Influence of Biomass Reburning on NO x Reductions during

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Influence of biomass reburning on NOx reductions during pulverized coal combustion Huiyong Zhuang, Yanqing Niu, Yanhao Gong, Yu Zhang, Yanru Zhang, and Shi'en Hui Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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Influence of biomass reburning on NOx reductions during pulverized coal combustion Huiyong Zhuang1,2, Yanqing Niu3, Yanhao Gong3, Yu Zhang3, Yanru Zhang2, Shien Hui3 1

Institute of Energy, Environment and Economy, Tsinghua University, Beijing 100084, China 2

3

National Bio Energy Co., Ltd., Beijing, 100052, China

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi, 710049, China

Abstract: The NOx reductions by reburning using sawdust, corn straw and cotton straw were studied in a lab-scale and self-heating drop-tube furnace with varied reaction temperatures, reburning fuel fraction, stoichiometric ratio, and residence time in reburning zone. Results show that NOx reduction efficiency increased with reburning biomass ratio, however, the NOx reduction efficiency increased first and then decreased with the increased reburning temperature, stoichiometric ratio, and residence time with an optimal point around 1100 °C, 0.6-0.7, and 1.88 s, respectively. Above 1100 °C, thermal-NOx originated from reburning biomass resulted in a decrease in NOx reduction efficiency. High stoichiometric ratio oxidized reducing radicals and thus decreased NOx reduction efficiency, but the NOx reductions were also inhabited when the excess air ratio was too low, which hindered the further conversion of the intermedias during NOx reductions. In fixed-size furnace, long residence time in reburning zone improved volatile release and NOx reductions, but shorted residence time of pulverized coal in primary zone resulted in incomplete burnout and excess oxygen which oxidized the reducing radicals and thus decreased NOx reduction efficiency. In addition, high volatiles and heating value (influencing furnace temperature distribution) of the reburning biomass caused high NOx reduction efficiency, and consequently, the woody biomass (sawdust) showed higher NOx reduction efficiency in comparisons with agricultural residues such as corn straw and cotton straw. Meanwhile, the Van Krevelen diagram where the fuel far from the zero point shows high NOx reduction efficiency may be as a guideline for the selection of reburning biomass.

1

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Keywords: Reburning; Biomass; NOx; Temperature; Reburning ratio; Excess air ratio

1. Introduction NOx emissions from coal combustion have resulted in smog and acid rain, as well as ozone depletion, and consequently severely endanger the eco-environment and human health1. Thus, varied de-NOx technologies such as low NOx burner 2, co-firing staging

5, 7-9

, reburning

7, 10

3-5

, exhaust gas recirculation 6, air and fuel

, selective non-catalytic reduction (SNCR) 4, selective catalytic reduction

(SCR)11, etc. have been employed singly or jointly. Among those, reburning is considered as one of the most promising and cost-effective NOx emission control strategies. In principal, a complete pulverized coal-fired furnace employing reburning technology consists of three distinct reaction zones along the height of the furnace, that is, primary-zone, reburning-zone, and burnout zone

7, 10

. In the

primary zone, about 80 % of fuel (heat basis) is burned with a lightly air excess; In the reburning zone downstream of the primary zone, about 20% of the reburning fuel is injected to generate a fuel-rich and oxygen-lean condition, and as a result, the NOx from the primary zone is reduced into N2; in the burnout zone downstream of the reburning zone, over-fire air is introduced to oxidize the unburned compounds. There exist varied fuels that can be used as reburning fuel such as pulverized coal 12, natural gas 7, and biomass

7, 10

. Among those, biomass is getting more attention worldwide due to the worsening

energy crisis and environmental issues, as well as being a sufficiently “green” renewable and CO2-neutral energy source 3. Although the NOx removal efficiency of biomass reburning (obviously higher than pulverized coal reburning 12) is 4-10% lower than that of natural gas reburning 13, the cost of NOx removal by biomass reburning is only 1/2 of that using natural gas, and 1/4 and 1/7 of the cost by means of SNCR and SCR 14. In addition, as a feasible biomass conversion option, in comparisons with biomass direct combustion (the biomass installed power-generating capacity has reached 13 GW (Gigawatt) and will increase to 30 GW in 2020 in China

15

; in Europe, it has taken up 70% of all

power generation from renewable fuel, and in USA it has reached 10 GW 16) which encounters serious ash-related issues such as slagging, agglomeration, and corrosion hindering its further popularization

2


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17-20

, biomass reburning in pulverized coal fired boiler not only weakens the influence of the

ash-related issues but also possesses higher power generation efficiency, lower investment, and weaker fuel dependence 22

co-firing capacity

3, 21

. Thus, most newly built power plants in EU reserve 10-20% biomass

. Biomass reburning is not only a cost-effective biomass conversion technology

but also an eco-friendly strategy assisting in coal clean combustion for power and heat generation. Being different from conventional biomass/coal blends co-firing technology, biomass reburning introduces biomass after primary burning zone and generates a zone with lean-oxygen and rich-fuel, where the CHi and NHi radicals pyrolyzed from biomass reduce NOx originated from the primary zone into N2 23, 24. Biomass possesses high volatiles (mainly hydrocarbons), meanwhile, the fuel-N in biomass can convert into NH3 during combustion, both that imporve the NOx reductions by reburning 3, 25, 26

.

Regarding of the major influencing factors such as biomass type, biomass particle size, reaction temperature in the reburning zone, reburning fuel fraction (reburning biomass/coal burned in primary zone), and stoichiometric ratio (air/fuel ratio in reburning zone), many studies on biomass reburning have been carried out. Lu et al.

10

reported that biomass type and heterogeneous reaction with char

have an important influence on NO reduction efficiency, and the NO reduction efficiency of biomass reburning increases with increased reaction temperature in the reburning zone, but it first increases and then decreases with a decrease of stoichiometric ratio or a increasing of reburning fuel fraction. Sun et al. 27 reported the similar results, meanwhile, they pointed out that NO reduction increased with increasing volatile content, initial NO concentration, and gas residence time. Meanwhile, Salzmann and Nussbaumer

9

conducted biomass staged combustion and found that low NOx emissions were

achievable at lower temperatures (900-1000 °C) and a stoichiometric ratio of 0.85 in the reduction zone. In speaking of particle size, Casaca and Costa

28

pointed out that oversize or undersize is

disadvantage to NOx reductions; however, Niu et al.

29

found that the NOx reduction efficiency

increased with decreased reburning fuel size, and Lu et al. 10 also found that small size facilitated NOx reductions. In addition, both combustion atmosphere and ash compositions show significant effects on NOx reductions by biomass reburning 10, 30-32. 3


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Although number of reseach on NOx reductions through biomass reburning during pulverized coal combustion have been conducted, the research are far from conclusive, moreover, information are imcomplete, for exzample, the NOx reduction characteristics in industrial practice which are different from the lab-scale electric-heating furnace, and the effect of reburning biomass characteristics or types. Thus, the NOx reductions during pulverized coal combustion by different biomass (sawdust as a represent of woody biomass, and corn stalk and cotton straw as represents of herb biomass) reburning with varied reaction temperatures (Tr), reburning fuel fraction (Frf), stoichiometric ratio (SR), and residence time (tr) in reburning zone were studied in a lab-scale and self-heating drop-tube furnace in detail.

2 Experiments 2.1 Experiment setups and methods Fig.1 Schematics of the drop-tube furnace experiment system As shown in Fig.1a, the experiments were conducted in a one denominational self-heating drop-tube furnace. The height of the furnace is 2.615 m, and the inner diameter of the reaction tube is 0.168 m. Corresponding to number X1 to X8, eight temperature testing holes and eight looking-fire-holes are opened in the front and rear walls, respectively; at the same level, eight sets of reburning biomass nozzles are furnished in the side walls. The compositions of the flue gas (NOx and O2) were analyzed by a flue gas analyzer (MRU 95/3CD) after economizer. The residual ash was collected at the outlet of the furnace by water-cooled probe and used to analysis the unburned carbon content. In order to build a comparable experiment system with utility boiler, besides that the furnace is self-heating by fuel burning instead of electric-heating, the burner is designed and produced on basis of power plant burner. In the burner, the air flow is divided into three parts, that is, primary airflow, inner secondary airflow, and outer secondary airflow. The primary airflow used to carry pulverized coal is direct flowed; whereas to build a high-temperature ignition zone at the outlet of the burner, the secondary airflows which consist of inner and outer air flow are swirl. The inner secondary air flows 4


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through axial spinning swirlers, and the outer secondary air passes through tangential vortex blades, both that build a high-temperature ignition zone at the out of the burner through entraining high temperature flue gas from the gas bath in the furnace. During experiments, the pulverized coal and reburning biomass were fed by home-made screw feeders, and they were calibrated before experiments. Corresponding to varied reburning biomass fraction of 0.0, 0.10, 0.15, 0.20, 0.25, 0.30, the coal feeding rate was 1.46, 1.31, 1.23, 1.16, 1.09, and 1.02 g·s-1. The biomass feeding rate observed energy balance, and the air flow rate observed chemical reaction. The excess air ratio in primary zone was 1.05. The primary air ratio was 0.2, and the air ratio of the inner and outer secondary air was 1:3, correspondingly, the swirling intensities were 0.95 and 1.08, respectively. In the burnout zone, the excess air ratio was set as 1.1 to provide enough oxygen to oxidize the unburned compounds. The reburning biomass was injected into the furnace through nozzles at the heights of X2, X3, X4, and X5, which correspond to a residence time in the reburning zone of 0.63, 1.25, 1.88, and 2.5 s, respectively. The temperature at the furnace height of X4 was defined as reburning temperature, and the maximum temperature in primary zone was designed as primary burning temperature. Being convenient to experimental analysis and industrial application, the temperature testing points are converted into dimensionless number, Y, according to Eq.1. Where L is the height of temperature testing point, and D is the burner diameter. Here, we adopt the inner diameter of the outer secondary air duct. Y=

L D

Eq.1

The NOx reduction efficiency, η, is defined according to Eq.2. [NO]zr denotes the NOx concentration in tail flue with biomass reburning, and [NO]jz denotes the NOx concentration without biomass reburning.

η =(1 −

[NO]zr ) × 100% [NO] jz

Eq.2

2.2 Fuels

5


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Table 1 Approximate and ultimate analyses of the fuelsa A typical coal (Shenmu bituminous) and three common biomasses (sawdust, corn straw, and cotton stalk) in China were selected and used in experiments. The approximate and ultimate analyses are listed in Table 1. Compared to coal, biomass present high volatile and oxygen content, whereas low carbon, fixed carbon, sulfur, and nitrogen content. The high volatile and low nitrogen, especially the former, imporve the NOx reductions in reburning stage

3, 25, 26

. Meanwhile, Seen from the Van

Krevelen diagram shown in Fig.2, not only between biomass and coal but also the biomass between each other present distinct properties. The sawdust (as a representation of woody biomass) shows the farthest distance away from the zero point in the Van Krevelen diagram, followed by herb biomass (cotton stalk and corn straw), and the coal shows the closest distance. As discussed below, the distinct fuel properties of biomass provide selecting guideline for reburning biomass. Fig.2 Van Krevelen diagram of the experimental fuels

3 Results and discussions 3.1 Effect of reaction temperature in reburning zone Tr Fig.3 Effect of reaction temperature in reburning zone on NOx reductions Fig.3 shows the effect of reaction temperature in reburning zone (varied from 800 °C to 1200 °C) on NOx reductions. With increased reaction temperature, NOx reduction efficiency increased. That can be explained from two aspects: 1) Increased reaction temperature in reburning zone accelerates the quick release of O, OH, CHi, HCN, and NHi radicals, resulting in high local concentrations and consequently high NOx reductions by homogeneous reactions Rs 1-3

10

; 2)

Meanwhile, the heterogeneous reduction of NOx by biomass char is enhanced. The intensive release of the volatiles from the biomass particle when the reaction temperature is increased causes the formation of a pore structure with a large surface area and more reactive sites within the char particle, and consequently enhances the heterogeneous reduction of NOx 27. CH i +NO  → HCN+ ⋅⋅⋅

R1

6


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HCN+O,OH  → N 2 + ⋅⋅⋅

R2

NH i +NO  → N 2 + ⋅⋅⋅

R3

However, when the reaction temperature was increased to 1200 °C, the NOx generation increased significantly, and the NOx reduction efficiency of sawdust and corn straw decreased, whereas the NOx reduction efficiency of cotton stalk continued to increase. When the reaction temperature is high enough, although the NOx reductions by both homogenous and heterogeneous reactions are improved, thermal-NOx originated from the reburning biomass generates9. Thus, the final NOx emissions depend on NOx reduction amount and externally generated thermal-NOx. Too high or too low temperature is disadvantage to NOx reductions, and there should be an optimal reaction temperature in the reburning zone which results in the maximum NOx reductions. In experiment conditions, it seems that 1100 °C is an optimal selection. Fig.4 Effects of the reburning temperature on furnace temperature distribution Meanwhile, the sawdust, as a representation of woody biomass, presented the highest NOx reduction efficiency, followed by cotton stalk and corn straw in turn. That is mainly the high volatile content of the sawdust. The higher volatile matters can produce more NHi and HCN radicals, enhancing homogeneous NOx reductions through reactions Rs 1-3 and resulting in higher NOx reduction efficiency 10. In addition, the effect of reburning on temperature distribution in primary zone is non-ignorable. Seen from Fig.4 which shows the effect of reburning temperature on furnace temperature distribution, increased reburning temperature resulted in high furnace temperature, and with a reburning temperature of 1200 °C the primary temperature in furnace was over 1300 °C and caused thermal-NOx formation. As a result, the NOx reduction efficiency decreased. Meanwhile, it can be seen that the furnace temperature with sawdust reburning was lower than that with herb biomass (corn straw and cotton stalk), lower furnace temperature causes less thermal-NOx formation, and thus higher NOx reduction efficiency. Thus, the high volatile content and lower temperature in primary burning zone of woody biomass than herb biomass result in higher NOx reduction efficiency, and the highest NOx reductions of sawdust, followed by cotton stalk and corn straw in turn. 7


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In addition, we could find that the fuel far from the zero point in the Van Krevelen diagram (as shown in Fig.2) presents high NOx reduction efficiency during pulverized coal combustion as reburning material. Thus, sawdust shows the highest NOx reduction efficiency, followed by cotton stalk and corn straw in turn. Meanwhile, probably attributed to this reason, biomass shows obviously higher NOx reduction efficiency than pulverized coal reburning 12. 3.2 Effects of reburning fuel fraction Frf Fig.5 Effects of reburning biomass fraction on NOx reductions Fig.5 shows the effects of reburning biomass fraction on NOx reduction at reburning temperature of 1100 °C. The NOx removal efficiency increased with an increasing of the reburning biomass fraction, especially when the reburning biomass fraction is above 0.15, the NOx reduction efficiency increased rapidly. The increase of reburning biomass fraction results in increased volatiles containing CHi, CO, H2 radicals, etc, which improves NOx reduction by homogeneous reactions. Meanwhile, increased biomass char concentration provides more available surface area for heterogeneous reduction of NOx 32. Similarly, because of the high volatile content and lower primary burning temperature during woody biomass reburning, the woody biomass (sawdust) showed higher NOx reduction efficiency compared to herb biomass (corn straw and cotton stalk). With a reburning fraction of 0.3, the NOx reduction efficiency by sawdust reburning reached at 52%. In addition, it can be seen that in comparisons of stoichiometric ratio of 0.8, the NOx reduction efficiency is higher with a stoichiometric ratio of 0.7. The increase of stoichiometric ratio decreased the NOx reduction efficiency becasue of the oxidation of the reducing radicals at certain content.

3.3 Effects of stoichiometric ratio Fig.6 Effects of stoichiometric ratio on NOx reductions Fig.6 shows the effects of stoichiometric ratio in reburning zone on NOx reductions. It can be seen that the NOx reduction efficiency decreased with an increase in the stoichiometric ratio.

8


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Well-known, the reducing radicals, taking CHi as an example, can reduce NOx into N2 in reducing atmosphere by reactions Rs 4-8; whereas in oxidizing atmosphere, they are oxidized by reaction R9. Thus, over high oxygen or stoichiometric ratio is disadvantages to NOx reduction, and the reducing atmosphere in the reburning zone is essential for NOx reduction. CH i + NO → HCN + ...

R4

HCN + O → HCO + H

R5

HCO + H → NH + CO

R6

NH + H → N + H 2

R7

N + NO → N 2 + O

R8

CH i + O → CO + H + ...

R9

On the other hand, when the stoichiometric ratio is low (for example sawdust and cotton stalk, the NOx reduction efficiency began to decrease with decreased stoichiometric ratio between 0.7-0.6), the reducing radicals can rapidly react with NOx originated from the primary zone into HCN according to R4; whereas due to the lack of oxygen, reaction R5 is slow, and subsequently the HCN is oxidized into NOx once entering into the burnout zone with excess oxygen, consequently, resulting in high NOx emissions. Thus, the stoichiometric ratio in the reburning zone cannot be too low, and a reducing atmosphere with certain of oxygen is important for high NOx reductions. Similarly, compared to herb biomass (corn straw and cotton stalk), the woody biomass (sawdust) showed higher NOx reduction efficiency attributed to high volatiles and less thermal-NOx generation influenced by furnace temperature. Meanwhile, cotton stalk shows higher NOx reduction efficiency than corn straw. The fuel being far from the zero point in Van Krevelen diagram should be responsible, while it needs further study. 3.4 Effects of residence time tr Fig.7 Effects of residence time in reburning zone on NOx reductions

9


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Fig.7 shows the effects of the residence time in reburning zone (varied from 0.63 to 2.5 sÅon NOx reductions. The NOx reductions presented a trend of first increasing and then decreasing with an increase of residence time in reburning zone. The longer the residence in the reburing zone is (that means a short residence time of the pulverized coal in primary zone with a constant burning zone or furnace height), the release of the reducing radicals is more complete, and consequently the NOx reduction reactions are more thoroughly because of the high reducing radicals and long reaction time. However, when the residence time in reburning zone is too long, such as above 1.88 s in the present experiment conditions, the residual oxygen due to short residence time and subsequent incomplete burnout of pulverized coal in primary zone oxidizes reducing radicals from reburning biomass, resulting in decreased NOx reduction efficiency. In addition, as a negative compensation on high NOx reduction efficiency, the burnout of char is not ideal. Wholly, the unburned carbon content in fly ash decreased with increased reburning temperature, stoichiometric ratio, and residence time, as well as decreased reburning biomass fraction. Among those, the stoichiometric ratio showed the maximum effect, and the unburned carbon content increased from 10.33 wt.% to 55.01 wt.% (corresponding to the stoichiometric ratio decreased from 1.0 to 0.6).

4 Conclusions On basis of the experimental study of NOx reductions during pulverized coal combustion by different biomass (sawdust, corn stalk, and cotton straw) reburning with varied reaction temperatures, reburning fuel fraction, stoichiometric ratio, and residence time in reburning zone, it is concluded that: 1) Being different from electric heating furnace, in self-heating furnace (or utility boiler) high biomass reburning temperature in reburning zone raised the temperature in primary zone and thus affected NOx emissions and reductions. An increasing of temperature in reburning zone fasted the NOx reduction rate, whereas when the temperature was over 1100 °C, thermal-NOx from reburning biomass formed and even resulted in decreased NOx reduction efficiency. 2) With increased reburning biomass fraction, the NOx reduction efficiency increased due to

10


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concentrated volatiles and char particles that improved the homogeneous and heterogeneous NOx reductions together. 3) Certain amount of oxygen or stoichiometric ratio in reburning zone is essential. Over high stoichiometric ratio oxidized the reducing radicals and resulted in low NOx reduction efficiency. Also, over low stoichiometric ratio inhibited the further conversion of intermedias and barriers NOx reductions. In experiment conditions, the optimal stoichiometric ratio was 0.6-0.7. 4) An adequate and not too long residence time is important for high NOx reduction efficiency. The NOx reduction efficiency first increased and then dropped down with increased residence time in reburning zone. 5) Biomass types show significant effects on NOx reduction. High volatiles in biomass were advantages for NOx reductions. Thus, woody biomass showed high NOx reduction efficiency compared to herb biomass. Meanwhile, the fuel properties distribution in Van Krevelen diagram should be considered and further studied.

Acknowledgements The present work was supported by National Natural Science Foundation of China (No.51406149) and the National Key Research and Development Program of China (No.2016YFC0801904).

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Energy Resources 2007, 25, (5), 1-5. 16. Li, L.; Yu, C.; Huang, F.; Bai, J.; Fang, M.; Luo, Z., Study on the Deposits Derived from a Biomass Circulating Fluidized-Bed Boiler. Energy & Fuels 2012, 26, (9), 6008-6014. 17. Niu, Y. Q.; Tan, H. Z.; Hui, S. E., Ash-related issues during biomass combustion: Alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Progress in Energy and Combustion Science 2016, 52, 1-61. 18. Niu, Y. Q.; Zhu, Y. M.; Tan, H. Z.; Hui, S.; Jing, Z.; Xu, W. G., Investigations on biomass slagging in utility boiler: Criterion numbers and slagging growth mechanisms. Fuel Processing Technology 2014, 128, 499-508. 19. Niu, Y. Q.; Du, W. Z.; Tan, H. Z.; Xu, W. G.; Liu, Y. Y.; Xiong, Y. Y.; Hui, S., Further study on biomass ash characteristics at elevated ashing temperatures: The evolution of K, Cl, S and the ash fusion characteristics. Bioresource Technology 2013, 129, 642-645. 20. Niu, Y.; Tan, H.; Wang, X.; Liu, Z.; Liu, H.; Liu, Y.; Xu, T., Study on fusion characteristics of biomass ash. Bioresource Technology 2010, 101, (23), 9373-9381. 21. Tatano, F.; Barbadoro, L.; Mangani, G.; Pretelli, S.; Tombari, L.; Mangani, F., Furniture wood wastes: Experimental property characterisation and burning tests. Waste Management 2009, 29, (10), 2656-2665. 22. Zhao, K.; Cui, D. W.; Xu, T. M.; Zhou, Q. L.; Hui, S.; Hu, H. L., Effects of hydrogen addition on methane combustion. Fuel Processing Technology 2008, 89, (11), 1142-1147. 23. Glarborg, P.; Jensen, A. D.; Johnsson, J. E., Fuel nitrogen conversion in solid fuel fired systems. Progress in Energy and Combustion Science 2003, 29, (2), 89-113. 24. Liu, C.; Hui, S.; Pan, S.; Zou, H.; Zhang, G.; Wang, D., Experimental Investigation on NOx Reduction Potential of Gas-Fired Coal Preheating Technology. Energy & Fuels 2014, 28, (9), 6089-6097. 25. Zhao, Z. S.; Matsuda, H.; Arai, N.; Hasatani, M., EFFECT OF COEXISTING CH4 ON GAS-PHASE FORMATION OF NO, N2O AND HCN THROUGH NH3. Kagaku Kogaku Ronbunshu 1993, 19, (2), 169-174. 13


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26. Zheng, A. Q.; Zhao, Z. L.; Chang, S.; Huang, Z.; He, F.; Li, H. B., Effect of Torrefaction Temperature on Product Distribution from Two-Staged Pyrolysis of Biomass. Energy & Fuels 2012, 26, (5), 2968-2974. 27. Shu, Y.; Wang, H. C.; Zhu, J. W.; Tian, G.; Huang, J. Y.; Zhang, F., An experimental study of heterogeneous NO reduction by biomass reburning. Fuel Processing Technology 2015, 132, 111-117. 28. Casaca, C.; Costa, M., NOx control through reburning using biomass in a laboratory furnace: Effect of particle size. Proceedings of the Combustion Institute 2009, 32, 2641-2648. 29. Shengli, N.; Chunmei, L.; Pan, G.; Kuihua, H.; Geng, P.; Zhongjie, C., NOx reduction using biomass as reburning fuel. Journal of Fuel chemistry and technology 2008, 36, (5), 583-587. 30. Lu, P.; Hao, J. T.; Yu, W.; Zhu, X. M.; Dai, X., Effects of water vapor and Na/K additives on NO reduction through advanced biomass reburning. Fuel 2016, 170, 60-66. 31. Haiyang, Y.; Yang, S.; Hai, Z.; Junfu, L., Discussion on mechanism of nitric oxide reduction by biomass reburning. Power system engineering 2008, 24, (1), 1-4. 32. Zhong, L. Experiments and study on chemistry kinetic mechanism of NO reduction by pulverized coal and char reburning. North China electric power university, Hebei, 2003.

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Energy & Fuels

Table Table 1 Approximate and ultimate analyses of the fuelsa ultimate analysis / wt.%

Approximate analysis / wt.%

Heat value

Fuel Cad

Had

Nad

Oad

St,ad

Mad

Aad

Vad

FCad

MJ·kg-1

Coal

82.9

3.38

1.2

12.3

0.22

5.42

6.34

29.54

58.7

27.54

Sawdust

50.55

3.66

0.35

45.43

0.01

8.94

2.02

74.24

14.8

17.55

Corn straw

62.51

3.44

0.45

33.55

0.05

11.19

4.26

69.26

15.29

12.43

Cotton stalk

56.94

4.27

0.52

38.23

0.04

9.58

3.45

72.31

14.66

13.36

a

ad: air dry basis.

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Figures

a. Drop tube furnace system

b. Burner Fig.1 Schematics of the drop tube furnace experiment system

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Energy & Fuels

Fig.2 Van Krevelen diagram of the experimental fuels

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Fig.3 Effect of reaction temperature in reburning zone on NOx reductions

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Energy & Fuels

Fig.4 Effects of the reburning temperature on furnace temperature distribution

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Fig.5 Effects of reburning biomass fraction on NOx reductions

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Energy & Fuels

Fig.6 Effects of stoichiometric ratio on NOx reductions

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Fig.7 Effects of residence time in reburning zone on NOx reductions

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Influence of Biomass Reburning on NO x Reductions during

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