A MODEL DEVELOPMENT ON USC-CFB BOILER SYSTEM FOR DYNAMIC SIMULATION OF COAL-FIRED POWER PLANT

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00140 A MODEL DEVELOPMENT ON USC-CFB BOILER SYSTEM FOR DYNAMIC SIMULATION OF COAL-FIRED POWER PLANT Hookyung Lee 1, Hyojun Kim 1, Kwanghun Jeong 1, Dirk Nitschke 2, Kihyun Lee 3* 1 Plant Dynamics Research Group, Corporate R&D Institute, Doosan Heavy Industries & Construction, Suji, Yongin, Korea 2 Europe R&D Center (Lentjes), Doosan Power Systems, Ratingen, Germany 3 System Engineering Team, Corporate R&D Institute, Doosan Heavy Industries & Construction, Suji, Yongin, Korea Presenting Author: hookyung.lee@doosan.com * Corresponding Author: kihyun.lee@doosan.com ABSTRACT Over the last decade there have been a growing number of environmental regulations aimed at controlling air emissions at coal-fired power plants, which have been developed towards higher thermal efficiency. CFB boiler is one of the alternatives for power generation in large scale. DHI has decided to develop an USC steam generator based on CFB combustion technology to be applied for a power plant of a capacity of commercial level. In the different design-works for plant demonstration, this paper reports dynamic simulation and analysis results for thermal process design of the USC-CFB boiler and the subsequent overall plant system. The system configuration was determined on its functional construction. According to mass and heat balance, the sizes of heating surface were defined with their function (economizer, evaporator, superheater, and reheater) and their order in the water/steam circuit to get target steam condition. Finally, specific geometry of the units was presented through a series of development stages. In order to consider physical behavior of air/gas flow and solid particles, hydrodynamics of solid particles and gas flow in the boiler, chemical reaction such as coal combustion, calcination, and desulfurization, and subsequent heat transfer are balanced as time progresses. The CFB boiler module is connected on the computational domain of ASIMPLE which is DHI s own dynamic plant simulation tool. In compared with design and operation data, dynamic characteristics gave us useful information predicting start-up and transient response by abrupt load changes in the system. KEYWORDS: CFB, Steam generator, Ultra supercritical, Power generation, Dynamic behavior 1. INTRODUCTION Over the last decade there have been a growing number of environmental regulations aimed at controlling air emissions at coal-fired power plants, which have been developed towards higher thermal efficiency. For steam generation in large-scale power generation, CFB (circulating fluidized bed) combustion technology is a promising method with effective NOx emission control and high-sulfur capture efficiency being one of its more attractive features when using low-grade coals.[1] For higher efficiency of the plant, higher steam temperature and pressure conditions are required based on thermodynamic laws. Ultimately, beyond supercritical state of steam introduced into a turbine. DHI (Doosan heavy industries and construction) has decided to develop an USC (ultra-supercritical) steam generator based on CFB combustion technology to be applied for a power plant of a capacity of commercial level. The boiler system is composed of largely furnace, cyclone, seal-pot, and external heat exchangers. For steam generation of supercritical or USC condition, water fed from the pump directly pass through the metal tube experiencing phase change to steam. The magnitude, arrangement, and locations of heating surfaces, such as evaporator, superheater, reheater, and economizer, in the boiler influence its thermal efficiency and output.[2] 1

2 Up to the present time, considerable work has been done on modeling of CFB reactors.[3] Most of the studies deal with the steady-state performance of CFB boilers and very few studies have reported the modeling of their dynamic responses. However, it is important to establish unsteady or dynamic models capable of being used for CFB boiler control and process.[4] In operating the power plant, the system is not on a steady-state always. The power system experiences initial cold/warm start-up, abrupt or planned load changes, abnormal operation by malfunctions and upset conditions, and shut-down. Therefore, understanding a transient behavior based on plant dynamic analysis is indispensable. Finally, it leads time shortening till commercial operation during commissioning, and it brings sufficient economic benefit in electricity sales. As one of a series of works to establish an USC-CFB reference boiler model, this paper reports dynamic simulation and analysis results for thermal process design of the USC-CFB boiler and the subsequent overall plant system. 2. METHOD 2.1 THERMAL PROCESS DESIGN AND MODEL DESCRIPTION The thermal power system including a coal-fired CFB boiler has largely two flow-paths, which are generally called air/gas side and water/steam side.[5] The air/gas side means the flow way of the flue gas produced by combustion from introduction of fuel and oxidant. As a working fluid, the water/steam side get the heat from the air/gas side at the heating surfaces, which are composed of a group of metal tubes, to attain a target steam condition finally. The sizes of heating surface are determined based on heat and mass balance. The design parameters such as furnace temperature and flue gas temperature are calculated, and then the values are used as a basis for the thermal process design of the boiler. The significant difference between the CFB boilers and the PC (pulverized coal) boilers is that the solids suspension concentration (including coal ash, sand, limestone) in CFB boilers is much higher than that in PC boilers. Figure 1 shows a schematic diagram of the typical CFB boiler. Because solid particles with high heat capacity relatively are circulated in the furnace, the applications of knowledge of heat transfer for the design of a boiler surface and load control are essential. In the boiler loop, recirculation of solid particles should be balanced with fuel inflow, conversion into flue gas, and ash removal (drain). The process of heat transfer in a CFB boiler involves four mechanisms: gas convection, radiation, solid particle convection, and radiation. There are heat transfer processes at water/steam side between the water/steam and the tube inner wall, at tube material, and at heating surface between the air/gas and the tube outer wall. In this study, all the models are based on conservation equations, principal physical laws, and physicochemical process equations associated with phenomenological phenomena in the CFB boiler. The chemical reaction and the heat transfer to furnace walls and tube banks are presented in detail.[6-10] Fig. 1 A schematic of a CFB boiler. Solid particles are circulated in the closed loop where is composed of furnace, cyclone (solid-gas separator), loop-seal, and other sub-units. 2

3 In this study, heat transfer in the CFB loop was defined with the cluster renewal model developed by Dutta and Basu.[10] Since the contact area between the solid particles and the wall surfaces is small, the direct heat transfer such as conduction through the point of contact is negligible. Therefore, the fractional wall coverage directly influences the heat transfer coefficient. The hydrodynamic models are used to calculate the concentration profile of gas and solid in the high concentration region (lower furnace), suspension region (upper furnace), cyclone, stand pipe, loopseal, and external fluidized bed heat exchangers. More information associated with model description is presented in Refs.[2,6] 2.2 INTEGRATION INTO A POWER PLANT SYSTEM The CFB boiler module, which was independently developed and validated with operating data in the commercial sub-critical CFB boiler, is connected on the computational domain of ASIMPLE (analysis and simulation for plant engineering) which is DHI s own dynamic plant simulation tool. The own program was specifically explained in the literature.[11,12] 3. RESULTS AND DISCUSSION For efficient operation at partial load conditions, it is essential to understand the property changes at dynamic situation. In this paper, some results associated with step change of fuel flow rate are presented. Figure 2 indicates gas-solid temperatures in the furnace, cyclone, and loopseal when the load decreases from 100% BMCR (boiler maximum continuous rating) condition to 75% MGR. Assuming that the coal feeding rate suddenly decreases to a certain value, the dynamic simulation shows transient behavior of gas-solid temperature. The bed temperature decreases due to lower combustion heat generation. It is known that the combustion temperature of a CFB boiler is considerably lower than that of a conventional PC boiler. To reach the steady-state downward, time delay is calculated and we could account for the phenomena with a concept such as time constant.[5] Figure 3 presents temperatures in the water/steam side at superheater and economizer, respectively. In decreasing the flue gas temperature by load down, they show reasonable transient behaviors associated with temperature change (Left). In the situation compared with instant decrease in air/gas temperature, continuous decrease of gas temperature for 5 min and 10 min linearly has influence on subsequent different time delay of water/steam temperatures at the economizer outlet (Right). Figure 4 shows the normalized values of the power generation capacity (MWe) during load changes from 100% BMCR condition to 56% MGR via 74% MGR. As the flow rates of fuel, oxidant, and feed water decrease, overall power generation capacity also decreases. Fig. 2 Transient variation of solid-gas temperatures according to load changes. 3

4 Fig. 3 Transient variation of water/steam temperatures according to load changes. Power Generation Load [%] Step load change (assumed situation) General load change Time [sec] Fig. 4 As the flow rate of fuel-coal decreases for target load changes, the power generation capacity also shows transient behavior. 4. CONCLUSIONS To understand the transient behavior of thermal process in the coal-fired CFB boiler, dynamic simulation was conducted based on governing conservation equations and physicochemical models. In the boiler system, air/gas side and water/steam side, which are coupled with hydrodynamics of gas-solid flow and consequent heat transfer mechanism, showed the reasonable behaviors according to load changes. In the overall plant system, they lead dynamic behavior in power generation capacity toward target condition. However, the model still requires to be validated by data from commercial CFB boilers. Finally, with model validation in the practical plant, we expect the mathematical model may contribute to control system tuning as well as process design of the USC-CFB boiler and overall power plant system. As a future work, the property curves drawn during start-up, shut-down, and abnormal operation by malfunction will be derived. 4

5 ACKNOWLEDGMENT The authors gratefully acknowledge support from the Doosan Heavy Industries & Construction. We especially thanks to Doosan Lentjes and Boiler R&D Center for discussion of the results. REFERENCE [1] Z. Man, B. Rushan, Y. Zezhong, J. Xiaoguo, Heat flux profile of the furnace wall of a 300 MWe CFB boiler, Powder Technol. 203 (2010) [2] P. Basu, Circulating Fluidized Bed Boilers., Springer International Publishing Switzerland, 2015: pp [3] J. Grace, A. Avidan, T. Knowlton, Circulating Fluidized Beds., Chapman & Hall Press, [4] Y. Chen, G. Xiaolong, Dynamic modeling and simulation of a 410 t/h pyroflow CFB boiler, Comput. Chem. Eng. 31 (2006) [5] H. Lee, H. Kim, K. Jeong, K. Roh, W. Jeon, W. Kim, H. Chi, K. Lee, Dynamic simulation and analysis of coal-fired thermal power plant., in: Proc. KSME spring division, 2016: pp [6] P. Basu, Combustion and gasification in fluidized beds, [7] B. Andersson, Effects of bed particle size on heat transfer in circulating fluidized bed boilers, Powder Technol. 87 (1996) [8] H. Lee, S. Choi, An observation of combustion behavior of a single coal particle entrained into hot gas flow, Combust. Flame 162 (2015) 2610~2620. [9] H. Lee, S. Choi, Motion of single pulverized coal particles in a hot gas flow field, Combust. Flame 169 (2016) 63~71. [10] A. Dutta, P. Basu, An improved cluster-renewal model for the estimation of heat transfer coefficients on the furnace walls of commercial circulating fluidized bed boilers, Trans. ASME 126 (2004) [11] H. Kim, K. Jeong, K. Roh, W. Jeon, Y. Kim, K. Lee, Dynamic simulation of coal-fired supercritical power plant with ASIMPLE based on mathematical models., in: Proc. Global Energy Technology Summit, [12] K. Jeong, H. Kim, K. Roh, W. Jeon, Y. Kim, K. Lee, Dynamic simulation of thermal power plant and preliminary controller tuning with ASIMPLE., in: Proc. PowerGen ASIA,