Sustainability: Humanity Perspective - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 778

undisturbed forest. For longer product life span or CO 2 capture and storage after biomass
conversion, a managed forest for biomass can turn the forest into a net CO 2 sink.
For a mature undisturbed forest, the total biomass storage is maximal at approximately the
same as the carrying capacity, i.e.

X SU ¼ X N (15.13)
where X SU is the total biomass storage for a mature undisturbed forest. The total “biomass
storage” for an optimally managed forest where harvesting is within the exponential growth
regime is given by

t L
t max þ

X max X 0
ln ðX max =X 0 Þ

t L
t max

X max X 0
t max

X ST ¼ X av þ P X t PS ¼ X 0 1 þ t PS (15.19)

where X ST is the total biomass storage for an optimal managed forest, X 0 is the standing
biomass right after a harvest, t L is the lag time from the last harvest to the exponential growth
start, X max is the standing biomass at optimal harvest time t max , and t PS is the average life
span of the harvested biomass from the time of harvest to the emission of CO 2 into the

atmosphere.
If the growth follows the logistic growth rate between the end of the lag phase and
harvesting point, the average standing biomass for a managed forest is given by

t L
t H þ

X N t L

t H

X av ¼ X 0 1 (15.20)

lnðX H =X 0 Þ
ln½ðX N X 0 Þ=ðX N X H Þ

1 þ

and the total “biomass storage” for an optimally managed logistic growth forest is

t L
t max þ X N

X max X 0 t L

t max

X max X 0
t max

X ST ¼ X 0 1 þ t PS (15.21)

lnðX max =X 0 Þ
ln½ðX N X 0 Þ=ðX N X max Þ

1 þ

Therefore, in general as depicted by Fig. 15.13 , the carbon storage will decrease and CO 2 in
the atmosphere will increase when we start to manage forest for energy, chemicals, and mate-
rials. The net increase in the CO 2 in atmosphere is due to the short average life span of the
harvested biomass before converted to CO 2 . Burning of biomass for electricity and/or heat
emits CO 2 shortly after harvesting. If one focuses on some special cases of applications, for
example long-lasting materials, the net effect can be the opposite: a net capture of CO 2
from atmosphere.
Converting grassland to forest can dramatically increase the CO 2 capture from atmo-
sphere to be stored as standing biomass. The net storage of biomass by grass and agricul-
tural crops is negligible as compared with forest or trees. For example, the CO 2 storage
due to forest growth in China and the developed world is substantial when compared
with their pre-industrialized past due to the shifting of low productive agriculture crop
land to forest.
Solar energy is vastly available and solar energy use appears to be less disturbing to the
ecosystems. It is clear that in the direct solar energy utilization cycle (other than the construc-
tion of the facility), there is no water, carbon dioxide, or any other substance emission