An upstream reboiler design for removal of noncondensable gases from geothermal steam for Kizildere geothermal power plant, Turkey

Kizildere geothermal power plant, Turkey, has an installed capacity of 20.4 MW_e. The field contains a high level of noncondensable gases (NCGs), changing from well to well, in amounts as high as 10–20% (with an average of 13% at the inlet of the turbine) by weight of steam. This amount of NCGs is being extracted from the condenser by gas compressors that consume about 17% of the total power production of the plant.An upstream reboiler process could be adopted to remove the NCGs from geothermal steam before they enter the turbine. Upstream reboilers therefore provide a cleaner and less corrosive steam supply to the turbine and condenser, increasing power generation performance for very high NCG contents.In this paper, upstream reboiler systems are investigated as an alternative to conventional gas extraction systems for Kizildere geothermal power plant. A vertical tube type reboiler has been designed and it is found that, as NCG content increases, the condensation heat transfer coefficient reduces steeply. It is concluded that vertical tube type reboilers are not efficient for fields that contain high levels of NCG (>15% by weight of steam). It is recommended that the use of direct contact reboilers be further investigated for this application.     

Oxygen-carrier selection and thermal analysis of the chemical-looping process for hydrogen production

The three-reactor chemical-looping process (TRCL) for the production of hydrogen from natural gas is quite attractive for both CO₂ capture and hydrogen production. The TRCL process consists of a fuel reactor, a steam reactor and an air reactor. In the fuel reactor, natural gas is oxidized to CO₂ and H₂O by the lattice oxygen of the oxygen carrier. In the steam reactor, the steam is reduced to hydrogen through oxidation of the reduced oxygen carrier. In the air reactor, the oxygen carrier is fully oxidized by air. In this process, the oxygen carrier is recirculated among the three reactors, which avoids direct contact between fuel, steam and air. In this study, various candidate materials were proposed for the oxygen carrier and support, and a thermal analysis of the process was performed. The oxygen carrier for the process must have the ability to split water into hydrogen in its reduced state, which is a different chemical property from that of the chemical-looping combustion medium. The selection of the oxygen carrier and support require careful consideration of their physical and chemical properties. Fe₂O₃, WO₃ and CeO₂ were selected as oxygen carriers. Thermal analysis indicated an expected hydrogen production of 2.64 mol H₂ per mol CH₄ under thermoneutral process conditions. The results indicated that hydrogen production was affected mainly by the steam-conversion rate. The solid-circulation rate and temperature drop in the fuel reactor were calculated for the selected oxygen carriers with different metal oxide contents and solid-conversion rates.     

Two novel oxy-fuel power cycles integrated with natural gas reforming and CO2 capture

Two novel system configurations were proposed for oxy-fuel natural gas turbine systems with integrated steam reforming and CO₂ capture and separation. The steam reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. Internal combustion is used, which allows a very high heat input temperature. Moreover, the turbine working fluid can expand down to a vacuum, producing an overall high-pressure ratio. Particular attention was focused on the integration of the turbine exhaust heat recovery with both reforming and steam generation processes, in ways that reduce the heat transfer-related exergy destruction. The systems were thermodynamically simulated, predicting a net energy efficiency of 50–52% (with consideration of the energy needed for oxygen separation), which is higher than the Graz cycle energy efficiency by more than 2 percentage points. The improvement is attributed primarily to a decrease of the exergy change in the combustion and steam generation processes that these novel systems offer. The systems can attain a nearly 100% CO₂ capture.     

Aspen Plus simulation of biomass integrated gasification combined cycle systems at corn ethanol plants

Biomass integrated gasification combined cycle (BIGCC) systems and natural gas combined cycle (NGCC) systems are employed to provide heat and electricity to a 0.19 hm³ y⁻¹ (50 million gallon per year) corn ethanol plant using different fuels (syrup and corn stover, corn stover alone, and natural gas). Aspen Plus simulations of BIGCC/NGCC systems are performed to study effects of different fuels, gas turbine compression pressure, dryers (steam tube or superheated steam) for biomass fuels and ethanol co-products, and steam tube dryer exhaust treatment methods. The goal is to maximize electricity generation while meeting process heat needs of the plant. At fuel input rates of 110 MW, BIGCC systems with steam tube dryers provide 20–25 MW of power to the grid with system thermal efficiencies (net power generated plus process heat rate divided by fuel input rate) of 69–74%. NGCC systems with steam tube dryers provide 26–30 MW of power to the grid with system thermal efficiencies of 74–78%. BIGCC systems with superheated steam dryers provide 20–22 MW of power to the grid with system thermal efficiencies of 53–56%. The life-cycle greenhouse gas (GHG) emission reduction for conventional corn ethanol compared to gasoline is 39% for process heat with natural gas (grid electricity), 117% for BIGCC with syrup and corn stover fuel, 124% for BIGCC with corn stover fuel, and 93% for NGCC with natural gas fuel. These GHG emission estimates do not include indirect land use change effects.     

Combined slow pyrolysis and steam gasification of biomass for hydrogen generation—a review

Hydrogen, the inevitable fuel of the future, can be generated from biomass through promising thermochemical methods. Modern‐day thermochemical methods of hydrogen generation include fast pyrolysis followed by steam reforming of bio‐oil, supercritical water gasification and steam gasification. Apart from the aforementioned methods, a novice technique of employing combined slow pyrolysis and steam gasification can be also engaged to produce hydrogen of improved yield and quality. This review paper discusses in detail about the existing hydrogen generation through thermochemical methods. It elaborates the merits and demerits of each method and gives insight about the combined slow pyrolysis and steam gasification process for hydrogen generation. The paper also elaborates about the various parameters affecting integrated slow pyrolysis and steam gasification process. Copyright © 2014 John Wiley & Sons, Ltd.     

An energy-exergy analysis of the Koppers-Totzek process for producing hydrogen from coal

Energy and exergy analyses of the Koppers-Totzek (KT) coal gasification process were performed. The KT process involves the partial oxidation of coal with oxygen and steam to produce a synthesis gas, and the treatment of the synthesis gas to yield hydrogen. The analyses were carried out using a process-simulation computer code which had been enhanced by the authors for exergy analysis. The exergy efficiency for the overall process was found to be 49%, and the energy efficiency 59%. The majority of energy losses was found to be associated with emissions of cooling water and stack gas. The majority of exergy losses was found to be due to internal consumptions, particularly within the gasification and steam generation systems. The losses associated with the cooling water and stack gas streams, although significant on an energy basis, are shown to be not that important on an exergy basis.     

超临界机组的合金管蒸汽侧氧化膜生长与剥落研究进展

超临界机组高温合金管氧化膜剥落问题是困扰机组安全与经济运行的难题,严重制约了机组蒸汽参数和效率的提高。特别是,在超临界机组采用给水加氧处理方式(OT)后,奥氏体不锈钢管内壁氧化膜大面积剥落事故屡见不鲜,尤以TP347H合金管为甚。本文总结了近年来国内外针对超临界机组合金管氧化膜研究的进展及相关成果,首先介绍了超临界蒸汽环境中合金管氧化机理和原子迁移机制,综述了铁素体和奥氏体合金表面氧化膜的形貌特征,分析了蒸汽溶氧对氧化膜生长速率、形貌和缺陷的影响。氧化膜完整性是决定合金抗腐蚀性能的重要因素,但在机组运行过程中氧化膜应力破坏了氧化膜完整性。进一步总结了国内外氧化膜应力和剥落研究的数值分析及实验研究情况,为我国超临界机组氧化膜剥落故障诊断研究提供参考。     

Main routes for the thermo-conversion of biomass into fuels and chemicals. Part 1: Pyrolysis systems

Since the energy crises of the 1970s, many countries have become interest in biomass as a fuel source to expand the development of domestic and renewable energy sources and reduce the environmental impacts of energy production. Biomass is used to meet a variety of energy needs, including generating electricity, heating homes, fueling vehicles and providing process heat for industrial facilities. The methods available for energy production from biomass can be divided into two main categories: thermo-chemical and biological conversion routes. There are several thermo-chemical routes for biomass-based energy production, such as direct combustion, liquefaction, pyrolysis, supercritical water extraction, gasification, air–steam gasification and so on. The pyrolysis is thermal degradation of biomass by heat in the absence of oxygen, which results in the production of charcoal (solid), bio-oil (liquid), and fuel gas products. Pyrolysis liquid is referred to in the literature by terms such as pyrolysis oil, bio-oil, bio-crude oil, bio-fuel oil, wood liquid, wood oil, liquid smoke, wood distillates, pyroligneous tar, and pyroligneous acid. Bio-oil can be used as a fuel in boilers, diesel engines or gas turbines for heat and electricity generation.     

Exergetic analysis of solar concentrator aided natural gas fired combined cycle power plant

This article deals with comparative energy and exergetic analysis for evaluation of natural gas fired combined cycle power plant and solar concentrator aided (feed water heating and low pressure steam generation options) natural gas fired combined cycle power plant. Heat Transfer analysis of Linear Fresnel reflecting solar concentrator (LFRSC) is used to predict the effect of focal distance and width of reflector upon the reflecting surface area. Performance analysis of LFRSC with energetic and exergetic methods and the effect, of concentration ratio and inlet temperature of the fluid is carried out to determine, overall heat loss coefficient of the circular evacuated tube absorber at different receiver temperatures. An instantaneous increase in power generation capacity of about 10% is observed by substituting solar thermal energy for feed water heater and low pressure steam generation. It is observed that the utilization of solar energy for feed water heating and low pressure steam generation is more effective based on exergetic analysis rather than energetic analysis. Furthermore, for a solar aided feed water heating and low pressure steam generation, it is found that the land area requirement is 7 ha/MW for large scale solar thermal storage system to run the plant for 24 h.     

Oxygen permeation and coal-gas-assisted hydrogen production using oxygen transport membranes

A tubular oxygen transport membrane (OTM) was developed to produce hydrogen via water splitting using fossil sources. In this study, two OTM materials, La_0.7Sr_0.3Cu_0.2Fe_0.8O_3−δ (LSCF) and BaFe_0.9Zr_0.1O_3−δ (BFZ), were prepared by a conventional solid-state technique. In tests with an LSCF thin-film tube (thickness ≈30 μm) as an OTM, hydrogen was produced by flowing simulated product streams from coal gasification on one side of the OTM and steam on the other side. In this method, the coal gas on the oxygen-permeate side drives the removal of oxygen from the other hydrogen-generation side of the OTM, where hydrogen and oxygen are produced by water splitting. With CO (99.5% purity) flowing on the oxygen-permeate side, the hydrogen production rate of the LSCF tube was measured to be ≈19.6 cm³/min at 900 °C, indicating that hydrogen can be produced at a significant rate by using product streams from coal gasification. Concentration polarization effects were found to lower the hydrogen production rate of the LSCF thin-film tube at high temperatures. This process also yields a CO₂-rich product stream that is ready for sequestration. The other candidate OTM material, BFZ, was tested by measuring its oxygen-permeation flux, DC conductivity, and hydrogen production, and by evaluating its microstructure. The dependences of the hydrogen production rate of BFZ disks (thickness, ≈1.6 mm) on water partial pressure and temperature were determined while flowing 80% CO₂/He over a graphite rod on the oxygen-permeate side and humidified N₂ on the hydrogen-generation side. Preliminary results indicate that BFZ is a promising OTM material.     

Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process

A mathematical model for the moving bed is developed to simulate the fuel and steam reactor in the TRCL (Three-Reactor Chemical-Looping) process. An ideal plug flow of the solid and gas is assumed in modeling the fuel and steam reactor in the TRCL process. The model considered the mass, heat balances, equilibrium, physical properties, such as the heat capacity and viscosity, and kinetics. From this model, the temperature, gas conversion and solid conversion profiles can be predicted for fuel and steam reactors. The oxygen carrier inventory (the mass of the oxygen carrier) in the fuel and steam reactor was calculated with variation of the solid inlet temperature, solid conversion, Fe₂O₃ content and steam feed rate. The temperature of the oxygen carrier to the reactor was the most sensitive parameter for determining the required inventory of the oxygen carrier. An increase in the solid inlet temperature was predicted to decrease the required inventory of the oxygen carrier. In the steam reactor, a solid inlet temperature increase over 1150 K will cause an increase in the inventory of the oxygen carrier due to the equilibrium conversion. An excessively low or high active material content will require a larger inventory of the oxygen carrier in the fuel reactor. In this study, approximately 20 wt.% of the Fe₂O₃ content was suitable for reducing the inventory of the oxygen carrier while achieving a solid conversion of 0.9 in the fuel reactor.     

Key experimental parameters in steam oxidation testing and their impact on interpretation of experimental results

Abstract

The acceptance of materials for extended duration, safety critical power generation applications usually requires several stages of testing and data generation. Simple, short term exposures under nominally constant atmospheres and temperatures can eliminate materials that are grossly unsuitable, but do not differentiate between materials that have broadly acceptable properties. To better differentiate between candidate materials it is desirable to tailor laboratory tests such that they more closely replicate in service conditions. In terms of components that are exposed to steam oxidation degradation mechanisms, this means replicating the steam conditions with an aim of producing oxide scale morphologies similar to that seen in service. Key experimental parameters have been identified, including water chemistry, pressure, steam delivery and flowrate, and a series of steam exposure tests on ferritic (P92), austenitic (Esshete 1250) and superalloy (IN740) material conducted to evaluate their effect on degradation rate and oxide scale morphology. The oxidation rate of the austenitic, and to a lesser extent the ferritic, material was found to be sensitive to the level of dissolved oxygen in the feed water, low (10 ppb) dissolved oxygen levels producing an increase in oxidation rate. The propensity to spall was also found to be reduced at low dissolved oxygen concentrations. In addition, the steam pressure and steam delivery method were shown to affect the oxidation rate and scale morphology for these materials.     

Existing large steam power plant upgraded for hydrogen production

This paper presents and discusses the results of a complete thermoeconomic analysis of an integrated power plant for co-production of electricity and hydrogen via pyrolysis and gasification processes fed by various coals and mixture of coal and biomass, applied to an existing large steam power plant (ENEL Brindisi power plant – 660 MW_e). Two different technologies for the syngas production section are considered: pyrolysis process and direct pressurized gasification. Moreover, the proximity of a hydrogen production and purification plants to an existing steam power plant favors the inter-exchange of energy streams, mainly in the form of hot water and steam, which reduces the costs of auxiliary equipment. The high quality of the hydrogen would guarantee its usability for distributed generation and for public transport. The results were obtained using WTEMP thermoeconomic software, developed by the Thermochemical Power Group of the University of Genoa, and this project has been carried out within the framework of the FISR National project “Integrated systems for hydrogen production and utilization in distributed power generation”.     

松木颗粒热解与高温蒸汽气化半焦产率及形貌特征分析

以成型松木颗粒为原料,进行低温热解,研究了热解温度和升温速率对生成的松木半焦产率及官能团的影响。以试验得到的松木半焦进行蒸汽气化试验,对比分析了温度对半焦重整气化形貌特征、比表面积和平均孔径的影响。研究表明：随着热解温度升高,松木半焦脂肪族结构峰消失转化为烃等小分子物质及气化气,进而降低半焦产率。升温速率升高,半焦产率呈先下降后升高的趋势,在800℃升温速率为30K/min时半焦产率最低。不同温度热解和蒸汽气化对比试验表明,温度相对较低时（500℃）热解和蒸汽气化半焦孔隙结构相近,随着温度的升高,蒸汽气化半焦结构发生明显变化,900°C时出现了更小的孔道结构且比表面积增加明显。蒸汽引入使松木半焦和水蒸气发生热解反应的同时发生了脱氢反应,气化半焦形貌出现熔融和烧结现象。     

Enhancement of hydrogen production in a modified moving bed downdraft gasifier – A thermodynamic study by including tar

A new approach on thermodynamic simulation of the gasification process is conducted by considering the formation of tar using Aspen Plus. The present model shows higher accuracy as compared to the conventional model in term of the composition of producer gas. The tar from pyrolysis process is successfully reduced with high reaction temperature in the combustion zone. A parametric study is performed by varying the split ratio of gasifying agents (steam/oxygen) through three different zones: (i) combustion zone, (ii) counter-current reduction zone, and/or (iii) co-current reduction zone. Introduction of the gasifying agents through the counter-current reduction zone has positive effects on the gasification performances in term of hydrogen concentration, cold gas efficiency, and gasification system efficiency. The effects of O₂ equivalence ratio and steam to carbon ratio (S/C) on the performance of gasification are also investigated. The gasification with oxygen provided the highest cold gas efficiency. A remarkable hydrogen production is achieved from gasification with both oxygen and steam.     

Fast starting fuel processor for automotive fuel cell systems

A fuel processor was constructed which incorporated two burners with direct steam generation by water injection into the burner exhaust. These burners with direct water vaporization enabled rapid fuel processor start-up for automotive fuel cell systems. The fuel processor consisted of a conventional chain of reactors: auto-thermal reformer (ATR), water gas shift (WGS) reactor and preferential oxidation (PrOx) reactor. The criticality of steam to the fuel reforming process was illustrated. By utilizing direct vaporization of water, and hydrogen for catalyst light-off, excellent start performance was obtained with a start time of 20 s to 30% power and 140 s to full power.     

Design of an integrated process for simultaneous chemical looping hydrogen production and electricity generation with CO2 capture

This study was aimed at proposing a novel integrated process for co-production of hydrogen and electricity through integrating biomass gasification, chemical looping combustion, and electrical power generation cycle with CO₂ capture. Syngas obtained from biomass gasification was used as fuel for chemical looping combustion process. Calcium oxide metal oxide was used as oxygen carrier in the chemical looping system. The effluent stream of the chemical looping system was then transferred through a bottoming power generation cycle with carbon capture capability. The products achieved through the proposed process were highly-pure hydrogen and electricity generated by chemical looping and power generation cycle, respectively. Moreover, LNG cold energy was used as heat sink to improve the electrical power generation efficiency of the process. Sensitivity analysis was also carried out to scrutinize the effects of influential parameters, i.e., carbonator temperature, steam/biomass ratio, gasification temperature, gas turbine inlet stream temperature, and liquefied natural gas (LNG) flow rate on the plant performance. Overall, the optimum heat integration was achieved among the sub-systems of the plant while a high energy efficiency and zero CO₂ emission were also accomplished. The findings of the present study could assist future investigations in analyzing the performance of integrated processes and in investigating optimal operating conditions of such systems.     

Syngas from sugarcane pyrolysis: An experimental study for fuel cell applications

The use of biomass for the production of electrical energy is a promising technological solution for those countries where there are problems with the disposal of agricultural waste and/or the production of low-cost energy. The gasification and/or pyrolysis of the biomass produces a gas rich in hydrogen that can be used in a fuel cell system to produce electrical energy with reduced environmental impact and significant energy recovery.In this work, a study of the pyrolysis of Brazilian sugarcane bagasse was carried out. The experimental process consisted of the pyrolysis of the biomass material in a batch pyrolysis reactor. In some runs the biomass was dry, while in others it was pre-treated by the addition of water. It was noted that the water added to the biomass before the pyrolysis process resulted in a decrease in the quantity of steam added to the fuel cell feeding gas, necessary to avoid carbon deposition, and in an increase in cell power, but, at the same time, caused a decrease in the quantity of syngas produced.Then, the composition of the gas obtained from the experimental pyrolysis of the sugarcane was inserted in a simulation tool of a molten carbonate fuel cell system in order to estimate the feasibility of the entire process in terms of operating conditions and electrical performance.The present study indicates that the syngas obtained from the sugarcane biomass (about 40%) can be converted into electricity using a fuel cell system with a high efficiency.     

Drying of biomass for second generation synfuel production

Drying is a major and challenging step in the pre-treatment of biomass for production of second generation synfuels for transport. The biomass feedstocks are mostly wet and need to be dried from 30 to 60 wt% moisture content to about 10–15 wt%. The present survey aims to define and evaluate a few of the most promising optimised concepts for biomass pre-treatment scheme in the production of second generation synfuels for transport. The most promising commercially available drying processes were reviewed, focusing on the applications, operational factors and emissions of dryers. The most common dryers applied now for biomass in bio-energy plants are direct rotary dryers, but the use of steam drying techniques is increasing. Steam drying systems enable the integration of the dryer to existing energy sources. In addition to integration, emissions and fire or explosion risks have to be considered when selecting a dryer for the plant. In steam drying there will be no gaseous emissions, but the aqueous effluents need often treatment. Concepts for biomass pre-treatment were defined for two different cases including a large-scale wood-based gasification synfuel production and a small-scale pyrolysis process based on wood chips and miscanthus bundles. For the first case a pneumatic conveying steam dryer was suggested. In the second case the flue gas will be used as drying medium in a direct or indirect rotary dryer.     

An advanced zero emission power cycle with integrated low temperature thermal energy

An innovative zero emission hybrid cycle named HICES (hybrid and improved CES cycle) is presented in this paper. It can utilize fossil fuel and low quality thermal energy such as waste heat from industrial processes and solar thermal energy for highly efficient electric power generation. In the HICES cycle, natural gas is internally combusted with pure oxygen. External low quality thermal energy is used to produce saturated steam between 70 and 250 °C as part of the working fluid. The thermodynamic characteristics at design conditions of the HICES cycle are analyzed using the advanced process simulator Aspen Plus. The influences of some key parameters are investigated. The results demonstrate that the thermodynamic performances of the HICES cycle are quite promising. For example, when the external heat produced saturated steam is at 70 °C, the net fuel-to-electricity efficiency is 54.18% even when taking into account both the energy penalties to produce pure oxygen and to liquefy the captured CO₂. The incremental low temperature heat to electric efficiency is as high as 14.08% at the same time. When the external heat produced saturated steam is at 250 °C, the net fuel-to-electricity efficiency reaches 62.66%. The incremental low temperature heat to electric efficiency achieves 48.92%.