Page 37 - Sustainable On-Site CHP Systems Design, Construction, and Operations

16 CHP B a s i c s

is oversized. Power sales to the electric utility are often not economical, and sometimes
not allowed. Also, in this case, a large portion of the thermal energy from the peak
electric load CHP plant may be wasted (i.e., rejected to the atmosphere) and regulated
CHP efficiency requirements may not be met (e.g., FERC).
Full utilization of a CHP plant, electrically and thermally, typically results in better
economic performance. An optimally sized CHP system uses as much of the recovered
useful thermal energy as possible, with minimum heat dumping or wasting of
recovered heat. Note it is possible, due to high electric demand utility charges, that a
cogeneration plant sized for the peak electric load could provide higher total life-cycle
cost savings (see Chap. 9), even though some of the recovered thermal energy is wasted
or the generator is not fully utilized during some portions of the year. As part of a CHP
general screening study, project engineers obtain historical data or develop estimates of
electrical and thermal energy use. It is critical to evaluate the daily profile of energy use
(i.e., energy use versus time). As noted, the relationship between electric energy demand
and coincident thermal energy demand is critical. For example, a facility that has high
electricity use during the daytime with little electricity use at night, and has high thermal
energy requirements at night (e.g., for space heating) with little thermal energy use during
the day is usually a poor cogeneration candidate unless a thermal storage strategy is incor-
porated. In general, CHP favors facilities that have coincident electric and thermal loads.
Environmental Impacts and Controls
Some key considerations in any CHP system are the following: What are the emissions from
the CHP engine? What emission control strategies are necessary to comply with local and
federal air quality regulations (not to mention to make the CHP plant as environmentally
friendly as is economically justified)? The following is a brief description of some of the
atmospheric pollutants that are found in combustion exhaust from CHP facilities:
Atmospheric pollutants. Pollutants generated by gas engines and turbine emissions
include nitrogen oxides (NO ), carbon monoxide (CO), hydrocarbons (HC), and sulfur
x
oxides (SO ). Aldehydes (CHO) and particulate matter 10 μm and smaller (PM ), are
x 10
also considered atmospheric pollutants. These atmospheric pollutants occur at extremely
low concentrations in gaseous fuel applications when compared to liquid fuel applications.
Nitrogen oxides. Nitrogen oxides (NO ) is formed in the combustion chamber by the
x
combination of high temperatures and the presence of nitrogen and oxygen. The reaction
between nitrogen (N ) and oxygen (O ) forms nitric oxide (NO) and nitrogen dioxide (NO )
2 2 2
collectively referred to as NO . NO is harmful to animals and humans because it limits
x 2
breathing capacity and the ability of blood to carry oxygen. In the lower atmosphere, when
exposed to sunlight, NO and NO act as precursors to the formation of ozone.
2
Carbon monoxide. Carbon monoxide (CO) is formed by the incomplete combustion
of fuel and oxygen. The complete combustion of a fuel, like methane (CH ), and oxygen
4
will produce carbon dioxide (CO ) and water. The incomplete combustion of methane
2
will form CO, CO , and water. Carbon monoxide is a poisonous gas. In the upper atmo-
2
sphere, it reacts with ozone (O ) to form CO , a greenhouse gas.
3 2
Hydrocarbons. Natural gas, which is comprised of methane, ethane, propane, butane,
and other heavier compounds, is a typical fuel for CHP facilities. Typically, a small amount
of hydrocarbons from the fuel source passes through the combustion chamber without
combusting. Nonmethane hydrocarbons (NMHC) can react with the nitrogen oxides in
the lower atmosphere and act as precursors to the formation of photochemical smog.
Sulfur oxides. Sulfur oxides (SO ) are formed when sulfur compounds in the fuel and
x
lube oil are oxidized in the combustion chamber. Sulfur oxides contained in the exhaust