Co-gasification of refused derived fuel and biomass in a pilot-scale bubbling fluidized bed reactor

Highlights

• Co-gasification of RDF with biomass was studied in an autothermal pilot-scale BFB.

• Stable conditions of operation were attained and defluidization was not observed.

• RDF addition led to enhanced gasification products and producer gas LHV increase.

• RDF is demonstrated as a promising feedstock for co-gasification with biomass.

Abstract

In this work, direct (air) co-gasification of refused derived fuel with biomass was demonstrated in an 80kW_th pilot-scale bubbling fluidized bed reactor. The influence of the process operating parameters, namely average bed temperature between 785 and 829 °C, equivalence ratio between 0.21 and 0.36 and refused derived fuel weight percentage in the fuel mixture (0, 10, 20, 50 and 100 wt%) was analyzed. For the operating conditions used, the process was demonstrated as autothermal and operating under steady-state conditions, with no defluidization phenomena observed. The increase of the refused derived fuel weight percentage in the fuel mixture led to an increase of the methane and ethylene concentration in the producer gas and, consequently, an increase of the producer gas lower heating value, reaching a maximum value of 6.4 MJ/Nm³. In terms of efficiency parameters, cold gas efficiency was found between 32.6 and 53.5% and carbon conversion efficiency between 56.0 and 84.1%. A slight increase of the cold gas efficiency was observed with the increase of the refused derived fuel weight percentage in the fuel mixture. Thus, refused derived fuel co-gasification with biomass was shown as a highly promising process for the valorization of wastes as an energetic resource.

Introduction

Waste management and sustainable energy supply are two of the main challenges of society. On one hand, waste management should be improved and integrated with environment and human health protection while promoting the principles of circular economy [1]. On the other hand, current worldwide energy supply mainly relies on finite fossil fuel resources (coal, oil and natural gas), resulting in its excessive extraction and consumption. These aspects lead to negative economic and environmental consequences, such as the depletion of fossil fuel resources and emission of large quantities of greenhouse gases (GHGs). This means that the current worldwide energy supply is associated to pollution despite providing economic development and life quality increase [2]. Furthermore, continuous industrialization, population growth and general increase of living conditions, led to higher worldwide energy requirements and waste production in the last decades. Energy recovery from municipal and industrial wastes can contribute to solve these issues, and to reduce the EU dependence on fossil fuel-based feedstocks imports, by representing a new source of sustainable energy to satisfy the increasing society energy demands. Thus, a transition to a more sustainable waste management and circular economy model is facilitated [1].

In this context, waste-to-energy (WtE) solutions can simultaneously contribute to overcome the problem of residues disposal and the reduction of GHGs emissions resulting from fossil fuels use [2]. WtE valorization options can be based on biochemical (e.g., hydrolysis, fermentation) or thermochemical conversion processes (e.g., combustion, pyrolysis, gasification). Municipal solid waste (MSW), after separation of the fraction that can be reused or recycled, are an interesting feedstock for WtE conversion [3]. MSW are a mixture of distinct organic and inorganic components generated from households, offices, commerce and public institutions and, despite constituting only between 7 and 10% of the total waste generated in EU, represent a significant challenge in terms of management [1]. In Portugal, MSW valorization as solid fuel is also considered as an opportunity to reduce GHGs emissions, minimize waste deposition in landfills, increase national energy independence and diversify the solid fuels supply [4].

Nonetheless, despite recent progresses in waste management, approximately 45% of the wastes generated (25% being MSW) in the EU are still disposed in landfill sites [5], [6]. A higher value (45%) is seen in Portugal for MSW disposition in landfill [7]. These values are significantly higher than the value of 10% that State Members should attain by 2035 [8]. Accordingly, in the EU, MSW production has increased from 150 million tons to 250 million tons between 1980 and 2017 [2], [9]. Thus, the development and improvement of WtE valorization options for MSW management is relevant [10].

In this respect, to improve the effective use of MSW for WtE processes, two types of solid fuels can be produced from wastes, namely Refuse Derived Fuel (RDF) and Solid Recovery Fuel (SRF) [11]. These fuels can be composed by a mixture of distinct non-hazardous solid wastes, such as plastics, textiles, paper, biomass packages and rubber [10], [11]. This composition can vary significantly depending on the waste origin, waste separation plant, season and the RDF/SRF production technique [10]. A more detailed description of the materials that typically compose these fuels can be found in the literature [12], [13]. To be defined as SRF, the solid fuel must be produced in compliance with the European standard EN 15,359 [14]. This means that the chlorine and mercury content and LHV of the solid fuel, must be determined [12], [13]. Both RDF and SRF can be used for energy conversion processes, however, it has been argued that the SRF has commercial advantages in comparison with the RDF, because it eases the trade between producers and users [12], thus, contributing to higher confidence in the market [12]. In this work, the MSW used in the gasification and co-gasification (G-CG) experiments are referred as RDF, because their mercury and chlorine content were not determined.

Amongst the WtE valorization methods based on thermochemical conversion processes, combustion is the most conventional and commercial process, while pyrolysis and gasification are still in a demonstration phase. Combustion technologies are commercially available with several distinct configurations employed at industrial scale. Currently, thermal treatment plants, such as incineration plants, are commonly used for MSW disposal [10]. In fact, at least 450 MSW mass burning incineration plants are in operation in Europe [15]. Heat recovery and district heating based on waste mass burning incineration is currently performed in the EU and considered as an important technology to provide environmentally friendly heat for residential and industrial sectors [16].

MSW pyrolysis and co-pyrolysis with biomass is under research and its potential is recognized, however, it is argued that the upscaling and commercialization of this process still requires substantial efforts [17].

On the other hand, gasification is recognized as a highly promising and flexible process for waste energetic valorization [18], that allows the production of various bioproducts that neither combustion nor pyrolysis are suited to provide. In this respect, gasification can perform the valorization of waste energy into a fuel gas with flexible application in different scenarios, such as the replacement of natural gas in boilers or kiln ovens, liquid fuel production by Fischer-Tropsch process [19] or methanol synthesis [20]. Furthermore, there are several advantages in handling a gas in comparison with a solid waste, such as transport, storage and application [18].

In this context, co-gasification of wastes with biomass is a process that has been drawing significant attention in recent years [2]. In general, co-gasification of biomass with non-biomass fuels can be used as a strategy to reduce the ash content of a feedstock [21] and may lead to benefits in terms of producer gas quality, char reactivity and tar formation, in comparison with gasification processes using only biomass [22]. This practice can take advantage of synergistic effects that can occur between the two feedstocks used for the co-gasification process [2], [23], for example, feedstocks with high ash content rich in alkali and alkali earth metals (e.g., sodium, potassium, magnesium and calcium) can have a catalytic effect during co-gasification with other fuels [21], [22], [24]. Furthermore, co-gasification of wastes with residual forest biomass (RFB) can valorize both wastes and RFB [25]. On one hand, in comparison with gasification plants using 100% biomass, adding wastes to the process avoids biomass excessive exploration and supply disruption. In fact, the high availability of wastes and its continuous generation, and the need of appropriate solutions for processing the non-recyclable fraction of organic wastes, turns wastes into an almost inexhaustible resource for gasification processes [18]. On the other hand, in comparison with gasification plants using 100% MSW, adding biomass may contribute to solve recognized process issues associated to the plastics gasification, such as feeding difficulties and contaminants formation [2].

In a recent literature review made by Ramos et al., [2], it is shown that only a small number of reports regarding co-gasification of biomass and wastes are currently available. The authors refer that the gasification products and yields are higher for co-gasification of MSW and biomass than for gasification processes of 100% biomass or 100% MSW, thus indicating synergy between these feedstocks. Accordingly, a recent co-gasification study of MSW and biomass [26] showed process benefits by adding wastes to the feedstock mixture, such as the increase of the LHV of the producer gas and the reduction of tar formation. Plastic addition to the fuel mixture in gasification processes has also been shown to lead to an increase in specific gas yield and LHV [27], [28], [29], [30]. It is argued that the thermal cracking of the plastic polymer chains leads to the production of diverse hydrocarbons with a wide range of molecular weight, including light hydrocarbons that contribute to the increase of both the gas yield and LHV [28]. Some studies also evaluate the potential of more advanced applications for the produced gas from wastes gasification, such as methanol [31] and hydrogen [32] production. In fact, a commercial biorefinery located in North America (Enerkem Alberta Biofuel) provides the industrial application of producing biofuels and renewable chemicals from non-recyclable waste and residues [33], [31]. Despite these advances, studies regarding wastes gasification are still extremely scarce, particularly for higher scales (pilot and industrial) [2], [34]. In this regard, further experimental research must be performed to characterize the effect of the addition of RDF on the producer gas quality and to evaluate potential synergistic effects.

Accordingly, in this work, distinct mixtures of RDF pellets (10, 20 and 50 wt%) were mixed with pine (chips and pellets) and used as feedstock in direct (air) co-gasification experiments in a bubbling fluidized bed (BFB) pilot-scale autothermal reactor. For comparison, gasification experiments with 100% RDF pellets and 100% pine (pinus pinaster, chips and pellets) were also performed. The influence of the RDF weight percentage on the operating conditions and process stability, in terms of temperature along time at several locations of the reactor and producer gas composition along time, was analyzed. Efficiency parameters were also determined to evaluate the influence of the RDF weight percentage on the process performance. This new information is relevant because it provides a systematic experimental analysis of the gasification of RDF and biomass, and their blends at different mixture ratios, in a bubbling fluidized bed at a pilot-scale level, for which there exists a recognized lack of studies. Thus, the obtained results provide new knowledge that may serve as a complementary tool to support decisions related to co-gasification projects, including the upscale of the process to the industrial scale and the development of co-gasification plants. This can ease the commercial breakthrough of this technology and consequently promote both waste management and energy supply sustainability in the future.