A Comprehensive, Three-Dimensional Analysis of a Large-Scale, Multi-Fuel, CFB Boiler Burning Coal and Syngas. Part 2. Numerical Simulations of Coal and Syngas Co-Combustion

The dispersion characteristics of fuel particles over the bed have always been a key consideration in the design of fluidized beds. However, the lateral fuel dispersion coefficients (Dsr) reported in the previous literature widely ranged from 10−4 to 10−1 m2/s, which were hard to be directly applied in large-scale CFB (circulating fluidized bed) boilers with wider bed width (close to 30 m) and variable coal feeding modes. To solve this problem, field tests were conducted on the first 600 MW CFB boiler in this paper, and the distributions of flue gas composition and flue gas temperature were obtained near the coal feeding port under four working conditions. The dispersion characteristics of fuel particles were analyzed in detail combined with the dispersion model. In addition, a new method based on bed heat balance was also proposed to estimate Dsr and applied in a 300 MW CFB boiler with a different coal feeding mode. The results revealed that Dsr through the local heat balance method ranged from 0.10–0.35 m2/s, which were 9–18% higher than the values through the dispersion model. Compared with the coal feeding port immersed in the bed, Dsr of the fuel particles fed into the splash zone increased by about 34.7%. Dsr under the coal feeding mode of a screw feeder with pressurized air ranged from 0.12–0.16 m2/s, which were lower than Dsr under the coal feeding method of importing fuel into circulating ash. The method and results in this paper could expand the applicability of fuel dispersion, and provide the most direct guidance for the design and optimization of coal feeding ports in large-scale CFB boilers.

The unknown NOx distributions inside large-scale CFB (circulating fluidized bed) boilers have always hindered the economy of the SNCR (selective non-catalytic reduction) process. In this study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensional distributions of flue gas composition at SNCR inlets for the first time. The penetration depth inside the horizontal flue pass was up to 7 m. The NOx distributions were analyzed in detail combining with the auxiliary test in the dilute phase zone. Key results show that the average O2 concentrations in #A and #C regions were 6.52% and 0.95%, respectively. The vertical NOx distributions of #A and #C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak value all appeared at 5 m depth, while the NOx distribution of #B SNCR inlet was basically increasing. Some local areas with extremely high NOx concentration (over 2000 mg/m3) were observed near the inclined edge of SNCR inlets, which has never been reported before. Based on this, the optimization of urea injections was conducted, which could save 15.7% of the urea solution consumption while ensuring ultra-low emission of NOx.

CFB (Circulating Fluidized Bed) boiler appeared in the second half of the 1970 s. CFB boiler attracted attention that it was possible for the stable burning for the low quality fuel such as biomass and sludge which wasn't possible to burn in the conventional combustion.From the beginning of 1980 s, CFB boiler started to use for coal burning. It has a potential to use low quality coal which has high ash, high moisture, low volatile matter and low heating value. The low NOx and in-furnace DeSOx were admitted as a new technology for the environment load reduction in the coal burning plant. Therefore, as for the point of the cost performance, it got high evaluation and became the mainstream of the coal combustion. Moreover, the development of a large scale and a high efficiency plant has been progressing in the coal combustion. The large scale CFB boiler for the biofuel applied coal combustion technology has been also carried forward.With regard to the electricity generation used the city garbage, the high efficiency process is now required as a CO2, reduction technology. Biomass burning experience was applied to this, because city garbage has a high moisture and a high volatile matter like a biomass. The commercial plant to achieve this has already built and operated by Foster Wheeler from the beginning of 1997. In this process, the city garbage is converted to RDF (Refuse Derived Fuel), which is to recovery resources and also for the easy feeding and high performance combustion.The biomass utilization technologies, which use Foster Wheeler CFB, are not only the above-mentioned direct burning. The gasification technology has been developing from the early days. The development of atmospheric CFB gasifier has started from the beginning of 1980 s, and also pressurized gasifier was from the middle of 1989 s. Both technologies are in the commercial use stage.In the Foster Wheeler CFB technology used for the energy recovery with the high performance, the biomass and RDF direct burning, the atmospheric gasification for the biomass co-firing with coal and pressurized gasification for biomass IGCC (Integrated Gasification Combined Cycle) are presented in this paper.

Circulating Fluidized Bed (CFB) boilers are desirable technology to utilize solid fuels, operate within a wide load range, and promote the high flexibility in burning a wide range of coal and other solid fuels, keeping high efficiency and low gaseous pollution. The paper presents thermal and flow modeling results of steam superheater operated in the large-scale coal-fired CFB boiler, both in steady and transient states. The mathematical model of the superheater wall was developed using Control Volume-based Finite Element Method. The heat transfer coefficient as the sum of radiative and convective heat transfer coefficients was considered in the presented approach to modeling thermal processes in a circulating fluidized bed. The overall heat transfer coefficient was computed for the analyzed type of the boiler at full load and steady-state conditions using available measuring data and results obtained using Computational Fluid Dynamics (CFD) modeling. Depending on the modeling approach adopted in the CFD model, the heat transfer coefficient at full boiler load varies between 159.96 and 189.63 W/(m2·K). In the analyzed period of boiler start-up, results of steam outlet temperature (455 °C–485 °C), heat transfer coefficients on the flue-gas side (100 W/(m2∙K) and 120 W/(m2∙K)), and heat transfer coefficients on the steam-side (925–2008 W/(m2∙K)) vary with time. The methods presented in the paper can be applied to modeling transient flow and thermal processes in superheaters with complicated flow arrangements. The advantage of the proposed method is a short computing time compared to detailed and complex 3D models. The developed model of the superheater with the transfer coefficient determination methods can be used to perform calculations in the full range of boiler load, both during steady and transient boiler operation.

Modeling of particle transport and combustion phenomena in a large-scale circulating fluidized bed boiler using a hybrid Euler–Lagrange approach

This paper presents an overview of advancements of circulating fluidized bed (CFB) technology in Thermal Power Research Institute (TPRI),including technologies and configuration and progress of scaling up. For devoloping large CFB boiler, the CFB combustion test facilities have been established, the key technologies of large capacity CFB boiler have been research systematically, the 100MW ∼330MW CFB boiler have been developed and manufactured. The first domestically designed 100MW and 210MW CFB boiler have been put into commericial operation and have good operating performance. Domestic 330MW CFB boiler demonstration project also has been put into commericial operation,which is H type CFB boiler with Compact heat exchanger. This boiler is China’s largest CFB boiler. The technical plan of domestic 600MW supercritical CFB boiler are also briefly introduced.

The current state of and progress in circulating fluidized bed (CFB) technology are reviewed. The CFB technology has been used industrially in the boilers of generating units with powers up to 600 MW. CFB boilers are most widely used in China. The steam parameters and unit powers of systems with CFB boilers are increasing. CFB technology has the advantage of being able to burn a wide range of fuels, including biomass and process wastes, and this is a further stimulus for the introduction of CFB boilers. Based on studies of the hydrodynamics and heat exchange in model CFB boilers and generalizations of published data on advances in the design of CFB boilers, recommendations for the basic dimensions and performance of ash collectors and recovery systems in CFB boilers have been developed at the JSC “VTI” to ensure reliable determination of the major characteristics of boiler designs. Design methods and programs have been formulated. Recent designs for advanced coal-fired central heating and power plants at VTI confirm the suitability of CFB boilers for widespread use. The first CFB boiler in Russia is being installed and tested at the 330 MW unit No. 9 of the Novocherkassk GRES State Regional Electric Power Plant. Experience with the installation and startup of this unit will create the conditions for widespread introduction of CFB technology in Russia.

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 MWe, a 50 MWe and a 135 MWe 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.

From a CFB reactor to a CFB boiler – The review of R&D progress of CFB coal combustion technology in China

Circulating fluidized bed (CFB) boiler has entered electric power industry field because of burning a wide range of fuels, while still achieving strict air emissions requirements. This study focuses on a 300MW CFB boiler, which will be one of the largest CFB boiler in the world. In a CFB boiler, fuels were burned with the addition of limestone to capture SO2 in a solid form. Therefore, the volume of ashes, both bottom ash and fly ash, discharged from a CFB boiler is much higher than the ashes discharged from a pulverized coal-fired (PC) boiler at the same capacity of the boiler. CFB boiler ash cannot be used as a cement replacement in concrete due to its unacceptably high sulfur content. The disposal in landfills has been the most common means of handling ash in circulating fluidized bed boiler power plants. However, for a 300MW CFB boiler power plant, there will be 600,000tons of ash discharged per year and will result in great volumes and disposal cost of ash byproduct. It was very necessary to solve the utilization of CFB ash and to decrease the disposal cost of CFB ash. The feasible experimental study results on the utilization of the bottom ashes of a 300MW CFB boiler in Baima power plant in China were reported in this paper. The bottom ashes used for test came from the discharged bottom ashes in a 100MW CFB boiler in which the anthracite and limestone designed for the 300MW CFB project was burned. The results of this study showed that the bottom ash could be used for cementitious material, road concrete, and road base material. The masonry cements, road concrete with 30MPa compressive strength and 4.0MPa flexural strength, and the road base material used for base courses of the expressway, the main road and the minor lane were all prepared with milled CFB bottom ashes in the lab. The better methods of utilization of the bottom ashes were discussed in this paper.

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.

According to the exergy efficiency analysis of circulating fluidized bed (CFB) boiler, various exergy loss rates are discussed in this paper. Combustion exergy loss and heat transfer exergy loss are the major losses of CFB boiler, accounting for more than 45% of the fuel chemical exergy by CFB boiler exergy analysis method. For 300MW and its below assembling unit, the exergy efficiency of CFB boiler is generally less than 50%. The boiler exhaust exergy loss rate is about 1%, which is much less than heat loss rate calculated by heat balance method, but there is still a great potentiality to be reduced. As far as CFB boiler is concerned, chemical incomplete combustion exergy loss rate and radiating exergy loss rate are both too small to be negligible. Unburned carbon exergy loss should be focused on due to its high energy quality. CFB boiler slag has a higher proportion of coal ash. Therefore, it is important to reduce the slag physical exergy loss for improving CFB boiler efficiency.

Circulating fluidized bed (CFB) boilers has realized the clean and efficient utilization of inferior coal like gangue and coal slime, high sulfur coal, anthracite, petroleum coke, oil shale and other resources. As a country with the largest amount of CFB boilers and the largest installed capacity in the world, China has 440 100–600 MWe CFB boilers with a total capacity of 82.29 GWe, including 227 units of 135 MWe, 95 units of 300 MWe, and 24 supercritical units. The statistics of typical 100–300 MWe CFB boilers showed that the average number of unplanned shut-down was only 0.37 times per year, among which the 135 MWe was 0.26 times per year and 300 MWe was 0.46 times per year. The auxiliary power ratio of some 300 MWe CFB boilers based on flow-pattern reconstruction can be reduced to about 4%, which is closed to the same level of pulverized coal (PC) boilers. This paper summarizes the development process and application status of China’s large-scale CFB boilers, analyzes the characteristics and technical performance of the iconic units, and introduces solutions to the problems such as water wall wear and bottom ash cooling.

For study of technical difficulties that make Botswana first CFB (Circulating Fluidized Bed) boiler can’t reach full load at beginning of loading process, first experiments were carried out as awful experimental cases to analyze influencing factors; Second experiments were carried out as tentative experimental cases to select useful solutions; Third experiments were carried out as targeted experimental cases to execute chose solutions. Results indicate that one prominent difficulty during CFB boiler reaching full load is that boiler operation bed temperature was too high which was a comprehensive effect with some other influencing factors. CFB boiler bed temperature MFT (Main Fuel Trip) set point slight increasing, grain size changing of both start-up material and feeding coal, limestone injecting, fluidizing air of seal pot and FBHE appropriate controlling and FBHE (Fluid Bed Heat Exchanger) gradually start working are taken simultaneously, all these effective solutions solve the technical difficulties and accelerate the boiler reach full load, but attentions still need to be paid for the boiler stable operation permanently in future.

A note on large-size supercritical CFB technology development