials. For example, the addition of organic additives such as coal tar pitch can substantially increase the cofusibility of the mixture beyond that expected if there was no interact ion between the components [19]. Thus, the fused aromatic material and/or volatile products formed by one coal could substantially affect the fusibility of other coals in a blend, and thus increase the fusibility of the mixture beyond that expected if there was no interaction.

Models that aim to predict the behaviour of blends in the absence of interaction can help to establish whether or not specific coals interact or not when blended. If strong interactions between some coals are found then this would indicate that these coals could be blended to produce a superior product.

5. CONCLUSIONS

(a) A model appropriate for predicting the thermoplastic behaviour of non- interact ing blends of coals as measured by Gieseler plastometry has been devised. Experimental determination of the plasticity of trial blends will then indicate whether any interact ion between them has occurred.

(b) This Gieseler " l inear model" seeks to take into account the influence of mineral mat ter and requires the following main assumptions:

(i) There is no interact ion between the component coals during pyrolysis.

(ii) The logarithm of Gieseler fluidity is l inearly additive.

268 R. Sakurovs et al./Fuel Processing Technol. 37 (1994) 255 269

(iii) Each coal added into the blend has a fusible and non-fusible component .

(iv) The infusible c o m p o n e n t consis ts of a mine ra l m a t t e r c o m p o n e n t and an o rgan ic component .

(v) The o rgan ic infusible c o m p o n e n t is a fixed f rac t ion of the ine r t in i t e con ten t of each of the coals in the blend.

(vi) The ra t io of fusible to non-fusible ma t e r i a l dur ing the pyrolys is is cons tan t .

(vii) Cons t ancy of vo lume f rac t ions of coal componen t s dur ing pyrolysis . (viii) Giese ler f luidity va lues below the no rma l r ange (<0.1 ddpm) can be

defined by ex t rapo la t ion . (c) A model a p p r o p r i a t e for p red ic t ing the the rmop las t i c b e h a v i o u r of non-

i n t e r ac t i ng blends of coals as measu red by P M R T A has been devised. E x p e r i m e n t a l d e t e r m i n a t i o n of fusibi l i ty of t r ia l b lends by P M R T A will t hen ind ica te whe t he r any i n t e r ac t i on has occurred.

(d) The P M R T A " l i nea r model" requi res only the fol lowing assumpt ions : (i) The re is no i n t e r a c t i o n be tween c o m p o n e n t coals dur ing pyrolysis .

(ii) P M R T A detec ts the same p ropor t ion of hydrogen p resen t in all the c o m p o n e n t coals.

(e) Both P M R T A and Giese ler models show t h a t in genera l fusibi l i ty of a coal blend, as defined by e i ther lmf or m in imum M2r16 value, is a non- l inear func t ion of blend composi t ion. As the difference in r a n k be tween components increases , the fusibi l i ty of the non- in t e rac t ing blend will be increasingly reduced compared to a mass -ave rage of t h a t of the components .

(f) The P M R T A l inear model and to a lesser ex ten t the Giese ler model provide rea l i s t ic me thods for p red ic t ing the the rmop las t i c p roper t i es of non-interac t ive coal blends, and thus m a k e possible the de tec t ion of i n t e r ac t ive effects, where present , in the pyrolys is of coal blends.

REFERENCES

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the prediction of the fusibility of coal blends

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