Study On The Pyrolysis Characteristics And Mechanism Of Mixed Sewage Sludge Containing Food Waste

Pyrolysis is one of the most prospective technologies for sludge treatment,which is widely used in Europe and the United States because of its high capacity reduction,low pollution and high efficiency.Recent years,under the new environment of waste separation,new food waste treatment methods of food waste shredding into the municipal sewage network have emerged in China and abroad.Shenzhen,Shanghai and many other cities are vigorously promoting the application of food waste shredders,resulting in a large amount of food waste debris sinks into sewage treatment plants.This makes the sludge composition significantly different from conventional sludge.At present,no studies have been conducted in china on the pyrolysis of mixed sewage sludge containing food waste,and the effects of pyrolysis conditions and different food waste contents on the distribution and components of pyrolysis products are not clear,nor are the components and generation patterns of pyrolysis pollutants.In this paper,the exploration and research are carried out,and the relevant characteristics and mechanisms are initially revealed.In this paper,food waste was crushed,fermented and dried to produce primary sludge,and it was divided into five groups of samples,PS,PS75RS25,PS50RS50,PS25RS75,RS,according to different mass ratios with residual sludge.Dried at 105°C for 24 hours and then crushed and sieved to make granules below 100 mesh.Subsequently,TG-FTIR experiments were performed at a ramp-up rate of 20°C/min within 30-850°C to investigate the pyrolytic weight loss properties,interaction relationships and escaping gas characteristics.The results show that the addition of PS can effectively improve the RS pyrolysis performance,and the interaction between the two substances is related to both the pyrolysis temperature and PS content.The medium temperature zone（300-500℃）promotes solid phase decomposition,and the high temperature zone（500-850℃）inhibits solid phase decomposition.The higher the PS content,the stronger the inhibition in the high-temperature region and the weaker the promotion in the medium-temperature region.From the point of view of interaction,low and medium temperature pyrolysis and low PS content can maximize the use of raw materials.The six gaseous products and functional groups of CH₄,CO₂,H₂O,C=O,CO and SO₂ are basically not released before 300°C.The yield increases with the increase of pyrolysis temperature from 300to 500°C.After 500°C,the yield of CH₄,C=O,CO and SO₂ decreases with the increase of temperature,and the yield of CO₂ and H₂O increases slowly with the increase of temperature and finally tends to level off.The three-phase yield of pyrolysis of mixed sewage sludge containing food waste was investigated using tube furnace pyrolysis experiments,focusing on the effects of pyrolysis temperature（500°C,550°C,600°C,650°C,700°C）and PS content（25%,50%,75%）on the three-phase yield.The experimental results showed that the pyrolysis temperature increased and the yield of liquid phase in the pyrolysis products showed a fluctuating decrease,a continuous increase in the yield of gas phase and a decrease in the yield of solid phase.As the PS content increased,the yield of liquid phase fluctuated and was lowest at 50%,the yield of gas phase increased and the yield of solid phase decreased.At the same time,the fitting equations for the three-phase yield at different pyrolysis temperatures and PS contents are given based on the experimental data for engineering applications.The liquid phase products of pyrolysis of mixed sewage sludge containing food waste were analyzed using thermal cracking-gas chromatography/mass spectrometry（Py-GC/MS）.The effects of pyrolysis temperature（500℃,550℃,600℃,650℃,700℃）and PS content（25%,50%,75%）on the yield and component distribution of the main pyrolysis products（Aliphatic,aromatic,oxygen-containing compounds,N-compounds,N-heterocyclic）were focused on,and the generation pattern of the main pyrolysis products was investigated.The experimental results showed that the relative yield of aliphatic increased and then decreased,oxygenated compounds decreased,and aromatic increased,as the pyrolysis temperature increased.An increase in PS content resulted in a significant increase in the relative yield of aliphatic,a small increase in oxygenated compounds,and a small decrease in aromatic.Combining the three-phase yield values,the absolute yield of aliphatic was highest at 25%PS content and pyrolysis temperature of 600-650℃,while the lowest was at 50%PS content and pyrolysis temperature of 500-550℃.The absolute yield of oxygenated compounds was highest for 25%PS content and 500-550℃pyrolysis temperature,and lowest for 75%PS content and 650-700℃pyrolysis temperature.The highest absolute yield of aromatic basically occurs at 25%PS content and 550-700℃working conditions.In order to obtain higher aliphatic hydrocarbons and reduce nitrogen-containing pollutants,600-700℃is the optimal pyrolysis temperature.The yield and component distribution and generation pattern of small molecule nitrogenous gaseous pollutants and large molecule nitrogenous pollutants were investigated using TG-FTIR and Py-GC/MS experiments.The experimental results showed that the yield of HCN and HNCO increased monotonically with the increase of temperature,and the yield of NH₃ showed an increasing and then decreasing trend,with a peak in the temperature range of500-550℃.HCN was the most abundant gaseous nitrogenous pollutant.The pyrolysis temperature was optimal at 550-600℃to reduce the release of gaseous nitrogenous pollutants.The yield of N-compounds decreases with increasing temperature and is the main nitrogenous pollutant,among which the yield of amines&amides,amino acids decreases with increasing temperature and the yield of nitrile substances fluctuates and increases.The effect of PS content on the yield of N-compounds was weak.The yield of N-heterocyclic increased slightly with increasing temperature,with yields of about 6-13%.It was found that the main macromolecular nitrogenous contaminants basically came from the decomposition of proteins,and the yield was less affected by the PS content,which can be inferred that PS is similar to the nitrogenous substances in RS.