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 quantità di alcuni composti che, a causa delle loro proprietà ossidanti o di incombustibilità, 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:
• reciprocating endothermal engines
• 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 €/kWh for the sold energy;
- Belgium: no subsidy for the realization, but the base revenue for the sale of the energy is 0.07 €/kWh besides a bonus of 0.05 € per thermal kWh for the district heating, thus reaching a total maximum revenue on the sold energy of 0.12 €/kWh;
- France: the energy transferred to the power network is paid only 0.05 €/kWh with consequent lack of interest in the farm sector;
- Holland: at present the revenue for the sale of the energy is 0.08 €/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 €/MWh; from 500,000 kWh to 1million kWh/year, 80 €/MWh; from 1 million kWh to 2 million kWh/year, 70 €/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 €/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 €/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.
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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. |
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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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