Ian W. Smith

The intrinsic reactivity of carbons to oxygen

Abstract The intrinsic reactivities, that is the reaction rates per unit area of pore surface in the absence of any mass transfer restrictions, for combustion of a wide range of carbons have been calculated from published data. The intrinsic reactivities, corrected to a common oxygen pressure of 101 kPa, were ascertained over the temperature range 580 to 2200 K. The reactivities of ‘non-porous’ carbons (diamond, soot, vitreous carbon and pyrolytic graphite) between 770 and 4000 K were also considered. Porous carbons of various origins show intrinsic reactivities that differ by up to 4 orders of magnitude at a given temperature. However, after high-temperature heat treatment, certain carbons of different origin (e.g. sugar and wood charcoals, nuclear and spectroscopic graphites) show closely similar reactivities when reacted with purified oxygen. The reactivities of the ‘non-porous’ carbons show a temperature dependence similar to the average temperature dependence of the porous carbons, but their reactivity expressed per unit external surface area is generally higher than the median intrinsic value of the porous materials.

Kinetics of combustion of a pulverized brown coal char between 630 and 2200°K

Measurements have been made of the combustion kinetics of size-graded fractions (89, 49, and 22 μm) of a char prepared from Yallourn brown coal. Reactions were carried out in an entrainment reactor (between 900 and 2200°K) at oxygen partial pressures of about 0.2 and 0.1 atm, and in a fixed-bed reactor (between 630 and 760°K) at about 0.1 atm oxygen. Above 900°K, the 89 and 49 μm fractions were subject to rate control by the coupled processes of pore diffusion and chemical reaction on the pore walls; the rate coefficient, Ra,c (the rate of carbon combustion per unit external area of the particle per (atmosphere of oxygen)0.5), being given by R a , c = 9.3 exp [ − 16 , 200 / ( ℝ T p ) ] , g / [ cm 2 sec ( atm ) 0.5 ] , where Tp is the temperature of the particle in °K and R is in cal/(mol °K). Below 760°K, the 89 and 49 μm fractions were found to react with rate control by chemical reaction alone. Above 900°K rate control for the 22 μm fraction lay intermediate between these two limits. At 1800°K the reactivity of the brown coal char is a factor of 4 higher than that of anthracite, and is 50% higher than that of a char from a low-rank bituminous coal; however, at 770°K the reactivity of the char is one to six orders of magnitude higher than the reactivities of other carbons.

Intrinsic reactivity of petroleum coke and brown coal char to carbon dioxide, steam and oxygen

The intrinsic reactivity of a petroleum coke and an Australian brown coal char to carbon dioxide, steam and oxygen have been measured in a fixed bed reactor using techniques which allow the reaction rate, activation energy and the order of reaction with respect to reactant concentration to be determined under conditions where chemical processes alone control the rate of reaction. Samples were recovered at various extents of reaction up to approximately 40% weight loss and changes in particle density and surface area determined. Reaction rates varied significantly as the reactions proceeded and these variations could be largely, but not completely, attributed to changes in surface area. The order of reaction was similar for the reactions of both carbons with all three reactant gases and remained essentially constant, at a value of approximately 0.6, during oxidation of up to 40% of the sample mass. The activation energy for reaction of both materials also remained unchanged during reaction and was similar for reaction of both carbons with CO 2 and H 2 O, where values in the range 214–242 kJ/mol were determined. Activation energies of 155 and 127 kJ/mol were measured for reaction with oxygen of petroleum coke and brown coal char respectively. The relative intrinsic reaction rates of petroleum coke at 1073 K and CO 2 , H 2 O and O 2 partial pressures of 0.2 atm were found to be 1, 0.2 and 0.9×10 5 respectively. The corresponding relativities for brown coal char were 1, 2, and 0.1×10 5 .

Combustion rate of bituminous coal char in the temperature range 800 to 1700 K

Abstract The combustion rates of three different sized fractions of char from a swelling bituminous coal have been measured in the temperature range 800 to 1700 K. The mass-median sizes of the fractions were 70, 35 and 18 μm. For each fraction the combustion rate was less than the limiting rate set by diffusion of oxygen to the particle. The chemical reaction rate coefficient (g/g s atm O 2 ), calculated from the measured rates and corrected for the external diffusion resistance, varied with temperature in a manner appropriate to an apparent activation energy of approximately 27 kcal/mol for the three fractions. The chemical reaction rate coefficient when expressed on the basis of unit external area of particle (g/cm 2 s atm O 2 ) increased with increasing particle size at 800 K: at 1250 K the coefficients for the 35 and 18 μm fractions were equal, and lower than those of the 70 μm fraction by a factor of 4. The densities of the particles did not vary appreciably with burn-off, except the density of the 70 μm fraction which increased sharply for values of burn-off between 0 and 0.01. It is probable that combustion occurred in a rate-control regime which was intermediate between that caused by chemical reaction alone and that caused by the combined effects of pore diffusion and chemical reaction.

Flash pyrolysis of coals: behaviour of three coals in a 20 kg h−1 fluidized-bed pyrolyser

Abstract Flash pyrolysis of Loy Yang brown coal, and Liddell and Millmerran bituminous coals has been studied using a fluidized-bed reactor with a nominal throughput of 20 kg h−1. The apparatus and its performance are described. The yields of tar and hydrocarbon gases are reported for each coal in relation to pyrolysis temperature, as also are analytical data on the pyrolysis products. The peak tar yields for the dry, ash-free Loy Yang and Millmerran coals were respectively 23% wt/wt (at ≈ 580 °C) and 35% wt/wt (at

Fluidizing feeders for providing fine particles at low, stable flows

600 °C). The tar yield from Liddell coal was 31% wt/wt at ≈ 580 °C. Hydro-carbon gases were produced in notable quantities during flash pyrolysis; e.g. Millmerran coal at 810 °C gave 6% wt/wt (daf) methane, 0.9% wt/wt ethane, 6% wt/wt ethylene, and 2.5% wt/wt propylene. The atomic H C ratios and the absolute levels of hydrogen in product tars and chars decreased steadily with increasing pyrolysis temperature.

Reactivity of petroleum coke to carbon dioxide between 1030 and 1180 K

Abstract Fluidizing feeders have been developed to provide stable flows of fine (−100 μ m) particles in the range 1–1000 g/h. Materials fed are coals, chars, cokes, alumina and silicon carbide. Feeders equipped with automatic pulse-feeding valves give steady flows of particles for periods of up to 10 hours. Descriptions are given of the feeders and important features are detailed. Data are presented to illustrate the operating conditions and performance of the feeders.

Experimental studies of ignition behaviour and combustion reactivity of pulverized fuel particles

Abstract Measurements were made of the reaction rate of three sizes (2.9, 0.9 and 0.22 mm) of petroleum-coke particles with carbon dioxide over the temperature range 1018–1178 K, and at carbon dioxide partial pressures between 26 and 118 kPa. A limited number of similar measurements were made on samples of a commercial aluminium-smelting anode, an experimental anode, and AGKSP graphite. The materials were all reacted under conditions of chemical rate control alone: there were no rate limitations due to transport processes without or within the carbon particles. The order of the rate with respect to carbon dioxide concentration was found to be close to 0.6 for the petroleum coke and anode carbons, and between 0.6 and 0.8 for the graphite. Activation energies in the range 203–237 kJ/mol were found for petroleum coke; 187–237 kJ/mol for electrode carbon; and 293 kJ/mol for the graphite. For the petroleum coke, the order was found to be constant up to 45% burn-off and the activation energy essentially constant between 21 and 45% burn-off. The reactivity ϱ s , based on unit pore surface area of the petroleum coke at a carbon dioxide pressure of 101 kPa, can be represented by: ϱ s = α exp [− E (RT) ] . For the 2.9 and 0.9 mm particles, α = 6.1 / sx 10 6 g/m 2 min and E = 215 kJ/mol; for the 0.22 mm particles the respective values are 1.8 / sx 10 7 and 222. The reactivity ϱ of the commercial electrode on a weight basis was within the range of those of the coke and experimental electrode. For AGKSP graphite, values of ϱ s were close to those found by Walker and Raats 14 .

The combustion of Loy Yang brown coal char

A pulse ignition technique, whereby a small mass of fine coal or char particles is fed into a drop-tube furnace, was used to evaluate the ignition characteristics of pulverized fuel particles. Combustion rates were estimated from the ignition temperatures and were compared with rates determined experimentally using a fixed-bed reactor, DTG, and an entrained-flow reactor. Three Australian coals, two chars prepared from these coals and a petroleum coke were used. Coal rank and particle size were found to influence the ignition temperatures of the coal and char particles. However, these effects were reduced at high oxygen concentrations. In pure oxygen, the ignition temperatures of the chars and their parent coals were similar. Heterogeneous and homogeneous ignition theories indicated that at low O2 concentrations ignition was controlled by a homogeneous mechanism, whereas at high O2 concentrations heterogeneous ignition became dominant. The combustion rates of the chars determined by the four techniques were compared at an oxygen partial pressure of 10.1 kPa. The results of the fixed-bed, DTG and entrained flow experiments were consistent with each other and spanned the range of kinetic control from regime I at low temperature (fixed bed) to regime II at high temperature (entrained flow). The drop-tube experiments indicated consistently higher combustion rates than did the other techniques. The pulse ignition measurements therefore, while providing a valid means of characterizing coal and char ignition behaviour, result in overestimation of the char combustion rate.

Statistical Methods for Deriving Seasonal Climate Forecasts from GCM’S

Abstract Combustion rate measurements of Loy Yang brown coal char particles (88 μm median size) over the temperature range 940–1420 K yielded a chemical rate coefficient Rc, expressed as R c = 12.6 exp [ −68.3 (RT p )] kg m 2 s ( kPa O 2 ) 0.4 . Particle size and density decreased with burn off, but in an unusual manner that requires further investigation. The value of the activation energy, the manner of change in size, and density of the particles during combustion, and the observed order in oxygen (0.4), suggest that the burning rate was controlled by the combined effects of pore diffusion and chemical reaction, with a tendency for chemical reaction to dominate.