Flameless Combustion of H 2 - Enriched Fuels: a CFD Aided Experimental Investigation

Transcription

1 Flameless Combustion of H 2 - Enriched Fuels: a CFD Aided Experimental Investigation C. Galletti 1, P. Gheri 2, G. Gigliucci 2, A. Parente 1, M. Schiavetti 2, S. Soricetti 1, L. Tognotti 1 1 DICCISM, University of 2 Enel Produzione Ricerca, 29 th Meeting on Combustion - Italian Section of the Combustion Institute 1

2 H 2 -enriched fuels Can H 2 be regarded as the energy carrier for the future? production (i.e. from renewable sources) transport storage conversion safety Conversion efficient utilization economical convenience of using not pure H 2, but mixtures (i.e. syngas) 29 th Meeting on Combustion - Italian Section of the Combustion Institute 2

3 H 2 -enriched fuels H 2 properties high laminar flame velocity S L (e.g. 2.7 m/s for H 2 /air) high adiabatic temperature T ad (2380 K) wide flammability range (4-75%) flashback in premixed flames flame stabilization with high fuel injection velocity long flames large (thermal) NOx emissions materials Conventional burners are unsuited! Alternative technologies high heating value (~30,000 kcal/kg) high reactivity COSTAIR (Continued Staged Air Combustion) TVC (Trapped Vortex Combustion) MILD (Moderate and Intense Low oxygen Dilution) or Flameless Combustion 29 th Meeting on Combustion - Italian Section of the Combustion Institute 3

4 Mild or flameless combustion Conditions T reactants > T self-ignition large entrainment of combustion products before reaction Characteristics flame dilution T [ C] stable flame unstable flame no flame volumetric combustion no T peaks air pre-heating does not affect NOx emissions Recirculation factor, k R HIGH EFFICIENCY with LOW NOx and SOOT emissions 29 th Meeting on Combustion - Italian Section of the Combustion Institute 4

5 Investigation of 2 burners placed at ENEL Ricerca laboratories of Livorno FLOX burner SOLO Stirling burner Flameless combustion regime when fed with NG How do the burners behave when H 2 is added to the fuel? 29 th Meeting on Combustion - Italian Section of the Combustion Institute 5

6 model validation CFD measurements understanding planning experimental campaign EXPERIMENTS predictions burner design improvements measurements CFD simulation of flameless combustion is not easy: Combustion models -Da1 mixed is burnt approaches are unsuited Turbulence models Kinetics model validation! 29 th Meeting on Combustion - Italian Section of the Combustion Institute 6

7 SOLO Stirling burner Heat /power micro cogeneration CHP Stirling Engine Fuel: NG and NG/ H 2 Measurements T (flue gases, Helium/flue gases heat exchanger) Flue gas composition (NO, O 2, CO 2, CO) Flow rates 29 th Meeting on Combustion - Italian Section of the Combustion Institute 7

8 Experimental campaign Enel laboratories of Livorno run 1 P He [bar] 40 Q NG [kg/s] Q H2 [kg/s] 0 XCO [ppmv] NO [ppmv] O2 [%vol] CO2 [%vol] 7.1 T flue gas [ C] 19.7 Load [kw] 10.8 Load H 2 [%] E E E E E E NOx slightly decrease when increasing H 2 concentration E E E th Meeting on Combustion - Italian Section of the Combustion Institute 8

9 CFD simulations CFX 5.7 by Ansys Inc. (DICCISM) The burner shows: Complex geometry: details CANNOT be neglected (e.g. inlet nozzles) Complex operation: heat exchange (e.g. finned heat exchanger with Helium) 6 fuel and 6 air injection nozzles 60 angular sector shell Helium heat exchanger flame tube outlet top air inlet axis air injection Helium heat exchanger fuel injection flame tube fuel inlet 29 th Meeting on Combustion - Italian Section of the Combustion Institute 9

10 Computational domain and mesh Hybrid mesh (unstructured+structured) ~250,000 cells fuel 29 th Meeting on Combustion - Italian Section of the Combustion Institute 10 air

11 Physical model Turbulence model k- Combustion model: Eddy Dissipation/Finite Rate model for CH 4 /H 2 oxidation Finite Rate model for NO formation Kinetics CH 4 /H 2 Oxidation: Jones and Lindstedt (1988) + Westbrook and Dryer (1981) NO formation: thermal (Zeldovich) and prompt (de Soete, 1974) Radiation Model Discrete Transfer (P1) Spectral Model Gray (WSGG) 29 th Meeting on Combustion - Italian Section of the Combustion Institute 11

12 Boundary conditions (some) Air Inlet: shell T calculated from pre-heater efficiency (SUBROUTINE) outlet top axis flame tube Helium heat exchanger air inlet fuel inlet Helium heat exchanger: negative heat source (SUBROUTINE) 29 th Meeting on Combustion - Italian Section of the Combustion Institute 12

13 Flow field 29 th Meeting on Combustion - Italian Section of the Combustion Institute 13

14 3-D D flame T > 1600 C 29 th Meeting on Combustion - Italian Section of the Combustion Institute 14

15 Radial profiles of temperature for increasing H 2 content 29 th Meeting on Combustion - Italian Section of the Combustion Institute 15

16 Temperature distribution 100%CH 4, P He = 120 bar Reduction of the region at T promoting thermal NOx formation with increasing H 2 content. Load H 2 [%] T>1600 C /Vtot [%] ,9%CH 4 14,1%H 2, P He = 120 bar NO calculated [ppmv] NO measured [ppmv] Internal recirculation [%] ,5%CH 4 19,5%H 2, P He = 120 bar th Meeting on Combustion - Italian Section of the Combustion Institute 16

17 Measuredeasured vs. predicted NOx Satisfactory agreement for large loads (for which the burner is designed) NOx are larger than those normally observed with flameless combustion. 29 th Meeting on Combustion - Italian Section of the Combustion Institute 17

18 FLOX burner self-recuperative burner (13 kw) radiation: Inconel shield and water jacket in the experimental rig coaxially to the burner measurements: T (flue gases, radiant tube and Inconel shield) Flue gas composition Flow rates water jacket combustion chamber Inconel shield radiant tube pre-heater 29 th Meeting on Combustion - Italian Section of the Combustion Institute 18

19 Griglia Computational di calcolo domain and mesh 3 windows which allow recirculation of exhaust gases 120 angular sector 1 fluid domain and 2 solid domains (flame and radiant tubes) Hybrid mesh (unstructured+structured) ~400,000 cells radiant tube flame tube 29 th Meeting on Combustion - Italian Section of the Combustion Institute 19

20 Physical model Turbulence model k- Combustion model: Eddy Dissipation/Finite Rate model for CH 4 /H 2 oxidation Finite Rate model for NO formation Kinetics CH 4 /H 2 Oxidation: Jones and Lindstedt (1988) + Westbrook and Dryer (1981) NO formation: thermal (Zeldovich) and prompt (de Soete, 1974) Radiation Model Discrete Transfer (P1) Spectral Model Gray (WSGG) 29 th Meeting on Combustion - Italian Section of the Combustion Institute 20

21 Boundary condition at the radiant Università tube SUBROUTINE to model the heat losses through radiation towards the surrounding Iterative solution water jacket T 3, 3 Q 12' 1 A A 2 T T T ' T Q A ' Q A Q k' A 2' 4 insulation Inconel shield insulation radiant tube T 2, 2 T 1, 1 29 th Meeting on Combustion - Italian Section of the Combustion Institute 21 Q2 3 Q1 2 combustion chamber Q2i 2e

22 Model validation Temperature profiles along the radiant tube This agreement was achieved only with a good reproduction of all burner details (no with 2D simulations) 29 th Meeting on Combustion - Italian Section of the Combustion Institute 22

23 with H 2 enriched fuels H 0 % (CH 100 %) Tmax = 2050K NO = 26 ppm H 5 % by wt. (29.6% by vol., 11.2% thermal load) One solution: to promote recirculation of combustion products by reducing the air inlet injection area. Tmax = 2720K A air R = 102% NO = 64 ppm 0.5A air R = 143% NO = 13 ppm 29 th Meeting on Combustion - Italian Section of the Combustion Institute 23

24 (1) CFD aided experiments have been performed on two burners operating in flameless regime fed with NG/H 2 mixtures. CFD issues: complex geometry and mesh; complex operation (internal heat exchanges) subroutines; combustion, turbulence and kinetics play a significant role; model validation is needed experiments and CFD should be planned jointly. 29 th Meeting on Combustion - Italian Section of the Combustion Institute 24

25 (2) For the SOLO Stirling burner a slight decrease of NOx with increasing H 2 content was observed experimentally and explained through CFD. For FLOX burner preliminary results have indicated that with H 2 enriched fuels, a larger recirculation of combustion products is needed to operate in flameless regime. Experimental work in progress on the FLOX burner Modelling work in progress: kinetics. 29 th Meeting on Combustion - Italian Section of the Combustion Institute 25