Coal-Derived Warm Syngas Purification and CO2 Capture-Assisted Methane Production

Robert Dagle1, David L. King1, Xiaohong Shari Li1, Rong Xing1, Kurt Spies1, Yunhua Zhu1, and Beau Braunberger2

1. Pacific Northwest National Laboratory, Richland, WA 2. Western Research Institute, Laramie, WY

Clean Coal Symposium 2014

August 21, 2014

1

Outline

Warm syngas cleanup CO2 sorbent material development

Sorbent integration with CO methanation reaction

Multi-unit process demonstration

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Coal Gasification for Fuels & Chemicals

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Coal

Gasifier

ASU

WGS Syngas cleanup

Synthesis, H2 production H2O

Slag Ash, particulates

syngas

H2S, COS, Cl, As, …

The order of wgs and syngas cleanup depends upon • gasifier type • whether water quench is employed • end use application

Driving Force For Warm Gas Cleanup

Current approaches use physical adsorption solvents to remove sulfur and other contaminants

Selexol -5 to 25oC Outlet sulfur content ≤ 5 ppmv Moderate CO2 slip

Rectisol -30 to -70oC Outlet sulfur < 100 ppb Complete CO2 removal

Both processes are inefficient due to the requirement to cool the syngas for purification and subsequent re-heat for synthesis or fuel cell use

4

Alternatives to Treating Gasifier Effluent: Water Quench

Quench system integrated with gasifier Water used to quench the slag Partial quench cools syngas to ~900oC

Allows use of sensible heat below 900oC for high P steam generation Impurity concentrations in syngas significantly higher than in full quench

Full quench produces syngas around 300oC Lower efficiency due to loss of sensible heat Particulates, majority of alkali, chlorides, metals, NH3 removed by water quench Facilitates subsequent wgs (generally sour shift) and increase in H2 concentration of syngas Remaining impurities requiring cleanup: H2S, COS, trace quantities of NH3, As, HCl Wastewater can be recycled in slurry-fed process

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Typical Content of Raw Syngas Produced by Coal Gasification Without Full Water Quench

Gas component Concentration (vol %) CO 30-60 H2 25-30 CO2 5-15 H2O 2-30 CH4 0-5 H2S 0.2-1 COS 0-0.1 HCN + NH3 0-0.3

6

Impurity HCl K Na AsH3 PH3 Hg Sb Se Pb Cd

ppmv 160 500 320 0.6 1.9 0.03 0.1 0.2 0.3 0.01

* Tars may also be present depending on gasifier type and mode of operation.

Warm Gas Cleanup Strategies

Chloride removal Sulfur (H2S and COS) removal

Trace contaminant removal

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Warm Gas Cleanup Approach (With Water Quench) For Generation of Syngas for Chemical Synthesis

Quench gasifier

HCl polishing sorbents

(Na2CO3-based)

Regenerable and polishing ZnO

sorbents for sulfur removal to <0.1 ppm

Metal sorbents for As, P, Sb, Se, HCl and S

deep removal

Sweet high T shift (optional)

Slag Wastewater containing particulates, chloride, alkali, non-volatile metals, NH3

Syngas with sulfur and trace other impurities

Sour shift CoMoS/Al2O3 (Optional)

Synthesis CH3OH, CH4, higher alcohols, ….

Solid Oxide Fuel Cell (H2, CO, CO2)

CO2 removal (as needed)

Sweet low T shift H2: PEM Fuel Cell (H2, CO2)

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Goal: ppb level impurities

HCl Removal - Na2CO3 Feed: 50% H2O, 13% CO, 10% CO2, 20% H2, 7% CH4, 100 ppm HCl 80,000 hr-1, 1 atm

Optimal Sorbent Capacity 450-500oC 9

450oC Ind. Eng. Chem. Res. 2013, 52, 8125-8138

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Less Than 50 ppb H2S Can Be Achieved Thermodynamically by ZnO Absorbent at 300oC

H2S Removal Thermodynamic Calculation

H2S Removal (in Syngas) with ZnO: Temperature Effect

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450oC optimal temperature

No detectable sulfur slip (< 40 ppb)

3000 ppm H2S in Syngas (38.4% CO, 38.4% H2, 3.2% N2, 20% H2O), 12,000 hr-1; ZnO Sorbent G-72D provided by Sud Chemie, contains small amount Al2O3 x

H2S Removal (in Syngas) with ZnO: Cycling Stability Evaluations

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Degradation of sorbent capacity after first couple cycles

5-10 ppm slip observed after first cycle

3000 ppm H2S in Syngas (38.4% CO, 38.4% H2, 3.2% N2, 20% H2O), 12,000 hr-1; ZnO Sorbent G-72D provided by Sud Chemie, contains small amount Al2O3 x

PH3 and AsH3 Removal From Warm Syngas by 28 wt.% Ni-Cu/SBA-16 Adsorbent (300oC)

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PH3

AsH3

Summary of Warm Inorganic Contaminant Cleanup

HCl sorption demonstrated with NaCO3 Sulfur removal

Fresh ZnO is capable of achieving ppb levels of H2S slip, in agreement with thermodynamics Regenerated ZnO does not achieve thermodynamically predicted levels of H2S slip

Typical slip is 5-10 ppm H2S Higher T operation (450oC) maximizes capacity of regenerated ZnO

Cause for difference in performance between fresh and regenerated ZnO is unclear

Sintering of ZnO crystals occurs Change in surface properties of ZnO may be responsible

A regenerated bed (450oC) followed by a fresh ZnO polishing bed (300oC) is predicted to provide a solution to bringing H2S slip to ppb levels

As, P sorption demonstrated with CuNi sorbent 14

Warm CO2 Capture

MgO-Based Double Salts: Facilitation by Molten Salts

CO2-Sorption Integrated with Catalytic

Methanation Reaction

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LiNaK-CO3 promoted MgO and MgO based double salt absorbents for CO2 removal at 300-500C Motivation and Background

• Capture CO2 from fossil fuel reforming/gasification • Eliminate a cooling/heating treatment of the gas stream, and improve the

thermal efficiency • Facilitate equilibrium-restricted processes, e.g. water-gas-shift reaction,

methane synthesis.

MgO and MgO based double salts(DS) • Thermodynamics predicts MgO to be effective absorbent below 380 °C:

𝑴𝑴𝑴 + 𝑪𝑴𝟐(𝑴) ↔ 𝑴𝑴𝑪𝑴𝟑 • MgO double salt carbonation temperature is increased up to 520 °C.

𝑴𝑴𝑴 + 𝑵𝑵𝟐𝑪𝑴𝟑 + 𝐂𝑴𝟐(𝑴) ↔ 𝐌𝐌𝐌𝐌𝟐(𝐂𝑴𝟑)𝟐

However, the reactions are limited by slow kinetics

Previous studies have found that the presence of NaNO3 significantly enhances the ability of MgO to capture CO2

NaNO3 is a strong oxidizing agent. Thus, the application of the NaNO3 promoted adsorbents is limited. Objective: Replace NaNO3 with non oxidizing molten salts such as molten carbonates.

Adv. Mater. Interfaces, 2014, 1, 1400030

Recently, our results indicate that the presence of Li-Na-K-CO3 can also significantly improve the ability of MgO and MgO based double salt to capture CO2.

Li2CO3-Na2CO3 -K2CO3 Phase Diagram

Adjusting the composition of the salt controls the temperature at which the molten phase forms.

LiNaKCO3 Li2CO3, 32.2wt% Na2CO3: 33.3wt% K2CO3: 34.5 wt% Melting point: 390C

CO2 absorption test of MgO and MgO +NaNO3 during heating in CO2 1

LiNaK-CO3 Promoted MgO Absorbents

Blue: 80% MgO, 20% LiNaKCO3 Green: MgO

TGA results of 80% MgO + 20% Li-Na-K-CO3 (350C calcined )

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Mass Change: 58.58 %[1]

[1]

100% CO2, 25C-425C

(5C/min)

Cyclic test: 360C (99 min in CO2) 390C (81min in N2)

High stable cyclic capacity (13mmol/g) was achieved (~50 wt.%)

Absorption rate: 4.5 mmol/g/min was observed at 360-370C

𝑴𝑴𝑴+𝐂𝑴𝟐 (𝑴)↔𝐌𝐌𝐂𝑴𝟑

Created with NETZSCH Proteus software

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Main 2013-10-07 11:59 User: TGA

Mass Change: -21.46 % Mass Change: 13.75 %[1]

[1]

• Molten carbonate promoted MgO-Na2CO3 absorbents have CO2 capacity of 2.5-4.5 mmol/g (~15-20 wt.%).

• Regeneration can be easily carried out both through PSA and TSA.

LiNaK-CO3 Promoted MgO-Na2CO3 Double Salt Absorbents

400C 100% PSW

Created with NETZSCH Proteus software

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Red: 390C CO2,450C air Green: 360C CO2, 400C air

𝑴𝑴𝑴+𝑵𝑵𝟐𝑪𝑴𝟑+𝐂𝑴𝟐 (𝑴)↔𝐌𝐌𝐌𝐌𝟐(𝐂𝑴𝟑 )𝟐

TGA cyclic test results of 44% MgO, 44% Na2CO3,12% LiNaK-CO3

Absorption performance and BET surface area of as a function of LiKNa-CO3

concentration

Illustration diagrams for the interfacial interactions of molten(A) and pre-molten(B) salt promoted gas-solid reaction process

Triple phase boundaries (TPB) are required for the molten salt promoted CO2+MgO reaction process

(A)

(B)

LiNaK-CO3 Promoted MgO and MgO Based Double Salt Absorbents

LiNaK-CO3 promoted

Absorb(Wt. %)

Operation temperature range, C

Operation condition

Cycling capacity

Absorption Desorption mmol/g

20%LiNaK-CO3@MgO 300-360 375-385

360C-390C combined swing 12-13

12%LiNaK-CO3@44%MgO/

44% Na2CO3 300-400 400-475

400C pressure swing 3.5--4.5

360-450C temperature swing 2.5-4.5

390-450C combined swing 3.4-4.5

By adjusting the absorbent’s composition and chosen different of molten salts, a series of absorbents which can be used for different applications were developed.

CO2-Sorption Enhanced Methanation (Methanation Reaction + CO2 Capture)

CO2-sorption enables enhanced selectivity to methane CO2 sorbent capacity = 24 wt.%

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Methanation-only 15% Ni/MgAl2O4 catalyst

Methanation + CO2 Capture

15% H2O, 40% H2, 32% CO, 3.0% CH4, 22% CO2,3.0% N2 1 bar; 3600C; 1800 hr-1 reaction, 46 hr-1 sorption OHCHHCO 2423 +↔+

222 HCOOHCO +↔+CO methanation:

WGS:

CO2-Sorption Enhanced Methanation Pressure Effect

Pressurized operation enhances CO2 sorption Enabling 99% CH4 Yield (gas phase)

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15% H2O, 40% H2, 32% CO, 3.0% CH4, 22% CO2, 3.0% N2 3600C; 1800 hr-1 reaction, 46 hr-1 sorption

CO2 Sorption Conclusions NaNO3 and molten carbonate can promote MgO and MgO-

based double salts to capture CO2 with a high cycling capacity.

Stable cycling CO2 capacity up to 13mmol/g was achieved MgO and MgO based double salts can capture CO2 with the presence of both molten and pre-molten salts.

A higher adsorption rate was observed at the temperature close to melting point.

By adjusting the adsorbent’s composition and chosen different of molten salts, a series of absorbents which can be used for different applications were developed. Non-corrosive sorbent was successfully integrated with catalytic methanation

Process Demonstration

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Multi-Unit Cleanup Process Train Demonstrated with water quenched coal-derived syngas obtained from the Western Research Institute

Disposable sorbents for HCl and trace contaminant removal (2 separate beds)

2 Regenerable bulk ZnO beds + ZnO sulfur polishing unit

Tar reformer

tars present because low T gasifier operation

R6LT-WGSCuZn-Al2O3

235○C

Wyoming coalsynthesis gas

R4Tar Reformer Ir/MgAl2O4

850○C

R3Trace Metal Polish

ZnO & CuNi/C300○C

R1HCL Removal

Na2CO3450○C

R2ADesulfurization

ZnO450○C

R2BDesulfurization

ZnO450○C

R5ACO2 Capture & SNG Double Salt & Ni/MgAl2O4

350○C/450 ○C

Clean, warmH2-rich syngas

Clean, warm, CH4-rich, CO2-lean syngas

R5BCO2 Capture & SNG Double Salt & Ni/MgAl2O4

350○C/450 ○C

Slip Stream

Process Flow Diagram

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2 Regenerable CO2-sorption enhanced methanation units demonstrate w/ slip stream

WGS bed used for warm cleanup demonstration

Demonstration Results for Warm Cleanup WRI Gasifier-Derived Syngas ~1 SLPM

Slight deactivation of WGS catalytic performance observed Ppm levels of sulfur found on front end of spent WGS catalyst Vast majority of contaminants removed from syngas (99% S removed)

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Raw Syngas Feed (Water Quenched)

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Considerable progress achieved from 2013 demonstration where significant deactivation occurred!

Demonstration Results CO2-Sorption Enhanced Methanation Reactors (2 Beds)

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CO2 Selectivity

CH4 SelectivityCO2 Sorbent Not

Yet ActivatedDeactviation

A B A Alternating/Regenerable Beds: Sorption 350oC Desorption 450oC (N2)

Mixed results – CO2 sorption and methanation reaction occurring simultaneously prior to gradual deactivation of sorbent

Summary

Warm gas cleanup is feasible and provides efficiency gains relative to ambient or sub-ambient liquid phase capture of impurities Increased benefit of warm gas cleanup will derive from continued development of warm CO2 capture technology in conjunction with syngas cleanup Absorption of CO2 by Na2CO3-MgO (forming double salt) is facilitated by molten salt

A regenerable CO2 capacity of ~20 wt.% is achieved with double salts using temperature swing (350oC sorption/450oC desorption)

Dissolution of some MgO into the molten salt, followed by reaction of CO2 at the triple phase boundary, provides basis for CO2 capture process

Combining CO2 capture with methanation in a single bed was demonstrated to yield 99+% (10 bar, 350oC)

Multi-contaminant removal process train was demonstrated for 100 hours with ~ 1 SLPM Wyoming coal-derived syngas (WRI-provided)

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Acknowledgments

Financial support by the US DOE Office of Fossil Energy (NETL), the State of Wyoming, and PNNL internal research funds is gratefully acknowledged Some of this work involves a collaboration with the National Energy Technology Laboratory (NETL), the Center For Clean Energy Engineering (University of Connecticut), and the Chinese Academy of Sciences (CAS) A portion of this work was carried out in the Environmental Molecular Sciences Laboratory (EMSL) at PNNL, a US DOE Office of Science user facility

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Questions?

Extra Slides

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Alternatives to Treating Gasifier Effluent

Radiant cooler Only radiant heat transfer cools the syngas Higher CO concentration in product gas Moisture content of syngas is low Somewhat prone to fouling Difficulties in scaling Favored for industrial gas production, CO production, IGCC where H2 purity not required to be high (no CO2 capture) IGCC: sulfur concentration <20ppmv Greater energy efficiency, but higher CAPEX Hot gas scrubber required to remove particulates, chlorides

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28.8 wt.% Ni-Cu/SBA-16 Functions Effectively as PH3 and AsH3 Absorbent

Ni + AsH3 =NiAs + 1.5 H2 (127 wt%) Kp = 8.85 x 1011 3Cu + AsH3 = Cu3As + 1.5H2 (39 wt%) Kp = 2.45 x 107 Ni + 2PH3 = NiP2 + 3 H2 (106 wt%) Kp = 1.01 x 1014 Cu + PH3 = Cu3P + 1.5 H2 (16 wt%) Kp = 5.6 x 1014

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COS dry 200 CCOS dry 450 CCOS wet 450 CH2S, COS wet 450 CTheoretical Maximum

H2S Removal (in Syngas) with ZnO COS Sorption

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Wet COS sorption favorable (COS+H2O H2S + CO2)

Dry COS sorption still feasible at warm temperatures (e.g., 450oC vs. 200oC)

1000 ppm COS in N2 (dry) or 80% N2 , 20% H2O (wet); 12,000 hr-1; ZnO Sorbent G-72E provided by Sud Chemie, contains small amount Al2O3 x

Crystal Size by XRD, nm

BET surface area, m2/g

Pore volume, cm3/g

Pore size, nm

Fresh ZnO 15 37.9 0.24 26 Regenerated

ZnO 50 6.7 0.19 114

Regenerated ZnO

ZnS

Fresh ZnO

Characterization of Fresh and Regenerated ZnO Sorbents

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Surface Adsorption Can Exceed Bulk Thermodynamic Performance

Initial work was carried out for sulfur removal, later extended to other impurities

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227 352 441 560 727 977 282 T, oC

H2S

(ppm

) in

H2

Ni + H2S ↔ Ni2S(surface) + H2

<5 ppb H2S in feed gas can be achieved at 350oC and lower J.G. McCarty and H. Wise, J. Chem. Phys. 1980, 72(12), 6332.

Challenge: utilize this concept while developing a regenerable adsorbent

NaNO3 is found to have a key impact on the performance*

* Zhang, K., Li, X. S., Duan, Y., Singh, P., King, D. L. and Li, L. (2013). Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO sorbent for intermediate temperature CO2 removal. Int. J. Greenhouse. Gas Control 12:351-358

3.4 mmol CO2/g

Comparative thermodynamics shows increased stability of double salt

The formation for Na2Mg(CO3)2 shifts the equilibrium towards higher temperature and enables regenerable CO2 uptake at 400 °C through PSA

TGA measurement of CO2 uptake over MgO + NaNO3

shows initiation of absorption on melting of nitrate salt*

* CO2 uptake on alkaline earth oxides catalyzed by nitrate salt is described in one of our manuscripts in preparation

CO2 absorption on MgO+NaNO3 confirms MgCO3 formation contributes to the high uptake observed in 1st peak during ramping CO2.

The absorption stops at 380-400 °C and desorption starts at higher temperature. This indicates the loss of high initial peak is due to high absorption temperature.

CO2 capacity of molten carbonate promoted dolomite absorbents increased from 5% to 21% after 24 carbonation-decomposition cycles, indicating a self-activating process

LiNaK-CO3 Promoted Dolomite Absorbents

360C 100% CO2-400oC 100% N2,

𝑴𝑴𝑴+Ca𝑪𝑴𝟑+𝐂𝑴𝟐 (𝑴)↔𝐌𝐌Ca(𝐂𝑴𝟑 )𝟐

TGA cyclic results of 80% dolomite, 20% LiNaK-CO3

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Dolomite + 20% Li-Na-K CO3

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In-situ XRD data of Na-Mg double salt absorbent with NaNO3

MgCO3 is formed during 1st cycle, and double salt during the 1st and subsequent cycles.

NaNO3 is not observed as it melts under absorption condition and becomes undectable by X-ray.

What is the role of NaNO3 in facilitating CO2 Capture by MgO and MgO-Based Double Salts?

MgO (and Na2CO3) are partially dissolved in molten NaNO3 and dissociate into their ionic components

Dissolved O2- ions react with CO2 to form CO32-

MgCO3 or Na2Mg(CO3)2 precipitate when solubility limit is reached CO2 is likely first adsorbed on MgO and interacts with O2- at the triple phase boundary

Proposed CO2 capture facilitated by nitrate salt at triple phase boundary “Phase Transfer Catalysis”

Cycling Temperature Sensitivity

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40% H2, 32% CO, 3.0% CH4, 22% CO2,3.0% N2, (15% H2O) 1 bar; 3600C; 1800 hr-1 reaction, 46 hr-1 sorption

CO2-sorption enhanced methanation 360C CO2-desorption 450C (1 hr)

Carbonated

Regenerated

XRD for R5A (A) and R5B (B) after integrated testing