The essential process stages are:
- Absorbent preparation and dosing
- Removal of SOx (HCl, HF) in the CFB absorber
- Solids removal
- Solids recirculation and product discharge
Calcium hydroxide is normally used as the absorbent, either directly as calcium hydroxide in the form of a dry powder (Ca(OH)2) or produced on site from quicklime (CaO) in a dry hydration plant. The vapours released in this process can be integrated into the FGD system or removed separately.
A special application is the installation of a CFB FGD with an upstream CFB boiler, where a primary desulpurisation takes place. In a best-case scenario no additional injection of calcium hydroxide into the CFB FGD is necessary.
The optimum reaction temperature in the CFB absorber is set through the injection of controlled water into the fluidised bed via controllable spillback nozzles. The fluidised bed in the CFB absorber requires the observance of certain parameters. In order to guarantee stable plant operation even during part load operation of the boilers, the clean gas is therefore recirculated in a controlled manner downstream of the FGD fan into the raw gas channel upstream of the CFB absorber. This is done via a pressure drop in the flue gas, and an additional blower is not required. With this procedure, the CFB FGD allows the realisation of any part load operation that may be required.
The discharge of solids from the CFB FGD system depends on pressure loss through the CFB absorber. The CFB FGD system requires a filter for solids recirculation and dust removal. If it is possible to use the desulphurisation product in conjunction with the fly ash of the boiler, then there is no need to remove the dust upstream of the CFB FGD.
If the boiler ash is marketed separately, the flue gas dust is removed in the existing filter. In such a case, the reaction product of desulphurisation must also be treated separately, so that the product discharge and the product silo can be designed smaller. The CFB FGD product has been utilized for various applications.
The core of a CFB FGD is the absorber. This in an insert-free, vertically arranged flue gas duct with one or more Venturi-type entry nozzles. Water injection takes place via an optimised nozzle arrangement above the Venturi nozzle(s). Both the absorbent dosing and the solids recirculation take place via pneumatic conveyor troughs. Unlike with water injection, the entry point is below the Venturi nozzle(s).