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Journal of Environmental Management 250 (2019) 109419
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Research article
Emission and conversion of NO from algal biomass combustion in O2/CO2 atmosphere
T
Bingtao Zhaoa, , Yaxin Sub ⁎
a
School of Energy and Power Engineering and Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai, 200093, China b School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai, 201620, China
ARTICLE INFO
ABSTRACT
Keywords: O2/CO2 atmosphere Algal biomass Direct combustion NO Emission and conversion
Environmental impacts of NO emissions from biomass combustion have become an important concern. To address NO emission and conversion from algal biomass combustion in O2/CO2 atmosphere, three typical algal biomass, Chlorella (Ch), Enteromorpha (En), and Sargassum (Sa), were used to investigate NO emission characteristics in a one-dimensional tube furnace. The effects of the combustion temperature and O2 concentration (21%, 25%, and 30%) on the NO emission were examined. It was found that the main peaks of NO positively are correlated to the O2 concentration and combustion temperature. The NO emission trends of each algal biomass are slightly affected by the O2 concentration at a given temperature. Roughly, the NO yield and conversion rate for each algal biomass increase with increasing O2 concentration at a given temperature. They first increase with the increasing temperature and then decrease beyond 800 °C with exception for Sa in 30% O2/CO2 atmosphere. However, 21% O2/CO2 atmosphere is at least effective to reduce NO emission from most algal biomass combustion compared to air-based atmosphere (21% O2/N2), by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. The possible conversion pathway of fuel-N implies that the NO emission from algal biomass combustion in O2/CO2 atmosphere is the result of the combined effect of the NO formation oxidized from Nprecursors and NO reduction by CO (converted from CO2) and other reductive components. These results may provide a positive reference for the control of NOx emissions from direct combustion or co-firing of algal biomass for energy utilization.
1. Introduction Algal biomass is a high-potential renewable resource different from fossil fuels and conventional biomass. Compared to the complex energy utilization methods including chemical extraction, biochemical fermentation and other thermochemical conversions (e.g., pyrolysis, gasification and liquefaction), direct combustion is a simple method of thermochemical utilization for algal biomass (Zhao et al., 2016; Chen et al., 2016; Wang et al., 2013). However, algal biomass combustion results in unavoidable NOx emissions which have become an important energy and environmental issue. To date, the researches on algal biomass combustion mainly focus on their combustion characteristics (Zhao et al., 2016; Chen et al., 2016; Wang et al., 2009; Wang et al., 2013; Sanchezsilva et al., 2013; Yu et al., 2008), as shown in Table 1. The great concern has been paid
⁎
to the combustion performances, particularly to the relationship between the combustion conditions (e.g., temperature and heating rate), the physicochemical properties of fuels (e.g., particle size of algal biomass) and the ash-forming elements (Mg, Ca and K etc.) (Karlström et al., 2017; Zhou et al., 2006; Zhan et al., 2017; Winter et al., 1999). In these investigations, most algal biomass combustion experiments had been carried out in an air-based atmosphere. It has been demonstrated that NOx formation and emission from algal biomass combustion highly depend upon the fuel composition and operating parameters in the system (Chen et al., 2016; Zhao et al., 2016) similar to that from pulverized coal combustion (Croiset et al., 2000; Yanik et al., 2018). Over the past decades, O2/CO2 atmospherebased combustion has gained widespread attention as an advanced and high-efficiency clean combustion technology which is different from traditional air-based combustion. Coupling O2/CO2 combustion tech-
Corresponding author. E-mail address: [email protected] (B. Zhao).
https://doi.org/10.1016/j.jenvman.2019.109419 Received 29 May 2019; Received in revised form 29 July 2019; Accepted 16 August 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Management 250 (2019) 109419
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Table 1 Combustion characteristics of algae biomass. Algae biomass
Parameters
Performances
Ref.
Chlorella, Enteromorpha and Sargassum C. vulgaris
Temperature of 700–900 °C, 100 ± 1 mg of biomass, air flow rate of 3 L/min, and particle size of 85–88 μm. In 20% O2/80% N2 atmosphere, temperature of 300–700 °C, 100 mg of sample, and flow rate of 0.3 L/ min. In 20% O2/80% N2 atmosphere, temperature of 1200 °C, 17 mg of biomass, heating rate of 20 °C/min, flow rate of 0.1 L/min, and particle size of 0.18 mm. Temperature of 900 °C, 15 mg of biomass, heating rate of 10–30 °C/min, flow rate of 0.12 L/min, and particle size of 0–0.48 mm.
Conversion ratio: 9.30–9.10% for En, 34.03–34.98% for Sa, and 6.73–5.90% for Ch. Less than 10 ppm NOx emissions at a temperature ≤500 °C; beyond 500 °C, the NOx yield increases with the temperature.
Zhao et al. (2016)
Five stages: dehydration and desiccation, release and combustion of volatiles, transition stage, combustion of char, and reaction at high temperature. The ignition temperature is not significantly reduced as the sample size decreases. The ignition point and the burnout temperature increase with the heating rate. The main burning weight loss ranges from 200 to 700 °C. The smaller the particle size, the faster the reaction. The combustion performance for Nannochloropsis gaditana was not significantly affected by the O2 flow rate. The effect of O2 concentration becomes highly pronounced at 480–600 °C. The addition of rice hull makes Enteromorpha combustion more fully. Co-burning of husk and Enteromorpha can reduce the emission of gas pollutants.
Wang et al., 2013
Enteromorpha and Sargassum Enteromorpha
Nannochloropsis gaditana
Heating rate of 40 °C/min and heating range of 125–1000 °C.
Enteromorpha clathrata
Temperature of 900 °C, 15 mg of biomass, heating rate of 10–30 °C/min, and air flow rate of 120 mL/min.
Chen et al. (2016)
Wang et al. (2009)
Sanchezsilva et al. (2013) Yu et al. (2008)
Table 2 Proximate analysis and ultimate analysis for the algal biomass (Zhao et al., 2016). Algae biomass
Ch En Sa
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Mad
Aad
Vad
FCad
Cad
Had
Oad
Nad
Sad
6.64 5.19 6.34
5.99 28.12 34.15
74.21 57.77 52.53
13.16 8.92 6.98
50.42 29.32 28.82
7.25 4.41 3.89
19.32 26.58 25.26
8.82 4.04 1.13
1.56 2.34 0.41
Although the NOx emission from O2/CO2-based combustion of conventional fuels (e.g., coal and general biomass) has been comprehensively reported, it is barely involved in the O2/CO2-based algal biomass combustion. Particularly, NOx emissions from co-combustion of coal/bagasse in O2/CO2 atmosphere were found to be lower than that in O2/N2 atmosphere, with reduction by 22%–39% (Kazanc et al., 2011), there are few reports on NOx emission and conversion from algal biomass combustion in O2/CO2 atmosphere. Moreover, the O2/CO2 atmosphere significantly reduces the emission of thermal-NOx when the temperature exceeds about 1300 °C (Kimura et al., 1995; Zhang et al., 2010). However, the role of the O2/CO2 atmosphere in reducing the emission of fuel-NOx for algal biomass in low temperature combustion (usually < 900 °C), remains unknown. Therefore, NOx emission and conversion characteristics in an O2/CO2 atmosphere still need to be addressed. Overall, in-depth understanding of NOx particularly NO formation and conversion combusted in O2/CO2 atmosphere are fundamental to the emission control and reduction strategy from algal biomass combustion or co-fired with coal. It would also provide basic information for NO contribution in principle for the industrial-scale oxy-fuel combustion system using algal biomass-based fuel or mixed
Bypass Combustion boat
Flue gas Filter
Flowmeter
Gas cylinder containing: 21vol.% O2+79vol.% CO2 or
Digital tube furnace (RT-1100 oC)
Flue gas analyzer (Testo Model 350)
25vol.% O2+75vol.% CO2 or 30vol.% O2+70vol.% CO2
Fig. 1. Schematic diagram of experimental system.
nology to biomass combustion or biomass/coal co-combustion is conducive to reducing fossil fuel consumption, improving boiler efficiency and, reducing follow-up CO2 capture costs. Also, it plays a positive role in both energy structure adjustment and air pollutants/greenhouse gases control relating to the combustion process.
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(a) 0.61
NO concentration (mg/L)
Fig. 2. Effect of O2 concentration on NO emission concentration from Ch combustion.
Ch at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
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fuel (e.g., travelling grate boiler and CFB boiler), although the ultimate NO emission in whole system partially depends on the device specific as well. The purpose of this work is to investigate the NO emission characteristics of three typical algal biomass in an O2/CO2 combustion atmosphere in terms of O2 concentration (21%, 25% and 30%) and combustion temperature (700 °C, 800 °C and 900 °C). On this basis, the NO yields and the conversion rate of fuel-N were comprehensively examined. Finally, the possible mechanisms of NO formation and inhibition were explored in this process.
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Three typical algal biomass were selected as fuel in this work, including Chlorella sp. (Ch, Chlorophyceae, Chlorophyta. from Shandong, China), Enteromorpha spp. (En, Chlorophyceae, Chlorophyta, from Zhejiang, China) and Sargassum muticum (Sa, Sargassaceae Phaeophyceae, from Hainan, China). All algal biomass were air-dried in advance and crushed using a powder muller to produce the powdered fuels with a mean particle diameter of 80 μm. Their proximate and ultimate analysis are shown in Table 2.
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2.2. Experimental system and O2/CO2 conditions A schematic diagram of the experimental system is shown in Fig. 1. It consists of gas cylinders with various O2/CO2 concentrations, a gas flowmeter, a digital electric heating tube furnace (OTF-1200X, China), a filter and a flue gas analyzer (Testo model 350, Germany). To protect the flue gas analyzer, a bypass tube was added between the probe and the exhaust of the tube furnace. The combustion temperature for each algal biomass was set as 700 °C, 800 °C, and 900 °C, and the O2 concentration in the O2/CO2 atmosphere was controlled at 21%, 25%, and 30% (vol%), respectively. The gas flow rate was 3 L/min. Once the temperature and gas flow rate were stabilized at the preset values, a combustion boat (L × W × H = 97 × 15 × 17 mm) loaded with 100 ± 1 mg of the algal biomass sample was rapidly fed into the central burning area of the furnace. Meanwhile, the NO concentration was measured in real time by the flue gas analyzer. During the on-site sampling process, the flue gas analyzer used 1 L/min flow rate and 1s sampling interval (but using data display control of 5s). The NO concentration data was obtained and processed in terms of the relationship between NO concentration and elapsed time. The NO yields, namely the total NO released during the whole process of algal biomass, then were calculated in terms on NO concentration curves. To ensure the reliability of the data, each experiment was repeated three times under the same conditions. Finally, the morphological characteristics of the ashes were characterized using a scanning electron microscope (Model: FEI Quanta
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Fig. 3. Effect of O2 concentration on NO emission concentration from En combustion.
En at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
450 FEG, USA). 3. Results and discussion
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3.1. NO emission characteristics in O2/CO2 atmosphere Figs. 2–4 show the effect of the O2 concentration on the NO emission from the three algal biomass combustion at 700–900 °C. It should be noted that the temperature in the present experiment is lower than the critical temperature of 1200 °C at which thermal-NO begins to generate (Sjaak and Jaap, 2002; Sereika et al., 2017), therefore it can be considered that NO emitted is converted only from fuel-N in the algal biomass. The main peaks of NO emission from each algal biomass combustion were found to be positively correlated to the both O2 concentration and combustion temperature in O2/CO2 atmosphere. At high O2 concentration, the peaks reach and even exceed those in air (i.e., 21%O2/79%N2 atmosphere), particularly for Sa. This phenomenon is attributed to the improved flame propagation due to the higher O2 concentration, which accelerates the oxidization of the nitrogen-containing volatile compounds (Gu et al., 2017). Moreover, the subordinate NO peaks for each algal biomass, if any, are remarkably lower than the main peaks. It is suggested that the NO is mainly emitted during the volatile combustion stage (Wang et al., 2013; Chen et al., 2013, 2016; Gao et al., 2016) and CO2 atmosphere plays an inhibitory role on NO formation. On the whole, the shape of the NO emission curve for each algal biomass is insignificantly affected by temperature. The algae Ch tends to be a unimodal distribution, possible resulting from its high volatile content and single cell structure (Chen et al., 2016). En presents a quasi-unimodal distribution, its inconspicuous second peak at 900 °C may be due to the decomposition of nitrogenous residues. Also, no significant subordinate peaks of NO appear to occur during entire combustion of Sa. Note that it is different from the emission characteristics of NO in air atmosphere which results from less initially-formed NO reduced as the char oxidation proceeds (Karlström et al., 2017). The phenomena imply that the reducing atmosphere indirectly generated by the CO2 atmosphere is effective to reducing NO on the char surface by catalysis (Wang et al., 2009; Zhang et al., 2011).
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Table 3 shows the NO yields and conversion rates. Roughly, as the O2 concentration increases the NO yield increases and even exceeds the values in air atmosphere. The result indicates that a high O2 concentration in O2/CO2 atmosphere does not necessarily reduce NO emission. At a given O2 concentration, the NO yield first increases with the increasing temperature and then decreases beyond 800 °C except for
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Fig. 4. Effect of O2 concentration on NO emission concentration from Sa combustion.
Sa at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
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Sa in 30% O2/CO2 atmosphere. The similar trends are found in the conversion rate of each algal biomass. In O2/CO2 atmosphere, CO2 is reduced to CO by both the reductive compounds in the volatile matter and the char, and NO is further reduced through NO–CO-char reactions. However, at a higher oxygen concentration the enhanced oxidizing atmosphere suppresses the reduction of NO and meanwhile increases the inner flame temperature and combustion intensity. As a result, the NO yield gradually increases (Pu et al., 2016). Combustion temperature is an important factor influencing the NO yield. However, the relationship between NO yield and temperature depended upon multi-factors. They are positively correlated before 800 °C in the present work. When this temperature is exceeded, NH3, as the main precursor for NO formation (Lane et al., 2014; Chen et al., 2017; Riaza et al., 2013; Mladenović et al., 2018), has decreased emission (Zhan et al., 2017). Meanwhile, the reducibility of the substances including CO (converted from CO2 atmosphere) and volatile compounds and char increases with the temperature (Do et al., 2017). These factors, in part, result in the reduction of NO emission, as reported by both Lian et al. (2014) and Duan et al. (2015). An exception is that Sa has a continuous NO yield as the temperature increases in 30% O2/CO2 atmosphere, implying that NO formation by oxidation is still dominant under this condition compared to its reduction. It may also have a high NO emission transition temperature (> 900 °C). Importantly, the NO yield for each algal biomass at a given temperature can be effectively reduced in a 21% O2/CO2 atmosphere compared to air atmosphere (21% O2/N2 atmosphere) by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. In this case, the oxidation effect is equivalent due to the same oxygen concentration. However, the presence of CO2 from O2/CO2 atmosphere plays an important role in NO reduction, while N2 from air has no effect because of its non-reactivity.
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Based on the fuel-N conversion mechanism of biomass combustion in air atmosphere (Zhou et al., 2006; Zhao et al., 2016), the possible conversion pathways for the combustion of algal biomass in O2/CO2 atmosphere are inferred as illustrated in Fig. 5. During algal biomass combustion, NH3 as well as HCN and HNCO in volatile-N are generated at 320–450 °C by decomposition of protein (Roslee and Munajat, 2018; Kim et al., 2013), which is the main source of fuel-N (Campanella et al., 2012; Roslee and Munajat, 2017; Wang et al., 2017). These precursors and char-N are oxidized to NO by O2 from O2/CO2 atmosphere via reaction (R1). CO2 from O2/CO2 atmosphere is reduced to CO by both the reductive compounds in the volatile matter (occurs during the devolatilization (Hong et al., 2017)) and the
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Table 3 NO yields and conversion rates from the algal biomass combustion. Temp.
Parameters
Algal biomass Ch
700 °C 800 °C 900 °C a b c
En
Sa
Atmosphere
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/ N2a
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/ N2a
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/N2a
Y(mg)b R(%)c Y(mg)b R(%)c Y(mg)b R(%)c
0.449 2.393 1.034 5.510 0.987 5.260
0.912 4.860 1.262 6.725 1.076 5.734
1.118 5.958 1.453 7.743 1.158 6.171
1.182 6.299 1.193 6.357 1.075 5.728
0.386 4.491 0.679 7.899 0.555 6.457
0.466 5.421 0.720 8.376 0.613 7.131
0.518 6.026 0.772 8.981 0.654 7.608
0.709 8.248 0.714 8.306 0.748 8.702
0.294 12.228 0.607 25.247 0.598 24.873
0.340 14.142 0.670 27.867 0.609 25.330
0.357 14.849 0.673 27.992 0.707 29.406
0.727 30.238 0.783 32.567 0.812 33.773
In air atmosphere (Zhao et al., 2016). t NO yields is calculated byY = Q t CNO (t ) dt , where CNO is the NO concentration, t is time and Q is gas flow rate. 0
N conversion rate is calculated by R = (14/30) Y / CN × 100%, where CN is the N content (wt. %) in the algal biomass.
(Volatile-N) + (Char-N) + O 2 CO 2 + C x H y + Char
NO
CO
R2
NO + Char CO NO + CO CO 2 1 2 N 2 NO + (NH3 + HCN + C x H y ) +Char
N2
4. Conclusions
R5
Char (R3) O2(R1)
Char-N
CO2 in O2/CO2 atmosphere
R3 R4
Volatile-N (NH3,HCN,HNCO)
N in algal biomass
2004; Sartor et al., 2014; Vodička et al., 2018; Gu et al., 2016; Bešenić et al., 2018; Ruzickova et al., 2019).
R1
NO
CxHy (R2) Char (R2) CO (R4)
In O2/CO2 atmosphere, the main peaks of NO for the three algal biomass Ch, En and Sa increase with the increasing O2 concentration and combustion temperature. At a high O2 concentration, the main peaks reach and even exceed those in air atmosphere. In addition, the trends of NO emission characteristics tend to be a unimodal/quasi-unimodal distribution and, are affected insignificantly by O2 concentration at a given temperature. The NO yield and conversion rate increase with the increasing O2 concentration at a given temperature, implying that high O2 concentration in O2/CO2 atmosphere does not necessarily reduce NO emission. However, 21% O2/CO2 atmosphere is at least effective to reduce NO emission from most algal biomass combustion compared to air-based atmosphere (21% O2/N2). NO emission can be decreased by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. Moreover, the evolution of the NO yield (and the corresponding conversion rate) at a given O2 concentration is reversed as the combustion temperature rises above 800 °C with exception for Sa in 30% O2/CO2 atmosphere. The conversion mechanism of fuel-N in algal biomass combustion is complex. Essentially, the NO emission from algal biomass combustion in O2/CO2 atmosphere is the result of the combined effect of the NO formation oxidized from N-precursors and NO reduction by CO (converted from CO2) and other reductive components.
N2
Char (R5) Reductive volatile NH3,HCN,CxHy (R5)
Fig. 5. Possible pathways of the fuel-N conversion for algal biomass in O2/CO2 atmosphere.
char by reaction (R2), and NO is further reduced through NO–CO-char reactions via reactions (R3) and (R4). This is the main reason why the presence of CO2 is able to reduce NO emission, as mentioned above. Also, the initially-formed NO is partly reduced via reaction (5) (Hu et al., 2000; Zhou et al., 2015). Moreover, an uneven surface is still observed and more pores appear on their surfaces, although the increase of oxygen concentration leads to increasing combustion intensity, as shown in Fig. 6. On the one hand, the loose and porous structure promotes the conversion of char-N into NO by the oxidation via the reaction (R1). On the other hand, it is beneficial to increase the reaction surface area for CO2–char reduction reaction (R2) (Hong et al., 2017). It also promotes the NO reduction via the reactions R3, R4 and R5 (Li et al., 1999). In addition, this advantage indirectly enhances the surface catalysis of char for NO reduction via the reaction (R4) (Molina et al.,
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(a) for Ch
(b) for En
(c) for Sa Fig. 6. Morphology of ashes for three algal biomass at 700 °C by SEM (from left to right: at 21%, 25% and 30% O2 concentration in O2/CO2 atmosphere).
Acknowledgments
Energy Inst. 90 (5), 806–812. Chen, H., Si, Y., Chen, Y., Yang, H., Chen, D., Chen, W., 2017. NOx precursors from biomass pyrolysis: distribution of amino acids in biomass and Tar-N during devolatilization using model compounds. Fuel 187, 367–375. Croiset, E., Thambimuthu, K., Palmer, A., 2000. Coal combustion in O2/CO2 mixtures compared with air. Can. J. Chem. Eng. 78, 402–407. Do, H.S., Bunman, Y., Gao, S., Xu, G., 2017. Reduction of NO by biomass pyrolysis products in an experimental drop-tube. Energy Fuel. 31 (4), 4499–4506. Duan, F., Sun, X., Zhang, Y., Hu, A., Chyang, C., 2015. Combustion behavior and NO emission characteristic of biomass in O2/CO2 atmosphere. J. Southeast Univ. 31 (2), 200–203 [In Chinese]. Gao, Y., Tahmasebi, A., Dou, J., Yu, J., 2016. Combustion characteristics and air pollutant formation during oxy-fuel co-combustion of microalgae and lignite. Bioresour. Technol. 207, 276–284. Gu, M., Wu, C., Zhang, Y., Chu, H., 2016. Study on combustion characteristics of two sizes pulverized coal in O2/CO2 atmosphere. J. CO2 Util. 7 (7), 6–10. Gu, M., Chen, X., Wu, C., He, X., Chu, H., Liu, F., 2017. Effects of particle size distribution and oxygen concentration on the propagation behavior of pulverized coal flames in O2/CO2 atmospheres. Energy Fuel. 30 (5), 5571–5580. Hong, Y., Chen, W., Luo, X., Pang, C.C., Lester, E., Wu, T., 2017. Microwave-enhanced pyrolysis of macroalgae and microalgae for syngas production. Bioresour. Technol.
This work was jointly sponsored by Natural Science Foundation of Shanghai (No. 17ZR1419300) and National Natural Science Foundation of China (No. 50806049). The assistance provided by Mr. Shengli Li in performing the emission experiments is appreciated. References Bešenić, T., Mikulčić, H., Vujanović, M., Duić, N., 2018. Numerical modelling of emissions of nitrogen oxides in solid fuel combustion. J. Environ. Manag. 215, 177–184. Campanella, A., Muncrief, R., Harold, M.P., Griffith, D.C., Whitton, N.M., Weber, R.S., 2012. Thermolysis of microalgae and duckweed in a CO2-swept fixed-bed reactor: bio-oil yield and compositional effects. Bioresour. Technol. 109, 154–162. Chen, Y., Han, H., Lu, J., Li, D., Li, J., Liu, S., 2013. Effects of alkali and alkaline earth metals on NOx reduction in coke combustion. Adv. Mater. Res. 634–638, 522–525. Chen, C., Chen, F., Cheng, Z., Chan, Q., Kook, S., Yeoh, G., 2016. Emissions characteristics of NOx and SO2 in the combustion of microalgae biomass using a tube furnace. J.
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B. Zhao and Y. Su 237, 47–56. Hu, Y., Naito, S., Kobayashi, N., Hasatani, M., 2000. CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases. Fuel 79, 1925–1932. Karlström, O., Perander, M., Demartini, N., Brink, A., Hupa, M., 2017. Role of ash on the NO formation during char oxidation of biomass. Fuel 190, 274–280. Kazanc, F., Khatami, R., Crnkovic, P.M., Levendis, Y.A., 2011. Emissions of NOx and SO2 from coals of various ranks, bagasse, and coal-bagasse blends burning in O2/N2 and O2/CO2 environments. Energy Fuel. 25, 2850–2861. Kim, S.S., Ly, H.V., Kim, J., Choi, J.H., Woo, H.C., 2013. Thermogravimetric characteristics and pyrolysis kinetics of alga Sagarssum sp biomass. Bioresour. Technol. 139, 242–248. Kimura, N., Omata, K., Kiga, T., Takano, S., Shikisima, S., 1995. The characteristics of pulverized coal combustion in O2/CO2 mixtures for CO2 recovery. Energy Convers. Manag. 37 (3), 805–808. Lane, D.J., Ashman, P.J., Zevenhoven, M., Hupa, M.M., Van Eyk, P.J., De Nys, R., Karlström, O., Lewis, D.M., 2014. Combustion behavior of algal biomass: carbon Release, nitrogen release, and char reactivity. Energy Fuel. 28 (1), 41–51. Li, Y.H., Radovic, L.R., Lu, G.Q., Rudolph, V., 1999. A new kinetic model for the NOcarbon reaction. Chem. Eng. Sci. 54 (19), 4125–4136. Lian, J., Wang, Z., Ji, W., Yang, G., Liu, L., Wu, J., Wang, E., Gou, X., 2014. Experimental study on biomass char combustion in O2/CO2 atmosphere. Appl. Mech. Mater. 694, 136–141. Mladenović, M., Paprika, M., Marinković, A., 2018. Denitrification techniques for biomass combustion. Renew. Sustain. Energy Rev. 82, 3350–3364. Molina, A., Eddings, E.G., Pershing, D.W., Sarofim, A.F., 2004. Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions. Combust. Flame 136 (3), 303–312. Pu, G., Wang, W., Peng, R., Zhu, W., Zhao, F., Fu, F., Long, Y., 2016. NO emission of cocombustion coal and biomass blends in an oxygen-enriched atmosphere. Energy Sources 38 (23), 3497–3503. Riaza, J., Álvarez, L., Gil, M.V., Pevida, C., Pis, J.J., Rubiera, F., 2013. Ignition and no emissions of coal and biomass blends under different oxy-fuel atmospheres. Energy Procedia 37, 1405–1412. Roslee, A.N., Munajat, N.F., 2017. Comparative study on the pyrolysis behaviour and kinetics of two macroalgae biomass (Gracilaria changii and Gelidium pusillum) by thermogravimetric analysis. IOP Conf. Ser. Mater. Sci. Eng. 257, 012037. https://doi. org/10.1088/1757-899X/257/1/012037. Roslee, A.N., Munajat, N.F., 2018. Comparative study on pyrolysis behavior and kinetics of two macroalgae biomass (Ulva cf. flexuosa and Hy. edulis) using thermogravimetric analysis. J. Teknologi (Sci. Eng.) 80 (2), 123–130. Ruzickova, J., Kucbel, M., Raclavska, H., Svedova, B., Raclavsky, K., Juchelkova, D., 2019. Comparison of organic compounds in char and soot from the combustion of biomass in boilers of various emission classes. J. Environ. Manag. 236, 769–783. Sanchezsilva, L., Lópezgonzález, D., Garciaminguillan, A.M., Valverde, J.L., 2013.
Pyrolysis, combustion and gasification characteristics of Nannochloropsis gaditana microalgae. Bioresour. Technol. 130 (1), 321–331. Sartor, K., Restivo, Y., Ngendakumana, P., Dewallef, P., 2014. Prediction of SOx and NOx emissions from a medium size biomass boiler. Biomass Bioenergy 65, 91–100. Sereika, T., Buinevičius, K., Puida, E., Jančauskas, A., 2017. Biomass combustion research studying the impact factors of NOx formation and reduction. Agron. Res. 15, 1223–1231. Sjaak, V.L., Jaap, K., 2002. The Handbook of Biomass Combustion and Co-firing. Twente University Press, Twente. Vodička, M., Haugen, N.E., Gruber, A., Hrdlička, J., 2018. NOx formation in oxy-fuel combustion of lignite in a bubbling fluidized bed – modelling and experimental verification. Int. J. Greenh. Gas. Control 76, 208–214. Wang, S., Jiang, X., Han, X., Liu, J., 2009. Combustion characteristics of seaweed biomass. 1. combustion characteristics of enteromorpha clathrata and sargassum natans. Energy Fuel. 23 (10), 5173–5178. Wang, S., Jiang, X., Wang, Q., Han, X., Ji, H., 2013. Experiment and grey relational analysis of seaweed particle combustion in a fluidized bed. Energy Convers. Manag. 66, 115–120. Wang, X., Sheng, L., Yang, X., 2017. Pyrolysis characteristics and pathways of protein, lipid and carbohydrate isolated from microalgae Nannochloropsis sp. Bioresour. Technol. 229, 119–125. Winter, F., Wartha, C., Hofbauer, H., 1999. NO and N2O formation during the combustion of wood, straw, malt waste and peat. Bioresour. Technol. 70, 39–49. Yanik, J., Duman, G., Karlström, O., Brink, A., 2018. NO and SO2 emissions from combustion of raw and torrefied biomasses and their blends with lignite. J. Environ. Manag. 227, 155–161. Yu, L., Wang, S., Jiang, X., Wang, N., Zhang, C., 2008. Thermal analysis studies on combustion characteristics of seaweed. J. Therm. Anal. Calorim. 93 (2), 611–617. Zhan, H., Yin, X., Huang, Y., Yuan, H., Xie, J., Wu, C., Shen, Z., Cao, J., 2017. Comparisons on formation characteristics of NOx precursors during pyrolysis of lignocellulosic industrial biomass wastes. Energy Fuel. 31, 9557–9567. Zhang, Y., Zhang, J., Sheng, C., Zhao, L., Xie, F., Chen, J., Liu, Y., 2010. NOx emission characteristics of pulverized coal combustion in O2/N2, O2/CO2 and O2/CO2/NO atmospheres. J. Chem. Ind. Eng. 61, 159–165. Zhang, J., Sun, S., Zhao, Y., Hu, X., Xu, G., Qin, Y., 2011. Effects of inherent metals on NO reduction by coal char. Energy Fuel. 25 (12), 5605–5610. Zhao, B., Su, Y., Liu, D., Zhang, H., Liu, W., Cui, G., 2016. SO2/NOx emissions and ash formation from algae biomass combustion: process characteristics and mechanisms. Energy 113, 821–830. Zhou, H., Jensen, A.D., Glarborg, P., Kavaliauskas, A., 2006. Formation and reduction of nitric oxide in fixed-bed combustion of straw. Fuel 85, 705–716. Zhou, H., Huang, Y., Mo, G., Liao, Z., Cen, K., 2015. Experimental investigations of the conversion of fuel-N, volatile-N and char-N to NOx and N2O during single coal particle fluidized bed combustion. J. Energy Inst. 90, 62–72.
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Research article
Emission and conversion of NO from algal biomass combustion in O2/CO2 atmosphere
T
Bingtao Zhaoa, , Yaxin Sub ⁎
a
School of Energy and Power Engineering and Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai, 200093, China b School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai, 201620, China
ARTICLE INFO
ABSTRACT
Keywords: O2/CO2 atmosphere Algal biomass Direct combustion NO Emission and conversion
Environmental impacts of NO emissions from biomass combustion have become an important concern. To address NO emission and conversion from algal biomass combustion in O2/CO2 atmosphere, three typical algal biomass, Chlorella (Ch), Enteromorpha (En), and Sargassum (Sa), were used to investigate NO emission characteristics in a one-dimensional tube furnace. The effects of the combustion temperature and O2 concentration (21%, 25%, and 30%) on the NO emission were examined. It was found that the main peaks of NO positively are correlated to the O2 concentration and combustion temperature. The NO emission trends of each algal biomass are slightly affected by the O2 concentration at a given temperature. Roughly, the NO yield and conversion rate for each algal biomass increase with increasing O2 concentration at a given temperature. They first increase with the increasing temperature and then decrease beyond 800 °C with exception for Sa in 30% O2/CO2 atmosphere. However, 21% O2/CO2 atmosphere is at least effective to reduce NO emission from most algal biomass combustion compared to air-based atmosphere (21% O2/N2), by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. The possible conversion pathway of fuel-N implies that the NO emission from algal biomass combustion in O2/CO2 atmosphere is the result of the combined effect of the NO formation oxidized from Nprecursors and NO reduction by CO (converted from CO2) and other reductive components. These results may provide a positive reference for the control of NOx emissions from direct combustion or co-firing of algal biomass for energy utilization.
1. Introduction Algal biomass is a high-potential renewable resource different from fossil fuels and conventional biomass. Compared to the complex energy utilization methods including chemical extraction, biochemical fermentation and other thermochemical conversions (e.g., pyrolysis, gasification and liquefaction), direct combustion is a simple method of thermochemical utilization for algal biomass (Zhao et al., 2016; Chen et al., 2016; Wang et al., 2013). However, algal biomass combustion results in unavoidable NOx emissions which have become an important energy and environmental issue. To date, the researches on algal biomass combustion mainly focus on their combustion characteristics (Zhao et al., 2016; Chen et al., 2016; Wang et al., 2009; Wang et al., 2013; Sanchezsilva et al., 2013; Yu et al., 2008), as shown in Table 1. The great concern has been paid
⁎
to the combustion performances, particularly to the relationship between the combustion conditions (e.g., temperature and heating rate), the physicochemical properties of fuels (e.g., particle size of algal biomass) and the ash-forming elements (Mg, Ca and K etc.) (Karlström et al., 2017; Zhou et al., 2006; Zhan et al., 2017; Winter et al., 1999). In these investigations, most algal biomass combustion experiments had been carried out in an air-based atmosphere. It has been demonstrated that NOx formation and emission from algal biomass combustion highly depend upon the fuel composition and operating parameters in the system (Chen et al., 2016; Zhao et al., 2016) similar to that from pulverized coal combustion (Croiset et al., 2000; Yanik et al., 2018). Over the past decades, O2/CO2 atmospherebased combustion has gained widespread attention as an advanced and high-efficiency clean combustion technology which is different from traditional air-based combustion. Coupling O2/CO2 combustion tech-
Corresponding author. E-mail address: [email protected] (B. Zhao).
https://doi.org/10.1016/j.jenvman.2019.109419 Received 29 May 2019; Received in revised form 29 July 2019; Accepted 16 August 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Combustion characteristics of algae biomass. Algae biomass
Parameters
Performances
Ref.
Chlorella, Enteromorpha and Sargassum C. vulgaris
Temperature of 700–900 °C, 100 ± 1 mg of biomass, air flow rate of 3 L/min, and particle size of 85–88 μm. In 20% O2/80% N2 atmosphere, temperature of 300–700 °C, 100 mg of sample, and flow rate of 0.3 L/ min. In 20% O2/80% N2 atmosphere, temperature of 1200 °C, 17 mg of biomass, heating rate of 20 °C/min, flow rate of 0.1 L/min, and particle size of 0.18 mm. Temperature of 900 °C, 15 mg of biomass, heating rate of 10–30 °C/min, flow rate of 0.12 L/min, and particle size of 0–0.48 mm.
Conversion ratio: 9.30–9.10% for En, 34.03–34.98% for Sa, and 6.73–5.90% for Ch. Less than 10 ppm NOx emissions at a temperature ≤500 °C; beyond 500 °C, the NOx yield increases with the temperature.
Zhao et al. (2016)
Five stages: dehydration and desiccation, release and combustion of volatiles, transition stage, combustion of char, and reaction at high temperature. The ignition temperature is not significantly reduced as the sample size decreases. The ignition point and the burnout temperature increase with the heating rate. The main burning weight loss ranges from 200 to 700 °C. The smaller the particle size, the faster the reaction. The combustion performance for Nannochloropsis gaditana was not significantly affected by the O2 flow rate. The effect of O2 concentration becomes highly pronounced at 480–600 °C. The addition of rice hull makes Enteromorpha combustion more fully. Co-burning of husk and Enteromorpha can reduce the emission of gas pollutants.
Wang et al., 2013
Enteromorpha and Sargassum Enteromorpha
Nannochloropsis gaditana
Heating rate of 40 °C/min and heating range of 125–1000 °C.
Enteromorpha clathrata
Temperature of 900 °C, 15 mg of biomass, heating rate of 10–30 °C/min, and air flow rate of 120 mL/min.
Chen et al. (2016)
Wang et al. (2009)
Sanchezsilva et al. (2013) Yu et al. (2008)
Table 2 Proximate analysis and ultimate analysis for the algal biomass (Zhao et al., 2016). Algae biomass
Ch En Sa
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Mad
Aad
Vad
FCad
Cad
Had
Oad
Nad
Sad
6.64 5.19 6.34
5.99 28.12 34.15
74.21 57.77 52.53
13.16 8.92 6.98
50.42 29.32 28.82
7.25 4.41 3.89
19.32 26.58 25.26
8.82 4.04 1.13
1.56 2.34 0.41
Although the NOx emission from O2/CO2-based combustion of conventional fuels (e.g., coal and general biomass) has been comprehensively reported, it is barely involved in the O2/CO2-based algal biomass combustion. Particularly, NOx emissions from co-combustion of coal/bagasse in O2/CO2 atmosphere were found to be lower than that in O2/N2 atmosphere, with reduction by 22%–39% (Kazanc et al., 2011), there are few reports on NOx emission and conversion from algal biomass combustion in O2/CO2 atmosphere. Moreover, the O2/CO2 atmosphere significantly reduces the emission of thermal-NOx when the temperature exceeds about 1300 °C (Kimura et al., 1995; Zhang et al., 2010). However, the role of the O2/CO2 atmosphere in reducing the emission of fuel-NOx for algal biomass in low temperature combustion (usually < 900 °C), remains unknown. Therefore, NOx emission and conversion characteristics in an O2/CO2 atmosphere still need to be addressed. Overall, in-depth understanding of NOx particularly NO formation and conversion combusted in O2/CO2 atmosphere are fundamental to the emission control and reduction strategy from algal biomass combustion or co-fired with coal. It would also provide basic information for NO contribution in principle for the industrial-scale oxy-fuel combustion system using algal biomass-based fuel or mixed
Bypass Combustion boat
Flue gas Filter
Flowmeter
Gas cylinder containing: 21vol.% O2+79vol.% CO2 or
Digital tube furnace (RT-1100 oC)
Flue gas analyzer (Testo Model 350)
25vol.% O2+75vol.% CO2 or 30vol.% O2+70vol.% CO2
Fig. 1. Schematic diagram of experimental system.
nology to biomass combustion or biomass/coal co-combustion is conducive to reducing fossil fuel consumption, improving boiler efficiency and, reducing follow-up CO2 capture costs. Also, it plays a positive role in both energy structure adjustment and air pollutants/greenhouse gases control relating to the combustion process.
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(a) 0.61
NO concentration (mg/L)
Fig. 2. Effect of O2 concentration on NO emission concentration from Ch combustion.
Ch at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
0.48
fuel (e.g., travelling grate boiler and CFB boiler), although the ultimate NO emission in whole system partially depends on the device specific as well. The purpose of this work is to investigate the NO emission characteristics of three typical algal biomass in an O2/CO2 combustion atmosphere in terms of O2 concentration (21%, 25% and 30%) and combustion temperature (700 °C, 800 °C and 900 °C). On this basis, the NO yields and the conversion rate of fuel-N were comprehensively examined. Finally, the possible mechanisms of NO formation and inhibition were explored in this process.
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NO concentration (mg/L)
Three typical algal biomass were selected as fuel in this work, including Chlorella sp. (Ch, Chlorophyceae, Chlorophyta. from Shandong, China), Enteromorpha spp. (En, Chlorophyceae, Chlorophyta, from Zhejiang, China) and Sargassum muticum (Sa, Sargassaceae Phaeophyceae, from Hainan, China). All algal biomass were air-dried in advance and crushed using a powder muller to produce the powdered fuels with a mean particle diameter of 80 μm. Their proximate and ultimate analysis are shown in Table 2.
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2.2. Experimental system and O2/CO2 conditions A schematic diagram of the experimental system is shown in Fig. 1. It consists of gas cylinders with various O2/CO2 concentrations, a gas flowmeter, a digital electric heating tube furnace (OTF-1200X, China), a filter and a flue gas analyzer (Testo model 350, Germany). To protect the flue gas analyzer, a bypass tube was added between the probe and the exhaust of the tube furnace. The combustion temperature for each algal biomass was set as 700 °C, 800 °C, and 900 °C, and the O2 concentration in the O2/CO2 atmosphere was controlled at 21%, 25%, and 30% (vol%), respectively. The gas flow rate was 3 L/min. Once the temperature and gas flow rate were stabilized at the preset values, a combustion boat (L × W × H = 97 × 15 × 17 mm) loaded with 100 ± 1 mg of the algal biomass sample was rapidly fed into the central burning area of the furnace. Meanwhile, the NO concentration was measured in real time by the flue gas analyzer. During the on-site sampling process, the flue gas analyzer used 1 L/min flow rate and 1s sampling interval (but using data display control of 5s). The NO concentration data was obtained and processed in terms of the relationship between NO concentration and elapsed time. The NO yields, namely the total NO released during the whole process of algal biomass, then were calculated in terms on NO concentration curves. To ensure the reliability of the data, each experiment was repeated three times under the same conditions. Finally, the morphological characteristics of the ashes were characterized using a scanning electron microscope (Model: FEI Quanta
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NO concentration (mg/L)
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Fig. 3. Effect of O2 concentration on NO emission concentration from En combustion.
En at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
450 FEG, USA). 3. Results and discussion
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3.1. NO emission characteristics in O2/CO2 atmosphere Figs. 2–4 show the effect of the O2 concentration on the NO emission from the three algal biomass combustion at 700–900 °C. It should be noted that the temperature in the present experiment is lower than the critical temperature of 1200 °C at which thermal-NO begins to generate (Sjaak and Jaap, 2002; Sereika et al., 2017), therefore it can be considered that NO emitted is converted only from fuel-N in the algal biomass. The main peaks of NO emission from each algal biomass combustion were found to be positively correlated to the both O2 concentration and combustion temperature in O2/CO2 atmosphere. At high O2 concentration, the peaks reach and even exceed those in air (i.e., 21%O2/79%N2 atmosphere), particularly for Sa. This phenomenon is attributed to the improved flame propagation due to the higher O2 concentration, which accelerates the oxidization of the nitrogen-containing volatile compounds (Gu et al., 2017). Moreover, the subordinate NO peaks for each algal biomass, if any, are remarkably lower than the main peaks. It is suggested that the NO is mainly emitted during the volatile combustion stage (Wang et al., 2013; Chen et al., 2013, 2016; Gao et al., 2016) and CO2 atmosphere plays an inhibitory role on NO formation. On the whole, the shape of the NO emission curve for each algal biomass is insignificantly affected by temperature. The algae Ch tends to be a unimodal distribution, possible resulting from its high volatile content and single cell structure (Chen et al., 2016). En presents a quasi-unimodal distribution, its inconspicuous second peak at 900 °C may be due to the decomposition of nitrogenous residues. Also, no significant subordinate peaks of NO appear to occur during entire combustion of Sa. Note that it is different from the emission characteristics of NO in air atmosphere which results from less initially-formed NO reduced as the char oxidation proceeds (Karlström et al., 2017). The phenomena imply that the reducing atmosphere indirectly generated by the CO2 atmosphere is effective to reducing NO on the char surface by catalysis (Wang et al., 2009; Zhang et al., 2011).
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Table 3 shows the NO yields and conversion rates. Roughly, as the O2 concentration increases the NO yield increases and even exceeds the values in air atmosphere. The result indicates that a high O2 concentration in O2/CO2 atmosphere does not necessarily reduce NO emission. At a given O2 concentration, the NO yield first increases with the increasing temperature and then decreases beyond 800 °C except for
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Fig. 4. Effect of O2 concentration on NO emission concentration from Sa combustion.
Sa at 700 oC 21% O2/79% CO2 25% O2/75% CO2 30% O2/70% CO2 21% O2/79% N2 (Air) (Zhao et al., 2016)
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Sa in 30% O2/CO2 atmosphere. The similar trends are found in the conversion rate of each algal biomass. In O2/CO2 atmosphere, CO2 is reduced to CO by both the reductive compounds in the volatile matter and the char, and NO is further reduced through NO–CO-char reactions. However, at a higher oxygen concentration the enhanced oxidizing atmosphere suppresses the reduction of NO and meanwhile increases the inner flame temperature and combustion intensity. As a result, the NO yield gradually increases (Pu et al., 2016). Combustion temperature is an important factor influencing the NO yield. However, the relationship between NO yield and temperature depended upon multi-factors. They are positively correlated before 800 °C in the present work. When this temperature is exceeded, NH3, as the main precursor for NO formation (Lane et al., 2014; Chen et al., 2017; Riaza et al., 2013; Mladenović et al., 2018), has decreased emission (Zhan et al., 2017). Meanwhile, the reducibility of the substances including CO (converted from CO2 atmosphere) and volatile compounds and char increases with the temperature (Do et al., 2017). These factors, in part, result in the reduction of NO emission, as reported by both Lian et al. (2014) and Duan et al. (2015). An exception is that Sa has a continuous NO yield as the temperature increases in 30% O2/CO2 atmosphere, implying that NO formation by oxidation is still dominant under this condition compared to its reduction. It may also have a high NO emission transition temperature (> 900 °C). Importantly, the NO yield for each algal biomass at a given temperature can be effectively reduced in a 21% O2/CO2 atmosphere compared to air atmosphere (21% O2/N2 atmosphere) by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. In this case, the oxidation effect is equivalent due to the same oxygen concentration. However, the presence of CO2 from O2/CO2 atmosphere plays an important role in NO reduction, while N2 from air has no effect because of its non-reactivity.
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Based on the fuel-N conversion mechanism of biomass combustion in air atmosphere (Zhou et al., 2006; Zhao et al., 2016), the possible conversion pathways for the combustion of algal biomass in O2/CO2 atmosphere are inferred as illustrated in Fig. 5. During algal biomass combustion, NH3 as well as HCN and HNCO in volatile-N are generated at 320–450 °C by decomposition of protein (Roslee and Munajat, 2018; Kim et al., 2013), which is the main source of fuel-N (Campanella et al., 2012; Roslee and Munajat, 2017; Wang et al., 2017). These precursors and char-N are oxidized to NO by O2 from O2/CO2 atmosphere via reaction (R1). CO2 from O2/CO2 atmosphere is reduced to CO by both the reductive compounds in the volatile matter (occurs during the devolatilization (Hong et al., 2017)) and the
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Table 3 NO yields and conversion rates from the algal biomass combustion. Temp.
Parameters
Algal biomass Ch
700 °C 800 °C 900 °C a b c
En
Sa
Atmosphere
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/ N2a
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/ N2a
21%O2/ CO2
25%O2/ CO2
30%O2/ CO2
21%O2/N2a
Y(mg)b R(%)c Y(mg)b R(%)c Y(mg)b R(%)c
0.449 2.393 1.034 5.510 0.987 5.260
0.912 4.860 1.262 6.725 1.076 5.734
1.118 5.958 1.453 7.743 1.158 6.171
1.182 6.299 1.193 6.357 1.075 5.728
0.386 4.491 0.679 7.899 0.555 6.457
0.466 5.421 0.720 8.376 0.613 7.131
0.518 6.026 0.772 8.981 0.654 7.608
0.709 8.248 0.714 8.306 0.748 8.702
0.294 12.228 0.607 25.247 0.598 24.873
0.340 14.142 0.670 27.867 0.609 25.330
0.357 14.849 0.673 27.992 0.707 29.406
0.727 30.238 0.783 32.567 0.812 33.773
In air atmosphere (Zhao et al., 2016). t NO yields is calculated byY = Q t CNO (t ) dt , where CNO is the NO concentration, t is time and Q is gas flow rate. 0
N conversion rate is calculated by R = (14/30) Y / CN × 100%, where CN is the N content (wt. %) in the algal biomass.
(Volatile-N) + (Char-N) + O 2 CO 2 + C x H y + Char
NO
CO
R2
NO + Char CO NO + CO CO 2 1 2 N 2 NO + (NH3 + HCN + C x H y ) +Char
N2
4. Conclusions
R5
Char (R3) O2(R1)
Char-N
CO2 in O2/CO2 atmosphere
R3 R4
Volatile-N (NH3,HCN,HNCO)
N in algal biomass
2004; Sartor et al., 2014; Vodička et al., 2018; Gu et al., 2016; Bešenić et al., 2018; Ruzickova et al., 2019).
R1
NO
CxHy (R2) Char (R2) CO (R4)
In O2/CO2 atmosphere, the main peaks of NO for the three algal biomass Ch, En and Sa increase with the increasing O2 concentration and combustion temperature. At a high O2 concentration, the main peaks reach and even exceed those in air atmosphere. In addition, the trends of NO emission characteristics tend to be a unimodal/quasi-unimodal distribution and, are affected insignificantly by O2 concentration at a given temperature. The NO yield and conversion rate increase with the increasing O2 concentration at a given temperature, implying that high O2 concentration in O2/CO2 atmosphere does not necessarily reduce NO emission. However, 21% O2/CO2 atmosphere is at least effective to reduce NO emission from most algal biomass combustion compared to air-based atmosphere (21% O2/N2). NO emission can be decreased by 8.2–62.0%, 4.9–45.6%, and 22.5–59.6% for Ch, En, and Sa, respectively. Moreover, the evolution of the NO yield (and the corresponding conversion rate) at a given O2 concentration is reversed as the combustion temperature rises above 800 °C with exception for Sa in 30% O2/CO2 atmosphere. The conversion mechanism of fuel-N in algal biomass combustion is complex. Essentially, the NO emission from algal biomass combustion in O2/CO2 atmosphere is the result of the combined effect of the NO formation oxidized from N-precursors and NO reduction by CO (converted from CO2) and other reductive components.
N2
Char (R5) Reductive volatile NH3,HCN,CxHy (R5)
Fig. 5. Possible pathways of the fuel-N conversion for algal biomass in O2/CO2 atmosphere.
char by reaction (R2), and NO is further reduced through NO–CO-char reactions via reactions (R3) and (R4). This is the main reason why the presence of CO2 is able to reduce NO emission, as mentioned above. Also, the initially-formed NO is partly reduced via reaction (5) (Hu et al., 2000; Zhou et al., 2015). Moreover, an uneven surface is still observed and more pores appear on their surfaces, although the increase of oxygen concentration leads to increasing combustion intensity, as shown in Fig. 6. On the one hand, the loose and porous structure promotes the conversion of char-N into NO by the oxidation via the reaction (R1). On the other hand, it is beneficial to increase the reaction surface area for CO2–char reduction reaction (R2) (Hong et al., 2017). It also promotes the NO reduction via the reactions R3, R4 and R5 (Li et al., 1999). In addition, this advantage indirectly enhances the surface catalysis of char for NO reduction via the reaction (R4) (Molina et al.,
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B. Zhao and Y. Su
(a) for Ch
(b) for En
(c) for Sa Fig. 6. Morphology of ashes for three algal biomass at 700 °C by SEM (from left to right: at 21%, 25% and 30% O2 concentration in O2/CO2 atmosphere).
Acknowledgments
Energy Inst. 90 (5), 806–812. Chen, H., Si, Y., Chen, Y., Yang, H., Chen, D., Chen, W., 2017. NOx precursors from biomass pyrolysis: distribution of amino acids in biomass and Tar-N during devolatilization using model compounds. Fuel 187, 367–375. Croiset, E., Thambimuthu, K., Palmer, A., 2000. Coal combustion in O2/CO2 mixtures compared with air. Can. J. Chem. Eng. 78, 402–407. Do, H.S., Bunman, Y., Gao, S., Xu, G., 2017. Reduction of NO by biomass pyrolysis products in an experimental drop-tube. Energy Fuel. 31 (4), 4499–4506. Duan, F., Sun, X., Zhang, Y., Hu, A., Chyang, C., 2015. Combustion behavior and NO emission characteristic of biomass in O2/CO2 atmosphere. J. Southeast Univ. 31 (2), 200–203 [In Chinese]. Gao, Y., Tahmasebi, A., Dou, J., Yu, J., 2016. Combustion characteristics and air pollutant formation during oxy-fuel co-combustion of microalgae and lignite. Bioresour. Technol. 207, 276–284. Gu, M., Wu, C., Zhang, Y., Chu, H., 2016. Study on combustion characteristics of two sizes pulverized coal in O2/CO2 atmosphere. J. CO2 Util. 7 (7), 6–10. Gu, M., Chen, X., Wu, C., He, X., Chu, H., Liu, F., 2017. Effects of particle size distribution and oxygen concentration on the propagation behavior of pulverized coal flames in O2/CO2 atmospheres. Energy Fuel. 30 (5), 5571–5580. Hong, Y., Chen, W., Luo, X., Pang, C.C., Lester, E., Wu, T., 2017. Microwave-enhanced pyrolysis of macroalgae and microalgae for syngas production. Bioresour. Technol.
This work was jointly sponsored by Natural Science Foundation of Shanghai (No. 17ZR1419300) and National Natural Science Foundation of China (No. 50806049). The assistance provided by Mr. Shengli Li in performing the emission experiments is appreciated. References Bešenić, T., Mikulčić, H., Vujanović, M., Duić, N., 2018. Numerical modelling of emissions of nitrogen oxides in solid fuel combustion. J. Environ. Manag. 215, 177–184. Campanella, A., Muncrief, R., Harold, M.P., Griffith, D.C., Whitton, N.M., Weber, R.S., 2012. Thermolysis of microalgae and duckweed in a CO2-swept fixed-bed reactor: bio-oil yield and compositional effects. Bioresour. Technol. 109, 154–162. Chen, Y., Han, H., Lu, J., Li, D., Li, J., Liu, S., 2013. Effects of alkali and alkaline earth metals on NOx reduction in coke combustion. Adv. Mater. Res. 634–638, 522–525. Chen, C., Chen, F., Cheng, Z., Chan, Q., Kook, S., Yeoh, G., 2016. Emissions characteristics of NOx and SO2 in the combustion of microalgae biomass using a tube furnace. J.
7
Journal of Environmental Management 250 (2019) 109419
B. Zhao and Y. Su 237, 47–56. Hu, Y., Naito, S., Kobayashi, N., Hasatani, M., 2000. CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases. Fuel 79, 1925–1932. Karlström, O., Perander, M., Demartini, N., Brink, A., Hupa, M., 2017. Role of ash on the NO formation during char oxidation of biomass. Fuel 190, 274–280. Kazanc, F., Khatami, R., Crnkovic, P.M., Levendis, Y.A., 2011. Emissions of NOx and SO2 from coals of various ranks, bagasse, and coal-bagasse blends burning in O2/N2 and O2/CO2 environments. Energy Fuel. 25, 2850–2861. Kim, S.S., Ly, H.V., Kim, J., Choi, J.H., Woo, H.C., 2013. Thermogravimetric characteristics and pyrolysis kinetics of alga Sagarssum sp biomass. Bioresour. Technol. 139, 242–248. Kimura, N., Omata, K., Kiga, T., Takano, S., Shikisima, S., 1995. The characteristics of pulverized coal combustion in O2/CO2 mixtures for CO2 recovery. Energy Convers. Manag. 37 (3), 805–808. Lane, D.J., Ashman, P.J., Zevenhoven, M., Hupa, M.M., Van Eyk, P.J., De Nys, R., Karlström, O., Lewis, D.M., 2014. Combustion behavior of algal biomass: carbon Release, nitrogen release, and char reactivity. Energy Fuel. 28 (1), 41–51. Li, Y.H., Radovic, L.R., Lu, G.Q., Rudolph, V., 1999. A new kinetic model for the NOcarbon reaction. Chem. Eng. Sci. 54 (19), 4125–4136. Lian, J., Wang, Z., Ji, W., Yang, G., Liu, L., Wu, J., Wang, E., Gou, X., 2014. Experimental study on biomass char combustion in O2/CO2 atmosphere. Appl. Mech. Mater. 694, 136–141. Mladenović, M., Paprika, M., Marinković, A., 2018. Denitrification techniques for biomass combustion. Renew. Sustain. Energy Rev. 82, 3350–3364. Molina, A., Eddings, E.G., Pershing, D.W., Sarofim, A.F., 2004. Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions. Combust. Flame 136 (3), 303–312. Pu, G., Wang, W., Peng, R., Zhu, W., Zhao, F., Fu, F., Long, Y., 2016. NO emission of cocombustion coal and biomass blends in an oxygen-enriched atmosphere. Energy Sources 38 (23), 3497–3503. Riaza, J., Álvarez, L., Gil, M.V., Pevida, C., Pis, J.J., Rubiera, F., 2013. Ignition and no emissions of coal and biomass blends under different oxy-fuel atmospheres. Energy Procedia 37, 1405–1412. Roslee, A.N., Munajat, N.F., 2017. Comparative study on the pyrolysis behaviour and kinetics of two macroalgae biomass (Gracilaria changii and Gelidium pusillum) by thermogravimetric analysis. IOP Conf. Ser. Mater. Sci. Eng. 257, 012037. https://doi. org/10.1088/1757-899X/257/1/012037. Roslee, A.N., Munajat, N.F., 2018. Comparative study on pyrolysis behavior and kinetics of two macroalgae biomass (Ulva cf. flexuosa and Hy. edulis) using thermogravimetric analysis. J. Teknologi (Sci. Eng.) 80 (2), 123–130. Ruzickova, J., Kucbel, M., Raclavska, H., Svedova, B., Raclavsky, K., Juchelkova, D., 2019. Comparison of organic compounds in char and soot from the combustion of biomass in boilers of various emission classes. J. Environ. Manag. 236, 769–783. Sanchezsilva, L., Lópezgonzález, D., Garciaminguillan, A.M., Valverde, J.L., 2013.
Pyrolysis, combustion and gasification characteristics of Nannochloropsis gaditana microalgae. Bioresour. Technol. 130 (1), 321–331. Sartor, K., Restivo, Y., Ngendakumana, P., Dewallef, P., 2014. Prediction of SOx and NOx emissions from a medium size biomass boiler. Biomass Bioenergy 65, 91–100. Sereika, T., Buinevičius, K., Puida, E., Jančauskas, A., 2017. Biomass combustion research studying the impact factors of NOx formation and reduction. Agron. Res. 15, 1223–1231. Sjaak, V.L., Jaap, K., 2002. The Handbook of Biomass Combustion and Co-firing. Twente University Press, Twente. Vodička, M., Haugen, N.E., Gruber, A., Hrdlička, J., 2018. NOx formation in oxy-fuel combustion of lignite in a bubbling fluidized bed – modelling and experimental verification. Int. J. Greenh. Gas. Control 76, 208–214. Wang, S., Jiang, X., Han, X., Liu, J., 2009. Combustion characteristics of seaweed biomass. 1. combustion characteristics of enteromorpha clathrata and sargassum natans. Energy Fuel. 23 (10), 5173–5178. Wang, S., Jiang, X., Wang, Q., Han, X., Ji, H., 2013. Experiment and grey relational analysis of seaweed particle combustion in a fluidized bed. Energy Convers. Manag. 66, 115–120. Wang, X., Sheng, L., Yang, X., 2017. Pyrolysis characteristics and pathways of protein, lipid and carbohydrate isolated from microalgae Nannochloropsis sp. Bioresour. Technol. 229, 119–125. Winter, F., Wartha, C., Hofbauer, H., 1999. NO and N2O formation during the combustion of wood, straw, malt waste and peat. Bioresour. Technol. 70, 39–49. Yanik, J., Duman, G., Karlström, O., Brink, A., 2018. NO and SO2 emissions from combustion of raw and torrefied biomasses and their blends with lignite. J. Environ. Manag. 227, 155–161. Yu, L., Wang, S., Jiang, X., Wang, N., Zhang, C., 2008. Thermal analysis studies on combustion characteristics of seaweed. J. Therm. Anal. Calorim. 93 (2), 611–617. Zhan, H., Yin, X., Huang, Y., Yuan, H., Xie, J., Wu, C., Shen, Z., Cao, J., 2017. Comparisons on formation characteristics of NOx precursors during pyrolysis of lignocellulosic industrial biomass wastes. Energy Fuel. 31, 9557–9567. Zhang, Y., Zhang, J., Sheng, C., Zhao, L., Xie, F., Chen, J., Liu, Y., 2010. NOx emission characteristics of pulverized coal combustion in O2/N2, O2/CO2 and O2/CO2/NO atmospheres. J. Chem. Ind. Eng. 61, 159–165. Zhang, J., Sun, S., Zhao, Y., Hu, X., Xu, G., Qin, Y., 2011. Effects of inherent metals on NO reduction by coal char. Energy Fuel. 25 (12), 5605–5610. Zhao, B., Su, Y., Liu, D., Zhang, H., Liu, W., Cui, G., 2016. SO2/NOx emissions and ash formation from algae biomass combustion: process characteristics and mechanisms. Energy 113, 821–830. Zhou, H., Jensen, A.D., Glarborg, P., Kavaliauskas, A., 2006. Formation and reduction of nitric oxide in fixed-bed combustion of straw. Fuel 85, 705–716. Zhou, H., Huang, Y., Mo, G., Liao, Z., Cen, K., 2015. Experimental investigations of the conversion of fuel-N, volatile-N and char-N to NOx and N2O during single coal particle fluidized bed combustion. J. Energy Inst. 90, 62–72.
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