Hydrogen Blend Causes Burner Instability

Hydrogen changes both the fuel properties and the burner aerodynamics. Its laminar flame speed is much higher than methane's, so the flame tends to move upstream toward the burner head. Its volumetric heating value is low, so for the same heat input the gas volume flow is higher; the blend LHV of about 31 MJ/m³ means roughly 15% more volume flow than natural gas. That changes the gas velocity at the burner ports and can disturb the air-fuel mixing that was set for methane. The flame is also more reactive, so it can stabilize closer to the metal, raising throat temperature.

The 0.6% O₂ is too low for stable combustion, especially with a fuel whose mixing is already disturbed. The 800 ppm CO and fluctuating flame signal show incomplete combustion and an unstable flame, not simply a dirty detector. The pull-back toward the burner head is the most serious symptom: it indicates a flashback risk. The existing gas train was not designed for hydrogen service, and the absence of hydrogen detection means a leak would not be detected by the current safety system.

The trial should not continue at 70% load in this condition. Reduce firing immediately and increase excess air toward 3–4% O₂ while watching CO, flame signal and throat temperature. If the flame signal remains unstable, if CO rises further, or if there is any sign of flame holding at the burner head, trip the burner and purge before any restart attempt. Do not try to compensate by raising gas pressure or by forcing more fuel; that increases the risk of flashback and uncontrolled combustion.

Before any further trial, the burner and gas train must be reviewed for hydrogen service. Check flame detector response, burner port velocity, gas pressure control, and whether the single shutoff valve arrangement meets the applicable gas safety standard for the blend. Add hydrogen detection and ventilation checks if the boiler house is enclosed. A controlled trial should start at low load with a written test procedure, stable flame signal, CO below the site limit and O₂ in the recommended band. Without these controls, the correct judgement is to stop the trial rather than accept a week of unstable operation.

First, obtain burner manufacturer approval for the maximum hydrogen fraction, including the allowable port velocity and flame speed margin. Re-tune the burner on the actual blend, starting at low fire, with calibrated gas and air flow measurement. Set conservative trip points for flame signal, CO, high throat temperature and low excess air. Confirm that the flame detector sees the hydrogen flame reliably; some detectors respond differently to hydrogen.

Second, review the gas train for the blend: check valve and regulator materials, tightness, venting and the low-pressure switch setting, which will change because the volumetric flow is higher. Add hydrogen detection and alarm in the boiler house, and verify ventilation. Third, establish operating limits: do not exceed the tested hydrogen fraction, load range or O₂ band. Record the flame signal, CO, NOx and throat temperature at each load. A retrial should be short, supervised and followed by an inspection of the burner head and refractory. If any limit is approached, stop and re-tune rather than widen the limit.