Abstract
Flame stability and pollution control are significant problems in the design and operation of any combustion system. Real-time monitoring and analysis of these phenomena require sophisticated equipment and are often incompatible with practical applications. This work explores the feasibility of model-based combustion monitoring and real-time evaluation of proximity to lean blowout (LBO). The approach uses temperature measurements, coupled with Chemical Reactor Network (CRN) model to interpret the data in real-time. The objective is to provide a computationally fast means of interpreting measurements regarding proximity to LBO. The CRN-predicted free radical concentrations and their trends and ratios are studied in each combustion zone. Flame stability and a blowout of an atmospheric pressure laboratory combustor are investigated experimentally and via a phenomenological real-time Chemical Reactor Network (CRN). The reactor is operated on low heating value fuel stream, i.e., methane diluted with nitrogen with N2/CH4volume ratios of 2.25 and 3.0. The data show a stable flame-zone carbon monoxide (CO) level over the entire range of the fuel-air equivalence ratio (Φ), and a significant increase in hydrocarbon emissions approaching blowout. The CRN trends agree with the data: the calculated concentrations of hydroxide (OH), O-atom, and H-atom monotonically decrease with the reduction of Φ. The flame OH blowout threshold is 0.025% by volume for both fuel mixtures. The real-time CRN allows for augmentation of combustion temperature measurements with modeled free radical concentrations and monitoring of unmeasurable combustion characteristics such as pollution formation rates, combustion efficiency, and proximity to blowout. This model-based approach for process monitoring can be useful in applications where the combustion measurements are limited to temperature and optical methods, or continuous gas sampling is not practical.
Highlights
Lean blowout can be defined as the phenomenon of flame extinction due to the reduction of the fuel-air ratio beyond a minimum threshold limit at which a given geometry can sustain a flame
The fuel is methane diluted with nitrogen, which, as stated above, allows for the combustion of reduced heating value fuels to be studied
We demonstrate a novel approach for monitoring combustor performance by computing difficultto-measure combustion parameters using a real-time model
Summary
Lean blowout can be defined as the phenomenon of flame extinction due to the reduction of the fuel-air ratio beyond a minimum threshold limit at which a given geometry can sustain a flame. The NOx and soot emissions can be controlled by lowering the flame temperature through leaning of the flame fuel-air equivalence ratio (Φ). By operating very lean, the combustors have limited margins between stable combustion and lean flame instabilities leading to lean blowout. Experimental and numerical studies have investigated species distribution in different combustor geometries, injector designs and fuel compositions e.g., [10,11,12,13,14,15,16,17]. The CFD models used to predict the species field are computationally
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