Production of biochar for potential catalytic and energy applications via microwave vacuum pyrolysis conversion of cassava stem Shin Ying Foong a , Noor Syazana Abdul Latiff a , Rock Keey Liew b , Peter Nai Yuh Yek a,c , Su Shiung Lam a,⇑ a Pyrolysis Technology Research Group, Institute of Tropical Aquaculture and Fisheries (Akuatrop), Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia b NV WESTERN PLT, No. 208B, Jalan Macalister, Georgetown 10400, Pulau Pinang, Malaysia c School of Engineering and Technology, University College of Technology Sarawak, Lot 88, Persiaran Brooke, 96000 Sibu, Sarawak, Malaysia article info Article history: Received 29 April 2020 Revised 4 August 2020 Accepted 4 August 2020 Available online 14 August 2020 Keywords: Microwave pyrolysis Vacuum Biomass waste Biochar Cassava stem abstract Cassava stem waste is abundantly available after starch extraction, hence posing disposal problem and potential risk to environmental pollution. Alternative ways are needed to dispose the waste instead of landfilling and open burning. Microwave vacuum pyrolysis was examined as an alternative method to convert the cassava stem waste into biochar. The cassava stem was first characterized by several analyses followed by conversion into biochar using microwave vacuum pyrolysis over different microwave power. The cassava stem contained high contents of carbon (49.5 wt%) and considerable amount of fixed carbon (27 wt%), indicating its suitability to be used as pyrolysis feedstock. The biochar yield from microwave power of 550 W, 650 W and 750 W were 77%, 73% and 70% respectively. The surface morphology of bio- char shows that it consists abundant pores on the surface, suggesting its suitability to be used as catalyst support for energy application, and also as adsorbent for use in adsorption-related process such as dye and heavy metal removal in wastewater treatment or nutrient retention in crops cultivation. The calorific value of the biochar ranged from 19.24 to 20.55 MJ kg 1 , showing high energy content for potential use as solid fuel. Our results show that cassava stem can be transformed into biochar for useful energy applica- tions rather than burned as low-grade fuel in boiler or disposed by landfilling. Ó 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc- nd/4.0/). 1. Introduction Cassava (Manihot esculenta Crantz) is one of the important source of dietary energy that originates from South America [1]. According to the statistic from World Federation of Food and Agri- culture Organization (FAO), the global production of cassava was increased from 226 million tonnes in year 2007 to 292 million ton- nes in year 2017. Nigeria occupied a prominent position in the pro- duction of cassava with 59 million tonnes produced in year 2017, followed by Democratic Republic of the Congo producing 31 mil- lion tonnes and Thailand producing 30 million tonnes [2]. Large amount of waste residue comprising mainly cassava stem have been generated during cassava processing. The cassava stems are commonly disposed by landfilling and open burning, occupying not only land resources but also causing pollution to the environ- ment [3]. Thus, a facile and environmentally friendly approach is needed to convert cassava stem into value-added products. Pyrolysis has been recognized as an effective technology to con- vert biomass and waste materials in limited or free of oxygen envi- ronment [4,5]. In particular, microwave pyrolysis has attracted much attention recently as a promising pyrolysis method for energy conversion of biomass and waste materials. The use of microwave radiation as a heat source shows advantages in provid- ing a rapid, energy efficient and targeted heating process compared to conventional pyrolysis [6]. Conventional pyrolysis and micro- wave pyrolysis show different heating mechanism with respect to the heat transfer between adjacent molecules. Conventional pyrolysis is normally performed with furnace as heating source, where the heat transfer would occur from the heating element of furnace to the outer surface to the inner core of the material being pyrolyzed, thereby limiting the heating efficiency and increasing the process time needed. The volatiles released diffuses from the inner core to the outer surface through a higher temperature region [7,8]. Microwave pyrolysis generates heat inside the mate- rial by collision between molecules of the material, which causes rapid and uniform heating of material, thus showing potential to shorten the process time and temperature. The volatiles released https://doi.org/10.1016/j.mset.2020.08.002 2589-2991/Ó 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ⇑ Corresponding author. E-mail address: lam@umt.edu.my (S.S. Lam). Materials Science for Energy Technologies 3 (2020) 728–733 CHINESE ROOTS GLOBAL IMPACT Contents lists available at ScienceDirect Materials Science for Energy Technologies journal homepage: www.keaipublishing.com/en/journals/materials-science-for-energy-technologies