Page 72 - Industrial Plant 2020

External Flue gas recirculation (EFGR) is a technique
where the combustion products are drawn back into
the burner. Typically, EFGR flow is branched off from
exhaust duct upstream stack, where temperatures are
lower, and are routed back to fan suction. Effect of
flue gas recirculation is primarily to reduce peak flame
temperature.

Figure 16- Principle of external flue gas recirculation

Common NOx reduction technology can be used
to compensate increase of NOx due to hydrogen

FGR effectiveness depends on many factors including
furnace dimension and flame interaction between burners.
Figure 17 shows EFGR trends and effect on NOx. As
example, 10% of FGR may reduce NOx of 50%.
Another way to reduce NOx emission is the steam injection.
It is a technique where steam is introduced in the burner to
reduce flame peak temperature; it is essentially expressed as
quantity of steam for unit of fuel (kg steam/ kg fuel).
Effect of steam injection is complex and shall be evaluated
case by case but NOx reduction could be in the range of
5%-10% for each 0,1 kg of steam/kg of fuel.
Figure 17- NOx reduction effect of flue gas recirculation

Hydrogen disadvantage on NOx formation may be Hydrogen has difficult confining since it is the
solved with well know NOx abatement techniques smallest element in universe, it is odourless and
like flue gas recirculation or steam injection or colourless.
combination of both. Therefore, for example, fuel gas piping components
As an example, considering a flue gas mixture (like shut off valves) shall be carefully designed.
containing 80% H volume, the use of approximately Also hazardous area classification and material
2
10% of recirculation or 0,5 kg/kg steam injection selection shall be duly evaluated during design of
could bring back the NOx emission to same level of boiler components.
natural gas.
The use of EFGR may significantly contribute to the
reduction of flue gas temperature at furnace outlet. Case history
As rule of thumb, 10% of flue gas recirculation could Macchi has been designing boiler firing hydrogen since
reduce furnace temperature of approximately 100 many years.
°C. In some cases, this could avoid furnace size Boilers for petrochemical plants burning refinery gases
increase. are the most common applications.
Refinery gas has typical concentration of H of
2
approximately 20-40 % volume. In the following
Other aspects diagram, hydrogen experience in industrial boiler is
Use of hydrogen shall deal with safety aspect. reported.

IndustrIal Plants - May 2020
70