我把最近写的论文，讨论部分拿出来，让大家拍砖，拍的越 ...？ 5.1 effect of wet .vs. dry feed fig. 6 shows the temperature distribution of the gasifier using dry coal as feedstock. the maximum temperature in this case is 23 3 0k, but is significantly lowered by about 150k with the wet slurry as feedstock. this decrement can be explained as follows. in the gasifier, coal reacts with oxygen and steam evaporated from the coal slurry and is partially oxidized at high temperature, after which it is converted into multi-component syngas. during this process, much more heat is used to evaporate the steam from the wet slurry and the gasification proceeds at a lower temperature than that with the dry coal as feedstock. table 1 is a summary of the predicted performance for the two- stage gasifier using wet and dry feed stock obtained from simulated results of the aspen plus program . in these two cases , the wet slurry corresponds to the baseline simulation (coal-water slurry containing 70% solids) while the pulverized dry coal employs nitrogen to transport the coal. in both cases , the fuel stream is assumed to be at the same temperature (t = 400k) and the coal specification i s datong coal. from table 1, it can be concluded that: (1) the syngas generated from the dry feed stock has a much higher mole fraction of the combustible component in the syngas, co+h2 is greater than 90%, but lower fraction of h2o and co2, than the wet feedstock . it leads to the heating value of the syngas generated from the dry feed is much greater than that generated from the wet feed stock . it should be emphasized that the fraction of h2o in wet feedstock is 18% higher than in dry feedstock. therefore, the sensible heat recovery process is necessary to be designed reasonably to prevent the latent heat of condensation from losing . furthermore, another problem, such as acid gas erosion in the surface of the pipe or equipment during the processes of start-up and shut-down, should be concerned. now, t he use of a fire-tube boiler for synthesis gas cooling was pioneered during the early development of the e-gas process, and now this proven design provides an additional cost and operations advantage to the e-gas technology. (2) the results of simulation for the two cases are quite similar in cc. the wet feedstock shows a slight reduction compared with the dry feedstock. it can be explained that, there is no high purity n2 ( 99%) in the e-gas gasifier system, it is very difficult to configure recycle process of fly ash same as shell gasifier (dry type) . simultaneously, 22% slurry is injected into the second stage of gasifier, which cooling the raw gas temperature generated in the first stage, therefore, t he cc in wet feedstock is about 2% lower than in dry feedstocks. it is easier to be known that the higher cge is convenient to improve the overall efficiency of igcc power plant, and provide the measures to decrease the size of the sensible heat recovery machine as well as the investment cost. however, the cge in slurry feedstock is 4% lower than in dry feedstock, it is one of the main reason that makes the overall efficiency of igcc plant in e-gas gasifier slightly decreased compared with in shell gasifier (dry type). (3) the experimental results of the gasification temperature, as represented by aspen plus, are the outlet temperature of the synthesis gas, while the simulation results are the maximum temperature in the two-stage gasifier. therefore, the difference between them increases significantly. (4) although the heating value of syngas increased for the dry feed stock , the lower syngas mass flow rate results in a slight reduction in cge. in light of the substantial change in syngas composition between the different feeds tocks , future study should examine the impact of using a dry feed stock on various downstream processes, such as water gas shift, gas clean up and gas turbines. 5.2 effect of system pressure a ltering the system pressure for a fixed geometrical configuration is a useful parametric study to investigate model behavior. however, it is of limited value for practical operation. in the plant design process, the system pressure for the gasifier is determined by the amalgamation of considerations for the various equipment and processes to be used in the igcc plant. once the operating pressure for the gasifier is determined, the gasifier is sized to provide the desired fuel conversion for the anticipated residence time and conditions within the gasifier. thus, in a real operation al gasifier , the steady state pressure should not be significantly altered from the original design condition. however, on the computer such tests can easily be performed by simulation . table 2 presents a summary of the predicted performance for the two- stage gasifier as a function of the system pressure within the gasifier. there is considerable advantage to gasifying under pressure, sufficiently so that practically all modern processes are operated at pressures of at least 10atm and up to as high as 80atm. the reasons for this are saving in compression energy and reduction of equipment size. the pressure in e-gas gasifier is therefore generally selected in accordance with the requirements of the process or equipment upstream or downstream of the gasifier. it is instructive to see how the gas composition is affected by the pressure. the selected pressures are 2 8, 5 0 and 70 atm , which corresponds to the baseline conditions, the approximate system pressure used by current generation coal gasifiers in igcc studies , and the approximate pressure reported for coal gasifiers used at coal-to-chemicals p lants, respectively. as can be seen f rom the table 2 , some conclusion can be reached: (1) t he increase of cge and that cc with increasing gasification pressure can be seen clearly. it can be explained that the slower gas velocities increase the particle residence time, which in turn increases cc. at 70 atm, the model predicts nearly complete fuel conversion. when the gasification pressure increases, the conversion from carbon to co or co2 proceeds keeping the cc and cge improves drastically. (2) in this study the pressure was increased by almost the tolerance limit of 20 atm , but the syngas composition only varied by a few percentage points. the yield of synthesis gas, mainly is co and h2, dropped with increasing pressure, while that of co2, h2o and ch4 increased. these trends can also be explained by the thermodynamic characteristics of gasification. in the principle chemical reactions: (i) generating co in reactions c+0.5o2→co, c+co2→2co and c+h2o→co+h2, and generating h2 in reaction co+h2o→co2+h2, are the volume increasing reactions, while (ii) generating ch4 in reactions c+2h2→ch4, co+3h2→h2o+ch4, and generating co2 in reactions c+o2→co2 and co+0.5o2→co2, are the volume decreasing reactions. the gasification process will shift to the reversed reaction with increasing pressure, which leads to the tendency of syngas variation that is shown in table 2. (3) it is instructive to see how the gasification temperature and gas composition change with pressure, as listed in table 2. the changes of gasification temperature and gas composition varied only to a very limited extent, less than 1%, with changing pressure. therefore, the effect of gasification pressure on the syngas composition can be neglected. even though the gasification pressure has little effect on the gasification results, it determines the gasifier dimensions. from the perspective of the entire system, the gasification pressure is determined by the downstream of the technological requirements, i.e., it reflects the dimension of gasification so that a larger scale needed with a higher pressure. 很多语法都用的不理想，不如前面的句子，下句想变换一种表达方式，但是不会。 [ ]查看更多
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