Alkali Sorption in Second-Generation Pressurized Fluidized-Bed Combustion

Abstract

Modern second-generation pressurized fluidized-bed combustion (2nd-Gen. PFBC) combined cycle power systems are developed to increase the efficiency of coal combustion. However, combined cycle power systems require reliable hot gas cleaning. The alkali metals sodium and potassium can lead to severe damage of the gas turbine blades, because of hot corrosion. This investigation was conducted to assess the potential for the reduction of alkali metals from hot gas by different aluminosilicate sorbents, such as silica, bauxite, bentonite, and mullite, under reducing atmospheres at a temperature of 750 °C. Using a flow channel reactor, an alkali chloride-laden gas stream was passed through a bed of aluminosilicate sorbents. Qualitative and quantitative analysis of the hot gas downstream of the sorbent bed was performed using high-pressure mass spectrometry (HPMS). Thus, the influence of different gas atmospheres on the alkali sorption was determined and conclusions were drawn concerning the chemical reactions that were occurring inside the sorbent bed. Moreover, annealing experiments were performed to determine the sorption capacity of the sorbent. Subsequent scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray diffraction (XRD) analyses showed that the phases formed inside the tested sorbent materials. Sorbent capacities for sodium and potassium of up to 110 mg/g have been achieved after 192 h for bentonite. Finally, thermodynamic computations were performed to scale the results of the laboratory experiments up to the conditions prevalent in 2nd-Gen. PFBC systems. The investigations revealed the possibility of reducing the overall alkali concentration in the hot gas under 2nd-Gen. PFBC conditions to values of <50 ppbv through the use of bentonite and activated bauxite. The amount of sorbent needed for a 500 MW el power plant was estimated to be ∼100 kg/h at a hot gas temperature of 700 °C.