Theoretical Study and Case Analysis of a Predried Lignite–Fired Power Plant with the Waste Heat Recovery System

Along with the construction of many coal-fired power plants, the supply of bituminous coal has become a key interest in China. An increasing number of coal-fired power plants consume lignite. However, lignite-fired power plants feature a very low plant thermal efficiency. But by means of lignite predrying, the efficiency of the lignite-fired power system can be improved, and the recovery of waste heat from the dryer can improve the plant thermal efficiency further. In the present article a theoretical model of the predried lignite–fired power plant with the waste heat recovery system based on basic thermal principles is developed and an existing case is analyzed. The above plant was shown to enhance the thermal efficiency of a conventional lignite-fired power plant by approximately 2.45%, which is 0.58% higher than predried lignite–fired power plant without waste heat recovery. Moreover, the influence of system parameters on the improvement of the plant thermal efficiency is determined. The results can be used in optimization of a waste heat recovery system in the thermodynamic process to increase the economy of lignite-fired power plants.     

Energy analysis of a lignite predrying power generation system with an efficient waste heat recovery system

Lignite has been extensively used for electricity generation in many regions worldwide. However, its high water content has obviously negative effect on plant thermal efficiency. Performance of lignite-fired power plant can be improved by predrying the lignite before combustion. In addition, recovery of waste heat from the dryer and the power generation system will enhance the plant thermal efficiency further. In the present study, a new lignite predrying power generation system integrated with an efficient waste heat recovery system was proposed. Both dryer exhaust waste heat and steam turbine exhaust latent heat were recovered to heat boiler feed water. Energy analysis indicates that system performance is improved significantly. The plant thermal efficiency increases linearly with drying degree and then increases at a lower rate. The generation of unused dryer exhaust changes the variation tendency of system performance with drying degree.     

Thermodynamic Analysis on a Lignite-Fired Power System Integrated with a Steam Dryer: Investigation on Energy Supply System of the Dryer

Lignite is becoming a competitive fuel for power generation with high security of supply and a low price. But a power plant firing lignite directly always has some weaknesses, including low thermal efficiency and high construction investment. Predrying lignite before feeding it to the boiler is a potential method to tackle these weaknesses, and low-pressure steam extracted from steam turbines is considered a competitive heat source for the drying process. In a lignite-fired power system integrated with a steam dryer, the steam extraction is led to the dryer via a connection pipe and depressurization valve. To obtain a stable drying temperature at a variable load of power, steam extraction is often used by the dryer after depressurization, which indeed causes exergy loss. To reduce the exergy loss, the steam extraction was proposed to be supplied to the dryer via a compressor or ejector. Thermodynamic models were developed to calculate the net efficiency of a power system with different energy supply systems. The energy saving boundary—that is, the net efficiency of power system could be increased in some ranges of drying temperature by some energy supply system of dryer—was obtained. Furthermore, the exergy efficiencies were calculated.     

Thermodynamic and economic analysis on a two-stage predrying lignite-fueled power plant

In this study, an improved configuration of lignite-fueled power plant integrated with a two-stage predrying system was proposed. The predrying system mainly consists of two fluidized-bed dryers and an additional feed water heater. Lignite is dried successively in the exhaust gas dryer and steam dryer. With boiler exhaust gas being the heat source of the first stage dryer, waste heat of a fraction of the boiler exhaust gas can be used. The exhaust gas of the second stage dryer was considered to be water vapor and its latent heat can be recovered by the additional feed water heater. The thermodynamic and economic analysis show that with the lignite drying degree being 0.1, 0.2, and 0.3?kg/kg, the power generation efficiency of the proposed power plant is 1.45, 2.12, and 2.81% higher than that of the conventional lignite power plant, respectively. Moreover, the annual net economic benefit will be 1.34, 2.03, and 1.60 M$/a during the lifetime of the drying system. The annual net economic benefit is not necessarily higher with higher power generation efficiency.     

Lignite drying with solar energy: Thermodynamic analysis and case study

As a clean, free, and nondepleting source, solar energy has become the focus of increasing attention in the drying industry. A lignite-fired power plant integrated with a solar dryer (LPPS), in which solar energy is used to dry lignite and the predried lignite is used to generate electricity, is analyzed theoretically in this paper. The aim of this study is to evaluate the energy performance of solar drying under different system parameters. Thermodynamic models, with which the second-law efficiency of the LPPS could be maximized, were developed. A reference case with three kinds of lignite as input fuel was analyzed to quantify the system performance. The first-law and second-law efficiencies were obtained. The solar-to-electric conversion efficiency in the LPPS is more than 34%. Therefore, solar drying is a potential technology that should be promoted in lignite-deposited areas. Moreover, the influence of main parameters on the performance of system was analyzed. Dryer efficiency is determined to have significant influence on the solar-to-electric conversion efficiency.     

Energy,exergy and economic analyses on heat pump drying of lignite

Abstract

Evaporative drying of lignite is an energy intensive process. In this study, the heat pump is integrated with a lignite drying system to decrease the energy consumption rate of lignite drying. The performance of heat pump drying is energetically and exergetically evaluated with developed models. Results show that the power consumption rates to dehydrate 1?kg of water from raw lignite in the heat pump drying system without and with lignite preheater are 660.82 and 585.62?kJ (kg H₂O)^?1, respectively. Exergetic analysis indicates that most exergy is destructed in the condenser and the evaporator in the heat pump drying. The case of lignite-to-electricity process (i.e., a lignite-fired power plant integrated with heat pump drying) is studied to examine additional benefits of heat pump drying to the downstream industrial processes that consume dried lignite. Thermodynamic and economic models are developed. Net efficiency of the lignite-to-electricity process can be increased by 1.4 and 1.57 percentage points for heat pump drying without and with lignite preheater, respectively. Preliminary economic analysis shows that the integration of heat pump drying without and with lignite preheater can earn additional 1.42 and 1.73 million USD, respectively. The influences of drying system and heat pump parameters are also analyzed.     

A Combined-Type Fluid-Bed Dryer Suitable for Integration Within a Lignite-Fired Power Plant: System Design and Thermodynamic Analysis

Lignite is becoming a competitive fuel for power plants, offering very high security of supply and cost-effectiveness. However, power plants firing lignite face some thorny issues, such as high carbon dioxide emissions, high investment in construction, etc. Lignite pre-drying is considered an attractive way to tackle these issues, but it consumes a lot of energy and has a high risk of ignition. Thus, a combined-type fluid-bed dryer, with which lignite could be dried safely using some waste from a power-generation process, is proposed in this paper. Both boiler exhaust flue gas and steam extraction of steam turbines are used as heat sources for this kind of dryer. To analyze its thermodynamics, a theoretical model was developed with which a reference case of a 1000 MW air-condensing power plant was performed. The results show that a dryer integrated within the power plant can evidently increase the plant efficiency by approximately 2.55%. The main factors, including the degree of pre-drying, dryer thermal efficiency, and the temperature of dryer exhaust, were analyzed. The results show that the degree of pre-drying has the most obvious influence. A 0.1 increase in pre-drying degree improves the plant thermal efficiency by about 0.62%, while a 10°C decrease of dryer exhaust temperature and a 10% increase in dryer thermal efficiency could improve the plant thermal efficiency by about 0.10% and 0.22%, respectively.     

Theoretical Study and Case Analysis for a Predried Lignite-Fired Power System

Lignite is a type of low-rank coal, which is uneconomically transported over long distances and does not efficiently generate electricity. As a result, lignite utilization and application is very limited; lignite is mainly used as a low-level fuel that generates electricity inefficiently and can be distributed only in areas near lignite mines. With increasing requirements in efficiency and environmental protection, studying a new lignite-fired power system to improve the efficiency of direct lignite-fired power plants is very important. A rotary-tube dryer is mature drying equipment used in a pre-dried lignite-fired power system (PLPS). However, a comprehensive model approach on the influence of PLPS parameters on power generation efficiency and optimization has not been previously investigated. In the current paper, a PLPS theoretical model was developed based on basic thermal principles, and a case analysis was performed using this model as the theoretical foundation. Parameter influence was also calculated and analyzed. Results show that the PLPS theoretical model can evidently increase the efficiency of a conventional lignite-fired power system (CLPS) by approximately 1.87% when the condensate is sent to the de-aerator, and by 1.72% when the condensate is sent to the condenser at the calculation benchmark condition listed in this paper.     

火电机组余热梯级供热技术的综合分析

针对传统供热技术平均?效率低的问题,提出了乏汽余热梯级供热技术。该技术有效地利用了机组乏汽余热,并减少了汽轮机的冷源损失,避免了中排抽汽参数过高造成的能量损失,提高了机组供热经济性。以某电厂为例,本文基于热力学定律与单耗分析理论,建立了乏汽余热梯级供热系统的单耗分析模型,应用该模型对其进行了?分析和附加燃料单耗分析,为节能降耗提供了理论依据。分析结果表明,发电煤耗率由改造前的249.69g/(kW·h)降低到149.9g/(kW·h),降低了99.79g/(kW·h),机组供热负荷较由改造前的788.4MW增加到1673.9MW,增加了885.5MW;改造后传热温差在非严寒期只有7℃、严寒期为26.94℃,比改造前大幅降低;在非严寒期和严寒期,加热器蒸汽入口平均比?比改造前分别降低了524.73kJ/kg、418.2kJ/kg,供热系统的平均?效率比改造前分别提高了51.35%、32.58%,供热系统的平均附加燃料单耗分别为3.11g/(kW·h)、7.98g/(kW·h)。可见通过“乏汽余热梯级供热技术”改造后,节能效果显著,该技术具有广阔的推广应用前景。     

Innovative Steam Drying of Empty Fruit Bunch with High Energy Efficiency

Empty fruit bunch (EFB) is one of the solid wastes from crude palm oil mills and has the lowest value for utilization compared to other solid wastes. To achieve an efficient utilization of EFB, drying is considered the first crucial process due to the high moisture content of EFB. In this study, EFB drying based on exergy recovery is proposed to achieve high energy efficiency. A fluidized bed is adopted as the main dryer. The proposed model is evaluated in terms of energy efficiency, especially regarding the influence of target moisture content and fluidization velocity. Up to 92% of the energy involved in the drying process can be recirculated. The total energy consumption for drying decreases as the target moisture content decreases, though there is no significant impact of fluidization velocity to total energy consumption. In addition, the required total length of the heat transfer tubes immersed inside the fluidized bed dryer is calculated because it relates to fluidization performance and economic issues. Lower target moisture content results in a longer heat transfer tube, and higher fluidization velocity leads to a shorter heat transfer tube.     

Exergy analysis of a spray dryer: Methods and interpretations

Exergy analysis has been used to assess the intrinsic exergy efficiency of a spray drying system modeled to produce 1.25?kg s^?1 of skim milk powder. From an exergy perspective, the dryer has a low exergy efficiency of 38% (on an evaporation basis), while the efficiencies associated with the mass transfer and heat transfer are 94% (thermomechanical efficiency) and 30% (transiting exergy efficiency), respectively. The improvement potential of 575?kW, of the 722?kW energy flow in the feed, also shows that the exergy efficiencies of spray dryers are intrinsically small. Reviewing exergy efficiency factors, there appears to be no universal efficiency factor for an exergy analysis. The inevitable (INE) exergy loss method is a potential shortcut technique based on the Carnot efficiency and first law analysis. There are some limitations on using the INE method for processes that are not exclusively thermal; in those cases, an entropy balance (second law property) is more appropriate. The INE method still shows potential as a starting basis of comparison because it shows the scale and the efficiency together, which is important for targeting areas for process improvement without doing a full exergy analysis. This work is a short review of the work on dryer exergy efficiency, mainly focusing on the various factors which are used, followed by a discussion and case study testing each factor to find a potential optimization method and a discussion on each factors merits.     

焦炉多余热回收系统的分析及优化

为了研究焦炉多余热回收系统中能量的利用情况，依据分析理论，通过对某焦化厂实际案例的计算，对现有的余热回收方案进行分析，指出3个子系统运行过程中的能量回收的薄弱环节。结果表明：干熄焦、荒煤气、烟气余热回收系统的效率分别为55.16%、17.18%、51.75%，干熄焦系统的损主要为换热过程中产生的不可逆换热损失，荒煤气系统的损主要为出口损以及不可逆换热损失，烟道系统的损主要为烟气出口损。在此基础上依据各等级能量匹配利用的原则对原方案进行优化，并使用分析理论对其计算并分析。结果表明：优化过后的余热回收系统的总效率为58.72%，相比优化之前提高了11.07%，系统总不可逆换热损降低了155.49MJ/t干煤。     

大型吸收式热电联产机组性能分析与对比

燃煤热电联产机组在电能与热能的协调和转换中起到至关重要的作用,并且在未来一段时间内仍是我国北方冬季采暖的主力热源。本文构建了包含电厂侧和热力站侧两部分在内的一种热电联产系统方案,并研究了机组背压、供给热电比、汽水系统?效率、热网换热器?效率、机组发电量、热网水泵功率在变工况下和单独改变供热功率、供水温情况下的变化规律,开创了电热价比的方法对电热两种捆绑式生产的热电联产机组来衡量其整体经济收益,最后通过热网水质量流量、热电比、背压、抽凝比和发电煤耗率这些指标将新方案与传统热电联产方案进行了对比。结果表明：对于热电联产机组,系统热力学参数随工况、供热负荷和供水温度变化呈现不同规律;本文新提出的电热价比方法能为热电联产机组提供运行决策和收益预测规划;新方案可通过提升热网供回水温差可以显著提高系统供热能力和能源利用效率：使得发电煤耗率减小3.22～7.00g/(kW·h),机组背压降低了65.07%,热网水流量减少了33.33%。     

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。     

A Comparative Study on Exergetic Performance Assessment for Drying of Broccoli Florets in Three Different Drying Systems

This article deals with the exergy analysis and evaluation of broccoli in three different drying systems. The effects of drying air temperature on the exergy destruction, exergy efficiency, and exergetic improvement potential of the drying process were investigated. The exergy destruction rate for the drying chamber increased with the rise in the drying air temperature at 1.5 m/s, both in the tray and the heat pump dryer. The highest exergy efficiency value was obtained as 90.86% in the fluid bed dryer in comparison to the other two drying systems and the improvement potential rate was the highest in the heat pump dryer during drying of broccoli at the drying air temperature of 45°C and the drying air velocity of 1.0 m/s.     

Inversion Temperature and Pinch Analysis,Ways to Thermally Optimize Drying Processes

This work studies the compatibility and suitability of a combined inversion temperature and pinch analysis with the process selection for air and superheated steam spray drying of milk solids. The inversion temperature is a good starting point for an energy analysis because it is a simplified rate-based approach to comparing the steam and air drying systems. pinch analysis enables process integration, at least on a heat recovery and heat exchanger network level.

The resulting inversion temperature for the studied system was estimated as 182°C for the dryer inlet temperature. However, mass and energy balances showed that a minimum inlet temperature for spray drying of 184°C was required for the superheated steam dryer in order to ensure that the outlet solids temperature above the dew point temperature.

The inversion temperature is still very relevant in the early stages of a design process because it allows a quick assessment of which drying medium should result in a smaller dryer. It was evident that the steam system is better from an energy perspective because of the recoverable latent heat of the water vapor carried out of the dryer with the recycled steam. The steam system has between 82 and 92% of thermal energy recovery potential as condensable steam, compared with 13–30% energy recovery of the air system. However, other important design and operational factors are not discussed here in detail.

Combining the inversion temperature and pinch analysis suggests that superheated steam drying both gives better energy recovery and is likely to give smaller dryers for all operational conditions.     

Performance Analysis and Assessment of an Industrial Dryer in Ceramic Production

In recent years, exergy analysis has been widely used in the design, operation, and performance assessment of various thermal systems, among which drying, which is an energy intensive operation, is of a great importance. In the ceramic industry, it is aimed at utilizing a minimum amount of energy in order to remove the maximum moisture for the desired final conditions of the product to be dried. In this study, energy and exergy analyses of a ceramic plant, located in Izmir, Turkey, with a yearly production capacity of 24 million m² were performed using the actual operational data over a period of 12 months. The drying system at the three stages was analyzed and the values for exergy destruction and efficiency for each component of the system and the whole system at a reference (dead state) temperature of 22°C were calculated. For the month of January, energy and exergy efficiencies for the spray dryer (SD) were determined to be 65.50 and 53.7%, respectively. Energy and exergy efficiency values of the vertical dryer (VD) were 45.12 and 43.3%, respectively, and those of the furnace (F) were 35.08 and 16%, respectively. Based on this one-year assessment, the energy efficiency values for the SD, VD, and F varied between 58.48 and 65.50%, 42.44 and 50.87%, and 30.44 and 36.99%, and the exergy efficiency values were in the range of 44.85–65.16%, 34.92–45.42%, and 12.73–16.41%, respectively.     

GASIFICATION OF GREEK LIGNITE IN AN INDIRECT HEAT (ALLOTHERMAL) ROTARY KILN GASIFIER

This work reports the performance results of a pilot-size lignite gasification plant. The feed material was Greek lignite (Megalopolis), currently being employed for electricity generation in pulverized lignite-fired thermoelectric stations. Low energy conversion efficiency, low station availability, and environmental issues call for developing improved processes, e.g., an IGCC (Integrated Gasification Combined Cycle). An indirect heat (allothermal) rotary kiln was selected as the lignite gasification reactor for developing an overall gasification process of improved efficiency. Weeklong gasification runs, at near atmospheric pressure and maximum temperature in the range 900-950°C, validated high DAF lignite conversions, i.e., 90-95%, and the production of a medium heating value synthesis gas (i.e., 11-13 MJ/Nm 3 dry basis), despite the use of air for burning recycled product gas for process heating. Gas composition is equivalent to that of autothermal gasifiers (e.g., Lurgi, Winkler, Koppers-Totzek), which operate on oxygen, under pressure and strict moisture and particle size specifications. Similarly, the kiln gas is comparable to that of an allothermal, high-pressure, fluidized bed gasifier running with a high rank coal feed. The data indicate satisfactory gasification efficiency and a good thermal efficiency that should be improved further through heat integration of a scaled-up process based on an indirect heat rotary kiln gasifier.     

Gasification of greek lignite in an indirect heat (allothermal) rotary kiln gasifier

This work reports the performance results of a pilot-size lignite gasification plant. The feed material was Greek lignite (Megalopolis), currently being employed for electricity generation in pulverized lignite-fired thermoelectric stations. Low energy conversion efficiency, low station availability, and environmental issues call for developing improved processes, e.g., an IGCC (Integrated Gasification Combined Cycle). An indirect heat (allothermal) rotary kiln was selected as the lignite gasification reactor for developing an overall gasification process of improved efficiency. Weeklong gasification runs, at near atmospheric pressure and maximum temperature in the range 900-950°C, validated high DAF lignite conversions, i.e., 90-95%, and the production of a medium heating value synthesis gas (i.e., 11-13 MJ/Nm 3 dry basis), despite the use of air for burning recycled product gas for process heating. Gas composition is equivalent to that of autothermal gasifiers (e.g., Lurgi, Winkler, Koppers-Totzek), which operate on oxygen, under pressure and strict moisture and particle size specifications. Similarly, the kiln gas is comparable to that of an allothermal, high-pressure, fluidized bed gasifier running with a high rank coal feed. The data indicate satisfactory gasification efficiency and a good thermal efficiency that should be improved further through heat integration of a scaled-up process based on an indirect heat rotary kiln gasifier.