🌿 Ammonia (NH₃) in waste gases — manual absorption method

Environment PN-EN ISO 21877

Overview

The emissions of ammonia come to a large extent from agriculture, from chemical processes (e.g. the manufacture of fertilisers), from power stations, cement works and waste incineration plants with SCR and non-SCR reactors (ammonia slip). ISO 21877:2019 specifies a manual measurement method — sampling and various analytical methods — for determining the mass concentration of ammonia in the waste gases of industrial plants, e.g. combustion plants or agricultural installations. The method determines all the compounds volatile at the temperature of sampling which during the absorption dissociate with the formation of ammonium ions — that is the volatile ammonia content of the gas. It is an independent measurement method (called in EN 14181 the standard reference method), validated in field tests in the range of about 8–65 mg/m³ under standard conditions; the lower limit was set by the operating conditions of the test plant, and the method may be applied at lower concentrations depending on the time and the volume of sampling and on the limit of detection of the analysis. It serves for periodic monitoring and for the calibration and the validation of fixed automated measuring systems for ammonia (in conjunction with ISO 17179). In Poland the standard is in force as PN-EN ISO 21877:2020-03.

Method principle

A representative sample is taken from the gas stream over a specified time at a specified flow; where there are droplets in the gas, isokinetic sampling is necessary. The probe is heated to a temperature ensuring the evaporation of the droplets and the absence of condensation of water vapour; the particles are retained by a specified filter — in-stack filtration (preferred with non-isokinetic sampling, because it does not require separate heating) or out-stack filtration, in which case the filter is heated to a set temperature. The ammonia passing through the filter is collected in an absorption system acidified with H₂SO₄ (a 0.05 M solution; the concentration may be increased at high concentrations of NH₃), and the mass of NH₄⁺ is determined after sampling by one of the analytical methods of clause 9 (spectrometry in accordance with ISO 7150-1, flow analysis CFA/FIA in accordance with ISO 11732, ion chromatography in accordance with ISO 14911). The volume of the gas is measured with a gas meter; the mass concentration is the quotient of the mass of ammonia in the solution (mg) and the volume of the gas (m³), with the conversion factor NH₄⁺→NH₃ fN = 0.944. The sampling system: a heated probe with an inlet of titanium, quartz, borosilicate glass or PTFE; a flat quartz filter; two gas-washing bottles with a sintered disc (porosity D1 or finer) at low flows, or impingers at high flows; a pump, a flow controller, a gas meter with a thermometer and a manometer, optionally a drying tower; all the components resistant to corrosion; the line to the gas-washing bottles heated to at least 15 K above the dew point of the gas, with the residence time minimised. In the presence of semi-volatile ammonium salts the temperature of sampling affects the gas/solid equilibrium; the analysis of the dust (total ammonia) is not covered by the standard.

Applications

• Measurement of ammonia slip downstream of SCR/SNCR plants in power stations, cement works and waste incineration plants
• Emissions of ammonia from chemical works (the manufacture of fertilisers) and agricultural installations
• Calibration and validation of fixed automated measuring systems for ammonia (ISO 17179, EN 14181)
• Periodic measurements for integrated permits and emission reporting
• Parallel sampling with other components in the side lines of an isokinetic system

Key parameters

Standard

Standard number

PN-EN ISO 21877:2020-03 (ISO 21877:2019)

Title (PL)

Emisja ze źródeł stacjonarnych — Oznaczanie stężenia masowego amoniaku — Metoda manualna

Title (EN)

Stationary source emissions — Determination of the mass concentration of ammonia — Manual method

Step-by-step procedure

1. Planning

   Establish the measurement site and the measurement points (EN 15259), the presence of droplets (→ isokinetic sampling), the expected concentration of NH₃ and the sampling time such that the mass of NH₄⁺ exceeds the limit of quantification of the analysis.

2. Assembly of the system

   Assemble the heated probe, the filter (in-stack or heated), two gas-washing bottles with 0.05 M H₂SO₄, a safety bottle, a drying tower (with a dry gas meter), the pump, the flow meter and the gas meter; heat the line to the gas-washing bottles to at least 15 K above the dew point; check the leak-tightness.

3. Sampling

   Sample over a specified time at a specified flow (with isokineticity — with a side line of a constant ratio of the flows, or without one); record the volume, the temperature and the pressure at the gas meter. Prepare a field blank.

4. Recovery of the samples

   Pour the solutions from both gas-washing bottles separately into glass, PP or PE bottles, rinse the components downstream of the filter with demineralised water and add the rinsings; assess the effectiveness of the absorption from the share of the second gas-washing bottle.

5. Analysis

   Determine NH₄⁺ by the chosen method (spectrometry, CFA/FIA or ion chromatography) against a calibration curve, taking the dilution and the blanks into account.

6. Calculations

   Calculate the mass of NH₃ (fN = 0.944), the volume of gas under standard conditions, the mass concentration in mg/m³ for dry gas and after the correction for reference O₂; draw up an uncertainty budget (≤ 20 % of the reference value).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sulfuric acid solution — protection of the eyes and the hands; the solutions after sampling contain ammonium salts and may contain acid components of the flue gas
• The heated probe and the filter are liable to cause burns; work on a stack platform with protection against falling
• Ammonia in the gases and at leaking ports irritates the respiratory tract — protection of the respiratory tract when opening the ports

Frequently asked questions