• Energy Research

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.

Diesel engine exhausts from a common rail 3.0 L F1C diesel engine were analyzed at two different load conditions of the WLTC testing cycle downstream of both the diesel particulate filter (DPF) and selective catalytic reactor (SCR) to verify their effect on the characteristics of carbon particulate matter. An array of chemical, physical and spectroscopic techniques (gas chromatography coupled with mass spectrometry (GC-MS), mobility analyzer, UV-Visible absorption and fluorescence spectroscopy) was applied for characterizing polycyclic aromatic hydrocarbons (PAH), heavy aromatic compounds and soot, constituting the particulate matter (PM) sampled from the exhaust. The engine was operated in half load (HL) (188 Nm, representing the more common condition for engine in urban traffic) and full load (FL) (452 Nm, representing the best performance of the engine operation) conditions, at the same engine speed (2000 rpm). Soot formation was enhanced in HL condition, with respect to FL, but, just because of the much lower soot amount, the after-treatment systems in this last condition resulted to be less efficient in the soot abatement. Indeed, the abatement through DPF was about 40% lower in the FL condition with respect to HL condition, and any significant further concentration decrease was found after SCR, in both conditions. By contrast, PAH concentration after DPF abatement was found to be higher in the HL with respect to FL condition. A further PAH concentration decrease of about 30% was found after the SCR in the HL condition whereas in FL the reduction was only about 5-6%. Also the heavy aromatic compounds having molecular weight above the GC-MS detection limit (300 u), were mitigated by SCR. Therefore, SCR did not cause a further soot reduction, whereas it was effective in largely reducing PAH and heavy aromatics emissions, especially in the lower temperature condition featuring the half-load condition, when combustion efficiency is worse. Moreover, SCR system reduced the emission of small particles probably due to an enhanced agglomeration of particles, with beneficial effect on the harmfulness to human health.

The present study investigates the influence of pressure on the products of fast pyrolysis of a lignocellulosic biomass (Walnut Shells).

HRTEM fringe analysis by means of our "Analyse Plan" (AP) tool and a new HRTEM image analysis procedure, based on mathematical morphological analysis (MA), was applied to soot sampled along the axis of sooting premixed benzene and ethylene flames. The layer length obtained by means of MA approach was found to be higher in respect to the layer length derived from AP method, as it better accounts for their local curvature and distortion. The mean layer length obtained by the MA method was then proved to agree well with the data evaluated by Raman spectroscopy. Moreover, the MA method was useful for extracting other soot nanostructural parameters related to the geometry of individual fringes as layer length, tortuosity, and local curvature radii, in turn correlated with the defective nature of soot structure. Some parameters provided by the MA method appeared to be more sensitive and informative about structural changes occurring as soot is formed. In particular, the percentage of fringes with high tortuosity and the percentage of nearly-straight fragments were found to decrease and increase, respectively, throughout the soot formation region. These trends suggested a better stacking of longer and more planar layers testifying the structural order improvement with soot aging. It was also observed that the soot nanostructural ordering occurred faster for benzene with respect to ethylene soot in agreement with H/C and absorption coefficients trends. The distributions of carbon in differently-sized aromatic structures were evaluated on the basis of the fringe length distribution to follow the aromatic growth. The method assigned aromatic sizes to the extracted HRTEM fringes, assuming that the fringes are in the shape of parallelograms. The distributions were found to evolve toward larger size aromatic systems demonstrating the occurrence of aromatic growth during soot formation. © 2014.

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.

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 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.