improved low emission and high efficiency wood stove with
TRANSCRIPT
Project ERA-NET Bioenergy “Stove 2020”
Workshop: “Wood Stoves 2020 - Towards high efficiency and low emissions”
Stockholm (Sweden), 13th of June 2017
Improved low emission and high
efficiency wood stove with integrated
PCM heat exchanger
Ingwald Obernberger, Christoph Mandl, Manuel Kössl
Contents
■ Background and objectives
■ Methodology and approach
■ Development of the new stove technology
■ Evaluation of test runs performed with the new stove
technology
■ Summary and conclusions
2
Background and objectives (I)
■ Today small-scale biomass combustion is one of the most relevant
bioenergy applications. Regarding the installed units, stoves show
the highest and steadily increasing numbers in Europe.
■ It is also well known that among the different residential biomass
combustion technologies logwood stoves show the highest CO,
OGC and fine particulate matter (PM) emissions.
■ Furthermore, even modern stoves show clearly lower thermal
efficiencies (in the range of 82% under test stand conditions) in
comparison to automatically fed and controlled biomass boilers
(e.g. pellet boilers).
■ Appliances with integrated heat storage unit may show
significantly increased efficiencies and therefore also contribute to
a better economy of stove technologies.
3
Background and objectives (II)
■ Moreover, they contribute to a better climate in the rooms as the
energy released to the room can be better distributed over a rather
long period in comparison to state-of-the-art wood log stoves.
■ The proper selection of phase change materials (PCM) can offer
energy and space efficient, innovative heat storage solutions.
■ Against this background, a low emission and high efficiency stove
with integrated PCM heat exchanger has been developed and
tested by RIKA and BIOS.
4
Methodology and approach (I)
General approach
■ Application of a new stove concept which ensures low gaseous
and particulate emissions
■ Definition of constraints regarding the implementation of the heat
exchanger for PCM application
■ Development of the basic geometry of the new stove concept with
integrated PCM heat exchanger
5
Flue gas fan
Main combustion chamber
Post combustion chamber
Grate
Window purge air
Flue gas collector
Steel plate
Isolation
Isolation
Primary air
PC
M h
eat
exch
an
ger
Methodology and approach (II)
■ Patented new low
emission stove concept
• Appropriate insulation of
main combustion
chamber and post
combustion chamber for
almost complete burnout
• Efficient mixing of the
flue gases with the
combustion air
• Application of air staging
• PCM heat exchanger to
maximise the efficiency
• Automatic control
6
■ PCMs (Phase Change Materials) enable a more efficient energy
recovery from the flue gas as due to the melting of the material at a
defined temperature the latent heat can be stored in addition to the
sensible heat.
■ Therefore, heat storage based on PCM can show valuable
advantages and potentials:
• increase the thermal efficiency
• make space efficient heat
storage possible
• supply heat at reasonably
high temperatures
Methodology and approach (III)
7
Development of the new stove technology (I)
■ Relevant steps
• Evaluation of possible phase change materials (PCM)
• Final selection of PCM and design of an appropriate heat exchanger
concept
• CFD (Computational Fluid Dynamics) based design of the stove with
integrated PCM heat exchanger
• Construction of testing plants
• Performance of test runs, evaluation and stepwise optimisation of the
technology
8
Development of the new stove technology (II)
■ Detailed evaluation of PCMs available on the market based on
literature reviews and data from manufacturers
• A high density and heat capacity of the PCM is important as the heat
storage unit should be realised in a compact way.
• PCMs have to be suitable for the application in a heat storage unit (e.g.
flue gas heat exchanger) of wood stoves.
– Melting temperature: 150 – 300 °C
– Degradation temperature: > 600 °C
– The price should be as low as possible
in order to be economically competitive
– The material should not be corrosive or
toxic under operation conditions
9
Based on the evaluation performed promising PCMs (salt mixtures) have
been selected for the PCM heat exchanger
corrosion test
■ The PCM heat exchanger should be integrated into the stove
concept in a way that a large share of the energy produced is
transferred to the PCM in order to ensure a high storage efficiency
Development of the new stove technology (III)
10
Flue gas inlet
(loading cycle)
Flue gas outlet
(loading cycle)
Air inlet (unloading cycle)
Air outlet (unloading cycle)
The release of the stored heat
(unloading cycle) can be
realised:
• Via convection air channels
which are opened at the end of
operation of the stove. Thus,
the stored heat can be
distributed even to different
rooms.
• Via slow natural convection
over a larger time duration
which is of advantage for the
comfort of living.
Casing +
isolation
■ Purpose of CFD-aided stove development and optimisation:
• Efficient CO burnout
• Efficient air staging
• Ensure a clean window
• Optimal integration of the PCM heat exchanger
– Regarding high utilization rate
– Regarding allowable application temperatures
• Reach high efficiency (low flue gas temperatures at stove outlet)
Development of the new stove technology (IV) –
CFD simulations
11
■ Iso-surfaces of air, flue gas and stove temperatures [°C]
Development of the new stove technology (V) –
CFD simulation results
12
Main combustion chamber
Post combustion chamber
PCM heat exchanger
Steel plate
Isolation
Flue gas collector
■ Iso-surfaces of air and flue gas velocities [m/s]
Development of the new stove technology (VI) –
CFD simulation results
13
■ Iso-surfaces of CO concentrations
[ppmv]
Development of the new stove technology (VII) –
CFD simulation results
14
>
■ Iso-surfaces of O2 concentrations
[m³ O2/ m³ flue gas w.b]
■ Iso-surfaces of temperatures of the PCM material [°C]
Development of the new stove technology (VIII) –
CFD simulation results
15
■ Air staging and window flushing has been optimized to ensure a
clean window and low O2 contents in the flue gas.
■ Good mixing of the flue gas with the combustion air indicated by
even distributions of O2 and CO at stove outlet.
■ Due to the high temperatures in the combustion chamber and the
efficient mixing of the flue gas with the combustion air a very good
gas phase burnout quality can be achieved, indicated by very low
CO emissions.
■ Due to the optimised positioning and improved insulation of the
PCM heat exchanger a complete melting of the PCM can be
achieved:
• Minimum temperature of the PCM is in the steady state 263 °C
above the melting temperature of the selected PCM
• Maximum temperature of the PCM is in the steady state 477 °C
lower than the degradation temperature of the selected PCM
Development of the new stove technology (IX) –
CFD simulation results
16
Evaluation of test runs performed with the new stove
technology (I)
■ Based on the development work a prototype of the stove with
integrated PCM heat exchanger was constructed.
17
Main combustion chamber
Flue gas duct
PCM heat
exchanger
Casing
Airbox
(with air dampers)
Chim
ney
O2, CO, CO2, OGC
Chimney draught
Flue gas temperature
measurement according
to EN 13240
Flue gas velocity with
hot wire anemometer or
Prandtl tube
• Determination of the total fly ash (TSP)
concentration in the flue gas downstream
the stove according to VDI 2066
• BLPI: Berner-type low pressure impactor
• ELPI: Electrical low-pressure impactor 18
Evaluation of test runs performed with the new stove
technology (II)
TSP BLPI ELPI
Evaluation of test runs performed with the new stove
technology (III)
■ Performance of test runs with the new low emission and high
efficiency stove with integrated PCM heat exchanger
• Performance of test runs with automated control of the stove including
emission measurements:
– Test run cycle: ignition batch, 4 full load batches, charcoal burnout batch
• Evaluation of the performance of the stove in terms of gaseous and
particulate emissions
• Evaluation of the performance of the PCM heat exchanger
■ Based on the evaluation of test runs performed the new stove
technology has been further developed and stepwise optimised
19
Evaluation of test runs performed with the new stove
technology (IV)
20 Explanations: Emissions related to dry flue gas and 13 vol.% O2
■ Test run – stove performance
Batch 1 (Ignition) Batch 2 Batch 3 Batch 4 Batch 5 Charcoal burnout
0
100
200
300
400
500
600
700
800
900
1,000
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
12:30 13:30 14:30 15:30 16:30 17:30
OG
C c
on
ten
t
CO
co
nte
nt
CO flue gas [mg/Nm³] OGC flue gas [mg/Nm³]
0
5
10
15
20
25
30
35
12:30 13:30 14:30 15:30 16:30 17:30
Vo
lum
e f
low
, O
2 c
on
ten
t
Total combustion air [Nm³/h] O2 flue gas [Vol% d.b.] Flue gas volume flow [Nm³/h]
Evaluation of test runs performed with the new stove
technology (V)
21 Explanations: Emissions related to dry flue gas and 13 vol.% O2
■ Test run – PCM heat exchanger loading and PM emissions
Batch 1 (Ignition) Batch 2 Batch 3 Batch 4 Batch 5 Charcoal burnout
0
10
20
30
40
50
12:30 13:30 14:30 15:30 16:30 17:30
TS
P, P
M 1
[m
g/N
m³]
TSP [mg/Nm³] PM1 (ELPI) [mg/Nm³] BLPI [mg/Nm³]
0
50
100
150
200
250
300
350
12:30 13:30 14:30 15:30 16:30 17:30
Te
mp
era
ture
s [
°C]
T-PCM bottom [°C] T-PCM middle [°C] T-PCM top [°C] T-flue gas (EN13240) [°C]
Evaluation of test runs performed with the new stove
technology (VI)
22
■ Energy balance for a complete test run
kW kWh %
Fuel power input 10.6 53.5 100.0%
Energy storage char coal 1.2 5.9 11.0%
Thermal output (radiation + convection) 3.9 19.9 37.1%
Energy storage stove 2.5 12.5 23.4%
Energy storage PCM heat exchanger 2.2 11.0 20.5%
Heat losses flue gas 0.7 3.7 6.9%
Total energy storage 23.5 44.0%
Thermal efficiency incl. charcoal combustion 93.1%
Thermal efficiency (EN13240) 92.6%
Duration of test run
Energy balance
5.07 h
Efficiencies over 90% are possible
More than 40 % related to the energy input with the fuel and
more than 50 % related to the useful heat can be stored
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
17:00 19:00 21:00 23:00 01:00 03:00 05:00 07:00 09:00 11:00 13:00 15:01
En
erg
y o
utp
ut
Energy output stove [kW] Energy output PCM heat exchanger [kW] Total energy output [kW]
Evaluation of test runs performed with the new stove
technology (VII)
23
■ Typical unloading cycle of the PCM heat exchanger
74% of stored energy
26% of stored energy
night (9 h) day
Explanations: Air flaps have been closed at the end of the test run; calculation performed based on energy
balances and measurements of surface temperatures during unloading
Slow heat release over a larger time duration which is of
advantage for the comfort of living
Evaluation of test runs performed with the new stove
technology (VIII)
24
■ Efficiency and emissions in comparison to relevant limits
Explanations: mean values of averaged emissions over entire batches 3 to 5 (from closing the door until opening
the door again for re-charging) according to prEN 16510 / DIN EN 13240
8075
90
0
10
20
30
40
50
60
70
80
90
100
[%]
efficiency
effi
cien
cy [
%]
1100
50 35
1000
8027
833
27
589
25 100
200
400
600
800
1,000
1,200
[mg/MJ] [mg/MJ] [mg/MJ]
CO OGC TSP
AUT: Art. 15a-B-VG
Ecodesign (2022)
GER: BimSchV
Low-emission stove
Summary and conclusions
■ With the new stove concept with integrated PCM heat exchanger, a
renewable CO2-neutral room heating technology which shows low
emissions and significantly increased efficiencies (> 90%) has been
developed.
■ The gaseous and especially the particulate emissions of the new
stove are on a very low level compared to state-of-the-art chimney
stoves.
■ The integrated PCM heat exchanger developed shows a good heat
storage capacity, a compact design and contributes to a better
room climate due to its slow heat release.
■ The final design of the new technology is currently ongoing. The
market introduction of the new stove technology with integrated
PCM heat exchanger is expected for 2018. 25
A new technological milestone regarding stove technology
has been achieved
Thank you for
your attention
Hedwig-Katschinka-Straße 4
A - 8020 Graz,
Tel.: +43 316 481300-22
mailto:[email protected]
www.bios-bioenergy.at
RIKA Innovative Ofentechnik GmbH,
Müllerviertel 20, A-4563 Micheldorf,
Tel.: +43 7582 686 253
mailto:[email protected]
www.rika.at