Conversion Of fuel-N Research Articles

Nitrogen Migration during Pyrolysis of Raw and Acid Leached Maize Straw

Solid biofuel is considered as a possible substitute for coal in household heat production because of the available and sustainable raw materials, while NOx emissions from its combustion have become a serious problem. Nitrogen-containing compounds in pyrolysis products have important effects on the conversion of fuel-N into NOx-N. Understanding these converting pathways is important for the environmentally friendly use of biomass fuels. The nitrogen migration during pyrolysis of raw and acid leached maize straw at various temperatures was investigated in this study. Thermal gravimetric analysis and X-ray photoelectron spectroscopy were used to investigate the performances of thermal decomposition and pyrolysis products from samples. The main nitrogen functional groups in biomass and biochar products were N-A (amine-N/amide-N/protein-N), pyridine-N, and pyrrole-N, according to the findings. The most common gaseous NOx precursor was NH3, which was produced primarily during the conversion of N-A to pyridine-N and pyrrole-N. The formation of HCN mainly came from the secondary decomposition of heterocyclic-N at high temperatures. Before the pyrolysis temperature increased to 650 °C, more than half of the fuel-N was stored in the biochar. At the same pyrolysis temperature, acid-leached maize straw yielded more gas-N and char-N than the raw biomass. The highest char-N yield of 76.39 wt% was obtained from acid-leached maize straw (AMS) pyrolysis at 350 °C. Low pyrolysis temperature and acid-leaching treatment can help to decrease nitrogen release from stable char structure, providing support for reducing nitrogenous pollutant emissions from straw fuel.

Exploring the sustainable production of ammonia by recycling N and H in biological residues: Evolution of fuel-N during glutamic acid gasification

In this work, the recycling of nitrogen and hydrogen from nitrogen-rich (N-rich) biological residues via autothermal gasification has been proposed as a process suitable for the sustainable production of ammonia (NH3). Two N-rich biological residues, sewage sludge and meat and bone meal, were used and analyzed in this work and glutamic acid was selected as the model compound of the protein-fuel-N in these residues. Glutamic acid gasification experiments were carried out in order to study the effect of temperature (800–900 °C) and steam-to-carbon ratio (0.5–1.0 g g−1) on the conversion of fuel-N into the most typical N-containing gasification products: NH3, molecular nitrogen (N2), hydrogen cyanide (HCN), nitrogen monoxide (NO), tar-N and char-N. Sewage sludge and meat and bone meal were also gasified under selected operating conditions with the main aim of assessing the NH3 production. The most abundant N-containing compounds obtained in the gasification of glutamic acid were NH3 (35–51% over fuel-N) and N2 (45–63% over fuel-N). The highest conversion of fuel-N to NH3–N in the glutamic acid gasification experiments (51%) was obtained at the lowest temperature (800 °C) and the lowest S/C ratio (0.5 g g−1). The increase in the temperature caused a decrease in the yield of NH3, as a consequence of its decomposition into N2. A similar fuel-N distribution was found when sewage sludge and meat and bone meal were gasified, obtaining joint yields of HCN–N, NO–N, tar-N and char-N lower than 5%, and being NH3–N (30–67%) and N2–N (28–68%) the majority products. The yields of NH3–N obtained from glutamic acid (51%), sewage sludge (30%), and meat and bone meal (67%) under the same gasification operating conditions were significantly different. These differences were attributed to the catalytic effect of the metals present in these residues and point to the need to optimize the operating conditions specifically for each residue. In summary, gasification of sewage sludge and meat and bone meal may be able to produce around 10% of the NH3 produced annually in Europe and between 102 and 262 GJ·ton−1 NH3 thanks to the combustion of the syngas generated.

Effect of the Primary Air Ratio on Combustion of the Fuel Preheated in a Self-preheating Burner

ABSTRACT The experiments with λ p (ratio of the primary air flow to the theoretical combustion air) were launched on a bench-scale coal preheating combustion test rig. The effect of λ p on operating characteristics of the self-preheating burner was investigated in detail. Besides, the combustion characteristics of the high temperature preheated fuel and NOX emission were discussed. The results showed that with increasing λ p, the temperatures in the self-preheating burner climbed and the main compositions of the coal gas (excluding N2) were CO, H2, CO2, and CH4. When λ p is 0.35 and 0.43, the conversion ratios of carbon, fixed carbon and volatile matter were both above 50%. The high combustion temperatures at the preheating fuel nozzle exit were caused by the rapid ignition of the coal gas and the recirculation of the high temperature flue gas. The unburned carbon content of the fly ash was slightly increased with the increase of λ p and the combustion efficiencies were all higher than 98%. No significant HCN was detected during the experiments, indicating that NH3 was the major precursor of NOX. On one hand, with the increase of λ p, more fuel-N was released during the preheating process, resulting in more fuel-N conversion to N2 and NH3. On the other hand, increasing λ p could elevate the combustion temperatures in the self-preheating burner, which would affect the heterogeneous reduction of NOX on the char surface in the DFC. Therefore, the exit NOX emission was the lowest for λ p was 0.35.

Gaseous emissions during oxy-fuel combustion of sewage sludge in a circulating fluidized bed

Emissions of NO, N2O, NO2, SO2 and CO were measured during oxy-fuel combustion of sewage sludge and a reference fuel (bituminous coal) in a 12 kW bench-scale CFB combustor. Combustion tests were conducted at temperature of 850 °C in air (base case) and O2 + CO2 mixtures with oxygen concentrations 21, 30 and 40%vol. The effects of oxygen concentration in oxidizing atmosphere and fuel type on gaseous emissions are evaluated. Instantaneous and average emissions of NO, N2O and SO2 for the combustion of sewage sludge in all atmospheres were much higher than those for the combustion of reference coal. As the oxygen concentration in O2/CO2 atmosphere increases, NO and SO2 emissions increase while N2O and CO concentrations decrease. The fractional conversions of fuel-N to NOx and N2O are calculated and compared for both fuels. Despite the high content of nitrogen in sewage sludge, the total conversion ratios of the fuel N to NOx and N2O for sewage sludge were much lower than those for coal.

Nitrogen conversion during the rapid pyrolysis of raw/torrefied wheat straw

Nitrogen functionalities play an important role on the conversion of fuel-N during pyrolysis of raw and torrefied biomass while the influences haven’t been clarified yet. The conversion of fuel-N during the rapid pyrolysis of wheat straw (WS) and different torrefied WS was investigated using a fixed-bed reactor at 600–1000 °C and the change of fuel characteristics and nitrogen-containing functional groups in samples were analyzed by TG and XPS. Results showed that the N content of samples increased gradually with torrefaction temperature. The main N functionality of WS was amide-N (N-A), while part of the N-A was converted to pyrrole-N (N-5), pyridine-N (N-6), and quaternary-N (N-Q) during torrefaction. CO2 and O2 can inhibit the transformation of N-A to N-5 and promote the transformation of N-A to N-6. During WS pyrolysis, NH3 was the main NOx precursors at 600–700 °C, while HCN became the main nitrogen-containing gas species at 800–1000 °C and the conversion of (NH3 + HCN)-N was 18.15%–32.0%. After torrefaction, the conversion of NH3-N decreased in varying degrees, especially at low pyrolysis temperature (600–700 °C). The conversion of HCN-N increased with temperature during torrefied WS pyrolysis which was similar to that of WS, but the conversion decreased greatly at 800–1000 °C. Torrefaction atmosphere has a substantial effect on the conversion of fuel-N. In particular, WS torrefied in CO2 of O2 atmosphere can promote the conversion of fuel-N to harmless N2 during pyrolysis.

Empirical Formula to Predict the NOx Emissions from Coal Power Plant using Lab-Scale and Real-Scale Operating Data

The use of fossil fuels has drastically increased throughout the world as the demand for energy increases. Accordingly, it has become critical that we reduce the oxides of nitrogen (NOx) and oxides of sulfur pollutants. Therefore, studies related to these activities have increased. This study was aimed at helping take pre-emptive action on NOx emissions by developing a formula that would predict NOx generation using factors related to the combustion characteristics and basic material properties of coal. In this study, the experiments were conducted using a drop tube furnace, and the correlation between coal’s major characteristics and NOx generation was analyzed and measured. Our results showed that the major factors affecting NOx generation are moisture, fixed carbon, and fuel ratio. Moisture tended to decrease NOx generation by delaying the ignition of coal and fixed carbon exhibited a tendency to be directly proportional to NOx generation. The R2 value for NOx of moisture and fixed carbon were derived as 0.7659 and 0.7063, respectively. Our results also showed that the fuel ratio had an exponential relation with the conversion of fuel-N to NOx. Based on the results of our analyses, we used moisture, fixed carbon, and fuel ratio as the major factors for creating an experimental formula. Through these results, we confirmed that the prediction formula reflects the actual amount of NOx emitted from the powerplants.

Experimental investigation of combustion characteristics and NOx formation of coal particles using laser induced breakdown spectroscopy

To study the mechanism of coal combustion and NOx formation, the combustion of coal particles in different atmospheres (O2/N2, O2/CO2) with different O2 concentrations was investigated using the CO2 laser as a heat source. The spatial distribution of atoms and groups (e.g., H at 656.2 nm, O at 777.3 nm, CN at 388.3 nm) relating to the combustion flame were measured simultaneously using laser induced breakdown spectroscopy (LIBS). The residual energy was measured during the collection of LIBS spectra in the combustion process, which could be characterized the temperature profiles of combustion flame due to the positive correlation with temperature. The combustion stage could be clearly discriminated by the emission of H and CN, along with the flame temperature. The residual energy obtained in different atmospheres indicated that the impact of combustion atmosphere on flame temperature is greater in the char combustion stage rather than volatile combustion stage. It was determined from the temporal and spatial distribution of residual energy and CN intensity that a higher flame temperature leads to a higher concentration of CN. The correlation between the generation of CN and the NOx formation was also investigated to show that the formation approaches of NOx are similar in the O2/CO2 and O2/N2 atmospheres, while the fuel-N conversion paths are different between volatile combustion and char combustion stages. The measurement of temporal and spatial distributions of LIBS spectra with varying flame temperatures is significant in revealing the mechanism of coal-particle combustion and NOx formation.

A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion

Despite the relatively low emissions in fluidized-bed combustion, NOx emission for biomass combustion is still a major concern because of increasingly stricter regulations. To realize NOx emission behavior in fluidized beds comprehensively, the effects of bed temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) are investigated in this study. In particular, three different types of operation are applied in staged combustion to find out the key parameter. The results indicate that NOx emissions increase with both bed temperature and excess oxygen, in which the influence of excess oxygen is greater than the other. Lowering bed temperature by water addition seems to be able to simultaneously reduce NOx emission and agglomerate formation, especially for fuels with high nitrogen content, but the pros and cons should be considered. The results in staged combustion infer that the residence time is much more critical than the stoichiometry in the bed. As for FGR, its impact appears to depend on the type of fuel. The correlation between NOx emission behavior and fuel characteristics is also scrutinized; it is concluded that the fuel-N conversion to NOx is essentially related to some features of fuels.

Conversion mechanism of fuel-N during pyrolysis of biomass wastes

To investigate the conversion of nitrogen bound in biomass fuel (abbreviated to fuel-N) during biomass pyrolysis, wheat straw (WS), rice straw (RS), spent coffee grounds (SCG) and palm kernel cake (PKC) were isothermally pyrolyzed in a horizontal tube reactor at the temperature range of 500–900 °C. The concentrations of light gaseous nitrogen containing species (gas-N) were measured online by a Fourier transform infrared (FTIR) spectroscopy gas analyzer, and the corresponding conversion rates were calculated. Results indicated that the conversion of fuel-N to gas-N as well as the evolution of N-functionalities in the portion of fuel-N that maintained in the char (char-N) were consistent regardless of the original N-functionalities in biomass samples. The conversion of fuel-N was found to be highly reliant on the temperature and fuel-N content. 17–47% of the fuel-N was finally retained in the char during pyrolysis. Therefore, the evolution of the N-functionalities from fuel-N to char-N was analyzed by X-ray photoelectron spectroscopy (XPS). Amide-N (N-A) was confirmed to be the dominant N-functionality in the raw biomass samples, and a small amount of pyrrolic-N (N-5), pyridinic-N (N-6) and quaternary-N (N-Q) were also identified. After pyrolysis, N-A was completely vanished in the char. However, only a very small fraction of N-A was decomposed into NH3 while most of it was preferentially converted to other gas-N (HCN, HNCO and NO) and N-5/N-6. For N-5/N-6, most of them preferred to retain in the char. As the temperature increased, a small amount of N-5/N-6 was converted to more stable N-Q and N-oxides (N-X) structures. Contrary to expected, N-A was also the main contributor to the formation of HCN, while the contribution from N-5/N-6 was less important. Finally, the conversion mechanism of fuel-N was concluded.

Experimental investigation on the conversion of fuel-N to NOx of quasi-particle in flue gas recirculation sintering process

Flue gas recirculation sintering process is a potential technology to decrease fuel consumption and NOx emissions compared with conventional sintering process. In present work, a vertical quartz tube reactor was used to investigate the combustion characteristics and conversion of fuel-N to NOx of quasi-particle. The mass conversion rate of quasi-particle increases with higher temperature. It was found that D1 model is more appropriate than other models to describe quasi-particle combustion process through comparing correlation coefficients calculated by different mechanism models. Effects of temperature, coke size and proportion, circulating flue gas components on the conversion of fuel-N to NOx of quasi-particle were studied. The conversion rate of fuel-N to NOx of quasi-particle increases with higher temperature. With increasing coke size and proportion, the conversion rate of fuel-N to NOx decreases obviously. O2 has a positive impact on the conversion of fuel-N to NOx of quasi-particle. CO could decrease the conversion rate of fuel-N to NOx by reducing NO directly or reacting with char to decrease NOx indirectly. CO2 has an obviously inhibitory effect on the conversion of fuel-N to NOx of quasi-particle because it reacts with char to generate CO. The results were conducive to further understanding the combustion behavior and NOx formation mechanism of quasi-particle during flue gas recirculation sintering.

RDF pyrolysis by TG-FTIR and Py-GC/MS and combustion in a double furnaces reactor

With the increasing depletion of fossil energy, the refuse-derived fuel (RDF) as an unavoidable by-product of human activities has been used as an alternative fuel in the precalciner cement kilns. Since the RDF combustion also brings the problems of NOx pollution, it is quite important to find ways to lower the NOx emission during RDF combustion in the precalciner. The pyrolysis characteristics and products of RDF were studied by TG-FTIR and Py-GC/MS. From TG-FITR and Py-GC/MS tests, various carboxylic acids and alkenes formed with NOx released at the RDF pyrolysis process at 200–550 °C. By simulating the temperature (700 °C, 800 °C and 900 °C) and O2 (12%, 14%, 16%, 18% and 21%) environment of the precalciner using a double furnaces reactor, the combustion processes and NOx formation characteristics of RDF combustion were studied. The results showed that the volatile-N was the dominant reactant source of fuel NOx during RDF combustion. The fuel-N conversion and NOx emission yield showed a continuous decreasing trend with temperature increasing from 700 to 900 °C. The fuel-N conversion and NOx emission yield showed a slight increasing trend with the oxygen concentration increase, and the optimum oxygen concentration for RDF combustion was 14%. In this study, the optimum temperature was 900 °C and oxygen concentration was 14% for de-NOx in the precalciner.

Study on the emission characteristics of nitrogen oxides with coal combustion in pressurized fluidized bed

Nitrogen oxides are one of the most significant pollution sources during coal combustion. This experimental study was conducted in a 15 kWth lab-scale pressurized fluidized bed (inner diameter = 81–100 mm, H = 2100 mm) firing with bituminous coals. The effects of operating parameters, including bed temperature (800 °C–900 °C), operating pressure (0.1–0.4 MPa), excess air level (16%–30%) and flow pattern on NOx and N2O emissions were systematically studied during the tests. During each test the interaction effects of all the operating parameters were properly controlled. The results show that most operating parameters have an opposite effect on NOx and N2O emissions, and the N2O emissions mainly depend on the bed temperature. Increasing the operating pressure can significantly suppress the fuel-N conversion to NOx but enhance its conversion to N2O. With the rise of the excess air level and fluidization number, NOx emissions grow distinctly while N2O emissions remain almost unchanged. Total nitrogen oxide emissions increase with the bed temperature while decrease with the operating pressure.

Advances in reduction of NO and N2O1 emission formation in an oxy-fired fluidized bed boiler

Fossil fuel combustion is one of the major means to meet the mounting global energy demand. However, the increasing NOx and N2O emissions arising from fossil fuel combustion process have hazardous effects. Thus, mitigating these gases is vital to attain a sustainable environment. Interestingly, oxy-fuel combustion in fluidized bed for carbon capture and minimized NOx emissions is strongly sustainable compare to the other approaches. It was assessed that NOx formation and fuel-N conversion have significant limitation under oxy-fluidized bed compared to air mode and the mechanism of NOx formation is still deficient and requires further development. In addition, this review paper discussed the potential of primary measure as low emission process with others supplementary techniques for feasible NOx reduction. The influences of combustion mode, operating parameters, and reduction techniques such as flue gas recirculation, oxygen staging, biomass co-firing, catalyst, influence of fluidized bed design and structure, decoupling combustion and their merges are respectively evaluated. Findings show that significant minimization of NOx emission can be achieved through combination of primary and secondary reduction techniques.

Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Abstract Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O were studied in a tube reactor at low temperature. In O2/N2, NO reaches the maximum value in the devolatilization stage and N2O reaches the maximum value in the char combustion stage. In O2/H2O, both NO and N2O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N2O in O2/N2 are obviously higher than those in O2/H2O, due to the reduction of H2O on NO and N2O. Temperature changes the trade-off between NO and N2O. In O2/N2 and O2/H2O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N2O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at = 30 vol% in O2/N2. In O2/H2O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N2O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N2O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N2O in both atmospheres. This work is to develop efficient ways to understand and control NO and N2O emissions for a clean and sustainable atmosphere.

Conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle under oxy-fuel fluidized bed conditions

The conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle of 7 mm in diameter were investigated in a laboratory-scale flow tube reactor under oxy-fuel fluidized bed (FB) conditions. The method of isothermal thermo-gravimetric analysis (TGA) combing with the coal properties was proposed to distinguish the devolatilization and char combustion stages of coal combustion. The results show that the char combustion stage plays a dominant role in NO and N2O emissions in oxy-fuel FB combustion. Temperature changes the trade-off between NO and N2O during the two stages. With increasing temperature, the conversion ratios of fuel-N to NO during the two stages increase, and the opposite tendencies are observed for N2O. CO2 inhibits the fuel-N conversions to NO during the two stages but promotes those to N2O. Compared with air combustion, the conversion ratios of fuel-N to NO during the two stages are lower in 21%O2/79%CO2, and those to N2O are higher. At <O2> = 21–50% by volume, the conversion ratios of fuel-N to NO during the two stages reach the maximum values at <O2> = 30% by volume, and those to N2O decrease with increasing O2 concentration. H2O suppresses the fuel-N conversions to NO and N2O during the two stages. A higher coal rank has higher total conversion ratios of fuel-N to NO and N2O. Fuel-N, volatile matter, and fixed carbon contents are the important factors on fuel-N conversions to NO and N2O during the two stages. The results benefit the understanding of NO and N2O emission mechanisms during oxy-fuel FB combustion of coal.

Effects of H2O on NO Emission during Oxy-coal Combustion with Wet Recycle

This study investigated the characteristics of NO emissions during oxy-coal combustion with wet-recycle, especially in the presence of high H2O concentrations. The oxy-combustion was carried out using two types of coal, namely Leiyang (LY) anthracite and Zhundong (ZD) bituminous coals, inside a 24 kW drop tube furnace under different O2/CO2/H2O atmospheres. The results showed that the NO conversion increased with decreasing CO2 concentration from 70% to 30% and decreased with increasing H2O concentration from 10% to 40%. Under the experimental conditions employed in this study, the fuel-N conversion for both LY and ZD coals under the oxy-coal wet recycled combustion was lower than that under the oxy-coal dry recycled combustion. The results indicated that H2O and CO2 showed a competitive effect on NO emissions, though both of them have positive effect on NO reduction. In order to investigate the effects of H2O/CO2 on recycled NO, oxy-coal combustion experiments were also performed with the initial additio...

Rapid pyrolysis and CO2 gasification of anthracite at high temperature

Anthracite could be burnt efficiently at high temperature utilizing oxy-coal technology. To clarify the effects of temperature and atmosphere on char porosity characteristics, char morphology, fuel-N conversion, and reducing products release, rapid pyrolysis and CO2 gasification of anthracite was carried out in a high temperature entrained-flow reactor to simulate the condition in a pulverized coal furnace. Developed pore structure was formed in the gasification chars, which could be contributed to charCO2 reaction at high temperatures. More mesopores were formed in internal carbon skeleton and retained against collapse and coalescent for gasification chars than pyrolysis chars. Compared with pyrolysis char, smoother and denser surface was observed in gasification char with the irregular bulges disappeared due to the destruction of external carbon skeleton. Char-N could be oxidized to NO in CO2 atmosphere and then reduced to N2 by (CN) on the char surface. Char-N release was greatly promoted due to gasification reaction along with poly-condensation at high temperature; and the preact release of char-N would result in a larger portion of NOx reduction in the following reduction zone with the oxygen-staging combustion technology compared with that in air-staging combustion. Complementally, homogeneous reduction in NOx emission would play a minor effect for anthracite in oxy-coal combustion because of the deficiency of CH4 and HCN, especially at high temperature.

NO and N2O Emissions during Devolatilization and Char Combustion of a Single Biomass Particle under Oxy-fuel Conditions at Fluidized Bed Temperature

Oxy-fuel fluidized bed combustion is a novel clean-biomass utilization technology. The NO and N2O emissions during oxy-fuel combustion of a single biomass particle at fluidized bed temperature were studied in a flow tube reactor. The method of isothermal thermogravimetric analysis was used to distinguish the devolatilization and char combustion stages of biomass combustion. This work is aimed to study the effects of temperature, CO2 concentration, atmosphere and O2 concentration, H2O vapor addition, and biomass type on the NO and N2O emissions during oxy-fuel combustion of a single biomass particle at fluidized bed temperature. In oxy-fuel combustion, NO is rapidly formed during the devolatilization stage, while N2O is mainly formed during the char combustion stage. In 30% O2/70% CO2 at T = 800 °C, the conversions of fuel-N to NO and N2O are 11.96% and 18.98%, respectively. The conversion of fuel-N to NO reaches the maximum value at T = 800 °C during the devolatilization stage, while it increases with inc...

Experimental study of the NO and N2O emissions during devolatilization and char combustion of a single biomass particle in O2/N2 and O2/H2O under low temperature condition

Oxy-H2O combustion is a novel oxy-fuel combustion technology, different from conventional and oxy-CO2 combustion. NO and N2O emissions during devolatilization and char combustion of a single biomass particle in O2/N2 and O2/H2O were investigated in a tube reactor under different temperatures (700°C, 800°C, and 900°C) and oxygen concentrations (21%, 30%, 40%, and 50% by volume). Single biomass pellets of poplar wood, rice husk, and corn stalk were selected. The method of isothermal thermo-gravimetric analysis was applied to distinguish the two stages of biomass combustion based on its proximate analysis. The presence of H2O vapor can improve the free radicals, H2 and CO concentrations, promoting NO and N2O reduction. The total conversions of fuel-N to NO and N2O in O2/N2 are over 2 times higher than those in O2/H2O. In O2/N2, NO is formed as the sample heated up, while N2O is mainly formed during char combustion stage. However, NO is formed after N2O in O2/H2O. No matter in O2/N2 or in O2/H2O, a higher fuel-N content in biomass results in higher NO and N2O yields during the two stages, whilst the conversions of fuel-N to NO and N2O during the two stages decrease with increasing fuel-N/ash content. In O2/N2, the total conversion of fuel-N to NO reaches the maximum value (14.35%) at T=800°C, whilst the total conversion of fuel-N to NO increases with increasing temperature in O2/H2O. The total conversions of fuel-N to N2O decrease with increasing temperature in O2/N2 and O2/H2O. The influence of oxygen concentration on NO and N2O is similar to that of temperature. In O2/N2, there is a maximum value of the total conversion of fuel-N to NO (14.35%) at 〈O2〉=30%, whilst there is no obvious change of the total conversion of fuel-N to NO with increasing oxygen concentration in O2/H2O. The total conversions of fuel-N to N2O decrease with increasing oxygen concentration in O2/N2 and O2/H2O. The results are beneficial to understand the NO and N2O emission mechanisms and favorable for the NO and N2O control in O2/H2O.

Nitrogen and sulfur conversion during pressurized pyrolysis under CO2 atmosphere in fluidized bed

Pressurized oxy-fuel combustion (POFC) is a promising technology for CO2 capture from coal-fired power plants, offering both high efficiency and a low penalty. However, the high partial pressure of CO2 in a POFC furnace has important impacts on fuel-N and fuel-S conversion during the coal pyrolysis process, and understanding this will help to achieve further control of SOx/NOx. In this study, coal pyrolysis experiments were conducted in a pressurized fluidized bed with the pressure range of 0.1–0.7MPa under N2 and CO2 atmosphere. The gaseous products were monitored by a Fourier transform infrared spectroscopy analyzer (FTIR) and the char residue was characterized by an X-ray photoelectron spectroscopy (XPS) analyzer in order to acquire the species information for S-containing and N-containing compounds. Results show that the enrichment of CO2 in the local atmosphere enhances the fuel-N conversion to HCN in the pyrolysis process, which serves as a favorable precursor to N2O. The generation of HCN and NH3 increase simultaneously with the increase of overall pressure. SO2 concentration in the gaseous product is relatively low, and as the pressure increases, the concentration decreases slightly due to CO reduction of SO2 to COS. Sulfur content in the char decreases as the pressure goes from 0.1MPa to 0.7MPa indicating higher CO2 pressure accelerates the decomposition of sulfur compounds in the coal, which is further confirmed by the XPS results.