CHAPTER 7 Numerical analysis of lignite drying in superheated steam
7.5 Influence of variability of fundamental parameters of superheated steam drying process
7.5.2 Sensitivity analysis of the drying kinetics in regard to process parameters
126
Figure 7.12 Numerically obtained correlation of drying rate and moisture content for 10 mm samples of Turow and Belchatow lignite dried at test temperature of 150 oC
127
The sensitivity analysis was performed mainly in regard to material properties of lignite, as listed in Tab. 6.1, however to obtain the proper overview, the factors independent from the material or even the object were taken into consideration as well.
The full list of parameters subject to investigation is presented in Tab. 7.9. The numbers gathered in the table are base (reference) values of the parameters. In the simulation attempts, each quantity was gradually increased by 10%, 30% and 50% at once, with other parameters remaining fixed (ceteris paribus rule). As a result, four sets of simulated drying characteristics were produced. The only exception concerned temperature of superheated steam. For analyzing the variability of other parameters, its value was fixed at 150 oC. However, the investigation of temperature modification itself was performed with reference value of 110 oC. Under such condition, all applicable cases fall into the range of actual experimental settings, with 165 oC as a maximum.
Table 7.9 Parameters used in sensitivity analysis of the numerical model
parameter reference value origin type
density (DBC) ρc 1536 kg m-3 averaged experimental
values
material properties initial water share WS0 51.25 mass%
thermal conductivity (DBC) λc 0.33 W m-1 K-1 commissioned research specific heat (DBC) cc 1246 J kg-1 K-1
apparent free water
transfer coefficient D 3×10-9 m2 s-1 empirical value sample diameter d 0.01 m
experimental conditions
extensive property superheated steam temperature Ta 150 (110) oC external parameter
128
The analysis was done basing on the time charts involving correlations of the moisture content, drying rate and temperature profile, similarly as in basic simulation.
In this particular part, each of the characteristic drying quantities was illustrated using a reference curve and three auxiliary curves, representing the increased values of a particular parameter.
The indicators selected for quantitative evaluation of the simulation outcome were the following quantities: drying time tdry, peak drying rate -(dX/dt)max, time of free water removal tfree and the maximum difference in temperature between center and surface of the sample ΔT1,51,max.
Fig. 7.13.1.A shows the influence of density of lignite on the drying kinetics.
Among major factors related to the density of coal, ash content may be distinguished.
Generally, significant content of mineral compounds is correlated with elevated value of specific weight. That observation is connected with relatively low density of lignite’s substantial ingredients: carbon, volatile matter and water in comparison to metal oxides that ash is mostly composed of. Still, the precise estimation of coal density should also take the ash composition into account, as normally several chemical compounds constitute its ash content [101]. An slight decline in the drying rate and a minor postponement of free water removal (from 33 to 36 minutes) is observed following the increase in density. Contrarily, the difference of surface and center temperature rises to 21.7 oC from the initial 20.6 oC. The total time of drying is extended by around 1/6 of the reference value, from 67.2 to 77.8 minutes.
In Fig. 7.13.2.B the influence of the initial water share on the process is exhibited. Note that curves of +30% and +50% equivalent to WS0 at the level of 66.6%
and 76.9%, respectively, may only have the theoretical meaning, because water share in lignite rarely exceeds 65%. The simulated incline in the water share, though, induces a
129
considerable rise in the peak drying rate. As the larger amount of water needs to be evaporated from the lignite, tfree exhibits approximately direct proportion to the initial water share. In the last cases, the temperature difference escalates to 29.9 oC.
The sensitivity analysis in regard to thermal conductivity as a parameter was presented in Fig. 7.13.2.C. The modifications of the reference value do not produce any changes in the top level of drying rate. The removal of free water and the termination of the process are remarked insignificantly sooner, as the transfer of heat through the sphere is simulated at a slightly higher, especially in the late stages of drying, when the dry part of coal possesses a greater share in the entire particle. The indicator most sensitive on thermal conductivity modification is the temperature gradient.
Abovementioned increase in heat transfer rate reduces the difference of temperature between the extreme layers. The values of this indicator were computed at 20.6, 19.7, 18.2 and 16.9 oC for base and auxiliary cases, respectively.
The impact of specific heat alterations was presented in Fig. 7.13.3.D. The rise in value of this property, even as high as 50% in regard to reference case, does not influence the drying indicators in a significant way. For instance, the drying time extends by around 8% in the maximal specific heat scenario. It should be taken into account even in this extreme case, the specific heat of dry coal (1869 J kg-1 K-1) is below half of the corresponding parameter of water (4180 – 4210 J kg-1 K-1 within the analyzed temperature range). This is the reason that the larger influence on heat accumulation during drying is exerted by wet phase of a lignite particle.
Among material properties analyzed in terms of sensitivity analysis, the apparent free water transfer coefficient, D, can be listed as well. Its value is related to the complexity of porous structure within lignite. The drying kinetics with regard to this parameter as a variable are exhibited in Fig. 7.13.3.E. The values of the indicators
130
remain virtually invariable, what is also reflected on their graphical representation. The instance of free water removal completion, which is the only indicator subject to any change, for subsequently increased value of D occurs 1%, 3% and 4% earlier than in the reference case. It can be explained by higher efficiency of free water transport to the vicinity of sample’s surface. The temperature of shallow layers is generally higher than the sample core, what elevates the rate of evaporation relating to the water stored inside and thus reduces the time required for free water removal.
Figure 7.13.1 Numerical simulation of drying parametrized for: A) density
131
Figure 7.13.2 Numerical simulation of drying parametrized for:
B) initial water share, C) thermal conductivity
132
Figure 7.13.3 Numerical simulation of drying parametrized for:
D) specific heat, E) apparent water transfer coefficient
133
Besides investigating the material properties, that can be classified as intensive parameters of the process, additional quantities independent from the structure of lignite were taken into consideration. The correlation of drying kinetics and sample diameter as well as superheated steam temperature have been previously discussed in a number of studies, in regard to experimental attempts [85,88,105] and relying on results of the numerical simulation [86,106]. Those studies, though, involved a range of experimental condition, as described in chapter 4. In this section, the base values were set for particle diameter and steam temperature at 10 mm and 110 oC, respectively. The pattern of stepwise increase was assumed the same as for material properties.
Drying curves for samples of various diameters are presented in Fig. 7.14.1.A, exhibiting strong dependence of size on the drying behavior. The rate of increase in the case of tfree and tdry in comparison to the base curve is similar. For the 11 and 13 mm scenarios both values are higher than reference case by 14% and 37%, respectively.
Only for the largest sample simulation, a small disparity was observed, evinced by increase of times required for the removal of free water by 65% and for the completion of drying by 62%. The drying rate achievable during CDRP was simulated at 3.52 s-1 in the case of 15 mm object, which is nearly 40% lower than base value. The least considerable impact of diameter incline concerns the extreme temperature difference. Its rise was simulated at 5%, 15% and 24%, respectively.
Fig. 7.14.2.B visualizes the influence of superheated steam temperature enhancement. Due to large disparity of analyzed instances in terms of drying time, the abscissae axis is limited to initial 100 minutes of the process. The ratio of peak temperature differences in extreme cases exceeds 16 (29.2 oC against 1.8 oC), which is the most considerable change among all indicators. The significant rise, regarding the period when the peak value of drying rate occurs as well as the value itself is also
134
observed. The predicted time required for the completion of drying is 329, 172, 79 and 50 minutes for 110, 121, 143 and 165 oC of superheated steam temperature, respectively.
On the whole, the degree to which variability of material properties influences drying kinetics is diversified. The most important in these terms is the total share of water within coal, followed by density and thermal conductivity. Regarding parameters independent from the material, the simulation of superheated steam drying displayed high sensitivity to changes both in object size and drying medium temperature. For more details, refer to [107].
Figure 7.14.1 Numerical simulation of drying parametrized for: A) sample diameter
135
Figure 7.14.2 Numerical simulation of drying parametrized for:
B) superheated steam temperature