NOx Abatement by Selective Catalytic Reduction

Chapter 14

NO Abatement by Selective Catalytic Reduction x

G. W. Spitznagel, K. Hüttenhofer, and J. K. Beer

Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch014

Power Generation GroupKWU,SiemensAG,Freyeslebenstrasse 1, 91058 Erlangen, Germany

Since 1986 more than 50 coal fired boilers in the FRG have been equipped with Siemens SCR catalytic converters for NOx-control. Especially in high dust applications plate type catalytic converters have proven to achieve optimum performance even under the worst flue gas conditions. In this connection, the proper design of catalytic converters concerning geometry and chemical composition is essential for reliable operation. This resulted in the development of a new poisoning resistant catalytically active material. Experience from more than five years of operation downstream of coal- fired boilers with various catalysts is presented. New application areas for catalysts, e. g. DeNOx air preheaters or combined dioxin and NOx control for waste incineration plants, and inital operating results with this new technology are presented.

The Federal Republic of Germany has an installed power plant capacity of 109.7 GWei. 23.6 GW or 22% of this total capacity is provided by nuclear power, 6.9 GW or 6% by hydroelectric power and 1.2 GW or 1% by renewable sources of energy. The lion's share, 78 GW or 71%, of the energy supply is based on fossil fuels (see table I), mainly coal (1). If one considers the emissions from a typical hard coal-fired 750 MW plant (see Figure 1), it soon becomes apparent that in a denselypopulated country such as the Federal Republic of Germany and especially at times when the environmental awareness of the citizens is clearly on the increase, legislators are forced to introduce strict environmental standards.

0097-6156/94/0552-0172$08.00/0 © 1994 American Chemical Society In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

14. SPITZNAGEL ET AL.

NQ Abatement by Selective Catalytic Reduction

Table I: Generating capacity in FRG (status 1990) without new federal countries according to (1)


Type of Power Source Nuclear Water Hardcoal Lignite Oil Gas Others Total

Generating Capacity [GW] 23.6 6.9 39.5 12.3 9.7 16.5 1.2 109.7

Percentage 21.5 6.3 36.0 11.2 8.9 15.0 1.1 100.0

Dust emissions from power plants (see Figure 2) were restricted as early as 1964 and as of 1975 the SO2 limits were reduced even further in North Rhine Westphalia. However, a definite step toward emission control was made when the Ordinance on Large Combustion Facilities came into force 1983 which prescribed a reduction in SO2 emissions to 400mg/m3 (140 ppm) by 1988 (by installing flue gas desulfurization plants) and by the Conference of Environmental Ministers 1984 which limited NOx emissions from power plants to 200 mg/m3 (97 ppm) as of 1988 to 1990 (2). In the meantime, licensing practice in urban areas in the FRG for new power plants demands levels of 50 mg S02/m3 (17.5 ppm) and 100 mg NOx/m3 (49 ppm). Up to now, approximately 150 power plants in the FRG with a combined electrical rating of 33,000 MW have been backfitted with SCR DeNOx equipment (see Figure 3). While primary measures are usually sufficient to meet NOx limits in lignite-fired plants, technology SCR DeNOx has established itself and been proven on an industrial scale in over 95% of hard coal-fired power plants (3), while other processes proved unsuitable for technical reasons (e.g. achievable abatement efficiency) or owing to problematic secondary emissions (e.g. N2O in SCNR (4)).

Catalytic Material and Shapes of Catalysts The heart of an SCR DeNOx plant is the catalytic converter (see Figure 4). It is usually a ceramic mass based on titanium dioxide, to which the active components, oxides of vanadium and tungsten or molybdenum, are added. Despite this significant similiarity between all SCR DeNOx catalytic converters there are great variations in their activity and geometry. When the SCR DeNOx technology came to Germany in 1984, catalytic converters were well-suited to Japanese power plant requirements

In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

173

ENVIRONMENTAL CATALYSIS


174

Without Flue Gas Cleaning

Figure 1:

With Flue Gas Cleaning

Media and end products of a 750 MW hard coal fired power plant with and without flue gas cleaning


SPITZNAGEL ET A L .


Regulations in NordrheinWestfalia

TA Luft Emission standards [dust: grain/eft] [SO , NO : ppm] x

x


1000

Further reduction by local authorities

500

Figure 2:

Development of emission standards in the FRG

MW

DeNO

0.1 % Active carbon 3 % SNCR

30 000

20 000 -

Y

— 97 % SCR

2 % Active carbon 4 % SCR

10 000 -

- f - 94 % Primary measures Hard coal

Figure 3:

Lignite

DeNOx measures in German coal fired power plants


175

ENVIRONMENTAL CATALYSIS


176

I

N , H 0, 0 , S0 (S0 ) 2

Figure 4:

2

2

2

3

Schematic view of a SCR DeNOx reactor



14. SPITZNAGEL ET AL.


(high-grade coals, base load operation, boiler with dry ash extraction) but these special conditions meant the manufacturers had only developed a few models (5). The special requirements of German power plants, such as frequent load changes in intermediate load power plants, slag tap fur naces with ash recirculation and the use of coal from all over the world (USA, Poland, Australia, etc.) or German high inerts coal, forced Siemens to develop the plate-type catalytic converter (6) in addition to li censing the honeycomb-type catalytic converter. This development made it possible to gain extensive know-how on the selection and fabrication of the correct type and design of catalytic converter, active surface and ma terial activity to suit the application (7). Plate-type catalytic converters are typically used in high-dust con figurations, i.e. immediately downstream of the boiler at a point where the flue gases have already reached the optimal temperature for the DeNOx reaction of about 350°C (660°F). The distance between the cata lyst plates is 4-6 mm which results in a specific surface area of 280380 m2/m3. Honeycomb-type catalytic converters, on the other hand, per form better in low-dust areas, i.e. downstream of dust collection and desulfurization of flue gases. In this configuration, they form very com pact DeNOx reactors with channel openings of 3-4 mm and specific sur face areas of 650-900 m /m3. Honeycomb-type catalytic converters are used less often in very high-dust applications (however in moderate dust laden flue gases both systems are comparable). Typical channel openings are 6 mm, which result in surface areas of 480 m /m3 (see table Π). 2

2

Table Π: Selection of catalytic converter according to operating conditions Fuel

Arrangement

Dust Tempera concentration ture Type range °F gr/scf

Spec. surface area Remarks m /m 2

3

High inerts Before ΑΡΗ (high dust) coal Other arrangements see hard coal

>6.5

610-810 Plate

285-350

Hard coal Before ΑΡΗ (high dust)

NOx Abatement by Selective Catalytic Reduction

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