A Mathematical Model for Differential—Velocity Circulating Fluidized Bed Boiler

INTRODUCTIONIt'ssoconspicuousofthedevelopmentofthecirculatingfluidizedbed(CFBC)recentlywhichistakenasamosteffectiveandcleancoalcombustiontechnology.ThermalEngineeringDepartmentofTsinghuaUniversityhasbeenlongengagedintheresearchofCFBCbothindesignandmo...     

Transient model of oxygen-starved proton exchange membrane fuel cell for predicting voltages and hydrogen emissions

Transfer (crossover) leaks initiated by the chemical deterioration of the PEM and the resulting performance degradation has been previously identified as one the primary life-limiting factors in fuel cells. The leaks result in reduced oxygen levels in affected cells, where a secondary factor intimately related to this is high hydrogen emissions in the cathode exhaust when some cells operate in fully-oxygen-starved conditions. This paper builds on previous work that developed a unified fuel cell model that predicts cell voltage behavior under driving (normal) and driven (oxygen-starved) conditions, where this latest analysis now explicitly includes hydrogen pumping and emissions release when operating under oxygen-depleted conditions. In addition to considering diffusion effects and electrochemical effects, the model tracks the evolution of hydrogen in the cell cathode when no oxygen remains to generate water. The voltage response of the model under normal (non-starved) conditions is first validated for steady-state and transient (current step-change) conditions against previously published experiments, and then the model is used to simulate the cell voltage and stack hydrogen emissions behavior measured from three different commercially available fuel cell stacks. In the first fuel cell stack, a 9-cell commercial short stack, only one cell was fully oxygen-starved. Excellent agreement is seen between the measured and simulated hydrogen release concentrations (where air injection was used downstream of the stack to ensure adequate oxygen levels for measurement with a catalytic hydrogen sensor and to condense water vapor in the exhaust), where the role of hydrogen pumping is seen to contribute significantly to the release behavior. The first fuel cell stack is then used transiently in comparison with testing performed where the hydrogen injection level in the cell is changed quickly, where the model gives good agreement with the measured emission response and cell voltage behavior. Further comparisons with test data from a second and third 10-cell commercial short stack models operated with stack inlet hydrogen injection show good agreement with measured emissions onset versus current, where the observed threshold of starvation and emissions occurs a few percent sooner in the third model than the simulation, but the overall behavior is well predicted.     

Combustion Model For Staged Circulating Fluidized Bed Boiler

INTRODUCTIONNumericalsimulationofcirculatingfluidizedbedcombustion(CFBC)isoneoftheimportantmeth-odsforthe0ptimizationofdesignandoperationandtheperformancepredictionofcirculatingfluidizedbedboiler.Researcheffortshavemademuchpr0gressinmodellingCFBfluiddynamics,heattransferandcom-bustioninthepastdecades.Duet0thecomplexity0fgassolidsfiowbehavi0rsinCFBcombustor,mostofthemodelsaresteadyone-dimensi0nal.Weissetal.(1987)[']devel0pedaCFBCcellmodel.ItdividestheCFBsystemintoaseriesofblockseach…     

A simple transient model for a high temperature PEM fuel cell impedance

We develop a pseudo two-dimensional, isothermal transient model for a high temperature proton exchange membrane fuel cell. It takes into account the dynamic change of oxygen concentration in the cathode gas diffusion layer and in the cathode channel. The model can be used to simulate and analyze electrochemical impedance spectra of the cell in both potentiostatic and galvanostatic modes, current interrupt results and step changes in the cell current or potential. The model is validated by fitting experimental data.     

A simple model of a high temperature PEM fuel cell

We develop a simple analytical model of a high temperature hydrogen fuel cell with proton exchange membrane. The model is validated against experimental results obtained in our group. The model is pseudo two dimensional, steady-state and isothermal, it accounts for the crossover of reactant gases through the membrane and it can be solved analytically. The role of the crossover is considered in detail.     

A new dynamic model for predicting transient phenomena in a PEM fuel cell system

In this paper, a mathematical model is developed to simulate the transient phenomena in a polymer electrolyte membrane fuel cell (PEMFC) system. At present many electrochemical models are available to the fuel cell designers to capture steady state behavior by estimating the equilibrium voltage for a particular set of operating conditions, but models capable of describing transient phenomena are scanty. In practical applications such as powertrains of land-based vehicles or submarines, the output power from the fuel cell system undergoes large variations especially during acceleration and deceleration. During such processes, many transient dynamic mechanisms become significant, while simple empirical models are unable to represent the transient dynamics caused by such as diffusion effect and double layer capacitance at the interface between the electrodes and the electrolyte. Hence, a novel dynamic fuel cell model is developed in this paper which incorporates the effects of charge double layer capacitance, the dynamics of flow and pressure in the anode and cathode channels and mass/heat transfer transient features in the fuel cell body. This dynamic model can predict the transient response of cell voltage, temperature of the cell, hydrogen/oxygen out flow rates and cathode and anode channel temperatures/pressures under sudden change in load current. The proposed model is implemented in SIMULINK environment. The simulation results are analyzed and compared to benchmark results. Lab tests are carried out at Connecticut Global Fuel Cell Center and a good agreement is found between tests and simulations. This model will be very useful for the optimal design and real-time control of PEM fuel cell systems.     

A simple electric circuit model for proton exchange membrane fuel cells

A simple and novel dynamic circuit model for a proton exchange membrane (PEM) fuel cell suitable for the analysis and design of power systems is presented. The model takes into account phenomena like activation polarization, ohmic polarization, and mass transport effect present in a PEM fuel cell. The proposed circuit model includes three resistors to approach adequately these phenomena; however, since for the PEM dynamic performance connection or disconnection of an additional load is of crucial importance, the proposed model uses two saturable inductors accompanied by an ideal transformer to simulate the double layer charging effect during load step changes. To evaluate the effectiveness of the proposed model its dynamic performance under load step changes is simulated. Experimental results coming from a commercial PEM fuel cell module that uses hydrogen from a pressurized cylinder at the anode and atmospheric oxygen at the cathode, clearly verify the simulation results.     

75 t/h电站循环流化床锅炉燃烧和污染物排放的数值模拟

以FLUENT软件为工具，运用数值模拟方法对75t/h电站循环流化床锅炉的炉内过程进行研究。计算和分析了炉内的温度分布，氧气、二氧化碳和一氧化碳的浓度分布，燃料颗粒的轨迹，氮氧化物(NOx)的排放。数值模拟的结果对循环流化床锅炉的设计和实际运行有一定的指导意义和参考价值。     

Co-combustion of refuse derived fuel with Korean anthracite in a commercial circulating fluidized bed boiler

The effect of co-combustion of Refuse Derived Fuel (RDF) with Korean anthracite on the combustion and environmental performance was observed in the Tonghae commercial Circulating Fluidized Bed (CFB) boiler. High contents of oxygen and CaO in RDF reduced the amount of air required for combustion and the limestone flow rate for SO₂ capture in the CFB boiler, respectively. The temperature in the furnace exit increased slightly due to re-combustion of volatiles which resulted in limiting the co-combustion ratio of RDF for the CFB boiler to operate stably. With the increasing co-combustion ratio of RDF, the output voltages of electrostatic precipitator (EP), which consists of 2-channels and 5-stages collecting plates, decreased linearly. Eventually, stability of the EP could not be maintained above 5% of the RDF co-combustion ratio. The emissions of NO_x, HCl and dioxin during co-combustion did not change appreciably as compared to the case when Korean anthracite was burnt alone, which were also low enough to meet Korean emissions limits. On the other hand, chlorine contents in ashes emitted from the CFB boiler increased gradually with the increasing co-combustion ratio, which implied that most of chlorines were fixed by limestone.     

Spatially resolved chemical species concentrations above the fuel bed of a small grate-fired wood-chip boiler

This work concerns the first stage of combustion in a small, air-staged, wood-chip boiler because comprehensive experimental data on the release of wood-volatiles during thermal conversion of solid biomass on grates are necessary to provide reliable data for the validation and optimization of fuel bed models and gas phase combustion models. Despite this importance, data especially from small-scale boilers are rare. This work provides detailed information about the spatial distribution of gaseous species, water and tar above the fuel bed of a commercial boiler with a nominal heat output of 40 kW. Furthermore, the influence of the primary air ratio and the wood-chip moisture on the fuel gas composition and the quantity of fine particulate matter emissions were investigated. Fuel gas volume fractions were: 16.6% CO, 3.6% H₂, 1.8% CH₄, 6.3% CO₂, 0.2% O₂, 28.3% H₂O, 43.2% N₂ for wood-chips with a fuel water content of 12% and a primary air ratio of 0.25. The corresponding tar content and the lower calorific value amounted to 78 g m⁻³ at standard temperature and pressure (STP) and 5.14 MJ kg⁻¹, respectively. The locally different concentration of species and temperature above the fuel layer resulted from the fuel bed movement. Strong gradients in species fraction were detected in direction of the fuel flow while species fraction gradients perpendicular to fuel bed movement were marginal. Particulate matter emissions of the investigated boiler were 7 mg m⁻³ (at STP and O₂ volume fraction of 13% in the dry exhaust gas) with aforementioned testing conditions.     

循环流化床锅炉采用强化福建Ⅱ类无烟煤燃烧的措施

1　引言燃用福建Ⅱ类无烟煤的循环流化床锅炉要想得到高的燃烧效率 ,必须按照福建Ⅱ类无烟煤的特性进行设计 ,采取相应的强化福建Ⅱ类无烟煤燃烧的措施 ,才能达到目的。2　福建Ⅱ类无烟煤在流化床内燃烧特性煤的沸腾燃烧是个很复杂的过程 ,它大致可分为挥发份燃烧和焦炭燃烧两个阶段。福建Ⅱ类无烟煤含碳量高 ,挥发份极低 (Ｖｄａｆ<5 %) ,着火点高(90 0℃以上 ) ,它的燃烧主要是通过碳粒的燃烧。其进入流化床后 ,首先有个物理加热升温至着火点的过程。在该过程中 ,其挥发份析出燃烧 ,但其所产生的热量远远不够提供该升温过程所需热量 ,需从…     

A cold model experimental study on the flow characteristics of bed material in a fluidized bed bottom ash cooler in a CFB boiler

IntroductionA fluidized bed bottom ash cooler is often used totreat high temperature bottom ash to reclaim heat andfine particles from the ash, and to have the ash easilytransported. Among the large CFB boilers in operation inthe world, there are many ash coolers that often workabnormally['-','].There are six fluidized bed bottom ash coolers in theimported 410im CFB boiler that was built and operatedin Gaoba power plant, Sichuan province, China in 1996N].High temperature slag-bond and jam …     

Optimization of emissions from fluidized bed boilers

The experiences of emissions of SO₂, NO_x and N₂O from fluidized bed boilers obtained from extensive measurements by the Department of Energy Conversion, Chalmers University of Technology, are briefly summarized. In addition, results from secondary methods to reduce NO and N₂O emissions are shown. With this background, possibilities for decreasing the N₂O emissions by the optimization of boiler parameters involving the use of secondary methods are discussed. The problem lies in the increase in the SO₂ and NO emissions when a measure is taken to reduce the N₂O emission. In particular, the influence of excess air and temperature is considered in comparison with selective noncatalytic reduction of NO.     

The use of a simple model in conjunction with a detailed carbon dioxide emissions model

We show how a simple model can be used in conjunction with an elaborate model to develop carbon dioxide emission scenarios. For both models, the carbon supply curves are the most important determinants. For the simple model, the carbon supply curves are endogenous while they are exogenous for the elaborate model. Two carbon dioxide emission scenarios have been developed by using the simple model in conjunction with the elaborate model.     

A particulate model of solid waste incineration in a fluidized bed combining combustion and heavy metal vaporization

This study aims to develop a particulate model combining solid waste particle combustion and heavy metal vaporization from burning particles during MSW incineration in a fluidized bed. The original approach for this model combines an asymptotic combustion model for the carbonaceous solid combustion and a shrinking core model to describe the heavy metal vaporization. A parametric study is presented. The global metal vaporization process is strongly influenced by temperature. Internal mass transfer controls the metal vaporization rate at low temperatures. At high temperatures, the chemical reactions associated with particle combustion control the metal vaporization rate. A comparison between the simulation results and experimental data obtained with a laboratory-scale fluid bed incinerator and Cd-spiked particles shows that the heavy metal vaporization is correctly predicted by the model. The predictions are better at higher temperatures because of the temperature gradient inside the particle. Future development of the model will take this into account.     

Performance and emissions of biodiesel in a boiler for residential heating

This paper presents the initial results of an experimental investigation of regulated and unregulated emissions of a boiler fueled with biodiesel and heating oil. In particular CO, SO₂, NO_x, particulate matter (PM), Polycyclic Aromatic Hydrocarbons (PAH), Volatile Organic Compounds (VOC) and aldehydes emissions were examined. When using biodiesel, a strong reduction in CO and PM emissions was recorded with respect to home heating oil. The PAHs contained in the PM in the case of biodiesel were nearly 13 times less toxic than in the oil case; the formaldehyde, on the contrary, is nearly double for biodiesel. The VOCs were very low for both fuels. The results indicate there may be benefits to using biodiesel in home heating or in industrial processes.     

Development of CO2 emission factors from a large circulating fluidized bed boiler

Greenhouse gas emissions of the nation should be precisely assessed in order to effectively tackle the problems stemming from climate changes. To do so, country-specific data that reflect a nation’s distinct characteristics must be applied to more precisely assess greenhouse gas emissions. In this research, carbon emission factors were assessed for emission sources making up over 53% of the domestic anthracite consumption with calorific values of fuels and elementary analysis. Furthermore, oxidation rates were assessed based on measurement results of unburned carbon produced from tested power plants to calculate CO₂ emission factors. Ultimately, the CO₂ emission factor was calculated at 106,747 kg/TJ, about 9% higher than the anthracite emission factor presented by the Intergovernmental Panel on Climate Control. Such differences are assumed to mainly come from disparities of anthracite properties along with combustion technology and differences of oxidation rates depending on the combustion conditions. Therefore, continued research on a wide variety of fuels and energy consumption facilities should be conducted in order to establish country-specific data, which will help more accurately assess greenhouse gas emissions and subsequently will lead to reliable greenhouse gas reduction goals.     

The development of a three-dimensional transient CFD model for predicting cooling ability of spent fuel pools

The objective of this research is to develop a numerical model for the evaluation of the cooling capability of spent fuel pool (SFP) in removing decay heat. Computational fluid dynamics (CFD) approach has been applied to develop a three-dimensional two-phase thermal hydraulic model. To simplify the analysis, the pressure drop of the coolant flowing through the fuel region in the pool is modeled as a porous-medium. The geometry as well as the boundary conditions of SFP of second nuclear power plant in Taiwan has been employed as sample to be analyzed and simulated numerically. The results show that the current configuration has enough cooling capability to meet the licensing regulations under normal operation and most kinds of conditions. In the case of extremely inappropriate layout of spent fuel arrangement, the computed results illustrate that slight local saturation on fuel rods may occur unless external cooling system is supplied.     

Heat transfer in a large-scale circulating fluidized bed boiler

Heat transfer of a furnace in a large-scale circulating fluidized bed (CFB) boiler was studied based on the analysis of available heat transfer coefficient data from typical industrial CFB boilers and measured data from a 12 MW_e, a 50 MW_e and a 135 MW_e CFB boiler. The heat transfer of heat exchanger surfaces in a furnace, in a steam/water cooled cyclone, in an external heat exchanger and in the backpass was also reviewed. Empirical correlation of heat transfer coefficient was suggested after calculating the two key parameters, solids suspension density and furnace temperature. The correlation approach agrees well with the data from the large-scale CFB boilers. __________ Translated from Journal of Power Engineering, 2006, 26(3): 305–310 (in Chinese) [译自: 动力工程]