Method of Purifying Flue Gases From Sulphur Dioxide

Gases can be purified from sulfur dioxide components present therein by contacting the gas to be treated with an absorbent in a liquid phase (wet method) or in a solid phase (dry method). The present invention relates to a substantially dry method. As the dry method includes reaction steps implying diffusion of sulfur dioxide components into a solid phase, one of its disadvantages is a relatively low reaction rate.

The object of the present invention is to reduce the impact of the said disadvantage. This object is achieved according to the invention characterized by the attached claim 1.

By effecting a liquid aqueous phase with a high solubility for the participating reactants (sulfur dioxide components) on the surface of the solid absorbent particles, the reaction steps, viz dissolving of the sulfur dioxide component, diffusion, separation of absorbent and precipitation of reaction product, proceed substantially more rapid than the corresponding reaction steps when only gaseous and solid reactants are present. The absorbent may be an oxide, a carbonate or a hydroxide of an alkali metal. Illustrative absorbents are calcium carbonate, magnesium calcium carbonate, sodium carbonate, calcium hydroxide, sodium hydroxide or mixtures thereof. Calcium hydroxide, sodium hydroxide or mixtures thereof are the preferred absorbents. The liquid phase, may be derived from a hygroscopic salt, which at the prevailing gas temperature and gas water content (water partial pressure) is a thermodynamically stable molten hydrate.

In recent years a process for absorbtion of sulphur dioxide from a hot flue gas has come into use which in the literature is referred as "dry scrubbing". The process substantially comprises a drying step and a filtering step, and it operates as follows: The hot gas is contacted in a drying apparatus with an absorption liquid in the form of an atomized aqueous suspension or an aqueous solution capable of reacting with sulphur dioxide. In the drying apparatus evaporation of the supplied water (the water component of the atomized aqueous suspension or solution) takes place and simultaneously a reaction between sulphur dioxide and the absorbent occurs to form a solid reaction product. As the gas leaves the drying apparatus, the major part of the supplied water has evaporated, the gas temperature has decreased, the water vapour content of the gas has increased, and part of the sulphur dioxide has been absorbed to form the solid reaction product. The gas thereafter passes a dust separator where the solid reaction product remaining and any other solid material is separated from the gas. Part of the separated dust may be further recycled into the absorption liquid in order to increase the utilization of the absorbent.

This process often results in incomplete utilization of the absorbent during the drying period. After said period the reaction between the sulphur dioxide and the absorbent proceeds in the dust separator. This reaction, however, proceeds relatively slowly, since the reaction steps occur in solid phases.

The present invention has also the object of bringing about a higher reaction rate during said latter process step. This is achieved by the method defined in the attached subclaims.

This higher reaction rate is obtained by adding to the gas a limited amount of hygroscopic substances, such as hygroscopic salts which under the conditions prevailing in the dust separator form on the surface of the solid particles a liquid aqueous phase with sufficient solubility for the reactants. The hygroscopic substance may be added separately to the flue gas or may be mixed with the absorption liquid prior to its atomization. As the reaction steps, viz. solution of sulphur dioxide, diffusion of reactants and precipitation of product, proceed substantially more rapid than the corresponding steps in a solid phase, the net effect thereof is an increase in the total reaction rate. In order to prevent the operation of the dust separator from being jeopardized, the volume of the liquid phase must be small in relation to the volume of the solid phases, and in order to achieve a good effect, the liquid phase must be well distributed over the surface of the solid phases. For example, the added hygroscopic substance may constitute a maximum of about 20 percent, and preferably is no greater than about 5 percent by volume of said solid phase present at said dust separator. Examples of hygroscopic substances, the hydrate of which has the property of being in the liquid phase under the conditions prevailing at the absorption of sulphur dioxide from flue gases of fossil-fired boilers, FeCl.sub.3, Fe(NO.sub.3).sub.3, Fe(NO.sub.3).sub.2, Al(NO.sub.3).sub.3, Ca(NO).sub.3).sub.2, Mg(ClO.sub.3).sub.2, MnCl.sub.2, K.sub.2 SO.sub.4 MgSO.sub.4, NaAl(SO.sub.4).sub.2, NaClO, Na.sub.3 PO.sub.4, Na.sub.2 SiO.sub.3, Na.sub.2 SO.sub.4 or Zn(NO.sub.3).sub.2.

The absorbent added to the drying apparatus may be at least partially extracted from fly ash or gaseous substances present in or separated from the flue gas.

In a pilot plant with a capacity of 10 000 Nm.sup.3 /h comprising a drying apparatus, into which atomized absorption liquid is injected, and a fabric filter as the dust separator, tests were carried out for determining the capacity with and without the addition of some of the aforesaid substances. Calcium hydroxide was used as absorbent which was added in an amount corresponding to a mole ratio Ca/SO.sub.2 =1,1 (calculated on ingoing gas). The ingoing gas had a temperature of 140.degree. C. and an SO.sub.2 -content of 850 ppm.

The filtering velocity of the fabric filter was 92 m/h. The gas temperature after passing through the fabric filter was 70.degree. C. The following SO.sub.2 absorption efficiencies were measured.