Current Organic Chemistry - Volume 20, Issue 23, 2016

Cellulose, which is a polymer of β-1→4 linked D-glucose units, comprises about half the components of lignocellulosic biomasses. A better understanding of the chemistry involved in cellulose pyrolysis provides valuable insights into the development of efficient pyrolysis-based conversion technologies of biomass into biofuels, biochemicals and biomaterials. This review focuses on the reactions and molecular mechanisms that determine the reactivity and product selectivity in pyrolysis and the related conversion technologies. This information is useful for understanding the processing technologies conducted at high temperatures, such as wood drying and the production of cellulose-plastic composite materials. Because the cellulose pyrolysis behavior changes drastically in the temperature range of 300–350 °C, this review is divided into low- and high-temperature regimes. The intermediates produced from cellulose pyrolysis are further converted into various gas, liquid or solid products. Hence, these processes are addressed as primary pyrolysis and secondary reactions in this review. The roles of the crystalline cellulose in pyrolysis are noted.

As a consequence of the shortage of traditional resources and escalating environmental constraints, the feedstocks for the production of energy and chemicals are swiftly changing. Biomass has received remarkable attention from both academia and industries as it is the most promising feedstock for these applications. Understanding the conversion mechanisms of such renewable low-value material to value-added products would lead to providing insights to enhance the product yield and/or quality and, in turn, help the new products compete with the traditional ones. In this regard, this paper provides an updated review on the processing of biomass for the production of valuable-products that could replace a part of the fossil fuel-based energy and chemicals. The common technologies that are performed in the conversion processes are demonstrated, and the thermochemical technique is emphasized. Several chemical reactors and their processes for fast pyrolysis applications are presented. The organic chemistry of pyrolysis of a biomass plant and its three constituents (i.e., cellulose, hemicellulose, and lignin) is debated. The effect of the heating mechanism, process parameters, loading of catalyst and other aspects are discussed. Eventually, the economics aspect of fast pyrolysis of biomass is evaluated.

Fast pyrolysis is an attractive platform technology to convert lignocellulosic biomass into fuels, energy, and other value-added products. However, most of the biomass materials, e.q., forest waste, agricultural waste, energy crops, and municipal solid waste, have low bulk and energy density and are difficult to store, transport and convert. Furthermore, the variations in feedstock composition and particle size among a few other factors all affect fast pyrolysis performance and product quality. Biomass densification, such as pelleting, generates a form of biomass feedstock with high mass and energy density and great homogeneity via blending of different feedstocks, providing unique opportunities to improve feedstock quality for fast pyrolysis. This paper summarizes the basic principles, processes and influencing factors, and the recent developments in both densification and fast pyrolysis fields with special focus on the combination of pelleting and fast pyrolysis. The challenges and perspectives are also provided for the development of an efficient, cost-effective and scalable fast pyrolysis technologies for generating fuels and chemicals.

Aiming at promoting the industrial application of biomass pyrolysis technology, many approaches have been explored to reveal pyrolysis reaction mechanism. Among which the thermal analysis kinetics based on mechanism scheme receives significant attention. The kinetic models developed in past decades are classified into five types: one-step global model based on model-fitting method, global model based on model-free method, multi-step successive model, semi-global model and distributed activation energy model. In this review, the computation processes are introduced, and the advantages and disadvantages of these kinetic models are discussed and compared in detail. To obtain a universal kinetic model and simplify the intricate behaviors of biomass pyrolysis, the pyrolytic kinetics of the three major components, namely cellulose, hemicellulose and lignin, have been studied individually. Their pyrolysis behaviors are dramatically different in view of kinetics. It is recommended that the fundamental theory progress which has been made in the solid reaction kinetics should be applied in the biomass pyrolysis kinetics, and a lot of related traditional knowledge in this area needs to be updated urgently.

Energetic ionic salts based on nitrogen-rich heterocycles are expected to usher in a new era in the fields of propellants, explosives, and pyrotechnics owing to their excellent combustion characteristics, green nature, and the ability to tailor them based on requirements. The review focuses on an important aspect of these compounds, other than the synthesis procedure and physico-chemical characterisation, that is frequently overlooked, i.e. the decomposition pathways and the associated chemical kinetic parameters, which are essential to elucidate and simulate their combustion characteristics. The reaction mechanisms of four major families of energetic ionic salts, explored by various experimental and numerical techniques, are reported in detail.