Method and apparatus for in situ gas concentration measurement

Apparatus for measuring concentration of a gas comprising:

• a tunable diode laser, said laser being tuned to project a laser beam at the wavelength of a spectral feature of said gas, said laser beam being modulated;
• a beamsplitter upon which said laser beam is projected, said beamsplitter splitting said laser beam into a reference beam and a sample beam;
• a sampling cavity having a first end and a second end, said first and second ends having opposed interior reflecting surfaces, said sampling cavity including a cylindrical metal body, said first sampling cavity end mounting to a first end of said body and said second sampling cavity end mounting to a second end of said body, said body having filters, said filters allowing exhaust gases from an exhaust stack to diffuse into said sampling cavity and preventing particulates from entering into said sampling cavity;
• a lens attached to said first end, said sample beam being projected from said beamsplitter into said sampling cavity through said lens, said lens deflecting a first surface reflection;
• a null detector receiving said first surface reflection, said null detector measuring intensity of said first surface reflection and producing an electrical null signal proportional to said intensity;
• a sample detector, said sample beam being reflected a plurality of times in said sampling cavity by said first and second ends and projected back through said lens to said sample detector wherein said lens helps focus said laser beam on said sample detector, said sample detector measuring intensity of said sample beam and producing an electrical sample signal proportional to said intensity, said sample detector being located near said null detector so that the path of said sample beam to said sample detector and the path of said sample beam to said null detector differ only by the path of the sample beam inside the sampling cavity;
• and
• a reference cell containing a predetermined concentration of said gas, said reference beam being projected from said beamsplitter through said reference cell;
• and
• a reference detector receiving said reference beam from said reference cell, said reference detector measuring intensity of said reference beam and producing an electrical reference signal proportional to said intensity, a third harmonic of said reference signal line locking said laser to said wavelength;
• a microprocessor receiving and comparing said sample signal and said null signal to eliminate laser beam variations and interference patterns between said laser and said lens and said fiber, said microprocessor comparing a second harmonic of said reference signal with said second harmonic of said sample signal and said second harmonic of said null signal to calculate concentration of said gas in said sampling cavity;
• an enclosure, said enclosure being attached to the outside of a wall of said exhaust stack and housing said laser, said reference cell, said reference detector, said sample detector, said null detector and said microprocessor;
• a weldment attached to said enclosure and projecting through said wall of said exhaust stack into the interior of said exhaust stack, said sampling cavity being mounted on said weldment on an end of said weldment furthest from said wall of said exhaust stack so that said sampling cavity is spaced away from said wall of said exhaust stack;
• and
• a calibration gas injection tube attached to and opening into an intermediate portion of said body of said sampling cavity at a first tube end and attached to said enclosure at a second tube end, with calibration gas flowing in said tube from said enclosure to said sampling cavity being heated by heat transfer from said exhaust gases to the temperature of said exhaust gases.

A method of measuring concentration of a gas in an exhaust stack comprising the steps of:

• tuning a laser to project a laser beam at the wavelength of a spectral feature of the gas;
• projecting the laser beam onto a lens which is attached to a sampling cavity containing exhaust gases from the exhaust stack;
• detecting a first portion of the laser beam at a first position, the first portion being a first surface refection of the laser beam off the lens;
• passing a second portion of the laser beam through the lens into the sampling cavity;
• reflecting the second portion of the laser beam a plurality of times between a first end of the sampling cavity and a second end of the sampling cavity;
• passing the second portion of the laser beam out of the sampling cavity and back through the lens;
• focusing the second portion of the laser beam as it passes back through the lens:
• detecting the second portion of the laser beam at a second position located near the first position detector so that the difference in the path of the first portion of the laser beam and the path of the second portion of the laser beam is the path of the second portion of the laser beam in the sampling cavity;
• and
• comparing the first and second portions of the laser beam to calculate said concentration of the gas in the exhaust stack.

A method of measuring concentration of a gas in an exhaust stack comprising the steps of:

• diffusing exhaust gases from the exhaust stack into a sampling cavity at the temperature and pressure of the exhaust gases while preventing particulates from entering the sampling cavity;
• tuning a laser to project a laser beam at the wavelength of a spectral feature of the gas;
• projecting the laser beam onto a lens which is attached to the sampling cavity;
• detecting intensity of a first portion of the laser beam at a first photodetector, said first portion being a first surface refection of the laser beam off the lens;
• converting said first portion of said laser beam into an electrical first signal proportional to said intensity;
• recording the DC value of the signal to properly scale the second harmonic signal;
• separating a second harmonic of the first signal with a first lock-in amplifier;
• passing a second portion of the laser beam through the lens into said sampling cavity;
• reflecting the second portion of said laser beam a plurality of times between a first end of the sampling cavity and a second end of the sampling cavity;
• passing the second portion of the laser beam out of the sampling cavity and back through the lens;
• focusing the second portion of the laser beam as it passes back through the lens;
• detecting intensity of the second portion of the laser beam at a second photodetector located near the first photodetector so that the difference in the path of the first portion of the laser beam and the path of the second portion of the laser beam is the path of the second portion of the laser beam in the sampling cavity;
• converting the second portion of the laser beam into an electrical second signal proportional to the intensity of the second portion of the laser beam;
• separating a second harmonic of the second signal with a second lock-in amplifier;
• projecting a third portion of the laser beam through a reference cell;
• and
• detecting the third portion of the laser beam to lock the laser to said wavelength.