Effects Of CO₂ On Distribution And Properties Of Three-phase Products During Low-rank Coal Pyrolysis

The pressurized fixed-bed dry bottom gasification is widely used for SNG because of its mature technology,flexible operation and relatively high CH₄ concentration in the crude gas.Excessive steam is used to ensure the ash discharged in solid form,resulting in the generation of large amounts of phenolic wastewater.Partially replacing steam by CO₂ is an effective way to solve this problem.It is inevitable that the chemical and physical reaction process will change when introduce CO₂ into the gasifier.This study focuses on the change of carbonization layer in gasifier when introduced CO₂ into it.Carbonization layer is the main bed stage of producing solid char,gas volatile,liquid and water,and tar which contain a lot of phenol compounds.Introducing CO₂ into pyrolysis process will inevitably affect the distribution and properties of the products.Therefore,it is of great benefit to explore this problem,which can provide theoretical support and basic data for the subsequent gasification operation,tar processing and phenolic wastewater treatment.The pyrolysis of Shengli and Yining coal?5-8 mm?under N₂ and CO₂ atmospheres were investigated from 400 ? to 700 ? in a fixed bed reactor,the effects of CO₂ on the distribution and properties of gas,liquid and solid phase products were analyzed.The char was analyzed by N₂ adsorption apparatus and Raman technique,GC/MS was used to analyze the composition of tar,and the releasing rules of pyrolysis gas was analyzed by gas analyzer.The results are as follows:?1?With increasing temperature,the char yields from Shengli and Yining coal pyrolysis decrease,the pyrolysis water and gas increase,and the tar yields increase first and then decrease.CO₂ atmosphere can promote the pyrolysis of Shengli and Yining coals,and resulting in higher yields of tar and water and lower yields of char and gas compared with the pyrolysis under N₂ atmosphere.?2?In the temperature range from 400 to 600 ?,CO₂ have little influence on the surface area and pore volume of the obtained chars,while at 700 ?,the chars prepared under CO₂ atmosphere have higher surface area and pore volume which can be attributed to the partial occur of char-CO₂ reaction.Raman charaterization results indicate that electron-rich group in char will decrease with increasing temperature.The ratios between small and big fused aromatic rings of the chars from pyrolysis in CO₂ decrease due to the consumption of 3-5rings of aromatic structures.?3?With increasing pyrolysis temperature both the release amounts of phenolic compounds from Shengli and Yining coals were increase first and then decrease,and the peak occurs at 500 and 600 ?,respectively.Below the peak temperature,CO₂ can inhibit the cracking of phenolic hydroxyl group and increase the content of phenolic compounds,above the peak temperature,it show the opposite effect.The effects of pyrolysis temperature and CO₂ on the release of naphthalene compounds were similar to that of phenols.?4?Compared with N₂ atmosphere,the effects of pyrolysis under CO₂ atmosphere on H₂,CO and CH₄ evolution are mainly happen at 600 and 700 ?.The amount of H₂ decrease under CO₂ atmosphere,it maybe that CO₂ can promote the H radicals transfer to the tar and water,the increase of CO is mainly due to the CO₂-char reaction,and the release amount of CH₄ is slightly higher than that in N₂ atmosphere.