1,720,975 research outputs found
Sulcis coal char reactivity under high-pressure H2
No full textThe hydrogasification reactivity of charN2 and charH 2, obtained from Sulcis coal pyrolysis at 900 °C in inert and reductive atmospheres, respectively, was investigated in a commercial microreactor under temperature programmed reaction (TPR) at high-temperature (700-800 °C) and high-pressure H2 (1.0-5.0 MPa) conditions. The flue gases were analyzed online by FTIR spectrometry for the identification of volatile species produced. The emissions of carbon (CO2 and CO) coming from both chars reached the maximum releasing rate below 500 °C, whereas nitrogen-based compounds (HCNO and NH3) were solely detected during the hydrogasification reaction of charN2. Meanwhile, the main product of char hydrogasification CH4 was detected at temperature as low as 450 °C. Generally, the carbon conversion values of charN2 were higher than those of charH2. At low-temperature and pressure reaction conditions the carbon conversion of charH2 rapidly went down. A first-order reaction rate dependence on H2 pressure at 800 °C was found for charN2, whereas the reaction order was almost double for charH2. The apparent activation energies of charN 2-H2 reaction at 1.0 and 5.0 MPa pressures were 96 and 78 kJ mol- 1, respectively, whereas the apparent activation energy for charH2-H2 reaction under 5.0 MPa pressure was 140 kJ mol- 1. The Sulcis coal reactivity during the hydrogasification process was significantly impacted by the pyrolysis conditions used in getting chars. © 2014 Elsevier B.V
TG-FTIR and kinetics of devolatilization of Sulcis coal
No full textThe N2-pyrolysis of low-rank Sulcis coal was investigated by thermogravimetric techniques (TG/DTG) in the temperature range ambient to 1000 C under dynamic heating conditions (50, 75, and 100 C min-1 heating rates). Little differences in the mass losses with heating rates were observed. From thermogravimetric analysis it was established that coal pyrolysis consisted of three main stages: water evaporation; devolatilization of thermally labile and more stable volatiles; and char formation. The evolved gas (EGA) by Fourier transform infrared spectrometry (FTIR) coupled to the thermobalance under 100 C min-1 heating rate was conducted for the identification of the gaseous species and their evolution profiles during coal thermal degradation. The temperatures of maximum rate of release of H2O, CO2, CO, COS, C2H4, as well volatile fragments originating from breaking of covalent bonds such as alkyl and ether groups, were in agreement with the temperature of maximum mass-loss rate around 466 C. Meanwhile the maximum releasing rates of SO2, CH4, and NH3 took place at 330, 575, and 690 C, respectively. An increase of CO emission intensity at 770 C was indicating in situ gasification with CO 2-bearing product of freshly formed char. The kinetic processing of non-isothermal TG data was performed by isoconversional method. In the coal conversion regions α = 5-40% the apparent activation energies were almost constant suggesting a single-step reaction path. The calculated average E value was 189 kJ mol-1. A kinetic compensation effect existed between E and ln A: the linear dependence provided an average pre-exponential factor A 0 value of 2 × 1011 min-1. With further increase of conversion degree a complex E dependence on α was evident as the coal thermal degradation process underwent a multi-step reactions. © 2013 Elsevier B.V. All rights reserved
Thermal stability and oxidizing properties of mixed alkaline earth-alkali molten carbonates: A focus on the lithium-sodium carbonate eutectic system with magnesium additions
No full textA comparative study on thermal behavior and oxygen solubility properties of eutectic 52/48 lithium/sodium carbonate salt containing minor additions of magnesium up to 10 mol% has been made in order to determine whether a general correlation between these two properties can be found or not. Consecutive TG/DSC heating/cooling thermal cycles carried out under alternating CO2 and N2 gas flows allowed to assign thermal events observed in the premelting region to a partial decarbonation process of the magnesium-alkali mixed carbonates. The observed decarbonation process at 460 C is believed to come from initial stage of thermal decomposition of magnesium carbonate resulting in the metastable formation of magnesium oxycarbonate-like phases MgO·2MgCO3, in a similar manner as previously reported for lanthanum. Reversible formation and decomposition of the magnesium carbonate phase has been observed under a CO2 gas atmosphere. The intensity of the decomposition process shows a maximum for a 3 mol% MgO addition that gives also the highest oxygen solubility, suggesting therefore that instability thermal analysis in the premelting region can be considered as providing an effective measure of the basicity/oxidizing properties of alkali carbonate melts with magnesium or, in more general terms, with cations that are strong modifiers of the carbonate melt basicity through formation of soluble oxycarbonate phases. © 2013 Elsevier B.V
The role of foreign cations in enhancing the oxygen solubility properties of alkali molten carbonate systems: Brief survey of existing data and new research results
No full textThis work presents a summary of experimental data on the oxygen solubility-speciation properties of alkali molten carbonates that have been obtained by the authors through the development of a highly sensitive analysis method. The purpose of this summary is to show in particular the effect of foreign cation additions on both oxygen solubility and dissolution mechanisms at 650°C. Our findings may be used to clearly indicate that rare earth and magnesium cations are the most effective in enabling oxygen solubility and basicity/oxidizing melt properties. The second part of this work reports new experimental results concerning the effect of simultaneous additions of La and Mg cations to an eutectic Li/Na carbonate system. A dramatic increase in oxygen solubility and active peroxide oxygen species has been found, thus revealing a strong synergistic effect of rare earth and alkaline earth cations on the molten carbonate oxygen chemistry. The results of this investigation suggest therefore that foreign cation addition is a potentially attractive option to design alkali molten carbonate salts with a high oxidizing power. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved
Analysis and distribution of volatile gases from catalytic pyrolysis of Sulcis low-rank coal
Full text linkCatalytic pyrolysis of Sulcis low-rank coal over naturally occurring olivine, and home-made 15 wt%Ni/γ-Al2O3 catalyst was conducted for the upgrading of coal pyrolysis volatile gases in the temperature range ambient-900 °C under atmospheric pressure. Raw coal and mixtures of coal-additive (90:10 wt%) were slowly heated in temperature programmed mode using a laboratory-scale quartz furnace coupled in parallel to Fourier transform infrared (FTIR) spectrometer and GC chromatograph for quantitative analysis of flue gas. Coal pyrolysis with and without additives was also conducted by TG/DTG/DSC analysis at different heating rate (β = 15, 20, 30 °Cmin−1). DSC results clearly indicated some extra exothermic events during catalytic coal pyrolysis. Quantitative gaseous products distribution with temperature showed yields significantly and selectively improved with additives. Generally, more CO and CO2 were emitted under catalytic coal pyrolysis. Meanwhile, nickel catalyst exerted a marked positive effect on H2 yield overall in the temperature range 400–500 °C. The light hydrocarbons such as methane, ethane, propane and n-hexane substantially remaining unchanged, whereas a remarkable increase of emitted ethene was originated from catalytic pyrolysis. A deeper SO2 evolution was observed over olivine, whereas the N-containing compounds (NH3, NOx) were also modified in catalytic pyrolysis. Formaldehyde was also monitored, which represents a fragment originating from polycyclic aromatic side chains. Reaction kinetic study by a model-free isoconversional method indicated a complex multiple-step mechanism of coal pyrolysis, exception made for conversion values between 5% and 50% where a single-step reaction path was operating. The calculated average Ea and the pre-exponential factor were markedly reduced by the presence of additives. Meanwhile the compensation effect was also existing
Influence of lanthanum carbonate additions on thermal stability of eutectic lithium-sodium carbonate near its melting point
No full textThermal behavior and stability of eutectic 52/48 Li/Na carbonate mixture containing various concentrations of lanthanum carbonate in the range 0.5-2.0 mol% was investigated by a combination of thermal analysis (DSC) and spectroscopic (EGA-FTIR) techniques under CO2 and N2 gas flows. Thermal decomposition of lanthanum carbonate with CO2 gas release was observed in the 470-480 °C range close to the melting point of eutectic alkali carbonate giving raise to formation of lanthanum oxycarbonate species. It was also found that the lanthanum carbonate phase promptly reformed, when the eutectic carbonate melt samples were cooled down to 440-460 °C under CO2 atmosphere. © 2012 Elsevier B.V. All rights reserved
High quality syngas production via steam-oxygen blown bubbling fluidised bed gasifier
No full textThis paper presents experimental and modelling results of fuel gas obtained in a steam-oxygen blown fluidised bed pilot scale (500 kWth) coal gasifier that is part of the ZECOMIX (Zero Emission of CarbOn with MIXed Technologies) research infrastructure run by ENEA. As there is poor information about start up and steady state operation of fluidised bed gasifiers at such a scale, in this work we investigated the influence of some important parameters such as coal ignition temperature, feeding rate of particulate bed material (olivine sand) and transition from oxidant to reducing environment. The experimental runs confirmed the crucial importance of S/O (steam/oxygen) ratio on the product gas composition. High quality syngas (low content of methane, below 1.0 v/v %, and high content of H2 and CO: CO + H2 61.0 v/v%) was obtained with S/O = 1.1.A shortcut model of the gasifier was also formulated, considering instantaneous coal pyrolysis, kinetics of coke steam reforming reactions in the fluidised bed, and achievement of thermodynamic equilibrium for the water gas shift reaction. The model takes into consideration the average residence time of coke particles into the gasifier: since this is a characteristic parameter of the reactor, the performance of different fluidised bed gasifiers can be simulated. © 2016 Elsevier Ltd
Novel synthesis of combined CaO-Ca 12 Al 14 O 33 -Ni sorbent-catalyst material for sorption enhanced steam reforming processes
No full textA properly CaO-Ca 12 Al 14 O 33 -Ni material with combined sorbent properties and catalyst activity was developed for H 2 production from hydrocarbons via sorption enhanced steam reforming (SE-SR) with simultaneous CO 2 capture. The combined sorbent-catalyst material (CSCM) was successfully prepared by multi-step approach method. At first a mixed calcium-aluminium oxide (CAO) ceramic was prepared by wet mixing/sintering method and used both as spacer for CaO-based sorbent and support for nickel catalyst. Subsequently, the sorbent and catalyst were prepared by wet mixing/sintering (900 °C) and wet impregnation/calcination (500 °C) methods, respectively. Then, an intimately powdery 1:1 mixture of two functional materials were cold-pressed and air-sintered at 900 °C obtaining the desired one-body sorbent-catalyst. The CSCM characteristics were investigated in detail by XRD, SEM-EDS, TG-DTG, BET physisorption, and TPR techniques. The CO 2 sorbent properties were assessed over 200th multiple sorption/desorption cycles and the stabilizing role of spacer Ca 12 Al 14 O 33 ceramic against sorbent decay was confirmed, whereas the presence of foreign Ni ions did not affect the sorbent CO 2 carrying capacity. A H 2 -rich gas (> 90%) with low concentrations of CO 2 and CO was produced over ten consecutive steam methane reforming (600 °C)/regeneration (750 °C) cycles at steam/carbon=3 molar ratio using CSCM. This good performance of SE-SR of methane process was attributed to the synergistic effect of high CO 2 capture capacity and catalytic activity, the latter thanks also to the facile surface NiAl 2 O 4 spinel to Ni° reduction in the low temperature range of 400–600 °C
Preparation of CaO-based sorbent from coal fly ash cenospheres for calcium looping process
No full textWith the aim to synthesize an inexpensive and high-stable sorbent for CO2 capture processes, an industrial waste-product derived from coal-fired power plant, the so-called coal fly ash (CFA) cenospheres, was employed as inert supporting material. The CaO-based sorbent derived from CFA cenospheres (mainly composed of mullite and quartz) was prepared via a solution-based citric acid method. The obtained slurry was decomposed at 500 °C in air for 2 h. From the XRD results the CaO-CFA500 sorbent was mainly composed of Ca12Al14O33 (mayenite) and Ca(OH)2 along with trace of γ-Al(OH)3 and CaCO3 and starting SiO2. A further air-heating at 900 °C was conducted because the CO2 sorbent is submitted at high temperature for regeneration. The CaO-CFA900 sorbent was made of free-CaO and two crystalline calcium-alumino-silicate phases, namely gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8). The optimized mesopore size particles belonging to CaO-CFA900 was reflected in a high stability over multiple cycles of carbonation/calcination. The initial CO2 capture capacity of the sorbent was 0.33 g CO2 g−1 sorbent, which was about three times the value of pure CaO (0.11 g CO2 g−1 CaO), and reduced to 0.22 g CO2 g−1 sorbent after 20 cycles remaining then stable over 200th cycles. From the present results it can be argued that CaL process could be easily scalable by re-using a coal-fired plant waste-product
Catalytic performance of Ni/CaO-Ca12Al14O33catalyst in the green synthesis gas production via CO2reforming of CH4
No full textDry Reforming of Methane over 15wt.% Ni/CaO-Ca12Al14O33 catalyst was performed in a microreactor in the temperature range 600-800°C under atmospheric pressure, at WHSV of 120 Lg-1h-1 and by time on stream of 12 h for producing synthesis gas. The novel catalyst was prepared by Ni wet impregnation of a mixed calcium-aluminum-oxide (CAO) ceramic support. Similarly, a Ni/γ-Al2O3 reference catalyst was prepared. Characterizations were conducted by TGA-FTIR, XRD, SEM-EDS, N2 physisorption, H2-TPR, CO2-TPD, and CO2-TPRn techniques. After calcination(500°C)/reduction(700°C) steps in situ formed CaO promoter was highly dispersed on Ca12Al14O33 carrier, which induced strong basicity. A good anchorage of NiO on CAO support was evidenced by reduction peaks at 490°C and 650°C on the H2-TPR profile. The reduced mesoporous catalyst presented high SBET, large pores volume, and unimodal pore size distribution. High reactants conversions, good H2 and CO selectivity, and H2/CO molar ratio close to unity at 800°C were achieved. Although the catalytic activity of Ni/γ-Al2O3 reference catalyst was slightly better than that of Ni/CaO-Ca12Al14O33 catalyst the stability was worse owing to the excessive carbon build-ups, whereas the novel catalyst displayed a very low carbon deposit on spent catalyst at 600 and 700°C, and negligible coke deposit at 800°C. It was established that the basicity of CaO-Ca12Al14O33 support can play a key role in preventing coke deposition during DRM. The Ni CaO-Ca12Al14O33 can serve as sorbent for CO2 capture and simultaneously for its catalytic conversion in a valuable fuel
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