Please explain the best approach to cutting NOx emissions from a gas turbine.Apr-2021
Greg Zoll, Becht, email@example.com
The technology utilised for gas turbine NOx reduction is well proven. It is almost universally achieved by use of selective catalytic reduction (SCR) which utilises a vanadium catalyst to reduce NOx to N2 and water. Ammonia is injected ahead of the catalyst as the reductant. For gas turbines operating in combined cycle applications, the SCR is embedded in the heat recovery steam generator to achieve the correct exhaust gas temperature. In simple cycle installations, cooling air is injected into the gas turbine exhaust ahead of the SCR. Alternatively, high temperature catalysts have been developed to eliminate or reduce the amount of cooing air required.
John Skelland, KBC (A Yokogawa Company), John.Skelland@KBC.global
NOx emissions can be reduced by injecting a small amount of water or steam into the combustion zone, known as wet low emission technology (WLE). This reduces the temperature in the combustion zone which reduces the rate of NOx production. However, this water or steam injection is a thermodynamic loss and so reduces the energy efficiency of the gas turbine. This technique also requires a supply of boiler feed water or steam to be provided. The technique has historically been the most common retrofit to existing gas turbines.
Alternatives to steam or water injection include:
• Dry low NOx combustion (DLN), a technology that uses staged combustion and lean premixed fuel/air mixtures. This requires that the gas turbine has several combustors rather than a single combustor. It is very effective in maintaining low NOx over the entire operating range. However, maintenance costs are generally higher than for a standard gas turbine and the control system is more complex. DLN upgrades are available as retrofits to some gas turbine models.
• Catalytic combustion: the turbine combines catalytic combustion of fuel and air in a catalyst bed followed by normal fuel combustion at a low enough temperature to prevent significant NOx formation. This technology is in the early stages of commercialisation.
• Post-combustion exhaust gas clean-up systems including selective catalytic reduction (SCR): this type of technology requires the use of NOx reducing chemicals (such as ammonia) in the presence of a catalyst. The advantages of this option include relatively low capital investment, a small footprint, and no need for modification to the gas turbine itself. However, operational expenses are high due to the cost of replacement catalyst. Also, the use of hazardous chemicals such as ammonia is required. This technology can usually be installed as a retrofit to existing gas turbines.
Of these, DLN is the most commonly used technology. DLN and SCR can be combined where really low NOx emissions are required.
Paul den Held, Petrogenium, firstname.lastname@example.org
As an introduction, the NOx in the gas turbine combustion system is formed by the following three mechanisms:
1. Thermal NOx: this is formed during fuel combustion by oxidation of molecular nitrogen (N2) in the combustion air. The formation of thermal NOx is highly dependent on the gas temperature.
2. Prompt NOx: radicals like CH and CH3 can attack free N2 in the air to form NOx.
3. Fuel NOx is produced by oxidation of nitrogen compounds contained in the fuel (up to 2% of the total NOx formed in the combustor).
Two key parameters are normally used to characterise the thermal NOx production during gas turbine combustion: equivalence ratio (fuel to air ratio) and adiabatic flame temperature. The relationship between the thermal NOx production and these two parameters is provided in Figure 1.
Selection of the appropriate NOx abatement design and the best approach for the gas turbine to reduce NOx depends on a number of factors:
1. Type of combustion system available, refit or new design
2. Type of fuel gas to be applied in the gas turbine
3. Diluent available (steam, water or nitrogen)
4. NOx limit requirements at the specific location
5. Manufacturers references on the typical fuel and fuel gas system
There are different primary NOx abatement options.
Conventional combustion systems
For a gas turbine using mainly natural gas as the fuel source and equipped with a conventional combustion system, the NOx abatement options are to apply a diluent and select either water injection or steam injection.
Each of these options has its specific issues. These include the impact on the maintenance factor, the reduced lifetime of the parts, and an obvious continuous loss of water or steam. This technique is applied on many gas turbines in operation that are not equipped yet with the dry low NOx systems. The diluent is applied to reduce the flame temperature and thereby reduce the NOx produced.
The application of a diluent needs certainly to be considered for gas turbines operating on low-Btu type fuels, syngas types of fuel gases with a high content of hydrogen and CO2, and for coal gasification applications. For such fuel gas applications, the DLN systems may not be offered by the manufacturers. Petrogenium can offer experience in the evaluation of the designs offered by the manufacturers.
Dry low NOx combustion systems
Dry low NOx (DLN) combustion systems are significantly more expensive (hardware costs are 30-40% more) than the standard combustor, and require a more complex staged fuel gas system and more intrinsically complex fuel gas nozzles. Since NOx production increases exponentially with high adiabatic flame temperatures, the NOx abatement technologies focus on reducing the peak flame temperatures. This however conflicts with the desire to have higher firing temperatures required to achieve turbine efficiency. Modern DLN combustors manage this conflict by designing combustors that can have lower peak flame temperatures whilst keeping the same firing temperatures. Combustion near stoichiometric conditions (equivalence ratio = 1) increases NOx production. Therefore DLN combustors are designed around 0.5 equivalence ratio. This, however, brings challenges, as combustors operating at this equivalence ratio often face risks of lean blow out (flame extinction). Current gas turbine manufacturers each offer their specific DLN combustion systems.
Catalytic combustor (future systems)
The catalytic combustor is still in the ‘proof-of-concept’ phase and requires substantial efforts before it is offered by manufacturers. Realistically speaking, it will be another 5-10 years or more before it would be commercially available for heavy duty gas turbines.
Secondary NOx abatement options
Selective catalytic reduction works on the principle of removing NOx from the gas turbine exhaust utilising, say, ammonia injection and a SCR catalyst that converts NOx to molecular nitrogen. This system is only considered in the case of DLN combustion systems and the selection of a required diluent is not an option. The SCR has some distinct limitations.
In summary, the best approach to reducing NOx emissions from the gas turbine depends on the NOx emission levels to be achieved, the gas turbine hardware currently installed, the application of the gas turbine, diluents available and the type of fuel gas used for the combustion.
Jan Zander, Petrogenium, email@example.com
A new gas turbine should have proven low NOx burners (nothing new here). Some attention should be given to the low NOx technology if a partial load of the gas turbine is required (such as in Oman LNG). For existing gas turbines it might be possible to retrofit low NOx burners. In extreme cases, steam injection may be required to reduce NOx. Water injection has been applied on gas turbines in refineries, but may cause higher maintenance costs (burners and blades).