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While biomass is a great renewable energy source, it is not a
good fuel, because it typically contains more than 70% air and
void space. This low volumetric energy density makes it
difficult to collect, ship, store and use.
Densification is a relatively new process in which the
air is squeezed out at very high pressure to make pellets (using
feed type machines), cubes (using alfalfa cuber) or logs (PrestoLog
etc.). Best of all, for many applications almost any biomass can
be used provided it is chopped fine: sawdust; agricultural
residues and even municipal solid waste.
Pellets
Cubes
Logs
Once densified, the fuel has many uses.
Co-firing with coal
Home Pellet Stoves Micro power plants WoodGas
Cooking
Mass and Volume Energy Densities: The High Heating
Value (kJ/kg or Btu/lb) is a mass energy density of the fuel.
(See biomass table of HHV).
However, for biomass a more important figure is the volume
energy density, (kJ/liter, MJ/m3 , Btu/ft3).
Because most biomass is so light, weight is not nearly as much a
factor as volume in collecting, shipping, storing and using.
The following table shows the dramatic difference between
high density and low density biomass fuels. The dense
biomass fuels are a factor of 3 to 4 times heavier than wood
chips, though not as dense as coal or diesel.
Mass and Volume Energy Densities of Some Fuels
|
FUEL
|
Bulk Density
kg/liter
|
Mass Energy Density
MJ/kg
|
Volume Energy Density
MJ/liter
|
|
Softwood chips ("Denver dry", 7% MCWB)
|
0.19
|
20
|
3.8
|
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Home Depot 1/4" sawdust pellets
|
0.68
|
20
|
13.6
|
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3/8" peanut shell pellets
|
0.65
|
19.8
|
12.9
|
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Corn
|
0.76
|
19.1
|
14.5
|
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Soybeans
|
0.77
|
21??
|
16.2
|
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Coconut shell (broken to 1/4 inch pieces)
|
0.54
|
20.5
|
11.1
|
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Coal (Bituminous)
|
1.1?
|
32.5
|
35.7
|
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Biodiesel
|
0.92
|
41.2
|
37.9
|
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Diesel
|
0.88
|
45.7
|
40.2
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[From various sources, especially Appendix A of our
"Thermal Data for Natural and Synthetic Fuels" (Gaur
and Reed, Dekker, 1998)]
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