Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser A review of operating parameters affecting bio-oil yield in microwave pyrolysis of lignocellulosic biomass S. Mutsengerere a, ⁎ , C.H. Chihobo a , D. Musademba a , I. Nhapi b a Fuels and Energy Department, Chinhoyi University of Technology, Private Bag 7724, Chinhoyi, Zimbabwe b Environmental Engineering Department, Chinhoyi University of Technology, Private Bag 7724, Chinhoyi, Zimbabwe ARTICLE INFO Keywords: Microwave pyrolysis Lignocellulosic biomass Bio-oil production MAP optimisation Value-added chemicals ABSTRACT The depletion of fossil fuels and the environmental concerns associated with their extraction and utilisation have heightened the need for alternative sustainable energy resources. Microwave-assisted pyrolysis (MAP) of bio- mass is one of the promising sustainable methods for the production of bio-oil and value-added chemicals. This conversion technique has been widely studied at lab-scale and has demonstrated good potential in producing bio-oils of high quality. The main objective of this review paper is to examine the main parameters that affect MAP yields, whilst assessing the options for optimising the identified parameters for maximum bio-oil pro- duction. The results show that factors such as pyrolysis temperature, microwave power, microwave absorbent type and catalysis greatly influence the bio-oil yield. Pyrolysis holding time, feedstock particle size, purging gas type and flowrate also affect the bio-oil yield, though to a lesser extent. It is also established that the MAP technique is highly scalable and has several environmental benefits. The review concludes that the inter- dependence of various factors must be studied in detail in order to optimise bio-oil yield. There is therefore a need to develop an optimised pilot scale MAP system in a bid to determine the techno-economic viability of upscaling the technology for commercialisation. The use of Response Surface Methodology (RSM) via the Central Composite Design (CCD) approach is recommended for the optimisation of MAP systems because it takes into account the interaction between the variables that affect bio-oil yield. This review could be used to guide future research related to the design, simulation, fabrication and operation of pilot scale MAP reactors. 1. Introduction Fossil fuels such as coal, natural gas and petroleum products, play a vital role in power generation, transportation and industrial sectors of most developing countries. Socioeconomic stability is a determinant of sustainable fuel energy supply [1]. The lack of sustainability of fossil fuels, coupled with their adverse environmental impacts, brings about the need for alternative sources of renewable and sustainable energy. Biomass has been identified as one of the promising sources of renew- able energy in that it can be converted into energy-efficient biofuels using agrochemical, biochemical, physical and thermochemical methods [2]. Pyrolysis is one of the thermochemical conversion methods that can be used for the conversion of biomass feedstock into high quality bio- fuels and value added chemicals. Pyrolysis is the destructive distillation of a solid, carbonaceous material in the absence of stoichiometric oxygen [3,4]. The pyrolysis process, which is a precursor to other thermochemical processes (gasification and combustion), can be treated as a stand-alone process. Products from biomass pyrolysis in- clude bio-oil (liquid), non-condensable gases and bio char (a solid re- sidue). Biomass pyrolysis has proved to be a viable thermochemical con- version pathway due to its ability to recover both the chemical and calorific value of the biomass feedstock [5]. This conversion method can produce bio-oil yields of up to 70–95% of the original biomass weight [6,7]. Typical operating temperatures for biomass pyrolysis range from 300° to 1000 °C [8]. Conventional pyrolysis systems are usually energy-intensive, thus compromising their economic viability in industrial applications. Over the past few decades, research has focused on the development of energy-efficient pyrolysis systems which can be adopted for biomass pyrolysis. The integration of microwave heating into the pyrolysis process has been identified as a promising solution. This has given birth to a new pyrolysis technique termed Microwave-Assisted Pyrolysis (MAP) [10–13]. Microwave energy can only heat selected reactants, reducing the energy requirements and associated costs for the MAP process, https://doi.org/10.1016/j.rser.2019.01.030 Received 11 May 2018; Received in revised form 4 January 2019; Accepted 12 January 2019 ⁎ Corresponding author. E-mail address: smutsengerere@cut.ac.zw (S. Mutsengerere). Renewable and Sustainable Energy Reviews 104 (2019) 328–336 1364-0321/ © 2019 Elsevier Ltd. All rights reserved. T