AU2005204297B8

AU2005204297B8 - A Method for Suppressing Sulphur Released During Combustion of Coal

Info

Publication number: AU2005204297B8
Application number: AU2005204297A
Authority: AU
Inventors: Dean John Anderson; Russell Wilson Anderson
Original assignee: YARRABOLDY BRIQUETTE Co Pty Ltd
Current assignee: YARRABOLDY BRIQUETTE Co Pty Ltd
Priority date: 2004-09-01
Filing date: 2005-08-29
Publication date: 2010-07-29
Anticipated expiration: 2025-08-29
Also published as: AU2005204297A1; AU2005204297B2

Description

1 A METHOD FOR SUPPRESSING SULPHUR RELEASED DURING COMBUSTION OF COAL FIELD OF THE INVENTION This invention relates to methods for the preparation of briquettes in 5 which the sulphur content of the briquettes is suppressed upon combustion. The invention will be described with reference to briquettes made from coal but it should be appreciated that the invention may extend to other types of industry fines. BACKGROUND ART 10 Briquetting of coal is a process whereby coal particles such as coal dust and the like can be agglomerated into large sizes making them more suitable for furnaces and other heating apparatus. One common manufacturing process for briquetting fine coals is where the mixture is fed into the briquetting under high temperatures (usually over 200*C). 15 Various binders such as pitch or bitumen can be used which melt at the operating temperature. Sometimes, the coal itself can contribute to the binding process at these high temperatures. A disadvantage with briquetting coal at high temperatures is the energy input required. For this reason, it is also known to have a "cold briquetting" process 20 which is a process usually carried out either with no heating, or with heating up to 100*C. The cold briquetting process requires the use of different types of binders which are able to hold the coal particles together to provide an acceptable briquette. The binders must also hold the briquette together during the combustion process to prevent premature, partial, or complete disintegration of the briquette and therefore 25 the loss of uncombusted coal through the furnace grate. It appears to be known to use molasses and lime as binders in a coal briquetting process. The molasses is usually diluted with water and is mixed with the coal particles. Lime is then added and the resultant mass is compressed into briquettes. A disadvantage with these binders is that the binder content is rather high 30 which reduces the energy value of the coal briquette. Merely reducing the binder content results in a briquette which is brittle and has other undesirable properties. In Australia, most coals presently mined contain less than 1 % sulphur 2 with most having a sulphur content of approximately 0.6% to 0.8%. This is one of the reasons that Australian coal is much sought after for export, as the sulphur content of coals from other countries is generally much higher. For example, China has vast reserves but prefers to import Australian coal as the native coals in China contain 5 higher sulphur levels, generally between 4% and 5%. When the Chinese coal is combusted, the sulphur in the coal is released and the majority of the 4% to 5% of sulphur is seen in stack emissions from the combustion facility. This sulphur when released into the atmosphere, is commonly known as acid rain, finds its way into water courses and other environmentally fragile 10 areas, causing damage. Our prior patent is directed towards achieving a briquetting process which uses a low binder content. This process included the steps of: (a) contacting coal particles with 0.25% - 4% molasses at a temperature of above 60*C, 15 (b) adding calcium oxide to the mixture at a sufficient rate to heat the product to drive off water, and (c) forming the mixture into briquettes. Whilst this process did indeed form briquettes having a lower than normal binder content, it also went some way towards suppressing the sulphur content 20 of the coal. Once the sulphur suppressing qualities of the process were realised, we realised that the sulphur suppression should be further investigated. OBJECT OF THE INVENTION It is an object of the invention to provide a method and a product which may overcome the abovementioned disadvantages or provide the public with a useful 25 or commercial choice. In one form, the invention resides in a method of manufacturing a coal briquette with suppressed sulphur content, the method including the steps of: (a) reducing the size of the coal to increase porosity of coal particles formed; 30 (b) contacting the coal particles with a first part of a sulphur suppressant, a lime part; (c) contacting the coal particles formed in step (b) with a second 3 part of a sulphur suppressant, up to 4% molasses at a temperature not greater than 60*C; and (d) forming the mixture into briquettes. The method has been developed to reduce or minimise sulphur 5 emissions from burning of the briquettes. Generally, the coal is mined and crushed to around minus 50 mm in the usual method. The coal may be washed or it may be run of mine (ROM). The -50mm coal is suitably then further ground, milled or crushed to approximately about -4mm. The smaller size fraction generally greatly affects the amount of binder required to 10 form the briquettes and also the effectiveness of the binder. While not wishing to be bound by theory, it appears that the size fraction to which the coal particles are reduced at this step allows the coal particles to more readily be penetrated by the various parts of the sulphur suppressant. Different coals have different porosities, even when milled to the same size fraction. The size of the coal particles is therefore 15 preferably milled to about -4mm as this may be an optimum size to allow the coal suppressant parts to be respectively absorbed into the labyrinthine pores inside the coal particles to form an interconnecting matrix of binder/suppressant. As the porosity of coals may vary from mine to mine, and from country to country, the final sizing of the crushed or milled coal particles may be varied to 20 achieve the optimum porosity. However, -4mm is an average and would suit most coal types. Generally, the crushed coal particles would be fed into container known in the process industry as a "surge bin". These bins are used to ensure continuity of supply to the process step following the surge bin. 25 The step of contacting the coal particles with a first part of a sulphur suppressant, calcium oxide may preferably take place in a mixer/blender. A mixer/blender should be chosen which will preferably result in the penetration of the respective sulphur suppressant parts into the pores of the coal particles instead of merely coating the surface of the coal particles. The material must either be brought 30 to the agitator of the mixer or the agitator should visit all parts of the mix. Suitably, a mixer with a high mixing index should be chosen. The performance of an industrial mixer is generally judged by the time required, the power load and the properties of 4 the product. The degree of uniformity of the mixed intermediate and final product, usually measured from analysis of a number of spot samples is also generally the most important quantitative measure of mixing effectiveness. Various types of lime may be used in step (b). For instance, quick lime 5 (calcium oxide), hydrated lime or calcium hydroxide may be used. Generally the coal particles will be homogenised in the mixer/blender with the first part of the sulphur suppressant. The combination of calcium oxide (quick lime) and molasses in the coal particles results in an exothermic reaction which raises the temperature of the 10 mixture to draw out moisture from the product. This heat can result in drying of the mixture. The use of quicklime may result in the final briquette product heating quickly and the formed briquettes may fracture into smaller fragments than those produced in the original formed shape of the briquette. This may be an advantage 15 where large quantities are to be produced and handling of smaller size material is not troublesome. However, if transfer points in the material handling system are a problem when dealing with smaller size or broken briquettes, intact or larger briquettes may be needed to satisfactorily convey the particles, and the use of hydrated lime may be required. 20 When hydrated lime is employed as the first part of the sulphur suppressant, according to the present invention, it may react more slowly with the later applied molasses. This may result in a more controlled heating of the mixture and the retention of full size product. When hydrated lime is used, greater space is generally required during any briquette curing phase. 25 Suitably, the lime part of the sulphur suppressant regardless of its actual form, will be retained in a dry condition and also it should be kept free from contaminants. As alluded to above, the amount of lime part used may preferably depend upon the size and/or porosity of the coal particles. The various parts of the 30 sulphur suppressant may suitably fully penetrate the coal pores in order to affix themselves to the sulphur present in the coal particles. It is therefore less preferred that the respective parts of the suppressant merely coat the coal particles as in that 5 case, only the sulphur which is present in the outer layer of the coal particle may be suppressed as the coal particle undergoes combustion. In some cases, it may be that simply coating larger particles of porous coal and thereby suppressing only the surface sulphur, will be sufficient to attain a 5 lowered sulphur reduction to satisfy local sulphur emission statutory requirements. However, this would not usually be the case. Generally, the maximum amount of lime required according to the method of the present invention may be approximately 8%. The second part of the sulphur suppressant is molasses. The molasses 10 is preferably added to the lime impregnated coal particles in sufficient quantities to bind the material up to an approximate maximum of 4%. Generally, the molasses will be added at between 0.25% and 4%. The molasses can be diluted with water at about a ratio of 30% water to 60% molasses, depending upon the viscosity of the molasses. The viscosity of molasses varies according to changing weather patterns during the 15 growing season and the cane or beet crushing method. While not wishing to be bound by theory, it appears that the quantity of molasses as a binder can be significantly reduced (typically to approximately 1% by weight) by maintaining the temperature of the molasses to between 40* C and 60*C. This seems to increase the viscosity of the molasses allowing it to penetrate the coal 20 particles more thoroughly to give better binding and a more uniform reaction between the suppressant parts at lower binder weights. The molasses may be the waste molasses from a sugar mill. This waste molasses (also called C-molasses) is unable to be further crystallised due in part to contaminants in the molasses which prevent crystallisation of the remaining sugar. 25 If the molasses is thoroughly homogenised with a fine particle sized coal, which has previously been thoroughly homogenised with a lime part of the suppressant, the sulphur fixing or sulphur retention in the ash is surprising and unexpectedly dramatically increased. The blended mixture of coal particles and now formed sulphur 30 suppressant is then preferably fed into a briquette forming apparatus. The most efficient briquette forming apparatus is a roll briquetter. This machine preferably has a pair of counter-rotating pocketed rolls, which rotate to form the required briquette 6 shape as the blended mixture is forced into the briquetter, generally with a screw auger from above the apparatus. The motor drives on the forming rollers and the feed auger are generally connected to very fine frequency drives, controlled using a programmable S logic controller (PLC) to ensure that the correct feed to roll speed is maintained for uniformity of product. The formed product is preferably transferred to a curing area. Good ventilation is preferred to allow the moisture from the curing product to dissipate. Curing will generally take approximately 48 hours and the formed product may then 10 be sufficiently well-formed for stockpiling. During stockpiling, it is preferred that the briquettes are stacked to allow an upward movement of air to move over the stockpiled product. Breakage of the product should be minimised during is may reduce the movement of air through the stockpile. While the invention is particularly suited for coal fines, coarser coal 15 particles may also be used such as particles of up to 1mm. Particles other than coal particles could be used such as other forms of industrial fines. Another aspect of the invention is directed to a method to speed up the steaming/curing process of the briquettes. In this method, formed briquettes are stockpiled, and as the briquettes are added to the stockpile, the briquettes are 20 subjected to a blast or flow of gas which causes the smaller particles to be distributed to the extremities of the stockpile while the larger particles are distributed in a central region of the stockpile. This can ensure that the central stockpile region is left unobstructed vertically thereby allowing the steaming/curing process to proceed with maximum speed. 25 The blast or flow of gas is typically in the form of an airstream. The airstream is such to blow smaller particles to the extremities of the stockpile. It is considered that a person skilled in the art would be able to determine the volume and velocity of the gas stream to achieve this result. Solar energy may be used to further assist the curing drying process. 30 Artificial heating may also be used to assist the curing/drying process. BEST MODE According to a preferred embodiment of the present invention, a 7 method of manufacturing a coal briquette with suppressed sulphur content is provided. The method includes the steps of: (a) reducing the size of the coal to increase porosity of coal particles formed; 5 (b) contacting the coal particles with a first part of a sulphur suppressant, a lime part; (c) contacting the coal particles formed in step (b) with a second part of a sulphur suppressant, up to 4% molasses at a temperature not greater than 60*C; and 10 (d) forming the mixture into briquettes. The method has been developed to reduce or minimise sulphur emissions from burning of the briquettes. Generally, the coal is mined and crushed to around minus 50 mm in the usual method. The coal may be washed or it may be run of mine (ROM). The -50mm 15 coal is then further ground, milled or crushed to approximately about -4mm. As the porosity of coals may vary from mine to mine, and from country to country, the final sizing of the crushed or milled coal particles may be varied to achieve the optimum porosity. However, -4mm is an average and would suit most coal types. 20 Generally, the crushed coal particles are then fed into a "surge bin". These bins are used to ensure continuity of supply to the process step following the surge bin. The step of contacting the coal particles with a first part of a sulphur suppressant, the lime part, takes place in a mixer/blender. The blender/mixer is 25 adapted to homogeneously mix the coal particles with the lime to promote penetration of the lime into the pores of the coal particles. Various types of lime may be used in step (b). The combination of lime and molasses in the coal particles results in an exothermic reaction which raises the temperature of the mixture to draw out moisture 30 from the product. This heat can result in drying of the mixture. The use of quicklime may result in the final briquette product heating quickly and the formed briquettes can fracture into smaller fragments than those 8 produced in the original formed shape of the briquette. This is advantageous where large quantities are to be produced and handling of smaller size material is not troublesome. When hydrated lime is employed as the first part of the sulphur 5 suppressant, according to the present invention, it may react more slowly with the later applied molasses. This results in a more controlled heating of the mixture and the retention of full size product. When hydrated lime is used, greater space is required during any briquette curing phase. Generally, the maximum amount of lime required according to the 10 method of the present invention is approximately 8%. The second part of the sulphur suppressant is molasses. The molasses is added to the lime impregnated coal particles in sufficient quantities to bind the material in amounts between 0.25% up to an approximate maximum of 4%. The molasses can be diluted with water at about a ratio of 30% water to 60% molasses, 15 depending upon the viscosity of the molasses. The quantity of molasses as a binder can be significantly reduced (typically to approximately 1% by weight) by maintaining the temperature of the molasses to between 40 C and 60*C. This increases the viscosity of the molasses allowing it to penetrate the coal particles more thoroughly to give better binding and a 20 more uniform reaction between the suppressant parts at lower binder weights. The blended mixture of coal particles and now formed sulphur suppressant is then fed into a briquette forming apparatus, the most efficient briquette forming apparatus being a roll briquetter. The motor drives on the forming rollers and the feed auger are 25 generally connected to very fine frequency drives, controlled using a programmable logic controller (PLC) to ensure that the correct feed to roll speed is maintained for uniformity of product. The formed product is then transferred to a curing area. Good ventilation is required to allow the moisture from the curing product to dissipate. 30 Curing will generally take approximately 48 hours and the formed product may then be sufficiently well-formed for stockpiling. During stockpiling, the briquettes are stacked to allow an upward movement of air to move over the stockpiled product.

9 Breakage of the product should be minimised during is may reduce the movement of air through the stockpile. It should be appreciated that various other changes or modifications may be made to the embodiments describes without departing from the spirit and 5 scope of the invention. For instance, the invention need not be limited to coal fines and other industry fines may also be used. The coal may be any bituminous or non bituminous coals, crushed coal, or coal fines recovered from a coal cleaning process (such as a washing process). Briquettes manufactured according to the method have been tested in 10 order to ascertain the effectiveness of the method and the results are contained in the following example: Example 1 The sample of briquettes, as well as a sample of the coal from which the briquettes had been prepared were received from the Yarraboldy Briquette 15 Company on 15th Feb 05. The quantities were approximately 20 kilograms of each. 1 Definition of Sulphur Fixation During the combustion of coal the sulphur contained in the coal can have two possible fates: (i) It can react with oxygen to produce gaseous oxides of sulphur that 20 are emitted to the atmosphere with the stack gases, or (ii) It can be absorbed by the coal ash, thus remaining as solid material and not causing atmospheric pollution. In practice, the sulphur in many coals has a mixed fate: some of it produces gaseous oxides while the remainder is absorbed by the ash. Usually the 25 major proportion produces gaseous oxides. From the viewpoint of atmospheric pollution, it is obviously desirable to maximise the proportion of the total sulphur absorbed by the ash. This proportion, when expressed as a percentage, is termed the sulphur fixation. The degree of sulphur fixation during combustion of coals is to a large 30 extent dependent on the properties of the coal ash. One of the aims of the Yarraboldy briquetting process is to produce ash with a maximum capacity to absorb the sulphur. 2 Method of Measuring Sulphur Fixation 10 The measurement of sulphur fixation first requires that the total sulphur content of the fuel be measured. Following this, there are two possible approaches: (i) Measure the quantity of sulphur oxides emitted with the combustion gases, or 5 (ii) Measure the quantity of sulphur remaining in the ash. Each method requires mass-balance calculations to determine sulphur fixation. For the results reported herein, the latter method was used. The steps taken were as follows: 10 (1) Prepare a representative laboratory sample of the briquettes, pulverised to -212 pm (nominally), (2) Measure the content of moisture, ash and sulphur in the laboratory sample, (3) Combust the laboratory sample in a laboratory muffle furnace at 15 815"C; retain the ash, (4) Analyse the ash to determine its sulphur content. The steps above are the essential steps. In addition to the above: (1) The laboratory briquette sample was also analysed for volatile matter content, 20 (2) The ash from the laboratory briquette sample was analysed for a number of elements (reported as oxides, as is the accepted practice). These analyses, though not essential, are a part of a normal identification of the test fuel. All of the above analyses were also performed on a laboratory sample of the parent coal. This would enable the properties of the briquettes to be compared 25 with the coal from which the briquettes were produced. 3 Laboratory Results The standard analysis results for coal and briquettes are given in Table 1. The results shown in bold were used to calculate the fixation of sulphur 30 during combustion of the briquettes. 4 Calculated Fixation of Sulphur Based on the sulphur mass-balance from the last column of Table 1, the 11 calculated sulphur fixation for the briquettes was 100%. Based on the analytical results for the parent coal in Table 1 it is also possible to calculate the sulphur fixation for combustion of the coal. In this case the result was 25%. 5 The implication of these results is: - For the unbriquetted coal, 75% of the sulphur was converted to gaseous oxides, whereas; - For the briquettes 0% of the sulphur was converted to gaseous oxides, since all of the sulphur was retained in the ash. Table 1: Standard Analysis of Chinese Coal and Briquettes Produced from the Coal Coal Analysis Coal Briquettes ProximateAnalysis ( adb) Moisture 1'"0 3.0 Ash 8.4 21.2 Volatile Matter 28:4 30.4 Fixed Carbon (by diff) 62.2 45.4 Total Sulphur As analysed (% adb) 2.01 1.85 Dry ash free basis (% daf) 2:22 2.44 Ash Analysis (expressed as oxides) SiO 2 36.4 12.3 A1 2 0 3 16.9 5.5 Fe 2

O

3 12.20 4.40 CaO 14.50 52.40 MgO 1.20 0.77 Na2O 0.63 0.23 K20 0.82 1.2 TiO 2 0.58 0.18 Mn 3

O

4 0.09 0.04

SO

3 15.20 22.80

P

2 05 0.08 0.09 BaO 0.09 0.04 SrO 1.80 0.58 ZnO 0.02 <0.01 Total 100.5 100.5 10 Laboratory tests were performed to determine the fixation of sulphur during combustion of briquettes in a laboratory muffle furnace at 815'C. The tests showed that 100 % of the sulphur in the briquettes was 12 fixated, that is it remained in the ash after combustion. This means that essentially no gaseous sulphur oxides were produced during combustion. The same procedure was used for combustion of the unbriquetted coal. In this case the fixation was 25%, meaning that 75% of the sulphur in the coal was 5 converted into gaseous sulphur oxides. Consequently it is concluded that the briquetting process was very effective in reducing the emission of gaseous sulphur oxides during combustion in the laboratory furnace.

Claims

1. A method of manufacturing a coal briquette with suppressed sulphur content, the method including the steps of: (a) reducing the size of the coal to increase porosity of coal particles formed; 5 (b) contacting the coal particles with a first part of a sulphur suppressant, a lime part; (c) contacting the coal particles formed in step (b) with a second part of a sulphur suppressant, up to 4% molasses at a temperature between 40'C and 60*C to increase the viscosity of the molasses allowing it to penetrate the coal particles 10 more thoroughly to give better binding and a more uniform reaction between the suppressant parts at lower binder weights; and (d) forming the mixture into briquettes.

2. A method of manufacturing a coal briquette according to claim I wherein the coal is mined and crushed to approximately minus 50 mm in a first 15 crushing step, the minus 50mm coal then further crushed to approximately about minus 4mm.

3. A method of manufacturing a coal briquette according to either claim 1 or claim 2 wherein the crushed coal particles are fed into surge bin to ensure continuity of supply to a process step following the surge bin. 20

4. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the first part of a sulphur suppressant is a calcium oxide and occurs in a mixer/blender.

5. A method of manufacturing a coal briquette according to claim 4 wherein the type of mixer/blender used enhances the penetration of the respective 25 sulphur suppressant parts into the pores of the coal particles.

6. A method of manufacturing a coal briquette according to any one of the preceding claims including at least one sampling step in which the degree of uniformity of the mixed intermediate and final product is measured by analysis of a number of spot samples from the mixer/blender. 30

7. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the lime part of the sulphur suppressant is chosen from the group of calcium oxide, hydrated lime or calcium hydroxide. 14

8. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the maximum amount of lime part used is approximately 8%.

9. A method of manufacturing a coal briquette according to any one of the 5 preceding claims wherein the molasses is added to lime impregnated coal particles formed in step (b) in sufficient quantities to bind the material up to an approximate maximum of 4%.

10. A method of manufacturing a coal briquette according to claim 9 wherein the molasses is added at between 0.25% and 4%. 10

11. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the temperature of the molasses is maintained between 400 C and 60*C, to increase the viscosity of the molasses allowing it to penetrate the coal particles more thoroughly to give better binding and a more uniform reaction between the suppressant parts at lower binder weights. 15

12. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the second part of the sulphur suppressant is thoroughly homogenised with a fine particle sized coal, which has previously been thoroughly homogenised with a lime part of the sulphur suppressant.

13. A method of manufacturing a coal briquette according to any one of the 20 preceding claims wherein step (d) includes a briquette forming apparatus.

14. A method of manufacturing a coal briquette according to claim 13 wherein the briquette forming apparatus is a roll briquetter having a pair of counter rotating pocketed rolls, which rotate to form a required briquette shape as the blended mixture formed in steps (a) to (c) is forced into the briquetter. 25

15. A method of manufacturing a coal briquette according to any one of the preceding claims further including a curing step following step (d) to allow moisture from the coal briquette to dissipate.

16. A method of manufacturing a coal briquette according to claim 15 wherein the curing step is performed for approximately 48 hours. 30

17. A method of manufacturing a coal briquette according to any one of the preceding claims wherein the formed briquettes are stockpiled in a manner to allow an upward movement of air to move over the stockpiled briquettes. 15

18. A method of manufacturing a coal briquette according to claim 17 wherein as the briquettes are added to the stockpile, the briquettes are subjected to a blast or flow of gas which causes any smaller particles to be distributed to the extremities of the stockpile while the larger particles are distributed in a central region 5 of the stockpile.

19. A method of accelerating the curing process of briquettes wherein formed briquettes are stockpiled, and as the briquettes are added to the stockpile, the briquettes are subjected to a blast or flow of gas which causes any smaller particles to be distributed to the extremities of the stockpile while the larger particles are 10 distributed in a central region of the stockpile.

20. A method of accelerating the curing process of briquettes according to claim 19 wherein the blast or flow of gas is in the form of an airstream.