EP2076466A2

EP2076466A2 - Method for purification of wastewater containing sulphur and nitrogen containing compounds

Info

Publication number: EP2076466A2
Application number: EP07825159A
Authority: EP
Inventors: Niels Ole Knudsen
Original assignee: Dong Energy Power AS
Current assignee: Dong Energy Power AS
Priority date: 2006-09-25
Filing date: 2007-09-21
Publication date: 2009-07-08
Also published as: WO2008038089A2; WO2008038089A3; WO2008038089B1

Abstract

The present invention relates to a method for elimination or reduction of sulphur and nitrogen compounds from wastewater, including particularly desulphurization wastewater from power plants by exposing said wastewater to acid hydrolysis under specific reaction conditions and simultaneously or subsequently to add a nitrite salt.

Description

METHOD FOR PURIFICATION OF WASTEWATER FOR SULPHUR AND NITROGEN CONTAINING COMPOUNDS

INTRODUCTION

Power plants fired with fossil fuel and/or biomass represent today a considerable part of the production of electricity and heat of the world. Sulphur, which in small amounts are common in oil and carbon, has for decades been recognized as the biggest source for emission of sulphur dioxide, which is one of the main reasons for "acid precipitation" and one of the most important reasons for air pollution in many urban areas as well as industrial areas. Purification of flue gas caused by the combustion is therefore of decisive importance for providing an environmentally desirable and stable energy production. The vast majority of combustion plants nowadays are thus equipped to absorb i.a. sulphur and chlorine compounds from flue gasses (desulphurization). However, in connection with a number of desulphurization processes used today substantial amounts of desulphurization water are formed containing i.a. chloride, fly ash and not least heavily degradable sulphur and nitrogen compounds (nitrilomono, nitrilodi and nitrilotri acids), also called NS compounds. Different methods for purification of such NS compounds in wastewater purifying plants - and thereby the possibility of i.a. recycling of the desulphurization wastewater - have earlier been described. However, the present invention provides a hitherto unknown, simple, economically profitable and industrially applicable method for elimination or reduction of NS compounds from desulphurization wastewater.

FIELD OF THE INVENTION

The present invention relates to the field of wastewater purification, including particularly methods for purification of desulphurization wastewater from power plants, which are peculiar in that the wastewater is subject to acid hydrolysis and addition of nitrite salt (NO₂ ^")-

BACKGROUND OF THE INVENTION

Sulphur dioxide emission contributes substantially to acidification of the external environment and has furthermore a direct impact on the health of humans and animals. Moreover, acidification and sulphur dioxide in the atmosphere can damage sensitive ecosystems, deteriorate the biodiversity and also have a negative impact on production of crops, growth of woods etc. Acid precipitation in the cities may also cause considerable damages on both modern as well as historical buildings.

Therefore, a still increasing part of today's power plants are equipped with devices for filtration and/or absorption of i.a. sulphur compounds contained in the flue gas, of which gypsum producing wet desulphurization plants are the most widespread. Due to the considerable formation of desulphurization wastewater in these plants, the main part of the power plants are nowadays equipped with specific wastewater purification plants. Hitherto known techniques for purification of desulphurization wastewater have, however, not been capable of eliminating or reducing, in an efficient way, the heavily degradable fraction of NS compounds.

DISCLOSURE OF PRIOR ART

Different types of treatment of desulphurization wastewater and corresponding contaminated types of wastewater have previously been described in the literature.

Thus, Buchheister F. et al., Chem. Eng. Technol. 23, (2000) describe a method for biological removal of nitrogen (denitrifϊcation) including particularly nitrogen compounds in the form of nitrate and nitrite, respectively (NO₃ ^" and NO₂ ^" respectively) from worn-out electrolyte solutions (wastewater) from electrochemical deburring processes. Due to the high content of inorganic compounds of the wastewater, the denitrification requires an external carbon source, including e.g. acetic acid, methanol etc. By use of the disclosed method a stable denitrification can only be achieved after dilution of the wastewater by adding e.g. non-toxic wastewater. Acid hydrolysis of nitrite (NO₂ ^") from wastewater by use of amidosulphonic acid is described as one possible embodiment. Thus, the purification method described in Buchheister F. et al. is based on a fundamentally different technology (i.a. comprising biological treatment and addition of external carbon sources) than the one according to the present invention and describes moreover neither a) the combination according to the present invention of acid hydrolysis under the specified reaction conditions and the addition of nitrite nor b) that the method can be used specifically for purification of desulphurization wastewater containing NS compounds.

Littlejohn D. and Chang S-G., Ind. Eng. Chem. Res., vol. 33(3) (1994) describe a method for oxidative decomposition of nitrogen sulphonates (the NS compound HONH(SO₃ )) from desulphurization liquids by treatment with the oxidation agents ozone (O₃), hydrogen peroxide

(H₂O₂) and nitrogen dioxide (NO₂), of which ozone proved superior in connection with the oxidation of both the nitrogen and sulphur compounds. The observed oxidation products comprised sulphate (SO₄ ^2"), nitrate (NO₃ ^"), nitrite (NO₂ ^") and possibly other nitrogen containing products. Consequently, the method according to the Littlejohn D. and Chang S-G article differs basically from the method according to the present invention, i.e. by using oxidative reactions. The method according to the article does not either relate to elimination or reduction of NS compounds according to the present invention, but relates on the contrary to a method for decomposition of nitrogen sulphonates to other sulphur and nitrogen containing compounds (oxidation products) including sulphate (SO₄ ^2"), nitrate (NO₃ ^") and nitrite (NO₂ ^"). These oxidation products cannot be said to be monosulphonated compounds corresponding to sulphamic acid (H₃NSO₃). Nor does it appear from the article that the oxidation products can be removed by addition of nitrite salt or compounds corresponding thereto. Finally, it appears explicitly from this article that the mentioned oxidation processes proceed optimally under alkaline conditions - in contrast to the method according to the present invention, which combines acid hydrolysis with addition of nitrite salt for purification of desulphurization wastewater.

Randolph et al. (1997) US 5,618,511 describe a method for regeneration of NS compounds from desulphurization wastewater in the form of the reaction product ammonium sulphate (NHLj)₂SO₄, which is provided in commercial quantity and quality. The method is to be precise a process involving several steps, whereby nitrogen and sulphur oxides in the desulphurization wastewater are converted into NS compounds via EDTA- (ethylenediaminetetraacetic acid, addition of iron and calcium. The NS compounds are hereafter precipitated as NS-containing calcium salts (in the form of slurries) and are subsequently hydrolyzed in boiling sulphuric acid (H₂SO₄). Hereafter the NS compounds are precipitated and can - after a further number of process steps, i.a. boiling with H₂SO₄ - be converted (regeneration) into ammonium sulphate (NH_t)₂SO₄. The mentioned reactions are therefore in contrast to the hydrolysis according to the present invention, which is performed under well-defined mild reaction conditions. The regeneration of the NS compounds in US 5,618,511 cannot either be analogized with elimination or reduction of NS compounds from desulphurization wastewater in the sense of the present invention.

Shen CH. and Rochelle G.T., Sci. TechnoL, vol. 32 (1998) describe i.a. the removal of nitrogen oxide (NO) from flue gas by oxidation of NO into nitrogen dioxide (NO₂) followed by absorption in sulphite containing (SO₃ ^2") limestone slurries (absorbent). The article discloses how NO₂ is exposed to acid hydrolysis and furthermore that NO₂ in the absorbent initiates the oxidation of sulphite in said slurry in the presence of oxygen. The method described in the articles is thus directed to removal by absorption of NO_x. and S0₂_ compounds in sulphite containing limestone slurries and thereby not distinctly elimination or reduction of NS compounds from desulphurization wastewater. Nowhere in the article it is described that the disclosed method is applicable in connection with removal of NS compounds from desulphurization wastewater. Furthermore, the method described in the article - contrary to the method according to the present invention - is based on oxidation of sulphite in limestone slurries in the presence of absorbed NO₂. Neither does it appear from the article that the possibly resulting monosulphonated compounds can be removed by addition of nitrite salt or equivalent compounds, as is the case by the method according to the present invention.

Wilburn R.T. and Wright T.L., PowerPlant Chemistry, vol. 6(5) (2004) describe selective catalytic reductions (SCR) of nitrogen oxides (NO_x) from desulphurization wastewater by reaction with ammonia (NH₃) and catalyst for providing simple nitrogen gas and water. Furthermore, the formation of ammonium (NH₄ ⁺) deposits provided by SCR is described as well as the ammonia provided by SCR and the distribution and dependence of the latter of the sulphur trioxide (SO₃) concentration. Thus, the document describes a technology based on ammonia for purification of nitrogen oxides from i.a. desulphurization wastewater. Ammonia is converted to aqueous solution or by reaction with acid immediately to ammonium (NH₄ ⁺), which subsequently forms ion pair with e.g. sulphate - being a primary reaction product in the desulphurization process - and thus forms ammonium sulphate ((NfL_t)₂SO₄). This is in contrast to the method according to the present invention, where the acid hydrolysis, being performed under specific, mild reaction conditions, secures the formation of sulphamic acid (H₃HSO₃) or corresponding monosulphonated compounds minimize the formation of ammonium sulphate ((NFL_t)₂SO₄).

WO2006/030398 describes i.a. a method for treatment of desulphurization wastewater, in which the decomposition of NS compounds occurs by spray evaporation at 140-150⁰C, optionally combined with acid hydrolysis. The process for decomposition of the NS compounds provides sulphate, whereas nitrogen oxides (NO_x) or free nitrogen gas (N₂) are provided by the nitrogen fraction of the NS compounds. The thus formed sulphates are precipitated upon redissolving as gypsum due to the high content of calcium ions in the desulphurization water. The acid sulphates formed according to the process of WO2006/030398 are monosulphonated reaction products and can possibly be analogized with the "sulphamic acid (H₃NSO₃) or corresponding monosulphonated compounds" formed according to the present invention. The reaction conditions, including particularly the temperature by which the mentioned sulphates are provided (140-150⁰C), are, however, not comparable with the reaction conditions of a method according to the present invention. Furthermore, it appears nowhere from WO2006/030398 that the monosulphonated reaction products can be degraded by addition of nitrite salt or equivalent compounds.

DESCRIPTION OF THE INVENTION

The present invention relates to a method for elimination or reduction of NS compounds from wastewater and is peculiar in that a) the wastewater is hydrolyzed under acid conditions and B) the thereby formed monosulphonated compounds are degraded by addition of a nitrite salt. The acid hydrolysis according to the present invention is thus performed under conditions, where the NS compounds are converted to sulphamic acid (H₃NSO₃) or corresponding monosulphonated compounds - in other words under so mild conditions that the sulphamic acid is, to a great extent, not hydrolyzed to ammonium sulphate ((NEU)₂SO₄), but still so strong conditions that the main part of the disulphonated compounds are removed. This process step is preferably performed at pH under 4, including particularly approx. pH 1, and temperatures below 100⁰C, including especially approx. 55⁰C.

Addition of nitrite salt, which can be made simultaneously or as a subsequent process step, eliminates or reduces the sulphamic acids or corresponding monosulphonated compounds formed by the acid hydrolysis.

DETAILED DESCRIPTION OF THE INVENTION

As will be well known to the skilled person, one of the fundamental components in desulphurization plants is the so-called absorber, into which flue gas, absorbent, water and air (oxygen) are fed. In the absorber the flue gas is thus brought into contact with a suspension of absorbents, including lime, calcium carbonate, limestone, TASP or corresponding compounds, whereby the flue gas is saturated with water. Hereafter, the absorber suspension is cooled to the adiabatic saturation temperature of scarcely 50° C.

During the contact between flue gas and the absorbent suspension SO₂ and HCl are transferred from the flue gas to the absorbent suspension (desulphurization). The gypsum thereby produced, which contains the desulphurization products SO₂ and HCl, can e.g. "be transported" from the desulphurization plant by taking out a stream from the absorber slurry, which is subsequently dewatered. Furthermore, the produced gypsum is usually washed, i.e. to reduce the content of chloride.

The water from the dewatering and the gypsum washing is typically returned to the absorber slurry, as, however, a minor partial stream from the dewatering can be taken out in order to maintain the content of particularly chloride but also fly ash components at an acceptable low level. This partial stream is lead to wastewater purification, where a precipitation of heavy metals occurs after adjustment of pH. Besides chloride and fly ash this wastewater contains a number of heavily degradable sulphur and nitrogen compounds, including nitrilomono, nitrilodi and nitrilotri acids, also called NS compounds. These NS compounds are partly derived from the reaction between gaseous NO₂ and sulphite salts in the absorber, but can also be added to the absorber with the absorbent, provided that TASP is used as absorbent. By use of TASP, containing in itself up to 1% NS compounds, the content of NS compounds can, however, be remarkably higher.

The method for elimination or reduction of NS compounds from wastewater, including particularly desulphurization wastewater according to the present invention is based on the wastewater being hydrolyzed under acidic conditions and the monosulphonated compounds thereby produced, including sulphamic acid being eliminated or reduced by subsequent or simultaneous addition of a nitrite salt. Therefore, it is decisive that the reaction conditions in respect of the acid hydrolysis are so that the NS compounds to a main extent are converted to sulphamic acid or corresponding monosulphonated compounds - i.e. so mild conditions that the sulphamic acid are to a main extent not hydrolyzed to ammonium sulphate ((NfL_t)₂SO₄). At the same time the acid hydrolysis must be performed under reaction conditions enabling that the main part of the disulphonated compounds are removed. Addition of nitrite salt (NO₂ ^"), which can take place simultaneously or as a subsequent process step in the method according to the present invention, eliminates or reduces the sulphamic acids or corresponding monosulphonated compounds formed by the acid hydrolysis.

The acid hydrolysis according to the present invention is provided by adding an acid (preferably mineral acid), including e.g. sulphuric acid, hydrochloric acid or corresponding acids to the given wastewater fraction. The acid used for the hydrolysis can also be produced by an autocatalytic hydrolysis process, i.e. no acid is added from outside.

Thus, methods according to the present invention can be performed at pH values between 0.2-6.0, preferably between 0.5-5.0, preferably between 0.7-4.0, more preferably between 0.9-3.0 and most preferably at pH 1.0.

In order to avoid formation OfNO₂ during the acid hydrolysis of the wastewater the present invention can furthermore be performed in such a way that the pH value is increased to between pH 1 and 5, preferably pH 1.5 to 3 in the process step between the acid hydrolysis step and the addition of nitrite salt.

Furthermore, methods according to the present invention can be carried out at temperatures between 20-100⁰C, i.e. between 40-65⁰C, preferably between 45-6O⁰C, more preferably between 48-57⁰C and most preferably at 55⁰C. The method according to the present invention is moreover peculiar in that the acid hydrolysis and the addition of nitrite salt can be carried out as a batch process, i.e. in the same reactor, in a continuous process or combinations thereof. In an alternative embodiment the acid hydrolysis and the addition of nitrite salt are performed in two or several separate reactors.

The acid hydrolysis of the desulphurization wastewater according to the present invention can in an alternative embodiment be combined with ozone, hydrogen peroxide or corresponding oxidation agents.

EXAMPLES

Example 1 It has turned out that there is a connection between the precipitation of gypsum (the gypsum production) and the elimination of NS compounds in relation to desulphurization, which has been exposed to a method according to the present invention. The production of gypsum is therefore a true estimate for the actual elimination or reduction of NS compounds in desulphurization wastewater being acid hydrolyzed and to which nitrite salt is added. Thus, Figure 1 shows the gypsum production by treatment of desulphurization wastewater according to the method of the present invention. The gypsum production by acid hydrolysis (curve with 0- symbolism) and by addition of nitrite salt (curve with B-symbolism) is depicted. The hydrolysis is performed at pH 1 and 55⁰C. The curve with 0-symbolism shows i.a. that convertion to monosulphonated compounds, including sulphamic acid reaches its maximum after approx. 15-17 minutes (corresponding to the production of 15-17 grams of gypsum per kg wastewater), whereafter the conversion decreases. After this period of time nitrite salt is added to remove the monosulphonated compounds produced, including sulphamic acid (curve with ..-symbolism). Addition of nitrite salt provides a further production of gypsum of 8-14 gram per litre wastewater. Thus, totally seen approx. 30 grams of gypsum per litre wastewater is removed (curve with A- symbolism).

Example 2

This example is performed as above, however, with simultaneous addition of acid (acid hydrolysis) and nitrite salt. The same amount of gypsum is produced, Le. approx. 30 gram gypsum per litre wastewater. References Buchheister F. et al., Chem. Eng. Technol. 23, vol. 11(2000)

Littlejohn D. and Chang S-G., Ind. Eng. Chem. Res., vol. 33(3), p. 515-518 (1994)

Randolph et al. (1997) US 5,618,511

Shen CH. and Rochelle G. T., Environ. Sci. Technol., vol. 32, p. 1994-2003 (1998)

Wilburn R.T. and Wright T. L., PowerPlant Chemistry, vol. 6(5), p. 295-304 (2004) WO2006/030398

Claims

1. A method for elimination or reduction of sulphur and nitrogen containing compounds from wastewater, characterized in that: a) the wastewater is subject to acid hydrolysis and b) a nitrite salt is added

2. Method according to claim 1, c harac t erize d in that the sulphur and nitrogen containing wastewater is desulphurization wastewater from a desulphurization plant.

3. Method according to claim 1, characterized in that process step a) is performed before process step b).

4. Method according to claim 1, characterized in that process step b) is performed before process step a).

5. Method according to claim 1, characterized in that the acid hydrolysis is provided by addition of a preferably inorganic acid and/or is provided by an autocatalytic hydrolysis process.

6. Method according to claim 1, c h a r a c t e r i z e d in that sulphamic acids or monosulphonated compounds corresponding thereto are formed by the acid hydrolysis.

7. Method according to claim 1, charac terized in that process steps a) and b) are performed as a continuous process, a batch process or combinations thereof.

8. Method according to claim 1, charac teriz ed in that process steps a) and b) are performed parallelly (simultaneously) or in series (after each other) in the same or different reactors.

9. Method according to claim 1, c harac terized in that process step a) and b) are performed in two or several separate reactors.

10. Method according to claim ^characterized in that the temperature by which process steps a) and b) are performed lies in the interval of 20-100⁰C.

11. Method according to claim 1, c h ara c t e r i z e d in that the pH value of the acid hydrolysis in step a) is between 0.2 and 6.0.

12. Method according to claim 1, characterized in that the pH value is raised to between pH 1 and 5 between step a) and step b).

13. Method according to claim ^characterized in that addition of ozone, hydrogen peroxide or corresponding oxidation agents are added to the wastewater in connection with step a).

14. Method according to claim 1, characterized in that steps a) and b) are performed as a partial stream purification.