In traditional industrial boilers and low-pressure power boilers, sodium sulfite is mainly added for chemical oxygen removal.
The deoxygenation ability of sodium sulfite was discovered by 1920, and it was widely used in chemical deoxygenation of power plants by 193 1. The reaction equation of sodium sulfite and oxygen is:
2Na2SO3 Ten O 2 → 2Na2SO4
Sodium sulfite is a traditional deoxidizer for boiler water, which has the advantages of low price and wide sources. However, there are obvious disadvantages: the reaction speed of sodium sulfite with oxygen is affected by PH value, temperature and catalyst, and it usually needs to be added in excess to cope with the fluctuation of boiler operation; According to the reaction formula of sodium sulfite and oxygen, at least 7.9ppm of sodium sulfite is consumed to remove 1ppm of oxygen. In order to make this reaction more thorough, it is usually necessary to maintain an excess of 20~40ppm in boiler water to ensure the oxygen removal effect. Because sodium sulfite reacts with oxygen to produce stable sodium sulfate, which increases the soluble solids in boiler water and deteriorates the water quality, the boiler must increase the number of blowdowns, leading to the waste of chemicals and the increase of fuel cost; When the working pressure of the boiler is higher than 6.2MPa, sodium sulfite will decompose to produce corrosive hydrogen sulfide and sulfur dioxide, which will be discharged with steam and cause corrosion to subsequent equipment:
Na2SO3 decahydrate 2H 2O → 2NaOH decahydrate sulfuric acid 3
H2SO4 → H2O → SO2
Moreover, sodium sulfite itself may undergo redox reaction to produce sodium sulfate and sodium sulfide;
Na2SO3 → 3Na2SO4 Ten Na2S
The produced sulfur dioxide and sodium sulfide are corrosive, so using sodium sulfite as deoxidizer is actually a kind of corrosion, not another kind; In addition, when the feed water containing sodium sulfite is used as desuperheating water and superheated steam is injected to adjust the temperature, sodium sulfate and other salts will be deposited in the superheated steam main pipe and steam turbine; Sodium sulfite has no passivation protection for metals.
2. Diammonium (N2H4)
With the emergence of large-capacity units and high-pressure boilers, sodium sulfite was gradually replaced by hydrazine (also known as hydrazine hydrate) in the 1950s and 1960s. The reaction formula between hydrazine and oxygen is:
N2H4 ten O2 → N2ten 2H 2O
Hydrazine reacts with oxygen to produce nitrogen and water, and excessive hydrazine will not produce soluble solids. Ammonia can improve the PH value of boiler water, which is beneficial to boiler protection. Hydrazine has corrosion inhibition effect. Hydrazine reacts with corrosion products of iron and copper to form Fe3O4 and Cu2O layers with passivation protection.
The final products of hydrazine reaction with oxygen and metal oxides are water and nitrogen, which will not increase the amount of dissolved solids in boiler water. The decomposition products of hydrazine are volatile gases.
However, hydrazine's deoxygenation efficiency is not as good as sodium sulfite's, and its deoxygenation speed is slower when the water temperature is lower. Only at higher temperature can it react effectively with oxygen to achieve the purpose of deoxygenation. The decomposition temperature is very high. At 3 16℃(9.8Mpa), hydrazine still enters the steam, and its toxicity makes the steam unable to be directly used in life. In particular, hydrazine is a highly toxic substance, which is easy to splash on eyes, skin or clothes during operation, and is easy to be inhaled, which brings serious harm to the operator's body and mind. And volatile, flammable and explosive. When the concentration of steam in the air reaches 4.7%, it will explode in case of fire, which will bring trouble to transportation, storage and use. Hydrazine is regarded as a suspected carcinogen and classified as a dangerous product by the Occupational Protection and Health Care Act (OSHA) of the United States. Direct contact between hydrazine and food has been banned, and countries such as Europe, America and Japan have also abandoned hydrazine and developed and applied new boiler water deoxidizers.
3. New deoxidizer
For the sake of health and safety, and in order to eliminate the disadvantages of using sodium sulfite and hydrazine in deoxygenation speed and efficiency, some new deoxidizers have been developed abroad. The new deoxidizer must have the characteristics of high deoxidization degree, high deoxidization speed, non-toxic or low toxicity, wide application range, convenient use and appropriate cost. Here is a brief introduction to some new deoxidizers developed abroad.
(1) hydroxylamine.
Hydroxylamine and its derivatives, such as diethylhydroxylamine, can be used as deoxidizers for boiler water, which is a patent published by Chemed Company in the United States in 1978. The final products of the reaction with oxygen are acetate, nitrogen and water. Cu2+ and hydroquinone can play a catalytic role, and the reaction speed is slightly faster than hydrazine.
2 carbohydrazide (carbohydrazide)
Carbonphthalazine, also known as diaminourea, is a derivative of diazonium. It is a patent published by Narko Chemical Company in the United States at 198 1. It is superior to hydrazine and hydroquinone in deoxidation effect and metal purification, and can play a catalytic role.
(3) hydroquinone.
Hydroquinone, as a deoxidizer of boiler water, is a patent published by Bates Experimental Company of the United States in 1980. Hydroquinone reacts with oxygen to produce hydrogen peroxide, and then quinone is further oxidized.
(4) dihydroxyacetone (1, 3- dihydroacetone)
1, 3- dihydroxyacetone is a patent published by Narko Chemical Company in the United States in 1982. Its reaction with oxygen can be catalyzed by benzoquinone and manganese.
5. Isoascorbic acid (isoascorbic acid)
Isoascorbic acid as an oxygen scavenger is a patent published by Narko Chemical Company in the United States at 198 1. It is an isomorphic substance of vitamin C(L- ascorbic acid), and its reaction with dissolved oxygen is very complicated, because it needs several intermediate steps, so its mechanism is not completely clear. Because of its safety, it is widely used as deoxidizer in food and feed, and it is used as boiler water in China. The decomposition at high temperature produces corrosive dissolved solids (data show that the decomposition products are 7 1.07% lactic acid, 20.48% acetate and 8.44% formate at 300℃); Only oxygen can be removed without passivation.
[6] Aminoguanidine compound (aminoguanidine)
Aminoguanidine compounds used as oxygen scavengers are patents published in 1984 by American Olin Company. They are nonvolatile derivatives of hydrazine.
(7) Oximeter: See the following instructions for details.
(8) Others: New deoxidants such as N- tetra-substituted phenylenediamine, n- isopropyl hydroxylamine and ethoxyquinoline have been developed abroad, but they are still under further research and practice, and there are few domestic studies.
4. Oxime deoxidizer
Oxime compounds (mainly dimethyl ketoxime, butanone oxime and acetaldehyde oxime), as a new type of deoxidizer, are patents published by Drew Chemical Company in the United States in 1984. They have low toxicity, high efficiency, high speed and passivation protection. Nolco Company (the world's largest water treatment company) and Drew Company of the United States have products of oxime boiler water deoxidizer, which have been widely used in developed countries such as Europe, America and Japan.
(1) Deoxygenation performance: Oxime compounds are organic compounds with oxime groups (C═N-OH). At present, the oximes used for boiler deoxidation and shutdown protection mainly include acetaldehyde oxime, dimethyl ketoxime (acetone oxime) and methyl ethyl ketoxime. Oxime compounds are highly reductive and easily react with oxygen.
Oxime compounds have good oxygen removal performance in a wide temperature and pressure range, and the most suitable temperature range is 138~336℃ and pressure range is 0.3~ 13.7Mpa. According to comparative experiments, under the same conditions, the oxygen removal speed and efficiency of oxime compounds are higher than hydrazine.
⑵ Corrosion inhibition and passivation: Oxime compounds can reduce oxides of high valence iron and copper to low valence oxides, and their aqueous solution can form a good magnetic oxide film on the steel surface, which has a good passivation and corrosion inhibition effect on the metal surface. Among them, dimethyl ketoxime has the best effect and the least dosage.
Comparative experiments show that oxime compounds have the same passivation and corrosion inhibition effect as hydrazine, which can significantly reduce the iron content in the solution and protect steel under high temperature and high pressure, among which dimethyl ketoxime has the best effect and the least dosage. At the same time, oxime compounds can clean the corrosion products of copper deposited in pipelines, economizers and so on. This is also the reason why the copper content in boiler water increased obviously at the initial stage of using oxime compounds.
⑶ Volatility: Oxime compounds are more volatile than hydrazine, DEHA, morpholine and cyclohexylamine. And is close to the volatility of NH3. When steam condenses, a certain amount of highly volatile deoxidizer will be dissolved in condensate water, which is beneficial to protect the metal materials of condensate system.
⑷ Decomposition: According to the decomposition experiment under high temperature and high pressure, the decomposition products of oxime compounds are NH3, N2, H2O and trace acetic acid, and no formic acid is produced, which has no adverse effect on the water vapor system.
5. Low toxicity: According to the comparison of LD50 data, the LD50 of hydrazine is 290mg/kg, acetaldehyde oxime is 1900mg/kg, methyl ethyl ketone oxime is 2800mg/kg, and dimethyl ketone oxime is 5500mg/kg. It can be seen that hydrazine is very toxic, while oxime compounds are very toxic and belong to low toxic compounds.
In a word, the research, development and application of new deoxidizers in China are paid more and more attention by scientific research units, manufacturers and users. In particular, the successful popularization and application of oximes such as dimethylketoxime and acetaldehyde oxime, and the application of erythorbic acid to a certain extent have achieved good results, making the use of new boiler water deoxidizers in China similar to those in developed countries in Europe, America and Japan.
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# Chemistry # Deoxidation