Experimental Study On Slurry Preparation And Gasification Of Urban Sewage Sludge And High Ash Fusion Temperature Coal

Urban sewage sludge as one of the by-products of municipal sewage treatment plant, its resource utilization and harmless disposal is an unevadable problem. Meanwhile, about 18 million tons per year of the domestic sludge cannot be directly used as an energy resource because of its characteristics of low calorific value and high contents of moisture and ash. On the other hand, coal as the most important energy resource in the next 50 years, more than half is high ash fusion temperature coal(ash fusion temperature> 1500℃),which cannot be directly applied to industrial entrained flow coal gasification. It is clear that the high mass fraction of alkali metals and alkaline earth metals rich in sewage sludge can efficiently lower the coal ash fusion temperature. Mixing with sewage sludge to produce eligible sludge coal water slurry (Sludge-CWS), high ash fusion temperature coal can be availably used in coal water slurry (CWS) entrained flow bed gasification, while sewage sludge can realize the resource ulilization by partly replacing water and coal consumption in CWS.Huainan coal (HN coal) as a typical coal with high ash fusion temperature from Huainan Pan-er coal mine and the sewage sludge (SS) collected from the sludge dewatering workshop of Huainan Shouchuang sewage treatment plant were used as experimental samples to prepare Sludge-CWS in this study. The slurribility effect of SS ratio on HN coal was investigated, and fusion temperatures, ash chemical and mineral compositions, zeta potential were detected separately by Ash melting temperature tester, XRF, XRD, SEM-EDX and System zeta potential. The home-made quartz spring thermal balance and TG-FTIR analyzer were both used to examine the CO2 gasification characteristics of Sludge-CWS. Furthermore, simulation and economic analysis of Sludge-CWS gasification were performed with Aspen Plus software. The main results were introduced as follows:There were nearly 90% SiO2 and Al2O3 and very low contents of alkali metals and alkaline earth metals (especially only 3% of CaO) in HN coal ash. This led to its high ash fusion temperature (> 1500℃) due to the high-temperatured formation of silicoaluminate mineral (mainly mullite), so that, HN coal cannot be used in commercial entrained flow bed gasifier. It was detected that nearly 40% CaO existed in SS ash, resulting in the addional quicklime in the procedure of sludge modification.It was revealed that the abundant SiO2 and Al2O3 in HN coal ash combined with CaO riched in SS, forming lower fusion tempereture minerals (mainly Anorthite) in high temperature, effectively preventing the formation of high fusion tempertrue mineral, such as mollite. It resulted in the decrease of HN coal ash fusion temperature. The ash flow temperature was lower than 1350℃ at 16.7% sludge ratio with 57% Sludge-CWS maximum slurry concertration.The Sludge-CWS preparation test showed that Ca2+riched in SS decreased the zeta potential of Sludge-CWS, which caused rising viscosity and decreasing maximum slurry concerntration at the fixed viscosity (1200mPa s). Sludge-CWS with good slurribily could be successfully produced. The maximum of slurry concerntration might reach 61% at the sludge ratio of 8% in 1200mPa s viscosity.The Sludge-CWS CO2 gasification experiments indicated that the increasing sludge ratio accelerated reaction velocity and lowered the reaction temperature for HN coal with a little decrease of CO content.The result of Sludge-CWS simulation showed that HN coal gasification temperature could be lowered to 1420℃ at 16.7% sludge ratio, while 18.1% oxygen and 19.9% water consumption was reducted, with the reduction 2.7% effective syngas mole fraction, compared with those in HN-CWS gasification.It was concluded that HN coal slurry with SS could be applied to CWS entrained flow bed gasification process, and massive SS resources utilization was achieved both environmentally and economically.