Page 69 - Industrial Plant 2020

Figure 6 - Mars II – Natural gas flame – Thermal Duty : 35 Figure 7- Mars II – Natural gas+ H2 (80% volume) flame – Thermal
MWth Duty : 35 MWth

800 mm in the furnace. The air velocity coming out burner ejector since less reactive. Natural gas flame
from inner part of burner generates localized low- result of dark blue colour with some yellows parts
pressure conditions close to fuel injectors, which whilst NG/H flame is of pale blue and more transparent
2
promote the flue gas recirculation inside furnace. (low flame emissivity). In the next pages, NOx emission
Recirculated flue gas will “mix” with fuel creating a lean will be discussed.
fuel mixture. This principle is a driver to reduce NOx
emission of burner.
Respect to MHM, MARS II burner has demonstrated to Boiler furnace design
reduce NOx emission of approximately 50% without A considerable quantity of energy is exchanged, mainly
any external flue gas recirculation or steam injection. by radiation, from flames to boiler inside furnace. In
natural gas units, this value is around 30-40% of total
The principle of internal flue gas recirculation is a key heat input. Using co-firing natural gas – hydrogen,
aspect of hydrogen firing. The formation of lean mixture furnace temperature is higher with lower heat
mitigate high reactivity, promoting sufficient delay of exchanged in combustion chamber.
combustion far from burner. Conversely, without this
mechanism the injection of high amount of hydrogen For the scope of this simplified analysis, the following
inside air path (see figure 3) improves combustion rate equation and assumptions have been applied:
leading to localized high temperatures not tolerable by
burner materials. Flash back is also another potential Q = ϭ * ε * S * (Tf^4 – Tw^4) * fb (heat transfer duty to furnace)
issue present in this type of burner. The use of burner ε = f (C / H) (flame emissivity)
technology where fuel and air are de-mixed provides
best solution to handle high level of concentration of Q: exchanged duty S: Furnace projected surface
hydrogen ruling out overheating and flash back issues. Ϭ : Stephan Boltzmann constant Tf : Furnace outlet temperature (FEGT)
In the following pictures, flames with natural gas and ε : flame emissivity – as function of ratio C/H Tw : tube wall temperature
natural gas figure 6 and 7 hydrogen are reported. (C,H: Carbon, Hydrogen content in fuel mixture) fb : flames-furnace interaction factor

The principle of internal flue gas
recirculation is a key aspect of Evaluation has been carried out on a bi-drum boiler
hydrogen firing. The formation of with following characteristics:
lean mixture mitigate high reactivity,
promoting sufficient delay of Steam production kg/h 145000 Natural gas emissivity 0,37
combustion far from burner Pressure, barg 47 Hydrogen gas emissivity 0,15

Steam temperature, °C 400 Flame-furnace interaction factor 1.0
Despite of high quantity of hydrogen (80% volume H )
2 Furnace volume m^3 336 Air excess % 10
flame is well developed downstream ejector (from
figure 07) and there aren’t signs of over-heating. As Furnace surface m^2 261 Ambient temperature °C 25
expected, natural gas flame is located further the Number of burners 3 Air humidity % 60

IndustrIal Plants - May 2020
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