Biomedical Engineering Reference
In-Depth Information
10.7.3.1 Grindability Index
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 rota-
tion, and energy input, it is necessary to know how much biomass would be
ground. HGI gives the comparative ease of grinding with reference to a stan-
dard coal. The higher the HGI the lower is the power requirements, and the
finer the particle size. It represents a fuel that is easier to grind.
An HGI-measuring machine is a miniature ball mill type of pulverizer.
Here, a standard mass (50 g) of coal is grounded for a given time in the mill
subjecting the balls to a known force. The resulting product is sieved to mea-
sure 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 Water, 1971; Joshi, 1978) a stan-
dard 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 et al. (2010).
The grindability index of torrefied biomass increases with torrefaction
temperature (Boskovic, 2013). It also depends on the type of biomass.
μ
10.7.4 Explosion and Fire
Dust explosion is a major problem in handling and conveying of fine dusts.
So, special attention is paid in PC-fired power plants where coal dust is
being conveyed or milled. Since pulverized biomass is being considered for
cofiring, one needs to explore this potential to ensure that presence of fine
dust of torrefied dust does not make the matter worse. In a typical explosive
situation, the dust could be ignited by an energy source, and it is followed
by rapid exothermic oxidation of the mixture. This leads to an increase in
temperature that further increases the reaction and the gas expands rapidly.
In a confined space like pipelines from the pulverizer to the burner, the pres-
sure increases with temperature. The combustion rate increases with temper-
ature and pressure, further aggravating the situation, which eventually leads
to explosion that could burst the pipeline or its confinement. Large buildup
of pressure leading to explosion is also possible even without a confinement.
The following factors favor dust explosion:
1. Fine particle size
2. High reactivity of dust materials
3. High concentration of dust in air
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