| The treatment and disposal of sewage sludge firstly should follow the principle that disposal determines treatment and treatment is suitable for disposal.In the process of determining the disposal method of sewage sludge,sludge property,local 1 conditions,environmental capacity,industrial structure and other factors should be considered synthetically to look for a safe and economical disposal method in order to determine the technical path of the whole process.The sludge of Hangzhou Qige sewage treatment plant was taken as an example.The results showed that the chemical composition had little fluctuation and the content of heavy metals basically meets the specification requirements.Therefore,drying and co-combustion(coal or cement)is a good choice for sludge treatment and disposal.In order to explore the drying characteristics of sludge,small disc and paddle recuperative sludge stirring dryer were used to do the drying experimental study for the sludge from Qige sewage treatment plant.In the condition of high temperature(180?),the sludge drying rate was high and the drying viscosity area was short.The high drying rate can maintain for a long time.The power output of dryer was stable and has no big fluctuation.In the condition of low temperature(160? and 140?),the sludge drying rate obviously became lower.The state was maintained for a long time when sludge came into viscous zone.At the end of drying,the sludge clusters were broken and the drying rate appeared to jump up but quickly decreased to zero.It was also found that the drying process is different for different types of sludge dryer.In paddle dryer,the differences between viscous zone and agglomerate zone of sludge drying was more obvious than that of disc dryer.At the same temperature in the same dryer,the different rotational speed also had some influences on sludge drying process.With the increase of rotational speed,the drying rate increased appropriately but the increasing degree became lower.The comprehensive analysis showed that moisture content in location of the sludge from Qige sewage treatment plant was 40%-70%.In the process of sludge drying,the components of the exhaust are complex.It not only contains common gases like ammonia,methane,organic acid,but also has harmful gases with high content such as HCN?HF?HCl etc.The precipitation content curves of HCN and ammonia with time showed a good positive correlation.Both of them interacted with each other in the process of their formation and precipitation.The results showed that the concentrations of COD and BOD5 in the sludge drying condensate were high,and the pH value of the condensate was alkaline.So the sludge drying condensate needs further treatment before discharge.The sludge combustion characteristics and co-combustion characteristics with coal were studied by thermo gravimetric experiments.According to the thermo-gravimetric curves,the combustion characteristics of sludge and coal were completely different.There were two kinds of different substances burning slowly at different temperature in the combustion process of sludge.While the volatile devolatilization and combustion of fixed carbon were carried out at the same time during the combustion of coal.The kinetic parameters of sludge,coal and their blends were calculated by Coats-Redfern integral method.Coal and blends were fitted well with first order reaction.The activation energy decreased with the increase of sludge amount in blends.The experiments of sludge combustion and co-combustion with coal were carried out on a vertical three section fluidized bed with electric heating.It was found that the volatile components of the sludge were mainly consisted of methane,hydrogen and carbon monoxide.The higher the furnace temperature,the more completely the sludge burned in the furnace.The temperature of the furnace to ensure full combustion of sludge was above 800?.The content of heavy metals in sludge was low.In the design of the separate combustion equipment,the control of semi volatile metal elements such as copper,zinc and lead should be taken into account.The effect of sludge co-combustion with coal on release of heavy metals was not significant.The content of chlorine in combustion residue was very small.The chlorine usually went into the atmosphere in the form of gaseous or small particles.The corrosion prevention measures should be considered in the design of the sludge combustion equipment.It is not necessary to consider the control of usual pollutants such as sulfur dioxide and nitrogen oxides when co-burn sludge in a coal fired boiler because the original control measures are sufficient to meet the national emission standards.With the increase of sludge mixing ratio,the dioxin emission increased gradually.Considering the fact that the dioxin removal efficiency of exhaust treatment facilities is more than 90%in actual industrial process,the final emission of dioxin will be lower than the national standard.The co-combustion of dried sludge in cement kiln was studied.External mixing test showed that the sludge content is greater than 1%,Internal mixing test showed that the scheme of replacing shale with sludge and sandstone and using sandstone to provide the main SiO2 to control the cement raw material formulation is not feasible.But if shale was replaced by the mixture of sludge and shale to participate in the formulation of raw materials and the amount of sandstone and limestone as well as copper slag was fine-tuned,the maximum mixed ratio of sludge can reach 12%on application base.The different design schemes for sludge treatment and disposal of Hangzhou Qige wastewater treatment plant were compared in details.Through technical and economic comparison,it was found that sludge transported to cement plant or thermal power plant for co-combustion after thermal drying in Qige is an optimal scheme.On the basis above,the engineering design of drying and co-combustion project of sludge is made in depth.The achievements of sludge drying and co-combustion technology have been applied in Fuyang Banqiao sludge combustion project and Fuyang South Cement Company.The test results show that the sludge drying and co-combustion projects run stably and meet the requirements of environmental protection.It has broad application prospects. |
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