Juan A. Conesa

Pyrolysis kinetics of almond shells and olive stones considering their organic fractions

The kinetics of thermal decomposition of two lignocellulosic materials (olive stones and almond shells) were studied using dynamic TG at heating rates between 2 and 25 K min−1 at atmospheric pressure. Different kinetic models were tested, that consider that the thermal decomposition of lignocellulosic materials can be grouped in two or three overall processes (each one being the result of a great number of reactions) corresponding to the hemicellulose, cellulose and lignin decompositions. The best results were obtained with a model that considers that the biomass decomposes via three independent reactions. There is a qualitative agreement between the decomposition of hemicellulose, cellulose and lignin with the peaks observed in the TG-DTG diagram, but the simple addition of the kinetics of isolated compounds cannot satisfactorily reproduce the kinetic behaviour of any raw material, and consequently some interactions must exist between the fractions considered.

Analysis of different kinetic models in the dynamic pyrolysis of cellulose

Abstract The kinetics of the thermal decomposition of cellulose in nitrogen were studied using dynamic TG at heating rates between 5 and 50 K min −1 . The most widely used models found in the literature which are usually applied in isothermal conditions were tested. Important differences were found between models that take into account the formation of an activated cellulose and those that assume the direct decomposition of cellulose to char, tar and gases. The best agreement between experimental and calculated values was found with models which consider the formation of activated cellulose and take into account the different behaviour at low and high temperatures. The simultaneous correlation of all the experimental data obtained in different conditions is proposed as a technique to prevent compensating effects between the pre-exponential factor and the activation energy, and thus to avoid erroneous conclusions.

Comments on the validity and utility of the different methods for kinetic analysis of thermogravimetric data

This work deals with the kinetic analysis of data obtained in thermobalance. It consists of a review of kinetic models used for material decomposition, and also of the methods used for the analysis. The topics presented comprise numerical problems related with kinetic analysis, best conditions for a kinetic run, discussion about correlation vs. actual models, nth order models, more complex models (models using a great number of reactions, models using various fractions), calibration of the temperature, position of the thermocouple, sample mass and particle size. Other topics treated are the validation of kinetic models using MS data and the different models available for kinetic studies in thermobalance.

Characterization of sewage sludges by primary and secondary pyrolysis

An apparatus formed by the combination of a primary pyrolyzer (Pyroprobe 1000) and a secondary reactor was used to study the thermal decomposition of three different chemical sewage sludges, using temperatures in the secondary reactor between 290 and 650°C and 700°C in the primary pyrolzer. Yields of 12 pyrolysis products were determined (methane, ethylene, ethane, propylene, propane, methanol, acetic acid, acetaldehyde, C4-hydrocarbons, CO, CO2 and water). The pyrolysis of chemical sewage sludge is an alternative to incineration because it presents the advantage of concentrating the heavy metals in the final residue, and of avoiding the formation of toxic organic compounds. This technique allows the liquids formed in primary pyrolysis to be transformed almost completely into light gases that do not present significant environmental problems. Only about 1–2% of liquids remains without cracking. The gases formed are mostly CO, CO2 and water. Hydrocarbons yields are only around 1–2% wt%, on a dry basis of the sludge. The paper presents the characterization of three sewage sludges by pyrolysis using simple equipment that can be useful for studying any type of wastes.

Thermogravimetric studies on the thermal decomposition of polyethylene

Abstract The thermal decomposition of polyethylene was analyzed by dynamic and isothermal experiments. Dynamic runs with different heating rates, exposed surface and initial mass were carried out. Isothermal runs of different initial mass were also carried out. Several kinetic models were tested and the best fit to the experimental thermogravimetry curves was obtained with a model involving a surface zero-order reaction.

Analysis of the pyrolysis and combustion of different sewage sludges by TG

Abstract The pyrolysis and combustion of seven samples of sewage sludge were studied by TG dynamic runs carried out at 15°C min −1 . Two different behaviors were observed. For some samples, considering the variations of weight loss versus temperature or time, the combustion thermograms are similar to those of pyrolysis but with a displacement of the corresponding curves of combustion to temperature intervals less than those of the pyrolysis runs. For the other samples, the pyrolysis thermograms coincide with the combustion thermograms, except in the last part of the combustion thermogram where a final decomposition appears as a consequence of the combustion of the char previously formed. The different behavior can be associated to aerobic stabilization of the samples. Some runs were also done with acid-washed samples to test the influence of the metal content.

Organic and inorganic pollutants from cement kiln stack feeding alternative fuels

In this work, an analysis of the emission of different pollutants when replacing partially the fuel type used in a cement kiln is done. The wastes used to feed the kiln were tyres and two types of sewage sludge. The increasing mass flow of sludge is between 700 kg h(-1) and 5,500 kg h(-1)1, for a total production of clinker of 150th(-1), whereas the fed tyres were in the flow range of 500-1,500 kg h(-1). Dioxins and furans, polycyclic aromatic hydrocarbons (PAHs) and other hydrocarbons, heavy metals, HCl and HF, CO, CO(2), NO(x) and other parameters of the stack were analyzed, according to the standard methods of sampling and determination, through more than 1 year in six series: one blank (no sewage sludge) and five more with increasing amount of sludge and/or tyres. The emission of PAHs and dioxins seems to increase with the amount of tyres fed to the kiln, probably due to the fed point used for this waste.

Kinetic model for the combustion of tyre wastes

Abstract The decomposition of tyre wastes at different heating rates in an oxidizing atmosphere is explained by means of a kinetic model including a first step of pyrolysis, which assumes three organic fractions not forming residues, and a step of combustion. The atmosphere used varied from 10 of oxygen to 20% (v/v). The activation energy for the step of combustion is in the range 221–235 kJ/mol, and there exists a dependence of the rate of decomposition on the partial pressure of oxygen.

Thermogravimetric analysis of olive stones with sulphuric acid treatment

Abstract The thermal decomposition of olive stones, with different time-lengths of 72% sulfuric acid treatment (from olive stone to klason lignin), was studied by thermogravimetry (TG-DTG). The TG curves indicate that the isolation of lignin begins with the easy elimination of hemicellulose and some of the cellulose, while the lignin fractions remain practically unaltered. More intensive treatment produced the progressive removal of the cellulose, but the formation of volatiles at relatively low temperatures indicated that thermal resistance had decreased due probably to the breakdown of the three-dimensional structure of lignin. However, even with prolonged treatment, it is difficult to remove all the cellulose and even when this is removed, the lignin structure is greatly altered. A method based on the kinetic analysis of the thermal decomposition is proposed to characterize the isolation degree of klason lignin and to monitor alterations in its structure.

Thermal decomposition of meat and bone meal

Abstract A series of runs has been performed to study the thermal behavior of meat and bone meal (MBM) both in inert and reactive atmosphere. Although they are actually burned, the thermal decomposition of such MBM wastes has not been studied from a scientific point of view until now. The aim of this work is to present and discuss the thermogravimetric behavior of MBM both in nitrogen and air atmospheres. A thermobalance has been used to carry out the study at three different heating rates. A kinetic scheme able to correlate simultaneously (with no variation of the kinetic constants) the runs performed at different heating rates and different atmospheres of reaction is presented.