Comparison between Regenerative Feed Water Heating and Regenerative Fuel Drying in Biomass Power Plant

Net power plant efficiency is the ratio of exportable electrical power output to the product of fuel consumption rate and fuel heating value. Conventionally, power plants are designed to attain the maximum net power plant efficiency by using regenerative feed water heating, in which extracted steam from steam turbine is used to increase feed water temperature. An important factor that determines the net power plant efficiency is the temperature of flue gas exhausted from the power plant. Decreasing this temperature increases the net power plant efficiency. However, exhaust flue gas temperature should not be lower than the lower limit in order to avoid the corrosion resulting from condensation of sulfuric acid vapor in flue gas on heat exchanger tubes. The lower limit can be raised by reducing fuel moisture content and raising air temperature at boiler inlet. In this paper, it is proposed that low-pressure extracted steam should be used for fuel drying in steam dryer, and vapor released from steam dryer should be used for air heating in heat recovery air preheater. By doing so, the lower limit of exhaust flue gas temperature is decreased, and the net efficiency of the power plant is increased. A case study of a 600-MW lignite-fired power plant is used to demonstrate that the integration of steam dryer and heat recovery air preheater is economically justified.

Conventionally, regenerative feed water heating is used to increase the net efficiency of a thermal power plant. In this method, extracted steam from the steam turbine is used to increase feed water temperature before feed water is sent to the boiler. Increasing the number of feed water heaters or the mass flow rates of extracted steam causes the net power plant efficiency to increase. However, there is a practical limit of the number of feed water heaters. Furthermore, there is an upper limit of the mass flow rate of extracted steam for a feed water heater. Consequently, there is an upper limit of the net power plant efficiency in a conventional design of power plant that relies only on regenerative feed water heating. In this paper, it is argued that the net power plant efficiency can be increased beyond this limit by using extracted steam to increase air temperature in steam coil air preheater before air enters the boiler. Models of boiler, steam turbine, and steam coil air preheater are used to demonstrate that the net efficiency of 50-MW power plant is increased by 2.32% due to the integration of steam coil air preheater having the area of 500 m 2 .

Parabolic trough collectors may be used to increase the temperature of a heat transfer fluid flowing inside absorber tubes. A method of solar-aided power generation in thermal power plants is using heat transfer from the high-temperature heat transfer fluid for feed water heating. In doing so, less steam is extracted from steam turbine, and power output is increased. This paper investigates an alternative method of solar-aided power generation that uses direct steam generating parabolic trough collectors for feed water heating in a small biomass power plant. Such a power plant consists of an open feed water heater, which is more commonly known as deaerator due to its capability to remove dissolved gases from feed water. Feed water heating by solar energy in this power plant is more easily implemented by using parabolic trough collectors with direct steam generation than parabolic trough collectors with a heat transfer fluid. The proposed integration of parabolic trough collectors into a biomass power plant requires the supply of saturated steam from parabolic trough collectors to the deaerator. The steam quality reaches the maximum value when the direct normal irradiance is maximum. The performance of this system depends on the average value of effective direct normal irradiance. Simulation results show that there exists the optimum extracted steam pressure, which results in the maximum electrical energy generated by the power plant.

ABSTRACTThis paper communicates detailed energy and exergy analysis of low-grade energy resource of solar-powered organic Rankine cycle (ORC) integrated with both the internal heat exchanger and open feed water heater, ORC incorporated with the internal heat exchanger, with open feed water heater and basic ORC, respectively. Results indicate that highest first law efficiency (11.9%), exergetic efficiency (51.88%) and lowest exergy destruction (1749 kW) are obtained for ORC integrated with both internal heat exchanger and open feed water heater among other considered ORCs. Moreover, zeotropic mixture (butane/R1234yf) shows better first law and exergetic efficiency and lower exergy destruction than pure fluid.

Cogeneration system in the cane sugar industry produces not only molasses and raw sugar but also exportable electrical power. Main components of the system are boiler, steam turbine, condenser, and sugar juice evaporation process. Bagasse is used as fuel for boiler. Bagasse is a by-product of sugar juice extraction process. It is characterized by a high moisture content, which leads to the inefficiency of energy conversion. The integration of steam dryer in cogeneration system to reduce bagasse moisture content before combustion will improve the system performance. The moisture content of bagasse is reduced in a steam dryer due to heat transfer from steam condensation. Saturated steam supplied to steam dryer is obtained by mixing superheated steam extracted from steam turbine with the appropriate amount of cooling water in desuper-heater. The objective of this paper is to evaluate the performance of this cogeneration system quantitatively. Models of boiler and steam dryer are used for this purpose. Simulation results show that the cogeneration system integrated with steam dryer generates more power output than the reference cogeneration system without steam dryer under the conditions that sugar juice processing capacity and bagasse consumption are the same. Furthermore, the cogeneration system integrated with steam dryer requires 20% less heating surface area than the reference cogeneration system under the condition that temperatures of flue gas exhausted from boilers of both systems are the same.

Thermoecology-based performance simulation of a Gas-Mercury-Steam power generation system (GMSPGS)

Conventionally, the thermal design a biomass power plant is aimed at maximizing the energy efficiency by using extracted steam to increase feed water temperature in feed water heaters and installing heating surface areas of heat exchangers in boiler to recover flue gas energy. The temperature of flue gas at the boiler outlet must not be so low that a risk of acid corrosion is unacceptable. The energy efficiency can also be improved by reducing fuel moisture content in steam dryer and increasing air temperature in steam-air preheater. However, both devices require extracted steam, which is already used for raising feed water temperature in an existing power plant. In this paper, it is hypothesized that retrofitting steam dryer and steam-air preheater to an existing power plant can improve the energy efficiency. Performances of the existing power plant and the retrofitted power plant integrated with steam dryer and steam-air preheater are compared. Simulation results indicate that, despite lower final feed water temperature in the retrofitted power plant, the retrofitted power plant is more energy efficient than the existing power plant. Economic assessment is carried under the condition that both power plants generate the same power output, which means that the retrofitted power plant consumes less fuel than the existing power plant. The payback period for the retrofitting investment is approximately 7 years.

This article determines the operating conditions leading to maximum work in a regenerative cycle with an open feed water heater through a procedure that combines the use of artificial neural networks (ANNs) and genetic algorithms (GAs). Water is an active fluid in the thermodynamical cycle; an objective function is obtained by using vapor enthalpy (a nonlinear function of operating conditions). Utilizing classical methods for maximizing the objective function usually leads to suboptimal solutions. Therefore, this article uses ANNs to estimate the steam properties as a function of operating conditions and GAs to optimize the thermodynamical cycle. The operating conditions are chosen with the aim of gaining maximum work in a boiler for a specific heat. To estimate the thermodynamic properties, an ANN was used to provide the necessary data required in the GA calculation.

Five configurations of regenerative Rankine cycle with two feed water heaters are compared in this paper. One configuration has two open feed water heaters (OFWH). The other four configurations have an open feed water heater placed before or after a closed feed water heater, which may be either closed feed water heater with drain pumped forward (FFWH) or closed feed water heater with drain cascaded backward (BFWH). The five regenerative Rankine cycles are OFWH-OFWH, OFWH-FFWH, OFWH-BFWH, FFWH-OFWH, and BFWH-OFWH cycles. The optimum extracted steam fractions that result in the maximum thermal efficiency can be determined for each cycle. For a case study of Rankine cycle with the maximum steam pressure, temperature, and flow rate of 6 MPa, 450 °C, and 50.46 kg/s, respectively, the maximum efficiency of OFWH-OFWH cycle is 34.64%, and the net power output is 39.74 MW. The maximum efficiencies of the other four cycles increase with feed water heater area. They reach 34.64% when feed water heater areas of OFWH-FFWH, OFWH-BFWH, FFWH-OFWH, and BFWH-OFWH cycles are, respectively, 94, 138, 137, and 198 m2. The installation costs of five cycles are also compared so that the condition under which each cycle has the lowest installation cost can be identified.

Steam turbines are used to meet a large percentage of the global electrical demand. In practice, a steam turbine is fed superheated H2O (steam) as a working fluid at a high pressure. Enthalpy is converted into output shaft work, and the working fluid exits at a lower pressure as either a superheated vapor or a liquid/vapor mixture with a sufficiently high quality to avoid mechanical erosion of the final stages of the turbine. To operate continuously, the working fluid must be returned to the turbine inlet state, with the primary goal of extracting net power. A Rankine cycle with modifications satisfies that requirement and forms the basis for virtually all steam-based stationary power plants. The turbine exit is condensed at the low pressure to liquid, then pumped to the high pressure needed at the turbine inlet, and then preheated, boiled, and superheated to the turbine inlet conditions. The source of the heat addition can be from the combustion of fossil fuels, nuclear energy, concentrated solar energy, incineration of municipal solid waste or geothermal energy, to name a few. In this chapter, several Rankine-based cycles are investigated, all scaled in size to generate a net power output of 1.0 GW, typical of large stationary power plants. The scaling factor is the mass flow rate of working fluid common to each device. First, an ideal Rankine cycle with operating parameters set by material-driven practical constraints is defined and analyzed. A second study introduces another device (reheater) that allows the cycle to operate at an overall higher pressure ratio without violating the practical restrictions. In the third study, an open feedwater heater is introduced, a device that preheats liquid from the pump using a portion of the steam extracted from an intermediate pressure from the turbine in a process called regeneration. A final study introduces realistic deviations from ideal behavior to more closely model actual power plants. The effect of each of these changes on the overall performance (mass flow rate, 1st and 2nd Law efficiencies), and the eXergy destruction in each device is compared.

For a biomass power plant, which uses a biomass fuel having a high moisture content, flue gas drying is an effective method of increasing the overall power plant efficiency. However, this method requires flue gas that has a high temperature. Most biomass power plants, however, are designed so that flue gas temperature is already reduced to a low value through the installation of economizer and air heater. This paper investigates the use of steam-air preheater to raise both air temperature and flue gas temperature. Therefore, steam-air preheater enhances the potential of flue gas dryer, and both devices can be used in conjunction to raise the overall efficiency of a biomass power plant. Models of power plant, flue gas dryer, and steam-air preheater are used in this paper to demonstrate by simulation that the integration of flue gas dryer and steam-air preheater results in 2.52% increase in electrical power output of a reference power plant that consumes a fuel with 52% moisture content (wet-basis) if the fuel moisture removal rate in flue gas dryer is 7140 kg/h, and the total heating surface area of steam-air preheater is 2000 m2. The simple payback period for the investment in this integration is 3.3 years.

Adequate seawater (SW) flow or cooling water flow through the condenser in a thermal power plant is a critical factor in maintaining the isentropic efficiency of the LP steam turbine, condenser pressure and moisture content at the turbine exhaust. At one of ACWA Power’s thermal sites, deviations from the design seawater flow rates were identified during operation, as compared to the original equipment manufacturers HMBs. The site was operating three pumps on two blocks instead of the required four pumps, two on each block due to the constraints in the SW pit level. An investigation was conducted using thermodynamic modeling software to assess the impact of reduced SW flow on the LP turbine’s performance and condenser pressure. The analysis indicated that the LP turbine’s isentropic efficiency and condenser pressure and exhaust moisture content were deviating from the design reference due to the inadequate SW flow. To validate these findings, actual plant data was analyzed. It was observed that when sufficient SW flow was restored, moisture content, condenser pressure and isentropic efficiency returned to the expected levels with the improvement of around 40–45 KJ/KWh in steam cycle heat rate.

Dry cooling of concentrating solar power (CSP) plants, an economic competitive option for the desert regions of the MENA region

Steam Rankine cycle (SRC), which is mainly utilised in power generation sector, faces external irreversibility in its daily operation causing inefficiency in the system. To address this issue, reheat Rankine cycle (RHRC) and regenerative Rankine cycle (RRC) have been widely studied and implemented in power plants to improve thermal efficiency and reduce external irreversibility of Rankine cycle. This study investigates the implementation of different RRC configurations in a combined heat and power plant, including RRC with modified thermal deaerator, RRC with open feed water heater (OFWH) and closed feed water heater (CFWH). A base case simulation model was first constructed using commercial simulation software Aspen HYSYS for the basic SRC system based on actual plant data. Various scenarios were then evaluated for their profitability and sustainability through techno‐economic analysis (TEA) and carbon footprint analysis (CFA). From both analyses, the scenario of RRC with CFWH showed the greatest long‐term potential, generating the highest annual profit of $ 771 691 and carbon footprint reduction of 14.63%, while RRC with modified thermal deaerator showed the greatest potential in the short run with the highest return of investment (ROI) of 201.51% and shortest payback period (PBP) of 0.50 year.

In this research a combined cycle power plant (CCPP) which can provide 80 MW of electrical power has been studied. The design parameters of the plant were chosen as: compressor pressure ratio, compressor isentropic efficiency, gas turbine isentropic efficiency, and turbine inlet temperature, pinch difference temperature, steam turbine inlet temperature, steam turbine isentropic efficiency, condenser pressure, and pump isentropic efficiency. In order to optimally find the design parameters a thermoeconomic approach has been followed. An objective function representing the total cost of the plant in terms of dollar per second was defined as the sum of the operating cost related to the fuel consumption, and the capital investment for equipment purchase and maintenance costs. Finally, the optimal values of decision variables were obtained by minimising the objective function using sequential quadratic programming (SQP) for, 50, 60, 70, and 80 MW of net power output.