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.     

Nitrogen chemistry in coal pyrolysis: Catalytic roles of metal cations in secondary reactions of volatile nitrogen and char nitrogen

The present review focuses on elucidating the chemistry of nitrogen release during coal pyrolysis, in particular, on making clear catalytic roles of inherent Ca and Fe ions in not only the partitioning of volatile-N to tar-N, HCN, NH₃ and N₂ but also the conversion of char-N to N₂.     

Pyrolysis of poly-l-leucine under combustion-like conditions

The protein poly-l-leucine has been used as a model compound for the nitrogen in biomass fuels. It was pyrolysed in a fluidised bed at 700 and 800 °C and the pyrolysis gases were analysed with a FT-IR spectrometer. HCN, NH₃ and HNCO were identified as the main nitrogen-containing species, while neither NO nor N₂O were found among the pyrolysis gases. At 700 °C, as much as 58% of the nitrogen content was converted into HCN and 31% into NH₃. The HCN/NH₃ ratio increased from about 1.9 at 700 °C to above 2.2 at 800 °C. Pyrolysis of another protein, poly-l-proline, at 800 °C gave a HCN/NH₃ ratio close to 10. This revealed that the protein's amino acid composition has a marked impact on the composition of the pyrolysate.     

Formation of N-containing gas-phase species from char gasification in steam

The formation of N-containing products during char-steam gasification has been investigated in a laboratory scale fixed bed reactor. Experiments were conducted at 1000 °C, 0.1-1.0 MPa, and 6-46% of H₂O in He base flow. Two very different coal chars, which were prepared from the rapid heating of Australian bituminous and sub-bituminous coals, were studied. The nitrogen-containing products released during the gasification were measured using an FTIR spectrometer (NH₃, HCN and HNCO) and gas chromatography (N₂). The major N-containing products formed during char-steam gasification are NH₃, HCN and N₂. Reactions of HCN in the same reactor were also studied; these experiments were conducted with HCN alone, HCN/steam, and HCN/steam/char. The results are consistent with a mechanism in which HCN is the primary N-containing product of the char-steam reaction, and the additional products result from further reactions of HCN either in the gas phase or promoted by the surface of the reactor or the char. Increasing concentrations of steam significantly influence the distribution of char-N to N-containing gas-phase products, resulting in the increase of NH₃ at the expense of N₂. Some differences in char behaviour are also observed, particularly on the distribution of N-containing products at 0.1 MPa total pressure.     

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.     

Thermal stability of polymer derived SiBNC ceramics

To study the thermal stability of polymer derived SiBNC ceramics, a polyborosilazane was pyrolyzed in N₂ and NH₃/N₂ atmosphere, respectively. The as-pyrolyzed products were annealed in N₂ atmosphere at 1200–1850 °C for 2 h. The chemical composition and phase structure of the as-pyrolyzed and annealed ceramics were investigated by element analysis, XRD and FT-IR. The results show that all the ceramics exhibit excellent high temperature stability. They are fully amorphous to 1700 °C, and only partial crystallization, giving a mixture of Si₃N₄, BN and SiC phases, is observed upon heating at 1850 °C. The N₂ pyrolyzed products show better stability than the NH₃/N₂ pyrolyzed products and have less tendency to crystalline at higher temperatures. Better retention of nitrogen at high temperatures is also observed for the N₂ pyrolyzed products.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

N2O decomposition over K-promoted Co-Al catalysts prepared from hydrotalcite-like precursors

N₂O decomposition was investigated over a series of K-promoted Co-Al catalysts. The activity tests showed that doping with K greatly enhanced the catalytic activity of the Co-Al catalyst, and the enhancement was critically dependent on the amount of K and the calcination temperature. When the catalyst had a K/Co atomic ratio of 0.04 and was calcined at 700–800 °C, a full N₂O conversion could be reached at a reaction temperature of 300 °C. Moreover, even under the simultaneous presence of 4% O₂ and 2.6% water vapor, such high-temperature treated K/Co-Al catalyst exhibited high reactivity and stability, with the N₂O conversion remaining at a constant value of 92% over 40 h run at 360 °C. In contrast, non-doped Co-Al catalyst showed a severe activity loss under such reaction conditions. A combination of characterization techniques was employed to reveal the promoting role of K and the effect of calcination temperature. The results suggest that doping with K increases the electron density of Co and weakens the Co–O bond, thus promoting the activation of N₂O on the Co sites and facilitating the desorption of oxygen from the catalyst surface. High-temperature calcinations made the desorption of O₂ proceed more readily.     

Accelerated formation of barium titanate by solid-state reaction in water vapour atmosphere

Barium titanate (BaTiO₃) powders were synthesized from commercially available raw materials (BaCO₃ and rutile) without particular mechanochemical processing by solid-state reactions in water vapour atmosphere. The formation rate of BaTiO₃ was accelerated by water vapour and single phase of BaTiO₃ was obtained by calcination at 700 °C for 4 h in water vapour atmosphere, though high temperature (850 °C for 2.5 h) was required by calcinations in air to complete the reaction. The formation kinetics followed the Valensi–Carter equation, which suggested that the reaction proceeded by a diffusion controlled process. The apparent activation energy for the formation of BaTiO₃ in air and water vapour atmosphere was estimated to be 361 ± 20 kJ/mol and 142 ± 17 kJ/mol, respectively. Water vapour is considered to enhance thermal decomposition of BaCO₃ and formation of BaTiO₃ by attacking surface Ti–O–Ti bonds in TiO₂, increasing partial pressure of Ba(OH)₂, and producing vacancies in the BaTiO₃ structure.     

Reactions of cyanamide, dicyandiamide and related cyclic azines in high temperature water

Cyanamide, dicyandiamide, and the related cyclic azines (melamine, ammeline, ammelide, and cyanuric acid) were reacted in water at 100–300 °C in a sealed 316 SS tube (275 bar) for the purpose of characterizing the hydrothermolysis chemistry of cyanamide. The conversion of cyanamide to dicyandiamide dominates at 100–175 °C. At 175–250 °C, when the reaction times are shorter than 15 min, the major pathway is hydrolysis of the cyanamide-dicyandiamide mixture to CO₂ and NH₃. A minor pathway is cyclization to higher azines (melamine, ammeline, ammelide and cyanuric acid). Above about 225 °C, hydrolysis of these cyclic azines to aqueous NH₃ and CO₂ occurs in a relative ratio which depends on the particular cyclic azine, and, to an extent, which increases with temperature. At 300 °C the conversion of all compounds to CO₂ and NH₃ is complete in 10 min. The hydrothermolysis chemistry of cyanamide and urea are compared.     

Regeneration of initial activity of a pitch-based ACF for NO–NH3 reaction at ambient temperature

The reactivity of adsorbed NO (including NO₂) and NH₃ in the presence of 4.0% oxygen in He was examined over a pitch-based ACF calcined at 800°C. Regeneration at 30°C by 4% O₂ in He without NH₃ was found to be optimum for the recovery of the initial activity with complete removal of NO within 3 h, with minimum leaks of adsorbed NO and NH₃. A higher temperature of 40°C for regeneration increased the liberation of adsorbed NO, and NH₃ over ACF was rather slow at a lower temperature of 25°C, slow regeneration being achieved. Oxygen appears necessary to regenerate the ACF through enhancing the reaction of adsorbed NO and NH₃ for the initial activity, which was ascribed to the catalytic activity for NO–NH₃ and adsorption of both NO and NH₃. NH₃ in the gas phase appears to inhibit the regeneration reaction of adsorbed species, by using the leaking amount during the regeneration.     

Use of elemental metals in different grade of activation for phthalocyanine preparation

Synthesis of non-substituted metal phthalocyaninates starting from phthalonitrile in various non-aqueous solvents in presence of a series of elemental metals in different grade of activation is described. Special attention is paid to phthalocyanine formation at relatively low temperatures (0–50 °C). In case of use of various forms of activated and non-active nickel, it is shown that its most active form causes rapid PcNi formation at 0–25 °C without addition of CH₃ONa.     

Low temperature synthesis of AlN by addition of various Li-salts

The effect of various Li-salts on the low temperature synthesis of AlN by direct nitridation of Al metal was investigated using four Al powders with average particle sizes of 3, 20, 100 and 150 μm. These were mixed with various Li-salts (LiNO₃, LiOH·H₂O and Li₂CO₃) in different concentrations and fired at various temperatures under flowing N₂. The as-received Al powders without Li addition showed AlN formation at about 600 °C in the 3 and 20 μm samples but no AlN formation up to 850 °C in the 100 and 150 μm samples. The crystallinity of the AlN products, where formed, was however low. By contrast, all the Al powders with added Li-salts showed AlN formation up to 800 °C, with LiOH·H₂O being especially effective. Thus, the AlN formation temperature can be significantly lowered in the coarser powders by the addition of Li-salts but the effect is less in the finer powders which undergo low temperature nitridation below the melting point of Al metal even without Li. The crystallinity of the AlN products was higher in the samples containing Li-salts than without Li-salts.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part X: Effects of volatile-char interactions on the conversion of coal-N during the gasification of a Victorian brown coal in O2 and steam at 800 °C

A novel fluidised-bed/fixed-bed reactor was used to study the effects of volatile-char interactions on the conversion of coal-N during the gasification of a Victorian brown coal at 800 °C. The reactor has the capability of controlling the extent and length of the interactions between volatiles and char. Our results indicate that in the absence of volatile-char interactions during gasification in O₂, the lack of abundant H radicals led to negligible formation of NH₃ and HCN from char-N. The presence of volatile-char interactions during the gasification of Victorian brown coal in O₂ at 800 °C drastically enhanced the formation of NH₃ and, albeit to a lesser extent, the formation of HCN. The enhanced conversion of char-N into NH₃ (and HCN) due to the volatile-char interactions is attributed to the presence of H radicals in the volatiles. H radicals in volatiles could “die off” as they pass through the nascent char bed during the course of volatile-char interactions.     

Thermal properties of the nitrogen-rich Ca-α-Sialons

Nitrogen-rich Ca-α-Sialon (Ca_xSi_12−2xAl_2xN₁₆ with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si₃N₄, AlN and CaH₂ powders in a hot press at 1800 °C using a pressure of 35 MPa and a holding time of 4 h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600–1800 °C using a pressure of 35 MPa and a holding time of 2 h. The α-Sialon phase was first observed at 1400 °C but the α-Si₃N₄ and AlN phases were still present at 1700 °C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800 °C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600 °C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325 °C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400 °C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.     

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.     

Thermal behaviour of Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides and characterization of formed oxides

Thermal behaviour of synthetic Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides (LDHs) with M^II/Mg/Mn molar ratio of 1:1:1 was studied in the temperature range 200–1100 °C by thermal analysis (TG/DTA/EGA), powder X-ray diffraction (XRD), Raman spectroscopy, and voltammetry of microparticles. Powder XRD patterns of prepared LDHs showed characteristic hydrotalcite-like phases, but further phases were indirectly found as admixtures. The Cu–Mg–Mn precipitate was decomposed at temperatures up to ca. 200 °C to form an XRD-amorphous mixture of oxides. The crystallization of CuO (tenorite) and a spinel type mixed oxide of varying composition Cu_xMg_yMn_zO₄ with Mn⁴⁺ was detected at 300–500 °C. At high temperatures (900–1000 °C), tenorite disappeared and a consecutive crystallization of 2CuO·MgO (gueggonite) was observed. The high-temperature transformation of oxide phases led to a formation of Cu^I oxides accompanied by oxygen evolution. The DTA curve of Ni–Mg–Mn sample exhibited two endothermic effects characteristic for hydrotalcite-like compounds. The first one with minimum at 190 °C can be ascribed to a loss of interlayer water, the second one with minimum at 305 °C to the sample decomposition. Heating of the Ni–Mg–Mn sample at 300 °C led to the onset of crystallization of oxide phases identified as Ni_xMg_yMn_zO₄ spinel, (Ni,Mg)O oxide containing Mn⁴⁺ cations, and easily reducible XRD-amorphous species, probably free Mn^III,IV oxides. At 600 °C (Raman spectroscopy) and 700 °C (XRD), the (Ni,Mg)₆MnO₈ oxide with murdochite structure together with spinel phase were detected. Only spinel and (Ni,Mg)O were found after heating at 900 °C and higher temperatures. Temperature-programmed reduction (TPR) profiles of calcined Cu–Mg–Mn samples exhibited a single reduction peak with maximum around 250 °C. The highest H₂ consumption was observed for the sample calcined at 800 °C. The reduction of Ni–Mg–Mn samples proceeded by a more complex way and the TPR profiles reflected the phase composition changing depending on the calcination temperature.     

Decomposition of ammonia with iron and calcium catalysts supported on coal chars

Decomposition of NH₃ to N₂ with Fe and Ca catalysts supported on brown coal chars has been studied with a cylindrical quartz reactor from a viewpoint of hot gas cleanup. The catalyst is prepared by pyrolyzing a brown coal with Fe or Ca ions added. In the decomposition of 2000 ppm NH₃ diluted with He at 750 °C and at a space velocity of 45,000 l/h, 2-6 wt% Fe catalysts are more active than not only 6 wt% Ca catalyst but also 8 wt% Fe catalyst loaded on a commercial activated carbon. The transmission electron microscope observations show that fine iron particles with the sizes of 20-50 nm account for the higher catalytic performances. When reaction temperature is increased to 850 °C, all of Fe and Ca catalysts on the chars achieve complete decomposition of NH₃. The co-feeding of H₂ with 2000 ppm NH₃ improves the performance of the 2% Fe catalyst at 750 °C, but contrarily the coexistence of syngas (CO/H₂=2) deactivates it remarkably, whereas the addition of CO₂ to syngas restores the catalytic activity of the Fe to the original state without syngas. The powder X-ray diffraction and temperature programmed desorption measurements strongly suggest that the Fe and Ca catalysts promote NH₃ decomposition through cycle mechanisms involving the formation of N-containing intermediate species and the subsequent decomposition to N₂.     

NOx storage properties of Pt/Ba/Al model catalysts prepared by different methods: Beneficial effects of a N2 pre-treatment before hydrothermal aging

The influence of a pre-treatment at 700 °C, either under a O₂/N₂ mixture or only under N₂, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N₂ adsorption, XRD, CO₂-TPD, H₂ chemisorption and H₂-TPR. The pre-treatment under N₂ largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H₂O and CO₂ in the inlet gas. The better NOx storage properties of the catalysts treated under N₂ before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity.