Energy - Environmental Science - page 320

Some analysts believe that liquid ethanol and
methanol produced from biomass could replace gaso-
line and diesel fuel when oil becomes too scarce
and expensive. Ethanol can be made from sugar and
grain crops (sugarcane, sugar beets, sorghum, sun-
flowers, and corn) by fermentation and distillation.
Gasoline mixed with 10-23% pure ethanol makes
gasohol, which can be burned in conventional gasoline
engines. Figure 13-42 lists the advantages and disad-
vantages of using ethanol as a vehicle fuel compared
to gasoline.
Methanol is typically made from natural gas but
can be produced at a higher cost from carbon dioxide,
coal, and biomass such as wood, wood wastes, agricul-
tural wastes, sewage sludge, and garbage. Figure 13-43
lists the advantages and disadvantages of using
methanol as a vehicle fuel compared to gasoline.
Chemist George A. Olah believes that establishing
a methanol economy is preferable to pursuing the highly
publicized hydrogen economy. He points out that
methanol can be produced chemically from carbon
dioxide in the atmosphere, which could slow projected
global warming. In addition, methanol can be con-
verted to other hydrocarbon compounds that can be

T rade-Offs

Solid Biomass

Advantages Disadvantages

Large potential
supply in some
areas

Nonrenewable if
harvested
unsustainably

Moderate to high
environmental
impact

Moderate costs

No net CO 2
increase if
harvested and
burned
sustainably

CO 2 emissions if
harvested and
burned
unsustainably

Low photosynthetic
efficiency

Plantation can
be located on
semiarid land
not needed for
crops

Soil erosion, water
pollution, and loss
of wildlife habitat

Plantations could
compete with
cropland

Plantation can
help restore
degraded lands

Often burned in
inefficient and
polluting open fires
and stoves

Can make use of
agricultural, timber,
and urban wastes

T rade-Offs

Ethanol Fuel

Advantages Disadvantages

Figure 13-41 Trade-offs: general advantages and disad-
vantages of burning solid biomass as a fuel. Critical thinking:
pick the single advantage and single disadvantage that you
think are the most important.

High octane Large fuel tank

needed

Lower driving range

Some reduction in
CO 2 emissions

Net energy loss

Producing Gaseous and Liquid Fuels
from Solid Biomass
Some forms of solid biomass can be converted into
gaseous and liquid biofuels.
Bacteria and various chemical processes can convert
some forms of biomass into gaseous and liquid biofu-
els. Examples include biogas (a mixture of 60% methane
and 40% CO 2 ), liquid ethanol, and liquid methanol.
In rural China, anaerobic bacteria in more than
500,000 biogas digesters convert plant and animal wastes
into methane gas that is used for heating and cooking.
After the biogas has been removed, the almost odorless
solid residue serves as fertilizer on food crops or on
trees. When they work, biogas digesters are very
efficient. Unfortunately, they are slow and unpre-
dictable, a problem that could be corrected by develop-
ing more reliable models. They also add CO 2 to the
atmosphere.

Much higher cost

Reduced CO
emissions

Corn supply limited

May compete with
growing food on
cropland

Can be sold as
gasohol

Higher NO emissions

Corrosive

Potentially
renewable

Hard to start in cold
weather

Figure 13-42 Trade-offs: general advantages and disadvan-
tages of using ethanol as a vehicle fuel compared to gasoline.
Critical thinking: pick the single advantage and single disad-
vantage that you think are the most important.