1. Introduction
Municipal solid waste (MSW) represents a valuable resource for energy recovery in the form of heat and electricity, syngas, char and pyrolysis oils. These strategies exist to reduce the quantity of municipal solid waste landfilled without energy recovery and improve alternative materials and energy use [
1]. One viable option is alternative fuel concepts such as RDF from MSW utilized in cement kilns, boilers and for power generation. RDF is produced from mixed combustible portions of waste, including household, commercial and industrial/trade waste [
2]. RDF from MSW comprises combustibles such as plastics, paper materials and wood. Research shows that RDF has improved properties such as heating value compared to mass MSW considering product [
3] and thermal processes [
4,
5]. Currently, RDF is largely produced and utilized in countries such as Germany, Italy, the US and the UK for fossil fuel substitution. A growing interest in utilizing RDF is also reported from developing countries such as India, Namibia and Mozambique [
6]. In Europe, 12 million tons of RDF replaced fossil fuel in the cement and waste-to-energy (WtE) plants in 2015 [
7]. According to reports [
8], RDF utilization has reached between 30% to 60% of fossil fuel and alternative material substitution in Europe. Other energy carriers such as fuel oil and gases are obtained from RDF through pyrolysis and gasification processes [
6].
There are several developing countries such as Ghana where MSW management continues to be a major environmental challenge, affecting sanitation, land use, water and natural resources [
9]. Growing urban populations, increased MSW generation per capita, insufficient financing, legislation gaps, lack of strategic planning and impacts of uncontrolled landfills are significant challenges in the waste sector [
10]. MSW generation in Ghana is estimated at around 14.5 kilo tons/day. The common practice of mixed MSW also decreases the recycling potential as a result of contamination [
11], hence the low rates recorded [
12]. The waste sector in Ghana was ranked third among sources of GHG emissions (3.2 MtCO
2eq), mainly methane and CO
2 from waste disposal and fuel use, while industrial processes accounted for 1.52 MtCO
2eq according to the 2019 inventory report [
13]. Similarly, the most energy consumed in Ghana derives from fossil fuels, followed by biomass in the form of firewood and charcoal and electricity, representing 50%, 34% and 17% respectively in 2021 [
14]. In addition, several industrial processes are powered by fossil fuels: coal, diesel and electricity from natural gas. None is reported to utilize any form of alternative fuels from MSW, coupled with the high cost of current fuel sources increasing the operational cost for industries [
15]. Currently, about nine different cement companies are in operation in Ghana [
16], with a few operating their kilns using fossil fuel sources of energy [
17]. This suggests the potential for dependable alternative fuel and other suitable raw material substitution in the sector.
The potential for waste-to-energy (WtE) from MSW has been assessed within the Ghanaian context over the years. However, reports argue that several studies undertaken on WtE favor biochemical processes due to the high portion of organics such as food waste in MSW with little emphasis on thermochemical treatments [
18]. Furthermore, Barnor et al. [
19] reported that nearly 700 GWh of biogas is possible annually from municipal solid waste with a plant availability of 97%. Thus, about 1.5% of Ghana’s total electricity demand represents savings of USD 5 million from MSW to electricity. The study recommended mechanical biological treatment (MBT) primarily because it can separate the organic component of MSW from the inorganic component with the possibility of obtaining non-biodegradable resources for other purposes. Using the MSW decision support tool, Brown et al. [
20] assessed the performance of municipal solid waste disposal operations in one of Ghana’s municipalities, Wa. The study recommended an integrated system consisting of separation, composting, incineration and RDF to landfill disposal within the municipality since it had a lesser impact on health. However, the study did not qualitatively or quantitatively assess the RDF potential. Afrane et al. [
21] also highlighted the dearth of data on the waste management industry in Ghana, which is also a limitation, especially for WtE.
In recent times, the presence of material recovery and recycling facilities has contributed to treating portions of MSW for compost and recycling. On the other hand, non-recyclables of combustible fractions end up at disposal sites as a result of contamination from mixed MSW, compromised physical state and low market value fractions, etc. Thus, there is room for considering combustible portions for thermochemical treatments via the RDF technique for alternative fuel. The availability of resources (MSW) and end-users such as cement kilns suggests that the production and use of RDF is viable. This may also improve on solid waste management in Ghana without major investment into new systems. The use of residual fractions involves separating noncombustible fractions, resizing in some cases, reducing moisture and forming a homogenous product suitable for specific needs [
22]. In this context, researchers [
3,
23,
24] have analyzed activities of various MBT plants and landfills to assess the usefulness of MSW residual fractions for RDF. Combustible materials of high calorific values (>20 MJ/kg) and compliance with standards were assessed. It is clear from the previous studies the characteristics of RDF vary considerably with location (regional and local variations), hence its implications. Furthermore, other variables of relevance, such as drivers and barriers, are imperative for decision-making recommendations. Currently, RDF from MSW and its utilization are relatively new concepts and are largely untapped in Ghana. RDF production as part of an integrated waste management process also falls in line with the waste management hierarchy.
In this context, the aim of this study was to investigate the potential utilization of MSW reject/residual fractions in RDF, the characteristics of RDF and the possible utilization of RDF as a substitute fuel. The study closes a knowledge gap with one of the first case studies to technically evaluate RDF as an improvement of current systems. As a result, this work serves as a reference for the various stakeholders in harnessing the energy potential of RDF from MSW.
4. Conclusions
MSW management remains a huge challenge in Ghana. Although efforts such as composting and recycling have emerged in recent years, landfilling without energy recovery, open dumping and indiscriminate disposal are prevalent. This study assessed RDF potential considering the existing material recovery facilities in Accra and Kumasi, the two largest cities of Ghana. The RF was made up of combustible materials including paper, plastics, textiles and wood, which are valuable components for RDF and can reduce landfill disposal. The study shows residual municipal solid waste is a good option for energy-efficient RDF. The RDF obtained in this study was classified as NCV 1, Cl 2 and Hg 2 under the EN 1539:2011 standard. Subsequently, it is estimated that RDF quantities can replace up to 25% of coal fuel utilized. The RDF shows higher LHV, lower moisture and lower ash, Cl, S and N than mass MSW. The findings show good potential for industrial fuel and the possibility of substituting for conventional fossil fuel use in Ghana. Diverting landfill waste disposal for refuse-derived fuel will also consequently protect soil/groundwater from contamination, microplastic contamination of water bodies and air pollution. Scalable systems and collaborations at small to medium scales will facilitate initiatives for the development of waste-to-energy. On the other hand, efforts and regulations from the government are necessary to implement such synergy towards the industrial use of alternative fuels such as RDF. Further studies with a focus on industrial symbiosis as well as sustainability assessment may provide the basis for collaborations and resource exchange between various industries. Research into the favorable conditions and determinants of symbiosis implementation is also of importance. Furthermore, studies towards pretreatment methods’ suitability to improve the overall fuel properties will enhance the development of the RDF concepts. This study provides valuable insights into RDF production as a sustainable component of an integrated MSW management system, especially for developing countries, towards achieving SGDs and a circular economy.