1、- 1 - 中文 3132字 英文原文 Limestone Effects on NOx Formation in CFB Combustors Abstract Circulating Fluidized Bed (CFB) combustion technology has been widely used in power generation with the considerations of its advantages in economically controlling SO2 and NOx emissions. However, it is found that NOX
2、emission increased up to 30% when limestone is added into the combustor for desulphurization, especially with rather high Ca/S ratio. The phenomenon of NOx augment was discussed based on associated mechanisms and chemical kinetics. The catalytic oxidization effect, preferential conversion effect fro
3、m HCN to NH3, and reduction effect on NOx were analyzed. It is found limestone can act as a catalyst, increasing the reaction rates for the reaction associated with NOx formation. Furthermore, CaO decomposed CaCO3 favors the conversion of HCN to NH3 when they are release in de-volatilization process
4、. Due to preferential oxidization of NH3 to NO rather than the unstable N2O, NOX emission increases with limestone injection. When high Ca/S is used, the reduction effect becomes dominated and reduces the overall NOX emission. Keywords CFB, limestone, desulphurization, NOX formation, chemical kineti
5、cs 1. Introduction Circulating Fluidized Bed (CFB) combustion technology has been widely used in power generation, because of its advantages in such as the high fuel flexibility for burning various kinds of coals and the high feasibility in economical emission control. Given that the combustion temp
6、erature in CFB combustors, e.g., boilers, is usually between 10201120K and secondary air can be injected at different ports along the axial direction, the NOX concentration in the flue gas can be controlled to be much lower than those using other combustion technologies. Shown in Fig. 1, NOx emissio
7、n from a CFB combustors are lowest, in the range of 100-220 ppm, compared with other kinds of combustors 1. - 2 - Fig. 1 NOX emissionlevel for different combustion systems 3 When coals with high surface content are burned, limestone is added into CFB boilers for desulphurization. Within the normal t
8、emperatures, the efficiency of limestone desulphurization can be up to 90% 1. In addition to the desulphurization, limestone may play several other positive roles in improving the boiler performance of such as combustion, heat transfer, material balance and ash separation in the separator. However,
9、observed in practical operation and experiments, limestone addition in CFB combustors might cause a negative effect on the NOX emissions. Figure 2 depicts the NOx emission of a commercial CFB boiler with limestone desulphurization, operating with bed temperature between 1150K and 1200K with Ca/s rat
10、io of 2.2. It can be seen, when limestone is injected, while N2O emission was little affected, the NO remarkably increased by 50 ppm or about 30%2. Figure 3 further depicts some experimental results of the influence Ca/S ratio on the NOx emission 3. When Ca/S ratio changes is smaller than 2, the NOX
11、 concentration increases with the Ca/S ratio. When Ca/S ratio is higher than 2, the NOX concentration decreases with the Ca/S ratio. Though the mechanisms for NOx formation in homogeneous reactions are rather clear, the studies on the mechanisms for NOx formation in heterogeneous atmosphere, especia
12、lly with the presence of limestone, are limited. - 3 - Fig.2 NOX emission with/without Fig.3 NOX concentration with differentlimestone desulphurization Ca/S ratios (Tb=1165K, O2=6% ) 2. NOx formation mechanisms and chemical kinetics in a CFB combustor There are mainly three well-known mechanisms cou
13、nting for the NOX formation in coal combustion 4: (1)Extended Zeldovich ( or thermal ) mechanism in which O, OH, and N2 species are in equilibrium values and N atoms are in steady state. (2) Prompt Mechanisms where NO is formed more rapidly than predicted by the thermal mechanism above, either by (i
14、) Fenimore CN and HCN pathways, or (ii) the N2O-intermediate route, or (iii) as a result of non-equilibrium concentrations of O and OH radicals in conjunction with the Extended Zeldovich scheme. (3)Fuel Nitrogen Mechanism, in which fuel-bound nitrogen is converted to NO. Apparently, for a CFB combus
15、tor, neither the Extended Zeldovich mechanism which depends strongly on temperature and only becomes significant when temperature is above 1750K, nor the Prompt mechanism which becomes significant only with abundant CHi radical and low O2 concentration is important. The dominate mechanism for NOX fo
16、rmation in CFB combustor is fuel nitrogen mechanism. Fuel Nitrogen mechanism is rather complicated. Part of nitrogen in coal is usually released in the form of HCN and NH3 as volatile nitrogen, and the rest remains as fixed nitrogen during the de-volatilization process. The ratio of volatile nitroge
17、n to fixed nitrogen, as a result of de-volatilization, depends on coal type, temperature and heating rate of coal particles 5. Normally, the oxidization of volatile nitrogen occurs in homogeneous gas-phase reactions immediately after de-volatilization, while the oxidization of fixd nitrogen in heterogeneous gas-solid reactions along with fixed carbon combustion6, 7.
