Optimizing renewable natural gas from waste for air quality and climate benefits

Abstract

Greenhouse gas (GHG) and air pollutant (AP) emissions threaten both the climate and human health. Food production, specifically agriculture, is a notable contributor of both kinds of emissions in Canada. Ontario, Canada produces a significant amount of livestock waste from its agriculture sector, as well as food waste from its population centers. The conventional waste management of livestock manure and food waste each significantly emit a number of GHGs and APs. One proposed solution is to divert these organic wastes into anaerobic digestion (AD) to produce renewable biogas.

Recently AD has been a growing topic of interest in research, with numerous life cycle assessments generally agreeing that biogas can offer environmental benefits. There is also recent interest in applying optimization to various components of system design, such as resource allocation and organic waste collection and transportation routes. There is a notable gap between research focused on the environment and health implications, and research on optimizing system design.

This study explores the air quality and climate change implications of optimally deploying renewable biogas in Ontario. We develop a mixed integer linear programming optimization model to address the questions: what are the GHG and AP benefits of a cost-minimized biogas system, what is the value and effect of minimizing instead for GHG and AP emissions, and finally, what is the overall optimal system when accounting for atmospheric impacts in cost estimation?

Available livestock manures and food waste under entirely conventional manure management (i.e., no biogas production) generates an estimated 3.6 megatonnes of carbon dioxide equivalent (Mt CO2e) GHG emissions and a cumulative 24.7 kilotonnes (kt) of AP emissions per year. Using the social cost of GHGs and spatially explicit benefits per ton (BPT) of air pollutants, we estimate these emissions to cause $2.8 billion (2026 CAD) in annual damages.

Diverting the waste entirely to AD could generate enough renewable natural gas to offset approximately 3% of Ontario's natural gas demand. Using the overall optimization based on social cost, we find the greatest reductions to social costs are realized with partial biogas implementation producing about 20% to 30% of the potential energy, depending on the discount rate applied.

We demonstrate the potential to increase benefits from biogas by spatially optimizing system design according to climate and air quality impacts. We also highlight the importance of considering spatially varying damages of air pollutants as it influences system design. The results of this study imply that AD may have net social benefits that could be incentivized with additional policies.