Co-hydrothermal carbonization of food waste-woody biomass blend towards biofuel pellets production
Introduction
Since the first industrial revolution in eighteenth century, the world has consumed all kinds of fossil energy (such as coal, oil, natural gas, etc.) at an astonishing speed in the world. The large-scale use of fossil energy makes human beings face two major crises of energy shortage and global warming. Therefore, it has become an important task for twenty-first century to develop renewable energy and carbon dioxide emission reduction (Zhao et al., 2014, Zhu et al., 2017). At the same time, the burning of fossil fuels has led to a large number of CO2 and NOx emissions into the atmosphere, so it is urgent to find renewable energy replace traditional fuels and to reduce environmental pollution (Zhang et al., 2016). As a renewable energy source, the exploitation and utilization of biomass energy not only helps to alleviate the energy crisis caused by the depletion of fossil fuels to the global economic development, but also reduces the emission of greenhouse gases, and helps to maintain the ecological balance and improve the human living environment (Wang et al., 2018b, Zhang et al., 2018).
Attention has been focused on the thermal techniques for conversion of biomass to biofuels such as biogas, bio-oil, and biochar during the past ten years (Wang et al., 2018b). Among these, hydrothermal carbonization (HTC) technique is special due to its high adaptability to the wet biomass like algae (Gai et al., 2015, Yu et al., 2011), food waste (Wang et al., 2018c), municipal sludge (Peng et al., 2016), animal manures (Cao et al., 2011), etc., and the water contained in the raw feedstock can be directly used as reaction medium (Prawisudha et al., 2012, Savage, 2012). Based on these, the solid product, named hydrochar, generally had improved calorific value, hydrophobicity and homogeneous properties process, since the occurrence of dehydration and decarboxylation reaction during HTC (Hoekman et al., 2011, Liu and Balasubramanian, 2014). In addition, the fuel properties like the fixed carbon content and heating values also depended much on the hydrothermal parameters mainly including the temperature, residence and time (Reza et al., 2012, Simsir et al., 2017, Wang et al., 2018a). In general, the hydrochar shows great potential to be an alternative of the solid fuel. The pelletization process in combination with the HTC is also proposed as an alternative to improve the fuel properties of biomass (Liu et al., 2014). To improve the energy density and mechanical strength of hydrochar fuel, Liu et al. used hydrochar produced from woody biomass under 250 °C to prepare pellets, and results indicated that hydrochar pellets had increased fixed carbon (20–30%) and heating values (4–6 MJ/kg) compared to raw biomass, and the liquid bridge enhanced the tensile strength of the hydrochar pellets (Liu et al., 2014). Meanwhile, in our previous study, results further showed that the hydrochar from 250 °C produced pellets with lower tensile strength than that from 200 °C due to the carbonized lignin which could not produce solid bridge, and the energy consumption was high (Wang et al., 2017). However, present studies focused mainly on the lignocellulose derived the hydrochar pellet, and food waste derived hydrochar has been rarely studied to prepare fuel pellets. One recent study showed that the food waste derived hydrochar pellets had poor mechanical strength since the limited lignin content because the main components in food waste were protein and carbohydrate like starch and glucose, and the binding ways in the hydrochar pellets relied on attraction forces between the hydrochar microspheres (Zhai et al., 2018). Thus, it can be concluded that the fuel properties and mechanical properties of hydrochar pellets depend much on the components of the raw materials and the preparing conditions during HTC. Considering these, since the lignocellulose-biomass derived hydrochar could produce pellets with excellent mechanical strength by providing solid bridge-type bonding within the hydrochar pellets and food waste were not suitable to form solid fuel with high mechanical strength, the woody biomass and food waste blend derived hydrochar pellets may generate a new approach to produce fuel pellets with enhanced fuel properties and mechanical strength.
In this study, hydrochar from varied food waste-woody biomass blend at 180–260 °C was used to prepare fuel pellets in combination with the pelletization process. The basic fuel properties of hydrochar proximate analysis, elemental analysis, and HHVs of the hydrochar were analyzed. Especially, the energy consumption in the pelletization process was investigated, and the mechanical strength and storage characteristics as well as the combustion characteristics of the hydrochar pellets were evaluated to assess the potential for solid biofuel production.
Section snippets
Materials and methods
Wood sawdust (WS), China fir (Cunninghamia lanceolata), was obtained from a furniture factory (Changsha, Hunan) and food waste (FW) was collected from the restaurants of Hunan University. Visual observation showed that the FW contained mainly cooked vegetables, rice, noodles, condiments, paper cups, and cooked meat, and the plastic and bones were separated out due to the processing limitations. The HTC process was carried out in a 500 mL 316 stainless steel reactor. Before this trial, about
Hydrochar yields and basic fuel properties of hydrochar
The hydrochar yield from different blend ratio of food waste and woody biomass is presented in Table 1. With the increase ratio of FW blend and temperature, it can be found that the hydrochar yield decreased consistently. For hydrochar prepared from 180 °C, the yield of hydrochar decreased approximately 40% with the increase ratio of FW, while this tendency was not obvious for hydrochar produced from 260 °C. It can be deduced that the FW was more easily carbonized than WS. As anticipated, the
Conclusions
High FW blended ratio enhanced the carbonization degree during HTC by mainly decreasing H/C of hydrochar with ratio from 0% to 50%, while mainly decreased the O/C from 75% to 100%. Increasing ratio of FW decreased the energy consumption during pelletization, whereas the Ts of pellets were increased about 2.4–5.5 MPa when WS ratio was increased. Moreover, hydrochar pellets made from high ratio FW achieved low ignition temperature and increased temperature range. For practical applications,
Acknowledgments
This research was financially supported by a project of the National Natural Science Foundation of China (No. 51679083), a scientific and technological project of Changsha City (KQ1602029), a key research and development project of Hunan Province (2018WK2011) and a project of Shenzhen Science and Technology Funds (JCYJ20160530193913646).
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