Elsevier

Fuel

Volume 79, Issue 11, September 2000, Pages 1379-1388
Fuel

Char porosity characterisation by scanning electron microscopy and image analysis

https://doi.org/10.1016/S0016-2361(99)00282-3Get rights and content

Abstract

No significant change in either the morphotype composition or the macroporosity (pores>5 μm) in the 0–30 wt.% char burnout interval were revealed by reflected light microscopy or image analysis. Two high temperature char series from a Tertiary South American coal (C1) and a Permian Gondwana coal (C2) were therefore examined by scanning electron microscopy to provide information on the combustion process up to ∼60 wt.% char burnout. This study documents a significant mesopore (∼0.1–5 μm) development on the fused chars in the burnout interval studied. A method to quantify the size and amount of the mesopores is described and both the parameters increased with increasing char burnout. Above a char burnout of ∼30 wt.% an increase in macroporosity was detected and ascribed to coalescence of mesopores to form larger pores. Although the measurement of mesoporosity is restricted to fused chars, i.e. pores in fragments and the char morphotypes inertoid, fusinoid and solid could not be measured, the consideration of mesoporosity seems to be fundamental in understanding, evaluating and modelling combustion processes in the char burnout interval studied.

Introduction

Pulverised coal combustion is widely used in power plants for the generation of electricity and heat. It is essential that a burnout close to 100% is reached during the combustion process for two main reasons: (1) as much energy as possible should be extracted from the coal; (2) a too high proportion of unburned char may render an ash product unsuitable for construction purposes etc. and, consequently, must be disposed of.

Combustion of coal can be subdivided broadly into two distinct stages. The first stage, pyrolysis, lasts in the order of 30–100 ms, whereas the second, char combustion, takes a considerably longer time, in the range of a second. The parent coal petrography and the processes during pyrolysis control the chars’ macroscopic morphology, which can have important consequences for subsequent char combustion [1], [2], [3], [4]. It has been shown that in some cases the porous tenuisphere and crassisphere chars can be correlated with the microlithotypes vitrite, clarite and vitrinertite V, whereas the more thick-walled char morphotypes, i.e. the crassinetwork/mixed/mixed network morphotype group, can be correlated with the microlithotypes inertite, durite, vitrinertite I, duroclarite, and clarodurite [2], [3]. However, in some cases the reflected light microscopic methods of char morphotype and macroporosity characterisation fail to explain the mass transport in the char burnout interval below ∼30 wt.% burnout (middle coal burnout interval, ∼50–70 wt.% coal burnout) [4]. This has encouraged more detailed studies of chars by scanning electron microscopy (SEM), as a means to understand the combustion process. The present paper focuses on a comparison of the results obtained from the SEM study with the results obtained from reflected light microscopic studies of the same char samples, and it describes a newly developed SEM method to characterise chars.

Section snippets

Sample material

Two char series produced in a laminar flow reactor, at temperatures in the range 1532–1692 K (∼1259–1419°C), heating rates of 104–105 K, and an oxygen concentration of 12 mol%, were selected for the present study [3], [5]. Four char samples at different burnout levels were available in each series (Table 1). The coal burnout and char burnout values were calculated on a dry, ash-free basis using the “ash-tracer method” [5]. The two char series were produced from the 106–125 μm fraction of a Tertiary

Experimental

The selected chars were characterised with respect to morphotype composition and macroporosity by reflected light microscopy, whereas finer structural details were studied both qualitatively and quantitatively by SEM.

Results and discussion

In the studied burnout interval the C1-derived chars showed only a minor change in morphotype composition, while the C2-derived chars did not show any significant change in morphotype composition (Table 4), i.e. it appears that the various morphotypes burn at similar rates in this interval. This is in agreements with the findings of Vleeskens et al. [6], but not with Bailey et al. [2] who found that the thin-walled chars disappeared more rapidly than the thick-walled chars.

The overall

Conclusions

Mesopores (defined as pores between ∼0.1–5 μm) have been identified in highly fused combustion chars in the studied char burnout interval (0–61 wt.%). A measurement technique has been developed to quantify surface mesoporosity in the combustion chars on an area basis, and to determine the size distribution of the mesopores. However, the method is extremely time consuming and needs to be further developed to make measurements of many more particles possible and thus improving the analytical

Acknowledgements

Elkraft A.m.b.a. is thanked for the financial support to this project. S. Sølberg (GEUS) is thanked for technical assistance. This paper is published with the permission of the Geological Survey of Denmark and Greenland.

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