Castel d'Aiano Wood 2008

8/13/2019 Castel d'Aiano Wood 2008

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THE ECOLOGICAL PLANT

IN CASTEL DAIANO

The CHP system of Casteld'Aiano school campus:wood chips gasification

and Stirl ing engine


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Wood chips CHP plant for a school campus in Castel DAiano, Italian Central Appenines,based on Stirling engine technology

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INDEX

1 Introduct ion ..................................................................................................... 32 The facilities served by the CHP plant .......................................................... 43 The plant configuration .................................................................................. 6

3.1 The gasifier .......................................................................................93.2 The combustion chamber ...............................................................103.3 The Stirling engine ..........................................................................113.4 The accumulator tanks ...................................................................143.5 The wood chips storage ..................................................................14

4 System dimensioning and main technical parameters ............................. 165 The syngas .................................................................................................... 186 Wood chips demand ..................................................................................... 197 Production of heat and power ..................................................................... 208 Environmental parameters ........................................................................... 229 The ash disposal ........................................................................................... 2310Main parameters ........................................................................................... 2411The work group ............................................................................................. 27


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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 CISA1in collaboration with COSEA, the local energy utility company,

in the territory of the Municipality of Castel DAiano, 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 DAiano 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.

1CISA stays for Centro per lInnovazione e la Sostenibilit Ambientale in Appennino, that is

Innovation Centre for the Environmental Sustainability in the Apennines


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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 ofthe 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 m3

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 m

3

53,675 kWhPeak daily consumption 194 m3 1,749 kWh


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School Swimming pool Sport centre


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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 DAiano 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


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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.


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3.1 The gasifier

This element consists of an outer steel chamber, lined with insulatingmaterial. 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.


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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.


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3.3 The Stirl ing 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 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 thisway 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 combust ion chamber

The head of the Stirling engine being placed inside thecombustion chamber


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The Stirling engine at the Technical University of Denmark in Lyngby

Pistons


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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.


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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. Its 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 loadingoperations.

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.


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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 feedsinto 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.


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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)

0C,1atm

Relative

density

(adimensional,

air density = 1)

0C,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

gases3% 0 4.77

Total 1.26 46.07 112.80

Total NCV = 1.26 kWh/Nm3


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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 dAiano, 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 below30% 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.


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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

65C, 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 75C. 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 lossesel= 17,5 %

th= 70 % total

= 87,5 %

Around 75 kg of wood chips(W=40%)


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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 theplant 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 protectionThytronic SVF5940 combined with a

bobbin, opening with a minimum tension


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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

CO2emission saved 200 t/year

Wood chip consumption 450 t/year

Ash production 6 m3/year


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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.


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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/hour7. 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 powerkWel (usable)

5,857 /kW


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Considering a 15 years long working l ife 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


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Compared with other renewable energy sources

(concerning only the electrical power)

210.000 kWh electricity produced

180 kW photovoltaic

1.500 m2of panels

140 kW wind power7 wind generators

20 kW each one

Time working 6000 h/y

Specific data

Time working 6000 h/y

210 MWhelectricity

produced

480 MWhtotal heat

allocated

1.200 MWhwood energy

consumption

450 t/yearW=40 %

20 families needs

68 families

needs

=

=

=


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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 dAiano

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)

Castel d'Aiano Wood 2008

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