Coal-Derived Warm Syngas Purification and CO 2 Capture-Assisted Methane Production

Transcription

1 Coal-Derived Warm Syngas Purification and CO 2 Capture-Assisted Methane Production Robert Dagle 1, David L. King 1, Xiaohong Shari Li 1, Rong Xing 1, Kurt Spies 1, Yunhua Zhu 1, and Beau Braunberger 2 1. Pacific Northwest National Laboratory, Richland, WA 2. Western Research Institute, Laramie, WY Clean Coal Symposium 2014 August 21,

2 Outline Warm syngas cleanup CO 2 sorbent material development Sorbent integration with CO methanation reaction Multi-unit process demonstration 2

3 Coal Gasification for Fuels & Chemicals Coal ASU H 2 O Gasifier syngas WGS Syngas cleanup Synthesis, H 2 production Slag Ash, particulates H 2 S, COS, Cl, As, The order of wgs and syngas cleanup depends upon gasifier type whether water quench is employed end use application 3

4 Driving Force For Warm Gas Cleanup Current approaches use physical adsorption solvents to remove sulfur and other contaminants Selexol -5 to 25 o C Outlet sulfur content 5 ppmv Moderate CO 2 slip Rectisol -30 to -70 o C Outlet sulfur < 100 ppb Complete CO 2 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

5 Alternatives to Treating Gasifier Effluent: Water Quench 5 Quench system integrated with gasifier Water used to quench the slag Partial quench cools syngas to ~900 o C Allows use of sensible heat below 900 o C for high P steam generation Impurity concentrations in syngas significantly higher than in full quench Full quench produces syngas around 300 o C Lower efficiency due to loss of sensible heat Particulates, majority of alkali, chlorides, metals, NH 3 removed by water quench Facilitates subsequent wgs (generally sour shift) and increase in H 2 concentration of syngas Remaining impurities requiring cleanup: H 2 S, COS, trace quantities of NH 3, As, HCl Wastewater can be recycled in slurry-fed process

6 Typical Content of Raw Syngas Produced by Coal Gasification Without Full Water Quench Gas component Concentration (vol %) CO H CO H 2 O 2-30 CH H 2 S COS HCN + NH Impurity HCl K Na AsH 3 PH 3 Hg Sb Se Pb Cd ppmv * Tars may also be present depending on gasifier type and mode of operation. 6

7 Warm Gas Cleanup Strategies Chloride removal Sulfur (H 2 S and COS) removal Trace contaminant removal 7

8 Warm Gas Cleanup Approach (With Water Quench) For Generation of Syngas for Chemical Synthesis Slag Metal sorbents for As, P, Sb, Se, HCl and S deep removal Quench gasifier Wastewater containing particulates, chloride, alkali, non-volatile metals, NH 3 Regenerable and polishing ZnO sorbents for sulfur removal to <0.1 ppm HCl polishing sorbents (Na 2 CO 3 -based) Syngas with sulfur and trace other impurities Sour shift CoMoS/Al 2 O 3 (Optional) Sweet low T shift Sweet high T shift (optional) Solid Oxide Fuel Cell (H 2, CO, CO 2 ) H 2 : PEM Fuel Cell (H 2, CO 2 ) Synthesis CH 3 OH, CH 4, higher alcohols,. Goal: ppb level impurities CO 2 removal (as needed) 20

9 HCl Removal - Na 2 CO 3 Feed: 50% H 2 O, 13% CO, 10% CO 2, 20% H 2, 7% CH 4, 100 ppm HCl 80,000 hr -1, 1 atm Ind. Eng. Chem. Res. 2013, 52, o C 9 Optimal Sorbent Capacity o C

10 H 2 S Removal Thermodynamic Calculation 25 Less Than 50 ppb H 2 S Can Be Achieved Thermodynamically by ZnO Absorbent at 300 o C

11 H 2 S Removal (in Syngas) with ZnO: Temperature Effect 3000 ppm H 2 S 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 Al 2 O 3 x 450 o C optimal temperature 11 No detectable sulfur slip (< 40 ppb)

12 H 2 S Removal (in Syngas) with ZnO: Cycling Stability Evaluations 3000 ppm H 2 S 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 Al 2 O 3 x Degradation of sorbent capacity after first couple cycles ppm slip observed after first cycle

13 PH 3 and AsH 3 Removal From Warm Syngas by 28 wt.% Ni-Cu/SBA-16 Adsorbent (300 o C) PH 3 AsH 3 33

14 Summary of Warm Inorganic Contaminant Cleanup 14 HCl sorption demonstrated with NaCO 3 Sulfur removal Fresh ZnO is capable of achieving ppb levels of H 2 S slip, in agreement with thermodynamics Regenerated ZnO does not achieve thermodynamically predicted levels of H 2 S slip Typical slip is 5-10 ppm H 2 S Higher T operation (450 o C) 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 (450 o C) followed by a fresh ZnO polishing bed (300 o C) is predicted to provide a solution to bringing H 2 S slip to ppb levels As, P sorption demonstrated with CuNi sorbent

15 Warm CO 2 Capture MgO-Based Double Salts: Facilitation by Molten Salts CO 2 -Sorption Integrated with Catalytic Methanation Reaction 15

16 LiNaK-CO 3 promoted MgO and MgO based double salt absorbents for CO 2 removal at C Motivation and Background Capture CO 2 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 + CO 2 (g) MMMO 3 MgO double salt carbonation temperature is increased up to 520 C. MMM + NN 2 CO 3 + CO 2 (g) MMMM 2 (CO 3 ) 2 However, the reactions are limited by slow kinetics

17 Previous studies have found that the presence of NaNO 3 significantly enhances the ability of MgO to capture CO 2 Li2CO3-Na2CO3 -K2CO3 Phase Diagram LiNaKCO 3 Li 2 CO 3, 32.2wt% Na 2 CO 3 : 33.3wt% K 2 CO 3 : 34.5 wt% Melting point: 390C CO 2 absorption test of MgO and MgO +NaNO 3 during heating in CO 2 1 NaNO 3 is a strong oxidizing agent. Thus, the application of the NaNO 3 promoted adsorbents is limited. Objective: Replace NaNO 3 with non oxidizing molten salts such as molten carbonates. Adv. Mater. Interfaces, 2014, 1, Adjusting the composition of the salt controls the temperature at which the molten phase forms. Recently, our results indicate that the presence of Li-Na-K-CO 3 can also significantly improve the ability of MgO and MgO based double salt to capture CO 2.

18 LiNaK-CO 3 Promoted MgO Absorbents TGA results of 80% MgO + 20% Li-Na-K-CO3 (350C calcined ) 100% CO 2, 25C-425C (5C/min) MgO+CO 2 (g) MgCO 3 Cyclic test: 360C (99 min in CO 2 ) 390C (81min in N 2 ) TG/% Temp. / C Mass Change: % Time /min [1] [1] Blue: 80% MgO, 20% LiNaKCO3 Green: MgO Absorption rate: 4.5 mmol/g/min was observed at C High stable cyclic capacity (13mmol/g) was achieved (~50 wt.%)

19 LiNaK-CO3 Promoted MgO-Na 2 CO 3 Double Salt Absorbents Created with NETZSCH Proteus software MgO+Na 2 CO 3 +CO 2 (g) MgNa 2 (CO 3 ) 2 TGA cyclic test results of 44% MgO, 44% Na 2 CO 3,12% LiNaK-CO 3 400C 100% PSW Red: 390C CO2,450C air Green: 360C CO2, 400C air TG/% Temp. / C TG /% Temp. / C 125 Mass Change: % [1] Mass Change: % [1] [2] [1] [1] Time /min Main :59 User: TGA Time /min Main :20 User: TGA [2] Created with NETZSCH Proteus software Molten carbonate promoted MgO-Na 2 CO 3 absorbents have CO 2 capacity of mmol/g (~15-20 wt.%). Regeneration can be easily carried out both through PSA and TSA.

20 Triple phase boundaries (TPB) are required for the molten salt promoted CO 2 +MgO reaction process 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 (A) (B)

21 LiNaK-CO 3 Promoted MgO and MgO Based Double Salt Absorbents LiNaK-CO 3 promoted Absorb(Wt. %) Operation temperature range, C Cycling capacity Operation condition Absorption Desorption mmol/g 20%LiNaK- CO C-390C combined swing %LiNaK- CO 44% Na 2 CO C pressure swing C temperature swing C combined swing 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.

22 CO 2 -Sorption Enhanced Methanation (Methanation Reaction + CO 2 Capture) 15% H 2 O, 40% H 2, 32% CO, 3.0% CH 4, 22% CO 2,3.0% N 2 1 bar; C; 1800 hr -1 reaction, 46 hr -1 sorption CO methanation: CO + 3 H 2 CH 4 + H 2O Methanation-only 15% Ni/MgAl 2 O 4 catalyst WGS: CO + H + 2O CO2 H 2 Methanation + CO 2 Capture 22 CO 2 -sorption enables enhanced selectivity to methane CO 2 sorbent capacity = 24 wt.%

23 CO 2 -Sorption Enhanced Methanation Pressure Effect 15% H 2 O, 40% H 2, 32% CO, 3.0% CH 4, 22% CO 2, 3.0% N C; 1800 hr -1 reaction, 46 hr -1 sorption Pressurized operation enhances CO 2 sorption Enabling 99% CH 4 Yield (gas phase) 23

24 CO 2 Sorption Conclusions NaNO 3 and molten carbonate can promote MgO and MgObased double salts to capture CO 2 with a high cycling capacity. Stable cycling CO 2 capacity up to 13mmol/g was achieved MgO and MgO based double salts can capture CO 2 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

25 Process Demonstration Multi-Unit Cleanup Process Train Demonstrated with water quenched coal-derived syngas obtained from the Western Research Institute 25

26 Process Flow Diagram Disposable sorbents for HCl and trace contaminant removal (2 separate beds) 2 Regenerable bulk ZnO beds + ZnO sulfur polishing unit Wyoming coal synthesis gas R1 HCL Removal Na 2 CO C R2A Desulfurization ZnO 450 C R2B Desulfurization ZnO 450 C Slip Stream Clean, warm, CH 4 -rich, CO 2 -lean syngas R3 Trace Metal Polish ZnO & CuNi/C 300 C R4 Tar Reformer Ir/MgAl 2 O C R5A CO 2 Capture & SNG Double Salt & Ni/MgAl 2 O C/450 C Tar reformer tars present because low T gasifier operation Clean, warm H 2 -rich syngas R6 LT-WGS CuZn-Al 2 O C R5B CO 2 Capture & SNG Double Salt & Ni/MgAl 2 O C/450 C 2 Regenerable CO 2 -sorption enhanced methanation units demonstrate w/ slip stream 26 WGS bed used for warm cleanup demonstration

27 Demonstration Results for Warm Cleanup WRI Gasifier-Derived Syngas ~1 SLPM Raw Syngas Feed (Water Quenched) 100 WGS Catalyst Performance 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) CO Conversion (%) CO Conversion Equilibrium CO Conversion Time-On-Stream (hrs) 27 CO Conversion (%) WGS Catalyst Performance (2013) Time-On-Stream (hrs) Considerable progress achieved from 2013 demonstration where significant deactivation occurred!

28 Demonstration Results CO 2 -Sorption Enhanced Methanation Reactors (2 Beds) Alternating/Regenerable Beds: A B A CO Conversion Sorption 350 o C Desorption 450 o C (N 2 ) CO 2 Sorbent Not Yet Activated CH 4 Selectivity Deactviation 60 % CO Conv (%) CH4 Sel (%) CO2 Sel (%) EQ CO2 Sel (%) EQ CH4 Sel (%) CO 2 Selectivity Time-On-Stream (hrs) Mixed results CO 2 sorption and methanation reaction occurring simultaneously prior to gradual deactivation of sorbent

29 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 CO 2 capture technology in conjunction with syngas cleanup Absorption of CO 2 by Na 2 CO 3 -MgO (forming double salt) is facilitated by molten salt A regenerable CO 2 capacity of ~20 wt.% is achieved with double salts using temperature swing (350 o C sorption/450 o C desorption) Dissolution of some MgO into the molten salt, followed by reaction of CO 2 at the triple phase boundary, provides basis for CO 2 capture process Combining CO 2 capture with methanation in a single bed was demonstrated to yield 99+% (10 bar, 350 o C) Multi-contaminant removal process train was demonstrated for 100 hours with ~ 1 SLPM Wyoming coal-derived syngas (WRI-provided) 29

30 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

31 31 Questions?

32 32 Extra Slides

33 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 H 2 purity not required to be high (no CO 2 capture) IGCC: sulfur concentration <20ppmv Greater energy efficiency, but higher CAPEX Hot gas scrubber required to remove particulates, chlorides 33

34 28.8 wt.% Ni-Cu/SBA-16 Functions Effectively as PH 3 and AsH 3 Absorbent Ni + AsH 3 =NiAs H 2 (127 wt%) Kp = 8.85 x Cu + AsH 3 = Cu 3 As + 1.5H 2 (39 wt%) Kp = 2.45 x 10 7 Ni + 2PH 3 = NiP H 2 (106 wt%) Kp = 1.01 x Cu + PH 3 = Cu 3 P H 2 (16 wt%) Kp = 5.6 x

35 H 2 S Removal (in Syngas) with ZnO COS Sorption 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 Al 2 O 3 x Sulfur Concentration, PPM COS dry 200 C COS dry 450 C COS wet 450 C H2S, COS wet 450 C Theoretical Maximum Sulfur fed, g S/g adsorbent Wet COS sorption favorable (COS+H 2 O H 2 S + CO 2 ) 35 Dry COS sorption still feasible at warm temperatures (e.g., 450 o C vs. 200 o C)

36 Characterization of Fresh and Regenerated ZnO Sorbents Regenerated ZnO ZnS Fresh ZnO Crystal Size by XRD, nm BET surface area, m 2 /g Pore volume, cm 3 /g Pore size, nm Fresh ZnO Regenerated ZnO

37 Surface Adsorption Can Exceed Bulk Thermodynamic Performance Ni + H 2 S Ni 2 S (surface) + H 2 H 2 S (ppm) in H 2 <5 ppb H 2 S in feed gas can be achieved at 350 o C and lower J.G. McCarty and H. Wise, J. Chem. Phys. 1980, 72(12), Challenge: utilize this concept while developing a regenerable adsorbent T, o C 227 Initial work was carried out for sulfur removal, later extended to other impurities 37

38 NaNO 3 is found to have a key impact on the performance * 3.4 mmol CO 2 /g * Zhang, K., Li, X. S., Duan, Y., Singh, P., King, D. L. and Li, L. (2013). Roles of double salt formation and NaNO 3 in Na 2 CO 3 -promoted MgO sorbent for intermediate temperature CO 2 removal. Int. J. Greenhouse. Gas Control 12:

39 Comparative thermodynamics shows increased stability of double salt The formation for Na 2 Mg(CO 3 ) 2 shifts the equilibrium towards higher temperature and enables regenerable CO 2 uptake at 400 C through PSA

40 TGA measurement of CO 2 uptake over MgO + NaNO 3 shows initiation of absorption on melting of nitrate salt* CO 2 absorption on MgO+NaNO 3 confirms MgCO 3 formation contributes to the high uptake observed in 1 st peak during ramping CO 2. The absorption stops at C and desorption starts at higher temperature. This indicates the loss of high initial peak is due to high absorption temperature. * CO 2 uptake on alkaline earth oxides catalyzed by nitrate salt is described in one of our manuscripts in preparation

41 LiNaK-CO3 Promoted Dolomite Absorbents MgO+CaCO 3 +CO 2 (g) MgCa(CO 3 ) 2 TGA cyclic results of 80% dolomite, 20% LiNaK-CO 3 360C 100% CO o C 100% N 2, CO2 capacity, wt% Dolomite Dolomite + 20% Li- Na-K CO Cycles Wt % Time, hrs CO 2 capacity of molten carbonate promoted dolomite absorbents increased from 5% to 21% after 24 carbonationdecomposition cycles, indicating a self-activating process

42 In-situ XRD data of Na-Mg double salt absorbent with NaNO 3 MgCO 3 is formed during 1 st cycle, and double salt during the 1 st and subsequent cycles. NaNO 3 is not observed as it melts under absorption condition and becomes undectable by X-ray.

43 What is the role of NaNO 3 in facilitating CO 2 Capture by MgO and MgO-Based Double Salts? MgO (and Na 2 CO 3 ) are partially dissolved in molten NaNO 3 and dissociate into their ionic components Dissolved O 2- ions react with CO 2 to form CO 3 2- MgCO 3 or Na 2 Mg(CO 3 ) 2 precipitate when solubility limit is reached CO 2 is likely first adsorbed on MgO and interacts with O 2- at the triple phase boundary

44 Proposed CO 2 capture facilitated by nitrate salt at triple phase boundary Phase Transfer Catalysis

45 Cycling 40% H 2, 32% CO, 3.0% CH 4, 22% CO 2,3.0% N 2, (15% H 2 O) 1 bar; C; 1800 hr -1 reaction, 46 hr -1 sorption Temperature Sensitivity CO 2 -sorption enhanced methanation 360C CO 2 -desorption 450C (1 hr) 45

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