Changes In Char Structure Research Articles

Investigations on NO reduction with biomass char: Char structural changes during the heat treatment in N2 and subsequent NO/O2 gasification

NO reduction with biomass char was studied from considering its structural properties. Raman, XPS and XRD were employed to characterize the char structure. Three types of biomass chars were used for NO reduction at a fixed bed reactor in NO/O2 at 900 ℃. The results showed that the heat treatment led to an obvious reordering of structures in all chars. There were significant differences between NO and O2 reaction in terms of the char structural changes in aromatic systems, surface groups and crystalline developments. New O-containing groups were formed and could be present as C-O and –COO groups in both the NO and O2 reactions. Two different pathways for char-NO and char-O2 reaction were given, respectively. It is proposed that the reaction channel of NO would be blocked as carbon molecular was firstly surrounded by O2 during NO/O2 reaction. The denitration performance of biomass char should be linked to the structural features of char-HT and the behaviors of char structural changes in the subsequent gasification in NO/O2.

Effect of cellulose–lignin interactions on char structural changes during fast pyrolysis at 100–350 °C

This study investigates the cellulose–lignin interactions during fast pyrolysis at 100–350 °C for better understanding fundamental pyrolysis mechanism of lignocellulosic biomass. The results show that co-pyrolysis of cellulose and lignin (with a mass ratio of 1:1) at temperatures < 300 °C leads to a char yield lower than the calculated char yield based on the addition of individual cellulose and lignin pyrolysis. The difference between the experimental and calculated char yields increases with temperature, from ∼2% 150 °C to ∼6% at 250 °C. Such differences in char yields provide direct evidences on the existence of cellulose–lignin interactions during co-pyrolysis of cellulose and lignin. At temperatures below 300 °C, the reductions in both lignin functional groups and sugar structures within the char indicate that co-pyrolysis of cellulose and lignin enhances the release of volatiles from both cellulose and lignin. Such an observation could be attributed to two possible reasons: (1) the stabilization of lignin-derived reactive species by cellulose-derived reaction intermediates as hydrogen donors, and (2) the thermal ejection of cellulose-derived species due to micro-explosion of liquid intermediates from lignin. In contrast, at temperatures ≥ 300 °C, co-pyrolysis of cellulose and lignin increases char yields, i.e., with the difference between the experimental and calculated char yields increasing from ∼1% at 300 °C to ∼8% at 350 °C. The results indicate that the cellulose-derived volatiles are difficult to diffuse through the lignin-derived liquid intermediates into the vapor phase, leading to increased char formation from co-pyrolysis of cellulose and lignin as temperature increases. Such an observation is further supported by the increased retention of cellulose functional groups in the char from co-pyrolysis of cellulose and lignin.

Brown coal char CO2‐gasification kinetics with respect to the char structure part II: Kinetics and correlations

AbstractCoal's physical structure is known to affect high‐temperature coal conversion processes such as gasification or combustion. This paper is Part II of the study of brown coal char structural changes during gasification. Firstly, German Lusatian brown coal was gasified in a laboratory‐scale fluidized bed reactor in CO2 within a temperature range of 800–950 °C at atmospheric pressure. Then, physical structure properties were extensively evaluated by means of various techniques. Char specific surface area and its changes during gasification reaction were evaluated using N2 and CO2 physical adsorption techniques. Adsorption isotherms were also interpreted employing unconventional methods in order to obtain the specific surface areas of pores of different sizes. Gasification kinetics were evaluated employing three widely applied kinetic models: the random pore model (RPM), the volume reaction model (VM), and the shrinking core model (SCM). Finally, the instantaneous gasification reaction rate was correlated with the char structural properties at the corresponding conversion degrees. The closest linear correlation appeared between the gasification reaction rate and the specific surface area of mesopores (as determined by N2 adsorption). Furthermore, correlations of the other structural properties with char conversion are provided. Observed structural changes were compared with the assumptions of the kinetic models.

The effect of carbon dioxide partial pressure on the gasification rate and pore development of Highveld coal chars at elevated pressures

Char structural changes occurring during gasification are normally only related to conversion and few data is published on the effect of carbon dioxide partial pressure on the extent of pore development. In this study, the char–CO2 reactivity and consequential pore development of different Highveld coal chars were investigated over a wide pressure range. Reactivity experiments were performed in a fixed bed reactor under reaction controlled conditions. The initial reaction rate was found to be solely a function of temperature and CO2 partial pressure and was well described by the Langmuir Hinshelwood rate equation.Surface analyses were performed on fresh and partially converted chars to quantify the effect of CO2 partial pressure on pore development. It was found that pores development more rapidly with conversion if the CO2 partial pressure is increased. This outcome has fundamental implications for the interpretations of gasification reaction kinetics. The term describing the total amount of active sites in the Langmuir Hinshelwood equation might not be constant at all partial pressures, and the possible impact of that is shown in this work. More work is required to further understand these effects and incorporate them appropriately into high pressure rate equations.

Experimental investigations on lignite char gasification kinetics using a pressurized drop tube reactor

A novel pressurized drop tube test facility, named KIVAN, for the investigation of heterogeneous gasification reactions under elevated pressures is presented. Facility configuration and possibilities of experimental test procedures are explained. The gasification behavior of a lignite char is investigated at a pressure of 21 bar. The effects of gasification temperature (1173–1323 K) and partial pressure of CO2 (5.25–15.75 bar) on the carbon conversion are determined. The measured temperature and concentration profiles inside the reactor, the settling velocity of the particles and the flow conditions along the reaction tube are used to determine kinetic parameters of the Boudouard reaction. Data on the char structural changes depending on the carbon conversion is provided and it is suitable for detailed numerical modeling of gasification processes. The determined kinetic parameters of the Boudouard reaction under these conditions are: activation energy 162 kJ·mol−1, reaction order 0.36 and pre-exponential factor 3.26·105 s−1 bar−036.

Changes in char structure during the low-temperature pyrolysis in N2 and subsequent gasification in air of Loy Yang brown coal char

This study aims to investigate the changes in char structure during the pyrolysis in N2 and subsequent gasification in air at low temperature in a thermogravimetric analyser. The chars produced from the gasification of Loy Yang brown coal in 15% H2O balanced with CO2 at 850 °C in a fluidised-bed/fixed-bed reactor were used in this work. The evolution of char structure during pyrolysis and in-situ gasification was characterised by FT-Raman spectroscopy. The char-O2 specific reactivity was measured as well. Our results show that, during further pyrolysis, the loss of O-containing structures led to the reduced observed Raman intensity from large aromatic rings and the formation of new cross-linking structures. During the subsequent gasification in air, the newly formed O-containing structures were more readily incorporated into less gasified char (0 min holding at 850 °C), but never reaching the same oxygenation level as those in the chars that had undergone extensive gasification (longer holding at 850 °C). The structural feature of the starting char (obtained at 850 °C) is the main factor determining the reaction pathway for the small/large aromatic ring systems during the low-temperature pyrolysis and in-situ gasification in air.

Changes in char structure due to inorganic matters during anthracite pyrolysis

To understand effects of inherent mineral matters on the evolution of char structure, the microcrystalline structures of chars from the pyrolysis of raw, HCl-washed (CW) and HCl&HF-washed anthracites (CFW) were investigated by X-ray diffraction (XRD) respectively. Stacking height (Lc), interlayer spacing (d002) and crystallite width (La) were used as parameters for char structure estimation. The results showed that with increasing contents of inherent mineral matters, Lc of char decreased, whereas d002 and La increased. To study effects of a single inorganic matter, CFW was impregnated with Al2O3, SiO2, and CaO respectively. It was found that all of them (oxides) have resulted in the decrease in Lc and increase in La and d002. Differing from Al2O3, SiO2, and CaO, the introduction of Fe2O3 has caused the reduction for both La and Lc values simultaneously, especially when the Fe2O3 contents in the coal were high.

The role of gasification reactions during pulverized solid fuel combustion: A detailed char combustion model based on measurements of char structure and kinetics for coal and pre-treated biomass

A detailed char combustion model including gasification reactions with emphasis on char structural evolution is developed and validated using combustion experiments from a lab-scale entrained flow reactor burning a bituminous coal and torriefied wood. The char combustion model is a single film model based on nth order Arrhenius type equations for char reactions. Diffusion through the particle boundary layer and inside the particle interior is considered by an effectiveness factor approach. Char reactivity towards O2, H2O and CO2 is measured for five different char samples from four different parent fuels, being two bituminous coals and two pre-treated biomasses. Char samples are generated at high heating rates in an entrained flow reactor. Reaction rates and intrinsic reactivity values are quantified using a thermo-gravimetric analyzer. Reaction rates for pre-treated biomass samples are one to two orders of magnitude higher compared with the coal samples. The oxygen reaction is around four to five orders of magnitude faster than gasification reactions in the chemically-controlled reaction regime. The char combustion model is able to accurately describe char conversion and char structural changes. Model results reveal that there might be fragmentation effects in early stages of char conversion, presumably for cenospherical char particles. Particle reaction rates and reaction regimes are shown to vary strongly among different particle sizes and trajectories, and even for a single particle along its profile. Therefore, mean values calculated from CFD results have to be interpreted with care and can be misleading. A parameter study on temperature and stoichiometry shows the importance of gasification reactions. Depending on the conditions, gasification reactions are responsible for 2 up to 60 wt.-% of char conversion. They become relevant in regions of high temperatures and oxygen depletion, e.g. inside the flame. For typical furnace conditions of 1300 °C and a slightly over-stoichiometric flame, the Boudouard reaction accounted for approximately 10 wt.-% and the heterogeneous water-gas shift reaction for 4 wt.-% of char conversion. Therefore, gasification reactions cannot be neglected when predicting fuel conversion in PF systems, particularly in modern staged combustion systems with reducing furnace conditions.

Changes in char structure during the thermal treatment of nascent chars in N2 and subsequent gasification in O2

This study aims to investigate the structural features of nascent char. Two nascent chars were prepared from the pyrolysis of Loy Yang brown coal and Collie sub-bituminous coal in a wire-mesh reactor by heating at 1000Ks−1 to 600°C. Structural changes in nascent chars were investigated by reheating the chars in N2 at low temperatures (≤600°C) followed by subsequent gasification in O2/N2 mixtures at 370°C. FT-Raman spectroscopy was applied to characterise the chemical structural features of the chars. Our results indicate that the nascent char from the brown coal is much less stable that from the sub-bituminous coal. The thermal treatment of Loy Yang brown coal nascent char even at ∼300°C caused obvious changes to its aromatic ring systems while the corresponding changes to the Collie nascent char were insignificant. During the gasification of Loy Yang nascent char in O2, oxygenation might preferentially happen on the larger rings, which activates and breaks the aromatic rings (to form dangling structures). With the opening of aromatic ring systems, O2 might be much easier to access into the char matrix and to improve the dispersion of alkali and alkaline earth metallic (AAEM) species, which in turn enhances the gasification reactivity. Collie nascent char showed much less changes in char structure during gasification. Additionally, the preferential breakage of cross-linking structures in O2 would also loosen the char structure and enhance the gasification reactivity for both nascent chars.

Evolution of chars during slow pyrolysis of citrus waste

Conversion of agro-wastes into energy can be key to a circular-driven economy that could lead to models for sustainable production. Thermochemical processing is an interesting alternative for the upgrading of agro-wastes to energy. However, owing to the complex and largely unknown set of reactions occurring during thermal breakdown, to ensuring consistent quality of the final products is still a goal to achieve at industrial level. The present study investigates the evolution of solid products of pyrolysis, to gain some insights in these complexities. Chars derived from slow pyrolysis (200–650°C) of citrus pulp in a horizontal reactor have been characterized by means of Fourier Transform Infrared spectroscopy (FT-IR), X-Ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM). Results are discussed also in light of similarities with coal thermal breakdown.At temperatures below 300°C, changes in solid matrix are mainly due to breaking of aliphatic compounds. Significant changes in char structure and behavior then occur between 300°C and 500°C mainly related to secondary char-tar reactions. Above 500°C, changes appear to occur mainly due to recombination reactions within matrix, which thereby becomes progressively less reactive.

Effects of volatile–char interactions on char during pyrolysis of rice husk at mild temperatures

In order to understand the sensitivity of volatile–char interactions to mild temperatures (600–800°C), in-situ rice husk char was prepared from fast pyrolysis (>103Ks−1) on a fixed-bed reactor. Retention of K in char, changes in char structure and char reactivity were determined. The results showed that volatile–char interactions did not cause obvious effect on the char yield but showed an inhibitory effect on char reactivity. The inhibition began only above 650°C and intensified with temperature rise, but kept almost unchanged at 700–800°C. Char structure and retention of K have a combined effect on char reactivity. The decreased reactivity was caused by additional volatilization of K from char matrix and transformation of relatively smaller aromatic ring systems to large ring systems (>6 benzene rings) above 650°C.

Effects of oxygen on the structure and reactivity of char during steam gasification of Shengli brown coal

The effects of oxygen on changes in char structure and reactivity during steam gasification of Shengli brown coal were examined. A stainless steel simulated entrained-flow reactor was employed to carry out gasification at 800 and 900°C in 8 atmospheres: pure N2, 15%H2O, 25%H2O and 35%H2O balanced with N2; and 1%O2, 15%H2O+1%O2, 25%H2O+1%O2 and 35%H2O+1%O2 balanced with N2. The structure of chars was measured by Raman spectrum, and the intrinsic reactivity of chars with air was measured using a thermogravimetric analyzer (TGA) at low temperature (300°C). The results show that the effects of oxygen on char structure are different at high temperature (900°C) and low temperature (800°C) in brown coal steam gasification. The volatilization of alkali and alkaline earth metallic (AAEM) is strongly influenced by char structure. The presence of O2 during steam gasification has a similar effect on the volatilization of Na to the evolution of structure at different temperatures. Experimental data provided evidence that a mixed H2O/O2 atmosphere could promote a decrease in subsequent char reactivity in air at 300°C, and that char reactivity could correlate well with the band area ratio, I(GR+VL+VR)/ID.

Structural transformation of nascent char during the fast pyrolysis of mallee wood and low-rank coals

The changes in char structure during the fast pyrolysis of three different feedstocks from 600°C to 1200°C were investigated. Western Australian Collie sub-bituminous coal, Victorian Loy Yang brown coal and Australian mallee wood were pyrolysed in a wire-mesh reactor at a heating rate of 1000Ks−1 with holding time ranging from 0s to 50s. FT-Raman/IR spectroscopy was used to characterise the structural features of the chars obtained at different temperatures. The combined use of a wire-mesh reactor and an FT-Raman/IR spectrometer has provided significant insights into the rapid changes in the chemical structure of nascent char during fast pyrolysis. Our results indicate that the three fuels began significant ring condensation at different temperatures. Mallee wood showed significant growth of large rings within 1s holding at 600°C; however Loy Yang and Collie coals showed significant ring condensation at 800°C and 900°C respectively. With increase in temperature to 1000°C and higher, most ring condensation occurred at <1s holding time. The decrease in the intensity of FT-IR spectrum at ~1600cm−1 confirmed the rapid growth in ring systems at relatively high temperatures.

A preliminary Raman spectroscopic perspective for the roles of catalysts during char gasification

How a catalyst would speed up gasification is fundamentally important for the development of efficient gasification technologies. It is particularly crucial for the efficient use of catalytic species inherently present in low-rank coal and biomass. This study aims to gain insights about the catalytic gasification reaction mechanisms through tracing the changes in char structure during gasification with FT-Raman spectroscopy. A Victorian brown coal was acid-washed to remove its inherent alkali and alkaline earth metallic species. The raw coal and the acid-washed coal were independently subjected to gasification at 800°C under different gasifying conditions (with oxidising and/or reducing atmosphere). Our FT-Raman results revealed that, regardless of the coal type, oxygenation that occurred during char gasification could increase the gasification rate. In addition, catalyst was shown to enhance the gasification rate by participating in the reaction between the char matrix and the radicals derived from the gasifying agents. The presence of a catalyst has made the gasification process to be less selective, due to both reduced selective consumption of char and reduced chance/time for ring condensation.

Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures

The gas release properties and changes in the char structure during lump coal pyrolysis were investigated in Ar and CO2 atmosphere. Fixed bed reactor and gas chromatography were employed to generate char at four pressures (0.1–1.5MPa in 0.5MPa increments) and measure gas composition from room temperature to 700°C in 50°C increments, respectively. The influences of pressure and atmosphere on char structural changes were examined by FT-IR and XRD. The results indicate that CO2 atmosphere is favorable to the release of H2O. The initial evolution temperature of gas species shows a notable dependency on atmosphere and pressure. Especially for CH4, CO2 atmosphere can promote its emission in advance and make the evolution profile subdued within 0.5MPa but hinder its evolution over the pressure range of 0.5MPa, and this inhibitive effect is more evident at a higher pressure. The chars prepared in CO2 atmosphere have more cycloalkane and aliphatic structures, and the role of atmosphere is particularly evident when the pressure exceeds 0.5MPa. There are more content of hydroxyl, secondary hydroxyl, olefinic CC bonds, aromatic CC structure left in chars prepared under CO2 atmosphere compared with those prepared under Ar atmosphere. The interplanar distance d002 indicates that Ar atmosphere is more helpful to the graphitization of char during pressurized coal pyrolysis, the stacking height Lc of char sample prepared in CO2 is always higher than that prepared in Ar atmosphere within 1.0MPa, CO2 atmosphere is beneficial to the growth of crystal layer in vertical position.

Inhibiting and other effects of hydrogen during gasification: Further insights from FT-Raman spectroscopy

Hydrogen as a gasification product gas is always present in a practical gasifier. Its inhibition of char gasification is always an important consideration in the design and operation of the gasifier. This study aims to provide further insights into the roles of hydrogen during gasification by characterising the chars at varying levels of conversion using FT-Raman spectroscopy. The char samples were prepared from the gasification of a Victorian brown coal at 800°C in 15% H2, 15% H2O or a mixture of 15% H2 and 15% H2O (all balanced with inert gas argon) in a fluidised-bed/fixed-bed reactor. In addition to its chemisorption on the char surface to reduce the availability of active sites for char to react with other gasifying agents such as H2O in this study, hydrogen also influenced the changes in char structure, including the oxygenation/deoxygenation of char. Despite the involvement of hydrogen in the gasification reactions, hydrogen has not caused permanent hydrogenation of char to form significant sp3-rich structures within the char.

Catalytic reforming of tar during gasification. Part IV. Changes in the structure of char in the char-supported iron catalyst during reforming

Our previous parts of this series have shown the char-supported iron catalysts to possess high activity for the reforming of tar during biomass gasification. This study aims to investigate the changes to the structure of char in the char-supported iron catalyst during the reforming of tar derived from the pyrolysis of mallee wood biomass. The char structure was characterised with Raman spectroscopy and its intrinsic reactivity at 400°C in a thermogravimetric analyser. The results showed that the Raman peak area in the range of 800–1800cm−1 of the catalysts changed slightly after being used for the steam reforming of biomass tar. The changes in the Raman band area ratio of ID/I(Gr+Vl+Vr) indicated that the relative ratio of larger and smaller aromatic ring systems increased after reforming at temperatures at 800°C or higher. The changes in the char-air reactivity before and after reforming provided further insights to the changes in char structure as well as the importance of alkali and alkaline earth metallic species deposited on the char-supported catalysts during reforming.

Evolution of char structure during steam gasification of the chars produced from rapid pyrolysis of rice husk

The structural evolution of rice husk char particles during steam gasification was studied by ultimate analysis, inductively coupled plasma atomic emission spectroscopy, scanning electronic microscope, N2 physisorption method and X-ray diffraction. Drastic changes in char structure occurred even when the char conversion was minimal. The C, H and O content decreased by more than 85%, 67% and 91%, respectively. The H/C atomic ratio always increased, while the O/C exhibited a “∼” shape. The maximum porosity appeared at char conversion of 48.6%. Ds and dave showed a good linear relationship. Aliphatic structures and smaller aromatic rings tended to be preferentially consumed to left the char more ordered and enriched with larger aromatic ring systems. The presence of steam favored the volatilization of Na. K, Mg and Ca showed a similar behavior and their concentrations reached maxima at 60.3%. Great loss of O-containing structures caused significant volatilization of K, Mg and Ca.

Changes in Char Structure during the Gasification of Mallee Wood: Effects of Particle Size and Steam Supply

The purpose of this study is to investigate the effects of the biomass particle size on the changes in char structure during the gasification in steam of mallee woody biomass. The experiments with continuous or sudden supply of steam were conducted using a fluidized-bed/fixed-bed quartz reactor. The structural features of chars were then characterized using Fourier transform (FT)-Raman spectroscopy. Our results indicate that the effects of the biomass particle size on char structure were minimal when the biomass was heated slowly while being in continuous contact with steam. On the contrary, when the biomass was first pyrolyzed in argon and then suddenly came into contact with steam, the biomass particle size could have a profound effect on the subsequent evolution of char structure during gasification. For the large biomass particles (>5 mm), the ratio between small and big aromatic ring systems in the char showed a maximum value after around 30 s of contact with steam.

Effects of gasifying agent on the evolution of char structure during the gasification of Victorian brown coal

Steam plays a vital role in the gasification process. This study aims to investigate the changes in char structure and reactivity during the gasification of Victorian brown coal. A Loy Yang brown coal sample was gasified at 800°C in a novel fluidised-bed/fixed-bed reactor in three different gasification atmospheres: 15% H2O balanced with argon, 4000ppm O2 balanced with CO2 and 4000ppm O2+15% H2O balanced with CO2. The intrinsic reactivities of chars with air were measured with a thermogravimetric analyser (TGA) at low temperatures (380 or 400°C). The char structural features were characterised using FT-Raman spectroscopy followed by spectral deconvolution. Our results indicate that steam, when it is present in the gasifying atmosphere, has a drastic effect on char structure and the subsequent reactivity of char with air at low temperatures. The presence of steam during the gasification at 800°C also impacts on the volatilisation of Mg and Ca by altering the char structure. Our data provide evidence that the char–H2O gasification follows a different reaction pathway from the char–CO2 gasification, at least for the gasification of Victorian brown coal under the current experimental conditions.