Environmental Engineering Reference
In-Depth Information
taBle 11.9
ner of coal and co-Fired Wood
Waste electricity Generation
ner
(electricity out/fossil
energy in)
electricity source
Coal
0.31
5% Co-fire
0.32
15% Co-fire
0.35
Source: Mann, M.K. and Spath, P.L., Clean Prod
Processes , 3, 81-91, 2001.
Mann and Spath's (2001) energy results were calculated in terms of a NER and ratio of energy
inputs to electricity produced, as shown in Figure 11.23. Coal consumption at the power plant is
not included in Figure 11.23. In this study, the NER was defined as the electricity delivered to
the grid divided by the system fossil energy inputs. Mann and Spath's (2001) NER values agree
very well with the willow NER reported by Heller et al. (2004). The NER increases with the
biomass co-fire fraction, as shown in Table 11.9, directly reflecting an increase in overall system
ef iciency.
Mann and Spath (2001) found coal transport to be highly energy-intensive compared with mov-
ing harvested biomass. Production of limestone and lime, key components of sulfur dioxide (SO 2 )
power plant emissions control, were also significant contributors to life-cycle energy requirements.
In this study, biomass co-firing at rates of 5% and 15% reduced total energy consumption by 3.5%
and 12.4%, respectively.
11.3.2.3 hybrid Poplar
Hybrid poplar was studied as a feedstock source for a hypothetical biomass gasification combined-
cycle (IGCC) power plant located in the midwestern United States (Mann and Spath 1997). The bio-
mass was assumed to be supplied to the power plant in the form of wood chips that were transported
by truck and rail. Mann and Spath (1997) found that transportation had a relatively minor effect on
life-cycle results compared with the agriculture and power generation stages. The study included
three major components: agricultural production (including farm capital equipment production and
use, fertilizer and herbicide production and use, and biomass preparation), transportation (including
production and use of equipment and fuel), and electricity production (including capital equipment
construction and use). Raw material extraction and waste disposal options were also included within
the life-cycle boundaries.
The poplar was assumed to be grown on land adjacent to the power plant, thus minimizing trans-
portation distance. The biomass yield was nearly identical to the value found by Heller et al. (2003).
Nitrogen, phosphorus, and potassium were all applied as fertilizers, and herbicide was also deemed
necessary. Poplar growth and harvest occurred on a 7-year rotation, with no fertilizer applied until
year 4. Transport of the harvested biomass was 70% by truck and 30% by train. A simulated gasifi-
cation combined-cycle power plant was assumed to receive the poplar biomass as wood chips and
operate for 30 years. The first batch of poplar was planted and harvested seven years before the
power plant came online. Additional acreage was assumed to be brought into production according
to a carefully sequenced schedule such that harvest cycles would occur annually and keep the power
plant sufficiently supplied.
When ignoring parasitic losses at the power plant, production of the poplar feedstock required
77% of life-cycle energy. The distribution of energy requirements is shown in Figure 11.24. Farm
 
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