Page 82 - Boiler plant and distribution system optimization manual

Combustion Analysis 67

Figure 7.9—The combustion slide rule

PARAMAGNETIC Zirconium Oxide Cell
In recent years, The zirconium oxide cell
The paramagnetic sensor takes advantage has become the most common oxygen sensor for
of the fact the oxygen molecules are strongly in- continuous monitoring of flue gases. The sensor
fluenced by a magnetic field. Because of this and was developed in the mid-1960s in conjunction
because other flue gas constituents, notably NO, with the U.S. space program and because of its
NO and certain hydrocarbons exhibit apprecia- inherent ability to make oxygen measurements
2
ble paramagnetic properties this instrument is in hot, dirty gases without sample conditioning,
usually limited to the laboratory. it was quickly accepted by industrial users. The
sampling element itself is a closed end tube or
disk made from ceramic zirconium.
WET ELECTROCHEMICAL INSTRUMENTS The zirconium oxide cell has several signif-
icant advantages over the other oxygen sensing
Wet electrochemical cells, of which there are methods. First, since the cell operates at high
many designs, all use two electrodes in contact temperatures, there is no need to cool or dry flue
with an aqueous electrolyte. Oxygen molecules gases before it is analyzed. Most zirconium oxide
diffuse through a membrane to the cathode where analyzers make direct oxygen measurements on
a chemical reaction occurs. the stack with nothing more than a filter to keep
The electrochemical cell is essentially a bat- ash away from the cell. The cell is not affected
tery with an electrical current that is directly by vibration and unlike other techniques, the
proportional to the flow of oxygen through the output actually increases with decreasing oxygen
membrane. concentration. In addition the cell has a virtually
These cells are designed to be replaced eas- unlimited shelf life.
ily, however as the flue gas sample is extracted
from the stack and brought to the sensor, sample
conditioning is required as well as periodic main- In SITU VS. CLOSE-COUPLED
tenance on the sampling system which becomes ExTRACTIVE ANALYzERS
fouled with combustion products and a high
moisture level. The high maintenance required There are two basic types of flue gas oxygen
for these sensors is a definite drawback. analyzers that employ the zirconium oxide cell: in
situ and closed coupled extractive.