Thermal conversion options
02-NOV-2005
Despite
all the novel thermal conversion options available today, conventional combustion
may well be the best option for mixed wet-and-dry municipal wastes, says Nickolas
Themelis
Broadly speaking, thermal treatment of materials is one of the oldest industries
on the planet. It was practised as early as 6000 years ago in copper-smelting
crucibles in the Negev Desert of present-day Israel. To this day, the metals
that form the bones, muscles and skin of civilization are made nearly exclusively
by high-temperature treatment of materials.
I spent most of my career in the extraction and refining of metals until I
became entangled in the integrated management of wastes. In the context of this
magazine, thermal treatment refers to the processing of solid wastes at elevated
temperatures with three objectives:
- to dispose of a variety of objects no longer wanted by humankind
- to recover useful materials ¿ commonly known as recycling
- to recover energy ¿ commonly known as waste-to-energy.
By far the dominant method of thermal treatment of solid wastes is combustion
in large furnaces to generate steam, which turns turbines to produce electrical
power and also can provide thermal energy for district heating. These facilities
have come to be known as waste-to-energy (WTE) plants (though there are other
ways of generating energy from wastes, such as anaerobic digestion and the use
of landfill gas).
The dominance of combustion technologies is indicated by the existence of nearly
400 WTE facilities in the EU and at least another 200 in all other countries.
Currently, about 35 countries have WTE facilities, ranging from the small island
of Bermuda to the mega-nation of China, and the number is growing year by year.
The entry of China to the WTE community in recent years was a surprise to some
since WTE is more costly to implement than modern landfilling. Uncontrolled
landfilling, still used in most parts of the world, is dirtcheap and also an
environmental disaster. Yet the Chinese must have figured out ¿ ahead of some
Western nations ¿ that the price of imported fuel will go up and up, so have
invested in nearly 40 WTE plants. Some of these plants are using imported grate
technologies while the rest are based on Chinese fluidized-bed technologies.1
Japan, after doing its utmost to increase recycling, combusts most of its MSW
in WTE plants. The bottom ash is used while the flyash must be processed in
some way, such as by vitrification in an electric arc furnace. The combination
of WTE and vitrification has increased the overall cost of WTE to the point
that some communities have opted for the alternative of high-temperature pyrolysis
of the MSW to produce a syngas-type of fuel and vitrified ash. Nevertheless,
the traditional combustion process remains dominant in Japan and in other countries
that use thermal treatment of wastes.
The novel processes that are competing with mass-burn combustion must first
pre-process the bulk MSW into a feedstock suitable for gasification or another
type of reaction. This requires bulk MSW ¿ which is wet and contains `particles¿
ranging in size from a coin to a suitcase and in hardness from cotton to glass
¿ to undergo shredding and separation of metal and glass. One does not need
to be a rocket scientist to see that, from a technical point of view, it is
much better to comminute and homogenize a solid feedstock and remove non-combustible
materials before feeding it into a combustion chamber or chemical reactor. This
is similar to what happened in coalfired boilers that advanced from stoker to
pulverized combustion. However, competent engineers who have worked in both
mass-burn and refuse-derived-fuel (RDF) combustion plants claim that the latter
are costly to build and operate. Therefore, for years I have challenged my graduate
students to come up with equipment and plant designs that would allow pre-processing
of the MSW at a reasonable cost. So far there were no takers, but I expect that,
with time, RDF technologies will be developed where the disadvantages of pre-processing
the MSW will be overshadowed by the benefits of combusting a small-size, homogeneous
fuel. These technologies would have smaller combustion chambers and higher thermal
efficiencies than present WTE plants.
Pre-processing of MSW at a reasonable cost is a challenge
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Future RDF processes will have to overcome another problem. In the case of
mass-burn WTE plants, the flyash constitutes only 2%¿4% of the mass of the combusted
MSW. When the MSW fuel is shredded to smaller particles, the flyash fraction
is much larger. It is well known that flyash is the collector of mercury and
other volatile metals and, also, of the minute amount of dioxins formed during
cooling of the combustion gases. Therefore, flyash must be deposited in hazardous
waste landfills (such as extinct salt mines) or processed in some costly way.
Therefore, it is desirable to keep the fly ash fraction as low as possible.
Generally a certain feedstock is needed to produce a certain product ¿ for
instance, you need copper concentrates or copper scrap to produce copper. Yet
developers of novel thermal treatment processes claim that the same feedstock
¿ mixed MSW ¿ works for processes that produce entirely different products.
For example, the feedstock of an anaerobic digestion (AD) process should be
source-separated food and plant wastes, as is the case in AD plants in Europe;
the same is true if the proposed process produces ethanol, although in this
case a high content of sugar would be preferable. On the other hand, the best
feedstock for a gasification or pyrolysis process would be non-recyclable plastic
wastes, most of which are presently landfilled in the US, despite an enormous
amount of effort by communities and petrochemical companies to recycle plastics.
In fact, after supervising several graduate theses on all types of MSW processing
methods and reading numerous papers on this subject, I have come to the conclusion
that the only all-forgiving process that will accept the lowly MSW feedstock
¿ which contains everything and anything discarded by people anywhere ¿ is combustion.
Furthermore, at this time the most accommodating of the existing types of combustion
processes seems to be mass-burn.
In closing, WMW readers may like to know that an academic¿industry organization
has been formed that aims to stimulate academic research on various aspects
of energy and materials recovery from municipal and other solid wastes, and
to disseminate research findings to professionals and the general public. It
is called the Waste-To-Energy Research and Technology Council (WTERT; www.columbia.edu/cu/wtert),
headquartered at Columbia University in the City of New York, US. It is a non-profit
organization that relies heavily on faculty and graduate students of several
universities concerned with integrated waste management and waste-to-energy.
WTERT seeks exchange of technical information and active collaboration with
academics who are concerned with the integrated management of wastes anywhere
in the world.
NICKOLAS J. THEMELIS is Director of the Earth Engineering
Center of Columbia University and is Chair of the Waste-to-Energy Research and
Technology Council.
e-mail: njt1@columbia.edu
web: www.columbia.edu/cu/wtert
NOTES
- A. Whitworth, M.S. Thesis, Columbia University, 2005; available in pdf
format on www.columbia.edu/cu/wtert
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