Design considerations for direct solid fuel chemical looping combustion systems

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1 Prof. Hermann Hofbauer Getreidemarkt 9/ Wien IEA GHG R&D Programme High Temperature Solid Looping Cycles Network 2nd Network Meeting, Alkmaar, September 15-17, 2010 Design considerations for direct solid fuel chemical looping combustion systems Tobias Pröll, Johannes Schmid, Christoph Pfeifer and Hermann Hofbauer

2 Outline What is chemical looping combustion (CLC)? CLC for gaseous fuels status and limitations CLC for solid fuels challenges and approaches Excursion: dual bed biomass gasification Improved dual circulating fluidized bed design Conclusion and outlook looping.at 2

3 Chemical looping combustion (CLC) N 2, (O 2 ) CO 2, (H 2 O) Cooling/ condensation CO 2 Air reactor (AR) MeO x MeO x-1 Fuel reactor (FR) H 2 O A new process for oxidizing fuels using metal oxides as oxygen carriers transporting oxygen from combustion air to fuel No mixing of combustion air and fuel, combustion products (CO 2 and H 2 O) not diluted by N 2 Air Global air/fuel ratio > 1 Fuel CLC shows unique potential for carbon capture because gas-gas separation is inherently avoided. Highly exothermal reactions in air reactor Global heat release equal to that of direct combustion looping.at 3

4 Critical issues in CLC Oxygen carrier particles thermodynamic suitability high reactivity sufficient transport capacity high mechanical stability cyclic stability of reactivity and transport capacity Reactor system excellent gas-solids contact in both reactors sufficient solids circulation rate looping.at 4

5 Dual fluidized bed systems Dual fluidized bed technologies apart from FCC Technology (Biomass) gasification Sorption enhanced reforming Carbonate looping for CO 2 capture Chemical looping Purpose of solids Heat transport, catalyst CO 2 and heat transport, catalyst CO 2 (and heat) transport Oxygen and heat transport Importance of gas solid contact Partially for tar reforming in the gas generator High in the reformer/carbonator, low in the re-calciner (heat-driven) High in the absorber/carbonator, low in the re-calciner (heat-driven) High in both reactors, no gas phase conversion without solids There is an ultimate requirement for good gas-solid contact in CLC systems looping.at 5

6 Dual circulating fluidized bed (DCFB) reactor system Global solids circulation is controlled by air reactor fluidization only (eg. air staging) exhaust AR exhaust FR Fuel reactor can be optimized towards fuel conversion Inherent stabilization of global solids hold up due to the direct hydraulic link between the reactors Low reactor volume compared to bubbling fluidized beds (i.e. low specific solids inventory) High potential for scale-up air air reactor (AR) LS LS fuel fuel reactor (FR) LS looping.at 6

7 CLC pilot plant looping.at 7

8 Status CLC for gaseuos fuels 140 kw DCFB pilot plant / NiO-based oxygen carrier: CH 4 conversion up to 99% CO 2 yield up to 96% based on total carbon in fuel good fuel conversion in spite of the limited riser heights Scale up to next size of about 10 MW possible Immidiate application for industrial steam generation Potential for power production limited because of competing gas turbine combined cycle technology Two development targets for efficient power production direct use of solid fuel steam cycle application pressurized operation for combined cycle application looping.at 8

9 CLC for solid fuels There is a huge potential in CLC for coal There is a lot of recent research on this topic There are three main challenges associated with direct solid fuel introduction: decomposition and oxidation of volatile compounds satisfactory conversion of char in the fuel reactor selective removal of fuel ash from the system Two of the challenges address fuel reactor design looping.at 9

10 Problem definition exhaust AR Volatiles form bubbles and move quickly upwards low contact time to oxygen carrier air air reactor (AR) LS LS exhaust FR steam fuel reactor (FR) solid fuel Force gas phase to pass dense regions before leaving the system Solids are well mixed immediate loss of unreacted char to air reactor Try to obtain tubular reactor like solids flow behavior or Selectively separate carbon from oxygen carrier looping.at 10

11 Theoretical consideration Depleted air global air ratio > 1 Condensation. Q AR MeO x end oxidizer CO 2 + H 2 O CO 2 Fuel reactor H 2 O (to compression) Air reactor devolatizer Solid fuel MeO (x-1) char gasifier Air Steam looping.at 11

12 Early design proposal from: Lewis W.K., Gilliland E.R. Production of pure carbon dioxide. U.S. Patent No. 2,665,972, looping.at 12

13 Early design proposal (2) If solid fuel is introduced between the "stages": solid fuel volatiles must pass through zones above char is led along with bed material through zones below Problems: too complicated geometry to be built at large scale, high pressure drop from: Lewis W.K., Gilliland E.R. Production of pure carbon dioxide. U.S. Patent No. 2,665,972, looping.at 13

14 Dual fluidized bed gasifier technology producer gas gasifier biomass steam flue gas loop seal riser additional fuel connecting chute air History: Experience at Vienna Univ. of Technol. since rd generation of 100 kw (fuel power input) DFB gasifier pilot plants currently in operation Large scale (8 MW fuel power) plant for CHP generation in successful operation since nd unit in operation since 2009 (10 MW) looping.at 14

15 Gasifier improvement efforts product gas exhaust gas product gas exhaust gas fuel gasifier ULS LLS char combustor G-ILS fuel gasifier ULS char combustor LLS steam air steam air looping.at 15

16 New dual CFB design exhaust AR exhaust FR Fuel reactor divided in vertical sections by areas of reduced cross section air reactor (AR) ULS fuel reactor (FR) LS solid fuel Fast fluidization regime in the reduced cross section, bubbling to turbulent regime in the zones between Consecutive dense zones LLS Gas-solid counter-current flow behavior air steam Particle size separation looping.at 16

17 Cold flow model testing looping.at 17

18 Local behavior pressure, qualitative Solids density (1 ) looping.at 18

19 Conclusion/Outlook Small geometrical change may be key to make dual bed systems significantly more efficient in fuel conversion Erosion seems manageable, expected velocities 1-5 m/s Significant acceleration effects around flow obstacles Ongoing: investigation at three different cold flow models with various particles Determination of suitable operating regimes Next step: kw pilot unit (gasification and possibly CLC) looping.at 19

20 Prof. Hermann Hofbauer Getreidemarkt 9/ Wien Acknowledgement: Financial support from the Austrian Government Climate and Energy Programme for the research project NE-IF "G-volution" is greatfully acknowledged. Contact: Tobias Pröll looping.at