Page 70 - Industrial Plant 2020

Keeping same furnace size, furnace flue gas outlet temperature
increases considerably especially for H content greater that 70%.
2
Higher flue gas temperature require careful evaluation of super-
heater thermal performance and re-evaluation of metallurgy of
tubes. The use of hydrogen, generally, requires more combustion
chamber surface to keep temperatures within typical industrial
limit (around 1400 °C-1450 °C). Nevertheless, in case of high
concentration, the furnaces size incresing may lead to expensive
design of boiler and external flue gas recirculation (EFGR) shall
be an unavoidable option. Principles and effects of EFGR will be
discussed in the next pages.

Figure 8- Effect of Hydrogen on furnace outlet temperature

Combustion control Final scope is to maintain overall air/fuel ratio
If hydrogen concentration in the fuel blend is within burner limits avoiding sub-stoichiometric
constant, combustion control system does not conditions.
require any particular precaution. Very often, its
content in fuel stream is variable and hence fuel Following diagrams show fuel mixture parameter as
mixture properties at boiler entrance changes with function of hydrogen concentration.
the time. For this reason, combustion control logics
shall consider more complex algorithms to evaluate The Figure 12 suggests the principle of combustion
the correct air-flow demand based on flowrate and control for hydrogen firing boiler.
characteristics of fuel. In conclusion on-line densimeter may provide evaluation

Figure 9- Combustion air demand for fuel mixture Figure 10- Fuel gas mixture density

In a fuel blend, specific air demand (required air for unit of
energy of fuel input) is not constant. Therefore, if hydrogen
content changes during boiler operation, air demand need
to be corrected with a factor depending on H content.
2
Since normal density of fuel mixture is linear with hydrogen
content and low calorific value, density value, obtained
through on-line densitometer, may provide signal to
combustion logic to perform required correction.

Figure 11- Low heat calorific value of fuel gas mixture vs density

IndustrIal Plants - May 2020
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