10,150 research outputs found
Mixing of a Lateral Gas Stream in a Two-Dimensional Riser of a Circulating Fluidized Bed
No full textA cold model of a circulating fluidized bed having a two-dimensional riser, with a 12 x 120 mm section and a 6.4 m height, was equipped with a device to inject a lateral gas stream along the riser. The apparatus was operated under conditions ranging from those characteristic of combustors to those of gas-conversion processes. Flow structures in the interaction region between the rising gas-solids suspension and the lateral gas stream were studied by means of a motion analysis system. Three main configurations were identified. The ratio between the momentum of the lateral gas stream and that of the rising suspension was found to be the parameter able to discriminate among the three configurations. A satisfactory agreement was found with mixing data obtained using a cylindrical riser
Mixing of a Lateral Gas Stream in a Two-Dimensional Riser of a Circulating Fluidized Bed
No full textA cold model of a circulating fluidized bed having a two-dimensional riser, with a 12 x 120 mm section and a 6.4 m height, was equipped with a device to inject a lateral gas stream along the riser. The apparatus was operated under conditions ranging from those characteristic of combustors to those of gas-conversion processes. Flow structures in the interaction region between the rising gas-solids suspension and the lateral gas stream were studied by means of a motion analysis system. Three main configurations were identified. The ratio between the momentum of the lateral gas stream and that of the rising suspension was found to be the parameter able to discriminate among the three configurations. A satisfactory agreement was found with mixing data obtained using a cylindrical riser
Mixing of a Lateral Gas Stream in a Two-Dimensional Riser of a Circulating Fluidized Bed
No full textA cold model of a circulating fluidized bed having a two-dimensional riser, with a 12 x 120 mm section and a 6.4 m height, was equipped with a device to inject a lateral gas stream along the riser. The apparatus was operated under conditions ranging from those characteristic of combustors to those of gas-conversion processes. Flow structures in the interaction region between the rising gas-solids suspension and the lateral gas stream were studied by means of a motion analysis system. Three main configurations were identified. The ratio between the momentum of the lateral gas stream and that of the rising suspension was found to be the parameter able to discriminate among the three configurations. A satisfactory agreement was found with mixing data obtained using a cylindrical riser
Hydrodynamics of a circulating fluidized bed operated with different secondary air injection devices
No full textA cold model of a circulating fluidized bed combustor having a 0.120 m i.d. and 5.75 m high riser, was operated by splitting the total gas flow rate in a primary air stream fed through the bottom distributor and a secondary air stream laterally injected at a fixed level along the riser. Two different feeding devices were used: in one case the injection of secondary air stream occurred over the whole circumference of the riser through a slot; in the other the injection occurred through four opposite ports. Solids used were 89 mu m Ballotini. Overall superficial gas velocity was fixed at 6 m/s. Solids mass flux was alternatively equal to 35 and 55 kg/(s m(2)). The ratio between secondary and primary air flow rates was kept at a value of 1. An isokinetic probe was used to measure radial profiles of upward and downward solids mass fluxes and those of tracer gas concentration. A stream of CO2 was mixed with the secondary air stream before the injection and used as tracer gas. For each set of operating conditions, axial profile of voidage and radial profiles of solids mass flux and CO2 concentration were measured along the riser. Results confirm that lateral air stream remarkably affects the hydrodynamics in the regions below and above the gas inlet. They also show that the mechanism of interaction between the secondary air stream and the gas-solids suspension rising from the primary zone completely changed when the injection device was changed, Solids and tracer gas distribution along the riser were noticeably different
Recupero di energia e materia da rifiuti solidi: i processi, le tecnologie, le esperienze, le norme
No full textTar removal during the fluidized bed gasification of plastic waste
No full textA recycled polyethylene was fed in a pilot plant bubbling fluidized bed gasifier, having an internal diameter of 0.381 m and a maximum feeding capacity of 90 kg/h. The experimental runs were carried out under various operating conditions: the bed temperature was kept at about 850 °C, the equivalence ratio varied between 0.2 and 0.35, the amount of bed material was between 131 and 215 kg, the fluidizing velocity was between 0.5 and 0.7 m/s, quartz sand and olivine were used as bed material, and air and steam were used as fluidizing reactants. The results confirm that the tar removal treatments applied inside the gasifier (primary methods) can eliminate or strongly reduce the need for a further downstream cleanup of the syngas. In particular, the utilization of a natural olivine as an in situ tar reduction agent remarkably improves the quality of the product gas, in terms of both high hydrogen volumetric fraction and larger syngas yield. © 2008 Elsevier Ltd. All rights reserved
An LCA answer to the mixed plastics waste dilemma: Energy recovery or chemical recycling?
No full textThe study focuses on mixed plastics waste (MPW), whose complex and unpredictable composition (due to high polymer heterogeneity, additives, and contaminants) makes its valorisation a true technical, environmental, economic, and regulatory challenge. Chemical recycling by means of advanced thermochemical treatments (ATT) could be a successful strategy, able to support the transition from a carbon intensive to a carbon negative sector, and alternative to the current treatments of energy recovery or mechanical downcycling. Some of these ATTs provide an efficient recovery of valuable resources, such as fuels and chemicals, but their role is mainly limited by time necessary to complete the process optimization and implement the required infrastructures. A reliable identification of the best alternatives is thus crucial. A specific LCA approach quantifies the environ-mental performances of a selected set of ATT technologies for resource recovery from MPW. It includes plastics -to-energy, by combustion or gasification; plastics-to-methane and plastics-to-hydrogen, by gasification; and plastics-to-oil, by thermal pyrolysis. The results highlight the crucial role of carbon capture and storage (CCS) units, which partially reduces that of the specific thermochemical treatment. The best performances, particularly for Climate Change category, are those of the MPW-to-hydrogen by gasification, followed by those of MPW-to-energy by combustion or gasification, all equipped with CCS. The sensitivity analysis considers the evolution of the European energy mix, characterised by a larger utilisation of renewable energy sources, and highlights the corresponding increased sustainability of chemical recycling by ATTs. This suggests that the MPW dilemma should be definitively solved in a close future
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