A new look at dual purpose, water and power, plants - economy and design features

In light of rapidly rising equipment and fuel costs, recent studies have shown some important results in the economy and optimum designs for dual purpose power/desalting complexes. The desalination cycle chosen for- detailed comparisons in this study is the well known multistage flash (MSF) evaporator, which uses brine recirculation and polyphosphate scale prevention. The multi- stage flash evaporator has found wide spread application in large plants throughout the world. Single units of up to 1400 m3/hr (9.5 MGD), acid dosing, have been built and are now in operation. In general due to corrosion problems, polyphosphate plants are now preferred over the acid one.

A number of schemes for the combined production of power and. water were chosen, mainly a combination of MSF Unit with each of the following power cycle:

• 1- Automatic extraction steam turbine

• 2- Simple gas turbine with waste heat boiler

• 3- Back pressure steam turbineThis paper presents a study of the economic aspects, thermodynamic features and optimization analysis of each of these combined power and water production plants.

The optimum value of the performance ratio and its effect on reducing the cost of water in each of the above mentioned schemes would be discussed in this paper.     

Systems analysis of dual-purpose nuclear power and desalting plants Part I. Optimization

The optimal process design of a dual purpose plant for producing power and water is investigated. A nuclear reactor and steam turbine power generator for steam and power production is coupled with two water plants, a multi-stage flash plant and a reverse osmosis plant. The total system cost for producing given levels of power and water is minimized. Optimal designs are presented for several combinations of water and external power demands ranging from 25 mgd to 150 mgd and from 50 MWe to 200 MWe.     

Dual purpose power/water plants utilizing both distillation and reverse osmosis

The energy consumption of several alternate dual purpose plants are compared for application in the range of 10–50MW with 1 to 20 MGD water production. This shows that the combined gas turbine-steam turbine system is considerably more energy efficient than a steam only system. Single stage R.O., used in conjunction with this combined cycle offers the minimum overall energy consumption but has the disadvantage of producing product water with high TDS. By utilizing both R.O. and distillation, energy consumption lower than with distillation alone is achieved and product water purity is acceptable. p]A specific design of a combined dual purpose plant is presented. This plant would have a net electrical output of 29,050 kw and 3.25 MGD of 440 ppm TDS, requiring 297.1 BTU/hr. The total capital costs of this combined plant is estimated at $41,150,000 and annual operating costs at $15,087,000. The unit production costs with fuel at $2.50/MM BTU would be 4.08¢/kw-hr and $2.44 per 1000 gal. This represents an annual savings of $1,961,000 over single purpose production or 44.5% reduction in water production costs with the same electrical production costs. p]It is concluded that the combined dual purpose plant presented is the most efficient, economical and flexible method of producing power and water in the range of 10 to 50 MW and 1 to 20 MGD.     

Fuel allocation in dual-purpose plants

Exergy provides the cost accounting equations for allocating energy in any plant. Sets of complex auxiliary equations can be derived to allocate energy cost for each plant item and system. Here a simplified alternate method has been developed utilizing the First Law of Thermodynamics with consideration of the cost factors to distribute fuel in dual-purpose boiler turbine generator (BTG)-MSF plants. The method is based on a formulation of an ideal point at which power and water products are equivalent.     

考虑水需求的水电联产海水淡化系统的优化设计

水电联产不仅能缓解淡水资源不足的问题,而且可有效降低能耗和淡化成本。建立了水电联产系统数学模型,将优化设计描述为一个混合整数非线性规划(MINLP)问题,并采用混合编码的遗传算法进行求解,结果表明,以水定电模式下水电联产系统最优操作模式为发电、多级闪蒸(MSF)和反渗透(RO)三者的集成,且MSF和RO的产水比存在最优值,发电采用背压式蒸汽轮机。随着淡水需求量的增加,联产系统的淡水成本逐渐降低,MSF与RO的产水比呈现出逐渐降低的趋势。     

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 η_c = 0.388, or by new combined gas/steam turbines power cycle GT/ST-CC of typical η_c = 0.54 under construction. The SWRO with energy recovery is assumed to consume typical 5.2 kWh/m³ electric energy.     

以生物质为燃料的SOFC和发动机热电联供系统：参数分析和性能优化

针对清洁高效能源转换技术需求,提出了一种以生物质为燃料的新型混合发电系统,该系统由生物质气化装置、固体氧化物燃料电池、发动机和余热回收子系统组成。采用Aspen Plus对系统进行了热力学建模,基于建模结果进行了参数分析,以确定关键参数对系统性能的影响。同时,通过ε-constraint的方法进行了效率最大和比发电成本最小的双目标优化。结果表明：随着蒸汽生物质比S/B的增加,系统发电效率从47.3%增加到50.3%;随着燃料利用率的增加,发电效率从45.5%增加到48.2%;入口生物质量和空气当量比的增加会使发电效率呈现下降趋势。在Pareto最优解的情况下,该混合系统可以同时达到系统效率为53.5%,比发电成本SEEC为0.0576 USD/（kW·h）,与标准电厂的能源成本（0.0546 USD/（kW·h））相当,而与以天然气为燃料的SOFC-发动机系统相比则降低了19.6%,说明该新型热电联供系统是一种清洁、高效、经济的能源转换技术。     

Optimization of potential and hydrogen utilization in an acid fuel cell

For a given cell configuration and a total pressure of 1 atm, a fuel cell is designed for a constant power load. This involves the determination of the extent of electrode area and the potential or average current density at which to operate the device. For a given air to fuel ratio, the optimum utilization of hydrogen is also determined on the basis of the resale value of unused fuel. The cost analysis is simplified to use a single value for capital cost and a single value for operating or fuel cost.     

An integrated self sufficient multipurpose plant for seawater desalination

A multipurpose plant is described, which produces drinking water and chemicals from seawater. The most important part of the plant consists in a combined MSF/VT evaporator, which operates on a high economy ratio and enables also a high concentration of the brine. These two facts are due to a special treatment of the evaporator feedwater by chemical agents produced from the concentrated brine. A surplus of these chemicals can be sold. The energy supply of the complex is assured by an adequate power station, which provides process steam and electricity at such a ratio as to minimize fuel cost.     

Cost optimization of a combined power and water desalination plant with exergetic,environment and reliability consideration

The present study deals with the multi-objective optimization for designing a combined gas turbine and multi stage flash desalination plant. In optimization approach, the exergetic, economic and environmental aspects have been considered, simultaneously. In order to achieve the optimal design, Multi-objective genetic algorithm (MOGA) is applied as a suitable optimization technique. The thermoenvironomic objective function is obtained by integrating the environmental impacts and thermoeconomic objective. By applying the optimization approach, this objective function is minimized, whereas system exergy efficiency is maximized. Moreover, equipment reliability using the state-space and the continuous Markov method is incorporated in optimization results to improve the products' cost values. The optimization results show that the cost of products and environmental cost impact are reduced by 13.4% and 53.4%, respectively, whereas a 14.8% increase happens in total exergy efficiency. Therefore, improvement in all objectives has been achieved using the optimization process, although the power and water productions have not changed much. Additionally, the sensitivity analysis shows the relationship between the fuel cost, pollution damage cost and the objective functions.     

热电联产及其燃气轮机蒸汽联合循环

阐述了各种热电联产及燃气轮机蒸汽联合循环的形式、适用燃料及效率,介绍了燃气轮机联合循环发电及供热在我国合成氨厂的应用,指出燃气-蒸汽联合循环热电联产将是热电联产的发展方向。     

氧化钙-二氧化碳循环系统脱碳的分析研究

采用燃料氧燃烧直接供热的氧化钙-二氧化碳双床循环系统已经被研究开发,用于从电厂尾气中脱碳。本研究基于实验和化学反应工程原理建立了分析模型,用于系统地研究循环特征,氧化钙活性衰减,燃料及其炉内热利用率的影响。基于模型推导获得了重要参数:最小循环热损失和最小热需求量,以及对应的固体循环比。它们都受供热燃料灰分和含硫量的影响,也受脱碳率的影响。显然最佳固体循环比介于二者之间。另一个重要参数是燃料在炉内的热利用率。高的热利用率不仅降低燃料需求量,降低其灰分和含硫的影响,降低氧的需求量及其辅助功,而且提高蒸汽循环的发电效率。一个发现是热需求量在临界固体循环比接近无穷大,这就限制了固体循环比的可操作范围, 以及燃料的灰分和含硫量。建立的分析模型和推导直接提供了这些变量之间的关系和范围。     

热能在乙烯厂生产中的合理利用

蒸汽动力系统是石化企业的重要组成部分,为整个生产过程提供蒸汽、电力等公用工程。由于生产中产品品种变化及季节不同,必然引起公用工程用量的变化,相应的蒸汽动力系统能否适应所发生的变化,是否具有可调性,将决定企业的产品成本、经济效益和能源利用程度。热能合理利用成为工厂深入挖潜、节能增效的一项重要工作。     

Hydraulic turbine energy recovery - R.O. System

In reverse osmosis desalination plants large flow rates of concentrated brine are discharged at high pressure from the membrane modules. Currently this pressure energy is wasted. This paper reviews the impact of soaring energy costs on the technical alternative of hydraulic turbine energy recovery. The capital costs, operating costs, and economics in electrical energy are evaluated for both sea-water and brackish water systems. Schemes of hydraulic turbine coupled with electric generator, tied to the electric power supply are considered for various plant sizes from 1 to 10 mgd. The analysis, in parametric form, presents the interrelationships between the cost of money, cost of electrical energy, and recovery factors for the different plant sizes and for sea-water and brackish water systems. The results will serve as a guide to determine when such a power recovery system should be seriously considered and evaluated in greater details for specific applications.     

Membrane processes for water supply and reuse - a question of energy consumption and cost

This paper is limited primarily to reverse osmosis which is the dominating membrane process in commercial plants. Desalination of brackish water and seawater with reverse osmosis, with special emphasis on costs and energy consumption, is the primary subject discussed in the paper. Some aspects of and development trends in industrial and domestic applications of membrane processes are also taken up, particularly with regard to by-product recovery and water reuse in connection with advanced wastewater treatment.The first RO plant to be brought into operation in Riyadh, Saudi Arabia, is located at Salbukh. The investment and total operation costs for this plant have been calculated in the paper. The water cost is at least twice as high as in a continental U.S. location. The main reason for this is the very high cost of civil and local works in Saudi Arabia. A similar calculation has been made for RO seawater desalination.Increased energy costs during the last decade have directed research and development work for all desalination methods towards reducing energy consumption. It is shown in the paper that energy recovery in connection with RO seawater desalination is particularly feasible. Different methods for energy recovery have been investigated and reported, the preferred methods depending on the size of the RO plant. A large underground RO plant for energy recovery, based on utilization of the static pressure instead of high pressure pumps, has also been studied.Another possible energy-saving, but also water quality improving method has been proposed, viz . a combined MSF-RO dual purpose plant. Excess power for reverse osmosis seems to be more and more available in Saudi Arabia due to the high power/water ratio in MSF dual purpose plants compared to the real demand for power and water.     

Technical and economic analysis for the integration of small reverse osmosis desalination plants into MAST gas turbine cycles for power generation

A technical and economic analysis concerning the integration of small reverse osmosis (RO) desalination plants into mixed air steam turbine (MAST) technologies for power generation was carried out. The simulation tool used is the computer aid reverse osmosis calculations optimization algorithm. This user-friendly software takes into account the capital cost, fuel cost and operation and maintenance requirements of each candidate RO desalination plan scheme and calculates the least-cost configuration. The results indicate that the integration of a RO desalination plant into MAST gas turbines has a minor effect on the final operating cost of the power plant.     

The Curacao KAE-LT-MED and auxiliary steam turbine project: A model for dual purpose MSF plants replacement

KAE, the water and power company of Curacao, has contracted to replace its older and less efficient MSF plants with a 10,000 m³/day Low Temperature, Horizontal Tube, Falling Film, Multi Effect Distillation (LT-MED) plant.The plant operates at a top brine temperature of 70 degrees C and requires steam at only 0.34 bar a.However, motive steam is available, as extraction from the power station's turbines, at 2.55 bar a, 155 °C, as required by the higher temperature MSF plant. This excess pressure potential is used to generate 3.1 MW of power in an auxiliary low pressure steam turbo-generator, thereby improving overall economy and lowering desalted water cost to a level unequalled by any other system. The paper describes the power station turbine, auxiliary low pressure turbo generator and LT-MED plant combined scheme, its operational features and economics.     

双压膨胀有机朗肯循环中低温余热发电系统的热力性能

基于热力学第一定律与第二定律分析法,结合动量定理与质量守恒定律构建一种双压膨胀有机朗肯循环（ORC）中低温余热发电系统热力性能的预测模型,研究热源温度、蒸发温度、夹点温差、闪蒸压力、循环倍率、喷射压缩器引射流体在混合室进口的马赫数、工质泵、向心透平及螺杆膨胀机的等熵效率对系统的净输出功率、热效率、余热利用率及(火用)效率等热力性能的影响。结果表明,系统的净输出功率与(火用)效率随着热源温度、蒸发温度、循环倍率及透平等熵效率的增大而升高,但随着夹点温差、闪蒸压力的增大而降低;系统的热效率随着蒸发温度、闪蒸压力及透平等熵效率的增大而升高;系统的余热利用率随着热源温度、循环倍率的增大而升高,但随着蒸发温度、夹点温差、闪蒸压力的增大而降低。当循环倍率k=2时系统的净输出功率、(火用)效率及余热利用率分别比常规单级ORC系统绝对提高了230.9kW、10.1%和17.9%。