1,720,986 research outputs found
Plant and process for the looping-type combustion of solid carbon-containing fuels
No full textThe invention relates to a plant and a process for the looping-type combustion of solid carbon-containing fuels with a carbon dioxide (CO2) flow output. Said process carries out the conversion of carbon without the help of solid carriers of the MyOx type, or of sulphate/sulphide type, and comprises the steps of: (i) Oxidation, wherein the carbon-containing solids are contacted with a gaseous flow comprising oxygen, for a time period and at a temperature sufficient to allow formation of a surface oxidized complex; (ii) Desorption, wherein the surface oxidized complexes generated by adsorption of oxygen in item (i) are released in a gaseous form by decomposition in the absence of O2
Impianto e processo per combustione di tipo looping di solidi carboniosi
No full textL’invenzione è relativa ad un impianto e processo per la “looping combustion” di combustibili solidi carboniosi con produzione di una corrente di anidride carbonica (CO2). Detto processo realizza la conversione del carbonio senza l’ausilio di vettori solidi di ossigeno del tipo , e comprende gli stadi di:
(i) Ossidazione, in cui i solidi carboniosi vengono posti in contatto con una corrente gassosa comprendente ossigeno, per un tempo e ad una temperatura sufficienti per formare un complesso superficiale ossidato;
(ii) Desorbimento, in cui i complessi superficiali ossidati generati per adsorbimento di ossigeno al punto i) sono rilasciati per decomposizione in carenza di O2 sotto forma gassosa
Assessment of the thermochemistry of oxygen chemisorption and surface oxide desorption during looping combustion of coal char
No full textCarbons have a pronounced tendency to chemisorb oxygen at moderate temperatures and to desorb surface oxides as combustion products (CO, CO 2) once the oxidized fuel is heated up under inert conditions. Based on this feature, a novel process of looping combustion of carbons (CarboLoop) has been recently proposed, and a proof-of-concept given. The process is based on alternated oxygen chemisorption on carbon and desorption of surface oxides carried out in dual bed reactors. One of the reactors, operated with an air feeding, acts as the fuel oxidizer. The second reactor, operated with partly recycled gaseous effluents (CO2 + impurities), acts as the fuel desorber. Further development of the CarboLoop concept requires the knowledge of the thermochemistry associated with the individual steps of carbon oxidation and of surface oxide desorption, which dictates the energy integration between the reactors. The present study moves along this path and provides a separate assessment of the thermochemistry of carbon oxidation and desorption stages. The procedure is based on complementary thermoanalytical techniques (TG, DSC, TPD) as well as on calorimetry. Experimental results obtained with a bituminous coal char indicate that both oxygen uptake (??H ??? -4 kJ/gc) and desorption (??H ??? -31 kJ/gc) are exothermic. The strong exothermicity of the desorption stage, at odds with published data on CO and CO2 abstraction from oxidized carbons, is explained in the light of the cocurrent progress of the stabilization of metastable surface oxides and of abstraction of CO and CO2 from oxidized carbon
The relevance of slag formation and wall burning to entrained-flow combustion and gasification of coal
No full textDevelopment of a dry bottom ash extraction/afterburning system from pulverized fuel co-fired utility boilers
No full textCo-firing of coal with waste/biomass in PF boilers is frequently associated with coarse unconverted waste/biomass particles reporting to the bottom ash. Accordingly, ash postcombustion may still be active during dry bottom ash extraction from utility boilers. This phenomenon has been frequently observed in plants equipped with dry ash belt extraction/conveying systems, such as the MAC (Magaldi Ash Cooler) proprietary technology. In the present work a model is presented that describes the fate of ash particles from a PC boiler co-fired with coal and RDF during their fall-out from the boiler and during the subsequent extraction on the belt-conveyor. The physico-chemical properties and the reactivity of ashes have been characterised experimentally. The model deals with the broad distribution of ash particles falling from the furnace on the belt and the establishment of a polydisperse granular bed on the conveyor. Modelling of heat transfer and combustion in the granular bed of ash is accomplished considering two distinct length- and time-scales: at the macroscale the deposited solids undergo transient heat transfer within the granular bed and with the environment; at the microscale ash particles of different size and temperature exchange heat among themselves, equalising their temperatures. Model results confirm that extensive carbon conversion can be achieved on the belt and that the MAC system can be effectively implemented to operate as a bottom ash afterburner
Preliminary Assessment of a Concept of Looping Combustion of Carbon
No full textA novel concept of looping combustion of carbon (CarboLoop) is presented. It is based on the feature of carbons to extensively uptake oxygen upon exposure to air at moderate temperatures. Surface oxides of carbon are eventually released as combustion products (CO, CO(2),) as the oxidized fuel is brought to moderate-to-high temperature in an oxygen-free atmosphere. This concept is pursued to the formulation of a preliminary scheme of a looping combustor of carbons based on a dual interconnected bed reactor system. One of the reactors is air-blown and acts as the fuel oxidizer. The second reactor, operated with partial recycle of gaseous effluents (CO(2),+ impurities), acts as the fuel desorber. Operating conditions of the oxidation and desorption stages may be properly tuned, leading to alternative looping strategies. The present study lays down the basic mechanistic background for analyzing the process, based on a simplified semiglobal approach to combustion and oxidation of solid carbons. The soundness of the CarboLoop concept has been verified by purposely designed experiments. The alternated oxidation/desorption stages typical of a looping combustor are simulated in a thermogravimetric analyzer. Experiments were directed to monitoring the oxidation and desorption steps under simulated looping conditions. Graphitized coke has been used as a surrogate carbon fuel. The experimental results confirm the soundness and potential of the CarboLoop concept and lay the path for its further development
Beneficiation of coal fly ashes by oxygen chemisorption
No full textThis paper addresses the issue of unburnt carbon in fly ashes from coal-fired power stations. It has been shown that carbon-in-ash has much lower combustion reactivity than the original coal, because of the extensive thermal annealing experienced in the boiler. Thermal annealing reduces the rate of the first reaction step of which carbon combustion is composed, namely oxygen chemisorption. In the present work experiments have been carried out in order to verify if a preconditioning stage consisting of mild pre-oxidation with air is able to promote oxygen chemisorption thus increasing the combustion reactivity of ashes. Fly ashes with high LOI have been exposed to air at temperature <400 °C for times up to 300. min. Results show that a satisfactory extent of oxygen chemisorption can be attained at 300-400 °C with 1-2. h holding time. After this conditioning, samples have been tested to check their combustion reactivity by means of non isothermal thermogravimetric analysis. Additionally, combustion experiments have been carried in a purposely designed suspension reactor at temperatures up to 1000 °C. Results confirm that ash pre-conditioning reduces the burn-out time of carbon in ash. The concept has been finalized into an international patent application
A Semi-Detailed Kinetic Model of Char Combustion with Consideration of Thermal Annealing
No full textThe present paper presents a semi-detailed kinetic model of coal char combustion which embodies consideration
of thermal annealing as a mechanism leading to the loss of char combustion reactivity along
burn off. The distinctive feature of this model is that deactivation induced by thermal annealing is followed
along with combustion. Thermodeactivation is modelled according to the power-law equation proposed by
Senneca and Salatino [1]. A semi-detailed combustion mechanism was taken after Hurt and Calo [2] and
includes three steps: formation of carbon–oxygen complexes (chemisorption), switch-over of surface oxides
and desorption of oxygen complexes to yield combustion products. Computation results allow to discuss
the impact of thermal annealing on char combustion under conditions of practical interest.
2010 The Combustion Institute
Relevance of structure, fragmentation and reactivity of coal to combustion and oxy-combustion
No full textThe nature and extent of the microstructural changes of coal upon oxidation under conditions typical of combustion systems, and the relation between microstructure and reactivity towards oxygen and carbon dioxide have been extensively addressed. The first major changes in morphology, structure and chemical composition of the solid fuel occur in the early instances of combustion/gasification processes, during heat up and devolatilization. At this stage particles can be broken into smaller fragments as a consequence of thermal stress and pressure generated by volatiles release. This phenomenon, regarded to as primary fragmentation, is considered responsible of important changes in the fuel particle size distribution within reactors.
At longer time scales the concurrent heating and heterogeneous reactions may further affect the structure and the reactivity of chars: on one side the prolonged heat exposure induces thermal annealing with progressive graphitization and loss of reactivity. On the other side, additional phenomena, referred to as secondary fragmentation and attrition by abrasion can occur in parallel with char burning. Eventually, for large extents of internal burning, the increase of particle voidage can induce percolative fragmentation. The distinction between the individual fragmentation processes, just as that between pyrolysis and thermal annealing, is more operational than conceptual and depends on the timescale of the processes and on the nature of the fuel. On the whole, the course of combustion/gasification of a solid fuel is ruled by the competition between heterogeneous reactions of the solid fuel and a number of complex phenomena that modify char structure and reactivity.
Limited information is available in the literature on the relationship among structure, fragmentation, and reactivity of coal in oxy-combustion conditions. The transferability of the lessons learnt under conventional combustion and gasification conditions to the case of oxy-combustion is here discussed to stress the need of additional investigation on this topic
GRAPHITE COMBUSTION IN HIGH ENTHALPY SUPERSONIC FLOW
No full textThe paper reports on the set up of an experimental protocol for the investigation of high temperature (1500K<T<2500K) graphite combustion in supersonic flow conditions. Cylindrical graphite specimens (3mm D, 100mm L) are exposed to supersonic flow of
nitrogen/oxygen mixtures in a small Planetary Entry Simulator, equipped with a plasma torch. The impact of the gas flow on the specimen determines a very sharp temperature rise. A fast IR camera allows to realize two-dimensional maps of the specimen temperature throughout the experiment. IR thermal images can also be of help to rebuild the consumption of the graphite rod. Results are checked against the sample weight loss and used to estimate the rate of carbon combustion and the oxygen concentration in the freestream
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