Many new global regulations require steam generators to reduce their carbon monoxide (CO) and carbondioxide (CO2) emissions. The two primary methods to reduce CO and CO2 emissions are toeither capture and sequester the carbon in the fuel gas orremove the carbon from the fuel before firing.Carbon capture from the fuel is becoming the more cost-effective method. Removing carbon before firing involves reforming natural gas – mainly methane (CH4) – and capturing the carbon atom while utilizing the hydrogen (H2)atoms as a fuel source.Capturing the carbon prior to combustion eliminates the need to outfit each boiler with costly equipment.
Fuel cost instability also pushes end users to consider alternative fuel sources they may already have, such asH2 leftover from various reforming and refining processes. TheH2 can be injected into the fuel gas stream to supplement the main fuel supply. Burning H2in steam generation systems can greatly reduce operating fuel costs while also meeting new carbon emissions regulations.
Burner Design
The combustion characteristics of H2 are vastly different from those of natural gas.The flame speed in H2combustion is approximately 5.7 ft/s, while natural gas’s flame speed is only 1.3 ft/s.H2 firing also has a higher stoichiometric adiabatic flame temperature(3860°F) thannatural gas(3518°F). These differences require engineers to evaluate proper burner types and construction materials.
Typical burner construction comprises metal components and a refractory throat or tile. The steel used to construct the nozzle, throat, and flame stabilizers needs to be a high-grade stainless or alloy to withstand elevated operating temperatures and prevent hydrogen embrittlement and high-temperature hydrogen attack. Furthermore, engineers need to evaluate the burner’s refractory and modify its composition to withstand elevated H2 temperatures.
Hydrogen’s flame speed is nearly five times that of natural gas.Burners utilizing premix, lean premix, or rapid premix designs are not suited for a fuel stream varying in H2 composition.As the composition of H2 increases in the fuel stream, these types of burners become more susceptible to flashback, which may damage burner components.
Emissions
Hydrogen’s high flame propagation speed allows for a more rapid combustion process thannatural gas.The rapid process releases combustion energy in a small area, leading to localized elevated near-flame region temperatures, which compound the effect of the inherentlyhighadiabatic flame temperatures on NOx emission rates.Any region with elevated temperatures above 2500°F is conducive to NOx formation.Field and test facility data haveshown that standard low-NOx burners firing H2 typically exhibit an increase in NOx emission rates up to a factor of three.
Flue Gas Recirculation (FGR), steam injection, and ultra-low-NOx(ULN) burner technology can decrease NOx.FGR is the process that diverts a portion of the flue gas exiting the boiler (typically after the economizer) and introduces it into the combustion air supply. The combustion air supply dilutes with spent combustion products, lowering peak flame temperature during combustion. Small quantities of carefully placed steam injection may also help polish NOx by cooling the flame and introducing a small amount of inerting.
Staged ULN burnersgenerally use both air and fuel staging mechanisms to decrease peak flame temperature.Properly staged fuel increases the amount of furnace gas able to entrain into the fuel stream before interacting with the air.Properly staging air within the combustion zone delays fuel and air mixing, stretching the combustion process over the furnace’s length.The drawn-out combustion process decreases the overall peak combustion temperatures, thereby reducingNOx formation.
The H2content in the fuel stream also has a significant impact on CO and CO2 emissions. As H2 replaces hydrocarbons in the fuel composition, the number of carbon atoms decreases. A fuel stream composed of 100% H2 cannot generate CO nor CO2 as a byproduct of combustion due to the lack of carbon in the combustion reaction. Therefore, the higher the H2 content of a fuel, the lower the overall CO and CO2 emissions.
Boiler Impact
When considering a new fuel, a boiler impact study can ensure no detriments to boiler performance.The combustion characteristics of H2may change where and how radiative and convective heat transfer occurs within the boiler, which can adverselyimpact steam generation rates and steam temperatures.
H2can operate with a lower excess air ratio than natural gas due to its higherflammability limit.A lower excess air ratio further reduces the required mass flow of air as compared to natural gas.High H2 firing temperaturesalso increase the Furnace Gas Exit Temperature (FEGT).
When firing H2, the resulting mass flow reduction through the boiler, combined with higher FEGT, can adversely impact the boiler’s convective heat transfer portions, jeopardizing both steam production and steam quality.However,the additional FGR mass flow lowers the FEGT and negates any adverse effects on convective heat transfer.
Instrumentation & Controls
Any burner designed for fuel composition spanning from natural gas to high H2 contentshould have a fullymetered combustion control system coupled with a Wobbe Index meter or Specific Gravity meter. The Wobbe Index meter monitors the varying fuel stream composition and provides the necessary input to the control system to properly adjust the fuel/air ratio. The inability to monitor the fuel stream composition and adjust the combustion control system accordingly can lead to a potentially unsafe, fuel-rich condition.
Engineers should also evaluate the fuel delivery equipment upstream of the burner for capacity constraints asH2 requires three times the volumetric fuel flow of natural gas to provide an equivalent heat release. Engineers must evaluate the sizes of pipes and fuel train components to ensure proper operation with any fuel – especially in combination with H2.
Flame detection is a critical burner safeguard, and selecting the proper equipment is crucial to burner performance. When H2 is present in the combustion process, it generates water vapor. As the H2 content approaches 80% in the fuel stream, most flame scanners available today have difficulty distinguishing and verifying the flame due to the high level of water vapor present.
Other considerations need to be analyzed to ensure the safe firing of H2while meeting the environmental limits of the operating jurisdiction. Consultation with an experienced burner supplier, well-versed in H2 firing, is essential in securing your success.
For more product related information please email [email protected]
Mr John Guarco with Bob Langstine and Mike Turner, Zeeco Connecticut, USA
