Abstract
BACKGROUND-AIM
This work is developed in the framework of the Green Deal project FRONTSH1P; one of the tasks includes the direct coupling of an in-house designed combustion chamber with a stationary fluidized bed gasifier operated with wood packaging waste (e.g., disused pallets). There are few studies reporting combustion strategies for renewable green producer gas, especially at very lean flame conditions. Hence, achieving a stable producer gas combustion (1.90 % CH4, 18.50 % H2, 8.85 % CO2, 22.20 % CO, 7.40 % H2O, and 41.15 % N2) at ultra-lean equivalence ratio (ER) is considered an interesting insight against rising clean thermal energy demand of industrial users at minimum cost of NOx pollutants.
METHODS
An experimental combustion chamber (size 820 mm × 520 mm × 520 mm) and a burner with a diameter of 68 mm were used. Thermal imaging based IR thermography technique is employed using IR thermal Camera FLIR - A700 of spectral range 7.5 – 14 μm with 30 Hz acquisition frequency. Intrusive flame temperature measurements were also conducted using N type thin wire thermocouples to calibrate the emissivity of the flame to be utilized in post-processing of IR images. Furthermore, the flue gas emissions were monitored using electrochemical cell-based flue gas analyzer, MRU (VARIO plus).
RESULTS
The time resolved flame images of producer gas compositions were viewed at 28 kW, 14 kW and 10 kW Thermal Loads (TL) for flame temperature field and topology analysis at ER ~ 0.20 - 0.85. The producer gas flame was quite stable and evenly distributed in the combustion chamber volume at ER = 0.85. Conversely, at lower ER conditions different outcomes were found. For instance, at ER = 0.43, the producer gas flame topology was strongly unstable due to several competing factors related to fluid dynamics and chemical kinetics. The decrement of flame temperature was noticed at lower ER conditions that may cause the reactions to be slower and may weaken the role of chemical kinetics associated with producer gas combustion process. In addition, the high flow rates of air as compared to fuel, contributed to shred the vortical structures that perturbed the flame by creating the various branches on flame surfaces. Moreover, the phenomenon of flame neck thinning was observed and became more evident when flame images were thermographically viewed at a ultra-lean ER = 0.2. Finally, the re-stabilization of producer gas flame was revealed with the addition of a small share of methane gas (up to 5% CH4) at ER = 0.2, which could represent a possible solution to achieve flame stability at ultra-lean combustion. Moreover, CO concentrations of 53 ppm and NOx emissions of 33 ppm were monitored at very lean ER = 0.20.
CONCLUSIONS
The combined impact of changing TL and lean ER conditions on the producer gas combustion parameters and flame topology are evaluated. Stable combustion operation of producer gas was observed at lean ER = 0.85 conditions; conversely, flame stability concerns at ultra-lean ER = 0.2, were encountered. It was revealed that a small share of methane could be a viable solution to overcome producer gas flame instability issues in these conditions. Furthermore, it was concluded that low NOx emissions ~ 35 ppm were produced mainly due to the fuel-prompt NOx mechanism.