NOx emission

The new gas turbines also utilize Low NO eombustors to reduee the NOx emissions, whieh otherwise would be high due to the high firing temperature of about 2300 °F (1260 °C). These low NOx eombustors require eareful ealibration to ensure an even firing temperature in eaeh eombustor. New types of instrumentation sueh as dynamie pressure transdueers have been found to be effeetive in ensuring steady eombustion in eaeh of the eombustors. 

Figure 1-33. Control of gas turbine NOx emissions over the years.

NOx Emissions. The amount NO emissions is very eritieal in most regions where gas turbines are being utilized for power generation. The present eap is about 22 ppm the aim is to go down to as low as 9 ppm. The teehniques offered here all are NOx emission friendly, in that they do not inerease the present levels of NOx, in faet in the ease of the injeetion systems, both steam, and heated and humidified eompressed air will lower the NOx emissions making the plant even more environmentally friendly, espeeially in this eritieal loea-tion. 

Basis for NOx Prevention. Emissions from turbines are a funetion of temperature and thus a funetion of the F/A ratio. Figure 10-20 shows that as the temperature is inereased the amount of NOx emissions are inereased and the CO, and the unburnt hydroearbons are deereased. The prineipal meehanism for NOx formation is the oxidation of nitrogen in air when exposed to high temperatures in the eombustion proeess, the amount of NOx is thus dependent on the temperature of the eombustion gases and also, to a lesser amount on the time the nitrogen is exposed to these high temperatures. 

Figure 10-21. Correlation of adiabatio flame temperature with NOx emissions.

Figure 10-22. Effect of fuel /air ratio on flame temperature and NOx emissions.

High pressure burners for gas turbines use pre-mixing to enable eombus-tion of lean mixtures. The stoiehiometrie mixture of air and fuel varies between 1.4 and 3.0 for gas turbines. The flames beeome unstable when the mixture exeeeds a faetor of 3.0 and below 1.4 the flame is too hot and NOx emissions will rise rapidly. The new eombustors are therefore shortened to reduee the time the gases are in the eombustor. The number of nozzles is inereased to give better atomization and better mixing of the gases in the eombustor. The number of nozzles in most eases inereases by a faetor of 5-10, whieh does lead to a more eomplex eontrol system. The trend now is to an evolution towards the ean-annular burners. For example, ABB GT9 turbine had one eombustion ehamber with one burner, the new ABB 13 E2 has 12 ean-annular eombustors and 72 burners. 

NOx emission requirements. This injeetion of steam reduees the temperature in the hot seetion, thus redueing the amount of NO produeed. When sprayed through the fuel nozzle, this steam ean impinge on the liner, thus ereating a temperature gradient, whieh ean lead to eraeks. Steam injeetion— whether it is required for NO eontrol or for extra power (5% steam by weight will produee 12% more work and inerease effieieney a few pereent)— must injeet steam into the eompressor diffuser to be safe and effeetive. This proeess will allow the steam to be fully mixed with the air before it enters the eombustor, redueing the ineidenee of liner failures due to steam injeetion. 

High flame temperature and high excess air increase NOx emissions. 

Benefit The use of LNB decreases the amount of NOx formation at the facility and therefore may help facilities meet state RACT or BA CT (40 CFR 52) requirements. Additionally, this technology may help facilities meet standards of performance for industrial-commercial-institutional steam generating units in 40 CFR 60, Subpart Db. A decrease in a facility s NOx emissions may decrease the possibility that a facility will meet the Nox emission threshold for an air permit under 40 CFR 70 and 71. 

NOx emission controls in large engines and turbines are based on the same principles. However, special designs must be applied to accommodate differences in the combustion process. Methods to control NfJ include the following. 

Nitrogen oxides (NOx) - thermal NOx is formed with gas as with other fuels, particularly if air preheat is practiced. There is very little fuel nitrogen compared with other fossil fuels so that the total NOx emissions are lower. 

Steam or water injection may also be used, and under some circumstances this can reduce NOx emissions without lowering plant efficiency. Such injection seems to operate in two ways. First, it cools the combustion and hence slows its rate (therefore acting in the same manner... 

Regenerators operating in full or partial combustion can utilize the benefits of the CO promoter. The addition of the promoter tends to increase the regenerator temperature and NOx emission. The metallurgy of the regenerator internals should be checked for tolerance of the higher temperature. 

Figure 2.2 Schematic of a modem four-pass, packaged horizontal FT boiler, with low NOx emission, showing path of hot combustion gas flow, and air/combustion gases recirculation system.

Low NOx emission burners also are available, typically maintaining NOx to below 20 to 60 ppm CO (corrected to 3% 02), equivalent to under 20 nanogram (ng) NOx,/J output. 

The use of FBC boilers results in lower NOx emissions compared with conventional boiler designs at the same furnace temperature. 

Nitrogen Dioxide (NO2) Is a major pollutant originating from natural and man-made sources. It has been estimated that a total of about 150 million tons of NOx are emitted to the atmosphere each year, of which about 50% results from man-made sources (21). In urban areas, man-made emissions dominate, producing elevated ambient levels. Worldwide, fossil-fuel combustion accounts for about 75% of man-made NOx emissions, which Is divided equally between stationary sources, such as power plants, and mobile sources. These high temperature combustion processes emit the primary pollutant nitric oxide (NO), which Is subsequently transformed to the secondary pollutant NO2 through photochemical oxidation. 

Estimates of urban NOx emissions and trends are generally limited to those provided by the developed countries which have the detailed emission Inventories. As In the case of other pollutants, the USA contributes the most on a per-country basis to the global NOx emissions per year. Because of the Inaccuracy of the data base used. It Is difficult to discern trends In these emissions. However, with new control technologies being Implemented for both stationary and mobile sources, downward trends In the developed countries may be more prevalent In the future years. Unfortunately, the opposite trend Is likely to occur In the developing countries. 

Due to its particular composition, biodiesel is biodegradable and allows reduced emissions, in terms of particulates and polycyclic aromatic hydrocarbons. Instead, the results of the combustion of biodiesel are contentious in relation to so-called NOx emissions, where it has been observed that such emissions are more or less increased, with respect to conventional diesel, depending on the characteristics of the engine in which it is used ... 

A gas turbine with power output of 10.7 MW and an efficiency of 32.5% burns natural gas. In order to reduce the NOx emissions to the environmental limits, 0.6 kg steam is injected into the combustion per kg of fuel. The airflow through the gas turbine is 41.6 kg-s 1. The composition of the natural gas can be assumed to be effectively 100% methane with a molar mass of 16 kg-kmoU1. The kilogram molecular volume can be assumed to occupy 22.4 m3 at standard conditions. 

As mentioned earlier, the most promising technologies for NOx emission after-treatment from diesel engines are mainly the following ... 

The previous section has evidenced that NH3-SCR technology has been used successfully for more than two decades, to reduce NOx emissions from power stations fired by coal, oil and gas, from marine vessels and stationary diesel engines. NH3-SCR technology for high-duty diesel (HDD) vehicles has also been developed to the commercialization stage and is already available as an option in the series production of several European truck-manufacturing companies starting from 2001. For mobile source applications, the preferred reductant source is aqueous urea, which rapidly hydrolyses to produce ammonia in the exhaust stream. 

SCR for heavy-duty vehicles reduces NOx emissions by 80%, HC emissions by 90% and PM emissions by 40% in the EU test cycles, using current diesel fuel (<350 ppm sulphur) [27], Fleet tests with SCR technology show excellent NOx reduction performance for more than 500000 km of truck operation. This experience is based on over 6 000 000 km of accumulated commercial fleet operation [82], The combination of SCR with a pre-oxidation catalyst, a hydrolysis catalyst and an oxidation catalyst enables higher NOx reduction under low-load and low-temperature conditions [83],... 

As a conclusion, with a pertinent densification of EGR gases, it is possible to decrease NOx emissions, while keeping under control the increase in particles emissions at a level compatible with the DPF treatment ability. 

If EuroV finally leads to a moderate reduction of the NOx emissions limit (s 20%), when regarding to US Tier 2-bin 5 levels3, we can presume that EuroVI will consist of much more stringent NOx limitation4. A NOx after-treatment system in combination with a DPF (which is mandatory since EuroV) in the exhaust line should become ordinary with EuroVI. The PM limit values will be unchanged from EuroV. 

The road transport seems to be the main source of NOx emissions (see Figure 7.4). 

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