The Right Mix for Clean Air

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Courtesy of Courtesy of Sulzer

Reducing air pollution is an increasingly important topic all around the world. Many governments—also in developing countries—set new limits for emissions such as nitrogen oxides in flue gas. Sulzer offers advanced mixing technology for flue gas cleaning. The installation in a large power plant shows that these high-performance mixers can cope with the most difficult requirements.

The flue gas generated in combustion processes in power plants and industry needs to be cleaned from harmful emissions such as nitrogen oxides (NOx), which are responsible for ozone formation. Selective catalytic reduction (SCR) is a method for reducing nitrogen oxides. By injecting ammonia (NH3) into the flue gas, NOx is converted into the environmentally neutral products nitrogen and water. This method is performed in so-called DeNOx systems and has a wide range of applications:

  • Thermal power plants fired by coal, oil, or gas
  • Refineries
  • Biofuel plants
  • Waste-to-energy plants

The location of the DeNOx system within the flue gas cleaning process depends on the type of fuel involved. In a tail-end configuration, the DeNOx system is at the end of the cleaning process. This allows a longer lifetime of the catalyst because the gas is already cleaner in this section. However, this configuration requires that the flue gas be reheated to reach the ideal temperature for the catalytic conversion.

Such a tail-end configuration can be found in many power plants like, for example, in the Grosskraftwerk Mannheim (GKM), Germany. GKM is one of Europe’s most efficient coal-fired power plants, with a total gross capacity of 1675 MW. Besides power and district heat generation, GKM is an important producer of single-phase power for the German railway. The plant consists of eight power station blocks. Two of the blocks use a common DeNOx plant with an SCR process in a tail-end configuration. Flue gas is reheated by natural gas surface burners to attain the necessary operating temperature (about 320°C) for the catalytic conversion. An injection system blows the ammonia/air mix into the flue gas flow. The operator faced some difficulties because, generally, only one boiler was in operation and the flow circumstances were not optimal. It was not possible to attain a satisfactory NOx concentration distribution.

Removing nitrogen oxides from flue gas

Selective catalytic reduction (SCR) technology requires the admixing of ammonia (NH3) to the flue gas before it enters the catalyst. Within the catalyst, the NOx reacts with the ammonia to nitrogen (N2) and water (H2O). The basic reactions of the SCR process are:

4 NO + 4 NH3 + O2 = 4 N2 + 6 H2O
2 NO2 + 4 NH3 + O2 = 3 N2 + 6 H2O

The dosing and mixing of the ammonia is a critical step within this process.

Demanding goals

GKM wanted to optimize the flue gas cleaning system. The goal was to reach a uniform temperature profile ahead of the catalytic converter and a uniform NOx concentration profile beneath the last catalytic convertor system. This would maximize the conversion of the ammonia and the NOx, in particular, in partial-load operation. Apart from that, GKM wanted to minimize the ammonia escape rate. Reducing the operating temperature of the flue gases and the ammonia would lower the inherent energy requirement (natural gas consumption).

The requirements were challenging because the available flow path was short. The system also needed to have the lowest possible pressure loss, cover a broad load range, and be reasonable in price.

All these optimizations required a suitable mixing system that doses ammonia correctly, mixes it uniformly with the flue gas, and ensures that no ammonia is emitted to the environment. Some suppliers only use finely spaced dosing grids with valves for the tuning of the flow through different parts of the grid. However, DeNOx systems with high conversion rates require better mixing quality and therefore need solutions with static mixers. GKM decided to retrofit with the newly developed Sulzer ContourTM static mixer. It is highly optimized for excellent mixing performance at very low pressure drops. It can be easily installed both in new and existing ducts.

Wing geometry for intense mixing

The Sulzer Contour mixer consists of two wings, which are joined in the center region of the duct. The two wings are positioned with a high angle of attack relative to the main flow in the duct but in opposite directions. The flows over the two wings are thereby deflected in opposite directions relative to the main flow, leaving a region with negative pressure in the center. Downstream of the Sulzer Contour wing pair mixers, strong large-scale vortices form quickly, which induce intense large-scale movements and turbulence for intense and highly effective mixing. The wing geometries can be optimized for low pressure drop and high mixing rate. It turned out that geometries that do not generate significant flow detachment are very efficient in mixing at low pressure drops. This explains why, with the Sulzer Contour mixers, it is possible to cause intense mixing with pressure drops below 1 mbar at typical flow conditions in flue gas ducts.

Often, large ducts are manufactured with rectangular cross-sections with large aspect ratios of up to between 3 and 6. In such ducts, an array of Contour mixers can be installed side by side to fill the cross-section. The arrangement of the mixers next to each other leaves an additional degree of freedom for the design of the mixers to be specified. It is possible not only to adapt the details of the wing geometries to tune the mixing performance, but also to change the rotation sense of the vortices produced.

The Sulzer Contour mixer is the only vortex-type mixer on the market that can generate vortices in corotational manner as well as in counterrotational manner. Corotational vortices feature strong intervortex mixing, which means that the homogenization across the longer side of the duct cross-section is possible. On the other hand, counterrotational vortices can produce slightly faster mixing within each of the vortices but there is hardly any exchange across the vortex boundaries. One of the unique features of the patented Sulzer Contour mixer designs is that the vortex pattern can be generated in an ideal manner for the duct at hand.

Sulzer carries out CFD simulations for the design and optimization of the mixer, the air injection grid, and the flow conditioning internals. The various possible configurations of the internals can be simulated at an early phase of the project and validated in physical model tests. Based on these results, Sulzer can offer tailor-made solutions to the customers.

Best choice for demanding applications

The Sulzer Contour static mixer met all of GKM’s requirements. After the revamp, the NOx distribution was much more uniform because of the Contour mixer. Much better temperature homogeneity was also achieved. Differences of up to 30°C from the mean temperature across the cross-section of the flue gas duct before had now been reduced to less than ±5°C.

The NOx concentration distribution after the mixer installation (right) is much better than before (left). The mean NOx concentration increased from 73 mg/m3 to 129 mg/m3.

The NOx concentration distribution after the mixer installation (right) is much better than before (left). The mean NOx concentration increased from 73 mg/m3 to 129 mg/m3.

As a result, the customer needed significantly less energy for heating. The equalization of the NOx and temperature profile made it possible to reduce the operating temperature of the DeNOx plant from 320°C to 290°C. This saving resulted in a payback time for the static mixers of just a few months.

This project was carried out in 2008 and was the first implementation of Sulzer’s Contour mixer. Since then, many successful installations have followed and have proved that the Contour mixer is the best choice for demanding conditions, such as:

  • SCRs in tight location with limited mixing distances
  • Need for good temperature homogenization
  • >90% NOx removal
  • Low pressure drop
  • Hard-to-measure or -predict inlet conditions in the field

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