Thermal Design And Off-design Performance Research On Heat Transfer System Of A Supercritical CO₂ Power Cycle

Allam-Z cycle is a simplified direct-fired supercritical CO₂ power cycle,using the mixture of the circulation CO₂ and combustion products as the working medium,with high cycle efficiency,simple heat recovery system,compact structure and the ability to capture high quality CO₂ by using the"cold energy"of fuel and liquid oxygen.A preliminary thermal design of the heat exchanger system of the supercritical CO₂ power cycle aiming at high efficiency power generation and CO₂ capture was carried out in this paper;and the effects of the structure parameters of heat exchangers on the cycle efficiency of Allam-Z cycle under100MW condition was analyzed.The variation in parameters of the heat exchangers and the variation in performance of the circulation system were analyzed under off-design operation conditions.Firstly,a thermodynamic simulation model suitable for the Allam-Z cycle is established in this paper by using thermodynamics,heat transfer,multi-components and multi-phase fluid dynamics and considering the thermodynamic characteristics and mechanism of each thermodynamic unit in the cycle.The problems existing in the practical use of the Allam-Z cycle system,such as the heat load of some heat exchangers（such as HX1）is large,and the design and manufacture of the multi-flow heat exchangers are difficult when NG（natural gas）and O₂ exchange heat with hot fluid at the same time.The heat transfer system was modified by dividing the multiflow heat exchanger into parallel heat exchangers,and the single heat exchangers with large temperature difference into series heat exchangers.The specific modifications to the design are that the HX1 is of three heat exchangers in series;the HX5 is of two parallel groups and two series heat exchangers for the cryogenic fluids;and the HX4 is also of two parallel groups and directly cooling the CO₂ from the HX3 by cryogenic fluids instead of the indirect cooling via the cooling water.Secondly,the helical baffle heat exchanger with high heat transfer coefficient and low pressure drop resistance was applied to the Allam-Z cycle system.Taking large heat exchanger HX1-1 and small heat exchanger HX5a-1 as examples,the heat transfer performance and structural parameter variation trends of the helical baffle heat exchangers are analyzed by changing the inner diameter of the shell as well as the outer diameter of the tube and center distance between the adjacent tubes.The schemes includes four different sized shell inner diameters D of heat exchangers HX1-1 and HX5a-1 at constant outer tube diameters d_o and center distances of adjacent tubes S,six schemes with combination of three different outer diameters d_o and two different center distances of adjacent tubes S at constant inner diameter of the shell D.The flow and heat transfer performances of the heat exchangers were analyzed by using the commercial heat exchanger design and check software HTRI with shell and tube side pressure dropsΔp_o andΔp_i,total heat transfer coefficient U,shell and tube side heat transfer coefficients h_o and h_i shell and tube side comprehensive indexes h_o/Δp_o^1/3 and h_i/Δp_i^1/3,tube length L,heat transfer area A and shell side flow rate v_o as parameters and indicators.The quantitative value of the influence of turbine back pressure on cycle efficiency determined by total shell pressure drop of heat exchangers is presented;and the thermal design of all the heat exchangers for the Allam-Z cycle heat transfer system is completed,which provides references to the thermal design and optimization of Allam-Z cycle heat transfer system.Thirdly,on the basis of the optimized design of the heat exchange system of the Allam-Z cycle,the change trends of the parameters of the heat exchangers and the overall cycle performances of the Allam-Z system were analyzed under six variable working conditions with turbine load between 50 MW and 100 MW with 10 MW as the step.With the outlet pressure of CO₂ in condenser HX4 is 7.21MPa as the target,the initial turbine pressure is iteratively calculated with Flugel formula and the turbine enthalpy drop is obtained with the constant changing of the turbine backpressure.The flow and heat transfer characteristics of the heat exchangers of the cycle system at the off-design working conditions from turbine exhaust to CO₂ condensation were iteratively calculated.The results show that the turbine backpressure is 7.52MPa～8MPa under off-design working conditions when the turbine load is 50MW～100MW.The variation trends of the shell and tube side mass flow rates G_o and G_i,temperature rise or drop of the shell/tube flows,logarithmic mean temperature difference,transferring heat Q,total heat transfer coefficient U,shell/tube side heat transfer coefficients,shell/tube side pressure drops,the enthalpy drop of turbine,and power generation output efficiency,net equivalent efficiency and other performance indexes were obtained.The results show that the logarithmic average temperature difference and the total heat transfer coefficient of each heat exchanger are decreasing;the turbine inlet pressure and turbine enthalpy drop,and the output efficiency and equivalent net efficiency of power generation are also decreasing with the decrease of turbine load.At 50%load,the output efficiency and equivalent net efficiency of power generation are reduced respectively to 78.8%and 77.2%of the value at the full load;and the total heat transfer coefficients of the involved heat exchangers are reduced to 80.6%～84.7%of the value at the full load.