WASTE-TO-ENERGY POWER PLANTS
What is Waste-to-Energy
Waste-to-Energy (WTE)is the sum of processes that produce electric energy from controlled waste incineration. Nowadays it has also become a safe and favorable form of energy recovery from the environmental point of view.
As such, it is currently regarded as an essential element in the mechanism of integrated waste management, in all industrialized Countries. In Italy too, after some 10 years of stasis, this energy recovery method is reporting a strong revival.
The new trend is supported by the safety granted by modern WTE technologies in terms of environmental impact, and so also in terms of effects on the population living in the surroundings.
Waste management in Italy
Every year in Italy, 25 million tons of Municipal Solid Waste (MSW) are produced. The resulting yearly average waste production of an Italian citizen is about 450 kg, more than one kilogram per day.
Such waste is almost completely disposed of in landfills: the percentage of waste separation aimed at recycling, composting or energy recovery is still totally negligible.
This situation definitely sets aside Italy from the rest of Europe, where percentages are reversed, as a matter of fact: the majority of waste is sent to WTE plants, while the rest is stored in landfills.
If Italy were to level up to other European countries, where about 30% of waste is used in WTE plants (Denmark 65%, France 42,3%, Germany 40%, Sweden 55%), recovered energy would be enough to make for 10% of the consumption of Italian families.
Municipal Solid Waste (MSW) 25.780.000 ton/y
Separate Collection 3,1%, Undifferentiated waste 93,7% Pre-Selected waste 3,1%
Landfill 87%
Waste-to-Energy6,7%
Composting  Refuse-Derived Fuel (RDF)

The role of WTE
The Waste-to-Energy philosophy, when correctly inserted into an integrated management strategy, aimed at maximizing the worth of waste, can achieve significant environmental and economical advantages.
First, it allows huge savings at the landfill: ashes and slag are a mere 10% in comparison to the original waste volume.
In WTE plants the entire energy contents of waste is recovered, save the unavoidable fraction that is lost through the flue gas, cooling devices, boiler walls and ashes.
Thanks to energy recovery, MSW can be considered a renewable energy source (as stated in law 9/1991 and 10/1991 on energy saving). Energy recovered from the waste is partially used for operating the plant itself (covering for about 50-60% of operating costs) and partially introduced in the national power grid or used in the whereabouts of the power station (for example, to heat schools or public buildings).

Energetic contents of MSW
During the last 30 years new life and consumption styles have remarkably
modified the composition of MSW. The quantity of organic waste has
decreased, while packaging-related waste has increased (as of now it is
about 40% of the total).
Such a massive presence of packaging material, like plastic, paper,
cardboard – all of which have high energy contents – has progressively raised
the overall Heating Value of MSW.
The following table shows the typical composition of Municipal Solid Waste:
COMPONENT
WEIGHT
%
LHV
(kcal/kg)
MOISTURE
%
Organic matter  31 1.000 75
Plastic and rubber  13 7.300 6
Paper and cardboard  24 2.900 30
Textile and wood  7 1.300 42
Glass and inert  8 - -
Metal  4 - -
Sieve scraps  13 1.200 45
TOTAL100 2.200 40
The Lower Heating Value is nowadays close to 2.000 – 2.200 kcal/kg. When
waste is being pre-selected, that is, when organic matter is subtracted and
sent to composting, the lower heating value of the remaining material raises
to about 3.000 kcal/kg.
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Contribution
of waste
energetic
content
The energetic content of waste is extremely important both for energetic and
management efficiency in a WTE plant.
When the Lower Heating Value of waste is lower than 1.200 kcal/kg,
combustion gets troublesome and it becomes necessary to take some
countermeasures. One is the increase of combustion air, which has the
significant disadvantage of lowering the combustion temperature and raising
the volume of flue gas to be cooled, treated and evacuated in the
atmosphere with all subsequent efficiency losses.
From the combustion of 1.000 kg of MSW with a mean Lower Heating Value it
is possible to produce about 2.500 - 3.000 kg of steam and thus about 500 -
600 kWh of electric energy.
Chemical
characteristics
of waste and
pollutants
production
The chemical composition of waste sentto WTE plants directly effects the
composition of the flue gas stream, although not with a linear correlation and
depending on the way the plant is run. The following tables show the mean
chemical composition of MSW and the mean concentrations of the major
macro-pollutants contained.
Mean chemical composition of Italian MSW:
Water Inerts Carbon Oxygen Hydrogen Sulphur Nitrogen Chlorine
25,5% 25,0% 24,2% 20,8% 3,4% 0,15% 0,50% 0,45%
Mean contents of major macro-pollutants:
Pollutant
Concentration
(mg/kg of MSW)
Pollutant
Concentration
(mg/kg of MSW)
Arsenic 0,6 Chlorobenzene 12,6
Cadmium 7,4 Chlorophenol (CP) 521,3
Chrome 215,6 Polychlorobiphenil (PCB) 79,8
Manganese 0,07 Dioxins 19,8
Mercury 247,6 Furans 2,3
Lead 161,8
Copper 1558,3
Zinc
Dioxins  The generic term “dioxins” refers to a whole family of organic chlorinated
compounds, namly “polychlorinated dibenzodioxins” (PCDD). 75 different
compounds belong to this family, each with a different degree of toxicity
depending on its chemical structure.
Dioxins are ubiquitously present in the environment. Global human exposition
to dioxins has been evaluated in a daily assumption of about 120 picograms.
In recent years, however, a significant reduction in the levels of environmental
dioxins has been noticed, and the trend is destined to continue afterwards.
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Dioxin formation
from MSW
combustion
Dioxins, like many harmful substances, are formed during the incomplete
combustion of organic matter in presence of chlorine, either organic or
inorganic.
MSW combustion leads to the formation of dioxins when temperatures are
below 850°C. Dioxins are originated both from chlorinated compounds found
in many types of waste (CPs, PCBs) and from carbon particles in presence of
chlorine, steam, oxygen and inorganic chlorides that act as catalysers in the
200°C – 400°C temperature range.
Dioxins are already present in waste in higher quantities that those found in
flue gas after combustion. This means that MSW combustion using the most
recent technologies allows, as a matter of fact, to reduce the emission of
dioxins into the environment.
Recent studies, based on accurate matter balances, have proved that
dioxins entering the WTE plant are 40 to 100 times those released by the flue
gas into the environment.
How does a WTE
plant work.
Scientific studies called for by growing worries about the safety of waste
combustion processes and the definition of strict emission standards have led
to advanced WTE technologies,  capable of ensuring adequate
environmental performances.
The following paragraphs will briefly describe the operation of a typical MWS-fired, WTE power plant, built following law prescriptions for safety and emission
control.
The plant is made up of the following main items: a furnace, an afterburning
chamber, a heat recovery steam generator and the emission control
equipment.
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Combustion  Inside the furnace the combustion of MSW takes place at about 1.100 °C in
modern plants. Italian laws require a minimum temperature of 950°C when
chlorine content is lower than 2%. If chlorine content exceeds 2% the required
minimum temperature is 1200°C.
The process itself takes place in three distinct steps:
•  Waste drying and precombustion.
•  Combustion of volatile substances.
•  Combustion of solid remnants and final conversion into ashes.
When combustion is carried on this way, it allows the destruction of toxic
substances released during the process, with an efficiency which is more than
or equal to 99,9%, as Italian laws decree.
Post-combustion
Combustion flue gas produced in the furnace flows to the post-combustion
chamber, where combustion processes complete, resulting in the complete
destruction of organic compounds.
Italian laws set the maximum levels for various process parameters which are
to be monitored and recorded in continuous. For MSW with chlorine content
less than 2%:
•  Free oxygen, higher than 6% in volume
•  Gas temperature, higher than 950°C
•  Gas speed, not less than 10 m/s
•  Mean stay in the post-combustion chamber, at least 2 seconds
Energy
recovery
Once through the post-combustion chamber, the flue gas enters the heat
recovery boiler, where steam is produced.
Steam can be used for industrial usage (process steam), for district heating or
for energy production in a steam turbine. State-of-the-art technologies can
achieve net electric efficiencies up to about 25%.
Flue gas
treatment
Cold flue gas outside the recovery boiler is processed through various
treatment systems, each dealing with a particular pollutant genre.
Remnants  After the combustion of undifferentiated waste, about 10-12% of the original
volume (which accounts for 20-25% in weight) remains on the bottom of the
furnace in the form of slag. This quantity can be lowered to 15% in weight if
the dry fraction is pre-selected. Another 5% in weight is left as ashes.
All such remnants are collected and sentto landfills. However, studies are
underway to make use them as road beds after detoxification.
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Emission
quality
Substances found in the flue gas of WTE plants (before treatment) can be
classified into three groups:
Dust– Dust is made of solid-state particles (metal oxides, carbon-based
particles, etc.) often referred to as“soot”, upon which dioxins and other
condensable micro-pollutants tend to fix.
Macro-pollutants– They are all substances present in gaseous form, like
carbon dioxide and monoxide, sulphurous anhydride and hydrochloric acid.
Micro-pollutants– They are substances present in very low concentrations,
such as heavy metals, organic chlorinated compounds like chlorophenols,
polychlorobiphenils, dioxins, furans and aromatic polycyclic hydrocarbons
(PAH).
A fraction of the stated pollutants, life for example carbon monoxide, carbon-based particles and traces of organic particles, is the result of incomplete
combustion.
Another fraction is made of heavy metals (mercury) and their chlorides (like
lead, cadmium).
Therefore, besides a proper management of the plant, it’s necessary to resort
to multi-staged flue gas treatment, depending on the physical-chemical
characteristics of pollutants.
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Emission
reduction
One first partial step in reduction of the polluting potential is taken in the heat
recovery boiler, thanks to the cooling processes that impede the formation of
dioxins.
Afterwards, before being released in the atmosphere, the flue gas is treated
to bring all pollutants levels below those set by environmental laws. This
treatment consists of several cleaning chemical-physical operations:
Dust removal– Dusting equipment, installed on tail-end after the recovery
boiler, can be of the cyclone or electrostatic kind. In the latter case flue gas
temperature has to be kept under 250°C to avoid dioxin forming, which is
fostered by sparks.
Once ashes are removed, the flue gas is stripped of acid components before
being released in the atmosphere.
Macro- and Micro-pollutants removal– Different technologies for macro- and
micro-pollutants are available: wet, semi-dry and dry.
In dry processes, the flue gas flows through bubblers with a solution of caustic
soda. This way, hydrochloric and hydrofluoric acids, sulphur dioxide, heavy
metals chlorides are all removed, while nitrogen oxides and most mercury are
not. Dioxins usually condensate on soot particles and are concentrated in the
sludge from the ash removal system.
After the bubblers, equipment can be installed for reducing nitrogen oxides to
gaseous nitrogen by injecting ammonia or urea. Mercury is removed with a
process involving sodium sulphide, while remaining dioxins are absorbed in
active carbon filters.
In semi-dry processesan aqueous suspension of calcium hydrate is used to
neutralize the acid potential in the flue gas. Calcium hydrate is effective on all
gases but nitrogen oxides.
In dry processesthe neutralizing agent is ventilated calcium oxide or sodium
bicarbonate.
Emission
control
Most recent technologies allow modern WTE plants to achieve emission levels
that not only satisfy all current regulations on the matter of environmental
performance, but also exceed them by far.
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MGM
experience and
services
WTE plants, also combined with district heating networks, are a part of the
know-how of MGM Engineering & Contracting. Our experience in the field
allows us perfectly tailoring investments on the customer’s needs.
MGM Engineering & Contractingcan offer the customers complete projects
and engineering services, thanks to managers and technicians with long time
proven experience in the energy area.
Consulting, engineering and expert services can be provided to energy and
utility companies, municipalities and power plant investors. Fields of expertise
include, integrated or separately, the following:
•  Technical and economical feasibility studies
•  Investment cost definition
•  Owner engineering
•  Conceptual and basic design
•  Front-end engineering
•  Detail engineering, including all related technical disciplines
•  Design review
•  Procurement, including purchasing, sub-contracting, expediting,
inspection and logistics
•  Tender evaluation, negotiation and contract preparation
•  Construction management, including field supervision and testing
•  Plant commissioning and start-up, including personnel training
•  Project management, planning and cost control
•  Proposal management and bid document preparation
•  Energy audit and services
•  Permits, authorizations and certifications