Lax nitrogen oxide limits for gas

The market forecasts for energy distribution in the future and the environmental protection efforts of the EU can be reduced to a simple formula: increased demand versus decreased emissions. This supposed contradiction quickly reveals questions about technological feasibility – especially since fossil resources will continue to play an important role in the global energy mix in the coming decades, despite all the progress made in renewables.

The International Energy Agency (IEA) forecasts in its World Energy Outlook 2018 that global energy demand will increase by more than a quarter by 2040. Industrial use of natural gas in particular is expected to grow by around 45 percent by then. China's demand alone will triple by 2040.

Germany's decision to phase out coal by 2038, with its gradual reduction of coal-fired thermal power plants, fits into this situation. Renewable energies will partly compensate for this, but in the event of weather-dependent supply gaps, there may be a need for gas-fired thermal power plants of 100 GW in Germany. With a status quo of 30 GW, the challenge is to minimize this discrepancy through advanced technology.

Lax nitrogen oxide limits for gas

The ultra-low NOx burner Atonox G, installed in a new gas-fired power plant in the Beijing area. In total, Saacke has so far installed a total volume of almost 1.300 MW of firing capacity in the Middle Kingdom. Figure. Saacke GmbH

Reducing emissions

This is where the EU comes in with the Industrial Emissions Directive. Since 2013, conclusions on best available techniques (BAT). The associated emission bandwidths are given a binding status. Licensing authorities must ensure within four years of publication of these conclusions that existing plants also comply with the associated emission levels.

This also applies to the conclusions of the 2017 Best Available Technology Reference (BREF) document on large combustion plants. By 2021, existing plants must also comply with values stated in the conclusion. These require, among other things, a significant reduction in NOx emissions for large combustion plants with a thermal capacity greater than or equal to 50 MW. Smaller plants may also be affected: Because the directive recognizes an "aggregation rule". Accordingly, the output of several combustion plants greater than or equal to 15 MW is added together, provided that their exhaust gases could be discharged via a common chimney.

Lax nitrogen oxide limits for gas

NOX emissions to be complied with according to the EU's BAT reference document compared with the current requirements of the 13. Federal Immission Control Ordinance (13. Bimschv) * Depends on the operating parameters of the heat generator ** 100-150 mg/Nm³ for 100-300 MW; 200 mg /Nm³ for 300 MW Table: Saacke GmbH

The German government has yet to implement the requirements of the conclusion into German law. The Federal Environment Agency (UBA) is preparing to do this. The specifications, including limits, are set out in the Large Combustion Plant Ordinance, the 13. Federal Immission Control Ordinance (BImSchV), incorporated.

It is still unclear, for example, what the limit values for NOx emissions will be. However, the BAT conclusions provide helpful guidance (s. Table). According to this, a reduction in permissible NOx emissions will primarily affect gas-fired existing and new plants. This is a circumstance that should not be underestimated, given the projected increase in global gas demand. Operators who have also switched from heavy and fuel oil boilers to natural gas for environmentally conscious reasons will face considerable challenges in this regard: Due to the sometimes very high combustion chamber heat load of existing systems, compliance with the statutory limit values is already proving difficult today.

Lax nitrogen oxide limits for gas

Material flows in an Atonox burner: yellow arrows = gaseous fuel blue arrows = combustion air; gray arrows = internal flue gas circulation. Image: Saacke GmbH

China is stricter

In general, there are two approaches to reduce NOX emissions: primary measures based on improved combustion (low NOX technologies), and secondary measures downstream of combustion. The latter go hand in hand with high investment costs and additional operating costs, so that many plant operators prefer less expensive low-NOX firing systems.

Although existing plants with low NOx technology can comply with the current limits with sufficient certainty, the new BAT values underline that the requirements will increase. A look at the bigger picture also shows this: although the BAT specifications may seem ambitious, stricter requirements are already in force in some places: China, for example, already requires NOx emissions below 30 mg/m³ for gas-fired new and existing plants in the Beijing area. The low-NOx technologies currently available on the market are reaching their limits in these areas.

Old technical tricks..

Typical primary measures are to increase exhaust gas recirculation or to operate with excess air. However, this leads to rising operating costs due to the necessary blowers, to efficiency losses due to flue gas losses and, in some cases, to unstable operation of the firing system. Only with the most advanced technology and a coordinated combination of various measures can low NOx emissions be reliably maintained, as in China's metropolitan areas.

Lax nitrogen oxide limits for gas

At packaging specialist Kunert Wellpappe in Biebesheim, Hesse, Saacke installed a Teminox GL ultra-low NOx natural gas burner (6 MW) in 2019. Nox emissions have since been below 30 mg/m3. The NOx emissions saved are roughly equivalent to those of 1.000 diesel passenger cars. Image: Saacke GmbH

For more than 80 years, Saacke GmbH has been continuously developing firing technologies in order to always be able to comply with the increasing requirements due to decreasing emission limits. Many Saacke combustion systems are in use worldwide. One example: At Beijing New International Airport, Saacke Atonox G series burners are installed on five boilers, each with a capacity of 62.5 MW. The ultra-low NOx technology has already proven its worth here, as in other projects in China. With a total volume of more than 1.200 MW of installed firing capacity falls below the NOx limit value of less than 30 mg/m³. In Germany, companies in the food and chemical industries are relying on these burners in order to be able to continue to produce in compliance with BAT in the future.

… And new tricks

The ultra-low NOx burner series Atonox G contains burners which, through staged combustion, meet the emission limits specified in the 13. This means that the maximum NOx emissions of 100 mg/m³ required by the Federal Immission Control Act (BImSchV) can be safely undercut. The improved performance of this series is achieved by intensifying the free-jet back suction of the exhaust gases from the combustion chamber back to the flame root. Similar to an external exhaust gas recirculation, the oxygen partial pressure in the flame as well as the combustion temperature is reduced, so that the NOx formation is additionally inhibited. However, because the exhaust gases are drawn internally from the combustion chamber, no or almost no external exhaust gas recirculation is necessary to keep NOx emissions well below the NOx limits still in force under the 13. Bimschv to be pressed. Under very demanding boundary conditions, such as those encountered in China, even the most stringent NOx limits can be undercut with additional external exhaust gas recirculation. Even heat generators with up to 60 percent higher combustion chamber heat loads (compared to conventional heat generators) can be operated below the legal limits of 30 mg/m³ based on sophisticated flue gas recirculation.

EU: limits too lax

Practice shows that the European legislator is on the right track with its BAT requirements, but has not exhausted the available technological possibilities. In the spirit of competition for the literal best available technologies for the benefit of the environment, policymakers should develop more confidence in the innovative power of engineers.

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