Experimental Study on Enhanced Control of NOx

Article pubs.acs.org/EF

Experimental Study on Enhanced Control of NOx Emission from Circulating Fluidized Bed Combustion Tuo Zhou,*,† Zhiqiang Gong,†,‡ Qinggang Lu,† Yongjie Na,† and Yunkai Sun† †

Institute of Engineering Thermalphysics, Chinese Acadamy of Sciences, Beijing 100190, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

‡

ABSTRACT: This paper explores possible measures of enhanced control of the nitrogen oxides (NOx) emission from combustion on a new circulating fluidized bed (CFB) test platform, which is a combined CFB and post-combustion chamber (PCC). Characteristics of Shenmu char combustion and NOx emission were studied on the conditions of the secondary air (SA) arrangement, lean oxygen combustion, and post-combustion, respectively. In comparison to the conventional methods (SA arrangement), post-combustion is a new method to improve control of the NOx emission during the combustion process. Experimental results show that the approach of modifying the SA arrangement is able to remove the NOx emissions by up to 30%, the lean oxygen combustion can reduce NOx emissions significantly but resulting in a declined combustion efficiency, and post-combustion is able to bring the final concentration of NOx emission down to only 51 mg/m3 (at 6% O2), meanwhile maintaining the high combustion efficiency of 98.6%.

1. INTRODUCTION Nitrogen oxides (NOx) from fossil fuel combustion contribute majorly to air pollution, and thermal power plants as the main source of NOx emission are subjected to increasingly strict environmental regulation. According to the latest version of Emission Standard of Air Pollutants for Thermal Power Plants (GB 13223-2011) in China, boilers of all thermal power plants are required to emit NOx no more than the maximum concentration of 100 mg/m3 (at 6% O2) from July 1, 2014. Circulating fluidized bed (CFB) boilers account for a considerable proportion in thermal power plants in China. Although CFB boilers can reduce initial NOx emission effectively by controlling the bed temperature,1,2 increasing the secondary air (SA) ratio,3−8 changing the ports position of the SA,9−12 and decreasing the air stoichiometric ratio,13−15 but these measures still fail to control the initial NOx emission concentration down below 100 mg/m3. Most CFB boilers equip themselves with selective non-catalytic reduction (SNCR) or selective catalytic reduction (SCR)16−18 to meet the emission standards but at the expense of their market competitiveness, which becomes crippled with the use of such technology. How to use the advantages of CFB in improved controlling of NOx emission from combustion is still an important research subject. Low-rank coals account for more than 55% of the coal reserves in China. The grade utilization of low-rank coals is effective coal pyrolysis at a low temperature range (500−700 °C), and pyrolysis gas and tar are used as high-grade raw materials, while the powdered char can be combusted in power plants. CFB boilers can be used to achieve the effective utilization of char.19 A new CFB test platform was established. This paper studies the combustion characteristics of Shenmu char and the emission characteristics of NOx in conditions of SA arrangement, lean oxygen combustion, and post-combustion, respectively, and also explores the possible measures of enhanced © 2015 American Chemical Society

control of the NOx emission from CFB boilers to reduce the concentration from CFB down below 100 mg/m3.

2. EXPERIMENTAL SECTION 2.1. Test Platform. The combustion experiments of Shenmu char were conducted on a CFB test platform (Figure 1), which consists of a

Figure 1. Schematic diagram of the CFB test platform. Received: March 11, 2015 Revised: May 11, 2015 Published: May 19, 2015 3634

DOI: 10.1021/acs.energyfuels.5b00519 Energy Fuels 2015, 29, 3634−3639

Article

Energy & Fuels

In the following discussion, the NO concentration has been normalized to dry flue gas with an oxygen concentration of 6%. Combustion temperatures (represented by the highest temperature in the furnace for each case) for cases 1, 2 and 3 are different at 870, 920, and 940 °C, respectively. The purpose of case 4 was to figure out NOx emission during extreme lean oxygen combustion conditions. The purpose of case 5 was to figure out NOx emission during postcombustion.

CFB, a post-combustion chamber (PCC), and an auxiliary system. The CFB is connected to the PCC through a flue duct of 1400 mm in length and 260 mm in diameter. The fuel power of this CFB test platform is 400−600 kW during normal operation. The main components of the CFB include a furnace (riser), a cyclone, a dual-exit ash distributor, a loop seal, an external heat exchanger (EHE), and two screw feeders. The CFB furnace is adiabatic, having an internal diameter of 430 mm and a height of 9000 mm. Two screw feeders are symmetrically arranged on both sides of the furnace with a height of 800 mm above the air distributor. The primary air (PA) is delivered into the furnace through the air distribution plate located on the bottom of the furnace. The SA is injected into the furnace from different ports, which are 1200, 2000, 4000, and 6000 mm above the air distributor. Each SA port is designed to make the SA enter down into the furnace at an angle of 45°, which can make SA with a long effective injection distance pass through the center of the horizontal plane,10 resulting in a good gas−solid mixing performance. The PCC is also adiabatic, with 5000 mm in height and 480 mm in inner diameter. There is a post-combustion air port mounted on the flue duct between the CFB and the PCC. There are six thermocouples at the furnace, one thermocouple at the cyclone, and four thermocouples at the PCC. There are five sampling points at the PCC and another one at the tail-end flue duct. The dried and filtered sampled gas is analyzed online with a Gasmet FTIR DX-400 analyzer and a KM9106 portable flue gas analyzer, and the concentration of oxygen carried by the fume stream is subject to monitoring of a CY-IS-G zirconia oxygen analyzer in the tail-end flue duct. 2.2. Fuel Characteristics. Shenmu char was made from the slow pyrolysis of Shenmu coal at a relatively low temperature between 600 and 700 °C. Proximate and ultimate analyses of Shenmu char are listed in Table 1. In comparison to the parent coal, the volatile content of Shenmu char decreases significantly and there is a clear increase in fixed carbon and ash content.

3. RESULTS AND DISCUSSION 3.1. SA Arrangement. Increasing the SA ratio and elevating its injection position is the primary way to control NOx emission during CFB boiler combustion. Until now, a great deal of research has been performed in this area.20−22 In this paper, the impact of changing the SA injection position under three levels of the bed temperatures on combustion and NOx emission is studied. The conditions of the experiments are cases 1, 2, and 3, as stated in Table 2. Temperature profiles along the axis of the CFB furnace at different SA injection positions are shown in Figure 2. Regardless of the experimental condition, the furnace temperatures experience a curve of ascending and then declining. The furnace temperatures are lower at the bottom because of the addition of PA, and the highest furnace temperatures are at a height of 2500 mm above the air distributor because of fuel combustion. From this point, the furnace temperatures decline under the cooling effect of the water-cooling tube arranged on the upper part of the furnace. Changing the SA injection position has some impact on the furnace temperature distribution. The carbon content in fly ash is shown in Figure 3. The role of the SA injection position in affecting the combustion efficiency is not distinct. By comparison, the furnace temperature is the main factor. When the highest furnace temperature is below 900 °C, the fly ash contains carbon more than 10%, and when the highest furnace temperature exceeds 940 °C, the carbon content in fly ash drops down below 3.5%. Thus, for Shenmu char, a higher furnace temperature (no lower than 900 °C) is more suitable for its combustion. The NOx emission in different cases is shown in Figure 4. Under the same furnace temperatures, the concentration of NOx drops in response to increasing the SA injection height. However, all of the SA arrangements are able to remove the NOx emissions by up to only 30%. The interesting result in Figure 4 is that the concentration of NOx drops as the furnace temperature rises. Generally, NOx emission increases with the increase of the combustion temperature, which has been verified by many researchers.13,15,20−22 However, the trend for Shenmu char is opposite, it was mentioned in the research results of another pilot-scale CFB.19 The Shemmu char produced from coal and most of the volatile has already been removed; thus, most NOx was converted from char N during the combustion process. At a higher combustion temperature, both the conversion rate of char N to NOx and the reduction rate of NOx increase, while the reduction rate of NOx is greater, resulting in a decrease in NOx emission.23,24 The N2O emission in different cases is also shown in Figure 4. All of these concentration are below 50 mg/m3, and the concentrations of N2O decreased with the furnace temperature increasing. 3.2. Lean Oxygen Combustion. As mentioned above, the modified SA arrangement cannot significantly reduce the NOx emission, which is far from the target of 100 mg/m3. Another way to reduce NOx emission is the adoption of a lower air

Table 1. Proximate and Ultimate Analyses of Shenmu Char item proximate analysis (wt %) moisturea volatile matterb volatile mattera fixed carbona asha ultimate analysisa (wt %) carbon hydrogen oxygen nitrogen sulfur low heating valuea (MJ/kg) a

Shenmu char 14.60 9.89 7.32 66.72 11.36 68.31 0.85 4.01 0.58 0.30 23.32

As received. bDry and ash-free basis.

2.3. Experimental Conditions. Experiments include changing bed temperature, modifying the SA arrangement, low air stoichiometric ratio combustion, and post-combustion. Table 2 lists the conditions of all of the experiments. The air stoichiometric ratio in the CFB furnace is λCFB; the air stoichiometric ratio in the postcombustion chamber is λPCC; and the SA ratio is β, which was defined as follows:

β=

SA f × 100% PA f + SA f + PCA f

(1)

where PAf is the primary air flow, SAf is the secondary air flow, PCAf is the post-combustion air flow. 3635


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Energy & Fuels Table 2. Operation Conditions of the Experiments air injection arrangement case 1

2

3

4 5

1.1 1.2 1.3 2.1 2.2 2.3 3.1 3.2 3.3

combustion temperature (°C)

λCFB

λPCC

β (%)

first SA

second SA

third SA

fourth SA

PCA

872 874 873 925 926 923 936 948 943 936 984

1.15 1.18 1.20 1.22 1.21 1.22 1.22 1.30 1.18 0.96 0.89

0 0 0 0 0 0 0 0 0 0 0.21

52 52 50 51 50 51 56 49 53 41 31

on on on on on on on on on on on

on off off on off off on off off on off

off on off off on off off on off off off

off off on off off on off off on off off

off off off off off off off off off off on

stoichiometric ratio in the combustion process. In case 4, the monitoring data exhibit the entire process of the oxygen concentration decreasing from 4.9 to 0% in the flue gas. The component emission of the flue gas is shown in Figure 5.

Figure 2. Temperature distribution along the axis of the CFB.

Figure 5. Component emission with decreasing the O2 concentration.

From Figure 5 as the oxygen concentration decreases, the concentration of NOx declines quickly, in contrast to a fast growing CO concentration, and the concentrations of NH3 and HCN as precursors of NOx increased as well. When the oxygen concentration is reduced to 0.9%, the NOx emission concentration descends from 592 to 99 mg/m3, with the CO concentration rising up to 3000 ppm. As the oxygen concentration slipped to 0.3%, the CO concentration moves to 5000 ppm and the NOx concentration reaches 0. The concentrations of N2O decreased with the oxygen concentration decreasing; as the oxygen concentration slipped to 1.1%, the N2O concentration reaches 0. In the meantime, the concentrations of NH3 and HCN in the flue gas increased with oxygen decreasing, but both were below 10 mg/m3. The NOx emission concentration in the flue gas can be significantly reduced or even eliminated by bringing the air stoichiometric ratio down to a quite low level during the combustion. The emission concentrations of NH3 and HCN are also maintained at a low level throughout the process, which means that most fuel N has been converted to N2 in a lean oxygen combustion environment. However, high CO emission concentration and a relatively higher carbon content in fly ash (38.99%, as detected after the experiment) lead to a substantial

Figure 3. Carbon content in fly ash in different cases.

Figure 4. NOx and N2O emission in different cases.

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temperature deviation at points that were 400, 1100, and 2100 mm away from the top of the PCC is only 2 °C, indicating the occurrence of a very uniform and stable combustion in the PCC. This condition is quite favorable for the control of NOx emission. The decrease in the temperature at the point 4100 mm away from the top of the PCC is mainly due to the combustion completion and that the flue gas temperature begins to drop with heat dissipation. Figure 8 shows the concentration trends of different components of the flue gas in the PCC. Figure 8a is the CO2

decline in the combustion efficiency. For this reason, it cannot be advisible to control NOx emission in the flue gas simply through lean oxygen combustion. 3.3. Addition of Post-combustion. The approach of lean oxygen combustion in the CFB furnace provides a possible way to control NOx emission at an extremely low level of oxygen in the furnace but requires post-combustion for CO and fine char particles. To this end, a post-combustion process following the CFB furnace was devised. For operating parameters of this process, see case 5 in Table 2. Figure 6 shows the temperature variations with time in the cyclone and PCC after PCA injection. In the figure, the word

Figure 6. Temperature variations with time in the cyclone and PCC after PCA injection. Figure 8. Concentration of different gases along the axis of the PCC.

“cyclone” represents the temperature inside the cyclone and others are temperatures measured in places having different distances from the top of the PCC. Before PCA injection, the temperature is 807 °C in the cyclone, 700 °C at the closest point, 400 mm away from the top of the PCC, and 617 °C at the farthest point, 4100 mm away from the top of the PCC. All flue gas temperatures measured in the PCC are lower than the temperature in the cyclone, which was caused by heat dissipation. With the PCA injection, CO and fine char particles incompletely burnt in the CFB continued to burn in the PCC. The temperatures inside the PCC rise and are stabilized after about 60 min of beginning post-combustion, in which state temperatures inside reach 956 °C at the point closest to the top of the PCC (distance of 400 mm away) and 899 °C at the farthest point (distance of 4100 mm). Figure 7 exhibits the temperatures along the axis of the PCC in stable conditions, with the maximum temperature deviation of only 57 °C. It is worth noting that the maximum

concentration curve, which increases as CO and char begin to burn in the PCC. Figure 8b is the CO concentration curve that starts from the initial level of more than 12 000 ppm, declines quickly as it burn, and finally fell down to 97 ppm at the outlet of the PCC, where CO is almost burnt out. Among these components, the concentration of NO shown in Figure 8c is very low along the axis of the PCC. As mentioned from Figure 5, there was no NOx and very low concentrations of the precursors of NOx, such as NH3 and HCN, in the flue gas because of lean oxygen combustion. During the post-combustion process, the precursors were partially converted to NO. For these reasons, the amount of NO generated was minimal and its final emission concentration was only 30 ppm. The concentration of NO2 along the furnace was declining in general, with the final emission concentration reaching 2 ppm, as shown in Figure 8d. The N2O concentration is very low, 4 ppm, as shown in Figure 8e. This phenomenon is mainly attributable to a higher combustion temperature in the PCC. The advantage has captured the attention of some researchers who intend to explore the possibility of making use of this advantage to control the emission of N2O from the CFB.25−27 According to the experimental results of case 5, the emission concentration of NOx (NO + NO2) is 51 mg/Nm3 (at 6% O2), far below the Chinese new emission limit, 100 mg/Nm3, the final emission concentration of CO is 97 ppm, the carbon content in fly ash is lower than 5%, and the calculated combustion efficiency is 98.6%. All of the data suggest that the combination of a CFB furnace in lean oxygen combustion and post-combustion can greatly cut down NOx emission without decreasing the combustion efficiency. Figure 9 is the schematic diagram of the new combustion process. Combustion is carried out in a CFB and a PCC. Coal

Figure 7. Temperatures along the axis of the PCC. 3637


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combustion, while the fine char particles and gas are delivered into the PCC. NOx generated in the PCC is mainly from the precursors (i.e., NH3 and HCN) carried by the flue gas. Because the fine char particles contain almost no fuel N, they not only generate no NOx but also reduce the generated NOx at high temperatures in the PCC.34−36 The reaction process in the PCC, which is represented by “③” in the figure, mainly consisted of the following reactions: NO + Cf → C(O) + 1/2N2

(10)

Cf , CO + O2 → CO2

(11)

The conversion ratio of fuel N to NOx is defined as the measured value of NOx in the exhaust from the PCC divided by the calculated value with all fuel N to NOx. In this experiment, the conversion ratio of fuel N to NOx is only 2.5%.

4. CONCLUSION This paper explores the possible measures with respect to the control of NOx emission from Shenmu char combustion on a CFB test platform, including the modified SA arrangement, furnace low air stoichiometric ratio combustion, and postcombustion. According to the experiments, the following can be concluded: (a) The modified SA arrangement approach plays a role in reducing NOx emission but fails to meet the new national standard limit of 100 mg/m3. (b) The combustion at a low air stoichiometric ratio in the furnace is able to cut down the emission concentration of NOx significantly. It even generates no NOx in the combustion process using an extremely lean oxygen concentration but results in low combustion efficiency. (c) The combination of lean oxygen combustion in the CFB furnace and post-combustion in the PCC can greatly reduce the final emission concentration of NOx and guarantee fuel burnout. The experimental results show that the final concentration of NOx in flue gas is only 51 mg/Nm3 and the combustion efficiency is as high as 98.6%. Furnace lean oxygen combustion followed by postcombustion is a new approach to improved control of initial NOx emission to CFB boilers. However, questions regarding the fuel combustion characteristics involved in the CFB and PCC, the NO x removal mechanism, and the furnace desulfurization require further studies in the future.

Figure 9. Schematic diagram of the fuel combustion process.

begins to burn under the PA in the dense phase zone of the CFB; carbon is converted into CO2 and CO because of the higher concentration of oxygen; a portion of fuel N is volatilized under the heat; and the precursors are oxidized to NOx . The reaction process in the dense-phase zone, represented by “①” in the figure, is considered to have mainly consist of the following reactions: C + O2 → CO2 , CO

(2)

fuel N → NH3, HCN, ...

(3)

NH3, HCN + O2 → NO

(4)

fuel N + O2 → NO

(5)

Some fuel particles in the dense-phase zone are carried away by the PA into the dilute phase zone and developed into coarse and fine char particles after a number of reactions and crushing. Meanwhile, because the air stoichiometric ratio in the CFB is below “1”, no O2 exists in the dilute-phase zone and the flue gas contains CO, NH3, and HCN in addition to CO2 and N2. Similarly, NOx generated in the dense-phase zone is completely reduced in the dilute-phase zone under a reducing atmosphere. The reduction of NO to N2 with char (in the following reactions, Cf is defined as carbon in char) and CO is known to be significant in fluidized bed combustion of coal.28−33 The reaction process in the dilute-phase zone, which is represented by “②” in the figure, is considered to have mainly consisted of the following reactions: CO2 + C → CO

(6)

fuel N → NH3, HCN, ...

(7)

NO + CO → CO2 + 1/2N2

(8)

NO + Cf → C(O) + 1/2N2

(9)

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-010-82543129. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant XDA07030100.

■

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The flue gas and char particles flow from the diluted-phase zone into the cyclone separator, where the coarse char particles are separated and returned to the dense-phase zone for further 3638


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Experimental Study on Enhanced Control of NOx

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