城市污水污泥的热解动力学特性研究

采用差示热天平获得上海程桥污水处理厂污泥的TG、DTG曲线，对干燥污泥的热解行为及其动力学规律采用热重分析法进行了研究。实验结果表明：干燥污泥的热解过程中有2个失重速率比较高的区域，这2个区域以挥发分的析出为主。通过热重分析法，求解了这2个区域的化学反应动力学参数——频率因子A和活化能E，对其反应机理进行了分析。     

城市污泥耦合锯末共热解特性及动力学分析

为实现城市污水污泥与锯末共热解的工业应用,利用热重分析仪对污泥耦合锯末共热解过程进行了实验与理论研究,揭示了锯末添加比例、升温速率对污泥热解特性的影响,并基于Coats-Redfern法,结合20种常见固体热解机理函数确定了污泥耦合锯末共热解过程最优热解动力学模型。结果表明:锯末相比污泥具有更低的表观活化能,最大失重速率是污泥的4倍;锯末的添加使得热重分析(TG)曲线向下偏移,最大失重速率明显增大,挥发份析出特性变强;随着升温速率的增大,固态残渣增加,最大失重速率减小,不利于热解反应的进行;按7∶3比例混合的污泥锯末耦合热解微分热重分析(DTG)曲线峰前(230~350℃)表观活化能为38.81 k J/mol,最优动力学模型为D_5-3D扩散模型;峰后(350~500℃)表观活化能为29.93 k J/mol,最优动力学模型为C~2-化学反应模型。     

污泥与木屑共热解特性研究

文章采用污泥、木屑为原料,在氮气气氛下进行热重实验,研究了升温速率和木屑添加量对污泥与木屑共热解特性的影响规律,并进行动力学分析。研究结果表明:随着升温速率的增加,样品挥发分析出阶段向高温方向移动,最大失重速率增加;随着木屑添加量的增加,样品总失重量及最大失重速率均显著增加。动力学分析认为:污泥热解的反应机理为三维扩散,机理函数为Z-L-T函数;污泥与木屑共热解的反应机理为成核和生长,机理函数为Avrami-Erofeev函数。污泥热解活化能为287.29～390.57 kJ/mol,污泥与木屑共热解活化能为170.16～277.05 kJ/mol,木屑的加入降低了污泥的热解活化能。     

污水污泥热解过程的能量平衡与反应热分析

依据能量守恒定律,通过分析污水污泥热解过程的能量利用、耗散和回收情况,提出了污泥热解制取三相产物处理系统的能量平衡模型,分析计算了污泥热解反应热,并利用回收率和耗能比对热解处理系统的耗能状况进行了评价.结果表明:能量利用过程中总存在能源的耗散,减少能耗的主要方式是尽可能提高产物的能值、降低热消耗和废弃的能量;在不同的热解工况下,热量损失的差别明显,热解停留时间长、升温速率慢、热解温度高均会导致输入能量和热损失增大;能耗评价也验证了热解技术能够回收更多的能量,可以获得更大的能耗比.     

石化污泥与烟煤混合共热解特性实验研究

选取石化污泥和烟煤,将两者按不同比例混合。采用热重分析法,在相同升温速率下,对各混合样品进行热解实验,探讨石化污泥与烟煤热解特性的差异及其共热解时石化污泥对烟煤热解过程的影响。结果表明:石化污泥与烟煤的热解过程和反应特征参数差异很大,主要表现在样品的挥发分析出温度区间、总失重率及失重速率上。同时,石化污泥的挥发分综合释放特性指数D远优于烟煤;在石化污泥和烟煤的共热解过程中,石化污泥的添加对烟煤的热解起到一定促进作用,当石化污泥掺入比例为10%时,混合物的热解特性最好。     

城市污泥热解及能量平衡分析

为了研究污泥热解特性,利用铝甑反应器对城市污泥进行了热解实验。实验结果表明:随着温度逐渐升高,热解程度逐渐加深,高温时热解液体的收率略有降低;热解过程可分为3个阶段,即水分析出阶段、挥发分析出阶段和残留有机物继续分解碳化阶段。在热解实验基础上进行能量平衡分析,结果发现,采用热解油气循环供热,既可以完全满足污泥热解过程的能量需求,还可为干燥过程节约51.43%的能量。     

基于TG和Py-GC-MS的污泥热裂解特性研究

利用热重分析法和热裂解-气相色谱-质谱方法(Py-GC-MS)对某市政污水处理厂污泥的热解特性进行了分析。污泥热解过程可以分为水分析出、挥发分析出和矿物质及焦炭分解三个阶段,提高升温速率可以促进污泥的热解转化效率,同时对挥发分析出温度、最大失重速率等热解特性参数有显著的影响。升温速率越高,污泥的最大失重速率越大,对应的失重峰宽增大。采用Py-GC-MS方法对污泥热裂解产物进行了分析,其中主要气体产物是CO_2,其余产物主要包括苯类及其化合物、含氮化合物、杂环化合物、酯类化合物、羧酸、含硅化合物和醚类化合物等。     

市政污泥与木屑共热解特性及动力学分析

采用热重分析法对市政污泥、木屑及其不同比例的混合样品热解特性进行了分析,研究了升温速率和混合比例对热解过程的影响。对污泥和木屑进行单独热解时,木屑比污泥的反应活性更高;随着升温速率的增大,二者的挥发分析出指数D均有增大,但升温速率对污泥挥发分析出的影响更大;污泥与木屑共热解改善了污泥热解过程的综合热解释放特性,有利于热解反应的进行;随木屑添加比例的增加D值呈指数增长,在木屑添加比例为80%时,D达到最大值118.18×10-8,但仍低于理论值141.67×10-8,说明存在竞争作用。文章采用FWO(Flynn-Wall-Ozawa)和Starink方法,分别计算木屑添加比例为80%的共热解表观活化能。当转化程度α为0.1～0.8时活化能变化较小,α为0.9时活化能分别突增到761.64,786.12 kJ/mol。这说明共热解过程可分为150～520℃和520～1 000℃两个温度阶段,与污泥单独热解相比,降低了转化率达到90%时的终温。     

城市污泥低温热解技术影响因素的分析

针对城市污泥处置引起的环境问题,介绍了污泥热解技术,对污泥的低温热解技术进行了探讨,分析了反应温度、反应时间、反应速率和催化剂等对污泥热解产物的影响.并对如何更好地进行污泥处置提出了意见.     

城市污水污泥热解特性及动力学研究

采用热重分析仪与傅里叶红外光谱仪对城市污水污泥进行实验,考察了反应过程及逸出气体产物,求解了热解表观动力学参数。研究表明,污泥样品在N2、CO2和N2+O2气氛中分别发生的热解、气化和燃烧反应,反应过程的特征参数不同;在N2中主要热解温度范围为200~560℃,反应过程在600℃基本完成;随着升温速率增加,热解最大失重速率提高;污泥样品在N2中的热解过程依次析出H2O、CO2、CH4和CO等气体;污泥样品热解不同反应阶段具有不同反应机理和动力学参数,表观活化能在60~100 kJ/mol范围内。     

Kinetic modeling and optimization of biomass pyrolysis for bio-oil production

Thermo-kinetic models for biomass pyrolysis were simulated under both isothermal and non-isothermal conditions to predict the optimum parameters for bio-oil production. A comparative study for wood, sewage sludge, and newspaper print pyrolysis was conducted. The models were numerically solved by using the fourth order Runge–Kutta method in Matlab-7. It was also observed that newspaper print acquired least pyrolysis time to attain optimum bio-oil yield followed by wood and sewage sludge under the identical conditions of temperature and heating rate. Thus, at 10 K/min, the optimum pyrolysis time was 21.0, 23.8, and 42.6 min for newspaper print, wood, and sewage sludge, respectively, whereas the maximum bio-oil yield predicted was 68, 52, and 36%, respectively.     

Pilot-scale pyrolysis experiment of municipal sludge and operational effectiveness evaluation

To promote engineering application of municipal sludge pyrolysis technology, pilot-scale sludge pyrolysis device systems integrated were set up to study the effectiveness of sewage sludge pyrolysis. With the application of various operating conditions, the optimal conditions and products were obtained. And comparing the operational effectiveness of pilot-scale experiment with the bench experiment, the optimal operating conditions and product characteristics coincide with those of the bench experiment. Through energy balance analysis, energy produced from pyrolysis oil and gas together or just from sewage-char is almost sufficient to supply the energy required for sewage sludge pyrolysis.     

The conversion of sewage sludge into biochar reduces polycyclic aromatic hydrocarbon content and ecotoxicity but increases trace metal content

The presence of contaminants considerably restricts the application of sewage sludge for the fertilisation and reclamation of soils. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment and vary widely in sewage sludge depending on the input of industrial effluents. The objective of the study was the investigation whether the pyrolysis affect (reduces or adds) the total quantity of PAHs in sewage sludge-derived biochars and whether the pyrolysis changes the PAHs spectrum in terms of relative contributions of more hazardous components. Additionally, the trace metal content was determined before and after pyrolysis as well as the ecotoxicological parameters test towards plant (Lepidium sativum), bacteria (Vibrio fischeri) and crustacean (Daphnia magna). Sewage sludges conversion to biochar significantly reduced the content of PAHs (from 8- to 25-fold depending on pyrolysis temperature and kind of sludge). The exception was the content of naphthalene. Naphthalene was predominant in sewage sludge-derived biochars. However the concentration of the most hazardous 5- and 6-rings PAHs in sewage sludge-derived biochars was much lower compared to sewage sludge. The pyrolysis of sewage sludges caused also a significant reduction of their toxicity towards the test organisms. Only in the case of crustacean it was observed that the extracts from some biochars, obtained at higher temperatures (600 °C and 700 °C) were more toxic to D. magna than extracts from sewage sludge. In turn, after pyrolysis an increase was noted for trace metals content (Pb, Cd, Zn, Cu, Ni and Cr).     

High temperature steam gasification of wastewater sludge

High temperature steam gasification is one of the most promising, viable, effective and efficient technology for clean conversion of wastes to energy with minimal or negligible environmental impact. Gasification can add value by transforming the waste to low or medium heating value fuel which can be used as a source of clean energy or co-fired with other fuels in current power systems. Wastewater sludge is a good source of sustainable fuel after fuel reforming with steam gasification. The use of steam is shown to provide value added characteristics to the sewage sludge with increased hydrogen content as well total energy. Results obtained on the syngas properties from sewage sludge are presented here at various steam to carbon ratios at a reactor temperature of 1173 K. Effect of steam to carbon ratio on syngas properties are evaluated with specific focus on the amounts of syngas yield, syngas composition, hydrogen yield, energy yield, and apparent thermal efficiency. The apparent thermal efficiency is similar to cold gas efficiency used in industry and was determined from the ratio of energy in syngas to energy in the solid sewage sludge feedstock. A laboratory scale semi-batch type gasifier was used to determine the evolutionary behavior of the syngas properties using calibrated experiments and diagnostic facilities. Results showed an optimum steam to carbon ratio of 5.62 for the range of conditions examined here for syngas yield, hydrogen yield, energy yield and energy ratio of syngas to sewage sludge fuel. The results show that steam gasification provided 25% increase in energy yield as compared to pyrolysis at the same temperature.     

Effect of wastewater treatment processes on the pyrolysis properties of the pyrolysis tars from sewage sludges

The pyrolysis properties of five different pyrolysis tars, which the tars from 1# to 5# are obtained by pyrolyzing the sewage sludges of anaerobic digestion and indigestion from the A2/O wastewater treatment process, those from the activated sludge process and the indigested sludge from the continuous SBR process respectively, were studied by thermal gravimetric analysis at a heating rate of 10 ℃/min in the nitrogen atmosphere. The results show that the pyrolysis processes of the pyrolysis tars of 1#, 2#, 3# and 5# all can be divided into four stages: the stages of light organic compounds releasing, heavy polar organic compounds decomposition, heavy organic compounds decomposition and the residual organic compounds decomposition. However, the process of 4# pyrolysis tar is only divided into three stages: the stages of light organic compounds releasing, decomposition of heavy polar organic compounds and the residual heavy organic compounds respectively. Both the sludge anaerobic digestion and the "anaerobic" process in wastewater treatment processes make the content of light organic compounds in tars decrease, but make that of heavy organic compounds with complex structure increase. Besides, both make the pyrolysis properties of the tars become worse. The pyrolysis reaction mechanisms of the five pyrolysis tars have been studied with Coats-Redfern equation. It shows that there are the same mechanism functions in the first stage for the five tars and in the second and third stage for the tars of 1#, 2#, 3# and 5#, which is different with the function in the second stage for 4# tar. The five tars are easy to volatile.     

Thermochemical conversion of sewage sludge: A critical review

The increasing levels of sewage sludge production demands research and development to introduce more commercially feasible options for reducing socio-economic and environmental problems associated with its current treatment. Sewage sludge may be processed to produce useful products or as a feedstock for energy generation. Initially, the characteristics of sewage sludge are discussed in terms of composition and the current options for its treatment with the associated environmental impacts. Processes to valorize sewage sludge are discussed, including heavy metal removal from sewage sludge, production of bio-char, production and use of activated carbon and use of sewage sludge combustion ash in cement and concrete. Thermochemical processes i.e., pyrolysis, co-pyrolysis and catalytic pyrolysis, also gasification and combustion for process intensification, energy and resource recovery from sewage sludge are then critically reviewed in detail. The pyrolysis of sewage sludge to produce a bio-oil is covered in relation to product bio-oil composition, reactor type and the use of catalysts. Gasification of sewage sludge focusses on the characteristics of the different available reactor types and the influence of a range of process parameters and catalysts on gas yield and composition. The selection and design of catalysts are of vital importance to enhance the selectivity of the selected thermochemical pyrolysis or gasification process. The catalysts used for sewage sludge treatment need more research to enable selectivity towards the targeted desired end-products along with optimization of parametric conditions and development of innovative reactor technologies. The combustion of sewage sludge is reviewed in terms of reactor technologies, flue gas cleaning systems and pollutant emissions. In addition, reactor technologies in terms of technological strength and market competitiveness with the particular application to sewage sludge are compared for the first time for thermochemical conversion. A critical comparison is made of the drying techniques, co-feedstocks and catalytic processes, reaction kinetics, reactor technologies, operating conditions to be optimized, removal of impurities, fuel properties, their constraints and required improvements. The emphasis of this review is to promote environmental sustainability for process intensification, energy and resource recovery from pyrolysis, gasification and combustion involving the use of catalysts.     

Sewage sludge-to-energy approaches based on anaerobic digestion and pyrolysis: Brief overview and energy efficiency assessment

Energy recovery from sewage sludge offers an opportunity for sustainable management of sewage sludge and energy. Anaerobic digestion and pyrolysis are among the most promising processes applicable for sewage sludge-to-energy conversion. Anaerobic digestion of sewage sludge forms methane-rich biogas, which can be utilized as fuel to offset heat and electricity consumption of the wastewater treatment sector. However, the digestion process has the limitation that it cannot sufficiently extract the energy in sewage sludge. The digested sludge is still energy profitable in that it contains considerable organic matter, but poor in biodegradability. Sludge pyrolysis is an innovative process that can convert both raw and digested sludge into useful bioenergy in the form of oil and gas, forming biochar as a byproduct that is environmentally resistant and holds potential for carbon sequestration and soil conditioning. It is expectable that sludge pyrolysis would step into practical deployment in the near future.This paper presents a brief overview of anaerobic digestion and pyrolysis in the application to bioenergy production from sewage sludge. An assessment of energy conversion efficiency of two parallel sludge-to-energy pathways is also presented. One pathway relies on an exclusive pyrolysis process (fed with raw sludge) while the other is based on anaerobic digestion followed by pyrolysis (fed with the digested sludge). The pathway via the combination of anaerobic digestion and pyrolysis could achieve higher energy efficiency compared to the pathway employing the pyrolysis alone.     

Pyrolysis of blends of different types of sewage sludge with one bituminous coal

Pyrolysis of sewage sludge samples from three Asturian urban wastewater treatment plants was carried out. One high volatile bituminous coal and its blends with 10 and 50 wt% of sludge were studied by thermogravimetry. The same operational conditions (a constant heating rate of 10 °C/min in the temperature range 25–800 °C and a N₂ flow of 200 cm³/min) were maintained throughout. The results indicate that sludge is formed by two organic fractions with different reactivity, whose devolatilisation processes partially overlap. Both fractions are more reactive than coal, since they decompose and devolatilise at temperatures lower than coal. Under oxidizing conditions, the action of oxygen during pyrolysis depends on the conditioning of sludge. If sludge is treated with FeCl₃, oxidative pyrolysis takes place. The behaviour of sludge–coal blends is intermediate between those of the coal and the corresponding sludge, without interactions between both blend components. In addition, a kinetic analysis was performed to fit thermogravimetric data, the global processes being considered as a series of consecutive first order reactions. A reasonable fit to the experimental data was obtained for all materials and their blends.     

Thermogravimetric analysis of the co-combustion of coal and paper mill sludge

The thermal behavior of semi-anthracite coal, paper sludge and their blends during pyrolysis and combustion processes was investigated in this study. The experiments were conducted in a differential thermogravimetric analyzer at different heating rates (10 K/min, 20 K/min and 30 K/min) and at temperatures ranging from 310 K to 1300 K. The results revealed that de-volatilization of paper sludge occurred earlier with a higher rate, and that the process was further accelerated under oxygen-enriched conditions. The blends had integrative thermal profiles that reflected both paper sludge and coal. In addition, the blends showed different ignition and combustion behavior depending on the percentage of sludge. Two types of non-isothermal kinetic analysis methods were applied to evaluate the combustion processes. The kinetic parameters of the blends confirmed the improved ignition characteristics. In addition, both the TG profiles and activation energy indicated that the combustion of their blends with low percentages of sludge, such as 10 wt.%, were similar to that of coal. These experimental results help explain and predict the behavior of coal and paper sludge blends in practical applications.