Legal Position on the Standard Oxygen Content for IC Engines When Setting Emission Limits

The third paragraph of clause 1.3 of the Emission Limit Values for Pollutants from Stationary Sources, approved by Order of the Ministry of Environmental Protection of Ukraine No. 309 of 27 June 2006, sets the standard oxygen content to which concentrations in gaseous combustion products are converted: 3 % for liquid and gaseous fuels, 6 % for solid fuels, and 15 % for gas turbines and diesel engines. Spark-ignition internal combustion engines running on petrol and natural gas are not named in the list.

In domestic practice, the absence in the third paragraph of clause 1.3 of Order No. 309 of a separate value for spark-ignition engines is sometimes taken as a settled position — that such engines fall into the “liquid and gaseous fuels” category with a standard oxygen content of 3 %. Given the widespread use of decentralised power sources — generator sets and cogeneration units with such engines — this question has acquired practical significance when setting emission limit values and assessing compliance with them.

Article 7 of the Law of Ukraine “On Protection of Atmospheric Air” and clause 1 of the Procedure approved by Resolution of the Cabinet of Ministers of Ukraine No. 1780 of 28 December 2001 define emission limit values as those set for dust-gas-air mixtures discharged from individual types of equipment and facilities.

Clause 2 of the Procedure for Developing and Approving Emission Limit Values for Pollutants from Stationary Sources, approved by Resolution of the Cabinet of Ministers of Ukraine No. 1780 of 28 December 2001, provides that emission limit values are determined taking into account the requirements of national legislation and the legislation of the European Union.

Letter of the Ministry of Environmental Protection No. 1123/11/10-09 of 11 February 2009 explains that, until technological emission standards are developed for process equipment and plant (installations), the measured emission concentration should be converted to the oxygen content provided for by European Union legislation (EU Directives, protocols, best available techniques reference documents, etc.), and only where no such legislation exists — in accordance with the third paragraph of clause 1.3 of Order No. 309. No technological emission standards for internal combustion engines have been approved in Ukraine.

European Union legislation, in all the forms named in that letter, contains a single standard oxygen content value for internal combustion engines — for all fuel types without exception:

1) The Gothenburg Protocol (1999) to the UNECE Convention on Long-range Transboundary Air Pollution. Scope: stationary engines — gas and dual-fuel engines with a rated thermal input above 1 MW, diesel engines above 5 MW. Annex V to the Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (Gothenburg, 1999, as amended in 2012, in force since 7 October 2019) to the Convention on Long-range Transboundary Air Pollution, to which Ukraine is a Party pursuant to Decree of the Presidium of the Verkhovna Rada of the Ukrainian SSR No. 231-X of 13 May 1980, sets nitrogen oxide emission limit values for new stationary engines — gas, dual-fuel and diesel — in the range of 95–225 mg/Nm³ at a standard oxygen content of 15 %.

Note. In the original 1999 version, Annex V set limit values of 250–600 mg/Nm³ for stationary engines at a standard oxygen content of 5 %. The 2012 amendments changed the reference oxygen content to 15 % without changing the stringency of the requirements: 500 mg/Nm³ at 5 % O₂ corresponds to 190 mg/Nm³ at 15 % O₂. The 2012 version is the one in force.

2) BAT conclusions for large combustion plants (Decision (EU) 2021/2326). Scope: combustion plants with a rated thermal input of 50 MW or more. The best available techniques (BAT) conclusions for large combustion plants (activity 1.1 of Annex I to Directive 2010/75/EU), established by Commission Implementing Decision (EU) 2021/2326, define in their general considerations the reference conditions for BAT-associated emission levels: dry gas, 273.15 K, 101.3 kPa; reference oxygen level — 3 % for combustion of liquid and/or gaseous fuels when not taking place in a gas turbine or an engine, and 15 % for combustion of liquid and/or gaseous fuels when taking place in a gas turbine or an engine.

3) Directive 2010/75/EU on industrial emissions (IED). Scope: combustion plants with a rated thermal input of 50 MW or more. Point 1 of Parts 1 and 2 of Annex V to Directive 2010/75/EU on industrial emissions provides that all emission limit values shall be calculated at a temperature of 273.15 K, a pressure of 101.3 kPa and after correction for the water vapour content of the waste gases and at a standardised O₂ content of 6 % for solid fuels, 3 % for combustion plants, other than gas turbines and gas engines, using liquid and gaseous fuels, and 15 % for gas turbines and gas engines.

Under Article 3(34) of Directive 2010/75/EU, “gas engine” means an internal combustion engine which operates according to the Otto cycle and uses spark ignition or, in the case of dual-fuel engines, compression ignition to burn fuel. Point 24 of the same Article defines fuel as any solid, liquid or gaseous combustible material. The notion of a “gas engine” is therefore a matter of engine design and combustion principle, not of the fuel type.

4) Directive (EU) 2015/2193 on medium combustion plants (MCPD). Scope: combustion plants with a rated thermal input equal to or greater than 1 MW and less than 50 MW. Annex II to Directive (EU) 2015/2193 on the limitation of emissions of certain pollutants into the air from medium combustion plants provides that emission limit values are defined at a temperature of 273.15 K, a pressure of 101.3 kPa and after correction for the water vapour content of the waste gases and at a standardised O₂ content of 6 % for plants using solid fuels, 3 % for plants, other than engines and gas turbines, using liquid and gaseous fuels, and 15 % for engines and gas turbines.

Directive (EU) 2015/2193 applies to combustion plants with a rated thermal input equal to or greater than 1 MW and less than 50 MW (Article 2(1)). The 5 MW threshold and the dates of 1 January 2025 and 1 January 2030 given in the Directive relate solely to the deadlines for bringing existing plants into compliance (Article 6(2) and (3)) and do not alter its scope. Table 3 of Part 1 and Table 2 of Part 2 of Annex II, which contain the emission limit values for engines and gas turbines, make no distinction by rated thermal input.

5) Commission Regulations (EU) No 813/2013 and No 814/2013 (Ecodesign). Scope: space heaters and water heaters (including cogeneration heaters and heat pumps with an internal combustion engine) with a rated heat output of up to 400 kW, with no limit on electrical capacity; the draft of the new regulation extends the scope to 1 MW. For installations whose capacity is below the scope of Directive (EU) 2015/2193, emissions from internal combustion engines are regulated by Commission Regulations (EU) No 813/2013 (Annex II, point 5) and (EU) No 814/2013 (Annex II), which set, for appliances with an internal combustion engine with a rated heat output of up to 400 kW and with no limit on electrical capacity, identical nitrogen oxide emission limit values — 240 mg/kWh of fuel input in terms of gross calorific value for gaseous fuels and 420 mg/kWh for liquid fuels, irrespective of the type of appliance and its capacity.

Converted to a concentration in dry flue gas under normal conditions using the specific volume of combustion products at stoichiometry (0.771 Nm³/kWh for natural gas, 0.822 Nm³/kWh for petrol; 0.828 for gas oil), these values amount to 89 and 146 mg/Nm³ at a standard oxygen content of 15 %. These values are consistent with the limit values set by Annex II to Directive (EU) 2015/2193 for new engines at 15 % O₂ — 95 mg/Nm³ for natural gas and 190 mg/Nm³ for liquid fuels.

The regulation prepared by the Commission to replace Regulation (EU) No 813/2013 extends the scope to appliances with a rated heat output of up to 1 MW, expressly excluding installations covered by Directives 2010/75/EU and (EU) 2015/2193.

In addition, point 4 of the national Technological Standards for Permissible Emissions of Pollutants from Thermal Power Installations, approved by Order of the Ministry of Environmental Protection No. 541 of 22 October 2008 (as amended by Order of the Ministry of Ecology No. 62 of 16 February 2018), defines the standard oxygen content to which the limit mass concentration of a pollutant is converted: 6 % for combustion of solid fuel, 3 % for combustion of gaseous and liquid fuel, and 15 % for combustion of gaseous and liquid fuel in gas turbines and stationary engines. The category “stationary engines” is defined without any limitation as to fuel type.

The European and domestic practice set out above, together with the physics of the process, leads to the conclusion that the standard oxygen content is a characteristic of the type of installation rather than of the fuel type — and on this reading the third paragraph of clause 1.3 of Order No. 309 acquires a coherent meaning. Accordingly, the 15 % standard oxygen content set by the third paragraph of clause 1.3 of Order No. 309 for gas turbines and diesel engines extends to all stationary internal combustion engines, including those running on any liquid or gaseous fuel, across the entire range of capacities covered by Order No. 309. Neither national legislation nor the legislation of the European Union sets any other standard oxygen content for internal combustion engines.