Environmentally Persistent Free Radical Generation And Mechanism During The Pyrolysis Of Biomasses

Environmentally Persistent Free Radicals (EPFRs) are a new kind of pollutant, and the lifetimes of EPFRs are longer than that of general radicals. EPFRs are tens of minutes to several hours or even longer, and with paramagnetic stability, which induce more oxidative organic matter by biological systems. EPFRs is widespread in the environment and has a stronger environmental sustainability and toxicity, which can impact on the human body and the environment. Yunnan is in the plateau area and the soils are rich in aluminium. Fenton reaction could happen under UV irradiation, and the intermediate products has free radicals also in the atmosphere, EPFRs formation resulted in a certain impact on the environment.Biochar is concerned much in recent years, which has a series of advantages, but it generate the radicals in the process of pyrolysis. It has different physical and chemical properties and stability because of different sources of raw materials, and generate the different EPFRs. So we use pine sawdust and rice straw widely cultivated in Yunnan to in situ reaction and prepare into biochar under different conditions of temperature and atmosphere. And we select lignin and cellulose which are the important component of biomass. However, in order to study on generating mechanism of free radicals in the biochar, the structure of lignin and cellulose is too complex. Therefore, we also select a series of single structure of the model compounds based on the lignin and cellulose structure. Three types:straight chain model compounds (high density polyethylene (HDPE), polyvinyl alcohol (PVA)); C-O heterocyclic model compounds (β-cyclodextrin); aromatic model compounds (polystyrene (PS), polystyrene ether (PPO)) to carry out in situ reaction and prepare into biochar under different conditions of temperature and atmosphere. The first to do is analyzation the physical and chemical properties and thermal stability of biochar by elemental analyzer, FTIR, and thermal gravimetric.In this study, the first to do is analyzation the properties of biochar. It is found that the biochar has more aromatic structure with increasing pyrolysis temperature. But for the two conventional biomass (rice straw, pine sawdust) which have inorganic minerals and organic matter, and the result is the two biomass carbonization degree lower than model compounds, because it can protect organic matter by the interaction of inorganic minerals and organic matter. Another result is the carbon carbonization rate in oxygen conditions is higher than nitrogen conditions. This is because sample combustion sharper with the increase of temperature, and make the combustion more intense by passing into oxygen, so that the samples are related to the mass loss, and once fuel burn out in, the sample quality and ash also remain.This study measures the free radicals of biochar by EPR. The results show that the content of free radicals of biochar is obviously increased with the increase of temperature; and the content of free radical in oxygen conditions is higher than in nitrogen conditions. The results show that temperature and condition are important factor for the content of free radical in biocha. For three model compounds, the content of free radicals straight chain model compounds≥ C-O heterocyclic model compounds> aromatic model compounds. And for the aromatic model compounds (PS,PPO), the content of free radicals of PS biochar> the content of free radicals of PPO biochar. The free radicals falling rate of PS> the free radicals falling rate of PPO by washing test, because the position of aromatic is different. In situ reaction by the electron paramagnetic (EPR), the results show:the biochar generate more free radicals at 300 ° C than 200 ° C, and free radical content under the conditions of oxygen is more than in nitrogen. In addition, due to the presence of oxygen, the new free radicals and oxygen combine to form the O-center free radical. O-center free radical is more stable than the C-center free radicals.All in all, the generation of free radicals in biochar is not only related to temperature and atmosphere, but also closely related to the structure of biomass. The experimental results show that the different structure of model compound generate different content and type of free radicals. However, the free radicals of biochar effect on the environmental needs more in-depth study.