CuO-based Al 2 O 3 -, MgAl 2 O 4 - or CeO 2 -supported oxygen carriers for chemical looping with oxygen uncoupling: synthesis and performance

Transcription

1 CuO-based Al 2 O 3 -, MgAl 2 O 4 - or CeO 2 -supported oxygen carriers for chemical looping with oxygen uncoupling: synthesis and performance Q. Imtiaz, M. Broda and C.R. Müller ETH Zurich, Laboratory of Energy Science and Engineering, Leonhardstrasse 27, 8092 Zürich, Switzerland muelchri@ethz.ch 1

2 Chemical looping with oxygen uncoupling (CLOU) Chemical looping with oxygen uncoupling (CLOU) is an emerging technology to efficiently combust complex (hydro-)carbonaceous fuels via free oxygen derived from the decomposition reaction of a metal oxide. CLOU is related to the chemical looping combustion (CLC) process. The advantage of the CLOU process over the conventional CLC process is that CLOU does not require a prior gasification reaction to convert a solid fuel, e.g. biomass or coal, into a synthesis gas. However, the CLOU process requires particular thermodynamic and kinetic characteristics of the oxygen carrier. Mattisson et. al. (2009) a were first to experimentally demonstrate the original idea proposed by Lewis and Gilliland (1951) b to combust a solid hydro-carbonaceous fuel by using gaseous oxygen derived from the decomposition reaction of CuO. a Int J Greenhouse Gas Control 2009, 3, 11. b Chem Eng Prog 1951, 47,

3 Chemical looping combustion (CLC) CH 4 Pure CO 2 for storage Condensed steam Fuel reactor: C n H 2m + (2n + m)me x O y nco 2 + mh 2 O + (2n + m) Me x O y-1 Me x O y Step 1 Me x O y-2 Step 2 Regenerator: ½O 2 + Me x O y-1 Me x O y Oxygen depleted air Air 3

4 Chemical looping with oxygen uncoupling (CLOU) Fuel reactor: Me x O y (s) Me x O y-2 (s) + O 2 (g) Pure CO 2 for storage Step 2 Solid fuel e.g. biomass, coal Condensed steam C n H 2m + (n + m/2) O 2 (g) nco 2 (g) + mh 2 O (g) Step 1 O 2 Me x O y Regenerator: Step 3 O 2 (g) + Me x O y-2 (s) Me x O y (s) Me x O y-1 Oxygen depleted air Air 4

5 Thermodynamics Co 3 O 4 CoO The transition metal oxides CuO, Mn 2 O 3 and Co 3 O 4 have suitable equilibrium partial pressures of gas-phase oxygen at temperatures relevant for CLOU. P O Mn 2 O 3 Mn 3 O 4 CuO, Co 3 O 4 and Mn 2 O 3 start to decompose at temperatures above 800 C, 675 C and 650 C, respectively Cu 2 O Temperature [ C] CuO Partial pressure of O 2 as a function of temperature : ( ) CuO/Cu 2 O, (----) Mn 2 O 3 /Mn 3 O 4 and ( ) Co 3 O 4 /CoO. Mass ratio of free oxygen to metal oxide: Oxygen carriers Ratio of free oxygen [g O 2 / g oxide] CuO Cu 2 O Mn 2 O 3 Mn 3 O Co 3 O 4 CoO

6 Previous reports Material (weight ratio) O 2 carrying capacity [a] Experimental Reference CuO/ZrO 2 (40/60) 4.04 % Fluidized bed reactor Energy Procedia 2009, 1, 447. CuO/Al 2 O 3 (60/40) CuO/ZrO 2 (60/40) 6.1 % TGA, Fluidized bed reactor [a] O 2 carrying capacity = (g O 2 / g oxygen carrier in oxidized form) x 100 Int. J. Green Gashouse Control 2009, 3, 11. CuO/ZrO 2 (40/60) 4.04 % Fluidized bed reactor Fuel 2009, 88, 683. Ca 0.97 Mn 1 x Ti x O 3 (x = 0.1, 0.125, 0.175, and 0.25) Mn 3 O 4 /Fe 2 O 3 (80/20) Mn 3 O 4 /Fe 2 O 3 (60/40) Mn 3 O 4 /NiO (80/20) Mn 3 O 4 /SiO 2 (80/20) MnO 2 /MgO (68.3/31.7) MnO 2 /MgO/Ca(OH) 2 (66.7/28.2/5.1) Mn 3 O 4 /MgO (65.4/34.6) MnO 2 /MgO/TiO 2 (60.2/31.9/7.9) CaMn Ti O % Fluidized bed reactor CuO/Al 2 O 3 (40/60) CuO/MgAl 2 O 4 (40/60) % TGA, Fluidized bed reactor Energy Fuels 2009, 23, NA Fluidized bed reactor Energy Fuels 2009, 23, NA Fluidized bed reactor Fuel 2011, 90, 941. Int. J. Green Gashouse Control 2011, 5, % Fluidized bed reactor Energy Fuels 2011, 25, CuO/MgAl 2 O 4 (60/40) 6.1 % Fluidized bed reactor (1.5 kw th ) Natural Cu ores (5.8, 63.3 and 87.3 % CuO) Int. J. Green Gashouse Control 2012, 6, 189. NA Fluidized bed reactor Energy Fuels 2012, 26, st August 2012 Laboratory of Energy Science and Engineering 6

7 CuO-based oxygen carriers CuO Cu 2 O is a promising oxygen carrier system for the CLOU process, because it has: A high oxygen uncoupling capacity (~ 10 wt. %). A high equilibrium partial pressure of oxygen at typical operating temperatures. Fast kinetics for the decomposition reaction. Low tendency for carbon deposition. CuO-based oxygen carriers are prone to thermal sintering and agglomeration due to the low Tammann temperatures of CuO and Cu 2 O of, respectively, 558 C and 481 C. a a Oil Gas Sci Technol 2011, 66,

8 Characterization and testing A co-precipitation technique was used to synthesize Cu-rich (80 wt. %), Al 2 O 3 -, CeO 2 - or MgAl 2 O 4 -stabilized oxygen carriers. a-b Calcined oxygen carriers were characterized using: Powder X-ray diffraction (XRD) Temperature programmed decomposition (TPD) N 2 adsorption analysis (BET) Breaking force measurements Scanning electron microscopy (SEM) The performance of the oxygen carriers synthesized was evaluated in a fluidized bed reactor. a Environ. Sci. Technol. 2012, 46, b ChemSusChem 2012, 5,

9 Synthesis protocol 14 ph of the nitrate solution Titration at a fixed ph Titration Volume of NaOH solution added [ml] Titration curves Aging for 150 min. Calcination at 1000 C for 2 h Drying at 100 C for 24 h 9

10 Experimental set-up supply of feed gases solenoid switch box pressure regulators flowmeters PG PG PG solenoid valves fluidized bed thermocouple sampling probe electrical furnace gas sampling and analysis pump CaCl 2 drying tube NDIR gas analyzers rotameter CH4 N 2 Air bed of Al 2O 3 + distributor oxygen carrier (1 g) Paramagnetic analyser The cyclic CLC and CLOU activity of the oxygen carriers was assessed in a fluidized bed reactor. The fluidized bed was made of quartz glass (i.d mm, length 460 mm) and heated in a tubular furnace. The temperature of the fluidized bed was controlled via a N-type thermocouple. 10

11 X-ray diffraction analysis Intensity [a. u.] (a) (b) (c) CuO CuAl 2 O 4 CuO CeO 2 CuO MgAl 2 O 4 XRD pattern of the oxygen carriers calcined at 1000 C: (a) CuO-Al 2 O 3, (b) CuO-CeO 2 and (c) CuO-MgAl 2 O 4. In MgAl 2 O 4 - and CeO 2 -stabilized oxygen carriers the formation of mixed oxides between the active compound, CuO, and the support was not observed. CuAl 2 O 4 was identified in Al 2 O 3 -stabilized oxygen carriers. Furthermore, Al 2 O 3 was not detected in the CuO-Al 2 O 3 oxygen carrier, indicating that either all the Al 2 O 3 formed CuAl 2 O 4 or the remaining Al 2 O 3 was present in an amorphous form θ [ ] 11

12 Temperature programmed decomposition (TPD) For all oxygen carriers, the decomposition reaction started at ~ 800 C and was completed at 1030 C, 950 C and 960 C for CuO-Al 2 O 3, CuO-CeO 2 and CuO- MgAl 2 O 4, respectively. The decomposition of CuO-Al 2 O 3 occurred in two steps due to the presence of CuAl 2 O 4. In the first step ( C) bulk CuO decomposed to Cu 2 O, whereas the decomposition of CuAl 2 O 4 occurred in the temperature range from C. Normalized mass % 2CuAl 2 O 4 2CuAlO 2 + Al 2 O 3 + ½O Temperature [ C] 2CuO Cu 2 O + ½O 2 80 % TPD profiles of the synthesized oxygen carriers: ( ) CuO-Al 2 O 3, ( ) CuO-MgAl 2 O 4, and ( ) CuO-CeO 2. 12

13 Mechanical and structural properties CuO content, BET surface area, BJH pore volume and crushing strength of the calcined oxygen carriers. Material CuO content [%] BET surface area [m 2 /g] BJH pore volume [cm 3 /g] Crushing strength [N] CuO-Al 2 O CuO-MgAl 2 O CuO-CeO <0.5 < The high calcination temperature of 1000 C resulted in low surface areas and pore volumes irrespective of the support material used. (a) 2 μm (b) 2 μm (c) 1 μm The morphology of the oxygen carriers was influenced by the support material. Scanning electron microscope (SEM) images of the calcined oxygen carriers: (a) CuO-CeO 2, (b) CuO-Al 2 O 3 and (c) CuO-MgAl 2 O 4. 13

14 CLC activity of the synthesized oxygen carriers A mixture of 10 vol. % CH 4 and 90 vol. % N 2 was used as the reducing gas and air for reoxidation. The oxygen carrying capacity of CuO-CeO 2 decreased rapidly with cycle number due to agglomeration. Cyclic redox performance at 900 C : ( ) CuO-Al 2 O 3, ( ) CuO-MgAl 2 O 4, and ( ) CuO-CeO 2. The dashed line corresponds to the theoretical amount of CO 2 formed assuming a CuO content of 80.0 wt. %. For Al 2 O 3 - or MgAl 2 O 4 stabilized oxygen carriers the amount of CO 2 produced was stable and close to the theoretical value. 14

15 CLOU activity of the synthesized oxygen carriers Cyclic oxygen uncoupling capacity studied at 950 C : ( ) CuO-Al 2 O 3, ( ) CuO-MgAl 2 O 4, and ( ) CuO-CeO 2. The dashed line corresponds to the theoretical amount of oxygen released assuming a CuO content of 80.0 wt. %. Decomposition was performed in N 2 and a mixture of 10.5 vol. % O 2 and 89.5 vol. % N 2 was used for re-oxidation. The oxygen uncoupling capacity depended strongly on the support used. For Al 2 O 3 -stabilized oxygen carrier, CuAl 2 O 4 is not fully reducible. The oxygen uncoupling capacity of CuO-Al 2 O 3 decreased with cycle number owing to attrition. CuO-CeO 2 showed a gradually decreasing oxygen uncoupling capacity due to agglomeration. CuO-MgAl 2 O 4 showed a high and stable oxygen uncoupling capacity 15

16 Conclusions We synthesized Cu-rich, Al 2 O 3 -, CeO 2 or MgAl 2 O 4 -stabilized oxygen carriers using a coprecipitation technique. XRD analysis revealed that CuO did not form mixed oxides with CeO 2 or MgAl 2 O 4. However, for CuO-Al 2 O 3 the formation of CuAl 2 O 4 was observed. TPD measurements indicated a two-step decomposition pathway for Al 2 O 3 -supported oxygen carriers due to the presence of CuAl 2 O 4. The second, high temperature, decomposition step resulted in a decrease in the oxygen uncoupling capacity of Al 2 O 3 - supported oxygen carrier at the operating conditions studied here (950 C). CeO 2 -supported oxygen carriers showed a high but continuously decreasing oxygen uncoupling capacity due to agglomeration. Only MgAl 2 O 4 -stabilized oxygen carriers possessed a stable and high oxygen carrying capacity close to the theoretical value of 2.51 mmol O 2 /g oxide. 16