1. Introduction
Using biomass as fuel for power plants is well accepted by many countries due to fossil fuel depletion and global warming. Indonesia, one of them, plans to reduce the proportion of fossil fuel power plants by 2025 and increase the proportion of biomass power plants. [References] As a fuel for power plants, various types of biomass such as wood, grain, and MSW(Municipal Solid Wastes) will be used. EFB(Empty Fruit Bunch), a kind of biomass, which occurs in palm oil production in Indonesia, seems to be the most suitable fuel. Because Indonesia is the largest producer of palm oil, it produces a large amount of EFB. Therefore, fuel prices are lower than other biomass, and it is easy to supply and secure fuel continuously. However, not all the EFB is suitable as fuel for power plants except when properly dried. Moisture content of biomass like EFB is typically 60%-70%. [Reference] this highly moisture content causes many problem such as lowering combustion temperature and stability of burning, higher CO and VOC emissions, difficulty of boiler operation. In addition the boiler efficiency is reduced by increasing the heat loss of the boiler such as flue gas loss, chemical unburned carbon loss, and mechanical unburned carbon loss. [Reference]
Having multiple disadvantages using raw-biomass as fuel for power plant can be solved by removing moisture from biomass. Drying method is broadly divided into mechanical drying and thermal drying. Mechanical drying methods can reduce moisture by up to 50 wt% through shredding, grinding, pressing and filtering. Thermal drying with direct or indirect dryer is used to lower the moisture to less than 50 wt%. Thermal drying requires large energy and cost because moisture have high specific heat. Recently drying processes have been integrated with power plants to increase energy efficiency and reduce costs. To dry the feedstock directly or indirectly, the waste heat and steam discarded from the plant are used appropriately.
The lower the moisture content of the biomass, the higher heating value and the boiler efficiency, but the energy and cost required for drying also increase. So it is important that determination of optimum drying level by trade-off among higher heating value of biomass, boiler efficiency, energy input and dryer cost. In many literature, the optimum moisture content of biomass is 10-20%. [References] Gebreegziabher et al. Studied the trade-off between drying level, dryer cost, energy consumption and boiler efficiency, When the heating value is limited to 15 MJ / kG (moisture content 17%), the operating conditions are mainly focused on drying temperature and particle size. Few discussions on how to obtain the optimum moisture content in a true sense. In this study, we have modeled a 10-MW EFB power plant incorporating economic evaluation. Through the process model, optimal drying conditions and optimal moisture content were determined by considering operating conditions such as drying temperature, drying time, and steam recirculation ratio. Even if the moisture content of the dried EFB was the same, the efficiencies could be different if they were reached under different drying conditions.
2 Process flow diagram description
A 10-MW small-scale biomass power plant under construction in Indonesia was simulated. The overall process consists of a shredding process, a drying process, a boiler, and a steam cycle (Figure 1). The amount of dried EFB to generate 10-MW will be less than raw-EFB, since the dried EFB increase the heating value and boiler efficiency. In other words, simulation was designed to vary the amount of EFB to 10-MW power generation. The process flow proceeds as follows.
1. EFB of 60% moisture content is finely crushed by shredder to 5mm size, Moisture content is lowered to 48%. EFB at 48% moisture content enters the hot air rotary dryer.
2. The air entering the rotary dryer rises the temperature by heat exchange with part of the steam coming out of the turbine. The heated air comes into direct contact with the EFB and the inside of the dryer, and moisture is evaporated through the material and heat exchange, reducing the moisture content of the EFB to 20%. Air and EFB are supposed to ideally mix well. The steam(191 ° C, 12atm) used for drying is fully condensed (188 ° C, 12atm) after preheating the air and the condensed water enters the Boiled Feed Water Tank (BFWT) to preheat the water.
3. The dried EFB enters the boiler and burns and the flue gas is discharged at 200 degrees Celsius. The heat generated by the burning of the EFB results in a steam of 433 degrees Celsius 60 atm.
4. The high-temperature and high-pressure steam is discharged at 0.107 atm after turning the turbine and produces 10 MW of electricity. A portion of the steam coming out of the turbine extraction valve is used for drying. The VLP discharged at 0.107atm recirculates the steam cycle when it is fully condensed by the condenser.
2.1 properties of EFB
As the results of industrial and elemental analyzes are required to model the biomass combustion reaction in Aspen Plus, industrial and elemental analyzes were conducted on EFB. (Fig. 2) If we know the water content of EFB and the high calorific value at that time, The high calorific value according to the water content can be estimated by AspenPlus (Figure 3) Estimation of the high calorific value according to the remaining water content using AspenPlus is very similar to the experimental value. Also, when the constant water content is exceeded, black out area where EFB does not burn occurs. [Consider the quotation]
 
3. Process model
The drying process consists of a direct dryer and an air heater. The material and energy balance for the following units are given below.
3.1.1 Dryer model
The dryer can be classified into co-current and counter-current types depending on the direction in which the solid and air flow. A counter-current dryer can achieve a solid with a lower moisture content. However, when the dryer operates at high temperatures, the driest solids contact the hottest air, which can cause a fire if the solids are flammable. So, co-current dryer was selected in this study because biomass generally has a risk of fire. In the dryer, the solid and hot air are in direct contact with each other, and the water of the solid moves to the air due to the transfer phenomenon. The evaporation process requires a large amount of energy because moisture has a high specific heat. The material and energy balance were calculated assuming that the dryer was in a steady state. Eq. (4) represents the material balance of water with respect to the solid, and Eq. (5) represents the material balance for air. Eq. (4) and Eq. (5) gives a material balance equation for moisture. [Reference] Eqs. (6) to (9) show the energy balance. By integrating the following Eqs (1) ~ (12), the moisture content and temperature can be calculated as the air and solids leave the dryer. If there is a difference from the conventional equation, we have to arbitrarily specify the difference between the outlet temperature of the solid and the air to calculate the material and energy balance. However, if you know Q and the drying rate, the equation below calculates the solid and air outlet temperatures, energy and material balance by Aspen Plus. [Picture presentation]
 
3.2 Drying kinetics
By modeling the drying curve, the limiting moisture content, the equilibrium moisture content and the heat and mass transfer coefficient for EFB in the drying kinetics, Q and the drying rate can be known and the energy consumption required for EFB drying can be predicted.
3.2.1 Drying curve
The drying process goes through several stages of drying. First, the solid is heated by a heat source like hot air. Then the moisture on the surface of the solid is evaporated, which is called the constant rate drying period. The drying rate is also the same regardless of the material if the drying conditions are the same because the moisture on the surface of the material evaporates. When the moisture on the surface of the material is removed, the moisture inside the solid is evaporated. This period is called the falling rate drying period, and the point at which the rate changes from the constant rate drying period to the falling rate drying period is called the critical moisture content. When drying continues, the moisture of the dry air equilibrates with the moisture of the solid and no more moisture evaporates. This point is called the equilibrium moisture content. Since the particle structure inside the material affects the drying rate during the falling rate drying period, the material has different drying curves in the falling rate drying period. In order to more accurately predict the evaporative behavior of EFB, we reflected a falling rate drying curve for the EFB on Aspen Plus.
The critical moisture content depends not only on the material and shape but also on velocity and temperature of the drying air. Since many factors affect the critical moisture content, we used the mean value of the critical moisture content shown in Table. The equilibrium moisture content is influenced by relative humidity and temperature of surrounding air. The equilibrium moisture content is usually close to zero at high temperatures and relative humidity.
3.2.2 Convective heat and mass transfer coefficient
Heat and mass transfer coefficient is calculated by using Eqs. (14) and (15).