Minimizing corrosion and build-up in a flue-gas system including a desulfurizer

Abstract

A method for minimizing corrosion and the build-up of deposits on surfaces of a flue-gas system exposed to moist substances and elevated temperatures, and which includes a desulfurizer and heat transfer means communicating with the desulfurizer, said method involving adding to the system at the heat transfer means a readily water soluble alkaline substance such as sodium hydroxide in amounts sufficient to produce with said flue-gas in said heat transfer means a minimum pH of about 5, preferably 7 or higher.

Description

[0001] The present invention minimizes corrosion and build-up in a flue-gas system including a desulfurizer and heat transfer means communicating with the desulfurizer where significant amounts of moisture and / or sulfuric acid are present by adding to the flue-gas while it is at a relatively high temperature a readily water soluble alkaline substance such as sodium hydroxide in an amount sufficient to produce with said flue-gases as they leave the heat transfer means a pH of a minimum of about 5, preferably 7 or higher.BACKGROUND OF THE INVENTION[0002] In most flue-gas systems, for safety and environmental reasons, as a means of conserving heat, the flue-gas leaving the furnace at relatively high temperatures is passed through a variety of treatment devices before escaping into the atmosphere. Among these devices are, usually in sequence, a boiler or heater, a precipitator, a heat transfer device such as a gas / gas heater, and a scrubber or desulfurizer, the flue-gas then returning t...

AI Technical Summary

Benefits of technology

[0017] Preferred alkaline substances for use in connection with the present invention are sodium hydroxide and sodium carbonate primarily because they are low cost materials, and the fact that the corrosion and build-up inhibition can be achieved with stoichiometric amounts of such low cost material is an exceedingly important advantage of the present invention. However, other substances having the same characteristics of alkalinity and ready water solubility could be employed. When certain chemicals often used in water treating systems are co-added with sodium hydroxide or sodium carbonate, this enhanced corrosion protection, and particularly helped to prevent clogging of the gas / gas heater openings even beyond that which was achieved with the use of either sodium hydroxide or sodium carbonate by itself. Particularly effective as co-additives were trisodium polyphosphate, trisodium phosphate, disodium monohydrogen phosphate, sodium borate, sodium di and polyborates, sodium silicates and sodium polysilicates, and in general water soluble sodium salts of the various phosphates, silicates and borates.

[0018] The effectiveness of the use of these basifying products having a high initial pH in minimizing corrosion or deposit build-up is shown by the following laboratory demonstration. In each of the following samples a mixture of 30 cc of water, 3 cc of 6 Normal sulfuric acid and 2 cc of the additive as described in Table I was observed after incubation at 130.degree. C. for one hour with the results as set forth in Table I.1TABLE I Basifying Agents to Prevent Corrosion of Gas / Gas Heaters Connected to a Desulfurizer Results After Incubating at 130.degree. for 1.5 Hours with Steel Metal Strip Exposed to Condensate from Sample Composition the Mixture of Dilute No. of Additive Sulfuric Acid and Additive 1 No additive Heavy corrosion, 1 / 8" of a de- posited, brown, layer on bottom of metal specimen. 2 14 cc of 1.2 N No corrosion, very slight, caustic solution scattered, brownish spots; trace of brown deposits on bot- tom 1 / 8" of metal specimen. 3 18 cc of 1.2 N No corrosion; deposit-free met- caustic solution al specimen, trace spotting on bottom 1 / 8" of metal specimen.

[0019] Without the basifying additive, rapid corrosion of the steel metal strips takes place. In each case, the additive increased the pH of the solution from below 3 to over 8.

[0020] In another series of experiments as shown in Table II, use of combining basifying chemicals with deposit modifying, or anti-corrosion, enhancing chemicals show a further improvement when this combination is used.2TABLE II Use of Combined Basifying Chemicals and Deposit-Modifying or Anti-Corrosion, or Enhancing Chemicals Are Used In Combination Results After Incubating at 130.degree. for 1.5 Hours with Steel Metal Strip Exposed to Condensate from Sample Composition the Mixture of Dilute No. of Additive Sulfuric Acid and Additive 1A None Metal completely covered with brown, rust-like, stain. Very heavy brownish / black coating on bottom 1 / 4" of metal specimen. 1B 18 cc of 1.2 N Essentially clear, very slight, caustic solution scattered, brown spots, trace deposit on bottom 1 / 8" of metal specimen. 1C 15 cc of 1.2 N Totally clear, deposit-free, caustic solution, metal strip, no trace of de- plus 0.70 cc of a posit on bottom 1 / 8" of metal 3% solution of specimen. trisodium phosphate 1D 15 cc of 1.2 N Totally clear, deposit-free, caustic solution, metal strip throughout. plus 0.70 cc of a 3% solution of sodium tripolyphosphate

[0021] The total amount of additives required is based on the flow rates of the flue-gas itself and the recirculating water solution from the scrubber 20, as well as the acidity present in the system. Basically, the feed rate of additive is determined primarily by the acidity of the stream and that amount of basifying agent, or basifying agent with modifier, that decrease the acid dew point of the stream as it leaves the gas / gas heater 16 from plus 300.degree. F. to 250.degree. F. or less.

[0022] With a boiler of 200 megawatts, an SO.sub.2 content of 6000 mg / Nm, and sulfuric acid content at the gas / gas heater of 30 ppm (112 mg / m.sup.3) and with a treatment rate of 600 ppm of a 5% solution of caustic, the following results were obtained. The acidity was reduced to 5.0 mg / Nm, or less than 2 ppm or 7.5 mg / m.sup.3. With the additive combination as shown in

Problems solved by technology

This in itself is a significant factor, since the prior art additives, generally insoluble or difficultly soluble, must be added in amounts substantially greater than the stoichiometric amounts in order to obtain the desired neutralization but by the same token increasing the build-up which is so undesirable.

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