Andrea Corti

A review of technologies and performances of thermal treatment systems for energy recovery from waste

The aim of this work is to identify the current level of energy recovery through waste thermal treatment. The state of the art in energy recovery from waste was investigated, highlighting the differences for different types of thermal treatment, considering combustion/incineration, gasification and pyrolysis. Also different types of wastes - Municipal Solid Waste (MSW), Refuse Derived Fuel (RDF) or Solid Refuse Fuels (SRF) and some typologies of Industrial Waste (IW) (sludge, plastic scraps, etc.) - were included in the analysis. The investigation was carried out mainly reviewing papers, published in scientific journals and conferences, but also considering technical reports, to gather more information. In particular the goal of this review work was to synthesize studies in order to compare the values of energy conversion efficiencies measured or calculated for different types of thermal processes and different types of waste. It emerged that the dominant type of thermal treatment is incineration associated to energy recovery in a steam cycle. When waste gasification is applied, the produced syngas is generally combusted in a boiler to generate steam for energy recovery in a steam cycle. For both the possibilities--incineration or gasification--co-generation is the mean to improve energy recovery, especially for small scale plants. In the case of only electricity production, the achievable values are strongly dependent on the plant size: for large plant size, where advanced technical solutions can be applied and sustained from an economic point of view, net electric efficiency may reach values up to 30-31%. In small-medium plants, net electric efficiency is constrained by scale effect and remains at values around 20-24%. Other types of technical solutions--gasification with syngas use in internally fired devices, pyrolysis and plasma gasification--are less common or studied at pilot or demonstrative scale and, in any case, offer at present similar or lower levels of energy efficiency.

Analysis of energy recovery potential using innovative technologies of waste gasification

In this paper, two alternative thermo-chemical processes for waste treatment were analysed: high temperature gasification and gasification associated to plasma process. The two processes were analysed from the thermodynamic point of view, trying to reconstruct two simplified models, using appropriate simulation tools and some support data from existing/planned plants, able to predict the energy recovery performances by process application. In order to carry out a comparative analysis, the same waste stream input was considered as input to the two models and the generated results were compared. The performances were compared with those that can be obtained from conventional combustion with energy recovery process by means of steam turbine cycle. Results are reported in terms of energy recovery performance indicators as overall energy efficiency, specific energy production per unit of mass of entering waste, primary energy source savings, specific carbon dioxide production.

Absorption of CO2 With Amines in a Semiclosed GT Cycle: Plant Performance and Operating Costs

A CO2 removal system, using aqueous solutions of amines, is applied to a Semi-Closed Gas Turbine/Combined Cycle (SCGT/CC) power plant.The SCGT/CC is interesting because of the possibility of achieving low emissions at the stack, with a decreased overall flow rate, lower NOx concentration and an increased CO2 concentration (over 15% in volume), which facilitates the removal treatment.Several compositions of the absorbing solution have been investigated, by means of simulations with ASPEN PLUS.A 18% DEA + 12% MDEA composition resulted the most convenient in terms of flow rate and energy requirement for the stripping.A cost analysis of the removal plant allows to estimate additional costs for CO2 removal with respect to conventional power plants.© 1998 ASME

Comparison of different biological treatment scenarios for the organic fraction of municipal solid waste

The possibility of applying anaerobic digestion (AD) process of the source-selected organic fraction (SS-OF) of municipal solid waste in the south of Tuscany (IT) territory was proposed and evaluated. With respect to a reference scenario, in which the overall amount of SS-OF and garden waste—expected in 2013—is addressed to aerobic composting process, three alternative scenarios were proposed, modeled, and compared by life cycle assessment (LCA). The first one is based on realizing six AD plants upstream of each already existing aerobic plant. The second one is based on realizing only three centralized anaerobic plants. The third alternative is based on co-digestion of a part of the SS-OF with sludge from waste water treatment plants in existing sludge AD plants; the remaining part of SS-OF is processed in devoted AD plants upstream of each already existing aerobic plant. LCA results show that AD application is always favorable with respect to the reference scenario in which organic wastes are aerobically processed. AD allows for net production of thermal and electric energy, generating negative impacts due to avoided emissions and avoided resource consumptions. Among the compared alternatives of AD application, the scenario based on decentralized plants (one anaerobic plant upstream of each existing aerobic plant) resulted to be most favorable, collecting the best value for almost all the considered environmental indicators.

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.

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

Biochemical methane potential tests of different autoclaved and microwaved lignocellulosic organic fractions of municipal solid waste.

The aim of this research was to enhance the anaerobic biodegradability and methane production of two synthetic Organic Fractions of Municipal Solid Waste with different lignocellulosic contents by assessing microwave and autoclave pre-treatments. Biochemical Methane Potential assays were performed for 21days. Changes in the soluble fractions of the organic matter (measured by soluble chemical oxygen demand, carbohydrates and proteins), the first order hydrolysis constant kh and the cumulated methane production at 21days were used to evaluate the efficiency of microwaving and autoclaving pretreatments on substrates solubilization and anaerobic digestion. Microwave treatment led to a methane production increase of 8.5% for both the tested organic fractions while autoclave treatment had an increase ranging from 1.0% to 4.4%. Results showed an increase of the soluble fraction after pre-treatments for both the synthetic organic fractions. Soluble chemical oxygen demand observed significant increases for pretreated substrates (up to 219.8%). In this regard, the mediocre results of methanes production led to the conclusion that autoclaving and microwaving resulted in the hydrolysis of a significant fraction of non-biodegradable organic substances recalcitrant to anaerobic digestion.

Wind tunnel experiments of tracer dispersion downwind from a small-scale physical model of a landfill

Wind tunnel experiments have been carried out on a small-scale physical model of a municipal waste landfill (MWL) in the CRIACIV (Research Centre of Building Aerodynamics and Wind Engineering) “environmental” wind tunnel in Prato (Italy). The MWL model simulates a landfill whose surface is higher than the surrounding surface, applying a 1:200 scaling factor. Modelling an area source such as landfill is a difficult task for numerical models due to turbulence phenomena that modifies the flow near the source increasing ground level concentration (GLC). For the specific task, a new set-up of the wind tunnel has been developed, with respect to previous studies carried out on line and point sources physical models. The tracer used in the experiments was ethylene, suitable for non-buoyant plume conditions, typical for MWL emissions. A detailed result database has been obtained in terms of GLC and concentration profiles as well as flow turbulence and velocity field characterisation.

Environmental impact from wet plumes in combined-cycle power plants

In the present study, we have carried out an investigation into some of the environmental effects of high energy efficiency power generation plants that produce electrical and thermal energy. The use of natural gas fuels may represent a success for the replacement of liquid or solid fuels, because of drastic reductions of sulphur oxides, carbon dioxide and particulates. An additional problem involves the study of the effects of vapour emissions from the combustion of fuels and from the cooling towers of the condenser device. Particulary, the problem of the analysis of vapour and drift diffusion, because of emissions from the wet cooling devices, is faced in terms of modelling the plume rise and mass loss of drift in the atmosphere, because of evaporation, in order to predict the increment of rain and the humidity effect. The results obtained show that the amount of water emitted in the forms of vapour and drift, in terms of local scale diffusion, do not create problems of high increments of relative humidity or dangerous rainfall phenomena: while the effects of vapour emission are shown as insignificant problems because of the deposition of a sensible amount of drift, correlated with the type of separator devices chosen can appear, but only on very limited areas.

Influence of Meteorological Input Parameters on Urban Dispersion Modelling for Traffic Scenario Analysis

The aim of this study is to evaluate the results from using parallel observation and numerical weather prediction data as inputs to the urban dispersion model SIRANE applied to traffic scenarios of NOX in the city of Florence. A detailed sensitivity analysis of the air quality modelling system against measurements from monitoring stations of the city of Florence has been carried out in order to identify the advantage and disadvantages of the use of meteorological observation and numerical weather prediction data.