Mixing and segregation in fluidized bed thermochemical conversion of biomass

The recent shift in policy intentions catalysed by COP21 is stimulating the much-needed global energy transition giving new momentum to the move towards a lower-carbon and more efficient energy system. Bio-based energy and chemicals are taking the lead in the progress toward extensive replacement of fossil resources with renewables. Fluidized bed thermochemical conversion of biomass (combustion, gasification, pyrolysis) displays a long record of successes, spanning from lab- to industrial scales, and stems out as the most viable pathway for the exploitation of biogenic fuels, either alone or in combination with fossil fuels. In spite of its diffusion, there are still open design and operational issues that are largely related to segregation and mixing of solid and gas phases in fluidized beds and effectiveness of multiphase contacting patterns. The common claim of fluidized beds being well stirred/well controlled environments for heterogeneous and gas-phase reactions falls short when applied to processing of biomass fuels. The lecture aims at providing a comprehensive framework of fundamental phenomena affecting mixing/segregation of phases during thermochemical processing of biomass, and of their interlinks. The basic processes include patterns and kinetics of biomass devolatilization, particle and volatile matter segregation along and across the reaction chamber, particle attrition/fragmentation and generation of fine particulates, the diversity of gasification patterns and rates, as related to chemical composition and morphology of the parent biogenic fuels. Segregation brings about important consequences in terms of temperature uniformity, of proper control of heterogeneous and gas-phase reaction pathways, of ash behaviour, of pollutant emissions, of plant operability and dependability. Measures to counteract segregation, including pre-processing of biomass and/or appropriate control of bed hydrodynamics, will also be surveyed from the fundamental and applied standpoints

INFLUENCE OF PROCESS PARAMETERS ON FLUIDISED BED DRYING OF POWDERED MATERIALS

The present study reports preliminary characterization about the fluidised bed drying of powdered materials. Tests were carried out in a Lexan® lab-scale fluidised bed with solids selected to effectively surrogate powders of interest in the manufacture of pharmaceuticals. The process was monitored to correlate the temperature and the flow rate of the fluidising gas, the temperature and the moisture level in the bed, the qualitative fluidisation patterns. Bed material was characterized to assess the modifications of the population of agglomerates as a function of the operating conditions

SHEAR-ASSISTED FLUIDIZED BED POWDER-COATING

This study addresses a novel concept of dense-fluidized bed coating of objects where the effectiveness of coating is promoted by the intentional and controlled establishment of shear flow around the object. The fluidized powder is sheared by the controlled oscillatory motion of the object with respect to the fluidized bed. The proof-of-concept is given with experiments carried out using a commercial powder specifically manufactured for dry coating applications in fluidized bed. Systematic analysis of the effect of different levels of shear rate on particle mobility/adhesion and effectiveness of coverage was performed. A simple descriptive model has been developed to provide a mechanistic framework for the interpretation of the results

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

Assessment of motion-induced fluidization of dense pyroclastic gravity currents

The paper addresses some fundamental aspects of the dynamics of dense granular flows down inclines relevant to pyroclastic density currents. A simple mechanistic framework is presented to analyze the dynamics of the frontal zone, with a focus on the establishment of conditions that promote air entrainment at the head of the current and motion-induced self-fluidization of the flow. The one-dimensional momentum balance on the current along the incline is considered under the hypothesis of strongly turbulent flow and pseudo-homogeneous behaviour of the two-phase gas-solid flow. Departures from one-dimensional flow in the frontal region are also analyzed and provide the key to the assessment of air cross-flow and fluidization of the solids in the head of the current. The conditions for the establishment of steady motion of pyroclastic flows down an incline, in either the fluidized or «dry» granular states, are examined

Dynamics of Stocks and Flows in a Regenerative Economy

This seminar will present a “trailer” for material in a book – “Living well on a Finite Planet: a systems approach to sustainability, society and economy” - by Roland Clift, George Martin and Simon Mair, to be published by Springer in 2023. The book will develop an approach to socio-economic restructuring that looks beyond the Circular Economy to envisage a repurposed economy addressing the three components of sustainability: economy, environment, and society. Rather than the usual economic concern with flows, the analysis takes an industrial ecology approach: it starts from demand for the services provided by the stock of products and materials in use and works out from there, through analysis of remanufacturing and recycling, to the associated material flows which are treated as responses rather than drivers. An earlier analysis, developed by Stahel and Clift, has been extended to stocks that change over time, to generate simple metrics accounting for the effect of stock growth on material demand allowing for product life. Applying the analysis to selected scarce metals shows how it can help to understand the development of “closed loop” systems. It also reveals why setting targets in terms of “circularity” can have perverse consequences

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

Numerical simulation of hydrogen production by chemical looping reforming in a dual interconnected fluidized bed reactor

Although exploitation of dual interconnected fluidized bed systems (DIFB) is currently being explored in various fields (1,2), DIFBs present some criticalities, mainly related to effective control of solids recirculation and to avoidance of gas leakage between the beds, extremely critical in chemical looping reforming (CLR) for hydrogen production. For the latter, the choice of the degree of oxygen carrier oxidation/reduction, operation temperature and loop design makes the design even more challenging. This paper aims at quantitative assessment of the influence of design variables by means of the numerical simulation of a DIFB-CLR process operated at steady state. The model couples a simple hydrodynamic simulation of a DIFB system equipped with non-mechanical valves for bed solids circulation with a 1D, dynamic and non-isothermal CLR model developed to determine temperature and oxidation degree of solids and gaseous species concentrations at the exit of both air and fuel reactors. The DIFB (Fig. 1), consisting of a riser and of a bubbling fluidized bed (BFB) as air and fuel reactors respectively, was modelled as a combination of interconnected blocks (riser, cyclone, downcomers, L-valve, BFB, loop-seal) after selection of constitutive equations. Methane and Nickel(II) oxide were selected as fuel and oxygen carrier. Results corresponding to steady operation are presented and the effects on the expected process performance of operating conditions are assessed. It is concluded that an appropriate choice of both operating temperature and oxidation/reduction degree of oxygen carrier is an essential prerequisite in order to achieve auto-thermal regimes while assuring process feasibility and good performances in terms of CH4 conversion and H2 selectivity. Please click Additional Files below to see the full abstract