Sorption procedures

SORPTION PROCEDURES

LUEHR FILTER has developed different procedures for the following applications:

  • Chemisorption of acidic pollutant gases such as HF, HCl and SO2, with Ca- or Na-based additives
  • Adsorption of PCDD / PCDF as well as Hg and Hg compounds by means of activated carbon / activated coke or other additives with large internal surface

It has been proven that the realization of high additive particle 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 ball rotor recirculation process enables the operationally reliable recirculation of large recirculation 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.

THE SORPTION PROCEDURES APPLIED BY US ARE BASED ON THE LUEHR CONDITIONING ROTOR-RECYCLE PROCESS (KUV).

PROCESS VARIANTS OF THE CONDITIONING ROTOR-RECYLCE PROCESS (KUV)

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.

Description of conditioning rotor

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

Conditioning Rotor-Recycle Process (KUV)

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.

Further Information

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.

Further Information

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.

Further Information

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.

Further Information

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

  • Heat recovery

The process is waste water-free.

Further Information

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 realized, including

  • Al secondary melting plants

  • Glass tanks

  • Tyre incineration

  • Domestic waste incinerations

  • RDF incineration

  • 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 injection of additive powders into the ducting and/or into reactor upstream filter

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.

Further Information

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