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biomass blends with solid heat carrier
FUPROC-04642; No of Pages 7 Fuel Processing Technology xxx (2015) xxx–xxx
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Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier Min Guo a,b, Ji-Cheng Bi a,⁎ a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 25 January 2015 Received in revised form 12 July 2015 Accepted 13 July 2015 Available online xxxx Keywords: Coal Biomass Co-pyrolysis Solid heat carrier Poly-generation
a b s t r a c t A series of experiments on co-pyrolysis of subbituminous coal/corn stalk blends with solid heat carrier (boiler ash) were carried out in a bench-scale moving bed pyrolyzer with different biomass blending ratios at different temperatures. Deviation of gaseous, liquid and char products yields between experimental values and calculated ones was discussed. Some characteristics of char and char-ash after co-pyrolysis were investigated, such as elemental analysis, heat value, ash melting point and ash composition. Results indicated that with increasing blending ratio of corn stalk and temperature, both gas and tar yields increased. Gas heat value decreased with increasing corn stalk blending ratio while increased with rising temperature. Compared with coal pyrolysis alone, more light-oil and water were found during the co-pyrolysis process. The char-ash showed little impact on boiler combustion ash at the blending ratio of biomass less than 30%. Application of co-pyrolysis in a polygeneration system was evaluated environmentally and economically. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Biomass is getting increasing attention as a potential resource of renewable energy due to high yield of volatile matter, possibility of being converted into liquid fuels and chemicals, low contents of sulfur and nitrogen as well as zero net emission of CO2. However, the drawbacks of biomass, such as low energy density, seasonal, difficult to collect and transport etc., limit its utilization greatly. One of the most promising solutions is to co-utilize with coal, such as co-combustion [1–4], co-gasification [5–8] and co-pyrolysis [8,9]. These solutions cannot only avoid the disadvantages of biomass and coal, but also exert the advantages of them simultaneously. Since co-pyrolysis process of coal and biomass can produce gaseous and liquid products with high added value, its research has been paid more and more attention in recent years. Many researchers used different fuels (such as sawdust [10,11], legume straw [12], lignite [13–17], and bituminous coal [18]), and different reactors (such as TGA [19–22], fluidized bed reactor [8], fixed-bed reactor [11], and free fall reactor [12]) under various operating parameters (such as temperature [23], heating rate [24], blending ratio, particle size, and contacting way of particles) to study the copyrolysis behaviors focusing on product distributions and product characteristics, as well as the possible existed synergetic effects. Although the conclusions are conflicting, but there are some certain laws: under the conditions of fast heating rate and good contact of ⁎ Corresponding author. E-mail address: [email protected] (J.-C. Bi).
coal/biomass particles, the synergistic effects are obvious; in contrast under the condition of slow heating rate and fluidized bed reactor, the synergistic effects are rarely found. But so far, there is scarcely report on application of co-pyrolysis of coal and biomass, especially by using coal-ash from a boiler as solid heat carrier. In order to realize the efficient and clean utilization of coal, a polygeneration process, which was coupled circulating fluidized bed (CFB) combustion (15 t/h coal and gangue feeding, 75 t/h steam output) with coal pyrolysis (5 t/h coal feeding), was finished in a pilot-scale [25]. In the poly-generation process, a pyrolyzer was set beside the CFB boiler and connected with it. High-temperature ash from the CFB boiler was quantitatively transported into the pyrolyzer to provide heat for coal pyrolysis, getting gas and tar. The char produced in the pyrolyzer was returned to the CFB boiler for combustion, getting heat or power. Thus, the poly-generation of gas, tar, heat and power can be obtained simultaneously in the process. The coal pyrolysis with the ash has been researched previously in our laboratory [26–28]. Because of the high content of volatile matter in the biomass, if adding some biomass into the pyrolyzer to co-pyrolyze with coal, it may produce more valuable gas and tar, which can improve the utilization of biomass and the economic benefits. Co-pyrolysis behaviors of coal and biomass with the hightemperature boiler ash as solid heat carrier were investigated in this work. The yields of gas, liquid, char and their characteristics were studied at different biomass blending ratios and at different pyrolysis temperatures since they were the most important parameters in the fast co-pyrolysis process. In addition, the application of co-pyrolysis in
http://dx.doi.org/10.1016/j.fuproc.2015.07.018 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
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M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
a poly-generation system was evaluated environmentally and economically. The results may provide useful information for the application of coal/biomass co-pyrolysis system. 2. Experimental 2.1. Materials The coal used was Fugu subbituminous coal (FSBC) from Fugu coal field, one of the main areas producing subbituminous coals in China. The biomass used was corn stalk (CS) from Taiyuan Shanxi because corn was one of the main food crops in the two provinces (Shaanxi and Shanxi) and a great deal of corn stalks were produced annually. The air-dried coal samples were crushed to the size of below 4.0 mm. In order to feed conveniently the air-dried corn stalks were first crushed to powder, and then granulated to cylinder form with diameter of 2.0 mm and length of 6.0 mm. The proximate analyses were performed according to GB/T 212–2001 (Chinese standards). The ultimate analysis was carried out using a CHNS/O Vario Micro Cube elemental analyzer. The results of the proximate and ultimate analyses of coal and granulous corn stalk particles are listed in Table 1. To eliminate the effect of moisture contained in the raw materials, prior to each test sufficient amounts of fuel samples were dried at 105 °C for more than two hours and then stored in a desiccator to prevent extra absorption of moisture from atmosphere. The FSBC/CS blends were prepared manually. The blending weight ratio of CS / (FSBC + CS) are 0%, 10%, 30%, 50%, 70%, 90% and 100%. The pyrolysis experiments were carried out at 600 °C and different blending ratios and also performed at CS blending ratio 70% and different temperatures. The solid heat carrier used in the study was high-temperature boiler ash fetched from a 75 t/h CFB boiler in a power plant. The composition of the boiler ash was analyzed by an X-ray fluorescence spectrometer Analyzer (S4 PIONEER Supplement, Bruker AXS). Table 2 shows some physical properties of the boiler ash. 2.2. Apparatus and procedure As shown in Fig. 1, the experimental setup consists of six essential sections. These sections are heat carrier feeder (Φ60 × 6 mm, 500 mm length), coal/biomass feeder (Φ50 × 6 mm, 200 mm length), pyrolyzer (Φ100 × 6 mm, 400 mm length), quenching tank (Φ140 × 6 mm, 120 mm length), cooling system and a temperature controller. In the upper part of the pyrolyzer, there is a mixer which makes the solid heat carrier and the coal/biomass mix fully to guarantee a rapid heat transfer between them. In order to make the condensable gas fully cooled, the cooling system consists of three condensers in series. The first is cooled by running water, and the second and third are further cooled by ice water placed in a stainless steel container. The temperature controller is connected by K-type thermocouples to the heat carrier feeder and pyrolyzer. The entire system is well sealed. For every experimental run, 1500 g of boiler ash was loaded into the heat carrier feeder, then was heated by the electrical heater to a set temperature, such as 800 °C, 750 °C or 700 °C, which made the
Table 1 Proximate and ultimate analyses (wt.%) of Fugu subbituminous coal and Corn stalk. Sample
FSBC CS
Proximate analysis (ad) M
A
V
4.82 3.54
3.20 5.50
36.35 73.94
HHV (MJ/kg)
29.91 15.75
Ultimate analysis (dry)
H/C
C
H
N
S
Oa
77.29 45.97
4.36 5.59
0.97 0.70
0.16 0.23
14.02 42.01
ad: air dried base; dry: dried base; M: Moisture; A: Ash; V: Volatiles. a By difference.
0.67 1.46
material pyrolyze at different temperature. The pyrolyzer was heated by the electrical heater to about 400 °C to make up for heat losses. Then 100 g of coal/biomass or their blends was loaded into the coal/ biomass feeder. Before experiments pure nitrogen was introduced into the whole setup to ensure air removed. After the heat carrier was heated to the desired temperature, the ash and the coal/biomass particles were dropped into the pyrolyzer at the same time by opening the valves under the two feeders. To prevent the loss of pyrolysis gases, the valves were instantly closed once the ash and the coal/biomass particles fell into the pyrolyzer. The ash and the coal/biomass particles were mixed sufficiently by gathering and scattering many times with the action of the internals in the mixer at the upper part of the pyrolyzer, which lasted a few seconds. At the same time, the coal/biomass particles were pyrolyzed preliminarily during the process of mixing. Then the mixed particles fell into the under part of the pyrolyzer and were pyrolyzed further. The volatile products flowed into the cooling system where the condensable volatiles were collected. The non-condensable gas at ambient temperature kept on flowing into a gas bag to collect for subsequent analysis. Experiments lasted for at least 20 min, until no further significant release of gas was observed. After the completion of pyrolysis, the residual solid blends of ash and char were discharged into the quenching tank to cool to the ambient environment and then were discharged to weigh and recorded. The solid char yield was calculated by the following formula: weight of char = A − (1500 − B), where A is the weight of the discharged blends of char and boiler ash, B is the weight of the leftover boiler ash in the heat carrier feeder. The gas was analyzed by the gas analyzer then to convert weight yield. The yield of liquid product was obtained by calculating weight increase of the cooling tubes which were weighed before test. In this study, each test was carried out at least two times to ensure the repeatability. 2.3. Products analysis method The chemical compositions of uncondensable gas from pyrolysis of the coal/biomass were analyzed by gas analyzers. Three types of gas analyzers were used to determine the composition of the gaseous product. The permanent gases, such as H2, CH4 and CO were analyzed with a SP-2305 gas chromatograph equipped with a TCD detector (5A molecular sieve column and pure argon as carrier gas). The hydrocarbon gases of C2–C4 were analyzed with a GC-1790 gas chromatography equipped with a FID detector (C18 column and pure nitrogen as carrier gas). The content of CO2 in the gas was analyzed by an Orsat gas analyzer. The liquid products included tar and water. Firstly, the liquid products were poured out from the condensers into a separating funnel where the liquid was separated into tar and water after setting at least 10 min. The tar remained in the condensers was rinsed by tetrahydrofuran (THF). Then THF was removed by a rotary ZFQ-85A evaporator at the conditions of 70 °C, atmospheric pressure and 50 rpm. The two parts of tar were blended together and then were extracted by n-hexane with an ultrasonic SB 25–12 DTDN extractor under the conditions of 40 kHz, 500 W, 25 °C, and 30 min. After extraction, the tar was separated into light-oil (n-hexane soluble) and asphaltene (n-hexane insoluble). Fig. 2 shows the analysis procedure of the liquid product. The characteristics of the solid char were analyzed including ultimate analysis, ash melting point and composition of the char-ash. Ultimate analysis of the char for carbon, hydrogen, nitrogen, and sulfur were carried out with a CHNS/O Elementary Analyzer (Vario Micro cube, Germany). The ash melting point was analyzed by an Ash Melting Point Analyzer (5E-AF-3, China). The composition of ash was analyzed by an XRF Analyzer (S4 PIONEER Supplement, Bruker AXS). In addition, to determine the possible synergetic effect during the co-pyrolysis process of FG/CS blends, the measured values were compared with the calculated ones from the additive Eq. (1) below.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
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Table 2 Physical properties of the boiler ash. Size (mm)
Density (kg/m3)
Composition (wt.%) SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
P2O5
0.2–4.0
1060
63.25
24.98
5.73
0.79
0.73
1.00
0.39
2.74
0.09
0.24
The additive equation assumed that there were no interactions between the FSBC and CS samples during the process of co-pyrolysis, so the calculated values were the sum of the values from individual samples with proportional to their blending weight ratio as Ycalc ¼ Ycoal ð1−BRÞ þ Ybiomass BR
ð1Þ
where Ycalc is the calculated values from the additive equation, Ycoal and Ybiomass are the measured values from the individual pyrolysis of coal and biomass, respectively, and BR is the blending ratio of biomass in the coal/biomass blends ranging from 0% to 100%.
temperature the yield of char decreases, and the yields of liquid and gas increase due to a greater decomposition of the samples at high temperature. But the change of the product yields is slow, because the range of pyrolysis temperature is not large. Fig. 3 demonstrates that over the whole blending ratio and pyrolysis temperature range, the yield of char is in good agreement with the calculated values basically. There is some deviation about the yields of liquid and gas between the experimental values and the calculated ones under the high corn stalk blending ratios and high temperatures. This indicated that the synergy effect happened in the volatiles, but not in the char.
3. Results and discussion
3.2. Properties of gas product
3.1. Distribution of product yields
As Fig. 4a shows, with the increase of corn stalk blending ratio, the content of CO2 and CO in the gas increase, while that of H2, CH4 and C2+ decrease because of the more and more oxygen contained in the blends as seen in the Fig. 5. The increasing of CO2 content from 25.52 vol.% to 35.18 vol.% results in the decrease of the gas calorific value from 32.07 MJ/m3 to 14.65 MJ/m3. As shown in Fig. 4b, at lower temperature 480 °C, CO2 and CO are the main gas component which contribute 57.24 vol.% and 31.78 vol.% of total gas product, respectively. This phenomenon can be attributed to the release of oxygenated gases from the decomposition of oxygenated functional groups at lower temperatures. In addition, the increase of the combustible gas with the increasing temperature leads to the increase of gas calorific value from 10.22 MJ/m3 to 16.86 MJ/m3. Fig. 4 also shows that the content
As Fig. 3a shows, when coal and corn stalk were pyrolyzed alone (0% and 100% of corn stalk, respectively) at the temperature of 600 °C, about 70 wt.% of raw corn stalk is converted into volatile matter, however, only about 15 wt.% of raw coal is converted into volatile matter. This is mainly because that the pyrolysis activity of biomass is higher than that of coal. The ether bonds (R–O–R) linked the macromolecular structure of biomass, are relatively weaker than the C_C bonds linked the dense polycyclic aromatic hydrocarbons of coal [12]. With increasing blending ratio, the char yields significantly decrease, but both the liquid and gas yields increase. This is because of the higher content of volatile matter (73.94 wt.%) in corn stalk. As Fig. 3b shows, with rising
Fig. 1. Schematic diagram of the experimental setup.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
Blends of tar/water
Blends of THF/tar
funnel
Tar
Tar
THF
90 80
CO2 : C2+ :
70
calc. calc. calc. calc. calc.
exp. exp. exp. exp. exp.
(a)
calorific value
35
30
60 25
50 40
20
30 20
15
10 0 0
Tar
Light-oil
Asphaltene
(n-hexane soluble)
(n-hexane insoluble)
Fig. 2. Analysis procedure of the liquid product.
90
(a)
70
exp. exp. exp.
20
30
40
50
60
70
80
90
10 100
90 80
CO2 : C2+ :
70
20
calc. calc. calc. calc. calc.
exp. exp. exp. exp. exp.
(b)
calorific value
3
H2 : CO : CH4 :
15
60 50 40 10
30 20 10 0
5
480
540
600
Pyrolysis temperature (°C)
60
Fig. 4. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the properties of gas products.
50 40
20
reactions in the volatiles, H2, CH4 and C2+ free radicals from biomass provides hydrogen donor to make the hydrogenation reactions of volatiles from coal [17].
10
3.3. Properties of liquid product
30
0 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) 90 80
(b)
70
char : liquid : gas :
calc. calc. calc.
exp. exp. exp.
60 50 40 30 20 10
As Fig. 6a shows, compared with coal pyrolysis alone, more tar, lightoil and water are found in the co-pyrolysis. Fig. 6b shows that the yields of tar and water increase with raising of temperature, but the yield of 45
Oxygen content in the blends (wt.%)
Yields of products (wt.%, dry)
calc. calc. calc.
char : liquid : gas :
Fraction of gas components (vol.%)
ultrasonic extraction with n-hexane
Yields of products (wt.%, dry)
10
Blending ratio of corn stalk (wt.%) 100
80
3
H2 : CO : CH4 :
Calorific value of gas product (MJ/m )
water
rotary evaporation
40
100
Calorific value of gas product (MJ/m )
of gaseous components is not consistent with the calculated value over the whole blending ratio range and the whole experimental temperature range. This is likely because that during the secondary
Fraction of gas components (vol.%)
4
40 35 30 25 20 15
0 480
540
600
Pyrolysis temperature (°C)
10 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) Fig. 3. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the product yields.
Fig. 5. Oxygen content of materials as a function of corn stalk blending ratio.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
Yields of water, tar and light-oil (wt.%, dry)
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
35
calc. calc. calc.
water : tar : light-oil :
30
another to produce secondary char and light gases [29]. Moreover, rapid evolution of atomic hydrogen from the biomass contributes to hydropyrolysis of coal and enhanced the production of light-oil [17]. In addition, the experimental values of water yield are less than the calculated ones either under the different blending ratios or at the different pyrolysis temperatures. This is likely due to that corn stalk pyrolysis produces a great deal of H2O and CO during the co-pyrolysis process, which promotes the water–gas shift reaction and consumes some water [11,29].
exp. exp. exp.
25 20 15 10
3.4. Properties of char
5
(a)
In the poly-generation system the char obtained from pyrolysis of raw material will be transported into the boiler for combustion. Thus, some of the properties of char such as elemental analysis, ash melting point and ash composition of char-ash were conducted in this study. As Table 3 lists, the content of carbon in the char decreases from 81.55 wt.% to 64.37 wt.% with increasing blending ratio from 0% to 100%, resulting in a decrease of caloric value from 28.39 MJ/kg to 22.71 MJ/kg. The content of ash in the char is concentrated from 4.02 wt.% to 19.28 wt.%. This is because that the content of biomass char in the mixing FSBC/CS chars increases with increasing blending ratio of corn stalk in the blends. In addition, the ash melting point of char-ash decreases with increasing blending ratio of corn stalk. This is attributed to the decreased content of Al2O3 and SiO2 (both of which can increase the ash melting point) and the increased content of K2O (which can decrease the ash melting point) in the char-ash. Coal is combusted in a circulating fluidized bed boiler usually below 1000 °C. Combustion operation may not be affected by the decrease in ash melting point of char-ash. As listed in Table 4, with raising the temperature, the content of carbon in the char increases from 72.70 wt.% to 75.62 wt.%, and the content of oxygen decreases from 13.55 wt.% to 8.95 wt.%. The caloric value of the char changes a little, just about 26–27 MJ/kg during the range of pyrolysis temperatures examined in this study. The content of ash in the char increases from 8.92 wt.% to 11.47 wt.% with increasing of the temperature from 480 °C to 600 °C. The table also shows that the ash melting point of the char-ash changes a little due to that the content of each component in the ash has a little change with increasing temperature.
0 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) Yields of water, tar and light-oil (wt.%, dry)
5
35
water : tar : light-oil :
30
calc. calc. calc.
exp. exp. exp.
25 20 15 10 5
(b)
0 480
540
600
Pyrolysis temperature (°C) Fig. 6. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the properties of liquid products.
light-oil reaches the maximum of 13.55 wt.% at the temperature of 530 °C, because under higher temperature, the more macromolecule matters (which are n-hexane insoluble) are obtained from the thermal decomposition of the coal/biomass blends. No matter at different blending ratios or at different temperatures, there are relatively large deviations of tar and light-oil yields between the experimental values and the calculated ones. This indicates that synergy effect occurred during the co-pyrolysis of coal and corn-stalk which not only increased the yield of tar, but also improved the tar quality. This is likely because that the volatiles escape from coal and corn stalks cracked secondly in the high temperature producing many active radical pieces. The relative small radicals from corn stalk volatiles providing hydrogen donor stabilize large radicals generated from coal volatiles. Stabilization causes these radicals to be released as tar rather than crosslinking with one
3.5. Properties of solid heat carrier In order to figure out whether the co-pyrolysis process or the materials has an effect on the heat carrier, some of the boiler ash was picked out by hand from the blends of char and ash to analyze after pyrolysis experiments. As Table 5 lists, the heat carriers have little change before and after the experiments. This illustrates that the high-temperature boiler ash just provides heat for the pyrolysis process, and it can be returned into the boiler and will not influence the operation of the boiler.
Table 3 Properties of char obtained with different CS contents. CS content (%) 0 10 30 50 70 90 100
Ultimate analysis (wt.%) C
H
N
S
O
81.55 81.27 79.17 78.41 75.62 68.29 64.37
1.83 2.70 2.09 1.97 2.49 2.18 2.23
1.30 1.31 1.27 1.20 1.20 1.10 1.04
0.14 0.15 0.16 0.19 0.26 0.38 0.43
11.15 10.48 11.37 9.65 8.95 12.24 12.64
Ash (wt.%)
HHV (MJ/kg)
Ash melting point (°C) DT
ST
HT
FT
Ash composition (wt.%) Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
4.02 4.08 5.93 8.57 11.47 15.79 19.28
28.39 29.65 27.82 27.58 27.24 24.06 22.71
1255 1228 1220 1187 1110 1050 1036
1268 1295 1256 1192 1132 1102 1118
1284 1322 1285 1218 1155 1122 1133
1342 1329 1314 1264 1202 1153 1160
18.93 15.51 12.60 9.75 7.10 5.19 3.59
10.90 10.93 11.20 9.83 10.50 10.00 11.80
4.60 6.16 4.58 4.76 5.53 8.05 3.85
0.69 4.77 9.36 13.25 16.39 18.50 21.14
1.81 3.03 4.63 5.52 6.69 7.39 9.03
0.46 0.82 1.50 1.70 1.82 2.02 1.60
0.40 0.70 1.33 1.71 2.03 2.26 2.02
7.75 7.10 7.15 6.86 6.97 6.35 5.13
1.13 0.86 0.68 0.51 0.33 0.23 0.15
50.09 45.22 45.37 43.64 41.50 38.72 31.91
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
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M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
Table 4 Properties of char obtained at different temperatures. Temperature (°C)
Ultimate analysis (wt.%) C
H
N
S
O
480 530 600
72.70 73.85 75.62
3.16 2.86 2.49
1.32 1.30 1.20
0.34 0.31 0.26
13.55 11.92 8.95
Ash (wt.%)
HHV (MJ/kg)
Ash melting point (°C)
Ash composition (wt.%)
DT
ST
HT
FT
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
8.92 9.75 11.47
26.55 26.76 27.24
1110 1120 1110
1126 1146 1132
1171 1172 1155
1218 1243 1202
6.67 7.40 7.10
10.92 10.40 10.50
4.51 4.89 5.53
15.57 15.79 16.39
6.83 6.78 6.69
2.08 2.06 1.82
2.08 2.07 2.03
7.18 7.07 6.97
0.24 0.37 0.33
41.63 41.50 41.50
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature.
Table 5 Properties of heat carrier (boiler ash) before and after the experiments. Heat carrier
Boiler ash (Original) Boiler ash (0%) Boiler ash (50%) Boiler ash (100%) Boiler ash (480 °C) Boiler ash (530 °C) Boiler ash (600 °C)
Ash melting point (°C)
Ash composition (wt.%)
DT
ST
HT
FT
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
1435 1438 1439 1436 1437 1433 1447
1494 1511 1508 1510 1486 1486 1519
1541 1541 1542 1550 1549 1550 1550
1550 1550 1550 1550 1550 1550 1550
24.98 25.41 25.46 24.93 25.49 25.43 25.08
0.79 0.76 0.77 0.70 0.68 0.72 0.73
4.73 4.30 4.06 4.70 4.12 4.06 4.25
2.74 2.85 2.80 2.80 2.90 2.88 3.00
0.73 0.77 0.75 0.75 0.77 0.77 0.77
0.09 0.09 0.08 0.06 0.12 0.10 0.07
0.24 0.20 0.20 0.18 0.16 0.17 0.16
0.39 0.35 0.24 0.36 0.26 0.29 0.27
1.00 1.03 1.06 1.01 1.04 1.02 1.02
64.25 64.24 64.50 64.58 64.14 64.26 64.51
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature; Boiler ash (50%): Heat carrier used when co-pyrolysis of FSBC/CS blends at the blending ratio of 50% CS; Boiler ash (480 °C): Heat carrier used when co-pyrolysis of FSBC/CS blends at the temperature of 480 °C.
4. Conclusions
Due to the different characteristics of biomass compared with coal, if the co-pyrolysis of coal and biomass is applied in a poly-generation system, it may affect the boiler or the whole poly-generation system. Owing to the higher content of volatile matter in biomass, the yields of high added value tar and gas obtained from co-pyrolysis of coal and biomass can increase, which may improve the economic benefits of the system. Because of the lower sulfur and nitrogen contents in biomass, it may reduce desulfurization and denitrogenation loads of the boiler. In addition, biomass is CO2 zero emission. These facts can improve the environmental benefits of the system. Due to the higher content of K and lower content of Si and Al in biomass, when the biomass char returns into the boiler burning, it may have an impact on the slagging of the boiler. In order to ensure feeding smoothly, about 30% biomass should be added in the pyrolysis reactor. So, the co-pyrolysis of 70% FSBC adding 30% CS at 600 °C is especially analyzed as an example to investigate the effect of the co-pyrolysis behavior on the poly-generation system or the boiler. As demonstrated in Fig. 7, compared with coal pyrolysis alone, copyrolysis of FSBC/CS increases the yield of tar by 36.77% and that of light-oil by 16.03%. Although the calorific value of gas decreases by 35.07%, the yield of gas increases by 112.98%. Considering these data comprehensively, it can be concluded that the co-pyrolysis of coal/biomass can enhance the economic benefits of the polygeneration system. After co-pyrolysis, the sulfur contained in coal and biomass is distributed in gas, liquid and solid products. As Fig. 7 shows, the sulfur distributed in char accounts for 72.22% after FSBC pyrolysis. But after co-pyrolysis of FSBC/CS, the sulfur distributed in char accounted for 61.88%, which can reduce the desulfurization load 14.32% for the boiler. Similarly, the denitrogenation load can be reduced 8.49%. In addition, co-pyrolysis of coal/biomass can reduce the CO2 net emission. All of these can improve the environmental benefits of the poly-generation system. Besides, it can be seen from Section 3.4 that the ash melting point of co-pyrolysis char-ash decreases a little in the co-pyrolysis of FSBC/CS at the blending ratio of 30% CS, which will not affect slagging of the boiler. These data can provide design and operation for the application of coal/biomass co-pyrolysis in a poly-generation system.
The experimental study of co-pyrolysis of Fugu subbituminous coal and corn stalk were conducted with high-temperature boiler ash as solid heat carrier. Both the corn stalk blending ratio and temperature showed influence on the yields and properties of three products. The Deviation of gaseous, liquid products yields between experimental values and calculated ones were large, but that of char yields almost no changed. The synergy effect occurred in the volatiles during the co-pyrolysis process, which made the yield of tar and light-tar increased, the gas yield decreased. The boiler ash just provided heat for the pyrolysis and its properties were almost not changed after the experiments. The application of co-pyrolysis in a poly-generation system may improve the economic and
35
100 90
30
80 25
70 60
3
20
Heat value of gas (MJ/m ) and Char (MJ/kg)
FSBC 70% FSBC + 30% CS
50 15
40 30
10
20 5
r eo va
at He
He
at
va
lu
lu
n ge tro ni
fc
fg eo
in
n ri lfu
0
ha
as
ar ch
ar ch
ar Ch su
Ga
il t-o Li
gh
r
0
s
10
Ta
Yields of products and distributed S and N in char (%)
3.6. Effect of co-pyrolysis on poly-generation system
Fig. 7. Characteristics of the products from FSBC pyrolysis and FSBC/CS co-pyrolysis.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
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Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier Min Guo a,b, Ji-Cheng Bi a,⁎ a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 25 January 2015 Received in revised form 12 July 2015 Accepted 13 July 2015 Available online xxxx Keywords: Coal Biomass Co-pyrolysis Solid heat carrier Poly-generation
a b s t r a c t A series of experiments on co-pyrolysis of subbituminous coal/corn stalk blends with solid heat carrier (boiler ash) were carried out in a bench-scale moving bed pyrolyzer with different biomass blending ratios at different temperatures. Deviation of gaseous, liquid and char products yields between experimental values and calculated ones was discussed. Some characteristics of char and char-ash after co-pyrolysis were investigated, such as elemental analysis, heat value, ash melting point and ash composition. Results indicated that with increasing blending ratio of corn stalk and temperature, both gas and tar yields increased. Gas heat value decreased with increasing corn stalk blending ratio while increased with rising temperature. Compared with coal pyrolysis alone, more light-oil and water were found during the co-pyrolysis process. The char-ash showed little impact on boiler combustion ash at the blending ratio of biomass less than 30%. Application of co-pyrolysis in a polygeneration system was evaluated environmentally and economically. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Biomass is getting increasing attention as a potential resource of renewable energy due to high yield of volatile matter, possibility of being converted into liquid fuels and chemicals, low contents of sulfur and nitrogen as well as zero net emission of CO2. However, the drawbacks of biomass, such as low energy density, seasonal, difficult to collect and transport etc., limit its utilization greatly. One of the most promising solutions is to co-utilize with coal, such as co-combustion [1–4], co-gasification [5–8] and co-pyrolysis [8,9]. These solutions cannot only avoid the disadvantages of biomass and coal, but also exert the advantages of them simultaneously. Since co-pyrolysis process of coal and biomass can produce gaseous and liquid products with high added value, its research has been paid more and more attention in recent years. Many researchers used different fuels (such as sawdust [10,11], legume straw [12], lignite [13–17], and bituminous coal [18]), and different reactors (such as TGA [19–22], fluidized bed reactor [8], fixed-bed reactor [11], and free fall reactor [12]) under various operating parameters (such as temperature [23], heating rate [24], blending ratio, particle size, and contacting way of particles) to study the copyrolysis behaviors focusing on product distributions and product characteristics, as well as the possible existed synergetic effects. Although the conclusions are conflicting, but there are some certain laws: under the conditions of fast heating rate and good contact of ⁎ Corresponding author. E-mail address: [email protected] (J.-C. Bi).
coal/biomass particles, the synergistic effects are obvious; in contrast under the condition of slow heating rate and fluidized bed reactor, the synergistic effects are rarely found. But so far, there is scarcely report on application of co-pyrolysis of coal and biomass, especially by using coal-ash from a boiler as solid heat carrier. In order to realize the efficient and clean utilization of coal, a polygeneration process, which was coupled circulating fluidized bed (CFB) combustion (15 t/h coal and gangue feeding, 75 t/h steam output) with coal pyrolysis (5 t/h coal feeding), was finished in a pilot-scale [25]. In the poly-generation process, a pyrolyzer was set beside the CFB boiler and connected with it. High-temperature ash from the CFB boiler was quantitatively transported into the pyrolyzer to provide heat for coal pyrolysis, getting gas and tar. The char produced in the pyrolyzer was returned to the CFB boiler for combustion, getting heat or power. Thus, the poly-generation of gas, tar, heat and power can be obtained simultaneously in the process. The coal pyrolysis with the ash has been researched previously in our laboratory [26–28]. Because of the high content of volatile matter in the biomass, if adding some biomass into the pyrolyzer to co-pyrolyze with coal, it may produce more valuable gas and tar, which can improve the utilization of biomass and the economic benefits. Co-pyrolysis behaviors of coal and biomass with the hightemperature boiler ash as solid heat carrier were investigated in this work. The yields of gas, liquid, char and their characteristics were studied at different biomass blending ratios and at different pyrolysis temperatures since they were the most important parameters in the fast co-pyrolysis process. In addition, the application of co-pyrolysis in
http://dx.doi.org/10.1016/j.fuproc.2015.07.018 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
2
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
a poly-generation system was evaluated environmentally and economically. The results may provide useful information for the application of coal/biomass co-pyrolysis system. 2. Experimental 2.1. Materials The coal used was Fugu subbituminous coal (FSBC) from Fugu coal field, one of the main areas producing subbituminous coals in China. The biomass used was corn stalk (CS) from Taiyuan Shanxi because corn was one of the main food crops in the two provinces (Shaanxi and Shanxi) and a great deal of corn stalks were produced annually. The air-dried coal samples were crushed to the size of below 4.0 mm. In order to feed conveniently the air-dried corn stalks were first crushed to powder, and then granulated to cylinder form with diameter of 2.0 mm and length of 6.0 mm. The proximate analyses were performed according to GB/T 212–2001 (Chinese standards). The ultimate analysis was carried out using a CHNS/O Vario Micro Cube elemental analyzer. The results of the proximate and ultimate analyses of coal and granulous corn stalk particles are listed in Table 1. To eliminate the effect of moisture contained in the raw materials, prior to each test sufficient amounts of fuel samples were dried at 105 °C for more than two hours and then stored in a desiccator to prevent extra absorption of moisture from atmosphere. The FSBC/CS blends were prepared manually. The blending weight ratio of CS / (FSBC + CS) are 0%, 10%, 30%, 50%, 70%, 90% and 100%. The pyrolysis experiments were carried out at 600 °C and different blending ratios and also performed at CS blending ratio 70% and different temperatures. The solid heat carrier used in the study was high-temperature boiler ash fetched from a 75 t/h CFB boiler in a power plant. The composition of the boiler ash was analyzed by an X-ray fluorescence spectrometer Analyzer (S4 PIONEER Supplement, Bruker AXS). Table 2 shows some physical properties of the boiler ash. 2.2. Apparatus and procedure As shown in Fig. 1, the experimental setup consists of six essential sections. These sections are heat carrier feeder (Φ60 × 6 mm, 500 mm length), coal/biomass feeder (Φ50 × 6 mm, 200 mm length), pyrolyzer (Φ100 × 6 mm, 400 mm length), quenching tank (Φ140 × 6 mm, 120 mm length), cooling system and a temperature controller. In the upper part of the pyrolyzer, there is a mixer which makes the solid heat carrier and the coal/biomass mix fully to guarantee a rapid heat transfer between them. In order to make the condensable gas fully cooled, the cooling system consists of three condensers in series. The first is cooled by running water, and the second and third are further cooled by ice water placed in a stainless steel container. The temperature controller is connected by K-type thermocouples to the heat carrier feeder and pyrolyzer. The entire system is well sealed. For every experimental run, 1500 g of boiler ash was loaded into the heat carrier feeder, then was heated by the electrical heater to a set temperature, such as 800 °C, 750 °C or 700 °C, which made the
Table 1 Proximate and ultimate analyses (wt.%) of Fugu subbituminous coal and Corn stalk. Sample
FSBC CS
Proximate analysis (ad) M
A
V
4.82 3.54
3.20 5.50
36.35 73.94
HHV (MJ/kg)
29.91 15.75
Ultimate analysis (dry)
H/C
C
H
N
S
Oa
77.29 45.97
4.36 5.59
0.97 0.70
0.16 0.23
14.02 42.01
ad: air dried base; dry: dried base; M: Moisture; A: Ash; V: Volatiles. a By difference.
0.67 1.46
material pyrolyze at different temperature. The pyrolyzer was heated by the electrical heater to about 400 °C to make up for heat losses. Then 100 g of coal/biomass or their blends was loaded into the coal/ biomass feeder. Before experiments pure nitrogen was introduced into the whole setup to ensure air removed. After the heat carrier was heated to the desired temperature, the ash and the coal/biomass particles were dropped into the pyrolyzer at the same time by opening the valves under the two feeders. To prevent the loss of pyrolysis gases, the valves were instantly closed once the ash and the coal/biomass particles fell into the pyrolyzer. The ash and the coal/biomass particles were mixed sufficiently by gathering and scattering many times with the action of the internals in the mixer at the upper part of the pyrolyzer, which lasted a few seconds. At the same time, the coal/biomass particles were pyrolyzed preliminarily during the process of mixing. Then the mixed particles fell into the under part of the pyrolyzer and were pyrolyzed further. The volatile products flowed into the cooling system where the condensable volatiles were collected. The non-condensable gas at ambient temperature kept on flowing into a gas bag to collect for subsequent analysis. Experiments lasted for at least 20 min, until no further significant release of gas was observed. After the completion of pyrolysis, the residual solid blends of ash and char were discharged into the quenching tank to cool to the ambient environment and then were discharged to weigh and recorded. The solid char yield was calculated by the following formula: weight of char = A − (1500 − B), where A is the weight of the discharged blends of char and boiler ash, B is the weight of the leftover boiler ash in the heat carrier feeder. The gas was analyzed by the gas analyzer then to convert weight yield. The yield of liquid product was obtained by calculating weight increase of the cooling tubes which were weighed before test. In this study, each test was carried out at least two times to ensure the repeatability. 2.3. Products analysis method The chemical compositions of uncondensable gas from pyrolysis of the coal/biomass were analyzed by gas analyzers. Three types of gas analyzers were used to determine the composition of the gaseous product. The permanent gases, such as H2, CH4 and CO were analyzed with a SP-2305 gas chromatograph equipped with a TCD detector (5A molecular sieve column and pure argon as carrier gas). The hydrocarbon gases of C2–C4 were analyzed with a GC-1790 gas chromatography equipped with a FID detector (C18 column and pure nitrogen as carrier gas). The content of CO2 in the gas was analyzed by an Orsat gas analyzer. The liquid products included tar and water. Firstly, the liquid products were poured out from the condensers into a separating funnel where the liquid was separated into tar and water after setting at least 10 min. The tar remained in the condensers was rinsed by tetrahydrofuran (THF). Then THF was removed by a rotary ZFQ-85A evaporator at the conditions of 70 °C, atmospheric pressure and 50 rpm. The two parts of tar were blended together and then were extracted by n-hexane with an ultrasonic SB 25–12 DTDN extractor under the conditions of 40 kHz, 500 W, 25 °C, and 30 min. After extraction, the tar was separated into light-oil (n-hexane soluble) and asphaltene (n-hexane insoluble). Fig. 2 shows the analysis procedure of the liquid product. The characteristics of the solid char were analyzed including ultimate analysis, ash melting point and composition of the char-ash. Ultimate analysis of the char for carbon, hydrogen, nitrogen, and sulfur were carried out with a CHNS/O Elementary Analyzer (Vario Micro cube, Germany). The ash melting point was analyzed by an Ash Melting Point Analyzer (5E-AF-3, China). The composition of ash was analyzed by an XRF Analyzer (S4 PIONEER Supplement, Bruker AXS). In addition, to determine the possible synergetic effect during the co-pyrolysis process of FG/CS blends, the measured values were compared with the calculated ones from the additive Eq. (1) below.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
3
Table 2 Physical properties of the boiler ash. Size (mm)
Density (kg/m3)
Composition (wt.%) SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
P2O5
0.2–4.0
1060
63.25
24.98
5.73
0.79
0.73
1.00
0.39
2.74
0.09
0.24
The additive equation assumed that there were no interactions between the FSBC and CS samples during the process of co-pyrolysis, so the calculated values were the sum of the values from individual samples with proportional to their blending weight ratio as Ycalc ¼ Ycoal ð1−BRÞ þ Ybiomass BR
ð1Þ
where Ycalc is the calculated values from the additive equation, Ycoal and Ybiomass are the measured values from the individual pyrolysis of coal and biomass, respectively, and BR is the blending ratio of biomass in the coal/biomass blends ranging from 0% to 100%.
temperature the yield of char decreases, and the yields of liquid and gas increase due to a greater decomposition of the samples at high temperature. But the change of the product yields is slow, because the range of pyrolysis temperature is not large. Fig. 3 demonstrates that over the whole blending ratio and pyrolysis temperature range, the yield of char is in good agreement with the calculated values basically. There is some deviation about the yields of liquid and gas between the experimental values and the calculated ones under the high corn stalk blending ratios and high temperatures. This indicated that the synergy effect happened in the volatiles, but not in the char.
3. Results and discussion
3.2. Properties of gas product
3.1. Distribution of product yields
As Fig. 4a shows, with the increase of corn stalk blending ratio, the content of CO2 and CO in the gas increase, while that of H2, CH4 and C2+ decrease because of the more and more oxygen contained in the blends as seen in the Fig. 5. The increasing of CO2 content from 25.52 vol.% to 35.18 vol.% results in the decrease of the gas calorific value from 32.07 MJ/m3 to 14.65 MJ/m3. As shown in Fig. 4b, at lower temperature 480 °C, CO2 and CO are the main gas component which contribute 57.24 vol.% and 31.78 vol.% of total gas product, respectively. This phenomenon can be attributed to the release of oxygenated gases from the decomposition of oxygenated functional groups at lower temperatures. In addition, the increase of the combustible gas with the increasing temperature leads to the increase of gas calorific value from 10.22 MJ/m3 to 16.86 MJ/m3. Fig. 4 also shows that the content
As Fig. 3a shows, when coal and corn stalk were pyrolyzed alone (0% and 100% of corn stalk, respectively) at the temperature of 600 °C, about 70 wt.% of raw corn stalk is converted into volatile matter, however, only about 15 wt.% of raw coal is converted into volatile matter. This is mainly because that the pyrolysis activity of biomass is higher than that of coal. The ether bonds (R–O–R) linked the macromolecular structure of biomass, are relatively weaker than the C_C bonds linked the dense polycyclic aromatic hydrocarbons of coal [12]. With increasing blending ratio, the char yields significantly decrease, but both the liquid and gas yields increase. This is because of the higher content of volatile matter (73.94 wt.%) in corn stalk. As Fig. 3b shows, with rising
Fig. 1. Schematic diagram of the experimental setup.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
Blends of tar/water
Blends of THF/tar
funnel
Tar
Tar
THF
90 80
CO2 : C2+ :
70
calc. calc. calc. calc. calc.
exp. exp. exp. exp. exp.
(a)
calorific value
35
30
60 25
50 40
20
30 20
15
10 0 0
Tar
Light-oil
Asphaltene
(n-hexane soluble)
(n-hexane insoluble)
Fig. 2. Analysis procedure of the liquid product.
90
(a)
70
exp. exp. exp.
20
30
40
50
60
70
80
90
10 100
90 80
CO2 : C2+ :
70
20
calc. calc. calc. calc. calc.
exp. exp. exp. exp. exp.
(b)
calorific value
3
H2 : CO : CH4 :
15
60 50 40 10
30 20 10 0
5
480
540
600
Pyrolysis temperature (°C)
60
Fig. 4. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the properties of gas products.
50 40
20
reactions in the volatiles, H2, CH4 and C2+ free radicals from biomass provides hydrogen donor to make the hydrogenation reactions of volatiles from coal [17].
10
3.3. Properties of liquid product
30
0 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) 90 80
(b)
70
char : liquid : gas :
calc. calc. calc.
exp. exp. exp.
60 50 40 30 20 10
As Fig. 6a shows, compared with coal pyrolysis alone, more tar, lightoil and water are found in the co-pyrolysis. Fig. 6b shows that the yields of tar and water increase with raising of temperature, but the yield of 45
Oxygen content in the blends (wt.%)
Yields of products (wt.%, dry)
calc. calc. calc.
char : liquid : gas :
Fraction of gas components (vol.%)
ultrasonic extraction with n-hexane
Yields of products (wt.%, dry)
10
Blending ratio of corn stalk (wt.%) 100
80
3
H2 : CO : CH4 :
Calorific value of gas product (MJ/m )
water
rotary evaporation
40
100
Calorific value of gas product (MJ/m )
of gaseous components is not consistent with the calculated value over the whole blending ratio range and the whole experimental temperature range. This is likely because that during the secondary
Fraction of gas components (vol.%)
4
40 35 30 25 20 15
0 480
540
600
Pyrolysis temperature (°C)
10 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) Fig. 3. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the product yields.
Fig. 5. Oxygen content of materials as a function of corn stalk blending ratio.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
Yields of water, tar and light-oil (wt.%, dry)
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
35
calc. calc. calc.
water : tar : light-oil :
30
another to produce secondary char and light gases [29]. Moreover, rapid evolution of atomic hydrogen from the biomass contributes to hydropyrolysis of coal and enhanced the production of light-oil [17]. In addition, the experimental values of water yield are less than the calculated ones either under the different blending ratios or at the different pyrolysis temperatures. This is likely due to that corn stalk pyrolysis produces a great deal of H2O and CO during the co-pyrolysis process, which promotes the water–gas shift reaction and consumes some water [11,29].
exp. exp. exp.
25 20 15 10
3.4. Properties of char
5
(a)
In the poly-generation system the char obtained from pyrolysis of raw material will be transported into the boiler for combustion. Thus, some of the properties of char such as elemental analysis, ash melting point and ash composition of char-ash were conducted in this study. As Table 3 lists, the content of carbon in the char decreases from 81.55 wt.% to 64.37 wt.% with increasing blending ratio from 0% to 100%, resulting in a decrease of caloric value from 28.39 MJ/kg to 22.71 MJ/kg. The content of ash in the char is concentrated from 4.02 wt.% to 19.28 wt.%. This is because that the content of biomass char in the mixing FSBC/CS chars increases with increasing blending ratio of corn stalk in the blends. In addition, the ash melting point of char-ash decreases with increasing blending ratio of corn stalk. This is attributed to the decreased content of Al2O3 and SiO2 (both of which can increase the ash melting point) and the increased content of K2O (which can decrease the ash melting point) in the char-ash. Coal is combusted in a circulating fluidized bed boiler usually below 1000 °C. Combustion operation may not be affected by the decrease in ash melting point of char-ash. As listed in Table 4, with raising the temperature, the content of carbon in the char increases from 72.70 wt.% to 75.62 wt.%, and the content of oxygen decreases from 13.55 wt.% to 8.95 wt.%. The caloric value of the char changes a little, just about 26–27 MJ/kg during the range of pyrolysis temperatures examined in this study. The content of ash in the char increases from 8.92 wt.% to 11.47 wt.% with increasing of the temperature from 480 °C to 600 °C. The table also shows that the ash melting point of the char-ash changes a little due to that the content of each component in the ash has a little change with increasing temperature.
0 0
10
20
30
40
50
60
70
80
90
100
Blending ratio of corn stalk (wt.%) Yields of water, tar and light-oil (wt.%, dry)
5
35
water : tar : light-oil :
30
calc. calc. calc.
exp. exp. exp.
25 20 15 10 5
(b)
0 480
540
600
Pyrolysis temperature (°C) Fig. 6. Effects of (a) blending ratio (at the same temperature 600 °C), (b) temperature (at the same CS blending ratio 70%) on the properties of liquid products.
light-oil reaches the maximum of 13.55 wt.% at the temperature of 530 °C, because under higher temperature, the more macromolecule matters (which are n-hexane insoluble) are obtained from the thermal decomposition of the coal/biomass blends. No matter at different blending ratios or at different temperatures, there are relatively large deviations of tar and light-oil yields between the experimental values and the calculated ones. This indicates that synergy effect occurred during the co-pyrolysis of coal and corn-stalk which not only increased the yield of tar, but also improved the tar quality. This is likely because that the volatiles escape from coal and corn stalks cracked secondly in the high temperature producing many active radical pieces. The relative small radicals from corn stalk volatiles providing hydrogen donor stabilize large radicals generated from coal volatiles. Stabilization causes these radicals to be released as tar rather than crosslinking with one
3.5. Properties of solid heat carrier In order to figure out whether the co-pyrolysis process or the materials has an effect on the heat carrier, some of the boiler ash was picked out by hand from the blends of char and ash to analyze after pyrolysis experiments. As Table 5 lists, the heat carriers have little change before and after the experiments. This illustrates that the high-temperature boiler ash just provides heat for the pyrolysis process, and it can be returned into the boiler and will not influence the operation of the boiler.
Table 3 Properties of char obtained with different CS contents. CS content (%) 0 10 30 50 70 90 100
Ultimate analysis (wt.%) C
H
N
S
O
81.55 81.27 79.17 78.41 75.62 68.29 64.37
1.83 2.70 2.09 1.97 2.49 2.18 2.23
1.30 1.31 1.27 1.20 1.20 1.10 1.04
0.14 0.15 0.16 0.19 0.26 0.38 0.43
11.15 10.48 11.37 9.65 8.95 12.24 12.64
Ash (wt.%)
HHV (MJ/kg)
Ash melting point (°C) DT
ST
HT
FT
Ash composition (wt.%) Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
4.02 4.08 5.93 8.57 11.47 15.79 19.28
28.39 29.65 27.82 27.58 27.24 24.06 22.71
1255 1228 1220 1187 1110 1050 1036
1268 1295 1256 1192 1132 1102 1118
1284 1322 1285 1218 1155 1122 1133
1342 1329 1314 1264 1202 1153 1160
18.93 15.51 12.60 9.75 7.10 5.19 3.59
10.90 10.93 11.20 9.83 10.50 10.00 11.80
4.60 6.16 4.58 4.76 5.53 8.05 3.85
0.69 4.77 9.36 13.25 16.39 18.50 21.14
1.81 3.03 4.63 5.52 6.69 7.39 9.03
0.46 0.82 1.50 1.70 1.82 2.02 1.60
0.40 0.70 1.33 1.71 2.03 2.26 2.02
7.75 7.10 7.15 6.86 6.97 6.35 5.13
1.13 0.86 0.68 0.51 0.33 0.23 0.15
50.09 45.22 45.37 43.64 41.50 38.72 31.91
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
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M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
Table 4 Properties of char obtained at different temperatures. Temperature (°C)
Ultimate analysis (wt.%) C
H
N
S
O
480 530 600
72.70 73.85 75.62
3.16 2.86 2.49
1.32 1.30 1.20
0.34 0.31 0.26
13.55 11.92 8.95
Ash (wt.%)
HHV (MJ/kg)
Ash melting point (°C)
Ash composition (wt.%)
DT
ST
HT
FT
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
8.92 9.75 11.47
26.55 26.76 27.24
1110 1120 1110
1126 1146 1132
1171 1172 1155
1218 1243 1202
6.67 7.40 7.10
10.92 10.40 10.50
4.51 4.89 5.53
15.57 15.79 16.39
6.83 6.78 6.69
2.08 2.06 1.82
2.08 2.07 2.03
7.18 7.07 6.97
0.24 0.37 0.33
41.63 41.50 41.50
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature.
Table 5 Properties of heat carrier (boiler ash) before and after the experiments. Heat carrier
Boiler ash (Original) Boiler ash (0%) Boiler ash (50%) Boiler ash (100%) Boiler ash (480 °C) Boiler ash (530 °C) Boiler ash (600 °C)
Ash melting point (°C)
Ash composition (wt.%)
DT
ST
HT
FT
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
P2O5
SO3
TiO2
SiO2
1435 1438 1439 1436 1437 1433 1447
1494 1511 1508 1510 1486 1486 1519
1541 1541 1542 1550 1549 1550 1550
1550 1550 1550 1550 1550 1550 1550
24.98 25.41 25.46 24.93 25.49 25.43 25.08
0.79 0.76 0.77 0.70 0.68 0.72 0.73
4.73 4.30 4.06 4.70 4.12 4.06 4.25
2.74 2.85 2.80 2.80 2.90 2.88 3.00
0.73 0.77 0.75 0.75 0.77 0.77 0.77
0.09 0.09 0.08 0.06 0.12 0.10 0.07
0.24 0.20 0.20 0.18 0.16 0.17 0.16
0.39 0.35 0.24 0.36 0.26 0.29 0.27
1.00 1.03 1.06 1.01 1.04 1.02 1.02
64.25 64.24 64.50 64.58 64.14 64.26 64.51
DT: Deformation temperature; ST: Softening temperature; HT: Hemispheric temperature; FT: Flow temperature; Boiler ash (50%): Heat carrier used when co-pyrolysis of FSBC/CS blends at the blending ratio of 50% CS; Boiler ash (480 °C): Heat carrier used when co-pyrolysis of FSBC/CS blends at the temperature of 480 °C.
4. Conclusions
Due to the different characteristics of biomass compared with coal, if the co-pyrolysis of coal and biomass is applied in a poly-generation system, it may affect the boiler or the whole poly-generation system. Owing to the higher content of volatile matter in biomass, the yields of high added value tar and gas obtained from co-pyrolysis of coal and biomass can increase, which may improve the economic benefits of the system. Because of the lower sulfur and nitrogen contents in biomass, it may reduce desulfurization and denitrogenation loads of the boiler. In addition, biomass is CO2 zero emission. These facts can improve the environmental benefits of the system. Due to the higher content of K and lower content of Si and Al in biomass, when the biomass char returns into the boiler burning, it may have an impact on the slagging of the boiler. In order to ensure feeding smoothly, about 30% biomass should be added in the pyrolysis reactor. So, the co-pyrolysis of 70% FSBC adding 30% CS at 600 °C is especially analyzed as an example to investigate the effect of the co-pyrolysis behavior on the poly-generation system or the boiler. As demonstrated in Fig. 7, compared with coal pyrolysis alone, copyrolysis of FSBC/CS increases the yield of tar by 36.77% and that of light-oil by 16.03%. Although the calorific value of gas decreases by 35.07%, the yield of gas increases by 112.98%. Considering these data comprehensively, it can be concluded that the co-pyrolysis of coal/biomass can enhance the economic benefits of the polygeneration system. After co-pyrolysis, the sulfur contained in coal and biomass is distributed in gas, liquid and solid products. As Fig. 7 shows, the sulfur distributed in char accounts for 72.22% after FSBC pyrolysis. But after co-pyrolysis of FSBC/CS, the sulfur distributed in char accounted for 61.88%, which can reduce the desulfurization load 14.32% for the boiler. Similarly, the denitrogenation load can be reduced 8.49%. In addition, co-pyrolysis of coal/biomass can reduce the CO2 net emission. All of these can improve the environmental benefits of the poly-generation system. Besides, it can be seen from Section 3.4 that the ash melting point of co-pyrolysis char-ash decreases a little in the co-pyrolysis of FSBC/CS at the blending ratio of 30% CS, which will not affect slagging of the boiler. These data can provide design and operation for the application of coal/biomass co-pyrolysis in a poly-generation system.
The experimental study of co-pyrolysis of Fugu subbituminous coal and corn stalk were conducted with high-temperature boiler ash as solid heat carrier. Both the corn stalk blending ratio and temperature showed influence on the yields and properties of three products. The Deviation of gaseous, liquid products yields between experimental values and calculated ones were large, but that of char yields almost no changed. The synergy effect occurred in the volatiles during the co-pyrolysis process, which made the yield of tar and light-tar increased, the gas yield decreased. The boiler ash just provided heat for the pyrolysis and its properties were almost not changed after the experiments. The application of co-pyrolysis in a poly-generation system may improve the economic and
35
100 90
30
80 25
70 60
3
20
Heat value of gas (MJ/m ) and Char (MJ/kg)
FSBC 70% FSBC + 30% CS
50 15
40 30
10
20 5
r eo va
at He
He
at
va
lu
lu
n ge tro ni
fc
fg eo
in
n ri lfu
0
ha
as
ar ch
ar ch
ar Ch su
Ga
il t-o Li
gh
r
0
s
10
Ta
Yields of products and distributed S and N in char (%)
3.6. Effect of co-pyrolysis on poly-generation system
Fig. 7. Characteristics of the products from FSBC pyrolysis and FSBC/CS co-pyrolysis.
Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018
M. Guo, J.-C. Bi / Fuel Processing Technology xxx (2015) xxx–xxx
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Please cite this article as: M. Guo, J.-C. Bi, Characteristics and application of co-pyrolysis of coal/biomass blends with solid heat carrier, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.07.018