Pyrolytic biochar stability assessed by chemical accelerating aging method

IN

DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS

STOCKHOLM SWEDEN 2020 ,

Pyrolytic biochar stability

assessed by chemical accelerating aging method

BINBIN CHEN

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Abstract

Now that the EU and Sweden have adopted a new climate policy framework to regulate net carbon emission. A new concept, negative CO

emission, has been considered to neutralize the CO

generated from necessary consumption of fossil fuel. Biochar, as a pyrolytic product from biomass, can store carbon in a relatively stable way. Therefore, it is one of the most promising and outstanding tools for carbon sink.

Biochar stability, defined as the ratio of remaining carbon in biochar after 100 years, is the most crucial factor when using biochar for carbon storage. So far, various approaches have been proposed to measure and predict biochar stability, such as elemental analysis, proximate analysis, accelerating aging methods.

Each method has its pros and cons. The reliability of these methods still needs to be verified. In this project, the chemical accelerating aging method has been selected for assessing biochar stability, because this method captures both chemical and physical properties of biochar. Besides, the gas, liquid, and solid products generalized during the chemical treatment are collected and analyzed separately in order to study the oxidation mechanism.

Biochar in this project is produced from miscanthus and seaweed at various pyrolysis temperature. It is found that biochar stability can be increased by enhancing pyrolysis temperature, and miscanthus biochar is more sensitive to pyrolysis temperature within the pyrolysis temperature range of 350-600℃.

The highest biochar stability (73%) has been achieved with miscanthus-derived biochar produced at

550 ℃, which demonstrates high potential as carbon sequestration tool.

ii

Sammanfattning

Nu när EU och Sverige har antagit en ny klimatpolitisk ram för att reglera nettokoldioxidutsläppen. Ett nytt koncept, negativt koldioxidutsläpp, har ansetts neutralisera den koldioxid som genereras av nödvändig förbrukning av fossila bränslen. Biokol, som en pyrolytisk produkt från biomassa, kan lagra kol på ett relativt stabilt sätt. Därför är det en av de mest lovande och enastående verktyg för kolsänka.

Biokolsstabilitet, definierad som förhållandet mellan återstående kol i biokol efter 100 år, är den viktigaste faktorn vid användning av biokol för kollagring. Hittills har olika metoder föreslagits för att mäta och förutsäga biokolsstabilitet, såsom elementär analys, proximate analys, accelererande åldrande metoder. Varje metod har sina för-och nackdelar. Tillförlitligheten hos dessa metoder måste fortfarande kontrolleras. I detta projekt har den kemiska accelererande åldrandemetoden valts ut för att bedöma biokolsstabilitet, eftersom denna metod fångar upp både kemiska och fysikaliska egenskaper hos biokol. Förutom, gasen, flytande, och fasta produkter generaliserade under den kemiska behandlingen samlas in och analyseras separat för att studera oxidation mekanism.

Biokol i detta projekt framställs av miscanthus och tång vid olika pyrolystemperatur. Det visar sig att

biokolsstabiliteten kan ökas genom att öka pyrolystemperaturen, och miscanthusbiokol är mer känsligt

för pyrolystemperatur inom pyrolystemperaturområdet 350-600°C. Den högsta biokolsstabiliteten

(73%) har uppnåtts med biokol som framställts vid 550°C och som visar stor potential som

kolbindningsverktyg.

Contents

1 Introduction ... 1

1.1 Biochar as a negative emission and its effect on the environment ...1

1.2 Biochar stability ...1

1.2.1 Carbon structure ...2

1.2.2 Elemental composition ...2

1.2.3 Pyrolysis conditions ...3

1.3 Methods to assess biochar stability ...3

1.3.1 Ultimate analysis and C structure analysis ...5

1.3.2 Labile/recalcitrant carbon content ...5

1.3.3 Incubation and modeling ...6

1.4 Objectives ...6

2 State of art of chemical accelerating aging methods ... 8

2.1 Biochar oxidation mechanism in environment ...8

2.2 Biotic degradation...8

2.3 Abiotic oxidation mechanism ...8

2.4 Accelerating aging method ...9

2.4.1 Hydrogen peroxide system (Edinburgh stability tool) ...9

2.4.2 NaOH and H

O

... 10

2.4.3 HNO

... 10

2.4.4 K

Cr

O

... 10

2.4.5 KMnO

... 11

2.4.6 (NH

)

S

O

... 11

2.5 Comparison in products surface chemistry ... 11

2.6 Summary ... 12

3 Experiments ... 13

3.1 Raw materials (biochar) ... 13

3.2 Method ... 13

3.2.1 Chemical oxidation to determine biochar stability ... 13

3.2.2 Elemental analysis ... 14

3.2.3 Liquid products analysis ... 14

3.2.4 Online gas analysis ... 15

3.3 Experimental Plan ... 15

4 Result and discussion... 16

4.1 Biochar stability influenced by feedstocks and temperature... 16

4.1.1 Solid residue... 16

4.1.2 Soluble organics ... 21

4.2 Online gas Analysis ... 22

4.3 Discussion on scientific, social and environmental sustainability of biochar as carbon sink22 4.4 Future work... 23

5 Conclusions ... 24

6 Acknowledgement ... 25

7 References ... 26

1

1 Introduction

1.1 Biochar as a negative emission and its effect on the environment

By definition, biochar is a carbon-rich product from agriculture and forestry wastes (also known as biomass), for example, wood, manure, leaves, rice straw. It is produced through pyrolysis, during which biomass is heated at relatively low temperatures (<700°C) in a closed converter with little or without an oxygen supply.[1] Afterward, the organic carbon in biomass is converted into a more stable charcoal- like structure with high aromaticity.

Biochar can be produced from a large number of feedstocks with heterogeneity in elemental composition and carbon structure. Therefore, it is difficult to fully understand the behavior of biochar.

But the shared characteristics at the elemental level and of carbon structures provide insights into the chemical and physical properties of biochar, which would be discussed later in detail.

The application of biochar in environment management is of great research interest, and it is expected to mitigate environmental stress in the following four aspects [1]: 1) biochar to reduce agriculture waste;

2) biochar to produce energy; 3) biochar as soil amendment; 4) biochar for carbon sink. And the last two effects closely related to the soil application of biochar (carbon sequestration).

Carbon sequestration is one of the critical ways to participate in the terrestrial ecosystem and greenhouse gas cycle. CO

in atmospheric circulation can be absorbed and converted into organic carbon by photosynthesis in plants. Therefore, the organic carbon in plant can be further sequestered in soil as biochar and excluded from the carbon cycle to reach the goal of negative carbon emission. A report on global warming of 1.5 ◦C released by the Intergovernmental Panel on Climate Change (IPCC) has identified the carbon sequestration as a promising carbon removal tool for climate change mitigation.

Besides, biochar contains nutrient elements such as N, P, Ca, K, etc. Biochar’s influences on the nitrogen cycle, such as increasing symbiotic biological N

fixation and plant N uptake, reducing soil N

O emission, have also been reported [2]. Furthermore, those elements can either be stored in soil together with recalcitrant carbon or gradually released into soil during carbon mineralization and act as a soil amendment. For example, significant changes in soil physical and chemical properties with high rate of biochar application and their enhancement effect in plant growth rate have been observed [3].

1.2 Biochar stability

In order to bring carbon sequestration into practice, it is critical to investigate whether and how long biochar would be stable in the soil. The application of carbon sequestration requires that the organic carbon in biochar is stable in soil for at least 100 years [4]. The potential of biochar as a carbon sequestration tool highly depends on its resistance to abiotic and biotic decomposition and the interaction with soil organic carbon, which is described as biochar stability. Up to now, considerable researches have been conducted to investigate biochar characteristics and propose indicators to predict biochar stability.

Biochar stability is closely affected by various factors, for instance, the physical properties (particle

size, surface area, etc.), chemical properties of biochar (elemental composition, surface functional

groups, phases, and carbon structures, etc.), the environmental conditions (pH, moisture, temperature,

etc.). Generally, the morphologies of biochar mostly contribute to the short-term decomposition, while

long-term stability (both thermal and chemical) is more related to the carbon aromaticity and the

formation of the organometallic complex [5]. Thus, the assessments of biochar stability mainly focus

on investigating elemental and chemical composition as well as carbon structures. In addition, pyrolysis

parameters play a significant role in determining the abovementioned factors, and it is also crucial to understand the influences of pyrolysis conditions.

1.2.1 Carbon structure

Carbon is the fundamental element in biochar, and it exists in both organic and inorganic phases.

Inorganic carbon appears as carbonate and bicarbonate, while organic carbon is comprised of aliphatic carbon, aromatic carbon, and functionalized carbon [6]. Understanding the aromatic structure and aromatic condensation of biochar provides insight into the fraction of stable carbon. It is considered as the most decisive factor for biochar stability because aromatic carbon can resist decomposition or decompose at a prolonged rate in soil compared with aliphatic compounds [7].

Additionally, the carbon in biochar can also be classified by its inherent resistance to decomposition.

In operation, labile carbon is defined as the carbon fraction that decomposed in incubation experiments firstly and at a rapid rate, while recalcitrant carbon, on the contrary, is used to describe the carbon that resists degradation with a lifetime of thousands of years[8]. Recalcitrant carbon is similar but not identical to aromatic carbon.

1.2.2 Elemental composition

C, H, O (and sometimes nitrogen is considered) are the most common elements and the framework in biochar. Their concentration and concentration ratio provide basic information and proxies for biochar stability. It is well-accepted to use C/H and C/O to assess biochar stability. C/H is an indication of the saturation for organic carbon. Thus, the high C/H ratio reflects low aromaticity in biochar. C/O ratio, in another way, gives information about biochar reactivity, because oxygen is closely related to the functional groups on the surface and potentially participate in degradation reaction. Biochar with an O/C ratio below 0.2, are commonly stable in soil, corresponding to a half-life more than 1000 years;

O/C ratio between 0.2-0.6 corresponds to half-lives of 100-1000 years, while O/C ratio above 0.6 is considered no practical value as carbon sequestration [9].

Biochar contains a wild range variation in nitrogen content across feedstocks. Biochar pyrolyzed from feedstocks with peptide bonds is relatively abundant in nitrogen, for example, 15% wt.% in casein, 10 wt.% in algae-derived biochar, 0.3-1 wt.% in wood-derived biochar [10, 11]. The organic nitrogen in biochar (also known as black nitrogen) mainly relates to amide-N at low pyrolysis temperature and gradually transforms into N hetero-aromatic compound with increasing pyrolysis temperature. Biochar rich in nitrogen demonstrates better thermal stability but worse chemical stability than biochar deficient in nitrogen [6]. C/N ratio is sometime in negligible when assessing biochar stability.

In addition to the major elements (C, H, O, N), biochar usually contains S, P, and metallic trace elements (Ca, K, Si, etc.). Those elements are of a small fraction and vary across feedstocks to a great extent, but provide specific properties to biochar, respectively.

Especially, trace elements (Ca, Fe, Mn, etc.), can have a positive or negative influence on biochar

stability. Al, Ca, Fe minerals can prevent chemical oxidation and stabilize biochar by forming

organometallic complexes such as Fe-O-C [12]. A loose correlation (r=0.74) exists between the content

of mineral (ash content) and chemical oxidation resistance by K

Cr

O

[13]. Similarly, topsoil in nature

with higher iron oxides content has found to be more oxidation-resistant compared with subsoil with

lower iron oxides content. A positive correlation is found between the oxidation-resistant organic

carbon and the concentration of iron oxides [14]. Except for the protective effect of Fe, it is reported

that C-Si interaction can form a physical encapsulation on biochar particles to improve the oxidation

resistance of biochar pyrolyzed from rice straw at 500 °C [15]. It is confirmed that mineral compositions

can stabilize biochar by forming organic-metal complexes, which provide chemical and/or physical

protection for oxidation.

3

However, on the other hand, negative influences on biochar stability can occur through various mechanisms: (1) to catalyze the formation of active functional groups; (2) to activate oxidative agents.

For example, K can catalyze the formation of O-containing functional groups, which accelerate the chemical reaction as well as thermal degradation of biochar and result in a reduction in biochar stability [16]. The existence of Fe ions can facilitate chemical degradation by activating the persulfate to produce oxidative free radical [17].

1.2.3 Pyrolysis conditions

Since biochar is produced from biomass through pyrolysis, both feedstocks of biomass and pyrolysis parameters can influence biochar properties to a great extent, both physically and chemically. Pyrolysis parameters include biomass feedstocks, pyrolysis temperature, residence time, heating rate, among which feedstocks and pyrolysis temperature exert a more significant impact on biochar stability.

Biomass with a high content of lignin (such as wood) leads to a higher concentration of carbon aromaticity and longer lifetime, followed by cellulose and hemicellulose [7].

Pyrolysis temperature (highest treatment temperature, known as HTT) during biochar production demonstrates the closest correlation with biochar stability in all studies, such as C/H ratio[18], biochar stability assessed by chemical oxidation [19]. Pyrolysis is a gradual process of dehydration and deoxidization, where aromatic carbon structures are slowly formed. Pyrolysis goes through four phases according to pyrolysis temperature:

(1) dehydration occurs together with the loss of volatile matters during 200 -300 °C . (2) small aromatic units start to form and arrange randomly during 300℃-600℃.

(3) carbonization is dominant during 600-700 °C.

(4) turbostratic char is formed above 700℃[12].

In addition to carbon structure, nitrogen structures are also sensitive to the pyrolysis temperature.

Amide-N has been found to reduce with the increase in pyrolysis temperature, while aromatic nitrogen that forms pyridine, pyrrole, and N-hetero-aromatic cycle increases [6].

Besides pyrolysis temperature, reaction residence time and heating rate may affect carbon structure as well in a similar way as pyrolysis temperature, but to a less extent [12].

1.3 Methods to assess biochar stability

Three categories of stability assessment methods have been proposed: 1) ultimate analysis and C structure analysis. 2) labile/recalcitrant carbon fraction. 3) incubation and modeling[20]. Summary of biochar assessment methods has been presented in Table 1-1.

Table 1-1 Summary of biochar assessment methods[20].

Classification Method Indicators Advantages Disadvantages

Ultimate analysis Elemental analyzer

C/H Easy; low cost;

well-accepted as standard

not accurate enough C/O

C/N

Carbon structure analysis with

NMR

Aromatic and nonaromatic carbon content

Quantitatively assess aromatic

Wild variation caused by

instruments XRD

The degree of aromatic condensation:

Δδ = δsorbed benzene − δneat benzene)

structure and measure aromaticity condensation;

Strong correlation with MRT

equipment

NEXAFS，XPS, etc.

Functional groups Promising to be developed

Expensive;

qualitative analysis;

need to be validated

Benzene polycarboxylic

acids(BPCA)

Total BPCA/TOC;

B6CAs/total BPCA

Promising to be developed

Not accurate for biochar pyrolyzed at low temperature;

need to be validated

Pyrolysis GC-MS TSi = ∑(𝑅𝑃%𝑖∗

𝑛

𝑖

𝑇𝑆𝐼𝑝𝑖)/100 Promising to be developed

need to be validated

Proximate

analysis FC, VM ^VM_FC, ^VM

VM+FC Easy; low cost

Always utilized as supplement to O/C,

H/C ratio

Thermal

oxidation TPO

𝑅50= 𝑇50,𝑋

𝑇50,𝑔𝑟𝑎𝑝ℎ𝑖𝑡𝑒

Easy; low cost

Robustness need to be further investigated GS = ^{𝑅50,𝑋−𝑅50,𝐶𝐸𝐿𝐿}

𝑅50,𝑃𝐶𝐸𝐿𝐿−𝑅_{50,𝐶𝐸𝐿𝐿)},

Chemical

oxidation Chemical oxidant Æ =Br × BrC × 100%

Bt × BtC

Easy; low cost;

fully capture the physical and chemical

Robustness need to be further investigated

5

properties of biochar

Incubation and

modelling \ MRT accurate Time consuming

1.3.1 Ultimate analysis and C structure analysis

As the abovementioned significance of elemental composition, the content of C, H, O, N determined by ultimate analysis is the most applicable indicator for the biochar stability. Biochar with an O/𝐶

ratio of maximum 0.4 and a H/𝐶

ratio of maximum 0.7 is considered as standard-compliant biochar by the European Biochar Certificate (EBC) [21].

Carbon aromaticity (defined as the fraction of aromatic carbon) and degree of condensation (defined as the size and purity of aromatic rings) have been found in a strong relationship with biochar stability, which can be qualitatively and/or quantitatively analyzed by instrumental methods and molecular markers [20], such as Nuclear magnetic resonance (NMR) analysis, X-ray diffraction (XRD) analysis, Near-edge X-ray absorption fine structure (NEXAFS), etc.

1.3.2 Labile/recalcitrant carbon content Fixed carbon/ volatile matter

During pyrolysis, aliphatic carbons gradually transform into aromatic moieties through cycling reaction.

Those aromatic carbons are in the form of graphene-like structures and interconnected across layers through aliphatic chains [22]. They are also described as fixed carbon(FC). Volatile matter (VM) is defined as those intermediate products that entrapped inside the aromatic carbon framework [22].

Therefore, proximate analysis measures the fixed carbon (FC) and volatile matter(VM), which are considered to reveal the labile/recalcitrant carbon content.

Along with the ultimate analysis, VM < 80% and O/𝐶

 > 0.2 or H/𝐶

 > 0.4 may indicate moderate sequestration ability; VM < 80% and O/𝐶

 < 0.2 or H/𝐶

 < 0.4 may indicate high C sequestration potential [23]. Besides,

FC

[9, 24] or

VM + FC

[25, 26] have been found correlated with O/Corg ratio and proposed as assessment indexes.

FC

ratio of <0.88 (reflecting O/C < 0.2) can correspond to a half-life of >1000 years, while 0.88 < 

FC

 < 3.0 (reflecting 0.2 < O/C < 0.6) can correspond to a half-life of 100–

1000 years [24].

However, both ultimate analysis and proximate analysis can only measure the elemental composition and chemical properties of biochar. In order to directly simulate the biotic and abiotic oxidation in soil, thermal oxidation and accelerating aging methods have been developed.

Thermal oxidation

The thermal oxidation method is to measure the weight loss of carbon when heating with the existence of oxygen, where oxygen acts as the oxidant. Temperature-programmed oxidation (TPO), such as thermogravimetry (TG), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) are some applicable approaches to assess biochar stability[20].

Recalcitrant index defined as

𝑅

=

50,𝑔𝑟𝑎𝑝ℎ𝑖𝑡𝑒

, (1)

is a wildly-applicable TPO index, where the temperature value 𝑇

corresponding to 50%

oxidation/volatilization of biochar, by 𝑇

, an external standardization factor ( 𝑇

= 885 °C)[27].

An improvement to 𝑅

assessment method is to use “gained stability” (GS) calculated by GS =

50,PCELL−R_50,CELL)

, (2)

where𝑅

, 𝑅

and 𝑅

are the R

values of biochar, cellulose and cellulose char produced at HTT 750 °C for 1 h, respectively[28]. Strong correlation (R = 0.97) was observed between the

“gained stability” index and the accelerated aging test results obtained by the chemical accelerating aging method[19].

Chemical accelerating oxidation

Accelerating aging with chemical agents is another promising method to assess biochar stability. For example, H

O

- and heat-assisted methods is wildly used because it combines thermal and chemical accelerating method. Carbon stability is defined as the fraction (Æ) of carbon that resists the oxidation treatment:

Æ =

Bt×BtC

, (3)

where BrC and BtC represent the carbon content in the residual and total biochar respective and Br and Bt the mass before and after the treatment [19].

Optional chemical oxidants include H

O

[19], HNO

[29], (NH

)

S

O

[30], dichromate (K

Cr

O

) oxidation[31], permanganate (KMnO

) oxidation[26], etc. The advantages and disadvantages of each oxidant have been justified in the next chapter (State of art).

The accelerating aging methods capture more interfering factors such as morphologies, particle size, chemical reaction site, etc.

1.3.3 Incubation and modeling

Incubation and modeling is to directly test biochar degradation rate in soil, and then extrapolate the prediction of mean residence time from the experiment data (such as efflux CO

).

However, biochar decomposes at a rapid rate initially and gradually slows down. It can take more than one year in order to accumulate reliable data. Besides, the extrapolation from experimental data by modeling relies heavily on the fitting model. According to the number of carbon pools, they can be classified into one-, two-, three-pool models, among which the two-pool model (labile and recalcitrant carbon) is the most used one[20]. Furthermore, environmental conditions, such as pH and moistures of soil, temperature, etc., are closely related to the result. Results obtained by different experiments cannot be used to compare directly.

1.4 Objectives

The main object of this project is to predict how long biochar can persist in soil and to investigate the potential influence on the environment, considering the Swedish environment and circumstances.

Assuming the similarity between the laboratory-based chemical oxidation and the natural abiotic and biotic degradation in soil, this project utilizes chemical oxidation and elemental analysis to investigate the stability of biochar pyrolyzed from miscanthus and seaweed. For the purpose of justifying the robustness of the chemical oxidation approach and understanding the oxidation mechanism, solid, liquid, and gas production during oxidation will be collected and analyzed.

1. Use the chemical oxidation approach (Edinburgh stability tool) and element analysis to determine

the fraction of recalcitrant C, N, S.

7

2. Use the GC/MS instrument to analyze the water-soluble organic carbon after oxidation in order to understand the oxidation mechanism and oxidized products.

3. Use the micro-GC to monitor the reaction and detect gas products during oxidation, both qualitatively and quantitatively.

4. Build correlations between biochar stability by chemical oxidation and other indicators (for example,

O/C and H/C ratio) to investigate the robustness and feasibility of the method.

2 State of art of chemical accelerating aging methods

2.1 Biochar oxidation mechanism in environment

Biochar in soil undergoes aging through mineralization and oxidation, and decompose into CO

or other organics. The oxidation of carbon would be caused by biotic and abiotic mechanisms. Abiotic process plays a more important role in initial rapid oxidation of biochar, during which most of labile carbon is removed. Biotic degradation is relatively a long-term process, and it is less constrained by the recalcitrance of aromatic carbon structure [32].

2.2 Biotic degradation

It has been confirmed that charcoal can go through microbial degradation in nature. The interaction between biochar and microorganism can result in the some or all of the following aspects: 1) biochar as food/energy source for microorganism.2) biochar oxidation conducted or catalyzed by oxidants released from microorganism’s activity. 3)biochar as habitat for microorganism.4) priming effect activated by biochar [33].

Biochar potential as a food source for microorganisms is determined by the extracellular enzymes' ability to decompose organic carbon. Extracellular peroxidase, esterase, and phenoloxidase enzymes [34], as well as manganese peroxidase system [35] have been observed to participate in the degradation process of coals. Several fungi groups demonstrate the ability to metabolize organic carbon and use biochar as the one and only source. In addition, aromatic structure in biochar can be oxidized by hydroxylase with the presence of fungi [36]. Evidence shows that bacteria in soil can produce hydrogen peroxide during microbial action [37]. Such researches prove that biochar is not biologically stable and can be involved in microbial degradation.

Incubation experiments with carbon isotopic labelling is an approach to study biotic degradation.

However, biotic degradation of biochar is a very complicated process and influenced by a variety of factors, including production methods, soil properties, and incubation conditions [33]. Besides feedstocks and pyrolysis parameters of the production of biochar, the functional groups on the surface, the surface area, the porous structure, the morphology of biochar are crucial as well. Environmental conditions also play crucial roles in biologically decomposing biochar, such as temperature, moisture, the type and activity of enzymes, etc. In addition, C/N (sometimes C/N/P) ratio is remarkably indispensable in analyzing the biotic degradation process because those nutrient elements are also essential in biological processes and the cycling of the ecosystem [10]. A comment to add is that incubation experiments cannot separate biotic degradation and abiotic degradation.

2.3 Abiotic oxidation mechanism

Most chemical oxidation in the soil is caused by free radicals. Researchers show that carboxylate carbon undergoes degradation in the environment with the existence of H

O

. It can be catalyzed by light and/or metallic elements and generate reactive oxygen species (ROS), including singlet oxygen (

O

), superoxide anion, and hydroxyl radicals (OH) in solar light [38]. It has been demonstrated that singlet oxygen (

O

) released by photocatalytic can oxidize a wild variety of organics, including aromatic hydrocarbons and pronuclear aromatic nitrogen heterocycles.

OH radical is more reactive than oxygen atoms to graphite at 298K and produce CO/CO

, and the reaction of which can be catalyzed by carbon itself [39]. The hydroxyl radical can add to the carbon- carbon bond in the aromatic conjugated system, and then lose hydrogen to form alcohol or ketone [40].

The oxidation degradation is closed influenced by the physical and chemical properties of biochar, the

surface morphology of biochar, the particle size, the chemistry of soil, temperature, and other

environmental conditions.

9

2.4 Accelerating aging method

In order to understand the aging behavior of biochar in the environment, many researches about chemical accelerating aging methods have been conducted to simulate the natural oxidation in the laboratory using oxidants. Hydrogen peroxide, nitric acid, (NH

)

S

O

, K

Cr

O

or KMnO

are the most common systems. Comparisons of chemical agents are collected in Table 2-1.

Table 2-1 Summary of chemical accelerating aging method.

Accelerating aging

system Feedstocks (pyrolysis temperature(℃)) Advantages Disadvantages

H2O2

Oak, Applewood branch, rice residues, straw and husk(400,500,600,800)[42];

Corn stalk(350,500,650)[54];

mannan-rich ivory nut(350,500)[55];

Sugarcane bagasse,green waste(350,450,550), sugarcane trash(550),wheat chaff, oil mallee, poultry manure(450,550)[19];

Pine, rice husk, wheat straw (350,450,550,650)[12];

pine, mized larch and spruce wood chips, softwood(350,450,550)[56];

corn stalk[47](250,500,650) rice straw[46]((200,300,400,500);

sewage sludge(300,600)[57]；

peanut shell, bamboo, saw dust, reed stalk, furfural residues, seaweed degumming residues and Enteromorpha prolifera(500)[58]

Strong oxidation ability;

Similar oxidation mechanism to biochar degradation;

No introduction of functional groups of other elements;

Relatively large sample pools

Results sensitive to H2O2

concentration ； Lack of corresponding soil incubation experiment.

HNO 3

Woody charcoal

(300,350,400,450,500,550)[59];

rice straw (350, 500,700)[29];

sapwood (400)[44]

Strong oxidation ability;

Approach to simulate biochar degradation in acid soil

Introduction of nitric group;

No direct relation to biochar stability

NaOH-H 2 O 2 alfalfa meal, maize stover, cellulose(500, 700)[43]

Approach to simulate biochar degradation in neutral or alkane soil

No direct relation to biochar stability

K2Cr2O7

wheat straw (200,250,300,400,500);[48]

pine, poplar, willow (400,550)[45];

bamboo (300,400,500,600,700)[60];

rice straw (200,300,400,500)[46];

corn stalk (250,500,650)[47];

sewage sludge(300,600)[57];

peanut shell, bamboo, saw dust, reed stalk, furfural residues, seaweed degumming residues, Enteromorpha prolifera (500)[58]

Fast (2h);

Results directly correlated with incubation;

Oxidation capacity can be quantified and controlled

Small sample group

KMnO4 pine, poplar,willow (400,550)[45] Rapid reaction Weak oxidation stability;

Small database

(NH4)S2O8 Almond shells[52]

Strong oxidation ability;

Similar oxidation mechanism to hydroxyl radical; Remains original pore structure

No direct relation with biochar stability

2.4.1 Hydrogen peroxide system (Edinburgh stability tool)

Edinburgh stability tool provides a laboratory-based method to assess relatively long-term biochar stability. In this system, 5% H

O

is added to the biochar sample with 0.1g C, and the system is heat at 80℃ for 2 days. Biochar stability is defined as the fraction (Æ) of carbon that resists the oxidation treatment: Æ =

Bt×BtC

, where BrC and BtC represent the carbon content in the residual and total biochar respective and Br and Bt the mass after and before the treatment [19].

It is found that 5% H

O

can remove 1.7% carbon in laboratory-produced biochar, and the loss corresponds to 25-161 years in nature based on a range of high and low residence time[19]. The projection from chemical stability and mean residence time can be done in such a way.

But the projection is based on the following assumptions: 1) the laboratory-produced biochar shares the

same or similar structure with natural wildfire charcoal produced at 300-600℃[41]. 2) the accelerating

aging methods can fully simulate the oxidation mechanisms in nature.

The remained carbon after oxidation decreases drastically with more oxidant utilized. Besides, the oxidation ability of H

O

is closely related to the concentration.0.03 mol H

O

can remove more than 80% carbon, while 0.01 mol H

O

only removes around 40% [19]. Which concentration can provide the best analog to natural oxidation still remains to be investigated.

Though the abovementioned considerations, it is expected to be a reliable method to quantify and compare biochar stability in neutral soil. Correlations with other indexes are investigated in a comparative study, where the Edinburgh tool biochar stability (Æ) demonstrates a tight correlation with C/H (r=-0.78), VM(r=-0.88), C/O(-0.64) and pyrolysis temperature(0.74) [42].

2.4.2 NaOH and H

O

20g biochar sample is added to 50mL 1M NaOH solution, 1mL 30% H

O

is then added to the system once a week. The duration lasts for 4 months[43].

NaOH provides an alkaline aqueous condition for the following reactions to occur and form singlet oxygen as the main oxidant (Fe as an example for electron acceptor) [43]:

H–O–O

+ Fe

→ H–O–O• + Fe

H–O–O• + Fe

+ H

O → O–O: + H

O

+ Fe

H–O–O–H + Fe

→ Fe

+ H–O• +

O–H

Singlet oxygen can oxidize both aliphatic and aromatic carbon for its high reactivity. This mechanism has been proved to be one of the main biochar aging mechanisms in the environment as mentioned above, which makes it a promising method to assess biochar stability. But the relationship between carbon loss after oxidation and mean residence time has not been built yet.

2.4.3 HNO

HNO

is utilized as a stronger oxidant to biochar compared with H

O

[44], and it has been wildly used as a surface modifier for biochar to improve the absorption capability of heavy metals.

HNO

in combination with H

SO

was used to simulate the aging process in acidic environments (acid rain), because NO

and SO

were the main anions in acid rain in China[29]. But new functional groups might be introduced while oxidizing. Nitronium ions formed in nitric acid mixed with concentrated sulfuric acid can enter the carbon matrix by electrophilic substitution of the hydrogen[29].

It would cause increasing difficulty in measuring C/N ratio during the aging process, which would be a reason why it is not generally utilized for measuring biochar stability.

2.4.4 K

Cr

O

Using potassium dichromate in sulfuric acid as an oxidant is a well-accepted method to remove labile carbon and thus measure black carbon content in soil and in nature[31]. Nonetheless, the use of potassium dichromate has long been questioned for the following two reasons: (1) recalcitrant carbon could be underestimated because aromatic carbon is attacked by chemical oxidation[31]; (2) and it could be overestimated because some fraction of labile carbon is prevented by resistance to acid[45]. It has been reported that treatment with K

Cr

O

removes more fraction of carbon than with H

O

[46, 47].

The following method was then improved to build a correlation between oxidation and incubation in soil[48]. Two groups of biochar samples are used, one is labeled with

C, and the other is not. 50-80mg unlabeled biochar sample is oxidized in 40mL 0.1M K

Cr

O

/H

SO

(various concentration) system, heated at 100 ℃ for 2h. The oxidation capacity can be controlled by adjusting the concentration of H

:

Reaction: Cr

O

 + 14H

+ 6e

  ⇋ _2Cr

+ 7H

O Nernst equation: E = 𝐸

−

𝑛𝐹

ln (

2

𝑐(𝐶𝑟₂𝑂₇²⁻)𝑐(𝐻⁺)¹⁴

)

The

C labeled sample is utilized for incubation and directly simulate mineralization of biochar in the

soil at 1-year scale. The correlation between the cumulative biochar-derived CO

-C emission and

11

incubation time has been built, from which the fraction of mineralized biochar carbon can be extrapolated.

Results demonstrate that the oxidized biochar by 0.1 M K

Cr

O

/0.1M H

SO

solution for and 2-h oxidation achieved the best match with potentially mineralized CO

-C in the soil at a 100-year scale (n=3, R=0.9959, RMSE = 2.53, RD = 15.3).

It is believed to be a simple and fast method to predict biochar stability based on oxidation results. But only wheat straw derived biochar has been tested. Thus, the sample group is too small. Whether it remains credible for biochar derived from other feedstocks, such as wood, manure, algae, etc., still needs further examination.

2.4.5 KMnO

KMnO

has also been used to remove labile carbon and investigate the stability of wood-derived biochar, according to the methodology proposed by Tirol-Padre and Ladha[49]. KMnO

demonstrates a strong correlation with C/H ratio (0.91) and tight correlation with thermal analysis stability (0.6), while K

Cr

O

is strongly correlated with C/O ratio (r=0.95) and thermal analysis stability(r=0.85) in comparison [45].

In terms of oxidizing ability, some labile fraction of carbon, such as sugar, amino, organic carbon acid, and compounds containing glycol groups, can be rapidly oxidized within 1 h in KMnO

. But cellulose is found to be decomposed by soil microbial enzymes, while not oxidized by KMnO

. Such difference challenged the reliability of KMnO

as a measure of labile carbon[49]. Carbon oxidized by KMnO

can only be referred to as permanganate-utilizable C (POC) when assessing biochar stability[49].

2.4.6 (NH

)

S

O

(NH

)

S

O

is considered one of the strongest oxidation agents for its strong and non-selective oxidation ability for organic matters. One of the oxidation mechanisms is as blow with the standard oxidation- reduction potential of 2.1V: S

O

+ 2H

+ 2e

=> 2 HSO

. Another possible mechanism is to release sulfate radicals (SO

·) (E=2.6V) by the activation to persulfate, which provides similar oxidation ability and oxidation mechanism to hydroxyl radical[30].

(NH

)

S

O

in H

SO

1M (1 g of carbon/10 mL of solution) has been used to substitute the HNO

method to provide an acid condition for its more robust ability to produce acid groups[50]. Additionally, oxidation with (NH

)

S

O

does not destroy the original pore structure and surface area in biochar, which can ensure the accuracy of the following assessments, such as absorption capability.

2.5 Comparison in products surface chemistry

Both biotic and abiotic oxidation initially occurs as biochar surface and gradually go into the interior [32]. Utilizing various oxidant approaches for biochar oxidation can cause specific changes in surface chemistry. Comparison between gas and liquid phase oxidation demonstrates that oxidation in the liquid phase results in a higher concentration of carboxylic acids on biochar surface, while oxidation in the gas phase results in an increase of hydroxyl and carbonyl groups[51].

Most chemical accelerating aging methods are conducted in liquid phases, where an increase in surface

acidity due to the formation carboxyl group has been observed, together with lactone, phenol, carbonyl,

hydroquinone, Quinone-like carboxyl-carbonate structures[50]. Generally, carboxyl groups are fixed

onto the biochar surface after oxidation with H

O

, (NH

)S

O

, and HNO

, while ketone and ether

groups are also detected in biochar treated with HNO

[52]. In addition, the nitrogen element is

introduced onto the surface during oxidation for nitro-oxygen bond (‒O‒NO

), and the NO

bond is

detected by FTIR[50]. Treatment with KMnO

would cause strong hydrogen bonds arisen from

hydroxyl and carbonyl groups. Those surface functional groups influence biochar hydrophilicity,

solubility in soil, absorption capability, which can further influence the interaction with microorganisms[32].

As for surface morphology, (NH

)S

O

can remain the original pore structure, while treatment with NaOH-H

O

and HNO

-H

SO

increase the surface area drastically (126% and 226% respectively)[53]

and generates more mesopores. Such an increase surface area could provide a possible reaction bed for microorganisms during biotic degradation[53].

2.6 Summary

In conclusion, H

O

and K

Cr

O

are two effective chemical oxidants with strong oxidation ability for

biochar. They could probably be a powerful tool to predict biochar stability, but the robustness of these

methods should be further justified.

13

3 Experiments

3.1 Raw materials (biochar)

Biochar produced from miscanthus and seaweed is used in this project. Miscanthus is able to absorb CO

in the atmosphere, and the liquid and solid pyrolyzed products have the possibility to be used as biofuels and carbon sink. In addition, it is one of the rich feedstocks in Sweden. On the other hand, macroalgal communities in Sweden and around the world capture and store a great amount of carbon, which contribute to “blue carbon” and have great potential to mitigate climate change through long- term carbon sink. Production and utilization of biochar pyrolyzed from miscanthus and seaweed can be of great economic and environmental value. However, limited information about its biochar stability in soil has been provided.

Miscanthus biochars pyrolyzed at 350℃, 450℃ and 550℃ (marked as M350, M 450, M550) and seaweed biochar pyrolyzed at 400℃, 500℃, 600℃ (market as S400, S500, S600) have been used to study the effects of pyrolysis temperature (highest treatment temperature, known as HTT) on biochar stability.

The biochar used for gas emission analysis is provided by Envigas AB, Sweden. The biochar was produced from softwood at a pyrolysis temperature of 550 °C.

3.2 Method

First, biochar is treated with chemical oxidation. Afterward, solid, liquid, and gas products are qualitatively and quantitatively analyzed to assess biochar stability. The experimental flow chart is shown in Figure 3-1.

Figure 3-1 Experimental flow chart.

3.2.1 Chemical oxidation to determine biochar stability

The chemical oxidation method is used to determine biochar stability as demonstrated in Figure 3-2.

Biochar samples were dehydrated in a desiccator for 1 hour. Each biochar sample containing 0.1g C was weighted (𝑤

) and added into a test tube and 7mL 5% H

O

solution is then added to the test tube.

The test tube was stirred for 2 h until the biochar sample was sufficiently immersed and became suspension. Afterward, the test tube was heated to 80 °C in water-silicone oil bath and held for 48 hours.

After 48 hours of chemical oxidation, the test tube was dried in desiccator overnight and then

reweighted (𝑤

). 10 mL deionized water was added into the test tube and stirred to dissolve the residual

sample. The insoluble oxidized residue was separated from the solution by filtration, and thereafter,

dried and weighted(𝑤

).

The analysis of soluble, insoluble and gas products provides a comprehensive understanding of biochar stability in application, in terms of the interaction with environments and footprints of carbon cycle.

The fraction of solid residues is directly related to biochar stability, representing the recalcitrant compounds that resist decomposition. The soluble organics would gradually leak into soil with the existence of water, the composition of which would influence the properties of soil. Besides, the gas products during decomposition would reenter the atmospheric circulation.

Figure 3-2 Experiment setup.

Biochar stability of each element is calculated as below (carbon as an example):

carbon stability =

1×𝑤₁𝐶

× 100%, (4)

Where 𝑤

and 𝑤

𝐶 is the weight and carbon content in biochar before oxidation, 𝑤

and 𝑤

𝐶 is the weight and carbon content in biochar residues after oxidation.

3.2.2 Elemental analysis

The concentration of C, N, H, S is determined by the Elemental Analyzer EL Cube Vario (Elementar, Germany). Both biochars before oxidation and solid residues after oxidation are analyzed.

3.2.3 Liquid products analysis

Liquid products were analyzed with a GC/MS instrument (Agilent 7890A GC coupled with Agilent

5975C MS). The column is DB1701. The GC program was set as 1) started holding time at 45°C for 5

minutes.2) heat to 70°C by 3°C/min heating rate, hold for 5 minutes. 3) heat to 90 °C with a heating

rate of 3 °C/min and hold for 5 minutes. 4) heat to 250 °C with a heating rate of 3 °C/min and hold for

5 minutes. The atom mass unit range was set from 45 to 500. Chemstation was used incorporation with

NIST11 to identify the peaks and calculate the peak area.

15 3.2.4 Online gas analysis

For gas analysis and online product detection, the Agilent 490 micro-GC has been employed. This micro-GC consists of 4 columns and thermal conductivity detectors. The calibration was done for CH

, C

H

, C

H

, C

H

, C

H

, CO, CO

, H

, N

and O

.

3.3 Experimental Plan

The experiental plan Table 3-1 Experimental plan.is consisted of three sample groups: miscanthus- derived biochar, seaweed-derived biochar and biochar purchased from Envigas.

Table 3-1 Experimental plan.

Feedstock Pyrolysis temperature (℃)

Elemental

analysis GCMS Online gas

analysis

Miscanthus

350, 450, 550

(M350, M450, M550)

 

Seaweed

400, 500, 600 (S400, S500, S600)



Biochar from

Envigas (BE)  

4 Result and discussion

4.1 Biochar stability influenced by feedstocks and temperature

4.1.1 Solid residue

The elemental analysis of biochar pyrolyzed from miscanthus before and after oxidation is presented in Table 4-1 and from seaweed is presented in Table 4-2. One thing to mention is that above 90% mass fraction of M350 is removed by oxidation, and solid residue after filtration is too little to collect and analyze. This result corresponds to the similarity of thermal degradation behavior between M350 and miscanthus biomass, where the degradation begins at 250 °C and ends at 400 °C with 70% mass loss [61]. It is assumed that at 350 °C pyrolysis temperature the carbon in biochar is hardly to be transformed into the aromatic structure and corresponds to be of little practical value as a carbon sink.

Table 4-1 Elemantal analysis of miscanthus biochar before and after oxidation.

Temperature Mass Fraction

N [%] C [%] H [%] S [%] O+Ash [%]

H/C atomic ratio

N/C atomic ratio

O/C atomic ratio

S/C atomic ratio

350 °C Raw 100% 0.5200 77.5000 3.8000 0.0600 18.0000 0.5884 0.0058 0.1742 0.0003

Filtrate 6.16% NA NA NA NA NA NA NA NA NA

Elemental

Storage rate

NA NA NA

450 °C Raw 100% 0.5800 82.1000 3.1000 0.0700 14.2000 0.4531 0.0061 0.1297 0.0003

Filtrate 35.57% 0.3200 59.0700 3.1700 0.0000 37.4400 0.6440 0.0046 0.4754 0.0000

Elemental Storage rate

19% 26% 2%

550 °C Raw 100% 0.6100 86.9000 2.3000 0.0700 10.1000 0.3176 0.0060 0.0872 0.0003

Filtrate 81.47% 0.7600 77.5500 2.7100 0.0300 18.9500 0.4193 0.0084 0.1833 0.0001

Elemental Storage rate

101% 73% 38%

17

Table 4-2 Elemantal analysis of seaweed biochar before and after oxidation.

Temperature Mass Fraction

N [%] C [%] H [%]

S [%] O + Ash [%]

H/C atomatic ratio

N/C atomatic ratio

O/C atomatic ratio

S/C atomic ratio

400℃ Raw 100% 3.37% 40.61% 2.17% 0.99% 52.87% 0.6420 0.0710 0.9764 0.0091

Filtrate 44.05% 3.22% 35.67% 2.37% 0.15% 58.59% 0.7973 0.0774 1.2319 0.0016

Elemental Storage rate

42.15% 38.69% 6.71%

500℃ Raw 100.00% 2.87% 38.00% 1.45% 1.11% 56.57% 0.4579 0.0647 1.1165 0.0109

Filtrate 53.47% 2.49% 31.46% 1.51% 0.38% 64.16% 0.5776 0.0678 1.5295 0.0045

Elemental Storage rate

46.39% 44.27% 18.32%

600℃ Raw 100% 2.33% 32.69% 0.84% 0.76% 63.39% 0.3069 0.0611 1.4546 0.0087

Filtrate 46.41% 2.99% 41.72% 1.34% 0.98% 52.97% 0.3856 0.0614 0.9523 0.0088

Elemental Storage

rate 59.49% 59.24% 60.11%

Carbon stability of biochar in this project is compared to that of biochar pyrolyzed from sugarcane bagasse and green waste at 350°C, 450°C, 550°C [19] (Figure 4-1). Miscanthus-derived biochar outperforms the other two biochar at 550°C, while it largely underperforms at 350°C and 450°C.

Miscanthus-derived biochar pyrolyzed at high temperatures demonstrate the potential as a carbon sink.

On the other hand, seaweed biochar demonstrates less sensitivity to temperature.

Figure 4-1 Carbon stability of biochars pyrolyzed from miscanthus, sugarcane bagasse, green waste[19].

Correlations between elemental ratios and biochar stability are presented in Figure 4-2 and Figure 4-3.

Ultimate analysis demonstrates that H/C ratio has a negative linear relationship with temperature, with R

larger than 0.9 both in miscanthus biochar and seaweed biochar. H/C ratios of M350, M450, M550, S500, S600 are below 0.6 and satisfy the threshold value proposed by the European Biochar Certificate (EBC) [21]. O

/C ratio is not analyzed in this project, because oxygen content cannot be separated from ash content by Elemental Analyzer.

In miscanthus biochar, N/C ratio and S/C ratio have no significant change within pyrolysis temperature from 350℃-550℃, and they are not directly related to carbon stability (Figure 4-2). However, in seaweed biochar, N/C ratio demonstrates a negative correlation with biochar stability with R

=0.84 (Figure 4-3). The difference implies that nitrogen would exist in different forms in biochar.

In miscanthus biomass, nitrogen mainly exists in peptide bonds, and these results demonstrate that N and S cannot be removed in the pyrolysis process, but gradually transform into N-, S- containing aromatic structure with temperature increasing.

0 10 20 30 40 50 60 70 80

300 350 400 450 500 550 600 650

biocharstability(%)

Temperature (℃)

miscanthus sugarcane bagasse green waste seaweed

19

Figure 4-2 Correlation between biochar stability H/C, N/C, S/C ratios in miscanthus biochar.

Figure 4-3 Correlation between biochar stability of H/C, N/C, S/C ratios in seaweed biochar.

N/C ratio of M450 decreases after oxidation, revealing that the majority of nitrogen in M450 is connected to labile carbon and is oxidized in the treatments (Figure 4-4). On the contrary, the N/C ratio

R² = 0.9733 R² = 0.2724

R² = 0.0581

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.0000 0.2000 0.4000 0.6000 0.8000 1.0000

N/C, S/C atomic ratio

biocharstability(%)

H/C atomic ratio

H/C ratio of miscanthus biochar N/C ratio of miscanthus biochar S/C ratio of mscanthus biochar

R² = 0.904

R² = 0.1696

R² = 0.8402

0 0.02 0.04 0.06 0.08

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

N/C, S/C atomic ratio

biocharstability(%)

H/C atomic ratio

H/C ratio of seaweed biochar S/C ratio of seaweed biochar N/C ratio of seaweed biochar

of M550 increases after oxidation, and the nitrogen stability is close to 100%. N/C ratio in seaweed biochar is very much higher than that of miscanthus biochar and increase after oxidation regardless of the pyrolysis temperature.

It is assumed that the majority of nitrogen element in miscanthus biochar is converted into or connected to aromatic structures as side chain, because nitrogen in the form of side chain is expected to have little influence to biochar stability. On the contrary, considering the negative correlation between N/C ratio and biochar stability in seaweed biochar as shown in Figure 4-3, nitrogen in seaweed biochar would exist as heterocycles. It is found that N in the forms of heterocycles would modify the electronic band and provide active sites for oxidation reaction, thus reduce the chemical resistance[10, 62].

S/C ratios for miscanthus biochar and seaweed biochar share similar characteristics (shown in Figure 4-5). It can be spectulated that, the most of sulfur-containing groups when pyrolyzed at low temperature are connected to labile carbon and removed in oxidation, while an increasing fraction of them exists as heterocycles and survives oxidation when enhancing pyrolysis temperature.

Figure 4-4 Comparison of N/C atomic ratio before and after oxidation.

0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900

M450 M550 S400 S500 S600

N/Cratio

N/C ratio before oxidation N/C ratio after oxidation

21

Figure 4-5 Comparison of S/C atomic ratio before and after oxidation.

4.1.2 Soluble organics

The water-dissolved filtrates of M350, M450, M550 are collected and shown in Figure 4-6. Obvious color gradient from brawn to transparent with increasing temperature can be observed. Only Acetic acid has been detected in M350, M450 by GC-MS. The absolute peak area of acetic acid in M350 is larger than it in M450.

A note to be mentioned is that the GCMS results cannot explain the difference of filtrate colors caused by various pyrolysis temperature. Due to the mechanisms and limitations of GCMS, it only applies to those small-molecular, volatile, thermal stable compounds. In order to fully investigate other possible substances in filtrate, which are likely to be involatile and macromolecular, another solution should be developed to analyze the soluble organics.

Figure 4-6Liquid products dissolved in deionized water after oxidation. From left to right:

550℃,450℃, 350℃.

0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120

M450 M550 S400 S500 S600

S/Cratio

S/C ratio before oxidation S/C ratio after oxidation

4.2 Online gas Analysis

The online gas analysis results of BE is presented in Figure 4-7. Only O

and CO

have been detected during the oxidation process. O

undergoes rapidly release initially, reaches its climax after approximately 24 minutes of oxidation, and gradually decreases to the level below detectable limit at 100 minutes. O

is released by the decomposition of H

O

and it represents the decomposition rate of H

O

. It reveals that the activation and the effectiveness of H

O

mainly occur within the first hour. The curve of CO

demonstrated similar characteristics as that of O

with its maximum value at 21 minutes after oxidation. It’s suggested that the oxidation of biochar is synchronized with the decomposition of the oxidant.

However, a small amount of CO

continues to release for 5 hours, which is obviously more extended than the decomposition of H

O

. The slow oxidation of biochar may be caused by the minute quantity of oxidant dissolved in the solution. It is believed that the oxidation process ceased due to the fact that the oxidant has been depleted. But whether the oxidant is sufficient to fully react with the labile carbon needs to be further verified through repeated oxidation experiments.

Figure 4-7 10h online gas analysis of biochar oxidation.

4.3 Discussion on scientific, social and environmental sustainability of biochar as carbon sink

Carbon sink with miscanthus and seaweed biochar is expected to be a cost-effective approach for carbon negative emission. M550 reaches the highest biochar stability of 73%, surpassing sugarcane bagasse and green wastes biochar pyrolyzed at the same temperature. It proves that miscanthus biochar can store carbon in a stable way and demonstrates its high potential for carbon sink. On the other hand, seaweed biochar can serve as a soil amendment due to the high nitrogen content. It is more suitable for carbon sink in nitrogen-poor soil.

Besides, biochar carbon sink is economically feasible in terms of production costs and energy costs.

The feedstocks of miscanthus and seaweed are cheap, accessible and environmental friendly. And the

0 20000 40000 60000 80000 100000 120000 140000

0 2000000 4000000 6000000 8000000 10000000 12000000 14000000 16000000

0 200 400 600 800

CO₂peak intensity O₂peak

intensity

Time/minutes

O2 CO2

23

pyrolysis of biochar can be manufactured using off-the-shelf equipment in many power plants, which can greatly reduce the initial investment in production facilities.

From the market side, the major power supply for industrial processes comes from the use of fossil fuels due to technological and economical limitations. As a result, industrial sectors are account for approximately 40% CO

emissions of total CO

emissions around the world, mostly from traditional industries, i.e. iron and steel, cement, chemicals. In order to achieve the net zero carbon emission, related companies are obligated to purchase equal amount of carbon negative emission. One thing to be noticed is that accessible and cheap negative emission approach would decrease the internal motivation for developing sustainable technologies to reduce carbon emissions. Thus, it is suggested that specific measures should be taken to encourage reduction in positive emissions, at the same time, regulations should be formulated to limit the maximum amount of negative emission that companies can purchase.

4.4 Future work

This project assessed miscanthus-derived and seaweed-derived biochar stability by chemical accelerating the aging method. In this project, oxidation treatments are conducted with 5% H

O

for 48h. Although this method is wildly used to investigate biochar stability, as a newly proposed one, some challenges remain to be solved to improve accuracy and precision.

1. Due to the fact that H

O

undergoes rapid decomposition initially and the concentration decreases drastically within the first hour, measures should be taken to ensure sufficient oxidant for oxidizing labile carbon. For example, oxidized residues can be added to 5% H

O

solution for repeated oxidation.

2. Due to the limitations of GCMS, soluble organics should be further analyzed with another approach.

3. The biochar stability measured with H

O

should be cross-checked with the results obtained from other assessment methods, for example, R

, oxidation with another chemical agent (K

Cr

O

, KMnO

, etc.), incubation and modeling.

4. A life cycle assessment of the process to produce biochar production as carbon sink should be

made.