To allow the reliable re-circulation of particles separated in the filter into the flue gas flow upstream filter, the Conditioning Rotor-Recycle Process proved to be advantageous for many applications.
The rotor is a hollow cylinder, made of a perforated plate with openings of approx. 30 x 30 mm. Up to 10% of its volume is filled with balls made of heat- and wear-resistant ceramics. The rotor is continuously rotating with approx. 1 rpm by means of a geared motor. The rotation causes the balls to move relatively to each other inside of cylinder and to the perforated shell. The rotor is passed through by the flue gas around its axis of rotation at first in downwards and finally in upwards direction.
The main functions of the conditioning rotor are:
Prior to being discharged out of the filter, the particles separated in the filter are repeatedly re-introduced into the reactor by means of a conveying screw. The particle recycle rate can be adjusted and, if needed, controlled e.g. subject to the current volume flow.
Compared to alternative, e.g. pneumatically working re-circulation systems, the Conditioning Rotor-Recycle Process offers the following advantageous features. These are among other things:
With regard to the temperature range of 100°C – 220°C, customary for fabric filters, the following sequence of reaction results when using Ca-based additive powders:
SO3 > HF >> HCl >>> SO2
The dry temperature as well as the absolute and relative humidity have a decisive influence on the HCl and SO2 separation, however, the separation of SO3 and HF does not present any problems within the stated temperature range. In order to save additive powder, it is often useful to cool down the crude gas temperature upstream reactor to optimal operating temperatures by means of recuperative heat exchange or preferably by using an evaporative cooler. The min. admissible operating temperature has to be chosen that way that adhesion and blockages especially due to hygroscopic characteristics of the CaCl2 particles in the plant will be avoided.
As a result of the increase in the absolute and relative humidity of the flue gas, the gas conditioning has a positive effect on the sorption output. However, a good additive powder efficiency, especially for the separation of SO2 can only be achieved if the water steam partial pressure close to the recycled particulate lies at least for a short time in the range of the saturation steam pressure. This will be achieved by using the chemisorption with particle conditioning. Regarding this type of process, the recycled particulate is wetted prior to re-introduction into the reactor. The wetting causes an increase in the water steam content at the surface of the additive powder particles, thus improving the reactivity in comparison to the acid crude gas components.
Due to the limited proportional wetting of the recycled particulate and depending on the gas temperature upstream reactor, it might be useful to install an upstream located evaporative cooler for the adjustment of optimum reaction conditions.
In case of very high pollution gas contents for HCl and SO2, the stoichiometry of the basic variant of the conditioned dry sorption has partly to be increased definitely above a typical base value of 2 without any additional measures, in order to observe reliably the emission limit values. At rising pollution gas contents it will therefore be advisable to apply a graded additive powder injection, thus using in addition the reaction chamber of evaporative cooler / spray absorber when indicated. The illustration above shows some corresponding process variants. With regard to all concepts, the main quantity of additive powder is in the nominal case injected into the reactor downstream evaporative cooler. The injection of additive powder upstream or inside of evaporative cooler / spray absorber mainly serves for the corrosion protection and particularly in case of pollution gas peaks for the pre-separation of acid pollution gas components.
A simple hybrid process for increased requirements on emission limit values
The combination SNCR - Conditioned dry sorption – Wet scrubber (TwinSorp – process) allows the observance of very low emission limit values i.a. for NOx, NH3, acid crude gases such as HCl and SOx, Hg and other heavy metals as well as dioxins/furanes in an economical way.
The conditioned dry sorption of this process variant is operated in that way that the crude gas downstream this stage will as far as possible comply with the requirements of e.g. 17 BImSchV. or EU Directive 2000/76/EC. Depending on the application in question, the downstream installed fine cleaning stage serves for the
The process is waste water-free.
LUEHR FILTER has a comprehensive knowledge regarding the use of the dry sorption process with NaHCO3. Since many years plants have been realised for different applications, i.a.
Regarding this process, special attention has to be paid to
Comprehensive studies at a domestic waste incinerator in France have been realised by us for the optimisation of the additive powder efficiency. One of the main results was that the multiple particle re-circulation helps to achieve a definite improvement of the additive powder efficiency. The influence of the temperature however, is rather low as of a value larger than 140°C.
It may be remarked in addition that the evaporative cooler shown in the illustration will only be used if the gas temperature has not been adjusted in an optimal way with regard to the application.
As far as no particle re-circulation has been provided for the optimisation of the additive powder efficiency, the fields injection of additive powder in the ducting as well as design of reaction line upstream filter will be optimised by using computer simulation programmes.