The generation of VOC s from stationary gas turbines and the impact on oxidation catalyst performance

Transcription

1 The generatin f VOC s frm statinary gas turbines and the impact n xidatin catalyst perfrmance Prject 1 Abstract The vlatile rganic cmpund (VOC) emissins frm natural gas fired gas turbines as used fr cmbined cycle pwer plants are examined with regard t the split between saturated and unsaturated hydrcarbns. Hw the saturated t unsaturated hydrcarbn rati in the emissins influences the perfrmance f the xidatin catalyst is investigated and fund t be an imprtant criteria in meeting stringent surce emissin criteria fr nn methane, nn ethane VOC s. Natural gas cmbustin emissins f hydrcarbns are dminated by alkanes, alkenes, aldehydes and armatics. The alkanes (typically methane, ethane, prpane and butane) are all saturated hydrcarbns, the mst stable f the hydrcarbn family, and result frm unburned fuel that is carried ver t the exhaust. Alkenes (typically ethene, prpene and butene), aldehydes (typically frmaldehyde) and armatics (typically benzene and tluene) are all unsaturated hydrcarbns that are prducts f incmplete cmbustin as they are nt typically fund in the natural gas fuel. Natural gas cmbustin is shwn t result in less than 50 % f nn methane, nn ethane VOC s t be cmpsed f saturated hydrcarbns. Platinum catalysts are used fr the xidatin f carbn mnxide (CO) and VOC s frm cmbined cycle pwer plants and shw different remval efficiencies fr different hydrcarbn species. It is shwn that unsaturated VOC s typically exceed the saturated species fr nn methane, nn ethane VOC s, and imprtantly, unsaturated VOC s are mre readily xidized by the catalyst befre release t the atmsphere as CO 2 and H 2 O. The general classificatin f VOC s frm mst difficult t easiest t xidize is: Methane (hardest) Nn methane alkanes Armatics Alkenes Alchls and Ethers (easiest) The saturated species are methane and the ther alkanes while armatics and alkenes are unsaturated. Alchls and ethers are saturated as they d nt cntain duble r triple bnds r rings, althugh they are xygenated s are readily xidized and are rarely fund in natural gas cmbustin prducts. Hence, it is cncluded that unsaturated VOC s are preferred t saturated VOC s in the gas turbine exhaust in rder t enhance the xidatin catalyst efficiency. Further investigatin is required t accunt fr variatins in VOC cmpsitin and magnitude due t changes in cmbustin such as ccur during part lad and start up cnditins f the gas turbine. Additinally, methane emissins are nt currently regulated by the US EPA as it des nt play a rle in trpspheric zne frmatin, hwever, this may change due t its rle as a greenhuse gas. This 1

2 Prject 1 change will lead t a significant shift in the requirement f xidatin catalysts t cntrl the mst difficult t xidize f the saturated hydrcarbns, methane. Intrductin Cmbined cycle pwer plant emissins include Vlatile Organic Cmpunds (VOC s) frm cmbustin within the gas turbine. Hwever, envirnmental regulatins fr new fssil fuel pwer plants may limit the VOC emissins t a value less than that cntained in the flue gas f the cmbustin prcess. T achieve the emissins limits an xidatin catalyst may be used t treat the flue gas befre release t the atmsphere. The VOC emissins frm the gas turbine f a natural gas fired cmbined cycle pwer plant are investigated with a fcus n the split between saturated and unsaturated VOC s and the impact n the xidatin catalyst perfrmance. The VOC distributin f saturated and unsaturated emissins frm a gas turbine will be investigated including the rle f fuels, cmbustin temperature, and relatinship t CO emissins. Furthermre, the impact f the VOC distributin n the xidatin catalyst perfrmance shall be reviewed. Gas Turbines with Natural Gas Cmbustin A cmbined cycle pwer plant cnsists f a gas turbine exhaust flw exiting t the atmsphere thrugh a heat recvery steam generatr fr the supply f steam t a steam turbine. A gas turbine cmbusting natural gas is cnsidered, where natural gas is nminally methane (CH4) with ther minr cmpnents as shwn in Table 1. Table 1 Pipeline Quality Natural Gas (USA) [1] Cmpnent Minimum (ml %) Maximum (ml %) Methane (CH 4 ) 75 Ethane (C 2 H 6 ) 10 Prpane (C 3 H 8 ) 5 Butanes (C 4 H 10 ) 2 Pentanes plus (C 5 H 12 +) 0.5 Nitrgen (N 2 ) and ther inerts 4 Carbn dixide (CO 2 ) 4 The cmbustin f methane may ideally be cnsidered t prduce carbn dixide and water nly as shwn in Equatin 1. Hwever, the reactin pathway includes 325 elementary steps invlving 53 species [3] with the majr chemical pathways shwn in Figure 1 and 2

3 Prject 1 Figure 2. It is nted that varius hydrcarbns frm including ethane (C 2 H 6 ), a higher hydrcarbn than the riginal reactant methane (CH 4 ). Less than ideal cmbustin may result in traces f the intermediate hydrcarbns being present in the gas turbine flue gas. The cntrl f these nn methane VOC emissins by the xidatin catalyst is investigated. Equatin 1 CH 4 + 2O 2 > CO 2 + 2H 2 O Tw mechanisms fr VOC frmatin in the exhaust f natural gas fired units are incmplete destructin f rganic cmpunds in the fuel and thse resulting frm reactins between precursrs [2]. The first mechanism indicates that the hydrcarbns present in the fuel will als be present in the exhaust, with Table 1 shwing that natural gas is dminated by the lighter hydrcarbns (C3 r less). The secnd mechanism may result in the frmatin f cmpunds that are nt present in the fuel, with a pssible shift frm methane dminated fuel t heavier hydrcarbns (C2 C6) in the exhaust. Figure 1 High temperature reactin pathway diagram fr cmbustin f methane in a well stirred reactr at T = 2200 K, P = 1 atm and 0.1 s residence time [3] 3

4 Prject 1 Figure 2 Lw temperature (< 1500 K) reactin pathway diagram fr the cmbustin f methane in a well stirred reactr at T = 1345 K, P = 1 atm and 0.1 s residence time [3] VOC Emissins frm Cmbustin The US Envirnmental Prtectin Authrity (EPA) regulates six criteria pllutants thrugh the Natinal Ambient Air Quality Standards (NAAQS); these are carbn mnxide, lead, nitrgen dixide, zne, particulate pllutin and sulfur dixide [4]. It is nted that VOC s are nt a criteria pllutant, hwever, trpspheric, r grund level zne is nt emitted directly int the air but is created by atmspheric reactins f xides f nitrgen (NOx) and VOC s [5], which are ften created simultaneusly in cmbustin prcesses. Hence VOC s are regulated emissins as they are precursrs t grund level zne. The EPA definitin f a VOC is any cmpund f carbn, excluding carbn mnxide, carbn dixide, carbnic acid, metallic carbides r carbnates, and ammnium carbnate, which participate in atmspheric phtchemical reactins [6]. This includes any such rganic cmpund but excludes methane, ethane and thers cnsidered nn reactive in the atmsphere [6]. The EPA definitin f VOC s as excluding methane and ethane is used in this reprt. It is nted that methane, while currently excluded frm VOC regulatin in the USA, is the secnd mst prevalent greenhuse gas (after CO2) emitted in the United States frm human activities [7]. Hence, future regulatin as a greenhuse gas rather than zne precursr is a pssible scenari fr methane. It will be shwn, that f the hydrcarbns emitted frm a natural gas fired cmbined cycle plant, methane is by far the largest cmpnent. 4

5 Prject 1 Furthermre, emissin regulatins cntinue t change internatinally, with the trend twards lwer values especially frm pint surces such as fssil fuel pwer plants. Current USA regulatins fr new cmbined cycle pwer plants includes prjects with 2 ppm limits fr NOx, CO, VOC and NH3 slip, als knwn as regulatin [8]. The Electric Pwer Research Institute (EPRI) [8] nted there is little rm fr further reductins befre reaching the ambient cncentratins f these pllutants as shwn in Figure 3. Figure 3 CO ambient air quality trend ( ) fr the USA [9] The Califrnia Air Resurces Bard (CARB) strategy fr reducing ambient zne cncentratin includes mdeling f the atmspheric transprt and chemistry f zne frmatin using VOC and NOx speciatin prfiles frm varius surces [2]. The nitrgen xide species include NO and NOx while the VOC s which can participate in zne frmatin reactins include [2]: Alkanes Alkenes Dilefins Alkynes Cyclanes Armatics Alchls, Aldehydes, Ketnes, Ethers Chlrinated hydrcarbns The families f hydrcarbns with examples are given in Appendix A. As nted previusly, the cmbustin f natural gas can result in a shift frm the hydrcarbns present in the fuel t heavier mlecules in the exhaust. Hence the frmatin f rganic cmpunds within the 5

6 Prject 1 exhaust that are nt present in the fuel and shwn in Table 2. All fuel cmpnent species increased the cncentratin within the exhaust except the majr cnstituent, methane, that was reduced. The CARB reprt [2] did nt prvide a cmparisn f fuel t emissins hydrcarbn species fr a gas turbine, s the internal cmbustin engine using natural gas shuld be cnsidered indicative nly fr a gas turbine engine. The CARB reprt [2] classified emissins as shwn in Table 2, hwever, it des nt differentiate between saturated and unsaturated cmpunds. Fr example, C2 may include ethane (single bnd), ethene (duble bnd) and ethyne (triple bnd), s cannt be used directly t prvide a saturated t unsaturated rati f VOC emissin data. Table 2 Cmparisn f fuel (natural gas) and emissins rganic cmpsitin fr internal cmbustin engines [2] Case 1 Case 2 Hydrcarbn Species Fuel (%) Exhaust (%) Fuel (%) Exhaust (%) C C C C C Acetaldehyde N/A 0.38 N/A 0.12 Frmaldehyde N/A 2 N/A 0.91 Acrlein N/A 0.1 N/A 0.03 PAH N/A 0.01 N/A 0.01 Benzene Tluene Xylenes Prpene Demany et al [10] analyzed the emissins frm a natural gas fired industrial burner fr C2 C10 aliphatic hydrcarbns, armatic hydrcarbns, and aldehydes. The VOC s where chsen because f their abundance in urban atmspheres and their rle in the frmatin f trpspheric zne. Demany et al [10] hypthesized that the industrial burners examined with fuel lean cnditins prduce bth lcally lean and rich fuel air mixture packets within turbulent eddies. Thse that are t lean r t rich d nt ignite and pass int the pst cmbustin zne resulting in emissins f unburned fuel. Similarly, igniting lean packets may result in the xidatin f lighter hydrcarbns int aldehydes while rich packets can result in alkenes, alkynes and armatics. Furthermre, partial xidatin may ccur when the ignited fuel air packets fail t reach cmpletin due t quenching frm a cler surface (thermal quench) r rapid dilutin with surrunding air (mixing quench), resulting in partially xidized emissins. Further reactins may ccur dwnstream f the cmbustin zne as VOC s, unlike NOx, d nt freeze shrtly after entering the pst cmbustin zne [10]. Unreacted r partially reacted packets may thermally decmpse r react with ther gaseus r surface materials in the dwnstream flw depending n temperature, pressure and residence time. 6

7 Prject 1 The VOC emissins prfile f Demany et al [10] included mstly methane, lighter (C2 C5) alkanes, alkenes and alkynes, as well as aldehydes (frmaldehyde and acetaldehyde) and trace amunts f armatics (benzene and tluene). Presence f alkanes (C2 C5) suggest unburned fuel. The alkenes (especially ethene, prpene and isbutene) and alkynes (especially acetylene), nne f which were detected in the natural gas supply, may result frm the pyrlytic dehydrgenatin f the alkanes. The natural gas cmbustin emissin species are shwn in Table 3, nte that case 5 is a stressed flame near its high swirl stability limits. Table 3 Natural gas cmbustin emissins frm an industrial lw NOx burner [10] Of the nn methane, nn ethane emissins presented in Table 3, the saturated t unsaturated hydrcarbn rati (nt including thse belw the lwer detectin limit) is given in Table 4. The nnmethane, nn ethane saturated t unsaturated VOC rati is shwn t vary frm 0 t Sme f the variatin may be due t the data belw the lwer detectable limit fr that species being ignred; this may be significant since when the data was available it was f nminally the same magnitude. Fr example, prpane was nt detected fr cases 1 and 2 with a lwer detectable limit f 4 ppbv but was measured at 13, 5 and 94 ppv fr the remainder f cases. Similarly, nn detected species where 7

8 Prject 1 recrded fr 22 and 5 f the 26 nn methane, nn ethane VOC species in case 1 and case 2 respectively, indicative f a pssible errr in the summatin f saturated and unsaturated cmpnents. Unsaturated nn methane, nn ethane VOC s exceeded the saturated fr all cases. Table 4 Nn methane, nn ethane saturated and unsaturated VOC emissins frm a natural gas fired industrial burner Case 1 Case 2 Case 3 Case 4 Case 5 Saturated (ppbv) Unsaturated (ppbv) Saturated/Unsaturated Thelke and Friedrich [11] categrized VOC emissin species fr sectrs including slvent use, transprtatin and statinary cmbustin. Althugh nt specific t cmbustin f natural gas in gas turbines, the results fr statinary cmbustin surces are presented in Figure 4. The saturated t unsaturated VOC rati is 29 % as nly alkanes are saturated. Thelke and Friedrich [11] measured nnmethane VOC s, hence ethane has been included in the alkanes categry. Therefre, the nn methane, nn ethane saturated VOC s are expected t be less than the reprted 29 % represented as alkanes. Figure 4 Speciatin f the VOC emissins frm statinary cmbustin surces in Germany 1998 [11] CARB [2]; Demany et al [10]; and Thelke and Friedrich [11] prvided VOC emissins data frm natural gas cmbustin in industrial applicatins [2, 10] and statinary cmbustin surces [11], hwever, the specific applicatin f a statinary gas turbine using natural gas has nt been cnsidered. 8

9 Prject 1 The Electric Pwer Research Institute (EPRI) [12] cnducted an emissin testing prgram n a cmbined cycle Siemens Westinghuse 501F gas fired cmbustin turbine with lean pre mix lw NOx cmbustrs. The prgram included measurement f frmaldehyde, benzene and ther vlatile rganic cmpunds at base lad, part lad and transients (including start up and shut dwn) f the gas turbine. Hwever, the results are f limited value fr this study f the emissin rati f nn methane, nn ethane saturated t unsaturated VOC s as it did nt measure alkanes ther than methane. The VOC s f mst interest t the EPRI investigatin were Frmaldehyde, benzene, tluene, ethylbenzene and xylenes as reflected in the results presented in Table 5. Table 5 Gas turbine (Siemens Westinghuse 501F) emissins at full lad [12] Oxidatin Catalysts fr the Cntrl f VOC s As nted previusly, VOC emissins frm statinary gas turbine surces f nn methane, nn ethane VOC s are typically limited t apprximately 2 ppmvd@15% O2 within the USA [8]. Bth cmbustin and pst cmbustin means are emplyed t meet the VOC limits, pst cmbustin methds include 9

10 Prject 1 xidatin catalysts. The xidatin catalyst is used t cnvert CO and VOC s t CO 2 and H 2 O befre release t the atmsphere. The xidatin efficiency f VOC s with a platinum catalyst at a given temperature is a functin f the chemical nature f the cmpund. As a general rule, the classificatin f VOC s frm mst difficult t easiest t xidize is [13]: Methane (hardest) Nn methane alkanes Armatics Alkenes Alchls and Ethers (easiest) Furthermre, temperatures f activatin fr xidatin f VOC s in a platinum catalyst are lwer fr unsaturated hydrcarbns than saturated nes [12]. The ease f xidatin is reflected in the aut ignitin temperatures (AIT) f VOC s with larger mlecules within a VOC family having lwer AIT s than the smaller mlecules. Hwever, VOC families shw negligible changes f AIT s between the crrespnding mlecules f nn methane alkanes, alkenes, alkynes and armatics. AIT s f selected VOC s are prvided in Table 6. Table 6 Autignitin temperatures f selected VOC's in air [15] Species Frmula VOC Family Aut ignitin temperature (C) Methane CH 4 Alkane 600 Ethane C 2 H Prpane C 3 H Butane C 4 H Ethlene C 2 H 4 Alkene 490 Prplene C 3 H Butlene C 4 H Ethyne C 2 H 2 Alkyne 305 Prpyne C 3 H 4 N/A Benzene C 6 H 6 Armatic 498 Tluene C 7 H Methanl CH 3 OH Alchls 385 Ethanl C 2 H 5 OH 363 Prpanl C 3 H 7 OH 413 Frmaldehyde CH 2 O Aldehydes 430 Additinal Wrk Future wrk may include the measurement f VOC speciatin fr particular gas turbine mdels using natural gas and ther fuels. The VOC emissin data cllected fr this reprt was f a general nature 10

11 Prject 1 such as natural gas cmbustin but in industrial burners instead f gas turbines, and gas turbine emissins but with limited VOC species measured. VOC emissins are als strngly dependent upn cmbustin characteristics s a gas turbine at full lad under steady perating cnditins is likely t have a different VOC prfile than the same machine under start up r transient cnditins. As emissins regulatins evlve it is predicted that start up emissins will need t be accunted fr s data ver a wide range f perating cnditins will be required. Methane was shwn t be the largest cmpnent f VOC emissins frm natural gas cmbustin. Hwever, current US EPA VOC regulatin des nt accunt fr methane (r ethane) due t its nnreactive nature in the atmsphere. It is als the mst difficult species t xidize in the catalyst. Hwever, the rle f methane as a greenhuse gas wuld make it likely that it will be regulated in the future. This wuld imply a future shift in catalyst design frm ne suited t nn methane VOC gases t a methane xidatin catalyst. Cnclusin Cmbustin f natural gas in a gas turbine results in VOC emissins due t unburned fuel and partially xidized prducts leaving the cmbustin zne. Envirnmental regulatins ften dictate that the VOC s cntained in the flue gas must be further reduced in the pst cmbustin zne, prir t release t the atmsphere. A prven means fr xidizing the VOC s is a platinum catalyst whse effectiveness is a functin f VOC species, catalyst temperature, residence time, platinum dispersin within the catalyst, platinum supprt structure and thers. The generatin f the VOC species and their impact n the xidatin catalyst was examined. Saturated VOC s are mre difficult t destry within an xidatin catalyst than unsaturated VOC s. Als, the nn methane, nn ethane VOC emissins frm natural gas cmbustin has been shwn t be dminated by unsaturated species. Hence, the practice f treating nn methane, nn ethane VOC emissins frm statinary gas turbine exhausts as a maximum f 50 % saturated VOC s is a reasnable cmmercial practice in rder t prvide guaranteed xidatin catalyst perfrmance, when actual VOC speciatin is nt prvided by the gas turbine manufacturer. References 1. Fss M. M., 2004, Interstate Natural Gas Quality Specificatins & Interchangeability, accessed 2 Octber 2013, Specificatins_and_Interchangeability.pdf 2. Califrnia Envirnmental Prtectin Agency: Air Resurces Bard, 1995, NOx and VOC Species Prfiles fr Gas Fired Statinary Cmbustin Surces, Vlume 1, Final Reprt, Cntract N. A Turns S. R., 2012, An Intrductin t Cmbustin: Cncepts and Applicatins, 3 rd editin, McGraw Hill, New Yrk. 11

12 Prject 1 4. United States, Envirnmental Prtectin Authrity (US EPA), 2013, Natinal Ambient Air Quality Standards, accessed 13 Octber, 5. United States, Envirnmental Prtectin Authrity (US EPA), 2013, Grund Level Ozne: Basic Infrmatin, accessed 13 Octber, 6. US Gvernment Printing Office, 2013, Title 40: Prtectin f Envirnment, Part 51, Subpart F, accessed 18 Octber 2013, bin/textidx?c=ecfr&rgn=div8&view=text&nde=40: &idn=40 7. United States, Envirnmental Prtectin Authrity (US EPA), 2013, Overview f Greenhuse Gases: Methane Emissins, accessed 19 Octber, 8. Electric Pwer Research Institute (EPRI), 2002, CT Experience and Intelligence Reprt, Octber issue. 9. United States, Envirnmental Prtectin Authrity (US EPA), 2013, Air Trends: Carbn Mnxide, accessed 19 Octber, Demany T. N., Miyasat M. M., Samuelsen G. S., 1998, Hazardus Air Pllutant and Ozne Precursr Emissins frm a Lw NOx Natural Gas Fired Industrial Burner, Twenty Seventh Sympsium (Internatinal) n Cmbustin/The Cmbustin Institute, pages Thelke J., Friedrich R., 2007, Cmpilatin f a Database n the Cmpsitin f Anthrpgenic VOC emissins fr Atmspheric Mdeling in Eurpe, Atmspheric Envirnment, 41, pages Electric Pwer Research Institute (EPRI), 2004, Frmaldehyde and VOC Emissins frm a Siemens Westinghuse 501F Cmbustin Turbine with Lean Pre Mix Cmbustrs and SCR and CO Catalysts, , Final Reprt, January. 13. Windawi H., Wyatt M., 1993, Catalytic Destructin f Halgenated Vlatile Organic Cmpunds: Mechanisms f Platinum Catalyst Systems, Platinum Metals Review, 37 (4), pages Windawi H., Zhang Z. C., 1996, Catalytic Destructin f Halgenated Air Txins and the Effects f Admixture with VOC s, Catalysis Tday, 30, pages Natinal Fire Prtectin Authrity (NFPA), 2012, NFPA 497 Recmmended Practice fr the Classificatin f Flammable Liquids, Gases, r Vaprs and f Hazardus (Classified) Lcatins fr Electrical Installatins in Chemical Prcess Areas 2012 Editin, NFPA, Quincy. 16. Ku K. K., 2005, Principles f Cmbustin, 2 nd editin, Jhn Wiley & Sns, Hbken. 17. Wikipedia, 2013, Saturatin (chemistry), accessed 13 Octber 2013, 12

13 Appendix A Prject 1 Hydrcarbns, Alchls and ther rganic cmpund families [3, 16] Family Name Other Designatin Mlecular Frmula Functinal Grup Alkanes Paraffins C n H 2n+2 Single C H bnds and single C C bnds Saturated r Unsaturated Saturated Examples Methane CH4 Ethane CH3 CH3 Prpane CH3 CH2 CH3 Alkenes Olefins C n H 2n Unsaturated Ethene Prpene C3H6 Alkynes Acetylenes C n H 2n 2 Unsaturated Ethyne Prpyne C3H4 Dilefins Diene Unsaturated 1,2 Prpadiene Cyclanes Cyclalkanes, Cyclparaffins, Napthenes C n H 2n r (CH 2 ) n Unsaturated Cyclprpane C3H6 Cyclhexane C6H12 13

14 Prject 1 Armatics Benzene family CnH2n 6 Unsaturated Benzene C6H6 Alchls ROH Saturated Methanl CH 3 OH Aldehydes Unsaturated Frmaldehyde CH 2 O Ketnes Unsaturated Acetne C 3 H 6 O Ethers R O R Saturated Dimethyl ether CH 3 O CH 3 R is the parent hydrcarbn radical A saturated cmpund has n duble r triple bnds r ring [17]. 14