Characteristics and mechanism of soot formation during the fast

Subscriber access provided by UNIV TEXAS SW MEDICAL CENTER

Biofuels and Biomass

Characteristics and mechanism of soot formation during the fast pyrolysis of biomass in an entrained flow reactor Yan Li, Houzhang Tan, Xuebin Wang, Shengjie Bai, Junyu Mei, Xiaoqing You, Renhui Ruan, and Fuxin Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00752 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Characteristics and mechanism of soot formation during the fast pyrolysis of biomass in an entrained flow reactor Yan Li1, Houzhang Tan1*, Xuebin Wang1*, Shengjie Bai1, Junyu Mei2, Xiaoqing You2, Renhui Ruan1, Fuxin Yang1

1. MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China 2. Center for Combustion Energy, Tsinghua University, Beijing 100084, China * Phone: +86-029-82668703. E-mail: [email protected]

ABSTRACT: To understand the effects of biomass origin and temperature on the characteristics of soot formation from biomass, fast pyrolysis of wheat straw and sawdust was performed in an entrained flow reactor in the temperature range of 900 to 1300 oC. The produced soot, permanent gas and tar were sampled and characterized by transmission electron microscopy (TEM) and gas chromatography/gas chromatography-mass spectroscopy (GC/GC-MS) with respect to yield, morphology, structure and composition. Moreover, soot formation was modeled in a plug flow reactor (PFR) with a detailed reaction mechanism. Results indicate that the woody biomass produced a significant higher quantity of soot (0.34%-6.30%(dry biomass, db)) than that from straw (0.28%-2.40%(db)),

and the woody soot has a more ordered structure. The reason was primarily

ascribed to the combined effects of high contents of lignin, cellulose and low content of ash in woody biomass. The carbonization of soot occurred at about 1100 oC when primary spherical

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

particles were formed with concentrically stacked graphitic layers. All of the collected tar species were deoxygenated aromatic components, of which benzene and naphthalene were the characteristic species. The aromatic species in sawdust tar were much more heterogeneous than those of straw. Due to the soot formation reactions occurred above 900 oC from the secondary decomposition of light CxHy gases, carbon conversions of the two biomasses declined. When the temperature reached 1100 o

C, the CO generation reactions were strengthened remarkably, resulting in the re-increase of carbon

conversion. A reasonable agreement between the observed and the predicted soot yield was obtained. In this studied case, the HACA (hydrogen abstraction carbon addition) route is the dominated route for soot formation, while the contribution from CPDyl (cyclopentadienyl) dimerization is small. KEYWORDS: Biomass, Pyrolysis, Soot formation, Carbon conversion, Simulation


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1. INTRODUCTION Soot is a carbonaceous solid material that is formed during pyrolysis and flame combustion. All of the hydrocarbon-based fuels have the potential to form soot. Soot in flames significantly enhances the radiative heat transfer with its large surface area. The near-burner flame temperature could be lowered by several hundred degrees due to the heat transfer to the surrounding walls1, which in turn affects the thermal formation of gaseous pollutants (NOx, SOx, etc.). However, the escaped soot from flames is one of important components in combustion-derived PM2.5. Soot along with its adhered PAHs and inorganic materials have been identified to be harmful to human health and have direct or indirect effects on the atmospheric climate2, 3. Millions tons of soot particles are emitted to atmosphere each year. It is estimated that nearly 30% to 40% of the global soot emissions were from biomass burning4. Therefore, it is important to study the properties and mechanisms for soot formation in biomass combustion systems. Until now, there have been extensive experimental studies on soot formation process in diesel engines and in diffusion flames at various combustion conditions. Detailed mechanisms of soot have been developed for simple hydrocarbon fuels. However, little attention has been given to the soot mechanisms from biomass. There is a limited literature available concerning soot particle formation from biomass. Zhang et al.5, 6 described tar destruction and soot formation during rapid pyrolysis and gasification of a woody waste in a drop tube furnace (DTF) over a wide temperature range from 600 to 1400 oC. The soot was defined as the solid particles passing through the char hopper as well as the membranous solids that deposited on the reactor. This could result in an over determination of soot as a portion of fly ash and char particles could also pass through the char hopper. The soot yield in Zhang’s research reached 16% (db) at 1100 oC. Trubetskaya et al.7, 8 also investigated the effects of


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

biomass origin on the yield, nanostructure and reactivity of soot in a DTF operated at 1250 and 1400 o

C. A PM2.5 cyclone was used to separate soot from char particles and soot was composed of

inorganic and organic fractions. The inorganic fraction in fact is the fly ash produced during pyrolysis. It was found that soot yields were almost lower than 9% (daf). All the soot samples exhibited a well-ordered graphitic structure and there were no observed structural changings between soot from different temperatures. Wiinikka et al.9, 10 characterized the soot produced from a real gasifier burning woody powder under different operating conditions with regards to morphology, nanostructure and reactivity. The results indicated that soot particles had less ordered nanostructure as temperature increased. As summarized from the previous soot studies of biomass, it can be found that most are focused on the characterization of physicochemical properties for soot, lacking of deep discussions on soot formation mechanism, particularly with respect to soot modeling. Taking into account that a high level of volatiles is produced during biomass pyrolysis, the secondary reactions of the hydrocarbons in volatiles are perceived as the precursors to soot. Thus three main routes are proposed for biomass soot formation11-15: (1) through conventional small hydrocarbon mechanism that is suggested to follow the HACA (hydrogen abstraction carbon addition) route; (2) through dimerization of two resonance stabilized cyclopentadienyl (CPDyl) radicals to generate aromatic rings, i.e. the CPD route. The CPDyl is largely derived after cracking of lignin monomer fragments; (3) through the direct condensation and transformation of aromatic rings. Up to now, it is still unclear that which ones dominate biomass soot formation. Fitzpatrick et al.16-18 suggested that the main contributions for biomass soot were via the HACA and CPD routes. Wijayanta et al.

19, 20

simulated the evolution of

biomass/coal volatiles (including soot) during O2/CO2 gasification in a plug flow reactor (PFR) using


Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

a detailed soot formation mechanism which emphasized the PAH condensation pathway to soot nuclei. The established confidence through the comparison of the simulation results with the experiments indicated the importance of PAH association for soot inception. In this study, pyrolysis of wheat straw and sawdust was conducted in an entrained flow reactor over the temperature range from 900 to 1300 oC to understand the effects of biomass origin and temperature on biomass soot formation characteristics. The soot was separated from char particles via a PM2.5 cyclone and was defined as the carbonaceous matter in PM2.5. Considering that soot chemistry is closely combined with the complex process of biomass decomposition, byproducts such as liquidus tar and permanent gas were collected and measured to help analyzing the formation process. Further, the simulation was conducted by using the KM2 mechanism, a detailed mechanism on soot formation with improved CPD sub-mechanism. Contributions of different routes on biomass soot formation were evaluated through comparing the prediction results between the full mechanism and the CPD route omitted mechanism. 2. EXPERIMENTAL METHODS 2.1. Feed Materials Wheat straw (WS) and poplar sawdust (SD) from the Baoji district were used as the feedstock in this study. The materials were milled and sieved to less than 100 µm and dried at 90 oC over night before use. The properties of the fuels and their ash compositions (analyzed by XRF) are shown in Table 1. The compositional analysis for biomass was based on Van Soest analysis according to H.S.S. Sharma21. The extractives mainly consist of protein, resin and fatty acids, waxes, and phytosterols. Table 1 Proximate, ultimate and compositional analyses of fuels


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fuel

Wheat straw

Proximate analysis (wt.% dry) Moisture* 7.66 Ash 9.93 Volatile 74.40 Fixed carbon 15.67 Ultimate analysis (wt.% dry ash free) Cdaf 45.53 Hdaf 5.26 Ndaf 0.78 Odaf 48.15 St,daf 0.28 Compositional analysis (wt.% dry) Cellulose 26.75 Hemicellulose 30.78 Acid-insoluble lignin 6.14 Extractives 25.75 Major ash components (wt.%) K2 O 29.81 CaO 19.00 SiO2 29.72 Fe2O3 5.59 Cl 8.16 S 2.43 P2O5 1.83 Al2O3 1.39

Page 6 of 30

Sawdust 30.54 2.89 82.30 14.81 50.82 5.56 0.14 43.39 0.09 43.96 34.33 18.25 2.00 23.91 55.41 6.63 4.54 0.47 1.69 3.95 1.23

* Moisture is measured as received 2.2. Apparatus and Procedures 2.2.1. Entrained flow reactor More details about the entrained flow reactor has been described in the literature22. It is composed of a feeding unit, gas supply unit, reactor and furnace heating unit, and a products sampling unit. The cylindrical reactor tube made from alumina ceramic is 1200 mm in length and 54 mm in diameter and is heated by three zones of temperature control system with a constant-temperature region about 500 mm long. Biomass particles were fed by a micro-scale fluidized-bed feeder and were injected directly into the constant-temperature region of the reactor


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

through a water-cooled feeding probe. The feeding rate was set as 200 or 100 mg/min. Nitrogen was used to maintain the inert atmosphere in the reactor and the total flow rate was 4 L/min. The gas residence time was estimated to be 2-4 seconds over the temperature range between 900 to 1300 oC. The sampling process at each condition was conducted in duplicate to ensure the repeatability. 2.2.2. Soot sampling and determination When biomass particles dropped into the reactor tube, they quickly devolatilized and decomposed into gas, tar, char, etc. Most of pyrolytic products were collected from the flue tail and then were sent for measurement. Based on the different size scale between soot and char particles23, 24

, a cyclone with cut size of 2.5 µm was employed for the separation7, 25. Fine particles passed

through the cyclone and then were captured from the gas flow by a quartz or a metal filter. To avoid tar condensation, the whole soot sampling system was heated up to 230 oC. Further, an additional hot dilution gas of 6 L/min was introduced into the sampling system to ensure the separation efficiency of the cyclone. The captured fine particles mainly contain soot and ash particles. Then the mixed particles were combusted at 550 oC in a muffle furnace to determine the carbon and ash content. The soot is defined as the carbonaceous matter in fine particles. Soot samples for TEM (Hitachi HT7700) analysis were prepared by ultrasonicating small amounts of the soot powder in dichloromethane solvent. After centrifugalizing three times, the suspensions were deposited on copper grid for TEM observation. 2.2.3. Gas and tar sampling and measurement The gas products were collected in a gas bag after a serial of dust and liquid removal equipment and the gas composition was measured by a gas chromatograph (GC-2014, Shimadzu), which is equipped with two types of columns (MS-13X and Porapak-N) as well as double thermal


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

conductivity detectors (TCD). The column and the detector temperatures were set at 60 and 100 oC respectively. Using this method, the O2, N2, CO, CO2, CH4, H2 and C2 species in gas can be detected separately. The method for tar sampling in this study is based on the one described in Yi Su’s research26, with some modifications. The heated flue gas first passed through the cyclone and the filter to remove particles and then was condensed and trapped in four impinger bottles. Two of the bottles were exposed to air and the rest were placed in a methanol–CO2 ice bath. The dichloromethane was applied as the absorber. The quantity of absorbed tar was determined by gravimetric analysis26. After dehydration by anhydrous sodium sulfate, a small volume (1 µL) of tar sample was injected into a gas chromatography-mass spectrometer (GC-MS QP2010 Ultra, Shimadzu), which used the Rxi-5Sil MS as column. The temperatures for both the injection port and interface were 280 °C. A flow of helium was used as the carrier gas (2.3 mL/min). The heating program for column oven was 50 °C for 2 min, then ramped up to 300 °C at 8 °C/min and stay for 10 min. The mass detector temperature was 230 °C. The electron-impact mass (EI-MS) spectra were recorded over the range of 15–400 m/z. Carbon conversion during pyrolysis (η) is defined as the mass fraction (wt.%) of carbon in the feedstock converted to gaseous products (including CO, CO2,CH4 and C2–C3 species), the formula is given by:

η=

C (CO)+C (CO 2 )+C (C x H y ) C (biomass)

×100% (Formula 1)

Where C(CO), C(CO2), C(CxHy), C(biomass) is the mass fraction of carbon in gaseous CO, CO2, light hydrocarbon (including CH4 and C2–C3 species) and biomass, %.


Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. Schematic view of the entrained flow reactor and the products collection system

2.2.4. Numerical calculation of soot formation The moments with interpolative closure (MoMIC) method was employed to predict the soot volume fraction along this reactor27, 28. The first three moments of the particle size distributions were coupled with the gas-phase chemistry simulated by the KM2 mechanism, which contains 202 species and 1351 reactions29. The KM2 mechanism includes detailed reaction pathways for the formation and growth of PAH molecules up to coronene, among which 17 reactions are associated with the CPD route in forming larger aromatics. In this numerical calculation, particle surface growth was based on HACA mechanism, while soot inception was defined as dimerization of two pyrene molecules30-32. The plug flow reactor (PFR) program in the CHEMKIN software package was employed to calculate the soot formation in our tube reactor 33. A composition of biomass volatiles pyrolyzed at 900 oC was implemented as the inlet reactants to predict the soot formation at varied temperatures (1000-1300 oC). The input species included H2, CO, CO2, CH4, C2H2, C2H4 and the tar species with mass fraction larger than 4% in Table 2. The


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

calculated soot volume fraction was converted to mass yield assuming a density of 1.8 g/cm3. The tar yield (δtar (%,db)) was obtained by mass balance: δtar =100%–δchar –δgas –δsoot– δmin–δH2O where, the δchar (char yield) at 900 oC were measured as 4.27% (db) for wood and 17.60% (db) for straw, respectively. The δgas, δsoot, δmin (yields of gas, soot, and minerals in fine particles) were measured using the above methods. The δH2O (H2O yield) was assessed as about 20% from the previous similar studies [5, 34]. Thereby, the calculated tar yield was 16.21% (db) for wood and 9.64% (db) for straw. 3. RESULTS AND DISCUSSION 3.1. Soot Yield The yields of soot from the two biomass fuels at different temperatures are showed in Figure 2.The soot yield increased with the increase of temperature. A rapid increase in soot yield was observed when the temperature increased from 900 to 1100 oC; then the increase slowed down, staying at 2.40% and 6.30% (db) at 1300 oC for wheat straw and sawdust respectively. The soot formation process needs sustained dehydrogenation and polymerization of aromatic radicals to form larger PAH radicals. During the process a high energy barrier has to be overcome. The increase of temperature favors soot formation. However the higher temperature also aggravates the competitive cracking reaction of aromatics as well as the self-gasification reaction between soot and H2O or CO2, leading to the slowdown of soot formation at higher temperatures. The soot-temperature profile obtained in the present experiment was different from that of some biomass gasification cases. The latter presented a bell shape with the peak temperature around 1100 or 1200 oC6, 25. The difference is mainly attributed to the changes in atmosphere. The level of self-gasification of soot in inert


Page 10 of 30

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

condition is extremely lower than that in gasification condition, resulting in sustained increase in soot production with temperature. In Figure 2, it was clear that sawdust exhibited a faster and earlier increase than wheat straw in the studied temperatures. The wood created a substantially higher soot production, roughly 1.6 times higher. Results from literature [7-9, 25, 35] also showed that woody samples have higher soot yields than those of herbaceous fuels. The result can be explained by the differences in composition between the two biomasses. As showed in Table 1, sawdust contains nearly three times as much lignin as straw. A high content of lignin can generate a significant fraction of soot-precursor PAHs during pyrolysis concerning the hydroxycinnamyl alcohol monomers in lignin. In addition, a variety of cracking side chains undergoes dehydration and cyclization, forming aromatic rings. Therefore, it can be suggested that lignin is the main source of biomass soot. Another factor for the high soot yield of sawdust should be the high level of cellulose, which is 1.7 times higher than that of straw. In previous literature36-38, pyrolysis experiments were performed under low temperatures (≤900 oC), and cellulose largely decomposed to volatiles fraction and did not contribute significantly to soot formation. However, with the increase of temperature the hydrocarbons in cellulose volatiles are subsequently pyrolyzed to generate soot. Wilson et al. provided the evidence that in diffusion flame cellulose also contributed to the soot formation in different routes with lignin39. Yu et al. also reported that produces from the secondary reaction of cellulose and hemicellulose tars, such as benzene, derivatives of benzene, furan and toluene etc., could further react and condense to form PAHs with a rise in temperature40. Consequently, cellulose and henmicellulose can also contribute to the soot formation. Apart from organic constituents in biomass, the inorganic ash content can affect the soot formation in large extent. Potassium species are demonstrated to have catalytic effect on tar


Energy & Fuels

decomposition, thus lower the soot precursor production41, 42.

7 6

WS SD

5

δsoot (%,db)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

4 3 2 1 0 900

1000

1100

1200

1300

o

T ( C)

Figure 2. Soot yields as a function of temperature for the two biomass fuels

3.2. Soot Morphology Figure 3 presents the typical TEM images of soot particles produced by the two biomasses at different temperatures. The soot particles were agglomerated by a batch of primary spherical particles with a broader particle size distribution from 20 to 200 nm, which were similar to those obtained from other biomass smokes8, 9. The prolonged residence time (about 1-4 s) in a reduced atmosphere allows significant soot surface growth, resulting in a larger particle size when compared to soot particles from gaseous flames. At lower temperatures (≤1000 oC), the soot particles showed a disordered, amorphous structure, similar to the tar-like precursor particles. When temperature was higher than 1100 oC, spherical particles became shaped with their structure being more organized with temperature. This transition is related to the carbonization process of soot. Higher temperatures provide sufficient thermal activation to surmount the activation energies for growth and reorientation of the carbon lamella43,44, leading to soot with much more structural order. At 1000 oC, the near-spherical shaped particles can be identified in sawdust soot while in wheat straw soot the totally


Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

amorphous structure. This indicates that woody soot has relative higher maturity, which corresponds to its high soot yield. 1100oC

1000oC

1300oC

200nm

200nm

200nm

(a) 1100oC

1000oC

1300oC

200nm

200nm

200nm

(b) Figure 3. TEM images of typical soot particles at different temperatures for (a) wheat straw, (b) sawdust

3.3. Tar Conversion Figure 4 shows the effects of temperature on tar yield for the two biomass fuels. Temperature strongly enhances tar decomposition. A temperature above 1200 oC is necessary to completely decompose tar components. The tar species analyzed by GC-MS are shown in Table 2. Tar generated from biomass pyrolysis above 900 oC consisted of deoxygenated aromatic compounds, among which benzene was the most notable species, with relative mass portion beyond 50 wt.%, followed by


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

naphthalene, acenaphthylene, pyrene, etc. These stable species are typical tar constituents during hydrocarbon fuel pyrolysis and are not completely destructible even at 1100 oC. On the contrary, large PAHs with three or more rings disappeared gradually when temperature increased, implying their conversion to solid soot. In addition to six-membered ring species, a significant number of five-membered ring containing species of different size were observed in tar molecules. The presence of five-membered rings could lead to positive curvature of the graphene layers, thus affecting the chemical reactivity of soot43. Phenol was among the characteristic products in some biomass pyrolysis cases. Majority of phenol is derived from the cracking of lignin monomer fragments45. However, phenol was not detected in this study, which was probably attributed to the relative longer residence time. The aggravation of pyrolysis process, such as increasing temperature or extending the reaction time, was able to remove the oxygen functionalities completely6, 46. The primary reaction of thermal decomposition of phenol is proved to be the production of CPD and CO through47: C6H5OH ↔c-C5H6+CO

(R764)

Once CPD formed, it dissociates unimolecularly to cyclopentadienyl (CPDyl) radical: c-C5H6 →c-C5H5+H

(R782)

Two CPDyl radicals combine and rearrange to form naphthalene11: +

+H+ H

(R1071)

Similar to the naphthalene formation process, a phenanthrene can also be formed based on the combination of CPDyl with indenyl (R1072). Therefore, large aromatics can be originated from these resonance stabilized radicals. Research from Fitzpatrick et al.17 suggested that the CPD route was an important PAH growth route in biomass soot formation.


Page 14 of 30

Page 15 of 30

As can be seen from Figure 4 and Table 2, the sawdust generated a higher yield of tar than that from the straw at 900 oC, coupled with much more heterogeneous aromatic species. PAHs are important precursors to soot. The tar formation behavior in sawdust pyrolysis corresponded with its high yield of soot. 2.0 1.6

Tar yields (%,db)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

WS SD

1.2 0.8 0.4 0.0 900

1000

1100

1200

1300

o

T ( C)

Figure 4. Tar yields as a function of temperature for the two biomasses Table 2 Relative mass portion of tar species at 900-1100 oC for the two biomasses

MW

Formula

Name

Sawdust

Structure

Straw

900℃

1000℃

1100℃

900℃

1000℃

1100℃

78

C6H6

Benzene

55.87

68.4

92.80

68.28

75.78

96.49

92

C7H8

Toluene

1.03

0.68

1.23

0.94

0.74

1.46

102

C8H6

Phenylacetylene

1.03

0.30

0.98

0.37

104

C8H8

Styrene

0.63

0.25

0.46

16

C9H8

Indene

0.50

0.29

0.25

128

C10H8

Naphthalene

12.14

7.34

0.84

14.44

6.38

152

C12H8

Biphenylene

8.01

3.91

4.49

6.21

10.44


1.52

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

154

C12H10

Biphenyl

0.23

166

C13H10

Fluorene

0.38

166

C13H10

1H-Phenalene

0.17

178

C14H10

Phenanthrene

6.43

178

C14H10

Anthracene

0.97

190

C15H10

4H-Cyclopenta[def]phenanthrene

0.42

202

C16H10

Fluoranthene

202

C16H10

226

0.18

2.35

2.14

1.08

4.57

4.85

2.18

1.33

Pyrene

5.18

8.32

4.12

3.89

C18H10

Benzo[ghi]fluoranthene

0.41

0.58

226

C18H10

Cyclopenta[cd]pyrene

0.79

2.39

228

C18H12

Benzo[c]phenanthrene

0.25

252

C20H12

Perylene

0.59

252

C20H12

Benzo[e]pyrene

0.41

0.46

0.53

0.34

3.4. Carbon Conversion The pyrolysis process and main gas reactions that may take place are outlined in Table 3. In pyrolysis process, the decomposition of methoxyl groups and aliphatic side chains from parent


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

molecules is the principal formation sources of light hydrocarbon gases. The secondary decomposition of these light hydrocarbon gases and certain tar species produces soot, leading to the decline of the energy conversion efficiency. Figure 5 shows the yields of the main gaseous products versus temperature (In order to depict the rules more clearly, the experiments started from 700 oC). It can be seen that the yields of CO and H2 increased steadily with temperature, while CO2 relatively stabilized with initial temperatures then decreased apparently when temperature increased above 1100 oC. The stability indicated that there was a balance between the CO2 consumption reactions (CO2 reforming and gasification) and production reaction (WGS). A higher temperature (above 1100 o

C) enhances the CO2 reforming and gasification reactions significantly, creating the decrease in CO2

content. For CxHy, the decrease in CH4 and C2H2 above 1000 oC coincided with the increase in soot at the same temperature range, implying their conversion to soot. Due to the cracking reaction, C2H4 suffered a gradual decrease with temperature. Based on the Formula(1), the calculated carbon conversions for the two biomasses are shown in Figure 6. Profiles of carbon conversion from two biomasses were characterized with the same trend. It initially increased with the increasing of temperature, peaking at 55.4% (db) at 900 oC for wheat straw and 59.3% (db) at 1000 oC for sawdust. Then, the data began to decline, reaching the bottom of 47.4% (db) and 55.8% (db) at 1100 oC. As the temperature increased further, the data increased again and finally stood at 58.3% (db) and 60.0% (db) at 1300 oC. The results generally consistent with Zhang’s results6 except for some variations in break temperatures. The difference may be due to the different temperature fields in reactors. The initial increase of carbon conversion at low temperatures is well understood as the extent of the primary pyrolysis is enhanced by temperature. When the temperature further increased above 900 oC, the secondary decomposition of light hydrocarbon gases


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

that forms soot (reaction (℃)) occurred, leading to the decline of carbon conversion. On the other hand, the increase of temperature favors the hydrocarbon reforming reactions (℃, ℃) and the carbon gasification reactions (℃, ℃), which produce CO and H2, contributing to the increase in carbon conversion. Consequently, the value is dependent on the balance between the soot forming reaction (℃) and the CO forming reactions (including reaction (℃-℃)). When the temperature reached 1100 o

C, the tar had almost completely decomposed, and the extent of CO forming reactions exceeded the

soot forming reaction, resulting in the re-increase in carbon conversion. From Figure 6, it can be found that sawdust presented a higher carbon conversion than wheat straw. The reason is due to the high gas yield (showed in Figure 5) which is associated with the high cellulose content in woody biomass. Table 3. Gas reactions occurred in biomass pyrolysis48 No

Reaction

∆H 0298 (kJ/mol)

℃

Hydrocarbon polymerization CmHn→(n/2)H2+mCsoot Steam reforming

endothermic

℃

CmHn+H2O→(m+n/2)H2+mCO

endothermic

℃ ℃ ℃ ℃

CO2 reforming CmHn+mCO2→2mCO+(n/2)H2 Steam gasification C+H2O↔H2+CO CO2 gasification C+CO2↔2CO Water gas shift(WGS) CO+H2O↔H2+CO2

endothermic +131.29 +172.46

a. CmHn can be tar or light gases


-41.16

Gas yields (mol/kg,db)

80 70 60 50 40 20 15 10 5 3 2 1 0 30

Total

CO

CO2

20 10 0 4 3 2 1 0 2.0 1.5 1.0 0.5 0.0 2.0 1.5 1.0 0.5 0.0

WS SD

H2

CH4 C2H2

C2H4 700

800

900

1000

T (oC)

1100

1200

1300

Figure 5. Gas yields of two biomasses at different temperatures 60

Carbon Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Gas yields (%,db)

Page 19 of 30

WS SD

55

50

45

40 700

800

900

1000

1100

1200

1300

T (oC)

Figure 6. Carbon conversions versus temperature

3.5. Modeling and mechanism analysis on soot formation 3.5.1 Modeling on soot formation The predicted soot yields at temperatures higher than 1000 oC with KM2 mechanism are showed in Figure 7. In order to assess the impact of CPD route on soot formation, the reactions involving CPDyl for aromatics formation were omitted from KM2 mechanism to compare with the


Energy & Fuels

full KM2 mechanism. As seen from Figure 7, the calculated soot yields using both full and omitted KM2 mechanisms agreed well with the measured ones. This indicates that under the conditions of this study (900-1300 oC), the HACA route is the dominated route for PAH formation and growth, whereas the contribution from CPD route is small, especially at high temperatures. Note that there might be some limitations on this conclusion. In this calculation, the volatiles generated at 900 oC were set as the input reactants, when phenol had already decomposed at such a high temperature. However, phenol is probably an important source for CPD production during biomass pyrolysis through C6H5OH → C5H6+CO

(R764)

and this has been discussed in section 3.3. Thus the contribution of CPD to PAH formation in this calculation may be underestimated. Considering that few aromatic ring-ring condensation reactions are included in KM2 mechanism, our calculation results indicate that the PAH condensation route for PAH growth may be not important during biomass pyrolysis. 10

Soot yields (%,db)

10

Soot yields (%,db)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

1 KM2 KM2 without CPD Experimental

1 KM2 KM2 without CPD Experimental

0.1

0.1 1000

1100

1200

1300

1000

T (oC)

1100

1200

1300

o

T ( C)

(a)

(b)

Figure 7. Comparison between experimental and calculated soot yield for (a) wheat straw, (b) sawdust 3.5.2 Mechanism analysis on soot formation based on gas product evolution and ROP analysis


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

In order to demonstrate the soot formation mechanism, the profiles of soot volume fraction and the relevant gas contents (H2, C2H2, C2H4, CH4, A1, A2, A3) along the reactor length at 1100

o

C and

1300 oC are plotted and compared in Figure 8. The ROP (rate of production) analysis on these relevant gas species based on gas reactions (not involving surface reactions) is conducted and shown in Figure 9 to demonstrate its relation to soot formation. As shown in Fig.8, along the reactor length, the soot volume keeps increasing at 1100 oC, while at 1300 oC, its increase becomes slow after 20 cm (~1.1 s), which indicates the significant promotion of high temperature on soot formation. At both temperatures, the soot formation is accompanied with the release of H2 and the consumption of C2H2, C2H4, CH4, A1, A2 and A3. Note that the mole fraction of C2H2 increases first and then decreases, and its decrease lags behind the decrease of C2H4 and CH4. These phenomena can be explained through the ROP analysis results in Figure 9. In this numerical calculation, soot inception is defined as dimerization of two pyrene molecules33. 2A4 => 32C(B) + 20H(se) + 28.7487open(se) (surface reaction 1) The soot nuclei continue grow on their surface through HACA mechanism, with the consumption of C2H2 and the release of H2. H+H(se) → open(se)+H2

(surface reaction 2)

open(se)+C2H2 → H(se)+2C(B)+H

(surface reaction 3)

As indicated from the ROP analysis in Figures 9(g)-(h), besides of C2H2 addition on A3, the self-addition of C9H7 also plays an important role in A4 formation. C9H7+C9H7 →A4+C2H2+H2

（R1152）

A3 and its derived species are mainly consumed by the formation of A4. The formation of A3 is


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

not only from C2H2 addition on A2, but also from C2H2 addition on biphenyl (R890) and C3H3 reacting with A2CH2 (R1146). P2-+C2H2 →A3+H A2CH2+C3H3→A3+H+H

（R890）（R1146）

As to the key species of benzene (A1), the ROP analysis in Figures 9(a)-(b) indicates that the self-addition of C3H3 (R298) is important for benzene formation. C3H3+C3H3→A1

（R298）

Through the ROP analysis results on A1-A4, besides of C2H2, these derived radicals from CH4 and C2H4 are also very important for the formation of PAHs. Consequently, we further conducted the ROP analysis on CH4, C2H2 and C2H4 in Figures 9(i)-(n). The ROP analysis indicates that the formation of CH3 radical is through CH4 dehydrogenation, and the addition of C1 and C2H2 is necessary for C3 formation. With the formation of soot, C2H2 is largely consumed, however, at the started stage, C2H2 can also be produced from C2H4 and other derived radicals as shown in Figures 9(m)-(n). This can explain why the concentration of C2H2 first increases and then decreases in Figure. 8. Based on the ROP analysis above, the formation pathway of PAHs and soot is proposed in Figure 10.


Page 22 of 30

10

10

0 12

0

10

(×10-3) C2H2 (×10-3) CH4 (×10-3) C2H4 (×10-3) H2

8 6 4

60

24

(×10-8) Particle volume fraction

18

(×10-4) A1 (×10-5) A2 (×10-7) A3

12 6

Mole fraction

20

Volume fraction

20

30

30

(×10-8) Particle volume fraction (×10-4) A1 (×10-5) A2 (×10-7) A3

30

Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0

30 15

0 16

0

12

(×10-3) C2H2 (×10-3) CH4 (×10-3) C2H4 (×10-3) H2

8 4

2

45

Volume fraction

Page 23 of 30

0 0

10

20

30

40

50

60

0

10

20

30

40

Distance (cm)

Distance (cm)

(a) 1100 oC

(b) 1300 oC

50

60

Figure 8. Parameters of soot formation and calculated mole fractions of different reactants along reactor

A1+CH3=A1-+CH4 R772

WS SD

C3H3+C3H3=>A1 R298

A1-+C2H4=A1+C2H3 R948

WS SD

A1+H=C4H5-2+C2H2 R472 A1+OH=A1-+H2O R688

A2-1+A1=>FLTN+H+H2 R1013

A1+C2H=A1C2H+H R784

A1+A1-=P2+H R883

A1+C3H3=C9H8+H R1125 A1+A1-=P2+H R883

A1+CH3=C6H5CH3+H R759

C3H3+C3H3=>A1 R298

A1-+C2H4=A1+C2H3 R948

A1+CH3=C6H5CH3+H R759

A1+H=A1-+H2 R690 -1.0

-0.5

o

A1-1100 C 0.0

0.5

1.0

A1+H=A1-+H2 R690 -1.0

-0.5

0.0

Relative ROP

Relative ROP

(a) A1-1100 oC

(b) A1-1300 oC

A2CH3+H=A2+CH3 R982

WS SD

A2-2+H=A2 R965

A2+H=A2-1+H2 R1196

A2+H=A2-1+H2 R1196

A2-2+CH4=A2+CH3 R1169

A2-2+CH4=A2+CH3 R1169

-0.5

A2-1100oC 0.0

0.5

1.0

A2+H=A2-2+H2 R1198 -1.0

-0.5

A2-1300oC 0.0

Relative ROP

Relative ROP


1.0

WS SD

A2-1+CH4=A2+CH3 R1167 A2-2+H=A2 R965

A2+H=A2-2+H2 R1198

0.5

A2C2HB+H=A2+C2H R834

A2-1+CH4=A2+CH3 R1167

-1.0

A1-1300oC

0.5

1.0

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

(c) A2-1100 oC

(d) A2-1300 oC

A2CH2+C3H3=A3+H+H R1146

WS SD

A3-1+H=A3 R1026

A2CH2+C3H3=A3+H+H R1146 P2-+C2H2=A3+H R890

P2-+C2H2=A3+H R890

A3-1+H=A3 R1026

A3-1+CH4=A3+CH3 R1173

A3-1+CH4=A3+CH3 R1173

A3+H=A3-1+H2 R1202

A3+H=A3-1+H2 R1202

A3+H=A3-4+H2 R1200 -1.0

A3-1100oC

-0.5

0.0

0.5

1.0

WS SD

A3+H=A3-4+H2 R1200 -1.0

A3-1300oC

-0.5

0.0

Relative ROP

(e) A3-1100 oC WS SD

A4-2+CH4=>A4+CH3 R1177 A4-1+CH4=>A4+CH3 R1175

A3C2H2=A4+H R876

WS SD

A4-4+H=A4 R882 A4-2+H=A4 R880 A4-1+H=A4 R878

A4-4+CH4=>A4+CH3 R1179

A4-2+CH4=>A4+CH3 R1177

A4+H=>A4-4+H2 R1208

A4-1+CH4=>A4+CH3 R1175

A4+H=>A4-1+H2 R1204

A4-4+CH4=>A4+CH3 R1179

A3-4+C2H2=>A4+H R873

A4+H=>A4-4+H2 R1208

A3C2H2=A4+H R876

A4+H=>A4-1+H2 R1204

A4+H=>A4-2+H2 R1206

C9H7+C9H7=>A4+C2H2+H2 R1152

C9H7+C9H7=>A4+C2H2+H2 R1152 -0.5

1.0

(f) A3-1300 oC

C6H5CH2+C9H7=>A4+H2+H2 R1145

-1.0

0.5

Relative ROP

A4-1100oC 0.0

0.5

A4-1300oC

A4+H=>A4-2+H2 R1206 -1.0

1.0

-0.5

Relative ROP

0.0

0.5

1.0

Relative ROP

(g) A4-1100 oC

(h) A4-1300 oC

CH4+OH=CH3+H2O R125

WS SD

A2-1+CH4=A2+CH3 R1167 A3-1+CH4=A3+CH3 R1173

A2-1+CH4=A2+CH3 R1167

WS SD

C2H4+CH3=C2H3+CH4 R261 CH4+OH=CH3+H2O R125

A1C2H*+CH4=A1C2H+CH3 R1227 A1C2H*+CH4=A1C2H+CH3 R1227

C2H4+CH3=C2H3+CH4 R261

A2-2+CH4=A2+CH3 R1169

CH4+H=CH3+H2 R123 A2-2+CH4=A2+CH3 R1169 -1.0

-0.5

CH4-1100oC 0.0

0.5

1.0

CH4+H=CH3+H2 R123 -1.0

Relative ROP

-0.5

CH4-1300oC 0.0

Relative ROP

(i) CH4-1100oC

(j) CH4-1300oC


0.5

1.0

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

C2H5(+M)=C2H4+H(+M) R249

WS SD

A1-+C2H4=A1C2H4 R947

A1-+C2H4=A1+C2H3 R948 C2H4+CH3=C2H3+CH4 R261

A1-+C2H4=A1+C2H3 R948

C4H6-13=H2CC+C2H4 R179

C2H4(+M)=H2+H2CC(+M) R248

C2H4(+M)=H2+H2CC(+M) R248

C2H4+CH3=C2H3+CH4 R261

C2H5(+M)=C2H4+H(+M） R249

C2H4+H=C2H3+H2 R250 -1.0

-0.5

C2H4-1100oC 0.0

0.5

1.0

WS SD

C2H4+H=C2H3+H2 R250 -1.0

-0.5

C2H4-1300oC 0.0

Relative ROP

Relative ROP

(k) C2H4-1100oC

(l) C2H4-1300oC

A1C2H+H=A1-+C2H2 R786

WS SD

c-C6H4=C4H2+C2H2 R662

0.5

1.0

H2CC(+M)=C2H2(+M) R154

WS SD

c-C5H5(+M)=C2H2+C3H3(+M) R1096 c-C5H5=C3H3+C2H2 R748 C2H3(+M)=C2H2+H(+M) R155

C6H5CH2=c-C5H5+C2H2 R1055

C6H5CH2=c-C5H5+C2H2 R1055

C2H2+CH3=PC3H4+H R170

c-C6H4=C4H2+C2H2 R662 C4H2+H=C2H2+C2H R166

H2CC(+M)=C2H2(+M) R154

C2H+H2=H+C2H2 R142

C2H3(+M)=C2H2+H(+M) R155 -1.0

-0.5

C2H2-1100oC 0.0

0.5

C2H2+CH3=PC3H4+H R170

1.0

-1.0

-0.5

C2H2-1300oC 0.0

Relative ROP

Relative ROP

(m) C2H2-1100oC

(n) C2H2-1300oC

Figure 9. ROP analysis of different species


0.5

1.0

Energy & Fuels

C3H3 -C 2H3

-H

+H -H2

2

2

+2C 2H2 -H

A1

H +C 2 -H

+C H 3 -CH

CH3

4

A2

+H -H2 +CH4

+H -C2 H

-CH3

A2CH2 2

-H

A3

+2 C

2

H

2

,H

+C 2H

+C3 H3

+C2 H4

+A1

-2H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

+H +CH4

CH2

+C3 H3

-CH3 +H

-2H

-H2 +C

2

-H

H

2

A4 +

C9H7 +

Inception

-2C2 H2 ,-H2

Figure 10. Formation pathways from benzene to pyrene for biomass pyrolysis

4. CONCLUSIONS In this work, fast pyrolysis of wheat straw and sawdust was carried out in an entrained flow reactor from 900 to 1300 oC. The analysis of pyrolytic products including soot, permanent gas and tar , as well as the simulation with a highly detailed soot formation reaction mechanism was performed to investigate the mechanisms of biomass soot formation. The results are summarized as follows: (1) The soot yields for wheat straw and sawdust were 0.28%-2.40% and 0.34%-6.30% (db) respectively. The soot yields increased with the increase of temperature. The woody biomass produces twice more soot than that of straw. The reason is associated with the high contents of lignin, cellulose and low content of ash in sawdust. The carbonization of soot occurs at 1100 oC, when the


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

soot transitions from amorphous to graphitic structure. Soot from sawdust has a more ordered structure. (2) Carbon conversions of the two biomasses has similar performances with the temperature. It initially increased with temperature, then declined at ~900

o

C due to the soot formation process.

When the temperature was higher than 1100 oC, the CO generation reactions were strengthened greatly, resulting in the re-increase of carbon conversion. (3) The yield of tar drastically went down with the increase of temperature. It is necessary to increase the temperature above 1200 oC to completely decompose tar in gas. All of the tar species were deoxygenated aromatic components, of which benzene, toluene, and naphthalene were the most stable species while aromatics with three and more rings decomposed gradually with the increasing temperature. The wood showed a much more heterogeneous tar species than straw. (4) The KM2 mechanism is sufficient to predict soot formation from biomass pyrolysis. The predicted soot yields by KM2 mechanism agreed well with the experimentally results. In this studied case, the HACA route is the dominated route for soot formation, while the contribution from CPD route is small. A reaction pathway of soot formation during biomass high-temperature pyrolysis is proposed based on the ROP analysis of soot and critical gas species. 5. ACKNOWLEDGEMENTS This study was supported by the National Key Research and Development Plan of China (No. 2016YFB0600605) and National Natural Science Foundation of China (Nos. 51676157 and 5161101654).

6. REFERENCES


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Lau, C. W.; Niksa, S., The impact of soot on the combustion characteristics of coal particles of various types.

Combustion and Flame 1993, 95, (1–2), 1-21. 2.

Lin, J. C.; Matsui, T.; Pielke, R.; Kummerow, C., Effects of biomass℃burning℃derived aerosols on precipitation

and clouds in the Amazon Basin: A satellite℃based empirical study. Journal of Geophysical Research: Atmospheres 2006, 111, (D19), 2156-2202. 3.

Kaivosoja, T.; Jalava, P. I.; Lamberg, H.; Virén, A.; Tapanainen, M.; Torvela, T.; Tapper, U.; Sippula, O.; Tissari, J.;

Hillamo, R.; Hirvonen, M. R.; Jokiniemi, J., Comparison of emissions and toxicological properties of fine particles from wood and oil boilers in small (20–25 kW) and medium (5–10 MW) scale. Atmospheric Environment 2013, 77, 193-201. 4.

Bond, T. C.; Streets, D. G.; Yarber, K. F.; Nelson, S. M.; Woo, J. H.; Klimont, Z., A technology℃based global

inventory of black and organic carbon emissions from combustion. Journal of Geophysical Research: Atmospheres 2004, 109, (D14), 1-43. 5.

Zhang, Y.; Kajitani, S.; Ashizawa, M.; Miura, K., Peculiarities of rapid pyrolysis of biomass covering medium-and

high-temperature ranges. Energy & fuels 2006, 20, (6), 2705-2712. 6.

Zhang, Y.; Kajitani, S.; Ashizawa, M.; Oki, Y., Tar destruction and coke formation during rapid pyrolysis and

gasification of biomass in a drop-tube furnace. Fuel 2010, 89, (2), 302-309. 7.

Trubetskaya, A.; Jensen, P. A.; Jensen, A. D.; Llamas, A. D. G.; Umeki, K.; Glarborg, P., Effect of fast pyrolysis

conditions on biomass solid residues at high temperatures. Fuel Processing Technology 2016, 143, 118-129. 8.

Trubetskaya, A.; Jensen, P. A.; Jensen, A. D.; Garcia Llamas, A. D.; Umeki, K.; Gardini, D.; Kling, J.; Bates, R. B.;

Glarborg, P., Effects of several types of biomass fuels on the yield, nanostructure and reactivity of soot from fast pyrolysis at high temperatures. Applied Energy 2016, 171, 468-482. 9.

Wiinikka, H.; Weiland, F.; Pettersson, E.; Öhrman, O.; Carlsson, P.; Stjernberg, J., Characterisation of submicron

particles produced during oxygen blown entrained flow gasification of biomass. Combustion and Flame 2014, 161, (7), 1923-1934. 10. Wiinikka, H.; Toth, P.; Jansson, K.; Molinder, R.; Broström, M.; Sandström, L.; Lighty, J. S.; Weiland, F., Particle formation during pressurized entrained flow gasification of wood powder: Effects of process conditions on chemical composition, nanostructure, and reactivity. Combustion and Flame 2018, 189, 240-256. 11. Richter, H.; Howard, J. B., Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways. Progress in Energy and Combustion Science 2000, 26, (4–6), 565-608. 12. Fletcher, T. H.; Ma, J.; Rigby, J. R.; Brown, A. L.; Webb, B. W., Soot in coal combustion systems. Progress in Energy and Combustion Science 1997, 23, (3), 283-301. 13. Brown, A. L.; Fletcher, T. H., Modeling soot derived from pulverized coal. Energy & fuels 1998, 12, (4), 745-757. 14. Ma, J.; Fletcher, T. H.; Webb, B. W., Conversion of coal tar to soot during coal pyrolysis in a post-flame environment. Symposium (International) on Combustion 1996, 26, (2), 3161-3167. 15. Ma, J.; Fletcher, T. H.; Webb, B. W., Effect of flame environment on soot formation in coal combustion. Coal Science and

Technology 1995, 24, (15), 869-879.

16. Fitzpatrick, E. M.; Ross, A. B.; Bates, J.; Andrews, G.; Jones, J. M.; Phylaktou, H.; Pourkashanian, M.; Williams, A., Emission of Oxygenated Species from the Combustion of Pine Wood and its Relation to Soot Formation. Process Safety and Environmental Protection 2007, 85, (5), 430-440. 17. Fitzpatrick, E.; Jones, J.; Pourkashanian, M.; Ross, A.; Williams, A.; Bartle, K., Mechanistic aspects of soot formation from the combustion of pine wood. Energy & Fuels 2008, 22, (6), 3771-3778. 18. Fitzpatrick, E.; Bartle, K.; Kubacki, M. L.; Jones, J.; Pourkashanian, M.; Ross, A.; Williams, A.; Kubica, K., The mechanism of the formation of soot and other pollutants during the co-firing of coal and pine wood in a fixed bed combustor. Fuel 2009, 88, (12), 2409-2417. 19. Wijayanta, A. T.; Alam, M. S.; Nakaso, K.; Fukai, J., Numerical investigation on combustion of coal volatiles under


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

various O2/CO2 mixtures using a detailed mechanism with soot formation. Fuel 2012, 93, 670-676. 20. Wijayanta, A. T.; Saiful Alam, M.; Nakaso, K.; Fukai, J.; Shimizu, M., Optimized combustion of biomass volatiles by varying O2 and CO2 levels: A numerical simulation using a highly detailed soot formation reaction mechanism. Bioresource Technology 2012, 110, 645-651. 21. Sharma, H., Compositional analysis of neutral detergent, acid detergent, lignin and humus fractions of mushroom compost. Thermochimica acta 1996, 285, (2), 211-220. 22. Wang, X.; Hu, Z.; Adeosun, A.; Liu, B.; Ruan, R.; Li, S.; Tan, H., Particulate matter emission and K/S/Cl transformation during biomass combustion in an entrained flow reactor. Journal of the Energy Institute 2017, https://doi.org/10.1016/j.joei.2017.10.005. 23. Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. F., Influence of pyrolysis conditions on the structure and gasification reactivity of biomass chars. Fuel 2004, 83, (16), 2139-2150. 24. Tumolva, L.; Park, J.Y.; Kim, J. S.; Miller, A. L.; Chow, J. C.; Watson, J. G.; Park, K., Morphological and elemental classification of freshly emitted soot particles and atmospheric ultrafine particles using the TEM/EDS. Aerosol Science and Technology 2010, 44, (3), 202-215. 25. Qin, K.; Jensen, P. A.; Lin, W.; Jensen, A. D., Biomass gasification behavior in an entrained flow reactor: gas product distribution and soot formation. Energy & Fuels 2012, 26, (9), 5992-6002. 26. Su, Y.; Luo, Y.; Chen, Y.; Wu, W.; Zhang, Y., Experimental and numerical investigation of tar destruction under partial oxidation environment. Fuel Processing Technology 2011, 92, (8), 1513-1524. 27. Frenklach, M.; Harris, S. J., Aerosol dynamics modeling using the method of moments. Journal of Colloid and Interface Science 1987, 118, (1), 252-261. 28. Frenklach, M., Method of moments with interpolative closure. Chemical Engineering Science 2002, 57, (12), 2229-2239. 29. Wang, Y.; Raj, A.; Chung, S. H., A PAH growth mechanism and synergistic effect on PAH formation in counterflow diffusion flames. Combustion and Flame 2013, 160, (9), 1667-1676. 30. Frenklach, M.; Wang, H., Detailed modeling of soot particle nucleation and growth. Symposium (International) on Combustion 1991, 23, (1), 1559-1566. 31. Appel, J.; Bockhorn, H.; Frenklach, M., Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C2 hydrocarbons. Combustion and Flame 2000, 121, (1), 122-136. 32. Saggese, C.; Ferrario, S.; Camacho, J.; Cuoci, A.; Frassoldati, A.; Ranzi, E.; Wang, H.; Faravelli, T., Kinetic modeling of particle size distribution of soot in a premixed burner-stabilized stagnation ethylene flame. Combustion and Flame 2015, 162, (9), 3356-3369. 33. Workbench,

R.,

15131.

Reaction

Design:

San

Diego

2013.

http://www.reactiondesign.com/support/help/help_usage_and_support/how-to-cite-products/. 34. Septien, S.; Valin, S.; Dupont, C.; Peyrot, M.; Salvador, S., Effect of particle size and temperature on woody biomass fast pyrolysis at high temperature (1000–1400°C). Fuel 2012, 97, 202-210. 35. Trubetskaya, A.; Jensen, P. A.; Jensen, A. D.; Steibel, M.; Spliethoff, H.; Glarborg, P., Influence of fast pyrolysis conditions on yield and structural transformation of biomass chars. Fuel Processing Technology 2015, 140, 205-214. 36. Lanza, R.; Dalle Nogare, D.; Canu, P., Gas phase chemistry in cellulose fast pyrolysis. Industrial & Engineering Chemistry Research 2009, 48, (3), 1391-1399. 37. Xin, S.; Yang, H.; Chen, Y.; Wang, X.; Chen, H., Assessment of pyrolysis polygeneration of biomass based on major components: Product characterization and elucidation of degradation pathways. Fuel 2013, 113, 266-273. 38. Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C., Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, (12), 1781-1788. 39. Wilson, J. M.; Baeza-Romero, M. T.; Jones, J. M.; Pourkashanian, M.; Williams, A.; Lea-Langton, A. R.; Ross, A.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

B.; Bartle, K. D., Soot formation from the combustion of biomass pyrolysis products and a hydrocarbon fuel, n-Decane: An Aerosol Time Of Flight Mass Spectrometer (ATOFMS) Study. Energy & Fuels 2013, 27, (3), 1668-1678. 40. Yu, H.; Zhang, Z.; Li, Z.; Chen, D., Characteristics of tar formation during cellulose, hemicellulose and lignin gasification. Fuel 2014, 118, 250-256. 41. Trubetskaya, A.; Larsen, F. H.; Shchukarev, A.; Ståhl, K.; Umeki, K., Potassium and soot interaction in fast biomass pyrolysis at high temperatures. Fuel 2018, 225, 89-94. 42. Umeki, K.; Häggström, G.; Bach-Oller, A.; Kirtania, K.; Furusjö, E., Reduction of tar and soot Formation from entrained-flow gasification of woody biomass by alkali impregnation. Energy & Fuels 2017, 31, (5), 5104-5110. 43. Vander Wal, R. L.; Tomasek, A. J., Soot nanostructure: dependence upon synthesis conditions. Combustion and Flame 2004, 136, (1–2), 129-140. 44. Vander Wal, R. L.; Bryg, V. M.; Hays, M. D., Fingerprinting soot (towards source identification): Physical structure and chemical composition. Journal of Aerosol Science 2010, 41, (1), 108-117. 45. Wang, S.; Dai, G.; Yang, H.; Luo, Z., Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review. Progress in Energy and Combustion Science 2017, 62, 33-86. 46. Wu, W.; Luo, Y.; Su, Y.; Zhang, Y.; Zhao, S.; Wang, Y., Nascent biomass tar evolution properties under homogeneous/heterogeneous decomposition conditions in a two-stage reactor. Energy & Fuels 2011, 25, (11), 5394-5406. 47. Horn, C.; Roy, K.; Frank, P.; Just, T., Shock-tube study on the high-temperature pyrolysis of phenol. Symposium (International) on Combustion 1998, 27, (1), 321-328. 48. Hernández, J. J.; Aranda, G.; Barba, J.; Mendoza, J. M., Effect of steam content in the air–steam flow on biomass entrained flow gasification. Fuel Processing Technology 2012, 99, 43-55.


Page 30 of 30

Characteristics and mechanism of soot formation during the fast

Recommend Documents