Influence of coal thermoplastic properties on coking pressure generation: Part 2 – A study of binary coal blends and specific additives

Duffy, J. J., Mahoney, M. R. and Steel, K. M. (2010) Influence of coal thermoplastic properties on coking pressure generation: Part 2 – A study of binary coal blends and specific additives. Fuel, 89 7: 1600-1615. doi:10.1016/j.fuel.2009.08.035

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Author Duffy, J. J.
Mahoney, M. R.
Steel, K. M.
Title Influence of coal thermoplastic properties on coking pressure generation: Part 2 – A study of binary coal blends and specific additives
Journal name Fuel   Check publisher's open access policy
ISSN 0016-2361
1873-7153
Publication date 2010-07-01
Year available 2009
Sub-type Article (original research)
DOI 10.1016/j.fuel.2009.08.035
Open Access Status File (Author Post-print)
Volume 89
Issue 7
Start page 1600
End page 1615
Total pages 16
Place of publication Oxford, United Kingdom
Publisher Elsevier
Language eng
Subject C1
Abstract A number of coal blends and pitch/coal blends were evaluated using rheometry, thermogravimetric analysis and microscopy to confirm and further elucidate the coking pressure mechanism previously proposed by Duffy et al. (2007) [1]. We confirm that blending a low rank, high fluidity, low coking pressure coal, with a high rank, low fluidity, high coking pressure coal can significantly reduce the coking pressure associated with the latter. Interestingly, blending does not necessarily result in a fluidity that is midway between that of the two coals; sometimes the fluidity of the blend is less than that of the low fluidity coal, especially when the coals are significantly different in rank. This occurs because the increase in complex viscosity (η*) through resolidification of the low rank, high fluidity coal counteracts the reduction in η* resulting from softening of the high rank, low fluidity coal. It has also been confirmed that the η* of the resultant blend can be estimated from the η* of each component coal using a logarithmic additivity rule commonly employed for polymer blends. Polarised light microscopy has indicated that the degree of mixing between coals of different rank is minimal, with fusion restricted to the particle surface. It is therefore inappropriate to think of such a coal blend in the same way as a single coal, since each component coal behaves relatively independently. This limited fusion is important for understanding the coking pressure mechanism for blends. It is proposed here that the lower rank coal, which softens at lower temperature, is able to expand into the interparticle voids between the high rank coal that is yet to soften, and these voids can create channels for volatiles to traverse. Then, and importantly, when the high rank coal begins to expand, the pore structure developed in the resolidified structures of the low rank coal can facilitate removal of volatiles, while the resolidified material may also act as a suitable sorbent for volatile matter. This is considered to be the primary mechanism by which coal blending is able to alleviate coking pressure, and applies to addition of inert material also. Addition of a coal tar pitch was found to increase fluidity but also to extend the thermoplastic range to lower temperatures. This caused an increase in the swelling range, which was accompanied by a long plateau in η*, a feature which has previously been observed for certain high fluidity, high pressure coals. Elasticity and η* at the onset of expansion were also higher for both the pitch impregnated coals and the high pressure blends, which supports previous findings for singly charged high pressure coals, and confirms the potential use of such criteria for identifying potentially dangerous coals/blends. © 2009 Elsevier Ltd. All rights reserved.
Keyword Coal
Coking pressure
Rheology
Carbonization
Rheometry
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

 
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Created: Tue, 24 Nov 2009, 19:24:56 EST by Dr Karen Steel on behalf of School of Chemical Engineering