Management of off-specification compost by using co-hydrothermal carbonization with olive tree pruning. Assessing energy potential of hydrochar

Highlights

• Energy valorisation of the off-specification compost (OSC) waste was accomplished.

• Hydrothermal carbonization (HTC) can convert OSC into a lignite-like coal product.

• 75% of OSC and 25% of biomass blend presented the highest energy recovery.

• Higher heating values were upgraded from 17.0 to 26.2 MJ/kg in the selected blend.

Abstract

In this work the management of a waste called off-specification compost (OSC) was proposed via hydrothermal carbonization (HTC). The composition of this residue makes it not suitable for agronomic purposes because of the Spanish regulation requirements. Therefore, a way of management and/or valorisation needs to be found. The energy recovery through co-HTC with olive tree pruning (OTP) was evaluated. Blending of OSC with lignocellulosic biomass allows to obtain a coal-like product with physicochemical properties similar to those of a lignite, characterised by its high carbon content. Blends of 25, 50 and 75% of OSC with OTP were analysed. The individual OSC does not present good parameters for being used as solid fuel based on its chemical composition, however, the blend of 75% of biomass with 25% of OSC does. With a higher heating value of 26.19 MJ/kg, this blend shows the best energy yield and energy densification ratio. Thermogravimetric and kinetic analysis reveal that as biomass content in the blend increases, the more the hydrochar behaves as a solid fuel, therefore OSC can be used for energy purposes while its current use of landfill disposal can be reduced.

Introduction

Urban population growth along with the gross domestic product (GDP) growth is causing an increment in the generation of large amounts of municipal solid wastes. By 2030, the world is expected to generate 2.59 billion tonnes of waste annually and by 2050, waste generation across the world is expected to reach 3.40 billion tonnes (Kaza et al., 2018). More than 50% of these wastes is estimated to be biodegradable (Nigussie et al., 2017). For years municipal waste was disposed in landfills, however, nowadays strict regulations concerning the environmental impact allow landfilling only under special consideration (Campuzano and González-Martínez, 2016). The current tendency is to avoid, reduce, reuse, recycle, recover and treat this kind of waste and, if nothing else is possible, to dispose of the remaining (Campuzano and González-Martínez, 2016).

The organic fraction of this residue usually undergoes biological treatments such as composting or anaerobic digestion, which are considered as environmentally friendly technologies (Akindele and Sartaj, 2018, Lou and Nair, 2009). The organic transformation into compost presents different benefits like decreasing the amount of waste dumped, which results in an important contribution to the reduction of the landfill volume created by biodegradables (Neri et al., 2018) at the same time that pathogenic organisms are destroyed (Saldarriaga et al., 2019). However, it is frequently found that treatment plants present a waste collection and separation system that makes not possible to ensure that the organic matter submitted to composting is adequate.

As an example, in the municipal solid waste treatment plant of San Román de la Vega (León, Spain), there are some different steps in the waste classification chain. The first step is a bulky treatment line. Here, high size products (i.e., glass, plastic, cardboard, or metals) are separated for its further recycling. After this first separation, the remaining residue is considered as organic waste and two primary treatment lines are found, in order: 1) aerobic fermentation (fermentation and maturation tunnels) and 2) anaerobic fermentation (biomethanization). The product obtained from the aerobic stabilization is what we know as compost, commonly used as organic soil amendment, helping in the mitigation of greenhouse gases (GHGs) emissions by reducing the need for synthetic fertilisers and sequestering carbon in soil (Lou and Nair, 2009). Nevertheless, due to the poor source separation of municipal wastes, this product does not meet the conditions for being considered as compost -with interesting specifications-. In Spain some requirements are needed to use this product as solid amendment, such as to present (in mass basis): (1) an organic matter content >35%; (2) a maximum moisture <40%; (3) a C/N relation <20; (4) the stones and loose stones with a 5 mm or higher size should not exceed the 2% and (5) the inert matter as metals, glasses and plastics with a 2 mm or higher size should not exceed the 1.5% of the total sample (RD 506/2013, 2013). These requirements make a large part of the compost generated in the urban waste treatment plants not be appropriate for this use and, therefore, to consider it as off-specification compost (OSC) (EWC 19.05.03). OSC can be defined as the bio-stabilised product derived from different input types including various solid wastes, sludges and liquors, and/or the oversize material resulting from screening such product (Norther Ireland Environment Agency, 2012). OSC consists of a mixture of heterogeneous compounds, including organic and inorganic materials, used as covering material for landfill (Neri et al., 2018). Although OSC is considered as an organic waste, this product has been barely studied because of its composition. Due to its high incombustible content (moisture and ash), additionally to landfill disposal, this product is usually treated by controlled incineration, which implies further associated costs (Malat’ak et al., 2018). Therefore, this work aims to find a way to avoid the environmental impacts generated by OSC using HTC as a process to homogenize and convert it into a solid biofuel. Because of the high amount of inert matter present in OSC, blending with another material previously studied may be necessary. This can lead to improve the energy recovery of the product. Therefore, in this work, a lignocellulosic biomass was selected as the blending material as it was proved to be a material that can be transformed into a carbon-rich product by thermal treatments, including HTC (González-Arias et al., 2020, Khan et al., 2019).

As a Mediterranean country, Spain is covered by about 2.6 million of hectares of olive groves (Gobierno de España, 2018). It is calculated that a hectare of olive trees generates about 1 dry ton of pruning per year, showing an interesting potential for energy recovery (Spinelli and Picchi, 2010). Nowadays this pruning is usually scattering in fields or eliminating by burning (Pérez et al., 2018). The low energy density due to the high moisture content in raw biomass leads to high costs of handling, transportation and storage (Iáñez-Rodríguez et al., 2019).

Therefore, with the aim of using both wastes on thermal purposes, some treatments (including thermo-chemical, chemical and biological) may be carried out to improve the combustion characteristics of both materials (Osman et al., 2019): the olive tree pruning (OTP) and the compost that does not meet the specifications as soil amendment (OSC). Due to the characteristics of these residues, some of the most interesting treatments are thermal processes such as pyrolysis or hydrothermal carbonisation (HTC), which have been demonstrated to be adequate for converting organic wastes into a carbonaceous biofuel (Smith et al., 2018, Wilk et al., 2017). For pyrolysis, torrefaction and gasification treatments, a moisture content lower than 10–15% is essential, otherwise the energy required for evaporating the water contained in the sample could make the process be energetic and economically inviable (Basu, 2018). Volpe et al. concluded in their work that at a fixed temperature, hydrochars presented higher energy densification than the char obtained from pyrolysis, relating this to the active role of water in HTC (Volpe et al., 2016). HTC is a thermochemical conversion technique conducted with pressurised water, that converts the organic matter into a carbon-based product characterised by its high heating value (HHV) and its high carbon content (Düdder et al., 2016, Lucian and Fiori, 2017). In the HTC process, the biomass is heated in the water-environment at temperatures between 180 and 290 °C while the pressure is maintained above saturation pressure to ensure the liquid state of water (Wilk et al., 2019). This technology has shown promising results with various residues producing three different phases: a solid phase which is a coal-like char (called hydrochar); a liquid phase containing water soluble organic compounds; and a gaseous phase which consists mostly of CO₂ (Rodriguez Correa et al., 2017). The process also leads to obtain other byproducts which could be used for power generation and the recovery of useful nutrients (Kumar et al., 2018). The major benefits of the hydrochar are the high energy content, good grindability and high hydrophobicity when compared to the solid products obtained from other thermal methods (Wilk et al., 2019). Moreover, HTC can be used for treating a high variety of wastes. Table 1 summarises some of them and their applications, along with the different treatments for the organic fraction of municipal solid wastes (OFMSW). This residue was selected as the most similar waste to OSC.

Although many works report the use of OFMSW, in this work this is not the product to treat, but OSC, which is a product scarcely studied. To the best of our knowledge, apart from Basso et al. who used HTC to evaluate if OSC could be used as solid fuel (Basso et al., 2015), no other works treating this product have been found in literature. Consequently, the analysis and use as biofuel of hydrochar produced from the co-HTC of OSC and biomass blends has not been previously evaluated.

Therefore, the main aim of this work was the search for a new management way of OSC through HTC looking for its energy valorisation. With this objective, OSC was thermally treated by co-HTC with OTP to improve the energy efficiency of the samples by reducing the ash content in the final product and increasing the fixed carbon content, thus improving the energy content. As HTC allows to obtain a homogeneous product, a coal-like hydrochar was selected as solid biofuel. Finally, to evaluate the synergies of the OTP and the OSC when blending and to select the optimal proportions of the components in the blend, thermogravimetric and kinetic studies were carried out. These studies allow to assess the thermal behaviour of the samples during combustion, reporting some useful information when actual applications for these wastes are considered.