Environmental Engineering Reference
In-Depth Information
Organic matter in terrestrial plants and soil is, among other things, a temporary storage system
for solar energy. The conversion of atmospheric CO
2
and H
2
O to organic matter by photosynthetic
reactions in plants stores the energy of visible light from the sun in the form of chemical energy
of the organic matter. The latter may be utilized in the same manner as fossil fuels, releasing CO
2
back to the atmosphere. Thus no net emissions of CO
2
to the atmosphere result from this cycle.
However, the efficiency of conversion of solar radiation to biomass energy is very low, less than 1
%, with the terrestrial average energy conversion rate amounting to 0.5 W/m
2
.
The food energy stored in agricultural crops like grains is only a fraction of that in the entire
crop mass, so that agricultural crop residues are potential sources of biomass energy. It has been
estimated that crop and forest residues in the United States have a heating value equal to 12% of
fossil fuel consumption, but only about one-fifth of this is readily usable.
3
The energy required
to collect, store, and utilize this residue further reduces the amount of energy available from it to
replace fossil fuels. Furthermore, crop residues are often used to build soil mass and fertility by
composting, during which some, but not all, of the residue carbon is released to the atmosphere as
CO
2
. Using residues as fuel reduces the amount of carbon uptake in soil.
The overall photosynthetic process by which water and carbon dioxide are combined to form
carbohydrate molecules in plants may be summarized as
n
CO
2
+
m
H
2
O
→
C
n
(
H
2
O
)
m
+
n
O
2
(7.1)
The first step of this overall process is the photosynthetic one, in which solar radiation provides
the energy needed to start the process that ends in the production of carbohydrates, such as sugar,
starch, or cellulose. Subsequent steps dissipate some of that energy, storing the rest in chemical
form. The process of conversion of solar energy into biomass is called primary production. All
living systems depend ultimately upon this process to maintain their viability.
Agricultural crops supply humans with food and materials (e.g., lumber, paper, textiles) as
well as energy. On an energy basis, the price to consumers of these commodities exceeds that of
fossil fuels, often by a considerable amount. To compete economically with traditional agricultural
commodities for scarce agricultural resources, biomass energy supplies must use otherwise under-
valued byproducts and waste streams or efficient, low capitalization facilities that convert primary
biomass to higher value fuels.
The principal processes that utilize the energy content of primary biomass are as follows:
•
Combustion
•
Gasification
•
Pyrolysis
•
Fermentation
•
Anaerobic digestion
Woody plants and grasses can be burned directly in stoves, furnaces, or boilers to provide cooking,
space, or process heat and electric power. Alternatively, biomass may be converted to a gaseous
fuel composed of H
2
and CO in a thermal process that conserves most of its heating value, with the
fuel being combustible in boilers and furnaces. Pyrolysis, the thermal decomposition of biomass,
3
D. Pimentel et al., 1981.
Science
212,
1110.
