• Energy Research

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 condensed products of pyrolysis embrace a wide range of compounds, ranging from relatively low molecularweight (MW) hydrocarbons ("light tar") to progressively heavier species, including large PAHs ("heavy tar"),ending up with fine carbonaceous particles (soot) of nanometric and, in some cases, even micrometric size. Whilemost part of "light tar" from either coal or biomass pyrolysis can be effectively detected by conventional gaschromatography/mass spectrometry (GC/MS), a detailed analysis of "heavy tar" and soot turns out to be rathercumbersome and more susceptible of errors, especially when the techniques, originally developed for fossil fuels-derived products, are applied to the oxygen rich biomass products. Size exclusion chromatography (SEC) coupledwith UV-Visible (UV-Vis) absorption detector is applied to both coal and biomass "heavy tar", providing evidenceof high mass components reaching MWs of thousands of u, similar to incipient soot. Carbon particles reach themicrometric scale in some coal pyrolysis products, so that "ad hoc" solvent-based separation procedures havebeen developed in order to distinguish soot from char fines. In biomass pyrolysis products, carbon rich partic-ulate remains confined to the nano-size range. The latter exhibits fluorescence emission in the green wavelengthrange, with a relatively high quantum efficiency, and can therefore be regarded as carbon dots. Recent progressshows that the molecular mass and size of the heaviest species in biomass heavy pyrolysis products can beassessed by SEC with a fluorescence detector. However, the technique still requires refinement due to the lack ofSEC calibration data valid for all tar species. Laser desorption ionization time of flight mass spectrometry (LDITOFMS) turns out to be a valid support to evaluate the molecular range of such ill-defined products.

The present work focuses on the quality of char and primary tar produced from fast pyrolysis of lignocellulosic biomasses in both nitrogen (N2) and carbon dioxide (CO2) atmosphere. Three biomasses are investigated: Walnut Shells, the richest in Lignin, Straw, the richest in Hemicellulose, Pinewood, the richest in Cellulose. Heat treatment has been carried out in a heated strip reactor (HSR) at 1573 and 2073 K with holding times of 3 s and heating rate of 104 K/s. The equipment allows quenching the volatiles as soon as they are emitted from the particles and collecting them for further chemical analyses. The char samples are also collected and analysed by thermogravimetric analysis in air. Primary tar produced from pyrolysis of Walnut shells results to be the richest in monosaccharides among the samples investigated. Surprisingly monosaccharides are scarce in the tar produced from Straw, despite its high content of Hemicelluloses. PAHs and high molecular weight tar are present in the primary products of all the biomasses investigated, but particularly in tar from Walnut Shells, the biomass with the largest lignin fraction. The chars of the three investigated biomasses appear to be all constituted by multiple components. After heat treatment in N2 at 1573 K, the most reactive char is the one obtained from Straw, and the least reactive is the Walnut Shells char. More severe heat treatment and the presence of CO2 in the atmosphere generate additional char components with both higher and lower reactivity. Altogether the quality of the pyrolysis products does not correlate with the content of Cellulosde/Hemicellulose/Lignin of the original biomasses.

Fluorescent carbon was produced from fast pyrolysis of lignocellulosic biomass in a modified wire-mesh reactor named heated strip reactor (HSR). The metal grid, usually employed as a sample holder in a wire-mesh reactor, is replaced in the HSR by a pyrolytic graphite foil. HSR can achieve temperatures up to 2073 K with a high heating rate (104 K/s). The volatiles produced by the HSR pyrolysis of a lignocellulosic biomass sample were immediately quenched in the surrounding low temperature environment, so avoiding the occurrence of secondary reactions of the volatiles. Volatiles were condensed in form of a tar-like material on a pyrex glass bridge located above HSR, whereas the residue solid (char and soot) remained on the strip. The tar-like material recovered and separated with different solvents in fractions of various characteristics showed blue and/or green fluorescence typical of fluorescent carbon dots (CDs). It was thus shown that fast pyrolysis of carbon resources as biomasses, can be employed as one-step approach to synthesize different classes of carbon materials, assimilable to CDs. The different CDs could be separated and isolated choosing appropriate organic solvents and constitute very promising materials for applications in photonics, electro-optics, chemical sensing, and other material science areas.

This research study aims to investigate the causes of the particles emitted by a spark ignition engine fueled with hydrogen. The experiments were carried out on a single cylinder 250 cm3 direct injection spark ignition engine. Two operating conditions at 2000 rpm both full and low load, representative of typical urban conditions, were investigated. A physical characterization of the particles, size and number, was performed through an Engine Exhaust Particle Sizer coupled to a single diluter. Chemical characterization was carried out on the condensed exhaust. The simultaneous analysis of the physical properties and their chemical characterization allows to point out not only the role of the oil on the particle emissions but also to give an important information on its state/composition, if it was unburned and oxidized. Particles were detected with conventional spectrometer at low load while at high load the noise signal ratio is too high to distinguish the presence of particles. More detailed chemical techniques highlighted the presence of PAH, alkyl-PAHs, oxy-PAHs and unburned hydrocarbon in the exhaust due to the mineral oil.

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 1573 K, t > 1 s), instead, CO2 somewhat hampers thermal annealing.

In the present work fast pyrolysis of coal in N2 and CO2 atmospheres was studied using two different systems: a drop tube reactor (DTR) and a heated strip reactor (HSR). The two systems allow comparable particles heating rates with different temperature of the gas phase. The tars produced in the two systems have been analyzed with the aim to study the role of gas tar reactions in soot inception. It was found that in DTR, where volatiles react in a hot atmosphere, polycyclic aromatic hydrocarbons (PAH) formation and growth is larger than in the HSR, where the volatile species are quickly quenched. The results provide evidence that secondary reactions of the tar are a source of PAH, which in turn, can be soot precursors, even in pyrolytic conditions. Also soot formation is highly limited in the HSR system, confirming the role of tar in soot formation mechanism. The effect of CO2 is not straightforward and varies according to pyrolysis time and temperature.

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.