• Energy Research

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.

The kinetics of the steam-assisted gasification for three different agro-industrial solid wastes (sawdust, olive and plum pits) was studied by macro thermo-gravimetric analysis (macro-TGA) at different heating rates (5, 10 and 15 K/min). The progressive CO release was moreover monitored to fully identify each step of the global gasification process. A single-step kinetics modelling was applied by using the Coats-Redfern method, with both a first order model for pyrolysis and a Ginstling - Brounstein 3D-diffusion model for the gasification stages, respectively. A comparison between macro-TGA and previous TGA results for the same bio-wastes was performed. Results indicated that the reaction proceeds in three well-defined and subsequent stages, involving water evaporation [298-473 K], biomass de-volatilization [473-648 K] with the highest production of CO, and char gasification as final step. Reaction rate parameters of the Arrhenius equation were determined for both the pyrolysis and gasification steps.

Abstract Solar pyrolysis of a carbonaceous feedstock (coal, biomass and wastes) is a process in which carbon-containing feedstocks are used as chemical reactants and solar energy is supplied as high-temperature process heat. This process has the potential to produce higher calorific value products with lower CO 2 emissions than conventional pyrolysis processes. As a consequence, the intermittent solar energy is chemically stored in the form of solar fuels. Solar pyrolysis was first demonstrated in an indoor environment using a solar simulator (image furnace) for exploring the fundamental mechanisms of carbonaceous feedstock pyrolysis under severe radiative conditions (high temperatures and heating rates) in comparison to conventional pyrolysis. More recently, low-temperature solar pyrolysis has been demonstrated to be a good technology for bio-oil production. Our high-temperature solar pyrolysis process produces more combustible gas products than other processes. This paper reviews developments in the field of solar pyrolysis processing by considering fundamental mechanisms, experimental demonstrations, models and challenges.

Abstract The growing interest in syngas production from biomass has extended research towards solar-driven pyrolysis taking place at high temperature and heating rate, due to its high efficiency and environmental advantages. The aim of this work is to experimentally study and compare the effect of pellet size (5, 10 and 15 mm height) on product yields (liquid, char and gas), gas composition (H2, CO, CO2, CH4) and tar secondary reactions during fast solar pyrolysis of sawdust pellets (800, 1200 and 1600 °C and heating rates of 10 and 50 °C/s). Additionally, the influence of pellet height on syngas quality is analyzed by parameters such as H2/CO, CH4/H2 ratios, mechanical gas efficiency and carbon conversion efficiency, focused on methanol production. An analysis based on characteristic times and dimensionless numbers is also performed to estimate the rate-controlling phenomena. Experimental results showed that, for temperatures below 1200 °C, there were no significant differences in the gas yield for the three pellet heights. However, the gas product distribution was influenced by the pellet size even at 800 °C. A further increase in temperature emphasized the influence of the pellet height on both gas yield and gas composition. Therefore, it can be argued that, in order to improve the gas yield and the quality of the syngas, the fast pyrogasification of large particles is advisable.

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.

Fil: Soria, Jose Miguel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Patagonia Norte. Instituto de Investigacion y Desarrollo en Ingenieria de Procesos, Biotecnologia y Energias Alternativas. Universidad Nacional del Comahue. Instituto de Investigacion y Desarrollo en Ingenieria de Procesos, Biotecnologia y Energias Alternativas; Argentina

Solar pyrolysis of pine sawdust, peach pit, grape stalk and grape marc was conducted in a lab-scale solar reactor for producing fuel gas from these agricultural and forestry by-products. For each type of biomass, whose lignocellulose components vary, the investigated parameters were the final temperature (in the range 800°C–2000 °C) and the heating rates (in the range 10–150 °C/s) under a constant sweep gas flow rate of 6 NL/min. The parameter influence on the pyrolysis product distribution and syngas composition was studied. The experimental results indicate that the gas yield generally increases with the temperature and heating rate for the various types of biomass residues, whereas the liquid yield progresses oppositely. Gas yield as high as 63.5wt% was obtained from pine sawdust pyrolyzed at a final temperature of 2000 °C and heating rate of 50 °C/s. This gas can be further utilized for power generation, heat or transportable fuel production.