GB2076851A - Process for the combustion of coal in the presence of calcium- containing material, Reducing SO2 emission - Google Patents

Abstract

Process for the combustion of particulate coal wherein the coal is combusted with an oxygen-containing gas in the presence of a particulate calcium- containing material, the process being also carried out in the presence of a tin (Sn)-containing or copper (Cu)-containing material. <IMAGE>

Description

SPECIFICATION Process for the combustion of coal in the presence of calcium-containing material The invention relates to a process for the combustion of coal wherein a particulate coal is combusted with an oxygen-containing gas in the presence of a particulate calcium-containing material.

Combustion of various coals results in sulphur dioxide emissions in excess of governmental standards.

Alkali impregnation of coal has been shown to be an inexpensive approach to reducing the SO2 emissions from combustion of sulphur-containing coal, and, under some conditions, may be economically competitive with stack gas scrubbing. CaO reacts with S02 from oxidation of coal sulphur compounds, ultimately forming CaSO4which is retained largely in the coal ash.

An alternate approach to the wet alkali coal impregnation technique is dry blending calcium-containing materials, e.g. limestone, with coal before or during combustion. The commercial viability of this approach will depend in part on maximizing the SO2 capture efficiency of the additive. While the type and origin of the calcium-containing additive is known to be an important factor in determining S02 capture efficiency, the effectiveness of the best calcium-containing additive has not been sufficient to reduce the SO2 emissions to governmental requirements at practical loadings of limestone. Therefore, a need has existed for improving the capture efficiency of calcium-containing materials in coal combustion methods. The invention satisfies that need.

Accordingly, the invention provides a process for the combustion of coal wherein a particulate coal is combusted with an oxygen-containing gas in the presence of a particulate calcium-containing material, characterized in that the combustion is also carried out in the presence of a tin-containing or a copper-containing material. The oxygen-containing gas is preferably air. Mixtures of tin-containing and copper-containing materials may be used.

Preferably, the process is carried out by blending the coal, the calcium-containing material, and the additive (the tin-containing and/or copper-containing material) prior to introduction into the burner.

However, simultaneous introduction of the materials, pre-blending of the coal and the calcium-containing material followed by concomitant introduction of the additive into the burner, and staged addition of the materials are clearly within the contemplation of the invention.

Any suitable manner of blending the coal and capture materials may be employed. For example, the calcium-containing material, e.g., particulate limestone, may be dry-blended with the particulate coal, and the mix may then be wetted lightly with a tin- and/or copper-containing solution. The copper-containing material may be copper-containing rods or elements.

The type of coal employed in the process according to the invention is much a matter of economics, but it is an advantage of the invention that low rank sulphur-conaining coals or lignites may be used. Accordingly, the term "coal", as used herein, includes such low rank materials as sub-bituminous coals and lignites. Very good results have been obtained wih beneficiated coal, i.e. coal from which pyritic sulphur has been removed and of which the ash content has been lowered. Similarly, the choice of calcium-containing materials is widely variable, the sole exception being, of course, calcium sulphate. CaCI2 may be used. As used herein, the term "reactive calcium-containing material" is understood to include any calciumcontaining material which would provide calcium to react with SO2 produced during combustion.In general, calcium-containing materials which are principally, or which decompose in the burner to provide CaO, are preferred. Limestones (principally CaCO3), because of their low cost and wide availability, are a preferred source of a CaO-yielding material. However, such unusual sources as limes, oyster shells, etc., if reduced to appropriate size, may be employed. Whatever the case, the reactive calcium-containing material will be supplied in the coal in an amount sufficient to capture or react with at least the bulk of the sulphur present in the coal. In general, the calcium-containing material or compound will be present in an amount of from about 1 per cent to normally about 50 per cent, preferably from about 5 per cent to 20 per cent (all by weight) based on the weight of the coal.Generally, the calcium-containing material will be employed in a particle size similar to that of the coal upon admission to the burner. Normally, the material will have a particle size of from 50 to 400 mesh, preferably 100 to 200 mesh (U.S. sieve series).

As indicated, the efficiency of the calcium-containing material is enhanced by the addition of an effective amount of an additive containing tin and/or copper. The type of tin-containing and of copper-containing material does not appear critical. Tin compounds, such as the oxide, chloride, sulphide, etc. may be used.

Very good results have been obtained with SnO. Tin-containing ores or tailings may be used. Copper compounds, such as the oxides (for example Cu2O), chlorides, sulphides, etc. may be employed. Very good results have been obtained with copper ores and copper oxides.

In general, the tin and/or copper-containing material will be present in an amount effective to improve the efficiency of the capture of or reaction of the SO2 generated during combustion. In general, the tin-containing material and the copper in the copper-containing material will be present in an amount of at least 0.01 per cent, and normally from 0.01 per cent to 10 per cent, preferably from 0.05 per cent and particularly from 0.1 per cent to 10 per cent, most preferably not more than 5 per cent (all by weight), calculated on coal.

It has been found that the capture efficiency of calcium-containing materials is further improved when the combustion is carried out in the presence of Cr203 as well. The combination will be employed, as indicated, in an effective amount, and the amount of the combination of tin-containing material and Cr203 employed will be similar to that of tin-containing material alone. The molar ratio of tin-containing material to Cr203 will preferably be in the range of from 0.2 to 1. It has further been found that the presence of BaO tends to stabilize the present combinations of tin- or copper-containing material and Cr203. If BaO is added, the amounts of tin-containing material and Cr203 remain the same. If BaO is added, the ratio of tin-containing material to Cr203 to BaO will range from (0.2 to 1): 0.05 to 0.3.If the additive is added as a particulate solid, the particle size will normally be similar to that of the coal.

The following Examples further illustrate the invention.

Examples 1-5 To test the concept that tin-containing materials would increase the capture efficiency of calciumcontaining materials by increasing the rate of reaction of SO2 to SO3, a simple flow apparatus utilizing a simuiated flue gas and realistically high temperatures were employed.

The composition of the feed was (%v): SO2 0.2, 022.4, H2O 2.4, CO2 9.7 and N2 85.3. The feed flow rate was 250 cm3/min. and the catalyst was diluted with 1.0 g quartz chips (40/100 mesh). The total contact time was 0.03 sec. and the run time 2.0 hours. The temperature was 800"C. Table I shows which catalysts were used in what amounts and presents the results.

TABLE I Example Catalyst Weight SO2 conversion g 1 SnO 0.055 8.0 2 Cr203 (60%m)/SnO (40%m) 0.055 11.0 3 Cr203 (57%m)/SnO (38%m)/BaO (5%m) 0.1 9.8 4 ditto 0.055a) 9.8/1 0.7b) 5 Cr203 (47.5%m)/SnO (47.5%m)/BaO(5%m) 0.055 9.2 a) doubling flow rate to 500 cm3/min. resulted in 6.9% conversion; b) repeat preparation of catalyst.

Examples 6 and 7 To test the concept that additive material would increase the SO2 capture efficiency of dolomitic limestones or limestones (CaCO3), mixtures of the additives with limestone/coal blends were prepared and subjected to two small scale burn tests (Examples 6 and 7). In these tests to a mixture of unbeneficiated Texas lignite (70 9,1.48% sulphur) and a locally available good quality limestone, Round Rock Limestone (Blum, Texas, 10 g, total calcium content 5.9%w) were added 2.8 g of a mixture consisting of 38%m SnO, 57%m Cr203 and 5%m BaO (4.0%, based on the weight of the lignite).

The first test (Example 6) was carried out using a micro-combustor (1 150"C, 1 second residence time, 3-11% 02). The second test (Example 7) was carried out in a hot tube (1050"C, 5 minutes residence time) and was a less stringent burn, in terms of sintering temperature.

The results are presented in Table II. They show that in Examples 6 and 7 SO2 capture efficiencies of around 67% and 70%, respectively, were obtained.

TABLE II SO2 emission % SO2 g SO21MJ Example 6 32.8-33.2 0.48-0.49 Example 7 29.6 0.43 Comparative Experiment 1 53.7-60.0 0.79-0.88 Comparative Experiment 2 39.6 0.58 Comparative Experiments 1 and2 The experiments of Examples 6 and 7 were modified in that no additive was added (Comparative Experiments 1 and 2, respectively). This resulted in a considerably higher SO2 emission, as shown in Table II.

Example 8 Commercial application of CaO scavenging of SO2 may be coupled with a prior beneficiation of the lignite to remove pyritic sulphur and to lower the ash content. The lowering of the intrinsic ash level will permit the addition of higher levels of limestone or CaO. To illustrate this approach, a 42.4 g sample of beneficiated Texas lignite (1.37%w sulphur, 13.6%w ash) dry blended with 7.6 g of Round Rock limestone (Blum, Texas, total calcium content 6.5%w) and 0.38 g of a mixture consisting of 38%m SnO, 57%m Cr203 and 5% BaO was prepared, the additive content being 0.90%w, calculated on lignite.The experiment was carried out with a microcombustor (1150"C, 1 second residence time, 3-11% 02). The results are presented in Table III.

TABLE Ill S02 emission % S02 g S021MJ Example 8 30.9-33.7 0.34-0.36 Comparative Experiment 3 41.4-55.6 0.45-0.60 This Table shows that the SO2 emission level was close to 0.35 g SO2/MJ.

Comparative Experiment 3 The experiment of Example 8 was modified in that no additive was added. This resulted in an SO2 emission level close to 0.52 g SO2/MJ, see Table lil. It can be calculated from these results that the addition of only 0.90%w of the additive resulted in a decrease of the SO2 emission level of about 33%.

Examples 9- 11 To test the concept that copper-containing materials would increase the SO2 capture efficiency of dolomitic limestones or limestones (CaCO3), mixtures of Cu2O with limestone/coal blends were prepared and subjected to small scale burn experiments. The limestone/coal blend consisted of unbeneficiated Texas lignite (70 g, 1.48% sulphur) and dolomitic limestone (10 g, Rochester, New Jersey).

The experiment of Example 9 was carried out using a micro-combustor (1 150"C, 1 second residence time, 3-11% 02). The experiments of Examples 10 and 11 were carried out in a hot tube and were less stringent burns, in terms of sintering temperature.

Table IV shows how much of the Cu2O was used and presents the results.

TABLE IV Amount of Cu2O S02 emission %w on lignite % S02 g 5021MJ Example 9 4.0 48.0-52.4 0.70-0.77 Example 10 4.0 27.3 0.40 Example 11 2.0 34.3 0.50 Comparative Experiment 4 0 53.7-60.0 0.79-0.88 Comparative Experiment 5 0 39.6 0.58 Comparative Experiments 4 and 5 The experiments of Examples 9 and 10 were modified in that no additive was added (Comparative Experiments 4 and 5, respectively). The results are presented in Table IV.

Comparison of the results of Example 9 and Comparative Experiment 4 shows that a loading of 4.0%w Cu2O results in a reduction of the SO2 emission of 7 absolute percent.

Comparison of the results of Example 10 and Comparative Experiment 5 shows that a loading of 4.0%w Cu2O has resulted in a reduction of the SO2 emission of 12 absolute per cent or of 32% of the SO2 emission in this Comparative Experiment.

Examples 12 and 13 In the experiments of these Examples the copper-containing material was added to a mixture of unbeneficiated Texas lignite (70 g, 1.48% sulphur) and a locally available good quality limestone, Round Rock limestone (Blum, Texas, 10 g, calcium content 5.9%w). The copper-containing material consisted of CuO-CuCr204/BaO, was formed by co-precipitation of the solid from a solution of compounds of these metals and was used in an amount of 2.8 g, corresponding to 4.0%w CuO-CuCr204/BaO, calculated on coal.

The experiment of Example 12 was carried out using a micro-combustor (1 1500C, 1 second residence time, 3-11 % 02) and that of Example 13 in a hot tube (1050"C, 5 min. residence time). The results presented in Table V show that the addition of 4.0%w additive on coal resulted in an SO2 capture efficiency of around 68%, see Example 12.

TABLE V S02 emission % 802 g 5021MJ Example 12 29.7-33.8 0.43-0.49 Example 13 20.4 0.30 Examples 14 and 15 and Comparative Experiment 6 Commercial application of CaO scavenging of SO2 may be coupled with a prior beneficiation of the lignite to remove pyritic sulphur and to lower the ash content. The lowering of the intrinsic ash level will permit the addition of higher levels of limestone or CaO. Accordingly, four 42.4 g samples of beneficiated Texas lignite (1.37%w sulphur, 13.6%w ash) dry-blended with 7.6 g of Round Rock limestone (Blum, Texas, total calcium content 6.5%w) were prepared.Four experiments were carried out using a microcombustor (1150"C, 1 second residence time, 3-11 % 02). In the first experiment the sample as such was burnt (Comparative Experiment 6). In the other two experiments different amounts of CuO-CuCr204/BaO were added to the samples; this additive was prepared in the same manner as in Examples 12 and 13.

Table IV shows how much of the additive was added and presents the results. It can be calculated therefrom that the addition of 3.5%w of the additive results in a reduction in SO2 emission of about 54%.

TABLE VI Weight ofadditive S02 emission g, %w on lignite % S02 g SO21MJ Comparative Experiment 6 0 0 41.4-55.6 0.45-0.60 Example 14 1.5 3.5 22.0-23.0 0.24-0.25 Example 15 0.38 0.90 30.0-35.0 0.33-0.37

Claims (14)

CLAIMS:

1. A process for the combustion of coal wherein a particulate coal is combusted with an oxygencontaining gas in the presence of a particulate calcium-containing material, in which the combustion is also carried out in the presence of a tin-containing or a copper-containing material.

2. A process as claimed in claim 1, in which the calcium-containing material is limestone.

3. A process as claimed in claim 1 or 2, in which the coal is beneficiated coal.

4. A process as claimed in any one of the preceding claims, in which the copper-containing material is a copper ore.

5. A process as claimed in any one of claims 1 to 3, in which the copper-containing material is a copper oxide.

6. A process as claimed in any one of the preceding claims, in which the copper is used in an amount in the range from 0.01 to 10% by weight, calculated on coal.

7. A process as claimed in any one of the preceding claims, in which the tin-containing material is SnO.

8. A process as claimed in any one of claims 1-3 and 7, in which the tin-containing material is used in an amount in the range from 0.01 to 10% by weight, calculated on coal.

9. A process as claimed in any one of the preceding claims, in which the combustion is also carried out in the presence of Cr203.

10. A process as claimed in claim 9, in which the combustion is carried out using a molar ratio of tin-containing material to Cr203 in the range of from 0.2 to 1.

11. A process as claimed in any one of the preceding claims, in which the combustion is also carried out in the presence of BaO.

12. A process as claimed in claim 11, in which the combustion is carried out using a molar ratio of (SnO : Cr203) : BaO from (0.2 to 1): 0.05 to 0.3

13. A process as claimed in any one of claims 9 to 12, in which the tin-containing material and Cr203 are used in a total amount in the range of from 0.1 to 5% by weight, calculated on coal.

14. A process as claimed in claim 1, substantially as hereinbefore described with reference to any one of the Examples.