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denser. No raw materials or cheniicals are used (other than ammonia for the reduction of nitrogen oxides). and no waste products or waste water are produced. The process was developed primarily in Europe, and it w a s introduced in the United States as a damonstration system inslalled at Ohio Edison Cnmpany’s Niles plant under the Department of Energy’s (DOE1 Clean Coal I1 program. .kec:ently. Ohio Edison announced its continued ust! of SNOX at the Niles plant after the dcinonstration has been ~:nmplcted.This marks the achievement of a gnal of all advocates of new d r a n coal tei:hnnlogy es: the adoption of the tern by a utility as a n integral part of i t s facilities. Funding f o r tht: $:$I inillion project was prnvidt:d hv DOE. tht: Ohio Coal Di:vrlupment Oi’fiix,
No. 2, 1994
27 88A$04 50,O
1994 Amerlcan Chemlcal
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ABB Environmental Systems, 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 OF.
Demonstrationplant The Niles SNOX System is installed on a 78,000 scfm (standard cubic feet per minute, equivalent to 35 MW) flue gas slipsimam 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 SO, and NO, 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 (78,000scfm, 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 SO, a n d NO, 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, removal. Downstream of the SCR reactor, the flue gas temperature is increased approximately 50 O F by using a second stage of heating to obtain the optimum reaction temperature in the SO, reactor where SO, is converted to SO,. The discharged gas is cooled through the gas-gas heat exchanger, converting SO, to H,SO, (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 c h e m i s e 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 g-as 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,+ 0.25 0,= N, + 1.5 H,O + 410 kJ/mol The gas is then heated by the support burner before it enters the SO, reactor. About 96% of the SO, is oxidized on an SO,-oxidizing catalyst to SO, by the following reaction:
SO, + 0.5 0,= S O , + 98 kJ/mol Ammonia and most carbonaceous combustibles in the gas are oxidized
Envimn. Sci. TBchnol., VoI. 28. No. 2. 1994 89 A
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completely. At the same time, practically all the remaining dust in the gas is retained in the catalyst bed, which must be cleaned at intervals. Cleaning is done by sifting the catalyst. This procedure may be necessary once or several times a year, depending on the efficiency of the dust removal equipment (e.g., a fabric filter or an electrostatic precipitator upstream of the process]. The oxidized flue gas is then cooled in its second pass through the gas-gas heat exchanger, and the sulfur trioxide in the gas is hydrated into sulfuric acid (gas): SO, + H,O = H,SO,[gl+ 100 kJ/mol The gas that is still above the sulfuric acid dew point temperature enters the WSA condenser, where it is further cooled in vertical glass tubes by air to a temperature of about 210 "F. The sulfuric acid vapor is condensed and concentrated to more than 93% purity i n the WSA condenser, and heat is transferred to the cooling air as follows: H,SO,(gI = H,SO,(l]
+ 69 kJ/mol
Plans for the future The SNOX process is fully automated and can be operated and maintained with minimal personnel. Unlike conventional scrubbers, the SNOX process does not require substantial raw material and waste handling systems. At this time we have no experience with the NO, and SO, catalyst replacement cycle and cost. Although our experience to date is limited, we have found the SNOX process easy to operate and maintain. Demonstration of the system has been highly successful. ABB Environmental Systems and Ohio Edison are making certain mechanical modifications to the Niles SNOX system to enhance its usefulness for the remaining life of the Niles plant and to reduce operating costs. Ohio Edison plans to take over the system in January 1995. Sher M. Durmni, a penerol oroiect engineer ;vi& Ohio Edison Company, --L\ , . has in-deDth exDeI Y I rience in'the mbnagement of large generating-plan t projects. He has a bachelor's degree in electrical and mechanical engineering from the Universitv of Karachi. Pokistan, and is a regist6redprofessional engineer in Ohio.
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