WO2006030398A1

WO2006030398A1 - Method of operating a flue gas cleaning plant

Info

Publication number: WO2006030398A1
Application number: PCT/IB2005/053042
Authority: WO
Inventors: Niels Ole Knudsen
Original assignee: Elsam Engineering A/S
Priority date: 2004-09-16
Filing date: 2005-09-15
Publication date: 2006-03-23

Abstract

In a flue gas cleaning plant (wet desulfurising plant), In which flue gas from a plant fired with fossil fuel, preferably coal, and/or biomass fuel is cleaned by passing through an air preheater, where it is cooled, an electrofilter or baghouse filter for removing particles and an absorber, where air, water, and lime, limestone or SDAP are added, in which absorber the sulfur is separated out as gypsum which is dewatered and washed free of chloride, a part of the heated air from the air preheater is used for the evaporation of the water from the dewatering,. Hereby, the excess of hot air from the air preheater is utilised, and simultaneously the chloride salts of the waste water can be utilized as de-icing salt.. Furthermore, the waste water content of SN compounds (sulfamic acid derivatives) is degraded during the evaporation.

Description

METHOD OF OPERATING A FLUE GAS CLEANING PLANT

FIELD OF THE INVENTION

The invention relates to a method of operating a flue gas cleaning plant (wet desulfurising plant), in which flue gas from a plant fired with fossil and/or biomass fuel is cleaned by passing through an air preheater, where it is cooled, an electrofilter or baghouse filter for removing particles, and an absorber, where air, water and an absorbent (lime, limestone or SDAP) are added (SDAP is the acronym for Spray Dry Absorption Product, a product which appears by desulfurising with Ca(OH)₂ in a spray absorber. It contains about 50-80% CaSO₃ and CaSO₄ as well as about 20-50 % Ca(OH)₂ and CaCO₃). In the absorber, the sulfur is separated out as gypsum which is separated and washed free of chloride.

DISCLOSURE OF PRIQR ART

Coal fired power plants today represent a very substantial part of the world's production of electricity. Since coal contains chlorine and sulfur, power plants contribute to the acidifying of the environment

Desulfurising plants

Therefore, an increasing part of the power plants are provided with desulfurising plants, where gypsum producing wet desulfurising plants are the most widespread.

As an example of the present art, the desulfurising plant of NJV3 (block 3 on the power station of Nordjyllandsvasrket) can be mentioned.

The heart of the plant is the absorber, into which flue gas, absorbent, water, and air (oxygen) are fed. However, the flue gas is first passed through an electrofilter for removing particles. Before the flue gas coming from the electrofilter with a temperature of typically 110-140⁰C enters the absorber, this is heat exchanged with the cleaned outlet gas from the absorber in a regenerative heat exchanger (gas preheater). Hereby the temperature of the flue gas is lowered, typically by well above 20⁰C, and the cleaned flue gas is reheated correspondingly. In the absorber, the cooled flue gas is contacted with a suspension of limestone, whereby the flue gas is saturated with water and cooled to the adiabatic saturation temperature of just under 50⁰C which is the typical absorber temperature.

The limestone suspension is introduced together with the flue gas in the top and falls down in the absorber sump - the contact is made more effective by the use of tower packings (grid). During the contact between the flue gas and the limestone suspension, SO₂ is transferred from the flue gas to the limestone suspension, whereby the flue gas is cleaned for its contents of acid components. In the wet desulfurising plant, SO₂ is converted to gypsum which can be dewatered and sold, whereas HCI after neutralisation with limestone is converted to a weak calcium chloride solution which till now has been taken out continuously from the plant as waste water. Further, after the contact the flue gas passes several drip catchers, before it is conducted through the gas preheater to the stack.

However, the electrofilter works best when the temperature of the flue gas is not too high. Therefore, the temperature is first lowered in an air preheater. In the air preheater, the flue gas is cooled down from about 373°C to about 121⁰C, cf. the scheme under Operating data", and coincidentally the combustion air is heated from about 33⁰C to about 361⁰C.

The European patent publication No. 1 ,075,627 (Alstom) teaches that the lowering of the temperature of the flue gas to a desired value for the function of the electrofilter requires a larger amount of air than the amount to be used for the combustion and therefore, air is taken out from the air preheater. The air taken out is used, for example, to reheat the saturated stack gas from the flue gas scrubber to above the dew point to reduce the visibility of the plume of smoke and to help the buoyancy.

Air preheater An air preheater may be a rotating regenerative air preheater based on the heat transmission occurring between air and the material of the rotating heat exchanger. At NJV3 it is constructed relatively complicated, consisting of 4 chambers, see figure 2.

However, also air preheaters with two chambers do exist. A general layout problem by air preheaters is that the ratio between the heat capacities for flue gas and fresh-air is about 1.2, which often makes it difficult to maintain a sufficiently low flue gas temperature (cf. the above mentioned European patent publication No. 1,075,627). Additional removal of hot secondary air allows increasing the temperature for fresh-air intake and maintaining the temperature of the flue gas or maintaining the temperature of the fresh-air and coincidentally lowering the temperature of the flue gas. This is illustrated in the example below.

Waste water from washing of gypsum

The produced gypsum is taken out from the plant by taking out a stream from the absorber sump and sending it to dewatering.. Here it occurs after preseparation in a hydrocyclone on a vacuum band filter, but in many places centrifuges are used instead. The produced gypsum is normally washed in order to obtain the desired low content of chloride.

The water from the dewatering and the washing of the gypsum is returned to the absorber sump, yet a minor partial stream from the dewatering being taken out to maintain the content of in particular chloride, but also fly ash components, at an acceptable level. This partial stream is conducted to waste water cleaning, where the pH is adjusted, and a precipitation of heavy metals occurs.

The outlet of this waste water is inappropriate for many reasons, in particular because of the content of calcium chloride and SN compounds:

• At domestic power plants, the waste water is often discharged to rivers, whereby the contents of salts and SN compounds are increased, which is inappropriate in relation to fauna, drinking water supply, etc.

• At coastal based plants, SN compounds and other nitrogen compounds are discharged.

• By outlet through municipal cleaning plants, the content of salts and their variations may impede the biological activity in the cleaning plants. Table 1 : SN compounds

NTS HATS

HO₃S HO₃S N SO₃H NO-SO₃H HO₃S HO₃S

IDS HADS

AS HAODS

HAMS

HAOMS

1 ) Gutberlet et al,: Bildung von Schwefel- und Schwefel-Stickstoff- Verbindungen in Rauchgasentschwefelungsanlageπ und ihr Einfluβ auf Oxidationskinetik von Sulfit, VGB Kraftwerkstechnik 76 (1996), Heft 2

ESV and ENV are Esbjergvaerket and Enstedvεerket, where the SN compounds were found. They occur in all wet desulfurising plants. At waste combustion plants, evaporators based on spray evaporators or the like are known. These employ hot, uncleaned flue gas for the evaporation, which gives a residual product mainly consisting of fly ash and thus suited for deposition only

DESCRIPTION OF THE INVENTION

It appears now that the hot air removed from the air preheater can be used for the evaporation of the waste water from the flue gas cleaning process.

Thus, the method of the invention is characterised in that a part of the heated air from the air preheater is used for the evaporation of waste water from the gypsum dewatering.

The soluble part of the evaporation brine consists of >95% of a mixture of CaCI₂,

MgCI₂, and NaCl. It is known to use such brines for thawing purposes, but it has not earlier been described that they are prepared by spray evaporation of waste water from desulfurising.

Liquid de-icing salt

The invention relates also to a method for the preparation of de-icing salt, by which the waste water from the dewatering of gypsum is evaporated.

By evaporation of the waste water in a spray evaporator, a dry powder is obtained which is very hygroscopic because of the content of CaCI₂. This powder is, therefore, difficult to store. Since CaCI₂ for thawing purposes is used primarily as liquid solutions, it might be suitable to store the de-icing salt in liquid state. Therefore, the largest evaporation capacity is obtained by evaporating 80-90% (determined by a mass balance, such that the desired final concentration is obtained) of the waste water to a dry powder and immediately thereafter to redissolve the powder in a bypass-stream of waste water. Hereby, both higher capacity and easier handling are obtained.

Since repeated tests have shown that the produced liquid de-icing salt may have a pH down to about 1 (which also is in accordance with the above stipulated reaction mechanism), it is preferred to add NaOH or another appropriate base to obtain a suitable pH about 8 in the final brine.

The repeated tests have shown that up to 25% of the dry powder from the spray evaporator may consist of gypsum (in particular when the SN content is high), as well as minor amounts of fly ash from the air preheater air. This has implied that we had to reconstruct the final production plant such that the liquid de-icing salt is filtered through a band filter to remove suspended gypsum and fly ash from the de-icing salt.

The idea of the above is in part the favourable energy economy by using secondary air from air preheater as mentioned previously. In a preferred embodiment, 3-10% hot air are drawn out from the air preheater in proportion to the amount of flue gas. As it appears from example 2 under Operating data", it is a question of a very cheap energy supply, since it is possible to take out up to 3 times the amount of heat in proportion to necessary additional firing of additional coal/energy. Furthermore, it is obtained that discharged air can be used for reheating the flue gas, so that fall-out of drops may be avoided (cf. the above mentioned European patent publication No. 1,075,627).

Recirculation of waste water

The determination of which part of waste water to be evaporated, and which part to be recirculated, can for example be carried out by measuring the conductance of the waste water. A concentration of chloride of 2-5% in the waste water is preferred.

By low-chloride coal there is only a limited need for removing waste water to maintain the low chloride concentration. In this case, it is necessary to increase the removal of waste water to keep the amount of other impurities low, such as fly ash, silica, CaF₂ etc. It may be suitable to recirculate waste water which is cleaned for the above mentioned components, to the absorber to diminish the amount of waste water to be evaporated..

The invention relates further to a method of decreasing the content of SN compounds in water from dewatering of gypsum, and this method is characterised in that the water from the dewatering is evaporated. As it appears from example 1, SN compounds are degraded in spray evaporators.. Evaporation of waste water is therefore particularly useful in combination with the use of SDAP, since the waste water by the use of SDAP can contain an amount of SN compounds up to 3 times as large as by using limestone.

The SN compounds are degraded by thermic decomposition by the temperature about 140 - 150⁰C occurring during the evaporation and in the particle separator of the evaporation plant, probably combined with acid hydrolysis. The components recovered in the evaporated waste water are acid sulfates which by redissolving are precipitated as gypsum due to the high content of calcium ions in the waste water. Thus, the vigorous precipitation of gypsum occurring during the evaporation is caused by the conversion of SN compound into sulfate. The nitrogen in the SN compounds ends up either as NO_x or as free N₂-

The mechanism for the decomposition is not known, and its explanation is impeded by the occurrence of a series of different SN compounds with their respective reaction paths. A possible decomposition path for HATS (see the formula in table 1 ) might be:

5 Na₃HATS + 7 H₂O + NaNO₃ + heat ^■» 14 NaHSO₄ + Na₂SO₄ + 3 N₂

This equation is in accordance with the fact that the best decomposition is obtained in the waste water (test 2) where the content of nitrate in the waste water is highest. By redissolving is obtained:

2 NaHSO₄ + CaCI₂ -» CaSO₄ + 2 HCI

According to another embodiment of the invention, a certain upconcentration of the waste water is first carried out in a conventional evaporator and then the concentrate from here can be evaporated in a spray evaporator.

in a few places in the world, the evaporation of waste water is carried out in multiple step evaporators, but it is a very expensive method, both with regard to plant and to operation because of the steam consumption, and because there has to be used exotic materials in the evaporator due to the corrosion-promoting properties of chloride. in one embodiment, the flue gas coming from the air preheater is cooled down, by means of two concentrical spray arrangements provided with water of different quality, at the inlet to the absorber. By so doing, the water balance in the plant is maintained such that excess of streams of water containing salts does not arise. A water stream containing salts can be reused for cooling down flue gas, as long as it does not contact solid surfaces. By this embodiment is used process water (tap water) in the nozzles nearest the sides of the absorber whereas in the "inner parts" of the absorber, "salt- containing water" can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained further with reference to the drawings, in which Figure 1 is a process diagram for a plant, which can be used in the method of the invention.

Figure 2 shows the construction of the air preheater in NJV3.

Figure 3 shows a block diagram for and the gross reaction in a typical wet desuifurising plant.

In the plant of figure 1, cold combustion air 1 enters a regenerative air preheater 5 and is heated by means of hot air 4 from a boiler (not shown). A part 2 of the heated air is used for the combustion, whereas another part 3 is conducted to an evaporator 20.

The cooled flue gas 6 is conducted to a filter 7 (baghouse fitter or electrofilter) for removing dust, from where dustless flue gas 8 is conducted to an absorber 12. Hereto are also conducted process water 9 and absorbent 11 (Le, a chemical containing substantial amounts of CaCOe, CaO and/or Ca(OH)₂, such as limestone, slaked lime and SDAP).

From the absorber 12 comes a slurry 13, containing i.a. gypsum. The gypsum is dewatered to about 90% dry matter on a band filter in a gypsum dewatering unit 14, whereafter the band filter farther away is sprinkled with water, whereby the chloride is washed out, and the dewatered gypsum 15 is taken out, whereas the waste water 16 is cleaned for particles and heavy metals in the waste water cleaning 17, from where a part of the cleaned waste water 18 is conducted to waste water evaporation 20, whereas another part of the cleaned waste water 19 is conducted to a tank 24 for redissolving of evaporation residue. From the waste water evaporation 20, air and dry residual product (i.e.. particularly de-icing salt) 21 are conducted to a particle separator 22. The dry evaporation product from the waste water evaporation 20 and from the particle separator 22 is conducted to tank 24 for redissolving of dry product, from where the produced liquid de-icing salt 25 is discharged. Hot air 26 coming from the particle separator 22 is mixed with and thereby heats the moist cleaned flue gas from the absorber 12, and the flue gas 28, which is in this way reheated and cleaned, is discharged through a stack 29,

EXAMPLES

Example 1

Evaporation of brine from coal-fired power plant

Table 3: ESV3 brine component balances (g)

Total N 11,0 0,2 1 ,1 2,3 -8 -69%

Total S 37,0 1,6 34,2 3,6 -1 -2%

Total N {not 5,1 0, 1 0,8 0,8 nitrate)

Total S (not 30,2 1,2 6,4 3,5 sulfate)

The amounts of dry ESV filter cake, ESV water and NJV water were determined by minimising the sum of deviation squares excl. the contribution from SO₄ ^2" and NO₃ ^". The input columns show at the top the contents in grammes of Ci^', SO₄ ^2", SO₄ ^2' (calculated as S)₁ NO₃ ^', NO₃ ^" (calculated as total N), Ca, K, Mg and Na in 11 ,9 liters of ESV water and 1 ,1 liters of NJV water, respectively. Furthermore, the contents of total N and of total S compounds are shown. Since the total content of N and S is far higher than corresponding to the respective contents of nitrate and sulfate, this proves that the waste water contains substantial amounts of sulfur-nitrogen compounds.

After evaporation of the waste water (the output column), it appears that the content of total N has decreased by about 70%, both for nitrate N and other N. This can only result from the conversion of nitrate N and SN N compounds into volatile nitrogen compounds which are released with the discharged air.

It appears further that the amount of sulfate is almost trebled during the evaporation process. This is a further evidence that during evaporation the SN compounds are converted into volatile nitrogen and sulfate.

Example 2

Evaporation of brine from Nordjyllandsvaerket

Sample 1 and sample 2 are waste water taken out from Nordjyllandsvaerket, in sample 2, about 1000 mg of additional nitrate per liter are added. The waste water is evaporated in a spray evaporator at about 150⁰C and the dried powder is redissolved in deionized water to approximately the same concentration as the original waste water. Hereafter, the contents of SN compounds etc. are analyzed.

Table 4: Evaporation tests sample 1

It appears for sample 1 that all SN compounds except sulfamic acid are degraded by between 90 and 100%,. The reason why sulfamic acid is not degraded is obviously a higher thermic stability. That the amount is increased must imply that other SN compounds are converted into the stable SN compound sulfamic acid during heating. By the evaporation tests in example 2, higher decomposition rates for SN compounds are observed. This may be explained by the evaporation temperature being a little higher, i.e. 15O⁰C compared to upwards of 140⁰C in example 1. Table 5: Evaporation tests sample 2

It appears that all SN compounds except sulfamic acid are degraded by more than 95%. The reason why sulfamic acid is not degraded is obviously a higher thermic stability. That the amount is increased must imply that other SN compounds are converted into the stable SN compound sulfamic acid during heating.

Conclusion

Thus, the results obtained in examples 1 and 2 support the following:

By spray drying the water from the dewatering of the gypsum and subsequent mixing, a sodium-calcium chloride brine containing 24-25% dry matter is obtained, but coincidentally substantial amounts of solids are precipitated in the form of gypsum, sodium and calcium chloride. During the spray drying, the SN compounds are degraded under liberation of SO₄ ^2*, which gives rise to the precipitation of gypsum.

It does not appear from the present data what occurs to the N content, only that a substantial loss occurs by decomposition of the SN compounds during the spray drying under liberation of N components (probably N₂) to the discharged air. Example 3

Reconstruction of block 3 on Nordjyllandsvaerket (NJV3)

This project comprises plants for the receipt and processing of SDAP, The plant is laid out for about one week's SDAP consumption, i.e. about 800 tons SDAP. The SDAP is suspended in water and used as replacement for lime sludge. Corresponding plants are operated at Studstrupvεerket (SSV), Fynsvasrket (FYV) and ESV.

The waste water from the desulfurising plant is expected to be evaporated in a spray evaporator by a method resembling that of the spray absorption plants at SSV and FYV. For the evaporation, hot air from of the air preheater of the boiler is used, which has been taken into account in an ongoing renovation project for the desulfurising plant which exactly comprises reheating flue gas with hot air, whereas the gas preheater is removed.

The gas preheater is a rotating gas preheater which is used to cooling down the flue gas from about 130 to 100⁰C and to reheating the flue gas after cleaning from 50 to 8O⁰C. Constructionally, it is a troublesome component, so it has been decided to remove it and replace it by sprinkling with water and reheating with hot air preheater air.

When the gas preheater is removed, an alternative system to cooling of the flue gas must be established. To avoid coatings on surfaces in the plant, it is desired to use clean water for the cooling down. At the same time it is desired to maintain the water balance in the plant, this means that the consumption of added clean water should preferably be minimised. By the use of clean water near to surfaces in the plant and of desulfurising slurry far from surfaces, an efficient cooling is obtained, and the water balance is maintained,

Operating data

For a typical full-load operating situation on NJV3, the following typical values have been read: Table 6

In the table is:

NTU = "Number of Transfer Units", i.e. number of equilibrium steps. The heat exchanger has the same efficiency as 8.6 series connected cross heat exchangers, lni = the logarithmic mean temperature difference over the heat exchanger. This is calculated from the temperature differences in both ends of the heat exchanger, 373-36 TC = 12^qC in the hot end and 121-33°C = 88⁰C in the cold end. θ = the thermic efficiency of the heat exchanger, θ = 1 corresponds to an ideal countercurrent heat exchanger, θ can be calculated directly from NTU.

After take out of 17 kg/sec additional secondary air, the following changes in the air and flue gastemperatures are obtained:

Table 7

The lower temperature of the combustion air (from 361 to 357⁰C) will increase the coal consumption of the power plant for the same electrical output. The additional consumption can in this example be calculated to 1.6 MJ/sek. On the other hand, it can be calculated that the heat content in the taken out hot air stream (17 kg/sec and 357°C) amounts to about 5 MJ/sek. Economically, this represents therefore a very cheap providing of energy.

Claims

PATENT CLAIMS

1. A method of operating a flue gas cleaning plant (wet desulfurising plant), in which flue gas from a plant fired with fossil and/or biomass fuel is cleaned by passing through an air preheater, where it is cooled, an electrofilter or baghouse filter for removing particles and an absorber, where air, water, and lime, limestone or SDAP are added, in which absorber the sulfur is separated out as gypsum, which is dewatered and washed free of chloride, characterised in that a part of the heated air from the air preheater is used for the evaporation of the water from the dewatering,

2. The method according to claim 1 , characterised in that flue gas from a coal fired plant is cleaned.

3. The method according to claim 1 , characterised in that the flue gas coming from the air preheater is furthermore cooled down by means of two concentric spray arrangements provided with water of different quality.

4. The method according to claim 1 , characterised in that a certain upconcentration of the waste water is first carried out in a conventional evaporator and the concentrate from here is then evaporated in a spray evaporator.

5. The method according to claim 1, characterised in that 3-10% hot air are drawn out from the air preheater in proportion to the amount of flue gas.

6. The method according to claim 1, characterised in that a minor part of the water from the dewatering is conducted past the evaporation plant and thereafter mixed with the evaporation product.

1. A method for reducing the content of SN compounds in water from dewatering of gypsum in a flue gas cleaning plant operated by the method according to claim 1 , characterised in that the water from the dewatering is evaporated.

8. A method for the preparation of de-icing salt in a flue gas cleaning plant operated by the method according to claim 1, characterised in that the water from the dewatering of gypsum is evaporated.