Copyright 1988 by ASME. Experience with Supplementary Combustion Systems to Maximize Steam Production in Gas Turbine Cogeneration Plants

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., Ne York, N.Y GT-318 The Society shall not be responsible for statements or opinions advanced in papers or in discssion at meetings of the Society or of its Divisions or Sections, or printed in its pblications. Discssion is printed only if the paper is pblished in an ASME Jornal. Papers are available from ASM E for fifteen months after the meeting. Printed in USA. Copyright 1988 by ASME Experience ith Spplementary Combstion Systems to Maximize Steam Prodction in Gas Trbine Cogeneration Plants J. C. BACKLUND E. E. FIORENZA Coen Company. Inc. Brlingame, California ABSTRACT Topping cycle cogeneration plants, consisting of a relatively small gas trbine generator (3-5 MW) and a spplementary fired heat recovery steam generator designed to maximize steam prodction, are becoming a very attractive system design. Special brners designed to fire into ater all frnaces can add sbstantial steam prodction capability to the system in a very efficient manner, tilizing the oxygen available in the trbine exhast. This paper examines recent operating experience at to U.S. installations, sing gas trbines rated at 3.3 MW ( Iso) and 4. 8 MW ( Iso). Gas fired brner designs are presented. Performance parameters sch as response to load, maximm steam prodction, brner rangeability, and emissions of NOx and CO are discssed. CONVERSION FACTORS TO U.S. CUSTOMARY UNITS BTU = KCAL x F 9/5 c + 32 LB KG x 2.25 FT M x IN MM X.394 PSI = KG/CM2 x Throgh the Pblic Utility Reglatory Policies Act of 1978 (PURPA), the federal government has given economic incentives to cogeneration projects and created a frameork for cooperation ith gas and electric tilities. In those geographic areas here tilities need additional generating capacity, the economic incentives are attractive, and ne projects have florished. The availability and competitive pricing of natral gas for the foreseeable ftre has served to encorage the installation of combstion trbines ith exhast heat recovery as the most poplar type of "medim sized" ( 2 to loo MW electrical) cogeneration plant. The short lead time, large degree of modlarity, ide choice of electrical and thermal capacities, reasonable cost, minimal environmental impact, and high energy efficiency have also contribted to the rapidly groing demand for combstion trbine based plants. A schematic of a typical spplementary fired gas trbine based cogeneration plant, consisting of a combstion trbine, exhast bypass, dct brner, and heat recovery boiler is shon in Figre 1. i i l STACK FXHAUST l I INTRODUCTION In recent years, cogeneration (the seqential prodction of electrical and thermal poer from a single fel sorce) has seen significant groth as a poer prodction method. There are many ne installations in the indstrial energy sector (chemical processing, plp & paper, etc.) and large commercial or instittional bilding complexes (niversities, hospitals, etc.). To date, most of the recent U.S. projects have been in California and the Glf Coast, bt increased activity is seen in the Midest and Northeast. GAS TURBIN!: FIGURE 1: TURAIN[ EXHAUST SILENCER l., / +}J L - y (, I BYPASS OAMPFR "i GENERATOR COEN DUCT BURNER t.rfs 7 - -, I TYPICAL SUPPLEMENTARY FIRED GAS TURBINE COGENERATION SYSTEM Presented at the Gas Trbine and Aeroengine Congress Amsterdam, The Netherlands-Jne 6-9, 1988 Donloaded From: on 6/19/218 Terms of Use:

2 SUPPLEMENTARY FIRING EQUIPMENT For fll load operation, the otlet temperatre of an indstrial gas trbine typically ranges from 45 to 54 C, and the oxygen level ranges from 13 to 16 percent by volme. Spplementary firing directly into the trbine exhast gases (TEG) yields important advantages. It provides a means to increase and control thermal otpt of the system (e.g., steam flo, sperheat temperatre, etc.) so that an optimm match can be made beteen engine and process needs. Overall system energy efficiency is increased, since all energy added by the brner is extracted by the heat recovery eqipment. See Figre 2 for a typical example of plant efficiency improvement ith increasing spplementary firing in the exhast of a 2.8 MW (Iso) gas trbine. Parallel Flo Brners in Water Wall Frnaces For installations reqiring relatively high steam otpt, and an extended srface boiler ith an adiabatic frnace cannot be sed de to temperatre limitations, the spplementary firing can be accomplished in a special brner/boiler arrangement. The trbine exhast is dcted to an inslated plenm (indbox) and is sed as preheated combstion air for the brners. The spplementary firing can then raise the combstion prodcts to an adiabatic temperatre in excess of 1, 65 C hile loering the oxygen level to as lo as 3 percent. Figre 3 is a typical arrangement of to parallel flo type brners firing into a ater all frnace, folloed by a convection section and economizer. 1 KG CM', SAT STEAM PRODUCTION 1 KG HR SUPPLE'MENTARY FIRING 1" KCAL HR FIGURE 2: COGENERATION PLANT EFFICIENCY WITH SUPPLEMENTARY FIRING FIGURE 3: PICTORIAL VIEW OF DUAL PARALLEL FLOW BURNERS APPLIED TO WATER WALL HEAT RECOVERY BOILER In addition, some brners can fire a ider range of fels than those sitable for combstion trbines (e.g., residal oil and lo-btu gas ). This capability adds flexibility to the system, and can greatly improve operating economics. There are to basic arrangements for spplementary firing. The arrangement sed depends on the desired qantity of the steam otpt relative to gas trbine poer prodction. Dct Brners in Adiabatic Frnaces For installations reqiring relatively lo amonts of steam otpt, the spplemental heat is added to the trbine exhast gases by inline "dct brners" hich are installed in the inslated dcting beteen the gas trbine and an extended srface aste heat boiler. See Figre 1. With this arrangement, the spplementary firing is limited to a maximm donstream temperatre of approximately 1, C. The pper temperatre limit is dictated by design of the aste heat boiler, and dct lining system. The parallel flo brner is designed to be monted in the refractory lined front all of a ater tbe boiler, very similar to a brner on a conventional fresh air fired boiler. The parallel flo brner is designed to provide complete combstion ith very lo excess air. There are no adjstable lovers or moving parts. Trbine exhast gas is distribted evenly into the spray pattern of the oil atomizer or into the gas stream issing from the gas spds. Increasing degrees of firing (higher adiabatic flame temperatres and greater tilization of 2 in the trbine exhast ) is achieved by the grid brner, and parallel flo brner, respectively. Each brner type, hoever, reqires more pressre drop on the TEG side to achieve the necessary mixing. Pressre drop across a grid type brner is typically less than 1 MM W. C., hereas a parallel flo brner reqires 6-9 MM W. C. for proper mixing and complete combstion. 2 Donloaded From: on 6/19/218 Terms of Use:

3 OPERATING RESULTS System Descriptions At present there are to similar systems in operation. System 1, denoted by a (), consists of an Allison 51KB5 gas trbine, dal Coen parallel flo gas fired brners, and a Deltak ater all heat recovery boiler. System 2, denoted by an (X), consists of an Allison 57K gas trbine, dal Coen parallel flo gas brners, and an ERI/Nebraska ater all heat recovery boiler. Basic thermal specifications for the to systems are shon belo in Table 1. Thermal Performance In spplementary fired topping cycle cogeneration systems sch as these, all the fel combsted in the trbine exhast is converted to steam. In addition, de to the increased flo of boiler feeda ter throgh the economizer, the stack temperatre is redced by 2-5 C from the nfired operating stack temperatre, frther increasing thermal efficiency. With its higher gas side temperatre, a spplementary fired system is more thermally efficient as heat inpt is increased, as shon in Figre 2. The se of ater cooled frnaces permit the fll tilization of this thermal energy, p to the limits of the available oxygen in the engine exhast. A very important operational consideration in the design of sch systems is brner operating range ( trndon). A brner ith high trndon ill allo the system to follo plant load variation throgh a ide range of steam flos by brner modlation, rather than by on-off brner control. System 1 & 2 have a total trndon of 3: 1. Each brner has a range of 15:1, and ith to brners per boiler, the total range is 3: 1. The importance of high trndon ca pa bili ty is shon in Figre 4. For comparison, an operating characteristic for a system ith a 4: 1 trndon is also shon. It can be seen that a brner ith 4: 1 trndon cold case serios operating problems if plant steam load as in the 1-2 KG/HR range, hereas a system ith 3: 1 trndon old not. Excessive on/off cycling of the 4: 1 trndon brner, and resltant nsteady steam prodction, old occr. Emissions of NOx Spplementary firing of natral gas in a trbine exhast stream has the potential for either adding to the trbine-generated NOx or, nder certain conditions, redcing the gas trbine NOx and prodcing no net increase in emissions. This redction (or marginal increase) can take place provided conditions exist to promote the process of "rebrning" FIGURE 4: ',p",',-p Brner #1 a)nd #2 on,pr Brner #1 on Unfired j :-;; \?'/{" -7',p,-P IDEAL SYSTEM --DUAL BURNER. 3 1 TD SINGLE BURNER, 4 1 TD Q-t P STEAM DEMAND, 1 KG/HR LOAD FOLLOWING CAPABILITY OF BURNER SYSTEMS WITH 3: 1 AND 4: 1 TURNDOWN Thermal NOx Formation Assming that there is no fel bond nitrogen in the fel gas, the only NOx formation mechanism is thermal reaction of N 2 and 2 in the trbine exhast as it is heated by the flame. This "thermal NOx" is generated only at very high temperatres. Some refinery gases contain componds sch as ammonia (NH 3 ). In lo mole percents, the conversion of sch molecles to NOx is very high. As the concentration of NH 3 in the fel gas increases, the conversion rate decreases, bt total NOx prodction overall increases. NOx Redction by Rebrning The conditions necessary for rebrning to occr are as follos: 1. The spplemental flame mst prodce a high temperatre fel-rich zone. The fel-rich zone mst have a high concentration of hydrocarbons to promote the localized rebrning process A portion of the NOx in the trbine combstion prodcts mst be injested into this localized fel-rich zone at sch a rate that this zone remains rich for a sfficient time for the rebrning reaction to take place. For the rebrning reaction spplementary fel shold nitrogen. to be effective, the have no fel bond The trbine exhast mst have sfficient time and temperatre in this fel-rich zone for this rebrning process to convert the trbine NOx to N2, C 2 and H2. It is very likely that the NOx is serving as a sorce of oxygen in this type of reaction. TABLE 1 - BASIC THERMAL SPECIFICATIONS FOR SYSTEMS 1 & 2 SYSTEM TURBINE EXHAUST (ISO) BURNER HEAT RECOVERY STEAM GENERATOR 1 () 15 KG/Sec at 51 c 15% 2 (dry) 2 (X) 18 KG/Sec at 58 c 15% 2 (dry) 2 X 1 6 KCAL/HR (LHV) Natral Gas 25 X 1 6 KCAL/HR (LHV) Refinery Gas 45,4 KG/HR 36 KG/CM 2 (g), 245 C 5, KG/HR 32 KG/CM 2 (g), 345 C 3 Donloaded From: on 6/19/218 Terms of Use:

4 Not all installations provide the operating conditions necessary to prodce these retrning reactions. Generally, those installations ith a relatively high degree of fel gas spplementary firing, in conjnction ith sfficient trbine NOx, have good potential for a significant amont of rebrning. A densely spaced grid brner, or a parallel flo type brner, ill expose a large portion of the TEG stream to the flame zone, here the hydrocarbon radicals in the flame case retrning of trbine prodced NOx. An example of an installation here a significant amont of retrning as observed is discssed in Reference 1. NOx formation and redction mechanisms in grid type spplemental brners is discssed in Reference 2. Operating Data from the Parallel Flo Brner NOx prodction data from the to operating installations is presented in Figre 5. Becase NOx emission data in gas trbine systems is freqently corrected to 15% 2, the data is presented in this manner in Figre 6. Cation shold be exercised hen sing "corrected" data, as this is a mathematical conversion only. Actal NOx mass prodction is increasing ith increased brner firing, bt on a corrected basis, one cold incorrectly assme that NOx is decreasing z :::> 8 :; e. 6 ::;;; x z 4 2 Brner Off / , FIGURE 5: BURNER NOx PRODUCTION (UNCORRECTED ) VS. FIRING RATE <;?- "' :;- 6 e. ::;;; x 4 Brner Off - -- z : FIGURE 6: BURNER NOx PRODUCTION (CORRECTED TO 15% 2 ) VS. FIRING RATE Another freqently sed method of expressing brner NOx prodction rate is LB/1 6 BTU (KG/1 6 KCAL) based on gross heating vale of the fel. A typical garantee vale for spplementary fired systems is. 1 LB/1 6 BTU (.18 KG/1 6 KCAL). Table 2 is a comparison of data from the to installations. In addition, typical vales from conventional fresh air (not TEG) fired boilers sing parallel flo brners, '.dth and ithot air preheat, is inclded for comparison. It is interesting to note the mch loer NOx prodction rates ith TEG as combstion air, compared to fresh air, especially considering the 54 C "preheat". NOx prodction is relatively lo for to reasons. First, as discssed previosly, some of the NOx prodced by combstion in the engine is converted o N2, C2, and H2 by re brning. Second, there is simply less 2 available in the engine exhast to form thermal NOx. Modern engines, ith high combstor :iring temperatres, and eqipped ith ater injection systems for NOx control, yield an exhast that is re la ti vely lo in 2, and high in C2 and H2. The higher inert content of the TEG reslts in loer flame temperatres. If the same engine is operated at redced load, prodcing a higher 2 exhast, NOx prodction from spplementary firing increases significantly. The NOx prodction rates approach the rates for preheated air firing. TABLE 2 - COMPARISON OF NOx PRODUCTION RATES WITH PARALLEL FLOW BURNER SYSTEMS SYSTEM BURNER NOx MASS PRODUCTION RATE System 1 () System 2 (X ) Parallel Flo Brner, Ambient Combstion Air Parallel Flo Brner, 15% Fle Gas Recirclation, Ambient Combstion Air Parallel Flo Brner, 2 9 C Preheated Combstion Air. 8 LB/ 1 6 BTU.8 LB/1 6 BTU. 18 LB/1 6 BTU. 9 LB/1 6 BTU. 45 LB/1 6 BTU (.14 KG/1 6 KCAL) (. 14 KG/1 6 KCAL) (. 32 KG/1 6 KCAL ) (.16 KG/1 6 KCAL) (. 81 KG/1 6 KCAL ) 4 Donloaded From: on 6/19/218 Terms of Use:

5 Emissions of Carbon Monoxide Many gas trbine engines, hen ater injected for NOx control, prodce increased levels of carbon monoxide (CO) and nbrned hydrocarbons (UHC). Withot ater injection, CO is sally negligible, less than 1 PPM (parts per million). Very heavy ater injection can increase CO to greater than 1 PPM. It is often difficlt to strike a balance that ill minimize both NOx and CO emissions, and satisfy permitting athorities. For Systems 1 and 2, gas trbine CO prodction as in the 5-75 PPM range. De to the excellent fel/teg mixing characteristics of the parallel flo brner, and the high adiabatic firing temperatre capability, mch or all of the gas trbine CO can be oxidized by the brner. The degree of CO incineration is dependent pon the brner firing rate, as can be seen in Figre 7. As a general rle, given sfficient oxygen, CO oxidizes very rapidly at temperatres above 8 c. s:: e_ ::;; o._ o._ <( FIGURE 7: CO REDUCTION VS. BURNER FIRING RATE CONCLUSION Using a parallel flo brner design to spplementary fire gas trbine exhast to high temperatres in a ater cooled boiler frnace has many advantages. These can be smmarized as follos: Very high steam prodction rates Very high overall plant efficiency Fll tilization of available preheated oxygen in the engine exhast - Wide brner operating range (trndon) - NOx prodction rates less than.1 LB/1 6 BTU (.18 KG/1 6 KCAL) - Redction of CO in the engine exhast Other advantages, not discssed in this paper, are the capability to fire liqid fels, and the ability to operate on fresh air backp shold the trbine be ot of operation. The primary disadvantage is increased pressre drop across the brner, as compared to an in-line dct brner design. REFERENCES 1. Brechm, G.V., P.L. Langsjoen, et al., "Control of NOx Emissions at a 22 MW Combined Cycle Poer Plant," EPRI Report CS-168, Research Project Backlnd, J.C., et al., "Experiences ith NOx Formation/Redction Cased by Spplementary Firing of Natral Gas in Gas Trbine Exhast Streams", ASME Pblication 85-JPGC-GT Donloaded From: on 6/19/218 Terms of Use: