Waste to energy
• process of generating energy in the form of electricity
and/or heat from the primary treatment of waste.
• form of energy recovery. WtE processes can produce
electricity and/or heat directly (combustion), or produce
a combustible fuel commodity, such as methane,
methanol, ethanol, synthetic (gas/liquid) fuels.
• various technologies applicable to all types of waste
(urban, industrial, etc…)
• some processes allow the additional recovery of
secondary raw materials that can be reused in industrial
production or other environmental applications
A PARADIGMATIC SHIFT IS REQUIRED
WtE is part of the “Urban Mining Process”
..which is part of “Circular Economy”
generic term for an urban/industrial economy that is producing no
waste and pollution, by design or intention, and in which material
flows are of two types, biological nutrients, designed to re-enter the
biosphere safely, and technical nutrients, which are designed to
circulate in the production system without entering the biosphere.
It is restorative and regenerative by design.
Circular economy doesn’t just “happen”.
Proper economic, regulatory and public awareness frameworks must
exists and be locally and globally consolidated (i.e. competition with
traditional economy, raw materials and product standards, public
acceptance of waste-derived products).
Circular economy - Basic principles
Waste is (someone’s/something’s) food
Waste does not exist… the biological and technical components
(nutrients) of a product are designed by intention to fit within a
materials cycle, designed for disassembly and re-purposing. Non-
toxic biological nutrients can be simply returned to the biosphere.
Technical nutrients – polymers, alloys and other man-made materials
are designed to be used again with minimal energy input.
Diversity is strength
Modularity, versatility and adaptiveness are to be prioritised in an
uncertain and fast evolving world. In working toward the circular
economy, we should focus on longer-lasting products, developed for
upgrade, ageing and repair, considering strategies like emotionally
Circular economy - Basic principles (2)
Energy must come from renewable sources
As in life, any system should ultimately aim to run on ‘current
sunshine’ and generate energy through renewable sources.
The ability to understand how things influence one another within a
whole. Systems thinking usually refers to nonlinear systems: systems
where through feedback and imprecise starting conditions the
outcome is not necessarily proportional to the input and where
evolution of the system is possible, like in are all living systems.
Prices or other feedback mechanisms should reflect real costs
Prices are “messages”, and therefore need to reflect full costs in
order to be effective, including those of negative externalities.
HAVE A PLACE
WtE / WtR
Combustion of organic material with energy
recovery, is the most common and old WtE
technique (first plant in Nottingham, 1874).
Modern incinerators reduce the volume of
the original waste by 95-96 percent,
depending upon composition and degree of
recovery of materials (e.g. metals) from ash.
• They are commonly used for MSW,
including dried WWTP sludge, disposal.
• May emit fine particulate, heavy metals,
trace dioxin and acid gas, even though
these emissions are relatively low in
A critic is that incinerators destroy valuable resources and they
reduce incentives for recycling. The issue, however, is open, since EU
countries recycling the most (up to 70%) also incinerate all their
residual waste to avoid landfilling.
Incinerators have electric efficiencies of 14-28%. The rest of the
energy, (thermal cogeneration) can be used for district heating or
industrial uses. Total efficiencies of cogenerating plants are typically
higher than 80% (based on the lower heating value –LHV- of waste).
Proper flue gas treatment is necessary to comply with air emission
Incinerators are among the least accepted environmental
installations by the public.
CURRENT TECHNOLOGY - MOVING GRATE INCINERATORS
typical for MSW. A moving grate enables movement of waste through
the combustion chamber, optimised to allow more efficient and
complete combustion. A moving grate boiler can handle up to 35 tons
of waste per hour, operate 8,000 hours/yr.
Must be designed to ensure that flue gases reach a temperature of at
least 850 °C for 2 seconds in order to ensure proper breakdown of toxic
organic substances (dioxins). It is therefore required to install backup
auxiliary burners into the boiler in case the waste LHV becomes too low
to reach this temperature.
Flue gases are then cooled in superheaters, where heat is transferred
to steam, for the electricity generation in a turbine. After, the flue gas
at a temperature of around 200 °C is passed to the cleaning system.
CURRENT TECHNOLOGY - MOVING GRATE INCINERATORS
CURRENT TECHNOLOGY - FLUIDIZED BED
The fluidized bed is created by a strong airflow forced through a sandbed,
seeping through the sand until the point where sand particles separate to let
the air through and mixing and churning occurs. Fuel and waste are then
Sand and pre-treated waste and/or fuel are kept suspended on pumped air
currents and take on a fluid-like character. The bed is violently mixed and
agitated keeping small inert particles and air in a fluid-like state. This allows all
of the mass of waste, fuel and sand to be circulated through the furnace.
CURRENT TECHNOLOGY - ROTARY-KILN
Used by municipalities and by large industrial plants. This design of incinerator
has 2 chambers. The primary chamber in a rotary kiln incinerator consist of an
inclined refractory lined cylindrical tube. Movement of the cylinder on its axis
facilitates movement of waste.
In the primary chamber, there is conversion of solid fraction to gases, through
volatilization, destructive distillation and partial combustion reactions. The
secondary chamber is necessary to complete gas phase combustion reactions.
Clinkers spill out at the end of the
cylinder. A tall flue-gas stack, fan,
or steam jet supplies the needed
draft. Ash drops through the
grate, but many particles are carried
along with the hot gases. These
particles and any combustible gases
may be combusted in an
Process that converts organic carbonaceous materials into carbon
monoxide, hydrogen and carbon dioxide, by reacting the material at
high temperatures (>700 °C), without combustion, with a controlled
amount of oxygen and/or steam.
The resulting gas mixture is called syngas (synthesis gas) is itself a fuel.
The power derived from gasification and combustion of the syngas is
considered to be renewable energy if the gasified compounds were
obtained from waste or renewable biomass.
In the XIX century, so called “city gas” was obtained with gasification of
Waste gasification has the following advantages over incineration:
• flue gas cleaning may be performed on the syngas instead of the much larger
volume of combustion flue gas.
• electric power may be generated in engines and gas turbines, which are much
cheaper and more efficient than the steam cycle used in incineration. Fuel cells
could potentially be used, but they have stringent requirements regarding the
purity of the syngas.
• Chemical processing (Gas to liquid) of syngas may produce other synthetic (liquid)
fuels instead of electricity.
Major challenge for gasification technologies is to reach a positive gross electric
efficiency. The high efficiency of converting syngas to electric power is counteracted
by significant power consumption in waste pre-processing, consumption of large
amounts of pure oxygen (which is often used as gasification agent), and gas cleaning.
Another challenge apparent in real life is to obtain long service intervals in the plants,
so that it is not necessary to shut down the plant frequently for cleaning the reactor.
Some people consider gasification an "incineration in disguise“, and arguing that the
technology is still dangerous to air quality and public health.
In a gasifier, carbonaceous material undergoes several different reactions:
Dehydration at around 100 °C. The resulting steam is mixed into the gas flow and may be
involved with subsequent reactions
Pyrolysis of carbonaceous matter. Pyrolysis (or devolatilization) process occurs at around
200-300 °C. Volatiles are released and char is produced, resulting in up to 70% weight loss
Combustion occurs as volatile products and some of the char react with oxygen to
primarily form carbon dioxide and small amounts of carbon monoxide, which provides
heat for the subsequent gasification reactions. Basic reaction
C + O2 → CO2
Gasification occurs as the char reacts with steam to produce carbon monoxide and
C + H20 → H2 + CO
In addition, reversible gas-phase water-gas shift reaction reaches equilibrium
very fast at the temperatures in the gasifier. This balances concentrations of
carbon monoxide, steam, carbon dioxide and hydrogen
CO + H2O ↔ CO2 + H2
In practice, a limited amount of oxygen (air) is introduced into the reactor to
allow some of the organic material to be "burned" to produce carbon dioxide
and energy, which drives a second reaction that converts further organic
material to hydrogen and additional carbon dioxide.
Further reactions occur to form methane and excess carbon dioxide
4 CO + 2 H2O → CH4 + 3 CO2
in reactors with high residence time of gases and organic materials, heat and
pressure. Catalysts are used in sophisticated reactors to improve reaction
rates, moving the system closer to equilibrium.
CURRENT TECHNOLOGY – NIPPON STEEL
A patented spurious process derived from the blast furnace iron
making technology, 33 operating plants in Japan and 2 in S. Korea.
It is a fixed bed system, where coal (about 5% in mass) is added to
the waste .
Temperature at the bottom of the furnace reaches in excess of
1300oC, yielding molten metals and vitrified
(inert) slag as final residues.
Organic waste content is decomposed
in the mid section of the furnace into
Syngas, that is burned in a
subsequent plant section.
Overall energetic efficiency
CURRENT TECHNOLOGY – FLUIDIZED BED MELTING FURNACE
Thermochemical decomposition of organic material at elevated
temperatures in the absence of oxygen (or any halogen). Involves the
simultaneous change of chemical composition and physical phase, and is
In general, pyrolysis of organic substances produces gas and liquid
products and leaves a solid residue richer in carbon content, char. Extreme
pyrolysis, which leaves mostly carbon as the residue, is called
• has been used since ancient times for turning wood into charcoal on
industrial scale, and into coke for steelmaking
• carbon fiber items are often produced from fibers of a suitable
polymer, pyrolyzed at high temperature (1500–3000 °C)
• basis of methods developed for producing fuel from biomass
• anhydrous pyrolysis used to produce diesel-like fuel from plastic waste
• pyrolysis of waste tires allows the high energy content of the tire to be
recovered as fuel: 38-56% oil (benzene, diesel, kerosene, fuel oil and
heavy fuel oil), 10-30% gas (natural-gas like) and 14-56% char.
Microwave-assisted pyrolysis can be used for further valorization of
urban waste sludge, otherwise destined to final disposal, with high
yields of a biodiesel-like product.
Max Temp. oC Time at TMAX [min]
% oil to
% oil to sludge
TQ 60 = = 3.57* 7*
TEST 1 270 20 55 9.68 19
TEST 2 180 28 50 3.30 7
TEST 3 400 5 55 8.64 17
TEST 4 490 1 54 10.25 21
TEST 5 600 3 56 8.71 17
TEST 6 400 6 46 11.79 24
TEST 7 500 9 51 7.63 15
TEST 8 650 - 60 7.38 15
TEST 9 280 2 8 12.52 25
TEST 10 400 2 18 10.77 22
Yields grater than 10% oil to sludge (in volume) where obtained
using pre-digested sludge.
Based on the oil energy content, the process –once optimized- can
yield energy recoveries greater than 100%.
1/ 270 49.36 13.96 35.40 1.7164 56.6 114.67 159.89
2/ 180 65.94 23.82 42.11 1.7894 59.1 89.63 140.35
3/ 400 47.22 25.08 22.14 1.5621 51.5 109.06 232.61
4/ 490 76.07 31.85 44.22 1.0775 35.6 46.80 82.54
5/ 600 44.09 21.53 22.56 1.1365 37.5 85.05 166.22
6/ 400 90.64 62.7 27.94 1.8413 60.8 67.09 217.61
7/ 500 60.19 18.79 41.40 1.022 33.7 55.99 81.40
8/ 650 94.51 33.31 61.20 0.86221 28.4 30.05 46.40
9/ 280 92.99 19.17 73.82 1.4137 46.7 50.22 63.26
10/ 400 134.62 33.34 101.28 1.4077 46.6 34.61 46.01
Simultaneous digestion of a homogenous mixture of two or more
substrates, e.g. manure or sewage sludge, mixed and digested
together with amounts of a single, or a variety of additional
substrates. The expression is applied independently of the types and
ratio of the respective substrates used simultaneously.
Merits of Co-digestion
Ecological, technological and economical advantages:
Improved nutrient balance. Digestion of a variety of substrates
instead of a single waste type improves the nutrient ratio of TOC:N:P
(optimally should be around 300:5:1), and maintains a good mix of
minerals (Na, K, Mg, Mn, ...) and balanced composition of trace
metals, helping maintaining stable and reliable digestion
performance and good fertilizer quality of the digestate.
Optimisation of rheological qualities. Wastes with poor fluid
dynamics, aggregating, particulate or bulking, and floating wastes can
be much easier digested after homogenisation with dilute substrate
such as sewage sludge or liquid manure. Mixing of different
substrates allows flexibility to compensate for seasonal mass waste
fluctuations, thus underloading/overloading of digesters can be
avoided and digestion process can be maintained at constant rate.
Merits of Co-digestion (cont’d)
Gate fees and biogas recovery. Application of co-substrates can
considerably improve the economics (payback time) of a plant. Gate
fees are significantly lower at farm-scale digestion plants than at an
incineration or composting facility (usually by a factor of 3-4). The
operator takes credit of the increased biogas production and the
income from the gate fee. Provided there is sufficient farmland
available, digestate can be directly recycled as a fertilizer at
Energy crops as co-substrate. In a limiting situation of other wastes,
energy crops might become an interesting alternative, especially
when plants are grown on fallow or set-aside land which attracts
subsidies. A number of crops (food AND non-food) demonstrate good
Feed waste’s C:N ratio
is the main factor in
determining final biogas
Nowadays, a small part of codigested organic waste consists of
sewage sludge residues in the EU.
composting consists of making a heap of wetted organic matter
(green waste: leaves, food waste) and waiting for the materials to
break down into humus after a period of weeks or months. Modern,
composting is a closely monitored process with measured inputs of
water, air, and carbon- and nitrogen-rich materials. The
decomposition is aided by shredding the plant matter, adding water
and ensuring aeration by regularly turning the mixture.
Compost is rich in nutrients and thus it is used in gardens,
landscaping, horticulture, and agriculture. The compost itself is
beneficial for the land in many ways, including as a soil conditioner, a
fertilizer, addition of vital humus or humic acids, and as a natural
pesticide for soil.
In ecosystems, compost is useful for erosion control, land and stream
reclamation, wetland construction, and as landfill cover (see compost
However, among the possible negative effects of compost application
to cropland is the potential release of toxic heavy metals into the
environment, and the transfer to these elements from the soil into
the food chain.