Company redesigns their SCR unit and decreases the amount of NOx emissions with at least 90%

A Chinese coal-fired power plant registered increasing NOx emissions. The power plant was already equipped with an SCR flue gas cleaning system (Selective Catalytic Reduction) claiming a NOx reduction efficiency of at least 90%.

To comply with the statutory requirements for NOx emissions they increased the amount of injected ammonia several times. As this is a very expensive solution, they decided to investigate the interior of the SCR unit. They were shocked to find the catalyst layer completely eroded and realised that they had to replace it.

The power plant replaced the catalyst several times and after failed attempts to solve the emission issue using three different subcontractors, FORCE Technology was asked to look into the matter.

Ammonia concentration right before the flow reaches the catalyst

The existing SCR unit
The existing SCR unit
Optimized unit
Optimized unit

From the above, it is clear that, the optimization performed by FORCE Technology has improved the homogeneity of the flue gas and ammonia mixture. Looking at the geometry also shows that the guide vane setup has been simplified in the optimized case, meaning that manufacturing and installation of the guide vanes is less expensive and increases efficiency of the catalysts.

The installed SCR unit did not match the original design

A thorough examination of the construction drawings revealed that additional components such as guide vanes had been added to the construction. Hence there was a mismatch between the original design of the SCR unit and the SCR unit actually installed. This was never discovered by the three previous subcontractors.

Based on the design actually installed, FORCE Technology created a CFD model (Computational Fluid Dynamics) that simulated the flow inside the SCR unit. The results were very close to what the power plant had detected visually themselves. Major parts of the catalyst had in fact eroded due to the high velocity areas of the flue gas. This explained reduced performance of the SCR unit.

Velocity field in the symmetry plane of the inlet duct and reactor

The existing SCR unit
The existing SCR unit
Optimized SCR unit

This shows how the baseline design causes the flow to heavily swirl right before it reaches the catalysts resulting in a very uneven distribution of the flow that enters the catalysts.

CFD and EFD confirmed the redesign

FORCE Technology then optimised the SCR unit using CFD and suggested a redesign. A comparison of the CFD results for the redesign with the actual installed design showed very promising improvements.

Then an EFD model (Experimental Fluid Dynamics) with the suggested redesign was built (scale 1:15) to give a solid basis for decision before implementation. This model proved that the redesign would fit the needs of the Chinese power plant reducing the NOx emissions considerably making it comply with the environmental regulations without having an excessive consumption of ammonia.

The Chinese coal-fired power plant implemented the redesign changes in their SCR unit and is now compliant with the environmental regulations of the Chinese authorities without frequent extensive repair costs and with a more reasonable usage of ammonia.

geometry scr unit
Optimisation of a SCR unit can consist of many different designs that needs to be simulated and tested to find the optimum flow. The goal in this process is to obtain a homogeneous mixture of the flue gas and ammonia when it reaches the catalyst layers and thereby increases the efficiency of the catalysts. It is also important to achieve an even flow distribution as the flow reaches the catalyst layers, since high-velocity-areas will quickly corrode the catalyst.