Kinetic Study of Hydrated Lime Reaction with HCl

Abstract

Hydrochloride (HCl) is an acidic pollutant present in the flue gas of most municipal or hazardous waste incinerators. Hydrated lime (Ca(OH)2) is often used as a dry sorbent for injection in a spray reactor to remove HCI. However, due to the short residence time encountered, this control method has generally been found to have low conversion efficiencies which results in the high lime usage and generates large amount of fly ash as solid wastes. A fundamental study was carried outto investigate the kinetics of HCl-lime reaction under simulated flue gas conditions in order to better understand the process thereby providing a basis for an optimized lime usage and reduced fly ash production. The initial reaction rate and conversion of three limes were studied using a thermogravimetric analyzer by varying the gas flow rate, temperature (170-400 degrees C), and HCI concentrations (600-1200 mg/m3) as well as the associated particle size and surface area of the limes. The initial lime conversions were found to rely mostly on the residence time, while the ultimate lime conversions were strongly influenced by temperature and the reaction products. CaOHCI was found to be the primary product in most cases, while for one specific lime, CaCl2 was the ultimate conversion product after an extended time period. The true utilization of lime in flue gas cleanup is thus higher when CaOHCl is considered as the final product than those based on CaCl2 as the final product, which has been commonly used in previous studies. The initial reaction was controlled by diffusion of HCl in gas phase and the subsequent reaction by gaseous diffusion through the developing product layer. Increasing the HCI concentration raised the initial rate as well as conversion. However, overloading the lime with excessive HCI caused clogging at its surface and a drop in the ultimate conversion. Limes with smaller particle diameters and higher surface areas were found to be more reactive. The effect of gas-phase mass transfer was minimized when an optimum flow rate was chosen, and in the absence of internal diffusion the reaction was found to be first order with respect to HCI concentration.

HCl absorption by CaO/Ca 3 Al 2 O 6 sorbent from CO 2 capture cycles using calcium looping

“…It has been widely reported that the calcium-based sorbents with larger surface area and pore volume exhibit better HCl absorption behavior [37][38][39]. Gupta and Fan [51] found that sufficient surface area and pore volume benefited the gaseous diffusion and led to the rapid reaction kinetics of CaO-HCl.…”

“…The CO 2 capture capacity of the CaO/Ca 3 Al 2 O 6 sorbent prepared from carbide slag, Al(NO 3 ) 3 ·9H 2 O, and glycerin water solution by the combustion synthesis could achieve 1.8 times as high as that of the carbide slag after 50 cycles due to the more porous structure, larger surface area, and pore volume in the multiple cycles [29]. The calcium-based sorbent with more porous structure, larger surface area, and pore volume could achieve higher HCl absorption capacity [37][38][39]. Therefore, the CaO/Ca 3 Al 2 O 6 sorbent may be used to sequentially capture CO 2 and HCl from the flue gases of the biomass-fired and the RDFs-fired boilers.…”

Influence of CO and H2 Concentrations on Coal Gas Dechlorination by Supported Manganese Oxide Sorbent

“…Popular choices for H 2 S removal include the Ca- (Li et al, 2007), Fe- , Mn- (Bakker et al, 2003;Ko et al, 2005), Zn-based sorbent (Bu et al, 2008), and mixed metal oxide sorbent (Ko et al, 2007;Lou et al, 2009;Chang et al, 2012); those for HCl removal include the Ca- (Yan et al, 2003;Chin et al, 2005), Na-based sorbent (Nunokawa et al, 2008), and industrial catalyst mixed with multiple components (Dou et al, 2005). Due to the fact that a high percentage of the pollution emitted by coal gasification comes from sulfides, research activities focusing on desulfurization have been widely studied; while relatively little attention has been paid to dechlorination.…”

High-Temperature Cleaning for Chlorine-Containing Coal Gas by Supported Manganese Oxide Sorbent

“…Popular candidates for desulfurization include Ca-based sorbent (Li et al, 2007), Fe-based sorbent (Ko et al, 2006), Mn-based sorbent (Bakker et al, 2003;Ko et al, 2005), Zn-based sorbent (Bu et al, 2008), and mixed metal oxide sorbent (Tseng et al, 2008;Lou et al, 2009;Chang et al, 2012). As for dechlorination, absorption by Ca-based sorbents has always been a prevalent choice (Yan et al, 2003;Partanen et al, 2005). Others like Na-based sorbents (Nunokawa et al, 2008) and industrial catalysts mixed with multiple components (Dou et al, 2007) were also developed.…”