The carbon credentials of hydrogen gas networks and supply chains

Highlights

• Quantification and analysis of various hydrogen production supply chains.

• The range of emissions is extremely large, from − 371 to 642 gCO₂eq/kW h_H2.

• Fossil routes must include CCS to lower emissions compared to current gas grids.

• Supply chains are major contributors to emissions and large variations across routes.

• Only lowest carbon hydrogen routes compete with low carbon electricity heat pumps.

Abstract

Projections of decarbonisation pathways have typically involved reducing dependence on natural gas grids via greater electrification of heat using heat pumps or even electric heaters. However, many technical, economic and consumer barriers to electrification of heat persist. The gas network holds value in relation to flexibility of operation, requiring simpler control and enabling less expensive storage. There may be value in retaining and repurposing gas infrastructure where there are feasible routes to decarbonisation. This study quantifies and analyses the decarbonisation potential associated with the conversion of gas grids to deliver hydrogen, focusing on supply chains. Routes to produce hydrogen for gas grids are categorised as: reforming natural gas with (or without) carbon capture and storage (CCS); gasification of coal with (or without) CCS; gasification of biomass with (or without) CCS; electrolysis using low carbon electricity. The overall range of greenhouse gas emissions across routes is extremely large, from − 371 to 642 gCO₂eq/kW h_H2. Therefore, when including supply chain emissions, hydrogen can have a range of carbon intensities and cannot be assumed to be low carbon. Emissions estimates for natural gas reforming with CCS lie in the range of 23–150 g/kW h_H2, with CCS typically reducing CO₂ emissions by 75%. Hydrogen from electrolysis ranges from 24 to 178 gCO₂eq/kW h_H2 for renewable electricity sources, where wind electricity results in the lowest CO₂ emissions. Solar PV electricity typically exhibits higher emissions and varies significantly by geographical region. The emissions from upstream supply chains is a major contributor to total emissions and varies considerably across different routes to hydrogen. Biomass gasification is characterised by very large negative emissions in the supply chain and very large positive emissions in the gasification process. Therefore, improvements in total emissions are large if even small improvements to gasification emissions can be made, either through process efficiency or CCS capture rate.

Introduction

Natural gas networks have historically been a relatively lower carbon route to heat or electricity generation, because combustion emissions are lower than other fossil fuels: approximately 50 gCO₂/MJ HHV heat, compared to 90 gCO₂/MJ for coal. However, meeting 1.5–2 °C climate targets requires much deeper decarbonisation. Extensive natural gas infrastructure exists in many countries, resulting from many years of investment. If nations are to contribute to climate stabilisation, these gas networks must either be decarbonised or become stranded assets.

Decarbonisation pathways have typically involved reducing dependence on gas grids via greater electrification of heat using heat pumps or even electric heaters. However, there are several technical, economic and consumer related barriers to electrification of heat e.g. [1], [2], [3]. Additionally, the gas network may hold significant value in relation to its flexibility of operation and supply, requiring simpler control and enabling less expensive storage [4]. Therefore, there could be value in retaining and repurposing gas infrastructure if there are feasible routes to decarbonisation. There are several options for decarbonisation, including blending or replacing natural gas with hydrogen [5].

Hydrogen can be used as an alternative to natural gas for heat, electricity or transport and unlike natural gas, hydrogen combustion produces no direct CO₂ emissions. Nevertheless, the supply chain associated with hydrogen production and delivery is likely to be more complicated compared to natural gas, which may result in emissions and/or a loss in efficiency. Additionally, some infrastructure changes are required to replace hydrogen with natural gas due to the difference in physical properties, which will result in additional costs and system shutdown periods while the transition is made.

This study investigates the greenhouse gas impacts associated with the conversion of gas grids to deliver hydrogen. This review builds upon on an extensive evidence-based assessment produced by the Sustainable Gas Institute [6]. The paper focuses on the options associated with decarbonising the source gas within the network rather than reducing demand or decarbonising at the point of use. Previous studies have estimated GHG emissions associated with hydrogen production via specific feedstocks and production processes, whilst a few have reviewed a selection of processes in aggregate [7], [8], [9], [10], [11]. This study goes further by reviewing a large range of production options and supply chains, combining fossil and renewable feedstocks, including renewable electrolysis and biomass gasification, the use of carbon capture and storage, as well as the most important current and prospective production processes. Particular focus is on emissions associated with the upstream supply chain, in order to understand the contribution to the large range of emissions seen in the literature and give insight into how emissions may be reduced in the future.

The following first describes the options, current status and potential for producing hydrogen via various feedstocks and processes. For each option, Section 3 reviews the evidence on associated GHG emissions. Section 4 discusses the importance of supply chain emissions associated with the different routes and the potential to reduce emissions, prior to concluding remarks in Section 5.