Modelling fast pyrolysis in a fluidized bed reactor: the role of heterogeneous secondary reactions and char loading

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Characterization of fluidized bed pyrolysis of sewage sludge by time-resolved pressure measurements

The management of sewage sludge in an economically and environmentally acceptable manner is one of the critical issues facing society today. Due to industrialization and urbanisation, production of wastewater sludge has dramatically increased in the last years and this is expected to continue in the future. The environmental legislation is becoming more and more restrictive as regards landfilling of this biodegradable waste and the use of sewage sludge in agriculture is often hindered due to the possible presence of heavy metals and pathogens. The disposal of wastewater sewage sludge by means of thermochemical conversion appears to be a potentially useful strategy to avoid landfill disposal and, at the same time, to exploit sludge as a source of energy and valuable chemicals. Fluidization technology applied to thermochemical processes, like combustion, gasification and pyrolysis, is an attractive option, due to its favorable characteristics: inherent operational flexibility, high efficiency, low pollutant emissions, ability to effectively accomplish destruction of micro-pollutants and pathogens (Werther and Ogada, 1999). Devolatilization of sewage sludge granules during thermochemical processing in fluidized beds plays a crucial role in the design and performance of fluidized bed converters. Uneven axial and radial distribution of volatile matter in the fluidized-bed combustor/gasifier is commonly experienced in industrial units and is determined by in-bed emission of volatile matter which is responsible for the enhancement of axial fuel particle segregation. On the other hand, the competition between fuel devolatilization and radial solids mixing crucially affects the radial distribution of volatile matter across the reactor and emphasizes the relevance of the devolatilization kinetics to volatile matter segregation. Short devolatilization times promote the release of volatile matter above the bed and close to the fuel feeding points. Please click Additional Files below to see the full abstract

Particle-wall interactions in entrained-flow slagging gasifiers

This paper aimed at the development of a phenomenological model of the fate of coal/ash particles in entrained-flow slagging coal gasifiers, which considers the establishment of a particle segregated phase in the near-wall region of the gasifier. In particular, near-wall phenomena were investigated and mechanistic understanding of particle–wall interaction patterns in entrained-flow gasifiers was pursued using the tool of physical modeling. Montan wax was used to mimic, at atmospheric conditions, particle-wall interactions relevant in entrained-flow gasifiers. As a matter of fact, this wax had rheological/mechanical properties resembling under molten state, those of a typical coal slag and, under solid state, those of char particles. Experiments have been carried out in a lab-scale cold entrained-flow reactor, equipped with a nozzle whence molten wax atomized into a mainstream of air to simulate the near-wall fate of char/ash particles in a real hot environment. The four particle-wall interaction regimes were investigated. The partitioning of the wax droplets/particles into the different phases was characterized by their selective collection at the reactor exhaust. Results showed that the particlewall interaction mechanisms and segregation patterns are deeply affected by the stickiness of both the wall layer and the impinging particle. In particular, the micromechanical interaction of a particle with a sticky wall enhances particle transport to the wall and the tendency to reach a segregationcoverage regime with the formation of a dense-dispersed phase in the near-wall region of the reactor. Furthermore, increasing the mainstream air flow rate induces particle segregation and accumulation phenomena

Laser Diagnostics of Hydrodynamics and Gas-Mixing in the Splash Zone of Gas-Fluidized Beds

The hydrodynamic patterns of gas flow associated with bubbles bursting at the surface of gas-fluidized beds have been investigated by means of planar laser induced fluorescence using acetone as diffusive gas tracer. The flow structures generated by the eruption of an isolated bubble have been characterized as a function of bed material size and of bubble injection level

Hydrodynamics of compartmented fluidized beds for concentrated solar power applications

Application of fluidized beds to collection and thermal storage of solar radiation is beneficial in Concentrated Solar Power (CSP) systems thanks to their well-known inherently good thermal performances. Non-conventional design and operation of fluidized beds based on uneven or unsteady (pulsed) fluidization (1), may further enhance their thermal performances improving the potential for applications in the very demanding CSP systems. Dense gas-fluidized beds have the potential to effectively accomplish three complementary tasks: the collection of incident solar radiation; the heat transfer of the incident power to immersed tube bundles of high-efficiency steam and/or organic Rankine cycles (ORC) and the thermal energy storage equalizing the inherent time-variability of the incident radiation for stationary CHP generation. A novel concept of solar receiver for CHP (combined heat and power) generation consisting of a compartmented dense gas fluidized bed has been proposed (2). The present study addresses the hydrodynamics of a dense gas-fluidized bed operated at ambient conditions and equipped with a compartmented windbox. Figure 1 outlines the experimental apparatus which consists of a nearly-2D fluidization column (2850x1860x200mm) equipped with two sparger-type gas distributors extending along 20 and 80% of the fluidized bed width. The regions of the bed above the two spargers were marked as compartments A and NA, respectively. The bed material was fine silica sand with a mean Sauter diameter of 145 µm. A pressure measurement system was used to monitor pressures and pressure gradients at different locations inside the fluidized bed. A pressure gradient exceeding a threshold of 0.11mbar/mm was assumed to mark the onset of local fluidization. A procedure was developed to draw the separation boundary (dashed line) between fluidized and non-fluidized regions for different bed heights (0.55, 0.95, 1.39 and 1.85m) as the gas superficial velocity was varied in either regions of the bed (UA and UNA=0-4Umf). Selected fluidization maps are shown in figure 2 where separation boundaries at different values of UA and UNA for a static bed height of 1.85 are reported. Figure 3 reports the fractional extension of the fluidized region at a level of 400mm for different values of UA e UNA as the static bed height was varied. Results indicate that a perfectly compartmented fluidized bed cannot be obtained simply using a compartmented windbox, but a proper choice of the operating conditions enables good control of the local fluidization conditions and of the gas cross-flow between the compartments. Please click Additional Files below to see the full abstract

HYDRODYNAMICS OF UNCONVENTIONAL FLUIDIZED BEDS: SOLIDS FLOW PATTERNS AND THEIR INFLUENCE ON MIXING/SEGREGATION OF A LARGE FLOTSAM PARTICLE IN A BED OF FINER SOLIDS

Gross solids circulation of solid phase and its influence on mixing/segregation of a large flotsam particle in beds of finer solids in unconventional fluidized beds has been investigated. A tapered two-dimensional fluidization column and a fluidization column equipped with a diverging cone as gas distributor have been adopted. The hydrodynamics of the gas-solid suspension in the two apparatus has been qualitatively assessed by visual observation and the trajectories of the centre-of-gravity of large flotsam particles have been evaluated to assess the extent of mixing/segregation

Modelling of a chemical looping combustion system equipped with a two- stage fuel reactor

The proper selection of the oxygen carrier and the correct design of the fuel reactor represent the main criticalities for the success of the chemical looping combustion (CLC) process for solid fuels. In a previous work (1) a two-stage fuel reactor (t-FR), consisting of two bubbling beds in series (bottom bed and top bed) (Fig. 1), has been proposed in order to overcome the limitations of a single-stage fuel reactor (poor char conversion, slip of unburnt volatiles, extensive elutriation of char fines). A mathematical model has been developed with the aim of assessing the performances of the two-stage fuel reactor varying operating conditions in comparison with a benchmark case consisting of a single-stage fuel reactor equipped with and without carbon stripper. The t-FR showed the best performances in terms of combustion efficiency, volatile matter and char conversion, carbon-to-CO2 conversion efficiency and loss of elutriated carbon for all the operating conditions investigated. In the present work a further enhancement of the model has been developed in order to study the hydrodynamics of the proposed multiple interconnected fluidized beds (MIFB) system for the CLC of solid fuels. The modelled system consists of the two-stage fuel reactor, a riser (Air Reactor) and non-mechanical valves for the regulation of the solid circulation between the two reactors. The different parts are considered as separate blocks mutually interconnected (Fig. 1). The operation of the system has been simulated by considering chemical looping combustion of a bituminous coal with an oxygen carrier consisting of CuO supported on zirconia. The numerical simulation has been addressed to evaluate (at steady state) the solid circulation rate, the temperature and oxidation degree of solids and concentrations profiles of gaseous species at the exit of both air and fuel reactors, with the utilization of proper constitutive equations for each block. Specific attention has been paid to the fluid dynamic behaviour of the t-FR. Results of the CLC-MIFB system with the t-FR are presented and the effects on the feasibility of the process of a variation in operating conditions are commented. Please click Additional Files below to see the full abstract

Hydrodynamics of a Loop-seal Operated in a Circulating Fluidized Bed: Influence of The Operating Conditions on Gas and Solid Flow Patterns

Hydrodynamic features of a loop-seal operated as solids re-injection device in a labscale cold CFB apparatus are studied. Gas flow patterns are characterized by means of gas tracing experiments with continuous injection of CO2 in the loop-seal chambers. Solids flow patterns are characterized by impulsive injection of dyecoloured particles into the supply chamber, followed by particle tracking

Thermal behaviour of fluidized beds directly irradiated by a concentrated solar radiation

Directly-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

Simulating particle-wall micromechanical interaction in entrained-flow slagging gasifiers by cold impact experiments

This paper deals with particle–wall interaction phenomena in entrained-flow slagging coal gasifiers. Different micromechanical char–slag interaction patterns may establish, depending on the stickiness of the wall layer and of the impinging char particle. Micromechanical interaction patterns were studied by means of an appropriate experimental apparatus which permitted to record a single particle/droplet impact on a flat surface. Montan wax was used to simulate, at nearly ambient temperature, the char–slag rebound characteristics upon colliding the wall. Particle–wall collision was described in terms of the particle restitution coefficient. In particular, the influence of the particle temperature and the impact angle as well as the impact velocity and the target surface on the rebound characteristics was studied. Results highlighted that the particle restitution coefficient decreased when enhancing operating conditions (temperature and impact velocity) able to promote plastic deformation upon particle impact