Abstract
The existence of inert gases such as N2 and CO2 in biogas will reduce the proportion of combustible components in syngas and affect the combustion and NOX formation characteristics. In this study, ANSYS CHEMKIN-PRO software combined with GRI-MECH 3.0 mechanism was used to numerically simulate the effects of different CO2 concentrations (CO2 volume ratio in biogas is 0–41.6%) on flame combustion temperature, flame propagation speed and nitrogen oxide formation of complex biogas with low calorific value. The results showed that when the combustion reaches the chemical equilibrium, the flame combustion temperature and flame propagation speed decrease with the increase of CO2 concentration, and the flame propagation speed decreases even more slowly. Meanwhile, the molar fraction of NO at chemical equilibrium decreases with the increase of CO2 concentration and the decrease is decreasing, which indicates that the effect of CO2 concentration in biogas on NO is simpler. While the molar fraction of NO2 does not change regularly with the change of CO2 concentration, the effect of CO2 concentration in biogas on NO2 is complicated. The highest molar fraction of NO2 was found at chemical equilibrium when the CO2 concentration was 33.6%, when the target was a typical low calorific value biogas.
Highlights
Biomass is a kind of clean and convenient energy with rich reserves, which is a promising green renewable energy (Zhang et al, 2005)
For typical low calorific value biogas, when the volume ratio of CO2 is 33.60%, it can be used as a control group to compare the influence of CO2 concentration change on flame combustion temperature
When the combustion reaches chemical equilibrium, the final temperature of the reaction keeps decreasing with the increase of CO2 concentration
Summary
Biomass is a kind of clean and convenient energy with rich reserves, which is a promising green renewable energy (Zhang et al, 2005). Direct combustion is a main utilization method for biomass (Yang et al, 2020). It has the disadvantages of low utilization ratio, massive content of dust and NOX in flue gas (Zhao and Su, 2019; Wang et al, 2020). The biogas obtained from pyrolysis or gasification of biomass can be combusted with pulverized coal as boiler fuel, which reduces the consumption of coal, and decreases the NOX emission significantly (Zhang et al, 2017). The composition of biogas is complex, especially the existence of inert gases such as CO2 and N2 will have a great impact on the combustion characteristics of flame and the formation of NOX (Chu et al, 2021)
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