Guangxi Yue

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

Hydrodynamic characteristics of a two-dimensional jetting fluidized bed with binary mixtures

Abstract A cold model of a two-dimensional jetting fluidized bed with a cross section 300 mm ×50 mm was employed to study the jet penetration depth and flow regime transitions. Four types of particles and four kinds of binary mixtures were used as bed materials in the test. In addition, a new multi-channel pitot tube system was developed to investigate the penetration depth. Experimental results indicate that the penetration depth decreases when the annular gas flow rate is increased from 1.0 Q mf to 2.5 Q mf , while it remains constant for annular gas flow rate ranging from 2.5 Q mf to 3.0 Q mf (annular gas flow rate including fluidization air from the plenum chamber and the V type gas distributor). A new correlation is presented for predicting the penetration depth for different annular gas flow rate. The effects of particulate density, particulate diameter, jet velocity, static bed height, nozzle diameter, and annular gas flow rate on flow regime transitions, i.e. jet, transition flow regime and spout, were systematically examined. A correlation to predict flow regime transitions was also developed, which was in good agreement with the experimental data from the present study and other references.

Effects of the Updated National Emission Regulation in China on Circulating Fluidized Bed Boilers and the Solutions To Meet Them

The advantage of circulating fluidized bed (CFB) boilers in China is their ability to utilize low rank coal with low cost emission control. However, the new National Emission Regulation (NER) issued in early 2012 brings much more stringent challenges on the CFB industries, which also causes much attention from other countries. Based on the principle of a CFB boiler and previous operating experience, it is possible for the CFB boilers to meet the new NER and maintain the advantage of low cost emission control, while, more influences should be considered in their design and operation. To meet the requirement of the new NER, the fly ash collector should adopt a bag house or combination of electrostatic precipitator and bag filter to ensure dust emissions of less than 30 mg · Nm(-3). For SO2 emission control, the bed temperature should be strictly lower than 900 °C to maintain high reactivity and pores. The limestone particle size distribution should be ranged within a special scope to optimize the residence time and gas-solid reaction. At the same time, the injecting point should be optimized to ensure fast contact of lime with oxygen. In such conditions, the desulfurization efficiency could be increased more than 90%. For lower sulfur content fuels (<1.5%, referred value based on the heating value of standard coal of China), increasing Ca/S enough could decrease SO2 emissions lower than that of the new NER, 100 mg · Nm(-3). For fuels with sulfur content higher than 1.5%, some simplified systems for flue gas desulfurization, such as flash dryer absorber (FDA), are needed. And the NOx emissions of a CFB can be controlled to less than 100 mg · Nm(-3) without any equipment at a bed temperature lower than 900 °C for fuels with low volatiles content (<12%), while for fuels with high volatiles, selective non-catalytic reduction (SNCR) should be considered. Due to the unique temperature in CFB as well as the circulating ash, the efficiency of SNCR could reach as high as 70%. The Hg emission of CFB is very low for the new NER due to its innate property.

Effect of boundary layer reactions on the conversion of char-N to NO, N2O and HCN at fluidized-bed combustion conditions

The effect of boundary layer reactions on the conversion of char-N to NO, N 2 O, and HCN was studied by a coupled experimental/theoretical approach. In the experimental part, the evolution of HCN at conditions of a batch fluidized-bed reactor was studied. The results show that the char-N conversion to HCN is very low under normal oxidizing conditions. This explains why it was not detected by early studies of this system. The results also suggest that the release of HCN is related to gases released when the char is oxidized rather than to late devolatilization reactions. The product distribution is modeled using heterogeneous reactions on the char surface coupled with homogeneous reactions in the gas phase. Three cases were analyzed. In Case A, only the heterogeneous production of N 2 O and NO was considered. In Case B, HCN and NO heterogeneous production was considered. Finally, Case C combined cases A and B by allowing the heterogeneous production of NO, N 2 O, and HCN. Cases A and B predict most of the qualitative trends for the char-N conversion to N 2 O reported in the literature. However, Case A fails to predict the experimentally observed release of HCN when oxidation around the particle is suppressed. Among the three cases, Case C is the one that closely predicts experimental observations. Finally, the predictions obtained by the model are used to define further experiments, particularly on the evolution of gas concentration in the boundary layer with char burnout, which can be useful to obtain more insight into the mechanism of nitrogen oxide formation during char oxidation.

Measurement of Heat Transfer in a 465t/h Circulating Fluidized Bed Boiler

The study of heat transfer and thermal boundary layer in the combustor of a circulating fluidized bed (CFB) is important to the boiler design and operation. Both heat transfer coefficient between the solid-gas flow and the water-wall and the thickness of thermal boundary layer are key data to determine the amount and layout of the tube walls in a CFB furnace. A series of experiments was conducted on a 465t/h commercial CFB boiler, which operated at bed temperature between 850 and 900°C, and at superficial gas velocity between 5.2 to 5.9m/s. Local bed to water wall heat transfer coefficients and temperature profiles near the wall were measured at a set of test ports at different heights of the sidewall. In the same time, the local solid bulk density near the wall was also measured. Special tools such as heat flux probe, solid bulk density sampling probe and temperature probe were developed for the experiments and their structures were introduced. The experimental results were compared with the data from previous studies. Theoretical analysis of the factors that play important role in heat transfer in a CFB boiler was also performed. The relationship between heat transfer and thermal boundary layer was also discussed. Furthermore, a simple model correlating the local heat transfer coefficients with bulk density was developed.Copyright

Operational Performance and Optimization of a 465t/h CFB Boiler in China

In the last three years in China, more than 80 units of 135MWe circulating fluidized bed (CFB) boilers were ordered, and about two dozens of them have been put into operation. So far, the experience and performance evaluation of the boilers with such large capacity are very limited. A series of cold and hot tests were carried out on the boiler in order to optimize the operation and provide more information to the future design. The influence of coal properties, bed material fluidization, air distribution, bed temperature and pressure drop on the boiler performance such as carbon content in fly ash was assessed and discussed. Some problems of the boiler, including the bottom ash system, milling system, abrasion of the heating surface in the furnace, refractory stability, and exhaust fuel gas temperature are reported and suggestions are given for the future improvement and design.Copyright

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

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80 × 30 mm two-dimensional quartz fluidized bed and in an 82 mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO2, Al2O3, Na2O, K2O, and SiO2 than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.

An Experimental Study on N2O Reduction Over Circulating Ashes of CFB Boilers

An experimental study on nitrous oxide (N2 O) reduction with circulating ashes of circulating fluidized bed (CFB) boilers was conducted for the development of an effective and economical technology to reduce the N2 O emission from CFB boilers. The experiments were conducted with three kinds of circulating ashes, by using a fixed bed reactor with a diameter of 25mm, under various conditions of different operational temperatures, initial N2 O concentrations, NH3 additions and O2 concentrations. The effects on N2 O reduction were compared that with quartz sand. The experimental results showed that circulating ashes may possess remarkable catalytic effect on N2 O reduction and the intensity of the catalytic effect strongly depends on operational parameters such as reaction temperature and O2 concentration. It was also found nitric oxide (NO) was produced during the process of N2 O reduction and its concentration also strongly affected by reaction temperature and O2 concentration. The mechanisms of N2 O reduction were discussed. The study confirmed the feasibility of injecting NH3 at the cyclone entrance of CFB boiler to form a selective catalytic reduction (SCR) process for N2 O emission without using extra catalyst and provided some guidance to choose the operational parameters for N2 O reduction with circulating ashes.Copyright

Research on the Anthracite Pyrolysis Property by Thermogravimetric Analysis

Experiments were performed in a TGA apparatus to investigate the pyrolysis properties of one kind of anthracite from Leiyang Power Plant, which has been used by several boilers. The effects of pulverized coal particle size, heating rate and final pyrolysis temperature on the volatile release property are presented. Experimental results confirmed that high heating rate could promote the volatile release rate at the primary period of pyrolysis. However, the complete pyrolysis still needs enough time. The final pyrolysis temperature also strongly affects the amount of volatile matters, while the effect of particle size can be ignored. So the total volatile yields are effected not only by the heating rate but also by the final pyrolysis temperature. The conclusion is different from the former results that volatile yields are independent of the heating rate. These results indicate the anthracites volatile matter come out parallel with the char combustion, which are similar for the different size pulverized coal particles.