The operator shall use the methods for monitoring flue gas flow set out in Article 43(5) of this Regulation for measuring the flue gas flow for N₂O emissions monitoring. For nitric acid production, the operator shall apply the method in accordance with point (a) of Article 43(5) unless it is technically not feasible. In that case and upon approval by the competent authority, the operator shall apply an alternative method, including by a mass balance methodology based on significant parameters such as ammonia input load, or determination of flow by continuous emissions flow measurement.

The flue gas flow shall be calculated in accordance with the following formula:

V_{flue gas flow} [Nm³/h] = V_air * (1 – O_2,air)/(1 – O_{2,flue gas})

Where:

V_air

Total input air flow in Nm³/h at standard conditions;

O_2,air

Volume fraction of O₂ in dry air [= 0,2095];

O_{2,flue gas}

Volume fraction of O₂ in the flue gas.

The V_air shall be calculated as the sum of all air flows entering the nitric acid production unit.

The operator shall apply the following formula, unless stated otherwise in its monitoring plan:

V_air = V_prim + V_sec + V_seal

Where:

V_prim

Primary input air flow in Nm³/h at standard conditions;

V_sec

Secondary input air flow in Nm³/h at standard conditions;

V_seal

Seal input air flow in Nm³/h at standard conditions.

The operator shall determine V_prim by continuous flow measurement before the mixing with ammonia takes place. The operator shall determine V_sec by continuous flow measurement, including where the measurement is before the heat recovery unit. For V_seal the operator shall consider the purged airflow within the nitric acid production process.

For input air streams accounting for cumulatively less than 2,5 % of the total air flow, the competent authority may accept estimation methods for the determination of that air flow rate proposed by the operator based on industry best practices.

The operator shall provide evidence through measurements under normal operating conditions that the flue gas flow measured is sufficiently homogeneous to allow for the proposed measurement method. Where non-homogeneous flow is confirmed through these measurements, the operator shall take that into account when determining appropriate monitoring methods and when calculating the uncertainty in the N₂O emissions.

The operator shall adjust all measurements to a dry gas basis and report them consistently.