煤中吡咯与吡啶类氮热解的分子动力学模拟

采用基于ReaxFF反应力场的分子动力学研究了不同温度下吡咯与吡啶的热解机理。结果表明,两者的主要含氮产物与中间产物均为 HCN 和 CN,其他主要产物为 H2、C2,H2以及 C3,H4等;随着温度的升高和时间的增长,两者热解的产物数量与种类也越来越多,产物中的氮逐渐从HCN向NH3和N2转移,但吡啶的热解产物要远少于吡咯,且其热解时间也大大晚于吡咯。模拟结果与相关实验数据一致,说明反应力场分子动力学计算可以合理有效地用于吡啶与吡咯热解机理的研究。     

傅立叶红外光谱法对煤中吡咯型氮的热解规律研究

选取吡咯为煤中吡咯氮的模型化合物，在石英管流动反应器中550℃～1020℃温度范围内研究了其热解规律，利用傅立叶红外光谱仪(Ft—Ir)对反应产物进行检测。试验结果表明：吡咯高温热解时，含氮产物最终几乎全部以HCN的形式存在，也能观测到不饱和腈类物质的生成与转化。图4表1参6     

生物质热解过程中氮转化规律的试验研究

采用TG-FTIR联用技术在氩气氛围下研究稻草、麦秆、杨木3种生物质热解过程中4种主要的含氮组分NH3、HCN、HNCO及NO的释放特性,并考察焦炭-N的产率。结果表明:在所选取的3种生物质热解过程中,4种含氮组分的释放趋势均与TG-DTG曲线一致,热解后期(温度高于500℃)释放较少;不同生物质热解,4种含氮组分的释放规律有较大差异,且相对产量分布不同。3种生物质NH3和HCN的相对产量明显高于HNCO和NO,NH3最高,HCN次之,两者之和占70%以上。随着生物质中H/N质量比的增大,NH3的相对产量增加,HCN的相对产量先增加后减少,HCN/NH3物质的量之比逐渐减小。杨木热解过程中气相氮释放较少,燃料氮81.50%存在于焦炭中,而稻草和麦秆中大部分燃料氮随挥发分析出,焦炭氮产率分别为29.97%和33.45%。     

不同煤种挥发氮析出过程的数值模拟与试验研究

采用数值模型研究了烟煤、贫煤和无烟煤等不同煤种热解、燃烧过程中挥发分氮的析出、中间含氮产物HCN的生成以及转变为NO的过程。利用有限体积方法对质量、化学组分、动量和热量守恒方程进行离散求解。计算获得了不同煤种的颗粒着火时间、热解过程、孔隙率、HCN和NO生成率等数据,并与沉降炉试验结果进行了比较,分析。     

废橡胶和废纸热解过程NOx前驱体的生成

在固定床反应器中热解废橡胶和废纸,考察在不同热解终温和加热方式下NOx前驱体(HCN和NH3)的生成特性.研究结果表明,随着热解终温的升高,原料中的N转化为HCN和NH3的比例不断增加.以废橡胶为例,HCN和NH3的产率从400℃的0.5%分别增加到1000℃的7%和9%;加热方式对HCN和NH3的产率有明显的影响.实验条件下,非等温热解时,废橡胶在所有热解温度下HCN和NH3的产率均高于等温热解时的产率.     

CO_2气氛中超细煤粉热解含氮气体释放规律

在固定床反应器上进行了CO2气氛中内蒙古褐煤的热解/气化试验,对含氮气体和CO析出特性进行了连续在线测量,考察了温度、粒径和气氛等因素的影响.结果表明,气氛对于气化反应特性和含氮气体的析出特性有较大影响且在高温区更为明显;煤样与CO2的气化反应在550,℃之后显著加快.CO2气氛下煤中氮主要以NH3、HCN和N2O形式析出,而N2气氛下主要是以NH3、HCN和NO形式析出;不同粒径的煤粉热解时HCN和NH3析出曲线相似且差距不大,N2O析出量随粒径增大稍有增加.     

淖毛湖煤加氢转化过程中硫和氮的迁移规律

基于硫、氮含量对新疆低变质煤热解炼制油气品质的重要影响作用,本实验通过建立多功能煤热解转化实验台,并结合XPS先进测试方法,充分考察了不同温度对新疆淖毛湖煤加氢转化中硫、氮迁移规律的影响.研究结果表明,原煤和低温半焦中的硫主要为噻吩硫、硫砜和硫酸盐等.低温热解条件下主要为不稳定有机硫的分解;当热解温度高于600℃时煤焦表面形成大量硫化物;随着温度的进一步升高,硫酸盐在高温热力环境下逐渐分解.原煤中的氮主要为吡咯氮及少量氮氧化物.低温热解阶段吡咯向吡啶转化,高温阶段吡咯和吡啶向季氮快速转化.当热解温度升高至800℃时,吡咯氮转化完全,季氮和吡啶氮为半焦中氮的主要形式.     

裂解温度对生物质热解焦油成分的影响

以锯末粉体为生物质热解焦油研究对象,研究了热解温度对焦油产量和焦油化学成分的影响规律,结果表明,热解温度为500℃时,生物质热解产生的焦油量最大,温度过高或过低都有利于焦油的减少。不同热解温度下,焦油中碳氢化合物的成分主要是芳香烃和少量的脂肪烃,含氧化合物主要是苯酚及其烷基衍生物,含氮化合物主要是吡啶、吡咯及其烷基衍生物等杂环化合物。     

煤质特性对快速热解中HCN释放的影响

在管式半携带半固定床反应器中，在600—1000℃范围内对6种煤质各异的原煤进行了快速热解HCN释放的研究．结果表明，热解温度、煤的灰分和氢含量对HCN的释放具有很大的影响，而煤氮向HCN的转化并不依赖于煤的含氮量．随着热解温度的升高，煤氮向HCN的转化率显著提高．原煤中氢含量越高，煤氮向HCN的转化率越高，说明热解过程中煤的供氢能力是决定HCN释放的重要因素．原煤中灰分越多，则煤氮向HCN的转化率越低，说明煤灰能够抑制HCN的生成或者能够促进HCN的分解．对原煤添加NaCl、CaCO3和二茂铁Fe(C5H5)2的热解研究表明，Na和Ca对于HCN释放的影响不明显，而Fe则显著降低了HCN的释放．     

煤焦水蒸气气化过程中含N气体产物的析出特性

在实验室规模的固定床反应器中研究煤焦水蒸气气化过程中含N产物的形成规律。利用FTIR测量气化过程中所释放的NH3、HCN和HNCO,GC测量N2。煤焦水蒸气气化过程中形成的含N产物主要是NH3、HCN和N2。同时,还研究了HCN在气化过程中的反应,这些实验分别采用HCN、HCN/蒸汽和HCN/蒸汽/煤焦进行。结果表明:HCN都是焦与水蒸气反应中的最初的含N产物,而其他含N产物则是由HCN在气相中或者煤在反应器壁以及焦表面进一步反应生成。     

Formation mechanism of HCN and NH3 during indole pyrolysis: A theoretical DFT study

Coal is a major contributor to the global emission of nitrogen oxides. The NO_x formation during coal utilisation typically derives from thermal decomposition of N-containing compounds pyrrole, which usually combines with an aromatic ring in the form of indole. NH₃ and HCN are common precursors of NO_x from the decomposition of N-containing compounds. In this study, possible pathways of indole pyrolysis to form HCN and NH₃ are investigated using the density functional theory (DFT) method. Calculation results indicate that indole pyrolysis has two type of possible initial reactions, which are internal hydrogen transfer and hydrogen homolysis reaction, respectively. The initial reaction mode of indole has a great impact on the subsequent pyrolysis pathway. Additionally, it is shown that indole can produce two nitrogen-containing products, i.e. HCN and NH₃. Five pathways will result in the formation of HCN (path-1, path-3, path-a, path-b, path-c), and another two pathways will lead to the NH₃ (path-2, path-4). Furthermore, among all the reaction mechanisms of indole pyrolysis, the path-1 is the optimal reaction pathway. During which, indole is converted to a diradical intermediate, then the intermediate undergoes a synergy ring-opening transition state to form a new intermediate. Afterwards, the new intermediate decomposes into CN by homolysis of the C–C bond.     

Formation of HNCO, HCN, and NH3 from the pyrolysis of bark and nitrogen-containing model compounds

Bark pellets have been pyrolyzed in a fluidized bed reactor at temperatures between 700 and 1000 °C. Identified nitrogen-containing species were hydrogen cyanide (HCN), ammonia (NH₃), and isocyanic acid (HNCO). Quantification of HCN and to some extent of NH₃ was unreliable at 700 and 800 °C due to low concentrations. HNCO could not be quantified with any accuracy at any temperature for bark, due to the low concentrations found. Since most of the nitrogen in biomass is bound in proteins, various protein-rich model compounds were pyrolyzed with the aim of finding features that are protein-specific, making conclusions regarding the model compounds applicable for biomass fuels in general. The model compounds used were a whey protein isolate, soya beans, yellow peas, and shea nut meal. The split between HCN and NH₃ depends on the compound and temperature. It was found that the HCN/NH₃ ratio is very sensitive to temperature and increases with increasing temperature for all compounds, including bark. Comparing the ratio for the different compounds at a fixed temperature, the ratio was found to decrease with decreasing release of volatile nitrogen. The temperature dependence implies that heating rate and thereby particle size affect the split between HCN and NH₃. For whey, soya beans, and yellow peas, HNCO was also quantified. It is suggested that most HCN and HNCO are produced from cracking of cyclic amides formed as primary pyrolysis products. The dependence of the HNCO/HCN ratio on the compound is fairly small, but the temperature dependence of the ratio is substantial, decreasing with increasing temperature. The release of nitrogen-containing species does not seem to be greatly affected by the other constituents of the fuel, and proteins appear to be suitable model compounds for the nitrogen in biomass.     

Mechanism study on the effect of alkali metal ions on the formation of HCN as NOx precursor during coal pyrolysis

Coal, as a typical fossil fuel, is a current major contributor to the global emission of nitrogen oxides (NO_x). The NO_x formation process during coal utilisation can be described as the thermal decomposition of N-containing model compounds into NO_x precursors followed by NO_x formation. The existence of alkali metal ions, Na⁺ and K⁺, during the coal utilisation process has a significant influence on the formation of NO_x species. However, the information about this influence is currently lacking within the available literature. Within this research, the effect of Na⁺ and K⁺ on the formation mechanism of NO_x during pyrrole pyrolysis were investigated using density functional theory (DFT). A hydrogen migration occurs from the meta-position of pyrrole-N is transferred to the ortho-position, and then pyrrole-N disconnected from the ortho-position C, which makes the ring opened. Lastly, in a concerted mechanism, a long carbon bond breaking between the migrating hydrogen and the carbon, nitrogen atoms. It was found that Na⁺ and K⁺ have a catalytic effect on the internal hydrogen transfer and ring-opening of pyrrole but have an inhibitory effect on internal hydrogen isomerization and concerted decomposition reaction. It was also found that those alkali metal ions (Na⁺ and K⁺) have strong interactions with pyrrole and its derived compounds (HCN and propyne molecules), those interactions are much larger than the existing attractive interactions among HCN, propyne molecules and their complexes. Additionally, it was found that both Na⁺ and K⁺ inhibit the HCN formation step from pyrrole pyrolysis, with Na^+, has a higher inhibition effect than that of K⁺. Furthermore, the mechanisms discussed in this research may well play a role in the thermal decomposition of other coal compounds such as indole and carbazole.     

Theoretical study of the effect of hydrogen radicals on the formation of HCN from pyrrole pyrolysis

As a typical fossil fuel, coal is a major contributor to nitrogen oxide (NO_x) pollution. The detailed mechanism of NO_x generation from coal pyrolysis need to be clarified. Within this research, we used density functional theory (DFT) to investigate the formation mechanism of HCN as a NO_x precursor during pyrolysis of pyrrole in the presence of hydrogen (H) radicals. Firstly, three different reaction positions for hydrogen radical attacking were compared. It was identified that hydrogen radical initially reacts with pyrrole at the location adjacent to N through a single elementary reaction step with an activation energy of 77.12 kJ/mol. Additionally, to examine the role of hydrogen radical in the pyrrole pyrolysis to form HCN, 12 subsequent reaction pathways were theoretically investigated. It was found that one of the pathway (Pathway a-4) involving hydrogen transfer followed by carbon-carbon cleavage processes is the route with the lowest energy barrier of all of the mechanisms reported, thus it plays an important role in the formation of HCN from the pyrrolic components of coal. These results further indicated that the hydrogen radicals significantly reduce the energy barrier of the pyrrole pyrolysis.     

Distribution of Nitrogen Species During Vitrinite Pyrolysis and Gasification

The formation of HCN and NH₃ during pyrolysis in Ar and gasification in CO₂ and steam/Ar was investigated. Vitrinites were separated and purified from different rank coal from lignite to anthracite. Pyrolysis and gasification were carried out in the drop-tube/fixed-bed reactor at temperatures of 600–900°C. Results showed that with increase of reaction temperature the yield of HCN increased significantly during pyrolysis and gasification. Decrease of coal rank also increased the yield of HCN. Vitrinite from lower rank of coal with high volatile content released more HCN. The yield of NH₃ was the highest at 800°C during pyrolysis and gasification. And the yield of NH₃ from gasification in steam/Ar was far higher than that from gasification in CO₂, where the hydrogen radicals play a key role. Nitrogen retained in char was also investigated. The yield of char-N decreased with an increase of pyrolysis temperature. Vitrinite from lower rank coal had lower yield of char-N than that from the high rank coal.     

生物质快速热解气相成分析出规律

利用恒温沉降炉对秸秆、稻壳、木屑及一种烟煤煤粉在900、1000、1100℃ 3个温度进行了快速热解试验,对4种燃料在快速热解过程中气相成分析出的规律进行了研究.生物质成分中高的挥发分、氧、H/C决定了其快速热解会取得比煤粉高的气相产率,木屑的气相产物产量最多,秸秆次之,稻壳最低.4种燃料热解气相产物中的主要成分是CO、H_2、CO_2、CH_4,少量的G_2H_4、C_2H_6、NO、HCN、COS,生物质和煤粉在快速热解及短的停留时间内,其析出的氮前驱物为HCN.快速热解析出的气相成分产量及组分分布与燃料种类、热解温度、热解停留时间相关.几种物料共同的规律是随停留时间的延长,气相产物的量不断地增加,当气相产物的产量趋于平稳时,相应的气相产物的各组分趋于恒定,这一停留时间标志着热解过程的结束,相同温度条件下煤粉的热解速率要慢于3种生物质.     

Effects of CaO on nitrogen transformation during pyrolysis of soybean protein

To elucidate the effects of CaO on nitrogen transformation during sludge pyrolysis, transformation behavior from char N into NO_x precursors during pyrolysis of soybean protein (SP) with CaO at 600–700°C was investigated. Results showed that CaO inhibited the transformation of pyridine N and quaternary N into HCN and promoted HCN conversion into NH₃ at 600–700°C. CaO inhibited the conversion of protein N and tar N into NH₃ at 600°C but promoted it at 700°C. NO_x precursor yield was the lowest when SP was pyrolyzed with CaO/N of 5.5 at 600°C (reduced by 11.66% compared with raw SP pyrolysis).     

Effects of Fe2O3 on pyrolysis characteristics of soybean protein and release of NOx precursors

The effects of ferric oxide (Fe₂O₃) on the pyrolysis characteristics of soybean protein and the release of precursors to nitrogen oxides (NOx) were studied using thermogravimetry and mass spectrometry. The results show that, as the content of Fe₂O₃ increases, there is no major difference between initial and peak temperatures of protein pyrolysis samples. Moreover, between the temperature range of 204 and 550°C where weight loss mainly occurs, total weight-loss rate decreases before increasing, with obvious weight loss occurring around the temperature of 650°C. Fe₂O₃ displays both inhibiting and promoting effects on the precipitation of nitrogen-containing gases such as ammonia (NH₃), hydrogen cyanide (HCN), isocyanic acid (HNCO), and acetonitrile (CH₃CN), with the inhibition effect prevailing over promotion effect on the whole.     

Mechanism study of nitric oxide reduction by light gases from typical Chinese coals

Coal devolatilization plays an important role in NO formation and reduction. In this study, the coal pyrolysis experiment was performed in an entrained flow reactor to obtain the light gas release characteristics. Six typical Chinese coals with volatile content ranged from 8.8% to 38.3% were studied. The pyrolysis temperature was in the range from 600 to 1200 °C. A significant rank dependence of HCN, CO and C₂H₂/C₂H₄/C₂H₆ was observed and their release for high volatile coals was higher than that for low volatile coals. The HCN–N/NH₃–N ratio ranged from 0.00 to 0.66 for anthracite coals and ranged from 1.63 to 3.90 for high volatile coals. Based on the experimental results, the effect of coal pyrolysis gas on NO reduction in a plug flow reactor at reducing atmosphere was kinetically calculated. The optimal excess air ratio(α_opt) corresponding to the maximum NO removal efficiency decreased with an increase in reduction temperature. For the light gas from the HL coal pyrolyzed at 800 °C, the α_opt decreased from 0.73 to 0.17 when the reduction temperature increased from 927 to 1327 °C. The rate of production analysis indicated that NO removal efficiency was determined by 3 competing reaction paths: NO reduction, NO formation and oxygen consumption by combustible species.