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•Mass loss rate, temperature, components in flue gas are measured simultaneously.•Pyrolysis dominates the loss of mass and is heat transfer controlled.•Mass loss and flue gas ...composition identify heterogeneous and homogenous combustion.•High content of H2 measured in the pyrolysis step.•In extruded pellets. axial chemical and thermal wave prevails on the radial one.
Aiming 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.
The exploitation of pine bark, a byproduct of timber industry, as biochar or activated carbons has been demonstrated. It can be successfully charred, and further activated, either by physical and ...chemical activation. The biochar yield and quality (amount of carbon) is mainly determined by the maximum temperature. Temperatures above 300°C were required to convert pine bark into biochar (by means of cracking and volatiles release), and yield stabilized at 32% at the maximum temperature tested (850°C). With uniform heating, the heating rate and the particle size did not affect significantly the biochar yield.
The carbon loss in biochar activation was found to be dramatic with steam at 850°C, while chemical activation significantly reduced it, preserving most of the biochar. The activated carbons obtained have been tested for adsorption of CO2 (at 90%) and toluene (a model VOC, between 500ppm and 2.5%). Significant differences in VOC loading capacity have been measured and explained with the different microstructure of the activated carbons produced. BET specific surface up to 3342m2/g has been measured, in carbons activated with H3PO4, and 1499m2/g with K2CO3, and total pores volume up to 2.57cm3/g.
The chemically activated carbons were found very effective in VOC capture, up to 50% (5.5mmol/g) of the original solid mass. Among them, the one activated by K2CO3 shows the highest loading capacity (32%, or 3.5mmol/g) even at quite low toluene concentration (500ppm), thanks to its largest share of micropores.
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•The bark of pine can be efficiently converted into biochar and activate carbons.•The chemical activation leads to produce carbons with specific surface up to 3340m2/g.•The chemically activated carbons show an excellent VOC adsorption capacity from air.•The adsorption capacities are uniquely connected to the micropores abundance.
ABSTRACT Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and ...particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases (<3), indicating that the former are dominant for energy dissipation. Current sheets corresponding to very high PVI (>5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas.
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•Cameroon sawmill residue show a remarkable C content, worth exploitation as biochar.•Good quality biochar, approaching coal, can be produced even at 500 °C.•The heating rate, between ...0.1 and 70°/min, does not affect biochar yield.•Optimal conditions result in short (<1h) production time per batch.•Pyrolysis in CO2 flow produces the best biochar yield and quality.
The 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.
•Horse manure (HM) samples from several origin, management and age were compared.•Heating values, ash and melting temperatures are compatible with combustion.•The very high moisture content in HM ...prevents its burning as collected.•Drying is required and dry HM burns as easily as woody biomass.•Drying procedure and rate are not obvious; HM texture plays a dramatic role.
We investigated the exploitation of horse manure for energy recovery by combustion. First, the impact of the waste origin and management (storage conditions) was evaluated in terms of elemental analysis, moisture content, heating value, and ash melting temperature. Besides some carbon loss over the time, horse manure origin and management policy do not impact its profitable exploitation by combustion. More relevant, the ignition tests disproved the current industrial opinion that combustion of manure alone is difficult, without the addition of auxiliary fuel, like wood shavings. It has been demonstrated through ignition tests that the major limitation in manure combustion is just the high moisture content (approx. 60% w.b.). The study focused on overcoming the detrimental effect of the large moisture content, and specifically its removal efficiency. The latter requires the understanding of manure permeability which is determined by the bedding material and the stable management. Here we provide indications of the impact of the manure composition and its sample size on the efficiency of moisture removal, whose energy impact must be minimized for an effective energy recovery. The energy balance confirms that the total removal of 62% w.b. moisture reduces by 23% the potential energy content of the manure; this drying penalty on energy is largely offset by the dramatic improvement in reactivity of the residual solid fuel.
We 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.
•The smouldering of biomass can be effectively controlled by inert additives.•Inerts in the biomass modify the texture and thermal properties of the bed.•Inerts in the biomass shape a well-defined and controllable front.•The front propagation rate is controlled by the flow rate of O2.•The pressure drop is a sharp diagnostic of smouldering onset and evolution.
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•Steps in development of PGMs-free catalysts for automotive emissions control.•La1-xAxCo0.5Cu0.5O3 catalysts are tested, x≤0.5, A=K, Sr, Ba.•Synthetic, realistic automotive exhausts ...mixtures used for activity tests.•La substitution affects structure, surface segregation, catalytic activity.•Higher catalytic activity and lower cost by substituting 50% La with Sr.
Several perovskites of the type La1-xAxCo0.5Cu0.5O3 (A=K, Sr, Ba; x=0, 0.25, 0.5) and a Sr-doped understoichiometric one, have been prepared by citrate method aiming at application as noble metals free catalysts for automotive application. The catalysts have been characterized by BET, X-ray diffraction (XRD), Temperature Programmed Reduction (TPR), X-Ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscopy (SEM) and the effect of A-doping has been investigated. The catalytic activity was studied in model reactions (CO oxidation, CO assisted NO reduction), and in a complex three way catalysts (TWC) mixture approaching automotive exhaust composition at both stoichiometric and O2-limiting conditions and the obtained results are discussed in relation with characterization results. A-doping induces the formation of highly dispersed cubic CuO particles, as observed in the understoichiometric samples. Doping also influences the surface segregation. La segregation is observed in the undoped LaCo0.5Cu0.5O3 sample whereas K is surface segregated in La0.75K0.25Co0.5Cu0.5O3; Co segregates in the Ba-doped perovskite and Sr in the understoichiometric La0.35Sr0.35Co0.5Cu0.5O3. The reducibility (TPR) is altered by A-doping. The reactivity results suggest that in simple CO+O2 and CO+NO mixtures the activation temperature is increased by any doping, but at 400°C the conversion in the CO assisted NO reduction is not significantly different with respect to the one obtained in the undoped catalyst. Activity with a more complex mixture, simulating actual engine exhaust, reveals the interesting activity of the Sr-doped catalysts, that outperform LaCo0.5Cu0.5O3 at both stoichiometric and O2-lean conditions. NO reduction can be achieved at lean O2 conditions, where it is quantitative from 400°C. The possibility to reach similar or better activity by replacing the use of La, which is a Rare Earth Element, by using Sr is particularly interesting. Stability at high temperatures and at fast fluctuations of O2 in inlet stream for 50% Sr-doped catalyst confirms these results.
During the lifetime of the Cluster mission, the inter‐spacecraft distances in the solar wind have changed from the large, fluid, scales (∼104 km), down to the scales of protons (∼102 km). As part of ...the guest investigator campaign, the mission achieved a formation where a pair of spacecraft were separated by ∼7 km. The small distances and the exceptional sensitivity of the search coil magnetometer provide an excellent data set for studying solar wind turbulence at electron scales. In this study, we investigate the intermittency of the magnetic field fluctuations in the slow solar wind. Using 20 time intervals with different constellation orientations of Cluster we cover spatial scales between 7 and 104 km. We compare time‐lagged increments from a single spacecraft with spatially lagged increments using multiple spacecraft. As the turbulent cascade proceeds to smaller scales in the inertial range, the deviation from Gaussian statistics is observed to increase in both temporal and spatial increments in the components transverse to the mean field direction. At ion scales, there is a maximum of kurtosis, and at sub‐ion scales, the fluctuations are only weakly non‐Gaussian. In the compressive component the deviation from Gaussian statistics is variable: it may increase throughout the inertial and sub‐ion ranges, but also, it may have a maximum at magnetohydrodynamic scales associated with large scale magnetic holes. The observations show differences in kurtosis of time and space increments when the spacecraft pairs are transverse to the flow, indicating its spatial anisotropy.
Plain Language Summary
Turbulence in the slow solar wind is investigated using multi‐spacecraft measurements for different satellites configurations. Twenty time intervals of more than 1 hr are analyzed. We compare differences in two time‐delayed magnetic field measurements (time‐lags) and of magnetic field measurements between spacecraft pairs (space‐lags). Space‐lags give the fluctuations along different satellite baseline directions and scales (from 7 to 9,000 km) while time‐lags give the fluctuations along the flow direction. The magnetic field fluctuations' intermittency, which can be thought of as the “patchiness” or “roughness” is investigated. Differences are observed between the time‐lagged measurements and the spatially lagged measurements when the spacecraft pairs are transverse to the flow.
Key Points
Multipoint Cluster magnetic field data (from 2003 to 2015) are used to investigate intermittency in solar wind plasma
Two different methods are used to calculate fluctuations: temporal increments and spatial increments
Spatial scales from ∼101 to ∼104 km are covered
Data from the Cassini spacecraft identify strong electron acceleration as the solar wind approaches the magnetosphere of Saturn. This so-called bow shock unexpectedly occurs even when the magnetic ...field is roughly parallel to the shock-surface normal. Knowledge of the magnetic dependence of electron acceleration will aid understanding of supernova remnants.
•TPO may easily induce a thermal wave in the catalyst.•Dispersions in small-scale monolith diverge significantly from adiabatic assumption.•Hysteresis changes when the position of the thermocouple ...that controls heating is moved.•Hysteresis in X(T) appears very different if IN, OUT or internal T is considered.
Experiments 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 °C/min) have been used to reproduce both ignition and extinction stages, up to 300 °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.
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