利用衝擊波管研究甲醇高溫熱解的反應速率常數及其反應機制

Abstract

我們利用活塞型衝擊波管-原子共振吸收光譜(ARAS)技術來研究1452 K至1605 K溫度範圍之甲醇(100-1000 ppm)熱解反應動力學。以電腦程式模擬適解由實驗測得之氫原子濃度變化，進而得到在該反應條件下，甲醇熱解的速率常數(ktotal)。此結果結合本實驗室之前的較低濃度實驗數據(0.48 ppm 甲醇 / 1000 ppm H2、 1 ppm 甲醇 / 100 ppm H2及10 ppm甲醇於1660 – 2050 K之熱解反應)6得到：

ktotal = (1.73±1.27)×10-7exp[-(31600±1134)/T] cm3 molecule-1s-1 ，

上述誤差值均為1σ。本實驗成功延伸至更低溫度的甲醇熱解實驗數據(1605-1452 K)，而且我們的實驗值(ktotal)和Jasper, A. W.實驗組20的理論計算結果較為接近，此外本實驗成功以新的反應機制(結合GRI3.0反應機制與本實驗室前篇研究6 使用之36條反應)來進行模擬，可成功地解釋100 ppm甲醇熱解實驗值所得。當我們進一步將此機制用於1000 ppm甲醇熱解實驗裡，反應時間在0-150μsec間，測量所得H原子的濃度低於模擬值，也就是有較長之潛伏期(incubation period)，因此，我們進行敏感度測試(sensitivity analysis)，並調整反應機制中之參數，以期獲得較佳之結果，但模擬所得仍無法與實驗值有滿意的符合，因此需更進一步的研究探討模擬所用的反應機制。

A diaphramless shock tube coupled with atomic resonance absorption spectrophotometry (ARAS) was employed to study the dissociation of methanol between 1452 and 1605 K. The rate constants of the 100 ppm methanol thermal decomposition in this temperature range obtained in this study have been combined with the result of our previous measurements ( 0.48 ppm CH3OH / 1000 ppm H2、1 ppm CH3OH /100 ppmH2 and 10ppm CH3OH，T = 1660 – 2050 K) 6 to give :

ktotal = (1.73±1.27)×10-7exp[-(31600±1134)/T] cm3 molecule-1s-1

Highly sensitive detection system in our laboratory has enabled to supply reliable experimental information in the lower temperature range (down to1605-1452 K). Our results agree with that predicted by Jasper, A. W.20 et al.Detailed reaction scheme by combining GRI-Mech 3.0 mechanism with the previously proposed simple reaction scheme6 (composed of 36 elementary reactions to simulate experiment results of 0.48-10 ppm CH3OH) is constructed to simulate the pyrolysis CH3OH at higher concentrations. The new reaction schemes agree the experimental time dependence of [H] in the pyrolysis of 100 ppm CH3OH. We also have conducted experiments at 1000 ppm CH3OH. Reaction time was limited 150μs due to poor detection of H atoms at high concentrations ( > to 1×1014 molecule / cm3). The observed temporal profiles of [H], especially in the early stage, do not agree with the results of simulations. Sensitivity analysis has been conducted and parameters of rate constants in the mechanism have been adjusted accordingly. The results are not satisfactory and further investigations are necessary to solve the discrepancy.