DYNAMIC MODELING OF THE ISAB ENERGY IGCC COMPLEX

Transcription

1 DYNAMIC MODELING OF THE ISAB ENERGY IGCC COMPLEX F. Pisacane R. Domenichini L. Fadabini Foster Wheeler Italiana S.p.A. Via Caboto CORSICO, Italy Abstract During the execution of detailed engineering of ISAB Energy Project, a Dynamic Simulation Study was performed. It was aimed to check the design and to optimize the control and the operability of the IGCC Complex. Dynamic Simulation was conducted to investigate the integration between the Gasification section and the CCU section which is essential for the correct and safe operation of the Plant. The IGCC model was exercised to predict the transient behaviour of the IGCC Complex subsequent to planned (i.e. loading/unloading) or unplanned (i.e. Gasifier trip, Gas Turbine trip, Steam Turbine trip etc.) disturbances of the steady state operation. The paper describes the steps followed in the dynamic modeling and some significant results obtained from the simulations. Introduction The IGCC Complex of ISAB Energy Project is presently in an advanced phase of erection at Priolo Gargallo (Sicily). The Dynamic Simulation Study executed as a part of detailed engineering, demonstrated to be a calculation tool necessary to check and finalize the design and the control strategy of the overall plant. Purpose of the IGCC Dynamic Simulation Study is to analyze the behaviour of the complex subsequent to a planned or unplanned disturbance of the steady-state operation. Check of mechanical design: the equipment dimensions defined on the basis of one or more operating and design cases, shall be suitable to withstand the transients which might prove more critical than the steady state operations. Check of control valve size and characteristics.

2 Definition and check of the IGCC control system; this includes definition of an ad hoc control philosophy to solve particular problems such as the control of interfaces between the CCU and Gasification Sections, and to ensure that no undesirable or unsafe conditions are expected during transients. Selection of safe operating procedures such as rate of load changes. Estimate of controller parameters (i.e. set point values, proportional, derivative and integral time constants), allowing a shorter tuning on field. This predictive parametrization is particularly useful for the control loops and logics that could hardly be tuned on the operating plant (i.e. controllers dedicated to island operation). The Dynamic Simulation Study consists in building a dynamic model describing the sections of the plant which are dynamically significant. The disturbance is imposed and the model is used to predict the caused transient behaviour of plant variables such as temperature, pressure, flow. The complete plant response is evaluated for its acceptability. Based on plant operating philosophy and operating experiences accumulated in similar units, planned events (i.e. IGCC loading/unloading) and unplanned events (i.e. trip of Gas Turbine, Steam Turbine, Gasifier etc.) have been identified in order to evaluate the IGCC dynamic response. This paper describes the sequential phases of the Dynamic Simulation Study, i.e. the model preparation, the execution and evaluation of the simulations, and the implementation of some modifications to the control logics arisen by the transients analysis. Process and Control Description The ISAB Energy IGCC Plant is designed to process heavy oil residues (i.e. Asphalt, Visbroken Vacuum Residue, Fuel Oil, etc) coming from the adjacent refinery. Figure 1 is a schematic of the IGCC Complex, showing the interfaces among the different Units. The Plant which has a design capacity of 560 MWe gross power output, is composed mainly of the following sections: Gasification: two Texaco Partial Oxidation Reactors using steam as moderator and oxygen as oxidant, of direct water quench type, each followed by one Scrubber, to remove the soot and ash from syngas. Carbon Recovery and Recycle to recover soot from soot water and recycle it to the Gasifiers.

3 Syngas Heat Recovery section where raw gas from Gasification is cooled by generating steam and hot water, with separation and condensation of most water vapour. The catalytic hydrolysis of COS to H 2 S is also achieved in this section. Acid Gas Removal where raw gas is scrubbed by means of formulated MDEA in order to selectively remove H2S, minimizing CO2 co-absorption. Purified gas is reheated, expanded by producing additional electric power, and humidified with water heated in the above mentioned Heat Recovery Section. Finally syngas enters the Combined Cycle Unit composed of two identical trains consisting of the Gas Turbine, the heat recovery steam generator with post-combustion, the Steam Turbine. In other words the syngas train is divided by the Expander into two main sections: High pressure gasification section, where raw syngas is produced by the Gasifiers, cooled by producing steam and hot water, and H 2 S washed, finally entering the Expander. Low Pressure gasification section where clean syngas coming out from the Expander is saturated with water and delivered to the CCU. In addition to these main sections, the IGCC Complex includes the Metals Recovery Section, the Sulphur Recovery and Tail Gas Treatment Section and all the Utility Systems required for the operation of the Plant.

4 Acid Gas HP Steam HP Oxygen Wet Syngas Asphalt Gasification and Carbon Recovery & Recycle Syngas Heat Recovery Acid Gas Removal Syngas Expansion & Saturation Dry Syngas Combined Cycle Unit Electric Power Circulating Water BFW Heavy Metal Recovery MP Steam Filter Cake LP Steam LP Steam MP Steam VLP Steam Sulphur Recovery LP Oxygen Utilities and Distribution Sulphur Figure 1 IGCC Block Flow Diagram The IGCC Complex control system is aimed to manage the electric power production of the five power generators (two Gas Turbines, two Steam Turbines and one Expander) connected to the national electric distribution grid. The IGCC Complex operates either in the power control mode or in the feed control mode. In the power control mode the amount of power produced by the complex is a set point defined by the management of the power distribution grid. In the feed control mode the amount of power produced by the complex is limited by the amount of available feed up to the maximum throughput capability of the complex. In essence the feed control mode is a specific case of the power control mode where the specified power output is the maximum possible. Then, during normal operation a fixed amount of electric power shall be produced or a fixed quantity of asphalt shall be destroyed. Variations in both requirements promote an unbalance between the syngas production and the syngas consumption. Due to the Gas Turbine requirement the pressure in the low pressure gasification section has to be maintained as much as possible constant. This is made by acting on the Expander admission valves and/or bypass valves. As a consequence the unbalance causes a fluctuation of the HP Gasification Section pressure, utilizing as a buffer for capacity adjustment, the large gas inventory existing in the system operating at high pressure.

5 The control philosophy is based on these main concepts: the Gasifiers operate under flow control; the CCU operates under power control; both in power production and in feedstock consumption control modes. This is obtained by suitably resetting the undirect controller set point on the basis of HP Gasification section pressure error. I.e.: - in case of power production control mode, the power control of the CCU is directly defined by the set point selected by the operator, while the feedstock flowrate set point is reset by the HP Gasification section pressure control. - in case of feedstock consumption control mode, the Gasifier load is directly defined by the set point selected by the operator, while the production set point is reset by the HP Gasification Section pressure control. In both cases the error of HP Gasification Section is used to match the balance between the syngas production and the syngas consumption by re-establishing the pressure itself. In addition, both in power production and feedstock consumption control modes, a feedforward control is active, resetting respectively the feedstock flowrate or the power production set point, based on the calculated required clean syngas or feedstock thermal power. A schematic of the IGCC Control System is shown in Figure no. 2. Feedstock Consumption OR Power Production SET POINT Gasification Master Controller Feedstock Consumption IGCC Master Controller Power Production Combined Cycle Master Controller Gasification Syngas Train PC Syngas Train PC CCU HP Section LP Section Expander Figure 2 IGCC Normal Operating Control Modes

6 Dynamic Model Preparation The dynamic model of the ISAB Energy IGCC Plant describes the whole process involving syngas, starting from its generation in the Gasifiers, through cooling, H 2 S washing, expansion, humidification and combustion in the CCU. Three main subsystems are identified: the Gasifiers, the syngas train section, the CCU. The subsystems are connected from the operating point of view through the modeling of the main controllers (IGCC Master Controller, CCU Master Controller, Gasification Master Controller etc.). The other ancillary sections (i.e. Metal Recovery, Sulphur Recovery and Utility Systems) are not simulated as the associated dynamics are not significant. The model for each section describes the main components, the piping and the associated control system; it integrates all the information necessary to evaluate the mass, thermal, hydraulic balances, predicting dynamically stream flows, temperatures and pressures during the transient. Data Gathering The following plant and equipment data have been assembled to build the dynamic model: a) Process flow diagrams of the plant b) Equipment physical data. This includes volumes, surfaces, dimensions, geometric arrangements and design characteristics of mechanical equipment in order to simulate offdesign component behaviour for gasification and combined-cycle components and associated valves and piping. c) Operating point data. Heat and mass balance for base-load operating condition. This includes all stream information (mass flow rates, pressures, temperatures, enthalpies, and compositions). d) Controls and logic drawings for the equipment and plant. Control valves and controllers data. e) Plant operating philosophy. Model Preparation The model have been built using a commercial dynamic simulation software of modular type. Some additional modules have been customized to describe adequately the ISAB Energy IGCC Plant. The following steps have been followed:

7 a) First, a model schematic is generated. This involves laying out the process which defines the scope of the model. b) A diagram is then created which depicts the selected software modules and their connections used to simulate the process. c) Most components can be simulated using modules from the standard software library. New modules for unique components are developed, as necessary. d) The next step is to superimpose a process heat balance with enough information to define the pressure, flowrate, enthalpy, and composition of each stream at operating point condition. e) Drawings of control strategies are developed from the operating procedures and plant controls and logic drawings. f) With the plant scope defined, all modules selected, all data gathered, a dynamic model of the plant is configured. The model includes all the main components (e.g. Gasifiers Scrubbers, exchangers, drums, absorber, Expander, saturator, combustors, gas and Steam Turbines, heat recovery steam generators, valves and all associated piping) in the plant as a series of resistance and volume modules connected together in a thermal/hydraulic network. g) Once the model is created and all appropriate variables initialized, a quick next step is to test the model at steady-state conditions to choose if the model variables match the heat balance at the operating condition, both in design condition and in offdesign condition. h) The next step is to dynamically test the model. Test disturbances are introduced into the model and the system's response in terms of flows, pressures and temperatures observed. The system should pass from the original steady-state condition to a different final steadystate condition through a transient which can be properly discussed. Evaluation of Plant Transients The selection of the planned, unplanned and upset events to be dynamically evaluated, is aimed to check all the normal and the emergency operations of the IGCC Complex. Planned events are the IGCC load variations. The simulation study is aimed to define the faster load ramp accepted by the equipment, minimizing the impact on their life. The expected upsets of the IGCC operation are: trip of HRSG postcombustion system; trip of one Steam Turbine;

8 Gas Turbine load rejection; trip of one Gas Turbine; trip of one Gasifier; trip of two Gasifiers; sudden disconnection of the CCU from the 380 kv national network; CCU island operation feeding only the CCU auxiliaries; trip of Expander; failure of saturator operation. For these emergency situations, the Dynamic Simulations are aimed to study their effects on the whole Plant operation, by checking the acceptability of the following process variable variations during the transients: LP Gasification Section pressure absolute value and gradient; HP Gasification Section pressure absolute value; Wet syngas water content absolute value and gradient; Steam/BFW interfaces (pressure, temperature and flowrate) between CCU and Gasification sections paying particular attention to HP steam pressure. Infact the HP export to the Gasifiers is essential for the IGCC operation (i.e. loss of HP steam moderator to Gasifier causes the IGCC shutdown). Once the planned and unplanned plant events have been selected, the model was exercised for each of the transients. Complete plant responses (stream flows, temperatures and pressures) to these events have been predicted and graphically presented. These responses have been evaluated for their reasonableness and acceptability, with the conclusions and considerations herebelow presented. Mechanical Design. The IGCC mechanical design (i.e. geometric dimensions, design temperature and pressure of equipment) checked during steady state and transient operations, demonstrates to be adequate. IGCC Control System. The control philosophy defined for the management of the integrated operation of the Gasification and CCU sections and translated in the logics of the Master Controllers (i.e. IGCC Master Controller, CCU Master Controller, Gasification Master Controller), has been validated through the Dynamic Simulation Study. The planned loading/unloading procedures and the relevant selected rates (2%/minute) are safe and reliable: no significant stress to either machines or equipment is expected. The logics dedicated to withstand the emergency situations (i.e. trip of Gas Turbine, HRSG postfiring, Steam Turbine, Gasifier, Expander, etc.) have been subject to some modifications on the basis of the Dynamic Simulation results.

9 Control Valves. All the main control valve sizes and characteristics have demonstrated to be adequate. Results All the transients subsequent to the planned and unplanned events above described have been studied and the significant plant parameters plotted to review their behaviours. Typical parametric plots for three transient conditions are presented in the following sections. IGCC Unloading/Loading In these simulations the IGCC operates in power production control mode. In the Simulation n 1 the power production set point is decreased from 100% to 50% through a ramp of 2%/minute load variation. In the Simulation n 2 the power production set point is increased from 50% to 100% through the same ramp. Figures 3 and 4 (Simulation n 1), 5 and 6 (Simulation n 2) show the power production set point, the total power output, the Gas Turbine and Steam Turbine power output and the postfiring load during the transients.

10 Figure 3 Simulation no. 1 Figure 4 Simulation no. 1

11 Figure 5 Simulation no. 2 Figure 6 Simulation no. 2

12 The plots demonstrate, as expected, the quick response of the Gas Turbines, and the large inertia of the steam cycle. This is particularly evident when the Gas Turbines are at maximum load and the syngas production variation modifies the HRSG postfiring load, influencing directly the Steam Turbine power production. In that case a variation of power production set point with the rate of 2%/minute determines a variation of power output with a rate of 1.6%/minute. Figure 7 (Simulation n 1) presents the HP steam drum pressure and the HP steam production: during unloading the pressure is sliding in the range bar in order to optimize the heat recovery by ensuring at the same time the HP steam export to the Gasification. Figure 7 Simulation no. 1

13 Trip of One Gas Turbine In Simulation n 3, the trip of one Gas Turbine occurs when the IGCC Plant is operating at its design capacity. As a consequence of the Gas Turbine trip, the entire train is shutdown. The reduction of the Gasifiers load with the maximum ramp (i.e. 4%/minute) is automatically activated by the control system. The excess syngas produced in the transient has to be discharged to flare. Suitable pressure control valves (PCV) are located in different sections of the syngas train, i.e. at Scrubbers outlet (raw syngas), at Saturator inlet (dry clean syngas), at Gas Turbine inlet (wet clean syngas). Originally the PCV s set points were selected in order to discharge preferentially dry syngas to flare. On the contrary, the dynamic simulations demonstrate that it is convenient to discharge the syngas through the dedicated PCVs, either at the Scrubbers outlet, or at the Gas Turbines inlet, instead of at the Saturator inlet. Discharge at the extremes of the Syngas Train allows to maintain a balanced flow of syngas through the different sections, i.e. cooling and saturation, ensuring an as much as possible constant Low Heating Value of wet syngas fed to Gas Turbines. Figure 8 shows the pressure of wet syngas at Gas Turbine inlet which has a first increase and then is well controlled at a value higher than the original steady state due to the reduced line pressure drop. Figures 9, 10 and 11 present the opening of the three PCVs discharging to flare. In Figure 12 the wet syngas water content is depicted: the decrease in the transient is due to the decrease of recirculating water temperature consequent to the syngas train unloading, even if a feedforward reset of the LP steam generator pressure is activated in order to reduce the heat absorbed by steam generation and ensure the heating of recirculating water. Anyway, the gradient is acceptable by the Gas Turbine. Finally Figure 13 shows the trend of pressure of the HP steam from CCU to Gasification. After a first sudden decrease of HP steam export pressure which is within the possible fluctuation limits, the nominal pressure is quickly restored. Acknowledgments The authors are grateful to Dr. Robert Giglio, who contributed to the computer programming at Foster Wheeler Development Corporation. References 1. Maderni L., Icardi G. and Fontana M., Control System for a Combined Cycle, ASME International Gas Turbine & Aeroengine Congress & Exposition, Toronto - Canada. (1989). [conference paper] 2. Ahluwalia K.S. and Domenichini R., Dynamic Modeling of a Combined Cycle Plant, ASME International Gas Turbine & Aeroengine Congress & Exposition, Toronto - Canada (1989). [conference paper] 3. Giglio R., Cerabolini M. and Pisacane F., The Dynamic Simulation of the Progetto Energia Combined Cycle Power Plants, International Joint Power Generation Conference, Houston - Texas (1996). [conference paper]

14 4. Domenichini R., Dynamic Simulation Study: an Engineering Tool to Optimize ISAB Energy IGCC Plant Design, Control and Operability, Gasification Technology in Practice, Milan - Italy (1997). [conference paper]

15 Figure 8 Simulation no. 3

16 Figure 9 Simulation no. 3 Figure 10 Simulation no. 3

17 Figure 11 Simulation no. 3

18 Figure 12 Simulation no. 3 Figure 13 Simulation no. 3