Plasma arc recycling
by Chris Woodford. Last updated: February 20, 2017.
Humans are machines for turning the world into waste—at least that's how it
seems. On average, every single person in the United States produces about 2kg
(5lb) of trash per day, which adds to up three quarters of a ton, per
person, each year! What are we to do with all this junk? Recycling is
one option, but not everyone does it and there are lots of things
(such as electronic circuit boards) made from multiple materials that cannot be easily broken down and
turned into new things. That's why much of our waste goes where it's always
gone, buried beneath the ground. But we're running out of landfill
space too—and that problem is bound to get worse. Another
possibility is to incincerate waste, as though it were a fuel, and
use it to produce energy, but incinerators are deeply unpopular with
local communities because of the air pollution they can produce. A new type
of waste treatment called plasma arc recycling
(sometimes referred to as "plasma recycling," "plasma
gasification," "gas plasma waste treatment," "plasma
waste recycling," and various other permutations of the words
plasma, gas, arc, waste, and recycling) aims to change all this. It
involves heating waste to super-high temperatures to produce gas that
can be burned for energy and rocky solid waste that can be used for building.
Supporters claim it's a cleaner, greener form of waste treatment, but
opponents argue it's simply old-fashioned incineration dressed up in
new clothes. What exactly does plasma recycling involve? Let's take a
Photo: Plasma torches like this are the heart of a plasma recycling plant. They can create temperatures of over 10,000 degrees—enough to blast waste materials apart into their constituent atoms so they can be reassembled into less harmful materials. Photo by Ames Laboratory courtesy of US Department of Energy: Digital Photo Archive.
What kind of waste do we make?
Chart: Most of the waste we produce is relatively harmless paper, metal, plastic, and so on (blue), but about 20 percent of it is much more problematic (orange and yellow). This high-level waste is a mixture of hazardous material, incinerator ash, and a very tiny amount (less than 1 percent) medical waste.
Over three quarters of our trash is ordinary, relatively harmless
household waste made up of paper, card,
glass, plastics of various
kinds, metals (mostly
steel and aluminum), and food waste. In many
countries, much of this is now separated and recycled or (in the case
of food waste) composted or fed into an
anerobic digester, although
quite a lot still goes to landfill or incineration. Simple household
waste aside, there's quite a lot of other trash that can't be treated
so easily. For example, batteries and other toxic chemical waste, and
medical waste from hospitals. And some conventional forms of waste
treatment (recycling plants and incinerators) themselves generate
waste products that have to be disposed of safely: things that
cannot be recycled or highly toxic "bottom ash" from
incinerators that needs to be disposed of somehow. Plasma recycling
claims to be able to tackle all these kinds of waste safely and with
little or no harm to the environment.
What is plasma arc recycling?
To answer that question, it helps to understand how plasma recycling
differs from conventional incineration: simply tossing waste on a
fire. Incineration makes use of the chemical reaction called
combustion, in which fuel (in this case, household trash) burns with
oxygen to release waste gases (typically carbon dioxide, steam, and
various kinds of air pollution) and
heat energy; a conventional
energy-from-waste incinerator is really just a polite version of
that. The main differences between a simple bonfire and a waste
incineration plant are: 1) the waste is burned in a closed container
at extremely high temperatures (to destroy as many toxic chemicals as
possible); 2) pollution from the smokestack (chimney) may be trapped
and "scrubbed" clean before it's released (using an
electrostatic smoke precipitator);
3) a very tall smokestack is used, (theoretically) to disperse any remaining pollution in the wind;
and 4) the energy released by burning the waste is captured and used to boil water,
drive a steam turbine, and generate electricity.
Artwork: Although plasma recycling processes vary, most work in broadly this way. Raw waste is processed to remove any recyclable materials before being fed, with gas, to the plasma arc. This vaporizes the waste to produce syngas (which has to be scrubbed clean) and aggregate.
Plasma arc recycling doesn't involve combustion. Instead of simply burning
the waste (at a few hundred degrees), the waste is heated to much
higher temperatures (thousands of degrees) so it melts and then vaporizes.
This is done by an electrical device known as a plasma arc, which is a kind of super-hot "torch" made by passing gas through an electrical spark. Think of the spark you get from the sparking plug in a car: electricity
feeds into the plug from the battery, makes a lightning-like spark
leap across a small air gap between two contacts, and the spark
ignites the fuel that powers your engine. A plasma arc is a much
bigger version of the same thing, with a gas (such as oxygen,
nitrogen, or argon) blowing through it to create a kind of super-hot
plasma torch (like a giant welding torch).
The plasma arc in a waste plant heats the waste to temperatures anywhere
from about 1000–15,000°C (1800–27,000°F), but typically in the
middle of that range, melting the waste and then turning it into
vapor. Simple organic (carbon-based)
materials cool back down into relatively clean gases; metals and
other inorganic wastes fuse together and cool back into solids. In
theory, you end up with two products: syngas (an energy-rich mixture
of carbon monoxide and hydrogen) and a kind of rocky solid waste not
unlike chunks of broken glass. The syngas can be piped away and
burned to make energy (some of which can be used to fuel the plasma
arc equipment), while the "vitrified" (glass-like) rocky
solid can be used as aggregate (for roadbuilding and other
construction). In practice, the syngas may be contaminated with toxic
gases such as dioxins that have to be scrubbed out and disposed of
somehow, while the rocky solid may also contain some contaminated material.
Where is plasma recycling being used?
Although plasma recycling is still relatively new, there's a huge amount of interest in the technology, and plants are starting to appear all around the world. Here's a small selection of what's currently up and running:
One of the first European plasma plants was a small demonstration site built in Swindon, England, and operated by Advanced Plasma Power (APP) since 2007. According to APP, the plant has an amazingly low environmental impact: it's the same size as a soccer pitch, looks much like an ordinary factory or warehouse, and has a modest smokestack (chimney) that rises only 10m (~33ft) above its roof (the smokestack on a typical incinerator would rise about 6–7 times higher). A full-scale plant built to a similar design could process 150,000 tonnes of ordinary household and commercial waste per year, diverting some 98 percent of waste that would otherwise end up in landfill. It would produce enough power for 17,500 homes and enough waste heat for 700. While it would be possible to build much bigger plants, it makes much more sense—politically, environmentally, and economically—to construct many small plants geared to local communities, removing their waste and producing power for them at the same time. Having proved that its process works, APP has since started work on a significantly bigger 6MW plasma plant in Birmingham, England. That's roughly the same output as three wind turbines working at full tilt, but still only a tiny amount of power generation: you'd need about 300 plasma sites like this to make as much power as one big coal-fired power plant!
US energy company InEnTec has been operating small-scale plasma plants for two decades, and now has sites in Washington state, Nevada and Oregon; it even has a tiny transportable plasma system that operates from the back of a couple of flatbed trucks. The United States Air Force (USAF) is also at the cutting edge of gas plasma technology, with a keen interest in reducing the waste it generates in war zones. It's been operating a prototype gas plasma plant at Hurlburt Field Air Force base in Florida for the last few years.
British-based APP won a contract to build a 20MW gas plasma plant for Port Fuels and Materials Services
in Hamilton, Ontario, Canada in 2014, which they estimate would provide enough energy to power 17,000 homes.
There are probably more plasma plants in Asia than anywhere else in the world. InEnTec has sold plants to Taiwan, Japan, and Malaysia, for example. In China, the Wuhan Kaidi company has been operating a prototype plant since 2013, using plasma technology supplied by US firm Westinghouse Plasma and
AlterNRG, a Canadian plasma firm that has also built a plant in Shanghai.
AlterNRG has also helped to build plants at Pune, India and both Mihama-Mikata and Utashinai in Japan.
Find out more
Pros and cons
Like every other waste-treatment process, plasma arc recycling has its
pros and cons. But it's important to remember that most of us produce
a significant amount of waste that must be disposed of somehow.
Waste is a problem that needs a solution; it's not something we can just ignore.
In other words, plasma recycling has to be judged not in isolation ("Is it good or bad?") but in comparison with the various alternatives ("Is it better or worse?").
Supporters of the technology claim that it's cleaner and greener than
incineration, because waste is "rearranged" into different
substances rather than burned to release pollution. Properly
designed, a plasma plant theoretically produces no air pollution and
no ash or dust; it's only real waste product is the solid, vitrified
aggregate that can be used in construction (APP claim that their
version, known as Plasmarok®, is "environmentally inert" and "leach resistant.")
In practice, every kind of waste treatment produces toxic heavy metals and other residues
that cannot be disposed of completely. In a plasma plant, they can at least be
separated out, melted down, and reused; they're not simply being blown
into the air as incinerator ash or stuffed underground in a landfill and left there
to cause problems for future generations.
Unlike virtually any other kind of waste treatment, plasma recycling can
cope with virtually any kind of waste, including the most hazardous,
high-grade, and hard-to-treat forms (toxic incinerator ash, hazardous
medical waste, toxic metals, electronic components, and so on). Where
landfilling squanders valuable material and—at best—produces small
amounts of methane energy, plasma recycling produces much more energy
with no land-take. Indeed, some plasma recycling companies have even
proposed "mining" existing landfills to use as raw fuels
for plasma plants; that raises the prospect that we could eventually be
able to clean up the toxic legacy of decades of landfill. Although plasma plants use a
significant amount of energy, roughly two thirds of what they make is
fed into the grid, which makes them, overall, carbon negative (they
have an overall benefit where global warming is concerned). A typical
plant would produce enough electricity to power up to 10,000 homes
and enough waste steam, as a byproduct, to heat or provide hot water
to maybe 500-1000. It's important to remember that plasma plants
produce syngas as a fuel, which can either be burned to make energy
in a conventional power plant or separated into hydrogen and carbon
monoxide, with the hydrogen collected and stored for use in fuel-cell
Photo: What will we do with our waste when we run out of landfill space? Photo by David Parsons courtesy of US Department of Energy.
Opponents of the technology are concerned that it's largely untried and its
drawbacks aren't yet known. No-one really knows whether it's safe or
whether it's more economic than other forms of waste treatment. One
concern is that it's simply a new way of dressing up something that is
little better than incineration. Although the waste isn't burned, it is
heated and some harmful products (including heavy metals and toxic dioxins) are left
over at the end of the process. The solid aggregate waste has been
billed as a useful construction material, but no-one can yet be certain
precisely what it would contain, how safe it would prove, or whether it could indeed release toxic
chemicals into the environment over time.
One argument against conventional incinerators is that they undermine
drives to reduce and recycle waste. If commercially operated
incinerators need (and indeed profit from) steady supplies of waste,
what is the incentive to reduce packaging in grocery stores and all
the other things we routinely send to the trash?
Is it a good thing?
Plasma recycling is still a new technology and it's too early to say whether
its benefits (the potential to supply energy, reduce fossil fuel
consumption, and reduce or restore landfills) will outweigh its
drawbacks (any toxic gases or solids that remain after treatment, the high
cost of investment in a relatively untried technology, and any
potential impacts on local communities). But with ever-increasing
consumption, growing pressure on the environment, and the local
unpopularity of incineration, landfill, and digestion, governments
are bound to see plasma recycling as a relatively clean solution
to a dirty problem that simply won't go away.