Malfunction
Conditions.
No matter how much data has been collected to build a prediction model, which may have covered almost every imaginable operating scenario, there will still be malfunction conditions which will effect emission levels in a manner that cannot be predicted. For example, in a gas turbine which utilizes water injection for NOx control, CO emissions will typically vary from about 400 ppmvd at minimum load (30%) to <10 ppmvd at base load (100%).
One of the parameters measured in a gas turbine with a can-annular arrangement of combustors is blade path spread (BPS). BPS is the difference between the average blade path temperature and the individual blade path temperature which is farthest from the average. In other words, if the average of 14 sets of blade path thermocouples was 800 °F, and the farthest reading from the average was 845 °F, then the BPS value would be 45 °F. Typical BPS levels range from 15 to 50 °F during normal operation, with a trip point set at approximately 90 °F.
Should
one or more of the water injection nozzles become plugged, or water
distribution throughout the combustor baskets becomes uneven, then the BPS
value will climb. As a direct consequence, CO emissions can increase
significantly, and unpredictably. A BPS value of 65 °F at 70% load on one day may
translate into 300 ppmvd CO, and a day later the same 65 °F BPS value at 70%
load, with similar ambient meteorological conditions, could result in actual CO
levels of over 500 ppmvd. There are ways around the dilemma. By using routines
such as input parameter validation, boundaries are set for certain operating
parameters that can have an effect on emission rates. If the unit operates outside
of the boundaries determined during the model building effort, then an alarm
could flag the data as questionable.
In the
BPS case mentioned above, problems could also have occurred if the unit was
equipped with a CEM system. If the unit had a CO monitor with a range from
0-500 ppmvd, and the BPS value climbed above the normal operating range, it is
possible that CO emissions would exceed the 500 ppmvd level and not be
monitored by the CEM system.
Malfunction
conditions and off-normal operating conditions will occur on occasion, and they
will contribute to excess emissions. When these situations occur, they may be
unmeasurable (for the CEM analyzer range) and unpredictable (outside the
boundaries established during the model building). Prompt investigation and
corrective action in accordance with air permit conditions is necessary
regardless of the type of monitoring system employed at the facility.
