Leonardo Tognotti

Devolatilization and pyrolysis of refuse derived fuels : characterization and kinetic modelling by a thermogravimetric and calorimetric approach

Abstract A characterization of pyrolysis behaviour of different refuse derived fuels (RDFs) under heating rates typical of conventional pyrolysis processes is presented. The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (d.s.c.) on different RDFs, and on some materials which have been considered ‘key components’ towards thermal degradation characteristics of RDFs, are reported. The RDF weight loss curve presents two distinct weight loss steps attributable, respectively, to cellulosic materials and plastic degradation. Samples from different plants and different municipal solid waste (MSW) feedstocks show the same qualitative behaviour. In interpretation of the experimental results, the assumption has been made that the pyrolysis rate of thermal degradation of RDF can be considered as the sum of the rates of the main RDF components: paper (cellulose), wood-like materials (cellulose, lignin and hemicellulose), plastics mainly polyethylene (PE); and that each component contributed to the formation of this sum to an extent proportional to its contribution to the composition of the RDF. On the basis of these data and by means of a simple mathematical approach a method is proposed which provides a simple tool for RDF characterization and, in particular, evaluation of plastic content. A kinetic model has been developed, based on the assumption that the RDF degradation rate is the weighed sum of the rates of primary reacting species: cellulose, lignin, hemicellulose, PE. Simplified kinetic pathways were used for the description of the degradation processes of RDF components. The model allows quantitative prediction of RDF weight loss and char yield at least at the heating rates used in present work.

Ignition and Combustion of Single, Levitated Char Particles

The paper focuses on the experimental measurement and model prediction of burnout times and ignition temperatures of single char particles. Experimental temperature-time histories of single, submillimetric char particles are compared with the predictions of a simple model that takes into account all the experimental findings obtained on a model char with an electrodynamic balance (EDB). The particular features of the experimental technique, allow the measurement of the temperature of the surface of a single levitated particle as the high temperature reaction proceeds. The particle is heated to reaction conditions with a laser while the surrounding gases are maintained at room temperature. Particle ignition temperatures and burnout times are easily detected from experimental curves. Even with the high temperature (2500-3000 K) and low characteristic time of the reaction (30-100 ms), reproducible experimental data were obtained, confirming that the EDB is a viable tool for studying high-temperature heterogeneous gas-solid reactions. Experimental results are compared with the predictions of a simple model developed for the description of high temperature combustion of single char particles. Even if particle morphological modifications occurring at high conversion degrees are not accounted for, the model well describes the particle ignition process and predicts with sufficient accuracy ignition temperatures and burnout times.

Biomass gasification: experimental and modelling activities at CRIBE

Biomass gasification can be operated in various reactor configurations and under different operating conditions. The process is versatile as for biomass input as well as for the product (electricity, cogeneration, syngas, hydrogen, Fischer-Tropsch fuels) and it is an option to promote distributed power generation. Despite this technology was proposed and studied starting from the mid 20 th century, it is still not widespread due to some technological barriers that need to be addressed [1]. The major technological barriers can be classified in two groups:

Development of an integrated procedure for comprehensive gasification modelling

Biomass gasification is an attractive process to convert a solid fuel into a gaseous product. Although gasification is a relatively old process, the versatility of the process (with production of syngas, electricity, hydrogen or chemicals) and the multiplicity of technological solutions (fixed beds, moving beds, fluidized beds and entrained flow reactors) make it a current topic of investigation. In spite of all these differences, most process studies in the literature modeled the gasifier as an equilibrium reactor (see for instance [1]). This approach is indeed fundamental for a preliminary study but hardly suitable for process analysis and optimization procedures. A detailed approach allows the operating conditions to be related with a heat balance and introduce a thermal profile. Also by-products in the syngas (e.g., CH4 and CO2) and residual char and tar, that are generally underestimated in equilibrium models, can be quantified on the basis of the operating conditions. Finally, a detailed model can simulate each step of the gasification according to the gasifer configuration for the specific optimization. Therefore a “gasifier model” should be developed instead of a “gasification model”. So, the aim of this work is the development of a procedure for modeling different gasifiers and show some examples of gasifier models.