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.
