Elsevier

Bioresource Technology

Volume 246, December 2017, Pages 135-141
Bioresource Technology

Review
The prospective use of biochar as adsorption matrix – A review from a lifecycle perspective

https://doi.org/10.1016/j.biortech.2017.08.041Get rights and content

Highlights

  • This paper reviews the available studies focused on biochar-pyrolysis systems.

  • The potential use of biochar as an alternative adsorption matrix was evaluated.

  • Diverse feedstocks were compared in terms of LCA environmental impact categories.

  • Biochar for adsorption showed better environmental results than activated carbon.

  • Energy potential provided lignocellulosic materials with the best performance.

Abstract

Biochar is obtained from the thermochemical conversion of biomass in an oxygen-limited environment. Beyond its use for soil amendment and carbon sequestration, other value-added applications, such as a green environmental sorbent for different types of pollutants, are also of increasing interest.

This paper reviews the available literature on the use of various feedstocks for biochar production as an initial step to compile the required inventory data from material flow data analysis. The environmental evaluation of different schemes of biochar production in a life cycle perspective reports the environmental impacts of the different alternatives, with more sustainable profiles than the use of activated carbon as conventional adsorption material. Moreover, most systems were validated as negative carbon processes with favorable performance regarding climate change mitigation. In particular, materials based on lignocellulosic waste were responsible for the highest environmental benefits, mainly due to their largest energy potential in comparison with other feedstocks.

Introduction

Biochar is defined as “a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment” and can be analogous to charcoal found in nature (IBI, 2013). The fine-grained, porous biochar used as soil amendment can improve its structure and fertility, nutrient availability, soil-water retention capacity and it is also a mechanism for carbon sequestration (Lehmann and Joseph, 2009).

Biochar production processes go from traditional kilns and earth mounds, as well as engineered systems based on flat bed or fluidised reactors for pyrolysis or gasification (Roy and Dias, 2017). Three major subclasses of pyrolysis can be identified: slow, fast and flash pyrolysis (Lenton and Vaughan, 2013, Roy and Dias, 2017, Qambrani et al., 2017, Qian et al., 2015). At small scale, the preferred alternative to produce biochar is downdraft gasification with air, which produces a gas that is immediately burned in an engine to produce heat and electricity (Brownsort, 2009a, Brownsort, 2009, IBI, 2014, Qambrani et al., 2017, Qian et al., 2015).

There is an extensive range of potential feedstocks for biochar production; e.g. wood and agricultural wastes, manure, rice husks and straw, leaves, food waste, paper sludge, bagasse and many others (Ahmad et al., 2014, Chan et al., 2007, Galbraith et al., 2006, Nanda et al., 2016, Thornley et al., 2009a, Thornley et al., 2009b). The utilization of energy crops for biochar may encounter emissions from land-use change. In this sense, biofuels from switchgrass are reported to increase emissions by 50%, which highlights the interest of using waste products (Searchinger et al., 2008). Waste of crops and wood are energy rich feedstocks and therefore, suited to pyrolysis (Lehmann and Joseph, 2009, Yang et al., 2007). Forest residues may offer a new resource of biomass, although care must be taken not to cause any soil nutrient degradation by over removal (Gabrielle and Gagnaire, 2008). Other types of waste such as manure or sewage sludge can be potentially applied for pyrolysis provided the excess process heat is used to dry the feedstock (Hospido et al., 2005, Huang et al., 2015). Among the different types of animal waste, chicken litter has proved a valuable soil amendment once pyrolysed, due to its high nutrient content (Chan et al., 2007).

Beyond the targets of carbon storage and soil amendment, there is an increasing interest in the use of biochar for other value-added applications, such as filler for composites, catalysts or material for electronic and bioenergy applications (Lehmann and Joseph, 2009, Nacu et al., 2016, Nanda et al., 2016, Qambrani et al., 2017, Qian et al., 2015). Versatile properties of biochar also render possible its use in metallurgical processes as well as chemical and pharmaceutical industries (Nanda et al., 2016).

Moreover, biochar has found multiple applications as an adsorption material for pollutant removal, likewise activated carbon (Lehmann and Joseph, 2009, Nanda et al., 2016, Qambrani et al., 2017, Qian et al., 2015); however, biochar can be activated to enhance its sorption capacity by increasing both surface area and microporosity (Ahmad et al., 2014, Lima et al., 2008, Lima et al., 2010, Mohan et al., 2014). A technical steam activation of biochar was found to improve its positive effects on nutrient retention and uptake by plants in comparison with non-activated biochar (Ahmad et al., 2014, Borchard et al., 2012); similarly, steam-activated biochar was also demonstrated to increase its adsorption ability regarding organic and inorganic contaminants removal from aqueous solutions (Ahmad et al., 2014, Mohan et al., 2014). Hydrothermal carbonization (Regmi et al., 2012), magnetization (Chen et al., 2011), oxidation by K2MnO4, hydrogen peroxide (Cho et al., 2010) and O3 (Sánchez-Polo and Rivera-Utrilla, 2002) are also other approaches that can enhance biochar adsorption capacity.

The prospective use of biochar for soils polluted with heavy metals, polycyclic aromatic hydrocarbons or pesticides relies on its capacity to reduce pollutants mobility and uptake by plants (Karami et al., 2011, Méndez et al., 2012, Yao et al., 2012). The mechanisms of heavy metal removal with biochar amendment might be attributed to electrostatic interactions, precipitation and formation of metal complexes (Dong et al., 2011, Jiang et al., 2012, Lu et al., 2012, Peng et al., 2011), while the removal of organic pollutants by biochar can be mainly attributed to surface adsorption (Kwon and Pgnatello (2005)).

Life Cycle Assessment (LCA) is a methodology defined according to ISO standards, that aims to evaluate the environmental burdens associated with a product, process or activity throughout its life cycle by quantifying energy, resources and emissions (ISO, 2006a, ISO, 2006b). A substantial number of LCA studies on biochar systems have been published to date, focusing specifically its use as soil amendment and the valorization of bio-oil and syngas for energy generation (Chan et al., 2016, Dang et al., 2014, Zhang et al., 2014, Gaunt and Lehmann, 2008, Hammond et al., 2011, Huang et al., 2015, Ibarrola et al., 2012, McCarl et al., 2009, Peters et al., 2015a, Roberts et al., 2010, Zhong et al., 2010).

As an example, Hammond et al. (2011) evaluated the carbon abatement potential of pyrolysis biochar systems from eight different feedstocks as basis for the development of further research within the UK framework; they found pyrolysis systems as carbon negative electricity technologies, although with lower energy rates than competing alternatives. Similarly, Gaunt and Lehmann (2008) also calculated the reduction of carbon emissions associated with slow pyrolysis from energy crops and crop wastes according to different purposes, either for biochar or energy production. The outcomes of their study highlighted the environmental benefits from the addition of biochar to agricultural soils instead of its valorization as energy, provided that biochar must retail at US$47/t to replace the economic profits which could have been made from the additional energy generation if biochar production had not been prioritized.

Roberts et al. (2010) analyzed the net GHG emissions of biochar made from different sources: crop residues (corn stover) and energy crops (switchgrass). While crop wastes achieved larger reductions in GHG emissions (in agreement with Dang et al., 2014) than energy crops, the land use model adopted also affected the results significantly. In line with this, Ibarrola et al. (2012) reported a carbon abatement ranging from 0.07t CO2eq for cardboard to 1.25t CO2eq per ton of wood waste, when biochar was used for soil amendment.

Peters et al. (2015a) modelled a slow pyrolysis system for generating heat and biochar from lignocellulosic feedstocks and compared its environmental performance with that of direct biomass combustion. Negative greenhouse gas emissions, indicative of significant carbon abatement potential, were obtained for the biochar system. Moreover, biochar application to low-quality soils where high yield increases are expected would show a more favorable performance in terms of climate change mitigation.

Later, Peters et al., 2015b, Chan et al., 2016 evaluated the environmental performance of bio-oil production through pyrolysis in comparison with alternative treatments through life cycle assessment. According to the results, the impacts from pyrolysis process were mainly associated with related energy requirements from pretreatment stages.

Furthermore, most authors (Zhong et al., 2010) mainly focused attention on potential environmental benefits from pyrolysis biochar systems using alternative feedstocks to produce renewable energy in combination with other value-added products; despite remaining hotspots to be addressed, environmental-friendly results were obtained in most cases.

This review paper aims to address and compare the environmental impacts of producing biochar from different feedstocks for its further use in substitution of activated carbon for pollutant removal. However, in contrast to previous related studies that either relied on the compilation or discussion of results from other authors, this study makes use of primary inventory data as a starting point for the development of a comparative LCA study based on common methodological principles. In this way, environmental results are not directly collected from literature, but they are calculated from processing inventory information. To this aim, an extensive literature search was conducted to collect inventory data from those studies focused on pyrolysis processes for biochar production. From this information, the LCA analysis of the different biochar processes was performed.

Section snippets

Biochar production from a Life Cycle Assessment (LCA) approach

The following feedstocks were selected for comparison in this study: sewage sludge (Hospido et al., 2005), yard waste (Roberts et al., 2010), wood chips (Bergman and Gu, 2014), corn stover (Dang et al., 2014), forest residue (Zhang et al., 2014), poultry litter (Huang et al., 2015), poplar (Peters et al., 2015a,b) and oil palm empty fruit bunch (EFB) (Chan et al., 2016).

Standardization is recommended to carry out an accurate comparative procedure (Andrae, 2011, Cherubini et al., 2009). However,

Relative contributions to the environmental impact by life cycle stage

Environmental results regarding different feedstocks and impact categories are shown in Fig. 2. Positive values refer to damaging burdens while negative ratios represent environmental credits that partially offset the environmental impacts. A common tendency can be observed in the different impact categories, regardless the feedstock considered, but with some variations. As an example, energy requirements play a significant role on environmental damages for all categories; however, while this

Biochar as an alternative to activated carbon under an environmental approach

Table 3 displays a comparison of the global characterization results associated with the different systems to each impact category as well as the environmental impacts derived from the conventional production of activated carbon. According to the results, biochar production and its use for the adsorption of pollutants present lower impacts than activated carbon (Table 3). Particular attention should be paid to those feedstocks with higher energy potential, including forest residue, wood chips

Conclusions

Comparative environmental results show that biochar as adsorption material is a more sustainable alternative than conventional activated carbon. The main findings that encourage the application of biochar for environmental purposes are associated with the net mitigation of carbon emissions from avoided processes, which results in environmental credits, especially in terms of CC. Moreover, the significantly beneficial influence was evidenced in the other environmental categories due to avoided

Acknowledgements

The authors belong to the Galician Competitive Research Group GRC 2013-032 as well as to CRETUS (AGRUP2015/02), programs funded by FEDER and Xunta de Galicia.

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