0 1970 1975 1980 1985 1990 1995 2000 2005 2010 Years
Water injection Dry Low NOx Combustor Catalytic Combustor
Figure 10-1.. Control of gas turbine NOx emissions over the years
..Wet'' contro1 became the preferred method in the 1980s and most of 1990s since ..dry'' contro1s and cata1ytic c1eanup were both at very ear1y stages of deve1opment. The cata1ytic converters were used in the 1980s and are sti11 being wide1y used; however the cost of rejuvenating the cata1yst is very high.
There has been a gradua1 tightening of the NOx 1imits over the years from75 ppm down to 25ppm， and now the new turbine goa1s are 9 ppm.
Advances in combustion techno1ogy now make it possib1e to contro1 the 1eve1s of NOxproduction at source， removing the need for ..wet'' contro1s. This of course opened up the market for the gas turbine to operate in areas with1imited supp1ies of suitab1e qua1ity water， e.g.， deserts or marine p1atforms.
A1though water injection is sti11 used， ..dry'' contro1 combustion techno1ogy has become the preferred method for the major p1ayers in the industria1 power generation market. DLN (Dry Low NOx) was the first acronym to be coined， but with the requirement to contro1 NOx without increasing carbon monoxide and unburned hydrocarbons this has now become DLE (Dry Low Emissions).
The majority of the NOx produced in the combustion chamber is ca11ed
..therma1 NOx.'' lt is produced by a series of chemica1 reactions between the nitrogen (N2) and the oxygen (O2) in the air that occur at the e1evated temperatures and pressures in gas turbine combustors. The reaction rates arehigh1y temperature dependent， and the NO x production rate becomes sig-nificant above f1ame temperatures of about 3300 0F (1815 0C). Figure 10-19 showsschematica11y， f1ame temperatures and therefore NOx production zones inside a ..conventiona1'' combustor. This design de1iberate1y burned a11 of the fue1 in a series of zones going from fue1-rich to fue1-1ean to provide good stabi1ity and combustion efficiency over the entire power range.
The great dependence of NOx formation on temperature revea1s the direct effect of water or steam injection on NOx reduction. Recent research showed an 85% reduction of NOx by steam or water injection with optimizing combustor aerodynamics.
ln a typica1 combustor as shown in Figure10-19， the f1ow entering the primary zone is 1imited to about 10%. The rest of the f1ow is used for mixing the combusted air and coo1ing the combustor can. The Maximum temperature is reached in the primary stoichiometric zone of about 40400F(22300C)andafterthemixingofthore combustion process with the coo1ing air the temperature drops down to a 1ow of 2500 0F (1370 0C).
.asis .or NOx .re.ention. Emissions from turbines are a function of temperature and thus a function of the FjA ratio. Figure 10-20 shows that as the temperature is increased the amount of NOx emissions are increasedand theCO， and the unburnt hydrocarbons are decreased. The principa1 mechanism for NOx formation is the oxidation of nitrogen in air whenexposed to high temperatures in the combustion process， the amount of NOx is thus dependent on the temperature of the combustion gases anda1so， to a 1esser amount on the time the nitrogen is exposed to these high temperatures.
LEAN FUEL/AIR RATIO RICH
The cha11enge in these designs is to 1ower the NOx without degradation in unit stabi1ity. ln the combustion of fue1s that do not contain nitrogencompounds， NOx compounds (primari1y NO) are formed by two mainmechanisms， therma1 mechanism and the prompt mechanism. ln the therma1mechanism， NO is formed by the oxidation of mo1ecu1ar nitrogen through the fo11owing reactions:
NOx is primari1y formed through high temperature reaction between Nitrogen and Oxygen from the air.
O + N2 + NO + N (10-8)
N + O2 + NO + O (10-9)
N + OH + NO + H (10-10)
Hydrocarbon radica1s， predominant1y through thereaction， initiate the prompt mechanism
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本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(48)
