23,000
3%
CO
8%
O
22,000
17%
N
21,000
H O
20,000
19,000
72%
18,000
0 10 20
30 40 50
100 60 70 80 90
Vol% H in fuel
2%
15% excess air, blended
100% CH stoich air
O
N
can react with the carbon to form methane, which can cre- ate pressure and cracking.⁸ Fuel gas delivery systems are typically not operating at high enough pressure for HTHA to be a concern. High-strength steels with high hardness numbers can be susceptible to HE. According to API 571, the effect is pronounced at temperatures from ambient to about 300˚F, corresponding to the range of most fuel gas systems. 8 While the softer carbon steel piping typically used is gen- erally not susceptible to HE, welds, bolted connections, and burner internals made of high-strength steel may be areas of concern. Careful review of the fuel gas delivery system should be performed before switching to H₂ fuel. Safety and controls Hydrogen has a wide range of flammability in air, approx - imately 4-75% by volume. 9 These properties give H₂ the National Fire Protection Association’s (NFPA) highest flam - mability rating of 4. 9 While methane also has a flammability rating of 4, it requires more energy to ignite and is easier to detect. Due to its low molecular size, H₂ may be susceptible to leakage in fuel gas piping designed for hydrocarbon fuels. H₂ gas is colourless and odourless. Due to its high diffu - sion rate in air, H₂ does not work well with the compar - atively heavier mercaptans added to methane to give it a smell. Therefore, H₂ leak detection instruments may need to be added to a fuel gas system before switching to 100% H₂ firing. H₂ fuel also requires significantly less air to combust sto - ichiometrically compared to methane. This is a key point to remember, especially if switching between H₂ and methane fuels while the heater is in operation. When firing H₂, the heater controls should be adjusted to reduce the combustion air. However, if a switch back to methane fuel is made quickly, without sufficient adjustments to increase combustion air, the methane will not fully combust and create a potentially unsafe, fuel-rich environment. Figure 6 demonstrates this Figure 6 Volumetric combustion air flow rate with increasing vol% H 2. Assumes a 100 MMBtu/hr (fired duty) heater, balance of fuel is methane, and 15% excess air
H O
31%
67%
phenomenon. The horizontal line represents the airflow required for stoichiometric combustion (just enough to combust the fuel with no excess air) of methane, while the declining line represents the airflow with 15% excess air at varying H₂ percentages. As shown, although excess air is at 15%, operation above approximately 90 vol% H₂ represents a sub-stoichiometric region for methane. A fuel gas system that accurately measures fuel gas com- position and controls combustion air accordingly is thus critical. Burner piping and fuel gas skid sizing For a given heat release, 100% H₂ fuel requires significantly less mass flow rate than methane. However, the volumetric flow rate required is more than three times as high. As such, the fuel gas skid and burner piping leading to the heater should be carefully reviewed to ensure proper pipe sizing for acceptable hydraulics. Stack plume The key variable contributing to the visibility of flue gas exiting a stack is its water content. 10 After exiting the fired heater, hot flue gas cools down, and the water present in the flue gas condenses, often creating a visible, white ‘fog’. Although not necessarily harmful, this visible water plume may become a nuisance to neighbouring communities and act as a transport medium for other pollutants in the gas. Figure 7a (top) Vol% flue gas composition with 100% methane fuel and 15% excess air; 7b (bottom) Vol% flue gas composition with 100% H₂ fuel and 15% excess air
52
PTQ Q3 2023
www.digitalrefining.com
Powered by FlippingBook