Reduction in Fuel Consumption in Biomass-Fired Power Plant Using Hybrid Drying System

Abstract

Fuels used in biomass power plants usually have high moisture contents. Two methods of fuel drying that have been proposed are steam drying and flue gas drying. Steam drying requires extracted steam as its energy source, whereas flue gas drying requires flue gas leaving the boiler as its energy source. Previous works have mostly been concerned with the integration of either dryer in a power plant. There have been a few investigations on the integration of both dryers. This paper proposes a novel hybrid drying system that uses a steam dryer to dry a portion of the fuel. Exhaust vapor from the steam dryer is then used for the heating of combustion air, which increases the flue gas temperature. The higher flue gas temperature increases the potential of the flue gas dryer, which is used to dry another portion of the fuel. It is shown that the hybrid drying system is capable of reducing fuel consumption to 7.76% in a 50 MW power plant. Furthermore, the integration of hybrid drying is shown to be economically justified because the simple payback period is 4.28 years.

1. Introduction

A thermal power plant converts the chemical energy of a fuel to electrical energy. The net power plant efficiency is the ratio of the net electrical energy output to the chemical energy. The primary objective of the thermal design of a power plant is to maximize the net efficiency subject to a set of constraints. The net efficiency of a power plant depends on boundary conditions of the power plant. Increasing the steam pressure, increasing the steam temperature, or decreasing the condensing pressure can increase the power plant efficiency. For a given set of boundary conditions, there are two methods of increasing the power plant efficiency: regeneration and heat recovery. Regeneration uses internal heat exchange between processes in the power plant to reduce the requirement of external heat input. Heat recovery is the use of the thermal energy of flue gas before it leaves the power plant. In fact, the flue gas temperature has to be minimized in order to maximize the power plant efficiency.

In the thermal design of a power plant, a commonly used method of regeneration is feed water heating. Heat is transferred from extracted steam to feed water in this method. An air heater and economizer are conventional heat recovery devices that decrease flue gas temperature. There are limits to the conventional methods of regeneration and heat recovery. The feed water temperature cannot be larger than the saturation temperature because water vaporization is detrimental to the downstream pump. This means that there is an upper limit to the extracted steam flow rate in regenerative feed water heating. Regenerative air heating [1] or regenerative fuel drying [2] may be used to circumvent this limit. An economizer is usually placed before the air heater in a thermal power plant. Therefore, the flue gas temperature is lowest at the air heater outlet. The flue gas temperature must be above the temperature at which sulfuric vapor starts to condense, which is the acid dew point. However, the temperature of flue gas at the air heater outlet has to be more than the acid dew point due to the conduction resistances between flue gas and air. Hence, there is a lower limit of the temperature of flue gas that is exhausted by a power plant [1].

The potential of biomass energy has been widely acknowledged for some time. One of the most common applications of biomass energy is using it as a fuel in biomass power plants. However, biomass fuels typically have low heating values due to their high moisture content. This moisture reduces the power plant efficiency when the fuel is combusted. To increase energy efficiency, drying the fuel is an effective method. Two established drying methods are steam drying and flue gas drying. Steam drying utilizes extracted steam from a steam turbine, while flue gas drying uses high-temperature flue gas as its energy source. Chantasiriwan [2] noted that steam drying increases the net power plant efficiency more than feed water heating. On the other hand, flue gas drying can decrease the flue gas temperature to the acid dew point, and using a flue gas dryer as an additional heat recovery device can result in a higher power plant efficiency than using only an air heater and an economizer. Most previous studies of drying systems have focused solely on either the steam dryer or flue gas dryer. Andersson et al. [3] found that the flue gas dryer was the most attractive option for biomass drying in a pulp mill. Sosa-Arnao and Nebra [4] determined the optimum configuration of heat recovery devices that included a flue gas dryer in a biomass boiler. Pang and Mujumdar [5] reviewed technologies for biomass drying using a conveyor dryer, rotary dryer, and pneumatic dryer. Liu et al. [6] considered using a rotary-tube dryer for steam drying, and found that the efficiency of a lignite-fired power plant could be increased by 1.87%. Li et al. [7] evaluated the economic feasibility of using a flue gas dryer in a biomass power plant. Liu et al. [8] compared flue gas drying and steam drying, and concluded that steam drying is a better option for revamping a lignite-fired power plant. Han et al. [9] showed that power plant efficiency could be increased by 1.51% using flue gas drying with waste heat recovery. Liu et al. [10] proposed using a compressor or ejector to enhance the performance of a steam drying system. Xu et al. [11] proposed using a supplementary steam cycle to improve the performance of a steam dryer in a power plant. Liu et al. [12] performed energy and exergy analyses of a power plant integrated with a steam dryer at full and partial loads. Zhu et al. [13] designed a system to recover exhaust vapor from a steam dryer for feed water heating. Han et al. [14] presented an improved concept of flue gas drying, in which water recovery and exhaust gas recirculation were incorporated. Liu et al. [15] compared flue gas drying and steam drying, and found that steam drying yielded better economic benefits. Chantasiriwan [16] proposed using a steam dryer to improve the performance of a cogeneration system. Li et al. [17] showed that the energy efficiency of a power plant that used municipal solid waste as fuel could be increased by using a steam dryer with heat recovery. Yan et al. [18] suggested that wet flue gas could supply energy for a steam dryer instead of extracted steam. Chantasiriwan [19] performed an analysis to demonstrate that the integration of a steam dryer and a steam–air preheater into an existing biomass power plant is economically feasible. Chantasiriwan [20] showed that the net efficiency of a coal-fired power plant could be increased by using a steam dryer with heat recovery.

Investigations on the use of two different dryers to reduce fuel moisture content have been carried out by a few researchers. Gebreegziabher et al. [21] proposed a two-stage drying system with a hot air dryer placed before a steam dryer. Medium-pressure steam extracted from the steam turbine is supplied to the steam dryer. Low-pressure steam from the steam dryer is used to increase the air temperature before hot combustion air is supplied to the air dryer. Liu et al. [22] showed that a fluidized-bed dryer using both extracted steam and flue gas as its heat sources could increase the efficiency of a lignite-fired power plant. Xu et al. [23] presented a two-stage drying system consisting of a flue gas dryer and a steam dryer. The integration of this system was shown to result in a higher power plant efficiency than the integration of only a steam dryer. An improved two-stage drying system was proposed by Zhu et al. [24]. Exhaust vapor from the steam dryer in this system is used for raising feed water temperature. Yan et al. [25] designed a novel drying system that uses a fluidized bed dryer and a rotary tube dryer. Extracted steam is supplied to a heat pump to produce hot water for fuel drying in the fluidized bed dryer. Extracted steam is also supplied to the rotary tube dryer.

In this study, a hybrid drying system is presented. The proposed system is designed to reduce the fuel consumption rate of a biomass power plant with the net power output unchanged. The three main contributions of this study to energy research are as follows.

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A novel hybrid drying system consisting of a steam dryer installed in parallel with a flue gas dryer is proposed. This system differs from the systems proposed by Xu et al. [23] and Zhu et al. [24], in which the flue gas dryer is placed after the steam dryer.

-

The proposed system uses vapor from the steam dryer to increase the combustion air temperature, which enhances the capacity of the flue gas dryer. By contrast, the systems proposed by Xu et al. [23] and Zhu et al. [24] use exhaust vapor from the steam dryer for feed water heating, which does not affect the performance of the flue gas dryer.

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It is demonstrated that the proposed hybrid drying system is capable of improving the power plant efficiency more than previously proposed drying systems.

Furthermore, this method is economically justified because the return on investment in the proposed system is attractive.

2. Reference Power Plant

The reference biomass power plant is depicted in Figure 1. The conversion of the chemical energy of fuel to thermal energy in the boiler (B) causes the transformation of feed water to superheated steam. Mechanical power is generated by the steam turbine (ST) as a result of steam expansion. Steam is extracted from the steam turbine at pressures p₁ and p₂ for the feed water heaters (FWH1 and FWH2). The exhaust steam at pressure p₃ becomes saturated feed water after condensation in the condenser (C). The feed water pressure is increased from p₃ to p₂ by the first pump before the feed water enters FWH2. The feed water is mixed with the extracted steam at pressure p₂ in FWH2. The output is sent to the second pump. After the feed water pressure is increased from p₂ to p₁, the feed water enters FWH1. The feed water is mixed with the extracted steam at pressure p₁ in FWH1. The output is sent to the third pump. The feed water pressure is increased from p₁ to p₀ before the feed water enters the boiler.

There are three stages of steam turbine expansion. The mass flow rates in the 1st, 2nd, and 3rd stages are, respectively, m_s, m_s − m₁, and m_s − m₁ − m₂. Steam pressure is decreased from p₀ to p₁, p₂, and p₃, respectively, in the first, second, and third stages. The steam expansion results in the following power output:

$$P_{out}=m_s\left(h_0-h_1\right)+\left(m_s-m_1\right)\left(h_1-h_2\right)+\left(m_s-m_1-m_2\right)\left(h_2-h_3\right)$$

(1)

$$h_i=h_{i-1}-{\eta }_{ti}\left(h_{i-1}-h_{i,s}\right)$$

(2)

where h_i−1 is the specific enthalpy of the incoming steam, and h_i,s is the specific enthalpy of the outgoing steam after isentropic expansion. Let η_p be the pump efficiency. The three pumps require the power of P_in:

$$P_{in}=m_s{w}_{p,1}+\left(m_s-m_1\right)w_{p,2}+\left(m_s-m_1-m_2\right)w_{p,3}$$

(3)

$$w_{p,i}=\frac{v_i\left(p_{i-1}-p_i\right)}{{\eta }_p}$$

(4)

where v_i is the specific volume of saturated water at pressure p_i. It can be shown that the maximum efficiency of this power plant occurs when the outputs of FWH1 and FWH2 are saturated water, which corresponds to the following extracted steam flow rates:

$$m_1=m_s\left(\frac{h_1-h_{sat,2}-w_{p,2}}{h_{sat,1}-h_{sat,2}-w_{p,2}}\right)$$

(5)

$$m_2=\left(m_s-m_1\right)\left(\frac{h_2-h_{sat,3}-w_{p,3}}{h_{sat,2}-h_{sat,3}-w_{p,3}}\right)$$

(6)

where h_sat,i is the specific enthalpy of the saturated water at pressure p_i.

The generator converts the power output of the steam turbine to electrical power. Let η_g be the generator efficiency. The gross power output is as follows:

$$P_{gross}={\eta }_g\left(P_{out}-P_{in}\right)$$

(7)

The power input in Equation (3) does not account for the auxiliary power (P_aux) required by auxiliary electrical devices in the power plant. The net power output is as follows:

$$P_{net}=P_{gross}-P_{aux}$$

(8)

Let LHV be the lower heating value of the as-received fuel. The net power plant efficiency is defined as follows:

$${\eta }_{net}=\frac{P_{net}}{m_{fi}LHV}$$

(9)

Assume that 30% excess air is used for complete fuel combustion. If the heating surface areas of the heat exchangers in the boiler are known, it can be shown, using a model of an industrial boiler [26], that the fuel consumption rate (m_fi) decreases with the temperature of the exhaust flue gas or final flue gas temperature (T_gf). A sufficiently high value of T_gf is needed to avoid corrosion at the inlet section of the air heater caused by condensing sulfuric acid vapor.

3. Modified Power Plant with Hybrid Drying System

It should be noted that, for given values of net power output and final flue gas temperature, there are extracted steam flow rates that cause the outputs of FWH1 and FWH2 to be saturated liquid water. These extracted steam flow rates result in the maximum value of η_net and the minimum value of m_fi. These extracted steam flow rates are the upper limits because higher extracted steam flow rates result in water vaporization at the outlets of FWH1 and FWH2, which have negative effects of the second and third pumps. This means that it is impossible to raise the net power plant efficiency and reduce the fuel consumption rate by increasing the extracted steam flow rates beyond the upper limits. However, the net power plant efficiency can be further increased, and the fuel consumption rate may be further decreased, by using extracted steam for fuel drying in a steam dryer [2]. A further reduction in fuel consumption may be attained by decreasing the final flue gas temperature using a flue gas dryer.

An illustration of the modified power plant is shown in Figure 2. Three additional components in this power plant are a steam dryer (SD), a flue gas dryer (FD), and an air preheater (AP). Steam extracted from a steam turbine at pressure p₂ is supplied to FWH2 and SD. The extracted steam flow rate for SD is m_sd. The extracted steam flow rates result in saturated water at the outlets of the feed water heaters (FWH1 and FWH2). Equation (5) is used to determine m₁. Equation (6) for m₂ must be changed in the modified power to the following equation:

$$m_2=\left(m_s-m_1\right)\left(\frac{h_2-h_{sat,3}-w_{p,3}}{h_{sat,2}-h_{sat,3}-w_{p,3}}\right)+m_{sd}\left(\frac{h_2-h_{sat,2}}{h_{sat,2}-h_{sat,3}-w_{p,3}}\right)$$

(10)

Extracted steam condenses in the SD, and becomes saturated water that is mixed with the output of FWH2. The operation of SD results in evaporating moisture or exhaust vapor that must be vented continually from SD. Exhaust vapor is saturated vapor at atmospheric pressure. Guo et al. [27] investigated using exhaust vapor for feed water heating. Havlik and Dlouhy [28] analyzed a system that used exhaust vapor for external consumption. In this paper, exhaust vapor is supplied to AP. Increased combustion air temperature leads to increased final flue gas temperature. The FD is used to recover the energy of flue gas before being exhausted to the environment.

The fuel is divided into three portions. The first and second portions are dried in the steam and flue gas dryers. They enter the boiler after leaving the dryers. The third portion is sent directly to the boiler. The extracted steam flow rate at pressure p₂ supplied to the steam dryer is m_sd. Assume that the steam dryer is designed to reduce the fuel moisture content from x_Mi to x_Md. Normally, moisture is removed quickly from very moist biomass [29]. As the moisture content of biomass decreases, it takes more time to reduce the moisture content. Since the steam dryer is designed for a relatively short drying time, x_Md must be sufficiently large. Inspection of the drying curves of an empty fruit bunch reveals that it does not take a long drying time to decrease the moisture content from 50% to 30% [29]. Therefore, x_Md is set at 0.3. It can be shown using the model of a steam dryer proposed by Chantasiriwan [16] that the mass flow rate of the first fuel portion is as follows:

$$m_{fi1}=\frac{\left(1-{\epsilon }_{sd}\right)m_{sd}{h}_{vl,2}}{\left[\left(1-x_{Mi}\right)c_{pf}+x_{Mi}{c}_{pw}\right]\left(T_{sat,a}-T_a\right)+\left(x_{Mi}-x_{Md}\right)h_{vl,a}/\left(1-x_{Md}\right)}$$

(11)

where c_pf and c_pw are the specific heat capacities of the dried fuel and water, ε_sd is the heat-loss factor of the steam dryer, h_vl,2 is the latent heat of condensation at p₂, T_sat,a is 100 °C, and h_vl,a is 2256 kJ/kg, which is the latent heat condensation at 1 atm. A rotary-tube dryer may be used as steam dryer. Steam condenses inside steam tubes in the dryer, whereas fuel is moved from the inlet to the outlet due to the inclination and rotation of the dryer. Heat is transferred from the steam to the fuel through the tube walls. The heat loss in the dryer is due to heat transfer between the outer surface of the dryer and the surrounding air. The heat loss depends on several factors, and is subject to uncertainty. In this paper, it is assumed that ε_sd is 0.1.

The mass flow rate of exhaust vapor from the steam dryer is m_vap, which equals the difference between m_fi1 and m_f1. The condensation of the exhaust vapor in the air preheater releases thermal energy for combustion air heating. The temperature of combustion air at the outlet of the air preheater is as follows:

$$T_{ai}=T_{ai}+\frac{\left(1-{\epsilon }_{ap}\right)m_{vap}{h}_{vl,a}}{m_a{c}_{pa}}$$

(12)

where m_a is the mass flow rate of combustion air, c_pa is the specific heat capacity of air, and ε_ap is heat-loss factor of the air preheater, which is assumed to be 0.05.

The flue gas dryer is designed to reduce the fuel moisture content from x_Mi to x_Md. Figure 2 shows that flue gas temperature decreases from T_gf to T_gd in the flue gas dryer. T_gd should be sufficiently above 100 °C, which is the temperature at which moisture in fuel evaporates. It is assumed that T_gd is 120 °C. The model of a flue gas dryer proposed by Chantasiriwan [26] is used to obtain the following expression of the mass flow rate of the second fuel portion:

$$m_{fi2}=\frac{\left(1-{\epsilon }_{fd}\right)m_g{c}_{pg}\left(T_{gf}-T_{gd}\right)}{\left[\left(1-x_{Mi}\right)c_{pf}+x_{Mi}{c}_{pw}\right]\left(T_{sat,a}-T_a\right)+\left(x_{Mi}-x_{Md}\right)/\left(1-x_{Md}\right)\left[h_{vl,a}+c_{pv}\left(T_{gd}-T_{sat,a}\right)\right]}$$

(13)

where c_pg and c_pv are the specific heat capacities of the flue gas and water vapor, m_g is the mass flow rate of the flue gas, and ε_fd is heat-loss factor of the flue gas dryer. A flash dryer may be used as a flue gas dryer. Flue gas and fuel flow in the same direction in the dryer. Heat is transferred directly from the flue gas to the fuel. Heat loss in the dryer occurs due to heat transfer between the outer surface of the dryer and the surrounding air. Normally, there is insulation to reduce heat loss. In this paper, it is assumed that ε_fd is 0.05.

The three portions of fuel are mixed in a mixing tank (MT) before the mixed fuel is sent to the boiler. The mass flow rate of the mixture is m_fi, the moisture content of the mixture is x_M, and the mixture temperature is T_f. Once m_fi1 and m_fi2 are known, the net power output of the modified power plant depends on the mass flow rate of the third portion of fuel (m_fi3). It can be shown that the net power output increases monotonically with m_fi3. Since the aim of the integration of the hybrid drying system is to lower the fuel consumption rate, m_fi3 is chosen so that the net power output of the modified power plant is the same as that of the reference power plant. The fuel consumption rate of the modified power plant, the fuel moisture content, and the fuel temperature are expressed as follows:

$$m_{fi}=m_{fi1}+m_{fi2}+m_{fi3}$$

(14)

$$x_M=\frac{x_{Md}\left(m_{f1}+m_{f2}\right)+x_{Mi}{m}_{fi3}}{m_{f1}+m_{f2}+m_{fi3}}$$

(15)

$$T_f=T_r+\frac{\left(m_{f1}+m_{f2}\right)\left[\left(1-x_{Md}\right)c_{pf}+x_{Md}{c}_{pw}\right]\left(T_{sat,a}-T_r\right)+m_{fi3}\left[\left(1-x_{Mi}\right)c_{pf}+x_{Mi}{c}_{pw}\right]\left(T_a-T_r\right)}{\left(m_{f1}+m_{f2}+m_{fi3}\right)\left[\left(1-x_M\right)c_{pf}+x_M{c}_{pw}\right]}$$

(16)

where T_r is 25 °C.

4. Economic Analysis

The installation costs of a steam dryer, a flue gas dryer, and an air preheater have to be compared with the quantity of saved fuel when evaluating the economic feasibility of the hybrid drying system. The costs of both dryers depend on the moisture removal rates in the dryers. The moisture removal rate (in kg/h) in each dryer is the difference between the inlet fuel flow rate and the outlet fuel flow rate multiplied by 3600.

$$M_1=3600\left(m_{fi1}-m_{f1}\right)$$

(17)

$$M_2=3600\left(m_{fi2}-m_{f2}\right)$$

(18)

According to Couper et al. [30], the cost function of a steam dryer depends on the heating transfer area of the steam dryer (A_sd).

$$C_{sd}=\mathrm{60,000}{A}_{sd}^{0.6}$$

(19)

Assume that 1 m² of A_sd is required to remove 10 kg/h of fuel moisture. Equation (19) is rewritten as follows:

$$C_{sd}=\mathrm{15,071}{M}_1^{0.6}$$

(20)

The cost function of a flue gas dryer is given by Sztabert and Kudra [31] as follows:

$$C_{fd}=3029\hspace{0.17em}\hspace{0.17em}\left(30+\frac{36.74M_2}{17.77+T_{gf}}\right)$$

(21)

Note that a factor of 3029 is used to account for inflation. Although air was mentioned as the drying medium by Sztabert and Kudra [31], it is assumed in this paper that the cost function is unchanged when air is replaced by flue gas.

The air preheater cost depends on the heating surface area (A_ap), which is determined as follows:

$$A_{ap}=\frac{m_a{c}_{pa}}{U_{ap}}\ln \left(\frac{T_{sat,a}-T_a}{T_{sat,a}-T_{ai}}\right)$$

(22)

An estimation of the overall heat transfer coefficient (U_ap) is provided by Kakac et al. [32]. The installation cost of an air preheater is assumed to be identical to that of an air-cooled condenser. The cost function of an air-cooled condenser according to Shamoushaki et al. [33] is as follows:

$$C_{ap}=\log \left(A_{ap}\right)+0.01764A_{ap}^2+617.4A_{ap}+3.31\times {10}^4$$

(23)

If the hybrid drying system leads to greater energy efficiency, installing it in the modified power plant can be economically viable. Even if the net power output remains the same, improved energy efficiency can reduce fuel consumption. To determine whether the integration is financially feasible, the cost of adding equipment to the power plant must be compared to the value of fuel saving. Fuel costs represent a variable expense of electricity generation, and the value of fuel can be related to the amount of electricity produced from a given amount of fuel. Assuming fuel accounts for half of the cost of generating electricity, the unit monetary value of fuel (c_fuel) can be calculated from the unit monetary value of electricity (c_el). If ΔM_f represents the annual reduction in fuel consumption, the annual income from fuel saving is equal to the product of ΔM_f and c_fuel. The simple payback period (t_pb) is the ratio of the costs of installing a steam dryer, flue gas dryer, and air preheater to the annual income.

$$t_{pb}=\frac{C_{sd}+C_{fd}+C_{ap}}{\Delta M_f{c}_{fuel}}$$

(24)

5. Results and Discussion

The simulation results in this section were obtained from Fortran codes written by the author and operated in the Windows 11 operating system. The design parameters of the reference power plant are p₀ = 6.5 MPa, T₀ = 495.2 °C, and m_s = 56.38 kg/s. The turbine efficiencies are 79.8%, 82.1%, and 82.8% for η_t1, η_t2, and η_t3, respectively. The pump efficiency is 75%. The extracted steam and exhaust pressures are p₁ = 1615 kPa, p₂ = 217 kPa, and p₃ = 9 kPa. The mass flow rates of extracted steam for FWH1 and FWH2 are, respectively, 7.54 kg/s and 6.36 kg/s. The generator efficiency is 99%, and the auxiliary power input is 2% of the net power output. The net power output is found to be 50.00 MW. The fuel in the power plants is bagasse. The composition of the fuel consumed by the power plant is shown in

Table 1. Fuel composition.

Carbon	Hydrogen	Oxygen	Nitrogen	Sulfur	Ash	Moisture
24.39%	2.26%	21.57%	0.15%	0.03%	1.60%	50.00%

. The lower heating value of the as-received fuel is 7364 kJ/kg. The ambient temperature is 30 °C. The final flue gas temperature is 148.9 °C. The fuel consumption rate is 22.75 kg/s. The net power plant efficiency is 29.85%.

In the modified power plant, 1 kg/s of extracted steam at 217 kPa is supplied to the steam dryer. The steam dryer reduces the moisture content of the first portion of fuel, with a mass flow rate of 2.383 kg/s from 50% to 30%. The mass flow rate of the exhaust vapor from the steam dryer is 0.681 kg/s. The combustion air temperature at the air preheater outlet is 47.48 °C. The final flue gas temperature is 148.8 °C. The flue gas temperature is reduced to 120 °C in the flue gas dryer. Therefore, the mass flow rate of the second portion of fuel is 3.775 kg/s. The moisture content of this fuel portion is decreased from 50% to 30%. In order for the modified power plant to generate 50.00 MW of net power output, the mass flow rate of the third portion of fuel has to be 15.628 kg/s. Consequently, the resulting fuel moisture is 45.61%, the total fuel consumption rate is 21.786 kg/s, and the net power plant efficiency is 31.17%. As a result of the lower fuel moisture content and higher combustion air temperature of the modified power plant compared with the reference power plant, the mass flow rate of steam is decreased to 56.21 kg/s, the steam temperature is increased to 502.2 °C, the mass flow rate of the extracted for FWH1 is decreased to 7.48 kg/s, and the mass flow rate of the extracted steam for FWH2 is increased to 7.20 kg/s. Therefore, the integration of the hybrid drying system does not change the net power plant, but decreases the fuel consumption rate by 4.24%.

Table 2. Comparison of parameters of the reference and modified power plants.

	Reference Power Plant	Modified Power Plant
ps (MPa)	6.5	6.5
ms (kg/s)	56.38	56.21
Ts (°C)	495.2	502.2
m1 (kg/s)	7.54	7.48
m2 (kg/s)	6.36	7.20
msd (kg/s)	0.0	1.00
mvap (kg/s)	0.0	0.681
xMi (%)	50.00	50.00
xM (%)	50.00	45.61
Ta (°C)	30.00	30.00
Tai (°C)	30.00	47.48
mfi (kg/s)	22.750	21.786
Pnet (MW)	50.00	50.00
ηnet (%)	29.85	31.17

compares important parameters of the modified and reference power plants.

The moisture removal rates in the steam dryer and the flue gas dryer are, respectively, 2451 kg/h and 3883 kg/h. The air preheater area is 350 m². Consequently, the capital cost of the hybrid drying system is USD 4.806 × 10⁶. Both the reference and the modified power plants are assumed to operate for 7920 h annually. Therefore, the mass of fuel saved by the hybrid drying system is 2.75 × 10⁴ ton. The fuel value is estimated from the net power output and the fuel consumption rate of the reference power plant. The quantity of electricity generated by 1 ton of fuel in the reference power plant is 610 kW.h of electrical energy. Assume that the value of one unit of electricity (1 kW.h) is USD 0.12 and the value of 1 ton of fuel is estimated as 36.6 USD/ton (half of the monetary value of generated electrical energy). Therefore, the average annual income due to saved fuel is USD 1.007 × 10⁶, and the simple payback period is 4.77 years.

It is interesting to see how the system parameters vary with the extracted steam flow rate in the steam dryer (m_sd). Therefore, simulation results are obtained for a range of m_sd. Figure 3 shows that the moisture removal rate in the steam dryer (M₁) increases from 1225 kg/h to 8578 kg/h, the moisture removal rate in the flue gas dryer (M₂) increases from 3556 kg/h to 5978 kg/h, and the fuel moisture content (x_M) decreases from 46.79% to 37.86% as m_sd increases from 0.5 kg/s to 3.5 kg/s. Increasing M₁ results in an increasing mass flow rate of exhaust vapor sent to the air preheater, which causes the combustion air temperature (T_ai) to increase from 38.7 °C to 94.3 °C, and the air preheater area (A_ap) to increase from 162 m² to 2930 m², as shown in Figure 4. Due to decreasing x_M and increasing T_ai, the steam flow rate (m_s), the extracted steam flow rate at pressure p₂ (m₂), and the steam temperature (T_s) increase from 56.15 kg/s, 6.76 kg/s, and 500.5 °C to 56.46 kg/s, 9.38 kg/s, and 511.2 °C, as shown in Figure 5. However, the extracted steam flow rate (m₁) at pressure p₁ decreases slightly from 7.47 kg/s to 7.46 kg/s. These changes in the steam flow rates and steam temperature lead to a reduction in the fuel consumption rate as m_sd increases to maintain the same value of net power output. Figure 6 shows that m_fi decreases from 22.00 kg/s to 20.69 kg/s. The variation in the simple payback period (t_pb) with m_sd is also shown in Figure 6. Figure 4 shows that increasing m_sd is accompanied by an increasing heating surface area of the air preheater. Figure 4 implies that the power output increases at a decreasing rate as the heating surface area increases. Eventually, the return of increasing the heating surface area is outweighed by the cost of doing so. This is why the curve in Figure 6 exhibits the minimum. The optimum value of m_sd that minimizes t_pb is 2.85 kg/s. The corresponding percentage increase in net power plant efficiency is 7.76%, and the minimum value of t_pb is 4.28 years.

Table 3. Comparison of percentage increases in power plant efficiency resulting from the integration of different drying systems.

Source	Drying System	Percentage Increase in Power Plant Efficiency
Ref. [8]	Flue gas drying	3.6%
Ref. [8]	Steam drying	5.1%
Ref. [9]	Flue gas drying	3.5%
Refs. [10,11,12]	Steam drying	4.4%
Ref. [23]	Two-stage drying	3.1%
Ref. [25]	Two-stage drying	3.2%
This paper	Hybrid drying	7.76%

compares the percentage increases in power plant efficiency due to the integration of a drying system from previous studies and the current study. It can be seen that the proposed hybrid drying system results in a significantly greater increase in power plant efficiency.

An additional benefit of the hybrid drying system is the decreased mass flow rate of condensed steam in the condenser. The mass flow rate of condensed steam in the reference power plant is 42.47 kg/s. The specific enthalpy of exhaust steam leaving the steam turbine is 2342 kJ/kg. Since the condensing pressure is 9 kPa, and condensation is complete, the cooling load of the condenser is 91.69 MW. In the modified power plant in which 1 kg/s of extracted steam at 217 kPa is supplied to the steam dryer, the condensed steam flow rate is 41.54 kg/s, which is 2.2% lower than that of the reference power plant. The reduced cooling load enables the size of the condenser to be decreased. This additional benefit is, however, not quantified economically in this paper.

6. Conclusions

A hybrid drying system consisting of a steam dryer, a flue gas dryer, and an air preheater is proposed and analyzed in this paper. Extracted steam is used as the heat source for the steam dryer. Exhaust vapor from the steam dryer is used to raise the temperature of combustion air in the air preheater. The flue gas dryer uses high-temperature flue gas as its energy source. The hybrid drying system results in decreasing moisture content of the fuel and increasing temperature of the combustion air. The integration of the hybrid drying system is capable of reducing the fuel consumption rate of a power plant without altering the net power output. Simulation results are obtained for a case study of a 50.00 MW reference power plant. With a supply of 1.0 kg/s of extracted steam for the steam dryer, the moisture content of fuel is decreased by the steam and flue gas dryers from 50.00% to 45.61%, and the fuel consumption rate is reduced by 4.24%. The simple payback period is 4.77 years. Increasing the mass flow rate of the extracted steam supplied to the steam dryer to 2.85 kg/s results in a reduction in the fuel consumption rate of 7.76% and a simple payback period of 4.28 years. These simulation results lead to the conclusion that the integration of the hybrid drying system is economically justified. This paper is part of a series of papers on thermal power plant optimization by the author [1,2,16,19,20,26,34,35,36,37]. It is recommended that future works consider the optimal design of a thermal power plant that incorporates all three heat recovery devices (an air heater, an economizer, and a flue gas dryer) and all three regenerative devices (a steam–air preheater, feed water heaters, and a steam dryer).