Elsevier

Renewable Energy

Volume 139, August 2019, Pages 988-999
Renewable Energy

Development and validation of mass reduction model to optimize torrefaction for agricultural byproduct biomass

https://doi.org/10.1016/j.renene.2019.02.106Get rights and content

Highlights

  • High correlation between the heating value and mass reduction at biomass was found.

  • Mass reduction model was proposed for optimizing torrefaction of woody biomass.

  • Reaction rate constants can be obtained based on the empirical Arrhenius equation.

  • Thermogravimetry was used to obtain frequency factor and activation energy of biomass.

  • Possible to predict torrefaction for different process times and temperatures using the developed mass reduction model.

Abstract

Global warming and environmental pollution are accelerating due to the continued use of fossil fuels. To address these problems, biomass is being considered as an alternative and renewable energy source. In this study, the use of agricultural byproducts as a new bioenergy resource was investigated considering their conversion into a fuel via the torrefaction process. In particular, pepper stems (Capsicum annuum) were selected as a renewable agricultural byproduct for this energy conversion process. The longer residence time and the higher temperature, The larger amount of mass reduction and the higher heating value by torrefaction, which showed that there was a correlation between the heating value and mass reduction (R2 = 0.9621). The reaction rate constants for the mass reduction model were then derived from the empirical Arrhenius equation.

Thermogravimetric analysis was conducted to obtain frequency factors and activation energies of the biomass at different heating rates. The experimental results were in good agreement with simulated results (r2 = 0.9639; root mean square error = 0.0363) when a heating rate of 7.5 °C/min was used, which also corresponded well with the rate of temperature increase inside the biomass.

Introduction

Throughout history, humans have used woody biomass as a source of heat energy [1]. However, the use of untreated woody biomass has steadily decreased to the point of being essentially negligible [[2], [3], [4]]. Recently, the used of biomass from agricultural and forestry sectors began after homogenization and standardization techniques were introduced via preprocessing (e.g., wood chips, particles, and pellets). However, this preprocessing technology is specialized in terms of transportation and storage, but still biomass has problems as fuel such as low heating value and high moisture content. To address these challenges, this study aimed to promote the application of biomass by demonstrating the use of torrefaction as a pretreatment process.

Torrefaction is a thermal pretreatment step in which lean oxygen conditions at low temperature (230–300 °C) over a relatively short period of time (10–60 min) are employed [[5], [6], [7], [8]]. The heating value of the torrefied product is increased to a value similar to that of solid fossil fuels, and also the storage and transport of the product are facilitated by enhancing its water resistance and reducing its weight [9]. However, it is difficult to make efficient use of agricultural and forestry byproducts in the torrefaction process due to varying densities and water contents [[10], [11], [12], [13]].

To solve this problem, Director & Sinelshchikov (2019) developed an efficient torrefaction process through one-dimensional model [14] and Goffé & Ferrasse (2018) suggested calculating method of efficiency of biomass conversion through stoichiometry [15]. As a similar approach the torrefaction severity index (TSI) was applied to account for the degree of biomass reduction at different experimental conditions, and used to select the optimized torrefaction process [[16], [17], [18]]. As mentioned above, high correlation was found between the mass reduction and the degree of torrefaction. Therefore, the objective of this study was to develop a torrefaction reduction prediction model based on characteristics such as the moisture and ash content, process temperature and time, in order to efficiently use the biomass.

Section snippets

Materials

This study was conducted using pepper stem (Capsicum annuum), which, excluding herbaceous plants, is one of the largest fractions of agricultural biomass produced in Korea. But pepper stem, lignocellulosic biomass of agricultural by-products, is not utilized but is burned or abandoned. Therefore, it can be effectively utilized in the torrefaction process. The elemental and industrial analytical data of the pepper stems employed in this study are shown in Table 1.

Thermal conversion of biomass

The thermal conversion rate is

TGA of pepper stems

The total required experimental time was ∼21–313 min, depending on the heating rate (Table 2). Shifts in the peak temperature for each sample indicated that the mass reduction varied with the heating rate. The burnout temperature is defined as the temperature where the rate of mass loss falls below 1%/min [59,60]. The lower heating rates resulted in better torrefaction performance than the higher heating rates.

As shown in Fig. 3, the mass reduction in the low temperature range (lower than

Conclusions

In this study, model optimization and experimental validation of a torrefaction process for woody agricultural byproducts with different characteristics were conducted. Comparing the experimental and simulated results, the most suitable reaction rate constant was derived when a heating rate of 7.5 °C/min was applied, with an R-squared value of 0.9639 and an RMSE of 0.0363. In conclusion temperature boundary layers are generated from the application of different heating rates and material

Acknowledgements

Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (IPET) through Agriculture, Food and Rural Affairs Research Center Support Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (717001-07).

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