Combustion
 
Combustion Mechanism of Single Non-porous Carbon Sphere
J. Chen, J. Pohl, V. Rudolph University of Queensland, Brisbane Australia; and D Harris CSIRO, Brisbane
This paper describes the measurements of the combustion rates of single carbon spheres. Carbon spheres, 100 mm in size, are heated in air by a laser to 1500 ~ 2000 K with a heating rate of 104 K/s. The surface temperature and weight loss of the carbon spheres are measured simultaneously during combustion. The change of particle size was recorded by two video cameras, which viewed the burning particle from two different directions. These measurements show that the combustion reaction at these conditions is controlled by the combination of diffusional and kinetic rate (regime II). Particle-to-particle variation of combustion rates could result partly from differences in the porous microstructure of each individual particle and partly from the change of combustion mode throughout the burn-off process. As reaction proceeds, the combustion rates first increases and seems to reach its maximum at around 50-60% burn-off and then decreases at higher burn-off levels. Initial results on individual coal maceral combustion rates are also presented in this paper.

Reactivity of Chars Prepared from the Pyrolysis and Gasification of Loy Yang Lignite under a Wide range of Conditions
H Wu, D Mody, C Li CRC for Clean Power from Lignite, Melbourne; J Hayashi and T Chiba,  Hokkaido University, Japan
A Loy Yang lignite sample was pyrolysed under a wide range of experimental conditions using a wire-mesh reactor, a fluidised-bed reactor, a drop-tube reformer and a thermogravimetric analyser (TGA). The reactivity of these char samples in CO2 and air was measured in the TGA as well as in the fluidised-bed reactor. A sample prepared by the physical impregnation of NaCl into the lignite was also used in order to investigate the effect of NaCl in the lignite on the reactivity of the resulting char. Our experimental results indicate that, due to the volatilisation of a substantial fraction of Na in the lignite substrate during pyrolysis, the true catalytic activity of the Na in the lignite substrate should be evaluated by measuring the sodium content in the char after pyrolysis. The char reactivity measured in situ in the fluidised-bed reactor was compared with that of the same char measured separately in the TGA after re-heating the char sample to the same temperature as that in the fluidised-bed. It was found that the re-heating of the char in the TGA reduced the char reactivity.
 
 

The Effect of Operations Conditions and Coal Type on Char Reactivity and Morphology during Combustion in a Drop Tube Furnace
R. Barranco, M Cloke and E Lester, University of Nottingham UK
Three coals from Colombia were chosen for a series of experiments using a Drop Tube Furnace (DTF). The research was undertaken to investigate possible effects of the operating temperature, particle size and coal type on char characteristics during combustion. The DTF is capable of reproducing high heating rates, high reaction temperature and the oxygen level can be regulated to simulate atmospheres similar to those found during combustion in full scale boilers. The coals were characterised using standard methods and image analysis techniques. The changes in the characteristics of the char produced were assessed using a number of different techniques including intrinsic reactivity analysis and automatic char test by image analysis.
The results showed that the operating temperature has a substantial effect on the intrinsic reactivity of the char. There is also a contribution of the particle size but this was found to be statistically significant only for one of the coals. It is clear from the results that standard characterisation techniques make poor combustion predictors. Conversely, a reactivity parameter, derived from a grey-scale histogram obtained by image analysis of the coal, provides an important parameter to predict coal combustion behaviour. This reactivity index is based on the premise that coal material which exhibits the highest reflectance is slowest to burn. On the other hand, char morphology and intrinsic reactivity play an important role when assessing coal combustion performance. Finally it may be concluded form the results that the Colombian coals used in this study burn cleanly and efficiently and therefore, they are good for combustion purposes.

Determining Coal Reactivity Parameters at Elevated Pressures using Bench-Scale Techniques
M  Kelly, D Roberts, C  Mill, D Harris, CSIRO Energy Technology, Brisbane; J  Stubington, University of NSW, Sydney; Y Otake University of Queensland, Brisbane; and T Wall, University of Newcastle, Newcastle
International interest in Australian coals for use in gasification applications is set to rise.  Already there are several coal-fired IGCC demonstration plants overseas and the use of these technologies is expected to increase, particularly in Asia and Europe.  Performance data for coals under the extreme conditions found in pressurised gasification technologies are difficult to obtain.  Fundamental coal reactivity parameters, such as volatile yield and char reactivity, need to be measured under conditions that are relevant to enable them to be successfully implemented into predictive gasification models.
 
This paper describes a bench scale technique that can be used to measure volatile yield and intrinsic char reactivity data for coals in high-pressure environments containing O2, CO2 and H2O. Coal volatile yield under fast heating rates and high pressures is determined using a wire-mesh reactor, and the intrinsic reactivity of the resulting chars at high pressure is determined using a pressurised thermogravimetric analyser.  The application of these techniques is demonstrated through the determination of volatile yield and char reactivity data at increased pressures for a suite of Australian export thermal coals.  This demonstrates the usefulness of the method as a tool for coal assessment for use in advanced utilisation technologies.

Intrinsic Gasification Kinetics at Increased Pressure: the Applications of the nth Order Rate Equation
D Roberts, D Harris  CSIRO Energy Technology, Brisbane;  and T Wall University of Newcastle, Newcastle
Pressurised char gasification models used for coal assessment and gasifier design require the input of reliable intrinsic kinetic data.  It is suggested in the literature that the nth order rate equation (that has been used extensively at atmospheric pressure) is not suitable for describing such data over a wide range of pressures.  However, there is no comprehensive analysis of char reactivity at increased pressures and low temperatures which can be used to clarify this suggestion.  Consequently, pressurised char gasification models use rate equations based on accepted reaction mechanisms.  Whilst this is a fundamentally correct approach, such expressions are complicated, poorly-quantified and difficult to apply to different coals.
 
This paper summarises the results of a wider study into the intrinsic reactivity of coal chars to O2, CO2 and H2O at pressures of 1–30 atm.  It was found that the nth order rate equation is applicable to the char–O2 reaction for pressures up to (at least) 16 atm.  For char–CO2 and char–H2O reactions, however, the reaction order (n) was found to decrease with pressure such that at above approximately 30 atm it was almost zero—consequently the nth order equation was not suitable.  The CO2 and H2O results were analysed using a TPD technique, which estimated the relative number of surface complexes on the char as a function of reaction pressure.  These results were used to model a reaction order that decreased with pressure.  This effect was incorporated into the nth order rate equation to improve its predictive capabilities over a range of pressures, and therefore simplify the incorporation of kinetic data into char gasification models.
 
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