1、 大连交通大学 2009 届本科生毕业设计外文翻译 1 Controlling the Furnace Process in Coal-Fired Boilers The unstable trends that exist in the market of fuel supplied to thermal power plants and the situations in which the parameters of their operation need to be changed (or preserved), as well as the tendency toward the
2、economical and environmental requirements placed on them becoming more stringent, are factors that make the problem of controlling the combustion and heat transfer processes in furnace devices very urgent. The solution to this problem has two aspects. The first involves development of a combustion t
3、echnology and,accordingly, the design of a furnace device when new installations are designed. The second involves modernization of already existing equipment. In both cases,the technical solutions being adopted must be properly substantiated with the use of both experimental and calculationstudies.
4、 The experience Central Boiler-Turbine Institute Research and Production Association (TsKTI) and ZiO specialists gained from operation of boilers and experimental investigations they carried out on models allowed them to propose several new designs of multifuel and maneuverablein other words, contro
5、llablefurnace devices that had been put in operation at power stations for several years. Along with this, an approximate zero-one-dimensional, zonewise calculation model of the furnace process in boilers had been developed at the TsKTI, which allowed TsKTI specialists to carry out engineering calcu
6、lations of the main parameters of this process and calculate studies of furnaces employing different technologies of firing and combustion modes . Naturally, furnace process adjustment methods like changing the air excess factor, stack gas recirculation fraction, and distribution of fuel and air amo
7、ng the tiers of burners, as well as other operations written in the boiler operational chart, are used during boiler operation.However, the effect they have on the process is limited in nature. On the other hand, control of the furnace process in a boiler implies the possibility of making substantia
8、l changes in the conditions under which the combustion and heat transfer proceed in order to considerably expand the range of loads, minimize heat losses, reduce the extent to which the furnace is contaminated with slag, decrease the emissions of harmful substances, and shift to another fuel. Such a
9、 control can be obtained by making use of the following three main factors: (i) the flows of oxidizer and gases being set to move in the flame in a desired aerodynamic manner； (ii) the method used to supply fuel into the furnace and the place at which it is admitted 大连交通大学 2009 届本科生毕业设计外文翻译 2 theret
10、o； (iii) the fineness to which the fuel is milled. The latter case implies that a flame-bed method is used along with the flame method for combusting fuel.The bed combustion method can be implemented in three design versions: mechanical grates with a dense bed, fluidized-bed furnaces, and spouted-be
11、d furnaces. As will be shown below, the first factor can be made to work by setting up bulky vortices transferring large volumes of air and combustion products across and along the furnace device. If fuel is fired in a flame, the optimal method of feeding it to the furnace is to admit it to the zone
12、s near the centers of circulating vortices, a situation especially typical of highly intense furnace devices. The combustion process in these zones features a low air excess factor ( 1) and a long local time for which the components dwell in them, factors that help make the combustion process more s
13、table and reduce the emission of nitrogen oxides . Also important for the control of a furnace process when solid fuel is fired is the fineness to which it is milled； if we wish to minimize incomplete combustion, the degree to which fuel is milled should be harmonized with the location at which the
14、fuel is admitted into the furnace and the method for supplying it there, for the occurrence of unburned carbon may be due not only to incomplete combustion of large-size fuel fractions, but also due to fine ones failing to ignite (especially when the content of volatiles Vdaf 20%). Owing to the poss
15、ibility of pictorially demonstrating the motion of flows, furnace aerodynamics is attracting a great deal of attention of researchers and designers who develop and improve furnace devices. At the same time, furnace aerodynamics lies at the heart of mixing (mass transfer), a process the quantitative
16、parameters of which can be estimated only indirectly or by special measurements. The quality with which components are mixed in the furnace chamber proper depends on the number, layout, and momenta of the jets flowing out from individual burners or nozzles, as well as on their interaction with the f
17、low of flue gases, with one another, or with the wall. It was suggested that the gas-jet throw distance be used as a parameter determining the degree to which fuel is mixed with air in the gas burner channel. Such an approach to estimating how efficient the mixing is may to a certain degree be used
18、in analyzing the furnace as a mixing apparatus. Obviously, the greater the jet length (and its momentum), the longer the time during which the velocity gradient it creates in the furnace will persist there, a parameter that determines how completely the flows are mixed in it. Note that the higher th
19、e degree to which a jet is turbulized at the outlet from a nozzle or burner, the shorter the distance which it covers, and, accordingly, the less completely the components are mixed in the furnace volume. Once through burners have advantages over swirl ones in this respect. 大连交通大学 2009 届本科生毕业设计外文翻译
20、3 It is was proposed that the extent to which once through jets are mixed as they penetrate with velocity w2 and density 2 into a transverse (drift) flow moving with velocity w1 and having density 1 be correlated with the relative jet throw distance in the following way Where ks is a proportionality
21、 factor that depends on the pitch between the jet axes (ks= 1.51.8). The results of an experimental investigation inwhich the mixing of gas with air in a burner and then in a furnace was studied using the incompleteness of mixing as a parameter are reported in 5. A round once through jet is intensiv
22、ely mixed with the surrounding medium in a furnace within its initial section, where the flow velocity at the jet axis is still equal to the velocity w2 at the nozzle orifice of radius r0.The velocity of the jet blown into the furnace drops very rapidly beyond the confines of the initial section, an
23、d the axis it has in the case of wall-mounted burners bends toward the outlet from the furnace. One may consider that there are three theoretical models for analyzing the mixing of jets with flowrate G2 that enter into a stream with flowrate G1. The first model is for the case when jets flow into a
24、free space (G1= 0),the second model is for the case when jets flow into a transverse (drift) current with flowrate G1 G2， and the third model is for the case when jets flow into a drift stream with flowrate G1G2. The second model represents mixing in the channel of a gas burner, and the third model
25、represents mixing in a furnace chamber. We assume that the mixing pattern we have in a furnace is closer to the first model than it is to the second one, since 0 G1/G2 1, and we will assume that the throw distance h of the jet being drifted is equal to the length S0 of the free jets initial section.
26、 The ejection ability of the jet being drifted then remains the same as that of the free jet, and the length of the initial section can be determined using the well-known empirical formula of G.N. Abramovich 6 ：S0= 0.67r0/a, (2)where a is the jet structure factor and r0 is the nozzle radius. At a =
27、0.07, the length of the round jets initial section is equal to 10 r0 and the radius the jet has at the transition section (at the end of the initial section) is equal to 3.3 r0. The mass flowrate in the jet is doubled in this case. The corresponding minimum furnace cross-sectional area Ff for a roun
28、d once through burner with the outlet cross-sectional area Fb will then be equal to and the ratio Ff/Fb20. This value is close to the actual values found in furnaces equipped with once through burners. In furnaces equipped with swirl burners, a= 0.14 and Ff/Fb10. In both cases, the interval between the burners is equal to the jet diameter in the transition section d tr , which differs little from the value that has been established in practice and recommended in 7. The method traditionally used to control the furnace process in large boilers consists of
