Brown Coal Char Research Articles

A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars

This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters.

Two-stage steam gasification of waste biomass in fluidized bed at low temperature: Parametric investigations and performance optimization

Steam gasification of waste biomass has been studied in a two-stage fluidized bed reactor, which has the primary pyrolysis fluidized bed using silica sand as bed material and the secondary reforming fixed bed with catalyst. The main objectives are parametric investigation and performance improvement especially at low temperature of around 600 °C using the wood chip and the pig manure compost as feedstock. Main operating variables studied are pyrolysis temperature, catalytic temperature, steam/biomass-C ratio, space velocity and different catalyst. Reaction temperatures and steam/C ratio have important role on the gasification process. About 60 vol.% H2 (dry and N2 free) and about 2.0 Nm3/kg biomass (dry and ash free basis) can be obtained under good conditions. Compared to Ni/Al2O3, Ni/BCC (Ni-loaded brown coal char) has a better ability and a hopeful prospect for the stability with coking resistance.

Development of a Ni-Loaded Brown Coal Char Catalyst for Fluidized Bed Biomass Gasification at Low Reaction Temperatures

The use of nickel-loaded brown coal char (Ni/BCC) as a catalyst for tar reforming has been studied under mild conditions in a laboratory-scale fluidized bed gasifier using steam as the gasifying agent and nitrogen as the product gas carriers. Characterization of the catalyst was performed in a fixed-bed reactor under various conditions such as Ni/BCC particle size in the range of 0.5 to 2 mm and pyrolysis temperature in the range of 823 to 1023 K in order to investigate the effect of both catalyst particle size and pyrolysis temperature on crystallite size of Ni/BCC. The Ni/BCC catalyst was consumed at various steam feed rate so as to determine the effect of steam feed rate on the size of the catalyst. The XRD pattern of the catalyst showed that the size of the Ni particles increases as the particle size of the catalyst decreases, pyrolysis temperature increases, and steam feed rate increases. SEM images and BET surface area were used to characterize the surface structure and surface area of the Ni/BCC catalyst, respectively. The space velocity and catalytic tar reforming temperature as a function of gas yields were investigated. The catalyst showed high and stable catalytic activity and produced a high-quality product gas with space velocities of 10000 and 4000 h−1 at 923 and 873 K, respectively.

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O 2 and CO 2

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 O 2 balanced with argon and pure CO 2. 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 CO 2 at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O 2 also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O 2 and pure CO 2) 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.

Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char

Catalytic gasification of a woody biomass, Japanese cypress, was investigated under a prepared nickel-loaded brown coal (LY-Ni) char in a two-stage fixed-bed reactor. The nickel-loaded brown coal was prepared by ion-exchange method with a nickel loading rate of 8.3 wt.%. Nickel species dispersed well in the brown coal, and the LY-Ni char via devolatilization at 600 °C showed a great porous property with a specific surface area of 382 m 2 g − 1 . The LY-Ni char was confirmed to be quite active for the Japanese cypress volatiles gasification at a relatively low-temperature range from 450 to 650 °C. For example, at 550 °C, 16.6 times hydrogen gas and 6.3 times total gases were yielded from the catalytic steam gasification of Japanese cypress volatiles under the LY-Ni char, compared with the case of non-catalyst. The biomass tar decomposition showed a dependence on catalyst temperatures. When the catalyst temperature was higher than 500 °C, Japanese cypress tar converted much efficiently, high gas yields and high carbon balances were obtained.

Biomass Gasification Using Nickel Loaded Brown Coal Char in Fluidized Bed Gasifier at Relatively Low Temperature

Our work focuses on developing nickel loaded brown coal char as a new catalyst to decompose tar and to enhance quality of product gas delivered from woody biomass pyrolysis at relatively low temperatures of 823 K and 923 K. It is carried out in a two-stage fixed-bed reactor and a lab scale fluidized bed gasifier (FBG) under various conditions. Inside of gasifier is constructed by two beds, the primary one is a fluidized bed with sand, and the second one is a catalyst bed. The catalyst bed is used to evaluate and to compare catalytic activity between the new catalyst and a conventional Ni/Al2O3 catalyst. The new catalyst is prepared by ion exchange method, dried at 380 K in nitrogen for 24 h, and is then calcined at 923 K in nitrogen for 90 min. The temperature as a function of gas yield and the effect of catalysts on gas yield in presence and absence of steam are investigated in this study. The new catalyst has shown high catalytic activity and stable activity and given the high quality of product gas in presence of steam.

Pressurized Pyrolysis and Gasification of Chinese Typical Coal Samples

This paper aims to understand the pyrolysis and gasification behavior of different Chinese coal samples at different pressures. First, the pyrolysis of four typical Chinese coals samples (Xiaolongtan brown coal, Shenfu bituminous coal, Pingzhai anthracite coal, and Heshan lean coal) were carried out using a pressurized thermogravimetric analyzer at ambient pressure and 3 MPa, respectively. The surface structure and elemental component of the resultant char were measured with an automated gas adsorption apparatus and element analyzer. It was observed that higher pressure suppressed the primary pyrolysis, while the secondary pyrolysis of coal particles was promoted. With respect to the resultant solid char, the carbon content increased while H content decreased; however, the pore structure varied greatly with increasing pressure for different coal samples. For Xiaolongtan brown coal (XLT) char, it decreased greatly, while it increased obviously for the other three char types. Then, the isothermal gasification behavior of solid char particles was investigated using an ambient thermal analyzer with CO2 as the gasifying agent at 1000 °C. The gasification reactivity of solid char was decreased greatly with increasing pyrolysis pressure. However, the extent of change displayed a vital relation with the characteristics of the original coal sample.

Preparing and characterizing the active carbon produced by steam and carbon dioxide as a heavy oil hydrocracking catalyst support

Active carbon was prepared from Yallourn brown coal char using steam and carbon dioxide activation in a laboratory rotary kiln. The activation rate with steam was faster than that with carbon dioxide. The pore structure of the active carbons was characterized using the nitrogen isotherms at 77 K. The pore volume and specific surface area of the active carbon increased with the carbon burn-off, and compared to carbon dioxide, steam activation produced active carbon that was richer in mesopores by increasing the pore size from micropores to mesopores. The porosity of the active carbons was related to the ability to adsorb maltene, the normal hexane-soluble fraction, in the vacuum residue of petroleum crude. The steam-activated carbon rich in mesopores had a greater ability to adsorb maltene, which consists of large-molecular-weight hydrocarbons.

Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal

Abstract The brown coal in the Latrobe Valley, Victoria, Australia, has many unique structural features and properties. The brown coal has a very low ash yield and contains highly dispersed alkali and alkaline earth metallic (AAEM) species, either as carboxylates forming part of its organic matter or as NaCl dissolved in its moisture. Owing to its unique structural features and properties, the brown coal behaves very differently from many other solid fuels such as biomass, bituminous coals and anthracites. For example, the highly reactive nature of its volatiles, the vulnerable nature of its nascent char and the presence of finely distributed AAEM species mean that the volatile–char interactions, a common phenomenon in all gasifiers, especially in the fluidised-bed gasifiers, would influence almost every aspect of its pyrolysis and gasification behaviour. Some recent progress in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal will be reviewed in this paper. After a brief account of the effects of AAEM species on the pyrolysis yields, the factors influencing the volatilisation of AAEM species will be summarised. This will be followed by the discussion of the factors influencing the reactivity of brown coal char and the catalytic reforming/cleaning of volatiles and gasification products by char-supported catalysts. The effects of dewatering/drying on the pyrolysis behaviour of Victorian brown coal and the conversion of pollutant-forming elements will be mentioned briefly. The progress in the fundamental understanding of the pyrolysis and gasification behaviour of Victorian brown coal has laid solid foundation for the further development of advanced gasification technologies for the clean and efficient utilisation of this cheap but important resource.

An Active Carbon Catalyst Prevents Coke Formation from Asphaltenes during the Hydrocracking of Vacuum Residue

Active carbons were prepared by the steam activation of a brown coal char. The active carbon with mesopores showed greater adsorption selectivity for asphaltenes. The active carbon was effective at suppressing coke formation, even with the high hydrocracking conversion of vacuum residue. The analysis of the change in the composition of saturates, aromatics, resins, and asphaltenes in the cracked residue with conversion demonstrated the ability of active carbon to restrict the transformation of asphaltenes to coke. The active carbon that was richer in mesopores was presumably more effective at providing adsorption sites for the hydrocarbon free-radicals generated initially during thermal cracking to prevent them from coupling and polycondensing.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VII. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air

FT-IR/Raman spectroscopies have been used to identify the structural features of Victorian brown coal chars during the gasification in air at 400 °C. The deconvolution of the Raman spectra has allowed us to identify the main structural sites in char where preferential reaction with O 2 takes place. The presence of Na and Ca catalysts is shown to alter the reaction pathways between char and O 2. In the absence of a catalyst, the O-containing functional groups formed in char during gasification were closely associated with the aromatic structure and thus tended to loosen the aromatic structure. The non-catalysed gasification was slow and took place on some specific (especially sp 3-rich or sp 2–sp 3 mixture) sites distributed throughout the char. In the presence of a catalyst (Na or Ca), the O-containing functional groups were not closely associated with the main aromatic structure throughout the char. The catalytic gasification reactions were localised on the sites associated with the catalysts. The preferential removal of smaller aromatic ring systems and the persistence of cross-linking structures in the presence of a catalyst mean that the large aromatic ring systems were increasingly concentrated with little flexibility, affecting the dispersion of catalyst.

Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals

A novel hot gas cleanup method to decompose a low concentration of NH 3 to N 2 with metal cations present inherently in low rank coals has been studied with a quartz reactor under the conditions of 750–850 °C, 0.1 MPa and high space velocity of 45,000 h −1. Each coal is pyrolyzed at 900 °C to prepare the char, which is subjected to the decomposition of 2000 ppm NH 3 after pretreatment with H 2. All of five chars examined promote NH 3 decomposition in inert gas, but the promotion effect depends strongly on the kind of char and can correlate more closely with the Fe content than with the Ca content. This result may indicate that the Fe plays a crucial role in the reaction. A commercial activated carbon with a very low Fe content of <0.05 wt% exhibits lower conversion of NH 3 to N 2 than five chars. The TEM pictures reveal the formation of nanoscale particles of Fe and Ca components on a brown coal char that provides the largest catalytic performance. The char maintains the high conversion level of 80% during 25 h reaction at 750 °C and achieves the complete decomposition of NH 3 at 850 °C. The co-feeding of a mixture of H 2, CO, and CO 2 does not change significantly the catalytic activity of the char at a steady state, whereas the coexistence of 2000 ppm H 2S lowers it in the whole range of time on stream. It is proposed by combining the XRD and TPD observations with our previous results that the catalytic decomposition of NH 3 in inert gas with the chars derived from low rank coals proceeds through two cycle mechanisms involving iron metal, iron nitrides, CaO and CaCN 2.

Mechanism of coal char burning at a low oxygen content in the feed stream

Evolution of the internal pore structure during the combustion of two different types of coal chars is compared. In this study brown coal char prepared from brown coal (coal-mine Novaky) by drying and devolatilization, and commercial black coal char (provided by U.S. Steel Kosice) were used. Particles were combusted to different degrees of burnout at an initial temperature of 800°C in a low oxygen containing atmosphere (5 vol%). It was shown that the combustion of both types of coal chars proceeds via the same shell progressive mechanism, despite differences in their original internal pore structure. The internal surface area of the brown coal char particles mainly belongs to the region of micropores, while for the black coal char is typical its macropore structure. Inside the brown coal particle cores pore structure evolution was observed. This change of the structure was caused by the reaction between solid carbon and carbon dioxide, due to which the specific surface area in the region of micropores significantly increased.

04/00402 The influence of high-temperature crystallite growth and petrography of pulverized char on combustion characteristics: Xu, X. et al. Fuel, 2003, 82, (7), 853–858

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.

Decomposition of ammonia with iron and calcium catalysts supported on coal chars

Decomposition of NH 3 to N 2 with Fe and Ca catalysts supported on brown coal chars has been studied with a cylindrical quartz reactor from a viewpoint of hot gas cleanup. The catalyst is prepared by pyrolyzing a brown coal with Fe or Ca ions added. In the decomposition of 2000 ppm NH 3 diluted with He at 750 °C and at a space velocity of 45,000 l/h, 2–6 wt% Fe catalysts are more active than not only 6 wt% Ca catalyst but also 8 wt% Fe catalyst loaded on a commercial activated carbon. The transmission electron microscope observations show that fine iron particles with the sizes of 20–50 nm account for the higher catalytic performances. When reaction temperature is increased to 850 °C, all of Fe and Ca catalysts on the chars achieve complete decomposition of NH 3. The co-feeding of H 2 with 2000 ppm NH 3 improves the performance of the 2% Fe catalyst at 750 °C, but contrarily the coexistence of syngas (CO/H 2=2) deactivates it remarkably, whereas the addition of CO 2 to syngas restores the catalytic activity of the Fe to the original state without syngas. The powder X-ray diffraction and temperature programmed desorption measurements strongly suggest that the Fe and Ca catalysts promote NH 3 decomposition through cycle mechanisms involving the formation of N-containing intermediate species and the subsequent decomposition to N 2.

Adsorption of MIB by activated carbons produced using several activation techniques

Water treatment authorities use activated carbon as the best available technology to remove low molecular weight organic compounds from potable water. In Australia, pollutants of concern include secondary metabolites from bacterial and cyanobacterial blooms which are highly odorous and, in some cases, toxic. Of these compounds, 2-methylisoborneol (MIB) is one of the most common and its unpleasant musty earthy odour can be detected at or above approximately 10 ng/L. Difficulties in using activated carbon to target such small organic compounds arise when the water has high concentrations of natural organic matter (NOM), as these compounds also adsorb on activated carbon. The adsorption of NOM on activated carbon increases the cost of using this material in water treatment due to competition with the target organic compounds, reducing the capacity of the activated carbon for the latter. The surface of activated carbon can be tailored during production to provide physical and chemical characteristics that can either aid or hinder the adsorption of particular compounds. One source of activated carbon currently under investigation at RMIT University is brown coal char waste from power stations. This waste, currently disposed of to landfill, is potentially an option for activated carbon production. This material has the advantage that it has already been carbonised at around 500°C in the power generation process. This means that less energy is required to produce activated carbon from power station char compared to coal, making the final product cheaper to produce. Previous work at RMIT has shown that steam activated power station char can remove organic compounds from water. Production of a range of activated carbons from power station char (PSC) was undertaken using different activation methods, including steam activation, steam activation with acid pre-treatment, alkali heat treatment, and Lewis acid heat treatment. The different activation methods produced activated carbons with different pore size distributions, in particular, the acid pre-treatment increased the surface area and porosity significantly compared with steam activation, and the alkali treatment increased the microporosity. Adsorption of MIB on these activated carbons was evaluated to determine the relationship between physical and chemical interactions of the activated carbon and adsorption. Adsorption of MIB on these activated carbons was found to be dependent on the secondary micropore volume. Lewis acid treatment and alkali treatment was not involved in the generation of many of these secondary pores, hence carbons from these treatments did not perform well in adsorption tests. The best adsorption results were achieved with steam activated or acid treated steam activated samples which performed comparably to commercial products. Initial results showed that competition from NOM adsorption was lowest with the PSC activated carbons, allowing greater adsorption of MIB, compared with the commercial activated carbons.

1-25. The effect of Ca-loaded brown coal char on desulfurization during gasification

1-25. The effect of Ca-loaded brown coal char on desulfurization during gasification

Char-Nitrogen Functionality and Interactions between the Nitrogen and Iron in the Iron-Catalyzed Conversion Process of Coal Nitrogen to N2

Iron catalysts precipitated on brown and bituminous coals promote conversion reactions of coal nitrogen to N 2 during pyrolysis at 900 °C, and the N 2 evolved from the brown coal arises mainly from char nitrogen (char-N). The present study, thus, focuses on clarifying the effect of the iron on char-N functionality and elucidating interactions between the two mainly with X-ray photoelectron spectroscopy (XPS). The N 1s spectra of brown coal chars revealed that the catalyst lowered the ratio of pyrrolic-N/pyridinic-N irrespective of the depth of the char, indicating the preferential formation of N 2 from pyrrolic-N. In the Fe 2p and N 1s XPS spectra of the Fe-bearing chars, Ar sputtering removed surface iron oxides to expose the metallic phase and concurrently increased the proportion of pyridinic-N with the corresponding decrease in quaternary-N. It is suggested that there are strong interactions between the iron and char-N even in the process of N 2 formation. Iron particles were more finely dispersed on the brown coal char and in closer contact with it, which accounts for higher conversion of char-N to N 2 . A schematic mechanism for this conversion process is discussed in terms of solid-solid reactions of metallic iron and char-N.

Selective Conversion of Fuel-bound Nitrogen to N2 with Iron Nanoparticles.

The highlight of this review article is a novel method to remove fuel-bound nitrogen as N2 during coal pyrolysis in an inert atmosphere. When a Cl-free iron catalyst is precipitated on Australian brown coal from FeCl 3 solution using Ca(OH) 2 , N2 yield increases with increasing pyrolysis temperature and reaches 50% at 900°C, whereas it is < 5% without the iron. A low loading (1 wt% Fe) is sufficient for the remarkable formation of N2, and the use of H2 instead of inert He is unnecessary. The iron catalyst decreases the partitioning to not only volatile nitrogen (tar, HCN, and NH3) but also char nitrogen. A smaller catalytic effect of the precipitated iron is observed with bituminous coal. The iron catalyst after pyrolysis exists as fine particles with a smaller size of 20-30 nm for the brown coal char, Fe3C and graphitized carbon being also present. It is likely that N2 originates mainly from solid phase reactions involving the formation and subsequent decomposition of iron nitrides. In the pyrolysis of different coals at 1000°C, N2 yields for low-rank coals from China and Germany reach 50-60% despite the absence of catalyst. Demineralization with HCI washing removes mainly Fe and Ca elements from these coals and drastically decreases N2 yield with a corresponding increase in nitrogen retention in the chars. The Ca ion-exchanged with the demineralized coals is inactive for N2 formation. Iron nanoparticles, derived from Fe-containing minerals, catalyze predominantly solid phase reactions to extract N2 from char and/or precursors.

97/04604 Iron catalysts for CO 2 pulsed gasification of Xianfeng brown coal char : Zhu, Z. et al. Ranliao Huaxue Xuebao, 1997, 25, (3), 218–222. (In Chinese)

97/04604 Iron catalysts for CO 2 pulsed gasification of Xianfeng brown coal char : Zhu, Z. et al. Ranliao Huaxue Xuebao, 1997, 25, (3), 218–222. (In Chinese)