“…At 500 °C, potassium is not able to regenerate as an active species and remains as carbonate. For the low-ash briquette (Figure b), the smaller CO 2 peak means a lower fraction of carbonated potassium and, thus, more catalyst in an active form, able to take part in the oxidation−reduction cycle reported in the literature. , 9 TPR molar flow ( n ) profiles for briquettes with high and low ash content: (a) A3‘-3.6; (b) A3-4.7.…”
This paper reports the NO reduction activity of several potassium-containing coal briquettes obtained from different coal precursors (ranging from anthracite to lignite) with the purpose of understanding the effect of coal rank. Also, two fractions of a bituminous coal, with two very distinct ash contents, were selected to determine the influence of the mineral matter content of coals in NO reduction. The catalytic effect of potassium in this reaction was evaluated in a fixedbed flow reactor at atmospheric pressure using two types of experiments: (i) temperature programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. The effect of coal rank is manifested in two ways: larger amounts of potassium incorporated in coal briquettes and higher NO reduction activity as the coal rank precursor decreases, for all of the reaction temperatures tested. Ash contents of the coals (quartz, silicates) seem to have a negative effect in the NO-carbon reaction, acting as a sink that sinters and deactivates the potassium, forming potassium carbonate and silicates, both of them inactive for the process.
“…At 500 °C, potassium is not able to regenerate as an active species and remains as carbonate. For the low-ash briquette (Figure b), the smaller CO 2 peak means a lower fraction of carbonated potassium and, thus, more catalyst in an active form, able to take part in the oxidation−reduction cycle reported in the literature. , 9 TPR molar flow ( n ) profiles for briquettes with high and low ash content: (a) A3‘-3.6; (b) A3-4.7.…”
This paper reports the NO reduction activity of several potassium-containing coal briquettes obtained from different coal precursors (ranging from anthracite to lignite) with the purpose of understanding the effect of coal rank. Also, two fractions of a bituminous coal, with two very distinct ash contents, were selected to determine the influence of the mineral matter content of coals in NO reduction. The catalytic effect of potassium in this reaction was evaluated in a fixedbed flow reactor at atmospheric pressure using two types of experiments: (i) temperature programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. The effect of coal rank is manifested in two ways: larger amounts of potassium incorporated in coal briquettes and higher NO reduction activity as the coal rank precursor decreases, for all of the reaction temperatures tested. Ash contents of the coals (quartz, silicates) seem to have a negative effect in the NO-carbon reaction, acting as a sink that sinters and deactivates the potassium, forming potassium carbonate and silicates, both of them inactive for the process.
“…In general, the presence of oxygen increases the activity of potassium by almost 3 times. Of course, a drawback of this effect is the concurrent increase in carbon burnoff (compare columns 4 and 6); this is not surprising, considering that alkali metals are good catalysts for carbon gasification by oxygen. − Paralleling the different NO x reduction capacities, the various samples show different burnoff levels during reaction. The potassium-loaded bituminous coal char A7w-8.1 and the low-temperature lignite chars have undergone the highest burnoff, while the potassium-loaded char A7w-4.6 and the high-temperature lignite and subbituminous coal chars were the least consumed.…”
The use of inexpensive potassium-containing coal chars for NO x reduction in a fuel-lean (oxygenrich) environment has been investigated. Coals of different rank have been used as char precursors. Two types of experiments were performed to study the kinetics of the NO x /carbon reaction: (i) temperature-programmed reaction in a 15 NO(0.5%)/O 2 (5%)/He mixture and (ii) isothermal reaction at 300 and 350°C. NO x adsorption at room temperature was also carried out using an identical mixture. Two characteristic temperature regimes were identified: (i) At temperatures below 200°C, NO x adsorption on the char surface is the more important phenomenon. (ii) At temperatures above 200°C, the true NO x reduction by carbons starts with N 2 and CO 2 being the main reaction products. A very active potassium-containing coal char has been prepared by using the following char preparation conditions: pyrolysis at 900°C and a KOH/coal ratio of 0.5/1.
“…It is reasonable to assume that this species, due to its stability, does not present catalytic activity. As previously reported in the literature, ,, the catalytic activity of potassium (and also other metals) is based on a redox cycle in which the catalyst is consecutively oxidized by gas species (NO x or O 2 ) and reduced by carbon. The catalytically active species of potassium have postulated to be mainly suboxides with unknown stoichiometry (K x O y ).…”
The influence of SO 2 in the reduction of NO x by different potassium-containing coal pellets with catalyst loading between 7.9 and 16.8% was studied. For comparative purpose, a catalystfree sample was also checked. At several temperatures between 350 and 550 °C, 2-h isothermal reactions were carried out under NO x /O 2 /N 2 and NO x /SO 2 /O 2 /N 2 gas mixtures, and the results revealed that SO 2 inhibits drastically the potassium-catalyzed NO x -carbon reaction. The amounts of SO 2 retained on samples indicate that there is a direct relationship between these quantities and the potassium loading on samples. X-ray fluorescence, Fourier transform infrared-in situ, and X-ray diffraction techniques were used in order to prove the presence of sulfur on samples after the reactions with NO x /SO 2 /O 2 /N 2 and to elucidate the nature of the sulfur species formed. The cause of the loss in activity seems to be related to the SO 2 irreversible chemisorption on the catalyst, originating the formation of potassium-sulfur compounds.
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