1,720,988 research outputs found
Modeling gasification of waste-derived fuels in a rotary kiln converter operated with oxygen staging
No full textThermal conversion of waste-derived fuels is gaining a clear role in the general frame of the circular economy as one pathway to close the recycle loop when a material or chemical recycle is impossible or economically unfeasible. Sewage sludge derived from the treatment of urban wastewaters is currently facing rapidly increasing production volumes and severe restrictions of the conventional disposal options: thermal conversion stems out as the most viable strategy, entailing large reduction of the sludge volume and thermal destruction of the toxic organic constituents. In the frame of thermochemical processing of waste-derived fuels, pyrolysis/gasification presents several advantages over the direct waste-to-energy combustion path, mostly related to the generation of syngas and condensable species which can be easily transported, burned or even exploited in gas-to-liquid fuel or chemical processes. The present study addresses the development of a process for oxy-pyrolysis of sewage sludge in a rotary kiln converter. The aim of the process is the production of syngas from devolatilization of a waste-derived fuel, with oxygen playing the role of promoting autothermal operation of the pyrolyzer by controlled oxidation of volatile compounds. The specific concern of the study is the assessment of the effectiveness of staged oxygen feeding, as opposed to localized feeding at the reactor inlet, as a tool to selectively promote desired secondary reactions occurring in gas phase, like partial oxidation of tars. The converter consists of a rotary kiln in which the oxidizer is fed at multiple coordinates along the reactor axis, so as to obtain a reactant contacting pattern resembling that of a Zwietering reactor. The reactor is modelled at steady state using a 1.5D frame. Material and energy balances are set up considering a semi-lumped kinetic mechanism that was purposely developed to represent the complex chemical pathways of the solid fuel, of the gaseous compounds, of different tar components and of soot. Model results are analyzed with a focus on the effect of axial staging of the oxidizer on the quality of the produced gas and on the performance of the reactor
Fluidized bed reactors for thermochemical storage of concentrated solar power
No full textExtensive R&D is in progress to exploit the huge amount of solar energy falling on Earth (105 TW). A fast growing technology developed to pursue this goal is Concentrating Solar Power (CSP): a field of optical sun tracking mirrors is used to concentrate the solar energy onto a receiver to eventually produce electricity or to drive an endothermic chemical reaction. The key to success of CSP is the integration with thermal energy storage systems, which provide a mean to overcome the inherent limitations associated with the variable/seasonal availability and spatial non uniformity of the primary energy source. Incident fluxes in CSP can reach and exceed 3000 kW m–2. For this reason, the receiver has a crucial role in CSP systems, as it must collect and transfer the incident solar energy while ensuring low heat losses and minimizing local overheating. Gas−solid Fluidized Beds (FB) can be conveniently applied to CSP thanks to their large heat transfer coefficients and effective thermal diffusivities. The interaction between the incident radiative flux and the FB can occur in an indirect way, focusing the radiation onto an exposed surface which transfers the heat to the FB, or in a direct way, providing the FB with transparent walls or windows. Direct absorption of solar energy permits operating temperatures high enough to perform thermochemical storage processes with high energy density. A critical issue in FB receivers is the bed surface overheating −induced by high concentrated solar radiations− that can cause sintering and/or degradation of the fluidized particles, hence a strong reduction of the thermochemical cycles efficiency. In the first part of the present work, the dynamics of a directly irradiated FB exposed to a highly concentrated simulated solar radiation has been investigated. Analysis of local temperature fluctuations in time and frequency domains has been performed. Conditioning of bed hydrodynamics close to the surface has been investigated as a mean to improve the interaction between the incident radiative flux and the bed. In the second part of the work, the application of a solar irradiated reactor to endothermal reactions has been demonstrated with reference to solar driven limestone calcination, followed by autothermal recarbonation of lime. Solar driven calcination has been investigated with the twofold perspective of: a) accomplishing thermochemical energy storage by a reversible high enthalpy and high temperature chemical reaction; b) performing solar aided CO2 capture from flue gas to be embodied in carbon capture and sequestration schemes such as calcium looping
A model for oxy-pyrolysis of solid fuels in a rotary kiln reactor
No full textA mathematical model of a rotary kiln oxy-pyrolyzer of sewage sludge is
presented. The specific feature of the model is the axial staging of O2 feeding to the
reactor as one important key to the quality of the syngas. The steady operation of
the oxy-pyrolyzer is analyzed in terms of fluxes of the solid fuel and of the gaseous
species, temperature of the solid and of the gas phases along the reactor. The
performance of the reactor is characterized in terms of process rate and chemical
composition of the produced syngas, heating value and thermal power
Thermal behaviour of fluidized beds directly irradiated by a concentrated solar radiation
No full textDirectly-irradiated fluidized bed reactors are very promising in the context of concentrated solar power applications as they can be operated at process temperatures high enough to perform thermochemical storage with high energy density. The present study aims at experimentally investigating the direct interaction between a concentrated simulated solar radiation and a fluidized bed by measuring the time-resolved bed surface temperature with an infrared camera under different fluidization gas velocities. The effect of a localized generation of bubbles was investigated too, by injecting a chain of bubbles through a nozzle located just at the centre of the concentrated solar beam. The obtained results encourage the localized generation of bubbles, just at the larger value of the impinging radiative heat flux, as a strategy to reduce the overheating of the bed surface and, as a consequence, the energy losses related to fluidizing gas and radiative re-emission
Modelling oxy-pyrolysis of sewage sludge in a rotary kiln reactor
No full textA mathematical model of a rotary kiln oxy-pyrolyzer of sewage sludge is presented. The specific feature of the model is the axial staging of oxygen feeding to the reactor as one important key to the quality of the syngas. The fate of the gaseous components and of the tar is followed using a simplified lumped-kinetic mechanism that was purposely developed. The steady operation of the oxy-pyrolyzer is analyzed in terms of fluxes of the solid fuel and of the gaseous species, extent of fuel devolatilization, temperature of the solid and of the gas phases along the reactor. The performance of the reactor is characterized in terms of process rate and chemical composition of the produced syngas, heating value and thermal power
Experimental investigation of Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR) for thermochemical processes
No full textThe integration of Concentrating Solar Thermal technology (CST) with thermochemical processes is regarded as a frontier innovation, with potential applications to energy storage and chemical industry. In this context, the development of novel multiphase reactors for CST becomes a strategic goal. This study aims to investigate the potentiality of a Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), that incorporates a solar particle receiver/reactor and a solid-solid heat exchanger for heat recovery. A laboratory prototype is tested with a high-flux solar simulator. The bed inventory is continuously recycled to a reservoir, that can also be operated as a secondary reactor. Black proppant, mullite and doped silica are used for inert experiments. Solid circulation rates in the range 0.5–2.5 g/s and heat transfer coefficients in the range 300–900 W/(m2 K) are estimated. Calcium Looping process is chosen for reactive experiments, using a mixture of limestone and black proppant
Heat transfer in directly irradiated fluidized beds
No full textDirectly-irradiated fluidized bed solar reactors are very promising in the context of solar chemistry and concentrated solar power (CSP) applications. With proper choice of the bed solids, fluidized bed reactors can be operated at fairly high process temperatures that enable thermochemical storage with high energy density and production of solar chemicals and fuels. Bed surface overheating upon irradiation is one key to the efficiency of the fluidized bed as thermal receiver and may be responsible for sintering and/or degradation of the fluidized particles. Tailoring the hydrodynamics of the bed close to the region where the incident power is concentrated may disclose effective measures to improve the interaction between the incident radiative flux and the bed and mitigate bed surface overheating.
In the present study radiative heat transfer from a concentrated simulated solar radiation source to a fluidized bed is investigated by time-resolved infrared mapping of the bed surface temperature. A fluidized bed of silicon carbide particles (0.127 mm), whose cross-sectional area is 0.78 × 0.78 m, was directly irradiated by highly concentrated simulated solar radiation, emitted by a 4 kWel short-arc Xe lamp coupled with an elliptical reflector. The experimental apparatus is also equipped with a movable nozzle coupled with a bubble generation system located coaxially to the concentrated simulated solar beam. The interaction of the concentrated radiative flux with the fluidized particles moving under the action of bubble bursting was assessed by characterizing the time-resolved bed surface temperature as the fluidization gas velocity was varied. The effect of localized generation of bubbles was also investigated by injecting chains of multiple bubbles from the nozzle located at variable distance from the bed surface
Modeling calcium looping for CO2 capture with a solar energy-driven calciner
No full textCalcium Looping (CaL) is an interesting post-combustion CO2 capture and storage technique, but it requires the operation of an endothermal calciner through the oxy-combustion of an auxiliary fuel. The idea behind the present work is to couple a CaL process with a concentrated solar power system, in order to supply all the thermal energy required by the calciner through a renewable source. The integration of a CaL cycle with a concentrated solar power system would offer many technical, economical and environmental advantages. This integration must however account for the absence of solar energy during the night. Therefore, in the present work a process with some peculiarities has been designed, including two additional storage vessels, and making use of a different looping strategy between day and night. A model of the described system has been here developed. It consists of a population balance model on the sorbent particles in the whole system, during both day and night operation, which also takes into account the occurrence of sorbent attrition/fragmentation. The influence of the main operating parameters (sorbent residence time, sorbent/CO2 inlet molar ratio, fluidization velocity) on the CO2 capture efficiency in the carbonator, on the sorbent loss by elutriation and on the calciner thermal power demand is assessed, together with a discussion concerning the solar power system integration, with an indication of the required surface of the heliostats field per bed cross-sectional area
Investigation of a calcium looping-concentrated solar power integrated process
No full textThe integration of a Calcium Looping (CaL) cycle with a Concentrated Solar Power system can represent an important climate¬-change mitigation technology. In this work, the solar CaL process has been experimentally investigated by the use of a solar Fluidized Bed (FB) reactor. Three short arc Xe lamps of 4 kWel each, coupled with elliptical reflectors, were used as solar simulator obtaining a peak flux of nearly 3000 kW m–2. Several calcination carbonation tests were carried out on a commercial limestone sample, to evaluate the sorbent performances in terms of CO2 capture capacity increasing the number of cycles. Results show that, for the limestone sorbent at hand, the higher temperatures obtained on the FB surface do not produce a severe worsening of the reactive material properties, thus encouraging the research on the solar-driven CaL process
Calcium looping process integrated with a concentrated solar power system: assessment of limestone performance
No full textThe use of Carbon Capture and Sequestration (CCS) technologies and the exploitation of renewable sources are two of the main strategies to strongly reduce the CO2 emissions. Among them, the Calcium Looping (CaL) cycle is an important post-combustion technology for CCS that has reached the maturity of the demonstration stage, while the Concentrated Solar Power (CSP) system is a fast-growing renewable technology which relies on the use of solar energy and that owns some unquestionable advantages with respect to the most developed photovoltaic technology. The integration between a CSP system and a CaL cycle, in order to use a renewable source to supply the energy required by the calciner, would improve the performance of the CaL technology by overcoming most of its major drawbacks (e.g., an auxiliary fuel and an air separation unit would not be needed anymore). However, the role that high concentrated radiation can have on the sorbent properties in the CaL cycle is still matter of concern. In this work, the solar CaL process has been investigated by means of experimental tests through the use of a directly irradiated fluidized bed reactor. The solar simulation task is carried out by three short-arc Xe-lamps of 4 kWel each, coupled with elliptical reflectors, capable of producing a concentrated solar beam on the fluidized bed surface with a peak flux of nearly 3000 kW m–2 and a total power of nearly 3 kW. Several calcination and carbonation tests have been performed on samples of commercial limestone, in order to establish the evolution of the CO2 capture capacity of the sorbent with increasing number of cycles. A comparison between results obtained with and without the use of the solar simulator has also been performed, to better understand and optimize the exploitation of concentrated solar radiations
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