GT2017-63572

SMALL RADIAL SWIRLER LOW NOX COMBUSTORS FOR MICRO GAS TURBINE APPLICATIONS Gordon E Andrews and Myeong Kim Clean Combustion Research School of Chemical and Process Engineering University of Leeds Leeds LS2 9JT UK ABSTRACT There is a growing interest in micro gas turbines for distributed electric power production This work examines the scale down of a successful low NOx radial swirler from a combustor of 140mm diameter to one of 76mm diameter suitable for micro gas turbines in the 30 100kW electric size range A 40mm outlet eight bladed radial swirler was investigated for four radial swirler vane depths from 30 5mm to 12 2mm and a constant pressure loss of 2 7 at different reference Mach numbers or residence times For a 740K inlet temperature and 0 6 equivalence ratio these gave combustor thermal loadings at atmospheric pressure from 33 to 62 kW and heat release of 7 14 MW m2bar The small radial swirlers had lean stable flames with near lean flammability weak extinction The vane passage single hole fuel injection achieved NOx emissions of 1 5ppm at 15 oxygen 740K air inlet temperature and 1700K primary zone temperature with 10ppm CO The 1ppm NOx was for the smallest vane depth and lowest thermal power The higher NOx for the larger heat release swirler designs were due to the longer flame development and a greater formation of prompt NOx due to the high UHC for most of the flame development INTRODUCTION The current interest in micro gas turbines MGTs in the 100kW class is in distributed electric power generation This avoids the typically 8 thermal efficiency loss of conventional high voltage power distribution from large GW power generator sites 1 2 Also they can be used alongside renewable energy such as solar to give continuous local power generation 3 4 They have multi fuel capability and with biomass 5 6 and in a biogas solar MGT system 7 offer completely renewable electricity throughout the day Another potential application of MGTs is as a range extender for automotive electrical vehicles where the MGT is used to generate electricity for on board battery charging 8 MGT generators in the 40kW or lower power range are being developed for this 8 For low cost MGTs automotive turbochargers are being used with a cylindrical combustor connecting the compressor and turbine using the same connections as for reciprocating engines 8 A key drawback of simple cycle MGTs is the low electric power thermal efficiency relative to diesel electric power generation However MGTs have a major advantage on power to weight ratio and power to footprint area ratio MGTs also offer much lower NOx emissions without expensive deNOx catalysts and in this paper a low single digit NOx combustion system is demonstrated The best way to improve the thermal efficiency of MGTs is to add heat recovery to a simple cycle MGT so that it is a combined heat and power system CHP where overall thermal efficiencies of 80 can be achieved 9 However these require a thermal power demand similar to the electrical power output In summer this is often a problem and hence other techniques to improve the electrical power generation efficiency have been developed Firstly MGT can be combine with solar as discussed above secondly a recuperator can be added to recover exhaust heat and raise the combustor inlet temperature and thus require less fuel to achieve the design power 10 thirdly water or steam can be added so that there is a higher mass flow through the turbine which generates more power 9 11 12 Steam is generated from water using an exhaust heat exchanger 13 This technique has demonstrated electrical power thermal efficiency improvements of about 10 of the simple cycle thermal efficiency with close to 30 achieved for an 85kW MGT 13 14 In the present work both simple cycle and recuperator MGT combustors are simulated by investigating a range of inlet air temperatures Water or steam injection was not investigated but the ability of a combustion system to sustain combustion in the presence of high levels of inerts requires an inherently good flame stability The present work will demonstrate well mixed flame stability down to the fundamental flammability limit and this will enable at least as much water steam as air into the 1Copyright 2017 ASME Proceedings of ASME Turbo Expo 2017 Turbomachinery Technical Conference and Exposition GT2017 June 26 30 2017 Charlotte NC USA GT2017 63572 combustor primary zone and more into the dilution zone without compromising flame stability or NOx MGT combustors can be annular 15 or a single cylindrical combustor 8 10 13 14 16 The cylindrical combustors have advantages in terms of the lower surface to volume ratio for wall cooling Also they are easier to remove for maintenance Both types of combustor are in production for MGTs 14 15 16 but the cylindrical combustor is the only one suitable for matching to automotive turbochargers Vick et al 9 have also shown that a cylindrical combustor can be used in a compact recuperated MGT with a central blockage of the combustor outlet to generate an annular feed into the turbine The present work uses a cylindrical combustor with radial swirlers and vane passage fuel injection 17 21 This type of low NOx GT combustor are used in seven production gas turbines in the 0 25 60 MWe range This is similar to some existing MGT designs 10 22 25 but all these designs have swirlers that are too small relative to their enclosure if the swirlers were to pass a large proportion of the total combustion air 17 Alternatively the swirlers have combustors that are larger than they need to be and this compromises the overall size and weight of the MGT Most of these designs use double radial swirler passages and often do not fuel the radial swirlers in a way that will generate good fuel and air mixing which is required for low NOx Small combustors for gas turbines of the size used in the present work may also be used as pilot well mixed burners in the centre of a larger radial or axial swirler array 21 26 They may also be used in arrays of swirlers in a larger can combustor for applications to much larger gas turbines as is currently used by several manufacturers 17 This principle is used in one commercial 175 kWe MGT downsized from larger GT low NOx combustors using four axial swirlers in a single can combustor Each swirler had 44 kWe generation capacity and was similar to the size in the present work However this design had a poor flame stability and the combustor in the well mixed mode did not have a stable flame below 1750K The present single small swirlers have about 50 kW thermal at 1 bar If used as 9 swirlers per can in a 10 can large scale GT could at 20 bar pressure and 60 combined cycle thermal efficiency fire a 54MWe CCGT SMALL RADIAL SWIRLER DESIGN FOR MGTs The small radial swirler used in the present work was a scaled down version of the successful ultra low NOx radial swirler flame stabilizer with vane passage fuel injection and a pilot fuel injector located centrally on the rear face of the radial swirler This was first developed in 1988 1990 by Andrews and Alkabie 17 20 and the achievement of low single digit NOx for both gas and liquid fuels was demonstrated With a single fuel hole at the entrance to the converging curved vane passage radial swirler the flame stability was excellent with weak extinction slightly leaner than the lean flammability limit 28 The original work of Alkabie and Andrews on low NOx radial swirler combustion was for relatively short blade depths with limited air flow capacity 18 20 but was scaled up to higher flow capacity and large thermal loadings at the same pressure loss by increasing the vane passage depth 29 and similar ultra low NOx results were demonstrated The influence of the vane passage design was shown by Escott et al 30 to have a small but significant effect on the NOx emissions Seven industrial gas turbine manufacturers now use radial swirlers with vane passage fuel injection for their DLN GT products The range of production engines using single flow direction radial swirlers covers the power range 250kW 60MW and the configurations used are similar to those developed by Andrews and co workers 17 Scaling of low NOx combustors can be a problem for NOx if the number of vane passages is held constant This is because the scale of turbulence and the length of shear layers increase if the combustor is made larger The consequence for this is that mixing is slower as a function of distance from the stabilizer and this can increase thermal NOx emissions This has led many gas turbine manufacturers to scale up combustors by increasing the number of the same size of basic low NOx flame stabilizer using more combustor cans in some cases Reducing the scale of a combustor as in the present work which reduces the turbulent length scale and reduces the size of turbulent shear layer mixing zones is less of an issue for NOx compared with scaling up The reduced size of shear layers and the reduced turbulent length scales are more of an issue in reducing the rate of the axial development of the flame and of the flame being attached to the flame stabilizer which will be shown to be a problem in the present work However scale effects that act on thermal NOx are of less importance for lean combustion where thermal NOx has been eliminated as in the present work An important feature of the original low NOx radial swirler work of Andrews and co workers 17 was the use of a simple single fuel hole on the centerline of the radial passage inlet just downstream of the inlet The radial curved passages had a larger inlet flow area than exit area so that a 3 2mm OD fuel injection tube did not obstruct the vane passage flow and increase the pressure loss This fuel injection method was used in the present work The principle of fuel injection at the radial vane passage inlet was adopted by one manufacturer 31 32 who used radial swirlers in the low NOx primary zone with converging radial passages of greater inlet area then exit area so that the fuel spoke did not increase the pressure loss However they increased the number of fuel holes per vane passage to three and six for counter rotating radial swirlers Other users of radial swirlers with vane passage fuel injection have injected the fuel through the upstream rear face of the passage for reverse flow combustors Considerable effort had then to be spent on optimizing the number size and location of the fuel holes to achieve sufficient mixing for low NOx to be demonstrated 33 Agbonzikilo et al 34 have recently shown the sensitivity of vane passage fuel and air mixing to the wall fuel injector design They investigated between 2 and 6 fuel holes per vane passage plane and counter bored holes were investigated An optimum design improved fuel and air mixing in the passage by 2Copyright 2017 ASME 60 compared with the initial 2 hole design Another manufacturer 35 36 has used one fuel injection hole on the rear wall close to the inlet of the vane passage The mixing of fuel and air is that of a jet in a crossflow The disadvantage is that the penetration varies with fuel flow or engine power output The sensitivity to fuel hole diameter was shown by a rear wall fuel hole size change from 2 2mm to 1 4mm that reduced the NOx at 1600K from 40ppm to 3ppm 36 The present work demonstrates 1ppm at 1600K with a single 1 6 mm diameter hole fuel injector on the centerline of the vane passage close to the inlet By injecting the fuel in the same direction as the air in the vane passages the wake of the injector assists in the mixing and no fuel flows along the vane passage walls CFD studies by King et al 37 38 supported by internal gas analysis traverses of the flames show that the fuel injected through a single hole on the centerline of the radial passage inlet was not fully mixed at the passage outlet but provided a local richer zone at the base of the shear layer which aided flame stability The mixing was completed in the radial expanding shear layer before the peak temperature is achieved Radial swirlers are suitable for reverse flow combustors and all of the current applications are for reverse flow combustors where the air flows into the radial swirler passages after backside cooling the combustor as first suggested by Andrews et al 39 using impingement backside cooling Andrews and Kim 40 showed the importance for ultra low NOx of having zero film or effusion cooling in low NOx primary zones This work used radial swirlers as the baseline combustor to which effusion cooled combustors were compared with no film cooling The NOx increased linearly with the proportion of film cooling air used as the primary zone operated with less air and with a higher temperature as air was taken from the primary zone to feed effusion cooling 40 For this reason no film cooling was used in the present work The original radial swirler design was a 140mm combustor and 76mm radial swirler outlet diameter 18 20 and the thermal power and combustion intensity was varied by changing the combustor flow rate at the same pressure loss by increasing the vane depth with no change in the outlet diameter d or the combustor diameter D The combustor reference Mach number M1 was increased as the vane depth was increased for the same pressure loss The relationships between pressure loss M1 and combustion intensity MW m2bar are given by Eqs 1 and 2 17 P P 0 5 Cd2 M12 A1 A2 1 MW PA1 105 S M1CV RT 0 5 MW bara m2 2 A1 is the combustor cross sectional area based on the diameter D and A2 is the minimum flow area of the vane passages which is a rectangular area of the blade minimum separation distance h times the vane depth L times the number of vanes 8 in the present work and 12 in some of the industrial applications of the design The advantage of radial swirlers is that the air flow capacity can be increased by increasing L without changing the swirler outlet diameter d The downstream aerodynamics are controlled by the expansion ratio D d and the swirl number S Thus the swirler flow capacity can be changed without changing the swirler outlet diameter whereas for axial swirlers the swirler diameter has to increase if the flow capacity or S is increased 41 Equation 2 for a 740K air inlet temperature a CV of 50 MJ kg for methane and a low NOx condition with 0 6 or lower has a combustion intensity of 13 4 MW bar m2 for a primary zone Mach number of 0 03 corresponding to 60 of the combustion air passing through the swirler For one bara and a 140mm combustor diameter this gives a thermal power of 207 kW For an industrial gas turbine operation with 50 CCGT thermal efficiency this would give an electrical power output that would depend on the operating pressure and the number of combustors For 8 combustors at 40 bar this is 33MWe and for one combustor at 3 bar it is 310 kW Larger or smaller powers could be achieved if the combustor diameter Figure 1 Radial swirler geometrical configuration Table 1 Characteristics of the small 8 bladed 45o radial swirlers Swirler A1 A2 A3 A4 R1 mm 46 46 46 46 R2 mm 54 54 54 54 R3 mm 33 33 33 33 L mm 30 5 21 5 15 0 12 2 h mm 8 0 8 0 8 0 8 0 do mm 76 76 76 76 d mm 40 40 40 40 D mm 76 76 76 76 D d 1 9 1 9 1 9 1 9 Swirl Number S 0 54 0 63 0 77 0 86 A1 A2 2 32 3 30 4 73 5 81 Ad A2 0 644 0 914 1 31 1 61 M1 0 03 0 025 0 019 0 016 Cd 0 36 0 40 0 46 0 47 P P 2 7 2 7 2 7 2 7 Thermal kW 0 6 1 atm 61 8 51 4 39 2 32 9 MWth bar m2 0 6 740K 13 4 11 2 8 51 7 16 Electrical power kW Cycle eff 0 3 34 PR 5 34 91 76 57 49 L h 330mm 3Copyright 2017 ASME was different but the above range is close to the range available in the market for radial swirler low NOx CCGTs In the present work a smaller 45o 8 vane passage radial swirler was used in a combustor with diameter D of 76mm and a radial swirler outlet diameter d of 40mm as shown in Fig 1 This is roughly half the previous 140mm combustor diameter and swirler outlet diameter which is 25 of the flow area and for the same reference Mach number this will give roughly 50kW thermal input at 1 bara and 740K air temperature for the same combustion intensity The geometrical details of the design are given in Table 1 The vane passage flow area A2 is 8Lh and is reduced from swirlers A1 to A4 by reducing the vane depth L Equation 2 for the same 13 4 MW bar m2 gives for swirler A1 a thermal power for methane at one bara of 61 8 kW for 0 6 A single combustor for MGT applications with a pressure ratio of 5 bara 40 41 and simple cycle thermal efficiency of 0 3 40 41 would give a MGT with 91kWe power output for swirler A1 as shown in Table 1 At 3bara and 20 thermal efficiency 10 it would be 37 kWe The four radial swirlers in Table 1 were designed to achieve the same pressure loss of 2 7 at different air flows that gave different primary zone reference Mach numbers M1 M1 was varied and the pressure loss was kept constant by reducing the vane depth L For M1 0 03 the simulated proportion of the total combustion air entering the primary zone was 60 as the reference Mach number for 100 if the air flow is typically 0 05 17 The primary zone length to the dilution zone position was 330mm in the present work and in the work of Alkabie and Andrews for the larger 140mm diameter combustors 18 20 However no dilution air was present in the present work which only evaluated the low NOx primary zone combustion performance The aim was to show that combustion was complete by the position of the dilution air so that no CO or UHC quench would occur The primary zone is operated for low NOx and low CO at around 1800K at full engine power The turbine entry temperature is then met by using the surplus air for dilution In the present work only the primary zone was investigated but dilution flows of 40 64 were simulated by reducing M1 Essentially this models turbine entry temperatures 1800K For 740K inlet temperature the primary zone T was 1060K and 40 dilution would give 1376K turbine entry temperature for swirler A1 For the other swirler