Elsevier

Bioresource Technology

Volume 275, March 2019, Pages 239-246
Bioresource Technology

Effect of pretreatment on chemical characteristic and thermal degradation behavior of corn stalk digestate: Comparison of dry and wet torrefaction

https://doi.org/10.1016/j.biortech.2018.12.044Get rights and content

Highlights

  • Dry and wet torrefaction characteristics of corn stalk digestate were compared.

  • Compared to DT, the reactivity of WT was higher but lower temperature sensitivity.

  • Torrefied temperature above 260 (DT) or 220 °C (WT) was not suited for CSD pyrolysis.

  • The organic coverings after WT change the burning type showing a premise of oxidation.

  • WT visibly narrow the burning range of CSD while DT gradually reduce the burning rate.

Abstract

In this study, the dry torrefaction (DT) and wet torrefaction (WT) were compared at different temperatures to explore the effect of these pretreatments on the characteristic of corn stalk digestate (CSD) and the thermal degradation behaviors. The results indicated the both torrefactions improved the fuel properties of CSD, including the lower volatiles content, higher carbon content and HHV. The WT showed higher removals of organics and alkali metals, while DT retained more carbon and ash. The SEM, FTIR and XRD analyses indicated the existence of organic coverings of WT samples, and showed richer surface functional groups, relative complete lignin structure and higher crystallinity compared to DT samples. The thermogravimetric analysis displayed the torrefied temperature above 260 °C (DT) or 220 °C (WT) was not suitable for CSD pyrolysis. Besides, the WT samples showed the more concentrated combustion range, while DT tends the burning of CSD to the behavior of coal.

Introduction

The promotion and application of biomass energy is considered an effective way to reduce the dependence on fossil energy and the burden on ecological environment. In recent years, the integration of different biomass conversion technologies is a new concept proposing, which could avoid the shortcomings of single process and improve overall energy recovery (Feng and Lin, 2017). Anaerobic digestion (AD) is one of the main biodegradable technologies transforming lignocellulosic biomass into bioenergy (Wang et al., 2016). But, after AD, amounts of solid digestate byproducts are generated, which bear the potential risk of environmental pollution. Due to the high content of lignocellulose and carbon, the lignocellulosic digestates could be used as a promising feedstock to thermo-chemical transformations, including combustion and pyrolysis (Monlau et al., 2015b). The investigation of Kratzeisen et al. (2010) indicated that the digestate pellets could be used as a fuel for combustion, which was considered an outstanding alternative solid fuel for wood. Meanwhile, the lignocellulosic degestate also exhibits extremely high pyrolysis potential, such as the high selectivity of phenol products in bio-oil (Liang et al., 2015), high adsorption performance of bio-char for heavy metals (Inyang et al., 2012). Hence, the cascaded process of AD and thermochemical treatment can simultaneously reduce the cost management of solid digestate and elevate the efficiency for bioenergy.

However, the lignocellulosic biomass is not suitable for direct utilization as solid fuel due to the inherent properties such as fibrous nature, high oxygen content, low heating value and low ash melting temperature (Bach and Skreiberg, 2016). A suitable pre-treatment is necessary to improve the physics and chemistry properties for biomass. The torrefaction was widely discussed due to the capacity to overcome the drawbacks forementioned. Both the wet torrefaction (WT) and dry torrefaction (DT) can enhance the fuel qualities, including the hydrophobicity, grindability, homogeneity, calorific value, carbon content, etc. (Wilk and Magdziarz, 2017, Yan et al., 2017, Kambo and Dutta, 2015).

WT, also referred to hydrothermal carbonization (HTC), is defined as a hydrothermal pretreatment under mild temperatures (180–250 °C) and pressure (2–10 MPa) with liquid water (Nizamuddin et al., 2017). Due to the presence of water, WT is highly suitable for treating the extreme wet digestate to avoid the drying energy consumption. During WT, hemicellulose and cellulose can be effectively removed by hydrolysis at relative low temperature (Funke and Ziegler, 2010). Meanwhile, the process water can also dissolve part of alkali and alkaline earth metals to mitigation the slagging of subsequent combustion. Hence, the liquid reaction media is beneficial to lowering reaction temperature and improve the fuel properties of lignocellulose. However, the high concentration of organic phase and metal ion in process water also partly limits the engineering development of WT, resulted in some problems of reactor by corrosion, deposition and clogging. Besides, the requirements of temperature and pressure also increase the investment cost of the relevant equipment (Bach and Skreiberg, 2016).

Compared to WT, DT is a more traditional thermo-pretreatment, which is carried out in the absence of oxygen at a temperature range of 200–300 °C for a residence time of 30 min to a couple hours. The up-scaling of DT process is less complicated than WT because the reaction media is inert gases with atmospheric pressure (Bach et al., 2017). For the solid digestate, a major obstacle for DT is the high energy consumption of drying brought by its high moisture content. The separated solid digestate contains up to 70% moisture (Monlau et al., 2015a), while the moisture content of DT feedstock is required to be less than 10% (Bach and Skreiberg, 2016). However, the investigation by Monlau et al. (2015a) indicated that the heat surplus of AD process was enough to cover solid digestate drying. Meanwhile, Doddapaneni et al. (2018) studied the techno-economic feasibility of the integrating of torrefaction and AD, which was better than standalone torrefaction. Therefore, DT is also considered a promising pretreatment for lignocellulosic digestate.

The torrefaction of solid digestates has a positive effect on the subsequent thermochemical conversion, which is benefited to identify the better option for the combination of the AD and thermochemical degradation to further increase the recovery efficiency of bioenergy. However, the researches of the thermal pretreatment on the digestate were limited, only a few papers have studied its DT and/or WT (HTC) performances and the physicochemical properties of torrefied samples (Mumme et al., 2011, Sawatdeenarunat et al., 2018). Few investigations focused on the comparative assessment of lignocellulosic digestate on the WT/DT performances and the subsequent thermal degradation behaviors. Therefore, the main objective of this study was to systematically investigate the WT and DT characteristics of corn stalk digestate (CSD) and the followed thermal degradation behavior of the torrefied CSD. Meanwhile, the fuel properties and the torrefied degradation pathways were compared in their respective temperature regions. Besides, to clarify the internal relations between the thermal pretreatment and the following pyrolysis or combustion behaviors, the torrefied digestates were studied by thermogravimetric (TG) technology in both nitrogen and air atmosphere.

Section snippets

Materials

The CSD was obtained from AD batch experiments at 35 °C. The total solid content was 10%. After 50 days, the solid residues were pressed and stored in sealed bags. Before torrefaction and analysis, the CSD were dried at 105 °C for 24 h and pulverized to particle sizes below 1 mm. The proximate and ultimate analyses of CSD are listed in Table 1.

Torrefaction experiments

The dry torrefaction experiment was carried out in a horizontal fixed bed tubular reactor (55 mm inner diameter, 680 mm length). In each run, 5 g of CSD

Mass and energy yields of torrefied CSD

As shown in Fig. 1, for both dry and wet torrefaction, the mass and energy yields of torrefied samples are decreased with the increase of temperature. But compared to DT, the yields of WT were clearly lower at the same treatment temperature. The mass yields of DT220 and DT240 were 85.02% and 76.82%, respectively, while the yields of WT220 and WT240 were 58.38% and 48.65%. As reported of previous work, the hydrolysis of hemicellulose occurs at around 180 °C while the cellulose starts decomposing

Conclusion

Both torrefactions improved the fuel properties of CSD. WT showed higher removals of organics and alkali metals, while DT retained more carbon and ash. Compared to DT, the reactivity of WT was higher but lower temperature sensitivity. With the torrefied temperature increasing, the structure and thermal degradation behaviors of WT samples were relatively close until 240 °C. Contrarily, the changes of DT samples were more gradual. The organic coverings after WT increased the devolatilization peak

Acknowledgements

The authors gratefully thank the support for this research from The National Natural Science Foundation of China (51536009, 51276103), Distinguished Expert of Taishan Scholars Shandong Province, Higher Education Superior Discipline Team Training Program of Shandong Province.

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