Combustion characteristics of waste sludge at air and oxy-fuel

Fuel 170 (2016) 92–99
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Fuel
journal homepage: www.elsevier.com/locate/fuel
Combustion characteristics of waste sludge at air and oxy-fuel
combustion conditions in a circulating fluidized bed reactor
Ha-Na Jang a, Jeong-Hun Kim b, Seung-Ki Back a, Jin-Ho Sung a, Heung-Min Yoo a,b, Hang Seok Choi a,
Yong-Chil Seo a,⇑
a
b
Dept. of Environmental Engineering, Yonsei University, Wonju 220-710, Republic of Korea
National Institute of Environmental Research, Inchon 404-708, Republic of Korea1
h i g h l i g h t s
Comparative combustion performance of waste sewage sludge in air and oxy-fuel conditions.
Pressure drop and temperature distribution in air and oxy-fuel combustion.
Comparison of ash composition in air and oxy-fuel combustion.
Comparison of flue-gas composition in air and oxy-fuel combustion.
a r t i c l e
i n f o
Article history:
Received 5 November 2014
Received in revised form 14 December 2015
Accepted 15 December 2015
Available online 28 December 2015
Keywords:
Circulating fluidized bed
Oxy-fuel combustion
Waste sewage sludge
Carbon dioxide
Carbon capture and storage
a b s t r a c t
Oxy-fuel combustion and circulating fluidized bed (CFB) technologies were applied to waste sewage
sludge using a cold-bed and a 30 kWth CFB pilot test bed. In the cold-bed tests, the minimum fluidization
velocity (umf) and the superficial velocity were determined as 0.120 m/s and 2.5 m/s, respectively. In pilot
tests, the combustion characteristics of waste sewage sludge in oxy-fuel condition were very distinctive
compared with that in air condition in terms of operation parameters such as the distribution of pressure
drop and temperature, flue-gas temperature, and the composition of ash and flue-gas. Based on cold-bed
and pilot bed tests, the oxygen injection rate was optimized in the range from 21% to 25% in oxy-fuel condition for waste sludge combustion to apply oxy-fuel combustion and CFB technologies with considering
technologies for stable and economic operation.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, the demand for renewable energy has increased
worldwide, because the releases of greenhouse gases from fossil
fuel will not be acceptable in the near future. As one of the renewable energy resources, the generation of waste sewage sludge in
Korea has gradually increased over the years. Finally, it will reach
over 10 million tons in 2015. Since ocean dumping was inhibited
due to the London Convention, the combustion of waste sewage
sludge has emerged as one of the alternative disposal options, with
the added potential benefit of energy recovery technologies for
waste to energy. Waste sewage sludge combustion has also been
utilized in the paper and pulp industries. In most plants to combust
such industrial waste sludge, fluidized bed combustion technology
⇑ Corresponding author. Tel.: +82 10 5373 2114; fax: +82 33 760 2571.
1
E-mail address: seoyc@yonsei.ac.kr (Y.-C. Seo).
Current address.
http://dx.doi.org/10.1016/j.fuel.2015.12.033
0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
was adopted in order to produce superheated steam to convert into
electricity or heat. Fluidized bed combustion (FBC) has been considered as a common technology for waste sewage sludge due to
the fuel-like characteristics of waste sewage sludge composed of
lots of organic components. There are lots of researches of combustion characteristics of waste sewage sludge using FBC technology.
Ogada and Werther [1] studied combustion characteristics of
dewatered sludge using laboratory and semi-pilot scale bubbling
FBC reactor. They observed that sludge combustion had distinctive
phenomenon compared with coal combustion due to lots of moisture and volatiles in the sludge. Sludge combustion undergoes series of reaction processes such as drying, de-volatilization, and char
combustion. Time spans were different with each combustion process, which was indicated the location these processes occurred.
Latva-Somppi et al. [2] compared the combustion characteristics
from co-combustion of paper sludge with biomass using industrial
BFBC and CFBC with fuel injection capacity of 10.8 ton/h and
10.4 ton/h, respectively. The bed temperature of BFBC was slightly
H.-N. Jang et al. / Fuel 170 (2016) 92–99
higher than that of CFBC. In addition, the temperature difference
from bed region to freeboard region was higher in BFBC than that
in CFBC because circulating material distributed the heat temperature efficiently along with CFBC reactor. Numerous researchers
focused on the combustion of volatiles of waste sewage sludge.
According to those researches, de-volatilization was occurred particular location such as flame zone and freeboard in which were
occurred significant temperature difference in the reactor. Moreover, fluidization velocity and more even temperature distribution
would be important for the homogenous combustion and heat
recovery [3–6]. In addition, ash and metals emission in waste sewage sludge combustion would be important because waste sewage
sludge contained significant portion of ash in which mostly consisted of heavy and trace metals. Along with this, researchers
focused on emission characteristics of major elements such as Al,
Ca, Na, K, Pb, Cd, Cr, Cu, and Ni from waste sewage sludge combustion using FBC reactor. Rink et al. [7] studied combustion characteristics of paper sludge using 300 kW fluidized bed combustion
facility with bubbling fluidization mode. There were locally high
temperature regions in the reactor using bubbling FBC. In this
region, high temperature caused the melting process of refractory
metals and alkali metals in waste sewage sludge with the formation of alkali-silicates in those temperature ranges. Char carbon
concentration gradually decreased as oxygen concentration and
bed temperature increased. Cenni et al. [8] studied behavior of
metals Cr, Mn, Ni, Pb, Zn by co-firing of drying sewage sludge during pulverized coal combustion using 500 kW pulverized fuel combustion chamber. Metal contents in the ash increased as sludge
mixing rate increased because the composition of flue-gas could
affect the condensation temperature of metal species by blending
rate of sewage sludge with coal. Corella and Toledo [9] studied
behavior of six metals such as Cd, Ni, Cr, Zn, Cu, and Pb using BFBC
pilot plant. Lopes et al. [10] studied heavy metal distribution from
co-combustion of sewage sludge and coal using pilot BFBC. They
observed that heavy metals were differently distributed by the
ash types such as bottom ash and fly ash. Amand and Leckner
[11] studied metal emission from co-combustion of sewage sludge
with coal or wood using 12 MWth commercial CFB boiler. The
amounts of ash and major elements increased as the sludge mixing
rate increased. Barbosa et al. [12] studied chemical and ecotoxicological properties of ashes by the co-combustion of coal
and sewage sludge. Co-combustion of sewage sludge occurred high
concentration of metals in bottom ash and fly ash. In series of those
researches, the composition of heavy metals in ash was unstable
because the difference of temperature distribution in the BFBC
reactor was higher than that in CFBC. Most researches focused on
the combustion characteristics of BFBC or co-combustion of waste
sewage sludge as second fuel in CFBC. The capacity of commercial
FBC plants ranged from 100 to 200 tons of waste sewage sludge
combustion per day. Those FBC plants have utilized excess air for
combustion, and have generated lots of carbon dioxide which is
one of the important greenhouse gases. Due to this global warming, CCS technology has been developed in order to reduce carbon
dioxide from stationary emission sources primarily from the fossil
fuels combustion. There are different types of carbon dioxide capture technologies, such as pre-combustion, post-combustion, and
oxy-fuel combustion. Oxy-fuel combustion is in the demonstration
phase and uses high purity oxygen in a combustion medium for
coal fired power plants [13]. During oxy-fuel combustion, a combination of oxygen typically with a greater than 95% purity level and
recycled flue gas are utilized for the combustion of the fuel. By
recycling the flue gas, a gas consisting primarily of carbon dioxide
is generated with a carbon dioxide concentration over 90%, which
is ready for sequestration without stripping of the carbon dioxide
from the gas stream [14]. Therefore, oxy-fuel combustion technology for newly constructed and retrofitted waste sewage sludge FBC
93
plants would be necessary to generated high purity carbon dioxide
in the flue-gas. In an earlier study, oxy-fuel combustion was
demonstrated by the pilot test performance of a pulverized coal
power plant. The earliest study of coal oxy-fuel combustion on a
pilot scale was conducted for the Argonne National Laboratory
(ANL) by the Energy and Environmental Research Corporation
(EERC) in their 3 MWth pilot facility. The results of that study indicated that oxy-fuel combustion would be capable of being successfully applied as a retrofit to a wide range of utility boiler and
furnace systems [15–17]. A pilot scale study conducted by the
International Flame Research Foundation (IFRF) evaluated the
combustion of pulverized coal in a mixture of O2 and recycled flue
gas with the primary consideration of retrofitting an existing boiler, while increasing the carbon dioxide concentration to above
90% for carbon dioxide capture [15]. A study by the IHI company
demonstrated combustion characteristics such as the flame temperature as the function of oxygen concentration, NOx conversion
rate, and SOx emission during oxy-fuel combustion in a 1.2 MWth
test furnace [15,18]. A study by the Air Liquide and the Babcock &
Wilcox (B&W) company demonstrated the combustion process
based on O2 enriched flue gas recirculation in order to provide an
easy-to-implement option for multi-pollutant control, including
carbon dioxide capture suitable for retrofitting an existing boiler.
The test showed that the oxy-fuel combustion generated less
NOx than air combustion. In addition, it showed the effective
removal performance of SOx with wet flue-gas desulfurization
(FGD) equipment and a significant reduction of mercury emissions
[15]. CANMET has been developing oxy-fuel combustion technology by conducting many theoretical and practical studies. In a
0.3 MWth capacity pilot-scale combustor, the coal combustion
behavior in various mixtures of oxygen and carbon dioxide were
studied in order to demonstrate the effects of several factors on
the combustion performance including the oxygen concentration,
recycled ratio, and burner performance, etc. [15,19]. In addition,
the CANMET organization designed a 0.8 MWth pilot plant for
the demonstration and evaluation of the oxy-fuel CFB combustion
process. The feedstock utilized in the tests included a pet-coke, a
bituminous coal, and a sub-bituminous coal. Along with pulverized
coal power plants, fluidized bed coal power plants with oxy-fuel
combustion have been demonstrated by pilot test performance
[20]. The VTT Technical Research Centre conducted a 0.1 MWth
pilot test under air and oxy-fuel combustion conditions using
seven different fuels including anthracite coal, bituminous coal,
lignite coal, and 2 different types of biomass such as wood pellet
and straw pellet. The flue gas emissions were measured and ash
samples were collected in different tests in order to evaluate the
combustion efficiency and emission performance. The primary flue
gas emissions, such as carbon dioxide, CO, NO, and SO2 were measured during all of the tests [21]. In addition, there were several
on-going demonstration projects for oxy-fuel combustion by several institutes worldwide [22–24]. However, all of the pilot and
demonstration projects were conducted primarily using coal fossil
fuel to demonstrate the oxy-fuel combustion of coal-fired power
plants because it has been most abundant fuel to be utilized for
energy production. There have been very limited studies reported
on oxy-fuel combustion technology to be applied any waste feedstocks. Major waste feedstock for oxy-fuel combustion has done
by wood biomass due to its abundance and convenient transporting. Arias et al. [25] studied combustion characteristics of coal and
biomass by comparing air and oxy-fuel combustion. The ignition
temperature and burnout rate were different by blending rate of
biomass with coal and oxygen injection rate in oxy-fuel combustion. According to Fryda et al. [26] deposition and fouling rate in
oxy-fuel combustion were significantly different between coal
and co-combustion of biomass with coal. In several researches,
they observed that oxygen injection rate was significantly
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H.-N. Jang et al. / Fuel 170 (2016) 92–99
dependent with the type of fuel since combustion characteristics of
waste feedstocks were very different in comparison with coal combustion [25–27]. Since CFBC technology has lots of advantages to
apply for heat recovery facility, waste sewage sludge combustion
should be retrofitted or newly commercialized to CFBC facility.
Besides, commercial FBC facility for waste sewage sludge should
be retrofitted to carbon dioxide reduction facility with applying
oxy-fuel combustion technology as carbon capture and storage
(CCS) technology. In this study, the concept of CFB with oxy-fuel
combustion technology as a means of waste to energy technology
and carbon capture has been applied to waste sewage sludge combustion. Cold-bed tests in a CFB model were conducted to simulate
the combustion of waste sewage sludge using the 30 kWth oxyfuel pilot test bed. To optimize oxy-fuel combustion conditions of
waste sewage sludge, pilot tests using 30 kWth CFB test bed was
designed, constructed, and operated to compare the combustion
performances and flue-gas composition with different oxygen
injection rate.
2. Facilities and experimental methods
2.1. CFB cold-bed
The preliminary simulation test for operating a 30 kWth CFB
oxy-fuel pilot test bed was conducted with the CFB cold-bed consisting of a riser, a cyclone, a down-comer, and a loop-seal. The
CFB cold-bed has a riser with an inner diameter of 0.15 m and a
height of 6.4 m, which was the same size as used for the 30 kWth
CFB oxy-fuel pilot test bed. The fluidization material for the coldbed simulation was the sand with a diameter of 315 lm and a density of 1461 kg/Nm3. The fluidization air was fed into a riser and a
loop-seal in the CFB cold-bed. The pressure drops along with the
height were observed in order to optimize the fluidization of the
sand material in the CFB. At steady state, the entire solid flow from
cyclone into down-comer was measured to determine solid circulating rate through the CFB loop seal. In the measuring column of
down-comer on cold-bed tests, circulated fluidization material
(sand) was measured during time constant. Solid circulation rate
was able to be measured by the analysis of bulk density and measured time. Table 1 shows the experimental conditions of each
design factor in the CFB cold-bed.
2.2. Sampling and analysis
The heating value of the waste sewage sludge was analyzed by
an AC-350 calorimeter from the LECO Corporation in St. Joseph, MI,
USA. The proximate analysis was conducted by TGA-601 from
LECO Corporation in MI, USA. The elemental analysis was conducted by 2400 seriesIICHNS/O analyzer from Perkin Elmer Company in MA, USA. The analysis of the gaseous components such
as carbon dioxide, carbon monoxide, and oxygen in the flue-gas
was conducted by a portable gas analyzer. For the calculation of
mass balance, the assumption of the conversion of fuel carbon to
carbon monoxide and carbon dioxide enable to calculate by measuring flue-gas flow rate and composition in terms of carbon
monoxide and carbon dioxide. In addition, carbon contents of bottom ash and fly ash were measured. The temperature of test bed
maintained isothermal condition for operation.
2.3. 30 kWth CFB oxy-fuel pilot test bed
The pilot test was conducted in the CFB combustion system
consisting of a riser, a cyclone, a down-comer, and a loop-seal.
The CFB combustor has a riser with an inner diameter of 0.15 m
and a height of 6.4 m. The CFB combustor was surrounded by
Table 1
Experimental conditions in the CFB cold-bed.
Design factor
Value
Bed diameter (m)
Sand material diameter (lm)
Sand material density (kg/m3)
Operation temperature (°C)
0.15
315
1461
25
heating materials in order to deliver the combustion heat to the
waste sewage sludge during the air and oxy-fuel combustion.
The optimum temperature for the waste sewage sludge combustion was determined to be 800 °C. The feeding rate of the waste
sewage sludge was optimized at 13 kg/h. The experimental tests
were carried out in the 30 kWth CFB combustor operating with
air and oxy-fuel conditions. The oxygen injection rate in oxy-fuel
conditions was varied from 21% to 40%, which was fed into the
riser. Once test bed was initiated, test operation was maintained
for 3 h per day with constant sludge feeding rate, oxygen injection
rate, and solid circulating rate to verify stable combustion conditions. Series of tests in air and oxy-fuel conditions with 21%, 25%,
30%, and 40% of oxygen injection rates were conducted for a week.
Those tests in air and oxy-fuel conditions with 21%, 25%, 30%, and
40% of oxygen injection rate were repeated in three times for three
weeks. Constant operation conditions and stability of CFB system
were constantly observed by data acquisition system and continuous analyzing systems. All data were measured during each test
period. Fig. 1 shows a schematic diagram of the 30 kWth CFB
oxy-fuel pilot test bed. Table 2 shows the experimental conditions
using the 30 kWth CFB oxy-fuel pilot test bed.
3. Results and discussion
3.1. Cold-bed test
3.1.1. Minimum fluidization velocity (umf) [28–30]
Experiments were conducted in a CFB cold-bed in order to figure out the proper operating conditions for combustion in a
30 kWth CFB oxy-fuel pilot test bed. Fig. 2 shows the variation in
the pressure drop from the bottom to the top of the riser as a function of the superficial velocity. If the gas flow rate through the fixed
bed was increased, the pressure drop due to the fluid drag continued to rise as per Eq. (1), until the superficial velocity reached a
critical value known as the minimum fluidization velocity, umf.
The pressure drop per unit height of a packed bed, Dpfr/Lm of uniformly sized particles, dp is correlated as follows [28]:
Dpfr
ð1 em Þ2 luo
1 em qg u20
g c ¼ 150
þ 1:75
2
3
Lm
em
e3m /s dp
ð/s dp Þ
ð1Þ
At the onset of fluidization the frictional pressure drop equals
the pressure drop caused by the weight of particles. The superficial
velocity at minimum fluidizing conditions, umf, is found by combining Eqs. (1) and (3). In general, for isotropic-shaped solids, umf is
correlated as follows [29,30]:
1:75
e3mf us
2
dp umf qg
l
þ
150ð1 emf Þ dp umf qg
e3mf u2s
l
dp qgðqs qg Þg
3
¼
l2
ð2Þ
1:75
e3mf us
Re2p;mf þ
150ð1 emf Þ
e3mf u2s
Rep;mf ¼ Ar
ð3Þ
Based on the equations given above, the minimum fluidization
velocity was calculated and compared with the experimental
results from the pressure drop changes with the fluidization air
H.-N. Jang et al. / Fuel 170 (2016) 92–99
95
Fig. 1. Schematic diagram of the 30 kWth CFB oxy-fuel pilot test bed.
Table 2
Experimental conditions using the 30 kWth CFB oxy-fuel pilot test bed.
Design factor
Value
Bed diameter (m)
Fuel feeding rate (kg/hr)
Solid fuel mixing rate (%)
Oxygen injection rate (%)
Combustion temperature (°C)
Flow rate (L/min)
0.15
13
0–30
21–40
800
900
3.1.2. External solid circulation rate
In circulating fluidized bed boilers, hot solids circulate around
whole furnace carrying heat from burning fuel to heat-absorbing
surfaces, and finally leave the furnace with the flue-gas. As shown
Fig. 3, the fast fluidization was observed to initiate from 2.5 m/s of
air superficial velocity when the solid circulation rate was above
10 kg/m2 s. In fast fluidization in CFB boilers, external solid circulation rate of Geldart particle B was ranged from 10 to 50 kg/m2 s [30].
According to Horio et al., the optimized solid circulation rate in CFB
reactor was ranged from 10 to 30 kg/m2 s [31]. Therefore, in our
work, fast fluidization is considered to have initiated from 2.5 m/s.
3.2. Test results for the 30 kWth CFB oxy-fuel pilot test bed
Fig. 2. Pressure drop of the riser as a function of the superficial velocity.
velocity. The calculated value of minimum fluidization velocity
was 0.097 m/s which was 19% below the experimental value. As
shown in the figure, the minimum fluidization velocity (umf) was
determined as 0.120 m/s.
3.2.1. Characteristics of waste sewage sludge
Table 3 shows the basic characteristics of the waste sewage
sludge. In the proximate analysis, the volatile and ash fraction in
the waste sewage sludge were 45.11% and 35.04%, respectively.
The calorific values of the sludge and coal were 3008 and
5966 kcal/kg, respectively. Due to the higher ash fraction, the total
fraction of organic materials composed of carbon, hydrogen, and
oxygen in the sludge primarily concerned in the combustion reaction were lower than those of typical coal. The nitrogen fraction in
the waste sewage sludge was higher than that of coal. The sulfur
fraction in the sludge and the coal was 0.43% and 0.58%, respectively. The theoretical oxygen demand based on the elemental
analysis of the sludge was calculated to be 835 L/min when feeding
13 kg/h of waste sewage sludge in the 30 kWth CFB oxy-fuel pilot
test bed. Based on the theoretical oxygen demand, the input flow
rate of air and oxy-fuel mixture was set at 900 L/min for the present study.
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H.-N. Jang et al. / Fuel 170 (2016) 92–99
Fig. 3. Solid circulating rate as a function of the superficial velocity.
Table 3
Results of the basic characteristic analysis of waste sewage sludge.
Proximate analysis (wt.%)
Moisture
Volatile
Fixed carbon
Ash
–
Calorific value (kcal/kg)
Element analysis (wt.%)
7.32
45.11
12.25
35.04
–
3008
Selected metals analysis
Heavy metals analysis (ppm)
Al
Ca
K
Na
–
–
–
Carbon
Hydrogen
Nitrogen
Oxygen
Sulfur
Chloride
28.14
4.74
4.43
23.90
0.43
0.053
Trace metals analysis
(ppm)
21,700.0
11,204.7
8249.0
5874.9
–
–
–
Zn
Cu
Pb
Cr
Ni
As
Cd
635.4
305.2
44.1
42.5
30.8
6.1
1.6
Fig. 4. Pressure profiles along the CFB pilot plant with air and oxy-fuel combustion.
3.2.2. Temperature and pressure profile
Fig. 4 shows that the pressure profile from the distributor to the
loop-seal in the reactor during the waste sewage sludge combustion, when air and oxy-fuel mixtures with oxygen injection rate
ranged from 21% to 40% were fed into the reactor. As shown in
the figure, the pressure profile was depicted as a typical pressure
trend of the CFB at each point in air and those of oxy-fuel combustion. The pressure drop of air combustion was higher than that of
oxy-fuel combustion with oxygen injection rate ranged from 21%
to 40%. Also, for oxy-fuel combustion, the pressure drop tended
to increase as the oxygen injection rate was increased. It was indicated that the pressure drop was resulted from the loss of dynamic
energy from the pathway of fluidization air which was related to
those parameters such as kinematic viscosity (t), density (q),
length of tube (L), and flow rate (Q). Table 4 shows the physical
gas properties of fluid flow for components in the fluidizing media
which was utilized as fluidization air such as oxygen, nitrogen, and
carbon dioxide. The kinematic viscosity of air combustion, which
fluidization air was composed of nitrogen and oxygen, was higher
than that of oxy-fuel combustion. The pressure drop was increased
as the kinematic viscosity of fluidization air was increased. Also,
the pressure drop of oxy-fuel combustion, when fluidization air
was composed of carbon dioxide and oxygen, was increased as
the oxygen rate was increased from 21% to 40% because the kinematic viscosity of fluidization air was increased as the oxygen rate
was increased. Fig. 5 shows the temperature profile in the riser
during the waste sewage sludge combustion along the height of
the reactor in the air and oxy-fuel combustion with oxygen injection rate ranged from 21% to 40% in a combustion medium. The
temperature profile in the riser was uniformly depicted as a function of the height in the air and oxy-fuel combustion. The temperature profile of the air combustion was relatively higher than that
of 21% of oxy-fuel combustion. As shown Table 4, the heat capacity
of carbon dioxide was much higher than that of nitrogen. It was
considered that flame temperature of 21% oxygen injection rate
in waste sewage sludge combustion was lower than that of air
combustion because waste sewage sludge contained lots of volatiles, and de-volatilization and total ignition time of waste sewage
sludge at this oxygen rate were delayed due to larger capacity of
carbon dioxide substituted for nitrogen in waste sewage sludge
combustion. However, the temperature trend of oxy-fuel combustion above 25% was higher than that of air and 21% oxy-fuel combustion. In addition, the temperature trend was gradually
increased as oxygen injection rate increased. It was indicated that
oxy-fuel combustion up to 25% occurred to high flame temperature
compared to air and 21% of oxy-fuel combustion in waste sewage
sludge combustion, and de-volatilization and total ignition time
were rapidly decreased with increasing oxygen rate.
3.2.3. Analysis of flue-gas and selected elements
Fig. 6(a) shows comparative concentration of selected heavy
and trace metals in the fly ash from the air and oxy-fuel waste sewage sludge combustion with oxygen injection rate ranged from 21%
to 40%. As shown figure, the species of Aluminum (Al), Potassium
(K) and Calcium (Ca) were dominant in the fly ash from waste
sewage sludge combustion in the air and oxy combustion. It was
Table 4
Physical gas properties of fluid flow utilized as fluidization air.
Density (q) (kg/m )
Thermal conductivity (k) (W/m k)
Specific heat capacity (cp) (J/mol °C)
Kinematic viscosity (m2/s)
3
H2O
O2
N2
CO2
Ratio, CO2/N2
0.157
0.136
45.67
3.20e04
0.278
0.087
36.08
2.09e04
0.244
0.082
34.18
2.00e04
0.383
0.097
57.83
1.31e04
1.6
1.2
1.7
0.7
H.-N. Jang et al. / Fuel 170 (2016) 92–99
Fig. 5. Temperature profiles along the height of the riser from the air and oxy-fuel
combustion.
considered that those species are concerned with particle formation mechanism such as vaporization, condensation, and coagulation, which caused the growth of fly ash from waste sludge
combustion. In waste sewage sludge combustion, agglomeration
and fouling are highly associated with Ca compounds and alkali
(K, Na) compounds as species of chlorides and sulfates, which inhibit fluidization and heat exchange in CFB boiler. Al compounds are
related with chlorine-induced corrosion, which occurs mainly at
the convective heat exchangers in CFB waste sewage sludge combustion facility. This chlorine-induced corrosion problem associated with Al compounds causes serious damage of convective
material and highly increases whole maintenance cost of CFB
waste sewage sludge combustion facility. Fig. 6(b) shows comparative concentration of selected heavy and trace metals in the bottom ash from the air and oxy-fuel waste sewage sludge
combustion with oxygen injection rate ranged from 21% to 40%.
97
As shown figure, the concentrations of Al, K and Ca compounds
in bottom ash were more decreased in the range from 21% to
25% of oxy-fuel condition than air and oxy-fuel condition above
30%. It was indicated that oxy-fuel combustion with the range from
21% to 25% would mitigate agglomeration, fouling and corrosion
problems from waste sewage sludge combustion, and economically beneficial in terms of long time operation of waste sewage
sludge combustion facility. As shown figure (a) and (b), Zn and
Cu compounds in bottom ash and fly ash from waste sewage
sludge combustion showed as similar trends with heavy metal
compounds in both ashes, which were more decreased in the range
from 21% to 25% of oxy-fuel combustion than air combustion.
Those results are related the difference of combustion characteristics in air and oxy-fuel combustion. In oxy-fuel combustion, the
composition of surrounding gases composition was totally different with air combustion. As shown Table 4, carbon dioxide which
was injected as dilution gas in oxy-fuel combustion has more heat
capacity than nitrogen as dilution gas in air combustion. Therefore,
carbon dioxide was able to absorb more heat in oxy-fuel condition
than that in air condition, which causes the drop-down of the adiabatic flame temperature in combustion surroundings. Eventually,
lower adiabatic temperature in oxy-fuel condition occurs to delay
ignition time of waste sewage sludge. As a result, it would occur to
delay of particle growth and formation, which was explained as
pathway in nucleation, vaporization, condensation and coagulation
mechanisms of those metals. However, Cr and Ni compounds in
both ashes showed as a different trend with previous metals,
which were more increased in those ranges of oxy-fuel combustion
than air combustion. It could be explained that the particle size
became finer and smaller in oxy-fuel condition because particle
have not enough time to grow due to the delay of ignition time.
It could attribute much concentration of Cr and Ni in bottom and
fly ash from waste sludge combustion in oxy-fuel condition
because those metals have high melting temperature which was
Fig. 6. Concentrations of heavy and trace metals in ash from air and oxy-fuel waste sewage sludge combustion: (a) fly ash and (b) bottom ash.
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H.-N. Jang et al. / Fuel 170 (2016) 92–99
Fig. 7. Flue-gas composition and temperature in the air and oxy-fuel combustion.
higher than adiabatic flame temperature in air and oxy-fuel conditions. Fig. 7 shows the flue gas temperature and composition from
the air and oxy-fuel combustion with oxygen injection rate ranged
from 21% to 40%. In the case of the flue-gas temperature, the temperature was relatively higher with the oxy-fuel combustion with
oxygen injection rate ranged from 21% to 25% than that of air and
oxy-fuel combustion up to 30%. It was considered that steam and
carbon dioxide in flue-gas had higher heat capacity than nitrogen
and oxygen as shown Table 4. As a result, the temperature of
flue-gas retaining larger steam and carbon dioxide was higher than
that of typical air and oxy-fuel combustion over 30% in waste sewage sludge combustion. In addition, waste sewage sludge has lots
of volatiles, and the heat released by their combustion in waste
sewage sludge was transferred further along with CFB combustion
system with those range than that of the air and oxy-fuel combustion at oxygen injection rate over 30% in waste sludge combustion.
In air combustion, the emission of carbon dioxide was 15.3% with
waste sewage sludge combustion. In oxy-fuel combustion, carbon
dioxide concentration in flue-gas was reached higher than 80% at
the range from 21% to 25% of oxygen injection rate. The concentration of excess oxygen in flue-gas was lower than 10% at the range
from 21% to 25% input oxygen. Based on experimental results of
this study, it was indicated that oxy-fuel combustion at the range
from 21% to 25% would be more beneficial to apply CCS technology
and to operate long time period than other cases in waste sewage
sludge combustion with considering operation parameters such as
distribution of pressure drop and temperature, solid circulating
rate, flue-gas temperature, ash and selected metal composition,
and flue-gas composition. As next step, flue-gas recirculation test
will be conducted to achieve higher purity over than 90% to verify
the possibility of oxy-fuel combustion technology as CCS ready
technology in an oxy-fuel CFB combustion system for burning
waste sewage sludge.
4. Conclusions
The comparative tests in air and oxy-fuel combustion with different oxygen injection rate using a cold-bed and a 30 kWth CFB
pilot test bed were conducted in order to verify the applicability
of a CCS technology with CFB system for waste sewage sludge combustion. Based on those experimental tests, major results are summarized as follows.
1. The flue-gas temperatures of the oxy-fuel combustion with oxygen injection rate ranged from 21% to 25% were higher than the
other cases. It was considered that steam and carbon dioxide
had higher heat capacity than nitrogen and oxygen. In addition,
the temperature of flue-gas retaining larger steam and carbon
dioxide was higher than that of air and oxy-fuel combustion
over 30% in waste sewage sludge combustion.
2. In heavy metal analysis, Al, K and Ca compounds in bottom ash
and fly ash from waste sewage sludge combustion were more
decreased in the range from 21% to 25% of oxy-fuel combustion
than air combustion and other cases. It was acknowledged that
oxy-fuel combustion with the range from 21% to 25% would
mitigate agglomeration, fouling and corrosion problems from
waste sewage sludge combustion.
3. Carbon dioxide concentration in flue-gas from waste sewage
sludge combustion was achieved over than 80% of purity with
the range from 21% to 25% of oxy-fuel combustion. Also, the
concentration of excess oxygen in flue-gas was lower than
10% at those ranges.
4. Based on those experimental results from this study, it was
indicated that the range from 21% to 25% of oxy-fuel combustion was more beneficial to apply a CCS technology, an efficient
energy recovery system, and an economical operation for longtime period for waste sewage sludge combustion using CFB
system.
Acknowledgements
This work was supported by the New & Renewable Energy Core
Technology Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) Granted financial resource from
the Ministry of Trade, Industry & Energy, Republic of Korea (No.
20113010130050, No. 20143010091790). This work is financially
supported by Korea Ministry of Environment (MOE) as Waste to
Energy and Recycling/Knowledge-based Environmental Service
Human Resource Development Project as well.
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