Hydrothermal carbonization of waste biomass

Hydrothermal carbonization (in acronym, HTC) is a thermochemical conversion process through which it is possible to directly transform wet organic substrates into a carbonaceous material, referred as hydrochar. Hydrochar has chemical and physical characteristics that make it similar to fossil peats and lignite. Depending on the process conditions, mostly temperature and residence time, this material can be enriched in its carbon content, modifying its structure and providing it interesting characteristics that make it possible to be used for several applications, such as for energy production, as a soil conditioner and improver, for carbon dioxide sorption and sequestration, and some others reported in literature. HTC is a different process, if compared to other common thermochemical processes, such as pyrolysis, torrefaction, gasification, etc., because it works in wet conditions (humidity content higher than 60%). As a matter of fact, biomass is transformed into hydrochar because of the properties of hot pressurized water, that acts both as a reactant and as a catalyst. The HTC process has been studied from many years, although at present not all the chemical reactions that occur during the process are completely known. Moreover, the application of this quite new process to different substrates can bring to different results. Even though HTC can be applied to any kind of organic material (of both animal and vegetable derivation), the possible uses of hydrochar can strongly be influenced by the characteristics of the feedstock. This, for example, can be due to legislative constraints. In Chapter 1, an overview of the existing literature is presented. To get insights on this process, a small bench scale batch reactor has been designed and built at the Department of Civil, Environmental and Mechanical engineering of the University of Trento, Italy. This reactor has been tested, prior to be used with real substrates. In Chapter 2 the reactor and the preliminary tests done are described. In this work, the HTC process applied to three different substrates have been studied: grape marc, the EWC 19.05.03 residue and the EWC 19.12.12 residue. In Chapter 3 the three raw substrates are described. Grape marc is produced by the winery industries or by distilleries. This feedstock is composed by woody seeds and holocellulosic skins and it presents an average humidity content of about 60%. At present, it is used for the production of animal food or it is landfilled. In this case, the application of HTC can be an interesting alternative to these end uses because, through this process, grape marc can be recovered, for example, for energy production. The hydrochar produced from this feedstock could be even used as a soil conditioner. In Chapter 4 several analyses on the hydrochar, on the process water and on the gaseous phase obtained during the carbonization tests are presented. The EWC 19.05.03 residue is a by-product of the composting treatment applied to the organic fraction of municipal solid waste (MSW). In collaboration with Contarina S.p.A., a company that collects and treats MSW in the province of Treviso, in the North-East of Italy, this by-product was carbonized and tested both as a soil conditioner and for energy production. Results of the analyses on the solid, liquid and gaseous phases produced by the HTC process are reported in Chapter 5. The EWC 19.12.12 residue is a by-product of the refuse derived fuel (RDF) production, from the residual fraction of the MSW. This substrate was provided by Contarina S.p.A. and preliminary tests on the exploitability of the hydrochar for energy production are reported in Chapter 6, together with analyses on both the liquid and gaseous phases. A rigorous energy balance has been proposed in Chapter 8, based on the experimental data obtained for grape seeds. In this chapter, all the hypotheses and the assumptions taken to evaluate the enthalpy of the HTC reaction at different process conditions (namely, three different temperatures and three residence times) are described. In Chapter 8 a kinetic model is proposed, based on a two-step reaction mechanism. The activation energy and pre-exponential factor of the various degradation reactions were determined by means of least square optimization versus the experimental data of grape marc. A thermo fluid model is even proposed in this chapter. The model integrates mass, momentum and heat equations within the reactor domain by means of the finite volumes method (f.v.m.) approach. Convective and radiative exchange between the reactor and the fluid within the reactor have been implemented in the f.v.m. model. Under two strong assumptions (mono-component and mono-phase fluid, which fulfils the reactor), it was possible to estimate the behaviour of an equivalent fluid (eq_fluid), in terms of thermal properties of the fluid (thermal capacity, thermal conductivity and thermal diffusivity). Moreover, a simplified dynamic analytic model is also presented – based on lumped capacitance method – in order to simulate the thermal behaviour of the system, using the actual temperature profile imposed by the reactor external heater. A resistance-capacitance network was used to describe the system. Finally, the Henry’s law has been applied to assess the amount of gas really produced during the HTC process. In Chapter 9, the main conclusions of this work are reported