Annex: Energy from waste technologies
Mechanical Biological Treatment (MBT)
Mechanical Biological Treatment (MBT) is the term
for the integration of mechanical and biological processing within
a single facility to deal with a mixed solid municipal waste stream,
such as autoclaving and anaerobic digestion (see below). The waste
stream is sorted—by water or by mechanical systems, such
as magnets or screening by size, in conjunction with manual sorting.
Fine material is separated from larger material such as metal
and plastic, and then the fine fraction is further separated into
lighter biodegradable material and heavier material such as glass
and grit. Usually the waste is sorted into recyclable, biodegradable
and fuel materials, as well as a reject stream to be landfilled.
The fuel stream can be used in an incinerator to generate energy
from waste, to fuel energy intensive industrial processes such
as cement kilns or co-firing with coal in power stations. Possible
biological treatments of the biodegradable waste stream include
composting or anaerobic digestion (see below).
Mechanical Heat Treatment/ Autoclaving
Thermal treatments were originally used to treat
clinical and laboratory waste on a small scale. They can be used
as an initial stage in MBT, following the removal of unsuitable
materials, such as metal objects and rubble. In the most common
system, the mixed solid municipal waste is shredded and then treated
with steam at 140-160°C in a thermal autoclave. After the
steam has been injected the pressure is maintained for 30 to 40
minutes, sterilising the waste and breaking down biodegradable
materials. Other types of mechanical heat treatment systems include
non-pressurised heating in a rotating kiln. Autoclaving is most
suited to facilities that generate heat and power through incineration
or anaerobic digestion.
Incineration
An incinerator is a furnace for burning waste at
high temperatures. Incineration not only reduces the volume of
the original waste by over 90%, but also has the potential to
convert the waste to energy. Heat can be recovered from the flue
gases. This recovered heat can then be used as process heat for
industry, for a district heating system or to generate steam for
electricity generation via a turbine (or both heat and power in
a Combined Heat and Power system). According to Danish estimates,[212]
incinerating 1 tonne of waste produces around 2MWh of heat
and around 0.67 MWh of electricity (enough electricity to power
approximately 670 homes for an hour). About one fifth of the waste
is estimated to be non-combustible in the form of metals and glass
which can be collected from the ash and recycled. There are different
types of incineration technology, including:
Moving
Grate Incineration—sometimes called
Municipal Solid Waste Incineration. The movement of the grate
through the furnace allows the waste to be combusted more efficiently.
Air is supplied from below through holes in the grate (which also
cools the grate) and at high speed through nozzles above the grate,
which facilitates complete combustion. The EU Directive on Waste
Incineration (2000/76/EC) states that the flue gas must reach
temperatures of 850°C for two seconds to ensure the appropriate
breakdown of organic toxins in the waste. Auxiliary backup oil
burners are often installed to ensure this. The flue gases are
then cooled and cleaned.
Fluidised
Bed Incineration—in this type of
furnace, pre-treated waste is put under such conditions that it
behaves as a fluid, allowing it to flow freely and fully circulate
throughout the combustion chamber. This process is achieved by
forcing pressurised air through a sandbed until a turbulent mixture
forms and a 'fluidised bed' is created. In the waste pre-treatment
process, non-combustible components are removed and the waste
shredded to produce coarse Refuse Derived Fuel (cRDF) which has
a higher calorific (energy) value than untreated waste. The cRDF
is then fed into the fluidised bed, coming into much greater contact
with the air than would be the case if the waste were untreated.
A wide variety of wastes can be incinerated, from sludge of high
water content to plastic refuse. By controlling the amount of
air and waste or fuel introduced to the fluidised bed, materials
of different densities can be made to float or sink in the mixture.[213]
The material to be combusted is then introduced, with the flue
gases following the same route as described above.
Rotary-Kiln—these
incinerators have a primary ignition chamber and a secondary afterburner
chamber. The primary chamber is a rotating, inclined cylindrical
vessel. Waste material including contaminated sludge and soils
are fed into the primary ignition chamber, where the rotation
facilitates movement and mixing of the waste. The high temperature
causes the contaminants and other waste to vaporise into a gas,
which is then burnt at a higher temperature (760-980°C) in
the secondary afterburner chamber.[214]
Anaerobic Digestion
Anaerobic Digestion is the biological processing
of biodegradable waste (such as kitchen and garden waste, farm
waste or sewage sludge) in the absence of air. Bacteria degrade
the material in an enclosed reaction tank, with attached heating
and mixing systems, to produce biogas, a mixture of methane and
carbon dioxide, and small amounts of some other gases. Anaerobic
digestion is normally carried out at about 35°C. Three groups
of bacteria are involved in the anaerobic digestion: the hydrolytic
and fermentative bacteria; the acetogenic bacteria; and the methanogenic
bacteria. The hydrolytic and fermentative bacteria break down
longer chain molecules to produce long and short chain organic
acids. The acetogenic bacteria feed on these organic acids to
produce short chain organic molecules and carbon dioxide. In turn,
the methanogenic bacteria use these molecules as a substrate for
the production of methane. A balance between the populations of
the types of bacteria is essential for the stability of the process.
Advanced waste treatment technologies can produce
biogas with 55-75% methane, depending on the system design and
feedstocks used. This can be concentrated and purified to the
same standards as fossil natural gas and distributed in the normal
gas grid, although this has yet to happen in the UK. Alternatively,
the methane can be used as fuel to generate electricity and heat
(combined heat and power generation, CHP) on site, and distributed
via available local networks.
The solid residue from the process (digestate) can
also be further processed into compost and liquid residues used
as a fertiliser. Anaerobic digestion can be carried out in small
scale systems, for example located on the farm and operated by
farmers, or to serve businesses (or clusters of businesses) with
large food waste arisings. Alternatively it can be carried out
in large centralised systems, for example to treat municipal food
waste being diverted from landfill by local authorities. Anaerobic
digestion is widely used for sewage sludge treatment and there
is spare capacity to process municipal biodegradable waste at
Waste Water Treatment Plants. However, to comply with EU regulations,
such as the EU Animal By-products Regulation (EC 1774/2002), this
may involve fitting additional treatment units, such as pasteurisation,
to these facilities. A number of purpose built Anaerobic Digestion
facilities have begun operations in recent years.
Gasification
Gasification is a process that can extract energy
from many types of organic material, including biomass. This is
subjected to high temperatures (above 700°C) in low-oxygen
environment within a closed tank to produce synthetic gas, known
as 'syngas'. This is primarily a mixture of carbon monoxide and
hydrogen. The 'syngas' itself can also be used as a fuel to generate
heat or electricity (which can be more efficient than directly
burning the original fuel, as well as less polluting) or undergo
further reactions to produce methane.[215]
It can also be processed further to make liquid fuels. The wastes
used in gasification and pyrolysis (see below), do not need to
be sorted but do need to be crushed, incurring increased costs
and energy use. Advanced gasification is a more efficient version
of this process.
Pyrolysis
The pyrolysis process is similar to gasification,
except that the fuel is subjected to high temperatures (400-800°C)
in a closed, virtually oxygen free (as opposed to low oxygen)
environment. Around 75% of the pyrolysis product is in the form
of a biofuel oil, with the remainder being gaseous and waste products
similar to gasification. The resultant gases can then be burned
in the presence of oxygen to generate heat or ultimately electricity.
The pyrolysis process is more widespread in industry than gasification,
which is not yet a commercially established technology. Advanced
pyrolysis is a more efficient version of this technique.
Plasma Waste Disposal
This technology uses electricity to break down waste
and convert it into fuel. High voltage, high current electricity
is passed between two electrodes, creating an arc. An inert gas
(such as helium, neon or argon) is pumped between the electrodes—where
it breaks down and forms a plasma (a very hot, ionised gas), with
temperatures between the electrodes reaching 13,900°C—and
through to a sealed waste vessel where temperatures a few feet
from the arc can reach 2,800 to 4,400°C. These high temperatures
break most types of waste down into a gaseous form that can used
for fuel. Small facilities are in operation in Canada,[216]
and Taiwan,[217] and
there are test facilities in Swindon,[218]
and the USA.[219]
Composting—Windrow and In-vessel
Composting is a natural biological process in which
organic matter is decomposed by fungi and bacteria in the presence
of oxygen to produce compost. Five key aspects need to be controlled
in the composting process: temperature, moisture content, oxygen
concentration, air circulation and the ratio of carbon to nitrogen
rich materials. These can be controlled by adding bulking materials
to the compost material or by mechanical means.
The composting process is designed according to the
types of waste accepted by the facility. Windrow composting consists
of piling biodegradable waste, such as garden waste, in long rows
(windrows) in open facilities. The rows are generally turned to
improve porosity and oxygen content, mix in or remove moisture,
and redistribute cooler and hotter portions of the pile. However,
facilities which accept mixed biodegradable waste including food
wastes are subject to more stringent regulation. To avoid the
transfer of diseases from meat wastes, the composting process
and the compost outputs are controlled in line with the requirements
of the EU Animal By-products Regulation (EC 1774/2002). The composting
is carried out in enclosed reactors (in-vessel composting), such
as metal tanks or concrete bunkers, in which air flow and temperature
can be controlled. Generally the air circulation is metered via
buried tubes that allow fresh air to be injected under pressure,
with temperature and moisture conditions monitored using probes
to allow maintenance of optimum conditions. Once the pathogens
have been eliminated, a further stage of composting takes place
in open facilities. Only compost originating from biowastes segregated
at source can be used for commercial applications, as composting
residual waste arising from mixed municipal solid waste produces
poorer quality compost.
212 Waste-to-Energy in Denmark, RenoSam and
Rambøll 2006. Back
213
Fluidized Bed Type Incinerator, Global Environment Centre, Japan. Back
214
Rotary Kiln Solid Waste Disposal System, HiTemp Technology Corporation,
USA. Back
215
An Overview of Incineration and EFW Technology as Applied to
the Management of Municipal Solid Waste (MSW), A. Knox, University
of Western Ontario, February 2005. Back
216
Zero Waste Ottowa, Plasco Energy Group. Back
217
PEAT International, National Cheng Kung University, Taiwan. Back
218
Swindon Plant, Advanced Power Plasma. Back
219
WPC Pilot Plant Pennsylvania, Westinghouse Plasma Corporation.
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