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Electricity Generation

Methane Digesters (Large)

This sewage sludge digester fermenter facility, sewage treatment plant works, produces methane biogas as a bi-product.

Agricultural, industrial, and human digestion processes create an ongoing (and growing) stream of organic refuse. Without thoughtful management, organic wastes can emit fugitive methane gases as they decompose. Methane creates a warming effect 34 times stronger than carbon dioxide over one hundred years.

One option is to control decomposition of organic waste in sealed tanks called anaerobic digesters. They harness the power of microbes to transform scraps and sludge and produce two main products: biogas, an energy source, and solids called digestate, a nutrient-rich fertilizer. The digestion process unfolds continuously, so long as feedstock supplies are sustained and the microorganisms remain happy.

When produced at industrial scales, biogas can displace dirty fossil fuels for heating and electricity generation. When cleaned of contaminants, it can be used in vehicles that would otherwise rely on natural gas. On the solids side, digestate supplants fossil fuel-based fertilizers while improving soil health.

Germany leads the way among established economies with nearly eight thousand methane digesters as of 2014—almost 4,000 megawatts of installed capacity in total. Adoption is increasing in the United States, including use at the waste water treatment plant for the nation’s capital.

References

Alessandro Volta…“air from marshy soil”: Wolfe, Ralph S. “A Historical Overview of Methanogenesis.” In Methanogenesis: Ecology, Physiology, Biochemistry & Genetics, edited by James G. Ferry. Dordrecht, The Netherlands: Springer Science+Business Media, 1993.

methane in a pistol: Sethi, Anand Kumar. The European Edisons: Volta, Tesla, and Tigerstedt. New York: Palgrave Macmillan, 2016.

scientists [discovered] microbes were responsible: Wolfe, “Methanogenesis.”

methane [vs.] carbon dioxide: Myhre, Gunnar, Drew Shindell, François-Marie Bréon, William Collins, Jan Fuglestvedt, Jianping Huang, Dorothy Koch et al. “Anthropogenic and natural radiative forcing.” In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, and New York: Cambridge University Press, 2013.

[history of] organic waste as an energy resource: Insam, Heribert, Ingrid Franke-Whittle, and Marta Goberna, eds. Microbes at Work: From Wastes to Resources. Heidelberg, Germany: Springer, 2010.

[use in] Germany: Buckley, Pearse, ed. IEA Bioenergy Annual Report 2015. Dublin, Ireland: IEA Bioenergy Secretariat, 2015.

rural China…digester gas: REN21: Renewables 2016 Global Status Report. Paris: REN21 Secretariat, 2016.

view all book references

Errata

p. 26

Correction: The cumulative result: 10.3 gigatons of carbon dioxide emissions avoided at a cost of $217 billion.

view all errata

Technical Summary

Methane Digesters (Large)

Project Drawdown defines methane digesters (large) as: large methane digesters associated with agriculture, manure, and wastewater facilities that produce biogas to be used for electricity generation in dedicated biogas or combined heat and power plants. This solution replaces conventional electricity-generating technologies such as coal, oil, and natural gas power plants.

Methane digesters have been installed throughout the world and at relatively high rates in China, the European Union, and Southeast Asia in the past 20 years. Large bio-digesters can be installed at dairy and hog farms, wastewater facilities, and landfills to produce electricity and heat for use on-site, or to provide electricity or gas to the grid.

Methodology

This analysis models the impacts of the adoption of methane digesters (large) associated with agriculture, manure, and wastewater facilities for the production of biogas to be used for electricity generation in dedicated biogas or combined heat and power plants.

Total Addressable Market [1]

The total addressable market for biogas units for electricity generation is based on projected global electricity generation in terawatt-hours from 2020-2050, with current adoption [2] estimated at 0.25 percent of generation (56 terawatt-hours) (IRENA, 2016).

Adoption Scenarios [3]

Impacts of increased adoption of methane digesters (large) from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.

Based on the evaluation of several global energy system modelling scenarios, the three scenarios were built upon five ambitious scenarios from the EU project AMPERE, [4] the 2°C Scenario of the International Energy Agency’s Energy Technology Perspectives, and the Greenpeace Energy [R]evolution Scenario, using a high growth trajectory. These sources do not clearly depict biogas technologies for electricity generation adoption pathways: their results combine biomass and waste for electricity generation. Therefore, a few assumptions were considered to obtain future adoption: biogas represents approximately 20 percent of total electricity generation from bioenergy worldwide, and the feedstocks covered within this solution represent 70 percent of total biogas. Landfill methane accounts for the remaining 30 percent (AEBIOM, 2012; WBA, 2013).

For methane digesters (large), three scenarios were developed. Each follows the same adoption trajectory, but varies in the percentage of electricity generation provided by biogas.

  • Plausible Scenario: In this scenario, biogas represents 1.09 percent of the total electricity generated in 2050.
  • Drawdown Scenario: In this scenario, biogas represents 1.04 percent of the total electricity generated in 2050.
  • Optimum Scenario: In this scenario, biogas represents 1.08 percent of the total electricity generated in 2050.

Financial Modeling

Several data points were analyzed to determine the average capital cost: it is recognized that costs can vary significantly by region, but exhaustive regional data was not available to calculate an average cost weighted by installation size. Available data points were mainly from Organisation for Economic Co-operation and Development (OECD) countries, reflecting the preponderance of present-day biogas installations in the European Union and United States. The financial inputs used in the model assume average installation costs of US$5,901 per kilowatt (Karellas et al, 2010; BiogasIN, 2011; Biogas Association, 2013; IEA, 2015; Energinet, 2012). [5]

Due to the maturity of this technology, a learning rate of 2 percent was considered, similar to the one applied to conventional technologies such as coal and natural gas power. An average capacity factor of 92 percent was used for the solution, compared to 55 percent for conventional technologies. An average fixed operation and maintenance cost of US$50.6 per kilowatt, and variable operation and maintenance cost of US$0.05 per kilowatt-hour, were considered for this solution, compared to US$33.0 and US$0.004, respectively, for the conventional technologies.

Integration [6]

Through the process of integrating all electricity generating technologies in development, the total addressable market for electricity generation was adjusted to account for reduced demand resulting from the growth of more energy-efficient technologies, [7] as well as increased electrification from other solutions like electric vehicles and high-speed rail. Grid emissions factors were calculated based on the annual mix of different electricity generating technologies over time. Emissions factors for each technology were determined through a meta-analysis of multiple sources, accounting for direct and indirect emissions.

In addition, large methane digesters have a noteworthy impact on methane and nitrous oxide emissions avoidance that would have occurred due to anaerobic degradation, which is also accounted for in the analysis.

Results

Comparing the results from the three modeled scenarios to the Reference Scenario allows us to estimate the climate and financial impacts of increased adoption of biogas units for electricity generation. The Plausible Scenario projects 1.09 percent of total electricity generation worldwide coming from biogas by 2050 (i.e. 565 terawatt-hours, against 129 terawatt-hours in the Reference Scenario). In the Drawdown and Optimum Scenarios, the market shares are similar: 1.04 percent and 1.08 percent, respectively.

The climate and financial impacts for the increased adoption of large methane digesters with biogas power plants are both substantial. Plausible Scenario adoption avoids a total of 8.4 gigatons of carbon dioxide-equivalent greenhouse gas emissions from 2020-2050. Large digesters have a net operational savings of US$201.41 billion; while operational costs due to feedstock purchase, maintenance, and operational staff salaries are significant, this is still lower than the fuel cost of conventional technologies (coal and natural gas power plants). The Drawdown and Optimum Scenarios yield results in a similar order of magnitude, with reductions in greenhouse gas emissions over 2020-2050 of 8.1 and 8.3 gigatons of carbon dioxide-equivalent, respectively.

Discussion

The conversion of waste material in bio-digesters into biogas has several positive financial and environmental impacts for different levels of stakeholders: farmers, industries, municipalities, and governments. These systems enable the capture and use of methane while also addressing waste management and nutrient recovery needs. They can also realize several revenue streams and cost savings for owners.

Appropriate feedstock for electricity-generating biogas plants is available in adequate quantities across the world from sewage sludge and agriculture systems. However, there is significant uncertainty associated with the future adoption of these technologies, since they are linked to other waste management solutions including waste-to-energy, landfill methane capture, recycling, and composting, that could affect the balance between the available waste for each solution.


[1] For more about the Total Addressable Market for the Energy Sector, click the Sector Summary: Energy link below.

[2] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.

[3] To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Energy Sector-specific scenarios, click the Sector Summary: Energy link.

[4] The adoptions from three AMPERE models (GEM E3, MESSAGE, and IMAGE) on their 450 scenarios were used.

[5] All monetary values are presented in US2014$.

[6] For more on Project Drawdown’s Energy Sector integration model, click the Sector Summary: Energy link below.

[7] For example: LED lighting and heat pumps.

Full models and technical reports coming in late 2017.

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