Page 38 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Overview 17

stream combines with water vapor in the atmosphere to form sulfurous acid (H SO )
2 3
and sulfuric acid (H SO ). These acids are released from the atmosphere as acid rain.
2 4
Limiting and reducing emissions from CHP plants is an important element of
sustainability. However, as discussed, the implementation of CHP by itself versus
conventional methods (i.e., buying power from the utility company and burning gas in
a boiler to make hot water or steam) reduces source fuel consumption and overall total
emissions. Further, as mentioned, the use of biofuels may further negate the impact of
CHP plant emissions as CO , for example, is absorbed as crops are grown for fuel.
2
Emission controls are discussed in detail in Chap. 7, and the type of emission con-
trol system used depends upon the type of prime mover used. For example, reciprocating
IC engines are either rich-burn or lean-burn, and the type of engine has an effect on the
emission controls used to reduce emissions. In general, except for NMHC, the lean-
burn combustion engine provides much lower levels of atmospheric pollutants. The
lean-burn combustion engine is capable of producing lower emissions than a rich-burn
engine before the aid of exhaust treatment and fuel-air ratio controllers.
As the amount of thermal NO generated is related linearly to the amount of time
x
that the hot gases are at flame temperature in the combustor, and exponentially to the
temperature of the flame, some CTG emission control systems work to cool the flame
temperature. For example, wet injection is an emission control technique in which water
or steam is injected into the combustor to lower the flame temperature, which lowers
the formation of NO . Steam injection can increase the power output of a turbine by
x
increasing the mass flow rates.
Exhaust gas treatment involves further reducing the levels of atmospheric pollutants
present in exhaust by “cleaning” these pollutants from the exhaust stream. Catalysts are
a common method of reducing the amount of atmospheric pollutants present in exhaust
gas. Catalysts are used to reduce pollutants in exhaust emissions by chemically converting
them into naturally occurring compounds. A catalyst sustains a chemical reaction
without being chemically changed. The catalyst will either oxidize or reduce chemical
compounds. Common catalyst types include three-way catalysts and selective catalytic
reduction (SCR).

Key Issues Facing Industry Today
As this book was being written, the world experienced extreme energy price volatility
that in part led to food riots around the world as commodity prices surged. Crude oil
peaked around $150 per barrel in summer of 2008, and in the United States, natural
gas reached more than $14 per million Btu (decatherm), but is now less than $4 per
decatherm. The sharp decline of the world economy in late 2008 sent crude oil prices
below $40 per barrel in the span of just about 4 months. In the global economy, we are
apparently all connected, and CHP plants, which today typically use fossil fuels,
experienced economic challenges as fuel prices escalated and utility electricity rates
lagged fuel price increases. Utility escalation rates often lag fuel prices due to the
inherent system inertia, regulatory requirements, and political hurdles that utilities
face in obtaining a rate increase. High fuel prices and relatively low electricity prices
hurt the economic viability of existing and proposed CHP systems. As many utilities use
fossil fuel as their main energy input, eventually electricity rates rise to reflect the cost
of the fuel source, or as it is today that when fuel prices fall, operating CHP plants
benefit from their investments.