Panagiotis Grammelis

Pyrolysis characteristics and kinetics of biomass residuals mixtures with lignite

Abstract Biomass residues in the Mediterranean region come mainly from agricultural and agro-industrial activities and forest byproducts. The paper presents the results of kinetic parameters and main devolatilisation characteristics of three biomass materials, when these are used either alone or in conjunction with Greek lignite. Namely, olive kernel, forest and cotton residues were pyrolysed in a thermogravimetric analyser, under dynamic conditions. The effect of material particle size and heating rate was investigated both on the pyrolysis behavior and reaction kinetics, over the temperature range of 25–850 °C. Furthermore, experiments with blends of lignite and biomass were conducted under the same conditions at the lower heating rate. The biomass materials presented higher thermochemical reactivity than lignite. Their decomposition was successfully modeled by three first-order independent parallel reactions, describing the degradation of hemicellulose, cellulose and lignin. No significant influence of the particle size was detected, both on the devolatilisation characteristics and kinetics. The effect of the heating rate on the pyrolysis behavior was more pronounced for biomass materials rather than lignite. A comparison between slow and fast heating rate tests reveals a small displacement of the DTG profiles to higher temperatures. It was concluded that such ‘solid bio-fuels’ could support the combustion of poor coals, because of the faster and in much higher quantity release of volatile compounds.

Thermogravimetric studies of the behavior of lignite-biomass blends during devolatilization

The behaviour of Greek pre-dried lignite, four biomass materials and their blends in the devolatilization stage was investigated by thermogravimetry. Biomass was added in the percentages of 5, 10 and 20% wt. in the fuel blend. All the tests were carried out in nitrogen atmosphere under dynamic conditions at a heating rate of 10 °C/min. The kinetic parameters for the thermal conversion of the pure fuels were determined through the independent parallel, first-order, reaction model. No significant interaction was detected in the solid phase between the components of the coal–biomass blends, under the same experimental conditions.

Cultivation and Characterization of Cynara Cardunculus for Solid Biofuels Production in the Mediterranean Region

Technical specifications of solid biofuels are continuously improved towards the development and promotion of their market. Efforts in the Greek market are limited, mainly due to the climate particularity of the region, which hinders the growth of suitable biofuels. Taking also into account the increased oil prices and the high inputs required to grow most annual crops in Greece, cardoon (Cynara cardunculus L.) is now considered the most important and promising sources for solid biofuel production in Greece in the immediate future. The reason is that cardoon is a perennial crop of Mediterranean origin, well adapted to the xerothermic conditions of southern Europe, which can be utilized particularly for solid biofuel production. This is due to its minimum production cost, as this perennial weed may perform high biomass productivity on most soils with modest or without any inputs of irrigation and agrochemicals. Within this framework, the present research work is focused on the planning and analysis of different land use scenarios involving this specific energy crop and the combustion behaviour characterization for the solid products. Such land use scenarios are based on quantitative estimates of the cropsproduction potential under specific soil-climatic conditions as well as the inputs required for its realization in comparison to existing conventional crops. Concerning its decomposition behaviour, devolatilisation and char combustion tests were performed in a non-isothermal thermogravimetric analyser (TA Q600). A kinetic analysis was applied and accrued results were compared with data already available for other lignocellulosic materials. The thermogravimetric analysis showed that the decomposition process of cardoon follows the degradation of other lignocellulosic fuels, meeting high burnout rates. This research work concludes that Cynara cardunculus, under certain circumstances, can be used as a solid biofuel of acceptable quality.

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.

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.

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...

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...

Co-firing Solid Recovered Fuels (SRFs) with brown coal in large-scale pulverised fuel power plants – a simulation approach

The results of the numerical simulations performed in a pulverised fuel boiler co-firing coal and Solid Recovered Fuels (SRFs) are presented. SRFs are initially modelled as a mixture of two different fractions (biogenic and plastic) and further simplifications are proposed to save computational expenses. The SRF submodels are validated with available data obtained from a laboratory-scale combustion facility. The furnace of a 600 MWe brown coal plant is further simulated. The respective boundary conditions are obtained from a performed large-scale measurement campaign during an SRF co-firing demonstration in the specific boiler. The results of the baseline case are in good agreement with the available experimental data for the large-scale boiler. A number of fuel and process-related combustion parameters is further examined, including the co-firing thermal share, injection locations and the Particle Size Distribution (PSD) of SRF particles. Computational Fluid Dynamics (CFD) modelling in large-scale boilers can be a reliable and efficient tool towards predicting specific changes of the combustion behaviour. It is particularly valuable when the intended investigations cannot be easily realised on a large scale, such as increasing the co-firing thermal share or modifying the injection locations of alternative fuels.

Experience on Combustion and Co-Combustion of Greek Brown Coal in Fluidized Bed Facilities

The paper aims to present the experience gained from the combustion trials of Greek brown coal in different installations, both in semi-industrial and laboratory scale. Specifically, these research activities are separated in two parts, i.e. combustion tests using only brown coal and co-combustion tests with brown coal and biomass. Combustion tests with Greek lignite were realised in three different Circulating Fluidized Bed Combustion (CFBC) facilities. Low rank lignite was burned in a pilot scale facility of approx. 100kW thermal capacity, located in Athens (NTUA) and a semi-industrial scale of 1.2 MW thermal capacity, located at RWE’s power station Niederaussem in Germany. The results include the determination of operating conditions to achieve proper fuel burnout, the examination of the influence of air staging on the temperature distribution inside the reactor and the investigation of the combustion behaviour of the particular fuel type and emitted pollutants. Several conclusions are drawn concerning the necessary modifications and requirements of the plant layout when a large scale CFBC installation is designed to utilize low grade brown coal. Co-combustion tests with Greek xylitic lignite and waste wood were carried out in the 1 MWth CFBC installation of AE&E, in Austria. During the tests, oxygen concentration and CO, SO2 , N2 O and NOX emissions were continuously monitored. Ash samples were collected and analysed for heavy metals content in ICP-AES spectrophotometer. The improved combustion behaviour of this lignite type was more than evident, since it has lower moisture content and increased calorific value. In all co-combustion tests, low emissions of gaseous pollutants were obtained and metal element emissions were lower than the corresponding values anticipated by the guidelines. In addition, lab-scale co-combustion tests of Greek pre-dried lignite with biomass were accomplished in a bubbling fluidised bed. The main purpose of these experiments was to examine ash melting problems and differentiation to the emitted pollutants due to biomass addition. The obtained results of all aforementioned activities showed that fluidised bed is the appropriate combustion technology to efficiently exploit the low quality Greek brown coal either alone or in conjunction with other biomass materials.Copyright