Environmental Engineering Reference
In-Depth Information
coal input energy. Here, the carbon emission savings would be only on the order of 5-10%.
While a CCGT has a higher efficiency than a single-cycle plant, coal gasification requires
some of the coal energy to be spent on gasification.
10.4.3
CO
2
Capture
In general, the volume fraction of CO
2
in the flue gas of fossil fuel electric power plants ranges
from 9% to 15%, depending on fuel (e.g., gas versus coal) and excess air used for combustion.
Because a relatively small fraction of one gas (CO
2
) needs to be separated from the majority of
other gases (N
2
,H
2
O, and excess O
2
), the capture of CO
2
is quite difficult and expensive, and it
requires extra energy, which reduces the thermal efficiency of a power plant.
The capture of CO
2
is only worthwhile in large power plants, especially those burning coal
(because coal emits more CO
2
than oil or gas). A 1000-MW coal-fired power plant emits between
6and8Mty
−
1
of CO
2
. The capture of CO
2
from all the world's large coal-fired power plants
would make a significant dent in the global carbon emissions. The following technologies for CO
2
capture from power plants are being developed:
•
Air separation-CO
2
recycling
•
Solvent absorption
•
Membrane gas separation
10.4.3.1 Air Separation-CO
2
Recycling
This method is based on combustion of the fossil fuel in pure oxygen, instead of air. A plant using
this method requires an air separation unit (ASU). Here, the gas separation shifts to precombustion,
rather than postcombustion. This does not save much energy in gas separation, but there are several
other beneficial effects to this method. First, the ASU produces useful byproducts, such as nitrogen
and argon. Second, part of the oxygen produced in the ASU can be used for coal gasification, thus
enabling the power plant to operate in a combined cycle gas turbine mode with a somewhat higher
thermal efficiency than a single-cycle coal combustion-steam turbine mode. (Some efficiency is
lost on account of using coal energy for gasification.) Third, the combustion of “syngas” (a mix of
CO and H
2
) with pure oxygen releases no SO
2
,NO
x
, or particulate matter. All the contaminants in
coal are removed during and after the coal gasification process. The combustion products of syngas
with pure oxygen are almost entirely CO
2
and H
2
O. The water vapor is condensed, and CO
2
is
captured. Thus, there is no exhaust gas. In fact, the power plant itself requires no smoke stack.
A schematic of an integrated air separation coal gasification combined cycle power plant with
CO
2
capture is shown in Figure 10.9. The ASU is a standard commercial unit, as employed on
a large scale in the steel industry and for coal gasification. After nitrogen separation (depending
on use, the separated nitrogen can be either liquid or gaseous), part of the oxygen is used for coal
gasification (see Chapter 5), and the rest is used for combustion. The fuel (syngas) comes from
the gasification unit. Because the combustion of syngas with pure oxygen would yield too high
a temperature for conventional materials, the flame needs to be cooled. Therefore, a part of the
captured CO
2
is recycled into the combustion chamber in order to achieve tolerable combustion
temperatures. The combustion gases expand in a gas turbine, producing a part of the power. The
residual heat is recovered in a heat recovery steam generator (HRSG). The generated steam runs a
steam turbine, producing more power. After heat exchange in the HRSG, water is condensed from
