Grinding studies, and the resulting HGI, allow many different types of coal users to evaluate how coals will perform in their mills, allowing them to estimate grinding power requirements and throughput capacities.
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HGI correlates to VM content, and the relationship is empirically specified for most of the hard coals and given with VM from 10% to 38% (db) by Eqs. (11.6) and (11.7):(11.6)HGI=173.03.30(VM)forVM21.72%(11.7)HGI=37.02.96(VM)forVM<21.72%
Grindability is an important factor for the characteristic of the brittle raw materials by the design the mills. As grindability depends on many unknown factors, hardgrove grindability index (HGI) is determined empirically using a sample mill, according to the following procedure. The device to measure the hardgrove grindability consists of a top rotating ring with eight balls of 1in. (25.4mm) diameter. A load of 640.5lb. (29.50.23Kg) is applied on the top rotating ring. The HGI is defined as:
HGI measures the ease of size reduction of a coal. HGI is generally related to coal rank -increasing with rank through the bituminous range but decreasing rapidly above 90%C (daf) in the anthracitic range.
Table 2 compares HGI, drop strength, abrasion resistance (Tumble Test) and water resistance data for the 4 mm coal only briquettes. These data distinguish the briquettes of coals A and B, which are two of the three lowest HGI coals in the set, as having significantly lesser abrasion and water resistance than the higher HGI coals. Also coal A briquettes have a much lower drop strength than the others. The dependence of compression strength on HGI plotted in Figure 1 again shows the low HGI coal briquettes to be of lower strength. However for high HGI coals, the briquette compression strength decreases with HGI. Reinforcement by bagasse fibres contributes to the greater compressive strength of coal/bagasse composite briquettes as is also shown in Figure 1. In contrast to the coal-only briquettes, compression strength for the composite briquettes increases monotonically with HGI. These data indicate that the strongest briquettes are formed with coals of HGI in the range 6590 a result consistent with earlier reports .
The grindability of coal is important for entrained-flow gasifiers, which require pulverized coal for dry feeding systems or coal with a wide range of particle size distribution for slurry feeding systems. Because grinding is an inefficient process, choosing coals that are easier to grind can increase the overall efficiency of the process.
There are several methods to estimate the grindability of coals. One of the most commonly used methods is the Hardgrove Grindability Index (HGI). HGI for coal is determined by grinding 50g of sample in a ball and ring type mill for 60 revolutions, after which the product is sieved through a 200 mesh sieve (75 micron screen) (Speight, 1994). HGI is computed by measuring the undersize of the product. Coals with higher HGI are easy to grind (Speight, 1994).
The influence of coal rank on grindability is not straightforward due to the complexity of maceral composition, moisture, volatile matter, and associated mineral matter content (Hower, 2006; Sengupta, 2002). Separating each of those factors is not easy. Moreover, HGI is difficult to interpret, particularly for low-rank coals. This is because the moisture content, which influences the HGI, can vary during the test. Although various factors can influence grindability, an approximate correlation exists between grindability and maceral content, as shown in Fig. 2.1. Vitrinite was shown to positively influence HGI, while liptinite and micrinite (an inertinite maceral) were shown to affect HGI negatively (Jorjani and Mesroghli, 2008). Similarly, coal rank can also affect HGI, as low-volatile and medium-volatile bituminous coals are easy to grind compared to lignite and anthracite coals (Berkowitz, 1979). The correlation between HGI and coal rank (or vitrinite reflectance) is shown in Fig. 2.2.
We note from the above paragraphs that torrefaction reduces the energy required for grinding biomass. This section discusses the grinding issue further. Torrefaction temperature is the most influential parameter affecting grinding of the terrified biomass. The higher the temperature at which torrefaction is performed, the lower is the energy requirement for grinding, or for a given energy input, more amount of fine particles are obtained after grinding. Grinding of torrefied biomass gives smaller and uniform size distribution of the product (Phanphanich and Mani,2011). The grinding energy requirement for a specified level of grinding decreases with torrefaction temperature. Herein, one notes that the specific energy consumption reduced from about 250kWh/ton for raw biomass to about 50kWh/ton for that torrefied at 280C.
Grindability index is a measure of the ease of grinding of a given feed. Utility industries use the Hardgrove grindability index (HGI) to express this parameter. In the direct cofired system, the existing mills designed for coal are used for grinding the cofired biomass. So for a given mill, given rotation, and energy input, it is necessary to know how much biomass would be ground. HGI gives the comparative ease of grinding with reference to standard coal. The higher the HGI the lower is the power requirements, and the finer the particle size. HGI represents a fuel that is easier to grind.
An HGI-measuring machine is a miniature ball mill type of pulverizer. Herein, a standard mass (50g) of coal is grounded for a given time in the mill subjecting the balls to a known force. The resulting product is sieved to measure amounts dropping below 75-m size. This amount is compared against some specified standards to define the parameter, HGI. As the HGI ball mill works on the same principle as pulverizing mills, the index obtained from this could give a fair assessment of the grinding capability of torrefied biomass.
For biomass, one should use (Agus and Waters,1971; Joshi,1979Agus and Waters,1971Joshi,1979) a standard volume of sample instead of mass to compare the grinding ease to coal and torrefied biomass. Thus an equivalent HGI was used to define the grinding ease of torrefied biomass. More details are given in Bridgeman etal., (2010).
The grindability of coal (i.e., the ease with which coal may be ground fine enough for use as pulverized fuel) is a composite physical property embracing other specific properties such as hardness, strength, tenacity, and fracture. Several methods of estimating relative grindability utilize a porcelain jar mill in which each coal may be ground for, say, 400 revolutions and the amount of new surface is estimated from screen analyses of the feed and of the ground product. Coals are then rated in grindability by comparing the amount of new surface found in the test with that obtained for a standard coal.
The test for grindability (ASTM D409) utilizes a ball-and-ring-type mill in which a 50g sample of closely sized coal is ground for 60 revolutions after which the ground product is screened through a 200-mesh sieve and the grindability index is calculated from the amount of undersize produced using a calibration chart. The results are converted into the equivalent Hardgrove grindability index. High grindability index (GI) numbers indicate easy-to-grind coals. There is a rough relationship between volatile content and grindability in the low-, medium-, and high-volatile bituminous rank coals. Among these the low-volatile coals exhibit the highest values of the grindability index, often over 100. The grindability indexes of the high-volatile coals range from the mid-50s to the high-30s. There is also a correlation of sorts between friability and grindability. Soft, easily fractured coals generally exhibit relatively high values of the grindability index.
Grindability is normally measured by determining the number of revolutions in a pulverizer that are required to achieve a given size reduction. This is an indirect measure of the amount of work required for that size reduction. Grindability increases slowly to a maximum at 90% carbon and decreases rapidly beyond this point. Hardness is ascertained by measuring the size of the indentation left by a penetrator of specified shape pressed onto the coal with a specified force for a specified time. It changes only slightly between 70 and 90% carbon and increases rapidly beyond 90% carbon.
Grindability refers to the ease of pulverizing a coal sample compared to certain reference coals. The standard method of coal grindability is called Hardgrove (ASTM D409), though this method does not provide reproducible and repeatable results for hard coal. Another method that can be accepted for petroleum coke is the test method ASTM D5003. According to the Hardgrove method, a prepared and sized sample receives a definite amount of grinding energy in a miniature pulverizer and the size of the pulverized product is determined by sieving. The resulting size distribution is used to produce an index relative to the ease of grinding.
During handling and preparation, physical changes (seam moisture) may occur to high-volatile bituminous and subbituminous coal and lignite. These changes are often sufficient to alter the grindability characteristics of the samples tested in the laboratory, thus producing different indices. The drying conditions and the moisture level can cause inconsistencies in results concerning repeatability and reproducibility.
The grindability test (ASTM D409, ISO 5074) uses a ball-and-ring type of mill in which a sample of closely sized coal is ground for a specified number (usually, 60) of revolutions. Then the ground product is sieved and the grindability index is calculated from the amount of undersize produced using a calibration chart. The results are converted into the equivalent Hardgrove grindability index. High grindability indices refer to easily ground coals. Prior to the experiments, each Hardgrove machine should be calibrated with reference samples of coal. The reference indices used are 40, 60, 80and 100.
In some cases, data fall outside the experimentally allowable limits. This is due to the following factors: (1) there is no equilibrium with the sample moisture and the laboratory atmosphere; (2) the sample may have been over dried or under-air-dried; (3) excessive dust loss may have occurred during screening due to a loose-fitting pan and cover on the sieve; or (4) the sample may not have had an even distribution of particles (Speight, 2005).
The Hardgrove Grindability Index (HGI) is used to determine the grindability of the coal. This index is important for gasifiers that require fine coal as feed, as with entrained flow gasifiers. For slurry-fed entrained flow gasifiers there is a trade-off regarding the grindability of a coal; a higher solid concentration with a coarser coal slurry may be produced, but larger particles will not gasify as well as smaller particles (Collot, 2006). Apart from milling and grinding, the hardness of a coal is not very relevant for gasification.
Hardgrove Grindability Index analysis is the ASTM method of determining the hardness of coal when pulverized for steam generation or size reduction. Grindabilities typically range on coal from 35 110. Harder and less grindable coal rates a smaller HGI index number.
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