Hamworthy Heating Boiling Point Autumn - Winter 2019
AUTUMN / WINTER 19 6 Feature Legislation is not merely about energy efficiency anymore but has long seen a focus on air quality as well, specifically NO x emissions. For this reason, we look at what they are, why we should be concerned about them, what system is in place to reduce them and what we can do as a manufacturer. What are NO x emissions? NO x is a collective term that stands for nitrogen oxides which mainly consist of nitric oxide (NO) or nitrogen dioxide gas (NO 2 ). NO x emissions can produce ozone (O 3 ) in the presence of sunlight can have harmful effects on throat and lungs (including aggravation of asthma and emphysema) but also irritate eyes and nose and mouth. Nitrogen oxides themselves mostly affect the lungs, reducing immunity to lung infections. The effects are similar to those of ozone. What affects the production of NO x emissions in commercial gas boilers? There are different types of NO x formation in combustion (thermal, prompt and fuel) but for condensing gas boilers, thermal NO x is the most important one. For a fully premixed burner, NO x emissions are fundamentally correlated to the flame temperature and depend on the following factors; • Air dilution λ value Higher λ values lower the flame temperature and reduce NO x emissions. • Burner & gas air ratio controls An optimised air and fuel mix can improve NO x emissions. • Modulating characteristic Modulating boilers adjust their fire/gas use according to the heat load, making them more efficient and ‘clean’. For this reason, the prescribed method to assess NO x emissions according to BS EN15502 is a weighted average of the boiler operating at different modulations. Emission samples are taken across the entire modulation range when the boilers are working at lower or higher fire (i.e. spring, summer, autumn and winter heat loads) and then adjusted to reference conditions of 0% O 2 (dry air free). Not following this process means more favourable conditions can yield ‘better’ NO x results. Dry air figures ensure a level playing field to compare manufacturers’ NO x figures. If the boiler is under 70kW, the manufacturer’s product fiche should have a section detailing emissions, this would be in line with this methodology. This should be checked against published data as this could be quoting either wet NO x levels (not dry air free) or levels at minimum modulation. IT’S A HARD NO X LIFE All of the published data of Hamworthy products is defined in this way (0% excess oxygen, dry air). We must be careful when analysing different manufacturer’s products to ensure we are comparing apples for apples. By using the British Standard, we can ensure accurate comparisons are made. • Efficient heat transfer The more efficient the heat transfer away from the combustion chamber, the lower the flame temperature and NO x emissions can be. • Gas group Various gas types have different calorific values (CV) which indicate how much energy (in megajoules) they can produce per unit of volume (m³). Fuels with a higher CV create a higher intensity flame and hence higher NO x . These measures are all applicable to conventional premixed steel, metal fibre and ceramic burner types, in combination with a well-designed gas air ratio control system. This allows NO x emission levels low enough for the next generation of domestic and small commercial boilers. A trade-off between efficiency and NO x ? Taking our Upton modular boiler as an example, gas and air are premixed before entering the burner which ensures a fully mixed combustion gas. A metal fibre surface distributes the combustion gas evenly over the surface of the burner. This, in combination with a high efficiency condensing heat exchanger, helps to keep the combustion temperature low and inhibits the formation of thermal NO x . While this burner/heat exchanger design helps to cut NO x emissions up to a certain point, lowering the combustion temperature further can reduce the seasonal efficiency of the boiler. However, Ecodesign and CO 2 budgets have been the main driver to increase seasonal efficiency to reduce carbon emissions. As a result, there would be a trade-off in efficiency to achieve substantially lower NO x levels.