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Influence of Turbulence-Chemical Interaction on CFD Pulverized Coal Mild Combustion Modelling
No full textMILD (Moderate and Intensive Low oxygen Dilution) combustion is a novel approach to reducing NOx
emissions and improving combustion efficiency in fossil fuels power plants. It is characterized by elevated
temperature and high dilution of reactants and strong recirculation inside the combustion chamber
which produce a low temperature increase, thus reducing NOx formation. The main differences with conventional
combustion concern the chemical reactions that take place in almost the entire volume of the
combustion chamber and the uniformity of both temperature and the chemical species concentration. For
this reason advanced turbulence-chemistry interaction models with detailed kinetic mechanisms are
required to accurately simulate MILD by means of CFD calculations.
The main aim of this work is to deepen the influence of turbulence-chemistry interaction on pulverized
coal MILD combustion and to understand which models are more accurate and suitable to reproduce the
process. In particular, two turbulence-chemistry interaction models are analyzed. On one hand, a conventional
model based on infinitely fast chemistry Eddy Dissipation Model with a two-step global kinetic
mechanism is considered. On the other hand, an advanced model based on finite rate chemistry Eddy
Dissipation Concept is considered and used with both a global and detailed kinetic mechanisms. The
results are finally compared with an experimental test-case.
From the comparison, advanced turbulence-chemistry models used with complex kinetic mechanisms
give, as expected, the best agreement with numerical results, despite the higher computational resources
required
Numerical simulation of pulverized coal oxy-combustion with exhaust gas recirculation
No full textCapturing CO2 from a conventional coal-fired power plant is a very expensive approach
for reducing CO2 emissions because of the low CO2 partial pressure in combustion products.
One option to reduce the capture cost is to burn coal with O2 and recirculated exhaust gas.
In this way the main products of combustion are H2O and CO2, which can be easily separated.
Furthermore, NOx emissions are smaller owing to the lower N2 fraction in the gas and the lower
flame temperature, while producing pure O2 is also highly expensive.
In this paper the performances of pulverized coal combustion with exhaust gas recirculation
are evaluated by means of the CFD commercial code Fluent, using advanced mathematical
models for coal devolatilization and for turbulence-chemistry interaction in the gas phase.
Advanced combustion models, accounting for finite reaction rates, are required to simulate
pulverized coal oxycombustion since the high fraction of CO2 reduces the flame temperature
due to the higher specific heat, thus determining combustion instabilities related to a low reaction
rate, such as extinction and ignition. The pulverized coal combustion with recirculated
exhaust gas is evaluated by considering the same burner geometry and operating condition used
for the conventional air combustion test-case in IFRF no.1 furnace. The secondary air stream is
replaced by a O2/CO2 mixture with the same O2 volume fraction and the same molar flow rate,
and keeping the same velocity profile.
The performances of oxy-coal combustion with exhaust gas recirculation are evaluated and
compared with conventional air combustion in terms of burner efficiency, flame characteristics,
emissions and other combustion parameters. Two oxycombustion cases are considered,
characterized by the same O2-to-coal ratio: in the first case the secondary molar flow rate is
kept constant as in the air combustion case (21%O2-79%CO2 by volume), in the second one
the secondary mass flow rate is kept constant (30%O2-70%CO2 by volume). The first case is
characterized by similar fluid dynamic behaviour of the air combustion, but by a lower flame
temperature related to the lower CO2 specific heat. The second case has similar flame temperature,
but fluid dynamic behaviour is different because of the different inlet velocity of secondary
stream. Oxycombustion flame are deeply different respect to conventional flame because of the
different physical properties of the O2-CO2 mixture. Therefore they require an optimization of
the burner geometry and of the O2-CO2 mixture composition in order to improve flame characteristics
and reduce pollutants emissions
Sviluppo e validazione di un modello CFD avanzato per la combustione del polverino di carbone
No full textThis work reports the development of an advanced CFD model for pulverizing coal combustion, performed with the commercial
CFD code Fluent. Coal combustion is characterized by complex chemical-physical phenomena, that require to be modeled through an
advanced mathematical approach. Two different approaches are considered. The first one , defined as EDM-SR, is based on a simple
single-rate (SR) empirical devolatilization model and on the Eddy Dissipation Model (EDM) for chemical-turbulence interactions,
considering initely fast chemical reactions. It represents a standard approach used for pulverized coal combustion simulations. The
second approach, defined as EDC-CPD, is based on advanced Chemical Percolation Devolatilization (CPD) model, and on Eddy
Dissipation Concet (EDC) for chemical-turbulence interactions. The results are compared with experimental data performed on IFRF
no.1 furnace. The EDC-CPD approach shows the best agreement with experimental data in terms of velocities, temperature and
species concentrations. The use of this approach is therefore more suitable for modeling pulverized coal combustion, nevertheless
the higher CPU time required
Sviluppo e validazione di un modello CFD avanzato per la combustione del polverino di carbone
No full textIn questo articolo è presentato un modello CFD avanzato per la combustione del polverino di carbone, realizzato con il software commerciale
Fluent. La combustione del carbone è caratterizzata da complessi fenomeni chimico-fisici, che richiedono di essere descritti attraverso specifici
modelli matematici. Si considerano due differenti approcci per la simulazione della combustione del carbone. Il primo approccio, definito
EDM-SR, si basa su un semplice modello di volatilizzazione empirico ad una velocità (single rate SR) e sul modello EDM per l’interazione tra
il flusso turbolento e le reazioni chimiche, basato sull’ipotesi di reazioni chimiche infinitamente veloci. Esso rappresenta l’approccio standard
utilizzato per la modellazione della conversione termo-fisica dei combustibili solidi. Il secondo approccio
EDC-CPD si basa sul modello avanzato di volatilizzazione CPD, e sul modello EDC per l’interazione tra chimica e turbolenza. I due approcci
sono validati considerando i dati sperimentali della fornace IFRF no.1. Dal confronto dei risultati numerici con quelli sperimentali risulta che il
secondo approccio fornisce delle previsioni maggiormente accurate in termini di velocità, temperature e concentrazione delle specie chimiche.
Il suo utilizzo è quindi fortemente consigliato, nonostante richieda delle risorse ed un tempo di calcolo notevolmente superiori
Comprehensive CFD Model of Air-Blown Coal-Fired Updraft Gasifier”
No full textA comprehensive CFD model has been developed to simulate the gasification process within an air-blown
updraft coal gasifier. Updraft fixed bed gasification processes are characterized by complex behavior,
since they involve different space- and time-dependent sub-processes where coal preheating, drying,
de-volatilization and char reactions take place. Simplified models, such as non-dimensional ones, useful
for preliminary gross mass and energy balance, are unable to correctly simulate the overall gasification
phenomena and more sophisticated approaches are required.
In particular, CFD models could be used to describe in a detailed way the complex time- and spacedependent
phenomena involved in the gasification process. Considering the high volume fraction of
the solid phase, close to the packing condition, the Euler–Euler approach is required to model this multiphase
flow. The solid phase is considered as a continua according to the kinetic and plastic theory of
granular flows.
The operation of a Wellman–Galusha gasifier is investigated, considering a non-continuous loading of
coal and extraction of the ash, with the aim of characterizing the space- and time-dependent behavior of the process
Comparative analysis of hydrogen combustion power plants integrated with coal gasification and CO2 removal
No full textTwo power generation systems with pre-combustion CO2 capture fuelled with hydrogen from coal gasification are analyzed and compared from a thermodynamlc and economic standpoint. The first solution, referred as Integrated Gasification Combined Cycle with CO2 Removal (IGCC-CR), Is fuelled with hydrogen produced by the integrated gasification section. The second, referred as Integrated Gasification Hydrogen Cycle (IGHC), is based on the oxycombustlon of hydrogen, producing steam that expands through an advanced high temperature steam turbine. The two H2 production sections are similar for both power plants, some minor modifications having been made to achieve better Integration with the corresponding power sections. System performance is investigated using coherent assumptions to enable comparative analysis on the same basis. The plants have overall efficiencies of around 39.8% for IGCC-CR and 40.6% for IGHC, slightly lower than conventional IGCCs (without CO2 capture) with a CO2 removal efficiencies of 91% and 100% respectively. Lastly a preliminary economic analysis shows an Increase In the cost of electricity compared to conventional IGCCs of about 44% for IGCC-CR and 50% IGHC. Copyright © 2006 by ASME
Zero emission hydrogen-oxygen internal combustion steam cycles with coal gasification and CO2 removal
No full textThis paper analyses the performance of an advanced zero-emission power plant based on a
stoichiometric hydrogen-oxygen combustion steam turbine cycle, integrated with coal gasification
and physical absorption and liquefaction of carbon dioxide. The power cycle is a sort of
innovative combined cycle, with steam as working fluid. The topping cycle is a steam Brayton
Cycle, the bottoming one a supercritical Hirn cycle.
The attention is focused on optimisation of the power cycle and evaluation of the power requirements
for hydrogen production. In particular, two different technology levels for the inlet
turbine temperatures are investigated: the first level scenario reproduces the inlet temperature
of the modern gas turbine and ultra-critical steam cycle, while the second represents the temperatures
that could be attained in the near future. The best configuration showed a net thermal
efficiency of approximately 42% with zero-emission
Two-dimensional CFD model of air-blown coal-fired updraft gasifier
No full textA CFD two-dimensional model has been developed for simulating the gasification process
within an air-blown updraft coal gasifier. Fixed-bed gasification processes are characterized by a
complex behaviour since they involve different space- and time-dependent sub-processes where
coal preheating and drying, devolatilization and char reactions take place. Simplified models,
such as non-dimensional ones, useful for preliminary gross mass and energy balance, are unable
to correctly simulate in detail the overall gasification phenomena and more sophisticated CFD
models are required for their understanding. The complexity of the physical processes in the
updraft gasifier is compounded by the multiphase nature of the flow and by the interphase processes.
Considering the high volume fraction of the solid phase, close to the packing condition,
the Euler-Euler approach is required to model the interpenetrating phases. The solid phase is
considered as a continuum according to the kinetic theory of granular flows. The aim of this
work is to characterize the spatial and time-dependent behaviour of updraft gasifiers in terms of
gas velocity, temperature and species concentration. In particular, the dynamic behaviour of the
process is fundamental to understanding the time required for complete coal conversion
Influence of Turbulence-Chemical Interaction on Modelling of a Pulverized Coal Oxycombustion Flame Operating at Low O2 Concentration
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