Germán Mazza

Local CFD kinetic model of cadmium vaporization during fluid bed incineration of municipal solid waste

The emissions of heavy metals during incineration of Municipal Solid Waste (MSW) are a major issue to health and the environment. It is then necessary to well quantify these emissions in order to accomplish an adequate control and prevent the heavy metals from leaving the stacks. In this study the kinetic behavior of Cadmium during Fluidized Bed Incineration (FBI) of artificial MSW pellets, for bed temperatures ranging from 923 to 1073 K, was modeled. FLUENT 12.1.4 was used as the modeling framework for the simulations and implemented together with a complete set of user-defined functions (UDFs). The CFD model combines the combustion of a single solid waste particle with heavy metal (HM) vaporization from the burning particle, and it takes also into account both pyrolysis and volatiles combustion. A kinetic rate law for the Cd release, derived from the CFD thermal analysis of the combusting particle, is proposed. The simulation results are compared with experimental data obtained in a lab-scale fluidized bed incinerator reported in literature, and with the predicted values from a particulate non-isothermal model, formerly developed by the authors. The comparison shows that the proposed CFD model represents very well the evolution of the HM release for the considered range of bed temperature.

Coupling scales for modelling heavy metal vaporization from municipal solid waste incineration in a fluid bed by CFD

Municipal Solid Waste Incineration (MSWI) in fluidized bed is a very interesting technology mainly due to high combustion efficiency, great flexibility for treating several types of waste fuels and reduction in pollutants emitted with the flue gas. However, there is a great concern with respect to the fate of heavy metals (HM) contained in MSW and their environmental impact. In this study, a coupled two-scale CFD model was developed for MSWI in a bubbling fluidized bed. It presents an original scheme that combines a single particle model and a global fluidized bed model in order to represent the HM vaporization during MSW combustion. Two of the most representative HM (Cd and Pb) with bed temperatures ranging between 923 and 1073K have been considered. This new approach uses ANSYS FLUENT 14.0 as the modelling platform for the simulations along with a complete set of self-developed user-defined functions (UDFs). The simulation results are compared to the experimental data obtained previously by the research group in a lab-scale fluid bed incinerator. The comparison indicates that the proposed CFD model predicts well the evolution of the HM release for the bed temperatures analyzed. It shows that both bed temperature and bed dynamics have influence on the HM vaporization rate. It can be concluded that CFD is a rigorous tool that provides valuable information about HM vaporization and that the original two-scale simulation scheme adopted allows to better represent the actual particle behavior in a fluid bed incinerator.

Nonisothermal drying kinetics of biomass fuels by thermogravimetric analysis under oxidative and inert atmosphere

ABSTRACT In depth investigation of nonisothermal drying kinetic, the first stage of thermal decomposition was conducted using thermogravimetric analysis, to deepen the thermal processes’ knowledge. The studied biomass wastes were peach pits, marc, and stalk from the canning, jam, and wine industries, respectively. The experimental data have been obtained under oxidative and inert atmospheres at different heating rate (5, 10, and 15u2009K/min), to fit to different isoconversional models to describe drying behavior of agro-industrial wastes. These models were evaluated based on different statistical parameters. The best fitting for all experiments were showed by Jander’s model. It is assumed that the three-dimensional diffusion is the drying rate controlling step. The calculated activation energy values are between 20.31 and 48.41u2009kJ/mol for all agro-industrial wastes at different experimental conditions. Calculated kinetic parameters for the nonisothermal drying under nitrogen atmosphere are generally higher than those for this phenomenon under air atmosphere. Different physicochemical phenomena are produced, which cause this variation during the drying under different atmospheres. Heating rates have a slight effect on the activation energy since the kinetic rate of drying phenomenon is controlled by the physical transformation occurrence, which is dependent on temperature and it is not on mass dependent.

Exergy Analyses of Onion Drying by Convection: Influence of Dryer Parameters on Performance

This research work is concerned in the exergy analysis of the continuous-convection drying of onion. The influence of temperature and air velocity was studied in terms of exergy parameters. The energy and exergy balances were carried out taking into account the onion drying chamber. Its behavior was analyzed based on exergy efficiency, exergy loss rate, exergetic improvement potential rate, and sustainability index. The exergy loss rates increase with the temperature and air velocity augmentation. Exergy loss rate is influenced by the drying air temperatures and velocities because the overall heat transfer coefficient varies with these operation conditions. On the other hand, the exergy efficiency increases with the air velocity augmentation. This behavior is due to the energy utilization was improved because the most amount of supplied energy was utilized for the moisture evaporation. However, the exergy efficiency decreases with the temperature augmentation due to the free moisture being lower, then, the moisture begins diffusing from the internal structure to the surface. The exergetic improvement potential rate values show that the exergy efficiency of onion drying process can be ameliorated. The sustainability index of the drying chamber varied from 1.9 to 5.1. To reduce the process environmental impact, the parameters must be modified in order to ameliorate the exergy efficiency of the process.

Pyrolysis and Combustion of Regional Agro-Industrial Wastes: Thermal Behavior and Kinetic Parameters Comparison

ABSTRACT The thermal decomposition of six regional agro-industrial wastes under inert and oxidative atmosphere at different heating rates was studied using thermogravimetric analysis. The kinetic parameters were calculated using the DAEM (distributed activation energy model) and FWO (Flynn–Wall–Ozawa) methods. The obtained thermogravimetric and differential thermogravimetric curves show similar shapes, considering the pyrolysis and combustion phenomena. Nevertheless, the biomass weight loss is slower and smaller under inert than oxidative atmosphere. The activation energy average values (E) in inert atmosphere for the active pyrolysis stage is about 133.90–275.57 kJ/mol and 136.51–261.10 kJ/mol for DAEM and FWO, respectively. The E average values for the devolatilization stage under oxidative atmosphere are about 104.11–125.73 kJ/mol and 108.40–128.81 kJ/mol using DAEM and FWO methods, respectively. The results calculated by FWO and DAEM methods show differences between activation energies for the same waste below 8%; thus, they were reliable and predictive in this study. The variation of E values with progressing conversion for two studied processes indicated the existence of a complex multi-step mechanism that occurs during the degradation process. Moreover, during the devolatilization stage of combustion, the lower value of E indicates that the combustion is produced in parallel to pyrolysis; the oxygen reacts with the remaining solid.

Kinetic analysis of regional agro-industrial waste combustion

ABSTRACT Cuyo Region generates a significant quantity of agro-industrial wastes. To exploit this waste for energy production, a combustor was installed in this region. To improve its design and operation, a kinetic study of six agro-industrial wastes combustion, using thermogravimetric analysis, was made. The results show that this phenomenon occurs in four stages: drying, devolatilization, char combustion and residual combustion. Maximum weight loss occurs during the devolatilization stage, followed by the char combustion. The contraction geometrys model describes the devolatilization, indicating that the degradation rate is controlled by the resulting reaction interface progress toward the solid center. Moreover, the first order reaction model describes the char combustion stages, showing that the reaction rate is proportional to remaining reactant(s) fraction. The highest energy activation values were obtained for sawdust at heating rate equal to 10 K/min and plum pits at heating rate equal to 15 K/min for devolatilization and char combustion, respectively, presenting slower reaction rate and more difficulty of a reaction starting. For both analyzed stages, the activation energy values vary slightly with the heating rate. This variation can be due to the kinetic rate being controlled by the occurrence of physical transformation. It does not depend on mass but it depends on temperature.

On-sun first operation of a 150 kWth pilot solar receiver using dense particle suspension as heat transfer fluid

A 50-150 kWth pilot solar rig comprising the key equipments of a real plant and that uses silicon carbide Dense Particles Suspension as the heat transfer fluid has been tested at the 1 MW solar furnace at Odeillo-Font Romeu, France. The tests were carried out under large ranges of operating parameters and controlling the mass flow rate when higher temperature was required and when changes on DNI (direct normal irradiation) occurred. This paper presents experimental results on particle outlet temperature, dynamic response of the system to solid mass flow rate and solar power variations, and receiver thermal efficiency (η). Mean and maximum particles’ temperature up to 585°C and 720°C respectively was reached. The receiver thermal efficiency was measured in the range 50-90%.

Prediction of regional agro-industrial wastes characteristics by thermogravimetric analysis to obtain bioenergy using thermal process

The use of energy from biomass is becoming more common worldwide. This energy source has several benefits that promote its acceptance; it is bio-renewable, non-toxic and biodegradable. To predict its behavior as a fuel during thermal treatment, its characterization is necessary. The experimental determination of ultimate analysis data requires special instrumentation, while proximate analysis data can be obtained easily by using common equipment but, the required time is high. In this work, a methodology is applied based on thermogravimetric analysis, curves deconvolution and empirical correlations for characterizing different regional agro-industrial wastes to determine the high heating value, the contents of moisture, volatiles matter, fixed carbon, ash, carbon, hydrogen, oxygen, lignin, cellulose and hemicellulose. The obtained results are similar to those using standard techniques, showing the accuracy of proposed method and its wide application range. This methodology allows to determine the main parameters required for industrial operation in only in one step, saving time.

Pyrolysis kinetics of regional agro-industrial wastes using isoconversional methods

ABSTRACT The thermochemical conversion, under inert atmosphere, at high heating rate of agro-industrial wastes was studied using thermogravimetric analysis. Two reaction mechanisms were supposed: (a) The thermal solid decomposition is carried out through a single reaction; or (b) this occurs through several independent parallel reactions based on its main components. Kinetic isoconversional methods were applied to both proposed mechanisms. The best fit was obtained for the single-reaction models. Nevertheless, to study the influence of pseudocomponent decomposition in the global process, the kinetic behavior of each of them was analyzed. The R2 and D3 models represent the thermal decomposition of biomass wastes and their pseudocomponents. The first model supposes that the reactions tend at first order, and the second assumes that the diffusion is the predominant phenomenon. In all cases, the model with the best fit for the cellulose decomposition is the same for the single global reaction model. The enthalpy variations are positive, indicating that the reactions are endothermic. The entropy variations have negative values, signifying that these processes are slow. The Gibbs free energy variations exhibited positive values, revealing that the total system energy increased at the approach of the reagents and the formation of the activated complex.