Comprehensive evaluation of various pyrolysis reaction mechanisms for pyrolysis process simulation

https://doi.org/10.1016/j.cep.2018.05.011Get rights and content

Highlights

  • Modeling of fast pyrolysis process for biomass is studied.

  • The fast pyrolysis reaction mechanisms are applied to the modeling of reactor.

  • To evaluate reaction mechanism, fast pyrolysis experiment is compared.

  • The mechanism including secondary cracking of tar is similar to experimental data.

Abstract

In recent years, an acceleration of global warming and various environmental pollution problems has been observed due to the increase in energy demand. Biomass, which has several advantages over more typical sources, is very popular as a green energy source. This study focuses on process modeling and analysis of biomass fast pyrolysis. The fast pyrolysis reaction mechanisms, proposed by previous researchers, are applied to the modeling of a fast pyrolysis reactor. This process analysis is performed according to operating conditions such as the reaction temperature, residence time, type of biomass and reactor type. To evaluate these results, the results of the process analysis of other researchers and the GC/MS (gas chromatograph-mass spectrometry) data of fast pyrolysis experiment are compared.

Introduction

In recent years, an acceleration of global warming and various environmental pollution problems has been observed due to the increase in the global energy demand. In response to this, there are now many international efforts to solve the ongoing environmental problems. In many countries, clean and renewable energy development policies have been established and implemented to reduce carbon dioxide emissions. Biomass, a bioenergy source, accumulates carbon dioxide through the carbon fixation process, and so is carbon neutral. In addition, biomass has several other advantages, including a stable supply and low sulfur content. These characteristics mean that biomass has become very popular as a potential green energy source [[1], [2], [3], [4]].

The conversion methods applied to utilize biomass as an energy source include biological conversion methods, such as anaerobic digestion and alcohol fermentation, and thermochemical conversion methods, such as combustion, gasification and pyrolysis. Thermochemical conversion methods are generally faster than biological methods, and the processes are relatively simple [5]. Fast pyrolysis is an existing thermochemical conversion methods for transforming biomass into useful products. In fast pyrolysis, biomass decomposes into three main products – non-condensable gas (NCG), tar and char – at approximately 750 K, in the absence of oxygen. After the condensation of the produced tar, a dark brown liquid is formed, which has a heating value of about half that of conventional oil fuels [6]. The reactor, in which the fast pyrolysis reaction takes place, is the core facility of the fast pyrolysis process. Since fast pyrolysis is a thermal treatment method requiring a short heating rate of 1000 ∼ 10,000 ℃ / s and a residence time of the pyrolysis product is only 1 ∼ 2 s, various types of reactors capable of realizing these conditions have been developed and applied [7]. Fast pyrolysis can produce a higher oil yield compared to that of other conversion methods, such as gasification, carbonization or torrefaction [6]. In fast pyrolysis, the yield of the bio-oil depends on various operation conditions, such as reaction temperature, residence time, reactor type, etc [6]. Therefore, many experimental studies have been conducted to determine the optimum conditions to obtain bio-oil of higher yield and good quality [[8], [9], [10], [11], [12], [13]].

The results of the fast pyrolysis process are also influenced by the type of biomass. The numerous different polymers present in the organic fraction of the biomass are divided in three main groups; Cellulose, Hemicellulose, Lignin. The distribution ratios of these three components varies depending on the type of wood, but they are generally distributed in the order of cellulose > lignin > hemicellulose, whereby cellulose is the main component of the biomass. Cellulose consists of long polymers built around a C6-monomer base structure and begins to thermally degrade around 620 K. Hemicellulose is, in most aspects, similar to cellulose, but is a more branched polymer. This results in a lower thermal stability; the thermal degradation of hemicellulose starts around 540 K. Lignin is the least homogeneous biomass compound and consists of many different chemical structures. However, lignin has the highest energy content of the three biomass components. The thermal degradation of lignin, which starts around 660 K, takes place over a wide temperature range, due to the range of chemical structures present [14,15]. The chemical composition of biomass is very diverse and as such, the actual fast pyrolysis reaction process is very complex. Modeling the reactor to account for the true reaction is currently very difficult due to the high computing costs. Therefore, it is necessary to study the simple, but accurate, reaction mechanism of fast pyrolysis in order to design the reactor. Until now, only a small number of commercialized chemical plants have adopted the fast pyrolysis bio-oil production process, and most research focuses on lab-scale experiments. Based on this research, however, the scaling-up of chemical plants for the commercialization should be optimized through process analysis, from which it can save up to 30% of the project cost [16].

Currently, although numerical studies have been performed using various reaction mechanisms of fast pyrolysis [[17], [18], [19], [20], [21], [22], [23], [24], [25]], it is very hard to find full critical reviews for the merits and demerits of these mechanisms through process simulation of biomass fast pyrolysis. Considering the design stage of the fast pyrolysis system, the in-depth comparisons and analysis of most of known reaction mechanisms is noteworthy as it may enable designers to select the most appropriate mechanism. Therefore, in the present study, fast pyrolysis reaction mechanisms, proposed by the previous researchers, were applied to the modeling of the reactor and a process analysis study was performed according to the changing operating conditions. In addition, to evaluate the process analysis results of this study, the process analysis results of other researchers [17,18] and the GC/MS (gas chromatograph-mass spectrometry) data of fast pyrolysis experiment [[8], [9], [10]] were compared. In terms of the operating conditions, the reaction temperature was varied from 673 K to 873 K, the residence time changed from 0.5 to 5.0 s and three types of biomass, each with a different ratio of cellulose, hemi-cellulose and lignin, were considered. Both batch reactor (BR) and continuous stirred-tank reactor (CSTR) models were assessed in this study. Finally, various fast pyrolysis reaction mechanisms were fully incorporated and evaluated in terms of future scale-up design.

Section snippets

Overview of the fast pyrolysis process

Fig. 1 shows a schematic diagram of the fast pyrolysis process. The process consists of a biomass feeder, heater, fast pyrolysis reactor, cyclone, condenser and electrostatic precipitator. First, the biomass feedstock is fed into the fast pyrolysis reactor through the feeder and is then pyrolyzed in the reactor. Here, the biomass decomposes into NCG, tar and char. These fast pyrolysis products move to the cyclone together with a fluidizing gas, such as nitrogen, and the char, which is mainly

Results and discussion

In the present study, the fast pyrolysis process was modeled as a lumped model using the governing equations and reaction mechanisms, above. Table 10 shows the major component ratios of biomass selected for the fast pyrolysis process analysis [17,18].

Conclusions

In the present study, fast pyrolysis reaction mechanisms, such as three step mechanisms, two-stage semi global mechanisms, the Broido-Shafizadeh mechanism, and multi-step mechanisms [[31], [32], [33], [34]] were applied to model a fast pyrolysis reactor. A process analysis study was performed according to the change in the operating conditions; the reaction temperature was varied from 673 K to 873 K, the residence time was changed from 0.5 to 5.0 s, and three types of biomass, each with a

Acknowledgement

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT, and Future Planning (MSIP) of Korea (NRF-2017R1A2B4009340).

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20164030201250).

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