NO Emissions from Oxidizer-Staged Combustion of Superfine

Jul 25, 2014 - combustion shows a vast number of advantages to be explored commercially in the near future. However, unexpected problems, such as bad ...
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NO Emissions from Oxidizer-Staged Combustion of Superfine Pulverized Coal in the O2/CO2 Atmosphere Jiaxun Liu, Xiumin Jiang,* Jun Shen, and Hai Zhang School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: The CO2 control technologies have been studied extensively in recent years, among which the oxy-fuel combustion shows a vast number of advantages to be explored commercially in the near future. However, unexpected problems, such as bad combustion characteristics and serious slagging and depositing issues, show up with the replacement of N2 by CO2. These inherent disadvantages in normal O2/CO2 combustion can be restrained via combining the superfine pulverized coal and oxy-fuel combustion technology. The axial NO emission characteristics of this new technology were focused here. The effects of the oxidizer staging were also studied in detail. Results indicate that the axial NO emissions of the unstaged O2/CO2 combustion basically showed “M” type of distributions along the furnace. The “M” type can be divided into the main homogeneous and heterogeneous reaction zones. The oxidizer-staged O2/CO2 combustion can mitigate NO emissions effectively. Coals with smaller particle sizes and higher volatiles are more advantageous for eliminating NO in the staged O2/CO2 combustion technology. The superfine pulverized coal used with certain low NO combustion technologies shows significant superiority in both combustion performance and NO abatement.

1. INTRODUCTION The ecological environment has drawn increasing concerns with the development of modern society. However, the greenhouse effect, caused by emissions of carbon dioxide from fossil fuel combustion, especially coal combustion, has attracted more and more attention. The carbon capture and sequestration (CCS) technology is regarded as a promising route to achieve a meaningful reduction in CO2 emissions during fossil fuel combustion in the near term.1 There are numerous technological options that are compatible with CCS activity. Among them, the current commercial or nearcommercial technologies generally include pre-combustion, post-combustion, oxy-enriched combustion, and chemical looping combustion (CLC) technologies.2,3 The CLC is still at the initial stage of exploration, which is not yet suitable for the commercial deployment worldwide. All three other technologies have been developed through planned demonstrations. However, oxy-fuel combustion is considered to be a more attractive method with lower CO2 capture costs, which has few technological barriers to be implemented to the existing power plants. The oxy-fuel technology has numerous advantages, such as producing an almost pure CO2 stream (>90%) that is easy to capture. It can also achieve lower NOx and SOx emissions and higher combustion efficiency compared to conventional air firing. Additionally, partly recycled hot flue gas (RFG) can lower the temperature and oxygen availability of the flame region, which can promote the NOx abatement. Moreover, recycled NOx can be further destroyed through reduction reactions because of the longer residence time.4 Thus far, several pilot-scale studies have been put into practice to evaluate the oxy-fuel combustion technology. The Ishikawajima-Harima Heavy Industries (IHI) in Japan has performed a number of works in demonstrations. Kimura et al.5 tested the © 2014 American Chemical Society

coal combustion characteristics in O2/CO2 mixtures using a 1.2 MW tunnel furnace. Nozaki et al.6 analyzed the effectiveness of direct oxygen injection through numerical simulations and tests. The results confirmed that the NOx emissions in conventional air combustion were higher than those of O2/CO2 combustion. In addition, the International Flame Research Foundation (IFRF, Ijmuiden, Netherlands)7 performed the oxy-fuel combustion in an existing 2.5 MW boiler and compared the furnace performance to the air combustion technique. Also, the Institute of Combustion and Power Plant Technology (IFK) has performed some interesting works about the corrosion behaviors of alloys under oxy-fuel combustion in a 3 MW test facility.8 Furthermore, the most near-commercial oxy-fuel project was operated at Schwarze Pumpe. Vattenfall Europe built a 30 MW oxy-fuel pilot facility at Schwarze Pumpe Power Station, and the first tonne of CO2 was purified and separated in September 2008.9 Despite these superiorities compared to other CO2 capture technologies, many problems remain unsolved. The replacement of N2 by CO2 in the reacting atmosphere may result in worse combustion performance, such as the decrease of flame propagation velocities, reduction of flame stabilities, retarded coal particle ignition points, lower temperatures, and unstable flames.10,11 Additionally, the corrosion phenomena and fouling issues become more serious in O2/CO2 combustion, because the fusion points of coal ashes become lower in the reductive atmosphere. The depositing of coal ashes on heating transfer surfaces will cause several problems. The importance of particle size effects on combustion are addressed with the proposal of superfine pulverized coal Received: May 2, 2014 Revised: July 23, 2014 Published: July 25, 2014 5497

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technology. Our previous research12,13 shows that the combustion performance of superfine pulverized coal is significantly improved. Better flame stability, pollutant emissions, coal ignition, char burnout characteristics, etc. are also achieved. Moreover, the fusion temperatures of superfine coal ashes increase evidently, which plays a significant role in the alleviation of ash and slag buildup in boilers. As a result, the oxy-fuel combustion of superfine pulverized coal technology can solve certain inherent disadvantages of conventional O2/ CO2 combustion, such as the low combustion efficiency, high energy loss, and slagging issues. On the other hand, it is proven that the oxy-fuel combustion technique can realize additional abatement of NOx compared to conventional air combustion.6,14,15 The NO evolution characteristics during superfine pulverized coal combustion in the O2/ CO2 atmosphere has been investigated in our previous work.16 The results indicated that, under conventional combustion conditions, superfine pulverized coal was even adverse for alleviating NO emissions. However, once combined with certain de-NOx methods, e.g., staging combustion, superfine pulverized coal will show significant superiority in both combustion properties and NO elimination. Therefore, we mainly focus on the oxidizer-staged combustion of superfine pulverized coal in O2/CO2 mixtures in this paper. The axial NOx emission characteristics will also be studied, and more detailed de-NOx mechanisms will be discussed. It has been widely accepted that the air-staged combustion technology can mitigate NOx emissions effectively in practical facilities. Different stoichiometric ratios are applied to hinder the NOx formation in two separate burning zones. It is reasonable to apply this staging method to the O2/CO2 combustion technology, which may further reduce NOx from the system. A primary oxygen-lean combustion zone is formed by limiting the initial supply of the oxidizer. The additional oxidizer known as the overfire oxidizer is sprayed into the burnout zone to ensure the complete combustion.17 Hu et al.18 studied the NOx emissions during coal combustion and claimed that the staging and reburning combustion methods could be introduced into the O2/recycled flue gas combustion. Liu et al.19 experimentally investigated pulverized coal combustion in different atmospheres. The results confirmed that oxidantstaging combustion in the O2/CO2 mixtures was as effective as air staging in reducing NOx. However, Kiga et al.20 studied the oxidizer staging in a burner and found that the NOx abatement was not so effective as the air-staging combustion. Recently, the staged O2/CO2 combustion technique had been studied by Watanabe and co-workers, and the NOx evolution mechanisms were discussed.21 The results reflected that the conversion ratio of NOx in O2/CO2 combustion was lower by 40% than air firing, when certain staging combustion was adopted. They concluded that the staged combustion in the O 2 /CO 2 atmosphere could fulfill significant reductions in NO x emissions. However, it is important to note that all of the conclusions are achieved using a flat CH4 flame, where NH3 is treated as fuel N. The actual situations of coal combustion are not involved in their work. To summarize, few studies report superfine pulverized coal combustion in the O2/CO2 atmosphere, not to mention oxidizer-staged combustion. In this work, the oxidizer-staged combustion of superfine pulverized coal in the O2/CO2 atmosphere was investigated in detail. The influences of coal particle sizes, stoichiometric ratios in the primary zone, burnout port positions, etc. were discussed. The present study mainly

focused on the knowledge of axial NOx emissions and the oxidizer-staged combustion of superfine pulverized coal in the O2/CO2 atmosphere, especially the effects of coal particle sizes. The flue gas recirculation in oxy-fuel combustion will not be discussed here.

2. MATERIALS AND METHODS 2.1. Preparation of Raw Superfine Pulverized Coal Samples. Two different kinds of coal from Shenhua (SH) and Neimongol (NMG) basins in China were selected here. Eight different mean particle sizes were obtained using a superfine pulverized jet mill (China). The equivalent mean particle sizes of SH samples are 14.7, 17.4, 21.3, and 44.2 μm, while those of NMG samples are 12.5, 14.9, 25.8, and 52.7 μm. The properties of the coal samples were discussed in the previous work12 and listed in Table S1 of the Supporting Information for a reference. They both belong to bituminous coals. SH coal has a higher extent of coalification than NMG coal, while the volatile content of NMG coal is much larger. 2.2. Experimental Apparatus. The experiments were performed in a one-dimensional drop-tube furnace system. A detailed description can be found elsewhere.16 The furnace was heated electrically with about 12 kW heating power. A cylindrical quartz glass tube was adopted as the reaction chamber (length of 2000 mm and inner diameter of 60 mm). A movable thermocouple was applied to measure the axial temperature distributions of the furnace, which was set to 1100 °C. The temperature profiles in the main reaction zone were basically stable, as shown in Figure 1.22 The staged combustion

Figure 1. Temperature profile along the furnace axis.22 experiments were performed using the multipath inlets that were mounted along the reaction tube. The configuration of the staged injectors was ignored because they had little influence on the NOx evolution.23 Four branch pipes were arranged along the axial positions of the chamber. The positions of these inlets were evenly distributed (250, 500, 750, and 1000 mm) between the injecting ports and the roof. There were seven other branch pipes on the other side of the chamber that acted as measuring points along the furnace axis. NOx emissions along the furnace in O2/CO2 combustion of superfine pulverized coal were studied through these points. A regulated stream of 0−1 g/min coal samples was entrained downward into the reaction tube by a simulated gas flow, which was mixed with high-purity O2 (99.999%) and CO2 (99.999%). The compositions of outlet gases, such as CO2, CO, and NO, were measured by an online Fourier transform infrared spectrometer (GASMET DX-4000, Finland).

3. RESULTS AND DISCUSSION 3.1. Centerline Axial NO Emissions of Unstaged O2/ CO2 Combustion of Superfine Pulverized Coal. The nitrogen transformation during O2/CO2 combustion of superfine pulverized coal can be better understood through the investigations of axial NO emission characteristics along the furnace. Homogeneous and heterogeneous reduction mechanisms are further analyzed in detail, which can provide a reference for refining the oxy-fuel combustion theory and use. 5498

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The NO concentration was calculated on a 6% O2 dry basis throughout the paper. The conversion ratio (CR) was introduced to evaluate the conversion degree of fuel N to NO, expressed as [N]NO/fuel N, where [N]NO was the total amount of nitrogen of NO in the exhaust gases. 3.1.1. Influence of the Temperature. The influence of the temperature on the centerline axial NO evolution in the O2/ CO2 atmosphere (30% O2/70% CO2) is illustrated in Figure 2.

Figure 3. Influence of the atmosphere on the centerline axial CO emissions (air and O2/CO2 combustion).

effects of CO2. The CO emissions reach the maximum at around 350 mm. Partial released NO are reduced by CO through homogeneous reactions. The existence of CO can also promote the reducing effects of coal chars. Thus, certain amounts of NO are removed on char surfaces under the catalysis effects. Therefore, a small valley of NO emission appears during this stage around 650 mm. With the further consumption of CO, NO emissions increase to the maximum around 950 mm. To verify this hypothesis of the gasification effects in O2/ CO2 combustion, an air combustion experiment was carried out under the conditions of 21% O2/79% N2, λT = 1.2, and 800 °C. The axial distribution of NO along the furnace is shown in Figure S1 of the Supporting Information. It can be observed that the centerline axial NO emissions in air combustion also show “M” type of distributions along the furnace. However, there is a monotonically decreasing trend to the minimum point of 1250 mm after NO emissions reach the maximum in the main homogeneous reaction region. No small valley around 650 mm appears because there is no apparent process of reduction introduced by CO in air combustion. The axial distribution of CO along the furnace in air combustion is also shown in Figure 3. The CO evolution is quite different from that of O2/CO2 combustion. The yields of CO reach the maximum promptly once the coal particles enter the furnace and then decrease monotonically afterward. There is no increasing trend of CO induced by gasification reactions that occur in O2/CO2 combustion. Thus, the process of reduction introduced by CO does not exist in air combustion. Therefore, the effects of gasification processes in O2/CO2 combustion cannot be neglected. More experimental data will be needed to support this speculation in the future. Figure 2 suggests that the axial NO emissions can be further divided into four stages: the progressively increasing homogeneous stage (region I), the progressively decreasing homogeneous stage (region II), the progressively increasing heterogeneous stage (region III), and the progressively decreasing heterogeneous stage (region IV). It is ascertained that the increasing homogeneous stage is mainly caused by the oxidizing of the released volatile N, while the decreasing homogeneous stage is the result of the reducing effects of gaseous matter. It is reasonable to infer that the increasing heterogeneous stage is mostly attributed to the combustion of char N. The decreasing

Figure 2. Influence of the temperature on the centerline axial NO emissions in O2/CO2 combustion.

The NMG_12.5 pulverized coal samples were chosen, and the stoichiometric ratios (SRs) were kept steadily at λT = 1.2. The temperatures were raised to 800, 900, and 1000 °C. The error bars were added in the figures to represent the variations of repeated tests. The centerline axial NO emissions in the O 2 /CO 2 atmosphere basically present “M” type of distributions along the furnace. Under fuel-lean combustion conditions, once the coal samples enter the chamber, the volatile nitrogen is released and then reacts with the ambient oxygen immediately. Therefore, NO emissions reach the maximum rapidly in the initial devolatilization processes. The quantities of NO increase with the temperatures in region I, because more volatile N is released at higher temperatures. Thus, there are more precursors that react with oxygen to form NO. With the large consumptions of ambient oxygen, a reduced atmosphere appears in certain parts of the reactor. Some of the released NO is reduced, resulting in the drop of NO emissions in region II. The NO emissions reach the minimum around 1250 mm from the burner (the entrance). These two regions belong to the initial pyrolysis stage of combustion, and large amounts of gaseous components are released as volatile matter. Although parallel heterogeneous reactions may also exist, the homogeneous reactions dominate here, including the homogeneous oxidizing region (region I) and the homogeneous reducing region (region II). Therefore, this area is defined as the main homogeneous reaction zone. Furthermore, it can be observed that there is a small valley around 650 mm in this region, which may be caused by the reducing effects of CO released during O2/CO2 combustion. The centerline axial CO emissions in O2/ CO2 mixtures are shown in Figure 3. There is an increasing trend of CO yields after the coal particles enter the furnace. Some are eliminated through the reactions with oxygen. However, more CO is released because of the gasification 5499

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concentrated. Then, there are more opportunities to react with oxygen, emitting more NO. In region III, NO emissions of NMG_14.9 increase rapidly and exceed those of NMG_12.5. This phenomenon is also due to the higher nitrogen content in NMG_14.9 parent coal, and more char N is converted into NO. In region IV, there is a significant decreasing trend for NMG_12.5 in the range of our experiments, while no apparent declining trend appears for NMG_14.9. The NMG_12.5 coal samples with smaller particle sizes have more prominent heterogeneous reduction effects. The pore network structures in coal play a key role here. With decreasing the particle sizes, the heat- and mass-transfer resistance decreases, which is due to the smoother pore surfaces and simpler pore structures.16 Thus, the transportation of reactants becomes easier. Additionally, the specific surface area increases significantly for smaller coal particles, and more active sites are available. Hence, the char/ NO reduction reactions are enhanced. As a result, heterogeneous reduction reaction prevails for smaller coal particles, which promotes the NO abatement. The influences of stoichiometric ratio and oxygen concentration on centerline axial NO emissions in the O2/CO2 atmosphere are reflected in Figures S2 and S3 of the Supporting Information. With increasing stoichiometric ratios, axial NO increases initially before λT = 1.2 and decreases after λT = 1.4. The reason is that char N is involved earlier with the increase of stoichiometric ratios. The main homogeneous reaction zones are shortened under fuel-lean combustion conditions. Similarly, the centerline axial NO emissions rise before the inlet oxygen concentration reaches 30%, and then there is a slight decline afterward. The detailed discussions can be found in the Supporting Information. 3.2. Staged O2/CO2 Combustion of Superfine Pulverized Coal. Air-staging combustion is one of the most sophisticated low-NOx combustion technologies, which has been widely adopted around the world. The furnace is divided into two zones to inhibit NOx formation. In the primary combustion zone, only a partial oxidizer is sprayed for delivering pulverized coal, forming an initial fuel-rich combustion zone.26 The primary oxidizer rate is calculated through the ratio of the main oxidizer to the total oxidizer, f M (M stands for main). The additional oxidizer known as the overfire oxidizer (staged oxidizer) is added in the burnout zone (secondary combustion zone) to ensure the complete combustion. The staged-oxidizer rate is recognized as the ratio of the burnout oxidizer to the total oxidizer, f S (S denotes staged). The total stoichiometric ratio is expressed as λT (T stands for total). Hence, the stoichiometric ratios of the main combustion zone and the staged combustion zone are λM = λT f M and λS = λT f S, separately, where f M + f S = 1.23 Therefore, if the staged combustion technology can be introduced into the O2/CO2 combustion, better NOx emission characteristics could be expected on this oxidizer-staged O2/CO2 combustion technology. As analyzed before, the application of superfine pulverized coal to certain low NOx combustion technologies (e.g., staged combustion) will show remarkable elevation in both combustion performance and NO elimination. The combination of staged combustion, O2/CO2 combustion, and superfine pulverized coal combustion technology can make full use of their respective advantages and solve the inherent disadvantages of each technology. The oxidant-staged O2/CO2 combustion technology has broad prospects of controlling CO2 and NOx emissions in the future.

heterogeneous stage is largely caused by the running out of the coal chars and the heterogeneous reduction effects. Moreover, it is interesting to notice that the influence of temperatures on separate stages is quite different. In region I, NO emissions increase with the temperatures going up. The higher the temperature, the more volatile N released, and thus, more precursors react with oxygen to form NO. In region II, when the temperature rises, the reducing rates of NO increase. The NO emissions at the end of the region are higher at 800 °C than those at 1000 °C. It is concluded that the higher the temperature, more effective the homogeneous reduction will be. This is because the release of volatile matter is more concentrated and faster with the increase of the temperature. There will be a longer residence time for NO to be reduced effectively, which makes the reducing rates of NO emissions increase. Moreover, higher temperatures can help to improve the reaction rates and promote the homogeneous reducing reactions. In region III, the increasing rates of NO emissions accelerate with the temperature. Higher temperatures promote the char N conversion ratios, and thus, more chars are burnt to release NO. In region IV, with the increase of the temperature, the declining rates of NO emissions decrease, which suggests that higher temperatures may inhibit the heterogeneous reducing reactions in certain circumstances. The promoting effects of chars on the heterogeneous de-NOx reactions become less significant when the ambient temperature is high.24 Similar conclusions were drawn by Teng,25 who discovered that a weak inverse relationship existed between the temperature and the ability of chars to react with NO. 3.1.2. Influence of the Particle Size. The effect of particle sizes on the centerline axial NO evolution in the O2/CO2 atmosphere (30% O2/70% CO2) is reflected in Figure 4. The NMG_12.5 and NMG_14.9 pulverized coal samples were chosen, and the SRs were kept steady at λT = 1.2. The temperature was raised to 1000 °C.

Figure 4. Influence of the particle size on the centerline axial NO emissions along the furnace.

In region I, the NO emissions of NMG_14.9 are higher than those of NMG_12.5, because of the higher nitrogen content in NMG_14.9 parent coal. Thus, the combustion of volatile N releases more NO. In region II, exhausted NO of NMG_14.9 is much lower than that of NMG_12.5, whose volatile content is higher. Under fuel-lean combustion conditions, volatile nitrogen in smaller coal particles releases faster and is more 5500

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3.2.2. Influence of the Burnout Port Position. The influence of the burnout port position on the NO formation in the O2/ CO2 atmosphere (30% O2/70% CO2) is shown in Figure 6.

3.2.1. Influence of the Stoichiometric Ratio in the Primary Combustion Zone. Figure 5 reveals the influence of the

Figure 5. Influence of the primary stoichiometric ratio on the NO emissions.

Figure 6. Influence of the burnout port position on the NO emissions.

primary stoichiometric ratio on the NO evolution in O2/CO2 mixtures (30% O2/70% CO2). The NMG_12.5 and SH_14.7 pulverized coal samples were chosen, and the temperature was kept at 1000 °C. The third layer of burnout ports (750 mm) was adopted as the staged oxidizer injectors. The total SRs were maintained steadily as λT = 1.2, and the primary combustion oxidizer rates f M were increased from 60 to 100%. The corresponding primary stoichiometric ratios λM were increased from 0.72 to 1.2. The similar trends are observed for both coal samples that NO emissions decline with the decrease of primary SRs. The initial oxygen-lean combustion condition in the primary zone is crucial to inhibit the NO formation. Additionally, the sub-stoichiometric atmosphere lowers the combustion temperature, which is also helpful for the elimination of NO. The influence of primary stoichiometric ratios on the degree of reduction in NO emissions is shown in the Figure S4 of the Supporting Information. For the SH_14.7 and NMG_12.5 pulverized coal, the largest degree of reduction in NO emissions can reach 58.4 and 69.9%, separately. Coal with higher volatile matter has better effects of reduction on NO evolution, after applying the oxidizer-staged combustion. The release of nitrogen species into the gaseous phase for higher volatile coal can be reduced in a deficiency of the oxidizer, which is similar to the air-staged combustion.27 Furthermore, it can be observed from Figure 5 and Figure S4 of the Supporting Information that a best primary stoichiometric ratio exists for each coal, which makes NO emissions the lowest and the degree of NO reduction the largest. For NMG_12.5 coal, the best primary stoichiometric ratio is around 0.85. Meanwhile, it is smaller than 0.7 for SH_14.7 coal, which does not show up in the experimental ranges. With the decrease of primary stoichiometric ratios, more NO is reduced in the primary zone, while more precursors, such as NH3, HCN, and char N, are formed. More NO will be released in the burnout zone with more oxidizer being added. Additionally, low stoichiometric ratios are harmful for the combustion stability. Therefore, in the practical applications, the specific primary stoichiometric ratios must be confirmed for different coal, particle sizes, and combustion equipment.

The NMG_12.5 and SH_14.7 pulverized coal samples were chosen, and the temperatures were kept at 1000 °C. The total SRs were maintained steadily as λT = 1.2, and the primary stoichiometric ratios λM were kept as 0.84. The burnout port positions from 250 mm (port 1) to 1000 mm (port 4) were selected. With the burnout port positions being lowered, NO decreases for both coal samples. Figure S5 of the Supporting Information shows the influence of burnout port positions on the degree of reduction in NO emissions. The largest degrees of NO reduction can reach 50.1 and 53.3%, separately, both of which are obtained at injecting ports 4. When the burnout port positions are lowered, the residence time of the primary combustion zone is prolonged. Volatile N has more sufficient time to be reduced, and NO emissions decrease. However, with further descent of the burnout ports, more precursors of NO and char N will be burned in the burnout zone and, thus, more NO will be released. Therefore, NO emissions show no continuous decrease with further lowering the burnout ports. Plus, this may also cause more heat loss because of the unburned carbon and gas. The optimum burnout port positions are distinct for different coals, particle sizes, combustion equipment, and combustion atmospheres. Normally, the staged oxidizer is injected when the primary combustion is basically finished. The injecting position is around the trailing edge of the flame profile, which is formed under the unstaged conditions. 3.2.3. Influence of the Coal Particle Size. The influence of the coal particle size on the degree of reduction in NO yields in O2/CO2 mixtures (30% O2/70% CO2) is reflected in Figure 7. The NMG and SH coal particles with different sizes were chosen, and the temperature was kept at 1000 °C. The total SRs were maintained steadily as λT = 1.2, and the primary stoichiometric ratios λM were kept as 0.84. The burnout port positions from 250 mm (port 1) to 1000 mm (port 4) were selected. It was observed that coal with smaller particle sizes reduced NO emissions significantly in the staged O2/CO2 combustion. NO emissions decrease with the burnout port positions descending. The largest degree of NO reduction for all coal samples was obtained at injecting port 4. The degree of NO 5501

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Figure 7. Influence of the particle size on the degree of reduction in NO at different burnout port positions.

reduction increases substantially with decreasing particle sizes, especially for NMG coal. The reason is that staged combustion plays a key role in inhibiting the formation of NOx from volatile N. More intense and concentrated volatile matter is released earlier for smaller coal particles. Large amounts of volatile N react with the reducing gaseous components being eliminated. Additionally, the reducing abilities of chars increase with decreasing the sizes. NMG coal has more volatile content and higher reactivity, which causes the larger degree of NO reduction. The impact of coal particle sizes on the NO evolution is further analyzed for more effective NMG coal in Figure 8.

Figure 9. Influence of the primary stoichiometric ratio on the NO emissions from coals with different particle sizes.

positions were fixed at port 3 (750 mm). The primary stoichiometric ratios λM were changed from 0.72 to 1.2. All of the NO emissions and CRs declined with the decrease of primary stoichiometric ratios. There is a significant dividing line in the figure, where the primary stoichiometric ratio is about 0.9. When the ratio is lower (λM < 0.9), NO emissions decline with the particle sizes decreasing. NO released from smaller pulverized coal is less than that from larger pulverized coal. It is quite different when the primary stoichiometric ratio is higher (λM > 0.9). There is a trend that NO emissions of smaller pulverized coal increase faster than those of larger coal particles, especially for NMG_14.9 coal, whose nitrogen content is higher. It is suggested that the amounts of NO released from homogeneous reactions are positively related to the nitrogen content in parent coal.28 Under fuel-lean combustion conditions (when λM > 0.9), a larger quantity of prompt released volatile N from smaller coal particles is converted into NO, which induces the increase of CRs. Moreover, the flame temperatures increase with the decrease of particle sizes, facilitating the NO formation. On the other hand, under fuel-rich combustion conditions (when λM < 0.9), the volatile nitrogen in smaller pulverized coal is released more concentrated and faster and, thus, has more opportunity and residence time to be reduced. In addition, as analyzed before, the specific surface areas increase significantly for smaller coal particles, and more reaction areas and active sites are provided.

Figure 8. Influence of the burnout port position on the NO emissions from coals with different particle sizes.

When the sizes of coal particles are decreased, NO emissions decline tremendously under the staged combustion conditions. The ability of NO reduction is raised significantly for coal samples with smaller particles, especially at injecting ports 3 and 4. It is concluded that coal with smaller particle sizes and higher volatile content has more potential for NO elimination, through applying the staged O2/CO2 combustion technology. To understand the effects of particle sizes better, Figure 9 reflects the influence of primary stoichiometric ratios on the NO emission characteristics obtained from different coal particles in the O2/CO2 atmosphere (30% O2/70% CO2). The NMG pulverized coal samples with distinct sizes were chosen, and the temperature was kept at 1000 °C. The total SRs were maintained steadily as λT = 1.2, and the burnout port 5502

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Notes

Hence, the char/NO reduction reactions are enhanced. Consequently, for the staged O2/CO2 combustion technology, a relatively higher reduction efficiency of NO can be expected for superfine pulverized coal. Therefore, it can be concluded that, under conventional combustion conditions, superfine pulverized coal was adverse for the abatement of NO. Once combined with certain de-NOx methods (e.g., fuel reburning, air/oxidizer staging, and flue gas recirculation), superfine pulverized coal shows prominent superiority in both combustion properties and NO elimination.

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

Information on detailed descriptions of the coal properties (Table S1), influences of the temperature (Figure S1), stoichiometric ratio (Figure S2), and inlet oxygen concentration (Figure S3) on the centerline axial NO emissions, and influences of the primary stoichiometric ratio (Figure S4) and burnout port position (Figure S5) on the degree of NO reduction. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grants 51306116 and 51376131). The authors are grateful for the help from Prof. Weidong Fan (School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China) with the experimental platform.

4. CONCLUSION (1) The centerline axial NO emissions of unstaged O2/CO2 combustion basically present “M” type of distributions along the furnace. There are main homogeneous and heterogeneous reaction zones, which can be further divided into four stages. The homogeneous reduction effect of NO is more efficient at higher temperatures, while it is adverse to the heterogeneous reduction effect. (2) In regions I and III, coal with a higher nitrogen content has higher NO emissions. In region II, smaller coal particles release more NO under fuel-lean combustion conditions. In region IV, the heterogeneous reduction process prevails in the small coal fractions. (3) With elevating stoichiometric ratios, there is an increase in the NO yields before λT = 1.2 and a decline as SRs exceed around λT = 1.4. Similarly, the centerline axial NO emissions rise initially before inlet oxygen concentrations reach 30% and decrease slightly afterward. (4) For the oxidizer-staged O2/CO2 combustion technology, the amounts of NO decline with the decrease of primary stoichiometric ratios. However, the best primary stoichiometric ratios are not the same for different coals. Coal with higher volatile matter has better reduction effects on NO, after applying the oxidizer-staged combustion. NO emissions decrease with lowering the burnout port positions. There are different optimum burnout port positions for different coals. (5) Coal with smaller particle sizes and higher volatile matter has greater potential for removing NO in the staged O2/CO2 combustion technology. When the primary stoichiometric ratio is lower (λM < 0.9), NO declines with the decrease of coal particle sizes. On the other hand, NO emissions of smaller pulverized coal grow faster than those of larger fractions, once the primary stoichiometric ratio is higher (λM > 0.9). (6) Under conventional combustion conditions, superfine pulverized coal was adverse to alleviating NO emissions. On the other hand, once combined with certain de-NOx methods, superfine pulverized coal shows significant superiority in both combustion properties and NO elimination.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-3420-5681. E-mail: [email protected]. cn. 5503

dx.doi.org/10.1021/ef5009924 | Energy Fuels 2014, 28, 5497−5504

Energy & Fuels

Article

(26) Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F. Prog. Energy Combust. Sci. 2009, 35, 385−397. (27) Spliethoff, H.; Greul, U.; Rüdiger, H.; Hein, R. G. Fuel 1996, 75, 560−564. (28) Pershing, D. W.; Wendt, J. O. L. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 60−67.

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dx.doi.org/10.1021/ef5009924 | Energy Fuels 2014, 28, 5497−5504