Assessment of Chlorine−Alkali−Mineral Interactions during Co-Combustion of Coal and Straw

Abstract

During co-combustion of coal and straw, ash deposition and pollutants (HCl, aerosols, etc.) formation is significantly influenced by the behavior of chlorine and alkali metals. On the bases of measurement data, the effect of inherent minerals in blended fuels on the release and retention of chlorine and alkali metals is investigated through the equilibrium analysis tool FACT-Win. Si, Al, Ca, Mg, and S may greatly affect the behavior of Cl, K, and Na, where the other minor elements, such as Fe, Ti, and Mn have not much influence. According to measurement data, very low amounts of chlorine and sulfur are retained in bottom, air preheater, and cyclone ash in a pulverized fuel combustor. But in filter ash, the chlorine and sulfur contents are relatively high and the ratio (K + Na)/(Cl + 2S) is close to 1. According to equilibrium calculations, in hard coal co-combustion with less than 50% straw, most of the potassium is combined with aluminosilicates to form KAlSi2O6(s,s2), etc., which can prevent ash depositing on the furnace surface. Besides the formation of NaAlSi3O8(s2), some sodium is released as NaCl(g) and NaOH(g). In brown coal co-combustion, most of the potassium is released as KCl(g), KOH(g), and K2SO4(g) because aluminum can be combined in the Ca and Mg compounds, such as Ca3Al2O6(s), CaAl2O4(s), and MgAl2O4(s). Increasing the straw fraction reduces KAlSi2O6(s2) and increases K2Si4O9(liq). Most of the sodium is released as NaOH(g), NaCl(g) and Na2SO4(g). Na2SO4(liq) is formed between 1200 and 1400 K for conditions with less than 50% straw. In pure straw combustion, KCl(g) and K2Si4O9(liq) are the main species. During the gas cooling, KCl(g) and KOH(g) may react with SO2(g) and H2O(g) to form K2SO4(g) and then a lot of aerosols are formed. Finally, the possible reactions involving chlorine and alkali metals are analyzed.

A Review of Thermal Co-Conversion of Coal and Biomass/Waste

“…Most of the studies involving co-firing coal and straw have focused on the speciation of alkali metals and the reactions of alkali metal with chlorine to determine the properties of fly ash deposits [284][285][286][287]; although equilibrium models have some limitation (no consideration is given to kinetics, gas phase is assumed ideal, perfect mixing prevail that all elements are available for reaction). Significant amounts of K are released in the gas phase as KCl and KOH [284] as well as K 2 SO 4 [284,[288][289][290] have studied reactions involving the reactions of these species with other species present in the gas phase. Zheng et al [273] observed that the K content in the fly ash is almost the same as that in the fuel ash during combustion and co-combustion involving straw.…”

“…They concluded that the total K in the fly ash can be predicted when the ash composition of the fuel and straw share are known. Na is likely to be released as NaCl, NaOH, NaSO 4 and Na [284] and react with the gas phase compounds. These observations corroborate with the fuel characterization based on the reactive and non-reactive ash-forming matter.…”

Study of ash deposition during coal combustion under oxyfuel conditions

“…At this point the fouling index F i [29,30] is introduced for a first assessment of the fouling behavior of the two coals. The fouling in dex is given in Eq.…”

Behaviour of gaseous chlorine and alkali metals during biomass thermal utilisation

“…have significant influence on the behaviour of chlorine and alkali metals [24][25][26][27][28][29][30][31]. In fact, it may be affected by many factors, such as composition of the fuel, combustion or gasification conditions, and low or high pressure.…”