Mechanistic Study On The Torrefaction Of Typical Food Waste And Its Effect On Pyrolysis/Gasification Characteristics

Food waste,as a majority composition of municipal solid waste,its utilization technology has attracted widely attention from many researchers.While,the disadvantages of food waste such as high moisture content,low energy density and poor grindability make it hard to collection,trasprotation,stortage and thermal utilization.So,a new food waste disposal technology(torrefaction-gasification)was proposed to solve above problems.The key scientific and theoretical issues are as follows:the torrefaction mechanism of food waste;the effect of torrefaction on char gasification reactivity;the effect of torrefaction on high temperature pyrolysis characteristic of food waste;the effect of torrefaction on gasification performance of food waste.In order to solve above scientific issues,four typical food wastes were chosen,including leftover rice(staple food)?lefover cabbage(vegetables)?lefover pork(meats)and watermelon peel(fruits).Then,a systematic research about the torrefaction mechanism and its effect on pyrolysis/gasification process were conducted.Firstly,four typical food wastes were torrefied in a fixed-bed furnace to compare the torrefactoin performace by characterizing their torrefaction products,and two-dimensional perturbation correlation infrared spectroscopy(2D-PCIS)method was used to reveal the evolution of char structure during torrefaction.The results show that torrefaction can improve the high heating value,but decrease the O/C and H/C ratio greatly,and 280°C is an optimal torrefaction temperature for food wastes.Majority of oxygen can be removed by dehydration and decarboxylation reactions in the form of water and CO₂,and part of oxygen was removed in the form of tar(i.e.,anhydrosugars for leftover rice,aldehydes/ketones for leftover pork,and acids/aldehydes/ketones for leftover cabbage and watermelon peel);Unlike oxygen,majority of carbon(71.8-82.3%)was retained in the solid product.In addition,For rice and cabbage/watermelon peel,a serious cracking of pyran/furan sugar ring and glucosidic bond happened at 260°C and 220°C,respectively;For pork,at tmperatures below 240°C,the main reaction pathway was the removal of methyl groups,followed by the cracking of carboxyl group,primary hydroxyl group and peptide bond at high temperatures.Secondly,the effect of torrefaction on pore structure,graphitization degree,content and form of alkali/alkaline earth metal(AAEM)for high temperature pyrolysis char were first compared.Then,the steam gasification reactivity for different chars were compared in a self-designed thermogravimetry.The results indicate that the char from torrefied sample has a poor porosity,mainly attributed the formation of micropores were inhibited.Torrafaction decrease the graphitization degree of char from cabbage and watermelon peel,while have a limited influence on the graphitization and cross-linked degree of char from leftover rice and pork.In addition,torrefaction can enhance the transformation from smaller aromatic ring systems of 3-5 fused benzene rings to large aromatics ring systems with no less than 6 fused benzene rings in char for all samples investigated.The content of active K(water-soluble and CH₃COONH₄-soluble form)was higher in the chars from torrfied cabbage and watermelon peel,while the content of active Ca had an opposite tendency.Moverover,torrefaction decrease the content of active K in pork char.Finally,torrefaction can enhance the gasification reactivity for torrefied cabbage and watermelon char,but a decreace tendency was found for torrefied rice and pork char.The effects were more serious at low temperatures(i.e.,800°C).Thirdly,in order to discuss the effect of torrefaction on the formation of products during devolatilizaiotn stage,a high temperature pyrolysis(600-1000°C)experiments were conducted in a continuous sample feeding vertical fixed-bed furnace to compare the pyrolysis performace by characterizing their pyrolysis products.The results show that the pyrolysis products of rice?pork and cabbage/watermelon peel were mainly consist of gas(69.2%)?tar(40.1%)and char(34.1-36.1%)at 600°C,respectively.Torrefacion decrease the gas yield for all samples,especially for rice,followed by cabbage,watermelon peel and pork.In addition,the formation of H₂,CO and C₂H₄ for rice and CO,CH₄and C₂H₄ for cabbage/watermelon peel were inhibited by torrefaction.While,a higher yield of CH₄,H₂and C₂H₄ was found for torrefied pork at>700°C;The effect of torrefaction on the formation of tar products is mainly controlled by temperature.At low temperatures(i.e.,700°C),torrefaction increase the formation of 2-3 rings and 3-5 rings aromatic sturctures in tar for rice and pork,respectively.In contrast,torrefaction inhibit the formation of 2-4 rings aromatic sturctures in tar for cabbage/watermelon peel.Finally,gasification experiments of raw and torrefied food wastes were conducted in the same fixed-bed reactor at 600–1000°C with a steam/carbon(S/C)molar ratio of 2 to compare the gasification performace by characterizing their gasification products.Compared with the existing date from other feedstocks demonstrate that the raw rice,cabbage and watermelon peel are high-quality gasification feedstock,which can can achieve a high CGE(70-93%)at a low gasification temperature(i.e.,700°C).Torrefaction mainly changed the yield and composition produced at devolatization stage,but have a limited effect on the tar reforming stage.The results show that the gasification of torrefied rice and pork at high temperatures(i.e.,1000°C)generate high quality syngas suitable for Fischer-Tropsch synthesis,and also have a high CGE(85.4-91.1%).While for torrefied cabbage and watermelon peel,a lower gasification temperature(i.e.,800°C)is more suitable for achieve a high CGE(90.4-95.6%)and H₂ content(?55%).