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Biogas plants installations - Rota Guido

Biogas plants installations - Rota Guido

equipment for zootechnics, facilities for zootechnics, Biogas plants installations, Biogas installations, cogenerator, anaerobic digesto
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Home page > Products > Biogas plants
We have been developing systems for the energy production by means of
renewable energy sources for years. Biogas belongs to those energey
sources. Thanks to the new regulations about self-production, to the
recognition of the environmental value of the energey production from
renewable sources and to a proven technology, today it is possible to
produce biogas for the cogeneration of heat and electricity under
advantageous conditions. The Material and Environment Research Centre
of ENEL (National Electricity Board) together with the Animal
Production Research Centre in Reggio Emilia, conducted, at the
beginning of the Nineties, a large-scale research in the Po valley
about the potential of the biogas that can be produced from the
zootechnical sewage. The research revealed that the cogeneration of
electricity and heat by means of biogas can lead to evident advantages
in both the energy and the environmental field. The cogeneration can
fit suitably in farm engineering, especially when special works are
required to meet the more and more pressing normative obligations
about the sewage disposal. The purpose is to promote the biogas as a
renewable energy source by pointing out the normative and
technological elements so as to allow a realistic and useful view of
the biogas use in the modern zootechnics.
______________________________________________________________________
THE BIOLOGICAL PROCESS OF ANAEROBIC DIGESTION
The anaerobic digestion is an elaborate biological process by which,
in the absence of oxygen, the organic matter turns to biogas (or
biological gas) consisting mainly of methane and carbon dioxide. The
percentage of methane in the biogas changes according to the organic
matter and to the process conditions, from a minimum of 50% to about
80%.
To make the process possible it is necessary that many groups of
microoganisms turn the organic matter into intermediates, mainly
acetic acid, carbon dioxide and hydrogen that can be used by the
methane microorganisms that end the process by producing methane. The
anaerobic microorganisms have a slow growth pace and a slow reaction
pace; it is therefore necessary to keep the optimum conditions of the
reaction environment, as far as possible. Nevertheless, the process
time is relatively long compared to other biological processes; but
the advantage of this process is that the complex organic matter is
turned into methane and carbon dioxide, that is to say that its final
production is a renewable energy source in the form of a high-grade
combustible gas. In order to allow the contemporaneous growth of all
necessary microorganisms, the reaction environment, usually called
digester (or anaerobic reactor) shall be able to combine the different
requirements of every microbic group. For instance, the optimum pH is
about 7/7.5. The optimum process temperature is around 35°C in case of
mesophylic bacteria, or around 55°C in case of thermophilic bacteria.
The following bacteria take part in the process:
* hydrolithic bacteria that break the biodegradable macromolecules
into more simple substances;
* acidogenic bacteria that use the simple organic matters liberated
by the hydrolithic bacteria as substrate and they produce
short-chain organic acids that represent the substrate for the
next bacteria;
* acetogenic bacteria, obligate hydrogen producing acetogens (OPHA),
that use the products of the acidogenic bacteria as substrate to
produce acetate, hydrogen and carbon dioxide;
* homacetogenic bacteria that synthesize the acetate starting from
the carbon dioxide and the hydrogen;
* methanogenic bacteria divided into two groups:
a) those that produce methane and carbon dioxide starting from
acetic acid, called acetoclastic bacteria;
b) those that produce methane starting from carbon dioxide and
hydrogen, called hydrogenotrophic bacteria.
While the methane is liberated almost totally as a gas because of
its little solubility in water, the carbon dioxide takes part in
the balance of the carbonates in the biomass under reaction. The
different bacterial species have close interactions and the
products of the metabolism of some species can be used by other
species as substrate or growing factors.
______________________________________________________________________
INFLUENCE OF THE QUALITY OF THE SEWAGE TO BE DISPOSED ON THE YIELD IN
BIOGAS
The overall sewage biodegradability analysed at the sewers collection
tank can considerably change, between 60% and 80%, according to both
the sewage `age' and the feeding type. An other classification of the
biodegradable contents allows to distinguish between a readily
biodegradable fraction (about 20% of the SSV) and a more slowly
biodegradable one within the soluble content, and between an easily
hydrolyzable suspended fraction and a slowly hydrolyzable one within
the suspended content.
Estimate of the biogas quantity that can be produced with the
anaerobic fermentation
starting from different remaining organic matters
Type of matter Content
of s.s. (%)
Organic matter
(% s.s.)
Biogas yield
m_/l organic matter
Farming
Cattle sewage 6-11 68-85 200-260
Cattle manure 11-25 65-85 200-300
Pig sewage 2.5-9.7 60-85 260-450
Pig manure 20-25 75-90 450
Bird sewage 10-29 75-77 200-400
Bird droppings 32.0-32.5 70-80 400
Sheep manure 25-30 80 240-500
Horse manure 28 75 200-400
Agriculture
Stover 34 86 350-390
Ensiled grass 26-82 67-98 300-500
Hay 86-93 83-93 500
Clover 20 80 300-500
Straw 85-90 85-89 180-600
Cornstalks 86 72 300-700
Agro-industry
Waste of apple distillat. 2.0-3.7 94-95 330
Molasses 80 95 300
Whey 4.3-6.5 80-92 330
Vegetable waste 5-20 76-90 350
If we apply the values of the previous table we can reckon, by way of
an example, the following yield of cogenerated biogas and energy
referred to the average values per product unit:
Data resulting from long-term lab tests, under normal conditions of
the anaerobic reactor and with limited hydraulic residence times,
reach transformation limits of the organic matter into biological gas
that change from 70% and 90% of the top biodegradability depending on
the sewage condition. Low levels of transformation into biogas can be
due to low temperatures, to too short hydraulic retention times (or
excessive organic loads) depending on the process temperature, to bad
hydrodynamic functioning of the reactor with formation of dead areas
and bypass flows between the inlet and the outlet or to the presence
of high-concentrated antibiotic or inhibiting substances.
A futher 12,5% reduction in the organic matter that can be turned into
biogas results from the sewage pretreatment operations (screening)
required to remove the rough solids that could form crusts on the
surface of the unmixed reactors. In order to reckon the yield of
biogas it is necessary to use the stoichiometric analysis: we produce
0.35 l of methane from every g of COD under standard conditions
(volume reckoned at 0°C and absolute pressure of 1 atm).
Actually, this value must be rectified as a fraction quantifiable on
the average of 5% of the destroyed COD is used for the cellular growth
of the anaerobic biomass in charge of the process.
Product Volume (m_) Weight (t) Biogas (m_) Electricity (Kwh) Heat
energy (Kwh)
Cattle sewage 1 1 15 27 54
Cattle manure 1 0.3 10.1 18 36
Pig sewage 1 1 15.6 28 56
Pig manure 1 0.3 23.5 42 84.6
Bird sewage 1 1 44.5 80 160
Bird droppings 1 0.3 29.3 52 105
Sheep manure 1 0.3 21.1 38 76
Horse manure 1 0.3 18.9 34 68
Stover 1 0.625 67.6 121 243
Ensiled grass 1 0.5 89 160 320
Hay 1 0.35 137.8 248 496
Clover 1 0.3 64 115 230
Straw 1 0.04 12 21 49
Cornstalks 1 0.4 123.8 222 445
Apple waste 1 0.3 2.6 4.6 9.4
Molasses 1 0.3 68.4 123 246
Whey 1 1 15.3 28 56
Vegetable waste 1 0.4 14.5 26 52
Tomato peel 1 0.4 29.8 53.6 107
Oil mill waste 1 0.5 357 642.6 1285
Citrus paste 1 0.3 36.8 65.8 131.7
The conversion factor lowers therefore to 0.33. Since the biogas is
usually measured at different temperature and pressure from the
standard conditions, this value shall be moltiplied by a factor equal
to (273 + T)/273, where T is the temperature in °C, and divided by a
factor equal to (10.33 + P)/10.33, where P is the pressure expressed
in mm of water column (follow the reverse procedure to change a
measure taken under the reactor's conditions into a measure under
standard conditions). But as previously mentioned, the yields of
biogas are often estimated with parameters that can be defined more
easily from the zootechnical point of view, and anyway related to COD,
such as the organic matter in the sewage.
WHY CARRY OUT THE ANAEROBIC TREATMENT OF THE SEWAGE?
Within the framework of constant and extreme energy requirements and
high environmental risk, today the anaerobic treatment with biogas
recycling represents a very interesting system having various
advantages:
1) Production of energy: the anaerobic treatment under controlled
conditions leads to the degradation of the organic matter and to the
production of biogas. The cogeneration of electricity and heat by
means of the biogas burning is economically advantageous for both the
company private consumption and the transfer to a third party which is
stimulated by the new regulations about the production of energy from
renewable sources.
2) Abatement of smells and polluting emissions (NH3 and CH4): the
smelly substances that can result from the process (hydrogen sulphide,
mercaptan, ammonia) are burnt together with the biogas.
3) Sewage stabilization: the abatement of the carbonaceous organic
load resulting from the anaerobic digestion gives the sewage a
sufficient stability also in the next storing periods; the slowdown in
the degradation and fermentation processes leads to a reduction in the
production of smelly compounds.
4) Reduction in the pathogenic charge: the anaerobic digestion in
mesophily can partially reduce the pathogenic charge in the sewage, if
any. But in thermophily it is possible to get the full hygienization
of the sewage with the total destruction of the pathogens.
______________________________________________________________________
TYPES AND WORKING OF THE BIOGAS PLANTS
The most common biogas plants can be referable to 3 different types
having separate special characteristics and therefore suitable to
different specific company requirements:
1) "Plug-flow"-type ditch plant
The "plug-flow" plant is characterized by the greatest simplicity of
installation.
Main features: this anaerobic digestion process can be effectively
used for both the livestock sewage treatment and the stabilization of
the sludge resulting from the flotation of the agricultural-livestock
sewage wastes. In case of use for the livestock sewage, it is
necessary to separate in advance the rough solids that are not
technically biodegradable within a reasonable time requirement, and to
use the manure liquid content only in the anaerobic process.
Consequently, the digester is lacking in any internal mixing devices
and the ditch conformation shall be preferred.
In case of flotation sludge, the digester stages will not be
separated. In case of very livestock sewage wastes, the separation
effect of the settleable solids from the sewage liquid content, due to
the lack of agitation in the digester, will bring about an
advantageous rise in the retention time of the solid content compared
to the liquid content. Actually, this phenomenon will allow to remove
the liquid content - with the substances readily available for the
digestion - quickly from the digester and to keep in the digester the
more complex molecules for long, by allowing the bacteria to break
them down and to make them available for the transformation into
biogas. Anyway, the solids will reach the digester outlet by means of
the rising motions due to the biogas and the heating coil near the
bottom of the digester, with the forward motion caused by the
positioning, in the starting and final sections of the digester, of
the induction and drain piping of the fresh and digested sewage.
Suitable for: essentially for medium- and large-sized farmings that
plan to produce energy for their direct requirements and to sell it to
the network managing company in case of surplus only. Generally
speaking, it is also suitable for those who shall considerably reduce
the environmental impact of their zootechnical activity by means of
the flotation and the biological purification of the wastes to be
dumpled into surface hydric bodies.
Process stages: in order to get the highest production of biogas it is
necessary that the digester receives `fresh' sewage; for that reason,
the sewage produced in the farming shall be removed from the livestock
shelters as quick as possible. The sewage is conveyed to a collection
pre-tank and then moved to the separation stage by means of a special
pumping station.
The mechanical separation of the liquid content from the rough solids
is almost always necessary to remove from the sewage those parts that
cannot be biodegraded within the expected digestion times, such as the
vegetable waste and the coat that tend to come to the surface because
of the biogas rising and to form a twisted cellulose crust on the
sewage surface that occupies useful volume and can clog the digester
in the long run. The solid fraction separated before the digester can
be composted or collected and used as amender on the farmlands, while
the liquid fraction, rich in organic matters, will feed the digester
which has usually a rectangular cross-section with one or more
parallel ditches. The anaerobic digestion of the sewage takes place in
a special digestion thanks to the activity of bacteria that crush the
complex molecules by producing methane, carbon dioxide, water and
hydrogen sulphide.
The aforesaid biological activities depend on many factors, such as:
pH, temperature and residence time of the sewage in the digester. In
particular, when the digestion temperature decreases it is necessary
to assure a longer residence time (HRT) of the sewage in the digester.
Consequently, in psychrophilic conditions, it is advisable to provide
for a HRT of at least 60 days, while in mesophilic conditions it is
possible to provide for a HRT of 18-20 days.
According to the above conditions, the energy yield of the plant gives
very good results in any season. In order to work under thermally
controlled conditions, the digester walls shall be suitably insulated
and the inside of the digester shall be heated and kept at the process
temperature by a heat exchanger situated near the bottom and
consisting of stainless steel pipes where the hot water resulting from
the burning of the cogenerated biogas flows.
The produced biogas is directly collected in the upper part of the
digester by means of a gasometric dome covering and, if necessary,
other gas-collecting pressure-switch dome coverings.
The gasometric dome has a semicylindrical shape or spheric cap and it
consists of three superimposed membranes made of PVC-coated polyester
fibres and welded with a high-frequency electronic system.
The inner membrane is used to hold the biogas in a chamber in contact
with the sewage; the side edges of the intermediate one are in contact
with the outside and it prevent the biogas from mix with the air
between the intermediate membrane and the external one which is always
blown up.
The air chamber is kept under pressure by a control unit and by valves
that let the air come in or out by keeping always the biogas at a
pressure of 200 mm H2O, regardless of the quantity of biogas. The
feeding of the burners is therefore regular and the external membrane
is always taut, thus bringing about the predictable advantages as to
wind, water or snow.
The pressure-switch membrane covering system has also the following
advantages:
* it spares the separate manufacture of a gasometer;
* it make the maintenance of the digester easier, as it can be
easily removed;
* it assures a high degree of insulation of the digester top;
* it can fit existing tanks;
* it allows to store the biogas at the use pressure of the burners,
by sparing the installation of compressors for the gas;
* it makes it possible to get a more flexible management of the
biogas users thanks to the high volume contained;
* it makes the dehumidification of the gas easier, especially during
the colder months, by means of the condensation water in contact
with the dome wall.
Through a special pipe connected with the gas-collecting covering of
the digester, the gas produced and recycled is conveyed to a
cogeneration plant that burns the biogas and produces electricity and
heat. Some heat is then reutilized to thermostat and to keep the
temperature in the digester. Finally, the sewage coming out of the
digester, stabilized and deodorized, will be collected in one or more
storing tanks, waiting for its agronomic use.
2) Mixed up-flow cylindrical plant
Main features: this anaerobic digestion process uses the manure
as-it-is (liquid content + solid content); consequently, the
cylindrical digestor will be provided with a helical mixing system, an
external timed recirculation pump and a nozzle system on the bottom to
assure the sewage movement and the `up-flow' and crust-breaking
effect. The digester will be daily fed with `fresh' sewage, while the
digested sewage will come out after an average residence time in the
tank of about 20/25 days.
The positioning of the digester below ground level can, within certain
limits, replace the insulation..
Suitable for: farmings that plan to manage the sewage as sole
homogeneous product and to make the best of it from the energetic and
economic point of view, as this system keeps the whole solid fraction
of the manure, thus increasing the biogas production. It can also be
suitable for medium-sized farmings having yet biomasses to be added
and digested together with the manure. This plant too has considerable
environmental advantages, but it is necessary to consider that: the
not-separated sewage shall be managed with suitable machinery during
the pumping stage; the digester needs more electromechanical
components; the plant has a higher power consumption and any additions
of substances containing nitrogen involve the need of a larger land
for the company balance provided for by the agronomic use plan.
Up-flow plant diagram
Process stages: in order to get the highest production of biogas, it
is essential that the digester is fed with `fresh' sewage: for that
reason, the sewage produced in the farming shall be removed from the
livestock shelters as quick as possible. The sewage is conveyed to a
collection, levelling, mixing and lifting pre-tank provided with mixer
and grinding pump, where a modest quantity of biomass in fixed amounts
can be added so as to get a pumpable mixture, with a solid content not
higher than 10% that enriches the sewage for the digester with organic
matter. The anaerobic digestion of sewage with modest quantities of
biomasses takes place in a special digester thanks to the activity of
bacteria that crush the complex molecules by producing methane, carbon
dioxide, water and hydrogen sulphide.
The aforesaid biological activities depend on many factors, such as:
pH, temperature and residence time of the sewage in the digester. In
case of Up-Flow digesters fed with a sewage-biomass mixture, it is
advisable to assure residence times of at least 30-40 days and
temperatures in the mesophilic and thermophilic field. It is also
possible to subdivide the digestion volume into two reactors - a
primary one and a secondary one - which allow more controlled
methanogenic and acidogenic stages.
3) Super-flow plant for ultrathick biomasses
Main features: the anaerobic digestion process uses the manure
"as-it-is" (liquid content + solid content) with addition of some
biomass, even in large quantities, beyond the pumpability limit.
Usually, the plant has two digesters: a primary one and a secondary
one. The cylindric primary digester is provided with a special
horizontal-axis mixer that assures the complete mixing of the sewage
and the biomass. The primary digester is non-stop fed with `fresh'
sewage and biomass according to a fixed feeding plan depending on the
compositions and the characteristics of the various intake
complements; the digested sewage will leave the tank after an average
residence time of about 20 - 30 days and it'll be conveyed to the
secondary digester, where it'll be remixed so as to recover the
remaining biogas. The residence time in the secondary digester is
about 30 - 40 days, with an overall average residence of about 60
days.
In-line digesters assure the best production of biogas in super-flow
plants
The good mixing is crucial in order to get high density in the
digester. The mass can be heated by means of pipings fastened onto the
digester walls.
Suitable for: firms and zootechnical centres having lands set-aside or
anyway a constant availability of biomass over the year that allows to
increase considerably the biogas production and, consequently, the
electric power, thus optimizing the process yield.
Detail of the gas overpressure valve Digester of the gasometric dome
full of biogas
Process stages: in order to get the highest production of biogas, the
sewage reaching the primary digester should be "fresh" and the
energetic structure quality of the biomass should be entire. The
produced sewage is conveyed to a collection, levelling, mixing and
lifting pre-tank provided with mixer and grinding pump that makes the
sewage uniform and feeds the cylindrical primary digester. A special
dosing augered hopper feeds the biomass and, by means of a specific
programme, loads in the digester the necessary quantity of matter to
assure a good digestion process. The biogas produced in anaerobic
conditions is directly collected in the upper part of the digester/s
through a gasometric dome covering having a globe shape with variable
volume. Through a pipe connected with the gas-collecting covering of
the digesters, the produced and collected gas is balanced, cooled,
dehumidified, filtered and conveyed to the cogeneration units that
burn the biogas, thus producing electrical power and heat. Then, the
outgoing sewage stabilized and deodorized will be collected as it is -
or after separation - in one or more storing tanks, waiting for its
agronomic use.
Dosing caisson to automate the digester feeding operations Side mixing
augers and bottom auger in the loading hopper to feed the biomass in
Watertight doors allowing the access to the digester for check and/or
maintenance.
______________________________________________________________________
BIOGAS PROCESSING AND USE
Biogas purification
The biogas processing is crucial to assure the correct working of
cogenerators.
Nel biogas sono presenti piccole quantitą di alcuni composti che, a
causa delle loro proprietą ossidanti o di incombustibilitą, devono
essere eliminati per favorire un buon processo di combustione mediante
le seguenti tecniche:
* Filtering with gravel or sand filters, necessary to remove the
suspended solids which consist basically of organic matter, fats
and any foams, before the suction of the recirculation compressors
or the auxiliary compressors of the boiler and the gas engines;
* Dehumidification; the temperature of the biogas coming out of the
digester is at least 35°C, with a high humidity degree that causes
the water vapour to condense; as a consequence, condensation traps
and sumps are arranged along the piping. But to prevent the water
vapour from condensing in the burning chamber it is necessary to
remove the humidity drastically by means, for instance, of a
condensation plant consisting of a direct-expansion refrigerator,
a tube-bundle water/biogas exchanger and a coalescent filter where
the steam is condensed and then removed through a manual or
automatic drainage;
* Desulphurization; necessary to abate the sulphur-based compounds,
it can be carried out by means of chemical filters full of iron
oxides that cause the precipitation of the compounds and,
consequently, their extraction; by means of washing towers that
execute a reverse-flow scrubbing of the gas with water and ferric
oxide; or by means of biological desulphurization with the direct
injection of some air - about 5-10% of the gas - in the digester,
so as to allow certain bacterial strains to trigger a sulphur
biological precipitation reaction.
The adjustment and control systems are crucial to assure the best
performance of the plant.
Uses of the biogas
After the necessary treatments, the biogas can have two applications:
a) the production of heat only;
b) the cogeneration of electrical power and heat.
Burning for the only production of heat:
Plants with simple technologies are used; it is enough to use a common
gas heat generator consisting of a burner where the combustible
material and the comburent burn by producing thermal energy and a heat
exchanger where the results of the burning transfer the produced heat
to a heat-transfer fluid. The biogas is treated like the natural gas,
but some changes are made in the burner for the gas inlet, the
combustible material/comburent mixing and the use of more resistant
materials to corrosion for the heat exchanger and the burner..
Cogeneration for the contemporaneous production of electrical power
and heat:
This is the contemporaneous production of heat and mechanic energy
immediately transformed into electric power (it's an only integrated
system called `total energy system') starting from the primary energy.
This energy production system allows a considerable energy saving
compared to the separate production of the same quantities of heat and
electrical power/mechanical energy; it is possible to exceed a 90%
efficiency (30% of electrical efficiency and 60% of thermal
efficiency).
Two different machinery are used:
o reciprocating endothermal engines
o microturbines
For the cogeneration with reciprocating endothermal engines we use
engines with Otto cycle or modified Diesel cycle, consisting of the
following components:
* reciprocating endothermal engine that, besides producing the
mechanical energy, produces also the thermal energy;
* alternator - usually asynchronous - for the production of
three-phase alternating current;
* thermal regenerator consisting of a heat exchanger that recovers
the heat produced by the whole system, from both the waste gas and
the engine cooling circuit and the lubricating oil;
* electric panel that allows to use the produced electrical power
and to interface with the national electric power line.
For the cogeneration with microturbines we use innovatory small-sized
gas turbomachinery deriving from aeronautics and consisting of the
following main components:
* gas turbine and regenerator;
* current generator electrical system;
* heat exchanger on the exhaust gases;
* operation and control system.
For powers over 50 Kw, the transfer to the electric network is usually
admitted for average voltage only.
Cogenerators can work as follows:
* parallel to the public mains: all energy produced by the
continuous-running engine at maximum power is transferred to the
company network connected with the external line. The magnetizing
energy is totally absorbed by the network, there are no problems
about peak loads and controls on the produced electric power
concern solely the voltage and the frequency that shall be kept
constant. A drawback of this system is that the cogenerator cuts
out in case of power failure in the mains;
* independent island of the electric power line: typical of places
without public network, or in case of applications that can be
separated from the company network - e.g. purification plants. It
is necessary to have a self-excited generator with a
battery-connected engine starter. The advantage of this system is
that the electric power is assured also in case of blackout in the
public network, but it has two drawbacks: the necessary oversizing
of the cogenerator, as it has to overcome the startings of the
different applications and the necessary uninterruptible power
supply units in case of electronic circuits or equipment that
cannot be disconnected, as the current is temporarily cut off when
the system starts;
* on standby: in case of regular working the cogenerator is
parallel-connected with the public network, while in case of
blackout the cogenerator goes on assuring the power to
preferential lines, after disconnecting automatically from the
network, and it supplies power according to the application's
requirement; once the public network is reactivated, the system
reverts to the parallel connection.
______________________________________________________________________
THE EUROPEAN SITUATION
In Europe, the spread of the anaerobic digestion started in the sector
of the sewage sludge stabilization and at present there are about
2,000 working digesters, about 400 biogas producing plants for the
disposal of the industrial wastewater with high organic load and 500
biogas recovery plants from urban waste dumps. Besides, about 2,500
anaerobic digesters work on livestock sewage in the EU Countries, in
particular in Germany (about 2,000), Denmark, Austria, Italy and
Sweden. In the last year, the anaerobic digestion are becoming more
common in the treatment of the organic content from the separate urban
waste collection, mixed with industrial waste and livestock sewage. In
Denmark, for instance, 25 centralized co-digestion plants are
currently working and they dispose of about 1 million tons of
livestock sewage and 325,000 t of industrial organic residues and
urban waste every year. As to grants and funding for the realization
of biogas plants within the European Countries, the current situation
is outlined below:
* Luxembourg: a subsidy equal to 60% of the investment cost and up
to 0.10 EUR/kWh for the sold energy;
* Belgium: no subsidy for the realization, but the base revenue for
the sale of the energy is 0.07 EUR/kWh besides a bonus of 0.05 EUR
per thermal kWh for the district heating, thus reaching a total
maximum revenue on the sold energy of 0.12 EUR/kWh;
* France: the energy transferred to the power network is paid only
0.05 EUR/kWh with consequent lack of interest in the farm sector;
* Holland: at present the revenue for the sale of the energy is 0.08
EUR/kWh, but the regulations that should come into force within
this year provide for incentives similar to the German ones;
* Germany: this is the European Country where the anaerobic
digestion has been given the most substantial boost thanks to
incentives starting from a minimum of 25% of the investment cost
and prices for the electric power produced from biogas assured for
20 years that are summed up in the following table:
REVENUES fUp to
150 kw
From 150 to 500 kw From 500 kw to 5 Mw
Base 0,11 0,09 0,08
Biomass 0,06 0,06 0,06
District heating 0,02 0,02 0,02
Technological efficiency 0,02 0,02 0,02
MAXIMUM POTENTIAL REVENUE 0,21 0,19 0,18
The strong biogas production incentive focuses entirely on the
energetic goals, without seeing to the environment and, in
particular, to the resulting impact of nitrates on the land,
also thanks to the less concentration of livestock compared to
Italy.
______________________________________________________________________
THE ITALIAN SITUATION
In our Country, especially in those areas with a strong inclination to
zootechnics, the situation is quite different: the environment and, in
particular, the nitrogen load have a substantial importance and can
play an essential role in prmoting or limiting the development of the
`zootechnical' biogas. Many reasons - related to both environmental
issues and energy efficiency - can drive the farmer to the biogas,
with the possibility to use also the biomasses resulting from
set-aside surfaces, thus exploiting them with no-food cultivations
such as fodder maize, grass, sorghum and the like.
In 1999 a research showed that in Italy 72 biogas plants worked with
livestock sewage. Five of which were centralized plants and 67 were
company plants. Almost all plants are situated in the Northern regions
(39 in Lombardy, 7 in Emilia-Romagna, 12 in Trentino Alto Adige).
At the end of 2004, the plants were more than 100, among which
approximately 70 belong to the low-cost simplified type with a plastic
gasometric dome on the storing tank of the livestock sewage.
Most of the plants currently working in Italy have been designed and
consequently dimensioned depending on a standard of energy saving for
the farming, using all the energy produced during the digestion
process for the energy requirements of the farming and, usually, the
relevant house applications.
The installation of cogeneration units in quite spacious room makes
all maintenance easier.
For that reason there are cogenerators that work "in isolation", i.e.
without the possibility of interfacing with the national electric
network in case of energy surplus.
At present, the development of new plant-engineering techniques and
the biomass digestion have improved and increased the biogas
production and the new energy regulations boost the production and
consequently the sale of the energy from renewable sources, thus
modifying the design of the new plants.
Also because of the greater requirement of the farming, farmers
usually choose a network-parallel connection so as to sell the energy
surplus.
This involves other advantages that can be easily assessed, as the
sale of the energy surplus and the green certificates (having a 8-year
validity in Italy) allows the owner to get yearly revenues that reduce
or even halve the investment return time compared to the old plants.
Biomasses Quantity:
Animals' manure 180.000 t/years
Agro-industrial waste 12.000.000 t/years
Abattoir waste 2.000.000 t/years
Sewage sludge 2-3.000.000 t/years
Organic content of urban waste 9.000.000 t/years
Cultivation waste 10.000.000 t SS/years
Energy cultivations 230.000 ha set a side
The above biomasses are currently available in our country: about 120
plants use the sewage sludge coming from the urban sewage treatment
plants, 7 plants use the organc content of the urban waste and some
plants use the agro-industrial waste. In Italy, the regulations and
the incentives to the production of electric power from renewable
sources should boost the development of biogas plants; consequently,
it is necessary to strengthen and to rationalize the systems
exploiting the anaerobic codigestion processes of various biomasses
(livestock and agro-industrial biomasses, energy cultivations and
cultivation waste, sewage sludge and organic content from separate
urban waste collection).
It would be better to boost also the development of anaerobic/aerobic
integrated systems for the combined treatment of sewage and urban
organic waste and other biomasses.
The insertion of vegetable substances with a high dry-matter content
in the fermenter involves technical problems that shall be
specifically faced right from the design phase, as to the biomass
feeding equipment and the requirement of suitable mixing and grinding
systems. The use of ensilages requires a special attention, as they
cause a fall in the pH value within the digester and possible
corrosion of the feeding equipment.
It should be noted that the installation of a biogas production plant
is the same as adding a new breeding with a different type of animals:
the microorganisms, with their own environmental and food
requirements. The farmer shall develop a correct management suitable
to the specific treatment requirements, so as to avoid inefficiency,
failure and disappointment about the revenues from the investment.
How to sell the energy
The energy from renewable sources is sold, upon the producer's
request, to the power network company at the wholesale price of the
electric power transferred from the sole buyer to the supplying
companies. The produced energy can be used as follows:
1) On the market:
* by selling the energy to a suitable end customer or to a
wholesaler by means of a bilateral contract
* by selling the energy on the stock exchange
2) Sale of the produced electric power to the relevant power network
company. A special agreement between Federenergia, Enel Distribuzione,
GRTN, Acquirente Unico and the producers' associations has the purpose
to define the contractual, economic and technical conditions for the
transfer of the electric power.
This agreement has one-year validity and can be renewed; the producer
gives the power network company all the produced energy except for
that used for private consumption.
According to the agreement for renewable source energy plants with
power up to 1MW, the network companies assure the following minimum
prices subdivided into classes: for the first 500,000 kWh/year, 95
EUR/MWh; from 500,000 kWh to 1million kWh/year, 80 EUR/MWh; from 1
million kWh to 2 million kWh/year, 70 EUR/MWh; over 2 million
kWh/year, the price of the Acquirente Unico (according to times or
undifferentiated).
The application of the aforesaid minimum prices assure the covering of
the expenses of the small-sized plants that use renewable sources but
cannot compete or take part in the market; besides, since the power
collected by the network companies is paid according to the real
input, the producers are not bound to submit the production plans and
consequently the amounts due for the unbalance are not applied. The
costs to be paid to the network company are 120 EUR/year for each
plant so as to cover the administrative costs and 0,5% of the exchange
value of the collected energy; if the network company manages also the
contracts, it is necessary to pay 120 EUR/year for each plant and 0,5%
of the exchange value of the collected energy.
Green certificates
The green certificates are real negotiable securities on the energy
market, issued and controlled by the National Conversion Network
Company (GRTN); they are used to boost the production of energy from
renewable sources and to certify that the energy at issue comes from
renewable source plants, such as sun, wind, water, geothermal sources
and the conversion of vegetables or organic and inorganic waste into
electric power.
To be entitled to the certification, the plants shall be qualified by
GRTN as "Renewable Energy Source Plant" (IAFR); the biogas plants
using organic waste and/or vegetables to produce the electric power
are therefore entitled to the IAFR qualification and to the green
certificates that can be obtained according to the following
procedure:
* ask the GRTN for the IAFR qualification;
* after getting the qualification, ask the GRTN for the issue of
green certificates for the current year;
* the next years, together with the application, it is necessary to
submit the UTF a declaration proving the real production.
To get a green certificate, it is necessary to produce at least 50,000
kWh every year, but for small-sized plants a production of electric
power over 25,000 kWh/year is enough for a green certificate. The
certificates are given to the qualified plant for 8 years after its
coming into use; it is possible to get new certificates for another
8-year period subject to modernization and strengthening of the plant.
At present, a proposal is under consideration: a 12-year period for
the issue of the green certificates. The application for certificates
is provided for by the law, since the national power system is bound
to receive a share of energy from renewable sources equal to 2.35%,
increased by a yearly 0.35% from 2004 to 2006, while the increase will
be the same or higher for the next three-year periods 2007-2009 and
2010-2012.
The technical condition for the transfer of the energy shall be agreed
upon with the supplying agency.
The price of the green certificates is variable and it is set from
year to year according to the granted incentives; as to 2004, the set
value was 9.739 eurocent/kWh and it applies to the whole production,
for both private consumption and transfer. It should be noted that the
green certificates in possession of the biogas plant owner and the
energy produced by the same plant can be separately sold, as the green
certificates are paid according to the whole energy production, while
only the electric power sold to the national power network is paid;
the national network profits by the dispatching priority as this
energy is produced from renewable sources. Besides, the construction
of an IAFR plant are considered as utilities, according to the
legislative decree 387/03. Therefore, after getting the CPI from the
Fire-prevention Department of the Ministry of the Internal Affairs,
all works related to the construction and management are subject to a
sole permission which is issued by the regional government or by
another delegated party; furthermore, the IAFRs with a power below 3MW
are considered as minor air polluting activities.
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