Development of integrated stove control
systems based on temperature sensors
Christoph Mandl, Ingwald Obernberger, Manuel Kössl
Project ERA-NET Bioenergy “Stove 2020”
Workshop: “Wood Stoves 2020 - Towards high efficiency and low emissions”
Stockholm (Sweden), 13th of June 2017
Contents
■ Background
■ Basics of automated control systems for stoves
■ Stove control based on temperature measurement
• Fundamentals
• Implementation
• Evaluation of the integrated stove control system based
on temperature sensors
■ Summary and conclusions
2
Background
■ All over Europe there is a growing awareness that residential wood
fuel appliances are potentially responsible for a great deal of
environmental hazards. At the same time the performance of stoves
and the knowledge about proper stove operation are progressing.
■ The end user can today choose between much better stove
products than in the past. But above all it is the end user's heating
behaviour (i.e. fuel selection, stove operation and maintenance)
which is most decisive for achieving high efficiencies and low
emissions.
■ Integrated automated control systems provide the basis for a low
emission stove operation at increased efficiency since they
contribute to a minimisation of user induced operation errors.
■ Therefore, the introduction of such systems, which are presently
not widely-used, can have a huge impact on emission reduction
from stoves.
3
Basics of automated control systems for stoves (I)
■ Common logwood stove concepts are manually controlled.
Therefore, process control efforts are usually limited to a change of
the combustion air distribution at the end of the ignition phase.
■ An automated control system for a stove can control and optimise
the operation of the stove but cannot influence the fuel used.
■ Advantages of an automated control system
• Reduces the user influences (operating errors)
• Increases the operational comfort for the user
• Provides the possibility to react on the changing process conditions
throughout the entire batch
• Reduces emissions (only gaseous) and increases the thermal
efficiency
• Reduces standing losses (by closing combustion air flaps)
4
Basics of automated control systems for stoves (II)
■ Challenges and requirements of automated control systems
• robust sensors are needed
• the technical solution has to be economically competitive
• a 230 V electrical connection is required
• the automated control concept needs to be suitable for different fuel
qualities and loads
■ Technical solutions for automated control systems
• thermo-mechanically operated primary air flap
– simplest way but with restricted accuracy and effects
• electronic sensor driven automatic control
– more efficient but also more costly
– the temperature (for example in the post combustion chamber) or the oxygen
concentration of the flue gas can be applied as guiding parameter for automated
adjustments of the combustion air flow and combustion air distribution by flaps
5
Stove control based on temperature measurement –
Fundamentals (I)
■ As the control concept should be cheap and robust, the following
basic strategy has been chosen:
• The different combustion phases can be identified by temperature
changes and since temperature sensors are the cheapest sensors
available and also rather robust they offer a suitable opportunity for
stove control
 furnace temperature based control
• The combustion air supply can be easily controlled by appropriate
dampers (air box)
 temperature controlled combustion air supply
■ Thus, the integrated control concept for the automated control of
logwood fired stoves of RIKA is based on a temperature
measurement in the combustion chamber and air flaps for the
combustion air supply control.
6
Stove control based on temperature measurement –
Fundamentals (II)
■ Control strategy
• Ignition phase
– Mainly primary air and a low amount of window purge air is injected in
order to facilitate a quick ignition and rapid increase of the combustion
chamber temperatures
• Transition to main combustion phase
– As soon as the temperature in the combustion chamber exceeds a
certain level the primary air damper is closed to avoid excessive burning
rates
– At the same time the window purge air flow is increased to maintain
adequate combustion air supply
– During the main combustion phase the window purge air flow should be
kept rather constant
7
Stove control based on temperature measurement –
Fundamentals (III)
■ Control strategy (cont.)
• Transition to charcoal burnout phase
– When the furnace temperature starts to drop below a certain value, the
amount of window purge air should be reduced to keep the temperature
at a reasonably high and nearly constant value until the end of the batch
– Thereby, excess oxygen is kept low and too much cooling of the
combustion chamber is prevented.
– As soon as the flames extinguish the CO and OGC emissions strongly
increase. Thus, re-charging of fuel should be performed as soon as the
flames extinguish.
• The air flaps should be closed at the end of the stove operation
in order to reduce standing losses.
8
Stove control based on temperature measurement –
Fundamentals (IV)
■ Effects
• Shorter ignition phase
• With combustion air flow control during the main combustion and
burnout phase more stable O2 concentrations in the flue gas can be
achieved
• Generally, lower O2 levels as well as sufficiently high temperatures can
be achieved
• Duration of char coal burnout with high gaseous emissions can be
reduced
 Lower emissions and higher efficiencies result
9
Stove control based on temperature measurement –
Fundamentals (V)
■ Automatically controlled stove:
scheme of a combustion batch
■ Conventional uncontrolled stove:
scheme of a combustion batch
•
•
•
•
T
T
CO
CO
IP
MCP
CBP
time
IP
shorter ignition phase
higher temperatures
lower emissions (CO, OGC, dust)
lower average O2 content in the flue
gas
MCP
CBP
time
10
Stove control based on temperature measurement –
Implementation (I)
■ At the stove
• Primary air through the grate and
• Window purge air are supplied.
■ Primary air and the window purge
air have to be separately
controlled by electronically
driven dampers.
■ The combustion air flows are
controlled in dependence of the
furnace temperature (measured
by a flame temperature sensor)
and calculated time dependent
temperature gradients.
11
Stove control based on temperature measurement –
Implementation (II)
■ The automated control has been integrated into a new low emission
wood stove with integrated PCM heat exchanger of RIKA.
Flue gas duct
Casing
PCM heat
exchanger
Flame temperature sensor
Main combustion chamber
Airbox
(with air dampers)
Explanations: The stove concept is protected by a patent; nominal fuel power input: 9 kW
12
Chimney
Evaluation of the integrated stove control system based
on temperature sensors (I)
Flue gas velocity with
hot wire anemometer or
Prandtl tube
Flue gas temperature
measurement according
to EN 13240
Chimney draught
O2, CO, CO2, OGC
TSP
• Flue gas composition downstream the
stove using standard flue gas analysers for
O2 (paramagnetic sensor), CO (NDIR) and
OGC (FID)
• Determination of the total fly ash (TSP)
concentration in the flue gas downstream
the stove according to VDI 2066
13
Evaluation of the integrated stove control system
based on temperature sensors (II)
■ General operation conditions
• Constant draught of 12 Pa over the stove
• Test fuel: hardwood (beech) without bark, moisture content: 12 -16 wt%
w.b.
■ Performance of gaseous and PM emission measurements
• Gaseous emissions (CO, OGC) and O2: continuous measurement from
before ignition of batch 1 until the end of batch 5
• PM emissions: over the whole batch (from closing the door until
opening it again)
■ Mode of re-charging as defined by the manufacturer
• Number of logs: 2 for batch 2 to 5 (3 for ignition batch)
• Mass per batch: 2.4 kg w.b. (full load)
• Dimensions of firewood pieces: 25 cm length
14
Evaluation of the integrated stove control system based
on temperature sensors (III)
Automated control
30
25
20
15
10
5
0
12:40
13:10
13:40
14:10
14:40
Total combustion air [Nm³/h]
CO content
Batch 1 (Ignition)
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
12:40
15:10
15:40
16:10
O2 flue gas [Vol% d.b.]
Batch 2
16:40
17:10
Flue gas volume flow [Nm³/h]
Batch 4
Batch 3
Batch 5
1,000
900
800
700
600
500
400
300
200
100
0
13:10
13:40
14:10
14:40
CO flue gas [mg/Nm³]
15:10
15:40
16:10
16:40
OGC content
Volume flow, O2 content
35
17:10
OGC flue gas [mg/Nm³]
Explanations: O2 related to dry flue gas; emissions related to dry flue gas and 13 vol% O2
15
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
Automated control
30
25
Batch 3
20
Batch 4
15
10
5
0
14:28
14:58
Total combustion air [Nm³/h]
15:28
15:58
O2 flue gas [Vol% d.b.]
16:28
CO flue gas [mg/Nm³]
Volume flow, O2 content
35
Manual control
30
25
20
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
Batch 3
Batch 4
15
10
5
0
14:10
14:40
Total combustion air [Nm³/h]
15:10
O2 flue gas [Vol% d.b.]
15:40
CO content
Volume flow, O2 content
35
CO content
Evaluation of the integrated stove control system based
on temperature sensors (IV)
16:10
CO flue gas [mg/Nm³]
Explanations: O2 related to dry flue gas; emissions related to dry flue gas and 13 vol% O2; dashed line: start of
16
batch (door closed)
Evaluation of the integrated stove control system based
on temperature sensors (V)
■ Advantage of automated control system
• Noticeable reduction of gaseous emissions
• Lower standard deviations regarding CO and OGC
• Minimisation of user induced errors
Automated control
100
90
80
70
60
50
40
30
20
10
0
CO - content
1,000
800
32
32 %
% reduction
reduction
600
400
45
45 %
% reduction
reduction
200
0
CO flue gas
OGC flue gas
TSP
mg/MJ
mg/MJ
mg/MJ
OGC - content, TSP
Manual control
1,200
Explanations: mean values and standard deviations of averaged emissions over entire batches 3 to 5 (from
closing the door until opening the door again for recharging) according to prEN 16510 / DIN EN
13240
17
Evaluation of the integrated stove control system based
on temperature sensors (VI)
■ Advantage of automated control system
• Higher thermal efficiency due to lower O2 contents in the flue gas
100
90
80
70
60
50
40
30
20
10
0
Automated control
89.9
88.0
12.2
10.1
Efficiency (EN13240)
O2 flue gas
%
vol% d.b.
20
18
16
14
12
10
8
6
4
2
0
O2 - content
Efficiency
Manual control
Explanations: mean values of O2 over entire batches 3 to 5 (from closing the door until opening the door again for
18
re-charging); calculation of efficiency according to prEN 16510 / DIN EN 13240)
Summary and conclusions (I)
■ An integrated automated stove control system based on
combustion air control in dependence of the combustion chamber
temperature has been developed and proven as suitable concept
for stoves to lower emissions and to increase the efficiency.
■ Test run results show that a considerable reduction of the gaseous
as well as of the PM emissions is possible by an appropriate
automated control of the air supply of the stove:
• High furnace temperatures and consequently lower emissions can be
reached within a shorter time during the ignition phase by a proper
balancing of primary and window purge air
• More stable O2 concentrations in the flue gas, generally lower O2-levels
as well as sufficiently high temperatures for improved burnout (= lower
emissions) during the main combustion phase as well as during the
burnout phase can be achieved
• Re-charging after “flame off”
19
Summary and conclusions (II)
■ By the implementation of the automated control the thermal
efficiency could be increased (up to 2% points) mainly due to lower
O2-levels in the flue gas.
■ Furthermore, standing losses can be reduced by an automated
closure of the air flaps at the end of the stove operation.
■ By the introduction of an integrated control system it could be
shown that such control systems, which are presently not widelyused, can have a huge impact on emission reduction and efficiency
increase.
■ Therefore, advanced automated control systems provide the basis
for a low emission stove operation at increased efficiencies. In
addition, they also contribute to a minimisation of user induced
operation errors and improve the comfort for the user.
20
Summary and conclusions (III)
■ The outcomes of the investigations regarding the
improvement of wood stoves by the application of
automated control concepts are summarized in:
Guidelines for automated control systems for stoves
■ The guidelines (as well as the proceedings of the
workshop) will be provided as download on the
Woodstoves2020 webpage:
http://www.tfz.bayern.de/cms08/en/index.php
21
Thank you for
your attention
Hedwig-Katschinka-Straße 4
A - 8020 Graz,
Tel.: +43 316 481300-22
mailto:mandl@bios-bioenergy.at
www.bios-bioenergy.at
RIKA Innovative Ofentechnik GmbH,
Müllerviertel 20, A-4563 Micheldorf,
Tel.: +43 7582 686 253
mailto:koessl@rika.at
www.rika.at
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