001.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 83, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-81-5; ISSN 2283-9216 Environmental Performance and Techno-Economic Feasibility of Different Biochar Applications: An Overview Yee Van Fana,*, Jiří Jaromír Klemeša, Chew Tin Leeb aSustainable Process Integration Laboratory – SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of Technology - VUT BRNO, Technická 2896/2, 616 69 Brno, Czech Republic bDepartment of Bioprocess Engineering, School of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 UTM Johor Bahru, Johor, Malaysia fan@fme.vutbr.cz Biochar has a broad application owning to its physicochemical properties and low cost by origin (by-products or generated from waste). Wide research attention received is the application to the soil as carbon sequestration. In contrast to the efficiency evaluation, the overall cost feasibility and environmental performance of the application still deserves broader analysis and discussion. This study aims to enumerate the advantages, and potential drawbacks of the different applications, in term of cost and the environmental footprints. The other alternatives (e.g. the conventional methods) in achieving the same purpose are compared. Special attention is given to the application of biochar to the soil as carbon sequestration. This study could serve as a guideline in evaluating the pros and cons of biochar application to ensure the unburdening footprints can offset the burdening footprint associated with the generation and other downstream processes towards sustainability. 1. Introduction The various application of biochar is attributed by its features, including high carbon content, large specific surface area, stable structure, and cation exchange capacity (Wang and Wang, 2019). The characteristics are varying with the types of substrates and the generation methods. The feedstock of biochar can be divided into forestry (e.g. wood), dedicated biomass (e.g. switchgrass) and residues (e.g. corn stover, husk, sawdust) or waste (e.g. food waste). Feedstock with high volatile content generally results in the low biochar yield. Char can be produced via carbonisation processes including pyrolysis, gasification, hydrothermal carbonisation, flash carbonisation or torrefaction as summarised in Figure 1. Biochar yields via the gasification (higher process temperature and different gaseous condition) are usually lower than the pyrolysis process because the gaseous products are the targeted products. The most common process in preparing biochar is pyrolysis and temperature is among the most influential operating parameters (Zhu et al., 2019) affecting the quality and yield of biochar. These including the pH, cation exchange capacity, specific surface area, ash content, volatile matter content, elemental composition and the yield (Li et al., 2019). Zhao et al. (2013) summarised that carbon sequestration, fixed carbon and minerals were determined by feedstock while the pH, specific surface area and recalcitrance were depending by temperature. The other novel pyrolysis methods, including microwave-assisted pyrolysis, co-pyrolysis, wet pyrolysis and pyrolysis with modification have been summarised by Wang et al. (2020). Bong et al. (2020) suggested that soil amendment and adsorption material are the suitable application for biochar derived from lignocellulosic biomass and food waste. However, the conclusion is mainly on the effectiveness based on physiochemical properties rather than from the environmental and economic perspectives. Modification of biochar to revise the surface properties received significant research interests recently. Huang et al. (2021) review the research efforts on biochar modification to enhance the specific application and cost, focusing on laboratory works instead of mature field application reports. There have been a lot of research and review studies related to improving the properties of biochar for an effective application. Barriers to biochar application in real-world setting deserve more attention, especially the techno-economic DOI: 10.3303/CET2183079 Paper Received: 16/10/2020; Revised: 06/11/2020; Accepted: 15/11/2020 Please cite this article as: Fan Y.V., Klemeš J.J., Lee C.T., 2021, Environmental Performance and Techno-Economic Feasibility of Different Biochar Applications: An Overview, Chemical Engineering Transactions, 83, 469-474 DOI:10.3303/CET2183079 469 challenges and the undesirable adverse environmental consequences. Tan (2016) highlighted that it remains uncertain whether biochar as carbon sequestration is scalable and cost-effective to mitigate climate change. Belmonte et al. (2017) proposed process system engineering to facilitate the planning of large-scale biochar system. The potential disadvantages of biochar application to the soil include albedo effect due to soil darkening, excessive pH elevation, and degrading the soil quality due to contaminants such as heavy metals. There are still conflicting opinions in the scientific literature regarding the sustainability of biochar. This study aims to enumerate the advantages, and potential drawbacks, in term of cost and the environmental footprints, of different biochar application. Figure 1: Different thermochemical conversion methods for biochar/char generation. Information extracted from Wang and Wang (2019). 2. Potential uses The development of biochar has received research attention and application in different fields including environment, energy, agriculture and materials. Figure 2 summarised the potential uses of biochar and different approaches to enhance effectiveness. Different researchers have studied structure-application relationships. For effective absorbent (water and air pollutants), high specific surface area, pore fraction and functional groups on its surface are important. The adsorption capacity depends on raw material, pyrolysis process and solution pH (Cha et al., 2016). One of the issues which limit the large-scale application of biochar as absorbent (e.g. wastewater treatment) is the subsequent separation. Magnetic biochar is introduced to enable the application of biochar in environmental remediation (Yi et al., 2020), facilitating the separation. Figure 2: Potential uses of biochar and the approaches to enhance the effectiveness Biochar has been often modified to revise its surface properties, so the performance of specific application with modified biochar can be enhanced at a reduced cost. The modification methods including acid, alkaline, oxidising agents, metal oxides and steam/gas are to remove impurities, increase surface area/porosity or introduce certain functional groups (hydroxyl, carboxyl) onto biochar surface (Huang et al., 2021). Mašek et al. 470 (2019) reported that potassium doping on biochar could increase carbon sequestration. Other than the pre- and post-treatment, CO2 looping in pyrolysis could also enhance the biochar properties (higher specific surface area) (Shen et al., 2017). Xiang et al. (2020) highlighted that engineered biochar is critical in improving the effectiveness of environmental protection, but low-cost modification technology is yet to be developed. Most of the application is still limited to laboratory scale and remains a challenge to perform upscale. 3. The sustainability of biochar application The applicability of biochar in the various area has been well proven. However, sustainability remains uncertain with no consensus. Table 1 shows the sustainability performance for some of the biochar application. The selected studies show positive impacts in term of sustainability performance; however, it is not questionable. The comparison basis is critical. The cost and environmental feasibility of biochar as activated carbon are generally promising; however, more in-depth comparison to the activated carbon generated from waste is required. Application of functionalised biochar should be given priority in future research (Lu et al., 2020). In general, it can be suggested that an application that focuses on the uses of functional group and surface area (activated carbon, materials, absorbent, catalyst, additives) of biochar is generally feasible. Table 1: The sustainability performance of the selected biochar application Biochar Application Sustainability Remarks Reference To produce activated carbon • The lowest in cost and environmental footprint compared to biochar as a soil amendment or to displace coal • The cost and environmental footprint of activated carbon from biochar are not compared to the activated carbon which could produce from another waste stream Kulas et al. (2019) • −0.9 kg CO2eq/kg and 6.6 kg CO2eq/kg for biochar and activated carbon • Cost of biochar (200 USD/kg) lower than activated carbon (1,240 USD/kg) to adsorb chromium and zinc, comparable to adsorb lead and copper. • Considered only the environmental impacts of energy demand • Biogenic carbon and potential soil fluxes are not accounted Alhashimi and Aktas (2017) For wastewater treatment • Low adsorption capacity wood biochar had more benefits for global warming, respiratory effects, and non-carcinogenic. • However, it exhibited higher impacts than powdered activated carbon in eutrophication, carcinogenic, ecotoxicity, acidification, ozone depletion due to higher biochar dose requirements to reach the treatment objective. • Economic feasibility is not assessed • The compared powdered activated carbon is coal- based Thompson et al. (2016) To improve agriculture activities • Scenario with a higher increase in grain yield (+0.18 to +1.00 t/ha/y) shows the positive net present value (NPV). • However, when the increase in yield is in the range of +0.07 to +0.28 t/ha/y, the NPV is negative even when the benefits time span was indefinitely stretched. • The estimated increase in crop yield is critical • Depend on the type of crop, efficiency of biochar, longevity of agronomic benefits and optimal application rate. Dickinson et al. (2015) Enhance anaerobic digestion (AD) • 3.60 - 4.10 €/ L for enzyme, 13-16 €/L for nutrients, 0.6 - 20 USD/kg for activated carbon, 0.2 - 0.5 USD/kg for biochar • Less environmental impacts than the conventional AD improvers • The efficiency of different alternatives are not assessed in detail Chiappero et al. (2020) However, the assessment results for soil amendment, carbon sequestration and energy generation have a contradiction. Sustainability of biochar as a carbon soil conditioner/soil amendment and carbon sequestration is still ambiguous, and the efficacy is rather a long term. Dickinson et al. (2015) highlighted that when the 471 increase in crop yield is 0.07 to 0.28 t/ha/y, the net present value is negative no matter how the benefits time span extended significantly. Lu et al. (2020) suggested that a better understanding of structure elucidation, reactivity of biochar, precise functioning as well as evaluation of long-term field application are required. This evaluation is critical as without the sufficient understanding and in-depth assessment; the techno-economic and environmental assessment could not be representative due to inaccurate or non-representative functional unit. Lee et al. (2020) found that the use of biochar for energy is favourable in term of carbon abatement. However, if the indirect land use, addition fertiliser application or biogenic carbon (Fan et al., 2020a) are considered, it could reverse the potential abatement. The sustainability of biochar application as energy also depends on the selected baseline scenario. Homagain et al. (2016) suggested that without the carbon credit accounted from carbon sequestration of biochar, bioenergy alone could not provide an economical alternative to fossil fuel energy production. Fan et al. (2020b) suggested that pyrolysis for energy (biochar to energy) is generally more preferable in term of emission-cost performance unless the biochar (biochar to soil) can be used as a soil amendment, beyond carbon sequestration. Table 2 further presents the different results reported on the sustainability of biochar applied to soil as carbon sequestration. Based on Table 2, the effect of biochar to the soil is generally positive. However, most of the studies have focused on a specific issue, and the comparison basis is not standardised. An assessment which provides a complete picture considering the entire life cycle assessment is still lacking. When life cycle assessment is conducted, some of the sources of environmental footprint, e.g. fluxes, biogenic carbon, land-use change and the life cycle cost is not considered. The sustainability under the different circumstance is yet to be confirmed. Table 3 shows the cost feasibility of biochar production reported in the selected studies. There are cases where biochar production is reported as economically feasible (+). Table 2: The impact of biochar application on the soil as carbon sequestration Remarks Reference + Decrease CH4 (132 %), increase soil organic carbon sequestration (16 %), increase corn yield (7.4 %) Yang et al. (2020) + Enhance carbon utilisation by soil microbial community and soil organic carbon sequestration of paddy soils Liu et al. (2019) + Increase soil inorganic content by 7.8-62.3 % (Long term application, 20 – 90 t ha-1) Dong et al. (2019) + More effective in sequestration soil carbon compared to crop residues Majumder et al. (2019) + Suppressing N2O and CH4 emission (21.7 - 62.3 %) in the rice paddy soils Sun et al. (2019) + Decrease soil N2O fluxes by 30.92 % He et al. (2017) - Increase soil CO2 fluxes by 22.14 % (suppressed when biochar added to fertilised soil) He et al. (2017) + Have useful negative emission potential (0.7 Gt/y) Smith (2016) Table 3: Cost feasibility of biochar production Description Reference N/A Slow pyrolysis to biochar pathway required a lower carbon price (subsidy). More financially attractive than fast pyrolysis (fuels and electricity route). Frank et al. (2020) N/A Profitable when the price of CO2 avoided is at a minimum of 85.76 USD/t to 118.13 USD/t Thengane et al. (2020) N/A Production cost range from 75 - 1,272 $/t of biochar depends on biomass type and technology. 0.05 – 0.15 $/m2 for soil application Struhs et al. (2020) - Auger based pyrolysis (biochar and biofuel production): 68% of outcomes resulted in financial loss, NPV of -24.3 x 106 $. However, if Renewable Identification Numbers credits of biofuel generation are considered, the likelihood of financial loss can be reduced to 50 % Campbell et al. (2018) + Hearth based pyrolysis (biochar with no liquid fuel): NPV of 41.5 x 106 $, 20 % likelihood resulting in a net loss Campbell et al. (2018) + Biochar-based bioenergy system with biochar land application offers a 5 - 9 % return on investment, breakeven at about 12-13 y Homagain et al. (2016) In some cases, it is either not feasible (-) or assessed, but comparative studies (N/A) is not conducted. Homagain et al. (2016) stated that pyrolysis process accounts for the highest share (36 %) of the total 472 production cost, followed by land application (14 %), feedstock collection (12 %) and transportation cost (9 %). The cost of land application plays a critical role. Struhs et al. (2020) highlighted that capital and operating costs of pyrolysis is relatively stable (lowest sensitivity impact), however biochar selling price and biomass management is critical serving as influencing factors for the overall cost feasibility. Carbon pricing and subsidise are also playing an important role, as reported in Thengane et al. (2020). 4. Conclusions This overview suggests observations as below: (i) The sustainability of biochar application as activated carbon, materials, absorbent, catalyst and additives are generally favourable. However, it also depends on the selected baseline scenarios. Efficiency is one of the critical factors in enhancing the sustainability of the biochar application that make use of the surface area and functional group. Engineered biochar is an essential field of study. (ii) The sustainability of biochar application as an energy source and to the soil, e.g. for carbon sequestration is relatively controversial, especially as the carbon intensity of current energy source is lower. Non-CO2 footprints, application cost and low carbon pricing are the key factors that could overturn the sustainability for the biochar application to soil. Comprehensive and systematic assessment for a sustainable biochar application is still lacking. Definition of a baseline scenario for a fair comparison is challenging, and a standardise life cycle framework consider different source of an environmental footprint (Čuček et al., 2012) is required. 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