Gas Turbine Units and Their Impact on the Environment – Part 2

Part 1

As discussed in Part 1 of this blog, Part Two will delve into various development strategies aimed at reducing emissions and enhancing gas turbine performance. Several approaches are currently being explored or employed to mitigate exhaust gas toxicity, as outlined below:

1. Injection of water or steam into the combustion chamber of a gas turbine unit to boost power and reduce NOx content.
2. Creation of low-emission multi-zone combustion chambers with variable geometry, pneumatic nozzles, and special flame stabilization.
3. The use of catalytic combustion chambers or coherent afterburning systems.
4. Use of environmentally friendly fuel – hydrogen as the main and additional fuel.

 

As previously mentioned, the toxicity and composition of exhaust gases from a gas turbine plant depend on the type of fuel used. For instance, a critical factor in understanding the mechanism of NOx generation in fuels is the content of chemically bound nitrogen [N]. However, NOx and CO emissions exhibit opposite dependencies on most parameters in the combustion zone (temperature, residence time, volume of the combustion zone, air flow, etc.), prompting the search for a compromise solution to minimize them.

As an illustration, Figure 5a depicts the dependencies of mass emissions of NOx and CO, and the excess air factor when using burner devices with diffusion mixture formation in afterburning chambers [9]. At α = 1.7…2.0, NOx and CO emissions are minimal. Figure 5b illustrates the dependence of NOx and CO emissions on temperature.

Injection of Water or Steam into the Combustion Chamber of a Gas Turbine

The injection of steam into the combustion chamber of a gas turbine unit has been a common practice for NOx control, as illustrated in Figure 6. The amount of steam that can be added is limited due to combustion concerns, typically restricted to about 2–3% of the airflow. This addition can provide an extra 3–5% of the rated power. The dual fuel nozzles on many industrial turbines can be easily retrofitted to accommodate steam injection, with the steam produced using a steam generator [18].

Gas turbines with steam injection exhibit high efficiency and specific power, making them competitive in various applications, including floating production storage and offloading vessels. However, in conventional systems, the steam, along with combustion products, is usually released into the atmosphere, resulting in heat loss from water evaporation. In installations like the “Aquarius” type (Figure 7), this drawback is addressed by incorporating a special contact condenser, which condenses the vapor in the exhaust gases, allowing the water to return to the installation cycle. “Aquarius” gas-steam turbine units prove promising in locations where large amounts of boiler water preparation pose challenges, as they not only enable the return of boiler water to the cycle but also yield excess distilled water through the condensation of moisture produced by fuel combustion [17].

Creation of Low-emission Multi-zone Combustion Chambers with Variable Geometry, Pneumatic Nozzles, and Special Flame Stabilization

One approach for achieving low-emission combustion is the use of a two-zone combustion chamber, consisting of a reserve (diffusive) zone and a basic (homogeneous) zone. The reserve zone stabilizes the flame and provides heat influx to the basic zone, where a premixed lean air-fuel mixture is burned at a lower temperature, reducing NOx formation. However, this introduces a risk of lean flameout and incomplete combustion, potentially increasing CO and unburned hydrocarbon (UHC) emissions. Striking a compromise in air and fuel distribution between the two zones is crucial to maintaining low emissions and combustion completeness under all operating conditions [14].

Another approach involves variable geometry within the combustion chamber, allowing adjustment of airflow and fuel injection based on engine load and speed. This enhances mixing and combustion efficiency, as well as flame stability and durability. Variable geometry can be achieved using movable components such as swirler vanes, nozzle flaps, or injector valves, altering the shape and size of air and fuel passages. For instance, a miniature gas turbine engine with a variable geometry swirler demonstrated a substantial reduction in NOx and CO emissions compared to a fixed geometry swirler [15].

Pneumatic nozzles, as another type of variable geometry device, control fuel injection and atomization in the combustion chamber using pressurized air to create a fine fuel droplet spray. This enhances fuel mixing and evaporation. Adjusting air pressure and flow rate allows control over spray characteristics, such as droplet size, velocity, and angle. Pneumatic nozzles can also reduce coking and fouling of fuel injectors, impacting combustion chamber performance and reliability. Special flame stabilization employs various mechanisms like recirculation, bluff-body, swirl, or plasma to prevent the flame from being blown off or extinguished by high-speed airflow in the combustion chamber [16].

Use of Catalytic Combustion Chambers or Coherent Afterburning Systems

Catalytic combustion is a chemical process that utilizes a catalyst to accelerate oxidation reactions of fuel and reduce pollutant emissions[1]. Discovered in the 1950s by Catalytic Combustion LLC[ 19], this technique can be applied to gas turbine technology to significantly decrease the formation of undesired products, especially pollutant nitrogen oxide gases (NOx), well below levels achievable without catalysts. Gas turbine engines employing catalytic combustion systems can produce emissions with significantly lower levels of noxious gases compared to those from conventional flame-fired engines. The gas turbine catalytic combustion system developed by Johnson Matthey comprises three essential zones through which gases flow successively (Figure 9). Key mechanisms within these zones include fuel preparation, such as splitting long molecules into shorter ones; fuel oxidation to release heat energy; and the destruction of pollutant gases in the exhaust [2]. The chamber must be redesigned to ensure a satisfactory fluidic flow system, aligning the catalyst with the engine [20].

Use of Environmentally Friendly Fuel – Hydrogen

Hydrogen can serve as a fuel in gas turbines, offering a potential pathway to decarbonize them by replacing natural gas fuel with hydrogen, which contains no carbon and, consequently, emits no COx in the exhaust[21]. The primary byproduct of combusting hydrogen is H2O, making it a truly COx emission-free fuel[2]. GE gas turbines have demonstrated operational experience with hydrogen content ranging from 5% (by volume) up to 100%[3]. However, hydrogen presents unique challenges related to supply and infrastructure. Its physical properties make production, storage, and transportation more challenging compared to natural gas. Safety concerns, such as a larger flammability range, lower vapor density, and faster flame speed, must be considered in system design[22]. More detailed information about the design features of gas turbines associated with the use of hydrogen fuel can be found in [23].

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Part 1

References:

1. https://www.britannica.com/technology/gas-turbine-engine/Development-of-gas-turbine
2. https://www.softinway.com/the-evolution-of-gas-turbines-from-the-first-designs-to-the-latest-environmentally-friendly-development-trends-part-1/
3. Analysis of Gas Turbine Systems for Sustainable Energy Conversion. – Marie Anheden, – Royal Institute of Technology Stockholm, Sweden 2000 TRITA-KET R112 ISSN 1104-3466 ISRN KTH/KET/R–112–SE.
4. York, M. Hughes, J. Berry, T. Russell, Advanced IGCC/hydrogen gas turbine development, Final Technical Report, DE-FC26-05NT42643 (2015) submitted to US Department of Energy
5. Reduction of nitrogen oxides in gas turbine exhaust gases, Postnikov A.M. – Publishing house of the Samara Scientific Center of the RAS. – 2002 – 286 pages.
6. http://energetika.in.ua/ru/books/book-3/part-1/section-3/3-9
7. https://www.cambridge.org/core/journals/aeronautical-journal/article/abs/performance-analysis-of-an-aero-engine-with-interstage-turbine-burner/FB31C38A3C51C5EE83FEAF4E3112FFE1
8. Manushin E.A. Gas turbines: problems and prospects M.: Energoatomizdat, 1986. – 168 p.
9. https://dspace.library.khai.edu/xmlui/bitstream/handle/123456789/1623/Gerasim.pdf?sequence=1
10. https://myengineeringworld.net/2013/06/gas-turbine-combustor-concepts-low-emissions.html
11. https://ela.kpi.ua/bitstream/123456789/39487/1/2020-7.pdf
12. https://en.wikipedia.org/wiki/Integrated_gasification_fuel_cell_cycle
13. https://vibrationacoustics.asmedigitalcollection.asme.org/GT/proceedings/GT1999/78590/V002T02A067/248338
14. https://link.springer.com/article/10.3103/S1068799811020103
15. https://www.mdpi.com/1996-1073/13/19/5230
16. https://www.researchgate.net/publication/346054984_Review_of_Gas_Turbine_Combustion_Chamber_Designs_to_Reduce_Emissions
17. https://www.hindawi.com/journals/ijce/2022/9123639/
18. https://www.sciencedirect.com/topics/engineering/steam-injection
19. https://en.wikipedia.org/wiki/Catalytic_combustion
20. https://technology.matthey.com/article/23/4/134-141/
21. https://www.ge.com/content/dam/gepower-new/global/en_US/downloads/gas-new-site/future-of-energy/hydrogen-overview.pdf
22. https://www.turbomachinerymag.com/view/the-future-of-hydrogen-as-a-gas-turbine-fuel
23. https://www.softinway.com/the-evolution-of-gas-turbines-from-the-first-designs-to-the-latest-environmentally-friendly-development-trends-part-2/