1. Introduction
In recent years, the circulating fluidized bed (CFB) technology has developed rapidly because of its advantages in fuel flexibility and load regulation [
1,
2,
3,
4]. The low-temperature combustion inside the furnace also provides sufficient benefits for de-NO
x combustion [
5,
6] and high-efficient desulfurization [
7]. Meanwhile, the operating temperature of the cyclone ranges from 800 to 950 °C, which is very suitable for the selective non-catalytic reduction (SNCR) system. The high-velocity rotation and strong turbulence intensity of flue gas promote the mixing of NO
x with reduction agents. In general, the NO
x emission can be limited below 100 mg/m
3 with SNCR technology [
8,
9]. However, with the increasingly stringent requirements of environmental protection, the conventional treatments have been difficult to meet the updated NO
x emission standard (50 mg/m
3 at 6% O
2) [
10].
Thus, various de-NO
x combustion technologies emerged as the times required, including air-staging [
11], improvement of cyclone efficiency [
12], fluidization state specification [
13], flue gas recirculation [
14] and optimization of urea injection [
15]. Most utility boilers could meet the ultra-low emission standards after retrofit, while the problems of high NH
3 escape and low NO
x-removal efficiency existed in a large number of power plants.
Including but not limited to CFB boiler, many scholars focused on the mechanism of SNCR reactions to improve de-NOx efficiency. Taking the cyclone of a 660 MW CFB boiler as the object, Kang et al. [
16] obtained a simplified 18-element mechanism using CHEMKIN software; the optimal temperature window was then proposed by combining with CFD simulation. When the gas temperature was not sufficient, the NO
x-removal efficiency could also be improved to a certain extent by increasing the ratio of ammonia to nitrogen (NSR). Yao et al. [
17] compared the effect of CO and CH
4 on NO reduction in a tubular reactor with simulated flue gas. At low temperatures, the additive of C
3H
8 was the most efficient in enhancing SNCR process. At pressurized oxy-combustion condition, kinetic modeling was carried out by Rahman et al. [
18] to simulate and optimize the SNCR process. The de-NO
x efficiency increased as the pressure increased from 1 to 10 atm. At high pressure, the generation of NNH showed to be the most promoting reaction. Świeboda et al. [
19] reviewed the application of SNCR technology in pulverized coal-fired boilers and pointed out that the measurement of exhaust gas parameters was the most direct validation for SNCR optimization.
It is generally believed that significant non-uniform input parameters exist in large-scale CFB boilers, including primary air and secondary air [
20,
21], fuel particle dispersion [
22], coal feed distribution [
23,
24], recirculating ash and heat absorption deviation in multi-parallel loops. Therefore, the flue gas composition at furnace outlets should also be uneven. Generally speaking, the smaller the uniformity deviation of NSR distribution is, the higher NO
x-removal efficiency is. On the contrary, the worse the mixing of urea solution with NO
x will be, which not only affects the NO
x-removal efficiency, but also may generate a sharp increase of NH
3 escape in some local areas. Therefore, it is particularly important to obtain the actual NO
x distributions at SNCR inlets for accurate urea injections. In the selective catalytic reduction (SCR) system commonly adopted in pulverized coal boiler, the non-uniform NO
x distribution and flow field can be directly calculated [
25] or even measured [
26] due to low solid concentration and gas temperature. For example, Liu et al. [
27] proposed a prediction-assisted feed-forward to enhance outlet NO
x control; 22 key operating parameters were considered in the MLR (multi-layer perception) method and the original dynamic NO
x emission was predicted successfully.
However, due to stronger thermal inertia and harsher in-furnace processes, it is difficult to dynamically monitor the NO
x distributions at the SNCR inlets of CFB boilers. Thus, on-site measurement shows to be the key means to solve this problem. Niklasson et al. [
28] performed on-line measurements with zirconia cell probe successfully. Hartge et al. [
29] designed novel probes to measure flue gas at various levels inside a 235 MW CFB boiler. The maximum penetration depth reached 3 m. A wall region with a thickness of 0.5 m was observed in the test, and the non-uniformity of gas composition in front and rear walls was attributed to fuel distribution. With a self-made sampling device system, the authors once measured the gas along the horizontal direction of secondary air ports with a maximum sampling depth up to 4 m [
30]. The oxygen distribution and jet penetration at the lower part of the furnace were basically predicted.
The above literature indicates that many studies are available concerning the SCR process or the reaction mechanism during SNCR process, but few are related to the non-uniformity of NO
x concentration at the SNCR entrance of CFB boilers. In particular, relevant field tests have rarely been reported. Nonetheless, the accumulation of the studies in [
28,
29,
31] inspires the scholars to further explore more detailed NO
x distributions in large-scale CFB boilers through the field test approach. Thus, the present contribution reports a preliminary investigation into a typical 300 MW CFB boiler, focusing on measuring the characteristics of NO
x distributions at the SNCR inlets. By comparison with the auxiliary test in the dilute phase, the non-uniformity of NO
x distribution was analyzed in detail. In addition, local regions with ultra-high NO
x concentration were captured at the inclined edge of the SNCR inlets for the first time. Based on the measured NO
x distributions, the regulation of precise urea injections was proposed, which could save 15.7% of the urea consumption while ensuring ultra-low emission of NO
x. It is the first time to achieve the sampling depth up to 7 m, and the detailed two-dimensional distributions of NO
x concentration were obtained. The results provide the lasting source for the improvement of CFB combustor model, and a direct reference for the prediction of NO
x distribution at SNCR entrance to enhance de-NO
x efficiency in large-scale CFB boilers.
4. Conclusions
In this study, field tests with deep penetration sampling were conducted on a typical large-scale CFB boiler. The horizontal and vertical distributions of flue gas composition at each SNCR inlet was successfully obtained, and the differences of NOx concentration were analyzed in detail combined with air/coal distribution as well as one auxiliary test in dilute phase zone. Although some horizontal ports were inaccessible due to limitation of site layout, the measurements provided a complete picture of the NOx distribution characteristics in a large utility CFB boiler for the first time.
The vertical NOx distribution at SNCR inlet was basically consistent with that in the dilute phase zone, which depended mainly on the initial fuel dispersion along depth direction. In addition, the vertical NOx concentration at the SNCR inlet on both sides presented a parabolic distribution but increased along height direction in the middle one. Particularly, some local areas with extremely high NOx concentration (over 2000 mg/m3) were captured near the inclined edge of SNCR inlets, which might be related to the potential reactions in tubes or local uneven combustion and is worthy of further study.
As a practical conclusion, the preliminary regulation of precise urea injection could save 15.7% of urea solution consumption based on the obtained two-dimensional NOx distributions. Follow-up work should be focused on three aspects: (1) the initial distribution of fuel particles over the bed or even without bed materials; (2) applications of the latest SNCR technologies in large-scale CFB boilers, such as new urea-based ammonia-releasing reduction agents from ERC Technik, Selective Cooling and TWIN-NOx® technologies from M&S, acoustic gas temperature measurement and advanced control systems; and (3) more comprehensive measurements in dilute phase zone to verify their relationship with gas distribution at the corresponding SNCR inlet, thus to predict the level of original NOx distribution and the subsequent SNCR process.