Repurposing Natural Gas Pipelines into 4th Generation District Heating Network Pipes

Abstract

The European Union aims to achieve a net-zero greenhouse gas emissions economy by 2050 through interconnected strategies such as energy transition, material transition, and circular economy practices. District heating is crucial for decarbonizing the heating sector, integrating renewable energy and waste heat. Nevertheless, previous studies have indicated that the steel or polymer distribution pipes contribute the most to the environmental impact of the infrastructure, calling for improved design options. As the energy transition renders much fossil fuel infrastructure obsolete, it presents an opportunity for material reuse that has been little explored. This thesis explores whether repurposing steel natural gas pipelines to build 4th Generation District Heating networks is an effective option to reduce the overall impact of the infrastructure. The diameter of the natural gas pipelines to be repurposed depends on the availability of the stock. Therefore, this study conducts a technical and environmental assessment by comparing two functionally equivalent networks: one using conventional pipes sized specifically during the design phase, and the other employing repurposed steel pipes of varying diameters. Results show that while reusing these pipelines is technically feasible, accurate diameter selection is crucial. A 10% reduction in pipe diameter doubles the energy needed for water pumping, while larger diameters demand thicker insulation. Life Cycle Assessment results suggest that repurposing pipes with diameters close to the design specifications, combined with polyethylene foam insulation and high-density polyethylene casing, can cut CO2 emissions by up to 40%. Conversely, repurposing pipes with non-ideal diameters, along with factors such as a short service life, long transport distances, or unsuitable insulation and casing, may may reduce or eliminate these benefits. The findings of this research are synthesized into a decision flowchart, designed to guide district heating planners in evaluating the most sustainable options for repurposing natural gas pipelines.