Shinji Kambara

Relation 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 NH 3 HCN 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.

NOx Removal Using Ammonia Radicals Prepared by Intermittent Dielectric Barrier Discharge at Atmospheric Pressure

The NOx removal was made at a temperature of 950°C by using ammonia radicals. The radicals were efficiently produced by flowing 1–3 % NH3 in Ar through the dielectric barrier discharge of a one-cycle sinusoidal (OCS)-wave power source. The discharge was intermittently made between coaxial cylindrical electrodes with a space of 1.5 mm at an applied peak-to-peak voltage of 10–35 kV. At the mixing zone in the reaction chamber the spectrum of the NH radical was observed at 336.7 nm. The radicals were introduced to a reaction chamber, and were mixed with 1000 ppm NOx in N2. The NOx reduction increased with increasing the concentration of the NH radical. The dependence of NOx reduction on the duty cycle of OCS voltage was also discussed.

Reduction of nitrogen oxide in N2 by NH3 using intermittent dielectric barrier discharge

NO in N2 gas was removed by injecting ammonia radicals, which were externally generated by flowing the NH3 gas diluted with Ar gas through dielectric barrier discharge with a one-cycle sinusoidal-wave power source. The NO reduction for changes in both the applied voltage and the repetition rate was well correlated with the discharge power, which was proportional to the total discharge time per unit time. There was an optimum NH3 concentration in the narrow concentration window for the energy efficiency of NO reduction at a fixed discharge power. The maximum energy efficiency was obtained at small values of the NH3 concentration and the discharge power. The low NH3 concentration effectively increased the energy efficiency by drastically decreasing the discharge-firing voltage.

Correlation of energy efficiency of NO removal by intermittent DBD radical injection method

Ammonia radicals are produced by a dielectric barrier discharge (DBD) in a chamber separate from the chamber that NO gas flows, and are injected in the NO gas flowing field to decompose NO gas. The power source for generating the DBD is a one-cycle sinusoidal (OCS) waveform so as to easily control the power consumed in the DBD plasma. The fundamental frequency of the OCS power source is 150 kHz. The correlation of the DeNOx characteristics was discussed, where the residence time of the ammonia gas in the radical injector and the power density consumed in the DBD plasma were considered. Their product was called residence energy density (RED). It was confirmed that the parametric data of the DeNOx energy efficiency were clearly correlated by the RED. Currently, the energy efficiency of 250 g/kWh was attained at a NO gas temperature of 600/spl deg/C. In order to obtain a high-energy efficiency in this system, the suppression of the energy consumed in the DBD plasma is effective, and instead, the ammonia flow rate decreases, compensating the accepted energy of the ammonia particles by the residence in the radical injector.

Molar ratio and energy efficiency of DeNO/sub x/ using an intermittent DBD ammonia radical injection system

Ammonia radicals are produced by a dielectric barrier discharge (DBD) in a chamber, called radical injector, which is separate from the chamber that NO gas flows. The radicals are injected into the mixing zone in NO gas flow field to decompose NO gas. The power source for generating the DBD is a one cycle sinusoidal (OCS) waveform so as to easily control the electrical power consumed in the DBD plasma. The fundamental frequency of the OCS power source is 150 kHz. Based on the molar ratio of ammonia particles to NO particles in a unit time, NO removal characteristics were discussed. By increasing the DBD consumed energy, the molar ratio approaches 1 showing the stoichiometric DeNO/sub x/ by NH/sub 2/ radicals. A high energy efficiency is found by reducing the consumed energy in the radical injector, where the molar ratio is higher than 1. In this case, the excess ammonia gas without converting into ammonia radicals in the radical injector is directly injected into the reaction zone, and contributes to decompose the NO gas. Oxygen gases with a concentration from 5% to 15% included in the NO gas significantly contribute to decompose NO gases, which brings a decomposition of NO with a low molar ratio.

Optimum conditions for NO reduction using intermittent dielectric barrier discharge at atmospheric pressure

NO in N2 gas was removed by injecting ammonia radicals, which were externally generated by flowing NH3 gas diluted with Ar gas through a dielectric barrier discharge (DBD) with a one-cycle sinusoidal-wave power source. The discharge was intermittently formed between coaxial cylindrical electrodes at an applied peak-to-peak voltage of 2–15 kV. The generated radicals were introduced in a reaction chamber and reacted with NO. In order to find optimum parameters for NO reduction and energy efficiency, the reaction temperature in the mixing zone, the voltage applied to the gap of the electrodes for DBD generation and its repetition rate, the NO gas concentration, and the ammonia concentration and flow rate were varied. A maximum energy efficiency of 140 g/kWh at a NO reduction of over 99% is obtained at a voltage slightly higher than the discharge firing voltage and a repetition rate of 5 kHz, which corresponds to a duty cycle of 5%. Thus it is found that the use of the intermittent power source is an advantage for obtaining a high energy efficiency of NO reduction.

DeNOx Characteristics Using Two Staged Radical Injection Techniques

Ammonia radical injection using pulsed dielectric barrier discharge (DBD) plasma has been investigated as a means to control NOx emissions from combustors. When DBD plasma-generated radicals (NH2, NH, N, and H) are injected into a flue gas containing nitrogen oxide (NOx), NOx is removed efficiently by chain reactions in the gas phase. However, because the percentage of NOx removal gradually decreases with increasing oxygen concentrations beyond 1% O2, improvement of the DeNOx (removal of nitrogen oxide) characteristics at high O2, concentrations was necessary for commercial combustors. A two-staged injection of the DeNOx agent was developed based on the detailed mechanisms of electron impact reactions and gas phase reactions. A concentration of H radical was observed to play an important role in NOx formation and removal. The effects of applied voltages, oxygen concentrations, and reaction temperatures on NOx removal were investigated under normal and staged injection. NOx removal was improved by approximately 20% using staged injection at O2, concentrations of 1 to 4%.

Leaching Characteristic of Arsenic in Coal Fly Ash

The leaching characteristics of arsenic from six CFA (coal fly ash) samples collected from a large scale power plant in Japan were investigated to evaluate more fully the rate of leaching of arsenic and related factors on determining arsenic leaching from different type of CFAs. The procedure of standard leaching tests according to Environmental Agency of Japan Notifications No. 13 was employed in this work. The results indicate that the leaching fractions of arsenic were low levels below 15%, and it was affected by CaO content in CFA. Leaching test results were compared with solution equilibrium calculation to consider the leaching mechanisms: however, experimental results were significantly lower than the equilibrium calculation results. To elucidate the leaching mechanisms, the leaching rate was investigated by extending the leaching tests for a long-term. The concentration of arsenic in the leachate was increased with time, and equilibrium between the solid phase (ash) and the leaching solution was reached in approximately 120 days. It is found that the constant a has a good relationship with CaO content in CFAs, in which the constant a (indicated leaching rate of arsenic) was decreased with an increase of CaO content in fly ash. Therefore, the value of the rate constant a can be said to be the main factor determining arsenic leaching.

Direct Quantitative Analysis of Arsenic in Coal Fly Ash

A rapid, simple method based on graphite furnace atomic absorption spectrometry is described for the direct determination of arsenic in coal fly ash. Solid samples were directly introduced into the atomizer without preliminary treatment. The direct analysis method was not always free of spectral matrix interference, but the stabilization of arsenic by adding palladium nitrate (chemical modifier) and the optimization of the parameters in the furnace program (temperature, rate of temperature increase, hold time, and argon gas flow) gave good results for the total arsenic determination. The optimal furnace program was determined by analyzing different concentrations of a reference material (NIST1633b), which showed the best linearity for calibration. The optimized parameters for the furnace programs for the ashing and atomization steps were as follows: temperatures of 500–1200 and 2150°C, heating rates of 100 and 500°C s−1, hold times of 90 and 7 s, and medium then maximum and medium argon gas flows, respectively. The calibration plots were linear with a correlation coefficient of 0.9699. This method was validated using arsenic-containing raw coal samples in accordance with the requirements of the mass balance calculation; the distribution rate of As in the fly ashes ranged from 101 to 119%.

Arsenic Leachability of Coal Fly Ashes from Different Types of Coal Fired Power Plants

The leaching behavior of arsenic (As) in coal fly ash collected from two different types of coal fired power plants (unit 1 and unit 2: 600 MWe) has been investigated to understand their behavior during combustion and effect of different boiler types on arsenic leachability. To determine dominant factors on arsenic leaching from coal fly ash, the change of arsenic chemical during coal combustion was predicted from the perspective of thermodynamic equilibrium and leaching test under alkaline condition. It found that, arsenic leaching fractions in unit 1 were higher than that of unit 2, it is associated with the amount of reactive CaO (calcium oxide) containing in coal fly ash from unit 1 was lower than that from unit 2. As2O3(gas) formed in the boiler reacts with CaO in the fly ash to form calcium arsenate Ca3(AsO4)2 which is thermodynamically stable calcium-arsenic compound. Hence the coal fly ash from unit 2 having higher CaO/Ash ratios generates more Ca3(AsO4)2 and has lower As leaching fraction than that from unit 1. CaO/Ash ratios were a promising index to reduce arsenic leachability from fly ash.