Environmental Engineering Reference
In-Depth Information
TABLE 5.6
Alkali Content and Slagging Potential of Biofuels
Fuel
Total Alkali (lb/MMBtu)
Slagging Potential
Wood
Pine chips
0.07
Minimal
White oak
0.14
Minimal
Hybrid poplar
0.46
Probable
Urban wood waste
0.46
Probable
Clean tree trimmings
0.73
Probable
Pits, nuts, shells
Almond shells
0.97
Certain
Refuse-derived fuel
1.60
Certain
Grasses
Switch grass
1.97
Certain
Wheat straw (average)
2.00
Certain
Wheat straw (high-alkali)
5.59
Certain
Rice straw
3.80
Certain
Source:
Adapted from Miles, T.R. et al., Alkali slagging problems with biomass
fuels. In:
First Biomass Conference of the Americas: Energy, Environment,
Agriculture, and Industry
, Vol. 1, National Renewable Energy Laboratory,
Burlington, VT, 1993.
a day to two, necessitating combustion system shutdown. A method to predict slag-
ging and fouling from combustion of biomass fuels has been adapted from the coal
industry. The method involves calculating the weight in pounds of alkali (K
2
O +
Na
2
O) per million Btu in the fuel as follows:
110
6
×
lb Alka
li
MMBtu
×
%
Ash%Alkali of theash
×
=
Btu/lb
This method combines all the pertinent data into one index number. A value below
0.4 lb/MMBtu (MMBtu = thousand thousand Btu) is considered a fairly low slag-
ging risk. Values between 0.4 and 0.8 lb/MMBtu will probably slag with increasing
certainty of slagging as 0.8 lb/MMBtu is approached. Above 0.8 lb/MMBtu, the fuel
is virtually certain to slag and foul (see Table 5.6).
Another process for biomass is the application of anaerobic digestion to produce
biogas (methane) for electricity generation. Recall that anaerobic digestion involves
the controlled breakdown of organic wastes by bacteria in the absence of oxygen.
Major agricultural feedstocks for anaerobic digestion include food processing wastes
and manure from livestock operations. The Energy Information Agency has also
projected a significant increase in the generation of electricity from municipal waste
and landfill gas—to about 0.5% of U.S. electricity consumption (EIA, 2006).
