Sensing system for gas turbine power plant exhaust

A gas composition sensor for use in an exhaust duct of a gas turbine power plant, the gas composition sensor comprising:

• a first pipe comprising:
• a first end portion connected to the exhaust duct;
• and
• a second end portion positioned within the exhaust duct at a location where gas composition is to be sensed;
• a transmitter configured to emit:
• a first laser beam to sense a first gas composition within the first pipe;
• and
• a second laser beam to sense gas composition in the exhaust duct outside of the second end portion;
• a receiver configured to receive the first laser beam and the second laser beam to determine a first composition signal and a second composition signal;
• and
• a controller configured to subtract the first composition signal from the second composition signal to determine gas composition at the location.

A method of determining composition of exhaust gas in a gas turbine power plant having an emissions reduction system configured to dispense reductant onto a catalyst bed, the method comprising:

• emitting a first laser beam into a first sensing tube extending into an exhaust duct;
• emitting a second laser beam into the first sensing tube and into the exhaust duct;
• sensing a first reductant level in the first sensing tube using the first laser beam;
• sensing a second reductant level in the exhaust duct using the second laser beam;
• subtracting the first reductant level from the second reductant level to determine a first reductant value;
• performing a comparison of the first reductant value to an expected reductant value;
• and
• adjusting a reductant valve to adjust an amount of reductant dispensed onto the catalyst bed based on the comparison.