Study On Flocculation Structure And Stability Mechanism Of The Co-slurry Of Sludge And Petroleum Coke

Sewage sludge generated from the treating process of urban living sewage produces huge pollution and destruction to ecological environment. Petroleum coke is a byproduct from delayed coking unit in petrochemical industry. Due to its high sulfur content, petroleum coke will aggravate load and operation cost of the boiler desulfurization unit when using directly as fuel. Clean and efficient utilization of the high sulfur petroleum coke has become a hot point in energy field. Sludge mixed with petroleum coke is a kind of slurry to be gasified. The flocculation structure of sludge is benifitial for improving the stability of the petroleum coke water slurry (PCWS), as well as saving the required heat consumption exhausted in sludge dewatering processing. However, the co-slurry concentration of the petroleum coke sludge slurry (PCSS) is low, limiting its large scale unilization in industry and the co-slurry interaction mechanisms need further research.In this paper, the effects of two modifiers and sludge addition on rheological property, stability, co-slurry concentration and Zeta potential of the PCSS were investigated. A theoretical model is deduced of how the sludge can improve the stability of PCWS with the help of rotational viscometer and torque rheometer. The three-dimensional spatial structure strength of the PCSS is predicted quantitatively. The influences of pre-treating sludge on rheological property and stability of the PCSS were explored by means of SEM-EDX (Scanning Electron Microscope and Energy Dispersive X-ray Detector) and FTIR(Fourier Transform Infrared Spectrometry). The co-slurry interaction mechanisms of sludge and petroleum coke particles were studied by using FBRM (Focused Beam Reflectance Measurement).Firstly, sludge was modified by Fe2(SO4)3 and Ca(OH)2 respectively, which reduce the apparent viscosity of the PCSS. With the increase of modifier addition, the electric charge of slurry and the electrostatic repulsion between particles decreases, which lessens the stability and increase the drainage rate. Considering the rheology and stability of the PCSS, the optimum adding amount of Fe2(SO4)3 and Ca(OH)2 modifier was determined to be 3% and 5% respectively. After adding the sludge, the maximum concentration of the PCSS is 62%. The maximum concentration of the PCSS was improved after sludge modification.Secondly, a sludge-coke-slurry spatial structure strength model was established and it predicted that energy consumption per-unit-quality of shearing the PCSS is as 2 to 3 times bigger as that of the PCWS. It can be concluded that it is the sludge that ameliorates stability of the PCWS. Also, sludge improved the thixotropic loop area of the PCSS. The slurry stability law reflected by the thixotropic loop area was consistent with the conclusions of theoretical models. The analysis results of SEM-EDX show that the petroleum coke particles are wrapped by the sludge flock during their co-slurry process to form a spatial network structure with some intensity. Meanwhile, composite coke are combined into a stable structure through the bonding effect which comes from the hydrogen bonding formed by -NH group,-CH group and-OH group of the sludge, thus improving the stability of the PCSS.Finally, in order to explore the flocculating mechanism in the process of co-slurry of the modified sludge and petroleum coke particles, FBRM instrument was employed to investigate the relationship between particle size distribution and total numbers of particles, as well as the stability of the slurry. The results show that the number of large size flocculation particles increased, while the number of small particles and the total particles decreased owing to the pellet coagulation of sludge and petroleum coke. Large size flocculation particles are connected with each other, forming a stable network structure, which prevents the settlement of petroleum coke particles effectively, thus improving the stability of slurry. Aggregates are formed between the particles through the form of chain loop and chain tail, which has some certain shear strength. The average particle size reduces with the increase of shear force, reaching the dynamic balance of flocculation and re-flocculation under the corresponding rotating speed. When the shear stress recovers, the number of total particles grows up and the average particle size drops down owning to the destruction of flock, which cannot realize the re-flocculation.