With new legislation coming into force to further reduce emissions, Steve O’Neill, Technical Specifications Engineer at burner solutions provider EOGB Energy Products Ltd, looks at the small changes that can be made stay within the limits…

Directive 2015/2193 will become UK law in December 2017.

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

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 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.

Updating combustion equipment

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.

Every facility has a unique mix of equipment and regulatory requirements. Therefore, the most cost-effective NOx/SO_X control technology, or combination of technologies, will be different for each application.

In addition to complying with the NOx/SO_X 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.

Flue gas re-circulation

One relatively inexpensive method of reducing NO_X 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 NO_X is diminished thus reducing the propensity for NO_X 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.

Zeldovitch mechanism

It is a fact that the formation of thermal NO_X increases exponentially as the flame temperature increases. The quantity of thermal NOx produced can be predicted by a calculation known as the extended Zeldovitch mechanism.

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

• 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 thermodynamic principals. The thermodynamic 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 NO_X but is also one of the three factors that affect burning velocity, which is critical in ensuring the aforementioned stability. The other critical factors are temperature and pressure.

Effectively the rate of reaction or burning velocity of the mixture is reduced. This all adds up to the reduction in both flame temperature and thermal NO_X 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.

Finding the best solution

FGR can be the perfect solution for operators of lower temperature applications in achieving lower emission levels. In fact, 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 its normal levels.

Before adopting the FGR technique, it’s important to involve your burner supplier right from the beginning of the design stage. 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.

EOGB offers a free no-obligation ‘health check’ service to give companies an honest evaluation of how their current heating system is performing and illustrate what savings can be made with some very minor adjustments.