Study On Sewage Sludge Pyrolysis Characteristic And Technology

A significant amount of sludge is generated in wastewater treatment processes.There are a variety of constituents, such as organic matter, metals, pathogens,parasitic ovum and etc. making it unstable and prone to turn septic. Hence, wastewatersludge can be the source of secondary contamination of surrounding environmentwithout proper handling and treatment. On the other hand, sludge can be a sort ofresource after certain types of pretreatment, e.g. heat pyrolysis. The reclamation ofwastewater sludge pyrolysis products can minimize the threat to environment as atype of waste recycle and reuse.In this study, coagulants, e.g. ferric and aluminum salts recycled from biosolidsare examined for chemical enhanced primary wastewater treatment. Mass balance ofsludge thermal hydrolysis products in three phases are studied with lab-scale batchfeed and continuous thermal hydrolysis reactors, fate and transport of all kinds ofelements are also evaluated. Energy balance of thermal hydrolysis is established bymeasuring heat value and reaction heat at different temperatures. Mechanism study isconducted to all kinds of constituents in biosolids in the thermal hydrolysis process.Fitting of individual section in thermal hydrolysis reactions is conducted by apparentkinetics. Industrial scale equipments are developed, economical analysis is conducted.The optimal condition for ferric salt in chemical enhanced primary wastewatertreatment is: pH at1.5, stirring time at60min. For aluminum salt, the optimalcondition is: pH at2.5, stirring time at60min. Recovery rate are79.2%and83.5%respectively, biosolid reduction rate are50.85%and35.51%. Multiple stage recoveryrates get reduced from83.78%an84.51%to70.06%and78.43%, respectively. Theperformance of reclaimed ferric and aluminum salts are close to or better than regularsalts on turbidity removal. The UV254transmittance of reclaimed salts treatedwastewater is slightly better than regular ones, COD and SCOD removal rates aresimilar to regular ones, total phosphorous removal rate is always higher than97%,color removal rate reduces from51%to41%.The main factor in the sludge pyrolysis process is temperature, production ratesof pyrolysis liquid and gaseous products are proportional to temperature. Temperatureincreasing rate and time are not strong factors. Higher temperature is beneficial formass transfer to gaseous phase of a variety of elements, such as carbon, hydrogen,oxygen, nitrogen and sulfur. Continuous process gives better pyrolysis extent thanbatch feed process, at high temperature, more carbon, nitrogen and oxygen aretransferred from sludge to other phases, less sulfur gets transferred because sulfurgets involved in reactions that immobilize it in the solid phase. Reduction rate otherelements is slightly higher in continuous process, the reason is gaseous product generation rate is higher in continuous process, micro particles in aerosol are takenaway by gaseous products.Combustion heat of pyrolysis products reduce significantly with pyrolysistemperature. Combustion heat of liquid products hydrolyzed under450-600℃canreach as high as26433kJ/kg. Optimal temperature of liquid pyrolysis productsutilization is600℃. Highest combustion heat of gaseous pyrolysis products isachieved at temperature of700-800℃. Aiming at energy utilization, batch feedreactors are suitable for high combustion heat biofuel in liquid, continuous flowreactors are suitable for biogas. The two types of pyrolysis processes can achieve77.50%energy recovery in terms of chemical energy under optimal conditions fromeither liquid or gaseous pyrolysis products. With the pyrolysis temperature increases,the reaction absorbs then releases energy, the turning point of energy absorb-releaseis588.56℃, and the turning point of energy lose-gain is809.56℃. Under differenttemperatures, the maximum energy absorbed is as low as637.24J/g, only3.94%ofsludge chemical energy. Hence, the pyrolysis of sludge is energy positive.Protein, lipid, polysaccharides, and humic substances in wastewater sludgeaccount for29.78%,16.62%,11.08%and11.77%, respectively. Within25-180℃, itis mainly gasification of moisture, volatile organics, carbonhydrate and sulfur hydride.When temperature is higher than240℃, protein, liquid and polysaccharides start topyrolysis. When temperature goes up to400℃, secondary pyrolysis happens,gasification rate increases immediately. In wastewater sludge, pyrolysis ofpolysaccharides ends at428℃, lipids gasification ends at464℃, pyrolysis of proteinfinishes at538℃.In apparent kinetics, the pyrolysis reaction fits first order intemperature range of148-220℃, second order in220-475℃and first order in475-630℃.Sludge pyrolysis products generation rate and elemental composition is similarto lab scale. Energy cost can be higher because of heat radiation under hightemperature. As shown in economic analysis, sludge pyrolysis process as resourcereclamation has excellent benefit output, it has multiple advantages, such as shortpay-off period, high rate of capital return, etc.