| The aquatic plant duckweed could absorb numerous nutrients such as N and P during its growth,and degrade/absorb toxic substances in water such as heavy metals,which has potential advantages in wastewater treatment.Using pyrolysis technology to treat the harvested duckweed as an energy product provides a new way to solve the water eutrophication and energy crisis,realize the double benefits of environmental control and energy preparation.In this paper,the biomass of duckweed was selected as the object of study.First,thermogravimetric experiments were carried out to investigate the weight loss characteristics of duckweed and the effect of phosphorus on the weight loss characteristics,the pyrolysis process was also analyzed.Secondly,in situ pyrolysis experiments of duckweeds before and after pickling and soaked in phosphoric acid were carried out by Py-GC/MS technology.The effects of alkali,alkaline earth metals(AAEMs)and phosphorus on the characteristics of duckweed in situ pyrolysis oil were investigated.Finally,fixed-bed pyrolysis experiments were carried out in a vertical reactor to investigate the pyrolysis characteristics of duckweed and the effect of phosphorus on product distribution.The main research results of this paper are as follows:(1)Thermogravimetric technology was used to conduct pyrolysis weightlessness experiments on experimental samples at high temperature.It was found that the pyrolysis range of duckweed was mainly fasten at 145 ~520 ℃,and the pyrolysis process of pickled duckweed was mainly concentrated at 155 ~510 ℃.The pyrolysis temperature zone was small and narrow compared to that of the as received sample,meanwhile the maximum pyrolysis temperature of the two weight loss peaks was decreased by 25 ℃ and 15 ℃,respectively.Phosphoric acid impregnation can promote biomass thermal dehydration reactions and reduce the energy required for biomass decomposition,such that the maximum weight loss peak temperature of pyrolysis decreases after impregnation and the initial temperature of pyrolysis decreases from 5 ~ 10℃.The FWO and KAS methods were used for kinetic analysis,The activation energy of duckweed was decreased by pickling;However,the activation energy of duckweed increased in different degrees after soaking with phosphoric acid.Using the principal curve method,it was found that the pyrolysis of several duckweed samples conformed to the n-order reaction model.(2)The in-situ pyrolysis test of different duckweed samples was carried out using Py-GC/MS technology.It is showed that there was no difference in nitrogen content of duckweed pyrolysis oil before and after pickling at 400 ~ 600 °C,Acid washing significantly increased the content of L-glucosone.Nitrogen content of phosphoric acid impregnated duckweed was higher at low temperature than acid-washed duckweed,the opposite is true at high temperature.And the content of levoglucosenone also increased.In addition,phosphoric acid impregnated duckweed is beneficial to phenol generation.(3)Pyrolysis experiments of duckweed and pickled duckweed were carried out in a fixed bed of a vertical reactor.It was found that the solid and liquid products of duckweed decreased with the increase of temperature,but the gas yield increased.Acid washing significantly reduced the CO2 production rate in pyrolysis gas.The pyrolysis gas caloric value is effectively increased by acid washing,compared with the original sample,it increased by 5%~ 155%.The C content in coke was significantly increased by acid washing,and the caloric value of coke was increased compared with the original coke.Besides,the content of aromatic hydrocarbons and phenols in pyrolysis oil also increased.(4)Pyrolysis of duckweed after pickling and phosphoric acid impregnation in a fixed bed reactor.The results indicated that the yield of solid and gas of Phosphoric acid impregnated duckweed is higher than that of acid-washed duckweed,but the yield of liquid is significantly lower.The yield of 20% impregnation is the highest,and that of 5% impregnation is the lowest.The H2 production increased after phosphoric acid impregnation,and the H2 production increased the most when the impregnation ratio was 20%.The ash content of pyrocarbon increased significantly after phosphoric acid impregnation,and the specific calorific value of 5% impregnated coke was slightly higher than that of pickled duckweed.The content of phenols was increased obviously by phosphoric acid impregnation,especially by 76.11% at 700 ℃. |
