Pressurised steam drying of Australian low-rank coals: Part 2. Shrinkage and physical properties of steam dried coals, preparation of dried coals with very high porosity

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Abstract

The shrinkage of brown coal during high pressure steam drying and the physical structure of the product have been investigated by mercury porosimetry, CO2 surface area analysis and transmission electron microscopy (TEM). The porosity of the coal can be tailored by controlling the temperature-time profile of the steam drying and samples with very high porosity can be obtained when drying is carried out under mild conditions. These examples also showed high surface area values as determined from CO2 adsorption measurements. The TEM measurements showed no evidence for tar mobilisation other than at 254°C, the highest temperature used. A simple conceptual model has been proposed to explain these findings.

Introduction

An extensive body of literature exists concerning high-pressure steam drying of low rank coals 1, 2, 3, 4, 5. In a previous paper [5], the relationship between the steam drying process conditions used by us and the equilibrium moisture content of the resulting coals was established. The extent of low temperature pyrolysis was measured and the composition of the resulting organic material determined.

In this paper, the physical structure and properties of the resulting steam dried coals are described. These physical characteristics may have important implications for their gasification, combustion, carbonisation and liquefaction. In particular, surface area and pore size distributions have important influences on the rates of reaction of product carbons.

Section snippets

Coals

The same coals were used as in our previous study and crushed to −4 mm [5]. Their as-mined moisture content, atomic H/C ratio (dried coals), surface areas and intruded volumes are summarised in Table 1.

Steam drying procedure

The steam drying apparatus has been described previously [5]. In summary, the raw coal (80 g) was dried under a pressure regulated superheated steam atmosphere in a batch autoclave system. Drying temperatures ranged from 180°C to 260°C and pressures from 1.0 to 2.5 MPa. At the completion of the

Changes in the porosity of the coal as a result of steam drying

In Part 1 [5], it was shown that the moisture content of Loy Yang low-ash (LY LA) coal decreases from 62% at the steam saturation temperature with less than 10°C of superheat and asymptotically approaches a moisture content of about 6% at high temperature. Such drying can have an important effect on low-rank coal porosity, with shrinkage of the coal structure during coal drying being well documented 3, 4, 7, 8, 9, 10, but the general relationship between extent of drying and change of porosity

Conclusion: a particle model of coal drying and shrinkage

A conceptual particle model to explain the results obtained from the three drying conditions used in this study is shown in Fig. 5. Significant amounts of pore collapse and cross-linking occur during drying 16, 17even at 105°C, with a significant hysteresis between desorption and re-adsorption of water. The hysteresis is caused by the limitation of water access to the pore structure of the coal. Cross-linking reactions may have sealed pores or limited the ability of collapsed pores to be

Acknowledgements

The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for New Technologies for Power Generation from Low-Rank Coal, which is established and supported under the Australian Government's Cooperative Research Centres program.

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