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DEFRA introduces Medium Combustion Plant Directive (MCPD)

The Medium Combustion Plant Directive

EMISSION LIMIT VALUES REFERRED TO IN ARTICLE 6 All emission limit values set out in this Annex 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 medium combustion plants using solid fuels, 3 % for medium combustion plants, other than engines and gas turbines, using liquid and gaseous fuels and 15 % for engines and gas turbines.

With reference to the above section of Annex II of the MCPD, it is imperative that plants are accurately monitoring emissions and these emissions must be monitored at certain conditions, including specific oxygen levels.


Carefully monitoring oxygen and controlling the boiler/ O2 trim can effectively reduce your NOx emissions.

Gas Analysis Probe (GAP)






SST Sensing’s Gas Analysis Probe (GAP)


Air contains about 20.9% oxygen. It is the primary source of oxygen for combustion. The amount of oxygen remaining after combustion is an indication of how much air is present during the combustion. Too little air in the combustion prevents all the fuel from being burned; wasting fuel and creating a potential for explosion. Conversely, too much air (excess air) requires extra heat to maintain the combustion reaction.

The availability of the extra nitrogen found in excess air and the additional heat required to maintain combustion temperatures are favourable conditions for the formation of thermal NOx.

It is necessary to keep some excess air for safe and efficient boiler operation; fuel has surface area, and takes time to burn. Keeping the excess air volume near and slightly above stoichiometric values for the particular fuel minimizes the formation of NOx, and keeps fuel usage at a minimum.


It is a general rule of thumb that a 10% increase in excess air will increase fuel usage by 1%. The fuel usage is greatly increased when the boiler is operating in oxygen deficient conditions. Too much excess air has implications on boiler fuel usage and safety, but also on emissions, specifically related to the creation of NOx.


Variations in the burning process require minor changes to the fuel / air mixture to maintain the correct ratio and efficient boiler operation. The process of adjusting the fuel or air is called boiler/O2 trim. It is critical, for safety and efficiency, to know the amount of oxygen in the process.


Common types of extraction O2 measurement are electrochemical cell and paramagnetic, but given the constraints of ash, lag time, and moisture content in the flue gas, these types of measurements can be problematic when used for continuous boiler process control.


SST Sensing offer Zirconia O2 measurement devices that continuously measure oxygen content near the combustion zone which allows the measurement to be taken directly into the flue gas without extraction.

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