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CO Precursor Gas Monitoring Training Overview. Solomon Ricks, U. S. EPA Office of Air Quality Planning and Standards Ambient Air Monitoring Group. Outline. Introduction Methodology Burdens Creek Operation Cautions Instrument Operation Interferences and Operational Issues Summary.

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Solomon ricks u s epa office of air quality planning and standards ambient air monitoring group

CO Precursor Gas Monitoring Training Overview

Solomon Ricks, U. S. EPAOffice of Air Quality Planning and Standards Ambient Air Monitoring Group


Outline
Outline

  • Introduction

  • Methodology

  • Burdens Creek Operation

  • Cautions

  • Instrument Operation

  • Interferences and Operational Issues

  • Summary


Introduction
Introduction

  • Why analyze for Carbon Monoxide (CO) as a precursor gas?

  • CO is formed from incomplete combustion

  • Values across the nation are falling below the sensitivity of current CO methods

  • Characterize typical urban/rural concentrations for long-term trends analysis.


Methodology
Methodology

Measurement Principles – Non-Dispersive Infrared (NDIR) based on Beers Law

T = I/Io = e(-axC)

where:

T = Transmittance of light through the gas to the detector

I = light intensity after absorption by CO

Io = light intensity at zero CO concentration

a = specific CO molar absorption coefficient

x = path length, and

C = CO concentration


Methodology1
Methodology

Measurement Principles – Non-Dispersive Infrared (NDIR) based on Beers Law

I/Io = e(-axC) Rearranging this equation:

ln I/Io = axC Rearranging this equation:

C = (ln I/Io)/(a*x)

I and Io are measured by detector (i.e. The M/R or S/R ratios of the instruments readout)

“a” is a constant CO molar absorption coefficient at a wavelength of 4.6 micrometers

“x” is the path length of the detection cell


Methodology2
Methodology

  • What makes a precursor gas CO instrument?

  • The Precursor CO instrument is identical to a non-precursor CO analyzer with the following exceptions:

    • Required sample stream dried using permeation or Nafion™ Dryer

    • Analyzer baseline determined and corrected using heated or other catalytic converter

    • Frequent auto-zero

    • The instrument has an ultra-sensitive or “hot” detector


Methodology3
Methodology

Diagram of a Precursor Gas CO instrument


Burdens creek operations
Burdens Creek Operations

T-API 300 EU TEI 48C-TLE


Responses to daily checks 10 25 2006 thru 05 22 2007
Responses to Daily Checks10-25-2006 thru 05-22-2007


Cautions
Cautions

  • Bench Cleaning

  • Inspections

  • Cylinders, handling, shipping, storage

  • CO is poisonous!!!

  • The Catalytic Converter and Bench are operated at 48 – 50o C. It is Hot!


Instrument operation
Instrument Operation

ItemSchedule

Replace particle filter Bi-weekly

Diagnostics Checks Bi-weekly

Perform Level I check Daily

Replace IR source As needed

Leak Check and Pump Check Out Annually

Inspect Pneumatic Lines Semi-annually

Clean inside of Chassis As needed

Rebuild or replace pump As needed

Clean optic bench As needed

Replace wheel motor As needed


Interferences and operational issues
Interferences and Operational Issues

  • Water Vapor

    • Water absorbs strongly in many IR spectra

    • Water must be removed using permeation or Nafion dryer

  • Carbon Dioxide

    • CO2 absorbs in several of the same regions as CO

    • Baseline adjustment via Auto-zero (Azero) or Auto-reference (Aref) Cycle


Interferences and operational issues1
Interferences and Operational Issues

  • Why auto-reference or auto-zero?

    • Continuous measurements at low concentrations may be limited by gradual changes in baseline

  • What cause the changes?

    • Environmental changes, i.e., temperature

    • Drift related to the age of optical bench components

    • Interferents, such as CO2


Interferences and operational issues2
Interferences and Operational Issues

  • Auto-reference or auto-zero cycle

  • TEI 48C-TLE

  • Performs an auto-zero, or “Azero”, cycle - 10 minutes through the Carolite scrubber.

  • Length of test not adjustable

  • Frequency is adjustable

  • Adjusts the baseline of the instrument

  • T-API 300EU

  • Performs an auto-reference, or “Aref”, cycle - <15 minutes through the heated palladium catalytic converter

  • Length of test not adjustable

  • Frequency is adjustable

  • Adjusts the measurement/reference (M/R) ratio


Interferences and operational issues3
Interferences and Operational Issues

  • Auto-Reference Cycle

  • API 300EU

  • Instrument goes into “Aref” cycle (<15 minutes)

    • 3-minute dwell period before and after auto-ref measurements

    • Auto-ref measurement lasts 9 minutes

  • Sample air re-routed to pass through converter - all CO is converted to CO2

  • Analyzer examines the M/R Ratio

  • Since CO is scrubbed out, any change in this ratio from last Aref cycle must mean that CO2 or H20 are interfering

  • Analyzer CPU adjusts the M/R ratio to compensate and the offset is changed

  • Note: API recommends that you set this to once every 24 hours for ambient conditions. For process control, more often. We have set it to once every 12 hours


Interferences and operational issues4
Interferences and Operational Issues

Auto-Zero Cycle, TEI 48C-TLE

  • Sample stream re-routed through internal scrubber

  • 10-minute duration

  • Adjusts the zero background

  • Not intended to replace routine calibration

  • Front panel display does not freeze


Summary
Summary

  • Carbon Monoxide in rural and suburban settings can be an indicator for VOCs

  • OAQPS continues operating instruments at its on-site monitoring station, Burdens Creek: TEI 48C-TLE and T-API 300EU

  • Other instrument(s) are available: Ecotech

  • Auto-reference or auto-zero sequence is an operational issue

  • Water vapor and CO2 can cause interference. The dryer and CO scrubber must be plumbed and working properly or baseline shifts occur