ON / OFF OPERATION OF CO2 CAPTURE

By : Sepideh Ziaii Fashami

Supervisors:Dr. Gary T. RochelleDr. Thomas F. Edgar Research Review Meeting January 11th ,2008

OUTLINE

Motivation Concept Texas Electricity Supply Power plant and CO2 Capture Processes Dynamic Strategies and Control Dynamic Modeling and Simulation Future Work

MOTIVATION

CO2 capture facilities: High energy requirement results in loss of power production With on/off strategies for CO2 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

Coal

Nuclear

Natural Gas

Wind

*

ENERGY PRICE ( TEXAS)

August 9,2006

0

5

10

15

20

25

30

35

12:00 AM 2:24 AM 4:48 AM 7:12 AM 9:36 AM 12:00 PM 2:24 PM 4:48 PM 7:12 PM 9:36 PM 12:00 AM

Time

Ener

gy P

rice(

¢/K

WH

)

STEAM TURBINES AND CO2 CAPTURE

BoilerHP MP LP Generator

Power Steam Turbines

Let down Steam Turbine

Condensate

StripperWater

Flue Gas From FGD

Absorber

Steam

Gas Out

Lean Rich

CO2

ON/OFF STRATEGIES IN CO2 CAPTURE

Option 1Run absorber continuously at 90% removal, Turn offstripper at peakFeatures : 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 CO2 CAPTURE

Option 2Run absorber at constant reduced level of removal during peak loadFeatures : 1. Reduced steam flow rate 2. Reduced solvent rate 3. No storage for rich and lean solvent 4. More CO2 released to the atmosphere (Lose values of CO2 capture)

ON/OFF STRATEGIES IN CO2 CAPTURE

Option 3Run 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 CO2 released to the atmosphere (Lose values of CO2 capture)

CONTROL OBJECTIVES

Optimum operating strategies : Minimize time for startup/ shutdown or other

dynamic strategies Minimize energy lost during startup/ shut down: - Drive 1st 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

Stripp

erLPS

FC FT

TC TT

SPPT

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

Stripp

erLPS

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 phase7. Molar and energy holdups are included for both phasesSolvent : 7 m MEASoftware: 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