I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
4.7 Emissions
Bound nitrogen in liquid fuel is contained in the long carbon-chain molecules. Distillate oil is the most com- mon oil fired in duct burners as a liquid fuel. The fuel- bound nitrogen content is usually low, in the range of 0.05 wt.%. Conversion to NOx is believed to be 80%–
90%. For No. 6 oil, containing 0.30 wt.% nitrogen, the conversion rate to NOx would be about 50%. Other heavy waste oils or waste gases with high concentra- tions of various nitrogen compounds may add rela- tively high emissions. Consequently, fuel NOx can be a major source of nitrogen oxides and may predominate over thermal NOx.
The impact of temperature on NOx production in duct burners is not as pronounced as in, for example, fired heaters or package boilers. One reason is that both the bulk fired temperature and the adiabatic flame tem- perature are lower than in fired process equipment.
In the formation of NOx, the equations are similar to formation of thermal NOx and are presented as such:
d
dt Ae
E
RT eq
(NO)=2 − (O2) (N2) (4.3)
and
( )
( ). ( ). O2 0 5 O20 5
eq k eq
= RTo (4.4)
One generally accepted practice is to assume (O2) in equilibrium with (O) and (O2) concentration using the Westenberg3 results for ko for (O2) equilibrium and Zeldovich constants, A and E, as measured by Bowman.4
When used to provide supplementary firing of tur- bine exhaust, duct burners are generally considered to be “low NOx” burners. Because the turbine exhaust contains reduced oxygen, the peak flame temperature is reduced and the reaction speed for O2 and N+ to form NOx is thus lowered. The burners also fire into much lower average bulk temperatures—usually less than 1600°F (870°C)—than process burners or fired boilers.
The high-temperature zones in the duct burner flames are smaller due to large amounts of flame quenching by the excess TEG. Finally, mixing is rapid and there- fore retention time in the high-temperature zone is very brief.
The same duct burner, when used to heat atmospheric air, is no longer considered “low NOx,” because the peak flame temperature approaches the adiabatic flame temperature in air.
Clearly, operating conditions have a major impact on NO formation during combustion. To properly assess NOx production levels, the overall operating regime must be considered, including TEG composition, fuel composi- tion, duct-firing temperature, and TEG flow distribution.
4.7.3 CO, uBHC, SOx, and Particulates
A general discussion of pollutant emissions is given in Volume 1, Chapter 14.
4.7.3.1 Carbon Monoxide
Carbon monoxide (CO), a product of incomplete com- bustion, has become a major permitting concern in gas turbine-based cogeneration plants. Generally, CO emis- sions from modern industrial and aero-derivative gas turbines are very low, in the range of a few parts per million (ppm). There are occasional situations in which CO emissions from the turbine increase due to high rates of water injection for NOx control or operation at partial load, but the primary concern is the sometimes large CO contribution from supplementary firing. The same low-temperature combustion environment that suppresses NOx formation is obviously unfavorable for complete oxidation of CO to CO2. Increased CO is pro- duced when fuels are combusted under fuel-rich con- ditions or when a flame is quenched before complete burnout. These conditions (see Figure 4.19) can occur if there is poor distribution of TEG to the duct burner, which causes some burner elements to fire fuel-rich and others to fire fuel-lean, depending on the efficiency of the TEG distribution device. The factors affecting CO emissions include
• TEG distribution
• Low TEG approach temperature
• Low TEG oxygen content
• Flame quench on “cold” screen tubes
• Improperly designed flame holders that allow flame quench by relatively cold TEG
• Steam or water injection
TEG temperature, °F
CO emissions, ppm
500 1100
0 1200
CO emissions are depressed by higher oxygen content in the
TEG and with lower (25–75 fps) TEG
velocities
Figure 4.19
Effect of conditions on CO formation.
For utilization and performance prediction, kinetic data can be utilized from the literature. For instance, for CO destruction, several kinetic data are available such as5
d
dt e
RT [CO]= − . − , (CO O)( ) (0. H O)0.
1 8107 25 000
2 5
2 5
RT
P
2
(4.5) Most published CO rates involve H2O because CO destruc- tion requires the (OH)−1 radical to produce the reaction.
4.7.3.2 UHCs
In the same fashion as carbon monoxide generation, UHCs are formed in the exhaust gas when fuel is burned without sufficient oxygen or if the flame is quenched before com- bustion is complete. UHCs can consist of hydrocarbons (defined as any carbon–hydrogen molecule) of one carbon or multiple carbon atoms. The multiple carbon molecules are often referred to as long-chain hydrocarbons. UHCs are generally classified in two groups:
1. UHCs as methane
2. Non-methane hydrocarbons or VOCs
The reason for the distinction and greater concern for VOCs is that longer-chain hydrocarbons play a greater role in the formation of photochemical smog. VOCs are usually defined as molecules of two carbons or greater and are sometimes considered to be three carbons or greater. These definitions are set by local air quality con- trol boards and vary across the United States.
UHCs can only be eliminated by correct combustion of the fuel. However, hydrocarbon compounds will always be present in trace quantities, regardless of how the HRSG system is operated.
For HC and VOC incineration, several sources are available such as Barnes et al.6
In general, d
dt
( ) . ( ) . ( ) ( ) /
( , )
C Ha b =−5 52 108 −0 815T C Ha b . O mol cm
12 200
0 5 2 3
P e T ss
(4.6) 4.7.3.3 Sulfur Dioxide
Sulfur dioxide (SO2) is a colorless gas that has a charac- teristic smell in concentrations as low as 1 ppm. SO2 is formed when sulfur (S) in the fuel combines with oxygen (O2) in the TEG. If oxygen is present (from excess of com- bustion) and the temperature is correct, the sulfur will further combine and be converted to sulfur trioxide (SO3).
These oxides of sulfur are collectively known as SOx.
Except for sulfur compounds present in the incoming particulate matter (PM), all of the sulfur contained in the fuel is converted to SO2 or SO3. Sulfur dioxide will pass
through the boiler system to eventually form the familiar
“acid rain” unless a gas-side scrubbing plant is installed.
Sulfur trioxide can, in the cooler stages of the gas path, combine with moisture in the exhaust gas to form sul- furic acid (H2SO4), which is highly corrosive and will be deposited in ducts and the economizer if the exhaust gas is below condensing temperatures. Natural gas fuels are fortunately very low in sulfur and do not usually cause a problem. However, some oil fuels and plant gases can be troublesome in this respect.
4.7.3.4 PM
Particulate emissions are formed from three main sources: ash contained in liquid fuels, unburned carbon in gas or oil, and SO3. The total amount of particulate is often called TSP (total suspended particulate). There is concern for the smaller-sized portion of the TSP, as this stays suspended in air for a longer period of time.
The PM-10 is the portion of the total PM that is less than 10 μm (10 × 10−6 m) in size. Particles smaller than PM-10 are on the order of smoke. Typical NOx and CO emis- sions for various fuels are shown in Table 4.1.
For particulate oxidation, an equation can be developed from fundamental principles utilizing a combination of diffusion of oxygen and surface reactivity as follows:
dm dt =
+
(12 ) 1 1
C A Km Kr
og p
(4.7)
where
m is the mass of particle t is time
C is the molar density A is the surface area
Km is the diffusion coefficient of oxygen in nitrogen Kr is the reaction coefficient of the form Ae−E/RT A is the frequency factor
E is the activation energy R is the universal gas constant T is the temperature
The equation can be integrated for constant density par- ticles and using particle tracking in time steps with con- stant or varying oxygen and temperature. An excellent source of char rate data is available by Smith and Smoot7.
Then, in all cases, one can post-process thermal map data in some discrete volume form and/or insert into a CFD code using the Rayleigh flux theorem as follows:
∂
∂t
∫
n dv=∫
n V da⋅cv cs
ρ ρ( ) (4.8)
where
n is the chemical in mass units t is the time
ρ is the density v is the volume a is the area
V is the velocity vector
where described in words, the formation of (n) through the volume surface is equal to the integrated rate of for- mation over the control volume.
It is a simple extrapolation to extend this concept for even coarse volumes as follows: