Characterization of Fly Ashes from Circulating Fluidized Bed Combustion (CFBC) Boilers Cofiring Coal and Petroleum Coke

Abstract

The chemistry, mineralogy, morphology, and particle size distribution were investigated in fly ashes from the burning of Datong (ShanXi, China) bituminous coal and the cofiring of Mideast high-sulfur petroleum coke (PC) with 30:70 (cal %) and 50:50 (cal %) blends of Datong bituminous coal in two commercial CFBC boilers. With the exception of CaO, the amounts of major oxides in the fly ashes from cofiring PC and coal were close to those of the common coal fly ashes. The PC−coal fly ashes were enriched in Ni, V, and Mo, implying these trace elements were mainly derived from PC. Ni and V, along with several other elements, such as Cr, Cu, Se, Pb, U, Th, and possibly As and Cd, increased in content with a decrease in temperature of the electrostatic precipitator (ESP). The results of chemistry, mineralogy, and morphology studies suggested that the desulfurization rate of the CFBC boilers at current conditions was low, and the PC tends to coarsen the fly ash particles and increase the loss on ignition (LOI) values, making these fly ashes unsuitable for use as a cement additive or a mineral admixture in concrete. Further studies on the combustion status of the CFBC boilers are needed if we want to be able to increase the desulfurization rate and produce high-quality fly ashes for broader and full utilization.

Investigation on the hydration of CFBC fly ash

“…Typical Ca/S molar ratios are often 2-2.5, as a result, there are significant amounts of unreacted CaO and desulphurization product CaSO 4 remaining in the ash [1,2]. Fly ash from a CFBC boiler (CFBC fly ash) is very different in mineralogical composition, chemical composition, and morphology from coal ash from traditional pulverized fuel firing because of many differences in their combustion processes [2][3][4], and therefore, the utilization of CFBC fly ash is restricted in many fields in which the pulverized fly ash often can be used. Many attempts to use CFBC fly ash or bed ash as cement or concrete additive have been reported [2,5,6].…”

“…This means CFBC fly ash can set and harden, similar to Portland cement, after addition of water [3,4,[11][12][13][14]. The self-cementitious strength of CFBC fly ash is affected by its fineness and chemical compositions, in particularly its f-CaO and SO 3 contents.…”

Synthesis of thermostable geopolymer from circulating fluidized bed combustion (CFBC) bottom ashes

“…As a 'young' member of bottom ash family, CBAs not only contain reasonable alumina and silica contents, but also possess a unique thermal history of heating at 800-900 • C, far lower than those of conventional bottom ashes (ca. 1200-1400 • C) [19,20]. Besides, CBAs have a retention time of several hours at the heating temperature, which represents an almost ideal condition needed for thermal activation of kaolinic source materials [8,12].…”

Low-reactive circulating fluidized bed combustion (CFBC) fly ashes as source material for geopolymer synthesis

“…This is possibly due to the low geopolymeric reactivity of CFAs, which results from the much lower amorphous fraction deriving from the lower firing temperature, making CFAs also an unfavorable raw material for direct geopolymerization. Nevertheless, CFAs contain a substantial amount of silica and alumina (Li et al, 2006). Therefore, it is reasonable to deduce that, via certain activation process, CFAs might also be utilized as an alternative source material for geopolymer production.…”