to calculate Lewis, Schmidt and Reynolds number are obtained through stream analysis of Aspen Plus. The diameter L in the original mass transfer coefficient and Reynolds number equation was replaced by the sauter mean diameter and the median particle diameter of EFB, respectively. As shown in Eqs. (14) ~ (21), it can be seen that the heat, mass transfer coefficient and evaporation are better at higher drying temperature, velocity and smaller particle size. The air velocity in the dryer was typically 0.5 m / s to 1.5 m / s, and the mean value was used to calculate the Reynolds number.
3.6 Net power generation
Net power generation is to subtract work consumed by pump, blower from work produced by turbine.
3.7 Integration of economic evaluation
Since the equipment cost is influenced by various factors such as usage, capacity, and materials, it is necessary to roughly estimate the equipment cost using equation. Total capital investment in the power plant neglected labor costs, installation costs, transportation costs, and pipeline installation costs except for some equipment costs. The equipment cost only considers devices such as Dryer, Shredder, and Furnace that are affected by the amount of EFB. Turbines are not considered because they are the same at 10 MW power generation, and detailed things like a pump blowers are ignored. The year in which the equipment cost estimate for dryers and shredders was developed was 2003 and the furnace was developed in 2002. Eqs 28 ~ 30 reflect the cost index in 2017.
3.7.1 Dryer cost
The cost of the dryer is affected by the area. Knowing the mass flow rate and drying time of the EFB, before estimating the dryer area, can estimate the volume of the dryer. It is assumed that the volume occupied by the EFB in the dryer is 20% and the density of the EFB is 500 kg / m3. If the EFB does not shrink during the drying process, the volume occupied by the EFB in the dryer is Eq. 24.
When the dryer operates at a load of 20%, the volume of the dryer is calculated by Eq. 25. The L / D ratio of the dryer should be known to determine the area of the dryer. Typical rotary dryers have an L / D ratio of 4-10. The L / D ratio was selected as the average value of 7. The diameter of the dryer can be expressed in equation of volume like Eq. (26), and then the area is calculated by Eq. (27) [dryer cost]
3.7.2 Shredder cost
The shredder which can be crushed by 5mm size is hammer mill. The hammer mill is related to the mass flow rate of the EFB and the equipment cost is estimated Eq. 28
3.7.3 Furnace cost
Ignoring detailed cost such as piping, steam drum, soot blower, fan, deaerator, and pump included in the boiler, only combustion was considered. The furnace cost is varied by the heat transfer rate and can be estimated by Eq. (30).
In this study, we only considered the fuel cost of EFB including the transportation cost of EFB as the operating cost except maintenance, labor, drug, and reprocessing costs necessary to operate the remaining power plants. EFB fuel cost is $ 14.08 / ton.
4. Objective function
Considering depreciation as 20 years of life for a typical power plant, annual equipment cost is estimated as Eq. (31). Annual operating costs are calculated by Eq. (32) that multiplies the EFB fuel cost by the annual usage. The objective function was to minimize the annual total cost (AC) that annual equipment costs plus annual operating costs.
Depreciation cost =
Annual working expenditure = 14.08$/ton*(annual usage)
AC = Depreciation cost + Annual working expenditure
5. Results
A case study was conducted by changing the steam recirculation ratio and drying time to find the optimum point. In all cases, EFB of 60% moisture content was applied by shredder to the condition that the particle size was 5mm and moisture content was 48%. When the EFB from the shredder is not dried, the base case is set to produce 10 MW of power.
5.1 Case study result except economic evaluation
Case study, without consideration of capital cost and operating cost, only compared fuel consumption for the same 10MW power generation. The results for the case study are shown in Table. 3. Figure 4 shows a three-dimensional graph of fuel consumption for the same power generation of 10 MW when the steam recirculation ratio is varied from 0 to 0.9 and the drying time is varied from 1 to 60 minutes. In the base case, the hot air mass flow rate is zero. This is because the steam recirculation ratio used for drying is zero. And to produce 10MW electric power, EFB of 60% moisture content was demanded 19,851kg / hr. The amount of EFB from the shredder was 15,270 kg / hr. Even without the energy used in the drying process, the amount of work required to operate the steam cycle was 95 kW. In case 1, 2 and 3, the change of the parameter value according to the steam recirculation ratio was compared at the same drying time, and in case 4, 5 and 6, the parameter values according to the drying time change were compared at the same ratio. Table. 3, the relative efficiency is the difference between the fuel consumption of the base and the other case. In cases 1, 2 and 3, the efficiency increases gradually as the ratio increases. In case 2, the efficiency becomes maximum at 8.15%. After Case 2, the efficiency decreases gradually and becomes -0.63% lower than Case 3 without drying. In other words, Case 3 uses too much steam for drying. Comparing Case 4, 5, and 6, it can be seen that efficiency increases as the drying time becomes longer. Because it can evaporate moisture from the EFB without additional energy consumption until the relative humidity of the air reaches 99%. Therefore, in the three-dimensional graph of figure 4, the point with the lowest fuel consumption was at the point where the drying time was 60 minutes and the steam recirculation ratio was 0.26. However, since the increase in drying time is much greater than the amount of EFB fuel reduction after a certain point, the cost of the dryer will increase as a result. The other change is that the longer the drying time, the smaller the difference between the EFB and the outlet temperature of the air.
5.2 Case study result including economic evaluation
We have conducted a case study to reflect the equipment and operation costs and compared it with the drying process which power plant currently being constructed in Indonesia. This drying process reduce moisture content of EFB from 48% to 20%. figure. 5 is the objective function graph for ratio and time. The point at which the objective function represents the smallest value is the optimal point. In the optimal point, the steam recirculation ratio, drying time, and moisture content of the dried EFB were 0.25, 22 min, and 9.79%, respectively. Although there were a little differences according to the drying time, most of the cases with steam recirculation ratio above 0.38 were not feasible to dry. The yellow dot in figure 5 shows the points where the moisture content of the dried EFB is 20%. Among them, the red and green dots represent the worst and the best conditions, respectively, which can reach a moisture content of 20%. The black dot represents the best condition among the conditions that can reach the moisture content of 15, 25, 30, and 35%, and the blue dot is the minimum value of objective function. Table 4 shows the data for each point. The moisture content of Case 1,6 was almost the same moisture content under different conditions, but showed a large difference in the cost savings. It does not matter how much moisture is dried, and it is important to know how to dry it. Cases 1, 2, 3, 4, and 5 select the best drying conditions at each moisture content. In terms of water efficiency, 9.79% EFB is the most cost-effective, but it does not show any significant difference in cost savings even when using EFB, which is 10-20%, as fuel for the power plant. From 20% or more, it can be seen that the difference in the cost saving rate becomes so large that it can not be ignored.
6. Conclusion
The EFB 10MW power plant process model created by Aspen Plus was used to optimize drying process operation. The model also considers the drying kinetics and material balance and energy balance of the dryer depending on the material properties of EFB. The model balanced the balance between steam, drying time, capital investment and operating costs used for drying. The optimal drying conditions were found to be 25, 22min, and 9.79%, respectively. Also the objective function graph shows that the same moisture content can be obtained, it can be seen that the cost savings can be different if the drying condition are different. This indicates that how to dry it is more important than how much to dry it. Another observation is that it was an optimal point when the water content was 9.79%, but there is no big difference between drying and 15 ~ 20%. so