Could we vaporise all this trash and get clean energy? (Image: Steve
Wilkes / Getty)
From trash to gas
To burn or to zap?
AT FIRST glance, 303 Bear Hill Road in Waltham,
Massachusetts, doesn't look like the scene of an environmental revolution.
But packed into a shipping container in the car park of this modest suburban
commercial building is a compact piece of technology that its maker IST
Energy insists can turn even the filthiest waste into clean, green
energy. "Trash will move from being a liability to an asset, providing
a clean source of energy that can be used right where it is produced,"
says Stuart Haber, the company's CEO.
IST is not alone in this revolution. It is
one of a growing number of companies and research groups around the world
working on gasification - a process that zaps household waste into energy
and which, its advocates say, produces few or no harmful emissions. Yet
as pilot gasification plants begin to spring up around the world, this
apparent environmentalist's dream is not being universally welcomed.
Opponents argue that the process is far from
clean and that its track record in terms of energy efficiency and emissions
can hardly be considered green. Not to mention the fact that it encourages
the throwaway society that the environmental movement has been trying so
hard to get rid of. So what is the real story? Is vaporising trash the
to our energy and waste-disposal woes, or an environmental wolf in
The idea of converting waste into energy has
been around for decades. Heat from garbage-fuelled incinerators can generate
steam that drives a turbine that in turn drives an electrical generator.
Now fears over energy security and climate change, combined with the rising
cost of dealing with the world's waste, are raising the possibility
of disposing of household trash using higher-energy methods once reserved
for hazardous materials such as medical waste and asbestos.
Gasification, and its cousin plasma gasification,
involve heating waste to a high temperature inside a sealed chamber. This
is done in the near absence of oxygen, so organic components in the waste
do not burn but instead reform into syngas, a mixture of carbon monoxide
and hydrogen. This can be filtered and chemically "scrubbed" to remove
toxic particles and gases, and then burned to produce energy or converted
into other fuels such as methane, ethanol or synthetic diesel. All that's
left to dispose of at the end is ash, dirty filters and chemicals from
the scrubbing process, which can be treated and sent to landfill or into
Gasification yields more energy per volume
of trash than incineration, but the possibilities don't end there. Adding
an arc of superheated plasma to the mix can increase that yield further.
Plasma gasification vaporises waste at much higher temperatures - up to
10.000°C compared with up to 1.600°C for normal gasification -
which ensures that more of the organic waste is gasified.
In this kind of gasification, plasma arcs
are created by passing a high-voltage current through a chamber filled
with an unreactive gas such as nitrogen (see diagram). As the current flows
through the enclosed space, it tears electrons from the gas to form a superheated
plasma that rips apart the molecules in whatever is fed into the chamber.
"It's like a continuous bolt of lightning that disintegrates almost
anything that crosses its path," says Daniel
Cohn of the Massachusetts Institute of Technology, who has been working
on plasma gasification since the 1980s and now sits on the board of InEnTec,
another waste-to-energy company.
Plasma gasification is like a continuous bolt
of lightning that disintegrates almost anything in its path
A further advantage of this technique is that
the very high temperatures cause the waste to end up not as fine ash but
as a glassy solid, which could in principle be used as filler in the construction
industry. And while the power required to run InEnTec's pilot plant in
Richland, Washington, amounts to one-third to half of the power it produces,
Cohn insists that the process is financially viable. He says syngas can
be converted to ethanol and synthetic diesel at costs that can compete
with petroleum-based equivalents. "We think we can produce fuel at a
cost of about $2 a gallon of gas equivalent," he says. If
he's right, trash could become the new oil.
Pilot gasification plants are being set up
at various sites in the US, Canada, France, the UK and Portugal, most of
them using the plasma technique. Japan already has two commercial plasma
plants, but these are focused primarily on simply disposing of household
waste rather than generating energy from it.
While these new plants will all be large installations,
IST Energy believes that small is the way to go. Its container-sized non-plasma
GEM system (short for Green Energy Machine) can convert almost 3 tonnes
of municipal waste a day into enough syngas to heat and power an office
building holding 500 people.
Keeping the system small and avoiding the
expense of creating plasma makes it affordable for businesses to deploy:
excluding the gas burner, the system costs $850,000 and, according to Haber,
will pay for itself in four years through savings on electricity, heating
and waste disposal charges.
Haber says the entire system can save the
equivalent of about 500 tonnes of carbon dioxide emissions a year through
reductions in landfill gases, fossil-fuel use and the transport of waste.
Haber also claims that, compared with traditional incineration, the quantity
of toxic gases produced by the GEM system is negligible. "It's really
a night-and-day difference," he says.
Gasification is not without its detractors,
partly because early attempts to gasify garbage were environmentally and
financially disastrous. A series of economic and environmental problems
at an early commercial gasification plant in Karlsruhe, Germany, including
a leak of toxic gases that temporarily closed the plant in 2000, caused
the facility to shut down for good in 2004. These troubles have tainted
the reputation of gasification - and, by association, plasma gasification
- ever since.
One objection that sceptics raise is that
gasification still produces CO2 emissions. Neil Tangri of the Global
Alliance for Incinerator Alternatives (GAIA) dismisses gasification
plants as glorified incinerators. "There is an intermediate step with
gasification, but the end result is always combustion," he says.
Another concern is that the waste gas from
gasification may contain dioxins, which form when organic material is heated
to high temperatures in the presence of chlorine-containing compounds,
which are ubiquitous in municipal waste.
"Any attempt to turn garbage into energy
will most likely cause the production of significant amounts of dioxin,
which many consider the most significant carcinogen known to science,"
says Ron Saff, a physician in Tallahassee, Florida, and a member of Physicians
for Social Responsibility.
Clean and green?
Others say that chlorine can cause an additional
problem in the extremely hot, oxygen-starved environment of a plasma gasification
chamber. "If you pass mixed waste with chlorine in it through a plasma
arc, you get metal in the [syn]gas that otherwise shouldn't be there,"
says Thomas Cahill,
an emeritus professor of physics and atmospheric science at the University
of California, Davis. These metal pollutants could escape into the environment
when the gas is burned, he argues.
Companies already running gasification systems
point out that the process is as clean as you make it: what matters is
how efficiently the syngas is scrubbed and how effectively the ash is disposed
of. They also say that they operate to strict national or regional standards
governing emissions from waste-to-energy power generation.
"The regulations that they have to comply
with are much more stringent and focus on a wider range of toxins than
for a conventional power plant," says Marc Wolman of the Massachusetts
Department of Environmental Protection in Boston. "If they don't meet
these limits they get shut down, period."
On the issue of dioxins, at least one waste-to-energy
company is making reassuring noises. Andreas Tsangaris of the Plasco
Energy Group in Ottawa, Canada, which has been running an 85-tonne-per-day
waste-to-energy pilot plant since September 2007, says: "We remove virtually
all the chlorine before combustion. There is no chance for dioxins to form."
The company's own
monitoring shows that its emissions, including those of dioxins and
heavy metals, have remained at or below the most stringent regulatory limits
America and Europe.
Nevertheless, a newspaper article by Cahill,
based in part on his studies of emissions from the smouldering remains
of the World Trade Center in New York - which he says are "eerily similar"
to those from gasification plants - plus a strongly worded editorial by
Saff, had a direct impact on two proposals for high-profile commercial
plasma gasification plants in the US. A plant in St Lucie, Florida, has
been scaled back significantly, partly in response to environmental concerns,
and plans for a similar plant in Sacramento, California, have been delayed
Another question mark over the green credentials
of waste gasification concerns just how efficient these plants are at producing
energy and minimising greenhouse gas emissions compared with other methods
of waste disposal. A
recent study by the Tellus Institute, an independent think tank based
in Boston, compared gasification with landfill sites where methane is captured
to be burned for energy. It concluded that while gasification produces
six times as much energy per tonne of waste as landfill sites, landfills
with methane recapture systems save two-and-a-half times as much CO2
equivalent as the combination of gasification and syngas burning. The Tellus
report also found that the energy saved by recycling a given amount of
waste is 3.4 times the energy that can be produced through gasifying it.
Some are opposed to gasifiers on principle.
They say their very existence discourages efforts to tackle the garbage
crisis at its source. "Once you build a gasifier, you have to feed it,"
says Tangri. "It creates a financial disincentive to do waste reduction
Ultimately, it may be some time before we
realise the full effects, for good or bad, of zapping our rubbish. Few
long-term independent studies have been carried out into emission levels,
dioxin contaminants and the potential for toxins to leach out from waste
ash. Nor is it clear how much energy can be created by gasifying various
types of waste, or how reliable energy generation can be, given variations
in the waste stream from day to day and in different parts of the world.
So far, though, the indications are that gasification is neither the panacea
for our waste and energy woes that some are claiming it to be, nor an environmental
catastrophe waiting to happen.
For Kevin Whiting of Juniper,
a British waste-processing consultancy based near Dursley in Gloucestershire,
the way forward may be on some kind of middle ground. "If there is a
market for recyclables, we should recycle as much as is practicable and
not take resources from our great-grandchildren," he says. "But
if waste can't be recycled, it has an energy value. And the more energy
you can generate [from it], the better."
gasification needs more light and less heat
Voir notre répertoire Energie