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Biomass pyrolysis for biochar or energy applications? A life cycle assessment Jens Peters, Diego Iribarren, and Javier Dufour Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5060786 • Publication Date (Web): 01 Apr 2015 Downloaded from http://pubs.acs.org on April 5, 2015

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Environmental Science & Technology

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BIOMASS PYROLYSIS FOR BIOCHAR OR

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ENERGY APPLICATIONS? A LIFE CYCLE

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ASSESSMENT

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Jens F. Peters1, Diego Iribarren1,* and Javier Dufour1,2 1

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Systems Analysis Unit. Instituto IMDEA Energía. Móstoles 28935 (Spain).

Department of Chemical and Energy Technology. Rey Juan Carlos University. Móstoles 28933

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(Spain).

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* Corresponding author: Tel.: +34-91 737 11 19; Fax: +34-91 737 11 40; E-mail address:

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[email protected]

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Abstract

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The application of biochar as a soil amendment is a potential strategy for carbon sequestration. In

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this paper, a slow pyrolysis system for generating heat and biochar from lignocellulosic energy

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crops is simulated and its life-cycle performance compared with that of direct biomass

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combustion. The use of the char as biochar is also contrasted with alternative use options: co-

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firing in coal power plants, use as charcoal, and use as a fuel for heat generation. Additionally,

ACS Paragon Plus Environment

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the influence on the results of the long-term stability of the biochar in the soil, as well as of

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biochar effects on biomass yield, is evaluated. Negative greenhouse gas emissions are obtained

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for the biochar system, indicating a significant carbon abatement potential. However, this is

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achieved at the expense of lower energy efficiency and higher impacts in the other assessed

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categories when compared to direct biomass combustion. When comparing the different use

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options of the pyrolysis char, the most favorable result is obtained for char co-firing substituting

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fossil coal, even assuming high long-term stability of the char. Nevertheless, a high sensitivity to

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biomass yield increase is found for biochar systems. In this sense, biochar application to low-

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quality soils where high yield increases are expected would show a more favorable performance

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in terms of global warming.

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Keywords: biochar; carbon sequestration; life cycle assessment; process simulation; slow

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pyrolysis; soil amendment

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Introduction

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Mitigating global warming requires significant reductions of greenhouse gas (GHG)

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emissions.1 Apart from promoting renewable energy and reducing overall energy consumption,

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the permanent sequestration of carbon dioxide from the atmosphere is one of the options to

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achieve this goal. Besides storing CO2 directly underground (e.g., in depleted oil or gas wells),

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biochar is often proposed as an option for permanent carbon storage in the soil.2–8 Biochar is the

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carbonaceous solid product (char) obtained from the slow pyrolysis of biomass and applied to the

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soil. The effects of biochar on soil properties are numerous, but yet little understood.9,10 Apart

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from possible yield increases due to increased soil carbon content, biochar is considered of


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interest due to its high stability in the soil and the corresponding ability to create long-term

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carbon sinks. Several works have investigated slow pyrolysis and biochar as an effective way for

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carbon sequestration and found significant GHG abatement potential.2,3,11–13 Nevertheless, the

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pyrolysis char can be used in other ways, mainly for energy purposes, as it is a type of charcoal

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with high energy content.14,15 Under environmental aspects, the use with the lowest impact

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should be favored, taking into account not only GHG emissions but also other potential

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environmental impacts. Hence, an assessment not only of the biochar system alone is required,

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but also of the possible alternative uses of the pyrolysis char. For this purpose, Life Cycle

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Assessment (LCA) is the methodology of choice, as it evaluates the potential impacts of a system

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for a wide set of impact categories regarding the whole life cycle of the product. 16,17 Previous

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studies found biochar systems to result in high GHG emission savings.11,18 Significant savings of

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fossil energy and GHG emissions were also reported for biochar systems in comparison with the

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use of the char as a fuel in cooking stoves7 or power plants.19 However, slightly worse results

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were found in comparison with direct biomass combustion and when including the avoided

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emissions due to replaced fossil fuel.2 Nevertheless, these studies are generally limited to global

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warming and energy demand and do not assess the main sources of uncertainty affecting biochar

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systems. This paper thoroughly evaluates a slow pyrolysis system for biochar production from an

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LCA perspective in order to quantify its environmental benefits as well as to facilitate the

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identification of the environmentally most favorable use of the pyrolysis char.

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Methodology

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Goal. The assessed slow pyrolysis system generates heat and pyrolysis char, which is applied

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to the soil as biochar for soil amendment and carbon sequestration. Figure 1 shows a block

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diagram of the system. The goal of this study is to evaluate the environmental impacts and the


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GHG reduction potential of the system (base case study). Furthermore, the influence of the

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assumed long-term stability of the biochar as well as of the assumed biomass yield increase on

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the results is evaluated and alternative use options for the pyrolysis char are assessed: energetic

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use substituting fossil coal, natural gas or charcoal. Finally, the system is compared with heat

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generation via direct combustion of the biomass.

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Figure 1. Block diagram of the biochar system

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Functional unit. An input-oriented functional unit (FU) is used for the assessment: 1 ha of

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agricultural area, used for one year. The results are therefore averaged over the plantation

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lifetime, showing the mean environmental benefit that can be obtained annually from the

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processing of the biomass per hectare. This facilitates giving a recommendation about the most

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promising way for using a given bioenergy potential.

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Avoided burden approach. An avoided burden approach is followed. All products of a

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system are assumed to avoid the production of their corresponding conventional equivalent. This

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avoids allocation in case of multifunctional systems and allows for a straightforward comparison

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of systems with different functions.20 For assessing alternative char use options, the substitution

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of (i) conventional charcoal, (ii) fossil coal in power plants, and (iii) natural gas for heat

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generation is considered. The heat obtained from the pyrolysis plants as well as from direct

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biomass combustion avoids the generation of the corresponding amount of heat from natural gas.

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System boundaries. The environmental assessment includes all processes from agricultural

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production until the final product substitution. The substitution takes place at the point where all


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downstream processes and emissions of the substituted product and the substitute are identical.

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Capital goods are not included. The production is assumed to be situated in Spain, and therefore

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all secondary data such as agricultural input, electricity mix and fossil fuel origin are specific for

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Spain as far as available.

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Impact assessment. The environmental characterization of each system is carried out

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according to the CML method21 and considers the following impact potentials: abiotic depletion

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(ADP), acidification (AP), eutrophication (EP), and global warming (GWP; 100-year

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perspective). Additionally, the cumulative non-renewable (fossil and nuclear; CEDnr) and total

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(renewable and non-renewable; CEDt) energy demand are quantified.22,23 SimaPro is the

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software used for the computational implementation of the inventories.24 A more detailed

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description of the impact categories is available in the online Supporting Information (SI).

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Agriculture. Hybrid poplar produced under irrigated intensive cultivation in short-rotation

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plantations in Central Spain is assumed to be the common feedstock for the assessment. Poplar

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has been identified as one of the energy crops with high potential in Spain, the country with the

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third highest agricultural bioenergy potential in the EU-27.25 Data about yields and inputs for

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poplar cultivation in Spain are taken from the literature,26–30 but corrected by the calculated

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nutrient uptake of the biomass. The latter is estimated by the average nutrient trace element

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content of the feedstock retrieved from the Phyllis database,31 multiplied with the assumed yield

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of 14.5 t·ha-1 (dry basis). This takes into account the positive yield effects of the biochar26 on a

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plantation with an average yield of 13.5 t·ha-1 under conventional practice (i.e., without biochar

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application). Emissions and nutrient leaching due to fertilizer application are estimated according

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to the ecoinvent methodology, using the correction factors for Spanish conditions.32 The lifetime


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of the plantations is assumed to be 15 years.26,27 More details and the inventory data for the

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agricultural phase can be found in the SI.

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Biomass transport. The biomass is harvested in form of coarse wood chips (50-100 mm

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particle size), which are stored in piles at the plantation site without further drying (an average

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50% water content as delivered to the plant is assumed).28,31,33,34 From these piles the biomass is

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shipped to the plant site by truck just in time. The average transport distance from the plantation

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to the pyrolysis plant is estimated to be 15.5 km.28

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Slow pyrolysis plant. The raw biomass is dried to 7% water content, ground to 3 mm particle

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size and converted by slow pyrolysis into gas, tars and a char product. The produced gases and

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tars are burned on site for heat generation, satisfying the heat demand of the pyrolysis reactor

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and generating heat for industrial or residential use. The char product (a fine powder) is

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quenched with water for easier handling, obtaining a char slurry for application to the soil as

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biochar. The pyrolysis plant is simulated in Aspen Plus,35 based on a kinetic reaction model for

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the predictive calculation of the pyrolysis products depending on feedstock composition and

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reactor conditions.28,36–38 The slow pyrolysis plant is described in detail in the SI.

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Biochar application. The biochar slurry is transported from the pyrolysis plant to the field by

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truck and spread like manure with a vacuum spreader before harrowing. Since the char is spread

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as slurry and plowed under afterwards, it is assumed that no further dust emissions occur due to

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biochar application.

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Biochar stability. A high uncertainty exists regarding the long-term stability of the

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biochar.9,10,39 In order to assess the impact of the assumptions concerning biochar properties, five

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types of biochar are considered, with an average lifetime of 15 years (BC-15), 300 years (BC-

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300), 500 years (BC-500), 1,000 years (BC-1k) and 10,000 years (BC-10k). In the base case, the


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biochar shows an average decomposition time of 1,000 years (BC-1k), with 90% of the char still

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remaining in the soil after 100 years, a value comparable to those used in other works.2,11

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Biochar effects on soil. Numerous publications exist dealing with biochar soil application and

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its effects on soil, agricultural yields and nitrogen leaching and emissions.6,40–43 The interest

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received by biochar is principally due to its potential for storing C in the soil over long time

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periods.2,8,13,44–46 Apart from that, biochar is said to increase agricultural yields, water use

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efficiency and nutrient retention in the soil, while also decreasing nitrate leaching and N2O

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emissions.47–50 Nevertheless, little consensus exists regarding these effects and the corresponding

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assumptions are subject to high uncertainty. A good overview about the effects of biochar

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application to soil is given in the works of Verheijen et al.9 and Sohi et al.10 In accordance with

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them, a linear yield increase is assumed, with a maximum 10% yield increase reached at 50 t·ha-1

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biochar application.9 4.5 t·ha-1 of biochar are obtained annually from the pyrolysis plant and

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applied to the plantations. For the BC-1k system, the accumulated biochar content averaged over

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the plantation lifetime is 35.5 t·ha-1. This results in a mean yield increase of 7.1% compared to a

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plantation without biochar application, giving a final biomass yield of 14.5 t·ha-1 (dry basis).

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Reduced nitrate leaching and N2O emissions are accounted for by assuming that the share of N

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provided by the biochar does not cause N2O emissions or nitrate leaching, while the mineral

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fertilizer emissions are unchanged.5 The amount of mineral fertilizer is constant in all cases, and

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the yield increases are thus attributed exclusively due to the biochar application. More details

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about char application and accumulation can be found in the SI.

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Char for energy use. Since the application of the biochar to the soil is not the only char use

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option, it is compared with alternative uses for energy purpose. In the CC (charcoal substitution)

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system, the pyrolysis char is assumed to be used in biomass stoves and boilers or for barbecue,


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substituting conventional charcoal on the existing market. It is assumed that the composition of

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the pyrolysis char does not differ significantly from that of otherwise produced charcoal. Thus,

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substitution is considered to take place at the retail store, with a transport distance of 150 km to

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the store, based on the average distance from the assumed plant site to a major urban area. In the

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FC (fossil coal substitution) system, the pyrolysis char is assumed to be used for co-firing in an

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existing coal power plant. Hence, it substitutes the combustion of fossil coal in the average

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Spanish coal power plant for electricity generation. The transport distance to the coal power

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plant is considered to be 200 km, based on the spatial distribution of existing coal power plants

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around the assumed plant site in Central Spain.20 In order to obtain the inventory data for this

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system, the combustion is simulated in Aspen Plus. In the NG (natural gas substitution) system,

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the pyrolysis char is burned on site for heat generation, substituting natural gas. For estimating

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the emissions from char combustion, the combustion process is simulated in Aspen Plus. More

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details concerning the simulation of the combustion processes and the corresponding inventory

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data can be found in the SI.

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Direct biomass combustion. The most straightforward way of using the biomass is its direct

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combustion for heat generation, therefore it is included as an alternative to the pyrolysis system.

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In the DBC (direct biomass combustion) system, the biomass is ground to 3 mm particle size and

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burned directly in an industrial furnace, generating heat which substitutes heat from natural gas.

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The biomass combustion plant is simulated in Aspen Plus and described in detail in the SI. Size

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and transport distances are assumed to be the same as for the pyrolysis plant.

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Results and discussion

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Biochar system (base case). The life-cycle performance of the biochar system (BC-1k; char

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used as biochar with a decomposition time of 1,000 years) is assessed in detail. The FU is 1 ha of


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short-rotation plantation, equivalent to 14.5 t of dry biomass. The heat output obtained from this

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amount of biomass in the slow pyrolysis plant is 95.69 GJ. Figure 2 shows the characterization

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results and the contribution of each sub-process to the results in each impact category. The

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avoided natural gas appears as negative (i.e., favorable) contributions to the different impact

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categories. Additionally, carbon fixation in the biomass during growth appears as a negative net

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global warming impact from the agricultural phase.

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Under global warming aspects, negative GHG emissions are obtained for the biochar system (-

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17.73 t CO2 eq·ha-1 or -1.22 t CO2 eq·t-1 dry biomass feedstock), i.e. it effectively removes

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carbon dioxide from the atmosphere while generating heat. This is a value at the upper end of

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those published in other LCA studies on biochar, which report GHG abatement potentials of

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0.79–1.25 t CO2 eq per metric ton of dry biomass feedstock (excluding land use change

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effects),11,19 though considering other feedstock types (wood waste, switchgrass and corn stover)

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and different product substitution assumptions. Approximately half of the C fixed in the biomass

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during growth is released into the atmosphere due to combustion in the pyrolysis plant, while the

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other half remains retained in the biochar. Other processes such as electricity consumption and

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transport only contribute a small share to GWP.


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Figure 2. Characterization results of the biochar system and contribution of the sub-processes to

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the potential impacts (FU = 1 ha of plantation). ADP: abiotic depletion potential; AP:

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acidification potential; EP: eutrophication potential; GWP: global warming potential; CEDt:

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cumulative total energy demand; CEDnr: cumulative non-renewable energy demand

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The dominating factor for abiotic depletion is the negative contribution due to the avoided

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natural gas, resulting in a high net reduction of abiotic depletion. The principal positive

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contributors to abiotic depletion are the electricity required by the plant and the agricultural

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activity (with electricity for irrigation contributing the main share).

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On the other hand, the avoided natural gas has a much lower relevance for acidification, since

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the combustion of natural gas is generally associated with low impacts in this category. Here,

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electricity for the pyrolysis plant and electricity for the agricultural phase are the main

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contributors, giving a net increase in acidification. Direct emissions from the combustor in the

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pyrolysis plant also contribute a considerable amount.


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Impacts in eutrophication are closely linked to the agricultural activity and the electricity

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consumption of the pyrolysis plant. Within the agricultural stage, nutrient leaching, electricity

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consumption for irrigation and fertilizer production contribute the main shares. The avoided

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natural gas plays a relatively minor role in this category, resulting in a net increase in

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eutrophication.

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Finally, the cumulative total energy demand is dominated by the biomass plantation phase (as

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the biomass is the main energy input of the system). Nevertheless, the cumulative non-renewable

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energy demand is negative, indicating savings of non-renewable energy. This is associated

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mainly with the avoided natural gas, giving a picture very similar to that obtained for abiotic

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depletion. The electricity requirements of both the pyrolysis plant and the agricultural activity in

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the biomass plantation (mainly irrigation) are the main unfavorable contributors to the

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cumulative non-renewable energy demand.

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Overall, the slow pyrolysis biochar system effectively sequesters carbon as biochar, but at the

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expense of a low energy efficiency. Electricity consumption in the pyrolysis plant and for

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irrigation of the plantations is found to be the key for further improving the overall

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environmental performance of the system.

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Comparison with direct biomass combustion. Table 1 presents the characterization results

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of the biochar system (BC-1k) along with those of direct biomass combustion (DBC) and those

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of alternative slow pyrolysis systems with the char substituting charcoal (CC), natural gas (NG)

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and fossil coal (FC). This section focuses on the comparison of the BC-1k system with the DBC

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system for heat generation.

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When compared with direct biomass combustion under global warming aspects, the biochar

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system achieves higher GHG emission savings. On the other hand, when considering the overall


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energy demand, direct biomass combustion shows a better result, leading to a better life-cycle

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energy efficiency score. This is reflected also in the cumulative non-renewable energy demand,

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where direct biomass combustion shows a substantially higher saving potential. Contrasting

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these results with those reported by Roberts et al.,2 who assessed a biochar system in terms of

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global warming and cumulative energy demand, the high GHG and energy savings identified by

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them can be confirmed, although they obtained slightly higher GHG emission savings for direct

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biomass combustion when including the avoided fossil fuel in the assessment.

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In the remaining assessed categories (abiotic depletion, acidification and eutrophication), direct

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biomass combustion also gives significantly better results. Since the conversion efficiency of

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BC-1k is lower (a significant amount of the biomass is finally applied to the field as biochar

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without using it energetically), less natural gas is substituted per ha of plantation and the

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corresponding benefits are lower. Furthermore, the slow pyrolysis plant is more complex than

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the direct combustion plant, requiring higher auxiliary energy inputs, mainly electricity.

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Both DBC (direct biomass combustion system) and BC-1k (biochar system) obtain positive

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values for acidification and eutrophication. Substituting conventional heat generation from

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natural gas by either of these two bioenergy systems brings along additional environmental

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impacts in these two categories, which is associated mainly with the agricultural activity required

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for producing the biomass and the electricity demand of the conversion plants.

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Table 1. Characterization results of the alternative systems (FU = 1 ha). BC-1k: base-case

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system (1,000-year stable biochar); DBC: direct biomass combustion; CC: char for charcoal

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substitution; NG: char for natural gas substitution; FC: char for fossil coal substitution; ADP:

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abiotic depletion potential; AP: acidification potential; EP: eutrophication potential; GWP:


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global warming potential; CEDt: cumulative total energy demand; CEDnr: cumulative non-

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renewable energy demand

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Category ADP AP EP GWP CEDnr CEDt

Unit

BC-1k

DBC

CC

NG

FC

kg Sb eq

-38.19 29.27 10.30 -17.73 -48.17 238.47

-121.80 21.55 8.74 -15.74 -221.48 45.59

-29.75 27.53 9.41 -7.96 -30.94 -67.31

-108.92 29.47 9.37 -13.95 -193.23 75.33

-140.21 -122.29 -18.21 -19.09 -190.46 77.27

kg SO2 eq 3-

kg PO4 eq t CO2 eq GJ GJ

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Overall, the high GHG emission savings found for the biochar system are achieved at the

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expense of a lower energy efficiency. This lower efficiency leads to a lower amount of fossil

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natural gas avoided and to higher environmental impacts in the remaining categories, giving a

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trade-off situation principally between GWP and fossil energy savings.

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Alternative char uses. When comparing the biochar system (BC-1k) with alternative slow

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pyrolysis systems (Table 1), substituting charcoal (CC) is a char use option with relatively poor

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results in most of the assessed categories. This is in line with the findings of Sparrevik et al.,7

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who assessed a biochar system in a tropical environment from a life-cycle perspective and

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identified the biochar system to be more favorable than the use of pyrolysis char as charcoal in

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terms of global warming. However, a negative total energy demand (CEDt) is obtained for CC

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(charcoal substitution). This is a surprising result, since it indicates total net energy savings of

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the system, a result of the product substitution. CC substitutes conventional charcoal, produced

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traditionally in little efficient kilns from forest wood. Avoiding its production saves high

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amounts of biomass primary energy, resulting in a negative CEDt result. In contrast, the non-

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renewable energy savings and the reduction in abiotic depletion are the lowest of all assessed

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options, again due to the fact that the char substitutes a renewable energy product (biomass in the

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form of charcoal).


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In the NG system (natural gas substitution), all pyrolysis products are used for substituting

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natural gas. In this sense, it is comparable to direct biomass combustion, but with an additional

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pyrolysis pre-processing stage. It hence shows the same general picture as direct biomass

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combustion: high savings in abiotic depletion and cumulative non-renewable energy demand, but

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positive values (increase of impacts) in acidification and eutrophication. Nevertheless, since the

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direct biomass combustion system requires less input and produces lower losses, it scores better

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in all assessed categories. Therefore, the use of slow pyrolysis instead of direct combustion has

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to be justified by the additional benefit obtained from the pyrolysis char. In other words, slow

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pyrolysis requires that a high environmental benefit is obtained from the use of the pyrolysis

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char. Otherwise, direct combustion of the biomass gives better results, since it is a more efficient

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conversion technology and avoids more fossil fuels per amount of biomass processed.

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FC (fossil coal substitution by co-firing) shows the highest reduction of environmental impacts

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among the slow pyrolysis systems in most of the assessed categories. This is due to the high

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environmental impacts associated with mining and combustion of fossil coal. Substituting fossil

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coal by pyrolysis char avoids these impacts and gives, in spite of the impacts caused by the

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agricultural activity and the electricity consumption, a net reduction for the co-firing system. FC

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also shows significantly better results than direct biomass combustion in all assessed categories

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(except for the cumulative energy demand results). Nevertheless, it has to be taken into account

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that co-firing in an average existing coal power plant is assumed. Fossil coal substitution in a

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power plant with a more efficient gas cleaning system would lead to lower environmental

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benefits.20

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In comparison with the biochar base-case system (BC-1k), co-firing in coal power plants gives

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a better life-cycle performance in all assessed categories. On the other hand, the use of the


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pyrolysis char for other energetic purposes such as substitution of natural gas or charcoal can be

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considered less favorable than its use as biochar under global warming aspects. In general, the

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overall energy demand is lower when all pyrolysis products are used for energy purpose. These

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findings differ from those published by Roberts et al.,2 who stated that coal substitution scores

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worse and direct biomass combustion better than biochar in terms of global warming, which is

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probably linked to the different product substitution approach.

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Influence of biochar stability. The characterization results obtained for the five types of

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biochar with different stability are given in Table 2. The biochar stability affects mainly the C

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sequestration and in consequence the result for the global warming impact category. Its influence

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on the remaining impact categories is generally small. It should be noted that acidification

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impacts increase marginally with higher char stability. This is an effect of the increased

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electricity consumption per ha of plantation in the processing plant, being electricity the main

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contributor to acidification.

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The different biochar stabilities affect the biomass yield increases. These converge quickly for

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chars with stabilities significantly higher than the plantation lifetime and therefore are very

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similar for all char types except for BC-15 (the least stable biochar considered), resulting in very

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similar characterization results. More details about the influence of char stability on yield

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increase can be found in the SI.

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Generally, there is a high uncertainty regarding the long-term stability of the biochar and the

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effects that it has on yield, GHG emissions from the soil and nitrate leaching. Assumptions

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favorable for biochar are made in this assessment, accounting for reduced N2O emissions, yield

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increase and reduced leaching, although scientific evidence is often not given. Under these


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circumstances, biochar shows a high GHG saving potential, with BC-10k (the most stable

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biochar considered) achieving a value very close to that of FC (fossil coal substitution).

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Hence, the assumptions concerning char stability are of high significance for the global

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warming results. Only for a very long-term stable char (> 10,000 years) its application to the soil

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as biochar would be among the most efficient strategies for C abatement, requiring that

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significantly less than 10% of the C contained in the biochar is decomposed within 100 years

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after application. Nevertheless, considering that biochars use to contain a significant share of

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volatile matter and that in short-term laboratory experiments a release of 3.1–11.9% of the C

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contained in the chars within 115 days was observed,51 it is seen probable that the actual

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decomposition will be higher than 10% in 100 years. For instance, Roberts et al. 2 and Ibarrola et

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al.11 assumed 80% of the biochar to be long-term stable, Masek et al.41 found stable C content in

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biochars to be between 40-90%, and Zimmermann52 states the average C loss to be of 3–26%

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within 100 years. Under these conditions, biochar would be less favorable than fossil coal

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substitution also in terms of global warming, and co-firing the biochar substituting fossil coal

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would be the best option.

331

Table 2. Characterization results of the biochar (BC) system with different biochar stabilities

332

(15, 300, 500, 1,000 and 10,000 years). ADP: abiotic depletion potential; AP: acidification

333

potential; EP: eutrophication potential; GWP: global warming potential; CEDt: cumulative total

334

energy demand; CEDnr: cumulative non-renewable energy demand Category

Unit

BC-15

BC-300

BC-500

BC-1k

BC-10k

ADP AP EP GWP CEDnr CEDt

kg Sb eq

-37.18 28.83 10.23 -5.60 -46.27 234.33

-38.15 29.26 10.29 -14.60 -48.10 238.32

-38.17 29.27 10.29 -16.39 -48.14 238.40

-38.19 29.27 10.30 -17.73 -48.17 238.47

-38.20 29.28 10.30 -18.94 -48.20 238.53

kg SO2 eq 3-

kg PO4 eq t CO2 eq GJ GJ

335


16

Page 17 of 25


336

Influence of biomass yield increase. The biomass yield effects reported in the literature vary

337

by orders of magnitude and the corresponding assumptions are thus associated with high

338

uncertainty (more details about this aspect can be found in the SI). In order to take into account

339

this source of uncertainty, a sensitivity analysis is performed varying the potential yield effects.

340

Biomass yield increases of 0%, 10% (base case), 20%, 50% and 100% at 50 t·ha-1 biochar

341

application are assessed for a 1000-year stable biochar (BC-1k). The results of the assessment

342

are presented in Table 3.

343

Table 3. Characterization results of the biochar system (BC-1k) with different biomass yield

344

effects (0%, 10%, 20%, 50% and 100% yield increase at 50 t·ha-1 biochar application). ADP:

345

abiotic depletion potential; AP: acidification potential; EP: eutrophication potential; GWP:

346

global warming potential; CEDt: cumulative total energy demand; CEDnr: cumulative non-

347

renewable energy demand Category ADP AP EP GWP CEDnr CEDt

Unit

BC 0%

BC 10%

BC 20%

BC 50%

BC 100%

kg Sb eq

GJ

-35.08 27.84 10.04 -16.61 -42.33

-38.19 29.27 10.30 -17.73 -48.17

-41.79 30.87 10.55 -18.97 -54.97

-56.88 37.61 11.63 -24.47 -83.41

-121.53 66.82 16.54 -47.80 -205.25

GJ

225.60

238.47

253.24

315.21

581.65

kg SO2 eq 3-

kg PO4 eq t CO2 eq

348 349

The yield effect of the biochar has a significant influence on the results, with increasing

350

reductions in GHG emissions, abiotic depletion and non-renewable energy demand. Under these

351

aspects, the use of biochar for amending low quality soils arises as an attractive option,

352

especially in terms of GHG emission savings. Nevertheless, even with 100% yield increase, the

353

reductions observed in abiotic depletion and non-renewable energy demand are similar to those

354

of direct biomass combustion or fossil coal substitution. Moreover, acidification and


17


Page 18 of 25

355

eutrophication impacts increase with higher yields, mainly due to the higher amount of biomass

356

to be transported and processed per hectare.

357

Overall picture. A normalization of the results supports the identification of the system with

358

the most favorable life-cycle performance by providing insight into the relevance of the different

359

categories. It is carried out by dividing the characterization results by the corresponding impacts

360

of a fixed reference, e.g. the annual impacts caused by a region. Figure 3 shows the results of all

361

assessed systems normalized according to the factors for West Europe.21

362 363

Figure 3. Normalized environmental results for each system. FC: char for fossil coal

364

substitution; DBC: direct biomass combustion; CC: char for charcoal substitution; NG: char for

365

natural gas substitution; BC-“x”: biochar system with different biochar stabilities; BC “x”%:

366

biochar system (1,000-year stability) with different biomass yield increases (“x”% at 50 t·ha -1 of

367

biochar application); ADP: abiotic depletion potential; AP: acidification potential; EP:

368

eutrophication potential; GWP: global warming potential


18

Page 19 of 25


369

Co-firing is the only option that shows a reduction in acidification and eutrophication, since in

370

this case the char avoids the mining and combustion of fossil coal, generally associated with very

371

high impacts in these categories. It gives the best results for abiotic depletion, acidification and

372

eutrophication, while showing a very high GHG saving potential. Despite the additional

373

pyrolysis process, it achieves better results than direct combustion of the biomass in all assessed

374

categories, due to the high environmental benefits obtained from the substitution of fossil coal.

375

Only biochar systems with high biomass yield increases (> 20% at 50 t·ha-1 of biochar

376

application) achieve significantly higher GHG emission savings.

377

In conclusion, co-producing biochar and heat by slow pyrolysis shows significant GHG

378

abatement potential. It gives significantly higher GHG savings than direct biomass combustion,

379

basically due to biochar effects on biomass yield and carbon sequestration in the soil. In this

380

respect, biochar application on low-quality soils should be favored due to increased biomass

381

yields and therefore increased GHG emission savings. Nevertheless, the energy efficiency of the

382

pyrolysis system is lower and the impacts in other assessed categories are higher than those of

383

direct biomass combustion.

384

When comparing the biochar system with other possible char uses, biochar can be an

385

interesting option for reducing GHG emissions, which is favored by increasing biochar stability

386

and biomass yield rises. Nevertheless, even though assumptions favorable for biochar are made,

387

co-firing the char in a power plant for substituting fossil coal is generally the best option under

388

environmental aspects. Only in terms of global warming, biochar can achieve better results

389

provided that high biomass yield increases are achieved. It is concluded that, under

390

environmental aspects, the use of pyrolysis char for energy purpose (e.g., replacing fossil coal in

391

power plants) can be more recommendable than its use as biochar.


19


Page 20 of 25

392

Acknowledgements

393

This research has been partly supported the Spanish Ministry of Economy and

394

Competitiveness (ENE2011-29643-C02-01 and IPT-2012-0219-120000).

395

Supporting Information Available

396

The available supporting information contains detailed information about the Aspen Plus

397

simulations used for the LCA and the methodology used for obtaining the inventory data. The

398

inventory data are provided, including all inputs and outputs of the processes. It further contains

399

a more detailed discussion about biochar effects on soil and yield and the corresponding

400

assumptions made in this paper. This information is available free of charge via the Internet at

401

http://pubs.acs.org/.

402

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