Formation mechanism of NOx precursor during organic waste pyrolysis coupled with hydrothermal pretreatment
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摘要: 以城市污泥（SS）、中药药渣（HTW）和硅藻（DT）为对象，在水平管式反应器上对比研究了水热处理前后样品在热解过程中NOx前驱物的生成特征，并结合热重（TGA）和X射线光电子能谱（XPS）表征分析了该耦合过程对NOx前驱物的影响机制。结果表明，在240 ℃下进行水热预处理能直接或间接地影响样品燃料N在不同热解阶段时的转化路径，从而在整体层面上降低NOx前驱物的释放量，例如当热解温度为900 ℃时，源于水热焦燃料N的NOx前驱物为55.0%（SS240）、48.1%（HTW240）和51.2%（DT240），比未经处理样品的NOx前驱物释放量分别少9.5%（SS）、6.0%（HTW）和15.4%（DT），但若以原料燃料N为基准，源于水热焦的NOx前驱物则比未经处理样品的NOx前驱物释放量分别少90.1%（SS）、41.9%（HTW）和59.8%（DT），并且对NH3的抑制效果高于HCN。进一步根据热失重曲线及其半焦N官能团的演变规律可以推测，水热预处理对NOx前驱物的两条影响机制，即含N官能团的脱除（对于初次反应的NH3释放）与含N官能团的稳定化（对于二次反应的HCN释放），可为废弃物的清洁利用提供理论参考。Abstract: Taking the high moisture-containing sewage sludge (SS), herbal tea waste (HTW) and diatom (DT) as the feedstock, the characteristics of NOx precursors during pyrolysis with or without hydrothermal pretreatment in a horizontal tubular reactor were compared. The formation mechanism of NOx precursors in pyrolysis coupled with hydrothermal pretreatment was also investigated by means of TGA and XPS techniques. The results show that the hydrothermal pretreatment can affect the formation pathways related to NOx precursors at different pyrolysis stages and reduce the release amount of NOx precursor on the whole level. For example, when the pyrolysis temperature is 900 ℃, the NOx precursor yield derived from the hydrothermal treated coke is 55.0% for SS240, 48.1% for HTW240 and 51.2% for DT240, which is 9.5%, 6.0% and 15.4% less than that for SS, HTW and DT untreated sample, respectively. But if calculating based on the amount of N content in feedstock, the released NOx precursor from the hydrothermal treated coke is 90.1%, 41.9% and 59.8% less than that for SS, HTW and DT untreated sample, respectively. The inhibition effect on NH3 formation is higher than that on HCN formation. Meanwhile, two influencing pathways caused by hydrothermal pretreatment were further elaborated, i.e., the removal of N functionalities that leads to a decrease in NH3 on the primary reaction and the stabilization of N functionalities that leads to a decrease in HCN on the secondary reaction.
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Key words:
- organic wastes /
- hydrothermal pretreatment /
- pyrolysis /
- NOx precursor /
- regulatory mechanism
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图 2 原料及水热焦的热解曲线(升温速率10 ℃/min；氩气气氛)
Figure 2 TG-DTG curves of biowastes and their corresponding hydrochars at Ar atmosphere with a heating rate of 10 ℃/min
图 3 热解温度对原料及水热焦中NOx前驱物生成的影响
Figure 3 Effect of pyrolysis temperature on the formation of NOx precursors derived from biowastes and their corresponding hydrochars
图 4 NOx前驱物中HCN与NH3的比例变化
Figure 4 Variation of the ratio of HCN-N to HN3-N derived from sample
图 5 原料与水热焦在不同热解温度下半焦XPS(N 1s)谱图(x轴：结合能E/eV；y轴：相对强度/cps)
Figure 5 N 1s XPS spectra of char-N derived from feedstock and hydrochar at different temperatures
图 6 原料与水热焦中各种含N官能团随热解温度的变化
Figure 6 Variation of N functionalities with pyrolytic temperature
— ☆ —: char; : N-X; : N-Q; : N-IN; : N-5;: N-A; : N-6
图 7 水热预处理对NOx前驱物的调控机制
Figure 7 Mechanisms for hydrothermal pretreatment on the release of NOx precursor
表 1 原料与水热焦的基本特性
Table 1 Properties of feedstock and the corresponding hydrochar
Sewage sludge Herbal tea waste Diatom SSraw SS240 HTWraw HTW240 DTraw DT240 Yield w/% - 70.27 - 53.95 - 45.77 Ultimate analysis w/% Carbon 21.57 12.65 45.08 53.18 31.92 26.27 Hydrogen 3.67 1.82 6.07 5.46 4.45 2.80 Oxygen 14.22 7.07 32.48 12.71 22.04 15.10 Nitrogen 3.43 1.16 2.72 1.79 5.22 2.74 Sulfur 0.51 0.31 0.26 0.12 1.14 0.22 Proximate analysis w/% Volatile matters 36.57 19.51 69.24 44.69 52.39 23.68 Ash 56.60 76.99 13.39 26.74 35.23 52.87 Fixed carbon 6.83 3.50 17.37 28.57 12.38 23.45 Coalification degree Atomic ratio of O/C 0.49 0.42 0.54 0.18 0.52 0.43 Atomic ratio of H/C 2.04 1.72 1.62 1.23 1.67 1.28 Fuel properties QHHV/(MJ·kg-1) 9.67 8.25 19.37 22.11 13.67 14.44 Energy density ratio 1 0.85 1 1.14 1 1.06 ERE/% - 59.91 - 61.57 - 48.33 note: HHV, higher heating value; ERE, energy recover efficiency 
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表 2 快速热解中挥发分的组分分析
Table 2 Volatile compositions of sample during fast pyrolysis at 800 ℃
Sewage sludge Herbal tea waste Diatom SSraw SS240 HTWraw HTW240 DTraw DT240 Organic acids /% 5.3 4.4 7.5 4.1 20.1 16.6 Aldehydes & ketones /% 3.1 1.3 16.6 10.9 2.3 1.5 Benzene & phenols /% 47.1 65.4 53.6 69.3 33.7 46.8 Furfuran derivations /% 8.1 0.0 7.1 3.3 1.0 0.0 N-related compounds /% 36.4 28.9 15.2 12.4 42.9 35.1 note: the fast pyrolysis is carried out with 104 K/min and argon atmosphere
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