The Medium Combustion Plant Directive – How EOGB & Baltur can help you comply with increased legislation

Directive 2015/2193 provides guidance and limitations on the emissions of pollutants such as Sulphur dioxide (SO2), oxides of nitrogen (NOx) and particulate matter into the air from the operation of medium combustion plant (between 1MW and 50MW).

The directive became UK Law on the 19th of December 2017. 

The directive bridges the gap between the Industrial Emissions Directive 2010/75/EU (the IED) and the “Eco Design” Directive 2009/125/EC.   

The Emission limits stated within the document vary according to the type of plant and the fuel being used with the overall aim being the reduction of air bound pollutant substances entering the atmosphere from the combustion process.   

As the demand for more stringent control of harmful emissions increases, appliance manufacturers are being challenged to both develop and bring existing combustion equipment into compliance with increased legislation in the most cost effective way possible for their clients.  

It is a fact that every facility has a unique mix of equipment and regulatory requirements. 

Therefore, it is fair to say the most cost-effective NOx/SOX control technology, or combination of technologies, will be different for each application.  

In addition to complying with the NOx/SOX emissions requirements, operating companies must also be able to install the equipment to meet compliance deadlines, which can affect productivity and place a financial strain on the companies themselves.  

One relatively inexpensive method of reducing NOX levels is to recirculate a specified percentage of the stack gases back into the combustion air duct prior to mixing with the fuel. This technique is known as flue gas recirculation (FGR). 

Flue gas recirculation (FGR) is by no means a new technology but it has been proven to be a highly effective technique for lowering NOx emissions from burners in lower temperature applications.   

By introducing a calculated quantity of the relatively inert flue gases into the fresh combustion air supply prior to mixing with the fuel, the combustion air becomes oxygen deficient or vitiated. 

By reducing the oxygen (O2) content in the fuel air mixture one of the main constituents in NOX is diminished thus reducing the propensity for NOX formation. 

Due to the fact that the fuel/air mixture has been further removed from its stoichiometric equivalence ratio it stands to reason that the flame temperature will be reduced by a certain amount.  

The resultant reduction in flame temperature leaves the designer/operator with the dilemma that the burner will encounter a potentially significant loss in power, which must be calculated for.  

This could potentially manifest itself in the form of a larger burner/appliance being required to achieve the desired heat input. 

It is a fact that the formation of thermal NOX increases exponentially as the flame temperature increases. (See extended Zeldovitch mechanism). 

The extended Zeldovitch mechanism identifies three factors to be considered when predicting the formation of thermal NOX 

• Fuel/air ratio 
• Residence time at high temperature 
• Time 

The level of flame cooling will depend upon the percentage reduction in O2 content within the fuel/air mixture and can be quantified via the use of thermo-dynamic principals. 

The above mentioned thermo dynamic principals also identify the potential for the impairment of burner stability. The use of FGR reduces flame temperature by altering the fuel/air mixture.  

The fuel/air mixture is not only a key factor in the formation of thermal NOX but is also one of the three factors that affect’s burning velocity, which is critical in ensuring the aforementioned stability. 

• Fuel/air ratio 
• Temperature 
• Pressure 

Effectively the rate of reaction or burning velocity of the mixture is reduced. It is a fact that this all adds up to the reduction in both flame temperature and thermal NOX formation but it does allow for the formation of other unburned hydrocarbon species (UHC’s) and consideration must be given to this fact at the design stage. 

It is key to involve your burner supplier from the beginning of the design stage of the FGR and control system. The system must allow safe operation throughout the full turndown range of the plant under all circumstances. It may be necessary to further insulate the flue components and consideration will have to be made to ensure the safety of all operating personnel. 

Early research into FGR identified that by, recirculating up to 25% of the flue gases through the burner, thermal NOx formation could potentially be reduced to as little as 25% of their normal levels.  

Unfortunately FGR is not conducive to all processes.  

FGR relies upon comparatively low stack temperatures approximately 3000C and low O2 (<5%) content within them to achieve thermal NOX reductions, so it stands to reason that it is not necessarily conducive to higher temperature applications.  

 Contact our Technical Team now by sending an email to technical@eogb.co.uk or by calling 01480 477066 to see how we can match your requirements.