Fuel-Flexible Low-Swirl Combustion System for Gas Turbines in Clean Coal Power Plants

Transcription

1 Fuel-Flexible Low-Swirl Combustion System for Gas Turbines in Clean Coal Power Plants Kenneth O. Smith, Peter L. Therkelsen, Sy Ali, Robert K. Cheng, and David Littlejohn Environmental Energy Technologies Division Lawrence Berkeley National Laboratory Berkeley, CA Research supported by NETL, Office of Fossil Energy, U.S. Department of Energy 2010 Gasification Technologies Conference, Washington, D.C. November 3, 2010

2 Summary: Combustion Issues at IGCC Power Plants Nt Natural gas, syngas, high h hd hydrogen fuel lhave different combustion properties Need method to utilize all fuels effectively without excessively complicated system Seek fuel flexible flexible low emission combustor design that is adaptable to turbines

3 Introduction Low emission industrial and utility gas turbines are based on lean premixed combustion (dry low NO x ) technologies; not intended to be fuel flexible Developed dfor pipeline quality natural gas to meet stringent ti temissions i regulations Variations in fuel composition affect combustion performance and fuel delivery pressure Gas turbines for Integrated Gasification Combined Cycle power plants with carbon capture and sequestration (IGCC CCS) require fuel flexible operation Nt Natural gas for startup t and as a backup fuel Unshifted syngas for transition or baseload Shifted high hydrogen fuel for baseload Need to redesign and/or modify IGCC GT combustion systems to: Transition smoothly between natural gas, unshifted syngas, and shifted syngas Provide low emissions and high efficiency Maintain turbine durability and operability

4 Objectives Of This Study Motivation: Laboratory studies show that the low swirl injector (LSI) concept has potential for fuel flexible flexible operation in gas turbines Demonstrated operability and ultra low NO x emissions from natural gas, syngas and hydrogen under simulated gas turbine conditions Objective: Conduct a preliminary feasibility and conceptual study of a LSI based fuel flexible combustion system for IGCC gas turbines Consider properties and combustion characteristics of 3 fuels: natural gas, unshifted syngas and shifted high hydrogen fuel to configure the layout of the fuel handling and delivery circuit Propose a conceptual design of the 3 fuel combustor for a typical F frame gas turbine gas turbine Identify technical and engineering challenges associated with the development of such a system

5 Low Swirl Background 1 burner (5 kw, 17 KBtu/hr) LSI stabilizes lean premixed flames in a divergent non recirculating flow LSI swirler has two flow passages 30% of the premixture bypasses swirl annulus through a center tube Unswirled bypass mitigates recirculation and promotes divergence LSI swirler is scalable Engineering guidelines developed Basic configuration and swirl number (0.4 < S < 0.55) independent of size Demonstrated in gas turbines 100kW microturbine 7 MW industrial gas turbine (Turbo Expo GT ) Commercialized for industrial heating 26 burner High turndown (up to 60:1 in lab tests) Guarantee 4 7 ppm NOx (@3%O2) (44 MW, 150 MBtu/hr)

6 Fuel-Flexibility Studies with LSI Rig tests with natural gas, syngas and H 2 at simulated gas turbine conditions (0.101 < P 0 < MPa, 298 < T 0 < 580K, 18 < U 0 < 60 m/s) Fuel flexible operation does not require movable swirler components High diffusivity and reactivity of H 2 affects flame stability and flame stabilization mechanism (Proc. Combust. Institute 32, 2009, GT ) NO x emissions from all fuels show log linear dependence on adiabatic flame temperature, T ad, for T ad > 1700K NO x emissions from H 2 flames show a 1 ppm NO x floor for T ad < 1700K NO x emissions from HC flames decrease log linearly for T ad < 1700K Analytic model was developed to illustrate fuel effects on flame properties Coupling of self similar diverging flowfield and linear correlation of d l b l fl d l fl bh d displacement turbulent flame speed explains flame behavior at turndown Upstream shift of H 2 flame positions due to higher correlation constant of the turbulent displacement flame speed

7 Insights from Fuel-Flexibility Studies & Approach for This Work LSI swirler is amenable to fuel flexible operation Basic combustion process of the LSI not affected by variations in fuel composition expected during IGCC operation cycle Four steps to assess the feasibility of a LSI based fuel flexible combustion system for IGCC turbines Fuel elanalysis interchangeability of the fuel elinjection system sstem Combustion system characteristics impact due to differences in fuel mass flow rates Conceptual fuel injector design balance between fuel flexibility and power output Operational considerations staging and piloting

8 Fuel Analysis Natural Gas Methane 97% Pure Hydrogen Medium Hydrogen High Hydrogen Hydrogen 100% 30% 81% Carbon Monoxide 56% 3% Carbon 3% 3% Dioxide Diatomic Nitrogen 3% 10% 12% Water 1% 1% LHV (kj/m 3 ) 32,676 10,059 9,687 8,457 Coal derived syngases have wide range of compositions and thus combustion properties Two representative coal derived gaseous fuels have been selected for the analysis, but neither fuel has a strictly defined molecular composition medium hd hydrogen fuel l(mhf) represents unshifted syngas supplied by the gasifier high hydrogen fuel (HHF) represents gas turbine feedstock after syngas has undergone watergas shift reaction for CO 2 sequestration

9 Specific gravity - γ Molecular weight kg/kmol Fuel Interchangeability Estimated by the Wobbe Index Wobbe Index, WI, provides Natural 100% H 2 Medium High H 2 Gas H 2 information on fuel forwarding/control distribution systems but not the combustion process Defined as the volumetric heat content WI = LHV vol of a given fuel γ (MJ/m3) Generally interchangeable if WI are 120,580 11,193 27,842 (51,840) (4,812) (11,970) within approximately +/ 10% Significantly lower WI indicates STP kg/m 3 (lbm/ft 3 ) (0.041) (0.005) (0.054) (0.019) LHV kj/kg (Btu/lbm) 47,450 (20,400) LHV kj/m 3 32,676 10,059 9,687 8,457 (Btu/ft 3 ) (877) (270) (260) (227) Wobbe Index MJ/m 3 (Btu/ft 3 ) 44.2 (1,186) 38.3 (1,028) 11.4 (306) (227) unacceptably high fuel system pressure drop Interchangeability of natural gas and hydrogen has been demonstrated in small scale gas turbines 17.0 (456)

10 Dual Fuel Injection Systems May Satisfy 3 Fuel Operation Requirements Good interchangea ability (+/ 10 0%) Margin nal interchan ngeability (10 to 15% var riation) Natural Gas High Hydrogen Pure Hydrogen Medium Hydrogen First fuel circuit for natural gas and/or pure H 2 Second fuel circuit for medium and high hydrogen fuels despite WI difference of 30 % Compositions of MHF and HHF vary with the feedstock and the gasification process WI also varies with temperature and pressure Wobbe Index

11 Combustion System Characteristics Natural Medium High Gas Hd Hydrogen Hd Hydrogen Fuel Temp (K) Primary Zone Temp (K) Mass Flow Fuel (kg/s) 1,839 1,839 1, Mass Flow Air (ks/s) Primary Zone Fuel/Air Total Mass Flow (kg/s) Normalized Mass Flow Preliminary design study for a representative 200 MW F class gas turbine Assumptions: MHF used as the design baseline Pressure ratio of 19 Primary zone temperature of 1840K to control NO x emissions Turbine inlet temperature of 1700 K with fixed air flow split of 84.4% Results show MHF requires significantly higherfuelflowrates flow rates than natural gas and HHF Lower fuel side pressure for natural gas and HHF operations

12 Fuel Injector Design Considerations Adopted approach Redesign of the lean premixed (LP) injectors to include two fuel circuits Provides flexibility for multi fuel operation Preserves power output and efficiency of the turbine Other options to accommodate MHF and HHF in combustors designed for NG Using existing fuel injection system Operate engine at derated power due to increased fuel pressure above design point Undesirable from efficiency and output perspectives Addition of fuel injectors Common practice for conventional ldiffusion i combustors to accommodate low Btu fuels Difficult to implement in LP combustor due to size restrictions

13 Desired Features of the Dual Circuit Fuel Injection System Incorporating new injectors and fuel circuit with minimum co po at g e jecto s a d ue c cut t u changes to existing lean premixed combustor geometry Resolving the higher risk of flashback and autoignition Maintaining an acceptable fuel pressure drop Continuing to meet single digit NOx emissions standards Having the ability to co burn with NG during fuel transfer and gasifier startup Maximizing design commonality between MHF and HHF injection systems

14 Conceptual Fuel Injector Design Targeting F class gas turbines with 14 combustor cans, each with several injectors Assumed five injectors and a central diffusion pilot in each combustor can Pilot needed for start up, fuel transition, and mitigating combustion stability issues Primary fuel (syngas) delivery to swirling air via axial spokes Second sets of spokes upstream of the swirler for natural gas operation Provisional pilot/ central burner Injector (1 of 5) 13 cm diam. Syngas MAIN Natural Gas Integrated fuel spokes and LSI vanes 63 cm diam. Combustor liner (1 of 14) Natural Syngas Gas PILOT Fuel spokes

15 Fuel Spoke Geometries and Flow Characteristics for Individual Injector NG MHF HHF # of Injectors Provisional pilot/ central burner Injector (1 of 5) 13 cm diam. Fuel Mass Flow (kg/min) Fuel Temp. p.( (K) Compr. Disch. Pres. (atm) Compr. Disch. Temp. (K) Primary Zone Air Flow (kg/s) Injector Delta Pres. (kpa) Oifi Orifice Count Syngas MAIN Natural Gas 63 cm diam. Combustor liner (1 of 14) Integrated fuel spokes and LSI vanes Orifice Diam. (mm) Fuel Line Diam. (cm) Natural Syngas Gas PILOT Fuel spokes

16 Autoignition Considerations MHF has short autoignition times ( τ ai ) about K, 20 atm, Φ 0.3 Dependent upon hydrogen concentration and inlet temperature HHF τ ai are even shorter 5 ms residence time from fuel linjection to flame is adopted dfor this study PIV measurements provide more realistic estimate than using bulk flow velocity Non uniform axial and radial velocity distributions with lowest velocities along the LSI axis Velocity at leading edge of flame brush is 30% of U 0 Maximum premixing length of 10 cm is established Length is within the LSI swirler recess design guideline Φ 0.3 Syngas MAIN Natural Gas Integrated fuel spokes and LSI vanes τai ~ 80 msec τai design <5 msec Natural Syngas Gas PILOT Fuel spokes 10 cm

17 Operational Considerations Dual fuel circuits and pilot provide fuel and operational flexibility Natural gas from one fuel circuit can be used during: Engine start up and when the gasifier is offline Gasifier startup and shutdown, removing the need for flaring Central diffusion pilot can be: Fueled with NG when starting the gas turbine and while transitioning to syngas operation Used with NG and/or syngas to ensure combustion stability, typical for low emission engines Hydrogen can be used in the NG circuit if available. NG and hydrogen have similar WI, but the 5 ms residence time criteria needs to be considered

18 Challenges Preliminary analyses demonstrate the feasibility of a multi fuel LSI gas turbine Refinement of the injection design needs to be conducted based upon specific turbine Potential system integration Issues: Combustor and turbine nozzle cooling: Three fuels produce different product gas temperatures and compositions. Combustor liner and turbine vanes and blades will experience different heat transfer rates. Proper cooling airflow needs to be established. Fuel injector length: Minimization of fuel residence time is needed to avoid damage due to auto ignition. iti Near homogeneous mixing i must be balanced with this need. Fuel spoke design: Optimization of the number, placement, and size of fuel spokes and fuel orifices must be made. Combustor volume: NG LP combustors employ large combustors to mitigate CO emissions. Hydrogen burns faster than NG. Smaller combustor liners areneeded to limit NOx emissions. Hydrogen burns faster than NG. Smaller combustor liners are needed to limit NOx emissions. Acoustic oscillations: Potential for generating undesirable acoustic pressure oscillations is unknown. Large mass flow: Increased flow through the turbine, due to higher fuel mass flows of syngas fuels, could cause compressor surge. Natural gas fuel circuit transition: The interchangeability of the fuel with natural gas degrades as hydrogen content decreases. A cost benefit analysis is needed to establish the optimum system hydrogen concentration level.

19 Thank you Questions? info/