Microwave pyrolysis of sewage biosolids: Dielectric properties, microwave susceptor role and its impact on biochar properties

Highlights

• Dielectric properties of biosolids depend on moisture content and frequency.

• Sample size impacts on distribution and intensity of microwave field.

• Microwave susceptor impacts significantly on pyrolysis heating rate.

• Microwave susceptor affects specific surface area of resultant biochar.

• Biochar produced with activated carbon has the highest carbon stability.

Abstract

Microwave assisted pyrolysis (MWAP) is an alternative heating approach to convert biosolids into value-added products, such as biochar, biogas and bio-oil. Studying the dielectric properties of biosolids is fundamental to understand the behaviour of this material under microwave irradiation and to design microwave assisted pyrolysis systems. This study examined the dielectric properties of biosolids with changes in moisture content and applied microwave frequency. Results demonstrated that the dielectric constant decreases with decreasing moisture content and with increasing microwave frequency, but the dielectric loss factor of dry biosolids is almost zero. Simulations demonstrated that moisture content of biosolids impacts on the distribution and intensity of electromagnetic field. Because of the poor dielectric properties of dry biosolids, a microwave susceptor must be added to the biosolids to attract microwave energy so that the materials can reach temperatures required for pyrolysis. Therefore, this study also investigated the impact of four microwave susceptors (activated carbon, charcoal, biochar and glycerol) on biosolids pyrolysis and on biochar properties produced from biosolids via microwave assisted pyrolysis at 600 °C. The choice of microwave susceptor influences the heating rate of biosolids and the specific surface area of the resultant biochar. Results show that activated carbon favours the heating process, increases surface area, and the biochar produced with activated carbon has the highest carbon stability and energy value.

Introduction

The recovery and recycling of resources is a highly important issue currently facing the world. The global population and non-renewable resource consumption per capita are increasing at a greater rate. Peak oil and peak phosphorus are probably the major concerns in the near future; while oil can be replaced by other forms of energy, for example hydrogen, phosphorus has no substitute. Peak phosphorus will occur in the next 20 years, consequently a problem of food security may arise because this element is fundamental for plant growth and cannot be chemically synthesised or produced [1]. Biosolids could be a sustainable source of energy and nutrients, such as nitrogen and phosphorus. Further, biosolids production rate is increasing rapidly due more restricted environmental regulations, urbanisation and population standards [2], increasing the availability of this feedstock. Recovering nutrients, energy and chemicals from biosolids could be a viable solution to manage the ever-increasing amounts of biosolids and to replace vital nutrients for plant growth.

Microwave pyrolysis heats biomass via dielectric heating, which occurs through one of two mechanisms. The first, dipole polarization, occurs when the poles of polar molecules attempt to align with the microwave field, which oscillates at 2.45 GHz, releasing heat due to the intramolecular resistance to this motion [3]. The second is Maxwell-Wagner depolarization, which occurs at the boundary between two materials with different dielectric properties or in materials where electrons can freely move, most notably in some carbon materials with delocalized π electrons [4]. The efficiency of a material to convert the electromagnetic radiation to thermal energy depends upon the dielectric properties, including the dielectric loss tangent (tanδ). The tanδ is the ratio between the efficiency with which microwave energy is converted into thermal energy (dielectric loss factor – ε″) and the ability of the materials molecules to be polarized (dielectric constant – ε′) [5]. Some materials do not have the dielectric properties necessary to be heated in a microwave field – this selective heating is considered an advantage in MWAP as the microwave energy only heats the biomass and not the chamber or pyrolysis atmosphere, unlike with conventional heating [4].

Most biomass do not heat in a microwave field and hence need to be mixed with high-tanδ materials, termed microwave absorbers or susceptors. A microwave susceptor acts as a hot spot, undergoing dielectric heating and transforming this into thermal heating. A material with tanδ > 0.2 is considered an efficient absorber [6]. By homogenously distributing a susceptor through the biomass, the sample can be uniformly heated by the penetrating microwave field, causing the centre of the sample to be hotter than the outside, due to the surface losing heat to the pyrolysis atmosphere via conventional heat transfer mechanisms [7]. Microwave susceptor particles are hotter than biosolids and transfer heat to the cooler surroundings by conduction. Different susceptors have been used in MWAP studies, the most common being carbon susceptors, such as activated carbon, MWAP-derived biochar, graphite and silicon carbide [8], [9], [10]. The choice of susceptor is an important consideration when designing a MWAP process as the susceptor influences the distribution of the products between char, oil and gas, and the properties of the final products, in addition to affecting the heating rate and maximum temperature [11].

Microwave assisted pyrolysis of sewage sludge and/or biosolids has been studied by several researchers, but little attention has been given to dielectric properties of biosolids [10], [12], [13], [14]. The dielectric properties of materials are fundamental to the design of microwave systems as well as understanding the behaviour of materials under microwave irradiation during heating and pyrolysis. Therefore, the first objective of this study was to understand the variation of dielectric properties of biosolids with moisture content and microwave frequency. The second objective was to assess the impact of different microwave absorber materials (activated carbon, charcoal, biochar and glycerol) on the microwave pyrolysis process of biosolids and biochar properties. The impact of microwave susceptors on bio-oil production has been studied, but the impact of these materials on biochar properties has not been explored yet [11]. MWAP of glycerol has been explored [15], [16], but glycerol has not been studied as a microwave susceptor for biomass MWAP. Finding new applications for glycerol is fundamental, since this product is produced in large quantities as a waste product from biodiesel production [17].