HomeMy WebLinkAboutCastel d'Aiano plant pubblication
THE ECOLOGICAL PLANT
IN CASTEL D’AIANO
The CHP system of Castel
d'Aiano school campus:
wood chips gasification
and Stirling engine
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
2
INDEX
1 Introduction ..................................................................................................... 3
2 The facilities served by the CHP plant .......................................................... 4
3 The plant configuration .................................................................................. 6
3.1 The gasifier .......................................................................................9
3.2 The combustion chamber ............................................................... 10
3.3 The Stirling engine .......................................................................... 11
3.4 The accumulator tanks ................................................................... 14
3.5 The wood chips storage .................................................................. 14
4 System dimensioning and main technical parameters ............................. 16
5 The syngas .................................................................................................... 18
6 Wood chips demand ..................................................................................... 19
7 Production of heat and power ..................................................................... 20
8 Environmental parameters ........................................................................... 22
9 The ash disposal ........................................................................................... 23
10 Main parameters ........................................................................................... 24
11 The work group ............................................................................................. 27
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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1 INTRODUCTION
The cogeneration system based on the gasification of wood biomass and a
Stirling engine for the combined production of heat and power (CHP), has been
realized by CISA1 in collaboration with COSEA, the local energy utility company,
in the territory of the Municipality of Castel D’Aiano, in the Italian Central
Apennines.
The combined generation of heat and power in small size plants using renewable
primary energy sources such as wood chips is still not very widespread in Europe.
Furthermore the plant in Castel D’Aiano is doubtlessly an innovative pilot plant
and one of the first example of this kind in Europe.
A small size CHP plant offers a number of advantages both in terms of
environment and sustainability: installing a centralized thermal plant for the
electricity and the heat demand of small settlements, school campuses, sport or
recreational facilities, promotes the diffusion of the technology with consequent
benefits on the local economy, the valorization of local mountain resources, the
distributed generation of energy and the creation of a short agro-wood biomass
supply chain.
1 CISA stays for Centro per l’Innovazione e la Sostenibilità Ambientale in Appennino, that is
“Innovation Centre for the Environmental Sustainability in the Apennines”
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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2 THE FACILITIES SERVED BY THE CHP PLANT
The main buildings served by the plant are part of a quite large school campus
including primary and secondary school. Before the installation of the plant the
campus was heated by an old generation, low efficiency, natural gas boiler; the
action on the plant, beside having turned the system into a renewable energy
based one, also has brought overall remarkable energy savings; furthermore in
general, it offers a new modern way of looking to the energy supply.
The campus includes areas dedicated to sport activities, didactics, recreational
activities and the lunchroom. Close to the campus there is the municipal outdoor
swimming pool. In summer the air temperature is warm; nonetheless the users of
the swimming pool complain for the low temperature of the water, as usually
happens at this altitude. Therefore, the thermal energy produced in summer is
directed to increase the water temperature of few degrees centigrade, with an
increase of the satisfaction of the swimming pool users.
The cogeneration plant will also heat the locker rooms of the sports ground.
In order to reach the above mentioned buildings and premises a small district
heating network was realized in subsequent steps: at the beginning the main
buildings were connected, such as school building and the gym; subsequently the
locker room and the swimming pool have been connected.
A table with the main figures related to the heat demand of the campus before the
installation of the CHP system is shown below. The year consumption of methane
gas was estimated through the analysis of historical data.
Gym
Yearly average consumption of natural gas 22,879 m
3
kWh/year for space heating 211,402 kWh/year
Peak monthly consumption 4030 m3 37,237 kWh
Peak daily consumption 134 m3 1,213 kWh
School
Yearly average 29,082 m3
kWh/year for space heating 268,713 kWh/year
Peak monthly consumption 5,809 m3 53,675 kWh
Peak daily consumption 194 m3 1,749 kWh
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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School Swimming pool Sport centre
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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3 THE PLANT CONFIGURATION
The CHP system can be described as based on two different stages connected by
the mass flow; each stage is linked to the next as it provides the input material for
the subsequent one, in a close chain of reactions.
One of the main characteristic of the plant is in fact the continuity of the operation
and the necessity to keep a certain set of parameters constant, whatever the
condition; in particular it is necessary to keep the electricity generation – which,
being fed into the grid is not influenced by intensity fluctuations - independent
from the heat generation – which, on the contrary, is influenced by the little
changes of a small district heating, trying to dissipate as less heat as possible.
Most components of woods may be gasified, this process is called gasification
because it transforms the solid fuel in gas fuel with a low calorific power. The
process is based on the partial combustion of wood, by filtering through the wood
chips a hot blend of flue gas coming from the combustion process and fresh air;
the air enriches the blend with oxygen in low percentage. The flue gas acts as
inert carrier or gasification agent while the oxygen acts as the comburent of the
partial combustion. The combustion can be kept under control by controlling the
amount of oxygen in the mixture. The gas obtained is commonly known as
syngas.
The plant of Castel D’Aiano relies on an up-draft gasifier; the syngas is directly
burned into a combustion chamber and consequentially is allowed to avoid the
complex purification process of the syngas, which has a content of particles and
tars; moreover another advantages of this kind of gasifire is that it can be fed with
wood chips with a quite high percentage of moist.
The gas obtained at the end of the first stage is then mixed with pre-heated air at
high temperature, and burns in the combustion chamber.
The high temperature gases produced are fluxed through the heat exchanger of
the Stirling engine, which accomplishes its thermodynamic cycle producing
mechanical energy which, on its turn, is transferred to a shaft and providing the
thermal energy to heat up the water for the district heating demand. The kinetic
energy is converted into electricity by an asynchronous generator. The exhaust
gases are then partly directed to the gasifyer (as mentioned before) which will use
them to complete its thermodynamic needs, closing in this way the cycle, partly
are directed through an economizer to cool them down before discharging
through the chimney; the recovered heat is also sent to the district heating.
During the combustion the flame temperature can reach extremely high values
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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such as about 1,250 °C while the heat exchanger temperature of the Stirling
engine can reach about 700 °C. Such elevated temperature allows the complete
oxidation of the gasification residues, leading to extremely clean exhaust gases,
with very low content of dust and negligible uncombusted residues.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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3.1 The gasifier
This element consists of an outer steel chamber, lined with insulating
material. The wood chips are fed to the top of the updraft gasifier with a
screw conveyor. The gasifier is heated by the heat recovered from the combustion
chamber up to 800 °C or more; air is supplied as to reach about the 10% of
oxygen in the blend and the biomass
gasification process starts. This produces
syngas which is extracted from the top of the
gasifier and carried to the combustion
chamber.
The syngas, drafted from the top, is obliged to pass through the wood chips,
extracting the wood
moisture and at the
same time cooling
down itself; this is
one of the reason
why this kind of
plant can be fed
with wood chips
with a high value of
moisture.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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3.2 The combustion chamber
The combustion chamber is a highly technological steel cylinder, which is
lined by an inert and insulating material, resistant up to really high
temperatures. The combustion chamber is directly in contact with the Stirling
engine.
A flow of syngas and preheated air passes through a special burner; the
combustion process is optimized by controlling the mixture components’
percentage. The air is pre-heated passing through an heat exchanger placed
inside the combustion chamber. In the chamber the temperature can reach values
up to 1,250 °C.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
11
3.3 The Stirling engine
The Stirling engine works with external combustion. It is a four-cylinder
engine and has an electric power output of 35 kW; each piston in the
cylinders occupies ¼ of a complete cycle of the engine shaft (90° phase shift).
Therefore, with 4 pistons, each will work as hot piston in the upper part of its cycle
and as cold piston in the lower part (“double acting cylinder”); the upper part of the
cylinder will be the
expansion zone while the
lower part will be the
compression zone. In this
way the connection duct is
totally inside the
combustion chamber,
receiving the radiation
from the combustion and
works as heat exchanger.
Stirling heat exchanger seen through the combustion chamber
The head of the Stirling engine being placed inside the
combustion chamber
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
12
The Stirling engine at the Technical University of Denmark in Lyngby
Pistons
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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3.4 The accumulator tanks
Like any biomass plant also in this case the system needs to be backed up
with big thermal storage, in this case two 3,000 l water tanks. This allows
to store the hot water produced and to use it when needed introducing it in the
district heating network. The tanks are
connected in sequence to each other in order to
optimize the internal stratification.
3.5 The wood chips storage
The wood chips storage room is nearby the thermal plant, separated by a
wall of bricks and armed concrete so
complying with the REI 120 standard. The
storage volume is 100 m3 (25m2 floor
surface by 4m height). The maximum stored
weight is then 30.7 t of wood chips and the
stored potential energy is 81,430 kWh.
The wood chips are carried directly to the
gasifier moved by a screw conveyors
system which is powered by two electrical
gear motors.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
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4 SYSTEM DIMENSIONING AND MAIN TECHNICAL
PARAMETERS
The plant is placed in an isolated area, far from the school premises, in a green
area behind the school building. It’s been installed in a wooded area without
altering the environmental characteristics, in order to demonstrate how this kind of
plants can be placed in a green area safely and with very low impact.
A totally independent access to the thermal plant has been realized in order to
avoid any inconvenience to the school activities during wood chip loading
operations.
The plant has been dimensioned so that the largest part part of the heat produced
is dedicated to room heating, minimizing the heat loss during the winter.
Considering the natural gas average consumption, the need for a thermal capacity
of 140kW was estimated; then, as a function of this capacity the volume of the
heat storage has been calculated in order to guarantee the energy need of the
heated buildings.
The following graph shows the hourly thermal need when the natural gas plant
was working; with the new system in operation the heat production will be
regulated in a different way in order to meet the CHP plant requirements.
Nevertheless the old thermal plant will not be dismantled as it could be exploited
to cover peak of heat demand in case of a particularly cold season or when
maintenance operations will be carried out.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
17
The optimal dimension of the plant is 200kWth gasifire with two 3,000 l tanks;
these will store the heat in form of hot water at 75 °C and provide the heat for the
early morning peak demand.
The system, designed as described above, can satisfy the whole heat demand of
the buildings, providing heat at low temperature to the school during the night in
order to increase the thermal inertia of the structure, cutting in this way the usual
morning peak of heat demand. In order to do so, the pumps of the district heating
area, have to work for a quite high number of hours; the pumps are electronically
controlled and the buried pipes of the system have been engineered as to
minimize the electrical needs for the hot water recirculation.
In these operational conditions the net electric power that the plant directly feeds
into the low tension grid (subtracting the power absorbed for the plant needs) - is
35kW; so we can consider this power the actual power directly sent into the grid.
The plant works continuously, without need for staff presence on site;
maintenance is planned every 4,000 working hours.
The ashes are taken out from the gasifier through a screw conveyor; this limits the
presence of staff to 1 hour/ week for a general check of all devices and to empty
the ash box.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
18
5 THE SYNGAS
The term stays for the gas produced in the
up-draft gasifier, obtained from wood chips
biomass. It has a low calorific power and
contains mainly carbon dioxide and nitrogen.
The table below shows the main
components responsible for the combustion
energy potential and the expected share of
each, when the system runs steadily.
Syngas characteristics
Component
Type
of
gas
Expec
ted
share
Density
(g/l)
0°C,1atm
Relative
density
(adimensional,
air density = 1)
0°C,1atm
NCV (net
calorific
value)
kWh/Nm3
Fuel gas
flow rate
Nm3/h
Inert gas
flow rate
Nm3/h
CO Fuel 20% 1.25 0.967 3.27 31.77
H2 Fuel 4% 0.089 0.069 2.997 6.35
CH4 Fuel 5% 0.716 0.554 9.7 7.94
CO2 Inert 16% 1.976 1.523 0 25.42
N2 Inert 55% 0 82.61
Other inert
gases 3% 0 4.77
Total 1.26 46.07 112.80
Total NCV = 1.26 kWh/Nm3
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
19
6 WOOD CHIPS DEMAND
The quality of the biomass used in the plant is certified by the providers, the local
forest owners of the nearby territory, and by the supervision of the Municipality of
Castel d’Aiano, which supports the birth of the forest biomass local supply chain.
The wood type provided to the plant are expected to be chestnut, fir tree, beach
tree and to a lesser extent downy oak, for an overall calorific power ranging from
2.5 to 3.4 kWh/kg.
The moisture content may be different, depending on the season, from 30% to
45%; lower content is not likely to occur. In fact in order to dry the wood below
30% of moisture content it should be dried in an oven; this would be complex and
expensive.
The wood consumption, considering 30% of moisture content and a low content of
small branches and bark, is estimated as 60 kg/h. If we consider to provide wood
with a lower quality, i.e. with more branches and bark and 50-60% of moisture
content, the hourly consumption could increase up to 95 kg/h. In the designing
phase an average situation has been taken into account, fixing the consumption
to 75kg/h. Initially the number of working hours of the plant has been considered
6000 hours which would bring to an average yearly consumption of 450 t of wood
chips.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
20
7 PRODUCTION OF HEAT AND POWER
According to the operational conditions mentioned above, the electricity fed into
the grid is estimated in 210 MWh/year, while the heat, in form of hot water at
65°C, is 480 MWh/year. The thermal energy is provided to the nearby public
buildings, or stored in the two 3,000l tanks.
The heat recovered from the cooling down of the Stirling engine and from the
exhaust is carried to the building through a couple of buried tubes, one for
carrying the water onwards, the other to bring it back, at the maximum
temperature of 75°C. The heat will be provided to the buildings through a plate
heat exchanger, which absorbs heat from the pipes of the district heating simply
having the water of the pre-existing thermal plant flowing through the heat
Main plant data
35 kWel
140 kWth
25 kW losseshel = 17,5 %
hth = 70 %htotal = 87,5 %
Around 75 kg of wood chips
(W=40%)
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
21
exchanger; in so doing the water of the heating system of the buildings is kept
separated from that of the cogeneration plant. This choice also allows to start the
old thermal plant whenever it is necessary, e.g., for an accidental interruption or
for planned maintenance. The district heating tubes laid underground are in PEX,
a flexible plastic material supplied in rolls, thermally insulated and with a
protective lining; this avoids having junctions between the tubes, guaranteeing a
longer life.
As for the production of electricity, it has to be considered that the plant is
expected to work continuously – as the table below shows – and this is perfectly
compatible with the thermal users connected to the district heating: in fact the
plant will be in condition to supply for a long and continuous time all the produced
heat, therefore avoiding the necessity to decrease the load as a consequence of a
decrease in the heat demand. A capacity loss is nonetheless expected only at the
beginning and at the end of the cold season.
In synthesis the electrical energy produced can be estimated as 210 MWh/year.
Electrical characteristic of the plant
Type of plant Renewable energy CHP plant
Fuel Wood biomass from short supply chain
Nominal capacity 35kW
Phases 3
Volts 400V
Frequency 50Hz
Current intensity 69 Ampere
Power factor 0.95-1
Maximum current intensity
needed at start of operation for
about 3 seconds
125 A
Grid protection Thytronic SVF5940 combined with a
bobbin, opening with a minimum tension
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
22
8 ENVIRONMENTAL PARAMETERS
One of the main objectives of the realization of this plant is to demonstrate a
sensible and positive improvement of the environmental conditions and, in
particular, a reduction of the green house gases emissions, as a consequence of
the use of a renewable energy source.
As briefly mentioned above, the design of the plant and of its processes are such
that it cannot have any negative impact on the air quality. Previously, a fossil fuel,
i.e. natural gas, was used to heat up the buildings while the CHP plant burns a
similar gas, but produced from local biomass, thus making the mountain territory
independent from fossil fuel; furthermore the overall efficiency increases as a
consequence of the combined production of electricity and heat.
Below the main environmental indicators:
Fossil fuels saved 91.3 toe/year
CO2 emission saved 200 t/year
Wood chip consumption 450 t/year
Ash production 6 m3/year
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
23
9 THE ASH DISPOSAL
The Italian law classifies the ash produced from the combustion of virgin wood as
non hazardous waste, therefore it must be disposed in a landfill. The chemical
characteristics of the ash are such that it may be used in agriculture as fertilizer,
as it is commonly done in Switzerland and Austria. The ash in fact is the organic
waste of the combustion and contains Calcium, Potassium, Phosphorus,
Magnesium and Sodium, and its spreading on the soil brings positive effects as
fertilizer or conditioner; the function of fertilizer is played by the ash as it brings to
the soil relevant quantity of nutritional elements, of which it is deprived by the
growing of the vegetation.
The function of conditioner is intended for acid soils, the ash contains alkaline-
earth elements (Ca, Mg) and alkaline metals (Na, K), which increase the soil pH
factor.
In Italy the spreading of the ash coming from wood combustion on farming or
wood land is forbidden by the decree 22/97 therefore it must be disposed in
landfill as non dangerous waste.
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
24
10 MAIN PARAMETERS
Synthesis of main technical and working parameters of the plant
1. Total installed capacity 200kW
2. Thermal efficiency 70% equals to 140 kWth
3. Electric efficiency 17,5% equals to 35kWel
4. Loss 12,5%
5. Overall efficiency 87,5%
6. Wood chip consumption (40% moist) 75 kg/hour
7. Planned working hour 6,000 hours /year
8. Thermal energy produced (140 kW x 6,000hour/year) = 840
MWhth/year
9. Electrical energy produced (35kW x 6,000 hour/year) = 210
MWhel/year
10. Wood chip consumption (75kg/hour x 6,000 hour/year) = 450
t/year
Synthesis of the plant cost
Building works 70,000 €
District heating: pipes and connections 50,000 €
Energy production technology 205,000 €
Piping, electrical connections and devices 90,000 €
Other technical services
(engineering, grid connection, permission costs and
other technical costs)
45,000 €
Total costs 460,000 €
Cost compared with the thermal power kWth (usable) 3,285 €/kWth
Costs compared with the electrical power kWel (usable) 13,142 €/kWel
Technology costs compared with the electrical power
kWel (usable) 5,857 €/kW
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
25
Considering a 15 years long working life of the plant:
Total thermal energy usable 140kW x 6,000h/y x 15y = 12,600 MWhth
Electrical energy usable 35kW x 6,000h/y x 15y = 3,150 MWhel
Synthesis of the economical operation data
Biomass costs (wood chips) 60 €/Ton = 27,000 €/y
Income from heat supply, based on the
district heating demand during winter,
excluding the swimming pool
75 €/Ton = 36,000 €
Income from electrical power
sent to the local grid, (actual price for
biomass fuelled plants)
0.28 €/kWh = 58,800 €/y
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
26
Compared with other renewable energy sources
(concerning only the electrical power)
210.000 kWh electricity produced
180 kW photovoltaic
1.500 m2 of panels
140 kW wind power
7 wind generators
20 kW each one
Time working 6000 h/y
Specific data
Time working 6000 h/y
210 MWh
electricity
produced
480 MWh
total heat
allocated
1.200 MWh
wood energy
consumption
450 t/year
W=40 %
20 families needs
68 families
needs
=
=
=
Wood chips CHP plant for a school campus in Castel D’Aiano, Italian Central Appenines,
based on Stirling engine technology
27
11 THE WORK GROUP
The plant has been done thanks to the hard work of a group of people, that
believes in new technology and renewable energy, as a way for building a new
future.
All the technology equipment is supplied from Stirling Danmark
The plant is realized from a joint venture between three public organizations, the
Co.Se.A. Consortium, the Municipality of Castel d’Aiano
and CISA
The local project is made by two engineers that followed also the erection works
Ing. Filippo Marini: plant project
Ing. Sergio Palmieri: civil project
This report is realized by the work of:
Filippo Marini, technical support and documentation
Riccardo Giacobazzi, text support and photography
Arianna Cecchi, ASTER, English translation (within the activities of the FP6
project ASTECH)