Experimental study of the bio-oil production from sewage sludge by supercritical conversion process

Environment-friendly treatment of sewage sludge has become tremendously important. Conversion of sewage sludge into energy products by environment-friendly conversion process, with its energy recovery and environmental benefits, is being paid significant attention. Direct liquefaction of sewage sludge into bio-oils with supercritical water (SCW) was therefore put forward in this study, as de-water usually requiring intensive energy input is not necessary in this direct liquefaction. Supercritical water may act as a strong solvent and also a reactant, as well as catalyst promoting reaction process. Experiments were carried out in a self designed high-pressure reaction system with varying operating conditions. Through orthogonal experiments, it was found that temperature and residence time dominated on bio-oil yield compared with other operating parameters. Temperature from 350 to 500 °C and reaction residence time of 0, 30, 60 min were accordingly investigated in details, respectively. Under supercritical conversion, the maximum bio-oil yield could achieve 39.73%, which was performed at 375 °C and 0 min reaction residence time. Meanwhile, function of supercritical water was concluded. Fuel property analysis showed the potential of bio-oil application as crude fuel.     

超临界水氧化法处理含油污泥

采用间歇式超临界水氧化(SCWO)反应器处理采油过程产生的含油污泥,考察了反应参数对污泥COD去除率的影响。实验结果表明,当初始COD为1 000 mg/L、反应温度为420℃、反应时间为10 min,反应压力为24 MPa、溶液pH为10、过氧比为400%时,SCWO法对含油污泥的COD去除率为92.20%,收集液COD低于GB8978—1996规定的一级排放标准(100 mg/L)。     

好氧颗粒污泥脱氮性能研究

研究了序批式反应器的污泥沉降时间、曝气速率、水力停留时间、进水方式等参数对好氧颗粒污泥的性能以及COD,NH4^*—N,TN去除率的影响。研究结果表明,在一个运行周期内,反应器运行工艺参数,如DO质量浓度和pH,具有一定的变化规律,并且其变化与反应器内各物质的转化存在严格的对应关系。因此,参照反应器运行工艺参数的特征点,对反应过程进行必要的控制,可达到较理想的处理效果。     

Study on optimal energy efficiency of a sludge drying-incineration combined system

Thermal drying is a frequently used technology to further remove the water in dewatered sludge. However, it is an expensive solution due to its highly energy consumption. The combination of sludge drying and incineration system, in which, the energy generated from sludge incineration is reused to sludge drying, can largely save the energy consumption of sludge treatment facilities. A bench-scale paddle sludge dryer was built to study the drying characteristics of sludge. Results show that, a significant fluctuation of sludge drying rate and stirring power emerges at the moisture content of 55–65 %. An energy model was established based on a sludge drying and incineration project. The most reasonable dryness of sludge outlet from sludge dryer and input to sludge incinerator was analyzed, in the purpose of achieving optimal energy efficiency. The mono-incineration of dry sludge can be achieved at 850 °C combustion temperature, when sludge lower heating value (LHV) is about 11213 kJ/kg and moisture content is about 60 % w/w. The effect of operation conditions, including sludge moisture content, LHV, and operation load were analyzed based on the energy model. This energy model could be applied for the improvement of energy efficiency of sludge drying and incineration combined system.     

Reduction of tar using cheap catalysts during sewage sludge gasification

Solid-fuel conversion or gasification study of sewage sludge and energy recovery has become increasingly important because energy recovery and climate change are emerging issues. Various types of catalysts, such as dolomite, steel slag and calcium oxide, were tested for tar reduction during the sewage sludge gasification process. For the experiments on sewage sludge gasification reactions and tar reduction using the catalysts, a fixed bed of laboratory-scale experimental apparatus was set up. The reactor was made of quartz glass using an electric muffle furnace. The sewage sludge samples used had moisture contents less than 6%. The experimental conditions were as follows: sample weight was 20 g and reaction time was 10 min, gasification reaction temperature was from 600 to 800°C, and the equivalence ratio was 0.2. The quantity of catalysts was 2–6 g, and temperatures of catalyst layers were 500–700°C. As the reaction temperature increased up to 800°C, the yields of gaseous products and liquid products increased, whereas char and tar products decreased, showing effects on gas product compositions. These results were considered to be due to the increase of the water-gas reaction and Boudouard reaction. In the case of experiments with catalysts, dolomite (4 g), steel slag (6 g) and calcium oxide (6 g) were used. When the temperature of catalysts increased, the weight of the tar produced decreased with different cracking performances by different catalysts. Reforming reactions were considered to occur on the surface of dolomite, steel slag and calcium oxide, causing cracking of the hydrocarbon structure, which eventually showed reduced tar generation.     

Catalytic gasification of tars from a dumping site

The work deals with catalytic gasification, pyrolysis and non-catalytic gasification of tar from an industrial dumping site. All experiments were carried out in a vertical stainless steel gasification reactor at 800 °C. Crushed calcined dolomite was used as the gasification catalyst. Parameters such as addition of water and air, and the influence of the catalyst in regard to the composition of the process gas were investigated. The catalytic gasification experiment in the steady state produced process gas with the composition: 56 % of H₂, 9 % of CO, 11 % of CH₄ and 12 % of CO₂ (mol.%). Concentration of the C₂ fraction was lower than 1 mol.%. Volume flow of air was later changed from 120 to 230 ml min^?1 reducing the amount of hydrogen to 51 mol.% and that of methane to 10 mol.%. Process gas created in a non-catalytic gasification process contained 26–30 mol.% of methane, 13–15 mol.% of carbon monoxide and 15–17 mol.% of the C₂ fraction and lower amounts of hydrogen (20 mol.%) and carbon dioxide (2–3 mol.%). The highest apparent conversion of tar was reached in the catalytic gasification processes. A higher rate of catalyst deactivation can be observed when water or air is not added.     

Hydrogen-rich synthesis gas production from waste wood via gasification and reforming technology for fuel cell application

The purpose of this study was to establish a fuel process for an advanced power generation system in which hydrogen-rich synthesis gas, as the fuel for the molten carbonate fuel cell (MCFC), can be extracted from biomass via gasification and reforming technologies. Experiments on waste wood gasification were performed using a bench-scale gasification system. The main factors influencing hydrogen generation in the noncatalytic process and in the catalytic process were investigated, and temperature was identified as the most important factor. At 950°C, without employing a catalyst, hydrogen-rich synthesis gas containing about 54 vol% hydrogen was extracted from feedstock with appropriately designed operation parameters for the steam/carbon ratio and the equivalence ratio. However, by employing a commercial steam reforming catalyst in the reforming process, similar results were obtained at 750°C.     

Clay-sewage sludge co-pyrolysis. A TG-MS and Py-GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Wastewater sewage sludge was co-pyrolyzed with a well characterized clay sample, in order to evaluate possible advantages in the thermal disposal process of solid waste. Characterization of the co-pyrolysis process was carried out both by thermogravimetric-mass spectrometric (TG-MS) analysis, and by reactor tests, using a lab-scale batch reactor equipped with a gas chromatograph for analysis of the evolved gas phase (Py-GC).Due to the presence of clay, two main effects were observed in the instrumental characterization of the process. Firstly, the clay surface catalyzed the pyrolysis reaction of the sludge, and secondly, the release of water from the clay, at temperatures of approx. 450-500 °C, enhanced gasification of part of carbon residue of the organic component of sludge following pyrolysis.Moreover, the solid residue remaining after pyrolysis process, composed of the inorganic component of sludge blended with clay, is characterized by good features for possible disposal by vitrification, yielding a vitreous matrix that immobilizes the hazardous heavy metals present in the sludge.     

Lactic Acid Production by Hydrolysis of Poly(Lactic Acid) in Aqueous Solutions: An Experimental and Kinetic Study

Poly(lactic acid) (PLA) is increasingly utilized as an alternative to petroleum-based polymers in order to reduce their impact on the environment. The monomer of PLA is mainly produced from corn, which, in addition to its food utilization, can be also used for the production of bioethanol or biofuels. In this work the depolymerization (chemical recycling) of PLA pellets in a batch reactor at temperatures near the melting temperature of solid PLA has been investigated to produce lactic acid. New experimental data are presented and a kinetic model is provided for a first analysis. With a residence time less than 120 min, a yield of lactic acid greater than 95 % has been obtained at temperatures of 160 and 180 °C for pressure equal to water vapour pressure and a water/PLA ratio by weight equal ~10.     

Supercritical water oxidation of landfill leachate

In this paper, ammonia as an important ingredient in landfill leachate was mainly studied. Based on Peng-Robinson formulations and Gibbs free energy minimization method, the estimation of equilibrium composition and thermodynamic analysis for supercritical water oxidation of ammonia (SCWO) was made. As equilibrium is reached, ammonia could be totally oxidized in SCW. N(2) is the main product, and the formation of NO(2) and NO could be neglected. The investigation on SCWO of landfill leachate was conducted in a batch reactor at temperature of 380-500 °C, reaction time of 50-300s and pressure of 25 MPa. The effect of reaction parameters such as oxidant equivalent ratio, reaction time and temperature were investigated. The results showed that COD and NH(3) conversion improved as temperature, reaction time and oxygen excess increased. Compared to organics, NH(3) is a refractory compound in supercritical water. The conversion of COD and NH(3) were higher in the presence of MnO(2) than that without catalyst. The interaction between reaction temperature and time was analyzed by using response surface method (RSM) and the results showed that its influence on the NH(3) conversion was relatively insignificant in the case without catalyst. A global power-law rate expression was regressed from experimental data to estimate the reaction rate of NH(3). The activation energy with and without catalyst for NH(3) oxidation were 107.07 ± 8.57 kJ/mol and 83.22 ± 15.62 kJ/mol, respectively.     

Gasification characteristics of sewage sludge combined with wood biomass

The gas products from gasification processes have been considered to have some limitations in gas composition and heating value from the previous studies. Gasification characteristics of sewage sludge and wood mixture were investigated using different mixing ratios with the purpose of better quality of gas product suitable for energy/power generation. The gasification experiment was performed by an indirectly heated fluidized bed reactor. As reaction temperature increased from 600 to 900 °C, the yield of gas product increased with higher generation of CO, H₂ and CH₄ by more activated gas conversion reactions. As the equivalence ratio increased from 0.2 to 0.4, composition ratio of CO₂ increased while CO, CH₄, H₂ decreased as expected. Several operating variables including mixing ratio of wood with dried sludge were also tested. From this initial stage of experiment, optimal operating conditions for the bubbling fluidized bed gasifier, could be considered 900 °C in temperature; 0.2 in equivalence ratio and 40 % in wood mixing ratio within test variables range. These results will be more thoroughly investigated for the application to the larger scale pilot system.     

Effect of pretreatment techniques on food waste solubilization and biogas production during thermophilic batch anaerobic digestion

The purpose of this study was to optimize the alkaline, ultrasonication, and thermal pretreatment in order to enhance the solubilization of food waste (FW) for the production of volatile fatty acids, hydrogen, and methane in thermophilic batch anaerobic digestion. Initially, the effect of pretreatment techniques in the acidogenic phase was studied, and the optimal combinations of different conditions were determined. It was found that each pretreatment technique affected food waste solubilization differently. Alkaline pretreatment increased hydrogen yield in the acidogenic sludge by four times over control. COD solubilization was increased by 47 % when FW pre-heated at 130 °C for 60 min. Ultrasonication at 20 kHz and 45 min reduced processing time to 38 h from the 60–80 h needed in normal operation. Response surface methodology (RSM) was used to optimize a combination of alkaline, ultrasonication, and thermal pretreatment. Optimized conditions were applied to methanogenic single-stage thermophilic AD process, and their impact on biogas production was monitored. Results showed that FW heated at 130 °C for 50 min geminates biogas production compared to control experiment. In conclusion, a short thermal pretreatment regime could significant affect biogas production in single-stage thermophilic AD.     

Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes

The influence of the lipid concentration on the anaerobic co-digestion of municipal biomass waste and waste-activated sludge was assessed by biochemical methane potential (BMP) tests and by bench-scale tests in a mesophilic semi-continuous stirred tank reactor. The effect of increasing the volatile solid (VS) concentration of lipid from 0% to 75% was investigated. BMP tests showed that lipids in municipal biomass waste could enhance the methane production. The results of bench-scale tests showed that a lipids concentration of 65% of total VS was the inhibition concentration. Methane yields increased with increasing lipid concentration when lipid concentrations were below 60%, but when lipid concentration was set as 65% or higher, methane yields decreased sharply. When lipid concentrations were below 60%, the pH values were in the optimum range for the growth of methanogenic bacteria and the ratios of volatile fatty acid (VFA)/alkalinity were in the range of 0.2–0.6. When lipid concentrations exceeded 65%, the pH values were below 5.2, the reactor was acidized and the values of VFA/alkalinity rose to 2.0. The amount of Brevibacterium decreased with increasing lipid content. Long chain fatty acids stacked on the methanogenic bacteria and blocked the mass transfer process, thereby inhibiting anaerobic digestion.     

Production of hydrogen by steam gasification of dehydrochlorinated poly(vinyl chloride) or activated carbon in the presence of various alkali compounds

Steam gasification of dehydrochlorinated poly(vinyl chloride) (PVC) or activated carbon was carried out in the presence of various alkali compounds at 3.0 MPa and 560°C–660°C in a batch reactor or in a semi-batch reactor with a flow of nitrogen and steam. Hydrogen and sodium carbonate were the main products, and methane and carbon dioxide were the minor products. Yields of hydrogen were high in the presence of sodium hydroxide and potassium hydroxide. The acceleration effect of the alkali compounds on the gasification reaction was as follows: KOH > NaOH > Ca(OH)₂ > Na₂CO₃. The rate of gasification increased with increasing partial steam pressure and NaOH/C molar ratio. However, the rate became saturated at a molar ratio of NaOH/C greater than 2.0.     

Comparative assessment of municipal sewage sludge incineration,gasification and pyrolysis for a sustainable sludge-to-energy management in Greece

For a sustainable municipal sewage sludge management, not only the available technology, but also other parameters, such as policy regulations and socio-economic issues should be taken in account. In this study, the current status of both European and Greek Legislation on waste management, with a special insight in municipal sewage sludge, is presented. A SWOT analysis was further developed for comparison of pyrolysis with incineration and gasification and results are presented. Pyrolysis seems to be the optimal thermochemical treatment option compared to incineration and gasification. Sewage sludge pyrolysis is favorable for energy savings, material recovery and high added materials production, providing a ‘zero waste’ solution. Finally, identification of challenges and barriers for sewage sludge pyrolysis deployment in Greece was investigated.     

Gasification and its emission characteristics for dried sewage sludge utilizing a fluidized bed gasifier

Various research has attempted to determine the proper treatment of sewage sludge, including thermal technologies. Efficient thermal technologies have been focused on because of their energy saving/energy recovery. Gasification technology can be considered one of these approaches. In this study, the characteristics of gasification reactions were investigated with the aim of finding fundamental data for utilizing sewage sludge as an energy source. For the experiments on sewage sludge gasification reaction characteristics, a laboratory-scale experimental apparatus was set up with a fluidizing bed reactor of 70-mm inner diameter and 600-mm total height using an electric muffle furnace. The experimental materials were prepared from a sewage treatment plant located in Seoul. The reaction temperature was varied from 630 to 860°C, and the equivalence ratio from 0.1 to 0.3. The gas yields, compositions of product gas, and cold gas efficiencies of product gas were analyzed by GC/TCD and GC/FID installed with a carboxen-1000 column. The experimental results indicated that 800°C, ER 0.2 was an optimum condition for sewage sludge gasification. The maximum yield of product gas was about 44%. Producer gas from experiments was mainly composed of hydrogen, carbon monoxide, carbon dioxide, and methane. The cold gas efficiency of sewage sludge gasification was about 68%. The H₂/CO ratio and CO/CO₂ ratio were about 1.1 and 1.4, respectively, in optimum reaction conditions. Gaseous pollutants such as SO₂, HCl, NH₃, H₂S, and NO₂ were also analyzed at various gasification/combustion conditions, and their gaseous products were compared, showing significantly different oxidized product distributions.     

催化超临界水氧化法去除焦化废水中的氨氮

采用催化超临界水氧化技术处理焦化废水.实验结果表明:升高反应温度、增加反应压力、延长反应时间可提高废水中氨氮去除率;在反应时间为60 s、反应压力为30 MPa、反应温度为460℃的最佳实验条件下,未加入催化剂时的氨氮去除率为53.7％,加入催化剂后,氨氮去除率大幅提高,以MnO2为催化剂时氨氮去除率为86.9％,以CuSO4为催化剂时氨氮去除率为92.4％.     

Hydrogen-rich fuel gas production from refuse plastic fuel pyrolysis and steam gasification

In this study, experimental conditions were optimized to maximize the production of hydrogen gas from refuse plastic fuel (RPF) by pyrolysis and steam gasification processes conducted in a laboratory-scale reactor. We carried out gasification using 10-g RPF samples at different temperatures (700°-1000°C) with and without steam. The effect of the amount of steam (0–0.25 g/min) for RPF steam gasification was also studied. The effect of K₂CO₃ as a catalyst on these processes was also investigated. Experimental results showed that the hydrogen gas yield increased with temperature; with respect to the gas composition, the hydrogen content increased mainly at the expense of other gaseous compounds, which highlights the major extension of secondary cracking reactions in the gaseous fraction at higher temperatures.     

Gasification of dried sewage sludge: status of the demonstration and the pilot plant

The disposal of sewage sludge from municipal waste water treatment plants is suffering from raising costs.The gasification is an alternative way of treatment, which can reduce the amount of solid residues that must be disposed from a water treatment plant. The produced gas can be used very flexible to produce electrical energy, to burn it very cleanly or to use it for upgrading.The gasification in the fluidised bed and the gas cleaning with the granular bed filter has shown successful operation. A demonstration plant in Balingen was set up in 2002 and rebuilt to a larger throughput in 2010. As a next step a demonstration plant was built in Mannheim and is now at the end of the commissioning phase. Nowadays the product gas is blended with biogas from sludge fermentation and utilized in a gas engine or combustion chamber to produce heat. In the future the process control for a maximized efficiency and the removal of organic and inorganic impurities in the gas will be further improved.     

Gasification applicability study of polyurethane solid refuse fuel fabricated from electric waste by measuring syngas and nitrogenous pollutant gases

To recycle polyurethane foam waste generated from electric appliance recycling centers for use as fuel in a gasification process, polyurethane solid refuse fuel fabricated as pellets was analyzed for the characteristics of elemental composition, proximate analysis, heating value, and thermo-gravimetric testing. It has a high heating value of 29.06 MJ/kg with a high content of combustibles, which could be feasibly used in any thermal process. However, the nitrogen content, of up to 7 %, was comparably higher than for other fuels such as coal, biomass, and refuse-derived fuel, and may result in the emission of nitrogenous pollutant gases of HCN and NH₃. By conducting gasification experiments on polyurethane solid refuse fuel in a fixed-bed reactor, a syngas with a heating value of 9.76 kJ/m³ and high content of both H₂ and CO were produced with good gasification efficiency; carbon conversion 54 %, and cold gas efficiency 60 %. The nitrogenous pollutant gases in syngas were measured at the concentrations of 160 ppm hydrogen cyanide and 40 ppm ammonia, which may have to be reduced using proper cleaning technologies prior to the commercialization of gasification technology for polyurethane waste.