Abstract
A fluidized bed reactor pyrolysis process with recycled non-condensable gases was designed for a capacity of 5 kg/h based on previous basic studies of key parts. The main components of the pyrolysis process were introduced, and the performance was appraised. Initial experiments were conducted between 450 °C and 550 °C to characterize the bio-oil at different temperatures, using non-condensable gas as fluidizing medium. Simultaneously, the contents and higher heating value of non-condensable gases were determined. The results showed that the excellent efficiency of cooling and of capturing the organic compounds, contributing to a high yield of bio-oil and clean non-condensable gases. At 500 °C, the highest yield of bio-oil reached 55.6 wt%, and the yields of bio-char and gas were 23.4 wt% and 21 wt%, respectively. Non-condensable gas not only carried the organic compounds, but also participated in fast pyrolysis. However, temperature was the major factor affecting the chemical components of bio-oil. The heavy bio-oil mainly included long chain macromolecules and phenols. Higher temperature favored degradation and gas purification.
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Fast Pyrolysis of Birch Wood in a Bubbling Fluidized Bed Reactor with Recycled Non-condensable Gases
Ning Li,a,b Zixiang Gao,a,b Weiming Yi,a,b Zhihe Li,a,b,* Lihong Wang,a,b Peng Fu,a,b Yongjun Li,a,b and Xueyuan Bai a,b
A fluidized bed reactor pyrolysis process with recycled non-condensable gases was designed for a capacity of 5 kg/h based on previous basic studies of key parts. The main components of the pyrolysis process were introduced, and the performance was appraised. Initial experiments were conducted between 450 °C and 550 °C to characterize the bio-oil at different temperatures, using non-condensable gas as fluidizing medium. Simultaneously, the contents and higher heating value of non-condensable gases were determined. The results showed that the excellent efficiency of cooling and of capturing the organic compounds, contributing to a high yield of bio-oil and clean non-condensable gases. At 500 °C, the highest yield of bio-oil reached 55.6 wt%, and the yields of bio-char and gas were 23.4 wt% and 21 wt%, respectively. Non-condensable gas not only carried the organic compounds, but also participated in fast pyrolysis. However, temperature was the major factor affecting the chemical components of bio-oil. The heavy bio-oil mainly included long chain macromolecules and phenols. Higher temperature favored degradation and gas purification.
Keywords: Fluidized bed reactor; Birch wood; Fast pyrolysis; Non-condensable gas; Bio-oil
Contact information: a: School of Agricultural Engineering and Food Science, Shandong University of Technology, Zibo, Box 255049, China; b: Shandong Research Center of Engineering and Technology for Clean Energy, Zibo, Box 255049, China; *Corresponding author: lizhihe@sdut.edu.cn
INTRODUCTION
The high demand for fossil fuels (oil/coal/natural gas) has generated an energy crisis along with serious environmental pollution and climate change. This represents a persistent need for alternative energy resource to substitute for fossil fuels. Biomass can be acquired from plant material or derived matter through the photosynthesis reaction, including woody biomass, agricultural residues, aquatic biomass (Tripathi et al. 2016); animal wastes such as all types of animal manure digestate (Vassilev et al. 2010); and industrial wastes. As a precursor of fossil fuels, biomass can be utilized by different and efficient treatments according to its intrinsic physiochemical characterization. Fast pyrolysis is a common thermal decomposition technology that rapidly converts biomass into crude bio-oil with byproducts, bio-char, and gas at a moderate temperature and in the absence of oxygen. The relative amount of each product is in the range of liquid (50 to 70%), char (12 to 15%), and gas (13 to 25%) depending on operating parameters, properties of biomass, and type of pyrolysis reactor (Isahak et al. 2012). Bio-oil as a dark brown fluid liquid is composed of numerous of oxygenated organic compounds that include acids, alkanes, aromatic hydrocarbons, phenols, sugars derivatives, and small amounts of ketones, esters, ethers, and amines (Isahak et al. 2012). In the case of high oxygen content, corrosiveness, and high viscosity of bio-oil, additional upgrading steps are applied with catalysts to improve the quality for heat and power supply, transportation fuels, and chemical production (such as food additives, farm insecticide, and solvents). Bio-char is also used in the tire industry as carbon soot, activated charcoal, or soil ameliorant to enhance the activity of microorganisms and absorb some harmful substances (Collard and Blin 2014; Hagner et al. 2015). Furthermore, it is economically viable for direct burning with the pyrolytic gas to supply heat or as the inert carrier gas for large scale plants (Lu et al. 2008).
With the deepening of research and improvement of the pyrolysis process, commercial plants have adopted simpler procedures and finer operation to reduce production cost and environmental impact (Kan et al. 2016). A fluidized bed reactor is well known in the petrochemical industry due to its characteristics of simple construction, operation, and scale-up as well as high heat/mass transfer (conduction as the main mode) (Zhang et al. 2010). By using an appropriate size of fluidizing bed media (sand, quartz, or catalyst) and particles (normally below 1 mm), the residence time of particles can be controlled in the range of 0.5 to 2 s by changing the gas-flow rate, which results in high yield of bio-oil (Bridgwater 2012; Isahak et al. 2012). The effects of pyrolysis parameters on the yield and quality of bio-oil by a micro-device under either N2 or Ar atmosphere have been investigated (Heo et al. 2010). However, in pilot plants or commercial units, it is necessary to use non-condensable gas (NCG) to avoid the need for high-cost nitrogen as the initial inert carrier gas and burn the by-product char for heat supplementation; such use of NCG will improve the commercial potential of the pyrolysis process. Lu et al. (2008) designed a fluidized bed reactor with a capacity of 120 kg/h, which is similar to the circulating fluidized bed reactor. Their reactor is completely located within the combustion chamber, where char recycled from cyclones is burnt. However, the rate of char/air should be carefully controlled to ensure combustion efficiency. Yi et al. (2011) and Liu et al. (2009) investigated the effect of fluid bed medium and temperature on the yield and components of bio-oil under the atmosphere of hot flue gas. The fluidized gas was generated by burning anthracite with oxygen. To ensure absence of oxygen, a pivotal point was to control the ratio of air. Li et al. (2014) studied the effect of oxygen on products in an autothermal fluidized bed reactor. Autothermal fast pyrolysis is practical and simplifies the equipment requirements, as there is no need for external heat input. The addition of oxygen increases the production of gases such as CO and CO2. This also changed bio-oil properties, reduced its heating value, increased its oxygen content, and increased its viscosity and phenolics concentration.
A typical NCG is composed of CO, CO2, CH4, and H2, and light hydrocarbon gases, such as C2-C4. Theoretically, recycled NCG for fluidization can affect the decomposition of biomass in the role of a catalyst due to the reductive and oxidative gas atmosphere (Mante et al. 2012). Zhang et al. (2011) investigated biomass fast pyrolysis in a fluidized bed reactor under N2, CO, CO2, and CH4 atmospheres. A CO atmosphere inhibited the bio-oil and CO and H2 atmospheres converted much more oxygen into CO2 and H2O. However, the reductive atmosphere increased the HHV of bio-oil. Jung et al. (2008) conducted an experiment in a fluidized bed reactor under N2 for rice straw and bamboo sawdust pyrolysis. The content of CO, CH4, and other hydrocarbons increased with increasing temperature, and the utilization of by-product gas as a fluidizing medium enhanced the bio-oil yield. Sellin et al. (2016) investigated banana leaves oxidative fast pyrolysis in a fluidized bed reactor with a capacity of 12 kg/h. O2 was the carrier gas and partially involved in combustion to maintain the energy required for biomass pyrolysis, and the inert gas ensured pyrolysis atmosphere. Furthermore, syngas was burnt to heat the air and to reduce energy consumption. Bio-char has an intrinsic value for the activated carbon due to its high yield of char of 23.3%. Controlling the ratio of oxygen and feedstock to maximize the yield of bio-oil is an important and difficult point. Jung et al. (2012) studied samples of waste square timber and ordinary plywood in a bench-scale fluidized bed reactor that was equipped with gas a circulation system. They found that the amount of products is affected by NCG. However, why and how the results can be obtained still remains unclear. Heo et al. (2010) investigated different parameters on the effects of furniture fast pyrolysis in a small-fluidized bed reactor. The optimal temperature of fast pyrolysis was 450 °C, and the NCG increased the yield of bio-oil. The effects of temperature and particle size on the yield of bio-oil in a fluidized bed reactor with hot vapor filtration unit were investigated by Pattiya and Suttibak (2012). At 475 °C, a maximum yield of bio-oil of 69.1 wt% was obtained, and the utilization of a hot filtration unit is better for the quality of bio-oil in terms of viscosity, solids content, ash content, and stability. The addition of methanol reduces the viscosity of bio-oil and slows the ageing rates of the bio-oil.
The effect of recycled NCG in the catalytic pyrolysis of hybrid poplars using FCC catalyst was investigated in the research of Mante et al. (2012). The results indicated that NCG improves the content of organic liquid fraction and the quality of bio-oil. Simultaneously, the presence CO and CO2 enhanced deoxygenation reaction, thus reducing the formation of char. Krishna et al. (2016) studied the pyrolysis of Cedrus saw-mill shavings in hydrogen and nitrogen atmosphere. Their results showed that the addition of hydrogen improved the systematic decomposition of macromolecular of biomass, which enhanced the yield of catechol. Furthermore, this promoted the efficiency of biomass fast pyrolysis. Phan et al. (2014) set up a fluidized bed reactor at a capacity of 60 g/h. Four types of Vietnamese biomass resources were investigated. The liquid yields were all above 50%, and the analytical results of the quality of bio-oil were satisfied, which could be directly used as combustion fuel or chemicals through upgrading. Above all, many different types of fluidized bed pyrolysis processes have been built and many parameters and different biomass were investigated. However, studies on bench-scale continuous fluidized bed pyrolysis with recycled bio-gas as the fluidized medium remain scarce.
Many problems are associated with the process such as the dynamic analysis of quench cooling (Li et al. 2017), the stability of the bed, and fluidizing flow rate (Li et al. 2015), which have been determined in previous studies. In this paper, a fast pyrolysis process based on a fluidized bed reactor coupled with a NCG circulating system was designed. The aims of this paper were (a) to build a stable and continuous process for bio-oil, for eventual scaling up; (b) to check the stability of processes according to key parameters, e.g., temperature, pressure, and char separation; and (c) to characterize the produced bio-oil under different operational temperatures.
EXPERIMENTAL
Biomass Preparation and Characterization
A birch wood sample grown in the Guizhou Province of China was purchased from a wood facility in Hebei Province, China. Prior to pyrolysis, it was ground and meshed to a particle size mainly in the range of 0.18 to 0.6 mm, and basic tests to characterize the feedstock were conducted for the subsequent evaluation of biomass pyrolysis. Proximate analysis was carried out according to the ASTM E871-82 (2019), ASTM E872-82 (2019) and ASTM E1755-01 (2019) standards. Fixed carbon was calculated via the difference based on the content of water, volatile matter, and ash. Ultimate analysis was performed with an automatic elemental analyzer (Euro Vector, model EA 3000, Pavia, Italy) to quantify the carbon (C), hydrogen (H), and nitrogen (N) contents. Oxygen (O) content was calculated according to the formula O% =1-(C+H+N)% based on dry basis. The higher heating value (HHV) was measured via automatic isoperibol calorimeter (IKA, model C2000, Staufen, Germany) based on ASTM D2015-00 (2000). Every test was repeated in triplicate, and the results were averaged. Thermo-gravimetric (TG) and differential thermo-gravimetric (DTG) analyses were performed with a thermogravimetric analyzer (STA 499 F5, Netzsch, Selb, Germany). A sample of 10 mg was placed in a crucible and heated from 25 °C to 800 °C with the heating rate of 10 °C/min. Purified nitrogen (99.999%) at a flow rate of 20 mL/min was used as the carrier gas to provide an inert atmosphere and to remove the volatiles rapidly.
Pyrolysis Process Set-up and Operation
Process set-up
A pilot scale plant was designed for continuous operation with a capacity of up to 5 kg/h of biomass. Figure 1 shows the schematic diagram of the plant, which was composed of the following parts: feeding system, fluidized bed reactor, gas-solid phase separation system (two-stage cyclones), cooling and retain system, gas circulation system (roots blower, gas container and flowmeter), and automatic monitoring system.
The feeding system consisted of a hopper and two-stage screws. The hopper was designed for a throughput of 30 kg. A pipe provided a connection with the gas tank to maintain the inner pressure equilibrium and to ensure anaerobic atmosphere, when birch wood particles were continuously poured into the feeder container. An electromagnet knocker was used to avoid feedstock blockage in the hopper. The knocker was located in the wall of the hopper. The frequency of knocking the wall was altered depending on the status of feeding. The first screw was controlled to maintain feeding rate by frequency transformation. To prevent blocking, the first screw had a variable-pitch. The rotational speed of the second screw was constant at 1440 r/min, and a water-cooling tube was installed at the inlet of second screw to avoided advanced reactions before feedstock entering into the reactor. In the conjunctive location of them, a view-port was installed where the state of the discharge could be checked and where it was convenient to service the unit.
The key part of the pilot scale plant was the fluidized bed reactor, which was made of stainless steel (316S) and had an inner diameter of 120 mm. There were two main parts: an upper reaction chamber (high of 850 mm) and a lower preheater chamber (length of 700 mm) that was filled with a ceramic wafer to sufficiently preheat the carrier gas. The filler materials were randomly arranged without prejudice to the flow of gas. Quartz sand was selected as bed material with a particle size ranging between 0.6 mm and 0.8 mm. The quantity of sand and the flow rate of gas were determined in a previous study (Li et al. 2015). The height of the bed material was 70 mm, and the flow rate was 6 m³/h to 8 m³/h.
Fig. 1. The schematic diagram of 5kg/h laboratory scale plant. 1. Hopper; 2. Two-stage screw; 3. Bubbling fluidized bed reactor; 4/5. Two-stage cyclones; 6. Spray tower; 7. ESP; 8. Filter; 9. Bio-oil tank; 10. Bio-oil pump; 11. Plate heat exchanger; 12. Roots blower; 13. Gas container; 14. Rotary flowmeter; 15. Gas analyser; 16. Electric controlling and monitoring system
A cyclone is cost-effective for the separation of bio-char and vapors in industrial production. According to Fernandez-Akarregi et al. (2013), two-stage cyclones more effectively remove bio-char from bio-oil than one cyclone. In this paper, two-stage cyclones were connected in series, and the particle size of separated bio-char is shown in Table 2. In theory, bio-char particles bigger than 10 µm were collected. The reactor and cyclones were heated by a heating wire, which was pushed through a ceramic column. Five electric heaters were tied in the reactor and cyclones used one. Cyclones were retained at 400 °C to avoid early vapor cooling and second cracking. An electromagnetic vibrator was applied to prevent char adhering to the tube wall.
Cooling and retaining systems were used to capture organic compounds, store bio-oil, and clean NCG. The spray tower was similar to that used in a previous study (Li et al. 2017); however, only one spiral nozzle was used (Zhang 2012). Deionized water, ethanol, or a mixture of both was chosen as coolant contacting with hot vapor. The hot vapors were mostly collected by the first condenser at a liquid temperature of 20 °C. The bio-oil was allowed to flow to the oil pot. The non-condensable vapor of aerosols and tar was captured viaelectrostatic precipitator, which was supplied by 20 kV DC. Due to the high speed of the gas, another filter (polypropylene fiber, 0.22 µm) was used to clean the NCG. Clean NCG was sent back to the reactor via root blower.
All thermal sections were wrapped up in ceramic fiber material at a thickness of 150 mm, which effectively minimized heat loss. The key parts of the process were detected with a K-type thermocouple, which is shown in Fig. 1. According to the feedback information, a temperature controller equipped with proportion integration differentiation (PID) controller adjusted the appropriate temperature by changing output voltage based on the thyristor module. Considering the increasing content of gas, an electromagnetic valve mounted in the buffer tank opened automatically to vent excess gas when the pressure of system reached a fixed value. All temperature and pressure information was saved in distributed control systems (LabView and ZTIC software, Beijing, China). The concentration and HHV of NCG were recorded by a specific gas analyzer software (Wuhan, China).
Unit operation
Prior to process operation, feedstock and cooling medium were loaded, and soap bubbles were used to check the leak-tightness of conduit joints in the fast pyrolysis system. The heating system was run for fluidized bed reactor and cyclones, and the gas analyzer was started. When the temperature reached 400 °C, the root blower was operated at a low frequency and a flow rate about 4 m³/h to improve heat exchange. Before that, the second screw should be continuously worked to prevent sand from regurgitation, which caused it to get stuck in the mechanism. After the goal temperature remained steady, the flow rate was increased to 7 to 8 m³/h, and a small feedstock was conveyed to deplete oxygen, according to data from the gas analyzer. During the experiment, the system pressure changed with the increased gas, and the process was kept stable by altering the rotation speed of the fan. After the experiment, the first screw and the heat system were stopped; secondly, the process was run until the temperature of reactor and cyclone fell to 200 °C. After the bio-oil and bio-char were collected, cyclones were heated to partially burn adherent carbon, while the condenser system was cleaned for the next experiment.
Mass balance
The mass yield (%) of products was determined based on the ratio between the quantity of each product generated after fast pyrolysis and the quantity of the fed biomass. Before the experiment, the mass of feedstock and cooling medium were weighed as mfs and mcm. The quantity of bio-char was collected and weighed from a two-stage cyclone. The carbon boxes were detected to be mcb10, mcb20, mcb11, and mcb21 before and after the experiment. Liquids were determined by weighing the respective mass that was obtained from spray tower (mLs), ESP (mLe), and filter (mLf) after the experiments. Every experiment is given that it was at mass balance. Therefore, the pyrolysis product yields were determined with the following equations.
Y(Bio-oil) = ((mLs+mLe+mLf–mcm)/mfs)×100 (1)
Y(Bio-char) = ((mcb11+mcb21–mcb10–mcb20)/mfs)×100 (2)
Y(Bio-gas) = (1-Y(Bio-oil)–Y(Bio-char))×100 (3)
Bio-oil and NCG analysis
The liquid fraction obtained under different temperatures was composed of light and heavy bio-oils. Chemical components were characterized via gas chromatography coupled with mass spectrometry (GC/MS, Agilent, Model 5973-6890N, Santa Clara, CA, USA). The content and HHV (higher heating value) of NCGes (CO, CO2, CH4, H2, and light carbons) were measured via Gasboard-3100 gas analyzer (Wuhan Cubic Optoelectronics Co., Ltd., Wuhan, China). The HHV also can be calculated by the following formula,
HHV= q1·[CO]+ q2·[CH4] + q3·[H2] (4)
Where q1, q2, q3 are the HHV of CO, CH4 and H2, respectively (MJ/m³); [CO], [CH4] and [H2] are the volume concentration of gas components, respectively (vol. %).
RESULTS AND DISCUSSION
Feedstock Characterization
Table 1 lists the characteristics of the birch wood sample. The proximate analysis showed similar amounts of volatile matter and fixed carbon to woody biomass (Vassilev et al. 2010) of 75.8 % and 17.2 %, respectively. However, the ash content was lower than the agricultural biomass of 0.6 wt.%. This result indicated that a small amount of impurities was present, especially metal-salts, which act as catalyst and contribute to decrease of bio-oil yield and changes in the properties (Vassilev et al. 2010). The moisture content without drying was 6.6 %. A moisture content below 10% and a low ash content can improves heat transfer in favor of thermal conversion and reduces additional energy needs (Choudhury et al. 2014; Hu et al. 2015). Elemental analysis showed that carbon and oxygen were the fundamental components (48.52 % and 45.4 %, respectively). The HHV was 18.55 MJ/kg, which is typical for lignocellulosic biomass (Abraham et al. 2013). Cellulose and hemicellulose were the main components; however, the content of lignin was less compared with previous research (Jung et al. 2012), which could be attributed to the removal of bark in the birch wood feedstock.
Table 1. Main Characteristics of the Birch Wood Sample
a:By the difference
The TG/DTG curve of birch wood in Fig. 2 indicates three thermal degradation stages between 25 °C and 800 °C. The first stage occurred prior to 200 °C, which was caused by the evaporation of water. The weight loss reached 6.24%. There are two main peaks (the temperature of peak 291 °C and 363 °C) in the second stage, which corresponded to hemicellulose devolatilization and cellulose decomposition, respectively. The maximum weight loss reached 75.4% at 520 °C. A similar peak variation and weight loss were reported by Khelfa et al. (2013). The third stage showed lignin decomposition and carbonization between 520 °C to 800 °C, and the loss of weight was up to 4.6%.