A study on experimental characteristic of microwave-assisted pyrolysis of microalgae
Highlights
► 1500 W and 2250 W are the best power to gain the maximum yield of bio-oil and gas, respectively. ► The maximum heating rate and temperature became higher as microwave power increased. ► High microwave power and catalysts promoted the pyrolysis of Chlorella vulgaris. ► Activated carbon exhibited the best catalysis effect followed by the solid residue. ► The optimal content of activated carbon was 5% with the bio-fuel (gas and bio-oil) yield of 87.47%.
Introduction
As social economy developed and living standard improved, energy consumption increased gradually. According to “China Statistical Yearbook 2010” (China, 2011), the total energy consumption in 2009 was 3.06647 billion tons standard coal, the amount which was 2.1 times as in 2000. The data of last 30 years showed that energy consumption of fossil fuels accounted for about 90% of the total energy consumption in China. Fossil fuels are non-renewable resources and their prices rise continually. Moreover, their uses significantly promote greenhouse gas (GHG) emission to the environment (Ruan et al., 2011). Therefore, developing new energy to substitute for fossil fuels is imperative (Yaman et al., 2006). The emerging energy such as solar energy, nuclear energy, wind energy and tidal energy still cannot replace the traditional fossil fuels so far because of the limitation of resource and technology. Biomass energy, which results in zero emission of CO2, is the most potential one among alternative energies. Being an emerging energy mostly applied in developing countries, biomass energy can not only alleviate the energy crisis but also reduce the pollution on the ecological environment (Guehenneux et al., 2005).
Compared with other biomass, microalgae have many advantages: (1) their growth cycle is short and generally their quantity can increase doubly within 24 h (Chisti, 2007); (2) their biomass production are higher, 5–30 times of traditional oil crops per unit surface area (Ruan et al., 2011, Schenk et al., 2008); (3) they are extremely rich in oil, over 60% by weight of dry biomass in some species (Gouveia and Oliveira, 2009, Ruan et al., 2011); (4) they do not pose a threat to traditional agricultural resources as they can be cultivated on non-arable land or on waste water without occupying agricultural land (Oh et al., 2010). So far, because of many advantages microalgae have been recognized as the most potential biomass resources to replace fossil fuels (Chisti, 2007). Therefore the study of microalgae energy utilization is very important.
Biomass can be converted into solid, liquid and gaseous products by thermo-chemical methods such as pyrolysis (Corma et al., 2006, Shie et al., 2010). Pyrolysis includes microwave-assisted pyrolysis and conventional pyrolysis such as muffle furnace, tube furnace, fixed bed and fluidized bed reactor (Czernik and Bridgwater, 2004, Meier and Faix, 1999, Mohan et al., 2006). Compared with conventional pyrolysis, microwave-assisted pyrolysis, which has shown advantages such as fast heating, even heating, easy control and selective heating, has been in good graces by many researchers. The studies of microwave-assisted pyrolysis of coffee hulls (Menendez et al., 2007), microalgae (Ruan et al., 2011), rice straw (Tsai et al., 2006) and corn stalk bale (Zhao et al., 2010) focus on the effects of temperature on the product yields and characteristics of the pyrolysis products. And there are some studies discuss the influence of microwave-assisted pyrolysis on pine wood (Chen et al., 2008) and corn stover (Wan et al., 2009) with the help of catalysts. Furthermore, many researchers believe that microwave-assisted pyrolysis will have a greater developing prospect in the transformation of biomass energy (Miura et al., 2004, Omar et al., 2011, Ruan et al., 2011, Ruan et al., 2007). However, there are few discussions on the effects of temperature rising rate, different contents of catalysts. Therefore, under microwave-assisted pyrolysis, the study on the effects of temperature rising rate, different contents of activated carbon and solid residue of microalgae has a significant contribution to harnessing its energy potential.
This paper investigated the microwave-assisted pyrolysis of Chlorella vulgaris under different microwave power levels, catalysts and contents of activated carbon and solid residue. The products, pyrolysis temperature and temperature rising rate were analyzed in order to obtain the optimal conditions and maximize bio-fuel formation.
Section snippets
Materials
In this study, C. vulgaris was provided by Jiangmen Yuejian Biotechnologies Co. Ltd. (Guangdong Province, China). The elemental analysis, proximate analysis and low calorific value are shown in Table 1. The elemental analysis and proximate analysis were based on ASTM D5373 standard and GB212-91 standard, respectively. ASTM D240-92, ASTM D4809-95, ISO 1928 and BSI 1016 standards provided foundation for detecting the low calorific value. Pre-treatment of raw materials were carried out before the
Different microwave power levels
The weight of each sample was 30 g, and three microwave power levels were 750 W, 1500 W and 2250 W. Under different microwave power levels, the curves of pyrolysis temperature measured by thermocouple, the curves of temperature raising rate and product fractional yields are shown in Fig. 2(a)–(c), respectively.
As shown in Fig. 2(a), the entire microwave-assisted pyrolysis temperature of C. vulgaris is less than 200 °C when the microwave power is 750 W. However, the pyrolysis temperature increases
Conclusions
In this study, C. vulgaris was pyrolyzed in a microwave oven under different situations. The microwave power of 1500 W and 2250 W were found to be optimal for obtaining the maximum bio-oil yield of 35.83% and the maximum bio-fuel yield of 74.93%, respectively. The research indicated that the maximum temperature rising rate and pyrolysis temperature became higher as the microwave power increased. Furthermore, it has been shown that the pyrolysis reaction can be enhanced by mixing C. vulgaris with
Acknowledgements
The authors are grateful to Guangdong Key Laboratory of Clean Energy Technology (2008A060301002) and the Fundamental Research Funds for the Central Universities, SCUT (No. x2dlD2105280) for the financial support.
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