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
Coal Pulverizer 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 (Hardgrave Grind ability 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 which grinding by
Coal mill 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 grinding powder embitter in his 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, embitter coal powder obtained by embrittling coal through a pressure release of 0.2-0.3 MPa 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 (HGI 50) which is difficult to ine-pulverize by rounding before and after the rounding process, Tetsuo Ono Department of Chemical Energy Engineering, Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI)* Improved Coal Pulverization Method Using the Embrittlement due to Cracks Generated in Pores of Coal
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 embitterment 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 with
coal ball mill Furthermore, a discharge pressure at the embitterment treatment can control the particle size of the pulverized coals.
Included in the graph is an analysis of B coal (HGI 55) which provides a high-quality CWM 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 coals 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-vulnerable particles can be pulverized but first must be transform into a ready-to crack form. For this purpose, embitterment of such particles via an embitterment 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.
