1,721,194 research outputs found
On the mechanism of single pellet smouldering combustion
No full textAiming at improving the design of wood pellet stoves and boilers, we developed original experiments to study chemical and physical processes occurring in a single pellet. We investigated medium temperature (<550 °C) and low air flowrate, where smouldering prevails. Two setups have been used, allowing to monitor the pellet weight loss, internal temperature, and the exhaust gas composition (H2, CO, CO2, O2, VOC), and modify the temperature and composition of the environment surrounding the pellet and its size. The study allowed to identify the sequence of phases involving drying, pyrolysis, partial and total oxidation of volatiles and char. The maximum rate of conversion is controlled by heat conduction, in the gas surrounding the solids and within the porous pellet, thus, it is linearly proportional to the environment temperature. At the higher temperature, the char yield decreases, and the useful heat must be extracted from volatiles combustion (in the gas phase) rather that embers (char combustion). An axial smouldering front, from the fractured edges prevails over a radial one, from the side of the pellet, due to a less permeable layer left by the extrusion process. Experiments in a nearly stagnant atmosphere clarified that both volatiles and char oxidations occur under O2 starvation; the rate is proportional to O2 available. Also, evidence of a transition from 1- to 2-films char combustion are identified, suggesting that O2 mass transfer controls the heterogeneous char oxidation first, and then the CO oxidation in the gas phase, at higher surface temperature
Emissions reduction from wood pellet stoves by uniform feeding
No full textThe instantaneous fuel-to-air ratio in pellet stoves may vary because of the feeding mechanism. The pellet feed flow rate variability of a standard screw feeder was quantitatively assessed with real-time measurements. Temporary excess or deficit of pellets commonly occur. The variability can be significantly reduced with a new feeder, by approx. 70%, both at nominal (full) and partial (half) load operation. The analysis of the instantaneous emissions reveals that the fuel feed instabilities cause temporary sub-optimal fuel-to-air ratio and, in turn, peaks of pollutants emissions. The impact on the emissions of a more regular pellet feeder is impressive. A reduction of CO emission by 46% and 37.5%, and of PM by 52% and 44% at nominal and partial load, respectively was measured. NOx emissions are less affected by a more uniform feed rate, being predominantly from fuel nitrogen
Optimization of biochar quality and yield from tropical timber industry wastes
No full textThe conversion of timber industry waste to biochar was investigated, based on residues from local tropical hardwoods in Cameroon. Closed and flow reactors, loaded with few grams of the sample and fluxed with different inert gases, were used with the aim of converting these wood residues to a safe and efficient solid fuel that can be exploited by the local community for cooking purposes. The pyrolysis temperature had the highest impact on the process up to approximately 700 °C as biochar yields decreased from 87 to 23.5% while increasing the temperature. A significant concentration of carbon required temperatures greater than 500 °C for the biochar to become a good solid fuel, approaching the heating value of coal. The role of heating rate appeared marginal even in a broad range: 0.1 to 70 °C/min. The dwell time (in the range 0.5 to 5 h) at maximum temperature revealed to be of little influence on the charring which occurred mostly during the first few minutes of the isothermal phase. Thus, actual production time can be <1 h/batch (up to 500 °C at 10 °C/min). Stepwise heating revealed a sequence of devolatilization reactions following their activation energies. Use of CO2 as inert gas increased the char yield. The inert gas flow rate had two contrasting effects on the biochar yield, in relation to volatile components residence time and the biomass temperature. The reactivity of biochar in combustion suggests a trade-off between heating content and reactivity. Charring in a closed, pressurized reactor produced biochar of low value as fuel, with tars absorbed in the porous solid residue
Biogas upgrading by 2-steps methanation of its CO2 – Thermodynamics analysis
No full textA thermodynamic analysis of the methanation of CO2 within biogas sets the limits to the achievable perfor-mances, directing process and catalyst development. Practical indications of thermodynamics are elaborated. The large presence of CH4 in the biogas does not hinder the achievement of an almost complete CO2 methanation, even with CH4/CO2 = 3. The smaller the temperature, the higher the CH4 concentration in the dry biomethane, and the lower the residual H2. Results comply with some grid specification even operating at 1 bar, below 400 degrees C, where active commercial catalysts are available. CO in the product is never a concern, in this temperature range. There is no advantage in operating above approx. 400 degrees C. The H2 slip can be further reduced increasing the pressure, but the improvement is most effective with a few bars; 15 bar is already quite good. H2 in excess of the stoichiometric is not useful. The process can be completely autothermal, but adiabatic operation must be avoided. Significant improvements in biomethane purity can be achieved with two methanation steps, with steam condensation in between. Residual H2 can be reduced from 7% (single stage, 1 bar, 300 degrees C), to 1.8% (two stages, at 300 degrees C, and 1 and 15bars, respectively). Partial steam condensation allows to limit coking in the second step, without a remarkable increase of residual H2. The second step cannot be autothermal above 150 degrees C, but the first step provides heat in excess, at 300 degrees C, to support also the second step
Manure drying optimization
No full textHorse manure is a poor feedstock whose energetic content is comparable to that of herbaceous biomass. However, it can be effectively burnt provided that it is previously desiccated, being its moisture content very high (60% wb). This work reports an experimental investigation to identify optimal conditions for horse manure drying in a bench-scale direct heating rotary drier, to be sequentially combined with its combustion. The relative impact of the variables affecting the drying rate is assessed by an extensive sensitivity analysis which addresses both the role of solid motion and the drying fluid. Results show that the effectiveness of the gas-solid contact has a remarkable effect on the moisture release rate, and the drying air temperature is the controlling parameter. Tests using flue gases as drying medium confirm that they can be profitably used, without significant loss of drying capacity, suggesting that combined drying and combustion is a viable option. Setting a target residual moisture at 30% wb, by sensitivity analysis the time required could be cut to 12 min, for a reference quantity of 1.2 kg, with a maximum moisture removal capacity of 70 (kg/h)/m(3). (C) 2022 Published by Elsevier Ltd
Investigation of thermal effects on heterogeneous exothermic reactions and their impact on kinetics studies
No full textExperiments and modelling were performed to investigate CO oxidation over a Pd-Rh monolith. We focused on thermal effects and hysteresis, to validate by modelling a thermal explanation of the results. Different feed composition (0.07-4% vol. CO) and heating rates (0.5-5 degrees C/min) have been used to reproduce both ignition and extinction stages, up to 300 degrees C, thus measuring the catalyst activity under transient conditions. The heating rate plays a marginal role in producing hysteresis, whereas the reactants concentration appears the real cause, because of its effect on the rate of heat production. A significant increase of the monolith temperature compared to the inlet gas is measured after ignition. The local overheating of the catalyst surface explains the hysteresis observed. When the reactor thermal control is based on the internal temperature, instead of the inlet one, the hysteresis appears dramatically different. The choice of the temperature used to control the oven, and report the activity results, may induce very misleading indications, including inexistent multiple steady-states. A model accounting for the thermal dynamics of the solid predicts the observed hysteresis, even with simple rate equations. That supports the thermal explanation for the direct hysteresis. The estimated activation energy is quite reasonable and compare well with literature. Preexponential factors accommodate for the weakening of the adiabatic channel critical assumption, less and less realistic as the reaction heat increases. The overall conclusion is that the local temperature can vary widely, in time and space, and any kinetic study not accounting for a precise knowledge of that will inevitably produce poorly representative parameter estimates. The limitation can be overcome with spatially-resolved measurements
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Onset and propagation of smouldering in pine bark controlled by addition of inert solids
No full textWe investigated the smouldering onset and propagation in a biomass (pine bark) bed to mitigate or control the process. By the addition of sand, the bed porosity has been modified. Sand of different average particle size, in different arrangements and proportions (from 30 to 70%) has been used, modifying the bed porosity (between 0.44 and 0.67). Experiments have been carried out in a flow reactor, with controlled O2 content in the feed. The packed bed was large enough to allow the development of a reaction front, as expected in large scale applications. Pressure drop measurements revealed as a useful indication to monitor the smouldering onset and the front evolution. The process appears always controlled by the flux of O2, as expected for smouldering. Properly tuning the bed porosity with inert solids, allows to control the self-heating propagation, in view of its exploitation for low temperature heat production with simultaneous controlled conversion. The addition of sand supports a well-defined reaction front. The inert solid preserves the bed texture when the biomass is progressively consumed and allows to evenly distribute O2 to the biomass, thus regulating the heat dissipation and keeping the smouldering front confined
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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