Hardgrove Grindability Index (HGI) Vs. Anthracite Quality?

Hardgrove Grindability Index (HGI) Vs. Anthracite Quality?

PostBy: lsayre On: Sun Aug 26, 2012 11:18 am

Is there any known correlation between the HGI of anthracite and its quality (meaning its analyticals as well as its potential burning characteristics)? I believe the lower the HGI value, the harder the coal.

I was also wondering if a simple (poor mans) hardness test of one coal vs. another would be possible, such as putting like size pieces of each back to back (or coal on coal) in a vice and slowly close the jaws, watching which piece crushes first in the process. The one that crushes first (or most) would presumably be the softer of the two coals. Would that indicate a higher fixed carbon and/or lower volatilities or ash content for the one that crushes last (or least) in such a coal on coal vice-crush test?
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Blaschak Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

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Re: Hardgrove Grindability Index (HGI) Vs. Anthracite Quality?

PostBy: Rob R. On: Fri Aug 31, 2012 9:02 am

lsayre wrote:I believe the lower the HGI value, the harder the coal.

Generally speaking, that is correct. I believe the test was developed to be an indication of how hard it is to grind the coal for power-plant type combustion.

For our home-heating purposes, I wouldn't worry about it too much. Just because one source of anthracite is harder than other doesn't mean you would be happier with the burning characteristics. Lehigh has some very hard anthracite, and it tends to have a lower volatile content as well. I purchase a load from them a few years ago and the coal looked like broken volcanic glass. I burned it in hand-fired equipment, and it was noticeably harder to get the coal ignited and burning...but it burned to a powder and burned a long time. In underfed or flat bed stokers an exceptionally hard coal will probably require an air adjustment, and might be more sensitive to outfires. In an AA or AHS you might see more unburned coal around the edge of the combustion tube.

I have burned some coal from the Western end of the Southern anthracite field that had a noticeable sulfur smell when burned, and seemed to be softer than average. I'm guessing the volatile content was higher than average, but it burned very hot and was easy to light/keep burning...so no complaints from me.
Rob R.
Stoker Coal Boiler: EFM DF520
Hand Fed Coal Stove: Hitzer 50-93
Coal Size/Type: Lehigh Rice
Other Heating: Dad's 1953 EFM Highboy