Torrefaction of cultivation residue of Auricularia auricula-judae to obtain biochar with enhanced fuel properties

Highlights

• Torrefied biochars are derived from cultural residue of Auricularia auricula-judae.

• Lower temperature torrefaction do not apparently enhance biochars energy density.

• Effect of residence time on increasing of HHV is obvious at high temperature range.

• Devolatilization and condensation improve CRAA biochars combustion efficiency.

• Maximum reactivity corresponds to the degradation of cellulose crystalline region.

Abstract

Auricularia auricula-judae, commonly known as the wood ear, is a species of edible Auriculariales fungus. Torrefactions of cultivation residues of A. auricula-judae (CRAA) at different temperatures and residence times are carried out. Effect of different torrefaction conditions on the mass and energy yields, elemental composition, heating value, thermal properties; and chemical, crystal and morphological structures of biochars are analyzed. Low torrefaction temperatures could not enhance the fuel properties as expected. Crystallinities of biochars derived from mild torrefaction conditions do not have a suitable variation trend because of the unstable degrees between degradation and recrystallization. Torrefaction of CRAA should be processed under severe conditions, such as 320 °C for 60 min and 280 °C for 120 min, in order to obtain enhanced biochar properties with DTG_max at low peak temperatures of 367 and 361 °C, as well high burnout temperatures of 528 and 530 °C, respectively.

Introduction

Exhaust fumes from combustion of fossil fuels causes aggravation of environmental pollution coupled to climate change leading therefore to urgent demand for renewable and sustainable resources (Baños et al., 2011). Lignocellulosic biomass is considered as a potential fossil fuel substitute because it is an abundant resource and has a carbon-neutral property (Wu et al., 2010). However, the utilization of lignocellulosic biomass is limited because of the following drawbacks: high moisture and oxygen contents, low density and heating values, and off-gas emission (Yang et al., 2014, Pimchuai et al., 2010). These issues can be overcome by using a torrefaction process. Torrefaction is a mild thermochemical technology which pretreates biomass at temperature ranging from 200 to 300 °C under inert environment. This process mainly decomposes hemicellulose and is responsible for the depolymerization and devolatilization of both cellulose and lignin (Mafu et al., 2016, Shoulaifar et al., 2014). As a result, a more homogeneous biochar with lower oxygen content, higher carbon content, and higher energy density is obtained (Rousset et al., 2011, Pimchuai et al., 2010). For a typical mass and energy balance of torrefaction, it is reported that 70% of the mass is retained during the torrefaction process, 90% of the initial energy remains in the solid product (Bergman et al., 2005). The torrefaction process releases volatiles of water, carbon dioxide, carbon monoxide etc. from the initial feedstocks, resulting in a lower O/C and H/C ratio (Wilk et al., 2016, Pimchuai et al., 2010). The generated volatiles occupied 30% of the mass, but contained only 10% of the energy. More energy is fixed in the solid product after feedstocks undergoing torrefaction (Bergman et al., 2005). Thus, torrefaction is an appropriate pretreatment method to yield solid-biomass feedstocks with a high energy density (Chen et al., 2012, Sadaka and Negi, 2009).

Auricularia auricula-judae belongs to the edible fungi species and has been widely used in China for food and drugs. From early 1970s, most A. auricula-judae is commercially cultivated in substitute cultivation instead of the natural trunks and branches of trees because of the high survival rates and short harvesting cycles (Wang Qiuling, 2011). The substitute cultivation is made of a PVC bag which is filled with mixture of wood sawdust (carbon source), wheat bran and soybean residue (nitrogen source) and other addictives (Gao et al., 2015, Wang Qiuling, 2011) in order to supply nutrients to the fruiting body. With over 2.85 billion bags of A. auricula-judae cultivation used for A. auricula-judae production, the by-products of Auriculariales fungus, namely the cultivation residue of A. auricula-judae (CRAA), are considered to be an abundant lignocellulosic resource (Wang Qiuling, 2011).

Most CRAA has currently being collected and directly burned, with dramatic consequences for the environment. The combustion of lignocellulosic biomass generates smoke and has a low burning conversion due to its high moisture and oxygen contents (Pimchuai et al., 2010). Many studies on the torrefaction of different woody and agricultural residues, such as rice straw (Huang et al., 2012), wheat straw (Shang et al., 2012), corn stover (Liu and Han, 2015), pomaces and nut shells (Chiou et al., 2015) can be found in the literatures. However, no research has been found on the torrefaction of cultivation residues of edible fungi. Therefore, in the scope of achieving a more energy-efficient utilization of CRAA, this work provides a deep insight into the technique of torrefaction, especially applied to CRAA. Effect of the torrefaction temperature and resident time on thermal properties, chemical and microtopography structures changes of CRAA biochars is shown in this study.