Investigation of Spontaneous Combustion Behaviour of Brown Coal and the Effect of Densification process

2017-03-14T05:34:58Z (GMT) by Mohammad Reza Parsa
Victorian brown coal (VBC), as one of Victorian’s main energy sources, is going to continue serving the Australian economy in to the foreseeable future. VBC is an attractive source of energy because of the enormity of the resource, the low (open-cut) mining cost, high reactivity and low inorganic content. However, the use of VBC is limited by its high moisture content (50 to 70 percent) which provides some serious challenges such as high CO<sub>2 </sub>emissions, low heating value and high transportation costs as a barrier to export. Thus, if VBC is to be transported over significant distances, it needs to be dried or significantly dewatered first. Generally, though drying the coal increases the risk of spontaneous combustion. <br>    Spontaneous combustion continues to be a major issue in the world's coal industry causing safety issues and material management challenges through each stage of coal handling and utilisation. It is a consequence of self-heating of the coal, where the heat released due to chemical and/or physical processes within coal particles accumulates faster than it dissipates into the environment. Although much valuable work has been carried out to describe the spontaneous combustion behaviour of coal, studies focused on developing a fundamental scientific understanding of its physical and chemical origins are rare, especially for brown coal. <br>    In the present work, to achieve a better understanding of brown coal spontaneous combustion mechanism(s), a variety of experimental strategies were applied. The knowledge obtained was also applied to create a new useful and potentially valuable product from brown coal. <br>    The densification of coal, where a mixture of coal and water (and optionally other additives) is kneaded prior to extrusion and subsequent dried at ambient conditions, was employed to modify the coal physico-chemical properties and then to evaluate their effects on the coal spontaneous combustion. The wire basket method, by which the critical ignition temperature, T<sub>cr</sub> is determined, was employed as the principal method for determining the spontaneous combustion propensity. <br>    The addition of small amounts of NaOH, 0.5 M (1.75 wt% dried basis), where the pH remained < 7, assisted oxidation. The NaOH reacted mostly with strong acidic groups of the brown coal and thereby disrupted the hydrogen bonding network between carboxylic groups. The disruption of the hydrogen bonding network between carboxylic groups makes the coal more susceptible to oxidation and shifts its oxidation/decomposition in air to lower temperature. In contrast, acid washing postponed carboxylic group oxidation/decomposition to higher temperature, probably because the replacement of coal cations with hydrogen enhanced the hydrogen bonding network. <br>    The addition of NaOH at higher concentration, 1 and 1.5 M (3.5 and 5.25 wt% dried basis), has a different effect that inhibits the oxidation. At high pH ≥ 7, the polarity of the acid functional groups was increased by ion exchange. Even weaker acidic groups were ion-exchanged with sodium ions, so that the coal structure was ‘tightened’ by cross-linking between coal-oxygen functionalities via strong electrostatic interactions. This decreased CO<sub>2</sub> surface area value and the pore volume measured by mercury intrusion porosimetry (MIP), it also increased the hardness of the products. <br>    The differential thermogravimetry (DTG)/ differential thermal analysis (DTA) techniques were used to study the detail of densified product oxidation in air, and the DTG curves were deconvoluted to facilitate better interpretation of the data. The DTG/DTA curves of all samples densified with 1 and 1.5 M NaOH, showed the proportion of mass loss in high temperature stages increased significantly during oxidation, compared to the raw coal. <br>    The overall effect of these factors, mentioned above, caused the reduction of coal oxidation rate by reducing the accessibility of active sites on the coal surface at which oxygen molecules can react and postponing spontaneous combustion of coal densified with 1 and 1.5 M NaOH to higher temperature with a maximum increase in the Tcr value determined by the wire basket method, of up to 20<sup>o</sup>C. <br>    The densification of brown coal with a range of other alkali salts including NH<sub>4</sub>OH, Ca(OH)<sub>2</sub> and KOH also reduced the CO<sub>2</sub> surface area, the pore volume and significantly increased the hardness of densified products. The TGA results revealed that the interaction of Ca(OH)<sub>2</sub>, KOH and NaOH with brown coal caused a reduction of mass loss proportion occurring at lower temperature exothermic stages in association with an increase in T<sub>cr</sub>. The T<sub>cr</sub> of Morwell coal densified with 1 M NH<sub>4</sub>OH was similar (slightly lower) to that for Morwell coal densified without additive. Although the interaction of Ca cations with brown coal led to lower meso and macropore volumes, the micropore volume did not change significantly, probably due to the bigger charge of the Ca divalent cations which makes it more difficult for crosslinks to form. <br>    The DTG/DTA curves of non-extruded sample prepared with 1 M NaOH were similar to those for extruded sample, but the CO<sub>2</sub> surface area value and micropore volume were higher which can be associated with lower T<sub>cr</sub> value for the non-extruded sample. Using NaCl as additive, at coal’s natural acidic pH, resulted in a very brittle densified product, with higher macro and mesopore volumes than the coal densified prepared without additive. However, the T<sub>cr</sub> value increased probably due to physical blockage of internal surface area and the inhibitory effect of chloride ions on the coal’s reactivity. <br>    The low temperature (25-110 °C) thermal behaviour of two Victorian brown coals was studied by differential scanning calorimetry (DSC). A clear irreversible and exothermic event was detected with an enthalpy  < -13.8 J/g, starting at temperature around 53-80<sup>o</sup>C (depending on the sample). This exothermic event was suggested to be associated with cross-linking reactions in the coal structure, probably involving the free radical species in low-rank coals, and had the potential to increase the temperature of coal by around 8.5<sup>o</sup>C. As a result of heating, the CO<sup>2</sup> surface area value and the MIP micropore volume decreased and the surface morphology of some coal particles changed from spongy and porous to smooth. The formation of CO<sup>2</sup> gas suggested that at least some cross-linking (polymerisation) reactions involved decarboxylation of oxygen functional groups. The results also indicated that the water and gases produced during the cross-linking reactions, when maintained in the reaction system in a batch (closed) DSC test, can have a significant impact on initiating or accelerating the exothermic reactions. <br>    The exothermic event appeared even in the absence of oxygen, although oxygen increased the heat released, probably due to an increase of free radical concentration. Adding oxygen and water individually did not reproduce the exothermic heat flow, but the exposure of the sample after experiencing the heat treatment, to oxygen and water simultaneously (similar to weathering) reproduced the exothermic event probably due to the formation of new active sites. This is consistent with reports that the spontaneous combustion accidents have often been reported after rain. <br>    Wet-oxidation, by UV/H<sub>2</sub>O<sub>2</sub>, of Morwell brown coal caused a reduction in the CO<sub>2</sub> surface area and the micropore volume, probably as a result of the increase of carboxylic group content and the development of a strengthened hydrogen bonding network. In addition, the degree of aromatic condensation increased as a result of radical polymerisation reactions. It seems that <sup>•</sup>OH radicals may non-selectively oxidise inherently susceptible sites leading to a lower tendency toward oxidation in air for these products. The T<sub>cr</sub> value of oxidised coal increased with increase of H<sub>2</sub>O<sub>2</sub> concentration consistent with the other trends observed. <br>    [...]<br>