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

Abstract

a b s t r a c tA 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 (g*) through resolidification of the low rank, high fluidity coal counteracts the reduction in g* resulting from softening of the high rank, low fluidity coal. It has also been confirmed that the g* of the resultant blend can be estimated from the g* 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 g*, a feature which has previously been observed for certain high fluidity, high pressure coals. Elasticity and g* 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.

Importance of Biomass and Binder Selection for Coking Briquette Preparation. Their Effect on Coal Thermoplastic Properties

“…The use of additives has been common practice for many years in the cokemaking industry as a means to reduce costs, widen the range of raw materials that can be introduced into blends used for the preparation of metallurgical coke and recycle wastes produced in-situ and ex situ. [1][2][3][4][5][6] In recent years, the use of biomass as an additive to coal blends, in order to produce metallurgical coke, has been envisaged as a possible solution for reducing the generation of non-renewable CO2 emissions by the steel industry and for promoting the use of lowcost raw materials to replace expensive prime coking coals. [7][8][9] The direct addition of sawdust to industrial coal blends has been investigated but the resultant decrease in bulk density diminishes the quality of the coke.…”

Associations of physical, chemical with thermal changes during coking as coal heats – Experiments on coal maceral concentrates

“…A number of techniques have been previously employed to investigate individual events involved in the physical, chemical and thermal changes as coal heats, for instance, Gieseler Plastometer [10,11] and Proton Magnetic Resonance Thermal Analysis (PMRTA) [12,13] for measuring fusibility and Rheometry [14][15][16] for viscosity; crucible swelling [10] and Audibert-Arnu dilatometer [10,11] for evaluating swelling; Thermogravimetric Analysis (TGA), Gas Chromatography (GC), Mass Spectrometry (MS), Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) [9,[17][18][19][20][21][22] for analysing chemical changes; and Differential Scanning Calorimetry (DSC) [23][24][25] and Differential Thermal Analysis (DTA) [26] for estimating thermal changes.…”

The influence of addition of coal tar pitch (CTP) and expired pharmaceuticals (EP) on properties and composition of pyrolysis products for lower and higher rank coals

“…CTP contains a large amount of similar PAHs. CTP was added to poor caking coals in order to improve their caking power and coke-making properties [9,10] because this additive increased the index of maximum fluidity (MF) determined by the Gieseler method [11,12,14]. However, it has not been determined quantitatively how the addition of CTP to heated coals influences the amount of the aforementioned PAHs in their plasticity state.…”