Daniele Fiaschi

A two-phase one-dimensional biomass gasification kinetics model

Abstract A mathematical model of biomass gasification kinetics in bubbling fluidized beds has been developed. It is one-dimensional, as it is capable of predicting temperature and concentration gradients along the reactor axis, and considers two phases, a bubble and a dense phase. In addition to the reaction kinetics in the dense phase, mass transfer between the two phases and a quantitative estimation of local bubble and particle properties are included in the model. A theoretical optimization with respect to ER, pressure, bed height and gas velocity has also been performed. Finally, a comparison with experimental data from the literature was done, which showed a largely satisfactory agreement, though further validation is still required.

Exergy analysis of the semi-closed gas turbine combined cycle (SCGT/CC)

Abstract In the present paper, an exergy analysis of the SCGT/CC cycle is presented. Exergy destruction has been analysed at the component level in order to identify the critical plant devices, considering several operating conditions. The power-plant configuration is similar to that presented in previous works, with the possibility of total or partial water injection in the combustion chamber at peakload conditions. Combustion, heat recovery steam generator (HRSG), water injection/mixing and water recovery system are the main sources of losses, representing globally more than 80% of the overall exergy destruction. The second-law efficiency ranges between 49% and 53%, moving from the fully injected to the not injected condition. These values are close to those of standard open cycles, which means a good potential for the SCGT/CC cycle, which has several advantages from the point of view of containment of emissions and capability of peakload shaving. Peakload operation (with partial or total water re-injection) involves additional waste of exergy, but is attractive as it can be very extended for this plant configuration. Some critical components, such as condensing heat exchanger, show some sensitivity to the operating parameters, which however only marginally affects the cycle performance.

SCGT/CC : An innovative cycle with advanced environmental and peakload shaving features

Abstract An innovative gas turbine cycle is studied, which can offer several advantages from the point of view of environmental friendship and peakload shaving capabilities. The basic idea of SCGT/CC is of cooling down the exhaust to temperatures as low as to allow full condensation of the water vapor; a large part of the exhaust gases is then recirculated to the compressor; the condensed water can be reinjected by means of a pump at compressor delivery. For maximum performance it is convenient not to inject this water flow, but rather to use it for other purposes; however, water injection produces a power boosting effect (at the expense of a small decrease in efficiency) which can be useful for peakload shaving applications. The working gas composition in the GT cycle is that corresponding to stoichiometric combustion, which opens the possibility of applying techniques for CO 2 recycling and general exhaust gas treatment. The cycle guarantees a high level of efficiency, and its adoption should imply minor modifications to existing equipment.

The Recuperative Auto Thermal Reforming and Recuperative Reforming Gas Turbine Power Cycles With CO2 Removal—Part II: The Recuperative Reforming Cycle

The relatively innovative gas turbine based power cycles R-ATR and R-REF (recuperative-auto thermal reforming GT cycle and recuperative-reforming GT cycle) here proposed, are mainly aimed to allow the upstream CO 2 removal by the natural gas fuel reforming. The second part of the paper is dedicated to the R-REF cycle : the power unit is a gas turbine (GT), fuelled with reformed and CO 2 cleaned gas, obtained by the addition of several sections to the simple GT cycle, mainly: ○ reformer section (REF), where the reforming reactions of methane fuel with steam are accomplished: the necessary heat is supplied partially by the exhausts cooling and, partially, with a post-combustion, ○ water gas shift reactor (WGSR), where the reformed fuel is, shifted into CO 2 and H 2 with the addition of water, and ○ CO 2 removal unit for the CO 2 capture from the reformed and shifted fuel. No water condensing section is adopted for the R-REF configuration. Between the main components, several heat recovery units are applied, together with GT cycle recuperator, compressor intercooler, and steam injection into the combustion chamber. The CO 2 removal potential is close to 90% with chemical absorption by an accurate choice of amine solution blend: the heat demand for amine regeneration is completely self-sustained by the power cycle. The possibility of applying steam blade cooling (the steam is externally added) has been investigated: in these conditions, the R-REF has shown efficiency levels close to 43-44%. High values of specific work have been observed as well (around 450-500 kJ/kg). The efficiency is slightly lower than that found for the R-ATR solution, and 2-3% lower than CRGTs with CO 2 removal and steam bottoming cycle, not internally recuperated. If compared with these, the R-REF offers higher simplicity due to absence of the steam cycle, and can be regarded as an improvement to the simple GT In this way. at least 5-6 points efficiency can be gained, together with high levels of CO 2 removal. The effects of the reformed fuel gas composition, temperature, and pressure on the amine absorption system for the CO 2 removal have been investigated, showing the beneficial effects of increasing pressure (i.e., pressure ratio) on the specific heat demand.

Thermoeconomic evaluation of the SCGT cycle

Abstract The analysis of the SCGT (Semi-Closed Gas Turbine cycle) is extended to the treatment of acid condensation (sulphur compounds) at the exit of the separator (SEP), with reference to different possible configurations already studied from the thermodynamic and environmental points of view. This detailed analysis was considered necessary because the natural gas fuel can contain a small amount of H 2 S which, reacting with air, can form SO 2 and finally sulphuric acid. This can represent a problem (mainly from the economic point of view) because of the possibility of sulphuric acid condensation at the exit of the separator, where the temperature can reach values below the acid dew point of the mixture. The data obtained from ENI publications were used for the natural gas composition, and a 0.005% H 2 S molar fraction was additionally hypothesized. With these assumptions, about 0.1% SO 2 can be found in the exhaust gases at the separator inlet. Aspen Plus was used in order to evaluate the chemical effects of the acidity of the condensate produced in the separator. An evaluation about costs of the devices to be used for condensation of the recirculated flue gas humidity has been performed, considering use of the special materials necessary for reducing the aggressive effects of acid water condensation. A final evaluation of the overall conversion system plant is also produced, showing the economic balance in terms of resulting cost of the unit of electrical energy produced and of inlet power in terms of fuel. The results are also evaluated in terms of CO 2 emissions, considering the ratio between the global cost of the power generation plant and the global carbon dioxide emissions, compared to other types of energy conversion open cycle solutions.

Semi-Closed Gas Turbine/Combined Cycle With Water Recovery and Extensive Exhaust Gas Recirculation

This innovative gas turbine cycle can offer several advantages over conventional cycles from the point of view of environmental friendship. The basic idea of SCGT/CC (Semi-Closed Gas Turbine/Combined Cycle with water recovery) is to cool down the exhaust temperatures to allow full condensation of the water vapor, and recirculate a large part of the exhaust gases to the compressor. The condensed water can then be reinjected by means of a pump at compressor delivery. The working gas composition is thus close to that corresponding to stoichiometric combustion, which opens the possibility of applying techniques for CO2 recycling and general exhaust gas treatment. An increase in work output is connected to water injection, while a high level of efficiency is maintained as the compressor work is reduced and the cycle parameters are tuned for the exhaust of this turbine.Copyright

A Biomass Combustion-Gasification Model: Validation and Sensitivity Analysis

The aim of the present paper is to study the gasification and combustion of biomass and waste materials. A model for the analysis of the chemical kinetics of gasification and combustion processes was developed with the main objective of calculating the gas composition at different operating conditions. The model was validated with experimental data for sawdust gasification. After having set the main kinetic parameters, the model was tested with other types of biomass, whose syngas composition is known. A sensitivity analysis was also performed to evaluate the influence of the main parameters, such as temperature, pressure, and air-fuel ratio on the composition of the exit gas. Both oxygen and air (i.e., a mixture of oxygen and nitrogen) gasification processes were simulated.

Exergy Analysis of Two Second-Generation SCGT Plant Proposals

Two different power plant configurations based on a Semi-Closed Gas Turbine (SCGT) are analyzed and compared in terms of First and Second Law analysis. SCGT plant configurations allow the application of CO2 separation techniques to gas-turbine based plants and several further potential advantages with respect to present, open-cycle solutions. The first configuration is a second-generation SCGT/CC (Combined Cycle) plant, which includes inter-cooling (IC) between the two compression stages, achieved using spray injection of water condensed in a separation process removing vapor from the flue gases. The second configuration (SCGT/RE) combines compressor inter-cooling with the suppression of the heat recovery steam generator and of the whole bottoming cycle; the heat at gas turbine exhaust is directly used for gas turbine regeneration.The SCGT/CC-IC solution provides good efficiency (about 55%) and specific power output figures, on account of the spray inter-cooling; however, with this configuration the cycle is not able to self-sustain the CO2 removal reactions and amine regeneration process, and needs a substantial external heat input for this purpose.The SCGT/RE solution is mainly attractive from the environmental point of view: in fact, it combines the performance of an advanced gas turbine regenerative cycle (efficiency of about 49%) with the possibility of a self-sustained CO2 removal process. Moreover, the cycle configuration is simplified because the HRSG and the whole bottoming cycle are suppressed, and a potential is left for cogeneration of heat and power.© 1998 ASME

The Recuperative-Auto Thermal Reforming and the Recuperative-Reforming Gas Turbine Power Cycles With CO2 Removal—Part I: The Recuperative-Auto Thermal Reforming Cycle

The relatively innovative gas turbine based power cycles R-ATR and R-REF (Recuperative-Auto Thermal Reforming GT cycle and Recuperative-Reforming GT cycle) here proposed are mainly aimed to allow the upstream CO 2 removal by the way of natural gas fuel reforming. The power unit is a gas turbine (GT), fueled with reformed and CO 2 cleaned syngas produced by adding some basic sections to the simple GT cycle: ○ auto thermal reforming (ATR) for the R-ATR solution, where the natural gas is reformed into CO, H 2 , CO 2 , H 2 O, and CH 4 ; this endothermic process is completely sustained by the heat released from the reactions between the primary fuel (CH 4 ), exhausts and steam. ○ water gas shift reactor (WGSR), where the reformed fuel is, as far as possible, shifted into CO 2 and H 2 by the addition of water. ○ water condensation, in order to remove a great part of the fuel gas humidity content (this water is totally reintegrated into the WGSR). ○ CO 2 removal unit for the CO 2 capture from the reformed fuel. Among these main components, several heat recovery units are inserted, together with GT cycle recuperator, compressor intercooler, and steam injection in combustion chamber. The CO 2 removal potential is close to 90% with chemical scrubbing using an accurate choice of amine solution blend: the heat demand is completely provided by the power cycle itself. The possibility of applying steam blade cooling by partially using the water released from the dehumidifier downstream the WGSR has been investigated: in these conditions, the R-ATR has shown an efficiency range of 44-46%. High specific work levels have also been observed (around 450-550 kJ/kg). These efficiency values are satisfactory, especially if compared with ATR combined cycles with CO 2 removal, more complex due to the steam power section. If regarded as an improvement to the simple GT cycle, R-ATR shows an interesting potential if directly applied to a current GT model; however, partial redesign with respect to the commercially available version is required. Finally, the effects of the reformed fuel gas composition and conditions on the amine CO 2 absorption system have been investigated, showing the beneficial effects of increasing pressure (i.e., pressure ratio) on the thermal load per kg of removed CO 2 .

Semi–Closed Hat (SC-HAT) Power Cycle

This new power cycle is derived from a simplified HAT cycle, with a partial recirculation of the exhaust gases added with respect to the traditional HAT configuration. The basic idea of applying recirculation to the HAT cycle stems from the interesting performance levels and general environmental advantages obtainable applying this technique to combined-cycle (SCGT/CC) and regenerative GT solutions (SCGT/RE); these power plants all share the integration with CO2 chemical scrubbing of the exhaust stack in order to reduce greenhouse effects.A relevant advantage of the proposed configuration over the original HAT solution is the possibility of complete water recovery from the separator before the recirculation node; here the temperature level is necessarily very low, allowing thus condensation of water produced by the natural-gas combustion process. This allows the self–sustainement of the HAT cycle, from the water consumption point of view, without any external supply. For the water separator, two thermodynamic models were developed (respectively simulating a single- and a multiple temperature condensation process), which have provided similar results.The whole cycle is modeled using a modular code, thoroughly tested against the performance of a large set of existing GTs. The layout is derived from an existing HAT configuration, with suppression of the economizer section in the regenerator and the possible practice of external (non-recuperative) intercooling between the two compressors. The first choice is imposed by the presence of an additional low-temperature heat load for the CO2 removal plant, while the second is sometimes necessary depending on the compressor pressure ratios and the possibility of including inside the cycle low-temperature internal cycle regeneration.The expected performance of the plant is relatively high and close to those typical of HAT, SCGT/RE and SCGT/CC cycles: a LHV-based efficiency level exceeding 50% inclusive of CO2 separation and delivery at ambient pressure and temperature; the specific work levels — in the range of 680 kJ/kg for the basic configuration — are lower than those of the HAT cycle but larger than for SCGT/CC and SCGT/RE solutions; the cycle requires relatively high overall pressure ratios (35–40). A notable improvement in specific work can be obtained with reheat.Copyright