Full paperRelation between functional forms of coal nitrogen and NOx emissions from pulverized coal combustion☆
Abstract
NOx emissions during pulverized coal combustion, the thermal decomposition behaviour of fuel-bound nitrogen during rapid pyrolysis and the functional forms of coal nitrogen were investigated to develop the general index estimating NOx levels for coals covering a wide range of rank. NOx levels under excess air and two-stage combustion strongly depended on coal type. The effect of nitrogen content in the parent coal on NOx levels was not a continuous relationship, seemingly because of the yields of volatile nitrogen species which evolve during the early stage of combustion. An improved model of NOx formation was proposed to explain the influence of coal type. The dominant factors for NOx reduction were derived on the basis of the improved model. The volatile nitrogen yield and the ratio strongly affect NOx formation. An NOx index to predict NOx levels was proposed based on the relation between the functional forms of coal nitrogen and the yields of nitrogen-containing species. The index involves the proportion of quaternary, pyrrole- and pyridine-type nitrogen.
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Cited by (154)
Transformation simulation of N-containing functional groups in coal pyrolysis and combustion processes by using ReaxFF
2024, Chemical Engineering ScienceIn order to explore the formation mechanism of NOx precursors and NOx during coal pyrolysis and combustion, four typical N-containing functional groups in coal, including pyridine-N (N-6), pyrrole-N (N-5), protonation-N (N-Q) and oxidized pyridine-N (N-X), were taken for study. Firstly, the thermal reaction processes of N-containing functional groups under different conditions were simulated via ReaxFF, then the transformation processes of N-containing functional groups to NOx precursors were obtained via Ovito, finally the reaction networks of NOx precursors and NOx were built via ReacNetGenerator. According to the study results, we speculated that the transformation process of N-containing functional groups to NOx precursors involved 4 steps, including the ring opening of N-6 and N-5, the shift of N atom to the edge, the shortening of carbon chain and the formation of NOx precursors. Besides, we found that the increasing of temperature greatly promoted the transformation processes of NO to HNO and HO2N.
Due to the high content of volatile nitrogen in sewage sludge, co-combustion of coal and sewage sludge easily leads to high emission of nitric oxides (NOx). To solve this problem, we applied the high-temperature preheating technology to promote the advance conversion of fuel-nitrogen into N2 so as to reduce NOx emission. In this study, a two-stage drop-tube furnace system was employed. The effects of sludge proportion, excess air ratio in the preheating zone (λ1), preheating temperature, and combustion temperature on NO emission and burnout were investigated. Our results showed that with the preheating treatment, increasing the sludge proportion reduced NOx emission and promoted char burnout. NO emission was the lowest when λ1 increased to 0.5. Under this condition, 62% of the fuel nitrogen could be converted to the gaseous species, in which the portion of N2 was 89.7%. By controlling λ1 to 0.5 and sludge proportion to 15%, the maximum NO reduction of 62% was achieved. The emitted NO and unburnt carbon in fly ash were reduced by 48.3% and 43.2%, respectively, when the preheating temperature increased to 1200 °C. The effect of combustion temperature strongly depended on the stoichiometry ratio. In oxygen-deficient regions, NO emission decreased with the increase of temperature, while an opposite trend appeared in oxygen-rich regions. The critical excess air ratio in our experiments was around 0.8.
Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace
2023, Proceedings of the Combustion InstituteUtilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NOx emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NOx emissions by reducing the production of char-NOx and fuel(NH3)-NOx simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NOx emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NOx emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH3/air and the prominent denitration effect of NH3 in the vicinity of the NH3 downstream injection location. In addition, NOx emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NOx emissions is closely associated with the denitration efficiency of the NH3. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO2 reduction without extensive retrofitting works. And the NOx emission can be reduced by producing a locally NH3 flame zone with a high equivalence ratio as well as ensuring adequate residence time.
In order to avoid the formation of NO from the nitrogen-containing pyrolysis components in the main combustion zone under preheating combustion technology, an improved preheating combustion technology was proposed. A two-stage drop-tube furnace was built to study the NO emission and combustion characteristics of high-temperature char (HTC) under the improved preheating combustion technology. The char was first heated to the initial pyrolysis temperature(600℃,800℃ and 1000℃) in the upper furnace under the atmosphere of nitrogen, before being burned in the second furnace. The influence of important operating parameters such as HTC temperature(600℃,800℃ and 1000℃), combustion temperature (1200℃,1300℃ and 1400℃)and excess air ratio (0.6 ∼ 1.4)were analyzed. The results show that raising the HTC temperature helps to reduce NO emissions. The maximum NO reduction efficiency is 21.1 % when the excess air ratio(α) = 1.0, which differs from the reported literature results of room-temperature char (RTC). In HTC, there is no absolute correlation observed between the BET area and NO reduction. The release of C and N is altered by pre-ignition of char at HTC; additionally, the evolution of nitrogen functional groups during the pyrolysis process is more key influencing factors. The stoichiometric ratio has a considerable influence on the effects of combustion temperature on NO emission. When there is a lack of oxygen, NOx emissions decrease as the temperature rises, whereas under oxygen-rich situations the converse is true. With the increase in HTC temperature from 600 °C to 1000 °C, the critical excess air ratio steadily increases from 0.75 to 0.85. The carbon concentration of fly ash can be reduced by increasing both the HTC and the combustion temperature. The largest reduction in carbon content in fly ash is 28.6 % at α = 1.0.
Pressurized O2/H2O combustion is a promising oxy-fuel technology for reducing CO2 emissions while improving the overall efficiency of power plants. However, this process is complex, and the associated char-N conversion are not well understood. In the present work, a pressurized horizontal furnace reactor was used to investigate the effects of different factors on the migration and transformation of char-N. Flue gas analysis and X-ray photoelectron spectroscopy were employed to assess the effects of pressure (over the range of 0.1–1.3 MPa), steam concentration (0%–60%) and residence time (60–240 s) on NO emissions and the transformation of char-N. Pressure was found to affect NO emissions and the lowest relative migration of char-N to NO was at 0.4 MPa. With increases in pressure, the proportions of N-5 and N-X groups increased and decreased, respectively·H2O promoted the migration of char-N to NO and the formation of N-X functional groups. As the reaction time prolonged, the proportion of N-5 groups decreased in the early stage of the reaction, that of N-X groups increased in the late stage and N-6 and N-Q groups remained stable.
Insights into the interaction between NO and char(N) containing different functional forms: Mechanistic, thermodynamic and kinetic studies
2022, Combustion and FlameChar-bound nitrogen [char(N)] is confirmed to be the real intermediate in heterogeneous reduction during coal combustion. Aiming to have a deep insight into the interaction mechanism between NO and char(N) containing different functional forms of nitrogen, a comprehensive theoretical exploration with density functional theory at M06–2X/6–31G(d,p)//def-TZVP level is performed. The detailed electronic descriptions of char(N) surface based on spin density, Mulliken atomic charge and electrostatic potential exhibit the attack sites and electron donating capability during NO chemisorption in the following order: char(N-5) < char(N-X) < char(N-6). Oxygen surface complex, an intermediate for N2O formation and a catalyst for NO reduction, can weaken the connected C
N bond and promote N2 separation. The migration of oxygen atoms to adjacent active sites is thermodynamically conducive to reduction reactions, leading to the preferred pathway of N2 release on char(N-5) and char(N-X) surfaces. Contiguous active sites remaining along char edge are unfavorable for N2O detaching, while beneficial to NO further chemisorption for consecutive reactions, resulting in the great contributions of char(N-5) in reducing NO. The lower energy barriers for NO reduction follow the sequence coincident with the results of electronic property analysis. The kinetically favorable temperatures and activation energies of different char(N) for reducing NO are determined, which is consistent with previous experiments. The theoretical results provide evidences to explain microscopic mechanism of NO-char(N) interaction, making contributions to effectively minimize NOx emissions in the future.
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Presented at ‘Coal Utilization and the Environment’, 18–20 May 1993, Orlando, USA