Supercritical water gasification of Victorian brown coal: Experimental characterisation

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Abstract

Supercritical water gasification is an innovative thermochemical conversion method for converting wet feedstocks into hydrogen-rich gaseous products. The non-catalytic gasification characteristics of Victorian brown coal were investigated in supercritical water by using a novel immersion technique with quartz batch reactors. Various operating parameters such as temperature, feed concentration and reaction time were varied to investigate their effect on the gasification behaviour. Gas yields, carbon gasification efficiency and the total gasification efficiency increased with increasing temperature and reaction time, and decreasing feed concentration. The mole fraction of hydrogen in the product gases was lowest at 600 °C, and increased to over 30 % at a temperature of 800 °C. Varying parameters, especially reaction time, did not improve the coal utilisation for gas production significantly and the measured data showed a large deviation from the equilibrium level.

Introduction

Victorian brown coals are an important energy resource for electricity production in Australia, but their high moisture content (up to 65%) makes their utilisation in conventional power stations very inefficient, resulting in high carbon intensity for the electricity produced. The problem of high moisture has long been recognised (“the brown coal of the Latrobe Valley would be an excellent fuel if it could be rid of water” [1]), and numerous methods for drying have been investigated [2]. However, thermally efficient drying techniques tend to suffer from poor economics, scale-up limitations and/or disposal problems for the effluent stream.

One alternative for the utilisation of Victorian brown coal which avoids the need for any water removal is “supercritical water gasification”. Supercritical water gasification (SCWG) is a novel thermochemical conversion, which utilises water above its critical point (T = 374 °C and P = 22.4 MPa), namely supercritical water (SCW). SCWG is well suited to wet feedstocks for the production of valuable fuel gases such as methane and hydrogen, because the water participates in the chemical reaction. As hydrogen has become of growing interest for use as an alternative transport fuel in recent years, most SCWG studies have focused on its production from agricultural wastes such as cornstarch, potato waste and wood sawdust [3], clover grass and corn silage [4], and wood [5] while sewage sludge categorised as industrial and municipal wastes was investigated by several researchers [6], [7], [8].

Because it avoids an energy-intensive drying process, SCWG has the potential to achieve a significantly higher total efficiency in both electricity generation and heat utilisation than the current thermal processes for wet feedstocks [9]. SCW plays an effective role in the SCWG reaction due to its gas-like liquid characteristics. One of the unique properties of SCW is that it possesses a non-polar solvent ability in which organic compounds and gases can be miscible, allowing the reaction to proceed homogenously. This leads to the rapid and complete gasification of feedstocks due to an absence of mass transfer limitations. More details of the properties of SCW can be found in [10], [11].

Additionally, almost complete separation of carbon dioxide from the product gas stream can be achieved by simply manipulating temperature and pressure during the let-down process [12]. This simple approach allows the SCWG process to be coupled with emerging sequestration technologies for zero carbon emissions. Furthermore, SCWG provides selectivity for the production gas species as it can be operated at low-temperature (200–400 °C) or high temperature (600–700 °C), optimising for methane and hydrogen production, respectively [13]. The high-temperature SCWG process enables a large amount of hydrogen to be produced, potentially exceeding a hydrogen efficiency of 100% [14] because the water participates in the reaction as a hydrogen supplier.

Another potential benefit of the SCWG process is as an alternative method for recycling water. Victoria consumed 71.2 million tonnes of brown coals in 2005–2006 [15], implying that 42.7 Giga-Litres of water was evaporated to atmosphere (based on 60 wt% water content) in the utilisation of the fuel. According to Antal et al. [3] who conducted SCWG of various plant-based biomasses, the effluent water product was clean and almost odour-free with a neutral pH value while the total organic carbon in the effluent was almost completely destructed. Clear water was also visually observed in the high-temperature SCWG of sewage sludge [9]. Thus, SCWG potentially offers not only an effective energy transformation pathway for clean energy production but also a new concept for water recycling from brown coal utilisation.

The SCWG of brown coal was previously investigated in batch cylindrical autoclaves by some researchers [16], [17]. Recently, a novel experimental approach was proposed by Potic et al. [18], namely the high throughput screening technique. This method utilises micro-quartz capillaries as batch reactor which are immersed in a fluidised-bed maintained at a desired reaction temperature. Unlike the autoclave method, this technique enables rapid heating and cooling to be achieved. Furthermore, the method allows SCWG reactions to be conducted in an environment free of catalytic surfaces. Hence, it is possible to precisely characterise the SCWG reaction. In this study, we applied this immersion technique for the SCWG of a Victorian brown coal to investigate its gasification characteristics.

Section snippets

Coal sample

The coal sample employed in this study was an 80:20 mixture of Loy Yang and Yallourn coals from the Latrobe Valley in Victoria. Particle size was in the range 20–50 μm. Table 1 shows the ultimate analysis on dry basis (d.b.) for this blended coal. Thus, the chemical formula can be expressed as CH0.879O0.254N0.007S0.002, which defines the molecular weight of the coal sample in this study.

Experimental apparatus and procedure

SCWG experiments were carried out by using the immersion technique proposed by Potic et al. [18], in which

Visual observations

The reactor contents were visually observed by photographing the quenched reactors after the SCWG reaction. Fig. 5 presents the top and bottom parts of the quenched reactors after 180 seconds SCWG reaction time for the higher coal feed concentration conducted at 600, 700 and 800 °C. A blackish solid product was clearly observed in the bottom part of all the reactors. It was also observed that the inner wall near the bottom of the reactor becomes brownish at 600 and 700 °C due to some material

Conclusions

The non-catalytic SCWG of Victorian brown coal was characterised by conducting gasification in micro-quartz batch reactors under various reaction conditions. Conclusions are as follows:

  • 1.

    Visual observations revealed that the coal sample is partially converted into yellow, brown and black products. At high temperatures, only the black product remains.

  • 2.

    Gas yields, carbon gasification efficiency and the total gasification efficiency increase with increasing temperature and reaction time and

Acknowledgements

Authors gratefully acknowledge the CSIRO Division of Petroleum Resources for providing the gas analysis instrumentation. The Department of Civil and Environmental Engineering of the University of Melbourne is acknowledged for the financial support of the experimental works and analyses. We are finally grateful for Dr. Adam Kosminski of the University of Adelaide for supplying the brown coal samples for this study.

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