NO Reduction by Potassium-Containing Coal Briquettes. Effect of Mineral Matter Content and Coal Rank

Abstract

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.

Behavior of different soot combustion catalysts under NOx/O2. Importance of the catalyst–soot contact

“…For the case of the uncatalysed reaction, it can be considered that only O 2 -carbon reaction is occurring, since the activity of NO in carbon oxidation in the absence of NO 2 is low compared to that of O 2 [23,24]. The mechanism of soot oxidation is well-accepted as initiated by the formation of surface oxygen complexes (SOCs) acting as intermediates, according to Mul et al [25] and Tabor et al [26] and then leading to CO and CO 2 , which can be formed from either the thermal decomposition of SOCs or from the reaction of SOCs with oxygen.…”

Potassium-Containing Coal Chars as Catalysts for NOx Reduction in the Presence of Oxygen

“…The parent bituminous coal (A) and the potassium-free char (A7) show the presence of SiO 2 (α-quartz), while the unwashed potassium-rich char (A7uw-25.5) reveals the presence of potassium carbonate (K 2 CO 3 ) and potassium silicate (KHSi 2 O 5 ). Potassium carbonate/silicate formation is favored in potassium-rich samples due to their high ash content (see Table ): interaction between potassium and mineral matter is known to result in silicate formation; − absence of SiO 2 peaks is confirming evidence that this reaction takes place. The presence of carbonate and silicate species may be responsible for the low activity of potassium-rich bituminous coal chars (see Figures and ): potassium carbonate does not decompose in the presence of carbons until at least 600 °C, and, at reaction temperatures ( T < 500 °C), the fraction of potassium present as carbonate is not active.…”

NOx Reduction by Potassium-Containing Coal Briquettes. Effect of NO2 Concentration

“…Samples. The briquetting process to obtain the coal briquettes has been described extensively elsewhere. , Basically, the method consists of mixing a bituminous coal (A3) with potassium-containing humic acid, followed by pressing (1−2 kg/cm 2 ) and pyrolyzing in N 2 for 2 h at 700 °C. The coal briquette is designated by the parent coal used, followed by their potassium content in weight percent K (i.e., sample A3-4.7).…”

“…Recently, a special collection of publications concerning this problem has been assembled in two different journals ( Energy Fuels and Fuel ). Both special issues contain publications devoted to the basic study of NO reduction over potassium-containing carbon, , which shows the peculiarity that is conformed in briquettes. The molded carbons present considerable advantages over conventional powdered or granular carbons …”

“…The present paper studies the NO x −carbon reaction using NO/O 2 atmospheres with the same NO x percentage value but with different NO/NO 2 ratios, with the purpose of linking the study of NO reduction by potassium-containing coal briquettes, performed in the absence of oxygen, − with that performed in the presence of oxygen. Therefore, the aim of this paper is analysis of the NO x −carbon reaction modifying the composition of the reaction mixture (NO + O 2 ) and, as a consequence, both the NO/NO 2 ratio and the NO 2 concentration.…”