Review pubs.acs.org/CR
Fates of Chemical Elements in Biomass during Its Pyrolysis Wu-Jun Liu, Wen-Wei Li, Hong Jiang,* and Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China ABSTRACT: Biomass is increasingly perceived as a renewable resource rather than as an organic solid waste today, as it can be converted to various chemicals, biofuels, and solid biochar using modern processes. In the past few years, pyrolysis has attracted growing interest as a promising versatile platform to convert biomass into valuable resources. However, an efficient and selective conversion process is still difficult to be realized due to the complex nature of biomass, which usually makes the products complicated. Furthermore, various contaminants and inorganic elements (e.g., heavy metals, nitrogen, phosphorus, sulfur, and chlorine) embodied in biomass may be transferred into pyrolysis products or released into the environment, arousing environmental pollution concerns. Understanding their behaviors in biomass pyrolysis is essential to optimizing the pyrolysis process for efficient resource recovery and less environmental pollution. However, there is no comprehensive review so far about the fates of chemical elements in biomass during its pyrolysis. Here, we provide a critical review about the fates of main chemical elements (C, H, O, N, P, Cl, S, and metals) in biomass during its pyrolysis. We overview the research advances about the emission, transformation, and distribution of elements in biomass pyrolysis, discuss the present challenges for resource-oriented conversion and pollution abatement, highlight the importance and significance of understanding the fate of elements during pyrolysis, and outlook the future development directions for process control. The review provides useful information for developing sustainable biomass pyrolysis processes with an improved efficiency and selectivity as well as minimized environmental impacts, and encourages more research efforts from the scientific communities of chemistry, the environment, and energy.
CONTENTS 1. Introduction 2. CHEMISTRY OF BIOMASS PYROLYSIS 2.1. Mechanism for Biomass Pyrolysis 2.2. Effects of Reaction Conditions on Biomass Pyrolysis 3. FATES OF CARBON, OXYGEN, AND HYDROGEN IN BIOMASS PYROLYSIS 3.1. Formation of Bio-oil, Biochar, Gas, and Tar 3.2. Formation of Persistent Organic Pollutants 4. FATES OF NITROGEN IN BIOMASS PYROLYSIS 4.1. Emission of NOx and Its Precursors 4.2. Nitrogen Species in Bio-oil and Biochar 5. FATES OF OTHER NON-CARBON ELEMENTS IN BIOMASS PYROLYSIS 5.1. Phosphorus 5.2. Sulfur 5.3. Chlorine 6. FATES OF METALS IN BIOMASS PYROLYSIS 6.1. Alkali and Alkaline-Earth Metals 6.2. Heavy Metals 7. IMPLICATIONS OF UNDERSTANDING ELEMENT FATES ON OPTIMIZATION OF BIOMASS PYROLYSIS TOWARD SELECTIVE RESOURCE RECOVERY AND POLLUTION ABATEMENT 8. CONCLUSIONS AND FUTURE PERSPECTIVES Author Information © 2017 American Chemical Society
Corresponding Authors ORCID Notes Biographies Acknowledgments References
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1. INTRODUCTION Biomass is a naturally abundant and sustainable clean resource on the earth. Until now, biomass-based energy is the fourth most widely used energy, amounting for about 10% of the global primary energy supply. The amount of global biomass annual production is estimated to be more than 10 billion tons (dry basis), and the energy contained in biomass is more than that of 2 billion tons of standard coal.1,2 For example, China is estimated to have generated more than 2.4 billion tons of biomass in 2010, an increase of 18.1% over the previous year.3 This large-scale biomass, if mishandled, will become a major source of environmental pollution. However, if it can be properly handled, biomass waste will become an abundantly available resource. For instance, the energy contained in the biomass waste of China amounts to that of 1 billion tons of standard coal, whereas the fertilizer value of the nitrogen (N)
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Received: September 19, 2016 Published: March 24, 2017 6367
DOI: 10.1021/acs.chemrev.6b00647 Chem. Rev. 2017, 117, 6367−6398
Chemical Reviews
Review
and phosphorus (P) contained in biomass waste is more than that of the total chemical fertilizer produced in China. However, considering the low economic benefits and harsh regulations for environmental protection, how to effectively recycle the waste biomass in an economically feasible way is still a big challenge.4 Currently, in many developing countries, the waste biomass is mainly treated by landfilling or direct incineration, bringing about serious pollution of the environment. For example, large-area landfilling sites are required for biomass landfilling, producing leachate with various contaminants (e.g., heavy metals, toxic organic compounds, NH4+, and NO3−)5−8 that may lead to severe soil and groundwater pollution. Moreover, the biomass incineration results in severe air pollution because of the emissions of heavy metals, fine particles, and persistent organic pollutants (POPs).9−12 Therefore, an environmentally friendly and economically feasible approach should be developed for the disposal of large-scale biomass waste. There are various methods available for the resource utilization of biomass waste, such as microbially mediated transformations and thermochemical conversion. The microbially mediated transformations, including anaerobic and aerobic digestion, are widely used to convert biomass waste into useful products.13−15 However, microbially mediated transformations of biomass generally involve a series of metabolic reactions such as hydrolysis, acidogenesis, and methanogenesis, which are time-consuming and difficult to control.16 Even worse, microbially mediated transformations of biomass release large amounts of greenhouse gases such as CH4 and CO217 into the atmosphere, which may greatly accelerate global climate warming. Alternatively, conversion of waste biomass into biofuels or chemical feedstocks through a thermochemical way is one of the prospective approaches,18−20 because it can be efficiently and easily conducted. Pyrolysis,21−27 gasification,28−33 and hydrothermal liquefaction34−37 are the main thermochemical technologies which have been widely employed to convert organic solid waste into fuels or chemicals. Among them, pyrolysis, as an environmentally friendly and cost-effective technology for recycle of the biomass, has several advantages over incineration and landfilling because of its low energy consumption (only approximately 10% of the energy content of biomass is consumed for the pyrolysis itself). Meanwhile, the harmful gas emission in biomass pyrolysis is remarkably weaker than that in incineration.38 Pyrolysis can be defined as the thermal biomass decomposition at a mediate temperature (e.g., 600−900 K) under limited O2 to yield bio-oil, gases, and biochar.39 The yields and properties of the bio-oil, biochar, and gas are influenced by many parameters including vapor residence time (VRT), temperature, and heating rate. For example, low temperature and heating rate, and long VRT, favor the formation of carbonaceous char, while high temperature and heating rate, and long VRT, increase gas yields. Moderate temperature, high heating rate, and short VRT are beneficial to higher pyrolysis oil yields.40 The pyrolysis process, based on the heating rate and reaction time, can be classified into three types, namely, slow, fast, and flash pyrolyses.41 The primary parameters for these three types of pyrolysis are summarized in Table 1. Slow pyrolysis, also called carbonization, is a conventional thermochemical process with a slow heating rate (approximately 5−7 K/min) and long reaction time (more than 1 h), in
Table 1. Main Features of Slow, Fast, and Flash Pyrolyses slow pyrolysisa
fast pyrolysisa
flash pyrolysisa
heating rate pyrolysis temp (K) feedstock size (mm) feedstock residence time reactor
5−7 K/min 500−1200
300−800 K/min 600−900
>1000 K/min 600−1200
5−50
