A review on the microwave-assisted pyrolysis technique

https://doi.org/10.1016/j.rser.2013.08.008Get rights and content

Abstract

Pyrolysis is a promising bioconversion technique for energy recovery, waste management, and converting biomass into useful energy products which has attracted considerable attention during the past decades. Char/carbonaceous residue, bio-oil, and syngas are the three main products of the pyrolysis process. The pyrolysis technique is one of the major barriers for large-scale commercialization of this method. This study strives to extensively review the recent work on microwave-assisted technology applied to the pyrolysis process as a way of cost reduction. The fundamentals of microwave irradiation and a brief background of pyrolysis are presented. Additionally, biomass resources which can be the raw material for pyrolysis process have been categorized and reviewed in this paper. The effectual parameters of the microwave-assisted pyrolysis process and advantages of this technique have been summarized. It is concluded that microwave-assisted technology is an effectual method to reduce the pyrolysis reaction time and increases the quality of value-added products from different kinds of feedstocks. In addition, this technique can overcome the needs of feedstock shredding and improves the quality of heating as well. Therefore, it can be a suitable method for decreasing the pyrolysis processing cost and a pathway out of poverty for developing countries.

Introduction

Energy is the most important necessity of humans existence on the earth. It is involved in most economic sectors, such as transportation, agriculture, industry and electricity generation, as well as food [1], [2]. Generally, there are three main categories of energy resources: fossil fuels, nuclear resources, and renewable resources [3]. There are several factors such as population growth and economic growth which affect the quality and quantity of energy use in the world. Recently, the world′s population has been increasing at an enormous rate. Although the growth rate for some nations is negative, the total world population is forecasted to increase by an average rate of 0.9% per year from 6.7 billion in 2008 to 8.5 billion in 2035, as illustrated in Table 1 [4]. Growth domestic product (GDP) affects the amount and type of energy use; the statistics show that GDP growth increases the demand for energy. The real GDP increased at an average rate of 2.4% annually from 2009 to 2011 [5]. Consequently, the total consumption and production of major sources of energy in the world such as liquid fuels, natural gas, and coal have increased in the past decades [6], [7]. On the other hand, fossil fuel consumption is known as the main reason for global warming and climate change. Carbon intensity of energy supplies and energy intensity of economic activities, which are the main factors that influence the carbon dioxide emission levels, are increasing day by day [8], [9]. The limited supply of fossil-based resources, yearly increases in the price of fossil fuels, increasing demand for energy due to rises in the population growth rate and GDP, and finally climate change, global warming, and the growing rate of carbon emission have stimulated interest in finding new alternative and cheaper energy resources. These alternative energy resources comprise renewable sources, such as geothermal energy [10], solar power [11], wind [12], hydropower [13], and biomass [14], [15].

Renewable technologies are considered clean sources of energy derived from natural processes and the optimal use of these resources minimizes environmental impacts and produces minimum secondary wastes; thus they are sustainable resources based on current and future economic and societal needs [16]. The limited supply of petroleum resources, global warming issues, increasing greenhouse gas emissions and increasing yearly prices of petroleum based fuels have motivated scientists to find new alternative and cheap energy resources [17]. One of the main strategies to decrease fossil fuel dependence is based on reducing energy consumption by applying energy saving programs focused on energy demand reduction, as well as using renewable energy sources, not only for large-scale energy production, but also for stand-alone systems as replacements for petroleum based fuels. Therefore, the use of renewable energies in the world will increase in order to have a more sustainable energy mix, reduce greenhouse gas emissions and a lower dependency from fossil fuels [18], [19].

About 14% of the total world energy demand is supplied by renewable energy sources, meaning that they still represent a small part of today′s global energy picture. Renewable energy resources include biomass, hydropower, geothermal, solar, wind and marine energies [16], [20], [21], [22]. Forest origins and wastes, agricultural crops and residues, and animal wastes are some potential natural energy sources of biomass. Three different phases of fuels can be produced from biomass: liquid-based, solid-based, and gas-based [23], [24], [25], [26]. Geothermal resources have been used to generate power in many countries around the world; some of these works related to geothermal power generations are discussed elsewhere [27], [28], [29], [30], [31], [32]. Solar energy has been utilized by many countries and has become more popular as a valuable energy source. It is one of the most important renewable resources with a high potential for solving the energy crisis in the world [33], [34], [35]. Wind energy is one of the greenest renewable resources for electricity generation, via mechanical power in windmills, and pumping water in wind pumps, and also sailing ships; in addition, it does not generate pollution or greenhouse gases [18], [36], [37].

Section snippets

Biomass resources

Biomass is known as living or recently living biological materials which can be used as a source of energy and seems to have a high potential for use as an energy source [38], [39]. Goran et al. believed that biomass has the potential to even be one of the main global primary energy sources during the next century [40]. It is one of the most promising alternative sources which can alleviate the energy crisis, reduce pollution and global warming, and it can contribute to sustainable development

Fundamentals of microwave heating

Von Hippel formulated the basic understanding of macroscopic microwave interactions with matter for the first time [61]. The application of microwave technology in the thermal treatment of biomass has increased from the mid-nineties. This technique not only reduces the energy consumption and processing time, but also enables the use of new chemistry (unique internal heating phenomenon associated with microwave energy). It can also enhance the overall production quality [62], [63].

Microwave

General overview and principles of pyrolysis process

It is essential to select the appropriate technology for the conversion of potential biomass resources into value-added products in order to moderate the energy crises and their consequent environmental degradation. Combustion or incineration (complete oxidation), gasification (partial oxidation), and pyrolysis (thermal degradation in the absence of oxygen) are the three main thermochemical conversion routes for the conversion of biomass resources into heat, electricity, fuels, chemicals, or

Variables affecting the microwave-assisted pyrolysis process

The yield and quality of produced value-added products are affected by some critical parameters in the microwave-assisted pyrolysis process. In order to obtain the highest quality and maximum conversion yield, these variables should be optimized. The most important variables in microwave-assisted pyrolysis reaction processes are listed below:

  • Type and size of input biomass/materials

  • Moisture and water content of input biomass/materials

  • Reaction temperature

  • Reaction time (residence time)

  • Microwave

Advantages of microwave-assisted pyrolysis technology

Microwave-assisted pyrolysis technology has the potential for energy and cost reduction. It has been proven as a powerful tool in waste reduction, material recovery, and converting biomass and bio wastes into value-added products. However, this method has not been industrialized yet, although it is an attractive option for further industrialization. In this section, the advantages of microwave-assisted pyrolysis technology are summarized as an efficient alternative method to conventional

Future directions

Considerable attention has been given to renewable substitutes for fossil fuels due to the twin problems of fossil fuel depletion and environmental degradation, increasing the population, and tremendous amount of wastes. Both thermochemical and biochemical processes can be applied to upgrade the biomass. Pyrolysis is a thermochemical method of energy recovery from biomass resources and offers higher energy production efficiency and less pollution. It allows higher energy recovery from biomass

Conclusions

In conclusion, pyrolysis is one of the most important bioconversion routes to convert biomass virgin resources, biomass residues and wastes (non-conventional sources), and municipal solid wastes and sewage sludge into value-added products. It involves the well-established thermochemical decomposition of biomass (hydrocarbon or organic materials) at elevated temperatures in the absence of oxygen, and can provide a variety of energy-dense products in three different phases: solid, liquid, and

Acknowledgement

The authors appreciate the financial assistance from New Brunswick Soil and Crop Improvement Association, New Brunswick Agricultural Council, Agriculture and Agri-Food Canada.

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