15-01-2013, 12:22 PM
Energy Conversion Technologies
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Introduction
In these and subsequent notes, we will describe the
infrastructure that is available to be considered in the
generation and planning functions. We classify this
information by
• Energy conversion, transport, and storage
• Technologies available now, and those likely to be
available in the future.
Some qualifications:
• We primarily consider only technologies which
facilitate the conversion, transport, and storage of
bulk (large) quantities of energy. There will be one
exception to this: small-scale distributed generation.
• By “energy conversion,” we mean the conversion of
energy into some form into electric energy.
• By “available now,” we mean that the technology is
available now at a cost that is reasonably competitive.
Pulverized coal power plants
There are three kinds of pulverized coal plants:
• Subcritical
• Supercritical
• Ultra-supercritical
In a PC plant, steam is admitted to the steam turbine
at 1000° F and 2400 psi for subcritical and 3500 psi
for supercritical [1]. (The water critical temperature
and pressure are 705 ºF (374 ºC) and 3210psi (217.7
atm), respectively. When temperature exceeds 705
ºF, and pressure are above these values, water can
exist only in the gaseous phase [2].) The pulverized
coal is burned in a steam generator constructed of
membrane waterwalls and tube bundles which absorb
the radiant heat of combustion producing steam that
is fed into a steam turbine generator [3]. The steam
expands in the turbine, and this expansion work
drives the turbine and generator to produce
electricity. The expanded steam is condensed to
water in the condenser and then returned to the steam
generator (or boiler).
Flue-gas from the combustion of the coal in the steam
generator is passed through an electrostatic
precipitator to remove particulates. The flue-gas then
passes through a flue-gas desulfurization (FGD) unit
(or scrubber1)
bed coal plants
In fluidized bed combustion (FBC), solid fuels are
suspended on upward-blowing jets of air during the
combustion process. The result is a turbulent mixing
of gas and solids. The tumbling action, like a
bubbling fluid, provides more effective chemical
reactions & heat transfer [6].
Fluidized-bed combustion evolved from efforts to
find a combustion process able to control SO2
emissions without scrubbers. The technology burns
fuel at temperatures of 1400-1700° F, well below the
threshold where nitrogen oxides form (at
approximately 2500° F, the nitrogen and oxygen
atoms in the combustion air combine to form
nitrogen oxide pollutants). The mixing action of the
fluidized bed brings the flue gases into contact with a
sulfur-absorbing chemical, such as limestone.
CO2 Capture and Sequestration for coal
Any coal-fired generation technology will require
CO2 capture and sequestration in order to
significantly reduce its CO2 emissions.
There are two ways to perform CO2 capture for PC or
for CFB plants: post-combustion capture and oxygen
based combustion. A third way is called precombustion
and involves IGCC, to be discussed in
Section 5 below. The three ways are illustrated in
Fig. 6b [10].
Simple Cycle Combustion turbines
Simple cycle combustion turbines (CTs), Fig. 8, [3]
generate power by compressing and heating ambient
air and then expanding those hot gases through a
turbine which turns an electric generator. They are
also referred to as a “gas turbine” and identical to jet
engines in theory of operation. CTs, a mature
technology, have low capital cost, short design and
installation schedules, rapid startup times, and high
reliability. On the other hand, they have high
operations and maintenance costs when compared to
combined cycle units and are therefore only used for
peaking operation. Sizes are typically less than 300
MW.
Natural Gas Combined Cycle power plants
Combined cycle combustion turbines [3], Fig. 10,
generate power by compressing and heating ambient
air and then expanding those hot gases through a
turbine which turns an electric generator. In addition,
heat from the hot gases of combustion is captured in
a heat recovery steam generator (HRSG) producing
steam which is passed through a steam turbine
generator. NGCC units have low emissions and
significantly higher efficiency than CTs. But their
capital cost is higher than CTs. Compared to standard
baseload plants, they are subject to the volatility of
natural gas prices. Their O&M costs are higher than
PC plants.