Decarbonisation Technology May 2022 Issue

Risks of firing hydrogen and high hydrogen blends vs methane only Any time an operator changes fuel sources, the implication on process safety is an important consideration. This is especially true when switching to pure hydrogen and high hydrogen fuels. In particular, hydrogen has several physical and combustion properties that differ significantly from methane and other hydrocarbon-based fuels, as shown in Table 1 . While some of these differences may impact the burner directly, others can be monitored using flue gas analysis to ensure safe and efficient combustion control. From the standpoint of physical properties, hydrogen is a very light and fast diffusing gas. Compared with methane, diatomic hydrogen is a very small molecule with 8X less mass than methane, which contributes proportionally to its 8X lower density. Because of its low molecular weight, hydrogen also exhibits very fast molecular speeds that are almost 3X faster than methane at the same temperature, meaning that hydrogen diffuses nearly 3X faster than methane. Altogether, these properties depict the inherent nature of hydrogen to move very quickly in a confined space, such as a combustion chamber. From a safety perspective, unburnt or leaked hydrogen would move much faster in the firebox than methane in the event of a fuel leak or loss of flame, and flue gas analysis is one option to detect and respond to these unsafe conditions. From the view of combustion properties, hydrogen is a very reactive and fast-burning gas. Notably, hydrogen displays extremely fast flame speeds that are 10X faster than the flame speed of methane, partly driven by its very fast molecular speeds. Hydrogen flames are also much shorter and hotter burning than methane, which could increase NOx emissions and impact any metal parts used within the burner throat. Hydrogen also has an extremely low minimum pre-ignition energy threshold, which poses a risk of flashback if run at high enough concentrations, especially in premix burners. From an operational standpoint, high hydrogen fuels have several implications at the burner and in the combustion control system. In particular, the fuel flow rate of hydrogen fuel will change considerably if measuring the flow

Property

Hydrogen

Methane

Molecular weight (g/mol)

2.02

16.04 0.717 1365 5-15

Density (kg/m³)*

0.0899 3848 4-75 12,109 142,081

Avg. molecular speed (m/sec) ** Flammability limits (vol%) Gross heating value (kJ/m³) Gross heating value (kJ/kg) Flame speed (cm/sec) Adiabatic flame temp. (°C) Autoignition temp. (°C) Min. ignition energy in air (mJ)

37,669 55,384

300

2254 585 0.02

1963

600

0.29

* At standard temperature (0°C) and pressure (1 atm, 101.325 kPa, 14.7 psia ** Based on the root mean squared molecular speed at 926°C

Table 1 Physical and combustion properties of hydrogen compared to methane

combustion processes provide heat, power, and steam to the entire plant. Electrification provides a window to remove emission sources entirely, but it also presents a challenge with existing plants that have many combustion processes (especially processes with very high temperature) or limited availability of external power sources. As indicated, the challenges of decarbonisation often arise when the combustion processes are small, decentralised, and scattered across many point sources and when large amounts of energy are consumed for high-temperature process heating. For this reason, many operators are considering the use of hydrogen fuels to reduce the carbon emissions from their combustion processes. In many cases, hydrogen fuels are more often readily implementable for existing equipment and more affordable to decarbonise than other options. Operators may consider firing pure hydrogen or blending the hydrogen with their natural gas to achieve their near- term and long-term emission targets. Some plants may still need to modify their burners or upgrade their piping material to handle high hydrogen fuels. However, with these modifications, operators can continue to use their existing assets and leverage the hydrogen fuel to offset their carbon emissions directly to atmosphere.