Hydrogen fuels and gas-fired appliances

In today’s eco-conscious environment, companies must reduce greenhouse gas emissions and invest in new technologies to minimize their environmental impact. In the appliances sector, an energy transition is ready to start with the target of zero carbon dioxide (CO2) emissions and reducing the use of natural gases. The main challenge is to increase safety and efficiency of the appliances while operating with new fuel types, like hydrogen.

As the world takes steps to decarbonize its energy, hydrogen is being considered as a key contributor to decarbonization efforts. This is specifically applicable for fuel gas appliances — both those using hydrogen as a blend with natural gas and potentially as a standalone fuel gas. Globally there are hundreds of projects assessing feasibility, performance and limits — not just for the effects on appliances but for the ability of the distribution infrastructure to deliver blended fuel to consumers safely. Many utilities are at the early stages, working to characterize the amount of hydrogen naturally occurring in their natural gas while piloting hydrogen blends in the low to mid-single digits. At the other end of the spectrum are projects like Leeds City Gate in the United Kingdom (U.K.), which is studying a transition to essentially pure hydrogen. Based on studies and reports examining the effects on performance for existing natural gas appliances, results show general favorability for approximately 20% hydrogen blends with natural gas. The volume and variety of these projects are motivated by interests as numerous as the utility companies themselves. They are influenced by of various factors, ranging from government targets and international agreements to environmental, social and governance (ESG) interests and corporate social responsibility (CSR), company metrics for carbon reduction and shareholder activism. These interests are underpinned by a need for the utilities to provide safe and reliable energy sources. Gas-fired appliance manufacturers are trying to design products capable of performing safely across a spectrum of outcomes while recognizing that multiple solutions may be necessary for different ranges of hydrogen blends. Additionally, this is all occurring in front of a backdrop of interests that support electrification rather than gaseous fuels as the way to reduce carbon emissions.

Why replace natural gas (methane) with hydrogen?
At the stoichiometric level, the combustion of a methane molecule (CH4) in the presence of oxygen (O2) creates two products: CO2 and water vapor (H2O). Stoichiometry is a sort of “perfect world” calculation, allowing chemistry theory to predict the reaction that will occur. Other carbon-based emissions may also manifest, such as carbon monoxide (CO), the creation of which can be influenced by appliance design and environmental conditions. Natural gas is otherwise a cleaner alternative to other carbon-based fuels, such as coal. Still, recent attention to reducing carbon emissions has focused on creating carbon dioxide when burning methane and, thus, an effort to reduce or eliminate the amount of CO2 created during the combustion process.

CH4  +  2O2  →  CO2  +  2H2O

When burning hydrogen in the presence of oxygen, a similar stoichiometric assessment results in one product: water vapor. Utilities are looking to replace percentages of natural gas with hydrogen as a way to reduce end-use carbon emissions, focusing on maintianing the safety and performance levels of new and existing appliances.

2H2  +  O2  →  2H2O

What are potential performance concerns for hydrogen blending?
Numerous studies have been conducted to understand the impacts of blending hydrogen with typical gaseous fuels and primarily natural gas, predominantly methane. As the lightest element on the periodic table, hydrogen is less dense than methane, with a lower calorific (heating) value of about 1/3 compared to a similar volume of methane. The flame temperature for hydrogen is also about 10% hotter when compared to methane. This combination of factors supports the industry consensus that about 20% hydrogen or lower mixtures are compatible with existing burner/appliance designs. However, as the H2 percentage increases, so does the potential for undesirable phenomena and risk. Hydrogen is less dense than typical fuel gases, with a higher flame speed. For a burner/appliance optimized for performance with a certain type of gas, introducing a gas with a higher flame speed can result in the flame burning closer to the port. This can increase the burner’s temperature. More severe circumstances allow the flame to propagate inside the burner. The condition is called light back, or flashback and will lead to incomplete combustion of the gas, significantly diminished performance and potential damage to the burner and appliance. Light back can be verified visually and sometimes audibly with a noticeable rumble as the flame burns inside the port. Certain product types incorporate flame supervision devices (FSDs) that interrupt the gas flow if the burner is inadvertently extinguished. An FSD that senses temperature is not expected to be impacted. However, devices that rely on other principles, such as radiation generated as part of the combustion process, may be affected. From the appearance standpoint, hydrogen flames emit little visible light, which contrasts the signature blue natural gas flame. The effect is relatively limited for lower percentage hydrogen blends but will be more noticeable for higher concentrations of hydrogen. Where safety is partly managed by the ability to see the flame, such as for a hob or cooktop burner, additional safeguards may be needed at the appliance to promote safe appliance operation when using elevated levels of hydrogen. For gas-fired appliances, a performance assessment is necessary to examine the effects resulting from the burner’s design. Of particular interest are the flame characteristics, which are related to port sizing or the burner openings intended to support a flame. Ports are deliberately spaced apart from each other but are also carefully sized to accommodate gas supplied at a particular pressure. That pressure results in a flow of gas through the burner’s ports that accounts for two velocities: 1) the speed of the gas exiting the burner through the port; and 2) the speed of the flame created from a continuous supply of gas. When designed correctly, a balance results between these two velocities, manifesting in the presence of a stable flame at each port.

What comes next?
It is difficult to recall an era marked by as much disruption as what gas-fired appliance manufacturers face today. Possible gaseous fuels span from no hydrogen to potentially all hydrogen, with various blended percentages in between. The mix of gas-fired and electric appliances may also differ in as little as five years, with some willing to bypass the discussion of hydrogen altogether in deference to electric alternatives. Is it possible that some gas utilities will consider another interim step from the manufactured gas era? Also known as town gas or coal gas, this fuel was widely available over a century ago but gradually fell out of favor, partly due to containing elevated percentages of carbon monoxide. Today there is also much focus placed on where fuel comes from and how it is created, so it’s not necessarily that coal gas will make a comeback. But manufactured gas also contained approximately 50% H2, which could provide some historical perspective for transporting and using something closer to a 50:50 blend of hydrogen and methane. As manufacturers and their suppliers seek to develop solutions, research and test laboratories can be important contributors and validators, simulating and blending gases to determine how burners/appliances will operate with a range of current and future fuel gases. These test results can be used to inform regulators and safety standard technical committees where changes are necessary in installation codes and gas-fired appliance standards. While some combustion engineers rely upon reference materials from decades ago, there may be tools even further in our past that hold keys to our future. Not every innovation will be as simple as the old becoming new, but it is clear that the hydrogen fuel era will mark another cycle of change for the appliance industry. Partnering with UL Solutions, a global safety science protagonist, for advisory, benchmarking and certification services can help build customer confidence and trust in products that manufacturers are launching on the market.