Bioenergy and emission characterizations of catalytic ...

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Renewable Energy 148 (2020) 1074e1093

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Renewable Energy

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Bioenergy and emission characterizations of catalytic combustion andpyrolysis of litchi peels via TG-FTIR-MS and Py-GC/MS

Chao Liu a, Jingyong Liu a, *, Fatih Evrendilek b, c, Wuming Xie a, Jiahong Kuo a,Musa Buyukada d

a Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, Guangdong Key Laboratory of Environmental Catalysis and Health RiskControl, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology,Guangzhou, 510006, Chinab Department of Environmental Engineering, Abant Izzet Baysal University, Bolu, 14052, Turkeyc Department of Environmental Engineering, Ardahan University, Ardahan, 75002, Turkeyd Department of Chemical Engineering, Abant Izzet Baysal University, Bolu, 14052, Turkey

a r t i c l e i n f o

Article history:Received 7 April 2019Received in revised form23 August 2019Accepted 29 September 2019Available online 30 September 2019

Keywords:Catalytic combustionEmission gas analysisTG-FTIR-MSPy-GC/MSBiofuel

* Corresponding author.E-mail addresses: [email protected], www053991

https://doi.org/10.1016/j.renene.2019.09.1330960-1481/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

This study characterized the catalytic combustions and emissions of litchi peels as a function of fivecatalysts as well as the effect of the best catalyst on the pyrolysis by-products. Na2CO3 and K2CO3

accelerated the devolatilization but delayed the coke burnout, while Al2O3 enhanced the coke oxidationrate. Both comprehensive combustion index and average activation energy dropped with the addedcatalysts. CO2, CO, and H2O were the main combustion gases between 300 and 510 �C. CO2, C-H, C¼O, andC-O were generated from the pyrolysis between 200 and 430 �C above which CO2 and CH4 were slightlyreleased. Total H2O, CO2, CO, NOx and SOx emissions declined with the added catalysts among whichK2CO3 performed better. The main pyrolytic by-products at 330 �C were terpenoids and steroids (71.87%),phenols (15.51%), aliphates (9.95%), and small molecules (2.78%). At 500 �C, terpenoids and steroids(78.35%), and small molecules (3.20%) rose, whereas phenols (12.87%), and aliphates (5.83%) fell. Fattyacid, and ester decreased, while terpenoids, and steroids increased with MgCO3 at 330 �C. Litchi peelsappeared to be a promising biowaste, with MgCO3 as the optimal catalytic option in terms of the bio-energy performance, and emission reduction.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Due to the exhaustion of fossil fuels, and the escalation ofenvironmental concerns, the use of solid biowaste from agriculture,forestry, petroleum, food and textile industries, and municipalitieshas become significantly important as the world’s fourth largestenergy resource [1]. Not only do large amounts of solid biowasteoccupy land, but also cause environmental pollution. Unlikemunicipal sewage sludge with high moisture, low calorific value,and complex composition, biowaste poses less problems with en-ergy generation and even has a higher calorific value than dolignite, bituminous coal, and oil shale [2e5]. Biowaste is consideredmore CO2-neutral than even fuel crops during the generation ofelectricity, and environmentally friendly by-products such as raw

@126.com (J. Liu).

materials, and enzymes [6,7].As a typical tropical fruit, litchi (Litchi chinensis Sonn.) produc-

tion occupies a cultivation area of over 500,000 ha in southernChina, with an annual rate of over 2 million tons and meets about70% of its global production [8,9]. The hot and humid harvest sea-sons rapidly deteriorate the quality of harvested litchi which in turnmakes its processing to produce wine and juice essential [10]. In sodoing, a large amount of litchi peels (LP), pericarps, and seeds aregenerated as the solid biowaste that threatens public health due toits strong infectivity and pathogenicity [11]. The recent focus aboutlitchi fruit has been on the extraction of its natural active in-gredients with anti-oxidant and -inflammatory effects such aspolyphenols, and oligomeric procyanidins [12e15], the utilizationof its carbonmaterials [16], and its preparation as an adsorbent [17]and a biofuel [18]. LP was also reported to possess a high calorificvalue of 19.6MJ/kg [7].

The main technologies of generating bioenergy from biowasteinclude destructive carbonization, thermochemical conversions

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Nomenclature

Symbols/abbreviationsTi ignition temperature (�C)Tb burnout temperature (�C)Tv initial devolatilization temperature (�C)-Rp maximum weight loss rate (%$min-1)-Rv average weight loss rate (%$min-1)DT1/2 temperature interval at half value of -Rp (�C)Mf residue massCi ignition index (%$min�1$�C-3)Cb burnout index (%$min�1$�C-4)Dv volatiles release index (%$min�1$�C-3)

S comprehensive combustibility index(%2 $min�2 $�C-3)

T absolute temperature (K)Tp ignition temperature (�C)k(T) reaction rate constanta conversion degree (%)f(a) differential reaction functiong(a) integral reaction functionb heating rate (�C$min-1)A apparent pre-exponential factor (s-1)R universal gas constant (J$mol�1$K-1)E apparent activation energy (kJ$mol)Ea average apparent activation energy (kJ$mol)

C. Liu et al. / Renewable Energy 148 (2020) 1074e1093 1075

(combustion, gasification, and pyrolysis), liquefaction, hydrolysis,and biological conversions (anaerobic digestion, composting, andfermentation) [19]. The high-tech thermal treatments of biowastewith coal [20], sludge [21], and oil shale [22] have been demon-strated to perform better in terms of waste and emission re-ductions, bioenergy generation, and value-added by-products[23e25]. By 2050, bioenergy is expected to provide about 38% and17% of theworld’s fuel use and electricity, respectively [26,27]. (Co-)combustions of biomass with coal or sludge can compensate for thedifference in moisture content, density and calorific value, reduceNOx and SOx emissions [28] but aggravate the issues of ash depo-sition, corrosion, and particulate matter emission [29]. As a softthermochemical conversion process, pyrolysis (500-1000 �C) wasfound not only to alleviate these issues but also to produce liquid(e.g., biodiesel, sugars, and bioethanol), solid (e.g., charcoal) and gasby-products (e.g., CH4, H2, and CO) [26].

Rapid pyrolysis is a technique to obtain high-yieldbio-oils atabout 500 ℃ with a high heating rate >200 �C s�1, and a shortresidence time [30]. Associated with the fast pyrolysis is the chal-lenge that the complex compositions of bio-oils significantly varywith the biofeedstock type, and the operational conditions [31].Bio-oils contain a great variety of oxygenated derivatives, thusresulting in a high water content, a low calorific value, and anincreasing corrosiveness and instability [31]. Typically, strongacidity and viscosity of these reactive oxygenated moieties, andtheir vulnerability to polymerization even under the ambienttemperature storage hinder the storage and transportation of bio-oils. The co-pyrolysis of lignocellulosic biomass and other feed-stocks with a low oxygen content, and a high calorific value (such ascoal, and synthetic polymer) was found to effectively improve thequality of bio-oils [32]. Catalytic (co-)pyrolysis can also improve thequality of bio-oils by the deoxygenation reactions (such as dehy-dration, decarboxylation, and decarbonylation). Natural catalystssuch as calcite, dolomite, and zeolite have been widely used owingto their relatively high activity and availability [33]. Yields of cat-alytic pyrolysis by-products depend on temperature, heating rate,flow rate of the carrier gas, feedstock type, particle size, and catalysttype [34]. Thus far, there has been no study about the catalyticcombustion and pyrolysis performances and by-products of LP.

Metal ions in biowaste ash were found to significantly affectboth thermal degradation characteristics and pyrolysis by-products[35e41]. The commonly used catalysts include alkali/alkaline earthmetals (AAEMs) (e.g., Mg, Na, and K), metal oxides (e.g., acid, Al2O3,SiO2, andMgO), and zeolites (ZSM-5, Fe/ZSM-5, and Ni/ZSM-5) [42].Generally, most metal ions are retained in solid char or ash and notevaporated during the pyrolysis or combustion under the moderatetemperatures [43]. AAEMs catalyze the devolatilization, char

oxidation, and the emissions of CO2, N, and S. Zhao et al. [38] foundthat potassium salts lowered both maximum decomposition tem-perature and apparent activation energy (E). Fuentes et al. [39]pointed out that K catalyzed the release of volatiles as well as thechar oxidation by shifting the mass loss peaks to lower tempera-tures. Huang et al. [36] reported that K2CO3 enhanced the com-bustion performances and reduced the flue gas emissions, whileNa2CO3 reduced E during the co-incineration of municipal sludgeand water hyacinth. Zhou et al. [37] indicated that exogenous Ksalts advanced tobacco’s decomposition in the low temperaturezone between 150 and 350 �C, but the thermal stability of charresidues was strengthened, with emissions being cut down in thehigh temperature zone. Cheng et al. [44] pointed out that sodiumacetates had higher catalytic activity than did calcium acetatestowards the sludge pyrolysis. Adding an appropriate catalyst intothe pyrolysis was shown to selectively optimize the distribution ofdesirable by-products such as hydrocarbons, and anhydrous sugarsas well as to alter the reaction pathways of biowaste degradation[45e49].

There still remains no study about how the thermochemicalproperties of LP change in response to the uses of differentadvanced technologies and catalysts. The thermochemical proper-ties such as combustion performance, kinetic and thermodynamicparameters, emission patterns, pyrolysis by-products, and slaggingrisk are essential to the design and optimization of incinerators, andexhaust gas purification devices [50] as well as to the cost-efficientand eco-friendly production of energy and by-products from bio-feedstocks. Therefore, the objectives of this study were to (1)quantify both combustion characteristics and gas emissions of LP inresponse to five common catalysts; and (2) characterize pyrolysisby-products with(out) MgCO3.

2. Experimental set-up

2.1. Materials

LP was collected from a fruit market in Guangzhou, China. Afterbeing washed with deionized water and dried naturally, it wasground using a Wiley mill (Marconi MA 048, Piracicaba, S~ao Paulo,Brazil) and sieved to obtain a particle size of less than 74 mm.Generally, biowaste substrate must be acid-or water-washed toobtain a catalyst- and inorganic contaminant-free sample inbiomass ash. However, this treatment alters the degree of poly-merization and crystallinity of the raw biowaste [51] and producesthe secondary pollutants. In this study, K2CO3 (�99.0%), Na2CO3(�99.8%), MgCO3 (83.68e98.32%), Al2O3 (�99.0%), and MgO(�98.0%) were adopted as the representative carbonates or oxides

C. Liu et al. / Renewable Energy 148 (2020) 1074e10931076

with alkali metal or alkaline earth metal and were chosen as thecatalysts. 5% of the catalysts were blended with LP powder, fullyground in an agatemortar and sieved to obtain a particle size of lessthan 74 mm. The blends were oven-dried at 105 �C for 24 h untilreaching a constant weight and maintained in a desiccator for thefollowing TG, TG-FTIR, TG-MS and Py-GC/MS experiments. Proxi-mate analysis was carried out according to the Chinese standard ofGB/T 212e2008 [52]. Ultimate analysis, and calorific value test werealso performed using an elemental analyzer (Vario EL Cube, Ger-many) and a microcomputer calorimeter (WZR-1T-CII, Bent In-strument Co., Ltd., China). Main elements such as Na, K, Al, Ca, P,and Ti were measured using a ZSX Primus II sequential X-rayfluorescence (XRF) spectrometer (Rigaku, Japan). Trace elementssuch as Fe, Mn, and Zn were detected using an atomic absorptionspectrometer, while Cr, Ni, Co, and Ba were tested using a graphitefurnace atomic absorption spectrometer by using an Integratedflame/graphite furnace atomization system (WFX210, Beijing RuiliAnalytical Instrument Co., Ltd., China).

2.2. Measurements

2.2.1. TG analysisGenerally, thermogravimetric (TG) experiments are conducted

using a smaller weight sample, a low heating rate, and a large flowrate of carrier gas in order to minimize the mass and heat transferlimitations, and the secondary reactions. Thus, 7.0± 0.1mg of LPwere used in each experimental run. Thermogravimetric analysiswere conducted using a simultaneous Thermal Analyzer (NetzschSTA 409 PC, Germany) from ambient temperature to 1000 �C at theheating rates of 10, 20, 25 and 30 �C$min-1 under the air atmo-sphere, with a gas flow rate of 80mL$min-1. The baseline driftcaused by gas buoyancy and instrumental errors was eliminatedbefore the start of the experiment. The same batch was randomlyreplicated for three times to ensure data reproducibility and reli-ability, and the resultant margin of errors of ±3%.

2.2.2. Combustion parametersTo evaluate the combustion performance, the three character-

istic parameters of ignition, peak, and burnout temperatures (Ti, Tp,and Tb, respectively) were introduced in this study. Ti was deter-mined using the (D)TG tangent method [53], reflecting the diffi-culty of the ignition. Tb represents the temperature when theweight loss reaches 98% of the total weight loss [54]. Also, thefollowing four indices of volatiles matter release (Dv), ignition (Ci),burnout (Cb), and comprehensive combustibility (S) [55,56] wereadopted as follows:

Dv ¼ RpTp � Tv � DT1=2

(1)

Ci ¼Rp

ti � tp(2)

Cb ¼ RpDT1=2 � tP � tb

(3)

S ¼ Rp � RvT2i � Tb

(4)

where Rp is maximumweight loss rate (%$min-1); and Rv is averageweight loss rate (%$min-1). The higher Dv value means a moreconcentrated combustion and better burnout performance. Thehigher the S value is, the more vigorous and the faster the burning

is [57].

2.2.3. Kinetic analysisKinetics of non-isothermal decompositions and heterogeneous

reactions of solid state are subject to Arrhenius law. The funda-mental reaction rate can be described using the Arrhenius equationthus:

dadT

¼Abexp

�� ERT

�f ðaÞ (5)

where T is absolute temperature (K); R is the universal gas constant(J$mol�1$K�1); b is heating rate (�C/min); and E, A, and f (a) are alsoknown as the kinetic triplets. The conversion degree (a) can bewritten as follow:

a¼w0 �wt

w0 �wf(6)

wherew0,wt, andwf refer to initial, instantaneous and final masses,respectively. The integral rearrangement of Eq. (5) yields thefollowing:

g

a

!¼ða0

1f ðaÞda¼

ðTT0

expð�ERT

!dT ¼A

bj

E; T

!(7)

where j (E, T) is the temperature integral without analytical so-lution but can be replaced only with approximate formulas, andg(a) is an integral form of f(a). Not only are the model-free methodsconsidered very convenient to estimate E without the requirementto know an exact f(a) or g(a) value, but also they reduce the effectsof mass transfer limitations by the use of multiple heating rates. Inthis study, Ewas estimated using the two common iso-conversionalmodels of Flynn-Wall-Ozawa [58] and Kissinger-Akahira-Sunose[59] based on the following equations:

lgb¼ lg�

AERgðaÞ

��2:315� 0:4567

ERT

(8)

ln�b

T2

�¼ ln

�AR

EGðaÞ�� ERT

(9)

The slopes of the best-fit linear regression lines of lg b versus 1/Tand ln(b/T2) versus 1/T can be used to estimate Ea based on FWOand KAS, respectively.

2.2.4. TG-FTIR analysis of pyrolytic and combustion functionalgroups

TG coupled with Fourier infrared (FTIR) spectrometry can pro-vide real-time information about the type and quantity of gasesreleased from the thermal decomposition of solid fuels. The TG-FTIR experiments were performed using a TG analyzer (209 F1,Netzsch, Germany) interfaced to the Nicolet IS 50 FTIR (Thermo-fisher) spectrophotometer. About 10± 0.1mg of LP were heated inTG analyzer at a heating rate of 10 �C$min-1 from room temperatureto 800 �C. Gases evolved can directly purged into the infrared de-tector at a flow rate of 80mL/min with high purity N2 as the py-rolysis gas and with N2/O2 (64/16) as the combustion gas.Temperatures of the gas transfer line and the infrared detector werekept at 280 and 250 �C, respectively, to prevent the condensation ofgases. A continuous scan modewas adopted in the range of 4000 to600 cm�1 at a resolution of 4 cm�1 with 16 scans per sampling.


2.2.5. TG-MS analysis of evolved gasesGas evolutions were monitored using a MS with an electron

ionization source (70 eV) coupled to a TG analyzer (Thermo MassPhoto TG-DTA-PIMS 410/S, Japan). About 3.5± 0.1mg of LP wereloaded in the TG analyzer and heated from room temperature to1000 �C at a heating rate of 20 �C$min-1 under 20% O2/80% He at-mosphere. Pure helium (99.999%) was used as the carrier gas with aflow rate of 300mL$min-1. About 1% of the effluent gas from the TGfurnace was allowed to pass to the mass spectrometer through thecapillary heated to 250 �C. Our study focused only on the selectedratios of mass to charge (m/z) of 2, 16, 18, 28, 30, 44, 46, 64, and 80amu that correspond to H2, CH4, H2O, CO, NO, CO2, NO2, SO2, andSO3, respectively. The mass spectra intensities were normalizedusing the initial sample mass since the sample mass of eachexperimental run slightly differed, and the baseline was also sub-tracted from the measurements for the experiments [60].

2.2.6. Py-GC/MS analysis of pyrolytic by-products with(out) MgCO3

The fast pyrolysis (Py) analyzer (FrontLab Frontier 2020id-AltraDSQII) was coupled with a gas chromatography-mass spectrometry(GC-MS) (Agilent 7890A GC, 5975C MS) system to determine thedistributions of the by-product with(out) 5% MgCO3 under thecharacteristic pyrolysis temperature between 330 and 500 �C.About 0.7± 0.1mg of LP were loaded in a small crucible anddropped vertically into the furnace for a rapid cracking for a resi-dence time of 24 s when the furnace reached the pre-set temper-ature. The pyrolysis by-products were eluted at a flow rate of1.0mL$min-1 high purity helium. The chromatographic separationwas performed with a HP-5MS column(30m� 0.25mm� 0.25 mm). The chromatographic temperatureramp was scheduled thus: an initial temperature of 50 �C for 2min;7 �C$min-1 up to 300 �C; 250 �C kept for 10min. The interface andion source temperature was operated at 300 �C, while the injectorand transfer line temperatures were maintained at 280 �C,respectively. The pyrolysis by-products were determined referringto the NIST 14 library and related literature [61e67], as well as tosemi-quantitative analysis of the relationship between absoluteintegrated area and mass. The instrumentation details of theexperimental analyses are shown in Table Se1.

3. Results and discussion

3.1. Proximate, ultimate and ash analyses

The physicochemical properties of solid biowaste are essentialto the fuel pretreatment, thermal conversion, reactor design, andpollution control. The proximate and ultimate analyses, higherheating values (HHVs), and ash compositions of LP are given inTable 1.

The moisture content of LP (5.75%) was below 10%, consideredfavorable for both energy generation and biowaste quality [70]. Therelatively high volatiles (75.53%) and fixed carbon (15.42%) con-tents of LP indicated the ease of its ignition in the initial combustionstage [71]. The HHVs of LP (19.12MJ/kg) was comparable to that ofthe commercial wood pellets and higher than that of some inferiorfossil fuels such as bituminous, lignite, and oil shale [72]. The lowerO/C and H/C ratios indicate the higher energy content as well as thehigher aromaticity and carbonation features of a given biowastematerial. The H/C and O/C ratios of 1.60 and 0.58, respectively,estimated in this study were lower than those of most biomasstypes [73], comparable to those of peat [74] and near those of coalas a reference solid fuel [75]. Its higher O content (36.81%) indicatedits higher hydrophilicity with more O2-containing functionalgroups such as -OH, C¼O and -COOH groups [76]. Generally, manyimportant intermediate products (e.g., CH and OH) of the reactions

between hydrocarbons and atomic or molecular O2 during theinitial devolatilization stage accelerate the devolatilization rate,thus improving the thermal reactivity. However, pyrolysis oil maycontain more O2-containing functional groups, not conducive tofurther utilization. The low N and S contents of LP meant lower NOx

and SOx emissions, thus reducing the difficulty with the purificationof the pyrolysis oil.

The lower ash content of LP (3.30%) than some woody andherbaceous species, agricultural crops, livestock manures, and coalpointed to its ability to reduce the waste stream, and the adverseeffects on bio-oil quantity and quality, and the incinerators such asfouling, erosion, and slagging. However, its relatively high AAEMcontents such as K, Ca, and Mg (Table 1) can lead to low-melting-point eutectics, resulting in alkali-induced slagging [77], equip-ment malfunction, environmental issues, efficiency reduction inoverall heat exchange, and erosion acceleration [78]. Therefore,both its ash content and composition should be considered toassess the risk of slagging [79]. Si compounds were shown to causeabrasion damage or silicate melt-induced slagging during thecombustion process [80]. The acceptable Si contents of solid fuelsfor thermal treatments were reported to range from 0.16 to 20%[81,82]. During the combustion, P compounds were shown tovolatilize, thus increasing the ash deposition, and decreasing theboiler efficiency [83]. Overall, the determined physicochemicalproperties appeared to provide a great advantage for LP to be usedin bioenergy generation.

3.2. TG analysis and IR spectrum

The (D)TG/DSC profiles obtained for the LP combustion at10 �C$min-1, and IR spectra of the control group are shown in Fig. 1.Generally, the biowaste combustion process includes the stages of(disruptive) devolatilization, char formation, and combustion. Thethermal degradation of LP was divided into four main stages. Stage1 at below 150 �Cwas the evaporation of physically adsorbed water.Stage 2 between 150 and 365 �C included the glass transition, andthe cellulose depolymerization to form active cellulose followed bythe rapid pyrolysis degradation induced by the volatiles. Variouscomplicated reactions occurred such as the breaking of the C-C andC-heteroatom bonds in side chains linked to aromatic or naph-thenic structures which results in polymerization and poly-condensation of aromatics [84,85]. With a large mass loss, largeamounts of small molecular compounds (e.g., H2O, CO, and CO2)were produced due to the cleavage of bonds from (hemi)cellulose,and lignin (e.g., O-H, C-O, and C¼O) during the fast devolatilizationprocess [86]. Stage 3 in the narrow range of 365e500 �C with thehighest mass loss rate corresponded to the oxidation of carbona-ceous materials during which a minor peak appeared at 410 �C dueto the degradation of remaining lignin which had complex, amor-phous and 3-D polymers with phenylpropane structure and highthermal stability [87,88]. Stage 4 at above 500 �C was probably dueto the decomposition of inorganic minerals and the stabilization ofcarbonaceous materials retained in the ash. No obvious peak ofmass loss, or endothermic peak in the air atmosphere occurred dueto the decomposition of CaCO3 in the ash between 600 and 850 �C[89,90] despite the high Ca and Na contents of the ash.

The typical combustion behaviors and peaks of the degradationof the lignocellulosic biomass were exhibited in our experiments.Hemicellulose with the worst thermal stability was decomposedfirst in the range of 220e315 �C, while cellulose composed ofcrystals linearly formed by (C6H10O5)n was degraded quickly ataround 315 �C. Ligninwith the most thermal stability was degradedslowly in a wider range of 160e900 �C due to its constituents suchas benzene ring, and various chemical bonds (b-O-4, a-O-4, and b-5,5-5) between benzene ring and oxygenated functional groups

Table 1Ultimate and proximate analyses, higher calorific value, and ash components of LP(air-dried basis, wt.%) (adapted from [43]).

Proximate analysis (wt%)Mad 5.75Vad 75.53FCad 15.42Aad 3.30Vad/(VadþFCad) 0.83Ultimate analysis (wt%)C 47.39H 6.32aO 36.81N 0.81S 0.019H/C 1.60O/C 0.58HHVS (MJ/kg) 19.12Ash composition (%)SiO2 1.683Al2O3 0.298Na2O 1.801MgO 3.219K2O 43.421CaO 37.591TiO2 0.022P2O5 6.004LOI 94.039Trace elements (mg/kg)Fe 217.55Mn 124.28Zn 11.02Cr 0.16Ni 0.04Co 1.35Cu 19.69Ba 137.09

M, moisture; V, volatile matters; A, ash; FC, fixed carbon; HHVS, higher heatingvalue; aO, calculated by O¼100%eCeHeNeSeMeA [68, 69].

Fig. 1. (a) (D)TG-DSC curves in air atmosphere at 10 �C$min-1; and (b) IR spectra of LP.


(methoxy, hydroxyl, carboxyl, carbonyl, and acetyl). Thus, thetemperature ranges of 200e250 �C and 250e350 �C for the shoul-ders and peaks were attributed to the degradation of hemicellulose,and cellulose, respectively [87,91]. The shoulder and main peaks ofthe DTG curve appeared at 230 and 307 �C, respectively.

Generally, the peak’s height and temperature of the DTG curvescan be positively and negatively correlated to biomass reactivity,respectively [92]. The relatively weaker decomposition energy ofthe volatiles than the char oxidation contributes to higher reactivity[93]. The weight loss at below 500 �C was 96.2% of which thedevolatilization process, and the coke combustion accounted for59.4% and 36.6%, respectively, while the moisture and ash contentselucidated only about 4.0%. The high burning rate, and the lowtemperature that occurred with the maximum weight loss wereindicative of a better burnout performance and high reactivity.According to the DSC curve, both devolatilization and coke com-bustion were exothermic with their peaks being in line with theweight loss peaks, while the former released more heat than thelatter. The higher rate of the coke oxidation than devolatilizationmay be attributed to its higher thermochemical reactivity, itshigher surface area, and its higher concentration of possible Hradicals from biochar precursors than those of coal chars [94].Biochars are generally more reactive towards O2, CO2, and H2O thancoal chars due to their efficient external mass transfer, and theinteraction between pore diffusion and kinetics [95]. The largeamount of AAEMs in the ash was also reported to catalyze thegasification of the biochars [96]. The ignition (237.8 �C) and burnout(474.0 �C) temperatures of LP were indicative of its excellent igni-tion and burnout performances.

As descripted by the van-Krevelen-type diagram, the H/C and O/

C ratios highly influences the bonding energies in fuels as the en-ergy of the C-C bond is higher than that of the C-O bond [97]. Thebetter ignition and burnout performances of LP can result from thegeneration of more O, H, OH, HO2 and CH3 radicals from volatilesduring the ignition induction period that are susceptible to spon-taneous ignition (e.g., dehydrogenation reactions, and classicalchain-branching mechanism). This can in turn suppress the sec-ondary reactions, promote the fragmentation of aromatic hydro-carbons and reduce the secondary product formations [98], whilehigher ash amount can impede oxygen diffusion, and transfer ef-ficiency of heat andmass [99]. The onset of devolatilization, and thepeak temperatures of the DTG curves shifted to the higher tem-peratures (Table 2). This showed that the increased heating ratepostponed the thermal degradation due to the heat- and mass-transfer limitations [100,101]. Due to the poor thermal conductiv-ity of biomass materials, the temperature gradient was generatedbetween the surface and core of the sample particles. With theincreased heating rate, the thermal degradation rates rose due tothe enhancement of heat and mass transfers, whereas the residuemass fell [102]. The high heat fluxes were shown not only todecrease the viscosity of the melted solid materials but also to formvolatiles [103,104].

The IR spectra bands of LP (Fig. 1b) were assigned according tothe related literature [105e110] and summarized in Tables Se2. Themarked absorbance at around 3408 cm�1 was due to the vibration

Table 2Combustion parameters of LP at four heating rates.

b Tv Ti Stage2 Stage3 Rp Rv Tb DT1/2 Mf

Tp2 Rp2 W2 Tp3 Rp3 W3

10 205.5 237.8 306.4 5.37 57.7 448.9 5.66 35.2 5.4 2.0 480.2 104.1 3.420 199.3 245.7 311.8 10.96 55.8 449.0 9.38 36.3 10.9 3.8 507.3 102.0 3.130 193.8 247.3 315.9 13.28 54.9 447.4 9.39 36.1 13.7 4.6 515.3 106.1 3.040 190.2 249.0 319.2 16.76 55.2 449.0 11.32 37.2 16.8 5.3 531.5 105.6 2.8

Tv: initial devolatilization temperature, �C; Ti: ignition temperature, �C; Tp2, Tp3: peak temperatures of the second and third stages, �C; Rp2, Rp3: values of the second and thirdpeaks, %$min-1; Rv: average weight loss rate, %$min-1; Tb: burnout temperature, �C; DT1/2: temperature interval at the half value of Rp, �C;Mf: residue mass, %;W2,W3: weightloss percentages of the second and third stages, %.

Fig. 2. (a) TG and (b) DTG curves of LP with catalysts at 20 �C$min-1.


of -OH groups, especially associated -OH. The characteristic absor-bance values at 2924 and 2854 cm�1 in the range of 3000 to2800 cm�1 were assigned to the symmetric and asymmetricstretchings of C-H from the ring and -CH2OH and -CH3 groups,indicative of -CH2 in cellulose and hemicellulose, respectively. TheC-H in-plane deformation vibrations at 1443 cm�1 and 1383 cm�1

indicated the presence of aliphatic hydrocarbons. No obvious C-Hvibration absorbance readings of the acetylenic (3300 cm�1) andaromatic (3030 cm�1) structures were found due to the interfer-ence from the broad -OH stretching absorption bands. The absor-bance peak range of 1900 to 1650 cm�1 (1735, 1650 cm�1) wasrepresentative of carbonyl compound such as aldehyde, acids, andketones. The reaction activity of C¼O conjugated (at below1700 cm�1) with the aromatic ring was lower than that of C¼Ounconjugated (at above 1700 cm�1) [111,112]. The relatively lowabsorbance intensity of C¼O when compared to -OH and C-Oindicated the lower hemicellulose content of LP due to its monomerstructures rich in acetyl, or carboxyl groups [113]. 1617 and1520 cm�1 corresponded to the C¼C stretching in the aromatic ring,and the aromatic skeletal vibrations in lignin, respectively.

The C-H in-plane bending (1475e1300 cm�1), the O-CH3

stretching vibration (1443 cm�1), and the out-plane bending vi-bration of aromatic ring C-H (870e675 cm�1) indicated the higheraromatic content of the ring structure. The C-O stretching bands(1300e950 cm�1), and the C-H bending (800 cm�1) showed thecharacteristic vibration of lignin [114]. The C-O stretching vibrationhad a strong absorption band (1300e950 cm�1) corresponding toalcohol (1060 cm�1), ether (1104 cm�1), and phenol (1247 cm�1).The higher amounts of C-O-C and C-OH structures, and the C-O-Cstretching vibration (1145 cm�1) may be attributed to pyranose ringskeletal. Overall, LP contained low hemicellulose, but high lignin,C¼C skeleton, C-OH and C-O-C.

3.3. Effect of catalysts on combustion

Fig. 2 illustrates (D)TG curves of the LP combustionwith the fivecatalysts at 20 �C$min-1, Table 3 lists the corresponding combustionparameters, and Fig. S-1 shows the weight loss curves of the indi-vidual catalysts under the same conditions. The residue contentsincreased significantly as a function of Al2O3, MgO, Na2CO3, K2CO3,and MgCO3, with the weight losses of 3.2, 3.1, 7.2, 10.9 and 57.0%,respectively. The theoretical weight losses of LP with the catalystswere 90.09 (Al2O3), 90.08 (MgO), 90.29 (Na2CO3), 90.48 (K2CO3),and 92.78% (MgCO3). Thus, with Na2CO3 and K2CO3, LP had anactual weight loss greater than its theoretical value which sug-gested a certain catalytic effect. With Al2O3, MgO, and MgCO3, theactual residue masses increased. Until 350 �C, Al2O3, MgO, andMgCO3 did not significantly affect the volatilization and combus-tion of LP in the early stage. Considering Ti, Tp2, and DT1/2 (Table 3),Na2CO3, and K2CO3 promoted the devolatilization of LP at the lowertemperatures and reduced Ti and DT1/2, respectively. This wasmainly due to the electron donating-effect of the alkali metal, and

the complex formed by the combination of the alkali metal andoxygen which acted as an oxygen carrier and accelerated thedevolatilization process. These oxides also had a strong absorptioncapacity for oxygen [36]. K2CO3 exhibited a better devolatilizationperformance than did Na2CO3 as a function of their differentstrengths of electron-donation and reactivity. All the five catalystsreduced the devolatilization rate (Rp2) of LP.

At above 350 �C, the LP combustion was mainly the oxidation ofresidual lignin and coke. Both Al2O3 and MgO moved the com-bustion peak of coke to a lower temperature, while Al2O3 signifi-cantly increased the oxidation rate (Rp3) of coke. These catalysis

Table 3Combustion parameters for LP with catalysts at 20 �C$min-1.

Samples Ti Stage2 Stage3 DT1/2 Rv Tb Wf 107 � Dv 102 � Ci 103 � Cb 107 � S

Tp2 Rp2 Tp3 Rp3

LP 245.7 311.8 10.97 449.0 9.38 102.0 3.80 500.7 94.66 3.890 5.825 6.719 2.002LP þ Al2O3 245.3 311.7 10.46 436.8 10.89 100.2 3.61 499.0 89.33 3.774 5.563 6.523 1.821LP þ MgO 245.1 310.5 10.28 443.5 8.96 102.8 3.58 502.2 90.05 3.631 5.503 6.319 1.766LP þ MgCO3 246.7 311.7 10.13 476.0 7.81 104.8 3.59 512.5 92.63 3.491 5.361 5.846 1.712LP þ K2CO3 231.2 294.4 10.42 473.2 5.58 89.8 2.77 855.2 91.66 4.578 6.183 4.263 1.007LP þ Na2CO3 241.7 306.4 10.26 468.3 8.40 93.7 3.54 834.8 91.27 4.089 5.660 3.866 1.235

Ti: ignition temperature, �C; Tp2, Tp3: the temperatures of the second and third peaks, �C; Rp2, Rp3: values of the second and third peaks, %$min-1; DT1/2: temperature interval atthe half value of Rp, �C; Rv: average weight loss rate, %$min-1; Tb: burnout temperature, �C;Wf: weight loss, %; Ci: ignition index, %$min$-1$�C-3; Cb: burnout index, %$min-1$�C-4;Dv: volatiles release index, %$min-1$�C-3; S: comprehensive combustibility index, %2$min-2$�C-3.


effects were attributed to the catalytic basic ions reacting with theoxygenic base on the char surface to form the complex salts,resulting in a continuous oxidation-reduction cycle of Al. Thismechanism enabled the continuous transport of oxygen to thecarbon surface and promoted the combustion as follows: 2Al(OH)3/ Al2O3 þ 3H2O; 2Al2O3 þ C / 4AlO þ CO2; 4AlO þ O2 / 2Al2O3[115,116]. On the contrary, Na2CO3, K2CO3, and MgCO3 shifted thedecomposition to a higher temperature, decreasing the weight lossrate. Thus, Na2CO3, K2CO3, and MgCO3 weakened the combustionintensity of coke and postponed its combustion. Both Na2CO3 andK2CO3made theminor peaks of the lignin decomposition disappearor overlap with the major peak of the coke oxidation as one com-bined peak. The combination of K and Na with some componentshindered the decomposition of lignin. K2CO3 also promoted thenew weight loss peaks in the range of 510e800 �C since it wastightly coupled with C [117]. Similarly, Zhou et al. [118] found thatthe catalytic effect of K on the O-H, C-O and C¼O structures in therange of 220e280 �C accelerated the devolatilization and earlydegradation of tobacco, while the degradation was slowed downdue to the protective effect of C-H, C¼C, and aromatic structuresbetween 350 and 500 �C. Similar to our results, they also reported anew lag peak between 490 and 550 �C.

At above 800 �C, the DTG curves of LP with Na2CO3 and K2CO3still showed a slow weight loss due to the high temperaturedecomposition (endothermic peak of the DSC curve) of the cata-lysts [119,120]. Unlike Rp3, the change in the average weight lossrate (Rp2) did not depend on the catalyst type. The strength of thecatalyst effect on Rp3 was of the following order:Al2O3> LP>MgO>Na2CO3>MgCO3>K2CO3. Na2CO3, and K2CO3raised the burnout temperature by 334.1, and 354.5 �C, respectively.Even though Na2CO3 and K2CO3 enhanced the devolatilization, theygrew harmful to the coke burnout in the later stage. The reason forthis may be the high temperature decomposition of the catalyst.Alternatively, Na, K, and the other alkali metals as the electronicdonor can be easily combined with the coke components to formrecalcitrant materials (such as C-K2O) with high thermal stabilitythat in turn participate in the high temperature reaction of the ashminerals. The mineral contents of the catalysts can lead to ashdeposition on the heated surface [121], and thus, to the burnouttemperature rise. This can hinder the coke oxidation, shift thecombustion to a higher temperature region and weaken the com-bustion intensity [117,122]. Some bimetallic carbonates fromK2CO3/Ca(OH)2 such as Na2Ca(CO3)3, and K2Ca(CO3)2 [123] mayhave also formed during the reaction of char with the alkali metalsat the high temperatures [124,125]. However, the dominating stepsof the catalysts are mainly the devolatilization and combustionstages of the organic matter.

Na2CO3 and K2CO3 increased Dv, whereas the other catalystsdecreased Dv (Table 3). K2CO3 increased Ci, while all the catalystsreduced Cb. Unlike Al2O3 and MgO, Na2CO3 and K2CO3 promoted

the devolatilization, with K2CO3 performing better. Al2O3 exerted astrong catalytic effect on the coke oxidation. All the catalystsdecreased Rv. The carbonate catalysts significantly postponed the LPcombustion by weakening its intensity compared to the oxidecatalysts. The S value of the catalysts was of the following order:LP> Al2O3>MgO>MgCO3>Na2CO3> K2CO3. This suggested thatthe carbonates played a greater role in the overall combustion of LPthan did the oxides. In particular, Na2CO3 and K2CO3 deterioratedthe burnout performance, thus sharply decreasing the compre-hensive combustion performance.

3.4. Interactions between LP and catalysts at 25 �C/min

The comparison of the experimental (TGexp) versus calculated(TGcal ¼ hlpTGlp þ hcatTGcat) values is shown in Fig. 3a. DTG (TGexp -TGcal) and DDTG (DTGexp - DTGcal) were introduced to represent thestrength of the synergistic effect [126] (Fig. 3bec). The influence ofthe catalysts on DTG was markedly divided into the two stages at375 �C. The DTG value of LP was basically zero with Al2O3, MgO, andMgCO3 until 375 �C and �2.87% and �8.44% with Na2CO3 andK2CO3, respectively, at 310 �C. Na2CO3 and K2CO3 accelerated themass loss by the devolatilization since Na and K enhanced theelectron transfer rate, and the oxygen transfer efficiency. In therange of 375e800 �C, all the DTG values were positive indicatingthat all the catalysts inhibited the degradation in the followingorder: K2CO3>Na2CO3>MgCO3>MgO> Al2O3. Na2CO3 and K2CO3improved the devolatilization more than did Al2O3, MgO, andMgCO3, but were extremely disadvantageous to the coke combus-tion and burnout. Therefore, in the range of 400e800 �C, Na2CO3and K2CO3 exerted a more significant effect on the LP combustion.

Currently, the catalytic combustion effect of the alkali metals ona given biomass material still remains unclear due to thecomplexity of the combustion reaction and the material. The alkalimetals were shown to enhance pyrolysis, combustion, gasification[127e130], adsorption on the catalyst [131], and the oxygentransfer efficiency [128,129]. The detailed reaction mechanismswere reported about K2CO3-catalyzed CO2 gasification of coal char,and Kþ- and Ca2þ-catalyzed pyrolysis of rice husk [119,130]. Forexample, in the case of K2CO3, K was tightly bonded with O2-con-taining functional groups due to the electron-donating effect toform COK complexes with coal char, or CnK clusters with graphiteby the cross-linking reaction between COK intermediates and Cmatrix which changed the charge distribution. As an O2 carrier, anunstable intermetallic metal oxide continuously transferred O2 tothe C atoms. The accelerated oxidation rate of C, and thus, therapidly formed CO2 weakened the intermolecular interaction be-tween the C-C or C-H bonds and improved themigration of electronfrom O2 to the surface of carbonaceous material so as to facilitatethe combustion [132]. The catalytic devolatilization effect of Al2O3,and Fe2O3 was less than that of the alkaline earth metal salts. Since


the alkali metal salts are mostly soluble in water with a betterdiffusion capacity, the contact surface with the material growsrelatively large [133]. The catalytic activity also depends on thestrength of electron donation and activity. However, there still

Fig. 3. (a) Comparison of calculated (hollow symbol) versus experimental (solidsymbol) TG curves; and (b) DTG and (c) DDTG with catalysts.

remain uncertainties regarding the mechanism of AAEMs-catalyzed biomass combustion.

The presence of O2 tended to render K thermally stable on Csurface, tightening the complex O-C-K bonds at the high temper-atures. Ketene group (C¼C¼O) was reported to be an intermediateof the soot combustion proceeded via an oxygen spillover mecha-nism in the NOx/O2 atmosphere [134]. The interactions between O2and surface K species form surface-activated O2 on K sites (Kþ-O*)which in turn affect C¼C on soot to form the intermediates ofC¼C¼O. NOx suppressed the soot combustion at higher tempera-tures, with a more adverse effect than did the higher K amount[135]. The coordination between O2 atoms and AAEMS [136e138]weakened the H bonds during the low-temperature pyrolysis [139].This resulted in the instability of the glycosidic bonds and OHgroups, thus probably promoting the cleavage of the glycosidicbonds, and the dehydration reactions. Although alkali metals have abetter devolatilization capacity, they also hinder the combustion ofcoke. In our study, the new weight loss peak between 510 and800 �C can be interpreted as a tightly bonded interaction betweenchar and K2CO3 that formed complex precursors with O, K, andintermediates with high thermal stability. A similar finding wasreported by Feng et al. [130] that K had a strong protective effect onthe aromatic C¼C and surface C¼O structures in biochar. Therefore,given the effects of the catalysts on the gas emissions to be dis-cussed in Section 3.7, the effects of K2CO3 and Na2CO3 on the LPcombustion related mainly to the mechanisms of O2 transfers andcomplex formations.

3.5. Kinetic analysis

Apparent activation energy, and its a-dependency are illustratedin Fig. 4aeb. The a range of 0.1e0.9 at an interval of no less than0.05 was used in this study to provide accurate E estimates. The R2

values based on the FWO and KASmethods were high (0.9613e1.0),with a maximum difference of only 3.8 kJ/mol in Ea between themethods. Thus, the E values with higher accuracy based on the KASthan FWO method were only discussed below. Since the R2 valuesin the range of 0.8e0.9 for Al2O3 were quite low, the E values in thisrange were not used. This was further justified considering theerror propagation associated with the iso-conversion model, or thestrong catalytic effect on the char oxidation (Fig. 2). The competi-tive relations were found between the mechanisms changing Eeven though such mechanisms were proved to be difficult to becharacterized [140]. The relationships between E and a with thecatalysts are shown in Fig. 4b.

In the conversion range of 0.1e0.65, the E values indicated thatthe multiple consecutive and competitive reactions related to thepyrolysis of hemicellulose, cellulose, and lignin [141]. The increasedE in the range of 0.1e0.25 (235e285 �C) was mainly caused by thehemicellulose decomposition due to the breaking of the weak linkof the molecular chain of the polymer degradation. In other words,the low E value was observed at the initial decomposition, but thecleavage of the subsequent chemical bonds on the lineal chain ofhemicellulose gradually increased E [142,143]. At a¼ 0.25, the Evalue slightly decreased until a¼ 0.65 at 390 �C due to the degra-dation of active cellulose with a lowmolecule weight formed in themain devolatilization stage which required lower energy [144].

The downward trend in E was owing to the breakdown of cel-lulosewhose E decreased with the increased a [145]. Ball et al. [146]stated that the charring of the lignin pyrolysis was an exothermalprocess with a low E value. The significant decrease in the E valuecan be also associated with the significant amount of heat and themore activated fractions of volatiles in LP [147]. Our results sup-ported the previous findings [148]. During the third stage, E rosesharply due to the combustion of the temperature-resistant


pyrolytic fraction with a stronger chemical bond that requiredhigher energy and higher temperature to terminate [149]. A largeamount of coke with low reaction activity was formed in this stage,thus rapidly increasing E [150].

The E value underwent the two increasing trends of the minorand major peaks of mass loss in the third stage during the ranges of

Fig. 4. (a) Estimation of activation energy (E) by the KAS method; and (b) relationshipbetween E and a with catalysts and (c) between Ea and catalysts.

0.65e0.8, and 0.8-0.95 due to the decreased reactivity of the re-sidual materials, respectively. Emin and Emax were observed ata¼ 0.65 at 390 �C and a¼ 0.85 at 450 �C, respectively. The Ea2(182.0 kJ/mol) and Ea3 (163.2 kJ/mol) values for the second andthird stages (Tables Se3) showed more energy requirement for thecombustion of volatiles than fixed carbon [151]. Given the highvolatiles content (75.53%) (Table 1), the IR analysis (Fig. 1b), and thepyrolysis functional group analysis (Fig. 6bed), it can be speculatedthat a large amount of -OH, C-O, and C¼O groups were generatedduring the pyrolysis. These active molecules participated in thecomplex gas-phase and gas-solid reactions and produced moreactive molecules, thus intensifying the emissions, and the energyrequirements.

The higher E values of the volatiles combustion at a> 0.85 canbe attributed to the resistant pyrolytic fraction with high thermalstability from the secondary reactions. Solidification between thesecondary char and tar resulted in the formation of aromatic semi-coke through condensation and cross-linking, thus inducing thestrong poly-condensation effect between polycyclic aromatic nu-cleus and residual ash [152]. Although the energy of C¼C bond ishigher than that of the C-H, C-O and C-C bonds, the heat release bythe volatiles combustion in the early stage facilitated the combus-tion of fixed carbon, thus rendering Ea3 smaller than Ea2 [153].

The Na2CO3 and K2CO3-increased E values at the ignition point(az 0.1), and the MgO, Al2O3, and MgCO3-reduced E values resul-ted from the Na2CO3 and K2CO3-enhanced devolatilization, andthus, the formation of the high-concentration active moleculesreducing the difficulty of ignition. The change in Ea2 was closelyrelated to the catalyst type. K2CO3 increased Ea2, while MgO andMgCO3 reduced it, with Al2O3 and Na2CO3 having no significanteffect. All the catalysts decreased Ea3. The carbonates affected Emore than did the oxides in the third stage, while K2CO3 as thestrongest catalyst decreased Ea3 most for the coke oxidation, as wasconsistent with Section 3.3. The relationship between Ea and thecatalysts (Fig. 4c) showed that Ea of the control LP sample (176.4 kJ/mol) dropped to 175.3, 160.6, 154.9, 156.8 and 136.6 kJ/mol withK2CO3, Al2O3, MgO, Na2CO3, and MgCO3, respectively.

3.6. Gas evolutions of LP combustion and pyrolysis

TG-FTIR was applied to characterize the gas evolutions withtime and temperature in order to overcome the drawback that Py-GC/MS allows only for the non-continuous analysis. Thus, theamount and identity of the gases of the pyrolysis and combustioncould be matched with the thermal degradation characteristics[154]. Fig. 5 displays the Gram-Schmidt curve, the (D)TG curves,and the 3D infrared spectrum of the LP combustion and pyrolysis at10 �C$min-1. The temporal evolution of the gases showed that theirmaximum concentrations coincided with the peak temperatures ofthe DTG curve. Compared to the pyrolysis, the presence of O2allowed the decomposition to advance fiercely. The IR spectra wereselected according to the characteristic temperatures of the maindecomposition stages of 230 �C (shoulder), 330 �C (peak) and430 �C (tail) for the pyrolysis, and of 250 �C (shoulder), 330 �C (peak1), 400 �C (peak 2) and 453 �C (peak 3) for the combustion(Fig. 6aeb). IR spectra with temperatures are depicted in Fig. 6cedto detail the evolutions of gases or functional groups. In theassignment of the functional groups to the evolved gases, relatedliterature [155e160] was referred to.

The combustion process produced a large amount of CO2

(2400e2240 cm�1, 1340, and 670 cm�1) due to the fracturing orreforming of C¼O and C-O-C [161]; of water (4000e3500 cm�1, and1875 to 1275 cm�1) due to the evaporation; and of CO(2230e2000 cm�1) due to the breakage of ether bridge connectinglignin structural units and diaryl ether linkages [162,163] (Fig. 6a).

Fig. 5. (a) (D)TG and Gram-Schmidt curves; and 3D infrared spectrum for (b) com-bustion and (c) pyrolysis of LP at 10 �C$min-1.


The obvious C¼O band (1880e1650 cm�1) indicated aldehydes,ketones, and carboxylic acids [164]. The fingerprints absorbancebetween 1600 and 650 cm�1 came from alkanes, alcohols, phenols,ethers, and lipids [165e167]. The characteristic wavelength ab-sorption peaks of Fig. S-2 showed that the evaporation, and the CO2release appeared at below and above 180 �C, respectively, with thelatter ceasing at 510 �C. At 300 �C, the main gas releases were CO2,and H2O, the former of which formed during the cracking of theC¼O and -COOH groups in hemicellulose at the low temperatures.The absorption bands for CO (2230e2000 cm�1), stretching vibra-tion (3100e2850 cm�1), in-plane bending vibration(1475e1350 cm�1), and in-plane bending vibration (720 cm�1)

were attributed to -C-Hn. The absorption bands for C¼O(1850e1600 cm�1), and C-O (1250e900 cm�1) increased gradually.This indicated that the slight amounts of aliphatic hydrocarbons,C¼O and C-O groups were generated at the low temperatures.However, the disappearance of these functional groups at above400 �C indicated the production of organic compounds in the earlycombustion and pyrolysis stages. The coke was burned violently at453 �C during which the absorptions of CO2, CO, and H2O peaked,while CH4 (3016 cm�1) began to appear. The ranges of room tem-perature to 180 �C, and 200e400 �C belonged mainly to the evap-oration and main decomposition stages, respectively, with the by-products of H2O, CO2, and CO having a similar trend (Fig. 6c).Also, a small amount of aliphatic hydrocarbons, C-O, and C¼Oemerged and peaked at 280 �C whose main combustion stage wasbetween 360 and 510 �C where CO2, and H2O were produced in alarge amount. At above 510 �C, there existed only weak absorptionpeaks of CO2, and H2O.

There were various kinds of functional groups generated duringthe pyrolysis whose complex by-products had a weaker absorptionintensity than did the complex combustion ones (Fig. 6bed). In therange of room temperature to 180 �C, H2O, and CO2 were formed ina small amount due to the decomposition of hemicellulose, and thede-polymerization of cellulose. At above 200 �C, the absorptionbands assigned to C¼O (1900e1650 cm�1), C-O, C-H, and C¼Cbegan to appear. In this range, esters (1780e1735 cm�1), aldehydes(1780e1705 cm�1), and carboxylic acids (1725e1700 cm�1) allshowed absorption peaks which suggested that carbonyl com-pounds may contain more esters and aldehydes. CO appeared atabove 230 �C later than the other functional groups. The absorptionintensity of saturated C-H (3000e2700 cm�1), unsaturated C-H(3100e3010 cm�1), H2O, CO, CO2, C-O, and C¼O peaked at 330 �Cand then declined gradually.

At 450 �C, the absorption bands of C¼O, C¼C, and C-O dis-appeared. Only the absorption peaks of CO2, and the small amountsof CO, saturated C-H, and unsaturated C-H (3100e3010 cm�1, and1650 to 1450 cm�1) were attributed to the charring process thatreleased CO2, H2O, and CH4. The absorption peak of CH4 appeared at330 �C and peaked at 530 �C due to the breakage of the -O-CH3,-CH3 and C¼C groups [168], in particular, the breakage of the -O-CH3 bond in the lignin structure. The absorption peaks of SO2 andNH3 at 1342 cm�1 and 966 cm�1, respectively, were very weak dueto the low S and N contents, with the higher absorption intensity ofNH3 than SO2 (Fig. S-2). During the pyrolysis, SO2 and NH3 had onlyone absorption peak due to the decomposition of volatiles in therange of 200e450 �C. During the combustion, SO2 and NH3 showedtwo obvious absorption peaks due to the decomposition of vola-tiles, and the coke oxidation at 300 and 450 �C, respectively. SO2and NH3 had the same release temperatures during the pyrolysisand combustion. This suggested that N and S had the same releasepatterns, consistent with the TG-MS gas analysis in Section 3.7.

3.7. Emission analysis of LP combustion with catalysts

The weight loss curves of the TG-MS experiments are shown inFig. S-3, while the gas emission characteristics of the catalyticcombustion are shown in Fig. 7. H2 and CH4 had similar emissionpatterns with two peaks of different intensities which decreasedwith the catalysts. The carbonates played a more significant rolethan did the oxides.

H2O emission (m/z¼ 18) showed three peaks in three stagesamong which the decomposition of volatiles produced the mostefflux of water vapor. The addition of the carbonates reduced itsintensity making water evaporate at the lower temperatures. MgOand MgCO3 are known to transform very easily into the basicmagnesium carbonate (crystal water) {(MgCO3)4Mg(OH)2$5H2O}

Fig. 6. FTIR at characteristic temperatures for (a) combustion and (b) pyrolysis; and gas evolutions during (c) combustion and (d) pyrolysis.


by absorbing H2O and CO2. Since the evaporation temperatures ofthe basic magnesium carbonate and Mg(OH)2 were 282.8 and428 �C, respectively, the H2O emission intensity of MgO and MgCO3declined. Since Na2CO3 and K2CO3 involved numerous chemicalreactions such as the carbonate decomposition, the char-M2CO3reactions, and the formation of some thermally stable bimetalliccarbonate, as discussed in Section 3.3, water may have participatedin these complex reactions. The lower emission intensity of CO (m/z¼ 28) than CO2 was attributed to the cleavage of C¼O and C-O-Cgroups at 300 �C, the char-by-M2CO3 interactions (2C þ M2CO3 %

2COM þ CO) [36,119,130], or the Boudouard-reaction(C þ CO2¼ 2CO) at 500 �C. The catalytic reduction of NOdecreased CO emission which ceased at above 500 �C [169,170].

The distributions of NOx and SOx depended on the N, S andvolatiles contents of the fuel, the heating rate, the final tempera-ture, the surrounding atmosphere, and the interaction among ele-ments and the fuel. Some AAEMs and complex mineral metalsystems participated in the heterogeneous catalytic reduction ofNOx in the coke pore systems or generated the high temperaturestable salts to reduce SOx emissions. The higher emission of NO (m/z¼ 30) than NO2 (m/z¼ 46) was indicative of NOx emissions beingdominated by NO [171]. The emission curves of NOx and CO2 hadthe similar patterns. The majority of the biomass N was easilyreleased with the combustion of volatiles [172], while theremaining fraction remained bound to the char structure and wasreleased with the char oxidation. Therefore, the emissions of

volatiles N and char N at 320 and 475 �C, respectively, releasedmore NO in the devolatilization than coke oxidation. The trans-formation of char-N to NO was generally found to be 50% lowerwith the O2/N2 than volatiles-N due to the decomposition of amines(e.g., proteins, and amino acids) [173e175].

The explanation for the decreased NO emission despite theincreased weight loss rate and mass in the devolatilization stagemay be that the additions of Na and K reduced the concentrations ofHCN and NH3, the precursors of NO. Also, Na and K may havecombined with some organic structures in coke to form a variety ofcomplexes whose high surface area facilitated the fuel deposition.The enhanced adsorption capacity of the carbon surface activatedby alkali metal K was most likely to cause more volatiles N pre-cursors to remain in the carbon skeleton structure until they werereleased at higher temperatures [176]. The reduction of the NOemission intensity during the coke oxidation may closely relate tothe CO formation on the hot char surface. The catalytic reductionreaction on the carbon surface of 2NO þ 2(eCO) % N2 þ 2CO2, orthe reduction at a free carbon site of NO þ (eC) % (eCO) þ1/2 N2were both beneficial to the decrease in NO emission [169,170]. Thecatalysts decreased NOx emission. Na2CO3, and K2CO3 shifted theoverall emission temperature of NOx to the low temperature zone,while MgCO3 and Al2O3 moved the peak emission temperature ofcoke-type N to the high temperature zone; namely, delayed therelease of char N. The NOx emission peaks with Na2CO3 and K2CO3at above 520 �C were attributed to the interaction among Na2CO3,

Fig. 7. Gas emission curves of LP combustion with catalysts as obtained by TG-MS at 20 �C$min-1.


Table 4Normalized intensities of LP with catalysts at 20 �C$min-1.

Samples Normalized intensities (10�7 A/g)

K2CO3 Na2CO3 MgCO3 MgO Al2O3 LP

m/z¼ 2 209.22 281.26 267.33 383.81 402.82 421.61m/z¼ 16 1.01 546.18 574.95 1619.51 2025.37 2124.41m/z¼ 17 999.29 1454.98 1146.97 1626.13 1673.92 1709.07m/z¼ 18 4027.85 5876.59 4615.00 6381.95 6615.41 6781.83m/z¼ 27 36.30 47.45 73.58 123.64 120.07 124.54m/z¼ 28 3337.56 3308.29 3558.91 5647.84 4254.50 5093.03m/z¼ 30 191.24 223.45 218.59 262.61 269.19 273.69m/z¼ 44 12600 13500 12300 14000 13800 14300m/z¼ 46 55.05 63.20 60.71 71.10 72.11 73.52m/z¼ 64 1.31 3.21 3.82 3.98 4.65 4.89m/z¼ 80 0.31 0.45 0.54 1.24 0.95 1.14Total 21459.14 25305.06 22820.40 30121.81 29238.99 30907.73


K2CO3, and the coke active sites, which formed the thermally stablecomplexes, thus delaying the release of some char N until itsoxidation at the higher temperatures.

The CO2 evolution at below 520 �C with K2CO3 resulted mainlyfrom the volatiles decomposition, and the coke oxidation(exothermic peak), while the CO2 evolution between 520 and700 �C (with a peak at 610 �C) corresponded to the oxidation ofcarbonaceous materials (exothermic peak) (Figs. S-1 and S-3). Thelatter stage led to the various complexes formed by the combina-tion of the alkali metal and coke active sites, and the char-by-M2CO3 interaction (CþM2CO3% C-M2Oþ CO2, withM referring toNa and K). At above 700 �C, the CO2 evolution was ascribed to thestabilization of the recalcitrant residues such as carbonate andchelating bidentate carbonate (endothermic peak).

The CO2 evolution with Na2CO3 between 510 and 700 �C stem-med from the oxidation of the complexes, and the char-by-M2CO3interaction, while the later peak was due to the decomposition ofcarbonates [119]. Some bimetallic carbonates with the high thermalstability may be retained in the combustion residues [172]. CO2 waseasily absorbed by the substances containing Ca, K, and Na at thehigh temperatures to form salts. Na2CO3 and K2CO3 lowered thetemperatures of the CO2 emission peaks, in particular, the firstpeaks at 338 �C by 25 and 32 �C, respectively. Similar to the NOx

emissions, the decreased CO2 emission with the increased weightloss rate in the devolatilization stage suggested that some C ele-ments were not lost in the form of CO2, or parts of CO2 wereabsorbed again. The overlapped peaks of the lignin decomposition,and the coke oxidation demonstrated that the decomposition ofsome C structures was hindered or delayed which agreed with theNOx emission patterns. However, the higher reduction degree of NOthan CO2 by Na2CO3 and K2CO3 during the coke combustion wasmainly due to the simultaneous generation of CO2 during the het-erogeneous catalytic reduction of NO. The higher CO2 emissionpeaks at above 520 �C with Na2CO3 than K2CO3 pointed to thestronger protection effect of Na2CO3 on the C structures. Overall,the CO, CO2 and NOx emissions were closely coupled and related tothe catalytic performances of Na2CO3 and K2CO3 in the differentcombustion stages of LP.

The SOx emissionwas concentrated in the devolatilization stage,as was supported by the previous study [177]. Aliphatic and aro-matic S compounds were reported to be conducive to the formationof S-containing gases in the low temperature via the cleavage of C-Sbond, thus forming the odor pollutants such as H2S, COS, and CS2[178,179]. The alkali addition was demonstrated to be an efficientway to cut down these odor pollutants during the pyrolysis ofsewage sludge at low temperatures [180,181]. K, Na, and Ca in theforms of the oxides or the carbonates were found to combine withSOx in an oxidizing atmosphere to form stable sulfates in the solidphase between 400 and 1200 �C, thus inhibiting SO2, whilesimultaneously generating CO2 which was released in smallamounts at above 500 �C [182e184].

The decomposition of CaSO3 releasing SO2 was reported to occuronly at above 800 �C [185]. CaSO3 when completely oxidized toCaSO4 did not decompose in the air atmosphere at below 1150 �Cand remained in the final product [186]. At the same time, H2Sreactedwith K, Mg and Ca to form sulfides [187]. The high P contentwas reported to affect the SOx capture in the co-incineration ofsludge with biowaste due to the competitive reaction of P with Caor Mg [182,188]. K2CO3 showed a stronger capture effect on SO2than did Na2CO3 and MgCO3 due to its higher alkalinity and cata-lytic activity. Our finding that the alkali metals (Na and K) exhibiteda more significant effect on the reduction of SOx than did thealkaline earth metal (Mg) was in agreement with the results byCheng et al. [181].

Due to the S fixation effect of Na2CO3 and K2CO3, the SO2

emission decreased in the devolatilization stage. The sulfate andcomplex minerals formed may not have broken down within thistemperature range in our experiment, thus leaving one obviousdevolatilization peak for SO2 release. Al2O3 exerted a more signif-icantly catalytic effect on the coke oxidation, with a slightly higherSO2 emission at 500 �C than did the other catalysts. The catalystsdecreased the peak emission temperature of SO3 by 40 �C for all thetreatment groups.

Table 4 shows the normalized intensities of the emissions of theselected gases from the catalytic combustion with respect to tem-perature. The dominant gas emissions of CO2, CO and H2O, and thelower emission intensity of SOx and NOx than CO2 emission wereconsistent with the results of the elemental analyses (Table 1). TheNOx emission was higher than the SOx emission during the LPcombustion, as was consistent with the results of the elementalanalysis. NOx was emitted mainly as NO. Overall, all the catalystsreduced the gas emissions by the varying degrees according towhich the carbonates performed better than the oxides. K2CO3reduced the gas emissions (except for CO and CO2) most, whileMgCO3 brought down the CO2 emission most. K2CO3 produced H2Oleast and inhibited CH4 emission, while Na2CO3 produced CO least.Among the carbonate catalysts, K2CO3 andMgCO3 performed betterin reducing the gas emissions from the LP combustion.

3.8. Effects of temperature and MgCO3 on pyrolysis by-products

The detailed pyrolysis mechanism of hemicellulose, cellulose,and lignin becomes quite complicated due to their different sourcesand compositions. Yet, some common pyrolytic by-products haveappeared recurrently from the related literature [189e192]. Thereactions that generated the main by-products of the thermaldegradation of biomass (e.g., LG, 5-HMF, FF, HA, HAA, GA, and PA)included dehydration, ring fragmentation, ring contraction, ringopening, and isomerization. The depolymerization of glucose withlowactivation energy tend to produce LG due to its thermally stablestructure, while it produces HAA via ring fragmentation. Thedehydration of hydroxyl groups at C1 and C6 can directly form py-ranose, while the dehydration of glucofuranose via the tautomeri-zation of glucose can directly produce furanose. Althoughhemicellulose has a very complicated structure composed of a largenumber of sugar monomers, its main pyrolysis by-products aredehydrated furanose, pyranose, aldehyde, ketone, alcohol, acid, andother small molecular structures with usually less than six C atoms.Since the C-O-C and C-C bonds play a major role in connectinglignin monomers, its pyrolysis by-products are usually dominatedby lignin-derived phenolic compounds with -O-CH3 structuresamong which p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S)are the most important.

Fig. 8. (a) Pyrolytic by-products of LP at 500 �C; (b) total ion current chromatogram; and (c) by-product distributions with different ranges of m/z.


The structures of the LP pyrolytic by-products at 330 and 500 �C,and with 5% MgCO3 at 330 �C are shown in Fig. 8. LP had fewertypes of pyrolysis by-products than did the other biomass materials(Table 5). The raised pyrolysis temperature, and the catalysts didnot significantly change the by-product types (Fig. 8b). Forexample, the LP pyrolytic by-products at 500 �C were categorizedinto the following four main groups, as was similarly reported byIbrahim et al. [193]: (1) a large number of terpenoids and steroids;(2) a certain amount of lignin-derived phenolic products; (3)aliphatic compounds with long chain; and (4) a small amount ofbenzene, furan, pyran, alicyclic ketone, alcohol, acid, and othersmall molecules.

LP appeared to contain high levels of lignin and extractives. Theextractives of the lignocellulosic biomass can be grouped into thefollowing three categories: (1) aliphates (fats and pwaxes); (2)terpenoids and steroids; and (3) phenols. The ratio of these threehighly depends on the type and geographical location of biomass[194]. As the two typical biomarkers, terpenoids and steroids arethe natural products of the physiological activity of the animals andplants. Terpenoids and steroids are ubiquitous in crude oil andplants and identified using the characteristic fragment ions of ter-penoids such as m/z¼ 191, 217 [195e198]. The chromatograms ofm/z¼ 191, 217, and the mass spectra of the corresponding terpe-noids and steroids with the retention times of 23.77, 37.95 and40.67min are shown in Fig. S-4.

Terpenoids and steroids are dominated by tetracyclic diterpane(#44, 45, 48, 55, 56, 58 to 60, 62), sterane (#57), and pentaclo-triterpane (#63 to 66). All contained the main polycyclic structureof tetracyclic diterpane (#44) and pentaclotriterpane (#63, 64)marked in red in Fig. 8a. The slight difference was due to thenumber of C atoms, the state of hydrogen atoms, and the side chainstructure. The phenolic by-products (#14, 18, 20, 21, 23 to 29, 31 to43) mainly consisted of the two typical structures of guaiacyl-lignin(G) and syringyl-lignin (S). The aliphatic compounds (#46, 47, 49 to53) were mainly the long chain fatty acids and esters, while smallmolecular substances (#1 to 13, 15, 22, 35, 42) were mostly furans,alicyclic ketones, and aldehydes. The fractions of the four groups inLP were estimated at 71.87, 15.51, 9.95, and 2.78% at 330 �C and78.35, 12.87, 5.83, and 3.20% at 500 �C, respectively.

The elevated temperature increased the contents of terpenoids,steroids, and small molecules but decreased phenols and thealiphatic compounds. MgCO3 at 330 �C changed the fractions of thefour groups to 74.12, 15.34, 7.59, and 3.37%, respectively, whichslightly changed phenols, and the small molecular compounds.MgCO3 reduced the aliphatic compounds and increased terpenoidsand steroids. The content of molecules withm/z of 50e100was verylow, but the contents of molecules withm/z of 100e300 and 400 to500 decreased, while those with m/z of 50e100 and 300 to 400increased with the increased temperature (Fig. 8c). This was mostlikely due to the fracture of long chain at the high temperature, and

Table 5Identification and relative molar abundance (%) of compounds released from LP with(out) 5% MgCO3 at a characteristic temperature based on Py-GC-MS.

No. Compounds Major ions (m/z) LP (330 �C) LP þ MgCO3

(330 �C)LP (500 �C)

ART(mim)

Area(%)

ART(mim)

Area(%)

ART(mim)

Area(%)

1 Acetic acid, C2H4O2 (G4) 60 / / / / 1.50 0.182 2-methyl-2-butenal, C5H8O (G4) 55,65,84 / / / / 2.16 0.063 Toluene, C7H8 (G4) 57,63,74,91 / / / / 2.33 0.104 Propanal, C3H6O (G4) 58,91 / / / / 2.49 0.045 2,3-Dihydro-5-methyl-furan, C5H8O (G4) 54,69,84 2.69 0.02 2.74 0.06 2.68 0.036 Furfural, C5H4O2 (G4) 50,67,95 3.16 0.11 3.16 0.06 3.17 0.257 Furfuryl alcohol, C5H6O2 (G4) 69,72,98 3.57 0.05 3.59 0.09 3.60 0.088 Dimethybenzene, C8H10 (G4) 55, 86,91,106 / / / / 3.79 0.119 2-ethenyl-2-Butenal, C6H8O (G4) 51,68,81,96 4.08 0.05 4.07 0.06 4.08 0.0410 2-methyl-2-Cyclopenten-1-one, C6H8O (G4) 53,67,78,96 / / / / 4.51 0.0311 2-Acetylfuran, C6H6O2 (G4) 51,67,81,95,110 / / / / 4.62 0.0112 3,4-dihydro-2H-pyran, C5H8O (G4) 55,69,84 / / / / 4.74 0.0713 2-hydroxy-2-Cyclopenten-1-one, C5H6O2 (G4) 55,69,98 5.00 0.11 4.99 0.28 5.06 0.2414 Phenol, C6H6O (G2) 51,66,79,94 6.43 0.08 6.47 0.08 6.47 0.2915 4-Piperidinemethanamine,C6H14N2 (G4) 58,65,84,96,114 6.56 0.51 6.61 0.58 6.64 0.2516 2-methyl-cyclohexanone, C7H12O (G4) 55,68,84,97,112 7.02 0.15 7.07 0.19 7.08 0.1717 3-methyl-1,2-cyclopentanedione, C6H8O2 (G4) 55,69,83,97,112 7.62 0.18 7.66 0.21 7.47 0.1818 2-methyl-phenol, C7H8O (G2) 55,63,79,95,108 8.2 0.12 8.25 0.09 8.20 0.1019 4-hydroxy-cyclohexanone, C6H10O2(G4) 55,67,81,96,114 8.4 0.14 8.49 0.27 8.40 0.1020 3-methyl-phenol, C7H8O (G2) 55,63,77,95,108 8.73 0.08 8.72 0.10 8.73 0.1521 2-methoxy-phenol, C6H5O2 (G2) 53,63,81,95,109,124 8.84 0.56 8.87 0.78 8.86 1.1622 4-Mercaptophenol, C6H6OS (G4) 55,71,97,111,126 9.50 0.03 9.53 0.08 9.52 0.0423 2,3-Dimethylhydroquinone,C8H10O2 (G2) 67,71,91,107,123,138 11.01 0.01 11.02 0.01 11.01 0.1424 Creosol, C8H10O2 (G2) 67,77,95,123,138 11.43 0.31 11.46 0.39 11.37 0.5725 4-methoxybenzene-1,2-diol, C7H8O3 (G2) 51,79,97,125,140 13.14 0.19 13.15 0.18 13.08 0.1826 4-ethyl-2-methoxy-phenol, C9H12O2 (G2) 77,91,122,137,152 13.38 0.57 13.40 0.55 13.36 0.4327 2-methoxy-4-vinyl-phenol, C9H10O2 (G2) 63,77,107,135,150 14.17 2.42 14.18 2.92 14.18 1.9928 2,6-dimethoxy-phenol, C8H10O3 (G2) 68,94,111,139,154 15.01 0.64 15.03 0.58 15.03 0.6529 2-methoxy-4-(1-propenyl)-phenol, C10H12O2 (G2) 65,77,91,121,149,164 15.11 0.42 15.12 0.45 15.11 0.3330 a-ethyl-4-methoxy-Benzenemethanol, C10H14O2 (G4) 57,77,91,122,137,166 15.33 0.11 15.34 0.12 15.32 0.1131 Vanillin, C8H8O3 (G2) 53,81,109,123,152 16.06 0.41 16.09 0.39 16.10 0.2832 2-methoxy-5-(1-propenyl)-phenol, C10H12O2 (G2) 55,77,91,103,149,164 16.19 0.16 16.20 0.18 16.19 0.2133 2-methoxy-4-(1-propenyl)-phenol, C10H12O2 (G2) 77,91,103,149,164 17.07 2.29 17.08 2.35 17.08 1.9134 Apocynin, C9H10O3 (G2) 65,77,108,123,151,166 17.96 0.37 17.96 0.37 17.90 0.2935 2,5-Dimethoxybenzoic acid, C9H10O4 (G4) 60,79,91,107,151,167,182 18.63 1.00 18.64 0.80 18.63 0.3436 Guaiacylacetone, C10H12O3 (G2) 60,77,94,122,137,180 18.77 0.81 18.78 0.52 18.77 0.5037 2-tert-Butyl-4-methoxy-phenol, C11H16O2 (G2) 77,91,122,137,165,180 19.40 1.20 19.41 1.06 19.41 0.6938 2,6-dimethoxy-4-allyl-phenol,C11H14O3 (G2) 65,77,91,119,151,179,194 20.12 0.26 20.12 0.26 20.12 0.1839 2,6-dimethoxy-4-allyl-phenol, C11H14O3 (G2) 65,77,91,119,137,151,194 21.04 0.30 21.05 0.32 21.04 0.2640 Coniferyl alcohol, C10H12O3 (G2) 63,77,91,124,137,180 21.47 0.62 21.47 0.50 21.49 0.3941 (E)-2,6-dimethoxy-4-propenyl-phenol, C11H14O3 (G2) 65,77,91,119,151,179,194 21.95 1.50 21.94 1.30 21.95 0.9842 2,3,4-trimethoxy-benzaldehyde, C10H12O4 (G4) 60,77,107,135,178,196 22.59 0.74 22.60 0.57 22.60 0.2543 (E)-Coniferyl alcohol, C10H12O3 (G2) 65,77,91,124,137,180 22.71 1.67 22.72 1.57 22.73 1.5144 Tetracyclic diterpane, C19 (G1) 69,81,121,136,191,207,217,245 23.77 5.87 23.77 5.98 23.76 4.0745 3-Ethyl-3-hydroxyandrostan-17-one, C21H34O2 (G1) 55,81,95,107,149,191,203,217,242 24.77 0.37 24.77 0.50 24.76 0.3346 Methyl-5,8,11-tetradecatrienoate, C16H26O2 (G3) 55,67,81,93,107,121,135,153,175,194,236 26.14 1.12 26.13 1.25 26.13 1.1247 n-Hexadecanoic acid, C16H32O2 (G3) 55,69,83,97,115,129,157,171,185,199,213,227,256 26.49 1.60 26.49 1.81 26.52 0.8948 1-Testosterone, C19H28O2 (G1) 55,81,93,107,135,191,204,256 26.61 0.92 26.60 0.90 26.60 0.5349 i-Propyl6,9,12-hexadecatrienoate, C19H32O2 (G3) 55,67,81,91,107,121,135,147,161,175,243 26.72 0.96 26.69 0.49 26.71 0.6150 Methyl-3-cis,9-cis,12-cis-octadeca trienoate, C19H32O2

(G3)55,67,81,91,107,121,135,152,161,202,256 27.15 2.78 27.17 0.55 27.18 0.54

51 Methyl-6,9,12-pentadecatrienoate, C17H28O2 (G3) 59,81,93,107,121,135,153,177,217,260 28.30 1.31 28.37 0.82 28.38 0.9052 6-Octadecenoic acid, C18H34O2 (G3) 55,67,81,97,109,123,137,152,165,181,194,220,264,282 29.09 1.49 29.15 1.25 29.15 0.9753 Ethyl hexadecanoate, C18H36O2 (G3) 55,69,83,97,115,129,152,185,199,216,241,284 29.42 0.23 29.47 0.22 29.47 0.1954 1,2-Benzenedicarboxylic acid, 1,2-butyl ester,

C16H22NO4 (G3)57,70,91,104,121,131,149,167,207,254,279 34.63 0.46 34.64 1.20 34.63 0.61

55 Androst-5-en-4-one, C19H28O (G1) 55,77,91,115,128,152,169,185,197,211,239,272 35.95 1.44 35.95 0.72 35.96 0.8656 Androst-5,7-dien-3-ol-17-one, C19H26O2 (G1) 55,69,77,91,115,131,151,165,183,207,253,286 37.24 0.34 37.24 0.17 37.24 0.5857 Sterane, C25 (G1) 69,81,95,121,137,191,207,217,231,341 38.01 5.72 38.04 26.45 38.03 23.9758 Cholesta-3,5-diene, (G1) 57,69,91,105,121,145,191,207,218,247,353,368 38.69 1.34 38.68 0.14 38.68 2.1959 6,16-Dimethylpregn-4-ene-3,20-dione, C23H34O3 (G1) 58,77,94,106,122,137,165,189,207,218,265,280,344 38.96 1.34 38.95 0.42 38.96 2.1760 (3b,5b,16a)-3,17-dihydroxy-16-methyl- Pregnan-20-

one, C22H36O3 (G1)55,69,91,105,121,137,175,191,207,218,253,356,392 40.01 2.42 40 0.63 40.01 2.57

61 Nornoehopene, (G1) 57,69,81,109,121,137,177,192,203,217,259,381,396 40.67 38.93 40.67 31.46 40.65 29.9562 Lathosterol, C27H46O (G1) 57,69,79,91,105,133,191,207,218,253,275,301,353,386 41.14 1.35 41.13 0.51 41.13 1.2963 Lupane, (G1) 55,81,91,121,137,177,191,207,218,255,315,412 42.29 2.15 42.28 0.76 42.28 1.9964 Oleanane, (G1) 55,69,81,91,105,119,133,159,191,207,218,255,300,351,412 42.64 5.51 42.64 3.20 42.62 4.6465 Hopane, (G1) 55,69,81,91,107,121,137,175,191,207,218,255,341,381,412 43.23 3.40 43.22 1.92 43.21 2.5266 Pentacyclotriterpane, C30 (G1) 57,69,81,95,107,121,137,149,177,191,207,218,257,341,412 43.68 0.77 43.67 0.36 43.67 0.69

Terpenoids and steroids (G1); phenolic compounds (G2); aliphatic compounds (G3); small molecular substances (G4).



the shedding of some side chain in phenols and polycyclic terpe-noids. MgCO3 slightly changed the small molecule products withm/z of 50e200, significantly decreased the products with m/z of200e300, and terpenoids with m/z of 400e500 and significantlyincreased thosewithm/z of 300e400. MgCO3 played a catalytic rolein long chain acids, esters, and the macromolecular terpenoid andsteroid compounds. This in turn promoted the cleavage of the sidechains of the macromolecular terpenoids and steroids or thedeoxygenation of the long chain fatty acids and esters. The deox-ygenation of these oxygenates produced olefins as well as the morestable polycyclic compounds by the dehydrogenation and poly-merization [199].

Harrison et al. [200] adopted sesquiterpene to produce a biofuelwith lower viscosity and higher volumetric heat of combustion bythe hydrogenation, dehydration, and isomerization reactions andsuggested the sustainable multicyclic sesquiterpenes as the versa-tile feedstock alternative to the ultra-performance fuels that com-bined the high densities with high cetane numbers. The main by-products of the LP pyrolysis were polycyclic terpenoids and ste-roids rich in C and H. LP appeared to have the potential to be used asan advanced biofuel [201,202].

The structures of most LP components were not completelydecomposed between 330 and 500 �C during the flash pyrolysis.This may be due to its high content of lignin and extractive com-positions such as terpenoids, and steroids. Since most extractivesfeatured thermal instability and were prone to the decompositionat low temperatures. The typical decomposition temperatures forthe extractives, hemicellulose, cellulose, and lignin were 205, 270,327 and over 327 �C, respectively [203]. Lignin was prone to theproduction of coke via the polymerization and aromatization re-actions during the pyrolysis. MgCl2 was reported to promote thethermal cracking of cellulose to produce more coke rather than tar[204]. Kþ and Ca2þ were shown to increase the amounts of thearomatic C¼C structures, and the surface C¼O groups in biochar[130]. Therefore, during the fast pyrolysis, only some parts of thethermo-unstable biomass components and extractive composi-tions were decomposed to generate the by-products of terpenoids,steroids, and phenols. Since the pyrolysis by-products are directlyrelated to the biomass type, part and location, their analyses bycontrolling for the pyrolysis parameters remain to be exploredthrough the use of a fixed-bed reactor.

4. Conclusion

The TG-FTIR, TG-MS and Py-GC-MS analyses were performed tocharacterize the catalytic combustion and pyrolysis of litchi peels,and their by-products in terms of their bioenergy performancesand emission reductions. The main conclusions derived from thepresent experimental study can be summarized thus:

(1) Owing to its low ash, N and S contents, high volatiles andcalorific value, and high-value pyrolytic by-products of ter-penoids, and steroids, LP appeared to possess the clear ad-vantages for bioenergy generation.

(2) Na2CO3 and K2CO3 exerted a greater influence on each stageof the LP combustion than did MgCO3, MgO, and Al2O3. Theyplayed a main role in accelerating the devolatilization anddelaying the coke burnout. Among the five catalysts, Al2O3

most strongly catalyzed the coke oxidation. The catalystsreduced the comprehensive combustion index (S) and theaverage activation energy (Ea) to the different extents.MgCO3 decreased Ea the most.

(3) The absorption peaks of C¼O, C-O, and C-H were observed atthe initial stages (<300 �C) of both LP combustion and py-rolysis. The gas evolutions were CO2, CO, and H2O during the

main combustion stage between 300 and 510 �C and CO2,aliphatic C-H, C¼O, and C-O during the main pyrolysis stagebetween 200 and 430 �C above which only the smallamounts of CO2 and CH4 were released. The types of thefunctional groups released from the pyrolysis by-productswere identified using Py-GC/MS analysis.

(4) The catalysts reduced the H2O, CO2, CO, NOx and SOx emis-sions in different amounts. K2CO3 decreased the NOx and SOx

emissions the most. Both Na2CO3 and K2CO3 had a certaincatalytic reduction and fixation effect on the releases of CO2,NOx, and SOx.

(5) The main pyrolytic by-products at 330 �C were terpenoidsand steroids (71.87%), lignin-derived phenols (15.51%), ali-phates (9.95%), and small molecules (2.78%) (e.g., benzeneseries, aldehydes, ketones, and acids). At 500 �C, terpenoids,steroids, and the small molecule compounds increased,while phenols, and aliphates decreased. The addition of 5%MgCO3 decreased the aliphatic compounds but increasedterpenoids, and steroids. MgCO3 promoted the side chainsshedding the macromolecular terpenoids and steroids aswell as the secondary reactions such as the polymerizationand condensation of the aliphatic groups.

(6) The catalysts exerted the different effects on the combustionand pyrolysis of LP. In terms of the catalytic bioenergy per-formances, and the emission reductions, MgCO3 appeared tobe the best choice among the five catalysts.

Acknowledgements

This research was financially supported by the National NaturalScience Foundation of China (No. 51978175, 51608129, 51506032),the Science and Technology Planning Project of Guangdong Prov-ince (No. 2018A050506046 & 2019B020208017).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.renene.2019.09.133.

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