Solving Noise Problems in the Process Industry

Combustors have been known to “sing” or to “roar” in various applications under diverse
operating conditions. This paper is aimed at investigating the underlying physics controlling
noise in combustion equipment. Last year, we presented a paper at the virtual AFRC meeting
discussing the thermal-acoustic coupling inside a Reaction Furnace of a Sulfur Recovery Unit
used in refineries to recover elemental sulfur [1]. Early work related to thermo-acoustic coupling
was observed by glass blowers when they heated a blub of gas joined to a cooler tube. [2] As
reported in our earlier CFD analysis, we identified noise frequency for this experiment of 8-9 Hz
frequency (see Figure 1) which compared well to the calculated resonant frequency of 8.6 Hz.
Coupling between combustion reactions and the natural acoustic behavior of a reactor is known
to generate noise inside combustion equipment in the process industry.
During my career, I have observed this “roaring” phenomenon that literally shook an
incinerator off its stand during startup. The vendor eliminated the problem by adjusting the
burner tip location thus altering the resonate frequency inside the chamber. Previous CFD
analysis of an incinerator identified conditions that led to reverse flow through the stack when
combustion gases from the incinerator cooled as they rose up the stack which increased their
density (and weight) which caused the heavier gas to flow back into the incinerator. This puffing
created uneven refractory cooling that increased refractory wear and associated maintenance
costs. LES based CFD transient analysis has allowed us to investigate difficult noise problems
not possible using RANS based CFD steady state analysis.
This paper presents recent work involving an enclosed combustion device with nearby
structures that altered wind conditions at the stack exit and created low frequency “combustion
roar”. Previous work to eliminate high noise created in a catalytic reactor of a nitric acid plant
required us to design and install a Helmholtz resonator to cancel reactor noise at the stack. In the
present work, we used the LES CFD code C3d to simulate various operating conditions to match
observed noise levels and frequency. Our analysis confirmed no combustion noise for no wind
conditions and moderate noise that died away over time for uniform wind conditions but when
nearby structures were included together with their impact on wind speed at the stack exit (i.e.,
non-uniform wind from ground level to the stack exit elevation), noise levels increased by three
times and the predicted noise level and frequency matched observations. We used the CFD code
to identify design changes that decoupled the wind from combustion noise to solve this problem.
Our proprietary CFD code, initially used to model pool fires [3], has been tailored for analysis of flares, incinerators and process heaters. [4] [5] With it, we solve the most difficult problems
related to thermal-acoustic phenomena in combustion equipment.