DYNAMIC MODELING OF THE ISAB ENERGY IGCC COMPLEX

F. PisacaneR. Domenichini

L. FadabiniFoster Wheeler Italiana S.p.A.

Via Caboto 120094 CORSICO, Italy

Abstract

During the execution of detailed engineering of ISAB Energy Project, a Dynamic SimulationStudy was performed. It was aimed to check the design and to optimize the control and theoperability of the IGCC Complex. Dynamic Simulation was conducted to investigate theintegration between the Gasification section and the CCU section which is essential for the correctand safe operation of the Plant. The IGCC model was exercised to predict the transient behaviourof the IGCC Complex subsequent to planned (i.e. loading/unloading) or unplanned (i.e. Gasifiertrip, 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 resultsobtained from the simulations.

Introduction

The IGCC Complex of ISAB Energy Project is presently in an advanced phase of erection atPriolo Gargallo (Sicily).The Dynamic Simulation Study executed as a part of detailed engineering, demonstrated to be acalculation tool necessary to check and finalize the design and the control strategy of the overallplant.

Purpose of the IGCC Dynamic Simulation Study is to analyze the behaviour of the complexsubsequent 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 moreoperating and design cases, shall be suitable to withstand the transients which might prove morecritical than the steady state operations.

• Check of control valve size and characteristics.

• Definition and check of the IGCC control system; this includes definition of an ad hoc controlphilosophy to solve particular problems such as the control of interfaces between the CCU andGasification Sections, and to ensure that no undesirable or unsafe conditions are expectedduring transients.

• Selection of safe operating procedures such as rate of load changes.

• Estimate of controller parameters (i.e. set point values, proportional, derivative and integraltime constants), allowing a shorter tuning on field. This predictive parametrization isparticularly useful for the control loops and logics that could hardly be tuned on the operatingplant (i.e. controllers dedicated to island operation).

The Dynamic Simulation Study consists in building a dynamic model describing the sections of theplant which are dynamically significant. The disturbance is imposed and the model is used topredict 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 dynamicresponse.

This paper describes the sequential phases of the Dynamic Simulation Study, i.e. the modelpreparation , the execution and evaluation of the simulations, and the implementation of somemodifications 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, VisbrokenVacuum Residue, Fuel Oil, etc) coming from the adjacent refinery. Figure 1 is a schematic of theIGCC Complex, showing the interfaces among the different Units.

The Plant which has a design capacity of 560 MWe gross power output, is composed mainly ofthe following sections:

• Gasification: two Texaco Partial Oxidation Reactors using steam as moderator and oxygen asoxidant, of direct water quench type, each followed by one Scrubber, to remove the soot andash from syngas.

• Carbon Recovery and Recycle to recover soot from soot water and recycle it to the Gasifiers.

• Syngas Heat Recovery section where raw gas from Gasification is cooled by generating steamand hot water, with separation and condensation of most water vapour. The catalytic hydrolysisof COS to H2S 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 withwater 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 byproducing steam and hot water, and H2S washed, finally entering the Expander.

• Low Pressure gasification section where clean syngas coming out from the Expander issaturated with water and delivered to the CCU.

In addition to these main sections, the IGCC Complex includes the Metals Recovery Section, theSulphur Recovery and Tail Gas Treatment Section and all the Utility Systems required for theoperation of the Plant.

Gasification and Carbon Recovery

& Recycle

Syngas Heat Recovery

Acid GasRemoval

Syngas Expansion

& Saturation

Combined CycleUnit

Wet Syngas

Dry Syngas

HP Oxygen

Asphalt

Electric Power

HP Steam

Heavy MetalRecovery

SulphurRecovery

Filter Cake

LP Oxygen

Circulating Water

MP Steam

VL

P St

eam

Utilities and Distribution

Sulphur

BFW

LP Steam

Aci

d G

as

LP

Stea

m

MP

Stea

m

Figure 1IGCC Block Flow Diagram

The IGCC Complex control system is aimed to manage the electric power production of the fivepower generators (two Gas Turbines, two Steam Turbines and one Expander) connected to thenational 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 bythe management of the power distribution grid.In the feed control mode the amount of power produced by the complex is limited by the amountof available feed up to the maximum throughput capability of the complex. In essence the feedcontrol mode is a specific case of the power control mode where the specified power output is themaximum possible.

Then, during normal operation a fixed amount of electric power shall be produced or a fixedquantity of asphalt shall be destroyed. Variations in both requirements promote an unbalancebetween the syngas production and the syngas consumption. Due to the Gas Turbine requirementthe pressure in the low pressure gasification section has to be maintained as much as possibleconstant. 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 systemoperating at high pressure.

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 bysuitably resetting the undirect controller set point on the basis of HP Gasification section pressureerror. I.e.:

- in case of power production control mode, the power control of the CCU is directly defined bythe set point selected by the operator, while the feedstock flowrate set point is reset by the HPGasification section pressure control.

- in case of feedstock consumption control mode, the Gasifier load is directly defined by the setpoint selected by the operator, while the production set point is reset by the HP GasificationSection pressure control.

In both cases the error of HP Gasification Section is used to match the balance between the syngasproduction and the syngas consumption by re-establishing the pressure itself.In addition, both in power production and feedstock consumption control modes, a feedforwardcontrol 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.

Gasification MasterController

OR Power Production

Feedstock

Consumption

SET POINT

Feedstock Consumption

IGCC Master

Controller

Power Production Combined CycleMaster Controller

CCUGasification

SyngasTrain

HP Section

PC Syngas Train

LP Section

PC

Expander

Figure 2IGCC Normal Operating Control Modes

Dynamic Model Preparation

The dynamic model of the ISAB Energy IGCC Plant describes the whole process involvingsyngas, starting from its generation in the Gasifiers, through cooling, H2S 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 viewthrough 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 notsimulated as the associated dynamics are not significant.

The model for each section describes the main components, the piping and the associated controlsystem; it integrates all the information necessary to evaluate the mass, thermal, hydraulicbalances, 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, geometricarrangements and design characteristics of mechanical equipment in order to simulate off-design component behaviour for gasification and combined-cycle components and associatedvalves and piping.

c) Operating point data. Heat and mass balance for base-load operating condition. Thisincludes all stream information (mass flow rates, pressures, temperatures, enthalpies, andcompositions).

d) Controls and logic drawings for the equipment and plant. Control valves and controllersdata.

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 IGCCPlant.The following steps have been followed:

a) First, a model schematic is generated. This involves laying out the process which defines thescope of the model.

b) A diagram is then created which depicts the selected software modules and their connectionsused to simulate the process.

c) Most components can be simulated using modules from the standard software library. Newmodules for unique components are developed, as necessary.

d) The next step is to superimpose a process heat balance with enough information to definethe pressure, flowrate, enthalpy, and composition of each stream at operating pointcondition.

e) Drawings of control strategies are developed from the operating procedures and plantcontrols and logic drawings.

f) With the plant scope defined, all modules selected, all data gathered, a dynamic model of theplant 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 ofresistance 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 testthe model at steady-state conditions to choose if the model variables match the heat balanceat 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 themodel 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 steady-state 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 tocheck 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 fasterload 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;

• 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 thewhole Plant operation, by checking the acceptability of the following process variable variationsduring 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 Gasifiersis essential for the IGCC operation (i.e. loss of HP steam moderator to Gasifier causes theIGCC shutdown).

Once the planned and unplanned plant events have been selected, the model was exercised for eachof the transients. Complete plant responses (stream flows, temperatures and pressures) to theseevents have been predicted and graphically presented. These responses have been evaluated fortheir reasonableness and acceptability, with the conclusions and considerations herebelowpresented.

Mechanical Design. The IGCC mechanical design (i.e. geometric dimensions, design temperatureand pressure of equipment) checked during steady state and transient operations, demonstrates tobe adequate.

IGCC Control System. The control philosophy defined for the management of the integratedoperation of the Gasification and CCU sections and translated in the logics of the MasterControllers (i.e. IGCC Master Controller, CCU Master Controller, Gasification MasterController), has been validated through the Dynamic Simulation Study. The plannedloading/unloading procedures and the relevant selected rates (2%/minute) are safe and reliable: nosignificant stress to either machines or equipment is expected.The logics dedicated to withstand the emergency situations (i.e. trip of Gas Turbine, HRSGpostfiring, Steam Turbine, Gasifier, Expander, etc.) have been subject to some modifications onthe basis of the Dynamic Simulation results.

Control Valves. All the main control valve sizes and characteristics have demonstrated to beadequate.

Results

All the transients subsequent to the planned and unplanned events above described have beenstudied 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° 1the power production set point is decreased from 100% to 50% through a ramp of 2%/minuteload variation. In the Simulation n° 2 the power production set point is increased from 50% to100% 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 loadduring the transients.

Figure 3Simulation no. 1

Figure 4 Simulation no. 1

Figure 5 Simulation no. 2

Figure 6 Simulation no. 2

The plots demonstrate, as expected, the quick response of the Gas Turbines, and the large inertiaof the steam cycle. This is particularly evident when the Gas Turbines are at maximum load andthe syngas production variation modifies the HRSG postfiring load, influencing directly the SteamTurbine power production.In that case a variation of power production set point with the rate of 2%/minute determines avariation 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 135-95 bar in order to optimize the heatrecovery by ensuring at the same time the HP steam export to the Gasification.

Figure 7Simulation no. 1

Trip of One Gas Turbine

In Simulation n° 3, the trip of one Gas Turbine occurs when the IGCC Plant is operating at itsdesign capacity. As a consequence of the Gas Turbine trip, the entire train is shutdown. Thereduction of the Gasifiers load with the maximum ramp (i.e. 4%/minute) is automatically activatedby 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. atScrubbers outlet (raw syngas), at Saturator inlet (dry clean syngas), at Gas Turbine inlet (wetclean syngas). Originally the PCV’s set points were selected in order to discharge preferentiallydry syngas to flare. On the contrary, the dynamic simulations demonstrate that it is convenient todischarge the syngas through the dedicated PCVs, either at the Scrubbers outlet, or at the GasTurbines inlet, instead of at the Saturator inlet. Discharge at the extremes of the Syngas Trainallows to maintain a balanced flow of syngas through the different sections, i.e. cooling andsaturation, ensuring an as much as possible constant Low Heating Value of wet syngas fed to GasTurbines.Figure 8 shows the pressure of wet syngas at Gas Turbine inlet which has a first increase and thenis well controlled at a value higher than the original steady state due to the reduced line pressuredrop.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 thedecrease of recirculating water temperature consequent to the syngas train unloading, even if afeedforward reset of the LP steam generator pressure is activated in order to reduce the heatabsorbed by steam generation and ensure the heating of recirculating water. Anyway, the gradientis acceptable by the Gas Turbine.Finally Figure 13 shows the trend of pressure of the HP steam from CCU to Gasification. After afirst 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 atFoster Wheeler Development Corporation.

References

1. Maderni L., Icardi G. and Fontana M., “Control System for a Combined Cycle”, ASMEInternational Gas Turbine & Aeroengine Congress & Exposition, Toronto - Canada. (1989).[conference paper]

2. Ahluwalia K.S. and Domenichini R., “Dynamic Modeling of a Combined Cycle Plant”, ASMEInternational Gas Turbine & Aeroengine Congress & Exposition, Toronto - Canada (1989).[conference paper]

3. Giglio R., Cerabolini M. and Pisacane F., “The Dynamic Simulation of the Progetto EnergiaCombined Cycle Power Plants”, International Joint Power Generation Conference, Houston -Texas (1996). [conference paper]

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

Figure 8Simulation no. 3

Figure 9Simulation no. 3

Figure 10 Simulation no. 3

Figure 11Simulation no. 3

Figure 12 Simulation no. 3

Figure 13 Simulation no. 3