1. Introduction
Currently, Brazil is one of the largest generators of municipal solid waste (MSW) in Latin America, reaching 79 million tons of waste in 2019, corresponding to a per capita generation of 379 kg/inhabitant/year [
1]. When looking into the associated greenhouse gases (GHG) emissions, in 2016 a total of 91.97 million tons (Mt) of CO
2 equivalent (CO
2-eq) were emitted, with 57.5% coming from the final disposal of MSW [
2]. According to Gouveia [
3], due to the various composition of MSW and the inappropriate final treatment (resulting in pollutants dispersion in air and water, and soil contamination), selective collection activity is the main alternative to waste management processes.
Another waste management option is the use of refuse-derived fuel (RDF), which refers to the fuel fraction recovered from non-hazardous solid waste of diverse origins such as organic matter present in MSW, various types of plastics, biodegradable waste, and considerable amounts of inorganic material, such as pieces of metal and glass [
4]. Considering the need to standardize the information about MSW generation and management Brazilian, a list of solid waste was created, where the RDF is classified as combustible waste and identified by code 19.12.10 [
5].
The inappropriate disposal of MSW in the environment has various impacts, affecting air quality due to GHG emissions, as well as water and soil contamination. These impacts can be minimized by converting MSW into energy through different Waste-to-Energy (WtE) technologies, such as incineration, pyrolysis, and gasification [
6]. In this way, energy recovered from WtE technologies can be described as the conversion of non-recyclable waste materials into heat, electricity, or fuel and other products with high added value, such as those resulting from biochemical treatments [
7].
Gasification is considered the thermochemical conversion process of carbonaceous materials into fuel gas, where a controlled amount of a gasification agent (air, oxygen, steam, CO
2) or a mixture of them is used, leading to an alteration of the chemical structure of fuel particles due to the high temperatures reached (above 700 °C) [
8]. This gaseous fuel, known as syngas, is generated from the gasification process with air in the temperature range between 770 °C and 1000 °C. The syngas produced from gasification with air mixtures enriched with oxygen, steam, and CO
2, offers an improved calorific value [
9].
According to Couto et al. [
10], gasification offers a good solution to the recovery of chemical energy present in MSW, producing syngas that can be used in various applications for the chemical industry or as fuel for the efficient production of electricity or heat, becoming an interesting option for the management of MSW. One of the technologies used for power generation involves gas microturbines (GM), which produce a nominal power between 25 kWe and 300 kWe [
11]. The structural configuration of turbines may vary according to the manufacturer and its application [
12], but GM usually operates in a single-shaft configuration with a regeneration system, reaching efficiencies of up to 35%. In the case of GM without a regeneration mechanism, the electrical efficiency can reach values up to 17% [
13].
The GM operating principle is based on the Brayton regenerative cycle and its components, which includes a centrifugal compressor coupled to a radial turbine that operates at a high rotation, and requires a digital power controller to manage the power output and can utilize both liquid and gaseous fuels [
14]. The main operational concerns of GM are associated with damage to components subjected to high temperatures, susceptible to severe degradation when there is an excess of contaminants in the fuel and the supply air. Liquids found in gaseous fuels can also cause structural damage to the microturbine [
15]. According to Capstone [
16], the fuel must enter the Capstone C200 GM with a pressure of 5.5 bar and a temperature between −20 and 50 °C, which indicates a need for fuel conditioning before usage in a GM.
Life Cycle Assessment (LCA) is a methodology used for the evaluation and quantification of environmental impacts attributable to the life cycle of the process, or service. Likewise, LCA involves the compilation and assessment of inputs, outputs, and associated potential environmental impacts throughout the lifecycle of products [
17]. The structure, principles, requirements, and guidelines that a study must consider for LCA methodology are based on the ISO 14.040 and ISO 14.044 standards. Thus, the LCA study includes objective and scope, allocation method, inventory, impact assessment, and finally interpretation [
18].
Rabou et al. [
19] analyzed the performance of GM using syngas and identified that the lower limit for stable GM operation was obtained by using a syngas with a lower heating value (LHV) of approximately 8 MJ/Nm
3. Corrêa et al. [
20] studied the effect of mixtures of syngas and natural gas use in a GM. The results showed that the efficiency drops about 13% (compared to using only natural gas) when the GM was powered with a mixture of 50% natural gas and 50% syngas. This behavior is associated with the composition of fuel gases and their LHV since the syngas/natural gas mixture contained lower concentrations of CH
4 (1.8%vol.) and H
2 (5.1%vol.) due to the presence of N
2 (57.7%vol.), while for natural gas the values of CH
4 and H
2, were 8.6%vol. and 38.1%vol., respectively. Additionally, the temperature analysis showed that the temperature of the gases released by the GM undergoes insignificant variations, despite the different fuel compositions.
Lozano et al. [
21] carried out an energetic and economic evaluation of the RDF gasification process coupled to an internal combustion engine, operating with air as a working fluid at 1.0 bar and 25 °C. The maximum cold gas efficiency of 57–60% was obtained for equivalence ratio values between 0.25 and 0.3, where the associated syngas (with LHV of 5.8 MJ/Nm
3) was burned in the engine, reaching an electrical power of 50 kW at 20% engine efficiency. The economic analysis showed that the project is feasible for a power greater than 120 kW, for which an investment of approximately
$300,000 is required. In addition, Dong et al. [
22] compared seven systems that involve the thermochemical conversion of MSW (highlighting pyrolysis, gasification, and incineration) and subsequent syngas energy use in prime movers (steam cycle, gas turbine/combined cycle, internal combustion engine) through the LCA methodology. The results indicated that pyrolysis and gasification together with the gas turbine/combined cycle, have the potential to reduce environmental loads. Furthermore, the heterogeneity of the purification technologies of MSW and syngas are the most relevant impediments for WtE systems based on pyrolysis or gasification.
This work aims to perform an energetic and environmental evaluation of refuse-derived fuel gasification as an alternative for solid waste energy recovery. To simulate the gasification process, two scenarios were considered. In the first scenario, the air is used as the gasification agent while a mixture of air enriched with oxygen (60% O2–40% N2) is utilized in the second scenario. The model of the gasification process was validated and different parameters such as gasification temperature, cold-gas efficiency, syngas LHV, and syngas composition were evaluated. The usage of syngas in a gas microturbine to generate electricity was also modeled. Thus, in order to determine energy recovery potential from RDF conversion, an analysis of power generated and gas microturbine efficiency at different operational conditions was performed. For the environmental evaluation of the gasification/electricity generation process, the LCA methodology was applied by using the SimaPro software, being possible to quantify the potential environmental impacts of the analyzed system. Therefore, this paper provides an energetic evaluation and a life cycle analysis of the RDF gasification and power generation process, which are essential to determine the viability of RDF thermochemical conversion as well as the associated environmental impacts.
4. Conclusions
This study involves an economic and environmental evaluation of refuse-derived fuel gasification/electricity generation. In this work, a chemical equilibrium model was developed for RDF gasification and the Life Cycle Assessment methodology was performed. Two scenarios were considered in RDF gasification modeling with different gasification agents: air and oxygen-enriched air (with 60% O2–40% N2). Key results show that in an oxygen-enriched RDF gasification environment, the syngas LHV value reaches 8.0 MJ/Nm3 with ER equals 0.3, which produces 79.6 kW of electrical power when used in the GM.
Environmental impacts in various scenarios were assessed using a life cycle assessment. Emissions from the pretreatment and gasification stages of the MSW are the dominant ones with a value of 2474.49 kg CO2 eq. The electricity generation stage had positive impacts for all categories analyzed through the LCA. The environmental impacts determined by the ReCiPe method indicate that pollutant emissions from the burning of fossil fuels present greater impacts for climate change, ozone depletion, terrestrial ecotoxicity, and fossil depletion categories. Therefore, gasification can be considered a promising technology for the management and use of MSW, it enables the production of useful syngas for different applications and with low environmental impact when compared to traditional MSW management methods, such as incineration. Taking into account the results of environmental impacts, concerning emissions from the MSW transport stage, a potential way to reduce these negative impacts would be to re-dimension the MSW collection routes, optimize the collection of fractions and reduce fuel consumption. The processes for transforming and obtaining RDF are highly energy-intensive, therefore, an alternative to reduce the negative environmental impact of RDF production could be the reduction of energy consumption in these operations with the use of more efficient equipment in the energy context and use of green energy (use of clean sources of electricity generation such as solar photovoltaics mainly for MSW drying and crushing steps). These are certain initiatives that depend on the assessment of economic feasibility among other variables. In summary, several factors will affect the quality and characteristics of the RDF. The use of urban solid waste also requires continuous work on environmental education for citizens and changes in consumption habits, which will allow greater advances in the future in terms of sustainable management of waste and preservation of the environment.