Emmanuel Kakaras

Computer simulation studies for the integration of an external dryer into a Greek lignite-fired power plant

Abstract Brown coal is considered to be a competitive primary energy source for power generation in parts of Central and Eastern Europe due to the economically recoverable reserves of this fuel in these regions. On the other side, it is well known that brown coal contains significant amounts of moisture which decreases the overall plant thermal efficiency due to the increased flue gas thermal loses. In order to keep the competitive edge of brown coal, an improvement of the power plant efficiency is required. One way to achieve this target is to dry the moist brown coal before combustion. Conventionally, the necessary moisture extraction process is integrated into milling, where a recirculated flue gas stream is used as a drying agent. External brown coal drying offers the potential to increase the net thermal efficiency of the entire power generation process by up to 5% points. This paper presents a description of the technical fundamentals of drying techniques developed and successfully tested so far. Furthermore, it presents computer simulation studies for the integration of an external dryer into a steam cycle of a Greek brown coal-fired power plant as a typical test to demonstrate the potential of the drying technologies for power generation. The major objectives of these studies are to formulate basic drying schemes which might be integrated into a power production steam cycle. Three dryers using steam as a drying agent, e.g. the tubular dryer, the fluidised bed dryer and the mechanical thermal dewatering process are compared to the conventional flue gas drying technology using a computer simulation.

Advantages and Possibilities of Solid Recovered Fuel Cocombustion in the European Energy Sector

The 1999/31 Elemental Carbon Directive sets strict rules on the disposal of untreated municipal solid waste in the European Union countries and forces a reduction of the biodegradable quantities disposed off to landfills up to 35% of the amount produced in 1995 in the coming decade. More environmentally friendly waste management options shall be promoted under the framework of the Community Waste Strategy ([96] 399 Final). In this context, the production and thermal use of solid recovered fuels (SRFs), derived from nonhazardous bioresidues and mixed- and mono-waste streams, could be a key element in a future waste management system. Within the scope of the European Demonstration Project, RECOFUEL, SRF cocombustion was demonstrated in two large-scale lignite-fired coal boilers at RWE power station in Weisweiler, Germany. As a consequence of the high biogenic share of the cocombusted material, this approach can be considered beneficial following European Directive 2001/77/EC on electricity from renewable energy sources (directive). During the experimental campaign, the share of SRF in the overall thermal input was adjusted to approximately 2%, resulting into a feeding rate of approximately 25 t/hr. The measurement campaign included boiler measurements in different locations, fuel and ash sampling, and its characterization. The corrosion rates were monitored by dedicated corrosion probes. The overall results showed no significant influence of SRF cocombustion on boiler operation, emissions behavior, and residues quality for the thermal shares applied. Also, no effect of the increased chlorine concentration of the recovered fuel was observed in the flue gas path after the desulfurization unit.

Production of special activated carbon from lignite for environmental purposes

A treatment technique involving three sequential stages (demineralisation, activation and sulphur dispersion) was developed for the production of suitable activated carbons from Greek lignite. Demineralisation included three steps of acid treatment and samples received were characterised by X-ray diffraction (XRD). A two-stage activation procedure (pyrolysis under nitrogen, followed by activation under carbon dioxide atmosphere) was used for the production of activated samples. Sulphur impregnation of activated carbons was performed by heating with high purity elemental sulphur flakes under nitrogen flow at temperatures up to 600 jC. SEM and line scattering techniques were used to evaluate sulphur distribution in the impregnated activated carbons. Adsorption of N2 at 77 K and CO2 at 298 K was used for the characterisation of products. Sulphur impregnated activated carbon samples were proven unreactive and stable at the flue gases temperature. D 2002 Elsevier Science B.V. All rights reserved.

Thermal exploitation of wastes with lignite for energy production

Abstract The thermal exploitation of wastewood with Greek lignite was investigated by performing tests in a laboratory-scale fluidized bed reactor, a 1-MWth semi-industrial circulating fluidized bed combustor, and an industrial boiler. Blends of natural wood, demolition wood, railroad sleepers, medium-density fiberboard residues, and power poles with lignite were used, and the co-combustion efficiency and the effect of wastewood addition on the emitted pollutants were investigated. Carbon monoxide, sulfur dioxide, and oxides of nitrogen emissions were continuously monitored, and, during the industrial-scale tests, the toxic emissions (polychlorinated dibenzodioxins and dibenzofurans and heavy metals) were determined. Ash samples were analyzed for heavy metals in an inductively coupled plasma-atomic emission spectroscopy spectrophotometer. Problems were observed during the preparation of wastewood, because species embedded with different compounds, such as railway sleepers and demolition wood, were not easily treated. All wastewood blends were proven good fuels; co-combustion proceeded smoothly and homogeneous temperature and pressure profiles were obtained. Although some fluctuations were observed, low emissions of gaseous pollutants were obtained for all fuel blends. The metal element emissions (in the flue gases and the solid residues) were lower than the legislative limits. Therefore, wastewood co-combustion with lignite can be realized, provided that the fuel handling and preparation can be practically performed in large-scale installations.

Techno-Economic Comparison of CO2 Capture Technologies Employed With Natural Gas Derived GTCC

Carbon Capture and Storage can either concern the removal of carbon as CO2 in flue gases (post-combustion option) or before its combustion in a Gas Turbine (pre-combustion option). Among the numerous CO2 capture technologies, amine scrubbing (MEA and MDEA), physical absorption (Selexol™ and Rectisol™) and H2 separator membrane reactors are investigated and compared in this study. In the pre-combustion options, the final fuel combusted in the GT is a rich-H2 fuel. Process simulations in ASPEN Plus™ showed that the case of H2 separation with Pd-based membranes has the greatest performance as far as the net efficiency of the energy system is concerned. The economic assessment reveals that the technology is promising in terms of cost of CO2 avoided, provided that the current high membrane costs are reduced.Copyright

Renewable and Conventional Electricity Generation Systems: Technologies and Diversity of Energy Systems

In this chapter, the primary technical aspects of conventional and renewable energy systems are presented. The description focuses on commercial systems installed across the world, together with a brief introduction to some promising technologies currently under development, such as Carbon Capture and Storage (CCS). Conventional energy systems include power plants using fossil fuels (natural gas, coal, etc.), while renewable energy systems include solar, wind, geothermal, biomass, and small-hydropower applications. These technologies are briefly described accompanied by economic figures (installation cost, fuel cost, specific cost of electricity, etc.) and emissions data (where applicable). Some insight on the energy strategy in specific countries is provided and how this can be related to local conditions and electric power requirements.

Optimization of Computational Performance and Accuracy in 3‐D Transient CFD Model for CFB Hydrodynamics Predictions

This work aims to present a pure 3‐D CFD model, accurate and efficient, for the simulation of a pilot scale CFB hydrodynamics. The accuracy of the model was investigated as a function of the numerical parameters, in order to derive an optimum model setup with respect to computational cost. The necessity of the in depth examination of hydrodynamics emerges by the trend to scale up CFBCs. This scale up brings forward numerous design problems and uncertainties, which can be successfully elucidated by CFD techniques. Deriving guidelines for setting a computational efficient model is important as the scale of the CFBs grows fast, while computational power is limited. However, the optimum efficiency matter has not been investigated thoroughly in the literature as authors were more concerned for their models accuracy and validity. The objective of this work is to investigate the parameters that influence the efficiency and accuracy of CFB computational fluid dynamics models, find the optimum set of these paramet...

Coal combustion with simulated gas turbine exhaust gas and catalytic oxidation of the unburnt fuel

Abstract One method of improving the overall efficiency of a power plant or increasing the power output is to re-power the existing boilers, using gas turbines in a combined cycle system. For the case of coal-fired plants, this option known as gas turbine topping is limited by the difficulties in coal burnout due to the low oxygen content (12 vol%) in the exhaust gas of modern gas turbines. A possible option to resolve the problem could be the introduction of an oxidation catalyst in order, first, to avoid the incomplete combustion of the coal and, second, to decrease the CO and CxHy emissions. This paper presents the results of coal combustion tests in an existing atmospheric fluidized bed with simulated gas turbine exhaust flue gas. The purposes of the trials were the examination of the possibility to achieve the combustion of low-grade lignite with about 12–14 vol% O2 content in the turbine exhaust gas and the experimental investigation of the catalytic oxidation on the burnout behaviour as well as the resulting CO, SO2, N2O, CxHy and NOx emissions. Two series of measurements were carried out, with and without catalysts. The influence of the catalysts on the burnout of coal and the combustion temperature is examined and discussed. The main conclusion of this experimental investigation is that it is possible to achieve combustion of the low-grade lignite in combined cycle applications, using turbine exhaust gas as a fluidizing and oxidant medium with the support of the catalysts examined in this study, as is shown by the drastic reduction of CO and CxHy emissions.

Emissions during the Co-combustion of Lignite and Waste Wood in a Fluidised Bed Reactor

Co-combustion tests were performed in a lab-scale fluidised bed reactor, in order to define (a) the optimum percentage for substituting Greek lignite by waste wood, and (b) the operation conditions ensuring complete burnout of the fuel blends. Tests were performed at the experimental facility of the NTUA’s Steam Boilers and Thermal Plants Laboratory (NTUA-LSB). Pre-dried lignite, from Ptolemais reserve, and various waste wood species, i.e. uncontaminated wood, demolition timber and railway sleepers, were used to prepare the fuel blends. In all tests, the emissions in flue gases - CO, SO2, N2O, NOx, NO, NO2 and CXHY — were continuously monitored.

Efficient CHP-Plant Configuration for District Heating Systems Utilizing Low-Rank Coals

AbstractThe present study assesses different lignite-fired plant configurations for combined heat and power (CHP) production for use in district heating (DH) networks, in terms of environmental, te...