Hydrogen as a Path to Sector-Coupled Deep Decarbonization
Authors: Prasad Vinayaka Nakul 
Published: November 2020

Abstract
Hydrogen is an essential feedstock for a variety of chemical and industrial processes. Refineries with a global share of over 30% are amongst the largest consumers. Traditional methods of generating hydrogen involve the reformation of fossil-fuel sources with the help of steam. These methods release CO2 as a side product and are thus carbon-intensive. A zero-carbon approach of producing hydrogen can be achieved via electrolysis of water powered by surplus renewable energy sources, which in return helps to balance the intermittency of solar photovoltaic (PV) or wind. Hydrogen is also often a by-product of industrial processes. These synthetic waste gases have typically been flared in the past. Burning these gases in gas turbines instead can significantly boost the economic case and reduce carbon emissions compared to flaringbecause of the utilization of the waste energy.
Gas turbines are typically designed for natural gas operation, and accommodating high levels of hydrogen poses significant challenges due to its different physical properties. First, hydrogen is the lightest molecule with a lower volumetric energy content and higher diffusivity. This has an impact on the fuel delivery system, as sealings and piping materials need to be upgraded. Second, local mixing between fuel and air may not be perfect as hydrogen flames tend to stabilize further upstream, where mixing quality is lower and are more compact. As hydrogen has a higher flame temperature, local hotspots can lead to higher NOx emissions. This, in return, may require performance adaptations to meet the local emissions standards.
Perhaps the most challenging aspect of hydrogen use in turbines is its significantly higher reactivity. Hydrogen has a substantially higher flame speed (up to 10 times) and lower ignition delay time than natural gas, which increases the risk of flashback and explosions.
To overcome all these challenges and guarantee safe operation with high hydrogen fuels, focused development is required, particularly with regards to the combustor. Following an iterative rapid prototyping approach, the design optimization is typically achieved via high-fidelity ComputerAided Engineering(CAE) simulations coupled with validation through high-pressure testing at engine conditions. Here, the use of Additive Manufacturing (AM) for generating prototypes has been a critical success factor in recent years. In addition to reducing the overall lead time by up to 70%, AM offers the opportunity to generate and manufacture more efficient aero designs for e.g., cooling and fuel routing schemes.
This paper focuses on the use of hydrogen in gas turbines and discusses the required development steps. General challenges of accommodating hydrogen in gas turbines and the implications on design modification and operation will be examined in detail. Examples of achieving up to 100% hydrogen operation on a 25MW scale gas turbine from recent testing programs will be subsequently presented. Use cases of gas turbines operating with high hydrogen fuels in industrial processes will alsobe discussed.