Hydrogen: potential superfuel? High hydrogen fuels have the potential to dramatically reduce CO 2 emissions compared to conventional fuels, depending on how the hydrogen is produced
Chuck Baukal, Bill Johnson, Michel Haag, Gilles Theis and Matt Whelan John Zink Hamworthy Combustion, a Koch Engineered Solutions Company
H ydrogen is a colourless, odorless, non-toxic, highly reactive, flammable, diatomic gas that is the most abundant element in the universe. The global demand for hydrogen is increasing rapidly. 1 Interest in hydrogen as a fuel continues to grow, mostly because it generates water when combusted, with little if any carbon dioxide (CO 2 ) generated, depending on how it is produced. Figure 1 shows the more H 2 in the fuel, the less CO 2 produced. In the past, using hydrogen as a fuel was only economical in certain applications. Reducing CO 2 emissions and concerns about fossil fuel depletion are two major recent drivers for considering high hydrogen fuels for more applications. 2 Hydrogen has the potential to be an important fuel depending on how it is generated. 3 Today, large scale hydrogen production is by steam methane reforming (SMR). SMR furnaces generate CO 2 emissions. If those emissions are captured, this is referred to as ‘blue hydrogen’. Oil and gas producers are particularly well positioned to produce blue hydrogen because natural gas is relatively low cost and because those producers have the infrastructure to produce hydrogen. 4 If the CO 2 is not captured, this is referred to as ‘grey hydrogen’. Hydrogen made with renewable energy, such as wind or solar energy powering an electrolysis process, is referred to as ‘green hydrogen’. While there is increased interest in using hydrogen as a fuel, high levels of hydrogen have been used in some applications such as boilers and fired heaters for many years. For example, the off- gas from ethylene crackers using natural gas as the feedstock is 70-85% H 2 which is used as fuel for the cracking furnaces. There are important issues to consider for new applications using high H 2 in the fuel. For example, hydrogen has a higher potential for leaking from fuel delivery systems, a very wide flammability range,
80% 90% 100%
CH RFG C H
40% 30% 20% 10% 60% 50% 70%
0%
0%
20% 40%
60%
80%
100%
H in fuel gas (vol. %)
a higher flame speed, a lower volumetric heating value, less air required per unit heating value, and a higher adiabatic flame temperature compared to other fuels. These issues impact the combustion system, which means it may not be a simple conversion from existing hydrocarbon fuels to high hydrogen fuels. Hydrogen combustion Hydrogen is an element whose natural state at Figure 1 CO 2 reduction with hydrogen addition to various fuels (RFG = refinery fuel gas = 20% H 2 , 55% CH 4 , 10% C 2 H 6 , 10% C 3 H 8 , 5% C 4 H 10 ). Assumes no CO 2 is produced making the hydrogen
Property
H 2
CH 4
C 3 H 8
16 44 35.8 91.2 50.0 46.3 1878 1904 44.8 46.4 5-15 2.1-9.5
Molecular weight LHV, MJ/Nm 3 LHV, MJ/kg Adiabatic flame temperature*, °C
2 10.8
119.8 2048
325 4-74.2
Max. flame speed, cm/s Flammability limit, vol% air
* based on 10% excess air, air temperature: 20°C.
Table 1
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