Page 147 - Sustainable On-Site CHP Systems Design, Construction, and Operations

120 CHP B a s i c s

catalytic oxidation system, when CO passes over a catalyst, usually a noble metal, it is
oxidized to CO at efficiencies of up to 90 percent. A catalytic converter also oxidizes
2
unburned or partially burned hydrocarbons and produces CO and H O. This type of
2 2
exhaust gas treatment is generally used for engines fueled by diesel.
Combustion Turbines
As stated earlier, emissions from combustion turbines without exhaust gas treatment are
lower than those from reciprocating engines. As discussed in Chap. 3, there are three
classes of combustion turbines: aero-derivative, stationary or industrial, and microtur-
bines. Emissions from microturbines (30 to 400 kW) are slightly higher than those resulting
from operation of larger-capacity aero-derivative and industrial turbines. Without
exhaust gas treatment, NO emissions from combustion turbines range from approxi-
x
mately 25 to 120 ppm (by volume) with conventional burners.
There are two options for reducing NO emissions from combustion turbines:
x
1. Combustion system modifications
2. Post-combustion exhaust gas treatment

Combustion System Modifications
The primary objective of combustion system modifications is to reduce the production
of thermal NO , which increases with increase in peak combustion temperature, resi-
x
dence time and pressure. There are two primary approaches commercially used in this
technology category:
1. Water/steam injection
2. Dry low NO combustion
x
In the water/steam injection approach, water or steam is injected into the com-
bustion chamber during combustion. Water or steam injection reduces NO produc-
x
tion by reducing the peak combustion temperature. In addition to reducing NO
x
production, steam injection also increases the mass flow rate of the hot products of
combustion entering the turbine and increases its power output without increasing
the load of the air compressor (which uses about two-thirds of the turbine output).
This emission control technology can reduce NO emissions by 70 to 90 percent of that
x
without water/steam injection.
The use of dry low NO (DLN) combustion technology has become a primary goal
x
in recent development efforts for combustion turbines. This category of technology
uses various options of premixing the fuel and combustion air. Traditional combustion
takes place within diffusion flames, where mixing and combustion takes place simulta-
neously resulting in high temperature peaks and correspondingly high thermal NO .
x
Premixing serves to reduce the peak temperatures and residence time. Several patented
DLN technologies have now become commercially available with proven NO emission
x
results in the range of 15 to 25 ppm (at 15 percent O ), even though NO emissions as
2 x
low as 5 ppm have been reported. One DLN combustor has the potential of reducing
NO emissions by as much as 90 percent as shown in Fig. 7-2.
x
Furthermore the beneficial effect of premixing (DLN) on the emissions of NO and
x
CO for natural gas and fuel oil are also shown in Figs. 7-3 and 7-4, respectively.