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Elucidating the mechanism underpinning ultra-clean coal production from Victorian brown coal and its application as a gasification fuel
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posted on 16.02.2017by Wijaya, Niken Audrey
As a cheap and abundant fuel, coal has been the single largest energy source for electricity generation (>85% in Victoria) in Australia. Such a situation will continue in the foreseeable future. However, irrespective of the utilisation process, coal conversion results in the
emission of greenhouse gases (principally CO2) and a variety of air pollutants including SOx,NOx and particulate matter. Burning low-rank lignite/brown coal is worse. The presence of abundant moisture in Victorian brown coal incurs a CO2 emission rate of approximately 1.2-1.5 tonnes CO2-e/MWh sent out, relative to 0.8~1.1 for the high-rank bituminous coal.
Ultra clean coal (UCC) is a coal-derived solid fuel with overall ash content in the order of 0.1 wt %. It is supposed to be one of the promising solutions to achieve the near-zero emission utilisation of brown coal in the carbon-constraint future. As the mineral matter is removed significantly, UCC has the potential to burn directly in gas turbine combined cycle system and direct carbon fuel cell (DCFC), resulting in a net power generation efficiency of no less
than 48% and 60%, respectively, on the higher heating value basis. Consequently, CO2
emission mitigation by 25 – 35% compared to conventional coal-fired power stations is
plausible. With the removal of ash-forming metals, the requirement for post-combustion flue gas cleaning is also minimised and even eliminated. Its gasification-derived synthetic gas is more value-added, which can be used as a hydrogen source for high-efficiency electricity generation with zero-emissions and as a feedstock for the synthesis of value-added chemicals
and liquid fuels. In summary, a successful use of UCC will significantly reduce the carbon footprint of brown coal industry and increase the quality and competitiveness of Victorian brown coal in national and international energy markets.
The ultimate goal of this research is to discover a novel method to produce UCC from
Victorian brown coal through the use of cheap/biodegradable reagents and mild leaching conditions for demineralisation. A variety of core technical/scientific issues underpinning the proposed novel upgrading process for Victorian brown coal have been investigated. A preliminary study on sulphur and mineral characterisation was carried out to provide better insight on their modes of occurrences in coal and selectively screen the suitable reagents for demineralisation. The chemical structures of individual ash-forming elements, their interactions with neighbouring inorganic elements and transformation upon demineralisation
treatment and/or pyrolysis were extensively investigated, through an array of advanced instruments including synchrotron-based X-ray Absorption Near Edge Structure (XANES),
Computer-Controlled Scanning Electron Microscopy (CCSEM), X-Ray Photoelectron
Spectroscopy (XPS), and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES).
Except for sodium and potassium, the modes of occurrence of major elements present in the coal are highly dependent on the coalification extent of the parent coal. For sodium and potassium in two of the brown coals tested, they are mainly water-soluble and organically bound. Their extraction from the coal is extremely rapid and easy. Potassium is also commonly found to have strong affinity with alumina-silicate compounds whose elution is more challenging and controlled by the intraparticle diffusion. The modes of occurrence of
Ca, Mg, and Fe span from water soluble to crystallised oxides. The controlling-step of their extraction is consequently different. This study showed that for Ca and Mg in coal A, their activation energies were well above 40 kJ/mol, indicating the dominant control of surface reaction on the removal of water-soluble and organically bound Ca and Mg. Their extraction in coal B, however, was found to be a function of the surface reaction and intraparticle diffusion as their activation energies were reduced to approximately 20 kJ/mol. Al, Si, and Ti
are more likely to be found in oxide forms, however, a considerable amount of chlorides,
sulphate and ion-exchangeable Al, amorphous Si, and organo-Ti that are both water and weak acid soluble might also present in brown coal.
Advanced synchrotron study on sulphur characterisation showed the prevalence of
heterocyclic thiophene and sulphidic sulphur in Victorian brown coal. Upon hydrothermal
treatment with acids, the sulphidic sulphur was reduced, as a result from the fragmentation of the light aliphatic hydrocarbons hosting the sulphidic sulphur. Sulphur vaporisation as a result of pyrolysis was initiated from the volatilisation of the mono/poly-sulphidic sulphur at 400 oC which is then followed by the more thermally stable thiophene as the pyrolysis temperature increases. Increasing the pyrolysis temperature to 800 oC favoured the sulphation of these volatilised sulphurs by the acid-soluble metals forming acid-soluble inorganic sulphate in the raw coal char. A prior acid washing of coal, on the other hand, restrained the
formation of the inorganic sulphate.
Upon understanding the detailed structures of ash-forming elements in brown coal, the study proceeds to conclude the optimum and most efficient condition for acid leaching to produce the targeted UCC, by addressing the key parameters that greatly affect the demineralisation performance. In this PhD, the author has successfully concluded the optimum leaching condition for UCC generation by varying a number of key parameters including particle size
(i.e. < 53 μm, 106-153 μm, 153-300 μm, and 300-600 μm), leaching temperature (i.e. room
temperature to 200oC), leaching time (1, 2, 5, 30 minutes and 1, 2 and 3 h) and an array of leaching reagents (1M citric acid, 1M pyroligneous acid, and 0.1M Na-EDTA + 1M
Two Loy Yang coals, annotated A and B, were investigated throughout this research study.
For Coal A, a room temperature washing with 1 M citric acid for 5 min was proven capable
to produce a UCC with an overall ash content of as low as 0.12 db-wt %, non-detectable Na
and ~20 mg/kg of Ca, thus, meeting the requirement for a gas turbine fuel. Coal B, on the other hand, a coal particle size between 153 and 300 μm and leaching temperature of 120°C were found to be the optimum conditions to achieve the maximum demineralisation extent in 5 min. Although the ash content in the resulting fuel is still beyond the criteria (0.2 wt %) for gas turbine fuel, the CCSEM study showed that the minerals remaining in the UCC were
smaller than 5 μm and Na in the coal was completely removed. Accordingly, the corrosion and erosion of gas turbine blades may not be of concern for this coal. Alternatively, it may be suitable for diesel engines requiring ash content up to 2% for very fine soft ash such as clay.
This study has presented breakthrough findings of the potential use of weak acids as a cheap, biodegradable leaching reagent for the UCC generation derived from brown coal, which is an answer to a more sustainable electricity generation.
Finally, the UCC produced was tested in a low-temperature catalytic gasification process to essentially assess its potential as a gasification fuel. Gasification at 750oC was found to be the optimum temperature for the steam gasification of Victorian chars to achieve high
gasification rate and hydrogen-rich syngas. A complete drying and upgrading of wet brown
coal was also integrated in the proposed gasification technology via mild pyrolysis prior to entering the gasifier. Prior pyrolysis of the produced UCC at 400oC was proven ideal to achieve gasification at high rate (i.e. similar to or higher than the chars derived from their respective raw coals) while minimizing the evolution of undesirable by-product gases to low
or trace level. Such mild pyrolysis temperature was also proven to avoid sulphation reaction of the volatilised sulphur while maintaining minimum energy requirement. Acid washing also works favourably in promoting the water gas shift reaction leading to higher H2/CO ratio.
Mass balance calculation from raw material to end product (i.e. hydrogen-rich syngas) was performed to provide an insight into the amount of energy generated from coal gasification with and without acid treatment. The calculation was performed on the basis of 1 tonne raw coal. Energy generated was based on the amount of hydrogen gas produced. Mass balance
was performed across acid treatment, filtration, pyrolysis (400oC), and catalytic steam gasification (750oC). It is important to note that for Victorian chars, the AA washed-400oC char produces comparable amount of hydrogen in the syngas as its counter raw-400oC char. A total energy of 13.7 MJ generated from this fuel is similar to the energy produced from char without prior acid treatment. Increasing pyrolysis temperature to 800oC showed similar if not less energy generated.
This PhD has confirmed the great potential of ammonium acetate washed chars derived at
400oC as gasification feedstock whose low ash-precursor minerals content relative to the corresponding raw chars, has put this fuel as an alternative high quality feedstock for advance electricity generation. Utilisation of this fuel in place of raw coals is highly expected to
increase the electricity generation efficiency to greater than 48% while maintaining high gasification rate, high hydrogen content syngas, and high energy generation.