Page 295 - Synthetic Fuels Handbook

FUELS FROM CROPS 281

and 10 years in the Corn Belt, Lake States, Northeast, and Northern Plains regions. Thus,
if it were planted in the spring of 2000, switchgrass could be harvested in 2000 or 2001,
willow could be harvested in 2004, and poplars could be harvested in 2006, 2008, or 2010,
depending on the region.
The use of cellulosic biomass in the production of ethanol also has environmental ben-
efits. Converting cellulose to ethanol increases the net energy balance of ethanol compared
to converting corn to ethanol. The net energy balance is calculated by subtracting the energy
required to produce a gallon of ethanol from the energy contained in a gallon of ethanol
(approximately 76,000 Btu). Corn-based ethanol has a net energy balance of 20,000 to 25,000
Btu/gal, whereas cellulosic ethanol has a net energy balance of more than 60,000 Btu/gal.
In addition, cellulosic ethanol use can reduce greenhouse gas emissions. Cellulosic
ethanol can produce an 8 to 10 percent reduction in greenhouse gas emissions when used
in E10 and a 68 to 91 percent reduction when used in E85.

9.4 OTHER ALCOHOLS

9.4.1 Methanol
Methanol is a colorless, odorless, and nearly tasteless alcohol and is also produced from
crops and is also used as a fuel. Methanol, like ethanol, burns more completely but releases
as much or more carbon dioxide than its gasoline counterpart. The balance is often seen
as the various biprocesses that draw carbon dioxide from the atmosphere so there is no net
modern release, as there is for fossil fuels.
Methanol and other chemicals were routinely extracted from wood in the nineteenth
and early twentieth centuries. However, the original route for methanol recovery from bio-
mass was quite different to current routes. Methanol was originally recovered from wood
as a by-product of charcoal manufacture, and was often called “wood alcohol.” Pyrolysis
(heating wood in the absence of air) to above 270°C in a retort causes thermal cracking
or breakdown of the wood and allows much of the wood to be recovered as charcoal. The
watery condensate leaving the retort contained methanol, amongst other compounds.
In 1923, commercial production of methanol from synthesis gas by a catalytic process
was commenced. Now almost all of the methanol used worldwide comes from the process-
ing of natural gas.
In general, methanol production from natural gas feed consists of three steps: (a) synthesis
gas (syngas) generation—in the case of natural gas feed, syngas production consists of con-
verting methane (CH ) into carbon monoxide (CO) and hydrogen (H ) via steam reforming;
4
2
(b) synthesis gas upgrading—primarily removal of CO , plus any contaminants such as sulfur;
2
and (c) methanol synthesis and purification—reacting the carbon monoxide, hydrogen, and
steam over a catalyst in the presence of a small amount of CO and at elevated temperature
2
and pressure. The methanol synthesis is an equilibrium reaction and excess reactants must be
recycled to optimize yields.
Modern methods proposed for the production of methanol from biomass involve the
conversion of the biomass to a suitable synthesis gas, after which processing steps are
very similar to those developed for methanol production from natural gas. However, the
gasification techniques proposed are still at a relatively early stage of development using
biomass feed and the methods are based on similar techniques used widely already with
natural gas as feed.
Before biomass can be gasified it must be pretreated to meet the processing constraints
of the gasifier. This typically involves size reduction, and drying to keep moisture contents
below specific levels. Thereafter, biomass gasification involves heating biomass in the