Coal/Water
 
On the Shear Dependent Flow Properties of Victorian Brown Coal
T Kealy and C Tiu  CRC for Clean Power from Lignite, Melbourne
Victorian brown coal has complex rheological properties exhibiting gradual, irreversible changes from granular solid to stiff paste through the application of shearing forces.  It has been suspected for some time that these interesting rheological changes take place as a result of the liberation of intraparticle water, contained in the void spaces of porous, friable coal particles.
 
This work outlines a new technique for lignite rheology assessment, based on torque measurement at the shaft of a laboratory scale, instrumented kneading mixer for a typical brown coal, mined in the Latrobe Valley, Victoria, Australia.  It is demonstrated that this technique yields reliable and reproducible data, allowing prediction of the sheared coal product flow properties even at low shear rates where previous attempts at characterisation have been unsuccessful or unreliable.
 
Typical samples of run-of-mine lignite have been subjected to shearing forces in the apparatus and the effects of processing parameters such as particle size and the duration and intensity of shearing have been analysed in terms of their effect on the behaviour of lignite.  Properties such as the point at which the mixture may be said to change from a solid to a stiff paste and the interdependence of this phenomenon with particle size and measured flow properties, are documented and examined in terms of the time and power input required to obtain them.  It has been found that particle size has decreased by several orders of magnitude after low power inputs, to 7.2 mm.  Decreases are at a much reduced rate thereafter.  Tentative explanations regarding the interdependence of rheology and processing parameters are suggested.  A three-stage model is proposed, with grinding, grinding/shearing and shearing stages apparent from the experimental evidence.

Steam Reforming of Rapid Pyrolysis Char from Brown and Sub-bituminous Coals
J Hayashi, H Takahashi, M Iwatsuki, K Morishita Hokkaido Universtiy, Japan; C Li Monash University, Melbourne; A Tstsumi and T Chiba University of Tokyo, Japan
Steam gasification of nascent rapid pyrolysis char from low-rank coals was studied at 1123 – 1223 K employing a novel drop-tube/fixed-bed reactor, into which a small amount of pulverized coals was injected and it was rapidly pyrolyzed on a filter as a gas/solid separator.  The volatiles were swept in-situ out of the reaction zone through the filter by a forced flow of atmospheric nitrogen/stream (47/53 in vol/vol) while the particles remaining on the filter being exposed to the flow for a desired period.  Char from Yallourn brown coal underwent so rapid steam gasification at 1173 K that its conversion reached 26 and 32% on the basis of carbon in the nascent char in 5 and 10 s, respectively.  It was also found that the gasification decelerates drastically at the char conversion around 80%, leaving residue much less reactive that the initial char. Char from Taiheiyo sub-bituminous coals was also steam-gasified rapidly and its conversion reached about 70 and 80% in 60 s at 1173 and 1223 K, respectively.  The results of the rate analysis suggested that the rapid pyrolysis of the coals forms so-called ‘rapid carbon’ in nascent char that transforms into less reactive material in a few hundred seconds at 1173 K
 

Estimation of Pore Size for Low Rank Coals Sorbing Water Based on its Molecular Mobility
N Kudo, J Hayashi, and T Chiba  Hokkaido University, Japan  and K Norinaga Tohuku University Japan
This paper proposes an application of proton magnetic resonance relaxation analysis (MRRA) for estimating the size of pores in water-sorbing coals on the basis o f the transverse relaxation characte4ristics of water condensed in the po4res as the molecular probe.  Raw Yallourn brown coal (water content; 1.46 g-water/g-mf coal and Beulah Zap lignite (0.53) were employed as the starting materials and were partially or completely dried at 303K.  The samples with known water contents were subjected to the MRRA using a Carr-Purcel/Meiboom-Gill(CPMG) pulse sequence.  The results of the analysis revealed that the initial amplitude of the CPMG signal corresponds quantitatively to the total amount of ‘mobile’ proton.  It was also found that there are three components with different relaxation times (T2) that are attributed to free water, pore water (freezable bound water and non-freezable water) and mobile coal hydroxyls/carboxyles. The relaxation characteristics the components where further analyzed based on a theory proposed by Brownstein and Tarr, and finally the following conclusions could be reached. (i) pores filled with pore water are slit-like rather than cylindrical in shape; (ii) the dimensions of pores are about 3 nm and 2 nm for the raw Yallourn and Beulah Zap, respectively; (iii) the pore dimension decreases linearly with the content of pore water.

The Future of Brown Coal Drying Technologies for Power Generation Comparison of Products from Various Processes
G Favas, A Chaffee and W Jackson   Monash University, Melbourne
The deregulation of the Australian electrical supply industry, the increase in competition between all electricity generators (hydro-electricity, natural gas, black and brown coal) across the national grid and the need to reduce greenhouse emissions have necessitated improvements in brown coal utilisation for power generation. The competitiveness of brown coal power generation will in large part depend on the implementation of energy efficient drying technologies for existing brown coal boilers, which currently burn coal with a moisture content of 60%.
Three major drying technologies, Steam Drying (SD), Hydrothermal Dewatering (HTD) and Mechanical Thermal Expression (MTE), were investigated in batch and bench scale units.
 
Steam drying is an evaporative drying technology that utilises superheated steam to remove water from high moisture coals at much lower temperatures than HTD and MTE; however, the metal ash forming elements remain in the final product. HTD and MTE are non-evaporative energy efficient drying technologies that remove water from the coal as a liquid, thus saving energy. An added advantage of these process is that some of the water soluble inorganics, in particular Na, are leached out from the coal, thus decreasing the inorganic content of the product coal. The temperatures used in the HTD process are higher than in SD and MTE (i.e. up to 320°C). In HTD, soluble organic material can also leach out into the wastewater that, therefore, may require substantial treatment. In the MTE process, in addition to heat, a mechanical force is also applied to the system to facilitate the removal of water and destroy the internal pore structure of the coal. A disadvantage of this system is the high mechanical forces (e.g., 12MPa); but processing temperatures are much lower (180°C to 200°C) than for HTD.
 
Products from each process have been subjected to a range of analyses, including proximate and ultimate analysis, the composition of the inorganics, helium density, surface area (CO2 adsorption), pore volume (mercury porosimetry) and calorific value. The effectiveness of each of the drying technologies in relation to the chemical and physical structure of the coal has been evaluated.

Production of High Quality Coal Product from a Low Quality Coal using a Modified Hydrothermal Dewatering Technique
G Favas and W Jackson Monash University, Melbourne
A novel method of producing a very low porosity coal material from highly porous Latrobe Valley raw brown coals has been developed using a combination of hydrothermal and evaporative drying. Low porosity coal was made in three different batch autoclave systems at temperatures of 320°C. Higher temperatures (up to 350°C) gave a small additional decrease in porosity but these conditions were found undesirable as the water vapour pressure and the loss of organic material were significantly increased. Residence times as low as 5min were found to be sufficient to give a densified coal product. The total organic carbon leached into the wastewater during the process was significantly lower than under pure hydrothermal dewatering conditions. The low porosity coal product was found to give a coal water mixture with a maximum coal concentration for a pumpable slurry of greater than 60%wt.d.b. in comparison to 45%wt.d.b. using conventional hydrothermal dewatering. This paper will discuss the method of preparation, the chemical transformations of the raw coal and the production of very highly concentrated coal water slurries, with and without the use of chemical additives.

Bound Water in Brown Coal
L Clemow W Jackson, CRC for clean power from lignite, Monash University, Melbourne;
R Sakurovs CSIRO Division for Energy Technology, Sydney; and D Allardice Allardice Consulting Victoria
A well known characteristic of brown coals is their strong affinity to water, which is demonstrated by their high moisture content as mined, high monolayer water capacity and high non-freezing water content.  It is therefore important to establish how these quantities relate to the coal structure.  For several coals, monolayer water capacity was measured by equilibration at 15% relative humidity at 30°C and the non-freezing water content by 1H nuclear magnetic resonance spectroscopy (nmr) and differential scanning calorimetry (DSC).  These properties have been correlated with structural characteristics of the same coals (elemental analysis, carboxyl and phenol content determined by aqueous titration).  It was found that both of these properties and their ratio were primarily dependent on the carboxylic acid concentration of the coal.  The phenol groups were less significant in this regard.  Therefore to improve the characteristics of brown coals, attention should be concentrated on processes which enhance decarboxylation.

The Chemistry of Coal-Water Interactions: and Theoretical and Experimental Investigations
A Chaffee, T Vu CRC for Clean Power from Lignite, Melbourne; and I Yarovshy RMIT University, Melbourne
Compared to other coals, Victorian lignite is remarkably clean, typically possessing low concentrations of inorganics and mineral matter. Unfortunately, the high moisture content (60 – 70% wet basis) means that a pre-drying process is required to remove most of the water in the coal prior to combustion. Drying processes that remove the water evaporatively require substantial energy, reducing the efficiency of power generation from this fuel.
To assist the optimisation of drying processes it is important that details of the coal structure and coal-water interactions be understood in as full detail as possible.
 
Since lignite is a very complex heterogeneous material, the complexity of the problem has been reduced through the use of ‘megafossil’ samples taken from the coal. Megafossils are the discrete macroscopic remains of wood, leaves, resins, etc that can be visually identified and hand-picked from the coal seam. Such megafossils are chemically less heterogeneous than lignite as a whole.
 
The physical structure of these fossil woods has been probed by techniques such as helium pycnometry, mercury porosimetry and gas adsorption. Likewise, the organic structure has been probed by elemental analysis, DRIFT-IR, SS-NMR and TGA. On the basis of these results, structural models of fossil wood are constructed and coal-water interactions are simulated.  The simulated results are compared with measured isotherms of water adsorption.

Upgrading of Coal Derived Oil as Transportation Fuels
S Azuma and S Wasaka New Energy and Industrial Technology Development Organization, Japan
The New Energy and Industrial Technology Development Organization (NEDO) has been developing coal liquefaction technology as part of the “New Sunshine Programs” planned by the Agency of Industrial Science and Technology, a division of the Ministry of International Trade and Industry in Japan.  The developed “NEDOL” process showed advantages such as applicability to a wide range of coal ranks, a high oil yield and reliability of plant facilities.
 
Parallel to the development of the coal liquefaction process, development of product upgrading technology has been proceeding to make use of the liquefied products as transportation fuels.  Based on the laboratory study of the upgrading process, a 40 bbl/d process development unit (PDU) was designed and constructed.  The PDU is composed of a two-stage hydrotreating unit, an atmospheric distillation unit and a reforming unit to produce gasoline and diesel fractions.  First-stage hydrotreating of coal derived crude oil was carried out successfully in the PDU.
 
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