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城市污泥是污水處理過程中的副產(chǎn)物，其含水率高，成分復雜，常規(guī)處理方法無害化難度大。超臨界水氧化技術(shù)可以直接處理濕污泥，具有處理徹底、反應(yīng)迅速、無二次污染等優(yōu)點。本文圍繞城市污泥預熱過程中的亞臨界水解特性及反應(yīng)過程中的超臨界水處理規(guī)律展開研究，主要研究內(nèi)容和創(chuàng)新性成果如下：
污泥預熱至200 °C左右首先會發(fā)生亞臨界水解，本文研究了溫度、初始含水率和預熱時間對水解的液相產(chǎn)物、固相產(chǎn)物及液固分離效果的影響規(guī)律。研究結(jié)果表明，溫度對水解的影響最大，當溫度從140 °C升高至210 °C時，離心后固相含水率從75.1 %降至46.0 %，清液TOC濃度從36378 mg•L-1升高至44510 mg•L-1。170 °C時超過一半的有機物從固相進入了液相。最佳水解參數(shù)為87 %含水率污泥，溫度為170 °C，預熱時間為40 min，這一條件下水解后的固相產(chǎn)物與原污泥的燃燒性能相當。
污泥進一步預熱至300 °C時會生成生物油并可能產(chǎn)生結(jié)焦現(xiàn)象，本文研究了污泥種類、反應(yīng)強度（用于描述反應(yīng)溫度和時間）、反應(yīng)壓力、污泥含水率、回收油相所用有機溶劑和添加劑等因素對預熱過程中產(chǎn)物產(chǎn)率的影響，并采用Mathematica軟件建立了用于預測升溫和恒溫過程中產(chǎn)物產(chǎn)率的反應(yīng)動力學模型。研究結(jié)果表明，初沉污泥的產(chǎn)油率最高，300 °C、60 min時高達29.7 %。反應(yīng)強度0.1–100時，油相產(chǎn)率最高（20.1–30.9 %），這一區(qū)間容易發(fā)生結(jié)焦堵塞。生物油由污泥通過脫水反應(yīng)和脫羧基反應(yīng)直接轉(zhuǎn)化而來，這一過程活化能僅為60 kJ•mol-1，而焦油主要通過脫水反應(yīng)產(chǎn)生。生物油主要由脂肪烴和脂肪酸組成，中等極性。壓力對產(chǎn)物產(chǎn)率的影響較小，85 %含水率污泥的產(chǎn)油率最高。添加K2CO3、Na2CO3、CH3COOH、MoO3-CoO/γ-Al2O3和Ru/C添加劑可以明顯降低產(chǎn)油率，抑制結(jié)焦，其中Ru/C效果最明顯。
當溫度超過水的臨界點（374 °C）以后，超臨界水氣化和氧化反應(yīng)占主導，本文通過實驗和Aspen Plus模擬研究了含水率、壓力、溫度和氧化系數(shù)對超臨界水氣化/氧化產(chǎn)物性質(zhì)的影響規(guī)律，并推測其反應(yīng)路徑，同時對超臨界水氣化–超臨界水氧化組合工藝進行了分析。研究結(jié)果表明，當含水率從87 %增至95 %時，氣相能量回收率降低0.5 %，87 %含水率的污泥75 °C時粘度低于6500 mPa•s，作為反應(yīng)原料較為合適。壓力對污泥超臨界水氣化產(chǎn)物的影響不大，控制在25 MPa即可。溫度從400 °C升高至600 °C時，氣化率從10.4 %增至37.1 %。450 °C時，氧化系數(shù)0時H2和CH4的氣相產(chǎn)率最高。450 °C以上時，液相中92 %以上的氮都以NH3–N形式存在，只有當溫度達到600 °C，NH3–N才能有效降解。600 °C，氧化系數(shù)為4.0時，固相產(chǎn)物質(zhì)量僅為初始污泥總質(zhì)量的3.5 %，減量化效果明顯。污泥在超臨界水氣化（450 °C）后的液相產(chǎn)物再進行超臨界水氧化（600 °C）處理，總氧化系數(shù)1.0時，最終COD和NH3–N分別降至354 mg•L-1和47 mg•L-1，基本可達到600 °C，氧化系數(shù)4.0條件下的處理效果。
污泥超臨界水處理后會生成NH3–N，酚類和吡啶類等難降解的中間產(chǎn)物，這類物質(zhì)與活潑物質(zhì)共存時，其氧化速率提高，稱為共氧化現(xiàn)象。本文采用實驗和Chemkin計算的方法進行了污泥與活潑醇類（甲醇、乙醇和異丙醇）的超臨界水共氧化研究。研究結(jié)果表明，醇類的添加對污泥中TOC和NH3–N的降解具有共氧化作用。原因一是醇類可提供OH•和HO2•這兩種活性自由基，原因二是醇類的添加抑制了產(chǎn)物中較難降解的芳香族化合物的產(chǎn)生，而促進了易降解的不含氮開鏈化合物的產(chǎn)生。乙醇可以在最短的時間內(nèi)產(chǎn)生最高濃度的HO2•自由基，異丙醇其次，甲醇最慢產(chǎn)生濃度最低，因此共氧化效果乙醇最好，異丙醇次之，甲醇最弱。
材料腐蝕問題制約著超臨界水處理技術(shù)的商業(yè)化應(yīng)用，本文研究了不銹鋼（316、316L）、鎳基合金（Incoloy 800和825、Inconel 600和625以及Hastelloy C276）以及鈦合金（TA–10）在污泥和亞/超臨界水環(huán)境中的腐蝕特性。研究結(jié)果表明，臨界點附近，350 °C時金屬經(jīng)歷活化路徑，表面及側(cè)面形貌平整，400 °C時經(jīng)歷鈍化路徑，首先生成氧化物，隨后氧化物發(fā)生脫落和溶解。超臨界水環(huán)境下，溫度高于450 °C時，化學腐蝕占主導，由于擴散速率Fe＞Ni＞＞Cr，鐵基合金生成Fe3O4外層和FeCr2O4內(nèi)層的氧化膜結(jié)構(gòu)，而鎳基合金生成外層NiO，內(nèi)層Cr2O3，最后通過固溶反應(yīng)生成NiCr2O4的氧化膜結(jié)構(gòu)。350 °C和400 °C時推薦316L作為選材，450 °C時推薦Incoloy 825作為選材，520 °C時推薦Inconel 625作為選材。
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- [ 1 ] 西安交通大學能源與動力工程學院
英文摘要
Municipal sewage sludge is generated by wastewater treatment with high moisture contents and complicated compositions. Conventional sludge treatment methods not only require a energy-inefficient pre-drying procedure but can also generate secondary pollution. Supercritical Water Oxidation (SCWO) of sludge can efficiently realize the maximization reduction and harmless disposal with low cost by direactly treating wet sludge. This thesis studies the subcritical hydrolysis characteristics and supercritical water oxidation of municipal sewage sludge. The major innovative results are shown as follows:
Influences of temperature, initial moisture content and preheating time on liquid and solid products as well as their separation are studied with respect to subcritical hyrolysis reactions below 200 °C. The results show that temperature plays the most important role in hydrolysis. Moisture contents of solid products decrease from 75.1 % to 46.0 % and the liquid TOC contents increase from 36378 mg•L-1 to 44510 mg•L-1 with an increase in temperature from 140 °C to 210 °C. More than half organics transfer from the solid phase to the liquid phase at 170 °C. The optimum hydrolysis parameters are 87 % moisture content, 170 °C and 40 min. Under this condition, the combustion performance of solid products are comparable to original sludge.
Biocrude produces and cooking occurs with furture heating to higher than 300 °C. Influence of sludge type, reaction intensity which describes the reaction temperature and holding time, pressure, moistuere content of sludge, recovery solvent and additives on the products fraction yields are investigated. Reaction kinetic models are established using the Mathematica Software to predict the products fraction yields during the heating and isothermal processes. The results show that primary sludge produces the maximum biocrude yield as high as 29.7 % at 300 °C and 60 min. Highest biocrude yields (20.1–30.9 %) are achieved with the reaction intensity of 0.1–100, which is prone to coking and plugging. Biocrude is directly produced from sludge through dehydration and decarboxylation reactions. The activation energy of this process is only 60 kJ•mol-1. By contrast, tar is generated by the dehydration reaction. Biocrude is mainly composed of aliphatic hydrocarbons and fatty acids with medium polarity. Pressure plays a small effect on the product fraction yields. Highest biocrude yields are produced from sludge with 85 % moisture contents. Added K2CO3, Na2CO3, CH3COOH, MoO3-CoO/γ-Al2O3 and Ru/C significantly decreases the biocrude yield. Ru/C is the most effective additive to avoid cooking.
Supercritical Water Gasification (SCWG) and SCWO domains when the temperature exceeds the critical point of water (374 °C). Effects moisture contents of sludge, pressure, temperature and oxidation coefficient are explored experimentally and theoretically with the Aspen Plus Software to deduce the reaction pathway. Feasibility of a SCWG–SCWO combined process is also analyzed. The results show that under SCWG conditions, energy recovery rates of gaseous products decrease by 0.6 % as the moisture contents increase from 87 % to 95 %. The opitimum moisture content is 87 % because its viscosity is lower than 6500 mPa•s at 75 °C. Pressure has little influence on the gaseous and liquid products, therefore, a pressure just over the critical pressure (22.05 MPa), such as 25 MPa is reasonable. A temperature increase from 400 °C to 600 °C results in a gasification efficient rise from 10.4 % to 37.1 %. An oxidation coefficient of 0 produces the highest H2 and CH4 yields at 450 °C. 92 % of nitrogen in liquid products is in the form of NH3–N at a temperature higher than 450 °C. 600 °C or higher temperature is essential for a significant degradation of NH3–N. At 600 °C and with an oxidation coefficient of 4.0, the mass of solid products is only 3.5 % of the sludge initial mass. If coupling SCWG at 450 °C and SCWO at 600 °C with a total oxidation coefficient of 1.0, COD and NH3–N contents of liquid phase are 354 mg•L-1 and 47 mg•L-1, respectively, which are basically comparable to the water quality obtained at 600 °C and with an oxidation coefficient of 4.0.
Refractory species such as NH3–N, phenols and pyridines can be produced by supercritical water process. The oxidation rate of these species can be accelerated with reactive species, which is referrd to co-oxidation phenomena. Effects of reactive alcohols (methanol, ethanol and iso-propanol) on SCWO of sludge are studied experimentally and theoretically with the Chemkin Software. The results show that there are co-oxidation effects between alcohols and sludge on the degration of TOC and NH3–N. One reason is that alcohos can provide OH• and HO2• radicals. Another reason is that adding alcohols not only suppresses forming recalcitrant aromatic products but also promotes producing reactive products (non-nitrogen open chain compounds). Production speeds and amounts of HO2• radicals by alcohols are in the following order: ethanol＞ iso-propanol＞ methanol, so ethanols is the best co-oxidaiton additives, followed by iso-propanol and methanol.
Corrosion problems restrict the commercial application of supercritical water treatment technology. Corrosion behaviors of stainless steel (316 and 316L), nickel base alloy (Incoloy 800, Incoloy 825, Inconel 600, Inconel 625 and Hastelloy C276) and titanium alloy (TA–10) in subcritical and SCW environment are explored. The results show that at temperatures near the critical point of water, surface and side morphologies are smooth because alloys undergo activation pathway at 350 °C. By constrast, the alloys form oxide and follow by the spallation and dissolution of oxide because the alloys undergoes passive pathway at 400 °C. In SCW environment, chemical corrosion domains when temperature is higher than 450 °C. For the differences in the diffusivities of alloy elements (Fe＞Ni＞＞Cr), duplex oxide layer structures of Fe-based alloys are formed with an outer Fe3O4 layer and an inner FeCr2O4 layer. For nickel based alloys, NiCr2O4 oxide is formed by solid solution reaction of a duplex-layer oxide scales (an outer NiO layer and an inner Cr2O3 layer). 316 SS is the optimum material at 350 °C and 400 °C, Incoloy 825 can be chosen at 450 °C and Inconel 625 can be used at this temperature.
英文關(guān)鍵詞
[Biocrude, Corrosion, Oxidation, Sludge, Supercritical water]
