(20)
3.3 Air heater drying air temperature optimization
As shown in Fig.2, the air exchanges heat with the part of steam coming out of the turbine. Air and steam flow counter-current to get the maximum heat from the steam. Air is 191 ° C and 12atm of steam is completely condensed, the heat is obtained at 188 ° C and 12atm when it exits the heat exchanger. The temperature of the hot air leaving the heat exchanger can’t exceed 191 ° C, the temperature at which the steam enters and leaves. Specific heat with temperature of the air is calculated by Aspen Plus.
3.3.1 Drying air temperature optimization
The temperature of the air exiting the heat exchanger depends on the air flow rate. The possible temperature range of hot air is 30 ° C to 191 ° C. As shown in Figure 3, the hotter air took a shorter time to get a lower moisture content of EFB. Therefore, it can be seen that the most efficient drying air temperature is the highest in the possible temperature range of air.
The highest temperature among the available temperature range is 191 ° C, but the highest temperature the air can reach is limited at 188 ° C in consideration of the heat loss of the heat exchanger. If the steam recirculation ratio increases, the flow rate of dry air also increases to match 188 ° C. In other words, the flow rate of dry air is designed to vary with the steam recirculation ratio.
3.4 Steam cycle model
In the steam cycle shown in fig 1, the feed water at 47 ° C and 2 atm rises to 12 atm by pump 1 and enters the feed water preheat tank. In the tank the feed water is preheated by completely condensed water by heat exchange with air. The preheated water rises to 67 atm by the pump 2. Next, the high-pressure water enters the boiler, and steam of 433 ° C and 60atm is produced. The high-temperature, high-pressure steam turns the turbine and part of it recirculates to heat the air. After producing 10MW electricity, the steam coming out of the turbine is discharged at 47 ° C, 0.107atm and is completely condensed by the condenser. The condensed water is returned to the pump at 47 ° C at 2 atm and recirculated.
3.5 Boiler efficiency
The boiler efficiency varies with the moisture content of the EFB. We reflected the boiler efficiency according to the moisture content of the wood in process model.