TGA Investigation of CO2 Gasification of Hydrothermally Treated Biomass (Corn Cob - Coconut Shell Mixture)

Gasification is one option for producing cleaner fuel from biomass. A gaseous mixture of H₂, CO, CH₄, and CO₂ is produced through the partial oxidation of biomass with a gasifying agent such as air, pure O₂, steam, CO₂, or a mixture of these. This method is capable of handling a wide range of inhomogeneous biomass (including forest, agricultural, and organic processing residues) and converting them into a homogeneous gas with a considerably higher level of applicability. In this research, the CO₂ gasification of hydrothermally treated biomass has been studied using TG-DTA analyzer (Bruker TG DTA 2000SA) apparatus. The biomass treated was a mixture of corn cob and coconut shell (weight ratio of 1:1). This raw biomass was firstly subjected to hydrothermal treatment at three different temperatures (200, 240, and 270°C-denoted as H-200, H-240, and H-270) using a batch autoclave prior to being gasified by CO₂ under atmospheric pressure in the TGA apparatus. The experimental results show that the weight loss of hydrochar was resulted mostly from the process of devolatilization (82.92-86.16%). Hydrochar obtained from higher hydrothermal temperatures demonstrated a lower reactivity of gasification, due to the lower amount of moisture and volatile matter. In addition, higher-temperature hydrochar contained lower potassium content and thus shifted the conversion of gasification reaction to a higher temperature.

Coconut shell; Corn cob; CO2 gasification; Hydrothermal treatment; Reactivity

The rapid development of the industry and transportation sector has boosted the need for energy. However, there has been a steady decrease in the supply of energy derived from fossil fuels such as coal, petroleum, and natural gas. This is taking place alongside growing concern for environmental matters such as the effects of greenhouse gases, acid rain, and global warming, which are associated with large emissions of CO₂, NO_x, and SO_x from the ongoing use of fossil fuels. Thus, the search for alternative energy sources that are more environmentally friendly has become a critical issue worldwide. For this reason, more attention is being paid to the exploration of renewable energy, especially biomass, which offers the greatest potential (Mangut et al., 2006).

Gasification offers a potential means of producing cleaner fuel from biomass. A gaseous mixture of  H₂, CO, CH₄, and CO₂  is produced through the partial oxidation  of  biomass with a gasifying agent such as air, pure O₂, steam, CO₂, or a mixture of these. This method is able to handle a wide range of inhomogeneous biomass (including forest, agricultural, and organic processing residues) and convert them into a homogeneous gas with a considerably higher level of applicability. Depending on its composition, the gas product can be used to generate heat and power (as fuel) (Sridhar et al., 2001; Yin et al., 2002; Rodrigues et al., 2003), to produce H₂ (Wei et al., 2008; Tavasoli et al., 2009; Acharya et al., 2009; Demirbas, 2009;), or to synthesize various chemicals and liquid fuels (Tijmensen et al., 2002; Datar et al., 2004).

There are many factors that influence the characteristics of biomass gasification, two of which are the biomass type and pre-treatment method. Different types of biomass, with their characteristically wide-ranging component compositions, vary in their gasification reactivity from one to another (Kumar & Gupta, 1994; Moilanen et al., 2009). Attempts have been made to use many different biomass samples as feedstock for gasification, including rice husk (Gibran et al., 2018) and oil palm frond (Sulaiman et al., 2012). However, raw biomass usually has a low energy density, high moisture content, and high O/C ratio, leading to lower gasification efficiency. The optimum temperature for raw biomass gasification is below 700°C, much lower than the temperature of gasification in practice. Meanwhile, choking and blockage of the gasifier are two commonly occurring problems due to the formation of condensable tar from biomass (Prins et al., 2006). Therefore, it would be advantageous for the biomass to be treated prior to gasification. Much work has been conducted in the area of dealing with biomass preparation; however, most of it employed pyrolysis (Chen et al., 1992; Kumar et al., 1992; Cetin et al., 2005) or torrefaction (Prins et al., 2006; Couhert et al., 2009) as a pre-treatment method. To our knowledge, only a few publications have provided information on the application of hydrothermal treatment as a feedstock preparation method for biomass gasification. Hence, the effect of hydrothermal pre-treatment on the CO₂ gasification of biomass was investigated in this study. A mixture of corn cob and coconut shell was hydrothermally treated in the range of 200-270°C using a batch autoclave, and subsequently gasified by CO₂ under atmospheric pressure in TGA apparatus.

Although biomass is a clean and renewable fuel for gasification, it is not ideal for use in a gasifier due to its low content of fixed carbon and high moisture and volatile matter content. Hydrothermal treatment can reduce the volatile matter content of biomass which tends to form tar in the subsequent gasification process. However, the increased temperature of hydrothermal treatment leads to a decrease in the reactivity of gasification. Since the moisture and volatile matter contents of hydrochar are still relatively high (moisture content of 3.08-4.21% and volatile matter content of 75.64-80.61%), the overall process was dominated by devolatilization. Only 13.85-16.47 wt.% of hydrochar was converted through gasification by CO₂.

The authors gratefully thank the Department of Chemical Engineering, Faculty of Engineering, Universitas Gadjah Mada for the funding of this research. In addition, the partial financial support from AUN SEED Net/JICA through the Short Term Research Program in Japan is highly acknowledged.

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