🌿 Nitrogen oxides (NOx) in waste gases — chemiluminescence (reference method)

Environment PN-EN 14792

Overview

EN 14792 specifies the standard reference method (SRM) based on the principle of chemiluminescence for the determination of nitrogen oxides (NOx = NO + NO₂, expressed as NO₂) in waste gases from ducts and stacks; it covers the gas sampling and conditioning system and the analyser. The standard gives the performance characteristics and the criteria for measuring systems, it is applied to periodic monitoring and to the calibration and control of automated measuring systems (AMS), and it also lays down criteria for the equivalence of alternative methods (EN 14793). It was validated in field tests at incineration plants, co-incineration plants and large combustion plants and on a test rig for sampling periods of 30 min in the range 0–1300 mg/m³ NO₂ (large combustion plants) and 0–400 mg/m³ (incineration plants); the limit values of the directives are expressed in mg/m³ NO₂ for dry gas, at a specified oxygen content, at 273 K and 101.3 kPa. In gases from conventional combustion systems the nitrogen oxides consist in more than 95 % of NO, the remainder being mainly NO₂; in other processes the ratio may be different. In Poland the regulation on the measurement of the quantity of emissions indicates PN-EN 14792 as the reference method for NOx in the control of continuous measurements.

Method principle

Chemiluminescence is based on the reaction of nitrogen monoxide with ozone: 2 NO + 2 O₃ → NO₂ + NO₂* + 2 O₂, NO₂* → NO₂ + hν. Part of the NO₂ formed is in an excited state and, returning to the ground state, emits light of an intensity depending on the content of NO (and on the pressure and the presence of other gases). The gas is fed at a constant flow into the reaction chamber of the analyser with an excess of ozone; the radiation emitted, filtered by a selective optical filter, is converted by a photomultiplier into an electrical signal proportional to the NO. For the determination of NO₂ the gas is passed through a converter reducing NO₂ to NO — the signal then corresponds to the sum of NO + NO₂, and the NO₂ is calculated from the difference; a two-channel analyser measures NO and NOx at the same time, a single-channel one — alternately. Interference by CO₂ is possible (especially together with water vapour) through the quenching of the chemiluminescence — corrected by means of the manufacturer's curves or by calibration with gases of a similar CO₂ content; a vacuum analyser reduces this error; where the NH₃ is above 20 mg/m³ a correction may be needed. The sampling system consists of a probe, a filter, a line and a conditioning system in one of four configurations: a condensation cooler, a permeation dryer, dilution with dry air or nitrogen, or maintenance of the temperature of the line as far as a heated analyser; with the first three the dew point at the outlet is ≤ 4 °C.

Applications

• Periodic measurements of NOx emissions from power boilers, turbines, engines, waste incineration plants and industrial furnaces
• Calibration and control (QAL2, AST in accordance with EN 14181) of automated NOx analysers on stacks — the reference method under the regulation on measurements of emissions
• Separate determination of NO and NO₂, e.g. downstream of SCR/SNCR installations and in processes with an increased share of NO₂
• Demonstration of the equivalence of alternative methods (EN 14793)
• Checking of the effectiveness of flue gas denitrification installations

Key parameters

Standard

Standard number

PN-EN 14792:2017-04 (EN 14792:2017)

Title (PL)

Emisja ze źródeł stacjonarnych — Oznaczanie stężenia masowego tlenków azotu — Standardowa metoda odniesienia: chemiluminescencja

Title (EN)

Stationary source emissions — Determination of mass concentration of nitrogen oxides — Standard reference method: chemiluminescence

Step-by-step procedure

1. Planning

   Plan the measurement in accordance with EN 15259; establish the expected share of NO₂ in the NOx and the content of NH₃, CO₂ and H₂O, in order to choose the configuration of the conditioning (where NO₂ > 20 mg/m³ or NO₂/NOx > 25 % do not use a cooler without evidence of low losses) and the range of the analyser.

2. Assembly of the system

   Install the probe, the heated filter and the heated line of materials not reacting with NOx, the conditioning system and the analyser; heat the parts above the dew point; check the leak-tightness.

3. Adjustment and checks

   Feed zero gas and span gas (about 80 % of the range) directly to the analyser and through the probe (the test gas port), check the effectiveness of the NO₂→NO converter, the response time and the absence of leaks; assess the influence of CO₂/H₂O and apply a correction if necessary.

4. Measurement

   Record NO and NOx continuously (or alternately) over the averaging period (e.g. 30 min); check the dew point at the outlet of the conditioning (≤ 4 °C), the temperature of the line and the flow; record the O₂ and the humidity for the conversions.

5. Drift check

   After the end of the measurement period check the zero and the span again; compare the drift with the criteria of the standard.

6. Calculations

   Convert the volume concentrations to mg/m³ NO₂, to dry gas under standard conditions and to the reference oxygen content; with a cooler correct for the residual humidity (the table from EN 14790); calculate the NO₂ from the difference NOx − NO; draw up an uncertainty budget (k = 2).

Required equipment and apparatus

Health and safety (OHS)

• The ozone generated in the analyser is toxic — check the leak-tightness and the discharge of the outlet gas through an ozone destructor
• Cylinders with test gases (NO in N₂) — NO is poisonous; the cylinders secured, the room ventilated
• The heated parts of the system and work on the stack platform — heat-resistant gloves, protection against falling

Frequently asked questions