Abstract
The energy requirements for converting one tonne (1,000 kg) of Chlorella slurry of 20 wt% solids via fast pyrolysis, microwave-assisted pyrolysis (MAP), and hydrothermal liquefaction (HTL) were compared. Drying microalgae prior to pyrolysis by using a spray drying process with a 50% energy efficiency required an energy input of 4,107 MJ, which is higher than the energy content (4,000 MJ) of raw microalgae. The energy inputs to conduct fast pyrolysis, MAP, and HTL reactions were 504 MJ (50% efficient), 1,057 MJ (~25% efficient), and 2,776 MJ (50% efficient), respectively. The overall energy requirement of fast pyrolysis is theoretically about 1.6 times more than that of HTL. The energy recovery ratios for fast pyrolysis, MAP, and HTL of microalgae were 78.7%, 57.2%, and 89.8%, respectively. From the energy balance point of view, hydrothermal liquefaction is superior, and it achieved a higher energy recovery with a less energy cost. To improve the pyrolysis process, developing drying devices powered by renewable energies, optimizing the pyrolysis process (specifically microwave-assisted), and improving the energy efficiency of equipment are options. Citation: Zhang, B., Wu, J., Deng, Z.,  Yang, C., Cui, C., and Ding, Y. (2017). A Comparison of Energy Consumption in Hydrothermal Liquefaction and Pyrolysis of Microalgae. Trends in Renewable Energy , 3(1), 76-85. DOI: 10.17737/tre.2017.3.1.0013
Highlights
Thermochemical conversion of microalgae can be divided into pyrolysis of dry algae and hydrothermal liquefaction (HTL) of algal slurries [1]
The proposed harvesting process is using a series of mechanical unit operations to dewater the microalgae media to a level of ~20% dry solids, which is considered as a less energy intensive processing option than completely drying microalgae for pyrolysis purpose
This paper provides a simple comparison between the energy consumptions in pyrolysis of microalgae and hydrothermal liquefaction of microalgae
Summary
Thermochemical conversion of microalgae can be divided into pyrolysis of dry algae and hydrothermal liquefaction (HTL) of algal slurries [1]. The microalgal culture has a very dilute concentration of 0.1-1% dry solids. Drying is one of most dominant costs for algae harvest and may account for 30% of the total product costs, and the power consumption was equivalent to 15.8% of the energy of the recovered hydrocarbon [2]. Because of this energy consumption barrier, pyrolysis is considered as a kind of hopeless technologies for microalgae and only limited to laboratory investigations [3]. Researchers recognized the advantages of the pyrolysis of microalgae (such as higher quality of pyrolytic bio-oil than that of cellulosic biomass) [4] and the merits of pyrolysis technology (such as lower capital cost than HTL) [5, 6]
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