Environmental Engineering Reference
In-Depth Information
Coal is delivered to a plant already sized to meet the feed size of the pulverizing mill (see
below), in the order of a few to ten centimeters per coal lump. In the United States and many other
countries, coal is washed at the mine. Washing of coal removes much of the mineral content of the
coal (including pyritic sulfur), thus reducing its ash and sulfur content and improving its heating
value per unit mass. In preparation for washing, the coal is crushed at the mine mouth to less than
a centimeter per
nut
or
slack
.
Most modern steam power plants fire pulverized coal. The raw coal from the stockpile is
delivered on a conveyor belt directly to a pulverizing mill. Such mills are either of the rotating ring,
rotating hammer, or rotating ball type. The mill reduces the raw coal lumps to particles smaller
than 1 millimeter. The pulverized coal is stored in large vertical silos from whence it is blown
pneumatically into the burners at a rate demanded by the load of the plant.
In oil-fired power plants, oil is stored in large tanks (the “tank farm”), to which oil is delivered
either by pipeline, by railroad tankers, or by tanker ship or barge if the plant is located near navigable
waters. Power plants like to have at least a 30-day supply of oil in their tanks. For a 1000-MW
plant with 35% efficiency, this can amount to over 1E(5) metric tons of oil. The oil is purchased
from refineries in the form it is combusted in the burners, with specified sulfur, nitrogen, and ash
content as well as other properties, such as viscosity and vapor pressure.
In natural gas-fired power plants, gas is delivered to the power plant by pipeline at high
pressure (compressed natural gas, CNG). Some gas-fired power plants use liquefied natural gas
(LNG). Liquefied gas is transported in huge (up to 1.25E(5) m
3
) refrigerated tankers at
164
◦
C.
−
The LNG is stored in refrigerated tanks until used.
5.2.2
Burner
The role of the burner is to provide a thorough mixing of the fuel and air so that the fuel is completely
burned. Ignition is accomplished by a spark-ignited light oil jet until the flame is self-sustaining.
In the combustion chamber a pulverized coal particle or atomized oil droplet burns in a fraction
of a second. The coal particle or oil droplet burns from the outside to the core, leaving behind
incombustible mineral matter. The mineral matter is called ash. In modern pulverized coal and
atomized oil fired power plants, more than 90% of the mineral matter forms the so-called
fly ash,
which is blown out of the boiler by forced or natural draft and is later captured in particle collectors.
About 10% of the mineral matter falls to the bottom of the boiler as
bottom ash
. When the bottom
of the boiler is filled with water, the bottom ash forms a wet sludge, which is sluiced away into
an impoundment. Some of the fly ash, however, is deposited on the water pipes lining the boiler.
This forms a slag which hinders heat transfer. The slag needs to be removed from time to time by
blowing steam jets against it or by mechanical scraping.
Coal burns relatively slowly, oil burns faster, and gas burns the fastest. For complete combustion
(
carbon burn-out
), excess air is delivered—that is, more air than is required by a stoichiometric
balance of fuel and the oxygen content of air. Pulverized coal requires 15-20% excess air; oil and
gas 5-10%.
A typical burner for pulverized coal is depicted in Figure 5.1. The central coal impeller carries
the pulverized coal from the silo in a stream of primary air. Tangential doors (
registers
) built into
the wind box allow secondary air to be admixed, generating a fast burning turbulent flame. The
impeller is prone to corrosion and degradation and has to be replaced once a year or so.
The burners are usually arranged to point nearly tangentially along the boiler walls. In such
a fashion a single turbulent flame ensues from all four burners in a row, facilitating the rapid and
