- Title
- A fundamental study on char creation from coal tailings (‘chailings’) and its application as a soil amendment
- Creator
- Tremain, Priscilla Tuisuva
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Food security and hence access to environmentally sustainable food resources is one of the key global challenges in the 21st century. In this context and from an agronomic perspective, the role of soil amendments (e.g. fertilisers) for enhanced food production cannot be overstated. One such soil amendment is biochar: a carbon-rich organic substance derived from biomass that has great potential to improve soil health by improving nutrient retention, particularly in coarsely textured soils. In addition, biochar can be used as a carbon sink and thus can be considered as an alternative means of reducing atmospheric carbon dioxide. Meanwhile, with the ever-increasing rates of coal production in Australia and around the world, management of mine waste, in particular coal tailings, is becoming a pressing issue. Given that both coal and biomass are organic substances with somewhat similar characteristics, the concept of ‘chailings’, a char created from coal tailings rather than biomass, for use as a soil amendment was proposed by our research team in 2009 and formed the basis of this thesis. As a result, the primary objectives of this thesis were to produce, characterise and apply chailings to soil and ultimately assess the concept for future work and scaled-up production. Chailings were produced via a slow pyrolysis process from tailings sourced from two coal mines (Mine A and Mine B) in New South Wales, Australia. Pyrolysis conditions were varied in terms of maximum heating temperature (400 °C to 850 °C) and holding time (0–6 hours) to create chailings with varying properties. Chailings were characterised using a number of techniques. X-ray diffraction and x-ray fluorescence techniques identified the primary mineral constituents as silica (i.e. quartz) and aluminosilicates (i.e. kaolinite or illite). Clear morphologic changes were observed via optical and scanning electron microscopy for increasing pyrolysis temperature, with evidence of particle swelling and devolatilisation apparent at high temperatures (>600°C). Proximate analyses indicated near complete devolatilisation was apparent at 800 °C for both mines, with thermogravimetric analysis revealing that peak devolatilisation occurred at 454 °C for Mine A and 464 °C for Mine B. Increases in pH and decreases in electrical conductivity (EC) and total acidic capacity were observed when pyrolysis temperature increased primarily due to the volatilisation of acidic compounds. A substantial increase in surface area with increasing pyrolysis temperature was observed for Mine A chailings, from 2.7 m²/g at 400 °C to 75.3 m²/g at 800 °C, because of the development of microporosity. However, a decrease was observed for Mine B chailings, from 2.4 m²/g at 400 °C to 1.2 m²/g at 800 °C, attributed to macroporosity and aggregation of particles. Overall, characterisation of chailings revealed that the physical and chemical properties of chailings were highly dependent on tailings sources and pyrolysis temperature, while holding time had a minimal effect. Devolatilisation of coal particles and differences in coal rank of the two tailings samples were the primary cause of the transformations observed. Properties of high-temperature (> 600 °C) chailings from both Mine A and Mine B, namely, surface area, porosity and pH, offered the most promise regarding the application of chailings to soil. Chailings produced at different pyrolysis temperatures were applied to three different soil types (sandy, clay and clay loam) at varying application rates (4–50 t/ha) in a series of glasshouse pot trials. The greatest plant response was for high-temperature (MA700(0) and MB700(0)) chailings application to a clay soil. Increases in overall plant yields were observed due to greater root propagation as a result of the decrease in bulk density offered by chailings application at 50 t/ha. The greatest soil responses to chailings application were observed in a sandy soil treated with high application rates (20–50 t/ha) of high-temperature chailings (produced at temperatures > 600 °C). For such treatments, significant changes to soil pH and EC were observed. Near linear increases in soil moisture were also observed, with a maximum increase (above the control value) of 485% for the MB800(2) 50 t/ha treatment. It was concluded that chailings produced at high temperatures (> 600 °C) offered beneficial properties to sandy soils, namely, increases in soil moisture and changes to soil pH. The effect of chailings amendment on hydraulic properties of a sandy soil was investigated as a result of the improved water retention observed upon application of high-temperature chailings. Experimental investigations revealed chailings application reduced water infiltration rate, saturated hydraulic conductivity (Ksat) and increased moisture retention of a sandy soil. The improvements observed were to a lesser extent than those observed in pot trials. The experimentally determined Ksat values were compared with nine different empirical correlations for predicting saturated hydraulic conductivity in sandy soils. This comparison was conducted to determine whether the chailings-amended soils behaved in a way predictable by these correlations. Only three correlations offered acceptable predictions of Ksat for the control soil; these three methods also correctly predicted Ksatsat values were in good agreement with the experimental data. The Kozeny–Carman (Chapuis) method offered the best prediction for Ksat for both Mine A and Mine B chailings amendments. A greenhouse gas (GHG) assessment and analysis of the gross energy requirement of the chailings life cycle was completed using life cycle analysis methodologies. The entire chailings life cycle from resource extraction to soil application and ensuing harvest was assessed. The overall global warming potential (GWP) of the chailings life cycle was found to be 2.06 t CO₂-e/t chailings produced and the energy requirement was 7.46 GJ/t chailings produced. The primary source of GHG emissions was from fugitive methane emissions during the coal extraction process. This was followed by the chailings production process, and chailings application to soils had the least impact on GWP and energy demand. Although the overall chailings life cycle resulted in a net flux of GHG emissions to the atmosphere, this was seen as a reasonable compromise, considering the benefits to agriculture and the reduction in economic and environmental liability obtained by removing tailings from a mine site. Ultimately, the investigations presented in this thesis showed that chailings produced at pyrolysis temperatures above 600 °C from the coal tailings have beneficial properties for soil application. These findings justify future work as chailings ultimately provide a new way to utilise coal tailings that not only reduces the burden on coal mines but may also provide benefits to agriculture.
- Subject
- coal tailings; pyrolysis; chailings; biochar
- Identifier
- http://hdl.handle.net/1959.13/1315128
- Identifier
- uon:22899
- Rights
- Copyright 2016 Priscilla Tuisuva Tremain
- Language
- eng
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