Mild Combustion of non-conventional and liquid fuels. Marco Derudi Dipartimento di Chimica, Materiali e Ingegneria Chimica / CFALab

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1 Mild Combustion of non-conventional and liquid fuels Marco Derudi Dipartimento di Chimica, Materiali e Ingegneria Chimica / CFALab

2 INTRODUCTION 2 Combustion processes are essential for power generation, thus the development of a combustion technology able to accomplish improvement of efficiency with reduction of pollutants emissions, as NOx and soot, is a main concern. To improve the thermal efficiency of combustion processes, in the 80 lots of studies were focused on Heat Exchange and Heat Recovery problems.

3 THE BIRTH OF MILD COMBUSTION 3 Excess enthalpy combustion 1 Heat-recirculating combustion: the exhausts can be used to preheat the reactants upstream the flame region 2 1 Lloyed, Weinberg, Nature, 251, Weinberg, Combust. Sci. Technol., 121, 1996

4 PRELIMINARY ISSUES 4 Advantages Thermal efficiency is increased Reduced fuel consumption Possibility to burn low-calorific fuels Drawbacks Very high peak temperatures High NOx emissions (thermal-nox) Critical design of the burner

5 NOx CONTROL STRATEGIES 5 Flame control - Temperature - Stoichiometry - Species dilution and scavenging Post-flame control Post-flame NOx reduction by - Reburning - Non-catalytic selective reduction - Catalytic selective reduction (SCR) Choi, Katsuki, Energy Conv. Management, 42, 2001

6 LOW-NOx BURNERS 6 NOx-control strategies by burner design: - Staging - Swirling - Recirculation These techniques effectively control: - Flame core stoichiometry - Peak flame temperature

7 RECIRCULATION vs FLAMMABILITY 7 Wünning & Wünning, Prog. Energy Combust. Sci., 23, 1997 %fuel LFL UFL Auto Ignition FP AIT T

8 DILUTION, PREHEATING, HIGH-MOMENTUM JETS 8 Traditional flame Mild Derudi et al., 6 th HiTACG Symposium, 2005 Milani, Saponaro, La Termotecnica, 1, 2000

9 ADVANCED LOW-NOx TECHNOLOGY 9

10 REGENERATIVE BURNERS

11 REGENERATIVE BURNERS

12 SELF-RECUPERATIVE BURNERS 12 combustion chamber recuperator FLOX burner, WS GmbH

13 ONE TECHNOLOGY, MANY NAMES 13 Heat-recirculating combustion Preheated air combustion (PAC) Diluted Combustion Noiseless combustion High temperature air combustion (HiTAC) Flameless oxidation MILD combustion (Moderate or Intense Low oxygen Dilution)

14 WHY FLAMELESS? 14 Natural Gas 48MWth Fuel Fuel Fuel TEA swirl burner (ANSALDO) internal recirculation

15 MILD COMBUSTION - DEFINITION 15 = 1500 K = 1000 K = 550 K T decreases by increasing dilution Derudi, Villani, Rota, Proceedings of the Combustion Institute, 31, 2007 Traditional combustion T Self-Ignition MILD COMBUSTION T inlet > T SI and T < T SI [K] Cavaliere, de Joannon, Prog. Energy Combust. Sci., 30, 2004

16 TYPICAL EXPERIMENTAL TRENDS 16 Krishnamurthy et al., Proceedings of the Combustion Institute, 32, 2009 MILD Szego et al., Combust. Flame, 154, 2008

17 LAB-SCALE MILD BURNER 17 Laboratory-scale burner is constituted by three main sections: feeding and pre-heating burner sampling and measurements Lab-scale burner advantages: low-cost low-time high flexibility

18 LAB-SCALE BURNER: DILUTION RATIO 18 Industrial burner Lab-scale burner k v M e = M + M a M e = recycled exhaust gases stream M a = primary air inlet stream M f = fuel inlet stream f k v R SA IA (1 + R) = SA (1 + FA) (1 + SA) SA = 2 ary /1 ary air volumetric ratio IA = inert/1 ary air volumetric ratio FA = fuel/air ratio R = recycle factor

19 DILUTION RATIO - K v 19 SN: 25% of air external to the nozzle k V = A ρ v E m& F E da SN: 80% of air external to the nozzle Dilution ratio induced by the jets as a function of the dimensionless distance from the air nozzle

20 HYDROGEN ENRICHED FUELS 20 H 2 -enriched fuels are interesting: greenhouse gases from solid fuels (e.g. coal/biomasses gasification) byproducts of industrial processes (e.g. coke oven gas) BUT H 2 shows properties that make conventional burners unsuited: alternative technologies (MILD) need of research (little work on H 2 enriched fuels )

21 RESULTS: CH 4 /H 2 21 Temperature OH (flame marker) CH 2 O (ignition marker) Measured T [K] Predicted T [K] K Y OH 5.0E-4 Y CH2O 9.3E K 0 0

22 RESULTS: CH 4 /H 2 22 Temperature predicted with EDC and DRM-19 flame 300K 2190 K 1270 K 2000 K 1270 K 1440 K MILD 1270 K 1390 K

23 MILD COMBUSTION OF H 2 ENRICHED FUELS: NOx 23 Contribution of different formation routes to the total NO emissions (dry basis) with CH 4 /H 2 mixture for different dilution ratios, kv NNH intermediate mechanism: N 2 + H NNH NNH + O NH + NO Galletti, Parente, Derudi, Rota, Tognotti, Int. J. Hydrogen Energy, 34, 2009

24 TOWARD A MILD COMBUSTION OF LIQUID FUELS 24 Retrofit a lab-scale burner developed for mild combustion of gases in order to use liquid fuels - Different nozzles configuration - Atomizer for the direct liquid fuel feeding Definition of an experimental procedure for tests with liquid fuels (n-octane, n-dodecane, ) Evaluate main characteristic parameters for mild combustion of liquids requirements for real-size burners design Preliminary definition of operating maps for pure (noctane) and practical (kerosene) fuels

25 LAB-SCALE MILD BURNER LAYOUT 25 Upper oven for heat maintenance Exhausted gases outlet Quartzwool insulation Thermocouple Liquid Fuel inlet Gaseous Fuel inlet Refractory insulation Preheating oven Secondary air inlet Primary air + inert inlet

26 LIQUID FUEL ATOMIZATION 26 A pneumatic atomizer (supported by a nitrogen stream), water-cooled to avoid fuel pyrolysis, was developed to create a short fuel jet Cooling water inlet Cooling water outlet Liquid fuel inlet Nitrogen inlet The liquid fuel is sprayed directly into the burner

27 EXPERIMENTAL PROCEDURE 27 The burner cannot be fired directly with a liquid fuel, so the fuel feed is varied once stable mild combustion conditions are achieved with a gaseous fuel (CH 4, C 2 H 6 ). Average Furnace Temp. Flame Combustion Mixed zone B C Thermal NO x Mild clean Extinction D High CO emissions CD BA Preheating Firing Increase Transition with of to the CHmild 4 dilution or C 2 H 6 Gaseous Gaseous Fuel 2ary fuel 2ary air air 1ary Air air A Kv can be modulated in a wide range K V Kv = ( M + M + M ) R M a1 f i a2 M + M + M a1 a2 f

28 MIXED ZONE: ROAD TO MILD 28 T [ C] A C T top B T half 600 T bottom Kv Thermal gradients smoothing A Flame Combustion B Transition C onc e ntra tion [ppm ] A B C Kv NO CO C Clean mild combustion NO < 30 ppm CO < 50 ppm

29 GAS TO LIQUID TRANSITION 29 Dual nozzle (DN) Single nozzle (SN) Liquid fuel Dual nozzle (DN) Gaseous fuel 1ary air Liquid fuel Gaseous fuel 1ary air HYBRID FUEL FEED: GAS and LIQUID 1ary air PURE METHANE OR ETHANE PURE LIQUID FUEL

30 n-octane COMBUSTION 30 K V =8.75, air excess=14%, air 950 C T [ C] T half 800 T top SN % n-octane DN Temperature increase due to the different thermal input Low thermal gradient Concentration [ppm] % n-octane NO CO Stable Mild clean regime during and after the gas to liquid fuel transition NO < 30 ppm and no CO emissions during the transition

31 TEMPERATURE PROFILES 31 K V =8.75, air excess=14%, air 1100 C K V =5.6 Preheated air nozzle Exhausts exit K V =2.7

32 MILD CLEAN REGION: n-octane C 1000 T [ C] K V = C Kv

33 REFERENCE COMPONENTS IN REAL FUELS 33 Concawe, Fuel quality, vehicle technology and their interactions, Report 99-52, 1999 Typical composition of a kerosene 3% Indanes Indenes and Tetralines 16% Alkylbenzenes 5% 2-3 ring aromatics 20% iso-alkanes 34% Cyclo-alkanes 22% n-alkanes Ranzi, Energy&Fuels, 20, 2006

34 INFLUENCE OF REFERENCE COMPONENTS Branched hydrocarbons NOx (ppm dry) n-c8 n-c8/i-c8 95/5 n-c8/i-c8 60/ liquid fuel (%) Cyclic hydrocarbons NO Cyc-C6 10% Cyc-C6 20% Cyc-C6 CO [ppm] Air excess [%]

35 KEROSENE 35 Kerosene contains not only linear chain and branched molecules, but also alkylbenzenes and other cyclic compounds. GC-MS Analysis C 12 C 11 Main fraction: C 10 - C 13 hydrocarbons C 10 C 13 C 14 C 9 C 15 C 8 C 16

36 KEROSENE COMBUSTION 36 CO [ppm] CO (1100 C) CO (950 C) OPTIMAL AIR EXCESS Lower CO emissions found for lower air preheating temperatures (it is increased the reactants residence time into the combustion chamber) Air excess [%] No CO emissions for air excess larger than 20% Concentration [ppm] SN NO CO SOx % kerosene DN CH 4 to KEROSENE TRANSITION NO < 30 ppm and CO < 1 ppm during the transition SO X emissions (max 2 ppm) K V = 9, air excess=20%, air 1100 C

37 MILD CLEAN REGION: KEROSENE C 1000 T [ C] K V = C Kv

38 CONCLUSIONS 38 Successful retrofit of a mild combustion burner designed for gaseous fuels to operate with liquid fuels. These tests evidenced that it is possible to change, also during a run, the fuel feeding strategy without affecting the mild sustainability. Separated nozzles can help to realize mild combustion at lower Kv with respect to the SN layout, thus foreseeing the possibility to obtain mild combustion of liquid wastes and/or high reactive fuels (at moderate/high air temperatures and velocities) at low dilution ratios. Operating parameters for burners design

39 DEVELOPMENTS 39 Mild combustion of liquid wastes Coal mild combustion Biomass mild combustion Stadler et al., Proceedings of the Combustion Institute, 32, 2009

40 AND FUTURE PERSPECTIVES 40 OXY-fuel mild combustion High-pressure mild combustion (?) Kim et al., Proceedings of the Combustion Institute, 31, 2007 OXY-coal mild combustion (?)