Influence of Biochar Addition on Nitrogen Transformation during

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Energy and the Environment

Influence of biochar addition on nitrogen transformation during co-pyrolysis of algae and lignocellulosic biomass Wei Chen, Haiping Yang, Yingquan Chen, Kaixu Li, Mingwei Xia, and Hanping Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02485 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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Environmental Science & Technology

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[Title Page]

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Influence of biochar addition on nitrogen transformation during co-pyrolysis of

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algae and lignocellulosic biomass

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Wei Chen, Haiping Yang*, Yingquan Chen*, Kaixu Li, Mingwei Xia, Hanping

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Chen

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State Key Laboratory of Coal Combustion, School of Power and Energy

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Engineering, Huazhong University of Science and Technology, 430074 Wuhan,

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China

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E-mail: [email protected], [email protected], [email protected],

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[email protected], [email protected], [email protected].

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Correspondence information: Haiping Yang, [email protected]; 1037 Luoyu

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Road, 430074 Wuhan, P. R. China; Yingquan Chen, [email protected];

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1037 Luoyu Road, 430074 Wuhan, P. R. China; Tel: +086+027-87542417-8109; fax:

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+086+027-87545526.

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ACS Paragon Plus Environment


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Influence of biochar addition on nitrogen transformation during co-pyrolysis of

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algae and lignocellulosic biomass

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Wei Chen, Haiping Yang*, Yingquan Chen*, Kaixu Li, Mingwei Xia, Hanping Chen

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State Key Laboratory of Coal Combustion, School of Power and Energy Engineering,

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Huazhong University of Science and Technology, 430074 Wuhan, China

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Abstract

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Algae are extremely promising sustainable feedstock for fuels and chemicals, while

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they contain high nitrogen content, which may cause some serious nitrogen emission

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during algae pyrolysis utilization. In this study, we proposed a feasible method to

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control the nitrogen emission during algae pyrolysis by introducing lignocellulosic

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biomass and biochar addition. Nitrogen transformation mechanism was investigated

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through quantitative analysis of N-species in the pyrolysis products. Results showed that

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co-pyrolysis of algae and lignocellulosic biomass greatly increased nitrogen in solid char

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with large amount of NH3 and HCN releasing (~20 wt.%), while nitrogen in bio-oil

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decreased largely. With biochar addition, NH3, HCN, and N-containing intermediates

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(amines/amides and nitriles) reacted with higher active O-species (O-C=O, -OH, and –

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COOH) in biochar addition, and formed large amounts of amine/amide-N, pyridinic-N,

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pyrrolic-N, and quaternary-N on the surface of biochar addition, which led to most

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nitrogen being enriched in char product and biochar addition (over 75 wt.%) at the

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expense of amines/amides, nitriles, and N-containing gas (only 3 wt.% NH3 and HCN

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emission; decrease of 85%). These results demonstrated that biochar addition showed a

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positive influence on fixation of N-species during thermochemical conversion of algae

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and could convert nitrogen to valuable N-doped biochar materials.

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Keywords: Nitrogen emission control; Biochar addition; Co-pyrolysis; Algae;

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Lignocellulosic biomass.

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1. Introduction

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The emission of greenhouse and fine particulates caused by fossil fuel consumption

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has led to an urgent need for the development of alternative and renewable resources.1

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Algae are considered as a promising sustainable feedstock for fuels, chemicals, and

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other valuable products due to their high CO2 fixing capability, cultivation on off-shore

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sites or in wastewater, fast growth rates, and nearly all-year-round harvest.2, 3 Algae

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pyrolysis can simply and efficiently obtain bio-fuels with high yield and heating values

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and valuable chemicals; hence, it has attracted increasing attention.4, 5 However, algae

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also contain a large amount of nitrogen (up to 10 wt.%), which might lead to serious

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nitrogen-related environmental issues.6-9 Hence, a feasible technology to minimize

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nitrogen emission is necessary for the better utilization of algae resources.

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Many studies have focused on nitrogen emission control (mainly of NOx precursors)

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during coal pyrolysis.10-15 Tsubouchi et al.10, 12, 15 pointed out that metallic Fe and Ca

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could promote the conversion of nitrogen to N2 through solid-solid interactions during

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coal pyrolysis at about 1000 °C. Yan et al.11 reported that K, Na, Si, and Al could also

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promote conversion of nitrogen to N2 during high-temperature coal pyrolysis. These

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studies were performed at very high temperature (>750 °C), as the main existing forms

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of nitrogen in coal were pyridinic-N, pyrrolic-N, and quaternary-N, all of which had a

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high thermal stability and could efficiently decompose only at high temperature.16-19

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However, greatly different from coal, protein-N is the dominant nitrogen form in algae

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and has lower thermal stability. Therefore, algae pyrolysis is usually performed at lower

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temperature (500-600 °C) for high bio-oil yield.20, 21 Thus, simply adopting the methods

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of coal pyrolysis cannot help nitrogen emission control of algae pyrolysis. Although

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some studies have reported nitrogen transformation during pyrolysis of algae, terrestrial

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biomass, or sewage sludge (protein-N is the major nitrogen form in these as well),

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nitrogen emission control was rarely involved.8, 21-23 Besides, most studies have focused

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on nitrogen emission control via converting nitrogen in coal to N2 (released

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directly).24-26 However, for algae, converting nitrogen to N2 would largely reduce their

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utilization value, as they contain high nitrogen content, much higher than that in coal.7

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If nitrogen can be fixed in solid char products to obtain valuable N-containing char

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materials (which can be widely applied for catalysis, pollutants adsorption, and energy

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storage),6, 27-29 it would not only control the nitrogen emission, but also greatly improve

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the utilization value of algae. However, there have been no reports on nitrogen emission

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control by transforming nitrogen into N-containing char materials yet. Therefore, it is

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necessary to investigate the nitrogen fixing approach and mechanism of nitrogen

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conversion during algae pyrolysis for decreasing nitrogen emission.

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Our previous studies20, 21 found that algae direct pyrolysis only could fix limited

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nitrogen in char product, while large amounts of nitrogen transformed into N-containing

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compounds in bio-oil, leading to problem of removing nitrogen in bio-oil. But we also

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more nitrogen in char product, and decrease nitrogen content in bio-oil, but more

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N-containing gas would be released. While Figueiredo et al.31 pointed out that

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O-containing groups of biochar materials can act as catalytic active sites for reactant

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chemisorption.32, 33 Furthermore, Zhang et al.34 found that biochar could react with

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N-containing gas (NH3) to enrich nitrogen in biochar and form numerous

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N-heterocyclic groups on the biochar surface. This indicated that biochar could control

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nitrogen emission by converting nitrogen to valuable N-containing biochar materials.

found that introducing bamboo waste (Ba) during algae pyrolysis could efficiently fix

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Besides, biochar is the by-product of lignocellulosic biomass fast pyrolysis, which is

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very cheap and could be obtained in large scale.35 However, the influence of biochar on

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the nitrogen transformation during co-pyrolysis of algae and lignocellulosic biomass

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and the related changes in biochar are still unknown. Therefore, it is critical to explore

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the mechanism of nitrogen emission control during algae pyrolysis using biochar in

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depth.

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In this study, we proposed a simple and green method to control nitrogen emission

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during co-pyrolysis of algae and lignocellulosic biomass using biochar addition. First,

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co-pyrolysis of algae and lignocellulosic biomass was performed to enrich more

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nitrogen in char product, and decrease nitrogen content in bio-oil, while releasing more

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N-containing gas; hence, biochar was then added to further trap the N-containing gas to

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realize nitrogen emission control and obtain N-containing biochar materials.

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Specifically, the influence of biochar addition (by-product of Ba fast pyrolysis) on the

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transformation of N-containing species during co-pyrolysis of algae (Spirulina platensis

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(SP), Nannochloropsis sp. (NS), or Enteromorpha prolifera (EP), with different nitrogen

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content) and lignocellulosic biomass (Ba or cellulose (Ce)) was investigated in a

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fixed-bed reactor system. The formation and evolution mechanism of N-containing

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species in solid char, liquid bio-oil, gas products, and added biochar were explored

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quantitatively. The adaptability of the method of nitrogen emission control on different

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algae and different lignocellulosic biomass was also discussed. Further, the possible

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mechanism of biochar addition for nitrogen emission control was proposed, which was

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important to improve the valuable utilization of nitrogen in algae and the green

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utilization of algae.

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2. Materials and methods

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2.1 Materials

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Ba was collected locally. SP and EP were provided by China Agricultural

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University. While NS was from Yantai. Cellulose (Ce) was purchased from

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Sigma-Aldrich Co., Ltd. Biochar addition was obtained from Ba pyrolysis at 600°C.36, 37

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Detailed properties of the materials are provided in SI Table S1.21, 30, 37 N content of SP

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reaches 10.99 wt.%, and that of NS and EP is 6.97 and 1.92 wt.%, respectively, as SP

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contains more proteins 65.2 wt.% than those of NS (40.5 wt.%) and EP (11.8 wt.%).21

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Furthermore, NS contained higher lipids content (30 wt.%), and EP contained much

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higher carbohydrates content (51.4 wt.%). Moreover, nitrogen content of Ba (Ce content

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is 46.5 wt.%) is only 0.27 wt.%.36 Besides, nitrogen content of biochar addition is very

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low (0.31 wt.%), while its oxygen content is higher (6.72 wt.%), and its ash content is

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3.38 wt.%.

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2.2 Co-pyrolysis experiment

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Co-pyrolysis of algae and lignocellulosic biomass was carried out in a fixed-bed

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system shown in Figure 1.30 Before experiment, the feedstock mixed well (~2 g, mass

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ratio of algae and lignocellulosic biomass was 1:1) was put in the quartz basket, which

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was placed at the top of the reactor, while biochar addition (~1 g) was in stage

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the atmosphere was Ar (99.999%, 200 mL/min).30, 37 When temperature reached 600 °C,

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feedstock was promptly placed in stage

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non-condensing gases were absorbed by NaOH (0.2 mol/L) and H2SO4 (0.1 mol/L)

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solutions, respectively.21 Furthermore, to precisely quantify the nitrogen content in

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bio-oil, each experiment was replicated with liquid nitrogen condensing unit to collect

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bio-oil.30

for 30 min.20,

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, and

HCN and NH3

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Char yield was calculated by the solid residue after experiment, while bio-oil yield

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was calculated by the mass increase of liquid nitrogen condensing unit.21, 37 Besides, to

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investigate the effect of co-pyrolysis and biochar addition on nitrogen emission,

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individual pyrolysis and co-pyrolysis of the samples were carried out at the same

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condition. Furthermore, to explore the mechanisms for nitrogen emission control in

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depth, co-pyrolysis of algae and Ce with/without biochar addition was also performed

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under the same experiment condition. Each experiment was performed over triplicate,

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and the mass balance was 97.45-102.33 wt.%.

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2.3 Characterization

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Nitrogen content in feedstock, bio-oil and char product was determined using a

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CHNS/O elementary analyser (Vario Micro Cube, Germany). N and O-containing

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species on the surface of char products and biochar addition were analysed using X-ray

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photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, UK) with an Al Kα (15 kV,

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10 mA, 150 W) radiation source.21

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The main N-species in bio-oil were identified using gas chromatography-mass

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spectroscopy (GC-MS, HP7890 series GC, HP5975 MS detector) with a capillary

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column (Agilent: HP-5MD, 19091s-433; 30 m×0.25 mm i.d.×0.25 µm d.f.).21 The

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detailed method could be found in our previous study.20, 21, 30

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NaOH and H2SO4 solutions absorbing HCN and NH3 (CN- and NH4+, respectively),

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were quantified by ion chromatograph (881 Compact IC Pro, Metrohm, Switzerland).21,

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3. Results and discussion

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3.1 Nitrogen distribution properties in co-pyrolysis products

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Figure 2 shows the nitrogen distribution in co-pyrolysis products of algae and

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lignocellulosic biomass with/without biochar addition. For SP and NS pyrolysis, about

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65 wt.% nitrogen transformed into bio-oil, with ~20 wt.% nitrogen in char product and

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~15 wt.% nitrogen in gas products. EP fixed more nitrogen in char product (42.57 wt.%)

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perhaps due to its higher ash content hindering the nitrogen release.16, 21 Compared with

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individual pyrolysis of SP, co-pyrolysis of SP + Ba promoted the fixation of nitrogen in

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solid char, and nitrogen in gas phase with diminishing of nitrogen in liquid. Moreover,

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co-pyrolysis of NS + Ba and EP + Ba showed similar tendency with SP + Ba, but the

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decrement of nitrogen in bio-oil was much higher (over 10 wt.%).

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With biochar addition, it was found that nitrogen content in gas decreased largely to

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1/2-1/7 (only 3-8 wt.% left), and nitrogen in liquid phase also strongly decreased, but no

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obvious change was observed in solid char nitrogen content. Furthermore, a large

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amount of nitrogen was collected in biochar addition (15-33 wt.%). This might be

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attributed to the interaction between biochar addition and N-containing volatiles

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evolved from co-pyrolysis of algae and lignocellulosic biomass; as a result of this

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interaction, nitrogen was attached to the surface of biochar matrix and fixed down. In

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addition, the lower nitrogen content in algae materials, the more nitrogen was fixed in

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char product and biochar addition, and the lesser nitrogen released. In other words, for

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algae with high nitrogen content, nitrogen emission might be controlled efficiently by

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increasing lignocellulosic biomass content.

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3.2 Release properties of N-containing gas products

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NH3 and HCN are the two major N-containing gases formed during co-pyrolysis,

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and their release properties are shown in Figure 3. 8.8 wt.% and 5.82 wt.% nitrogen as

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NH3 and HCN were released during SP pyrolysis, respectively. In addition, NS released

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less nitrogen as NH3 (6.72 wt.%) and EP released much more nitrogen as NH3 (10.51

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wt.%). Moreover, HCN release showed a tendency similar with NH3. NH3 was mainly

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derived from labile amino acids cracking in algae, the decomposition of amides and

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amines intermediates, and the reactions of H radicals with N-heterocyclic groups in char

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products.21, 38, 39 HCN mainly resulted from the secondary decomposition of nitriles

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intermediates and N-heterocyclic species in char products.21, 38 Besides, the higher NH3

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yield of NS may be ascribed to the more lipids (30 wt.%) in NS, as NH3 would easily

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react with long-chain fatty acids from lipids decomposition to generate long-chain

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amides.20, 21, 30 With Ba mixing, the amount of NH3 and HCN from co-pyrolysis of SP +

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Ba increased greatly, and the total nitrogen content reached ~24 wt.%. Moreover,

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co-pyrolysis of NS + Ba and EP + Ba showed similar release properties, leading to

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significantly increasing NH3 and HCN yields. This might be because co-pyrolysis of

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algae and lignocellulosic biomass enhanced the cracking of large molecules of

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N-containing intermediates with more N-containing gas formation.21, 30 Furthermore, Ce

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and lignin in Ba promoted the formation of more NH3.23, 40 The increase in NHCN yield

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might be because co-pyrolysis promoted the decomposition of nitriles intermediates.21

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Besides, the increase in N-containing gas yields achieved by co-pyrolysis of NS + Ba

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and EP + Ba was obviously larger than that achieved by co-pyrolysis of SP + Ba. This

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may be ascribed to that carbohydrates in NS (19.2 wt.%) and EP (54.1 wt.%), which

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were much higher than that in SP (12 wt.%), also promoted N-containing gas release by

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providing a low-temperature pathway for amino acid decomposition during

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co-pyrolysis,21, 41 as the structure of the carbohydrates was similar to that of Ce in Ba.42

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With biochar addition, N and NHCN yields decreased largely. For co-pyrolysis of

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SP + Ba, only 5.28 wt.% nitrogen in NH3 and 1.51 wt.% nitrogen in HCN were released.

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Co-pyrolysis of NS + Ba and EP + Ba also showed a similar tendency, and N and

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NHCN yields of the latter EP + Ba were much lower, with only 2.21 wt.% nitrogen in

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NH3 and 1.07 wt.% nitrogen in HCN getting released. This indicated that biochar

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addition could efficiently decrease NH3 and HCN emission during co-pyrolysis of algae

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and lignocellulosic biomass, probably because NH3 and HCN, or their precursors

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(amines/amides and nitriles) reacted with O-containing species in biochar addition to

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form more stable N-containing species.31 Besides, the decrease of HCN was more

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obvious than that of NH3, perhaps because HCN had higher reactivity and could be

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removed more easily. In addition, Ngas yield was slightly higher than that of NH3 and

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HCN. This might be due to inorganic constituents in samples promoting the conversion

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of a little nitrogen to NOx and N2.11, 21

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3.3 Distribution of N-species in bio-oil products

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The main N-species in bio-oil are amines/amides (such as hexadecanamide), nitriles

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(such as hexadecannitrile), and N-heterocyclics (such as pyridines, pyrroles, indoles),

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whose evolution properties are shown in Figure 4. The content of amines/amides

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(11.75%), nitriles (15.54%) and N-heterocyclics (14.37%) from SP pyrolysis was higher

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than that of NS and EP, owing to SP containing more proteins (65.2 wt.%)21. The

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presence of amines/amides was mainly ascribed to the reactions between NH3/NH2*

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from proteins decomposition and long-chain fatty acids intermediates from lipids

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cracking.20, 21 Nitriles mainly originated from the dehydration reactions of amides, and

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cracking reactions of amino acids.21, 22, 43 N-heterocyclics mainly originated from direct

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cracking of some amino acids, and secondary cracking of N-heterocyclic groups in char

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product.21, 44, 45 N-species content from co-pyrolysis of SP + Ba decreased largely from

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41.66 % to 12.53% as compared with N-species content from individual pyrolysis of SP.

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Moreover, compared with SP + Ba, NS + Ba and EP + Ba also presented similar trends,

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that is, the content of all N-containing components decreased sharply. This might be

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because co-pyrolysis inhibited the reactions of NH3/NH2* with long-chain fatty acids to

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generate long-chain amides,30,

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caused an increase of NH3 emission (see Figure 3)). Furthermore, co-pyrolysis fixed

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more N-heterocyclic species in char, and inhibited their release, leading to a decrease of

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N-heterocyclics in bio-oil.21,

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N-containing species in bio-oil obtained from co-pyrolysis of NS + Ba and EP + Ba was

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larger than that obtained from co-pyrolysis of SP + Ba, indicating that carbohydrates

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have significant effects on nitrogen transformation during co-pyrolysis (which inhibited

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the generation of N-containing species in bio-oil). This was consistent with the results

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of N-containing gas release (Figure 3).

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thus decreasing amines/amides content (this also

Besides, it was observed that the decrease of

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Besides, with biochar addition, for three samples, the content of amines/amides,

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and nitriles decreased further; however, N-heterocyclics content increased (reached

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~10%). This might be because amines/amides reacted with O-containing species in

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biochar addition and formed more stable N-containing species (N-heterocyclic species)

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and retained in biochar addition.31 Moreover, nitriles might have higher activity to

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easily react with biochar addition (see Figure 6). N-heterocyclics might mainly originate

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from the cyclization reaction of N-containing intermediates and the secondary

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decomposition of N-heterocyclic species formed in biochar addition. N-heterocyclics

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may mainly come from two aspects: 1) Biochar addition promoted the cyclization

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reactions of N-containing intermediates, and formed some N-heterocyclics in bio-oil.21,

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intermediates by dehydration, decarbonylation, decarboxylation, and cyclization

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reactions, and the pores of biochar addition may also promote the remove of side chain,

As the active O-containing groups in biochar addition would react with N-containing

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thus further promoted the N-heterocyclics formation.30,

34, 36

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decomposition of N-heterocyclic species in biochar addition generated some

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N-heterocyclics. As NH3, HCN and N-containing intermediates would react with

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biochar addition and formed lots of N-heterocyclic species in biochar addition (see

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Figure 6), while some of these N-heterocyclic species may further decompose into some

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N-herterocyclics in bio-oil.21 For example, the secondary decomposition of pyridinic-N,

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pyrrolic-N, and quaternary-N in biochar may form pyridines, pyrroles and indoles,

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respectively.21 Besides, the total content of N-species increased after biochar addition;

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however, Nbio-oil yield decreased (see Figure 2), which might be because of two reasons:

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bio-oil yield decreased largely with biochar addition (the decrease was about 15 wt.%),

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thus leading to the decrease of Nbio-oil yield; and biochar addition might also promote the

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cracking of large-molecule N-containing intermediates to more small-molecule

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N-species, which could be detected by GC-MS, leading to the increase of the relative

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content of N-species.

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3.4 Evolution properties of N-containing species in co-pyrolysis char product

2) The secondary

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To reveal the nitrogen transformation mechanism of co-pyrolysis, the evolution

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properties of N-containing groups (pyridinic-N (398.5 ± 0.3 eV), protein-N (399.8 ± 0.3

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eV), pyrrolic-N (400.5 ± 0.3 eV), and quaternary-N (401.2 ± 0.3 eV)) in solid char were

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explored by XPS,21, 30 as shown in Figure 5 and SI Figure S1. It should be noted that, to

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better understand the evolution mechanism of N-containing groups, the absolute yield

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of N-containing groups was determined from nitrogen yield in char product and relative

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content of N-containing groups on the surface of char product. That is, the distribution

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of N-containing groups in the bulk of char product was considered the same as that on

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the surface of char product; since the nitrogen distribution in feedstock was uniform. It

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was found that only a small amount of protein-N was retained in char products from SP,

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NS, and EP pyrolysis, while large amounts of pyridinic-N, pyrrolic-N, and quaternary-N

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generated, which mainly resulted from the deamination or dehydrogenation reactions of

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amino acids.21 Compared with SP pyrolysis, co-pyrolysis of SP + Ba greatly increased

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pyridinic-N, pyrrolic-N, and quaternary-N yields, but decreased protein-N yield.

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N-containing species obtained by the co-pyrolysis of NS + Ba and EP + Ba showed

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similar tendencies to that obtained from the co-pyrolysis of SP + Ba. Furthermore, for

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all samples, pyridinic-N and quaternary-N were the major N-containing species and

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both of them accounted for about 8 wt.%, 10 wt.%, and 15 wt.%, respectively.

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Protein-N decreasing may result from that Ce in Ba, providing a low-temperature

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pathway for amino acid cracking in algae, and contributed to more pyridinic-N

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generation.21, 47 Besides, the O-containing species in Ba might directly react with some

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amino acids (such as leucine, lysine, proline, and arginine) to form large amounts of

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pyridinic-N and pyrrolic-N by dehydration, dehydrogenation, or decarbonylation

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reactions (Maillard reactions), followed by more pyridinic-N converting to

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quaternary-N.21, 30 Moreover, N-species content in char product from the co-pyrolysis of

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EP + Ba was much higher owing to the hindrance in nitrogen release from ash in EP.

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Further, after biochar addition, the N-containing species in solid char from co-pyrolysis

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showed no obvious change; thus, the results are not presented here.

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3.5 Structure evolution of biochar addition

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To further explore the influence of biochar addition on co-pyrolysis of algae and

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lignocellulosic biomass, N-containing and O-containing species in biochar addition

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before or after used are shown in Figure 6. Here, the N-containing groups and

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O-containing groups in biochar addition are only shown as relative content, but not

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absolute yield, as their distribution in the bulk of biochar addition after use may not be

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the same as that on the surface of biochar addition, because the reactions of biochar

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addition with pyrolytic intermediates may mainly occur on the surface of biochar

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addition. Regarding biochar addition before use, nearly no N-species were observed;

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however, it was rich in O-containing groups. There were large amounts of C=O

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(531.0-531.9 eV), O-C=O/-OH (532.3-532.8 eV), O-C=O/C-O (533.1-533.8 eV),

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-COOH (534.3-535.4 eV) in biochar addition (Figure 6a and SI Figure S2).37, 48, 49 This

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indicated that the biochar addition in this study had higher activity.31, 32

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After use, many N-containing species appeared on the surface of biochar addition

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(Figure 6c and SI Figure S3, there was no N-containing groups in biochar addition

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before used), which not only contained pyridinic-N, pyrrolic-N, and quaternary-N, but

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also amine/amide-N (400.0±0.3 eV);34 however, there was no protein-N. Figure 6d

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shows pyridinic-N was the dominant component in all biochar addition (over 40%).

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Furthermore, combined with nitrogen yield in biochar addition (Figure 2), it was found

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that N-containing species in biochar addition increased greatly with decreasing nitrogen

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content in the feedstock (nitrogen yield in biochar addition increased), which may be

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ascribed to the complete reaction of N-containing intermediates with biochar addition

325

for low-nitrogen feedstock. Accordingly, a significant change was also observed for

326

O-containing species. It can be seen that O-C=O/-OH, O-C=O/C-O, and -COOH

327

content reduced greatly, while the content of C=O-containing species increased

328

significantly. This might be attributed to: the reactions of O-C=O and -COOH in biochar

329

addition with N-containing intermediates (amines/amides or NH3) to generate amide-N

330

(R-CO-NH2) through dehydration reactions (Equation (1));37 reactions of -OH in

331

biochar addition with N-containing intermediates to form amine-N (R-NH2) by

13


Page 14 of 32

Page 15 of 32


332

dehydration reactions (Equation (2)); and possible reactions of -OH with nitriles or

333

HCN (they contained C ≡ N, which had higher activity and could easily undergo

334

addition reactions with biochar addition) to form amide-N (Equation (3)).30, 36 These

335

reactions could significantly increase the content of C=O and amine/amide-N, while

336

largely decrease that of O-C=O/-OH, O-C=O/C-O, and -COOH, thereby decrease the

337

nitrogen emission efficiently; this was consistent with the results of NH3 and HCN (see

338

Figure 3). Furthermore, most amine/amide-N would further convert to the more stable

339

pyridinic-N and pyrrolic-N through cyclization reactions (Equation (4), and some

340

pyridinic-N could further transform into quaternary-N (Equation (5)).27, 30 Meanwhile,

341

some pyridinic-N and pyrrolic-N may also undergo secondary cracking reactions and

342

transform into bio-oil, leading to an increase of N-heterocyclics in bio-oil (see Figure

343

4).

344

(R − NH )/NH + (O − C = O)/(−COOH) → (R − CO − NH ) + H O (1)

345

(R − NH )/NH + (−OH) → (R − NH ) + H O (2)

346

(R − C ≡ N)/HCN + (−OH) → R − CO − NH (3)

347

(R − CO − NH )/(R − NH ) → (pyridinic − N)/(pyrrolic − N) + CO (4)

348

pyridinic − N → quaternary − N (5)

349

3.6 Possible mechanisms of nitrogen emission control

350

To further explore the effect of biochar addition on nitrogen emission, Ba was

351

replaced by Ce (the dominated component in Ba). Nitrogen distribution in the products

352

from co-pyrolysis of algae and Ce with biochar addition is shown in SI Figure S4.

353

Similar to the case of Ba, the co-pyrolysis of algae and Ce also promoted nitrogen

354

fixation in char product significantly and decreased Nbio-oil yield; but increased the

355

release of N-containing gas. After introducing biochar addition, a large amount of

14



356

nitrogen (reached 35.42 wt.%) was trapped by biochar addition, leading to significant

357

decrease of nitrogen in bio-oil and N-containing gas products, especially in the latter

358

(even only 2.4 wt.% release). In addition, the releasing properties of NH3 and HCN are

359

shown in SI Figure S5. Co-pyrolysis of algae and Ce increased N and NHCN yields

360

greatly, while introduction of biochar addition decreased N and NHCN yields sharply,

361

with only 1.31-4.87 wt.% nitrogen in NH3 and 0.82-2.43 wt.% nitrogen in HCN. This

362

indicated that the co-pyrolysis of algae and lignocellulosic biomass (rich in Ce) with

363

biochar addition could efficiently decrease nitrogen emission.

364

Based on the above discussion, the influence of biochar addition on nitrogen

365

transformation during co-pyrolysis of algae and lignocellulosic biomass is shown in

366

Figure 7. Co-pyrolysis of algae and lignocellulosic biomass greatly increased Nchar yield

367

and promoted the decomposition of protein-N as well as formation of pyridinic-N,

368

pyrrolic-N, and quaternary-N. Simultaneously, co-pyrolysis decreased Nbio-oil yield, and

369

inhibited the formation of amines/amides, nitriles, and N-heterocyclics. However, it

370

increased NH3 and HCN yields, leading to an increase of nitrogen emission.

371

Furthermore, with biochar addition, N-containing intermediates (amines/amides and

372

nitriles) as well as NH3 and HCN reacted strongly with O-containing species (O-C=O,

373

-OH, and –COOH) in biochar addition to form large amounts of amine/amide-N. Most

374

of the amine/amide-N then transformed into pyridinic-N and pyrrolic-N by cyclization

375

reactions, while some pyridinic-N further converted into quaternary-N through

376

condensation reactions. Thus, this caused significant nitrogen transformation on the

377

surface of biochar addition, and greatly increased the content of amine/amide-N,

378

pyridinic-N, pyrrolic-N, and quaternary-N. Meanwhile, it significantly decreased Nbio-oil

379

yield further, and inhibited amines/amides and nitriles formation but promoted

15


Page 16 of 32

Page 17 of 32


380

N-heterocyclics generation due to the secondary reaction of N-containing intermediates

381

and decomposition of N-heterocyclic species in biochar addition. Besides, biochar

382

addition sharply decreased NH3 and HCN release, which controlled nitrogen emission

383

efficiently and converted most nitrogen into high-valued N-doped solid char materials.

384

This also indicated that the inexpensive and green biochar addition (by-product of fast

385

pyrolysis) could be used to efficiently control nitrogen emission during algae pyrolysis.

386

Associated content

387

Supporting information

388

Properties of materials (Table S1). Evolution of N-containing groups in char from

389

co-pyrolysis, and N-containing and O-containing species in biochar addition after use

390

for co-pyrolysis of algae and lignocellulosic biomass (Figures S1-S3). Nitrogen

391

distribution in the products, and NH3 and HCN releasing properties from co-pyrolysis of

392

algae and Ce (Figures S4-S5).

393

Author information

394

Corresponding Author

395

*Phone: +086-027-87542417-8109; email: [email protected].

396

*Phone: +086-027-87542417-8109; email: [email protected].

397

Notes

398

The authors declare no competing financial interest.

399

Acknowledgements

400

We want to express sincere thanks to the financial support from the National Nature

401

Science Foundation of China (51622604 and 51861130362), the Fundamental Research

402

Funds for the Central Universities, the technical support from Analytical and Testing

403

Center in Huazhong University of Science & Technology (http://atc.hust.edu.cn). We

16



404

are also grateful for Dr. Zhidan Liu of China Agricultural University to provide algae

405

samples.

406

References

407

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408

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538

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539

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540

49. Zhou, J. H.; Sui, Z. J.; Zhu, J.; Li, P.; Chen, D.; Dai, Y. C.; Yuan, W. K.,

541

Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and

542

FT-IR. Carbon 2007, 45 (4), 785-796.

543 544

22



545

Figure captions

546

Abstract Graphic.

547

Figure 1 Schematic diagram of co-pyrolysis system for nitrogen emission control.

548

Figure 2. Nitrogen distribution properties in co-pyrolysis products.

549

Figure 3. N-species distribution in bio-oil.

550

Figure 4. The releasing properties of NH3 and HCN from co-pyrolysis.

551

Figure 5. N 1s spectra of N-containing species in co-pyrolysis char product (a) and

552

N-containing species distribution properties in char products (b).

553

Figure 6. O and N 1s spectra of biochar addition before (a) and after use (b-c), and their

554

distribution properties (d).

555

Figure 7. Influence of biochar addition on nitrogen transformation during co-pyrolysis

556

of algae and lignocellulosic biomass.

557

23


Page 24 of 32

Page 25 of 32


H2 O

NH3

CH4 CO2 H2

HCN

CO

Mechanism

558 559

Graphical abstract

560 561

24



Mass flow controller 200

Ar

Page 26 of 32

Quartz basket Feedstock

Valve Stage ¢ñ Quartz reactor C600

Biochar addition

T M

Temperature controller Stage ¢ ò Electric furnace

Gas bag Ice-water mixture

NaOH H 2SO 4

Ⅰ

Discharge into air Ice-water mixture or liquid nitrogen

Color changing silica gel Ⅱ

562 563

Figure 1 Schematic diagram of co-pyrolysis system for nitrogen emission control.

564

25


Page 27 of 32


100

N yields (wt.%)

80

60

40

20

0 SP Cop. Bio.

NS Cop. Bio.

SP+Ba

565

2.

Nitrogen

NS+Ba Samples

distribution

EP Cop. Bio.

EP+Ba

566

Figure

properties

567

(Ngas=100%-Nchar-Nbio-oil-Nbiochar addition).

568

addition.

569

respectively. SP + Ba, NS + Ba, EP + Ba: mean co-pyrolysis of SP and Ba, NS and Ba,

570

EP and Ba, respectively. Following figures represent the same meaning.

: Nchar;

in

co-pyrolysis

: Nbio-oil;

: Ngas;

products : Nbiochar

Cop., Bio.: mean the results of co-pyrolysis without and with biochar addition,

571

26



Page 28 of 32

N-containing gas yields (wt.%)

25

20

15

10

5

0 SP Cop. Bio.

SP+Ba

572

NS Cop. Bio.

NS+Ba Samples

EP Cop. Bio.

EP+Ba

573

Figure 3. The releasing properties of NH3 and HCN from co-pyrolysis.

574

NHCN.

575

27


: N ;

:


Relative content (area %)

Page 29 of 32

15

10

5

0 SP Cop. Bio.

SP+Ba 576

NS Cop. Bio.

EP Cop. Bio.

NS+Ba Samples

EP+Ba

577

Figure 4. N-species distribution in bio-oil. : amines/amides; : nitriles; ▲:

578

N-heterocyclics.

579

28



a

SP+Ba

H N

Intensity (a.u.)

Pyrrolic-N

Pyridinic-N

Protein-N

Quaternary-N

404

402

400

398

396

Binding Energy (eV)

580 N-containing species content (wt.%)

b 20 15

10

5

0 SP SP+Ba

NS NS+Ba

EP EP+Ba

Samples

581 582

Figure 5. N 1s spectra of N-containing species in co-pyrolysis char product (a) and

583

N-containing species distribution properties in char products (b). : protein-N; :

584

pyridinic-N; ▲: pyrrolic-N; ▼: quaternary-N.

585

29


Page 30 of 32


SP+Ba-biochar O-C=O/-OH Before use

b

C=O

After use

O-C=O/C-O -COOH

538

536

534

532

530

O-C=O/-OH O-C=O/C-O -COOH

538

536

Binding energy (eV)

Intensity (a.u.)

c

Amine/amide-N

SP+Ba-biochar After use Pyrrolic-N Qauternary-N

Pyridinic-N

Before use 404

402

400

398

534

532

530

Binding energy (eV) 50

d 50

N-containing species content (%)

586

C=O

SP+Ba-biochar

Intensity (a.u.)

Intensity (a.u.)

a

40 40 30 30 20

20

10

10

396

before

after

0

O-containing species content (%)

Page 31 of 32

0 SP+Ba NS+Ba EP+Ba Biochar SP+Ba NS+Ba EP+Ba

Binding Energy (eV)

Samples

587 588

Figure 6. O and N 1s spectra of biochar addition before (a) and after use (b-c), and their

589

distribution properties (d). : amine/amide-N; : pyridinic-N; ▲: pyridone-N; ▼:

590

quaternary-N; ☆: C=O; ○: O-C=O/-OH; △: O-C=O/C-O; ▽: -COOH. O: means the

591

according oxygen atom of O-containing species.

592

30



593 594

Figure 7. Influence of biochar addition on nitrogen transformation during co-pyrolysis

595

of algae and lignocellulosic biomass.

596

31


Page 32 of 32

Influence of Biochar Addition on Nitrogen Transformation during

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