Abstract

Rural areas can benefit from the development of biorefineries for the valorization of endogenous feedstocks. In this study, a pre-feasibility assessment of an integrated multi-product biorefinery to produce essential oils and biomethane is carried out considering current technical and economic conditions. The proposed concept is based on the steam distillation of forestry biomass for the extraction of essential oils (2900 L/y) followed by biomethane production via syngas methanation using the spent biomass as feedstock (30.4 kg/h). In parallel, the anaerobic treatment of WWTP sludge (5.3 kg/h) is used to produce additional biomethane for mobile applications. The results show that the intended multi-product biorefinery delivers attractive benefits for investors as described by the calculated financial indicators: NPV of EUR 4342.6, IRR of 18.1%, and PB of 6 years. Overall, the pre-feasibility analysis performed in this study demonstrates that the proposed biorefinery concept is promising and warrants further investment consideration via cost and benefit analysis, ultimately promoting the implementation of multi-product biorefineries across Europe.

Keywords: biorefineries; gasification; essential oils; biomass wastes; biomethane

1. Introduction

Biorefineries have been suggested to decrease the environmental and social issues caused by fossil resources by replacing fossil feedstocks with biological resources. In these infrastructures, biomass is fractionated into a multitude of value-added products and energy vectors capable of sustainably satisfying the energy and material needs of several industry sectors [1]. To achieve this wide range of products, biorefineries, such as conventional oil refineries, require the integration of different processes and technologies in a single facility, preferably.

Regarding feedstock for biorefineries, there have been many studies testing forestry biomass wastes, agricultural wastes, sludges from various sources, or municipal solid wastes (MSWs) [2,3,4]. Forestry biomass residues have received greater attention in this application, mostly as a response to the increasing global energy demand but also for their potential in the reduction in greenhouse gas (GHG) emissions [2]. These biomass wastes are renewable energy sources, and they are perceived as recycling carbon instead of removing it from long-term storage [3]. Another very promising feedstock for biorefineries is sludge, particularly sludges from wastewater treatment facilities (WWTPs). These materials are solid waste residues rich in organic compounds such as cellulose, which can represent approximately 20–50% of the influent suspended solids in WWTPs [5,6].

Because of its location and climate, Portugal is well-suited to forest growth, which covers about 35% of the territory. In this context, forestry wastes are a potential renewable feedstock for the country [7,8]. WWTP sludges are also very representative, constituting another potential waste to be used in biorefineries. For example, according to Santos et al. (2022), these sludges can be considered a valuable material source after proper treatment, contributing to the sustainable circular economy of the wastewater treatment sector [9]. Overall, several industries are producing very significant amounts of waste with good biorefining potentials such as food, chemical, textile, paints, resins, pharmaceuticals, tanneries, paper, metallurgy, and mining [9].

Processing biomass and wastes in biorefineries may require the integration of several technological processes, such as separation processes, chemical or biochemical conversions, and thermochemical conversions. Thus, biorefineries can be classified according to the type of technological process involved and defined in different platforms: biochemistry, thermochemistry, biodiesel, and biogas. The thermochemical platform involves the decomposition of biomass via gasification or pyrolysis, using heat and catalysts. Current developments require the improvement of thermochemical processes to higher operation efficiency, advancements in new equipment, and coupling with other technologies, such as electrolysis, methanation, or anaerobic digestion (AD), to expand the biomass feedstocks that can be used and the array of end products. With this more complex approach, also known as multi-product biorefineries, these infrastructures can yield energy, biofuels, and added-value products. One example of a multi-product biorefinery is the extraction of essential oils (EOs) from forestry biomass and the use of waste biomass from the process to produce biomethane via gasification and syngas methanation. In parallel, it is also possible to use other feedstocks in an anaerobic digestor to produce and upgrade biogas into biomethane, enhancing renewable gas production. The merged biomethane flows may then be used in mobility applications or for heat and electricity production. Consequently, this conceptual biorefinery concept based on technologically mature technologies would yield several marketable products, a low amount of generated waste, and improved yields.

EOs are one of the most interesting products that can be obtained in biorefineries using forestry biomass wastes. These compounds have been thoroughly studied throughout the years due to several pharmacological properties given by their main bioactive compounds (e.g., isoprenoids) [10]. In addition, EOs also present antimicrobial, antioxidant and anti-inflammatory properties, which explain the considerable interest in their extraction, as described by several authors [11,12,13,14,15,16,17]. Due to their features, EOs extracted from different feedstocks are commercialized and used in many applications such as food packaging, edible films and coatings [18,19,20,21,22,23], microencapsulation [24], biomedicine applications [25,26], and agricultural applications [27,28,29,30]. The high market value of essential oils could enable the use of waste forestry resources to be economically viable.

Usually, EOs are extracted by cold pressing, steam distillation—SD (which includes dry steam, direct steam, and hydro distillation), solvent-assisted extraction, ultrasonic-assisted extraction, supercritical fluid extraction, or solvent-free microwave extraction [31]. SD is the most conventionally used technique for EO extraction, albeit presenting lower yield and efficiency and higher extraction time than the other referred methods. Furthermore, SD has low capital and operational costs, making this technique very interesting for biorefinery integration [31,32]. Kant and Kumar (2022) analyzed conventional EO extraction techniques from rosemary and oregano and determined that production costs for EO extraction using SD varied between 14.90 and 71.93 EUR/kg [31]. EOs from rosemary and oregano were also studied by Moncada et al. (2016). The authors used water distillation (conventional) and supercritical fluid extraction (non-conventional) and concluded that energy integration played a relevant role in the pricing of EOs. Oregano EOs showed the lowest production costs by using supercritical fluid extraction with full energy integration (6.31 EUR/kg), while rosemary EOs had lower production using water distillation with full energy integration (6.18 EUR/kg) [33].

Gasification is the conversion of organic or carbonaceous raw materials at high temperatures. The process mainly produces gaseous products, including hydrogen (H₂), carbon monoxide (CO), small amounts of carbon dioxide (CO₂), nitrogen (N₂), water (H₂O), and hydrocarbons (C_nH_m) [34,35]. Biomass gasification is an old and economical alternative for the production of renewable gases. For example, the production of hydrogen can be achieved by the partial oxidation of wood particles using oxygen as the gasifying agent, yielding a hydrogen fraction directly in the syngas, which can be enhanced through the water–gas shift (WGS) reaction [36]. Low-temperature catalytic gasification is also an interesting alternative for hydrogen production from an energy point of view, as it requires a relatively low heat input, and gas treatment is not necessary. Both from an input–output point of view and the complexity involved in the process, low-temperature catalytic gasification becomes more attractive and viable than high-temperature gasification [37]. Furthermore, several processes are used to clean and condition the syngas to the quality needed, not only for hydrogen production but also for further chemical synthesis. Mature technologies (commercially available for syngas cleaning and upgrading) include the above-mentioned WGS reaction, scrubbers, membrane separation, or pressure swing adsorption (PSA).

AD is the current technological benchmark for biomethane production. The process uses microorganisms to convert organic compounds such as carbohydrates, proteins, and lipids into methane, carbon dioxide, water, and other vestigial compounds. AD is a well-established and mature technology used to treat sludges and other organic effluents [38,39]. Biogas, the main product resulting from the process, has enough methane content to contribute as a renewable energy vector; simultaneously, digestate can be used as a fertilizer due to its high nutrient concentration (N and P) [39]. Methanation, on the other hand, has also been receiving a lot of attention as a thermochemical pathway for biomethane production. Two main reactor concepts represent the state of the art in methanation technologies: adiabatic or cooled fixed-bed reactors and fluidized bed reactors. Adiabatic fixed beds are commercially available but typically increase the complexity of the process setup due to their inherent heat vulnerability. On the other hand, fluidized beds can avoid localized hot spot formation and increase the tolerance to unsaturated hydrocarbon traces in the feed gas, although they still lack technological maturity.

Despite their great potential to be a common point between different productive chains and industrial processing lines, biorefineries have not been widely implemented worldwide [40]. This is evident when collecting information on techno-economic analysis for multi-product bio-refineries. There is still a shortage of information regarding the costs involved in the implementation of biorefineries, more so when considering multiple technologies and multiple products. Despite this, some studies share relevance with the present work [2,41]. Michailos et al. (2020), for example, evaluated the techno-economic performance of a Power-to-Gas (P2G) system which closes the energy and material loops of an AD plant and produces high-purity methane from sewage sludge in a real wastewater plant (WWTP). The authors considered four production scenarios: biomethanation, biomethanation + gasification of the digestate for hydrogen production, biomethanatiom (with increased hydrogen and carbon dioxide) + gasification of the digestate for hydrogen production, and biomethanation + gasification of the digestate + integrated gasification combined cycle. The energy efficiency of the proposed concepts was found to be between 26.5% and 35.5%, with a minimum selling price (MSP) for biomethane between 154.8 and 209.8 EUR/MWh, with the possibility of being reduced by 34–42% with the implementation of some process improvements and by considering revenues from the process’s by-products [41].

In this paper, the pre-feasibility of an integrated multi-product biorefinery yielding EOs and biomethane as major products is assessed. The concept involves the use of SD to fractionate mixtures of forestry biomass (mainly E. globulus and C. ladanifer) and the gasification of the resulting biomass to obtain syngas. This syngas is further cleaned and processed via catalytic methanation to obtain biomethane, while in parallel, an anaerobic digestor processes WWTP sludge to produce additional biomethane after biogas upgrading. The final biomethane uses considered in the study are mobility (e.g., heavy freight transportation) and heat and electricity production (e.g., solid oxide fuel cells).

2. Proposed Biorefinery Concept

The biorefinery concept considered in this work starts with the forest management practices from which biomass wastes are produced: pre-cleaning, cleaning, classification, transport, and final separation via particle dimension. After collection, part of the biomass, namely eucalyptus (E. globulus) and rockrose (C. ladanifer), is subjected to a steam distillation process in a 200 kg/h reactor to extract the EOs. These forest species were chosen considering their abundance in Portugal and the strong potential to become feedstocks in a biorefinery for the production of multiple products [42]. Steam for the SD process is obtained from the thermal energy produced in the gasification reactor. The spent biomass wastes from the extraction of EOs are then grounded and pelletized for subsequent gasification in a 1000 kg/h fluidized-bed gasifier at 800–95 °C. After gasification, the producer gas is cleaned through a cyclone filter and condenser, yielding char and ash (for soil applications) and condensates (which will be further introduced into the AD process). This gasifier has the particularity of operating with 50 vol.% oxygen produced by an electrolyzer (23.4 kg/h of hydrogen) coupled with photovoltaic panels. Finally, in the methanation reactor (fixed bed), carbon dioxide from the burning of the syngas is mixed with this green hydrogen and transformed into methane (30.42 kg/h).

In parallel, an AD reactor is fed with WWTP sludge, achieving a biogas production rate of 5.3 kg/h. WWTP sludge is also an extremely abundant and under-valorized waste in Portugal. Biogas upgrading proceeds through PSA, and the biomethane produced in the two technological pathways are combined and used for mobile applications or in an SOFC for the production of thermal and electrical energy. The following flowchart presents the multi-product biorefinery described above (Figure 1):

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