WO2001070377A1

WO2001070377A1 - Use, method and composition of organic acids in gypsum producing desulphurising plants

Info

Publication number: WO2001070377A1
Application number: PCT/DK2000/000133
Authority: WO
Inventors: Folmer Fogh
Original assignee: Elsamprojekt A/S
Priority date: 2000-03-21
Filing date: 2000-03-21
Publication date: 2001-09-27
Also published as: AU3275100A; PL361626A1

Abstract

In gypsum producing flue gas desulphurizing plants, limestone and spray dry absorption product can be used in arbitrary mixing ratios when one or more organic acids having a buffer capacity between pH 3 and 7, preferably adipic acid, is added.

Description

USE, METHOD AND COMPOSITION OF ORGANIC ACIDS IN GYPSUM PROD_{UCING D}ES_UL_PH_URISIN_G PLANTS

TECHNICAL FIELD

The present invention relates to the use of one or more organic acids in gypsum producing flue gas desulphurizing (FGD) plants in which spray dry absorption product (SDAP) and limestone are used in arbitrary mixing ratios. 0

BACKGROUND ART

For flue gas desulphurizing, two different processes are currently in common use, the gypsum process (the wet desulphurizing process), and the spray dry absorption process, which are both used in power plants in the Danish power pool ELSAM. 5

In the gypsum process, SO₂ from the flue gas is absorbed in an aqeous slurry of lime/limestone, the pH of which may be between 5 and 6. Under these conditions the absorbed SO₂ is present in the slurry as hydrogen sulphite (HSO₃ ^"), which is readily oxidized to sulphate (SO ^~) and this is in turn combined with the calcium ions from 0 the limestone giving an aqeous slurry containing gypsum which is recovered.

SDAP is produced in the spray dry absorption (SDA) process. The SDA process adopted in the ELSAM area, uses slaked lime that is atomized and spray dried with hot (140-150°C) and ash free (less than 100 mg/Nm³ particulate) flue gas. S0₂ in the 5 flue gas is absorbed in the atomized droplets of slaked lime. Simultaneously with the evaporation of water from the atomized droplets, chemical reactions take place and SDAP is produced as a fine (d₉₀ < 40 μm, d₅₀ = 15 μm), white, and dry (1-2% H₂0) powder which is trapped in a bag filter. SDAP is a mixture of calcium sulphite, calcium chloride, Ca(OH)₂, CaC0₃, gypsum, and particulate matter originating from 0 the flue gas (fly ash) and the lime. The SDAP is difficult to utilize, mainly due to its sulphite content and, formerly, mostly had to be disposed of.

German Offenlegungsschrift no. 3,925,879 discloses a process by which the limestone used in gypsum producing FGD plants can be partly, e.g. 15-35%, 5 replaced by SDAP when this is used in an amount corresponding to 1.2 to 1.8 times the amount of limestone to be replaced. Hereby the remaining lime (Ca(OH)₂) and limestone (CaCO₃) in the SDAP is used and converted into gypsum. When using SDAP and limestone in admixture, however, problems have arisen in the form of AIF_X inhibition of the normal absorbent, i.e. the limestone. AIF_X inhibitions are characterized by an inadequate dissolution rate of the limestone resulting in a drop in operating pH in spite of surplus limestone addition. It is generally accepted that AIF_X complexes with x in the 4-6 range form apatite type minerals (e.g. CaAIF₃(OH)₂ ^«CaF₂) that inhibit limestone dissolution. In our experience, AIF_X inhibitions occur when the content of mobile aluminium in the absorber slurry is above 8-15 ppm.

Later on, F. Fogh: "SDAP an Efficient Absorbent for Wet Desulphurisation", VGB- Konferenz "Kraftwerk und Umvelt" (VGB Conference on "Power Plant and Environment"), April 4-5, 2000, has shown that using SDAP alone as absorbent for desulphurizing in the FGD plant solved the problem of AIF_X inhibition.

However, the available amount of SDAP is limited by the capacity of the spray dry absorption plants, and is not always sufficient. It is, therefore, desirable to mix with limestone. This leads, however, to a pronounced risk for AIF_X inhibition.

Furthermore, N.O. Knudsen, F. Fogh: "Measures by aluminium induced limestone inhibition at gypsum producing desulphurisation plants", VGB Chemie in Kraftwerk 1999, have explained the AIF_X inhibition chemistry in more detail, and shown that moderate AIF_X inhibition in gypsum producing FGD plants can be avoided by the addition of an organic acid such as adipic acid. This disclosure has been made in a desulphurizing plant in normal operation with limestone without the use of SDAP.

DISCLOSURE OF THE INVENTION

It has now surprisingly been found that SDAP and limestone can be employed in arbitrary mixing ratios in gypsum producing FGD plants without the risk of AIF_X inhibition when one or more organic acids having a buffer capacity between pH 3 and 7 is/are added.

Hereby, the pH value is stabilized at a level where aluminium ions are not liberated, and AIF_X inhibition is avoided. Accordingly, the invention comprises the use of one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6, in gypsum producing FGD plants in which SDAP and limestone are used in arbitrary mixing ratios.

Furthermore, the invention comprises a process for the desulphurizing and the production of gypsum in FGD plants in which SDAP and limestone are used in arbitrary mixing ratios, characterized by the addition of one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6, as well as a composition for avoiding AIF_X inhibition in gypsum producing FGD plants in which SDAP and limestone are used in arbitrary mixing ratios, characterized by comprising one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6.

Preferably, said SDAP comprises at least 20% sulphite and has an alcalinity of at least 20% measured as equivalent CaCO₃.

Preferably, said acid is formic acid, acetic acid, propionic acid, sulfopropionic acid, hydroxypropionic acid, lactic acid, phthalic acid, benzoic acid, adipic acid, succinic acid, maleic acid, or a mixture of two or more of these acids, in particular adipic acid. Preferably, said acid is biologically degradable by the microbiological activity in the waste water purifying plant.

The invention has the advantage of making a use for the large amounts of SDAP, which formerly had to be disposed, by turning this by-product into a useful material, gypsum, and in addition it reduces the amount of limestone used in the wet limestone gypsum process and, furthermore, the process of the invention can be carried out even by varying supplies of SDAP.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail in the experimental part below and with reference to the accompanying figures, of which

Figure 1 is a graph showing the desulphurizing rate during the operation on SDAP/limestone (•) according to the invention with the addition of adipic acid and during the operation with limestone alone (❖) according to the prior art, resp., Figure 2A through C show the gypsum quality during the operation according to the invention, whereby Figure 2A shows the content of moisture and of residual limestone in the gypsum, Figure 2B shows the content of soluble chloride (Cl ) and total sulfite (SO₃ ^2") in the gypsum, and

Figure 2C shows the purity of the gypsum.

EXPERIMENTAL Introduction The experiments were carried out from August 4 to September 10, 1999, at Vestkraft Unit 3 (400 MW_e) (VKE3), which is one of the six coal fired units operated by the ELSAM power pool.

Materials In the test period, the coal was mainly russian coal containing 0.3-0.4% sulphur corresponding to a S0₂ concentration at the desulphurizing plant inlet of 600-800 mg/Nm³.

The limestone used is "Grena Miljøfiller", a commercial limestone which was obtained from Faxe Kalk A/S, Grønnegade 20, DK-8500 Grenaa, Denmark.

The SDAP was obtained from the Midtkraft Units 3 and 4 (MKS3 and MKS4), other coal fired units operated by the ELSAM power pool using the spray dry absorption process.

Operating conditions

In the first part of the test period with high ratio of SDAP to limestone, the supplies of

SDAP was not always sufficient and, therefore, bypass operation was necessary.

The desulphurizing plant was entirely out of operation from August 29, 1999 (morning) to September 1 , 1999 (afternoon) due to an unforeseen boiler breakdown.

In the period from August 4 to August 19, 1999 the adipic acid concentration in the absorber slurry was gradually increased to 500 mg/l by daily batch additions during SDAP operation. During the initial 10-12 days the adipic acid concentration was gradually and quite carefully increased to 200 mg/l and, since process dissturbances were not observed the adipic acid concentration was then increased to 500 mg/l in one step and maintained in the 400-500 mg/l range by daily batch additions (approx 100 kg/d) during the remaining test period

Thus, the concerns about a decreased solubility rate and a decreased oxidation rate of SDAP (sulphite) resulting from the pH stabilizing effect of the adipic acid appeared to be groundless

Table I shows the ratio of the absorbents, SDAP and limestone, during the test period with remarks

Table I

Note The limestone demand for the desulphurisation process is covered by the equivalent amount of limestone in SDAP (30-35%) and limestone (Grena Miljøfiller)

Desulphurizing capability

The desulphurizing capability has - apart from periods with bypass operation - been unaffected by the ratio of SDAP and limestone and, as it appears from the bypass filtered data, cf figure 1 , the S0₂ emission is stable and below 25-30 mg/Nm³, independent of the SO₂ load

Bypass filtered data from January and the first half of February 1999 when there was a comparable SO₂ load, but Grena Miljøfiller was used alone without the addition of adipic acid, show a more varying SO₂ emission on a higher level and a SO₂ emission increasing with the SO₂ load, cf figure 1

The mean value and the standard deviation of the S0₂ emission at SO₂ loads below 900 kg/h was determined and is shown in table II

Table II

Gypsum gua tv As it appears from the figures 2A through C, the gypsum quality is greatly unaffected by the addition of adipic acid and the shifts carried out in the ratios of SDAP and limestone

Adipic acid consumption Measures of adipic acid show that the adipic acid loss primarily is due to the waste water outlet but, besides, a certain degradation loss occurs

The adipic acid let out with the waste water was degraded by microbiological activity in the waste water purifying plant

CONCLUSION

• The addition of adipic acid did not affect the solubility of SDAP and the oxidation capacity negatively

• Operation with varying ratios SDAP/hmestone is possible

• The addition of adipic acid ameliorates the desulphurizing capability compared with operation on SDAP alone

• The addition of adipic acid does not affect the gypsum quality noticeably

• The consumption of adipic acid is primarily determined by the waste water outlet, and secondarily by a degradation loss of 4 to 6 percent per day - in total approximately hundred kilogrammes per day

Claims

1. Use of one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6, in gypsum producing FGD plants in which SDAP and limestone are used in arbitrary mixing ratios.

2. The use according to claim 1 , characterized in that said SDAP comprises at least 20% sulphite and has an alcalinity of at least 20% measured as equivalent CaCO₃.

3. The use according to claim 1 , characterized in that said acid is formic acid, acetic acid, propionic acid, sulfopropionic acid, hydroxypropionic acid, lactic acid, phthalic acid, benzoic acid, adipic acid, succinic acid, maleic acid, or a mixture of two or more of these acids, preferably adipic acid.

4. A process for the desulphurizing and the production of gypsum in FGD plants in which SDAP and limestone are used in arbitrary mixing ratios, characterized by the addition of one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6.

5. The process according to claim 4, characterized in that said SDAP comprises at least 20% sulphite and has an alcalinity of at least 20% measured as equivalent CaCO₃.

6. The process according to claim 4, characterized in that said acid is formic acid, acetic acid, propionic acid, sulfopropionic acid, hydroxypropionic acid, lactic acid, phthalic acid, benzoic acid, adipic acid, succinic acid, maleic acid, or a mixture of two or more of these acids, preferably adipic acid.

7. A composition for avoiding AIF_X inhibition in gypsum producing FGD plants in which SDAP and limestone are used in arbitrary mixing ratios, characterized by comprising one or more organic acids having a buffer capacity between pH 3 and 7, preferably between pH 4 and 6.

8. The composition according to claim 7, characterized in that said SDAP comprises at least 20% sulphite and has an alcalinity of at least 20% measured as equivalent CaC0₃.

9. The composition according to claim 7, characterized in that said acid is formic acid, acetic acid, propionic acid, sulfopropionic acid, hydroxypropionic acid, lactic acid, phthalic acid, benzoic acid, adipic acid, succinic acid, maleic acid, or a mixture of two or more of these acids, preferably adipic acid.