Objectif
The development and construction of a Walther flue gas desulphurization demonstration plant to prove that this process permits the reheating of cleaned gas as well as the spray drying of the ammonium sulphate solution produced during the scrubbing sequence. By the reheating of scrubbed flue gases using a regenerative heat exchanger, +/- 1,800 Kg/h of fuel oil is saved under full load operation.
Difficulties were almost exclusively in the conventional part of the plant and related to the "flowability" of the ammonium sulphate powder.
The rubber coated steel scrubbing system consists of two scrubbers arranged in series. In the upper part, the scrubbing solution containing ammonia is injected into the flue gas in order to absorb SO2, which is absorbed in the first scrubber. The second serves to bind the ammonia gas still contained in the flue gas, thus preventing exhaust of gaseous ammonia from the plant.
Scrubbing solution PH value in scrubber 1 is 5.6 - 6.2 and between 4-4.5 in scrubber 2.
The scrubbing process is such that SO2/ammonia ratio is over-stoichiometric up stream of the last scrubber so that, due to reaction with the SO2, ammonium content in the second scrubber is reduced to such an extent that raw gas concentration of ammonia is extremely low.
Cleaned flue gases are reheated with the regenerative heat exchanger with less than 5% leakage. SO2 concentration of the reheated clean gas at the stack entry is the same as that of the clean gas after the last scrubber.
Use of the heat exchanger proved that under operational conditions it is possible to reheat flue gas cooled during the washing process without the use of auxiliary energy. Heat taken from the flue gas by cooling it from 130 deg.C to 72 deg.C prior to entry into scrubber 1 is given up to the flue gas after passage in the scrubbing system, reheating it from 45 deg.C to 100 deg.C.
A single stage axial blower of 830,000 Nm3 capacity at maximum pressure difference of 66 Mbar, is located up stream of the washing system. It operated trouble-free for some time until ammonium chloride deposits were discovered on the impellers. The problem was solved by installation of an injection device before the blower.
It also became necessary to reduce the concentration of aerosols to make the flue gas trail invisible when operating the boiler with the FGD plant. The separating system chosen was ableto restrict aerosol concentration to less than 15mg/m3. A large scale filter was installed within the flue gas channel up stream of scrubber 2. This however, was not successful since the glass fibre used was not resistant to flouride aerosols.
Product treatment after scrubbing caused problems partially as a result of the large scale use of the Walther process to pelletise ammonium sulphate. This was solved by installation of a second pelletising pan, spray drier adjustments and installation of a faster controlling system and optimization of drier gas distribution. The discharge system is also equipped with a pulveriser to prevent sporadic conveyor clogging.
Ater a two year operating period the flue gas plant using the Walther process reached expectations in regard to both effective cleaning and end-product quality. SO2 concentration below the guaranteed value of 180mg/Nm3 dry was reached in the case of SO2 inlet concentrations up to 4,200 mg/Nm3 dry.
The flue gas desulphurization plant consists of the following systems and/or main components :
- ammonia storage and evaporator station
- flue gas scrubbing and reheating
- oxidization and spray drying
- product treatment.
The system has other important advantages apart from those already noted :
- no waste water
- production of the marketable nitrogen fertilizer ammonium sulphate.
The Walther desulphurisation system is connected to a new 475 Mwe coal fired plant for combined heat/ electricity production, and is located in the Grosskraftwerk (Mannheim). The system is designed for a flue gas SO2 content of 150-200mg/m3 and for demonstration, 50% of one boiler's flue gas (750,000m3/h) will be desulphurized.
The system consists of a rotating regenerative heat exchanger, two SO2 washing towers, oxidization tank, spray dryer and equipment for treatment of the fertilizer end-product.
The heat exchanger serves both to cool and to reheat flue gas entering and leaving the FGD plant. When ammonia is added to the flue gas before entry into the heat exchanger, a reaction occurs in the first washing tower producing ammonium-sulphate-bisulphate. The flue gases are then subjected to another fresh water wash in the second tower to eliminate salt solution particles and any ammonia remaining.
Concentrated washing liquid is extracted from the washing circuit of the first washing tower, pumped to an oxidization tank where the ammonium-sulphite and bisulphite are converted to ammonium sulphate. This is then pulverised in a spray dryer by hot flue gases extracted from the boiler off-gas in front of the air heater, and collected in a tank. The product is then sent to a granulator. It is then transferred to a moving-bed dryer, and subsequently sieved, milled and stored in a tank prior to transportation.
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Coordinateur
51069 Köln
Allemagne
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