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- Improvement on fuel characteristics of biomass and refused derived fuel with torrefactionPublication . Azpilicueta, Ander; Paudel, Pramod; Sousa, NelsonThe search for sustainable energy alternatives has been increasingly important, not only because the depletion of fossil fuels, but also due to the necessary reduction in the emission of greenhouse gases. The use of renewable energy has contributed to the mitigation of this problem, but this doesn’t always adapt to the requirements of energy demand. The energy of biomass, in particular by the combustion of vegetal biomass, is a form already in use and with broad application, from domestic heating equipment to thermoelectric power plants. An alternative energy source is the refuse derived fuels, RDF, which offer a double benefit, given they provide an energy resource and avoid the occupation of landfill space. However, the use of biomass or RDF has limitations, due to the fuel quality. As received, it presents low heating value, low adiabatic flame temperature and low density. The improvement of the properties of these fuels may be obtained by torrefaction. This is a low-temperature pyrolysis process, where the heating gradient, top temperature and process duration must be controlled. In this study, an experimental prototype was developed for torrefaction trials. During the torrefaction experiments the heating rate and maximum temperature were controlled, and were measured sample temperatures, mass decay and also the composition of the gases released. Later, the higher heating value, density and hydrophilicity of biomass and RDF samples, as received and with various stages of torrefaction, were measured, confirming the advantage of applying this thermal process in the improvement of the biomass and RDF fuel properties.
- Modeling biomass particle drying, devolatilization and combustion in a grate fired combustorPublication . Sousa, Nelson; Azevedo, João L.T.The paper presents the application of a numerical model to describe the evolution of a particle in a grate fired combustion system. Due to the particle dimensions, important temperature gradients exist inside the particles and reactions occur within the particles. A model using layers is used to describe the processes inside the particles. The biomass composition is expressed as fractions of moisture, unreacted biomass, char and ash. Biomass conversion is described by a competitive reaction model leading to the formation of light volatiles, tars and char. Tar is subject of secondary reactions inside the particles forming either light gases or char. Transport equations are solved for the gases within the particle including oxygen from the environment that reacts with gases or char. The model is applied to different heating rates leading to different amounts of tars and char in accordance with data from the literature. The model is also applied to simulate the combustion of a single trunk standing in a heated gas stream and comparisons are done for the temperature and mass loss.
- Study of biomass particle combustion model to use in multi-particle simulation codesPublication . Sousa, Nelson; Azevedo, João L.T.This paper presents an analyses of the modelling specifications required in the simulation of the conversion of a single biomass particle to be used in multi-particles computer simulation codes. It considers the transport and reaction of gases, the heterogeneous combustion and heat transfer within the biomass. The biomass conversion is described by a competitive reaction mechanism specifying the composition of the volatile species and tar properties. This approach can represent the influence of the heating conditions on biomass conversion and produce acceptable compositions for the final products. Tar decomposition is considered within the particle as well as combustion reactions for all combustibles: volatile, tar and char. The study of several approaches show the level of numerical approach required to simulate the thermal conversion of the biomass particle and further reactions such as tar decomposition and oxidation of the combustibles gases within the particle. The validation of the numerical code is accomplished with the conversion of a 50 mm trunk in a hot gas stream by measuring the mass loss and temperature along the combustion process. Parametric tests are carried out to investigate the conversion of tars and oxidation of gases within the particle and it is concluded that these can be neglected for particle diameters smaller than 20 mm with an error less than 1%. For combustible particles such as wood chips and pellets with 6 mm diameter, all intern gradients may be neglected.
- Paragens de autocarro inteligentes e inclusivasPublication . Rodrigues, Joao; Pires Rosa, Manuela; Viegas, Micael; Turner, Daniel; Veiga, Ricardo; Sousa, NelsonApresentam-se diferentes conceitos que determinam uma paragem inteligente e inclusiva, criando-se assim uma paragem de autocarros inovadora desde à geração da sua própria energia, tornando-a autossuficiente e sustentável, à integração na rede de autocarros para comunicação da quantidade de utilizadores à espera, até à criação de um ecrã interativo que se adapta às necessidades de qualquer utilizador através de uma inovadora framework desenvolvida no âmbito do projeto ACCES4ALL.
- Modelo de escoamento e combustão de biomassa em grelhaPublication . Sousa, Nelson; Azevedo, João L. T.Este trabalho apresenta a formulação de um modelo de simulação de combustão de biomassa em grelha. O modelo inclui várias aproximações para cada um dos processos considerados, nomeadamente, o movimento dos sólidos já apresentado noutro artigo e o escoamento de gases através de meio granular, considerando a equação de Forchheimer. Para descrever a evolução das propriedades nos sólidos utiliza-se um modelo unidimensional para aproximar as partículas de combustível que é comparado com um modelo detalhado. Apresenta-se ainda o algoritmo em desenvolvimento para acoplar os diversos submodelos e a inclusão de balanços de energia e de massa com recção química.
- Hydrodynamic model for a biomass grate fired systemPublication . Sousa, Nelson; Azevedo, João L.T.The present paper describes a hydrodynamic model for the solids motion in a grate fired combustion system. The overfed bed material is considered in a Lagrangian referential until the particles stop in a position over the bed or exit the domain. The solid material in the grate is then considered as an incompressible continuous media. The momentum balances are applied in an Eulerian referential to particle elements to calculate their velocity in the grate direction. For the conditions considered the calculated velocity of the elements increase always from the start of the grate towards the exit, so the motion in the vertical direction is always downward. This motion is calculated from continuity and the two components enable the definition of the solids flow within the bed. The application of the model for solids motion is shown to be representative of different situations that are analysed for a vibrating grate working with wood pellets. The distribution of solids in the bed is visually and computationally characterised for three situations: i) feeding particles above the bed over an inclined still grate, ii) vibrating an initial bed promoting the motion of particles and iii) vibrating the bed and feeding above the grate in order to achieve a continuous evolution. The comparison of the results show that the model provides a good representation of reality although it can be improved by adjusting model parameters. The model allows for the generation of solids flow patterns in the bed and is a base for the development of a model for grate fired combustion systems.
- Photovoltaic solar system for a smart bus shelter at Faro AirportPublication . Sousa, Nelson; Pires Rosa, Manuela; Baioa, Luís; Costa, CláudioIn this work, a photovoltaic system solution is studied for power-supply in an accessible and smart bus stop. In addition to lighting, the bus stop is equipped with a digital kiosk and a computer system to offer information to passengers and provide aid to people with reduced mobility. Several approaches in the integration of photovoltaic energy have been explored, demonstrating that the best solution is not a stand-alone system, but a smaller photovoltaic installation with the backing of the public power grid for periods of low photovoltaic production. The conclusions presented are based on economic analysis and environmental performance, particularly on avoided CO2 emissions, in order to ensure the sustainability of the recommended solution.
- Model simplifications on biomass particle combustionPublication . Sousa, Nelson; Azevedo, João L. T.Particle combustion modeling has been widely studied, although some considerations on the conversion processes are yet to be clarified. Thermal decomposition and final yield are addressed differentially in the literature, giving attention to the heating rate or to the peak temperature. It is assumed that the volatized gases are expelled immediately from the particle, neglecting homogeneous reactions, and in more simplified models that the particle is thermal and chemically uniform.This study presents an analysis of the modeling specifications required in the simulation of the conversion of a single biomass particle. This comprehensive approach shows the model requirements for more exact outcomes regarding the biomass particle size. The thermal model is discretized in concentric layers and may be used in cylindrical or parallelepiped geometry. Biomass decomposition is described by a competitive reaction model, justified by the analysis of the published experimental data. Temperature distribution is studied inside the particle as well as the gas flow, following a Darcy equation. Gas flow considers the diffusivity of oxygen to the interior of the particle and its reaction with the combustible gases and char.The validation of the numerical code is accomplished by the conversion of a 50 mm diameter wood trunk in a hot gas stream, measuring the mass loss and the temperature along the combustion process.Parametric tests were carried out to investigate the conversion of tars and oxidation of gases within the particle in typical combustion conditions. For biomass particles with a diameter more than 50 mm, the oxidation of combustible gases is considerable inside the particle, and therefore it is important to modulate the gas flow. Regarding the tar decomposition, the reactions involved are significant for particle diameters larger than 20 mm. In typical combustion conditions, small particles such as wood pellets, all properties can be considered uniform. The number of layers in the model can also be encountered considering the pyrolysis number.This study concludes that some process modulations are irrelevant for small particles. This is important when an expeditious method is required for the implementation in a multiparticle numerical code. (C) 2016 Elsevier Ltd. All rights reserved.