Low-Temperature Selective Catalytic Reduction NO Control

Chapter 16

Low-Temperature Selective Catalytic Reduction NO Control x

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 29, 2016 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch016

Phillip A. Lowe Intech Inc., 11316 Rouen Drive, Potomac, MD 20854-3126

The early German applications of SCR NOx control technology to slagging coal-fired boilers were based upon Japanese technology. Unanticipated problems such as catalyst poisoning by arsenic led the Germans to commercially develop a different SCR configuration, called the tail end or cold side application. Initial tail end applications used the Japanese/German SCR technology developed for 300-400 °C operation for an SCR application where the flue gas was at 90-150 °C. This resulted in a considerable economic penalty to reheat the flue gas to the SCR operating temperature. This paper discusses operational experience and new catalyst formulations that suggest that the tail end configuration may now be more economic than other SCR configurations, especially for U.S. coal-fired retrofit service.

Low Temperature SCR Experience A companion paper(1) describes the development of SCR technology in Japan and Germany. Essentially, three configurations have been developed for locating the SCR reactor in the flue gas: the high dust design where it is located before the particulate control and is subjected to all of the contaminants in the flue gas but where the flue gas is in the catalyst's operating range; the low dust design where a high temperature particulate control is installed and the SCR can be located downstream of that device; and the tail end design where the SCR reactor is located downstream of the sulfur control process where the resulting clean flue gas temperature is below 150 °C. The configurations are shown in Figure 1. The Germans operated over 70 pilot plants to test Japanese SCR designs under German coal-fired utility boiler operating conditions. From these tests and a few of the early operating systems on several wet bottom 0097-6156/94/0552-0205S08.00/0 © 1994 American Chemical Society In Environmental Catalysis; Armor, John N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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

TAIL END

LOW OUST

HICH DUST

V

SG

V

SG

V

SG

AH

PC

SCR

PC

1.

NH3

AH

SCR

Figure 1.

W

FGD

AH

W

PC

τ

ι

3

NH

HEAT

SCR Location Configurations

LECEND» W W SG - STEAM GENERATOR AH - AIR HEATER PC - PARTICULATE COLLECTOR FCO - FLUE CAS OESULPHURIZATION SCR - SELECTIVE CATALYTIC REDUCTION C - CHIMNEY

3

NH

S



16. LOWE

Low-Temperature Selective Catalytic Reduction NQ Control

(e.g., slagging boilers) boilers, an unexpected premature deactivation of the catalyst was observed. It subsequently was found that the practice of recycling the fly ash catch from the particulate control back to the boiler to slag it also recycled the trace impurities of the coal ash. Alkali materials and especially arsenic were concentrated in the flue gas by this process, and their resulting concentrations poisoned the SCR catalysts. One solution to this problem was to place the SCR catalyst reactor downstream of all of the flue gas treatment processes (the particulate and the acid gas controls). At this location, the SCR treated a relatively clean flue gas. However, the flue gas had to be reheated from its final temperature back to the SCR operating temperature. This resulted in a 1-4% penalty in the overall thermal efficiency of the power plant, a very significant cost. In addition, the initial cost estimates assumed that the tail end catalysts would have the same operating life as those that operated in the contaminated high dust flue gas configuration, and in fact the early applications used the same SCR catalyst formulations rather than reformulating the catalyst to take advantage of the very clean operating environment of the tail end configuration. For a number of site specific reasons, the initial tail end SCR systems cost nearly three times the initial capital than did the initial high dust systems. These economic conditions (the failure to optimize the tail end SCR to its operating conditions, the flue gas reheat penalty, and the high capital costs for the initial tail end applications) resulted in a conventional wisdom that the high dust configuration is more economic and is the preferred SCR configuration. As is often the case with conventional wisdom, it does not keep up with new developments and thus may not generally be applicable to many new situations. There have been a number of specific problems with the high dust configuration. It is recognized that in each case where a problem has been identified the designers and plant operators have made adjustments to allow the continued operations of the plant. Thus, many of these problems have been "solved", although if the same conditions do not exist at the next plant the solution may not be applicable or at least optimized. Thus, it is instructive to consider these issues and how they impact high dust configuration applications. These include: 1. The high dust system may significantly restrict the operating range of the boiler since the flue gas must be delivered at above 300 °C or else catalyst poisoning by bisulfates can occur in addition to the failure of the catalyst to provide the required level of NOx reduction. To address this, boiler economizer bypass ducts have been installed, especially if the plant is to cycle to low power levels, When this is done, the plant thermal efficiency deteriorates significantly at low power conditions, a fact usually not addressed in cost comparisons between the high dust and tail end designs. Of course, if a low sulfur fuel is used or if boiler cycling is not required, this concern may not be relevant.


207

208


2.

3.

ENVIRONMENTAL CATALYSIS

The high dust system requires the installation of soot blowers and the use of a dummy initial catalyst section. The soot blowers are required to control the dust buildup on the catalysts, which can block the flow distribution and reduce the overall NOx reduction while increasing the ammonia bypass. The dummy catalyst layer is a sacrificial element which the initial dust particles impact and erode rather than impacting and eroding the catalyst itself. In the U.S., the catalyst is composed of metals that are considered as being toxic, thus any erosion of the catalyst may lead to serious air toxic emission problems. However, it is noted that new catalyst formulations are more resistant to erosion and if the fly ash is soft enough they may eliminate the need for sacrificial material. However, when sacrificial material and soot blowing are required, the resulting larger more complicated reactor system increases its capital and installation costs (soot blowing has been reported to increase the capital costs by about five percent). In a high dust system the most highly reactive catalysts can not be used because such catalysts also convert S 0 to S0 . Sulfur trioxide can form sulfuric acid which attacks the flue gas ducting and downstream equipment, it also forms ammonium bisulfate, a sticky material that plugs and fouls downstream equipment. The use of less reactive catalysts results in the need for larger SCR reactors with their inherent impact on capital costs and places very stringent control requirements on the ammonia injection system (ammonia bypass of the SCR reactor must be less than 5 ppm and often less than 1-3 ppm). Restricting ammonia flow means that the upper limit of NOx control is effectively less than 90% and probably less than 80-85%. A net result of these conditions has been that the catalysts have an operating life of about 2-4 years (some plants have operated for 5 years before additional catalyst material was required). It is noted that more reactive catalysts that still limit the conversion of S 0 to S 0 has been an area of active research. Thus, new catalyst formulations may reduce the impact of this concern. In Japan over time, the operational control of the startup period at one plant became lax (not an unusual condition in a power plant). As a result, unburned fuel left the boiler and become imbedded in the dust on the SCR reactor. Subsequently, the SCR reactor oxidized the fuel and the resulting fire caused considerable damage. Although this problem in not an inherent problem with high dust SCR designs, it does demonstrate the coupling between the boiler and the high dust configuration SCR reactor, which can cause restrictions on the boiler operation. Usually in retrofit conditions, there is very limited space available near the boiler for locating the high dust SCR reactor. When the SCR is to be connected to the duct work, the plant outage is long and may take over several months to complete. If the boiler walls must be penetrated in order to install bypass ducting, the costs and time delays can be significant. Plant shutdown increases costs as replacement 2

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16. LOWE

Low-Temperature Selective Catalytic Reduction N(\ Control

power must be purchased during the plant shutdown period. Most economic comparisons between high dust and tail end configurations ignore this important cost element. Although new construction sequencing concepts can reduce the down time required, high dust systems are inherently more difficult and time consuming to install in retrofit than are tail end configurations. 6. The use of slagging boilers that increased the concentration of arsenic in the flue gas lead to catalyst poisoning. On of the initial solutions was to introduce the tail end configuration or to modify the boiler operation to eliminate the recycling of fly ash to the boiler (now some new catalyst formulations are more resistant to arsenic poisoning and can be used in high dust, slagging boiler applications). In addition, some slagging boilers have been found to produce SiF , which under some part load conditions can deposit silicon on the catalyst of a high dust system. The silicon acts as a poison. This illustrates that all of the potential deposition/poison conditions may not be recognized before plant operations occur. While the same concern can be stated for tail end configurations, their inherently cleaner operating environment lowers the potential for such an unanticipated problem to occur. There are a number of specific advantages, and some problems, associated with the tail end configuration: 1. As noted above, the flue gas must be reheated to the SCR operating temperature, which can cause up to a 4% reduction in the plant efficiency. As a result, several catalyst vendors and plant operators are developing low temperature SCR catalysts. With the newer catalyst formulations becoming available, it appears that the penalty can be limited to a 1 % or less decrease in the plant thermal efficiency. 2. In a tail end system, the SCR is decoupled from the boiler and thus all part load operations can be performed as needed. Also, very importantly, boiler upsets and changes in the trace impurities of the fuel do not apply a direct impact on the SCR catalyst. Thus, the tail end design is inherently more flexible in terms of being able to successfully accommodate future (and presently unanticipated) changes in the power cycle or fuel. Since the system may have to be applied for over 40 years, this provides a significant increase in the plant owner's ability to accommodate changes that though presently unanticipated will nevertheless occur. 3. Tail end SCR designs can take advantage of the relatively clean flue gas (small trace element concentrations, low S 0 concentrations) to use a highly reactive catalyst. This decreases the catalyst volume; at present the volume is typically one half of that required for a high dust design(13). Also, the catalyst life in clean flue gases has been demonstrated to be 10-15 years. These conditions significantly decrease the initial capital costs of the system as well as the ongoing operating costs. 3

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4.

Ammonia bypass can be set by environmental considerations and not to control the formulation of bisulfates (however, if a recuperative heat exchanger is used the ammonia slip and overall NOx removal efficiency of the SCR system is lowered). However, a larger allowable ammonia discharge means that the catalyst can be operated with confidence over the full operating life at 904-% NOx removal efficiencies. 5. Aside from the very first few installations, the net impact of all of these considerations has been that the tail end SCR costs, in practice, have been the same or lower than similar sized high dust systems. Perhaps the best indication of the desirability of a given SCR design is the choice of the market place. Krueger(2) has identified the installed capacity (MW thermal) and the number of individual SCR units used at German and non German VGB affiliates. The details of the non German affiliates were not provided, but Table I shows the results for the German utilities. It shows that the initial applications selected the high dust configuration. But more of the newer plants have used the tail end configuration. New Operating Information and SCR Designs Some German plantsO) reheat the flue gas using a rotary heat reheater that introduces a small amount of hot untreated flue gas into the flue gas downstream of the desulfurization unit but upstream of the SCR unit. The introduced impurities have led to contamination of the SCR catalysts. The solution has been to improve the thermal efficiency of non contact reheaters or to use natural gas or some other clean fuel to reheat the flue gas prior to its being treated by the SCR system. However, the loss of catalyst activity in tail end applications has been much less than that experience by high dust systems(4), and the loss of catalyst activity in many of the high dust systems has been less than originally expected(5). Plants in Japan with SCR reactors treating flue gas with a cleanliness similar to that found in tail end plants have experienced 11-15 year catalyst lives(6). The high dust SCR system has been found to affect the flue gas desulfurization (FGD) process at some plants(7). Sulfuric acid formed by the SCR catalysts has oxidized manganese in the FGD process, making the FGD sulfate product unacceptable to the building industry. Mercury oxidized by the SCR catalysts becomes soluble and ends up in the FGD waste water, complicating its treatment process. Tail end configurations do not have these complications. One interesting approach to controlling the sulfuric acid formation in high dust systems is to burn coals that have alkali ash or else a small amount of lime is added to the coal. The theory is that the lime or alkali neutralizes the acid as it is formed. One of the early promising low temperature SCR designs was developed by Stadtwerke Duesseldorf (8,9,10). In this process, a two reactor system based upon using activated lignite coke or activated coke was



16. LOWE

Low-Temperature Selective Catalytic Reduction NQ^ Control 211

T a b l e I. G e r m a n S C R Utility

Year

Applications

High Dust SCR Application Number Capacity, MW t

2

Tail End SCR Application Number Capacity, M W t

1985

1468

3

-

-

1986

2100

3

-

-

1987

3352

5

2142

6

1988

11580

14

3398

5

1989

11421

19

16230

25

1990

8691

12

9764

33

1991

1916

3

241

5

1992

-

-

900

2

1993

1637

2

-

-


212


designed and installed at the 1000 MW Lausward electric power plant, the 200 MW Garath district heating plant, and the 200 MW Flingern refuse-toenergy plant. The utility Stadtwerke Duesseldorf has installed the system at 11 plants that treat over 5 million m of flue gas(11). In the Duesseldorf two reactor process, the first reactor bed is downstream of the FGD unit. It acts as an S 0 polisher in that it removes the final S 0 contamination from the flue gas. It also removes fine dust, metals such as mercury, other acid gases such as HCI, and for the incinerators it removes dioxins and furans. Since the metals and other toxic materials are removed in the first 100-200 mm of the bed, the bed is segmented to collect the trace materials in the leading section and the sulfur loaded carbon in the second section. This aids in subsequent disposal or recovery treatment of the spent activated carbon. Ammonia is added downstream of the first activated carbon reactor and the second reactor acts as a SCR catalyst removing the NOx. These reactors operate at 90-120 °C, thus they eliminate the need for flue gas reheating and the 1-4% thermal penalty associated with that process. Stadtwerke Duesseldorf has found that the operating costs of the activated carbon total process system is about one half of that of a similar metallic SCR catalyst system, both operating in a tail gas system. The operating costs are similar to those of a high dust system, but because of the ability to construct the tail gas system while the plant was operating (and thus avoiding a long down time to install the SCR system), the activated carbon system's capital costs are less than those estimated for a high dust system(8). In this design, as the first reactor becomes loaded with sulfur or trace elements such as mercury, it is discharged and the spent bed material is sent to the boiler where it is blended with the fuel. Thus, the particulate and FGD systems are the sole points for removal of toxic and acid materials, and their design can be optimized for such service. Fresh activated carbon is added to the first reactor from the SCR reactor, and fresh activated carbon is added to the system at the SCR reactor. This system reduces the NOx from 1,800 to 200 mg/m (about 900 to 100 ppm), under special operating conditions 150 mg/m was achieved(9). However, the utility needs to achieve 100 mg/m NOx discharges (which are achievable by the metallic SCR catalysts), so they are examining other catalysts to replace the activated carbon NOx control portions of the system. They are examining zeolite and titanium oxides which will operate at 100-120 °C (zeolite) or 170-250 °C (titanium oxides). These catalysts will require some flue gas reheating. Zeolite can be considered as a potential catalyst because of the clean nature of the flue gas. In a high dust environment, earlier German pilot plant tests showed that the zeolite became plugged and then failed prematurely. Although it is more expensive to operate the catalysts at the high end of their temperature range due to the need to reheat the flue gas, the zeolite and titanium catalysts are more efficient at reducing the NOx at high temperatures. In tests(9) at 170 °C, NOx reductions of 95% have been observed; at 140-150 °C, zeolite and 3

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16. LOWE Low-Temperature Selective Catalytic Reduction NQ Control 213 titanium dioxide produced 75% and 80% NOx reductions, respectively. If the stoichiometric balance of ammonia to NOx is increased above 1, some N 0 production is formed(12) with the zeolite catalysts. Keeping the ammonia to NOx ratio at 0.9 eliminates this problem(9). Other manufacturers are reported to be developing catalysts! 11) that operate at 100-150 °C. The Shell International Chemie(13,14) has been developing a titanium/vanadium catalyst that will operate at 120-350 °C for 90% NOx conversion. The greater the inlet NOx concentration, the greater the NOx percent reduction achieved. It has been applied commercially at chemical plants in Europe and at a refinery in Los Angeles. It requires a low dust and S 0 environment, making it potentially suitable for the tail gas SCR configuration for coal- or waste-fired plants.


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U.S. Application Considerations and Conclusions

A key issue for U.S. coal-fired SCR applications is that the U.S. coals contain as much as 2-6 times the amount of sulfur as compared to coals burned in Japan and Germany. The resulting additional S 0 in the flue gas is a serious concern, since the addition of ammonia and the effects of the catalyst itself both lead to the production of ammonia bisulfate. The bisulfate is a serious problem since it plugs and fouls downstream equipment and it can severely shorten the catalyst life. Sulfate aerosols often are not adequately controlled by wet FGD scrubbing, and may be discharged from the plant stack. German operating experience with fuels with higher sulfur levels has shown that the SCR system can produce sulfuric acid which results in acid attack of downstream equipment and the emission of sulfuric acid aerosols, even if a wet FGD system is used. Solutions to this type of problem have included lowering the boiler flue gas outlet temperature, adding lime to the flue gas, changing soot blowing and cleaning schedules, and limiting the boiler operating range. Since U.S. coals may have even greater sulfur contents, this could be an area of specific concern for SCR applications to U.S. coal-fired service conditions. A key lesson learned is that just monitoring the SCR effectiveness by catalyst activity measurements is insufficient to control sulfur-related problems(5). Although specialized high dust SCR catalysts can be developed and demonstrated for U.S. operating conditions, it would be simpler and there would be a greater assurance that the design used at one plant could be transferred to another plant if tail gas SCR systems were developed. However, it is recognized that with proper engineering and development, either the high dust or the tail gas system should be a suitable NOx control for U.S. conditions. 3

Literature Cited

1. Lowe, P.; Ellison, W.; "Foreign Experience With Selective Catalytic Reduction NOx Controls", American Chemical Society Meeting, Denver, March-April 1993, to be published in the meeting proceedings. In Environmental Catalysis; Armor, John N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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2.

Krueger, H.; "Status of NOx Reduction in Power Plants of VGB Members", VGB Kraftwerkstechnik, 4, 1991.

3.

Ellison, W.; Weiler, H.; "Stack Gas Cleaning Optimization Via German Retrofit Wet FGD Operating Experience", EPRI/EPA Joint Symposium on SO Control", Washington, D.C., March 1991. 2


4.

Hjalmersson, A-K; "NOx Control Technologies for Coal Combustion", IEACR/24, June 1990.

5. Rummenhohl, V.; Weiler,H.;Ellison,W.;"Experience Sheds Light on SCRO&MIssues", Power, September 1992. 6.

Lowe, P.; Ellison, W.; "Understanding the German and Japanese CoalFired SCR Experience", Proceedings EPRI/EPA Joint Symposium on Stationary Combustion NOx Control, Washington, D.C., March 1991.

7.

Gutberlet, H.; Dieckmann, H.J.; Schallert, B.; "Auswirkungen von SCR-DENOX-Anlagen auf nachgeschaltete Kraftwerkskomponenten (Effects of SCR DENOX Plants on Downstream Plant Components), VGB Kraftwerkstechnik, 71 (6), June 1991.

8.

Kassebohm, B.; "The Semi-Dry Flue Gas Desulphurisation System in Duesseldorf", The Institute of Energy, London, September 1990.

9.

Kassebohm, B.; Wolfering, G.; "Catalytic Flue GasDenitrificationat Low Temperature i.e. Without Reheating", VGB Kraftwerkstechnik, 4, 1991.

10.

Power

11.

Kassebohm, B.; Wawrzik, U.; "The Three Stage Flue Gas Cleaning System of Duesseldorf", Materials and Energy From Refuse, 4, Oostende, Belgium, March 1992.

12.

Hjalmersson, A-K; "Interactions in Emissions Control For Coal-Fired Plants", IEACR/47, March 1992.

13.

EPA, "Evaluation and Costing of NOx Controls for Existing Utility Boilers in the NESCAUM Region", EPA453/R-92-010,December 1992.

International, "Activated-Coke Filters Pioneered for Refuse-, Coal-Fired Plants", September 1991.

RECEIVED

September 30, 1993


Low-Temperature Selective Catalytic Reduction NO Control

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