Hardgrove grindability index (HGI)

1. PRESENTATION TOPIC HARDGROVE GRINDABILITY INDEX AND EFFECT OF COAL BLENDIN ON IT PRESENTATED BY KOMAL AROOSH (2010-2012) DEPARTMENT CENTER FOR COAL TECHNOLOGY PUNJAB UNIVERSITY LAHORE

2. GRINDABILITY OF COAL The measure of the coal resistance to crushing is said to be the grindability of a coal. Generally, lignite and anthracites are more resistant to grinding than bituminous coals. Two major factors, the moisture and ash contents of a coal are affecting the grindability. One commonly used method for assessing grindability is the hard grove test.

3. COMMON TEST SERVINGS TO DETERMINE THE GRINDABILITY FACTOR On industrial scale, WI work index by bond and HGI tests serving are used to determine the value of grindability. The Hard grove method is mostly used to determine the grindability of coal, but also in determining grindability of other raw materials. Bond’s method is used to evaluate the rock grindability.

4. HARDGROVE GRINDABILITY INDEX (HGI)   The Hardgrove Grindability index is designed to determine the ease with which coal can be pulverized. It measures the grindability of coal and is a qualitative measure of coal. It provides information for determining grinding power consumption and pulverizing capacities. HGI of coal depends on the coal composition.

5. It can be used by industry to determine power consumption during the grinding process. Usually softer coals have higher HGI. Some physical properties of coal such as hardness, tenacity and fracture can also be shown by it. Between grindability and rank of the coal there exists a fixed relationship. Coal easier to grind have low volatile matter and lower moisture. The moisture content and the humidity in which HGI has been conducted affect the results.

6. HARDGROVE GRINDABILITY INDEX (HGI)

7. FEATURES OF HGI MACHINE N O OF BALLS = 8 B ALLS MATERIAL = Iron D IAMETER OF EACH B ALL = 25.4±0.003 mm N O OF R EVOLUTIONS = 60 N O OF LOADED RINGS = 3 W T OF EACH LOADED EING = 600gm

8. PROCEDURE TO DETERMINE THE HGI VALUE The experimental procedure for the determination of hard grove grindability index HGI is as follow according to the ASTM standard ; The 50gm sample of prescribed size -16+30 mesh is taken in the ball mill of HGI machine along with 8 iron balls. Close the mouth of the ball mill , and set to rotate for exactly 60 revolutions. After the requisite rotation , the machine stopped automatically. The sample left in the ball mill is then collected along with any powdered substance sticking to the surface of the machine with the help of a brush. This sample is then put in a sieve of 75μ size and is shaken for about 10 minutes. The sample which passed through 75μ size and retained on 300 μ size is weighed on the balance.

9. CALCULATIONS The hard groove grind ability index of coal is than calculated using the following formula. HGI = 13 + 6.93 w   Where, w = weight of the test sample passing through 75μ sieve and retained on 300 μ sieve after grinding in the HGI machine.

10. F ACTORS AFFECTING THE GRINDABILITY VALUES 1. PETEROGRAPHIC COMPOSITION The hard grove grind ability index value is influenced by petrographic composition of coal.

11. 2. COMBUSTIBLE MATTER Commonly, the increase of contents of volatile combustible matter improves the grindability up to the contents of volatile combustible matter of approximately 30%, beyond which the grindability deteriorates. Similarly the HGI value increases with the growth of carbon contents, the grindability then drops rapidly with the contents of carbon exceeding approximately 92%.

12. 3. MASCERALS a. LITHOTYPES The lithotypes of black coal with similar contents of volatile combustible matter include differences in the HGI values. b. DURITE Durite (dull coal) is a lithotype characteristic by low HGI and is generally the toughest.

13. c. FUSITE AS LITHOTYPE Labeling fusite as lithotype with the highest HGI value is disputable, because its extreme fragility is caused by origination of significant amount of fine fractions rather during screening that in the course of grinding in the testing device ( hower , 1988). d. GLITTERING VITRINITE LITHOTYPES Glittering vitrinite lithotypes in black coal have significantly higher HGI values than durites in the same coal group. The difference in grindability of individual lithotypes allows for selective grinding. It is generally valid that the occurrence of vitrinite in coal increases the HGI value, whereas the micronite and liptinite macerals decrease the grindability.

14. COAL PROPERTIES COMPLEMENTARY TO HGI Coal calorific value, moisture content and volatile matter content/reactivity can be complementary properties to HGI. If the volatile matter content or reactivity is higher than normal, then coal particles somewhat larger will burn out successfully in the normal furnace volume. This coarser product requirement allows the mill to provide more output. Alternatively it allows the use of coal with a lower HGI with effective particle burnout. Thus a combination of coal properties is necessary to ensure relatively complete combustion/reaction in normal boiler furnaces

15. Coal with less volatile matter content or lower reactivity will require more particles to be less than 75 microns for fairly complete reaction. This applies whether the reaction is: • combustion in a boiler to generate power • pulverized coal injection in a cement kiln to make cement clinker • pulverized coal injection in a blast furnace to assist economic iron making • entrained particle gasification to make syngas for power generation • entrained particle gasification to make syngas for chemical production.

16. EFFECT OF BLENDING ON HGI Personally, I determined the HGI value for some of the coal blends of Kotali and Afghan coal in the lab of CENTER FOR COAL TECH PUNJAB UNIVERSITY LAHORE under the guidance of Dr . Shafqat Nawaz and got the following graph , while comparing the calculated and measured values on the basis of wt blending ratio of Afghan coal %.

17. The results of this study indicate :- Experimentally determined values of HGI of coal blends differed by about 0.682 to 2.414 with the weighted average computed values. These results are supported by the studies Vuthaluru et al who studied the effect of moisture and coal blending on Hard Groove Grindability Index of Western Australian coal where it has been observed that measured HGI values of binary and tertiary blends corresponded well with the weighted average values of HGI within ±2 .

18. Rubeira et al found that the HGI value have linear relationship when blending of two different hard coals were considered. The difference between weighted averages computed values and experimental values are found to be between 1.88 to 3.522 . Contrary to the above stated findings of researchers, findings of Conroy and Bennett state that even if coals of similar HGI were blended, the measured HGI values of blends were lower than the calculated values, in some cases lower than the either blended components. Thus it was concluded that there is no general method of predicting the HGI of the coal blend, which must be determined experimentally on case- to case basis.

19. According to the above study, the blend no 3 shows the optimum result for cement factories with approximately HGI 45 and sulphur content 1.054 , while blend no 5 is suitable for power sectors with approximately HGI 66 and sulphur content 1.469 . The value of the blends for HGI and moisture content shows the inverse relation, as the moisture content increases HGI content going to decreases. It is generally valid that the occurrence of vitrinite in coal increases the HGI value, whereas the micronite and liptinite macerals decrease the grindability.

20. REFFERENCES [1] http://www.britannca.com/EBchecked/topic/286/grindability [2] http://gse.vsb.cz/2008/LIV-2008-1-27-32.pdf [3] http://wiki.answers.com/Q/What_is_Hardgrove_grindability_index [4] http://gse.vsb.cz/2008/LIV-2008-1-27-32.pdf [5] Hardgrove Grindability Index; www.acarp.com.au/Downloads/ACARP Hardgrove Grindability Index.pdf. [6] C. Ashley., B. Phill ., Evaluate combustion behavior of Australian export and overseas low rank coal blends; < www.acarp.com.au/abstracts.aspx? RepId =C3097 accessed on 20 Oct 2012. [7] T. Wall., L. Elliott., D. Sanders., C. Ashley, Technology Assessment Report 14, Cooperative Research Centre For Black Coal Utilisation , NSW Australia, 2001, p. 22. [8] J. T. Riley., S. R. Gilleland ., R. F. Forsythe., H. D. Graham., F. J. Hayes, Non-additive analytical values for coal blends, Proceedings of the 7th international conference on coal testing, Charleston, WV, USA, 21-23 Mar. 1989. Ashland, KY, USA, Coal Testing Conference, Standards Laboratories, Technical Services Division, pp. 32-38, 1989.