Elsevier

Bioresource Technology

Volume 343, January 2022, 126112
Bioresource Technology

Valorization of oat husk by hydrothermal carbonization: Optimization of process parameters and anaerobic digestion of spent liquors

https://doi.org/10.1016/j.biortech.2021.126112Get rights and content

Highlights

  • Integration of HTC and AD was performed for oat husk valorization.

  • HTC process optimization was carried out using the response surface methodology.

  • Validation results at optimal conditions showed < 1% error.

  • Oat husk-spent liquor was a suitable substrate for AD yielding 144 NmLCH4/gCOD.

  • Hydrochar addition during AD boosted the methane production by 17%.

Abstract

The hydrothermal carbonization (HTC) optimization of oat husk was performed using a response surface methodology. Furthermore, anaerobic digestion (AD) of spent liquor and hydrochar addition were evaluated in the biomethane potential (BMP) test. Results found that temperature influences the most in the studied responses (i.e., mass yield (MY) and higher heating value (HHV)). Optimal hydrochar MY (53.8%) and HHV (21.5 MJ/kg) were obtained for 219.2 °C, 30 min, and 0.08 of biomass/water ratio. A successful prediction capability of the optimization approach was observed, archiving an error < 1% between predicted and validated responses. The BMP experiment showed the feasibility of spent liquor as a potential substrate to be treated by AD (144 NmLCH4/gCOD). Hydrochar boosted the methane production of spent liquor increasing up to 17% compared to digestion with no hydrochar addition. These findings provide new insights regarding oat husk valorization by integrating HTC and AD for energy production.

Introduction

Agricultural waste is one of the primary sources of biomass that can be used for cleaner energy production (Heidari et al., 2019). In this context, the production of oat (a crop showing positive effects on health, e.g., for obesity and blood pressure control (Chang et al., 2013)) in the world was ∼ 25 million tons in 2020 led by the European Union, Russia, and Canada (IndexMundi, 2021). Considering that 50% of the grain weight is husk (de Oliveira et al., 2017), a massive amount of this lignocellulosic waste is widely produced. In Chile, oat production represents 2.4% of the global production, with about 319 000 tons of oat husk generated in 2020 (IndexMundi, 2021), which is usually burned in agricultural land leading to air pollution and uncontrolled fires in rural sectors (Gómez-González et al., 2019). Hydrothermal carbonization (HTC) is a biomass conversion process capable of positively contributing to the recovery of agricultural, forestry, municipal solid wastes, animal manure, sewage sludge, etc. (Heidari et al., 2019). The possibility of using wet feedstock is one of the advantages of this technology, which comprises of treating biomass with subcritical water at a relatively low-temperature range (180–250 °C). Under subcritical conditions, biomass constituents (i.e., hemicellulose, cellulose, and lignin) are decomposed and converted by hydrolysis, dehydration, decarboxylation, condensation polymerization, and aromatization reactions (Funke and Ziegler, 2010). A solid carbonaceous product, also called hydrochar, is obtained from this process, whereas a liquid (i.e., spent liquor) and a gas phase are formed as by-products. In terms of energy production, there is an increasing interest in hydrochar because of its improved carbon content compared to raw biomass (Libra et al., 2011). In this regard, the effect of HTC on properties and kinetics of combustion behavior of lignocellulosic waste such as pine sawdust, rapeseed meal, oat husk and straw, olive cake, grape cake, and corn leaves was investigated, showing promising results (Monedero et al., 2019). In another work, oat husks were converted by HTC for hydrochar utilization in the treatment of pharmaceutical wastewaters, and improved adsorption of carbamazepine was observed using oat husk-based hydrochar (Aghababaei et al., 2021).

Although the HTC of oat husk has been addressed, its main process parameters' optimization is lacking. Hence, an optimization approach of the HTC of oat husk considering critical process variables such as reaction temperature, residence time, and biomass/water ratio was first provided in this work. The reaction temperature is well-known for being the most crucial parameter in HTC as it enables biomass matrix degradation for hydrochar production (Libra et al., 2011). About residence time, besides affecting the basic fuel properties, it is also known for increasing energy consumption at higher retention times (Nakason et al., 2018). Thus, determining a proper residence time results in a more efficient process. On the other hand, the biomass/water ratio must be controlled to ensure a correct biomass dispersion into the reaction medium. Therefore, this study provides deeper insights into the impact of individual process parameters and their interactions into the output variables using a response surface methodology (RSM). This classical methodology allows obtaining accurate models without performing too much and unnecessary experimental runs.

Previous studies using the RSM methodology optimized the HTC response variables (i.e., mass yield (MY) and higher heating value (HHV)) individually, but the simultaneous optimization of both response variables is not yet addressed. A study carried out by Gan et al. (2019) obtained one model for MY and another for HHV in the HTC of palm kernel shell, which means different optimized conditions for each response variable. On the other hand, Kang et al. (2019) determined optimal conditions for energy yield instead of MY and HHV; however, energy yield may be more influenced by MY than establishing an equilibrated contribution from MY and HHV. In this regard, Reza et al. (2013) found the highest energy yields correspond to those having the highest MY values. Since higher MY are expected at lower reaction temperatures and residence times, the carbonization degree will be lower, and hydrochar energy densification may not probably be enough. Hence, the RSM approach combined with the optimization of multiple responses technique (i.e., simultaneous optimization of MY and HHV) was provided in this work for the first time in HTC to find a balanced contribution from MY and HHV.

Nonetheless, if the HTC process optimization of oat husk is investigated towards an industrial application, the study should not be focused only on hydrochar production. Still, it must consider a strategy for the spent liquor valorization, which is the other significance of this study. Depending on raw biomass composition and HTC operational conditions, several compounds such as organic acids, aldehydes, phenols, furfurals, aromatic amines, among other compounds produced by biomass decomposition, can be found in the liquid phase (Heidari et al., 2019). Several works have been recently published about the valorization of the HTC process water by anaerobic digestion from different feedstock such as sewage sludge (Aragón-Briceño et al., 2021), maize silage digestate (Cao et al., 2020), macroalgae (Brown et al., 2020), cow manure (Marin-Batista et al., 2020) and the organic fraction of municipal solid waste (Lucian et al., 2020). The methane potential of HTC-spent liquors obtained from lignocellulosic waste (Pagés-Díaz et al., 2020) (i.e., pine sawdust, canola waste, olive waste, vineyard waste) has also been previously reported. Nevertheless, although some studies have already reported the methane production of process water from HTC, the methane potential from oat husk spent liquors and the impact of oat husk-derived hydrochar on the methane yield are not available yet in the literature. Thus, the process must be evaluated for different biomasses to optimize the operating parameters in different scenarios.

Based on this background, this work investigates the conversion of oat husks by HTC, considering the effect of reaction temperature, residence time, and biomass/water ratio on both MY and HHV. For a potential industrial scale-up of this process, simultaneous optimization of these two response variables was conducted to keep hydrochar with an improved calorific value, including reasonable mass yields. Besides, an integration scheme of HTC and AD for the spent liquor valorization obtained from the hydrochar production at optimal conditions was proposed, including an analysis of the overall energy potential. To this end, the methane potential of oat husk-spent liquor and the impact of oat husk-derived hydrochar on the methane yield was additionally investigated.

Section snippets

Materials

Oat husks were provided by “Molinera Gorbea”, an agro-industrial plant located in the Araucania Region (Southern Chile). Biomass was stored and treated as received.

Hydrothermal carbonization experiments

Hydrothermal carbonization experiments were carried out in a high-pressure 5 L stainless steel reactor (model HiPR-SF5L). Raw biomass and water were loaded inside the reactor according to each experiment biomass/water ratio. The surrounding air was removed from the free headspace of the reactor using a gaseous N2 stream to ensure the

Statistical analysis of the relationship between factors and responses

To evaluate the fit of the experimental results with the proposed models, an analysis of variance was performed, as shown in Table 2. First, a p-value < 0.0001 from the quadratic models confirmed the models were highly significant, which was in line with an insignificant p-value of 0.2814 and 0.4080 from the lack of fit tests for MY and HHV, respectively. About the fit statistics, the results indicated high R2 values (0.9954 and 0.9878, respectively) and adjusted R2 values (0.9896 and 0.9722,

Conclusions

The optimization of HTC of oat husk showed the dominant effect of temperature on MY and HHV. Simultaneous maximization of response variables led to carbon enrichment of 11%, mainly associated with a low reaction temperature but also due to a low residence time and a high biomass/water ratio. BMP results exhibited the possibility of coupling AD to HTC to environmentally take advantage of spent liquor for methane production. The hydrochar use from oat husk was also demonstrated to improve the

CRediT authorship contribution statement

Herman A. Murillo: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Jhosané Pagés-Díaz: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Funding acquisition. Luis A. Díaz-Robles: Conceptualization, Supervision, Writing - review & editing, Funding acquisition. Fidel Vallejo: Conceptualization, Investigation, Writing – original draft. César Huiliñir: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was financially supported by the project BMBF-CONICYT 150067 and the postdoctoral project FONDECYT N°3190176 (ANID/CONICYT). Herman Murillo and Fidel Vallejo acknowledge the support of “Secretaría de Educación Superior, Ciencia, Tecnología e Innovación” (SENESCYT). Fidel Vallejo acknowledges the support of CONICYT National Doctorate Grant Folio 21170340.

References (48)

  • K. Kang et al.

    Microwave-assisted hydrothermal carbonization of corn stalk for solid biofuel production: optimization of process parameters and characterization of hydrochar

    Energy

    (2019)
  • S. Kannan et al.

    Optimization and characterization of hydrochar produced from microwave hydrothermal carbonization of fish waste

    Waste Manag.

    (2017)
  • Q. Lang et al.

    Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: hydrochar properties and heavy metal transformation behavior

    Bioresour. Technol.

    (2018)
  • M. Lucian et al.

    Hydrothermal carbonization coupled with anaerobic digestion for the valorization of the organic fraction of municipal solid waste

    Bioresour. Technol.

    (2020)
  • E. Monedero et al.

    Effect of hydrothermal carbonization on the properties, devolatilization, and combustion kinetics of Chilean biomass residues

    Biomass and Bioenergy

    (2019)
  • J. Mumme et al.

    Use of biochars in anaerobic digestion

    Bioresour. Technol.

    (2014)
  • K. Nakason et al.

    Hydrothermal carbonization of unwanted biomass materials: Effect of process temperature and retention time on hydrochar and liquid fraction

    J. Energy Inst.

    (2018)
  • S. Nizamuddin et al.

    An overview of effect of process parameters on hydrothermal carbonization of biomass

    Renew. Sustain. Energy Rev.

    (2017)
  • I. Oliveira et al.

    Hydrothermal carbonization of agricultural residues

    Bioresour. Technol.

    (2013)
  • J. Pagés-Díaz et al.

    Anaerobic bio-methane potential of the liquors from hydrothermal carbonization of different lignocellulose biomasses

    Renew. Energy

    (2020)
  • J. Pagés-Díaz et al.

    Valorization of the liquid fraction of co-hydrothermal carbonization of mixed biomass by anaerobic digestion: effect of the substrate to inoculum ratio and hydrochar addition

    Bioresour. Technol.

    (2020)
  • M.T. Reza et al.

    Production, characterization, and biogas application of magnetic hydrochar from cellulose

    Bioresour. Technol.

    (2015)
  • M.T. Reza et al.

    Reaction kinetics of hydrothermal carbonization of loblolly pine

    Bioresour. Technol.

    (2013)
  • E. Sabio et al.

    Conversion of tomato-peel waste into solid fuel by hydrothermal carbonization: influence of the processing variables

    Waste Manag.

    (2016)
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