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Biomass treatment strategies for thermochemical conversion Qiaoming Liu, Stephen C. Chmely, and Nourredine (Nour) Abdoulmoumine Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00258 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017
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Energy & Fuels
Biomass treatment strategies for thermochemical conversion Qiaoming Liua, Stephen C. Chmelyb and Nourredine Abdoulmouminea,b a Biosystems Engineering and Soil Science Department, University of Tennessee b Center for Renewable Carbon, University of Tennessee Abstract Biomass is among the most promising renewable resources to provide a sustainable solution to meet the world’s increasing using biochemical and thermochemical conversion technologies. Thermochemical conversion processes (pyrolysis, gasification, and combustion) thermally convert biomass into energy dense intermediates that can be in turn converted into power, liquid fuels and chemicals. The performance of the processes and quality of the intermediates are strongly affected by endogenic and technogenic inorganics. This review highlights investigations on the effect and the fate of inorganics during pyrolysis, gasification, and combustion of lignocellulosic biomass and critically and comprehensively presents pretreatment and posttreatment approaches for inorganic removal. During pyrolysis process, the inorganic contents can have significant catalytic effects and change the thermal degradation rate, chemical pathway, and bio-oil yield. During combustion process, the inorganic contents can bring various technological problems, environmental risks, and health concerns. During gasification process, the inorganic contents cause diversified downstream hazards. In recent years, several pretreatment (mechanical, thermal, and chemical pretreatment) and posttreatment (gas product and liquid product posttreatment) approaches have been employed to control and diminish the impact of inorganics during thermochemical conversion. Effective pretreatment technologies exist to remove inorganic contaminants to lower concentration limits. However, the main drawbacks of these pretreatments are that they (i) reduce the overall efficiency due to the need of further drying process of wet biomass after pretreatment and (ii) increase chemicals, facilities and drying costs. Posttreatment technologies are utilized to meet the strict levels of cleanup demands for the downstream applications. A great number of technologies exist to purify the raw synthesis gas stream that is produced by thermochemical conversion of biomass.
Keywords: Inorganic remediation; Thermochemical conversion; Inorganic contaminants; Lignocellulosic biomass
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1. Introduction
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The excessive consumption, finite reserves, and established contribution to the greenhouse
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effect of fossil fuels is motivating the development of renewable technologies for as sustainable
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long term solution. These technologies rely on biomass, solar, wind, water or geothermal
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resources as their primary energy source. Among all these renewable resources, biomass is the
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only that produce power, liquid fuels and chemicals thus making it an attractive option for
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countries with abundant biomass resources.1 Biomass can be converted into intermediates that
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can be used to produce power, liquid fuels and chemicals through biochemical (enzymatic
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hydrolysis, sugar fermentation) or thermochemical (combustion, pyrolysis, gasification) routes.
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The biochemical route seeks to convert carbohydrates in lignocellulosic biomass into energy
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carriers for power2, ethanol and butanol as liquid fuels3, and other platform chemicals4-6. In the
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thermochemical platform, biomass is converted thermally into energy dense intermediates that
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can be in turn converted into power, liquid fuels, and chemicals.7 Specifically, biomass can be
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converted into thermal energy during direct combustion; into thermal energy and a mixture of
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flammable gas known as syngas during gasification; and into mostly an energy-rich liquid
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known as bio-oil as well as small amount of syngas and solid biochar during pyrolysis.7 The
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presence of biomass inorganics is detrimental to processes in both routes. For example, the
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presence of inorganic compounds during biochemical conversion has been associated with a
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number of issues that resulted in inhibition of biological growth or productivity through the
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biochemical conversion route.8-10 Issues related to inorganics during thermochemical conversion
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include equipment corrosion, fouling of surfaces, catalysts deactivation, and bed agglomeration
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in reactors.11, 12
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In recent years, recognizing the challenges associated with inorganics during biomass
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conversion, several review papers attempted to organize the growing body of knowledge on the
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fate of inorganics during conversion, their effects on the processes and pretreatment and
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posttreatment strategies to mitigate those effects.13,
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pretreatment techniques in the context of biochemical conversion technologies15 and
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posttreatments reviews focused solely on gasification.11, 12 This review focuses only on the main
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thermochemical conversion processes – namely pyrolysis, gasification and combustion – and
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comprehensively reviews recent work to elucidate the fate of inorganics during these processes
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and summarizes pretreatment and posttreatment techniques for controlling these elements.
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However, past reviews focused on
2. The origin, nature, and variability of inorganics in lignocellulosic biomass
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Besides the structural carbohydrates and lignin, lignocellulosic biomass also contains a small
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amount of extraneous components that do not serve structural functions. These extraneous
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components are present in and outside the cell wall but are generally not bound to it.16-18
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Extraneous components are grouped into extractives and inorganics, the latter being of interest in
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this review. Inorganic elements in lignocellulosic biomass have authigenic - formed in biomass -,
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detrital - formed outside biomass, but fixed in or on biomass -, and technogenic - formed outside
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biomass – origins (see Table 1).19, 20
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Table 1. Origin of inorganic matter in lignocellulosic biomass Process Origin Time of formation Formation mechanism Authigenic Syngenetic Inorganics that are the result of biogenic Natural processes during plant growth (e.g. photosynthesis, diffusion, adsorption, osmosis, pinocytosis, exocytosis, endocytosis, hydrolysis, precipitation, etc.). Epigenetic Inorganics generated from natural processes after plants died (evaporation, precipitation) Detrital Pre-syngenetic Fine (~ 1µm) inorganic particulates suspended in water and transported into the plant during syngenesis (endocytosis). Syngenetic, Small (
