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14:00   Session 19: Large-scale ORC units II
Chair: Tobias Erhart
20 mins
Vittorio Tola
Abstract: A performance assessment of natural gas-fueled combined cycle (NGCC) power plants and coal-fired steam power plants, both equipped with a CO2 removal system and integrated with an Organic Rankine Cycle (ORC), was performed. For large scale power plants (in this paper a fuel chemical power input equal to 1000 MWt was assumed as reference for both NGCC and steam plant), post-combustion CO2 removal systems based on chemical solvents like amines are expected to reduce the net plant efficiency between 9-12 percentage points at 90% overall CO2 capture. For NGCC+CCS power plants, to improve the capture efficiency and reduce capture equipment costs, exhaust gas recirculation (EGR) has been firstly proposed, assuring a gain of plant overall efficiency in the range of 1-1.5 percentage points. The recovery of low temperature heat, available from the solvent-based CO2 removal systems and related process equipment, can be performed in order to further increase the plant efficiency. In particular low temperature heat is available in flue gas coolers that are required upstream of the CO2 capture unit and, in case of NGCC also for exhaust gas recirculation. Gas at the hot end of the syngas coolers shows a temperature in the order of 80-100 °C for the NGCC and of about 120 °C for the steam plant. Additional available low temperature heat sources are the amine condenser of the CO2 desorption column, which operates at around 100-110 °C and the amine reboiler water cooling that reaches temperatures of 130-140°C. The thermal energy of these various sources could be utilized in different low-temperature heat recovery systems. This paper evaluates low temperature heat recovery by means of an Organic Rankine Cycle (ORC) that can convert heat into electricity at very low temperatures. By producing additional electrical power by the ORC, the global performance of the above mentioned power plants can be improved. This study shows that the integration of CCS with the steam plant allows to recover a larger amount of waste heat in comparison to NGCC (more than 200 MW versus 110 MW). As a consequence, integrating ORC technology with a post-combustion capture system leads to an increase of efficiency of about 1-1.5 percentage points for the NGCC plant and of about 2 percentage points for the steam plant, depending on the amount of low temperature heat available. Different ORC configurations (in series, in parallel or cascaded) were analyzed for thermal energy recovery. Among several organic fluids available and analyzed, N-Butane was assumed as organic operating fluid. Optimum cycle operating temperatures and pressures were identified in order to evaluate the most efficient approach for low temperature heat recovery.
20 mins
Tobias Erhart, Jürgen Gölz, Ursula Eicker, Martijn van den Broek
Abstract: The results in this work show the influence of long-term operation on the decomposition of working fluids in eight different power plants (both heat-led and electricity-led) in a range of 900 kWel to 2 MWel. All case study plants are using Octamethyltrisiloxane (MDM) as a working fluid. The case study plants are between six to 12 years old. On one system detailed analyses, including the fluid distribution throughout the cycle, have been conducted. All fluid samples have been analysed via Head Space Gas Chromatography Mass Spectrometry (HS-GC-MS). Besides the siloxane composition, the influence of contaminants such as mineral oil based lubricants (and its components) has been examined. In most cases the original main working fluid has degraded to fractions of siloxanes with a lower boiling point (low-boilers) and fractions with a higher boiling point (high-boilers). As a consequence of the analyses, a new fluid management system has been designed and tested in one case study plant (case study number 8). The measures include fluid separation, cleansing and recycling. Pre-post comparisons of fluid samples have proved the effectiveness of the methods. The results show that the recovery of used working fluid offers an alternative to the purchase of fresh fluid, since operating costs can be significantly reduced. For large facilities the prices for new fluid range from 15€ per litre (in 2006) to 22€ per litre (in 2013), which is a large reinvestment, especially in the light of filling volumes of 4000 litres to 7000 litres per cycle. With the above mentioned method a price of 8€ per litre of recovered MDM can be achieved.
20 mins
John Harinck, Ludovico Calderazzi, Piero Colonna, Hugo Polderman
Abstract: Gas processing plants are characterized by large energy flows. Therefore it is key to maximize energy efficiency and to optimize utility balances. In the gas to liquid (GTL) complexes operated by Shell in Malaysia and Qatar, the highly exothermic Fischer Tropsch process is applied to convert gas into liquid hydrocarbon products. Most of the available thermal energy is used to cogenerate steam and to preheat feed streams, but still substantial additional cooling is required to reduce the temperature of intermediate streams for further processing. In the Qatar GTL plant this duty is in the order of 600 MWth. By a detailed investigation that included simulations and cost evaluation of both commercial ORC systems and dedicated advanced ORC concepts, it could be established that heat recovery by means of low-temperature ORC units is a feasible option. Prerequisite is that the ORC unit is directly coupled to the process, without an intermediate thermal fluid loop. A parallel study focused on application of ORC systems in LNG plants. These plants waste large quantities of thermal energy in the form of high temperature exhaust gas from gas turbines used for power generation and gas compression. A similar evaluation of current ORC technology for the recovery of this high-temperature heat led to the conclusion that ORC systems can be more attractive than steam cycles for waste heat recovery from both mid-range gas turbine installations and for larger systems in remote or arid locations where steam cycles are impractical. This still is a large scope of deployment.
20 mins
Henrik Ohman, Per Lundqvist
Abstract: A review of the thermodynamic performance of ORC’s from public, as well as non-public sources has revealed a correlation suitable to be used as a “rule of thumb” for high-level performance estimation of ORC power generators. Using the correlation, the limited amount of available test data can be generalised leading to a high level evaluation of the commercial benefits of any potential application for ORC’s. Power generators using ORC-technology exist in relatively low numbers. Furthermore, field installations seldom imply comparable boundary conditions. As ORC’s generally operate at low temperature differences between source and sink it has been shown that their relative sensitivity to variations in temperatures i.e. the finiteness of source- and sink, is larger than the sensitivity of power generators operating with large temperature differences. Therefore the establishing of practical “rule of thumb” performance estimation, similar to the term of merit Coefficient Of Performance, COP, as used in refrigeration and air conditioning industry, has previously not been successful. In order to arrange field data in a manner suitable for comparison a refinement of suitable terms of merit was required. The suggested, refined terms are presented and explained as well as critically evaluated against the most common efficiency terms traditionally used. The current lack of a performance “Rule of thumb” leaves room for less serious vendors and laymen to make performance claims unrealistic to practical achievements. Scrutinizing such questionable statements requires detail process simulations and a multitude of technical assumptions. Hence argumentation becomes ineffective. If a suitable “rule of thumb” can be established argumentation against dubious claims would become significantly more forceful. This paper suggests a new term to be used as “rule of thumb” and explains a method on how to use it.
20 mins
Cong-Toan Tran, Assaad Zoughaib
Abstract: In industrial processes, a large amount of energy is usually lost as waste heat. This waste source reduces not only the energy efficiency of industrial process but also contributes to greenhouse gases emissions and thermal pollution. In this context, The CERES-2 project (CERES denotes “Energy paths for energy recovery in industrial systems”), supported by the French National Research Agency, aims at developing a decision-making tool to identify the optimal solutions of industrial waste heat recovery. This platform leans on energy integration and multi-objectives optimization to identify and design the best waste heat recovery solutions, according to technical and economic criteria, for a given industrial process. The solutions gather direct heat recovery, heat pumping and electricity production technologies. This paper presents how the developed multi scale methodology helps optimizing the integration and the architecture of an ORC in an industrial process. On the process scale, CERES platform uses a MILP algorithm that uses Grand Composite Curve of the industrial process to specify the best integration location of the ORC in a systematic manner. The algorithm is based on exergy criteria and a simplified modeling of the ORC. This algorithm tests every possible couple of temperature level and chooses the best ones for the location of the heat recovery systems. Once the ORC operating conditions defined, its detailed design and optimization is performed thanks to a model developed in Modelica language permitting to design the working fluid and the heat exchangers. The multi-objectives optimization of the cycle is performed by using self-adaptive version of Strength Pareto Evolutionary Algorithms 2 (SPEA2) implemented in CERES platform.