Page 83 - Boiler plant and distribution system optimization manual

68 Boiler Plant and Distribution System Optimization Manual

In situ Analyzer to measure on a dry basis. The difference between
In an in situ analyzer, the zirconium oxide wet and dry measurements can result in readings
cell is located at the end of a stainless steel probe that may differ by as much as 0.5 percent oxygen.
that is inserted directly into the flue gas stream. A If the condition occurs where the combus-
small heating element encompasses the cell, and tibles concentration increases to a point where
a thermocouple provides feedback to an external there is no net oxygen in the flue gas, it becomes
temperature control circuit. a sensor of net combustibles. The voltage gener-
ated by the cell increases sharply as the flue gas
Close-coupled Extractive Analyzer changes from a net oxygen to a net combustibles
A close-coupled extractive analyzer is de- condition. This property of a zirconium cell is
signed somewhat differently. The zirconium ox- extremely useful on some combustion processes
ide element and temperature controlled furnace because it permits measurement on both sides
are housed in an insulated enclosure mounted of stoichiometric combustion, either excess air or
outside, but immediately adjacent to, the flue gas excess fuel.
stack or duct.
While in situ analyzers are limited to flue
gas temperatures of about 1100 F [593 C] or less, RELATIVE HUMIDITY
close-coupled sensors can be used with high tem-
perature probe materials up to 3,200 F [1760 C]. Relative humidity can change the amount of
In general, close-coupled units respond much oxygen in air at 70 F [21 C] from 20.9% at 0% RH
faster to changes in the flue gas stream because to 20.4% at 100% RH. This 0.5% change will effect
they do not rely on diffusion to carry the sample excess air settings and efficiency (0.2% or more).
to the sensing cell. A close coupled sensor can be
fitted with a catalytic combustibles sensor in the
same flow loop as the oxygen cell, thus making a OxYGEN DEFICIENCY AND SAFETY
combination oxygen/combustibles analyzer.
In measuring oxygen O , here are some
2
important points to keep in mind. Normal air
NET OxYGEN VS. GROSS OxYGEN contains 20.9% oxygen and 79.1% nitrogen; the
MEASUREMENTS usual alarm point is 19.5% unconsciousness oc-
curs at 15%; brain damage at 10%, and death at
As burners cannot mix fuel and air perfectly, 5%. (These are approximate figures and may vary
both oxygen and unburned combustibles are in with the individual.
the flue gas. zirconium oxide analyzers indicate
net oxygen; i.e., the oxygen left over after burning
whatever combustibles are present on the hot zir- CARBON MONOxIDE AND COMBUSTIBLES
conium oxide cell. Orsat, paramagnetic and wet MEASUREMENT METHODS
cell oxygen analyzers measure gross oxygen.
Usually the difference between net and gross There are three prevalent methods for on line
measurements are small since combustibles are monitoring of flue gas combustibles: wet electro-
generated in the parts per million range. Occa- chemical cell, catalytic element, and non-disper-
sionally conditions may occur where net and sive infrared absorption.
gross readings are significantly different. Differ-
ences may also occur because zirconium oxide Electrochemical cell
measures oxygen on a wet basis; i.e. the flue gas The wet electrochemical cell technique is
contains water vapor. The other measuring tech- used only for carbon monoxide. It works on the
niques all require cool, dry samples and are said principle that current flowing between the anode