The role of CO2 and H2O on ignition chemistry in MILD (diluted, pre-­heated) combustion conditions. (Contributo in atti di convegno)

Type
Label
  • The role of CO2 and H2O on ignition chemistry in MILD (diluted, pre-­heated) combustion conditions. (Contributo in atti di convegno) (literal)
Anno
  • 2013-01-01T00:00:00+01:00 (literal)
Alternative label
  • M. de Joannon, P. Sabia, R. Ragucci, A. Cavaliere (2013)
    The role of CO2 and H2O on ignition chemistry in MILD (diluted, pre-­heated) combustion conditions.
    in CM0901 Detailed Chemical Models for Cleaner Combustion IV Annual Meeting, Perugia, Italy, 16-18 September 2013
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • M. de Joannon, P. Sabia, R. Ragucci, A. Cavaliere (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio 80, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio 80, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio 80, 80125 Napoli, Italy D.I.C.M.A.P.I, Università degli Studi di Napoli Federico II, P.le V. Tecchio 80, 80125 Napoli, Italy (literal)
Titolo
  • The role of CO2 and H2O on ignition chemistry in MILD (diluted, pre-­heated) combustion conditions. (literal)
Abstract
  • It is currently unquestionable that the use of new combustion technologies based on dilution and pre-heating of fuel and/or oxidant, such as MILD combustion [1], is among the most favorable methods to reduce the formation and/or fully eliminate some classes of pollutants. This is related to the peculiar reacting modalities from pre-heated and diluted reactants, characterized by relatively low, nearly homogeneous temperature and concentration distributions. These features also imply a strong flexibility with respect to the fuel thus allowing the use of low calorific values mixtures with high efficiency. In this framework MILD combustion appears to be one of the most suitable modes to recover energy from biofuels. The high content of diluent species, deriving either from presence of diluent in low calorific values fuels or from recirculation of flue gases to reach MILD combustion regime, namely H2O and/or CO2 makes the role of these species relevant in the oxidation chemistry in such a non standard condition [2]. The aim of the present work is to highlight the effect of the H2O and CO2 on ignition kinetics of two reference fuels. More specifically, ignition delay time has been evaluated from experimental point of view in a tubular flow reactor [2] using propane and a model gas surrogating the gaseous fraction of biomass pyrolysis products containing C1-C2 species (biogas), CO and CO2. The experimental analysis has been carried out at atmospheric pressure and inlet temperature ranging from 1000K to 1300K in presence of high dilution of either N2, or CO2 or H2O. In particular, different dilution level (90-97%) and C/O ratio (0.05-1.5) have been analyzed. As example, the ignition delays of the biogas, measured for C/O=0.2 and a dilution of 90% of the different diluent considered have been reported in the fig.1 with symbols. It has been found that for a fixed inlet temperature the ignition delay time measured in presence of CO2 or H2O is longer than the one measured using nitrogen as diluent and the extent of increase depends on fuel and diluent itself. However, such an increase of ignition delay does not prevent the use of MILD combustion processes, even if they rely on massive CO2 and H2O recirculation, in practical applications. Numerical simulations for studying the evolution both of propane and the model gaseous fuel oxidation process in the same working conditions of experimental tests have been carried out by means of several kinetic models available in literature. It has been shown that models are able to reproduce the main features of ignition and oxidation process also in such a not standard condition, even though they significantly underestimate ignition delay values. This behavior is well shown by the numerical results reported in fig.1 with solid and dashed line. Thus, a detailed kinetic analysis has been carried out with the aim to evidence the main routes in which CO2 and H2O could interfere with. CO2 and H2O can modify the evolution of the combustion process for both kinetic and thermodynamic reasons. Beside a different heat capacity of such species, they can be involved in direct or indirect way in the fuel oxidation reaction. Figure 1. Experimental and numerical ignition delay of biogas in presence of N2 or CO2 or H2O as diluent The numerical analysis has evidenced the complex effect of the diluent typology on evolution of chemical process that depends on the temperature range considered. More specifically, is has been shown that CO2 and H2O strongly alter the competition between the oxidation and recombination/pyrolytic reaction channels also acting on H2/O2 oxidation systems. [1] A. Cavaliere, M. de Joannon, Progress in Energy and Combustion Science 30-4 (2004) [2] P. Sabia, M. de Joannon, A. Picarelli, R. Ragucci, Combustion and Flame 160-1 (2013) (literal)
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