Page 146 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Carbon Footprint—Envir onmental Benefits and Emission Contr ols 119

3. Oxidation of unburnt hydrocarbons (C H ) to carbon dioxide and water:
x y
2C H + (2x+y/2)O → 2xCO + yH O
x y 2 2 2
The efficiency of a three-way catalyst system for reducing emissions of NO and CO is in
x
the range of 85 to 95 percent, depending on the exhaust gas temperature, air-fuel ratio, and
catalyst volume. A three-way catalyst can reduce NO emissions to as low as 0.5 lb/MWh. 11
x
Some examples of the installed and operating costs and cost-effectiveness (cost of
removing 1 ton of NO ) of three-way catalyst systems for engine capacities ranging
x
from 250 to 4000 kW are shown in Table 7-4.
Lean burn operations use fuel-air ratios that are lower than the stoichiometric ratio.
The energy efficiency of lean burn engines is slightly higher than those for the rich burn
engines. Without any treatment of the exhaust gases, NO emissions from lean burn
x
engines are in the range of 2 to 6 lb/MWh (42 to 127 ppm at 15 percent oxygen). Many
installations using lean burn engines do not require exhaust treatment. If exhaust treat-
ment is needed to reduce NO emissions, the most common treatment is the use of
x
selective catalytic reduction (SCR).
An SCR system selectively reduces NO emissions by injecting ammonia (either in the
x
form of liquefied anhydrous ammonia or aqueous ammonium hydroxide) into the exhaust
gas upstream of the catalyst. NO , ammonia (NH ), and oxygen (O ) in the air react on the
x 3 2
surface of the catalyst to form nitrogen (N ) and water (H O). For the SCR system to operate
2 2
properly, the exhaust gas must be within a particular temperature range (typically between
450 and 850°F). The temperature range is dictated by the selected catalyst surface charac-
12
teristics where reactions occur. Typically these catalysts are manufactured from noble metal
oxides of vanadium or titanium, or from a zeolite. SCR is most effective for engines operat-
ing at constant loads. At variable loads, it is less effective and some of the ammonia might
go through the system unreacted. This is known as ammonia slip and is regulated by many
local air-quality management agencies.
An SCR treatment system has the potential to reduce NO emissions from natural
x
gas–fired, diesel-fired, and dual-fuel lean burn reciprocation engines by up to 90 per-
cent. Some examples of the installed and operating costs and cost-effectiveness (cost of
removing 1 ton of NO ) for SCR treatment systems for engine capacities ranging from
x
250 to 4000 kW are shown in Table 7-4.
In some applications, it might be necessary to reduce emissions of CO in the exhaust
of lean burn engines. It is accomplished by catalytic oxidation of the exhaust gases. In a

Annual Operating Cost Effectiveness
Installed Cost ($) Cost ($) ($/ton) of NO Removed
x
Engine Three-Way Three-Way Three-Way
Capacity (kW) Catalyst SCR Catalyst SCR Catalyst SCR
250 20,000 310,000 10,000 140,000 290–310 4,380–4,810
1,000 42,000 340,000 27,000 180,000 200–220 1,320–1,490
4,000 130,000 470,000 96,000 310,000 180–190 580–660

Source: EPA (July 1993); Combined Heating, Cooling, and Power Handbook (2002).

TABLE 7-4 Examples of Installed and Operating Costs, and Cost-Effectiveness of Emission Control
Technologies Commercially Available for Reciprocating Engines