The application of microwave heating in bioenergy: A review on the microwave pre-treatment and upgrading technologies for biomass

Abstract

Bioenergy, derived from biomass and/or biological (or biomass-derived) waste residues, has been acknowledged as a sustainable and clean burning source of renewable energy with the potential to reduce our reliance on fossil fuels (such as oil and natural gas). However, many bioenergy processes require some form of pre-treatment and/or upgrading procedure for biomass to generate a modified residue with more suitable properties and render it more compatible with the specific energy conversion route chosen. Many of these pre-treatments (or upgrading procedures) involve some form of substantive heating of the biomass to achieve this modification. Microwave (MW) heating has attracted much attention in recent years due to the advantages associated with dielectric heating effects. These advantages include rapid and efficient heating in a controlled environment, increasing processing rates and substantially shortening reaction times by up to 80%. However, despite this interest, the growth of industrial MW heating applications for bioenergy production has been hindered by a lack of understanding of the fundamentals of the MW heating mechanism when applied to biomass and waste residues. This article presents a review of the current scientific literature associated with the application of microwave heating for both the pre-treatment and upgrading of various biomass feedstocks across different bioenergy conversion pathways including thermal and biochemical processes. The fundamentals behind microwave heating will be explained, as well as discussion of the imperative areas which require further research and development to bridge the gap between fundamental science in the laboratory and the successful application of this technology at a commercial scale.

Revealing low temperature microwave‐assisted pyrolysis kinetic behaviors and dielectric properties of biomass components

“…The high temperature could accelerate the chemical reaction, and showed up as the increased weigh loss rate at the corresponding temperature range. When microwave was applied to biomass components, swelling and fragmentation effects occurred within the materials and changed the structure disruption of the polymers, which resulted in the change of dielectric properties . In general, the absorbed power for material was proportional to the dielectric loss factor, and the temperature increased as both the dielectric constant and dielectric loss factor increased for all samples measured .…”

“…Among various pyrolysis processes, microwave heating (MH) is a promising technology used for biomass conversion with the advantages of rapid, volumetric and selective heating, which could reduce the limitations of heat transfer with conventional heating (CH), and offer high heating rates with a reduced process time. The most important advantage of microwave‐assisted pyrolysis (MAP) is that the pyrolysis can occur at relatively lower temperatures and with a lower activation energy compared to conventional pyrolysis (CP), which means less reaction limitation and energy consumption . The MAP of wheat straw for hydrocarbons took place at reduced temperatures with the release of bio‐gases at 120–180°C, compared with CP at 250–400°C to achieve the same gas composition .…”

“…The dielectric constant ( ɛ ′) determines how much electromagnetic energy is reflected or absorbed, while dielectric loss factor ( ɛ ″) expresses the conversion efficiency of electromagnetic energy to thermal energy. The dielectric loss tangent (tan δ = ɛ ″/ ɛ ′) indicates the transfer ability of electromagnetic energy into thermal energy at a given temperature . The dielectric properties may vary with biomass composition, temperature, and even density.…”

Recent Progress on Microwave Processing of Biomass for Bioenergy Production

“…Pretreatment processes based on steam explosion, ammonia fiber/freeze explosion, ammonia recycle percolation, hydrothermolysis, milling, microwave irradiation, electron beam irradiation, and pretreatments with alkalis, acids, supercritical carbon dioxide, lime, white rot fungi, peroxide and organic solvents (organosolvolysis) have been extensively investigated 4) ,21) 23) . Microwave irradiation or heating is expected to provide an alternative and reliable option to conventional pretreatment systems based on the use of different chemistry due to the internal heating, and rapid heating rates up to the target temperature 22) . Numerous studies on pretreatment of various raw materials have been reported 24) 30) .…”

“…3, is also an important required preprocessing step to improve the net energy balance of bioenergy production. Reduction of water content from raw materials both reduces weight directly, and critically affects the energy efficiency of subsequent biomass processing, quality of products, and longevity of the product shelf life 22) . Microwave drying is also an effective energy-saving method.…”