Semi-Dry Method with CFB Absorber
This method uses the principle of dosing the Ca(OH)2 sorbent into the fluid absorber.
Process water to cool down flue gases is separately supplied to the absorber. As a result of heat generated by the flue gases, all the supplied process water evaporates.
The absorber consists of an upper large-diameter cylinder part and a lower small-diameter part, where both these parts are interconnected by a reduction in the shape of a Venturi tube. The fluid layer is composed of a fresh sorbent and recirculated product. Due to the high speed of flue gases at the inlet into the lower part of the absorber, loose particles cannot fall down but are carried by the flue gas flow upwards. Inside the Venturi tube, the loose particles are uniformly dispersed into the flue gas flow and accelerated in the vertical direction. In the upper large-diameter part of the absorber, the flue gas flow speed is not that high anymore and the drag force of the flue gas flow is no longer able to keep all the loose material in the uplift and transport. This results in a continuous return flow of heavier particles of the fluid material near the internal surface of the casing of the absorber. As soon as such sinking loose mass gets nearer to the lower Venturi tube, it is quickly put into the uplift here again by the fast flue gas flow and continues to rise up. Fine fractions leave the absorber in its highest part and are captured in the bag filter. The thus-captured product is recirculated from the bag filter into the absorber and the smaller part is evacuated into the desulphurisation product silo.
Water and sorbent are supplied separately to the process and, therefore, no operationally unpleasant suspensions are present here. The technology operates with temperatures above the dew point. For this reason, in contrast to limestone wet scrubbing, no special materials are required. It is not necessary to address problems concerning waste water because this technology only works with solid desulphurisation product.
Chemical reactions taking place in the absorber
Ca(OH)2 + SO2 → Ca SO3 * 0,5H2O + 0,5H2O
Ca(OH)2 + SO3 → Ca SO4 * 0,5H2O + 0,5H2O
Ca(OH)2 + 2HCl → CaCl2 + 2H2O
Ca(OH)2 + 2HF → CaF2 + 2H2O
Ca(OH)2 + CO2 → CaCO2 + H2O
Basic technological units
- fluid-bed boiler
- bag filter
- sorbent storage and dosing
- product circulation
- transport of the product to silo
- water management
- smoke stacks, flue-gas ventilator
Product handling and utilisation
The desulphurisation product resulting from this method can be used in many ways:
- The desulphurisation product mainly contains the mixture of calcium sulphate CaSO4, calcium sulphite CaSO3 and calcium hydroxide Ca(OH)2. It also contains a sufficient quantity of calcium carbonate CaCO3. The desulphurisation product itself cannot be used.
- In the mixing centre, the desulphurisation product can be mixed with flue ash from boilers and mixing water into the so-called stabilised mixture which can be further utilised in the construction industry for subgrade reinforcement, for dumping site and waste depository sealing layers, for landscape reclamation, etc.
- The stabilised mixture at the outlet from the mixing centre is a wet mix with optimum moisture suitable for immediate processing. It is advisable to process the stabilised mixture in one to six hours from its production.
- After setting, the technical properties of the stabilised mixture are fundamentally different from the original non-treated feed materials and its properties are similar to those of weak concrete.
- The stabilised mixture meets all of the requirements, not only technical (physical and chemical) requirements, but also health and environmental protection requirements according to the applicable legislation of the EU and the Czech Republic.
Advantages of the method
- circulation of the particles in the absorber, their mutual abrasive action, removal of the treated layer and exposure of non-treated material
- the sorbent is carried into the reactor in dry powder form which eliminates problems with preparation, storage and injection of the suspension transport routes
- water and sorbent are added separately into the process which eliminates problems with cleaning the pipeline routes; all water is evaporated as a result of heat generated by the flue gases
- Ca/S ratio = 1.25 – 1.9 in the broad range of input SO2
- concentration recirculation of the desulphurisation product, utilisation of the residual hydrate in the product
- desulphurisation efficiency over 90%, suitable for medium-sized to large sources
- flue gas temperature is above the dew point which eliminates the need for the installation of special materials
- a solid desulphurisation product which eliminates problems with waste water