Combined SNCR and SCR

Combined SNCR and SCR is not a new invention – far from it! In ScandeNOx’s founder worked on a pilot plant with this technology as early as in 1987.

The first trials were conducted on a system with SCR catalyst installed in a rotating regenerative air preheater (Ljungström type), where the upper (hottest) 500 mm of heat exchange elements were replaced with catalyst modules.

The idea was to facilitate utilising the full potential of the SNCR technology by using an SCR catalyst to eliminate the ammonia slip.  When the SNCR technology was new, the ammonia slip at high NOx reduction figures was often unacceptable due to the high SO2 concentration in the flue gas and consequent condensation of ammonium bisulphate.

By installing the catalyst in the air preheater, no reactor was needed, and costly downtime during installation could be greatly reduced. Since the catalyst replaced some of the heat exchange elements, the only operational cost caused by the catalyst was a slightly increased pressure loss, and an insignificantly reduced heat transfer potential.

The results

…may not seem impressive at a first glance, but the catalyst did in fact remove app. 90 % of the injected ammonia.

At district heating plant Boliden in Uppsala, Sweden, the following results were obtained with a urea-based SNCR system:

At this plant only 5 [ppmv, d, 0 % O2] of ammonia slip was permitted, so without a “Slip Brake”, only about 50 % NOx reduction could be achieved. With a catalyst to remove 90 % of the ammonia slip, the allowable slip from the SNCR system would be 50 [ppmv…] – so the NOx reduction by SNCR could be increased to some 65 – 70 %. Since the initial NOx concentration was low, the 45 [ppmv…] of ammonia removed by the catalyst would increase the over all NOx reduction to more than 90 %.

For a number of reasons it is not always possible to install the SCR catalyst in the air preheater.

At the power plant Fyriskraft in Uppsala, Sweden, the air preheater was turned “upside down” due to height restrictions – so the catalyst would have to be placed in the flue gas duct outside of the air preheater to reduce the mechanical stress on the modules and to facilitate cleaning of the heat exchange elements without having to take all elements out of the air preheater:

CFD & physical modelling of such systems is obviously crucial for the performance, so ScandeNOx uses leading modelling experts for this task and cooperates closely with the catalyst manufacturers.

ScandeNOx’s founder has worked with SN+CR technology since it was first introduced and has participated in the continued development of this technology ever since. As late as in the spring of 2010, he invented the SN+CR concept for cement plants presented below.

ScandeNOx’s founder and CEO was for a period of 9 years employed in the World’s leading manufacturer of cement plants, where he advanced to the position of Emissions Technology Manager and held the following subordinate positions:

ScandeNOx’s founder thus has the necessary experience to advice our clients on virtually all emission related matters concerning cement plants.

Problems with raw meal derived ammonia emissions on a number of cement plants inspired him to combine the Slip Brake concept known from the power plant industry with the medium dust SCR technique recently tested in Italy.

The result was the SN+CR concept for cement plants shown in the right column:

Since the raw materials absorb most of the ammonia slip when the raw mill is ON, the catalyst can often be installed in the raw mill bypass .

Ammonia in the flue gas inhibits the formation of Dioxins/Furans through the denovo synthesis, and the catalyst can burn out the Dioxins/Furans as well as VOC, TOC, PAH - and oxidize Mercury.

The oxidized Mercury is easier to capture in the dust filter and can thus largely be bled out of the system with the dust admitted to the cement mill.