Changes In Char Structure Research Articles

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.

Changes in Char Structure during the Gasification of a Victorian Brown Coal in Steam and Oxygen at 800 °C

Char structure is an important factor influencing its reactivity during gasification. This study aims to investigate the changes in char structure during the gasification of brown coal. A Victorian brown coal was gasified in a fluidized-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H2O, 2000 ppm O2, or 15% H2O and 2000 ppm O2, respectively. Although the char gasification in 2000 ppm O2 was mainly rate-limited by the external diffusion of O2, the char−H2O reaction was mainly rate-limited by the chemical reactions. The structural features of char at different levels of char gasification conversion were examined with FT-Raman spectroscopy. Our results show that the chars from the gasification in the mixture of 2000 ppm O2 and 15% H2O had almost the same features as the chars from the gasification in 15% H2O alone when the same levels of char conversion were achieved. Both the thermal decomposition of char and the char gasification reactions could result in changes in char structure during gas...

Evolution of biomass char structure during oxidation in O2 as revealed with FT-Raman spectroscopy

FT-Raman spectroscopy has been used to identify structural features and evaluate the structural evolution of biomass chars during gasification with air. Chars prepared from the pyrolysis of a cane trash sample with a fast particle heating rate in a novel fluidised-bed/fixed-bed reactor at 500, 700 and 900 °C were oxidised at 400 °C in air in a TGA. The data derived from the spectral deconvolution of Fourier Transform — Raman spectra suggest that the 500 °C char showed very different structural features after pyrolysis and during oxidation from the 700 and 900 °C chars, while the differences between the latter two chars were small. Preferential consumption by O 2 of smaller aromatic rings and structures of somewhat aliphatic characteristics left the char more enriched with larger aromatic ring systems. The changes in char structure are in agreement with the observed reactivity measured in O 2 in a thermogravimetric analyser.

Drastic changes in biomass char structure and reactivity upon contact with steam

An Australian cane trash biomass was pyrolysed by heating at a slow heating rate to 700–900 °C in an inert gas atmosphere. The chars were then gasified in situ with steam. Our results indicate that the gasification of char with steam, even only for 20 s when the char conversion was minimal, resulted in drastic reduction in the intrinsic reactivity of char with air at 400 °C. The decreases in the char reactivity were not mainly due to the possible volatilisation of inherent catalysts during gasification in steam. Instead, the FT-Raman spectroscopy of the chars showed that the gasification of char with steam resulted in drastic changes in char structure including the transformation of smaller ring systems (3–5 fused rings) to large ring systems (⩾6 fused rings). It is believed that the intermediates of char–steam reactions, especially H, penetrated deep into the char matrix to induce the ring condensation reactions.

FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal

FT-Raman spectroscopy with a 1064 nm laser was used to investigate chemical structural changes of char during the pyrolysis of Victorian Loy Yang brown coal samples. The chars were diluted with KBr in order to record Raman spectra with acceptable quality. The interpretation of the Raman spectral data for these highly disordered and heterogeneous chars differs distinctly from that for the highly condensed/graphitised carbon materials. The FT-Raman spectra of chars in this study over the range of 800–1800 cm −1 were curve-fitted with 10 bands representing major structures in the chars. This has given information about the size of aromatic rings and the nature of substitutional groups and cross-links in char. The observed Raman intensity of a char is governed by its Raman scattering ability and its light absorptivity for both excitation laser and Raman scattering. The overall Raman intensity (peak area) as well as the ratios among the intensities of some major Raman bands has allowed some semi-quantitative evaluation of changes in char structure with increasing temperature during pyrolysis. The presence of ion-exchangeable Na and Ca in brown coal greatly affects the char-forming reactions during pyrolysis.

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.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam

The purpose of this study is to investigate the major factors influencing the Na-catalysed and non-catalysed gasification reactivity of a Victorian brown coal in steam. An acid-washed (H-form) sample and a Na-exchanged (Na-form) sample prepared from the same Loy Yang brown coal were gasified in 15% steam in a novel two-stage fluidised-bed/fixed-bed reactor. All C-containing species in the gasification product gas were converted into CO 2 that was monitored with a mass spectrometer continuously to determine the in situ gasification reactivity. While the volatile-char interactions were responsible for the volatilisation of Na when the coal was continuously fed into the reactor, the physical entrainment by gas of agglomerated Na-containing crystalline species (likely to be Na 2CO 3 or Na 2O) from char surface was the main mechanism for the loss of Na during char gasification. The Raman spectroscopy of char showed the preferential release of smaller aromatic ring system to be more significant during the non-catalysed char gasification than the Na-catalysed gasification. The dispersion of Na in char appeared to deteriorate with the enrichment of large aromatic ring systems in char, greatly affecting the char gasification reactivity. The char gasification reactivity showed a maximum with increasing conversion with the maximum to shift towards lower conversion with increasing temperature. Increasing temperature does not always lead to increases in the in situ char gasification reactivity.

Effects of volatile–char interactions on the reactivity of chars from NaCl-loaded Loy Yang brown coal

In a fluidised-bed gasifier, char particles are in constant contact with the volatiles and the products from the gasification and thermal cracking of volatiles. The highly reactive nature of volatiles as well as the vulnerable structure of char from brown coal means that there are strong interactions between volatiles and char. The purpose of this study is to investigate the effects of volatile–char interactions on the reactivities of chars from a Victorian brown coal. NaCl-loaded and acid-washed Loy Yang brown coal samples were pyrolysed in a novel fluidised-bed/fixed-bed reactor at 900 °C that has been specially designed to investigate the volatile–char interactions. Char reactivity in air was measured in a thermogravimetric analyser (TGA) under conditions minimising mass transfer limitations. The oxidation of char with air in the TGA showed apparent kinetic compensation effects between the apparent activation energies and pre-exponential factors. The presence of the apparent kinetic compensation effect is a reflection of the heterogeneous structure of char having sites of a wide range of energy levels. Our results have clearly shown that volatile–char interactions can lead to drastic decreases in char reactivity due to the volatilisation of Na and the changes in char structure. The reactivities of chars from the pyrolysis of the catalyst-free H-form Loy Yang brown coal provided unequivocal evidence for the changes in char structure after volatile–char interactions. For chars from the NaCl-loaded Loy Yang brown coal, it appears that the condensation of ring structures in char as a result of volatile–char interactions could have also led to changes in the dispersion of Na catalyst.

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.

Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion

Char fragmentation during pulverized coal combustion was studied using an Australian bituminous coal. The coal was combusted with air in a drop tube furnace at a gas temperature of 1300°C. The char samples were collected at different levels of char burnout, and their structure was examined using scanning electron microscopy. Approximately 40% of the char particles formed after pyrolysis were cenospheres with a highly non-uniform porous structure and a large central void. A large number of fine particles were also observed in the char samples with burnout levels between 30 and 50 vol%, which suggests that significant fragmentation occurs during the early combustion stage. A mathematical model was developed relating the fragmentation of cenospherical char particles with the macropores in the particle shell. The formation of these macropores partially results because of the carbon removal from the surface of the thin shell due to surface oxidation. A percolation model was used to simulate the char structural changes during combustion in regime III, and the predicted particle size distributions qualitatively agreed with the experimental measurements.