Changes in combustion properties of Natural gas when mixed with Hydrogen

Transcription

1 Changes in combustion properties of Natural gas when mixed with Hydrogen PARISA SAYAD, ALESSANDRO SCHÖNBORN AND JENS KLINGMANN DEPARTMENT OF ENERGY SCIENCES, LUND UNIVERSITY

2 Hydrogen-containing fules in gas turbine combustors o Fuel mixtures comprising Hydrogen and Methane as main constituents are of increasing interest to power generation using gas turbines: Hydrogen has been proposed as storage medium for intermittent renewable energy produced from wind or solar power Hydrogen introduction into existing natural gas grids has been proposed as a means of storage and distribution Using synthesis gases as part of the fuel in gas turbines

3 Hydrogen-containing fules in gas turbine combustors ohydrogen is one of the main reactive components of synthesis gas o Synthesis gas (syngas) can be obtained from renewable sources such as biomass, or from traditional fuels like coal or heavy oil. o The composition of syngas may vary according to the particular gasification process and feed-stock used

4 Hydrogen-containing fules in gas turbine combustors Vol% Indirect gasification Oxygen/steam blown wet dry wet dry CO H2O N CO H CH

5 Fuel interchangability in gas turbine combustors o Fuel handling and injection systems o Reaching the required combustion temperature o Wobbe index Normalised heating value which is used to compare the combustion energy output of different composition fuel gases in an appliance If two fuels have identical Wobbe indicies, given the same pressure and valve settings, the energy output will also be identical

6 Premixed or diffusion flames Premixed: Diffusion flames: Fuel and air is mixed prior to combustion Combustion may occur at non-stoichiometric conditions Combustion is controlled by diffusion and reaction rates (chemical kinetics). Fuel and air is mixed and combusted at the same time Combustion occurs at close to stoichiometric conditions Combustion is controlled by mixing (diffusion)

7 Emissions at different equivalence ratio Emissions Blow out CO Sweet spot NOx f =1 Lean premixed combustion can show low CO and NOx emissions at the same time Temperature or Equivalence ratio, f

8 Fuel interchangability in gas turbine combustors oflame holding and combustion stability Flashback Blowout Autoignition Dynamic instability oemissions NOX CO

9 Flame holding and operability/flashback o Flashback occurs when the flame propagates upstream from the combustion chamber into the premixing section. Boundary layer flame propagation (critical velocity gradient) Turbulent flame propagation in core flow Combustion instabilities Upstream flame propagation induced by combustion induced vortex breakdown

10 Flame holding and operability/blowout o Blowout is the process of flame extinction under lean conditions, by means of the flame leaving its stable anchored position and being drawn downstream into the combustor where it extinguishes. oit is likely to occur under part-load aconditions and during transient operation asuch as during turbine start-up.

11 Flame holding and operability/autoignition o Autoignition occurs when the residence time of the fuel-air mixture in the premixing section exceeds the critical time necessary to cause self ignition

12 Flame holding and operability/influencing parameters o Fuel properties Reactivity and chemical kinetics Transport properties o Flow-filed in the combustor Turbulence levels The combustion behavior of blended fuels, may be completely different from their separate components, and cannot be estimated by an arithmetic mean based on proportions Turbulent flame speed Fluid dynamics effects (turbulent intensity) Laminar flame speed Diffusion Reaction rates

13 Flame holding and operability/hydrogen properties o Compared to Methane, Hydrogen has: Significantly higher reactivity Higher adiabatic flame temperature at stochiometric conditions in air Higher laminar flame speed. A hydrogen flame propagates five times faster than a methane flame at atmospheric conditions Different thermal and mass diffusion (Lewis number) which affects the flame behavior in terms of resistance to stretch

14 Flame holding and operability/hydrogen properties o Compared to Methane, Hydrogen has: More prone to flashback Significantly higher reactivity Higher adiabatic flame temperature at stochiometric conditions in air Higher risk of autoignition Higher laminar flame speed. A hydrogen flame propagates five times faster than a Stable combustion at lower methane flame at atmospheric conditions equivalence ratios Different thermal and mass diffusion (Lewis number) which affects the flame behavior in terms of resistance to stretch

15 Why experimental approach? o The physics behind blowout and flashback is very complicated due to their unstable nature which is an interaction between chemical kinetics and turbulence o For this reason it is not possible to study these phenomena using CFD modeling o Conducting blowout and flashback experiments in a real facility using Hydrogen containing fuels is not a practical option

16 The atmospheric variable-swirl burner/schematic Combustion intensity: 2 MW/m 3.atm Thermal power: 17 KW Volume:.9 m 3 Air mass flow rate: 3.5 gr/sec

17 The atmospheric variable-swirl burner/layout Air.6 MPa Air, 293 K.6 MPa Strainer Mass-flow meter, pressure sensor, Thermocouple Mass-flow meter, pressure sensor, Thermocouple Air, K.1 MPa Air K.1 MPa Swirl mixer Radial flow Axial flow.1 MPa Burner Spar k plug Hot exhaust Gases.1-1 MPa Ignition circuit Exhau st gas analysi s rack Mass-flow meters, pressure sensor, Thermocouple Solenoidcontrolled shut-off valves Pressure regulators.4 MPa CH 4 CO H2

18 Measurement techniques o Velocity measurements using LDA In order to determine the swirl number for each flow condition, the axial and tangential velocity profiles were measured 1 mm above the dump plane of the combustor using LDA. o Emission measurements The CO measurements in the exhaust gas were carried out using a non-dispersive infrared (NDIR) gas analyzer (Horiba).

19 Measurement techniques o High-speed OH* chemiluminesence The occurrence of flashback and unsteady propagation of the flame in the premixing tube was recorded using high speed OH * chemiluminescence imaging. This was done using a high-speed camera (Vision Research Phantom V 611) equipped with an image intensifier (Hamamatsu C4598), a band-pass filter (Acton Research 31.5±5.75nm) and a phosphateglass lens (UV-Nikkor 15 mm, f/4.5) to photograph OH * chemiluminescence of the flame around 36 nm at high speed.

20 Experimental variables o Flow Parameters Flow-field (Swirl Number) Inlet Temperature Total Mass Flow rate o Fuel Composition Syngas (H 2 + CO + CH 4 + N 2 +CO 2 ) Hydrogen enriched methane Wet mixtures(h 2 + CH 4 + CO + H 2 O)

21 Experimental variables/ Swirl number o By adjusting the mass flow rate through the respective axial- and radial flow paths, and hence varying the ratio between tangential and axial momentum through the combustor, different flow patterns can be achieved o In order to characterize different flow cases, the swirl number is introduced Where R is the radius of the swirler, G t is the axial flux of tangential momentum and G a is the axial flux of axial momentum

22 Experimental results/ Velocity measurements Axial velocity/u bulk S=.6 S=.13 S=.24 S=.34 S=.53 S= Y [mm].7 Tangential velocity/u bulk S=.6 S=.13 S=.24 S=.34 S=.53 S= Y [mm].6.5 Swirl number Axial momentum/ Tangential momentum

23 Experimental results/ Flashback limits 1 [CH ]=., 4 S=.66 [CO] =., S=.66 [CO]=[CH 4 ], S=.66 Flashback equivalence ratio [CH 4 ]=., S= H 2 molar content [%]

24 Experimental results/ Blowout limits Blowout equivalence ratio [CH ]=., 4 S=.66 [ CO]=., S=.66 [CO]=[CH ], S=.66 4 [CH ]=., 4 S= H 2 molar content [%]

25 Experimental results/ Stability range 1 [CH ]=., 4 S=.66 [CO] =., S=.66 [CO]=[CH 4 ], S=.66.8 [CH 4 ]=., S=.53 Stability range H 2 molar content [%]

26 t =.888 ms Z [m m] Z [mm ] t =.444 ms t = ms t = 2.22 ms Z [mm] Z [mm] t = ms t = 3.18 ms t = ms Z [mm] Y [mm ] -2 2 Y [m m] t = ms t = ms t = ms t = 1.36 ms t = ms t = ms t = ms -2 2 Y [mm] -2 2 Y [mm] -2 2 Y [m m] Y [mm] t = 2.72 ms -2 t = ms t =. ms -2 Z [mm] Flashback due to combustion induced vortex breakdown t =. ms 6 Flashback in the boundary layer Experimental results/ Flashback visualisation

27 Experimental results/ Autoignition t =.148 ms Z [mm] Z [mm] t =.74 ms t =.296 ms t =.37 ms Z [mm] Z [mm] t =.222 ms t =.518 ms t =.592 ms Z [mm] Y [mm] -2 2 Y [mm] t = ms t = 1.48 ms t = ms t = 2.72 ms t = ms t = ms t = 2.96 ms -2 2 Y [mm] -2 2 Y [mm] -2 2 Y [mm] Y [mm] t =.888 ms -2 t =.444 ms t =.592 ms -2 Z [mm] Flashback and blowout caused by auto-ignition in the premixing tube t =. ms 6

28 Experimental results/ Autoignition oestimation of the bulk residence time in the premixer in combination with gas-phase chemical kinetic modeling (Chemkin) suggested that the residence time of the reactants in the premixer (.1 s) was significantly shorter than the autoignition delay time calculated from chemical kinetic modeling under these conditions (> 1.5 s).

29 Experimental results/ Autoignition orecirculation zones can significantly increase the effective residence time of the reactants, and thereby assist autoignition. o Surface reactions on the walls of premixing tubes can significantly reduce autoignition delays below what is predicted by gas phase kinetic modeling.

30 Characterization of fuels