Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part IV. Pyrolysis of a set of Australian and Chinese coals

The formation of HCN and NH₃ from the pyrolysis of a small set of Chinese and Australian coals were studied using a novel fluidised-bed/fixed-bed reactor and a fluidised-bed/tubular reactor. The fluidised-bed/fixed-bed reactor has some features of a fluidised-bed reactor and of a fixed-bed reactor, allowing the evaluation of the effects of coal properties on the formation of HCN and NH₃ to be carried out on a similar basis for a wide range of coals. The thermal cracking of volatiles was investigated in a tubular reactor in tandem with the fluidised-bed/fixed-bed reactor where the nascent volatiles were generated in situ from the pyrolysis of coal. Our experimental results indicate that, in addition to coal rank, the petrographic composition and/or geographic origin of the coal are important factors influencing the formation of HCN and NH₃ during pyrolysis. Among the few Chinese and Australian coals studied, the inertinite-rich Chinese coals tend to give more NH₃ during pyrolysis than the Australian coals of similar carbon contents. It is believed that the structure of inertinites of less caking properties favours the formation of H radicals in the pyrolysing solid over a ‘correct’ temperature range to overlap with the activation and subsequent hydrogenation of the N-containing ring systems for the formation of NH₃ in the solid. If the coal properties favour the release of coal-N as volatiles, the formation of HCN in the gas phase is more likely. Under the current experimental conditions, where volatiles may be deposited on the reactor wall, the formation and destruction of the sooty materials on the reactor wall play an important role in the formation of HCN from the cracking of volatiles.     

Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part II. Effects of experimental conditions on the yields of NOx and SOx precursors from the pyrolysis of a Victorian brown coal

A Victorian brown coal was pyrolysed in a quartz reactor. The reactor has some features of a drop-tube reactor and of a fixed-bed reactor, capable of operating at fast and slow heating rates. The yield of HCN was found to change with gas flow rate and coal feeding rate, indicating that HCN and/or its precursors could interact significantly with the nascent char to be incorporated into char as soot or to form N₂. Experimental results indicated that HCN does not significantly convert to NH₃, either on the char surface or in the gas phase, at least during the pyrolysis of the brown coal in this study. The yields of HCN and NH₃ were both sensitive to changes in heating rate. The reduction in the yields of HCN and NH₃ with decreasing heating rate is mainly due to the lack of radicals at the slow heating rate, which are required to initiate the opening of the N-containing rings. The carbonisation/condensation reactions also make the N-containing heteroaromatic ring systems increasingly stable during the extended holding at high temperatures at the slow heating rate. Experimental results appear to suggest that there are two types of organic sulphur-containing structures in the brown coal with very different thermal stability. The first type could be converted into H₂S at low temperatures (<600°C). The other type was stable at temperatures up to 1000°C. The changes in heating rate or coal feeding rate did not affect significantly the formation of H₂S.     

Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part III. Further discussion on the formation of HCN and NH3 during pyrolysis

The formation of HCN and NH₃ from the pyrolysis of coal (and biomass) is discussed based on our experimental data as well as the data in the literature, including the pyrolysis of N-containing pyrrolic and pyridinic model compounds reported in the literature. The pyrolysis of the model compounds and the thermal cracking of coal pyrolysis volatiles appear to be in good qualitative agreement in terms of the onset decomposition temperature, the main intermediates and the final N-containing product (HCN). The formation of NH₃ requires the presence of condensed phase(s) of carbonaceous materials rich in hydrogen. Direct hydrogenation of the N-sites by the H radicals generated in situ in the pyrolysing solid is the main source of NH₃ from the solid. The initiation of the N-containing heteroaromatic ring by radical(s) is the first step for the formation of both HCN and NH₃. While the thermally less stable N-containing structures are mainly responsible for the formation of HCN, the thermally more stable N-containing structures may be hydrogenated slowly by the H radicals to NH₃. The formation of NH₃ and the formation of HCN are controlled by the local availability of radicals, particularly the H radicals, in the pyrolysing solid. The increased yield of NH₃ (and HCN) with increasing heating rate can be explained by the rapid generation of the H radicals at high heating rates, favouring the formation of NH₃ (and HCN) over the combination of N-containing ring systems within the coal/char matrix. The size of the N-containing heteroaromatic ring systems and the types of substitutional groups also play important roles in the formation of HCN and NH₃.     

Fate of coal nitrogen during combustion

A total of 21 coals covering all ranks have been burned under a wide variety of conditions to ascertain the impact of coal properties on the fate of fuel nitrogen. Fuel NO was identified with a nitrogen-free oxidant consisting of Ar-O₂CO₂. In general, under fuel-lean conditions fuel NO formation increases with increasing fuel nitrogen content; however, other fuel properties also significantly affect the fate of fuel-bound nitrogen during combustion. In particular, fuel nitrogen conversion appears to be greater with coals containing a high fraction of volatile reactive nitrogen. Under fuel-rich conditions measurements of first-stage and exhaust-species concentrations suggest that the optimum stoichiometry for minimum emissions is a function of fuel composition. As first-stage stoichiometry is decreased, the NO formed in the first stage decreases, but other oxidizable gas nitrogen species increase as does nitrogen retention in the char. Total fixed nitrogen generally increases with increasing fuel nitrogen and correlates well with excess air exhaust emissions. The distribution of the total fixed nitrogen species leaving the first stage is strongly dependent upon the coal composition. Of the 12 coals tested in detail, only 1 (the high-volatile B bituminous from Utah) produced high HCN concentrations. The low-volatile Pennsylvania anthracite formed almost no HCN or NH₃ even under extremely fuel-rich conditions. In general, the first stage NO percentage decreased significantly with decreasing coal rank from anthracite to lignite. Conversely, the relative importance of NH₃ grew with decreasing rank. HCN was greater than NH₃ in all bituminous tests, but less than NH₃ with all subbituminous and lignite coals.     

Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation

Pyrolysis of 10 coals with carbon contents of less than 80 wt%(daf) has been studied with a fixed bed quartz reactor to examine mainly nitrogen release from char-N without volatile matters. When temperature is raised from 1000 to 1350 °C, N₂ yield increases but char-N decreases for all the coals used. There is a strong reverse correlation between N₂ and char-N, which points out that most of N₂ arises from char-N via solid phase reactions. NH₃ is also formed from char-N at high temperatures of ≥1000 °C. In the pyrolysis of low rank coals, demineralization by HCl washing increases yields of tar-N, HCN and char-N, but decreases NH₃ and N₂. The addition of 3 wt% Ca to the demineralized coals shows almost the reverse effect. The XRD measurements after pyrolysis at 1000–1350 °C reveal that the Ca exists predominantly as CaO with the average crystallite size of 25–65 nm and promotes carbon crystallization. As the extent of crystallized carbon increases, N₂ yield increases remarkably. It is likely that the highly dispersed CaO catalyzes efficiently conversion reactions of char-N to N₂ in the process of carbon crystallization. The reaction mechanism is discussed in term of interactions between CaO particles and char-N.     

A theoretical investigation on the thermal decomposition of pyridine and the effect of H2O on the formation of NOx precursors

Pyridine is one of the main nitrogen-containing compounds in coal, and its pyrolytic mechanism to generate NO_x precursors (mainly NH₃ and HCN) remains unclear. In this work, the possible pathways for the pyrolysis of pyridine to form HCN and/or NH₃ were investigated by the density functional theory method, and the effects of H₂O on pyridine pyrolysis were also investigated. The results show that there are two possible reactions for the initial pyridine pyrolysis, i.e., internal hydrogen transfer and C–H bond homolysis, and that internal hydrogen transfer is more favorable. Nine possible reaction pathways following internal hydrogen transfer are obtained and analyzed. Among these pathways, pyridine prefers to produce HCN instead of NH₃. The existence of H₂O has significant effects on the decomposition of pyridine, as it participates in pyridine pyrolysis to form NH₃ rather than HCN as the major product.     

Nitrogen evolution during rapid hydropyrolysis of coal

The behavior of nitrogen evolution during rapid hydropyrolysis of coal has been investigated at temperatures ranging from 923 to 1123 K and hydrogen pressure up to 5 MPa using a continuous free fall pyrolyzer. Three coals have been tested in this study. The dominant nitrogen gaseous species is ammonia, together with a little amount of HCN because most of HCN is converted to NH₃ through secondary reactions. The results show that the evolution of nitrogen in coal is caused mainly by devolatilization at temperatures below 973 K, while the evolution of volatile nitrogen in char is accelerated with increasing temperature and hydrogen pressure. The mineral matter in coal act as catalysts to promote the evolution of volatile nitrogen in char to N₂ apparently at high temperatures of 1123 K, as found during pyrolysis of coal by Ohtsuka et al. A pseudo-first-order kinetic model was applied to the evolution of nitrogen in coal during rapid hydropyrolysis. The model shows the activation energy for the nitrogen evolution from coal is 36.6–58.6 kJ/mol while the rate of the nitrogen evolution depends on hydrogen pressure in the order of 0.16–0.24.     

铬革屑热解NO、NH3和HCN的释放特性

采用傅里叶变换红外光谱仪对铬革屑热解过程中NO、NH₃以及HCN的释放特性规律进行研究,探讨了随着时间的变化,热解温度对这3种含氮气体释放特性的影响。结果表明,热解过程中NO、NH₃以及HCN的释放随温度的升高呈现不同的规律。在热解温度较低的情况下含N气体的释放较少,其中NH₃的释放占主导。随着温度的升高,NO、HCN的释放量上升,尤其在550℃,HCN的释放率反超NH₃,释放的浓度达到500 ml·m^-3以上,NH₃的释放率维持在10%~15%。高温区HCN的释放存在迅速升高的过程,NO的释放量相对较少。NH₃的释放主要与胶原蛋白中氨基结构的破坏有关,如酰胺及酰胺带结构,而HCN主要来自于焦含氮结构的二次裂解,以及热解液体中含氮杂环物质的分解。NO的释放主要与C=O,C-O/C-O-C等含氧基团的变化以及原料中的N/O比例有关。     

无烟煤化学链燃烧过程燃料氮转化释放特性

基于赤铁矿石载氧体,在小型单流化床反应器上,开展煤挥发分和焦炭的化学链燃烧研究,探讨挥发分氮和焦氮在化学链燃烧过程中的转化特性。研究表明:燃料氮释放的中间产物HCN和NH₃与铁矿石载氧体具有较高的化学反应亲和性,易于被载氧体氧化生成N₂和NO。淮北无烟煤挥发分氮转化过程中,NO是唯一的氮氧化物,反应器出口中间产物NH₃的释放份额略高于HCN。在煤焦化学链燃烧还原过程中,部分燃料氮释放的中间产物HCN和NH₃被铁矿石氧化导致少量NO的生成,还原过程中无N₂O的释放;较高的还原反应温度加速了NO的生成。减少进入载氧体氧化再生过程的焦炭量可减少空气反应器NO和N₂O的生成。     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH3 and HCN

Our results indicate that the gas atmosphere surrounding coal/char particles can greatly affect the formation of NH₃ and HCN through its influence on the availability of H radicals. Based on our results, it is believed that the chemisorption of CO₂ on the nascent char surface can consume H radicals or block the access of N-sites by H radicals for the formation of NH₃ and HCN. For the chars whose thermal cracking generates little H radicals, the gasification of char by CO₂ can also generate additional H radicals, enhancing the formation of NH₃. However, even gasification of char in CO₂ at 950 °C does not lead to the formation of HCN. The oxidation of coal with 4% O₂ at low temperatures (400-600 °C) leads to the formation of HCN as well as NH₃ due to the enhanced formation of (H) radicals. The gasification of coal with 15% H₂O drastically enhances the formation of NH₃ due to the greatly enhanced availability of H as an intermediate between the reactions of H₂O and char. These results support our reaction mechanisms proposed previously, emphasising the importance of H on the formation of NH₃ and HCN during pyrolysis, which can also be extended to the conversion of coal-N during gasification.     

Change of nitrogen functionality of N-enriched condensation products during pyrolysis

Changes in the nitrogen functionality of ¹⁵N-enriched condensation products prepared from glucose and ¹⁵N-glycine were investigated during pyrolysis at 600–1000 °C. The structural changes in the condensation products were studied by means of solid-state ¹³C and ¹⁵N NMR spectroscopies. During pyrolysis, the aliphatic moieties of the condensation products decomposed and evolved as gas and tar. At pyrolysis temperatures above 600 °C, almost all the carbon in the chars were converted to aromatic carbon. After pyrolysis, large amounts of nitrogen remained in the chars as char nitrogen (char-N), and about 30% of the nitrogen was eliminated from the chars as HCN and NH₃. With increasing temperature, the production of HCN and NH₃ increased and the amount of char-N decreased. By combining X-ray photoelectron spectroscopy and NMR results, detailed results for nitrogen fractions in chars were obtained. During pyrolysis, the fraction of unsubstituted pyrrole-N decreased and the fraction of quaternary-N increased. The fraction of pyridine-N remained almost constant at temperatures below 800 °C, but at 900 °C and above, the fraction of pyridine-N decreased. The fraction of substituted pyrrole-N showed minimum at 800 °C. On the basis of these results, structural changes of nitrogen functional groups during pyrolysis are discussed.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VIII. Effects of pressure on the formation of NH3 and HCN during the pyrolysis and gasification of Victorian brown coal in steam

Effects of pressure on the formation of HCN and NH₃ during the pyrolysis and gasification of Loy Yang brown coal in steam were investigated using a pressurised drop-tube/fixed-bed reactor. The NH₃ yield increased with increasing pressure during both pyrolysis and gasification. Increasing pressure selectively favours the formation of NH₃ at the expenses of other N-containing species. The changes in the yield of NH₃ with increasing pressure were mainly observed in the feeding periods both during pyrolysis and gasification and were closely related to the formation and subsequent cracking of soot both as a result of intensified thermal cracking of volatile precursors inside the particles and as a result of volatile-char interactions after the release of volatiles. While the corresponding HCN yield during pyrolysis showed little sensitivity to changes in pressure, the HCN yield during gasification in steam showed some increases with increasing pressure. Our data indicate that the direct hydrogenation of char-N by H radicals, favoured by the presence of steam, is the main route of NH₃ formation during pyrolysis and gasification. The direct conversion, either through hydrogenation or hydrolysis, of HCN into NH₃ on char surface during the pyrolysis and gasification of brown coal is not an important route of NH₃ formation.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VII. Pyrolysis and gasification of cane trash with steam

Biomass-nitrogen conversion during the pyrolysis and gasification of a cane trash in steam was investigated using a fluidised-bed/fixed-bed reactor and a fluidised-bed/tubular reactor. Our results indicate that the thermal cracking of volatile-N is the main route of HCN formation although the thermal cracking of char-N also contributes to the formation of HCN. There are three major routes of NH₃ formation: ‘hydrolysis’ of N-containing structures in the solid phase during the primary pyrolysis, thermal cracking and gasification of solid nascent char as well as the thermal cracking and reforming of volatile-N. Under the current experimental conditions, the hydrolysis of HCN does not appear to be an important route of NH₃/HNCO formation.     

Formation of HCN and NH3 during coal macerals pyrolysis and gasification with CO2

Three coal macerals with high purities were separated from Pingshuo gas coal. The formation rules of HCN and NH₃ during macerals pyrolysis and gasification were investigated. Experiments were carried out in a tubular quartz reactor at atmospheric pressure. The reactor allowed coal particles to be heated up rapidly and held for a prespecified period of time at a peak temperature. The amount of HCN and NH₃ were quantified by ion chromatography. The influence of temperature and macerals type on the formation rules of HCN and NH₃ was discussed. Results showed that the formation of HCN was mainly due to the thermal cracking of volatile, and NH₃ formed both from the thermal cracking of volatile and the cracking of nascent char. The HCN yield increased with an increase in pyrolysis temperature. For three coal macerals (liptinite, vitrinite and inertinite), the yield of HCN depended not only on their volatile contents but also nitrogen-containing functional groups, in which more pyrrole-type nitrogens would form more amount of HCN at lower temperature. The yield of NH₃ depended on the ability of forming ‘H’ radical. Under the experiment condition in this study, inertinite could convert more nitrogen into NH₃ than vitrinite and liptinite. The yield of HCN during gasification was almost the same as that during pyrolysis, the yield of NH₃ during gasification was little higher than that during pyrolysis.     

Releases of NO and its precursors from coal combustion in a fixed bed

Experiments have been carried out to investigate the emissions of nitrogen species including NO and its precursors during temperature-programmed coal combustion by TG/EGA method. Experimental results show that the conversion ratio of fuel nitrogen to NO is the highest, followed by that of fuel nitrogen to HCN and the conversion ratio to NH₃ is negligibly small. Nitrogen is retained in the char and released mainly as NO at the later stages of coal combustion. HCN and NO are both primary products from coal char oxidation. Coal rank, heating rate, indigenous minerals and external additives are the major influential factors of the nitrogen species release. Higher rank coals with higher fuel ratio have higher NO releases. HCN release decreases as fuel ratio increases for most coals. The fuel nitrogen conversion to NO increases and the fuel nitrogen conversion to HCN decreases with the increase of heating rate, which may imply that the char nitrogen prefers to react with oxygen to form NO instead of HCN while coal char is combusted at higher temperatures. Different metallic additives show different effects on nitrogen species emission and the effects of indigenous minerals on nitrogen release can be qualitatively estimated by ash analyses.     

流化床煤燃烧过程不同气氛下的气态氮释放特征

利用微型流化床反应装置,结合快速过程质谱仪,在850~940℃操作温度下,研究了三种不同粒度分布烟煤和无烟煤在热解、气化和燃烧反应条件下四种主要气态氮产物HCN、NH₃、NO和NO₂的释放规律。结果表明,微型流化床可以实时检测挥发分氮和焦炭氮的动态释放序和类型,热解、气化和燃烧反应气氛的改变主要影响HCN和NH₃的释放量。热解产物的气态氮主要是来自于挥发分,燃烧反应的HCN和NH₃的释放量与温度有明显关系,而气化反应的各类气态氮释放量随温度变化波动不大。煤颗粒尺寸和温度变化对烟煤和无烟煤中各类气态氮释放量产生影响比较复杂,其中NH₃的释放特性是区分挥发分N释放和半焦N释放的重要特征。     

Effect of rank,temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor

Pyrolysis of 11 coals with carbon contents of 77–93 wt.% (daf) and corresponding demineralized samples has been studied in a fixed bed quartz reactor with a heating rate of 20 K/min to examine rank, demineralization, temperature and inherent mineral species dependences of nitrogen distribution. Nitrogen mass balances fall within 92.5–104.6%. The results indicate that the chars derived from the coals with higher rank show larger nitrogen retention. Demineralization suppresses volatile nitrogen emission during coal pyrolysis, especially for low rank coals. Coal-N conversion to tar-N reaches the asymptotic values at 600 °C. HCN yields are lower than NH₃ yields during coal pyrolysis. The trends in HCN and NH₃ emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N₂ starts emitting at 600 °C, and as the temperature increases the conversion increases linearly with a corresponding reverse change of char-N. With the catalysts added, N₂ formation is prompted with the sequence of Fe>Ca>K>Ti≫Na≫Si≈Al, meanwhile, char-N decreases correspondingly. Fe, Ca, K, Na, Si and Al increase coal-N conversion to NH₃ with the sequence of Fe>Ca>K≈Na≫Si≈Al in the pyrolysis. Na addition prompts HCN formation; however, the presence of Ti and Ca decrease the HCN yields with small value. The other catalysts have no notable influence on HCN emission in the pyrolysis. Demineralization and Ti addition increase coal-N conversion to tar-N slightly whereas K, Ca, Mg, Na, Si and Al additions decrease tar-N yield weakly, other catalysts hardly influence tar nitrogen emission. N₂ emits mainly from char-N with slight contribution of volatile nitrogen. The mechanism of different N-containing species formation and catalysts influence in the pyrolysis is also discussed in the paper.     

Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification

Six coals with different ranks and different ash contents have been used to study the effect of demineralization on N₂ formation during coal pyrolysis. Chars obtained after pyrolysis have been also gasified with carbon dioxide at 1000 °C to investigate the influence of the demineralization on char gasification reactivity. The pyrolysis results show that the demineralization by acid washing drastically changes N₂ formation profiles and decreases nitrogen conversion to N₂ for low rank coals; on the other hand, the demineralization has little effect on N₂ formation for high rank coals. Addition of 0.5 wt% Fe promotes N₂ formation from the demineralized coals, but the catalytic effect depends on the coal type. It is found that the Fe remarkably promotes N₂ formation from the demineralized low rank coals, but the effect is much smaller for high rank demineralized coals. These observations suggest that the existing state of Fe-containing minerals and added Fe catalyst is important for catalytic N₂ formation during coal pyrolysis. Gasification results show that the demineralization lowers char gasification reactivity not only for low rank coals but also for high rank coals.     

城市污泥慢速热解过程中氮的转化规律

利用热重-质谱联用(TG-MS)技术研究城市污泥慢速热解特性及含氮气体产物的生成规律,同时利用原位红外光谱仪实时检测固体表面官能团的变化。研究结果表明:初沉污泥在500℃之前热解已基本完成,二沉污泥由于添加了矿物质盐类,在700℃左右仍有一个较大的失重峰;二沉污泥热解过程HCN和NH₃总生成量均小于初沉污泥,即二沉污泥所加矿物质抑制了HCN和NH₃释放;但温度大于400℃时所加矿物质对HNCO生成具有一定促进作用;污泥中蛋白质热分解会产生环酰胺类物质、含氮杂环化合物和腈类物质,并最终转化为HCN,这是污泥热解过程中HCN的主要来源;400℃以下NH₃主要来自铵盐分解和HCN转化,蛋白质热分解对于NH₃生成贡献很小;400℃以上基本检测不到NH₃生成,即较高温度下挥发分二次反应对NH₃生成几乎没有影响;300~480℃,污泥中木质素裂解产生了大量含氧自由基,促使HCN转化为N₂O,HNCO则最终转化成了NO。     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part V. Pyrolysis of a sewage sludge

A sewage sludge sample from a wastewater treatment plant in China was pyrolysed in a fluidised-bed/fixed-bed reactor and in a fluidised-bed/tubular reactor. HCN was found to be the main NO_x precursor, representing up to about 80% of the nitrogen present in the sludge. The thermal cracking of volatiles is the main route of HCN formation. NH₃ was also an important NO_x precursor formed during the pyrolysis of the sewage sludge. The experimental results indicate that there are at least two distinctive stages of NH₃ formation during the pyrolysis of the sewage sludge at a fast heating rate. The formation of NH₃ at temperatures lower than 400-500 °C is at least partly due to the amino structures in the sludge. The reactions of volatiles in the gas phase make negligible contributions to the observed NH₃ yield.