Techno-economic analysis of using corn stover to supply heat and power to a corn ethanol plant – Part 2: Cost of heat and power generation systems

This paper presents a techno-economic analysis of corn stover fired process heating (PH) and the combined heat and power (CHP) generation systems for a typical corn ethanol plant (ethanol production capacity of 170 dam³). Discounted cash flow method was used to estimate both the capital and operating costs of each system and compared with the existing natural gas fired heating system. Environmental impact assessment of using corn stover, coal and natural gas in the heat and/or power generation systems was also evaluated. Coal fired process heating (PH) system had the lowest annual operating cost due to the low fuel cost, but had the highest environmental and human toxicity impacts. The proposed combined heat and power (CHP) generation system required about 137 Gg of corn stover to generate 9.5 MW of electricity and 52.3 MW of process heat with an overall CHP efficiency of 83.3%. Stover fired CHP system would generate an annual savings of 3.6 M$ with an payback period of 6 y. Economics of the coal fired CHP system was very attractive compared to the stover fired CHP system due to lower fuel cost. But the greenhouse gas emissions per Mg of fuel for the coal fired CHP system was 32 times higher than that of stover fired CHP system. Corn stover fired heat and power generation system for a corn ethanol plant can improve the net energy balance and add environmental benefits to the corn to ethanol biorefinery.     

Energy efficiency analysis and impact evaluation of the application of thermoelectric power cycle to today’s CHP systems

High efficiency thermoelectric generators (TEG) can recover waste heat from both industrial and private sectors. Thus, the development and deployment of TEG may represent one of the main drives for technological change and fuel substitution. This paper will present an analysis of system efficiency related to the integration of TEG into thermal energy systems, especially Combined Heat and Power production (CHP). Representative implementations of installing TEG in CHP plants to utilize waste heat, wherein electricity can be generated in situ as a by-product, will be described to show advantageous configurations for combustion systems. The feasible deployment of TEG in various CHP plants will be examined in terms of heat source temperature range, influences on CHP power specification and thermal environment, as well as potential benefits. The overall conversion efficiency improvements and economic benefits, together with the environmental impact of this deployment, will then be estimated. By using the Danish thermal energy system as a paradigm, this paper will consider the TEG application to district heating systems and power plants through the EnergyPLAN model, which has been created to design suitable energy strategies for the integration of electricity production into the overall energy system.     

Black liquor gasification—consequences for both industry and society

The pulp and paper industry consumes large quantities of biofuels to satisfy process requirements. Biomass is however a limited resource, to be used as effectively as possible. Modern pulping operations have excess internal fuels compared to the amounts needed to satisfy process steam demands. The excess fuel is often used for cogeneration of electric power. If market biofuel availability at a reasonable price is limited, import/export to/from a mill however changes the amount of such biofuel available for alternative users. This work compares different mill powerhouse technologies and CHP plant configurations (including conventional recovery boiler technology and black liquor gasification technology) with respect to electric power output from a given fuel resource. Different process steam demand levels for different representative mill types are considered. The comparison accounts for decreased/increased electricity production in an alternative energy system when biofuel is imported/exported to/from the mill. The results show that black liquor gasification is in all cases considered an attractive powerhouse recovery cycle technology. For moderate values of the marginal electric power generation efficiency for biofuel exported to the reference alternative energy system, excess mill internal biofuel should be used on mill site for gas turbine based CHP power generation. The remaining excess biofuels in market pulp mills should be exported and used in the reference alternative energy system in this case. For integrated pulp and paper mills, biofuel should be imported, but only for cogeneration usage (i.e. condensing power units should be avoided). If biofuel can be used elsewhere for high efficiency CHP power generation, mill internal biofuel should be used exclusively for process heating, and the remainder should be exported.     

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 CO₂ emission permit prices. The developed model is a suitable decision making tool when evaluating the trade-offs between investments, profits, and fossil CO₂ 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.     

CPH systems for cogeneration of power and hydrogen from coal

Three layouts with an integrated coal gasifier hydrogen production and a small powerplant section have been modelled using a computer code (ASPEN PLUS^TM${\mathrm{PLUS}}^{\mathrm{TM}}$). The integration allows to eliminate or to reduce the losses at the condenser of the powerplant: the steam is reheated and fed to the gasifier. The resulting counterpressure operation of the powerplant is justified like in a co-generator of heat and power (CHP). In this case we have a co-generation of power and hydrogen (CPH). Therefore the efficiency of the power plant is not high, but it shows an “apparent” efficiency very high. Even if the concept has been demonstrated, further work is required because power generation is very small with respect to the hydrogen production.     

Extending existing combined heat and power plants for synthetic natural gas production

In this work, integration of a synthetic natural gas (SNG) production process with an existing biomass CHP steam power cycle is investigated. The paper assesses two different biomass feedstock drying technologies—steam drying and low‐temperature air drying—for the SNG process. Using pinch technology, different levels of thermal integration between the steam power cycle and the SNG process are evaluated. The base case cold gas efficiency for the SNG process is 69.4% based on the lower heating value of wet fuel. The isolated SNG‐related electricity production is increased by a factor of 2.5 for the steam dryer alternative, and tenfold for the low‐temperature air dryer when increasing the thermal integration. The cold gas efficiency is not affected by the changes. Based on an analysis of changes to turbine steam flow, the integration of SNG production with an existing steam power cycle is deemed technically feasible. Copyright © 2011 John Wiley & Sons, Ltd.     

Sustainability assessment of cogeneration sector development in Croatia

The effective and rational energy generation and supply is one of the main presumptions of sustainable development. Combined heat and power production, or co-generation, has clear environmental advantages by increasing energy efficiency and decreasing carbon emissions. However, higher investment cost and more complicated design and maintenance sometimes-present disadvantages from the economical viability point of view. As in the case of most of economies in transition in Central and Eastern Europe, Croatia has a strong but not very efficient co-generation sector, delivering 12% of the final energy consumption. District heating systems in the country's capital Zagreb and in city of Osijek represent the large share of the overall co-generation capacity. Besides district heating, co-generation in industry sector is also relatively well developed. The paper presents an attempt to assess the sustainability of Croatian co-generation sector future development. The sustainability assessment requires multi-criteria assessment of specific scenarios to be taken into consideration. In this respect three scenarios of Croatian co-generation sector future development are taken into consideration and for each of them environmental, social and economic sustainability indicators are defined and calculated. The assessment of complex relationships between environmental, social and economic aspects of the system is based on the multi-criteria decision-making procedure. The sustainability assessment is based on the General Sustainability Index rating for different cases reflecting different criteria and their priority. The method of sustainability assessment is applied to the Croatian co-generation sector contributing to the evaluation of different strategies and definition of a foundation for policy related to the sustainable future cogeneration sector development.     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.     

采用燃气—蒸汽联合循环冷热电三联供能源系统在焦炉煤气制液化天然气工厂的应用研究

日益恶化的全球环境问题,要求中国天然气消费与生产保持高速增长,资源的特征决定了须从多种渠道解决天然气来源问题,焦炉煤气制液化天然气作为煤制天然气的一个路线,也将逐步地成为一个新兴的行业而快速成长与发展,国家已将此技术列入了近年重点示范推广的关键技术。在这个生产过程中冷热电能消耗都比较大,本文研究的是运用燃气轮机,进行燃气—蒸汽联合循环,实现"冷热电"三联供的典型分布式能源系统,使系统的循环效率从22～23%提高到40%以上。     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.     

Analysis of cooling,heating, and power systems based on site energy consumption

Feasibility of cooling, heating, and power systems frequently is based on economic considerations such as energy prices. However, a most adequate feasibility of CHP systems must be based on energy consumption followed by economic considerations. CHP systems designs must yield economical savings, but more importantly must yield real energy savings based on the best energy performance. For CHP systems, energy savings is related to primary energy and not to site energy. This paper presents a mathematical analysis demonstrating that CHP systems increase the site energy consumption (SEC). Increasing the SEC could yield misleading results in the economic feasibility of CHP systems. Three different operation modes are evaluated: (a) cooling, heating, and power; (b) heating and power; and (c) cooling and power, to represent the operation of the system throughout the year. Results show that CHP systems increase site energy consumption; therefore primary energy consumption (PEC) should be used instead of SEC when designing CHP systems.     

基于旁路系统提升热电机组风电消纳能力研究

针对中国三北地区冬季供热期弃风现象严重的问题,提出利用高低压旁路供热解耦传统热电机组的电热强耦合关系,并基于旁路系统供热的热电机组电热特性,建立风电消纳能力数学模型,根据电网调峰需求,给出热电机组的运行策略。结果表明:高低压旁路系统参与供热可大幅提升热电机组的风电消纳能力和供热能力。为了保证高低压旁路供热安全,要注意高低压旁路蒸汽流量的匹配关系。采用"传统抽汽+高低压旁路"切换方式供热,风电消纳能力最强。以某330 MW热电机组为例,采用高低压旁路供热可进一步提升供热能力90.56%;在满足额定供热量的前提下,采用"传统抽汽+高低压旁路"切换方式供热可进一步提升热电机组风电消纳能力324.46%。     

The role of policy instruments for promoting combined heat and power production with low CO2 emissions in district heating systems

Policy instruments clearly influence the choice of production technologies and fuels in large energy systems, including district heating networks. Current Swedish policy instruments aim at promoting the use of biofuel in district heating systems, and at promoting electric power generation from renewable energy sources. However, there is increasing pressure to harmonize energy policy instruments within the EU. In addition, natural gas based combined cycle technology has emerged as the technology of choice in the power generation sector in the EU. This study aims at exploring the role of policy instruments for promoting the use of low CO₂ emissions fuels in high performance combined heat and power systems in the district heating sector. The paper presents the results of a case study for a Swedish district heating network where new large size natural gas combined cycle (NGCC) combined heat and power (CHP) is being built. Given the aim of current Swedish energy policy, it is assumed that it could be of interest in the future to integrate a biofuel gasifier to the CHP plant and co‐fire the gasified biofuel in the gas turbine unit, thereby reducing usage of fossil fuel. The goals of the study are to evaluate which policy instruments promote construction of the planned NGCC CHP unit, the technical performance of an integrated biofuelled pressurized gasifier with or without dryer on plant site, and which combination of policy instruments promote integration of a biofuel gasifier to the planned CHP unit. The power plant simulation program GateCycle was used for plant performance evaluation. The results show that current Swedish energy policy instruments favour investing in the NGCC CHP unit. The corresponding cost of electricity (COE) from the NGCC CHP unit is estimated at 253 SEK MWh^?1, which is lower than the reference power price of 284 SEK MWh^?1. Investing in the NGCC CHP unit is also shown to be attractive if a CO₂ trading system is implemented. If the value of tradable emission permits (TEP) in such as system is 250 SEK tonne^?1, COE is 353 SEK MWh^?1 compared to the reference power price of 384 SEK MWh^?1. It is possible to integrate a pressurized biofuel gasifier to the NGCC CHP plant without any major re‐design of the combined cycle provided that the maximum degree of co‐firing is limited to 27–38% (energy basis) product gas, depending on the design of the gasifier system. There are many parameters that affect the economic performance of an integrated biofuel gasifier for product gas co‐firing of a NGCC CHP plant. The premium value of the co‐generated renewable electricity and the value of TEPs are very important parameters. Assuming a future CO₂ trading system with a TEP value of 250 SEK tonne^?1 and a premium value of renewable electricity of 200 SEK MWh^?1 COE from a CHP plant with an integrated biofuelled gasifier could be 336 SEK MWh^?1, which is lower than both the reference market electric power price and COE for the plant operating on natural gas alone. Copyright © 2005 John Wiley & Sons, Ltd.     

Exergetic comparison of efficiency indicators for combined heat and power (CHP)

Legislative regulations in favor of combined heat and power (CHP) production have been implemented in many countries. Although these regulations put different emphasis on power production vs. process heat production, they are based on energy quantities and not on exergy. In order to analyze and compare the exergetic consequences of the various legislations, a relative avoided irreversibility (RAI) is defined. This can be regarded as the exergy loss that is avoided when reference plants with separate production are replaced by an actual CHP plant. Some series of industrial and district heating CHP plants, under varying operational conditions, are used as test cases. It is seen that some, but not all, CHP cases are exergetically beneficial to separate generation. Comparison with the RAI allows a quantitative assessment of the various performance indicators. It is seen that exergetic improvements were only captured to a limited degree by the various energy-based efficiency indicators. Some legislatively defined indicators even appear to discourage thermodynamic improvements.     

Combined heat and power considered as a virtual steam cycle heat pump

The first aim of this paper is to shed light on the thermodynamic reasons for the practical pursuit of low temperature operation by engineers involved in the design and the operation of combined heat and power (CHP) and district heating (DH) systems. The paper shows that the steam cycle of a combined heat and power generator is thermodynamically equivalent to a conventional steam cycle generator plus an additional virtual steam cycle heat pump. This apparently novel conceptualisation leads directly to (i) the observed sensitivity of coefficient of performance of CHP to supply and return temperatures in associated DH systems, and (ii) the conclusion that the performance of CHP will tend to be significantly higher than real heat pumps operating at similar temperatures. The second aim, which is pursued more qualitatively, is to show that the thermodynamic performance advantages of CHP are consistent with the goal of deep, long-term decarbonisation of industrialised economies. As an example, estimates are presented, which suggest that CHP based on combined-cycle gas turbines with carbon capture and storage has the potential to reduce the carbon intensity of delivered heat by a factor of ∼30, compared with a base case of natural gas-fired condensing boilers.     

垃圾焚烧电厂热电联产的经济性分析

合理利用垃圾资源进行热电联产,是节能减排、改善环境的有力措施。以某2×750 t·d^-1垃圾焚烧电厂为例,通过模型研究发现热电联产可以减少垃圾焚烧电厂的冷源损失,提高全厂热效率;利用一抽蒸汽进行热电联产可实现蒸汽品质的梯级利用,获得较高的经济效益;供热量为30 t·h^-1,垃圾热值由4185.9 kJ·kg^-1增加至8371.7 kJ·kg^-1时,发电量越多,供热能力越强,年热电联产经济效益由7822.76万元增加到14641.07万元;垃圾热值为8371.7 kJ·kg^-1,供热量从10 t·h^-1增加到60 t·h^-1时,垃圾焚烧电厂热效率从28.96%增加到48.50%,年经济效益从13602.74万元增加到15455.66万元。当该地区垃圾热值较高并具备供热条件时,实现垃圾热电联产具有较高的收益。     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.     

A novel approach for treatment of CO2 from fossil fired power plants,Part A: The integrated systems ITRPP

The environmental issues, due to the global warming caused by the rising concentration of greenhouse gases in the atmosphere, require new strategies aimed to increase power plants efficiencies and to reduce CO₂ emissions.This two-paper work focuses on a different approach for capture and reduction of CO₂ from flue gases of fossil fired power plant, with respect to conventional post-combustion technologies. This approach consists of flue gases utilization as co-reactants in a catalytic process, the tri-reforming process, to generate a synthesis gas suitable in chemical and energy industries (methanol, DME, etc.). In fact, the further conversion of syngas to a transportation fuel, such as methanol, is an attractive solution to introduce near zero-emission technologies (i.e. fuel cells) in vehicular applications.In this Part A, integrated systems for co-generation of electrical power and synthesis gas useful for methanol production have been defined and their performance has been investigated considering different flue gases compositions. In Part B, in order to verify the environmental advantages and energy suitability of these systems, their comparison with conventional technology for methanol production is carried out.The integrated systems (ITRPP, Integrated Tri-Reforming Power Plant) consist of a power island, based on a thermal power plant, and a methane tri-reforming island in which the power plants' exhausts react with methane to produce a synthesis gas used for methanol synthesis. As power island, a steam turbine power plant fuelled with coal and a gas turbine combined cycle fuelled with natural gas have been considered.The energy and environmental analysis of ITRPP systems (ITRPP-SC and ITRPP-CC) has been carried out by using thermochemical and thermodynamic models which have allowed to calculate the syngas composition, to define the energy and mass balances and to estimate the CO₂ emissions for each ITRPP configuration.The repowering of the base power plants (steam turbine power plant and gas turbine combine cycle) is very high because of the large amount of steam produced in the tri-reforming island (in the ITRPP-SC is about of 64%, while in the ITRPP-CC is about of 105%).The reduction in the CO₂ emissions has been estimated in 83% (15.4 vs. 93.4 kg/GJ_Fuelinput) and 84% (8.9 vs. 56.2 kg/GJ_Fuelinput) for the ITRPP-SC and ITRPP-CC respectively.     

The influence of water and steam injection on the performance of a recuperated cycle microturbine for combined heat and power application

Microturbines are promising power sources for small scale combined heat and power (CHP) systems. However, the power output and efficiency of microturbines decreases much as the ambient temperature increases. As a remedy to minimize the performance penalty at hot ambient conditions, the injection of water or steam into a microturbine CHP system was analyzed in this work. An analysis program to simulate the operation of a microturbine CHP system was set up and validated by using measured test data. The injection of hot water, which is generated at the heat recovery unit, at two different locations inside the microturbine was predicted. The generation of steam through the same heat recovery unit and its injection at the two locations was predicted as well. All the four cases provide sufficiently enhanced power output. Injection at the recuperator inlet exhibits a higher efficiency than injection at the combustor in both water and steam injections. Steam injection provides a higher power generation efficiency than water injection on the average. The injection of steam at the recuperator inlet is most promising in terms of power generation efficiency. However, water injection at the recuperator also enhances power generation efficiency while still providing thermal energy to some extent.     

Investigation of green hydrogen production and development of waste heat recovery system in biogas power plant for sustainable energy applications

In this study, biogas power production and green hydrogen potential as an energy carrier are evaluated from biomass. Integrating an Organic Rankine Cycle (ORC) to benefit from the waste exhaust gases is considered. The power obtained from the ORC is used to produce hydrogen by water electrolysis, eliminate the H₂S generated during the biogas production process and store the excess electricity. Thermodynamic and thermoeconomic analyses and optimization of the designed Combined Heat and Power (CHP) system for this purpose have been performed. The proposed study contains originality about the sustainability and efficiency of renewable energy resources. System design and analysis are performed with Engineering Equation Solver (EES) and Aspen Plus software. According to the results of thermodynamic analysis, the energy and exergy efficiency of the existing power plant is 28.69% and 25.15%. The new integrated system's energy, exergy efficiencies, and power capacity are calculated as 41.55%, 36.42%, and 5792 kW. The total hydrogen production from the system is 0.12412 kg/s. According to the results of the thermoeconomic analysis, the unit cost of the electricity produced in the existing power plant is 0.04323 $/kWh. The cost of electricity and hydrogen produced in the new proposed system is determined as 0.03922 $/kWh and 0.181 $/kg H₂, respectively.