Process of Purifying Exhaust Gas

The present invention is in a process of purifying an exhaust gas which, in addition to dust, contains gaseous pollutants, such as SO.sub.2, SO.sub.3, HCl, HF and nitrogen oxides, by the addition of sorbents to the gas stream and a dry separation of substantially all pollutant-laden sorbents together with the dust.

Such exhaust gases are formed, e.g., in glass-melting processes and in dependence on the nature and composition of the raw materials and fuels employed will contain, per standard cubic meter, 200 to 1000 mg dust, 500 to 3000 mg SO.sub.2 and SO.sub.3, wherein the SO.sub.3 content is 5 to 10%, about 100 mg HCl, 20 to 50 mg HF and 1500 to 3000 mg nitrogen oxides. Cloth filters and electrostatic precipitators are available, inter alia, for collecting the dust. The gaseous pollutants can be separated by the addition of sorbents and reactants, possibly with the aid of catalysts. The added sorbents may be used in dry, wet and quasi-dry processes.

In the dry processes, sorbent powders are added to the gas stream and after a reaction with, or an attachment of, the pollutant, are separated from the gas stream in most cases together with the dust. The separated substances are often re-introduced into the gas stream in order to improve the utilization of the sorbents and to maintain a higher solids concentration in the gas stream so that the probability of a contact between solid particles and pollutant gas molecules is increased. Dry processes afford the advantage that they exert virtually no influence on the temperature of the exhaust gas.

In wet processes the sorbents are introduced into the gas stream as a solution or suspension. While such a process results in an improved reaction and in an improved separation of pollutants, the exhaust gas stream is inevitably cooled to a high degree. Such wet processes involve considerable heat losses and in many cases require a reheat of the purified exhaust gas stream to the lowest temperature which is permissible in the chimney. The wet processes also require an expensive processing of the sorbent liquor.

The quasi-dry processes are a combination of dry and wet processes. In the quasi-dry processes the sorbents are added as a solution or suspension, as in the wet processes, and the rate of liquid is so controlled that the liquid is entirely evaporated in the exhaust gas stream and the pollutant-laden sorbents can be separated in a dry state together with the dusts. In that case the sorbent liquor need not be processed but the temperature of the exhaust gases is greatly reduced so that heat losses occur.

For various reasons, wet process cannot be used to purify exhaust gases emanating from glass-melting tanks and which contain the above-mentioned pollutants. In practice it is particularly undesirable that the sewage must be treated in a plant which is often described as a separate chemical factory, which involves operating costs and gives rise to unpredictable difficulties.

Whereas dry processes can be used for a satisfactory separation of the dusts and of the SO.sub.2 as well as the HF, they do not achieve satisfactory separation of HCl. This relationship is virtually inverted in a certain way as regards the results of the quasi-dry processes, in which all pollutants other than the HF can be separated from the exhaust gas to the prescribed residual concentration with a reasonably low expenditure.

For this reason there is a need for a process for the purification of such exhaust gases which does not have the above-described disadvantages and with which produces a purified exhaust gas containing the pollutants within the legally described limits at an economically justifiable expenditure.

The above-stated objects and others are obtained in accordance with the process of the present invention. In the invention an exhaust gas containing dust and gaseous pollutants is treated by the addition of an absorbent (sorbent) which is a dry powder in an amount of 70 to 90% of the total amount of sorbent required. A turbulence is subsequently produced in the gas stream, and 10 to 30% of the total amount of sorbent required is then introduced as a solution or suspension into the turbulent gas stream.

The dry sorbent can be Na.sub.2 CO.sub.3 or Ca(OH).sub.2 or a mixture thereof. The solution can contain Na.sub.2 CO.sub.3 or NaOH or a mixture thereof. When added to water Ca(OH).sub.2 can form a suitable suspension.

Preferably the amount of sorbent introduced as solution or suspension is determined based on the HCl content of the gas stream. The amount of the solution or suspension introduced should be controlled so that the temperature of the exhaust gas does not drop by more than 10.degree. to 15.degree. C. due to the introduction of the solution or suspension.

The sorbents should be added in an amount such that the treated exhaust gas contains, on a standard cubic meter basis, not in excess of about 400 mg SO.sub.2, 5 gm SO.sub.3, 30 mg HCl and 5 mg HF.

The sorbents will become laden with the chemically bound pollutants and, along with the dusts, can be collected in dust collection equipment, such as an electrostatic precipitator (esp). The collected substances can be recycled to the exhaust gas. The gas leaving the esp can be introduced into a catalytic converter or subject to a catalytic treatment for elimination of the NO.sub.x components.

FIG. 1 is a schematic representation of the process in accordance with the invention.

A reaction vessel 1 is shown which may contain internal fixtures for producing a turbulence in the gas stream and which has such dimensions that the exhaust gas will be treated for the desired time. The cross-section will determine the gas velocity and the height will determine the residence or treating time. Reactor 1 is fed with the gas to be purified through supply line 2. Supply line 2 tangentially feed into reactor 1 so that a certain turbulence will be produced in the entering gas stream. 70 to 90% of the total amount of required sorbent is fed in a dry state through line 6 into the incoming exhaust gas stream before it enters the reaction vessel. The remaining 10 to 30% of the sorbent entering the reactor 1 through line 7 is sprayed as a solution or suspension into the reactor. The gas which has been treated flows through line 3 into a dust-collecting electrostatic precipitator 4, which is represented only by its region and by part of the means 5 for withdrawing dust.

A portion of the mixture of dust and pollutant-laden sorbents which becomes available in the dust-collecting electrostatic precipitator 4 may optionally be recycled through recycle line 8 into the reaction vessel 1 in order to obtain a higher solids concentration therein and to improve the utilization of the sorbents.

The processs of the invention affords the advantage that all pollutants can be separated from the exhaust gas except for the permissible residual contents whereas a wet-chemical processing of sewage is not required and high heat losses will not occur. The exhaust gases, which normally become available at 350.degree. to 450.degree. C., are cooled only by 10.degree. to 15.degree. C. so that the nitrogen oxides can subsequently be eliminated by catalytic processing in a temperature range which is favorable for such processing without a need for additional measures.

It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.