Experimental study assessment of mitigation of carbon formation on Ni/YSZ and Ni/CGO SOFC anodes operating on gasification syngas and tars

Transcription

1 Experimental study assessment of mitigation of carbon formation on Ni/YSZ and Ni/CGO SOFC anodes operating on gasification syngas and tars Clean Coal Technologies Conference May 2009 Joshua Mermelstein 1, Marcos Millan 1, Nigel Brandon 2 Imperial College of London 1 Dept. of Chemical Engineering 2 Dept of Earth Sciences and Engineering Page 1

2 Presentation Agenda Introduction to the project Tar analogs for biomass gasification Solid Oxide Fuel Cells (SOFCs) Experimental Conditions Results Conclusions Page 2

3 Introduction to the Project Key Questions - How do tars interact with the Nickel catalysts found in SOFCs - What is the interaction of tars with SOFC anodes and the mechanism for carbon formation from biomass gasification tars? - How do tars interact and affect the SOFC operation and its performance under conditions of a synthetically generated syngas comprising of H2, CO, CO2, CH4, H2O? - Experimentally, what is an appropriate model tar to describe these affects and how does it compare with other model tars and an actual biomass tar? Page 3

4 Introduction to the Project Why Combined Biomass Gasification and Fuel Cell Technology? - Biomass fuels have a substantial potential as a carbon neutral renewable energy source that can be used for the production of electric power, steam, and heat. - Biomass materials have the most favorable thermo-chemical conversion to renewable energy. - Biomass gasification emits low levels of sulfur and nitrogen compounds compared with coal gasification Page 4

5 Biomass Gasification Syngas Properties Table 2: Comparison of measured particulate and tar levels from different biomass gasifier designs [2] Fixed Bed Gasifier Type Low Particulate Loading (g/nm 3 ) High Representative Range Min. Tar Loading (g/nm 3 ) Max. Representative Range Downdraft Updraft Moving Bed Fluidized Bed < Circulating Fluidized Bed < Page 5

6 Tar Analog for Experimental Studies Amount and composition of the tars is dependant on the properties of the biomass, pressure, temperature, and residence time of the gasification process, and the type of gasifier. Compound Table 5: Mixture to represent tars as suggested by Singh et al. Class Composition (wt. %) Toluene or Benzene: representing all one-ring compounds 3 65 Naphthalene: representing 2-ring compounds 4 20 Phenol: representing phenolic and other heterocyclic compounds 2 10 Pyrene: representing 3-ring and higher compounds 5 5 Page 6

7 Solid Oxide Fuel Cells Definition - A fuel cell is an electrochemical energy conversion device that converts chemical energy from a fuel directly into electricity and heat Technology - SOFCs comprised of all solid state materials - Ceramic make up allows the FC to operate at temperatures up to 1000 C - Nickel catalysts in the anode allows for internal reforming of hydrocarbons ***Fuel flexibility*** Solid Oxide Fuel Cell (SOFC) Anode Characteristics (NiO-YSZ, NiO-CGO) - Yittria-stabilized zirconia (YSZ) ceramic support added to maintain stability of conductive Ni particles - Ceramic support acts as a partial ionic conductor Anode H 2 + O 2- H 2 O +2e - O 2- ions through electrolyte Cathode ½O 2 + 2e - O 2- Load Page 7 H 2 + ½O 2 H 2 O

8 Experimental Set-up Mass Flow Controllers H 2 CO CO 2 NC RE CE WE SE NC Autolab PGSTAT302 Exhaust CH 4 Represents Heated Lines N 2 Operating Conditions 765 C Humidifier Syringe Pump 15% H 2 balance N 2 Up to 7.5% humidified steam 15 g/m 3 tars Page 8

9 Results - Is benzene an appropriate tar analog? Cell Performance (dry conditions) - Sharply reduced irreversible losses in cell kinetics - No clear distinction between different tar compounds with load of < 75 ma/cm 2 Impedance Characteristics (dry conditions) - C 6 H 6 and C 7 H 8 ~ the same spectra, low freq. response of toluene is slightly higher - Tar Mix has ~20% smaller resistance Potential (V) No Tars Benzene Toluene Tar Mix Z" (Ω cm²) Benzene Toluene Tar Mix Current Density (ma/cm 2 ) Z' (Ω cm²) Page 9

10 Results - Current Effects on SOFC Impedance Characteristics - As the current drawn from the SOFC increases, the flux of oxygen ions across the electrolyte increases, allowing for partial oxidation of carbon. C x H y + yo 2- _ y H 2 O + xco 2 +2ye Increase in current density shows reduced degradation indicating a reduction of carbon formation * Note: Impedance results taken at OCV for data comparison ma/cm 2 5 ma/cm 2 10 ma/cm 2 20 ma/cm ma/cm ma/cm Z" (Ω cm²) Z' (Ω cm²) R p (ohm cm 2 ) Current Density (ma/cm 2 ) Page 10

11 Results - Steam Effects on Carbon Deposition NiO/YSZ Anode Impedance - S/C > 1 inhibits carbon formation by steam reforming however cell damage still occurred with S/C <3 - S/C = 2 with a load of 50 ma did not show any significant change in the ohmic resistances of the cell. Partial oxidation removes carbon as it forms on the surface of the catalyst. C n H m m + nh2o nco + n + H Initial S/C = 1 S/C = 2 S/C = 3 S/C = ma/cm 2 Z" (Ω cm²) Page Z' (Ω cm²)

12 Results - Steam Effects on Carbon Deposition NiO/CGO Anode z"(ω cm 2 ) z"(ω cm 2 ) t = 0 min With Tar t = 30 min z'(ω cm 2 ) z"(ω cm 2 ) z'(ω cm 2 ) 7.5% steam 5.0% steam z"(ω cm 2 ) t = 0 min With Tar t = 30 min z"(ω cm 2 ) t = 0 min With Tar t = 30 min z'(ω cm 2 ) 2.5% steam 1.0% steam t = 0 min With Tar t = 30 min Change in Anode Performance (%) ma/cm ma/cm ma/cm Humidified Steam (%) - Decrease in R s at 7.5% steam associated with microstructural changes and minor carbon deposits from initial exposure - Above thermodynamic stable region of carbon formation (>2.5% steam) the electrochemical response is unaffected, performance is increased - Operation <2.5% steam led to decreased performance and carbon formation Page 12

13 Results - Internal CO 2 Reforming Temperature Programmed Oxidation - Carbon formation reduced with high CO 2 feed in dry conditions. Common peaks at 650 C and 800 C indicating amorphous and whisker type carbon respectively. - Significant reduction in carbon operating in 2.5% steam - Further increase in steam and CO 2 >10% increases endothermic reforming reactions causing a localized cooling effect. Thus allowing the Boudouard reaction to take place forming carbon from CO produced in RWGS reaction. - Cooling effect can be reduced by heat produced from exothermic oxidation reactions while operating the fuel cell under load CO 2 Fraction % CO 2 25% CO 2 10% CO 2 Carbon Deposited (mg) % steam 2.5% steam 5.0% steam % 20% 30% 40% 50% Temperature (C) Feed CO 2 Page 13

14 Results - Long Term Operation on Biomass Gasification Syngas, 100mA/cm 2, 3 hours Syngas Compositions 1) 15% H2 2) 15% H2 + 10% CO2 3) 15% H2 + 10% CO2 + 25% CO 4) 15% H2 + 10% CO2 + 25% CO + 2% CH4 Degradation - Syngas of H 2 only maintained the longest stability (~140 min) before degradation occurred. - Addition of CO 2, CO, and CH 4 incurred degradation of the anode in < 75 minutes Anode Potential (V) Time (minutes) H 2 H 2 + CO 2 H 2 + CO 2 + CO H 2 + CO 2 + CO + CH 4 - Highest amount of degradation and carbon formation occurred with typical biomass gasification syngas (4) 1) 2) 3) 4) Page 14

15 Results - Long Term Operation on Biomass Gasification Syngas, 300mA/cm 2, 3 hours Tar concentrations tested Results Syngas composition (4) (a) 15 g/m 3 (b) 5 g/m 3 (c) 2 g/m 3 - Minor dusting of carbon occurred in (a) in localized region of the anode. Performance/microstructure unaffected - No carbon present in (b) or (c). - High over-potential caused localized heating and minor degradation to cell performance Anode Potential (V) Time (minutes) 15 g/m 3 5 g/m 3 2 g/m 3 a) b) c) Page 15

16 Conclusions - Increase in steam reduced anode degradation by carbon, with Ni/CGO being more resilient to carbon deposition than Ni/YSZ. - Dry reforming of tar in high CO 2 had similar effects to inhibiting carbon deposition compared to 2.5% steam. However, high steam and CO 2 >10% led to increased carbon attributed to localized cooling such that the Boudouard reaction could take place. - Long term operation in 100 ma/cm 2 showed notable amounts of carbon for each syngas. Highest degradation occurred with the typical gasification syngas. - Long term operation of typical syngas at 300 ma/cm 2 led to insignificant carbon deposition with 15 g/m 3 tar. Carbon deposition did not occur with tars < 5 g/m 3. - Not recommended to feed > 10% CO 2 in steam >5% unless proper heat management is implemented. - The fuel cell at a minimum should be operated with an anode current density of at least 300 ma/cm 2 to inhibit carbon formation with no more than 5 g/m 3 biomass gasification tars Page 16

17 Thank you for your attention More Information: Joshua Mermelstein +44 (0) Page 17

18 Results - Current Effects on SOFC Performance - Performance curves show that kinetic effects of carbon deposition are reduced at 10 ma/cm No tar 10 ma/cm 2 0 ma/cm Potential (V) Current Density (ma/cm 2 ) Page 18

19 Results - TPO of Tars exposed to Ni-O catalysts - Benzene showed to be the most reactive depositing the highest amount of carbon - Two distinct peaks at ~600 C and ~690 C representing amorphous char and graphitic whisker type carbon respectively Benzene Toluene Tar Mix CO 2 Fraction Temperature ( o C) Page 19

20 Results - Surface microstructure of fuel cells exposed to tar compounds - Extensive damage to each cell shown from exposure to tar compounds, resulting to deactivation of the catalyst and reduced performance - Presence of pyrolytic, encapsulating and whisker carbon Figure 5 (a) Micrograph of reduced Ni-YSZ anode before exposure to tar species, x 5k resolution Figure 5 (c) Micrograph of SOFC exposed to 15 g/m 3 toluene for 30 min in 15% H 2 /N 2, x 35k resolution Page 20 Figure 5 (b) Micrograph of SOFC exposed to 15 g/m 3 benzene for 30 min in 15% H 2 /N 2, x 35k resolution Figure 5 (d) Micrograph of SOFC exposed to 15 g/m 3 tar mix for 30 min in 15% H 2 /N 2, x 36k resolution

21 Results Thermodynamic Current Effects on SOFC Impedance Characteristics - N j (mol/cm 2 s) = O 2 4 F Current Density (ma/cm 2 ) - C(s) is maximum at OCV - Carbon deposition no longer thermodynamically stable at current density of 361 ma/cm 2 (fuel utilization of 12%) Threshold Current Density - Past the threshold current density, [CO 2 ] rapidly increases, CO production decreased and H 2 consumption increases with increased H 2 O production. - Thermodynamics therefore indicate that in low steam concentrations, industrial SOFC systems running on syngas containing gasification tars (<15 g/m3) should be operated at fuel utilizations > 15% to avoid carbon deposition on the surface of the anode. Equilibrium Composition (mol/s) 1.2x x x x x x H2(g) CO(g) CO2(g) H2O(g) CH4(g) C(s) Fuel Utilization (%) Page 21

22 Table 2: Series and polarization resistance of Ni/CGO anode before, while, and after operating on 15 g/m 3 benzene model tar in 15% H 2 atmosphere in humidified steam for 30 minutes. Humidified Steam Initial Operating with Tar After tar exposure Series Resistance (ohm cm 2 ) 7.5% % % % Polarization Resistance (ohm cm 2 ) 7.5% % % % Page 22

23 t = 0 min t = 180 min -0.8 t = 0 min t = 180 min Anode Potential (V) Anode Potential (V) Current density (ma/cm 2 ) Figure 14a: Polarization curve of Ni/CGO anode operating at 765 C in a 15% H 2, 10%CO 2, 25% CO, 2% CH 4 syngas in 5% humidified steam exposed to 15 g/m 3 benzene for 180 minutes at 300 ma/cm Current density (ma/cm 2 ) Figure 14b: Polarization curve of Ni/CGO anode operating at 765 C in a 15% H 2, 10%CO 2, 25% CO, 2% CH 4 syngas in 5% humidified steam exposed to 5 g/m 3 benzene for 180 minutes at 300 ma/cm t = 0 min t = 180 min Anode Potential (V) Current density (ma/cm 2 ) Figure 14c: Polarization curve of Ni/CGO anode operating at 765 C in a 15% H 2, 10%CO 2, 25% CO, 2% CH 4 syngas in 5% humidified steam exposed to 2 g/m 3 benzene for 180 minutes at 300 ma/cm 2. Page 23