Circulating Fluidized Bed Units Research Articles

Modified active disturbance rejection control based on gain scheduling for circulating fluidized bed units

Circulating fluidized bed (CFB) units are extensively operated in China due to their wide fuel adaptability, low emissions and high combustion efficiency. Nevertheless, CFB units are facing many control challenges due to their strong nonlinearity and large lag characteristics. To handle these control challenges, a modified active disturbance rejection control based on gain scheduling is structured in this paper. Firstly, a decentralized active disturbance rejection control strategy based on gain scheduling is designed by analyzing control difficulties of CFB units. Then, the parameter tuning and scheduling methods are provided, and the convergence of the extended state observer during the scheduling process is derived theoretically. The advantages of the proposed method in tracking performance, disturbance rejection performance, and ability to reject fuel quality fluctuation and uncertain heat transfer coefficients are illustrated by comparative simulations. Finally, the proposed control strategy is practically applied to the main steam pressure system of a 300 MW CFB unit. Running data demonstrate that it has a faster tracking performance and better disturbance rejection ability in [50∼100]% of the rated load, where the hourly average integral absolute error index has decreased obviously, and it shows a good field application prospect.

Data-driven optimization of pollutant emission and operational efficiency for circulating fluidized bed unit

To achieve the economic and environmentally friendly operation of circulating fluidized bed (CFB) units, it is imperative to conduct optimization to obtain an economical mode of pollutant removal. This article focuses on the multi-objective optimization between SO2-NOx emissions and thermal efficiency for CFB units. According to the operation data and production mechanism of pollutants, models of SO2-NOx emission concentration, bed temperature, and oxygen content based on a convolutional neural network–bidirectional long short-term memory–attention mechanism (CNN-BiLSTM-Attention) were established. Then, an improved quantum genetic algorithm was used to find the optimal input variables of the SO2-NOx emission model. The proposed modeling method was evaluated, and it more accurately simulated the trends of actual operation data than other models under different operating conditions. Combining these models with economic calculations, the operating costs under typical conditions were reduced by 3.20% and 1.82% respectively, and the thermal efficiency increased by 0.72% and 1.07%, which contributes to economical and intelligent operation of the unit.

Discussion on the Feasibility of Deep Peak Regulation for Ultra-Supercritical Circulating Fluidized Bed Boiler

In order to meet the flexibility operation needs of coal-fired units under the goal of carbon peak and carbon neutralization, it is imperative for circulating fluidized bed (CFB) units to participate in deep peak regulation. By systematically summarizing deep peak regulation operation practice of existing SC and subcritical-parameter-levels CFB units, the feasibility of deep peak regulation technology of an ultra-supercritical (USC) CFB unit under development and being built is analyzed and demonstrated; meanwhile, the deep peak regulation capacity of the boiler is also predicted. The results show that by analyzing the structural characteristics and design performance of the USC-CFB boiler, for technical problems such as stable combustion under low load, hydrodynamic safety, denitration performance under wide load, and rapid boiler load change rate existing in deep peak regulation, technical measures were implemented by selecting advanced boiler furnace type, adopting good design technology of the secondary rising water wall and uniformity design of bed temperature and bed pressure, strengthening the reducing atmosphere inside the furnace, improving the performance of wear-resistant refractory materials, quickly controlling the furnace bed material stock under variable load, optimizing the control strategy of CFB unit, and so on. The boiler achieved good operation characteristics and good deep peak regulation performance, and the pollutant emissions can steadily achieve ultra-low emission standards. When the USC-CFB unit participates in deep peak regulation, the minimum stable combustion load of the boiler can reach 20~30% BMCR, and a boiler load change rate under 30% BMCR or above could reach 1.5~2% BMCR/min, while that below 30% BMCR could reach 1% BMCR/min. The research results can provide references for the deep peak regulation of in-service supercritical (SC) CFB units and design optimization of similar USC-CFB units.

Dynamic model for subcritical circulating fluidized bed boiler-turbine units operated in a wide-load range

As the proportion of renewable energy generation increases in power grid, most of circulating fluidized bed (CFB) boiler-turbine units are required to operate in a wide-load range to maintain the stability of power grid. However, few studies focus on modeling of CFB units operated in a wide-load range, especially in ultra-low load. This study develops a dynamic model for subcritical CFB boiler-turbine units operated in a wide-load range, which can provide foundation for designing flexible operation mode. In this work, different characteristics of subcritical CFB units in ultra-low load were analyzed. Then, the model structure with balance between complexity and accuracy was derives from mass and energy conservation laws as well as thermodynamic principles. The parameters and functions in model were identified based on running data from a 330 MW subcritical CFB unit. Validation results show that the of model has satisfactory accuracy in wide-load range, from ultra-low load to full load. Besides, the model can reflect essential dynamic characteristics of the unit. Therefore, the model can be feasible for performance analysis and testing control strategy.

A dynamic nonlinear model used for controller design of a 600 MW supercritical circulating fluidized bed boiler-turbine unit

• Energy storage characteristic and variable coal quality are considered in modeling. • Parameters in model change with different load condition to improve model accuracy. • The model is validated by a 600 MW supercritical once-through CFB unit. • Open-loop step responses show the different inertia and nonlinearity of CCS. • Transfer function matrix of CCS is derived from the dynamic nonlinear model. Large-scale circulating fluidized bed (CFB) power generation units with high steam pressure and temperature have been put into operation in order to obtain high cycle efficiency and less emissions. To ensure the flexible operation of such CFB units, it is necessary to build a model for the design of coordinated control system (CCS). However, few studies focus on modeling of such CFB units. Thus, this work develops a mechanistic dynamic nonlinear model of a large-scale supercritical CFB unit, which is suitable for CCS design because of its relatively low complexity. Firstly, model structure is derived from mass and energy conservation laws, combined with analysis of operational characteristics of a 600 MW supercritical CFB unit. Then, unknown parameters and nonlinear functions in model are identified based on running data by using regression analysis and optimization algorithm. Validation results show that the model has satisfactory accuracy and it can capture essential dynamic characteristics of the unit and nonlinearity of CCS. More importantly, the model accuracy is further improved by proposed variable dynamic parameters and correction of coal quality. After this, transfer function matrix of CCS is derived from dynamic nonlinear model, which can be applied to controller design and simulation analysis.

Migration Behaviors of As, Se and Pb in Ultra-Low-Emission Coal-Fired Units and Effect of Co-Firing Sewage Sludge in CFB Boilers

The migration behaviors of As, Se and Pb in ultra-low-emission coal-fired units were investigated and the effect of co-firing of sewage sludge and coal was explored in circulating fluidized-bed (CFB) boiler units. Samples of feed fuel including coal and sewage sludge, fly ash, bottom slag and desulfurization slurry were collected from five CFB units with a capacity between 150 MW and 350 MW and two pulverized coal boiler (PC) units with a capacity of 350 MW and 600 MW. The majority of As, Se and Pb in coal is released during coal combustion and enriched in fly ash. As, Se and Pb in fly ash and bottom slag are associated with inorganic matter, of which As, Se and Pb associated with silicates and aluminosilicates account for more than 60%. In the wet flue gas desulfurization (WFGD) unit, more than half of the As, Se and Pb migrates to the solid fraction and the concentrations of As, Se and Pb in fine solid particles are much higher than those in coarse solid particles and the liquid fraction. The proportions of exchangeable As, Se and Pb or associated with organic matter in gypsum are higher than those of fly ash and bottom slag. According to the two studied CFB units with blending ratios of sewage sludge below 10%, co-firing with sewage sludge has little effect on the migration and transformation characteristics of As, Se and Pb. Under a long-term acidic leaching environment, Pb in combustion by-products is in the range of low risk, while As and Se are in the range of medium risk.

Study on economic operating mode for combined desulfurization system of circulating fluidized bed unit

With the stricter emission regulation taking effect, most of circulating fluidized bed (CFB) power generation units operates with conventional desulfurization in CFB furnace and flue gas desulfurization (FGD) technology to meet the requirements of ultralow SO2 emission. Therefore, it is urgent to analyse operating cost of the combined desulfurization system and explore the low-cost operating mode for CFB units. In this paper, the operating cost model of combined desulfurization system was established, in which the influence of desulfurization in furnace on thermal efficiency of the CFB boiler was considered. Then, the operating cost of combined desulfurization system under different load conditions for a 300MW subcritical CFB unit was analysed, combined with measured data. The operating cost optimization result shows that, under different loads, when Ca/S is in the range of 0.4 to 0.5, the operating cost of combined desulfurization system reaches the minimum. In addition, as the unit load increases, the higher proportion of SO2 removal in FGD is required to achieve low-cost operating mode for CFB units.

Physics-informed energy-balanced modeling and active disturbance rejection control for circulating fluidized bed units

Circulating fluidized bed (CFB) units, with many advantages such as low emission and wide fuel adaptability, have been extensively applied to the power and thermal supply. To analyze their dynamic characteristics and design a suitable control strategy, a physics-informed model is derived, which is suitable for the direct energy balance (DEB) structure. By dividing the unit into three modules, a 9th-order nonlinear dynamic model is obtained based on energy conservation and mass conservation principles. The model accuracy is verified by the field running data. The nonlinearity, coupling property and high-order dynamics of the coordinated control system (CCS) are analyzed, and then the advantages of the DEB structure for CFB units are derived theoretically. Finally, the control performance of the DEB structure with proportional–integral–derivative control and active disturbance rejection control (ADRC) is compared based on the field running data. This physics-informed model can offer a solid foundation for a control strategy design under the DEB structure. The advantages of ADRC are validated quantitatively, which shows a promising future in power units.

Performance evaluation of a supercritical circulating fluidized bed boiler for power generation under flexible operation

It is well known that the Łagisza power plant in Poland is the world’s first supercritical circulating fluidized bed (CFB) boiler, whose commercial operation started on June 2009. It has attracted a great deal of interest and operational data are publicly available, therefore it has been chosen as the object of the present study aimed at assessing load and fuel flexibility of supercritical CFB plants. First, the thermal cycle was modelled, by means of the commercial code Thermoflex®, at nominal and part load conditions for validation purposes. After having verified the validity of the applied modelling and simulation tool, the advantage of having supercritical steam combined with CFB boiler over subcritical steam and pulverized coal (PC) boiler, respectively, was quantified in terms of electric efficiency. As a next step, the designed fuel, i.e. locally mined hard coal, was replaced with biomass: 100% biomass firing was taken into account in the case of subcritical CFB boiler whereas the maximum share of biomass with coal was set at 50% with supercritical CFB boiler, consistently with the guidelines provided by the world leading manufacturers of CFB units. A broad range of biomass types was tested to conceive mixtures of fuel capable of preserving quite high performance, despite the energy consumption in pretreatment. However, the overall efficiency penalty, due to biomass co-firing, was found to potentially undermine the benefit of supercritical steam conditions compared to conventional subcritical power cycles. Indeed, the use of low-quality biomass in thermal power generation based on steam Rankine cycle may frustrate efforts to push the steam cycle boundaries.

What are the environmental advantages of circulating fluidized bed technology? ——A case study in China

In this paper, the environmental cost and environmental benefit evaluation system of circulating fluidized bed (CFB) technology is established by analyzing the environmental damage caused by coal-fired power generation and comparing circulating fluidized bed boilers with ordinary coal-fired boilers. Among them, the environmental cost is mainly the cost of desulfurization equipment and sellout equipment. Environmental benefits include internal environmental benefits and external environmental benefits. The internal environmental benefits mainly include the cost savings from desulfurization and denitrification to reduce the emission of polluting gases and the benefits from the comprehensive utilization of the ash from the circulating fluidized bed boiler. The external environmental benefits mainly include the reduction of occupied farmland, the emission of waste gas caused by spontaneous combustion of coal gangue and the pollution of water resources. After measuring the environmental costs and benefits of CFB technology, this paper further estimated its value. On this basis, particle swarm optimization (PSO) with extreme learning machine (ELM) intelligent algorithm was used to predict the scenario of thermal power generation and evaluate the environmental benefits of CFB units in the future through further calculation.

Construction of a regional inventory to characterize polycyclic aromatic hydrocarbon emissions from coal-fired power plants in Anhui, China from 2010 to 2030.

The infrastructures of coal-fired power plants in China have changed significantly since 2010, but the magnitude and characteristics of polycyclic aromatic hydrocarbon (PAH) emissions remain to be updated. In the present study, a unit-based PAH emission inventory for coal-fired power plants between 2010 and 2017 was constructed for Anhui Province, China. Atmospheric PAH emissions from pulverized coal (PC) and circulating fluidized bed (CFB) units in 2017 were 8600kg and 7800kg, respectively. The emission rates and intensities for CFB units (7.2kg ton-1 and 2.1kgMW-1) were significantly higher than those for PC units (1.1kg ton-1 and 0.19kgMW-1), primarily because CFB boilers were operated at lower combustion temperatures and poor combustion conditions compared to PC boilers. The distribution patterns of PAH emissions across different age groups largely reflected the time periods for constructing coal-fired units in Anhui and for the transition of small units to large ones. The accomplishment of ultralow emission technologies and phase-out of outdated coal-fired units were responsible for the decreasing trend of PAH emissions between 2012 and 2017. The warmer summer in 2013 and 2017 and colder winter in 2011 compared to other years probably caused increased use of air conditioners, resulting in increased electricity consumption and PAH emissions. Future PAH emissions would decrease by 45-57% during 2017-2030, benefitting from power plant fleet optimization, i.e., phasing out outdated coal-fired units and replacing them with large ones. With the best available optimized power plant fleets and end-of-pipe control measures accomplished in Anhui's CFPPs, PAH emissions in 2030 would potentially be reduced by 56-65%.

Modeling for the bed temperature 2D-interval prediction of CFB boilers based on long-short term memory network

Circulating fluidized bed (CFB) units play an important role in thermal power generation system in China. Because of advantages of wide fuel flexibility and low pollutant emissions, the proportion of CFB units is increasing constantly. For an accurate bed temperature changing trend prediction in advance, sequence prediction is needed, and accurate bed temperature change interval prediction is also required, a sequence-interval prediction indicates the 2D-interval prediction. This paper presents a bed temperature sequence interval prediction model for typical 300 MW CFB unit using long-short term memory network (LSTM) based on actual operation unit, and the coal feed rate, primary air rate and secondary air rate are selected as input variables using grey relational analysis. Previous bed temperature and automatic generation control instruction are introduced to the prediction models, and the length of input variables sequences are optimized using genetic algorithm. Several model patterns are compared and discussed, and the effect of introducing of automatic generation control instruction is investigated. The results reveal that the model structure could effectively described the characteristic of bed temperature of CFB unit and the model could achieve an accurate 2D-interval trend prediction of bed temperature.

Control-oriented modelling and investigation on quick load change control of subcritical circulating fluidized bed unit

The flexibility enhancement of circulating fluidized bed (CFB) power plant contributes to both power grid stability and renewable energy accommodation. The considerable energy storage makes it possible for CFB units to perform quick load change, while there are many CFB units cannot meet the basic requirement of load change speed from power grid. The study aims at realizing the quick load change control of subcritical CFB unit. With reasonable assumptions, a mechanism-based boiler-turbine system model of subcritical CFB unit is established to design control strategy. Through the analysis on energy storage utilization, a control strategy based on dynamic matrix control, which can overcome the large delay and inertia of CFB unit, is designed to realize the quick load change control. The simulation results show that the proposed control strategy can shorten the response time of the main steam pressure and accelerate the load change speed, while the traditional direct energy balance control strategy is unsuitable for quick load change control. Finally, a case study on the 300 MW subcritical CFB unit reveals that the load change speed can be raised to 3% of rated power output per minute and it almost takes no impact on operational safety.

Fuzzy logic and bed-to-wall heat transfer in a large-scale CFBC

PurposeThe purpose of this paper is to first present the key features of the fuzzy logic (FL) approach as a cost-effective technique in simulations of complex systems and then demonstrate the formulation and application of the method.Design/methodology/approachThe FL approach is used as an alternative method of data handling, considering the complexity of analytical and numerical procedures and high costs of empirical experiments. The distance from gas distributor, the temperature and the voidage of the bed, flue gas velocity and the load of the boiler are the input parameters, whereas the overall heat transfer coefficient for the membrane walls constitutes the output. Five overlapping sigmoid and constant linguistic terms are used to describe the input and the output data, respectively. The Takagi–Sugeno inference engine and the weighted average defuzzification methods are applied to determine the fuzzy and crisp output value, respectively.FindingsThe performed FL model allows predicting the bed-to-wall heat transfer coefficient in a large-scale 670 t/h circulating fluidized bed (CFB) boiler. The local heat transfer coefficients evaluated using the developed model are in very good agreement with the data obtained in complementary investigations.Originality/valueThe performed model constitutes an easy-to-use and functional tool. The new approach can be helpful for further research on the bed-to-wall heat transfer coefficient in the CFB units.

Investigation on energy storage and quick load change control of subcritical circulating fluidized bed boiler units

The energy storage of circulating fluidized bed (CFB) boilers on fuel side cannot be ignored due to the special combustion type different from pulverized coal boilers. The sizable energy storage makes it possible for CFB units to enhance the quick load change ability and to increase the scale of new energy power connected into grid. Through mechanism analysis, the model of energy storage of subcritical CFB boilers has been established for the first time. Then by the project practice, the quantitative analysis is demonstrated for the capacity and control characteristics of energy storage on fuel side and steam water side. Based on the control characteristics and the transformation of the energy storage, a coordinated control system (CCS) control strategy named advanced energy balance (AEB) is designed to shorten the response time through the use of energy storage and to accelerate the load change speed of subcritical CFB units. Finally, a case study on a 300MW CFB unit proves the feasibility of the proposed control strategy.

The theory and practice of NOx emission control for circulating fluidized bed boilers based on the re-specification of the fluidization state

The NOx emission of circulating fluidized bed (CFB) boilers in China is more and more stringently controlled due to the updated national emission regulation (NER). Based upon the theory of CFB combustion and previous research results related to NOx formation mechanism, a technical route was proposed by means of re-specifying the fluidization state in CFB boilers, to achieve the ultra-low NOx emission without installing the deNOx equipment additionally. The main method is to increase the solid circulating rate and decrease the bed inventory as well as the mean size of bed material, which would influence the bed temperature and oxidizing/reducing atmosphere distribution, and thereby benefit for the reduction of NOx emission. Besides, the reasonable control of bed temperature and air supply should be considered. This technique has been demonstrated to be effective in three CFB units with different capacities (150t·h−1, 260t·h−1 and 560t·h−1) and burning different types of coal. No obvious decrease of combustion efficiency was found in the field tests. The technical route provides a feasible approach to further control the NOx emission for CFB boilers.

The Non-Iterative Estimation of Bed-to-Wall Heat Transfer Coefficient in a CFBC by Fuzzy Logic Methods

The Fuzzy Logic (FL) approach is proposed in this paper to determine the overall heat transfer coefficient in the combustion chamber of a large-scale 670 t/h circulating fluidized bed (CFB) boiler. The FL model for the prediction of bed-to-wall heat transfer coefficient was successfully validated against data from the boiler. The model can be easily applied by scientists and engineers for simulations and optimizations of CFB units.

Simulation of mass balance behavior in a large-scale circulating fluidized bed reactor

We determine using a compound model the influence of the mass of granular matter on the behavior of a supercritical circulating fluidized bed (CFB) reactor. Population balance enables a stationary-regime modeling of the mass flow of granular matter inside a CFB unit in a large-scale. The simulation includes some important dynamic processes of gas-particle flows in fluidized bed such as attrition, fragmentation, elutriation, and fuel combustion. Numerical calculations with full boiler loading were performed of operational parameters such as furnace temperature, furnace pressure, feeding materials mass flows, and excess air ratio. Furthermore, three bed inventory masses were adopted as experimental variables in the simulation model of mass balance. This approach enables a sensitivity study of mass flows of granular matter inside a CFB facility. Some computational results from this population balance model obtained for a supercritical CFB reactor are presented that show consistency with the operational data for large-scale CFB units.

A generalized model of SO2 emissions from large- and small-scale CFB boilers by artificial neural network approach: Part 1. The mathematical model of SO2 emissions in air-firing, oxygen-enriched and oxycombustion CFB conditions

Since the complexity of sulfur capture and release during solid fuel combustion in circulating fluidized bed (CFB) boilers, especially in the oxycombustion conditions is still not sufficiently recognized, the development of a simple SO2 emission model for wide range of operating conditions is of practical significance.The paper introduces the artificial neural network (ANN) approach for the prediction of SO2 emissions from CFB boilers. The model considers a wide range of parameters influencing SO2 emissions. The [16-1-6-1] ANN model was successfully applied to predict SO2 emissions from coal combustion in several large- and small-scale CFB boilers, over a wide range of operating conditions, both in air-firing as well as oxygen-enriched and oxycombustion conditions.Since the method constitutes a quick and easy to run technique this approach makes a complementary tool in relation to the experimental procedures and the programmed computing approach. Therefore, the model can be easily applied by scientists and engineers for simulations and optimizations of CFB units.

Three-dimensional modeling of fuel flow with a holistic circulating fluidized bed furnace model

To study the mixing and flow of fuel inside commercial scale circulating fluidized bed (CFB) furnaces, a new model was introduced to solve the flow of fuel in an existing semi-empirical, steady state, three-dimensional Eulerian model frame. The fuel flow model is a simplified momentum equation which is based on the momentum balance considering the fuel inertia, gravity, and drag force from gaseous and solid phase. The model improves the prediction capabilities of fuel flow patterns in the existing model frame for conventional and especially renewable fuels, such as biomass. The fuel flow model improves the analysis of CFBs and can be utilized in the design and development of commercial CFB units.