Kazutomo Ohtake

Influence of coal type on evolution characteristics of alkali metal compounds in coal combustion

The influence of coal type on the evolution characteristics of alkali metal compounds, especially sodium compounds, on the supposition of pressurized fluidized-bed coal combustion was elucidated experimentally in this study by using a rapidly heated electrical batch reactor. The evolution fraction of sodium was evaluated quantitatively by analyzing the sodium content in the burnt particles. Water and ammonium acetate extractions were carried out to classify the form of sodium compounds in the raw coals, and the ion components in the water-extracted solution were also analyzed by ion chromatography. Additionally, the relation between the existing locations of sodium and other elements at the cross section in the particle of raw coals was analyzed by an energy-dispersive X-ray (EDX) system and was quantified by means of the cross-correlation method between the locations of two elements. The results show that the evolution characteristics of sodium are influenced by the coal composition/structure. The water-soluble sodium was the largest fraction in all of the coals tested. Most of the sodium evolved was classified as water-soluble sodium. From the results of the cation and anion components in the water-extracted solution, the sodium in the coals with sodium chloride as a major sodium compound was evolved more easily than that in other coals. The distributions of sodium, silicon, and aluminum contributed to the evolution characteristics of sodium. The coals with high cross-correlation coefficients between sodium and silicon/aluminum had a low evolution fraction of sodium.

Separation of the basic factors affecting no formation in pulverized coal combustion

Separated influences of various basic factors influencing NO formation behavior in pulverized coal combustion have been derived through experiments in a one-dimensional laminar furnace and the simplified theoretical analysis of flame structural change around coal particles. The effect of particle size on combustion history of coal particles the influences of oxygen-fuel stoichiometric ratio and particle size on NO formation behaviors in the furnace, and the individual separated effects of the stoichiometric ratio, flame temperature and coal properties on the conversion ratio from fuel N to exhaust NO were clarified. When the particle size is large enough, there appears the region in the early stage of combustion process where the flame zone is established away from the particle surface, while for small particles both volatile matter and fixed carbon burn on the particle surface or in the pores of particles through the combustion process. The NO formation rate in the early stage of combustion process increases with the stoichiometric ratio for large particles and decreases with the particle size. The rate is less affected by the stoichiometric ratio for small particles. The conversion ratio from fuel N to NO decreases with the nitrogen content and increases with the volatile matter content under the excess oxygen condition. The conversion ratio to NO increases with the stoichiometric ratio especially for high volatile coals, and temperature has little effect on it except for fuel rich condition for high volatile coals. These results on the conversion ratio could be well explained by the fractions of gas phase N-species converted to NO, HCN and NH 3 from fuel N.

Study on characteristics of self-desulfurization and self-denitrification in biobriquette combustion

The characteristics of self-desulfurization and self-denitrification in biobriquettes were studied experimentally and numerically in this paper. The biobriquette was produced by mixing coal, biomass, desulfurizer, and/or denitrificater under a high-compression pressure condition. The combustion process of biobriquette appears in two stages, namely the volatile combustion stage and the char combustion stage. It was proved that limestone, wasted scallop shell, and calcium hydroxide have effective self-desulfurization capability in biobriquette combustion and that desulfurization mainly happens in the char combustion stage. Comparatively among the three desulfurizers used, calcium hydroxide has the highest desulfurization capability because of its lower calcination temperature, and scallop shell the second because of the larger porosity after calcination. A desulfurization efficiency as high as about 80% can be reached for some kinds of coals using scallop shell as desulfurizer with Ca/S over 3.A modified shrinking-core model was developed to predict the desulfurization efficiency in the char combustion stage, and an approximate agreement was obtained between the predictions and the experiments. It was also found that pulp black liquid, an industrial waste to roll as binder in the biobriquetting process, has both self-denitrification and self-desulfurization capabilities in biobriquette combustion. A denitrification efficiency of about 40% can be obtained by adding the pulp black liquid into biobriquette with about 15% in mass.

Fundamental study on N2O formation/decomposition characteristics by means of low-temperature pulverized coal combustion

N 2 O formation/decomposition characteristics and its mechanisms in pulverized coal combustion, especially at low temperature, are discussed by using a one-dimensional electrically heated laminar drop furnace for various coal types The behavior of nitrogen compounds along the furnace axis is studied by analyzing both the sampled burning particles and combustion gas and by calculating the mass balance of nitrogen. Additionally, the effects of the combustion efficiency and the ratio of fixed carbon to volatile matter content (fuel ratio) on N 2 O formation/decomposition characteristics are elucidated experimentally. As a result, coals that evolve more HCN than NH 3 produce higher N 2 O concentration. In the down-stream region beyond the point where volatile matter combustion is complete, N 2 O concentration increases but NO concentration decreases gradually. Increase of N 2 O in this region may be caused not only by the reactions between NO and carbon in char (NO+char-C→NCO and NCO+NO→N 2 O+CO) but also by the direct heterogeneous reaction between nitrogen in char and NO (char-N+NO→N 2 O). For the coals with low fuel ratio, N 2 O conversion decreases because of high flame temperature surrounding the coal particles caused by strong volatile matter combustion. The decomposition reaction of N 2 O by H radicals produced by the oxidation reaction of CO by OH radicals also contributes near the particle surface. Thereafter, the combustion atmosphere and temperature surrounding the coal particles affects, N 2 O formation/decomposition characteristics. The exit N 2 O concentration increases with decreasing combustion efficiency and decreasing fuel ratio. Data from testing nine different types of coal show that the exit N 2 O concentration has a good correlation with the combustion efficiency and the fuel ratio.

Study on N2O formation/destruction characteristics in coal combustion under a wide temperature range

Characteristics of N 2 O formation/destruction in coal combustion are studied experimentally and theoretically over a wide range of combustion temperature. The combustion experiments are carried out in pulverized (1200–1700 K) and bubbling fluidized bed (1000–1200 K) coal combustors to find the influence of combustion temperature, combustion air ratio and coal type on N 2 O formation. The effect of catalytic reactions by char, CaO and stainless steel particles on the N 2 O destruction reaction are also tested by using a packed bed reactor. In order to get a deeper understanding of the mechanisms of N 2 O formation/destruction, numerical simulations are carried out for homogeneous volatile combustion and for combustion of single coal particles. The controlling reactions of N 2 O formation/destruction are analyzed. The experimental and numerical study found that N 2 O was mainly formed from the volatiles combustion. The low temperature and oxidizing atmosphere favor N 2 O formation. N 2 O concentration increases with decreasing temperature and with increasing air ratio. HCN and NH 3 , evolved as volatile-N species, make important contributions to the formation of N 2 O as well as NO. Especially, HCN contributes more to the formation of N 2 O than NH 3 in the low temperature range. On the other hand, H radicals produced by the oxidation reactions of CO and H 2 promote the destruction of N 2 O. Char, CaO and stainless steel particles also act as N 2 O destruction catalysts.

Gas exchange between the bubble and emulsion phases during bubbling fluidized bed coal combustion elucidated by conditional gas sampling

A method for gas composition measurements in bubble and emulsion phases has been developed and applied to bubbling fluidized bed coal combustion. Resolution of the bubbles from the emulsion phase is based on detection of the time change of the pressure difference between two points along the path of bubbles. Cold model experiments confirm that the conditional sampling performs as expected. The probe isdemonstrated to resolve bubbles from the emulsion phase correctly. In addition, with single CO 2 bubbles formed in the fluidized bed under the minimum fluidization condition by air, gas exchange between the bubble and emulsion phases is characterized by using this sampling system. Gas in a bubble tends to remain in the bubble provided that the ratio of bubble rising velocity to gas velocity in the emulsion phase exceeds a certain value. This sampling method is also applied to coal combustion conditions in order to elucidate the roles ofbubble and emulsion phase in the combustion process. For fluidizing material of smaller particle diameter, the combustion of volatile matter mainly takes place in the bubble phase. NO concentration in the bubble phase becomes lower than that in the emulsion phase, while N 2 O shows the opposite tendency. This might be caused by homogeneous reactions of NO to form N 2 O in the bubble phase.

Reduction of pollutants from diesel exhaust by high pressure gas injection

Abstract In order to reduce the pollutants from diesel exhaust, high-pressure gas was injected during combustion into a modified pre-chamber of an IDI diesel engine at a varied load and gas injection timing. The result shows that high-pressure gas injection can reduce the particulates and NO x simultaneously, with little influence on the engine performance. The combustion process in the pre-chamber was visualized by means of an ultra high-speed video system and image analysis of the pictures obtained. The results of the visualization show that by injecting gas, the flame is first quenched, followed by the reduction of NO x in the pre-chamber by hydrocarbon. Although high-pressure gas injection could have caused a substantial increase in HC as a result of temperature quenching, this seems to be controlled by the consumed portion of HC during the recycling of NO x . Chemical analysis of polycyclic aromatics compounds was carried out for the purpose of establishing its variations with gas injection. It was established that gas injection not only reduces PM by oxidation of solid carbon, but also the PAH component.