oxygen reactivity measurement

Use of the differential thermal gravimetric carbon/oxygen reactivity measurement

analyser

for

Brian R. Stanmore Department of Chemical Engineering, University of Queensland, (Received 26 June 7988; revised 78 February 7997)

St Lucia 4067,

Australia

Two carbonaceous materials of known combustion kinetics were burned in a differential thermal gravimetric analyser. The temperature of the sample was raised at a rate of 15 K min-’ with a flow of excess air passing over it, and the combustion rates (based on external area) determined. The rates were similar to the accepted values obtained by other techniques. (Keywords: reactivity; carbon/oxygen;

DTG analyser)

Measurement of the combustion kinetics of carbonaceous materials has customarily been performed in fixed beds at low temperature or as single particles in entrained flow devices (e.g. drop tube furnaces) at high temperatures. The effect of mass diffusion is uncertain in fixed beds and the high costs have limited the extent of high temperature testing. The differential thermal gravimetric analyser (DTG) is finding increasing use for the measurement of char and coal reactivities. The instrument is run either in an isothermal mode with a step change in oxygen concentration from zero to initiate combustion or in a non-isothermal mode with a pre-set rate of temperature rise, e.g. the burning profile’. However to date there has been difficulty in extracting absolute kinetic data from the output because many investigators found the results to be specific to the instrument and the operating conditions. Modern DTG instruments permit the use of small samples and some designs have good thermal contact between thermocouple and sample. This is necessary with a strongly exothermic reaction like combustion to ensure that the thermocouple measures actual particle temperature. It is now possible to show that such instruments can produce reactivity data which can be used to give absolute kinetic parameters. This paper illustrates this fact for two materials whose combustion kinetics had previously been determined by the usual methods. BACKGROUND The reasons for the increase in interest in the DTA analyser are that the instrument: . is relatively inexpensive and compact; . is amenable to automatic data-logging a@l processing;

. gives a reliable indication of particle temperature; . can be operated under kinetic control with negligible external diffusion effects2,3; . can easily be used to prepare samples at known burn-out for subsequent analysis. In spite of these advantages experience suggests that there are significant reservations about the accuracy of the data produced, particularly with respect to coal. In addition to the fact that results are often instrument-specific there are the following objections: . The temperatures of operation are often significantly lower than those used in commercial systems. . The heating rates are low compared to commercial systems so that for coals the devolatilization and char formation processes (and hence the char yield and properties) will be different. . The combustion stages are difficult to identify. The deficiencies are illustrated by the inability in many instances to fit a straight line to the Arrhenius plots of reaction rate versus reciprocal temperature. This is the case for some chars and many coal&*. Morgan et ~1.’ showed that the chars from various maceral components can be related to differing reactivities. They demonstrated a correlation between DTG results and performance in a pilot scale combustor and also with mathematical modelling of large furnace performance. Their previous work on coal was less satisfying6 in that particle size effects were apparent, particularly with respect to the mode of evolution of volatiles. They claim that oxidation rates at higher temperatures cannot be reliably determined by simple extrapolation of Arrhenius plots, and caution that ‘the temptation to overinterpret Arrhenius plots is best avoided’.

In spite of these demonstrated deficiencies encouraging results are now emerging from DTG studies, suggesting that some attempt should be made to evaluate the usefulness of the technique for rates of measuring the oxidation carbonaceous materials. The questions requiring answers include: . Does the DTG give fundamental kinetic properties, i.e. Arrhenius parameters? . Can reactivities measured at low temperatures be extrapolated to higher (entrainment combustion) temperatures? The work reported here addresses these questions by measuring the combustion rate of two carbonaceous materials which have been extensively studied and well characterized. Coals are best avoided because of the complexity of pyrolytic destruction and volatiles evolution. The materials used were a petroleum coke and a Millmerran char which have been reported elsewhere9-‘*. Both samples were obtained from the CSIRO Division of Coal and Energy Technology, and their properties are listed in Table I. The kinetic data given in Tuhle 1 for the petroleum coke differ from that reported elsewhere by Young and Smith’ as they describe only the petroleum coke used in these tests and not the combined results from the two different cokes used earlier. The correlation was obtained from 69 data points taken from Young’s work on the same sample and fitted by regression analysis.

EXPERIMENTAL The burning profile mode of operation, i.e. a linear increase in temperature with time is commonly adopted. Morgan et u[.~ investigated the influence of sample mass, crucible configuration, heating rate

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Short Communications Table 1

Properties

of test materials Petroleum coke

Property Ash (% dry basis) Volatile matter (% dry basis)

(kg mm2 s-’

26 20 1120 90 80.2 28.0 0.5” 0.15

0.6

0.2 1 660 60,135 119.5 214 0.5 0.09

Initial particle density (kg mm3) Initial mean diameter (,m) Activation energy (kJ mol-‘) Pre-exponential factor Reaction order n

Millmerran char

(kPam0,5)

Area modifier yh “Assumed “Measured during test

and particle size and recommended a set of conditions. Some of the limits appear to be set by the type of temperature sensing in use. The conditions used in the present tests were those of CummingL3 except that the air flow rate was 100 ml min- ‘, the maximum value found not to disturb the balance mechanism. The sample size was 15-l 7 mg and the rate of temperature rise was 15 K min- ’ The samples had been screened to a limited size range, - 150 + 106 pm or - 75 + 45 pm for petroleum coke and - 106 + 73 pm for Millmerran char. The instrument used had a thermocouple sensor integral with the sample support pan. This system will give better temperature sensing than a thermocouple in the gas space adjacent to the sample but not as good as a thermocouple within the sample itself. The comparatively slow rate of temperature rise assists in keeping the thermocouple close to the sample temperature. The maximum temperature difference between the particles and sensor is estimated to be of the order of 2 K. The change in particle size as burn-out progressed was traced by quenching the combustion with nitrogen and removing the sample for analysis in a particle size analyser. The change in diameter from the initial value d, was fitted by: d=d,(l

RESULTS The DTG traces obtained are depicted in Figure I, The highly reactive Millmerran char exhibits a pronounced volatile ‘spike’, with an area consistent with the volatile matter content found by proximate analysis. This mass and that of the ash was deducted from the sample mass to give the mass of active material. The

FUEL,

1991,

Petroleum coke

I

I

200

1

300

I

I

I

1

I

400 500 600 700 Temperature (“C)

800

Figure 1 Burning profiles for petroleum and Millmerran char

coke

- -2.0 U? 0 3 & I E -4.0

Millmerran char

0) Y Q $ _I -6.0

-8.0

Petroleum coke

I

0.5

1.0

1.5

-u)

where u is burn-out and 51is an empirical exponent. The values of z found are given in T&r I. The global rate of carbon oxidation was derived from the rate of mass loss following the method of Cummings. External mass diffusion effects were assumed to be absent. The external surface area was calculated from the measured diameters as burn-out proceeded.

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Millmerran char

Vol 70, December

middle portion of the burning process and then increases again. The initial burning in these tests apparently takes place in regime I conditions, where all the interior surface of the particle is accessible to oxygen. The activation energy is around 200 kJ mol-’ which is higher than found by Tyler’* who reported a value of 158.6 kJ mol-‘. However the magnitude of p calculated on an intrinsic basis is similar to that found by Tyler, around 2 x lo-’ kg mm* s-’ (kPa)mo.5. At a temperature of 830 K the activation energy falls to 88 kJ mol- ‘, consistent with regime II combustion. Under these conditions the reaction rate is so high that pore diffusion cannot completely supply the interior of the particle with oxygen. The value of the Thiele modulus calculated at the point of change was about three which is of the same order as would be expected for a change from regime I to regime 11 combustion’4. Also shown in Figure 2 are correlations found by other groups examining the same materials using different techniques. (The CSIR09 group used burn-out in a drop tube furnace, Sandia Laboratories” used a temperature measurement in a flat flame and the Technical University of Wroclaw” used particle temperature at ignition in a drop tube furnace.) The Millmerran char shows an almost constant gradient on the Arrhenius (constant activation energy) plot during burn-out and agreement between the present work and the other studies is excellent. It apparently burns in regime II conditions at all temperatures tested. The appearance of constant kinetics over a wide temperature range parallels the findings of Smith and Tyler for a brown coal char” when the data were corrected to intrinsic values. A further study on coals seems warranted if well-characterized coal samples can be obtained, but bearing in mind the difficulties mentioned above.

lIT(K-’ x 10-z) Figure 2 Arrhcnius plot for petroleum coke (135 and 60 pm) and Millmerran char. -, Young and Smith’: pm-) Rybak”; -‘-.-, Mitchell” petroleum coke showed an initial rapid combustion rate increase which was followed by a prolonged period of almost constant rate of mass loss with increasing temperature. Values for the combustion rate p (kgm-‘s’ (kPa)-‘-“) were calculated at 25 K intervals from the traces (using the dashed portion during volatiles evolution). The value of n was assumed to be 0.5, consistent with the value found by Young” and adopted by Tyler” and Rybak’ ’ The data are plotted in Arrhenius form in Ficgure 2. The petroleum coke activation energy decreases during the

CONCLUSIONS For two different carbonaceous materials, the DTG analyser applied to the measurement of combustion kinetics under burning profile conditions gave similar values to other techniques in use. The arrangement of the temperature sensing of the sample is important. ACKNOWLEDGEMENTS The author thanks the Queensland Electricity Commission for the use of test equipment and B. Young of CSIRO for assistance. REFERENCES

1 2

Wagoner, C. L. and Winegartner, E. C. J. Eny. Pow. 1973, April, 119 Saha, R., Levendis, Y. A., Flagan, R. C. ef ul. Fuel 1988, 67, 215

Short Communications 3 4 5 6 I 8

Tseng, H. P. and Edgar, T. F. Fuel 1984, 63, 385 Smith, S. E., Neavel, R. C., Hippo, E. J. e/ al. Fuel 198 I, 60, 458 Cumming, J. W. Fuel 1984,63. 1436 Morgan, P. A., Robertson, S. D. and Unsworth, J. F. Fuel 1986, 65, 1546 Morgan, P. A., Robertson, S. D. and Unsworth. J. F. Fud 1987. 66, 210 Serageldin, M. A. and Pan, W. P.

9

IO

11

Therm&in?. Acta 1983, 71, 1 Young, B. C. and Smith, I. W. ‘Proc. 18th Symp. (Int.) on Combustion’, The Combustion Institute, Pittsburgh, 1981, p. 124 Mitchell, R. E. and McLean, W. J. ‘Proc. 19th Symp. (Int.) on Combustion’, The Combustion Institute, Pittsburgh, 1982, p. I 113 Rybak, W., Zembruski, M. and Smith.

12 13 14 15

I. W. ‘Proc. 2lst Symp. (Int.) on Combustion’, The Combustion Institute. Pittsburgh. 1986. 0.231 Tyler, R. J. F%l 1986, 65, 235 Cumming, J. W. and McLaughlin, J. Thermochim. Acru 1982, 57, 253 Stanmore, B. R. Trams. I. Chml. E. 1980. 58, 66 Smith, I. W. and Tyler, R. J. Con&. Sci. Technol. 1974, 9, 87

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