Low temperature upgrading of moist agroindustrial wastes for subsequent energy uses

Abstract

The research work developed in the present thesis deals with the upgrading of moist agroindustrial wastes by torrefaction and hydrothermal carbonization for their subsequent use as bioenergy feedstocks in industrial and domestic systems. The Mediterranean region concentrates the main producer countries of olive oil, canned foodstuffs and juices worldwide, and thus produces large amounts of moist agroindustrial wastes needing suitable management treatments. Unfortunately, their use in energy applications is not directly an efficient option, since this kind of wastes present moisture contents higher than 65% and low energy density. Therefore, the main objective of the present work is to convert moist agroindustrial wastes in profitable materials that can be used in energy purposes. Low temperature treatments such as torrefaction (TF) and hydrothermal carbonization (HTC) were selected in order to maximize the solid recovery and minimize energy requirements and costs. On the one hand, torrefaction of olive mill waste was studied. On the other, hydrothermal carbonization of olive mill waste, artichoke wastes and orange juice wastes was studied as a potential alternative to dry treatments. Through these investigations, it was possible to bear out that HTC is more energy beneficial to treat moist agroindustrial wastes than TF, leading to energy savings up to 50%. Then, since HTC was considered more appropriate than TF to manage this kind of wastes, following investigations were focused on a deeply study of the HTC process, hydrochar properties and its use in possible energy applications. The effect of lipids on the HTC process and hydrochar properties was studied and it was found that lipids, which mainly remain on the hydrochar, improve the hydrophobicity of the solid and thus reduce its moisture content, decreasing the total energy requirements of the global low temperature upgrading process. In addition, it was found that lipids retain antimicrobial liposoluble substances on the solid and thus improve the biodegradability of the HTC-liquor. Then, it was concluded that, although HTC is an effective treatment regardless the lipids loading, HTC is energetically and enviromentally more advantageous as the amount of lipids contained in the raw wastes increases. When considering the use of hydrochars as renewable fuels for both industrial and domestic applications, ashes behaviour and related emissions are important concerns. To improve ash fusibility characteristics of hydrochars, the composition of their ashes was modified with rice hulls and/or mineral additives. It was found that rice hulls and kaolin are able to increase the hydrochar ash fusion characteristic temperatures to values higher than 1500ºC. The high cost of kaolin could limit its application. Thus, rice hulls were considered the best option among the additives studied, since consisting of a waste, allows reaching economic and sustainability criteria. Emissions associated with the combustion of hydrochars were assessed and compared with those associated with raw materials, torrefied materials (TF-chars) and conventional fuels (i.e., anthracite and wood). Experiments were conducted in both a lab-scale furnace and a commercial domestic pellet stove. Lab-scale experiments showed that hydrochar and TF-char lead to 40% lower emissions of light hydrocarbons but 50% higher emissions of SVOCs and PAHs than OMW. As compared with conventional fuels emissions of hydrochar and TF-chars were substantially higher than those associated with anthracite, whereas the toxicity factor of hydrochar was similar to that of wood. Experiments conducted in the domestic pellet stove showed that the toxicity associated with hydrochar was very similar to that of OMW, although it duplicated that of wood pellets. Hence, it is mandatory to ensure good combustion conditions to avoid the release of toxic compounds to the environment. Finally, the life cycle analysis (LCA) of HTC coupled with energy recovery from the hydrochar was conducted to determine its environmental implications and compare them with the impacts associated with current management options. Results indicated that HTC and subsequent energy generation is more beneficial than biological treatments. However, this scenario was less advantageous than direct incineration, due to the energy losses that occur during HTC and the discharge of HTC-liquor to the environment. Thus, it is recommended that future research efforts focus on the evaluation of appropriate HTC-liquor treatments and methods to improve the energy retention efficiencies of the hydrochar.