Article pubs.acs.org/EF
Experimental Investigation on NOx Reduction Potential of Gas-Fired Coal Preheating Technology Changchun Liu, Shien Hui,* Su Pan, Hao Zou, Geng Zhang, and Denghui Wang School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: A gas-fired coal preheating (GFCP) technology, offering a flexible method to reduce NOx, could be used with other de-NOx combustion technology such as air staging to seek maximum NOx reduction. The preheating chamber key apparatus of GFCP was investigated with the help of infrared camera. The results show that devolatilization and partial oxidation (combustion) of coal occurred in the preheating chamber, and this may prove the main heat source of preheating chamber is combustion of coal volatiles and gas is only used to prevent flameout. A self-sustaining combustion drop furnace was used to investigate the NOx reduction potential of GFCP with air staging. Gas species concentrations along furnace are plotted for several runs, offering details to further study and analyze the GFCP. With GFCP, much HCN, CiHj, and soot were produced in the preheating chamber, so under the similar air staging condition, the NO destructed by HCN and soot was stronger than that without. NOx reduction could archive up to 72% with GFCP and air staging, if the residence time in the combustion zone could be prolonged, the NOx reduction will be even higher. may be difficult to realize literal MILD13 combustion in our tests. However, some features of MILD combustion13 such as high injecting momentum of the combustion air and a relatively long distance between a fuel nozzle and air nozzles, were used in our experiment to study the NOx reduction potential of GFCP. And the combustion was stable under MILD. The MILD denotes secondary air only through two outer secondary air nozzles into the furnace in this paper. There are three NO sources in coal combustion: the fuel NO, the thermal NO, and the prompt NO. Compared with the fuel NO, the prompt NO and the thermal NO formation are unimportant in pulverized coal combustion.18 During PC firing in furnace, coal nitrogen is expelled in three stages.19 Initially, during primary devolatilization, fuel nitrogen is liberated as HCN and a structural element in heavy, aromatic compounds collectively called tar. Devolatilization of the PC usually occurs within 100 ms. Next, the secondary pyrolysis of the volatile in hot fuel-rich gases converts tar-nitrogen to HCN. Eventually, in the third stage, oxygen contacts with the char, liberating additional nitrogen either by direct chemical conversion to NO or by thermal dissociations induced by the higher particle temperatures associated with char combustion. So it is interesting to observe the behavior of preheating chamber and get the detailed gas species concentrations along the furnace height.
1. INTRODUCTION Coal is the principal primary energy source in the world; however, coal combustion could cause serious environmental problems, such as acid rain and photochemical smog.1 Development of a combustion method with low NOx emissions and high fuel efficiency has been the objective of the combustion community. Many de-NOx combustion techniques, such as air staging, low NOx burner, reburning, and exhaust gas recirculation, have been used to reduce NOx emissions.2 Postcombustion de-NOx techniques such as Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) are high NOx reduction efficiencies, but they are expensive in investment and operation.3 Conventional combustion de-NOx techniques have achieved great success in NOx emissions reduction, but these techniques are not enough to meet more stringent NOx emissions standards.4 Therefore, it is desirable to develop a novel combustion de-NOx method. A gas-fired coal preheating (GFCP) method is that a pulverized coal (PC) stream enters the chamber where flue gas from gas combustion is used to heat the PC stream to high temperature before coal combustion. The GFCP method developed for utility PC boilers by the AllRussian Thermal Engineering Institute (VTI) has been verified to be an effective de-NOx method.5−7 This approach also increases the degree of the freedom of NOx control strategies, since it can be used with conventional combustion de-NOx techniques such as air staging, low NOx burner, reburning, and moderate and intensive low-oxygen dilution (MILD). For modern coal-fired utility boilers, close-coupled over fire air (CCOFA) and separated over fire air (SOFA) are used to reduce NOx emissions.8 So it could be useful to study effects of CCOFA and SOFA when working with GFCP. MILD has been proven to be a successful method to reduce NOx emissions for gas fuel.9 The essence of this technology is the fuel is oxidized in an environment that contains a substantial amount of inert gas. The research in recent years has also indicated high NOx reduction potential of this technology for solid fuels.10−17 It is © 2014 American Chemical Society
2. EXPERIMENT A schematic of the experimental apparatus is shown in Figure 1. The drop furnace was designed to allow self-sustaining combustion, without external heating, in a configuration which was representative of practical furnaces characteristic times and temperature. The furnace consisted of specially shaped refractory bricks; these bricks were rectangles with a circle inside. Namely, specially shaped refractory Received: May 23, 2014 Revised: August 6, 2014 Published: August 7, 2014 6089
dx.doi.org/10.1021/ef501175v | Energy Fuels 2014, 28, 6089−6097
Energy & Fuels
Article
Figure 1. Schematic diagram of the experimental system. sampling line were heated to 150 °C before measuring. Samples were then placed in a Fourier transform infrared (FTIR) analyzer (GASMET FTIR Dx4000), where the concentrations of NO, NO2, N2O, CO, CO2, CH4, NH3, HCN, and H2O were measured with an accuracy of ±2% in volume concentration. O2 was measured by MRU electrochemical flue gas analyzer with an accuracy of ±2%. The fly ash was sampled at the stack, as shown in Figure 1. The flow rates of propane, inner and outer primary air, inner and outer secondary air, and CCOFA and SOFA, were regulated and measured by rotameters. Thermocouples located on the furnace center monitored temperature profiles of the drop furnace. Experiments were performed with Huangling (HL) bituminous coal, whose properties are listed in Table 1. In order to facilitate the analysis of experimental data, relative position X is used to describe the sampling point location, defined as
bricks were used to form a circular furnace, with an inner diameter of 160 mm and a height of 2600 mm. Typical residence time in the drop furnace was ∼1 s. Refractory bricks of the furnace were lined with 90mm-thick silicate fiber. The preheating chamber had an inner diameter of 79 mm and a height of 980 mm. To view the combustion, a part of the preheating chamber was made of quartz glass and the rest made of stainless covered by 30-mm-thick silicate fiber. Four premixed propane burner nozzles with bluff body were mounted on the top of the preheating chamber, and the primary air, consisting of inner primary air and outer primary air, was located in the center of propane burner nozzles. The pulverized coal burner was mounted on the top of the furnace, which consisted of two outer secondary air nozzles, inner secondary axial vanes, a preheating mixture nozzle, etc. The experimental apparatus was operated in 35 kW thermal load summing of the heat input of coal and that of propane. Pulverized coal was supplied pneumatically from a screw feeder to the preheating chamber. It was directly preheated by the premixed propane burner, and devolatilization and partial oxidation occurred in this preheating chamber. The residence time of pulverized coal in the preheating chamber was near 0.7 s. The produced mixture was introduced into subsequent drop furnace via the coal burner. At the top of the drop furnace, the inner secondary air was introduced by axial vanes, and the outer secondary was introduced by two straight pipes located equidistantly on a pitch circle around the inner secondary air. The close coupled over fire air (CCOFA) and the separated over fire air (SOFA) are shown in Figure 1 and their relative positions are listed in Table 2 (presented later in this work). Of course, if preheating (GFCP) was not used, coal particles would traverse the preheating chamber and then be devolatilized and oxidized in the drop furnace. To avoid condensation, samples were withdrawn from the furnace with a stainless steel air-cooled probe, and the dust filter and the gas
X=
x L
(1)
where x is the distance between the sampling point of interest and the burner mouth and L is the total length of the drop furnace. These relative positions of furnace are listed in Table 2. Table 3 gives the experimental conditions. The total stoichiometric ratio is λ = 1.3, ∼5% O2 in the flue gas. The primary air for these runs with GFCP included propane combustion air accounting for ∼10% of the total air flow, and ∼5% of the total value directly devoted to accompany coal suspension. Therefore, the primary air rate with GFCP was similar to those runs without GFCP at the preheating mixture nozzle. The total coal residence time in the furnace was also listed for all the runs. The primary air rate of these runs with GFCP included propane combustion air, which account for ∼10% of the total air flow. 6090
dx.doi.org/10.1021/ef501175v | Energy Fuels 2014, 28, 6089−6097
Energy & Fuels
Article
of the entire system for case (a)-1, ∼12% for case (a)-2, ∼9% for case (a)-3, and ∼6% for case (a)-4. In addition, the mass flow rate of pulverized coal remains constant all the time. As shown in Figure 3a, the temperature peaks are located at the center of the preheating chamber, and this indicates that devolatilization and partial oxidation occurred in this preheating chamber. In the pretests with good thermal insulation, when the preheating chamber temperature was higher than 750 °C, the temperatures that could be maintained even rise when the premixed propane burners were enclosed. The heat released from the propane burners accounted for
