Page 84 - Boiler plant and distribution system optimization manual
P. 84
Combustion Analysis 69
and cathode is directly proportional to the flow of systems requiring CO/combustibles measure-
carbon monoxide through the membrane. There ment as an active input generally utilize infrared
are problems that occur with this type, flow rate carbon monoxide analyzers.
as affected by the ambient pressure, temperature
and humidity. Furthermore, the membrane can
become coated with flue gas condensation, thus INFRARED CO MEASUREMENT
reducing its effectiveness. Because of this, these
sensors are prone to zero and span drift. Carbon monoxide is one of many gases that
are known to absorb infrared energy at specific
discrete wavelengths. The amount of energy ab-
CATALYTIC COMBUSTIBLES SENSOR sorbed is a measure of the concentration of car-
bon monoxide.
Catalytic element sensors have been widely There are two types of carbon monoxide an-
used for detecting combustible gases in ambient alyzers: off-stack (sampling) and across the stack
air in mine shafts, parking garages, and other (in situ).
closed areas. High quality sensors with carefully
selected elements, can be used to measure carbon
monoxide in flue gas. Catalytic sensors are avail- OFF-STACK CO ANALYzERS
able with full scale ranges as sensitive as 0-2000
ppm combustibles and with accuracies of (+/–) Off-stack analyzers are housed in enclosures
100 ppm or better. suitable for the environmental conditions and are
The principle behind all catalytic sensors is usually located at easily accessible places near
the same, if combustibles and oxygen are both the combustion process. In most cases a sampling
present in a gas stream, they will not normally system is required to clean, dry, and cool the sam-
burn together unless the temperature is elevated ple before it enters the analyzer. Provision for the
something above 1000 F [638 C] However, if the introduction of calibration gases are usually an
same gas mixture comes in contact with a solid integral part of the design of the sample condi-
catalyst, such as platinum, combustion will occur tioning system.
at temperatures as low as 400 F [204 C].
There are two elements present, one with a
catalyst in an inert binder the other is inert. The ACROSS-THE-STACK CO ANALYzERS
entire housing is heated to over 400 F, when the
flue gas sample containing both oxygen and com- Across-the-stack CO analyzers are based on
bustibles pass through the housing, combustion the same technical principles as off-stack analyz-
occurs on the active element but not on the refer- ers but their design is somewhat different. The
ence element. This causes the temperature of the infrared source is housed in an enclosure that
active element to rise and its resistance to change. mounts directly on the stack or duct. The infrared
Some close-coupled extractive oxygen analyzers beam generated by the source passes completely
have been modified to incorporate a catalytic through the stack into a similar enclosure mount-
combustibles sensor in addition to the oxygen ed on the other side.
sensor. There are two major advantages of the
The catalytic sensor has the advantage of be- across-the-stack systems. First, the speed of re-
ing both low cost and sensitive to hydrogen and sponse is nearly instantaneous. Off-stack sys-
carbon monoxide. These sensors make it ideal tems, conversely, can take several minutes to re-
for flue gas monitoring and recording. However, spond to a change in flue gas conditions. Second,
the zero and span stability of the sensor is not as across-the-stack systems provide a measurement
good as that of the infrared sensor. Combustion of the average CO concentration in the stack.
