Plasma-assisted combustion of hydrogen swirling flames: Extension of lean blowout limit and NOx emissions

Abstract

A key challenge in the use of hydrogen in practical combustors is to stabilize the flame at equivalence ratios close and even below the LBO limit. Plasma-Assisted Combustion (PAC) has been shown to improve the ignition, stabilization, and blowout margins of combustion systems for a great variety of fuels. In this work, Nanosecond Repetitively Pulsed (NRP) discharges are applied to extend the lean blowout limit (LBO) of a partially premixed hydrogen–air combustor atmospheric test rig equipped with a special unit injecting gaseous hydrogen in crossflow into a swirling air stream. The plasma is formed between this injector and an electrode placed in the central recirculation zone. The LBO limit of a 2.2-kW, V-attached flame is extended from 0.220 to 0.177 (20% reduction) using NRP discharges with a plasma power of about 1% of the nominal flame power. Spectroscopic measurements indicate that the plasma operates in the non-equilibrium spark regime: a low ionization degree is observed, and the temperature in the interelectrode gap is moderate (3500 K on average). It is found that plasma discharges increase the OH ∗ concentration at the flame foot, in the vicinity of the plasma location. Burnt gases analyzed at the outlet of the system indicate that low concentrations are being produced, even at stoichiometric conditions where the Emission Index reaches its maximum value of 1.1 g /kgH 2 . The application of the NRP discharges increases the concentration at all equivalence ratios. To minimize the power consumption and the increase, the NRP discharges are applied with a duty cycle where the plasma is discharged during about half of the time. It is shown that the concentration scales linearly with the plasma power and thus that emission induced by the plasma can be mitigated by 60% with this technique while still preserving the LBO extension.

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