The effect of hydrogen containing fuel blends upon flashback in swirl burners. Nick Syred Cardiff School of Engineering Wales, U.K.

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1 The effect of hydrogen containing fuel blends upon flashback in swirl burners. Nick Syred Cardiff School of Engineering Wales, U.K.

2 Objectives Investigate flashback behaviour in swirl combustors representative of industrial practice, with an emphasis on alternative fuel blends, especially those containing significant quantities of hydrogen 5 different fuel blends investigated with 3 different swirl numbers Correlations sort with existing data from the era of town gas before advent of large supplies of natural gas

3 The Market Many different sorts of process gas produced as by-products of many industrial process: Carbon Black waste gas Coke Oven Gas BOS gas (steel works) Blast Furnace gas Refinery gases including gases with very high hydrogen levels Heating values vary considerable from very low (~ 1.5 MJ/m^3) to that of Natural gas (~35 MJ/m^3) or more. Hence substantial variation in air fuel ratio from 0.5:1 (Carbon Black waste gas) to ~10:1 for natural gas occur (all by volume). Hence quite different burner set ups are needed, such as mode of fuel entry. Premixing of fuel and air is important (or in reality often partial premixing), especially for gas turbines, if low NOx is to be achieved and this can give rise to problems of flashback when high turndowns are needed or if fuel blends with significant hydrogen content are used

4 Why swirl combustors? Excluding reciprocating engines at least 70 to 80% of fuels pass through a combustor with flame stabilization and/or mixing achieved by some element of swirl, cyclonic or vortex flow: Coal fired boilers-front wall fired, cyclonic, tangential fired Virtually all gas turbine combustors Many combustors on industrial furnaces Commonly used with poor quality, low calorific value fuels Even with reciprocating engines swirl is widely used to enhance mixing

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6 Designs of swirl burner for Efficient combustion of poor quality waste gases from Carbon black plants Heating values range from 1.5 to 2 MJ/Nm^3 (Natural gas 35 MJ/m^3) Fuel components: ~5% H 2 : ~5% CO 50% steam, temp. ~ 190 o C Rest N 2, traces hydrocarbons Fuel/air premixed, flashback can be problem as heating Values >2MJ/m^3

7 Above large power station Residual Fuel oil Flame, left schematic of large coal fired swirl burner

8 Detail of large swirl combustor for 230Mw power plant, natural gas fired, gas oil start up-ansaldo manufacture- 8 combustors used

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11 Various types of vaned swirlers widely used in GT combustors, burners for cement plant, systems where very compact swirlers are needed. Note use of curved vanes to improve efficiency and reduce separation losses. Swirl also used in liquid fuel atomisers

12 Typical flashback damage in an Industrial burner

13 Fuel Name %CH4 %H2 %CO %N2 Pure Methane Pure Hydrogen %H %H Coke Oven Gas Table 2: Fuels Blends and their composition

14 Swirl Burner Configuration Fig.1. Actual swirl burner all parts Fig. 2. Generic swirl burner diagram Parameter known as the Swirl Number, S, generally used for characterisation-can be defined from geometry-ratio of Axial flux of angular momentum to axial flux of axial momentum, normalised by the exit radius, defined at burner exhaust plane Generally as low a swirl number as possible commensurate with forming a stable central recirculation zone and giving the desired mixing rate is desired. Gives lowest pressure drop as increases in S cause in pressure drop

15 Detail of Swirl Combustor Fig.3. Swirl Burner Inserts-Left hand, 4 inlets S I =1.47 Right hand, 9 inlets, S I =1.04. Fig. 4. Swirl Burner II Assembly, 9 inlets, S II =1.04.

16 Swirl Burner detail for S=1.46 Swirl Burner name I II III Geometrical swirl number Exhaust Sleeve NO Yes Yes Table 1: Swirl Burners and their specifications

17 Two different modes of flashback found depending on Swirl number and exhaust configuration. The higher swirl number gives worse flashback except for hydrogen doped fuels (>60% H 2 ) and equivalence ratios > 0.6 Figure 6a Photo of flame surrounding central fuel injector at S I =1.47, just before radial flashback Figure 6b Photo of flame just before flashback through outer wall boundary layer, S II =1.04

18 a) S I =1.47 b) S II =1.04 Figure 3. Flashback Limits of the Generic Swirl Burners with three different Swirl Numbers for 5 different fuels. Note: The area under the curve for each fuel is the flashback region, here stable combustion may occur undesirably inside the burner. c) S III =0.8

19 Critical Boundary Velocity gradient used to define flashback from Lewis & von Elbe G f =4 *V/Π*r^3 (V=vol. flowrate, r exhaust radius) Derived from simple laminar flow analysis of pipe flow, but despite limitations very useful for data comparison-large literature from pre natural gas era With methane flashback significantly reduced with use of swirl burner S=0.8 Remember higher pressure drop with swirl combustor than laminar flames!

20 Comparison Flashback Limits H2 for lewis and von Elbe and cardiff S= Lewis von Elbe Cardiff S=0.8 H2 E quiv alence R a tio Critical velocity gradient Swirl Burner S=0.8 not doing so well with H 2 for weak equivalence ratios laminar results better-opposite for rich combustion. Interest is of course in weak combustion to get reduced average flame temperature for low NOx Work needed to find out ways of improving weak flashback limit.

21 Methane Blow Off and Hydrogen and COG Flashback Comparison S= Total M as s Flow Lean Blow Off methane FlashBack hydrogen Flashback COG Equivalence ratio Dual Fuelling Problems. What happens when you want to switch form natural Gas (i.e. methane say) to COG or indeed pure H 2? Difficult with same combustor even for COG as can see than COG and H2 flashback limits beyond the lean Blow off limit for methane. Hence in practice manufacturers resort to partial Premixing with high hydrogen content fuels

22 Conclusions Flashback behaviour of 5 different fuel gases has been investigated as a function of swirl number and exhaust geometry Low swirl numbers give best flashback resistance with methane fuels up to 30% hydrogen content. With Coke Oven Gas (COG) the hydrogen starts to significantly change behaviour such that the lower swirl number of 0.8 only gives best flashback resistance for equivalence ratios less than ~0.6. This behaviour continues for pure hydrogen The flashback results are not in accord with the differences in laminar flame speed between hydrogen and methane and seem to be affected by the augmentation of turbulent flame speed at the high velocities, hence kinetic energy of turbulence found with hydrogen based fuel mixes Two different flashback modes were found and significantly affected flashback behaviour for all results Especially for gas turbines lower swirl numbers are desirable as they give lower pressure losses and hence better thermodynamic cycle performance