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Environment Protection Engineering
Vol. 42 2016 No. 3
DOI: 10.5277/epe160308
MAREK JUSZCZAK1
COMPARISON OF CO AND NOx CONCENTRATIONS
FROM A 20 kW BOILER FOR PERIODIC
AND CONSTANT WOOD PELLET SUPPLY
The impact of the fuel feeding mode (continuous or periodic with various stand-by/operation time
ratios) on carbon monoxide (CO) and nitrogen oxides (NO, NOx) concentration values in the flue gas
was analyzed for coniferous wood pellet firing. Experiments were performed using a 20 kW water
boiler equipped with an under-fed (retort) wood pellet furnace located in a full scale heat station sim-
ulating real-life conditions. Impact of oxygen concentration and temperature in the combustion cham-
ber on the concentrations of carbon monoxide and nitrogen oxide have been examined. It was con-
cluded that the commonly used periodic fuel supply does not necessarily cause an important increase
of the concentrations of carbon monoxide compared to those in the continuous fuel feeding mode.
Continuous fuel supply can even imply higher concentrations of carbon monoxide when fuel and air
streams are not chosen properly.
1. INTRODUCTION
Low heat output heating boilers (<50 kW), typically used in domestic heat stations,
in comparison with high heat output boilers (>1 MW), present a much higher emission
of incomplete combustion products (carbon monoxide, hydrocarbons and sooth) per
produced energy unit. It is estimated that in Germany in the year 2000 the share of small
scale wood combustion systems contributing to the emission of incomplete combustion
products was between 16 and 40%, although their total energy production was only
about 1% [1]. These numbers have already changed as currently more modern boilers
are being used in Germany, it depicts however the scale of the problem. In Poland, this
_________________________
1Poznan University of Technology, Institute of Environmental Engineering, Division of Heating, Air
Conditioning and Air Protection, Piotrowo 3A, 60-965 Poznań, Poland, e-mail: [email protected]
96 M. JUSZCZAK
proportion is probably similar or even higher, as customers more frequently use cheap
boilers of simple and old design.
The reason behind it is mostly the fact that these boilers, in order to be limited in
size and low-cost, are equipped with small combustion chambers. Their walls function
directly as heat exchange surfaces and are reached by the flue gas much too fast. As
a consequence, the flue gas is cooled and the combustion process is hampered. This
problem is especially visible in the case of biomass, as in the first stage of burning it
releases as much as 80 wt. % of volatile substances, as opposed to hard coal where it
is only 20 wt. %, which makes the flue gas reach the cold heat exchange surfaces and
be cooled even quicker. Additionally, biomass is characterized by a smaller lower
heating value in comparison with hard coal which leads frequently to a lower temper-
ature in the combustion chamber. All these factors create conditions that favor gener-
ation of considerable amounts of incomplete combustion products.
The most useful and easily controllable pollutant emission indicator for incom-
plete combustion products is CO concentration. The Polish-European law [2] is quite
liberal in that aspect as it permits the CO concentration to be even 3000 mg/m3 for
high heat efficiency heating boilers with a heating output of less than 300 kW. How-
ever, internal legislation of some countries, e.g. Sweden, limit the permitted CO con-
centration to 2000 mg/m3. The criteria are even stricter in order for a boiler to be
granted an ecological certificate. For example, to obtain the Nordic countries’ Svan
Mark certificate it cannot exceed 1000 mg/m3 and for the German Bauer Engel certif-
icate it needs to be lower than 100 mg/m3 [3]. Obviously, to reach these values one
needs to use high quality deciduous wood pellets with no contamination, such as sand
or silicates. The most modern wood pellet furnaces equipped with the oxygen probe
and continuous fuel supply can give as low CO concentrations as 10–50 mg/m3 [4].
These values, however, can only be reached probably in stable laboratory conditions,
with a high heat output, and definitely not over a longer period of time in a real-life
heat station during operation. In the case of a heat station with conditions resembling
the real ones (periodic fuel supply, no oxygen probe air stream regulation) the con-
centrations vary from 300 to 800 mg/m3 [5–8]. It does not specify the permitted value
for nitrogen oxide concentrations, however, producers of small heating boilers must
comply with NOx concentration of below 400 mg/m3, in order to obtain the Polish
ecological certificate [10], which facilitates boiler commercialization. All concentra-
tions are presented for 10% O2 content in flue gas.
Limited availability of high quality and cheap wood pellets in Poland (that are cur-
rently used in power plants), aroused interest in agricultural biomass pellets as fuel for
heating boilers with low heat output. Agricultural biomass is described by low ash melting
temperature which is why it should be fired at a temperature lower than 700 °C [11],
in order to avoid the production of slag that hampers furnace operation and the com-
bustion process. Also chlorine content is an important factor in biomass combustion
as chlorine can form KCl melting at a low temperature. Therefore, it is recommended
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 97
to use additives such as Ca(OH)2 or dolomite that react with chlorine and impede KCl for-
mation [12–14]. Interesting results of study in the area of pollutant emission from firing
biomass and other fuels can be found in the papers by Musialik--Piotrowska et al. [15, 16]
and Hardy et al. [17].
An important factor that could influence pollutant emissions of wood pellet furnaces
is the mode of automated pellet supply, namely: whether it is continuous or periodic,
which is precisely the topic of this study. The majority of wood pellet furnaces with low
heat output, even those produced by renowned manufacturers, do not enable fluent pel-
let stream regulation. Instead, they use periodic fuel supply, which means that fuel
stream is regulated manually by modifying the operation and stand-by time of the fixed-
-speed screw feeder, as well as the stream of air for combustion in the beginning of
firing process, in order to obtain the desired heat output of the boiler. The fact that air
stream is constant for the required boiler heat output and is not reduced during the stand-
by period in pellet feeding causes lowering of the temperature in the combustion cham-
ber and excessive increase of oxygen concentration, and as a result an increase of CO
concentration. This is why continuous pellet feeding system would seem to be much
more beneficial. Nevertheless it is barely introduced as a technical solution in low heat
output heating boilers with wood pellet furnaces due to its elevated cost. For periodic
fuel supply systems described above, the settings would differ depending on wood type.
Furnace producers do suggest recommended settings (operation time, stand-by time, air
stream), however only for good quality 100% deciduous wood pellets as deciduous
wood is considered to be the best biomass type for firing due to its low nitrogen
(0.1–0.2%), ash (1%), and negligible sulfur content. These settings are not always ade-
quate for coniferous or mixed wood pellets.
2. EXPERIMENTAL SET UP AND PROCEDURE,
MEASURING EQUIPMENT, MATERIAL
In the experimental study, the emission of pollutants has been examined during co-
niferous wood pellet firing in a low heat output boiler with under-fed furnace. The aim
of performed experimental study was to determine:
whether the concentration of CO is much higher for periodic wood pellet supply
than for continuous fuel supply,
at which value of oxygen concentration one can achieve the lowest possible CO
concentration (a parameter which is necessary to determine the adequate air stream for
combustion, especially if oxygen probe is to be used in the future),
the fluctuations of concentrations of nitrogen oxides and carbon monoxide de-
pending on the temperature and oxygen concentration in the combustion chamber upon
changing one of these parameters,
98 M. JUSZCZAK
The experiments were carried out in a full scale heat station equipped with a 20 kW
boiler with an under-fed wood pellet furnace (Fig. 1). Unlike many studies performed
by accredited laboratories that examine boilers in idealized conditions, in this study,
a special attention was paid to ensure conditions similar to the real ones and therefore
simulate domestic boiler operation where heat demand is variable. The boiler lacked an
automatic device of air stream with an oxygen probe (lambda sensor) in the flue gas
downstream the boiler. Air stream was modified manually at the beginning of the firing
process by fan speed regulation, ranging from 10 to 100% of its maximum value. Pellets
were supplied from the storage to the furnace by means of a fixed-speed screw feeder.
The furnace contained a horizontal fixed-speed screw pellet dispenser that subsequently
introduced pellets to the burning region. The furnace was equipped with an electric ig-
nition device.
Fig. 1. A view of 20 kW pellet supplied boiler (Biomax, Lumo type, Poland) (left),
and under-fed (retort) furnace (Bioline 20, Ecotec type, Sweden) (right)
Originally the boiler worked with a periodic pellet supply system. However, for the
purpose of this study, a continuous pellet supply system was arranged by installing an
inverter that enforced fluent regulation of screw feeder resolutions. The boiler was con-
nected to a 900 dm3 water heat storage through a special mixing and pumping device,
Laddomat 21 (Swedish production) composed with a pump and three thermal valves
that open or close automatically depending on incoming water temperature. This device
enabled water flow in the boiler after the temperature of 64 °C was reached. The water
at the inlet of the boiler was maintained at temperature over 64 °C in order to keep the
combustion chamber walls hot and therefore minimize emission of incomplete combus-
tion products.
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 99
The gas pollutant concentrations in the flue gas downstream the boiler as well as
the flue gas temperature were measured using a Vario Plus (MRU) flue gas analyzer
(Germany). Concentrations of O2, NO, and NO2 were measured with electrochemical
cells. Gas analyser calculated NOx concentration as a total of NO (transformed to NO2)
and NO2 concentration. CO and CxHy concentrations were measured using infrared ra-
diation. The flue gas analyser also calculated chimney loss. Temperature in the com-
bustion chamber was measured ca. 0.30 m above the flame with a radiation shielded
thermocouple PtRhPt. The measurements of temperature in the flame made with a py-
rometer showed that the temperature measured with the thermocouple is about 250 °C
lower than the temperature in the flame. Unfortunately, it was not possible to make
structural changes in the boiler to take flue gas samples directly from the flame area.
Dust concentration in the chimney was measured using a gravimetric dust meter equipped
with isokinetic aspiration. Heat received by the boiler water and boiler heat output were
measured with an ultrasonic heat meter. Fuel stream was measured using a Sartorius lab
balance. Pellet lower heating value was measured using the calorimetric method and heat
efficiency was calculated as heat transferred to the boiler water divided by fuel mass multi-
plied by fuel lower heating value. The obtained values were confronted with heat efficien-
cies calculated based on chimney loss and other estimated heat losses.
The study was performed using “Ecopellet” wood pellets certified to the highest
European quality standard DIN plus no. 7A105, corresponding with the recent European
standard E DIN EN 14961-2:2010-07, ordered from the Polish Company Barlinek. The
pellets 6 mm in diameter, up to 40 mm long, were made entirely from coniferous wood
sawdust. Their moisture was 10%, ash content 0.7%, lower heating value 18.5 MJ/kg,
nitrogen content 0.35%.
In the study, pollutant concentrations were examined for three different fuel
streams (3.0, 4.5 and 6.0 kg/h) and related three different speeds of screw pellet dis-
penser. For each fuel stream, six different fuel feeding modes (five periodic and one
continuous mode) were applied. Each of the feeding modes was performed for 5 con-
tinuous hours. Experimental design is presented in Table 1. For fuel stream of
3.0 kg/h, the operation/stand-by time ratio was 2:3, for 4.5 kg/h 2:1 and for 6.0 kg/h
8:1. Such settings result from the necessity of obtaining good furnace operation. Air
stream was chosen empirically in order to obtain the lowest possible CO concentration
while observing gas analyzer indications in terms of CO concentrations and watching
the flame in the combustion chamber through a sight glass. For the examined fuel
streams of 3.0 , 4.5 and 6.0 kg/h, the air stream values were set at about 30, 50 and
100% of their maximum, respectively. Gas pollutant concentrations, temperature in
combustion chamber and oxygen concentration were measured continuously and
transmitted in real time to a personal computer where they got recorded every 2 s,
mean values were calculated every 10 s and analyzed on diagrams (Figs. 3–8). For
gas pollutant concentrations (CO, NO, NOx), uncertainty intervals were calculated
with a 0.95 probability (Table 1).
100 M. JUSZCZAK
Tab
le 1
. M
ean
par
amet
er v
alu
es m
easu
red
fo
r al
l fu
el f
eed
ing m
od
es
Fu
el
mas
s
stre
am
[kg/h
]
Fu
el
feed
ing
mo
de
Tim
e o
f
op
erat
ion
:
stan
d-b
y m
od
e
[s]
Po
llu
tan
t co
nce
ntr
atio
ns
for
10
% O
2 i
n f
lue
gas
[mg/m
3]
Tem
per
atu
re
in c
om
bu
stio
n
cham
ber
a
[°C
]
O2
con
cen
trat
ion
[%]
Air
exce
ss
rati
o
Hea
t
ou
tpu
t
[kW
]
Hea
t
effi
cien
cy
[%]
CO
N
O
NO
x
3.0
con
tin
uo
us
44
3
32
29
0
17
44
6
27
46
1
9
1.8
1
2.6
8
2
per
iodic
20
:30
23
6
26
24
2
9
37
3
14
48
2
8
1.6
1
3.0
8
4
30
:45
41
8
33
25
7
22
39
5
36
42
5
8
1.6
1
3.3
8
6
40
:60
24
6
28
23
9
10
36
9
16
46
2
8
1.6
1
2.8
8
3
50
:75
26
6
22
27
3
4
42
5
8
44
7
9.5
1
.8
12
.7
85
60
:90
28
4
36
28
5
10
44
5
15
43
0
9
1,9
1
3.1
8
4
4.5
con
tin
uo
us
32
9
49
31
3
35
42
3
54
44
0
10
1.9
1
9.2
8
3
per
iodic
20
:10
31
0
34
27
2
37
42
3
58
43
7
10
1.9
1
9.0
8
2
30
:15
23
7
28
24
7
34
38
2
53
46
2
7
1.5
1
9.2
8
3
40
:20
23
1
31
25
4
38
39
1
5
45
5
7
1.5
1
9.2
8
3
50
:25
29
9
41
25
6
4
39
4
6
43
6
7
1.5
1
9.2
8
3
60
:30
35
0
39
24
0
20
36
9
33
43
7
7
1.5
1
9.7
8
5
6.0
con
tin
uo
us
39
2
46
24
1
14
37
1
22
43
1
7.5
1
,6
26
.5
86
per
iodic
40
:5
30
2
44
26
7
5
41
5
8
46
3
8
1.6
2
5.9
8
4
80
:10
29
3
67
27
9
13
43
2
20
45
9
7.5
1
.6
25
.6
83
12
0:1
5
32
9
38
28
1
13
43
5
18
43
3
7.5
1
.6
25
.3
82
16
0:2
0
33
5
28
28
2
18
43
7
28
43
9
9.5
1
.8
25
.0
81
20
0:2
5
34
7
26
25
4
3
39
2
5
43
7
7.5
1
.6
25
.6
83
a Tem
per
atu
re i
n t
he
com
bu
stio
n c
ham
ber
(m
easu
red
wit
h a
th
erm
oco
up
le)
is
ca.
25
0 °
C l
ow
er t
han
th
e te
mp
erat
ure
in
th
e fl
ame
(mea
sure
d w
ith
a p
yro
met
er).
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 101
3. RESULTS AND DISSCUSION
Figures 2, 3, 5, 7 present time dependences of gas pollutant concentrations and of
other parameters. All of them seem to display a similar shape. Figures 4, 6, 8 show
dependences of CO, NO and NOx concentrations on temperature in the combustion
chamber and on oxygen concentration in the flue gas for 3 kg/h continuous fuel feeding
and some periodic feeding modes.
Fig. 2. CO concentrations in correlation with fuel mass stream
and fuel feeding mode, according to Table 1
Fig. 3. Time dependences of pollutant concentrations
and other parameters for 3 kg/h continuous fuel supply
0
50
100
150
200
250
300
350
400
450
500
20:30 30:45 40:60 50:75 60:90 20:10 30:15 40:20 50:25 60:30 40:05 80:10 120:15 160:20 200:25
CO
co
nce
ntr
atio
ns
[mg/m
3]
for
10
% O
2
con
tin
uo
us periodic
3.0 [kg/h]
periodic
con
tin
uo
us
con
tin
uo
us periodic
4.5 [kg/h] 6.0 [kg/h]
1
10
100
1000
00
:00
00
:34
01
:08
01
:42
02
:16
02
:50
03
:24
03
:58
04
:32
05
:06
05
:40
06
:14
06
:48
07
:22
07
:56
08
:30
09
:04
09
:38
10
:12
10
:46
11
:20
11
:54
12
:28
13
:02
13
:36
14
:10
14
:44
15
:18
15
:52
16
:26
17
:00
17
:34
18
:08
18
:42
19
:16
19
:50
20
:24
20
:58
21
:32
22
:06
22
:40
23
:14
23
:48
24
:22
24
:56
25
:30
26
:04
26
:38
27
:12
27
:46
28
:20
28
:54
29
:28
Gas
co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
O2 [%]
Air axcess ratio
Flue gas temperature [°C]
Time [mm:ss]
NO [mg/m3] (10% O2)CO [mg/m3] (10% O2) NOx [mg/m3] (10% O2)
102 M. JUSZCZAK
Correlations obtained for other fuel feedings were similar to those presented in the
paper. Rough measurements of dust concentration showed the values between 30 and
50 mg/m3. All pollutant concentrations were calculated for 10% O2 content in flue gas.
The values of uncertainty intervals for CO, NO, NOx concentrations indicate the mag-
nitude of fluctuations of these parameters for a certain fuel feeding method.
Fig. 4. Dependences of CO and NO, NOx concentrations on temperature in the combustion chamber (a),
and on oxygen concentration in the flue gas (b) for continuous fuel feeding and fuel mass stream of 3.0 kg/h
y = 277.27e-3E-04x
R² = 0.0017
y = 616.15e-0.003x
R² = 0.4491
y = 939.82e-0.02x
R² = 0.4498
0
50
100
150
200
250
300
350
400
450
500
400 410 420 430 440 450 460 470 480 490 500
Temperature in a combustion chamber [C]
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
NO [mg/m3] for 10% O2 CO [mg/m3] for 10% O2
NOx [mg/m3] for 10% O2
a)
y = 17.76x2 - 298.23x + 1467.4R² = 0.444
y = -0.6149x2 + 21.936x + 70.631
R² = 0.8519
y = -0.9626x2 + 33.924x + 107.43R² = 0.8523
0
50
100
150
200
250
300
350
400
450
500
5 6 7 8 9 10 11 12
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
O2 concentration [%]
NOx [mg/m3] for 10% O2
CO [mg/m3] for 10% O2
NO [mg/m3] for 10% O2
b)
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 103
Fig. 5. Time dependences of pollutant concentrations and other parameters
for 3 kg/h periodic fuel supply: operation time 20 s
CO concentrations measured during all tests were below 450 mg/m3 (Table 1).
Although it would seem that CO concentrations for continuous fuel supply mode
should be lower than those for periodic mode, it was quite the opposite for most cases
(Table 1). The highest value of CO concentration was measured for 3 kg/h continuous
supply, probably due to too low air stream. Combustion in all cases was performed at low
temperatures, and during continuous feeding most probably the pellets had not enough
time to be completely fired and CO concentration was relatively high. Table 1 and Fig. 2
show that the increase of stand-by period is not always accompanied by an increase
of CO concentration. In order to decrease CO concentration, the air stream for com-
bustion needs to be adjusted as well. However, CO concentration decreases upon in-
creasing oxygen concentration to a certain minimum value, after which CO concen-
tration starts to decrease (Figs. 4, 6, 8). Determining oxygen concentration at which
CO concentration is the lowest might be helpful in the case when an automatic air
stream regulation oxygen probe downstream the boiler is installed. For the studied
furnace with periodic fuel supply, it can be assumed that an increase of CO concen-
tration above the minimum value is caused also by an important decrease in tempera-
ture in the combustion chamber during the stand-by in pellet supply. Nitrogen oxide
(NO and NOx) concentrations depend mainly on the amount of nitrogen introduced
into the furnace. As coniferous wood presents higher nitrogen content than deciduous
wood, these concentrations were significant and varied from 239 (NO) and 369 (NOx)
mg/m3 to 313 (NO) and 423 (NOx) mg/m3.
1
10
100
10000
0:0
0
00
:34
01
:08
01
:42
02
:16
02
:50
03
:24
03
:58
04
:32
05
:06
05
:40
06
:14
06
:48
07
:22
07
:56
08
:30
09
:04
09
:38
10
:12
10
:46
11
:20
11
:54
12
:28
13
:02
13
:36
14
:10
14
:44
15
:18
15
:52
16
:26
17
:00
17
:34
18
:08
18
:42
19
:16
19
:50
20
:24
20
:58
21
:32
22
:06
22
:40
23
:14
23
:48
24
:22
24
:56
25
:30
26
:04
26
:38
27
:12
27
:46
28
:20
28
:54
29
:28
NOx [mg/m3] (10% O2)
Flue gas temperature [°C]
Air axcess ratio
O2 [%]
Gas
con
cen
trat
ion
[m
g/m
3]
for
10
% O
2
Time [mm:ss]
CO [mg/m3] (10% O2)
NO [mg/m3] (10% O2)
104 M. JUSZCZAK
Fig. 6. Dependences of CO and NO, NOx concentrations on temperature
in the combustion chamber (a) and on oxygen concentration in the flue gas (b)
for fuel mass stream 3.0 kg/h and periodic fuel supply: operation time 20 s, stand-by time 30 s
y = 4105.9e-0.006x
R² = 0.2237
y = 886.51e-0.003x
R² = 0.2812
y = 1490.1e-0.003x
R² = 0.2975
0
50
100
150
200
250
300
350
400
450
500
425 430 435 440 445 450 455 460 465 470
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
Temperature in combustion chamber [C]
NOx [mg/m3] for 10% O2
NO [mg/m3] for 10% O2
CO [mg/m3] for 10% O2
a)
y = 7.0028x2 - 98.034x + 559.8R² = 0.7654
y = 166.23e0.0485xR² = 0.7394
y = 252.31e0.0509xR² = 0.7604
0
50
100
150
200
250
300
350
400
450
500
6 7 8 9 10 11 12
CO [mg/m3] for 10% O2
NOx [mg/m3] for 10% O2
NO [mg/m3] for 10% O2
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
O2 concentration [%]
b)
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 105
Fig. 7. Time dependences of pollutant concentrations and other parameters
for 3 kg/h periodic fuel supply: operation time 60 s, stand-by time 90 s
A slight increase of NO and NOx concentrations was observed when oxygen con-
centration increased and a subtle decrease when temperature in the combustion chamber
increased. This means probably that in the same time when temperature in combustion
chamber increased combustion process was more intensive and oxygen concentration
decreased. For all the cases presented in Table 1 for an almost-real life conditions, a rel-
atively high boiler heat efficiency of above 80% was obtained. Throughout the course
of the experiments, the presence of hydrocarbons in the flue gas was not detected,
whereas soot was observed in the damper. This could mean that the combustion process
was being partially hampered on the cool surfaces of the relatively small combustion
chamber in spite of applying the Laddomat 21 mixing and pumping device. Dust con-
centrations of 30–50 mg/m3 were much below the officially permitted values of
150 mg/m3 [2].
4. CONCLUSIONS
Applying periodic fuel supply instead of continuous feeding mode does not nec-
essarily have to come with a radical increase in CO concentration. Actually, the study
has shown that in most conditions it was quite the opposite and CO concentrations
were lower for periodic fuel supply. Generally, in order to keep CO concentration
levels low one has to carefully select appropriate settings: fuel mass stream and air
stream for both fuel feeding modes and additionally stand-by/operation time ratio for
periodic fuel supply.
1
10
100
1000
00:0
0
00:3
8
01:1
6
01:5
4
02:3
2
03:1
0
03:4
8
04:2
6
05:0
4
05:4
2
06:2
0
06:5
8
07:3
6
08:1
4
08:5
2
09:3
0
10:0
8
10:4
6
11:2
4
12:0
2
12:4
0
13:1
8
13:5
6
14:3
4
15:1
2
15:5
0
16:2
8
17:0
6
17:4
4
18:2
2
19:0
0
19:3
8
20:1
6
20:5
4
21:3
2
22:1
0
22:4
8
23:2
6
24:0
4
24:4
2
25:2
0
25:5
8
26:3
6
27:1
4
27:5
2
28:3
0
29:0
8
29:4
6
NOx [mg/m3] (10% O2)
Air axcess ratio [-]
Flue gas temperature [°C]
O2 [%]
Gas
co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
Time [mm:ss]
NO [mg/m3] (10% O2)
CO [mg/m3] (10% O2)
106 M. JUSZCZAK
Fig. 8. Dependences of CO and NO, NOx concentrations on temperature in the combustion chamber (a)
and on oxygen concentration in the flue gas (b) for fuel mass stream 3.0 kg/h
and periodic fuel supply: operation time 60 s, stand-by time 90 s
As boiler producers only indicate adequate settings for pure deciduous wood pellets,
in the case of coniferous or mixed wood pellets or any other new kind of fuel these
settings need to be determined experimentally for a specific boiler and furnace type.
y = 6851.8e-0.007x
R² = 0.1512
y = 424.31e-1E-03x
R² = 0.0299
y = 676.99e-0.001x
R² = 0.0309
0
100
200
300
400
500
600
420 425 430 435 440 445 450 455
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
Temperature in the combustion chamber [C]
NO [mg/m3] for 10% O2
NOx [mg/m3] for 10% O2
CO [mg/m3] for 10% O2
a)
y = 16.648x2 - 288.74x + 1527.5
R² = 0.7385y = 184.28e0.0402x
R² = 0.5488
y = 278.87e0.0431x
R² = 0.585
0
100
200
300
400
500
600
8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
NOx [mg/m3] (10% O2)
Po
llu
tant co
nce
ntr
atio
n [
mg
/m3]
for
10
% O
2
O2 concentration [%]
NO [mg/m3] for 10% O2
CO [mg/m3] for 10% O2
b)
CO and NOx concentrations from a 20 kW boiler for wood pellet supply 107
ACKNOWLEDGEMENTS
The author thanks the technical workers and students of Poznan University of Technology for their
help during the research. This work was carried out as a part of the research project PB-13/615/08BW
sponsored by Poznan University of Technology.
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