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Steam co-gasification of biomass and coal in decoupled reactors
Fuel Processing Technology 141 (2016) 61–67
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Steam co-gasification of biomass and coal in decoupled reactors Yalkunjan Tursun, Shaoping Xu ⁎, Chao Wang, Yahui Xiao, Guangyong Wang State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 28 February 2015 Received in revised form 22 April 2015 Accepted 29 June 2015 Available online 23 July 2015 Keywords: Co-gasification Decoupled gasification Moving bed Biomass Coal
a b s t r a c t Steam co-gasification of pine sawdust and bituminous coal was conducted in a lab-scale external circulating radial-flow moving bed (ECRMB) gasification system. The system is composed of three decoupled reactors, i.e., a gas–solid countercurrent moving bed pyrolyzer, a radial-flow moving bed gasifier and a riser-type combustor. Calcined olivine was used as circulating heat carrier and in-situ tar destruction catalyst as well. The influences of biomass blending ratio (BR), pyrolyzer temperature, gasifier temperature and steam to carbon mass ratio (S/C) on the gasification performance were investigated. The results indicated that the gas and tar yields increased with the increase of BR. Synergetic effect based on gas composition was found during co-gasification. At the S/C range of 0 to 1.3, the gas yield and H2 content in product gas increased but CO2 decreased with the increase of S/C. Higher gasifier temperature promoted the gas yield, H2 + CO in product gas, carbon conversion and chemical efficiency of the process. A gas yield of 0.60 Nm3/kg daf and a tar yield of 5.8 g/Nm3 in the gas were obtained at the gasifier temperature of 850 °C, BR of 50% and S/C of 1.3. Pyrolyzer temperature at the range of 500 to 700 °C had no remarkable influence on the product gas composition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the method of reducing carbon dioxide emissions from fossil fuels is becoming a worldwide issue. Biomass received a lot of interest in recent years because carbon dioxide from biomass conversion is regarded as neutral [1]. Biomass can be an alternative for the fossil fuels since it is a renewable energy source with abundant source in some extent. Biomass gasification is one of the promising processes among all biomass conversion processes [2]. The product gas from biomass gasification can be used in many applications, for example generation of electric power, Fischer–Tropsch synthesis for liquid fuels and fuel cells [1]. The gasification process must be supplied by energy since the gasification reactions are endothermic. In the conventional gasification processes, air is used as gasification agent and the energy for the gasification reactions provided by partial combustion of the fuels. However, air gasification produces a gas with lower heating value due to dilution by nitrogen in the air. Product gas with higher heating value could be achieved using the pure oxygen as gasifying agent; but the production of pure oxygen increases the operational cost. Dual fluidized-bed system can also produce high-quality product gas which contains very small amount of nitrogen. In this gasification process, gasification reactor and combustion reactor are isolated and controlled separately. The energy for the endothermic gasification reactions is provided from the combustion reactor by circulating bed materials. In this case, the product gas achieved from the gasification reactor and the flue gas exited from combustion reactor are separated [3]. ⁎ Corresponding author. E-mail address: [email protected] (S. Xu).
http://dx.doi.org/10.1016/j.fuproc.2015.06.046 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Specifically, when the combustion of the residual char does not provide enough energy to satisfy the endothermic gasification reactions in dual-bed biomass gasification systems, additional feedstock with higher carbon content is needed. Co-gasification of biomass with coal can be an alternative to pure biomass gasification since it has several advantages and may compensate their weakness with each other [4]. Co-gasification of biomass and coal has been received much attention in recent years. Saw et al. [5] and Kern et al. [6] investigated co-gasification of coal and biomass in dual fluidized bed. Saw et al. [5] found effective synergy during co-gasification of biomass and coal. Kern et al. [6] reported that increasing lignite ratios in the lignite–biomass blends enhanced the reduction of tars in the product gas with the catalytically active ash content from lignite. Synergistic effects were also reported by Miccio et al. [7] during co-gasification of coal and biomass in ICFB (internal circulating fluidized bed) gasification system. They stated that synergistic effect is exerted by the greater abundance of char. However, no synergetic effect during co-gasification was also reported by Aigner et al. [1] who investigated co-gasification of coal and wood in a 100 kW dual fluidized bed gasifier that gas composition and tar yield showed linear correlation with the changing coal to biomass ratio. Co-gasification biomass and coal can not only reduce air pollutants such as NOx and SOx [8–10] but also improve gasification reactivity [9,11] and the gasification efficiency [12]. Some components such as alkali and alkaline earth metals in the biomass act as catalysts and promoted the carbon gasification reactions during co-gasification [13,14]. Most of co-gasification investigations mainly focus on thermogravimetric analysis [15,16], fixed bed gasifier [17,18], drop tube furnace [19, 20] and fluidized bed gasifier [12,22–24], however, in the reviewed literature there is limited study dedicated to the decoupled moving-bed
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co-gasification for biomass and coal, especially for the pyrolysis and gasification isolated system. In generally, gasification process involves a series of reaction such as fuel pyrolysis, tar cracking, char gasification and combustible matter combustion. In a conventional coupled gasification process such as the entrained flow gasification process these reactions take place in single reactor. In a decoupled gasification process, the reaction process is divided into several steps and is carried out in two or more reactors [25–27]. This paper presents results of the co-gasification of sawdust and bituminous coal in a decoupled gasification system, namely external circulating radial-flow moving bed (ECRMB) gasifier, at atmospheric pressure with the gasifying agent of steam. The influences of four reaction parameters, i.e., biomass ratio (BR), pyrolyzer temperature, gasifier temperature, and steam to carbon mass ratio (S/C), on the performance of the co-gasification were taken into account. 2. Experimental 2.1. Experimental apparatus The ECRMB gasification system is a further development of the so-called ECCMB technology (external circulating concurrent moving bed) which was described by Wei [28], Tursun [29], and Zou [30]. Fig. 1 shows the principle of the ECRMB steam gasification process. The schematic of the lab-scale ECRMB gasification facility is shown in Fig. 2. The gasification system includes three isolated reactors, i.e., a gas–solid crosscurrent radial-flow moving-bed gasifier (3), a solid– solid cocurrent and gas–solid countercurrent moving-bed pyrolyzer (4) and a riser-type combustor (1). The pyrolyzer (40 mm i.d. and 330 mm high) operates between 500 °C and 700 °C where the solid fuels are pyrolyzed to produce volatiles and chars. In the gasifier, cracking and steam reforming of the volatiles take place at a relatively higher temperature. The combustion reactor (28 mm i.d and 1956 mm high) allows combustion of the residential chars and regenerates the deactivated bed materials/catalysts. Every reactor is made of stainless steel and electrically heated independently. As it is shown in Fig. 2, the radial-flow moving bed gasifier has an annular bed with a height of 220 mm, an i.d. of 24 mm, and an o.d. of 108 mm. This specific designed radial-flow moving bed gasifier provides longer residence time for steam reforming of the volatiles to yield a product gas with lower tar content. The gasifier also acts as a dust filter to produce a gas with less dust. The residual chars from the pyrolyzer along with the bed material are transported to the combustor by a screw conveyor (5) where the char and carbon deposit on the surface of catalysts burned off. The
circulating rate of bed material can be adjusted by varying the rotation speed of the screw conveyor. The flue gas separated from bed materials in the cyclone (2) at the top of the gasifier before the bed material returned to gasifier. Energy balance has not been considered in the evaluation of the results since the gasification system is electrically heated, not only for startup, but also to compensate for heat losses to the surroundings. 2.2. Fuel feedstock and bed materials The biomass feedstock used in this test is pine sawdust from Dalian City, Liaoning Province, China and the coal is bituminous coal from Changji City, Xinjiang Province, China. Their proximate and ultimate analyses are presented in Table 1. Before test, the fuel samples were sieved to 0.38–0.83 mm, and dried for 4 h at 105–110 °C. The BR is defined as the mass ratio of the sawdust feed rate to the total feed rate of sawdust and coal as follows: BR ð%Þ ¼
sawdust feed rate ðg=hÞ 100%: ðsawdust þ coalÞ feed rate ðg=hÞ
ð1Þ
The olivine of particle size of 0.38–0.83 mm calcined at 900 °C for 4 h was used as bed materials and in-situ tar destruction catalyst. The olivine came from Yichang City, Hubei Province, China and the XRF (X-ray Fluorescence) analyses of the calcined olivine are listed in Table 2. 2.3. Procedure and product analysis The reactors electrically heated up to desired temperature after about 5 kg of the calcined olivine as bed materials were loaded into the reaction system. The bed materials were fluidized by the preheated pressurized air (4.2–4.6 Nm3/h) in the riser-type combustor. The bed materials were circulated in the system continuously with the help of the screw conveyor between the bottom of the pyrolyzer and that of the combustor. Fuel particle feeds into pyrolyzer by two screw feeders (7) at a fixed feeding rate of 200 g/h. The S/C ratio could be controlled by changing of the steam flow rate of steam generator (6). Table 3 shows the operating parameters of the gasification system. The gaseous products are cooled in four sequential glycol-cooled (−20 °C) condenser and tar traps (10) to separate the condensable components. The liquids (tar and water) obtained are collected in the tar traps. The tar traps were washed with ethyl acetate after each experiment. The tar was recovered by evaporating the solvent at 45 °C in a rotary evaporator. Detailed information relating to tar sampling, analysis and composition for this test is available on our previous work [31]. The product gas is sampled when the system reached a steady state that kept for 2 h. The produced gas was collected in a gas bag (13) every 10 min and the main components (H2, CO, CO2, CH4, C2H4 and C3H6) were off-line analyzed by a GC-7890II gas chromatograph coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Gas compositions reported in this paper are averaged values with respect to time on stream. Chemical efficiency, carbon conversion and steam to carbon mass ratio (S/C) were determined by the following equations (Eqs. (2)–(4)): Chemical efficiency ð%Þ 3 LHV of product gas kJ=Nm gas yield Nm3 =kg ¼ 100% ð2Þ LHV of fuel feedstock into system ðkJ=kgÞ
Carbon conversion ð%Þ ¼ Fig. 1. The principle of the ECRMB steam gasification process.
gasified carbon in the product gas ðgÞ 100% carbon of feedstock fed into system ðgÞ
ð3Þ
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Fig. 2. Schematics of the lab-scale gasification facility. (1) Combustor, (2) cyclone, (3) gasifier, (4) pyrolyzer, (5) screw conveyor, (6) steam generator, (7) screw feeder, (8) feed hopper, (9) filter, (10) cooling system, (11) absorbent cotton filter, (12) vacuum pump, (13) gas bag, (14) gas meter, (15) gas holder.
S=C ¼
Msteam : CMbiomass þ CMcoal
ð4Þ
In Eq. (4), Msteam is the mass flow rate of steam introduced into the system (g/min), CMbiomass and CMcoal are the carbon mass flow rate of biomass and coal into the system (g/min), respectively. 3. Results and discussions 3.1. Effect of BR In this study, the gasification experiments were conducted at the BR of 0%, 25%, 50%, 75% and 100%. During this test pyrolyzer temperature, gasifier temperature and S/C were fixed at 600 °C, 800 °C and 1.3, respectively. Fig. 3 shows effect of BR on gas yield and tar yield. The gas yield increases steadily from 0.31 to 0.84 Nm3/kg daf with the increase of BR. Tar yield increased from 3.7 g/Nm3 for the gasification Table 1 Proximate and ultimate analysis of feedstock. Sample Proximate analysis (wt.%, ad) Moisture Ash Volatile matter Fixed carbon Ultimate analysis (wt.%, daf) Carbon Hydrogen Oxygena Nitrogen Sulfur LHVb (MJ/kg) a b
By difference. Lower heating value.
Pine sawdust
Bituminous coal
8.26 0.61 78.40 12.73
10.47 3.66 28.88 56.99
47.75 6.98 44.84 0.07 0.36 19.05
76.97 4.09 17.64 0.70 0.60 28.01
of pure coal to 11.1 g/Nm3 for the gasification of pure biomass with the increase of BR. Similar trend on tar content was also observed in the study of Kern et al. [6] that the tar content was increased from 3.0 to 16.8 g/Nm3 with the increasing BR from 0 to 100%. The tar yield in this study is generally low due to the presence of the catalytically active bed material of olivine. Furthermore, the residence time of pyrolysis gas (3–4 s) in the radial-flow moving bed gasifier is relatively longer than other dual fluidized bed steam gasifiers, which is favorable to tar cracking. Woei et al. [5] suggested that the highly porous lignite char and the presence of high Ca and Fe found in the lignite ash showed catalytic activity for further cracking the tars during co-gasification of lignite and wood pellets in a 100 kW dual fluidised bed steam gasifier. Masnadi et al. [14] also stated that the potassium in biomass enhanced the carbon gasification reaction during co-gasification of biomass with fossil fuels. However, the catalytic effect of fuel ashes to carbon gasification and tar cracking reactions could be neglected due to separation of pyrolyzer and gasifier in the ECRMB process. It is generally agreed that biomass or coal gasification occurs through three steps namely the fuel pyrolysis, the secondary reactions of volatile and the gasification reactions of the char [21]. In the ECRMB process the pyrolyzer and gasifier are separated, and the product gas mainly comes from the volatile matter of the fuel. Therefore, devolatilization of fuel and secondary reaction of volatiles play critical roles in determining the product distribution both for biomass and coal gasification. Hence, the biomass produced higher gas and tar yields since biomass releases large amount of volatiles. Fig. 4 displays the gas composition at different BRs. With the increase of BR in the fuel blend, the H2 increases from 37.4% for the gasification of pure coal to 40.4% for the pure biomass and the CO and CH4 increase Table 2 Chemical composition of the calcined olivine. Component
MgO
SiO2
Fe2O3 Al2O3 CaO
Composition (wt.%) 42.95 44.77 9.57
1.00
NiO
SO3
TiO2 K2O
1.09 0.38 0.13 0.06 0.05
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Table 3 Operation parameters of the gasification system. Total bed material inventory (kg) Circulating amount of bed material (kg/h) Fuel feeding rate (kg/h) Steam to carbon ratio (g/g) Gasifier temperature (°C) Pyrolyzer temperature (°C) Combustor temperature (°C) Air flow rate of combustor (m3/h) Operation pressure (MPa)
5.0 4.5 0.20 0.3–1.9 750–850 500–700 850 4.2–4.6 0.1
from 5.2 to 12.5% and 5.0 to 10.3%, respectively with the increase of BR. The CO2 decreases from 50.7 to 34.0% with the increment of BR. The CO2 concentration is significantly higher in all tests and it may be related with the capability of transferring oxygen of olivine. Lancee et al. [32] stated that the capability of transferring oxygen of olivine increases the CO2 content in the gasification zone. The dashed lines in Fig. 4 show the calculated value of gas composition. The non-linear changes in the gas composition show synergy during co-gasification in the present study. Without synergy between the biomass and coal, the gas composition would be expected to change linearly with increasing BR. The synergy during co-gasification has a positive contribution to the gas composition in which the concentrations of H2 and CH4 are higher, and that of CO2 is lower, than the calculated values as seen in Fig. 4. Non-linear changes on gas composition were also found by Saw et al. [5], who investigated co-gasification of blended lignite and wood pellets in a 100 kW dual fluidized bed steam gasifier. They suggested that synergetic effect during the co-gasification resulted from the large effective pore diameter and cracks in the blended chars with the presence of catalytic metals. In the ECRMB gasification process, the residence time of pyrolysis gases in the gasifier is relatively longer, therefore, the synergetic effect on gas composition in this test may attribute to gas–gas interaction [33] or gas–solid interactions in the presence of catalytically active bed material. LHV, chemical efficiency and carbon conversion increase constantly with increasing BR as it is shown in Fig. 5. The LHVs of the product gases range from 6900 to 11,400 kJ/Nm3 with the increasing BR. The LHV of product gas for pure biomass obtained in this study is approximately double the value of the typical LHV of air biomass gasification. The increment on chemical efficiency is in accordance with the aforementioned tendency on gas yield and LHV with increasing BR. Similar results on the thermal efficiency were obtained by Lapuerta et al. [34] who reported exponential increase of cold gas efficiency from 15% to 40% with the increase of BR from 0 to 100%. It is noteworthy that carbon
Fig. 3. Effect of BR on gas and tar yields (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
Fig. 4. Effect of BR on gas composition (solid line: experimental; dashed line: calculated) (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
conversion is significantly low for all tests. In this decoupled gasification system, the temperature of pyrolyzer is relatively low (600 °C) and the residual char was transported into combustor to generate heat for the process. Therefore, the char is hardly involved in the gasification reaction (steam–carbon, Boudouard reactions) which leads to lower carbon conversion in the gasifier, especially for pure coal gasification. Similar result was also found by Seo et al. [23] who investigated co-gasification coal/biomass blend in a dual circulating fluidized bed reactor, where the carbon conversion increased from 39% to 79% with the increase of BR from 0 to 100%. The author suggested that lower carbon conversion is related with lower gasifier temperature (800 °C). 3.2. Influence of pyrolyzer temperature Experiments at the pyrolyzer temperature of 500, 600 and 700 °C were carried out with a fixed S/C of 1.3, a BR of 50% and a gasifier temperature of 800 °C. The results are listed in Table 4. It can be seen that within the investigated range, the gas yield increases with the increase in pyrolyzer temperature at the cost of the tar and char yield, which leads to increment of carbon conversion. The increase of the gas yield attributes to further devolatilization of char at higher temperature. The char yield varies from 45.9 to 40.1 wt.% daf in the pyrolyzer temperature range of 500 to 700 °C. According to the simulation of Schuster et al. [3] based on a dual fluidized-bed steam gasifier, with
Fig. 5. Effect of BR on LHV, chemical efficiencies and carbon conversion (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
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Table 4 Effect of pyrolyzer temperature on gasification performance. Pyrolyzer temperature (°C)
500
600
700
Gas composition (vol.%) H2 CO CO2 CH4 C2H4 C2H6 C3H6 Gas yield (Nm3/kg daf) Tar yield (g/Nm3) Char yield (wt.% daf) Carbon conversion (%)
40.62 8.10 40.44 9.27 1.25 0.29 0.03 0.49 10.6 45.9 29.4
40.57 8.22 39.72 9.89 1.23 0.29 0.08 0.53 7.8 43.5 31.7
40.60 8.98 39.29 9.80 1.03 0.27 0.03 0.58 6.1 40.1 34.4
42.6 wt.% of the carbon leaving the gasification zone as char, no product gas has to be recirculated to the combustion reactor to provide the energy for the gasification reactions. Gas compositions have shown unremarkable changes within the temperature interval. This indicated that in this decoupled gasification system the further reaction of the pyrogas in the gasifier in the presence of the catalytic bed materials, such as steam reforming reaction and tar creaking reaction, dominates the product gas composition. 3.3. Influence of gasifier temperature
Fig. 7. Effect of gasifier temperature on chemical efficiency and carbon conversion (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
reactions in this decoupled gasification system. These data of gas composition confirm the better catalytic activity of calcined olivine at higher temperature. These results are in good agreement with other published experimental observation [35]. 3.4. Effect of S/C
The influence of the gasifier temperature on gas yield, tar content, chemical efficiency and carbon conversion at the temperatures of gasifier from 700 to 850 °C at an S/C ratio of 1.3, a pyrolyzer temperature of 600 °C and a BR of 50% is shown in Figs. 6 and 7. The gas yield, chemical efficiencies and carbon conversion increase with the increase of the gasifier temperature, whereas the tar yield decreases. This is because that at higher temperatures and with olivine as catalytic circulating bed material, the cracking and steam reforming of tars were improved, which leads to less tar formation and higher gas yield. A gas yield of 0.60 Nm3/kg daf and a tar content of 5.8 g/Nm3 dry gas were obtained at 850 °C. Fig. 8 displays the product gas composition obtained by variation of the gasifier temperature. With the increase of gasifier temperature, H2 and CO increase from 34.2 to 42.4% and 5.6 to 10.2%, respectively. At the same time, CO2 dramatically decreases from 46.6 to 36.6%. The product gas with concentration of 42.4% H2 and 10.2% CO can be produced at 850 °C. The CH4 composition and other hydrocarbons decrease with the increasing gasifier temperature due to further cracking and reforming reactions. It is indicated that at the higher temperature in the gasifier the steam reforming and dry reforming reactions are the dominant
The amount of steam as gasification agent in the gasification reactor is essential for system performance and product gas quality [36]. The effect of the S/C on the gas and tar yields at the gasifier temperature of 800 °C, pyrolyzer temperature of 600 °C, BR of 50% and S/C ranging from 0 to 1.9 is shown in Fig. 9. As shown in Fig. 9, the gas yield smoothly increases with the increase of S/C from 0 to 1.3, and no remarkable change on gas yield has shown further increment on S/C. Slight increase of gas yield at higher S/C has also been shown in other published experimental observations [36]. The tar content decreases sharply from 16.9 to 6.7 g/Nm3 with an increase in the S/C from 0 to 1.9. This result indicates that the presence of steam leads to more tar participating in reforming reaction. The effect of the S/C on the gas composition is shown in Fig. 10. The result shows that the gas composition depends on the S/C. With the increase in S/C, the tar cracking and steam reforming reactions are enhanced to increase H2 and decrease CO2, CO, and CH4 compositions
Fig. 6. Effect of gasifier temperature on gas and tar yields (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
Fig. 8. Effect of gasifier temperature on gas composition (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
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increased with the increasing BR. The gas yield and carbon conversion of biomass were higher than coal due to higher volatile content of biomass. The gas yield increased and the tar yield decreased with the increasing pyrolyzer temperature. Pyrolyzer temperature has little impact on gas composition within the pyrolyzer temperature range of 500 to 700 °C. The H2 + CO, gas yield, chemical efficiency and carbon conversion are greatly increased with the increasing of gasifier temperature. Gas yield and H2 increased and CO2 decreased greatly at the S/C range of 0–1.3, but slightly influenced by the further increment on the S/C. Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 50776013) and the National High Technology Research and Development Program (“863” Program) of China (No. 2008AA05Z407). Fig. 9. Effect of S/C on gas and tar yields (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, BR: 50%).
as shown in Fig. 10. H2 increases from 21.6 to 40.6% while CO2, CO and CH4 decrease from 50.8 to 41.4%, from 14.8 to 7.2% and from 12.5% to 9.0%, respectively. Slight increment on other hydrocarbons has been shown with the increment of S/C, which may attribute to shorter residence time of hydrocarbons in the gasifier caused by large amount of steam. When the S/C is higher than 1.3, the gas composition is slightly influenced by increasing steam addition to gasifier. As we know, the separation steam from producer gas easily by condensation and dryness, however, more energy was consumed to produce excess steam. Therefore, it is necessary to select an appropriate S/C ratio according to different application. 4. Conclusion Co-gasification of coal and pine sawdust was investigated in an external circulating radial-flow moving bed gasification system. Calcined olivine was used as the bed material as well as in-situ tar destruction catalysis. The tar and gas yields increased with increasing BR due to the higher volatile content of the biomass but the tar yield was generally low due to the presence of the catalytically active bed material of olivine. Concentration of H2, CO and CH4 increased while CO2 decreased with the increasing BR. The non-linear changes in the gas composition showed synergy during co-gasification in the present study. Carbon conversion and chemical efficiency were a constant
Fig. 10. Effect of S/C on gas composition (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, BR: 50%).
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Steam co-gasification of biomass and coal in decoupled reactors Yalkunjan Tursun, Shaoping Xu ⁎, Chao Wang, Yahui Xiao, Guangyong Wang State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 28 February 2015 Received in revised form 22 April 2015 Accepted 29 June 2015 Available online 23 July 2015 Keywords: Co-gasification Decoupled gasification Moving bed Biomass Coal
a b s t r a c t Steam co-gasification of pine sawdust and bituminous coal was conducted in a lab-scale external circulating radial-flow moving bed (ECRMB) gasification system. The system is composed of three decoupled reactors, i.e., a gas–solid countercurrent moving bed pyrolyzer, a radial-flow moving bed gasifier and a riser-type combustor. Calcined olivine was used as circulating heat carrier and in-situ tar destruction catalyst as well. The influences of biomass blending ratio (BR), pyrolyzer temperature, gasifier temperature and steam to carbon mass ratio (S/C) on the gasification performance were investigated. The results indicated that the gas and tar yields increased with the increase of BR. Synergetic effect based on gas composition was found during co-gasification. At the S/C range of 0 to 1.3, the gas yield and H2 content in product gas increased but CO2 decreased with the increase of S/C. Higher gasifier temperature promoted the gas yield, H2 + CO in product gas, carbon conversion and chemical efficiency of the process. A gas yield of 0.60 Nm3/kg daf and a tar yield of 5.8 g/Nm3 in the gas were obtained at the gasifier temperature of 850 °C, BR of 50% and S/C of 1.3. Pyrolyzer temperature at the range of 500 to 700 °C had no remarkable influence on the product gas composition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the method of reducing carbon dioxide emissions from fossil fuels is becoming a worldwide issue. Biomass received a lot of interest in recent years because carbon dioxide from biomass conversion is regarded as neutral [1]. Biomass can be an alternative for the fossil fuels since it is a renewable energy source with abundant source in some extent. Biomass gasification is one of the promising processes among all biomass conversion processes [2]. The product gas from biomass gasification can be used in many applications, for example generation of electric power, Fischer–Tropsch synthesis for liquid fuels and fuel cells [1]. The gasification process must be supplied by energy since the gasification reactions are endothermic. In the conventional gasification processes, air is used as gasification agent and the energy for the gasification reactions provided by partial combustion of the fuels. However, air gasification produces a gas with lower heating value due to dilution by nitrogen in the air. Product gas with higher heating value could be achieved using the pure oxygen as gasifying agent; but the production of pure oxygen increases the operational cost. Dual fluidized-bed system can also produce high-quality product gas which contains very small amount of nitrogen. In this gasification process, gasification reactor and combustion reactor are isolated and controlled separately. The energy for the endothermic gasification reactions is provided from the combustion reactor by circulating bed materials. In this case, the product gas achieved from the gasification reactor and the flue gas exited from combustion reactor are separated [3]. ⁎ Corresponding author. E-mail address: [email protected] (S. Xu).
http://dx.doi.org/10.1016/j.fuproc.2015.06.046 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Specifically, when the combustion of the residual char does not provide enough energy to satisfy the endothermic gasification reactions in dual-bed biomass gasification systems, additional feedstock with higher carbon content is needed. Co-gasification of biomass with coal can be an alternative to pure biomass gasification since it has several advantages and may compensate their weakness with each other [4]. Co-gasification of biomass and coal has been received much attention in recent years. Saw et al. [5] and Kern et al. [6] investigated co-gasification of coal and biomass in dual fluidized bed. Saw et al. [5] found effective synergy during co-gasification of biomass and coal. Kern et al. [6] reported that increasing lignite ratios in the lignite–biomass blends enhanced the reduction of tars in the product gas with the catalytically active ash content from lignite. Synergistic effects were also reported by Miccio et al. [7] during co-gasification of coal and biomass in ICFB (internal circulating fluidized bed) gasification system. They stated that synergistic effect is exerted by the greater abundance of char. However, no synergetic effect during co-gasification was also reported by Aigner et al. [1] who investigated co-gasification of coal and wood in a 100 kW dual fluidized bed gasifier that gas composition and tar yield showed linear correlation with the changing coal to biomass ratio. Co-gasification biomass and coal can not only reduce air pollutants such as NOx and SOx [8–10] but also improve gasification reactivity [9,11] and the gasification efficiency [12]. Some components such as alkali and alkaline earth metals in the biomass act as catalysts and promoted the carbon gasification reactions during co-gasification [13,14]. Most of co-gasification investigations mainly focus on thermogravimetric analysis [15,16], fixed bed gasifier [17,18], drop tube furnace [19, 20] and fluidized bed gasifier [12,22–24], however, in the reviewed literature there is limited study dedicated to the decoupled moving-bed
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co-gasification for biomass and coal, especially for the pyrolysis and gasification isolated system. In generally, gasification process involves a series of reaction such as fuel pyrolysis, tar cracking, char gasification and combustible matter combustion. In a conventional coupled gasification process such as the entrained flow gasification process these reactions take place in single reactor. In a decoupled gasification process, the reaction process is divided into several steps and is carried out in two or more reactors [25–27]. This paper presents results of the co-gasification of sawdust and bituminous coal in a decoupled gasification system, namely external circulating radial-flow moving bed (ECRMB) gasifier, at atmospheric pressure with the gasifying agent of steam. The influences of four reaction parameters, i.e., biomass ratio (BR), pyrolyzer temperature, gasifier temperature, and steam to carbon mass ratio (S/C), on the performance of the co-gasification were taken into account. 2. Experimental 2.1. Experimental apparatus The ECRMB gasification system is a further development of the so-called ECCMB technology (external circulating concurrent moving bed) which was described by Wei [28], Tursun [29], and Zou [30]. Fig. 1 shows the principle of the ECRMB steam gasification process. The schematic of the lab-scale ECRMB gasification facility is shown in Fig. 2. The gasification system includes three isolated reactors, i.e., a gas–solid crosscurrent radial-flow moving-bed gasifier (3), a solid– solid cocurrent and gas–solid countercurrent moving-bed pyrolyzer (4) and a riser-type combustor (1). The pyrolyzer (40 mm i.d. and 330 mm high) operates between 500 °C and 700 °C where the solid fuels are pyrolyzed to produce volatiles and chars. In the gasifier, cracking and steam reforming of the volatiles take place at a relatively higher temperature. The combustion reactor (28 mm i.d and 1956 mm high) allows combustion of the residential chars and regenerates the deactivated bed materials/catalysts. Every reactor is made of stainless steel and electrically heated independently. As it is shown in Fig. 2, the radial-flow moving bed gasifier has an annular bed with a height of 220 mm, an i.d. of 24 mm, and an o.d. of 108 mm. This specific designed radial-flow moving bed gasifier provides longer residence time for steam reforming of the volatiles to yield a product gas with lower tar content. The gasifier also acts as a dust filter to produce a gas with less dust. The residual chars from the pyrolyzer along with the bed material are transported to the combustor by a screw conveyor (5) where the char and carbon deposit on the surface of catalysts burned off. The
circulating rate of bed material can be adjusted by varying the rotation speed of the screw conveyor. The flue gas separated from bed materials in the cyclone (2) at the top of the gasifier before the bed material returned to gasifier. Energy balance has not been considered in the evaluation of the results since the gasification system is electrically heated, not only for startup, but also to compensate for heat losses to the surroundings. 2.2. Fuel feedstock and bed materials The biomass feedstock used in this test is pine sawdust from Dalian City, Liaoning Province, China and the coal is bituminous coal from Changji City, Xinjiang Province, China. Their proximate and ultimate analyses are presented in Table 1. Before test, the fuel samples were sieved to 0.38–0.83 mm, and dried for 4 h at 105–110 °C. The BR is defined as the mass ratio of the sawdust feed rate to the total feed rate of sawdust and coal as follows: BR ð%Þ ¼
sawdust feed rate ðg=hÞ 100%: ðsawdust þ coalÞ feed rate ðg=hÞ
ð1Þ
The olivine of particle size of 0.38–0.83 mm calcined at 900 °C for 4 h was used as bed materials and in-situ tar destruction catalyst. The olivine came from Yichang City, Hubei Province, China and the XRF (X-ray Fluorescence) analyses of the calcined olivine are listed in Table 2. 2.3. Procedure and product analysis The reactors electrically heated up to desired temperature after about 5 kg of the calcined olivine as bed materials were loaded into the reaction system. The bed materials were fluidized by the preheated pressurized air (4.2–4.6 Nm3/h) in the riser-type combustor. The bed materials were circulated in the system continuously with the help of the screw conveyor between the bottom of the pyrolyzer and that of the combustor. Fuel particle feeds into pyrolyzer by two screw feeders (7) at a fixed feeding rate of 200 g/h. The S/C ratio could be controlled by changing of the steam flow rate of steam generator (6). Table 3 shows the operating parameters of the gasification system. The gaseous products are cooled in four sequential glycol-cooled (−20 °C) condenser and tar traps (10) to separate the condensable components. The liquids (tar and water) obtained are collected in the tar traps. The tar traps were washed with ethyl acetate after each experiment. The tar was recovered by evaporating the solvent at 45 °C in a rotary evaporator. Detailed information relating to tar sampling, analysis and composition for this test is available on our previous work [31]. The product gas is sampled when the system reached a steady state that kept for 2 h. The produced gas was collected in a gas bag (13) every 10 min and the main components (H2, CO, CO2, CH4, C2H4 and C3H6) were off-line analyzed by a GC-7890II gas chromatograph coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Gas compositions reported in this paper are averaged values with respect to time on stream. Chemical efficiency, carbon conversion and steam to carbon mass ratio (S/C) were determined by the following equations (Eqs. (2)–(4)): Chemical efficiency ð%Þ 3 LHV of product gas kJ=Nm gas yield Nm3 =kg ¼ 100% ð2Þ LHV of fuel feedstock into system ðkJ=kgÞ
Carbon conversion ð%Þ ¼ Fig. 1. The principle of the ECRMB steam gasification process.
gasified carbon in the product gas ðgÞ 100% carbon of feedstock fed into system ðgÞ
ð3Þ
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Fig. 2. Schematics of the lab-scale gasification facility. (1) Combustor, (2) cyclone, (3) gasifier, (4) pyrolyzer, (5) screw conveyor, (6) steam generator, (7) screw feeder, (8) feed hopper, (9) filter, (10) cooling system, (11) absorbent cotton filter, (12) vacuum pump, (13) gas bag, (14) gas meter, (15) gas holder.
S=C ¼
Msteam : CMbiomass þ CMcoal
ð4Þ
In Eq. (4), Msteam is the mass flow rate of steam introduced into the system (g/min), CMbiomass and CMcoal are the carbon mass flow rate of biomass and coal into the system (g/min), respectively. 3. Results and discussions 3.1. Effect of BR In this study, the gasification experiments were conducted at the BR of 0%, 25%, 50%, 75% and 100%. During this test pyrolyzer temperature, gasifier temperature and S/C were fixed at 600 °C, 800 °C and 1.3, respectively. Fig. 3 shows effect of BR on gas yield and tar yield. The gas yield increases steadily from 0.31 to 0.84 Nm3/kg daf with the increase of BR. Tar yield increased from 3.7 g/Nm3 for the gasification Table 1 Proximate and ultimate analysis of feedstock. Sample Proximate analysis (wt.%, ad) Moisture Ash Volatile matter Fixed carbon Ultimate analysis (wt.%, daf) Carbon Hydrogen Oxygena Nitrogen Sulfur LHVb (MJ/kg) a b
By difference. Lower heating value.
Pine sawdust
Bituminous coal
8.26 0.61 78.40 12.73
10.47 3.66 28.88 56.99
47.75 6.98 44.84 0.07 0.36 19.05
76.97 4.09 17.64 0.70 0.60 28.01
of pure coal to 11.1 g/Nm3 for the gasification of pure biomass with the increase of BR. Similar trend on tar content was also observed in the study of Kern et al. [6] that the tar content was increased from 3.0 to 16.8 g/Nm3 with the increasing BR from 0 to 100%. The tar yield in this study is generally low due to the presence of the catalytically active bed material of olivine. Furthermore, the residence time of pyrolysis gas (3–4 s) in the radial-flow moving bed gasifier is relatively longer than other dual fluidized bed steam gasifiers, which is favorable to tar cracking. Woei et al. [5] suggested that the highly porous lignite char and the presence of high Ca and Fe found in the lignite ash showed catalytic activity for further cracking the tars during co-gasification of lignite and wood pellets in a 100 kW dual fluidised bed steam gasifier. Masnadi et al. [14] also stated that the potassium in biomass enhanced the carbon gasification reaction during co-gasification of biomass with fossil fuels. However, the catalytic effect of fuel ashes to carbon gasification and tar cracking reactions could be neglected due to separation of pyrolyzer and gasifier in the ECRMB process. It is generally agreed that biomass or coal gasification occurs through three steps namely the fuel pyrolysis, the secondary reactions of volatile and the gasification reactions of the char [21]. In the ECRMB process the pyrolyzer and gasifier are separated, and the product gas mainly comes from the volatile matter of the fuel. Therefore, devolatilization of fuel and secondary reaction of volatiles play critical roles in determining the product distribution both for biomass and coal gasification. Hence, the biomass produced higher gas and tar yields since biomass releases large amount of volatiles. Fig. 4 displays the gas composition at different BRs. With the increase of BR in the fuel blend, the H2 increases from 37.4% for the gasification of pure coal to 40.4% for the pure biomass and the CO and CH4 increase Table 2 Chemical composition of the calcined olivine. Component
MgO
SiO2
Fe2O3 Al2O3 CaO
Composition (wt.%) 42.95 44.77 9.57
1.00
NiO
SO3
TiO2 K2O
1.09 0.38 0.13 0.06 0.05
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Table 3 Operation parameters of the gasification system. Total bed material inventory (kg) Circulating amount of bed material (kg/h) Fuel feeding rate (kg/h) Steam to carbon ratio (g/g) Gasifier temperature (°C) Pyrolyzer temperature (°C) Combustor temperature (°C) Air flow rate of combustor (m3/h) Operation pressure (MPa)
5.0 4.5 0.20 0.3–1.9 750–850 500–700 850 4.2–4.6 0.1
from 5.2 to 12.5% and 5.0 to 10.3%, respectively with the increase of BR. The CO2 decreases from 50.7 to 34.0% with the increment of BR. The CO2 concentration is significantly higher in all tests and it may be related with the capability of transferring oxygen of olivine. Lancee et al. [32] stated that the capability of transferring oxygen of olivine increases the CO2 content in the gasification zone. The dashed lines in Fig. 4 show the calculated value of gas composition. The non-linear changes in the gas composition show synergy during co-gasification in the present study. Without synergy between the biomass and coal, the gas composition would be expected to change linearly with increasing BR. The synergy during co-gasification has a positive contribution to the gas composition in which the concentrations of H2 and CH4 are higher, and that of CO2 is lower, than the calculated values as seen in Fig. 4. Non-linear changes on gas composition were also found by Saw et al. [5], who investigated co-gasification of blended lignite and wood pellets in a 100 kW dual fluidized bed steam gasifier. They suggested that synergetic effect during the co-gasification resulted from the large effective pore diameter and cracks in the blended chars with the presence of catalytic metals. In the ECRMB gasification process, the residence time of pyrolysis gases in the gasifier is relatively longer, therefore, the synergetic effect on gas composition in this test may attribute to gas–gas interaction [33] or gas–solid interactions in the presence of catalytically active bed material. LHV, chemical efficiency and carbon conversion increase constantly with increasing BR as it is shown in Fig. 5. The LHVs of the product gases range from 6900 to 11,400 kJ/Nm3 with the increasing BR. The LHV of product gas for pure biomass obtained in this study is approximately double the value of the typical LHV of air biomass gasification. The increment on chemical efficiency is in accordance with the aforementioned tendency on gas yield and LHV with increasing BR. Similar results on the thermal efficiency were obtained by Lapuerta et al. [34] who reported exponential increase of cold gas efficiency from 15% to 40% with the increase of BR from 0 to 100%. It is noteworthy that carbon
Fig. 3. Effect of BR on gas and tar yields (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
Fig. 4. Effect of BR on gas composition (solid line: experimental; dashed line: calculated) (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
conversion is significantly low for all tests. In this decoupled gasification system, the temperature of pyrolyzer is relatively low (600 °C) and the residual char was transported into combustor to generate heat for the process. Therefore, the char is hardly involved in the gasification reaction (steam–carbon, Boudouard reactions) which leads to lower carbon conversion in the gasifier, especially for pure coal gasification. Similar result was also found by Seo et al. [23] who investigated co-gasification coal/biomass blend in a dual circulating fluidized bed reactor, where the carbon conversion increased from 39% to 79% with the increase of BR from 0 to 100%. The author suggested that lower carbon conversion is related with lower gasifier temperature (800 °C). 3.2. Influence of pyrolyzer temperature Experiments at the pyrolyzer temperature of 500, 600 and 700 °C were carried out with a fixed S/C of 1.3, a BR of 50% and a gasifier temperature of 800 °C. The results are listed in Table 4. It can be seen that within the investigated range, the gas yield increases with the increase in pyrolyzer temperature at the cost of the tar and char yield, which leads to increment of carbon conversion. The increase of the gas yield attributes to further devolatilization of char at higher temperature. The char yield varies from 45.9 to 40.1 wt.% daf in the pyrolyzer temperature range of 500 to 700 °C. According to the simulation of Schuster et al. [3] based on a dual fluidized-bed steam gasifier, with
Fig. 5. Effect of BR on LHV, chemical efficiencies and carbon conversion (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, S/C: 1.3).
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Table 4 Effect of pyrolyzer temperature on gasification performance. Pyrolyzer temperature (°C)
500
600
700
Gas composition (vol.%) H2 CO CO2 CH4 C2H4 C2H6 C3H6 Gas yield (Nm3/kg daf) Tar yield (g/Nm3) Char yield (wt.% daf) Carbon conversion (%)
40.62 8.10 40.44 9.27 1.25 0.29 0.03 0.49 10.6 45.9 29.4
40.57 8.22 39.72 9.89 1.23 0.29 0.08 0.53 7.8 43.5 31.7
40.60 8.98 39.29 9.80 1.03 0.27 0.03 0.58 6.1 40.1 34.4
42.6 wt.% of the carbon leaving the gasification zone as char, no product gas has to be recirculated to the combustion reactor to provide the energy for the gasification reactions. Gas compositions have shown unremarkable changes within the temperature interval. This indicated that in this decoupled gasification system the further reaction of the pyrogas in the gasifier in the presence of the catalytic bed materials, such as steam reforming reaction and tar creaking reaction, dominates the product gas composition. 3.3. Influence of gasifier temperature
Fig. 7. Effect of gasifier temperature on chemical efficiency and carbon conversion (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
reactions in this decoupled gasification system. These data of gas composition confirm the better catalytic activity of calcined olivine at higher temperature. These results are in good agreement with other published experimental observation [35]. 3.4. Effect of S/C
The influence of the gasifier temperature on gas yield, tar content, chemical efficiency and carbon conversion at the temperatures of gasifier from 700 to 850 °C at an S/C ratio of 1.3, a pyrolyzer temperature of 600 °C and a BR of 50% is shown in Figs. 6 and 7. The gas yield, chemical efficiencies and carbon conversion increase with the increase of the gasifier temperature, whereas the tar yield decreases. This is because that at higher temperatures and with olivine as catalytic circulating bed material, the cracking and steam reforming of tars were improved, which leads to less tar formation and higher gas yield. A gas yield of 0.60 Nm3/kg daf and a tar content of 5.8 g/Nm3 dry gas were obtained at 850 °C. Fig. 8 displays the product gas composition obtained by variation of the gasifier temperature. With the increase of gasifier temperature, H2 and CO increase from 34.2 to 42.4% and 5.6 to 10.2%, respectively. At the same time, CO2 dramatically decreases from 46.6 to 36.6%. The product gas with concentration of 42.4% H2 and 10.2% CO can be produced at 850 °C. The CH4 composition and other hydrocarbons decrease with the increasing gasifier temperature due to further cracking and reforming reactions. It is indicated that at the higher temperature in the gasifier the steam reforming and dry reforming reactions are the dominant
The amount of steam as gasification agent in the gasification reactor is essential for system performance and product gas quality [36]. The effect of the S/C on the gas and tar yields at the gasifier temperature of 800 °C, pyrolyzer temperature of 600 °C, BR of 50% and S/C ranging from 0 to 1.9 is shown in Fig. 9. As shown in Fig. 9, the gas yield smoothly increases with the increase of S/C from 0 to 1.3, and no remarkable change on gas yield has shown further increment on S/C. Slight increase of gas yield at higher S/C has also been shown in other published experimental observations [36]. The tar content decreases sharply from 16.9 to 6.7 g/Nm3 with an increase in the S/C from 0 to 1.9. This result indicates that the presence of steam leads to more tar participating in reforming reaction. The effect of the S/C on the gas composition is shown in Fig. 10. The result shows that the gas composition depends on the S/C. With the increase in S/C, the tar cracking and steam reforming reactions are enhanced to increase H2 and decrease CO2, CO, and CH4 compositions
Fig. 6. Effect of gasifier temperature on gas and tar yields (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
Fig. 8. Effect of gasifier temperature on gas composition (pyrolyzer temperature: 600 °C, BR: 50%, S/C: 1.3).
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increased with the increasing BR. The gas yield and carbon conversion of biomass were higher than coal due to higher volatile content of biomass. The gas yield increased and the tar yield decreased with the increasing pyrolyzer temperature. Pyrolyzer temperature has little impact on gas composition within the pyrolyzer temperature range of 500 to 700 °C. The H2 + CO, gas yield, chemical efficiency and carbon conversion are greatly increased with the increasing of gasifier temperature. Gas yield and H2 increased and CO2 decreased greatly at the S/C range of 0–1.3, but slightly influenced by the further increment on the S/C. Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 50776013) and the National High Technology Research and Development Program (“863” Program) of China (No. 2008AA05Z407). Fig. 9. Effect of S/C on gas and tar yields (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, BR: 50%).
as shown in Fig. 10. H2 increases from 21.6 to 40.6% while CO2, CO and CH4 decrease from 50.8 to 41.4%, from 14.8 to 7.2% and from 12.5% to 9.0%, respectively. Slight increment on other hydrocarbons has been shown with the increment of S/C, which may attribute to shorter residence time of hydrocarbons in the gasifier caused by large amount of steam. When the S/C is higher than 1.3, the gas composition is slightly influenced by increasing steam addition to gasifier. As we know, the separation steam from producer gas easily by condensation and dryness, however, more energy was consumed to produce excess steam. Therefore, it is necessary to select an appropriate S/C ratio according to different application. 4. Conclusion Co-gasification of coal and pine sawdust was investigated in an external circulating radial-flow moving bed gasification system. Calcined olivine was used as the bed material as well as in-situ tar destruction catalysis. The tar and gas yields increased with increasing BR due to the higher volatile content of the biomass but the tar yield was generally low due to the presence of the catalytically active bed material of olivine. Concentration of H2, CO and CH4 increased while CO2 decreased with the increasing BR. The non-linear changes in the gas composition showed synergy during co-gasification in the present study. Carbon conversion and chemical efficiency were a constant
Fig. 10. Effect of S/C on gas composition (gasifier temperature: 800 °C, pyrolyzer temperature: 600 °C, BR: 50%).
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