灰化温度和热解气氛对污泥灰热力学特性的影响

Effects of Ashing Temperature and Pyrolysis Atmosphere on Thermodynamic Properties of Sewage Sludge Ash

通过对污泥灰样微观形貌、化学组成、矿物质转化、热力学特性以及加热过程中潜在的物理化学变化进行分析和探讨,获得了灰化温度以及热解气氛等对污泥灰样理化特性及热力学特性的影响规律。实验结果表明,污泥灰样呈典型的多孔特性,在热处理过程中易发生变形、烧结和熔融。灰样的主要化学成分为SiO_2、Al_2O_3、P_2O_5、CaO、Fe_2O_3和MgO,并以石英、硬石膏、白磷钙石、磷酸钙、磷酸铁钙、硅线石以及钙长石等矿物质形式存在。污泥灰样中的含磷矿物质及其转化过程对污泥灰矿物质演变和热力学特性影响显著,过量的磷酸盐离子会优先结合硬石膏中原本与硫酸根离子结合的钙离子,形成高熔点的白磷钙石和易分解的硫酸铝,从而导致硬石膏的分解并以SO_2或SO_3的形式释放S元素。同步热分析实验表明,污泥灰样A在空气气氛条件下经历了4个热力过程,分别为外在水分蒸发,白云母脱羟基和结晶水析出,白云石、方解石和白云母分解以及矿物质熔融和硬石膏分解,而污泥灰样B则表现为3个热力过程,分别为外在水分蒸发,以晶体转化、非晶体或无定形物质形成为主的微质量损失过程以及硬石膏分解和气相产物释放。灰样在氮气气氛中的热力学特性与空气气氛中相似,但总质量损失低于空气气氛中的总质量损失。

Microscopic morphology, mineral evolution, thermodynamic properties and potential physicochemical changes of sewage sludge ashes were analyzed in order to investigate the effects of ashing temperatures and atmospheres on the physicochemical and thermodynamic properties of sewage sludge ashes. Results showed that sewage sludge ash has the typical porous structure, which is prone to deform and melt during thermal treatment. The major chemical compositions of the ash samples include SiO2, Al2O3, P2O5, CaO, Fe2O3 and MgO. The main crystalline phases in the ash samples are quartz, anhydrite, whitlochite, calcium aluminum phosphate, calcium iron phosphate, sillimanite and anorthite. Phosphorus in ash samples has an important effect on mineral evolution. An excess of phosphate may contend for calcium ions chemically combined with sulphate in the anhydrite, forming calcium phosphate with high melting point and readily decomposable aluminium sulphate, thereby causing decomposition of anhydrite and release of sulphur in the form of SO2 or SO3. The thermal behavior of ash sample A in air atmosphere includes four thermal processes: evaporation of external moisture;dehydroxylation of muscovite and crystal water loss;decomposition of dolomite, calcite and muscovite;and mineral melting and decomposition of anhydrite. While ash sample B illustrates three thermal processes: evaporation of external moisture;crystal transformation or amorphous substance formation;and decomposition of anhydrite and gaseous product release. The thermal behaviors of the ash samples in N2 atmosphere are similar with those in air atmosphere, while the weight loss in air atmosphere is more significant than in N2 atmosphere.