Junfu Lu

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

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

Predictive control of coal mills for improving supercritical power generation process dynamic responses

The paper is to study new control strategies for improvement of dynamic responses of a supercritical power generation process through an improved control to the associated fuel preparation performed by the coal milling process. Any control actions taking for the milling process will take a long time to show their influences onto the boiler, turbine and generator responses as the whole process experiences coal transmission, grinding, drying and blowing to the furnace. The control philosophy behind the work presented in the paper is to develop a control strategy to achieve prediction of the future demand for fuel input and implement control actions at the earliest possible time. The paper starts from description of the nonlinear mathematical model developed for the supercritical coal fired power plant and then moves onto control strategy development. Finally, the simulation study has been carried out to demonstrate the effect of the new predictive control.

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

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

Destruction of N2O over Different Bed Materials

Since under fluidized bed conditions N2O is produced as a by product of the De-NOx process, the thermal decomposition of N2O was investigated under conditions relevant to those in FBC installations. Laboratory experiments were made in a current of nitrogen using a fixed bed of pure quartz sand or sand with 10% (wt.) of the solids tested, CaO and Fe2O3. With a sand bed the decomposition was slightly faster than in the empty reactor and the reaction was first order with respect to [N2O]. Both fresh CaO and Fe2O3 strongly catalysed N2O decomposition. Their effectiveness diminished after they were heated to temperatures typical for FBC, but they still retained appreciable activity. This activity went down with increasing particle size. The flue gas components investigated were O2, water vapour and CO2. Their presence appeared to interfere with N2O decomposition and increased with the concentration of the additive. The observations indicated that this could only be due to heterogeneous effects. Thus the effects of the bed solids and of the gas phase components are opposed. The effects associated with N2O decomposition have proved to be surprisingly complex and instead of supplying simple answers, this work uncovered more problems.

Novel CFB Boiler Technology with Reconstruction of its Fluidization State

Compared with a conventional pulverized coal fired boiler, the combustion efficiency of a CFB boiler is lower while the self-consumed service power is 1–2% higher. The solution of these problems is the key research topic for researchers and manufacturers of CFB boilers. Based on the State Specification Design Theory of CFB boilers, Tsinghua University proposed a novel CFB technology by reconstruction of the fluidization state in the furnace by adjusting the bed inventory and bed quality. Theoretical analyses show that there is an optimal bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan power consumption. The proposed novel process was implemented in a 75t/h CFB boiler. The results of field tests on this boiler validated the theoretical analyses.

Mineral Matters in High-AAEM Zhundong Coal—In Comparison with High-calcium PRB Coal

Mineral contents in two Zhundong coals and PRB coal were determined by XRF (X-ray fluorescence), XRD (X-ray diffraction) and TG-DSC (thermo gravimetry–differential scanning calorimetry). Effect of ashing temperature was assessed by comparing ash element compositions and mineral patterns at three temperature levels. Element compositions of the ashes showed that Zhundong coals have higher slagging and fouling tendency than PRB coal. It was also found that quartz and kaolinite are main mineral patterns in Zhundong coals and PRB coal. Quartz barely changes with ashing temperature because of its high reaction and melting temperature. Kaolinite begins to dehydrate at ~673 K and then decomposes as temperature further increases. The mineral patterns strongly depend on ashing temperature because of the different reactions among minerals. Based on the mineral pattern and transformation analyses using TG-DSC, calcite was the main form of calcium in one Zhundong coal, while anhydrate was the main form of calcium in the other Zhundong coal and PRB coal.

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.