Back Pressure Steam Research Articles

외기조건변화에 따른 CHP 성능 해석

본 연구에서는 가스터빈과 증기터빈, 열회수증기 발생 장치와 지역난방 열교환기로 열병합 발전 시스템을 구성하여 복수기가 없이 증기 터빈 중압단에서 추기된 증기와 배기 증기를 지역난방 열교환기의 열원으로 사용하는 추기 배압식을 적용하였다. 구성된 시스템에 대하여 필요로 하는 열부하량과 발전 출력 조건을 만족 시키기 위한 최적 설계 성능 해석을 하였으며, 이와 함께 겨울철 외기 온도 조건의 변화에 대한 시스템의 부분부하 해석을 하였다. 해석을 위해 상용 프로그램인 Thermoflex를 사용하였다. 시스템의 해석 결과, 기준 조건에서 수요처의 요구를 만족 시키는 최적 설계를 기준으로 각 외기 온도 변화에 대한 부분부하 성능 해석의 결과를 얻을 수 있었다. 그 결과 열부하량이 고정된 상태에서, 가스터빈과 전체 시스템의 출력은 외기온도가 감소함에 따라서 증가하였지만, 열원인 배기가스의 온도 감소로 인하여 증기터빈의 출력은 이와 반대로 감소하는 경향을 나타내었다. 그러나 전체 시스템에서 가스터빈의 차지하는 비중이 크기 때문에 전체 시스템의 출력의 경향은 가스터빈과 동일한 결과를 얻을 수 있었다. The co-generation system consisted of gas a turbine, a steam turbine, heat recovery steam generator and a heat exchangers for district heating was investigated in the present study. A back-pressure steam turbine (non-condensing type) was used. A partial load analysis according to the outdoor temperature in winter was conducted and optimal thermal load and power conditions was examined using the commercial computing software Thermoflex. As a result, under a constant thermal load, the power outputs of gas turbine and overall system increased as an outdoor temperature decreased. On the other hand, the reduction in exhaust gas temperature led to the decrease in output of steam turbine. Considering the portion of gas turbine in overall system in terms of the power output, it can be known that the tendency in power output of overall system was similar to that of the gas turbine.

Modeling of Combined Heat and Power Generating Power Plant Units Using Markov Processes with Continuous Time Parameter and Discrete State Space

Power plant units with extraction and back-pressure turbines cogenerating heat and power play a decisive role in the heat and power generation of the former centrally planned Eastern European countries. Cogeneration requires that for a certain portion of the operating period, extraction-condensing and back-pressure steam turbine power plant units make less than their nominal capacity available to the power system. Consequently, the two-state reliability description is not suitable for differentiated reliability modeling of cogeneration power plants. This article briefly sets out a three-or-more-state reliability description of these power plant units using Markov processes with continuous time parameter and discrete state space.

A novel power block for CSP systems

Concentrating Solar Thermal Power (CSP) and in particular parabolic trough, is a proven large-scale solar power technology. However, CSP cost is not yet competitive with conventional alternatives unless subsidized. Current CSP plants typically include a condensing steam cycle power block which was preferably designed for a continuous operation and higher operating conditions and therefore, limits the overall plant cost effectiveness and deployment. The drawbacks of this power block are as follows: (i) no power generation during low insolation periods (ii) expensive, large condenser (typically water cooled) due to the poor extracted steam properties (high specific volume, sub-atmospheric pressure) and (iii) high installation and operation costs. In the current study, a different power block scheme is proposed to eliminate these obstacles. This power block includes a top Rankine cycle with a back pressure steam turbine and a bottoming Kalina cycle comprising another back pressure turbine and using ammonia–water mixture as a working fluid. The bottoming (moderate temperature) cycle allows power production during low insolation periods. Because of the superior ammonia–water vapor properties, the condensing system requirements are much less demanding and the operation costs are lowered. Accordingly, air cooled condensers can be used with lower economical penalty. Another advantage is that back pressure steam turbines have a less complex design than condensing steam turbines which make their costs lower. All of these improvements could make the combined cycle unit more cost effective. This unit can be applicable in both parabolic trough and central receiver (solar tower) plants. The potential advantage of the new power block is illustrated by a detailed techno-economical analysis of two 50 MW parabolic trough power plants, comparing between the standard and the novel power block. The results indicate that the proposed plant suggests a 4–11% electricity cost saving.

A contribution for thermoeconomic modelling: A methodology proposal

This work presents a thermoeconomic optimization methodology for the analysis and design of energy systems. This methodology involves economic aspects related to the exergy conception, in order to develop a tool to assist the equipment selection, operation mode choice as well as to optimize the thermal plants design. It also presents the concepts related to exergy in a general scope and in thermoeconomics which combines the thermal sciences principles (thermodynamics, heat transfer, and fluid mechanics) and the economic engineering in order to rationalize energy systems investment decisions, development and operation. Even in this paper, it develops a thermoeconomic methodology through the use of a simple mathematical model, involving thermodynamics parameters and costs evaluation, also defining the objective function as the exergetic production cost. The optimization problem evaluation is developed for two energy systems. First is applied to a steam compression refrigeration system and then to a cogeneration system using backpressure steam turbine.

Modelling the power production of single and multiple extraction steam turbines

A thermodynamic model for the prediction of the operating performance of back pressure steam turbines with single and multiple extractions is presented. The approach takes the advantage of previous models reported in the literature and incorporates some improvements that allow for the prediction of the shaft work under changes in the operating conditions such as steam flow rate and pressure of each of the extractions. Simulation under changed operating conditions is conducted through the use of expressions that correlate the temperature and enthalpy of the steam as a function of the extraction pressure. The model incorporates the calculation of the isentropic efficiencies of each stage. The validity of the model is demonstrated by comparing the results with those of commercial turbines reported in the open literature.

Strategic simulation of the energy management in a Kraft mill

The economic assessment of an energy enhancement program envisaged for a Kraft wood pulping mill has been performed. The retrofit projects which have been analysed include reduction of the process steam demand, steam exportation to a district heating network and cogeneration. An automated MILP optimisation model has been developed to quantify the economic benefit of the projects independently and in combinations. Three cogeneration systems were considered: single back-pressure steam turbine, single steam condensing turbine and, a two-turbine system. A sensitivity analysis has been conducted for two modalities of key cost parameters (steam demand reduction level, fuel and power purchase costs, steam and power sales price) to determine the impact of those parameters on the economics of co-generation options. Results show that the configuration with the single back-pressure steam turbine has the shortest payback time, but the combination of two turbines produces more power and gives higher benefits in the long term.

Supercritical steam cycles and biomass integrated gasification combined cycles for sugarcane mills

Abstract Back in 1970s and 1980s, cogeneration plants in sugarcane mills were primarily designed to consume all bagasse, and produce steam and electricity to the process. The plants used medium pressure steam boilers (21 bar and 300 °C) and backpressure steam turbines. Some plants needed also an additional fuel, as the boilers were very inefficient. In those times, sugarcane bagasse did not have an economic value, and it was considered a problem by most mills. During the 1990s and the beginning of the 2000s, sugarcane industry faced an open market perspective, thus, there was a great necessity to reduce costs in the production processes. In addition, the economic value of by-products (bagasse, molasses, etc.) increased, and there was a possibility of selling electricity to the grid. This new scenario led to a search for more advanced cogeneration systems, based mainly on higher steam parameters (40–80 bar and 400–500 °C). In the future, some authors suggest that biomass integrated gasification combined cycles are the best alternative to cogeneration plants in sugarcane mills. These systems might attain 35–40% efficiency for the power conversion. However, supercritical steam cycles might also attain these efficiency values, what makes them an alternative to gasification-based systems. This paper presents a comparative thermoeconomic study of these systems for sugarcane mills. The configurations studied are based on real systems that could be adapted to biomass use. Different steam consumptions in the process are considered, in order to better integrate these configurations in the mill.

Energy saving opportunities in heat integrated beverage plant retrofit

This paper presents practical applications of mathematical programming for energy integration in a large beverage plant. The opportunities of heat integration between batch operations were analysed by a mixed integer linear programming (MILP) model, which was slightly modified by considering specific industrial circumstances. The feasibility of combined electricity, heating and cooling production was studied using a simplified MILP model, developed for the selection of an optimal polygeneration system. The superstructure includes cogeneration systems with different prime movers (steam turbine and gas turbine), and a trigeneration system with a back-pressure steam turbine. The proposed heat integration scheme and the selected cogeneration system may improve a company’s economic performance and reduce its environmental impact.

Alternative primary energy for power desalting plants in Kuwait: the nuclear option II — The steam cycle and its combination with desalting units

In the first part of this study, it was shown that the use of nuclear option to fuel the cogeneration power desalting plants (N-CPDP) in Kuwait is more economical than the most efficient gas/steam turbines combined cycle GSCC using oil or natural gas. The power cost produced by N-CPDP was found to be at least 35% less than that of the GSCC. Furthermore, the use of fossil fuel in Kuwait would consume all of its oil reserves in less than 30 years if its present rate of fuel consumption prevails. The very high cost of oil fuel and the emission of greenhouse gases due to its burning (with its negative environmental effects) favor the use of nuclear energy. It was found that Kuwait, Saudi Arabia, Egypt, and United Arab Emirates satisfy the conditions required to consider the nuclear option in terms of: (1) needed additional power capacity, (2) needed seawater desalting capacity, (3) size of the electricity grid, and (4) the basic infrastructure required to build the N-CPDP. The use of a light water pressurized water reactor, the AP-600 (600 MW nominal power output), in N-CPDP was anticipated for Kuwait. This paper gives the details of the AP-600 steam cycle and its combination with thermal desalting plants with multi-effect distillation (MED), multi-stage flash (MSF), and thermal vapor compression (TVC) desalting systems. The water costs due to the coupling of MED, MSF, or TVC to the AP-600 nuclear power plant (NPP) were also calculated. Based on the required water-to-power ratio, either a back pressure steam turbine (BPST) or an extraction condensing steam turbine (ECST) was chosen. For the BPST, a maximum water-to-power ratio of 97 MIGD to 451 MW was obtained. Then, the use of ECST was chosen with a seawater desalting capacity of 50 MIGD. The results show that the cost of desalinating water with nuclear power is cheaper than that produced by fossil-fired plants, given the high cost of fossil fuel. Further, the estimated costs of producing electricity and water with MED+NCPP are lower than MSF+NCPP and TVC+NCPP. The unit product cost of the desalted water was calculated to be in the range of $0.87- 1.4 per m3 of product water based on a plant capacity of 227.3×103 m3/d. The presented technoeconomic results for the different desalination scenarios can help decision makers in choosing the best option that is suitable for the Kuwaiti conditions.

Exergetic, Thermal, and Fuel Savings Analyses of a Heat-matched, Bagasse-based Cogeneration Plant in Sugar Industries

ABSTRACT The sugar industry employs different cogeneration schemes to satisfy process steam demand, and generates surplus power by upgrading the steam inlet parameters. A wide range of steam inlet parameters is used in Indian sugar industries, and varies from 21 bar to 110 bar in pressure and 330°C to 540°C in temperature. Most of the people understand that higher pressure (HP) steam inlet conditions result in more efficient power plant. In this article, exergy methods, in addition to more conventional energy analysis, are employed to evaluate overall and component efficiencies and to identify and assess thermodynamic losses. The analysis is carried out for a heat-matched bagasse-based cogeneration plant of a typical 2500 tcd sugar factory using a backpressure steam turbine and an extraction condensing steam turbine for a wide range of steam inlet conditions; selected around the sugar industry's export cogeneration plant for better choice of cogeneration plant configuration. The results indicate that a ba...

Exergy analysis of cogeneration power plants in sugar industries

In this paper, exergy analysis of a heat-matched bagasse-based cogeneration plant of a typical 2500 tcd sugar factory, using backpressure and extraction condensing steam turbine is presented. In the analysis, exergy methods in addition to the more conventional energy analyses, are employed to evaluate overall and component efficiencies and to identify and assess the thermodynamic losses. The analysis is carried out for a wide range of steam inlet conditions selected around the sugar industry’s export cogeneration plant. The results show that, at optimal steam inlet conditions of 61 bar and 475 °C, the backpressure steam turbine cogeneration plant perform with energy and exergy efficiency of 0.863 and 0.307 and condensing steam turbine plant perform with energy and exergy efficiency of 0.682 and 0.260, respectively. Boiler is the least efficient component and turbine is the most efficient component of the plant.

On the reduction of desalting energy and its cost in Kuwait

All seawater desalting processes, multi-stage flash (MSF), multi-effect boiling (MEB), mechanical vapor compression (MVC) and seawater reverse osmosis (SWRO) consume significant amounts of energy. The recent increase of fuel oil cost raises the cost of energy consumed for desalting water and the final water cost, and creates more interest in using more energy efficient desalting systems.The most used desalting systems by distillation (MSF and MEB) are usually combined with power plants in what is called co-generation power desalting plants, CPDP. Fuel is supplied to the CPDP to produce both desalted water D and power W, and the fuel cost is shared between D and W. Exergy analysis and equivalent work are among the methods used to determine the fuel energy charged to each product. When desalting systems, such as SWRO and MVC, are not combined with a power plant, the fuel energy can be directly determined from its electrical power consumption.In this paper, the fuel energy cost charged to desalting seawater in the presently used CPDP in Kuwait is calculated based on exergy analysis. The MSF, known by its high energy consumption, is the only desalting method used in Kuwait. The MSF units consume 258 kJ/kg thermal energy by steam supplied to the brine heater BH, 16 kJ/kg by steam supplied to steam ejectors, and 4 kWh/m3 mechanical energy for pumping. These MSF units are operated either by:(1)Steam extracted from extraction/condensing steam turbines EC/ST as in as in Doha West, Azzour, and Sabbiya CPDP. This practice is used in most Gulf area.(2)Steam supplied directly from boilers as occurred in single purpose desalting plants as Al Shuwaikh plant; or in winter time when no steam turbines are in operation in the CPDP to supply steam to the desalting units.The CPDP have limited water to power production ratio. While they can cope with the increase of power demand, it cannot satisfy the water demand, which is increasing with higher pace than the power demand.The case of steam CPDP used in Kuwait is presented in this paper as a reference plant to evaluate the amount of fuel energy consumed to desalt water in MJ/m3, its cost in $/m3. The resulted high fuel cost calls for some modifications in the reference CPDP to lower the energy cost, and to increase its water to power ratio. The modifications include the use of an auxiliary back-pressure steam turbine ABPST supplied with the steam presently extracted to the MSF units. The power output of the ABPST operates MVC or SWRO desalting units; while the ABPST discharged steam operates LT-MEB desalting unit. The desalting fuel energy costs when applying these modifications are also calculated by the exergy analysis and compared with that present situation.It is also suggested to increase desalted water output by using separate SWRO desalting units operated by the existing power plants of typical ηc = 0.388, or by new combined gas/steam turbines power cycle GT/ST-CC of typical ηc = 0.54 under construction. The SWRO with energy recovery is assumed to consume typical 5.2 kWh/m3 electric energy.

Modeling an industrial energy system: Perspectives on regional heat cooperation

Through energy efficiency measures, it is possible to reduce heat surplus in the pulp and paper industry. Yet pulp and paper mills situated in countries with a heat demand for residential and commercial buildings for the major part of the year are potential heat suppliers. However, striving to utilize the heat within the mills for efficient energy use could conflict with the delivery of excess heat to a district heating system. As part of a project to optimize a regional energy system, a sulfate pulp mill situated in central Sweden is analyzed, focusing on providing heat and electricity to the mill and its surrounding energy systems. An energy system optimization method based on mixed integer linear programming is used for studying energy system measures on an aggregated level. An extended system, where the mill is integrated in a regional heat market (HM), is evaluated in parallel with the present system. The use of either hot sewage or a heat pump for heat deliveries is analyzed along with process integration measures. The benefits of adding a condensing unit to the back-pressure steam turbine are also investigated. The results show that the use of hot sewage or a heat pump for heat deliveries is beneficial only in combination with extended heat deliveries to an HM. Process integration measures are beneficial and even increase the benefit of selling more heat for district heating. Adding a condensing turbine unit is most beneficial in combination with extended heat deliveries and process integration.

Suggested modifications of power-desalting plants in Kuwait

Modifications are suggested to the Kuwaiti cogeneration power-desalting plants CPDP. The suggestions would increase their desalting capacity, use more efficient desalting systems, and solve the acute problem of matching the installed desalted water capacity with the ever increasing demands of potable water. In year 2003, 93% of potable water was satisfied by the desalted water. The suggested modifications for a 300 MW steam turbine combined with two MSF units of 6 MIGD each include: (1) Substitute some of the old and in-efficient MSF units with more efficient multi effect desalting MED system of almost the same capacity, and gain ratio. The MED system can use low pressure steam of 75EC saturated temperature as heat source, and about 2 kWh/m 3 pumping energy (about 50% less than that of MSF units). (2) Adding a back pressure steam turbine BPST. Steam, now extracted from the steam turbine, at pressures ranging from 4.74 to 3.5 bar to the MSF units, is directed to an added BPST to produce more work before its discharge at 0.4 bar (about 75EC saturation temperature), as needed by the suggested MED units. (3) Adding seawater reverse osmosis SWRO desalting system. The work output from the BPST can be used to operate a seawater reverse osmosis SWRO desalting plant to produce more desalted water. Details of the modification are given in the paper. The modifications increase the power output of the plant without adding any more fuel. It can add up to three times the desalting capacity if SWRO system, and without adding any more fuel.

Optimisation of environmental gas cleaning routes for solid wastes cogeneration systems. Part II – Analysis of waste incineration combined gas/steam cycle

In the first paper of this paper (Part I), conditions were presented for the gas cleaning technological route for environomic optimisation of a cogeneration system based in a thermal cycle with municipal solid waste incineration. In this second part, an environomic analysis is presented of a cogeneration system comprising a combined cycle composed of a gas cycle burning natural gas with a heat recovery steam generator with no supplementary burning and a steam cycle burning municipal solid wastes (MSW) to which will be added a pure back pressure steam turbine (another one) of pure condensation. This analysis aims to select, concerning some scenarios, the best atmospheric pollutant emission control routes (rc) according to the investment cost minimisation, operation and social damage criteria. In this study, a comparison is also performed with the results obtained in the Case Study presented in Part I.

Cogeneration potential in the Columbian palm oil industry: Three case studies

Abstract The palm oil mills are characterized by the availability of considerable amounts of by-products of high-energy value such as empty fruit bunches (EFB), fibers, shells and liquid effluents with high content of organics called palm oil mill effluent (POME). A palm oil mill produces residues equivalent to almost three times the amount of oil produced by biomass, showing a huge potential for increasing the power efficiency of the plants and installed power, mainly by the use of by-products in cogeneration plants with high steam parameters and by reducing steam consumption in process. The objective of this paper is to present the results of the study about the cogeneration potential for three representative palm oil mills located in two important palm oil producing regions in Colombia (South-America), fifth palm oil producers of the world. The sizing of the cogeneration system was made assuming it operation during the greatest possible number of hours throughout the year (based on the seasonal availability of fruit) considering parameters for the steam at 2 MPa and 350 °C, using a condensing-extraction turbine. The balance of mass and energy was made by using the Gate Cycle Enter Software, version 5.51, to estimate the potential of electricity generation. The results showed that for fresh fruit bunch (FFB) processing capacities between 18 and 60 t FFB h −1 , it is possible to have surplus power ranging between 1 and 7 MW, if the plants are self-sufficient in electric energy and steam for process. With an average Capacity Factor (approximately 0.4), it is possible to expect a generation index of 75 and 160 kWh t −1 FFB when the processing plant is operating or shutdown, respectively, 3 or 4 times better than when a traditional system with a back-pressure steam turbines is used. This analysis used up to 60% of EFB produced in plant as fuel, considering its value as fertilizer for the palm crop. Several economic conditions were considered to estimate the economic and technical feasibility of cogeneration systems in palm oil mill for Colombian palm oil sector.

Steam driven large multi effect MVC (SD MVC) desalination process for lower energy consumption and desalination costs

It has been published and demonstrated in operating plants that the low-temperature multi effect desalination process (LT MED) when operated on turbine backpressure steam requires minimal energy consumption yielding specific desalination energy costs approaching values of optimal RO energy costs. However due to variations in electricity demand, the operation of large steam turbines at base load is in most cases impractical. As a result, in order to allow variations in electricity supply and full capacity of the desalination extraction, steam extracted from the turbines at higher pressures than the back pressure is commonly used. The extraction steam pressure of these turbines is in the range of 1.5–4.5 barg. (Typically as required for process steam at refineries and for MSF desalination plants). The LT MED process when utilizing such extraction steam incorporates thermo compressors (steam ejectors) in order to take advantage of the excessive extraction steam pressure, thereby increasing the economy ratio of the MED (decreasing energy consumption). Thermo compressors (ejectors) as a rule have a relatively low adiabatic efficiency, thus limiting the potential of the MED to increase the economy ratio even more. In this paper an approach of using a novel large centrifugal compressor driven by an auxiliary steam turbine utilizing the extraction steam is incorporated instead of the thermo compressor for large LT MED plants. The centrifugal compressor and auxiliary turbine having a much higher efficiency than the thermo compressor results in significant energy savings, thus lowering the desalination costs. This paper will also describe the thermodynamic advantages, the specific investment and desalination costs comparison, and the resulting desalination cost reduction potential of the steam-driven multi effect MVC process.

A MINLP model including the pressure levels and multiperiods for CHP process optimisation

In this paper a mixed integer nonlinear programming (MINLP) model for small-scale combined heat and power (CHP) plant synthesis and operation is presented. The model includes also the modelling of pressures levels and part load operation unlike the model presented in earlier work. Also, a detailed model of a back-pressure steam turbine is added to the model, taking into account the efficiency changes in turbine stages and the steam extraction pressure dependence on the steam mass flow rate. The sensitivity analysis shows that the developed model can be solved fairly reliably with commercial local MINLP solvers. The model corresponds well with a simulation model based on an existing plant. The model is able to find improved small-scale CHP designs that have higher efficiencies and that are profitable for wide ranges of electricity prices and fossil CO 2 emission permit prices. The developed model is a suitable decision making tool when evaluating the trade-offs between investments, profits, and fossil CO 2 emissions in small-scale CHP plant synthesis and operation. However, the formulations of the model are not limited to small-scale CHP processes and can be used to model also larger CHP processes and energy systems.

Thermodynamic and thermoeconomic comparison of combined cycle cogeneration systems

In this paper, several configurations of combined cycle cogeneration systems are investigated by energy, exergy and thermoeconomic analyses. In each of these configurations, varying steam demand is considered rather than fixed steam demand. Among the different approaches for thermoeconomic analysis in the literature, the specific exergy costing method is applied. Since the systems have more than one product (process steam and electrical power), systems are divided into several sub-systems and cost balances are applied together with the auxiliary equations. Hence, the cost rate of each product is calculated. Comparison of the configurations, in terms of performance assessment parameters and costs per unit of exergy, are also given in this study. In conclusion, it is found that combined cycle cogeneration systems with extraction condensing steam turbines are thermodynamically and thermoeconomically more advantageous when compared to those with back-pressure steam turbines.

Integrating renewable sources of energy into an existing combined heat and power system

In recent years, renewable energy sources have played an increasingly important role in potential energy production. The integration of renewables into energy production plants has therefore become a major challenge for many organizations. This study concerns the modernization of a small power plant in a large hospital. The design criteria include the possibility of utilizing renewable energy sources and providing a potential increase in heat production (with additional heat being supplied to a nearby university campus). The existing boiler conditions (i.e. controls, efficiency, etc.) are unable to satisfy the desired requirements and therefore require an extensive retrofit. The purpose of this study was to create an effective methodology for the development of an optimal solution to an integrated renewable energy retrofit. This retrofit would supply the current requirements for steam and in addition provide heat to a newly built local facility. Various alternatives were assessed as part of this study including: a retrofit without a complete concept change; a combined heat and power system using natural gas and making use of alternative cogeneration devices; a system utilizing biomass without the production of electricity; and a system producing energy from biomass with combined heat and power. All alternatives were required to maximize the utilization of existing equipment. In considering the combined heat and power production alternative, the plant would be required to reduce the consumption of imported electrical energy in the original building. After considering the technical and economic factors of the various design solutions obtained, and taking into account additional criteria such as the global trend of the price of crude oil and natural gas, electricity costs, and the political support for electric production from renewable sources, the two best alternatives were determined. The preferred design utilizes an energy production plant with a biomass boiler. An alternative choice includes the use of a combined heat and power energy plant that utilizes biomass as a fuel combined with a backpressure steam turbine for extraction.