Low temperature oxidation of coals—a calorimetric study

Low temperature oxidation calorimetric study Ryuichi Hitachi

Kaji, Yukio Research

Hishinuma

Laboratory,

Hitachi

and Yoichi Ltd, 4026

of coals-a

Nakamura”

Kuji-cho,

Hitachi-shi,

12 Japan * Kasado Works, Hitachi Ltd, 794 Higashitoyoi, Kudamatsu-shi, (Received 1 July 1986; revised 28 August 7986)

lbaraki-ken,

Yamaguchi-ken,

319Japan

The rates of heat liberation and oxygen consumption due to coal oxidation were measured in the temperature range 2@17o”C using coals ranging from subbituminous to anthracite. It was found that the Elovitch equation fit the results for the heat generation rate excellently when it was modified slightly to include a corrective term representing the heat generation rate at the steady state. The oxygen consumption rate at a given temperature was found to be proportional to the product of the internal surface area and oxygen content of the coals, indicating that the oxygen containing surface groups are acting as reactive sites. Using these results. the heat evolved per mole of oxygen at steady state was calculated to be 75-90 kcal/mol. (Keywords: oxidation of coal; thermal analysis; Elovitch equation)

Design of safe systems for transporting and storing coal to prevent self-heating and thermal runaway is a major priority for coal utilization. It is also important for economic reasons because low temperature, atmospheric oxidation decreases the calorific value of coal. Spontaneous ignition occurs when the heat generation from coal undergoing an oxidation reaction at near ambient temperatures becomes greater than the cooling capacity of or the heat loss from the system. Thus, knowledge of the rates of the oxidation reaction and the heat generation accompanying it is very important in constructing the safe systems. Heat generation has been said to be proportional to the oxygen consumption by coal’. However, few data have been reported, all of which show a wide diversity in values, from about 50 to 100kcal/mo12-6. The discrepancies may have resulted in part from the difficulties in measurements. In the experiments for obtaining the heat generation, coal samples are usually pretreated by heating either in oucuo or in inert atmosphere, then they are brought into contact with oxygen to measure the oxygen consumption and the heat generation rates under either adiabatic or isothermal conditions. This pretreatment is expected to change the coal surface properties greatly, decomposing the surface oxygen-containing groups and leaving vacant active sites for the low temperature oxidation reaction7. Therefore, the heat generation thus measured is a complex phenomenon, the first stage of which is dominated by the rapid chemisorption of oxygen to the vacant sites created by the decomposition of the oxygencontaining groups during the pretreatment, generating a large quantity of heat of chemisorption. The heat generated per mole of oxygen at this stage is considered to be the heat of oxygen chemisorption to the vacant sites on the coal surface. The rate of heat generation decreases gradually as the rate of chemisorption decreases with the increase in 0016-2361/87/02015404$3.00 0 1987 Butterworth & Co. (Publishers)

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surface coverage with oxygen, finally leading to a steady state where the decomposition of the chemisorbed oxygen to oxidation products such as CO,, CO and H,O is expected to proceed at a rate equal to the oxygen chemisorption onto the active sites thus formed. The heat evolved per mole of oxygen at the steady state is considered to be the heat of oxidation reaction, which is the heat of oxygen chemisorption superimposed on the heat loss due to the decomposition of the chemisorbed oxygen. It is this steady state value which corresponds to the rate of heat generation due to low temperature oxidation actually occurring in the coal yard. However, it appears that it takes quite a long time for the reaction to reach a steady state when pretreated coals are used’. The heat generation decreases gradually over a long period of time and sometimes to a level below the sensitivity of the apparatus. The difficulty resides in obtaining the values for the steady state in which all the reactions proceed at constant rates. The purpose of this paper is to describe a new technique for obtaining the steady state heat generation rate for coal oxidation at low temperatures, using the Elovitch equation modified to include a corrective term for the steady state heat generation, and to report some results of the experiments undertaken to determine the heat of reaction. EXPERIMENTAL Materials

Coals of various ranks, ranging from subbituminous to anthracite, were used: B coal (bituminous, Australia), D coal (bituminous, China), T coal (bituminous, Japan), W coal (subbituminous, USA) and H coal (anthracite, Vietnam). The analyses of the five coals were listed in a previous paper7. They were ground and sieved using wire mesh screens. For calorimetric measurements, fractions having a diameter of 0.105-0.5 mm were used. They were

Low temperature

oxidation of coals: R. N. Kaji et al.

passing it through a quartz tube packed with calcium chloride. Calorimetric measurements were conducted at atmospheric pressure and temperatures between 20 and 65°C for periods of up to 10 h. The rate of heat evolution was obtained by graphical integration of the thermograms. The thermogram was divided into several segments which corresponded to several time intervals. The area above the base line for each time interval was measured by cutting out the chart and weighing it. Calibration was made against a standard thermogram obtained by feeding a fixed amount of electricity to the calibration heater. The rate of 0, consumption was measured using an apparatus with a cylindrical stainless steel reactor (53.5 mm i.d., 120mm length), described in detail in a previous paper, with the gas phase oxygen concentration of 21.2’74’. Specific surface area was measured using Carlo Erba Model 120 and Model 200 mercury intrusion porosimeters. Details of the procedure were described elsewhere’. of the calorimeter:

Figure 1 Schematic drawing cell; 4, reference cell; 5, gas reservoir; 8, thermo-module; thermostatic air bath

1, 2, cocks; 3, sample

reservoir; 6, drying tube; 7, flexible air 9, calibration

heater:

10, heat sink;

11.

0

0

\

\

2.2

lJ

I

I

I

I

I

2.4

2.6

2.8

3.0

3.2

1 / Tx103

( K-’

3.4

)

Figure 2 Arrhenius plot of the oxygen consumption 0, B coal; 0, H coal

rates: 0, W coal;

dried for 30min at 120°C in nitrogen and stored in airtight containers until use. Fractions with a diameter of 0.5-l.Omm were used without any pretreatment for the measurements of 0, consumption rates. Apparatus and procedure Calorimetric measurements were carried out using a twin type conduction calorimeter available from Tokyo Riko (Model MPC-11). The measuring part of the apparatus is shown in Figure 1. About one gram of coal was introduced into the sample cell. The cell unit containing the measuring and reference cells and the gas reservoir was then outgassed at 120°C for 15 min, rapidly cooled down to room temperature and transferred, in vacua, into the calorimeter. After a thermal equilibrium was attained, air was introduced into the cells by opening cocks 1 and 2. The air had been previously dried by

RESULTS

AND DISCUSSION

Oxygen consumption rate The oxygen consumption rates for W, B and H coals were measured using a flow type reactor described earlier’, with the gas phase oxygen concentration of 21.2 %. The rate at each temperature decreased gradually with time and reached an asymptotic value, which was taken as the steady state oxygen consumption rate. An Arrhenius plot of the oxygen consumption rates for W, B and H coals is shown in Figure 2. As can be seen from the figure, the coals have almost the same activation energy, despite their wide range of ranks. The activation energies obtained from the results are summarized in Table 1. These values are also very close to the activation energies for the CO, and CO formation reactions’. In Figure 3, the steady state oxygen consumption rates at 100°C are plotted against S x Fo,, where S (m2/g dry coal) is the surface area of the pores with radii greater than 100 A and Fo, (wt % d.a.f.) is the oxygen content of the coal. The product S x Fo, can be considered as representing the number of oxygen containing groups on the coal pore surface. The oxygen consumption rates correlate fairly well with S x Fo, with a slope very close to one, indicating that they increase in proportion to the number of surface oxygen and that the oxygen containing surface groups are acting as reactive sites for the oxidation reaction. These results seem to provide further evidence for the previous conclusions7 : 1. the variation in the oxidation rate with coal rank is attributed to difference in the number of active sites 2. the reactivities, towards oxygen, of the active sites of coals of different rank are approximately the same and Table 1 Activation generation

energies for oxygen consumption

reaction and heat

E, (kJ/mol) Coal

0,

W B H

53.2 51.1 54.0

consumption

Heat generation 48.5 50.3”

’ Results for D and T coals are included

FUEL, 1987, Vol 66, February

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temperature

oxidation

R. Al. Kaji et al.

of coals:

Although there are still some ambiguities with regard to the physical picture that the equation suggests”, the Elovitch equation was shown to be satisfactorily applicable to a large body of rate of chemisorption data’,’ ‘. The equation is expressed by:

dq

0

1

dt=ccexp(-P.q) where q is the amount of gas adsorbed, and LXand p are constants. The value of LX is interpreted as the initial rate of chemisorption for q=O. But since, in the case of coal oxidation, the initial rate of oxygen chemisorption is expected to be influenced by the sample conditioning, it should be regarded as a mere curve-fitting parameter. Equation (1) can also be written in the form:

0 :

/

dq l/P dt-t+t, I”

1 S x F02

( m2. % of dry coal /g dry coal )

Figure 3 Relationship between the oxygen S x Fo,: 0, W coal; 0, B coal; 0, H coal

consumption

rate

and

similar reactions are taking place, irrespective of the coal rank 3. the oxygen containing groups on the pore surface are acting as active sites for oxygen chemisorption, with a fresh sorption site being created by the evolution of the gaseous oxidation products during the oxidation of coal. Rate of heat generation Figure 4 shows the rates of heat generation for B coal at various temperatures as plotted against time. Time zero indicates the time at which air is introduced into the cells. Similar profiles are obtained for other coals. The rate of heat generation is initially high and decreases with time, rapidly at first and gradually afterwards. The initial high rate of heat generation may be due to a rapid adsorption of oxygen to the active sites created by the pretreatment of the sample. Evacuating and heating the sample at 120°C should have caused decomposition of some of the surface oxygen-containing groups, leaving a number of vacant active sites for oxygen chemisorption. Therefore, the initial rate of heat generation at a given temperature may be affected by the conditions of the sample preparation, since the initial availability of the active sites are determined by them. Raising the pretreatment temperature to where the decomposition of volatile matter occurs, e.g. above about 200°C would produce a completely different result as a consequence of the change in basic coal composition and surface characteristics. However, a preliminary experiment using a Curie-point pyrolyser and a gas chromatograph confirmed that, at 120°C the decomposition of only the oxygen-containing groups are appreciable. The initial fast adsorption is then followed by a slow process, and the heat generation rate gradually decreases due to the decreased rate of adsorption as the surface coverage by oxygen increases. There have been some attempts to correlate adsorption of oxygen on coals using the Elovitch equation which is one of the commonly used equations to represent gas-solid reaction kineticsY.

156

FUEL, 1987, Vol 66, February

(2)

where to = l/c~P. If it is assumed that the active sites on the coal surface for oxygen chemisorption are uniform in nature, and hence that the heat of chemisorption is a constant at any stage of the process, then the left-hand side of Equation (2) can be interpreted as representing the rate of heat generation. Here, Equation (2) suggests that a plot of the reciprocal heat generation rate against time should give a straight line. However, in actually plotting the data, appreciable deviation from a straight line was seen. Abrupt changes of gradient in the straight line in applying the Elovitch equation to experimental data are not uncommonlo14. As stated earlier, the surface oxygen containing groups are acting as active sites for coal oxidation. The sites are activated by the thermal decomposition of the oxygen containing groups, deactivated by the formation of a surface oxygen complex, and then reactivated by the thermal decomposition of the complex. If that is the case, then it is expected that the heat generation rate at each temperature would attain a certain limiting value at a steady state when the rate of oxygen chemisorption becomes balanced with that of decomposition of the oxygen complexes to the gaseous products, while Equation (2) predicts it to be zero at t+cc. Then, a term representing this steady-state heat generation rate should be incorporated in the equation, (3) where

(dq,/dt),

is the rate

of heat

generation

due to

30 -

“0

50

loo

150

200

Time Figure 4 Heat generation rates 63.3”C; 0, 55.O”C; 0, 23.8”C

250

300

350

( min )

for B coal

(0.105SO.21 mm+):

0,

Low 5

.E E

z E

oxidation

of coals:

R. N. Kaji et al.

Figure 5 for the heat generation rate of B coal at 40°C. The best lit equation is given by:

385 dq -------+ dt-t+27.5

4

z 0 t -0 m .

temperature

3

2

G . g 1

a

I

I

I

1

2

4

6

8

(mine1

1 /(t+to)x103 Figure 5 A plot of heat generation coal (0.10550.21 mm4)

10

1

rate at 40°C against

l/(t + to) for B

l-

xl;;, -1 5 -2

-3

2.9

I

I

I

I

I

3.0

3.1

3.2

3.3

3.4

1/Tx103

Figure 6 Arrhenius plot of the steady-state W coal; 0, B coal; n , D coal: 0, T coal

3.5

1.02

for this particular case with the correlation coefficient of 0.996. Similar analyses were made for the other results. It was confirmed that Equation (3) provides excellent fits to all the data with correlation coefficients greater than 0.995. The consistency of all the data with Equation (3) is amazingly good. Its broad validity fully justifies its applicability to the present results. Arrhenius plots of the (dq/dt), thus obtained for W, B, D and T coals are shown in Figure 6. The heat generation rate of H coal was also measured. However, its steady state values were below the sensitivity of the apparatus and too small to be distinguishable from zero. Allowing for the probable uncertainties of the extraporation, the plots show fairly good linear relations. The points for B, D and T coals seem to fall along a single straight line. This can be expected from the oxygen consumption rates previously stated. The values of S x Fo, for D and T coals are 25.6 and 18.8 rn’.% of dry coal/g dry coal, respectively. From these values and the results shown in Figure 3, these coals are expected to have oxygen consumption rates similar to that of B coal, resulting in similar heat generation rates. Effect of particle size was also investigated using B coal having diameters of 0.074-O. 105, 0.105-0.21, 0.35-0.5 and 0.84-l .O mm. Particle size was found to have no effect on the rate within the range tested. This is in accord with the previous results for CO2 and CO formation reactions. The activation energies obtained using the least squares method are listed in Table 1. They are in very good agreement with those obtained from the oxygen consumption rates. The implication of all these facts is that the heat generation obtained here corresponds to that due to the steady state oxidation reaction which involves oxygen chemisorption and desorption of the gaseous products. Heat evolved per mole of oxygen can be calculated from the results of Figures 2 and 6. It has a value between 75 and 90 kcal/mol for the examined coals, which is close to the values obtained by Lamplough and Hil12.

(K-l)

heat generation

rates: 0,

oxidation reaction at the steady state, i.e. at t-co. The first term in the right-hand side of Equation (3) represents the heat generation rate for oxygen chemisorption initially. The rate of chemisorption decreases with and the observed heat generation oxygen uptake, gradually shifts to the heat generation due to oxidation reaction at steady-state at which the rates of deactivation and reactivation of the surface sites are equal. The best value of t, was determined by trial using the least squares method to give the correlation coefficient closest to one. The values of l//I and (dq/dt), were obtained as the slope and intercept of the best correlations. An example of the best fit line is shown in

REFERENCES 1 2

Schmidt, L. D., ‘Chemistry of Coal Utilization’ (Ed. H. H. Lowry), Vol. 1, Ch. 18, Wiley, New York, 1945 Lamplough, F. E. E. and Hill, A. M. Trans. Instn. Min. Engrs. 1912-1913,45,629

9 10 11 12 13 14

Winmill, T. F. Trans. Instn. Min. Engrs. 19141915, 48, 508 Sevenster, P. G. Fuel 1961, 40, 7 van Doornum, G. A. W. J. Inst. Fuel 1954, 27, 482 Guney, M. CollierJj Guardian 1968, 105 Kaji, R., Hishinuma, Y. and Nakamura, Y. Fuel 1985, 64, 297’ Nordon, P., Young, B. C. and Bainbridge, N. W. Fuel 1979,58, 443 Taylor, H. A. and Thon, N. J. Am. Chem. Sot. 1952, 14, 4169 Harris, J. A. and Evans, D. G. Fuel 1975, 54, 276 Landsberg, P. T. J. Chem. Phys. 1955, 23, 1079 Newman, J. 0. H., Stanley, L., Evans, P. L., Coldrick, A. J. T‘. and Kempton, T. J. Nature 1967, 280 Wood, T. J. Appl. Chem. 1958,8, 565 Carpenter, D. L. and Sergeant, G. D. Fuel 1966, 45, 311

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