Some of the most promising, attention-getting energy
alternatives aren't revolutionary ideas. We all know about
windmills and waterwheels, which have been around for centuries.
Today, a variety of improvements, including innovative turbine
designs, are transforming these ancient machines into cutting-
edge technologies that can help nations satisfy their energy
There's another old process -- one you probably don't know much
about -- that's gaining in popularity and may join wind and
hydropower in the pantheon of clean, renewable energy. The
process is known as
gasification, a set of chemical reactions that uses
limited oxygen to convert a carbon-containing feedstock into a
synthetic gas, or syngas.
It sounds like combustion, but it's not. Combustion uses an
abundance of oxygen to produce heat and light by burning.
Gasification uses only a tiny amount of oxygen, which is
combined with steam and cooked under intense pressure. This
initiates a series of reactions that produces a gaseous mixture
composed primarily of carbon monoxide and hydrogen. This syngas
can be burned directly or used as a starting point to
manufacture fertilizers, pure hydrogen, methane or liquid
Believe it or not, gasification has been around for decades.
Scottish engineer William Murdoch gets credit for developing the
basic process. In the late 1790s, using coal as a feedstock, he
produced syngas in sufficient quantity to light his home.
Eventually, cities in Europe and America began using syngas --
or "town gas" as it was known then -- to light city
streets and homes. Eventually, natural gas and electricity
generated from coal-burning power plants replaced town gas as
the preferred source of heat and light.
Today, with a global climate crisis looming on the horizon and
power-hungry nations on the hunt for alternative energy sources,
gasification is making a comeback. The Gasification Technologies
Council expects world gasification capacity to grow by more than
70 percent by 2015. Much of that growth will occur in Asia,
driven by rapid development in China and India. But the United
States is embracing gasification, as well.
Let's take a closer look at how this process works. We're going
to start with coal gasification, the most common form of the
The heart of a coal-fired power plant is a boiler, in which coal
is burned by combustion to turn water into steam. The following
equation shows what burning coal looks like chemically: C + O2
--> CO2. Coal isn't made of pure carbon, but of carbon bound
to many other elements. Still, coal's carbon content is high,
and it's the carbon that combines with oxygen in combustion to
produce carbon dioxide, the major culprit in global warming.
Other byproducts of coal combustion include sulfur oxides,
nitrogen oxides, mercury and naturally occurring radioactive
The heart of a power plant that incorporates gasification isn't
a boiler, but a
gasifier, a cylindrical pressure vessel about 40 feet (12
meters) high by 13 feet (4 meters) across. Feedstocks enter the
gasifier at the top, while steam and oxygen enter from below.
Any kind of carbon-containing material can be a feedstock, but
coal gasification, of course, requires coal. A typical
gasification plant could use 16,000 tons (14,515 metric tons) of
lignite, a brownish type of coal, daily.
A gasifier operates at higher temperatures and pressures than a
coal boiler -- about 2,600 degrees Fahrenheit (1,427 degrees
Celsius) and 1,000 pounds per square inch (6,895 kilopascals),
respectively. This causes the coal to undergo different chemical
reactions. First, partial oxidation of the coal's carbon
releases heat that helps feed the gasification reactions. The
first of these is pyrolysis, which occurs as coal's volatile
matter degrades into several gases, leaving behind char, a
charcoal-like substance. Then, reduction reactions transform the
remaining carbon in the char to a gaseous mixture known as
Carbon monoxide and hydrogen are the two primary components of
syngas. During a process known as gas cleanup, the raw syngas
runs through a cooling chamber that can be used to separate the
various components. Cleaning can remove harmful impurities,
including sulfur, mercury and unconverted carbon. Even carbon
dioxide can be pulled out of the gas and either stored
underground or used in ammonia or methanol production.
That leaves pure hydrogen and carbon monoxide, which can be
combusted cleanly in gas turbines to produce electricity. Or,
some power plants convert the syngas to natural gas by passing
the cleaned gas over a nickel catalyst, causing carbon monoxide
and carbon dioxide to react with free hydrogen to form methane.
This "substitute natural gas" behaves like regular
natural gas and can be used to generate electricity or heat
homes and businesses.
But if coal is unavailable, gasification is still possible. All
you need is some wood.
Coal gasification is sometimes called "clean coal"
because it can be used to generate electricity without belching
toxins and carbon dioxide into the atmosphere. But it's
still based on a nonrenewable fossil fuel. And it still requires
mining operations that scar the Earth and leave behind toxic
wastes of their own. Wood gasification -- or
biomass gasification, to be more technically correct --
may provide a viable alternative. Biomass is considered a
renewable energy source because it's made from organic
materials, such as trees, crops and even garbage.
Biomass gasification works just like
gasification: A feedstock enters a gasifier, which cooks
the carbon-containing material in a low-oxygen environment to
produce syngas. Feedstocks generally fall into one of four
Agricultural residues are left after farmers harvest a commodity
crop. They include wheat, alfalfa, bean or barley straw and corn
stover. Wheat straw and corn remnants make up the majority of
Energy crops are grown solely for use as feedstocks. They
include hybrid poplar and willow trees, as well as switchgrass,
a native, fast-growing prairie grass.
Forestry residues include any biomass left behind after timber
harvesting. Deadwood works well, too, as do scraps from
debarking and limb-removal operations.
Urban wood waste refers to construction waste and demolition
debris that would otherwise end up in a landfill. Pallets --
flat transport structures -- also fall into this category.
The choice of feedstock determines the gasifier design. Three
designs are common in biomass gasification with
mechanical casting: updraft, downdraft and crossdraft. In
an updraft gasifier, wood enters the gasification chamber from
above, falls onto a grate and forms a fuel pile. Air enters from
below the grate and flows up through the fuel pile. The syngas,
also known as producer gas in biomass circles, exits the top of
the chamber. In downdraft or crossdraft gasifiers, the air and
syngas may enter and exit at different locations.
The choice of fuel and gasifier design affects the relative
proportions of compounds in the syngas. For example, wheat straw
placed in a downdraft gasifier produces the following:
17 to 19 percent hydrogen gas
14 to 17 percent carbon monoxide
11 to 14 percent carbon dioxide
Virtually no methane
But charcoal placed in a downdraft gasifier produces the
28 to 31 percent carbon monoxide
5 to 10 percent hydrogen gas
1 to 2 percent carbon dioxide
1 to 2 percent methane
One attractive quality of gasification is its scalability. The
Polk Power Station just southeast of Tampa is a gasification
plant covering 4,300 acres (1,740 hectares). It converts 100
tons (90.7 metric tons) of coal an hour into 250 million watts
of power for about 60,000 homes and businesses [source: Folger].
But you don't have to be a giant public utility to
experiment with gasification. You can build a simple, small
gasifier with materials you find around the house. YouTube
features several videos of these homemade units. One video, for
example, shows a paint can playing the role of the pressure
vessel in which gasification reactions occur. As the syngas is
produced inside the sealed can, it moves through some simple
plumbing fittings to a burner can, where the gas can be ignited.
Another interesting video shows a small team assembling and
operating a wood gasifier based on plans prepared by the U.S.
Federal Emergency Management Agency (FEMA) and the Oak Ridge
National Laboratory. FEMA developed these plans in 1989
specifically for small-scale gasification in the event of a
petroleum emergency. The agency's report includes detailed,
illustrated instructions for the fabrication, installation and
operation of a downdraft biomass
gasifier generator. (A link to the report is included in
the source list under "Lafontaine" on the next page.)
The unit requires a galvanized metal trash can, a small metal
drum, common plumbing fittings and a stainless steel mixing bowl
and can be mounted on a vehicle to provide syngas for internal
combustion. With the gasifier in place, the vehicle can run
reliably using wood chips or other biomass as the fuel.
If you're interested in gasification, but aren't the
do-it-yourself type, then you might want to consider buying a
gasification unit from a manufacturer. For example, New Horizon
Corporation distributes gasification systems that can be
installed in a home environment. These biomass gasification
boilers can heat houses, garages and other buildings and can use
a variety of fuels, including seasoned wood, corn cobs, sawdust,
wood chips and any kind of pellet.
Either way, gasification will likely emerge as one of the most
important energy alternatives in the coming decades. It's
the cleanest way to use coal, but also works efficiently with
renewable energy sources, such as biomass. And, because one of
the primary products of gasification is hydrogen, the process
offers a stepping stone to producing large quantities of
hydrogen for fuel cells and cleaner fuels.
Keep reading for more links to the future of energy and green
- Created: 10-12-21
- Last Login: 10-12-21