ES&T VIEWS
The SNOX Process: A SUCCESS STORY BY
SHER
M.
DURRANI
T
he SNOX process is a catalytic process for the combined removal of sulfur oxides and nitrogen oxides from flue gases p r o d u c e d by a coal-fired boiler. In the SNOX process, sulfur dioxide is oxidized catalytically to sulfur trioxide, and nitrogen oxides are reduced to free nitrogen and water by catalytic reduction with ammonia. The sulfur trioxide is converted to sulfuric acid, which is recovered in the WSA (wet sulfuric acid) condenser. No raw materials or chemicals are used (other than ammonia for t h e r e d u c t i o n of nitrogen oxides), a n d n o w a s t e p r o d u c t s or waste water are produced. The process was developed primarily in Europe, and it was introduced in the United States as a demonstration system installed at Ohio Edison Company's Niles plant under t h e D e p a r t m e n t of E n e r g y ' s (DOE) Clean Coal II program. Recently, Ohio Edison announced its continued use of SNOX at the Niles plant after the demonstration has been c o m p l e t e d . This marks t h e achievement of a goal of all advocates of new clean coal technology processes: the adoption of the system by a utility as an integral part of its facilities. F u n d i n g for t h e $ 3 1 m i l l i o n project was provided by DOE, the O h i o Coal D e v e l o p m e n t Office, Views are insightful commentaries on timely environmental topics, represent an author's opinion, and do not necessarily represent a position of the society or editors. Contrasting views are invited.
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0 0 1 3 - 9 3 6 X / 9 4 / 0 9 2 7 - 8 8 A $ 0 4 . 5 0 / 0 © 1994 American Chemical Society
The Niles SNOX system
Niles boiler No.2
Existing ESP
Existing air preheater Ambient
Vent
\mbient" •-air \
Hot air
WSA-condenser
air Cooling air fan Flue gas
Support burner
Ambiant air ; Natural gas
Stack Existing facilities
Slipstream Sulfuric acid Flue gas fan
Baghouse
Gas-gas heat exchanger
Fan Ambient air Ammonia tank
so 2 reactor SCR reactor
New facilities
Acid drum Acid • Acid cooler >umps
Support burner
Acid storage tank
Natural gas
ABB E n v i r o n m e n t a l S y s t e m s , Snamprogetti USA, and Ohio Edison Company. The WSA condenser is a unique air-cooled falling-film condenser. Flue gas flows through vertical glass tubes and, because the temperature difference across the gas film inside the tubes and the turbulence of the gas are carefully controlled, sulfuric acid is condensed essentially without formation of acid mist. Moving downward in counter flow with the hot flue gas, the acid film is concentrated to more than 93 wt % sulfuric acid. The cooling air leaves the condenser at about 400 °F. Demonstration plant The Niles SNOX System is installed on a 78,000 scfm (standard cubic feet per minute, equivalent to 35 MW) flue gas slipstream from a 115-MW cyclone-fired boiler burning primarily Ohio coal of approximately 3% sulfur content. Construction was completed in late 1991, and the originally scheduled demonstration testing is nearing completion. However, testing has been extended into 1994 in conjunction with plans to make system modifications for long-term operation at the plant. The system has consistently demonstrated substantially better than 90% removal of S 0 2 and NO x from the flue gas while producing 28 tons (at full load operation) per day of concen-
trated sulfuric acid with 93-95% purity (commercial grade). A portion (78,000 scfm, SNOX design capacity) of flue gas is diverted from the Unit 2 flue gas duct between the air preheater and the electrostatic precipitator and is heated before passing through a fabric filter. The flue gas temperatures at various stages are critical to the chemical reactions for S 0 2 and N O x reduction and for the formation of sulfuric acid. A flue gas fan with an inlet vane control regulates the flow of gas through the process. A heat pipe gas—gas heat exchanger, using naphthalene as the heat transfer medium, further raises the temperature of the incoming gas by transferring the heat from higher temperature flue gas from downstream in the process. Ammonia is injected in the flue gas before passing through the selective catalytic reduction (SCR) reactor for NO x removal. Downstream of the SCR reactor, the flue gas temperature is increased approximately 50 °F by using a second stage of heating to obtain the optimum reaction temperature in the SO z reactor where S 0 2 is converted to S0 3 . The discharged gas is cooled through the gas—gas heat exchanger, converting S 0 3 to H 2 S0 4 (vapor). The WSA condenser further cools the gases below the acid dew point temperature and condenses sulfuric acid. The sulfuric acid leaves the
condenser and is cooled before it is pumped to a storage tank. The flue gas is reheated to above the acid dew point temperature en route to the plant chimney. The air used to cool the flue gas passing through the WSA condenser is vented to the atmosphere in the demonstration plant. In a fullscale plant, where the SNOX system is fully integrated with the boiler, the heat generated by the exothermic reactions (as shown under process chemistry below) and the heat recovered from condensing sulfuric acid are returned to the boiler cycle by using the WSA condenser cooling air discharge as combustion air. Process chemistry After the initial pass through the gas—gas heat exchanger, the flue gas is mixed with ammonia upstream of the SCR reactor. Here the nitrogen oxides are removed by selective catalytic reduction with ammonia, according to the scheme: NO + NH 3 + 0.25 0 2 = N2 + 1.5 H 2 0 + 410 kj/mol The gas is then heated by the support burner before it enters the S0 2 reactor. About 96% of the S 0 2 is oxidized on an S0 2 -oxidizing catalyst to S 0 3 by the following reaction: S 0 2 + 0.5 O z = S 0 3 + 98 kj/mol Ammonia and most carbonaceous combustibles in the gas are oxidized Environ. Sci. Technol., Vol. 28, No. 2, 1994
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Environ. Sci. Technol., Vol. 28, No. 2, 1994
completely. At the same time, practically all t h e remaining dust in t h e gas is retained in t h e catalyst bed, w h i c h must be cleaned at intervals. Cleaning is done by sifting t h e catalyst. This procedure may be necessary once or several times a year, dep e n d i n g o n t h e efficiency of t h e dust removal equipment (e.g., a fabric filter or an electrostatic precipitator upstream of t h e process). T h e o x i d i z e d flue gas is t h e n cooled in its second pass through the gas—gas heat exchanger, a n d the sulfur trioxide in t h e gas is hydrated into sulfuric acid (gas): S 0 3 + H 2 0 = H 2 S0 4 (g) + 100 kj/mol The gas that is still above t h e sulfuric acid d e w p o i n t t e m p e r a t u r e enters t h e WSA condenser, w h e r e it is further cooled in vertical glass t u b e s by air to a t e m p e r a t u r e of about 210 °F. T h e sulfuric acid vapor is condensed a n d concentrated to m o r e t h a n 9 3 % p u r i t y i n t h e WSA condenser, a n d heat is transferred to t h e cooling air as follows: H 2 S0 4 (g) = H 2 S0 4 (1) + 69 kj/mol Plans for the future The SNOX process is fully automated and can be operated and m a i n t a i n e d w i t h m i n i m a l personnel. Unlike conventional scrubbers, the SNOX process does not require substantial r a w material a n d waste handling systems. At this time w e have n o experience w i t h t h e N O x and S 0 2 catalyst replacement cycle a n d cost. Although our experience to date is limited, w e have found t h e SNOX process easy to operate a n d maintain. Demonstration of t h e system h a s been highly successful. ABB Environmental Systems a n d Ohio Edison are making certain mechanical modifications to t h e Niles SNOX system to enhance its usefulness for t h e r e m a i n i n g life of t h e Niles plant a n d to reduce operating costs. O h i o E d i s o n p l a n s to take over t h e system in January 1995. Sher M. Durrani, a general project engineer with Ohio Edison Company, has in-depth experience in the management of large generating-plant projects. He has a bachelor's degree in electrical and mechanical engineering from the University of Karachi, Pakistan, and is a registered professional engineer in Ohio.
