1,721,020 research outputs found
Process analysis and performance evaluation of updraft coal gasifiers
No full textCoal gasification is becoming commercially even more import due to its potential application in hydrogen, ammonia, methanol and other chemicals and clean fuels production, other than power generation, together with carbon dioxide capture and sequestration. In this framework the technological development is also addressed, with a renewed interest, to simplified processes and plant solutions based, for example, on gasification with air (or air enriched with oxygen) and on moving or fluidised bed gasifiers, of interest for small and medium scale plants. The design, analysis and performance evaluation of the overall system (gasification, gas clean-up, desulphurisation, CO-shift conversion, CO2 and hydrogen separation, etc.) require a preliminary estimation of gasifier mass and energy balances and raw gas composition, which influence the whole downstream gas clean-up and treatment systems. The present study reports a process analysis and performance evaluation of updraft moving bed gasifiers, which have been carried out by a computer simulation model developed using the Aspen Plus 12.1 software, The model schematises the gasifier in several different zones: coal preheating and drying, devolatilization, gasification, combustion and oxidant preheating, under the hypothesis of char gasification at thermodynamic equilibrium. The model allows to appraise the mass and energy balance of the gasifier and the main characteristics of the syngas produced by the gasification process (composition, mass flow, temperature, lower heat value, etc.), being assigned coal composition and coal, steam and oxidant (air eventually enriched with oxigen) mass flows. In this paper the model is applied to predict the performance of two updraft moving bed gasifiers (sized respectively for 35 kg/h and 700 kg/h of low sulphur coal and high sulphur coal (Sulcis). The gasifiers are part of a small pilot gasification and gas treatment plant for hydrogen production under construction at the Sotacarbo Research Centre in Sardinia
Power generation plants with carbon capture and storage: a techno-economic comparison between coal combustion and gasification technologies
No full textComparative performance analysis of internal and external reforming in SOFC-MGT hybrid power plants
No full textSOFC-MGT hybrid power plants are a very attractive near
term option, as they achieve efficiencies of over 60% even for
small power outputs (200-400 kW). The SOFC hybrid systems
currently developed are fuelled with natural gas, which is
reformed inside the same stack at about 800-900 °C. However,
the use of alternative fuels with a lower reforming temperature
can lead to enhanced energy management, and hence enhanced
performance of the hybrid power plant as a whole.
This paper reports a comparative performance analysis of
SOFC-MGT power plants fuelled by methane and methanol,
whose reforming temperature is in the range 250-300 °C. For
the methanol fuelled plant both internal and external reforming
have been examined. The performance analysis has been
carried out by considering different values for the most
important operating parameters of the fuel cell.
The comparative analysis has demonstrated that simply
replacing methane with methanol in SOFC-MGT power plants
with internal reforming slightly reduces the efficiency.
However, the use of methanol in SOFC-MGT power plants
with external reforming enhances efficiency significantly (by
about 4 percentage points). The use of methanol with external
fuel reforming raises efficiency of the stack tanks to the
improved heat management and to the higher hydrogen partial
pressure at the anode inlet
Performance Evaluation of an Organic Rankine Cycle Fed by Waste Heat Recovered from CO2 Capture Section
Full text linkNatural gas-fueled combined cycle (NGCC) allows to reach the best performance among power plants fed by fossil fuels, but causes considerable CO2 emissions. With the aim of reducing greenhouse gases impact, NGCC could be integrated with post-combustion CO2 removal systems, typically based on chemical solvents like amines, that cause very large net efficiency penalties (about 9-12 percentage points at 90% overall CO2 capture). To reduce these high capture penalties, exhaust gas recirculation (EGR) has been studied. To further enhance the overall plant efficiency, the recovery of available low temperature heat from the solvent-based CO2 removal systems could be also performed. Low temperature heat is available in flue gas coolers (80-100°C), in the amine reboiler water cooling (130-140°C) and in the splitter condenser (100-130 °C). This waste thermal energy could be recovered by means of an Organic Rankine Cycle (ORC) that is able to convert heat into electricity efficiently even at comparably low temperatures. N-Butane was found to be as the most promising organic working fluid for the cycle operating temperatures and pressures. ORC produces additional electrical power improving the global performance of the power plant, for example, up to 1-1.5 percentage points in efficiency
Externally reformed SOFC-MGT hybrid systems fuelled by methanol and DME
No full textSOFC-MGT hybrid power plants are based on the integration of a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200-500 kW). The SOFC-MGT systems currently developed are fuelled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and DME (200-350 °C) is significantly lower than that of natural gas (700-900 °C) the reformer can be sited even outside the stack. External reforming in SOFC-MGT plants fuelled by methanol and DME enhances efficiency on account of improved exhaust heat recovery and of the higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio, SCR) must be carefully chosen in order to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fuelled hybrid plant is around 240 °C, giving efficiencies of about 67-68% with a SOFC temperature of 900 °C (the efficiency is about 72-73% at 1000 °C). Similarly, for DME the optimum reforming temperature is around 280 °C with efficiencies of about 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around 1 for methanol and 1.5-2 for DM
SOFC-MGT hybrid power plants fuelled by methanol and DME
No full textSOFC-MGT hybrid power plants are based on the integration of a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200-500 kW). The SOFC-MGT systems currently developed are fuelled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. Simply replacing methane with methanol, ethanol or DME in internally reformed SOFC-MGT systems slightly reduces efficiency and power output. However, owing to the lower reforming temperature of methanol and DME (250-300 °C), the reformer can be sited even outside the fuel cell stack. On the contrary, methane and ethanol are unsuitable for externally reformed SOFC-MGT systems, owing to their higher reforming temperature. External reforming in SOFC-MGT plants fuelled by methanol and DME enhances efficiency on account of improved exhaust heat recovery and of the higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio, SCR) must be carefully chosen in order to optimise the hybrid plant performance. In particular, the optimum reforming temperature is around 240 °C for methanol (with hybrid plant efficiencies of about 67%) and around 290 °C for DME (with hybrid plant efficiencies of about 65%). Using methanol and DME in externally reformed SOFC-MGT hybrid plants can lead to efficiency improvement with respect to internally reformed hybrid plants fuelled by methane, especially for the higher values of the fuel utilization factor. In particular, under the assumptions adopted here, the efficiency of externally reformed hybrid plants is higher that that of the internally reformed methane-fuelled plant for UF higher than 67-69% for methanol and 79-80% for DME
Externally reformed solid oxide fuel cell-micro-gas turbine (SOFC-MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)
No full textSOFC-MGT hybrid power plants are based on the integration of a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200-500 kW). The SOFC-MGT systems currently developed are fuelled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and DME (200-350 °C) is significantly lower than that of natural gas (700-900 °C) the reformer can be sited even outside the stack. External reforming in SOFC-MGT plants fuelled by methanol and DME enhances efficiency on account of improved exhaust heat recovery and of the higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio, SCR) must be carefully chosen in order to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fuelled hybrid plant is around 240 °C, giving efficiencies of about 67-68% with a SOFC temperature of 900 °C (the efficiency is about 72-73% at 1000 °C). Similarly, for DME the optimum reforming temperature is around 280 °C with efficiencies of about 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around 1 for methanol and 1.5-2 for DME
Comparative performance assessment of CRGT power plants fuelled by hydrogen energy carriers
No full textChemically Recuperated Gas Turbines (CRGT) perform the exhaust heat recovery through the endothermic steam reforming process of the primary fuel, which produces an H2-rich fuel. In particular, methanol, ethanol and DME are very suitable fuels for CRGT power plants owing to their low reforming temperature, which leads to a very effective waste heat recovery, especially in comparison with CRGT solutions fuelled by natural gas.
This paper reports a comparative performance analysis of CRGT power plants fuelled by methanol, ethanol and DME. The performance analysis has been carried out with reference of two 40 MW class commercial turbines (the aeroderivative GE LM6000 unit and the heavy-duty GE PG6561 one). The best operating conditions have been pointed out by considering different operating parameters for the reforming process (pressure, temperature and water/fuel molar ratio).
Thermochemical recuperation in CRGT power plants allows to achieve a significant performance improvement in comparison with the performance of the reference gas turbines without any exhaust heat recovery. In particular, the CRGT solutions here considered show maximum efficiencies in the range 54-57%, according to the primary fuel considered, with a corresponding increase of the power output and a decrease of the specific CO2 emissions
A steady state model for predicting performance of small-scale up-draft coal gasifiers
No full textSmall-scale fixed-bed coal and biomass gasifiers represent an attractive option for distributed combined
heat and power generation. As known, gasification phenomena are very complex, involving drying,
devolatilization, pyrolysis, heterogeneous and homogenous reactions, with a large number of intermediate
and final products. Gasification processes are also influenced by reaction kinetics and fluiddynamical
effects, such as temperature and concentration gradients. For this reason, simulation models
are able to predict gasifiers performance under the assumption of thermodynamic equilibrium only if the
gasification process takes place at a known temperature and the reaction time is lower than the reactants
residence time. As a consequence, for fixed-bed gasifiers equilibrium models must consider drying and
devolatilization taking place at lower temperature in the heat transfer zone, where solid feed is heated
by syngas. Therefore, moisture and volatiles are not involved in the gasification reactions since they
are released before reaching the reaction zone.
Several models based on steady-state and one-dimensional representations have been developed to
reproduce gasification processes in fixed-bed reactors. These models have been found adequate to provide
information for engineering design and process optimization. In this framework a steady-state simulation
model has been developed at the Department of Mechanical Chemical and Materials Engineering
(DIMCM) of the University of Cagliari by using the Aspen Plus computer code for predicting performance
of small-scale up-draft fixed-bed coal gasifiers. The model can be used to evaluate the mass and energy
balance in each zone of the gasifier and the main characteristics of the syngas produced by the gasification
process (composition, mass flow, temperature, heating value, etc.).
This paper describes the model and presents the main results of a parametric analysis, which shows
how the gasification process is influenced by the main operating parameters. Moreover, the results of
the model have been compared with the experimental results of an up-draft gasifier fed with an lignite
from Alaska. The above-mentioned gasifier is part of a pilot gasification and gas treatment plant built at
the Sotacarbo Research Centre in Sardinia, Italy. The comparison shows that the model well represents
the performance of the pilot-scale unit
Solar assisted Ultra Supercritical steam power plants with Carbon Capture and Storage
No full textThis paper focuses on the evaluation of the potential benefits arising from the integration of concentrating solar systems into coal-based Ultra Supercritical (USC) power plants with Carbon Capture and Storage (CCS). In particular, different solutions for the solar field, based on direct steam generation (DSG) with parabolic trough and linear Fresnel collectors, and for using the steam in the Rankine cycle were analyzed and compared.
The comparative analysis was carried out with reference to a USC plant based on a double reheated Rankine cycle with four steam turbines and nine regenerative steam extractions. The USC plant was also integrated with a post-combustion CO2 removal process (USC-CCS) based on chemical absorption with an aqueous solution of MEA. The CO2 removal section requires a large amount of steam to be extracted from the low pressure turbine in order to satisfy the reboiler thermal power requirement, reducing performance of USC plant (about 10.5 percentage points of efficiency reduction for a CO2 removal of 90%). The performance of the reference USC-CCS power plant were evaluated by means of simulation models specifically developed through Aspen-Plus and Gate-Cycle software platforms.
In order to offset the efficiency penalty introduced by CO2 removal, the USC-CCS plant was integrated with a concentrating solar field with direct steam generation (DSG) based on parabolic trough and linear Fresnel collectors. The solar field is based on several lines of collectors connected in parallel to achieve the required steam mass flow and therefore the required thermal power output. Owing to the difficulties related to the storage of steam, the presence of a thermal energy storage was not considered. Performance of the solar field were evaluated on a yearly basis by means of a specifically developed simulation model. In particular, performance of collectors were evaluated as a function of solar radiation and solar position, for given values of the main geometrical and technical characteristics of collectors. The study was carried out by using a data set for a typical meteorological year for the site of Cagliari, in Sardinia (Italy).
Direct solar energy is available only for a limited number of hours in the year, due to cloudiness and nights; moreover, during most of the solar field operating hours the DNI is below its design value. Therefore, the design conditions of the USC-CCS plant considered here refer to the absence of steam production by the solar field. As a consequence during periods of solar energy availability, steam production from the solar field increases mass flow rate of the steam turbines leading to the off-design operation mode of the USC plant, with a corresponding efficiency penalty. For this reason an in-depth analysis of USC performance due to solar integration was carried out to assess in particular the influence of sliding pressure on steam turbine performance. A preliminary cost analysis was also performed.
Overall, integration with concentrating solar collectors allows to reduce USC efficiency penalization due to CO2 capture, assuring an efficiency increase up to 1-2 percentage points, depending on solar field size. On the other hand, solar fields require a very large land availability (about 1-1.5 Km2 for 300 MW of thermal power, depending on collectors technology). The results of the comparative performance assessment demonstrate that owing to their better optical efficiency, the use of parabolic troughs gives better performance than linear Fresnel collectors, even if the latter give the higher energy production per m2 of occupied land
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