Volumetric Combustion of Biomass in Boiler for CO2 and NOx Reduction
1
Wlodzimierz Blasiak1 , Weihong Yang*1, Jun Li1, A. Ponzio2
Division of Energy and Furnace Technology, Royal Institute of Technology(KTH)
Brinellvägen 23, 10044 Stockholm, Sweden
Tele: +46 8 7908402 ; Email: weihong@kth.se
2
Jernkontoret, Box 1721, 111 87 Stockholm
ABSTRACT:
To achieve the urgent targets in environment, biomass substituting coal was considered as
an effective and promising method during last decades. In this paper, the idea is further
developed for a 100% pulverized biomass combustion in a boiler. A new concept of
volumetric combustion technology was proposed, the properties of wood pellets
combustion in high temperature oxidizer were investigated firstly. Then an Aspen model
was used to describe and studied volumetric combustion system with three different kinds
of fuel, and compared the CO2 and NOx emissions. At last, two application concepts of
volumetric combustion were discussed. The results shows that wood pellets were ignited
and combusted much faster than coal pellets, when the oxidizer was preheated to 1000°C,
the ignition time to be almost independent of oxygen concentration; NOx emissions
decreased with the recirculation ratio of flue gas increasing, and there is almost no NOx
emission when recirculation ratio approaches to 10% in all fuel conditions; high percent of
biomass combust in volumetric combustion system with high boiler efficiency, and high
emissions reduction. So, it is an attractive and applicable technology in future.
Key words: Volumetric combustion, Wood pellets, CO2, NOx
-1 1. INTRODUCTION
At year 2007 the EU adopted a far-reaching energy policy aimed at achieving a
number of ambitious targets by the year 2020: to reduce greenhouse gases by 20 %
compared to 1990, to raise the share of renew-able energy in the EU to 20 % and to
improve energy efficiency by 20 %[1] .
Biomass and coal co-combustion represents a near-term, low-risk, low-cost,
sustainable, renewable energy option that promises reduction in effective CO2 emissions,
reduction in SOx and often NOx emissions, and several societal benefits [2, 3]. For
example, a 5% cofiring of biomass and coal in the globe power generation, we can get 40
GW, about 300 Mton CO2/year.
Over the past 5-10 years there has been remarkably rapid progress over in the
development of cofiring. Several plants have been retrofitted for demonstration purposes,
while another number of new plants are already being designed for involving biomass coutilization with fossil fuels[3]. Approximate 40 pulverized coal fired power plants in the
worldwide that cofiring biomass on a commercial basis, the average number is about 3%
energy input. This leads to a 3.5 Mton of coal substituted, and make around 10 Mton of
CO2 reduction[4]. Furthermore, The co-firing potential is huge, EU contributes about
50,000MWe and world contributes 160,000 MWe (50-160 nuclear power plants)[5].
Biomass is able to penetrate all energy sectarian markets, but economical constraints
still limits its general deployment. Therefore, demanding a promote, efficient but
sustainable biomass schemes. The economics of biomass cofiring are significantly
impacted by the twin factors of deregulation and environmental requirement. Uncertainties
in both arenas make investment in most, or all, biomass cofiring technologies difficult to
justify. Consequently, biomass cofiring has become a tool for use in niche applications
where relatively high coal costs and relatively abundant biomass converge. Such niches
also require favorable regulatory treatment without the threat of New Source Review.
Further on, new regulations force power sector to use biomass of agricultural origin, for
examples, LCPD (Large Combustion Plant Directive), NECD (National Emission Ceilings
Directive) and IPPCD (Integrated Pollution Prevention and Control Directive)[6]. Those
regulations promot obviously large percentage (above 40 %) biomass co-firing will be used
by power sector.
-2 Combustion behaviors of biomass are different from coal’s. Biomass yields a much
higher fraction of its mass through devolatilization than that does coal. Typically biomass
of the size and under the heating rtes typical of pc-cofiring yields 90-95% of its dry,
inorganic-free mass during devolatilization, compared with 55-60% for most coals.
However, if biomass particles are too lager or dense, they may be not entrained into
the flues gas, and enter the bottom ash stream with little or no conversion beyond drying if
the biomass injection is not well organized. Additionally, the low biomass particles
densities help biomass particles oxidizer at rates much higher than coal. Further on, if
boilers do not mix flue gases effectively in furnace section, it will format striated flows,
which is a common feature of biomass-coal combustion.Therefore, researching and
development of an advanced combustion technology is a technical challenge for biomass,
and / or larger percentage of cofiring with coal utilization with aims at a maximum
efficiency and minimum emissions in larger scale electricity generation.
The idea is to develop a combustion concept independently on available fuels
including biomass. Concept of the Volumetric Combustion has been proposed by Prof.
Blasiak[7]. Volumetric Combustion can be characterized as a very intensive mixing and
internal recirculation inside of the combustion chamber. In this case, combustion occurs in
a very larger volume with relatively uniform distribution of reacting species, temperature
and heat fluxes.
In this paper, the idea is further developed a new combustion concept for a 100%
pulverized biomass combustion in a boiler---Volumetric Combustion. In order to control
the combustion process of biomass in boiler, the properties of wood pellets combustion in
high temperature oxidizer were investigated. Flame phenomenon, ignition time of wood
pellets combustion were studied experimentally. Furthermore, the properties of volumetric
combustion system were systematically studied by thermodynamic analysis method.
Emissions reduction during volumetric combustion process was focused with three
different kinds of fuel, namely, biomass only, co-combustion and coal only. At last, several
possible technologies for volumetric combustion application were discussed. This work are
not only to explain the probably for 100% biomass combustion boiler without reduction of
efficiency, also to declare the emissions reduction properties with volumetric combustion
technology.
-3 2. Development Testing and Thermodynamic Analysis
2.1 Concept of volumetric combustion in a boiler
Traditional air staging systems are not uncommon in utility and district heating boilers
independent of firing configuration or combustion technology. Typical organization of
staged combustion is shown in Figure 1. In order to prevent formation of nitrogen oxides
from fuel-bound nitrogen, the primary combustion zone is operated under substoichiometric conditions with excess air number () less than one.
To complete
combustion a secondary air is introduced into the upper furnace by means of air supply
system called Over-Fire Air (OFA). The secondary combustion zone is operated with
excess air number () above one. Interaction between the two separated combustion zones
is difficult to control in large scale combustion chambers particularly when boiler’s load
and operational parameters change.
Negative effects of such staged combustion are usually: too high content of carbon in
fly ash (called Loss on Ignition, LOI) and carbon monoxide. Further on, NOx reduction for
traditional systems is limited. Other known negative effects include a drop of steam
temperature and water wall wastage. Reason of all negative effects is poor mixing inside
combustion chamber. It results often in uncontrolled flow pattern typically characterized by
so called “chimney” flow transporting part of gases and fuel particles through combustion
chamber with too high velocity and thus too short residence time.
Figure 1. Traditional staged combustion concept
Flue gas recirculation is commonly used to design low-NOx combustion process [8,
9]. It is applied to design low-NOx burners and low-NOx combustion in large scale
combustion chambers. In traditional approach external or internal flue gas recirculation is
used. External recirculation has obvious drawbacks as for example drop of efficiency and
high investment costs. Internal flue gas recirculation is usually very limited because the
-4 traditional secondary air systems (OFA) are designed just to deliver the secondary air to
complete combustion only. Usually just 10-15 % of air is supplied as a secondary air.
In the developed Volumetric Combustion, a very intensive mixing and internal
recirculation inside of the combustion chamber has been used as schematically shown in
Figure 2, it is also commonly combined with air staging concept. Aim is to prevent
formation of NOx originating mainly from fuel nitrogen.
Figure 2. Volumetric Combustion
Contrary to traditional OFA system, Volumetric Combustion allows to increase
amount of secondary air up to about 30-40% without causing problems with combustion
stability or incomplete combustion. The new concept promotes intensive internal
recirculation of flue gases from the level of secondary air injection down to primary
combustion zone. Combination of air staging and internal flue gas recirculation changes not
only in-furnace flow but also affects the combustion. Due to intensive recirculation and
good mixing between secondary air and flue gas the combustion volume is larger than with
traditional staged combustion. Such combustion is termed “volumetric” combustion. The
importance of extensive flue gas recirculation on combustion and formation of so called
“flameless” combustion in industrial furnaces has been reported. Volumetric combustion of
gas and oil has also been reported for conditions of High-temperature Air CombustionHiTAC[10], or flameless Oxidation[11]. It has been also used for oxygen enriched and
oxyfuel combustion air [12].
2.2 Experimental set-up and process
Test facility is the batch type HiTAC/G facility at KTH, the introductions about this
test facility and detailed test methods were reported by A. Ponzio[13]. Here, experiments
with wood pellets were performed using 5g or 30 g of 8 mm wood pellets inserted in to a
-5 small basket. A single wood pellet of 15mm diameter (4-5g) was attached to the certain
piston with thermocouple wire, proximate and ultimate analysis of wood pellet is presented
in Table 1. Experiments were carried out for three different oxygen concentrations in the
oxidizer (5, 10 and 21 %) and three different temperatures of the oxidizer (600, 800 and
1000C). The total oxidizer flow was 10 Nm3/h in all 5g experiments and 4Nm3/h in both
30g and the single pellet experiments. The experimental cases are presented In Table 2.
Table 1. Wood pellets properties
Proximate analysis (av,%)
Ultimate analysis(av,%)
Moisture
7.7
C
51.0
Volatile
74.7
H
6.3
Fixed carbon
16.9
O
41.8
Ash
0.7
N
0.2
LHV (kJ/kg)
17270
S
0.01
Table 2. Experimental conditions in the experiment with wood pellets
Oxidizer
temperature
1000°C
800°C
600°C
Oxygen concentration in oxidizer
21%
5g
15 s
30 s
45 s
1 min
/
2 min
/
3 min
5 min
5g
45 s
1 min
2 min
3 min
5 min
5g
1 min
2 min
3 min
5 min
10%
Single pellet
15 s
/
/
1 min
1.5 min
2 min
2.5 min
3 min
/
30 g
/
/
/
/
5 min
/
30 g
/
/
/
/
/
/
/
/
5 min
/
/
-6 5g
15 s
30 s
45 s
1 min
/
2 min
/
3 min
5 min
5g
45 s
1 min
2 min
3 min
5 min
5g
1 min
2 min
3 min
5 min
5%
Single pellet
/
30 s
/
1 min
1.5 min
2 min
/
3 min
/
30 g
/
/
/
/
5 min
/
5g
15 s
30 s
45 s
1 min
/
2 min
/
3 min
5 min
5g
45 s
1 min
2 min
3 min
5 min
5g
1 min
2 min
3 min
5 min
2.3 Model description
The volumetric combustion system process was built using Aspen plus, including
decomposition, volatile and chars combustion, and internal flue gas recirculation. Figure 10
shows Aspen Plus flowsheet of volumetric combustion. In this model, the CO2 and NOx
emissions are focused when using three different kinds of fuel, namely, biomass only, cocombustion and coal only.
Biomass
decompostion
Fuel Feed
Primary
Combustion
Zone
Air Supply
Heat
Exchanger
Coal
decompostion
Secondary
Combustion
Zone
High Temperature
Flue Gas Recirculation
Figure 3. Aspen plus flowsheet of volumetric combustion with biomass and (or) coal
The main model assumptions as follows, 1) steady state flow; 2) potential and kinetic
energies are negligible; 3) environment state at T=298K and P=1atm; 4) the gas obey the
ideal gas relations.
Firstly, biomass and coal were decomposited in two independent blocks, ‘BIOPYRO’
and ‘COALPYRO’ respectively. Specify the yield distribution according to the proximate
and ultimate analysis of biomass and coal used in this study. The proximate and ultimate
analysis of the used biomass and coal are given in Table 4 [14], which is different from the
data in Table 1. The main reason is for comparing the results with reference data
conveniently, and improving validation of this model. C, H, O and N are the weight
-7 fractions of carbon, hydrogen, oxygen and nitrogen respectively in the used biomass and
coal.
Biomass and coal consists of mainly C, H, N, O, S, ash, and moisture. H, N, O, S and
part of C constitute the gas phase, the fixed carbon and ash comprise of the solid phase after
decomposition process. And then the stream was feed into the next block, ‘PC-ZONE’, in
which fuel rich combustion happened with excess air ratio (λ) less than one, called this
process as primary combustion in this study. After primary combustion, the stream was
feed into the next block, ‘SC-ZONE’, in which oxygen rich combustion happened with
excess air ratio (λ) above one, this process was called secondary combustion. After
secondary combustion, some of flue gas was recycled back into the bottom of furnace for
diluting oxygen concentration of combustion zone, and the others flowed into back-pass of
boiler to exchange heat in economizer and air preheated.
Table 4. Fuel analysis[14]
Items
Coal
Biomass
Moisture content (wt %)
9.20
8.80
Volatile matter
29.20
73.50
Fixed carbon
52.55
15.30
Ash
9.05
2.40
C
67.37
47.0
H
4.37
5.0
O
7.99
36.23
N
1.11
0.49
S
0.60
0.08
HHV (kJ/kg, ar)
27712 18969
Proximate analysis (wt%, ar)
Ultimate analysis (wt% , ar)
3. RESULTS AND DISCUSSIONS
-8 3.1 Flame phenomena of wood pellets combustion
In Figure 4, typical flame appearances for three different oxygen concentrations
oxidizers are showed, and all the oxidizers were preheated to 1000C. The photos are
ordered from left to right by increasing oxygen concentration, 5%, 10% and 21%. When the
oxygen concentration is 5%, 4s and 640ms after the experiments ignition stared smoothly, a
long flame that wrapped the pellet developed and persisted to the end of the 3 minutes
experiment; when the oxygen concentration is 10%, ignition started at 1s 760ms, the
ignition started at the front of the pellet and a long flame appeared that wrapped the pellet.
The flame extinguished at 2min and 11s; when air (21%) preheated to 1000°C was used to
combust wood pellets, ignition occurred at 2s and 640ms after the start of the experiment,
the flame was shorter than that of the diluted oxidizer cases, and the flame extinguished at 2
min 8s and 800ms.
5%
21%
10%
Figure 4. Typical flame appearances in three different oxygen concentration oxidizers preheated to
1000°C
From the flame phenomenon of Figure 4, it is clear that the lower oxygen
concentration, the lager combustion volume, and the longer combustion time. It is very
satisfied with the features of typical volumetric combustion, which is a large volume of
process with relatively uniform distribution of reacting species, temperature and heat
fluxes.
3.2 Ignition time of wood pellets
In this work, there are two different approaches were used to determine ignition time:
1) by using a stop watch and the video-recording of the experiments and 2) by equalizing
ignition time to the time of a temperature increase in the gases downstream from the
sample. The latter method was used as a compliment of the stop-watch method due to the
-9 difficulty to determine exactly when the flame appeared in the diluted cases because of its
weak color. The results both of the two methods are showed in Figure 5. It can be noted
that the two methods differ only significantly for 5% oxygen conditions in which the stopwatch method indicated longer ignition time, where presumably the flame was there for
some time before it was actually visible. The properties of coal pellets combustion have
been investigated by A. Ponzio (2008) [15]. In comparison to coal[15], wood ignited much
faster than coal at low temperatures. However, the tendency of the ignition time to be
almost independent of oxygen concentration at high temperatures (1000C) and highly
dependent on oxygen concentration at low temperature (600C), which is same with
coal[15]. For example, 5% oxygen concentration of oxidizer and 600C, the wood pellets
did not ignite in five minutes.
300
300
5% O2
10% O2
21% O2
21% O2 (30 g)
10% O2 (30 g)
5% O2 (30 g)
200
150
100
200
150
100
50
50
0
500
5% O2
10% O2
21% O2
250
Ignition time, tV [s]
Ignition time,tT [s]
250
600
700
800
900
1000
0
500
1100
600
700
800
900
1000
1100
Toxidizer [oC]
Toxidizer [oC]
(a)
(b)
（a: temperature increase record method；b: video record method）
Figure 5. The effect of oxidizer temperature on ignition time of wood pellets
3.3 Mass loss properties of wood pellets
The wood pellets combustion experiments were performed with a higher flow-rate of
oxidizer and smaller samples both in weight and size, so a direct comparison should be
avoided. However, one cannot but note the striking difference in reactivity between
biomass and coal. In Figure 6, the wood showed a large difference between the mass loss
rate in the “devolatilisation-zone” and the mass loss rate in the “char-zone”, and the mass
loss rate in the “devolatilisation-zone” is much bigger than that in the “char-zone”.
According to the coal pellets combustion results of A. Ponzio [15], there was little
difference in mass loss rate between these two zones for coal pellets for 5-21% oxygen and
almost no visible difference for 30-100% oxygen. This would suggest that the
devolatilisation of wood is much faster than the devolatilisation of coal.
-10 The explanations for this may be several. First and foremost, there were much higher
oxygen to fuel ratio in the wood experiments due to higher flow-rate of the oxidizer and
lower mass of the sample. However, when 30g wood samples were used, the five minutes
conversions were between 84 and 89% for 800-1000°C and 5-21% oxygen concentration as
showed in Table 3. The properties of lower density and higher porosity of wood than coal
seem to be the second possible explanation for the quick mass loss rate. Moreover, the high
volatile content of the biomass particle leaves a more accessible char that could be readily
passed by reactants and products of all kinds.
1,0
Wood pellets 1000C
0,8
Moisture
1,0
Volatiles
0,8
Wood pellets 1000C
Moisture
1 min
Volatiles
Mass [-]
Mass [-]
3 min
0,6
21% O2
10% O2
5% O2
0,4
5 min
0,6
0,4
0,2
0,2
Char
Char
0,0
0,0
0
1
2
3
4
5
6
7
8
9
0
5
Time [min]
10
15
20
25
30
Oxygen concentration [%]
(a) Mass as function of time for different oxygen
(b) Mass as function of oxygen concentration in
concentrations in the oxidizer.
the oxidizer for different duration of experiment.
Figure 6. Influence of oxygen concentration on wood pellet combustion for Toxidizer=1000C
Table 3. Five minutes conversion of wood pellets (8 mm, 30 g) samples.
Cases
1000C
5%
Conversion 0.86
800C
10%
21%
5%
10%
21%
0.84
0.86
0.84
0.89
0.87
In Figure 7, the mass-time curves for wood experiments performed with a single 15
mm pellet are shown. As can be seen, the mass loss rate was a slightly lower than in the
cases for wood pellets presented above. Differences in the ratio between surface area and
mass as well have been the explanation. However, with respect to coal[15], the results in
Figure 7 confirm that wood pellets were combusted much faster than the coal pellets. The
differences in density for the wood pellets and the coal pellets were significant.
-11 Single wood pellet (15 mm), 1000C
Moisture
Volatiles
Mass [-]
21% O2
10% O2
Char
0
0
1
2
3
4
5
6
7
8
9
Time [min]
Figure 71. Influence of oxygen concentration on single 15 mm
wood pellet combustion for Toxidizer=1000C
The devolatilisation rate appeared to be independent of oxygen concentration when
oxidizer was preheated to 1000C, while at 800C it seemed to be determined by it. In fact,
the mass loss rate in the “devolatilisation zone” was significantly increased, mainly
between 10 and 21%. This was also evidenced by an inclined 1 min curve in Figure 8b. A
plausible interpretation is that at 800C, the heat of combustion is indeed an important
promoting factor for the devolatilisation process. As was pointed out above, a similar, but
less significant, tendency towards increased importance of oxygen concentration for low
oxygen concentration and in the initial part of the experiment was seen in the coal case. The
behavior of mass loss of wood was little dependent on temperature of the oxidizer, at least
in the very initial part of the experiment.
Wood pellets, 800C
1,0
0,8
Moisture
1,0
Volatiles
0,8
Wood pellets 800C
Moisture
1 min
Volatiles
0,6
Mass [-]
Mass [-]
3 min
21% O2
10% O2
5% O2
0,4
5 min
0,6
0,4
0,2
0,2
Char
Char
0,0
0,0
0
1
2
3
4
5
6
7
8
9
0
Time [min]
5
10
15
20
25
30
Oxygen concentration [%]
(a) Mass as function of time for different oxygen
(b) Mass as function of oxygen concentration in
concentrations in the oxidizer.
the oxidizer for different duration of experiment.
Figure 8. Influence of oxygen concentration on wood pellet combustion for Toxidizer 800C
There was very little dependence on the mass loss on oxygen concentration at 600C.
This is in fact no surprise: a look at the ignition times reported above confirms that most
-12 part of the experiment is merely devolatilisation (no significant combustion occurring).
Flaming ignition occur only 1min30s to 2min30s into the process depending on the oxygen
concentration, and if only 5% oxygen is used, there is no ignition at all. No or little
combustion equals no or little heat release, thus no influence of the oxygen concentration
on the mass lost, something confirmed by the almost flat curves in Figure 9b.
1,0
Wood pellets, 600C
0,8
Moisture
1,0
Volatiles
0,8
Wood pellets 600C
Moisture
1 min
Volatiles
0,6
Mass [-]
Mass [-]
3 min
21% O2
10% O2
5% O2
0,4
0,2
5 min
0,6
0,4
0,2
Char
Char
0,0
0,0
0
1
2
3
4
5
6
7
8
9
0
Time [min]
5
10
15
20
25
30
Oxygen concentration [%]
(a) Mass as function of time for different oxygen
(b) Mass as function of oxygen concentration in
concentrations in the oxidizer.
the oxidizer for different duration of experiment.
Figure 9. Influence of oxygen concentration on wood pellet combustion for Toxidizer 600C
3.4 Model validation of volumetric combustion system
The model was validated against the power plant boiler operating data B. Higgins et
al. [14], which can reach 55MWe and provide 179MWt of heating output with maximum
continuous output. The input data for both coal only and co-combustion was used in this
paper [14],
Table 5. The main input data in Aspen Plus
Unit
Coal only[14]
Co-combustion[14]
Biomass only
Load
MWt
179
179
179
Fuel
Heat input
100% coal
55% coal +45%biomass 100% biomass
Excess air
%
20
20
20
Excess O2
% dry
3.6
3.6
3.6
Coal flow
t/h
23
13
0
Biomass flow
t/h
0
15
35
Total air flow
t/h
254
245
234
-13 Results were found to be good agreement with the reported results. According to this
model results, the NOx emission was 145mg/Nm3 in co-combustion condition; the NOx
emission was 320mg/Nm3 in coal only without flue gas recirculation, which was an
acceptable result with the operation result (300mg/Nm3). The under or over of operation
result is a common problem for Aspen plus[16], because it is a steady state model of
combustion process, and there are many influence factors affect the real operation result.
Here, the main purpose is to obtain the CO2 and NOx emissions regular in different
recirculation ratio conditions in thermodynamic method.
3.5 Emissions reduction of volumetric combustion
Biomass as a renewable resource and the CO2 released during biomass combustion
will be re-captured by the re-growth of the biomass through photosynthesis[17]. The
neutrality of biomass CO2 has been recognized for many years by an abundance of studies
and is universally accepted by agencies, institutions, regulations and legislation. CO2
emission from combustion of biomass fuel source is not assumed to increase net
atmospheric CO2 levels[18]. Therefore, co-combustion is an effective pathway to reduce
the emission of CO2, and the more percent of biomass replace coal (in heat), the less CO2
releases.
In this work, three kinds of percents of biomass in total feed fuel were used in
volumetric combustion, 0%, 45% and 100% as showed in Table 5. The properties of CO2
emission in three different kinds of fuel combustion process are compared, as showed in
Table 6.
Table 6. CO2 emission in three different kinds of fuel
CO2 emission
Coal only
Co-combustion
Biomass only
(kg/h)
56815.4
32113.0
0
As mentioned before, internal flue gas recirculation in the volumetric combustion not
only contributes to intense in-furnace flow, and beneficial to format lower oxygen
concentration for reduction NOx. Therefore, the recirculation ratio R is an important factor
in volumetric combustion. Here, R represents the fraction of recirculation part in total
combustion productions. For example, R=0 means that there is no flue gas recirculation flue
-14 gas back to furnace chamber, R=0.1 means that there are 10% flue gas back into primary
combustion zone.
Figure 10 shows the effect of recirculation ratio on NOx emission at different
combustion conditions. One condition is un-staging combustion, in which the combustion
air is feed in one position. Another is air staging combustion, in which the combustion air is
divided into primary air and secondary air. Therefore, the combustion zone was divided
into a primary zone run with a deficiency of air and a second burnout zone run with excess
air. In this case, the solid fuel consists of 55% coal and 45% biomass; the combustion air
temperature was kept in 493K both in un-staging combustion and air staging combustion
conditions; the primary air ratio λ=0.7 in air staging combustion.
Figure 10. Effects of recirculation ratio on NOx emission at different combustion conditions
Obviously, the amount of NOx emission in un-staging combustion process linear
decreased slightly with the R increasing. From Figure 10, the total amount of NOx
decreased from 449mg/Nm3 to 350 mg/Nm3 with R range from 0 to 30%. The same
decreased trend was obtained by Baltasar J. et al by experimental methods[9]. Compared to
un-staging combustion, the amount of NOx emission reduced significantly in air staging
combustion, the mechanistic pathways for formation and reduction of nitrogen oxides
during air staging combustion were reported by Beer[19]. More interesting, the NOx
emission firstly rapid increased then deceased sharply with the R increasing during air
-15 staging combustion, the NOx almost approach 0mg/Nm3 when R=4%. The possible
explaining for the NOx emission increasing firstly as follows, when the small part of high
temperature flue gas recycled into boiler furnace, and it mainly contributed to furnace
temperature but had less influence on formation enough low oxygen concentration zone.
Thermal NOx formation is highly dependent on temperature, whereby higher temperature
promotes NOx formation. Then with the recirculation ratio increased, more flue gas
contributed to decrease oxygen concentration, and therefore the NOx emission is decreased.
Fuel nitrogen is released from fuel as HCN and sometimes as NHi with i=1, 2, 3, these
intermediate radicals subsequently react to form NOx in oxidizing environment and N2 in
reducing environment[2]. Therefore, the higher flue gases recirculation ratio, the lower
oxygen concentration in furnace, then was formatting a good reducing environment for
reduction NOx. The result showed flue gas recirculation is a better method for reduction
NOx. Especially, it plays an important role for reducing NOx emission during air staging
combustion process, there is almost no NOx emission when recirculation ratio approached
to 4%.
Figure 11. The effect of recirculation ratio on NOx emission in different fuel conditions
The effects of recirculation ratio on NOx emissions in three different kinds of fuel type,
namely, biomass only, co-combustion and coal only are showed in Figure 11. From Figure
11, the same result was obtained, NOx emission increased firstly then decreased sharply,
there is almost no NOx emission when recirculation ratio reaches to 10% in all three fuel
-16 conditions, and the results further certificated the above explanation for the characteristic of
NOx emission. More important things, the level of NOx emission during biomass
combustion process is strikingly lower than that of coal combustion process, and the level
of NOx emission in co-combustion is lower than coal combustion, but higher than biomass
combustion. The first and obvious reason is the nitrogen component in biomass is lower
than that of coal (Table 4). Munir (2010) analyzed the mechanism of the reduction NOx in
biomass co-combustion[2]. According to Munir, there are two more possible explanations.
Firstly, the volatile matter in biomass is higher than that in coal, and thus, the more percent
of biomass replaced coal as fuel, the more volatile was released, the formation of NH3 and
HCN increased with increasing fuel volatility, so the predominant combustion is gas-phase
reactions. Secondly, as biomass contain less carbon and are highly oxygen compared coal
(Table 4), the amount of stoichiometric air is less than coal combustion, it can cause locally
stronger reducing environment with addition of biomass without changing the air supply
conditions.
When considering the flue gas recirculation in volumetric combustion system, the
recirculation ratios corresponding to NOx emission reach to 0mg/Nm3 are different, 10%, 4%
and less 3% in coal only combustion, co-combustion and biomass only, respectively.
Obviously, the more percent of biomass in co-combustion, the less flue gas need to recycle
for realizing zero NOx emission. In other words, in order to realize NOx zero emission in
volumetric combustion, more drive power for recirculation is needed during coal
combustion process, and less drive power for recirculation is needed during biomass
combustion. There is no doubt that this is very interesting result, because volumetric
combustion of biomass is possible for realizing zero NOx emission with lower investment
cost and higher reliable.
4. APPLICATIONS OF VOLUMETRIC COMBUSTION
4.1 Application concepts
Biomass co-combustion in volumetric combustion system is different from
conventional co-combustion. In volumetric combustion, biomass combust blendly under a
high temperature oxygen deficient atmosphere led by a strong internal flue gas recirculation.
As analyzed before, volumetric combustion system has several benefits as bellow:

A lower NOx emission (both Fuel-NOx and Thermal-NOx),
-17-

A lower particle matter (PM) emissions,

A higher uniformity temperature profile, thus, a higher heat transfer, a higher
exergy efficiency,

A possibility for a bigger cofiring blends due to better combustion controlling,

A lower corrosion possibility

Keep the same steam parameters at the max.
Based on those benefits, 100% biomass as fuel combust in power plant boiler is more
realizable. Application concepts of volumetric combustion in a real boiler can be seen as
Figure 13. One is used rotating secondary air supplied via high velocity air nozzles in upper
furnace. Bigger momentum of high velocity air promotes intensive internal recirculation of
flue gases from the secondary combustion zone down to primary combustion zone. In this
way, cold secondary air swirl down along furnace wall, and hot flue gas from primary
combustion zone swirl up inside furnace, as showed in Figure 13(a). Another application
concept is located a wall tube inside furnace, the same with high velocity secondary air
injection, bigger momentum of high velocity air promotes intensive internal recirculation as
well. But, the differences from first concept are cold secondary air swirl down inside
furnace, and hot flue gas swirl up along furnace wall, as showed in Figure 13(b).
(a)
(b)
Figure 13. Two concepts of volumetric combustion in boiler
-18 4.2 Application example
A example of using volumetric combustion is the Rotating Opposed Fired Air
(ROFA), a boosted over-fire air system that includes a patented rotation process[20], which
is belong to the first application concept. For intensive recirculation and mixing in furnace,
maybe there are several methods. On a typical ROFA System installation, 25-40% of the
total combustion air is injected into the upper furnace through special asymmetricallyplaced air nozzles, which creates a sub-stoichiometric condition at the burner area, which
significantly decreases NOx formations.
Several different biomass fuels combustion with/without ROFA was operated and
tested in real boiler, the alternative biomass fuels were straw pellets, willow pellets and
wood pellets. Figure 14 shows that the results of biomass co-combustion test, with NOx
emissions and LOI (Loss on Ignition) in the fly ash as a function of BSR (Burner
Stoichiometric Ratio). In Figure 14, LNB-NOx means that coal combustion with LNB
(lower NOx burner) condition; ROFA-NOx means coal combustion with ROFA condition;
Biomass-NOx means 45% biomass and 55%coal co-combustion with ROFA condition.
Both the two operation conditions are same with that showed in Table 5. The NOx
emissions for the varying biomass fuels co-combustion shows significantly decreasing
compared to coal-only combustion.
Figure 14. NOx and LOI as a function of BCR for varying biomass fuel tests [14]
-19 5. CONCLUSIONS
A new concept of volumetric combustion was proposed in this paper, which intensive
internal recirculation and good mixing between secondary air and flue gas, the combustion
volume is larger than with traditional staged combustion. Several benefits showed in
volumetric combustion system, those benefits contribute the possibility of realizing 100%
biomass combustion in boiler. In this paper, the properties of wood pellets combustion in
high temperature oxidizer were investigated firstly. Then an Aspen model was used to
describe and studied volumetric combustion system with three different kind of fuel,
namely, biomass only, co-combustion and coal only, and compared the CO2 and NOx
emissions. At last, two application concepts of volumetric combustion were discussed, one
application example of volumetric combustion shows striking emissions reduction. The
main interesting conclusions are as follows:
1. Wood pellets were ignited and combusted much faster than coal pellets, when the
oxidizer was preheated to 1000°C, the ignition time to be almost independent of
oxygen concentration. However, the ignition time highly dependent on oxygen
concentration at low temperature (600°C).
2. NOx emissions decreased with the recirculation ratio of flue gas increasing, and
there is almost no NOx emission when recirculation ratio approaches to 10% in all
fuel conditions, namely, coal only, co-combustion and biomass only.
3. Volumetric combustion is a suitable technology for combusting high percent of
biomass with higher boiler efficiency, and higer reduction of CO2 and NOx
emission.
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7. ACKONWLEDGEMENTS
The authors would like to thank the IndComb AB Sweden and EIT/KIC Innoenergy for
the financial support of this work.
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