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Numerical Study on the Steam Reforming of Biomass Tar Using a Detailed Chemical Kinetic Model

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Numerical Study on the Steam Reforming of Biomass Tar Using a Detailed Chemical Kinetic Model 794 J. Jpn. Inst. Energy, Vol.Journal 94, No. of 8,the 2015 Japan Institute of Energy, 94, 794-804（2015） Special articles: Asian Conference on Biomass Science 特集：アジアバイオマス科学会議 Numerical Study on the Steam Reforming of Biomass Tar Using a Detailed Chemical Kinetic Model Narumon THIMTHONG ※1, Srinivas APPAR I ※2, Ryota TANAKA※2, Keita IWA NAGA ※1, Tomoaki NAMIOKA※3, Shinji KUDO ※2, Jun-ichiro HAYASHI ※1※2※4, and Koyo NORINAGA ※1※2 (Received November 18, 2014) Steam reforming (SR) and partial oxidation (POx) of nascent volatiles (NV) generated from fast pyrolysis of cedar wood chips in a two-stage reactor were studied numerically. A detailed chemical kinetic model (DCKM) consisting of more than 8000 elementary step-like reactions and more than 500 chemical species was used to simulate pyrolysis at 750 °C and reforming of the NV at 900 °C in the first and second stages, respectively. The molecular composition of the NV, which is one of the required boundary conditions for computations using the DCKM, was approximated based on analytical pyrolysis experiments. Global reactions accounting for the decomposition of the ill-defined portion of the NV and soot reforming were also tested to improve the model capabilities. The DCKM with the global reaction coupled with a plug-flow reactor model could fairly reproduce the experimentally observed trends for the effects of oxygen and steam partial pressures on the yields of major products such as hydrogen, carbon monoxide, and tar residual rate. 熱分解部と改質部で構成される二段反応器における，スギチップ熱分解で生成した揮発成分の無触媒水蒸気改質および部分 酸化特性に関する数値解析的研究を実施した。揮発成分に含まれる化学種の反応を網羅する8000 以上の素反応および500 以 上の化学種からなる詳細化学反応速度モデルをバイオマス急速熱分解生成物の水蒸気改質反応系に初めて適用し，反応特性の 予測を試みた。本速度モデルを用いたシミュレーションの場合，熱分 解生成物の分子組成を定義する必要がある。そこで，別途， 熱分解―ガスクロマトグラフィー実験を実施し，熱分解初期生成物中の 52 種類の化合物を同定，定量した。初期熱分解生成物に 含まれる未定義成分（ガスクロマトグラフィーで分離不可能な重質成分）の分 解やススのガス化反応を表現する総括反応モデルを 経験的に構築し，これらを詳細化学反応速度モデルに加えてシミュレーションを実施した。得られた反応速度モデルは，水蒸気およ び酸素の分圧が，水素，一酸化酸素などの主要生成物の収率に及ぼす影響ばかりでなく，微量副生成物であるタールの転換特性 に及ぼ す影 響も良 好に再 現した。 Key Words Biomass fast pyrolysis, Tar reforming, Kinetic model, Elementary reaction 1. Introduction greenhouse gas emissions as well as the consequences of Efficient and effective technologies are required to climate change, motivate the search for renewable energy promote the utilisation of renewable energy from biomass sources. Thermochemical conversions, such as pyrolysis resources. A report on world energy consumption predicts and gasification, are effective in converting biomass into an increased energy demand of 56% from 2010 to 2040 1), valuable fuels/products 2). The thermochemical processes which impacts fossil carbon fuel prices. Concerns about for biomass are diverse, yet some problems are always energy requirements and environmental effects, such as associated with biomass conversion. One of the major ※ 1 Interdisciplinary Graduate School of Engineering Sciences, ※ 3 Department of Mechanical Engineering, Chubu University Kyushu University Kasugai, Aichi 487-8501, Japan Kasuga 816-8580, Japan ※ 4 Research and Education Center of Carbon Resources, ※ 2 Institute for Materials Chemistry and Engineering, Kyushu University Kyushu University Kasuga, 816-8580, Japan Kasuga 816-8580, Japan J. Jpn. Inst. Energy, Vol. 94, No. 8, 2015 795 problems is unavoidable products or impurities from the the partial oxidation of the NV derived from cedar sawdust 22) gasification process, such as tars, solid particles, NH3, H2S, fast pyrolysis . However, there are no reports of using a HCl, and SO2, which affect the quality of syngas and cause DCKM to analyse steam reforming of the NV derived from problems in downstream applications 3) 4). biomass fast pyrolysis to predict the tar characteristics. Tar is a major problematic by-product. It consists The purpose of this study was to examine the of stable aromatic compounds, such as polycyclic capability of using an existing DCKM 6) 19) 21) 22) to predict aromatic hydrocarbons. Tars are formed during biomass the experimentally observed trends of steam reforming thermochemical conversion and condense at reduced (SR) of the NV derived from woody biomass (cedar temperatures. Although tar is only a minor component in chips) fast pyrolysis under various reforming conditions. biomass gasification, even small amounts can significantly This is the first such attempt. The DCKM is used to affect downstream applications by blocking and/or fouling describe the vapour phase reforming of the volatiles. The the process lines. Therefore, the removal or control of tar initial molecular compositions are required input for the is necessary before syngas can be used in any downstream DCKM computations, and were derived from analytical equipment 4) ～ 7). pyrolysis experiments 6). Global reactions accounting for There are several ways to remove tar 8), both the decomposition of the experimentally undefinable physically 9) 10) and chemically 11) ～ 14). Chemical methods using portion included in the NV and soot reforming were also a catalyst have been applied for potential tar elimination, tested to improve the model capabilities. Finally, the model but they are expensive and require good technology to was critically evaluated by comparing the predicted and manage and regenerate the deactivated catalyst 9) 15) 16). experimental results obtained using a two-stage reactor 15). Non-catalytic partial oxidation or steam reforming is a practical and effective method for tar removal from the 2. Methodology thermochemical conversion of biomass 11) 15). Many studies 2.1 Experimental have successfully applied a partial oxidation approach to 2.1.1 Biomass sample control the tar concentration 11) 13) 17) 18). In addition to partial Japanese cedar wood chips with particle sizes of 1.5- oxidation, steam reforming provides additional advantages 2.0 mm were used. The results of proximate and ultimate not only for tar removal, but also in terms of ensuring a analyses have been described elsewhere 15). hydrogen-rich content in the end product. 2.1.2 Reforming of nascent volatiles by steam or air Non-catalytic reforming of the nascent volatiles (NV) The experimental study of the partial oxidation (POx) derived from biomass fast pyrolysis is a complex process and steam reforming (SR) of NV to examine the effect of that likely consists of uncountable chemical reactions. A steam and air reagents on tar destruction by Wang et al. 15) deeper understanding of the complex reactions in the was used to critically evaluate the DCKM. It should be reforming process associated with tar formation and noted that no additional experiment was carried out for the consumption is required for better process design and POx and SR of NV, the experimental data was taken from optimisation. Wang et al. 15). The experiment was designed for pyrolysis A detailed chemical kinetic model (DCKM) consisting in the first reactor followed by reforming in the second of thousands of elementary step-like reactions of stable reactor. Biomass (1.0 g/min) together with carrier nitrogen species experimentally and theoretically established (1 NL/min) were continuously fed into the pyrolyser where for volatile components derived from biomass, as well the temperature was held at 750 °C. The generated NV as intermediates including radical species for biomass were immediately flowed into the reformer reactor and thermochemical conversion, has been developed to reacted with the reforming reagent (air/steam) at 900 °C. understand both the conversion of feedstocks and formation Products were collected from the sampling ports between of products 6) 19) ～ 22). The DCKM was developed to overcome the pyrolysis and reformer reactors and along the flow the limitations of the lumping approach, in which species direction inside the reformer reactor. Detailed descriptions are grouped into one or more different lumps and kinetic of the experimental set-up as well as the product analysis parameters are determined by numerical fitting 23) 24). In have been reported previously 15). contrast, the concentration of each individual molecule in the 2.1.3 Analytical pyrolysis gas phase can be used directly as input information for a The molecular composition of the NV is required for DCKM, and the kinetic parameters of individual elementary the DCKM computations and was derived from analytical reactions are provided based on experimental and pyrolysis experiments with an original set-up 6) 19). These theoretical studies. In a recent report, we used a DCKM for experiments were also used to monitor the secondary gas- 796 J. Jpn. Inst. Energy, Vol. 94, No. 8, 2015 phase cracking of the NV at different residence times. 2.2 Numerical simulation Unlike the experiments for the POx and SR of NV, these 2.2.1 Vapour phase cracking of NV in the UTSR experiments were done by us and the data given here is The vapour-phase cracking of NV derived from all original. In addition, these data were used to develop a cedar sample fast pyrolysis in the UTSR was numerically global reaction for the unidentified products to be applied simulated using the BATCH code in the DETCHEM along with the DCKM. The pyrolyser was designed as a program package. DETCHEMBATCH was designed for U-shaped two-stage tubular reactor (UTSR) divided into computational analysis of time-dependent homogeneous two zones by quartz wool filter, one for the fast pyrolysis reaction systems 25). Simulations were performed under of cedar wood chips and the other for the cracking of the isobaric and isothermal conditions for residence times NV. Approximately 1.0 mg of cedar sample wrapped with of 0 - 4.1 s. The boundary conditions required for the stainless steel (SUS) wire mesh was fixed by a magnet to computations are listed in Table 1. the upper part of the UTSR. After heating the UTSR to the Table 2 shows the product yields when the sample desired temperature, the sample was dropped to the bottom was fast pyrolysed with the UTSR at 750 °C and a vapour- of the first zone. Char product remained over the quartz frit phase residence time of 0.2 s. There were 52 identifiable at the reactor bottom and the char yield was determined by pyrolysates from the NV quantified by three different GC weighing method. The NV formed by fast pyrolysis were instrumental configurations.
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