• Energy Research

A mathematical model has been developed to predict fragmentation of particles under a wide range of pyrolysis and combustion conditions. The model is an upgrade of a previous one that took into account only fragmentation during the heat up and devolatilization stage. The model calculates the temperature and oxygen profiles within the particle, the evolution of internal porosity as a consequence of both devolatilization and carbon combustion, the mechanical stress caused by temperature gradients, and by volatiles-generated overpressure inside the particles. Eventually the model calculates the probability of rupture of the particle based on the Weibull (1939) theory. The model has been used to simulate heating of coal particles under inert conditions at different heating rates and temperatures showing good agreement with previous work. The model has been further used to simulate heating under oxidative conditions in order to highlight the role of combustion on fragmentation phenomena.

In oxy-fuel combustion, coal particles undergo pyrolysis in CO2 rich atmospheres. The composition of the gaseous atmosphere under which pyrolysis takes place may have important effects also on the formation of pyrolysis products. In the present work, pyrolysis experiments have been carried out with a medium rank high volatile bituminous coal substituting N2 with CO2. Experiments have been carried out in a laminar drop tube reactor as well as in a fixed bed reactor. CO2 chars prepared in the drop tube reactor turned out to be less reactive towards combustion than the corresponding N2 chars. Differently chars prepared at low heating rates and temperature have similar combustion reactivity regardless of the N2 vs CO2 pyrolysis atmosphere.

The present work focuses on the effects of pressure on the quality of char and primary tar produced from fast pyrolysis of lignocellulosic biomass. Heat treatment has been carried out in a heated strip reactor (HSR) at 1573 K in nitrogen at 2, 4, 8 bar, with holding times of 3 s and heating rate of 104 K/s. The equipment allows quenching the volatiles as soon as they are emitted from the particles and collecting them for further chemical analyses. The char samples are also collected for thermogravimetric analysis in air. The DTG curves in air of char prepared at 2 bar shows two resolved peaks. Increasing the pressure of heat treatment from 2 to 4 bar has a minor effect on char reactivity, whereas further increase to 8 bar drastically changes the char combustion patterns, and the DTG curves exhibit only one well defined peak. For all the process conditions investigated, Oxo-aromatics are the dominant species in the tar. Benzendiol prevails in the 2 bar tar, followed by oxo-aromatic compounds related to lignin structure, while PAHs are mainly present as Fluorene. When pressure increases, Phenols compounds drastically prevail, and PAHs as Anthracene and Pyrene appear.

The present study investigates the influence of pressure on the products of fast pyrolysis of a lignocellulosic biomass (Walnut Shells).

Pyrolysis is considered the most promising approach to convert biomass and to generate valuablebio-oils and biochars. This study aimed to investigate fast pyrolysis of different types of wastesfrom Mediterranean biomasses (Opuntia Ficus Indica, Pomegranate, Aleppo Pine Cones, and PinusPinea Cones) and compare the quality of bio-oils and chars.Biomasses were characterized by proximate and ultimate analyses, FTIR spectroscopy, X-ray diffraction,thermogravimetric and calorimetric analysis. Fast pyrolysis experiments were carried outin a heated strip reactor with a heating rate of 103 K/s and holding time of 3 s at 1000 K. Tar productswere readily quenched and collected on a cold pyrex bridge, positioned above the sampleholder at near ambient temperature, so as to minimize secondary reactions. Tar samples were characterizedby GC-MS analysis and the quality in terms of chemical families was discussed. Charsamples were recovered from the strip and analysed by SEM, elemental analysis, and XRD.The results show that the bio-oils and char composition vary sensibly:- Aleppo pine cones and pinus pinea cones, the hardest of the examined biomasses, upon fastpyrolysis produce a bio-oil that is made of oxygenated aromatic compounds and a bio-charwith high carbon content (>60 %), which could be envisaged for soil amendment and carbonsequestration.- Pyrolysis of Opuntia Ficus Indica wastes (peels and cladodes) generates chars with comparablylower carbon content, but particularly rich in potassium (K) and calcium (Ca).The bio-oil from the peels is still rich in aromatic compounds, similar to APC and PPC,while the bio-oil from the cladodes is almost exclusively made of anhydrosugars.- Pyrolysis of pomegranate peels generates a carbon-rich char, as APC and PPC, but the biooilis rich in anhydrosugars and also furfural.From the morphological point of view, all bio-chars seem to be made up of agglomerates of irregularelongated or semi-spherical shaped particles with large macropores.

A most interesting solution for the disposal of meat and bone meal (MBM) is co-feeding with coal in combustion plants. MBM, is however, quite different from any other traditional or alternative solid fuel in terms of chemical composition, ash content and microstructural properties. Its effects on the performance of a boiler are largely unexplored. The present paper addresses the characteristics of MBM as alternative solid fuel and the effects of co-feeding MBM (6%) and coal (94%) in a utility boiler. A first activity consisted in the characterisation of the physico-chemical properties and the reactivity of MBM. The experimental campaign included ultimate and proximate analysis, granulometric analysis, ICP, SEM, XRD. An extensive campaign of isothermal and non isothermal thermogravimetric experiments was carried out to assess the reactivity of MBM upon pyrolysis, combustion and gasification and to obtain appropriate kinetic expressions. A second activity focused on co-firing of MBM and coal. Bottom and fly ashes were collected from an industrial boiler operated with MBM and coal. Ash samples were characterised by SEM, XRD, ICP, TGA and granulometric analysis. Results of this activity showed that MBM contributes mostly to bottom ash, however also the fly ashes are different from those typically encountered when the boiler is operated with coal alone. Differences concern the chemical composition and particle size distribution of ashes, in particular a large population of very fine particles characterised by perfectly spherical shape and non negligible carbon content is observed.

Mechanistic studies of coal combustion have long highlighted the variety of reaction pathways along which gasification may take place. These involve chemisorption of reactants, formation of surface oxides, surface mobility of chemisorbed species, and product desorption. At the same time, exposure of the solid fuel to high temperatures is associated with solid-state thermally activated processes. Altogether, the course of gasification may be profoundly affected by the overlapping and interplay of heterogeneous oxidation with purely thermally activated solid-state reactions. In the present work the combustion of a South African bituminous coal is analyzed in the framework of a simplified reaction network that embodies heterogeneous oxidative and thermally activated processes (pyrolysis, thermal annealing, coal combustion, char combustion, oxygen chemisorption) active both on the raw coal and on its char. The kinetics of each process of the network is assessed by a combination of thermogravimetric and gas analysis on coal and char samples. The analysis is directed to the determination of the prevailing combustion pathway, established from the interplay of oxidative and solid-state thermally activated processes, as a function of combustion conditions (temperature, heating rate, particle size). © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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