A techno-economic assessment on biomass waste-to-energy potential in Cameroon

Biomass waste-to-energy (WtE) offers a critical solution to carbon neutrality through improving the resource recycling and recovery. This study comprehensively assessed how WtE can be implemented in generating electricity for Cameroon with an estimation to the energy potential of anaerobic digestion of three organic waste streams including municipal solid waste, wastewater sludge, and livestock manure. We assessed the energy potential in terms of the theoretical, technical, and economic potentials. The findings highlighted a theoretical energy potential of 936.37 TWh yr−1 in Cameroon. If only applied to a fraction of organic wastes, the technical potential could reach 48.64 TWh yr−1. Furthermore, considering the economic costs of technology installation, 17.06 TWh yr−1 could be generated, and this economic generation potential could supply to 38.9% of the country’s current electricity demand. This study implies that WtE would significantly reduce fossil fuels consumption and greenhouse gases emissions from poorly disposed wastes, to enable decarbonization transition and improve human health in African countries.


Introduction
The amount of waste generated worldwide has been rapidly increasing due to economic growth, population increase, urban development, industrialization rates, and living standards (Kaza et al 2018, Chen et al 2020, Yaqoob et al 2021).As a result, researchers have been exploring innovative ways to use organic waste as a renewable source of energy to meet energy targets, reduce dependency on fossil fuels, and decrease anthropogenic greenhouse gas emissions (GHGs) (Kaza et al 2018, Milbrandt et al 2018, Rasheed et al 2021).Waste-to-energy (WtE) utilization can reduce environmental stress by preventing waste disposal at landfills and improving waste management practices (REN21 2019, Cole-Hunter et al 2020, Yaqoob et al 2021).Despite the low adoption of WtE approaches worldwide, technologies such as anaerobic digestion (AD) are promising for the efficient use of ubiquitous organic wastes at low costs (Divya et al 2015, Milbrandt et al 2018, Aghbashlo et al 2019).The diversion of organic wastes from landfills enables both energy production and the recovery of nutrients as fertilizers (Istrate et al 2021).
From a global perspective (as seen in table 1), the potential of WtE utilization is undisputable, and several studies have assessed the specific potential of developing countries considering various waste streams.However, the adoption of WtE methods in Cameroon, which is a sub-Saharan African country (see figure 1), is far behind schedule.Despite the fact that less than a quarter of the waste disposed of in landfills is being exploited, the focus of local administrative authorities remains on waste  In light of this, it is critical that Cameroon implement an integrated waste management strategy that prioritizes sustainable waste management practices.This should include recovering valuable resources for recycling and energy generation, as well as promoting economic development and raising living standards.At the same time, Cameroon faces a severe energy poverty crisis, with only 62.7% of the population having access to the national grid, and rural areas are particularly affected, where only 23.03% of the population has access to electricity (World Bank 2018).Uneven generation and supply across regions rely on hydropower, petroleum, and natural gas resources.However, a rapidly growing population and economy demand urgent additional clean energy, including WtE.Cameroon's ministry of energy and water resources plans to produce 5000 MW by 2030 through energy supply network expansion and renewable energy utilization (Engo 2019).
Despite a few studies on Cameroon's renewable energy potential (Abanda 2012, Wirba et al 2015, Mboumboue and Njomo 2016, Muh et al 2018), no study has taken organic WtE potential into account, which is critical to tackling energy poverty and fostering sustainability.In this regard, the study aims to solve these issues by addressing the following questions: What is Cameroon's biomass WtE potential?In Cameroon, how may biomass WtE be used?The technique suggested in this research is noteworthy since it provides for uncertainty assessment and may be replicated in countries with comparable socioeconomic and environmental factors.The study relies on reliable data sources such as the Institute of Statistics of Cameroon, United Nations, and the World Bank, to provide a comprehensive overview of the potential of organic waste to generate electricity in Cameroon, including municipal solid waste, municipal wastewater, and livestock manure.It also offers a detailed analysis of the municipal level, where data on waste generation, collection, and WtE utilization are scarce.The study's results give vital insights into Cameroon's remarkable WtE potential and its implications for sustainable development and carbon neutrality.The study is divided into five sections: an introduction (section 1), methods (section 2), results (section 3), discussion of findings (section 4), and conclusion (section 5).

Method
This research examines Cameroon's waste treatment scenario and assesses the WtE potential of municipal solid waste (MSW) (figure 3), municipal wastewater sludge, and livestock manure (cattle, pigs, and poultry).The evaluation of WtE potential involved a bottom-up resource-focused approach in evaluating theoretical, technical, and economic aspects, taking into account key inputs such as annual waste generation rates projected from national statistics, waste composition analysis to determine the organic fraction suitable for AD (figure 2), energy conversion rates using waste properties and reactions, existing collection infrastructure as the baseline, technology and installation costs, and a 20 year financial analysis (see supplementary material).The study adopts a descriptive statistics to characterize waste quantities and sources and a cost-benefit modeling to evaluate technical and economic viability of WtE projects in Cameroon.

Theoretical WtE potential
The theoretical potential within a well-defined space and time was defined as the theoretical maximum energy generated from all physically available organic wastes, considering the structural and ecological boundaries of the study area (Batidzirai et al 2012).This potential depends on the total available organic material under consideration for each waste stream, the theoretically highest biogas production potential and consumption, and the maximum conversion efficiencies of biogas to electricity.

Estimation of theoretical electricity generation potential from MSW
The theoretical electricity potential (Th.PE MSW ) of biogas in the country was estimated by multiplying the total biogas yield from MSW per municipality, the conversion efficiency of biogas to electricity (94% as per Rios and Kaltschmitt (2016)), and the higher heating value (HHV = 24.7 MJ m −3 ), as stated in equation ( 1) below: where: θ HHV = upper heating value of biogas generated from waste (MJ m −3 ), η e = electricity conversion efficiency (%), and Th.BY mun = theoretical biogas yield per municipality (m 3 )

Estimation of theoretical electricity generation potential from municipal wastewater
To estimate the potential electricity production from wastewater, we considered the biogas production by AD of the sludge produced by wastewater treatments.In Cameroon, wastewater treatment is neither commercial nor industrial, as wastewater is still mostly collected in septic tanks across the country.In this study, upflow anaerobic sludge blanket reactors were used as a reference technology for wastewater treatment.To estimate the total sewage flow to anaerobic reactors in Cameroon, the equivalent population per region (table S2, supplementary material) and the annual theoretical potential electricity generation (Th.PE WW ) of biogas produced from wastewater treatments in the country was estimated by multiplying the volume of biogas generated per municipality (Th.BY mun ), HHV, and efficiency of electricity conversion, as shown in equation ( 2) where: θ HHV = upper heating value of biogas generated from waste (MJ m −3 ), η e = electricity conversion efficiency (%), and Th.BYmun = theoretical biogas yield from municipal wastewater(m 3 ).

Estimation of theoretical electricity generation potential from livestock manure
To evaluate the theoretical electricity production potential of biogas from livestock manure (cattle, pigs, and poultry), we assumed that animal waste would be processed in anaerobic digesters.Data on herd size and type from different municipalities across the country, daily production of manure per animal type, biogas yield per manure type, and organic matter in dry matter (DM) per manure type were obtained from the literature (table S3, supplementary material).Based on these data, the yearly theoretical potential of electricity generated from livestock manure (Th.PE a ) was calculated using equation ( 3)

Technical WtE potential
The technical potential represents the achievable energy generation considering the practical limitations and constraints associated with the implementation of AD technology.
where: T.PE mun = technical potential of electricity generation per municipality; T.BY mun = technical biogas yield from municipal solid waste per municipality (m 3 ); OP = annual operation of power plant; θ LHV = LHV of biogas (MJ m −3 biogas); and η e = electricity conversion efficiency (%).

Estimation of technical electricity potential from municipal wastewater
To estimate the technical electricity potential from municipal wastewater, additional parameters were considered to evaluate the net value of energy released during AD, including the population associated with wastewater sludge collection services, efficiency of sewage collection, and LHV.The technical potential of electricity generation from municipal wastewater (T.PE ww ) was estimated using equation ( 5) where: T.PE ww = technical potential of electricity generation from wastewater; Th.BY mun = theoretical biogas yield of municipal wastewater; OP = annual operation of power plant; θ LHV = LHV of biogas (MJ m −3 biogas); and η e = Electricity conversion efficiency (%).

Estimation of technical electricity potential from livestock manure
From a technical perspective, not all theoretically available organic wastes can be used for energy production.For instance, there are no technical methods for the collection of all livestock slurry, especially considering grazing behaviors (Rios and Kaltschmitt 2016).Therefore, we assumed that all livestock manure technically available per municipality was collected and transported to a biogas plant where they were anaerobically digested for energy production.The technical potential of electricity generation from livestock manure (T.PE agr ) in Cameroon was estimated using equation ( 6) where T.PE a = technical potential of electricity generated from livestock manure per municipality; T.SW a = annual livestock manure generated per municipality; φ DM = dry matter (DM) in manure produced per animal type (a) (%); φ Org,a = organic fraction of livestock manure per animal type; F gas,a = biogas production factor per animal type (a) (m 3 Mg −1 ); OP = annual operation of power plant; θ LHV = LHV of biogas generated from waste (MJ m −3 ); and η e = electricity conversion efficiency (%).

Economic potential of biomass WtE projects
The economic potential represents the energy generation that is economically viable based on the costs associated with technology installation, operation, and maintenance.The economic potential analysis considered capital costs, operational costs, revenue from electricity generation and bio-slurry, and other relevant economic factors (see supplementary material for details).The difference between the present cash inflow and outflow within an economic lifetime yields the net present value (NPV) (equation ( 7)) where, C net is the net cash flow of the biogas project (C in − C out ), C o is the initial investment costs of the biogas project, r is the interest rate by year (y), and n is the economic life of the project.This study adopted 20 y as the life of biogas projects, excluding the construction period as suggested by ADB (2016) for projects with periodic maintenances and short-term replacement of equipment.The financial internal rate of return was estimated for NPV = 0.The economic potential is the fraction of the technical potential that meets these economic standards.For NPV > 0, the share of technical potential is equal to the economic potential, which implies that the uncertainty of the technical potential was included in this economic assessment.The economic potentials of electricity generated from MSW, municipal wastewater, and livestock manure were estimated using equations ( 8)-( 10 (10) Biogas plants of different sizes were investigated, including small (SBP), medium (MBP), and large biogas plants (LBP) with capacities of 500, 2000, and 10 000 m 3 , respectively.

Sensitivity analysis
The sensitivity analysis was conducted for each SBP, MBP, and LBP following the standard scenarios shown in table 2. In this assessment, we considered that revenue from sales of electricity and bio-slurry continued as usual.

Results
The results for the three different electricity generation potentials and associated uncertainties considering the most valuable sources of biomass waste in Cameroon are shown in table 3 below.

Theoretical potential
The theoretical electricity generation potential from biomass waste in Cameroon is extensive, with an estimated potential of approximately 936.37 TWh per year.Among the various waste sources, wastewater treatment shows the highest potential at 900.98 TWh per year, followed by livestock manure at 23.63 TWh per year.The livestock manure potential is further broken down into cattle (0.16 TWh y r−1 ), pig (7.77TWh y r−1 ), and poultry (15.70 TWh y −1 ).The assessment reveals that all ten regions in Cameroon have favorable conditions for biomass waste production, collection, and transformation into electricity (figure 4).The central region, which includes the capital and has a population of approximately 4038 347 inhabitants (table S2, supplementary material), has the highest biomass WtE potential at around 176.21 TWh per year.It is followed by the farnorth region (165.22TWh y r−1 ) and the Littoral region (143.40TWh y r−1 ), both of which have large areas, significant populations, and extensive livestock production.

Technical potential
The technical electricity generation potential from biomass wastes in Cameroon varies from 3.68 to 48.64 TWh per year.Wastewater treatment contributes the most to this potential, ranging from 3.60 to 48.12 TWh per year.Municipal solid wastes have a potential of 0.01-0.42TWh per year, while livestock manure ranges from 0.0011 to 0.11 TWh per year.These findings indicate that all municipalities in Cameroon have the technical conditions necessary for biomass WtE conversion.
However, it is important to note that the technical potential represents only about 5.2% of the theoretical potential.This significant decrease is attributed to various factors, including structural and ecological limitations, processing technologies, and energy transformation efficiency.These factors affect the availability of biomass wastes, the production of biogas, and the subsequent transformation into electricity using commercially available technology.

Economic potential
The electricity generation potential for Cameroon was estimated as 17.06 TWh y r−1 , which indicates that the minimum technical WtE potential is economically viable.The WtE potential from wastewater treatment showed the highest economic potential of 16.99 TWh y r−1 , followed by MSW at 0.05 TWh y r−1 , and livestock manure at

Waste type Potential
Amount of waste (1000 tonnes y r−1 ) Biogas (1000 m 3 y r−1 ) Electricity (TWh y  0.02 TWh y r−1 (table 3).The financial costs of biogas plants, including investment and operating costs, and the benefits of supplying electricity and fertilizers to the energy and agricultural markets, respectively, were converted to economic values (S2. supplemental results).The results revealed that nearly all regions in Cameroon have an WtE economic potential based on existing commercial technology.
Sensitivity tests were conducted to determine the changes on final economic values according to initial economic values for SBP, MBP, and LBP.The findings (table S25, supplementary material) revealed that in the three waste streams, a 10% increase in overall costs (investment and operational costs) and a 10% decline in benefits (sales of electricity and fertilizers) represented the most sensitive scenario for the WtE economic potential.In addition, a 10% increase in investment costs was the least sensitive parameter for WtE economic potential.These tests emphasize the impact of uncertainties on overall costs and benefits of biogas projects.

Discussion and implications based on WtE production potential assessment
The implications of biomass WtE utilization in Cameroon are vast and could have significant impacts on the country's energy sector and waste management practices.Cameroon is facing an energy crisis, with only 16% (789 MW) being met (Muh et al 2018).Given the increasing energy demand, biomass WtE utilization could provide a viable solution to address this crisis, increase electrification, and contribute to the country's overall energy consumption.
The study reveals that there is a substantial techno-economic potential for energy generation from organic waste streams, such as municipal solid waste, livestock manure, and municipal wastewater sludge (as seen in table 3).In fact, these waste streams alone could supply almost 39% of the country's energy demand.This finding highlights the relevance of WtE utilization in Cameroon's energy mix and its potential to significantly contribute to the country's energy consumption and electrification efforts.Additionally, the agricultural sector, which contributes significantly to the country's GDP, can provide additional waste streams for biomass WtE utilization, making it an important sector to consider in promoting sustainable waste management practices.
While the implementation of biomass WtE utilization, particularly from livestock manure, may face logistical challenges such as collection from distant sites, it presents an excellent opportunity for biogas use at the regional level.Moreover, the implications of biomass WtE utilization in Cameroon are not limited to the energy sector alone.It can also play a crucial role in alleviating energy poverty in the country.To fully realize these benefits, the development of policies that incentivize the establishment of biogas plants, create new energy markets, and invest in sustainable waste management practices is essential.
Such policies can significantly impact per capita electricity generation and the share of electricity consumption in Cameroon.

Policy implications of energy transition in Cameroon
In terms of policy implications, biomass WtE is critical for Cameroon's transition to a low-carbon future.The country's current energy mix, dominated by hydropower, oil, natural gas, and a small proportion of biomass, necessitates a shift towards more renewable energy sources.The lessons learned from the COVID-19 era further emphasize the importance of sustainable waste management practices, and the implementation of biomass WtE can contribute significantly to this aspect as well.
For the widespread adoption of biomass WtE technology, it is crucial to gain social acceptance and business support (Geels et al 2017).Cameroon already has economic policies in place to encourage the development of renewable energy technologies, and these can be applied to both small-scale residential installations and large-scale commercial projects (REN21 2019).Innovation policies like feedin tariffs (FIT), renewable energy auctions, and net metering systems show promise in promoting the diffusion of renewable energy sources in Cameroon.Furthermore, the study highlights the potential of exploiting biogas from organic waste at landfills for electricity generation, aligning with Cameroon's Long-Term Energy Sector Development Plan.
In terms of WtE project operation, policies such as cash compensation, credit rollover, and payout at avoidable costs can help energy suppliers generate revenue and improve their financial competitiveness (Silva Dos Santos et al 2018).While biogas projects are relatively new in the Cameroonian energy market, the assurance of renewable energy purchase can reduce economic risks for energy suppliers.Implementing policies similar to the FIT Act in Japan (Kimura 2017) could enhance the attractiveness of biogas projects for investors, providing investment security and encouraging significant returns.

Conclusions
This research identified biomass wastes generated in urban cities in Cameroon and their potential for electricity production.Biomass WtE utilization can create high-added-value products (electricity), provide adequate disposal of biomass wastes, and enable a transition to decarbonization.This study provided a review of the WtE potential for Cameroon to develop effective energy planning and sustainable public policies, which is a good reference for other African countries.The assessments highlighted the importance of evaluating the energy generation potential of various renewable sources and benefits and barriers related to their development and diffusion.Economic feasibility is one of the main barriers to the development of biomass WtE in Cameroon because the investment costs of WtE projects are generally high.Moreover, the lack of business models or effective innovation policies to support such investments owing to their potential to improve environmental and human health aspects is a major obstacle for the exploitation and deployment of the WtE potential.The adoption of new markets, including small-or micro-scale electricity generation and an open electricity market, is necessary for the realization of WtE in Cameroon.
There are some limitations that should be improved with further research.Firstly, WtE projects are complex, and their success depends on the plants' optimal size, location, and logistics, which remain an inherent risk associated with WtE projects that is beyond the scope of this study.The optimal biogas plant size for each municipality should consider the local environment, composition, quantity of waste, and infrastructure available.Secondly, this review of potential is based on biomass for electricity generation as an advanced approach, so the results of this study can be considered a kind of upper limit for biomass utilization.Diverse and flexible-designed solutions could be obtained for practical implementation.

Figure 1 .
Figure 1.Cameroon map showing location of various powerplants and landfills across the country's ten regions.
where, N a = number of heads of animal type (a) per year; R waste,a = manure production rate per animal type (a) (kg d −1 .unit);365 = number of days in a year; φ DM = organic content in DM of the manure generated per animal type (a) (%); φ Org,a = organic fraction of livestock manure per animal type; F gas,a = biogas production factor per animal type(a) (m 3 Mg −1 ); θ HHV = upper heating value of biogas generated from waste (MJ m −3 ); and η e = electricity conversion efficiency (%).

Figure 4 .
Figure 4. Theorical and technical potential for electricity production across ten regions in Cameroon.

Table 1 .
Global perspective of the energy generation potential of biomass waste.
The maximum and minimum values indicate the most likely range in which realistic values are expected to be found.Average values are not necessarily the arithmetic mean of the range, but they were calculated based on available information.Assumptions were used in the assessments in the absence of appropriate data.
Factors such as feedstock availability, technology efficiency, and operational considerations were taken into account to estimate the technical potential.Three scenariosminimum, average, and maximum-were used to account for uncertainties in this assessment.
Process sensitive analysis-fertilizer and electricity sales. 2.

Table 3 .
Summary of findings (organic wastes generated, collectable biogas flow, potential energy production).