Flowpac Pump-less High Efficiency FGD Scrubber

Transcription

1 Mega Symposium May 19-22, 2003 Washington, DC Flowpac Pump-less High Efficiency FGD Scrubber Paper # 166 Robert Agonis - Project Manager ALSTOM Power Environmental Control Systems 1409 Centerpoint Boulevard Knoxville, TN Kjell Nolin Maintenance Manager Mechanical Karlshamn Kraft Box 65 SE Karlshamn Sweden Donald Schreyer Project Engineering Manager ALSTOM Power Environmental Control Systems 1409 Centerpoint Boulevard Knoxville, TN Abstract With the introduction of recirculation tank in-situ oxidation in wet FGD, operators were faced with new challenges concerning density measurement & stratification, accurate level control, even foaming. In the process of resolving these issues, engineers were impressed with the ability to displace large masses of liquid using relatively modest amounts of pressurized air. Such was the birth of the Flowpac scrubber as originally invented by the owner at the Karlshamn, Sweden generating plant and developed by ALSTOM where it has been in service since There the 340 MW Flowpac installation continues to demonstrate very high sulfur removal efficiencies that consistently exceed 97% via a recirculating bubble bed reactor. Reagent recirculation is achieved via density differentials created by the oxidation air injected into the recirculation tank. Increased fan horsepower as needed to overcome the bubbling bed is easily offset by the absence of any large recirculation pumps. This paper presents the operating experience as well as the distinct maintenance advantages offered by the 340 MW Karlshamn unit with its greatly simplified equipment requirements Introduction Karlshamn Power Station is comprised of three units. The first of these was commissioned in 1969 and the other two in 1971 and These are the biggest Nordic Page 1 of 9

2 oil fired condensing units, each is rated 340 MW. Under normal circumstances hydropower and nuclear power meet the basic electric demands. However when difficulties arise like extremely cold weather, an extremely dry year, or the need to service a nuclear unit the Karlshamn units provide the security to the Swedish electric supply. The role of the Karlshamn units is to serve as peaking and standby power that can be started up and deliver electric power on short notice. Karlshamn 3, on the right, is equipped with the Flowpac FGD system The units were designed to fire oil with low sulfur concentrations. In the late 80 s the National Licensing Board imposed the growing environmental demands for Environment Protection. These regulations would become effective in 1997 and required the reduction of flyash, nitrogen oxides and sulfur. The utility was permitted and then taxed on emissions and upon this basis made the environmental investments at the power station. The National Licensing Board established the following emissions conditions for the Karlshamn Power Station block: Sulfur (S) 25 mg S/MJ or 175 mg SO2/NM3 or 62 ppm Nitrogen oxides (as NOx) - 70 mg/mj or 250 mg/nm3 or 120 ppm Flyash Karlshamn Unit 3 only 90% collecting efficiency before the desulfurization plant. Ammonia shall not exceed 5 ppm. Oil in water effluent, annual mean value shall not exceed 10 mg/l Page 2 of 9

3 Emissions exceeding these limits were charged emission fees (taxes) as follows: 40 SEK per kg NOx - 70 mg/mj at full load corresponds to 8000 SEK (~ $800) per hour 27 SEK per 0.1% Sulfur content and m3-25 mg/mj at full load corresponds to 2000 SEK (~ $200) per hour Karlshamn approached the reduction in emission fees in the following manner. Unit 1 is the oldest unit in the power station group so it is fired with 0.1% fuel oil and would be the last to be started. Unit 2 is started only when the demand for power has exceeded the capacity of Unit 3. Unit 2 was fitted with an SCR to remove the nitrosoxides from the flue gas. Unit 3 was fitted with a SCR and additional high efficiency flue gas treatment equipment for the removal of sulfur and particulate. The modification allows the unit to be fired with lower cost high sulfur oil. Rather than apply the conventional open spray tower WFGD system the owner had a better idea. The use of a sieve tray to obtain very efficient gas to liquid contact without the use of large recycle pumps. The plant personal are very familiar with fans and compressor and small pumps. Development work started in 1992 and ABB was invited to acquire the inventive bubble bed idea and participate in initial results in ABB subsequently acquired the technology. The technology was further developed by ABB Flakt Industria AB, Sweden and patented in Sweden. A contract was awarded to ABB Flakt Industria AB, Sweden to equip Karlshamn Unit 3 with the sieve tray absorber system in In 1996 the system went into operation and has operated successfully since. The sieve tray absorber technology was given the name Flowpac. The acquisition of certain ABB s power generation assets by ALSTOM moved the ownership to ALSTOM. Flow Diagram for Unit 3 Page 3 of 9

4 The Flowpac absorber differs in the flue gas contact zone. The preparation of the reagent lime or limestone for use in the absorber is the same as a conventional system open spray tower system. The dewatering of the oxidized slurry with hydro cyclones and belt filters is the same as a conventional open spray tower system. The radical change is in the heart of the WFGD system the gas contact zone. The flue gas enters the absorber under the sieve tray, flows through the sieve tray holes and then rises through a bed of limestone slurry. This is a very turbulent zone allowing intimate contact between liquid and gas, excellent conditions for SO2 absorption and natural oxidation. Absorber internals with empty reaction recycle tank. The sieve tray encircles the absorber recycle tank. The recycle tank is agitated and supplied with oxidation air. The airlift effect generated by the difference in densities when oxidation air is introduced into the recycle tank eliminates the need for large absorber recycle pumps. The expanding slurry rises in the recycle tank and flows out across the sieve tray and is suspended by the flue gas, when it reaches the circumference of the sieve tray it flows to the down comer where the dispersed gas is released causing an increase in density. This density difference of 5-10% between the slurry with dispersed air and the pure slurry generate the circulation of fresh slurry. Page 4 of 9

5 Section through prescrubber and absorber. One of the strengths of the Flowpac cross-flow sieve tray design is the excellent gasliquid distribution inherent in this design. Other mass transfer devices such as a spray tower require high L/G ratios to compensate for mal-distribution of gas and liquid (sneakage). Spray towers have used performance enhancement plates or wall rings to obviate the sneakage occurring at, for instance, the inner wall. The Flowpac sieve tray has no sneakage path thus all the flue gas must come in contact and pass through a slurry bed. Reagent addition to the Flowpac tray is directly to the absorption zone rather than a reaction tank. This in combination with the high turbulence occurring in the slurry bed on top of the sieve tray achieves very high SO 2 efficiencies and almost full utilization of the limestone with little sensitivity to ph changes, and complete sulphite oxidation. Contact of flue gas with slurry assures excellent removal efficiency greater than 98% and collecting efficiencies greater than 99.5% with the use of adipic acid. Page 5 of 9

6 Absorber and sieve tray at 80% flue gas flow and 90% slurry bed height. Intimate contact of the flue gas passing through the slurry bed on the sieve tray also has shown additional benefits of high SO3 removal, efficiencies of 70% reduction have been measured. The violent agitation produced by the flue gas passing through the slurry bed and the intimate contact of the flue gas to the liquid also captures fine particulate. The slurry on the top of the bubbling bed, call it froth or foam means fewer mist particles being carried from the bed of slurry on the sieve tray to the mist eliminator. This very low liquid entrainment produced a problem free mist eliminator operation with reduced flushing, and thus lowed auxiliary energy consumption compared to a conventional spray tower absorber. While the Karlshamn unit was installed with a two pass and a four-pass mist eliminator the two pass mist eliminator has not been required for operation. Page 6 of 9

7 Outside of Flowpac absorber at Karlshamn, showing two of the eight downcomers. The low overall height of the Flowpac due to lack of large recycle pumps and spray banks allows the inspection and service of the internals to be easily reached and inspected without scaffolding. The simple back to basics Flowpac design was brought forth from an owner perspective. The Flowpac doesn t require large recycle pumps with large motors, gear sets and mechanical seals. The space these pumps require is not only near the absorber but also in the storeroom for spare parts. These large pumps also generate a fair amount of noise, The pumps need to be piped to numerous spray nozzles and when a pump is removed from operational service the pump and piping need to be flushed, the slurry runback and flush water must be processed. The lack of large recycle pumps reduces operating and maintenance requirements. Karlshamn Flowpac Performance Design Measured Sieve tray bed height, inches Oil S content, % SO 2 Inlet, ppm SO 2 Outlet, ppm SO 2 Removal, % SO 3 Removal, % No Guarantee ~ 70 Page 7 of 9

8 Execution Schedule Development work had been done in 1992 and 1993 on the inventive bubble bed idea in the ABB Flakt Industria AB lab/technical center to prove the concept. In early 1994 additional pilot work was commenced at the power station. In early October 1994 the utility issued an inquiry, obtaining bids from various international WFGD suppliers. The bids were extensively reviewed and an award was made to ABB at the end of March Detail engineering began immediately. Flowpac s fewer pieces to design foundations for permitted the civil work in the field to commence in May. Vessel and duct fabrication commenced upon completion of design and material arrived for erection in the field by October. The mechanical erection was completed by July of 1996 and checkout commenced, the WFGD system was in operation by November of 1996, a rather impressive 18-month schedule for a new process Brief Project Schedule Summary The main characteristics of the new absorber are as follows: Very high SO 2 collecting efficiency, SO 2 emission of almost zero with adipic acid addition, High SO 3 and particle collection efficiency, No problems with scaling or corrosion, Lower auxiliary energy consumption than a spray tower absorber, Lower maintenance and supervision costs due to lower height, absence of large recycle pumps and piping, Minimum entrainment of droplets resulting in improved droplet eliminator performance with reduced flushing. Page 8 of 9

9 The squat profile of the absorber vessel and the fewer large pieces of rotating equipment to install, pipe and wire had a very positive impact in reducing the field man-hours to install the complete system. Karlshamn Unit 3 Acknowledgements The authors would like to acknowledge the technical competence, experience and determination of the Mati Maripuu, Lars-Erik Johansson, Rikard Hakansson and Sune Bengtsson from the original development team. References 1. K, Nolin, VGB Power Tech 11/2000, Newly Developed FGD Plant for oil fired condensing Power Station in Karlshamn, Sweden Page 9 of 9