1,721,002 research outputs found
Comparison of global models of sub-bituminous coal devolatilization by means of thermogravimetric analysis
No full textCoal gasification and combustion are strongly dependent on devolatilization step. Aim of this work is to obtain the parameters of global kinetics of devolatilization of a sub-bituminous coal with high sulfur content. The kinetic parameters are obtained by means of TG experimental data, and applying different approaches to extrapolate the data to industrial relevant conditions. The simpler method is a model-free one which supposes a single step process whose Arrhenius kinetic parameters (A and E a) have to be determined. Another common approach is the distributed activation energy model (DAEM) which assumes a series of first order parallel reactions occurring and sharing the same pre-exponential factor, A, with a continuous distribution of the activation energy. For the fitting of the experimental data, a numerical solution to DAEM and two approximate methods have been evaluated. Moreover, the results of these kinetic methods based on empirical approaches were compared with simulated data obtained using a more complex model based on percolation theory with cross-linking mechanism and vapor-liquid equilibrium (chemical percolation devolatilization, CPD model), which allows to simulate the coal pyrolysis from volatile yield data. © 2014 Akadémiai Kiadó, Budapest, Hungary
Experimental evaluation of Mg- and Ca-based synthetic sorbents for CO2 capture
No full textHydrogen with high purity can be directly derived from fossil and renewable energy sources, like natural gas, coal and biomass, by the so-called sorption-enhanced reforming (SER) and water gas shift (SEWGS) processes characterized by simultaneous CO2 capture.This paper deals with CO2 capture on a solid sorbent under cyclic (carbonation/decarbonation), industrially relevant conditions. Hydrotalcite-like compounds (double Mg/Al hydroxy-carbonates), in comparison to CaO-based sorbents, are characterized by a comparatively smaller energy demand for regeneration and the operating temperature range is much lower (200-400°C), making them particularly attractive for SEWGS processes.Different kinds of hydrotalcite were prepared and the Mg2+/Al3+ ratio, the effect of a promoter addition and the substitution of the bivalent cation (Ca instead of Mg) were investigated, with commercial and laboratory synthesized samples.Original and heat-treated hydrotalcite samples were characterized by XRD, BET and SEM-EDX in order to detect composition, crystalline phases and morphology. Sorption capacity was investigated under cyclic conditions by means of TGA-DSC. Step-response experiments were performed in a micro-reactor to evaluate the kinetic behavior, and a first-order-with-dead-time model for gas mixing was used to fit the results and work out the CO2 dynamic load on the solid phase. © 2013 The Institution of Chemical Engineers
Influence of temperature on oxygen permeation through ion transport membrane to feed a biomass gasifier
No full textOxygen-permeable perovskite membranes with mixed ionic-electronic conducting properties can play an important role in the high temperature separation of oxygen from air. A detailed design of a membrane test module is presented, useful to test mechanical resistance and structural stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) capillary membrane in the reactor environment. Preliminary experimental results of membrane permeation tests highlight the positive effect of temperature on perovskite materials. This behaviour is also confirmed by a computational model of char combustion with oxygen permeated through the membrane module, when it is placed inside a gasifier reactor to provide the necessary input of heat to the gasification endothermic process. The results show that the temperature affects the oxygen permeation of the BSCF membrane remarkably. © Published under licence by IOP Publishing Ltd
Sorption enhanced steam methane reforming on catalyst-sorbent bifunctional particles: A CFD fluidized bed reactor model
No full textSorption Enhanced Steam Methane Reforming (SE-SMR) has been proposed as an efficient novel technology to increase hydrogen yield and reduce the environmental footprint in comparison to state of art H2 production processes. Sorbent/catalyst materials characterized by stable behaviour over multiple reforming/calcination cycles may ensure to achieve almost stationary operating conditions utilizing a dual fluidized bed system (the reformer and the sorbent regenerator) with a solid circulation loop. Bifunctional, Combined Sorbent-Catalyst Materials (CSCM) are under development to integrate endothermic catalytic reforming and heterogeneous CO2 sorption in one particle, decrease mass and heat transfer resistances and reduce the solid hold-up in the reactors. This paper deals with the numerical simulation of a pilot scale bubbling fluidized bed SE-SMR reactor by means of a Two-Dimensional Computational Fluid-Dynamic (2D CFD) approach. The hydrodynamic picture is supplemented with a comprehensive Particle Grain Model (PGM) previously developed to describe the kinetics of catalytic and sorption functions, and successfully validated with micro-reactor reactivity tests and multi-cycle thermo-gravimetric sorption tests. The effect of repeated carbonation-calcination steps (the “history” of the granular material) is included in the computation of the reactor performance by utilizing the appropriate size of the sorbent grains in the carbonation rate expression. The numerical results show quantitatively the positive influence of carbon dioxide sorption on the reforming process, at different operating conditions, specifically the enhancement of hydrogen yield and reduction of methane residual concentration in the reactor outlet stream. A preliminary validation of CFD simulations is also carried out utilizing experimental data obtained from a pilot scale bubbling fluidized bed SE-SMR reactor (total bed mass ≈ 14 kg). An estimate is provided for the inward heat flow that would be required to operate the reactor in stationary temperature conditions: it is substantially reduced by the exothermic sorption process and could be satisfied by means of the solid circulation loop connecting the SE-SMR reactor to the high temperature calciner in the whole dual fluidized bed system. © 2017 Elsevier Lt
Self-activation and effect of regeneration conditions in CO2-carbonate looping with CaO-Ca12Al14O33 sorbent
No full textCO2 capture by solid sorbents through uptake-regeneration cycling is a promising option for high temperature removal of CO2 from combustion gases and synthesis/fuel gases. The present study investigates the influence of regeneration atmosphere and temperature on the CO2 uptake capacity during repeated cycling of CaO-based solid sorbents. The sorbents were synthesised to contain 75 and 85% w/w of active phase (CaO) and binder (Ca12Al14O33) and were then subjected to cycling tests with repeated CO2 uptake and release in a thermogravimetric analyser TGA for up to 200 cycles. Test conditions were chosen to test high temperature CO2 capture at 600°C in an atmosphere containing 14 and 25% v/v CO2 (N2 balance). Three different regeneration conditions were tested:(a)mild condition: regeneration at 900°C in 14% CO2 or 100% N2;(b)moderate condition: regeneration at 1000°C in 14% CO2; and(c)severe condition: regeneration at 1000°C in 86% CO2.Hydration of the sorbent during synthesis and prolonged carbonation prior to the cycling tests significantly improved the stability of the uptake capacity. Interestingly, the pretreated 75% w/w CaO synthetic sorbent maintained a good uptake capacity up to the 150th cycle under severe regeneration conditions and even showed continuously increasing CO2 uptake capacity throughout the 150 cycle test with 25% CO2. The 75% w/w CaO sorbent is thus an interesting candidate for future work on high temperature CO2 capture. © 2013 Elsevier B.V
Characterization and density separation of coal gasification residues generated from the Zecomix research infrastructure
No full textThis paper presents the results of characterization investigations carried out on the solid residues produced during coal gasification tests performed in the Zecomix (Zero Emission of CarbOn with MIXed technology) research infrastructure. In this pilot-scale plant, coal is gasified in a steam/oxygen-blown bubbling fluidized bed containing olivine. The solid residues, collected both directly from the solid bed (bed ash) and downstream from it (mixed ash), were characterized in terms of their main physical, chemical and mineralogical properties with the aim of identifying suitable management strategies for each of them within the Zecomix process. Thus, an experimental protocol was also developed to separate the organic and inorganic fractions of both ash types. The main constituents of the bed ash were Mg, Si and Fe, which represent the elemental components of olivine. The total organic carbon content of the bulk bed ash was of 5%, while that of the bulk mixed ash proved to be significantly higher (24-27%). Finally, the particle size and density separation procedure developed in this work showed to be effective for separating the organic and inorganic fractions of the bulk samples of both types of residues, allowing to reach separation efficiencies higher than 90%. © 2015 Elsevier B.V. All rights reserved
Increasing CO2 carrying capacity of dolomite by means of thermal stabilization by triggered calcination
No full textThe high-temperature, solid chemical looping for CO2 capture is a promising technology to mitigate greenhouse gases emission. The choice of a high-performance sorbent is a fundamental need to improve the CO2 uptake in solid regeneration systems. Calcium-based sorbents have demonstrated a good compromise between cost, performance and environmental impact. In particular, calcined dolomite is selected as CO2-acceptor in pre-combustion processes due to its good experimental capacity for CO2 uptake. Moreover, among the solid acceptors investigated in scientific literature, naturally occurring sorbents (e.g. calcite and dolomite) are not considered as potentially hazardous substances, as they are not toxic either to the environment or to humans. This work presents the effect on CO2 carrying capacity of different compositions of the calcination atmosphere, from 100% N2 to 50/50% CO2-N2, as well as a novel pre-treatment (here called triggered calcination) by means of half-calcination in CO2 with subsequent flash N2 calcination. This new decomposition method improves CO2 capture up to 24% in prolonged carbonation/calcination cycling (over 150 cycles). Other factors have been studied such as heating rate, CO2 concentration and carbonation time, as well as other pre-treatments. Increased and sustained rates of CO2 uptake can be explained as a result of changes in the internal structure of sorbent particles. In order to explain them, a study of the surface area has been carried out by means of an indirect method based on TGA experiments. © 2014 Elsevier B.V
Oxygen transport by ionic membranes: Correlation of permeation data and prediction of char burning in a membrane-assisted biomass gasification process
No full textThis paper addresses the important issue of feeding oxygen to a fluidized bed gasifier in an efficient way, in cases of small to medium scale units (a few MWth), to obtain a syngas free of nitrogen and with relatively high calorific value, without the need to utilize a complex dual fluidized bed system. To this scope, the application to biomass conversion systems of ion transport membrane (ITM) technology for oxygen separation from air is studied by coupling an oxygen transfer model to a gasification model that considers thermodynamic and kinetic constraints. Numerical evaluations are performed of char partial combustion with oxygen permeated through the membrane, in the gasifier region close to the tubular ITM surface, as a means to provide the necessary input of heat to biomass gasification, a globally endothermic process. The results show that the membrane surface needed to provide the required oxygen flow to the gasifier is small enough to be arranged inside the fluidized bed volume, assuring feasibility of an autothermal process. The model is also helpful to optimize the location of the membrane module and evaluate different options. Experimental investigations are needed to check the resistance and durability of ITM materials in the gasifier environment. © 2014 Elsevier B.V
Hydrogen by sorption enhanced methane reforming: A grain model to study the behavior of bi-functional sorbent-catalyst particles
No full textThis work utilizes a previously developed particle grain model (PGM) for carbon dioxide CaO-based sorbents, properly integrated to describe numerically the behavior of a single particle where some catalytic activity is combined to the sorption function. In this way, the model capability is extended to the investigation of a bi-functional sorbent-catalyst particle for sorption enhanced steam methane reforming (SE-SMR) processes to produce hydrogen.The kinetic description of carbon dioxide capture by calcium oxide is assumed to be that successfully validated in a previous work by means of dynamic carbonation data obtained with calcined dolomite particles of different size fluidized by a N2/CO2 gas mixture. Further simulations presented here show the ability of the sorption model to describe faithfully the additional influence of temperature, carbon dioxide concentration in the gas phase and number of solid carbonation cycles. A state of the art methane and water gas shift kinetic model is utilized to predict the particle catalytic activity in the sorption enhanced reaction process.A numerical procedure is developed in MATLAB® to integrate over time and particle radius the model equations, assuming that small particles, of the order of those of interest for fluidized bed reactors (dp=500 μm), are in contact with different gas phases of constant composition.The results show that conversion of the sorbent grains and the increasing thickness of the calcium carbonate layer around them make carbon dioxide sorption and methane reforming rate strong functions of residence time of particle in the reacting atmosphere, with different scenarios for the interaction between catalytic steam reforming and CO2 sorption.The model predicts that, with sufficient amount of calcium oxide inside the particle, conditions exist where the time averaged rates of carbon dioxide sorption, methane reforming and water gas shift, respectively, are such that a perfect balance exists between carbon dioxide captured by the solid phase and CO+CO2 produced by the reforming reactions. © 2016 Elsevier Ltd
Integration of a calcium looping process (CaL) to molten carbonate fuel cells (MCFCs), as carbon concentration system: First findings
No full textThe utilisation of coal as fuel for the production of energy will grow parallel with the increase of the cost of oil in the next years. This paper aims to investigate the integration between two clean coal technologies: Calcium Looping (CaL) process and Molten Carbonate Fuel Cell (MCFC) in order to produce a high CO2 concentrated stream. The main goal of this work is to find out the optimum working point of a system using CaL and MCFC technologies, coupled together, to produce decarbonised energy from coal as a primary energy source. The integrated system of CaL with MCFC presented in this paper, is fed with a raw syngas coming from coal gasification [1]. The raw syngas is decarbonised using calcium oxide as solid sorbent in the first reactor of CaL (carbonator), subsequently the clean syngas flowing out of the carbonator is used as anodic fuel for an MCFC. The thermal regeneration of solid sorbents occurs burning methane with air producing a CO2 reach gas, that feeds the cathodic compartment of MCFC. This configuration allows to concentrate the CO2 from cathode side to anode side of the MCFC, using internal electrochemical reactions of the cell, producing electric power at the same time. This work has been structured to tackle the coupling of CaL with MCFC using a combined numerical and experimental approach. Thus the investigation of the possible integration has been carried out starting with a lumped model simulating the whole calcium looping process. The model was used to investigate the behaviour of the CaL system when varying the amount of solid sorbent used in the process. Data coming from the model in terms of gas composition flowing out from CaL reactors were subsequently validated experimentally simulating different operating conditions in a MCFC single cell (81 cm2). Performance of the MCFC was monitored with polarisation curves and power density curves, aiming to integrate experimentally the electric behaviour of the whole system, in order to have a first validation of the two systems working together. © 2018 Elsevier Ltd. All rights reserved
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