NO and N2O Emissions during Devolatilization and Char Combustion

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NO and N2O emissions during devolatilization and char combustion of a single biomass particle under oxy-fuel conditions at fluidized bed temperature Hao Zhou, Yuan Li, Ning Li, Runchao Qiu, and Kefa Cen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 03 Jun 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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Energy & Fuels

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NO and N2O emissions during devolatilization and char combustion of a single

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biomass particle under oxy-fuel conditions at fluidized bed temperature

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Hao Zhou*, Yuan Li, Ning Li, Runchao Qiu, Kefa Cen

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State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,

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Zhejiang University, Hangzhou 310027, China.

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*Corresponding author. Telephone: +8657187952598; Fax: +8657187951616. E-mail:

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[email protected].

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Abstract: Oxy-fuel fluidized bed combustion is a novel clean-biomass utilization technology. The

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NO and N2O emissions during oxy-fuel combustion of a single biomass particle at fluidized bed

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temperature were studied in a flow tube reactor. The method of isothermal thermo-gravimetric

11

analysis was used to distinguish devolatilization and char combustion stages of biomass

12

combustion. This work is aimed to study the effects of temperature, CO2 concentration,

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atmosphere and O2 concentration, H2O vapor addition, and biomass type on the NO and N2O

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emissions during oxy-fuel combustion of a single biomass particle at fluidized bed temperature. In

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oxy-fuel combustion, NO is rapidly formed during devolatilization stage, whilst N2O is mainly

16

formed during char combustion stage. In 30%O2/70%CO2 at T = 800℃, the conversions of fuel-N

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to NO and N2O are 11.96% and 18.98%, respectively. The conversion of fuel-N to NO reaches the

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maximum value at T = 800℃ during devolatilization stage, while it increases with increasing

19

temperature during char combustion stage. In addition, the conversions of fuel-N to N2O decrease

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with increasing temperature during the two stages. In oxy-fuel combustion, CO2 suppresses the

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NO emission and promotes the N2O emission, and H2O vapor addition promotes the NO and N2O

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reductions. Compared with air combustion, lower conversions of fuel-N to NO and higher 1

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conversions of fuel-N to N2O are observed during the two stages in oxy-fuel combustion. During

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the two stages, the conversions of fuel-N to NO reach the maximum values at = 30%, and

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the conversions of fuel-N to N2O decrease with increasing O2 concentration. A higher fuel-N

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content in biomass leads to higher NO and N2O yields but lower conversions of fuel-N to NO and

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N2O during the two stages. The present results contribute to understanding the NO and N2O

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emission mechanisms during oxy-fuel fluidized bed combustion.

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Keywords: NO, N2O, Oxy-fuel, Single biomass particle, Fluidized bed temperature

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1. Introduction

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In most countries, coal is the main resource for power generation due to its abundant reserves

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and economic benefits. However, the respirable particulate and gaseous emissions derived from

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coal combustion have caused serious environmental problems.1 The utilization of alternate energy

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source and the improvement of combustion technology are considered to deal with the

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environmental problems of coal combustion. Biomass is a renewable energy source with the

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advantages of lower risk and capital required in energy generation when compared with other

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renewable sources. Compared with coal combustion, biomass combustion has the advantages of

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CO2-neutral, low SO2 and NOx emissions.2, 3 In UK, biomass as an alternate fuel has been

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applied in the large power plants.4 In China, the installed capacity of power generation using

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biomass is planned to reach 30 GW which accounts for 3% of the total power generation by 2020.5

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Oxy-fuel combustion is a feasible technology for capturing CO2 in power plants, in which air

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is replaced by a mixture of pure O2 and recycled flue gas (predominantly consisting of CO2 and

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H2O vapor).6 As a way of biomass utilizations, fluidized bed (FB) combustion represents an 2


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economic and environmentally acceptable technology for biomass combustion, including alone

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and co-combustion.7, 8 Oxy-fuel FB combustion is the technology combining oxy-fuel combustion

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and FB combustion, and it has the advantages of fuel flexibility, uniform temperature distribution,

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and low SO2 and NOx emissions.9-11 It was found that the O2 concentration of 30% in oxy-fuel FB

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combustion could maintain the same bed temperature as in air combustion and the O2

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concentration was about 29% with wet recirculation.12 Since CO2 and H2O vapor are the main

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components in oxy-fuel FB combustion, especially in wet oxy-fuel FB combustion, the effects of

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CO2 and H2O vapor on NO and N2O should be considered in both experiments and analysis. For

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oxy-fuel FB combustion of coal, lower NOx emission and higher N2O emission were detected

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compared with air combustion.13, 14 Zhao et al. investigated the NO formation during oxy-fuel

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combustion of biomass chars and obtained that CO2 could suppress NO formation through

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heterogeneous reactions.15 Moroń and Rybak demonstrated that the NOx emission during oxy-fuel

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combustion of biomass was lower than that in air combustion.3 However, the N2O emission was

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not considered. Moreover, H2O vapor could increase the concentrations of H/OH radicals, H2 and

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CO,16 and the reduction effect of H2O vapor on NO16,

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conventional conditions. It was found that H2O vapor could also reduce the NO emission under

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oxy-fuel conditions.3

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and N2O18 was investigated under

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In general, the combustion process of biomass can be divided into the devolatilization and

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char combustion stages. NO and N2O are formed via homogeneous19-21 and heterogeneous19, 21-23

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reactions. Some studies on the NO and N2O emissions of biomass combustion have been

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conducted in FB combustors.24-26 However, these studies only focused on the total NO and N2O

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emissions in the whole combustion process. This work is aimed to study the NO and N2O 3


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emissions during devolatilization and char combustion stages. Thus, it is necessary to decouple the

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stages of devolatilization and char combustion. Some research was carried out to study the NO

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and N2O formation and reduction mechanisms during the stage of devolatilization27-31 or char

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combustion32-36 separately. However, these studies did not consider the overall performance of

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biomass combustion. There are several studies using carbon conversion to distinguish the

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devolatilization and char combustion stages of biomass combustion and investigating the NO and

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N2O emissions during the two stages in conventional combustion.19, 37, 38

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Due to the high volatile content and low energy density of biomass, its combustion

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characteristics are significantly different from those of coals. Understanding the NO and N2O

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emission mechanisms of a single biomass particle during oxy-fuel combustion at FB temperature

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is a first step in understanding the NO and N2O emissions of biomass during oxy-fuel FB

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combustion. In addition, the gas-solid reactions of formed NO and N2O over char/ash are

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minimized during single biomass combustion. In this work, the NO and N2O emissions of a single

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biomass particle during oxy-fuel combustion at FB temperature were studied in a flow tube reactor.

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In order to deeply understand the NO and N2O emission mechanisms, the method of isothermal

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thermo-gravimetric analysis (TGA)39-41 was applied to distinguish the devolatilization and char

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combustion stages of single biomass particle combustion. Due to the wide use of biomass pellet

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fuels in China,5 three types of biomass pellet fuels were considered. The work is aimed to study

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the effects of temperature, CO2 concentration, atmosphere and O2 concentration, H2O vapor

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addition, and biomass type on NO and N2O emissions during devolatilization and char combustion

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stages under oxy-fuel combustion conditions at FB temperature.

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2. Experimental 4


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2.1. Biomass samples

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Experiments were carried out with single biomass particles prepared from biomass pellet

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fuels. Three kinds of biomass pellet fuels were considered: poplar wood (PW), rice husk (RH),

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and corn stalk (CS). The biomass pellet fuels were dried in a vacuum drying box at 60℃ for

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about 10 h prior to the tests. The original biomass pellets with cylindrical shape were carved to

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spherical shape with a diameter of 7 mm. Table 1 summarizes the proximate and ultimate analyses

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of the biomass samples. The compositions of the biomass ash are shown in Table 2.

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2.2. Experimental rig and procedure

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A laboratory-scale experimental rig was set up to study the NO and N2O emissions during

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oxy-fuel combustion of a single biomass particle at FB temperature. Figure 1 shows the schematic

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diagram of the experimental rig for single particle combustion. The experimental rig mainly

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consists of a flow tube reactor, an electronic balance, a gas supply system, a steam generator, and a

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flue gas analyzer. A quartz tube was used as the tube reactor. The inner diameter and the height of

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the tube reactor are 60 mm and 500 mm, respectively. The tube reactor was heated to the set

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temperature by an electric furnace whose temperature was controlled by a temperature controller

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with ±1℃ inaccuracy. A sample holder consisted of a platinum wire, in which the biomass

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particle was placed, was linked to the electronic balance by a platinum wire. In each test, the

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biomass particle was rapidly introduced into the reactor with the set temperature. The

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time-resolved weight of the biomass particle was recorded online by the electronic balance to

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monitor the mass conversion of the biomass particle in the combustion period. The electronic

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balance used in the tests was a METTLER TOLEDO AL204 with the readability of 0.0001 g, and

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its sampling time was set to 1 s. The flow rates of O2, CO2, N2, and Ar were controlled by four 5


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ALICAT mass flow controllers (MFCs) with ±0.5% inaccuracy of full scale. Moreover, a

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high-performance peristaltic pump was used to control the flow rate of deionized water. The

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deionized water was carried by the supplied gas towards the steam generator which was a tube

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furnace. In order to guarantee the complete evaporation of the deionized water, the temperature of

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the steam generator was set to 600℃. In addition, the supplied gas was also preheated in the steam

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generator before it was introduced into the reactor. The exhausted flue gas from the tube reactor

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was condensed through a condenser, and a conical receiver placed in an ice water bath was used to

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collect the condenser water. After the processes of drying and filtration, the concentrations of O2,

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CO, CO2, NO, NO2, and N2O in the flue gas were measured online by the GASMET FT-IR

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Dx4000, and its sampling time was set to 2 s.

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In order to study the effects of atmosphere and O2 concentration, CO2 concentration, and H2O

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vapor addition, the inlet gas compositions (“O2+N2”, “O2+CO2”, “O2+CO2+Ar”, and

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“O2+CO2+H2O”) were supplied to the reactor in the tests. In order to maintain the constant

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boundary condition for biomass combustion, the total flow rate of the inlet gas was set to a rate of

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8 L/min under standard temperature and pressure (STP). Thus, the biomass particle was

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combusted in a laminar flow (Reynolds number of supplied gas flow: Ref < 2100, and Reynolds

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number of particle: Rep < 500). The typical FB temperature (700℃, 800℃, and 900℃) and the O2

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concentrations of inlet gas (21%, 30%, 40%, and 50%) were considered in this work. In most

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cases, the O2 concentration of the flue gas was over the O2 range of the gas analyzer (0~25%),

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especially for the case in which the H2O vapor was removed through the condenser

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(“O2+CO2+H2O” atmosphere). Therefore, the flue gas was diluted by introducing N2 before it

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entered the gas analyzer. To control the dilution ratio accurately, the flow rates of the flue gas and 6


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the dilution gas (N2) were controlled by two ALICAT MFCs. The tests for each case were

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conducted at least 3 times to check the repeatability, and the relative standard deviation of the

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experimental results is less than 10%.

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2.3. Analysis method

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In the tests, the temperature of the reactor was set below 1000℃. Thus, the formations of

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thermal NOx and prompt NOx are neglected, and the NOx and N2O emissions are assumed to

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derive from the fuel-N in biomass. Moreover, the NO2 concentration measured was below 1 ppm,

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so it is neglected in the tests. Thus, the NO and N2O emissions are considered in this work. NO

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and N2O are supposed to derive from both the devolatilization and char combustion stages of

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biomass combustion. The NO or N2O yield from fuel-N in biomass during devolatilization or char

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combustion stage can be determined as follows:

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Y p ,i =

QM i Vm

∫

(E-1)

Ci dt

t∈t p

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where i denotes NO or N2O, p represents devolatilization or char combustion stage, Yp,i is the i

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yield during p stage, Ci is the i concentration by volume, Q is the flow rate of flue gas in the

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reactor under STP, Mi is the molar mass of i molecule, Vm is the molar volume of gas under STP, tp

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is the period of p stage, and t is the time. The conversion of fuel-N to NO or N2O during

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devolatilization or char combustion stage can be determined as follows:

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CVp ,i =

Y p ,i m0 N fuel

×

niMN × 100% Mi

(E-2)

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where CVp,i is the conversion of fuel-N to i during p stage, m0 is the initial mass of the biomass

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particle, Nfuel is the nitrogen content in biomass sample, ni is the number of nitrogen atoms in i

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molecule, and MN is the molar mass of nitrogen atom. 7


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3. Results and discussion

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3.1. NO and N2O release characteristics

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The weight of the particle and the NO and N2O concentrations in the flue gas are measured

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simultaneously in the biomass combustion process. The residual mass fraction is introduced to

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describe the mass conversion of the particle in the combustion process. The residual mass fraction

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of the particle is defined as follows:

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η=

mt m0

(E-3)

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where η is the residual mass fraction of the biomass particle, m0 is the initial mass of the biomass

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particle, and mt is the mass at time t.

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Figure 2a shows the NO and N2O release curves and corresponding mass reduction curve

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during oxy-fuel combustion of the CS particle at T = 800 ℃ (inlet gas composition:

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30%O2+70%CO2). Considering the dilution of N2 before measurement, the NO and N2O

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concentrations in Figure 2a have been translated to the real concentrations in the reactor rather

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than the values measured by the gas analyzer. After the biomass particle is introduced into the

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reactor, it undergoes the devolatilization and char combustion stages. In general, the

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devolatilization and char combustion stages are relatively independent with slight overlap. Thus, it

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is difficult to distinguish the two stages precisely. Combining with the proximate analysis of the

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biomass sample, the mass reduction characteristics are applied to distinguish the two stages in this

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work. In this work, it is supposed that the devolatilization stage consists of the moisture

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evaporation and volatile combustion. The turning point from devolatilization to char combustion

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is the point where η is equal to the value of 1-CM-CVM (CM is the moisture content of the sample

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and CVM is the volatile matter content of the sample). As shown in Figure 2a, the mass reduction 8


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rate of the biomass particle during devolatilization stage is much faster than that during char

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combustion stage, owing to rapid burning of volatiles. The NO concentration increases rapidly and

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reaches the peak quickly during devolatilization stage. During char combustion stage, the release

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rate of NO slows down and lasts for a long time. NH3 and HCN, released rapidly as the particle is

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heated up, are oxidized to NO through complex reaction mechanisms.20, 37, 42, 43 Meanwhile, NCO,

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converted from HCN by O/OH radicals, are oxidized to N2O, via the reaction

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NCO+NO→N2O+CO.20, 44 The N2O emission is delayed in the test, and N2O is mainly formed

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during char combustion stage. This behavior may be attributed to the fast destruction by H/OH

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radicals (R-1 and R-2) and the thermal decomposition (R-3),20

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N 2 O+H → N 2 +OH

(R-1)

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N2 O+OH → N 2 +HO2

(R-2)

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N2 O+M → N2 +O+M

(R-3)

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where M is a third body.

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Figure 2b shows the conversions of fuel-N to NO and N2O with corresponding mass

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conversion during oxy-fuel combustion of the CS particle (T = 800℃, inlet gas composition:

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30%O2+70%CO2). During devolatilization stage, the conversion of fuel-N to NO increases rapidly

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as the residual mass fraction decreases, whilst the conversion of fuel-N to N2O maintains at a very

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low level with no obvious increase. During char combustion stage, the conversions of fuel-N to

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NO and N2O increase quickly as the residual mass fraction decreases, and the growth rate of the

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conversion of fuel-N to N2O is significantly larger than that to NO. The conversions of fuel-N to

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NO during the two stages are 5.73% and 6.18%, respectively. Moreover, the conversions of fuel-N

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to N2O during the two stages are 0.52% and 18.46%, respectively. At the final stage, the 9


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conversion of fuel-N to N2O reaches 18.98%, which is about 1.6 times higher than that to NO

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(11.91%). It can be concluded that the devolatilization and char combustion stages have almost the

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same contributions to the NO emission during oxy-fuel combustion. The char combustion stage

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plays a major part in the N2O emission.

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3.2. Effect of temperature on NO and N2O emissions

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Figure 3 shows the conversions of fuel-N to NO and N2O varying with temperature during

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oxy-fuel combustion of the CS particles (inlet gas composition: 30%O2+70%CO2). The error bars

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denote the standard deviation of the data. It can be seen that the total conversion of fuel-N to NO

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increases with increasing temperature but there is no obvious growth tendency when the

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temperature exceeds 800℃. Base on Arrhenius law, temperature affects both the volatile release

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rate and the char combustion rate. A higher temperature leads to a higher reaction rate, higher free

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radicals concentrations, and more NH3 and HCN yields,27, 45 which are conducive to the NO

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formation. During devolatilization stage, the conversion of fuel-N to NO reaches the maximum at

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T = 800℃. Similar to the reactions in selective non-catalytic reduction (SNCR) process, NH3

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(including its related NHi radicals) is a reducing agent for NO via the global reaction (R-4).46

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4NH3 +4NO+O 2 → 4N 2 +6H 2 O

(R-4)

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At the O2 concentration ( = 30%), the reduction effect of NH3 on NO is strengthened at

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800~900℃ which is the optimal temperature window in SNCR.47 This may result in the slight

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decrease of the conversion of fuel-N to NO during devolatilization stage when temperature

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exceeds 800℃. During char combustion stage, a higher temperature will open more N-sites in the

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sample matrix,48 leading to a slightly higher conversion of fuel-N to NO.

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The conversions of fuel-N to N2O decrease dramatically with increasing temperature during 10


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the two stages. At high temperature, the free radicals (mainly H/OH radicals) are in high levels,

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promoting the fast destruction of N2O (R-1 and R-2). Moreover, the thermal decomposition of

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N2O (R-3) is also enhanced at high temperature. In addition, HCN tends to N2O at low

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temperature and NO at high temperature.44,

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enhanced the reduction of N2O over char.

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and N2O during oxy-fuel combustion: one emission level increases with the expense of the other.

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3.3. Effect of CO2 concentration on NO and N2O emissions

34

49

Wang et al. indicated that high temperature

Therefore, temperature changes the trade-off of NO

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In order to study the effect of CO2 concentration on NO and N2O emissions, the inlet gas was

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a mixture of O2, CO2 and Ar. Ar, an inert gas, is applied to adjust the CO2 concentration. Figure 4

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shows the conversions of fuel-N to NO and N2O varying with CO2 concentration during oxy-fuel

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combustion of the CS particles (T = 800℃, inlet gas composition: 30%O2+CO2+Ar). It can be

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seen that the conversion of fuel-N to NO decreases as the CO2 concentration increases during

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devolatilization stage and it decreases slightly with increasing CO2 concentration during char

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combustion stage. Chang et al. found that CO2 could change the fuel-N transformation routine in

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pyrolysis.50 The existence of CO2 inhibits NH3 formation and promotes HCN formation,51 whereas

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HCN oxidation is suppressed through the completion with O2 for H radicals, via the reaction

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CO2+H→CO+OH.52 A high CO2 atmosphere leads to a high CO concentration, via carbon

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gasification by CO2, which is conducive to the reduction of NO over char surface.15 Thus, CO2 has

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obvious suppression effect on NO emission during devolatilization stage and has slight inhibitory

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effect on NO emission during char combustion stage.

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The conversions of fuel-N to N2O increase with increasing CO2 concentration during the two

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stages. The promotion of HCN formation in a high CO2 atmosphere51 results in the increase of 11


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N2O emission. Leckner and Gómez-Barea indicated that the O2 concentration of 30% in oxy-fuel

243

FB combustion could maintain the same temperature as in air combustion.12 Thus, a high CO2

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concentration reduces the combustion temperature of biomass particles, weakening the fast

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destruction of N2O (R-1 and R-2) and the thermal decomposition of N2O (R-3). Although CO has

246

a reduction effect on N2O over char surface,34 temperature has a more intense effect on the

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reduction of N2O over char surface. A high CO2 concentration leads to the increase of N2O

248

emission with increasing CO2 concentration during the two stages. Therefore, CO2 promotes the

249

N2O emission during the two stages.

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3.4. Effects of atmosphere and O2 concentration on NO and N2O emissions

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Figure 5 shows the conversions of fuel-N to NO and N2O during the CS particles combustion

252

in different atmospheres (T = 800℃). The total conversion of fuel-N to NO in 21%O2/79%CO2

253

combustion is lower than that in air combustion, which is consistent with the results in the

254

literature.15 The conversion of fuel-N to NO during devolatilization stage in 21%O2/79%CO2

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combustion is significantly lower than that in air combustion, whilst the conversion of fuel-N to

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NO during char combustion stage in 21%O2/79%CO2 combustion is slightly lower than that in air

257

combustion. It suggests that CO2 obviously suppresses the NO emission during devolatilization

258

stage and slightly inhibits the NO emission during char combustion stage. The results can be

259

attributed to the effect of CO2 on fuel-N transformation routine in pyrolysis50 and the reduction

260

effect of CO on NO during char oxidation.15 CO2 has an inhibitory effect on NH3 formation51 and

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HCN oxidation.52 In addition, significant CO formed via carbon gasification by CO2 is conducive

262

to the NO over char surface.15 The total conversion of fuel-N to N2O in 21%O2/79%CO2

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combustion is significantly higher than that in air combustion, which is in line with the studies on 12


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coal.13, 53 During the two stages, the conversions of fuel-N to N2O in 21%O2/79%CO2 combustion

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are obviously larger than those in air combustion. It indicates that CO2 promotes the N2O emission

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during the two stages. Compared with air combustion, HCN formation is promoted in

267

21%O2/79%CO2 combustion,51 resulting in the increase of N2O emission. Moreover, the

268

combustion temperature of the biomass particles in 21%O2/79%CO2 is lower than that in air,

269

which is also conducive to N2O emission. The results indicate that CO2 suppresses the NO

270

emission and promotes the N2O emission during the two stages.

271

The conversions of fuel-N to NO during the two stages reach the maximum values at =

272

30%. Similar to the effect of temperature, a higher O2 concentration results in a higher reaction

273

rate and more free radicals which promote the NO emission. In SNCR process, a high O2

274

concentration can shift the temperature window to a low temperature.47 The reduction effect of

275

NH3 may be improved at a high O2 concentration, resulting in decrease of the conversion of

276

fuel-N to NO during devolatilization stage at = 30~50%. At = 30~50%, the decrease

277

of the conversion of fuel-N to NO during char combustion stage can be attributed to the

278

enhancement of NO over char surface. Due to the rise of char combustion temperature at a high

279

temperature, the reduction of NO over char surface may be enhanced via R-5 and R-6.15

280

2NO+2C → 2CO+N 2

(R-5)

281

char 2NO+2CO   → CO2 +N 2

(R-6)

282

During the two stages, the conversions of fuel-N to N2O decrease with increasing O2

283

concentration. The results can be explained that the destruction reactions of N2O with O/OH

284

radicals (R-1 and R-2) are strengthened at a higher O2 concentration. Moreover, the destruction of

285

N2O with O radicals is also improved via the reactions N2O+O→N2+O2 and N2O+O→2NO.54 In 13


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286

addition, a higher O2 concentration results in a higher combustion temperature, which is

287

conducive to the thermal decomposition of N2O (R-3).

288

3.5. Effect of H2O vapor addition on NO and N2O emissions

289

In order to study the recirculation of wet flue gas, the effect of H2O vapor addition on NO

290

and N2O was evaluated during the wet oxy-fuel combustion. Figure 6 shows the conversions of

291

fuel-N to NO and N2O varying with H2O vapor concentration during the wet oxy-fuel combustion

292

of the CS particles (T = 800℃, inlet gas composition: 30%O2+CO2+H2O). The conversions of

293

fuel-N to NO decrease steadily with increasing H2O vapor concentration during the two stages.

294

The addition of H2O vapor results in the significant H/OH radicals, H2 and CO contents, which are

295

conducive to NO reduction.16, 17 The formed O/OH radicals can accelerate the reduction reactions

296

of NO by NHi,55 and H2 and CO are conducive to NO reduction via R-7, R-8 and R-9.16

297

2NO+2H 2 → N 2 +2H 2 O

(R-7)

298

NO+CO → N+CO 2

(R-8)

299

NO+N → N 2 +O

(R-9)

300

During devolatilization stage, a slight decline trend is observed for the conversion of fuel-N to

301

N2O with increasing H2O vapor concentration. During char combustion stage, the conversion of

302

fuel-N to N2O decreases rapidly when H2O vapor concentration increases from 0 to 10% and

303

decreases steadily when H2O vapor concentration is over 10%. In the existence of H2O vapor, the

304

destruction of N2O by CO is via R-10.56

305

N 2 O+CO → N 2 +CO 2

(R-10)

306

The significant H/OH radical and CO contents, formed in a H2O atmosphere, enhance the

307

destruction reactions of N2O via R-1, R-2 and R-10.20, 56 The results indicate that H2O vapor 14


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Energy & Fuels

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addition has an effect on N2O reduction, especial for the char combustion stage. Moreover, the

309

reduction rate of N2O slows down when H2O vapor concentration is over 10%.

310

3.6. Effect of biomass type on NO and N2O emissions

311

The NO and N2O yields for different biomass types during oxy-fuel combustion are shown in

312

Figure 7 (inlet gas composition: 30%O2+70%CO2, T = 800℃). The fuel-N content of biomass

313

varies widely, and the fuel-N content in biomass is PW < RH < CS in this work. In conventional

314

combustion, Winter et al. showed that a biomass with high fuel-N content produced more NH3 and

315

HCN but possessed lower conversions to NH3 and HCN.37 It can be inferred that the amounts of

316

NH3 and HCN increase with increasing fuel-N content in oxy-fuel combustion. This may result in

317

the increase of NO and N2O yields for the biomass with high fuel-N content during the two stages.

318

The conversions of fuel-N to NO and N2O for different biomass types during oxy-fuel

319

combustion are shown in Figure 8. It is found that the total conversions of fuel-N to NO and N2O

320

decrease with increasing fuel-N content. During devolatilization stage, the conversion of fuel-N

321

to NO decreases with increasing fuel-N content, and the conversion of fuel-N to N2O

322

decreases slightly with increasing fuel-N content. During char combustion stage, the

323

conversions of fuel-N to NO and N2O are observed to decrease with increasing fuel-N content.

324

Since the biomass with higher fuel-N content can produce more NH3,37 the reduction effect of

325

NH3 on NO may be enhanced for the biomass with high fuel-N content, leading to a low

326

conversion of fuel-N to NO. Due to the reaction NCO+NO→N2O+CO,20, 44 a low conversion

327

fuel-N to NO may result in a low conversion of fuel-N to N2O for the biomass with high

328

fuel-N content. In this work, the ash content is just in accordance with fuel-N content in biomass,

329

PW < RH < CS. The catalytic reduction of ash on NO57 and N2O58 also contributes to the low 15


Energy & Fuels

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330

conversions of fuel-N to NO and N2O for the biomass with high ash content. Thus, the biomass

331

with high fuel-N content possesses high NO and N2O emissions whilst low conversions of fuel-N

332

to NO and N2O during the two stages.

333

4. Conclusions

334

Experimental study of the NO and N2O emissions during oxy-fuel combustion of a single

335

biomass particle at FB temperature was conducted in a flow tube reactor. The devolatilization and

336

char combustion stages of single biomass particle combustion were distinguished using the

337

method of isothermal TGA. The effects of temperature, CO2 concentration, atmosphere and O2

338

concentration, H2O vapor addition, and biomass type on NO and N2O emissions were studied. The

339

conclusions are as follows:

340

1) During oxy-fuel combustion of a single biomass particle at FB temperature, NO is rapidly

341

formed and reaches the peak during devolatilization stage, whilst N2O is mainly formed

342

during char combustion stage. In 30%O2/70%CO2 at T = 800℃, the conversions of fuel-N to

343

NO during devolatilization and char combustion stages are 5.73% and 6.18%, respectively.

344

The conversions of fuel-N to N2O during the two stages are 0.52% and 18.46%, respectively.

345

Thus, the total conversions of fuel-N to NO and N2O are 11.96% and 18.98%, respectively.

346

2) No obvious growth tendency for the conversion of fuel-N to NO is observed when the

347

temperature exceeds 800℃. The conversion of fuel-N to NO reaches the maximum at T = 800℃

348

during devolatilization stage, while the conversion of fuel-N to NO increases slightly with

349

increasing temperature during char combustion stage. Moreover, the conversion of fuel-N to

350

N2O decreases with increasing temperature during the two stages. Thus, temperature changes

351

the trade-off of NO and N2O during oxy-fuel combustion: one emission level increases with 16


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Energy & Fuels

352

the expense of the other.

353

3) The existence of CO2 obviously suppresses the NO emission during devolatilization stage and

354

slightly inhibits the NO emission during char combustion stage. Moreover, it promotes the

355

N2O emission during the two stages. Compared with air combustion, lower conversions of

356

fuel-N to NO and higher conversions of fuel-N to N2O are observed during the two stages in

357

oxy-fuel combustion. During the two stages, the conversions of fuel-N to NO reach the

358

maximum values at = 30%. Moreover, a higher O2 concentration leads to lower

359

conversions of fuel-N to N2O during the two stages.

360

4) The conversion of fuel-N to NO decreases steadily with increasing H2O vapor concentration.

361

H2O vapor addition promotes the NO reduction obviously during the two stages. The

362

conversion of fuel-N to N2O decreases obviously as H2O vapor concentration increases from 0%

363

to 10% and decreases steadily when H2O vapor concentration is over 10%. H2O vapor

364

addition mainly promotes the N2O reduction during char combustion stage, and the

365

destruction effect of H2O vapor addition on N2O during devolatilization stage is not obvious.

366

5) During the two stages, higher fuel-N content in biomass leads to higher NO and N2O yields.

367

However, the conversions of fuel-N to NO and N2O decrease with increasing fuel-N in

368

biomass during the two stages, due to the effect of NH3 and catalytic ash.

369

Acknowledgements

370

The project is supported by the National Basic Research Program of China (2015CB251501).

371

References

372

1.

373

coal combustion, their applicability in a thermal power plant and subsequent assessment of uncertainty

Roy, J.; Sarkar, P.; Biswas, S.; Choudhury, A., Predictive equations for CO2 emission factors for

17


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

374

in CO2 estimation. Fuel 2009, 88, (5), 792-798.

375

2.

376

a fuel for boilers. Renewable and Sustainable Energy Reviews 2011, 15, (5), 2262-2289.

377

3.

378

oxy-fuel atmospheres. Atmospheric Environment 2015, 116, 65-71.

379

4.

380

Department of Energy and Climate Change 2014.

381

5.

382

China. September, 2007.

383

6.

384

McDonald, D.; Myöhänen, K.; Ritvanen, J.; Rahiala, S.; Hyppänen, T.; Mletzko, J.; Kather, A.; Santos,

385

S., Oxyfuel combustion for CO2 capture in power plants. International Journal of Greenhouse Gas

386

Control 2015, 40, 55-125.

387

7.

388

pelletized biomass and waste-derived fuels. Combustion and Flame 2008, 155, (1-2), 21-36.

389

8.

390

Veijonen, K., Circulating fluidised bed co-combustion of coal and biomass. Fuel 2004, 83, (3),

391

277-286.

392

9.

393

circulating fluidized bed. Fuel Processing Technology 2006, 87, (6), 531-538.

394

10. Czakiert, T.; Sztekler, K.; Karski, S.; Markiewicz, D.; Nowak, W., Oxy-fuel circulating fluidized

395

bed combustion in a small pilot-scale test rig. Fuel Processing Technology 2010, 91, (11), 1617-1623.

Saidur, R.; Abdelaziz, E. A.; Demirbas, A.; Hossain, M. S.; Mekhilef, S., A review on biomass as

Moroń, W.; Rybak, W., NOx and SO2 emissions of coals, biomass and their blends under different

Stephenson, A. L.; MacKay, D. J., Life cycle impacts of biomass electricity in 2020. UK

Commission, N. D. a. R., Medium and Long-Term Development Plan for Renewable Energy in

Stanger, R.; Wall, T.; Spörl, R.; Paneru, M.; Grathwohl, S.; Weidmann, M.; Scheffknecht, G.;

Chirone, R.; Salatino, P.; Scala, F.; Solimene, R.; Urciuolo, M., Fluidized bed combustion of

Gayan, P.; Adanez, J.; de Diego, L. F.; Garcia-Labiano, F.; Cabanillas, A.; Bahillo, A.; Aho, M.;

Czakiert, T.; Bis, Z.; Muskala, W.; Nowak, W., Fuel conversion from oxy-fuel combustion in a

18


Page 18 of 34

Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

396

11. Jia, L.; Tan, Y.; Anthony, E. J., Emissions of SO2 and NOx during Oxy-Fuel CFB Combustion

397

Tests in a Mini-Circulating Fluidized Bed Combustion Reactor. Energy & Fuels 2010, 24, (2), 910-915.

398

12. Leckner, B.; Gómez-Barea, A., Oxy-fuel combustion in circulating fluidized bed boilers. Applied

399

Energy 2014, 125, 308-318.

400

13. Roy, B.; Chen, L.; Bhattacharya, S., Nitrogen oxides, sulfur trioxide, and mercury emissions

401

during oxy-fuel fluidized bed combustion of Victorian brown coal. Environmental science &

402

technology 2014, 48, (24), 14844-50.

403

14. de las Obras-Loscertales, M.; Mendiara, T.; Rufas, A.; de Diego, L. F.; García-Labiano, F.; Gayán,

404

P.; Abad, A.; Adánez, J., NO and N2O emissions in oxy-fuel combustion of coal in a bubbling fluidized

405

bed combustor. Fuel 2015, 150, 146-153.

406

15. Zhao, K.; Jensen, A. D.; Glarborg, P., NO Formation during Oxy-Fuel Combustion of Coal and

407

Biomass Chars. Energy & Fuels 2014, 28, (7), 4684-4693.

408

16. Li, S.; Wei, X.; Guo, X., Effect of H2O Vapor on NO Reduction by CO: Experimental and Kinetic

409

Modeling Study. Energy & Fuels 2012, 26, (7), 4277-4283.

410

17. Xu, H.; Smoot, L. D.; Hill, S. C., Computational model for NOx reduction by advanced reburning.

411

Energy & Fuels 1999, 13, (2), 411-420.

412

18. Löffler, G.; Wargadalam, V. J.; Winter, F.; Hofbauer, H., Decomposition of nitrous oxide at

413

medium temperatures. Combustion and Flame 2000, 120, (4), 427-438.

414

19. Winter, F.; Wartha, C.; Löffler, G.; Hofbauer, H., The NO and N2O formation mechanism during

415

devolatilization and char combustion under fluidized-bed conditions. Symposium (International) on

416

Combustion 1996, 26, (2), 3325-3334.

417

20. Wargadalam, V. J.; Loffler, G.; Winter, F.; Hofbauer, H., Homogeneous formation of NO and N2O 19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

418

from the oxidation of HCN and NH3 at 600-1000℃. Combustion and Flame 2000, 120, (4), 465-478.

419

21. Liu, H.; Gibbs, B. M., Modelling of NO and N2O emissions from biomass-fired circulating

420

fluidized bed combustors. Fuel 2002, 81, (3), 271-280.

421

22. Svoboda, K.; Cermak, J.; Hartman, M., Chemistry and emissions of nitrogen oxides (NO, NO2,

422

N2O) in combustion of solid fuels II. Heterogeneous reactions - N2O. Chemical Papers 2000, 54, (2),

423

118-130.

424

23. Svoboda, K.; Cermák, J.; Trnka, O., Chemistry and Emissions of Nitrogen Oxides (NO, NO2,

425

N2O) in Combustion of Solid Fuels. I. Heterogeneous Reactions - NO+NO2. CHEMICAL

426

PAPERS-SLOVAK ACADEMY OF SCIENCES 2000, 54, (2), 104-117.

427

24. Permchart, W.; Kouprianov, V. I., Emission performance and combustion efficiency of a conical

428

fluidized-bed combustor firing various biomass fuels. Bioresource Technology 2004, 92, (1), 83-91.

429

25. Chyang, C. S.; Qian, F. P.; Lin, Y. C.; Yang, S. H., NO and N2O emission characteristics from a

430

pilot scale vortexing fluidized bed combustor firing different fuels. Energy & Fuels 2008, 22, (2),

431

1004-1011.

432

26. Tarelho, L. A. C.; Neves, D. S. F.; Matos, M. A. A., Forest biomass waste combustion in a

433

pilot-scale bubbling fluidised bed combustor. Biomass and Bioenergy 2011, 35, (4), 1511-1523.

434

27. Becidan, M.; Skreiberg, Ø.; Hustad, J. E., NOx and N2O Precursors (NH3 and HCN) in Pyrolysis

435

of Biomass Residues. Energy & Fuels 2007, 21, (2), 1173-1180.

436

28. Tian, F.-J.; Yu, J.; McKenzie, L. J.; Hayashi, J.-i.; Li, C.-Z., Conversion of Fuel-N into HCN and

437

NH3 During the Pyrolysis and Gasification in Steam: A Comparative Study of Coal and Biomass†.

438

Energy & Fuels 2007, 21, (2), 517-521.

439

29. Ren, Q., NOx and N2O precursors from biomass pyrolysis. Journal of Thermal Analysis and 20


Page 20 of 34

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

440

Calorimetry 2013, 115, (1), 881-885.

441

30. Ren, Q.; Zhao, C., NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: interaction

442

between amino acid and mineral matter. Applied Energy 2013, 112, 170-174.

443

31. Ren, Q.; Zhao, C., NOx and N2O precursors from biomass pyrolysis: role of cellulose,

444

hemicellulose and lignin. Environ Sci Technol 2013, 47, (15), 8955-61.

445

32. Dong, L.; Gao, S.; Song, W.; Xu, G., Experimental study of NO reduction over biomass char. Fuel

446

Processing Technology 2007, 88, (7), 707-715.

447

33. Dong, L.; Gao, S.; Xu, G., NO Reduction over Biomass Char in the Combustion Process. Energy

448

& Fuels 2010, 24, (1), 446-450.

449

34. Wang, X.; Xiong, Y.; Tan, H.; Liu, Y.; Niu, Y.; Xu, T., Influences of CO and O2 on the Reduction

450

of N2O by Biomass Char. Energy & Fuels 2012, 26, (6), 3125-3131.

451

35. Karlström, O.; Brink, A.; Hupa, M., Time dependent production of NO from combustion of large

452

biomass char particles. Fuel 2013, 103, 524-532.

453

36. Zhao, K.; Glarborg, P.; Jensen, A. D., NO Reduction over Biomass and Coal Char during

454

Simultaneous Combustion. Energy & Fuels 2013, 27, (12), 7817-7826.

455

37. Winter, F.; Wartha, C.; Hofbauer, H., NO and N2O formation during the combustion of wood,

456

straw, malt waste and peat. Bioresource Technology 1999, 70, (1), 39-49.

457

38. Bai, J.; Yu, C.; Li, L.; Wu, P.; Luo, Z.; Ni, M., Experimental Study on the NO and N2O Formation

458

Characteristics during Biomass Combustion. Energy & Fuels 2013, 27, (1), 515-522.

459

39. Mermoud, F.; Golfier, F.; Salvador, S.; Van de Steene, L.; Dirion, J. L., Experimental and

460

numerical study of steam gasification of a single charcoal particle. Combustion and Flame 2006, 145,

461

(1-2), 59-79.

21


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

462

40. Wang, C.; Wang, J.; Lei, M.; Gao, H., Investigations on Combustion and NO Emission

463

Characteristics of Coal and Biomass Blends. Energy & Fuels 2013, 27, (10), 6185-6190.

464

41. Wang, C. B.; Shao, H.; Lei, M.; Wu, Y. H.; Jia, L. F., Effect of the coupling action between

465

volatiles, char and steam on isothermal combustion of coal char. Applied Thermal Engineering 2016,

466

93, 438-445.

467

42. Tan, L. L.; Li, C. Z., Formation of NOx and SOx precursors during the pyrolysis of coal and

468

biomass. Part I. Effects of reactor configuration on the determined yields of HCN and NH3 during

469

pyrolysis. Fuel 2000, 79, (15), 1883-1889.

470

43. Hansson, K.-M.; Samuelsson, J.; Tullin, C.; Åmand, L.-E., Formation of HNCO, HCN, and NH3

471

from the pyrolysis of bark and nitrogen-containing model compounds. Combustion and Flame 2004,

472

137, (3), 265-277.

473

44. Kilpinen, P.; Hupa, M., Homogeneous N2O chemistry at fluidized bed combustion conditions: A

474

kinetic modeling study. Combustion and Flame 1991, 85, (1), 94-104.

475

45. Li, C.-Z.; Tan, L. L., Formation of NOx and SOx precursors during the pyrolysis of coal and

476

biomass. Part III. Further discussion on the formation of HCN and NH3 during pyrolysis. Fuel 2000,

477

79, (15), 1899-1906.

478

46. Bae, S. W.; Roh, S. A.; Kim, S. D., NO removal by reducing agents and additives in the selective

479

non-catalytic reduction (SNCR) process. Chemosphere 2006, 65, (1), 170-5.

480

47. Kasuya, F.; Glarborg, P.; Johnsson, J. E.; Dam-Johansen, K., The thermal DeNOx process:

481

Influence of partial pressures and temperature. Chemical Engineering Science 1995, 50, (9),

482

1455-1466.

483

48. Duan, L.; Duan, Y.; Zhao, C.; Anthony, E. J., NO emission during co-firing coal and biomass in an

22


Page 22 of 34

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

484

oxy-fuel circulating fluidized bed combustor. Fuel 2015, 150, 8-13.

485

49. Pels, J. R.; Wojtowicz, M. A.; Kapteijn, F.; Moulijn, J. A., Trade-Off between NOx and N2O in

486

Fluidized-Bed Combustion of Coals. Energy & Fuels 1995, 9, (5), 743-752.

487

50. Chang, L.; Xie, Z.; Xie, K.-C.; Pratt, K. C.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z., Formation of NOx

488

precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the

489

formation of NH3 and HCN☆. Fuel 2003, 82, (10), 1159-1166.

490

51. Duan, L.; Zhao, C.; Ren, Q.; Wu, Z.; Chen, X., NOx precursors evolution during coal heating

491

process in CO2 atmosphere. Fuel 2011, 90, (4), 1668-1673.

492

52. Giménez-López, J.; Millera, A.; Bilbao, R.; Alzueta, M. U., HCN oxidation in an O2/CO2

493

atmosphere: An experimental and kinetic modeling study. Combustion and Flame 2010, 157, (2),

494

267-276.

495

53. Yoshiie, R.; Kawamoto, T.; Hasegawa, D.; Ueki, Y.; Naruse, I., Gas-Phase Reaction of NOx

496

Formation in Oxyfuel Coal Combustion at Low Temperature. Energy & Fuels 2011, 25, (6),

497

2481-2486.

498

54. Hayhurst, A. N.; Lawrence, A. D., The amounts of NOx and N2O formed in a fluidized bed

499

combustor during the burning of coal volatiles and also of char. Combustion and flame 1996, 105, (3),

500

341-357.

501

55. Lu, P.; Hao, J.; Yu, W.; Zhu, X.; Dai, X., Effects of water vapor and Na/K additives on NO

502

reduction through advanced biomass reburning. Fuel 2016, 170, 60-66.

503

56. Chen, Z.; Lin, M.; Ignowski, J.; Kelly, B.; Linjewile, T. M.; Agarwal, P. K., Mathematical

504

modeling of fluidized bed combustion. 4: N2O and NOx emissions from the combustion of char. Fuel

505

2001, 80, (9), 1259-1272.

23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

506

57. Karlström, O.; Perander, M.; DeMartini, N.; Brink, A.; Hupa, M., Role of ash on the NO

507

formation during char oxidation of biomass. Fuel 2017, 190, 274-280.

508

58. Zhang, J.; Yang, Y.; Hu, X.; Dong, C.; Lu, Q.; Qin, W., Experimental Research on Heterogeneous

509

N2O Decomposition with Ash and Biomass Gasification Gas. Energies 2011, 4, (12), 2027-2037.

510 511

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Energy & Fuels

512 513 514 515

Tables Table 1 Proximate and ultimate analyses of biomass. Polar wood (PW)

Rice husk (RH)

Corn stalk (CS)

9.03 74.93 14.88 1.16

8.03 58.19 15.46 18.32

6.77 52.10 8.61 32.52

48.05 4.86 36.58 0.24 0.08 17.73

39.21 4.23 29.62 0.51 0.08 13.53

32.01 3.44 24.02 1.02 0.22 11.87

Sample Proximate analysis (wt%, as received) Moisture Volatile matter Fixed Carbon Ash Ultimate analysis (wt%, as received) C H O N S Qar,net (MJ/kg) 516 517 518

25


Energy & Fuels

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519 520

Page 26 of 34

Table 2 Composition of biomass ash. Sample PW RH CS

Ash analysis (wt%) Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

TiO

Fe2O3

1.90 0.11 0.97

5.49 0.57 6.26

2.40 0.57 11.27

9.86 93.41 55.08

5.53 0.78 1.60

5.18 0.52 2.28

20.42 0.10 8.50

46.71 2.88 9.51

0.19 0.88 0.49

2.32 0.18 4.04

521 522 523

26


Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

524 525 526

Figures

527 528 529 530

Figure 1. Schematic diagram of the experimental rig for single particle combustion.

27


Energy & Fuels

devol.

char

100

350

Residual mass fraction (%)

80

300

250

70 60

200

50 150

40 30

100

NO and N2O (ppm/g fuel)

Residual mass fraction NO N2O

90

20 50 10 0

0 0

50

100

150

200

250

300

t (s)

531 532

(a)

22

devol.

char

20

fuel-N to NO fuel-N to N2O

18

Conversion of fuel-N (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

16 14 12 10 8 6 4 2 0 100

90

80

70

60

50

40

30

Residual mass fraction (%)

533 534 535 536 537 538

(b) Figure 2. Combustion characteristics during the oxy-fuel combustion of CS particles (T = 800℃, inlet gas composition: 30%O2+70%CO2), (a) NO and N2O release curves and corresponding mass reduction curve, (b) conversions of fuel-N to NO and N2O with corresponding mass conversion.

28


Page 29 of 34

539

Conversions of fuel-N to NO and N2O (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

devol.-NO char-NO devol.-N2O char-N2O

700

800

900

T (°C)

540 541 542 543 544

Figure 3. Conversions of fuel-N to NO and N2O varying with temperature (CS particles, inlet gas composition: 30%O2+70%CO2)

29


Energy & Fuels

545

22 20 Conversions of fuel-N to NO and N2O (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

18

devol.-NO char-NO devol.-N2O char-N2O

16 14 12 10 8 6 4 2 0 0

35

70

(%)

546 547 548 549 550

Figure 4. Conversions of fuel-N to NO and N2O varying with CO2 concentration (CS particles, T = 800℃, inlet gas composition: 30%O2+CO2+Ar)

30


Page 31 of 34

551 Air

Oxy-fuel

26 devol.-NO char-NO devol.-N2O

24 Conversions of fuel-N to NO and N2O (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

22

char-N2O

20 18 16 14 12 10 8 6 4 2 0 21

21

30

40

50

(%)

552 553 554 555 556

Figure 5. Conversions of fuel-N to NO and N2O at different atmospheres and O2 concentrations (CS particles, T = 800℃).

31


Energy & Fuels

557

22 devol.-NO char-NO devol.-N2O

20 Conversions of fuel-N to NO and N2O (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

18

char-N2O

16 14 12 10 8 6 4 2 0 0

10

20

30

40

(%)

558 559 560 561 562

Figure 6. Conversions of fuel-N to NO and N2O varying with H2O vapor concentration (CS particles, T = 800℃, inlet gas composition: 30%O2+CO2+H2O).

32


Page 33 of 34

563

3.5 devol.-NO

3.0

char-NO devol.-N2O char-N2O

NO and N2O yields (mg/g fuel)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.5 2.0 1.5 1.0 0.5 0.0 PW

RH

CS

Biomass type

564 565 566 567 568

Figure 7. NO and N2O yields for different biomass types (inlet gas composition: 30%O2+70%CO2, T = 800℃).

33


Energy & Fuels

569

40 devol.-NO

Conversions of fuel-N to NO and N2O (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

char-NO devol.-N2O

35

char-N2O

30 25 20 15 10 5 0 PW

RH

CS

Biomass type

570 571 572 573 574

Figure 8. Conversions of fuel-N to NO and N2O for different biomass types (inlet gas composition: 30%O2+70%CO2, T = 800℃).

34


NO and N2O Emissions during Devolatilization and Char Combustion

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