LUEHR FILTER has developed different procedures for the following applications:
It has been proven that the realisation of high additive particle re-circulation rates, especially when using Ca-based additives, leads to a significant improvement in the separation rate for acidic pollutant gas components such as HF, HCl and SO2 and/or to a reduction in the amount of additive added.
The LUEHR Conditioning Rotor - Recycle Process enables the operationally reliable re-circulation of large re-circulation quantities, even if problematic particles such as CaCl2 are present in larger quantities in the particle spectrum. A largely homogeneous mixing of the recycled particles with the flue gas flow is achieved. Pneumatic conveying systems, which are often susceptible to faults, are not required.
To allow the reliable re-circulation of particles separated in the filter into the 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:
Avoidance of particle deposits when reversing a particle-laden gas flow
Achievement of a homogeneous distribution of particles in the gas flow even with high particle loads
(for example up to n x 100 g/m3)
Disintegration of larger agglomerates with a descent velocity higher than the transport velocity in the ascending part of reactor
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:
Mechanical particle transport by means of reliable screw conveyors.
Discharge and intermediate storage of the recycled particulate prior to a new introduction into the reactor is not necessary.
Securing of a homogeneous distribution of recycled particulate during injection in the gas flow by using the conditioning rotor.
No increase of O2 in the gas due to introduction of conveying air.
In the temperature range between 100°C and 220°C, which is usual for filtering separators, the following reactivity sequence results when using Ca-based additives:
SO3 > HF >> HCl >>> SO2
While the deposition of SO3 and HF is unproblematic in the mentioned temperature range, the drying temperature as well as the absolute and relative humidity have a significant influence on the HCl and SO2 deposition. In order to save additives, it is therefore often advisable to reduce the gas temperature upstream of the reactor to optimum reaction temperatures by means of recuperative heat exchange or preferably by using an evaporation cooler. The minimum permissible operating temperature must be selected in such a way that caking and clogging are avoided, particularly due to the hygroscopic properties of the CaCl2 particles in the plant.
Gas conditioning has a positive influence on the sorption result by increasing the absolute and relative humidity in the flue gas. However, good additive utilization, especially for SO2 separation, can only be achieved if, at least temporarily, the water vapour partial pressure in the immediate vicinity of the circulation particles is close to the saturation vapour pressure. This is achieved by using chemisorption with particle conditioning. In this process, the circulating particles are moistened before being added to the reactor again. The moistening causes an increase in the water vapour content on the surface of the additive particles and thus improves the reactivity towards the acidic harmful gas components.
Due to the limited percentage humidification of the circulation particles, it may be advisable, depending on the gas temperature upstream of the reactor, to install an evaporation cooler upstream of the reactor to set the optimum reaction conditions.
In the case of very high pollutant gas contents for HCl and SO2, the stoichiometry of the basic process of conditioned dry sorption has to be raised, in some cases significantly, above a usual base value of 2 in order to ensure compliance with the emission limit values. With increasing pollutant gas contents, it is therefore recommended to add the additive in stages and thus to make additional use of the reaction space of the evaporation cooler/spray absorber if necessary. The figure above shows different process variants. In all concepts, the main amount of additive is added in the nominal case into the reactor after the evaporation cooler. In addition to corrosion protection, the addition of additive before or in the evaporative cooler/spray absorber serves to pre-separate acidic pollutant gas components, especially in the case of pollutant gas peaks.
The combination of SNCR - conditioned dry sorption - wet scrubber (TwinSorp® process) enables very low emission limits to be met for NOx, NH3, acidic pollutant gases such as HCl and SOx, Hg and other heavy metals and dioxins / furans, among others, in a cost-effective manner. In this process concept, conditioned dry sorption is operated in such a way that the exhaust gas after this stage largely meets the requirements of, for example, the 17th BlmSchV or EU Directive 2000/76/EC. Depending on the task, the following wet fine cleaning stage is left
the separation of NH3
Progressing reduction in emission values e.g. for the acid crude gas components
The process is waste water-free.
LUEHR FILTER has extensive know-how in the use of the dry sorption process with NaHCO3. For many years, plants for the most diverse applications have been realised, including
Al secondary melting plants
Domestic waste incinerations
Thermolysis for domestic waste
Biomass incinerations (waste wood)
Regarding this process, special attention has to be paid to
the selection of a suitable classifier mill to activate the NaHCO3
the homogeneous introduction of the additives into the pipeline or the reactor upstream filter
We carried out extensive investigations to optimise the utilisation of additives at a domestic waste incineration plant in France. One of the main results was that a significant improvement in additive utilisation can be achieved by multiple particle re-circulation. In contrast, the influence of the temperature is rather small from a value greater than 140°C.
It should also be noted that the evaporative cooler shown in the diagram is only used if the gas temperature is not optimally set depending on the task.
If no particle re-circulation is planned to optimise the additive utilisation, the area where the additive is introduced into the pipeline and the design of the reaction section upstream of the filter are optimised by using computer simulation programs.
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