Why CO and NOx are addressed for a boiler plant
A gas boiler is often perceived by the operator as a "clean" source. Compared with a coal or heavy fuel oil boiler that is usually true, but it does not mean that no obligations apply to the boiler plant. When burning natural gas, nitrogen oxides, designated as NOx, arise primarily, and carbon monoxide, i.e. CO, also arises during imperfect combustion.
NOx are related especially to combustion temperature, burner design, excess air, and boiler tuning. CO is a typical indicator of combustion quality. Elevated CO often signals a problem with the combustion process, poor tuning, insufficient combustion air supply, fouling of the heat exchanger, or another operational problem.
For a gas boiler plant, dust and sulphur are usually not addressed to the same extent as for solid or liquid fuels. The main subject of measurement is usually NOx and CO.
What matters is input, not just output
For classification of a boiler under the Air Protection Act, rated thermal input is essential, not only thermal output stated in common technical materials. Output states how much heat the boiler delivers to the system. Input expresses the energy supplied in the fuel.
If the manufacturer states only output and efficiency, input is determined approximately as the ratio of output to efficiency.
| Data | What it means | Why it matters |
|---|---|---|
| thermal output | heat usable for heating or technology | commonly stated in projects and data sheets |
| thermal input | energy supplied in fuel | determines source classification under air protection |
| efficiency | ratio between output and input | enables calculation of input if not stated directly |
| total boiler plant input | sum of relevant boilers according to rules of the law | determines source regime and obligations |
In practice: a boiler with an output of 280 kW may not be below the 300 kW input threshold. If, for example, it has 93% efficiency, its input is approximately 301 kW. That may already be decisive for source classification.
Small source and listed source
In common speech the term "small source" is still used. Current legislation, however, works mainly with the division into sources listed in Annex 2 to the Air Protection Act and sources not listed in that annex.
For boilers the key threshold is 0.3 MW total rated thermal input. Boilers from this threshold fall among listed stationary sources. For boilers above 5 MW the regime is further tightened, for example regarding requirements for operating rules and scope of documentation.
| Boiler plant type | Typical regime |
|---|---|
| boiler up to 0.3 MW | generally not a listed source under Annex 2, but correct operation and technical condition still apply |
| boiler plant from 0.3 MW to 5 MW inclusive | listed stationary source, typically code 1.1. for boilers |
| boiler plant above 5 MW | listed source with stricter regime and usually broader documentation |
| several boilers in one facility | sum of inputs may be assessed according to aggregation rules |
For several boilers in one boiler plant it is necessary to assess whether their inputs are aggregated. It is therefore not enough to assess each boiler in isolation. Common location, common chimney, or technical arrangement of sources may be decisive.
Fuel: why natural gas is simpler than solid fuels
Fuel type significantly affects the scope of obligations and expected pollutants. For natural gas, mainly NOx and CO are typically measured. For solid fuels, particulate matter and sulphur dioxide are additionally addressed. For liquid fuels the situation may differ according to fuel type and sulphur content.
| Fuel | Typically monitored substances | Practical note |
|---|---|---|
| natural gas | NOx, CO | common for gas boiler plants |
| LPG or other gaseous fuel | NOx, CO, possibly others according to fuel | specific fuel must be verified |
| light fuel oil / gas oil | NOx, CO, possibly SO2 | depends on fuel and source regime |
| solid fuels | NOx, CO, TSP, SO2 | usually broader scope of measurement and records |
| biomass | NOx, CO, TSP | depends on boiler type and fuel quality |
For a gas boiler, measurement may therefore be simpler than for a solid-fuel boiler, but it must still correspond to the legal regime of the source, emission limits, and operating permit conditions.
When CO and NOx emissions are measured
CO and NOx emissions are measured mainly at listed combustion sources, typically boiler plants with total rated thermal input from 0.3 MW. The exact scope and measurement interval depend on input, fuel, operating permit, and any exceptions or special conditions.
For combustion sources, one-off measurement is also carried out after first putting the source into operation, after a fuel change stated in the permit, or after intervention in construction or equipment that could change emissions. This is important, for example, when replacing burners, reconstructing the boiler plant, or changing combustion technology.
| Situation | What is usually addressed |
|---|---|
| new gas boiler from 0.3 MW | first measurement after putting into operation and setting obligations |
| boiler plant 1 to 5 MW on gaseous fuel | periodic measurement typically once every 3 calendar years |
| gas source up to 1 MW | for some sources pollution level may be determined by calculation |
| change of fuel or burner | need to verify whether emissions have changed |
| requirement from regional authority or CEI | measurement according to permit conditions or inspection |
| boiler plant above 5 MW | usually stricter regime and more frequent or broader obligations |
For gas boilers, measurement usually monitors NOx concentration, CO concentration, oxygen content, flue gas temperature, pressure, humidity, or other quantities needed to recalculate results to reference conditions. The result is not only the value on the analyser, but a report with recalculation to set reference conditions.
Periodic measurement is not a service inspection
Emission measurement under the Air Protection Act is not the same as a routine boiler service inspection. A service technician may check combustion, efficiency, safety, burner settings, and operational condition of the equipment. Emission measurement for air protection purposes must, however, be carried out in a manner corresponding to legal and methodological requirements, and the result is an emission measurement report.
Service measurement may be useful for tuning the boiler. Measurement usable for official purposes must, however, be carried out by an authorised person according to the requirements of the Air Protection Act.
The difference is practical:
| Check | Purpose |
|---|---|
| boiler service inspection | safety, efficiency, burner settings, operability |
| authorised emission measurement | demonstration of compliance with emission limits and air protection obligations |
| operating records | ongoing documentation of source operation, fuel, hours, and measurement |
| operating permit | legal framework for operation of a listed source |
Operating records for a boiler plant
For listed sources it is important to keep operating records. These should document how the source was operated, which fuel was used, what the operating hours were, and how operating permit conditions were met. For smaller gas boiler plants the scope of records is usually simpler than for large industrial sources, but basic operating data should not be missing.
Operating records usually contain especially data on source identification, boilers, fuel, operating hours, chimneys or exhausts, measurements carried out, faults, maintenance, and any technology changes. For gas boiler plants up to 5 MW burning exclusively natural gas some requirements may be simpler, but the specific permit conditions and current legal regime of the source must always be respected.
Practically important: operating records are not just an archive of reports. They are ongoing proof that the boiler plant was operated in accordance with the permit and that the source did not exceed the set regime.
Operating permit and documentation
If the boiler plant is a listed stationary source, an operating permit issued by the regional authority usually comes into play. For natural gas boilers up to 5 MW documentation may be simpler than for solid fuels or large boiler plants, but classification, input, fuel, chimney, emission limits, and method of determining emissions must still be determined correctly.
For a new boiler plant, boiler replacement, increase in output, fuel change, or reconstruction of the flue gas path it is advisable to verify whether a new operating permit or change of existing permit is needed. A common problem is that a boiler is technically replaced in operation, but air protection documentation remains set for the original equipment.
Most common mistakes at gas boiler plants
At gas boiler plants a frequent mistake is confusing output with input. For source classification, however, input is decisive. Another common problem arises with several boilers when aggregation rules are not checked and the operator assesses each boiler separately even though the boiler plant as a whole may fall under the listed source regime.
Reliance on a service record instead of an emission measurement report is also common. Service inspection is important, but on its own it may not fulfil the obligation of one-off or periodic emission measurement under air protection. At older operations a problem is often that the operating permit does not match current boilers, fuel, or chimneys.
How to proceed when assessing a boiler plant
First it is advisable to establish the actual technical state: how many boilers are installed, what their rated thermal input is, what fuel they burn, whether they work into a common chimney, and whether it is a main or backup source. Then classification under Annex 2 to the Air Protection Act and conditions of any operating permit are verified.
If the boiler plant has a valid permit, it is necessary to check whether it matches reality. If the source has no permit but already falls among listed sources by input, the legal status of operation must be addressed. Only then does it make sense to determine precisely whether CO and NOx emissions are to be measured, at what interval, and at which chimney or measuring location.
Summary
For a gas boiler, CO and NOx emissions are not measured only according to the general impression that it is a "small" or "clean" source. What matters is rated thermal input, fuel, number of boilers, method of flue gas discharge, classification under the Air Protection Act, and operating permit conditions.
Boilers from 0.3 MW total rated thermal input usually fall among listed stationary sources. For gaseous fuels, measurement of NOx and CO, operating records, and demonstration of compliance with emission limits are most often addressed in practice. For smaller gas sources the regime may be simpler, but there too it is necessary to distinguish service inspection, operating records, and emission measurement for air protection purposes.
Send us boiler types, their rated thermal input or output and efficiency, fuel type, boiler plant schematic, chimney information, and any operating permit. We will verify source classification and propose whether CO and NOx emission measurement, operating records, change of operating permit, or further documentation is needed.
Factual basis of the article
The article is based mainly on the following regulations:
- Act No. 201/2012 Coll., on air protection,
- Decree No. 415/2012 Coll., on permissible pollution level and its determination.
Act No. 201/2012 Coll. distinguishes stationary sources listed in Annex 2 and sources not listed in the annex. Annex 2 includes combustion of fuels in boilers with total rated thermal input from 0.3 MW to 5 MW inclusive and boilers above 5 MW. Decree No. 415/2012 Coll. sets intervals of one-off emission measurement, specific emission limits for combustion stationary sources, and requirements for operating and summary operating records.