Method and apparatus for automated isokinetic sampling of combustor flue gases for continuous monitoring of hazardous metal emissions

Abstract

An isokinetic sampling system and method of operation assures the taking of more valid data over a long period by automated isokinetic sampling of flue gases in the stack for a stationary combustor. Sample air is extracted from the flue gases by a sampling probe having a sampling nozzle of a precise inner diameter, a thermocouple to measure the air temperature in the stack, and a pitot tube that gives readings representative of differential pressure across sampling nozzle. A separate transducer senses absolute pressure of the flue gases. Temperatures, absolute pressures and differential pressures of the flue gases are measured and representations of the temperatures, absolute pressures and differential pressures are fed to a micromanometer which feeds signals representative of these representations to a computer that generates responsive control signals for a mass flow controller. The mass flow controller throttles the flow of sample air through it to a correct magnitude to achieve isokinetic extraction of sample air under the existing stack conditions of temperature, pressure, and velocity at the point of extraction at the sample nozzle for the selected nozzle opening, or diameter. Stack conditions can vary over time and this automated isokinetic sampling system immediately adjusts the flow rate of mass flow controller so that isokinetic sampling continues uninterrupted and valid data can be generated by monitoring sensors.

I claim: 
     
       1. A method of automated isokinetic sampling of flue gases for a continuous monitor of metal emissions comprising the steps of: extracting sample air from flue gases in the exhaust stack for a combustor with a heated sampling probe;   measuring temperatures, absolute pressures and differential pressures of said flue gases;   feeding signals representative of said temperatures, absolute pressures and differential pressures to a computer:   generating control signals in said computer based on said representative signals for a mass flow controller; and   controlling said mass flow controller with said control signals to affect the flow of said sample air through said heated sampling probe and a sampling interface for said continuous monitor of metal emissions.   
     
     
       2. A method according to claim 1 further including the step of: coupling representations of said temperatures, absolute pressures and differential pressures to a microprocessor micromanometer.   
     
     
       3. A method according to claim 2 further including the step of: throttling said flow of said sample air through a vacuum pump.   
     
     
       4. A method according to claim 3 further including the step of: correcting said flow rate to a magnitude through said mass flow controller to achieve isokinetic extraction of said sample air under the existing stack conditions of temperature, pressure, and velocity at the point of extraction of said sample air in said heated sampling probe.   
     
     
       5. A method according to claim 4 in which said step of extracting is done by a heated sampling probe having a sampling nozzle, a thermocouple to measure gas temperature, a pilot tube to measure differential pressures, and a transducer to measure absolute pressure. 
     
     
       6. A method according to claim 5 in which said microprocessor micromanometer quantifies said differential pressures. 
     
     
       7. An apparatus for automated isokinetic sampling of flue gases for a continuous monitor of metal emissions comprising: means for extracting sample air from flue gases in the exhaust stack for a combustor with a heated sampling probe;   means coupled to said extracting means for measuring temperatures, absolute pressures and differential pressures of said flue gases;   means coupled to said measuring means for feeding signals representative of said temperatures, absolute pressures and differential pressures to a computer;   means coupled to said feeding means for generating control signals in said computer based on said representative signals and connecting them to a mass flow controller; and   means coupled to said generating means for controlling the flow of said sample air through said heated sampling probe and a sampling interface for said continuous monitor of metal emissions.   
     
     
       8. An apparatus according to claim 7 further comprising: means for coupling representations of said temperatures, absolute pressures and differential pressures to a microprocessor micromanometer.   
     
     
       9. An apparatus according to claim 8 in which said mass flow controller includes means for throttling said flow of said sample air through said vacuum pump. 
     
     
       10. An apparatus according to claim 9 in which said throttling means corrects said flow rate to a magnitude through said mass flow controller to achieve isokinetic extraction of said sample air under the existing stack conditions of temperature, pressure, and velocity at the point of extraction of said sample air in said heated sampling probe. 
     
     
       11. An apparatus according to claim 10 in which said extracting means includes a heated sampling probe having a sampling nozzle, thermocouple, and a pitot tube, and a transducer to provide representations of said temperatures, absolute pressures and differential pressures. 
     
     
       12. An apparatus according to claim 11 in which said microprocessor micromanometer quantifies said differential pressures.