NOx pollution is a high-profile public health issue. The European Environment Agency said in a 2016 report that the UK alone suffered nearly 12,000 premature deaths related to NO2 pollution in 2013, with more than 70,000 deaths across Europe in the same year.
While diesel vehicles are believed to be the chief culprits, other forms of combustion are also under growing pressure to reduce NOx emissions, including industrial boilers. The latest Europe-wide clampdown arrives in the form of the Medium Combustion Plant Directive (MCPD), which is set to be written into national law across member states (including the UK) in December 2017.
It regulates emissions of SO2 and dust in addition to NOx. It also lays down rules to monitor emissions of carbon monoxide. Covering combustion plant ranging from 1 MWth to 50 MWth, the MCPD plugs the regulatory gap at EU level between large combustion plants (>50 MWth), which are covered under the Industrial Emissions Directive (IED), and smaller appliances (heaters and boilers <1 MWth) covered by the Ecodesign Directive.
When it comes to low-NOx, it’s more about the burner than the boiler design, although the boiler design does play a part
Chris Coleman, boiler house specialist, Spirax Sarco
As well as pan-European measures, more localised (and more stringent) initiatives are also being deployed to tackle NOx hotspots.
In the UK, for instance, the government is expected to crack down on diesel vehicles across a number of cities by designating them as Clean Air Zones. In addition, observers suspect that the boiler plant emissions limits ultimately making it into British law may be far tighter than the MCPD requirements.
“With the MCPD it’s not clear yet what’s going to happen, but I believe that the UK will lower the figures to make the regulation even more stringent,” says Chris Coleman, boiler house specialist with steam system company Spirax Sarco.
So the details of local and national emissions limits may vary across Europe, but the overall pressure on boiler operators is only pushing them in one direction, and that’s towards low-NOx solutions.
In basic terms, NOx is produced at high peak flame temperatures so the different strategies for minimising emissions all focus on maintaining flame temperatures at more moderate levels. This impacts on the design of boilers and the burners that fire inside them.
If boiler manufacturers don’t work on solutions to enable low-NOx on reverse-flame boilers, reverse-flame will become a thing of the past
Leigh Bryan, sales and marketing manager, Fulton
“When it comes to low-NOx, it’s more about the burner than the boiler design, although the boiler design does play a part,” says Coleman. “Naturally, the burner technology is fundamental to reducing NOx emissions and as such we work closely with all the leading burner companies to ensure that we optimise for all fuel types,” confirms Simon Tarr, head of industrial sales at Bosch Commercial and Industrial, which supplies a range of horizontal, three-pass boilers.
“However, with the burner representing just one component of a boiler system, there are a number of additional steps that can be taken to further reduce NOx levels.
“One such method is to ensure a large furnace area, which can reduce the furnace heat release rate. All of our boilers are of the horizontal, steel shell and tube type made with three-pass construction which helps optimise the furnace geometry to provide the most effective heat transfer. For our ultra-low emissions boilers this furnace area is increased further. These are available in both our steam and hot water models.”
Such three-pass horizontal boilers are the most prolific type of design for industrial applications, but there are also other common configurations.
For example, reverse-flame boilers use only two passes. Owing to the inherent chamber restrictions of their reverse chamber design, they are effectively right in the firing line for the low-NOx regulations.
“If the MCPD comes into force, many reverse flame boilers will not comply, which is fine for boilers already in the field for a while [thanks to the transition periods built into the directive] but we will not able to sell that new equipment. If boiler manufacturers don’t work on solutions to enable low-NOx on reverse-flame boilers, reverse-flame will become a thing of the past,” says Leigh Bryan, sales and marketing manager for Fulton [pictured above].
The company offers a variety of steam and hot water boiler configurations, including reverse-flame and three-pass horizontal systems, as well as a range of vertical boilers.
“We are working with burner manufacturers to develop solutions for reverse-flame boilers that use a system of flue gas recirculation to maintain the flame temperature. These may mean that we can upgrade existing systems, rather than replace them,” he adds.
Recirculating gases – either on a large scale or by creating small eddy currents within the flame – helps prevent problematic peak flame temperatures being reached.
The larger the furnace the lower the NOx, but you want to use as small a furnace as possible to keep the cost down. That’s where the art comes in
Sharon Kuligowski, managing director, Dunphy Combustion
However, Sharon Kuligowski, managing director with burner manufacturer Dunphy Combustion, cautions that it’s always going to be an uphill struggle to achieve low-NOx with a reverse-flame boiler, because the two-pass design makes it much harder to maintain eddy currents around the burner head: “If you’ve got a reverse-flame boiler you’ve only got one large chamber. Basically the flue gases come round on themselves and those eddy currents are virtually destroyed.”
Furthermore, she adds that the physical size of the furnace is also an important factor: “The larger the furnace the lower the NOx, but you want to use as small a furnace as possible to keep the cost down. That’s where the art comes in,” she adds. So installing a new burner may be enough to bring the performance of a three-pass boiler within the required emissions limits, but a reverse-flame boiler may need to be de-rated in order to pass muster.
Tarr believes that de-rating could prove to be a useful approach: “It’s not unusual to find large, previously-installed boilers that now supply smaller duties. This is particularly common where sites have reduced production or capacity, or perhaps become more efficient elsewhere in the plant. In this case it may be possible to de-rate the boiler and retro-fit a smaller burner.”
He also agrees that effective gas recirculation is a key tool for NOx prevention, but he adds that other approaches can help: “Other methods to consider, particularly in ultra-low NOx environments, include adding ancillary equipment such as economisers (flue gas heat exchangers to pre-heat feed water or return water), or a pressurised feed tank and deaerator to increase feed water temperature. Reducing flame temperature is key, so it’s important to consider the whole system and scope of supply rather than just one part.”
Horizontal boilers may be the most common industrial solutions for steam and hot water, but vertical boilers can also provide a space-saving alternative, according to Fulton, especially in the lower range of duties.
Last year the company developed a low-NOx version of its VSRT series of vertical steam boilers, with a mesh burner and furnace designed as a single component to deliver under 40 mg/ kWh of NOx.
The current models in the VSRT range are only rated to a modest 300 kW, which is suitable for applications such as microbreweries and some food and pharmaceutical installations.
However, Bryan says that Fulton is currently developing VSRT systems from 150 kW to 2,500 kW. By 2019 he predicts that the VSRT range will provide an effective alternative across the majority of the reverse-flame boiler market.
Meanwhile, could electric steam boilers provide a realistic option that completely negates any issues of NOx emission restrictions? The answer is yes.
“The vast majority of electric boiler sales are to universities and research facilities, pharmaceuticals and small food plants,” says Bryan, noting that electric boilers are always likely to be confined to smaller-scale installations, or sites where flueing or fuel supply restrictions may be an issue.
“The larger the electric boiler, the more onerous the electric supply requirements, at which point, clients need to consider sensible, conventional alternatives.”