Co-Combustion of Tannery Sludge in a Bench-Scale Fluidized-Bed

Sep 5, 2017 - State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China. ‡ Zhejiang Zheneng Changxing Electric P...
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Co-combustion of Tannery Sludge in a Bench-scale Fluidized Bed Combustor: Gaseous Emissions, Cr Distribution and Speciation Hao Dong, Xuguang Jiang, Guojun Lv, Fei Wang, Qunxing Huang, Yong Chi, Jianhua Yan, Weizhong Yuan, Xijiong Chen, and Weizhong Luo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01831 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Co-combustion of Tannery Sludge in a Bench-scale Fluidized Bed Combustor: Gaseous Emissions, Cr Distribution and Speciation

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Hao Dong,† Xuguang Jiang,*† Guojun Lv,† Fei Wang,† Qunxing Huang,†

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Yong Chi,† Jianhua Yan,† Weizhong Yuan,‡ Xijiong Chen,‡ Weizhong Luo‡

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State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou

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310027, China



Zhejiang Zheneng Changxing Electric Power Generation Co., Ltd., Huzhou 313000, China

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ABSTRACT: In this study, eleven coal mono-combustion tests and four co-combustion

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tests of coal and tannery sludge were conducted on a 35kW fluidized bed combustor. The

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combustion behavior and emission characteristics of the fuels were investigated on a

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bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of an

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excess air ratio, primary air rate, secondary air ratio and fuel type on flue gas emissions

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were studied. The results showed that the fluidization status and temperature distribution

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had direct influences on CO emission. Sufficient fluidization and high temperature

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effectively reduced CO emission. NOx emission was relatively sensitive to the excess air

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ratio and increased with increasing excess air ratio. By comparing BFB and CFB, we

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found that CFBs have an advantage in optimizing combustion and controlling emissions

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by enhancing mixing and increasing freeboard temperatures.

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The Cr speciation and distribution among different ash types were extensively

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investigated in four co-combustion tests. The results showed that the distribution modes

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of Cr in BFBs and CFBs were different and determined by separate fluid dynamics

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modes of sludge ash particles in the combustor. The extent of Cr oxidation in ash in CFB

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tests was higher than that in BFB tests, particularly for bottom ash and heat exchanger

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ash, due to longer residence times in high-temperature regions.

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Introduction

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The management of hazardous wastes that pose severe threats to the environment and

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human health have drawn worldwide concern. In China, 39.76 million tons of industrial

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hazardous waste were generated in 2015 according to the 2016 Environmental Statistics

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Annual Report by the Chinese Environmental Protection Department. A lack of sufficient

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disposal capabilities have made it challenging to compensate for ever-increasing amounts

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of hazardous waste.1

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Tannery sludge (TS), which is generated from tanning processes, contains a high

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content of Cr2 and is classified under code HW21-193-001-21 in China’s Hazardous

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Waste List. Only 60-70% of chromium salts used in the tanning process react with skins;

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the remaining 30-40% are disposed in the tanning exhaust bath and end up as TS.3 Cr

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toxicity is significantly influenced by its oxidation state. Hexavalent chromium (Cr(VI))

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is known to have 100-fold higher toxicity than trivalent chromium (Cr(III))4. Therefore,

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Cr speciation in waste streams is of special concern.

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The co-combustion of hazardous wastes in fluidized bed combustion (FBC) systems is

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a potential technology for effective disposal of hazardous wastes. Co-combustion in FBC

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systems has been widely applied to many kinds of wastes, such as biomass, municipal

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solid waste (MSW), refuse-derived fuel (RDF) and sewage sludge, and extensively

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characterized based on combustion, emission, ash deposition and trace element

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transformation.5-10 However, few studies have investigated the co-combustion of

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hazardous wastes in FBC systems.11,

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results.

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Thus, the need remains for more meaningful

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In this work, TS was co-combusted in a bench-scale FBC system for the first time.

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Fifteen tests were conducted under different operational conditions, including two modes

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of fluidization, namely, bubbling fluidized bed (BFB) combustion and circulating

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fluidized bed (CFB) combustion, two feedstock types, coal mono-combustion and

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co-combustion of coal and TS. Other operational parameters, such as the primary and

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secondary air flow rates and feeding rate, were also adjusted to determine the optimal

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combustion conditions. The emission characteristics and Cr distribution and speciation

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were investigated in this study.

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Experimental procedures

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Materials

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TS was obtained from a tannery in Haining. The TS was predried at 105°C for more

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than 24 h and subsequently milled in a ball miller. Proximate analysis, calorific value

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analysis and ultimate analysis were performed for TS and coal (Table 1). The proximate

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analysis was conducted using the Coal Industry Analysis Method (GB/T 212-2008). The

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lower heating value was quantified by the Chinese Standard Method of Determination of

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Calorific Value of Coal (GB/T 213-2008). The ultimate analysis was performed using the

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elemental analyzer 1ECO-CMNS932. The elemental contents of TS were quantified

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using X-ray fluorescence analysis (Table 2) with an ARL ADVADT’XIntelli Power

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TM4200 instrument.

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Procedures

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Experiments were conducted on a 35 kWth FBC test rig;13 the system scheme is shown

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in Figure 1. Tests were switched between BFB and CFB by controlling valves after the

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furnace, as shown in Figure 1. The preparation steps performed before conducting

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experiments, including cold fluidization and ignition tests, have been extensively

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described elsewhere.13 Feeding coal was sieved under 1 cm. Dried TS was ground to a

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mean diameter of less than 1 mm. For co-combustion tests, the percentage of TS in the

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samples was 10% by weight which referring to co-combustion of sludges of which

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caloric value are close to TS.8, 14 Because there was no heating surface in the bed area, the

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feeding rate of coal must be limited during BFB tests to control bed temperature. For this

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purpose, and due to limitation of capacity of feeder, we added 50%wt quartz sand in coal

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during BFB tests. For each test, the emissions of gaseous pollutants were monitored at the

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stack. The concentrations of gaseous products (CO and NOx) and moisture were

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measured using the FTIR spectrometer of a gas analyzer GASMET FTIR Dx4000 (Temet

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Instrument Oy, Finland). Ash samples were taken at four different places (marked in

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Figure 1):

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a. bottom ash (BA) from the combustor bed;

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b. heat exchanger ash (HA) from the heat exchanger;

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c. fabric filter ash (FA) from the baghouse; and

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d. quartz glass fiber ash (QA) from the stack.

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The concentrations of total Cr in the ash samples were measured using inductively

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coupled plasma-atomic emission spectrometry (ICP-AES) after microwave digestion of

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each sample in hydrochloric acid, nitric acid and hydrochloric acid. Cr(VI) in the ash

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samples was extracted using U.S. Environmental Protection Agency Method 3060A and

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quantified using the diphenylcarbazine method. Fifteen tests were conducted in this

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study; the conditions of these tests are listed in Table 3. Eleven of these tests involved the

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mono-combustion of coal and were labeled C1 through C11; the four other tests involved

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the co-combustion of coal and TS and were labeled S1 through S4.

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The actual excess air ratio was calculated using the measured levels of O2 and CO in the exhaust gas according to the following equation: Excess air ratio=

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21 21 − ( − 0.5 )

This equation is based on a study by Basu et al.15 H2 and CH4 gases had sufficiently

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low concentrations to be considered negligible.

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Results and discussion

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Temperature distribution

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Figure 2 shows that for the mono-combustion of coal in BFB, i.e., experiments C1-7,

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the temperatures above the distributor generally decreased as the excess air ratio

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increased. This effect occurred because the excess air had a cooling effect on the

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combustor. Tests C4-7 with excess air ratios ranging from 2.15-2.77 achieved more

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uniform temperature profiles than the other three tests, C1-3, because the fluidization

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velocity increased, and the combustion zone moved toward the top of the freeboard.16 For

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the co-combustion tests, the temperatures decreased within the same excess air ratio

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range because of the relatively low caloric value of TS. In addition, because the

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combustibles of TS consisted predominantly of volatiles and the amount of fixed carbon

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was negligible, the decreased temperatures were observed primarily within denser bed

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regions. For the CFB tests (Figure 3), the temperature distributions were relatively

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uniform because the particles were transported predominantly along the combustor.

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Gaseous emissions

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As an incomplete combustion product, the level of CO emissions reflects the

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combustion efficiency. Generally, a decreased CO emission indicates better combustion.

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The CO emissions of all BFB tests are shown in Figure 4a and Figure 5a, including data

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for tests C1-3 conducted at low fuel-feeding rates and low primary flow rates; tests C4-7

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conducted at high fuel-feeding rates and high primary flow rates; test 7 with secondary

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flow; and tests S1 and S2 with TS mixed in the feedstock. Enhanced mixing, sufficient

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residence time and high temperature were observed to be the dominant factors affecting

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complete combustion. As shown in Figure 4a for tests C1-3, the amount of CO emission

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decreased from 1852 mg/m3 to 1096 mg/m3 as the primary air flow rate increased from

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70 L/h to 78 L/h. However, when the primary air flow increased to 89 L/h, CO emissions

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increased to 1247 mg/m3. Because the excess air ratio positively correlated with the

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primary air supply when the feeding rate was constant, the change in CO emission with

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PA in Figure 5a was consistent with that for the excess air ratio for tests C1-3. The

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highest emission occurred at the lowest fluidizing air flow (70 L/h). This effect was due

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to poor fluidization at low fluidizing air velocity, resulting in poor mixing conditions.

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Defluidization has been reported to occur when the PA flow rate decreases to 67 L/h in

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present FB combustor.13 The decreased CO emission levels when the PA reached 78 L/h

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suggested that better fluidization and mixing conditions were achieved and that

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combustion was optimized. Subsequently, as the PA flow rate increased to 89 L/h, the

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amount of CO produced moderately increased. As mentioned in the discussion on

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temperature distribution, the temperature decreased with increasing excess air ratio

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cooling the combustor. Thus, as a result of the cooling effect of the excess air flow, the

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combustion reaction rate decreased. Moreover, with a higher PA flow rate, the residence

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time of combustibles in the combustor was shortened because the air superficial velocity

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was higher. Due to these low temperatures and short residence times, fewer CO

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molecules were converted to CO2, and more CO was emitted under higher PA flow rates

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in test C3. Similar phenomena have also been reported by Chen13, Madhiyanon et al.17

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and Varol et al.18

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For tests C4-6, as previously discussed, the temperature increased along the combustor

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due to the high fuel-feeding rate and high PA flow rate. A previous analysis indicated that

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high temperatures accelerate the oxidation of CO, whereas a high PA flow rate decreases

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the residence time of CO and other incomplete combustion products. As shown in Figure

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4a and Figure 5a, the levels of CO emissions for tests C4-6 were generally lower than

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those for tests C1-3, which indicated that temperature, rather than residence time, has a

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dominant impact on the conversion of CO to CO2. Unlike tests C1-3, the feeding rates of

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tests C4-6 were not kept constant at 14, 14 and 11 kg/h. Thus, the excess air ratios of tests

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C4-6 were not proportional to their PA flow rates, and the trends of CO emissions under

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different excess air ratios were different from that under the corresponding PA flow rates.

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As shown in Figure 5a, the changes in CO emissions with excess air ratios of tests C4-6

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followed similar changes as those of tests C1-3. At a minimal excess air ratio of 2.20, the

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lowest PA was observed, and the resultant fluidizing velocity was the lowest, which is

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not favorable for efficient fluidization and mixing. Poor mixing can be taken as the

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reason why the CO emission of test C4 was the highest among tests C4-6. Furthermore,

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because the mean diameter of the fuel particles was larger than that of the bed material, a

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higher critical fluidization velocity would be required with more fuel fed per unit time.

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Thus, the mixing condition was not readily improved, but the PA was maintained at a

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relatively high level, i.e., 91-99 L/h, during tests C4-6. This result can be observed by

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comparing test C4 and test C3. The PA flow rates of these two tests were nearly the same,

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at 91 and 89 L/h. The temperature of test C4 was higher than that of test C3, whereas the

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CO emission of test C4, which was 1242 mg/m3, was not significantly less than that of

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test C3, which was 1247 mg/m3, implying that the fluidization of test C4 was worse than

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that of test C3. At an excess air ratio of 2.45 in C5, a higher PA, 99 L/h, was introduced

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in the combustor, and fluidization and mixing were consequently improved. As a result,

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the CO emission was notably reduced to 621 mg/m3. At a higher excess air ratio of 2.77

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in test C6, the combustor was cooled by the excess air, and the CO emission increased to

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805 mg/m3. The calculated excess air ratio of test C5 with the highest air supply and

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lowest fuel fed was not the largest among tests C4-6 as expected. This result may have

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been caused by enhanced mixing, which promotes complete combustion. More directly,

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the CO emission decreased as the PA increased for tests C4-6, as shown in Figure 4a,

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which verified that the mixing conditions are dependent on the PA flow and play a major

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role in combustion during tests C4-6.

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Test C7 was the only BFB experiment in which the secondary air was added. Relative

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to test C4, which had the same PA flow rate and feeding rate, we observed reductions in

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the CO emissions and excess air ratio. Secondary air is known to enhance turbulence and

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mixing in the freeboard and to promote the combustion of incomplete combustion

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products.19 The results also correlate with the relatively high freeboard temperature of test

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C7 shown in Figure 2. Tests S1 and S2 involved the co-combustion experiments of TS.

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The various excess air ratios used in tests S1 and S2 were comparable with those of tests

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C4-6. However, more CO was emitted in the co-combustion experiments, as shown in

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Figure 5a. The air flow rates of tests S1 and S2 were less than those of tests C4-6,

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whereas the feeding rates were nearly the same. Thus, the mixing conditions of tests S1

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and S2 were not as optimal as those of tests C4-6, resulting in more CO being emitted.

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Because of the relatively low temperature of FBC (below 1000°C), NOx was mostly

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generated from organically bound nitrogen in the fuel and is termed fuel-N.20 The

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conversion scheme of fuel-N to NOx is shown in Figure 6.21 The first step is fuel-N

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devolatilization. The original nitrogen bound in the fuel thermally decomposes into

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volatiles (termed ash volatile-N), including tar and light gaseous species, such as HCN

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and NH3. The remaining nitrogen is retained in the char (termed char-N). Additionally, as

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the temperature increases, the further cracking of tar and char results in more light species

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released, including soot from tar. The subsequent step after devolatilization is the

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oxidation of volatile-N and char-N. For volatile-N, oxidation eventually occurs through

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the reactions of two light gaseous species, as shown in reactions 1 and 2.22

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 CN      (1)

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  N     (2)

200

,

,

,









,

,

As for char-N, the overall oxidation mechanism is similar to reaction 3, although the

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details remain to be clarified. 21

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( ) +  → NO + ()(3)

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where C(N) and C(O) denote species bound on the char surface containing nitrogen and

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oxygen, respectively. Note that the NO formed in char surface may also be reabsorbed

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and then reduced to N2. The reactions are shown in reactions 4 and 5.21 Cf denotes a free

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carbon site.

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2  +  → C(N) + ()(4)

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( ) +  →  + ()(5)

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As a result, the conversion of char-N to NOx is relatively limited by the reduction of

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NO on char. Overall, NOx formation may occur in bulk flow in the particle boundary

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layer or on the internal or external particle surface.

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Figure 4b and Figure 5b demonstrate NOx emissions under different PA rates and

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excess air ratios, respectively. As shown in Figures 5a and b, NOx increases with the PA

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rate and excess air ratio from 264 mg/m3 to 341 mg/m3 for tests C1-3. As the excess air

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ratio increases, the oxygen concentration in the flue gas increases, facilitating the

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generation of NOx from fuel-N.17, 18 Moreover, the higher PA rate enhances the mixing of

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the high-temperature dense bed, which was the main area for NO formation. Better

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mixing conditions of these dense beds ensures that oxygen can contact fuel-N species

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more sufficiently and provide more favorable kinetic conditions for NO formation.

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Similar changes with the excess air ratio are also observed in tests C4-6 and tests S1 and

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2. However, the NOx release in tests C4-6 does not increase with the PA rates as observed

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in tests C1-3, indicating that the air stoichiometry in the excess air ratio is a major factor

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rather than the mixing condition and temperature factors in NO formation.

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A comparison of tests C1-3 and tests C4-6 shows that the emission levels of NOx in the

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former three tests are all below those of the latter. Because the temperatures of tests C4-6

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show pronounced increases along the entire combustor relative to tests C1-3, the results

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can be interpreted in terms of volatile-N evolution and oxidation. Conceivably, the

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cracking of tar and char are facilitated under high temperatures. Experiments have

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verified that the thermal cracking of tar at temperatures below 1373°C yields only small

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amounts of light gaseous nitrogen.23 The concentrations of free oxygen, hydroxyl and

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hydrogen radicals increase at high temperatures, leading to increased gas-N species

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oxidation via reactions 1 and 2.22 However, the remarkable increase of NOx from tests

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C1-3 to tests C4-6 can be attributed to competition between NOx and N2O formation.

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Glarborg et al.21 found that increases in NO yields are correlated with similar-magnitude

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decreases in N2O yields. There are two major routes for N2O formation. In a

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homogeneous mechanism, fuel-N cracks to form the gas-phase product HCN.

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Subsequently, HCN is oxidized by oxygen and hydroxyl radicals to NCO, forming N2O

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with NO via reaction 224. In fact, HCN is the main precursor of N2O.

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 +  →   + (6)

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In a heterogeneous mechanism, reactions 7 and 8 occur on the char surface. C(NCO) is

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an oxidized char nitrogen species.25

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C(N) +   → ( )(7)

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C(NCO) + NO →  +   +  (8)



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The formation of N2O is significantly influenced by oxygen content and temperature.

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First, a conversion of HCN to NH3 occurs at low oxygen concentrations,26 i.e., higher

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oxygen concentrations promote the formation of HCN to act as a precursor to form N2O.

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Second, N2O is sensitive to high temperatures. When the reaction temperature is over

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900°C, N2O decomposes to N2.20 Thus, N2O formation can be inhibited by high

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temperatures.21 As a result, N2O formation decreases as the temperature increases, as has

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been reported in several studies.27, 28 FBC has a lower temperature than grate furnaces

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and pulverized coal fired boilers; therefore, N2O is typically formed at higher rates under

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fluidized bed conditions. Typical N2O concentrations from CFBs and pulverized coal

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fired boilers are 74 mg/Nm3 and