Conversion Of coal-N Research Articles

Experimental and numerical evaluation of coal-N conversion characteristic during preheating-combustion under O2/CO2 atmosphere

High-temperature preheating and O2/CO2 combustion have been proven effective for de-NOx and CO2 capture. In this study, we combined these two methods to investigate the transformation characteristics of volatile-N and to validate whether the reduction of nitrogen in volatile-N is enhanced during preheating of pulverized coal (PC) under the O2/CO2 atmosphere. The conversion of coal-N to NOx and its precursors during the preheating process under the O2/CO2 atmosphere was examined through experimental and numerical approaches. The results indicate that the gasification of CO2 during the preheating stage increases the volatilization of coal-N. Subsequently, in a high-temperature, strongly reducing atmosphere, up to 52.21% of the coal-N is directly converted into N2. Furthermore, the presence of CO2 also participates in gas-phase reactions, leading to changes in the relative proportions of free radicals such as O, H, and OH. Additionally, it increases the concentration of CO, thereby facilitating the reduction of NO during the combustion stage. The experimental findings indicate a synergistic effect between preheating and O2/CO2 combustion in reducing NO formation. Furthermore, by implementing further air staging, the NO emissions can be reduced by an additional 49.60%, with NO emissions down to 109.70 mg·m−3.

Influences of Hydrothermal Modification on Nitrogen Thermal Conversion of Low-Rank Coals

The influence of hydrothermal modification on thermal conversion of coal-N in low-rank coals was investigated in this paper. Hydrothermal modification has been performed using three typical Chinese low-rank coals at final modification temperatures of 200, 250, and 300 °C. An analytical pyro-probe coupled to a pyrolysis–gas chromatography mass spectrometry (Py-GC/MS) system was used to study the release characteristics of nitrogen-containing compounds from raw and modified coal. In addition, a tube furnace apparatus was used to study the release characteristics of nitrogen pollutants during pyrolysis and combustion of Zhaotong raw and modified coal. Results showed that hydrothermal modification affected the content and type of nitrogen-containing compounds. Some triple bonds present in the coal structure are broken to form amides. The presence of azoles decreases sharply after modification, but it increases as the hydrothermal modification temperature increases. During tube furnace pyrolysis, the HCN yield...

Experimental study on combustion characteristics and NOX emissions of pulverized anthracite preheated by circulating fluidized bed

A 30 kW bench-scale rig of pulverized anthracite combustion preheated by a circulating fluidized bed (CFB) was developed. The CFB riser has a diameter of 90 mm and a height of 1,500 mm. The down-fired combustion chamber (DFCC) has a diameter of 260 mm and a height of 3,000 mm. Combustion experiments were carried out using pulverized anthracite with 6.74% volatile content. This low volatile coal is difficult to ignite and burn out. Therefore, it requires longer burnout time and higher combustion temperature, which results in larger NOX emissions. In the current study, important factors that influence the combustion characteristics and NOX emissions were investigated such as excess air ratio, air ratio in the reducing zone, and fuel residence time in the reducing zone. Pulverized anthracite can be quickly preheated up to 800°C in CFB when the primary air is 24% of theoretical air for combustion, and the temperature profile is uniform in DFCC. The combustion efficiency is 94.2%, which is competitive with other anthracite combustion technologies. When the excess air ratio ranges from 1.26 to 1.67, the coal-N conversion ratio is less than 32% and the NOX emission concentration is less than 371 mg/m3 (@6% O2). When the air ratio in the reducing zone is 0.12, the NOX concentration is 221 mg/m3 (@6% O2), and the coal-N conversion ratio is 21%, which is much lower than that of other boilers.

HCN and NH3Formation during Coal/Char Gasification in the Presence of NO

Understanding the conversion of coal-N during gasification is an important part of the development of gasification-based power generation technologies to reduce NO(x) emissions from coal utilization. This study investigated the conversion of coal-N in the presence of NO during the gasification of three rank-ordered coals and their chars in steam and low-concentration O(2). Our results show that NO can be incorporated into the char structure during gasification. The inherent char-N and the N incorporated into the char from NO-char reactions behave very similarly during gasification. During the gasification in steam, significant amounts of HCN and NH(3) can be formed from the incorporated N structure in char, especially for the relatively "aged" chars, mainly due to the availability of abundant H radicals on the char surface during the gasification in steam. During the gasification in 2000 ppm O(2), the formation of HCN or NH(3) from the N structures in char, including those incorporated into the char from the NO-char reactions, was not a favored route of reaction mainly due to the lack of H on char surface in the presence of O(2).

Formation of NO x 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 O 2 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 O 2, the lack of abundant H radicals led to negligible formation of NH 3 and HCN from char-N. The presence of volatile–char interactions during the gasification of Victorian brown coal in O 2 at 800 °C drastically enhanced the formation of NH 3 and, albeit to a lesser extent, the formation of HCN. The enhanced conversion of char-N into NH 3 (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.

Release of fuel-N during entrained-flow gasification of fuel mixed with model compound

Model compound pyridine was added to diesel oil to simulate the existence of coal-N. A laboratory opposed multi-burner (OMB) gasifier was used to investigate the axial and radial concentration distribution of HCN, NH 3, NO and N 2. The maximum concentrations of nitrogen pollutants (HCN, NH 3 and NO) are produced at the nozzle plane and these concentrations decrease away from the plane. Moreover, N 2 participates in gasification and the order of concentrations of nitrogen compounds in exit is N 2>HCN>NH 3>NO. NO and N 2 increase as O 2/fuel ratio increases. HCN and NH 3 reach maximum when O 2/fuel ratio is 1.3, but largely decrease at 1.7 and 0.9. Flow field distribution make radial concentration profiles uniform toward the exit region and low near the side-wall in other axial position. The steam addition leads to increased formation of HCN and NH 3, whereas decreased formation of NO. The results may be useful for the further investigation of coal-N conversion mechanism.

Effects of volatile–char interaction on the formation of HCN and NH 3 during the gasification of Victorian brown coal in O 2 at 500 °C

In a coal gasifier interactions between volatiles and char are significant. The partly reducing conditions in a gasifier would mean the presence of high concentration of partial oxidation products and radicals surrounding the char particles. Currently, little is known about the effects of in situ volatile–char interactions on the conversion of char-N. This study examines the effect of in situ volatile–char interactions on the formation of HCN and NH 3 during the low temperature (500 °C) gasification of Loy Yang brown coal in oxygen. Two novel reactor systems were used. The reactor configurations allowed the quantification of HCN and NH 3 from char-N gasification, volatile-N oxidation and volatile–char interactions separately. Our results indicate that volatile–char interactions can have drastic effects on coal-N conversion during gasification by providing an important source of the radicals for the formation of HCN and NH 3 from char-N during gasification in 4% or 8% O 2 at 500 °C. In the presence of radicals and O 2, N-containing structures in the nascent char can be easily broken down to give HCN and NH 3 during the gasification of the char. In the absence of O 2, some of the nascent char-N structures may stabilise into structures less favourable for the formation of HCN and NH 3 and more favourable for the formation of other N-containing species such as NO x .

Pulverized coal flame structures at elevated pressures. Part 2. Interpreting NO X production with detailed reaction mechanisms

This study develops equivalent reactor networks from the CFD simulations for the pressurized coal flames described in Part 1, and quantitatively interprets the data by applying detailed reaction mechanisms across the reactor networks. These simulations depict all the important trends in the database with both coal quality and increasing pressure. The only quantitative discrepancy is that CO yields were under-predicted by roughly a factor of two throughout, although the predictions correctly indicate higher CO levels for coals of progressively lower rank. Once the reaction mechanisms were validated against the database, they were used to assess the impact of pressure on NO X production for realistic fuel injector configurations. These simulations, in conjunction with the test results, establish that (1) coal-N conversion to NO diminishes for progressively higher pressures under typical burner operating conditions; and (2) HCN tends to persist to higher stoichiometric ratios (S.R.) for progressively higher pressures, although it would not be present in flue gas prepared at a S.R. of 1.15 or greater at pressures to 3.0 MPa.

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 NH 3 yields during coal pyrolysis. The trends in HCN and NH 3 emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N 2 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, N 2 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 NH 3 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. N 2 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.

03/02096 Allotropic forms of carbon in coal: Alekseyev, A. D. et al. Fizika i tekhnika Vyoskikh Davlenii 2002, 12, (30, 63–69. (In Russian)

In a coal gasifier interactions between volatiles and char are significant. The partly reducing conditions in a gasifier would mean the presence of high concentration of partial oxidation products and radicals surrounding the char particles. Currently, little is known about the effects of in situ volatile–char interactions on the conversion of char-N. This study examines the effect of in situ volatile–char interactions on the formation of HCN and NH3 during the low temperature (500 °C) gasification of Loy Yang brown coal in oxygen. Two novel reactor systems were used. The reactor configurations allowed the quantification of HCN and NH3 from char-N gasification, volatile-N oxidation and volatile–char interactions separately. Our results indicate that volatile–char interactions can have drastic effects on coal-N conversion during gasification by providing an important source of the radicals for the formation of HCN and NH3 from char-N during gasification in 4% or 8% O2 at 500 °C. In the presence of radicals and O2, N-containing structures in the nascent char can be easily broken down to give HCN and NH3 during the gasification of the char. In the absence of O2, some of the nascent char-N structures may stabilise into structures less favourable for the formation of HCN and NH3 and more favourable for the formation of other N-containing species such as NOx.

Formation of NO x precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH 3 and HCN ☆

Our results indicate that the gas atmosphere surrounding coal/char particles can greatly affect the formation of NH 3 and HCN through its influence on the availability of H radicals. Based on our results, it is believed that the chemisorption of CO 2 on the nascent char surface can consume H radicals or block the access of N-sites by H radicals for the formation of NH 3 and HCN. For the chars whose thermal cracking generates little H radicals, the gasification of char by CO 2 can also generate additional H radicals, enhancing the formation of NH 3. However, even gasification of char in CO 2 at 950 °C does not lead to the formation of HCN. The oxidation of coal with 4% O 2 at low temperatures (400–600 °C) leads to the formation of HCN as well as NH 3 due to the enhanced formation of (H) radicals. The gasification of coal with 15% H 2O drastically enhances the formation of NH 3 due to the greatly enhanced availability of H as an intermediate between the reactions of H 2O and char. These results support our reaction mechanisms proposed previously, emphasising the importance of H on the formation of NH 3 and HCN during pyrolysis, which can also be extended to the conversion of coal-N during gasification.

Effect of alkali and alkaline earth metals on nitrogen release during temperature programmed pyrolysis of coal

Formation of HCN, NH 3, and N 2 during fixed-bed pyrolysis at 10K min −1 has been studied using coal samples after partial demineralization followed by addition of metal hydroxides from aqueous systems. Without additives, NH 3 is the predominant product at ≤ 700°C, showing the two peaks in the formation rate profile, whereas N 2 is the only product at ≥ 800°C. The presence of NaOH, KOH and Ca(OH) 2 promotes considerable NH 3 formation between 450 and 600°C, but in contrast suppresses HCN formation in this region. The Ca shows the largest effect on both the promotion and suppression. It is likely that the NH 3 increased by Ca addition arises partly from HCN, but mainly from secondary reactions of tar-N. These hydroxides affect N 2 formation in quite different manners: the Na decreases the rate between 700 and 950°C, and the K changes it less significantly than the Na, but the Ca remarkably increases the rate in a low temperature region of 550–700°C. These different features are discussed in terms of solid-phase reactions of alkali metal carbonates with char-N and secondary decomposition reactions of tar-N on CaO particles. As a result, total conversion of coal-N to HCN, NH 3 and N 2 up to 1000°C increases in the sequence of Na < none < K < Ca.

Key Factors for Formation of N2 from Low-Rank Coals during Fixed Bed Pyrolysis: Pyrolysis Conditions and Inherent Minerals

Low-rank coals have been pyrolyzed in high-purity He under different conditions with a fixed bed reactor, and factors controlling N2 formation have been examined. N2 is the dominant product at ≥800 °C after almost complete release of tar-N, HCN, and NH3. Heating rate (10-1400 °C/ min) affects N2 slightly. In contrast, conversion of coal-N to N2 increases remarkably with increasing temperature with a corresponding decrease in char-N and reaches 65-70% at 1200 °C. These data lead to a strong, reverse correlation between N2 and char-N, which shows that char-N and/or precursors are the major source of N2. Demineralization removes mainly Fe- and Ca-containing minerals and results in a drastic decrease in conversion to N2. Addition of a nanophase iron catalyst after demineralization promotes N2 formation, whereas Ca(OH)2 has no significant effect on it. Upon pyrolysis, Fe-containing minerals probably in the ion-exchangeable forms are transformed into fine particles of metallic iron, which are responsible for the remarkable formation of N2 from low-rank coals. Catalysis by the mineral-derived iron is discussed in terms of interactions with char-N in the solid phase.

Kinetic Modeling of Fuel-Nitrogen Conversion in One-Dimensional, Pulverized-Coal Flames

Abstract A detailed reaction mechanism for converting fuel nitrogen to nitric oxide and molecular nitrogen in gas flames is used to model nitrogen chemistry in fuel-rich coal-dust/oxidizer flat flames. In the devolatilization zone a simplified pyrolysis model is used and a hydrocarbon oxidation scheme supplement the nitrogen reactions. The predicted distributions of nitrogenous species (HCN, NH3, NO and N2) are compared to time-resolved experimental data obtained for two stoichiometrics and coal-types. The model accounts for the radical removal on particle surfaces, and various heterogeneous reactions for reduction of NO are considered. In the devolatilization flame zone the coal-N conversion mechanisms appear to be augmented by physical and chemical processes occurring in the vicmity of the devolatilizing particles. The current analysis supported by other investigations indicate that processes such as formation of fuel-rich volatile clouds and heterogeneous reduction of NO on surfaces of coal-particles a...