Modeling of secondary reactions of tar (srt) using a functional group model

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Modeling of secondary reactions of tar (srt) using a functional group model

  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 3, Issue 3, Septmebr - December (2012), pp. 123-136© IAEME: www.iaeme.com/ijmet.htmlJournal Impact Factor (2012): 3.8071 (Calculated by GISI) ©IAEMEwww.jifactor.com MODELING OF SECONDARY REACTIONS OF TAR (SRT) USING A FUNCTIONAL GROUP MODEL Ashraf Elfasakhany* Department of Mechanical Engineering, Faculty of Engineering, Taif University, Box 888, Al-Haweiah, Taif, Saudi Arabia * Corresponding author Tel.: +966 (02) 7272020; Fax: +966 (02)7274299 E-mail address: ashr12000@yahoo.com ABSTRACT During flash pyrolysis/gasification of biomass, tar is the major product, which reaches up to 70%. This tar causes a lot of problems that present significant impediments to the application of biomass gasification systems. The reduction or decomposition of tar at the gasification process becomes one of the most necessary and urgent problems. As a result, a great need exists for a comprehensive model that helps to understand the mechanism of tar decomposition/cracking (second reactions of tar), which will possibly improve biomass gasifiers and furnaces design. The purpose of this study was at detail modelling of the second reactions of tar (SRT) using a functional group model. Types of gasses, yields and kinetic rates during the SRT within temperature range of 500-1200 oC are modelled and validated with experimental measurement from literature. The model discusses the scenario of tar cracking mechanism, the quantitatively most important product from the SRT, and the best indicator for the tar conversion process. The model also helps to classify tar compounds into different categories with identifying quantitatively the most important compound from each category as well as the proper method to crack each one. The model can explain the complicated behavior of parallel, series and overlapping reactions of different tar compounds at the SRT conditions. Keywords: Functional group, Tar cracking, Numerical modelling, Gasification. 1. INTRODUCTION During fast biomass pyrolysis/gasification, relatively high amount of tar is produced. This tar, which may reach up to 70%, is an extremely complex mixture [1]. The term ’tar’ describes a lump involving thousands of single substances. Because of its complexity, there is no consistent and generally accepted definition for tar [2]. However, the tar could be classified, as a result of thermal biomass pyrolysis/gasification, into three major classes: primary, secondary and tertiary tars [3]. Primary tar is formed due to the presence of oxygen compounds in a temperature range of 400-700oC. Secondary tar is formed in a temperature range of 700-850 oC, and it includes phenolics and olefins. 123
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMETertiary tar products appear in the temperature regime of 850-1000 oC and are characterized byaromatics. Sometimes, these three main classes are divided into sub-classes as well. During thermal biomass pyrolysis/gasification, the tar classes are formed and cracked hereafter.However, some of such tar classes and in particular their compounds are not fully cracked atpyrolysis/ gasification process where some of which are left and they are so called non-reactive tars.Their values are mainly attributed to the structure of biomass and type of pyrolysis/gasificationprocess (slow or fast) [4]. For example, tar left after the gasification of beech wood and sweet gumhardwood is respectively about 22 and 11% [5-6]. Tar disposal during pyrolysis/gasification becomes one of the most necessary and urgent problems[7]. Tar in the product gases will condense in low temperature, and lead to clog or blockage in fuellines and corrosion in engines and turbines [8-9]. Tar also represents a not negligible energy loss and,in turn, that lowers the overall efficiency of the process. Therefore, the reduction or decomposition oftar in gasifiers is one of the biggest obstacles in its utilization for power generation and synthesis gasapplications [7, 10-11]. Attempts to remove the tar can mainly be divided into two main methods; treatments insidegasifiers (primary methods) and hot gas cleaning after the gasifiers (secondary methods). Primarymethods require the appropriate selection of operating parameters, the use of a bed additive orcatalyst, as well as modifications upon the design of gasifiers. These primary methods only remove orcapture the tar from product gases, while the energy in tar is lost [12]. Besides, bed additives orcatalysts have technical shortcomings, such as inactivation by carbon, soot and H2S [13-15]. Secondary methods involve tar-cracking after gasifiers, either thermally or catalytically using of aceramic filter, electrostatic precipitator and scrubber. Secondary methods have been extensively usedbut until now have not appeared to be sufficiently effective. Besides, the high costs associated withthe downstream equipment and their operation [12, 15-17]. Accordingly, simple, effective and cheapmethods to reach high tar conversion within and/or after gasifiers are needed [9, 18-19]. To achievesuch goal, a great need exists for further understanding the mechanism of tar decomposition/cracking(second reactions of tar) using modelling approach. There are different approaches found in the literature to model the second reactions of tar (SRT).The simplest approach has been the treatment of the numerous reactions taking place as a singlereaction following first order kinetics [20-21]. Nevertheless, this approach did not consider thecomplex nature of tar. In another approach, the SRT has been treated using a multiple reaction modelbased on the assumption of many parallel reactions following first order kinetics. This model is rathercomplicated to solve and it requires at least four experimental parameters; the results are uncertain asthey turn out that the different sets of parameters with different values are often found to fit equallythe data [22]. In a different approach, the tar compounds are lumped according to their chemicalstructures and pyrolysis behaviors. This approach leads to very complex reaction mechanisms with alarge number of parameters, whose determination is uncertain at best [23-25]. In summary, there is alack in a simple, accurate and general modelling for the SRT mechanism [5, 6, 26]. Besides, theinaccuracy of the SRT modelling is one of the main reasons for the discrepancy in the overall biomassgasification/combustion modelling [27]. The difficulties of the SRT modelling may be attributed tothat tar compounds under thermal cracking process interact to each other and, in turn, they arechanged rapidly to other compounds [28-29]. The objective of this work was at investigating a simple and accurate modelling tool for the SRTmechanism at high heating rate condition. This model, which uses several different functional groups,provides information regarding the change in quantity and composition during the thermal tarcracking process. The model also helps to explain the complicated behavior of parallel, series andoverlapping reactions of different tar compounds under the SRT condition.2. NUMERICAL MODEL There has not been an agreement in literature concerning the compounds used to represent tar onmodeling condition [30]. Some researchers considered tar is modeled using toluene and naphthalene 124
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEcompounds at high temperature and atmospheric pressure conditions [31]. Others considered tar ismodeled using benzene, naphthalene and heavy aromatic [32]. Simell et al. [33-35] used benzene andnaphthalene for a tar modeling in number of studies. Dou et al. [30] used Methylnaphthalene as tarmodeling. However, other researchers argued that naphthalene and phenol are the best compoundsused for tar modelling [2, 35-39]. In order to commonly characterize compound(s) used for tar modelling, it is needed to analyze tarcompounds firstly. Tar consists of 53 compounds [28-29]; but they might be classified into four maingroups under cracking conditions: non-condensable gasses (CO, CO2, CH4, H2, etc.), primary tar(Acetol, Acetic acid, Guaiacol, etc.), secondary tar (Phenol, Cresol, Ethyltoluene, etc.) and tertiary tar(Naphthalene, Benzopyrene, etc.). Typical representatives of primary, secondary and tertiary tars arepresented in detail in Milne et al. [2]. Yields of tar compounds are varied significantly, which dependon the working conditions, biomass source, etc.; hence some of tar compounds might be neglecteddue to their very small yields. In the current study, the four main groups of tar compounds are modelled as eleven differentfunctional groups. We considered only these eleven groups since the rest of tar compound are in verysmall quantities (< 0.002) under current working conditions and, in turn, they are neglected. Table 1summarizes the eleven functional groups, kinetic rate constants and the end products from the SRTexpressed in terms of weight percentage of the total tar yield. The rate of the SRT is modelled as a first order reaction based on the difference between theultimate yield of product mi∞ and the amount of product left mi of the functional group. Theformation/cracking of each tar group is described according to Eq. (1) as follows: dmi  E  ( ) = mi∞ − mi Ai exp − i RTg  Eq. (1) dt  Where mi∞ is the ultimate yield of the functional group (Kg); mi is the amount of the functional groupleft (Kg); E is the activation energy (kJ/mole); A is the frequency factor (s-1); Tg is the gastemperature (K); R is the universal gas constant (KJ/mole K). Table 1 List of functional groups and kinetic rate constants for the secondary reaction of tar (SRT) modelReactions A (s-1) E (kJ/mole) mi∞ Refs. k5 6Tar → Acetol 1.6×10 108.0 5.00 (presented) k6Tar → Acetic acid 2.5×105 107.5 1.00 (presented) k7Tar → Guaiacol 22×104 83.60 0.08 (presented) k9Tar → o-Cresola 3.7×107 118.00 6.7×10-2 (presented) k 10o-Cresola → o-Cresolb  1.6×105 107.50 3×10-2 (presented) k8Tar  → Phenol 4.22×106 110.07 0.17 (presented) k 11Tar → Naphthalene 9.53×104 93.300 0.130 [46] k 12Tar → Benzo[a]pyrene  2.3×104 80.000 1.8×10-3 [47]Tar k 1 CO → 1.55×105 87.6 12.4 [48] k2Tar → CO2 3.26×104 72.8 6.80 [49]Tar k 3 CH4 → 1.5×106 114.3 2.40 [50-51] k4Tar → H2 1.5×106 114.3 0.42 [46-51] a o-Cresol produced from tar b o-Cresol cracked at T> 850 oC 125
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME3. NUMERICAL METHODS The calculations are carried out using an in house CFD code. In the code, the gas phase transport isdescribed by the Navier-Stokes equations, the energy transport equation based on the first law ofthermodynamics, and the species transport equations based on the law of mass conservation ofchemically reacting gas mixture, together with the equation of state, the caloric equation of state, aswell as chemical reaction mechanisms and the related kinetic rates. A general form of the transportequations can be written as follows: ∂ρφ ∂ρu iφ ∂  µ t ∂φ  + =   + Sφ Eq. (2) ∂t ∂xi ∂xi  Pr ∂xi   Here φ represents the velocity components, enthalpy, and species mass fractions. Different terms,from left to right, represent the time evolution, the convection, diffusion and the source term,respectively. The source terms in the above transport equations, Sφ,, which are due to chemical species from tarcracking and reactions, are calculated using ‘chemical in cells’ method. When the turbulent intensityis low, the distribution of these source terms may be very un-smooth, which could lead to non-smoothresult or even non-convergence of the numerical solution. This can be avoided by using morechemical source groups and more amounts of injected tars. For further detail about the calculationmethods and the CFD code used, you may see [40-42].4. EXPERIMENTAL The experimental measurement of Morf el al. [29] is used to validate our modelling calculations.Although there are many experiments available in the literature, see e.g., [5-17], the currentexperiment has been chosen because it produces accurate measurements and overcomes some of thedrawbacks of other experiments. Examples of such drawbacks of other experiments are that duringbiomass pyrolysis, tar compounds are sequentially released into the gas phase. Thus, the amount oftar measured in other experiments is less than the real quantity. In addition, some of tar crackingproducts has not been considered. The Morf el al. [29] experiment overcame these drawbacks byusing a continuously fuel-fed pyrolysis process. The experiment was carried out in a laboratory reactor system where wood was fed to producepyrolysis products at a mean temperature of 380 oC. After the primary pyrolysis stage, the producedchar is separated, and the tar containing pyrolysis gas is swept to another conversion reactor wherehomogenous and/or heterogeneous tar reactions were measured. The tar conversion is operated atseveral temperature levels from 500-1100 oC. During the experiment, tar samples are taken and theconcentrations of non-condensable gasses (CO, CO2, CH4, and H2) are measured on-line. For moredetail, you may see the experiment.5. RESULTS AND DISCUSSIONS Results from the second reaction of tar (SRT) modelling are presented and discussed. Comparisonsbetween our model predictions and experimental results of Morf et al. [29] are shown in Figs. 1-4. Asseen, the yields and rates of the functional groups modelling (M-FG) are perfectly predicted theexperimental data (Exp). Fig. 1 presents acetol, acetic acid and guaiacol functional groups. Such functional groups areclassified among the primary tar category since they have similar trends within related temperatureregimes. This category is characterized by oxygenated compound that gives the primary tar its highreactivity. The quantitatively most important compound of such primary tar is acetol, which is about5% of total tar, as shown in Fig. 1a. As seen, acetol is almost constant within temperatures 500- 126
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME600oC, and slowly cracked between 600-700 oC. Hence after, it steeps cracked until reaches almostzero value at 1000 oC. The quantitatively second most important compound of primary tar is acetic acid, which is about1%. As shown in Fig. 1b, the acetic acid is kept almost stable in the temperature range of 500-800 oCand slowly cracked for the temperature range 800-850 oC. Hence after, it steeps cracked until reachesalmost zero value at 1200 oC. Guaiacol yield, as shown in Fig. 1c, is converted faster than other primary tar compounds (e.g.,acetol and acetic acid) to reach less than 0.01% at 850 oC. In the beginning, the yield of guaiacol intotal tar is very small, which is about 0.08%. Although such very low yield of guaiacol, it is one ofthe most important reasons for higher-molecular weight of tar fractions [43]. The rest of the primarytar compounds are rather smaller than guaiacol yields and, in turn, they are neglected in bothmodelling and experimental. Fig. 1a Yields of Acetol functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. Fig. 1b Yields of Acetic acid functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. 127
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Fig. 1c Yields of Guaiacol functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. o-Cresol yield behaves totally different from all other tar compounds. It increases with temperatureto reach the peak at about 850 oC. Then it steep decreases with a further temperature increasing untilvanished at about 1200 oC, as shown in Fig. 2a. However, other tar compounds only increase,decrease or remain constant with temperature. Fig. 2b shows the yield of phenol, and as seen it increases between temperatures 650-900 oC;however, before 650 oC and after 900 oC, there is no significant changes. Besides, a large scattering inthe experimental measurements is clearly shown. This is attributed to a technical problem in theexperiment, as explained by Morf et al. [29]. Our study believes that the phenol yield should bedecreased in a higher temperature condition (after 850 oC), similar to o-Cresol yield. This finding isconsistence with Abu El-Rub et al. [37], in which they showed that the conversion of phenoldominates at 900 oC, due to the thermal cracking. Fig. 2a Yields of o-Cresol functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. 128
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Phenol and o-Cresol are classified among secondary tar compounds where this category representsless than 1 % of total tar yields, as shown in Fig. 2. The secondary tar compounds are alsoresponsible, along with guaiacol, for higher-molecular tar weight [43]. Naphthalene and benzo[a]pyrene compounds are typically considered under the tertiary tarcategory. Naphthalene yield, as shown in Fig. 3a, increases with temperature. Naphthalene isquantitatively the most important compound in the tertiary tar (0.13% of total tar yield). However,benzo[a]pyrene is only about 0.002% of total tar yield, which is an almost negligible fraction. Theother tertiary tar compounds have smaller yields than benzo[a]pyrene and, therefore, they areneglected in the current study. Fig. 3b shows the yield of benzo[a]pyrene where it behaves similarlyto naphthalene yield, and all other tertiary tar compounds show similar qualitative behavior [29]. Fig. 2b Yields of Phenol functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. Fig. 3a Yields of Naphthalene functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. 129
  8. 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Comparing the behavior of the primary, secondary and tertiary tar components, it is seen that theprimary tar decreases with increasing temperature; however tertiary tar behaves in opposite criteria,which increases with increasing temperature. Secondary tar behaves as a combination between theprimary and tertiary tar compounds, which increases and decreases afterwards. It is also shown thatthe tertiary tar compounds have no significance in a temperature lower than 700 oC. However, after700 oC such tar compounds are formed rapidly while primary tar (especially acetol) is demolishedrapidly at this temperature. ×10-3 Fig. 3b Yields of Benzo[a]pyrene functional group modelling (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. Light hydrocarbons, e.g., CH4, CO, CO2 and H2, behave similarly to the tertiary tar compounds,as shown in Fig. 4. Fig. 4a shows the CO mass fraction versus temperature produced from the tarcracking. As seen, CO yield is kept almost constant until temperature reaches 700 oC, hereafter itincreases fast to reach its double value. CO increases till reach about 12.4 % and this clearly showsthat carbon monoxide is the most important gasses produced from homogenous tar cracking. Thismonotonic rising was investigated by other researchers to propose that the CO yield could be used asan indicator for the tar cracking process, see e.g., [44-46]. The CO2 yield is not varied significantly along all the temperature range (500-1200 oC), as shownin Fig. 4b. The carbon dioxide is almost stable due to that the tar cracking process is working on thecondition of being lacking in external supply of oxygen. On the other hand, the experimental datashow scattering in the results and this minor scattering may attribute to the inaccuracy in themeasuring devices. Fig. 4c illustrates the CH4 mass fraction varies with temperature. As seen, the CH4 yield does notchange significantly for a while and increases slowly when temperature reaches 700 oC; hereafter CH4increases significantly like CO. H2, similar to CO and CH4, does not change significantly for a while (from T=500-700 oC). ByT=1000 oC, it monotonically increases to reach eight times of its original value, as shown in Fig. 4d.Such a rapid boost is most probably due to water-gas shift reaction as follow: CO + H 2O → CO2 + H 2 Eq. (3) 130
  9. 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME According to that reaction, the water content in the gas phase decreases while H2 increases. Theexperimental measurement of Morf et al. [29] showed that the water yield in the gas phase decreaseswhen temperature reaches 600 oC and higher. Finally, this work may highlight some important findings as follows: It is important to note that tar compounds are not all thermally cracked, e.g., tertiary tar. Hence, primary and secondary tars may be considered as temporary products under thermal tar cracking conditions, however, the tertiary tar is not. Primary tar compounds occupy about 6 % of total tar; however, both secondary and tertiary tars occupy together about 0.4%. Although, this small yield of the secondary and tertiary tars, they are responsible for higher-molecular weight of tar fraction and that causes several problems in gasifiers. Fig. 4a Yields of CO functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. Fig. 4b Yields of CO2 functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. 131
  10. 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Acetol, phenol and naphthalene are the quantitatively most important product from primary, secondary and tertiary tars, respectively. Different tar compounds are cracked in the rate constants of range O104-O107 (s-1) for frequency factors (A) and 72-118 (kJ/mole) for activation energies (E). When considering the modelling of the SRT in the absence of oxygen, the complicated processes may be simplified into systems of relations (see Table 1); it is more than likely that the yields of functional groups at the SRT depend on temperature, heating rate and air/oxygenate compounds. Fig. 4c Yields of CH4 functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. Fig. 4d Yields of H2 functional group modelled (M-FG) and validated with experimental measurements (Exp) of Morf et al. [29]. 132
  11. 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME The behavior of SRT includes parallel, series and overlapping reactions for different tar compounds under thermal process. Secondary tars are overlapped with both primary and tertiary tars, however, most of primary tars (acetol and guaiacol) are demolished while tertiary tars are formed, i.e., series reactions. Tar compounds in each category (primary, secondary and tertiary) are reacted at the same temperature range, i.e., parallel reactions. The most significant temperatures for the SRT are 700 and 850 oC. At 700 oC, primary tar compounds are cracked significantly while tertiary tar compounds as well as non condensable gases are formed in a same rapid attitude. However, at 850 oC, secondary tar compounds reach almost their peak values and start to crack afterwards while primary tar is almost vanished. This may help to conclude that primary tar is cracked to secondary and tertiary tar as well as light hydrocarbons. Secondary tar, on the other hand, is cracked to tertiary tar and light hydrocarbons. Secondary tar vanishes at 1200 oC, however, tertiary tar is not. Hence, most of the problematic tar is the tertiary one since it is not cracked at even very high temperature and, in turn, causes a lot of problems that present the significant impediment to the application of biomass gasification systems. In order to decompose all tar compounds during pyrolysis/gasification, this work shows that increasing pyrolysis/gasification temperature and/or reaction time enhances tar aromaticity and reduces the yields of the functional groups. Nevertheless, this is not enough to decompose all tar compounds, as showed early, and, in turn, it is proposed to use oxygenate compounds during pyrolysis/gasification. Such oxygenate compounds should be in limited amounts not to oxidize other gases in gasifiers. Besides, oxygenate compounds should be added at a high temperature condition where the tertiary tar is left (>1000 oC). Homogenous tar conversion reactions without external supply of oxygen become important at temperature higher than 700 oC and that clearly indicated by increasing concentrations of CO, CH4 and H2. Boroson et al. [44-46] proposed that the CO yield could be used as an indicator for tar cracking process because it is stepping increased to reach its double value under the thermal tar cracking condition. However, Morf el al. [29] argued that the H2 is a better indicator of tar cracking process than CO because H2 value is in minor concentrations at temperature up to 700 oC. However, at higher temperature it is duplicated eight times because of tar cracking. In a conclusion, current study agreed with Morf el al. [29] argument since CH4 is also duplicated at higher temperature, i.e., similar to CO. On the other hand, tar cracking is generally indicated by increasing the concentrations of all gases, CO, CH4 and H2. CO is quantitatively the most important product from the homogenous tar conversion, and H2 is the best indicator for the tar conversion. Yields of some gases are not only from tar cracking but also from gas-phase reactions between themselves (water gas-shift reaction, for example).6. CONCLUSIONS Tar is an undesirable by product of biomass gasification because of its various problems. In thecurrent study, a general kinetic model was developed to describe the SRT process in gasifiers. Suchmodel can explain the complicated behavior of parallel, series and overlapping reactions of differenttar compounds on thermal condition. The model quantifies the tar cracked to eleven differentfunctional groups. A sequential method to estimate the rate constants in the lumped kinetic model wasadopted, which greatly simplifies the model using first order equation. Kinetic constants andactivation energies were determined and validated with experimental data. Results show that primary tar occupies about 6 % of total tar; however, secondary and tertiary tarsoccupy less than 0.5% for both. Acetol, phenol and naphthalene are the quantitatively most importantproducts from primary, secondary and tertiary tars, respectively. Primary and secondary tars areconsidered as temporary products under thermal tar cracking conditions, however, the tertiary tar isnot. 133
  12. 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME SRT without external supply of oxygen become important at temperature higher than 700 oC whereprimary tar is cracked significantly while tertiary tar as well as non condensable gases is significantlyformed in almost similar rapid attitude. When temperature reaches about 850 oC, secondary tar gets toits peak yield and starts to crack afterwards; however, primary tar is almost vanished. Secondary tar iscracked to tertiary tar and light hydrocarbons (CO, CO2, CH4 and H2). Primary tar, on the other hand,is cracked to secondary and tertiary tar as well as light hydrocarbons. Tar cracking is generallyindicated by increasing the concentrations of CO, CH4 and H2. Besides, CO is the largest by productof homogenous tar conversion, and H2 is the best indicator for the tar cracking process. Increasing pyrolysis/gasification temperature and/or reaction time enhances tar crackingsignificantly. Yet, this is not enough to decompose all tar compounds even at very high temperature.In turn, it is proposed using oxygenate compounds under certain circumstances for cracking such tarcompounds in gasifiers. Such oxygenate compounds should be in a limited quantity and added withinhigh temperature condition where the unbroken tars are left.REFERENCES[1] L. Fagbemi, L. Khezami, R. Capart, Pyrolysis products from different biomasses: application to the thermal cracking of tar, Applied Energy 69 (2001) 293-306.[2] T.A. Milne, N. Abatzoglou, R.J. Evan, Biomass gasifier ‘‘Tars”: their nature, formation and conversion, National Renewable Energy Laboratory (NREL), 1998.[3] R.J. Evans, T.A. Milne, Molecular characterisation of the pyrolysis of biomass, Fundamentals Energy Fuels 2 (1987) 123-138.[4] S.C. Bhattacharya, A.H. Siddique, H.L. Pham, A study on wood gasification for low-tar gas production, Energy 24 (1999) 285-296.[5] J. Rath, G. Steiner, M.G. Wolfinger, G. Staudinger, Tar cracking from pyrolysis of large beech wood particles, Journal of Analytical and applied Pyrolysis 62 (2002) 83-92.[6] J. Rath, G. Staudinger, Cracking reaction of tar from pyrolysis of spruce wood, Fuel 80 (2001) 1379-1389.[7] W.F. Fassinou, L.V. Steene, S. Toure, G. Volle, P. Girard, Pyrolysis of Pinus pinaster in a two- stage gasifier: Influence of processing parameters and thermal cracking of tar, Fuel Processing Technology 90 (2009) 75-90.[8] J.F. Gonzalez, S. Roman, G. Engo, J.M. Encinar, G. Martinez, Reduction of tars by dolomite cracking during two-stage gasification of olive cake, Biomass and Bioenergy 35 (2011) 4324- 4330.[9] L. Devi, K.J. Ptasinski, F.J. Janssen, A review of the primary measures for tar elimination in biomass gasification processes, Biomass Bioenerg 24 (2002) 125-140.[10] V. Nemanova, T. Nordgreen, K. Engvall, K. Sjöström, Biomass gasification in an atmospheric fluidised bed: Tar reduction with experimental iron-based granules from Höganäs AB Sweden, Catalysis Today 176 (2011) 253-257.[11] M.A. Uddin, H. Tsuda, S. Wu, E. Sasaoka, Catalytic decomposition of biomass tars with iron oxide catalysts, Fuel 87 (2008) 451-459.[12] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: An overview, Renewable and Sustainable Energy Reviews 12 (2008) 397- 416.[13] D. Sutton, B. Kelleher, J.R.H. Ross, Review of literature on catalysts for biomass gasification, Fuel Process Technology 73(2001) 155-173.[14] P.A. Simell, Catalytic hot gas cleaning of gasification gas, VTT Publication No. 330, 1997a.[15] Dayton, A review of the literature on catalytic biomass tar destruction, National Renewable Energy Laboratory, NREL Report TP-510-32815, 2002.[16] T. Young, J.O. Kim, J.W. Kim, J.S. Kim, Influence of operation conditions and additives on the development of producer gas and tar reduction in air gasification of construction woody wastes using a two-stage gasifier, Bioresource Technology 102 (2011) 7196-7203. 134
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