Article pubs.acs.org/EF
An Experimental Study on NOx Emissions in Combustion of Pulverized Coal Preheated in a Circulating Fluidized Bed Jianguo Zhu,*,† Ziqu Ouyang,†,‡ and Qinggang Lu† †
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of the Chinese Academy of Sciences, Beijing 100049, China
‡
ABSTRACT: This article describes an experimental study on NOx emissions in pulverized coal combustion after preheating and air staging. The process takes place in two stages: the pulverized coal is first preheated in a circulating fluidized bed, and then the resulting fuel gas and char particles are burned in a down-fired combustor under air-staging conditions. Experiments were conducted with three types of coal, three coal particle sizes, four preheating temperatures, and three air ratios in the circulating fluidized bed. With this process, combustion was stable, the temperature was uniform in the combustor after coal preheating, and the burning efficiency was high. The combination of air-staging combustion with pulverized coal preheated in a circulating fluidized bed was effective in reducing NOx emissions. NOx emissions from combustion of bituminous coal, anthracite coal, and semicoke were 368, 256, and 458 mg/N m3 (@6% O2), and the conversion rates of nitrogen in the fuel (fuel-N) were 28.66%, 23.94%, and 57.36%, respectively. The NO emissions and fuel-N conversion rate initially decreased then increased with increasing preheating temperature. The NOx emissions and fuel-N conversion rate decreased with increasing pulverized coal size and air ratio in the circulating fluidized bed.
1. INTRODUCTION
A previous study on pulverized anthracite coal combustion after preheating in a CFB was conducted to investigate the combustion characteristics and the mechanisms of NOx formation for a typical case.22 The effects of operating conditions in the down-fired combustor, including the air ratio, residence time in the reducing zone, and excess air ratio on NOx emissions in pulverized coal combustion preheated by a CFB also were previously investigated.24 However, NOx emissions have not been evaluated under differing operating conditions in the CFB. Therefore, the objective of the present experimental study was to evaluate the effects of operating conditions in the CFB on NOx emissions in pulverized coal combustion preheated by a CFB. Different coal types, pulverized coal sizes, preheating temperatures, and air ratios in the CFB were investigated in the experiments.
NOx emissions from coal combustion in power plants are responsible for atmospheric pollution such as acid rain and photochemical smog. In recent years, there has been substantial research and development focusing on reducing NOx emissions from combustion processes. Many combustion techniques aimed at NOx reduction such as high-temperature air combustion, fuel reburning, and air staging have been developed. High-temperature air combustion, with a uniform temperature profile in the reactor and a lower peak temperature preventing formation of thermal NOx, can reduce NOx emissions.1−5 Fuel reburning can reduce NOx emissions by NOx reduction reactions in the reburning zone.6−10 Air staging prevents NOx emissions by forming a reducing atmosphere in the primary zone.11−16 Use of a lowNOx burner is also a widely used technique for reducing NOx emissions.17−19 However, these techniques still have difficulty reducing NOx emissions to the levels required by regulations. More efficient but also expensive methods, such as selective catalytic reduction (SCR),20,21 must be installed and operated to reduce NOx emissions. Recently, a method of preheating pulverized coal in a circulating fluidized bed (CFB) has been proposed and adopted. With this method, the combustion is stable, the burning efficiency is high, and the NOx emissions are relatively low.22 The process takes place in two stages: the pulverized coal is first preheated under a low air ratio in a CFB, and then the resulting fuel gas and char particles are burned in a down-fired combustor under air-staging conditions. With the advantages of strong turbulence and fast heat transfer of the CFB,23 pulverized coal can be preheated quickly and steadily to a high temperature and then introduced into the down-fired combustor for combustion. © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Apparatus and Method. Figure 1 shows the schematic illustration of the test apparatus. The experimental system consisted of a CFB, a down-fired combustor (DFC), and some auxiliaries. A horizontal tube that was 48 mm in diameter and 500 mm in length was installed between the CFB and the DFC to convey the preheated fuels. Details of the structures and dimensions of the CFB and the DCF are shown in ref 22. The gas resulting from the preheating process is called high-temperature coal gas, and the composition was mainly CO, CH4, H2, N2, and CO2. The resulting char particles from the CFB are called preheated coal particles. The high-temperature coal gas along with the preheated coal particles were conveyed to the top center of the DFC through a nozzle for further combustion. Secondary air was supplied to the DCF through the nozzle, and tertiary air was injected into the DFC through a tertiary air nozzle 600 mm below the top. A reducing zone appeared Received: May 23, 2013 Revised: November 18, 2013 Published: November 19, 2013 7724
dx.doi.org/10.1021/ef402146w | Energy Fuels 2013, 27, 7724−7729
Energy & Fuels
Article
Table 2. Cases for Determining the Effects of Coal Type on NOx Emissions coal type coal feed rate (kg/h) preheating temperature (°C) λCFB λRZ λ
case 1
case 2
case 3
bituminous 5.13 900 0.25 0.7 1.3
anthracite 3.41 900 0.25 0.7 1.3
semicoke 4.60 900 0.25 0.7 1.3
excess air ratio, respectively. They were determined as follows:22 λCFB = λRZ = λ= Figure 1. Schematic of the test apparatus.
AI A stoi AI + AII A stoi
AI + AII + AIII A stoi
(1)
where AI, AII, and AIII, are the primary, secondary, and tertiary air flows and Astoi is the theoretical air requirement for pulverized coal combustion. In this group of experiments, λCFB, λRZ, λ, and the preheating temperature in the CFB were set to 0.25, 0.7, 1.3, and 900 °C, respectively. The size distributions of the three types of coal (shown in Figure 2) were obtained using a Malvern Mastersizer
between the top of the DFC and the tertiary air port. Details of the structures and dimensions of the secondary and tertiary air nozzles are shown in ref 24. There were eight thermocouples and eight sampling ports in the test apparatus, allowing the temperatures and compositions of the solid and gas samples to be analyzed in the experiments. The specific sampling and measurement methods in the experiments were introduced in ref 22. 2.2. Fuel Characteristics. Table 1 shows the proximate and ultimate analyses of the different coal types used in the experiments,
Table 1. Proximate and Ultimate Analyses of the Different Types of Coal coal type analysis proximate (wt %, air-dried) moisture ash volatiles fixed carbon ultimate (wt %, air-dried) carbon hydrogen oxygen nitrogen sulfur lower heating value (MJ/kg)
bituminous
anthracite
semicoke
1.87 26.14 27.46 44.53
2.40 8.64 6.74 82.22
1.20 15.46 8.20 75.14
58.28 3.74 8.61 1.04 0.32 22.61
82.08 3.13 1.87 1.18 0.70 30.70
76.82 1.41 3.99 0.76 0.38 24.0
Figure 2. Size distributions of the coals in cases 1−3.
2000 laser analyzer. The size distributions of the three types of coal were basically the same, and the diameters of the three coals were
