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Simultaneous Removal of SO2 and CO2 From Flue Gases at Large Coal-Fired Stationary Sources

Y. F. Khalil(1) and AJ Gerbino(2)

(1) Chemical Engineering Department, Yale University, New Haven, CT 06520

(2) AQSim, Inc., Glen Ridge, NJ 07028

OLI’s 24th User Conference

Hyatt Hotel, Morristown, NJ

October 23 – 24, 2007

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Presentation Outline

Motivation for developing alternative technologies for CO2 capture:

- U.S. GCCI

- Integrated control technologies (ICTs)

- Technical and economic barriers of CO2 capture using MEA

Research objectives

Research apporach for modeling CO2 and SO2 capture using:

- OLIs’ ESP

- ICEM (DOE model)

Results and discussion: IECM and ESP

Summary

Roadmap for future work

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Motivation #1: The U.S. Global Climate Change Initiative (GCCI)

GCCI is one of the primary drivers for CO2 emission reduction.

Between 2002 and 2012, this initiative targets 18% reduction in the greenhouse gases (GHGs) intensity.

A second goal of this initiative is to provide a portfolio of commercially-ready CO2 removal technologies for 2012 assessment.

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Motivation #2: Integrated Control Technologies (ICTs)

More cost effective compared to single-effect technologies

Less footprint and, hence, easier to retrofit

Possibility of sharing some unit operations

Possibility of shared raw materials

Example: simultaneous removal of CO2 and SO2

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Motivation #3: Monoethanolamine (MEA) Scrubber for CO2 Capture

MEA scrubbing is the conventional technology for CO2 capture from flue gases

Unfortunately this technology is energy-intensive -- a significant amount of energy is required for recovering the MEA solvent: ≅ 67% of the MEA plant operating cost is attributed to steam requirements for solvent regeneration and ≅ 15% of the cost is for MEA makeup.

MEA is corrosive and requires adding corrosion inhibitors

MEAHEX

MEA makeup

Some CO2 remains in the regenerated MEA

Additional drawback of MEA technology:

Low CO2 loading, i.e., grams of CO2 absorbed per gram of absorbent.

For a 500 MWth coal-fired plant, MEA makeup ~ 22.7 tons/hr

MEA recirculated ~ 6,599 tons/hr

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Total Energy Usage for Recovery & Compression: MEA System

3.4 million BTU/ton CO2

85.1%

4.5%

5.2%

5.2%

Absorption

Feed Compr

1st stage - 1- 10 atm

2nd stage - 10 - 100 atm

Total Energy: 3.41 MBtu/ton CO2

Slightly compress the feed gas to 1.2 bar 0.15 MBtu/ton CO2

Desorb CO2 in the stripper 2.9 MBtu/ton CO2

Compress the CO2 off-gas to 100 bar 2 stages at 0.18 MBtu/ton CO2 each

Source: J. L. Anthoney, Dept. of Chem. Eng, Kansas State U.

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Caustic (NaOH), $/ton: 624.7(needed for MEA reclaiming)

Note:In the proposed process, CaO will be produced in-situ.

Make-up could be in the form of CaCO3 or CaO to compensate for Ca loss as CaSO3or CaSO4

Activated carbon (AC) for MEA cleaning, $/ton: 1,322

Lime, $/ton: 72.01Corrosion inhibitor cost, $/ton: 258.6(20% of MEA cost)

Limestone cost, $/ton: 19.64MEA cost, $/ton: 1,293

Proposed process for CO2 removal by scrubbing with using Ca(OH)2 slurry

Conventional MEA scrubbing for CO2removal

Costs are based on 2005 dollars (as provided by the IECM program)

Cost of Raw Materials

5.1 kg MEA (pure solvent) per 1 kg CO2 removed

From reaction stoicheometry:

~ 1.16 kg Ca(OH)2 per 1 kg SO2 removed

~ 1.68 kg Ca(OH)2 per 1 kg CO2 removed

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Research Objectives

Model the simultaneous removal of SO2 and CO2 gases by chemi-sorption in a slurry of hydrated lime [Ca(OH)2].

Benchmark the performance/effectiveness of this proposed technology with:

- MEA scrubbing approach for CO2 removal

- Wet flue gas desulfurization (FGD) for SO2 removal

- These separate-effect technologies (MEA and FGD) are typically connected in series in a fossil-fired power plant

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Research Approach

1. Use OLI’s Environmental Simulation Program (ESP, v-7.0-55) to model the simultaneous removal of SO2 and CO2 gases by scrubbing into a slurry of hydrated lime [Ca(OH)2].

• Three hypothetical flue gas compositions are to be evaluated : CO2concentrations of 3%, 14%, and 25%; representative of exhaust streams of a NG-fired power plant, coal-fired power plant, and a cement production plant, respectively.

- Only the coal-fired plant (11 – 15% CO2) is discussed in this presentation

• Concentration of SO2 in the flue gas is assumed to be 2000 ppm

Three-Fold Approach:

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Research Approach

1. Use the OLI’s Environmental Simulation Program (ESP, v-7.0-55)to model the simultaneous removal of SO2 and CO2 gases by scrubbing into a slurry of hydrated lime [Ca(OH)2].

• Flue gas flow rate was kept constant at ~ 1.6x106 acfm (~ 2.7x106

m3/hr); such flow rate is typical of a 500 MWth coal fired power plant.

• The proposed process includes a SO2 scrubber, a CO2 scrubber, a calciner, a lime slaking reactor, and a few auxiliary unit operations such as heat exchangers, filters and dryers.

Three-Fold Approach (cont’d):

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Research Approach

2. Use the Integrated Environmental Control Model (IECM)software to predict the performance of a coal-fired plant that uses MEA scrubbing for CO2 capture and wet FGD unit for SO2removal

• IECM software has been developed by the Center for Energy and Environmental Studies, Carnegie Mellon University for DOE in 2007 (Current Version: 5.21; February 2, 2007)

3. Compare ESP predictions with IECM predictions for CO2 and SO2 removal

Three-Fold Approach (cont’d):

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Importance of the Proposed Integrated Technology

Fossil-fuel-based power generation stations; which contribute about 30% of the World’s CO2 emissions

Coal-fired gasification combined cycle (IGCC) turbines

Cement production plants

Petrochemical plants

The proposed integrated technology for simultaneous removal of CO2 and SO2 could be of interest to many industrial facilities including:

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CO2 Gas Absorption Reaction (carbonation reaction):

CO2 (g) + Ca(OH)2 CaCO3 + H2O ΔHo298 K ≅ -113 kJ/mole

Calcination Reaction:

CaCO3 CaO + CO2 (g) ΔHo298 K ≅ 178 kJ/mole

Lime Slaking Reaction:

CaO + H2O Ca(OH)2 ΔHo298 K ≅ -65 kJ/mole

Chemical Reactions for CO2 Removal

Lime Slaker

CalcinerCarbonator CO2

H2O

CO2 in flue gas

CaCO3

CaOCa(OH)2

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Carbonator: Exothermic ReactionΔ

G, k

J/m

ole

ΔGR ad ΔHR are calculated by HSC software

CO2 (g) + Ca(OH)2 CaCO3 + H2O

ΔGR at 298oK = -72.643 kJ/mole

ΔHR, k

J/m

ole

ΔHR at 298oK = -113.03 kJ/mole

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ΔG

, kJ/

mol

e

CaCO3 (s) CaO (s) + CO2 (g)

Calciner: Endothermic Reaction

ΔGR at 1198oK = -5.528 kJ/moleΔGR at 1273oK = -16.169 kJ/mole

ΔHR, k

J/m

ole

ΔHR at 1198oK = 164.949 kJ/moleΔHR at 1273oK = 163.207 kJ/mole

Typical Calciner Temperature Range 1220oK – 1420oK

ΔGR ad ΔHR are calculated by HSC software

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ΔG

, kJ/

mol

e

CaO (s) + H2O Ca(OH)2

Lime Slaker: Exothermic Reaction

ΔGR at 298oK = -57.804 kJ/mole

ΔHR, k

J/m

ole

ΔHR at 298oK = -65.145 kJ/mole

ΔGR ad ΔHR are calculated by HSC software

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SO2 Gas Absorption Reaction:

SO2 (g) + Ca(OH)2 CaSO3 (s) + H2O ΔHo298 K ≅ -163 kJ/mole

Forced Oxidation of CaSO3 to CaSO4:

CaSO3 (s) + 1/2O2 (g) CaSO4 (s) ΔHo298 K ≅ -556 kJ/mole

Chemical Reactions for SO2 Removal

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Lime Slaker

CalcinerCarbonator CO2

H2O

CO2 in flue gas

CaCO3

CaOCa(OH)2

Lime SlakerSO2 in flue gas

Ca(OH)2

CaSO3 or CaSO4

Makeup CaO to compensate for Ca lost in CaSO3 or CaSO4

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SO2 Absorption: Exothermic ReactionΔ

G, k

J/m

ole

ΔGR at 298oK = -114.736 kJ/mole

ΔGR ad ΔHR are calculated by HSC software

ΔHR, k

J/m

ole

ΔHR at 298oK = -162.509 kJ/mole

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Forced Oxidation of CaSO3: Exothermic ReactionΔ

G, k

J/m

ole

ΔGR at 298oK = -498.504 kJ/moleΔH

R, k

J/m

ole

ΔHR at 298oK = -556.469kJ/mole

ΔGR ad ΔHR are calculated by HSC software

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Co-Production of Lime and Syngas:

CaCO3 + CH4 (g) CaO + 2CO (g) + 2H2 (g) ΔHo 298 K ≅ 426 kJ/mole

Chemical Reactions for Co-Production of SynGas

Typical Calciner Temperature Range 1220oK – 1420oK

Hence, co-production of Syngas can take place within the calciner temperature range

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SynGas Production: Endothermic Reaction

ΔG

, kJ/

mol

e

ΔHR, k

J/m

ole

ΔGR ad ΔHR are calculated by HSC software

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Mitigation of Operating Risks of the Calciner

Lime Sintering (decrease in surface area and pore size of CaO)

Reducing the operating temperature of the calciner results in less sintering of the produced calcium oxide and, hence, more reactive lime (CaO) in the lime slaker.

Cost of CaO Makeup Due to Loss of Reactivity

Because calcium is used continuously in a cyclical manner, sintering and corresponding reduction in reactivity is a cumulative process that may require periodic makeup of calcium oxide. If calcium can be recycled say 500 times, then it may easily be considered to be cost effective.

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Process Flow Diagram (PFD) as Simulated in ESP

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Quench

SO2

Scr

ubbe

r

CO

2 S

crub

ber

CaSO3Filter

CaCO3 Filter

Dryer

SlakerQuench Water

Flue Gas

Quench Liquid Out

Quenched flue Gas

SO2 Scrubber Bottom

Lime Split 2

SO2 Scrubber Vent

CO2 Scrubber Bottoms

Vent Gas

Lime Split 1

Slaked Lime

CaCO3 Cake

Dry CakeCalciner

CO2

CaCO3 Filtrate

Blowdown

CaSO3 Blowdown

CaSO3 Filtrate

CaSO3 Blowdown

H2O/CaOH2 Feed

CC

V-1

CaO

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User defined

ΔHR = -84.6 kJ/mole CO2

ΔHR (30 wt% MEA in water) = -84.6 kJ/mole CO2 & Mass of MEA (30 wt%) to absorb 1 kg CO2 = 17 kg MEA solution

Simulation of CO2 Removal Using DOE/IECM

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Simulation of SO2 Removal Using DOE/IECM

User defined

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Simulation Results of MEA-Based Technology for CO2 Removal Using the Integrated Environmental

Control Module (IECM)

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Coal-Fired Boiler

Absorber• Remove heat of chemisorption • Cool lean regenerated MEA solvent by removing sensible heat

Stripper• Heat the rich MEA solvent by extracting sensible heat from the lean MEA solvent• Supply heat of desorption using steam in the reboiler

Possible Power Plant Capture Add-ons• Cool flue gas to absorber conditions (25oC)• Compress flue gas to overcome pressure drop in Absorber• Post compression of CO2 to desired product pressure

CO2

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3131CO2 removal = (2.667E6 tons/yr) / 6575 hrs/yr ~ 406 tons/hr for a 500 MWth coal-fired plant

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CO2 (mole%) in input flue gas = 2.048E4 lb-mole/hr / 1.706E5 lb-mole/hr ~ 12%

CO2 removal efficiency = 90% (user defined) and CO2 escape with flue gas = 10%

3333MEA scrubber plant cost about $281M / $700M ~ 37% of the 500 MWth plant cost

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Simulation Results of Wet-FGD Technology for SO2 Removal Using the Integrated Environmental

Control Module (IECM)

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Air Preheater

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ESP Simulation Results

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ESP Simulation ResultsFlue Gas Stream

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ESP Simulation ResultsFlue Gas Stream

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ESP Simulation ResultsFlue Gas Stream

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ESP Simulation ResultsUtility Water

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ESP Simulation ResultsFlue Gas Stream

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ESP Simulation ResultsFlue Gas Stream

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Summary

OLI’s ESP was a useful simulation tool for modeling CO2 and SO2 capture using Ca(OH)2 slurry

Other insights and opportunities for improving the ESP simulation capabilities

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Roadmap for Future Work

Simulate CO2 capture using the monoethanolamine technology

Compare performance/CO2 capture efficiency and raw materials requirements versus CO2 capture using Ca(OH)2 slurry

Calculate the energy requirements for the Ca(OH)2technology and compare to MEA energy requirements

Demonstrate improved Ca utilization in the proposed technology (i.e., Ca consumed to remove S and C)

Estimate calcium make-up requirements (tons/hr) for the simultaneous removal of CO2 and SO2