Sustainable Energy Production Using Solid Materials

This chapter introduces the issues related to the sustainable satisfaction of the global energy needs, and the use of biomass as a source of renewable energy, highlighting its advantages and aspects to be taken into consideration. The principal solid biomass characterisation techniques are then presented: ultimate analysis for the quantification of the main elements, proximate analysis for humidity, volatiles (including tar), fixed carbon and ash, and calorific value analysis. The biomass properties in the terms described above are then discussed, also with reference to its positioning on the van Krevelen’s diagram. Finally, the heating behaviour of a solid fuel particle is presented, which can also be understood as a manifesto of the topics that will be treated in this book.

Once the devolatilisation process is over, the biomass char particle is ready for the thermoconversion stages. Before going into the details of combustion and gasification (and related aspects), this chapter conceptually presents the diffusive, kinetic and reactoristic aspects related to heterogeneous processes involving char and a gaseous oxidising agent. The shrinking core model and the shrinking particle model will be presented, with analysis of the possible controlling regimes. The characterising equations will be obtained with reference to the main chemical-physical parameters involved. We will then move from the scale of the particle to that of the chemical reactor, showing the general design equations and addressing some case studies of interest in industrial practice.

After a presentation on the main kinetic laws that regulate the carbon combustion process, this chapter focuses on obtaining relations of practical interest for the operation of a combustor. In particular, expressions will be obtained for the flow rate of air to be fed together with the biomass, and for the flow rate (and chemical speciation) of the gases produced by combustion. The relations will be presented both in general terms and through directly applied examples.

Solid materials to be sent to combustion systems are typically characterised by a particle size distribution; therefore, this chapter opens with a presentation of the concepts of particle size distributions and characteristic mean diameters. As fluid dynamic precursors of fluidised bed reactors, fixed bed reactors are presented, up to the characteristic equations which determine the condition of minimum fluidisation of a bed of granular material. Fluidised beds are illustrated in their main characteristics (presence of dense phase/bubbles and their role, elutriation phenomena), highlighting in particular their advantages when considering solid–gas applications such as biomass combustion. The chapter closes with a technical-historical focus on fluidised bed combustors.

The possibility of performing in situ desulphurisation during fluidised bed combustion is one of the main advantages of this technology for energy production. The process is presented in its general characteristics and, with specific reference to limestone-based sorbent particles, the main attrition and fragmentation phenomena are discussed. Then follows a more detailed description of the reactive process which involves the capture of sulphur dioxide by calcium oxide. First of all, the grain model is presented, with the identification of the main degrees of conversion, the expressions for the sulphation rate, and the constitutive equations. Then we focus on the random pore model and its main equations. Finally, the concepts of fractal-like dynamics for the study of heterogeneous processes are presented, then applied to the case of the sulphation reaction, with the introduction of a time-dependent expression for the diffusion coefficient.

This chapter introduces the scientific and geo-social context linked to anthropogenic emissions of CO₂, and to the connected climate emergency. The theme is treated on the basis of data regarding CO₂ emissions, its concentration in the atmosphere, the temperature of the planet, and the melting of glaciers. Furthermore, a quantitative analysis is presented which relates socio-economic parameters and CO₂ emissions for all the Countries of the United Nations. Finally, the basic concepts concerning the capture of CO₂ aimed at its subsequent geological storage or reuse are introduced.

In this chapter, we present three technologies that are establishing themselves in the scientific and industrial panorama, as innovative methods for obtaining streams rich in CO₂, which can therefore be reused or stored, avoiding its direct introduction into the environment with a simultaneous greenhouse effect. Thinking specifically of combustion phenomena, we discuss here the concept of oxy-combustion, that of chemical looping combustion and, moving on to post-process CO₂ capture and concentration methods, we analyse the calcium looping concept.

This chapter deals with the topic of gasification, aimed at obtaining syn-gas to be used as a flexible energy carrier. The process is presented in its historical evolution; then, peculiar chemical reactions and operating conditions, and contact reactor schemes, are discussed. We also focus on integrated gasification cycles, syn-gas oxy-combustion techniques and the sorption-enhanced gasification process.

Biomass thermoconversion residues can have a fate other than landfilling. In this chapter, it is presented the case in which the ash can be used as an adsorbent material towards pollutants from fluid phases, thus contributing to increasing the “sustainable circularity” nature of the proposal (a residue is reused for purification operations). The ashes are discussed in their main characteristics, together with possible activation processes aimed at increasing their sorbent capacities. This is then followed by a general discussion on the fluid–solid adsorption process, and by an in-depth analysis of the thermodynamic and kinetic aspects, with the appropriate constitutive equations and methods for obtaining, from experimental data, the kinetic and diffusive parameters of the process. The chapter then continues with an extension in which the concepts of fractal dynamics are applied to the constitutive equations of adsorption kinetics.

Ash resulting from thermoconversion processes of solid fuels (biomass), and other residues encountered in this book (e.g., spent sorbent from calcium looping processes; mixtures of ash and spent sorbent from fluidised bed combustion processes with in situ desulphurisation), can find fields of reuse such as supplementary cementitious materials, pozzolanic additions, raw materials to replace natural ones, depending on the properties of the residues and the type of cement (for example, Portland, sulphoaluminate) to which one refers.

Among the methods for the sustainable production of energy using solid materials, there are innovative techniques for the exploitation of solar energy. In this chapter, the main concepts regarding solar energy and concentrated solar power systems will be presented.

The final chapter, on the basis of the reasoning developed in the opening rationale, focuses on an analysis (in terms of geographical distribution) of the review articles concerning some of the main themes that we have dealt with so far, and published up to year 2010.