Environmental Engineering Reference
In-Depth Information
5.2.9
Emission Control
If left uncontrolled, power plants can emit quantities of air pollutants that cause ambient pollutant
levels to exceed standards designed to protect human health and the environment. Suppose a
1000-MW power plant burns coal with a 10% mineral content and 2% sulfur content (not unusual).
It is a base-loaded plant that works at 100% capacity and 35% thermal efficiency. The coal has a
heating value of 12,000 Btu lb
−
1
(
28 MJ kg
−
1
). All the mineral content exits the smoke stack as
particles (fly ash), and the sulfur exits as sulfur dioxide SO
2
. This plant would emit 3.2E(5) t y
−
1
of particles and 1.3E(5) t y
−
1
of SO
2
(sulfur dioxide has twice the molecular weight of sulfur).
In addition, the plant would emit copious quantities of nitrogen oxides, products of incomplete
combustion (PIC), carbon monoxide, and volatile trace metals. Clearly, such an uncontrolled power
plant could present a major risk to human health and the environment. Therefore, in most countries,
environmental regulations require that the operator of the power plant install emission control
devices for these pollutants. These devices contribute significantly to the capital and operating cost
of the plant, and reduce to some degree the thermal efficiency, because the devices syphon off some
of the power output of the plant. These costs are passed on to the customers as added cost of the
electricity. The control devices also produce a stream of waste, because what is not emitted into
the atmosphere usually winds up as a solid or liquid waste stream.
≈
5.2.9.1 Control of Products of Incomplete
Combustion and Carbon Monoxide
The control of PIC and CO is relatively easy to accomplish. If the fuel and air are well-mixed, as is
the case in modern burners, and the fuel is burnt in excess air, the flue gas will contain very little,
if any, PIC and CO. It is in the interest of power plants to achieve a well-mixed, fuel-lean (air-
rich) flame, not only for reducing the emission of these pollutants, but also for complete burn-out
of the fuel, which increases the thermal efficiency of the plant. PIC and CO emissions do occur
occasionally, especially during start-ups and component breakdowns, when the flame temperature
and fuel-air mixture is not optimal. Under those conditions a visible black smoke emanates from the
smoke stack. These occurrences should be rare and should not contribute significantly to ambient
concentrations of these pollutants.
5.2.9.2 Particle Control
Particles, also called particulate matter (PM), would be the predominant pollutant emanating from
power plants were it not controlled at the source. This stems from the fact that coal, and even
oil, contains a significant fraction by weight of incombustible mineral matter. In older, stoker-fed
and cyclone burner plants, the mineral matter accumulates in the bottom of the boiler as bottom
ash and is discarded as solid waste or taken up in water and sluiced away. In modern pulverized
coal-fired plants the majority (
≈
90%) of the mineral matter is blown out from the boiler as
fly ash. The fly ash contains (a) a host of toxic metals, such as arsenic, selenium, cadmium,
manganese, chromium, lead, and mercury, and (b) nonvolatile organic matter (soot), including
polycyclic aromatic hydrocarbons (PAHs); these would pose a public health and environmental
risk if emitted into the atmosphere. For that reason, most countries instituted strict regulations on
particle emissions from power plants.
