ON / OFF OPERATION OF CO 2 CAPTURE By : Sepideh Ziaii Fashami Supervisors: Dr. Gary T. Rochelle Dr. Thomas F. Edgar Research Review Meeting January 11 th,2008

OUTLINE  Motivation  Concept Texas Electricity Supply Power plant and CO 2 Capture Processes  Dynamic Strategies and Control  Dynamic Modeling and Simulation  Future Work

MOTIVATION CO 2 capture facilities:  High energy requirement results in loss of power production  With on/off strategies for CO 2 capture, 1. Run capture with lower cost of energy 2. Turn off capture to meet peak demand

LOAD AND GENERATION (TEXAS) * ERCOT=Electricity Reliability Council of Texas *

ENERGY PRICE ( TEXAS) Energy Price(¢/KWH)

STEAM TURBINES AND CO 2 CAPTURE

ON/OFF STRATEGIES IN CO 2 CAPTURE Option 1 Run absorber continuously at 90% removal, Turn off stripper at peak Features : 1. Reduced steam consumption 2. Sufficient storage for rich & lean solvent 3. Larger equipment needed for stripping 4. Dynamic behavior and control of the stripper are independent of the absorber

ON/OFF STRATEGIES IN CO 2 CAPTURE Option 2 Run absorber at constant reduced level of removal during peak load Features : 1. Reduced steam flow rate 2. Reduced solvent rate 3. No storage for rich and lean solvent 4. More CO 2 released to the atmosphere (Lose values of CO 2 capture)

ON/OFF STRATEGIES IN CO 2 CAPTURE Option 3 Run absorber at variable level of removal Features : 1. Variable lean and rich loading 2. Variable steam flow rate 3. No storage for rich and lean solvent 4. More CO 2 released to the atmosphere (Lose values of CO 2 capture)

CONTROL OBJECTIVES Optimum operating strategies :  Minimize time for startup/ shutdown or other dynamic strategies  Minimize energy lost during startup/ shut down: - Drive 1 st stage compressor by letdown steam turbine - Minimize energy lost through control valves  Minimize fluctuations of process variables

CONTROL CONFIGURATION 1 (STRIPPER) Controlled variables: -Lean loading -Column pressure MP LP HPS Stripper LPS FC FT TC TT SP PT PC to 2nd stage 1st stage Rich Solution ST SC SP Lean Solution

CONTROL CONFIGURATION 2 (STRIPPER) Controlled variables: Lean loading No control on column pressure Less energy loss through control valves Lean Solution MP LP HPS Stripper LPS TC TT to 2nd stage 1st stage Rich Solution

WHY IS THE DYNAMIC MODEL OF A PLANT IMPORTANT? Dynamic modeling and simulation provide us tools for :  Evaluation of dynamic behavior of the plant during normal and transition operations  Exploring optimum operating strategies  Model-based ( steady-state or dynamic) controller design

DYNAMIC MODELING OF A SIMPLE STRIPPER A rate-based dynamic model for a stripper (packed column) uses the following assumptions: 1. Packed column is vertically divided into N segments (radial gradients are ignored) 2. Each phase is well-mixed 3. Vapor-liquid equilibrium forms at the interface 4. Solvent is not volatile 5. The condenser and reboiler are equilibrium stages

DYNAMIC MODELING OF A SIMPLE STRIPPER 6. Equilibrium reactions occur only in the liquid phase 7. Molar and energy holdups are included for both phases Solvent : 7 m MEA Software: ACM( Aspen Custom Modeler) - Proprietary product of Aspen Tech - Easy-to-use programming tool for creating, editing process units - Solves dynamic, steady-state & optimization problems in an equation-based manner

FUTURE WORK  Creation of a rate-based dynamic model for a simple stripper combined with the letdown steam turbine and compressors using ACM  Study different options for generating strategies with respect to steady-state and dynamic performance  Study proposed control configurations for each strategy and find optimum control design