Improved Coal Pulverization Method Using the Embrittlement due to Cracks Generated in Pores of Coal

Pulverized coals produced by a rounding method have a wide range of particle sizes. Even though the coals have the same Hardgrove Grindability Index (HGI), some coals are difficult to pulverize by the rounding method. There are many pores in the coal. When pore pressure is rapidly reduced, cracks are generated in the pores by the expansion of air and the coal becomes brittle. The purpose of the present work is to investigate the performance of coal pulverization by an improved rounding method specifically by the embrittlement of coal due to the cracks generated in the pores. The experimental results show that pulverized coals for CWM which have a wide range of particle size can be easily produced by the improved rounding method. Furthermore, a discharge pressure at the embrittlement treatment can control the particle size of the pulverized coals.


Introduction
When producing CWM (Coal Water Mixture) from pulverized coal, coal particle size distribution must be adjusted to a wider range consisting of a large quantity of finer particles.To this end, we developed a method for rounding pulverized coal particles to create large quantities of finer particles which will ease the necessary adjustment of particle size distribution.As stated in a previous report, 1 this technique makes it possible to produce a high quality CWM.However, another problem still remains, for even though certain brands of coal might have the same HGI (Hardgrove Grindability Index) value, some are more difficult than others to pulverize using this rounding method.Therefore, an improved pulverization technique must be developed in order to expand the number of coal types usable in making CWM.
To address this problem, we focused our attention on the numerous pores present within coal and, on this basis, proposed a method for embrittling coal via the generation and propagation of cracks.First, coal powder is loaded into a pressure vessel and subjected to high pressure.Then, the pressure is rapidly released, causing the rapid expansion of the gases residing in the coal pores.In short, this method utilizes gas expansion pressure to trigger crack generation and propagation.If the coal powder embrittled in this manner is fine-pulverized using the rounding method, it becomes easier to adjust particle size distribution so it consists of large numbers of extremely fine particles that will be suitable for CWM.We report here that, through experimentation, we have investigated and been able to verify that, indeed, embrittled coal powder obtained by embrittling coal through a pressure release of 0.2-0.3MPa is easily fine-pulverized to attain a particle size distribution suitable for CWM.

Coal Pores and Embrittlement of Coal
Fig. 1 shows the particle size distributions of A coal (HGI҃50) Ҁ which is difficult to fine-pulverize by rounding Ҁ before and after the rounding process,

Abstract
Pulverized coals produced by a rounding method have a wide range of particle sizes.Even though the coals have the same Hardgrove Grindability Index (HGI), some coals are difficult to pulverize by the rounding method.
There are many pores in the coal.When pore pressure is rapidly reduced, cracks are generated in the pores by the expansion of air and the coal becomes brittle.The purpose of the present work is to investigate the performance of coal pulverization by an improved rounding method specifically by the embrittlement of coal due to the cracks generated in the pores.
The experimental results show that pulverized coals for CWM which have a wide range of particle size can be easily produced by the improved rounding method.Furthermore, a discharge pressure at the embrittlement treatment can control the particle size of the pulverized coals.analyzed using a Laser Micron Sizer (Seishin, LMS-30) particle size distribution analyzer.Included in the graph is an analysis of B coal (HGI҃55) Ҁ which provides a high-quality CWM Ҁ after the rounding process.
In the graph, the post-rounding 10% diameter is approx.1.6 mm for both A and B coal; the post-rounding 50% diameter for B coal is approx.9 mm, while that for A coal is approx.16 mm; and the post-rounding 90% diameter for B coal is 33 mm, while that for A coal is 88 mm.In sum, it is apparent that A and B coal differ in terms of the size of their larger particles.It is possible that A coal's tend to be larger due to the difficulty of fine-pulverizing its relatively large particles during the rounding process, despite the fact that fine particles are also generated.
Relatively large, less-pulverable particles can be pulverized but first must be transform into a ready-tocrack form.For this purpose, embrittlement of such particles via an embrittlement technique appears to be a promising solution.
Ordinarily, a very brittle solid substance will have numerous internal voids and cracks.When subjected to an external force, such a substance will fracture and break along these voids and cracks, leading to its pulverization.In some cases, pulverization will occur when the crystalline grain interfaces within a solid substance are subjected to a force which produces a shearing strain.
Coal is considered an amorphous substance.Therefore, to increase the brittleness of coal particles, it is necessary to trigger cracks, etc., within them.As a means for triggering such cracks, we focused our attention on the pores present within coal particles.As shown in the SEM (Scanning Electron Microscope) photograph of a cross-section of a coal particle in Fig. 2, ordinarily both relatively wide and narrow cracks appear as striations inside the coal particle.Furthermore, numerous pores appear as black dots of various sizes.Founded on the porous nature of coal, our method relies on the generation and propagation of cracks.The preferred technique of crack generation and propagation is to utilize gas impact force by loading coal powder into a pressure vessel and, after subjecting it to high pressure, rapidly releasing the pressure to produce a rapid expansion of the gases within the coal pores.In this manner, the coal is embrittled by the generation of cracks started at the pores and by the cumulative force of the already started cracks.It is then possible, we believe, to obtain a particle size distribution suitable for CWM production by fine-pulverizing the embrittled coal using the rounding process.

Experiment
In our experiment, using a commercial cereal puffer to measure the effects, high pressure was exerted on coal powder then rapidly released, causing the gases in the coal pores to expand violently and, thereby, triggering the emanation of cracking from  3, measures 250 mm҂300 mm in diameter.One end of the vessel was fitted with a butterf ly-shaped lid in order to effect the rapid release of pressure.The other end of the vessel was equipped with a gauge to monitor the internal pressure.In pastry-making, pressure is increased by heating with a burner.For our experiment, however, nitrogen gas was pressure-fed through pressure-gauge piping to increase the internal pressure, as shown in Fig. 4. Loaded with coal powder, the vessel's internal pressure was increased to a predetermined level with the pressurized nitrogen gas.Then, the lid was unlocked and opened, which caused the rapid release of pressure.As a result, the coal powder within the vessel was discharged and carried away by the outward f low of gas.To capture the discharged coal powder, a 250-liter polyethylene bag with a built-in lid was placed on the vessel as a collection bag.The amount of coal powder discharged per gas release cycle was set to approx.400 ml.However, not all the coal powder was discharged from the vessel; a certain amount remained in the vessel in every discharge cycle because the discharge port measured only 150 mm in diameter.
Until a total of 2 liters of coal powder (an amount needed for the rounding stage of our experiment) had been embrittled, the above-mentioned sequence was repeated with our sample coal powder repeatedly filled into the vessel.As a result, coal powder accumulated in the vessel.Despite this fact, for our purposes, only the powder discharged from the vessel and captured in the collection bag was treated as embrittled coal powder because the discharged portion of powder was, we reasoned, subjected to the greatest expansion pressure.The experiment was repeated at each of four discharge pressures (0.2, 0.3, 0.5 and 0.7 MPa).
Next, in order to fine-pulverize the embrittled coal powder, the prototype rounding device shown in Fig. 5 was employed. 1The device consists of two disks (stationary disk and rotary disk) with meshing crusher teeth.The coal powder is fed through the center portion of the stationary disk.The coal particles sandwiched between the two crushing teeth are transported in circumferential, radial, and axial directions.During this process, the corners of our embrittled sample particles wore off by colliding with one another or through the friction of the crusher teeth, making the particles spherical and, at the same time, generating fine particles out of the shaved off corner portions.

Results of Experiment, Obser vations
We evaluated the effectiveness of the embrittlement of the coal powder by verifying crack occurrence through the inspection of SEM photographs of crosssections of the coal particles.In addition, after fine pulverization via the rounding process, we measured the particle size distribution of the fine powder product.

SEM Inspection
For comparison purposes, the cross-sectional SEM photographs of the coal particles both before and after embrittlement are shown in Fig. 6.Photos (a), (b), and (c) each represent a cross-sectional view of the coal particles before embrittlement, and photos (d), (e), and (f) each show a cross-sectional view of the coal particles after embrittlement.From these photos, each pre-embrittlement cross-sectional view exhibits innumerable pores as dots together with striated cracks.In contrast, each post-embrittlement cross-sectional view shows far fewer dots and an increased number of striated cracks.Moreover, the length of post-embrittlement striations is much longer and their width twice as large as the pre-embrittlement striations.
From these findings, it can be safely judged that subjecting coal powder to the rapid release of high pressure causes the gases residing in coal pores to expand violently, which results in the formation and spread of cracks.

Fine Pulverization by Rounding
Embrittled coal powder was fine-pulverized with the rounding device illustrated in Fig. 5.Then, the particle size of the fine-pulverized coal powder was again measured using the particle size distribution analyzer.The results of our analysis is graphed in Fig. 7 and Fig. 8. Fig. 7 shows the particle size distributions of coal powders that were embrittled with discharge pressures of 0.2, 0.3, 0.5 and 0.7 MPa using the cereal puffer, and then were fine-pulverized using the previously mentioned rounding technique.For comparison purposes, this diagram also illustrates the particle size distributions of a non-processed coal powder and a coal powder sample that was processed by rounding but was not embrittled.Fig. 8 illustrates our investigation into possible changes in coal powder particle size based on whether or not crack propagation affects particle size during the embrittlement process.From these findings, it is apparent that the particle size of the coal powder exhibited virtually no change even when cracking was triggered by effecting the rapid expansion of the coal pore gases.Nevertheless, this coal powder was easily fine-pulverized via the rounding process.The particle size after fine-pulverization, for 90% diameter, was no greater than 35 mm regardless of the level of pressure exerted in the embrittlement process.The particles at 50% diameter were 16 mm, without having undergone the embrittlement process.Although results varied somewhat depending on the amount of pressure exerted, the coal powder sample at 50% diameter after having undergone the embrittlement process, fell into the much lower range of 3 to 5 mm.Consequently, our method proved to be able to readily prepare coal powder that has a particle size distribution suitable for CWM formulation involving a large proportion of finer particles.Employing this method, a CWM of viscosity 1000 mPa-s at approx.65% concentration was successfully produced with A coal, which otherwise would have been a material difficult to pulverize.
Additionally, Fig. 9 illustrates the relationship between the pressure exerted and post-pulverization particle size Ҁ a greater discharge pressure led to a smaller 50% diameter.This means that by controlling the discharge pressure, the resultant post-rounding particle size can be controlled as required.

Conclusion
We proposed a coal powder embrittlement method by which the gases in coal particle pores are compressed and then rapidly released, causing the violent expansion of the gases and, thereby, the generation of cracks emanating from the coal pores.We also proposed a grounding method for fine-pulverizing the coal particles thus embrittled, and demonstrated the effect of the combination of these methods in our experiment.The results clearly show that the particle size of coal powder, even after the embrittlement procedure, remains virtually unchanged while, in contrast, fine-pulverization is readily achieved by the rounding procedure.
To sum up, by employing the method detailed herein, previously less pulverable coal brands that generate a lower amount of fine particles can be used to produce a large amount of finer particles.This method will contribute to the better production of high-quality CWM as well as to an increase in the range of coal brands usable in CWM manufacturing.In addition, the embrittlement method put forth here may make it possible to obtain the particle size needed for a given application, since controlling the discharge pressure during the embrittlement process changes the particle size produced in the rounding process.

Fig. 1
Fig. 1 Particle size distribution of pulverized coal

Fig. 6
Fig. 6 SEM photographs of cross-section of coal particles before and after embrittlement

Fig. 9
Fig. 9 Relation between pressure and particle diameter