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The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO

During the lifetime of coal and biomass particles in a reactor, severe changes in the carbonaceous structure occur. In the early stages of heat treatment, transformations at both the structural and...

A coal char has been oxidized isothermally at temperatures comprised between 300 and 1073 K. The pre-oxidized chars have been subjected to Temperature Programmed Desorption (TPD) and to core-level high-resolution X-ray photoelectron spectroscopy (XPS) analysis using Synchrotron radiation to infer the nature of the carbon oxides that populate the surface and their evolution throughout thermochemical processing. For low oxygen coverages and mild oxidation temperatures the prevailing carbon-oxygen moieties are epoxy. Raising the oxidation temperature up to ~723K the edge carbon oxygen complexes (ether-hydroxyl and carbonyl-carboxyl) increase. The amounts of CO + CO2 desorbed during TPD also increase with temperature and duration of oxidation for relatively mild oxidative treatments (temperature below ~723K). Upon further increase of the oxidation temperature the amount of CO + CO2 decrease and the ratio of CO/CO2 increases remarkably. Altogether, results suggest the existence of a strong link between a remarkable shift of surface oxides from epoxy to ether/carbonyl and the desorption of CO and CO2. Moreover, the CO/CO2 ratio during desorption can be well correlated with the relative abundance and stability of epoxy moieties with respect to the “edge” oxides. Results are analyzed in the frame of a semi-lumped kinetic model of carbon oxidation with a focus on the role and nature of surface oxides as intermediates in carbon gasification reactions.

The present study investigates the influence of pressure on the products of fast pyrolysis of a lignocellulosic biomass (Walnut Shells). Experiments were carried out using a special heated strip reactor (HSR) at temperature of 1573 and 2073 K, pressure up to 8 bar and heating rate of 104 K/s. Two atmospheres have been investigated: N2 and CO2. Increasing the pressure of heat treatment, the combustion pattern and the average combustion reactivity of the chars change remarkably. Multiple components with different combustion reactivity have been found in the char samples. Upon heat treatment in N2 the most reactive components are depleted more easily than upon heat treatment in CO2. Consequently, thermodeactivation is more severe in N2 than in CO2. Raman analysis of the chars shows that graphitic order develops only upon heat treatment at 2073 K at the pressure of 2 bar, but not upon heat treatment at 1573 K, not even at the higher pressure investigated (4 and 8 bar). The loss of reactivity induced by pressure is therefore ascribed to changes in the internal distribution of char components. The evolution of the solid components is accompanied by changes in the distribution of aliphatics/methoxy aromatic compounds in the produced tar. The interplay of temperature, pressure, and CO2 affects both mineral matter and the carbon matrix resulting into a rather complex behaviour.

The work reports preliminary results on the morphological changes that solid particles of different type and origin experience upon very fast heating in a heated strip reactor at T=1000-1600°C under inert atmosphere. Samples included a Colombian Coal, Naphthalene Pitch, a Paraffinic Wax, a natural lignocellulosic Biomass as well as biomass components, namely Cellulose, Hemicellulose (Xylan) and Lignin. During the heat up phase Biomass, Coal and Lignin particles mainly retain the original morphology and size. Wax, Naphthalene Pitch, Hemicellulose and Cellulose particles cluster or melt. Naphthalene Pitch forms columns of some millimeter height.

A novel experimental method based on periodic swinging of oxidation/desorption reaction stages is used to investigate the formation and release of surface oxides on carbon under mild oxidation conditions. The chemical nature of the carbon-oxygen complex and the key mechanistic features of the interaction between solid carbon and oxygen are assessed by continuous monitoring of CO/CO2 release during the experiments and by probing - via XPS - the chemical nature of oxygen moieties at different stages of the experiments. Sub-bituminous char is used as carbon substrate. The proposed technique turns out to be a simple though effective method to assess the nature and extent of surface oxides formed under different reaction conditions. Results show that oxygen is extensively chemisorbed on carbon as epoxy moieties below ~750 K. At higher temperatures, isomerization into “edge” oxides (ether-hydroxyl and carbonyl-carboxyl functionalities) takes place, followed by oxide decomposition and desorption as CO and CO2. Estimates of the oxygen chemisorption rate as a function of temperature are given. The study provides new insights into the extent and chemical nature of surface oxides on carbons, that represent one key to their successful application in several areas of energy conversion and storage. Oxides affect surface physico-chemical (polarity, wettability) and electrical (capacitance, resistivity) properties, relevant to application of carbons in batteries, supercapacitors and fuel cells. Moreover, understanding surface oxides on carbon is helpful in designing novel concepts of energy conversion from carbon through chemical looping, transient operation of stationary combustors, synthesis of carbon-based catalysts, beneficiation of ash.

A laminar drop tube reactor (DTR) was used to perform fast pyrolysis of walnut shells, a ligno-cellulosic biomass sample, in nitrogen and carbon dioxide atmospheres. The DTR reached the temperature of 1300 °C and the heating rate of 104–105 °C/s. Char samples collected at different residence times along the reactor were characterized by ultimate and proximate analysis and by SEM. Char combustion reactivity was then measured by non-isothermal thermogravimetric analysis (TGA) in air. The analyses show that at residence times of 66 ms pyrolysis in N2 is not complete, whereas it is complete in CO2. For residence times of 115 ms the differences between samples produced in N2 and CO2 atmospheres level off. The derivative thermogravimetric (DTG) curves of the char combustion show the existence of multiple peaks. Notably, early combustion peaks progressively fade in the chars collected at increasing reactor residence time, confirming the completion of pyrolysis. A kinetic model of char combustion is proposed which includes multiple parallel reactions.

Previous studies have shown that increased carbon dioxide concentrations upon heat up affects the products of coal pyrolysis and in particular that chars prepared under carbon dioxide rich atmospheres are less reactive than chars prepared in nitrogen, and consistently tars are more aromatic [1-4]. In the present work, this issue is investigated with reference to a biomass sample, namely walnut shells (WS), where the lignin component prevails over cellulose and hemicellulose. Preliminary experiments of thermal degradation have been carried out using a thermogravimetric (TG) apparatus, under constant heating rate conditions, in flows of either nitrogen or carbon dioxide. Derivative thermogravimetric (DTG) curves reveal the existence of multiple peaks, which are typically associated with the degradation of different lignocellulosic components. A multiple parallel reaction scheme has therefore been used to fit the experimental data and kinetic parameters have been obtained. Walnut shells were also pyrolyzed in a fixed bed reactor at 600 °C in either nitrogen or carbon dioxide so as to collect pyrolysis products in amounts sufficient for further analysis. Char and tar samples have been characterized using different techniques (e.g. GC-MS, elemental analyzer, TGA, SEM) revealing limited differences. Combustion rate of the chars has been measured by means of non-isothermal thermogravimetric experiments in air and again small differences have been observed between the samples prepared under carbon dioxide and nitrogen. It has been concluded that under the low heating rate conditions typical of the thermogravimetric apparatus and fixed bed reactor used in the work, the effect of carbon dioxide on liquid and solid products of biomass pyrolysis exist, but are less important than for coal. The work is complemented by a companion paper [5], which investigates the effect of carbon dioxide on biomass pyrolysis under high temperature and fast heating rate conditions typical of a drop tube reactor.

In a previous work [Heuer S et al. Fuel Process Technol 2016;150:41-9] a large production of a fluffy carbon-rich material was observed to accompany the char formed during the early stages of a medium rank (bituminous) coal pyrolysis carried out in a drop tube furnace, (1573 K, residence times < 130 ms). This peculiar material was found to be much more abundantly formed in CO2 than in N2 flow. SEM analysis showed that it contains a large portion of submicron soot-like particles mixed with particles of tenths of microns in size with the typical char morphology. The present work reports on the separation of the two differently sized fractions produced in CO2 and N2 flow and their subsequent analysis. The separation was performed dispersing the material in ethanol by ultrasonic mixing, followed by settling, and decanting to produce top and bottom products enriched in the fine and coarse particle fractions, respectively. The procedure was repeated several times and the size separation effectiveness was checked by SEM and laser granulometry sizing. Thermogravimetry, elemental and spectroscopic analysis were applied to the coarse and fine fractions to provide insights on their structural features. The fine soot particulate was almost ash- free suggesting that its formation occurs in the gas phase, as typically soot does, while the coarse fraction presented significant residues of coal inorganic matter typical of char. Both fine and coarse particulate resulted less reactive, and somewhat smaller in size, when produced in CO2 in comparison to N2/Ar pyrolysis conditions. Their lower reactivity is associated with higher aromaticity and structural order as well as with a lower presence of hydrogen and aliphatic functionalities.

The use of biomass templating materials with a cheap production method as an enhanced sorbent for CO2 uptake has been proposed recently. However, the attrition and fragmentation behaviour of this type of material, which is a vital parameter for calcium looping sorbents, has not yet been investigated in detail. In this work the fragmentation behaviour of biomass-templated sorbents is investigated. Three types of materials were prepared using a mechanical pelletiser: 1. lime and cement (LC); 2. lime and flour (LF); and 3. lime, cement and flour (LCF). These samples were heat treated in a pressurised heated strip reactor (PHSR) and in a bubbling fluidised bed (BFB) and changes in particle size distribution were measured to assess fragmentation. Results indicated that the addition of biomass enhances the propensity to undergo fragmentation. Upon heat treatment in the PHSR the particle size of LC was not modified significantly; on the contrary the mean particle diameter of LF decreased from 520 mu m to 116 mu m and that of LCF from 524 mu m to 290 mu m. Fragmentation tests in the BFB confirmed the trend: 67% of the particles of LF fragmented, against 53% of LCF and 18% of LC samples. The addition of cement to the LF samples partially counteracts this performance degradation with respect to attrition. However, calcium aluminate pellets (LC) showed the lowest rate of fragmentation amongst all of the samples tested. (C) 2016 Published by Elsevier Ltd.

In the present work fast pyrolysis of coal in N2 and CO2 atmospheres was studied in a drop tube reactor (DTR) and in a heated strip reactor (HSR). In the DTR the volatiles generated by coal pyrolysis were entrained in a hot gas stream and were collected at the reactor outlet by tar traps. In the HSR, the volatiles were ejected from the hot coal particles into a cool environment and the condensable species, including primary tar, deposited and/or condensed on a glass bridge located above the heated strip. The composition of tars produced in the two reactors was compared to study the role of gas tar reactions in soot inception, and reference compounds for each class of tar species produced were identified. In the DTR the formation and growth of polycyclic aromatic hydrocarbons (PAH) were found higher than in the HSR. Soot formation occurred only in the DTR, being negligible in the HSR. It was concluded that the hot gas environment of the DTR favours secondary tar reactions, formation of PAH and eventually soot, while in the HSR this path was prevented due to prompt cooling down of volatiles. The presence of large concentration of CO2 in the pyrolysis atmospheres further promoted formation of heavy PAH and soot in the DTR, but not in the HSR, where the cooler environment limits soot-CO2 reactions in the gas phase.

The paper explores the primary products from fast pyrolysis of biomass components: Lignin, Cellulose and Hemicellulose (Xylan). A heated strip reactor is employed at temperatures of 1573 K and 2073 K with N2 and CO2 atmospheres. Volatiles quench immediately after volatilization on a cold pyrex bridge, while char remains on the heated strip for 3 s. Tar, soot and char are collected and subject to chemical treatments and analyses, including gas chromatography-mass spectrometry and Size Exclusion Chromatography, Thermogravimetric analysis, Raman spectroscopy and Scanning Electron Microscopy. Fast pyrolysis of Lignin produces “Light tar” (soluble in acetone) and “Heavy tar” (soluble in NMP), char, a minor fraction of soot. The “Light tar” contains Vanillin, which can be considered the main primary depolymerization product, but also aliphatics and PAHs. Higher temperature enhances “Heavy tar” and graphitization of the char. Cellulose at 1573 K produces only “Light tar”, largely made of Levoglucosan, as the result of depolymerization. At higher temperature the tar becomes heavier. Hemicellulose has a peculiar behavior: it produces a “Light tar” which is chemically similar to that of Cellulose and, at high temperature also “Heavy tar”. Hemicellulose pyrolysis results also in the production of an atypical solid residue: swollen ad spongy at lower temperature, bright and glassy at higher temperature. CO2 affects the pyrolysis products, particularly those of Lignin, promoting tar cracking and oxygenation already at the stage of primary pyrolysis and hindering thermal annealing and structural ordering of the solid carbonaceous structure.

A drop tube reactor with high heating rates typical of pulverized boilers (>104 K/s) has been used to carry out experiments with coal in different atmospheres: N2, CO2, O2/N2 and O2/CO2. The reactor wall temperature was set at 1573 K and the particles’ residence time was kept below 130 ms. In O2/N2 and O2/CO2 atmospheres coal pyrolysis was complete and additional char conversion occurred. The degree of char conversion increased with oxygen concentration values but was further enhanced by the presence of carbon dioxide, suggesting a positive contribution of CO2 to the overall rate of conversion. Chemico-physical and structural analysis of chars revealed internal burning under regime II conditions and highlighted that the presence of CO2 favors the formation of lactones in the chars. In N2 and CO2 atmospheres the pyrolysis stage was completed, but char conversion was negligible. The combustion stage of the N2 and CO2 chars was investigated in a second stage by thermogravimetric (TG) analysis (in regime I conditions) and in a flat flame burner (in regime II conditions) to separate atmospheric effects on char formation from those on char combustion. In TG, the CO2 chars resulted to be less reactive then the N2 chars, but in the flat flame burner, the experimental rate of carbon conversion of the N2 char and the CO2 char were similar. The TG results were worked out to estimate the intrinsic kinetics of the N2 and CO2 chars towards oxygen, carbon dioxide and O2/CO2 mixtures. Kinetic rate expressions were extrapolated to regime II conditions after consideration of mass transfer limitations. Notably, the kinetic model developed for the CO2-char matched the observed rate of char (oxy-) combustion well, whereas the kinetic model of the N2-char overpredicted the reaction rate.

The paper explores changes in reactivity and chemico-physical characteristics of char and tar produced by severe heat treatment of lignite in both inert atmospheres and CO2 rich atmospheres. The role of mineral matter, in particular metal oxides, in catalysing chemical and physical transformations is also addressed. A Rhenish Lignite from the Garzweiler mine was studied and compared with: a) mineral-free synthetic carbon (HTC), obtained from cellulose; b) a synthetic carbon doped with iron oxide (Fe2O3). A heated strip reactor (HSR) was employed at temperatures of 1300 and 1800 °C in N2 and CO2 atmospheres. Liquid and solid products (tar and char) were analysed and compared. Tar composition was evaluated by extraction and gas chromatography-mass spectrometry, whereas the solid carbonaceous material produced by pyrolysis, mainly composed of char, was characterized regarding its thermal behaviour by thermogravimetric analysis and its structure by Raman spectroscopy and scanning electron microscopy. Results show that iron oxide exerts a catalytic influence on both pyrolysis and char oxidation. Upon severe heat treatment, it reduces char reactivity promoting graphitization and structural ordering. The overall effect on char reactivity is therefore not easy to predict.

The present work compares the primary tars produced by fast pyrolysis in nitrogen (N2) and carbon dioxide (CO2) atmospheres of coal (bituminous Colombian Coal), raw biomasses (Straw and Walnut Shells) and biomass macromolecules (Cellulose, Hemicellulose and Lignin). Primary tars have been obtained in a heated strip reactor (HSR). Particles are heated up to temperatures of 1573 and 2073 K with a heating rate of 104K/s, meanwhile the gas in the reactor is kept at near ambient temperature. Therefore, volatiles ejected from the hot particles are quenched immediately after volatilization, minimizing secondary reactions. Chemico-physical analyses as Gas Chromatography-Mass Spectrometry (GC-MS), Size Exclusion Chromatography (SEC) and Ultraviolet-visible spectroscopy (UV-Vis) have been performed to detect the compounds present in primary tars. Anhydrosugars (Levoglucosan) largely dominates the primary tars of Cellulose and Hemicellulose. Lignin primary tars are instead mainly composed of oxo-aromatics (Vanillin and Benzendiol). The chemical composition of tars from fast pyrolysis of two natural biomasses (Walnut Shells and Straw) cannot be obtained as linear combination of the chemical composition of the tars produced from pyrolysis of single lignocellulosic components. First of all, no anhydrosugars are present. Phenols and Benzofurans prevail in Straw tar, while Benzendiols and some compounds related to Lignin structure (Vanillin and Cinnamaldehyde) prevail in tars of Walnut Shells. At the higher temperature the results drastically change: oxo-aromatics are present in the form of Naphtalenols rather than Benzendiols and larger amounts of heavy polycyclic aromatic hydrocarbons PAHs (Indenopyrene and Benzofluorantene) appear. The primary tars of Coal pyrolysis are very different from those of biomass: heavy Aliphatics (C21-C30) are the most abundant class at 1573 K. At 2073 K large amounts of heavy PAHs (Benzophenanthrene and Benzopyrene) are produced. A table is provided with the indication of the main components of primary tars, which can be of interest for the development and validation of pyrolysis models for coal and biomass.

The high temperatures and heating rates typical of PF are known to induce thermal annealing of char and loss of its reactivity. Several authors investigated this effect for coals in inert atmospheres, while little is known about the effects of CO2-rich atmospheres, typical of oxy-combustion and gasification, on the course of thermal annealing. Thermal annealing of biomass has been scarcely investigated in the literature; however, available studies reported that also biomass can suffer from thermo-deactivation. The present study aims to provide further insight on thermal annealing of biomass in the context of gasification and oxy-combustion. A lignin-rich biomass (walnut shells) has been heat-treated in a heated strip reactor at temperatures of 1573-2073 K with a holding time of 3 s using atmospheres of either N2 or CO2. Similar experiments have been performed with a high volatile bituminous coal (Colombian coal) used as reference. The char samples have been analyzed by thermogravimetric analysis and Raman spectroscopy. Results have been further compared with those reported in previous studies where heat treatment of the same fuels were performed in fixed bed, fluidized bed, and drop tube reactors at lower temperature or shorter holding time. Two remarkable results have been obtained: (1) Loss of reactivity by thermal annealing and structural reorganization follow similar pathways for coal and biomass. (2) The effect of CO2 on pyrolysis and thermal annealing is non-monotonic along with heat treatment: in the early instances of heat treatment (T = 1573 K, t < 0.1 s), CO2 fosters pyrolysis and thermal annealing, increasing structural ordering. At longer holding times (T > 1573 K, t > 1 s), instead, CO2 somewhat hampers thermal annealing.

The mechanism of carbon oxidation by oxygen has been scrutinized by means of dynamic experiments consisting of cyclic oxidation of solid carbon at low-moderate temperature (200-300°C) followed by desorption of surface oxides at high temperature (700-800°C). The rapid shift between oxidation and desorption conditions is accomplished thanks to a purposely designed reactor consisting of a dual fluidized bed configuration equipped with a rapid transfer line that enables fast pneumatic conveying of the solid sample from one reactor to the other while keeping distinct reaction conditions in the two reactors. Results are analysed with a specific focus on the role and nature of surface oxides as intermediates in carbon gasification. The existence of surface oxides of different chemical nature and stability provides the starting point to elaborate a tentative mechanistic frame of the experiment. The key features of the phenomenology, namely the oxidation rate and the CO/CO2 ratio in the evolved gasification products, are interpreted in the light of the relative abundance and stability of "edge" oxides and epoxy moieties generated by carbon oxidation

Char emissivity of burning particles is an important factor for heat transfer calculations in pulverized fuel boilers. As the chemical composition is known to influence the emissivity in general, a coal sample has been prepared by a leaching method to reduce the mineral content. A flat flame burner was used for the combustion of the particles in oxyfuel atmosphere, providing boundary conditions comparable to pulverized coal applications. The burnout-dependent emissivity of the sample was measured in a defined spectral range and compared with data for an unleached sample of the same coal, indicating that the mineral content has minor effect for the investigated conversion levels, although clear changes in the emissivity show that conversion in general is not negligible.

The title compound, bis(4-butyloxyacetophenon)azine, exhibits a rich phase behavior. It has two different crystal phases and one liquid crystal phase before transition to the isotropic liquid phase. The liquid crystalline phase is nematic. All the phase transitions of the compound are reversible. In particular, the solid–solid transition at 83 °C is single-crystal-to-single-crystal as proven by optical and electron microscopy and X-ray diffraction analysis and it shows a remarkable degree of reversibility; single crystals can undergo several cycles of transition between the two phases without any damage. The crystal phase stable at lower temperature has been fully characterized by single crystal X-ray analysis. It shows an arrangement of the molecules in layers in the plane (a, b), with the layers piled up along c without interdigitation of the alkyl tails of molecules belonging to consecutive layers. The crystal phase stable at higher temperature is disordered.

In oxy-combustion, coal particles undergo devolatilization in CO 2 enriched atmospheres. Besides the well-known influence of thermal conditions, the composition of the pyrolysis atmosphere may also have important effects on the formation and properties of pyrolysis products. In an international collaboration, researchers from three institutions from Aachen, Bochum and Naples carried out pyrolysis experiments with a medium rank coal in a fixed bed, fluidized bed and drop tube reactor, substituting N 2 with CO 2 . The goal of the current study was to investigate the influence of increased CO 2 concentrations on the pyrolysis products (tar, gas and solids) when different heating rates, temperature and residence times are applied. Pyrolysis products were analyzed by several techniques to highlight differences in structure and chemical composition. At low heating rates and temperature, the differences between N 2 and CO 2 pyrolysis products were marginal. A CO 2 rich atmosphere, instead, impacted severely the properties of pyrolysis products under the fast heating-short residence time conditions typical of drop tube reactors. Upon prolonged exposure to severe treatment differences apparently leveled off.

Semicarbazides of p-substituted benzoic acids are a class of simple molecules endowed with H-bonding donor and acceptor groups capable of forming H-bonded rows along three linearly independent directions (orthogonal H-bonding synthons). When the acceptor group at the para position is strong (–NO2, –CN, –N=) the same crystal packing is observed, in which three linearly independent chains are formed, and the lattice parameters are easily predictable. In the case of halogen atoms at the para position (–F, –Cl) two different packings are observed in which only one or two H-bonded chains are present. Through ab initio periodic DFT-LCAO computations, we have calculated the lattice energy and density of each semicarbazide in each of the three packings, constructing the matrices Uij and ρij of the lattice energy and density. It is found that all of the different modes of packing observed experimentally for single members of the class correspond to minima of the lattice energy and to acceptable lattice densities for every member of the class.