Download The Generation of VOCs from Stationary Gas Turbines and the Impact on Oxydation Catalyst Performance

Stewart Wyatt MANE6960 AWPPCE Project 1 The generation of VOC’s from stationary gas turbines and the impact on oxidation catalyst performance Abstract The volatile organic compound (VOC) emissions from natural gas fired gas turbines as used for combined cycle power plants are examined with regard to the split between saturated and unsaturated hydrocarbons. How the saturated to unsaturated hydrocarbon ratio in the emissions influences the performance of the oxidation catalyst is investigated and found to be an important criteria in meeting stringent source emission criteria for non‐methane, non‐ethane VOC’s. Natural gas combustion emissions of hydrocarbons are dominated by alkanes, alkenes, aldehydes and aromatics. The alkanes (typically methane, ethane, propane and butane) are all saturated hydrocarbons, the most stable of the hydrocarbon family, and result from unburned fuel that is carried over to the exhaust. Alkenes (typically ethene, propene and butene), aldehydes (typically formaldehyde) and aromatics (typically benzene and toluene) are all unsaturated hydrocarbons that are products of incomplete combustion as they are not typically found in the natural gas fuel. Natural gas combustion is shown to result in less than 50 % of non‐methane, non‐ethane VOC’s to be composed of saturated hydrocarbons. Platinum catalysts are used for the oxidation of carbon monoxide (CO) and VOC’s from combined cycle power plants and show different removal efficiencies for different hydrocarbon species. It is shown that unsaturated VOC’s typically exceed the saturated species for non‐methane, non‐ethane VOC’s, and importantly, unsaturated VOC’s are more readily oxidized by the catalyst before release to the atmosphere as CO2 and H2O. The general classification of VOC’s from most difficult to easiest to oxidize is: o
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Methane (hardest) Non‐methane alkanes Aromatics Alkenes Alcohols and Ethers (easiest) The saturated species are methane and the other alkanes while aromatics and alkenes are unsaturated. Alcohols and ethers are saturated as they do not contain double or triple bonds or rings, although they are oxygenated so are readily oxidized and are rarely found in natural gas combustion products. Hence, it is concluded that unsaturated VOC’s are preferred to saturated VOC’s in the gas turbine exhaust in order to enhance the oxidation catalyst efficiency. Further investigation is required to account for variations in VOC composition and magnitude due to changes in combustion such as occur during part load and start‐up conditions of the gas turbine. Additionally, methane emissions are not currently regulated by the US EPA as it does not play a role in tropospheric ozone formation, however, this may change due to its role as a greenhouse gas. This 1 Stewart Wyatt MANE6960 AWPPCE Project 1 change will lead to a significant shift in the requirement of oxidation catalysts to control the most difficult to oxidize of the saturated hydrocarbons, methane. Introduction Combined cycle power plant emissions include Volatile Organic Compounds (VOC’s) from combustion within the gas turbine. However, environmental regulations for new fossil fuel power plants may limit the VOC emissions to a value less than that contained in the flue gas of the combustion process. To achieve the emissions limits an oxidation catalyst may be used to treat the flue gas before release to the atmosphere. The VOC emissions from the gas turbine of a natural gas fired combined cycle power plant are investigated with a focus on the split between saturated and unsaturated VOC’s and the impact on the oxidation catalyst performance. The VOC distribution of saturated and unsaturated emissions from a gas turbine will be investigated including the role of fuels, combustion temperature, and relationship to CO emissions. Furthermore, the impact of the VOC distribution on the oxidation catalyst performance shall be reviewed. Gas Turbines with Natural Gas Combustion A combined cycle power plant consists of a gas turbine exhaust flow exiting to the atmosphere through a heat recovery steam generator for the supply of steam to a steam turbine. A gas turbine combusting natural gas is considered, where natural gas is nominally methane (CH4) with other minor components as shown in Table 1. Table 1 Pipeline Quality Natural Gas (USA) [1] Component Methane (CH4) Ethane (C2H6) Propane (C3H8) Butanes (C4H10) Pentanes plus (C5H12 +) Nitrogen (N2) and other inerts Carbon dioxide (CO2) Minimum (mol %) 75 Maximum (mol %) 10 5 2 0.5 4 4 The combustion of methane may ideally be considered to produce carbon dioxide and water only as shown in Equation 1. However, the reaction pathway includes 325 elementary steps involving 53 species [3] with the major chemical pathways shown in Figure 1 and 2 Stewart Wyatt MANE6960 AWPPCE Project 1 Figure 2. It is noted that various hydrocarbons form including ethane (C2H6), a higher hydrocarbon than the original reactant methane (CH4). Less than ideal combustion may result in traces of the intermediate hydrocarbons being present in the gas turbine flue gas. The control of these non‐methane VOC emissions by the oxidation catalyst is investigated. Equation 1 CH4 + 2O2 ‐> CO2 + 2H2O Two mechanisms for VOC formation in the exhaust of natural gas fired units are incomplete destruction of organic compounds in the fuel and those resulting from reactions between precursors [2]. The first mechanism indicates that the hydrocarbons present in the fuel will also be present in the exhaust, with Table 1 showing that natural gas is dominated by the lighter hydrocarbons (C3 or less). The second mechanism may result in the formation of compounds that are not present in the fuel, with a possible shift from methane dominated fuel to heavier hydrocarbons (C2‐C6) in the exhaust. Figure 1 High temperature reaction pathway diagram for combustion of methane in a well stirred reactor at T = 2200 K, P = 1 atm and 0.1 s residence time [3] 3 Stewart Wyatt MANE6960 AWPPCE Project 1 Figure 2 Low temperature (< 1500 K) reaction pathway diagram for the combustion of methane in a well stirred reactor at T = 1345 K, P = 1 atm and 0.1 s residence time [3] VOC Emissions from Combustion The US Environmental Protection Authority (EPA) regulates six criteria pollutants through the National Ambient Air Quality Standards (NAAQS); these are carbon monoxide, lead, nitrogen dioxide, ozone, particulate pollution and sulfur dioxide [4]. It is noted that VOC’s are not a criteria pollutant, however, tropospheric, or ground level ozone is not emitted directly into the air but is created by atmospheric reactions of oxides of nitrogen (NOx) and VOC’s [5], which are often created simultaneously in combustion processes. Hence VOC’s are regulated emissions as they are precursors to ground level ozone. The EPA definition of a VOC is any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participate in atmospheric photochemical reactions [6]. This includes any such organic compound but excludes methane, ethane and others considered non‐reactive in the atmosphere [6]. The EPA definition of VOC’s as excluding methane and ethane is used in this report. It is noted that methane, while currently excluded from VOC regulation in the USA, is the second most prevalent greenhouse gas (after CO2) emitted in the United States from human activities [7]. Hence, future regulation as a greenhouse gas rather than ozone precursor is a possible scenario for methane. It will be shown, that of the hydrocarbons emitted from a natural gas fired combined cycle plant, methane is by far the largest component. 4 Stewart Wyatt MANE6960 AWPPCE Project 1 Furthermore, emission regulations continue to change internationally, with the trend towards lower values especially from point sources such as fossil fuel power plants. Current USA regulations for new combined cycle power plants includes projects with 2 ppm limits for NOx, CO, VOC and NH3 slip, also known as “2+2+2+2” regulation [8]. The Electric Power Research Institute (EPRI) [8] noted there is little room for further reductions before reaching the ambient concentrations of these pollutants as shown in Figure 3. Figure 3 CO ambient air quality trend (1980‐2012) for the USA [9] The California Air Resources Board (CARB) strategy for reducing ambient ozone concentration includes modeling of the atmospheric transport and chemistry of ozone formation using VOC and NOx speciation profiles from various sources [2]. The nitrogen oxide species include NO and NOx while the VOC’s which can participate in ozone formation reactions include [2]: o
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Alkanes Alkenes Diolefins Alkynes Cyclanes Aromatics Alcohols, Aldehydes, Ketones, Ethers Chlorinated hydrocarbons The families of hydrocarbons with examples are given in Appendix A. As noted previously, the combustion of natural gas can result in a shift from the hydrocarbons present in the fuel to heavier molecules in the exhaust. Hence the formation of organic compounds within the 5 Stewart Wyatt MANE6960 AWPPCE Project 1 exhaust that are not present in the fuel and shown in Table 2. All fuel component species increased the concentration within the exhaust except the major constituent, methane, that was reduced. The CARB report [2] did not provide a comparison of fuel to emissions hydrocarbon species for a gas turbine, so the internal combustion engine using natural gas should be considered indicative only for a gas turbine engine. The CARB report [2] classified emissions as shown in Table 2, however, it does not differentiate between saturated and unsaturated compounds. For example, C2 may include ethane (single bond), ethene (double bond) and ethyne (triple bond), so cannot be used directly to provide a saturated to unsaturated ratio of VOC emission data. Table 2 Comparison of fuel (natural gas) and emissions organic composition for internal combustion engines [2] Hydrocarbon Species C1 C2 C3 C4 C5 Acetaldehyde Formaldehyde Acrolein PAH Benzene Toluene Xylenes Propene Fuel (%) 86.12 3.49 3.71 3.38 3.27 N/A N/A N/A N/A 0.017 0.013 0.004 0.006 Case 1 Exhaust (%) 61 6.5 8.6 8.6 11.6 0.38 2 0.1 0.01 0.28 0.09 0.03 0.83 Fuel (%) 89.50 8.10 3.30 0.53 0.06 N/A N/A N/A N/A 0.00020 0.00008 0.00006 0.016 Case 2 Exhaust (%) 70 13.67 12.01 1.91 0.34 0.12 0.91 0.03 0.01 0.08 0.02 0.007 0.74 Demanyo et al [10] analyzed the emissions from a natural gas fired industrial burner for C2‐C10 aliphatic hydrocarbons, aromatic hydrocarbons, and aldehydes. The VOC’s where chosen because of their abundance in urban atmospheres and their role in the formation of tropospheric ozone. Demanyo et al [10] hypothesized that the industrial burners examined with fuel lean conditions produce both locally lean and rich fuel‐air mixture packets within turbulent eddies. Those that are too lean or too rich do not ignite and pass into the post‐combustion zone resulting in emissions of unburned fuel. Similarly, igniting lean packets may result in the oxidation of lighter hydrocarbons into aldehydes while rich packets can result in alkenes, alkynes and aromatics. Furthermore, partial oxidation may occur when the ignited fuel‐air packets fail to reach completion due to quenching from a cooler surface (thermal quench) or rapid dilution with surrounding air (mixing quench), resulting in partially oxidized emissions. Further reactions may occur downstream of the combustion zone as VOC’s, unlike NOx, do not freeze shortly after entering the post combustion zone [10]. Unreacted or partially reacted packets may thermally decompose or react with other gaseous or surface materials in the downstream flow depending on temperature, pressure and residence time. 6 Stewart Wyatt MANE6960 AWPPCE Project 1 The VOC emissions profile of Demanyo et al [10] included mostly methane, lighter (C2‐C5) alkanes, alkenes and alkynes, as well as aldehydes (formaldehyde and acetaldehyde) and trace amounts of aromatics (benzene and toluene). Presence of alkanes (C2‐C5) suggest unburned fuel. The alkenes (especially ethene, propene and isobutene) and alkynes (especially acetylene), none of which were detected in the natural gas supply, may result from the pyrolytic dehydrogenation of the alkanes. The natural gas combustion emission species are shown in Table 3, note that case 5 is a “stressed” flame near its high swirl stability limits. Table 3 Natural gas combustion emissions from an industrial low NOx burner [10] Of the non‐methane, non‐ethane emissions presented in Table 3, the saturated to unsaturated hydrocarbon ratio (not including those below the lower detection limit) is given in Table 4. The non‐
methane, non‐ethane saturated to unsaturated VOC ratio is shown to vary from 0 to 0.94. Some of the variation may be due to the data below the lower detectable limit for that species being ignored; this may be significant since when the data was available it was of nominally the same magnitude. For example, propane was not detected for cases 1 and 2 with a lower detectable limit of 4 ppbv but was measured at 13, 5 and 94 ppv for the remainder of cases. Similarly, non‐detected species where 7 Stewart Wyatt MANE6960 AWPPCE Project 1 recorded for 22 and 5 of the 26 non‐methane, non‐ethane VOC species in case 1 and case 2 respectively, indicative of a possible error in the summation of saturated and unsaturated components. Unsaturated non‐methane, non‐ethane VOC’s exceeded the saturated for all cases. Table 4 Non‐methane, non‐ethane saturated and unsaturated VOC emissions from a natural gas fired industrial burner Saturated (ppbv) Unsaturated (ppbv) Saturated/Unsaturated Case 1 0 24 0.00 Case 2 108 114 0.94 Case 3 0 62 0.00 Case 4 9 1030 0.01 Case 5 9096 49857 0.18 Theloke and Friedrich [11] categorized VOC emission species for sectors including solvent use, transportation and stationary combustion. Although not specific to combustion of natural gas in gas turbines, the results for stationary combustion sources are presented in Figure 4. The saturated to unsaturated VOC ratio is 29 % as only alkanes are saturated. Theloke and Friedrich [11] measured non‐
methane VOC’s, hence ethane has been included in the alkanes category. Therefore, the non‐methane, non‐ethane saturated VOC’s are expected to be less than the reported 29 % represented as alkanes. Figure 4 Speciation of the VOC emissions from stationary combustion sources in Germany 1998 [11] CARB [2]; Demanyo et al [10]; and Theloke and Friedrich [11] provided VOC emissions data from natural gas combustion in industrial applications [2, 10] and stationary combustion sources [11], however, the specific application of a stationary gas turbine using natural gas has not been considered. 8 Stewart Wyatt MANE6960 AWPPCE Project 1 The Electric Power Research Institute (EPRI) [12] conducted an emission testing program on a combined cycle Siemens‐Westinghouse 501F gas fired combustion turbine with lean pre‐mix low NOx combustors. The program included measurement of formaldehyde, benzene and other volatile organic compounds at base load, part load and transients (including start up and shut down) of the gas turbine. However, the results are of limited value for this study of the emission ratio of non‐methane, non‐ethane saturated to unsaturated VOC’s as it did not measure alkanes other than methane. The VOC’s of most interest to the EPRI investigation were Formaldehyde, benzene, toluene, ethylbenzene and xylenes as reflected in the results presented in Table 5. Table 5 Gas turbine (Siemens‐Westinghouse 501F) emissions at full load [12] Oxidation Catalysts for the Control of VOC’s As noted previously, VOC emissions from stationary gas turbine sources of non‐methane, non‐ethane VOC’s are typically limited to approximately 2 ppmvd@15% O2 within the USA [8]. Both combustion and post combustion means are employed to meet the VOC limits, post combustion methods include 9 Stewart Wyatt MANE6960 AWPPCE Project 1 oxidation catalysts. The oxidation catalyst is used to convert CO and VOC’s to CO2 and H2O before release to the atmosphere. The oxidation efficiency of VOC’s with a platinum catalyst at a given temperature is a function of the chemical nature of the compound. As a general rule, the classification of VOC’s from most difficult to easiest to oxidize is [13]: o
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Methane (hardest) Non‐methane alkanes Aromatics Alkenes Alcohols and Ethers (easiest) Furthermore, temperatures of activation for oxidation of VOC’s in a platinum catalyst are lower for unsaturated hydrocarbons than saturated ones [12]. The ease of oxidation is reflected in the auto ignition temperatures (AIT) of VOC’s with larger molecules within a VOC family having lower AIT’s than the smaller molecules. However, VOC families show negligible changes of AIT’s between the corresponding molecules of non‐methane alkanes, alkenes, alkynes and aromatics. AIT’s of selected VOC’s are provided in Table 6. Table 6 Autoignition temperatures of selected VOC's in air [15] Species Formula VOC Family Methane Ethane Propane Butane Ethlene Proplene Butlene Ethyne Propyne Benzene Toluene Methanol Ethanol Propanol Formaldehyde CH4 C2H6 C3H8 C4H10 C2H4 C3H6 C4H8 C2H2 C3H4 C6H6 C7H8 CH3OH C2H5OH C3H7OH CH2O Alkane Alkene Alkyne Aromatic Alcohols Aldehydes Auto ignition temperature (C) 600 472 450 288 490 460 385 305 N/A 498 480 385 363 413 430 Additional Work Future work may include the measurement of VOC speciation for particular gas turbine models using natural gas and other fuels. The VOC emission data collected for this report was of a general nature 10 Stewart Wyatt MANE6960 AWPPCE Project 1 such as natural gas combustion but in industrial burners instead of gas turbines, and gas turbine emissions but with limited VOC species measured. VOC emissions are also strongly dependent upon combustion characteristics so a gas turbine at full load under steady operating conditions is likely to have a different VOC profile than the same machine under start up or transient conditions. As emissions regulations evolve it is predicted that start up emissions will need to be accounted for so data over a wide range of operating conditions will be required. Methane was shown to be the largest component of VOC emissions from natural gas combustion. However, current US EPA VOC regulation does not account for methane (or ethane) due to its non‐
reactive nature in the atmosphere. It is also the most difficult species to oxidize in the catalyst. However, the role of methane as a greenhouse gas would make it likely that it will be regulated in the future. This would imply a future shift in catalyst design from one suited to non‐methane VOC gases to a methane oxidation catalyst. Conclusion Combustion of natural gas in a gas turbine results in VOC emissions due to unburned fuel and partially oxidized products leaving the combustion zone. Environmental regulations often dictate that the VOC’s contained in the flue gas must be further reduced in the post combustion zone, prior to release to the atmosphere. A proven means for oxidizing the VOC’s is a platinum catalyst whose effectiveness is a function of VOC species, catalyst temperature, residence time, platinum dispersion within the catalyst, platinum support structure and others. The generation of the VOC species and their impact on the oxidation catalyst was examined. Saturated VOC’s are more difficult to destroy within an oxidation catalyst than unsaturated VOC’s. Also, the non‐methane, non‐ethane VOC emissions from natural gas combustion has been shown to be dominated by unsaturated species. Hence, the practice of treating non‐methane, non‐ethane VOC emissions from stationary gas turbine exhausts as a maximum of 50 % saturated VOC’s is a reasonable commercial practice in order to provide guaranteed oxidation catalyst performance, when actual VOC speciation is not provided by the gas turbine manufacturer. References 1. Foss M. M., 2004, “Interstate Natural Gas – Quality Specifications & Interchangeability,” accessed 2 October 2013, http://www.beg.utexas.edu/energyecon/lng/documents/CEE_Interstate_Natural_Gas_Quality_
Specifications_and_Interchangeability.pdf 2. California Environmental Protection Agency: Air Resources Board, 1995, NOx and VOC Species Profiles for Gas Fired Stationary Combustion Sources, Volume 1, Final Report, Contract No. A132‐
104. 3. Turns S. R., 2012, An Introduction to Combustion: Concepts and Applications, 3rd edition, McGraw Hill, New York. 11 Stewart Wyatt MANE6960 AWPPCE Project 1 4. United States, Environmental Protection Authority (US EPA), 2013, “National Ambient Air Quality Standards,” accessed 13 October, http://www.epa.gov/air/criteria.html 5. United States, Environmental Protection Authority (US EPA), 2013, “Ground Level Ozone: Basic Information,” accessed 13 October, http://www.epa.gov/airquality/ozonepollution/basic.html 6. US Government Printing Office, 2013, “Title 40: Protection of Environment, Part 51, Subpart F,” accessed 18 October 2013, http://www.ecfr.gov/cgi‐bin/text‐
idx?c=ecfr&rgn=div8&view=text&node=40:2.0.1.1.2.3.8.1&idno=40 7. United States, Environmental Protection Authority (US EPA), 2013, “Overview of Greenhouse Gases: Methane Emissions,” accessed 19 October, http://epa.gov/climatechange/ghgemissions/gases/ch4.html 8. Electric Power Research Institute (EPRI), 2002, CT Experience and Intelligence Report, October issue. 9. United States, Environmental Protection Authority (US EPA), 2013, “Air Trends: Carbon Monoxide,” accessed 19 October, http://www.epa.gov/airtrends/carbon.html 10. Demanyo T. N., Miyasato M. M., Samuelsen G. S., 1998, “Hazardous Air Pollutant and Ozone Precursor Emissions from a Low‐NOx Natural Gas‐Fired Industrial Burner,” Twenty‐Seventh Symposium (International) on Combustion/The Combustion Institute, pages 1283‐1291. 11. Theloke J., Friedrich R., 2007, “Compilation of a Database on the Composition of Anthropogenic VOC emissions for Atmospheric Modeling in Europe,” Atmospheric Environment, 41, pages 4148‐4160. 12. Electric Power Research Institute (EPRI), 2004, Formaldehyde and VOC Emissions from a Siemens‐Westinghouse 501F Combustion Turbine with Lean Pre‐Mix Combustors and SCR and CO Catalysts, 1005408, Final Report, January. 13. Windawi H., Wyatt M., 1993, “Catalytic Destruction of Halogenated Volatile Organic Compounds: Mechanisms of Platinum Catalyst Systems,” Platinum Metals Review, 37 (4), pages 186‐193. 14. Windawi H., Zhang Z. C., 1996, “Catalytic Destruction of Halogenated Air Toxins and the Effects of Admixture with VOC’s,” Catalysis Today, 30, pages 99‐105. 15. National Fire Protection Authority (NFPA), 2012, NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas 2012 Edition, NFPA, Quincy. 16. Kuo K. K., 2005, Principles of Combustion, 2nd edition, John Wiley & Sons, Hoboken. 17. Wikipedia, 2013, “Saturation (chemistry),” accessed 13 October 2013, http://en.wikipedia.org/wiki/Saturation_(chemistry) 12 Stewart Wyatt Appendix A MANE6960 AWPPCE Project 1 Hydrocarbons, Alcohols and other organic compound families [3, 16] Family Name Alkanes Other Designation Paraffins Molecular Formula CnH2n+2 Alkenes Olefins CnH2n Functional Group Saturated or Unsaturated Single C‐H bonds Saturated and single C‐C bonds Alkynes Acetylenes CnH2n‐2 Diolefins Diene Cyclanes Cycloalkanes, CnH2n or Cycloparaffins, (CH2)n Napthenes Examples Methane CH4 Ethane CH3‐CH3 Propane CH3‐CH2‐CH3 Unsaturated Ethene Propene C3H6 Unsaturated Ethyne Propyne C3H4 Unsaturated 1,2‐Propadiene Unsaturated Cyclopropane C3H6 Cyclohexane C6H12 13 Stewart Wyatt Aromatics Benzene family MANE6960 AWPPCE Project 1 CnH2n‐6 Unsaturated Benzene C6H6 Alcohols ‐ ROH Saturated Methanol CH3OH Aldehydes ‐ Unsaturated Formaldehyde CH2O Ketones ‐ Unsaturated Acetone C3H6O Ethers o
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‐ R‐O‐R R is the parent hydrocarbon radical A saturated compound has no double or triple bonds or ring [17]. 14 Saturated Dimethyl ether CH3‐O‐
CH3