1,720,999 research outputs found
POLARITY INVERSION IN ELECTROHYDRODYNAMICS SPRAYING
No full textAn experimental study of polarity inversion effects in electrohydrodynamic spraying is developed by characterizing the several atomization regimes that can be observed by varying liquid flow rate and applied voltage. n -Heptane, doped with an antistatic additive, is atomized in a classical needle-to-plate system. The study allows for the determination of two-dimensional maps of the stability regimes occurring by varying the controlling parameters. These maps have been obtained by comparison of results of direct visualization, by means of a digital camera, and Particle Dynamics Analyzer (PDA) measurements of droplet size and velocity components. Results show that occurrence of the regimes is not qualitatively influenced by polarity inversion, but in the negative polarity case any modification on spraying modes needs higher variation in applied voltage, and so the regimes are stable in larger voltage ranges
Mechanical and thermal treatments of municipal solid waste organic fraction in small dehydration units
No full textLandfilling, composting and incineration represent the most common processes used for the disposal of
organic fraction of municipal solid waste (OFMSW). The evaluation of the best available technology used
for the exploitation of these processes has to take into account the environmental impact of the technology
itself and of the connected activities in addition to peculiar local aspects related to lands availability, legal
constrains and climatic conditions of the specific country. Different technological solutions and
management strategies have been proposed aimed to reduction of Green House Gas (GHG) emissions
and leachate release as well as to increase energy recovery efficiency of existing disposal plants.
The aim of the present study is to identify a feasible OFMSW disposal processing chain at apartment or
condominium scale capable to perform a proper dehydration and volume reduction of the organic waste
thus producing a material directly available for Waste to Energy processes. Four sub-processes have been
identified: mechanical treatment, separation of the aqueous phase, drying and effluent gas treatment. The
first three sub-processes have optimized on the basis of characteristic times, energy consumption and
adaptability to domestic scale
Thermal and mechanical stabilization process of the organic fraction of the municipal solid waste
No full textIn the present study a thermo-mechanical treatment for the disposal of the Organic Fraction of Municipal Solid Waste (OFMSW) at apartment or condominium scale is proposed. The process presents several advantages allowing to perform a significant volume and moisture reduction of the produced waste at domestic scale thus producing a material with an increased storability and improved characteristics (e.g. calorific value) that make it available for further alternative uses. The assessment of the applicability of the proposed waste pretreatment in a new scheme of waste management system requires several research steps involving different competences and application scales. In this context, a preliminary study is needed targeting to the evaluation and minimization of the energy consumption associated to the process. To this aim, in the present paper, two configurations of a domestic appliance prototype have been presented and the effect of some operating variables has been investigated in order to select the proper configuration and the best set of operating conditions capable to minimize the duration and the energy consumption of the process. The performances of the prototype have been also tested on three model mixtures representing a possible daily domestic waste and compared with an existing commercially available appliance. The results obtained show that a daily application of the process is feasible given the short treatment time required and the energy consumption comparable to the one of the common domestic appliances. Finally, the evaluation of the energy recovered in the final product per unit weight of raw material shows that in most cases it is comparable to the energy required from the treatment
Effect of feedstock demineralization on physico-chemical characteristics of arundo donax derived biochar
No full textSolid residue derived from thermochemical degradation of vegetal biomass, generally named biochar, is
currently under study for its potential agronomic value as soil amender and fertilizer. Feedstock pretreatments
aiming to a partial demineralization of the biomass could promote the development of char
porosity by inducing modifications of the chemical and physical characteristics of the feedstock. In the
present paper three different methods of feedstock demineralization have been applied on Arundo donax
samples. Steam assisted pyrolysis of raw and demineralized Arundo donax has been carried out in a
proper experimental apparatus up to 873 K, at pressure P=5×105
Pa and heating rate HR=5 K/min. A
comparison of the chemical and physical properties of produced chars and energetic content of gaseous
phase has been evaluated for its possible use as fuel to sustain biochar production itself
Hemicellulose, cellulose and lignin interactions on Arundo donax steam assisted pyrolysis
No full textSlow Steam pyrolysis of Arundo donax is proposed as possible process for the recovery of a solid material suitable to be used as biochar. In view of the evaluation of the feasibility of such a process, products yields, surface area of char and energetic content of gaseous products needed to assist energetically the process are relevant data to be evaluated in dependence on process operating conditions and biomass characteristics. Biomass main components (hemicellulose, cellulose and lignin) contribute to a different extent to the determination of products yield and characteristics both for their own intrinsic chemical nature and for the onset of possible interactions due to their simultaneous presence in a real biomass. Moreover, it is known that inorganic elements present in the biomass can affect pyrolysis mechanisms and consequently products yields and characteristics. In the present paper the influence of inorganic species and of possible interactions between biomass main components on pyrolysis of A. donax has been studied. To this aim steam assisted pyrolysis tests have been carried out on a mixture of xylan, cellulose and lignin resembling the composition of A. donax canes in a proper experimental apparatus up to 873 K, at pressure P = 5 × 105 Pa and heating rate HR = 5 K/min. Products yields, gas releasing rate as function of the temperature, gas composition and specific internal surface of char have been compared to the data computed from the superposition of the results obtained for the single components and to the behavior of untreated and demineralized samples of A. donax canes processed in the same experimental apparatus in the same operating conditions. Results obtained from this study show that primary pyrolysis of holocellulose is affected by the presence of inorganic species that depress the devolatilization of heavier compounds in favor of cracking reactions determining a higher release of light compounds, CO2 and CO. Moreover, interactions between lignin and cellulose are relevant in operating conditions where mass transfer phenomena cannot be neglected. Interesting results have been obtained for the solid residue as for the development of its internal surface that seems to be reduced from both interactions between biomass components and presence of inorganic ions
Dynamic behaviors in methane MILD and oxy-fuel combustion. Chemical effect of CO2
No full textThe oxidation process of CH4/O2 mixtures diluted in CO2 under moderate or intense low-oxygen dilution
(MILD) and oxy-fuel combustion conditions was numerically investigated in a perfectly stirred flow reactor at atmospheric
pressure. The analysis aimed to investigate the kinetics involved in fuel oxidation in systems highly diluted and strongly
preheated. Particular attention was focused on the effects of CO2 on oxidation routes because it can significantly alter the kinetic
pathways participating directly in key reactions or indirectly in termolecular reactions as a third body species. Furthermore,
adiabatic flame temperatures are lower with respect to air conditions because of the higher thermal heat capacity of CO2 in
comparison to that of N2, thus modifying the kinetics promoted by temperature in combustion processes. The analyses were
realized as a function of main system parameters, systematically changing inlet temperatures and mixture compositions. Results
showed the establishment of complex dynamic behaviors in terms of temperature oscillations for both lean and rich fuel mixtures
in both nonadiabatic and adiabatic conditions. Further numerical analyses were performed to highlight the kinetic aspects of the
problem. Simulations suggested that in fuel-lean conditions, the dynamics observed are related to the H2/O2 subsystem reactions
independent of diluent nature, while for fuel-rich mixtures diluted in carbon dioxide, the CO2 decomposition to CO and CH3
recombination to ethane are key reactions for the onset of temperature oscillations
Ammonia/Methane combustion: Stability and NOx emissions
No full textIn the framework of the energy shift toward Renewable Energy Sources, ammonia is considered a valuable energy vector, due to its very high hydrogen-density and well-established production processes. Despite the many potential advantages, its combustion features (narrow flammability limits, high auto-ignition temperature, potential high fuel-NOx emissions) may hinder its wide utilization. To overcome these issues in conventional combustion, “fuel enhancers” are also used. Differently from conventional systems, MILD Combustion already proved to be very effective in oxidizing ammonia in terms of stability and NOx emission. Nevertheless, a “fuel enhancer” can be also useful in MILD Combustion conditions to further improve the process characteristics. The present study focuses on the ammonia/methane combustion characteristics under MILD Combustion conditions in a lab-scale burner. Gaseous pollutant emissions (NOx, NH3, H2, CO) and process stability limits were analysed as a function of the equivalence ratio and NH3/CH4 fuel composition. Results showed that the use of NH3/CH4 blends extends the stable operational range of the system, in terms of both working temperatures and equivalence ratios, with respect to pure NH3. On the other hand, blends produce higher NOx emissions, with respect to both the pure NH3 and CH4 cases. Experimental data were compared with chemical kinetics modelling results. Chemical pathways and rate of production of main intermediate products highlighted that oxidation pathways of carbon and nitrogen-based species are essentially decoupled as experiments suggested. In contrast, the interplay of the methane and ammonia main chemical pathways affects the DeNOx channel in correspondence of the observed NOx emission increase
CO2 and H2O effect on propane auto-ignition delay times under mild combustion operative conditions
No full textThe auto-ignition process of propane/oxygen mixtures was experimentally and numerically studied over a range of temperatures (850-1250K) and mixture compositions (from fuel-ultra-lean to fuel-rich conditions) under MILD operative conditions. The mixtures were diluted in CO2 or H2O from 90 up to 97%. The experimental tests were realized in a Tubular Flow Reactor (TFR) at atmospheric pressure.Several combustion regimes were identified as a function of the mixture composition and inlet temperature. The experimental results showed that CO2 and H2O significantly alter the ignition process. In particular, a significant slowing of the system reactivity was observed with respect to the mixtures that were diluted in nitrogen.Numerical simulations were performed by commercial codes and detailed kinetic mechanisms. Comparisons between experimental and numerical results pointed out that kinetic models are not able to correctly reproduce system behaviors in all the experimental conditions.For CO2-diluted mixtures a good agreement between experimental and numerical analysis was obtained for fuel lean mixtures, whereas for stoichiometric and fuel-rich mixtures conditions the consistency of predicted data was less satisfactory.In the case of steam-diluted systems, the discrepancy between the experimental data and the predictions is about one order of magnitude for any mixture composition, but the model can reproduce the slight dependence of the ignition data on the mixture compositions.Further numerical analyses were performed to identify the reactions controlling the ignition process under MILD operative conditions in presence of CO2 and H2O. Results suggested that steam and carbon dioxide drastically alter the main branching mechanisms as third molecular species in termolecular reactions and/or by decomposition reactions
Diffusion Ignition Processes in MILD Combustion: A Mini-Review
No full textMILD combustion processes belong to new combustion technologies developed to achieve efficient and clean fuel conversion. The basic concept behind its implementation is the use of high levels of hot exhausted gas recirculation within the combustion chamber. They simultaneously dilute fresh reactants, to control system temperatures and pollutants emission, while promoting fuel complete oxidation. The combination of low maximum system working temperatures and high diluted mixtures with intense pre-heating delineates an oxidation process with unique chemical and physical features, such as uniformity of scalars at macroscale related to distributed reacting regions at microscale, extremely different from conventional flames. In turn, this requires the definition and characterization of new elementary processes, not ascribable to traditional deflagration or diffusive flame structures, which, in literature have been identified as “diffusion ignition.” The present mini-review reports on several literature characterizations of such reactive structures under steady and unsteady conditions combining evidences from numerical, experimental, and/or theoretical studies. Both premixed and non-premixed configurations were analyzed in terms of system temperature, heat release, and species distributions as key parameters to describe the intrinsic nature of such new elementary processes. Analyses were realized changing the main system external parameters (mixture pre-heating temperature, dilution level in several feeding configurations) moving from traditional to MILD conditions. Results highlighted the “distributed ignition” nature of igni-diffusive structures, with implication on the thickness of the oxidation structures in the mixture fraction space, the presence/absence of a pyrolysis region, and the correlation of the maximum heat release with the mixture stoichiometric
The Effect of Diluent on the Sustainability of MILD Combustion in a Cyclonic Burner
No full textThe present study investigates the characteristics of MILD/flameless combus- tion in a cyclonic lab-scale burner. Such a configuration is effective for achieving turbulent mixing in a very short time while allowing for a reasonably long residence time for the development of combustion reactions. These two constraints are mandatory in the case of MILD combustion processes (high inlet temperatures and diluted mixtures). Such operating conditions are achieved through massive heat/mass recirculation towards the fresh incom- ing mixtures by recycling the exhausted gases, featuring a process where chemical kinetics times are elongated because of the dilution levels. Thus, long residence times are needed to achieve a satisfying reaction progress, and the high inlet temperatures result in fast and effi- cient mixing between disproportionate flows to avoid the onset of oxidation reactions before achieving diluted conditions. Under these constraints, a lab-scale facility was designed and built. The oxidation processes of C3H8/O2 mixtures highly diluted in N2 or CO2 were inves- tigated by varying the external parameters of the system, namely, the inlet temperature (up to 1300 K) and the mixture composition (from lean to rich mixtures). Several combus- tion regimes were experimentally identified. When the MILD regime was established, the combustion process became homogeneous within the burner without luminous emissions. To investigate the distributed nature of the MILD combustion processes, chemical simula- tions were performed under the assumption of a well-stirred reactor. For both the diluents, good agreement between the experimental and numerical results was obtained for MILD combustion conditions
- …
