Subscriber access provided by READING UNIV
Review
Review of novel combustion techniques for clean power production in gas turbines Medhat Ahmed Nemitallah, Sherif S. Rashwan, Ibrahimm B. Mansir, Ahmed A. Abdelhafez, and Mohamed A. M. Habib Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03607 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Review of novel combustion techniques for clean power production in gas turbines By: Medhat A. Nemitallah. Sherif S. Rashwan, Ibrahim B. Mansir, Ahmed A. Abdelhafez and Mohamed A. Habib KACST and Mechanical Engineering Department, Faculty of Engineering, KFUPM, Dhahran 31261, Saudi Arabia
Abstract The tremendous increase in energy demand due to increased population and rapid economics results in increased level of atmospheric pollutants and global warming. The global shift to the use of renewable clean energies still has some restrictions in term of the availability of the advanced reliable technologies and the cost of application compared to conventional fossil fuels. Until we can have this full conversion to renewables, the development of novel techniques for clean combustion of fossil fuels is appreciated. Forced by the simultaneous increased pressure of strict emissions regulations and the target of limiting the global warming to 2 oC, gas turbine manufacturers developed novel combustion techniques for clean power production in gas turbines as per the present review study. These novel techniques depend either on the modification in the existing combustion system or developing novel burners for clean power production. In this review, different clean combustion techniques are presented including; flame type variability, burner design, and fuel and oxidizer flexibility. The combustion and emission characteristics of different flame types including; non-premixed/premixed, moderate or intense low-oxygen dilution (MILD) flameless combustion, colorless distributed combustion (CDC), and low-swirl injector (LSI) combustion flames are presented with their limitations for applications. Novel burner designs for clean burning in gas turbines are investigated in detail including; swirl stabilized, dry low NOx (DLN) and dry low emission (DLE), catalytic combustion, perforated plate, environmental vortex (EV), sequential environmental vortex (SEV), advanced environmental vortex (AEV), and lean direct injection (LDI) micromixer burners. As an effective technique to control combustion instabilities within the gas turbine combustor, fuel flexibility approach is studied considering mainly hydrogen-enrich combustion and the associated concerns about fuel variability technique are investigated. Oxidizer flexibility approach in gas turbines is also studied under premixed combustion mode considering lean premixed (LPM) air combustion and oxy-fuel combustion and both techniques are compared in terms of performance and emissions. Finally, the feasibility of the different clean combustion techniques is discussed along with the available market products utilizing such novel technologies. Keywords: Clean combustion; EV/SEV/AEV burners; DLN/DLE burners; Micromixer burners; Lean premixed combustion, fuel and oxidizer flexibility. Corresponding author: M. A. Nemitallah, E-Mail:
[email protected], Tel: +966138604959.
1 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1. Introduction Nowadays, power demand is growing globally, and access to reliable, affordable energy is a critical issue. The International Energy Agency (IEA) reported that by 2020, the global economy will grow by about 3.5% annually, and the total population will raise by about one billion. Based on such forecasts, the IEA expects the total energy demand to be raised more than 30% by 2040 [1]. Additional 6,700 GW of power is expected to be added in the next 25 years considering new-installed and retired gas power plants. Gas turbines for power production are characterized by high cycle efficiency, more than 50% in modern 250 MW-class combined-cycle, and low NOx in natural gas fueled turbines under premixed combustion conditions [2]. Gas turbines also accounts for all commercial aero-propulsion power production systems fueled by kerosene. This growing market for gas turbines encouraged the investments in this sector for clean power production. The pressure of strict regulations on emissions has pushed the manufacturers of gas turbine combustors to invent environmental combustors while maintaining high combustion efficiency and wider operability limits. NOx emissions are considered as the most serious kinds of pollutants. The production of NOx emissions within the combustor is mainly function of the combustion temperature and residence time of the combustion products in the elevated temperature zones. NOx can be produced within the combustor through various mechanisms [35]; however, the dominant mechanism within the zones of elevated temperatures is the thermal NOx (Zeldovich) mechanism [6]. Lowering the combustion temperature even by few degrees can result in notable reduction in NOx emissions [7]. For same given oxygen and nitrogen fractions, the produced amount of thermal NOx in few seconds at a temperature of the flame of 1800 K is in the same order of the produced amount of NOx in milli-seconds when the flame temperature is raised to 2100 K [6]. In the last three decades, intensive research and development work has been conducted to develop combustion technologies for clean power production especially for gas turbine applications. in 1990’s, the US Department of Energy (DOE) started a collaborative program including gas turbine manufacturers, Universities and laboratories to develop advanced turbine systems with improved cycle efficiency of up to 60% [8,9]. In 2000, another program called vision-21 was initiated by the US DOE to develop fuel-flexible electrical generation facilities with the main objective off reducing NOx emissions while capturing CO2 [10]. Different techniques have been proposed by researchers to reduce emissions out of gas turbines toward clean energy production for the control of global warming. Those techniques include either modifications in the existing combustion systems or inventing novel combustion burners to control the emissions. Some of those clean burning techniques have already been implemented in industrial gas turbines for electrical power production and the other are still under research and development. One of the most common clean burning techniques is the dry low NOx technology, which has been implemented in many industrial gas turbine units by different manufacturers. Those gas turbine units include Siemens KG2-3G, Ansaldo Energia AE94.3A, and General Electrics (GE) GE LM2500. However, research is still going trying to improve the performance of the gas turbine while adapting DLN technology to further reduce the emissions. The subject of the present review is to summarize the status of the research and development of novel techniques for clean power production in gas turbine. Those techniques include flame type variability, burner design, fuel flexibility, and oxidizer flexibility. In fact, different flame types have different combustion and emissions characteristics and changing flame mode can result in significant changes in the produced emissions. Non-premixed 2 ACS Paragon Plus Environment
Page 2 of 54
Page 3 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
flames have been used in gas turbines for power generation thanks to their strong stability behavior over wide ranges of loading conditions [11-13]. Non-premixed flames results in stoichiometric combustion zones within the combustor and, consequently, elevated temperature spots are created within the combustor, which raises the level of NOx emissions [14]. Converting the combustion mode from non-premixed to premixed prevents the creation of stoichiometric combustion zones within the combustor as the reactants are premixed upstream of the combustor. This results in reduction in combustion temperature and, accordingly, NOx emissions are reduced [15]. However, premixing the reactants upstream of the combustor results in fluctuations in the flow field which interacts with the pressure field resulting in various kinds of combustion instabilities, which adversely affect the engine operation [16-18]. Colorless distributed combustion (CDC) is another technique to control emissions through the control of flame type within the combustor. The idea here is to create a uniform mixture within the combustor through fast mixing between incoming fresh air, fuel and burned gases to eliminate the possibility of the creation of stoichiometric combustion zone to control flame temperature and, accordingly, control NOx emissions. CDC can result in improved performance of the gas turbine combustor in terms of uniform thermal field, low NOx and CO emissions and improved combustion stability at reduced noise level. Also, Low-Swirl Injector (LSI) combustion is an effective technique toward to the control of the flame to control the emissions. The low-swirl combustion technique exploits the same wavelike principal of premixed turbulent flames. The free burn of the premixed flame in a divergent flow region under reduced swirling flow results in a suspended flame wave “standing wave” without the need for flame anchoring technique. The flame is suspended in a location where the flame speed is the same as the flow velocity. This results in more stable flame and the possibilities for flashback and blow-out are reduced. The design of the burner is also an effective technique to control emissions out of gas turbine combustors. Different burner designs with different combustion and emissions characteristics are discussed in the present review. Those designs include swirl-stabilized burners, dry low NOx (DLE) burners, dry low emission (DLE) burners, burners for catalytic combustion, Perforated plate burners, Environmental burners including environmental vortex (EV), sequential environmental vortex (SEV) and advanced environmental vortex (AEV), and micromixer burners. The designs of these burners are introduced in this review with their performance in terms of flame stabilization and emissions. Fuel flexibility approach may be an option to control instabilities associated with premixed mode of combustion and improving the overall combustion and emissions characteristics. It is concluded that there is a rising interest in switching the operation of combined cycle power plants from natural gas to hydrogen-enriched methane or to syngas to increase the flame stability limits and control the emissions of NOx as well. The progress of the effects of hydrogen enrichment on premixed combustion and emissions characteristics are also reviewed in the present study. Also, oxidizer flexibility can play a significant role in controlling gas turbine emissions. Air combustion using hydrocarbon fuels is the main sources for both NOx and CO2 emissions. Due to the existence of nitrogen in air, NOx emissions will be created in the combustion zone and the rate of production will increase depending on the combustion temperature. There have been many approaches in reducing the CO2 emissions via carbon capture and sequestration (CCS) technologies. Among the current techniques, oxy-fuel combustion offers a brighter future for carbon-dioxide emission-free society as the fuel is burned using O2 to produce H2O and CO2. The CO2 is separated after H2O condensation. This technique of oxy-fuel burning can also eliminate NOx emissions due to the absence of nitrogen within the combustor. The application of the oxy-fuel combustion technique 3 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
in gas turbines is reviewed in this study considering the associated operability issues and their effects on the performance of the gas turbine combustor. Finally, the feasibility of the different technologies for controlling gas turbine emissions is summarized at the end of the current review including market products utilizing these technologies.
2. Adaptation of gas turbines to regulations of pollutant emissions In the 1970’s, when emission controls were first introduced, the pollutant of primary concern to regulators shifted to be concerned with NOx. For the relatively low levels of NOx emission reduction initially targeted, the injection of steam into the flame zone produced the targeted reduction in NOx emissions with insignificant effect on the performance. Also, the emissions of other pollutants including CO and volatile organic compounds (VOC) did not increase in significant amounts. During the 1980’s, more strict policies were imposed requiring significant reduction in NOx emissions. Based on that, further attempts were made to utilize steam injection to ensure achieving the targeted emission values. As a result, cycle performance and part lives are affected badly, and the emission rates for other pollutants including CO and VOC increased significantly. At this stage, the research was forced to develop other techniques for controlling emissions, which led to the development of the lean premixed (LPM) combustion technique [19]. 2.1 Emission regulatory overview 2.1.1 Clean air act (CAA) Congress enacted the clean air act (CAA) in 1970 to consider growing concerns about the quality of atmospheric air [20]. Based on this act, national ambient air quality standards (NAAQS) were established to control the emissions for criteria pollutants. Areas of the country that exceed the NAAQS are considered non-attainment for that pollutant. In non-attainment areas, the United States regulatory structure imposes more stringent air pollution control programs. The CAA is a Federal law covering the entire country. But, States and local governments are permitted to create and implement more stringent air pollution strategies than those required by the CAA. 2.1.2 New source performance standards (NSPS) In the early 1970’s, the emissions control requirements for nitrogen oxides (NOx) were first applied to control the emissions of gas turbines by the Los Angeles County air pollution control district (LAAPCD) and the San Diego air pollution control district (SDAPCD). As a solution to match these regulations, water was injected into the combustor in the flame zone in order to cool down the combustion temperature. When half as much water as fuel was injected into the flame zone, NOx emissions were reduced by about 40%. Under such operating conditions, the emission level achieved was approximately 75 ppmvd (parts per million by volume, dry) on oil. The United States EPA used these data and other data to develop new source performance standards that went into effect in September 1979. Turbines with heat input over 10 million Btu/hr, generating less than 30 MW electrical output, and supplying less than one-third of their electrical output to an electric utility, are required to meet a NOx emission standard of 150 ppm, corrected for efficiency. The details of this NSPS are presented in 40CRF60 Subpart GG. The emergency turbines are excused from this standard, as are special types of turbines. Electric utility turbines having heat input above 100 million Btu/hr should meet a NOx emissions standard of 75 ppm, corrected for efficiency. Today, most of the working turbines can achieve NOx emissions of 42
4 ACS Paragon Plus Environment
Page 4 of 54
Page 5 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
ppm to 25 ppm or less without any means for post-combustion control. Based on that, the existing NSPS is not typically a controlling regulation for gas turbines emissions [19]. In July 2004, EPA modified the NSPS for gas turbines in a direct final ruling. The significant modification is that the new LPM turbines that commence construction after July 8, 2004 are required to use a NOx continuous emissions monitoring system (CEMS). Also, the owners can monitor continuously the engine parameters that give indication when the turbine is working out of the lean premixed mode of combustion. On February 18, 2005, EPA suggested standards of performance for new stationary gas turbines in 40CFR60, subpart KKKK. The new standards reflect the changes in the turbine design and, accordingly, NOx emissions control technologies and are intended to make the emission limits up to date with the performance of present gas turbines. 2.1.3 New source review In addition to the AAQS, the CAA developed an air permitting program called new source review (NSR). The NSR is divided into two main programs: prevention of significant deterioration (PSD) and non-attainment NSR. Each of these programs applies to "major sources" and "major modifications". However, in non-attainment NSR at much lower thresholds, a source can be considered "major". 2.1.4 Best available control technology (BACT) If a source triggers PSD review, so the owner should define the appropriate level of emissions controls for the pollutants that exceed the specific limits. In attainment areas, the standard for evaluation is the BACT. The BACT determination put achievable emissions limitations considering environmental, energy and economic effects of applying the emission control technology required to meet that limitation. Now, the EPA asks for a "top-down" BACT analysis to be followed for all PSD permit applications [21]. Based on that, BACT is a "living" standard, which is more stringent over time unless new, low cost and more efficient emission control technologies are developed. For a certain application, BACT can only be defined in the form of current demonstrated technology. Generic BACT requirements do not exist as BACT determinations are site-specific. However, for gas turbines with power greater than 25 MW, there have been recent BACT determinations for NOx as low as 5 ppm to 2 ppm. 2.1.5 Lowest achievable emission rate (LAER) LAER determination is used in major sources/modifications in non-attainment areas. For class or source category, LAER is defined as the strictest emission control system achieved in practice. The main difference between BACT and LAER is that LAER does not take care of economic effects when evaluating emission control technologies of pollutants.
3. Type of flame The mode of the flame and how fuel and oxidizer are introduced to the combustor has direct effect on combustion temperature and emissions within the combustor. In this section, different flame modes are introduced toward the applications of clean power production in gas turbines.
5 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
3.1 Non-premixed/premixed flames In diffusion flames, a reaction sheet is created forming the flame border as shown in Figure 1. The Figure represents a co-axial swirling flow jet, the most commonly flow configurations which support diffusion flames. The diffusion flame starts at laminar mode, but it shifts to turbulent diffusion flame when a further increase of flow rate occurs. This could form a flicker at the flame top. The flame length is also increased due to the increase in turbulence level. The fuel concentration is highest on the centerline as the fuel is introduced from the centerline pipe and slightly decreases to reach zero at the flame reaction sheet. The concentration of oxidizer is highest at the wall and tends to zero at the flame reaction sheet.
Figure 1: Reaction zones of co-axial burner holding jet diffusion flame [22]. This diffusion (non-premixed) kind of flames creates stoichiometric combustion zones on the thin flame sheet resulting in elevated temperature spots within the combustor. This results in the dissociation of reacting species to produce emissions, mainly NOx. To solve this issue, flame mode can be turned to the premixed combustion mode. In lean premixed approach, fuel is mixed with air upstream within the mixing length. Many studies revealed that the application of the technique of premixed flames is a promising solution that can bring down the concentration of NOx by a single digit value in ppm [23-25]. However, the conversion of the flame from the diffusion type (non-premixed) to the premixed type results in an increased levels of flame instabilities which brings its own challenges to burner designers and manufacturer. The variations in flame temperature for both conventional diffusion and DLE premixed flames are presented in Figure 2 (left). The maximum temperature corresponds to the conventional diffusion 6 ACS Paragon Plus Environment
Page 6 of 54
Page 7 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
flame, while the minimum temperature corresponds to the lean premixed flame. The highest flue gas temperature occurs at the stoichiometric condition and, consequently, NOx emissions have a higher value at this operating condition. Decreasing temperature leads to reduction in NOx emissions and increase in CO emission while operating at lean extinction condition. To achieve ultra-low NOx emissions (
