Effects of volatile-char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam

The purpose of this study is to investigate the effects of volatile-char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam. A novel one-stage fluidised-bed/fixed-bed quartz reactor was employed to carry out the experiments in the presence and absence of volatile-char interactions. The effects of thermal annealing on char structure were also investigated under similar conditions. The structural features of char were evaluated using FT-Raman spectroscopy. The results indicate that the char structural features were considerably affected by volatile-char interactions, which was shown from the Raman band area or the ratios between the band areas. H radicals from the thermal cracking/reforming of volatiles are believed to play a vital role in the changes in char structure due to the volatile-char interactions. H radicals could penetrate into char matrix and favour the condensation of aromatic rings, which was the main reason for the decrease in the ratio of small (less than 6 fused rings) to large aromatic rings during the volatile-char interactions. The volatile-char interactions also greatly affected the concentrations of O-containing groups in char and thus significantly altered the observed Raman intensity of the char.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VI. Further investigation into the effects of volatile-char interactions

The purpose of this study is to further investigate the effects of volatile-char interactions on the volatilisation and dispersion of alkali and alkaline metallic species and changes in char structure during pyrolysis. Ion-exchanged (H-form, Na-form and Ca-form) Loy Yang brown coal samples were pyrolysed in a novel two-stage fluidised-bed/fixed-bed reactor over a wide temperature range of 500-930 °C. Our results indicate that soot formation and destruction on char (pore) surface during volatile-char interactions could be catalysed by Na and, to a lesser extent, Ca on char. Volatile-char interactions caused additional volatilisation of Na at temperatures higher than 700 °C although there are no effects on the volatilisation of Ca. The formation and simultaneous (catalytic) destruction of soot on char surface are closely linked to the volatilisation of Na from the char. Volatile-char interactions have also caused changes in char structure and/or changes in Na/Ca dispersion, as is reflected by the reduction in char reactivity. These results indicate that the volatile-char interactions are not limited on the char surface. It appears that H radicals must have penetrated into the char structure during volatile-char interactions.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char-H2O and char-O2 reactivities

Volatile-char interactions are an important consideration in the design and operation of a gasifier. This study aims to investigate the effects of volatile-char interactions on the in situ char-steam reactivity at 800 °C and the ex-situ char-O₂ reactivity at 400 °C. A Victorian brown coal was gasified in 15% steam at 800 °C in a one-stage novel fluidised-bed/fixed-bed quartz reactor, in which the extent of volatile-char interactions could be controlled. The chars after varying extents of volatile-char interactions and/or varying extents of char conversion in steam were also collected for the measurement of their reactivity with air at 400 °C in a thermogravimetric analyser. Our results show that the char-steam gasification reactions were greatly inhibited by the volatile-char interactions. It is believed that the H radicals generated from the thermal cracking/reforming of volatiles slowed the char gasification in three ways: occupying the char reactive sites, causing the char structure to re-arrange/condense and enhancing the release of catalytic species inherently present in the brown coal. The importance of volatile-char interactions to char-steam reactivity was further confirmed by the char-air reactivity.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part X: Effects of volatile-char interactions on the conversion of coal-N during the gasification of a Victorian brown coal in O2 and steam at 800 °C

A novel fluidised-bed/fixed-bed reactor was used to study the effects of volatile-char interactions on the conversion of coal-N during the gasification of a Victorian brown coal at 800 °C. The reactor has the capability of controlling the extent and length of the interactions between volatiles and char. Our results indicate that in the absence of volatile-char interactions during gasification in O₂, the lack of abundant H radicals led to negligible formation of NH₃ and HCN from char-N. The presence of volatile-char interactions during the gasification of Victorian brown coal in O₂ at 800 °C drastically enhanced the formation of NH₃ and, albeit to a lesser extent, the formation of HCN. The enhanced conversion of char-N into NH₃ (and HCN) due to the volatile-char interactions is attributed to the presence of H radicals in the volatiles. H radicals in volatiles could “die off” as they pass through the nascent char bed during the course of volatile-char interactions.     

Effects of volatile-char interactions on the volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass

An important feature of a fluidised-bed gasifier is the continuous contact between volatiles and char. The aim of this study is to experimentally investigate the effects of volatile-char interactions on the volatilisation of AAEM species during pyrolysis of two sugarcane industry wastes, bagasse and cane trash. A two-stage quartz fluidised-bed/fixed-bed reactor was used for this fundamental study. Our results indicate that the volatile-char interactions could lead to the additional volatilisation of alkali and alkaline earth metallic (AAEM) species, particularly if the volatile-char interactions have resulted in additional char weight losses. The monovalent Na and K behaved differently from the divalent Mg and Ca in biomass. Our results provide circumstantial but clear evidence that the AAEM species in biomass could behave distinctly differently from those in brown coal, largely due to the differences in the structure and composition between biomass and coal. The development of biomass gasification technologies must consider the special thermochemical characteristics of biomass. Furthermore, even the bagasse and cane trash grown in the same area behave drastically differently, at least partly due to the different microstructures of bagasse and cane trash.     

Effects of brown coal quality and process parameters on the hydroliquefaction of brown coal

During the development of the HVB process the influence of different parameters on the process have been investigated. In this Paper the influence of the properties of the brown coal on the process is reported. It is shown that: (1) the petrographical composition of Rhenish brown coal has virtually no effect on the HVB process; (2) the problems of sedimentation caused by the ash composition of the brown coal can be overcome by a particular thermal pretreatment of the coal; and (3) the moisture content of the brown coal is of minor importance for the technical realization of the HVB process.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity

The purpose of this study is to investigate the catalytic effects of Na as NaCl or as sodium carboxylates (-COONa) in Victorian brown coal on the char reactivity. A Na-exchanged coal and a set of NaCl-loaded coal samples prepared from a Loy Yang brown coal were pyrolysed in a fluidised-bed/fixed-bed reactor and in a thermogravimetric analyser (TGA). The reactivities of the chars were measured in air at 400 °C using the TGA. The experimental data indicate that the Na in coal as NaCl and as sodium carboxylates (-COONa) had very different catalytic effects on the char reactivity. It is the chemical form and dispersion of Na in char, not in coal, that govern the catalytic effects of Na. For the Na-form (Na-exchanged) coal, the char reactivity increased with increasing pyrolysis temperature from 500 to 700 °C and then decreased with pyrolysis temperature from 700 to 900 °C. The increase in reactivity with pyrolysis temperature (500-700 °C) is mainly due to the changes in the relative distribution of Na in the char matrix and on the pore surface. For the NaCl-loaded coals, when Cl was released during pyrolysis or gasification, the Na originally present in coal as NaCl showed good catalytic effects for the char gasification. Otherwise, Cl would combine with Na in the char to form NaCl during gasification, preventing Na from becoming an active catalyst. Controlling the pyrolysis conditions to favour the release of Cl can be a promising way to transform NaCl in coal into an active catalyst for char gasification.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part V. Combined effects of Na concentration and char structure on char reactivity

A set of NaCl-loaded Loy Yang brown coal was pyrolysed in a thermogravimetric analyser between 600 and 900 °C. The char sample after pyrolysis was cooled down directly for in situ reactivity measurement with air. The results indicated that the volatilisation of Na during pyrolysis is an important reason for the existence of catalyst loading saturation level with Na as a catalyst in char because the char prepared at high temperature had a limited holding capacity for Na. Under the experimental conditions in this study, the char reactivity showed good linear correlation with the Na concentration in the reacting char. Peak pyrolysis temperature, affecting the release of Cl and distribution of Na in char, is an important factor governing the correlation between the char reactivity and Na concentration in char. The catalytic activity of Na is a result of the interaction between Na and char and thus is greatly dependent on the char/carbon structure. At high char conversion levels where the char structure is more inert and highly condensed, the catalytic activity of Na is reduced compared with its activity at low char conversion levels. The catalytic activity of Na depends on the structure of char.     

Production of chemicals by cracking pyrolytic tar from Loy Yang coal over iron oxide catalysts in a steam atmosphere

Iron oxide catalysts containing metal promoters were tested to evaluate whether they could be applied to the coal tar obtained from the pyrolysis of Loy Yang coal. Catalytic cracking of the coal tar in a steam atmosphere was conducted in a fixed-bed reactor at 773 K and ambient atmospheric pressure. For iron oxide catalysts containing cerium, zirconium, and aluminium, the total yield of monocyclic aromatic hydrocarbons, phenols, and ketones exceeded 40 mol-C% on a tar basis, while about 97 wt.% of the initial heavy tar was decomposed. The combination of cerium, zirconium, and aluminium improved the activity of the iron oxide catalyst. Moreover, the addition of steam to the tar vapour increased the yield of ketones and made the catalyst more durable.     

Effects of effective stress changes on permeability of latrobe valley brown coal

One of the key issues with geological sequestration of carbon dioxide in coal seams is change of permeability caused by carbon dioxide (CO₂) injection, and especially any resulting reduction in injectivity. Injection causes changes in pressure and effective stress, with further changes caused by coal matrix swelling associated with adsorption of CO₂. In this paper we aim to study how the change in effective stress and coal swelling may influence the gas permeability in brown coal using natural coal and reconstituted coal specimens. Tests were conducted at different confining pressures to represent conditions at different depths. Different gas injection pressures were also employed at each confining stress stage. The test results clearly depicted an exponential reduction of coal permeability to CO₂ gas when effective stress increases. Based on the experimental results, an empirical correlation to represent the effect of stress on permeability was developed. The results also showed that increase in pore pressure can induce further swelling of the coal specimens, and this can lead to further decrease in permeability which can have important impact on field injectivity. Test results for natural brown coal specimens were compared with results of tests on reconstituted coal specimens made from compaction of coal particles obtained from crushing of blocks of natural coal. Observed permeability behaviour of gas in reconstituted coal was similar to the natural coal specimen permeability trend, when effective stress increases.     

Effects of solvent and iron-compounds on the liquefaction of brown coal

Hydrogen-donor solvents such as hydrophenanthrene are the most effective aromatic solvents for the liquefaction of brown coal. The hydrogen-donating ability of the solvent is more important for brown coals than for bituminous coals, because the thermal decomposition and subsequent recombination of the structure of the brown coals occurs rapidly. Three-ring aromatic hydrocarbons are more effective solvents than two-ring aromatics, and polar compounds are less effective solvents with brown coals than with bituminous coals. The thermal treatment of brown coal, accompanied by carbon dioxide evolution at temperatures > 300°C, in the presence of hydrogen-donating solvent did not affect the subsequent liquefaction reaction. However, thermal treatment in the absence of solvent strongly suppressed the liquefaction reaction, suggesting that the carbonization reaction occurred after the decarboxylation reaction in the absence of hydrogen donor. To study the effect of various iron compounds, brown coal and its THF-soluble fraction were hydrogenated at 450°C in the presence of ferrocene or iron oxide. The conversion of coal and the yield of degradation products are increased by the addition of the iron compounds, particularly ferrocene, and the yield of carbonaceous materials is decreased.     

褐煤氧化氨解反应工艺条件研究

以褐煤为原料,在较温和的反应温度(≤110℃)和氧压(≤0.5 MPa)条件下,考察了反应温度、氨水质量分数、氧压、含固体积分数及搅拌速率对褐煤氧化氨解反应产物中总氮及氨态氮质量分数的影响。结果表明,外部传递过程对氧化氨解过程影响显著,反应过程应保持足够高的搅拌强度;在一定反应时间内,反应温度和氨水质量分数的升高有利于总氮质量分数的提高,但对氨态氮质量分数影响不大;氧压升高,总氮质量分数和氨态氮质量分数均有所升高,但影响都不明显;含固体积分数对总氮质量分数和氨态氮质量分数基本无影响。实验得到了褐煤氧化氨解反应较合适的工艺条件。     

Influence of pyrolysis conditions on the structure and gasification reactivity of biomass chars

The physical and chemical structure as well as gasification reactivities of chars generated from several biomass species (i.e. pinus radiata, eucalyptus maculata and sugar cane bagasse) were studied to gain insight into the role of heating rate and pressure on the gasification characteristics of biomass chars. Char samples were generated in a suite of reactors including a wire mesh reactor, a tubular reactor, and a drop tube furnace. Scanning electron microscopy analysis, X-ray diffractometry, digital cinematography and surface area analysis were employed to determine the impact of operating conditions on the char structure. The global gasification reactivities of char samples were also determined for a range of pressures between 1 and 20 bar using pressurised thermogravimetric analysis technique. Char reactivities were found to increase with increasing pyrolysis heating rates and decreasing pyrolysis pressure. It was found that under high heating rates the char particles underwent plastic deformation (i.e. melted) developing a structure different to that of the virgin biomass. Pressure was also found to influence the physical and chemical structures of char particles. The difference in the gasification reactivities of biomass chars at pressure was found to correlate well with the effect of pyrolysis pressure on the graphitisation process in the biomass char structure.     

Generation of ultra-clean coal from Victorian brown coal — Sequential and single leaching at room temperature to elucidate the elution of individual inorganic elements

This paper addressed the probability of the generation of ultra-clean coal from chemical leaching of low-rank Victorian brown coal. Sequential leaching was employed to determine the modes of occurrence of the major elements in the two coals studied, including Na, K, Mg, Ca, Fe, Al, Ti, and Si. The results indicate that, the modes of occurrence of individual metals vary greatly with brown coal sample and elemental type. For one brown coal tested, it is dominated by water-soluble and ammonium acetate-soluble ion-exchangeable cations. Therefore, a single washing through the use of woody biomass-derived pyroligneous acid or citric acid easily reduced the concentrations of its overall ash and even sulphur and chlorine to meet the requirements for gas turbine fuel. The leaching of the organically bound cations in this coal was also very rapid and completed in 5 min. In contrast, another brown coal tested is mainly composed of quartz and/or clay compounds which remained intact even after being leached with 5 M nitric acid. These mineral grains possess two peak size ranges in the coal, 1.0-2.2 μm and 4.6-10 μm. The former size bin was embedded deeply in coal matrix, and hence, its leaching upon acids was very slow when compared with coarse particles which are mostly discrete grains residing separately from coal matrix. The Na-EDTA was found to be able to mobilise the small grains substantially through its Na ion to penetrate coal matrix to react with Al, forming acid-soluble Na aluminates. The ammonium acetate-insoluble Ti and Fe polyhedra were also mobilised by the EDTA. Accordingly, the overall ash content in coal residue accounts for ~ 1.5 wt.%, relative to 2.6 wt.% in the corresponding raw coal and 2.0 wt.% in the ammonium acetate-insoluble residue.     

Hydrothermal upgrading of Loy Yang Brown coal — Effect of upgrading conditions on the characteristics of the products

Hydrothermal treatment of the high-moisture containing (59%) Loy Yang brown coal was conducted in a 20 ml stainless steel autoclave at 200 to 350 °C for 30 and 180 min. Three treatment methods were employed: 1) conventional method (i.e., extra water was added before treatment); 2) as-received method (i.e., treatment without addition of extra water); and 3) separation method (i.e., treatment with physical separation of water and solid). Treatment using the as-received method revealed that hydrothermal conditions can be maintained in the autoclave without addition of extra water. Upgrading using the separation method yielded more effective drying especially during treatment at 350 °C, where the water content was reduced to 6%. Treatment above 250 °C led to progressive lower water content, increasing carbon and decreasing oxygen content. Calculations of the calorific values of the upgraded coals showed that hydrothermal treatment at 300 and 350 °C led to a net increase in the calorific value compared to the raw coal. Reductions in the volatile content and the changes in the physical properties of the upgraded coals significantly reduced self-ignition tendencies of the coal treated above 300 °C.     

Determination of microporosity of brown coal. Small angle x-ray scattering

The results obtained in this study indicate that small angle X-ray scattering can be used to characterize the micropore structure of brown coal irrespective of the moisture content of the sample. For dry coal, the values obtained are compared with those measured by gas adsorption. Surface areas calculated from the micropore data are discussed briefly.     

Effect of iron on the gasification of Victorian brown coal with steam:enhancement of hydrogen production

This paper reports the significant enhancement of hydrogen production during the gasification of Victorian brown coal with steam using iron as a catalyst. Iron was loaded into the acid-washed Loy Yang brown coal using ferric chloride aqueous solution. Gasification experiments were carried out using a quartz reactor at a fast particle heating rate. The yield of char was determined by directly weighing the reactor before and after each experiment. Gases were analysed using a GC with dual columns. The overall gasification rate of a char increases greatly in the presence of iron. The transformation of iron species during pyrolysis and gasification was examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that both reduced-iron (α-Fe and γ-Fe) and magnetite (Fe₃O₄) highly dispersed in a char can catalyse the gasification of the char with steam. In particular, the char from iron-loaded coal samples gives much higher yields of H₂ than a char from the acid-washed coal under similar conditions. The mechanism for the enhancement of hydrogen production in the presence of iron is discussed.     

Effect of MgO addition to upgraded brown coal on ash-deposition behavior during combustion

The objectives of this study were the evaluation of the effect of MgO addition to coal on the reduction of ash deposition during upgraded brown coal (UBC) combustion, and the elucidation of the mechanisms of the reduction of ash deposition. The melting temperature of UBC ash is 1494 K, which is lower than that of bituminous coal ash. Before the actual ash-deposition experiments, the molten slag fraction in the UBC ash was estimated by means of chemical equilibrium calculations for various mixing mass ratios of MgO to coal ash. The simulation results indicate that MgO addition plays a role in decreasing the molten slag fraction. It was confirmed that Mg formed solid composites with Si, Fe, Al, Ca, and Mn, and played a role in decreasing the molten slag fraction in the ash on the tube. As a next step, ash-deposition tests were conducted using a pilot-scale pulverized coal combustion furnace equipped with a refractory wall. The results showed that MgO addition contributed to decreasing the rate of ash deposition even for UBC. These calculations and experimental results suggested that one of the reduction mechanisms due to MgO addition involved the production of solid-phase aluminosilicates.     

Some physical properties of Canadian coals and their effects on coal reactivity

The pore volume, surface area and compressibility of eleven Canadian coals, varying in rank from lignite to semianthracite, have been determined by mercury porosimetry, gas adsorption method and relations derived from helium and mercury densities. The total pore volume was measured in the diameter range of 0.2 nm–2.98 μm, which was subdivided into two groups, namely the micropore region (< 0.0036 μm) and the combined meso- and macropore region (0.0036–2.98 μm). It has been determined that the porosity of the eleven coals studied varies from 2 to 39%. It has been found that the total pore volume, micropore volume, surface area and the apparent compressibility of these coals decrease with increase in the carbon content, or the rank of the coals. The effect of the total pore volume, micropore volume and surface area on chemical reactivity of the coal is discussed separately. A good correlation was obtained between the carbon content and helium density of the coal after correction is made for the mineral content.