By: Edwin H. Weaver and Stephane Bourgoin
Wet Scrubbing SystemSystem Operation
An industrial waste incineration facility located at the Schwedt oil refinery installed in northeastern Germany incorporates all of the integrated system design features required to address all air, liquid, and solid regulatory issues.
The facility is equipped with a spray dryer absorber, fabric filter, and wet scrubber. It is designed for the maximum following emissions:
- HCl, 10 mg/Nm3;
- SO2, 50 mg/Nm3;
- particulates, 10 mg/Nm3;
- mercury, 50 µg/Nm3;
- dioxin, 0.10 ng/Nm3.
Purge water from the wet scrubbing system's containing salts is dried in the spray dryer absorber and collected as a dry waste in the fabric filter following the spray dryer absorber.
This facility uses a 5-t/h rotary kiln to burn a variety of both on and off-site wastes, including water-treatment cakes, bottom waste from oil tanks, contaminated soils, sludges, paint, and oil-soaked rags.
The kiln's exhaust gases pass through a waste-heat boiler to recover 12 t/h of steam energy and reduce the flue-gas temperature. The gases then pass through a low-efficiency electrostatic precipitator, which removes most of the flyash generated in the kiln. The flue gas, before being exhausted, passes through a spray dryer absorber, fabric filter, wet scrubbing system, and heat exchanger.
The spray dryer is a downflow co-current design using dual fluid nozzles for the air-atomization of the liquid. Flue gas containing less than 100 mg/Nm3 particulate and acid gases enter the spray dryer at a maximum flue-gas temperature of 400°C. The flue gas then divides into five "flow tubes."
One proprietary dual fluid nozzle in each flow tube injects the liquid's containing the wet-scrubber bleed. This bleed primarily consists of solids (gypsum, CaSO3, CaF2) and dissolved salts (Cl-, F-, [SO4]2-, Ca2+). The dual fluid nozzle is a proprietary design that atomizes the liquid into fine droplets having the proper size distribution for drying of the material.
The nozzle is ceramic in areas where wear is anticipated and Hastelloy in areas exposed to the corrosive liquid. Other parts are of stainless steel. Sufficient liquid is injected through the spray nozzles to control the gas temperature to 160°C at the exit of the spray dryer.
The vessel has a diameter of 5500mm and a straight side height of 9150mm. This results in a drying time of approximately 11.5 sec. at the maximum design conditions. Dust removal is provided under the hopper of the spray dryer.
After the flue gas exits the spray dryer/absorber, coke is injected into the gas stream to reduced and control of mercury and dioxins. The flue gas then enters a four-module pulsejet fabric filter, which removes flyash and dried salts from the gas stream.
Each fabric filter module has a total of 462 P-84 bags. Each bag is 2000mm long and has an oval shape for a total cloth area of 0.714m2 per bag cloth area of 330m2 per module and 1320m2 total. The maximum gross and net air-to-cloth ratios are 0.84 m/min and 1.12 m/min, respectively.
Wet Scrubbing System
The wet scrubbing system basically consists of two scrubbing towers followed by a droplet separator. Flue gas enters the top of the first scrubbing tower. There it is immediately quenched from a gas temperature of approximately 160°C to a saturated gas temperature of approximately 63°C. This spray, by three proprietary LAB "G" nozzles located in the top of the vessel, has a dual flat cone shape and comprises a modicum of relatively narrow size-range droplets.
As the flue gas continues through the vessel, a series of 10 LAB "G" nozzles arranged vertically sprays liquid for acid gas scrubbing within the vessel. Lime removes the acid gas. The flue gas exits the side of the vessel near the bottom.
Scrubbing vessel construction is of FRP. The vessel diameter is 2550mm, resulting in a gas velocity of approximately 3 m/sec. Three 220-m3/hr pumps (two operating and one spare) provide the liquid for this scrubbing vessel.
An elbow between the two scrubbers maintains droplet separation and adequate pH control in both. Except for the absence of quench spray, the second scrubber is an inverted replica of the first, with the flue gas flow being vertically upward and exiting from the top of the vessel.
The flue gas then enters four droplet-separator Cycolabs operating in parallel, which are virtually plug proof. Each has a diameter of 520mm and an effective length of 3700mm. A LAB "F" nozzle, producing a very coarse spray at the exit of each Cycolab, ensures the removal of any remaining dissolved gypsum in the flue gas and avoids possible gypsum precipitation in the heat exchanger. A conventional Chevron-type mist eliminator is located downstream to remove any droplets that might be entrained from the "F" nozzles at the exit of the Cycolabs.
The last item through which the flue gas passes before the ID fan and the stack is the heat exchanger, which raises the gas temperature from 63°C to 115°C for plume reduction.
Liquid flow in the system is from the clean gas side (LAB "F" nozzle tank) to the dirty gas side (first scrubber module). Makeup water to account for evaporation losses and bleed losses is added at the "F" nozzle tank. Water from the tank overflows into the liquid loop for the second scrubber vessel. In addition to this overflow, lime is added to the second scrubber to maintain a pH of approximately 6. Bleed from the second scrubber flows to the first scrubber where lime again is added to maintain a pH of approximately 5. Bleed from this loop, based on salt concentration, is fed to the spray dryer for drying and collection in the fabric filter.
The system was placed into operation in July 1997. The air-pollution-control system essentially has operated reliably without problems since the initial startup. An internal inspection of the air-pollution-control system, performed in October 1997 showed all components to be in excellent condition.
The system is operating very close to the maximum flow conditions.
About the authors: Edwin H. Weaver and Stephane Bourgoin are associated with Belco Technologies Corporation, Parsippany, N. J. 07054 and LAB, S.A., Lyon, France, respectively.
This article is adapted from the paper 98-WA70.04, "Pollution Control System Design for Achieving Stringent Emissions Standards on Waste Incineration FacilitiesA Case Study," presented the Air & Waste Management Association's 91st Annual Meeting & Exhibition, June 14-18, 1998, San Diego, CA