Effect of the Air Temperature on Combustion Characteristics and NOx

Mar 23, 2012 - The results show that air temperature in the deep air staging has a significant effect on the flame stability, emissions of NOx, and un...
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Effect of the Air Temperature on Combustion Characteristics and NOx Emissions from a 0.5 MW Pulverized Coal-Fired Furnace with Deep Air Staging Zhengqi Li,* Yong Liu, Zhichao Chen, Qunyi Zhu, Jinzhao Jia, Jing Li, Zhenwang Wang, and Yukun Qin School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, People’s Republic of China ABSTRACT: This paper evaluates the effect of the air temperature on the combustion characteristics and NOx formation in a 0.5 MW laboratory furnace fired by a pulverized coal swirl burner with deep air staging. The temperature and compositions of flue gas and fly ash in the primary zone and the second burnout zone were sampled and measured. The results show that air temperature in the deep air staging has a significant effect on the flame stability, emissions of NOx, and unburnt carbon content in fly ash. When the air temperature is increased from 200 to 400 °C, in the primary zone (the stoichiometric ratio is 0.85), the ignition of flame is advanced, the flame stability improved significantly, the overall temperature level and combustion rate increased significantly, the CO concentration increased, the NOx concentration decreased significantly, and the carbon, hydrogen, and nitrogen release rates and the char burnout increased significantly. After the overfire air injection, in the second burnout zone, as the air temperature is increased, the NOx concentration decreased but the formation of NOx in the second burnout zone increased significantly, as well as the contribution of N species conversion to NOx. As the combustion reaction is completed, the final char burnout increased from 96.4 to 99.8%. Spliethoff et al.3 have carried out an investigation on air staging in an electrically heated tube reactor. The effects of the temperature for air staging with different types of coals were evaluated. The results show that higher temperatures with excess air result in higher NOx concentrations, but under the air-deficient conditions provided by air staging, the NOx emissions decrease. Fan et al.4 carried out an investigation on staged combustion in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of anthracite coal. They concluded that, the more the air is staged, the more NOx emissions are reduced. Costa and Azevedo5 conducted a study in a 300 MWe, frontwall-fired, pulverized coal utility boiler, which was retrofitted with boosted overfire air (OFA) injectors that allowed for the operation of the furnace under deeper staging conditions. Under these conditions, the authors reported NOx emissions below 500 mg N−1 m−3 at 6% O2 and acceptable (comparable to those obtained in a previous study in the same boiler with a conventional overfire system) CO emissions and overall particle burnout. Zabetta et al.6 describe a new method that they called “combined staging” (CS), which is combined with fuel staging, air staging, and selective noncatalytic reduction in synergistic combination rather than in sequence. The performance of CS was tested with detailed chemical kinetic models. Models indicate that CS can reduce over 40% NOx at lower temperatures and within shorter residence time than required by other techniques. Ribeirete and Costa2 conducted an investigation on the performance of a large-scale laboratory furnace fired by an industry-type pulverized coal swirl burner. Data about the

1. INTRODUCTION In recent years, the contribution to air pollution caused by NOx generated from the combustion of coal has become a concern to the international community. Currently, the NOx emission limits continue to tighten around the world. For example, in the European Union (EU), from 2008, the permissible emission limit from power plants over 500 MWth is 500 mg of NO2 N−1 m−3 at 6% O2, but from 2016, the limit allowed for these power plants will drop further to 200 mg of NO2 N−1 m−3 at 6% O2.1 In China, from 2012, the allowed NOx emission from coal-fired power plants over 300 MWth in city centers is 100 mg of NO2 N−1 m−3 at 6% O2 and the allowed NOx emission of the coalfired power plants over 300 MWth in suburbs is 200 mg of NO2 N−1 m−3 at 6% O2. This legislation urgently necessitates much more stringent NOx emission control techniques. Among existing techniques for NOx control, deep air staging combustion is one of the most efficient and attractive technologies for reducing NOx emissions, because it does not need expensive equipment, such as selective catalytic reduction (SCR) systems. Deep air staging combustion reduces the stoichiometric ratio of the primary combustion zone to below 0.9. When operating a boiler under deep air conditions, there are a number of processes that are not well-described, such as slagging, fouling behavior, high-temperature corrosion, and burner flame stability.2 In deep air staging combustion, the temperature and residence time in the primary zone play a key role in NOx reduction. The aim of the present investigation is to concentrate on the evaluation of the impact of the air temperature on flame stability, NOx emissions, particle burnout, and CO emissions from a 0.5 MW laboratory furnace fired by a pulverized coal swirl burner with deep air staging. Related studies include Spliethoff et al.,3 Fan et al.,4 Costa and Azevedo,5 Zabetta et al.,6 Ribeirete and Costa,2 and Li et al.7 © 2012 American Chemical Society

Received: February 9, 2012 Revised: March 21, 2012 Published: March 23, 2012 2068

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a flue gas dust removal system, an OFA system, and other additional components. The pulverized coal swirl burner was arranged at the top of the furnace, and the flame was an axial diffusion type. For the investigation of deep air staging, there are several possible places for adding burnout air to the furnace. The operating temperature range of the furnace is from 0 to 1600 °C, and the flue gas residence time in the furnace is in the range between 4 and 6 s. The internal pressure of the chamber is between −150 and −100 Pa.

influence of the axial position of the staged air injector, the primary zone stoichiometric ratio (λpz), the coal type, and the configuration of the staged air injector on pollutant emissions and particle burnout were obtained. It was concluded that NOx emissions decrease as the distance between the staged air injector and the burner exit increases and a reduction in λpz causes a decrease in both NOx emissions and particle burnout and an increase in CO emissions. Li et al.7 carried out an experimental study of air staging in a 1 MW (heat input power) tangentially fired pulverized coal furnace. The influences of air staging, including the air stoichiometric ratio of primary combustion zone (SR1), the locations of OFA nozzles along furnace height, and the ratio of the coal concentration of the fuel-rich stream to that of the fuellean stream (RRL) in the primary air nozzle, on NOx reduction efficiency and unburned carbon in fly ash were investigated. The results show that increasing the distance from the burner exit to the OFA injector reduces NOx emissions but the unburned carbon in fly ash and CO emission all increase. To study the impact of the air temperature on flame stability, NOx emissions, particle burnout, and CO emissions in a pulverized coal swirl burner furnace with deep air staging, three different experiments were performed on a 0.5 MW laboratory furnace, with the air temperature at 200, 300, and 400 °C. In all cases, the primary zone stoichiometric ratio is 0.85 and other operating conditions are fixed. The temperature and composition of flue gas and fly ash in the primary zone and second burnout zone were sampled and measured.

3. EXPERIMENTAL CONDITIONS AND PARAMETERS 3.1. Experimental Conditions. To avoid other factors affecting experimental results, various parameters in the experiments were monitored and strictly controlled. The experimental runs were only started once the control parameters had reached this required state. During the experiments, a thermocouple was used to measure the temperature. Flue gas composition and fly ash were sampled by a water-cooled collecting probe. This water-cooled stainless-steel probe (shown in Figure 2) consists of a water feed pipe, a water outlet pipe, a sampling pipe, and an outer pipe. The gas was sampled via the sampling pipe. When the flue gas entered the sampling pipe, the temperature deceased rapidly and the pulverized coal stopped burning. The samples were passed through filters into a Testo 350 M gas analyzer. The coke sample was obtained with a vacuum pump and sampling pipe, by means of a cyclone separator, coke collector, and flow meter. The accuracy of the Testo 350 M gas analyzer for each species measurement is 1% for O2, 3% for CO, and 5 ppm for NO and NO2. Calibration was carried out on each sensor before measurement. The repeatability of the flue gas data was on average within 5%. Char burnout was calculated using

ψ = [1 − (ω k /ω x )]/(1 − ω k )

(1)

where ψ is the char burnout, ω is the ash weight fraction, and the subscripts k and x refer to the ash contents in the input coal and char sample, respectively. The percentage release of components (C, H, and N) was calculated using

2. EXPERIMENTAL FACILITIES Figure 1 shows a schematic diagram of the 0.5 MW laboratory furnace. The furnace consists of a coal feeding system, an ignition system, a

β = 1 − [(ωi x /ωi k )(ωαk /ωαx )]

(2)

where ωi is the weight percentage of the species of interested, ωα is the ash weight percentage, and the subscripts k and x refer to the different contents in the input coal and char sample, respectively.8 The repeatability of the char burnout and release of components (C, H, and N) data was on average within 10%. 3.2. Experimental Parameters. Figure 3 shows the specific size of the swirl burner and OFA nozzle used in this experiment. The maximum diameter of the burner is 160 mm, and the OFA is injected inside the combustion chamber from the furnace axis through two horizontal nozzles. The feeding rate of pulverized coal in the test is 40 kg/h. The primary zone stoichiometric ratio is 0.85. The air preheater allows the air temperature to be regulated during the course of combustion, and the experimental arrangements are shown in Table 1, with other operating conditions being fixed. The coal used in the experiments was bituminous coal from China. The characteristics in Table 2 indicate that the coal is a typical bituminous coal with high-volatile content. The average particle size of the coal was 39.94 μm. The distribution of coal particle sizes is shown in Figure 4. In the tests, the measuring point was arranged in the primary zone and second burnout zone. The measuring points were arranged in four sections at 80, 240, 400, and 560 mm along the furnace height axis (Z). The measuring points were arranged along the radial positions (r) at 0, 16, 32, 48, 64, 80, 96, 128, 160, 240, 320, and 400 mm in each section. For each measuring point, the temperature and gas compositions were measured and the fly ash was sampled. Figure 1. Schematic diagram of the test facility.

4. RESULTS AND DISCUSSION Figures 5 and 6 show the comparative radial profiles of selected gas species concentration measurements (O2, CO, and NOx),

swirl burner, a furnace, a cooling system, a furnace temperature detection system, a furnace flue gas detection system, an air preheater, 2069

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Figure 2. Water-cooled stainless-steel probe.

Figure 4. Distribution of coal particle sizes. Figure 3. Schematic of the furnace, burner, and OFA system (dimensions in millimeters).

primary coal jet itself. For 50 ≤ r ≤ 100 mm, a maximum O2 concentration is achieved because of the mixing with the secondary air flow. For 125 ≤ r ≤ 400 mm, the gas temperatures decrease; however, the O2, CO, and NOx concentrations are relatively constant. This area is the external recirculation zone caused by high-temperature flue gas recirculating around the burner jet. At Z = 240 mm, the reaction zone is slightly wider than at Z = 80 mm, owing to the expansion of the central jet as the combustion process progresses. For 0 ≤ r ≤ 50 mm, the gas temperature rises, the O2 concentration decreases, the CO concentration increases significantly, and the NOx concentration decreases compared to the Z = 80 mm case. For 50 ≤ r ≤ 150 mm, compared again with Z = 80 mm, the gas temperature increases, the O2 concentration decreases, and CO and NOx concentrations increase. This is because the incoming secondary air is less sharply defined than at the section at Z = 80 mm. At Z = 400 mm, the O2 and NOx concentrations and gas temperature profiles reveal that intense combustion continues. At Z = 560 mm, the O2 and NOx concentrations and gas temperature profiles are relatively flat because of the entrainment of the accelerated combustion gases. Although most of the combustion has been completed at this stage, the mixing between the relatively cooler external recirculation zone gases and hotter air steam is continuing. As seen, for the given stoichiometric ratio of 0.85 in the primary zone with an increase in the air temperature from 200 to 400 °C, the flue gas temperature increases significantly, the O2 concentration decreases, the CO concentration increases, the NOx concentration decreases significantly, and the carbon, hydrogen, and nitrogen release rates and char burnout increase in all four sections. High air temperature accelerates the rate of combustion and results in a high flue gas temperature. This increase in flue gas temperature has two effects. First, increasing

Table 1. Experimental Arrangements case temperature of primary air (°C) temperature of secondary air (°C) temperature of OFA (°C) stoichiometric ratio in the primary zone (λpz) primary air mass flow rate (kg/h) inner secondary air mass flow rate (kg/h) outer secondary air mass flow rate (kg/h) OFA mass flow rate (kg/h) feed rate of pulverized coal into primary air (kg/h) distance from the burner exit to the OFA nozzle (mm) excess air ratio at the exit of the furnace (%)

1

2

3

200 200 200

300 300 300 0.85 117.12 72.36 108.72 127.90 40 1520

400 400 400

15

Table 2. Coal Characteristics

proximate analysis (wt %, as received) ultimate analysis (wt %, as received)

fixed carbon

ash

moisture

volatile matter

net heating value (kJ/kg)

52.07

7.46

8.45

32.02

25160

carbon

hydrogen

sulfur

nitrogen

oxygen

66.12

3.89

0.82

0.79

12.47

gas temperature, release rate of components (C, H, and N), and char burnout at four axial locations for different air temperatures. At Z = 80 mm, the char burnout, the component element (C, H, and N) release rate, and the temperature profiles show that a significant proportion of particle combustion and primary jet dispersion has already occurred. Very high CO and NOx and low O2 concentrations between 0 ≤ r ≤ 50 mm indicate that intense combustion is taking place inside the 2070

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Figure 5. Radial profiles of the temperature and gas species concentrations (O2, CO and NOx) at the four sections: (×) 200 °C, (○) 300 °C, and (△) 400 °C. (∗) The range of Testo 350 M for the CO concentration is from 0 to 100 000 ppm.

the temperature under fuel-rich conditions drives the reactions (as shown in Figure 7) toward the formation of N2 rather than fuel NO.9 Second, the increase in the temperature leads to increased devolatilization rates and yields. The higher volatile yield means that the combustibles in the gas phase increase, leading to a more fuel-rich gas phase that inhibits fuel NO formation from the volatile nitrogen species. Therefore, a higher air temperature created more fuel-rich conditions and increased the flame temperature in the fuel-rich zone, both reducing fuel NO formation. The enhanced devolatilization also yields less residual carbon that must be burned in the second stage, leading to lower NOx formation from char and lower unburned carbon in ash. Figure 8 shows photographs of flame patterns taken from the furnace window at three air temperatures. Observations of the flames, as the air temperature increases from 200 to 400 °C, show that the pulverized coal was more rapidly ignited and burnt, the ignition of flame was advanced, and the flame brightness and the flame stability were significantly improved. This indicates that a higher air temperature allows for a deeper air staging. For a coal-fired power plant, flame stability concerns typically limit the amount of OFA that can be used when the boiler is operated under part load conditions. Because one of

the advantages of higher air temperatures is significant improvement in flame stability, the plant was able to use the deep air staging with higher air temperatures at part loads. Figure 9 shows the temperature profile, gas species concentrations (O2, CO, and NOx), carbon, hydrogen, and nitrogen release rates, and char burnout rate for different air temperatures in the furnace center line. As the air temperature is increased from 200 to 400 °C, the flue gas temperature in the primary zone and in the second burnout zone increased significantly. The O2 concentrations have two peaks: the first peak is in the burner nozzle, and the second peak is near the OFA nozzle. As the air temperature is increased from 200 to 400 °C, the first peak value declines and the second peak value remains relatively constant. This is because the increased air temperature accelerates the combustion rate and the ignition point of the coal flame is brought forward. Therefore, the volatiles will be rapidly ignited and burnt in the primary zone and consume a larger amount of oxygen. The amount of oxygen in the primary zone will be less, thus leading to deeper oxygen-short combustion, efficiently inhibiting NOx emissions. The CO concentration remains high in the primary zone and declines quickly after the point of OFA injection, tending to zero as the combustion reaction is complete. As the air 2071

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Figure 6. Radial profiles of carbon, hydrogen, and nitrogen release rates and char burnout at the four sections: (×) 200 °C, (○) 300 °C, and (△) 400 °C.

declines more rapidly in the second burnout zone. This can be attributed to the fact that the air temperature effects the degree of conversion of char to carbon monoxide in the primary zone and a higher air temperature, corresponding to a higher char burnout rate, results in a higher CO formation in the fuel-rich conditions. In the second burnout zone, as the air temperature is increased, the flue gas temperature increased and the combustion at the last stage is further intensified, causing a rapid decline in the CO concentration. In the overall combustion process, the NOx concentrations can be divided into three stages: declining in the primary zone, increasing after the OFA injection, and stable as the combustion reaction is complete. In the primary zone, as the air temperature increases from 200 to 400 °C, the NOx concentration decreases significantly. At air temperatures of 200, 300, and 400 °C, the NOx concentration at the end of the primary zone downstream is 257 mg/m3 at 6% O2, 216 mg/m3 at 6% O2, and 171 mg/m3 at 6% O2, respectively. The reason for decreasing NOx formation as air temperature increases is that an increase in the release of volatile nitrogen and acceleration in the decomposition of fuel NO leads to a reduction in

Figure 7. NOx production pathways in coal combustion.9

temperature increases from 200 to 400 °C, it can be seen that the CO concentration increases in the primary zone but 2072

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Figure 8. Photographs of the flame pattern taken from the furnace window.

Figure 9. Profiles of temperature, gas species concentration (O2, CO, and NOx), carbon, hydrogen, and nitrogen release rates, and char burnout rate in the furnace center line: (×) 200 °C, (○) 300 °C, and (△) 400 °C. (∗) The range of Testo 350 M for the CO concentration is from 0 to 100 000 ppm.

Table 3. Formation of NOx in the Primary Zone and Second Burnout Zone

the amount of char N, which may be converted to fuel NO. A comparison of the temperature effects for deep air staging shows that accelerated decomposition is the dominant effect for reducing NOx formation. As the air temperature is increased from 200 to 400 °C, at the end of the second burnout zone, the NOx concentration decreased. For air temperatures of 200, 300, and 400 °C, the NOx concentration at the end of the second burnout zone downstream is 291 mg/m3 at 6% O2, 265 mg/m3 at 6% O2, and 233 mg/m3 at 6% O2, respectively. However, the formation of NOx in the second burnout zone increases. The percentage of NOx produced in the second burnout zone to total NOx emission increases from 11.6 to 26.6% (as shown in Table 3). This indicates that, as the air temperature is increased, the positive effect that the air temperature has on fuel NO decomposition in the fuel-rich primary zone may be compensated by higher conversion of N species to nitrogen oxide by the addition of OFA. As the air temperature is increased from 200 to 400 °C, in the primary zone, the carbon, hydrogen, and nitrogen release rates and char burnout rate are increased significantly. This can be attributed to the fact that an increase in air temperature

case

1

2

3

air temperature (°C) NOx concentration at the end of the primary zone downstream (mg/m3) at 6% O2 NOx concentration at the end of the second burnout zone downstream (mg/m3) at 6% O2 formation of NOx in the second burnout zone (mg/m3) at 6% O2 percentage of the NOx produced in the second burnout zone to total NOx emission (%)

200 257

300 216

400 171

291

265

233

34

49

62

11.6

18.4

26.6

accelerates the coal combustion rate. In the second burnout zone, as the air temperature is increased from 200 to 400 °C, the combustion at the last stage is further intensified, causing a gradual increase of the char burnout rate. As the combustion reaction is completed, for air temperatures of 200, 300, and 400 °C, the final char burnout is 96.4, 98.7, and 99.8%, respectively. However, in our experimental apparatus, the residence time of the flue gas in the second burnout zone is about 3 s, which is 2073

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(8) Costa, M.; Silva, P.; Azevedo, J. L. T. Measurements of gas species, temperature, and char burnout in a low-NOx pulverized-coalfired utility boiler. Combust. Sci. Technol. 2003, 175, 271−289. (9) Wendt, J. O. L. Mechanisms governing the formation and destruction of NOx and other nitrogenous species in low NOx combustion systems. Combust. Sci. Technol. 1995, 108, 323−344.

longer than the residence time in the burnout zone of a coalfired utility boiler. Therefore, it may be that the air temperature under deep air staging in a coal-fired utility boiler would have a greater impact on char burnout.

5. CONCLUSION Three different experiments were performed on a 0.5 MW furnace using the same pulverized coal swirl burner to burn bituminous coal at air temperatures of 200, 300, and 400 °C. The primary zone stoichiometric ratio (λpz) is 0.85. The temperature and composition of flue gas and fly ash in the primary zone and second burnout zone were sampled and measured. The main conclusions drawn are as follows: In the primary zone, as the air temperature is increased from 200 to 400 °C, the pulverized coal was more rapidly ignited and burnt, the ignition of flame was advanced, the flame brightness and the flame stability were significantly improved, the flue gas temperature increased significantly, the CO concentration increased, the NOx concentration decreased significantly, and the char burnout increased. In the second burnout zone, as the air temperature is increased from 200 to 400 °C, the NOx concentration decreased, the formation of NOx in the second burnout zone increased, and the contribution of N species conversion to NOx increased significantly. As the air temperature is increased from 200 to 400 °C and as the combustion reaction is completed, char burnout increased.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-451-86-41-88-54. Fax: +86-451-86-41-25-28. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Contract 51121004).

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