‘Unburnable carbon’ refers to fossil fuel reserves that cannot be used and the resulting greenhouse gases emitted if the world has a limited ‘carbon budget’. This situation leads to the question: what role does technology have in addressing these concepts and concerns related to them?

The International Energy Agency (IEA) and the Intergovernmental Panel on Climate Change (IPCC) both mention the role of carbon capture and storage (CCS) as a way to preserve the economic value of fossil fuels in carbon-constrained scenarios. Organisations such as Carbon Tracker Initiative (CTI), the Smith School Stranded Assets Programme (Oxford University), and University College London (UCL) have recently assessed these topics. These include assessments of the role of CCS, suggesting it will have an insignificant impact on the amount of the world's fossil fuel resources that can be utilised in a 2°C climate scenario. However, some of these reports view CCS from a resource-limited perspective, for example taking conservative views of the amount of CO2 storage capacity available and on availability of CCS before 2050.

Therefore, IEAGHG has commissioned a study on this topic to the Sustainable Gas Institute. The study has undertaken an assessment of the relevance of CCS in terms of the ‘unburnable carbon’ concept. This study does not aim to assess or provide evidence of the ‘unburnable carbon’ concept but rather to look at the role of CCS technologies in such concepts. This report will also not evaluate other approaches to reduce CO2 emissions from fossil fuel use other than CCS, such as hjosir efficiency low emission (HELE) technologies.

• The global ‘carbon budget’ in emission scenarios for climate change mitigation implies that a certain amount of fossil fuel reserves should not be used and their resulting greenhouse gases emitted to atmosphere. This concept is often referred to as ‘unburnable carbon’.
  
  
• As carbon capture and storage (CCS) is a technology that prevents or reduces the emissions of CO2 to the atmosphere, it has the potential to enable use of fossil fuels in carbon-constrained scenarios.
  
  
• In order to evaluate the potentially unburnable carbon of fossil fuel reserves, it is necessary to estimate the overall remaining fossil fuel reserves and compare them with the global carbon budget.
  
  
• Integrated assessment models (IAMs) are a good means to evaluate carbon budgets as they have a large coverage of technologies, geographical scope, economics and climate data. These models are widely used in publications of the Intergovernmental Panel on Climate Change (IPCC), the International Energy Agency (IEA) and academia, and most of them cannot achieve a 2°C or lower scenario without CCS. This report selects and investigates a subset of models that focus on technology options and include CCS.
  
  
• This study does not aim to assess or provide evidence of the ‘unburnable carbon’ concept but rather to look at the role of CCS technologies in this regard. It will assess the assumptions, methodologies, any contentious subjects and differences related to this topic.
  
  
• This study found that the impact of CCS on unburnable carbon appears to be material up to 2050 and further increases up to 2100. This applies especially to coal but also to gas to some extent
  
  
• Model assumptions and cost data availability do generally not limit uptake of CCS in IAMs. However, other reasons seem to limit CCS uptake in models, and the authors of this report hypothesise it could be that residual emissions from CCS, for which CO2 capture rates of 85-90% are usually assumed, are the reason. It is recommended to investigate this further and to give consideration in R,D&D to increasing capture rates.
  
  
• Uncertainties in IAMs and fossil reserve estimates can influence the total amount of carbon considered as unburnable.
  
  
  
• The authors review estimates of global CO2 geological storage capacity, and find that estimates obtained from volumetric approaches are large and well above the extent of the CO2 emissions related to fossil fuel reserves.
  
  
• Storage capacity estimates from dynamic approaches are likely to be lower, and hence further work on improving dynamic storage efficiency, such as pressure management by brine extraction, is required.
  
  
• The related additional costs for pressure and brine management should be considered in IAMs.