D. Vamvuka

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.

Thermal degradation studies and kinetic modeling of cardoon (Cynara cardunculus) pyrolysis using thermogravimetric analysis (TGA)

A key element in the design of sustainable pyrolysis processes is the thermal degradation kinetics of biomass. In this work, pyrolysis tests for cardoon (Cynara carduculus) stems and leaves were performed in a non-isothermal thermogravimetric analyzer (TGA) in order to determine the thermal degradation behavior of both stems and leaves. The kinetic parameters of the process were evaluated using three different kinetic models, the independent parallel reaction model, KAS and OFW iso-conversional model. Good agreement with the experimental TGA data was observed for all models, the best being with the independent parallel reaction model. A variance in the activation energy with conversion was observed when the KAS and OFW models were employed, which reveals that the pyrolysis of cardoon progresses through more complex and multi-step kinetics.

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.

The effect of chemical reagents on lignite flotation

Abstract The possibility of cleaning a lignite by the froth flotation method was studied. Laboratory tests were carried out and the influence of various surfactant additions, feed particle size, pulp density and system pH on process performance was investigated. The feasibility of separating coal and refuse was determined from the coal recovery–ash reduction curves, while the quality of the products was evaluated by proximate and ultimate analyses, as well as mineralogical and chemical analyses of the ashes. The phenomenon of reverse flotation was observed through the whole work. Within the range studied, the optimum particle size was −300+75 μm, the pulp density was 15% and the froth dosage was 0.4 g/kg coal. The use of kerosene alone as a collector neither improved the total recovery nor enhanced the selectivity. In contrast, addition of surfactants without any kerosene resulted in lower product ash levels. The ash/yield performance of cationic, non-ionic and anionic surfactants was higher in acidic, neutral and alkaline solutions, respectively, however the overall performance of cationic surfactants was better. Zeta potential measurements contributed to the understanding of these effects.

Energy Exploitation of Agricultural Residues in Crete

The island of Crete is a typical Mediterranean area with a high biomass potential, the major part of which is still unexploited or irrationally exploited, but at the same time has a problematic energy supply during the high touristic season. In this paper the energy content of the biomass potential is estimated, as a parameter to alleviate the energy system of the island. The exploitation of biomass is studied with reference to the following aspects: The major residue production (olive kernel, husks – citrus fruits, grapes), branches (olive tree, citrus tree, grape tree); The qualitative analysis (proximate, ultimate, calorific value, ash analysis) of samples of basic agricultural residues of the Cretan production (vineshoots, olive tree wood and citrus, olive kernel).

Development of a modified independent parallel reactions kinetic model and comparison with the distributed activation energy model for the pyrolysis of a wide variety of biomass fuels.

The pyrolysis of six waste biomass samples was studied and the fuels were kinetically evaluated. A modified independent parallel reactions scheme (IPR) and a distributed activation energy model (DAEM) were developed and their validity was assessed and compared by checking their accuracy of fitting the experimental results, as well as their prediction capability in different experimental conditions. The pyrolysis experiments were carried out in a thermogravimetric analyzer and a fitting procedure, based on least squares minimization, was performed simultaneously at different experimental conditions. A modification of the IPR model, considering dependence of the pre-exponential factor on heating rate, was proved to give better fit results for the same number of tuned kinetic parameters, comparing to the known IPR model and very good prediction results for stepwise experiments. Fit of calculated data to the experimental ones using the developed DAEM model was also proved to be very good.

Gasification of Waste Biomass Chars by Carbon Dioxide via Thermogravimetry—Effect of Catalysts

The effect of catalyst addition on thermal conversion characteristics and kinetics of waste biomass chars was investigated. The experiments were performed in a thermogravimetric analysis system, at non-isothermal heating conditions, under a carbon dioxide atmosphere. Reaction rates were determined by a power law model. The bulk of char gasification process occurred between 800 °C and 950 °C. Addition of alkali salts improved char conversion and increased the reactivity of the samples by lowering the reaction temperature. Peak values were reduced by 4 to 138 °C. The catalytic activity was enhanced when 10% Na2CO3, 10% CaCO3, and 10% Li2CO3 were used during the gasification of municipal solid wastes, sewage sludge, and waste paper, respectively. Alkali carbonates and especially binary mixtures of them increased the reactivity of the fuels by significantly reducing the activation energy. Activation energies ranged from 180 to 370 kJ/mol without catalysts, whereas from 82 to 353 kJ/mol with catalyst addition.

Study on Catalytic Combustion of Biomass Mixtures with Poor Coals

The effect of catalyst addition on ignition and combustion characteristics of biomass fuels from the Mediterranean region, one lignite and lignite/biomass blends, were investigated. Transition metal oxides and alkali/alkaline earth compounds were used as catalysts. The experiments were conducted in a thermogravimetric analysis system. The performance of the process was evaluated in terms of combustion rate, ignition, and combustion indices. The results showed that metal oxides enhanced the ignition and combustion performance of lignite, showed no catalytic activity to burn off of biomass fuels, but influenced the temperature sensitivity of their reaction in air. The relative activity was in the following order: V2O5>Fe2O3>CaO>NaCl>MgO>CuO>TiO2>LiCl. The combustion characteristics of the blends followed those of parent fuels in an additive manner. Blending lignite with biomass fuels increased its thermochemical reactivity. In terms of burnout, catalyzed combustion of blends was further improved.

Production scheduling of a lignite mine under quality and reserves uncertainty

The effect of uncertainty sources to the stochastic optimization of the combined project of a new surface lignite mine exploitation and power plant operation for electricity generation is investigated. Major sources of uncertainty that were considered are the reserves and the quality of the lignite. Since probability distribution functions for these uncertainties were estimated during the detailed exploration phase of the deposit, the overall goal is then to determine the optimal capacity of the power plant and consequently the optimal production rate of the mine over the time. The optimization objective that was selected is the maximization of the net present value of the project. Emphasis is placed on the sensitivity analysis for the investigation of the effect of quality and reserves uncertainty on project optimization, on the mathematical formulation of risk attitude strategy and on increasing the efficiency of the optimization process by creating a limited set of feasible solutions applying empirical rules. The developed methodology was applied for the determination of the optimal annual production rate of a new surface lignite mine in the area of Ptolemais–Amynteon in Northern Greece.

Investigation of the Combustion Performance of Residues from Vineyards and Processing Industry via Fluidized Bed Experiments, Factorial Design, and Modeling

ABSTRACT Residues from vineyards and wine/spirits industry, as being abundant in South European countries, were investigated for their application in thermal processes for energy production. The thermal behavior and the reactivity of the fuels and their blends were determined by thermogravimetric analysis tests. The combustion performance of the residues in terms of efficiency and emissions was studied by carrying out experiments in a fluidized bed unit. The effects of excess air ratio, fuel loading, and blending ratio were examined and a factorial design and modeling was developed in order to provide optimal values of process variables for minimizing emissions. The results showed that CO emissions were low, SO2 emissions were negligible, while NOx emissions from grape husks were high. A mitigation of CO and NOx levels could be achieved by decreasing excess air from 50% to 30% or reducing reactor loading. By increasing the percentage of grape husks in the mixtures, flue gas emissions were increased, however, combustion efficiency was maintained at high levels. Factorial design analysis and modeling indicated that the optimum combustion and emission performance was accomplished when the combustor was fed with vine shoots at reduced loading and excess air below 40%.