1,721,780 research outputs found
Particle Tracking and Deposition from CFD Simulations using a Viscoelastic Particle Model
No full textIn the present dissertation the mathematical modelling of particle deposition is studied and the solution algorithms for particle tracking, deposition and deposit growth are developed. Particle deposition is modelled according to mechanical impact and contact mechanics taking into account the dependency on time, temperature and particle-deposit composition explicitly. Indeed, such a model lies in the field of the rheology of visco-elastic solids which the author of this dissertation refers to in the following chapters. Particle adhesion is calculated by imposing an energy balance between kinetic energy, energy loss and the work of adhesion at the impact while the hard sphere approach is applied to model particle to particle collision. These calculations eventually return as result the particle tangential and normal to impact surface (energy) restitution coefficients. Particular attention was given to the implementation of the solution algorithms and the development of a computational strategy to investigate in detail both particle trajectories, properties and deposit locations. The development of the solution algorithms is twofold, to investigate both particle deposition and the deposit growth applying different computational strategies and algorithms which are usually employed separately (competitor algorithm solution). In the "integrated" approach proposed here, these strategies are coupled (staged partner algorithm solution) according to a sequential use (staged procedure), to provide detailed and time dependent result data. A novel computer program for Lagrangian particle tracking on unstructured meshes, was developed to investigate particle deposition in Computational Fluid Dynamics (CFD) data post-processing. Developing a particle tracker program as a separate and CFD independent computer code has overcome several limitations in particle modelling which are present in commercial CFD code (i.e. non-open source) even though, on the other hand, it required to develop a robust in-cell particle location algorithm as well as an accurate and efficient particle interpolation and integration time scheme. All these characteristics and requirements have driven the author in the development of the Particle Post-Processor software, nicknamed P³, which is capable of calculating particle trajectories and deposition, deposit growth and particle-particle interaction (hard spheres model). A specific particle in-cell detection algorithm, to locate the cell hosting the particle, was developed to upload and elaborate results from commercial CFD codes for hybrid-unstructured meshes. Three commercial CFD codes have been tested. Particle tracking on both Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) results was successfully performed and compared. Numerical results are substantially in good agreement with the experimentals.Process and EnergyMechanical, Maritime and Materials Engineerin
Alkali metals in combustion of biomass with coal
No full textGrowing demand for energy in the world, depletion of fossil fuels and green house effect require from us to utilize alternative, renewable sources of power. Biomass gained in the last few years more and more attention especially in Europe. Many research programs focused on the various forms of thermal biomass utilization have been launched and successfully accomplished expanding our knowledge and contributing to the, so-called, sustainable development. Utilization of straw, biomass present in Europe in large although spread quantities, is an interesting option among others for small decentralized CHP plants. On the other side, straw thermal utilization can cause serious problems resulting in power plant shut downs. The chemical composition of straw, especially high volatile alkali metals content in combination with other elements like chlorine causes corrosion and deposits formation problems, moreover, in combination with silica and calcium slagging and fouling problems. The main goal of this thesis was to investigate the mechanisms responsible for alkali metals release and sequestering during combustion of straw and the influence of co-combustion of straw with coal. The knowledge regarding these mechanisms is necessary to operate biomass fired power plants in a safe, efficient and profitable way. The research has been done by means of experiments and system modeling. The tests have been done using pilot scale CFB combustor and bench scale heated grid reactor together with the fundamental studies over KCl-kaolin interactions in TG reactor. The high alkali (HIAL) straws selected for the experiments were characterized by a broad range of potassium contents, from average values to extremely high potassium content. This in combination with certain ratios of Cl and Si would lead to corrosion and deposit formation problems mentioned above. Finding a way to capture alkali metals by additives in combustion systems, circulating fluidized bed in particular, is the next issue this thesis was aiming at. The screening of possible alkali metals sorbing additives was presented. Further more fundamental investigation of the most promising additive, aluminasilicate clay - kaolin, a natural constituent of coal ash, were shown and novel results were presented.Mechanical Maritime and Materials Engineerin
Mercury speciation in pulverized fuel co-combustion and gasification
No full textCoal based power generation is a significant source of mercury emissions to the atmosphere and this has attracted huge attention in the past decade. Recently, the concerns regarding global warming and need for new energy resources introduced the concept of cofiring of biomass and waste as secondary fuels in the power industry. The addition of a variety of secondary fuels will change the behaviour of mercury in the combustor which will then affect the design of emission control technology. This PhD thesis primarily focused on the impact of secondary fuels and the combustion conditions on mercury speciation in pulverized fuel co-combustion of secondary fuels with coal. The experimental work of this thesis mainly involves bench scale studies to investigate the speciation of mercury in pulverized fuel co-combustion under air-staging and at different combustor temperatures (1000 and 1300ºC). A new model is developed to predict different forms of mercury in the post-combustion zone upstream of a particulate control device. The model incorporates reactions of mercury with flue gas species and simultaneous adsorption of oxidized mercury (HgCl2) on fly ash particles in cooling of flue gases. Additional work is concentrated on understanding the mercury speciation under gasification conditions. In gasification processes, most of the mercury is exist in elemental form. A kinetic model has been developed to predict speciation of mercury in gasification product gas.Mechanical Maritime and Materials Engineerin
Steam boilers: Process models for improved operation and design
No full textBiomass combustion can be an economic way to contribute to the reduction of CO2 emissions, which are a main suspect of the so-called greenhouse effect. In order to promote a widespread utilization of biomass combustion, operational problems like fuel treatment, slagging, fouling and corrosion have to be solved. There are few research initiatives focused on the improvement of design and operation, for example combustion modeling, material research or equipment optimization. In this work, two aspects are considered: performance improvement due to the optimization of cleaning cycles to reduce slagging and the use of predictive dynamic models as an aid for control and equipment design. These objectives can be accomplished by using process models that share a number of characteristics. The first part includes the development and application of an online process-monitoring tool to recognize deposit phenomena during biomass combustion. Slagging and fouling refers to deposits of solids on heat exchanger surfaces. Since slagging and fouling is a local process highly dependent on time and temperature, the analysis of the heat transfer diagrams is used to study the deposit tendency in an early stage of the process. In this work, heat transfer transients of the evaporator, the economizer and the superheaters are analyzed on the basis of a physical model. Measured data from a biomass-fired power plant, e.g. from an acoustic pyrometry and the process control system, were used as input for the monitoring. A thermodynamic steady state model was developed and validated with data from several boiler types, i.e. pulverized fuel, fluidized bed and grate. The monitoring campaign was combined with fuel and ash measurements. The fuel from the power plants showed clear slagging tendencies. Measured material properties were analyzed and used to improve the modelâs accuracy. Deposits on the heat exchangers could be accurately detected and the overall soot blowing strategy could be optimized, something not possible without a process-monitoring model. The model has been verified and validation of the results proved the correct diagnosis of the deposit status inside the furnace. A predictive dynamic model was developed, based on the steady state results and is presented in the second part of this thesis. The steam boiler is one of the most complex components of a thermal power plant as far as the process control is concerned. A common boiler configuration is the once-through arrangement and this is the type of boiler considered in this work. A problem in two-phase systems modeling is the correct calculation of the phase boundary, because the position of the phase transition changes rapidly, depending on load conditions and temperature distribution along the walls. The prediction of the correct spatial distribution of the phases is crucial with respect to the accuracy of the model. A lumped parameters model with a moving boundary approach is developed insteadMechanical Maritime and Materials Engineerin
Biomass fuel characterization for NOx emissions in cofiring applications
No full textThis dissertation investigates the impact of various biomass fuels and combustion conditions on the NOx emissions during biomass co-firing. Fossil fuels dominated the energy scenario since the industrial revolution. However, in the last decades, increasing concerns about their availability and environmental risks related with their extensive utilization, together with rising oil prices are favouring the development of renewable energies. Increasing the share of renewable energy sources in the power generation sector is strongly promoted in the EU, and the European Commission has set as goal to double its current global share by 2010. Co-firing biomass and waste fuels with fossil fuels appears as a promising route to introduce renewable fuels in the power sector and reduce CO2 emissions. Biomass co-firing has been implemented at several coal-fired units, generally limited to about 5% of the thermal input. This practice offers several advantages. Besides CO2 emission reduction, it enables flexibility in fuel supply as power operators are not dependent on a constant supply of a particular fuel. Economically, co-firing is encouraged in many countries as fiscal incentives are available for power operators going "green" and emission trading spreads. In order to increase the fired biomass share, an important requirement that has to be met by power operators concerns NOx emissions, as future EU targets tighten. It is therefore crucial to characterize the combustion of biomass and waste fuels with pulverized coal, in order to minimize NOx formation. Experimental and modelling tools have been used to investigate the NOx formation from fuel-bound nitrogen of biomass fuels and coal/biomass mixtures.Mechanical, Maritime and Materials Engineerin
Characterisation of supplementary fuels for co-combustion with pulverised coal
No full textThe current and future energy policy aims at increasing the share of renewable energy in worlds energy supply. One possibility to enhance energy production by renewable sources within a short term is co-combustion. This means co-firing biomass and waste with fossil fuels at existing power plants originally designed to fire fossil fuels. In Central Europe, the main interest lies in co-firing biomass and waste at pulverised coal boilers, because these plants form the basis of the currently utilised thermal fuel conversion techniques. Co-combusting supplementary fuels with coal is an economical option to increase the share of renewable energy because usually no large investments are required before implementation at large-scale utilities. Moreover, in many countries fiscal advantages are granted for renewable fuels. Co-firing can contribute to the increasing waste disposal problem by offering alternatives to landfills and waste incineration plants. When striving after higher renewable fuel shares, supplementary fuel replacement ratios at power plants increase and various problems can arise due to the differences between coal and the secondary fuels. Technical barriers that must be studied and overcome include matters related to fuel supply, fuel handling, changed combustion conditions in the boiler, ash quality and emissions. For this, the fundamental combustion properties of supplementary fuels compared with those of coal must be carefully studied. In order to better understand and predict the (co-)combustion behaviour of different biomass and waste fuels, a combination of experimental work and modelling should be used. Therefore, in the first part of this dissertation, pyrolysis of supplementary fuels is experimentally investigated by laboratory scale set-ups i.e. thermogravimetric analyser and heated wire mesh. The second part of the thesis concentrates on Computational Fluid Dynamics (CFD) modelling of co-combustion in a bench-scale, electrically heated pulverised fuel combustion reactor. Combustion sub-models are validated against experimentally determined concentration profiles and particle burnout data. The CFD model is then applied to optimise co-firing regarding supplementary fuel type, share and particle size.Design, Engineering and Productio
Studies on high efficiency energy systems based on biomass gasifiers and solid oxide fuel cells with Ni/GDC anodes
No full textMechanical Maritime and Materials Engineerin
Characterisation and prediction of deposits in biomass co-combustion
No full textThis PhD thesis deals with the theoretical, experimental and modeling work which was performed to study deposition during biomass and waste co-combustion in pulverised coal facilities. Fossil fuels dominate the current energy scenario. Increasing concerns about fossil fuels availability and about the impact of their extensive use on the environment, had led public and governments to focus their attention on the utilisation of sustainable forms of energy. Reducing CO2 emissions, increasing the shares of renewable energy consumption and improving energy efficiency have become world wide targets. In this context biomass and waste fuels are being increasingly used. An economical option to introduce biomass fuels is to co-fire them together with coal in existing pulverised coal facilities. But formation of ash deposits from solid fuels combustion influences operation of utility boilers. Many studies have been done to characterise deposition trends of coal fuels. The behaviour of biomass and waste fuels differs considerably from that of coals given the distinct composition and association of their organic matter. A better knowledge of the behaviour of biomass and waste fuels is required to predict deposits formation. Different and complementary approaches are explored in this PhD work in order to contribute to the current knowledge on deposition of ashes from co-firing biomass and waste fuels with coal. A set of fuels fired in full scale facilities were available for this research including two different coals, pine chips, B-wood, corn and rape straw, plastic and green house residue, biomass mix, palm kernels, olive residue, pepper plant, chicken litter and meat and bone meal. In the experimental part of this work, a wide range of biomass and waste fuels has been analysed by standard methods and their ashes have been obtained at laboratory. A leaching procedure for the ashes has been developed based on chemical fractionation techniques widely used for biomass fuels. Thermo Mechanical Analysis (TMA) has been applied to the ashes and their leached solid residues to investigate their fusion characteristics. The TMA traces of the ashes and their successively leached solid residues are a fuel ‘foot print’ and depend strongly on the chemical composition of the samples. The difference between the TMA traces of biomass ashes before and after leaching them, clearly relates with the reactivity of the inorganic compounds and can be used to rank the fuels depending on the reactivity of their inorganic compounds during combustion and their tendency to form deposits. In parallel to this work on the leached solid residues, chemical analysis of the liquid leachates has been done and the results used to classify the fuels as having more or less reactive inorganics. Both approaches convey to the same conclusion about the fuel reactivity ranking. Chicken litter, MBM, olive residue and plastic and green house residue are predicted to be more prone to deposition, while pine chips and biomass mix appear to have the smaller presence of ‘problematic’ inorganic species and therefore will give less deposition. Experimental work has also been conducted to investigate the effects of deposits build up on heat transfer surfaces, as this is one of the biggest impacts associated to deposits in plant operation. This effect can be monitored on line using appropriate probes and thermocouples to evaluate the heat transferred to the cooling medium. Two different air cooled metal probes have been developed and tested. Deposition experiments have been done at a 50kWel pulverized fuel flow combustion reactor (CR) designed, commissioned and started up during this research work. At the CR different deposition probes have been tested and different fuels and fuel blends have been co-fired with coal, such as MBM, chicken litter, olive residue, B-Wood or palm kernels. The decay in thermal flow from the hot flue gases to the cold probe cooling air has been measured but characterisation of the different behaviour of fuels and fuels blends based only on heat transfer measurements was not possible. Samples of deposits were collected from the cooled metal probes and from the specially designed ceramic probes. Some fuels are found to produce more sintered deposits –chicken litter, palm kernels, olive residue- while the others appear less sintered. The results of these observations compare well with the results from TMA. Samples of the deposits collected in the probes are analysed to obtain their chemical composition. In the modeling part of this work chemical equilibrium models are used to investigate the occurrence of ash deposits due to co-firing of biomass and coal. Traditional chemical equilibrium calculations combined with an existing combustion model are used to model the deposition experiments done in the CR facility. The enrichment of some elements found in the deposits and the amount of melt phase obtained by modeling compare well with the experimental values. All combustion experiments performed at 1300°C give a percentage of melt above the 15%, meaning that sintered deposits form. The model takes into account data about reactivity of the inorganic species in each fuel as obtained by ash leaching analysis and the availability of some inorganics existing in the outer layers of the fuel particle. It has been observed that the results of ash melting by equilibrium calculations strongly depend on the fraction of inorganic components of the fuel that are accounted for as reactive/non reactive. The model has been applied to predict the formation of condensed fases for the fuels studied. The combination of modeling results and the information obtained from the chemical analyses performed on the deposit samples gives a better insight on the ash formation and deposition phenomena.Process and EnergyMechanical, Maritime and Materials Engineerin
Integrated modelling for improving the design and operation of steam power plants
No full textPower producing companies have faced a new situation following the development of unregulated electricity markets. Strong competition forces them to take power production costs into account more carefully in order to maintain or increase profits. Additionally, to promote the efforts to reduce CO2 emissions,legislation and taxation drives operators to increase the use of coal and renewable energy sources more efficiently and over a wider range. This results in an increasing interest in the use of solid-biomass-based fuels in pulverized coal fired power plants. Therefore, fuel flexibility due to the combustion of varying relative shares of coal and biomass poses new challenges for plant operators. The understanding of deposition formation and its behavior and consequences has become a key issue in optimizing plant operation and in securing plant performance and availability. The investigation presented in this dissertation was carried out as part of the EU-ADMONI project. The overall objective of the work presented here is the development of advanced monitoring models for solid fuel fired steam power plants. To obtain such advanced monitoring methods, three kinds of models are to be coupled with each other. They are: Process Monitoring Models, Dynamic Process Models and 3D Boiler Models. This coupling consists of the exchange of different boundary conditions (BCs) with each other. Although the development of an overall monitoring methodology has been outside the scope of this work, this dissertation contains descriptions and examples of the three separate modelling techniques needed to obtain the overall monitoring methodology which can be used for improving the operation of steam power plants. Process monitoring models have been developed to predict or monitor either heat transfer resistances of the heat exchanging equipment of the steam cycle (this is an indicator of the amount of deposit formation) or the overall steam cycle efficiency of the power plant at hand, using the process data provided by the Data Acquisition System (DAS) of the plant. The novelty of the models presented in this work is the investigation of the possible use of exergy analysis within the existing monitoring models to detect a possible decay of plant performance due to arising operational problems. Dynamic process models of the steam cycle have been developed to provide insight in the physics and plant behavior when (co)-firing varying biomass and coal blends. The results of these models can be very important for the design of the control system of plants which are co-firing these kinds of mixtures. Physically based models have been formulated, since no historical data is available in the design phase, and thus black box models were not an option. A methodology for the dynamic modelling of energy conversion systems has been proposed, and the modelling of a simple single pressure, small, steam power cycle, has been done as a "proof-of-concept". An accurate methodology for the simulation of 3D pf combustion has been developed to accurately predict the flow and temperature fields inside the boiler. To be able to accurately predict these phenomena, detailed combustion models are necessary. Various combustion models for solid fuel combustion have been proposed in literature, but an extensive validation of these models is hardly given. Therefore, several combustion models have been validated against experimental data and the most accurate model has been used for full scale 3D boilerMechanical Maritime and Materials Engineerin
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