Jiansheng Zhang

Design Theory of Circulating Fluidized Bed Boilers

Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this theory are presented in this paper. First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH’s studies validated that a CFB boiler can be generally described as the superposition of a fast bed in the upper part with a bubbling bed or turbulent bed in the bottom part. A concept model of material balance for the open system of CFB boiler was developed and later improved as a more comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into account. Some results of the models are discussed. Then the concept of State Specification of a CFB boiler is defined and discussed. The State Specification is regarded as the first step to design a CFB and a base to classify different style of CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted by major CFB boiler makers is summarized and associated importance issues are addressed. The heat transfer model originally developed by Leckner and his coworkers is adopted and improved. It is further calibrated with experimental data obtained on the commercial CFB boiler measurements. The principle, improvements and application of the model are introduced. Some special tools developed for heat transfer field test are also given. Also, combustion behaviors of char and volatile content are studied, and the combustion difference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile content and size distribution is discussed. The concept of vertical distribution of combustion and heat in CFB boiler furnace is introduced and discussed as well. In the last, the suggested design theory of CFB boiler is summarized.Copyright

A USM turbulence-chemistry model for simulating NOx formation in turbulent combustion

A unified second-order moment (USM) turbulence-chemistry model for simulating NOx formation in turbulent combustion is proposed. All the correlations, including the correlation of the reaction-rate coefficient fluctuation with the concentration fluctuation, are closed by the transport equations in the same form. This model abandons the series expansion approximation of the exponential term or the approximation of using a product of two single-variable PDFs instead of a joint PDF. The proposed model is used to simulate methane–air jet diffusion combustion and NOx formation. The combustion prediction results are compared with those using the EBU-Arrhenius model and other two versions of the second-order moment model. The NOx prediction results are compared with those using the pure presumed PDF model. Validation of predictions using the experimental data given by the Sandia National Laboratory, USA indicates that the proposed model gives better results than other models, and it is much economical than other refined models.

Simulation of swirling combustion and NO formation using a USM turbulence-chemistry model☆ ☆

Abstract A unified second-order moment (USM) turbulence-chemistry model is used to simulate methane-air swirling combustion and NO formation for different swirl numbers. The simulation results are compared with those using the EBU–Arrhenius (E–A) combustion model, the simplified PDF model of NO formation in turbulent flows and the corresponding experimental results. The comparison indicates that the USM model is obviously better than the E–A model and the simplified PDF model. The E–A model cannot reasonably simulate the finite-rate kinetics, while the simplified PDF model, using a product of two single-variable PDFs instead of a joint PDF, remarkably under-predicts the NO reaction rate. The USM model gives the best agreement with the experimental results. Predictions show that as the swirl number increases the total NO formation at first decreases and then increases, which is in agreement with the experimental results.

Studies on the effect of swirl on no formation in methane/air turbulent combustion

Most present studies on pollutant formation concentrate on chemical reaction kinetics. To understand the interaction between turbulence and chemistry in NO formation, the effect of swirl number on NO formation in methane/air turbulent combustion is studied by experiments, in which a small amount of ammonia is added to the fuel to simulate fuel nitrogen, and simultaneously by numerical simulation, using a second-order-moment PDF turbulence-chemistry model. The predicted results are in overall agreement with the measured results. Both predictions and experiments show that as the swirl number increases from 0 to 1, the thermal NO at first increases and then decreases. In contrast, the fuel NO at first decreases and then increases. The studies also show that the increase in swirl number first leads to a rapid decrease and then a slower increase in turbulence intensity, and first an increase and then a slight decrease of temperature near the exit. As the activation energy of thermal NO formation is much larger than that of fuel NO formation, these results imply that the thermal NO is predominantly affected by temperature, whereas the fuel NO is predominantly affected by species mixing via turbulence. The research results are expected to be used for developing low-NO x burners.

Determination of Ignition Temperature of Coal by Using Thermogravimetry

In this paper, the ignition temperatures (Ti ) of seven kinds of coals were measured by using thermlgravimetric analyzer (TGA). A TG-DTG method was suggested to determine the ignition temperature. This method is simple, convenient, standardized and with high repeatability. The relations between Ti and volatile content and active energy are analyzed. Compared with the ignition temperatures measured in a bench scale fluidized bed and boiler test, Ti s derived from TG-DTG method have nearly same tendency with changing volatile content, but have 100°C deviation below those measured in fluidized bed. The results strongly indicate that TG-DTG method can substitute the fluidized bed method and boiler test for measuring ignition temperature of a coal with an off-set adjustment.Copyright

Effect of the Additives on the Desulphurization Rate of Flash Hydrated and Agglomerated CFB Fly Ash

CFB fly ash from separators was mixed with water or the mixture of water and additives under the temperature of 363K by use of a blender. Then, this compound of fly ash and water or additives was pumped into a CFB combustion chamber by a sludge pump. Because the temperature of flue gas was high in CFB, the fly ash was hydrated fast and agglomerated in the same time. Through this process, the size of agglomerating fly ash is larger than the original particle and the relative residence time of agglomerated fly ash in CFB becomes longer. Therefore, the rate of utility of calcium in fly ash improves and the content of carbon in fly ash decreases. This results in a low Ca/S and low operational cost for CFB boiler. The additive is one key factor, which affects the rate of desulfurization of agglomerated fly ash. Effect of different additives on rate of desulfurization is not same. Cement and limestone are beneficiated to sulfur removal of agglomerated fly ash, but sodium silicate does not devote to the rate of sulfur removal of agglomerated fly ash.

Combustion Modeling of CFB Boiler Furnace Based on an Euler-Euler Approach

For the better design of large scale CFB boilers, reliable multi-dimensional modeling which can be used to predict the heterogeneous distributions of gas and solid concentration as well as temperature in the furnace is necessary and demanded. A model describing the complex combustion process in a CFB boiler furnace has been developed. The model consists of several essential sub-parts: the hydrodynamics of the bed, combustion of fuel, and overall mass balance of the furnace. In the computational fluid dynamics (CFD) study on hydrodynamics in a CFB boiler, the Euler-Euler approach is used, in which both gas and solid phases are considered as interpenetrating continua with the interaction through drag and energy dissipation caused by particle fluctuation. The constitutive equations for solid phase are derived from the kinetic theory of granular flow (KTGF). Some simplifications of the complicated theoretical equations with empirical correlations are adopted, to save computing time and skip the currently unknown phenomena. Drag coefficient between gas phase and solid phase is modified by the energy-minimization multi-scale (EMMS) principle. A simplified description of reaction process is also adopted. The present model was applied to predict the hydrodynamic and combustion behaviors in a commercial 135 MWe CFB boiler. Some primary results are obtained and discussed in comparison with the measured data. Prediction results agree with the experimental data in general, confirming the correctness of the model. More reliable experiments are needed for the model improvement in the future.Copyright

Thermodynamic Calculation of Moisture Separator Reheater in Nuclear Power Plant

MSR (moisture separator reheater) has an important effect on the safe and economic operation of the steam turbine unit in the nuclear power plant. The thermodynamic calculation is the basic of design and operation of MSR. In this study, a new method of thermodynamic calculation of MSR is proposed. The calculated results indicated that the method can be used for the primary design and the check of heating surface area in MSR. Furthermore, the results verify previous research on the condensation in horizontal tube.

A Molecular Dynamics Method for Atomic Models for Amorphous Material: an Example with SiO 2

With the scaling of MOSFET, the degradation of circuit performance caused by the interconnection network impairs signal propagation delay, power dissipation and signal integrity. Following the transition from aluminum to copper interconnects, the search for suitable low-k dielectrics is currently the most important and urgent step in the course of the continued miniaturization of device dimensions in the microelectronics industry. In addition to the dielectric constant, sufficient mechanical strength is a key to successful integration of the low-k materials. Theoretical investigations of dielectric and mechanical properties of low-k films would be useful for finding appropriate film component and suitable micro-structure of the film. Unfortunately, theoretical research on the property prediction of low-k films has been limited. Tajima et al. [1] developed a method to create molecular models of amorphous polymers with cross-links in which the chemically possible molecular structures were generated from a given atom-group composition. They have applied this method to the molecular modeling of a typical SiOCH low-k film which is made by plasma enhanced chemical vapor deposition. However, their structures were too small (containing 66 atoms) to reproduce the complexity of low-k material and the first principle calculation they applied to get dielectric property was costly in time. We followed this method and found that the direct extending of their method to larger structure will confront the difficulty in structure relaxation [2], as the totally random initial structure takes no essential of bonds (like bond length and bond angle) into account and causes the molecular dynamics (MD) relaxation hard to complete. This is exactly one important motivity of current work. Besides, fullerene based insulating materials with extremely low dielectric constants and good mechanical properties were designed by Zagorodniy et al. [3]. This kind of material is, however, something away from the state-of-art low-k film. Producing atomic structure for amorphous material by sudden increasing and decreasing simulated temperature in MD and by continuous local bonds rearrangement [4] are among the favorite methods of physicists. However, it is difficult to transplant these methods into low-k structures modeling because of the peculiarity of the material, e.g. porosity, and the effect of cross-link of atoms on the dielectric and mechanic properties. In this paper, we propose a MD based method to create amorphous atomic structure of several hundred atoms in which dielectric constants and mechanical properties can be predicted more efficiently comparing with first principle simulation. This method is also easy to deal with the porosity and complicated cross-link of atoms in low-k material with acceptable computational cost. It is noted that bonding-fixed (BF) potential, (i.e. the potential that needs not only the atom position but the bonds between the atoms to calculate the force on the atoms, and that never changes the bonds during the process) in MD calculation is robust and it can be used to relax the initial structures to stable ones. With a coarse but rational initial guess given, BF potential can be used to relax amorphous structure of several hundred atoms. And a further structure optimization using non-BF potential will give a more stable thus more reasonable final configuration. If the relaxation fails in this step, it suggests that it is the bonds restriction rather than things more physically meaningful makes the BF relaxed structure stable. Amorphous material is in the sub-stable state, and any artificial structures must be relaxed to rational one with the minimum energy. With polarizable potential used (e.g. shell model potential, the simplest while widely used classic potential that take polarization into account), we can get dielectric as well as mechanic properties from MD calculation. The flow chart of the process is shown in Fig.1. The BF-potential will maintain the bonding relation of the initial chemically possible structures that make it possible to study the effect of cross-link of atoms on the material properties. The BF potential offers capability to uphold the pores introduced in the initial guess that make the study of porous material possible.

Experimental Measurement on Bubble Parameters During Fluidized Bed Scaling

An experimental verification is reported on the bubble parameter similarity during fluidized bed scaling. Two bubbling fluidized beds were designed according to Horio’s scaling method, and bubble size and mean bubble rising velocity were measured and compared. It can be concluded from the results that with the increment of u/umf , the similarity between two beds increases with respect to the bubble diameter and rising velocity. The analysis of the experimental data confirms the applicability of Horio’s method on bubble fluidized bed. When given dynamic behavior prediction of a real boiler is desired, the Horio’s law is valid to establish a cold model and the analysis method introduced in the present paper can be used.Copyright