Auto-ignition of biomass synthesis gas in shock tube at elevated temperature and pressure
Abstract
Ignition delay times of multi-component biomass synthesis gas (bio-syngas) diluted in argon were measured in a shock tube at elevated pressure (5, 10 and 15 bar, 1 bar = 10<sup>5</sup> Pa), wide temperature ranges (1,100–1,700 K) and various equivalence ratios (0.5, 1.0, 2.0). Additionally, the effects of the variations of main constituents (H<sub>2</sub>:CO = 0.125–8) on ignition delays were investigated. The experimental results indicated that the ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa. The ignition delays were also found to rise as CO concentration increases, which is in good agreement with the literature. In addition, the ignition delays of bio-syngas were found increasing as the equivalence ratio rises. This behavior was primarily discussed in present work. Experimental results were also compared with numerical predictions of multiple chemical kinetic mechanisms and Li’s mechanism was found having the best accuracy. The logarithmic ignition delays were found nonlinearly decrease with the H<sub>2</sub> concentration under various conditions, and the effects of temperature, equivalence ratio and H<sub>2</sub> concentration on the ignition delays are all remarkable. However, the effect of pressure is relatively smaller under current conditions. Sensitivity analysis and reaction pathway analysis of methane showed that R1 (H + O<sub>2</sub> = O + OH) is the most sensitive reaction promoting ignition and R13 (H + O<sub>2</sub> (+M) = HO<sub>2</sub> (+M)), R53 (CH<sub>3</sub> + H (+M) = CH<sub>4</sub> (+M)), R54 (CH<sub>4</sub> + H = CH<sub>3</sub> + H<sub>2</sub>) as well as R56 (CH<sub>4</sub> + OH = CH<sub>3</sub> + H<sub>2</sub>O) are key reactions prohibiting ignition under current experimental conditions. Among them, R53 (CH<sub>3</sub> + H (+M) = CH<sub>4</sub> (+M)), R54 (CH<sub>4</sub> + H = CH<sub>3</sub> + H<sub>2</sub>) have the largest positive sensitivities and the high contribution rate in rich mixture. The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases sharply as the mixture turns rich. Therefore, the ignition delays become longer as the equivalence ratio increases.
