Production Of Hydrocarbons From Catalytic Pyrolysis Of Sewage Sludge And Routes Of Transformation

As the by-product of waste water treatment process,sewage sludge production is increasing along with the fast-developing waste water treatment.Sewage sludge contains large number of organic compounds with potential risk on environment that requires proper disposal.This study conducted catalytic pyrolysis of sewage sludge for hydrocarbons production.Based on the characteristics of sewage sludge thermal decomposition,the effects of operation parameters on products distributions and hydrocarbon selectivities were investigated and postulated triple-cycle transformation pathway was provided.The release of oxygen and nitrogen during pyrolysis and catalytic pyrolysis were also compared,and the removal pathways of oxygen and nitrogen during catalytic pyrolysis were proposed.Sludge pyrolysis was divided into three stages(25~150 ?C,150~728 ?C and 728~900 ?C),and most of sludge volatiles escaped at the second stage.FWO method and KAS method showed excellent fitting results on sludge TG curves with R2 over 0.969.In the process of sludge pyrolysis,apparent activation energy was low at low temperature but increased rapidly at high temperature.Above 750 ?C,the activation energy was above 500 kJ?mol-1 and the major volatiles release completed under 750 ?C.Based on the kinetics study of sludge pyrolysis,products distributions and hydrocarbons selectivities during catalytic pyrolysis of sludge were investigated and the results showed that ZSM-5 catalyst could convert sludge into aromatic hydrocarbons,olefins and alkanes.With the increase of reaction temperature and ZSM-5 to sludge ratio hydrocarbons yield increased and 18.1 g of hydrocarbons were obtained with 100 g of sewage sludge at 650 ?C with ZSM-5 to sludge ratio at 20:1.From 450 ?C to 750 ?C,selectivity of monocyclic aromatic hydrocarbons such as benzene increased from 14.7 % to 22.9 % while that of polycyclic aromatic hydrocarbons decreased from 24.8 % to 21.5 %.Small olefin such as ethylene and small alkane such as methane were more favored at higher temprerature.During catalytic pyrolysis of sewage sludge,aromatic hydrocarbons,olefins and alkanes could transform between each other in forms of “aromatic hydrocarbons cycle”,“olefins cycle” and “alkanes cycle”,meanwhile,different kinds of hydrocarbons could also transform between each other based on triple-cycle transformation pathway.During ex-situ catalytic pyrolysis,“olefins cycle” was more favored and more olefins of 17.5 % were obtained while “aromatic hydrocarbons cycle”and “alkanes cycle” were more favored during in-situ catalytic pyrolysis and more aromatic hydrocarbons of 24.0 % and alkanes of 8.7 % were obtained.The release of oxygen and nitrogen during pyrolysis and catalytic pyrolysis of amino acids were comparatively investigated to analyze the removal pathways of oxygen and nitrogen.The results showed that druing pyrolysis,carboxyl group is readily thermally decomposed into carbon dioxide.During pyrolysis of leucine and proline,51.7 % and 67.1 % of carbon dioxide were observed,respectively.However,during catalytic pyrolysis,carboxyl group were decomposed through decarbonylation into carbon monoxide while low yield of carbon dioxide was observed.During catalytic pyrolysis of leucine and proline,53.3 % and 50.4 % of carbon monoxide were obtained,respectively,while only 9.5 % and 13.4 % of oxygen were removed as carbon monoxide,respectively.During pyrolysis,amino in leucine is relatively active and readily decomposed into ammonia of 44.4 % while nitrogen-containing heterocycle group in proline was relatively thermally stable and less ammonia of 14.2 % was obtained.During catalytic pyrolysis of both leucine and proline,however,nitrogen was removed mainly in form of ammonia of 81.7 % and 59.7 %.