Thermal treatment under high-vacuum of tars relevant in combustion and material science

The composition of tars, typically derived from coal and heavy fuel processing or formed in fuel-rich combustion, determines their transformation into carbons relevant in combustion and environmental fields as well as for material production. The speciation of the huge number of aromatic components of tars, usually found in form of viscous black liquid or solid, is not straightforward because of the tar complexity and high molecular weight, spanning from few hundreds up to thousands of Da. To this regard, the pre-separation of tar in lighter and heavier fractions simplifies the further characterization of its composition. The present work reports a fractionation method of a typical sample of combustion-formed tar based on moderate heating in high-vacuum conditions (10-6 mbar). It was preliminarily tested on a single polycyclic aromatic hydrocarbon, coronene, and on synthetic mixtures of polycyclic aromatic hydrocarbons, presumed to be the basic aromatic moieties of tar components. Lighter components obtained by condensation/deposition as thin films and/or crystals, as well as the heavier residue, were analysed by optical microscopy and spectroscopy. The separation procedure allowed to get more information on the components distribution also inferring the self-organization in cluster assembly and/or crystal forms

DYNAMIC BEHAVIOUR OF METHANE OXIDATION IN PREMIXED FLOW REACTOR

Thermokinetic temperature oscillations related to oxidation of small hydrocarbons have not been extensively studied yet, because they occur in a temperature and pressure range not relevant for practical applications. Exploitation of new combustion methodologies such as Mild Combustion also indicated such a phenomenology in small-hydrocarbons oxidation. In this paper, experimental characterization of dynamic behavior occurring in methane mild combustion in premixed flow conditions reported in a previous work was extended and a comparative analysis with data obtained by means of numerical studies was performed. The experimental study was carried out in an atmospheric jet-stirred flow reactor at different inlet temperature and mixture compositions. Several typologies of temperature oscillations were identified whose amplitudes and frequencies strongly depend on the temperature and carbon/oxygen ratio considered. These dynamic behaviors were tentatively explained by means of a rate of production analysis performed by using different methane oxidation kinetic models available in the literature. It was shown that the CH 3 recombination path present in the methane oxidation mechanism plays a key role in modulation of temperature oscillations

HYDROGEN-ENRICHED METHANE MILD COMBUSTION IN A WELL STIRRED REACTOR,

The reduction of pollutants emission, such as NOx and soot, can be achieved by lowering and controlling the adiabatic temperature of the system. Mild Combustion processes employ great amounts of diluents and high inlet temperatures of reactants. The first aspect guarantees a high heat capacity of the system, hence lower working temperatures with respect to a traditional oxidation process; the latter feature comes out from the necessity to sustain the oxidation process with no-flammable mixtures working with so high dilution degree. This new kind of combustion presents several features that deserve a systematic study. Previous experimental works carried out in a jet stirred flow reactor showed a complex dynamic behavior of methane Mild Combustion. In practical applications, such instabilities do not allow to control the working temperature; thus they lower the efficiency of the combustion processes. In this paper, the effect of hydrogen addition to the methane combustion in Mild conditions was studied, with particular focus on the dynamic behavior detected in the past. The experimental results showed that the higher the inlet hydrogen content, the higher the increase of the system reactivity and a more significant reduction of the area in the Tin–C/O plane where thermo-kinetics oscillations were observed. However, hydrogen addition did not affect significantly the typology of oscillations. Furthermore, it was observed that in the Tin–C/O plane the dynamic region shifts towards lower temperatures as well as the irregular oscillation region extends as the hydrogen concentration increases. The experimental Tin–C/O maps were then compared with the numerical ones, obtained with different kinetic mechanisms available in the literature. An overall good agreement was found between experimental and numerical data. The evolution of the oscillations region and the increase of reactivity due to the hydrogen addition were predicted quite well

ANALYSIS OF PROCESS PARAMETERS FOR STEADY OPERATIONS IN METHANE MILD COMBUSTION TECHNOLOGYANALYSIS

The main process parameters affecting combustors of all types are analyzed in the range of interest concerning mild combustion processes for methane oxidation. They are studied by means of direct comparison between experimental measurements made in a Jet Stirred Flow Reactor and numerical predictions based on a kinetic scheme developed for general use. Wide ranges of both inlet temperature (875–1275 K) and oxygen/fuel ratio (C/O from 0.01 to 1.4) as well as narrow ranges of dilution (85–90% of nitrogen content) and residence time (0.35–0.5 s) were covered in relation to the identification of regimes either with multiple operating points (hysteresis), steady–unsteady behavior or stable–unstable evolution. It was assessed that the competition between oxidation and recombination channels is stressed under such conditions. The prevalence of acetylene formation and its stabilization with respect to the oxidation of the recombination products is responsible for exothermicity-damping at temperatures higher than 1175 K in rich conditions. This competition is responsible for temperature oscillation modulation in stoichiometric and lean conditions, even though the prevalence of acetylene formation still inhibits temperature increase at temperatures higher than 1300 K. The main practical conclusion derived from this is that diluted combustion processes cannot be designed without a preliminary, accurate analysis of the auto-ignition process. This has thus far been completely ignored for low mass molecular species of interest in natural gas, such as methane. In that case, relatively high air temperatures of above 1400 K must be reached for stable conditions for all air/fuel ratios, whereas temperatures above 1000 K may be high enough only when the richness is more than twice the stoichiometric value. Decreasing both residence time and dilution level in a narrow range of values is beneficial in suppressing temperature oscillations, according to the analysis in terms of inlet temperature and air/fuel ratio