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Gasification is the process by which carbon-based
materials are converted to a combustible syngas at high temperatures in a
controlled oxygen environment. The technology has been around since the
mid-1800s, developed originally as a means to convert coal to usable gas for
fuel, heat, and light. Through its evolution, gasifiers have been used for
producing heat, generating power, and making liquid fuels and chemicals.
How Gasification
Works
Gasification is the reaction of carbonaceous materials
with steam under high temperature (anywhere between 650 and 1,400 degrees
Celsius) to produce carbon monoxide and hydrogen. The basic chemistry is: C +
H2O à
H2 + CO, though other reactions take place based on the composition of the
feedstock and atmosphere in the reaction chamber. By-products of the process
can include ash and char or slag.
Gasification systems can vary in a number of different
ways. Most significantly is whether the heat for the reaction is supplied
internally through oxidizers (direct gasification) or from an external heat
source (indirect gasification). Pyrolysis is an indirect gasification process
in an inert atmosphere.
Image Credit: BVSDE
As It Stands Today
Major developments in gasifier technology today revolve
around Integrated Gasification Combined Cycle (IGCC) power plants. Instead of
directly combusting the fossil fuels, these facilities use the fuel to make
syngas which, after cleaning, can then be used at elevated temperatures and
pressures to heat steam to run a turbine. Electricity from syngas is an
attractive alternative to direct combustion, as it can be produced from low
value feedstock sources and has the potential to be cleaner and more efficient,
reducing emissions and creating useful or disposable byproducts from captured
pollutants.
 But the pipedream of sustainable gasifier technology
involves the use of municipal solid waste (MSW) as the feedstock, meaning it
acts as both a waste reduction and energy generation process. This is (in
concept) a much more desirable disposal option than landfill dumping and
produces more energy than waste-incineration plants.
Beneath the obvious benefits of this waste-to-energy
process lie a lot of ugly problems and logistical issues, however. For one, MSW
generally requires extensive pretreatment before it can be put into a gasifier.
This includes mechanical and manual sorting to remove recyclables and
undesirable materials, and crushers to grind the material into a usable form.
Secondly, the waste generally requires preheating to lower the moisture content
to a reasonable level. Effectively cleaning the dirty syngas is also a problem,
as MSW gasification will generate NOX, SOX, and various VOCs. Also, some types
of gasifiers cannot handle the inconsistent composition of MSW in the reaction
chamber, and those that can require energy from another source for startup.
Finally, the average carbon content of MSW is only around 20%, meaning a large
amount (tens to hundreds of tons) of trash per day would be needed to generate
a noticeable amount of electricity for a community.
The Promises of MAGS
Terragon Environmental
Technologies Inc. is tackling waste-to-energy on a smaller scale. Their Micro
Auto Gasification System (MAGS), designed alongside the Office of Naval
Research (ONR), is first and foremost a waste-disposal system. It was developed
to provide a simple operating, compact solution for handling waste streams in
naval vessels, short facilities, and isolated areas. The capacity of a single
MAGS system can handle the waste generation of about 1000 soldiers or a
community of 500 people.
Here
is a video describing the MAGS system.
The gasification method is
controlled pyrolysis, in which up to 40kg of waste is heated in a chamber at
750 degrees Celsius in an inert atmosphere. The result is a low emissions char
product along with syngas which is diverted through a cleaning system to a
combustion chamber to supply heat for the process. Glass and metal are left
intact and ready for recycling, and the wastewater and emissions of the process
are said to be well within the standards, according to developers.
"Decades ago, the idea of
harvesting energy from trash was just a side show in the environmental movement,"
said Steve McElvany, the MAGS program officer at ONR. "Now, the technology is
mature enough to where the Department of the Navy is seriously evaluating its
practical and tactical benefits."
They also suggest that MAGS technology
will be beneficial for small communities and businesses to reduce both waste
and energy costs. Needless to say, the history and past difficulties regarding
waste gasification are enough to draw plenty of skepticism towards the
technology. But small-scale developments like these could be the right approach
to making the process practical.
Sources:
ONR
- MAGS
BVSDE - Energy from
gasification of solid wastes
IJESD - Waste to Energy: A Case
Study
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