Modeling competitive adsorption of sulfur trioxide from flue gas for in -duct injection of hydrated lime

A technology currently being developed for mercury removal is injection of powdered activated carbon (PAC) into the flue gas. The low temperatures for effective mercury adsorption by PAC is below the dew point temperature for sulfuric acid. Sulfuric acid condenses in the ductwork within the temperature range of 120°–150°C. If left untreated, the sulfuric acid quickly corrodes the steel in the ductwork prior to the PAC injection system. This investigation studied the option of in-duct injection of hydrated lime to adsorb SO3 or H2SO4 upstream of the dew point temperature range.;Thermogravimetric analysis of the adsorption of acid gases on hydrated lime was performed to investigate the effectiveness of corrosion control through in-duct injection. Empirical models for the adsorption of SO3, H2SO4, SO2 and HCl on hydrated lime are determined within the temperature range of 120°C–180°C, gas compositions typical of flue gas, and typical in-duct residence times of 0.5–5 seconds.;Under this condition SO2 adsorption was found to be first order with respect to SO2 and dependent upon temperature and relative humidity. HCl was found to have functional dependencies of relative humidity and temperature, with a reaction order of 0.46. The adsorption rate for HF was found to have functional dependencies on CO2 concentration, HF concentration, temperature and relative humidity. Empirical models of the adsorption rates were derived as follows: rSO2=6.65CSO2Exp- 1809T+15.5HR mmolesec rHCl=0.054C0.46HClExp -469T+1.6HR mmolesec and RHF=0.01C0.18CO2C0. 12HFExp17.4HR mgsec with the terms C, T and HR denoted as concentration, temperature and relative humidity, respectively. Hydrated lime was found to be ineffective at adsorbing SO3 or H2SO4 under the experimental conditions of this study. This may be due to either slow adsorption kinetics or slow diffusion of the H2SO4 aerosols formed during reaction of SO3 with H2O. The reaction kinetics for HF is provided in terms μg of sorbent per second due to lack of prior research into the reaction products of Ca(OH)2 and HF. Recommendations are made for future research to develop more precise reaction kinetics for in-duct SO3 or H2SO4 control technologies through a central composite design approach and modification of the experimental apparatus to minimize possible diffusion limitations for H2SO4 adsorption.